Ultrastructural localization of ChAT-like immunoreactivity in the human vestibular periphery

KCNQ2/3 regulates efferent mediated slow excitation of vestibular afferents in mammals

Primary vestibular afferents transmit information from hair cells about head position and movement to the CNS, which is critical for maintaining balance, gaze stability and spatial navigation. The CNS, in turn, modulates hair cells and afferents via the efferent vestibular system (EVS) and its activation of several cholinergic signaling mechanisms. Electrical stimulation of EVS neurons gives rise to three kinetically- and mechanistically-distinct afferent responses including a slow excitation, a fast excitation, and a fast inhibition. EVS-mediated slow excitation is attributed to odd-numbered muscarinic acetylcholine receptors (mAChRs) on the afferent whose activation leads to the closure of a potassium conductance and increased afferent discharge. Likely effector candidates include low-threshold, voltage-gated potassium channels belonging to the KCNQ (Kv7.X) family, which are involved in neuronal excitability across the nervous system and are subject to mAChR modulation. Specifically, KCNQ2/3 heteromeric channels may be the molecular correlates for the M-current, a potassium current that is blocked following the activation of odd-numbered mAChRs. To this end, multiple members of the KCNQ channel family, including KCNQ2 and KCNQ3, are localized to several microdomains within vestibular afferent endings, where they influence afferent excitability and could be targeted by EVS neurons. Additionally, the relative expression of KCNQ subunits appears to vary across the sensory epithelia and among different afferent types. However, it is unclear which KCNQ channel subunits are targeted by mAChR activation and whether that also varies among different afferent classes. Here we show that EVS-mediated slow excitation is blocked and enhanced by the non-selective KCNQ channel blocker XE991 and opener retigabine, respectively. Using KCNQ subunit-selective drugs, we observed that a KCNQ2 blocker blocks the slow response in irregular afferents, while a KCNQ2/3 opener enhances slow responses in regular afferents. The KCNQ2 blockers did not appear to affect resting afferent discharge rates, while KCNQ2/3 or KCNQ2/4 openers decreased afferent excitability. Here, we show pharmacological evidence that KCNQ2/3 subunits are likely targeted by mAChR activation in mammalian vestibular afferents. Additionally, we show that KCNQ3 KO mice have altered resting discharge rate as well as EVS-mediated slow response. These data together suggest that KCNQ channels play a role in slow response and discharge rate of vestibular afferents, which can be modulated by EVS in mammals.

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.

Efferent innervation of turtle semicircular canal cristae: comparisons with bird and mouse

In the vestibular periphery of nearly every vertebrate, cholinergic vestibular efferent neurons give rise to numerous presynaptic varicosities that target hair cells and afferent processes in the sensory neuroepithelium. Although pharmacological studies have described the postsynaptic actions of vestibular efferent stimulation in several species, characterization of efferent innervation patterns and the relative distribution of efferent varicosities among hair cells and afferents are also integral to understanding how efferent synapses operate. Vestibular efferent markers, however, have not been well characterized in the turtle, one of the animal models used by our laboratory. Here we sought to identify reliable efferent neuronal markers in the vestibular periphery of turtle, to use these markers to understand how efferent synapses are organized, and to compare efferent neuronal labeling patterns in turtle with two other amniotes using some of the same markers. Efferent fibers and varicosities were visualized in the semicircular canal of red-eared turtles (Trachemys scripta elegans), zebra finches (Taeniopygia guttata), and mice (Mus musculus) utilizing fluorescent immunohistochemistry with antibodies against choline acetyltransferase (ChAT). Vestibular hair cells and afferents were counterstained using antibodies to myosin VIIa and calretinin. In all species, ChAT labeled a population of small diameter fibers giving rise to numerous spherical varicosities abutting type II hair cells and afferent processes. That these ChAT-positive varicosities represent presynaptic release sites were demonstrated by colabeling with antibodies against the synaptic vesicle proteins synapsin I, SV2, or syntaxin and the neuropeptide calcitonin gene-related peptide. Comparisons of efferent innervation patterns among the three species are discussed.

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.

Muscarinic acetylcholine receptor subtype expression in type vestibular hair cells of guinea pigs

Recent studies have demonstrated that five subtypes (M1-M5) of muscarinic acetylcholine receptor (mAChR) are expressed in the vestibular periphery. However, the exact cellular location of the mAChRs is not clear. In this study, we investigated whether there is the expression of M1-M5 muscarinic receptor mRNA in isolated type II vestibular hair cells of guinea pig by using single-cell RT-PCR. In vestibular end-organ, cDNA of the expected size was obtained by RT-PCR. Moreover, mRNA was identified by RT-PCR from individually isolated type II vestibular hair cells (single-cell RT-PCR). Sequence analysis confirmed that the products were M1-M5 mAChR. These results demonstrated that M1-M5 mAChR was expressed in the type II vestibular hair cells of the guinea pig, which lends further support for the role of M1-M5 mAChR as a mediator of efferent cholinergic signalling pathway in vestibular hair cells.

The muscarinic inhibition of the potassium M-current modulates the action-potential discharge in the vestibular primary-afferent neurons of the rat

There is consensus that muscarinic and nicotinic receptors expressed in vestibular hair cells and afferent neurons are involved in the efferent modulation of the electrical activity of the afferent neurons. However the underlying mechanisms of postsynaptic control in neurons are not well understood. In our work we show that the activation of muscarinic receptors in the vestibular neurons modulates the potassium M-current modifying the activity of afferent neurons. Whole-cell patch-clamp recordings were made on vestibular-afferent neurons isolated from Wistar rats (postnatal days 7-10) and held in primary culture (18-24 h). The M-current was studied during its deactivation after depolarizing voltage-clamp pulses. In 68% of the cells studied, those of larger capacitance, the M-current antagonists linopirdine and XE-991 reduced the amplitude of the M-current by 54%+/-7% and 50%+/-3%. The muscarinic-receptor agonist oxotremorine-M also significantly reduced the M-current by 58%+/-12% in the cells. The action of oxotremorine-M was blocked by atropine, thus indicating its cholinergic nature. The erg-channel blocker E-4031 did not significantly modify the M-current amplitude. In current-clamp experiments, linopirdine, XE-991, and oxotremorine-M modified the discharge response to current pulses from single spike to multiple spiking, reducing the adaptation of the electrical discharge. Our results indicate that large soma-size cultured vestibular-afferent neurons (most probably calyx-bearing neurons) express the M-current and that the modulation of this current by activation of muscarinic-receptor reduces its spike-frequency adaptation.

Expression of nicotinic acetylcholine receptor subunit alpha9 in type II vestibular hair cells of rats

AIM

To explore the cell specific existence of alpha 9 AChR in the vestibular type II hair cells (VHC II) of rats.

METHODS

To detect the expression of alpha 9 AChR messenger RNA (mRNA) in the vestibular endorgans and single VHC II of rats by using the reverse transcription polymerase chain reaction (RT-PCR) technique and the single cell RT-PCR technique, respectively.

RESULTS

It was shown that alpha 9 AChR mRNA was detected in the vestibular endorgans. By using single-cell RT-PCR, mRNA encoding alpha 9 AChR was also detected in the VHC II of the rats. Sequence analysis of the PCR products confirmed identity to corresponding cDNA sequence in the predicted region.

CONCLUSION

We established a method which could effectively detect the cell specific expression of mRNA in an individual VHC. Present data confirm that alpha 9 AChR mRNA is expressed in the VHC II of rats and indicates that alpha 9 AChR may function as a mediator of efferent cholinergic signaling in mammalian VHC.

The properties of ACh-induced BK currents in guinea pig type II vestibular hair cells

Molecular biological studies have demonstrated that both muscarinic receptor subtypes and nicotinic receptor subunits were located in mammalian vestibular sensorineural epithelium. However, the functional roles are still unclear, with the exception of the well-known alpha9-containing nicotinic ACh receptor (alpha9nAChR). In this study, the properties of acetylcholine (ACh)-induced currents were investigated by whole-cell patch clamp technique in isolated type II vestibular hair cells (VHCs II) of guinea pig. VHCs II displayed a sustained, non-inactivating current when extracellular application of ACh. ACh-induced currents restored gradually and it took about 60 s to get a complete recovery. ACh-induced current was not affected by extracellular Na(+), but strongly affected by extracellular K(+) and Ca(2+). Depletion of the intracellular Ca(2+) stores by intracellular application of inositol 1,4,5-trisphosphate (IP3) or blocking of the release of intracellular Ca(2+) stores by intracellular application of heparin failed to inhibit this current. ACh-induced currents were inhibited by nifedipine, Cd(2+), tetraethylammonium (TEA), charybdotoxin (CTX), iberiotoxin (IBTX), atropine and d-tubocurarine (DTC), respectively, but not by apamin. In conclusion, ACh stimulates a large conductance, Ca(2+)-activated K(+) current (BK) in guinea pig VHCs II by activation of the influx of Ca(2+) ions, which is mediated by an ACh receptor that could not be defined to be the odd-number muscarinic receptor.

Localization of efferent neurotransmitters in the inner ear of the homozygous Bronx waltzer mutant mouse

Naturally occurring mutant mice provide an excellent model for the study of genetic malformations of the inner ear. Mice homozygous for the Bronx waltzer (bv/bv) mutation are severely hearing impaired or deaf and exhibit a 'waltzing' gait. Functional aspects of cochlear and vestibular efferents in the bv/bv mutant mouse are not well known. The present study was designed to evaluate several candidates of efferent neurotransmitters or neuromodulators including choline acetyltransferase (ChAT), gamma-aminobutyric acid (GABA), and calcitonin gene-related peptide (CGRP) in the inner ear of the bv/bv mutant mouse. Ultrastructural investigations at both light and electron microscopic level were performed. Ultrastructural morphologic evaluations of the cochlea and the vestibular end-organs were also undertaken. It is demonstrated that ChAT, GABA and CGRP immunoreactivities are present in the cochlea and in vestibular end-organs of bv/bv mutant mice. In the organ of Corti, immunoreactivity of ChAT, GABA and CGRP is confined to the inner spiral fibers, tunnel-crossing fibers, and the vesiculated nerve endings synapsing with outer hair cells. Interestingly, immunoreactivity was detectable even where inner hair cells appeared missing. Results also revealed malformations of the outer hair cells with synaptic contacts to efferent nerve endings consistently intact. In the neurosensory epithelia of the vestibular end-organs, the presence of ChAT, GABA, and CGRP immunoreactivity was localized at the vestibular efferents, with the exception of the macula of saccule. In one 8-month-old macula of utricle where the depletion of hair cells appeared highest, ChAT immunostaining was still discernible. Ultrastructural investigation demonstrated that vesiculated efferent nerve endings make synaptic contact with the outer hair cells in the organ of Corti and with type II hair cells in the vestibular end-organs. The present study provides further support that the efferent system in the bv/bv mutant inner ear is morphologically as well as functionally mature. These findings also demonstrate that if and when the onset of efferent degeneration in the bv/bv mutant inner ear occurs, it transpires subsequent to pathological conditions in the hair cells. The present findings give further indication that the efferent systems of the bv/bv mutant inner ear are independent of the afferent systems in many aspects including development, maturation as well as degeneration.

Ultrastructural evaluation of calcitonin gene-related peptide immunoreactivity in the human cochlea and vestibular endorgans

Calcitonin gene-related peptide (CGRP) is a neuropeptide widely distributed in the peripheral and central nervous system. Demonstrated in the efferent systems of the mammalian cochlea and vestibule, immunoreactive patterns of CGRP may vary by species. There is, however, no information in the literature investigating CGRP localization in the human cochlea. In the present study, the ultrastructural localization of CGRP immunoreactivity was evaluated in the human inner ear with immunoelectron microscopy. It was found that, in human cochlea, CGRP immunoreactivity was located in unmyelinated nerve fibres of the spiral lamina, inner spiral fibres beneath inner hair cells, tunnel spiral fibres, tunnel crossing fibres and outer radial fibres. In endorgans of human vestibule, CGRP immunoreactivity was located in vesiculated nerve fibres and bouton-type nerve terminals which were seen to contact afferent nerve chalices surrounding type I sensory cells and afferent nerve fibres, or to form an en passant contact with afferent dendrites. CGRP immunoreactivity appeared to be confined to efferent systems in all cases. This study presents evidence that CGRP could serve a role in neurotransmission or neuroregulation in both cochlear and vestibular efferent systems of human.