Suppression of the TGF-ß signaling exacerbates degeneration of auditory neurons in kanamycin-induced ototoxicity in mice.

Transforming growth factor-ß (TGF-ß) signaling plays a significant role in multiple biological processes, including inflammation, immunity, and cell death. However, its specific impact on the cochlea remains unclear. In this study, we aimed to investigate the effects of TGF-ß signaling suppression on auditory function and cochlear pathology in mice with kanamycin-induced ototoxicity. Kanamycin and furosemide (KM-FS) were systemically administered to 8-week-old C57/BL6 mice, followed by immediate topical application of a TGF-ß receptor inhibitor (TGF-ßRI) onto the round window membrane. Results showed significant TGF-ß receptor upregulation in spiral ganglion neurons (SGNs) after KM-FA ototoxicity, whereas expression levels in the TGF-ßRI treated group remained unchanged. Interestingly, despite no significant change in cochlear TGF-ß expression after KM-FS ototoxicity, TGF-ßRI treatment resulted in a significant decrease in TGF-ß signaling. Regarding auditory function, TGF-ßRI treatment offered no therapeutic effects on hearing thresholds and hair cell survival following KM-FS ototoxicity. However, SGN loss and macrophage infiltration were significantly increased with TGF-ßRI treatment. These results imply that inhibition of TGF-ß signaling after KM-FS ototoxicity promotes cochlear inflammation and SGN degeneration.

Disturbance in the protein landscape of cochlear perilymph in an Alzheimer's disease mouse model.

Hearing loss is a pivotal risk factor for dementia. It has recently emerged that a disruption in the intercommunication between the cochlea and brain is a key process in the initiation and progression of this disease. However, whether the cochlear properties can be influenced by pathological signals associated with dementia remains unclear. In this study, using a mouse model of Alzheimer's disease (AD), we investigated the impacts of the AD-like amyloid ß (Aß) pathology in the brain on the cochlea. Despite little detectable change in the age-related shift of the hearing threshold, we observed quantitative and qualitative alterations in the protein profile in perilymph, an extracellular fluid that fills the path of sound waves in the cochlea. Our findings highlight the potential contribution of Aß pathology in the brain to the disturbance of cochlear homeostasis.

Murine cochlear damage models in the context of hair cell regeneration research.

Understanding the complex pathologies associated with hearing loss is a significant motivation for conducting inner ear research. Lifelong exposure to loud noise, ototoxic drugs, genetic diversity, sex, and aging collectively contribute to human hearing loss. Replicating this pathology in research animals is challenging because hearing impairment has varied causes and different manifestations. A central aspect, however, is the loss of sensory hair cells and the inability of the mammalian cochlea to replace them. Researching therapeutic strategies to rekindle regenerative cochlear capacity, therefore, requires the generation of animal models in which cochlear hair cells are eliminated. This review discusses different approaches to ablate cochlear hair cells in adult mice. We inventoried the cochlear cyto- and histo-pathology caused by acoustic overstimulation, systemic and locally applied drugs, and various genetic tools. The focus is not to prescribe a perfect damage model but to highlight the limitations and advantages of existing approaches and identify areas for further refinement of damage models for use in regenerative studies.

Quantitative profiling of cochlear synaptosomal proteins in cisplatin-induced synaptic dysfunction.

The disruption of ribbon synapses in the cochlea impairs the transmission of auditory signals from the cochlear sensory receptor cells to the auditory cortex. Although cisplatin-induced loss of ribbon synapses is well-documented, and studies have reported nitration of cochlear proteins after cisplatin treatment, yet the underlying mechanism of cochlear synaptopathy is not fully understood. This study tests the hypothesis that cisplatin treatment alters the abundance of cochlear synaptosomal proteins, and selective targeting of nitrative stress prevents the associated synaptic dysfunction. Auditory brainstem responses of mice treated with cisplatin showed a reduction in amplitude and an increase in latency of wave I, indicating cisplatin-induced synaptic dysfunction. The mass spectrometry analysis of cochlear synaptosomal proteins identified 102 proteins that decreased in abundance and 249 that increased in abundance after cisplatin treatment. Pathway analysis suggested that the dysregulated proteins were involved in calcium binding, calcium ion regulation, synapses, and endocytosis pathways. Inhibition of nitrative stress by co-treatment with MnTBAP, a peroxynitrite scavenger, attenuated cisplatin-induced changes in the abundance of 27 proteins. Furthermore, MnTBAP co-treatment prevented the cisplatin-induced decrease in the amplitude and increase in the latency of wave I. Together, these findings suggest a potential role of oxidative/nitrative stress in cisplatin-induced cochlear synaptic dysfunction.

Role of Oxidative Stress in Sensorineural Hearing Loss.

Hearing is essential for communication, and its loss can cause a serious disruption to one's social life. Hearing loss is also recognized as a major risk factor for dementia; therefore, addressing hearing loss is a pressing global issue. Sensorineural hearing loss, the predominant type of hearing loss, is mainly due to damage to the inner ear along with a variety of pathologies including ischemia, noise, trauma, aging, and ototoxic drugs. In addition to genetic factors, oxidative stress has been identified as a common mechanism underlying several cochlear pathologies. The cochlea, which plays a major role in auditory function, requires high-energy metabolism and is, therefore, highly susceptible to oxidative stress, particularly in the mitochondria. Based on these pathological findings, the potential of antioxidants for the treatment of hearing loss has been demonstrated in several animal studies. However, results from human studies are insufficient, and future clinical trials are required. This review discusses the relationship between sensorineural hearing loss and reactive oxidative species (ROS), with particular emphasis on age-related hearing loss, noise-induced hearing loss, and ischemia-reperfusion injury. Based on these mechanisms, the current status and future perspectives of ROS-targeted therapy for sensorineural hearing loss are described.

L-Ergothioneine slows the progression of age-related hearing loss in CBA/CaJ mice.

The naturally occurring amino acid, l-ergothioneine (EGT), has immense potential as a therapeutic, having shown promise in the treatment of other disease models, including neurological disorders. EGT is naturally uptaken into cells via its specific receptor, OCTN1, to be utilized by cells as an antioxidant and anti-inflammatory. In our current study, EGT was administered over a period of 6 months to 25-26-month-old CBA/CaJ mice as a possible treatment for age-related hearing loss (ARHL), since presbycusis has been linked to higher levels of cochlear oxidative stress, apoptosis, and chronic inflammation. Results from the current study indicate that EGT can prevent aging declines of some key features of ARHL. However, we found a distinct sex difference for the response to the treatments, for hearing - Auditory Brainstem Responses (ABRs) and Distortion Product Otoacoustic Emissions (DPOAEs). Males exhibited lower threshold declines in both low dose (LD) and high dose (HD) test groups throughout the testing period and did not display some of the characteristic aging declines in hearing seen in Control animals. In contrast, female mice did not show any therapeutic effects with either treatment dose. Further confirming this sex difference, EGT levels in whole blood sampling throughout the testing period showed greater uptake of EGT in males compared to females. Additionally, RT-PCR results from three tissue types of the inner ear confirmed EGT activity in the cochlea in both males and females. Males and females exhibited significant differences in biomarkers related to apoptosis (Cas-3), inflammation (TNF-a), oxidative stress (SOD2), and mitochondrial health (PGC1a).These changes were more prominent in males as compared to females, especially in stria vascularis tissue. Taken together, these findings suggest that EGT has the potential to be a naturally derived therapeutic for slowing down the progression of ARHL, and possibly other neurodegenerative diseases. EGT, while effective in the treatment of some features of presbycusis in aging males, could also be modified into a general prophylaxis for other age-related disorders where treatment protocols would include eating a larger proportion of EGT-rich foods or supplements. Lastly, the sex difference discovered here, needs further investigation to see if therapeutic conditions can be developed where aging females show better responsiveness to EGT.

Surface electrical stimulation of the auditory cortex preserves efferent medial olivocochlear neurons and reduces cochlear traits of age-related hearing loss.

The auditory cortex is the source of descending connections providing contextual feedback for auditory signal processing at almost all levels of the lemniscal auditory pathway. Such feedback is essential for cognitive processing. It is likely that corticofugal pathways are degraded with aging, becoming important players in age-related hearing loss and, by extension, in cognitive decline. We are testing the hypothesis that surface, epidural stimulation of the auditory cortex during aging may regulate the activity of corticofugal pathways, resulting in modulation of central and peripheral traits of auditory aging. Increased auditory thresholds during ongoing age-related hearing loss in the rat are attenuated after two weeks of epidural stimulation with direct current applied to the surface of the auditory cortex for two weeks in alternate days (Fernández del Campo et al., 2024). Here we report that the same cortical electrical stimulation protocol induces structural and cytochemical changes in the aging cochlea and auditory brainstem, which may underlie recovery of age-degraded auditory sensitivity. Specifically, we found that in 18 month-old rats after two weeks of cortical electrical stimulation there is, relative to age-matched non-stimulated rats: a) a larger number of choline acetyltransferase immunoreactive neuronal cell body profiles in the ventral nucleus of the trapezoid body, originating the medial olivocochlear system.; b) a reduction of age-related dystrophic changes in the stria vascularis; c) diminished immunoreactivity for the pro-inflammatory cytokine TNFα in the stria vascularis and spiral ligament. d) diminished immunoreactivity for Iba1 and changes in the morphology of Iba1 immunoreactive cells in the lateral wall, suggesting reduced activation of macrophage/microglia; d) Increased immunoreactivity levels for calretinin in spiral ganglion neurons, suggesting excitability modulation by corticofugal stimulation. Altogether, these findings support that non-invasive neuromodulation of the auditory cortex during aging preserves the cochlear efferent system and ameliorates cochlear aging traits, including stria vascularis dystrophy, dysregulated inflammation and altered excitability in primary auditory neurons.

Characteristics of spatial protein expression in the mouse cochlear sensory epithelia: Implications for age-related hearing loss.

Hair cells in the cochlear sensory epithelia serve as mechanosensory receptors, converting sound into neuronal signals. The basal sensory epithelia are responsible for transducing high-frequency sounds, while the apex handles low-frequency sounds. Age-related hearing loss predominantly affects hearing at high frequencies and is indicative of damage to the basal sensory epithelia. However, the precise mechanism underlying this site-selective injury remains unclear. In this study, we employed a microscale proteomics approach to examine and compare protein expression in different regions of the cochlear sensory epithelia (upper half and lower half) in 1.5-month-old (normal hearing) and 6-month-old (severe high-frequency hearing loss without hair cell loss) C57BL/6J mice. A total of 2,386 proteins were detected, and no significant differences in protein expression were detected in the upper half of the cochlear sensory epithelia between the two age groups. The expression of 20 proteins in the lower half of the cochlear sensory epithelia significantly differed between the two age groups (e.g., MATN1, MATN4, and AQP1). Moreover, there were 311 and 226 differentially expressed proteins between the upper and lower halves of the cochlear sensory epithelia in 1.5-month-old and 6-month-old mice, respectively. The expression levels of selected proteins were validated by Western blotting. These findings suggest that the spatial differences in protein expression within the cochlear sensory epithelia may play a role in determining the susceptibility of cells at different sites of the cochlea to age-related damage.

Preoperative Imaging in Cochlear Implants.

OBJECTIVE: To determine the utility of computed tomography (CT) and magnetic resonance imaging (MRI) in cochlear implant candidates. STUDY DESIGN: Retrospective case review. SETTING: Tertiary referral hospital. PATIENTS: A total of 207 cochlear implanted patients with CT and/or MRI. INTERVENTIONS: N/A. MAIN OUTCOME MEASURES: Age versus abnormal radiologic findings, imaging abnormality versus postoperative outcomes, postoperative outcomes versus electrode design, Cambridge Cochlear Implant Protocol (CCIP) status for imaging abnormalities, sensitivity and specificity of CT and MRI for round-window/cochlear occlusion, and MRI for incomplete partitions. RESULTS: A total of 207 patients with CT, MRI, or both were reviewed retrospectively. Less than half (15.5%) of CT scans had findings that might affect surgical intervention compared with 5.9% of MRI. No significant difference was found between children and adults for relevant imaging abnormalities (grade 4 or higher) with either CT (p = 0.931) or MRI (p = 0.606). CCIP status correlated with cochlear abnormalities (p = 0.040); however, only 46.2% of radiographic abnormalities on CT would be identified by these criteria. For detecting cochlear occlusion requiring surgical intervention, the sensitivity and specificity for CT were 40% (4 of 10; 95% confidence interval [CI], 12.16-73.76) and 95.73% (95% CI, 91.40-98.27), respectively. For MRI, the sensitivity and specificity were 33.33% (1 of 3; 95% CI, 0.84-90.57) and 96.97% (63 of 65; 95% CI, 89.32-99.63), respectively. There was no difference for postoperative AzBio scores for higher-grade imaging abnormalities (p = 0.6012) or for electrode designs (p = 0.3699). CONCLUSIONS: Significant radiographic abnormalities were relatively uncommon in cochlear implant patients on either CT or MRI at our single-center institution. If present, abnormal imaging findings rarely translated to management changes. CCIP status does not reliably predict which patients are likely to have abnormalities. Both MRI and CT have low sensitivity for round-window or cochlear occlusion, but detection likely leads to changes in surgical management.

Pneumococcal Meningitis Induces Hearing Loss and Cochlear Ossification Modulated by Chemokine Receptors CX3CR1 and CCR2.

PURPOSE: Pneumococcal meningitis is a major cause of hearing loss and permanent neurological impairment despite widely available antimicrobial therapies to control infection. Methods to improve hearing outcomes for those who survive bacterial meningitis remains elusive. We used a mouse model of pneumococcal meningitis to evaluate the impact of mononuclear phagocytes on hearing outcomes and cochlear ossification by altering the expression of CX3CR1 and CCR2 in these infected mice. METHODS: We induced pneumococcal meningitis in approximately 500 C57Bl6 adult mice using live Streptococcus pneumoniae (serotype 3, 1 × 105 colony forming units (cfu) in 10 µl) injected directly into the cisterna magna of anesthetized mice and treated these mice with ceftriaxone daily until recovered. We evaluated hearing thresholds over time, characterized the cochlear inflammatory response, and quantified the amount of new bone formation during meningitis recovery. We used microcomputed tomography (microCT) scans to quantify cochlear volume loss caused by neo-ossification. We also performed perilymph sampling in live mice to assess the integrity of the blood-perilymph barrier during various time intervals after meningitis. We then evaluated the effect of CX3CR1 or CCR2 deletion in meningitis symptoms, hearing loss, macrophage/monocyte recruitment, neo-ossification, and blood labyrinth barrier function. RESULTS: Sixty percent of mice with pneumococcal meningitis developed hearing loss. Cochlear fibrosis could be detected within 4 days of infection, and neo-ossification by 14 days. Loss of spiral ganglion neurons was common, and inner ear anatomy was distorted by scarring caused by new soft tissue and bone deposited within the scalae. The blood-perilymph barrier was disrupted at 3 days post infection (DPI) and was restored by seven DPI. Both CCR2 and CX3CR1 monocytes and macrophages were present in the cochlea in large numbers after infection. Neither chemokine receptor was necessary for the induction of hearing loss, cochlear fibrosis, ossification, or disruption of the blood-perilymph barrier. CCR2 knockout (KO) mice suffered the most severe hearing loss. CX3CR1 KO mice demonstrated an intermediate phenotype with greater susceptibility to hearing loss compared to control mice. Elimination of CX3CR1 mononuclear phagocytes during the first 2 weeks after meningitis in CX3CR1-DTR transgenic mice did not protect mice from any of the systemic or hearing sequelae of pneumococcal meningitis. CONCLUSIONS: Pneumococcal meningitis can have devastating effects on cochlear structure and function, although not all mice experienced hearing loss or cochlear damage. Meningitis can result in rapid progression of hearing loss with fibrosis starting at four DPI and ossification within 2 weeks of infection detectable by light microscopy. The inflammatory response to bacterial meningitis is robust and can affect all three scalae. Our results suggest that CCR2 may assist in controlling infection and maintaining cochlear patency, as CCR2 knockout mice experienced more severe disease, more rapid hearing loss, and more advanced cochlear ossification after pneumococcal meningitis. CX3CR1 also may play an important role in the maintenance of cochlear patency.

Gene expression analysis of oxidative stress-related genes in the apical, middle, and basal turns of the cochlea.

It can be observed from aminoglycoside-induced hair cell damage that the cochlea basal turn is more susceptible to trauma than the apex. Drug-induced hearing loss is closely related to oxidative damage. The basilar membrane directly exposed to these ototoxic drugs exhibits differences in damage, indicating that there is an inherent difference in the sensitivity to oxidative damage from the apex to the base of the cochlea. It has been reported that the morphology and characteristics of the cochlea vary from the apex to the base. Therefore, we investigated oxidative stress-related gene expression profiles in the apical, middle, and basal turns of the cochlea. The Oxidative Stress RT2 Profiler™ PCR Array revealed that three of the 84 genes (Mb, Mpo, and Ncf1) were upregulated in the middle turn compared to their level in the apical turn. Moreover, eight genes (Mb, Duox1, Ncf1, Ngb, Fmo2, Gpx3, Mpo, and Gstk1) were upregulated in the basal turn compared to their level in the apical turn. The qPCR verification data were similar to that of the PCR Array. We found that MPO was expressed in the rat cochlea and protected against gentamicin-induced hair cell death. This study summarized the data for the gradient of expression of oxidative stress-related genes in the cochlea and found potential candidate targets for prevention of ototoxic deafness, which may provide new insights for cochlear pathology.

Effect of 2.45 GHz Microwave Radiation on the Inner Ear: A Histopathological Study on 2.45 GHz Microwave Radiation and Cochlea.

BACKGROUND: The present study aims to determine the possible low dose-dependent adverse effects of 2.45 GHz microwave exposure and Wi-Fi frequency on the cochlea. METHODS: Twelve pregnant female rats (n=12) and their male newborns were exposed to Wi-Fi frequencies with varying electric field values of 0.6, 1.9, 5, 10 V/m, and 15 V/m during the 21-day gestation period and 45 days after birth, except for the control group. Auditory brainstem response testing was performed before exposure and sacrification. After removal of the cochlea, histopathological examination was conducted by immunohistochemistry methods using caspase (cysteine-aspartic proteases, cysteine aspartates, or cysteine-dependent aspartate-directed proteases)-3, -9, and terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL). Kruskal-Wallis and Wilcoxon tests and multivariate analysis of variance were used. RESULTS: Auditory brainstem response thresholds in postexposure tests increased statistically significantly at 5 V/m and above doses. When the number of apoptotic cells was compared in immunohistochemistry examination, significant differences were found at 10 V/m and 15 V/m doses (F(5,15)=23.203, P=.001; Pillai's trace=1.912, η2=0.637). As the magnitude of the electric field increased, all histopathological indicators of apoptosis increased. The most significant effect was noted on caspase-9 staining (η2 c9=0.996), followed by caspase-3 (η2 c3=0.991), and TUNEL staining (η2 t=0.801). Caspase-3, caspase-9, and TUNEL-stained cell densities increased directly by increasing the electric field and power values. CONCLUSION: Apoptosis and immune activity in the cochlea depend on the electric field and power value. Even at low doses, the electromagnetic field in Wi-Fi frequency damages the inner ear and causes apoptosis.

The cells of the sensory epithelium, and not the stria vascularis, are the main cochlear cells related to the genetic pathogenesis of age-related hearing loss.

Age-related hearing loss (ARHL) is a major health concern among the elderly population. It is hoped that increasing our understanding of its underlying pathophysiological processes will lead to the development of novel therapies. Recent genome-wide association studies (GWASs) discovered a few dozen genetic variants in association with elevated risk for ARHL. Integrated analysis of GWAS results and transcriptomics data is a powerful approach for elucidating specific cell types that are involved in disease pathogenesis. Intriguingly, recent studies that applied such bioinformatics approaches to ARHL resulted in disagreeing findings as for the key cell types that are most strongly linked to the genetic pathogenesis of ARHL. These conflicting studies pointed either to cochlear sensory epithelial or to stria vascularis cells as the cell types most prominently involved in the genetic basis of ARHL. Seeking to resolve this discrepancy, we integrated the analysis of four ARHL GWAS datasets with four independent inner-ear single-cell RNA-sequencing datasets. Our analysis clearly points to the cochlear sensory epithelial cells as the key cells for the genetic predisposition to ARHL. We also explain the limitation of the bioinformatics analysis performed by previous studies that led to missing the enrichment for ARHL GWAS signal in sensory epithelial cells. Collectively, we show that cochlear epithelial cells, not stria vascularis cells, are the main inner-ear cells related to the genetic pathogenesis of ARHL.

Antioxidative Stress-Induced Destruction to Cochlear Cells Caused by Blind Antioxidant Therapy.

OBJECTIVE: Verification that blind and excessive use of antioxidants leads to antioxidant stress which exacerbates cochlear cell damage. STUDY DESIGN: Basic research. SETTING: The Third Affiliated Hospital of Sun Yat-Sen University. METHODS: We compared and quantified hair cell-like house ear institute-organ of corti 1 (HEI-OC1) cell density, cell viability, and apoptosis caused by different concentrations of N-acetylcysteine (NAC) via Hoechst staining, Cell Counting Kit 8, Hoechst with propidium iodide staining, and Annexin V with propidium iodide (PI) staining. Apoptosis induced by high concentrations of M40403 and coenzyme Q10 in cochlear explants was analyzed and compared by cochlear dissection and activated caspase 3 labeling. RESULTS: With the increase of NAC concentration (0-1000 µmol/L), cell density decreased consequently and reached the lowest at 1000 µmol/L (****P ≤ .0001). Cell viability is also declining (**P < .01). The number of Annexin V-fluorescein isothiocyanate-labeled cells and PI-labeled cells increased with increasing NAC concentration after treatment of HEI-OC1 cells for 48 hours. The proportion of apoptotic cells also rose (*P < .05, **P < .01). Cochlear hair cells (HCs) treated with low concentrations of M40403 and coenzyme Q10 for 48 hours showed no damage. When the concentrations of M40403 and coenzyme Q10 were increased (concentrations＞30 µmol/L), HC damage began, followed by a dose-dependent increase in HC loss (*P < .001, **P < .0001). Activated caspase-3 was clearly apparent in cochlear explants treated with 50 µmol/L M40403 and coenzyme Q10 compared with cochlear explants without added M40403 and coenzyme Q10. CONCLUSION: These experimental results suggest that inappropriate application of antioxidants can cause severe damage to normal cochlear HCs.

Cochlear Otosclerosis and Secondary Hydrops (Ménière's Syndrome).

ABSTRACT: This article discusses a case of cochlear otosclerosis leading to secondary hydrops and near-complete hearing loss. Histopathological examination revealed advanced multifocal otosclerosis in both temporal bones, with specific focus on cochlear invasion and significant bone resorption. The severity of the case ruled out surgical intervention due to the risk of further hearing loss. The article emphasizes the challenges in managing otosclerosis-related hydrops and highlights the potential use of advanced imaging techniques for diagnosis. The study underscores the complexity of otosclerosis-induced hearing loss, contributing to the understanding of this pathology and its impact on auditory function.

Dynamic Molecular Markers of Otosclerosis in the Human Cochlea.

OBJECTIVE: To investigate the role and distribution of various molecular markers using immunohistochemistry and immunofluorescence to further elucidate and understand the pathogenesis of otosclerosis. METHODS: Archival celloidin formalin-fixed 20-micron thick histologic sections from 7 patients diagnosed with otosclerosis were studied and compared to controls. Sections in the mid-modiolar region were immunoreacted with rabbit polyclonal antibodies against nidogen-1, ß2-laminin, collagen-IX, BSP, and monoclonal antibodies against TGF ß-1 and ubiquitin. Digital images were acquired using a high-resolution light and laser confocal microscope. RESULTS: Nidogen-1, BSP, and collagen-IX were expressed in the otospongiotic regions, and to lesser extent, in the otosclerotic regions, the latter previously believed to be inactive. ß2-laminin and ubiquitin were uniformly expressed in both otospongiotic and otosclerotic regions. There was a basal level of expression of all of these markers in the normal hearing and sensorineural hearing loss specimens utilized as control. TGF ß -1, however, though present in the otosclerosis bones, was absent in the normal hearing and sensorineural hearing loss controls. CONCLUSIONS: Our results propose that the activity and function of TGF-1 may play a key role in the development and pathogenesis of otosclerosis. Further studies utilizing a higher number of temporal bone specimens will be helpful for future analysis and to help decipher its role as a potential target in therapeutic interventions.

Mitochondrial Regulation of Macrophages in Innate Immunity and Diverse Roles of Macrophages During Cochlear Inflammation.

Macrophages are essential components of the innate immune system and constitute a non-specific first line of host defense against pathogens and inflammation. Mitochondria regulate macrophage activation and innate immune responses in various inflammatory diseases, including cochlear inflammation. The distribution, number, and morphological characteristics of cochlear macrophages change significantly across different inner ear regions under various pathological conditions, including noise exposure, ototoxicity, and age-related degeneration. However, the exact mechanism underlying the role of mitochondria in macrophages in auditory function remains unclear. Here, we summarize the major factors and mitochondrial signaling pathways (e.g., metabolism, mitochondrial reactive oxygen species, mitochondrial DNA, and the inflammasome) that influence macrophage activation in the innate immune response. In particular, we focus on the properties of cochlear macrophages, activated signaling pathways, and the secretion of inflammatory cytokines after acoustic injury. We hope this review will provide new perspectives and a basis for future research on cochlear inflammation.

Intra-cochlear Flushing Reduces Tissue Response to Cochlear Implantation.

INTRODUCTION: Intraoperative trauma leading to bleeding during cochlear implantation negatively impacts residual hearing of cochlear implant recipients. There are no clinical protocols for the removal of blood during implantation, to reduce the consequential effects such as inflammation and fibrosis which adversely affect cochlear health and residual hearing. This preclinical study investigated the implementation of an intra-cochlear flushing protocol for the removal of blood. METHODS: Three groups of guinea pigs were studied for 28 days after cochlear implantation; cochlear implant-only (control group); cochlear implant with blood injected into the cochlea (blood group); and cochlear implant, blood injection, and flushing of the blood from the cochlea intraoperatively (flush group). Auditory brainstem responses (ABRs) in addition to tissue response volumes were analyzed and compared between groups. RESULTS: After implantation, the blood group exhibited the highest ABR thresholds when compared to the control and flush group, particularly in the high frequencies. On the final day, the control and blood group had similar ABR thresholds across all frequencies tested, whereas the flush group had the lowest thresholds, significantly lower at 24 kHz than the blood and control group. Analysis of the tissue response showed the flush group had significantly lower tissue responses in the basal half of the array when compared with the blood and control group. CONCLUSIONS: Flushing intra-cochlear blood during surgery resulted in better auditory function and reduced subsequent fibrosis in the basal region of the cochlea. This finding prompts the implementation of a flushing protocol in clinical cochlear implantation. LEVEL OF EVIDENCE: N/A Laryngoscope, 134:1410-1416, 2024.

The Breakdown of Blood-Labyrinth Barrier Makes it Easier for Drugs to Enter the Inner Ear.

PURPOSE: This study aimed to investigate dynamic change of permeability of blood-labyrinth barrier (BLB) after noise exposure and its effect on the drug delivery efficiency of systemic administration. METHODS: Gadopentetate dimeglumine (Gd-DTPA) and dexamethasone (DEX) were used as tracers, and magnetic resonance imaging (MRI) and immunofluorescence were used to observe the change of the BLB after strong noise exposure in guinea pigs. High-performance liquid chromatography-mass spectrometry (LC-MS) was used to observe the effect of the breakdown of BLB after noise exposure on the drug delivery efficiency of intravenous DEX. The guinea pigs were divided into 6 groups: normal group (N), 1, 3, 5, 8, and 12 days after noise exposure groups (P1, P3, P5, P8, P12), with 5 animals in each group. RESULTS: The BLB changes dynamically after noise exposure. Increased permeability of the blood-endolymph barrier, the endolymph-perilymph barrier, and the blood-nerve barrier was observed at days 1-3, 1-5, and 1-8, respectively, after noise exposure in guinea pigs. Higher drug concentration in the cochlear tissue was obtained by intravenous administration of DEX in guinea pigs during the time window of increased permeability of the BLB. CONCLUSION: After noise exposure, the increased BLB permeability makes it easier for drugs to enter the inner ear from blood. In guinea pigs, 1-8 days after strong noise exposure, the drug delivery efficiency of systemic administration increased. After 8 days, the efficiency gradually returned to normal level. 1-8 days after noise exposure may be the best intervention time for systemic administration. LEVEL OF EVIDENCE: NA Laryngoscope, 134:2377-2386, 2024.

Blast-Induced Central Auditory Neurodegeneration Affects Tinnitus Development Regardless of Peripheral Cochlear Damage.

Blast exposure causes serious complications, the most common of which are ear-related symptoms such as hearing loss and tinnitus. The blast shock waves can cause neurodegeneration of the auditory pathway in the brainstem, as well as the cochlea, which is the primary receptor for hearing, leading to blast-induced tinnitus. However, it is still unclear which lesion is more dominant in triggering tinnitus, the peripheral cochlea or the brainstem lesion owing to the complex pathophysiology and the difficulty in objectively measuring tinnitus. Recently, gap detection tests have been developed and are potentially well-suited for determining the presence of tinnitus. In this study, we investigated whether the peripheral cochlea or the central nervous system has a dominant effect on the generation of tinnitus using a blast-exposed mouse model with or without earplugs, which prevent cochlear damage from a blast transmitted via the external auditory canal. The results showed that the earplug (+) group, in which the cochlea was neither physiologically nor histologically damaged, showed a similar extent of tinnitus behavior in a gap prepulse inhibition of acoustic startle reflex test as the earplug (-) group, in which the explosion caused a cochlear synaptic loss in the inner hair cells and demyelination of auditory neurons. In contrast, both excitatory synapses labeled with VGLUT-1 and inhibitory synapses labeled with GAD65 were reduced in the ventral cochlear nucleus, and demyelination in the medial nucleus of the trapezoid body was observed in both groups. These disruptions significantly correlated with the presence of tinnitus behavior regardless of cochlear damage. These results indicate that the lesion in the brainstem could be dominant to the cochlear lesion in the development of tinnitus following blast exposure.