Cholinomimetics mimic efferent effects on semicircular canal afferent activity in 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.

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.

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.

Muscarinic signaling in the cochlea: presynaptic and postsynaptic effects on efferent feedback and afferent excitability

Acetylcholine is the major neurotransmitter of the olivocochlear efferent system, which provides feedback to cochlear hair cells and sensory neurons. To study the role of cochlear muscarinic receptors, we studied receptor localization with immunohistochemistry and reverse transcription-PCR and measured olivocochlear function, cochlear responses, and histopathology in mice with targeted deletion of each of the five receptor subtypes. M2, M4, and M5 were detected in microdissected immature (postnatal days 10-13) inner hair cells and spiral ganglion cells but not outer hair cells. In the adult (6 weeks), the same transcripts were found in microdissected organ of Corti and spiral ganglion samples. M2 protein was found, by immunohistochemistry, in olivocochlear fibers in both outer and inner hair cell areas. M3 mRNA was amplified only from whole cochleas, and M1 message was never seen in wild-type ears. Auditory brainstem responses (ABRs) and distortion product otoacoustic emissions (DPOAEs) were unaffected by loss of Gq-coupled receptors (M1, M3, or M5), as were shock-evoked olivocochlear effects and vulnerability to acoustic injury. In contrast, loss of Gi-coupled receptors (M2 and/or M4) decreased neural responses without affecting DPOAEs (at low frequencies). This phenotype and the expression pattern are consistent with excitatory muscarinic signaling in cochlear sensory neurons. At high frequencies, both ABRs and DPOAEs were attenuated by loss of M2 and/or M4, and the vulnerability to acoustic injury was dramatically decreased. This aspect of the phenotype and the expression pattern are consistent with a presynaptic role for muscarinic autoreceptors in decreasing ACh release from olivocochlear terminals during high-level acoustic stimulation and suggest that muscarinic antagonists could enhance the resistance of the inner ear to noise-induced hearing loss.

The role of defensins in the excitability of the peripheral vestibular system in the frog: Evidence for the presence of communication between the immune and nervous systems

Defensins are one of the major groups of endogenous peptides that are considered to be important antibiotic-like effectors of host innate and adaptive antimicrobial immunity. The current study investigated the electrophysiological effects of externally applied human and rabbit defensins (HNP-1 and RNP-1, correspondingly) on afferent neurotransmission in the frog semicircular canals (SCC). Application of HNP-1 and RNP-1 induces a concentration-dependent decrease in resting activity. Threshold concentrations for both substances were of the order of 0.0001 nM. The firing evoked by L-glutamate (L-Glu) and its agonists alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA), kainate and N-methyl-D-aspartate (NMDA) and (1S, 3R)-1-aminocyclopentane-trans-1,3-dicarboxilic acid (ACPD) could be inhibited by HNP-1, suggesting that defensins exert inhibitory control over both ionotropic and metabotropic glutamate receptors. HNP-1 considerably inhibited the L-glutamate/high Mg2+ -induced increase in frequency, thus, demonstrating its postsynaptic site of action. Acetylcholine (ACh) responses under HNP-1 did not differ from the frequency increase induced by ACh alone, and the ACh antagonist atropine left the response to HNP-1 intact. The specific opioid receptor antagonist naloxone (Nal) antagonized the inhibitory response evoked by HNP-1. The results obtained support the evidence for the recruitment of defensins in communication between the immune and nervous systems, and on the potential of sensory receptors to participate in the inflammatory response.

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.

Histamine (H3) receptors modulate the excitatory amino acid receptor response of the vestibular afferents

Although the effectiveness of histamine-related drugs in the treatment of peripheral and central vestibular disorders may be explained by their action on the vestibular nuclei, it has also been shown that antivertigo effects can take place at the peripheral level. In this work, we examined the actions of H3 histaminergic agonists and antagonists on the afferent neuron electrical discharge in the isolated inner ear of the axolotl. Our results indicate that H3 antagonists such as thioperamide, clobenpropit, and betahistine (BH) decreased the electrical discharge of afferent neurons by interfering with the postsynaptic response to excitatory amino acid agonists. These results lend further support to the idea that the antivertigo action of histamine-related drugs may be caused, at least in part, by a decrease in the sensory input from the vestibular endorgans. The present data show that the inhibitory action of the afferent neurons discharge previously described for BH is not restricted to this molecule but is also shared by other H3 antagonists.

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 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.

Transmitter release from Rana pipiens vestibular hair cells via mGluRs: a role for intracellular Ca(++) release

The response of the semicircular canal (SCC) to the group I mGluR-selective agonist dihydroxyphenylglycine (DHPG; 300 microM) - facilitation of afferent discharge rate - was dose-dependently reduced by the phospholipase C inhibitor U-73122 (1-100 microM; IC(50): 22 microM), the smooth endoplasmic reticulum Ca(++) ATPase inhibitor thapsigargin (100 nM-3 microM; IC(50): 500 nM), and xestospongin C (100 pM-1 microM; IC(50): 11 nM), an inositol trisphosphate receptor (IP(3)R) antagonist. Ryanodine, a modulator of Ca(++)-induced Ca(++) release, biphasically facilitated, then suppressed this response (1 nM-1 mM; approximate IC(50): 50 microM). 5 mM caffeine increased the amplitude (34.6+/-13.4%) and duration (453+/-169.8%; n=4) of the response of the SCC to DHPG, while 50 mM caffeine eliminated this response (n=2). The protein kinase C inhibitor bisindolylmaleimide I-HCl (10-100 microM; n=3) and the cyclic-ADP ribose antagonist 8-Br-cyclic-ADP ribose (1-10 microM; n=3) had no effect on the response of the SCC to DHPG. These data suggest that the increase in transmitter release following activation of group I mGluRs on vestibular hair cells is associated with intracellular Ca(++) release from both IP(3)-sensitive and ryanodine/caffeine-sensitive intracellular Ca(++) stores. Such positive feedback on transmitter release may serve to enhance the contrast between the spontaneous and stimulus-evoked modes of hair cell transmitter release, thereby optimizing signal discrimination at the synapse between hair cells and vestibular afferent fibers.

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.

Expression of gamma-aminobutyric acid(A) receptor subunits in the vestibular system

OBJECTIVES

Profile the expression of genes encoding GABAA receptor subunits in the vestibular end organs of a rat.

MATERIALS AND METHODS

Using a combination of reverse transcription followed by polymerase chain reaction (PCR) with gene-specific primers, expression of mRNAs encoding 13 individual GABA(A) receptor subunits was examined.

RESULTS

PCR amplification products representing subunit gene expression for alpha1-6, beta1-3, and gamma1-3, but not for delta, subunits were amplified, suggesting multiple molecular levels of regulation of vestibular GABA(A) receptor expression. Nucleotide sequencing confirmed the identity of rat vestibular end-organs subunit cDNAs.

CONCLUSION

These results give the most direct evidence to date that GABAA receptors composed of the detected subunits are expressed in the mammalian vestibular system, giving new support to previous investigations implicating GABA as a vestibular neurotransmitter.

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.

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.

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.

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.

The cholinergic pharmacology of the frog saccule

Stimulation of the efferent nerves to the vestibular organs of the frog's inner ear produces either facilitation or inhibition of afferent firing. Similarly, application of acetylcholine (ACH), the major transmitter of the efferents, can produce both facilitation and/or inhibition as previously reported [Guth et al. (1986) Acta Otolaryngol. 102, 194-204; Norris et al. (1988) Hear. Res. 32, 197-206]. The firing rates of afferent neurons of the semicircular canal (SCC) using multiunit recordings are generally facilitated by ACH. Conversely, the firing rates of afferent units innervating the saccule are generally inhibited by ACH. This latter inhibition is antagonized by strychnine more potently than by curare, which is more potent than atropine. When inhibition is antagonized by strychnine or curare an underlying facilitation is revealed. The inhibition of saccular afferents by ACH shows desensitization requiring about 20 min to recover. The ACH-induced inhibition is mimicked by nicotine at very high concentrations but not by dimethyl phenylpiperazinium or cytisine. The fact that multiunit afferent firing from the SCC is generally facilitated while that from the saccule is generally inhibited by ACH suggests a different distribution of ACH receptors and receptor types (i.e. muscarinic or nicotinic and their subtypes) in the two organs and demonstrates the usefulness of recording from multiple units simultaneously. The difference in distribution of ACH receptors may be important for understanding the physiology of vestibular efferents.

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.

Bilateral communication between vestibular labyrinths in pigeons

Extracellular action potentials from single horizontal semicircular canal primary afferent fibers were recorded in paralysed decerebrate pigeons during pulse mechanical stimulation of the contralateral horizontal semicircular canal. Clear responses to the contralateral membranous duct displacement stimuli were observed in 51% of the tested 158 horizontal semicircular canal afferents. Generally, three different types of responses were obtained in the primary afferent fibers including excitation, inhibition, and a few complex type neural activity profiles. Inhibitory responses were of larger amplitude and had longer time constants than did excitatory responses. The few complex type responses observed were characterized by an initial excitatory discharge followed by a longer duration decrease in the fiber's firing rate. The sensitivity to stimulation and type of response obtained for each afferent was significantly correlated with the fiber's coefficient of variation value. Regular firing afferents were less sensitive and exhibited primarily excitatory responses (71%) to contralateral canal stimulation. Irregular firing afferents were more sensitive and exhibited mostly inhibitory responses (84%). The present results demonstrate that a communication network for information exchange between the bilateral labyrinths exists in pigeons. The observed responses in primary afferent fibers to contralateral horizontal semicircular canal stimulation are proposed to be mediated by the vestibular efferent system, which could provide an anatomical pathway for information exchange from vestibular receptors on opposite sides of the head.

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.

The correlated blanching of synaptic bodies and reduction in afferent firing rates caused by transmitter-depleting agents in the frog semicircular canal

Synaptic bodies (SBs) associated with rings of synaptic vesicles and well-defined, pre- and post-synaptic membrane structures are indicators of maturity in most hair cell-afferent nerve junctions. The role of the SBs remains elusive despite several experiments showing that they may be involved in storage of neurotransmitter. Our results demonstrate that SBs of the adult posterior semicircular canal (SCC) cristae hair cells become less electron dense following incubation of the SCC with the transmitter-depleting drug tetrabenazine (TBZ). Objective quantification and comparison of the densities of the SBs in untreated and TBZ-treated frog SCC demonstrated that TBZ significantly decreased the electron density of SBs. This reduction in electron density was accompanied by a reduction in firing rates of afferent fibers innervating the posterior SCC. A second transmitter-depleting drug, guanethidine, previously shown to reduce the electron density of hair cell SBs, also reduced the firing rates of afferent fibers innervating the posterior SCC. In contrast, the electron density of dense granules (DG), similar in size and shape to synaptic bodies (SB) in hair cells, did not change after incubation in TBZ, thus indicating that granules and SBs are not similar in regard to their electron density. The role of SBs in synaptic transmission and the transmitter, if any, stored in the SBs remain unknown. Nonetheless, the association of the lessening of electron density with a reduction in afferent firing rate provides impetus for the further investigation of the SB's role in neurotransmission.

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.

Cyclic AMP modulates sensory-neural communication at the vestibular end organ

Adenosine 3':5'-cyclic phosphate (cAMP) is a second messenger that plays an important role in mediating neuronal interactions in many systems. A possible role for cAMP in sensorineural communication at the vestibular end organ was studied. The putative roles for cAMP action investigated here were: the ability of cAMP to act as the second messenger for the efferent transmitter, acetylcholine, and the possible involvement of cAMP in modulating spontaneous or mechanically-evoked afferent nerve firing. Levels of cAMP were increased pharmacologically with forskolin, 3-isobutyl-1-methyl xanthine (IBMX) and dibutyryl cAMP. Changes in multiunit afferent nerve firing measured from the ampullar nerve of the semicircular canal, and the transepithelial potential measured across the neuroepithelium of the semicircular canal were recorded. At selected doses, all drugs produced a similar increase in spontaneous multiunit afferent nerve firing with a concomitant decrease in the transepithelial potential. Mechanically-evoked hair cell activity and the response to exogenously applied acetylcholine were unaffected by these drugs. We are suggesting that the excitatory aspects of the acetylcholine response are not mediated via a cAMP-dependent mechanism. However, cAMP does play an important role in modulating spontaneous afferent nerve firing in the semicircular canal. The finding that spontaneous afferent nerve firing can be biochemically modulated without altering mechanically-induced afferent firing is novel and deserves further investigation.

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

Two cholinergically-induced modulations of membrane conductances have been identified in hair cells isolated from the crista ampullaris of the leopard frog (Rana pipiens), using the whole cell recording configuration of the patch clamp technique. Of 56 crista hair cells tested, 28 showed drug-induced changes in membrane current or membrane potential which were repeatable and could be reversed with washout of drug. The predominant effect (observed in 20 hair cells) of acetylcholine (Ach, 100 microM) to 1mM) or carbachol (1 microM to 50 microM) applied to these hair cells was the reduction of an outward current corresponding to a change in conductance of approximately -0.22 nS. This action by Ach on hair cells has been inferred from previous studies of afferent fiber discharge which reported an increase in firing rate with stimulation of efferent fibers or exogenous application of cholinomimetics (Bernard et al., 1985; Valli et al., 1986; Guth et al., 1986; Norris et al., 1988a). The Ach-induced reduction in outward current was associated with a depolarization of the zero-current membrane potential by approximately +2.5 mV. In a total of 8 hair cells, an Ach-induced reversible increase in outward current was recorded. Changes in conductance were approximately +0.13 nS and were associated with a hyperpolarization of the zero-current membrane potential by approximately -2.2 mV. This current increase is likely to be responsible for the inhibitory post-synaptic potentials (IPSPs) which have previously been recorded intracellularly from acoustico-lateralis hair cells during stimulation of the efferent innervation (Flock and Russell, 1976; Ashmore and Russell, 1982; Art et al., 1984, 1985). Of the remaining 28 hair cells, six cells failed to exhibit any change in membrane conductance or membrane potential in the presence of cholinomimetics while an additional 15 cells exhibited decreases, and 7 cells exhibited increases in outward conductance, during application of Ach or carbachol, which were neither reversible with washout nor repeatable. The Ach-induced decrease in outward current could be reversible blocked by removal of Ca2+ from the external solution. The antagonism of the Ach-induced decrease in outward current by atropine (10(-5) M) suggests that this current may correspond to a facilitatory, 'atropine-preferring' Ach receptor mediated response previously identified in the isolated semicircular canal (Norris et al., 1988a).(ABSTRACT TRUNCATED AT 400 WORDS)

Some properties of frog vestibular choline acetyltransferase and acetylcholinesterase

The amount and some properties of choline acetyltransferase (ChAT) and of acetylcholinesterase (AchE) were investigated in the frog vestibule. Enzyme activities were found to be of the same order of magnitude as in frog nervous tissue and various properties of vestibular ChAT (dependence on pH, chloride and Triton X-100 activation, phosphate sensitivity) and AchE (inhibition by eserine but not by Tetraisopropylpyrophosphoramide) were also similar as those of the homologous central nervous system enzymes. Although the precise localization of ChAT and AchE is not yet certain the efferent neurotransmitter in the vertebrate vestibular sensory periphery is believed to be acetylcholine and thus the enzymes responsible for its synthesis and degradation may participate in regulating inner ear function.

Histamine and related substances influence neurotransmission in the semicircular canal

Histamine and other imidazole-containing substances were found to increase ampullar nerve afferent firing rate while both H1 and H2 histamine antagonists effectively inhibited ampullar nerve activity. A specific inhibitor of histidine decarboxylase, the enzyme which catalyses the synthesis of histamine, reduced ampullar nerve firing in a dose-dependent manner. These observations suggest a physiological role for histamine in the inner ear. Maintenance of a response to histamine after de-efferentation of the crista ampullaris supports the hypothesis that the site of action is the hair cell; antagonism of the histamine response by a cholinergic antagonist, atropine, and antagonism of a cholinergically mediated facilitation by the histaminergic antagonist pyrilamine, indicate that the site of action may involve the acetylcholine receptor complex on the crista ampullaris hair cells. The observation that imidazole-containing compounds cause significant effects on semicircular canal neurotransmission provides an important finding with regard to the site of action of antihistamines used for the treatment of vertigo and motion sickness.

Three tests of the hypothesis that glutamate is the sensory hair cell transmitter in the frog semicircular canal

Three series of experiments were devised to test the hypothesis that glutamate is the transmitter released by sensory hair cells of the frog semicircular canal. These three tests were: 1 - The Tolerance experiment (i.e. making the preparation tolerant to injected Glu yet still capable of responding to endogenous transmitter). 2 - The Glu Decarboxylase experiment (i.e. bathing the preparation in sufficient enzyme to prevent the effects of exogenous Glu by degrading it without affecting the response to endogenous transmitter) and; 3 - The Diltiazem experiment (i.e. using the calcium channel antagonist, diltiazem, to prevent the effect of exogenous Glu and yet not to interfere with endogenous transmitter release and action). The Tolerance and Diltiazem experiments produced results indicative of a clear dissociation between exogenous Glu and natural transmitter. The Glu decarboxylase experiment results were not so clear, producing both evidence for and against the hypothesis.

The acetylcholine receptors of the semicircular canal in the frog (Rana pipiens)

We present preliminary evidence that acetylcholine has three distinct modes of action on the discharge rate of afferent fibers from the frog isolated semicircular canal. These are: (I) a facilitatory effect which is of mixed muscarinic-nicotinic type and is mediated by a receptor which we call "atropine-preferring'; (II) a suppressive effect unmasked by atropine and antagonized by strychnine whose action is mediated by a "strychnine-preferring" receptor; (III) a suppressive effect produced by the cholinesterase inhibitors eserine and echothiophate, which being antagonized by curare, is mediated by a "curare-preferring" receptor.

Adenosine is a modulator of hair cell-afferent neurotransmission

Adenosine has been implicated in neuromodulation in the central nervous system [(1985) Annu. Rev. Neurosci. 8, 103-124]. Its mechanism of action is thought to be a receptor-mediated inhibition of a transmitter release. To assess adenosine's role as a neuromodulator in the vestibular periphery, spontaneous activity of the afferent fibers in the ampullar nerve of the semicircular canal, in vitro, was used as the dependent variable. Afferent firing has been previously shown to depend on transmitter release by the hair cells [(1985) Brain Res. 330, 1-9]. Adenosine was shown to inhibit firing rate; the adenosine antagonist theophylline was shown to increase firing rate; the enzyme adenosine deaminase, which catabolizes adenosine to inosine, was shown to increase firing rate; the adenosine uptake inhibitor dipyridamole was shown to decrease firing rate; and adenosine was shown to be released from the isolated semicircular canal by electrical stimulation. All these findings are internally consistent and unreservedly support the hypothesis that adenosine has a neuromodulatory role in neurotransmission in the semicircular canal.