Age-related cochlear synaptopathy: an early-onset contributor to auditory functional decline

Targeted spiral ganglion neuron degeneration in parvalbumin-Cre neonatal mice

The spiral ganglion neurons (SGNs) are the primary afferent neurons in the cochlea; damage to the SGNs leads to irreversible hearing impairment. Mouse models that allow selective SGN degeneration while sparing other cell types in the cochlea are lacking. Here, we investigated a genetic ablation method of the SGN using a Cre-responsive adeno-associated virus (AAV) vector expressing diphtheria toxin subunit-A (DTA). We microinjected AAV2-retro-FLEX-DTA-mCherry driven by the EF1a or hSYN promoter in neonatal parvalbumin-Cre (PVCre) and wild-type strains via the posterior semicircular canal. Apoptotic markers were observed in the degenerating SGNs as early as 3 days. After 1 week, we assessed the SGN cell density, revealing an average degeneration of 60% for AAV-DTA driven by the EF1a promoter and 61% for that driven by the hSYN promoter. By 1 month, injected ears demonstrated a nearly complete loss of SGN, while hair cell morphology was intact. The auditory brain stem response result showed significantly elevated threshold shifts at 1 month, while the distortion-product otoacoustic emissions function remained intact. Furthermore, we show that our method did not effectively ablate SGN in adult PVCre mice. We generated a neonatal mouse model with primary SGN degeneration in PVCre mice, mimicking auditory neuropathy phenotype using an AAV Cre-dependent expression of DTA.

Middle-aged CBA/CaJ mice exhibit auditory dysfunction in background noise

Aging is associated with deficits in auditory functioning. Characterization of auditory deficits that originate in middle-age is crucial for understanding the initial age-related functional impairments and the spatio-temporal progression of age-related auditory pathophysiology. Early age-related deficits in auditory processing are evident in difficult listening conditions, such as background noise, before becoming evident in quiet. To investigate the effect of noise on age-related auditory dysfunction, we collected suprathreshold auditory brainstem responses (ABRs) from young, middle-aged, and aged CBA/CaJ mice in quiet and broad-band background noise. We utilized multiple ABR metrics, including phase locking value (PLV), a measure of neural synchrony correlated to speech-in-noise understanding in humans. Despite no differences in auditory processing in quiet between young and middle-aged mice, middle-aged mice exhibited a distinct auditory phenotype from both young and aged mice in background noise conditions. We found that noise significantly decreased amplitude in middle-aged mice more than in young and aged mice. Noise significantly increased latencies for wave I and V in young mice, but only affected wave V in middle-aged mice and did not affect aged latencies. Noise significantly decreased PLV in middle-aged mice to a greater extent than in young mice, but to a lesser extent in aged mice. These results show that middle-aged mice have a distinct, auditory dysfunction phenotype evident in background noise. Our data show that suprathreshold auditory function in noise can identify early age-related hearing loss and can be used as a sensitive tool for detecting auditory dysfunction in normal hearing animal models.

Do Disinfectants Induce Ototoxicity and Cause Structural Damage to the Inner Ear?

BACKGROUND

This study examines the impact of commonly used disinfectants on hearing and the structural integrity of the inner ear.

OBJECTIVE

To evaluate the effects of indophor, alcohol, and chlorhexidine on auditory function and cochlear morphology in mice.

METHODS

Sixty-four mice were evenly divided into four groups: saline, indophor, alcohol, and chlorhexidine. Each mouse received a 25 μL injection of the designated liquid into the left ear, while the right ear was remained untreated. Auditory brainstem response (ABR) testing was performed at three intervals: before injection, 24 h post-injection, and one week post-injection. Cochlear tissues from both ears were analyzed using immunofluorescence and scanning electron microscopy to observe morphological changes.

RESULTS

Chlorhexidine and alcohol significantly increased ABR thresholds in the left ear 24 h and one week post-injection (p < 0.001). Chlorhexidine caused severe damage to outer hair cells, inner hair cells, and spiral ganglion cells, while alcohol had a milder effect. Iodophor mainly affected outer hair cells, with partial recovery after seven days. In the right ear, only chlorhexidine caused a sustained ABR threshold increase (p < 0.001) and severe damage to outer and inner hair cells. Both alcohol and chlorhexidine induced demyelination of spiral ganglion cells in the left ear, but recovery was seen only in the alcohol group after one week.

CONCLUSION

Indophor, alcohol, and chlorhexidine can all affect hearing, with alcohol and chlorhexidine causing more severe and lasting damage. These findings provide crucial insights for selecting disinfectants in ear surgeries.

LEVEL OF EVIDENCE

NA Laryngoscope, 135:2154-2163, 2025.

Cochlear neural contributions to triple network changes in tinnitus, hyperacusis & misophonia? A perspective review

What do tinnitus, the perception of sounds without an internal or external source of noise, hyperacusis, the pathological hypersensitivity to noise, or misophonia, an intolerance to certain everyday noises, have in common, and what differentiates them? A large number of excellent studies focused in the last few decades on identifying the neural correlates of tinnitus, hyperacusis, or misophonia on the basis of central triple-network changes. In this perspective review we explicitly examine, possible differential and causal involvement of peripheral components as a presumptive trigger that may drive observed triple-network changes. Based on our results, we venture to hypothesize that: (i) tinnitus, hyperacusis, and misophonia can occur despite clinically normal hearing thresholds, and are likely causally independent of sex and age, (ii) tinnitus and hyperacusis, but possibly also misophonia are related to altered auditory processing that through desynchronized (tinnitus) or hyperactive (hyperacusis, misophonia) bottom-up ascending processing potentially explains the activity changes in, e.g., default or salient brain networks, as suggested in various studies of these different diseases. (iii) In misophonia a stress-induced top-down influence, as deep as the auditory nerve fibers, may be discussed as a contributor to generating misophonia-trigger sounds, a hypothesis that can be tested in future studies. We hope that the selective consideration of a possible interaction between peripheral and central components will help to minimize the greatest handicap of these pathologies to date towards successful therapy: the lack of clarification of the underlying causative mechanism of the diseases.

Can Mismatch Negativity Be Used as an Indicator to Predict Central Auditory Deficits in Individuals with Normal Hearing?

Background/Objectives: In the early stage of presbycusis, patients experience reduced speech perception in noisy environments despite normal audiometry, normally known as hidden hearing loss. Diagnostic indicators like the reduced amplitude of ABR wave I, elevated extended high-frequency threshold (EHT), and decreased middle ear muscle reflex (MEMR) amplitude aim to identify biomarkers of peripheral auditory pathology but remain inconsistent. Mismatch negativity (MMN) is a cortical auditory evoked potential generated when the brain detects sound changes. This study aimed to assess MMN as a diagnostic tool for hidden hearing loss in adults. Methods: Seventy-three subjects with normal hearing underwent an extended pure-tone audiogram examination ranging from 0.125 to 16 kHz and a subsequent MMN assessment with two different paradigms: a speech (ba/da) and a tone (1/2 kHz) paradigm. The MMN's amplitude and latency were measured and analyzed. Results: The outcome shows a significant age-related effect on MMN amplitude in the speech condition (χ² = 13.0, p = 0.002). Specifically, the MMN amplitude in the 25-30-year-old group was significantly smaller than in the 20-25-year-old group (p = 0.0015, Cohen's d = 0.63). Additionally, no further effects of age were observed on the cortical potentials examined. Also, neither tone nor speech paradigms showed a significant influence of EHT on the amplitude or latency of either MMN or P300. Conclusions: The application of MMN as an electrophysiological tool to diagnose hidden hearing loss in normal hearing adults has limitations. However, in contrast to MMN responses to tonal stimuli, the present study reveals that MMN amplitude obtained with speech stimuli may indicate early signs of central auditory deficits.

The Medial Olivocochlear Efferent Pathway Potentiates Cochlear Amplification in Response to Hearing Loss

The mammalian cochlea receives efferent feedback from the brain. Many functions for this feedback have been hypothesized, including on short timescales, such as mediating attentional states, and long timescales, such as buffering acoustic trauma. Testing these hypotheses has been impeded by an inability to make direct measurements of efferent effects in awake animals. Here, we assessed the role of the medial olivocochlear (MOC) efferent nerve fibers on cochlear amplification by measuring organ of Corti vibratory responses to sound in both sexes of awake and anesthetized mice. We studied long-term effects by genetically ablating the efferents and/or afferents. Cochlear amplification increased with deafferentation using VGLUT3-/- mice, but only when the efferents were intact, associated with increased activity within OHCs and supporting cells. Removing both the afferents and the efferents using VGLUT3-/- Alpha9-/- mice did not cause this effect. To test for short-term effects, we recorded sound-evoked vibrations while using pupillometry to measure neuromodulatory brain state. We found no state dependence of cochlear amplification or of the auditory brainstem response. However, state dependence was apparent in the downstream inferior colliculus. Thus, MOC efferents upregulate cochlear amplification chronically with hearing loss, but not acutely with brain state fluctuations. This pathway may partially compensate for hearing loss while mediating associated symptoms, such as tinnitus and hyperacusis.

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.

Evidence for the Auditory Nerve Generating Envelope Following Responses When Measured from Eardrum Electrodes

Steady-state auditory evoked potentials are useful for studying the human auditory system and diagnosing hearing disorders. Identifying the generators of these potentials is essential for interpretation of data and for determining appropriate clinical and research applications. Here we infer putative generators of a steady-state potential measured from an electrode on the eardrum and compare this potential with the traditional envelope following response (EFR) measured from an electrode on the high forehead (N = 18, 10 female). We hypothesized that responses from the eardrum electrode would be consistent with an auditory nerve (AN) compound action potential (CAP) evoked by each cycle of the stimulus envelope, resulting in a potential we call CAPENV. Steady-state potentials were evoked by a 90 dB peSPL, 3000-Hz puretone carrier whose envelope was modulated by a tone sweep with frequencies from 20 to 160 Hz or 80 to 640 Hz. We calculated group delay to infer potential generators. We also compared the empirically measured CAPENV with simulated CAPENV from a humanized model of AN responses. Response latencies and model simulations support the interpretation that CAPENV is generated by the AN rather than hair cell or brainstem generators for all modulation frequencies tested. Conversely, latencies for the traditional EFR were consistent with a shift from cortical to brainstem generators as the modulation frequency increased from 20 to 200 Hz. We propose that CAPENV may be a fruitful tool for assessing AN function in humans with suspected AN fiber loss and/or temporal coding disorders.

Developing a Calibration Method to Minimize Variability in Auditory Evoked Potentials

PURPOSE

To reduce amplitude variability of auditory evoked potentials (AEPs) we developed a circuit that generates an electric calibration pulse (CalPulse) following each evoking sound presentation. We aim to determine if external CalPulse signals can function as a reliable calibration reference for AEP amplitude measurements.

METHODS

The CalPulse circuit was integrated with an AEP recording montage. The amplitude and morphology of two CalPulse signals (square wave and sine wave) was first assessed in vitro with electrodes submerged in saline. Repeatability of the two signals was then compared in vivo using five (3 male/2 female) 4-month-old CBA/CAJ mice and four unique auditory brainstem response (ABR) configurations. Sine wave CalPulse amplitudes were subsequently used to adjust raw ABR wave-1 amplitudes in a sample of 38 (19 male/19 female) CBA/CaJ mice. Variability in adjusted wave-1 amplitudes was compared with raw amplitudes. Measurements were repeated every month for 4 months (8 to 11 months old) to evaluate its potential as a tool to detect age-related changes in auditory function.

RESULTS

Wave quality examinations indicate that both CalPulse signal types are stable in vitro, with the sine wave signal being more repeatable when recorded in vivo. Sine wave CalPulse amplitudes correlated positively with ABR wave-1 amplitudes. Normalizing wave-1 amplitudes with CalPulse measures significantly reduced within-subject variability. Normalized wave-1 amplitudes showed a significant decrease at 10 months of age consistent with age-related cochlear synaptopathy, while uncalibrated wave-1 amplitudes from the same recordings failed to detect this decrease.

CONCLUSION

Our new calibration circuit can be used to improve diagnostic sensitivity of AEP measures.

A Systematic Review: State of the Science on Diagnostics of Hidden Hearing Loss

Background/Objectives: A sizeable population of patients with normal pure-tone audiograms endorse a consistent difficulty of following conversations in noisy environments. Termed hidden hearing loss (HHL), this condition evades traditional diagnostic methods for hearing loss and thus is significantly under-diagnosed and untreated. This review sought to identify emerging methods of diagnosing HHL via measurement of its histopathologic correlate: cochlear synaptopathy, the loss of synapses in the auditory nerve pathway. Methods: A thorough literature search of multiple databases was conducted to identify studies with objective, electrophysiological measures of synaptopathy. The PRISMA protocol was employed to establish criteria for the selection of relevant literature. Results: A total of 21 studies were selected with diagnostic methods, including the auditory brainstem response (ABR), electrocochleography (EcochG), middle ear muscle reflex (MEMR), and frequency-following response (FFR). Measures that may indicate the presence of synaptopathy include a reduced wave I amplitude of ABR, reduced SP amplitude of EcochG, and abnormal MEMR, among other measurements. Behavioral measures were often performed alongside electrophysiological measures, the most common of which was the speech-in-noise assessment. Conclusions: ABR was the most common diagnostic method for assessing HHL. Though ABR, EcochG, and MEMR may be sensitive to measuring synaptopathy, more literature comparing these methods is necessary. A two-pronged approach combining behavioral and electrophysiological measures may prove useful as a criterion for diagnosing and estimating the extent of pathology in affected patients.

NHANES-based machine learning for cognitive impairment classification and blood and hearing threshold characterization in age-related hearing loss

OBJECTIVE

This study aims to develop a machine learning-based classification model for cognitive impairment (CI) in elderly deaf patients and analyze the contributions of blood indices and hearing characteristics in identifying CI.

METHODS

Blood and audiometric data from 833 elderly deaf patients across three NHANES cycles were used to build a classification model with five algorithms: Logistic Regression, Random Forest (RF), XGBoost, Artificial Neural Networks (ANN), and Support Vector Machine (SVM). The optimal model was selected to rank feature importance.

RESULTS

The RF model, with an AUC of 0.834, performed best. Key predictors of CI included gender, systolic blood pressure, PTA+3kHz, neutrophil percentage, calcium, 6kHz hearing threshold, glycated hemoglobin, lymphocyte count,etc.

CONCLUSION

Hematological markers and hearing thresholds, especially the 3kHz threshold, are significant in identifying CI in ARHL, suggesting the need for further clinical exploration.

In vitro generation of spiral ganglion neurons from embryonic stem cells

Spiral ganglion neurons (SGNs) are crucial for transmitting auditory signals from the inner ear to the brainstem, playing a pivotal role in the peripheral hearing process. However, SGNs are usually damaged by a variety of insults, which causes permanent hearing loss. Generating SGNs from stem cells represents a promising strategy for advancing cell-replacement therapies to treat sensorineural hearing loss. SGNs comprise two subtypes of neurons (types 1 and 2); however, it remains a challenge to regenerate SGN subtypes. This study aimed to investigate the generation and characterization of SGN subtype neurons induced from embryonic stem cells (ESCs) in vitro. ESCs were cultured and treated with retinoic acid, followed by neuronal induction. The differentiated cells showed protein expressions of multiple neuronal markers, suggesting the generation of neuron-like cells. Protein expressions of vGlut-1 and GATA-3 indicate the generation of glutamatergic otic neuron-like cells. ESC-derived neuron-like cells cultured for 6 days showed co-expressions of calretinin, calbindin, and POU4F1 antibodies, suggesting an early stage of SGN subtype induction. However, 14-day in vitro induction generated cells showing two distinct SGN subtypes: a group of cells expressed calretinin (subtype 1a/2 precursor), and the other group expressed calbindin and POU4F1 (subtype 1b/c). These results suggest that in vitro generation of SGN subtypes from ESCs is culture time dependent.

Effects of age, stimulus degradation, and sentence context on auditory temporal processing

This study examined how age, stimulus degradation, and sentence context affect sensitivity to speech temporal cues. Adults aged 20-72 with self-reported normal hearing completed a phonemic categorization task using unprocessed and 8-channel vocoded (simulating stimulus degradation in cochlear implants) versions of a dent/tent contrast with varying voice-onset times. Target words were embedded in sentences biased toward "dent" or "tent," or presented in a neutral context. Advancing age reduced voice-onset time sensitivity regardless of sentence context, although this effect was less pronounced for vocoded stimuli. These results suggest that age-related temporal processing deficits may be unaffected by top-down linguistic influences.

Impact of tinnitus on chirp-evoked auditory brainstem response recorded using maximum length sequences

Tinnitus is considered a potential consequence of cochlear synaptopathy. While animal studies have shown that this neural deafferentation reduces wave I amplitudes of the auditory brainstem response (ABR) at suprathreshold levels, studies in humans with tinnitus have reported conflicting results. To enhance the sensitivity of ABRs in detecting neurophysiological conditions associated with tinnitus, this study aimed to examine chirp-evoked ABRs in normal-hearing individuals with tinnitus using the maximum length sequence (MLS) technique. Chirp stimuli improve synchronous neural discharge during ABR recording, while the MLS technique enables the extraction of evoked responses from overlapping waveforms. We hypothesized that this combined approach would more effectively reveal ABR morphological characteristics associated with tinnitus. The results indicated no significant difference in noise exposure between the tinnitus and control groups. However, the tinnitus group exhibited significantly larger wave I amplitude, prolonged wave V latency, and extended interpeak interval in MLS responses to chirps at the lowest stimulus rate of 13.8/s. These findings identify unique characteristics of MLS responses to chirps in individuals with tinnitus. We interpret our findings in relation to the ongoing discussion about the neurophysiological mechanisms of tinnitus. Further studies should be conducted to investigate possible etiologies of tinnitus.

Masking Effects Caused by Contralateral Distractors in Participants With Versus Without Listening Difficulties

OBJECTIVES

To examine the effects of distractor sounds presented to the contralateral ear on speech intelligibility in patients with listening difficulties without apparent peripheral pathology and in control participants.

DESIGN

This study examined and analyzed 15 control participants (age range, 22 to 30 years) without any complaints of listening difficulties and 15 patients (age range, 15 to 33 years) diagnosed as having listening difficulties without apparent peripheral pathology in the outpatient clinic of the Department of Otolaryngology-Head and Neck Surgery, Tohoku University Hospital. Speech intelligibility for 50 Japanese monosyllables presented to the right ear was examined under the following three different conditions: "without contralateral sound," "with continuous white noise in the contralateral ear," and "with music stimuli in the contralateral ear."

RESULTS

The results indicated the following: (1) speech intelligibility was significantly worse in the patient group with contralateral music stimuli and noise stimuli; (2) speech intelligibility was significantly worse with contralateral music stimuli than with contralateral noise stimuli in the patient group; (3) there was no significant difference in speech intelligibility among three contralateral masking conditions (without contra-stimuli, with contra-noise, and with contra-music) in the control group, although average and median values of speech intelligibility tended to be worse with contralateral music stimuli than without contralateral stimuli.

CONCLUSIONS

Significantly larger masking effects due to a contralateral distractor sound observed in patients with listening difficulties without apparent peripheral pathology may suggest the possible involvement of masking mechanisms other than the energetic masking mechanism occurring in the periphery in these patients. In addition, it was also shown that the masking effect is more pronounced with real environmental sounds, that is, music with lyrics, than with continuous steady noise, which is often used as a masker for speech-in-noise testing in clinical trials. In other words, it should be noted that a speech-in-noise test using such steady noise may underestimate the degree of listening problems of patients with listening difficulties in their daily lives, and a speech-in-noise test using a masker such as music and/or speech sounds could make listening problems more obvious in patients with listening difficulties.

The potential of mitochondrially-targeted tetrapeptide in protecting against noise-induced hearing impairment

Noise-induced hearing loss (NIHL) constitutes a significant global health issue for which there is no effective treatment. The loss of cochlear hair cells and associated synaptopathy are common causes of hearing impairment. One primary mechanism implicated in NIHL is the accumulation of reactive oxygen species (ROS), which ultimately overwhelms cochlear cells. ROS are detected in the cochlea immediately after noise exposure and persist for at least a week. Within cells, ROS are primarily generated in mitochondria as byproducts of cellular metabolism. Elamipretide is a synthetic tetrapeptide known to concentrate in mitochondria, improving mitochondrial function and reducing ROS production. To test the hypothesis that elamipretide treatment mitigates NIHL, 16-week-old male and female CBA/J mice were exposed to 8-16 kHz octave-band noise (OBN) at 98 dB SPL for 2 hours. Elamipretide was administered intraperitoneally immediately after noise exposure and continued for 2 weeks. Efficacy was evaluated based on auditory brainstem response (ABR) thresholds, wave amplitudes, and wave latencies in treated and control groups. Results showed that OBN-exposed mice exhibited an elevation in ABR thresholds at 16 and 32 kHz and a reduction in ABR wave-I amplitude at 32 kHz, although wave-I latencies were not affected at 16 or 32 kHz. Elamipretide treatment prevented the OBN-induced elevation of ABR thresholds and the attenuation of wave-I amplitude. These findings provide proof of concept that mitochondrial-targeted elamipretide can prevent NIHL in a mammalian model and highlight its potential to protect against NIHL in humans.

Deciphering Compromised Speech-in-Noise Intelligibility in Older Listeners: The Role of Cochlear Synaptopathy

Speech intelligibility declines with age and sensorineural hearing damage (SNHL). However, it remains unclear whether cochlear synaptopathy (CS), a recently discovered form of SNHL, significantly contributes to this issue. CS refers to damaged auditory-nerve synapses that innervate the inner hair cells and there is currently no go-to diagnostic test available. Furthermore, age-related hearing damage can comprise various aspects (e.g., hair cell damage, CS) that each can play a role in impaired sound perception. To explore the link between cochlear damage and speech intelligibility deficits, this study examines the role of CS for word recognition among older listeners. We first validated an envelope-following response (EFR) marker for CS using a Budgerigar model. We then applied this marker in human experiments, while restricting the speech material's frequency content to ensure that both the EFR and the behavioral tasks engaged similar cochlear frequency regions. Following this approach, we identified the relative contribution of hearing sensitivity and CS to speech intelligibility in two age-matched (65-year-old) groups with clinically normal (n = 15, 8 females) or impaired audiograms (n = 13, 8 females). Compared to a young normal-hearing control group (n = 13, 7 females), the older groups demonstrated lower EFR responses and impaired speech reception thresholds. We conclude that age-related CS reduces supra-threshold temporal envelope coding with subsequent speech coding deficits in noise that cannot be explained based on hearing sensitivity alone.

Early transtympanic administration of rhBDNF exerts a multifaceted neuroprotective effect against cisplatin-induced hearing loss

BACKGROUND AND PURPOSE

Cisplatin-induced sensorineural hearing loss is a significant clinical challenge. Although the potential effects of brain-derived neurotrophic factor (BDNF) have previously been investigated in some ototoxicity models, its efficacy in cisplatin-induced hearing loss remains uncertain. This study aimed to investigate the therapeutic potential of recombinant human BDNF (rhBDNF) in protecting cells against cisplatin-induced ototoxicity.

EXPERIMENTAL APPROACH

Using an in vivo model of cisplatin-induced hearing loss, we investigated the beneficial effects of transtympanic administration of rhBDNF in a thermogel solution on hearing function and cochlear injury, using electrophysiological, morphological, immunofluorescence and molecular analyses.

KEY RESULTS

Our data showed that local rhBDNF treatment counteracted hearing loss in rats receiving cisplatin by preserving synaptic connections in the cochlear epithelium and protecting hair cells (HCs) and spiral ganglion neurons (SGNs) against cisplatin-induced cell death. Specifically, rhBDNF maintains the balance of its receptor levels (pTrkB and p75), boosting TrkB-CREB pro-survival signalling and reducing caspase 3-dependent apoptosis in the cochlea. Additionally, it activates antioxidant mechanisms while inhibiting inflammation and promoting vascular repair.

CONCLUSION AND IMPLICATIONS

Collectively, we demonstrated that early transtympanic treatment with rhBDNF plays a multifaceted protective role against cisplatin-induced ototoxicity, thus holding promise as a novel potential approach to preserve hearing in adult and paediatric patients undergoing cisplatin-based chemotherapy.

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.

The Inter-Phase Gap Offset Effect as a Measure of Neural Health in Cochlear Implant Users With Residual Acoustic Hearing

OBJECTIVES

The inter-phase gap (IPG) offset effect is defined as the dB offset between the linear parts of electrically evoked compound action potential (ECAP) amplitude growth functions for two stimuli differing only in IPG. The method was recently suggested to represent neural health in cochlear implant (CI) users while being unaffected by CI electrode impedances. Hereby, a larger IPG offset effect should reflect better neural health. The aims of the present study were to (1) examine whether the IPG offset effect negatively correlates with the ECAP threshold and the preoperative pure-tone average (PTA) in CI recipients with residual acoustic hearing and (2) investigate the dependency of the IPG offset effect on hair cell survival and intracochlear electrode impedances.

DESIGN

Seventeen adult study participants with residual acoustic hearing at 500 Hz undergoing CI surgery at the University Hospital of Zurich were prospectively enrolled. ECAP thresholds, IPG offset effects, electrocochleography (ECochG) responses to 500 Hz tone bursts, and monopolar electrical impedances were obtained at an apical, middle, and basal electrode set during and between 4 and 12 weeks after CI surgery. Pure-tone audiometry was conducted within 3 weeks before surgery and approximately 6 weeks after surgery. Linear mixed regression analyses and t tests were performed to assess relationships between (changes in) ECAP threshold, IPG offset, impedance, PTA, and ECochG amplitude.

RESULTS

The IPG offset effect positively correlated with the ECAP threshold in intraoperative recordings ( p < 0.001) and did not significantly correlate with the preoperative PTA ( p = 0.999). The IPG offset showed a postoperative decrease for electrode sets that showed an ECochG amplitude drop. This IPG offset decrease was significantly larger than for electrode sets that showed no ECochG amplitude decrease, t (17) = 2.76, p = 0.014. Linear mixed regression analysis showed no systematic effect of electrode impedance changes on the IPG offset effect ( p = 0.263) but suggested a participant-dependent effect of electrode impedance on IPG offset.

CONCLUSIONS

The present study results did not reveal the expected relationships between the IPG offset effect and ECAP threshold values or between the IPG offset effect and preoperative acoustic hearing. Changes in electrode impedance did not exhibit a direct impact on the IPG offset effect, although this impact might be individualized among CI recipients. Overall, our findings suggest that the interpretation and application of the IPG offset effect in clinical settings should be approached with caution considering its complex relationships with other cochlear and neural health metrics.

Age dependent deficits in speech recognition in quiet and noise are reflected in MGB activity and cochlear onset coding

The slowing and reduction of auditory responses in the brain are recognized side effects of increased pure tone thresholds, impaired speech recognition, and aging. However, it remains controversial whether central slowing is primarily linked to brain processes as atrophy, or is also associated with the slowing of temporal neural processing from the periphery. Here we analyzed electroencephalogram (EEG) responses that most likely reflect medial geniculate body (MGB) responses to passive listening of phonemes in 80 subjects ranging in age from 18 to 76 years, in whom the peripheral auditory responses had been analyzed in detail (Schirmer et al., 2024). We observed that passive listening to vowels and phonemes, specifically designed to rely on either temporal fine structure (TFS) for frequencies below the phase locking limit (<1500 Hz), or on the temporal envelope (TENV) for frequencies above phase locking limit, entrained lower or higher neural EEG responses. While previous views predict speech content, particular in noise to be encoded through TENV, here a decreasing phoneme-induced EEG amplitude over age in response to phonemes relying on TENV coding could also be linked to poorer speech-recognition thresholds in quiet. In addition, increased phoneme-evoked EEG delay could be correlated with elevated extended high-frequency threshold (EHF) for phoneme changes that relied on TFS and TENV coding. This may suggest a role of pure-tone threshold averages (PTA) of EHF for TENV and TFS beyond sound localization that is reflected in likely MGB delays. When speech recognition thresholds were normalized for pure-tone thresholds, however, the EEG amplitudes remained insignificant, and thereby became independent of age. Under these conditions, poor speech recognition in quiet was found together with a delay in EEG response for phonemes that relied on TFS coding, while poor speech recognition in ipsilateral noise was observed as a trend of shortened EEG delays for phonemes that relied on TENV coding. Based on previous analyses performed in these same subjects, elevated thresholds in extended high-frequency regions were linked to cochlear synaptopathy and auditory brainstem delays. Also, independent of hearing loss, poor speech-performing groups in quiet or with ipsilateral noise during TFS or TENV coding could be linked to lower or better outer hair cell performance and delayed or steeper auditory nerve responses at stimulus onset. The amplitude and latency of MGB responses to phonemes requiring TFS or TENV coding, dependent or independent of hearing loss, may thus be a new predictor of poor speech recognition in quiet and ipsilateral noise that links deficits in synchronicity at stimulus onset to neocortical activity. Amplitudes and delays of speech EEG responses to syllables should be reconsidered for future hearing-aid studies.

Sensory representations and pupil-indexed listening effort provide complementary contributions to multi-talker speech intelligibility

Multi-talker speech intelligibility requires successful separation of the target speech from background speech. Successful speech segregation relies on bottom-up neural coding fidelity of sensory information and top-down effortful listening. Here, we studied the interaction between temporal processing measured using Envelope Following Responses (EFRs) to amplitude modulated tones, and pupil-indexed listening effort, as it related to performance on the Quick Speech-in-Noise (QuickSIN) test in normal-hearing adults. Listening effort increased at the more difficult signal-to-noise ratios, but speech intelligibility only decreased at the hardest signal-to-noise ratio. Pupil-indexed listening effort and EFRs did not independently relate to QuickSIN performance. However, the combined effects of both EFRs and listening effort explained significant variance in QuickSIN performance. Our results suggest a synergistic interaction between sensory coding and listening effort as it relates to multi-talker speech intelligibility. These findings can inform the development of next-generation multi-dimensional approaches for testing speech intelligibility deficits in listeners with normal-hearing.

Auditory brainstem responses as a biomarker for cognition

A non-invasive, accessible and effective biomarker is critical to the diagnosis, monitoring and treatment of age-related cognitive decline. Recent work has suggested a strong association between auditory brainstem responses (ABR) and cognitive function in aging macaques. Here we show in 118 human participants (66 females; age range=18-92 years; hearing loss = -5 to 70 dB HL) that cognition is associated with both age and hearing level, but this triad relationship is mainly driven by the age factor. After adjusting for age, cognition is still significantly associated with both the ABR wave V amplitude (B, 0.110, 95% CI, 0.018- 0.202; p = 0.020) and latency (B, -0.101, 95% CI, -0.186- -0.016; p = 0.021). Importantly, this age-adjusted ABR-cognition association is primarily driven by older individuals and language-dependent cognitive functions. We also perform the area under the curve (AUC) of the receiver-operating-characteristic analysis and find that the ABR wave V amplitude is best for detecting good cognitive performers (AUC = 0.96) whereas the wave V latency is best for detecting poor ones (AUC = 0.86). The present result not only confirms the previous animal work in humans but also shows the clinical potential of using auditory brainstem responses to improve diagnosis and treatment of age-related cognitive decline.

In vivo AAV9-Myo7a gene rescue restores hearing and cholinergic efferent innervation in inner hair cells

In the mammalian cochlea, sensory hair cells are crucial for the transduction of acoustic stimuli into electrical signals, which are then relayed to the central auditory pathway via spiral ganglion neuron (SGN) afferent dendrites. The SGN output is directly modulated by inhibitory cholinergic axodendritic synapses from the efferent fibers originating in the superior olivary complex. When the adult cochlea is subjected to noxious stimuli or aging, the efferent system undergoes major rewiring, such that it reestablishes direct axosomatic contacts with the inner hair cells (IHCs), which occur only transiently during prehearing stages of development. The trigger, origin, and degree of efferent plasticity in the cochlea remains largely unknown. Using functional and morphological approaches, we demonstrate that efferent plasticity in the adult cochlea occurs as a direct consequence of mechanoelectrical transducer current dysfunction. We also show that, different from prehearing stages of development, the lateral olivocochlear - but not the medial olivocochlear - efferent fibers are those that form the axosomatic synapses with the IHCs. The study also demonstrates that in vivo restoration of IHC function using AAV-Myo7a rescue reestablishes the synaptic profile of adult IHCs and improves hearing, highlighting the potential of using gene-replacement therapy for progressive hearing loss.

Upregulation of the Cav1.3 channel in inner hair cells by interleukin 6-dependent inflammaging contributes to age-related hearing loss

Age-related hearing loss (AHL) is the most common sensory disorder amongst the older population. Inflammaging is a ≈chronic low-grade inflammation that worsens with age and is an early sign of AHL; however, the underlying mechanisms remain unclear. We used electrophysiological and genetic approaches to establish the importance of interleukin 6 (IL-6)-dependent inflammation in AHL. Elevated IL-6 in the cochlea enhanced Cav1.3 calcium channel function in the inner hair cell (IHC) synapse in mice with AHL. IL-6 upregulated the Cav1.3 channel via the Janus kinase-mitogen activated kinase pathway, causing neurotransmitter excitotoxicity and synapse impairment; IL-6 deficiency or the administration of a Cav1.3 channel blocker attenuated this age-related damage, and rescued hearing loss. Thus, IL-6-dependent inflammaging upregulated the Cav1.3 channel in IHCs, contributing to AHL. Our findings could help the comprehensive understanding of inflammaging's effects on AHL, aiding in early intervention to protect against hearing decline.

Rodent models in sensorineural hearing loss research: A comprehensive review

Sensorineural hearing loss (SNHL) constitutes a major global health challenge, affecting millions of individuals and substantially impairing social integration and quality of life. The complexity of the auditory system and the multifaceted nature of SNHL necessitate advanced methodologies to understand its etiology, progression, and potential therapeutic interventions. This review provides a comprehensive overview of the current animal models used in SNHL research, focusing on their selection based on specific characteristics and their contributions to elucidating pathophysiological mechanisms and evaluating novel treatment strategies. It discusses the most commonly used rodent models in hearing research, including mice, rats, guinea pigs, Mongolian gerbils, and chinchillas. Through a comparative analysis, this review underscores the importance of selecting models that align with specific research objectives in SNHL studies, discussing the advantages and limitations of each model. By advocating for a multidisciplinary approach that leverages the strengths of various animal models with technological advancements, this review aims to facilitate significant advancements in the prevention, diagnosis, and treatment of sensorineural hearing loss.

An electrophysiological early marker of age-related hearing loss in the Wistar rat model

The goal of the present study was to determine, through a detailed study of the auditory brainstem response (ABR) waves, the possible existence of an early functional marker for the onset of presbycusis in an animal model. Toward this goal, Wistar rats were divided into four age groups: 3-month-old (3M, n = 6, control), 9-month-old (9M, n = 6), 14-month-old (14M, n = 6), and 20-month-old (20M, n = 6). ABR recordings were performed at 0.5, 1, 2, 4, 8, 16, and 32 kHz. The novel result reported here is that wave amplitudes, particularly wave II, were significantly diminished in the 9M group, even though there was no evidence of significant age-related threshold shift at that age. A significant increase in auditory thresholds with age was first detected at 14M, which further progressed at 20M, confirming our previous findings. These findings suggest that measurable alterations in ABR waves may precede age-related threshold shift and could serve as early markers to detect the onset of age-related hearing loss. Upon translation to humans, they could be used to implement early objective diagnosis, crucial to prevent or mitigate the negative consequences of presbycusis, a common, progressive, and irreversible neurodegenerative age-related disorder. This may allow, for instance, a better preservation of residual hearing, thus delaying the progression of the disease and minimizing the impact of hearing loss, ultimately improving the quality of life for those who suffer from this neurodegenerative condition.

Semicircular canal drug delivery safely targets the inner ear perilymphatic space

Effective, reproducible, and safe delivery of therapeutics into the inner ear is required for the prevention and treatment of hearing loss. A commonly used delivery method is via the posterior semicircular canal (PSCC); however, its specific targeting within the cochlea remains unclear, impacting precision and reproducibility. To assess safety and target specificity, we conducted in vivo recordings of the pharmacological manipulations delivered through the PSCC. Measurements of auditory brainstem response (ABR), vibrometry, and vestibular behavioral and sensory-evoked potential (VsEP) revealed preserved hearing and vestibular functions after artificial perilymph injections. Injection of curare, a mechanoelectrical transducer (MET) channel blocker that affects hearing when in the endolymph, had no effect on ABR or VsEP thresholds. Conversely, injection of CNQX, an AMPA receptor blocker, or lidocaine, a Na+ channel blocker, which affects hearing when in the perilymph, significantly increased both thresholds, indicating that PSCC injections selectively target the perilymphatic space. In vivo tracking of gold nanoparticles confirmed their exclusive distribution in the perilymph during PSCC injection, supporting the pharmacological finding. Together, PSCC injection is a safe method for inner ear delivery, specifically targeting the perilymphatic space. Our findings will allow for precise delivery of therapeutics within the inner ear for therapeutic and research purposes.

Genetic and pharmacologic alterations of claudin9 levels suffice to induce functional and mature inner hair cells

Hearing loss is the most common form of sensory deficit. It occurs predominantly due to hair cell (HC) loss. Mammalian HCs are terminally differentiated by birth, making HC loss challenging to replace. Here, we show the pharmacogenetic downregulation of Cldn9, a tight junction protein, generates robust supernumerary inner HCs (IHCs) in mice. The ectopic IHC shared functional and synaptic features akin to typical IHCs and were surprisingly and remarkably preserved for at least fifteen months >50% of the mouse's life cycle. In vivo, Cldn9 knockdown using shRNA on postnatal days (P) P2-7 yielded analogous functional ectopic IHCs that were equally durably conserved. The findings suggest that Cldn9 levels coordinate embryonic and postnatal HC differentiation, making it a viable target for altering IHC development pre- and post-terminal differentiation.

Does age protect against loss of tonotopy after acute deafness in adulthood?

The mammalian auditory system develops a topographical representation of sound frequencies along its pathways, also called tonotopy. In contrast, sensory deprivation during early development results in no or only rudimentary tonotopic organization. This study addresses two questions: (1) How robust is the central tonotopy when hearing fails in adulthood? (2) What role does age play at time of deafness? To address these questions, we deafened young and old adult rats with previously normal hearing. One month after deafening, both groups were unilaterally supplied with cochlear implants and electrically stimulated for 2 h. The central auditory neurons, which were activated as a result of the local electrical intracochlear stimulation, were visualized using Fos staining. While the auditory system of young rats lost the tonotopic organization throughout the brainstem, the auditory system of the older rats mainly sustained its tonotopy. It can be proposed that plasticity prevails in the central auditory system of young adult rats, while network stability prevails in the brains of aging rats. Consequently, age may be an important factor in protecting a hearing-experienced adult auditory system from a rapid loss of tonotopy when suffering from acute hearing loss. Furthermore, the study provides compelling evidence that acute deafness in young adult patients should be diagnosed as early as possible to prevent maladaptation of the central auditory system and thus achieve the optimal hearing outcome with a hearing prosthesis.

Bats as instructive animal models for studying longevity and aging

Bats (order Chiroptera) are emerging as instructive animal models for aging studies. Unlike some common laboratory species, they meet a central criterion for aging studies: they live for a long time in the wild or in captivity, for 20, 30, and even >40 years. Healthy aging (i.e., healthspan) in bats has drawn attention to their potential to improve the lives of aging humans due to bat imperviousness to viral infections, apparent low rate of tumorigenesis, and unique ability to repair DNA. At the same time, bat longevity also permits the accumulation of age-associated systemic pathologies that can be examined in detail and manipulated, especially in captive animals. Research has uncovered additional and critical advantages of bats. In multiple ways, bats are better analogs to humans than are rodents. In this review, we highlight eight diverse areas of bat research with relevance to aging: genome sequencing, telomeres, and DNA repair; immunity and inflammation; hearing; menstruation and menopause; skeletal system and fragility; neurobiology and neurodegeneration; stem cells; and senescence and mortality. These examples demonstrate the broad relevance of the bat as an animal model and point to directions that are particularly important for human aging studies.

Resistance to age-related hearing loss in the echolocating big brown bat (Eptesicus fuscus)

Hearing mediates many behaviours critical for survival in echolocating bats, including foraging and navigation. Although most mammals are susceptible to progressive age-related hearing loss, the evolution of biosonar, which requires the ability to hear low-intensity echoes from outgoing sonar signals, may have selected against the development of hearing deficits in bats. Many echolocating bats exhibit exceptional longevity and rely on acoustic behaviours for survival to old age; however, relatively little is known about the ageing bat auditory system. In this study, we used DNA methylation to estimate the ages of wild-caught big brown bats (Eptesicus fuscus) and measured hearing sensitivity in young and ageing bats using auditory brainstem responses (ABRs) and distortion product otoacoustic emissions (DPOAEs). We found no evidence for hearing deficits in bats up to 12.5 years of age, demonstrated by comparable thresholds and similar ABR and DPOAE amplitudes across age groups. We additionally found no significant histological evidence for cochlear ageing, with similar hair cell counts, afferent and efferent innervation patterns in young and ageing bats. Here, we demonstrate that big brown bats show minimal evidence for age-related hearing loss and therefore represent informative models for investigating mechanisms that may preserve hearing function over a long lifetime.

A primate model animal revealed the inter-species differences and similarities in the subtype specifications of the spiral ganglion neurons

Type I spiral ganglion neurons are peripheral neurons essential for hearing perception. While they can be subdivided in mice based on characteristic gene expression patterns, detailed examinations of these subtypes in primates and humans are lacking. In this study, we investigated the developmental subtypes of spiral ganglion neurons in the common marmoset (Callithrix jacchus). We confirmed that Type I spiral ganglion can be divided based on the characteristic gene expression patterns of several marker genes. However, some combinations of these genes differ from those in rodents, suggesting common marmoset's suitability for advancing our understanding of human cochlear development. Additionally, identifying the essential time points for subtype specifications and subsequent maturation will aid in studying the primate-specific developmental biology of the inner ear. This could lead to new treatment strategies for hearing loss in humans and be valuable for studying age-related hearing loss, as well as designing regenerative therapies.

Pathological mechanisms and candidate therapeutic approaches in the hearing loss of mice carrying human MIR96 mutations

BACKGROUND

Progressive hearing loss is a common problem in the human population with no effective therapeutics currently available. However, it has a strong genetic contribution, and investigating the genes and regulatory interactions underlying hearing loss offers the possibility of identifying therapeutic candidates. Mutations in regulatory genes are particularly useful for this, and an example is the microRNA miR-96, a post-transcriptional regulator which controls hair cell maturation. Mice and humans carrying mutations in miR-96 all exhibit hearing impairment, in homozygosis if not in heterozygosis, but different mutations result in different physiological, structural and transcriptional phenotypes.

METHODS

Here we present our characterisation of two lines of mice carrying different human mutations knocked-in to Mir96. We have carried out auditory brainstem response tests to examine their hearing with age and after noise exposure and have used confocal and scanning electron microscopy to examine the ultrastructure of the organ of Corti and hair cell synapses. Bulk RNA-seq was carried out on the organs of Corti of postnatal mice, followed by bioinformatic analyses to identify candidate targets.

RESULTS

While mice homozygous for either mutation are profoundly deaf from 2 weeks old, the heterozygous phenotypes differ markedly, with only one mutation resulting in hearing impairment in heterozygosis. Investigations of the structural phenotype showed that one mutation appears to lead to synaptic defects, while the other has a much more severe effect on the hair cell stereociliary bundles. Transcriptome analyses revealed a wide range of misregulated genes in both mutants which were notably dissimilar. We used the transcriptome analyses to investigate candidate therapeutics, and tested one, finding that it delayed the progression of hearing loss in heterozygous mice.

CONCLUSIONS

Our work adds further support for the importance of the gain of novel targets in microRNA mutants and offers a proof of concept for the identification of pharmacological interventions to maintain hearing.

Pharmacological Approaches to Hearing Loss

Hearing disorders pose significant challenges to individuals experiencing them and their overall quality of life, emphasizing the critical need for advanced pharmacological approaches to address these conditions. Current treatment options often focus on amplification devices, cochlear implants, or other rehabilitative therapies, leaving a substantial gap regarding effective pharmacological interventions. Advancements in our understanding of the molecular and cellular mechanisms involved in hearing disorders induced by noise, aging, and ototoxicity have opened new avenues for drug development, some of which have led to numerous clinical trials, with promising results. The development of optimal drug delivery solutions in animals and humans can also enhance the targeted delivery of medications to the ear. Moreover, large genome studies contributing to a genetic understanding of hearing loss in humans combined with advanced molecular technologies in animal studies have shown a great potential to increase our understanding of the etiologies of hearing loss. The auditory system exhibits circadian rhythms and temporal variations in its physiology, its vulnerability to auditory insults, and its responsiveness to drug treatments. The cochlear clock rhythms are under the control of the glucocorticoid system, and preclinical evidence suggests that the risk/benefit profile of hearing disorder treatments using chronopharmacological approaches would be beneficial. If translatable to the bedside, such approaches may improve the outcome of clinical trials. Ongoing research into the molecular and genetic basis of auditory disorders, coupled with advancements in drug formulation and delivery as well as optimized timing of drug administration, holds great promise of more effective treatments. SIGNIFICANCE STATEMENT: Hearing disorders pose significant challenges to individuals and their overall quality of life, emphasizing the critical need for advanced pharmacological approaches to address these conditions. Ongoing research into the molecular and genetic basis of auditory disorders, coupled with advancements in drug delivery procedures and optimized timing of drug administration, holds the promise of more effective treatments.

Hidden hearing loss in a Charcot-Marie-Tooth type 1A mouse model

Hidden hearing loss (HHL), a recently described auditory neuropathy characterized by normal audiometric thresholds but reduced sound-evoked cochlear compound action potentials, has been proposed to contribute to hearing difficulty in noisy environments in people with normal hearing thresholds and has become a widespread complaint. While most studies on HHL pathogenesis have focused on inner hair cell (IHC) synaptopathy, we recently showed that transient auditory nerve (AN) demyelination also causes HHL in mice. To test the effect of myelinopathy on hearing in a clinically relevant model, we studied a mouse model of Charcot-Marie-Tooth type 1A (CMT1A), the most prevalent hereditary peripheral neuropathy in humans. CMT1A mice exhibited the functional hallmarks of HHL together with disorganization of AN heminodes near the IHCs with minor loss of AN fibers. These results support the hypothesis that mild disruptions of AN myelination can cause HHL and that heminodal defects contribute to the alterations in the sound-evoked cochlear compound action potentials seen in this mouse model. Furthermore, these findings suggest that patients with CMT1A or other mild peripheral neuropathies are likely to suffer from HHL. Furthermore, these results suggest that studies of hearing in patients with CMT1A might help develop robust clinical tests for HHL, which are currently lacking.

Gap detection ability declines with central auditory neurodegeneration following age-related cochlear synaptopathy

Age-related hearing impairment (ARHI) is commonly associated with decreased auditory temporal resolution caused by auditory neurodegeneration. Age-related deterioration in gap detection ability, resulting in poor temporal auditory processing, is often attributed to pathophysiological changes in both the peripheral and central auditory systems. This study aimed to investigate whether the gap detection ability declines in the early stages of ageing and to determine its usefulness in detecting peripheral and central auditory degeneration. The study used 1-month-old (1 M), 6-month-old (6 M) and 12-month-old (12 M) mice to examine changes in gap detection ability and associated auditory pathophysiology. Although hearing thresholds did not significantly differ between the groups, the amplitude of auditory brainstem response (ABR) wave I decreased significantly in an age-dependent manner, consistent with age-related cochlear synaptopathy. The relative ABR amplitude ratio of waves 2 and 5 to wave 1 was significantly increased in 12 M mice, indicating that the central auditory system had increased in relative neuroactivity. A significant increase in gap detection thresholds was observed in 12 M mice compared to 1 M mice. Although cochlear synaptopathy and central hyperactivity were positively correlated with gap detection thresholds, central hyperactivity strongly influenced gap detection ability. In the cochlear nucleus and auditory cortex, the inhibitory synaptic expression of GAD65 and the expression of parvalbumin were significantly decreased in 12 M mice, consistent with central hyperactivity. Evaluating gap detection performance may allow the identification of decreased auditory temporal resolution in the early stages of ARHI, which is strongly associated with auditory neurodegeneration.

Presynaptic Nrxn3 is essential for ribbon-synapse maturation in hair cells

Hair cells of the inner ear and lateral-line system rely on specialized ribbon synapses to transmit sensory information to the central nervous system. The molecules required to assemble these synapses are not fully understood. We show that Nrxn3, a presynaptic adhesion molecule, is crucial for ribbon-synapse maturation in hair cells. In both mouse and zebrafish models, the loss of Nrxn3 results in significantly fewer intact ribbon synapses. We show in zebrafish that, initially, Nrxn3 loss does not alter pre- and postsynapse numbers but, later, synapses fail to pair, leading to postsynapse loss. We also demonstrate that Nrxn3 subtly influences synapse selectivity in zebrafish lateral-line hair cells that detect anterior flow. Loss of Nrxn3 leads to a 60% loss of synapses in zebrafish, which dramatically reduces pre- and postsynaptic responses. Despite fewer synapses, auditory responses in zebrafish and mice are unaffected. This work demonstrates that Nrxn3 is a crucial and conserved molecule required for the maturation of ribbon synapses. Understanding how ribbon synapses mature is essential to generating new therapies to treat synaptopathies linked to auditory or vestibular dysfunction.

Midlife Speech Perception Deficits: Impact of Extended High-Frequency Hearing, Peripheral Neural Function, and Cognitive Abilities

OBJECTIVES

The objectives of the present study were to investigate the effects of age-related changes in extended high-frequency (EHF) hearing, peripheral neural function, working memory, and executive function on speech perception deficits in middle-aged individuals with clinically normal hearing.

DESIGN

We administered a comprehensive assessment battery to 37 participants spanning the age range of 20 to 56 years. This battery encompassed various evaluations, including standard and EHF pure-tone audiometry, ranging from 0.25 to 16 kHz. In addition, we conducted auditory brainstem response assessments with varying stimulation rates and levels, a spatial release from masking (SRM) task, and cognitive evaluations that involved the Trail Making test (TMT) for assessing executive function and the Abbreviated Reading Span test (ARST) for measuring working memory.

RESULTS

The results indicated a decline in hearing sensitivities at EHFs and an increase in completion times for the TMT with age. In addition, as age increased, there was a corresponding decrease in the amount of SRM. The declines in SRM were associated with age-related declines in hearing sensitivity at EHFs and TMT performance. While we observed an age-related decline in wave I responses, this decline was primarily driven by age-related reductions in EHF thresholds. In addition, the results obtained using the ARST did not show an age-related decline. Neither the auditory brainstem response results nor ARST scores were correlated with the amount of SRM.

CONCLUSIONS

These findings suggest that speech perception deficits in middle age are primarily linked to declines in EHF hearing and executive function, rather than cochlear synaptopathy or working memory.

The role of hidden hearing loss in tinnitus: Insights from early markers of peripheral hearing damage

Since the presence of tinnitus is not always associated with audiometric hearing loss, it has been hypothesized that hidden hearing loss may act as a potential trigger for increased central gain along the neural pathway leading to tinnitus perception. In recent years, the study of hidden hearing loss has improved with the discovery of cochlear synaptopathy and several objective diagnostic markers. This study investigated three potential markers of peripheral hidden hearing loss in subjects with tinnitus: extended high-frequency audiometric thresholds, the auditory brainstem response, and the envelope following response. In addition, speech intelligibility was measured as a functional outcome measurement of hidden hearing loss. To account for age-related hidden hearing loss, participants were grouped according to age, presence of tinnitus, and audiometric thresholds. Group comparisons were conducted to differentiate between age- and tinnitus-related effects of hidden hearing loss. All three markers revealed age-related differences, whereas no differences were observed between the tinnitus and non-tinnitus groups. However, the older tinnitus group showed improved performance on low-pass filtered speech in noise tests compared to the older non-tinnitus group. These low-pass speech in noise scores were significantly correlated with tinnitus distress, as indicated using questionnaires, and could be related to the presence of hyperacusis. Based on our observations, cochlear synaptopathy does not appear to be the underlying cause of tinnitus. The improvement in low-pass speech-in-noise could be explained by enhanced temporal fine structure encoding or hyperacusis. Therefore, we recommend that future tinnitus research takes into account age-related factors, explores low-frequency encoding, and thoroughly assesses hyperacusis.

Auditory brainstem response to paired clicks as a candidate marker of cochlear synaptopathy in humans

OBJECTIVE

This study aimed to evaluate whether auditory brainstem response (ABR) using a paired-click stimulation paradigm could serve as a tool for detecting cochlear synaptopathy (CS).

METHODS

The ABRs to single-clicks and paired-clicks with various inter-click intervals (ICIs) and scores for word intelligibility in degraded listening conditions were obtained from 57 adults with normal hearing. The wave I peak amplitude and root mean square values for the post-wave I response within a range delayed from the wave I peak (referred to as the RMSpost-w1) were calculated for the single- and second-click responses.

RESULTS

The wave I peak amplitudes did not correlate with age except for the second-click responses at an ICI of 7 ms, and the word intelligibility scores. However, we found that the RMSpost-w1 values for the second-click responses significantly decreased with increasing age. Moreover, the RMSpost-w1 values for the second-click responses at an ICI of 5 ms correlated significantly with the scores for word intelligibility in degraded listening conditions.

CONCLUSIONS

The magnitude of the post-wave I response for the second-click response could serve as a tool for detecting CS in humans.

SIGNIFICANCE

Our findings shed new light on the analytical methods of ABR for quantifying CS.

The curvature quantification of wave I in auditory brainstem responses detects cochlear synaptopathy in human beings

PURPOSE

Patients with age-related hearing loss complain often about reduced speech perception in adverse listening environment. Studies on animals have suggested that cochlear synaptopathy may be one of the primary mechanisms responsible for this phenomenon. A decreased wave I amplitude in supra-threshold auditory brainstem response (ABR) can diagnose this pathology non-invasively. However, the interpretation of the wave I amplitude in humans remains controversial. Recent studies in mice have established a robust and reliable mathematic algorithm, i.e., curve curvature quantification, for detecting cochlear synaptopathy. This study aimed to determine whether the curve curvature has sufficient test-retest reliability to detect cochlear synaptopathy in aging humans.

METHODS

Healthy participants were recruited into this prospective study. All subjects underwent an audiogram examination with standard and extended high frequencies ranging from 0.125 to 16 kHz and an ABR with a stimulus of 80 dB nHL click. The peak amplitude, peak latency, curvature at the peak, and the area under the curve of wave I were calculated and analyzed.

RESULTS

A total of 80 individuals with normal hearing, aged 18 to 61 years, participated in this study, with a mean age of 26.4 years. Pearson correlation analysis showed a significant negative correlation between curvature and age, as well as between curvature and extended high frequency (EHF) threshold (10-16 kHz). Additionally, the same correlation was observed between age and area as well as age and EHF threshold. The model comparison demonstrated that the curvature at the peak of wave I is the best metric to correlate with EHF threshold.

CONCLUSION

The curvature at the peak of wave I is the most sensitive metric for detecting cochlear synaptopathy in humans  and may be applied in routine diagnostics to detect early degenerations of the auditory nerve.

A free intravesicular C-terminal of otoferlin is essential for synaptic vesicle docking and fusion at auditory inner hair cell ribbon synapses

Our understanding of how otoferlin, the major calcium sensor in inner hair cells (IHCs) synaptic transmission, contributes to the overall dynamics of synaptic vesicle (SV) trafficking remains limited. To address this question, we generated a knock-in mouse model expressing an otoferlin-GFP protein, where GFP was fused to its C-terminal transmembrane domain. Similar to the wild type protein, the GFP-tagged otoferlin showed normal expression and was associated with IHC SV. Surprisingly, while the heterozygote Otof+/GFP mice exhibited a normal hearing function, homozygote OtofGFP/GFP mice were profoundly deaf attributed to severe reduction in SV exocytosis. Fluorescence recovery after photobleaching revealed a markedly increased mobile fraction of the otof-GFP-associated SV in Otof GFP/GFP IHCs. Correspondingly, 3D-electron tomographic of the ribbon synapses indicated a reduced density of SV attached to the ribbon active zone. Collectively, these results indicate that otoferlin requires a free intravesicular C-terminal end for normal SV docking and fusion.

Hidden hearing loss in a Charcot-Marie-Tooth type 1A mouse model

Hidden hearing loss (HHL), a recently described auditory neuropathy characterized by normal audiometric thresholds but reduced sound-evoked cochlear compound action potentials, has been proposed to contribute to hearing difficulty in noisy environments in people with normal hearing thresholds, a widespread complaint. While most studies on HHL pathogenesis have focused on inner hair cell (IHC) synaptopathy, we recently showed that transient auditory nerve (AN) demyelination also causes HHL in mice. To test the impact of myelinopathy on hearing in a clinically relevant model, we studied a mouse model of Charcot-Marie-Tooth type 1A (CMT1A), the most prevalent hereditary peripheral neuropathy in humans. CMT1A mice exhibited the functional hallmarks of HHL together with disorganization of AN heminodes near the IHCs with minor loss of AN fibers. These results support the hypothesis that mild disruptions of AN myelination can cause HHL, and that heminodal defects contribute to the alterations in the sound-evoked cochlear compound action potentials seen in this mouse model. Also, these findings suggest that patients with CMT1A or other mild peripheral neuropathies are likely to suffer from HHL. Furthermore, these results suggest that studies of hearing in CMT1A patients might help develop robust clinical tests for HHL, which are currently lacking.

Bilateral and symmetric glycinergic and glutamatergic projections from the LSO to the IC in the CBA/CaH mouse

Auditory space has been conceptualized as a matrix of systematically arranged combinations of binaural disparity cues that arise in the superior olivary complex (SOC). The computational code for interaural time and intensity differences utilizes excitatory and inhibitory projections that converge in the inferior colliculus (IC). The challenge is to determine the neural circuits underlying this convergence and to model how the binaural cues encode location. It has been shown that midbrain neurons are largely excited by sound from the contralateral ear and inhibited by sound leading at the ipsilateral ear. In this context, ascending projections from the lateral superior olive (LSO) to the IC have been reported to be ipsilaterally glycinergic and contralaterally glutamatergic. This study used CBA/CaH mice (3-6 months old) and applied unilateral retrograde tracing techniques into the IC in conjunction with immunocytochemical methods with glycine and glutamate transporters (GlyT2 and vGLUT2, respectively) to analyze the projection patterns from the LSO to the IC. Glycinergic and glutamatergic neurons were spatially intermixed within the LSO, and both types projected to the IC. For GlyT2 and vGLUT2 neurons, the average percentage of ipsilaterally and contralaterally projecting cells was similar (ANOVA, p = 0.48). A roughly equal number of GlyT2 and vGLUT2 neurons did not project to the IC. The somatic size and shape of these neurons match the descriptions of LSO principal cells. A minor but distinct population of small (< 40 μm2) neurons that labeled for GlyT2 did not project to the IC; these cells emerge as candidates for inhibitory local circuit neurons. Our findings indicate a symmetric and bilateral projection of glycine and glutamate neurons from the LSO to the IC. The differences between our results and those from previous studies suggest that species and habitat differences have a significant role in mechanisms of binaural processing and highlight the importance of research methods and comparative neuroscience. These data will be important for modeling how excitatory and inhibitory systems converge to create auditory space in the CBA/CaH mouse.

Age-Related Hearing Loss, Cognitive Decline, and Social Interaction: Testing a Framework

PURPOSE

Aging increases risk for hearing loss, cognitive decline, and social isolation; however, the nature of their interconnection remains unclear. This study examined the interplay between age-related hearing loss, cognitive decline, and social isolation in adults by testing the ability to understand speech in background noise, a challenge frequently reported by many older adults.

METHOD

We analyzed data collected from 128 adults (20-79 years of age, Mage = 51 years) recruited as part of the Aging Brain Cohort at the University of South Carolina repository. The participants underwent testing for hearing, cognition, and social interaction, which included pure-tone audiometry, a words-in-noise (WIN) test, a hearing questionnaire (Speech, Spatial and Qualities of Hearing Scale [SSQ12]), a social questionnaire (Patient-Reported Outcomes Measurement Information System-57 Social), and the Montreal Cognitive Assessment. We used a single pure-tone average (PTA) threshold value and a single WIN threshold value for each participant because there were no differences on average between the left and right ears.

RESULTS

Poorer hearing was significantly associated with cognitive decline, through both PTA and WIN thresholds, with a stronger association observed for WIN threshold. Adults with poorer hearing also exhibited greater social isolation, as evidenced by their WIN threshold and SSQ12 score, although not through PTA. This connection was more pronounced with the WIN threshold than with the SSQ12 score. Cognition was not related to social isolation, suggesting that social isolation is affected more by the ability to understand words in noise than by cognition in a nondemented population.

CONCLUSIONS

Understanding speech in challenging auditory environments rather than mere threshold detection is strongly linked to social isolation and cognitive decline. Thus, inclusion of a word-recognition-in-noise test and a social isolation survey in clinical settings is warranted.

SUPPLEMENTAL MATERIAL

https://doi.org/10.23641/asha.26237060.

Effects of Age on Responses of Principal Cells of the Mouse Anteroventral Cochlear Nucleus in Quiet and Noise

Older listeners often report difficulties understanding speech in noisy environments. It is important to identify where in the auditory pathway hearing-in-noise deficits arise to develop appropriate therapies. We tested how encoding of sounds is affected by masking noise at early stages of the auditory pathway by recording responses of principal cells in the anteroventral cochlear nucleus (AVCN) of aging CBA/CaJ and C57BL/6J mice in vivo. Previous work indicated that masking noise shifts the dynamic range of single auditory nerve fibers (ANFs), leading to elevated tone thresholds. We hypothesized that such threshold shifts could contribute to increased hearing-in-noise deficits with age if susceptibility to masking increased in AVCN units. We tested this by recording the responses of AVCN principal neurons to tones in the presence and absence of masking noise. Surprisingly, we found that masker-induced threshold shifts decreased with age in primary-like units and did not change in choppers. In addition, spontaneous activity decreased in primary-like and chopper units of old mice, with no change in dynamic range or tuning precision. In C57 mice, which undergo early-onset hearing loss, units showed similar changes in threshold and spontaneous rate at younger ages, suggesting they were related to hearing loss and not simply aging. These findings suggest that sound information carried by AVCN principal cells remains largely unchanged with age. Therefore, hearing-in-noise deficits may result from other changes during aging, such as distorted across-channel input from the cochlea and changes in sound coding at later stages of the auditory pathway.

Tissue engineering strategies for spiral ganglion neuron protection and regeneration

Cochlear implants can directly activate the auditory system's primary sensory neurons, the spiral ganglion neurons (SGNs), via circumvention of defective cochlear hair cells. This bypass restores auditory input to the brainstem. SGN loss etiologies are complex, with limited mammalian regeneration. Protecting and revitalizing SGN is critical. Tissue engineering offers a novel therapeutic strategy, utilizing seed cells, biomolecules, and scaffold materials to create a cellular environment and regulate molecular cues. This review encapsulates the spectrum of both human and animal research, collating the factors contributing to SGN loss, the latest advancements in the utilization of exogenous stem cells for auditory nerve repair and preservation, the taxonomy and mechanism of action of standard biomolecules, and the architectural components of scaffold materials tailored for the inner ear. Furthermore, we delineate the potential and benefits of the biohybrid neural interface, an incipient technology in the realm of implantable devices. Nonetheless, tissue engineering requires refined cell selection and differentiation protocols for consistent SGN quality. In addition, strategies to improve stem cell survival, scaffold biocompatibility, and molecular cue timing are essential for biohybrid neural interface integration.

Resistance to age-related hearing loss in the echolocating big brown bat ( Eptesicus fuscus )

Hearing mediates many behaviors critical for survival in echolocating bats, including foraging and navigation. Most mammals are susceptible to progressive age-related hearing loss; however, the evolution of biosonar, which requires the ability to hear low-intensity echoes from outgoing sonar signals, may have selected against the development of hearing deficits in echolocating bats. Although many echolocating bats exhibit exceptional longevity and rely on acoustic behaviors for survival to old age, relatively little is known about the aging bat auditory system. In this study, we used DNA methylation to estimate the ages of wild-caught big brown bats ( Eptesicus fuscus ) and measured hearing sensitivity in young and aging bats using auditory brainstem responses (ABRs) and distortion product otoacoustic emissions (DPOAEs). We found no evidence for hearing deficits in aging bats, demonstrated by comparable thresholds and similar ABR wave and DPOAE amplitudes across age groups. We additionally found no significant histological evidence for cochlear aging, with similar hair cell counts, afferent, and efferent innervation patterns in young and aging bats. Here we demonstrate that big brown bats show minimal evidence for age-related loss of peripheral hearing sensitivity and therefore represent informative models for investigating mechanisms that may preserve hearing function over a long lifetime.

Diabetes mellitus, hearing loss, and therapeutic interventions: A systematic review of insights from preclinical animal models

OBJECTIVES

The aim of this systematic review article is to evaluate the relationship between diabetes mellitus (DM) and sensorineural hearing loss (SNHL) utilizing preclinical animal models. The review focused on studies assessing SNHL in diabetic animal models, elucidating the mechanisms of DM-associated SNHL, and exploring the response of diabetic animal models to noise overexposure. We also discussed studies investigating the efficacy of potential therapeutic strategies for amelioration of DM-associated SNHL in the animal models.

METHODS

A protocol of this systematic review was designed a priori and was registered in the PROSPERO database (registration number: CRD42023439961). We conducted a comprehensive search on PubMed, Science Direct, Web of Science, Scopus, and EMBASE databases. A minimum of three reviewers independently screened, selected, and extracted data. The risk of bias assessment of eligible studies was conducted using the Systematic Review Center for Laboratory Animal Experimentation (SYRCLE) tool.

RESULTS

Following the screening of 238 studies, twelve original articles were included in this systematic review. The studies revealed that hyperglycemia significantly affects auditory function, with various pathological mechanisms contributing to DM-induced hearing impairment, including cochlear synaptopathy, microangiopathy, neuropathy, oxidative stress, mitochondrial abnormalities, and apoptosis-mediated cell death. Emerging interventions, such as Asiaticoside, Trigonelline, Chlorogenic acid, and Huotanquyu granules, demonstrated efficacy in providing otoprotection for preserving cochlear hair cells and hearing function.

CONCLUSIONS

Our systematic review delves into the intricate relationship between DM and hearing impairment in animal models. Future research should focus on targeted therapies to enhance cochlear mitochondrial function, alleviate oxidative stress, and regulate apoptosis. The association between SNHL and social isolation as well as cognitive decline underscores the necessity for innovative therapeutic modalities addressing yet undiscovered mechanisms. Translating findings from animal models to human studies will validate these findings, offering a synergistic approach to effectively manage DM-associated co-morbidities such as hearing impairment.