Cholinergically-induced changes in outward currents in hair cells isolated from the semicircular canal of the frog

A review of efferent cholinergic synaptic transmission in the vestibular periphery and its functional implications

It has been over 60 years since peripheral efferent vestibular terminals were first identified in mammals, and yet the function of the efferent vestibular system remains obscure. One reason for the lack of progress may be due to our deficient understanding of the peripheral efferent synapse. Although vestibular efferent terminals were identified as cholinergic less than a decade after their anatomical characterization, the cellular mechanisms that underlie the properties of these synapses have had to be inferred. In this review we examine how recent mammalian studies have begun to reveal both nicotinic and muscarinic effects at these terminals and therefore provide a context for fast and slow responses observed in classic electrophysiological studies of the mammalian efferent vestibular system, nearly 40 years ago. Although incomplete, these new results together with those of recent behavioral studies are helping to unravel the mysterious and perplexing action of the efferent vestibular system. Armed with this information, we may finally appreciate the behavioral framework in which the efferent vestibular system operates.

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.

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.

Two distinct channels mediated by m2mAChR and α9nAChR co-exist in type II vestibular hair cells of guinea pig

Acetylcholine (ACh) is the principal vestibular efferent neurotransmitter among mammalians. Pharmacologic studies prove that ACh activates a small conductance Ca2+-activated K+ channels (KCa) current (SK2), mediated by α9-containing nicotinic ACh receptor (α9nAChR) in mammalian type II vestibular hair cells (VHCs II). However, our studies demonstrate that the m2 muscarinic ACh receptor (m2mAChR) mediates a big conductance KCa current (BK) in VHCs II. To better elucidate the correlation between these two distinct channels in VHCs II of guinea pig, this study was designed to verify whether these two channels and their corresponding AChR subtypes co-exist in the same VHCs II by whole-cell patch clamp recordings. We found that m2mAChR sensitive BK currents were activated in VHCs II isolated by collagenase IA, while α9nAChR sensitive SK2 currents were activated in VHCs II isolated by trypsin. Interestingly, after exposing the patched cells isolated by trypsin to collagenase IA for 3 min, the α9nAChR sensitive SK2 current was abolished, while m2mAChR-sensitive BK current was activated. Therefore, our findings provide evidence that the two distinct channels and their corresponding AChR subtypes may co-exist in the same VHCs II, and the alternative presence of these two ACh receptors-sensitive currents depended on isolating preparation with different enzymes.

Modulation of hair cell efferents

Outer hair cells (OHCs) amplify the sound-evoked motion of the basilar membrane to enhance acoustic sensitivity and frequency selectivity. Medial olivocochlear (MOC) efferents inhibit OHCs to reduce the sound-evoked response of cochlear afferent neurons. OHC inhibition occurs through the activation of postsynaptic α9α10 nicotinic receptors tightly coupled to calcium-dependent SK2 channels that hyperpolarize the hair cell. MOC neurons are cholinergic but a number of other neurotransmitters and neuromodulators have been proposed to participate in efferent transmission, with emerging evidence for both pre- and postsynaptic effects. Cochlear inhibition in vivo is maximized by repetitive activation of the efferents, reflecting facilitation and summation of transmitter release onto outer hair cells. This review summarizes recent studies on cellular and molecular mechanisms of cholinergic inhibition and the regulation of those molecular components, in particular the involvement of intracellular calcium. Facilitation at the efferent synapse is compared in a variety of animals, as well as other possible mechanisms of modulation of ACh release. These results suggest that short-term plasticity contributes to effective cholinergic inhibition of hair cells.

Prestin and the cholinergic receptor of hair cells: positively-selected proteins in mammals

The hair cells of the vertebrate inner ear posses active mechanical processes to amplify their inputs. The stereocilia bundle of various vertebrate animals can produce active movements. Though standard stereocilia-based mechanisms to promote amplification persist in mammals, an additional radically different mechanism evolved: the so-called somatic electromotility which refers to the elongation/contraction of the outer hair cells' (OHC) cylindrical cell body in response to membrane voltage changes. Somatic electromotility in OHCs, as the basis for cochlear amplification, is a mammalian novelty and it is largely dependent upon the properties of the unique motor protein prestin. We review recent literature which has demonstrated that although the gene encoding prestin is present in all vertebrate species, mammalian prestin has been under positive selective pressure to acquire motor properties, probably rendering it fit to serve somatic motility in outer hair cells. Moreover, we discuss data which indicates that a modified α10 nicotinic cholinergic receptor subunit has co-evolved in mammals, most likely to give the auditory feedback system the capability to control somatic electromotility.

Muscarinic acetylcholine receptor subtype expression in avian vestibular hair cells, nerve terminals and ganglion cells

Muscarinic acetylcholine receptors (mAChRs) are widely expressed in the CNS and peripheral nervous system and play an important role in modulating the cell activity and function. We have shown that the cholinergic agonist carbachol reduces the pigeon's inwardly rectifying potassium channel (pKir2.1) ionic currents in native vestibular hair cells. We have cloned and sequenced pigeon mAChR subtypes M2-M5 and we have studied the expression of all five mAChR subtypes (M1-M5) in the pigeon vestibular end organs (semicircular canal ampullary cristae and utricular maculae), vestibular nerve fibers and the vestibular (Scarpa's) ganglion using tissue immunohistochemistry (IH), dissociated single cell immunocytochemistry (IC) and Western blotting (WB). We found that vestibular hair cells, nerve fibers and ganglion cells each expressed all five (M1-M5) mAChR subtypes. Two of the three odd-numbered mAChRs (M1, M5) were present on the hair cell cilia, supporting cells and nerve terminals. And all three odd numbered mAChRs (M1, M3 and M5) were expressed on cuticular plates, myelin sheaths and Schwann cells. Even-numbered mAChRs were seen on the nerve terminals. M2 was also shown on the cuticular plates and supporting cells. Vestibular efferent fibers and terminals were not identified in our studies. Results from WB of the dissociated vestibular epithelia, nerve fibers and vestibular ganglia were consistent with the results from IH and IC. Our findings suggest that there is considerable co-expression of the subtypes on the neural elements of the labyrinth. Further electrophysiological and pharmacological studies should delineate the mechanisms of action of muscarinic acetylcholine receptors on structures in the labyrinth.

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.

Pharmacology of acetylcholine-mediated cell signaling in the lateral line organ following efferent stimulation

Cholinergic efferent fibers modify hair cell responses to mechanical stimulation. It is hypothesized that calcium entering the hair cell through a nicotinic receptor activates a small-conductance (SK), calcium-activated potassium channel to hyperpolarize the hair cell. The calcium signal may be amplified by calcium-induced calcium release from the synaptic cisternae. Pharmacological tests of these ideas in the intact cochlea have been technically difficult because of the complex and fragile structure of the mammalian inner ear. We turned to the Xenopus laevis lateral line organ, whose simplicity and accessibility make it a model for understanding hair cell organ function in a relatively intact system. Drugs were applied to the inner surface of the skin while monitoring the effects of efferent stimulation on afferent fiber discharge rate. Efferent effects were blocked by antagonists of SK channels including apamin (EC50 = 0.5 microM) and dequalinium (EC50 = 12 microM). The effect of apamin was not enhanced by co-administration of phenylmethylsulfonyl fluoride, a proteolysis inhibitor. Efferent effects were attenuated by ryanodine, an agent that can interfere with calcium-induced calcium release, although relatively high (mM) concentrations of ryanodine were required. Fluorescent cationic styryl dyes, 4-di-2-asp and fm 1-43, blocked efferent effects, although it was not possible to observe specific entry of the dye into the base of hair cells. These pharmacological findings in the Xenopus lateral line organ support the hypothesis that effects of efferent stimulation are mediated by calcium entry through the nicotinic receptor via activation of SK channels and suggest the generality of this mechanism in meditating cholinergic efferent effects.

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

Frog vestibular organs are endowed with a prominent cholinergic efferent innervation whose stimulation results in several different effects, thereby suggesting diversity in the expression of postsynaptic acetylcholine (ACh) receptors. The application of ACh can mimic efferent stimulation in producing both an inhibition and a facilitation of afferent discharge which are thought to be mediated by at least two distinct ACh receptors present on vestibular hair cells, i.e., alpha9-containing nicotinic receptors (alpha9nAChR) and muscarinic receptors (mAChR), respectively. Using patch-clamp and multiunit vestibular afferent recordings, we demonstrate the presence of an additional excitatory hair cell nicotinic ACh receptor pharmacologically distinct from both alpha9nAChR and mAChR. In order of increasing potency, this distinct receptor was activated by ACh, carbachol, and particularly by the selective nicotinic agonist 1,1-dimethyl-4-phenyl-piperazinium (DMPP). This DMPP-sensitive nicotinic receptor (RDMPP) was antagonized by the classic nicotinic antagonist d-tubocurarine, but refractory to strychnine, atropine, and propylbenzilylcholine mustard, at concentrations that completely block alpha9nAChR and/or mAChR. Activation of RDMPP on application of ACh or DMPP to a subpopulation of isolated posterior semicircular canal (SCC) hair cells resulted in a large depolarization (18.0 +/- 1.2 mV). The current underlying this depolarization was typically small (80.1 +/- 21.6 pA) and showed an inward rectification starting around -45 mV. Given their respective EC50s (47 nM vs. 20 microM), RDMPP was nearly 400 times more sensitive to ACh than alpha9nAChR and thus responded to concentrations of ACh considered too low to be effective at stimulating alpha9nAChR. Despite this remarkable sensitivity, exogenous ACh readily stimulated the mAChR in the intact posterior SCC preparation but failed to activate RDMPP unless the acetylcholinesterase inhibitor physostigmine was present, or high concentrations of ACh were used (>3 mM). In frog, RDMPP most likely underlies the rapid excitatory response seen during efferent stimulation.

The effect of proteolytic enzymes on the alpha9-nicotinic receptor-mediated response in isolated frog vestibular hair cells

In frog vestibular organs, efferent neurons exclusively innervate type II hair cells. Acetylcholine, the predominant efferent transmitter, acting on acetylcholine receptors of these hair cells ultimately inhibits and/or facilitates vestibular afferent firing. A coupling between alpha9-nicotinic acetylcholine receptors (alpha9nAChR) and apamin-sensitive, small-conductance, calcium-dependent potassium channels (SK) is thought to drive the inhibition by hyperpolarizing hair cells thereby decreasing their release of transmitter onto afferents. The presence of alpha9nAChR in these cells was demonstrated using pharmacological, immunocytochemical, and molecular biological techniques. However, fewer than 10% of saccular hair cells dissociated using protease VIII, protease XXIV, or papain responded to acetylcholine during perforated-patch clamp recordings. When present, these responses were invariably transient, small in amplitude, and difficult to characterize. In contrast, the majority of saccular hair cells ( approximately 90%) dissociated using trypsin consistently responded to acetylcholine with an increase in outward current and concomitant hyperpolarization. In agreement with alpha9nAChR pharmacology obtained in other hair cells, the acetylcholine response in saccular hair cells was reversibly antagonized by strychnine, curare, tetraethylammonium, and apamin. Brief perfusions with either protease or papain permanently abolished the alpha9-nicotinic response in isolated saccular hair cells. These enzymes when inactivated became completely ineffective at abolishing the alpha9-nicotinic response, suggesting an enzymatic interaction with the alpha9nAChR and/or downstream effector. The mechanism by which these enzymes render saccular hair cells unresponsive to acetylcholine remains unknown, but it most likely involves proteolysis of alpha9nAChR, SK, or both.

A role for chloride in the hyperpolarizing effect of acetylcholine in isolated frog vestibular hair cells

Acetylcholine (ACh) is the dominant transmitter released from inner ear efferent neurons. In frog vestibular organs, these efferent neurons synapse exclusively with type II hair cells. Hair cells isolated from the frog saccule hyperpolarize following the application of 50 microM ACh, thereby demonstrating the presence of an ACh receptor. A role for Cl(-) in the response of hair cell-bearing organs to efferent nerve activation or ACh application was suggested some years ago. Perfusion with solutions in which most of the Cl(-) was replaced by large impermeant anions decreased the cholinergic inhibition of afferent firing in the cat and turtle cochleas, and frog semicircular canal. Our previous work in the intact organ demonstrated that substitution of large impermeant anions for Cl(-) or use of Cl(-) channel blockers reduced the effect of ACh on saccular afferent firing. Using the perforated-patch clamping technique, replacement of Cl(-) by methanesulfonate, iodide, nitrate, or thiocyanate attenuated the hyperpolarizing response to ACh in hair cells isolated from the frog saccule. The chloride channel blockers picrotoxin and 4,4'-dinitrostilbene-2,2'-disulfonic acid were also tested and found to inhibit the ACh response. Thus, the present work demonstrates that the effects of Cl(-) substitutions or Cl(-) channel blockers on the ACh response in the intact saccule can be explained completely by effects on the hair cell. Evidence is also presented for the presence of the messenger RNA for a calcium-dependent chloride channel in all hair cells but especially saccular hair cells. This channel may be involved in the response to ACh. The precise role for chloride in this response, whether as a distinct ion current, as a transported ion, or as a permissive ion for other components, is discussed.

Topographic distribution of nicotinic acetylcholine receptors in the cristae of a turtle

The neurochemical basis of cholinergic efferent modulation of afferent function in the vestibular periphery remains incompletely understood; however, there is cellular, biochemical and molecular biological evidence for both muscarinic and nicotinic acetylcholine (ACh) receptors (nAChRs) in this system. This study examined the topographic distribution of alpha-bungarotoxin (alpha-BTX) nAChRs in the cristae of a turtle species. Cristae were perfusion-fixed, cut at 20 micrometer on a cryostat and incubated with alpha-BTX or polyclonal antibodies raised against Torpedo nAChR. Light microscopy showed abundant specific labeling of nAChR in the central zone of each hemicrista on the calyx-bearing afferents surrounding type I hair cells and on the base of the type II hair cells. Within the peripheral zone, dense labeling of type II hair cells near the torus and sparse or no label was observed on type II hair cells near the planum. The alpha-BTX binding showed a similar pattern within the cristae. The similarity between the topographic distribution of alpha-BTX binding nAChR and of efferent inhibition of afferents supports the notion that the inhibitory effect of afferents is mediated by nAChR.

Autoradiographic demonstration of quinuclidinyl benzilate binding sites in the vestibular organs of the gerbil

Gerbil vestibular tissues were isolated by microdissection and incubated in vitro with 3H-quinuclidinyl benzilate (3H-QNB). Control tissues were incubated in medium containing unlabeled atropine to differentiate non-specific from specific binding. Autoradiographic grain densities were determined by morphometric techniques and evaluated by two-tailed t-test. The label densities of sensory epithelia from experimental preparations of ampulla, utricle and saccule were found to be significantly higher than those in the adjacent endolymphatic compartment and also higher than those of adjacent stromal tissue comprising connective tissue, nerve fibers and capillaries. In contrast, no tissue region in atropine controls showed label density significantly above that of the endolymphatic compartment. Label density of ampullar sensory epithelium incubated with 3H-QNB alone was significantly higher than that of sensory epithelium from utricle or saccule. Grain density was greater in the peripheral regions of the ampullar crista compared to the vertex. Appreciable label was also present in nerve bundles beneath the sensory epithelium of the ampulla. The current study demonstrates the existence of putative muscarinic neurotransmitter/neuromodulator receptor sites in mammalian vestibular sense organs at locations corresponding to efferent innervation, with particularly significant concentrations in the ampulla.

The metabotropic glutamate receptors of the vestibular organs

This research sought to test the presence and function of metabotropic excitatory amino acid receptors (mGluR) in the frog semicircular canal (SCC). The mGluR agonist +/- 1-aminocyclopentane-trans-1,3-dicarboxylate (ACPD) produced an increase in afferent firing rates of the ampullar nerve of the intact posterior canal. This increase was not due to a stimulation of cholinergic efferent terminals or the acetylcholine (ACh) receptor, since atropine, in concentrations which blocked the response to exogenous acetylcholine, did not affect the response to ACPD. Likewise, ACPD effects were not due to stimulation of postsynaptic NMDA receptors, since the NMDA antagonist D(-)-2-amino-5-phosphonopentanoate (AP-5) did not affect the response to ACPD, reinforcing the reported selectivity of ACPD for mGluRs. When the SCC was superfused with artificial perilymph known to inhibit hair cell transmitter release (i.e. low Ca-high Mg), ACPD failed to increase afferent firing. This suggests that the receptor activated by ACPD is located on the hair cell. Pharmacological evidence suggested that the mGluRs involved in afferent facilitation belong to Group I (i.e. subtypes 1 and 5). In fact, the Group III agonist AP-4 had no effect, and the ACPD facilitatory effect was blocked by the Group I mGluR antagonists (S)-4-carboxyphenylglycine (CPG) and (RS)-1-aminoindan-1,5-dicarboxylic acid (AIDA). Additional pharmacological evidence supported the presence of Group I mGluRs. Interestingly, the mGluR antagonists, AIDA and 4CPG, by themselves did not affect the resting firing rates of ampullar afferents. This may suggest that the mGluRs are not involved in resting activity but perhaps only in evoked activity (as suggested in Guth et al. (1991) Hear. Res. 56, 69-78). In addition, the mRNA for the mGluR1 has been detected in hair cells of both SCC, utricle, and saccule. In summary, the evidence points to an mGluR localized to the hair cell (i.e. an autoreceptor) which may be activated to produce a positive feedback augmentation of evoked but not resting transmitter release and thus affect afferent activity.

A role for chloride in the suppressive effect of acetylcholine on afferent vestibular activity

Afferents of the frog semicircular canal (SCC) respond to acetylcholine (ACh) application (0.3-1.0 mM) with a facilitation of their activity while frog saccular afferents respond with suppression (Guth et al., 1994). All recordings are of resting (i.e., non-stimulated) multiunit activity as previously reported (Guth et al., 1994). Substitution of 80% of external chloride (Cl-) by large, poorly permeant anions of different structures (isethionate, methanesulfonate, methylsulfate, and gluconate) reduced the suppressive effect of ACh in the frog saccular afferents. This substitution did not affect the facilitatory response of SCC afferents to ACh. Chloride channel blockers were also used to test further whether Cl- is involved in the ACh suppressive effect. These included: niflumic and flufenamic acids, picrotoxin, 5-nitro-2-(-3-phenylpropylamino)benzoic acid (NPPB), and 4,4'-dinitrostilbene-2,2'-disulfonic acid (DNDS). As with the Cl- substitutions, all of these agents reduced the suppressive response to ACh in the saccule, but not the facilitatory response seen in the SCC. The suppressive effect of ACh on saccular afferents is considered to be due to activation of a nicotinic-like receptor (Guth et al., 1994; Guth and Norris, 1996). Taking into account the effects of both Cl- substitutions and Cl- channel blockers, we conclude that changes in Cl- availability influence the suppressive effect of ACh and that therefore Cl- may be involved in this effect.

Acetylcholine, outer hair cell electromotility, and the cochlear amplifier

The dominant efferent innervation of the cochlea terminates on outer hair cells (OHCs), with acetylcholine (ACh) being its principal neurotransmitter. OHCs respond with a somatic shape change to alterations in their membrane potential, and this electromotile response is believed to provide mechanical feedback to the basilar membrane. We examine the effects of ACh on electromotile responses in isolated OHCs and attempt to deduce the mechanism of ACh action. Axial electromotile amplitude and cell compliance increase in the presence of the ligand. This response occurs with a significantly greater latency than membrane current and potential changes attributable to ACh and is contemporaneous with Ca2+ release from intracellular stores. It is likely that increased axial compliance largely accounts for the increase in motility. The mechanical responses are probably related to a recently demonstrated slow efferent effect. The implications of the present findings related to commonly assumed efferent behavior in vivo are considered.

Efferent neurons and vestibular cross talk in the frog

A galvanic stimulus (30- to 120-s, 0.3-mA constant current pulse) was used to depolarize the spike-generating region of horizontal and anterior canal afferent neurons. The galvanically induced spike activity from these neurons served as a driving input to the efferent vestibular system in the bullfrog. Efferent-mediated effects were assessed by intracellular recordings of posterior canal afferent spike activity, either ipsilateral or contralateral to the driving stimulus. Ipsilateral to the driving stimulus, efferent-mediated spike rate changes occurred in 62 (39%) of 158 posterior canal afferent neurons. Ipsilateral efferent-mediated effects were overwhelmingly excitatory (92%). Of responding units, 3% were inhibited during stimulus application and 5% showed mixed responses involving 3-20 s of inhibition followed by facilitation. Contralateral to the driving stimulus, efferent-mediated spike rate changes occurred in 18 (23%) of 77 posterior canal afferent neurons. Contralateral efferent-mediated effects were overwhelmingly inhibitory (95%). Only one unit was facilitated during stimulation and no mixed responses to contralateral stimulation were observed. Analysis of the coefficient of variation in interspike intervals (CV) before and during stimulation showed no significant efferent-mediated effects on spike train noise. Comparisons of resting spike rates between units showing efferent-mediated effects and those that did not were in general agreement with previous studies. Responding units had a lower mean spike rate (6.8 +/- 0.70 spikes/s, mean +/- SE) than did nonresponding units (10.7 +/- 0.42 spikes/s, mean +/- SE; P < 0.001; 2-tailed t-test of log-normalized data). Comparison between groups in the regularity of their resting spike rates, as quantified by CV, showed considerable overlap. When responding and nonresponding units with similar resting spike rates were compared, responding units had more irregular resting spike rates than did nonresponding units (P < 0.004; 2-tailed, paired t-test). In most cases (77%) the temporal pattern and general shapes of efferent-mediated responses mirrored the driving input of the galvanically activated afferent neurons. The other 23% of efferent-mediated responses exhibited a marked adaptation of the response. Adapting and nonadapting units were not significantly different in their mean resting spike rates or in the regularity of their resting spike rates.

Acetylcholine response in guinea pig outer hair cells. I. Properties of the response

The properties of the ACh (acetylcholine) response in guinea pig outer hair cells (OHCs) are not well understood. It has been shown that the response to ACh involves the activation of a Ca2+ dependent K+ selective conductance (referred to as Ksub where sub stands for suberyldicholine). In the present study, we examined the voltage dependence, the time dependence, and the desensitization of the ACh response. In addition, we examined the K+ selectivity of K(sub). These properties are important for aiding in the determination of the type of K+ channels activated by ACh. Patch-clamp technique in the whole-cell mode was used to record from single OHCs isolated from adult pigmented guinea pigs. ACh (100 microM) was applied to the voltage-clamped OHCs and the ACh induced currents (IACh) were measured. A voltage dependence of the ACh response was found with the ACh induced currents decaying monoexponentially at potentials positive to -30 mV. The decay of the ACh induced currents was faster soon after establishing the whole-cell mode of recording when compared to the decay of the currents some time later. This effect, referred to as the time dependence, was different from the desensitization of the response upon prolonged application of ACh. The desensitization of the ACh induced currents was about 50% after 2 min of continuous application of 100 microM ACh. The examined characteristics of the ACh response in guinea pig OHCs indicate a voltage and time dependence of the response and strong K+ selectivity of the Ksub.

The hair cell acetylcholine receptors: a synthesis

In this article the evidence concerning the nature of the acetylcholine (ACh) receptors on hair cells is reviewed. A schematic organization of these receptors is offered, based on the evidence as follows. (1) There are two kinds of ACh receptors on hair cells: muscarinic-like and nicotinic-like. (2) The nicotinic-like receptor mediates a hyperpolarizing response to ACh and a consequent reduction in afferent firing. (3) The muscarinic-like receptors mediate both a depolarization and a hyperpolarization of hair cells. (4) The hyperpolarization results in a reduction in afferent firing and (5) the depolarization results in an increase in afferent firing.

Acetylcholine activates a K+ conductance permeable to Cs+ in guinea pig outer hair cells

Acetylcholine (ACh), the major neurotransmitter released by efferent nerve fibers in the cochlea, has been shown to activate a Ca(2+)-dependent K+ conductance in outer hair cells (OHCs). Previously we reported that this ACh operated conductance is permeable to Cs+. The purpose of the present study was to characterize further this Cs(+)-permeable channel and its dependency on Ca2+ using isolated OHCs and the patch clamp technique in the whole cell configuration. The changes in the ACh response were examined when Cs+, Ba2+, Cd2+, N-methyl-D-glucamine (NMG+) and tetraethylammonium (TEA+) were placed in the external or internal solutions. Cs+ substituted for K+ in carrying the ACh-evoked Ca(2+)-dependent K+ current. When NMG+/TEA+ was substituted for internal K+ ACh-evoked an inward and an outward current, and Cs+ substituted for external K+ blocked the outward but not the inward current evoked by ACh suggesting it was carried by K+. In the NMG+/TEA+ condition, when the cell was held at different Vh values for an extended period of time, the ACh-induced K+ current rectified. In Ba2+ (3 mM) with zero Ca2+ ACh failed to induce any detectable current and the ACh response slowly recovered from the Ba2+ block, suggesting a block at an intracellular site. Cd2+ (1 mM) readily and reversibly blocked ACh-induced currents even when carried by Cs+. This data suggests that ACh opens a channel selective for K+, conductive to Cs+ and dependent on Ca2+.

Cholinergic agonists increase intracellular calcium concentration in frog vestibular hair cells

Acetylcholine (ACh) is usually considered to be the neurotransmitter of the efferent vestibular system. The nature and the localization of cholinergic receptors have been investigated on frog isolated vestibular hair cells (VHCs), by measuring variations of intracellular calcium concentration ([Ca2+]i), using calcium sensitive dye fura-2. Focal iontophoretic ACh (1 M, 300 nA.40 ms) application induced a rapid increase in [Ca2+]i, reaching a peak in 20 s and representing about 5-fold the resting level (from 61 +/- 6 to 320 +/- 26 nM). Applications of muscarinic agonists as methacholine and carbachol induced weaker calcium responses (from 78 +/- 25 to 238 +/- 53 nM) than the one obtained with ACh applications. These muscarinic agonists were efficient only in precise zones. Desensitization of muscarinic receptors to successive stimulations was significant. Perfusion of nicotine or 1,1-dimethyl-4-phenyl-piperazinium (DMPP), a nicotinic agonist, induced an increase in [Ca2+]i only in some cells (4/28 with DMPP). These results indicated the presence of cholinergic receptors on frog VHCs: muscarinic receptors were more responsive than nicotinic receptors. Presence of muscarinic and nicotinic receptors in the membrane of VHCs could indicate different modulations of VHCs activity mediated by [Ca2+]i and involving an efferent control which represents a central regulation of the vestibular afferent message.

Calcium signalling in hair cells: multiple roles in a compact cell

Ca2+ is critical for mechanosensory adaptation, frequency tuning, afferent synaptic transmission, and efferent modulation in hair cells. These four processes involve cytoplasmic Ca2+ in three independent signalling pathways. Recent work suggests that Ca2+ regulates a myosin adaptation motor, and that a mobile Ca2+ buffer is highly concentrated in hair cells. Focal Ca2+ entry and the cytoplasmic Ca2+ buffer help to separate these pathways by limiting the spread of Ca2+ signals.

The role of inositol trisphosphate on ACh-induced outward currents in bullfrog saccular hair cells

Acetylcholine (ACh) is considered as the most likely candidate for a neurotransmitter of the efferent synapse onto hair cell. In this paper, the nature of this cholinergic receptor mechanism on dissociated bullfrog saccular hair cell was examined by using whole cell recording and Ca2+ sensitive fluorophotometric technique. Both the ACh-induced current and the increase of [Ca2+]i were observed in an oscillatory manner, and were the largest around the basal part of the cell where the efferent synapse is thought to make a contact with the membrane. The reversal potential of ACh-induced current indicated that ACh activated a K+ conductance. The ACh-induced current was reversibly blocked by atropine, d-tubocurarine (dTC), apamin, tetraethylammonium (TEA) and quinine. Neither muscarine nor nicotine mimicked the ACh-induced current. When GTP gamma S was injected into a hair cell, the first ACh application induced an outward current of transient kinetics, but in subsequent trials ACh-induced current lost its decay phase. Intracellularly injected D-myo-inositol 1,4,5-trisphosphate (InsP3) generated outward currents. Intracellularly injected heparin suppressed ACh-induced currents, and lithium (Li+) increased ACh-induced currents. These results indicate that ACh activates a receptor coupled with a guanine nucleotide binding protein (G-protein) which triggers metabolic cascades of InsP3 and Ca2+ leading to the activation of the Ca(2+)-activated K+ channel.

Coexistence of calcitonin gene-related peptide and choline acetyltransferase in eel efferent neurons

We applied choline acetyltransferase, (ChAT) and calcitonin gene-related peptide (CGRP) immunocytochemistry to the efferent neurons that innervate the lateral line and the ear of the eel. Strong immunoreactivity to the ChAT antiserum was observed in neurons located within the octavolateralis efferent nucleus that could be distinguished, on the basis of their form, location and dendritic organization, from the ChAT-immunopositive motoneurons of the adjacent facial motor nucleus. Both facial motoneurons and efferent neurons were found to be immunopositive for CGRP, although the reaction was always stronger in the motoneurons. Double labelling experiments established the presence of both ChAT and CGRP in many efferent neurons. The results are evidence that cholinergic efferent neurons supplying end organs of different modalities may also produce calcitonin gene-related peptide.

In vitro pharmacologic characterization of a cholinergic receptor on outer hair cells

Acetylcholine (ACh) is the major neurotransmitter released from the efferent fibers in the cochlea onto the outer hair cells (OHCs). The type of ACh receptor on OHCs and the events subsequent to receptor activation are unclear. Therefore we studied the effect of agonists and antagonists of the ACh receptor on isolated OHCs from the guinea pig. OHCs were recorded from in whole cell voltage and current clamp configuration. ACh induced an increase in outward K+ current (IACh) which hyperpolarized the OHCs. No desensitization to ACh application was observed. Cs+ replaced K+ in carrying the IACh. The IACh is Ca(2+)-dependent, time and voltage sensitive, and different from the IKCa induced by depolarization of the membrane potential. When tested at 100 microM, several agonists also induced outward current responses (acetylcholine > suberyldicholine > or = carbachol > DMPP) whereas nicotine, cytisine and muscarine did not. The IACh response to 10 microM ACh was blocked by low concentrations of traditional and non-traditional-nicotinic antagonists (strychnine > curare > bicuculline > alpha-bungarotoxin > thimethaphan) and by higher concentrations of muscarinic antagonists (atropine > 4-DAMP > AF-DX 116 > pirenzepine). Pharmacologically, the ACh receptor on OHCs is nicotinic.

Effects of extracellular ATP on hair cells isolated from the guinea-pig semicircular canals

Externally applied ATP (100 microM) induced membrane currents in type I and type II vestibular hair cells enzymatically isolated from guinea-pig semicircular canals. In whole-cell voltage-clamp and with 140 mM K+ in the pipette solution, ATP evoked an inwardly directed current in 58% of the cells when held at potentials below -40 mV. In the remainder, external ATP produced an outward current. After block of the K currents, an inward current activated by ATP was revealed at -50 mV. Intracellular Ca2+ levels were monitored using the Ca2+ indicator Fura-2 and were found to rise in both hair cell types in response to ATP. These results strongly suggest that ATP directly controls the entry of Ca2+ into crista hair cells which can then further modulate K+ currents.

Receptor-mediated release of inositol phosphates in the cochlear and vestibular sensory epithelia of the rat

Various neurotransmitters, hormones and other modulators involved in intercellular communication exert their biological action at receptors coupled to phospholipase C (PLC). This enzyme catalyzes the hydrolysis of phosphatidylinositol 4,5-bisphosphate (PtdInsP2) to inositol 1,4,5-trisphosphate (InsP3) and 1,2-diacylglycerol (DG) which act as second messengers. In the organ of Corti of the guinea pig, the InsP3 second messenger system is linked to muscarinic cholinergic and P2y purinergic receptors. However, nothing is known about the InsP3 second messenger system in the vestibule. In this study, the receptor-mediated release of inositol phosphates (InsPs) in the vestibular sensory epithelia was compared to that in the cochlear sensory epithelia of Fischer-344 rats. After preincubation of the isolated intact tissues with myo-[3H]inositol, stimulation with the cholinergic agonist carbamylcholine or the P2 purinergic agonist ATP-gamma-S resulted in a concentration-dependent increase in the formation of [3H]InsPs in both epithelia. Similarly, the muscarinic cholinergic agonist muscarine enhanced InsPs release in both organs, while the nicotinic cholinergic agonist dimethylphenylpiperadinium (DMPP) was ineffective. The muscarinic cholinergic antagonist atropine completely suppressed the InsPs release induced by carbamylcholine, while the nicotinic cholinergic antagonist mecamylamine was ineffective. Potassium depolarization did not alter unstimulated or carbamylcholine-stimulated release of InsPs in either organ. In both tissues, the P2 purinergic agonist alpha,beta-methylene ATP also increased InsPs release, but the P1 purinergic agonist adenosine did not. These results extend our previous observations in the organ of Corti of the guinea pig to the rat and suggest a similar control of the InsP3 second messenger system in the vestibular sensory epithelia.(ABSTRACT TRUNCATED AT 250 WORDS)

Choline acetyltransferase immunoreactive neurons innervating labyrinthine and lateral line sense organs in amphibians

The goal of the present study was to investigate aspects of the central organization of the neurons belonging to the octavolateralis efferent system of amphibians. The perikarya of three genera, Pleurodeles, Xenopus, and Discoglossus, were located in the brainstem by applying retrograde tracers to the appropriate cranial nerves and choline acetyltransferase immunohistochemistry was used to identify cholinergic neurons. The efferent neurons supplying lateral line (Pleurodeles, Xenopus) and labyrinthine (Pleurodeles, Xenopus, and Discoglossus) end organs were found to intermingle in a single octavolateralis efferent nucleus. The neurons lie bilateral to the labelled nerves in Pleurodeles and ipsilateral in Xenopus and Discoglossus. Separate labelling of the anterior and posterior octavus rami provided no evidence for distinct groupings of efferent neurons that could be associated with auditory and vestibular end organs. In all three species many if not all octavolateral efferent neurons displayed immunoreactivity for choline acetyltransferase. They could be distinguished from the cholinergic facial motoneurons, with which they sometimes intermingle, on the basis of either their distinctive size and shape (Pleurodeles, Xenopus) or their location (Discoglossus). Double labelling in Xenopus confirmed the cholinergic nature of the efferent neurons.

Actions of cholinergic agonists and antagonists on the efferent synapse in the frog sacculus

Intracellular recordings were made from hair cells in the frog saccular epithelium isolated with its innervating nerves. Inhibitory post-synaptic potentials (IPSPs) were recorded from hair cells when the efferent fibers were activated by electrical stimulation. The effects of acetylcholine (ACh), cholinomimetics, and cholinergic antagonists on the efferent synapse were studied in a preparation where the IPSPs can be observed directly. ACh or carbachol (CCh) produced a transient membrane hyperpolarization with a decrease in input resistance followed by an abolition or reduction of the IPSP. In a low Ca2+ medium where efferent synaptic activity was abolished, ACh or CCh still induced hyperpolarization, though the response appeared to be smaller than that in normal medium. Neither nicotinic (dimethyl-4-phenyl-piperazinium (DMPP), phenyltrimethylammonium (PTMA) and nicotine) nor muscarinic (muscarine, methacholine, bethanechol and oxotremorine) agonists induced the membrane hyperpolarization, but the former drugs inhibited the IPSPs while the latter drugs did not. Both d-tubocurarine and atropine inhibited the IPSP, but the d-tubocurarine was more potent, causing inhibition even at a dose of 0.5 microM while 2 microM or more atropine was needed. The ACh- or CCh-induced hyperpolarization was inhibited completely by d-tubocurarine (5 microM), but only slightly by atropine (5 microM). These results may indicate that the IPSP and the effects of ACh or CCh are based on a direct interaction between ACh or CCh and ACh receptors on the hair cells.

Differential modulation of spontaneous and evoked neurotransmitter release from hair cells: some novel hypotheses

It has been generally accepted that even in the absence of mechanical stimulation of the transductional elements, a resting depolarizing current exists which is ultimately responsible for the spontaneous release of neurotransmitter. Movement of the transductional elements modulates this resting current and thereby the evoked release of neurotransmitter occurs. Recent data from our laboratory and others have led us to question whether the relationship between spontaneous and evoked neurotransmitter release is as simple as stated. Indeed, a variety of experimental manipulations appear to influence the two modes of release differently. Examination of our results and the results of others has led us to four hypotheses: 1. the two modes of neurotransmitter release are processed differently by the hair cells; 2. cyclic AMP is involved in spontaneous but not evoked neurotransmitter release; 3. there is a positive feedback step involving an excitatory amino acid and its receptor on the hair cell in evoked neurotransmitter release and; 4. different pools of calcium are involved according to the mode of release. Accordingly, there may be several biochemical steps between the transductional movement of the stereocilia at the apex of the hair cells and the ultimate release of the neurotransmitter at the base of these cells. Some of these biochemical steps are different depending on whether the mode of release is spontaneous or evoked. These biochemical steps may amplify or at least interact with the biophysical processes previously described in the hair cells.

The vestibular end organs: morphological and physiological diversity of afferents

Vestibular-nerve fibers, even those innervating a single end organ, have been found to differ in their branching patterns within the neuroepithelium. They also vary in their responses to head movements and to activation of efferent fibers, and in the central pathways to which they contribute. These results are enabling plausible inferences to be made about the peripheral mechanisms determining the discharge properties of physiologically distinguishable afferents, and about the contributions the different afferents make to the overall functioning of the vestibular system.

Direct measurement of the action of acetylcholine on isolated outer hair cells of the guinea pig cochlea

Acetylcholine has long been thought to be the neurotransmitter of the cochlear efferent system in mammals although the evidence is largely indirect. By using whole-cell recordings from isolated outer hair cells, we show that acetylcholine activates a large rapidly desensitizing outward potassium current. This corresponds to hyperpolarization of the membrane potential from rest. The half maximal dose for acetylcholine was 13.5 microM with a cooperativity of 2. The response was not due to a conventional muscarinic action of acetylcholine for it was not blocked by 0.1 microM atropine and muscarinic antagonists but it could be blocked by 0.1 microM curare, suggesting that it shared many properties of a nicotinic receptor. It was, however, inhibited by 10 microM strychnine. The potassium current activated by acetylcholine required external calcium and was characterized by a significant delay at room temperature. This points to the involvement of a second messenger system, possibly calcium itself.

Acetylcholine receptor localization in human adult cochlear and vestibular hair cells

The FITC technique using alpha-bungarotoxin visualized the staining pattern of acetylcholine (ACh) receptors in adult human cochlear and vestibular hair cells (HCs) in normal labyrinths and in cochleae with sensorineural hearing loss. Flourescence staining occurred in the cuticular plates of all HCs, indicating that the micromechanics of their suprastructures can act under cholinergic control. Quantitative differences of the fluorescence of ACh receptors occurred between the three rows of outer HCs at the same level in the cochlea and decreasing along a base-to-apex directed gradient. There is strong evidence that the subsurface cisterns are integrated in the efferent nerve system. In the degenerating organ of Corti an uncoupling of the efferent system takes places adjacent to disintegrating HCs, though the staining in the cuticular plates remains until a very late stage in HC disintegration. In vestibular HCs type I, fluorescence is emitted in the supranuclear area of the cytoplasm below the cuticular plate probably indicating an efferent guidance on the afferent nerve transmission directly via the HC itself.

Control of intracellular calcium by ATP in isolated outer hair cells of the guinea-pig cochlea

1. Intracellular calcium levels were monitored in isolated outer hair cells of the guinea-pig cochlea using the calcium-sensitive dye Fura-2. 2. The calcium in the cells was studied during application of ATP externally applied from a pipette. ATP induced a rise of intracellular calcium which could be separated into two components: a rapid rise, peaking in 20 s, localized around the apical end of the cell, and a slower rise, peaking in 50-150 s but spread throughout the cell. The effects were observed with 5, 25 and 100 microM-ATP concentrations. 3. In the absence of external Ca2+, ATP was still able to trigger a rise in Ca2+, but with a longer delay. Under these conditions, the cells did not show the initial rapid Ca2+ rise. The result suggests that ATP can mobilize intracellular stores. 4. A rise in intracellular Ca2+ was also observed when 5 mM-caffeine was applied to the bath. 5. Simultaneous measurements were made of whole-cell currents and intracellular calcium. ATP activated an inward current at resting potentials of -60 mV. Internal Ca2+ levels increased during the inward current. In current-clamped cells Ca2+ levels also increased during the associated depolarization produced by ATP. 6. Adenosine (150 microM) did not produce any measurable inward current. Acetylcholine (ACh, 100 microM-1 mM) produced only a small rise in Ca2+. However, applied simultaneously with ATP, ACh suppressed the rise in intracellular Ca2+ produced by ATP, with the kinetics of a competitive antagonist. 7. Intracellular Ca2+ increased with step depolarizations of the cell above -20 mV during whole-cell clamp. Large rises in Ca2+ were also observed on depolarizing the cell with isotonic KCl. 8. Calcium levels in supporting cells of the organ of Corti were sensitive to ATP. In these cells, rises in intracellular Ca2+ did not require the presence of extracellular Ca2+. 9. It is concluded that the organ of Corti contains receptors for ATP on a variety of the cells. ATP controls a direct entry of Ca2+ through the membrane and also may mobilize intracellular stores.

Acetylcholine, carbachol, and GABA induce no detectable change in the length of isolated outer hair cells

The mechanical and electrical properties of cochlear outer hair cells (OHCs) are suggested to modulate transduction by inner hair cells. These properties of OHCs are presumably regulated by efferent neurons which use several transmitters including acetylcholine (Ach) and gamma aminobutyric acid (GABA). Since it had been suggested that Ach causes isolated OHCs to shorten visibly, this study was designed to investigate whether GABA also alters the length of OHCs. OHCs were isolated from the guinea pig cochlea by mechanical dispersion after collagenase treatment. Cells were initially selected by strict morphological criteria. In addition they were only included in further studies if they attained a constant length during 10 min of superfusion with buffer solution. Neither GABA (20 microM: 100 microM), Ach (5 mM; 10 microM with 10 microM eserine) or carbachol (10 microM; 100 microM) altered OHC length when applied in iso-osmotic Hank's balanced salt solution (total number of cells tested, 72). If a change in length occurred it must have been smaller than 0.3 microns, our detection ability. In contrast, high potassium and variations in osmolarity changed hair cell length by 3-10% in agreement with other reports.