Characterization of Cerebellar Output Neurons in Fish
Thomas C. Harmon, David L. McLean, and Indira M. Raman
(see pages 3063–3074)
The cerebellar circuitry for movement, which is conserved across vertebrate species, is in place as early as 6 d postfertilization in zebrafish larvae. This week, Harmon et al. use electrophysiological recordings to describe the properties of eurydendroid neurons, the fish equivalent of mammalian cerebellar nuclei neurons, which carry cerebellar output.
Eurydendroid neurons receive sensory and motor inputs from excitatory granule cell parallel fibers and inhibitory Purkinje cells; the authors sought to demonstrate how the output cells integrate these inputs during movement. Eurydendroid neurons express the transcription factor olig2, so the researchers created a fish line in which olig2+ neurons were fluorescently labeled. Whole-cell recordings from cerebellar olig2+ neurons in immobilized fish showed spontaneous activity, and silent neurons became active following a brief hyperpolarization. Synaptic inputs produced spontaneous EPSCs and IPSCs in voltage clamp. IPSCs had a faster rise time than EPSCs and were larger with greater charge transfer, resulting in an estimated threefold to fourfold greater conductance from inhibitory inputs. Further analysis of IPSCs showed that between one and three Purkinje cells contact each eurydendroid neuron.
Purkinje cells produce simple spikes and complex spikes—which are about five times larger than simple spikes. To determine whether the spike types produce similar IPSCs (as they do in mammals), the researchers used optogenetics to express the inhibitory channel Archaerhodopsin (Arch) in Purkinje cells. Optical stimulation of Arch blocked Purkinje cell simple spikes, while leaving complex spikes intact. Surprisingly, this produced smaller, rather than larger, IPSCs in eurydendroid neurons, demonstrating that the incoming spike amplitude alone did not determine IPSC amplitude.
During spontaneous or evoked swimming, eurydendroid cells increased their firing rate. Both EPSCs and IPSCs increased during movement, but the timing and magnitude differed during spontaneous and evoked swimming. The authors determined that, despite the greater impact of inhibitory over excitatory inputs, eurydendroid neurons paradoxically increased their excitatory output during swimming, demonstrating the functional dominance of parallel fiber over Purkinje cell inputs. The findings illustrate the impact of timing and more subtle spike features on synaptic outcomes, as well as giving new insights into cerebellar synaptic communication.
Primary Auditory Neurons Characterized after Noise Exposure
Ben Warren, Georgina E. Fenton, Elizabeth Klenschi, James F. C. Windmill, and Andrew S. French
(see pages 3130–3140)
Loud noise causes preventable hearing loss, but the mechanisms behind the neural dysfunction are not understood—in part because vertebrate hair cells reside within the bony cochlea, making them difficult to study in vitro. Now Warren et al. provide a detailed characterization of primary auditory neurons after noise exposure using an increasingly popular animal model: the locust (Schistocerca gregaria), which has more accessible neurons than vertebrates.
Auditory neurons (green) and nuclei of all cells (magenta) in Müller's organ of a locust ear. Group III auditory neurons are highlighted by the white dotted circle. See Warren et al. for details.
Locusts were exposed to either a 126 dB tone (noise exposed) or a silent speaker (control) for 24 h. Hair cells move in response to sound, producing measurable movement of the tympani in locusts, as in mammals. The authors first measured these sound-evoked movements and found that tympani of noise-exposed locusts vibrated with displacement and mechanical gain—the amplitude of tympanic vibration relative to tone amplitude—about 10 times higher than controls. Electrophysiological recordings of tone-evoked spikes from the auditory nerve in an ex vivo preparation showed that neurons from noise-exposed and control locusts produced a similar number of spikes, but spikes from noise-exposed locusts had peak responses that were reduced by half. Spike generation was also slightly delayed, indicating a decrease in spike synchrony. Neurons from control and noise-exposed locusts did not differ morphologically or, surprisingly, in their individual electrophysiological properties. Discrete (or spontaneous) depolarizations were smaller in neurons from noise-exposed locusts, as were maximal transduction currents, which were also delayed compared with controls. Tone-evoked spikes measured from the dendrite or axon looked similar in both groups. Blocking half the mechanically sensitive channels with streptomycin in control locusts mimicked the electrophysiological effects seen in neurons from noise-exposed locusts. Finally, RNA analysis revealed no difference in expression level of the three candidate genes for mechanosensitive channels.
The authors considered whether the noise-evoked reduction in transduction currents could be explained by a decrease in the sound-induced force at the tympanum or by the expression of fewer mechano-sensitive channels, but they ruled out those possibilities based on their findings. Instead, they hypothesize that damage to support cells may disrupt the carefully maintained balance of cations in the specialized lymph fluid that bathes the mechanoreceptors, resulting in a loss of extracellular electrochemical potential. The study demonstrates that although auditory deficits result from dysfunctional signals from primary auditory neurons, those cells themselves are not damaged; rather, the reduced signals may be a protective response to increased metabolic demand.
Footnotes
This Week in The Journal was written by Stephani Sutherland, Ph.D.
