Vesicular Glutamatergic Transmission in Noise-Induced Loss and Repair of Cochlear Ribbon Synapses

Phenotypic changes of auditory nerve fibers after excitotoxicity

There is a substantial body of evidence elucidating the pathophysiological aspects of excitotoxicity in the mammalian cochlea. However, the question of whether the resultant damage is reversible remains unresolved. To replicate an excitotoxic event, we investigated the long-term effects of kainate application in gerbil cochleae. Surprisingly, despite persistent synapse loss, the compound action potential of the auditory nerve fully recovered. This functional retrieval was associated with a phenotypic change in auditory nerve fibers. Thresholds were improved along the tonotopic axis. High-spontaneous rate (SR) fibers largely populated the apical region, while low-SR fibers from the basal region exhibited sound-driven activity indistinguishable from control high-SR fibers. This functional phenotype change may support the full recovery of neural response thresholds and amplitudes after excitotoxicity. Furthermore, hyperresponsiveness of the auditory nerve fibers could be a crucial factor in the development of hyperactivity in the central auditory pathways, a common occurrence following acoustic overstimulation.

Female mice lacking GluA3 show early onset of hearing loss, cochlear synaptopathy, and afferent terminal swellings in ambient sound levels

AMPA-type glutamate receptors (AMPARs) mediate excitatory cochlear transmission. However, unique roles of AMPAR subunits are unresolved. Lack of subunit GluA3 (Gria3 KO ) in male mice reduced cochlear output by 8 postnatal weeks. Here, we studied the role of X-linked Gria3 in cochlear function and synapse anatomy in females. Auditory brainstem responses (ABRs) were similar in 3-week-old female Gria3 WT and Gria3 KO mice raised in quiet. However, after switching to ambient sound, ABR thresholds were elevated and wave-1 amplitudes were diminished at 5-week and older in Gria3 KO . A quiet vivarium precluded this effect. Paired synapses were similar in number, but lone ribbons and ribbonless synapses were more frequent, and swollen afferent terminals were observed only in female Gria3 KO mice in ambient sound. Synaptic GluA4:GluA2 ratios increased relative to Gria3 WT , particularly in ambient sound, suggesting an activity-dependent increase in calcium-permeable AMPARs in Gria3 KO . We propose that lack of GluA3 induces a sex-dependent vulnerability to AMPAR-mediated excitotoxicity.

Neural and behavioral binaural hearing impairment and its recovery following moderate noise exposure

Noise-induced cochlear synaptopathy has been studied for over 25 years with no known diagnosis for this disorder in humans. This type of "hidden hearing loss" induces a loss of synapses in the inner ear but no change in audiometric thresholds. Recent studies have shown that by two months post synaptopathy-inducing noise exposure, synapses in some animal species can regenerate. Animal studies to date have focused primarily on peripheral hearing measures to diagnose ribbon synapse loss, while suggesting binaural listening deficits such as speech-reception-in-noise result from this disorder, but haven't accounted for the possible regeneration of synapses. To address this, we measured binaural physiological and behavioral function, the latter utilizing the pre-pulse inhibition of acoustic startle method, in both male and female adult guinea pigs following exposure to noise that has been shown to induce cochlear synaptopathy. Physiological measurements extended to 2 months post noise exposure to characterize any deficit and subsequent recovery. While common audiological assessments showed temporary threshold shift, reduced evoked potential amplitudes indicative of synaptopathy and measurable binaural electrophysiological hearing deficits post exposure, all measures recovered by 2 months. Suspected regeneration of synaptic ribbons occurred by 2 months post exposure and cochlear histology revealed no synaptic loss 4 months post exposure. Our results show that the same noise exposure protocol demonstrated to cause synaptic loss in prior studies causes physiological binaural processing deficits in the brainstem and that the recovery of neural binaural processing coincides with the regeneration of synapses shown in previous studies and normal binaural hearing behavior.

Noise-induced ribbon synapse loss in the mouse basal cochlear region does not reduce inner hair cell exocytosis

Noise-induced hearing loss is one of the most common forms of hearing loss in adults and also one of the most common occupational diseases. Extensive previous work has shown that the highly sensitive synapses of the inner hair cells (IHCs) may be the first target for irreparable damage and permanent loss in the noise-exposed cochlea, more precisely in the cochlear base. However, how such synaptic loss affects the synaptic physiology of the IHCs in this particularly vulnerable part of the cochlea has not yet been investigated. To address this question, we exposed 3-4-week-old C57BL/6J mice to 8-16 kHz noise for 2 h under isoflurane anesthesia. We then employed hearing measurements, immunohistochemistry and patch-clamp to assess IHC synaptic function. Two noise sound pressure levels (SPLs) were used to evoke acute hearing threshold elevations with different levels of recovery 2 weeks post-exposure. Regardless of noise intensity, the exposure resulted in a loss of approximately 25-36% of ribbon synapses in the basal portions of the cochlea that persisted 2 weeks after exposure. Perforated patch-clamp recordings were made in the IHCs of the basal regions of the cochlea where the greatest synaptic losses were observed. Depolarization-evoked calcium currents in IHCs 2 weeks after exposure were slightly but not significantly smaller as compared to controls from age-matched non-exposed animals. Exocytic changes monitored as changes in membrane capacitance did not follow that trend and remained similar to controls despite significant loss of ribbons, likely reflecting increased exocytosis at the remaining synapses. Additionally, we report for the first time that acute application of isoflurane reduces IHC calcium currents, which may have implications for noise-induced IHC synaptic loss.

CaBP1 and 2 enable sustained CaV1.3 calcium currents and synaptic transmission in inner hair cells

To encode continuous sound stimuli, the inner hair cell (IHC) ribbon synapses utilize calcium-binding proteins (CaBPs), which reduce the inactivation of their CaV1.3 calcium channels. Mutations in the CABP2 gene underlie non-syndromic autosomal recessive hearing loss DFNB93. Besides CaBP2, the structurally related CaBP1 is highly abundant in the IHCs. Here, we investigated how the two CaBPs cooperatively regulate IHC synaptic function. In Cabp1/2 double-knockout mice, we find strongly enhanced CaV1.3 inactivation, slowed recovery from inactivation and impaired sustained exocytosis. Already mild IHC activation further reduces the availability of channels to trigger synaptic transmission and may effectively silence synapses. Spontaneous and sound-evoked responses of spiral ganglion neurons in vivo are strikingly reduced and strongly depend on stimulation rates. Transgenic expression of CaBP2 leads to substantial recovery of IHC synaptic function and hearing sensitivity. We conclude that CaBP1 and 2 act together to suppress voltage- and calcium-dependent inactivation of IHC CaV1.3 channels in order to support sufficient rate of exocytosis and enable fast, temporally precise and indefatigable sound encoding.

The therapeutic effect and underlying biochemical mechanism of methylprednisolone and D-methionine in "rescuing" noise-induced hearing loss in guinea pigs

OBJECTIVES

Currently, there are no approved therapeutics for noise-induced hearing loss (NIHL). Both oxidative stress and cochlear inflammation play important roles in the mechanism of NIHL. In this study, we evaluate the effect of D-methionine (D-met) and methylprednisolone (MP) on noise-induced hearing loss of guinea pigs.

DESIGN

One hundred and thirty-two male guinea pigs were evenly divided into eleven groups: control, saline, MP (15, 30, 45 mg/kg), D-met (200, 400, 600 mg/kg), and combinations of MP (15, 30, 45 mg/kg) with D-met (200, 400, 600 mg/kg) in increasing doses. Sixty minutes following a 6-hour exposure to continuous broadband white noise at a sound pressure level of 105 ± 2 dB, treatments were given every 12 h over the course of 3 days. Click-evoked auditory brainstem responses were evaluated one day before and fourteen days after noise exposure. The animals in the combination group were sacrificed 14 days after noise exposure, and cochlear lateral wall Na+, K+-ATPase and Ca2+-ATPase activities, and lipid peroxidation (LPO) were evaluated.

RESULTS

The mean permanent threshold shift (PTS) showed a dose-dependent rescue effect from low to high doses in both MP and D-met treatment groups. In the combination treatment groups, MP (45 mg/kg) with D-met (600 mg/kg) demonstrated a complete rescue response without a significant difference in PTS compared to the control group. The noise-induced decreases in Na+, K+-ATPase and Ca2+-ATPase activities demonstrated dose-dependent recoveries from the low to high dose combination groups. Specifically, the MP (45 mg/kg) with D-met (600 mg/kg) group achieved 84.8% and 95.5% recovery of Na+, K+-ATPase and Ca2+-ATPase activity levels, respectively, compared to the control group. The noise-induced increase in LPO levels exhibited dose-dependent alleviation from the low to high dose combination groups, showing only a 12.3% LPO increment in the MP (45 mg/kg) with D-met (600 mg/kg) group.

CONCLUSIONS

Noise-induced hearing loss was completely rescued in the MP (45 mg/kg) with D-met (600 mg/kg) treatment group. Significant decreases in cochlear lateral wall oxidative stress were demonstrated, along with the reversal of Na+, K+-ATPase and Ca2+-ATPase activity levels.

Local delivery of soluble fractalkine (CX3CL1) peptide restores ribbon synapses after noise-induced cochlear synaptopathy

Cochlear ribbon synapses between sensory inner hair cells (IHCs) and spiral ganglion neurons (SGNs) are vulnerable to rapid and primary damage and/or loss due to noise overexposure. Such damaged ribbon synapses can repair spontaneously in mouse and guinea pig. However, the mechanisms for synaptic repair are unclear. Previously, we have demonstrated a critical role for the fractalkine signaling axis (CX3CL1-CX3CR1) in synaptic repair, wherein noise-damaged ribbon synapses are spontaneously repaired in the presence of fractalkine receptor (CX3CR1) expressed by cochlear macrophages. Here, we examined whether local administration of chemokine fractalkine ligand (CX3CL1 or FKN) in the form of a peptide is effective in restoring synapses and hearing loss after noise-induced cochlear synaptopathy (NICS). Specifically, the efficacy of different isoforms of FKN was evaluated for restoration of loss of IHC ribbon synapses and hearing after NICS. A single transtympanic injection of soluble isoform of FKN (sFKN) peptide at 1 day after synaptopathic noise trauma for 2 hours at 93 decibel sound pressure level led to significant recovery of auditory brainstem response (ABR) thresholds, ABR peak I amplitudes and ribbon synapses in FKN knockout mice when compared to mice injected with membrane-bound FKN peptide (mFKN). Likewise, local treatment with sFKN peptide in FKN wild type mice restored synaptopathic noise-damaged ribbon synapses and ABR peak I amplitudes. Mechanistically, FKN regulates macrophage numbers in the damaged cochlea and in the absence of macrophages, sFKN failed to restore loss of synapses and hearing after NICS. Furthermore, sFKN treatment attenuated cochlear inflammation after NICS without altering the expression of CX3CR1. Finally, injected sFKN peptide was detectable inside the cochlea for 24 h localized to the basilar membrane and spiral lamina near the sensory epithelium. These data provide a proof-of-principle that local delivery of an immune factor, sFKN is effective in restoring ribbon synapses and hearing loss after NICS in a macrophage-dependent manner and highlights the potential of sFKN as an immunotherapy for cochlear synaptopathy due to noise.

Endolymphatic hydrops and cochlear synaptopathy after noise exposure are distinct sequelae of hair cell stereociliary bundle trauma

Endolymphatic hydrops, increased endolymphatic fluid within the cochlea, is the key pathologic finding in patients with Meniere's disease, a disease of episodic vertigo, fluctuating hearing loss, tinnitus, and aural fullness. Endolymphatic hydrops also can occur after noise trauma and its presence correlates with cochlear synaptopathy, a form of hearing loss caused by reduced numbers of synapses between hair cells and auditory nerve fibers. Here we tested whether there is a mechanistic link between these two phenomena by using multimodal imaging techniques to analyze the cochleae of transgenic mice exposed to blast and osmotic challenge. In vivo cochlear imaging after blast exposure revealed dynamic increases in endolymph that involved hair cell mechanoelectrical transduction channel block but not the synaptic release of glutamate at the hair cell-auditory nerve synapse. In contrast, ex vivo and in vivo auditory nerve imaging revealed that synaptopathy requires glutamate release from hair cells but not endolymphatic hydrops. Thus, although endolymphatic hydrops and cochlear synaptopathy are both observed after noise exposure, one does not cause the other. They are simply co-existent sequelae that derive from the traumatic stimulation of hair cell stereociliary bundles. Importantly, these data argue that Meniere's disease derives from hair cell transduction channel blockade.

Noise-induced cochlear synaptopathy in C57BL/6 N mice as a function of trauma strength: ribbons are more vulnerable than postsynapses

Noise-induced cochlear synaptopathy is characterized by irreversible loss of synapses between inner hair cells (IHCs) and spiral ganglion neurons (SGNs) despite normal hearing thresholds. We analyzed hearing performance and cochlear structure in C57BL/6 N mice exposed to 100, 106, or 112 dB SPL broadband noise (8-16 kHz) for 2 h. Auditory brainstem responses (ABRs) were assessed before, directly after, and up to 28 days post-trauma. Finally, the number, size, and pairing of IHC presynaptic (CtBP2-positive) ribbons and postsynaptic AMPA receptor scaffold (Homer1-positive) clusters were analyzed along the cochlea. Four weeks after the 100 dB SPL trauma, a permanent threshold shift (PTS) was observed at 45 kHz, which after the higher traumata extended toward middle to low frequencies. Loss in ABR wave I amplitudes scaled with trauma strength indicating loss of functional IHC synaptic connections. Latencies of wave I mostly increased with trauma strength. No trauma-related OHC loss was found. The number of synaptic pairs was reduced in the midbasal and basal cochlear region in all trauma conditions, with ribbon loss amounting up to 46% of control. Ribbons surviving the trauma were paired, whereas 4-6 unpaired postsynapses/IHC were found in the medial, midbasal, and basal regions irrespective of trauma strength, contrasting findings in CBA/CaJ mice. Our data confirm the susceptibility of ribbon synapses and ABR wave I amplitudes to a noise trauma of 100 dB SPL or larger. Notably, peripheral dendrites bearing IHC postsynapses were less vulnerable than presynaptic ribbons in C57BL/6 N mice.

Individual and combined effects of noise exposure and diabetes mellitus on hearing

Hearing loss (HL) is a prevalent health concern with a significant impact on society and the economy. Several factors contribute to the development of hearing impairment, with noise overexposure being the primary culprit. Diabetes mellitus (DM) is also a factor in hearing impairment, and studies have shown a positive correlation between DM and HL; however, the exact causal relationship and pathogenesis remain contentious. Given the ubiquity of noise exposure and the high incidence of DM, individuals may develop diabetes while being chronically exposed to noise. It is particularly important to explore the independent and combined effects of noise and DM on hearing, which can help healthcare professionals understand the potential risks posed by these factors and inspire prevention strategies and potential interventions for hearing impairment. This review summarizes the current research advancements in noise-induced HL and diabetes-related HL and discusses their characteristics and potential mechanisms. Furthermore, this review focuses on the combined effects of noise exposure and DM on hearing, setting the stage for further research and development of intervention strategies to address HL.

Electron Microscopic Mapping of Mitochondrial Morphology in the Cochlear Nerve Fibers

To enable nervous system function, neurons are powered in a use-dependent manner by mitochondria undergoing morphological-functional adaptation. In a well-studied model system-the mammalian cochlea, auditory nerve fibers (ANFs) display distinct electrophysiological properties, which is essential for collectively sampling acoustic information of a large dynamic range. How exactly the associated mitochondrial networks are deployed in functionally differentiated ANFs remains scarcely interrogated. Here, we leverage volume electron microscopy and machine-learning-assisted image analysis to phenotype mitochondrial morphology and distribution along ANFs of full-length in the mouse cochlea inner spiral bundle. This reveals greater variance in mitochondrial size with increased ANF habenula to terminal path length. Particularly, we analyzed the ANF terminal-residing mitochondria, which are critical for local calcium uptake during sustained afferent activities. Our results suggest that terminal-specific enrichment of mitochondria, in addition to terminal size and overall mitochondrial abundance of the ANF, correlates with heterogenous mitochondrial contents of the terminal.

C-Phycocyanin Attenuates Noise-Induced Cochlear Synaptopathy via the Inhibition of Oxidative Stress and Intercellular Adhesion Molecule-1 in the Cochlea

The synapses between inner hair cells (IHCs) and spiral ganglion neurons (SGNs) are the most vulnerable structures in the noise-exposed cochlea. Cochlear synaptopathy results from the disruption of these synapses following noise exposure and is considered the main cause of poor speech understanding in noisy environments, even when audiogram results are normal. Cochlear synaptopathy leads to the degeneration of SGNs if damaged IHC-SGN synapses are not promptly recovered. Oxidative stress plays a central role in the pathogenesis of cochlear synaptopathy. C-Phycocyanin (C-PC) has antioxidant and anti-inflammatory activities and is widely utilized in the food and drug industry. However, the effect of the C-PC on noise-induced cochlear damage is unknown. We first investigated the therapeutic effect of C-PC on noise-induced cochlear synaptopathy. In vitro experiments revealed that C-PC reduced the H2O2-induced generation of reactive oxygen species in HEI-OC1 auditory cells. H2O2-induced cytotoxicity in HEI-OC1 cells was reduced with C-PC treatment. After white noise exposure for 3 h at a sound pressure of 118 dB, the guinea pigs intratympanically administered 5 μg/mL C-PC exhibited greater wave I amplitudes in the auditory brainstem response, more IHC synaptic ribbons and more IHC-SGN synapses according to microscopic analysis than the saline-treated guinea pigs. Furthermore, the group treated with C-PC had less intense 4-hydroxynonenal and intercellular adhesion molecule-1 staining in the cochlea compared with the saline group. Our results suggest that C-PC improves cochlear synaptopathy by inhibiting noise-induced oxidative stress and the inflammatory response in the cochlea.

Cyclic AMP signaling promotes regeneration of cochlear synapses after excitotoxic or noise trauma

Introduction

Cochlear afferent synapses connecting inner hair cells to spiral ganglion neurons are susceptible to excitotoxic trauma on exposure to loud sound, resulting in a noise-induced cochlear synaptopathy (NICS). Here we assessed the ability of cyclic AMP-dependent protein kinase (PKA) signaling to promote cochlear synapse regeneration, inferred from its ability to promote axon regeneration in axotomized CNS neurons, another system refractory to regeneration.

Methods

We mimicked NICS in vitro by applying a glutamate receptor agonist, kainic acid (KA) to organotypic cochlear explant cultures and experimentally manipulated cAMP signaling to determine whether PKA could promote synapse regeneration. We then delivered the cAMP phosphodiesterase inhibitor rolipram via implanted subcutaneous minipumps in noise-exposed CBA/CaJ mice to test the hypothesis that cAMP signaling could promote cochlear synapse regeneration in vivo.

Results

We showed that the application of the cell membrane-permeable cAMP agonist 8-cpt-cAMP or the cAMP phosphodiesterase inhibitor rolipram promotes significant regeneration of synapses in vitro within twelve hours after their destruction by KA. This is independent of neurotrophin-3, which also promotes synapse regeneration. Moreover, of the two independent signaling effectors activated by cAMP - the cAMP Exchange Protein Activated by cAMP and the cAMP-dependent protein kinase - it is the latter that mediates synapse regeneration. Finally, we showed that systemic delivery of rolipram promotes synapse regeneration in vivo following NICS.

Discussion

In vitro experiments show that cAMP signaling promotes synapse regeneration after excitotoxic destruction of cochlear synapses and does so via PKA signaling. The cAMP phosphodiesterase inhibitor rolipram promotes synapse regeneration in vivo in noise-exposed mice. Systemic administration of rolipram or similar compounds appears to provide a minimally invasive therapeutic approach to reversing synaptopathy post-noise.

Thyroid hormone controls the timing of cochlear ribbon synapse maturation

Ribbon synapses in the cochlear hair cells are subject to extensive pruning and maturation processes before hearing onset. Previous studies have highlighted the pivotal role of thyroid hormone (TH) in this developmental process, yet the detailed mechanisms are largely unknown. In this study, we found that the thyroid hormone receptor α (Thrα) is expressed in both sensory epithelium and spiral ganglion neurons in mice. Hypothyroidism, induced by Pax8 gene knockout, significantly delays the synaptic pruning during postnatal development in mice. Detailed spatiotemporal analysis of ribbon synapse distribution reveals that synaptic maturation involves not only ribbon pruning but also their migration, both of which are notably delayed in the cochlea of Pax8 knockout mice. Intriguingly, postnatal hyperthyroidism, induced by intraperitoneal injections of liothyronine sodium (T3), accelerates the pruning of ribbon synapses to the mature state without affecting the auditory functions. Our findings suggest that thyroid hormone does not play a deterministic role but rather controls the timing of cochlear ribbon synapse maturation.

Synaptic ribbon dynamics after noise exposure in the hearing cochlea

Moderate noise exposure induces cochlear synaptopathy, the loss of afferent ribbon synapses between cochlear hair cells and spiral ganglion neurons, which is associated with functional hearing decline. Prior studies have demonstrated noise-induced changes in the distribution and number of synaptic components, but the dynamic changes that occur after noise exposure have not been directly visualized. Here, we describe a live imaging model using RIBEYE-tagRFP to enable direct observation of pre-synaptic ribbons in mature hearing mouse cochleae after synaptopathic noise exposure. Ribbon number does not change, but noise induces an increase in ribbon volume as well as movement suggesting unanchoring from synaptic tethers. A subgroup of basal ribbons displays concerted motion towards the cochlear nucleus with subsequent migration back to the cell membrane after noise cessation. Understanding the immediate dynamics of synaptic damage after noise exposure may facilitate identification of specific target pathways to treat cochlear synaptopathy.

Noise-induced synaptic loss and its post-exposure recovery in CBA/CaJ vs. C57BL/6J mice

Acute noise-induced loss of synapses between inner hair cells (IHCs) and auditory nerve fibers (ANFs) has been documented in several strains of mice, but the extent of post-exposure recovery reportedly varies dramatically. If such inter-strain heterogeneity is real, it could be exploited to probe molecular pathways mediating neural remodeling in the adult cochlea. Here, we compared synaptopathy repair in CBA/CaJ vs. C57BL/6J, which are at opposite ends of the reported recovery spectrum. We evaluated C57BL/6J mice 0 h, 24 h, 2 wks or 8 wks after exposure for 2 h to octave-band noise (8-16 kHz) at either 90, 94 or 98 dB SPL, to compare with analogous post-exposure results in CBA/CaJ at 98 or 101 dB. We counted pre- and post-synaptic puncta in immunostained cochleas, using machine learning to classify paired (GluA2 and CtBP2) vs. orphan (CtBP2 only) puncta, and batch-processing to quantify immunostaining intensity. At 98 dB, both strains show ongoing loss of ribbons and synapses between 0 and 24 h, followed by partial recovery, however the extent and degree of these changes were greater in C57BL/6J. Much of the synaptic recovery is due to transient reduction in GluA2 intensity in synaptopathic regions. In contrast, CtBP2 intensity showed only transient increases (at 2 wks). Neurofilament staining revealed transient extension of ANF terminals in C57BL/6J, but not in CBA/CaJ, peaking at 24 h and reverting by 2 wks. Thus, although interstrain differences in synapse recovery are dominated by reversible changes in GluA2 receptor levels, the neurite extension seen in C57BL/6J suggests a qualitative difference in regenerative capacity.

Hidden hearing loss: Fifteen years at a glance

Hearing loss affects approximately 18% of the population worldwide. Hearing difficulties in noisy environments without accompanying audiometric threshold shifts likely affect an even larger percentage of the global population. One of the potential causes of hidden hearing loss is cochlear synaptopathy, the loss of synapses between inner hair cells (IHC) and auditory nerve fibers (ANF). These synapses are the most vulnerable structures in the cochlea to noise exposure or aging. The loss of synapses causes auditory deafferentation, i.e., the loss of auditory afferent information, whose downstream effect is the loss of information that is sent to higher-order auditory processing stages. Understanding the physiological and perceptual effects of this early auditory deafferentation might inform interventions to prevent later, more severe hearing loss. In the past decade, a large body of work has been devoted to better understand hidden hearing loss, including the causes of hidden hearing loss, their corresponding impact on the auditory pathway, and the use of auditory physiological measures for clinical diagnosis of auditory deafferentation. This review synthesizes the findings from studies in humans and animals to answer some of the key questions in the field, and it points to gaps in knowledge that warrant more investigation. Specifically, recent studies suggest that some electrophysiological measures have the potential to function as indicators of hidden hearing loss in humans, but more research is needed for these measures to be included as part of a clinical test battery.

Personalized Porous Gelatin Methacryloyl Sustained-Release Nicotinamide Protects Against Noise-Induced Hearing Loss

There are no Food and Drug Administration-approved drugs for treating noise-induced hearing loss (NIHL), reflecting the absence of clear specific therapeutic targets and effective delivery strategies. Noise trauma is demonstrated results in nicotinamide adenine dinucleotide (NAD+) downregulation and mitochondrial dysfunction in cochlear hair cells (HCs) and spiral ganglion neurons (SGNs) in mice, and NAD+ boosted by nicotinamide (NAM) supplementation maintains cochlear mitochondrial homeostasis and prevents neuroexcitatory toxic injury in vitro and ex vivo, also significantly ameliorated NIHL in vivo. To tackle the limited drug delivery efficiency due to sophisticated anatomical barriers and unique clearance pathway in ear, personalized NAM-encapsulated porous gelatin methacryloyl (PGMA@NAM) are developed based on anatomy topography of murine temporal bone by micro-computed tomography and reconstruction of round window (RW) niche, realizing hydrogel in situ implantation completely, NAM sustained-release and long-term auditory preservation in mice. This study strongly supports personalized PGMA@NAM as NIHL protection drug with effective inner ear delivery, providing new inspiration for drug-based treatment of NIHL.

Cochlear Ribbon Synapses in Aged Gerbils

In mammalian hearing, type-I afferent auditory nerve fibers comprise the basis of the afferent auditory pathway. They are connected to inner hair cells of the cochlea via specialized ribbon synapses. Auditory nerve fibers of different physiological types differ subtly in their synaptic location and morphology. Low-spontaneous-rate auditory nerve fibers typically connect on the modiolar side of the inner hair cell, while high-spontaneous-rate fibers are typically found on the pillar side. In aging and noise-damaged ears, this fine-tuned balance between auditory nerve fiber populations can be disrupted and the functional consequences are currently unclear. Here, using immunofluorescent labeling of presynaptic ribbons and postsynaptic glutamate receptor patches, we investigated changes in synaptic morphology at three different tonotopic locations along the cochlea of aging gerbils compared to those of young adults. Quiet-aged gerbils showed about 20% loss of afferent ribbon synapses. While the loss was random at apical, low-frequency cochlear locations, at the basal, high-frequency location it almost exclusively affected the modiolar-located synapses. The subtle differences in volumes of pre- and postsynaptic elements located on the inner hair cell's modiolar versus pillar side were unaffected by age. This is consistent with known physiology and suggests a predominant, age-related loss in the low-spontaneous-rate auditory nerve population in the cochlear base, but not the apex.

Spatial patterns of noise-induced inner hair cell ribbon loss in the mouse mid-cochlea

In the mammalian cochlea, moderate acoustic overexposure leads to loss of ribbon-type synapse between the inner hair cell (IHC) and its postsynaptic spiral ganglion neuron (SGN), causing a reduced dynamic range of hearing but not a permanent threshold elevation. A prevailing view is that such ribbon loss (known as synaptopathy) selectively impacts the low-spontaneous-rate and high-threshold SGN fibers contacting predominantly the modiolar IHC face. However, the spatial pattern of synaptopathy remains scarcely characterized in the most sensitive mid-cochlear region, where two morphological subtypes of IHC with distinct ribbon size gradients coexist. Here, we used volume electron microscopy to investigate noise exposure-related changes in the mouse IHCs with and without ribbon loss. Our quantifications reveal that IHC subtypes differ in the worst-hit area of synaptopathy. Moreover, we show relative enrichment of mitochondria in the surviving SGN terminals, providing key experimental evidence for the long-proposed role of SGN-terminal mitochondria in synaptic vulnerability.

Oxidative Stress Plays an Important Role in Glutamatergic Excitotoxicity-Induced Cochlear Synaptopathy: Implication for Therapeutic Molecules Screening

The disruption of the synaptic connection between the sensory inner hair cells (IHCs) and the auditory nerve fiber terminals of the type I spiral ganglion neurons (SGN) has been observed early in several auditory pathologies (e.g., noise-induced or ototoxic drug-induced or age-related hearing loss). It has been suggested that glutamate excitotoxicity may be an inciting element in the degenerative cascade observed in these pathological cochlear conditions. Moreover, oxidative damage induced by free hydroxyl radicals and nitric oxide may dramatically enhance cochlear damage induced by glutamate excitotoxicity. To investigate the underlying molecular mechanisms involved in cochlear excitotoxicity, we examined the molecular basis responsible for kainic acid (KA, a full agonist of AMPA/KA-preferring glutamate receptors)-induced IHC synapse loss and degeneration of the terminals of the type I spiral ganglion afferent neurons using a cochlear explant culture from P3 mouse pups. Our results demonstrated that disruption of the synaptic connection between IHCs and SGNs induced increased levels of oxidative stress, as well as altered both mitochondrial function and neurotrophin signaling pathways. Additionally, the application of exogenous antioxidants and neurotrophins (NT3, BDNF, and small molecule TrkB agonists) clearly increases synaptogenesis. These results suggest that understanding the molecular pathways involved in cochlear excitotoxicity is of crucial importance for the future clinical trials of drug interventions for auditory synaptopathies.

GelMA/PEDOT:PSS Composite Conductive Hydrogel-Based Generation and Protection of Cochlear Hair Cells through Multiple Signaling Pathways

Recent advances in cochlear implantology are exemplified by novel functional strategies such as bimodal electroacoustic stimulation, in which the patient has intact low-frequency hearing and profound high-frequency hearing pre-operatively. Therefore, the synergistic restoration of dysfunctional cochlear hair cells and the protection of hair cells from ototoxic insults have become a persistent target pursued for this hybrid system. In this study, we developed a composite GelMA/PEDOT:PSS conductive hydrogel that is suitable as a coating for the cochlear implant electrode for the potential local delivery of otoregenerative and otoprotective drugs. Various material characterization methods (e.g., 1H NMR spectroscopy, FT-IR, EIS, and SEM), experimental models (e.g., murine cochlear organoid and aminoglycoside-induced ototoxic HEI-OC1 cellular model), and biological analyses (e.g., confocal laser scanning microscopy, real time qPCR, flow cytometry, and bioinformatic sequencing) were used. The results demonstrated decent material properties of the hydrogel, such as mechanical (e.g., high tensile stress and Young's modulus), electrochemical (e.g., low impedance and high conductivity), biocompatibility (e.g., satisfactory cochlear cell interaction and free of systemic toxicity), and biosafety (e.g., minimal hemolysis and cell death) features. In addition, the CDR medicinal cocktail sustainably released by the hydrogel not only promoted the expansion of the cochlear stem cells but also boosted the trans-differentiation from cochlear supporting cells into hair cells. Furthermore, hydrogel-based drug delivery protected the hair cells from oxidative stress and various forms of programmed cell death (e.g., apoptosis and ferroptosis). Finally, using large-scale sequencing, we enriched a complex network of signaling pathways that are potentially downstream to various metabolic processes and abundant metabolites. In conclusion, we present a conductive hydrogel-based local delivery of bifunctional drug cocktails, thereby serving as a potential solution to intracochlear therapy of bimodal auditory rehabilitation and diseases beyond.

The cochlear matrisome: Importance in hearing and deafness

The extracellular matrix (ECM) consists in a complex meshwork of collagens, glycoproteins, and proteoglycans, which serves a scaffolding function and provides viscoelastic properties to the tissues. ECM acts as a biomechanical support, and actively participates in cell signaling to induce tissular changes in response to environmental forces and soluble cues. Given the remarkable complexity of the inner ear architecture, its exquisite structure-function relationship, and the importance of vibration-induced stimulation of its sensory cells, ECM is instrumental to hearing. Many factors of the matrisome are involved in cochlea development, function and maintenance, as evidenced by the variety of ECM proteins associated with hereditary deafness. This review describes the structural and functional ECM components in the auditory organ and how they are modulated over time and following injury.

Fatty acid binding protein type 7 deficiency preserves auditory function in noise-exposed mice

Fatty acid-binding protein 7 (FABP7) is vital for uptake and trafficking of fatty acids in the nervous system. To investigate the involvement of FABP7 in noise-induced hearing loss (NIHL) pathogenesis, we used Fabp7 knockout (KO) mice generated via CRISPR/Cas9 in the C57BL/6 background. Initial auditory brainstem response (ABR) measurements were conducted at 9 weeks, followed by noise exposure at 10 weeks. Subsequent ABRs were performed 24 h later, with final measurements at 12 weeks. Inner ears were harvested 24 h after noise exposure for RNA sequencing and metabolic analyses. We found no significant differences in initial ABR measurements, but Fabp7 KO mice showed significantly lower thresholds in the final ABR measurements. Hair cell survival was also enhanced in Fabp7 KO mice. RNA sequencing revealed that genes associated with the electron transport chain were upregulated or less impaired in Fabp7 KO mice. Metabolomic analysis revealed various alterations, including decreased glutamate and aspartate in Fabp7 KO mice. In conclusion, FABP7 deficiency mitigates cochlear damage following noise exposure. This protective effect was supported by the changes in gene expression of the electron transport chain, and in several metabolites, including excitotoxic neurotransmitters. Our study highlights the potential therapeutic significance of targeting FABP7 in NIHL.

Ultrastructure of noise-induced cochlear synaptopathy

Acoustic overexposure can eliminate synapses between inner hair cells (IHCs) and auditory nerve fibers (ANFs), even if hair-cell function recovers. This synaptopathy has been extensively studied by confocal microscopy, however, understanding the nature and sequence of damage requires ultrastructural analysis. Here, we used focused ion-beam scanning electron microscopy to mill, image, segment and reconstruct ANF terminals in mice, 1 day and 1 week after synaptopathic exposure (8-16 kHz, 98 dB SPL). At both survivals, ANF terminals were normal in number, but 62% and 53%, respectively, lacked normal synaptic specializations. Most non-synapsing fibers (57% and 48% at 1 day and 1 week) remained in contact with an IHC and contained healthy-looking organelles. ANFs showed a transient increase in mitochondrial content (51%) and efferent innervation (34%) at 1 day. Fibers maintaining synaptic connections showed hypertrophy of pre-synaptic ribbons at both 1 day and 1 week. Non-synaptic fibers were lower in mitochondrial content and typically on the modiolar side of the IHC, where ANFs with high-thresholds and low spontaneous rates are normally found. Even 1 week post-exposure, many ANF terminals remained in IHC contact despite loss of synaptic specializations, thus, regeneration efforts at early post-exposure times should concentrate on synaptogenesis rather than neurite extension.

Glutamate-aspartate transporter dysfunction enhances aminoglycoside-induced cochlear hair cell death via NMDA receptor activation

Glutamate is a crucial neurotransmitter for hearing transduction in the cochlea, but excess glutamate is detrimental to the survival of cochlear sensory cells. Glutamate-aspartate transporter (GLAST) is the major transporter for glutamate removal; however, its role in aminoglycoside-induced hair cell loss is not well studied. In the present study, we first investigated the localization and expression of GLAST over the course of development of the mouse cochlea, and we found that inhibition of GLAST increased hair cell death. However, when the glutamate receptor NMDAR was inhibited by D-AP5, hair cell death was no longer increased by the GLAST inhibitor. Our results indicate that GLAST inhibition aggravates damage to cochlear hair cells, which may occur via NMDAR, and this suggests new clinical strategies for ameliorating the ototoxicity associated with the dysfunction of glutamate metabolism.

Synaptic activity is not required for establishing heterogeneity of inner hair cell ribbon synapses

Neural sound encoding in the mammalian cochlea faces the challenge of representing audible sound pressures that vary over six orders of magnitude. The cochlea meets this demand through the use of active micromechanics as well as the diversity and adaptation of afferent neurons and their synapses. Mechanisms underlying neural diversity likely include heterogeneous presynaptic input from inner hair cells (IHCs) to spiral ganglion neurons (SGNs) as well as differences in the molecular profile of SGNs and in their efferent control. Here, we tested whether glutamate release from IHCs, previously found to be critical for maintaining different molecular SGN profiles, is required for establishing heterogeneity of active zones (AZs) in IHCs. We analyzed structural and functional heterogeneity of IHC AZs in mouse mutants with disrupted glutamate release from IHCs due to lack of a vesicular glutamate transporter (Vglut3) or impaired exocytosis due to defective otoferlin. We found the variance of the voltage-dependence of presynaptic Ca2+ influx to be reduced in exocytosis-deficient IHCs of otoferlin mutants. Yet, the spatial gradients of maximal amplitude and voltage-dependence of Ca2+ influx along the pillar-modiolar IHC axis were maintained in both mutants. Further immunohistochemical analysis showed an intact spatial gradient of ribbon size in Vglut3-/- mice. These results indicate that IHC exocytosis and glutamate release are not strictly required for establishing the heterogeneity of IHC AZs.

Intrinsic mechanism and pharmacologic treatments of noise-induced hearing loss

Noise accounts for one-third of hearing loss worldwide. Regretfully, noise-induced hearing loss (NIHL) is deemed to be irreversible due to the elusive pathogenic mechanisms that have not been fully elucidated. The complex interaction between genetic and environmental factors, which influences numerous downstream molecular and cellular events, contributes to the NIHL. In clinical settings, there are no effective therapeutic drugs other than steroids, which are the only treatment option for patients with NIHL. Therefore, the need for treatment of NIHL that is currently unmet, along with recent progress in our understanding of the underlying regulatory mechanisms, has led to a lot of new literatures focusing on this therapeutic field. The emergence of novel technologies that modify local drug delivery to the inner ear has led to the development of promising therapeutic approaches, which are currently under clinical investigation. In this comprehensive review, we focus on outlining and analyzing the basics and potential therapeutics of NIHL, as well as the application of biomaterials and nanomedicines in inner ear drug delivery. The objective of this review is to provide an incentive for NIHL's fundamental research and future clinical translation.

Macrophages Promote Repair of Inner Hair Cell Ribbon Synapses following Noise-Induced Cochlear Synaptopathy

Resident cochlear macrophages rapidly migrate into the inner hair cell synaptic region and directly contact the damaged synaptic connections after noise-induced synaptopathy. Eventually, such damaged synapses are spontaneously repaired, but the precise role of macrophages in synaptic degeneration and repair remains unknown. To address this, cochlear macrophages were eliminated using colony stimulating factor 1 receptor (CSF1R) inhibitor, PLX5622. Sustained treatment with PLX5622 in CX3CR1 GFP/+ mice of both sexes led to robust elimination of resident macrophages (∼94%) without significant adverse effects on peripheral leukocytes, cochlear function, and structure. At 1 day (d) post noise exposure of 93 or 90 dB SPL for 2 hours, the degree of hearing loss and synapse loss were comparable in the presence and absence of macrophages. At 30 d after exposure, damaged synapses appeared repaired in the presence of macrophages. However, in the absence of macrophages, such synaptic repair was significantly reduced. Remarkably, on cessation of PLX5622 treatment, macrophages repopulated the cochlea, leading to enhanced synaptic repair. Elevated auditory brainstem response thresholds and reduced auditory brainstem response Peak 1 amplitudes showed limited recovery in the absence of macrophages but recovered similarly with resident and repopulated macrophages. Cochlear neuron loss was augmented in the absence of macrophages but showed preservation with resident and repopulated macrophages after noise exposure. While the central auditory effects of PLX5622 treatment and microglia depletion remain to be investigated, these data demonstrate that macrophages do not affect synaptic degeneration but are necessary and sufficient to restore cochlear synapses and function after noise-induced synaptopathy.SIGNIFICANCE STATEMENT The synaptic connections between cochlear inner hair cells and spiral ganglion neurons can be lost because of noise over exposure or biological aging. This loss may represent the most common causes of sensorineural hearing loss also known as hidden hearing loss. Synaptic loss results in degradation of auditory information, leading to difficulty in listening in noisy environments and other auditory perceptual disorders. We demonstrate that resident macrophages of the cochlea are necessary and sufficient to restore synapses and function following synaptopathic noise exposure. Our work reveals a novel role for innate-immune cells, such as macrophages in synaptic repair, that could be harnessed to regenerate lost ribbon synapses in noise- or age-linked cochlear synaptopathy, hidden hearing loss, and associated perceptual anomalies.

The hair cell analysis toolbox is a precise and fully automated pipeline for whole cochlea hair cell quantification

Our sense of hearing is mediated by sensory hair cells, precisely arranged and highly specialized cells subdivided into outer hair cells (OHCs) and inner hair cells (IHCs). Light microscopy tools allow for imaging of auditory hair cells along the full length of the cochlea, often yielding more data than feasible to manually analyze. Currently, there are no widely applicable tools for fast, unsupervised, unbiased, and comprehensive image analysis of auditory hair cells that work well either with imaging datasets containing an entire cochlea or smaller sampled regions. Here, we present a highly accurate machine learning-based hair cell analysis toolbox (HCAT) for the comprehensive analysis of whole cochleae (or smaller regions of interest) across light microscopy imaging modalities and species. The HCAT is a software that automates common image analysis tasks such as counting hair cells, classifying them by subtype (IHCs versus OHCs), determining their best frequency based on their location along the cochlea, and generating cochleograms. These automated tools remove a considerable barrier in cochlear image analysis, allowing for faster, unbiased, and more comprehensive data analysis practices. Furthermore, HCAT can serve as a template for deep learning-based detection tasks in other types of biological tissue: With some training data, HCAT's core codebase can be trained to develop a custom deep learning detection model for any object on an image.

Calcium signaling and genetic rare diseases: An auditory perspective

Deafness is a highly heterogeneous disorder which stems, for 50%, from genetic origins. Sensory transduction relies mainly on sensory hair cells of the cochlea, in the inner ear. Calcium is key for the function of these cells and acts as a fundamental signal transduction. Its homeostasis depends on three factors: the calcium influx, through the mechanotransduction channel at the apical pole of the hair cell as well as the voltage-gated calcium channel at the base of the cells; the calcium buffering via Ca²⁺-binding proteins in the cytoplasm, but also in organelles such as mitochondria and the reticulum endoplasmic mitochondria-associated membranes with specialized proteins; and the calcium extrusion through the Ca-ATPase pump, located all over the plasma membrane. In addition, the synaptic transmission to the central nervous system is also controlled by calcium. Genetic studies of inherited deafness have tremendously helped understand the underlying molecular pathways of calcium signaling. In this review, we discuss these different factors in light of the associated genetic diseases (syndromic and non-syndromic deafness) and the causative genes.

COCHLEAR GLIAL CELLS MEDIATES GLUTAMATE UPTAKE THROUGH A SODIUM-INDEPENDENT TRANSPORTER

Since glutamate is the primary excitatory neurotransmitter in the mammalian cochlea, the mechanisms for the removal of glutamate from the synaptic and extrasynaptic spaces are critical for maintaining normal function of this region. Glial cells of inner ear are crucial for regulation of synaptic transmission throughout since it closely interacts with neurons along the entire auditory pathway, however little is known about the activity and expression of glutamate transporters in the cochlea. In this study, using primary cochlear glial cells cultures obtained from newborn Balb/C mice, we determined the activity of a sodium-dependent and sodium-independent glutamate uptake mechanisms by means of High Performance Liquid Chromatography. The sodium-independent glutamate transport has a prominent contribution in cochlear glial cells which is similar to what has been demonstrated in other sensory organs, but it is not found in tissues less susceptible to continuous glutamate-mediated injuries. Our results showed that x_CG^- system is expressed in CGCs and is the main responsible for sodium-independent glutamate uptake. The identification and characterization of the x_CG^- transporter in the cochlea suggests a possible role of this transporter in the control of extracellular glutamate concentrations and regulation of redox state, that may aid in the preservation of auditory function.

Current Advances in Gene Therapies of Genetic Auditory Neuropathy Spectrum Disorder

Auditory neuropathy spectrum disorder (ANSD) refers to a range of hearing impairments characterized by an impaired transmission of sound from the cochlea to the brain. This defect can be due to a lesion or defect in the inner hair cell (IHC), IHC ribbon synapse (e.g., pre-synaptic release of glutamate), postsynaptic terminals of the spiral ganglion neurons, or demyelination and axonal loss within the auditory nerve. To date, the only clinical treatment options for ANSD are hearing aids and cochlear implantation. However, despite the advances in hearing-aid and cochlear-implant technologies, the quality of perceived sound still cannot match that of the normal ear. Recent advanced genetic diagnostics and clinical audiology made it possible to identify the precise site of a lesion and to characterize the specific disease mechanisms of ANSD, thus bringing renewed hope to the treatment or prevention of auditory neurodegeneration. Moreover, genetic routes involving the replacement or corrective editing of mutant sequences or defected genes to repair damaged cells for the future restoration of hearing in deaf people are showing promise. In this review, we provide an update on recent discoveries in the molecular pathophysiology of genetic lesions, auditory synaptopathy and neuropathy, and gene-therapy research towards hearing restoration in rodent models and in clinical trials.

GluA3 subunits are required for appropriate assembly of AMPAR GluA2 and GluA4 subunits on cochlear afferent synapses and for presynaptic ribbon modiolar-pillar morphology

Cochlear sound encoding depends on α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid receptors (AMPARs), but reliance on specific pore-forming subunits is unknown. With 5-week-old male C57BL/6J Gria3-knockout mice (i.e., subunit GluA3KO) we determined cochlear function, synapse ultrastructure, and AMPAR molecular anatomy at ribbon synapses between inner hair cells (IHCs) and spiral ganglion neurons. GluA3KO and wild-type (GluA3WT) mice reared in ambient sound pressure level (SPL) of 55-75 dB had similar auditory brainstem response (ABR) thresholds, wave-1 amplitudes, and latencies. Postsynaptic densities (PSDs), presynaptic ribbons, and synaptic vesicle sizes were all larger on the modiolar side of the IHCs from GluA3WT, but not GluA3KO, demonstrating GluA3 is required for modiolar-pillar synapse differentiation. Presynaptic ribbons juxtaposed with postsynaptic GluA2/4 subunits were similar in quantity, however, lone ribbons were more frequent in GluA3KO and GluA2-lacking synapses were observed only in GluA3KO. GluA2 and GluA4 immunofluorescence volumes were smaller on the pillar side than the modiolar side in GluA3KO, despite increased pillar-side PSD size. Overall, the fluorescent puncta volumes of GluA2 and GluA4 were smaller in GluA3KO than GluA3WT. However, GluA3KO contained less GluA2 and greater GluA4 immunofluorescence intensity relative to GluA3WT (threefold greater mean GluA4:GluA2 ratio). Thus, GluA3 is essential in development, as germline disruption of Gria3 caused anatomical synapse pathology before cochlear output became symptomatic by ABR. We propose the hearing loss in older male GluA3KO mice results from progressive synaptopathy evident in 5-week-old mice as decreased abundance of GluA2 subunits and an increase in GluA2-lacking, GluA4-monomeric Ca2+-permeable AMPARs.

Review of blast noise and the auditory system

The auditory system is particularly vulnerable to blast injury due to the ear's role as a highly sensitive pressure transducer. Over the past several decades, studies have used a variety of animal models and experimental procedures to recreate blast-induced acoustic trauma. Given the developing nature of this field and our incomplete understanding of molecular mechanisms underlying blast-related auditory disturbances, an updated discussion about these studies is warranted. Here, we comprehensively review well-established blast-related auditory pathology including tympanic membrane perforation and hair cell loss. In addition, we discuss important mechanistic studies that aim to bridge gaps in our current understanding of the molecular and microstructural events underlying blast-induced cochlear, auditory nerve, brainstem, and central auditory system damage. Key findings from the recent literature include the association between endolymphatic hydrops and cochlear synaptic loss, blast-induced neuroinflammatory markers in the peripheral and central auditory system, and therapeutic approaches targeting biochemical markers of blast injury. We conclude that blast is an extreme form of noise exposure. Blast waves produce cochlear damage that appears similar to, but more extreme than, the standard noise exposure protocols used in auditory research. However, experimental variations in studies of blast-induced acoustic trauma make it challenging to compare and interpret data across studies.

Putative COVID-19 therapies imatinib, lopinavir, ritonavir, and ivermectin cause hair cell damage: A targeted screen in the zebrafish lateral line

The biomedical community is rapidly developing COVID-19 drugs to bring much-need therapies to market, with over 900 drugs and drug combinations currently in clinical trials. While this pace of drug development is necessary, the risk of producing therapies with significant side-effects is also increased. One likely side-effect of some COVID-19 drugs is hearing loss, yet hearing is not assessed during preclinical development or clinical trials. We used the zebrafish lateral line, an established model for drug-induced sensory hair cell damage, to assess the ototoxic potential of seven drugs in clinical trials for treatment of COVID-19. We found that ivermectin, lopinavir, imatinib, and ritonavir were significantly toxic to lateral line hair cells. By contrast, the approved COVID-19 therapies dexamethasone and remdesivir did not cause damage. We also did not observe damage from the antibiotic azithromycin. Neither lopinavir nor ritonavir altered the number of pre-synaptic ribbons per surviving hair cell, while there was an increase in ribbons following imatinib or ivermectin exposure. Damage from lopinavir, imatinib, and ivermectin was specific to hair cells, with no overall cytotoxicity noted following TUNEL labeling. Ritonavir may be generally cytotoxic, as determined by an increase in the number of TUNEL-positive non-hair cells following ritonavir exposure. Pharmacological inhibition of the mechanotransduction (MET) channel attenuated damage caused by lopinavir and ritonavir but did not alter imatinib or ivermectin toxicity. These results suggest that lopinavir and ritonavir may enter hair cells through the MET channel, similar to known ototoxins such as aminoglycoside antibiotics. Finally, we asked if ivermectin was ototoxic to rats in vivo. While ivermectin is not recommended by the FDA for treating COVID-19, many people have chosen to take ivermectin without a doctor's guidance, often with serious side-effects. Rats received daily subcutaneous injections for 10 days with a clinically relevant ivermectin dose (0.2 mg/kg). In contrast to our zebrafish assays, ivermectin did not cause ototoxicity in rats. Our research suggests that some drugs in clinical trials for COVID-19 may be ototoxic. This work can help identify drugs with the fewest side-effects and determine which therapies warrant audiometric monitoring.

Synaptopathy in Guinea Pigs Induced by Noise Mimicking Human Experience and Associated Changes in Auditory Signal Processing

Noise induced synaptopathy (NIS) has been researched extensively since a large amount of synaptic loss without permanent threshold shift (PTS) was found in CBA mice after a brief noise exposure. However, efforts to translate these results to humans have met with little success—and might not be possible since noise exposure used in laboratory animals is generally different from what is experienced by human subjects in real life. An additional problem is a lack of morphological data and reliable functional methods to quantify loss of afferent synapses in humans. Based on evidence for disproportionate synaptic loss for auditory nerve fibers (ANFs) with low spontaneous rates (LSR), coding-in-noise deficits (CIND) have been speculated to be the major difficulty associated with NIS without PTS. However, no robust evidence for this is available in humans or animals. This has led to a re-examination of the role of LSR ANFs in signal coding in high-level noise. The fluctuation profile model has been proposed to support a role for high-SR ANFs in the coding of high-level noise in combination with efferent control of cochlear gain. This study aimed to induce NIS by a low-level, intermittent noise exposure mimicking what is experienced in human life and examined the impact of the NIS on temporal processing under masking. It also evaluated the role of temporal fluctuation in evoking efferent feedback and the effects of NIS on this feedback.

Multiple Sevoflurane Exposures During the Neonatal Period Cause Hearing Impairment and Loss of Hair Cell Ribbon Synapses in Adult Mice

Objectives

This study aims to investigate the effects of multiple sevoflurane exposures in neonatal mice on hearing function in the later life and explores the underlying mechanisms and protective strategies.

Materials and Methods

Neonatal Kunming mice were exposed to sevoflurane for 3 days. Auditory brainstem response (ABR) and distortion product otoacoustic emission (DPOAE) tests, immunofluorescence, patch-clamp recording, and quantitative real-time PCR were performed to observe hearing function, hair cells, ribbon synapses, nerve fibers, spiral ganglion neurons, and oxidative stress.

Results

Compared to control group, multiple sevoflurane exposures during the neonatal time significantly elevated ABR thresholds at 8 kHz (35.42 ± 1.57 vs. 41.76 ± 1.97 dB, P = 0.0256), 16 kHz (23.33 ± 1.28 vs. 33.53 ± 2.523 dB, P = 0.0012), 24 kHz (30.00 ± 2.04 vs. 46.76 ± 3.93 dB, P = 0.0024), and 32 kHz (41.25 ± 2.31 vs. 54.41 ± 2.94 dB, P = 0.0028) on P30, caused ribbon synapse loss on P15 (13.10 ± 0.43 vs. 10.78 ± 0.52, P = 0.0039) and P30 (11.24 ± 0.56 vs. 8.50 ± 0.84, P = 0.0141), and degenerated spiral ganglion neuron (SGN) nerve fibers on P30 (110.40 ± 16.23 vs. 55.04 ± 8.13, P = 0.0073). In addition, the V_half of calcium current become more negative (−21.99 ± 0.70 vs. −27.17 ± 0.60 mV, P < 0.0001), exocytosis was reduced (105.40 ± 19.97 vs. 59.79 ± 10.60 fF, P < 0.0001), and Lpo was upregulated (P = 0.0219) in sevoflurane group than those in control group. N-acetylcysteine (NAC) reversed hearing impairment induced by sevoflurane.

Conclusion

The findings suggest that multiple sevoflurane exposures during neonatal time may cause hearing impairment in adult mice. The study also demonstrated that elevated oxidative stress led to ribbon synapses impairment and SGN nerve fibers degeneration, and the interventions of antioxidants alleviated the sevoflurane-induced hearing impairment.

Gene Therapy Restores Auditory Functions in an Adult Vglut3 Knockout Mouse Model

Adeno-associated virus (AAV)-based gene therapy has been demonstrated to be extremely effective for treating genetic hearing loss over the past several years. However, successful gene therapies for hereditary deafness have not been well-studied in adult mice. To explore the possibility of gene therapy after peripheral auditory maturity, we used AAV8 to express Vglut3 in the cochleae of 5 w, 8 w, and 20 w Vglut3KO mice. Results indicated that AAV8-Vglut3 could mediate the exogenous expression of Vglut3 in all inner hair cells (IHCs). Auditory function was successfully restored, and the hearing threshold remained stable for at least 12 weeks after rescue. Moreover, the results revealed that the number of synaptic ribbons, as well as their morphology, were significantly recovered after gene therapy, potentially indicating the glutamate-dependent plasticity of IHCs. Taken together, our data introduces the possibility of gene therapy in adult mice and advances our knowledge of the role of Vglut3 in presynaptic plasticity.

Synaptic transmission at the vestibular hair cells of amniotes

A harmonized interplay between the central nervous system and the five peripheral end organs is how the vestibular system helps organisms feel a sense of balance and motion in three-dimensional space. The receptor cells of this system, much like their cochlear equivalents, are the specialized hair cells. However, research over the years has shown that the vestibular endorgans and hair cells evolved very differently from their cochlear counterparts. The structurally unique calyceal synapse, which appeared much later in the evolutionary time scale, and continues to intrigue researchers, is now known to support several forms of synaptic neurotransmission. The conventional quantal transmission is believed to employ the ribbon structures, which carry several tethered vesicles filled with neurotransmitters. However, the field of vestibular hair cell synaptic molecular anatomy is still at a nascent stage and needs further work. In this review, we will touch upon the basic structure and function of the peripheral vestibular system, with the focus on the various modes of neurotransmission at the type I vestibular hair cells. We will also shed light on the current knowledge about the molecular anatomy of the vestibular hair cell synapses and vestibular synaptopathy.

Animal-to-Human Translation Difficulties and Problems With Proposed Coding-in-Noise Deficits in Noise-Induced Synaptopathy and Hidden Hearing Loss

Noise induced synaptopathy (NIS) and hidden hearing loss (NIHHL) have been hot topic in hearing research since a massive synaptic loss was identified in CBA mice after a brief noise exposure that did not cause permanent threshold shift (PTS) in 2009. Based upon the amount of synaptic loss and the bias of it to synapses with a group of auditory nerve fibers (ANFs) with low spontaneous rate (LSR), coding-in-noise deficit (CIND) has been speculated as the major difficult of hearing in subjects with NIS and NIHHL. This speculation is based upon the idea that the coding of sound at high level against background noise relies mainly on the LSR ANFs. However, the translation from animal data to humans for NIS remains to be justified due to the difference in noise exposure between laboratory animals and human subjects in real life, the lack of morphological data and reliable functional methods to quantify or estimate the loss of the afferent synapses by noise. Moreover, there is no clear, robust data revealing the CIND even in animals with the synaptic loss but no PTS. In humans, both positive and negative reports are available. The difficulty in verifying CINDs has led a re-examination of the hypothesis that CIND is the major deficit associated with NIS and NIHHL, and the theoretical basis of this idea on the role of LSR ANFs. This review summarized the current status of research in NIS and NIHHL, with focus on the translational difficulty from animal data to human clinicals, the technical difficulties in quantifying NIS in humans, and the problems with the SR theory on signal coding. Temporal fluctuation profile model was discussed as a potential alternative for signal coding at high sound level against background noise, in association with the mechanisms of efferent control on the cochlea gain.

Bottom-Up and Top-Down Attention Impairment Induced by Long-Term Exposure to Noise in the Absence of Threshold Shifts

Objective

We aimed to assess the effect of noise exposure on bottom-up and top-down attention functions in industrial workers based on behavioral and brain responses recorded by the multichannel electroencephalogram (EEG).

Method

In this cross-sectional study, 563 shipyard noise-exposed workers with clinical normal hearing were recruited for cognitive testing. Personal cumulative noise exposure (CNE) was calculated with the long-term equivalent noise level and employment duration. The performance of cognitive tests was compared between the high CNE group (H-CNE, >92.2) and the low CNE group; additionally, brain responses were recorded with a 256-channel EEG from a subgroup of 20 noise-exposed (NG) workers, who were selected from the cohort with a pure tone threshold <25 dB HL from 0.25 to 16 kHz and 20 healthy controls matched for age, sex, and education. P300 and mismatch negativity (MMN) evoked by auditory stimuli were obtained to evaluate the top-down and bottom-up attention functions. The sources of P300 and MMN were investigated using GeoSource.

Results

The total score of the cognitive test (24.55 ± 3.71 vs. 25.32 ± 2.62, p < 0.01) and the subscale of attention score (5.43 ± 1.02 vs. 5.62 ± 0.67, p < 0.001) were significantly lower in the H-CNE group than in the L-CNE group. The attention score has the fastest decline of all the cognitive domain dimensions (slope = -0.03 in individuals under 40 years old, p < 0.001; slope = -0.06 in individuals older than 40 years old, p < 0.001). When NG was compared with controls, the P300 amplitude was significantly decreased in NG at Cz (3.9 ± 2.1 vs. 6.7 ± 2.3 μV, p < 0.001). In addition, the latency of P300 (390.7 ± 12.1 vs. 369.4 ± 7.5 ms, p < 0.001) and MMN (172.8 ± 15.5 vs. 157.8 ± 10.5 ms, p < 0.01) was significantly prolonged in NG compared with controls. The source for MMN for controls was in the left BA11, whereas the noise exposure group's source was lateralized to the BA20.

Conclusion

Long-term exposure to noise deteriorated the bottom-up and top-down attention functions even in the absence of threshold shifts, as evidenced by behavioral and brain responses.

A New Player in the Hippocampus: A Review on VGLUT3+ Neurons and Their Role in the Regulation of Hippocampal Activity and Behaviour

Glutamate is the most abundant excitatory amino acid in the central nervous system. Neurons using glutamate as a neurotransmitter can be characterised by vesicular glutamate transporters (VGLUTs). Among the three subtypes, VGLUT3 is unique, co-localising with other "classical" neurotransmitters, such as the inhibitory GABA. Glutamate, manipulated by VGLUT3, can modulate the packaging as well as the release of other neurotransmitters and serve as a retrograde signal through its release from the somata and dendrites. Its contribution to sensory processes (including seeing, hearing, and mechanosensation) is well characterised. However, its involvement in learning and memory can only be assumed based on its prominent hippocampal presence. Although VGLUT3-expressing neurons are detectable in the hippocampus, most of the hippocampal VGLUT3 positivity can be found on nerve terminals, presumably coming from the median raphe. This hippocampal glutamatergic network plays a pivotal role in several important processes (e.g., learning and memory, emotions, epilepsy, cardiovascular regulation). Indirect information from anatomical studies and KO mice strains suggests the contribution of local VGLUT3-positive hippocampal neurons as well as afferentations in these events. However, further studies making use of more specific tools (e.g., Cre-mice, opto- and chemogenetics) are needed to confirm these assumptions.

Metabotropic glutamate receptor: A new possible therapeutic target for cochlear synaptopathy

Objectives

Cochlear synaptopathy is a common cause of auditory disorders in which glutamate over-activation occurs. Modulating glutamatergic pathways has been proposed to down-regulate post-synaptic excitation.

Materials and Methods

12-guinea pigs as sham and test groups were exposed to a 4-kHz noise at 104 dB SPL, for 2 hr. Pre-exposure intra-tympanic injection with LY354740 and normal saline 9% was applied in the test and sham groups. The amplitude growth of ABR-wave-I and wave-III latency shift with noise were considered in pre- and post-exposure times. The synapses were observed by transmission electron-microscopy.

Results

ABR thresholds recovered 1-week post-exposure in both groups. The reduction of wave-I amplitude at 4, 6, and 8 kHz were statistically different between pre- and 1- day post-exposure and recovered mostly in the sham group. The amount of latency shift in masked ABR was different between pre- and all post-exposure, and the response could not be detected at higher than 50 dB SL noise. However, the response detectability increased to 60 dB SL noise, and the significance of differences between pre- and post-exposure persisted only at the high level of noise in the test group. In electron-microscopy of sham samples, the size of the ribbon was larger, spherical with an irregularity, and hollow. The post-synaptic density was thicker and missed its flat orientation.

Conclusion

The higher slope of the ABR-wave I amplitude, the more tolerance of noise in masked ABR, concomitant with the histological finding that revealed less synaptic damage, confirmed the therapeutic effect of LY354740 in cochlear synaptopathy.

Auditory-nerve responses in mice with noise-induced cochlear synaptopathy

Cochlear synaptopathy is the noise-induced or age-related loss of ribbon synapses between inner hair cells (IHCs) and auditory-nerve fibers (ANFs), first reported in CBA/CaJ mice. Recordings from single ANFs in anesthetized, noise-exposed guinea pigs suggested that neurons with low spontaneous rates (SRs) and high thresholds are more vulnerable than low-threshold, high-SR fibers. However, there is extensive postexposure regeneration of ANFs in guinea pigs but not in mice. Here, we exposed CBA/CaJ mice to octave-band noise and recorded sound-evoked and spontaneous activity from single ANFs at least 2 wk later. Confocal analysis of cochleae immunostained for pre- and postsynaptic markers confirmed the expected loss of 40%-50% of ANF synapses in the basal half of the cochlea; however, our data were not consistent with a selective loss of low-SR fibers. Rather they suggested a loss of both SR groups in synaptopathic regions. Single-fiber thresholds and frequency tuning recovered to pre-exposure levels; however, response to tone bursts showed increased peak and steady-state firing rates, as well as decreased jitter in first-spike latencies. This apparent gain-of-function increased the robustness of tone-burst responses in the presence of continuous masking noise. This study suggests that the nature of noise-induced synaptic damage varies between different species and that, in mouse, the noise-induced hyperexcitability seen in central auditory circuits is also observed at the level of the auditory nerve.NEW & NOTEWORTHY Noise-induced damage to synapses between inner hair cells and auditory-nerve fibers (ANFs) can occur without permanent hair cell damage, resulting in pathophysiology that "hides" behind normal thresholds. Prior single-fiber neurophysiology in guinea pig suggested that noise selectively targets high-threshold ANFs. Here, we show that the lingering pathophysiology differs in mouse, with both ANF groups affected and a paradoxical gain-of-function in surviving low-threshold fibers, including increased onset rate, decreased onset jitter, and reduced maskability.

Endolymphatic Hydrops is a Marker of Synaptopathy Following Traumatic Noise Exposure

After acoustic trauma, there can be loss of synaptic connections between inner hair cells and auditory neurons in the cochlea, which may lead to hearing abnormalities including speech-in-noise difficulties, tinnitus, and hyperacusis. We have previously studied mice with blast-induced cochlear synaptopathy and found that they also developed a build-up of endolymph, termed endolymphatic hydrops. In this study, we used optical coherence tomography to measure endolymph volume in live CBA/CaJ mice exposed to various noise intensities. We quantified the number of synaptic ribbons and postsynaptic densities under the inner hair cells 1 week after noise exposure to determine if they correlated with acute changes in endolymph volume measured in the hours after the noise exposure. After 2 h of noise at an intensity of 95 dB SPL or below, both endolymph volume and synaptic counts remained normal. After exposure to 2 h of 100 dB SPL noise, mice developed endolymphatic hydrops and had reduced synaptic counts in the basal and middle regions of the cochlea. Furthermore, round-window application of hypertonic saline reduced the degree of endolymphatic hydrops that developed after 100 dB SPL noise exposure and partially prevented the reduction in synaptic counts in the cochlear base. Taken together, these results indicate that endolymphatic hydrops correlates with noise-induced cochlear synaptopathy, suggesting that these two pathologic findings have a common mechanistic basis.

VGLUT3-p.A211V variant fuses stereocilia bundles and elongates synaptic ribbons

Abstract

DFNA25 is an autosomal‐dominant and progressive form of human deafness caused by mutations in the SLC17A8 gene, which encodes the vesicular glutamate transporter type 3 (VGLUT3). To resolve the mechanisms underlying DFNA25, we studied phenotypes of mice harbouring the p.A221V mutation in humans (corresponding to p.A224V in mice). Using auditory brainstem response and distortion product otoacoustic emissions, we showed progressive hearing loss with intact cochlear amplification in the VGLUT3^A224V/A224V mouse. The summating potential was reduced, indicating the alteration of inner hair cell (IHC) receptor potential. Scanning electron microscopy examinations demonstrated the collapse of stereocilia bundles in IHCs, leaving those from outer hair cells unaffected. In addition, IHC ribbon synapses underwent structural and functional modifications at later stages. Using super‐resolution microscopy, we observed oversized synaptic ribbons and patch‐clamp membrane capacitance measurements showed an increase in the rate of the sustained releasable pool exocytosis. These results suggest that DFNA25 stems from a failure in the mechano‐transduction followed by a change in synaptic transfer. The VGLUT3^A224V/A224V mouse model opens the way to a deeper understanding and to a potential treatment for DFNA25.

Key points

The vesicular glutamate transporter type 3 (VGLUT3) loads glutamate into the synaptic vesicles of auditory sensory cells, the inner hair cells (IHCs).

The VGLUT3‐p.A211V variant is associated with human deafness DFNA25.

Mutant mice carrying the VGLUT3‐p.A211V variant show progressive hearing loss.

IHCs from mutant mice harbour distorted stereocilary bundles, which detect incoming sound stimulation, followed by oversized synaptic ribbons, which release glutamate onto the afferent nerve fibres.

These results suggest that DFNA25 stems from the failure of auditory sensory cells to faithfully transduce acoustic cues into neural messages.

Estradiol Protects against Noise-Induced Hearing Loss and Modulates Auditory Physiology in Female Mice

Recent studies have identified sex-differences in auditory physiology and in the susceptibility to noise-induced hearing loss (NIHL). We hypothesize that 17β-estradiol (E2), a known modulator of auditory physiology, may underpin sex-differences in the response to noise trauma. Here, we gonadectomized B6CBAF1/J mice and used a combination of electrophysiological and histological techniques to study the effects of estrogen replacement on peripheral auditory physiology in the absence of noise exposure and on protection from NIHL. Functional analysis of auditory physiology in gonadectomized female mice revealed that E2-treatment modulated the peripheral response to sound in the absence of changes to the endocochlear potential compared to vehicle-treatment. E2-replacement in gonadectomized female mice protected against hearing loss following permanent threshold shift (PTS)- and temporary threshold shift (TTS)-inducing noise exposures. Histological analysis of the cochlear tissue revealed that E2-replacement mitigated outer hair cell loss and cochlear synaptopathy following noise exposure compared to vehicle-treatment. Lastly, using fluorescent in situ hybridization, we demonstrate co-localization of estrogen receptor-2 with type-1C, high threshold spiral ganglion neurons, suggesting that the observed protection from cochlear synaptopathy may occur through E2-mediated preservation of these neurons. Taken together, these data indicate the estrogen signaling pathways may be harnessed for the prevention and treatment of NIHL.

Sensory transduction is required for normal development and maturation of cochlear inner hair cell synapses

Acoustic overexposure and aging can damage auditory synapses in the inner ear by a process known as synaptopathy. These insults may also damage hair bundles and the sensory transduction apparatus in auditory hair cells. However, a connection between sensory transduction and synaptopathy has not been established. To evaluate potential contributions of sensory transduction to synapse formation and development, we assessed inner hair cell synapses in several genetic models of dysfunctional sensory transduction, including mice lacking transmembrane channel-like (Tmc) 1, Tmc2, or both, in Beethoven mice which carry a dominant Tmc1 mutation and in Spinner mice which carry a recessive mutation in transmembrane inner ear (Tmie). Our analyses reveal loss of synapses in the absence of sensory transduction and preservation of synapses in Tmc1-null mice following restoration of sensory transduction via Tmc1 gene therapy. These results provide insight into the requirement of sensory transduction for hair cell synapse development and maturation.