Osmotic stabilization prevents cochlear synaptopathy after blast trauma

Preclinical Models to Study the Molecular Pathophysiology of Meniere's Disease: A Pathway to Gene Therapy

Background: Meniere's disease (MD) is a set of rare disorders that affects >4 million people worldwide. Individuals with MD suffer from episodes of vertigo associated with fluctuating sensorineural hearing loss and tinnitus. Hearing loss can involve one or both ears. Over 10% of the reported cases are observed in families, suggesting its significant genetic contribution. The condition is polygenic with >20 genes, and several patterns of inheritance have been reported, including autosomal dominant, autosomal recessive, and digenic inheritance across multiple MD families. Preclinical research using animal models has been an indispensable tool for studying the neurophysiology of the auditory and vestibular systems and to get a better understanding of the functional role of genes that are involved in the hearing and vestibular dysfunction. While mouse models are the most used preclinical model, this review analyzes alternative animal and non-animal models that can be used to study MD genes. Methods: A literature search of the 21 genes reported for familial MD and the preclinical models used to investigate their functional role was performed. Results: Comparing the homology of proteins encoded by these genes to other model organisms revealed Drosophila and zebrafish as cost-effective models to screen multiple genes and study the pathophysiology of MD. Conclusions: Murine models are preferred for a quantitative neurophysiological assessment of hearing and vestibular functions to develop drug or gene therapy.

Circadian rhythm disruptions exacerbate inner ear damage in a murine endolymphatic hydrops model

Meniere's disease (MD) is an inner ear disease characterized by endolymphatic hydrops (EH). Maintaining a regular daily routine is crucial for MD patients. However, the relationship between circadian rhythms and MD remains unclear. Therefore, we investigated the effect of circadian rhythm on endolymphatic hydrops and its underlying mechanisms. Mice with endolymphatic hydrops were subjected to chronic jet lag (CJL) conditions to simulate the MD patients under circadian rhythm disruptions. We assessed whether this disruption would exacerbate inner ear damage with endolymphatic hydrops. RNA-seq of the inner ear and bioinformatic analysis were performed. Then, the expression of PER2, AQP2, AQP4, AQP5, and BDNF were assessed, and the morphological changes were evaluated in the inner ear. Our findings showed circadian rhythm disruption affected the cochlear internal clock genes in the inner ear, particularly in mice with EH. EH mice under CJL conditions exhibited exacerbated hearing impairment and an increased severity of EH. GO enrichment analysis revealed that the regulation of fluid homeostasis and neurotransmitter release at synapses were significantly enriched. Disruption of circadian rhythms disturbed the expression pattern of PER2, reduced BDNF levels, and affected the expression of aquaporins in the cochlea. Moreover, the disruption of circadian rhythm compromised inner hair cell synapses and auditory nerve fibers. This study indicated that disruption of circadian rhythms may exacerbate inner ear damage in endolymphatic hydrops mice by affecting the aquaporins and compromising synapses and auditory nerves in the inner ear. BDNF and PER2 may play a central role in these pathophysiological processes.

Cochlear Mechanics Are Preserved After Inner Ear Delivery of Gold Nanoparticles

Novel therapeutic delivery systems and delivery methods to the inner ear are necessary to treat hearing loss and inner ear disorders. However, numerous barriers exist to therapeutic delivery into the bone-encased and immune-privileged environment of the inner ear and cochlea, which makes treating inner ear disorders challenging. Nanoparticles (NPs) are a type of therapeutic delivery system that can be engineered for multiple purposes, and posterior semicircular canal (PSCC) infusion is a method to directly deposit them into the cochlea. We sought to assess PSCC infusion of gold NPs into the cochlea, including the NPs' distribution and effect on cochlear mechanics. We performed optical coherence tomography (OCT) imaging to monitor PSCC infusion of gold NPs into the cochlear chambers. OCT imaging demonstrated that the infusion specifically targeted the perilymphatic spaces within the cochlea. We assessed cochlear mechanics by using OCT vibrometry to measure sound-evoked movements of the basilar membrane. We found no changes in cochlear mechanics between measurements at baseline, after the PSCC canalostomy, immediately after the infusion, and 1 h after the infusion of gold NPs (p > 0.05, paired t-test). These findings validate the PSCC infusion approach for perfusing the cochlear perilymphatic space with a nanoparticle delivery system. Thus, PSCC infusion of nanoparticles is a feasible therapeutic delivery technique for treating inner ear disorders while preserving residual cochlear function.

Electrocochleography in Chronic Tinnitus: Correlations with Audiological Profiles and Psychological Distress

OBJECTIVES

This study aimed to explore the electrophysiological characteristics of patients with chronic tinnitus through electrocochleography (ECochG) findings and determine if these findings correlate with specific audiological patterns that could differentiate tinnitus patients from those without this condition.

MATERIALS AND METHODS

A retrospective analysis of medical records from patients who visited a tinnitus clinic at a tertiary university hospital between March 2020 and December 2023 was conducted. Inclusion criteria were non-pulsatile subjective tinnitus lasting over three months, and ECochG performed at initial evaluation. Audiological assessments and ECochG results were analyzed, with the SP/AP ratio being a focal point.

RESULTS

Among 256 patients, an elevated SP/AP ratio was observed in 37.5 % of patients. No significant difference in ECochG outcomes was noted based on tinnitus laterality. Patients with an elevated SP/AP ratio reported more sleep disturbances, higher depression scores, attention problems, and aural fullness. These patients also exhibited lower loudness discomfort levels and low-frequency hearing losses. Significant correlations were found between elevated SP/AP ratios and DPOAE responses.

CONCLUSIONS

The findings highlight the SP/AP ratio in ECochG as a valuable biomarker for assessing clinical and psychological aspects of tinnitus, indicating its potential utility in tailoring treatment strategies. Elevated SP/AP ratios were associated with sleep disturbances, depression, attention problems, aural fullness, hyperacusis, and low-frequency hearing loss, suggesting a complex interplay between cochlear pathology and tinnitus perception. This study underscores the need for a nuanced understanding of ECochG results in the clinical evaluation of tinnitus, potentially guiding more personalized management approaches.

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.

Multifunctional redox modulator prevents blast-induced loss of cochlear and vestibular hair cells and auditory spiral ganglion neurons

Blast wave exposure, a leading cause of hearing loss and balance dysfunction among military personnel, arises primarily from direct mechanical damage to the mechanosensory hair cells and supporting structures or indirectly through excessive oxidative stress. We previously reported that HK-2, an orally active, multifunctional redox modulator (MFRM), was highly effective in reducing both hearing loss and hair cells loss in rats exposed to a moderate intensity workday noise that likely damages the cochlea primarily from oxidative stress versus direct mechanical trauma. To determine if HK-2 could also protect cochlear and vestibular cells from damage caused primarily from direct blast-induced mechanical trauma versus oxidative stress, we exposed rats to six blasts of 186 dB peak SPL. The rats were divided into four groups: (B) blast alone, (BEP) blast plus earplugs, (BHK-2) blast plus HK-2 and (BEPHK-2) blast plus earplugs plus HK-2. HK-2 was orally administered at 50 mg/kg/d from 7-days before to 30-day after the blast exposure. Cochlear and vestibular tissues were harvested 60-d post-exposure and evaluated for loss of outer hair cells (OHC), inner hair cells (IHC), auditory nerve fibers (ANF), spiral ganglion neurons (SGN) and vestibular hair cells in the saccule, utricle and semicircular canals. In the untreated blast-exposed group (B), massive losses occurred to OHC, IHC, ANF, SGN and only the vestibular hair cells in the striola region of the saccule. In contrast, rats treated with HK-2 (BHK-2) sustained significantly less OHC (67%) and IHC (57%) loss compared to the B group. OHC and IHC losses were smallest in the BEPHK-2 group, but not significantly different from the BEP group indicating lack of protective synergy between EP and HK-2. There was no loss of ANF, SGN or saccular hair cells in the BHK-2, BEP and BEPHK-2 groups. Thus, HK-2 not only significantly reduced OHC and IHC damage, but completely prevented loss of ANF, SGN and saccule hair cells. The powerful protective effects of this oral MFRM make HK-2 an extremely promising candidate for human clinical trials.

Blast Exposure Alters Synaptic Connectivity in the Mouse Auditory Cortex

Blast exposure can cause auditory deficits that have a lasting, significant impact on patients. Although the effects of blast on auditory functions localized to the ear have been well documented, the impact of blast on central auditory processing is largely undefined. Understanding the structural and functional alterations in the central nervous system (CNS) associated with blast injuries is crucial for unraveling blast-induced pathophysiological pathways and advancing development of therapeutic interventions. In this study, we used electrophysiology in combination with optogenetics assay, proteomic analysis, and morphological evaluation to investigate the impairment of synaptic connectivity in the auditory cortex (AC) of mice following blast exposure. Our results show that the long-range functional connectivity between the medial geniculate nucleus (MGN) and AC was impaired in the acute phase of blast injury. We also identified impaired synaptic transmission and dendritic spine alterations within 7 days of blast exposure, which recovered at 28 days post-blast. Additionally, proteomic analysis identified a few differentially expressed proteins in the cortex that are involved in synaptic signaling and plasticity. These findings collectively suggest that blast-induced alterations in the sound signaling network in the auditory cortex may underlie hearing deficits in the acute and sub-acute phases after exposure to shockwaves. This study may shed light on the perturbations underlying blast-induced auditory dysfunction and provide insights into the potential therapeutic windows for improving auditory outcomes in blast-exposed individuals.

ROS-induced oxidative stress and mitochondrial dysfunction: a possible mechanism responsible for noise-induced ribbon synaptic damage

Evidence suggests that damage to the ribbon synapses (RS) may be the main cause of auditory dysfunction in noise-induced hearing loss (NIHL). Oxidative stress is implicated in the pathophysiology of synaptic damage. However, the relationship between oxidative stress and RS damage in NIHL remains unclear. To investigate the hypothesis that noise-induced oxidative stress is a key factor in synaptic damage within the inner ear, we conducted a study using mice subjected to single or repeated noise exposure (NE). We assessed auditory function using auditory brainstem response (ABR) test and examined cochlear morphology by immunofluorescence staining. The results showed that mice that experienced a single NE exhibited a threshold shift and recovered within two weeks. The ABR wave I latencies were prolonged, and the amplitudes decreased, suggesting RS dysfunction. These changes were also demonstrated by the loss of RS as evidenced by immunofluorescence staining. However, we observed threshold shifts that did not return to baseline levels following secondary NE. Additionally, ABR wave I latencies and amplitudes exhibited notable changes. Immunofluorescence staining indicated not only severe damage to RS but also loss of outer hair cells. We also noted decreased T-AOC, ATP, and mitochondrial membrane potential levels, alongside increased hydrogen peroxide concentrations post-NE. Furthermore, the expression levels of 4-HNE and 8-OHdG in the cochlea were notably elevated. Collectively, our findings suggest that the production of reactive oxygen species leads to oxidative damage in the cochlea. This mitochondrial dysfunction consequently contributes to the loss of RS, precipitating an early onset of NIHL.

Blast Overpressures as a Military and Occupational Health Concern

PURPOSE

This tutorial reviews effects of environmental stressors like blast overpressures and other well-known acoustic contaminants (continuous, intermittent, and impulsive noise) on hearing, tinnitus, vestibular, and balance-related functions. Based on the overall outcome of these effects, detailed consideration is given to the health and well-being of individuals.

METHOD

Because hearing loss and tinnitus are consequential in affecting quality of life, novel neuromodulation paradigms are reviewed for their positive abatement and treatment-related effects. Examples of clinical data, research strategies, and methodological approaches focus on repetitive transcranial magnetic stimulation (rTMS) and electrical stimulation of the vagus nerve paired with tones (VNSt) for their unique contributions to this area.

RESULTS

Acoustic toxicants transmitted through the atmosphere are noteworthy for their propensity to induce hearing loss and tinnitus. Mounting evidence also indicates that high-level rapid onset changes in atmospheric sound pressure can significantly impact vestibular and balance function. Indeed, the risk of falling secondary to loss of, or damage to, sensory receptor cells in otolith organs (utricle and saccule) is a primary reason for this concern. As part of the complexities involved in VNSt treatment strategies, vocal dysfunction may also manifest. In addition, evaluation of temporospatial gait parameters is worthy of consideration based on their ability to detect and monitor incipient neurological disease, cognitive decline, and mortality.

CONCLUSION

Highlighting these respective areas underscores the need to enhance information exchange among scientists, clinicians, and caregivers on the benefits and complications of these outcomes.

Automated Segmentation of Optical Coherence Tomography Images of the Human Tympanic Membrane Using Deep Learning

Optical Coherence Tomography (OCT) is a light-based imaging modality that is used widely in the diagnosis and management of eye disease, and it is starting to become used to evaluate for ear disease. However, manual image analysis to interpret the anatomical and pathological findings in the images it provides is complicated and time-consuming. To streamline data analysis and image processing, we applied a machine learning algorithm to identify and segment the key anatomical structure of interest for medical diagnostics, the tympanic membrane. Using 3D volumes of the human tympanic membrane, we used thresholding and contour finding to locate a series of objects. We then applied TensorFlow deep learning algorithms to identify the tympanic membrane within the objects using a convolutional neural network. Finally, we reconstructed the 3D volume to selectively display the tympanic membrane. The algorithm was able to correctly identify the tympanic membrane properly with an accuracy of ~98% while removing most of the artifacts within the images, caused by reflections and signal saturations. Thus, the algorithm significantly improved visualization of the tympanic membrane, which was our primary objective. Machine learning approaches, such as this one, will be critical to allowing OCT medical imaging to become a convenient and viable diagnostic tool within the field of otolaryngology.

Polymorphisms and NIHL: a systematic review and meta-analyses

Background

Noise-induced hearing loss (NIHL) has always been a global public health problem, which is related to noise exposure and genetic factors. Many researchers have tried to identify the polymorphisms that cause different individuals' susceptibility to NIHL. We conducted a meta-analysis of the most frequently studied polymorphisms to identify those genes that may be associated with NIHL and may provide value in risk prevention.

Methods

PubMed, China National Knowledge Infrastructure (CNKI) database, Embase, Wang Fang, Web of Science and Cochrane library were searched, and qualified studies on the correlation between polymorphism and NIHL susceptibility were screened, and then polymorphisms cited in at least three studies were selected for meta-analysis. Fixed- or random-effects models were used to calculate odds ratios and 95% confidence intervals. Statistical I2 tests and sensitivity analyses were used to detect interstudy heterogeneity and test the statistical stability of overall estimates, respectively. Egger's tests were applied to detect publication bias among included studies. All of the above analyses were performed using stata 17.0.

Results

64 genes were initially selected and introduced in 74 papers. Among them, 10 genes (and 25 polymorphisms) have been reported in more than 3 papers. Twenty five polymorphisms participated in the meta-analysis. Of the 25 polymorphisms, only 5 were significantly associated with the risk of AR: rs611419 (GRHL2) polymorphism and rs3735715 polymorphism (GRHL2), rs208679 polymorphism (CAT), rs3813346 polymorphism (EYA4) were significantly associated with the susceptibility of NIHL, rs2227956 polymorphism (HSP70) was significantly associated with the susceptibility of white population NIHL, and the remaining 20 gene polymorphisms were not significantly associated with NIHL.

Conclusion

We found polymorphisms that are valuable for the prevention of NIHL and polymorphisms that are not related to NIHL. This is the first step to establish an effective risk prediction system for the population, especially for high-risk groups, which may help us better identify and prevent the occurrence of NIHL. In addition, our research results contribute to the in-depth exploration of NIHL.

Systematic review registration

https://inplasy.com/inplasy-2023-6-0003/, identifier INPLASY202360003.

A chemo-mechanical cochleostomy preserves hearing for the in vivo functional imaging of cochlear cells

In vivo and real-time multicellular imaging enables the decoding of sensory circuits and the tracking of systemic drug uptake. However, in vivo imaging of the auditory periphery remains technically challenging owing to the deep location, mechanosensitivity and fluid-filled, bone-encased nature of the cochlear structure. Existing methods that expose the cochlea invariably cause irreversible damage to auditory function, severely limiting the experimental measurements possible in living animals. Here we present an in vivo surgical protocol that permits the imaging of cochlear cells in hearing mice. Our protocol describes a ventro-lateral approach for preserving external and middle ear structures while performing surgery, the correct mouse positioning for imaging cochlear cells with effective sound transmission into the ear, the chemo-mechanical cochleostomy for creating the imaging window in the otic capsule bone that prevents intracochlear fluid leakage by maintaining an intact endosteum, and the release of intracochlear pressure that separates the endosteum from the otic capsule bone while creating an imaging window. The procedure thus preserves hearing thresholds. Individual inner and outer hair cells, supporting cells and nerve fibers can be visualized in vivo while hearing function is preserved. This approach may enable future original investigations, such as the real-time tracking of ototoxic drug transport into the cochleae. The technique may be applied to the monitoring of sound-evoked functional activity in multiple cochlear cells, in combination with optogenetic tools, and may help to improve cochlear implantation in humans. The cochleostomy takes ~1 h and requires experience in surgery.

What Predictability for Animal Models of Peripheral Vestibular Disorders?

The different clinical entities grouped under the term peripheral vestibulopathies (PVs) or peripheral vestibular disorders (PVDs) are distinguished mainly based on their symptoms/clinical expression. Today, there are very few commonly accepted functional and biological biomarkers that can confirm or refute whether a vestibular disorder belongs to a precise classification. Consequently, there is currently a severe lack of reliable and commonly accepted clinical endpoints, either to precisely follow the course of the vertigo syndrome of vestibular origin or to assess the benefits of therapeutic approaches, whether they are pharmacological or re-educational. Animal models of PV are a good means to identify biomarkers that could subsequently be exploited in human clinical practice. The question of their predictability is therefore crucial. Ten years ago, we had already raised this question. We revisit this concept today in order to take into account the animal models of peripheral vestibular pathology that have emerged over the last decade, and the new technological approaches available for the behavioral assessment of vestibular syndrome in animals and its progression over time. The questions we address in this review are the following: are animal models of PV predictive of the different types and stages of vestibular pathologies, and if so, to what extent? Are the benefits of the pharmacological or reeducational therapeutic approaches achieved on these different models of PV (in particular the effects of attenuation of the acute vertigo, or acceleration of central compensation) predictive of those expected in the vertiginous patient, and if so, to what extent?

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.

The Role of Genetic Variants in the Susceptibility of Noise-Induced Hearing Loss

Noised-induced hearing loss (NIHL) is an acquired, progressive neurological damage caused by exposure to intense noise in various environments including industrial, military and entertaining settings. The prevalence of NIHL is much higher than other occupational injuries in industrialized countries. Recent studies have revealed that genetic factors, together with environmental conditions, also contribute to NIHL. A group of genes which are linked to the susceptibility of NIHL had been uncovered, involving the progression of oxidative stress, potassium ion cycling, cilia structure, heat shock protein 70 (HSP70), DNA damage repair, apoptosis, and some other genes. In this review, we briefly summarized the studies primary in population and some animal researches concerning the susceptible genes of NIHL, intending to give insights into the further exploration of NIHL prevention and individual treatment.

In vivo real-time imaging reveals megalin as the aminoglycoside gentamicin transporter into cochlea whose inhibition is otoprotective

Aminoglycosides (AGs) are commonly used antibiotics that cause deafness through the irreversible loss of cochlear sensory hair cells (HCs). How AGs enter the cochlea and then target HCs remains unresolved. Here, we performed time-lapse multicellular imaging of cochlea in live adult hearing mice via a chemo-mechanical cochleostomy. The in vivo tracking revealed that systemically administered Texas Red-labeled gentamicin (GTTR) enters the cochlea via the stria vascularis and then HCs selectively. GTTR uptake into HCs was completely abolished in transmembrane channel-like protein 1 (TMC1) knockout mice, indicating mechanotransducer channel-dependent AG uptake. Blockage of megalin, the candidate AG transporter in the stria vascularis, by binding competitor cilastatin prevented GTTR accumulation in HCs. Furthermore, cilastatin treatment markedly reduced AG-induced HC degeneration and hearing loss in vivo. Together, our in vivo real-time tracking of megalin-dependent AG transport across the blood-labyrinth barrier identifies new therapeutic targets for preventing AG-induced ototoxicity.

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.

Correlation of Histomorphometric Changes with Diffusion Tensor Imaging for Evaluation of Blast-Induced Auditory Neurodegeneration in Chinchilla

In the present study, we have evaluated the blast-induced auditory neurodegeneration in chinchilla by correlating the histomorphometric changes with diffusion tensor imaging. The chinchillas were exposed to single unilateral blast-overpressure (BOP) at ∼172dB peak sound pressure level (SPL) and the pathological changes were compared at 1 week and 1 month after BOP. The functional integrity of the auditory system was assessed by auditory brainstem response (ABR) and distortion product otoacoustic emissions (DPOAE). The axonal integrity was assessed using diffusion tensor imaging at regions of interests (ROIs) of the central auditory neuraxis (CAN) including the cochlear nucleus (CN), inferior colliculus (IC), and auditory cortex (AC). Post-BOP, cyto-architecture metrics such as viable cells, degenerating neurons, and apoptotic cells were quantified at the CAN ROIs using light microscopic studies using cresyl fast violet, hematoxylin and eosin, and modified Crossmon's trichrome stains. We observed mean ABR threshold shifts of 30- and 10-dB SPL at 1 week and 1 month after BOP, respectively. A similar pattern was observed in DPAOE amplitudes shift. In the CAN ROIs, diffusion tensor imaging studies showed a decreased axial diffusivity in CN 1 month after BOP and a decreased mean diffusivity and radial diffusivity at 1 week after BOP. However, morphometric measures such as decreased viable cells and increased degenerating neurons and apoptotic cells were observed at CN, IC, and AC. Specifically, increased degenerating neurons and reduced viable cells were high on the ipsilateral side when compared with the contralateral side. These results indicate that a single blast significantly damages structural and functional integrity at all levels of CAN ROIs.

Assessment of auditory and vestibular damage in a mouse model after single and triple blast exposures

The use of explosive devices in war and terrorism has increased exposure to concussive blasts among both military personnel and civilians, which can cause permanent hearing and balance deficits that adversely affect survivors' quality of life. Significant knowledge gaps on the underlying etiology of blast-induced hearing loss and balance disorders remain, especially with regard to the effect of blast exposure on the vestibular system, the impact of multiple blast exposures, and long-term recovery. To address this, we investigated the effects of blast exposure on the inner ear using a mouse model in conjunction with a high-fidelity blast simulator. Anesthetized animals were subjected to single or triple blast exposures, and physiological measurements and tissue were collected over the course of recovery for up to 180 days. Auditory brainstem responses (ABRs) indicated significantly elevated thresholds across multiple frequencies. Limited recovery was observed at low frequencies in single-blasted mice. Distortion Product Otoacoustic Emissions (DPOAEs) were initially absent in all blast-exposed mice, but low-amplitude DPOAEs could be detected at low frequencies in some single-blast mice by 30 days post-blast, and in some triple-blast mice at 180 days post-blast. All blast-exposed mice showed signs of Tympanic Membrane (TM) rupture immediately following exposure and loss of outer hair cells (OHCs) in the basal cochlear turn. In contrast, the number of Inner Hair Cells (IHCs) and spiral ganglion neurons was unchanged following blast-exposure. A significant reduction in IHC pre-synaptic puncta was observed in the upper turns of blast-exposed cochleae. Finally, we found no significant loss of utricular hair cells or changes in vestibular function as assessed by vestibular evoked potentials. Our results suggest that (1) blast exposure can cause severe, long-term hearing loss which may be partially due to slow TM healing or altered mechanical properties of healed TMs, (2) traumatic levels of sound can still reach the inner ear and cause basal OHC loss despite middle ear dysfunction caused by TM rupture, (3) blast exposure may result in synaptopathy in humans, and (4) balance deficits after blast exposure may be primarily due to traumatic brain injury, rather than damage to the peripheral vestibular system.

In Vivo Cochlear imaging provides a tool to study endolymphatic hydrops

Exposure to noise trauma, such as that from improvised explosive devices, can lead to sensorineural hearing loss and a reduced quality of life. In order to elucidate the mechanisms underlying noise-induced hearing loss, we have adapted optical coherence tomography (OCT) for real-time cochlear visualization in live mice after blast exposure. We demonstrated that endolymphatic hydrops develops following blast injury, and that this phenomenon may be associated with glutamate excitotoxicity and cochlear synaptopathy. Additionally, osmotic stabilization of endolymphatic hydrops partially rescues cochlear synapses after blast trauma. OCT is thus a valuable research tool for investigating the mechanisms underlying acoustic trauma and dynamic changes in endolymph volume. It may also help with the diagnosis and treatment of human hearing loss and/or vertigo in the near future.

Cochlear supporting cells require GAS2 for cytoskeletal architecture and hearing

In mammals, sound is detected by mechanosensory hair cells that are activated in response to vibrations at frequency-dependent positions along the cochlear duct. We demonstrate that inner ear supporting cells provide a structural framework for transmitting sound energy through the cochlear partition. Humans and mice with mutations in GAS2, encoding a cytoskeletal regulatory protein, exhibit hearing loss due to disorganization and destabilization of microtubule bundles in pillar and Deiters' cells, two types of inner ear supporting cells with unique cytoskeletal specializations. Failure to maintain microtubule bundle integrity reduced supporting cell stiffness, which in turn altered cochlear micromechanics in Gas2 mutants. Vibratory responses to sound were measured in cochleae from live mice, revealing defects in the propagation and amplification of the traveling wave in Gas2 mutants. We propose that the microtubule bundling activity of GAS2 imparts supporting cells with mechanical properties for transmitting sound energy through the cochlea.

Central and peripheral auditory abnormalities in chinchilla animal model of blast-injury

Exposure to blast overpressure or high-intensity sound can cause injuries to the auditory system, which leads to hearing loss or tinnitus. In this study, we examined the involvement of peripheral auditory system (PAS), and central auditory system (CAS) changes after exposure to blast overpressure (15-25 psi) on Day 1 and additionally during 7 days of post blast time period in chinchillas. Auditory brainstem response (ABR), distortion product otoacoustic emission (DPOAE), and cochlear hair cell changes were measured or identified in post-blast period within 7 days to detect injuries in the PAS. In the CAS, changes in NMDAR1 (excitatory receptor) and GABAA (inhibitory receptor) as well as changes in serotonin (5-HT_2A) and acetylcholine (AChR) receptors were examined in different brain regions: auditory cortex (AC), geniculate body (GB), inferior colliculus (IC) and amygdala by immunofluorescence staining. We observed the PAS abnormalities of increased ABR threshold and decreased DPOAE response in animals after blast exposure with hearing protection devices (e.g., earplug). Blast exposure also caused a reduction in both NMDAR1 and GABA_A receptor levels in acute condition (post-blast or Day 1) in AC and IC, while serotonin and acetylcholine receptor levels displayed a biphasic response at Day 1 and Day 7 post-exposure. Results demonstrate that the earplug can protect the tympanic membrane and middle ear against structural damage, but the hearing level, cochlear outer hair cell, and the central auditory system (levels of excitatory and inhibitory neurotransmitter receptors) were only partially protected at the tested blast overpressure level. The findings in this study indicate that blast exposure can cause both peripheral and central auditory dysfunctions, and the central auditory response is independent of peripheral auditory damage. The CAS dysfunction is likely mediated by direct transmission of shockwaves in all the regions of central nervous system (CNS), including nerves and surrounding tissues along the auditory pathways. Hence, targeting central auditory neurotransmitter abnormalities may have a therapeutic benefit to attenuate blast-induced hearing loss and tinnitus.

Mammalian Cochlear Hair Cell Imaging Using Optical Coherence Tomography (OCT): A Preliminary Study

OBJECTIVES

This study aimed to investigate the feasibility of using optical coherence tomography (OCT) to provide information about cochlear microanatomy at a cellular level, specifically of cochlear hair cells in mammals.

MATERIALS AND METHODS

A total of 10 Sprague-Dawley rats were divided into 2 experimental groups for comparing the arrangement of normal and damaged hair cells. Postnatal day 3 Sprague-Dawley rats were used to test the swept-source OCT system, and the images recorded were compared with fluorescence microscope images.

RESULTS

Intracochlear structures (the inner hair cells, outer hair cells, and auditory nerve fibers) were clearly visualized at the individual cellular level.

CONCLUSION

These images reflect the ability of OCT to provide images of the inner hair cells, outer hair cells, and auditory nerve fibers (ex vivo). OCT is a promising technology, and these findings could be used to encourage research in the area of cochlear microstructure imaging in the future.

Excess extracellular K+ causes inner hair cell ribbon synapse degeneration

Inner hair cell (IHC) ribbon synapses are the first synapse in the auditory system and can be degenerated by noise and aging, thereby leading to hidden hearing loss (HHL) and other hearing disorders. However, the mechanism underlying this cochlear synaptopathy remains unclear. Here, we report that elevation of extracellular K+, which is a consequence of noise exposure, could cause IHC ribbon synapse degeneration and swelling. Like intensity dependence in noise-induced cochlear synaptopathy, the K+-induced degeneration was dose-dependent, and could be attenuated by BK channel blockers. However, application of glutamate receptor (GluR) agonists caused ribbon swelling but not degeneration. In addition, consistent with synaptopathy in HHL, both K+ and noise exposure only caused IHC but not outer hair cell ribbon synapse degeneration. These data reveal that K+ excitotoxicity can degenerate IHC ribbon synapses in HHL, and suggest that BK channel may be a potential target for prevention and treatment of HHL.

Is cochlear synapse loss an origin of low-frequency hearing loss associated with endolymphatic hydrops?

There is a strong association between endolymphatic hydrops and low-frequency hearing loss, but the origin of the hearing loss remains unknown. A reduction in the number of cochlear afferent synapses between inner hair cells and auditory nerve fibres may be the origin of the low-frequency hearing loss, but this hypothesis has not been directly tested in humans or animals. In humans, measurements of hearing loss and postmortem temporal-bone based measurements of endolymphatic hydrops are generally separated by large amounts of time. In animals, there has not been a good objective, physiologic, and minimally invasive measurement of low-frequency hearing. We overcame this obstacle with the combined use of a reliable surgical approach to ablate the endolymphatic sac in guinea pigs and create endolymphatic hydrops, the Auditory Nerve Overlapped Waveform to measure low-frequency hearing loss (≤ 1 kHz), and immunohistofluorescence-based confocal microscopy to count cochlear synapses. Results showed low- and mid-(1-4 kHz) frequency hearing loss at all postoperative days, 1, 4, and 30. There was no statistically significant loss of cochlear synapses, and there was no correlation between synapse loss and hearing function. We conclude that cochlear afferent synaptic loss is not the origin of the low-frequency hearing loss in the early days following endolymphatic sac ablation. Understanding what is, and is not, the origin of a hearing loss can help guide preventative and therapeutic development.

Personal Audio System: Hearing Symptoms, Habits, and Sound Pressure Levels Measured in Real Ear and a Manikin

Purpose The aims of this study were to characterize hearing symptoms, habits, and sound pressure levels (SPLs) of personal audio system (PAS) used by young adults; estimate the risk of developing hearing loss and assess whether instructions given to users led to behavioral changes; and propose recommendations for PAS users. Method A cross-sectional study was performed in 50 subjects with normal hearing. Procedures included questionnaire and measurement of PAS SPLs (real ear and manikin) through the users' own headphones and devices while they listened to four songs. After 1 year, 30 subjects answered questions about their usage habits. For the statistical analysis, one-way analysis of variance, Tukey's post hoc test, Lin and Spearman coefficients, the chi-square test, and logistic regression were used. Results Most subjects listened to music every day, usually in noisy environments. Sixty percent of the subjects reported hearing symptoms after using a PAS. Substantial variability in the equivalent music listening level (Leq) was noted (M = 84.7 dBA; min = 65.1 dBA, max = 97.5 dBA). A significant difference was found only in the 4-kHz band when comparing the real-ear and manikin techniques. Based on the Leq, 38% of the individuals exceeded the maximum daily time allowance. Comparison of the subjects according to the maximum allowed daily exposure time revealed a higher number of hearing complaints from people with greater exposure. After 1 year, 43% of the subjects reduced their usage time, and 70% reduced the volume. A volume not exceeding 80% was recommended, and at this volume, the maximum usage time should be 160 min. Conclusions The habit of listening to music at high intensities on a daily basis seems to cause hearing symptoms, even in individuals with normal hearing. The real-ear and manikin techniques produced similar results. Providing instructions on this topic combined with measuring PAS SPLs may be an appropriate strategy for raising the awareness of people who are at risk. Supplemental Material https://doi.org/10.23641/asha.12431435.

Early Detection of Endolymphatic Hydrops using the Auditory Nerve Overlapped Waveform (ANOW)

Endolymphatic hydrops is associated with low-frequency sensorineural hearing loss, with a large body of research dedicated to examining its putative causal role in low-frequency hearing loss. Investigations have been thwarted by the fact that hearing loss is measured in intact ears, but gold standard assessments of endolymphatic hydrops are made postmortem only; and that no objective low-frequency hearing measure has existed. Yet the association of endolymphatic hydrops with low-frequency hearing loss is so strong that it has been established as one of the important defining features for Ménière's disease, rendering it critical to detect endolymphatic hydrops early, regardless of whether it serves a causal role or is the result of other disease mechanisms. We surgically induced endolymphatic hydrops in guinea pigs and employed our recently developed objective neural measure of low-frequency hearing, the Auditory Nerve Overlapped Waveform (ANOW). Hearing loss and endolymphatic hydrops were assessed at various time points after surgery. The ANOW detected low-frequency hearing loss as early as the first day after surgery, well before endolymphatic hydrops was found histologically. The ANOW detected low-frequency hearing loss with perfect sensitivity and specificity in all ears after endolymphatic hydrops developed, where there was a strong linear relationship between degree of endolymphatic hydrops and severity of low-frequency hearing loss. Further, histological data demonstrated that endolymphatic hydrops is seen first in the high-frequency cochlear base, though the ANOW demonstrated that dysfunction begins in the low-frequency apical cochlear half. The results lay the groundwork for future investigations of the causal role of endolymphatic hydrops in low-frequency hearing loss.

Cytokine Levels in Inner Ear Fluid of Young and Aged Mice as Molecular Biomarkers of Noise-Induced Hearing Loss

Sensorineural hearing loss (SNHL) is the most common sensory deficit worldwide, frequently caused by noise trauma and aging, with inflammation being implicated in both pathologies. Here, we provide the first direct measurements of proinflammatory cytokines in inner ear fluid, perilymph, of adolescent and 2-year-old mice. The perilymph of adolescent mice exposed to the noise intensity resulting in permanent auditory threshold elevations had significantly increased levels of IL-6, TNF-α, and CXCL1 6 h after exposure, with CXCL1 levels being most elevated (19.3 ± 6.2 fold). We next provide the first immunohistochemical localization of CXCL1 in specific cochlear supporting cells, and its presumed receptor, Duffy antigen receptor for chemokines (DARC), in hair cells and spiral ganglion neurons. Our results demonstrate the feasibility of molecular diagnostics of SNHL using only 0.5 μL of perilymph, and motivate future sub-μL based diagnostics of human SNHL based on liquid biopsy of the inner ear to guide therapy, promote hearing protection, and monitor response to treatment.