Inhibition of Gpx4-mediated ferroptosis alleviates cisplatin-induced hearing loss in C57BL/6 mice

Cisplatin-induced hearing loss is a common side effect of cancer chemotherapy in clinics; however, the mechanism of cisplatin-induced ototoxicity is still not completely clarified. Cisplatin-induced ototoxicity is mainly associated with the production of reactive oxygen species, activation of apoptosis, and accumulation of intracellular lipid peroxidation, which also is involved in ferroptosis induction. In this study, the expression of TfR1, a ferroptosis biomarker, was upregulated in the outer hair cells of cisplatin-treated mice. Moreover, several key ferroptosis regulator genes were altered in cisplatin-damaged cochlear explants based on RNA sequencing, implying the induction of ferroptosis. Ferroptosis-related Gpx4 and Fsp1 knockout mice were established to investigate the specific mechanisms associated with ferroptosis in cochleae. Severe outer hair cell loss and progressive damage of synapses in inner hair cells were observed in Atoh1-Gpx4-/- mice. However, Fsp1-/- mice showed no significant hearing phenotype, demonstrating that Gpx4, but not Fsp1, may play an important role in the functional maintenance of HCs. Moreover, findings showed that FDA-approved luteolin could specifically inhibit ferroptosis and alleviate cisplatin-induced ototoxicity through decreased expression of transferrin and intracellular concentration of ferrous ions. This study indicated that ferroptosis inhibition through the reduction of intracellular ferrous ions might be a potential strategy to prevent cisplatin-induced hearing loss.

Reactive Oxygen Species-Related Disruptions to Cochlear Hair Cell and Stria Vascularis Consequently Leading to Radiation-Induced Sensorineural Hearing Loss

Aims: Radiation-induced sensorineural hearing loss (RISNHL) is one of the major side effects of radiotherapy for head and neck cancers. At present, no effective clinical treatment or prevention is available for RISNHL. This study thus aimed to investigate the cochlear pathology so that the underlying mechanisms of RISNHL may be elucidated, consequently paving the way for potential protective strategies to be developed. Results: Functional and morphological impairment in the stria vascularis (SV) was observed after irradiation (IR), as indicated by endocochlear potential (EP) reduction, hyperpermeability, and SV atrophy. The expression of zonulae occludins-1 was found to have decreased after IR. The loss of outer hair cells (OHCs) occurred later than SV damage. The disruption to the SV and OHCs could be attributed to reactive oxygen species (ROS)-related damage. In addition, EP shifts and the loss of OHCs were reduced when ROS was reduced by N-acetylcysteine (NAC) in C57BL/6 mice, attenuating auditory threshold shifts. Innovation: The damage to the SV was found to occur before OHC loss. ROS-related damage accounted for SV damage and OHC loss. The incidences of SV damage and OHC loss were decreased through ROS modulation by NAC, subsequently preventing RISNHL, suggesting the possible role of NAC as a possible protective agent against RISNHL. Conclusion: The findings from this study suggest oxidative stress-induced early SV injury and late OHC loss to be the key factors leading to RISNHL. NAC prevents IR-induced OHC loss, and attenuates auditory brainstem response and EP shifts by regulating the level of oxidative stress. Antioxid. Redox Signal. 40, 470-491.

Predicting Atrophy of the Cochlear Stria Vascularis from the Shape of the Threshold Audiogram

Several lines of evidence have suggested that steeply sloping audiometric losses are caused by hair cell degeneration, while flat audiometric losses are caused by strial atrophy, but this concept has never been rigorously tested in human specimens. Here, we systematically compare audiograms and cochlear histopathology in 160 human cases from the archival collection of celloidin-embedded temporal bones at the Massachusetts Eye and Ear. The dataset included 106 cases from a prior study of normal-aging ears, and an additional 54 cases selected by combing the database for flat audiograms. Audiogram shapes were classified algorithmically into five groups according to the relation between flatness (i.e., SD of hearing levels across all frequencies) and low-frequency pure-tone average (i.e., mean at 0.25, 0.5, and 1.0 kHz). Outer and inner hair cell losses, neural degeneration, and strial atrophy were all quantified as a function of cochlear location in each case. Results showed that strial atrophy was worse in the apical than the basal half of the cochlea and was worse in females than in males. The degree of strial atrophy was uncorrelated with audiogram flatness. Apical atrophy was correlated with low-frequency thresholds and basal atrophy with high-frequency thresholds, and the former correlation was higher. However, a multivariable regression with all histopathological measures as predictors and audiometric thresholds as the outcome showed that strial atrophy was a significant predictor of threshold shift only in the low-frequency region, and, even there, the contribution of outer hair cell damage was larger.SIGNIFICANCE STATEMENT Cochlear pathology can only be assessed postmortem; thus, human cochlear histopathology is critical to our understanding of the mechanisms of hearing loss. Dogma holds that relative damage to sensory cells, which transduce mechanical vibration into electrical signals, versus the stria vascularis, the cellular battery that powers transduction, can be inferred by the shape of the audiogram, that is, down-sloping (hair cell damage) versus flat (strial atrophy). Here we quantified hair cell and strial atrophy in 160 human specimens to show that it is the degree of low-frequency hearing loss, rather than the audiogram slope, that predicts strial atrophy. Results are critical to the design of clinical trials for hearing-loss therapeutics, as current drugs target only hair cell, not strial, regeneration.

GRAIL gene knockout mice protect against aging-related and noise-induced hearing loss

BACKGROUND

Hearing loss is a global health issue and its etiopathologies involve complex molecular pathways. The ubiquitin-proteasome system has been reported to be associated with cochlear development and hearing loss. The gene related to anergy in lymphocytes ( GRAIL ), as an E3 ubiquitin ligase, has not, as yet, been examined in aging-related and noise-induced hearing loss mice models.

METHODS

This study used wild-type (WT) and GRAIL knockout (KO) mice to examine cochlear hair cells and synaptic ribbons using immunofluorescence staining. The hearing in WT and KO mice was detected using auditory brainstem response. Gene expression patterns were compared using RNA-sequencing to identify potential targets during the pathogenesis of noise-induced hearing loss in WT and KO mice.

RESULTS

At the 12-month follow-up, GRAIL KO mice had significantly less elevation in threshold level and immunofluorescence staining showed less loss of outer hair cells and synaptic ribbons in the hook region compared with GRAIL WT mice. At days 1, 14, and 28 after noise exposure, GRAIL KO mice had significantly less elevation in threshold level than WT mice. After noise exposure, GRAIL KO mice showed less loss of outer hair cells in the cochlear hook and basal regions compared with WT mice. Moreover, immunofluorescence staining showed less loss of synaptic ribbons in the hook regions of GRAIL KO mice than of WT mice. RNA-seq analysis results showed significant differences in C-C motif chemokine ligand 19 ( CCL19 ), C-C motif chemokine ligand 21 ( CCL21 ), interleukin 25 ( IL25 ), glutathione peroxidase 6 ( GPX6 ), and nicotinamide adenine dinucleotide phosphate (NADPH) oxidase 1 ( NOX1 ) genes after noise exposure.

CONCLUSION

The present data demonstrated that GRAIL deficiency protects against aging-related and noise-induced hearing loss. The mechanism involved needs to be further clarified from the potential association with synaptic modulation, inflammation, and oxidative stress.

Dopaminergic and cholinergic innervation in the mouse cochlea after noise-induced or age-related synaptopathy

Cochlear synaptopathy, the loss of or damage to connections between auditory-nerve fibers (ANFs) and inner hair cells (IHCs), is a prominent pathology in noise-induced and age-related hearing loss. Here, we investigated if degeneration of the olivocochlear (OC) efferent innervation is also a major aspect of the synaptopathic ear, by quantifying the volume and spatial organization of its cholinergic and dopaminergic components, using antibodies to vesicular acetylcholine transporter (VAT) and tyrosine hydroxylase (TH), respectively. CBA/CaJ male mice were examined 1 day to 8 months after a synaptopathic noise exposure, and compared to unexposed age-matched controls and unexposed aged mice at 24-28 months. In normal ears, cholinergic lateral (L)OC terminals were denser in the apical half of the cochlea and on the modiolar side of the inner hair cells (IHCs), where ANFs of low-spontaneous rate are typically found, while dopaminergic terminals were more common in the basal third of the cochlea and, re the IHC axes, were offset towards the habenula with respect to cholinergic terminals. The noise had only small and transient effects on the density of LOC innervation, its spatial organization around the IHC axes, or the extent to which TH and VAT signal were colocalized. The synaptopathic noise also had relatively small and transient effects on cholinergic innervation density in the outer hair cell (OHC) area, which normally peaks in the 16 kHz region and falls monotonically towards higher and lower frequencies. In contrast, in the aged ears, there was massive degeneration of OHC efferents, especially in the apical half of the cochlea, where there was also significant loss of OHCs. In the IHC area, there was significant loss of cholinergic terminals in both apical and basal regions and of dopaminergic innervation in the basal half. Furthermore, the cholinergic terminals in the aged ears spread from their normal clustering near the IHC basolateral pole, where the ANF synapses are found, to positions up and down the IHC somata and regions of the neuropil closer to the habenula. This apparent migration was most striking in the apex, where the hair cell pathology was greatest, and may be a harbinger of impending hair cell death.

The effects of noise-induced hair cell lesions on cochlear electromechanical responses: A computational approach using a biophysical model

A biophysically inspired signal processing model of the human cochlea is deployed to simulate the effects of specific noise-induced inner hair cell (IHC) and outer hair cell (OHC) lesions on hearing thresholds, cochlear compression, and the spectral and temporal features of the auditory nerve (AN) coding. The model predictions were evaluated by comparison with corresponding data from animal studies as well as human clinical observations. The hearing thresholds were simulated for specific OHC and IHC damages and the cochlear nonlinearity was assessed at 0.5 and 4 kHz. The tuning curves were estimated at 1 kHz and the contributions of the OHC and IHC pathologies to the tuning curve were distinguished by the model. Furthermore, the phase locking of AN spikes were simulated in quiet and in presence of noise. The model predicts that the phase locking drastically deteriorates in noise indicating the disturbing effect of background noise on the temporal coding in case of hearing impairment. Moreover, the paper presents an example wherein the model is inversely configured for diagnostic purposes using a machine learning optimization technique (Nelder-Mead method). Accordingly, the model finds a specific pattern of OHC lesions that gives the audiometric hearing loss measured in a group of noise-induced hearing impaired humans.

Primed to die: an investigation of the genetic mechanisms underlying noise-induced hearing loss and cochlear damage in homozygous Foxo3-knockout mice

The prevalence of noise-induced hearing loss (NIHL) continues to increase, with limited therapies available for individuals with cochlear damage. We have previously established that the transcription factor FOXO3 is necessary to preserve outer hair cells (OHCs) and hearing thresholds up to two weeks following mild noise exposure in mice. The mechanisms by which FOXO3 preserves cochlear cells and function are unknown. In this study, we analyzed the immediate effects of mild noise exposure on wild-type, Foxo3 heterozygous (Foxo3+/-), and Foxo3 knock-out (Foxo3-/-) mice to better understand FOXO3's role(s) in the mammalian cochlea. We used confocal and multiphoton microscopy to examine well-characterized components of noise-induced damage including calcium regulators, oxidative stress, necrosis, and caspase-dependent and caspase-independent apoptosis. Lower immunoreactivity of the calcium buffer Oncomodulin in Foxo3-/- OHCs correlated with cell loss beginning 4 h post-noise exposure. Using immunohistochemistry, we identified parthanatos as the cell death pathway for OHCs. Oxidative stress response pathways were not significantly altered in FOXO3's absence. We used RNA sequencing to identify and RT-qPCR to confirm differentially expressed genes. We further investigated a gene downregulated in the unexposed Foxo3-/- mice that may contribute to OHC noise susceptibility. Glycerophosphodiester phosphodiesterase domain containing 3 (GDPD3), a possible endogenous source of lysophosphatidic acid (LPA), has not previously been described in the cochlea. As LPA reduces OHC loss after severe noise exposure, we treated noise-exposed Foxo3-/- mice with exogenous LPA. LPA treatment delayed immediate damage to OHCs but was insufficient to ultimately prevent their death or prevent hearing loss. These results suggest that FOXO3 acts prior to acoustic insult to maintain cochlear resilience, possibly through sustaining endogenous LPA levels.

Umbilical Cord Mesenchymal Stromal Cell-Derived Exosomes Rescue the Loss of Outer Hair Cells and Repair Cochlear Damage in Cisplatin-Injected Mice

Umbilical cord-derived mesenchymal stromal cells (UCMSCs) have potential applications in regenerative medicine. UCMSCs have been demonstrated to repair tissue damage in many inflammatory and degenerative diseases. We have previously shown that UCMSC exosomes reduce nerve injury-induced pain in rats. In this study, we characterized UCMSC exosomes using RNA sequencing and proteomic analyses and investigated their protective effects on cisplatin-induced hearing loss in mice. Two independent experiments were designed to investigate the protective effects on cisplatin-induced hearing loss in mice: (i) chronic intraperitoneal cisplatin administration (4 mg/kg) once per day for 5 consecutive days and intraperitoneal UCMSC exosome (1.2 μg/μL) injection at the same time point; and (ii) UCMSC exosome (1.2 μg/μL) injection through a round window niche 3 days after chronic cisplatin administration. Our data suggest that UCMSC exosomes exert protective effects in vivo. The post-traumatic administration of UCMSC exosomes significantly improved hearing loss and rescued the loss of cochlear hair cells in mice receiving chronic cisplatin injection. Neuropathological gene panel analyses further revealed the UCMSC exosomes treatment led to beneficial changes in the expression levels of many genes in the cochlear tissues of cisplatin-injected mice. In conclusion, UCMSC exosomes exerted protective effects in treating ototoxicity-induced hearing loss by promoting tissue remodeling and repair.

Roles of Bak and Sirt3 in Paraquat-Induced Cochlear Hair Cell Damage

Paraquat, a superoxide generator, can damage the cochlea causing an ototoxic hearing loss. The purpose of the study was to determine if deletion of Bak, a pro-apoptotic gene, would reduce paraquat ototoxicity or if deletion of Sirt3, which delays age-related hearing loss under caloric restriction, would increase paraquat ototoxicity. We tested these two hypotheses by treating postnatal day 3 cochlear cultures from Bak^±, Bak^-/-, Sirt3^±, Sirt3^-/-, and WT mice with paraquat and compared the results to a standard rat model of paraquat ototoxicity. Paraquat damaged nerve fibers and dose-dependently destroyed rat outer hair cells (OHCs) and inner hair cells (IHCs). Rat hair cell loss began in the base of the cochlea with a 10 μM dose and as the dose increased from 50 to 500 μM, the hair cell loss increased near the base of the cochlea and spread toward the apex of the cochlea. Rat OHC losses were consistently greater than IHC losses. Unexpectedly, in all mouse genotypes, paraquat-induced hair cell lesions were maximal near the apex of the cochlea and minimal near the base. This unusual damage gradient is opposite to that seen in paraquat-treated rats and in mice and rats treated with other ototoxic drugs. However, paraquat always induced greater OHC loss than IHC loss in all mouse strains. Contrary to our hypothesis, Bak deficient mice were more vulnerable to paraquat ototoxicity than WT mice (Bak^-/- > Bak^± > WT), suggesting that Bak plays a protective role against hair cell stress. Also, contrary to expectation, Sirt3-deficient mice did not differ significantly from WT mice, possibly due to the fact that Sirt3 was not experimentally upregulated in Sirt3-expressing mice prior to paraquat treatment. Our results show for the first time a gradient of ototoxic damage in mice that is greater in the apex than the base of the cochlea.

Altered Outer Hair Cell Mitochondrial and Subsurface Cisternae Connectomics Are Candidate Mechanisms for Hearing Loss in Mice

Organelle crosstalk is vital for cellular functions. The propinquity of mitochondria, ER, and plasma membrane promote regulation of multiple functions, which include intracellular Ca2+ flux, and cellular biogenesis. Although the purposes of apposing mitochondria and ER have been described, an understanding of altered organelle connectomics related to disease states is emerging. Since inner ear outer hair cell (OHC) degeneration is a common trait of age-related hearing loss, the objective of this study was to investigate whether the structural and functional coupling of mitochondria with subsurface cisternae (SSC) was affected by aging. We applied functional and structural probes to equal numbers of male and female mice with a hearing phenotype akin to human aging. We discovered the polarization of cristae and crista junctions in mitochondria tethered to the SSC in OHCs. Aging was associated with SSC stress and decoupling of mitochondria with the SSC, mitochondrial fission/fusion imbalance, a remarkable reduction in mitochondrial and cytoplasmic Ca2+ levels, reduced K+-induced Ca2+ uptake, and marked plasticity of cristae membranes. A model of structure-based ATP production predicts profound energy stress in older OHCs. This report provides data suggesting that altered membrane organelle connectomics may result in progressive hearing loss.

Rapamycin but not acarbose decreases age-related loss of outer hair cells in the mouse Cochlea

Adding rapamycin or acarbose to diet at 9-10 months of age has been shown to significantly increase life span in both male and female UM-HET3 mice. The current study examined cochleae of male and female UM-HET3 mice at 22 months of age to determine if either treatment also influenced age-related loss of cochlear hair cells. A large loss of cochlear outer hair cells was observed at 22 months of age in untreated mice in both apical and basal halves of the cochlear spiral. Addition of acarbose to diet had no significant effect on the amount of outer hair cell loss at 22 months of age or in its pattern, with large loss in both apical and basal halves. The addition of rapamycin to diet, however, significantly reduced outer hair cell loss in the basal half of the cochlea at 22 months of age when compared to untreated mice. There was no significant difference between male and female mice in any of the conditions. Age-related outer hair cell loss in the apical cochlea precedes outer hair cell loss in the base in many mouse strains. The results of the present study suggest that rapamycin but not acarbose treatment can delay age-related loss of outer hair cells at doses at which each drug increases life span.

Psychophysical and modeling approaches towards determining the cochlear phase response based on interaural time differences

The cochlear phase response is often estimated by measuring masking of a tonal target by harmonic complexes with various phase curvatures. Maskers yielding most modulated internal envelope representations after passing the cochlear filter are thought to produce minimum masking, with fast-acting cochlear compression as the main contributor to that effect. Thus, in hearing-impaired (HI) listeners, reduced cochlear compression hampers estimation of the phase response using the masking method. This study proposes an alternative approach, based on the effect of the envelope modulation strength on the sensitivity to interaural time differences (ITDs). To evaluate the general approach, ITD thresholds were measured in seven normal-hearing listeners using 300-ms Schroeder-phase harmonic complexes with nine different phase curvatures. ITD thresholds tended to be lowest for phase curvatures roughly similar to those previously shown to produce minimum masking. However, an unexpected ITD threshold peak was consistently observed for a particular negative phase curvature. An auditory-nerve based ITD model predicted the general pattern of ITD thresholds except for the threshold peak, as well as published envelope ITD data. Model predictions simulating outer hair cell loss support the feasibility of the ITD-based approach to estimate the phase response in HI listeners.

Styrene enhances the noise induced oxidative stress in the cochlea and affects differently mechanosensory and supporting cells

Experimental and human investigations have raised the level of concern about the potential ototoxicity of organic solvents and their interaction with noise. The main objective of this study was to characterize the effects of the combined noise and styrene exposure on hearing focusing on the mechanism of damage on the sensorineural cells and supporting cells of the organ of Corti and neurons of the ganglion of Corti. The impact of single and combined exposures on hearing was evaluated by auditory functional testing and histological analyses of cochlear specimens. The mechanism of damage was studied by analyzing superoxide anion and lipid peroxidation expression and by computational analyses of immunofluorescence data to evaluate and compare the oxidative stress pattern in outer hair cells versus the supporting epithelial cells of the organ of Corti. The oxidative stress hypothesis was further analyzed by evaluating the protective effect of a Coenzyme Q₁₀ analogue, the water soluble Q_ter, molecule known to have protective antioxidant properties against noise induced hearing loss and by the analysis of the expression of the endogenous defense enzymes. This study provides evidence of a reciprocal noise-styrene synergism based on a redox imbalance mechanism affecting, although with a different intensity of damage, the outer hair cell (OHC) sensory epithelium. Moreover, these two damaging agents address preferentially different cochlear targets: noise mainly the sensory epithelium, styrene the supporting epithelial cells. Namely, the increase pattern of lipid peroxidation in the organ of Corti matched the cell damage distribution, involving predominantly OHC layer in noise exposed cochleae and both OHC and Deiters' cell layers in the styrene or combined exposed cochleae. The antioxidant treatment reduced the lipid peroxidation increase, potentiated the endogenous antioxidant defense system at OHC level in both exposures but it failed to ameliorate the oxidative imbalance and cell death of Deiters' cells in the styrene and combined exposures. Current antioxidant therapeutic approaches to preventing sensory loss focus on hair cells alone. It remains to be seen whether targeting supporting cells, in addition to hair cells, might be an effective approach to protecting exposed subjects.

Oral Ethanol Potentiates the Loss of Outer Hair Cells in Cisplatin-Exposed Rats

OBJECTIVE: The aim of this study was to examine the effect of oral ethanol on cisplatin ototoxicity.

STUDY DESIGN AND SETTING: Twenty-seven-week-old, female Fisher 344 rats were divided into 4 experimental groups. The animals were administered per os (PO) saline (group 1), PO ethanol (group 2), PO saline with intraperitoneal (IP) cisplatin (group 3), or PO ethanol with IP cisplatin (group 4). After 3 days, scanning electron microscopy and counts of outer auditory hair cells were performed.

RESULTS: A 2-fold increase in outer hair cell loss was obtained in the basal cochlear turn of rats receiving concomitant cisplatin and ethanol compared with animals receiving cisplatin and saline. No hair cell loss was observed in the middle cochlear turn of any experimental group.

CONCLUSION: Our findings support potentiation of ototoxicity when cisplatin is combined with oral ethanol.

SIGNIFICANCE: Contraindications for alcohol use in cancer patients receiving cisplatin are implicated.

The potential use of low-frequency tones to locate regions of outer hair cell loss

Current methods used to diagnose cochlear hearing loss are limited in their ability to determine the location and extent of anatomical damage to various cochlear structures. In previous experiments, we have used the electrical potential recorded at the round window -the cochlear response (CR) -to predict the location of damage to outer hair cells in the gerbil. In a follow-up experiment, we applied 10 mM ouabain to the round window niche to reduce neural activity in order to quantify the neural contribution to the CR. We concluded that a significant proportion of the CR to a 762 Hz tone originated from phase-locking activity of basal auditory nerve fibers, which could have contaminated our conclusions regarding outer hair cell health. However, at such high concentrations, ouabain may have also affected the responses from outer hair cells, exaggerating the effect we attributed to the auditory nerve. In this study, we lowered the concentration of ouabain to 1 mM and determined the physiologic effects on outer hair cells using distortion-product otoacoustic emissions. As well as quantifying the effects of 1 mM ouabain on the auditory nerve and outer hair cells, we attempted to reduce the neural contribution to the CR by using near-infrasonic stimulus frequencies of 45 and 85 Hz, and hypothesized that these low-frequency stimuli would generate a cumulative amplitude function (CAF) that could reflect damage to hair cells in the apex more accurately than the 762 stimuli. One hour after application of 1 mM ouabain, CR amplitudes significantly increased, but remained unchanged in the presence of high-pass filtered noise conditions, suggesting that basal auditory nerve fibers have a limited contribution to the CR at such low frequencies.

Effect of Corticosteroid on Salicylate-Induced Morphological Changes of Isolated Cochlear Outer Hair Cells

Our previous studies showed that pretreatment with corticosteroids, which inhibits release of arachidonic acid (precursor of prostaglandins and leukotrienes), partially prevented salicylate-induced hearing loss in vivo. The purpose of this study was to determine the effect of pretreatment with corticosteroid (dexamethasone sodium phosphate) on isolated cochlear outer hair cells (OHCs) exposed to salicylate in vitro. Isolated OHCs from the chinchilla cochlea were exposed to salicylate with or without pretreatment with dexamethasone. Images were stored and analyzed on the Image program. The OHCs exposed to salicylate demonstrated a significant shortening in cell length. The OHCs exposed to salicylate after pretreatment with dexamethasone exhibited no significant change in cell length. We conclude that corticosteroid treatment of isolated OHCs is effective in blocking the morphological changes induced by salicylate. This study gives additional evidence that salicylate ototoxicity is mediated by alteration in the levels of arachidonic acid metabolites.

Susceptibility of outer hair cells to cholesterol chelator 2-hydroxypropyl-β-cyclodextrine is prestin-dependent

Niemann-Pick type C1 disease (NPC1) is a fatal genetic disorder caused by impaired intracellular cholesterol trafficking. Recent studies reported ototoxicity of 2-hydroxypropyl- β-cyclodextrin (HPβCD), a cholesterol chelator and the only promising treatment for NPC1. Because outer hair cells (OHCs) are the only cochlear cells affected by HPβCD, we investigated whether prestin, an OHC-specific motor protein, might be involved. Single, high-dose administration of HPβCD resulted in OHC death in prestin wildtype (WT) mice whereas OHCs were largely spared in prestin knockout (KO) mice in the basal region, implicating prestin's involvement in ototoxicity of HPβCD. We found that prestin can interact with cholesterol in vitro, suggesting that HPβCD-induced ototoxicity may involve disruption of this interaction. Time-lapse analysis revealed that OHCs isolated from WT animals rapidly deteriorated upon HPβCD treatment while those from prestin-KOs tolerated the same regimen. These results suggest that a prestin-dependent mechanism contributes to HPβCD ototoxicity.

Effects of caffeic acid phenethyl ester on cisplatin ototoxicity

PURPOSE

To report histological and electrophysiological data in rats treated with cisplatin and caffeic acid phenethyl ester.

METHODS

We randomly divided 28 Wistar rats into four groups of seven, to be treated as follows: control (saline), cisplatin, CAPE and cisplatin-CAPE. Distortion product otoacoustic emission (DPOAE) measurements were performed on day one (before drug administration) and day five under anaesthesia. All animals were killed under general anaesthesia on day five after the DPOAE measurement. The cochleae of each rat were histopathologically and immunohistochemically evaluated.

RESULTS

The outer hair cells were mostly preserved in the control and CAPE groups. Moderate-to-severe and mild-to-moderate hair cell losses were detected in the cisplatin and cisplatin-CAPE groups, respectively. DPOAE assessments revealed significant deterioration in the cisplatin group (P < 0.05). The difference between the cisplatin and cisplatin-CAPE groups was statistically significant (P < 0.05).

CONCLUSION

CAPE prevents cisplatin ototoxicity.

Chronic Conductive Hearing Loss Leads to Cochlear Degeneration

Synapses between cochlear nerve terminals and hair cells are the most vulnerable elements in the inner ear in both noise-induced and age-related hearing loss, and this neuropathy is exacerbated in the absence of efferent feedback from the olivocochlear bundle. If age-related loss is dominated by a lifetime of exposure to environmental sounds, reduction of acoustic drive to the inner ear might improve cochlear preservation throughout life. To test this, we removed the tympanic membrane unilaterally in one group of young adult mice, removed the olivocochlear bundle in another group and compared their cochlear function and innervation to age-matched controls one year later. Results showed that tympanic membrane removal, and the associated threshold elevation, was counterproductive: cochlear efferent innervation was dramatically reduced, especially the lateral olivocochlear terminals to the inner hair cell area, and there was a corresponding reduction in the number of cochlear nerve synapses. This loss led to a decrease in the amplitude of the suprathreshold cochlear neural responses. Similar results were seen in two cases with conductive hearing loss due to chronic otitis media. Outer hair cell death was increased only in ears lacking medial olivocochlear innervation following olivocochlear bundle cuts. Results suggest the novel ideas that 1) the olivocochlear efferent pathway has a dramatic use-dependent plasticity even in the adult ear and 2) a component of the lingering auditory processing disorder seen in humans after persistent middle-ear infections is cochlear in origin.

[The experimental study on repair of noise-induced hearing loss in guinea pigs by bone marrow NTCSCs transplantation]

OBJECTIVE

To observe the repairing effects of bone marrow transplantation with nerve tissue committed stem cell (NTCSCs) on experimental rats with injury of noise-induced hearing loss.

METHOD

Guinea pigs were randomly divided into control group, noise exposure group and the transplanting group. A week after white noise exposure of 110 dB, NTCSCs and PBS were injected into guinea pigs of the noise exposure group and the transplanting group respectively. One week after noise exposure to four weeks continuous administration. ABR thresholds were measured respectively prior to the experiment, 1 week post-noise,1, 2 and 4 weeks post-drugs, The changes of cochlea hair cells were also observed by a scan electron microscope (SEM).

RESULT

The ABR threshold shifts in the transplanting group were significantly fewer than that in the noise exposure group. SEM showed that hear hair of the inner and outer hair cells in noise exposure group displayed mess, fusion and imperfections. In the transplanting treatment group, the hair cells displayed slight pathological changes, there wasn't significant differents comparied with normal group. The number of OHCs were relatively stable in the normal group, while the obvious OHC loss was observed in other groups. There was significant difference among the three groups, however, the OHC loss in the transplanting group was no significantly different to that in the noise exposure (P > 0.05).

CONCLUSION

The bone marrow NTCSCs which had been transplanted to rat cochlea could reduce the damage of the noise on the hair cell, and thus played a role in repairing the damage of auditory nerve.

[Effect of sildenafil on morphology to noise-induced hearing loss in guinea pigs]

OBJECTIVE

To study the effects of sildenafil on morphology to noise-induced hearing loss in guinea pigs.

METHOD

Guinea pigs were randomly divided into control group, noise exposure group and the sildenafil treatment group, 12 in each group. a week after white noise exposure of 110 dB, sildenafil (10 mg/kg x d) and NS (4 ml/kg x d) were injected into guinea pigs of the sildenafil treatment group and noise exposure group respectively for four continuous weeks. ABR thresholds were measured respectively prior to the experiment, 1 week post-noise, 1, 2 and 4 weeks post-drugs, the changes of cochlea hair cells were also observed with a scan electron microscope (SEM) and light microscope.

RESULT

The ABR threshold shifts in the sildenafil treatment group were significantly fewer than that in the noise exposure group. SEM showed that hear hair of the inner and outer hair cells in noise exposure group displayed mess, fusion and imperfections. In the sildenafil treatment group, the hair cells displayed slight pathological changes, there wasn't significant differents comparied with normal group. The number of OHCs were relatively stable in the normal group, while the obvious OHC loss was observed in other groups. There was significant difference among the three groups, however, the OHC loss in the sildenafil treatment group was not significantly different to that in the noise exposure (P > 0.05).

CONCLUSION

Sildenafil can significantly protect against noise-induced hearing loss.

A QTL on Chr 5 modifies hearing loss associated with the fascin-2 variant of DBA/2J mice

Inbred mouse strains serve as important models for human presbycusis or age-related hearing loss. We previously mapped a locus (ahl8) contributing to the progressive hearing loss of DBA/2J (D2) mice and later showed that a missense variant of the Fscn2 gene, unique to the D2 inbred strain, was responsible for the ahl8 effect. Although ahl8 can explain much of the hearing loss difference between C57BL/6J (B6) and D2 strain mice, other loci also contribute. Here, we present results of our linkage analyses to map quantitative trait loci (QTLs) that modify the severity of hearing loss associated with the D2 strain Fscn2 (ahl8) allele. We searched for modifier loci by analyzing 31 BXD recombinant inbred (RI) lines fixed for the predisposing D2-derived Fscn2 (ahl8/ahl8) genotype and found a statistically significant linkage association of threshold means with a QTL on Chr 5, which we designated M5ahl8. The highest association (LOD 4.6) was with markers at the 84-90 Mb position of Chr 5, which could explain about 46 % of the among-RI strain variation in auditory brainstem response (ABR) threshold means. The semidominant nature of the modifying effect of M5ahl8 on the Fscn2 (ahl8/ahl8) phenotype was demonstrated by analysis of a backcross involving D2 and B6.D2-Chr11D/LusJ strain mice. The Chr 5 map position of M5ahl8 and the D2 origin of its susceptibility allele correspond to Tmc1m4, a previously reported QTL that modifies outer hair cell degeneration in Tmc1 (Bth) mutant mice, suggesting that M5ahl8 and Tmc1m4 may represent the same gene affecting maintenance of stereocilia structure and function during aging.

Aminoglycoside ototoxicity and hair cell ablation in the adult gerbil: A simple model to study hair cell loss and regeneration

The Mongolian gerbil, Meriones unguiculatus, has been widely employed as a model for studies of the inner ear. In spite of its established use for auditory research, no robust protocols to induce ototoxic hair cell damage have been developed for this species. In this paper, we demonstrate the development of an aminoglycoside-induced model of hair cell loss, using kanamycin potentiated by the loop diuretic furosemide. Interestingly, we show that the gerbil is relatively insensitive to gentamicin compared to kanamycin, and that bumetanide is ineffective in potentiating the ototoxicity of the drug. We also examine the pathology of the spiral ganglion after chronic, long-term hair cell damage. Remarkably, there is little or no neuronal loss following the ototoxic insult, even at 8 months post-damage. This is similar to the situation often seen in the human, where functioning neurons can persist even decades after hair cell loss, contrasting with the rapid, secondary degeneration found in rats, mice and other small mammals. We propose that the combination of these factors makes the gerbil a good model for ototoxic damage by induced hair cell loss.

Predicting the location of missing outer hair cells using the electrical signal recorded at the round window

The electrical signal recorded at the round window was used to estimate the location of missing outer hair cells. The cochlear response was recorded to a low frequency tone embedded in high-pass filtered noise conditions. Cochlear damage was created by either overexposure to frequency-specific tones or laser light. In animals with continuous damage along the partition, the amplitude of the cochlear response increased as the high-pass cutoff frequency increased, eventually reaching a plateau. The cochlear distance at the onset of the plateau correlated with the anatomical onset of outer hair cell loss. A mathematical model replicated the physiologic data but was limited to cases with continuous hair cell loss in the middle and basal turns. The neural contribution to the cochlear response was determined by recording the response before and after application of Ouabain. Application of Ouabain eliminated or reduced auditory neural activity from approximately two turns of the cochlea. The amplitude of the cochlear response was reduced for moderate signal levels with a limited effect at higher levels, indicating that the cochlear response was dominated by outer hair cell currents at high signal levels and neural potentials at low to moderate signal levels.

[The expression of miR-183 family in the pathogenesis and development of noise-induced deafness]

OBJECTIVE

To detect the expression variation of microRNA-183 family in cochlea of animal model characterized by noise-induced deafness at various time points, and to explore the mechanisms responsible for noise-induced deafness.

METHOD

Fifty mice were randomly divided into 5 groups. In the experimental group, 40 mice were exposed to 2-4 kHz narrow band noise at 100 dB SPL 6h per day for 3 consecutive days. The rest 10 mice served as the control group without receiving any noise. Auditory brainsterm response (ABR) were examined at the 1st, 7th, 14th and 28th day compaired with the ABR before the experiment,to confirm noise lead to the permanent threshold shift. The pathological damage processes of hair cell were detected by the basilar membrane stretched techniques. Real-time reverse transcriptase-polymerase chain reaction (qRT-PCR) was apply to quantify the expression of microRNA183 family members. Statistical analysis was performed by the SPSS 17.0 software.

RESULT

The hearing of mice in the experimental group was significantly less than that in the control group. In the experimental group, the hearing of mice exposed to noise were markedly less when compared with the one exposure to null-noise. The hearing in the 1st day group was least among experimental groups, and the followed one was mice in the 7th day group. No statistical difference were observed between the 14th and 28th day groups (P > 0.05). The results of surface preparation showed that the outer hair cells were chaotic, deformational, and their number decreased is time-dependent. The missing of the outer hair cells occurred mainly in the first and second rows, while the inner hair cells were not pronouncedly missing. The qRT-PCR showed that the expressions of the three genes (miR-183/96/182)in the 1st day and 7th day group with exposure to noise were less than in the control group (P < 0.01), while no significant difference was found between 1st day and 7th day group (P > 0.05). The expressions rised in the 14th day experimental groups, whereas the 28th day group's expressions of the three genes decreased markedly which were more than that in the 1st day and 7th day group (P < 0.01).

CONCLUSION

After noise exposure for some time, the expressions of miRNA-183 family members have significant changes in animal model with noise-induced deafness, which indicated that the miRNA183 family members may play important roles in the pathogenesis and development of noise-induced deafness.

Prestin regulation and function in residual outer hair cells after noise-induced hearing loss

The outer hair cell (OHC) motor protein prestin is necessary for electromotility, which drives cochlear amplification and produces exquisitely sharp frequency tuning. Tecta^C1509G transgenic mice have hearing loss, and surprisingly have increased OHC prestin levels. We hypothesized, therefore, that prestin up-regulation may represent a generalized response to compensate for a state of hearing loss. In the present study, we sought to determine the effects of noise-induced hearing loss on prestin expression. After noise exposure, we performed cytocochleograms and observed OHC loss only in the basal region of the cochlea. Next, we patch clamped OHCs from the apical turn (9–12 kHz region), where no OHCs were lost, in noise-exposed and age-matched control mice. The non-linear capacitance was significantly higher in noise-exposed mice, consistent with higher functional prestin levels. We then measured prestin protein and mRNA levels in whole-cochlea specimens. Both Western blot and qPCR studies demonstrated increased prestin expression after noise exposure. Finally, we examined the effect of the prestin increase in vivo following noise damage. Immediately after noise exposure, ABR and DPOAE thresholds were elevated by 30–40 dB. While most of the temporary threshold shifts recovered within 3 days, there were additional improvements over the next month. However, DPOAE magnitudes, basilar membrane vibration, and CAP tuning curve measurements from the 9–12 kHz cochlear region demonstrated no differences between noise-exposed mice and control mice. Taken together, these data indicate that prestin is up-regulated by 32–58% in residual OHCs after noise exposure and that the prestin is functional. These findings are consistent with the notion that prestin increases in an attempt to partially compensate for reduced force production because of missing OHCs. However, in regions where there is no OHC loss, the cochlea is able to compensate for the excess prestin in order to maintain stable auditory thresholds and frequency discrimination.

A point mutation in the gene for asparagine-linked glycosylation 10B (Alg10b) causes nonsyndromic hearing impairment in mice (Mus musculus)

The study of mouse hearing impairment mutants has led to the identification of a number of human hearing impairment genes and has greatly furthered our understanding of the physiology of hearing. The novel mouse mutant neurological/sensory 5 (nse5) demonstrates a significantly reduced or absent startle response to sound and is therefore a potential murine model of human hearing impairment. Genetic analysis of 500 intercross progeny localized the mutant locus to a 524 kilobase (kb) interval on mouse chromosome 15. A missense mutation in a highly-conserved amino acid was found in the asparagine-linked glycosylation 10B gene (Alg10b), which is within the critical interval for the nse5 mutation. A 20.4 kb transgene containing a wildtype copy of the Alg10b gene rescued the mutant phenotype in nse5/nse5 homozygous animals, confirming that the mutation in Alg10b is responsible for the nse5/nse5 mutant phenotype. Homozygous nse5/nse5 mutants had abnormal auditory brainstem responses (ABRs), distortion product otoacoustic emissions (DPOAEs), and cochlear microphonics (CMs). Endocochlear potentials (EPs), on the other hand, were normal. ABRs and DPOAEs also confirmed the rescue of the mutant nse5/nse5 phenotype by the wildtype Alg10b transgene. These results suggested a defect in the outer hair cells of mutant animals, which was confirmed by histologic analysis. This is the first report of mutation in a gene involved in the asparagine (N)-linked glycosylation pathway causing nonsyndromic hearing impairment, and it suggests that the hearing apparatus, and the outer hair cells in particular, are exquisitely sensitive to perturbations of the N-linked glycosylation pathway.

Effect of metabolic presbyacusis on cochlear responses: a simulation approach using a physiologically-based model

In the presented model, electrical, acoustical, and mechanical elements of the cochlea are explicitly integrated into a signal transmission line where these elements convey physiological interpretations of the human cochlear structures. As a result, this physiologically-motivated model enables simulation of specific cochlear lesions such as presbyacusis. The hypothesis is that high-frequency hearing loss in older adults may be due to metabolic presbyacusis whereby age-related cellular/chemical degenerations in the lateral wall of the cochlea cause a reduction in the endocochlear potential. The simulations quantitatively confirm this hypothesis and emphasize that even if the outer and inner hair cells are totally active and intact, metabolic presbyacusis alone can significantly deteriorate the cochlear functionality. Specifically, in the model, as the endocochlear potential decreases, the transduction mechanism produces less receptor current such that there is a reduction in the battery of the somatic motor. This leads to a drastic decrease in cochlear amplification and frequency sensitivity, as well as changes in position-frequency map (tuning pattern) of the cochlea. In addition, the simulations show that the age-related reduction of the endocochlear potential significantly inhibits the firing rate of the auditory nerve which might contribute to the decline of temporal resolution in the aging auditory system.

Age-related hearing loss in rhesus monkeys is correlated with cochlear histopathologies

Audiometric hearing deficits are a common symptom of age-related hearing loss (ARHL), as are specific histopathological changes in the cochlea; however, very little data have been collected in non-human primates. To examine this relationship further, we collected auditory brainstem responses (ABRs) from rhesus monkeys spanning in age from 10 to 35 years old, and examined four different morphological features of their cochleae. We found significant correlations between ABR thresholds and the loss of outer hair cells and spiral ganglion cells, but not with the loss of inner hair cells or a reduced thickness of the stria vascularis. The strongest correlation with ABR thresholds was the number of different pathologies present. These findings show that while aged rhesus monkeys experience audiometric hearing deficits similar to that seen in humans, they are not correlated with a single peripheral deficit, but instead with a number of different underlying cochlear histopathologies, indicating that similar histopathologies may exist in geriatric humans as well.

Noise-induced nitrotyrosine increase and outer hair cell death in guinea pig cochlea

BACKGROUND

Modern research has provided new insights into the biological mechanisms of noise-induced hearing loss, and a number of studies showed the appearance of increased reactive oxygen species (ROS) and reactive nitrogen species (RNS) during and after noise exposure. This study was designed to investigate the noise exposure induced nitrotyrosine change and the mechanism of outer hair cells death in guinea pig cochlea.

METHOD

Thirty guinea pigs were used in this study. The experimental animals were either exposed for 4 hours per day to broadband noise at 122 dB SPL (A-weighted) for 2 consecutive days or perfused cochleae with 5 mg/ml of the SIN1 solutions, an exogenous NO and superoxide donor, for 30 minutes. Then the cochleae of the animals were dissected. Propidium iodide (PI), a DNA intercalating fluorescent probe, was used to trace morphological changes in OHC nuclei. The distribution of nitrotyrosine (NT) in the organ of Corti and the cochlear lateral wall tissue from the guinea pigs were examined using fluorescence immunohistochemistry method. Whole mounts of organ of Corti were prepared. Morphological and fluorescent changes were examined under a confocal microscope.

RESULTS

Either after noise exposure or after SIN1 perfusion, outer hair cells (OHCs) death with characteristics of both apoptotic and necrotic degradation appeared. Nitrotyrosine immunolabeling could be observed in the OHCs from the control animals. After noise exposure, NT immunostaining became much greater than the control animals in OHCs. The apoptotic OHC has significant increase of nitrotyrosine in and around the nucleus following noise exposure. In the normal later wall of cochleae, relatively weak nitrotyrosine immunolabeling could be observed. After noise exposure, nitrotyrosine immunoactivity became stronger in stria vascularis.

CONCLUSION

Noise exposure induced increase of nitrotyrosine production is associated with OHCs death suggesting reactive nitrogen species participation in the cochlear pathophysiology of noise-induced hearing loss.

Distortion-product otoacoustic emissions: a useful test for monitoring ototoxicity induced by pegylated interferon and ribavirin treatment in patients with chronic hepatitis C

Pegylated-interferon (peg-IFN) and ribavirin combination therapy for the treatment of hepatitis C virus (HCV) infection is well known to be associated with significant adverse effects. Several studies have investigated a possible auditory pathway involvement during IFN therapy, but a method to monitor the potential auditory involvement during treatment has not yet been described. The aim of this study is to evaluate possible modifications of the outer hair cell (OHC) function in HCV patients receiving peg-IFN and ribavirin combination therapy. Thirteen adult HCV patients (8 F/5 M, mean age 52∓12 years) treated with peg-IFN and ribavirin combination therapy underwent Pure Tone Audiogram and Distortion Product Otoacoustic Emission (DPOAE) tests. We compared mean auditory thresholds (PTA) and mean DPOAE amplitude before, at month 3 during, and at the end of treatment (T0, T3, and Tend, respectively), and 3 months after treatment discontinuation (Tfu). No significant differences were found in hearing levels at the different time points analyzed. During treatment, three patients developed tinnitus, which in 2 cases resolved spontaneously after the end of therapy. Compared to T0 (19.5±0.83), a statistically significant DPOAE increase at T3 (30±1,26) and Tend (28.6±2.16) was found (p<0.05 at both time points), while DPOAEs returned to pre-treatment levels at Tfu (19.3±1.3). In our group, none of the patients reported a permanent auditory impairment, excluding one patient with persistent tinnitus. Peg-IFN could produce an increase of motility of the OHCs by means of intracellular pathways. DPOAE test could be considered a new method for monitoring ototoxicity induced by IFN. On the basis of recent literature and our audiological results, physicians should be aware of the possible ototoxic effects of peg-IFN, requiring appropriate surveillance, and the patient should be informed of the potential side effects of IFN therapy on the auditory pathway.

Gentamicin rapidly inhibits mitochondrial metabolism in high-frequency cochlear outer hair cells

Aminoglycosides (AG), including gentamicin (GM), are the most frequently used antibiotics in the world and are proposed to cause irreversible cochlear damage and hearing loss (HL) in 1/4 of the patients receiving these life-saving drugs. Akin to the results of AG ototoxicity studies, high-frequency, basal turn outer hair cells (OHCs) preferentially succumb to multiple HL pathologies while inner hair cells (IHCs) are much more resilient. To determine if endogenous differences in IHC and OHC mitochondrial metabolism dictate differential sensitivities to AG-induced HL, IHC- and OHC-specific changes in mitochondrial reduced nicotinamide adenine dinucleotide (NADH) fluorescence during acute (1 h) GM treatment were compared. GM-mediated decreases in NADH fluorescence and succinate dehydrogenase activity were observed shortly after GM application. High-frequency basal turn OHCs were found to be metabolically biased to rapidly respond to alterations in their microenvironment including GM and elevated glucose exposures. These metabolic biases may predispose high-frequency OHCs to preferentially produce cell-damaging reactive oxygen species during traumatic challenge. Noise-induced and age-related HL pathologies share key characteristics with AG ototoxicity, including preferential OHC loss and reactive oxygen species production. Data from this report highlight the need to address the role of mitochondrial metabolism in regulating AG ototoxicity and the need to illuminate how fundamental differences in IHC and OHC metabolism may dictate differences in HC fate during multiple HL pathologies.

[Morphological changes associated with low-tone hearing loss in guinea pig models of early endolymphatic hydrops]

The present study was to explore the functional and morphological changes in cochleas of guinea pig models of early endolymphatic hydrops. Thirty albino guinea pigs were randomly divided into three groups: control, 4-week model and 8-week model groups. For each group, n = 10. Model groups were operated on the right ears to result in endolymphatic hydrops with the method of slight destruction of endolymphatic sac and duct from extradural posterior cranial fossa approach, and the animals in control group were sham operated. Electrocochleogram recorded by trans-tympanic approach and auditory brainstem response (ABR) were tested in preoperative model groups, control group, 4-week model group and 8-week model group to assess the hearing changes. Histologic morphometry was used to quantify hydrops by testing scala media area (SMA) ratio. Scanning electron microscope was used to assess the changes of cochlea hair cells. The results showed that the summating potential/compound action potential (SP/AP) ratio of electrocochleogram in 4-week model group (0.33 ± 0.14) and 8-week model group (0.43 ± 0.14) increased significantly, compared with that in control group (0.07 ± 0.06). The maximum SMA ratio in 4-week model group (2.64 ± 0.10) and 8-week model group (3.54 ± 0.13) increased significantly, compared with that in control group (1.06 ± 0.08). The results of maximum SMA ratio correlated with SP/AP ratio of electrocochleogram (r = 0.86). The results of hearing threshold of ABR revealed that the operated ears of model groups were higher than the preoperative results at frequencies of 2 kHz and 4 kHz. And the damage of cochlea hair cells in operated ears occurred in apical and subapical turns. These results suggest the increased SP/AP ratio of electrocochleogram can indicate early endolymphatic hydrops. There is low-tone hearing loss in guinea pig models of early endolymphatic hydrops, and it may be associated with the abnormalities of the stereocilia among the outer hair cells in operated ears which occurs in apical and subapical turns.

The protective effect of conditioning on noise-induced hearing loss is frequency-dependent

We compared the extent of temporary threshold shift (TTS) and hair cell loss following high level 4 kHz noise exposure with those preconditioned with moderate level 1 and 4 kHz octave band noise. Fifteen Male albino guinea pigs (300- 350 g in weight) were randomly allocated into three groups: those exposed to 4 kHz octave band noise at 102 dB SPL (group 1, n=5); those conditioned with 1 kHz octave band noise at 85 dB SPL, 6 hours per day for 5 days, then exposed to noise (group 2, n=5); those conditioned with 4 kHz octave band noise at 85 dB SPL, then exposed to noise (group 3, n=5). An hour and one week after noise exposure, threshold shifts were evaluated by auditory-evoked brainstem response (ABR) and then animals were euthanized for histological evaluation. We found that TTS and cochlear damage caused by noise exposure were significantly reduced by 1 kHz and 4 kHz conditioning (P<0.001). We also showed that 4 kHz protocol attenuates noise- induced TTS but no significant TTS reduction occurred by 1 kHz conditioning. Both protocol protected noise-induced cochlear damage. We concluded that lower tone conditioning could not protect against higher tone temporary noise-induced hearing loss, thus conditioning is a local acting and frequency-dependent phenomenon.

Biophysical mechanisms underlying outer hair cell loss associated with a shortened tectorial membrane

The tectorial membrane (TM) connects to the stereociliary bundles of outer hair cells (OHCs). Humans with an autosomal dominant C1509G mutation in alpha-tectorin, a protein constituent of the TM, are born with a partial hearing loss that worsens over time. The Tecta(C1509/+) transgenic mouse with the same point mutation has partial hearing loss secondary to a shortened TM that only contacts the first row of OHCs. As well, Tecta(C1509G/+) mice have increased expression of the OHC electromotility protein, prestin. We sought to determine whether these changes impact OHC survival. Distortion product otoacoustic emission thresholds in a quiet environment did not change to 6 months of age. However, noise exposure produced acute threshold shifts that fully recovered in Tecta (+/+) mice but only partially recovered in Tecta(C1509G/+) mice. While Tecta(+/+) mice lost OHCs primarily at the base and within all three rows, Tecta(C1509G/+) mice lost most of their OHCs in a more apical region of the cochlea and nearly completely within the first row. In order to estimate the impact of a shorter TM on the forces faced by the stereocilia within the first OHC row, both the wild type and the heterozygous conditions were simulated in a computational model. These analyses predicted that the shear force on the stereocilia is ~50% higher in the heterozygous condition. We then measured electrically induced movements of the reticular lamina in situ and found that while they decreased to the noise floor in prestin null mice, they were increased by 4.58 dB in Tecta(C1509G/+) mice compared to Tecta(+/+) mice. The increased movements were associated with a fourfold increase in OHC death as measured by vital dye staining. Together, these findings indicate that uncoupling the TM from some OHCs leads to partial hearing loss and places the remaining coupled OHCs at higher risk. Both the mechanics of the malformed TM and the increased prestin-related movements of the organ of Corti contribute to this higher risk profile.

Oxidative stress in spiral ganglion cells of pigmented and albino guinea pigs exposed to impulse noise

CONCLUSIONS

The results suggest that melanin inhibits formation of reactive oxygen species (ROS) and prevents apoptosis in spiral ganglion cells (SGCs) of pigmented guinea pigs following impulse noise.

OBJECTIVE

The stria vascularis of pigmented guinea pig cochlea contains melanocytes that produce melanin, which has a protective effect on noise-induced hair cell damage through its antioxidant property. ROS are involved in cochlear damage induced by impulse noise trauma. The purpose of the present study was to investigate the oxidative stress in SGCs of pigmented and albino guinea pigs after exposure to impulse noise.

METHODS

Pigmented and albino guinea pigs were exposed to impulse noise. Auditory thresholds were assessed by sound-evoked auditory brainstem response (ABR) before impulse noise exposure and 72 h after impulse noise exposure. 4-Hydroxynonenal (HNE) was used as a histochemical marker of ROS formation, and active-caspase-3 (cas-3) served as a marker for apoptosis. 4-HNE and cas-3 were determined immunohistochemically. Hair cell damage was analyzed by scanning electron microscopy.

RESULTS

The rates of 4-HNE-positive and cas-3-positive SGCs in pigmented guinea pigs were much less than those for albino guinea pigs. Correspondingly, there was less hair cell damage and reduced ABR threshold shifts in pigmented guinea pigs.

Evaluation of ear function after cardiopulmonary bypass with otoacoustic emissions: a pilot study

OBJECTIVES

During extracorporeal circulation (ECC) there is a great hemodynamic stress with possible impact on the microcirculation, including cochlear one. Previous studies have evaluated the effect of ECC on inner ear with contrasting results. The aim of this study is to evaluate possible modifications of the outer hair cells (OHC) function after open heart surgery (OHS) under ECC with transient evoked (TEOAEs) and distortion product otoacoustic emissions (DPOAEs).

METHODS

Ten patients (5 F and 5 M), undergoing OHS with ECC, were subjected to an audiological assessment pre- and postoperatively. We compared the pre-operative and post-operative mean auditory thresholds, mean TEOAEs reproducibility and amplitude, and mean DPOAEs amplitude. Student's t-test was used to compare different values.

RESULTS

No significant differences were found between pre- and post-operative audiological assessment both in hearing level and in otoacoustic emissions.

CONCLUSION

OHC function seems to be not affected by hemodynamic stress induced by ECC. Further studies on a larger scale will be necessary to confirm our preliminary data.

Anti-epileptic drugs delay age-related loss of spiral ganglion neurons via T-type calcium channel

Loss of spiral ganglion neurons is a major cause of age-related hearing loss (presbycusis). Despite being the third most prevalent condition afflicting elderly persons, there are no known medications to prevent presbycusis. Because calcium signaling has long been implicated in age-related neuronal death, we investigated T-type calcium channels. This family is comprised of three members (Ca(v)3.1, Ca(v)3.2, and Ca(v)3.3), based on their respective main pore-forming alpha subunits: α1G, α1H, and α1I. In the present study, we report a significant delay of age-related loss of cochlear function and preservation of spiral ganglion neurons in α1H null and heterozygous mice, clearly demonstrating an important role for Ca(v)3.2 in age-related neuronal loss. Furthermore, we show that anticonvulsant drugs from a family of T-type calcium channel blockers can significantly preserve spiral ganglion neurons during aging. To our knowledge, this is the first report of drugs capable of diminishing age-related loss of spiral ganglion neurons.

[Ototoxicity of kanamycin sulfate in adult rats and its underlying mechanisms]

The aim of the present study was to assess the ototoxicity of kanamycin sulfate (KM) in adult rats and its underlying mechanism. Forty male Sprague-Dawley rats (6-7 weeks old) were randomly divided into the experimental group and the control group. The animals in the experimental group were injected subcutaneously with KM (500 mg/kg per day) for two weeks, and the control group received equal volume of normal saline. To assess the ototoxicity of KM, the auditory brainstem response (ABR) was recorded to monitor the changes in hearing thresholds, and the density of spiral ganglion cells (SGCs) and morphology of cochlea were observed using surface preparations and frozen sections of cochlea. The results showed that the hearing threshold of rats in the experimental group was elevated by more than 60 dB across all the frequencies two weeks after the first administration of KM. And in the experimental group, the density of SGCs became lower, and organ of Corti suffered loss of hair cells. The loss of outer hair cells (OHCs) was more severe than that of inner hair cells (IHCs), correlated with the density decrease of SGCs. We conclude that the ototoxicity of KM in the adult rats was apparent and the underlying mechanism is associated with the loss of SGCs and hair cells.

Therapeutic window for ferulic acid protection against noise-induced hearing loss in the guinea pig

CONCLUSION

Our results are in agreement with the general idea that natural antioxidants achieve their best cytoprotective capacity if given before and soon after the stressor.

OBJECTIVE

We focused on ferulic acid (FA, 4-hydroxy 3-methoxycinnamic acid), a phenolic compound that is known to exhibit antioxidant properties. Our study was designed to evaluate the effectiveness of FA for different schedules of treatment to establish the 'therapeutic window' for FA protection.

METHODS

Guinea pigs were exposed to acoustic trauma (6 kHz at 120 dB for 60 min) and received a total dose of 600 mg/kg of FA. Group I, noise control; group II, noise + FA (150 mg/kg) for 4 days starting 24 h post exposure; group III, noise + FA (60 mg/kg) 1 h before and 9 days post exposure; group IV, noise + FA (60 mg/kg) given 3 days before and 7 days post exposure; group V, noise + FA (150 mg/kg) 1 h before and 3 days post noise exposure. Auditory brainstem response (ABR) test and immunohistochemical and morphological studies were performed.

RESULTS

Group V had significantly decreased noise-induced hearing loss at day 21 from noise exposure. The improvement of auditory function by FA was paralleled by a significant reduction in oxidative stress marker. The other schedules of drug administration showed a minor degree of protection.

[The hearing function in prematurely born children during the first year of life depending on the duration of gestation estimated by the otoacoustic technique]

A total of 239 premature infants of different gestational age remained under the observation throughout the first year of their life for the elucidation of dynamics of the parameters of DP-gram. It was shown that the absence of the distortion-product otoacoustic emission (DPOAE) response during the first months of life of the premature infants was due to immaturity of their cochlear receptor apparatus. The predominant peak was recorded at a frequency of 2 kHz, regardless of the gestational age. The amplitude of DP-gram values in premature infants lay in the low-frequency region (f2=1 kHz) and never reached the high level. The most intensive maturation of the cochlear receptor apparatus is observed during the first 6 months in premature infants of the gestational age below 28 weeks. After the sixth month, the "high-level" response must develop, regardless of the gestational age. By this time, the intensity of maturation of the cochlear receptor apparatus in all age groups of premature infants is virtually identical (p>0.05).

New insights into glutamate ototoxicity in cochlear hair cells and spiral ganglion neurons

CONCLUSION

Excess glutamate (Glu) exposure (20 mM) in the cochlear perilymph affects the physiological function of outer hair cells (OHCs) within a 2 h period and induces apoptosis in the modiolus spiral ganglion neurons (SGNs) in an apoptosis-inducing factor (AIF)-dependent manner.

OBJECTIVES

To determine whether high-dose Glu affects the function of OHCs and whether it induces AIF- and caspase-3-dependent apoptosis in the cochlear SGNs.

METHODS

Perilymphatic perfusions of Glu (20 mM) and artificial perilymph (AP) solutions were performed in adult guinea pig cochleae. Both cochlear microphonics (CM) and electrical auditory brainstem response (eABR) were measured before and 2 h after perfusions. The hair cell morphologies were examined using transmission electron microscopy. The expression of two apoptotic indicators, AIF and caspase-3, was examined 8 h after perfusions.

RESULTS

In contrast to AP perfusions, the perfusion of 20 mM Glu caused significant reduction in the CM and eABR amplitudes. Inner hair cells (IHCs) after Glu perfusion were deformed and exhibited vacuolization in the postsynaptic region, whereas the OHC system appeared unaffected. AIF expression was detected in the nuclei of SGNs 8 h after Glu exposure, but the expression of caspase-3 was not shown in any cochlear tissues.

Too much of a good thing: long-term treatment with salicylate strengthens outer hair cell function but impairs auditory neural activity

Aspirin has been extensively used in clinical settings. Its side effects on auditory function, including hearing loss and tinnitus, are considered as temporary. A recent promising finding is that chronic treatment with high-dose salicylate (the active ingredient of aspirin) for several weeks enhances expression of the outer hair cell (OHC) motor protein (prestin), resulting in strengthened OHC electromotility and enhanced distortion product otoacoustic emissions (DPOAE). To follow up on these observations, we carried out two studies, one planned study of age-related hearing loss restoration and a second unrelated study of salicylate-induced tinnitus. Rats of different strains and ages were injected with salicylate at a dose of 200 mg/kg/day for 5 days per week for 3 weeks or at higher dose levels (250-350 mg/kg/day) for 4 days per week for 2 weeks. Unexpectedly, while an enhanced or sustained DPOAE was seen, permanent reductions in the amplitude of the cochlear compound action potential (CAP) and the auditory brainstem response (ABR) were often observed after the chronic salicylate treatment. The mechanisms underlying these unexpected, permanent salicylate-induced reductions in neural activity are discussed.

Effects of age and sex on the expression of estrogen receptor alpha and beta in the mouse inner ear

CONCLUSION

Estrogen receptor (ER) alpha and beta were expressed in the inner ear, and expression decreased with increasing age. ERalpha may alter cochlear and vestibular sensory transduction, and ERbeta may have a neuroprotective function in the inner ear.

OBJECTIVE

Expression of ERalpha and ERbeta in the mouse inner ear and its alterations with sex and aging were analyzed.

MATERIALS AND METHODS

Male and female CBA/J mice aged 8 weeks and 24 months were used. The localization and the intensity of ERalpha and ERbeta immunoreactivity in the inner ear of young and old mice of both sexes were investigated by immunohistochemistry.

RESULTS

ERalpha and ERbeta were co-expressed in the inner ear, i.e. in the nuclei of stria vascularis, outer and inner hair cells, spiral ganglion cells and vestibular ganglion cells, vestibular dark cells and endolymphatic sac. Strial marginal cells, outer hair cells and type II ganglion cells showed less expression of ERalpha. No gender- or age-related difference was noted in the expression pattern of ERalpha or ERbeta, but fluorescence intensity of ERalpha was stronger in young female mice than in young male mice. In contrast, ERbeta revealed no significant difference. In the old mice, fluorescence intensities of both ERalpha and ERbeta were significantly decreased in both sexes.

[Electrophysiological evaluation of the hearing function following cisplatin administration]

Experiments on guinea pigs with the use of electrophysiological methods (short-latency auditory evoked potentials, SLAEPs, and distortion product otoacoustic emission, DPOAE) have demonstrated damage of outer hair cells following application of therapeutic doses of cisplatin. It is concluded that finding of the logarithmic amplitude-time dependence for the first SLAEPs peak permits to diagnose an injury to outer hair cells.

Mutations in LOXHD1, an evolutionarily conserved stereociliary protein, disrupt hair cell function in mice and cause progressive hearing loss in humans

Hearing loss is the most common form of sensory impairment in humans and is frequently progressive in nature. Here we link a previously uncharacterized gene to hearing impairment in mice and humans. We show that hearing loss in the ethylnitrosourea (ENU)-induced samba mouse line is caused by a mutation in Loxhd1. LOXHD1 consists entirely of PLAT (polycystin/lipoxygenase/alpha-toxin) domains and is expressed along the membrane of mature hair cell stereocilia. Stereociliary development is unaffected in samba mice, but hair cell function is perturbed and hair cells eventually degenerate. Based on the studies in mice, we screened DNA from human families segregating deafness and identified a mutation in LOXHD1, which causes DFNB77, a progressive form of autosomal-recessive nonsyndromic hearing loss (ARNSHL). LOXHD1, MYO3a, and PJVK are the only human genes to date linked to progressive ARNSHL. These three genes are required for hair cell function, suggesting that age-dependent hair cell failure is a common mechanism for progressive ARNSHL.

Organic solvents and hearing loss: The challenge for audiology

Organic solvents have been reported to adversely affect human health, including hearing health. Animal models have demonstrated that solvents may induce auditory damage, especially to the outer hair cells. Research on workers exposed to solvents has suggested that these chemicals may also induce auditory damage through effects on the central auditory pathways. Studies conducted with both animals and humans demonstrate that the hearing frequencies affected by solvent exposure are different to those affected by noise, and that solvents may interact synergistically with noise. The present article aims to review the contemporary literature of solvent-induced hearing loss, and consider the implications of solvent-induced auditory damage for clinical audiologists. Possible audiological tests that may be used when auditory damage due to solvent exposure is suspected are discussed.

The effects of acoustic environment after traumatic noise exposure on hearing and outer hair cells

Previous studies reported that exposure to non-traumatic level sounds after traumatic noise exposure reduced the degree of noise-induced hearing loss and hair cell stereocilia damage. The current study investigated the effects of a 3-day post-noise acoustic environment on the degree of noise-induced hearing loss and cochlear damage. Female chinchillas were exposed to traumatic continuous noise (4 kHz octave-band noise) at 107 dB SPL for 1h and then placed in either an augmented acoustic environment (AAE) or deprived acoustic environment (DAE) for 3 days. The AAE group was exposed to a broad-band noise (4-20 kHz) at 80 dB SPL and the DAE animals were fit with conventional earplugs to minimize the level of acoustic stimulation. Auditory brainstem responses (ABRs) were recorded before and 3 days after the traumatic noise exposure. The AAE group showed a significantly lower average threshold shift at the frequencies of 4 and 8 kHz (p<0.01). Correspondingly, significantly fewer missing and dying outer hair cells (OHCs) were observed in the AAE group than in the DAE group. Although the cochlear reduced and oxidized glutathione levels (GSH and GSSG, respectively) were essentially the same in two groups at day 3, significant correlations were found between GSSG levels and mean ABR threshold shift (1-16 kHz) in the AAE group; as well as GSSG and percentage of total OHC loss in the DAE group. The results suggest that post-noise acoustic environment influenced the degree of hearing loss and OHC deterioration after traumatic noise exposure.

[Role of caspase 12 in apoptosis of cochlea induced by intense noise in guinea pigs]

OBJECTIVE

To investigate the relationship between caspase 12 activation and endoplasmic reticulum stress mediated apoptosis of guinea pig cochlea cells induced by intense noise.

METHODS

Thirty-two guinea pigs were randomly divided into 4 groups. The guinea pigs in the experiment groups were exposed to 4 kHz narrow band noise at 120 dB SPL for 4 h. After the noise expose for 1, 4, 14 days of the experiment guinea pigs, auditory brainstem response (ABR) of the guinea pigs on experiment and control groups were tested before decapitated. Four guinea pig's cochleae of every group were taken to paraffin section, and the rest was extracted the total protein. Apoptosis was tested by terminal deoxynucleotidyl transferase (TDT)-mediated deoxyuridine triphosphate (d-UTP) nick and labeling method (TUNEL) and transmission electron microscopy. Expression of caspase 12, Bip/GRP78 was tested by immunohistochemistry and Western blot methods.

RESULTS

The observation by transmission electron microscopy showed the features characteristic of apoptotic cells in the Corti and SGC of 1d after the noise expose, but no in the control. There were higher expressions of Tunel-Positive cells in the OHC, SGC and SV of experiment groups, and there was significant differences compared with the control group (P < 0.01). Protein levels of Bip/GRP78 and caspase 12 were risen up after noise exposed, and there all were significant differences compared with the control group (P < 0.01).

CONCLUSION

Intense noise causes cochlea cell lesion by inducing apoptosis to result in and caspase 12 induced endoplasmic reticulum stress-related apoptosis plays an important role in the procedure of apoptosis.