Hair Cell Afferent Synapse Research Articles

Tonotopic distribution of short-term adaptation properties in the cochlear nerve of normal and acoustically overexposed chicks.

Cochlear nerve adaptation is thought to result, at least partially, from the depletion of neurotransmitter stores in hair cells. Recently, neurotransmitter vesicle pools have been identified in chick tall hair cells that might play a role in adaptation. In order to understand better the relationship between adaptation and neurotransmitter release dynamics, short-term adaptation was characterized by using peristimulus time histograms of single-unit activity in the chick cochlear nerve. The adaptation function resulting from 100-ms pure tone stimuli presented at the characteristic frequency, +20 dB relative to threshold, was well described as a single exponential decay process with an average time constant of 18.6+/-0.8 ms (mean+/-SEM). The number of spikes contributed by the adapting part of the response increased tonotopically for characteristic frequencies up to approximately 0.8 kHz. Comparison of the adaptation data with known physiological and anatomical hair cell properties suggests that depletion of the readily releasable pool is the basis of short-term adaptation in the chick. With this idea in mind, short-term adaptation was used as a proxy for assessing tall hair cell synaptic function following intense acoustic stimulation. After 48 h of exposure to an intense pure tone, the time constant of short-term adaptation was unaltered, whereas the number of spikes in the adapting component was increased at characteristic frequencies at and above the exposure frequency. These data suggest that the rate of readily releasable pool emptying is unaltered, but the neurotransmitter content of the pool is increased, by exposure to intense sound. The results imply that an increase in readily releasable pool size might be a compensatory mechanism ensuring the strength of the hair cell afferent synapse in the face of ongoing acoustic stress.

Otoferlin, Defective in a Human Deafness Form, Is Essential for Exocytosis at the Auditory Ribbon Synapse

The auditory inner hair cell (IHC) ribbon synapse operates with an exceptional temporal precision and maintains a high level of neurotransmitter release. However, the molecular mechanisms underlying IHC synaptic exocytosis are largely unknown. We studied otoferlin, a predicted C2-domain transmembrane protein, which is defective in a recessive form of human deafness. We show that otoferlin expression in the hair cells correlates with afferent synaptogenesis and find that otoferlin localizes to ribbon-associated synaptic vesicles. Otoferlin binds Ca(2+) and displays Ca(2+)-dependent interactions with the SNARE proteins syntaxin1 and SNAP25. Otoferlin deficient mice (Otof(-/-)) are profoundly deaf. Exocytosis in Otof(-/-) IHCs is almost completely abolished, despite normal ribbon synapse morphogenesis and Ca(2+) current. Thus, otoferlin is essential for a late step of synaptic vesicle exocytosis and may act as the major Ca(2+) sensor triggering membrane fusion at the IHC ribbon synapse.

The cochlea – new insights into the conversion of sound into electrical signals

The organ of hearing, the cochlea, has been under active investigation over the past 50 years. Recently new insights into gene defects producing deafness and into hair cell mechanisms and their control have been gained from a variety of experimental approaches. These approaches have relied on advances in electrophysiology, molecular biology, genetic screening and laser interferometry. Whereas once the cochlea was the private province of engineers and mathematicians, it is now open to all manner of investigation. We are gaining important insights into cochlear function at the molecular level (genes and proteins), cellular level (hair cells, supporting cells and neurones) and at the organ level. This Special Issue of The Journal of Physiology explores different aspects of our current understanding of cochlear function, from transduction and amplification and their control through to the encoding of sound into electrical signals that are rapidly transmitted across the inner hair cell afferent synapse to the brain.

The Glutamate–Aspartate Transporter GLAST Mediates Glutamate Uptake at Inner Hair Cell Afferent Synapses in the Mammalian Cochlea

Ribbon synapses formed between inner hair cells (IHCs) and afferent dendrites in the mammalian cochlea can sustain high rates of release, placing strong demands on glutamate clearance mechanisms. To investigate the role of transporters in glutamate removal at these synapses, we made whole-cell recordings from IHCs, afferent dendrites, and glial cells adjacent to IHCs [inner phalangeal cells (IPCs)] in whole-mount preparations of rat organ of Corti. Focal application of the transporter substrate D-aspartate elicited inward currents in IPCs, which were larger in the presence of anions that permeate the transporter-associated anion channel and blocked by the transporter antagonist D,L-threo-beta-benzyloxyaspartate. These currents were produced by glutamate-aspartate transporters (GLAST) (excitatory amino acid transporter 1) because they were weakly inhibited by dihydrokainate, an antagonist of glutamate transporter-1 (excitatory amino acid transporter 2) and were absent from IPCs in GLAST-/- cochleas. Furthermore, D-aspartate-induced currents in outside-out patches from IPCs exhibited larger steady-state currents than responses elicited by L-glutamate, a prominent feature of GLAST, and examination of cochlea from GLAST-Discosoma red (DsRed) promoter reporter mice revealed that DsRed expression was restricted to IPCs and other supporting cells surrounding IHCs. Saturation of transporters by photolysis of caged D-aspartate failed to elicit transporter currents in IHCs, as did local application of D-aspartate to afferent terminals, indicating that neither presynaptic nor postsynaptic membranes are major sites for glutamate removal. These data indicate that GLAST in supporting cells is responsible for transmitter uptake at IHC afferent synapses.

Structure and Function of the Hair Cell Ribbon Synapse

Faithful information transfer at the hair cell afferent synapse requires synaptic transmission to be both reliable and temporally precise. The release of neurotransmitter must exhibit both rapid on and off kinetics to accurately follow acoustic stimuli with a periodicity of 1 ms or less. To ensure such remarkable temporal fidelity, the cochlear hair cell afferent synapse undoubtedly relies on unique cellular and molecular specializations. While the electron microscopy hallmark of the hair cell afferent synapse — the electron-dense synaptic ribbon or synaptic body — has been recognized for decades, dissection of the synapse’s molecular make-up has only just begun. Recent cell physiology studies have added important insights into the synaptic mechanisms underlying fidelity and reliability of sound coding. The presence of the synaptic ribbon links afferent synapses of cochlear and vestibular hair cells to photoreceptors and bipolar neurons of the retina. This review focuses on major advances in understanding the hair cell afferent synapse molecular anatomy and function that have been achieved during the past years.

Hair cell synaptic ribbons are essential for synchronous auditory signalling

Hearing relies on faithful synaptic transmission at the ribbon synapse of cochlear inner hair cells (IHCs). At present, the function of presynaptic ribbons at these synapses is still largely unknown. Here we show that anchoring of IHC ribbons is impaired in mouse mutants for the presynaptic scaffolding protein Bassoon. The lack of active-zone-anchored synaptic ribbons reduced the presynaptic readily releasable vesicle pool, and impaired synchronous auditory signalling as revealed by recordings of exocytic IHC capacitance changes and sound-evoked activation of spiral ganglion neurons. Both exocytosis of the hair cell releasable vesicle pool and the number of synchronously activated spiral ganglion neurons co-varied with the number of anchored ribbons during development. Interestingly, ribbon-deficient IHCs were still capable of sustained exocytosis with normal Ca2+-dependence. Endocytic membrane retrieval was intact, but an accumulation of tubular and cisternal membrane profiles was observed in ribbon-deficient IHCs. We conclude that ribbon-dependent synchronous release of multiple vesicles at the hair cell afferent synapse is essential for normal hearing.

Evidence That Rapid Vesicle Replenishment of the Synaptic Ribbon Mediates Recovery from Short-Term Adaptation at the Hair Cell Afferent Synapse

We have employed both in vitro patch clamp recordings of hair cell synaptic vesicle fusion and in vivo single unit recording of cochlear nerve activity to study, at the same synapse, the time course, control, and physiological significance of readily releasable pool dynamics. Exocytosis of the readily releasable pool was fast, saturating in less than 50 ms, and recovery was also rapid, regaining 95% of its initial amplitude following a 200-ms period of repolarization. Longer depolarizations (greater than 250 ms) yielded a second, slower kinetic component of exocytosis. Both the second component of exocytosis and recovery of the readily releasable pool were blocked by the slow calcium buffer, EGTA. Sound-evoked afferent synaptic activity adapted and recovered with similar time courses as readily releasable pool exhaustion and recovery. Comparison of readily releasable pool amplitude, capture distances of calcium buffers, and number of vesicles tethered to the synaptic ribbon suggested that readily releasable pool dynamics reflect the depletion of release-ready vesicles tethered to the synaptic ribbon and the reloading of the ribbon with vesicles from the cytoplasm. Thus, we submit that rapid recovery of the cochlear hair cell afferent fiber synapse from short-term adaptation depends on the timely replenishment of the synaptic ribbon with vesicles from a cytoplasmic pool. This apparent rapid reloading of the synaptic ribbon with vesicles underscores important functional differences between synaptic ribbons in the auditory and visual systems.

A Role for Purinergic Receptors at the Inner Hair Cell-Afferent Synapse?

Previously published evidence is reviewed for a functional role of ATP and its receptors at the chemical synapse between the mammalian inner hair cell and the primary afferent dendrites of the VIIIth nerve. New findings are also presented, both from gross cochlear potentials and single neurone recordings during intracochlear perfusion of ATP-receptor antagonists and agonists. Both the previous and present results are consistent with the notion that endogenous ATP may act to regulate the excitability of the primary afferent dendrite, possibly acting on P2X₂ receptors.

Calcium Dependence of Exocytosis and Endocytosis at the Cochlear Inner Hair Cell Afferent Synapse

Release of neurotransmitter at the inner hair cell (IHC) afferent synapse is a fundamental step in translating sound into auditory nerve excitation. To study the Ca 2+ dependence of the underlying vesicle fusion and subsequent endocytosis, we combined Ca 2+ uncaging with membrane capacitance measurements in mouse IHCs. Rapid elevations in [Ca 2+] i above 8 μM caused a biphasic capacitance increase corresponding to the fusion of ∼40,000 vesicles. The kinetics of exocytosis displayed a fifth-order Ca 2+ dependence reaching maximal rates of >3 × 10 7 vesicle/s. Exocytosis was always followed by slow, compensatory endocytosis (τ ≅ 15 s). Higher [Ca 2+] i increased the contribution of a faster mode of endocytosis with a Ca 2+-independent time constant of ∼300 ms. These properties provide for rapid and sustained transmitter release from this large presynaptic terminal.

Kinetics of exocytosis and endocytosis at the cochlear inner hair cell afferent synapse of the mouse.

Hearing in mammals relies on the highly synchronous synaptic transfer between cochlear inner hair cells (IHCs) and the auditory nerve. We studied the presynaptic function of single mouse IHCs by monitoring membrane capacitance changes and voltage-gated Ca(2+) currents. Exocytosis initially occurred at a high rate but then slowed down within a few milliseconds, despite nearly constant Ca(2+) influx. We interpret the observed secretory depression as depletion of a readily releasable pool (RRP) of about 280 vesicles. These vesicles are probably docked close to Ca(2+) channels at the ribbon-type active zones of the IHCs. Continued depolarization evoked slower exocytosis occurring at a nearly constant rate for at least 1 s and depending on "long-distance" Ca(2+) signaling. Refilling of the RRP after depletion followed a biphasic time course and was faster than endocytosis. RRP depletion is discussed as a mechanism for fast auditory adaptation.

AMPA-type glutamate receptor subunits are expressed in the avian cochlear hair cells and ganglion cells.

The cellular localization of AMPA-type glutamate receptor subunits was examined in the pigeon inner ear using subunit specific polyclonal antibodies (GluR1-4). In the auditory ganglion cell bodies immunoreactivity for the subunits GluR2/3 and GluR4, but not for GluR1 was detected. The hair cells showed diffuse immunoreactivity for GluR4. Additionally, immunostaining for the subunits GluR2/3 and GluR4 was present below the hair cells. These results indicate that the AMPA type glutamate receptors play a role in neurotransmission at the hair cell afferent synapse in the avian auditory system.

Responses of auditory nerve fibers innervating regenerated hair cells after local application of gentamicin at the round window of the cochlea in the pigeon

Hair cells in the basilar papilla of birds have the capacity to regenerate after injury. There is also functional recovery of hearing after regeneration of the hair cells. The present study was undertaken to determine the effect of local aminoglycoside application on the physiology of auditory nerve fibers innervating regenerated hair cells. Collagen sponges loaded with gentamicin were placed at the round window of the cochlea in adult pigeons. The local application of gentamicin-loaded collagen sponges resulted in total hair cell loss over at least the basal 62% of the basilar papilla. According to the pigeon cochlear place-frequency map (Smolders, Ding-Pfennigdorff and Klinke, Hear. Res. 92 (1995) 151–169), frequencies above 0.3 kHz are represented in this area. Physiological data on single auditory nerve fibers were obtained 14 weeks after gentamicin treatment. The response properties showed the following characteristics when compared to control data: CF thresholds (CF=characteristic frequency) were elevated in units with CF above 0.15 kHz, sharpness of tuning ( Q 10dB) was reduced in units with CF above 0.38 kHz, low-frequency slopes of the tuning curves were reduced in units with CF above 0.25 kHz, high frequency slopes of the tuning curves were reduced in units with CF above 0.4 kHz, spontaneous firing rate was reduced in units with CF above 0.38 kHz, dynamic range of rate-intensity functions at CF was reduced in units with CF above 0.4 kHz and the slopes of these rate-intensity functions were elevated in units with CF above 0.4 kHz. Maximum discharge rate was the only parameter that remained unchanged in regenerated ears. The results show that the response properties of auditory nerve fibers which innervate areas of the papilla that were previously devoid of hair cells are poorer than the controls, but that action potential generation in the afferent fibers is unaffected. This suggests that despite structural regeneration of the basilar papilla, functional recovery of the auditory periphery is incomplete at the level of the hair cell or the hair cell-afferent synapse.

Role of Nitric Oxide in Kainic Acid-induced Elevation of Cochlear Compound Action Potential Thresholds

Nitric oxide (NO) has specifically been found to mediate the effects of excitatory amino acids in the central nervous system (CNS). Excitatory amino acids are the primary neurotransmitters at the cochlear hair cell-afferent nerve synapse. Recent studies in our laboratory demonstrate that NO synthase is an active enzyme in the spiral ganglion cells of the cochlea. Given our current understanding of neurotransmission in the cochlea, it is reasonable to postulate that the actions of NO in cochlear neuronal tissue are similar to the actions of NO in the CNS, and that NO acts as a neurotransmitter/neuromodulator in the cochlea. In addition, NO is implicated as a mediator of excitotoxicity in the CNS and may therefore play a similar role in excitotoxicity in the cochlea. To further elucidate the role of NO in cochlear excitotoxicity, this study investigated the effects of 7-nitroindazole (7-NI), a competitive inhibitor of neuronal nitric oxide synthase, with regard to kainic acid (KA)-induced elevation of compound action potential (CAP) thresholds. KA is a conformationally restricted analog of glutamate with well-known excitotoxic effects on SGC's and previously described inhibitory actions on cochlear CAP thresholds. In anesthetized gerbils, CAP thresholds were recorded before and after cochlear perfusions with control solutions of artificial perilymph solution and test solutions of KA. 7-NI was administered i.p. prior to KA perfusion in an effort to block its depolarizing and toxic effects. Results showed that cochlear perfusion with KA caused significant elevation (p<0.05) of the mean CAP threshold. This threshold shift was significantly reduced (p<0.05) in animals pretreated with 7-NI. These results indicate that NO is involved in the toxic effects on CAP thresholds elicited by KA in the cochlea.

Extracellular ATP-induced Ca2+ Mobilization of Type I Spiral Ganglion Cells from the Guinea Pig Cochlea

Intracellular calcium concentrations ([Ca2 +],) in type I cochlear spiral ganglion cells (SGCs) of the guinea pig were measured by digital imaging microscopy and the Ca2 +-sensitive dye Fura-2. Extracellular ATP induced elevation of [Ca2 +]i in type I SGCs in a concentration-dependent manner. The ATP-induced elevation of [Ca2 +]i in SGC was even evident in the Ca2 + -free solution, thereby suggesting that ATP induces a Ca2 + -release from intracellular stores in SGCs. Suramin and reactive blue 2, both antagonists for the P2-purinergic receptor, inhibited the [Ca2 +]i increase in SGCs induced by extracellular ATP in a dose-dependent manner. Adenosine did not induce any changes of [Ca2 +]i in SGC. These results suggest that type I SGCs may possess P2-purinergic receptor but not P1-purinergic receptor. Extracellular ATP induced a [Ca2 +]i increase in type I SGCs, with and without neuritic processes, while L-glutamate increased [Ca2 +]i in type 1 SGCs with neuritic processes, but not SGCs without neuritic processes. The ATP-induced [Ca2 +]i increase was almost equal in both soma and processes. Therefore, the distribution of P2-purinergic receptor in type I SGCs may be homogeneous in soma and processes. Based on these observations, we suggest that extracellular ATP may act as a neurotransmitter or neuromodulator of the hair cell-afferent nerve synapse in the guinea pig cochlea.

Off-oscillation in hair cell membrane is correlated with the decremental response at the hair cell-afferent fibre synapse in the goldfish sacculus.

In this study, we examined the decremental response in the hair cell-afferent fibre synapse of goldfish. This decremental response is a transient reduction in sound-evoked EPSPs associated with a step reduction in sound intensity. To determine its presynaptic correlates, a patch clamp study was performed on oscillatory-type hair cells isolated from the sacculus of goldfish. Injection of depolarizing current produced a damped oscillation in the membrane potential of such cells. When the current intensity was reduced in steps, instead of simply being turned off, very marked dips were produced as off-oscillation in the hair cell membrane potential. These dips correlated with the decremental response in EPSPs.

Identification of a glutamate / aspartate transporter in the rat cochlea

The neurotransmitter at the synapses between hair cells and spiral ganglion cells in the cochlea is probably L-glutamate or a similar excitatory amino acid. Glutamate uptake by nerve terminals and glial cells is an important component of neurotransmission at glutamatergic synapses of the central nervous system, for providing a reservoir of transmitter or transmitter precursors and the termination of the released glutamate. Hair cell synapses are not surrounded by glial cells, therefore, the uptake mechanism for glutamate in the cochlea may be unique. cDNA was synthesized from total RNA isolated separately from the rat organ of Corti, spiral ganglia, and lateral wall tissues. The expression of a glutamate/aspartate transporter (GLAST) was detected by DNA amplification with the polymerase chain reaction. The other two members of glutamate transporters in this family were not detected by this method. A partial cDNA encoding to GLAST was identified by sequence analysis in a rat cochlear cDNA library. Data concerning the expression and the molecular structure of the glutamate transporter GLAST in the cochlea may provide important information regarding the neurotransmission process at the hair cell-afferent synapses.

Presynaptic origin of a decremental response in hair cell-afferent fiber synapse in goldfish sacculus

Presynaptic origin of a decremental response in hair cell-afferent fiber synapse in goldfish sacculus

Response patterns to tone bursts in peripheral auditory system of anurans.

Response patterns to tone bursts in peripheral auditory system of anurans.A L Megela, and R R CapranicaA L Megela, and R R CapranicaPublished Online:01 Sep 1981https://doi.org/10.1152/jn.1981.46.3.465MoreSectionsPDF (2 MB)Download PDF ToolsExport citationAdd to favoritesGet permissionsTrack citations ShareShare onFacebookTwitterLinkedInWeChat Previous Back to Top Next Download PDF FiguresReferencesRelatedInformation Cited ByTemporally selective processing of communication signals by auditory midbrain neuronsTaffeta M. Elliott, Jakob Christensen-Dalsgaard, and Darcy B. Kelley1 April 2011 | Journal of Neurophysiology, Vol. 105, No. 4Synaptic Ribbon Enables Temporal Precision of Hair Cell Afferent Synapse by Increasing the Number of Readily Releasable Vesicles: A Modeling StudyJohn H. Wittig, and Thomas D. Parsons1 October 2008 | Journal of Neurophysiology, Vol. 100, No. 4New variations on the derivation of spectro-temporal receptive fields for primary auditory afferent axonsHearing Research, Vol. 189, No. 1-2New variations on the derivation of spectro-temporal receptive fields for primary auditory afferent axonsHearing Research, Vol. 186, No. 1-2Coding of amplitude-modulated signals in the cochlear nucleus of a grass frogAcoustical Physics, Vol. 48, No. 4Measurement of temporal regularity of spike train responses in auditory nerve fibers of the green treefrogJournal of Neuroscience Methods, Vol. 52, No. 2Representation of acoustic signals in the eighth nerve of the Tokay gecko: I. Pure tonesHearing Research, Vol. 76, No. 1-2Effects of spike discharge history on discharge probability and latency in frog basilar papilla unitsHearing Research, Vol. 69, No. 1-2Periodicity extraction in the anuran auditory nerveJournal of Comparative Physiology A, Vol. 172, No. 1Selectivity for temporal characteristics of sound and interaural time difference of auditory midbrain neurons in the grassfrog: A system theoretical approachHearing Research, Vol. 60, No. 2Sensitivity for interaural time and intensity difference of auditory midbrain neurons in the grassfrogHearing Research, Vol. 47, No. 3Basic properties of auditory-nerve responses from a ‘simple’ ear: The basilar papilla of the frogHearing Research, Vol. 47, No. 1-2Temperature-dependence of auditory nerve response properties in the frogHearing Research, Vol. 46, No. 1-2Peripheral auditory processing in the bobtail lizard Tiliqua rugosaJournal of Comparative Physiology A, Vol. 167, No. 1Influence of adaptation on neural sensitivity to temporal characteristics of sound in the dorsal medullary nucleus and torus semicircularis of the grassfrogHearing Research, Vol. 45, No. 1-2Modelling the response of auditory midbrain neurons in the grassfrog to temporally structured monaural stimuliHearing Research, Vol. 43, No. 2-3A model for the peripheral auditory nervous system of the grassfrogHearing Research, Vol. 41, No. 1Masked thresholds of auditory midbrain neurons in the frog Rana ridibundaHearing Research, Vol. 35, No. 2-3Sensitivity of neurons in the dorsal medullary nucleus of the grassfrog to spectral and temporal characteristics of soundHearing Research, Vol. 29, No. 2-3Auditory responses in the eighth nerve of a mating call-less toad, Bufo chilensis (Amphibia: Bufonidae)Comparative Biochemistry and Physiology Part A: Physiology, Vol. 84, No. 4Temporary threshold shift, adaptation, and recovery characteristics of frog auditory nerve fibersHearing Research, Vol. 17, No. 2The auditory system of anuran amphibiansProgress in Neurobiology, Vol. 22, No. 1Prediction of the responses of auditory neurons in the midbrain of the grass frog based on the spectro-temporal receptive fieldHearing Research, Vol. 10, No. 2 More from this issue > Volume 46Issue 3September 1981Pages 465-78 https://doi.org/10.1152/jn.1981.46.3.465PubMed7299429History Published online 1 September 1981 Published in print 1 September 1981 Metrics

Quantal analysis of the size of excitatory post‐synaptic potentials at synapses between hair cells and afferent nerve fibres in goldfish.

1. A statistical analysis has been made of the transmitter release at the hair cell afferent fibre synapse in the sacculus of the goldfish, using the amplitude of the excitatory post-synaptic potentials (e.p.s.p.s) in response to stimulus tone as a measure of the transmitter release under application of tetrodotoxin. 2. Application of binomial statistics allowed a direct calculation of the mean probability of release (p) and the readily available store (n), and the X2-test showed that the binomial predictions fitted fairly well with the observed distribution of the responses. 3. Adaptive rundown of e.p.s.p.s during sound stimulation, i.e. the successive rundown in the size of the mean quantal content (m), was found to be associated with a reduction in the size of parameter n, but not of p. 4. A marked negative correlation was demonstrated between the amplitude of two consecutive e.p.s.p.s, supporting the depletion hypothesis of the adaptive rundown of e.p.s.p.s. 5. The increase in the e.p.s.p. amplitude and the increase in the mean quantal content, m, brought about by an increase in the tone intensity were found mostly explicable in terms of an increase in the statistical parameter n. The probability parameter p was found largely in invariable, although in certain instances the increase in m was also accompanied by a slight increase in the parameter p.