Group i metabotropic glutamate receptors in spiral ganglion neurons contribute to excitatory neurotransmissions in the cochlea

G protein-coupled receptors in cochlea: Potential therapeutic targets for hearing loss

The prevalence of hearing loss-related diseases caused by different factors is increasing worldwide year by year. Currently, however, the patient’s hearing loss has not been effectively improved. Therefore, there is an urgent need to adopt new treatment measures and treatment techniques to help improve the therapeutic effect of hearing loss. G protein-coupled receptors (GPCRs), as crucial cell surface receptors, can widely participate in different physiological and pathological processes, particularly play an essential role in many disease occurrences and be served as promising therapeutic targets. However, no specific drugs on the market have been found to target the GPCRs of the cochlea. Interestingly, many recent studies have demonstrated that GPCRs can participate in various pathogenic process related to hearing loss in the cochlea including heredity, noise, ototoxic drugs, cochlear structure, and so on. In this review, we comprehensively summarize the functions of 53 GPCRs known in the cochlea and their relationships with hearing loss, and highlight the recent advances of new techniques used in cochlear study including cryo-EM, AI, GPCR drug screening, gene therapy vectors, and CRISPR editing technology, as well as discuss in depth the future direction of novel GPCR-based drug development and gene therapy for cochlear hearing loss. Collectively, this review is to facilitate basic and (pre-) clinical research in this area, and provide beneficial help for emerging GPCR-based cochlear therapies.

Diverse identities and sites of action of cochlear neurotransmitters

Accurate encoding of acoustic stimuli requires temporally precise responses to sound integrated with cellular mechanisms that encode the complexity of stimuli over varying timescales and orders of magnitude of intensity. Sound in mammals is initially encoded in the cochlea, the peripheral hearing organ, which contains functionally specialized cells (including hair cells, afferent and efferent neurons, and a multitude of supporting cells) to allow faithful acoustic perception. To accomplish the demanding physiological requirements of hearing, the cochlea has developed synaptic arrangements that operate over different timescales, with varied strengths, and with the ability to adjust function in dynamic hearing conditions. Multiple neurotransmitters interact to support the precision and complexity of hearing. Here, we review the location of release, action, and function of neurotransmitters in the mammalian cochlea with an emphasis on recent work describing the complexity of signaling.

Anatomy and Physiology of Metabotropic Glutamate Receptors in Mammalian and Avian Auditory System

Glutamate, as the major excitatory neurotransmitter used in the vertebrate brain, activates ionotropic and metabotropic glutamate receptors (iGluRs and mGluRs), which mediate fast and slow neuronal actions, respectively. mGluRs play important modulatory roles in many brain areas, forming potential targets for drugs developed to treat brain disorders. Here, we review studies on mGluRs in the mammalian and avian auditory system. Although anatomical expression of mGluRs in the cochlear nucleus has been well characterized, data for other auditory nuclei await more systematic investigations especially at the electron microscopy level. The physiology of mGluRs has been extensively studied using in vitro brain slice preparations, with a focus on the auditory circuitry in the brainstem. These in vitro physiological studies have demonstrated that mGluRs participate in synaptic transmission, regulate ionic homeostasis, induce synaptic plasticity, and maintain the balance between Excitation and Inhibition (E/I) in a variety of auditory structures. However, the modulatory roles of mGluRs in auditory processing remain largely unclear at the system and behavioral levels, and the functions of mGluRs in auditory disorders remain entirely unknown.

Acoustic Trauma Changes the Parvalbumin-Positive Neurons in Rat Auditory Cortex

Acoustic trauma is being reported to damage the auditory periphery and central system, and the compromised cortical inhibition is involved in auditory disorders, such as hyperacusis and tinnitus. Parvalbumin-containing neurons (PV neurons), a subset of GABAergic neurons, greatly shape and synchronize neural network activities. However, the change of PV neurons following acoustic trauma remains to be elucidated. The present study investigated how auditory cortical PV neurons change following unilateral 1 hour noise exposure (left ear, one octave band noise centered at 16 kHz, 116 dB SPL). Noise exposure elevated the auditory brainstem response threshold of the exposed ear when examined 7 days later. More detectable PV neurons were observed in both sides of the auditory cortex of noise-exposed rats when compared to control. The detectable PV neurons of the left auditory cortex (ipsilateral to the exposed ear) to noise exposure outnumbered those of the right auditory cortex (contralateral to the exposed ear). Quantification of Western blotted bands revealed higher expression level of PV protein in the left cortex. These findings of more active PV neurons in noise-exposed rats suggested that a compensatory mechanism might be initiated to maintain a stable state of the brain.

mGluR1 enhances efferent inhibition of inner hair cells in the developing rat cochlea

KEY POINTS

Spontaneous activity of the sensory inner hair cells shapes maturation of the developing ascending (afferent) auditory system before hearing begins. Just before the onset of hearing, descending (efferent) input from cholinergic neurons originating in the brainstem inhibit inner hair cell spontaneous activity and may further refine maturation. We show that agonist activation of the group I metabotropic glutamate receptor mGluR1 increases the strength of this efferent inhibition by enhancing the presynaptic release of acetylcholine. We further show that the endogenous release of glutamate from the inner hair cells may increase the strength of efferent inhibition via the activation of group I metabotropic glutamate receptors. Thus, before the onset of hearing, metabotropic glutamate signalling establishes a local negative feedback loop that is positioned to regulate inner hair cell excitability and refine maturation of the auditory system.

ABSTRACT

Just before the onset of hearing, the inner hair cells (IHCs) receive inhibitory efferent input from cholinergic medial olivocochlear (MOC) neurons originating in the brainstem. This input may serve a role in the maturation of the ascending (afferent) auditory system by inhibiting spontaneous activity of the IHCs. To investigate the molecular mechanisms regulating these IHC efferent synapses, we combined electrical stimulation of the efferent fibres with patch clamp recordings from the IHCs to measure efferent synaptic strength. By examining evoked responses, we show that activation of metabotropic glutamate receptors (mGluRs) by general and group I-specific mGluR agonists enhances IHC efferent inhibition. This enhancement is blocked by application of a group I mGluR1-specific antagonist, indicating that enhancement of IHC efferent inhibition is mediated by group I mGluRs and specifically by mGluR1s. By comparing spontaneous and evoked responses, we show that group I mGluR agonists act presynaptically to increase neurotransmitter release without affecting postsynaptic responsiveness. Moreover, endogenous glutamate released from the IHCs also enhances IHC efferent inhibition via the activation of group I mGluRs. Finally, immunofluorescence analysis indicates that the efferent terminals are sufficiently close to IHC glutamate release sites to allow activation of mGluRs on the efferent terminals by glutamate spillover. Together, these results suggest that glutamate released from the IHCs activates group I mGluRs (mGluR1s), probably present on the efferent terminals, which, in turn, enhances release of acetylcholine and inhibition of the IHCs. Thus, mGluRs establish a local negative feedback loop positioned to regulate IHC activity and maturation of the ascending auditory system in the developing cochlea.

GluA2-Containing AMPA Receptors Distinguish Ribbon-Associated from Ribbonless Afferent Contacts on Rat Cochlear Hair Cells

Mechanosensory hair cells release glutamate at ribbon synapses to excite postsynaptic afferent neurons, via AMPA-type ionotropic glutamate receptors (AMPARs). However, type II afferent neurons contacting outer hair cells in the mammalian cochlea were thought to differ in this respect, failing to show GluA immunolabeling and with many “ribbonless” afferent contacts. Here it is shown that antibodies to the AMPAR subunit GluA2 labeled afferent contacts below inner and outer hair cells in the rat cochlea, and that synaptic currents in type II afferents had AMPAR-specific pharmacology. Only half the postsynaptic densities of type II afferents that labeled for PSD-95, Shank, or Homer were associated with GluA2 immunopuncta or presynaptic ribbons, the “empty slots” corresponding to ribbonless contacts described previously. These results extend the universality of AMPAergic transmission by hair cells, and support the existence of silent afferent contacts.

Metabotropic glutamate receptors in auditory processing

As the major excitatory neurotransmitter used in the vertebrate brain, glutamate activates ionotropic and metabotropic glutamate receptors (mGluRs), which mediate fast and slow neuronal actions, respectively. Important modulatory roles of mGluRs have been shown in many brain areas, and drugs targeting mGluRs have been developed for the treatment of brain disorders. Here, I review studies on mGluRs in the auditory system. Anatomical expression of mGluRs in the cochlear nucleus has been well characterized, while data for other auditory nuclei await more systematic investigations at both the light and electron microscopy levels. The physiology of mGluRs has been extensively studied using in vitro brain slice preparations, with a focus on the lower auditory brainstem in both mammals and birds. These in vitro physiological studies have revealed that mGluRs participate in neurotransmission, regulate ionic homeostasis, induce synaptic plasticity, and maintain the balance between excitation and inhibition in a variety of auditory structures. However, very few in vivo physiological studies on mGluRs in auditory processing have been undertaken at the systems level. Many questions regarding the essential roles of mGluRs in auditory processing still remain unanswered and more rigorous basic research is warranted.

TRPC3 ion channel subunit immunolocalization in the cochlea

Canonical transient receptor potential (TRPC) subunits assemble as tetramers to form ion channels with high calcium (Ca(2+)) permeability. Here, we investigated the possibility that TRPC3 ion channels are broadly expressed in the adult guinea pig and mouse cochleae. Using immunofluorescence, pronounced labeling occurred in the spiral ganglion (SG) neurons, inner hair cells (IHC), outer hair cells (OHC) and epithelial cells lining scala media. TRPC3 expression was homogeneous in the SG throughout the cochlea. In contrast, there was marked spatial variation in the immunolabeling in the cochlear hair cells with respect to location. This likely relates to the tonotopy of these cells. TRPC3 immunolabeling was more pronounced in the IHC than OHC. Both basal region IHC and OHC had higher TRPC3 expression levels than the corresponding cells from the apical region of the cochlea. These data suggest that TRPC3 ion channels contribute to Ca(2+) homeostasis associated with the hair cells, with higher ion fluxes in more basal regions of the cochlea, and may also be a significant pathway for Ca(2+) entry associated with auditory neurotransmission via the SG neurons. TRPC3 expression was also identified within the spiral limbus region, inner and outer sulcus, but without evidence for spatial variation in expression level. Expression in these gap junction-coupled epithelial cells lining scala media is indicative of a contribution of TRPC3 channels to cochlear electrochemical homeostasis.

Loss of GABAB receptors in cochlear neurons: threshold elevation suggests modulation of outer hair cell function by type II afferent fibers

Despite pharmacological and immunohistochemical evidence for GABA as a neurotransmitter in the olivocochlear efferent bundle, a clear functional role of GABA in the inner ear has not emerged. To explore the role of metabotropic GABA(B) receptors, we characterized the cochlear phenotype of mice with targeted deletion of the GABA(B1) subunit and determined its tissue localization using a mouse line expressing a GFP-tagged GABA(B1) subunit under the endogenous promoter. Immunostaining revealed GABA(B1) expression in both type I and type II ganglion cells and in their synaptic terminals under inner and outer hair cells, respectively. No GABA(B1) expression was observed in hair cells. Mean cochlear thresholds, measured via both auditory brainstem responses and distortion product otoacoustic emissions (DPOAEs), were elevated by approximately 10 dB in GABA(B1)-deficient mice, consistent with outer hair cell dysfunction. Olivocochlear efferent function, assessed via DPOAE suppression during efferent electrical stimulation, was unaffected by GABA(B1) deletion. GABA(B1)-deficient mice showed increased resistance to permanent acoustic injury, with mean threshold shifts approximately 25 dB smaller than wild-types after exposure to 8-16-kHz noise at 100 dB for 2 h. In contrast, there was no vulnerability difference to temporary acoustic injury following exposure to the same noise at 94 dB for 15 min. Our results suggest that GABAergic signaling in type II afferent neurons may be required for normal outer hair cell amplifier function at low sound levels and may also modulate outer hair cell responses to high-level sound.

Ca2+ entry via AMPA-type glutamate receptors triggers Ca2+-induced Ca2+ release from ryanodine receptors in rat spiral ganglion neurons

Ryanodine receptor (RyR)-gated Ca2+ stores have recently been identified in cochlear spiral ganglion neurons (SGN) and likely contribute to Ca2+ signalling associated with auditory neurotransmission. Here, we identify an ionotropic glutamate receptor signal transduction pathway which invokes RyR-gated Ca2+ stores in SGN via Ca2+-induced Ca2+ release (CICR). Ca2+ levels were recorded in SGN in situ within rat cochlear slices (postnatal day 0-17) using the Ca2+ indicator fluo-4. RyR-gated Ca2+ stores were confirmed by caffeine-induced increases in intracellular Ca2+ which were blocked by ryanodine (100 microM) and were independent of external Ca2+. Glutamate evoked comparable increases in intracellular Ca2+, but required the presence of external Ca2+. Ca2+ influx via the glutamate receptor was found to elicit CICR via RyR-gated Ca2+ stores, as shown by the inhibition of the response by prior depletion of the Ca2+ stores with caffeine, the SERCA inhibitor thapsigargin, or ryanodine. The glutamate analogue AMPA (alpha-Amino-3-hydroxy-5-methylisoxazole-4-propionic acid) elicited Ca2+ responses that could be inhibited by caffeine. Glutamate- and AMPA-mediated Ca2+ responses were eliminated with the AMPA/Kainate receptor antagonist DNQX (6,7-dinitroquinoxaline-2,3-dione). These data demonstrate functional coupling between somatic AMPA-type glutamate receptors and intracellular Ca(2+) stores via RyR-dependent CICR in primary auditory neurons.

Peripheral metabotropic glutamate receptor 5 mediates mechanical hypersensitivity in craniofacial muscle via protein kinase C dependent mechanisms

We previously demonstrated that peripherally located N-methyl-D-aspartic acid (NMDA) receptors contribute to acute muscle nociception and the development of chronic muscular hyperalgesia. In the present study, we investigated the potential role of peripheral group I metabotropic glutamate receptors (mGluRs 1/5) in the development of muscular hypersensitivity to mechanical stimulation, and attempted to elucidate intracellular signaling mechanisms associated with the mGluR activation in male Sprague-Dawley rats. First, our Western blot analyses revealed that mGluR 5 protein, but not mGluR 1 protein, is reliably detected in trigeminal ganglia and the masseter nerve. Subsequent behavioral studies demonstrated that the group I mGluR agonist, R,S-3,5-dihydroxyphenylglycol (DHPG), significantly decreased the mechanical threshold to noxious stimulation of the masseter, and that the DHPG-induced mechanical hypersensitivity can be effectively prevented by pretreatment of the masseter with 2-methyl-6-(phenylethynyl)pyridine hydrochloride (MPEP), a selective mGluR 5 antagonist, but not by 7-(hydroxyimino)cyclopropa[b]chromen-1a-carboxylate ethyl ester (CPCCOEt), a selective mGluR 1 antagonist. Moreover, the DHPG-induced mechanical hypersensitivity was significantly blocked by inhibiting either the alpha or epsilon isoform of protein kinase C (PKC). Collectively, these data provide evidence that peripherally located mGluR 5 may play an important role in the development of masseter hypersensitivity, and that PKC activation is required for the modulatory effect of peripheral mGluR 5 in the craniofacial muscle tissue. Thus, selective targeting of peripheral mGluR 5 and PKCalpha, as well as PKCepsilon, might serve as an effective therapeutic strategy in the management of chronic muscle pain conditions, such as temporomandibular disorders.

Connexin29 is highly expressed in cochlear Schwann cells, and it is required for the normal development and function of the auditory nerve of mice

Connexins (Cxs) are a family of protein subunits constituting gap junctions, which facilitate exchanges of molecules important for cellular signaling and metabolic activities intercellularly or between different regions of the cytoplasm in the same cells. Mutations in Cxs are the major cause of nonsyndromic childhood deafness, which are mostly found in Cx26 and Cx30 expressed in cochlear supporting cells and fibrocytes. So far, little is known about the functional contribution of Cxs in other types of cochlear cells. Here, we show that Cx29 was highly expressed in the cochlea. The developmental expression time course of Cx29 was similar to that of a myelin marker [myelin associate glycoprotein (MAG)]. Immunolabeling identified Cx29 exclusively in the Schwann cells myelinating the soma and fiber of spiral ganglion (SG) neurons. The absence of the Cx29 gene in mice (Cx29(-/-) mice), with a penetrance of approximately 50%, caused a delay in the maturation of hearing thresholds, an early loss of high-frequency sensitivities, a prolongation in latency and distortion in the wave I of the auditory brainstem responses, and elevated sensitivity to noise damages. The morphology of sensory hair cells and otoacoustic emissions that depend on the integrity of hair cells were normal in Cx29(-/-) mice. In contrast, decreases in MAG expression and severe demyelination at the soma of SG neurons were found in Cx29(-/-) mice. Our findings demonstrated the requirement of Cx29 for normal cochlear functions and suggest that Cx29 is a new candidate gene for studying the auditory neuropathy.

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.

Uso do acamprosato no tratamento do zumbido: um estudo duplo-cego

O tratamento do zumbido é, ainda nos dias de hoje, um grande desafio para os otorrinolaringologistas. Várias lacunas persistem em sua fisiopatologia, tendo como resultado vários tipos de tratamento, com resultados muito irregulares. O acamprosato é uma droga utilizada no tratamento do alcoolismo, devido à sua ação reguladora da transmissão glutamatérgica e GABA-érgica, nunca tendo sido empregado no tratamento do zumbido. OBJETIVO: Avaliar a segurança e eficácia do uso do acamprosato, no tratamento do zumbido de causa neurossensorial. FORMA DE ESTUDO: ensaio clinico randomizado. MATERIAL E MÉTODO: 50 pacientes com zumbido de causa neurossensorial foram divididos em 2 grupos, 25 recebendo acamprosato e 25 placebo por 3 meses, em um estudo prospectivo duplo-cego, sendo analisados os efeitos terapêuticos e efeitos colaterais, de acordo com escala (nota) de 1 a 10, atribuída pelo próprio paciente. RESULTADOS: Foi observado algum grau de melhora sintomatológica em 86,9% dos pacientes, sendo que em 47,8% dos casos observamos melhora superior a 50%, dados estatisticamente significativos em relação ao placebo. A incidência de efeitos colaterais encontrada foi baixa (12%) e de intensidade leve, com boa tolerabilidade geral. CONCLUSÃO: O acamprosato, medicação utilizada no tratamento do alcoolismo, é eficaz e seguro para o tratamento do zumbido de causa neurossensorial, com percentual de melhora superior à maioria das medicações utilizadas para o tratamento do zumbido, constituindo uma excelente alternativa terapêutica.

Tinnitus treatment with acamprosate: double-blind study

Nowadays, the treatment of tinnitus is still a great challenge for the otolaryngologists. Many facts remain unknown in its pathophysiology, leading to many different therapies, with irregular results. Acamprosate is a drug used in alcoholism treatment, due to its regulating effects in glutamatergic and GABA neurotransmission, and has never been used before in the treatment of tinnitus Aim: To evaluate efficacy and safety of the acamprosate in the treatment of sensorineural tinnitus. Study design: randomized clinical trial. Material and Method: 50 patients with sensorineural tinnitus were divided into two groups: 25 received acamprosate and 25 placebo, for a period of 3 months, in a prospective double-blind study, being analyzed for its efficacy and safety by the subjective score from 1 to 10 given by the patient. Results: We found a high index of success in the relief of tinnitus, about 86.9%. In 47.8% of the cases we found more than 50% relief. The incidence of side effects was low, 12%, all of them mild. Conclusion: Acamprosate, a drug used in the treatment of alcoholism, is a safe and successful alternative for sensorineural tinnitus’ treatment.

Immunocytochemical and pharmacological characterization of metabotropic glutamate receptors of the vestibular end organs in the frog

Using immunocytochemistry and multiunit recording of afferent activity of the whole vestibular nerve, we investigated the role of metabotropic glutamate receptors (mGluR) in the afferent neurotransmission in the frog semicircular canals (SCC). Group I (mGluR1alpha) and group II (mGluR2/3) mGluR immunoreactivities were distributed to the vestibular ganglion neurons, and this can be attributed to a postsynaptic locus of metabotropic regulation of rapid excitatory transmission. The effects of group I/II mGluR agonist (1S,3R)-1-aminocyclopentane-trans-1,3-dicarboxylic acid (ACPD) and antagonist (R,S)-alpha-methyl-4-carboxyphenylglycine (MCPG) on resting and chemically induced afferent activity were studied. ACPD (10-100 microM) enhanced the resting discharge frequency. MCPG (5-100 microM) led to a concentration-dependent decrease of both resting activity and ACPD-induced responses. If the discharge frequency had previously been restored by L-glutamate (L-Glu) in high-Mg2+ solution, ACPD elicited a transient increase in the firing rate in the afferent nerve suggesting that ACPD acts on postsynaptic receptors. The L-Glu agonists, alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) and N-methyl-D-aspartate (NMDA), were tested during application of ACPD. AMPA- and NMDA-induced responses were higher in the presence than absence of ACPD, implicating mGluR in the modulation of ionotropic glutamate receptors. These results indicate that activation of mGluR potentiates AMPA and NMDA responses through a postsynaptic interaction. We conclude that ACPD may exert modulating postsynaptic effects on vestibular afferents and that this process is activity-dependent.