Candidate's thesis: enhancing intrinsic cochlear stress defenses to reduce noise-induced hearing loss

Redox homeostasis dysregulation in noise-induced hearing loss: oxidative stress and antioxidant treatment

Noise exposure is an important cause of acquired hearing loss. Studies have found that noise exposure causes dysregulated redox homeostasis in cochlear tissue, which has been recognized as a signature feature of hearing loss. Oxidative stress plays a pivotal role in many diseases via very complex and diverse mechanisms and targets. Reactive oxygen species are products of oxidative stress that exert toxic effects on a variety of physiological activities and are considered significant in noise-induced hearing loss (NIHL). Endogenous cellular antioxidants can directly or indirectly counteract oxidative stress and regulate intracellular redox homeostasis, and exogenous antioxidants can complement and enhance this effect. Therefore, antioxidant therapy is considered a promising direction for NIHL treatment. However, drug experiments have been limited to animal models of NIHL, and these experiments and related observations are difficult to translate in humans; therefore, the mechanisms and true effects of these drugs need to be further analyzed. This review outlines the effects of oxidative stress in NIHL and discusses the main mechanisms and strategies of antioxidant treatment for NIHL.

Role of mitochondrial dysfunction and oxidative stress in sensorineural hearing loss

Sensorineural hearing loss (SNHL) can either be genetically inherited or acquired as a result of aging, noise exposure, or ototoxic drugs. Although the precise pathophysiological mechanisms underlying SNHL remain unclear, an overwhelming body of evidence implicates mitochondrial dysfunction and oxidative stress playing a central etiological role. With its high metabolic demands, the cochlea, particularly the sensory hair cells, stria vascularis, and spiral ganglion neurons, is vulnerable to the damaging effects of mitochondrial reactive oxygen species (ROS). Mitochondrial dysfunction and consequent oxidative stress in cochlear cells can be caused by inherited mitochondrial DNA (mtDNA) mutations (hereditary hearing loss and aminoglycoside-induced ototoxicity), accumulation of acquired mtDNA mutations with age (age-related hearing loss), mitochondrial overdrive and calcium dysregulation (noise-induced hearing loss and cisplatin-induced ototoxicity), or accumulation of ototoxic drugs within hair cell mitochondria (drug-induced hearing loss). In this review, we provide an overview of our current knowledge on the role of mitochondrial dysfunction and oxidative stress in the development of SNHL caused by genetic mutations, aging, exposure to excessive noise, and ototoxic drugs. We also explore the advancements in antioxidant therapies for the different forms of acquired SNHL that are being evaluated in preclinical and clinical studies.

Antioxidant Therapy as an Effective Strategy against Noise-Induced Hearing Loss: From Experimental Models to Clinic

Cochlear redox unbalance is the main mechanism of damage involved in the pathogenesis of noise-induced-hearing loss. Indeed, the increased free radical production, in conjunction with a reduced efficacy of the endogenous antioxidant system, plays a key role in cochlear damage induced by noise exposure. For this reason, several studies focused on the possibility to use exogenous antioxidant to prevent or attenuate noise-induce injury. Thus, several antioxidant molecules, alone or in combination with other compounds, have been tested in both experimental and clinical settings. In our findings, we tested the protective effects of several antioxidant enzymes, spanning from organic compounds to natural compounds, such as nutraceuticals of polyphenols. In this review, we summarize and discuss the strengths and weaknesses of antioxidant supplementation focusing on polyphenols, Q-Ter, the soluble form of CoQ10, Vitamin E and N-acetil-cysteine, which showed great otoprotective effects in different animal models of noise induced hearing loss and which has been proposed in clinical trials.

D-methionine administered as late as 36 hours post-noise exposure rescues from permanent threshold shift and dose-dependently increases serum antioxidant levels

OBJECTIVE

To elucidate D-methionine's (D-met) dose and time rescue parameters from steady-state or impulse noise-induced permanent threshold shift (PTS) and determine D-met rescue's influence on serum and cochlear antioxidant levels.

DESIGN

Five D-met doses at 0, 50, 100, or 200 mg/kg/dose administered starting at 1, 24, or 36 hours post steady-state or impulse noise exposure. Auditory brainstem responses at baseline and 21 days post-noise measured PTS. Serum (superoxide dismutase [SOD], catalase [CAT],, glutathione reductaseand glutathione peroxidase [GPx]) and cochlear (Glutathione [GSH] and glutathione disulphide [GSSG]) antioxidant levels measured physiological impact.

STUDY SAMPLE

Chinchillas (10/study group; 6-8/confirmatory groups).

RESULTS

D-met significantly reduced PTS for impulse noise (100 mg [2, 6, 14 and 20 kHz]; 200 mg [2, 14 and 20 kHz]) and steady-state noise (all dosing groups, time parameters and tested frequencies). PTS reduction did not significantly vary by rescue time. D-met significantly increased serum SOD (100 and 200 mg for 24 hour rescue) and GPx (50 mg/kg at 24 hour rescue) at 21 days post-noise. Cochlear GSH and GSSG levels were unaffected relative to control.

CONCLUSION

D-met rescues from steady-state and impulse noise-induced PTS even when administered up to 36 hours post-noise and dose-dependently influences serum antioxidant levels even 21 days post-noise. D-met's broad and effective dose/time window renders it a promising antioxidant rescue agent.

The Use of Antioxidants in the Prevention and Treatment of Noise-Induced Hearing Loss

As of today, there are no therapeutic measures for the prevention or treatment of noise-induced hearing loss (NIHL). The current preventative measures, including avoidance and personal protective hearing equipment, do not appear to be sufficient because there is an increasing number of people with NIHL, especially in the adolescent population. Therefore, we must find a therapy that prevents the impact of noise on hearing. Antioxidants are a promising option in preventing the damaging effects of noise by targeting free radicals but further studies are needed to confirm their efficacy in humans.

Cochlear Preconditioning as a Modulator of Susceptibility to Hearing Loss

Significance: Acquired sensorineural hearing loss is a major public health problem worldwide. The leading causes of sensorineural hearing loss are noise, aging, and ototoxic medications, with the key underlying pathology being damage to the cochlea. The review focuses on the phenomenon of preconditioning, in which the susceptibility to cochlear injury is reduced by exposing the ear to a stressful stimulus. Recent Advances: Cochlear conditioning has focused on the use of mono-modal conditioning, specifically conditioning the cochlea with moderate noise exposures before a traumatic exposure that causes permanent hearing loss. Recently, cross-modal conditioning has been explored more thoroughly, to prevent not only noise-induced hearing loss, but also age-related and drug-induced hearing losses. Critical Issues: Noise exposures that cause only temporary threshold shifts (TTSs) can cause long-term synaptopathy, injury to the synapses between the inner hair cells and spiral ganglion cells. This discovery has the potential to significantly alter the field of cochlear preconditioning with noise. Further, cochlear preconditioning can be the gateway to the development of clinically deployable therapeutics. Therefore, understanding the underlying mechanisms of conditioning is crucial for optimizing clinical protection against sensorineural hearing loss. Future Directions: Before the discovery of synaptopathy, noise exposures that caused only TTSs were believed to be either harmless or potentially beneficial. Any considerations of preconditioning with noise must consider the potential for injury to the synapses. Further, the discovery of different methods to precondition the cochlea against injury will yield new avenues for protection against hearing loss in the vulnerable populations. Antioxid. Redox Signal. 36, 1215-1228.

Predicting secondary endolymphatic hydrops in patients with noise-induced hearing loss

BACKGROUND

Long-term noise exposure may damage the cochlea and endolymph resorption system, which induces episodic vertigo and/or fluctuating hearing loss in later years.

OBJECTIVE

This study adopted clinical symptoms, inner ear test battery, and/or magnetic resonance (MR) imaging to evaluate development of secondary endolymphatic hydrops (EH) in patients with noise-induced hearing loss (NIHL).

METHODS

Forty NIHL patients with secondary EH were assigned to Group A. Another 40 age-and sex-matched NIHL patients without EH were assigned to Group B. All patients underwent an inner ear test battery. MR imaging was performed when diagnosis of EH was equivocal via above testing.

RESULTS

Group A had significantly higher mean hearing levels (MHLs) than Group B at 1000, 2000, 4000, and 8000 Hz. Both groups displayed a significantly declining sequence of abnormality rates of the inner ear test battery. Under receiver operating characteristic (ROC) curve analysis, the cutoff threshold at 4 kHz for predicting the presence of secondary EH in NIHL patients was 52 dBHL, with a sensitivity of 62% and a specificity of 69%.

CONCLUSIONS

NIHL patients revealing a typical 4 kHz dip-type audiogram with dip threshold >52 dBHL may predict development of secondary EH. A longitudinal follow-up coupled with MR imaging is required for confirmation.

Preloaded D-methionine protects from steady state and impulse noise-induced hearing loss and induces long-term cochlear and endogenous antioxidant effects

Objective

Determine effective preloading timepoints for D-methionine (D-met) otoprotection from steady state or impulse noise and impact on cochlear and serum antioxidant measures.

Design

D-met started 2.0-, 2.5-, 3.0-, or 3.5- days before steady-state or impulse noise exposure with saline controls. Auditory brainstem response (ABRs) measured from 2 to 20 kHz at baseline and 21 days post-noise. Samples were then collected for serum (SOD, CAT, GR, GPx) and cochlear (GSH, GSSG) antioxidant levels.

Study sample

Ten Chinchillas per group.

Results

Preloading D-met significantly reduced ABR threshold shifts for both impulse and steady state noise exposures but with different optimal starting time points and with differences in antioxidant measures.

For impulse noise exposure, the 2.0, 2.5, and 3.0 day preloading start provide significant threshold shift protection at all frequencies. Compared to the saline controls, serum GR for the 3.0 and 3.5 day preloading groups was significantly increased at 21 days with no significant increase in SOD, CAT or GPx for any impulse preloading time point. Cochlear GSH, GSSG, and GSH/GSSG ratio were not significantly different from saline controls at 21 days post noise exposure.

For steady state noise exposure, significant threshold shift protection occurred at all frequencies for the 3.5, 3.0 and 2.5 day preloading start times but protection only occurred at 3 of the 6 test frequencies for the 2.0 day preloading start point. Compared to the saline controls, preloaded D-met steady-state noise groups demonstrated significantly higher serum SOD for the 2.5–3.5 day starting time points and GPx for the 2.5 day starting time but no significant increase in GR or CAT for any preloading time point. Compared to saline controls, D-met significantly increased cochlear GSH concentrations in the 2 and 2.5 day steady-state noise exposed groups but no significant differences in GSSG or the GSH/GSSG ratio were noted for any steady state noise-exposed group.

Conclusions

The optimal D-met preloading starting time window is earlier for steady state (3.5–2.5 days) than impulse noise (3.0–2.0). At 21 days post impulse noise, D-met increased serum GR for 2 preloading time points but not SOD, CAT, or GpX and not cochlear GSH, GSSG or the GSH/GSSG ratio. At 21 days post steady state noise D-met increased serum SOD and GPx at select preloading time points but not CAT or GR. However D-met did increase the cochlear GSH at select preloading time points but not GSSG or the GSH/GSSG ratio.

Oral D-methionine protects against cisplatin-induced hearing loss in humans: phase 2 randomized clinical trial in India

Objective: This exploratory Phase 2 clinical trial is the first determining safety and efficacy of oral D-methionine (D-met) in reducing cisplatin-induced ototoxicity.Design: Randomised parallel double-blind placebo-controlled exploratory Phase 2 study.Study samples: Fifty adult cancer patients received oral D-met or placebo before each cisplatin dose. Physical examination, blood collection and audiometry occurred at baseline and subsequent visits plus post-treatment audiometry. After attrition, final analysis included 27 patients.Results: Significant treatment group by ear and time (baseline vs. post-treatment) interactions occurred at 10 kHz and 11.2 kHz. Placebo and D-met groups differed in threshold shift for left ear at 11.2 kHz (mean difference = 22.97 dB [9.59, 36.35]). Averaging across ears, placebo group showed significant threshold shifts from baseline to post-treatment at 10 kHz (mean shift= -13.65 dB [-21.32,-5.98]), 11.2 kHz (-16.15 dB [-25.19,-7.12]), and 12.5 kHz (-11.46 dB [-19.18,-3.74]) but not 8 kHz (-8.65 dB [-17.86, 0.55]). The D-met group showed no significant threshold shifts (8 kHz: -1.25 dB [-7.75, 5.25]; 10 kHz:-3.93 dB [-8.89, 1.03]; 11.2 kHz:-4.82 dB [-11.21, 1.57]; 12.5 kHz:-3.68 dB [-11.57, 4.21]). Side effects did not significantly differ between groups.Conclusion: Oral D-met reduces cisplatin-induced ototoxicity in humans.

D-methionine immediate and continued rescue after noise exposure does not prevent temporary threshold shift but alters cochlear and serum antioxidant levels

OBJECTIVE

Determine if D-methionine (D-met) rescue prevents temporary threshold shift (TTS) from steady-state or impulse noise and determine D-met's impact on serum and cochlear antioxidant levels.

DESIGN

D-met at 50, 100 or 200 mg/kg/doses were administered 0, 6 and 18 hours-post noise. ABRs at baseline and 24 hours post-noise measured TTS. Serum (SOD, CAT, GR, GPx) and cochlear (GSH, GSSG) antioxidant levels measured physiological influence. Three control groups, with impulse or steady-state or without noise, were saline-injected.

STUDY SAMPLE

Ten Chinchillas/group.

RESULTS

D-met rescue did not significantly reduce TTS or impact serum CAT, SOD, GPx or GR levels vs. noise-exposed control groups, but TTS was greater in all groups relative to no-noise controls. D-met significantly elevated CAT at 50 mg/kg vs. steady-state controls and SOD at 200 mg/kg vs. impulse noise controls. D-met significantly reduced cochlear GSH/GSSG ratios in the 100 mg/kg D-met group vs. impulse noise controls.

CONCLUSIONS

While D-met rescue has reduced permanent threshold shift in previous studies, it did not reduce TTS in this study. However, D-met rescue did alter selective serum and cochlear oxidative state changes 24 hours post-noise relative to controls. Results demonstrate TTS studies do not always predict PTS protection in otoprotectant experimental designs.

Effect of Korean Red Ginseng on Noise-Induced Hearing Loss

Objective:

Noise-induced hearing loss (NIHL) is one of the most important problems affecting both social and professional life of patients. There is no treatment method considered to be successful on the hearing loss that has become a permanent nature. Aim of this study is to evaluate protective effect of Korean Red Ginseng (KRG) against NIHL in an animal model.

Methods:

Twenty-eight rats were separated into four groups [control saline (group I), control KRG (group II), saline + noise (group III), KRG + noise (group IV)]. Rats in the saline and KRG groups were fed via oral gavage with a dose of 200 mg/kg/day throughout for 10 days. Fourteen rats (group III and IV) were exposed to 4 kHz octave band noise at 120 dB SPL for 5 hours. Hearing levels of rats were evaluated by distortion product otoacoustic emissions (DPOAE) and auditory brainstem responses (ABR) at 4, 8, 12, 16 and 32 kHz frequencies prior to and on days 1, 7 and 10 after the noise exposure. Rats were sacrificed on 10th day, after the last audiological test. Cochlea and spiral ganglion tissues were evaluated by light microscopy.

Results:

Audiological and histological results demonstrated that after noise the group IV showed better results than group III. In the noise exposed groups, the most prominent damage was seen at the 8 kHz frequency region than other regions. After the noise exposure, DPOAE responses were lost in 1st, 7th and 10th measurements in both group III and IV. Thus, we were not able to perform any statistical analyses for DPOAE results.

Conclusion:

Our findings suggest that KRG seems to be an efficient agent against NIHL. There is need for additional research to find out about the mechanisms of KRG’s protective effect.

Cochlear hair cells of echolocating bats are immune to intense noise

Exposure to intense noise can damage cochlear hair cells, leading to hearing loss in mammals. To avoid this constraint, most mammals have evolved in relatively quiet environments. Echolocating bats, however, are naturally exposed to continuous intense sounds from their own and neighboring sonar emissions for maintaining sonar directionality and range. Here, we propose the presence of intense noise resistance in cochlear hair cells of echolocating bats against noise-induced hearing loss (NIHL). To test this hypothesis, we performed noise exposure experiments for laboratory mice, one nonecholocating bat species, and five echolocating bat species. Contrary to nonecholocating fruit bats and mice, the hearing and the cochlear hair cells of echolocating bats remained unimpaired after continuous intense noise exposure. The comparative analyses of cochleae transcriptomic data showed that several genes protecting cochlear hair cells from intense sounds were overexpressed in echolocating bats. Particularly, the experimental examinations revealed that ISL1 overexpression significantly improved the survival of cochlear hair cells. Our findings support the existence of protective effects in cochlear hair cells of echolocating bats against intense noises, which provides new insight into understanding the relationship between cochlear hair cells and intense noises, and preventing or ameliorating NIHL in mammals.

Hsp70/Bmi1-FoxO1-SOD Signaling Pathway Contributes to the Protective Effect of Sound Conditioning against Acute Acoustic Trauma in a Rat Model

Sound conditioning (SC) is defined as “toughening” to lower levels of sound over time, which reduces a subsequent noise-induced threshold shift. Although the protective effect of SC in mammals is generally understood, the exact mechanisms involved have not yet been elucidated. To confirm the protective effect of SC against noise exposure (NE) and the stress-related signaling pathway of its rescue, we observed target molecule changes caused by SC of low frequency prior to NE as well as histology analysis in vivo and verified the suggested mechanisms in SGNs in vitro. Further, we investigated the potential role of Hsp70 and Bmi1 in SC by targeting SOD1 and SOD2 which are regulated by the FoxO1 signaling pathway based on mitochondrial function and reactive oxygen species (ROS) levels. Finally, we sought to identify the possible molecular mechanisms associated with the beneficial effects of SC against noise-induced trauma. Data from the rat model were evaluated by western blot, immunofluorescence, and RT-PCR. The results revealed that SC upregulated Hsp70, Bmi1, FoxO1, SOD1, and SOD2 expression in spiral ganglion neurons (SGNs). Moreover, the auditory brainstem responses (ABRs) and electron microscopy revealed that SC could protect against acute acoustic trauma (AAT) based on a significant reduction of hearing impairment and visible reduction in outer hair cell loss as well as ultrastructural changes in OHCs and SGNs. Collectively, these results suggested that the contribution of Bmi1 toward decreased sensitivity to noise-induced trauma following SC was triggered by Hsp70 induction and associated with enhancement of the antioxidant system and decreased mitochondrial superoxide accumulation. This contribution of Bmi1 was achieved by direct targeting of SOD1 and SOD2, which was regulated by FoxO1. Therefore, the Hsp70/Bmi1-FoxO1-SOD signaling pathway might contribute to the protective effect of SC against AAT in a rat model.

Drug development for noise-induced hearing loss

INTRODUCTION

Excessive exposure to noise is a common occurrence that contributes to approximately 50% of the non-genetic hearing loss cases. Researchers need to develop standardized preclinical models and identify molecular targets to effectively develop prevention and curative therapies.

AREAS COVERED

In this review, the authors discuss the many facets of human noise-induced pathology, and the primary experimental models for studying the basic mechanisms of noise-induced damage, making connections and inferences among basic science studies, preclinical proofs of concept and clinical trials.

EXPERT OPINION

Whilst experimental research in animal models has helped to unravel the mechanisms of noise-induced hearing loss, there are often methodological variations and conflicting results between animal and human studies which make it difficult to integrate data and translate basic outcomes to clinical practice. Standardization of exposure paradigms and application of -omic technologies will contribute to improving the effectiveness of transferring newly gained knowledge to clinical practice.

Evaluating the Efficacy of L-N-acetylcysteine and Dexamethasone in Combination to Provide Otoprotection for Electrode Insertion Trauma

BACKGROUND

Electrode insertion trauma (EIT) during cochlear implantation (CI) can cause loss of residual hearing. L-N-acetylcysteine (L-NAC) and dexamethasone (Dex) have been individually shown to provide otoprotection albeit at higher concentrations that may be associated with adverse effects. Objective/Aims: The aim of this study is to determine whether L-NAC and Dex could be combined to decrease their effective dosage.

MATERIALS AND METHODS

The organ of Corti (OC) explants were divided into various groups: 1) control; 2) EIT; 3) EIT treated with different concentrations of Dex; 4) EIT treated with different concentrations of L-NAC; 5) EIT treated with L-NAC and Dex in combination. Hair cell (HC) density, levels of oxidative stress, proinflammatory cytokines and nitric oxide (NO) was determined.

RESULTS

There was a significant loss of HCs in explants subjected to EIT compared to the control group. L-NAC and Dex in combination was able to provide significant otoprotection at lower concentrations compared to individual drugs.

CONCLUSIONS AND SIGNIFICANCE

A combination containing L-NAC and Dex is effective in protecting sensory cells at lower protective doses than each compound separately. These compounds can be combined allowing a decrease of potential side effects of each compound and providing significant otoprotection for EIT.

Combination of antioxidants and NFAT (nuclear factor of activated T cells) inhibitor protects auditory hair cells from ototoxic insult

Hair cell (HC) degeneration causes hearing loss in millions of people worldwide. Aminoglycoside exposure is one major cause of sensory HC damage. Aminoglycosides generate free radicals within the inner ear, permanently damaging sensory cells, and thus causing hearing loss. Hearing protection requires strategies to overcome the apparently irreversible loss of HCs in mammals. The nuclear factor of activated T cells (NFAT) inhibitor 11R-VIVIT reportedly protects HCs from gentamicin toxicity. Here we investigated whether the combination of 11R-VIVIT with the antioxidant L-carnitine or N-acetylcysteine could protect mouse cochlear HCs from gentamicin damage. Compared to single-component treatment, combined treatment with 11R-VIVIT plus L-carnitine yielded significant protection from gentamicin, and 11R-VIVIT plus N-acetylcysteine provided almost complete protection of HCs from gentamicin. Caspase activity in organ of Corti was significantly reduced by combined treatment with 11R-VIVIT + N-acetylcysteine + gentamicin, compared to 11R-VIVIT + gentamicin or gentamicin alone. Analysis of relative gene expression by qPCR revealed down-regulation of the pro-apoptotic genes Fasl and Casp9, and up-regulation of the antioxidant genes Hmox1 and Nrf2 after treatment with 11R-VIVIT + N-acetylcysteine + gentamicin, compared to single-compound treatment or gentamicin alone in cultures. Selective NFAT inhibition by 11R-VIVIT may be a good strategy for preventing gentamicin-induced HC damage. L-carnitine and N-acetylcysteine, with their ROS-reducing properties, contribute to the synergistic effectiveness with 11R-VIVIT by decreasing ROS-induced NFAT translocation. Our data suggest that a combined approach of NFAT inhibition together with an antioxidant, like N-acetylcysteine, could be useful for hearing loss treatment and/or prevention. Cover Image for this issue: https://doi.org/10.1111/jnc.14759.

Octave band noise exposure: Laboratory models and otoprotection efforts

With advances in the understanding of mechanisms of noise injury, the past 30 years have brought numerous efforts to identify drugs that prevent noise-induced hearing loss (NIHL). The diverse protocols used across investigations have made comparisons across drugs difficult. A systematic review of the literature by Hammill [(2017). Doctoral thesis, The University of Texas at Austin] identified original reports of chemical interventions to prevent or treat hearing loss caused by noise exposure. An initial search returned 3492 articles. After excluding duplicate articles and articles that did not meet the systematic review inclusion criteria, a total of 213 studies published between 1977 and 2016 remained. Reference information, noise exposure parameters, species, sex, method of NIHL assessment, and pharmaceutical intervention details for these 213 studies were entered into a database. Frequency-specific threshold shifts in control animals (i.e., in the absence of pharmaceutical intervention) are reported here. Specific patterns of hearing loss as a function of species and noise exposure parameters are provided to facilitate the selection of appropriate pre-clinical models. The emphasis of this report is octave band noise exposure, as this is one of the most common exposure protocols across pharmacological otoprotection studies.

Effects of parenteral papaverine and piracetam administration on cochlea following acoustic trauma

Introduction

Noise exposure, the main cause of hearing loss in countries with lot of industries, may result both in temporary or permanent hearing loss. The goal of this study was to investigate the effects of parenteral papaverine and piracetam administration following an acoustic trauma on hearing function with histopathologic correlation.

Materials and Methods

Eighteen Wistar albino rats exposed to noise for 8 h in a free environment were included. We divided the study population into three groups, and performed daily intraperitoneal injections of papaverine, piracetam, and saline, respectively, throughout the study. We investigated the histopathologic effects of cellular apoptosis on inner hair cells (IHCs) and outer hair cells (OHCs) and compared the distortion product otoacoustic emissions (DPOAEs) thresholds among the groups.

Results and Discussion

On the 3^rd and 7^th days, DPOAE thresholds at 8 kHz were significantly higher both in papaverine and piracetam groups compared with the control group (P = 0.004 for 3^rd day, P = 0.016 and P = 0.028 for 7^th day, respectively). On the 14^th day, piracetam group had significantly higher mean thresholds at 8 kHz (P = 0.029); however, papaverine group had similar mean thresholds compared to the control group (P = 0.200). On the 3^rd and 7^th days following acoustic trauma, both IHC and OHC loss were significantly lower in both papaverine and piracetam groups. On the 7^th day, the mean amount of apoptotic IHCs and OHCs identified using Caspase-3 method were significantly lower in both groups, but the mean amount identified using terminal deoxynucleotidyl transferase dUTP nick end labeling method were similar in both groups compared to the control group.

Conclusion

We demonstrated the effects of papaverine and piracetam on the recovery of cochlear damage due to acoustic trauma on experimental animals using histopathologic and electrophysiologic examinations.

Advances and Challenges in Pharmaceutical Therapies to Prevent and Repair Cochlear Injuries From Noise

Noise induces a broad spectrum of pathological injuries to the cochlea, reflecting both mechanical damage to the delicate architecture of the structures of the organ of Corti and metabolic damage within the organ of Corti and lateral wall tissues. Unlike ototoxic medications, the blood-labyrinth barrier does not offer protection against noise injury. The blood-labyrinth barrier is a target of noise injury, and can be weakened as part of the metabolic pathologies in the cochlea. However, it also offers a potential for therapeutic intervention with oto-protective compounds. Because the blood-labyrinth barrier is weakened by noise, penetration of blood-borne oto-protective compounds could be higher. However, systemic dosing for cochlear protection from noise offers other significant challenges. An alternative option to systemic dosing is local administration to the cochlea through the round window membrane using a variety of drug delivery techniques. The review will discuss noise-induced cochlear pathology, including alterations to the blood-labyrinth barrier, and then transition into discussing approaches for delivery of oto-protective compounds to reduce cochlear injury from noise.

Targeting dysregulation of redox homeostasis in noise-induced hearing loss: Oxidative stress and ROS signaling

Hearing loss caused by exposure to recreational and occupational noise remains a worldwide disabling condition and dysregulation of redox homeostasis is the hallmark of cochlear damage induced by noise exposure. In this review we discuss the dual function of ROS to both promote cell damage (oxidative stress) and cell adaptive responses (ROS signaling) in the cochlea undergoing a stressful condition such as noise exposure. We focus on animal models of noise-induced hearing loss (NIHL) and on the function of exogenous antioxidants to maintaining a physiological role of ROS signaling by distinguishing the effect of exogenous "direct" antioxidants (i.e. CoQ₁₀, NAC), that react with ROS to decrease oxidative stress, from the exogenous "indirect" antioxidants (i.e. nutraceutics and phenolic compounds) that can activate cellular redox enzymes through the Keap1-Nrf2-ARE pathway. The anti-inflammatory properties of Nrf2 signaling are discussed in relation to the ROS/inflammation interplay in noise exposure. Unveiling the mechanisms of ROS regulating redox-associated signaling pathways is essential in providing relevant targets for innovative and effective therapeutic strategies against NIHL.

Otoprotectants: From Research to Clinical Application

There is an urgent need for otoprotective drug agents. Prevention of noise-induced hearing loss continues to be a major challenge for military personnel and workers in a variety of industries despite the requirements that at-risk individuals use hearing protection devices such as ear plugs or ear muffs. Drug-induced hearing loss is also a major quality-of-life issue with many patients experiencing clinically significant hearing loss as a side effect of treatment with life-saving drug agents such as cisplatin and aminoglycoside antibiotics. There are no pharmaceutical agents approved by the United States Food and Drug Administration for the purpose of protecting the inner ear against damage, and preventing associated hearing loss (otoprotection). However, a variety of preclinical studies have suggested promise, with some supporting data from clinical trials now being available as well. Additional research within this promising area is urgently needed.

Mitochondria-Targeted Antioxidants for Treatment of Hearing Loss: A Systematic Review

Mitochondrial dysfunction is associated with the etiologies of sensorineural hearing loss, such as age-related hearing loss, noise- and ototoxic drug-induced hearing loss, as well as hearing loss due to mitochondrial gene mutation. Mitochondria are the main sources of reactive oxygen species (ROS) and ROS-induced oxidative stress is involved in cochlear damage. Moreover, the release of ROS causes further damage to mitochondrial components. Antioxidants are thought to counteract the deleterious effects of ROS and thus, may be effective for the treatment of oxidative stress-related diseases. The administration of mitochondria-targeted antioxidants is one of the drug delivery systems targeted to mitochondria. Mitochondria-targeted antioxidants are expected to help in the prevention and/or treatment of diseases associated with mitochondrial dysfunction. Of the various mitochondria-targeted antioxidants, the protective effects of MitoQ and SkQR1 against ototoxicity have been previously evaluated in animal models and/or mouse auditory cell lines. MitoQ protects against both gentamicin- and cisplatin-induced ototoxicity. SkQR1 also provides auditory protective effects against gentamicin-induced ototoxicity. On the other hand, decreasing effect of MitoQ on gentamicin-induced cell apoptosis in auditory cell lines has been controversial. No clinical studies have been reported for otoprotection using mitochondrial-targeted antioxidants. High-quality clinical trials are required to reveal the therapeutic effect of mitochondria-targeted antioxidants in terms of otoprotection in patients.

Cysteine/Glutathione Deficiency: A Significant and Treatable Corollary of Disease

Glutathione (GSH) deficiency may play a pivotal role in a variety of apparently unrelated clinical conditions and diseases. Orally administered N-acetylcysteine (NAC), which replenishes the cysteine required for GSH synthesis, has been tested in a large number of randomized placebo-controlled trials involving these diseases and conditions. This chapter focused on developing a base of evidence suggesting that NAC administration improves disease by increasing cysteine and/or GSH in a variety of diseases, thereby implying a significant role for GSH deficiency in the clinical basis of many diseases. To develop this base of evidence, we systematically selected studies which considered the hypothesis that the therapeutic efficacy for NAC is an indication that cysteine and/or GSH deficiency is a pathophysiological part of the diseases studied. In this manner we focus this chapter on explaining the biological mechanisms of NAC therapy in a wide variety of disorders and demonstrate its ubiquitous role in improving disease that involves disrupted GSH and/or cysteine metabolism.

Toward Cochlear Therapies

Sensorineural hearing impairment is the most common sensory disorder and a major health and socio-economic issue in industrialized countries. It is primarily due to the degeneration of mechanosensory hair cells and spiral ganglion neurons in the cochlea via complex pathophysiological mechanisms. These occur following acute and/or chronic exposure to harmful extrinsic (e.g., ototoxic drugs, noise...) and intrinsic (e.g., aging, genetic) causative factors. No clinical therapies currently exist to rescue the dying sensorineural cells or regenerate these cells once lost. Recent studies have, however, provided renewed hope, with insights into the therapeutic targets allowing the prevention and treatment of ototoxic drug- and noise-induced, age-related hearing loss as well as cochlear cell degeneration. Moreover, genetic routes involving the replacement or corrective editing of mutant sequences or defected genes are showing promise, as are cell-replacement therapies to repair damaged cells for the future restoration of hearing in deaf people. This review begins by recapitulating our current understanding of the molecular pathways that underlie cochlear sensorineural damage, as well as the survival signaling pathways that can provide endogenous protection and tissue rescue. It then guides the reader through to the recent discoveries in pharmacological, gene and cell therapy research towards hearing protection and restoration as well as their potential clinical application.

Animal model studies yield translational solutions for cochlear drug delivery

The field of hearing and deafness research is about to enter an era where new cochlear drug delivery methodologies will become more innovative and plentiful. The present report provides a representative review of previous studies where efficacious results have been obtained with animal models, primarily rodents, for protection against acute hearing loss such as acoustic trauma due to noise overexposure, antibiotic use and cancer chemotherapies. These approaches were initiated using systemic injections or oral administrations of otoprotectants. Now, exciting new options for local drug delivery, which opens up the possibilities for utilization of novel otoprotective drugs or compounds that might not be suitable for systemic use, or might interfere with the efficacious actions of chemotherapeutic agents or antibiotics, are being developed. These include interesting use of nanoparticles (with or without magnetic field supplementation), hydrogels, cochlear micropumps, and new transtympanic injectable compounds, sometimes in combination with cochlear implants.

Inner ear test battery in guinea pig models - a review

OBJECTIVE

This study reviewed the development of the inner ear test battery comprising auditory brainstem response (ABR), and caloric, ocular vestibular-evoked myogenic potential (oVEMP), and cervical vestibular-evoked myogenic potential (cVEMP) tests in guinea pig models at our laboratory over the last 20 years. Detailed description of the methodology for testing the small animals is also included.

METHODS

Inner ear disorders, i.e. ototoxicity, noise exposure, or perilymph fistula were established in guinea pig models first. One to four weeks after operation, each animal underwent ABR, oVEMP, cVEMP, and caloric tests. Then, animals were sacrificed for morphological study in the temporal bones.

RESULTS

Inner ear endorgans can be comprehensively evaluated in guinea pig models via an inner ear test battery, which provides thorough information on the cochlea, saccule, utricle, and semicircular canal function of guinea pigs. Coupled with morphological study in the temporal bones of the animals may help elucidate the mechanism of inner ear disorders in humans.

CONCLUSIONS

The inner ear test battery in guinea pig models may encourage young researchers to perform basic study in animals and stimulate the progress of experimental otology which is in evolution.

Chronic lead exposure induces cochlear oxidative stress and potentiates noise-induced hearing loss

Acquired hearing loss is caused by complex interactions of multiple environmental risk factors, such as elevated levels of lead and noise, which are prevalent in urban communities. This study delineates the mechanism underlying lead-induced auditory dysfunction and its potential interaction with noise exposure. Young-adult C57BL/6 mice were exposed to: 1) control conditions; 2) 2 mM lead acetate in drinking water for 28 days; 3) 90 dB broadband noise 2 h/day for two weeks; and 4) both lead and noise. Blood lead levels were measured by inductively coupled plasma mass spectrometry analysis (ICP-MS) lead-induced cochlear oxidative stress signaling was assessed using targeted gene arrays, and the hearing thresholds were assessed by recording auditory brainstem responses. Chronic lead exposure downregulated cochlear Sod1, Gpx1, and Gstk1, which encode critical antioxidant enzymes, and upregulated ApoE, Hspa1a, Ercc2, Prnp, Ccl5, and Sqstm1, which are indicative of cellular apoptosis. Isolated exposure to lead or noise induced 8-12 dB and 11-25 dB shifts in hearing thresholds, respectively. Combined exposure induced 18-30 dB shifts, which was significantly higher than that observed with isolated exposures. This study suggests that chronic exposure to lead induces cochlear oxidative stress and potentiates noise-induced hearing impairment, possibly through parallel pathways.

HPN-07, a free radical spin trapping agent, protects against functional, cellular and electrophysiological changes in the cochlea induced by acute acoustic trauma

Oxidative stress is considered a major cause of the structural and functional changes associated with auditory pathologies induced by exposure to acute acoustic trauma AAT). In the present study, we examined the otoprotective effects of 2,4-disulfophenyl-N-tert-butylnitrone (HPN-07), a nitrone-based free radical trap, on the physiological and cellular changes in the auditory system of chinchilla following a six-hour exposure to 4 kHz octave band noise at 105 dB SPL. HPN-07 has been shown to suppress oxidative stress in biological models of a variety of disorders. Our results show that administration of HPN-07 beginning four hours after acoustic trauma accelerated and enhanced auditory/cochlear functional recovery, as measured by auditory brainstem responses (ABR), distortion product otoacoustic emissions (DPOAE), compound action potentials (CAP), and cochlear microphonics (CM). The normally tight correlation between the endocochlear potential (EP) and evoked potentials of CAP and CM were persistently disrupted after noise trauma in untreated animals but returned to homeostatic conditions in HPN-07 treated animals. Histological analyses revealed several therapeutic advantages associated with HPN-07 treatment following AAT, including reductions in inner and outer hair cell loss; reductions in AAT-induced loss of calretinin-positive afferent nerve fibers in the spiral lamina; and reductions in fibrocyte loss within the spiral ligament. These findings support the conclusion that early intervention with HPN-07 following an AAT efficiently blocks the propagative ototoxic effects of oxidative stress, thereby preserving the homeostatic and functional integrity of the cochlea.

Emerging Therapies for Sensorineural Hearing Loss

OBJECTIVE

To critically review and evaluate the proposed mechanisms and documented results of the therapeutics currently in active clinical drug trials for the treatment of sensorineural hearing loss.

DATA SOURCES

US National Institutes of Health (NIH) Clinical Trials registry, MEDLINE/PubMed.

STUDY SELECTION & DATA EXTRACTION

A review of the NIH Clinical Trials registry identified candidate hearing loss therapies, and supporting publications were acquired from MEDLINE/PubMed. Proof-of-concept, therapeutic mechanisms, and clinical outcomes were critically appraised.

DATA SYNTHESIS

Twenty-two active clinical drug trials registered in the United States were identified, and six potentially therapeutic molecules were reviewed. Of the six molecules reviewed, four comprised mechanisms pertaining to mitigating oxidative stress pathways that presumably lead to inner ear cell death. One remaining therapy sought to manipulate the cell death cascade, and the last remaining therapy was a novel cell replacement therapy approach to introduce a transcription factor that promotes hair cell regeneration.

CONCLUSION

A common theme in recent clinical trials registered in the United States appears to be the targeting of cell death pathways and influence of oxidant stressors on cochlear sensory neuroepithelium. In addition, a virus-delivered cell replacement therapy would be the first of its kind should it prove safe and efficacious. Significant challenges for bringing these bench-to-bedside therapies to market remain. It is never assured that results in non-human animal models translate to effective therapies in the setting of human biology. Moreover, as additional processes are described in association with hearing loss, such as an immune response and loss of synaptic contacts, additional pathways for targeting become available.

Secondary Endolymphatic Hydrops

HYPOTHESIS

A review of the most recent literature will provide clinicians with an update of secondary endolymphatic hydrops, aiding in diagnosis and treatment of affected patients.

BACKGROUND

Secondary endolymphatic hydrops is a pathologic finding of the inner ear resulting in episodic vertigo and intermittent hearing loss. It is a finding for which extensive research is being performed.

METHODS

A review of the most recent literature on secondary endolymphatic hydrops was performed using PubMed literature search.

RESULTS

Recent investigation of secondary endolymphatic hydrops has brought attention to traumatic and inflammatory insults as causes for secondary endolymphatic hydrops. Such etiologies, including postsurgical effects of cochlear implantation and endolymphatic sac ablation; otosclerosis and its operative intervention(s); acoustic and mechanical trauma; medications; and systemic inflammatory processes, have been determined as causes of secondary lymphatic hydrops. Histopathological slides for many of the etiologies of secondary endolymphatic hydrops are presented.

CONCLUSION

Through an understanding of the pathophysiology and etiologies of secondary endolymphatic hydrops, clinicians will gain a better understanding of this complex disease process, which will aid in treatment of patients with this disease process.

Thiol/disulphide homeostasis as a novel indicator of oxidative stress in sudden sensorineural hearing loss

OBJECTIVES

To investigate a novel oxidative stress marker, thiol/disulphide literature homeostasis, in patients with idiopathic sudden sensorineural hearing loss, and to compare the results with healthy controls for the first time.

METHODS

Thirty-two patients with idiopathic sudden sensorineural hearing loss and 30 healthy individuals were included in the study. Serum native thiol, total thiol and disulphide levels were measured, and disulphide/native thiol and disulphide/total thiol ratios were determined in all subjects.

RESULTS

Serum native thiol and total thiol levels were significantly lower in patients with sudden sensorineural hearing loss compared with controls (p < 0.05). Of the 32 patients, 25 had lower native thiol levels than controls (333.2 ± 73.9 vs 381.8 ± 35.6 μmol/l, p = 0.002) and 24 had lower total thiol levels (375.1 ± 74.3 vs 426.1 ± 39.3 μmol/l, p = 0.002).

CONCLUSION

The changes in oxidative markers evident in a significant number of patients may be associated with oxidative stress, which may, in turn, have caused sudden sensorineural hearing loss in those patients.

Noise-Induced Neural Degeneration and Therapeutic Effect of Antioxidant Drugs

The primary site of lesion induced by noise exposure is the hair cells in the organ of Corti and the primary neural degeneration occurs in synaptic terminals of cochlear nerve fibers and spiral ganglion cells. The cellular basis of noise-induced hearing loss is oxidative stress, which refers to a severe disruption in the balance between the production of free radicals and antioxidant defense system in the cochlea by excessive production of free radicals induced by noise exposure. Oxidative stress has been identified by a variety of biomarkers to label free radical activity which include four-hydroxy-2-nonenal, nitrotyrosine, and malondialdehyde, and inducible nitric oxide synthase, cytochrome-C, and cascade-3, 8, 9. Furthermore, oxidative stress is contributing to the necrotic and apoptotic cell deaths in the cochlea. To counteract the known mechanisms of pathogenesis and oxidative stress induced by noise exposure, a variety of antioxidant drugs including oxygen-based antioxidants such as N-acetyl-L-cystein and acetyl-L-carnitine and nitrone-based antioxidants such as phenyl-N-tert-butylnitrone (PBN), disufenton sodium, 4-hydroxy PBN, and 2, 4-disulfonyl PBN have been used in our laboratory. These antioxidant drugs were effective in preventing or treating noise-induced hearing loss. In combination with other antioxidants, antioxidant drugs showed a strong synergistic effect. Furthermore, successful use of antioxidant drugs depends on the optimal timing of treatment and the duration of treatment, which are highly related to the time window of free radical formation induced by noise exposure.

Neuroglobin immunoreactivity in the human cochlea

Neuroglobin (Ngb) is an oxygen-binding protein with a demonstrated role in endogenous neuroprotective mechanisms. It has been shown to function as a scavenger for reactive oxidizing species thereby assisting in cellular defense against oxidative stress. In the present study, we characterized the presence of Ngb in the human cochlea. Immunohistochemical staining was performed on formalin fixed celloidin human cochlea sections obtained from human temporal bones, using affinity purified polyclonal antibodies against Ngb. Thirty-six temporal bones were analyzed, 15 with normal otologic histories and 21 diagnosed with different inner ear pathologies. Ngb immunoreactivity (Ngb-IR) was consistently expressed in the neurons of spiral ganglia (SG) and supporting cells of the organ of Corti. There was a significant decrease of Ngb-IR in SGNs from specimens with inner ear pathologies when compared to normal specimens. In contrast, Ngb-IR in the organ of Corti did not show significant changes between pathological and normal specimens. The differential pattern of Ngb expression in these cochlear structures suggests that Ngb may participate in defense mechanisms in inner ear pathologies where oxidative stress is involved.

d-Methionine reduces tobramycin-induced ototoxicity without antimicrobial interference in animal models

BACKGROUND

Tobramycin is a critical cystic fibrosis treatment however it causes ototoxicity. This study tested d-methionine protection from tobramycin-induced ototoxicity and potential antimicrobial interference.

METHODS

Auditory brainstem responses (ABRs) and outer hair cell (OHC) quantifications measured protection in guinea pigs treated with tobramycin and a range of d-methionine doses. In vitro antimicrobial interference studies tested inhibition and post antibiotic effect assays. In vivo antimicrobial interference studies tested normal and neutropenic Escherichia coli murine survival and intraperitoneal lavage bacterial counts.

RESULTS

d-Methionine conferred significant ABR threshold shift reductions. OHC protection was less robust but significant at 20kHz in the 420mg/kg/day group. In vitro studies did not detect d-methionine-induced antimicrobial interference. In vivo studies did not detect d-methionine-induced interference in normal or neutropenic mice.

CONCLUSIONS

d-Methionine protects from tobramycin-induced ototoxicity without antimicrobial interference. The study results suggest d-met as a potential otoprotectant from clinical tobramycin use in cystic fibrosis patients.

Role of somatostatin receptor-2 in gentamicin-induced auditory hair cell loss in the Mammalian inner ear

Hair cells and spiral ganglion neurons of the mammalian auditory system do not regenerate, and their loss leads to irreversible hearing loss. Aminoglycosides induce auditory hair cell death in vitro, and evidence suggests that phosphatidylinositol-3-kinase/Akt signaling opposes gentamicin toxicity via its downstream target, the protein kinase Akt. We previously demonstrated that somatostatin-a peptide with hormone/neurotransmitter properties-can protect hair cells from gentamicin-induced hair cell death in vitro, and that somatostatin receptors are expressed in the mammalian inner ear. However, it remains unknown how this protective effect is mediated. In the present study, we show a highly significant protective effect of octreotide (a drug that mimics and is more potent than somatostatin) on gentamicin-induced hair cell death, and increased Akt phosphorylation in octreotide-treated organ of Corti explants in vitro. Moreover, we demonstrate that somatostatin receptor-1 knockout mice overexpress somatostatin receptor-2 in the organ of Corti, and are less susceptible to gentamicin-induced hair cell loss than wild-type or somatostatin-1/somatostatin-2 double-knockout mice. Finally, we show that octreotide affects auditory hair cells, enhances spiral ganglion neurite number, and decreases spiral ganglion neurite length.

Antioxidants reduce cellular and functional changes induced by intense noise in the inner ear and cochlear nucleus

The present study marks the first evaluation of combined application of the antioxidant N-acetylcysteine (NAC) and the free radical spin trap reagent, disodium 2,4-disulfophenyl-N-tert-butylnitrone (HPN-07), as a therapeutic approach for noise-induced hearing loss (NIHL). Pharmacokinetic studies and C-14 tracer experiments demonstrated that both compounds achieve high blood levels within 30 min after i.p injection, with sustained levels of radiolabeled cysteine (released from NAC) in the cochlea, brainstem, and auditory cortex for up to 48 h. Rats exposed to 115 dB octave-band noise (10-20 kHz) for 1 h were treated with combined NAC/HPN-07 beginning 1 h after noise exposure and for two consecutive days. Auditory brainstem responses (ABR) showed that treatment substantially reduced the degree of threshold shift across all test frequencies (2-16 kHz), beginning at 24 h after noise exposure and continuing for up to 21 days. Reduced distortion product otoacoustic emission (DPOAE) level shifts were also detected at 7 and 21 days following noise exposure in treated animals. Noise-induced hair cell (HC) loss, which was localized to the basal half of the cochlea, was reduced in treated animals by 85 and 64% in the outer and inner HC regions, respectively. Treatment also significantly reduced an increase in c-fos-positive neuronal cells in the cochlear nucleus following noise exposure. However, no detectable spiral ganglion neuron loss was observed after noise exposure. The results reported herein demonstrate that the NAC/HPN-07 combination is a promising pharmacological treatment of NIHL that reduces both temporary and permanent threshold shifts after intense noise exposure and acts to protect cochlear sensory cells, and potentially afferent neurites, from the damaging effects of acoustic trauma. In addition, the drugs were shown to reduce aberrant activation of neurons in the central auditory regions of the brain following noise exposure. It is likely that the protective mechanisms are related to preservation of structural components of the cochlea and blocking the activation of immediate early genes in the auditory centers of the brain.

The effect of gingko biloba on hearing in mice with noise-induced temporary threshold shift

Background and Objectives

Gingko biloba extract is known for enhancing blood circulation, scavenging free radicals, and antagonizing against platelet-activating factor. This study evaluated the effect of Gingko biloba on the noise-induced temporary threshold shift of hearing.

Materials and Methods

Temporary threshold shift was induced by exposing mice to 110 dB SPL sound for 1 hour. The experimental group consisted of mice fed Gingko biloba [3 mg/kg, 6 mg/kg, and 12 mg/kg in 0.5% carboxymethyl cellulose (CMC)] for 7 days before noise exposure. CMC solution without Gingko biloba was fed to control mice. Hearing threshold was measured by auditory brainstem response (ABR) and distortion product otoacoustic emission (DPOAE).

Results

The hearing threshold increased after noise exposure and recovered to normal within 5 days in all groups. Compared to control mice (fed CMC solution only), mice fed Gingko biloba showed more rapid recovery of ABR threshold at 16 kHz in all three experimental groups. At the other frequencies, there was no significant change in hearing recovery in the Gingko biloba groups. There was no difference in DPOAE between groups.

Conclusions

Temporary threshold shift of hearing after noise exposure was partly affected by oral Gingko biloba.

Therapeutic effects of orally administrated antioxidant drugs on acute noise-induced hearing loss

OBJECTIVE

The objective of this study was to investigate the dose-dependent therapeutic effect of the orally administrated antioxidant drugs [4-hydroxy alpha-phenyl-tert-butylnitrone (4-OHPBN) and N-acetyl-L-cysteine (NAC)] on acute noise-induced hearing loss because oral administration is the most commonly used method of drug administration due to its convenience, safety, and economical efficiency.

METHODS

Thirty chinchilla were exposed to a 105 dB octave band noise centered at 4 kHz for 6 h and randomly assigned to a control group (saline only) and three experimental groups [4-OHPBN (10 mg/kg) plus NAC (20 mg/kg), 4-OHPBN (20 mg/kg) plus NAC (50 mg/kg), and 4-OHPBN (50 mg/kg) plus NAC (100 mg/kg)]. The drugs were orally administrated beginning 4 h after noise exposure and then administered twice daily for the next 2 days. Permanent auditory brainstem response threshold shifts, distortion product otoacoustic emission threshold shifts, and the percentage of missing outer hair cell were determined.

RESULTS

The oral administration significantly reduced permanent hearing threshold shift, distortion product otoacoustic emission threshold shift, and the percentage of missing outer hair cell in a dose-dependent manner.

DISCUSSION

This result demonstrates that orally administered drugs can treat acute noise-induced hearing loss in a dose-dependent manner. This suggests that oral administration was effective in treating acute noise-induced hearing loss as in intraperitoneal administration.

Protective effect of silymarin on noise-induced hearing loss in Guinea pigs

Background

Hearing capability plays a principal role on human's communication. Noise-induced hearing loss (NIHL) caused by exposure to high noise levels is a serious socio-economic problem in modern societies. NIHL can either be reversible, resulting in a temporary threshold shifts (TTS) or irreversible, resulting in a permanent threshold shifts (PTS). PTS is often confirmed in the time span of between 2 - 6 weeks. NIHL may be prevented by avoidance of excessive amounts of noise or reducing the sound energy entering the inner ear using hearing protective devices. However, there are some conditions that such prevention is not possible such as noise exceeding the protective capabilities of the hearing protection device, working in military or the person does not tolerate the protection device. Thus the protective agent for preventing NIHL would be useful.

Objective

Free radical molecules and consequence oxidative stress have been shown to play a significant role in noise-induced hearing loss. Silymarin is an antioxidant flavonoid complex derived from the herb milk thistle has ability to mitigating the oxidative stress, scavenge free radicals. In the current study, we aimed to evaluate the protective effect of silymarin on noise induced hearing loss in guinea pig by auditory brain stem response.

Materials and Methods

Twenty guinea pigs randomly divided into 2 groups. The animals in the experimental group were intraperitoneally injected with 100 mg/kg/day silymarin dissolved in propylene glycol for 6 consecutive days. The control subjects were intraperitoneally injected with propylene glycol for 6 consecutive days. All animals were exposed to 4 kHz octave band noise at 120 dB SPL for 6 hours. Auditory brainstem responses (ABRs) at frequencies of 2, 4, 6, 8, 12, 16 and 20 kHz were precisely recorded before intervention and then on intervals of 0, 3, 10 and 15 days after noise exposure. Data were analyzed using repeated measures ANOVA.

Results

Threshold shifts for the experimental group at all frequencies immediately, 3, 10 and 15 days after noise exposure were significantly reduced compared to the control group (P < 0.01).

Conclusions

The findings indicate a protective effect of silymarin on temporary and permanent noise-induced hearing loss.

Pulmonary delivery of d-methionine is associated with an increase in ALCAR and glutathione in cochlear fluids

In animals, hearing loss resulting from cochlear mechanosensory cell damage can be mitigated by antioxidants such as d-methionine (d-met) and acetyl-l-carnitine (ALCAR). The systemic routes of administration of these compounds, that must of necessity transit trough the cochlear fluids, may affect the antioxidant levels in the cochlea and the resulting oto-protective effect. In this study, we analyzed the pharmacokinetics of [(14)C]d-met in the cochlea and four other tissues after intratracheal (IT), intranasal (IN), and oral by gavage (OG) administration and compared it to intravenous administration (IV). We then analyzed the effect of these four routes on the antioxidant content of the cochlear fluids after d-met or ALCAR administration, by liquid chromatography/mass spectrometry. Our results showed that the concentration of methionine and ALCAR in cochlear fluids significantly increased after their respective systemic administration. Interestingly, d-met administration also contributed to an increase of ALCAR. Our results also showed that the delivery routes differently affected the bioavailability of administered [(14)C]d-met as well as the concentrations of methionine, ALCAR and the ratio of oxidized to reduced glutathione. Overall, pulmonary delivery via IT administration achieved high concentrations of methionine, ALCAR, and oxidative-related metabolites in cochlear fluids, in some cases surpassing IV administration, while IN route appeared to be the least efficacious. To our knowledge, this is the first report of the direct measurements of antioxidant levels in cochlear fluids after their systemic administration. This report also demonstrates the validity of the pulmonary administration of antioxidants and highlights the different contributions of d-met and ALCAR allowing to further investigate their impact on oxidative stress in the cochlear microenvironment.

Targeting the somatostatin receptors as a therapeutic approach for the preservation and protection of the mammalian cochlea from excitotoxicity

The neuropeptide somatostatin (SST) is an important modulator of neurotransmission in the central nervous system (CNS) and binds to G-protein-coupled receptors (SSTR1-5) on target cells. Little is known about the expression and function of the somatostatinergic system in the mammalian cochlea. We analyzed the expression of SSTR1-SSTR5 in the immature mammalian cochlea. The peak in the expression of SSTR1 and SSTR2 at mRNA and protein level is around the onset of hearing to airborne sound, at postnatal day (P)14. This suggests their involvement in the maturation of the mammalian cochlea. We demonstrated that all five receptors are expressed in the inner hair cells (IHC) and outer hear cells (OHC) as well as in defined supporting cells of the organ of Corti (OC) in the adult mouse cochlea. A similar expression of the SSTRs in the IHC and OHC was found in cultivated P6 mouse OC explants as well as in neuroepithelial cell culture. In order to learn more about the regulation of SSTRs, we used mice with either a deletion of SSTR1, SSTR2 or SSTR1/SSTR2 double knock out (DKO). In DKO mice, SSTR5 was up-regulated and SSTR3 and SSTR4 were down regulated. These findings provide evidence of a compensatory regulation in the mammalian cochlea as a consequence of a receptor subtype deletion. In addition, we observed reduced levels of phospho-Akt and total-Akt in SSTR1 KO and DKO mice as compared to wild type (WT) mice. Akt is likely to be involved in hair cell survival. Most importantly, we found improved hair cell survival in somatostatin and octreotide treated OC explants that had been exposed to gentamicin compared to those explants exposed to gentamicin alone. These findings propose that the somatostatinergic system within the cochlea may have neuroprotective properties.

Sequence of vestibular deficits in patients with noise-induced hearing loss

This study utilized audiometry, and cervical vestibular-evoked myogenic potential (cVEMP), ocular VEMP (oVEMP) and caloric tests to investigate the sequence of vestibular deficits in patients with noise-induced hearing loss (NIHL). Thirty patients with NIHL underwent an inner ear test battery. Another 30 normal controls with age- and sex-matched were included for comparison. The abnormal percentages of the audiometry, and cVEMP, oVEMP and caloric tests were 100, 70, 57 and 33 % in NIHL patients, which showed significant differences from 13, 13, 7 and 3 % in normal controls, respectively. A significantly decreasing trend among the four tests, with the sequence of damage from the cochlea, followed by the saccule, utricle, and semicircular canals was noted in NIHL patients, but not in normal controls. In conclusion, the decreasing order of abnormal percentages in the function of the cochlea, saccule, utricle and semicircular canals after chronic noise exposure further supports that the pars inferior (cochlea and saccule) is more vulnerable to noise exposure than the pars superior (utricle and semicircular canals).

A new oral otoprotective agent. Part 1: Electrophysiology data from protection against noise-induced hearing loss

Background

Data from animal studies show that antioxidants can compensate against noise-induced stress and sensory hair cell death. The aim of this study was to evaluate the otoprotection efficacy of various versions of orally administered Acuval 400^® against noise damage in a rat animal model.

Material/Methods

Fifty-five Sprague Dawley rats were divided into 4 groups: A) noise-exposed animals; B) animals exposed to noise and treated with the Acuval; C) animals exposed to noise and treated with a combination of Coenzyme Q10 and Acuval; D) animals treated only with Acuval and Coenzyme Q10 and with no exposure to noise. All solutions were administered orally 5 times: 24 and 2 hrs prior to noise exposure, and then daily for 3 days. The auditory function was assessed by measuring auditory brainstem responses (ABR) in the range from 2 to 32 kHz at times =1, 7, 14 and 21 days after noise exposure.

Results

At low frequencies (click and 4 kHz) animals from both A and B groups showed significant threshold shifts in the majority of the tested frequencies and tested times. For the same frequencies, animals from group C presented threshold levels similar to those from group D. At frequencies ≥8 kHz the protective performance of the 2 Acuval groups is more clearly distinguished from the noise group A. At 32 kHz the 2 Acuval groups perform equally well in terms of otoprotection. Animals in Group D did not show any significant differences in the hearing threshold during the experiment.

Conclusions

The data of this study suggest that a solution containing Coenzyme Q10 and Acuval 400^®, administered orally, protects from noise-induced hearing loss.

Selective degeneration of synapses in the dorsal cochlear nucleus of chinchilla following acoustic trauma and effects of antioxidant treatment

The purpose of this study was to reveal synaptic plasticity within the dorsal cochlear nucleus (DCN) as a result of noise trauma and to determine whether effective antioxidant protection to the cochlea can also impact plasticity changes in the DCN. Expression of synapse activity markers (synaptophysin and precerebellin) and ultrastructure of synapses were examined in the DCN of chinchilla 10 days after a 105 dB SPL octave-band noise (centered at 4 kHz, 6 h) exposure. One group of chinchilla was treated with a combination of antioxidants (4-hydroxy phenyl N-tert-butylnitrone, N-acetyl-l-cysteine and acetyl-l-carnitine) beginning 4 h after noise exposure. Down-regulated synaptophysin and precerebellin expression, as well as selective degeneration of nerve terminals surrounding cartwheel cells and their primary dendrites were found in the fusiform soma layer in the middle region of the DCN of the noise exposure group. Antioxidant treatment significantly reduced synaptic plasticity changes surrounding cartwheel cells. Results of this study provide further evidence of acoustic trauma-induced neural plasticity in the DCN and suggest that loss of input to cartwheel cells may be an important factor contributing to the emergence of hyperactivity in the DCN after noise exposure. Results further suggest that early antioxidant treatment for acoustic trauma not only rescues cochlear hair cells, but also has impact on central auditory structures.