Cellular/Molecular
Targeting of Calcium Channels to Active Zones
A. Ashleigh Long, Eunju Kim, Hung-Tat Leung, Elvin Woodruff III, Lingling An, R. W. Doerge, William L. Pak, and Kendal Broadie
(see pages 3668–3682)
This week, Long et al. describe a newly discovered Drosophila protein, Fuseless, that is required for vesicle fusion. Fuseless is a transmembrane protein expressed in presynaptic membranes in retina and neuromuscular junctions. Synaptic transmission was impaired in fuseless mutants but was rescued by expression of the transgene exclusively in presynaptic cells. Although synapses appeared essentially normal in mutants, the number of synaptic vesicles was nearly double that in wild-type flies, indicating impairment of exocytosis. The amplitude of evoked excitatory junction potentials was greatly reduced in mutants, and raising extracellular calcium concentration increased the amplitude much less in mutants than in controls, suggesting that a defect in calcium entry underlies the defect in vesicle fusion. Indeed, the expression pattern of voltage-sensitive calcium channels (VSCCs) was disrupted in mutants—the channels were no longer clustered in active zones. Thus, Fuseless is necessary for proper targeting of VSCCs, which enables the localized calcium influx necessary for vesicle release.
Drosophila fuseless mutants (bottom) have more vesicles clustered near synaptic active zones and more docked vesicles (white arrowheads) than control flies (top). See the article by Long et al. for details.
Development/Plasticity/Repair
Could Botulinum Toxin Be Bad for You?
Flavia Antonucci, Chiara Rossi, Laura Gianfranceschi, Ornella Rossetto, and Matteo Caleo
(see pages 3689–3696)
Botulinum toxins (BoNTs) are used increasingly to treat maladies from spasms and migraines to obesity and wrinkles. It has been assumed that the toxin remains localized at the injection site, where it cleaves proteins involved in vesicle fusion, thereby blocking neurotransmitter release. But now Antonucci et al. demonstrate that BoNT/A is retrogradely transported along microtubules, transcytosed, and taken up by afferent terminals. When BoNT/A was injected into one hippocampus in rats, it cleaved its target — synaptosomal-associated protein of 25 kDa (SNAP-25)—in the contralateral hippocampus, resulting in reduced neuronal activity. Similarly, when BoNT/A was injected into the superior colliculus or whisker pads, SNAP-25 was cleaved in the retina and facial nucleus, respectively. In the retina, BoNT/A remained active for at least 25 d after injection. Although cleaved SNAP-25 was detected only in afferents that projected directly to the injection site, it is not clear whether further transcytosis would occur over time.
Behavioral/Systems/Cognitive
The Mystery of REM Atonia
Patricia L. Brooks and John H. Peever
(see pages 3535–3545)
It has long been assumed that glycinergic inhibition of motor neurons is responsible for decreasing muscle tone during rapid eye movement (REM) sleep. Brooks and Peever have now overturned this hypothesis. Microdialysis of the glycine antagonist strychnine into the trigeminal nucleus of rats resulted in increased tone in facial muscle during wakefulness and non-REM sleep, suggesting that tonic glycinergic inhibition occurs during these states. Tonic inhibition immediately switched to phasic inhibition when the rat entered REM sleep, however, and although strychnine increased the size of muscle twitches, it had no effect on atonia during REM sleep. When REM ended, strychnine effects on tone reappeared. Thus it appears that contrary to assumptions, glycine decreases muscle tone in all states except REM sleep. Intriguingly, GABA antagonists and AMPA were also unable to decrease muscle tone during REM sleep, indicating that neither GABAergic inhibition nor decreased glutamatergic excitation is responsible. What is responsible for REM atonia remains a mystery.
Neurobiology of Disease
Somatostatin Receptors That Regulate Epileptiform Activity
Cuie Qiu, Thomas Zeyda, Brian Johnson, Ute Hochgeschwender, Luis de Lecea, and Melanie K. Tallent
(see pages 3567–3576)
The neuropeptide somatostatin reduces the probability of seizures in part by activating the IM current through voltage-gated potassium channels. In experiments reported in this issue, Qiu et al. identified which of the four somatostatin receptors (SST1–SST4) expressed in the brain are responsible for antiepileptic effects of somatostatin by comparing SST2, SST3, and SST4 knock-out mice. Although each knock-out increased susceptibility to seizures induced by a GABA receptor blocker, the effect was most severe in SST4 knock-outs. Moreover, only SST4 knock-outs had more severe seizures than wild-type animals when seizures were induced by a glutamate agonist. The effects of somatostatin on bursting in hippocampal cultures in the presence or absence of IM blockers indicated that SST2 and SST4 are the main receptors mediating the antiepileptic effects of somatostatin. In addition, it appears that activation of SST4 increases IM, whereas activation of SST4 reduces epileptiform activity by a still unknown mechanism.
