Immunocytochemical localization of AMPA selective glutamate receptor subunits in the rat cochlea

Ca2+-Permeable AMPARs Mediate Glutamatergic Transmission and Excitotoxic Damage at the Hair Cell Ribbon Synapse

We report functional and structural evidence for GluA2-lacking Ca²⁺-permeable AMPARs (CP-AMPARs) at the mature hair cell ribbon synapse. By using the methodological advantages of three species (of either sex), we demonstrate that CP-AMPARs are present at the hair cell synapse in an evolutionarily conserved manner. Via a combination of in vivo electrophysiological and Ca²⁺ imaging approaches in the larval zebrafish, we show that hair cell stimulation leads to robust Ca²⁺ influx into afferent terminals. Prolonged application of AMPA caused loss of afferent terminal responsiveness, whereas blocking CP-AMPARs protects terminals from excitotoxic swelling. Immunohistochemical analysis of AMPAR subunits in mature rat cochlea show regions within synapses lacking the GluA2 subunit. Paired recordings from adult bullfrog auditory synapses demonstrate that CP-AMPARs mediate a major component of glutamatergic transmission. Together, our results support the importance of CP-AMPARs in mediating transmission at the hair cell ribbon synapse. Further, excess Ca²⁺ entry via CP-AMPARs may underlie afferent terminal damage following excitotoxic challenge, suggesting that limiting Ca²⁺ levels in the afferent terminal may protect against cochlear synaptopathy associated with hearing loss.SIGNIFICANCE STATEMENT A single incidence of noise overexposure causes damage at the hair cell synapse that later leads to neurodegeneration and exacerbates age-related hearing loss. A first step toward understanding cochlear neurodegeneration is to identify the cause of initial excitotoxic damage to the postsynaptic neuron. Using a combination of immunohistochemical, electrophysiological, and Ca²⁺ imaging approaches in evolutionarily divergent species, we demonstrate that Ca²⁺-permeable AMPARs (CP-AMPARs) mediate glutamatergic transmission at the adult auditory hair cell synapse. Overexcitation of the terminal causes Ca²⁺ accumulation and swelling that can be prevented by blocking CP-AMPARs. We demonstrate that CP-AMPARs mediate transmission at this first-order sensory synapse and that limiting Ca²⁺ accumulation in the terminal may protect against hearing loss.

Ablation of the auditory cortex results in changes in the expression of neurotransmission-related mRNAs in the cochlea

The auditory cortex (AC) dynamically regulates responses of the Organ of Corti to sound through descending connections to both the medial (MOC) and lateral (LOC) olivocochlear efferent systems. We have recently provided evidence that AC has a reinforcement role in the responses to sound of the auditory brainstem nuclei. In a molecular level, we have shown that descending inputs from AC are needed to regulate the expression of molecules involved in outer hair cell (OHC) electromotility control, such as prestin and the α10 nicotinic acetylcholine receptor (nAchR). In this report, we show that descending connections from AC to olivocochlear neurons are necessary to regulate the expression of molecules involved in cochlear afferent signaling. RT-qPCR was performed in rats at 1, 7 and 15 days after unilateral ablation of the AC, and analyzed the time course changes in gene transcripts involved in neurotransmission at the first auditory synapse. This included the glutamate metabolism enzyme glutamate decarboxylase 1 (glud1) and AMPA glutamate receptor subunits GluA2-4. In addition, gene transcripts involved in efferent regulation of type I spiral ganglion neuron (SGN) excitability mediated by LOC, such as the α7 nAchR, the D2 dopamine receptor, and the α1, and γ2 GABAA receptor subunits, were also investigated. Unilateral AC ablation induced up-regulation of GluA3 receptor subunit transcripts, whereas both GluA2 and GluA4 mRNA receptors were down-regulated already at 1 day after the ablation. Unilateral removal of the AC also resulted in up-regulation of the transcripts for α7 nAchR subunit, D2 dopamine receptor, and α1 GABAA receptor subunit at 1 day after the ablation. Fifteen days after the injury, AC ablations induced an up-regulation of glud1 transcripts.

Cobalt-Induced Ototoxicity in Rat Postnatal Cochlear Organotypic Cultures

Cobalt (Co) is a required divalent metal used in the production of metal alloys, batteries, and pigments and is a component of vitamin B12. Excessive uptake of Co is neurotoxic causing temporary or permanent hearing loss; however, its ototoxic effects on the sensory hair cells, neurons, and support cells in the cochlea are poorly understood. Accordingly, we treated postnatal day 3 rat cochlear organotypic cultures with various doses and durations of CoCl2 and quantified the damage to the hair cells, peripheral auditory nerve fibers, and spiral ganglion neurons (SGN). Five-day treatment with 250 μM CoCl2 caused extensive damage to hair cells and neurons which increased with dose and treatment duration. CoCl2 caused greater damage to outer hair cells than inner hair cells; damage was greatest in the base of the cochlea and decreased towards the base. CoCl2 increased expression of superoxide radical in hair cells and SGNs and SGN loss was characterized by nuclear condensation and fragmentation, morphological features of apoptosis. CoCl2 treatment increased the expression of caspase-3 indicative of caspase-mediated programmed cell death. These results identify hair cells and spiral ganglion neurons as the main targets of Co ototoxicity in vitro and implicate the superoxide radical as a trigger of caspase-mediated ototoxicity.

Deletion of Shank1 has minimal effects on the molecular composition and function of glutamatergic afferent postsynapses in the mouse inner ear

Shank proteins (1-3) are considered the master organizers of glutamatergic postsynaptic densities in the central nervous system, and the genetic deletion of either Shank1, 2, or 3 results in altered composition, form, and strength of glutamatergic postsynapses. To investigate the contribution of Shank proteins to glutamatergic afferent synapses of the inner ear and especially cochlea, we used immunofluorescence and quantitative real time PCR to determine the expression of Shank1, 2, and 3 in the cochlea. Because we found evidence for expression of Shank1 but not 2 and 3, we investigated the morphology, composition, and function of afferent postsynaptic densities from defined tonotopic regions in the cochlea of Shank1(-/-) mice. Using immunofluorescence, we identified subtle changes in the morphology and composition (but not number and localization) of cochlear afferent postsynaptic densities at the lower frequency region (8 kHz) in Shank1(-/-) mice compared to Shank1(+/+) littermates. However, we detected no differences in auditory brainstem responses at matching or higher frequencies. We also identified Shank1 in the vestibular afferent postsynaptic densities, but detected no differences in vestibular sensory evoked potentials in Shank1(-/-) mice compared to Shank1(+/+) littermates. This work suggests that Shank proteins play a different role in the development and maintenance of glutamatergic afferent synapses in the inner ear compared to the central nervous system.

Synaptic profiles during neurite extension, refinement and retraction in the developing cochlea

Background

During development, excess synapses form between the central and peripheral nervous systems that are then eliminated to achieve correct connectivity. In the peripheral auditory system, the developing type I spiral ganglion afferent fibres undergo a dramatic re-organisation, initially forming connections with both sensory inner hair cells (IHCs) and outer hair cells (OHCs). The OHC connections are then selectively eliminated, leaving sparse innervation by type II afferent fibres, whilst the type I afferent synapses with IHCs are consolidated.

Results

We examined the molecular makeup of the synaptic contacts formed onto the IHCs and OHCs during this period of afferent fibre remodelling. We observed that presynaptic ribbons initially form at all the afferent neurite contacts, i.e. not only at the expected developing IHC-type I fibre synapses but also at OHCs where type I fibres temporarily contact. Moreover, the transient contacts forming onto OHCs possess a broad set of pre- and postsynaptic proteins, suggesting that functional synaptic connections are formed prior to the removal of type I fibre innervation. AMPA-type glutamate receptor subunits were transiently observed at the base of the OHCs, with their downregulation occurring in parallel with the withdrawal of type I fibres, dispersal of presynaptic ribbons, and downregulation of the anchoring proteins Bassoon and Shank. Conversely, at developing type I afferent IHC synapses, the presence of pre- and postsynaptic scaffold proteins was maintained, with differential plasticity in AMPA receptor subunits observed and AMPA receptor subunit composition changing around hearing onset.

Conclusions

Overall our data show a differential balance in the patterns of synaptic proteins at developing afferent IHC versus OHC synapses that likely reflect their stable versus transient fates.

Two modes of release shape the postsynaptic response at the inner hair cell ribbon synapse

Cochlear inner hair cells (IHCs) convert sounds into receptor potentials and via their ribbon synapses into firing rates in auditory nerve fibers. Multivesicular release at individual IHC ribbon synapses activates AMPA-mediated EPSCs with widely ranging amplitudes. The underlying mechanisms and specific role for multivesicular release in encoding sound are not well understood. Here we characterize the waveforms of individual EPSCs recorded from afferent boutons contacting IHCs and compare their characteristics in immature rats (postnatal days 8-11) and hearing rats (postnatal days 19-21). Two types of EPSC waveforms were found in every recording: monophasic EPSCs, with sharp rising phases and monoexponential decays, and multiphasic EPSCs, exhibiting inflections on rising and decaying phases. Multiphasic EPSCs exhibited slower rise times and smaller amplitudes than monophasic EPSCs. Both types of EPSCs had comparable charge transfers, suggesting that they were activated by the release of similar numbers of vesicles, which for multiphasic EPSCs occurred in a less coordinated manner. On average, a higher proportion of larger, monophasic EPSCs was found in hearing compared to immature rats. In addition, EPSCs became significantly faster with age. The developmental increase in size and speed could improve auditory signaling acuity. Multiphasic EPSCs persisted in hearing animals, in some fibers constituting half of the EPSCs. The proportion of monophasic versus multiphasic EPSCs varied widely across fibers, resulting in marked heterogeneity of amplitude distributions. We propose that the relative contribution of two modes of multivesicular release, generating monophasic and multiphasic EPSCs, may underlie fundamental characteristics of auditory nerve fibers.

A corticotropin-releasing factor system expressed in the cochlea modulates hearing sensitivity and protects against noise-induced hearing loss

Noise-induced hearing loss is a highly prevalent occupational injury, yet little is known concerning the signals controlling normal cochlear sensitivity and susceptibility to noise-induced trauma. While the corticotropin-releasing factor (CRF) system is involved in activation of the classic hypothalamic-pituitary-adrenal axis, it is also involved in local physiological responses to stress in many tissues, and is expressed in the inner ear. We demonstrate that mice lacking the CRF receptor CRFR2 exhibit a significantly lower auditory threshold than wild type mice, but this gain of function comes at the price of increased susceptibility to acoustic trauma. We further demonstrate that glutamatergic transmission, purinergic signaling, and activation of Akt (PKB) pathways within the cochlea are misregulated, which may underlie the enhanced sensitivity and trauma susceptibility observed in CRFR2(-/-) mice. Our data suggest that CRFR2 constitutively modulates hearing sensitivity under normal conditions, and thereby provides protection against noise-induced hearing loss.

Spiral ganglion neurones: an overview of morphology, firing behaviour, ionic channels and function

The spiral ganglion cells provide the afferent innervation of the hair cells of the organ of Corti. Ninety-five percent of these cells (termed type I spiral ganglion neurones) are in synaptic contact with the inner hair cells, whereas about 5% of them are type II cells, which are responsible for the sensory innervation of the outer hair cells. To understand the function of the spiral ganglion neurones, it is important to explore their membrane properties, understand their activity patterns and describe the variety of ionic channels determining their behaviour. In this review, a brief description is given of the various experimental methods that allow the investigation of the spiral ganglion cells, followed by the discussion of their action potential firing patterns and ionic conductances. The presence, distribution and significance of the K(+) currents of the spiral ganglion cells are specifically addressed, along with the introduction of the putative subunit compositions of the relevant voltage-gated K(+) channels.

Persistence of Ca(v)1.3 Ca2+ channels in mature outer hair cells supports outer hair cell afferent signaling

Outer hair cells (OHCs) are innervated by type II afferent fibers of as yet unknown function. It is still a matter of debate whether OHCs perform exocytosis. If so, they would require presynaptic Ca2+ channels at their basal poles where the type II fibers make contacts. Here we show that L-type Ca2+ channel currents (charge carrier, 10 mM Ba2+) present in neonatal OHCs [postnatal day 1 (P1) to P7] decreased from approximately 170 to approximately 50 pA at approximately the onset of hearing. Ba2+ currents could hardly be measured in mature mouse OHCs because of their high fragility, whereas in the rat, the average Ba2+ current amplitude of apical OHCs was 58 +/- 9 pA (n = 20, P19-P30) compared with that of the inner hair cells (IHCs) of 181 +/- 50 pA (n = 24, P17-P30). Properties of Ba2+ currents of mature OHCs resembled those of neonatal OHCs. One exception was the voltage dependence of activation that shifted between birth and P12 by +9 mV toward positive voltages in OHCs, whereas it remained constant in the IHCs. Ca(v)1.3-specific mRNA was detected in mature OHCs using cell-specific reverse transcription (RT)-PCR and in situ hybridization. Ca(v)1.3 protein was stained exclusively at the base of mature OHCs, in colocalization with the ribbon synapse protein CtBP2 (C-terminal binding protein 2)/RIBEYE. When current sizes were normalized to the estimated number of afferent fibers or presynaptic ribbons, comparable values for IHCs and OHCs were obtained, a finding that together with the colocalization of Ca(v)1.3 and CtBP2/RIBEYE protein strongly suggests a role for Ca(v)1.3 channels in exocytosis of mature OHCs.

The signal transduction pathway for the dopamine D1 receptor in the guinea-pig cochlea

Dopamine released from lateral efferent fibers modulates the activity of the auditory nerve, but the signaling mechanism by which this is mediated is not known. The present study investigated the signal transduction pathway for the dopamine D1 receptor in the guinea-pig cochlea. D1 receptor immunolabeling was localized to the spiral ganglia neurons and at the base of the inner hair cells. Western immunoblotting on whole cochlear preparations revealed positive bands for the D1 receptor and for dopamine and the cyclic AMP-regulated phosphoprotein. The amplitude of the compound action potential was enhanced in the presence of the D1 receptor agonist, SKF 38393, an effect that was abolished by H89, a protein kinase A inhibitor. Conversely, SKF 83566, a D1 receptor antagonist decreased the amplitude of compound action potential, while forskolin, a protein kinase A activator prevented this effect. Furthermore, it was found that the level of glutamate receptor 1 phosphorylation at the protein kinase A site (Ser845) was increased by the D1 agonist, but decreased by D1 antagonist. Our results provide evidence that the D1 receptor is localized in the spiral ganglion neurons as well as the nerve endings under the inner hair cells and they can modulate auditory nerve function. One signal transduction pathway of D1 receptor in the auditory nerve is via protein kinase A-mediated glutamate receptor 1 phosphorylation.

Developmental regulation of neuron-specific P2X3 receptor expression in the rat cochlea

ATP-gated ion channels assembled from P2X3 receptor (P2X3R) subunits contribute to neurotransmission and neurotrophic signaling, associated with neurite development and synaptogenesis, particularly in peripheral sensory neurons. Here, P2X3R expression was characterized in the rat cochlea from embryonic day 16 (E16) to adult (P49-56), using RT-PCR and immunohistochemistry. P2X3R mRNA was strongly expressed in the cochlea prior to birth, declined to a minimal level at P14, and was absent in adult tissue. P2X3R protein expression was confined to spiral ganglion neurons (SGN) within Rosenthal's canal of the cochlea. At E16, immunolabeling was detected in the SGN neurites, but not the distal neurite projection within the developing sensory epithelium (greater epithelial ridge). From E18, the immunolabeling was observed in the peripheral neurites innervating the inner hair cells but was reduced by P6. However, from P2-8, immunolabeling of the SGN neurites extended to include the outer spiral bundle fiber tract beneath the outer hair cells. This labeling of type II SGN afferent fiber declined after P8. By P14, all synaptic terminal immunolabeling in the organ of Corti was absent, and SGN cell body labeling was minimal. In adult cochlear tissue, P2X3R immunolabeling was not detected. Noise exposure did not induce P2X3R expression in the adult cochlea. These data indicate that ATP-gated ion channels incorporating P2X3R subunit expression are specifically targeted to the afferent terminals just prior to the onset of hearing, and likely contribute to the neurotrophic signaling which establishes functional auditory neurotransmission.

Transient Ca2+-permeable AMPA receptors in postnatal rat primary auditory neurons

Fast excitatory transmission in the nervous system is mostly mediated by alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptors whose subunit composition governs physiological characteristics such as ligand affinity and ion conductance properties. Here, we report that AMPA receptors at inner hair cell (IHC) synapses lack the GluR2 subunit and are transiently Ca2+-permeable before hearing onset as evidenced using agonist-induced Co2+ accumulation, Western blots and GluR2 confocal microscopy in the rat cochlea. AMPA (100 microM) induced Co2+ accumulation in primary auditory neurons until postnatal day (PND) 10. This accumulation was concentration-dependent, strengthened by cyclothiazide (50 microM) and blocked by GYKI 52466 (80 microM) and Joro spider toxin (1 microM). It was unaffected by D-AP5 (50 microM), and it could not be elicited by 56 mM K+ or 1 mM NMDA + 10 microM glycine. Western blots showed that GluR1 immunoreactivity, present in homogenates of immature cochleas, had disappeared by PND12. GluR2 immunoreactivity was not detected until PND10 and GluR3 and GluR4 immunoreactivities were detected at all the ages examined. Confocal microscopy confirmed that the GluR2 immunofluorescence was not located postsynaptically to IHCs before PND10. In conclusion, AMPA receptors on maturing primary auditory neurons differ from those on adult neurons. They are probably composed of GluR1, GluR3 and GluR4 subunits and have a high Ca2+ permeability. The postsynaptic expression of GluR2 subunits may be continuously regulated by the presynaptic activity allowing for variations in the Ca2+ permeability and physiological properties of the receptor.

Glutamate induced currents in isolated inner hair cells from guinea-pig cochlea

We investigated the direct action of glutamate (Glu) on the membrane current of isolated inner hair cells of guinea-pig cochlea. Glu elicited inward currents at a holding potential of -70 mV. Eight of 13 cells showed a steady inward current, while five of 13 cells showed a fast and rapidly desensitized current. I-V relationships demonstrated that the reversal potential of Glu-induced current was near 0 mV. Glu-induced currents were dose-dependent, where the half maximum concentration (K(d)) was 41 microM and Hill coefficient (n) was 1.75.

Structure and innervation of the cochlea

The role of the cochlea is to transduce complex sound waves into electrical neural activity in the auditory nerve. Hair cells of the organ of Corti are the sensory cells of hearing. The inner hair cells perform the transduction and initiate the depolarization of the spiral ganglion neurons. The outer hair cells are accessory sensory cells that enhance the sensitivity and selectivity of the cochlea. Neural feedback loops that bring efferent signals to the outer hair cells assist in sharpening and amplifying the signals. The stria vascularis generates the endocochlear potential and maintains the ionic composition of the endolymph, the fluid in which the apical surface of the hair cells is bathed. The mechanical characteristics of the basilar membrane and its related structures further enhance the frequency selectivity of the auditory transduction mechanism. The tectorial membrane is an extracellular matrix, which provides mass loading on top of the organ of Corti, facilitating deflection of the stereocilia. This review deals with the structure of the normal mature mammalian cochlea and includes recent data on the molecular organization of the main cell types within the cochlea.

Neurotransmission in the vestibular endorgans--glutamatergic transmission in the afferent synapses of hair cells

In the sensory pathways the first synapse is that between hair cells and primary afferent neurons and its most likely neurotransmitter candidate has long been thought to be glutamate. A number of pharmacological and electrophysiological studies have lent credence to this theory (reviewed by Bledsoe et al. 1988, Bobbin 1979, Ehrenberger and Felix 1991, Puel et al. 1991; Puel 1995) as has recent neurochemical and immunocytochemical work (reviewed by Ottersen et al. 1998; Usami et al. 2000). These recent studies reveal that the afferent hair cell synapse resembles the central glutamate synapses in many ways. Of the proteins confirmed to be involved in signal transduction and transmitter metabolism at most central synapses, many are also seen in the afferent hair cell synapse, and have an analogous compartmentation. On the other hand, there are also important differences, especially those related to the molecular mechanisms that underlie transmitter release.

The AMPA receptors of auditory neurons

The ionotropic glutamate receptor (GluR) subtype known as the AMPA receptor, which mediates rapid excitatory synaptic transmission in many regions of the nervous system, is composed of four different protein subunits, termed GluRs 1-4. The functional properties of each AMPA receptor are determined by the relative levels of GluRs 1-4 and by post-transcriptional modifications of these proteins through mRNA editing and alternative exon splicing. The present paper reviews the published evidence for (1) localization of mRNAs and immunoreactivity for GluRs 1-4 in the cochlea and subcortical central nervous system auditory pathways of mammals and birds, and (2) involvement of AMPA receptors in synaptic transmission in the auditory system. Recent biochemical and electrophysiological evidence concerning the specialized properties of AMPA receptors on brainstem auditory neurons is also reviewed, along with data concerning how these properties emerge during normal development.

Contribution of glutamate receptors to spontaneous and stimulus-evoked discharge in afferent fibers innervating hair cells of the Xenopus lateral line organ

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.

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.

Synaptic mechanisms for coding timing in auditory neurons

Neurons in the cochlear ganglion and auditory brain stem nuclei preserve the relative timing of action potentials passed through sequential synaptic levels. To accomplish this task, these neurons have unique morphological and biophysical specializations in axons, dendrites, and nerve terminals. At the membrane level, these adaptations include low-threshold, voltage-gated potassium channels and unusually rapid-acting transmitter-gated channels, which govern how quickly and reliably action potential threshold is reached during a synaptic response. Some nerve terminals are remarkably large and release large amounts of excitatory neurotransmitter. The high output of transmitter at these terminals can lead to synaptic depression, which may itself be regulated by presynaptic transmitter receptors. The way in which these different cellular mechanisms are employed varies in different cell types and circuits and reflects refinements suited to different aspects of acoustic processing.

Auditory pathway and auditory brainstem response in mice lacking NMDA receptor epsilon 1 and epsilon 4 subunits

There is considerable evidence that the N-methyl-D-aspartate receptor (NMDAR) is a component of excitatory amino acid synapses in the ascending auditory pathway. The availability of mice that are defective in NMDAR epsilon 1 or NMDAR epsilon 4 subunit paves the way for investigations on the role of this receptor in auditory function. Non-radioactive in situ hybridization was used in the parent C57/6J wild strain to determine if these subunits are normally expressed in cochlear nucleus (CN) and superior olivary complex (SOC) and to confirm their absence in the respective mutant mice. Evoked auditory brainstem response (ABR) to normal acoustic stimulation was investigated to assess function. In situ hybridization revealed the expression of NMDAR epsilon 1 and epsilon 4 subunits mRNAs in major neuronal types in the CN and SOC of the wild type mice while epsilon 1 and epsilon 4 expression were absent in their respective mutant mice. The ABR threshold for the epsilon 1 mutant mice was similar to that of wild type mice however the threshold for the epsilon 4 mutant mice was significantly elevated. These results suggest a role for the NMDAR epsilon 4 in normal auditory functions while the NMDAR epsilon 1 may have a less critical function under normal conditions.

The glutamate receptor subunit delta1 is highly expressed in hair cells of the auditory and vestibular systems

In the inner ear, fast excitatory synaptic transmission is mediated by ionotropic glutamate receptors, including AMPA, kainate, and NMDA receptors. The recently identified delta1 and delta2 glutamate receptors share low homology with the other three types, and no clear response or ligand binding has been obtained from cells transfected with delta alone or in combination with other ionotropic receptors. Studies of mice lacking expression of delta2 show that this subunit plays a crucial role in plasticity of cerebellar glutamatergic synapses. In addition, these mice show a deficit in vestibular compensation. These findings and the nature of glutamatergic synapses between vestibulocochlear hair cells and primary afferent dendrites suggest that delta receptors may be functionally important in the inner ear and prompted us to investigate the expression of delta receptors in the cochlea and peripheral vestibular system. Reverse transcription and DNA amplification by PCR combined with immunocytochemistry and in situ hybridization were used. Our results show that the expression of delta1 in the organ of Corti is intense and restricted to the inner hair cells, whereas delta1 is expressed in all spiral ganglion neurons as well as in their satellite glial cells. In the vestibular end organ, delta1 was highly expressed in both hair cell types and also was expressed in the vestibular ganglion neurons. The prominent expression of delta1 in inner hair cells and in type I and type II vestibular hair cells suggests a functional role in hair cell neurotransmission.

Immunocytochemical localization of a high-affinity glutamate-aspartate transporter, GLAST, in the rat and guinea-pig cochlea

Glutamate transporters play an important role in the reuptake of glutamate after its release from glutamatergic synapses. Four such transporters have so far been cloned from the rat brain. One, the glutamate-aspartate transporter GLAST, has been detected in the mammalian cochlea, in which the principal afferent synapse of the auditory nerve, between the inner hair cells and neurites of type I spiral ganglion neurons, has been suggested to be glutamatergic. The distribution of GLAST was therefore investigated to provide clues to the handling of glutamate in the cochlea. This was studied using light and electron microscopic immunocytochemistry in rats and guinea pigs with antibodies raised against synthetic peptides based on the sequence for GLAST. Significant immunoreactivity was found in the myelin sheath formed by satellite cells surrounding the type I spiral ganglion neurons, and along the plasma membranes of supporting cells around the inner hair cells; other cells in both locations were only weakly labelled, if at all. The absence of substantial numbers of synapses in the spiral ganglion suggests that GLAST is unlikely to be associated with the uptake of synaptic glutamate after release in this region. Immunoreactivity associated with the inner hair cells is consistent with the utilization of glutamate at the afferent synapse.

Expression and function of adenosine receptors in the chinchilla cochlea

Previous studies indicate the presence of adenosine receptors in the cochlear tissues obtained from different animals. This study was initiated to determine the subtypes of adenosine receptor (AR) present in the chinchilla cochlea and to assess their function. Radioligand binding studies demonstrate the presence of both the A1AR and A3AR in membranes prepared from the cochlea, using the radioligands [3H]DPCPX and [125I]APNEA. Estimates of the number (Bmax) of A1AR and A1AR plus A3AR by saturation curves were 118 +/- 13 and 417 +/- 120 fmol/mg, respectively, with the respective equilibrium dissociation constants (Kd) averaging 2.7 +/- 0.2 and 26.3 +/- 13.8 nM. No significant number of A2aAR were detected using [3H]CGS21680. The nonhydrolyzable adenosine analog R-phenylisopropyladenosine (R-PIA, 1 microM) elicited a small but significant degree of inhibition of forskolin-stimulated adenylyl cyclase activity (10.4 +/- 2.5%) in cochlear membrane preparations, which was insensitive to blockade by theophylline (100 microM). Furthermore, R-PIA elicited an increase in inositol 1,4,5-trisphosphate production in dissociated cell preparations obtained from the cochlea. No significant effect of R-PIA was observed on auditory measures such as auditory brainstem evoked response, cochlear action potential and endocochlear potential following round window application. However, round window application of R-PIA elicited significant increases in the activities of antioxidant enzymes such as superoxide dismutase and glutathione peroxidase and significantly reduced the levels of malondialdehyde, a marker of lipid peroxidation. These results suggest a potential cytoprotective role of adenosine in the cochlea against oxidative damage.

Aspartate is enriched in sensory cells and subpopulations of non-neuronal cells in the guinea pig inner ear: a quantitative immunoelectron microscopic analysis

The cellular and subcellular localization of aspartate in the guinea pig inner ear was studied by means of quantitative postembedding cytochemistry. High levels of aspartate immunoreactivity were found in neuronal compartments, including all types of hair cells, and afferent and efferent nerve fibers. In addition, sub-basilar tympanic cells, interdental cells, and the mesothelial cells in the Reissner's membrane were strongly immunoreactive. The present immunocytochemical results are consistent with the idea that aspartate is involved in neurotransmission in the inner ear, but also point to possible metabolic roles of aspartate.

Organization of AMPA receptor subunits at a glutamate synapse: a quantitative immunogold analysis of hair cell synapses in the rat organ of Corti

Sensitive and high-resolution immunocytochemical procedures were used to investigate the spatial organization of AMPA receptor subunits (GluR1-4) at the synapse between the inner hair cells and the afferent dendrites in the rat organ of Corti. This is a synapse with special functional properties and with a presynaptic dense body that defines the center of the synapse and facilitates its morphometric analysis. A quantitative postembedding immunocytochemical analysis was performed on specimens that had been embedded in a metachrylate resin at low temperature after freeze substitution. Single- and double-labeling procedures indicated that GluR2/3 and GluR4 subunits were colocalized throughout the postsynaptic density, with a maximum distance of 300 nm from the presynaptic body and with higher concentrations peripherally than centrally. No receptor immunolabeling was found at extrasynaptic membranes, but some GluR4 subunits appeared to be expressed presynaptically. The synapses between outer hair cells and afferent dendrites were devoid of labeling. The present data indicate that AMPA receptor subunits are inserted into the postsynaptic membrane in a very precise manner and that their density increases on moving away from the center of the synapse.

High-resolution immunogold localization of AMPA type glutamate receptor subunits at synaptic and non-synaptic sites in rat hippocampus

The cellular and subcellular localization of the GluRA, GluRB/C and GluRD subunits of the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) type glutamate receptor was determined in the rat hippocampus using polyclonal antipeptide antibodies in immunoperoxidase and immunogold procedures. For the localization of the GluRD subunit a new polyclonal antiserum was developed using the C-terminal sequence of the protein (residues 869-881), conjugated to carrier protein and absorbed to colloidal gold for immunization. The purified antibodies immunoprecipitated about 25% of 3[H]AMPA binding activity from the hippocampus, cerebellum or whole brain, but very little from neocortex. These antibodies did not precipitate a significant amount of 3[H]kainate binding activity. The antibodies also recognize the GluRD subunit, but not the other AMPA receptor subunits, when expressed in transfected COS-7 cells and only when permeabilized with detergent, indicating an intracellular epitope. All subunits were enriched in the neuropil of the dendritic layers of the hippocampus and in the molecular layer of the dentate gyrus. The cellular distribution of the GluRD subunit was studied more extensively. The strata radiatum, oriens and the dentate molecular layer were more strongly immunoreactive than the stratum lacunosum moleculare, the stratum lucidum and the hilus. However, in the stratum lucidum of the CA3 area and in the hilus the weakly reacting dendrites were surrounded by immunopositive rosettes, shown in subsequent electron microscopic studies to correspond to complex dendritic spines. In the stratum radiatum, the weakly reacting apical dendrites contrasted with the surrounding intensely stained neuropil. The cell bodies of pyramidal and granule cells were moderately reactive. Some non-principal cells and their dendrites in the pyramidal cell layer and in the alveus also reacted very strongly for the GluRD subunit. At the subcellular level, silver intensified immunogold particles for the GluRA, GluRB/C and GluRD subunits were present at type 1 synaptic membrane specializations on dendritic spines of pyramidal cells throughout all layers of the CA1 and CA3 areas. The most densely labelled synapses tended to be on the largest spines and many smaller spines remained unlabelled. Immunoparticle density at type 1 synapses on dendritic shafts of some non-principal cells was consistently higher than at labelled synapses of dendritic spines of pyramidal cells. Synapses established between dendritic spines and mossy fibre terminals, were immunoreactive for all studied subunits in stratum lucidum of the CA3 area. The postembedding immunogold method revealed that the AMPA type receptors are concentrated within the main body of the anatomically defined type 1 (asymmetrical) synaptic junction. Often only a part of the membrane specialization showed clustered immunoparticles. There was a sharp decrease in immunoreactive receptor density at the edge of the synaptic specialization. Immunolabelling was consistently demonstrated at extrasynaptic sites on dendrites, dendritic spines and somata. The results demonstrate that the GluRA, B/C and D subunits of the AMPA type glutamate receptor are present in many of the glutamatergic synapses formed by the entorhinal, CA3 pyramidal and mossy fibre terminals. Some interneurons have a higher density of AMPA type receptors in their asymmetrical afferent synapses than pyramidal cells. This may contribute to a lower activation threshold of interneurons as compared to principal cells by the same afferents in the hippocampal formation.