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Poppi LA, Holt JC, Lim R, Brichta AM. A review of efferent cholinergic synaptic transmission in the vestibular periphery and its functional implications. J Neurophysiol 2019; 123:608-629. [PMID: 31800345 DOI: 10.1152/jn.00053.2019] [Citation(s) in RCA: 27] [Impact Index Per Article: 5.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/11/2022] Open
Abstract
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.
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Affiliation(s)
- L A Poppi
- School of Biomedical Sciences and Pharmacy, The University of Newcastle, Newcastle, NSW, Australia.,Hunter Medical Research Institute, New Lambton Heights, NSW, Australia.,Preclinical Neurobiology Research Group, The University of Newcastle, Newcastle, NSW, Australia
| | - J C Holt
- Department of Otolaryngology, University of Rochester Medical Center, Rochester, New York
| | - R Lim
- School of Biomedical Sciences and Pharmacy, The University of Newcastle, Newcastle, NSW, Australia.,Hunter Medical Research Institute, New Lambton Heights, NSW, Australia.,Preclinical Neurobiology Research Group, The University of Newcastle, Newcastle, NSW, Australia
| | - A M Brichta
- School of Biomedical Sciences and Pharmacy, The University of Newcastle, Newcastle, NSW, Australia.,Hunter Medical Research Institute, New Lambton Heights, NSW, Australia.,Preclinical Neurobiology Research Group, The University of Newcastle, Newcastle, NSW, Australia
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Muscarinic Acetylcholine Receptors and M-Currents Underlie Efferent-Mediated Slow Excitation in Calyx-Bearing Vestibular Afferents. J Neurosci 2017; 37:1873-1887. [PMID: 28093476 DOI: 10.1523/jneurosci.2322-16.2017] [Citation(s) in RCA: 31] [Impact Index Per Article: 4.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/21/2016] [Revised: 01/09/2017] [Accepted: 01/10/2017] [Indexed: 11/21/2022] Open
Abstract
Stimulation of vestibular efferent neurons excites calyx and dimorphic (CD) afferents. This excitation consists of fast and slow components that differ >100-fold in activation kinetics and response duration. In the turtle, efferent-mediated fast excitation arises in CD afferents when the predominant efferent neurotransmitter acetylcholine (ACh) activates calyceal nicotinic ACh receptors (nAChRs); however, it is unclear whether the accompanying efferent-mediated slow excitation is also attributed to cholinergic mechanisms. To identify synaptic processes underlying efferent-mediated slow excitation, we recorded from CD afferents innervating the turtle posterior crista during electrical stimulation of efferent neurons, in combination with pharmacological probes and mechanical stimulation. Efferent-mediated slow excitation was unaffected by nAChR compounds that block efferent-mediated fast excitation, but were mimicked by muscarine and antagonized by atropine, indicating that it requires ACh and muscarinic ACh receptor (mAChR) activation. Efferent-mediated slow excitation or muscarine application enhanced the sensitivity of CD afferents to mechanical stimulation, suggesting that mAChR activation increases afferent input impedance by closing calyceal potassium channels. These observations were consistent with suppression of a muscarinic-sensitive K+-current, or M-current. Immunohistochemistry for putative M-current candidates suggested that turtle CD afferents express KCNQ3, KCNQ4, and ERG1-3 potassium channel subunits. KCNQ channels were favored as application of the selective antagonist XE991 mimicked and occluded efferent-mediated slow excitation in CD afferents. These data highlight an efferent-mediated mechanism for enhancing afferent sensitivity. They further suggest that the clinical effectiveness of mAChR antagonists in treating balance disorders may also target synaptic mechanisms in the vestibular periphery, and that KCNQ channel modulators might offer similar therapeutic value.SIGNIFICANCE STATEMENT Targeting the efferent vestibular system (EVS) pharmacologically might prove useful in ameliorating some forms of vestibular dysfunction by modifying ongoing primary vestibular input. EVS activation engages several kinetically distinct synaptic processes that profoundly alter the discharge rate and sensitivity of first-order vestibular neurons. Efferent-mediated slow excitation of vestibular afferents is of considerable interest given its ability to elevate afferent activity over an extended time course. We demonstrate for the first time that efferent-mediated slow excitation of vestibular afferents is mediated by muscarinic acetylcholine receptor (mAChR) activation and the subsequent closure of KCNQ potassium channels. The clinical effectiveness of some anti-mAChR drugs in treating motion sickness suggest that we may, in fact, already be targeting the peripheral EVS.
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Maison SF, Rosahl TW, Homanics GE, Liberman MC. Functional role of GABAergic innervation of the cochlea: phenotypic analysis of mice lacking GABA(A) receptor subunits alpha 1, alpha 2, alpha 5, alpha 6, beta 2, beta 3, or delta. J Neurosci 2006; 26:10315-26. [PMID: 17021187 PMCID: PMC1806703 DOI: 10.1523/jneurosci.2395-06.2006] [Citation(s) in RCA: 69] [Impact Index Per Article: 3.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022] Open
Abstract
The olivocochlear efferent system is both cholinergic and GABAergic and innervates sensory cells and sensory neurons of the inner ear. Cholinergic effects on cochlear sensory cells are well characterized, both in vivo and in vitro; however, the robust GABAergic innervation is poorly understood. To explore the functional roles of GABA in the inner ear, we characterized the cochlear phenotype of seven mouse lines with targeted deletion of a GABA(A) receptor subunit (alpha1, alpha2, alpha5, alpha6, beta2, beta3, or delta). Four of the lines (alpha1, alpha2, alpha6, and delta) were normal: there was no cochlear histopathology, and cochlear responses suggested normal function of hair cells, afferent fibers, and efferent feedback. The other three lines (alpha5, beta2, and beta3) showed threshold elevations indicative of outer hair cell dysfunction. Alpha5 and beta2 lines also showed decreased effects of efferent bundle activation, associated with decreased density of efferent terminals on outer hair cells: although the onset of this degeneration was later in alpha5 (>6 weeks) than beta2 (<6 weeks), both lines shows normal efferent development (up to 3 weeks). Two lines (beta2 and beta3) showed signs of neuropathy, either decreased density of afferent innervation (beta3) or decreased neural responses without concomitant attenuation of hair cell responses (beta2). One of the lines (beta2) showed a clear sexual dimorphism in cochlear phenotype. Results suggest that the GABAergic component of the olivocochlear system contributes to the long-term maintenance of hair cells and neurons in the inner ear.
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Affiliation(s)
- Stéphane F Maison
- Department of Otology and Laryngology, Harvard Medical School and Eaton-Peabody Laboratory, Massachusetts Eye and Ear Infirmary, Boston, Massachusetts 02114, USA.
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Derbenev AV, Linn CL, Guth PS. Muscarinic ACh Receptor Activation Causes Transmitter Release From Isolated Frog Vestibular Hair Cells. J Neurophysiol 2005; 94:3134-42. [PMID: 16222072 DOI: 10.1152/jn.00131.2005] [Citation(s) in RCA: 12] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
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.
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Affiliation(s)
- Andrei V Derbenev
- Dept. of Biological Sciences, Western Michigan University, Kalamazoo, MI 49008, USA
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Dawkins R, Keller SL, Sewell WF. Pharmacology of acetylcholine-mediated cell signaling in the lateral line organ following efferent stimulation. J Neurophysiol 2005; 93:2541-51. [PMID: 15615825 PMCID: PMC2001307 DOI: 10.1152/jn.01283.2004] [Citation(s) in RCA: 26] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
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.
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Affiliation(s)
- Rosie Dawkins
- Eaton-Peabody Lab., Massachusetts Eye and Ear Infirmary, 243 Charles St., Boston, MA 02114, USA
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Jones AK, Elgar G, Sattelle DB. The nicotinic acetylcholine receptor gene family of the pufferfish, Fugu rubripes☆. Genomics 2003; 82:441-51. [PMID: 13679024 DOI: 10.1016/s0888-7543(03)00153-8] [Citation(s) in RCA: 34] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
Nicotinic acetylcholine receptors (nAChRs) mediate fast cholinergic synaptic transmission at nerve-muscle junctions and in the brain. However, the complete gene family of nAChRs has not so far been reported for any vertebrate organism. We have identified the complete nAChR gene family from the reference genome of the pufferfish, Fugu rubripes. It consists of 16 alpha and 12 non-alpha candidate subunits, making it the largest vertebrate nAChR gene family known to date. The gene family includes an unusual set of muscle-like nAChR subunits comprising two alpha1s, two beta1s, one delta, one epsilon, and one gamma. One of the beta1 subunits possesses an aspartate residue and N-glycosylation sites hitherto shown to be necessary for delta-subunit function. Potential Fugu orthologs of neuronal nAChR subunits alpha2-4, alpha6, and beta2-4 have been identified. Interestingly, the Fugu alpha5 counterpart appears to be a non-alpha subunit. Fugu possesses an expanded set of alpha7-9-like subunits and no alpha10 ortholog has been found. Two new candidate beta subtypes, designated beta5 and beta6, may represent subunits yet to be found in the human genome. The Fugu nAChR gene structures are considerably more diverse than those of higher vertebrates, with evidence of "intron gain" in many cases. We show, using RT-PCR, that the Fugu nAChR subunits are expressed in a variety of tissues.
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Affiliation(s)
- Andrew K Jones
- MRC Functional Genetics Unit, Department of Human Anatomy and Genetics, University of Oxford, South Parks Road, Oxford OX1 3QX, UK
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Holt JC, Lioudyno M, Guth PS. A pharmacologically distinct nicotinic ACh receptor is found in a subset of frog semicircular canal hair cells. J Neurophysiol 2003; 90:1526-36. [PMID: 12966175 DOI: 10.1152/jn.00273.2002] [Citation(s) in RCA: 22] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
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.
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Affiliation(s)
- Joseph C Holt
- Department of Pharmacology (SL83), Tulane University School of Medicine, New Orleans, Louisiana 70112, USA.
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Bailey GP, Sewell WF. Contribution of glutamate receptors to spontaneous and stimulus-evoked discharge in afferent fibers innervating hair cells of the Xenopus lateral line organ. Hear Res 2000; 144:8-20. [PMID: 10831861 DOI: 10.1016/s0378-5955(00)00023-x] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 11/27/2022]
Abstract
The relative contributions of NMDA (N-methyl-D-aspartate) and non-NMDA glutamate receptors to spontaneous and stimulus-evoked transmission at the hair cell/afferent fiber synapse were determined in the Xenopus laevis lateral line organ. The non-NMDA receptor antagonist, CNQX (6-cyano-7-nitroquinoxaline-2,3-dione), reversibly reduced both spontaneous and stimulus-evoked discharge rate with an EC(50) of 0.5 microM. NMDA receptor antagonism with the combination of chlorokynurenic acid (100 microM) and elevated magnesium (1.1 mM), or elevated magnesium alone, blocked responses to NMDA without significantly altering spontaneous or stimulus-evoked discharge rate or the responses to kainate. All non-NMDA receptor agonists tested increased discharge rate at low concentrations and, at higher concentrations, increased, then suppressed discharge rate. The EC(50)s were: domoic acid (2.4 mcM)<quisqualic acid (6 mcM)<kainic acid (18 mcM)<AMPA (82 mcM)<<glutamate (1150 mcM). NMDA and ibotenic acid also produced an increase in discharge followed by a suppression, but the suppressive phase of the response predominated and maximum increases in discharge rates were low compared to effects of the non-NMDA agonists. The EC(50)s were: NMDA (148 mcM)<ibotenic acid (463 mcM). The EC(50) for the suppression of afferent discharge that followed the initial excitatory effect was similar to the EC(50) for excitation. Perfusion with active concentrations of kainate, AMPA, or NMDA did not alter the threshold for electrical stimulation of these nerve fibers. We conclude that most of the postsynaptic signal normally seen in afferent fibers is mediated by non-NMDA receptors.
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Affiliation(s)
- G P Bailey
- Eaton-Peabody Laboratory of Auditory Physiology, Massachusetts Eye and Ear Infirmary, Boston 02114, USA
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Yamashita T, Ohnishi S, Ohtani M, Kumazawa T. Effects of efferent neurotransmitters on intracellular Ca2+ concentration in vestibular hair cells of the guinea pig. ACTA OTO-LARYNGOLOGICA. SUPPLEMENTUM 1993; 500:26-30. [PMID: 8095765 DOI: 10.3109/00016489309126173] [Citation(s) in RCA: 15] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 01/28/2023]
Abstract
The effects of putative efferent neurotransmitters on intracellular Ca2+ concentration ([Ca2+]i) in isolated vestibular hair cells (VHCs) of the guinea pig were determined using the Ca2+ sensitive dye Fura-2 and digital imaging microscopy. In the presence of 1 mM acetylcholine (ACh) there was a gradual increase in [Ca2+]i. In the presence of 10 microM ATP there was a rapid rise in [Ca2+]i. Thus, both ACh and ATP seem to be efferent neurotransmitters in VHCs of the guinea pig. When 100 microM Gamma-aminobutyric acid (GABA) was added there was a rapid increase in [Ca2+]i; thus GABA also seems to be an efferent neurotransmitter in VHCs. With the independent addition of 10 microM calcitonin gene-related peptide and 10 microM M- and L-enkephalin there were no significant increase in [Ca2+]i. We presume that these neuropeptides were functioning as efferent neuromodulators rather than as neurotransmitters.
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Affiliation(s)
- T Yamashita
- Department of Otolaryngology, Kansai Medical University, Osaka, Japan
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Abstract
To determine whether some of the substances that may be present in hair-cell sensory organs could affect neural activity in afferent fibers, we examined 56 compounds for the ability to alter the discharge rate of afferent fibers innervating hair cells in the lateral line organ of Xenopus laevis, the African clawed frog. These compounds included amino acids, glutamyl dipeptides, standard neurotransmitter candidates, and other constituents of tissues and body fluids. Substances found to be excitatory included some neutral amino acids (alanine, serine, threonine, asparagine, glutamine, and proline), ATP, carnosine, histidine, and barium chloride. Compounds that suppressed discharge included the aromatic amino acids (phenylalanine, tryptophan, and tyrosine), serotonin, and gamma-glutamyl dipeptides. GABA and acidic amino acids (glutamate, aspartate, and cysteine sulfinate) produced a brief excitation followed by a suppression of discharge rate. Several of these substances were active at sufficiently low concentrations that their presence in body fluids may affect afferent fiber discharge rate under normal or pathological conditions.
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Affiliation(s)
- E A Mroz
- Eaton-Peabody Laboratory, Massachusetts Eye and Ear Infirmary, Boston 02114
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