Signal Transmission by Auditory and Vestibular Hair Cells

We interact with the world around us by sensing a vast array of inputs and translating them into signals that can be interpreted by the brain. We have evolved many sensory receptors, each uniquely specialised to detect diverse stimuli. The hair cells are sensory receptors, initially developed to provide a sense of body position and movement, but later adapted to sense minute pressure waves in the environment that are perceived as sounds. As such, hair cells bestow a sense of hearing and balance, which are major advantages for survival. Mammals have four different types of hair cell, two of which are dedicated to hearing, the inner and outer hair cells, and the other two to balance, the type-I and type-II hair cells. While all hair cells employ common mechanisms to detect and relay signals from sound or motion, they also have unique attributes that specialise them for a specific functional role. In this chapter we describe the process of signal transmission in mammalian auditory and vestibular hair cells. Since mammalian hair cells do not regenerate, their loss results in permanent auditory or vestibular deficit. Efforts to regenerate or repair malfunctioning hair cells have recently intensified, mainly through gene, stem-cell and molecular therapy

Oxytocin Increases Phasic and Tonic GABAergic Transmission in CA1 Region of Mouse Hippocampus

Oxytocin is a neuropeptide that plays important peripheral and central neuromodulatory functions. Our data show that, following activation of oxytocin receptors (OtRs) with the selective agonist TGOT (Thr4,Gly7-oxytocin), a significant increase in frequency and amplitude of spontaneous inhibitory postsynaptic currents (sIPSC) occurred in hippocampal CA1 pyramidal neurons (PYR) in mice. TGOT affected also sIPSC deactivation kinetics, suggesting the involvement of perisynaptic GABAA receptors (GABAARs) as well. By contrast, TGOT did not cause significant changes in frequency, amplitude or deactivation kinetics of miniature IPSC, suggesting that the effects elicited by the agonist are strictly dependent on the firing activity of presynaptic neurons. Moreover, TGOT was able to modulate tonic GABAergic current mediated by extrasynaptic GABAARs expressed by PYRs. Consistently, at spike threshold TGOT induced in most PYRs a significant membrane hyperpolarization and a decrease in firing rate. The source of increased inhibition onto PYRs was represented by stuttering fast-spiking GABAergic interneurons (INs) that directly respond to TGOT with a depolarization and an increase in their firing rate. One putative ionic mechanism underlying this effect could be represented by OtR activation-induced up-modulation of L-type Ca2+ channels. In conclusion, our results indicate that oxytocin can influence the activity of a subclass of hippocampal GABAergic INs and therefore regulate the operational modes of the downstream PYRs by increasing phasic and tonic GABAergic transmission in CA1 region of mouse hippocampus

Stem Cell-Derived Human Striatal Progenitors Innervate Striatal Targets and Alleviate Sensorimotor Deficit in a Rat Model of Huntington Disease

Huntington disease (HD) is an inherited late-onset neurological disorder characterized by progressive neuronal loss and disruption of cortical and basal ganglia circuits. Cell replacement using human embryonic stem cells may offer the opportunity to repair the damaged circuits and significantly ameliorate disease conditions. Here, we showed that in-vitro-differentiated human striatal progenitors undergo maturation and integrate into host circuits upon intra-striatal transplantation in a rat model of HD. By combining graft-specific immunohistochemistry, rabies virus-mediated synaptic tracing, and ex vivo electrophysiology, we showed that grafts can extend projections to the appropriate target structures, including the globus pallidus, the subthalamic nucleus, and the substantia nigra, and receive synaptic contact from both host and graft cells with 6.6 ± 1.6 inputs cell per transplanted neuron. We have also shown that transplants elicited a significant improvement in sensory-motor tasks up to 2 months post-transplant further supporting the therapeutic potential of this approach

Rac1 and Rac3 GTPases Control Synergistically the Development of Cortical and Hippocampal GABAergic Interneurons

The intracellular mechanisms driving postmitotic development of cortical γ-aminobutyric acid (GABA)ergic interneurons are poorly understood. We have addressed the function of Rac GTPases in cortical and hippocampal interneuron development. Developing neurons express both Rac1 and Rac3. Previous work has shown that Rac1 ablation does not affect the development of migrating cortical interneurons. Analysis of mice with double deletion of Rac1 and Rac3 shows that these GTPases are required during postmitotic interneuron development. The number of parvalbumin-positive cells was affected in the hippocampus and cortex of double knockout mice. Rac depletion also influences the maturation of interneurons that reach their destination, with reduction of inhibitory synapses in both hippocampal CA1 and cortical pyramidal cells. The decreased number of cortical migrating interneurons and their altered morphology indicate a role of Rac1 and Rac3 in regulating the motility of cortical interneurons, thus interfering with their final localization. While electrophysiological passive and active properties of pyramidal neurons including membrane capacity, resting potential, and spike amplitude and duration were normal, these cells showed reduced spontaneous inhibitory currents and increased excitability. Our results show that Rac1 and Rac3 contribute synergistically to postmitotic development of specific populations of GABAergic cells, suggesting that these proteins regulate their migration and differentiation

The Properties of Synaptic Transmission in Adult Mammalian Vestibular Hair Cells Differs Between Type I and Type II Cells

Inner ear sensory synapses faithfully transduce information over a wide range of stimulus intensities for prolonged periods of time. The efficiency of such demanding and stringent exocytotic activity depends on the presence of specialised presynaptic ribbons in the sensory hair cells. Ribbons are electron dense structures able to tether a large number of releasable vesicles at the synaptic active zone and can maintain high rates of vesicle release. Calcium entry through CaV1. 3 (L-type) Ca2+ channels in response to cell depolarization causes local increases in Ca2+ at the ribbon synapses, which is detected by the exocytotic Ca2+ sensors. At ribbon synapses of mature vestibular hair cells (VHCs), the coupling between Ca2+ channels and the exocytotic Ca2+ sensor remains unclear. We studied the Ca2+ dependence of exocytosis and the release kinetics of different vesicle pool populations in mature synaptotagmin-4 (Syt-4) mouse VHCs using patch-clamp capacitance measurements under physiological recording conditions. Exocytosis in adult Type II VHCs showed a high order dependence on Ca2+ entry, which contrasts with the linear Ca2+ dependence observed in adult mammalian auditory inner hair cells (IHCs). The synaptic properties of mature Type II VHCs, including the characteristics of the Ca2+ current and dynamics of vesicle release, were not affected by an absence of Syt-4. By contrast, the release of synaptic vesicles from Type I VHCs was very small in both Syt-4 control and KO cells, even for long voltage steps, which prevented us from uncovering the Ca2+ dependence of release. Our findings show that the coupling between Ca2+ influx and neurotransmitter release at VHC ribbon synapses at Type II VHCs is described by a non-linear relation that is likely to be more appropriate for the faithful encoding of low frequency vestibular information, consistent with that observed in very low frequency mammalian IHCs. On the other hand synaptic vesicle release at mature Type I VHCs was very small suggesting that these cells favour faster non-quantal transmission in order to drive the most rapid reflex in the body, the vestibular-ocular reflex

Pimozide Increases a Delayed Rectifier K+ Conductance in Chicken Embryo Vestibular Hair Cells

Pimozide is a conventional antipsychotic drug largely used in the therapy for schizophrenia and Tourette’s syndrome. Pimozide is assumed to inhibit synaptic transmission at the CNS by acting as a dopaminergic D2 receptor antagonist. Moreover, pimozide has been shown to block voltage-gated Ca2+ and K+ channels in different cells. Despite its widespread clinical use, pimozide can cause several adverse effects, including extrapyramidal symptoms and cardiac arrhythmias. Dizziness and loss of balance are among the most common side effects of pimozide. By using the patch-clamp whole-cell technique, we investigated the effect of pimozide [3 μM] on K+ channels expressed by chicken embryo vestibular type-II hair cells. We found that pimozide slightly blocks a transient outward rectifying A-type K+ current but substantially increases a delayed outward rectifying K+ current. The net result was a significant hyperpolarization of type-II hair cells at rest and a strong reduction of their response to depolarizing stimuli. Our findings are consistent with an inhibitory effect of pimozide on the afferent synaptic transmission by type-II hair cells. Moreover, they provide an additional key to understanding the beneficial/collateral pharmacological effects of pimozide. The finding that pimozide can act as a K+ channel opener provides a new perspective for the use of this drug

Exocytosis at mammalian vestibular ribbon synapses shows a high-order Ca2+ dependence and does not require synaptotagmin-4

Inner ear sensory synapses faithfully transduce information over a wide range of stimulus intensities for prolonged periods of time. The efficiency of such demanding and stringent exocytotic activity depends on the presence of specialised presynaptic ribbons in the sensory hair cells. Ribbons are electron dense structures able to tether a large number of releasable vesicles at the synaptic active zone and can maintain high rates of vesicle release. Calcium entry through CaV1.3 (L-type) Ca2+ channels in response to cell depolarization causes local increases in Ca2+ at the ribbon synapses, which is detected by the exocytotic Ca2+ sensors. At ribbon synapses of mature vestibular hair cells (VHCs), the coupling between Ca2+ channels and the exocytotic Ca2+ sensor remains unclear. We studied the Ca2+ dependence of exocytosis and the release kinetics of different vesicle pool populations in mature synaptotagmin-4 (Syt-4) mouse VHCs using patch-clamp capacitance measurements under physiological recording conditions. Exocytosis in VHCs showed a high order dependence on Ca2+ entry, which contrasts with the linear Ca2+ dependence observed in adult mammalian auditory inner hair cells (IHCs). The synaptic properties of mature VHCs, including the characteristics of the Ca2+ current and dynamics of vesicle release, were not affected by an absence of Syt-4. Our findings show that the coupling between Ca2+ influx and neurotransmitter release at VHC ribbon synapses is described by a non-linear relation that is likely to be more appropriate for the faithful encoding of low frequency vestibular information, consistent with that observed in very low frequency mammalian IHCs

Pimozide Increases a Delayed Rectifier K⁺ Conductance in Chicken Embryo Vestibular Hair Cells

Pimozide is a conventional antipsychotic drug largely used in the therapy for schizophrenia and Tourette’s syndrome. Pimozide is assumed to inhibit synaptic transmission at the CNS by acting as a dopaminergic D2 receptor antagonist. Moreover, pimozide has been shown to block voltage-gated Ca2+ and K+ channels in different cells. Despite its widespread clinical use, pimozide can cause several adverse effects, including extrapyramidal symptoms and cardiac arrhythmias. Dizziness and loss of balance are among the most common side effects of pimozide. By using the patch-clamp whole-cell technique, we investigated the effect of pimozide [3 μM] on K+ channels expressed by chicken embryo vestibular type-II hair cells. We found that pimozide slightly blocks a transient outward rectifying A-type K+ current but substantially increases a delayed outward rectifying K+ current. The net result was a significant hyperpolarization of type-II hair cells at rest and a strong reduction of their response to depolarizing stimuli. Our findings are consistent with an inhibitory effect of pimozide on the afferent synaptic transmission by type-II hair cells. Moreover, they provide an additional key to understanding the beneficial/collateral pharmacological effects of pimozide. The finding that pimozide can act as a K+ channel opener provides a new perspective for the use of this drug

Supra-linear Ca2+ dependence of the neurotransmitter release at mammalian vestibular ribbon synapses

Poster N. 21 - In near-physiological conditions (1.3 mM extracellular Ca2+ and body temperature), the Ca2+-dependence of neurotransmitter release in mature mouse VHCs is high-order. This result differs from the linear coupling reported by Dulon et al. (2009), presumably due to the different experimental conditions they employed (5 mM extracellular Ca2+ and room temperature). The supralinear nature of the relationship between Ca2+ influx and exocytosis in VHCs, which resembles the one found in mature apical cochlear IHCs, is presumably apt to process low-frequency vestibular stimuli

Non conventional signal transmission at the mouse vestibular Type I hair cell - calyx synapse

Vestibular sensory epithelia of Amniotes contain two types of hair cells, Type I and Type II, which differ in electrophysiological properties and synaptic contacts. Type I hair cells alone express a low-voltage activated outward rectifying K+ conductance, named GK,L. Moreover, each Type II hair cell is contacted by several bouton afferent endings, while a single large calyceal afferent terminal encloses the basolateral membrane of Type I hair cells, where voltage-gated Ca2+ and K+ channels and the presynaptic sites for glutamate release are located. Besides vesicular transmission, a nonquantal transmission has been hypothesized to occur at the calyx synapse, whereby K+ exiting the hair cell directly depolarizes the calyx terminal. To investigate K+ involvement in signal transmission, we whole-cell recorded from in vitro mouse Type I hair cells or their associated calyx. We found that intercellular (in the synaptic cleft) K+ increased or decreased depending upon hair cell membrane potential as a consequence of GK,L negative voltage-range of activation . Moreover, we found evidence for the calyx inner membrane facing the synaptic cleft expressing voltage-gated K+ channels of the KV1 and KV7 type. Present results suggests a scenario where hair bundle deflection produces calyx depolarization or hyperpolarization by modulating K+ flux across the hair cell and through postsynaptic voltage-gated K+ channels