A pharmacologically distinct nicotinic ACh receptor is found in a subset of frog semicircular canal hair cells

Elucidating the role of muscarinic acetylcholine receptor (mAChR) signaling in efferent mediated responses of vestibular afferents in mammals

The peripheral vestibular system detects head position and movement through activation of vestibular hair cells (HCs) in vestibular end organs. HCs transmit this information to the CNS by way of primary vestibular afferent neurons. The CNS, in turn, modulates HCs and afferents via the efferent vestibular system (EVS) through activation of cholinergic signaling mechanisms. In mice, we previously demonstrated that activation of muscarinic acetylcholine receptors (mAChRs), during EVS stimulation, gives rise to a slow excitation that takes seconds to peak and tens of seconds to decay back to baseline. This slow excitation is mimicked by muscarine and ablated by the non-selective mAChR blockers scopolamine, atropine, and glycopyrrolate. While five distinct mAChRs (M1-M5) exist, the subtype(s) driving EVS-mediated slow excitation remain unidentified and details on how these mAChRs alter vestibular function is not well understood. The objective of this study is to characterize which mAChR subtypes drive the EVS-mediated slow excitation, and how their activation impacts vestibular physiology and behavior. In C57Bl/6J mice, M3mAChR antagonists were more potent at blocking slow excitation than M1mAChR antagonists, while M2/M4 blockers were ineffective. While unchanged in M2/M4mAChR double KO mice, EVS-mediated slow excitation in M3 mAChR-KO animals were reduced or absent in irregular afferents but appeared unchanged in regular afferents. In agreement, vestibular sensory-evoked potentials (VsEP), known to be predominantly generated from irregular afferents, were significantly less enhanced by mAChR activation in M3mAChR-KO mice compared to controls. Finally, M3mAChR-KO mice display distinct behavioral phenotypes in open field activity, and thermal profiles, and balance beam and forced swim test. M3mAChRs mediate efferent-mediated slow excitation in irregular afferents, while M1mAChRs may drive the same process in regular afferents.

The Cholinergic Lateral Line Efferent Synapse: Structural, Functional and Molecular Similarities With Those of the Cochlea

Vertebrate hair cell (HC) systems are innervated by efferent fibers that modulate their response to external stimuli. In mammals, the best studied efferent-HC synapse, the cholinergic medial olivocochlear (MOC) efferent system, makes direct synaptic contacts with HCs. The net effect of MOC activity is to hyperpolarize HCs through the activation of α9α10 nicotinic cholinergic receptors (nAChRs) and the subsequent activation of Ca²⁺-dependent SK2 potassium channels. A serious obstacle in research on many mammalian sensory systems in their native context is that their constituent neurons are difficult to access even in newborn animals, hampering circuit observation, mapping, or controlled manipulation. By contrast, fishes and amphibians have a superficial and accessible mechanosensory system, the lateral line (LL), which circumvents many of these problems. LL responsiveness is modulated by efferent neurons which aid to distinguish between external and self-generated stimuli. One component of the LL efferent system is cholinergic and its activation inhibits LL afferent activity, similar to what has been described for MOC efferents. The zebrafish (Danio rerio) has emerged as a powerful model system for studying human hearing and balance disorders, since LL HC are structurally and functionally analogous to cochlear HCs, but are optically and pharmacologically accessible within an intact specimen. Complementing mammalian studies, zebrafish have been used to gain significant insights into many facets of HC biology, including mechanotransduction and synaptic physiology as well as mechanisms of both hereditary and acquired HC dysfunction. With the rise of the zebrafish LL as a model in which to study auditory system function and disease, there has been an increased interest in studying its efferent system and evaluate the similarity between mammalian and piscine efferent synapses. Advances derived from studies in zebrafish include understanding the effect of the LL efferent system on HC and afferent activity, and revealing that an α9-containing nAChR, functionally coupled to SK channels, operates at the LL efferent synapse. In this review, we discuss the tools and findings of these recent investigations into zebrafish efferent-HC synapse, their commonalities with the mammalian counterpart and discuss several emerging areas for future studies.

Unraveling the Molecular Players at the Cholinergic Efferent Synapse of the Zebrafish Lateral Line

The lateral line (LL) is a sensory system that allows fish and amphibians to detect water currents. LL responsiveness is modulated by efferent neurons that aid in distinguishing between external and self-generated stimuli, maintaining sensitivity to relevant cues. One component of the efferent system is cholinergic, the activation of which inhibits afferent activity. LL hair cells (HCs) share structural, functional, and molecular similarities with those of the cochlea, making them a popular model for studying human hearing and balance disorders. Because of these commonalities, one could propose that the receptor at the LL efferent synapse is a α9α10 nicotinic acetylcholine receptor (nAChR). However, the identities of the molecular players underlying ACh-mediated inhibition in the LL remain unknown. Surprisingly, through the analysis of single-cell expression studies and in situ hybridization, we describe that α9, but not the α10, subunits are enriched in zebrafish HCs. Moreover, the heterologous expression of zebrafish α9 subunits indicates that homomeric receptors are functional and exhibit robust ACh-gated currents blocked by α-bungarotoxin and strychnine. In addition, in vivo Ca²⁺ imaging on mechanically stimulated zebrafish LL HCs show that ACh elicits a decrease in evoked Ca²⁺ signals, regardless of HC polarity. This effect is blocked by both α-bungarotoxin and apamin, indicating coupling of ACh-mediated effects to small-conductance Ca²⁺-activated potassium (SKs) channels. Our results indicate that an α9-containing (α9*) nAChR operates at the zebrafish LL efferent synapse. Moreover, the activation of α9* nAChRs most likely leads to LL HC hyperpolarization served by SK channels.SIGNIFICANCE STATEMENT The fish lateral line (LL) mechanosensory system shares structural, functional, and molecular similarities with those of the mammalian cochlea. Thus, it has become an accessible model for studying human hearing and balance disorders. However, the molecular players serving efferent control of LL hair cell (HC) activity have not been identified. Here we demonstrate that, different from the hearing organ of vertebrate species, a nicotinic acetylcholine receptor composed only of α9 subunits operates at the LL efferent synapse. Activation of α9-containing receptors leads to LL HC hyperpolarization because of the opening of small-conductance Ca²⁺-activated potassium channels. These results will further aid in the interpretation of data obtained from LL HCs as a model for cochlear HCs.

Spontaneous and Acetylcholine Evoked Calcium Transients in the Developing Mouse Utricle

Spontaneous calcium transients are present during early postnatal development in the mouse retina and cochlea, and play an important role in maturation of the sensory organs and neural circuits in the central nervous system (CNS). It is not known whether similar calcium transients occur during postnatal development in the vestibular sensory organs. Here we demonstrate spontaneous intracellular calcium transients in sensory hair cells (HCs) and supporting cells (SCs) in the murine utricular macula during the first two postnatal weeks. Calcium transients were monitored using a genetically encoded calcium indicator, GCaMP5G (G5), at 100 ms-frame⁻¹ in excised utricle sensory epithelia, including HCs, SCs, and neurons. The reporter line expressed G5 and tdTomato (tdT) in a Gad2-Cre dependent manner within a subset of utricular HCs, SCs and neurons. Kinetics of the G5 reporter limited temporal resolution to calcium events lasting longer than 200 ms. Spontaneous calcium transients lasting 1-2 s were observed in the expressing population of HCs at birth and slower spontaneous transients lasting 10-30 s appeared in SCs by P3. Beginning at P5, calcium transients could be modulated by application of the efferent neurotransmitter acetylcholine (ACh). In mature mice, calcium transients in the utricular macula occurred spontaneously, had a duration 1-2 s, and could be modulated by the exogenous application of acetylcholine (ACh) or muscarine. Long-lasting calcium transients evoked by ACh in mature mice were blocked by atropine, consistent with previous reports describing the role of muscarinic receptors expressed in calyx bearing afferents in efferent control of vestibular sensation. Large spontaneous and ACh evoked transients were reversibly blocked by the inositol trisphosphate receptor (IP₃R) antagonist aminoethoxydiphenyl borate (2-APB). Results demonstrate long-lasting calcium transients are present in the utricular macula during the first postnatal week, and that responses to ACh mature over this same time period.

Existence of nicotinic receptors in a subset of type I vestibular hair cells of guinea pigs

In mammals, vestibular hair cells (VHCs) are classified as type I and II according to morphological criteria. Acetylcholine (ACh) is identified as the primary efferent neurotransmitter. To date, cholinergic activities have been reported in mammalian type II VHCs, but similar activities in type I VHCs have not been pursued presumably because the body of type I VHCs were suggested to be totally surrounded by afferent nerve calyces. A few reports showed that part of type I VHCs were incompletely surrounded by calyces and received contact from the efferent nerve endings in the mammals studied. The possibility of the expression of cholinergic receptors, their subunit composition, and their function in mammals' type I VHCs are still unclear. In this study, nicotinic responses were investigated by the whole-cell patch clamp technique in isolated type I VHCs of guinea pigs. Of the cells, 7.3% were sensitive to cholinergic agonists and showed an excitatory current at -40 mV which was not sensitive to nifedipine, iberiotoxin (IBTX), and apamin. The main carriers of this current were Na⁺ and K⁺. The rank order of activation potency was nicotine > 1,1-dimethyl-4-phenyl-piperazinium (DMPP) > ACh. These nicotinic ACh receptors (nAChRs) were not blocked by strychnine and α-bungarotoxin (α-BTX), but sensitive to d-tubocurarine (dTC) and mecamylamine (Mec). The findings provide physiological evidence that some subtypes of nAChRs may be located in a subset of type I VHCs, which were different from α9α10 nAChRs.

Confirming a Role for α9nAChRs and SK Potassium Channels in Type II Hair Cells of the Turtle Posterior Crista

In turtle posterior cristae, cholinergic vestibular efferent neurons (VENs) synapse on type II hair cells, bouton afferents innervating type II hair cells, and afferent calyces innervating type I hair cells. Electrical stimulation of VENs releases acetylcholine (ACh) at these synapses to exert diverse effects on afferent background discharge including rapid inhibition of bouton afferents and excitation of calyx-bearing afferents. Efferent-mediated inhibition is most pronounced in bouton afferents innervating type II hair cells near the torus, but becomes progressively smaller and briefer when moving longitudinally through the crista toward afferents innervating the planum. Sharp-electrode recordings have inferred that efferent-mediated inhibition of bouton afferents requires the sequential activation of alpha9-containing nicotinic ACh receptors (α9^*nAChRs) and small-conductance, calcium-dependent potassium channels (SK) in type II hair cells. Gradations in the strength of efferent-mediated inhibition across the crista likely reflect variations in α9^*nAChRs and/or SK activation in type II hair cells from those different regions. However, in turtle cristae, neither inference has been confirmed with direct recordings from type II hair cells. To address these gaps, we performed whole-cell, patch-clamp recordings from type II hair cells within a split-epithelial preparation of the turtle posterior crista. Here, we can easily visualize and record hair cells while maintaining their native location within the neuroepithelium. Consistent with α9^*nAChR/SK activation, ACh-sensitive currents in type II hair cells were inward at hyperpolarizing potentials but reversed near −90 mV to produce outward currents that typically peaked around −20 mV. ACh-sensitive currents were largest in torus hair cells but absent from hair cells near the planum. In current clamp recordings under zero-current conditions, ACh robustly hyperpolarized type II hair cells. ACh-sensitive responses were reversibly blocked by the α9nAChR antagonists ICS, strychnine, and methyllycaconitine as well as the SK antagonists apamin and UCL1684. Intact efferent terminals in the split-epithelial preparation spontaneously released ACh that also activated α9^*nAChRs/SK in type II hair cells. These release events were accelerated with high-potassium external solution and all events were blocked by strychnine, ICS, methyllycaconitine, and apamin. These findings provide direct evidence that activation of α9^*nAChR/SK in turtle type II hair cells underlies efferent-mediated inhibition of bouton afferents.

ACh-induced hyperpolarization and decreased resistance in mammalian type II vestibular hair cells

In the mammalian vestibular periphery, electrical activation of the efferent vestibular system (EVS) has two effects on afferent activity: 1) it increases background afferent discharge and 2) decreases afferent sensitivity to rotational stimuli. Although the cellular mechanisms underlying these two contrasting afferent responses remain obscure, we postulated that the reduction in afferent sensitivity was attributed, in part, to the activation of α9- containing nicotinic acetylcholine (ACh) receptors (α9*nAChRs) and small-conductance potassium channels (SK) in vestibular type II hair cells, as demonstrated in the peripheral vestibular system of other vertebrates. To test this hypothesis, we examined the effects of the predominant EVS neurotransmitter ACh on vestibular type II hair cells from wild-type (wt) and α9-subunit nAChR knockout (α9^-/-) mice. Immunostaining for choline acetyltransferase revealed there were no obvious gross morphological differences in the peripheral EVS innervation among any of these strains. ACh application onto wt type II hair cells, at resting potentials, produced a fast inward current followed by a slower outward current, resulting in membrane hyperpolarization and decreased membrane resistance. Hyperpolarization and decreased resistance were due to gating of SK channels. Consistent with activation of α9*nAChRs and SK channels, these ACh-sensitive currents were antagonized by the α9*nAChR blocker strychnine and SK blockers apamin and tamapin. Type II hair cells from α9^-/- mice, however, failed to respond to ACh at all. These results confirm the critical importance of α9nAChRs in efferent modulation of mammalian type II vestibular hair cells. Application of exogenous ACh reduces electrical impedance, thereby decreasing type II hair cell sensitivity. NEW & NOTEWORTHY Expression of α9 nicotinic subunit was crucial for fast cholinergic modulation of mammalian vestibular type II hair cells. These findings show a multifaceted efferent mechanism for altering hair cell membrane potential and decreasing membrane resistance that should reduce sensitivity to hair bundle displacements.

Reviewing the Role of the Efferent Vestibular System in Motor and Vestibular Circuits

Efferent circuits within the nervous system carry nerve impulses from the central nervous system to sensory end organs. Vestibular efferents originate in the brainstem and terminate on hair cells and primary afferent fibers in the semicircular canals and otolith organs within the inner ear. The function of this efferent vestibular system (EVS) in vestibular and motor coordination though, has proven difficult to determine, and remains under debate. We consider current literature that implicate corollary discharge from the spinal cord through the efferent vestibular nucleus (EVN), and hint at a potential role in overall vestibular plasticity and compensation. Hypotheses range from differentiating between passive and active movements at the level of vestibular afferents, to EVS activation under specific behavioral and environmental contexts such as arousal, predation, and locomotion. In this review, we summarize current knowledge of EVS circuitry, its effects on vestibular hair cell and primary afferent activity, and discuss its potential functional roles.

Muscarinic Acetylcholine Receptors and M-Currents Underlie Efferent-Mediated Slow Excitation in Calyx-Bearing Vestibular Afferents

Stimulation of vestibular efferent neurons excites calyx and dimorphic (CD) afferents. This excitation consists of fast and slow components that differ >100-fold in activation kinetics and response duration. In the turtle, efferent-mediated fast excitation arises in CD afferents when the predominant efferent neurotransmitter acetylcholine (ACh) activates calyceal nicotinic ACh receptors (nAChRs); however, it is unclear whether the accompanying efferent-mediated slow excitation is also attributed to cholinergic mechanisms. To identify synaptic processes underlying efferent-mediated slow excitation, we recorded from CD afferents innervating the turtle posterior crista during electrical stimulation of efferent neurons, in combination with pharmacological probes and mechanical stimulation. Efferent-mediated slow excitation was unaffected by nAChR compounds that block efferent-mediated fast excitation, but were mimicked by muscarine and antagonized by atropine, indicating that it requires ACh and muscarinic ACh receptor (mAChR) activation. Efferent-mediated slow excitation or muscarine application enhanced the sensitivity of CD afferents to mechanical stimulation, suggesting that mAChR activation increases afferent input impedance by closing calyceal potassium channels. These observations were consistent with suppression of a muscarinic-sensitive K⁺-current, or M-current. Immunohistochemistry for putative M-current candidates suggested that turtle CD afferents express KCNQ3, KCNQ4, and ERG1-3 potassium channel subunits. KCNQ channels were favored as application of the selective antagonist XE991 mimicked and occluded efferent-mediated slow excitation in CD afferents. These data highlight an efferent-mediated mechanism for enhancing afferent sensitivity. They further suggest that the clinical effectiveness of mAChR antagonists in treating balance disorders may also target synaptic mechanisms in the vestibular periphery, and that KCNQ channel modulators might offer similar therapeutic value.SIGNIFICANCE STATEMENT Targeting the efferent vestibular system (EVS) pharmacologically might prove useful in ameliorating some forms of vestibular dysfunction by modifying ongoing primary vestibular input. EVS activation engages several kinetically distinct synaptic processes that profoundly alter the discharge rate and sensitivity of first-order vestibular neurons. Efferent-mediated slow excitation of vestibular afferents is of considerable interest given its ability to elevate afferent activity over an extended time course. We demonstrate for the first time that efferent-mediated slow excitation of vestibular afferents is mediated by muscarinic acetylcholine receptor (mAChR) activation and the subsequent closure of KCNQ potassium channels. The clinical effectiveness of some anti-mAChR drugs in treating motion sickness suggest that we may, in fact, already be targeting the peripheral EVS.

Nicotinic acetylcholine receptors regulate vestibular afferent gain and activation timing

Little is known about the function of the cholinergic efferents innervating peripheral vestibular hair cells. We measured vestibular sensory evoked potentials (VsEPs) in α9 knockout (KO) mice, α10 KO mice, α7 KO mice, α9/10 and α7/9 double KO mice, and wild-type (WT) controls. We also studied the morphology and ultrastructure of efferent terminals on vestibular hair cells in α9, α10, and α9/10 KOs. Both type I and type ll vestibular hair cells express the α9 and α10 subunits. The efferent boutons on vestibular cells in α9, α10, and α9/10 KOs appeared normal, but a quantitative analysis was not performed. Mean VsEP thresholds were significantly elevated in α9 and α9/10 KO animals. Some α9 and α9/10 KO animals, however, had normal or near-normal thresholds, whereas others were greatly affected. Despite individual variability in threshold responses, latencies were consistently shortened. The double α7/9 KO resulted in decreased variance by normalizing waveforms and latencies. The phenotypes of the α7 and α10 single KOs were identical. Both α7 and α10 KO mice evidenced normal thresholds, decreased activation latencies, and larger amplitudes compared with WT mice. The data suggest a complex interaction of nicotinic acetylcholine receptors (nAChRs) in regulating vestibular afferent gain and activation timing. Although the α9/10 heteromeric nAChR is an important component of vestibular efferent activity, other peripheral or central nAChRs involving the α7 subunit or α10 subunit and α9 homomeric receptors are also important. J. Comp. Neurol. 525:1216-1233, 2017. © 2016 Wiley Periodicals, Inc.

The mammalian efferent vestibular system plays a crucial role in the high-frequency response and short-term adaptation of the vestibuloocular reflex

Although anatomically well described, the functional role of the mammalian efferent vestibular system (EVS) remains unclear. Unlike in fish and reptiles, the mammalian EVS does not seem to play a role in modulation of primary afferent activity in anticipation of active head movements. However, it could play a role in modulating long-term mechanisms requiring plasticity such as vestibular adaptation. We measured the efficacy of vestibuloocular reflex (VOR) adaptation in α9-knockout mice. These mice carry a missense mutation of the gene encoding the α9 nicotinic acetylcholine receptor (nAChR) subunit. The α9 nAChR subunit is expressed in the vestibular and auditory periphery, and its loss of function could compromise peripheral input from the predominantly cholinergic EVS. We measured the VOR gain (eye velocity/head velocity) in 26 α9-knockout mice and 27 cba129 control mice. Mice were randomly assigned to one of three groups: gain-increase adaptation (1.5×), gain-decrease adaptation (0.5×), or no adaptation (baseline, 1×). After adaptation training (horizontal rotations at 0.5 Hz with peak velocity 20°/s), we measured the sinusoidal (0.2-10 Hz, 20-100°/s) and transient (1,500-6,000°/s(2)) VOR in complete darkness. α9-Knockout mice had significantly lower baseline gains compared with control mice. This difference increased with stimulus frequency (∼ 5% <1 Hz to ∼ 25% >1 Hz). Moreover, vestibular adaptation (difference in VOR gain of gain-increase and gain-decrease adaptation groups as % of gain increase) was significantly reduced in α9-knockout mice (17%) compared with control mice (53%), a reduction of ∼ 70%. Our results show that the loss of α9 nAChRs moderately affects the VOR but severely affects VOR adaptation, suggesting that the EVS plays a crucial role in vestibular plasticity.

Pharmacologically distinct nicotinic acetylcholine receptors drive efferent-mediated excitation in calyx-bearing vestibular afferents

Electrical stimulation of vestibular efferent neurons rapidly excites the resting discharge of calyx/dimorphic (CD) afferents. In turtle, this excitation arises when acetylcholine (ACh), released from efferent terminals, directly depolarizes calyceal endings by activating nicotinic ACh receptors (nAChRs). Although molecular biological data from the peripheral vestibular system implicate most of the known nAChR subunits, specific information about those contributing to efferent-mediated excitation of CD afferents is lacking. We sought to identify the nAChR subunits that underlie the rapid excitation of CD afferents and whether they differ from α9α10 nAChRs on type II hair cells that drive efferent-mediated inhibition in adjacent bouton afferents. We recorded from CD and bouton afferents innervating the turtle posterior crista during electrical stimulation of vestibular efferents while applying several subtype-selective nAChR agonists and antagonists. The α9α10 nAChR antagonists, α-bungarotoxin and α-conotoxin RgIA, blocked efferent-mediated inhibition in bouton afferents while leaving efferent-mediated excitation in CD units largely intact. Conversely, 5-iodo-A-85380, sazetidine-A, varenicline, α-conotoxin MII, and bPiDDB (N,N-dodecane-1,12-diyl-bis-3-picolinium dibromide) blocked efferent-mediated excitation in CD afferents without affecting efferent-mediated inhibition in bouton afferents. This pharmacological profile suggested that calyceal nAChRs contain α6 and β2, but not α9, nAChR subunits. Selective blockade of efferent-mediated excitation in CD afferents distinguished dimorphic from calyx afferents by revealing type II hair cell input. Dimorphic afferents differed in having higher mean discharge rates and a mean efferent-mediated excitation that was smaller in amplitude yet longer in duration. Molecular biological data demonstrated the expression of α9 in turtle hair cells and α4 and β2 in associated vestibular ganglia.

M2 muscarinic ACh receptors sensitive BK channels mediate cholinergic inhibition of type II vestibular hair cells

There are two types of hair cells in the sensory epithelium of vestibular end organ. Type II vestibular hair cell (VHC II) is innervated by the efferent nerve endings, which employ a cholinergic inhibition mediated by SK channels through the activation of α9-containing nAChR. Our previous studies demonstrated that a BK-type cholinergic inhibition was present in guinea pig VHCs II, which may be mediated by an unknown mAChR. In this study, BK channel activities triggered by ACh were studied to determine the mAChR subtype and function. We found the BK channel was insensitive to α9-containing nAChR antagonists and m1, m3, m4 muscarinic antagonists, but potently inhibited by the m2 muscarinic antagonist. Muscarinic agonists could mimic the effect of ACh and be blocked by m2 antagonist. cAMP analog activated the BK current and adenyl cyclase (AC) inhibitor inhibited the ACh response. Inhibitor of Giα subunit failed to affect the BK current, but inhibitor of Giα and Giβγ subunits showed a potent inhibition to these currents. Our findings provide the physiological evidence that mAChRs may locate in guinea pig VHCs II, and m2 mAChRs may play a dominant role in BK-type cholinergic inhibition. The activation of m2 mAChRs may stimulate Giβγ-mediated excitation of AC/cAMP activities and lead to the phosphorylation of Ca(2+) channels, resulting in the influx of Ca(2+) and opening of the BK channel.

Spatial and temporal expression patterns of nicotinic acetylcholine α9 and α10 subunits in the embryonic and early postnatal inner ear

The expression and function of nicotinic receptor subunits (nAChRs) in the inner ear before the onset of hearing is not well understood. We investigated the mRNA expression of the α9 and α10 nAChR subunits in sensory hair cells of the embryonic and postnatal rat inner ear. We mapped their spatial and temporal expression in cochlear and vestibular hair cells using qPCR, [35S] labeled cRNA in situ hybridization, and α-bungarotoxin (α-Bgt) to label the presumptive membrane-bound receptor on cochlear hair cells. The results suggest that (1) the mRNA expression of the α9 subunit precedes expression of the α10 subunit in both cochlear and vestibular hair cells, (2) the mRNA expression of both the α9 and α10 subunits occurs earlier in the vestibular system than in the cochlea, (3) the mRNA expression of both subunits is required for the assembled receptor complexes, and (4) the presumptive assembled receptor, at least in the cochlea, is associated with synapse formation and the onset of function.

Responses of pigeon vestibular hair cells to cholinergic agonists and antagonists

Acetylcholine (ACh) is the major neurotransmitter released from vestibular efferent terminals onto hair cells and afferents. Previous studies indicate that the two classes of acetylcholine receptors, nicotinic (nAChRs) and muscarinic receptors (mAChRs), are expressed by vestibular hair cells (VHCs). To identify if both classes of receptors are present in VHCs, whole cell, voltage-clamp- and current-clamp-patch recordings were performed on isolated pigeon vestibular type I and type II HCs during the application of the cholinergic agonists, acetylcholine and carbachol, and the cholinergic antagonists, D-tubocurarine and atropine. By applying in different combinations, these compounds were used to selectively activate either nAChRs or mAChRs. The effects of nAChR and mAChR activation on HC currents and zero electrode current potential (V(z)) were monitored. It was found that presumed mAChR activation decreased both inward and outward currents in both type I and type II HCs, resulting in either a depolarization or hyperpolarization. Conversely, nAChR activation mainly increased both inward and outward currents in type II HCs, resulting in a hyperpolarization of their V(z). nAChR activation also increased outward currents in type I HCs resulting in either a depolarization or hyperpolarization of their V(z). The decrease of inward and outward currents and the depolarization of the V(z) in type I pigeon HCs by activation of mAChRs represents a new finding. Ion channel candidates in pigeon vestibular HCs that might underlie the modulation of the macroscopic ionic currents and V(z) by different AChR activation are discussed.

Mechanisms of efferent-mediated responses in the turtle posterior crista

To study the cellular mechanisms of efferent actions, we recorded from vestibular-nerve afferents close to the turtle posterior crista while efferent fibers were electrically stimulated. Efferent-mediated responses were obtained from calyx-bearing (CD, calyx and dimorphic) afferents and from bouton (B) afferents distinguished by their neuroepithelial locations into BT units near the torus and BM units at intermediate sites. The spike discharge of CD units is strongly excited by efferent stimulation, whereas BT and BM units are inhibited, with BM units also showing a postinhibitory excitation. Synaptic activity was recorded intracellularly after spikes were blocked. Responses of BT/BM units to single efferent shocks consist of a brief depolarization followed by a prolonged hyperpolarization. Both components reflect variations in hair-cell quantal release rates and are eliminated by pharmacological antagonists of alpha9/alpha10 nicotinic receptors. Blocking calcium-dependent SK potassium channels converts the biphasic response into a prolonged depolarization. Results can be explained, as in other hair-cell systems, by the sequential activation of alpha9/alpha10 and SK channels. In BM units, the postinhibitory excitation is based on an increased rate of hair-cell quanta and depends on the preceding inhibition. There is, in addition, an efferent-mediated, direct depolarization of BT/BM and CD fibers. In CD units, it is the exclusive efferent response. Nicotinic antagonists have different effects on hair-cell efferent actions and on the direct depolarization of CD and BT/BM units. Ultrastructural studies, besides confirming the efferent innervation of type II hair cells and calyx endings, show that turtle efferents commonly contact afferent boutons terminating on type II hair cells.

Polysynaptic inputs to vestibular efferent neurons as revealed by viral transneuronal tracing

The Bartha strain of the alpha-herpes pseudorabies virus (PrV) was used as a retrograde transneuronal tracer to map synaptic inputs to the vestibular efferent neurons of the Mongolian gerbil, Meriones unguiculatus. Although previous experiments have shown that vestibular efferent neurons respond to visual motion and somatosensory stimuli, the anatomic connections mediating those responses are unknown. PrV was injected unilaterally into the horizontal semicircular canal neuroepithelium of gerbils, where it was taken up by efferent axon terminals. The virus was then retrogradely transported to efferent cell bodies, replicated, and transported into synaptic endings projecting onto the efferent cells. Thirty animals were sacrificed at approximately 5-h increments between 75 and 105 h post-infection after determining that shorter time points had no central infection. Infected cells were visualized immunohistochemically. Temporal progression of neuronal infection was used to determine the nature of primary and higher order projections to the vestibular efferent neurons. Animals sacrificed at 80-94 h post-inoculation exhibited immunostaining in the dorsal and ventral group of vestibular efferent neurons, predominately on the contralateral side. Neurons within the medial, gigantocellular, and lateral reticular formations were among the first cells infected thereafter. At 95 h, additional virus-labeled cell groups included the solitary, area postrema, pontine reticular, prepositus, dorsal raphe, tegmental, the subcoeruleus nuclei, the nucleus of Darkschewitsch, and the inferior olivary beta and ventrolateral subnuclei. Analysis beyond 95 h revealed virus-infected neurons located in the vestibulo-cerebellar and motor cortices. Paraventricular, lateral, and posterior hypothalamic cells, as well as central amygdala cells, were also labeled. Spinal cord tissue exhibited no labeling in the intermediolateral cell column, but scattered cells were found in the central cervical nucleus. The results suggest functional associations among efferent feedback regulation of labyrinthine sensory input and both behavioral and autonomic systems, and support a closed-looped vestibular feedback model with additional open-loop polysynaptic inputs.

Muscarinic ACh Receptor Activation Causes Transmitter Release From Isolated Frog Vestibular Hair Cells

In the frog, vestibular efferent fibers innervate only type-II vestibular hair cells. Through this direct contact with hair cells, efferent neurons are capable of modifying transmitter release from hair cells onto primary vestibular afferents. The major efferent transmitter, acetylcholine (ACh), is known to produce distinct pharmacological actions involving several ACh receptors. Previous studies have implicated the presence of muscarinic ACh receptors on vestibular hair cells, although, surprisingly, a muscarinic-mediated electrical response has not been demonstrated in solitary vestibular hair cells. This study demonstrates that muscarinic receptors can evoke transmitter release from vestibular hair cells. Detection of this release was obtained through patch-clamp recordings from catfish cone horizontal cells, serving as glutamate detectors after pairing them with isolated frog semicircular canal hair cells in a two-cell preparation. Although horizontal cells alone failed to respond to carbachol, application of 20 μM carbachol to the two-cell preparation resulted in a horizontal cell response that could be mimicked by exogenous application of glutamate. All of the horizontal cells in the two-cell preparation responded to 20 μM CCh. Furthermore, this presumed transmitter release persisted in the presence of d-tubocurarine at concentrations that block all known hair cell nicotinic ACh receptors. The effect on the detector cell, imparted by the carbachol application to the hair cell-horizontal cell preparation, was blocked both by 2-amino-5-phosphonopentanoic acid, a selective N-methyl-d-aspartate antagonist, and the muscarinic antagonist, atropine. Thus vestibular hair cells from the frog semicircular canal can be stimulated to release transmitter by activating their muscarinic receptors.

A transiently expressed SK current sustains and modulates action potential activity in immature mouse inner hair cells

From just after birth, mouse inner hair cells (IHCs) expressed a Ca(2+)-activated K(+) current that was reduced by intracellular BAPTA at concentrations >or= 1 mM. The block of this current by nifedipine suggests the direct involvement of Ca(v)1.3 Ca(2+) channels in its activation. On the basis of its high sensitivity to apamin (K(D) 360 pM) it was identified as a small-conductance Ca(2+)-activated K(+) current (SK), probably SK2. A similar current was also found in outer hair cells (OHCs) from the beginning of the second postnatal week. In both cell types the appearance of the SK current coincided with their becoming responsive to acetylcholine (ACh), the main efferent neurotransmitter in the cochlea. The effect of ACh on IHCs was abolished when they were simultaneously superfused with strychnine, consistent with the presence of nicotinic ACh receptors (nAChRs). Extracellular Ca(2+) either potentiated or blocked the nAChR current depending on its concentration, as previously reported for the recombinant alpha9alpha10 nAChR. Outward currents activated by ACh were reduced by blocking the SK current with apamin or by preventing SK current activation with intracellular BAPTA (>or= 10 mM). The endogenous mobile Ca(2+) buffer concentration was estimated to be equivalent to about 1 mM BAPTA, suggesting that in physiological conditions the SK channel is significantly activated by Ca(2+) influx through both Ca(v)1.3 Ca(2+) channels and alpha9alpha10 nAChRs. Current clamp experiments showed that in IHCs the SK current is required for sustaining a train of action potentials and also modulates their frequency when activated by ACh.