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This Week in The Journal

2009-07-15

Correction for Jin-A Lee et al., Inhibition of Autophagy Induction Delays Neuronal Cell Loss Caused by Dysfunctional ESCRT-III in Frontotemporal Dementia

2009-07-15

Does Asynchronous Neuronal Activity Average out on a Macroscopic Scale?

Boonstra, T. W., Houweling, S., Muskuslus, M. — 2009-07-15

ON Inputs to the OFF Layer: Bipolar Cells That Break the Stratification Rules of the Retina

Hoshi, H., Liu, W.-L., Massey, S. C., Mills, S. L. — 2009-07-15

The vertebrate retina is a distinctly laminar structure. Functionally, the inner plexiform layer, in which bipolar cells synapse onto amacrine and ganglion cells, is subdivided into two sublaminae. Cells that depolarize at light offset ramify in sublamina a; those that depolarize at light onset ramify in sublamina b. The separation of ON and OFF pathways appears to be a fundamental principle of retinal organization that is reflected throughout the entire visual system. We show three clear exceptions to this rule, in which the axons of calbindin-positive ON cone bipolar cells make ribbon synapses as they pass through the OFF layers with three separate cell types: (1) dopaminergic amacrine cells, (2) intrinsically photosensitive ganglion cells, and (3) bistratified diving ganglion cells. The postsynaptic location of the AMPA receptor GluR4 at these sites suggests that ON bipolar cells can make functional synapses as their axons pass through the OFF layers of the inner plexiform layer. These findings resolve a long-standing question regarding the anomalous ON inputs to dopaminergic amacrine cells and suggest that certain ON bipolar cell axons can break the stratification rules of the inner plexiform layer by providing significant synaptic output before their terminal specializations. These outputs are not only to dopaminergic amacrine cells but also to at least two ON ganglion cell types that have dendrites that arborize in sublamina a.

BMI1 Sustains Human Glioblastoma Multiforme Stem Cell Renewal

Abdouh, M., Facchino, S., Chatoo, W., Balasingam, V., Ferreira, J., Bernier, G. — 2009-07-15

Glioblastoma multiforme (GBM) is one of the most common and aggressive types of brain tumors. In GBM, a subpopulation of CD133-positive cancer initiating cells displays stem cell characteristics. The Polycomb group (PcG) and oncogene BMI1 is part of the Polycomb repressive complex 1 (PRC1) that regulates gene expression by modifying chromatin organization. Here we show that BMI1 is expressed in human GBM tumors and highly enriched in CD133-positive cells. Stable BMI1 knockdown using short hairpin RNA-expressing lentiviruses resulted in inhibition of clonogenic potential in vitro and of brain tumor formation in vivo. Cell biology studies support the notion that BMI1 prevents CD133-positive cell apoptosis and/or differentiation into neurons and astrocytes, depending on the cellular context. Gene expression analyses suggest that BMI1 represses alternate tumor suppressor pathways that attempt to compensate for INK4A/ARF/P53 deletion and PI(3)K/AKT hyperactivity. Inhibition of EZH2, the main component of the PRC2, also impaired GBM tumor growth. Our results reveal that PcG proteins are involved in GBM tumor growth and required to sustain cancer initiating stem cell renewal.

Heritability of Intracortical Inhibition and Facilitation

2009-07-15

The present twin study investigates heritability of motor cortex excitability, measured by the paired pulse transcranial magnetic stimulation technique. Specifically, intracortical facilitation (ICF) and inhibition (ICI) and corticospinal excitability were tested in monozygotic (MZ), dizygotic (DZ), and unrelated pairs (UP). Robust ICF and ICI effects were found, with a higher similarity of MZ than DZ and UP pairs. Heritability estimates (h²) were 0.80 for ICI and 0.92 for ICF. However, corticospinal excitability did not show significant differences between MZ and DZ pairs, whereas both significantly differed from UP. Hence, the study provides—for the first time—a clear evidence of heritable individual differences in motor cortex excitability.

BDNF-Mediated Cerebellar Granule Cell Development Is Impaired in Mice Null for CaMKK2 or CaMKIV

Kokubo, M., Nishio, M., Ribar, T. J., Anderson, K. A., West, A. E., Means, A. R. — 2009-07-15

The Ca²⁺/calmodulin-activated kinases CaMKK2 and CaMKIV are highly expressed in the brain where they play important roles in activating intracellular responses to elevated Ca²⁺. To address the biological functions of Ca²⁺ signaling via these kinases during brain development, we have examined cerebellar development in mice null for CaMKK2 or CaMKIV. Here, we demonstrate that CaMKK2/CaMKIV-dependent phosphorylation of cAMP response element-binding protein (CREB) correlates with Bdnf transcription, which is required for normal development of cerebellar granule cell neurons. We show in vivo and in vitro that the absence of either CaMKK2 or CaMKIV disrupts the ability of developing cerebellar granule cells in the external granule cell layer to cease proliferation and begin migration to the internal granule cell layer. Furthermore, loss of CaMKK2 or CaMKIV results in decreased CREB phosphorylation (pCREB), Bdnf exon I and IV-containing mRNAs, and brain-derived neurotrophic factor (BDNF) protein in cerebellar granule cell neurons. Reexpression of CaMKK2 or CaMKIV in granule cells that lack CaMKK2 or CaMKIV, respectively, restores pCREB and BDNF to wild-type levels and addition of BDNF rescues granule cell migration in vitro. These results reveal a previously undefined role for a CaMKK2/CaMKIV cascade involved in cerebellar granule cell development and show specifically that Ca²⁺-dependent regulation of BDNF through CaMKK2/CaMKIV is required for this process.

Netrin-DCC, Robo-Slit, and Heparan Sulfate Proteoglycans Coordinate Lateral Positioning of Longitudinal Dopaminergic Diencephalospinal Axons

2009-07-15

Longitudinal axons provide connectivity between remote areas of the nervous system. Although the molecular determinants driving commissural pathway formation have been well characterized, mechanisms specifying the formation of longitudinal axon tracts in the vertebrate nervous system are largely unknown. Here, we study axon guidance mechanisms of the longitudinal dopaminergic (DA) diencephalospinal tract. This tract is established by DA neurons located in the ventral diencephalon and is thought to be involved in modulating locomotor activity. Using mutant analysis as well as gain of function and loss of function experiments, we demonstrate that longitudinal DA axons navigate by integrating long-range signaling of midline-derived cues. Repulsive Robo2/Slit signaling keeps longitudinal DA axons away from the midline. In the absence of repulsive Robo2/Slit function, DA axons are attracted toward the midline by DCC (deleted in colorectal cancer)/Netrin1 signaling. Thus, Slit-based repulsion counteracts Netrin-mediated attraction to specify lateral positions of the DA diencephalospinal tract. We further identified heparan sulfate proteglycans as essential modulators of DA diencephalospinal guidance mechanisms. Our findings provide insight into the complexity of positioning far-projecting longitudinal axons and allow us to provide a model for DA diencephalospinal pathfinding. Simultaneous integrations of repulsive and attractive long-range cues from the midline act in a concerted manner to define lateral positions of DA longitudinal axon tracts.

Transcriptome Profiling Reveals TGF-{beta} Signaling Involvement in Epileptogenesis

2009-07-15

Brain injury may result in the development of epilepsy, one of the most common neurological disorders. We previously demonstrated that albumin is critical in the generation of epilepsy after blood–brain barrier (BBB) compromise. Here, we identify TGF-β pathway activation as the underlying mechanism. We demonstrate that direct activation of the TGF-β pathway by TGF-β1 results in epileptiform activity similar to that after exposure to albumin. Coimmunoprecipitation revealed binding of albumin to TGF-β receptor II, and Smad2 phosphorylation confirmed downstream activation of this pathway. Transcriptome profiling demonstrated similar expression patterns after BBB breakdown, albumin, and TGF-β1 exposure, including modulation of genes associated with the TGF-β pathway, early astrocytic activation, inflammation, and reduced inhibitory transmission. Importantly, TGF-β pathway blockers suppressed most albumin-induced transcriptional changes and prevented the generation of epileptiform activity. Our present data identifies the TGF-β pathway as a novel putative epileptogenic signaling cascade and therapeutic target for the prevention of injury-induced epilepsy.

Vestibular Signals in Macaque Extrastriate Visual Cortex Are Functionally Appropriate for Heading Perception

Liu, S., Angelaki, D. E. — 2009-07-15

Visual and vestibular signals converge onto the dorsal medial superior temporal area (MSTd) of the macaque extrastriate visual cortex, which is thought to be involved in multisensory heading perception for spatial navigation. Peripheral otolith information, however, is ambiguous and cannot distinguish linear accelerations experienced during self-motion from those resulting from changes in spatial orientation relative to gravity. Here we show that, unlike peripheral vestibular sensors but similar to lobules 9 and 10 of the cerebellar vermis (nodulus and uvula), MSTd neurons respond selectively to heading and not to changes in orientation relative to gravity. In support of a role in heading perception, MSTd vestibular responses are also dominated by velocity-like temporal dynamics, which might optimize sensory integration with visual motion information. Unlike the cerebellar vermis, however, MSTd neurons also carry a spatial orientation-independent rotation signal from the semicircular canals, which could be useful in compensating for the effects of head rotation on the processing of optic flow. These findings show that vestibular signals in MSTd are appropriately processed to support a functional role in multisensory heading perception.

Regulation of Early Neurite Morphogenesis by the Na+/H+ Exchanger NHE1

Sin, W.-C., Moniz, D. M., Ozog, M. A., Tyler, J. E., Numata, M., Church, J. — 2009-07-15

The ubiquitously expressed Na⁺/H⁺ exchanger NHE1 plays an important role in regulating polarized membrane protrusion and directional motility in non-neuronal cells. Using NGF-differentiated PC12 cells and murine neocortical neurons in vitro, we now show that NHE1 plays a role in regulating early neurite morphogenesis. NHE1 was expressed in growth cones in which it gave rise to an elevated intracellular pH in actively extending neurites. The NHE1 inhibitor cariporide reversibly reduced growth cone filopodia number and the formation and elongation of neurites, especially branches, whereas the transient overexpression of full-length NHE1, but not NHE1 mutants deficient in either ion translocation activity or actin cytoskeletal anchoring, elicited opposite effects. In addition, compared with neocortical neurons obtained from wild-type littermates, neurons isolated from NHE1-null mice exhibited reductions in early neurite outgrowth, an effect that was rescued by overexpression of full-length NHE1 but not NHE1 mutants. Finally, the growth-promoting effects of netrin-1, but not BDNF or IGF-1, were markedly reduced by cariporide in wild-type neocortical neurons and were not observed in NHE1-null neurons. Although netrin-1 failed to increase growth cone intracellular pH or Na⁺/H⁺ exchange activity, netrin-1-induced increases in early neurite outgrowth were restored in NHE1-null neurons transfected with full-length NHE1 but not an ion translocation-deficient mutant. Collectively, the results indicate that NHE1 participates in the regulation of early neurite morphogenesis and identify a novel role for NHE1 in the promotion of early neurite outgrowth by netrin-1.

"Referred Visual Sensations": Rapid Perceptual Elongation after Visual Cortical Deprivation

Dilks, D. D., Baker, C. I., Liu, Y., Kanwisher, N. — 2009-07-15

Visual perceptual distortion (i.e., elongation) has been demonstrated in a single case study after several months of cortical deprivation after a stroke. Here we asked whether similar perceptual elongation can be observed in healthy participants after deprivation and, crucially, how soon after deprivation this elongation occurs. To answer this question, we patched one eye, thus noninvasively and reversibly depriving bottom-up input to the region of primary visual cortex (V1) corresponding to the blind spot (BS) in the unpatched eye, and tested whether and how quickly elongation occurs after the onset of deprivation. Within seconds of eye patching, participants perceived rectangles adjacent to the BS to be elongated toward the BS. We attribute this perceptual elongation to rapid receptive field expansion within the deprived V1 as reported in electrophysiological studies after retinal lesions and refer to it as "referred visual sensations" (RVS). This RVS is too fast to be the result of structural changes in the cortex (e.g., the growth of new connections), instead implicating unmasking of preexisting connections as the underlying neural mechanism. These findings may shed light on other reported perceptual distortions, as well as the phenomena of "filling-in."

Single-Cell and Population Coding of Expected Reward Probability in the Orbitofrontal Cortex of the Rat

2009-07-15

The orbitofrontal cortex (OFC) has been implicated in decision-making under uncertainty, but it is unknown how information about the probability or uncertainty of future reward is coded by single orbitofrontal neurons and ensembles. We recorded neuronal ensembles in rat OFC during an olfactory discrimination task in which different odor stimuli predicted different reward probabilities. Single-unit firing patterns correlated to the expected reward probability primarily within an immobile waiting period before reward delivery but also when the rat executed movements toward the reward site. During these pre-reward periods, a subset of OFC neurons was sensitive to differences in probability but only very rarely discriminated on the basis of reward uncertainty. In the reward period, neurons responded during presentation or omission of reward or during both types of outcome. At the population level, neurons were characterized by a wide divergence in firing-rate variability attributable to expected probability. A population analysis using template matching as reconstruction method indicated that OFC generates a distributed representation of reward probability with a weak dependence on neuronal group size. The analysis furthermore confirmed that predictive information coded by OFC populations was quantitatively related to reward probability, but not to uncertainty.

Differential Dopaminergic Modulation of Neostriatal Synaptic Connections of Striatopallidal Axon Collaterals

Tecuapetla, F., Koos, T., Tepper, J. M., Kabbani, N., Yeckel, M. F. — 2009-07-15

Recent studies have demonstrated that GABAergic synaptic transmission among neostriatal spiny projection neurons (SPNs) is strongly modulated by dopamine with individual connections exhibiting either D₁ receptor (D₁R)-mediated facilitation or D₂ receptor (D₂R)-mediated inhibition and, at least in some preparations, a subset of connections exhibiting both of these effects. In light of the cell type-specific expression of D_1aR in striatonigral and D₂R in striatopallidal neurons and the differential expression of the other D₁ and D₂ family dopamine receptors, we hypothesize that the nature of the dopaminergic modulation is specific to the types of SPNs that participate in the connection. Here the biophysical properties and dopaminergic modulation of intrastriatal connections formed by striatopallidal neurons were examined. Contrary to previous expectation, synapses formed by striatopallidal neurons were biophysically and pharmacologically heterogeneous. Two distinct types of axon collateral connections could be distinguished among striatopallidal neurons. The more common, small-amplitude connections (80%) exhibited mean IPSC amplitudes several times smaller than their less frequent large-amplitude counterparts, principally because of a smaller number of release sites involved. The two types of connections were also differentially regulated by dopamine. Small-amplitude connections exhibited strong and exclusively D₂R-mediated presynaptic inhibition, whereas large-amplitude connections were unresponsive to dopamine. Synaptic connections from striatopallidal to striatonigral neurons exhibited exclusively D₂R-mediated presynaptic inhibition that was similar to the regulation of small-amplitude connections between pairs of striatopallidal cells. Together, these findings demonstrate a previously unrecognized complexity in the organization and dopaminergic control of synaptic communication among SPNs.

Neocortical Disynaptic Inhibition Requires Somatodendritic Integration in Interneurons

Hull, C., Adesnik, H., Scanziani, M. — 2009-07-15

In his theory of functional polarity, Ramon y Cajal first identified the soma and dendrites as the principal recipient compartments of a neuron and the axon as its main output structure. Despite notable exceptions in other parts of the nervous system (Schoppa and Urban, 2003; Wässle, 2004; Howard et al., 2005), this route of signal propagation has been shown to underlie the functional properties of most neocortical circuits studied so far. Recent evidence, however, suggests that neocortical excitatory cells may trigger the release of the inhibitory neurotransmitter GABA by directly depolarizing the axon terminals of inhibitory interneurons, thus bypassing their somatodendritic compartments (Ren et al., 2007). By using a combination of optical and electrophysiological approaches, we find that synaptically released glutamate fails to trigger GABA release through a direct action on GABAergic terminals under physiological conditions. Rather, our evidence suggests that glutamate triggers GABA release only after somatodendritic depolarization and action potential generation at GABAergic interneurons. These data indicate that neocortical inhibition is recruited by classical somatodendritic integration rather than direct activation of interneuron axon terminals.

Responses to Static Visual Images in Macaque Lateral Geniculate Nucleus: Implications for Adaptation, Negative Afterimages, and Visual Fading

McLelland, D., Ahmed, B., Bair, W. — 2009-07-15

Adaptation to static scenes is a familiar and fundamental aspect of visual perception that causes negative afterimages, fading, and many other visual illusions. To establish a foundation for understanding the neuronal bases of such phenomena and to constrain the contributions of retinal versus cortical processing, we studied the responses of neurons in the dorsal lateral geniculate nucleus during and after the presentation of prolonged static visual stimuli. We found that parvocellular (P) cells (the more numerous and color-sensitive pathway) showed response adaptation with a time constant on the order of tens of seconds and that their response after the removal of a visual stimulus lasting 1 min was similar in amplitude and time course to the response evoked by the photographic negative stimulus. Magnocellular (M) cells (the faster-conducting and achromatic pathway) had after responses that were substantially weaker than responses evoked by patterned visual stimuli. This difference points to the existence of an adaptive mechanism in the P-pathway that is absent or impaired in the M-pathway and is inconsistent with full adaptation of photoreceptors, which feed both pathways. Cells in both pathways often maintained a substantial tonic response throughout 1 min stimuli, suggesting that these major feedforward inputs to cortex adapt too slowly to account for visual fading. Our findings suggest that faster-adapting mechanisms in cortex are likely to be required to account for the dynamics of perception during and after the viewing of prolonged static images.

Mitochondria Are the Source of Hydrogen Peroxide for Dynamic Brain-Cell Signaling

2009-07-15

Hydrogen peroxide (H₂O₂) is emerging as a ubiquitous small-molecule messenger in biology, particularly in the brain, but underlying mechanisms of peroxide signaling remain an open frontier for study. For example, dynamic dopamine transmission in dorsolateral striatum is regulated on a subsecond timescale by glutamate via H₂O₂ signaling, which activates ATP-sensitive potassium (K_ATP) channels to inhibit dopamine release. However, the origin of this modulatory H₂O₂ has been elusive. Here we addressed three possible sources of H₂O₂ produced for rapid neuronal signaling in striatum: mitochondrial respiration, monoamine oxidase (MAO), and NADPH oxidase (Nox). Evoked dopamine release in guinea-pig striatal slices was monitored with carbon-fiber microelectrodes and fast-scan cyclic voltammetry. Using direct fluorescence imaging of H₂O₂ and tissue analysis of ATP, we found that coapplication of rotenone (50 nm), a mitochondrial complex I inhibitor, and succinate (5 mm), a complex II substrate, limited H₂O₂ production, but maintained tissue ATP content. Strikingly, coapplication of rotenone and succinate also prevented glutamate-dependent regulation of dopamine release, implicating mitochondrial H₂O₂ in release modulation. In contrast, inhibitors of MAO or Nox had no effect on dopamine release, suggesting a limited role for these metabolic enzymes in rapid H₂O₂ production in the striatum. These data provide the first demonstration that respiring mitochondria are the primary source of H₂O₂ generation for dynamic neuronal signaling.

Three Patterns of Oscillatory Activity Differentially Synchronize Developing Neocortical Networks In Vivo

Yang, J.-W., Hanganu-Opatz, I. L., Sun, J.-J., Luhmann, H. J. — 2009-07-15

Coordinated patterns of electrical activity are important for the early development of sensory systems. The spatiotemporal dynamics of these early activity patterns and the role of the peripheral sensory input for their generation are essentially unknown. We performed extracellular multielectrode recordings in the somatosensory cortex of postnatal day 0 to 7 rats in vivo and observed three distinct patterns of synchronized oscillatory activity. (1) Spontaneous and periphery-driven spindle bursts of 1–2 s in duration and ~10 Hz in frequency occurred approximately every 10 s. (2) Spontaneous and sensory-driven gamma oscillations of 150–300 ms duration and 30–40 Hz in frequency occurred every 10–30 s. (3) Long oscillations appeared only every ~20 min and revealed the largest amplitude (250–750 µV) and longest duration (>40 s). These three distinct patterns of early oscillatory activity differently synchronized the neonatal cortical network. Whereas spindle bursts and gamma oscillations did not propagate and synchronized a local neuronal network of 200–400 µm in diameter, long oscillations propagated with 25–30 µm/s and synchronized 600–800 µm large ensembles. All three activity patterns were triggered by sensory activation. Single electrical stimulation of the whisker pad or tactile whisker activation elicited neocortical spindle bursts and gamma activity. Long oscillations could be only evoked by repetitive sensory stimulation. The neonatal oscillatory patterns in vivo depended on NMDA receptor-mediated synaptic transmission and gap junctional coupling. Whereas spindle bursts and gamma oscillations may represent an early functional columnar-like pattern, long oscillations may serve as a propagating activation signal consolidating these immature neuronal networks.

NR2A at CA1 Synapses Is Obligatory for the Susceptibility of Hippocampal Plasticity to Sleep Loss

Longordo, F., Kopp, C., Mishina, M., Lujan, R., Luthi, A. — 2009-07-15

A loss in the necessary amount of sleep alters expression of genes and proteins implicated in brain plasticity, but key proteins that render neuronal circuits sensitive to sleep disturbance are unknown. We show that mild (4–6 h) sleep deprivation (SD) selectively augmented the number of NR2A subunits of NMDA receptors on postsynaptic densities of adult mouse CA1 synapses. The greater synaptic NR2A content facilitated induction of CA3-CA1 long-term depression in the theta frequency stimulation range and augmented the synaptic modification threshold. NR2A-knock-out mice maintained behavioral response to SD, including compensatory increase in post-deprivation resting time, but hippocampal synaptic plasticity was insensitive to sleep loss. After SD, the balance between synaptically activated and slowly recruited NMDA receptor pools during temporal summation was disrupted. Together, these results indicate that NR2A is obligatory for the consequences of sleep loss on hippocampal synaptic plasticity. These findings could advance pharmacological strategies aiming to sustain hippocampal function during sleep restriction.

Reduction of the Rate of Protein Translation in Patients with Myotonic Dystrophy 2

2009-07-15

Myotonic dystrophy 2 (DM2) is an autosomal dominant, multisystem disease, which primarily affects skeletal muscle. DM2 is caused by CCTGn expansion in the intron 1 of the ZNF9 gene. Expression of the mutant CCUGn RNA changes RNA processing in patients with DM2; however, the role of ZNF9 protein in DM2 pathology has been not elucidated. ZNF9 has been shown to regulate cap-dependent and cap-independent translation. We have examined a possible role of ZNF9 in the regulation of translation in DM2 patients. We found that ZNF9 interacts with the 5' UTRs of terminal oligopyrimidine (TOP) tract mRNAs encoding human ribosomal protein, RPS17, poly(A)-binding protein 1 (PABP1), and the elongation factors, eEF1A and eEF2. The binding activity of ZNF9 toward these TOP-containing 5' UTRs is reduced in DM2 muscle. Consistent with the reduction of this activity, the levels of RPS17, PABP, eEF1A, and eEF2 proteins are also diminished in DM2 muscle. The reduction of ZNF9 RNA-binding activity in DM2 correlates with a decrease of ZNF9 protein levels in cytoplasm of DM2 muscle cells. We found that the reduction of ZNF9 is caused by expression of the mutant CCUG repeats. This decrease of proteins of translational apparatus in DM2 correlates with a reduction of a rate of protein synthesis in myoblasts from DM2 patients. We found that the ectopic expression of ZNF9 in DM2 myoblasts corrects rate of protein synthesis, suggesting that the alterations in CCUG-ZNF9-TOP mRNAs pathway are responsible for the reduction of the rate of protein translation in DM2 muscle cells.

The Foveal Confluence in Human Visual Cortex

Schira, M. M., Tyler, C. W., Breakspear, M., Spehar, B. — 2009-07-15

The human visual system devotes a significant proportion of its resources to a very small part of the visual field, the fovea. Foveal vision is crucial for natural behavior and many tasks in daily life such as reading or fine motor control. Despite its significant size, this part of cortex is rarely investigated and the limited data have resulted in competing models of the layout of the foveal confluence in primate species. Specifically, how V2 and V3 converge at the central fovea is the subject of debate in primates and has remained "terra incognita" in humans. Using high-resolution fMRI (1.2 x 1.2 x 1.2 mm³) and carefully designed visual stimuli, we sought to accurately map the human foveal confluence and hence disambiguate the competing theories. We find that V1, V2, and V3 are separable right into the center of the foveal confluence, and V1 ends as a rounded wedge with an affine mapping of the foveal singularity. The adjacent V2 and, in contrast to current concepts from macaque monkey, also V3 maps form continuous bands (~5 mm wide) around the tip of V1. This mapping results in a highly anisotropic representation of the visual field in these areas. Unexpectedly, for the centermost 0.75°, the cortical representations for both V2 and V3 are larger than that of V1, indicating that more neuronal processing power is dedicated to second-level analysis in this small but important part of the visual field.

Proactive Inhibitory Control and Attractor Dynamics in Countermanding Action: A Spiking Neural Circuit Model

Lo, C.-C., Boucher, L., Pare, M., Schall, J. D., Wang, X.-J. — 2009-07-15

Flexible behavior depends on the brain's ability to suppress a habitual response or to cancel a planned movement whenever needed. Such inhibitory control has been studied using the countermanding paradigm in which subjects are required to withhold an imminent movement when a stop signal appears infrequently in a fraction of trials. To elucidate the circuit mechanism of inhibitory control of action, we developed a recurrent network model consisting of spiking movement (GO) neurons and fixation (STOP) neurons, based on neurophysiological observations in the frontal eye field and superior colliculus of behaving monkeys. The model places a premium on the network dynamics before the onset of a stop signal, especially the experimentally observed high baseline activity of fixation neurons, which is assumed to be modulated by a persistent top-down control signal, and their synaptic interaction with movement neurons. The model simulated observed neural activity and fit behavioral performance quantitatively. In contrast to a race model in which the STOP process is initiated at the onset of a stop signal, in our model whether a movement will eventually be canceled is determined largely by the proactive top-down control and the stochastic network dynamics, even before the appearance of the stop signal. A prediction about the correlation between the fixation neural activity and the behavioral outcome was verified in the neurophysiological data recorded from behaving monkeys. The proposed mechanism for adjusting control through tonically active neurons that inhibit movement-producing neurons has significant implications for exploring the basis of impulsivity associated with psychiatric disorders.

Enhanced Visual Motion Perception in Major Depressive Disorder

2009-07-15

Major depressive disorder (MDD) is a mood disorder that is not traditionally considered to affect the visual system. However, recent findings have reported decreased cortical levels of the inhibitory neurotransmitter GABA in occipital cortex. To explore possible functional consequences of MDD on visual processing, we applied a psychophysical visual motion processing task in which healthy young adults typically exhibit impaired perceptual discrimination of large high-contrast stimuli. It has been suggested that this phenomenon, spatial suppression, is mediated by GABAergic center–surround antagonism in visual pathways. Based on previous findings linking MDD to occipital GABA dysfunction, we hypothesized that MDD patients would exhibit decreased spatial suppression, leading to the counterintuitive hypothesis of better psychophysical performance. Indeed, motion perception for typically suppressed stimuli was enhanced in patients with MDD compared with age-matched controls. Furthermore, the degree of spatial suppression correlated with an individual's illness load; patients with greater lifetime duration of depression exhibited the least spatial suppression and performed the best in the high-contrast motion discrimination task. Notably, this decrease in spatial suppression persisted beyond recovery and without the confound of acute illness or treatment; all patients had been clinically recovered and unmedicated for several months at the time of testing, suggesting that depression has ubiquitous consequences that may persist long after mood symptoms have receded. This finding raises the possibility that spatial suppression may represent a sensitive endophenotypic marker of trait vulnerability in MDD.

{beta}-Amyloid Oligomers Induce Phosphorylation of Tau and Inactivation of Insulin Receptor Substrate via c-Jun N-Terminal Kinase Signaling: Suppression by Omega-3 Fatty Acids and Curcumin

2009-07-15

Both insulin resistance (type II diabetes) and β-amyloid (Aβ) oligomers are implicated in Alzheimer's disease (AD). Here, we investigate the role of Aβ oligomer-induced c-Jun N-terminal kinase (JNK) activation leading to phosphorylation and degradation of the adaptor protein insulin receptor substrate-1 (IRS-1). IRS-1 couples insulin and other trophic factor receptors to downstream kinases and neuroprotective signaling. Increased phospho-IRS-1 is found in AD brain and insulin-resistant tissues from diabetics. Here, we report Aβ oligomers significantly increased active JNK and phosphorylation of IRS-1 (Ser616) and tau (Ser422) in cultured hippocampal neurons, whereas JNK inhibition blocked these responses. The omega-3 fatty acid docosahexaenoic acid (DHA) similarly inhibited JNK and the phosphorylation of IRS-1 and tau in cultured hippocampal neurons. Feeding 3xTg-AD transgenic mice a diet high in saturated and omega-6 fat increased active JNK and phosphorylated IRS-1 and tau. Treatment of the 3xTg-AD mice on high-fat diet with fish oil or curcumin or a combination of both for 4 months reduced phosphorylated JNK, IRS-1, and tau and prevented the degradation of total IRS-1. This was accompanied by improvement in Y-maze performance. Mice fed with fish oil and curcumin for 1 month had more significant effects on Y-maze, and the combination showed more significant inhibition of JNK, IRS-1, and tau phosphorylation. These data indicate JNK mediates Aβ oligomer inactivation of IRS-1 and phospho-tau pathology and that dietary treatment with fish oil/DHA, curcumin, or a combination of both has the potential to improve insulin/trophic signaling and cognitive deficits in AD.

Impaired Balance of Mitochondrial Fission and Fusion in Alzheimer's Disease

Wang, X., Su, B., Lee, H.-g., Li, X., Perry, G., Smith, M. A., Zhu, X. — 2009-07-15

Mitochondrial dysfunction is a prominent feature of Alzheimer's disease (AD) neurons. In this study, we explored the involvement of an abnormal mitochondrial dynamics by investigating the changes in the expression of mitochondrial fission and fusion proteins in AD brain and the potential cause and consequence of these changes in neuronal cells. We found that mitochondria were redistributed away from axons in the pyramidal neurons of AD brain. Immunoblot analysis revealed that levels of DLP1 (also referred to as Drp1), OPA1, Mfn1, and Mfn2 were significantly reduced whereas levels of Fis1 were significantly increased in AD. Despite their differential effects on mitochondrial morphology, manipulations of these mitochondrial fission and fusion proteins in neuronal cells to mimic their expressional changes in AD caused a similar abnormal mitochondrial distribution pattern, such that mitochondrial density was reduced in the cell periphery of M17 cells or neuronal process of primary neurons and correlated with reduced spine density in the neurite. Interestingly, oligomeric amyloid-β-derived diffusible ligands (ADDLs) caused mitochondrial fragmentation and reduced mitochondrial density in neuronal processes. More importantly, ADDL-induced synaptic change (i.e., loss of dendritic spine and postsynaptic density protein 95 puncta) correlated with abnormal mitochondrial distribution. DLP1 overexpression, likely through repopulation of neuronal processes with mitochondria, prevented ADDL-induced synaptic loss, suggesting that abnormal mitochondrial dynamics plays an important role in ADDL-induced synaptic abnormalities. Based on these findings, we suggest that an altered balance in mitochondrial fission and fusion is likely an important mechanism leading to mitochondrial and neuronal dysfunction in AD brain.

Loss of Hsp70 Exacerbates Pathogenesis But Not Levels of Fibrillar Aggregates in a Mouse Model of Huntington's Disease

2009-07-15

Endogenous protein quality control machinery has long been suspected of influencing the onset and progression of neurodegenerative diseases characterized by accumulation of misfolded proteins. Huntington's disease (HD) is a fatal neurodegenerative disorder caused by an expansion of a polyglutamine (polyQ) tract in the protein huntingtin (htt), which leads to its aggregation and accumulation in inclusion bodies. Here, we demonstrate in a mouse model of HD that deletion of the molecular chaperones Hsp70.1 and Hsp70.3 significantly exacerbated numerous physical, behavioral and neuropathological outcome measures, including survival, body weight, tremor, limb clasping and open field activities. Deletion of Hsp70.1 and Hsp70.3 significantly increased the size of inclusion bodies formed by mutant htt exon 1, but surprisingly did not affect the levels of fibrillar aggregates. Moreover, the lack of Hsp70s significantly decreased levels of the calcium regulated protein c-Fos, a marker for neuronal activity. In contrast, deletion of Hsp70s did not accelerate disease in a mouse model of infectious prion-mediated neurodegeneration, ruling out the possibility that the Hsp70.1/70.3 mice are nonspecifically sensitized to all protein misfolding disorders. Thus, endogenous Hsp70s are a critical component of the cellular defense against the toxic effects of misfolded htt protein in neurons, but buffer toxicity by mechanisms independent of the deposition of fibrillar aggregates.

Modulation of Cerebellar Excitability by Polarity-Specific Noninvasive Direct Current Stimulation

Galea, J. M., Jayaram, G., Ajagbe, L., Celnik, P. — 2009-07-15

The cerebellum is a crucial structure involved in movement control and cognitive processing. Noninvasive stimulation of the cerebellum results in neurophysiological and behavioral changes, an effect that has been attributed to modulation of cerebello–brain connectivity. At rest, the cerebellum exerts an overall inhibitory tone over the primary motor cortex (M1), cerebello–brain inhibition (CBI), likely through dentate–thalamo–cortical connections. The level of excitability of this pathway before and after stimulation of the cerebellum, however, has not been directly investigated. In this study, we used transcranial magnetic stimulation to determine changes in M1, brainstem, and CBI before and after 25 min of anodal, cathodal, or sham transcranial direct current stimulation (tDCS) applied over the right cerebellar cortex. We hypothesized that anodal tDCS would result in an enhancement of CBI and cathodal would decrease it, relative to sham stimulation. We found that cathodal tDCS resulted in a clear decrease of CBI, whereas anodal tDCS increased it, in the absence of changes after sham stimulation. These effects were specific to the cerebello–cortical connections with no changes in other M1 or brainstem excitability measures. The cathodal effect on CBI was found to be dependent on stimulation intensity and lasted up to 30 min after the cessation of tDCS. These results suggest that tDCS can modulate in a focal and polarity-specific manner cerebellar excitability, likely through changes in Purkinje cell activity. Therefore, direct current stimulation of the cerebellum may have significant potential implications for patients with cerebellar dysfunction as well as to motor control studies.

Amygdala Activation Predicts Gaze toward Fearful Eyes

Gamer, M., Buchel, C. — 2009-07-15

The human amygdala can be robustly activated by presenting fearful faces, and it has been speculated that this activation has functional relevance for redirecting the gaze toward the eye region. To clarify this relationship between amygdala activation and gaze-orienting behavior, functional magnetic resonance imaging data and eye movements were simultaneously acquired in the current study during the evaluation of facial expressions. Fearful, angry, happy, and neutral faces were briefly presented to healthy volunteers in an event-related manner. We controlled for the initial fixation by unpredictably shifting the faces downward or upward on each trial, such that the eyes or the mouth were presented at fixation. Across emotional expressions, participants showed a bias to shift their gaze toward the eyes, but the magnitude of this effect followed the distribution of diagnostically relevant regions in the face. Amygdala activity was specifically enhanced for fearful faces with the mouth aligned to fixation, and this differential activation predicted gazing behavior preferentially targeting the eye region. These results reveal a direct role of the amygdala in reflexive gaze initiation toward fearfully widened eyes. They mirror deficits observed in patients with amygdala lesions and open a window for future studies on patients with autism spectrum disorder, in which deficits in emotion recognition, probably related to atypical gaze patterns and abnormal amygdala activation, have been observed.

Postsynaptic Mechanisms Govern the Differential Excitation of Cortical Neurons by Thalamic Inputs

Hull, C., Isaacson, J. S., Scanziani, M. — 2009-07-15

Thalamocortical (TC) afferents relay sensory input to the cortex by making synapses onto both excitatory regular-spiking principal cells (RS cells) and inhibitory fast-spiking interneurons (FS cells). This divergence plays a crucial role in coordinating excitation with inhibition during the earliest steps of somatosensory processing in the cortex. Although the same TC afferents contact both FS and RS cells, FS cells receive larger and faster excitatory inputs from individual TC afferents. Here, we show that this larger thalamic excitation of FS cells occurs via GluR2-lacking AMPA receptors (AMPARs), and results from a fourfold larger quantal amplitude compared with the thalamic inputs onto RS cells. Thalamic afferents also activate NMDA receptors (NMDARs) at synapses onto both cells types, yet RS cell NMDAR currents are slower and pass more current at physiological membrane potentials. Because of these synaptic specializations, GluR2-lacking AMPARs selectively maintain feedforward inhibition of RS cells, whereas NMDARs contribute to the spiking of RS cells and hence to cortical recurrent excitation. Thus, thalamic afferent activity diverges into two routes that rely on unique complements of postsynaptic AMPARs and NMDARs to orchestrate the dynamic balance of excitation and inhibition as sensory input enters the cortex.

This Week in The Journal

2009-07-08

Faithful Expression of Multiple Proteins via 2A-Peptide Self-Processing: A Versatile and Reliable Method for Manipulating Brain Circuits

2009-07-08

The Temporoparietal Junction as a Part of the "When" Pathway

Spierer, L., Bernasconi, F., Grivel, J. — 2009-07-08

Differential Modulation of Long-Term Depression by Acute Stress in the Rat Dorsal and Ventral Hippocampus

Maggio, N., Segal, M. — 2009-07-08

The ventral hippocampus (VH) was recently shown to express lower-magnitude long-term potentiation (LTP) than the dorsal hippocampus (DH). An exposure to acute stress reversed this difference, and VH slices from stressed rats expressed larger LTP than controls, whereas LTP in the DH was suppressed by stress. We have now used long-term depression (LTD)-generating trains of stimulation to examine whether this differential LTP reflects a genuine difference in synaptic modifiability between the two sectors of the hippocampus. Surprisingly, slices of DH and VH express similar magnitudes of LTD. However, while prior stress enhanced LTD in the DH, it actually converted LTD to slow-onset, robust LTP in the VH. These two effects of stress on LTD were blocked by glucocorticosterone receptor (GR) and mineralocorticosterone receptor (MR) antagonists, respectively. Acute exposure of slices to a GR agonist dexamethasone facilitated LTD in slices of both DH and VH, while activation of MRs by aldosterone converted LTD to LTP in both regions. Thus, differential activation of the two species of corticosterone receptors determines the ability of the two sectors of the hippocampus to undergo plastic changes in response to LTD-inducing stimulation.

{alpha}-Latrotoxin Stimulates a Novel Pathway of Ca2+-Dependent Synaptic Exocytosis Independent of the Classical Synaptic Fusion Machinery

2009-07-08

-Latrotoxin induces neurotransmitter release by stimulating synaptic vesicle exocytosis via two mechanisms: (1) A Ca²⁺-dependent mechanism with neurexins as receptors, in which -latrotoxin acts like a Ca²⁺ ionophore, and (2) a Ca²⁺-independent mechanism with CIRL/latrophilins as receptors, in which -latrotoxin directly stimulates the transmitter release machinery. Here, we show that the Ca²⁺-independent release mechanism by -latrotoxin requires the synaptic SNARE-proteins synaptobrevin/VAMP and SNAP-25, and, at least partly, the synaptic active-zone protein Munc13-1. In contrast, the Ca²⁺-dependent release mechanism induced by -latrotoxin does not require any of these components of the classical synaptic release machinery. Nevertheless, this type of exocytotic neurotransmitter release appears to fully operate at synapses, and to stimulate exocytosis of the same synaptic vesicles that participate in physiological action potential-triggered release. Thus, synapses contain two parallel and independent pathways of Ca²⁺-triggered exocytosis, a classical, physiological pathway that operates at the active zone, and a novel reserve pathway that is recruited only when Ca²⁺ floods the synaptic terminal.

Reassessment of Corticospinal Tract Regeneration in Nogo-Deficient Mice

2009-07-08

The myelin-derived neurite growth inhibitor Nogo has been proposed to play a major role in blocking axon regeneration in the CNS after injuries. However, past studies have produced mixed results regarding the regenerative phenotype of various Nogo-deficient mouse lines after experimental spinal cord injury. Two lines did not display enhanced corticospinal tract (CST) regeneration, and one displayed modest regeneration. A fourth line, a Nogo-A,B gene-trap mutant, was instead reported to exhibit extensive CST regeneration, but the results were later found to be inadvertently confounded with an axon labeling artifact. Of the four Nogo mutant lines studied so far, three continue to express some isoform(s) of Nogo, leaving open the question whether any remaining Nogo protein contributes to the modest regenerative phenotype reported in some. The remaining Nogo mutant line studied was confounded by the unexplained rescue of embryonic lethality associated with this mutation. To gain a better understanding of the contribution of Nogo as an inhibitor of regeneration of CNS axons, and particularly CST axons, we reanalyzed the Nogo-A,B gene-trap mutant line and analyzed a novel, fully viable Nogo deletion mutant line that is null for all known isoforms of Nogo. Our analyses failed to reveal any enhanced CST regeneration after experimental spinal cord injury in either line. These results indicate that Nogo alone does not account for lack of CST regeneration and have implications for current therapeutic development for spinal cord injury in humans by targeting Nogo.

Binge Drinking Upregulates Accumbens mGluR5-Homer2-PI3K Signaling: Functional Implications for Alcoholism

2009-07-08

The glutamate receptor-associated protein Homer2 regulates alcohol-induced neuroplasticity within the nucleus accumbens (NAC), but the precise intracellular signaling cascades involved are not known. This study examined the role for NAC metabotropic glutamate receptor (mGluR)–Homer2–phosphatidylinositol 3-kinase (PI3K) signaling in regulating excessive alcohol consumption within the context of the scheduled high alcohol consumption (SHAC) model of binge alcohol drinking. Repeated bouts of binge drinking (~1.5 g/kg per 30 min) elevated NAC Homer2a/b expression and increased PI3K activity in this region. Virus-mediated knockdown of NAC Homer2b expression attenuated alcohol intake, as did an intra-NAC infusion of the mGluR5 antagonist MPEP [2-methyl-6-(phenylethynyl)pyridine hydrochloride] (0.1–1 µg/side) and the PI3K antagonist wortmannin (50 ng/side), supporting necessary roles for mGluR5/Homer2/PI3K in binge alcohol drinking. Moreover, when compared with wild-type littermates, transgenic mice with an F1128R point mutation in mGluR5 that markedly reduces Homer binding exhibited a 50% reduction in binge alcohol drinking, which was related to reduced NAC basal PI3K activity. Consistent with the hypothesis that mGluR5–Homer–PI3K signaling may be a mechanism governing excessive alcohol intake, the "anti-binge" effects of MPEP and wortmannin were not additive, nor were they observed in the mGluR5^F1128R transgenic mice. Finally, mice genetically selected for a high versus low SHAC phenotype differed in NAC mGluR, Homer2, and PI3K activity, consistent with the hypothesis that augmented NAC mGluR5–Homer2–PI3K signaling predisposes a high binge alcohol-drinking phenotype. Together, these data point to an important role for NAC mGluR5–Homer2–PI3K signaling in regulating binge-like alcohol consumption that has relevance for our understanding of the neurobiology of alcoholism and its pharmacotherapy.

Therapeutic Administration of Plasminogen Activator Inhibitor-1 Prevents Hypoxic-Ischemic Brain Injury in Newborns

2009-07-08

Disruption of the integrity of the blood–brain barrier (BBB) is an important mechanism of cerebrovascular diseases, including neonatal cerebral hypoxia–ischemia (HI). Although both tissue-type plasminogen activator (tPA) and matrix metalloproteinase-9 (MMP-9) can produce BBB damage, their relationship in neonatal cerebral HI is unclear. Here we use a rodent model to test whether the plasminogen activator (PA) system is critical for MMP-9 activation and HI-induced brain injury in newborns. To test this hypothesis, we examined the therapeutic effect of intracerebroventricular injection of plasminogen activator inhibitor-1 (PAI-1) in rat pups subjected to unilateral carotid artery occlusion and systemic hypoxia. We found that the injection of PAI-1 greatly reduced the activity of both tPA and urokinase-type plasminogen activator after HI. It also blocked HI-induced MMP-9 activation and BBB permeability at 24 h of recovery. Furthermore, magnetic resonance imaging and histological analysis showed the PAI-1 treatment reduced brain edema, axonal degeneration, and cortical cell death at 24–48 h of recovery. Finally, the PAI-1 therapy provided a dose-dependent decrease of brain tissue loss at 7 d of recovery, with the therapeutic window at 4 h after the HI insult. Together, these results suggest that the brain PA system plays a pivotal role in neonatal cerebral HI and may be a promising therapeutic target in infants suffering hypoxic–ischemic encephalopathy.

Domain General Mechanisms of Perceptual Decision Making in Human Cortex

Ho, T. C., Brown, S., Serences, J. T. — 2009-07-08

To successfully interact with objects in the environment, sensory evidence must be continuously acquired, interpreted, and used to guide appropriate motor responses. For example, when driving, a red light should motivate a motor command to depress the brake pedal. Single-unit recording studies have established that simple sensorimotor transformations are mediated by the same neurons that ultimately guide the behavioral response. However, it is also possible that these sensorimotor regions are the recipients of a modality-independent decision signal that is computed elsewhere. Here, we used functional magnetic resonance imaging and human observers to show that the time course of activation in a subregion of the right insula is consistent with a role in accumulating sensory evidence independently from the required motor response modality (saccade vs manual). Furthermore, a combination of computational modeling and simulations of the blood oxygenation level-dependent response suggests that this region is not simply recruited by general arousal or by the tonic maintenance of attention during the decision process. Our data thus raise the possibility that a modality-independent representation of sensory evidence may guide activity in effector-specific cortical areas before the initiation of a behavioral response.

Positive AMPA Receptor Modulation Rapidly Stimulates BDNF Release and Increases Dendritic mRNA Translation

Jourdi, H., Hsu, Y.-T., Zhou, M., Qin, Q., Bi, X., Baudry, M. — 2009-07-08

Brain-derived neurotrophic factor (BDNF) stimulates local dendritic mRNA translation and is involved in formation and consolidation of memory. 2H,3H,6aH-pyrrolidino[2'',1''-3',2']1,3-oxazino[6',5'-5,4]-benzo[e]1,4-dioxan-10-one (CX614), one of the best-studied positive AMPA receptor modulators (also known as ampakines), increases BDNF mRNA and protein and facilitates long-term potentiation (LTP) induction. Several other ampakines also improve performance in various behavioral and learning tasks. Since local dendritic protein synthesis has been implicated in LTP stabilization and in memory consolidation, this study investigated whether CX614 could influence synaptic plasticity by upregulating dendritic protein translation. CX614 treatment of primary neuronal cultures and acute hippocampal slices rapidly activated the translation machinery and increased local dendritic protein synthesis. CX614-induced activation of translation was blocked by K252a [(9S,10R,12R)-2,3,9,10,11,12-hexahydro-10-hydroxy-9-methyl-1-oxo-9,12-epoxy-1H-diindolo[1,2,3-fg:3',2',1'-kl]pyrrolo[3,4-i][1,6]benzodiazocine-10-carboxylic acid methyl ester], CNQX, APV, and TTX, and was inhibited in the presence of an extracellular BDNF scavenger, TrkB-Fc. The acute effect of CX614 on translation was mediated by increased BDNF release as demonstrated with a BDNF scavenging assay using TrkB-Fc during CX614 treatment of cultured primary neurons and was blocked by nifedipine, ryanodine, and lack of extracellular Ca²⁺ in acute hippocampal slices. Finally, CX614, like BDNF, rapidly increased dendritic translation of an exogenous translation reporter. Together, our results demonstrate that positive modulation of AMPA receptors rapidly stimulates dendritic translation, an effect mediated by BDNF secretion and TrkB receptor activation. They also suggest that increased BDNF secretion and stimulation of local protein synthesis contribute to the effects of ampakines on synaptic plasticity.

Individual Differences in True and False Memory Retrieval Are Related to White Matter Brain Microstructure

2009-07-08

We sometimes vividly remember things that did not happen, a phenomenon with general relevance, not only in the courtroom. It is unclear to what extent individual differences in false memories are driven by anatomical differences in memory-relevant brain regions. Here we show in humans that microstructural properties of different white matter tracts as quantified using diffusion tensor imaging are strongly correlated with true and false memory retrieval. To investigate these hypotheses, we tested a large group of participants in a version of the Deese–Roediger–McDermott paradigm (recall and recognition) and subsequently obtained diffusion tensor images. A voxel-based whole-brain level linear regression analysis was performed to relate fractional anisotropy to indices of true and false memory recall and recognition. True memory was correlated to diffusion anisotropy in the inferior longitudinal fascicle, the major connective pathway of the medial temporal lobe, whereas a greater proneness to retrieve false items was related to the superior longitudinal fascicle connecting frontoparietal structures. Our results show that individual differences in white matter microstructure underlie true and false memory performance.

VEGFR-1 Regulates Adult Olfactory Bulb Neurogenesis and Migration of Neural Progenitors in the Rostral Migratory Stream In Vivo

2009-07-08

The generation of new neurons in the olfactory bulb (OB) persists into adulthood and is a multistep process that includes proliferation, fate choice, migration, survival, and differentiation. Neural precursor cells destined to form olfactory interneurons arise in the subventricular zone (SVZ) and migrate along the rostral migratory stream (RMS) to the OB. Recently, some factors classically known from their effects on the vascular system have been found to influence different steps of adult neurogenesis. In the present study, we report a modulatory function for the vascular endothelial growth factor receptor-1 (VEGFR-1) in adult olfactory neurogenesis. We identified expression of VEGFR-1 in GFAP-positive cells within regions involved in neurogenesis of the adult mouse brain. To determine functions for VEGFR-1 in adult neurogenesis, we compared neural progenitor cell proliferation, migration, and differentiation from wild-type and VEGFR-1 signaling-deficient mice (Flt-1TK^–/– mice). Our data show that VEGFR-1 signaling is involved in the regulation of proliferation of neuronal progenitor cells within the SVZ, migration along the RMS, and in neuronal differentiation and anatomical composition of interneuron subtypes within the OB. RMS migration in Flt-1TK^–/– mice was altered mainly as a result of increased levels of its ligand VEGF-A, which results in an increased phosphorylation of VEGFR-2 in neuronal progenitor cells within the SVZ and the RMS. This study reveals that proper RMS migration is dependent on endogenous VEGF-A protein.

How Ongoing Fluctuations in Human Visual Cortex Predict Perceptual Awareness: Baseline Shift versus Decision Bias

Wyart, V., Tallon-Baudry, C. — 2009-07-08

Visual perception fluctuates across repeated presentations of the same near-threshold stimulus. These perceptual fluctuations have often been attributed to baseline shifts—i.e., ongoing modulations of neuronal activity in visual areas—driven by top-down attention. Using magnetoencephalography, we directly tested whether ongoing attentional modulations could fully account for the perceptual impact of prestimulus activity on a subsequent seen–unseen decision. We found that prestimulus gamma-band fluctuations in lateral occipital areas (LO) predicted visual awareness, but did not reflect the focus of spatial attention. Moreover, these prestimulus signals influenced the decision outcome independently from the strength of the following visual response, suggesting that baseline shifts alone could not explain their perceptual impact. Using a straightforward decision-making model based on the accumulation of sensory evidence over time, we show that prestimulus gamma-band fluctuations in LO behave as a decision bias at stimulus onset, irrespectively of subsequent stimulus processing. In contrast, spatial attention suppressed prestimulus alpha-band signals in the same region, and produced a sustained baseline shift that also predicted the outcome of the seen–unseen decision. Together, our results indicate that prestimulus fluctuations in visual areas can influence the conscious detection of an upcoming stimulus via two distinct mechanisms: an attention-driven baseline shift in the alpha range, and a decision bias in the gamma range.

Human Variation in Overriding Attentional Capture

Fukuda, K., Vogel, E. K. — 2009-07-08

Attention can be directed either voluntarily based on the goals of the individual or involuntarily "captured" by salient stimuli in the immediate environment. Although involuntary capture is a critical means of directing attention, the completion of many common tasks requires our ability to ignore salient, but otherwise irrelevant stimuli while restricting our attention to stimuli that are related to our goals. Here, we report neurophysiological measures of spatial attention in humans that gauge an individual's ability to resist attentional capture from salient but irrelevant information. By measuring the rapid reallocation of spatial attention immediately after the onset of distractors, we observe that the ability to override attentional capture varies substantially across individuals and is strongly predicted by the specific working memory capacity of each person. High-capacity individuals were much more capable of resisting attentional capture than low-capacity individuals, who involuntarily reallocated spatial attention when distractors were present in the display. These results provide evidence that the poor attentional abilities associated with low memory capacity may stem from an inability to override attentional capture in the initial moments after the onset of distracting information.

The Positive Allosteric Modulator Morantel Binds at Noncanonical Subunit Interfaces of Neuronal Nicotinic Acetylcholine Receptors

Seo, S., Henry, J. T., Lewis, A. H., Wang, N., Levandoski, M. M. — 2009-07-08

We are interested in the positive allosteric modulation of neuronal nicotinic acetylcholine (ACh) receptors and have recently shown that the anthelmintic compound morantel potentiates by enhancing channel gating of the 3β2 subtype. Based on the demonstration that morantel-elicited currents were inhibited by the classic ACh competitor dihydro-β-erythroidine in a noncompetitive manner and that morantel still potentiates at saturating concentrations of agonist (Wu et al., 2008), we hypothesized that morantel binds at the noncanonical β2(+)/3(–) subunit interface. In the present study, we created seven cysteine-substituted subunits by site-directed mutagenesis, choosing residues in the putative morantel binding site with the aid of structural homology models. We coexpressed the mutant subunits and their respective wild-type partners in Xenopus oocytes and characterized the morantel potentiation of ACh-evoked currents, as well as morantel-evoked currents, before and after treatment with a variety of methanethiosulfonate (MTS)-based compounds, using voltage-clamp recordings. The properties of four of the seven mutants, two residues on each side of the interface, were changed by MTS treatments. Coapplication with ACh enhanced the extent of MTS modification for 3A106Cβ2 and 3β2S192C receptors. The activities of two mutants, 3T115Cβ2 and 3β2T150C, were dramatically altered by MTS modification. For 3β2T150C, while peak current amplitudes were reduced, potentiation was enhanced. For 3T115Cβ2, both current amplitudes and potentiation were reduced. MTS modification and morantel were mutually inhibitory: MTS treatment decreased morantel-evoked currents and morantel decreased the rate of MTS modification. We conclude that the four residues showing MTS effects contribute to the morantel binding site.

Phosphorylation of Prion Protein at Serine 43 Induces Prion Protein Conformational Change

2009-07-08

The cause of the conformational change of normal cellular prion protein (PrP) into its disease-associated form is unknown. Posttranslational modifications, such as glycosylation, acetylation, S-nitrosylation, and phosphorylation, are known to induce protein conformational changes. Here, we investigated whether phosphorylation could induce the conformational change of PrP because PrP contains several kinase motifs and has been found recently in the cytosol, in which kinases generally reside. Neuronal cyclin-dependent kinase 5 (Cdk5) phosphorylated recombinant PrP_23–231 at serine 43 (S43) in an in vitro kinase assay. Cdk5-phosphorylated PrP became proteinase K resistant, formed Congo Red-positive fibrils, and formed aggregates that were immunostained with anti-PrP and anti-phospho-PrP^S43 (anti-pPrP^S43). pPrP^S43 was detected in PrP/Cdk5/p25 cotransfected N2a cells. Roscovitine inhibition of Cdk5 activity or transfection of N2a cells with mutant PrP S43A eliminated the anti-pPrP^S43-immunopositive protein. Alkaline phosphatase-sensitive and proteinase K-resistant pPrP^S43 immunoreactivity was observed in scrapie-infected but not control-injected mice brains. These results raise the possibility that phosphorylation could represent a physiological mechanism of PrP conversion in vivo.

A Triplet Repeat Expansion Genetic Mouse Model of Infantile Spasms Syndrome, Arx(GCG)10+7, with Interneuronopathy, Spasms in Infancy, Persistent Seizures, and Adult Cognitive and Behavioral Impairment

2009-07-08

Infantile spasms syndrome (ISS) is a catastrophic pediatric epilepsy with motor spasms, persistent seizures, mental retardation, and in some cases, autism. One of its monogenic causes is an insertion mutation [c.304ins (GCG)₇] on the X chromosome, expanding the first polyalanine tract of the interneuron-specific transcription factor Aristaless-related homeobox (ARX) from 16 to 23 alanine codons. Null mutation of the Arx gene impairs GABA and cholinergic interneuronal migration but results in a neonatal lethal phenotype. We developed the first viable genetic mouse model of ISS that spontaneously recapitulates salient phenotypic features of the human triplet repeat expansion mutation. Arx^(GCG)10+7 ("Arx plus 7") pups display abnormal spasm-like myoclonus and other key EEG features, including multifocal spikes, electrodecremental episodes, and spontaneous seizures persisting into maturity. The neurobehavioral profile of Arx mutants was remarkable for lowered anxiety, impaired associative learning, and abnormal social interaction. Laminar decreases of Arx+ cortical interneurons and a selective reduction of calbindin-, but not parvalbumin- or calretinin-expressing interneurons in neocortical layers and hippocampus indicate that specific classes of synaptic inhibition are missing from the adult forebrain, providing a basis for the seizures and cognitive disorder. A significant reduction of calbindin-, NPY (neuropeptide Y)-expressing, and cholinergic interneurons in the mutant striatum suggest that dysinhibition within this network may contribute to the dyskinetic motor spasms. This mouse model narrows the range of critical pathogenic elements within brain inhibitory networks essential to recreate this complex neurodevelopmental syndrome.

Metaplastic Regulation of Long-Term Potentiation/Long-Term Depression Threshold by Activity-Dependent Changes of NR2A/NR2B Ratio

Xu, Z., Chen, R.-Q., Gu, Q.-H., Yan, J.-Z., Wang, S.-H., Liu, S.-Y., Lu, W. — 2009-07-08

In vivo experience induces changes in synaptic NMDA receptor (NMDAR) subunit components, which are correlated with subsequent modifications of synaptic plasticity. However, little is known about how these subunit changes regulate the induction threshold of subsequent plasticity. At hippocampal Schaffer collateral–CA1 synapses, we first examined whether a recent history of neuronal activity could affect subsequent synaptic plasticity through its actions on NMDAR subunit components. We found that prior activity history produced by priming stimulations (PSs) across a wide range of frequencies (1–100 Hz) could induce bidirectional changes in the NR2A/NR2B ratio, which governs the threshold for subsequent long-term potentiation/long-term depression (LTP/LTD). Manipulating the NR2A/NR2B ratio through partial NR2 subunit blockade mimicked the PS regulation of the LTP/LTD threshold. Our results demonstrate that activity-dependent changes in the NR2A/NR2B ratio can be critical factors in metaplastic regulation of the LTP/LTD threshold.

Minute Effects of Sex on the Aging Brain: A Multisample Magnetic Resonance Imaging Study of Healthy Aging and Alzheimer's Disease

2009-07-08

Age is associated with substantial macrostructural brain changes. While some recent magnetic resonance imaging studies have reported larger age effects in men than women, others find no sex differences. As brain morphometry is a potentially important tool in diagnosis and monitoring of age-related neurological diseases, e.g., Alzheimer's disease (AD), it is important to know whether sex influences brain aging. We analyzed cross-sectional magnetic resonance scans from 1143 healthy participants from seven subsamples provided by four independent research groups. In addition, 96 patients with mild AD were included. Estimates of cortical thickness continuously across the brain surface, as well as volume of 17 subcortical structures, were obtained by use of automated segmentation tools (FreeSurfer). In the healthy participants, no differences in aging slopes between women and men were found in any part of the cortex. Pallidum corrected for intracranial volume showed slightly higher age correlations for men. The analyses were repeated in each of the seven subsamples, and the lack of age x sex interactions was largely replicated. Analyses of the AD sample showed no interactions between sex and age for any brain region. We conclude that sex has negligible effects on the age slope of brain volumes both in healthy participants and in AD.

Maximal Voluntary Fingertip Force Production Is Not Limited by Movement Speed in Combined Motion and Force Tasks

Keenan, K. G., Santos, V. J., Venkadesan, M., Valero-Cuevas, F. J. — 2009-07-08

Numerous studies of limbs and fingers propose that force–velocity properties of muscle limit maximal voluntary force production during anisometric tasks, i.e., when muscles are shortening or lengthening. Although this proposition appears logical, our study on the simultaneous production of fingertip motion and force disagrees with this commonly held notion. We asked eight consenting adults to use their dominant index fingertip to maximize voluntary downward force against a horizontal surface at specific postures (static trials), and also during an anisometric "scratching" task of rhythmically moving the fingertip along a 5.8 ± 0.5 cm target line. The metronome-timed flexion–extension movement speed varied 36-fold from "slow" (1.0 ± 0.5 cm/s) to "fast" (35.9 ± 7.8 cm/s). As expected, maximal downward voluntary force diminished (44.8 ± 15.6%; p = 0.001) when any motion (slow or fast) was added to the task. Surprisingly, however, a 36-fold increase in speed did not affect this reduction in force magnitude. These remarkable results for such an ordinary task challenge the dominant role often attributed to force–velocity properties of muscle and provide insight into neuromechanical interactions. We propose an explanation that the simultaneous enforcement of mechanical constraints for motion and force reduces the set of feasible motor commands sufficiently so that force–velocity properties cease to be the force-limiting factor. While additional work is necessary to reveal the governing mechanisms, the dramatic influence that the simultaneous enforcement of motion and force constraints has on force output begins to explain the vulnerability of dexterous function to development, aging, and even mild neuromuscular pathology.

The gad2 Promoter Is a Transcriptional Target of Estrogen Receptor (ER) {alpha} and ER{beta}: A Unifying Hypothesis to Explain Diverse Effects of Estradiol

Hudgens, E. D., Ji, L., Carpenter, C. D., Petersen, S. L. — 2009-07-08

Estradiol (E₂) regulates a wide range of neural functions, many of which require activation of estrogen receptor (ER) and/or ERβ, ligand-gated transcriptional regulators. Surprisingly, very few neural gene targets of ERs have been identified, and these cannot easily explain the myriad effects of E₂. GABA regulates most of the same neural functions as E₂, and GABAergic neurons throughout the brain contain ER. Therefore, we examined whether E₂ directly regulates expression of glutamic acid decarboxylase 2 (gad2), the enzyme primarily responsible for GABA synthesis for synaptic release. Using dual luciferase assays, we found that E₂, but not other gonadal steroids, stimulated the activity of a 2691 bp rat gad2 promoter reporter construct. Activation required either ER or ERβ, and ERβ did not repress ER-mediated transactivation. Site-directed mutagenesis studies identified three estrogen response elements (EREs) with cell-specific functions. An ERE at –711 upstream of the gad2 translational start site was essential for transactivation in both MCF-7 breast cancer cells and SN56.B5.G4 neural cells, but an ERE at –546 enhanced transcription only in neural cells. A third ERE at –1958 was inactive in neural cells but exerted potent transcriptional repression in E₂-treated MCF-7 cells. Chromatin immunoprecipitation assays in mouse GABAergic N42 cells confirmed that E₂ induced ER binding to a DNA fragment containing sequences corresponding to the –546 and –711 EREs of the rat promoter. Based on these data, we propose that direct transcriptional regulation of gad2 may explain, at least in part, the ability of E₂ to impact such a diverse array of neural functions.

Role of Calcitonin Gene-Related Peptide in Light-Aversive Behavior: Implications for Migraine

2009-07-08

Migraine is a chronic neurological disorder characterized by recurrent episodes of severe unilateral throbbing head pain and associated symptoms, such as photophobia. Our current understanding of the mechanisms underlying migraine has been hampered by limitations in ascertaining migraine symptoms in animal models. Clinical studies have established the neuropeptide calcitonin gene-related peptide (CGRP) as a key player in migraine. Here, we establish a genetic model of photophobia by engineering increased sensitivity to CGRP in mice. These transgenic mice (nestin/hRAMP1) display light-aversive behavior that is greatly enhanced by intracerebroventricular injection of CGRP and blocked by coadministration of the CGRP receptor antagonist olcegepant. This behavior appears to be an indicator of photophobia and cannot be fully explained by gross abnormality of ocular anatomy or differences in general anxiety or motor activity. Our findings demonstrate that a single gene, receptor activity-modifying protein 1 (RAMP1), can be a modifier of photophobia and, by extension, suggest that genetic or epigenetic modulation of RAMP1 levels may contribute to migraine susceptibility. Moreover, they validate CGRP hypersensitive mice as a tool for exploring the neurobiology and novel therapies for migraine and other disorders involving photophobia.

Deletion of the {alpha}7 Nicotinic Acetylcholine Receptor Gene Improves Cognitive Deficits and Synaptic Pathology in a Mouse Model of Alzheimer's Disease

Dziewczapolski, G., Glogowski, C. M., Masliah, E., Heinemann, S. F. — 2009-07-08

It has been recently shown that the Alzheimer's disease (AD) pathogenic peptide amyloid β_1–42 (Aβ_1–42) binds to the 7 nicotinic acetylcholine receptor (7nAChR) with high affinity and the 7nAChR and Aβ_1–42 are both found colocalized in neuritic plaques of human brains with AD. Moreover, the intraneuronal accumulation of Aβ_1–42 was shown to be facilitated by its high-affinity binding to the 7nAChR, and 7nAChR activation mediates Aβ-induced tau protein phosphorylation. To test the hypothesis that 7nAChRs are involved in AD pathogenesis, we used a transgenic mouse model of AD overexpressing a mutated form of the human amyloid precursor protein (APP) and lacking the 7nAChR gene (APP7KO). We have shown that, despite the presence of high amounts of APP and amyloid deposits, deleting the 7nAChR subunit in the mouse model of AD leads to a protection from the dysfunction in synaptic integrity (pathology and plasticity) and learning and memory behavior. Specifically, APP7KO mice express APP and Aβ at levels similar to APP mice, and yet they were able to solve a cognitive challenge such as the Morris water maze test significantly better than APP, with performances comparable to control groups. Moreover, deleting the 7nAChR subunit protected the brain from loss of the synaptic markers synaptophysin and MAP2, reduced the gliosis, and preserved the capacity to elicit long-term potentiation otherwise deficient in APP mice. These results are consistent with the hypothesis that the 7nAChR plays a role in AD and suggest that interrupting 7nAChR function could be beneficial in the treatment of AD.

Activity-Dependent Codevelopment of the Corticospinal System and Target Interneurons in the Cervical Spinal Cord

Chakrabarty, S., Shulman, B., Martin, J. H. — 2009-07-08

Corticospinal tract (CST) connections to spinal interneurons are conserved across species. We identified spinal interneuronal populations targeted by the CST in the cervical enlargement of the cat during development. We focused on the periods before and after laminar refinement of the CST terminations, between weeks 5 and 7. We used immunohistochemistry of choline acetyltransferase (ChAT), calbindin, calretinin, and parvalbumin to mark interneurons. We first compared interneuron marker distribution before and after CST refinement. ChAT interneurons increased, while calbindin interneurons decreased during this period. No significant changes were noted in parvalbumin and calretinin. We next used anterograde labeling to determine whether the CST targets different interneuron populations before and after the refinement period. Before refinement, the CST terminated sparsely where calbindin interneurons were located and spared ChAT interneurons. After refinement, the CST no longer terminated in calbindin-expressing areas but did so where ChAT interneurons were located. Remarkably, early CST terminations were dense where ChAT interneurons later increased in numbers. Finally, we determined whether corticospinal system activity was necessary for the ChAT and calbindin changes. We unilaterally inactivated M1 between weeks 5 and 7 by muscimol infusion. Inactivation resulted in a distribution of calbindin and ChAT in spinal gray matter regions where the CST terminates that resembled the immature more than mature pattern. Our results show that the CST plays a crucial role in restructuring spinal motor circuits during development, possibly through trophic support, and provide strong evidence for the importance of connections with key spinal interneuron populations in development of motor control functions.

Selective Inhibition of Hypoxia-Inducible Factor (HIF) Prolyl-Hydroxylase 1 Mediates Neuroprotection against Normoxic Oxidative Death via HIF- and CREB-Independent Pathways

2009-07-08

Oxidative stress contributes to tissue injury in conditions ranging from cardiovascular disease to stroke, spinal cord injury, neurodegeneration, and perhaps even aging. Yet the efficacy of antioxidants in human disease has been mixed at best. We need a better understanding of the mechanisms by which established antioxidants combat oxidative stress. Iron chelators are well established inhibitors of oxidative death in both neural and non-neural tissues, but their precise mechanism of action remains elusive. The prevailing but not completely substantiated view is that iron chelators prevent oxidative injury by suppressing Fenton chemistry and the formation of highly reactive hydroxyl radicals. Here, we show that iron chelation protects, rather unexpectedly, by inhibiting the hypoxia-inducible factor prolyl 4-hydroxylase isoform 1 (PHD1), an iron and 2-oxoglutarate-dependent dioxygenase. PHD1 and its isoforms 2 and 3 are best known for stabilizing transcriptional regulators involved in hypoxic adaptation, such as HIF-1 and cAMP response element-binding protein (CREB). Yet we find that global hypoxia-inducible factor (HIF)-PHD inhibition protects neurons even when HIF-1 and CREB are directly suppressed. Moreover, two global HIF-PHD inhibitors continued to be neuroprotective even in the presence of diminished HIF-2 levels, which itself increases neuronal susceptibility to oxidative stress. Finally, RNA interference to PHD1 but not isoforms PHD2 or PHD3 prevents oxidative death, independent of HIF activation. Together, these studies suggest that iron chelators can prevent normoxic oxidative neuronal death through selective inhibition of PHD1 but independent of HIF-1 and CREB; and that HIF-2, not HIF-1, regulates susceptibility to normoxic oxidative neuronal death.

Congruence of BOLD Response across Intertemporal Choice Conditions: Fictive and Real Money Gains and Losses

Bickel, W. K., Pitcock, J. A., Yi, R., Angtuaco, E. J. C. — 2009-07-08

Intertemporal choice is predicated on the valuation of commodities with respect to delay until their receipt. Subjective value of a future outcome decreases, or is discounted, as a function of that delay (Bickel and Johnson, 2003). Although behavioral studies suggest no difference between the devaluation of real and fictive outcomes, no neuroimaging studies have investigated potential differences in the underlying deliberative process. Here, we compare behavioral and neural correlates of intertemporal valuation of real and hypothetical monetary gains as well as hypothetical losses, which have been posited to involve different mechanisms. Behavioral and neuroimaging sessions were conducted in which participants made intertemporal choice decisions in a gains condition using both real and hypothetical $100 money and in a loss condition using a fictive $100 money. Within-subject comparison of behavioral data revealed no significant difference between levels of discounting across the three conditions. Random-effects analysis of functional magnetic resonance imaging (fMRI) data of each of the three discounting conditions independently revealed significant signal change in limbic (anterior cingulate, striatum, posterior cingulate) and executive functioning areas (lateral prefrontal cortex), whereas a repeated-measures ANOVA failed to detect differences in signal change across the three discounting conditions after correcting for multiple comparisons. These data support a concordance between real and hypothetical conditions from delay-discounting studies and further suggest a congruence of the fMRI blood oxygen level-dependent signal across brain regions associated with the deliberative process of different forms of intertemporal choice.

Postnatal Expression Pattern of HCN Channel Isoforms in Thalamic Neurons: Relationship to Maturation of Thalamocortical Oscillations

2009-07-08

Hyperpolarization-activated cyclic nucleotide-gated cation (HCN) channels are the molecular substrate of the hyperpolarization-activated inward current (I_h). Because the developmental profile of HCN channels in the thalamus is not well understood, we combined electrophysiological, molecular, immunohistochemical, EEG recordings in vivo, and computer modeling techniques to examine HCN gene expression and I_h properties in rat thalamocortical relay (TC) neurons in the dorsal part of the lateral geniculate nucleus and the functional consequence of this maturation. Recordings of TC neurons revealed an approximate sixfold increase in I_h density between postnatal day 3 (P3) and P106, which was accompanied by significantly altered current kinetics, cAMP sensitivity, and steady-state activation properties. Quantification on tissue levels revealed a significant developmental decrease in cAMP. Consequently the block of basal adenylyl cyclase activity was accompanied by a hyperpolarizing shift of the I_h activation curve in young but not adult rats. Quantitative analyses of HCN channel isoforms revealed a steady increase of mRNA and protein expression levels of HCN1, HCN2, and HCN4 with reduced relative abundance of HCN4. Computer modeling in a simplified thalamic network indicated that the occurrence of rhythmic delta activity, which was present in the EEG at P12, differentially depended on I_h conductance and modulation by cAMP at different developmental states. These data indicate that the developmental increase in I_h density results from increased expression of three HCN channel isoforms and that isoform composition and intracellular cAMP levels interact in determining I_h properties to enable progressive maturation of rhythmic slow-wave sleep activity patterns.

Dlg1, Sec8, and Mtmr2 Regulate Membrane Homeostasis in Schwann Cell Myelination

2009-07-08

How membrane biosynthesis and homeostasis is achieved in myelinating glia is mostly unknown. We previously reported that loss of myotubularin-related protein 2 (MTMR2) provokes autosomal recessive demyelinating Charcot–Marie–Tooth type 4B1 neuropathy, characterized by excessive redundant myelin, also known as myelin outfoldings. We generated a Mtmr2-null mouse that models the human neuropathy. We also found that, in Schwann cells, Mtmr2 interacts with Discs large 1 (Dlg1), a scaffold involved in polarized trafficking and membrane addition, whose localization in Mtmr2-null nerves is altered. We here report that, in Schwann cells, Dlg1 also interacts with kinesin 13B (kif13B) and Sec8, which are involved in vesicle transport and membrane tethering in polarized cells, respectively. Taking advantage of the Mtmr2-null mouse as a model of impaired membrane formation, we provide here the first evidence for a machinery that titrates membrane formation during myelination. We established Schwann cell/DRG neuron cocultures from Mtmr2-null mice, in which myelin outfoldings were reproduced and almost completely rescued by Mtmr2 replacement. By exploiting this in vitro model, we propose a mechanism whereby kif13B kinesin transports Dlg1 to sites of membrane remodeling where it coordinates a homeostatic control of myelination. The interaction of Dlg1 with the Sec8 exocyst component promotes membrane addition, whereas with Mtmr2, negatively regulates membrane formation. Myelin outfoldings thus arise as a consequence of the loss of negative control on the amount of membrane, which is produced during myelination.

This Week in The Journal

2009-07-01

Correction for Gaston et al., Maturation of Ribbon Synapses in Hair Cells Is Driven by Thyroid Hormone

2009-07-01

Spike-Timing-Dependent Plasticity Induces Presynaptic Changes at Immature Hippocampal Mossy Fiber Synapses

Lanore, F., Rebola, N., Carta, M. — 2009-07-01

Mediobasal Hypothalamic Leucine Sensing Regulates Food Intake through Activation of a Hypothalamus-Brainstem Circuit

Blouet, C., Jo, Y.-H., Li, X., Schwartz, G. J. — 2009-07-01

In response to nutrient stimuli, the mediobasal hypothalamus (MBH) drives multiple neuroendocrine and behavioral mechanisms to regulate energy balance. While central leucine reduces food intake and body weight, the specific neuroanatomical sites of leucine sensing, downstream neural substrates, and neurochemical effectors involved in this regulation remain largely unknown. Here we demonstrate that MBH leucine engages a neural energy regulatory circuit by stimulating POMC (proopiomelanocortin) neurons of the MBH, oxytocin neurons of the paraventricular hypothalamus, and neurons within the brainstem nucleus of the solitary tract to acutely suppress food intake by reducing meal size. We identify central p70 S6 kinase and Erk1/2 pathways as intracellular effectors required for this response. Activation of endogenous leucine intracellular metabolism produced longer-term reductions in meal number. Our data identify a novel, specific hypothalamus–brainstem circuit that links amino acid availability and nutrient sensing to the control of food intake.

A Role for Blind DN2 Clock Neurons in Temperature Entrainment of the Drosophila Larval Brain

Picot, M., Klarsfeld, A., Chelot, E., Malpel, S., Rouyer, F. — 2009-07-01

Circadian clocks synchronize to the solar day by sensing the diurnal changes in light and temperature. In adult Drosophila, the brain clock that controls rest–activity rhythms relies on neurons showing Period oscillations. Nine of these neurons are present in each larval brain hemisphere. They can receive light inputs through Cryptochrome (CRY) and the visual system, but temperature input pathways are unknown. Here, we investigate how the larval clock network responds to light and temperature. We focused on the CRY-negative dorsal neurons (DN2s), in which light–dark (LD) cycles set molecular oscillations almost in antiphase to all other clock neurons. We first showed that the phasing of the DN2s in LD depends on the pigment-dispersing factor (PDF) neuropeptide in four lateral neurons (LNs), and on the PDF receptor in the DN2s. In the absence of PDF signaling, these cells appear blind, but still synchronize to temperature cycles. Period oscillations in the DN2s were stronger in thermocycles than in LD, but with a very similar phase. Conversely, the oscillations of LNs were weaker in thermocycles than in LD, and were phase-shifted in synchrony with the DN2s, whereas the phase of the three other clock neurons was advanced by a few hours. In the absence of any other functional clock neurons, the PDF-positive LNs were entrained by LD cycles but not by temperature cycles. Our results show that the larval clock neurons respond very differently to light and temperature, and strongly suggest that the CRY-negative DN2s play a prominent role in the temperature entrainment of the network.

Location of the {beta}4 Transmembrane Helices in the BK Potassium Channel

2009-07-01

Large-conductance, voltage- and Ca²⁺-gated potassium (BK) channels control excitability in a number of cell types. BK channels are composed of subunits, which contain the voltage-sensor domains and the Ca²⁺- sensor domains and form the pore, and often one of four types of β subunits, which modulate the channel in a cell-specific manner. β4 is expressed in neurons throughout the brain. Deletion of β4 in mice causes temporal lobe epilepsy. Compared with channels composed of alone, channels composed of and β4 activate and deactivate more slowly. We inferred the locations of the two β4 transmembrane (TM) helices TM1 and TM2 relative to the seven TM helices, S0–S6, from the extent of disulfide bond formation between cysteines substituted in the extracellular flanks of these TM helices. We found that β4 TM2 is close to S0 and that β4 TM1 is close to both S1 and S2. At least at their extracellular ends, TM1 and TM2 are not close to S3–S6. In six of eight of the most highly crosslinked cysteine pairs, four crosslinks from TM2 to S0 and one each from TM1 to S1 and S2 had small effects on the V₅₀ and on the rates of activation and deactivation. That disulfide crosslinking caused only small functional perturbations is consistent with the proximity of the extracellular ends of TM2 to S0 and of TM1 to S1 and to S2, in both the open and closed states.

Perirhinal Cortex Contributes to Accuracy in Recognition Memory and Perceptual Discriminations

O'Neil, E. B., Cate, A. D., Kohler, S. — 2009-07-01

The prevailing view of the medial temporal lobe (MTL) holds that its structures are dedicated to long-term declarative memory. Recent evidence challenges this position, suggesting that perirhinal cortex (PRc) in the MTL may also play a role in perceptual discriminations of stimuli with substantial visual feature overlap. Relevant neuropsychological findings in humans have been inconclusive, likely because studies have relied on patients with large and variable MTL lesions. Here, we conducted a functional magnetic resonance imaging study in healthy individuals to determine whether PRc shows a performance-related involvement in perceptual oddball judgments that is comparable to its established role in recognition memory. Morphed faces were selected as stimuli because of their large degree of feature overlap. All trials involved presentation of displays with three faces. The perceptual oddball task required identification of the face least similar to the other display members. The memory task involved forced-choice recognition of a previously studied face. When levels of behavioral performance were matched, we observed comparable levels of activation in right PRc for both tasks. Moreover, right PRc activity differentiated between accurate and inaccurate trials in both tasks. Together these results indicate that declarative memory demands are not a prerequisite for a performance-related engagement of PRc and that the introduction of such declarative memory demands in an otherwise closely matched perceptual task does not necessarily lead to an increase in PRc involvement. As such our findings show that declarative memory and perception are not as clearly separable at the level of MTL functioning as traditionally thought.

Selective Cortical Layering Abnormalities and Behavioral Deficits in Cortex-Specific Pax6 Knock-Out Mice

2009-07-01

The transcription factor Pax6 has been implicated in neocortical neurogenesis in vertebrates, including humans. Analyses of the role of Pax6 in layer formation and cognitive abilities have been hampered by perinatal lethality of Pax6 mutants. Here, we generated viable mutants exhibiting timed, restricted inactivation of Pax6 during early and late cortical neurogenesis using Emx1-Cre and hGFAP-Cre lines, respectively. The disruption of Pax6 at the onset of neurogenesis using Emx1-Cre line resulted in premature cell cycle exit of early progenitors, increase of early born neuronal subsets located in the marginal zone and lower layers, and a nearly complete absence of upper layer neurons, especially in the rostral cortex. Furthermore, progenitors, which accumulated in the enlarged germinal neuroepithelium at the pallial/subpallial border in the Pax6 mutants, produced an excess of oligodendrocytes. The inactivation of Pax6 after generation of the lower neuronal layers using hGFAP-Cre line did not affect specification or numbers of late-born neurons, indicating that the severe reduction of upper layer neurons in Pax6 deficiency is mostly attributable to a depletion of the progenitor pool, available for late neurogenesis. We further show that Pax6^fl/fl;Emx1-Cre mutants exhibited deficiencies in sensorimotor information integration, and both hippocampus-dependent short-term and neocortex-dependent long-term memory recall. Because a majority of the morphological and behavior disabilities of the Pax6 mutant mice parallel abnormalities reported for aniridia patients, a condition caused by PAX6 haploinsufficiency, the Pax6 conditional mutant mice generated here represent a valuable genetic tool to understand how the developmental cortical disruption can lead to a human behavior abnormality.

Differential Modulatory Influences between Primary Auditory Cortex and the Anterior Auditory Field

Carrasco, A., Lomber, S. G. — 2009-07-01

Neuroanatomical studies have revealed a vast network of corticocortical connections among the various fields that form cat auditory cortex. However, few studies have explored the functional communicative properties of these connections. The purpose of the present study was to examine the bidirectional processing contributions between the primary auditory cortex (A1) and the nonprimary anterior auditory field (AAF). Using acute recording techniques, multiunit neuronal activity was collected from the right hemisphere of nine mature cats. Cortical maps were generated, and the precise location of A1 and AAF was identified. Subsequently, the synaptic activity of A1 or AAF was suppressed with reversible thermal deactivation procedures while the neuronal response to tonal stimuli of the non-inactivated area (A1 or AAF) was measured. We examined response strength and latency, characteristic frequency, bandwidth, and neuronal threshold of A1 and AAF receptive fields before and during epochs of deactivation. Three major changes in A1 response properties were observed during AAF neuronal suppression: a decrease in response strength, an increase in neuronal thresholds, and a sharpening of receptive field bandwidths. In contrast, A1 deactivation did not produce any discernible changes in AAF neuronal responses. Collectively, these results suggest that the modulation of acoustic information between A1 and AAF in cat auditory cortex is dominated by a unidirectional AAF to A1 pathway.

Mice with Altered Myelin Proteolipid Protein Gene Expression Display Cognitive Deficits Accompanied by Abnormal Neuron-Glia Interactions and Decreased Conduction Velocities

2009-07-01

Conduction velocity (CV) of myelinated axons has been shown to be regulated by oligodendrocytes even after myelination has been completed. However, how myelinating oligodendrocytes regulate CV, and what the significance of this regulation is for normal brain function remain unknown. To address these questions, we analyzed a transgenic mouse line harboring extra copies of the myelin proteolipid protein 1 (plp1) gene (plp1^tg/– mice) at 2 months of age. At this stage, the plp1^tg/– mice have an unaffected myelin structure with a normally appearing ion channel distribution, but the CV in all axonal tracts tested in the CNS is greatly reduced. We also found decreased axonal diameters and slightly abnormal paranodal structures, both of which can be a cause for the reduced CV. Interestingly the plp1^tg/– mice showed altered anxiety-like behaviors, reduced prepulse inhibitions, spatial learning deficits and working memory deficit, all of which are schizophrenia-related behaviors. Our results implicate that abnormalities in the neuron-glia interactions at the paranodal junctions can result in reduced CV in the CNS, which then induces behavioral abnormalities related to schizophrenia.

Parallel ON and OFF Cone Bipolar Inputs Establish Spatially Coextensive Receptive Field Structure of Blue-Yellow Ganglion Cells in Primate Retina

2009-07-01

In the primate retina the small bistratified, "blue-yellow" color-opponent ganglion cell receives parallel ON-depolarizing and OFF-hyperpolarizing inputs from short (S)-wavelength sensitive and combined long (L)- and middle (M)-wavelength sensitive cone photoreceptors, respectively. However, the synaptic pathways that create S versus LM cone-opponent receptive field structure remain controversial. Here, we show in the macaque monkey retina in vitro that at photopic light levels, when an identified rod input is excluded, the small bistratified cell displays a spatially coextensive receptive field in which the S-ON-input is in spatial, temporal, and chromatic balance with the LM-OFF-input. ON pathway block with l-AP-4, the mGluR6 receptor agonist, abolished the S-ON response but spared the LM-OFF response. The isolated LM component showed a center-surround receptive field structure consistent with an input from OFF-center, ON-surround "diffuse" cone bipolar cells. Increasing retinal buffering capacity with HEPES attenuated the LM-ON surround component, consistent with a non-GABAergic outer retina feedback mechanism for the bipolar surround. The GABAa/c receptor antagonist picrotoxin and the glycine receptor antagonist strychnine did not affect chromatic balance or the basic coextensive receptive field structure, suggesting that the LM-OFF field is not generated by an inner retinal inhibitory pathway. We conclude that the opponent S-ON and LM-OFF responses originate from the excitatory receptive field centers of S-ON and LM-OFF cone bipolar cells, and that the LM-OFF- and ON-surrounds of these parallel bipolar inputs largely cancel, explaining the small, spatially coextensive but spectrally antagonistic receptive field structure of the blue-ON ganglion cell.

The Role of Human Orbitofrontal Cortex in Value Comparison for Incommensurable Objects

FitzGerald, T. H. B., Seymour, B., Dolan, R. J. — 2009-07-01

The human orbitofrontal cortex is strongly implicated in appetitive valuation. Whether its role extends to support comparative valuation necessary to explain probabilistic choice patterns for incommensurable goods is unknown. Using a binary choice paradigm, we derived the subjective values of different bundles of goods, under conditions of both gain and loss. We demonstrate that orbitofrontal activation reflects the difference in subjective value between available options, an effect evident across valuation for both gains and losses. In contrast, activation in dorsal striatum and supplementary motor areas reflects subjects' choice probabilities. These findings indicate that orbitofrontal cortex plays a pivotal role in valuation for incommensurable goods, a critical component process in human decision making.

Nonequilibrium Calcium Dynamics Regulate the Autonomous Firing Pattern of Rat Striatal Cholinergic Interneurons

Goldberg, J. A., Teagarden, M. A., Foehring, R. C., Wilson, C. J. — 2009-07-01

Striatal cholinergic interneurons discharge rhythmically in two patterns associated with different afterhyperpolarization timescales, each dictated by a different calcium-dependent potassium current. Single spiking depends on a medium-duration afterhyperpolarization (mAHP) generated by rapid SK currents that are associated with N-type calcium channels. Periodic bursting is driven by a delayed and slowly decaying afterhyperpolarization (sAHP) current associated with L-type channels. Using calcium imaging we show that the calcium transients underlying these currents exhibit two corresponding timescales throughout the somatodendritic tree. This result is not consistent with spatial compartmentalization of calcium entering through the two calcium channels and acting on the two potassium currents, or with differences in channel gating kinetics of the calcium dependent potassium currents. Instead, we show that nonequilibrium dynamics of calcium redistribution among cytoplasmic binding sites with different calcium binding kinetics can give rise to multiple timescales within the same cytoplasmic volume. The resulting independence of mAHP and sAHP currents allows cytoplasmic calcium to control two different and incompatible firing patterns (single spiking or bursting and pausing), depending on whether calcium influx is pulsatile or sustained. During irregular firing, calcium entry at both timescales can be detected, suggesting that an interaction between the medium and slow calcium-dependent afterhyperpolarizations may underlie this firing pattern.

Ethanol-Modulated Camouflage Response Screen in Zebrafish Uncovers a Novel Role for cAMP and Extracellular Signal-Regulated Kinase Signaling in Behavioral Sensitivity to Ethanol

2009-07-01

Ethanol, a widely abused substance, elicits evolutionarily conserved behavioral responses in a concentration-dependent manner in vivo. The molecular mechanisms underlying such behavioral sensitivity to ethanol are poorly understood. While locomotor-based behavioral genetic screening is successful in identifying genes in invertebrate models, such complex behavior-based screening has proven difficult for recovering genes in vertebrates. Here we report a novel and tractable ethanol response in zebrafish. Using this ethanol-modulated camouflage response as a screening assay, we have identified a zebrafish mutant named fantasma (fan), which displays reduced behavioral sensitivity to ethanol. Positional cloning reveals that fan encodes type 5 adenylyl cyclase (AC5). fan/ac5 is required to maintain the phosphorylation of extracellular signal-regulated kinase (ERK) in the forebrain structures, including the telencephalon and hypothalamus. Partial inhibition of phosphorylation of ERK in wild-type zebrafish mimics the reduction in sensitivity to stimulatory effects of ethanol observed in the fan mutant, whereas, strikingly, strong inhibition of phosphorylation of ERK renders a stimulatory dose of ethanol sedating. Since previous studies in Drosophila and mice show a role of cAMP signaling in suppressing behavioral sensitivity to ethanol, our findings reveal a novel, isoform-specific role of AC signaling in promoting ethanol sensitivity, and suggest that the phosphorylation level of the downstream effector ERK is a critical "gatekeeper" of behavioral sensitivity to ethanol.

Multisensory Integration in Dynamical Behaviors: Maximum Likelihood Estimation across Bimanual Skill Learning

Ronsse, R., Miall, R. C., Swinnen, S. P. — 2009-07-01

Optimal integration of different sensory modalities weights each modality as a function of its degree of certainty (maximum likelihood). Humans rely on near-optimal integration in decision-making tasks (involving e.g., auditory, visual, and/or tactile afferents), and some support for these processes has also been provided for discrete sensorimotor tasks. Here, we tested optimal integration during the continuous execution of a motor task, using a cyclical bimanual coordination pattern in which feedback was provided by means of proprioception and augmented visual feedback (AVF, the position of both wrists being displayed as the orthogonal coordinates of a single cursor). Assuming maximum likelihood integration, the following predictions were addressed: (1) the coordination variability with both AVF and proprioception available is smaller than with only one of the two modalities, and should reach an optimal level; (2) if the AVF is artificially corrupted by noise, variability should increase but saturate toward the level without AVF; (3) if the AVF is imperceptibly phase shifted, the stabilized pattern should be partly adapted to compensate for this phase shift, whereby the amount of compensation reflects the weight assigned to AVF in the computation of the integrated signal. Whereas performance variability gradually decreased over 5 d of practice, we showed that these model-based predictions were already observed on the first day. This suggests not only that the performer integrated proprioceptive feedback and AVF online during task execution by tending to optimize the signal statistics, but also that this occurred before reaching an asymptotic performance level.

Neurobehavioral Performance in Feline Immunodeficiency Virus Infection: Integrated Analysis of Viral Burden, Neuroinflammation, and Neuronal Injury in Cortex

2009-07-01

Human immunodeficiency virus (HIV) infection causes motor and neurocognitive abnormalities affecting >50% of children and 20% of adults with HIV/AIDS (acquired immunodeficiency syndrome). The closely related lentivirus, feline immunodeficiency virus (FIV), also causes neurobehavioral deficits. Herein, we investigated the extent to which FIV infection affected specific motor and cognitive tasks in conjunction with viral burden and immune responses within the brain. Neonatal animals were infected with a neurovirulent FIV strain (FIV-Ch) and assessed in terms of systemic immune parameters, viral burden, neurobehavioral performance, and neuropathological features. FIV-infected animals displayed less weight gain and lower blood CD4⁺ T-cell levels than mock-infected animals (p < 0.05). Gait analyses disclosed greater gait width with increased variation in FIV-infected animals (p < 0.05). Maze performance showed that FIV-infected animals were slower and made more navigational errors than mock-infected animals (p < 0.05). In the object memory test, the FIV-infected group exhibited fewer successful steps with more trajectory errors compared with the mock-infected group (p < 0.05). Performance on the gait, maze, and object memory tests was inversely correlated with F4/80 and CD3 expression (p < 0.05) and with viral burden in parietal cortex (p < 0.05). Amino acid analysis in cortex showed that d-serine levels were reduced in FIV-infected animals, which was accompanied by diminished kainate and AMPA receptor subunit expression (p < 0.05). The neurobehavioral findings in FIV-infected animals were associated with increased gliosis and reduced cortical neuronal counts (p < 0.05). The present studies indicated that specific motor and neurocognitive abilities were impaired in FIV infection and that these effects were closely coupled with viral burden, neuroinflammation, and neuronal loss.

ASIC2 Subunits Target Acid-Sensing Ion Channels to the Synapse via an Association with PSD-95

2009-07-01

Acid-sensing ion channel-1a (ASIC1a) mediates H⁺-gated current to influence normal brain physiology and impact several models of disease. Although ASIC2 subunits are widely expressed in brain and modulate ASIC1a current, their function remains poorly understood. We identified ASIC2a in dendrites, dendritic spines, and brain synaptosomes. This localization largely relied on ASIC2a binding to PSD-95 and matched that of ASIC1a, which does not coimmunoprecipitate with PSD-95. We found that ASIC2 and ASIC1a associated in brain, and through its interaction with PSD-95, ASIC2 increased ASIC1a localization in dendritic spines. Consistent with earlier work showing that acidic pH elevated spine [Ca²⁺]_i by activating ASIC1a, loss of ASIC2 decreased the percentage of spines responding to acid. Moreover, like a reduction of ASIC1a, the number of spine synapses fell in ASIC2^–/– neurons. These results indicate that ASIC2 facilitates ASIC1a localization and function in dendritic spines and suggest that the two subunits work in concert to regulate neuronal function.

I Heard That Coming: Event-Related Potential Evidence for Stimulus-Driven Prediction in the Auditory System

Bendixen, A., Schroger, E., Winkler, I. — 2009-07-01

The auditory system has been shown to detect predictability in a tone sequence, but does it use the extracted regularities for actually predicting the continuation of the sequence? The present study sought to find evidence for the generation of such predictions. Predictability was manipulated in an isochronous series of tones in which every other tone was a repetition of its predecessor. The existence of predictions was probed by occasionally omitting either the first (unpredictable) or the second (predictable) tone of a same-frequency tone pair. Event-related electrical brain activity elicited by the omission of an unpredictable tone differed from the response to the actual tone right from the tone onset. In contrast, early electrical brain activity elicited by the omission of a predictable tone was quite similar to the response to the actual tone. This suggests that the auditory system preactivates the neural circuits for expected input, using sequential predictions to specifically prepare for future acoustic events.

Calcium-Activated SK Channels Influence Voltage-Gated Ion Channels to Determine the Precision of Firing in Globus Pallidus Neurons

Deister, C. A., Chan, C. S., Surmeier, D. J., Wilson, C. J. — 2009-07-01

Globus pallidus (GP) neurons fire rhythmically in the absence of synaptic input, suggesting that they may encode their inputs as changes in the phase of their rhythmic firing. Action potential afterhyperpolarization (AHP) enhances precision of firing by ensuring that the ion channels recover from inactivation by the same amount on each cycle. Voltage-clamp experiments in slices showed that the longest component of the GP neuron's AHP is blocked by apamin, a selective antagonist of calcium-activated SK channels. Application of 100 nm apamin also disrupted the precision of firing in perforated-patch and cell-attached recordings. SK channel blockade caused a small depolarization in spike threshold and made it more variable, but there was no reduction in the maximal rate of rise during an action potential. Thus, the firing irregularity was not caused solely by a reduction in voltage-gated Na⁺ channel availability. Subthreshold voltage ramps triggered a large outward current that was sensitive to the initial holding potential and had properties similar to the A-type K⁺ current in GP neurons. In numerical simulations, the availability of both Na⁺ and A-type K⁺ channels during autonomous firing were reduced when SK channels were removed, and a nearly equal reduction in Na⁺ and K⁺ subthreshold-activated ion channel availability produced a large decrease in the neuron's slope conductance near threshold. This change made the neuron more sensitive to intrinsically generated noise. In vivo, this change would also enhance the sensitivity of GP neurons to small synaptic inputs.

The Functional Equivalence of Ascending and Parallel Fiber Inputs in Cerebellar Computation

Walter, J. T., Dizon, M.-J., Khodakhah, K. — 2009-07-01

At the center of the computational cerebellar circuitry are Purkinje cells, which integrate synaptic inputs from >150,000 granule cell inputs. Traditional theories of cerebellar function assume that all granule cell inputs are comparable. However, it has recently been suggested that the two anatomically distinct granule cell inputs, ascending and parallel fiber, have different functional roles. By systematically examining the efficacy of patches of granule cells with photostimulation, we found no differences in the efficacy of the two inputs in driving the activity of, or in producing postsynaptic currents in, Purkinje cells in cerebellar slices in vitro. We also found that the activity of Purkinje cells was significantly increased upon stimulation of lateral granule cells in vivo. Moreover, when we estimated parallel fiber and ascending apparent unitary EPSC amplitudes using photostimulation in cerebellar slices in vitro, we found them to be indistinguishable. These results are inconsistent with differential functional roles for these two inputs. Instead, our data support theories of cerebellar computation that consider granule cell inputs to be functionally comparable.

Sustained Conditioned Responses in Prelimbic Prefrontal Neurons Are Correlated with Fear Expression and Extinction Failure

Burgos-Robles, A., Vidal-Gonzalez, I., Quirk, G. J. — 2009-07-01

During auditory fear conditioning, it is well established that lateral amygdala (LA) neurons potentiate their response to the tone conditioned stimulus, and that this potentiation is required for conditioned fear behavior. Conditioned tone responses in LA, however, last only a few hundred milliseconds and cannot be responsible for sustained fear responses to a tone lasting tens of seconds. Recent evidence from inactivation and stimulation studies suggests that the prelimbic (PL) prefrontal cortex is necessary for expression of learned fears, but the timing of PL tone responses and correlations with fear behavior have not been studied. Using multichannel unit recording techniques in behaving rats, we observed sustained conditioned tone responses in PL that were correlated with freezing behavior on a second-to-second basis during the presentation of a 30 s tone. PL tone responses were also correlated with conditioned freezing across different experimental phases (habituation, conditioning, extinction). Moreover, the persistence of PL responses after extinction training was associated with failure to express extinction memory. Together with previous inactivation findings, the present results suggest that PL transforms transient amygdala inputs to a sustained output that drives conditioned fear responses and gates the expression of extinction. Given the relatively long latency of conditioned responses we observed in PL (~100 ms after tone onset), we propose that PL integrates inputs from the amygdala, hippocampus, and other cortical sources to regulate the expression of fear memories.

Perceptual Integration of Illusory and Imagined Kinesthetic Images

Thyrion, C., Roll, J.-P. — 2009-07-01

It is generally agreed that motor imagery involves kinesthetic sensations especially as far as first-person imagery is concerned. It was proposed to determine the extent to which motor imagery and vibration-induced illusory sensations of movement are integrated perceptually. Imagined and illusory hand movements were evoked both separately and in various combinations in 12 volunteers. After each trial, the participants were asked to draw the movement trajectory perceived. In all the subjects, propriomimetic vibration patterns applied to various wrist muscles induced spatially oriented or more complex illusory hand movements such as writing or drawing. Depending on the instructions, the subjects were also able to produce imagined hand movements in various directions and at two different velocities. When straight illusory and imagined movements were evoked simultaneously, all the subjects perceived a single movement trajectory, in which the direction and the velocity of the two ongoing sensations were exactly integrated. This perceptual integration also occurred in the case of more complex movements, such as writing and drawing, giving rise to the perception of original trajectories also combining the features of both motor images. Because these two kinesthetic images, the one intentionally and centrally induced and the other peripherally evoked, activate almost the same neural network including cortical sensory and motor areas, parietal regions, and the cerebellum, these results suggest that common processes may be involved in such a perceptual fusion. The nature of these common processes is discussed, and some fields of research in which these findings could potentially be applied are suggested.

Induction of Neuronal Vascular Endothelial Growth Factor Expression by cAMP in the Dentate Gyrus of the Hippocampus Is Required for Antidepressant-Like Behaviors

2009-07-01

The cAMP cascade and vascular endothelial growth factor (VEGF) are critical modulators of depression. Here we have tested whether the antidepressive effect of the cAMP cascade is mediated by VEGF in the adult hippocampus. We used a conditional genetic system in which the Aplysia octopamine receptor (Ap oa₁), a G_s-coupled receptor, is transgenically expressed in the forebrain neurons of mice. Chronic activation of the heterologous Ap oa₁ by its natural ligand evoked antidepressant-like behaviors, accompanied by enhanced phosphorylation of cAMP response element-binding protein and transcription of VEGF in hippocampal dentate gyrus (DG) neurons. Selective knockdown of VEGF in these cells during the period of cAMP elevation inhibited the antidepressant-like behaviors. These findings reveal a molecular interaction between the cAMP cascade and VEGF expression, and the pronounced behavioral consequences of this interaction shed light on the mechanism underlying neuronal VEGF functions in antidepression.

Inhibition of Autophagy Induction Delays Neuronal Cell Loss Caused by Dysfunctional ESCRT-III in Frontotemporal Dementia

Lee, J.-A, Gao, F.-B. — 2009-07-01

Autophagy is a conserved lysosomal protein degradation pathway whose precise roles in age-dependent neurodegenerative diseases remain largely unknown. Here we show that the autophagy inhibitor 3-methyladenine delays neuronal cell loss caused by dysfunctional endosomal sorting complex required for transport III (ESCRT-III), either through loss of its essential component mSnf7-2 or ectopic expression of the disease protein CHMP2B^Intron5, which is associated with frontotemporal dementia linked to chromosome 3. Neuronal loss was also delayed by reduced activity of the autophagy genes atg5 and atg7. However, the endosomal accumulation of ubiquitinated proteins induced by dysfunctional ESCRT-III was not significantly affected, further confirming the essential contribution of dysregulated autophagy pathway in neurodegeneration. These findings show that autophagic stress by excess accumulation of autophagosomes is detrimental to neuronal survival under certain neurodegenerative conditions.

Bistability and Non-Gaussian Fluctuations in Spontaneous Cortical Activity

Freyer, F., Aquino, K., Robinson, P. A., Ritter, P., Breakspear, M. — 2009-07-01

The brain is widely assumed to be a paradigmatic example of a complex, self-organizing system. As such, it should exhibit the classic hallmarks of nonlinearity, multistability, and "nondiffusivity" (large coherent fluctuations). Surprisingly, at least at the very large scale of neocortical dynamics, there is little empirical evidence to support this, and hence most computational and methodological frameworks for healthy brain activity have proceeded very reasonably from a purely linear and diffusive perspective. By studying the temporal fluctuations of power in human resting-state electroencephalograms, we show that, although these simple properties may hold true at some temporal scales, there is strong evidence for bistability and nondiffusivity in key brain rhythms. Bistability is manifest as nonclassic bursting between high- and low-amplitude modes in the alpha rhythm. Nondiffusivity is expressed through the irregular appearance of high amplitude "extremal" events in beta rhythm power fluctuations. The statistical robustness of these observations was confirmed through comparison with Gaussian-rendered phase-randomized surrogate data. Although there is a good conceptual framework for understanding bistability in cortical dynamics, the implications of the extremal events challenge existing frameworks for understanding large-scale brain systems.

Do You Feel My Pain? Racial Group Membership Modulates Empathic Neural Responses

Xu, X., Zuo, X., Wang, X., Han, S. — 2009-07-01

The pain matrix including the anterior cingulate cortex (ACC) mediates not only first person pain experience but also empathy for others' pain. It remains unknown, however, whether empathic neural responses of the pain matrix are modulated by racial in-group/out-group relationship. Using functional magnetic resonance imaging we demonstrate that, whereas painful stimulations applied to racial in-group faces induced increased activations in the ACC and inferior frontal/insula cortex in both Caucasians and Chinese, the empathic neural response in the ACC decreased significantly when participants viewed faces of other races. Our findings uncover neural mechanisms of an empathic bias toward racial in-group members.

Dopamine Signaling Is Required for Depolarization-Induced Slow Current in Cerebellar Purkinje Cells

Kim, Y. S., Shin, J. H., Hall, F. S., Linden, D. J. — 2009-07-01

Brief strong depolarization of cerebellar Purkinje cells produces a slow inward cation current. This current, called depolarization-induced slow current (DISC), is triggered by Ca influx in the Purkinje cell and is attenuated by a blocker of vesicular fusion. Previous work in other brain regions, such as the substantia nigra and ventral tegmental area, has shown that dopamine can be released from dendrites to produce paracrine and autocrine signaling. Here, we test the hypothesis that postsynaptic release of dopamine and autocrine activation of dopamine receptors is involved in DISC. Light immunohistochemistry showed that D₃ dopamine receptors, vesicular monoamine transporter type 2 (VMAT2), and dopamine plasma membrane transporters (DATs) were all expressed in cerebellar Purkinje cells. However, their expression was strongest in the gyrus region of cerebellar lobules IX and X. Comparison of DISC across lobules revealed that it was weak in the anterior portions of the cerebellum (lobules II, V, and VI) and strong in lobules IX and X. DISC was blocked by dopamine receptor antagonists (haloperidol, clozapine, eticlopride, and SCH23390). Likewise, DISC was strongly attenuated by inhibitors of VMAT (reserpine and tetrabenazine) and DAT (GBR12909 and rimcazole). These drugs did not produce DISC attenuation through blockade of depolarization-evoked Purkinje cell Ca transients. Purkinje cells in cerebellar slices derived from DAT-null mice expressed DISC, but this DISC ran down at a significantly higher rate than littermate controls. Together, these results suggest that strong Purkinje cell depolarization produces Ca-dependent release of vesicular postsynaptic dopamine that then excites Purkinje cells in an autocrine manner.

Cell Type-Specific Requirements for Heparan Sulfate Biosynthesis at the Drosophila Neuromuscular Junction: Effects on Synapse Function, Membrane Trafficking, and Mitochondrial Localization

Ren, Y., Kirkpatrick, C. A., Rawson, J. M., Sun, M., Selleck, S. B. — 2009-07-01

Heparan sulfate proteoglycans (HSPGs) are concentrated at neuromuscular synapses in many species, including Drosophila. We have established the physiological and patterning functions of HSPGs at the Drosophila neuromuscular junction by using mutations that block heparan sulfate synthesis or sulfation to compromise HSPG function. The mutant animals showed defects in synaptic physiology and morphology suggesting that HSPGs function both presynaptically and postsynaptically; these defects could be rescued by appropriate transgene expression. Of particular interest were selective disruptions of mitochondrial localization, abnormal distributions of Golgi and endoplasmic reticulum markers in the muscle, and a markedly increased level of stimulus-dependent endocytosis in the motoneuron. Our data support the emerging view that HSPG functions are not limited to the cell surface and matrix environments, but also affect a diverse set of cellular processes including membrane trafficking and organelle distributions.

TWIK-1 and TREK-1 Are Potassium Channels Contributing Significantly to Astrocyte Passive Conductance in Rat Hippocampal Slices

2009-07-01

Expression of a linear current–voltage (I–V) relationship (passive) K⁺ membrane conductance is a hallmark of mature hippocampal astrocytes. However, the molecular identifications of the K⁺ channels underlying this passive conductance remain unknown. We provide the following evidence supporting significant contribution of the two-pore domain K⁺ channel (K_2P) isoforms, TWIK-1 and TREK-1, to this conductance. First, both passive astrocytes and the cloned rat TWIK-1 and TREK-1 channels expressed in CHO cells conduct significant amounts of Cs⁺ currents, but vary in their relative P_Cs/P_K permeability, 0.43, 0.10, and 0.05, respectively. Second, quinine, which potently inhibited TWIK-1 (IC₅₀ = 85 µm) and TREK-1 (IC₅₀ = 41 µm) currents, also inhibited astrocytic passive conductance by 58% at a concentration of 200 µm. Third, a moderate sensitivity of passive conductance to low extracellular pH (6.0) supports a combined expression of acid-insensitive TREK-1, and to a lesser extent, acid-sensitive TWIK-1. Fourth, the astrocyte passive conductance showed low sensitivity to extracellular Ba²⁺, and extracellular Ba²⁺ blocked TWIK-1 channels at an IC₅₀ of 960 µm and had no effect on TREK-1 channels. Finally, an immunocytochemical study showed colocalization of TWIK-1 and TREK-1 proteins with the astrocytic markers GLAST and GFAP in rat hippocampal stratum radiatum. In contrast, another K_2P isoform TASK-1 was mainly colocalized with the neuronal marker NeuN in hippocampal pyramidal neurons and was expressed at a much lower level in astrocytes. These results support TWIK-1 and TREK-1 as being the major components of the long-sought K⁺ channels underlying the passive conductance of mature hippocampal astrocytes.

Endogenous Nitric Oxide Is a Key Promoting Factor for Initiation of Seizure-Like Events in Hippocampal and Entorhinal Cortex Slices

2009-07-01

Nitric oxide (NO) modulates synaptic transmission, and its level is elevated during epileptic activity in animal models of epilepsy. However, the role of NO for development and maintenance of epileptic activity is controversial. We studied this aspect in rat organotypic hippocampal slice cultures and acute hippocampal–entorhinal cortex slices from wild-type and neuronal NO synthase (nNOS) knock-out mice combining electrophysiological and fluorescence imaging techniques. Slice cultures contained nNOS-positive neurons and an elaborated network of nNOS-positive fibers. Lowering of extracellular Mg²⁺ concentration led to development of epileptiform activity and increased NO formation as revealed by NO-selective probes, 4-amino-5-methylamino-2',7'-difluorofluorescein and 1,2-diaminoanthraquinone sulfate. NO deprivation by NOS inhibitors and NO scavengers caused depression of both EPSCs and IPSCs and prevented initiation of seizure-like events (SLEs) in 75% of slice cultures and 100% of hippocampal–entorhinal cortex slices. This effect was independent of the guanylyl cyclase/cGMP pathway. Suppression of SLE initiation in acute slices from mice was achieved by both the broad-spectrum NOS inhibitor N-methyl-l-arginine acetate and the nNOS-selective inhibitor 7-nitroindazole, whereas inhibition of inducible NOS by aminoguanidine was ineffective, suggesting that nNOS activity was crucial for SLE initiation. Additional evidence was obtained from knock-out animals because SLEs developed in a significantly lower percentage of slices from nNOS^–/– mice and showed different characteristics, such as prolongation of onset latency and higher variability of SLE intervals. We conclude that enhancement of synaptic transmission by NO under epileptic conditions represents a positive feedback mechanism for the initiation of seizure-like events.

Reelin and Notch1 Cooperate in the Development of the Dentate Gyrus

Sibbe, M., Forster, E., Basak, O., Taylor, V., Frotscher, M. — 2009-07-01

The development of the hippocampal dentate gyrus is a complex process in which several signaling pathways are involved and likely interact with each other. The extracellular matrix molecule Reelin is necessary both for normal development of the dentate gyrus radial glia and neuronal migration. In Reelin-deficient Reeler mice, the hippocampal radial glial scaffold fails to form, and granule cells are dispersed throughout the dentate gyrus. Here, we show that both formation of the radial glia scaffold and lamination of the dentate gyrus depend on intact Notch signaling. Inhibition of Notch signaling in organotypic hippocampal slice cultures induced a phenotype reminiscent of the Reelin-deficient hippocampus, i.e., a reduced density of radial glia fibers and granule cell dispersion. Moreover, a Reelin-dependent rescue of the Reeler phenotype was blocked by inhibition of Notch activation. In the Reeler dentate gyrus, we found reduced Notch1 signaling; the activated Notch intracellular domain as well as the transcriptional targets, brain lipid-binding protein, and Hes5 are decreased. Disabled1, a component of the Reelin-signaling pathway colocalizes with Notch1, thus indicating a direct interaction between the Reelin- and Notch1-signaling pathways. These results suggest that Reelin enhances Notch1 signaling, thereby contributing to the formation of the radial glial scaffold and the normal development of the dentate gyrus.

Distinct Cerebellar Contributions to Intrinsic Connectivity Networks

2009-07-01

Convergent data from various scientific approaches strongly implicate cerebellar systems in nonmotor functions. The functional anatomy of these systems has been pieced together from disparate sources, such as animal studies, lesion studies in humans, and structural and functional imaging studies in humans. To better define this distinct functional anatomy, in the current study we delineate the role of the cerebellum in several nonmotor systems simultaneously and in the same subjects using resting state functional connectivity MRI. Independent component analysis was applied to resting state data from two independent datasets to identify common cerebellar contributions to several previously identified intrinsic connectivity networks (ICNs) involved in executive control, episodic memory/self-reflection, salience detection, and sensorimotor function. We found distinct cerebellar contributions to each of these ICNs. The neocerebellum participates in (1) the right and left executive control networks (especially crus I and II), (2) the salience network (lobule VI), and (3) the default-mode network (lobule IX). Little to no overlap was detected between these cerebellar regions and the sensorimotor cerebellum (lobules V–VI). Clusters were also located in pontine and dentate nuclei, prominent points of convergence for cerebellar input and output, respectively. The results suggest that the most phylogenetically recent part of the cerebellum, particularly crus I and II, make contributions to parallel cortico-cerebellar loops involved in executive control, salience detection, and episodic memory/self-reflection. The largest portions of the neocerebellum take part in the executive control network implicated in higher cognitive functions such as working memory.

Odor-Evoked Neural Oscillations in Drosophila Are Mediated by Widely Branching Interneurons

Tanaka, N. K., Ito, K., Stopfer, M. — 2009-07-01

Stimulus-evoked oscillatory synchronization of neurons has been observed in a wide range of species. Here, we combined genetic strategies with paired intracellular and local field potential (LFP) recordings from the intact brain of Drosophila to study mechanisms of odor-evoked neural oscillations. We found common food odors at natural concentrations elicited oscillations in LFP recordings made from the mushroom body (MB), a site of sensory integration and analogous to the vertebrate piriform cortex. The oscillations were reversibly abolished by application of the GABA_a blocker picrotoxin. Intracellular recordings from local and projection neurons within the antennal lobe (AL) (analogous to the olfactory bulb) revealed odor-elicited spikes and subthreshold membrane potential oscillations that were tightly phase locked to LFP oscillations recorded downstream in the MBs. These results suggested that, as in locusts, odors may elicit the oscillatory synchronization of AL neurons by means of GABAergic inhibition from local neurons (LNs). An analysis of the morphologies of genetically distinguished LNs revealed two populations of GABAergic neurons in the AL. One population of LNs innervated parts of glomeruli lacking terminals of receptor neurons, whereas the other branched more widely, innervating throughout the glomeruli, suggesting that the two populations might participate in different neural circuits. To test the functional roles of these LNs, we used the temperature-sensitive dynamin mutant gene shibire to conditionally and reversibly block chemical transmission from each or both of these populations of LNs. We found only the more widely branching population of LNs is necessary for generating odor-elicited oscillations.

Uncovering the Neural Signature of Lapsing Attention: Electrophysiological Signals Predict Errors up to 20 s before They Occur

2009-07-01

The extent to which changes in brain activity can foreshadow human error is uncertain yet has important theoretical and practical implications. The present study examined the temporal dynamics of electrocortical signals preceding a lapse of sustained attention. Twenty-one participants performed a continuous temporal expectancy task, which involved continuously monitoring a stream of regularly alternating patterned stimuli to detect a rarely occurring target stimulus whose duration was 40% longer. The stimulus stream flickered at a rate of 25 Hz to elicit a steady-state visual-evoked potential (SSVEP), which served as a continuous measure of basic visual processing. Increasing activity in the band (8–14 Hz) was found beginning ~20 s before a missed target. This was followed by decreases in the amplitude of two event-related components over a short pretarget time frame: the frontal P3 (3–4 s) and contingent-negative variation (during the target interval). In contrast, SSVEP amplitude before hits and misses was closely matched, suggesting that the efficacy of ongoing basic visual processing was unaffected. Our results show that the specific neural signatures of attentional lapses are registered in the EEG up to 20 s before an error.

Mrgprd Enhances Excitability in Specific Populations of Cutaneous Murine Polymodal Nociceptors

2009-07-01

The Mas-related G protein-coupled receptor D (Mrgprd) is selectively expressed in nonpeptidergic nociceptors that innervate the outer layers of mammalian skin. The function of Mrgprd in nociceptive neurons and the physiologically relevant somatosensory stimuli that activate Mrgprd-expressing (Mrgprd⁺) neurons are currently unknown. To address these issues, we studied three Mrgprd knock-in mouse lines using an ex vivo somatosensory preparation to examine the role of the Mrgprd receptor and Mrgprd⁺ afferents in cutaneous somatosensation. In mouse hairy skin, Mrgprd, as marked by expression of green fluorescent protein reporters, was expressed predominantly in the population of nonpeptidergic, TRPV1-negative, C-polymodal nociceptors. In mice lacking Mrgprd, this population of nociceptors exhibited decreased sensitivity to cold, heat, and mechanical stimuli. Additionally, in vitro patch-clamp studies were performed on cultured dorsal root ganglion neurons from Mrgprd^–/– and Mrgprd^+/– mice. These studies revealed a higher rheobase in neurons from Mrgprd^–/– mice than from Mrgprd^+/– mice. Furthermore, the application of the Mrgprd ligand β-alanine significantly reduced the rheobase and increased the firing rate in neurons from Mrgprd^+/– mice but was without effect in neurons from Mrgprd^–/– mice. Our results demonstrate that Mrgprd influences the excitability of polymodal nonpeptidergic nociceptors to mechanical and thermal stimuli.

This Week in The Journal

2009-06-24

Mean-Variance or Prospect Theory? The Nature of Value Representations in the Human Brain

Boorman, E. D., Sallet, J. — 2009-06-24

Functional Magnetic Resonance Imaging-Assessed Brain Responses during an Executive Task Depend on Interaction of Sleep Homeostasis, Circadian Phase, and PER3 Genotype

2009-06-24

Cognition is regulated across the 24 h sleep-wake cycle by circadian rhythmicity and sleep homeostasis through unknown brain mechanisms. We investigated these mechanisms in a functional magnetic resonance imaging study of executive function using a working memory 3-back task during a normal sleep-wake cycle and during sleep loss. The study population was stratified according to homozygosity for a variable-number (4 or 5) tandem-repeat polymorphism in the coding region of the clock gene PERIOD3. This polymorphism confers vulnerability to sleep loss and circadian misalignment through its effects on sleep homeostasis. In the less-vulnerable genotype, no changes were observed in brain responses during the normal-sleep wake cycle. During sleep loss, these individuals recruited supplemental anterior frontal, temporal and subcortical regions, while executive function was maintained. In contrast, in the vulnerable genotype, activation in a posterior prefrontal area was already reduced when comparing the evening to the morning during a normal sleep-wake cycle. Furthermore, in the morning after a night of sleep loss, widespread reductions in activation in prefrontal, temporal, parietal and occipital areas were observed in this genotype. These differences occurred in the absence of genotype-dependent differences in circadian phase. The data show that dynamic changes in brain responses to an executive task evolve across the sleep-wake and circadian cycles in a regionally specific manner that is determined by a polymorphism which affects sleep homeostasis. The findings support a model of individual differences in executive control, in which the allocation of prefrontal resources is constrained by sleep pressure and circadian phase.

Amyloid Reduction by Amyloid-{beta} Vaccination Also Reduces Mouse Tau Pathology and Protects from Neuron Loss in Two Mouse Models of Alzheimer's Disease

2009-06-24

Shown to lower amyloid deposits and improve cognition in APP transgenic mouse models, immunotherapy appears to be a promising approach for the treatment of Alzheimer's disease (AD). Due to limitations in available animal models, however, it has been unclear whether targeting amyloid is sufficient to reduce the other pathological hallmarks of AD—namely, accumulation of pathological, nonmutated tau and neuronal loss. We have now developed two transgenic mouse models (APPSw/NOS2^–/– and APPSwDI/NOS2^–/–) that more closely model AD. These mice show amyloid pathology, hyperphosphorylated and aggregated normal mouse tau, significant neuron loss, and cognitive deficits. Aβ_1–42 or KLH vaccinations were started in these animals at 12 months, when disease progression and cognitive decline are well underway, and continued for 4 months. Vaccinated APPSwDI/NOS2^–/– mice, which have predominantly vascular amyloid pathology, showed a 30% decrease in brain Aβ and a 35–45% reduction in hyperphosphorylated tau. Neuron loss and cognitive deficits were partially reduced. In APPSw/NOS2^–/– vaccinated mice, brain Aβ was reduced by 65–85% and hyperphosphorylated tau by 50–60%. Furthermore, neurons were completely protected, and memory deficits were fully reversed. Microhemorrhage was observed in all vaccinated APPSw/NOS2^–/– mice and remains a significant adverse event associated with immunotherapy. Nevertheless, by providing evidence that reducing amyloid pathology also reduces nonmutant tau pathology and blocks neuron loss, these data support the development of amyloid-lowering therapies for disease-modifying treatment of AD.

GABA-cAMP Response Element-Binding Protein Signaling Regulates Maturation and Survival of Newly Generated Neurons in the Adult Hippocampus

2009-06-24

Survival and integration of new neurons in the hippocampal circuit are rate-limiting steps in adult hippocampal neurogenesis. Neuronal network activity is a major regulator of these processes, yet little is known about the respective downstream signaling pathways. Here, we investigate the role of cAMP response element-binding protein (CREB) signaling in adult hippocampal neurogenesis. CREB is activated in new granule neurons during a distinct developmental period. Loss of CREB function in a cell-autonomous manner impairs dendritic development, decreases the expression of the neurogenic transcription factor NeuroD and of the neuronal microtubule-associated protein, doublecortin (DCX), and compromises the survival of newborn neurons. In addition, GABA-mediated excitation regulates CREB activation at early developmental stages. Importantly, developmental defects after loss of GABA-mediated excitation can be compensated by enhanced CREB signaling. These results indicate that CREB signaling is a central pathway in adult hippocampal neurogenesis, regulating the development and survival of new hippocampal neurons downstream of GABA-mediated excitation.

Detection of Interaural Time Differences in the Alligator

Carr, C. E., Soares, D., Smolders, J., Simon, J. Z. — 2009-06-24

The auditory systems of birds and mammals use timing information from each ear to detect interaural time difference (ITD). To determine whether the Jeffress-type algorithms that underlie sensitivity to ITD in birds are an evolutionarily stable strategy, we recorded from the auditory nuclei of crocodilians, who are the sister group to the birds. In alligators, precisely timed spikes in the first-order nucleus magnocellularis (NM) encode the timing of sounds, and NM neurons project to neurons in the nucleus laminaris (NL) that detect interaural time differences. In vivo recordings from NL neurons show that the arrival time of phase-locked spikes differs between the ipsilateral and contralateral inputs. When this disparity is nullified by their best ITD, the neurons respond maximally. Thus NL neurons act as coincidence detectors. A biologically detailed model of NL with alligator parameters discriminated ITDs up to 1 kHz. The range of best ITDs represented in NL was much larger than in birds, however, and extended from 0 to 1000 µs contralateral, with a median ITD of 450 µs. Thus, crocodilians and birds employ similar algorithms for ITD detection, although crocodilians have larger heads.

Complexin-I Is Required for High-Fidelity Transmission at the Endbulb of Held Auditory Synapse

2009-06-24

Complexins (CPXs I–IV) presumably act as regulators of the SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor) complex, but their function in the intact mammalian nervous system is not well established. Here, we explored the role of CPXs in the mouse auditory system. Hearing was impaired in CPX I knock-out mice but normal in knock-out mice for CPXs II, III, IV, and III/IV as measured by auditory brainstem responses. Complexins were not detectable in cochlear hair cells but CPX I was expressed in spiral ganglion neurons (SGNs) that give rise to the auditory nerve. Ca²⁺-dependent exocytosis of inner hair cells and sound encoding by SGNs were unaffected in CPX I knock-out mice. In the absence of CPX I, the resting release probability in the endbulb of Held synapses of the auditory nerve fibers with bushy cells in the cochlear nucleus was reduced. As predicted by computational modeling, bushy cells had decreased spike rates at sound onset as well as longer and more variable first spike latencies explaining the abnormal auditory brainstem responses. In addition, we found synaptic transmission to outlast the stimulus at many endbulb of Held synapses in vitro and in vivo, suggesting impaired synchronization of release to stimulus offset. Although sound encoding in the cochlea proceeds in the absence of complexins, CPX I is required for faithful processing of sound onset and offset in the cochlear nucleus.

Spatial Pattern Coding of Sensory Information by Climbing Fiber-Evoked Calcium Signals in Networks of Neighboring Cerebellar Purkinje Cells

Schultz, S. R., Kitamura, K., Post-Uiterweer, A., Krupic, J., Hausser, M. — 2009-06-24

Climbing fiber input produces complex spike synchrony across populations of cerebellar Purkinje cells oriented in the parasagittal axis. Elucidating the fine spatial structure of this synchrony is crucial for understanding its role in the encoding and processing of sensory information within the olivocerebellar cortical circuit. We investigated these issues using in vivo multineuron two-photon calcium imaging in combination with information theoretic analysis. Spontaneous dendritic calcium transients linked to climbing fiber input were observed in multiple neighboring Purkinje cells. Spontaneous synchrony of calcium transients between individual Purkinje cells falls off over ~200 µm mediolaterally, consistent with the presence of cerebellar microzones organized by climbing fiber input. Synchrony was increased after administration of harmaline, consistent with an olivary origin. Periodic sensory stimulation also resulted in a transient increase of synchrony after stimulus onset. To examine how synchrony affects the neural population code provided by the spatial pattern of complex spikes, we analyzed its information content. We found that spatial patterns of calcium events from small ensembles of cells provided substantially more stimulus information (59% more for seven-cell ensembles) than available by counting events across the pool without taking into account spatial origin. Information theoretic analysis indicated that, rather than contributing significantly to sensory coding via stimulus dependence, correlational effects on sensory coding are dominated by redundancy attributable to the prevalent spontaneous synchrony. The olivocerebellar circuit thus uses a labeled line code to report sensory signals, leaving open a role for synchrony in flexible selection of signals for output to deep cerebellar nuclei.

Visual Salience Affects Performance in a Working Memory Task

Fine, M. S., Minnery, B. S. — 2009-06-24

Many studies of bottom-up visual attention have focused on identifying which features of a visual stimulus render it salient—i.e., make it "pop out" from its background—and on characterizing the extent to which salience predicts eye movements under certain task conditions. However, few studies have examined the relationship between salience and other cognitive functions, such as memory. We examined the impact of visual salience in an object–place working memory task, in which participants memorized the position of 3–5 distinct objects (icons) on a two-dimensional map. We found that their ability to recall an object's spatial location was positively correlated with the object's salience, as quantified using a previously published computational model (Itti et al., 1998). Moreover, the strength of this relationship increased with increasing task difficulty. The correlation between salience and error could not be explained by a biasing of overt attention in favor of more salient icons during memorization, since eye-tracking data revealed no relationship between an icon's salience and fixation time. Our findings show that the influence of bottom-up attention extends beyond oculomotor behavior to include the encoding of information into memory.

Information about Complex Fingertip Parameters in Individual Human Tactile Afferent Neurons

Saal, H. P., Vijayakumar, S., Johansson, R. S. — 2009-06-24

Although information in tactile afferent neurons represented by firing rates has been studied extensively over nearly a century, recent studies suggest that precise spike timing might be more important than firing rates. Here, we used information theory to compare the information content in the discharges of 92 tactile afferents distributed over the entire terminal segment of the fingertip when it was contacted by surfaces with different curvatures and force directions representative of everyday manipulations. Estimates of the information content with regard to curvature and force direction based on the precise timing of spikes were at least 2.2 times and 1.6 times, respectively, larger than that of spike counts during a 125 ms period of force increase. Moreover, the information regarding force direction based on the timing of the very first elicited spike was comparable with that provided by spike counts and more than twice as large with respect to object shape. For all encoding schemes, afferents terminating close to the stimulation site tended to convey more information about surface curvature than more remote afferents that tended to convey more information about force direction. Finally, coding schemes based on spike timing and spike counts overall contributed mostly independent information. We conclude that information about tactile stimuli in timing of spikes in primary afferents, even if limited to the first spikes, surpasses that contained in firing rates and that these measures of afferents' responses might capture different aspects of the stimulus.

Searching for Targets within the Spatial Layout of Visual Short-Term Memory

Kuo, B.-C., Rao, A., Lepsien, J., Nobre, A. C. — 2009-06-24

Recent studies have revealed that the internal representations that we construct from the environment and maintain in visual short-term memory (VSTM) to guide behavior are highly flexible and can be selectively modulated according to our task goals and expectations. In the current study, we conducted two experiments to compare and contrast neural mechanisms of selective attention related to searching for target items within perceptual versus VSTM representations. We used event-related potentials to investigate whether searching for relevant target items from within VSTM representations involves spatially specific biasing of neural activity in a manner analogous to that which occurs during visual search for target items in perceptual arrays. The results, replicated across the two experiments, revealed that selection of a target object within a search array maintained in VSTM proceeds through a similar mechanism as that in the perceptual domain. In line with previous results, N2pc potentials were obtained when targets were identified within a perceptual visual-search array. Interestingly, equivalent N2pcs, with similar time courses and scalp distributions, were also elicited when target items were identified within a VSTM representation. The findings reinforce the notion of highly flexible VSTM representations that can be modulated according to task goals and suggest a large degree of overlap in the spatially specific neural mechanisms of target selection across the perceptual and VSTM domains.

Neuroligin 2 Controls the Maturation of GABAergic Synapses and Information Processing in the Retina

2009-06-24

In the present study, we investigated the role of Neuroligin 2 (NL2) in synaptic transmission and network function using the mouse retina as a model circuit. We show that NL2 is preferentially located at GABAergic rather than glycinergic or glutamatergic postsynapses. The absence of NL2 from the retina resulted in a severe reduction of GABA_A receptor clustering, and in subtle alterations of the retinal circuitry. Light processing was impaired accordingly, and retinal ganglion cells, the output neurons of the retina, showed increased basal activity and altered coding of visual information. Together, our data indicate that NL2 is essential for the functional integrity of GABAergic signaling and as a consequence, for information processing in the retina.

Visualization of Chemokine Receptor Activation in Transgenic Mice Reveals Peripheral Activation of CCR2 Receptors in States of Neuropathic Pain

2009-06-24

CCR2 chemokine receptor signaling has been implicated in the generation of diverse types of neuropathology, including neuropathic pain. For example, ccr2 knock-out mice are resistant to the establishment of neuropathic pain, and mice overexpressing its ligand, monocyte chemoattractant protein-1 (MCP1; also known as CCL2), show enhanced pain sensitivity. However, whether CCR2 receptor activation occurs in the central or peripheral nervous system in states of neuropathic pain has not been clear. We developed a novel method for visualizing CCR2 receptor activation in vivo by generating bitransgenic reporter mice in which the chemokine receptor CCR2 and its ligand MCP1 were labeled by the fluorescent proteins enhanced green fluorescent protein and monomeric red fluorescent protein-1, respectively. CCR2 receptor activation under conditions such as acute inflammation and experimental autoimmune encephalomyelitis could be faithfully visualized by using these mice. We examined the status of CCR2 receptor activation in a demyelination injury model of neuropathic pain and found that MCP1-induced CCR2 receptor activation mainly occurred in the peripheral nervous system, including the injured peripheral nerve and dorsal root ganglia. These data explain the rapid antinociceptive effects of peripherally administered CCR2 antagonists under these circumstances, suggesting that CCR2 antagonists may ameliorate pain by inhibiting CCR2 receptor activation in the periphery. The method developed here for visualizing CCR2 receptor activation in vivo may be extended to G-protein-coupled receptors (GPCRs) in general and will be valuable for studying intercellular GPCR-mediated communication in vivo.

Ca2+ Regulation of Dynamin-Independent Endocytosis in Cortical Astrocytes

Jiang, M., Chen, G. — 2009-06-24

Astrocytes release ATP and glutamate through vesicular exocytosis to mediate neuron–glial interactions. In contrast to exocytosis, the endocytic pathways in astroglial cells are poorly understood. Here, we identify a constitutive endocytic pathway in cultured astrocytes that is dependent on neither clathrin nor dynamin. This dynamin-independent endocytic pathway is regulated by Rab5, an early endosome protein. The endocytosed vesicles show fast transition from early endosomes to late endosomes and lysosomes within a few minutes. Interestingly, this clathrin- and dynamin-independent endocytosis in astrocytes is potently regulated by intracellular Ca²⁺. ATP and glutamate greatly enhance the dynamin-independent endocytosis through elevating the intracellular Ca²⁺. In addition, amyloid-β peptide (Aβ) also enhances the dynamin-independent endocytosis by inducing Ca²⁺ transients in astrocytes. These results demonstrate a novel endocytic pathway in glial cells that is dynamin independent but tightly regulated by intracellular Ca²⁺. The regulation by ATP, glutamate, and Aβ suggests an important role of the dynamin-independent endocytosis in both physiological and pathological conditions.

Phosphodiesterase 5 Inhibition Improves Synaptic Function, Memory, and Amyloid-{beta} Load in an Alzheimer's Disease Mouse Model

2009-06-24

Memory loss, synaptic dysfunction, and accumulation of amyloid β-peptides (Aβ) are major hallmarks of Alzheimer's disease (AD). Downregulation of the nitric oxide/cGMP/cGMP-dependent protein kinase/c-AMP responsive element-binding protein (CREB) cascade has been linked to the synaptic deficits after Aβ elevation. Here, we report that the phosphodiesterase 5 inhibitor (PDE5) sildenafil (Viagra), a molecule that enhances phosphorylation of CREB, a molecule involved in memory, through elevation of cGMP levels, is beneficial against the AD phenotype in a mouse model of amyloid deposition. We demonstrate that the inhibitor produces an immediate and long-lasting amelioration of synaptic function, CREB phosphorylation, and memory. This effect is also associated with a long-lasting reduction of Aβ levels. Given that side effects of PDE5 inhibitors are widely known and do not preclude their administration to a senile population, these drugs have potential for the treatment of AD and other diseases associated with elevated Aβ levels.

A Cholinergic-Dependent Role for the Entorhinal Cortex in Trace Fear Conditioning

Esclassan, F., Coutureau, E., Di Scala, G., Marchand, A. R. — 2009-06-24

Trace conditioning is considered a model of higher cognitive involvement in simple associative tasks. Studies of trace conditioning have shown that cortical areas and the hippocampal formation are required to associate events that occur at different times. However, the mechanisms that bridge the trace interval during the acquisition of trace conditioning remain unknown. In four experiments with fear conditioning in rats, we explored the involvement of the entorhinal cortex (EC) in the acquisition of fear under a trace-30 s protocol. We first determined that pretraining neurotoxic lesions of the EC selectively impaired trace-, but not delay-conditioned fear as evaluated by freezing behavior. A local cholinergic deafferentation of the EC using 192-IgG-saporin did not replicate this deficit, presumably because cholinergic interneurons were spared by the toxin. However, pretraining local blockade of EC muscarinic receptors with the M1 antagonist pirenzepine yielded a specific and dose-dependent deficit in trace-conditioned responses. The same microinjections performed after conditioning were without effect on trace fear responses. These effects of blocking M1 receptors are consistent with the notion that conditioned stimulus (CS)-elicited, acetylcholine-dependent persistent activities in the EC are needed to maintain a representation of a tone CS across the trace interval during the acquisition of trace conditioning. This function of the EC is consistent with recent views of this region as a short-term stimulus buffer.

GABAergic Neurons of the Medial Septum Lead the Hippocampal Network during Theta Activity

Hangya, B., Borhegyi, Z., Szilagyi, N., Freund, T. F., Varga, V. — 2009-06-24

Information processing in the hippocampus critically relies on its reciprocal interaction with the medial septum (MS). Synchronization of the septo-hippocampal system was demonstrated during both major hippocampal activity states, the regular theta rhythm and the large amplitude irregular activity. Previous experimental and modeling data suggest that the MS provides rhythmic drive to the hippocampus, and hippocampo-septal feedback synchronizes septal pacemaker units. However, this view has recently been questioned based on the possibility of intrahippocampal theta genesis. Previously, we identified putative pacemaker neurons expressing parvalbumin (PV) and/or the pacemaker hyperpolarization-activated and cyclic nucleotide-gated nonselective cation channel (HCN) in the MS. In this study, by analyzing the temporal relationship of activity between the PV/HCN-containing medial septal neurons and hippocampal local field potential, we aimed to uncover whether the sequence of events during theta formation supports the classic view of septal drive or the challenging theory of hippocampal pacing of theta. Importantly, by implementing a circular statistical method, a temporal lead of these septal neurons over the hippocampus was observed on the course of theta synchronization. Moreover, the activity of putative hippocampal interneurons also preceded hippocampal local field theta, but by a shorter time period compared with PV/HCN-containing septal neurons. Using the concept of mutual information, the action potential series of PV/HCN-containing neurons shared higher amount of information with hippocampal field oscillation than PV/HCN-immunonegative cells. Thus, a pacemaker neuron population of the MS leads hippocampal activity, presumably via the synchronization of hippocampal interneurons.

Nonmotor Symptoms of Parkinson's Disease Revealed in an Animal Model with Reduced Monoamine Storage Capacity

2009-06-24

Parkinson's disease (PD) is a progressive neurodegenerative disorder that is characterized by the loss of dopamine neurons in the substantia nigra pars compacta, culminating in severe motor symptoms, including resting tremor, rigidity, bradykinesia, and postural instability. In addition to motor deficits, there are a variety of nonmotor symptoms associated with PD. These symptoms generally precede the onset of motor symptoms, sometimes by years, and include anosmia, problems with gastrointestinal motility, sleep disturbances, sympathetic denervation, anxiety, and depression. Previously, we have shown that mice with a 95% genetic reduction in vesicular monoamine transporter expression (VMAT2-deficient, VMAT2 LO) display progressive loss of striatal dopamine, l-DOPA-responsive motor deficits, -synuclein accumulation, and nigral dopaminergic cell loss. We hypothesized that since these animals exhibit deficits in other monoamine systems (norepinephrine and serotonin), which are known to regulate some of these behaviors, the VMAT2-deficient mice may display some of the nonmotor symptoms associated with PD. Here we report that the VMAT2-deficient mice demonstrate progressive deficits in olfactory discrimination, delayed gastric emptying, altered sleep latency, anxiety-like behavior, and age-dependent depressive behavior. These results suggest that the VMAT2-deficient mice may be a useful model of the nonmotor symptoms of PD. Furthermore, monoamine dysfunction may contribute to many of the nonmotor symptoms of PD, and interventions aimed at restoring monoamine function may be beneficial in treating the disease.

Novelty Enhancements in Memory Are Dependent on Lateral Prefrontal Cortex

Kishiyama, M. M., Yonelinas, A. P., Knight, R. T. — 2009-06-24

Physiological evidence indicates that several brain regions, including the medial temporal lobes and prefrontal cortex (PFC), are involved in processing events that are novel or distinctive in their immediate context. However, behavioral studies that investigate whether these regions are critical for producing stimulus novelty advantages in memory are limited. For example, evidence from an animal lesion study indicated that the PFC is involved in stimulus novelty effects, but this has not been examined in humans. In the current study, we used a von Restorff novelty paradigm to test a large cohort of lateral PFC patients (n = 16). We found that patients with lateral PFC damage were impaired in recollection- and familiarity-based recognition, and they did not exhibit a normal memory advantage for novel compared with non-novel items. These results provide neuropsychological evidence supporting a key role for the lateral PFC in producing stimulus novelty advantages in memory.

Cholinergic Stimulation Enhances Neural Activity Associated with Encoding but Reduces Neural Activity Associated with Retrieval in Humans

Kukolja, J., Thiel, C. M., Fink, G. R. — 2009-06-24

The cerebral cholinergic system is centrally involved in memory formation. Studies in rodents suggest that cholinergic stimulation may facilitate encoding of new information but may interfere with retrieval. We investigated the effect of cholinergic stimulation on encoding and retrieval of episodic memory in humans. We also tested whether the putative benefit of cholinergic stimulation on memory function depends on individual baseline performance. Since such effects were expected to be greatest in an older population resulting from an age-related degeneration of the cholinergic system, we recruited 22 healthy older subjects (51–68 years) for an event-related functional magnetic resonance imaging experiment. In two separate scanning sessions, subjects encoded and retrieved items and their spatial context under cholinergic stimulation or placebo with the acetylcholine-esterase inhibitor physostigmine or saline being administered intravenously in a double-blind cross-over design. Baseline performance was recorded at a separate occasion without scanning. Cholinergic stimulation enhanced neural activity for successful versus unsuccessful spatial context encoding in the right hippocampus but reduced activity for successful versus unsuccessful spatial context retrieval in the right amygdala. These data may bridge the gap between rodent and human studies by showing that also in man cholinergic stimulation enhances encoding but interferes with retrieval on a neural level. Furthermore, baseline performance negatively correlated with the effect of cholinergic stimulation. Thus, participants who were worse at baseline benefited more from cholinergic stimulation than those who had better baseline values, indicating that a cholinergic deficit contributes to the memory decline even in healthy older subjects.

Focal Adhesion Kinase Acts Downstream of EphB Receptors to Maintain Mature Dendritic Spines by Regulating Cofilin Activity

Shi, Y., Pontrello, C. G., DeFea, K. A., Reichardt, L. F., Ethell, I. M. — 2009-06-24

Dendritic spines are the postsynaptic sites of most excitatory synapses in the brain and are highly enriched in polymerized F-actin, which drives the formation and maintenance of mature dendritic spines and synapses. We propose that suppressing the activity of the actin-severing protein cofilin plays an important role in the stabilization of mature dendritic spines, and is accomplished through an EphB receptor–focal adhesion kinase (FAK) pathway. Our studies revealed that Cre-mediated knock-out of loxP-flanked fak prompted the reversion of mature dendritic spines to an immature filopodial-like phenotype in primary hippocampal cultures. The effects of FAK depletion on dendritic spine number, length, and morphology were rescued by the overexpression of the constitutively active FAK^Y397E, but not FAK^Y397F, indicating the significance of FAK activation by phosphorylation on tyrosine 397. Our studies demonstrate that FAK acts downstream of EphB receptors in hippocampal neurons and EphB2–FAK signaling controls the stability of mature dendritic spines by promoting cofilin phosphorylation, thereby inhibiting cofilin activity. While constitutively active nonphosphorylatable cofilin^S3A induced an immature spine profile, phosphomimetic cofilin^S3D restored mature spine morphology in neurons with disrupted EphB activity or lacking FAK. Further, we found that EphB-mediated regulation of cofilin activity at least partially depends on the activation of Rho-associated kinase (ROCK) and LIMK-1. These findings indicate that EphB2-mediated dendritic spine stabilization relies, in part, on the ability of FAK to activate the RhoA–ROCK–LIMK-1 pathway, which functions to suppress cofilin activity and inhibit cofilin-mediated dendritic spine remodeling.

Glutamate Transporter Coupling to Na,K-ATPase

2009-06-24

Deactivation of glutamatergic signaling in the brain is mediated by glutamate uptake into glia and neurons by glutamate transporters. Glutamate transporters are sodium-dependent proteins that putatively rely indirectly on Na,K-ATPases to generate ion gradients that drive transmitter uptake. Based on anatomical colocalization, mutual sodium dependency, and the inhibitory effects of the Na,K-ATPase inhibitor ouabain on glutamate transporter activity, we postulated that glutamate transporters are directly coupled to Na,K-ATPase and that Na,K-ATPase is an essential modulator of glutamate uptake.

Na,K-ATPase was purified from rat cerebellum by tandem anion exchange and ouabain affinity chromatography, and the cohort of associated proteins was characterized by mass spectrometry. The 1–3 subunits of Na,K-ATPase were detected, as were the glutamate transporters GLAST and GLT-1, demonstrating that glutamate transporters copurify with Na,K-ATPases. The link between glutamate transporters and Na,K-ATPase was further established by coimmunoprecipitation and colocalization. Analysis of the regulation of glutamate transporter and Na,K-ATPase activities was assessed using [³H]d-aspartate, [³H]l-glutamate, and rubidium-86 uptake into synaptosomes and cultured astrocytes. In synaptosomes, ouabain produced a dose-dependent inhibition of glutamate transporter and Na,K-ATPase activities, whereas in astrocytes, ouabain showed a bimodal effect whereby glutamate transporter activity was stimulated at 1 µm ouabain and inhibited at higher concentrations. The effects of protein kinase inhibitors on [³H]d-aspartate uptake indicated the selective involvement of Src kinases, which are probably a component of the Na,K-ATPase/glutamate transporter complex. These findings demonstrate that glutamate transporters and Na,K-ATPases are part of the same macromolecular complexes and operate as a functional unit to regulate glutamatergic neurotransmission.

Impact of Serotonin 2C Receptor Null Mutation on Physiology and Behavior Associated with Nigrostriatal Dopamine Pathway Function

2009-06-24

The impact of serotonergic neurotransmission on brain dopaminergic pathways has substantial relevance to many neuropsychiatric disorders. A particularly prominent role has been ascribed to the inhibitory effects of serotonin 2C receptor (5-HT_2CR) activation on physiology and behavior mediated by the mesolimbic dopaminergic pathway, particularly in the terminal region of the nucleus accumbens. The influence of this receptor subtype on functions mediated by the nigrostriatal dopaminergic pathway is less clear. Here we report that a null mutation eliminating expression of 5-HT_2CRs produces marked alterations in the activity and functional output of this pathway. 5-HT_2CR mutant mice displayed increased activity of substantia nigra pars compacta (SNc) dopaminergic neurons, elevated baseline extracellular dopamine concentrations in the dorsal striatum (DSt), alterations in grooming behavior, and enhanced sensitivity to the stereotypic behavioral effects of d-amphetamine and GBR 12909. These psychostimulant responses occurred in the absence of phenotypic differences in drug-induced extracellular dopamine concentration, suggesting a phenotypic alteration in behavioral responses to released dopamine. This was further suggested by enhanced behavioral responses of mutant mice to the D₁ receptor agonist SKF 81297. Differences in DSt D₁ or D₂ receptor expression were not found, nor were differences in medium spiny neuron firing patterns or intrinsic membrane properties following dopamine stimulation. We conclude that 5-HT_2CRs regulate nigrostriatal dopaminergic activity and function both at SNc dopaminergic neurons and at a locus downstream of the DSt.

Functional Significance of Nonspatial Information in Monkey Lateral Intraparietal Area

Balan, P. F., Gottlieb, J. — 2009-06-24

Although the parietal cortex is traditionally associated with spatial perception and motor planning, recent evidence shows that neurons in the lateral intraparietal area (LIP) carry both spatial and nonspatial signals. The functional significance of the nonspatial information in the parietal cortex is not understood. To address this question, we tested the effect of unilateral reversible inactivation of LIP on three behavioral tasks known to evoke nonspatial responses. Each task included a spatial component (target selection in the hemifield contralateral or ipsilateral to the inactivation) and a nonspatial component, namely limb motor planning, the estimation of elapsed time, and reward-based decisions. Although inactivation reliably impaired performance on all tasks, the deficits were spatially specific (restricted to contralateral target locations), and there were no effects on nonspatial aspects on performance. This suggests that modulatory nonspatial signals in LIP represent feedback about computations performed elsewhere rather than a primary role of LIP in these computations.

Opioids Depress Cortical Centers Responsible for the Volitional Control of Respiration

2009-06-24

Respiratory depression limits provision of safe opioid analgesia and is the main cause of death in drug addicts. Although opioids are known to inhibit brainstem respiratory activity, their effects on cortical areas that mediate respiration are less well understood. Here, functional magnetic resonance imaging was used to examine how brainstem and cortical activity related to a short breath hold is modulated by the opioid remifentanil. We hypothesized that remifentanil would differentially depress brain areas that mediate sensory-affective components of respiration over those that mediate volitional motor control. Quantitative measures of cerebral blood flow were used to control for hypercapnia-induced changes in blood oxygen level-dependent (BOLD) signal. Awareness of respiration, reflected by an urge-to-breathe score, was profoundly reduced with remifentanil. Urge to breathe was associated with activity in the bilateral insula, frontal operculum, and secondary somatosensory cortex. Localized remifentanil-induced decreases in breath hold-related activity were observed in the left anterior insula and operculum. We also observed remifentanil-induced decreases in the BOLD response to breath holding in the left dorsolateral prefrontal cortex, anterior cingulate, the cerebellum, and periaqueductal gray, brain areas that mediate task performance. Activity in areas mediating motor control (putamen, motor cortex) and sensory-motor integration (supramarginal gyrus) were unaffected by remifentanil. Breath hold-related activity was observed in the medulla. These findings highlight the importance of higher cortical centers in providing contextual awareness of respiration that leads to appropriate modulation of respiratory control. Opioids have profound effects on the cortical centers that control breathing, which potentiates their actions in the brainstem.

Inosine Alters Gene Expression and Axonal Projections in Neurons Contralateral to a Cortical Infarct and Improves Skilled Use of the Impaired Limb

2009-06-24

Recovery after stroke and other types of brain injury is restricted in part by the limited ability of undamaged neurons to form compensatory connections. Inosine, a naturally occurring purine nucleoside, stimulates neurons to extend axons in culture and, in vivo, enhances the ability of undamaged neurons to form axon collaterals after brain damage. The molecular changes induced by inosine are unknown, as is the ability of inosine to restore complex functions associated with a specific cortical area. Using a unilateral injury model limited to the sensorimotor cortex, we show that inosine triples the number of corticospinal tract axons that project from the unaffected hemisphere and form synaptic bouton-like structures in the denervated half of the spinal cord. These changes correlate with improved recovery in animals' ability to grasp and consume food pellets with the affected forepaw. Studies using laser-capture microdissection and microarray analysis show that inosine profoundly affects gene expression in corticospinal neurons contralateral to the injury. Inosine attenuates transcriptional changes caused by the stroke, while upregulating the expression of genes associated with axon growth and the complement cascade. Thus, inosine alters gene expression in neurons contralateral to a stroke, enhances the ability of these neurons to form connections on the denervated side of the spinal cord, and improves performance with the impaired limb.

Subcellular Dynamics of Somatostatin Receptor Subtype 1 in the Rat Arcuate Nucleus: Receptor Localization and Synaptic Connectivity Vary in Parallel with the Ultradian Rhythm of Growth Hormone Secretion

Stroh, T., van Schouwenburg, M. R., Beaudet, A., Tannenbaum, G. S. — 2009-06-24

Growth hormone (GH) secretion in male rats exhibits a 3.3 h ultradian rhythm generated by the reciprocal interaction of GH-releasing hormone (GHRH) and somatostatin (SRIF). SRIF receptor subtypes sst₁ and sst₂ are highly expressed in GHRH neurons of the hypothalamic arcuate nucleus (ARC). We previously demonstrated an ultradian oscillation in binding of SRIF analogs to the ARC in relation to GH peaks and troughs. Here we tested the hypothesis that these ultradian changes in SRIF binding are due to differential plasma membrane targeting of sst₁ receptors in ARC neurons using immunocytochemistry and electron microscopy. We found that 87% of sst₁-positive ARC neurons also synthesized GHRH. Subcellularly, 80% of sst₁ receptors were located intracellularly and 20% at the plasma membrane regardless of GH status. However, whereas 30% of the cell-surface sst₁ receptors were located perisynaptically or subsynaptically following exposure to high GH secretion, this fraction was increased to 42% following a GH trough period (p = 0.05). Furthermore, the relative abundance of symmetric and asymmetric synapses on sst₁-positive dendrites also varied significantly, depending on the GH cycle, from approximately equal numbers following GH troughs to 70:30 in favor of symmetric, i.e., inhibitory, inputs after GH peaks (p < 0.02). These findings suggest that postsynaptic localization of sst₁ receptors and synaptic connectivity in the ARC undergo pronounced remodeling in parallel with the GH rhythm. Such synaptic plasticity may be an important mechanism by which sst₁ mediates SRIF's cyclical effects on ARC GHRH neurons to generate the ultradian rhythm of GH secretion.

The Formation of Recent and Remote Memory Is Associated with Time-Dependent Formation of Dendritic Spines in the Hippocampus and Anterior Cingulate Cortex

Restivo, L., Vetere, G., Bontempi, B., Ammassari-Teule, M. — 2009-06-24

Although hippocampal–cortical interactions are crucial for the formation of enduring declarative memories, synaptic events that govern long-term memory storage remain mostly unclear. We present evidence that neuronal structural changes, i.e., dendritic spine growth, develop sequentially in the hippocampus and anterior cingulate cortex (aCC) during the formation of recent and remote contextual fear memory. We found that mice placed in a conditioning chamber for one 7 min conditioning session and exposed to five footshocks (duration, 2 s; intensity, 0.7 mA; interstimulus interval, 60 s) delivered through the grid floor exhibited robust fear response when returned to the experimental context 24 h or 36 d after the conditioning. We then observed that their fear response at the recent, but not the remote, time point was associated with an increase in spine density on hippocampal neurons, whereas an inverse temporal pattern of spine density changes occurred on aCC neurons. At each time point, hippocampal or aCC structural alterations were achieved even in the absence of recent or remote memory tests, thus suggesting that they were not driven by retrieval processes. Furthermore, ibotenic lesions of the hippocampus impaired remote memory and prevented dendritic spine growth on aCC neurons when they were performed immediately after the conditioning, whereas they were ineffective when performed 24 d later. These findings reveal that gradual structural changes modifying connectivity in hippocampal–cortical networks underlie the formation and expression of remote memory, and that the hippocampus plays a crucial but time-limited role in driving structural plasticity in the cortex.

Hyperdopaminergia and NMDA Receptor Hypofunction Disrupt Neural Phase Signaling

2009-06-24

Neural phase signaling has gained attention as a putative coding mechanism through which the brain binds the activity of neurons across distributed brain areas to generate thoughts, percepts, and behaviors. Neural phase signaling has been shown to play a role in various cognitive processes, and it has been suggested that altered phase signaling may play a role in mediating the cognitive deficits observed across neuropsychiatric illness. Here, we investigated neural phase signaling in two mouse models of cognitive dysfunction: mice with genetically induced hyperdopaminergia [dopamine transporter knock-out (DAT-KO) mice] and mice with genetically induced NMDA receptor hypofunction [NMDA receptor subunit-1 knockdown (NR1-KD) mice]. Cognitive function in these mice was assessed using a radial-arm maze task, and local field potentials were recorded from dorsal hippocampus and prefrontal cortex as DAT-KO mice, NR1-KD mice, and their littermate controls engaged in behavioral exploration. Our results demonstrate that both DAT-KO and NR1-KD mice display deficits in spatial cognitive performance. Moreover, we show that persistent hyperdopaminergia alters interstructural phase signaling, whereas NMDA receptor hypofunction alters interstructural and intrastructural phase signaling. These results demonstrate that dopamine and NMDA receptor dependent glutamate signaling play a critical role in coordinating neural phase signaling, and encourage further studies to investigate the role that deficits in phase signaling play in mediating cognitive dysfunction.

Suppression of Spreading Depression-Like Events in Locusts by Inhibition of the NO/cGMP/PKG Pathway

Armstrong, G. A. B., Rodgers, C. I., Money, T. G. A., Robertson, R. M. — 2009-06-24

Despite considerable research attention focused on mechanisms underlying neural spreading depression (SD), because of its association with important human CNS pathologies, such as stroke and migraine, little attention has been given to explaining its occurrence and regulation in invertebrates. In the locust metathoracic ganglion (MTG), an SD-like event occurs during heat and anoxia stress, which results in cessation of neuronal output for the duration of the applied stress. SD-like events were characterized by an abrupt rise in extracellular potassium ion concentration ([K⁺]_o) from a baseline concentration of ~8 to >30 mm, which returned to near baseline concentrations after removal of the applied stress. After return to baseline [K⁺]_o, neuronal output (ventilatory motor pattern activity) from the MTG recovered. Unlike mammalian neurons, which depolarize almost completely during SD, locust neurons only partially depolarized. SD-like events in the locust CNS were suppressed by pharmacological inhibition of the nitric oxide/cyclic guanosine monophosphate/protein kinase G (NO/cGMP/PKG) pathway and were exacerbated by its activation. Also, environmental stressors such as heat and anoxia increased production of nitric oxide in the locust CNS. Finally, for the intact animal, manipulation of the pathway affected the speed of recovery from suffocation by immersion under water. We propose that SD-like events in locusts provide an adaptive mechanism for surviving extreme environmental conditions. The highly conserved nature of the NO/cGMP/PKG signaling pathway suggests that it may be involved in modulating SD in other organisms, including mammals.

FOXO3a Is Broadly Neuroprotective In Vitro and In Vivo against Insults Implicated in Motor Neuron Diseases

2009-06-24

Aging is a risk factor for the development of adult-onset neurodegenerative diseases. Although some of the molecular pathways regulating longevity and stress resistance in lower organisms are defined (i.e., those activating the transcriptional regulators DAF-16 and HSF-1 in Caenorhabditis elegans), their relevance to mammals and disease susceptibility are unknown. We studied the signaling controlled by the mammalian homolog of DAF-16, FOXO3a, in model systems of motor neuron disease. Neuron death elicited in vitro by excitotoxic insult or the expression of mutant SOD1, mutant p150^glued, or polyQ-expanded androgen receptor was abrogated by expression of nuclear-targeted FOXO3a. We identify a compound [Psammaplysene A (PA)] that increases nuclear localization of FOXO3a in vitro and in vivo and show that PA also protects against these insults in vitro. Administration of PA to invertebrate model systems of neurodegeneration similarly blocked neuron death in a DAF-16/FOXO3a-dependent manner. These results indicate that activation of the DAF-16/FOXO3a pathway, genetically or pharmacologically, confers protection against the known causes of motor neuron diseases.

NOS1AP Regulates Dendrite Patterning of Hippocampal Neurons through a Carboxypeptidase E-Mediated Pathway

2009-06-24

During neuronal development, neurons form elaborate dendritic arbors that receive signals from axons. Additional studies are needed to elucidate the factors regulating the establishment of dendritic patterns. Our work explored possible roles played by nitric oxide synthase 1 adaptor protein (NOS1AP; also known as C-terminal PDZ ligand of neuronal nitric oxide synthase or CAPON) in dendritic patterning of cultured hippocampal neurons. Here we report that the long isoform of NOS1AP (NOS1AP-L) plays a novel role in regulating dendrite outgrowth and branching. NOS1AP-L decreases dendrite number when overexpressed at any interval between day in vitro (DIV) 0 and DIV 12, and knockdown of NOS1AP-L results in increased dendrite number. In contrast, the short isoform of NOS1AP (NOS1AP-S) decreases dendrite number only when overexpressed during DIV 5–7. Using mutants of NOS1AP-L, we show that neither the PDZ-binding domain nor the PTB domain is necessary for the effects of NOS1AP-L. We have functionally narrowed the region of NOS1AP-L that mediates this effect to the middle amino acids 181–307, a region that is not present in NOS1AP-S. Furthermore, we performed a yeast two-hybrid screen and identified carboxypeptidase E (CPE) as a binding partner for the middle region of NOS1AP-L. Biochemical and cellular studies reveal that CPE mediates the effects of NOS1AP on dendrite morphology. Together, our results suggest that NOS1AP-L plays an important role in the initiation, outgrowth, and maintenance of dendrites during development.

Inhibition of the Mammalian Target of Rapamycin Signaling Pathway Suppresses Dentate Granule Cell Axon Sprouting in a Rodent Model of Temporal Lobe Epilepsy

Buckmaster, P. S., Ingram, E. A., Wen, X. — 2009-06-24

Dentate granule cell axon (mossy fiber) sprouting is a common abnormality in patients with temporal lobe epilepsy. Mossy fiber sprouting creates an aberrant positive-feedback network among granule cells that does not normally exist. Its role in epileptogenesis is unclear and controversial. If it were possible to block mossy fiber sprouting from developing after epileptogenic treatments, its potential role in the pathogenesis of epilepsy could be tested. Previous attempts to block mossy fiber sprouting have been unsuccessful. The present study targeted the mammalian target of rapamycin (mTOR) signaling pathway, which regulates cell growth and is blocked by rapamycin. Rapamycin was focally, continuously, and unilaterally infused into the dorsal hippocampus for prolonged periods beginning within hours after rats sustained pilocarpine-induced status epilepticus. Infusion for 1 month reduced aberrant Timm staining (a marker of mossy fibers) in the granule cell layer and molecular layer. Infusion for 2 months inhibited mossy fiber sprouting more. However, after rapamycin infusion ceased, aberrant Timm staining developed and approached untreated levels. When onset of infusion began after mossy fiber sprouting had developed for 2 months, rapamycin did not reverse aberrant Timm staining. These findings suggest that inhibition of the mTOR signaling pathway suppressed development of mossy fiber sprouting. However, suppression required continual treatment, and rapamycin treatment did not reverse already established axon reorganization.

Selecting for Memory? The Influence of Selective Attention on the Mnemonic Binding of Contextual Information

Uncapher, M. R., Rugg, M. D. — 2009-06-24

Not all of what is experienced is remembered later. Behavioral evidence suggests that the manner in which an event is processed influences which aspects of the event will later be remembered. The present experiment investigated the neural correlates of "selective encoding," or the mechanisms that support the encoding of some elements of an event in preference to others. Event-related MRI data were acquired while volunteers selectively attended to one of two different contextual features of study items (color or location). A surprise memory test for the items and both contextual features was subsequently administered to determine the influence of selective attention on the neural correlates of contextual encoding. Activity in several cortical regions indexed later memory success selectively for color or location information, and this encoding-related activity was enhanced by selective attention to the relevant feature. Critically, a region in the hippocampus responded selectively to attended source information (whether color or location), demonstrating encoding-related activity for attended but not for nonattended source features. Together, the findings suggest that selective attention modulates the magnitude of activity in cortical regions engaged by different aspects of an event, and hippocampal encoding mechanisms seem to be sensitive to this modulation. Thus, the information that is encoded into a memory representation is biased by selective attention, and this bias is mediated by cortical–hippocampal interactions.

Evidence of Action Sequence Chunking in Goal-Directed Instrumental Conditioning and Its Dependence on the Dorsomedial Prefrontal Cortex

Ostlund, S. B., Winterbauer, N. E., Balleine, B. W. — 2009-06-24

The current study investigated the contribution of the dorsomedial prefrontal cortex (dmPFC) to instrumental action selection. We found that cell body lesions of the dmPFC, centered on the medial agranular area, spared rats' ability to choose between actions based on either the value or the discriminative stimulus properties of an outcome. We next examined the effects of these lesions on action sequence learning using a concurrent bidirectional heterogeneous chain task in which the identity of the reward delivered was determined by the order in which the two lever press actions were performed. Although both lesioned rats and sham controls learned to perform the task, we found that they relied on different behavioral strategies to do so. In subsequent tests, rats in the sham group were able to withhold their performance of a sequence when either its associated outcome was devalued or the contingency between that sequence and its outcome was degraded by delivering the outcome noncontingently. Interestingly, lesioned rats failed to reorganize their performance at the action sequence level and, rather, were found to withhold their performance of the terminal response in the sequence that had earned the devalued outcome relative to the more distal response, suggesting that they represented the elements of the sequence as distinct behavioral units. These findings demonstrate that rats can use sequence-level representations, or action chunks, to organize their behavior in a goal-directed manner and indicate that the dmPFC plays a critical role in this process.

Histone Deacetylases 1 and 2 Form a Developmental Switch That Controls Excitatory Synapse Maturation and Function

2009-06-24

The structural assembly of synapses can be accomplished in a rapid time frame, although most nascent synapses formed during early development are not fully functional and respond poorly to presynaptic action potentials. The mechanisms that are responsible for this delay in synapse maturation are unknown. Histone deacetylases (HDACs) regulate the activity state of chromatin and repress gene expression through the removal of acetyl groups from histones. Class I HDACs, which include HDAC1 and HDAC2, are expressed in the CNS, although their specific role in neuronal function has not been studied. To delineate the contribution of HDAC1 and HDAC2 in the brain, we have used pharmacological inhibitors of HDACs and mice with conditional alleles to HDAC1 and HDAC2. We found that a decrease in the activities of both HDAC1 and HDAC2 during early synaptic development causes a robust facilitation of excitatory synapse maturation and a modest increase in synapse numbers. In contrast, in mature neurons a decrease in HDAC2 levels alone was sufficient to attenuate basal excitatory neurotransmission without a significant change in the numbers of detectable nerve terminals. Therefore, we propose that HDAC1 and HDAC2 form a developmental switch that controls synapse maturation and function acting in a manner dependent on the maturational states of neuronal networks.