Post-exposure treatment attenuates noise-induced hearing loss

Effects of Astragaloside IV on Hearing, Inflammatory Factors, and Intestinal Flora in Mice Exposed to Noise

Long-term exposure to noise can cause irreversible hearing loss. Considering that there is no effective drug treatment, it is important to seek preventive treatment for noise-induced hearing loss (NIHL). Although astragaloside IV (AS-IV) protects against NIHL by reducing serum inflammatory factors, there is scarce information on the regulation of inflammatory factors by AS-IV to prevent NIHL. We investigated the hearing thresholds and relationship between the serum levels of inflammatory cytokines and intestinal microbiota of c57bl/6j mice exposed to noise (103 dB SPL 4 h·d-1) for 7 days, treated with or without AS-IV. Our results revealed a lower hearing threshold and lower serum levels of TNF-α, TNF-γ, IL-6, IL-1β, and IFN-γ in the mice treated with AS-IV. Additionally, AS-IV increased the abundance levels of the phylum Firmicutes, class Bacillus, order Lactobacillus, and family Lactobacillus (p < 0.05), and decreased those of the phylum Bacteroidetes and order Bacteroidales (p < 0.05). Lactobacillus and Bacilli negatively correlated with TNF-α, TNF-γ, and IL-1β; Erysipelotrichaceae negatively correlated with INF-γ; and Clostridiales positively correlated with IL-1β. In conclusion, AS-IV reduces the elevation of hearing thresholds in mice, preventing hearing loss in mice exposed to noise, and under the intervention of AS-IV, changes in the levels of inflammatory factors correlate with intestinal flora. We suggest that AS-IV improves intestinal flora and reduces inflammation levels in c57bl/6j mice exposed to noise.

NADPH Oxidase 3: Beyond the Inner Ear

Reactive oxygen species (ROS) were formerly known as mere byproducts of metabolism with damaging effects on cellular structures. The discovery and description of NADPH oxidases (Nox) as a whole enzyme family that only produce this harmful group of molecules was surprising. After intensive research, seven Nox isoforms were discovered, described and extensively studied. Among them, the NADPH oxidase 3 is the perhaps most underrated Nox isoform, since it was firstly discovered in the inner ear. This stigma of Nox3 as "being only expressed in the inner ear" was also used by me several times. Therefore, the question arose whether this sentence is still valid or even usable. To this end, this review solely focuses on Nox3 and summarizes its discovery, the structural components, the activating and regulating factors, the expression in cells, tissues and organs, as well as the beneficial and detrimental effects of Nox3-mediated ROS production on body functions. Furthermore, the involvement of Nox3-derived ROS in diseases progression and, accordingly, as a potential target for disease treatment, will be discussed.

Apoptotic vesicles resist oxidative damage in noise-induced hearing loss through activation of FOXO3a-SOD2 pathway

BACKGROUND

Mesenchymal stem cell (MSC) transplantation is a promising therapeutic approach for noise-induced hearing loss (NIHL). As the indispensable role of apoptosis in MSC transplantation was raised, the benefits of MSC-derived apoptotic vesicles (apoVs) in several disease models have been proved. However, whether apoVs benefit in NIHL have not been studied yet.

METHODS

Female CBA/J mice and HEI-OC1 cells were used in this study. Flow cytometry, nanoparticle tracking analysis (NTA) and transmission electron microscopy (TEM) were used to characterize apoVs. Proteomic analysis was used to identify function proteins in apoVs. Immunofluorescence was used to reveal distribution pattern. Auditory brainstem response (ABR) test was used to measure the effect of apoVs treatment. DCFH-DA staining and MitoSOX staining were used to indicate oxidative damage. Western-blot and qRT-PCR were used to study the signaling pathways.

RESULTS

We found that apoVs can be endocytosed by hair cells through systemic administration. Importantly, apoVs administration effectively attenuated NIHL and reduced hair cell loss by resisting oxidative damage in vivo. Further, apoVs application activated forkhead box o3 (FOXO3a)-mitochondrial superoxide dismutase 2(SOD2) pathway, which may relate to signal transduction and activators of transcription 3 (STAT3) in apoVs.

CONCLUSIONS

These findings uncovered the role of apoVs in preventing NIHL and resisting oxidative damage, indicating that apoVs is a promising way for inner ear delivery and a prospective cell-free therapy for NIHL.

Sestrin 2 Deficiency Exacerbates Noise-Induced Cochlear Injury Through Inhibiting ULK1/Parkin-Mediated Mitophagy

Aims: Noise damage to auditory hair cells is associated with oxidative stress and mitochondrial dysfunction. This study aimed to investigate the possible effect of sestrin 2 (SESN2), an endogenous antioxidant protein, on noise-induced hearing loss (NIHL) and the underlying mechanisms. Results: We identified SESN2 as a protective factor against oxidative stress in NIHL through activation of Parkin-mediated mitophagy. Consistently, SESN2 expression was increased and mitophagy was induced during the early stage after a temporary threshold shift due to noise exposure or hydrogen peroxide(H2O2) stimulation; conversely, SESN2 deficiency blocked mitophagy and exacerbated acoustic trauma. Mechanistically, SESN2 interacted with Unc-51-like protein kinase 1(ULK1), promoting ULK1 protein-level stabilization by interfering with its proteasomal degradation. This stabilization is essential for mitophagy initiation, since restoring ULK1 expression in SESN2-silenced cells rescued mitophagy defects. Innovation and Conclusion: Our results provide novel insights regarding SESN2 as a therapeutic target against noise-induced cochlear injury, possibly through improved mitophagy. Antioxid. Redox Signal. 38, 115-136.

Association of dietary factors with noise-induced hearing loss in Korean population: A 3-year national cohort study

Noise-induced hearing loss (NIHL) is a hearing impairment (HI) caused by various clinical factors. Identifying the relationship between NIHL and nutrient consumption could help in reducing the prevalence of hearing loss. The aim of this study was to analyze the relationship between NIHL and dietary factors using data of the Korea National Health and Nutrition Examination survey (KNHANES). The data were collected from The Fifth KNHANES 2010-2012. The survey was taken by a total of 10,850 participants aged 20-65 years. Air conduction audiometry was measured at 500, 1000, 2000, and 4000 Hz in both ears. Metabolic syndrome, noise exposure, alcohol consumption, smoking, income level, marital status, and nutritional intake were evaluated. The differences between non-HI and HI participants in the noise-exposed group showed statistically significant differences in age, sex, marital and smoking status, alcohol consumption, and fasting glucose and triglyceride levels (p<0.05). In a multiple regression analysis of the noise-exposed group, age showed a significant association with HI (OR: 0.604; 95% CI: 0.538-0.678) after adjusting for confounders. In multivariate analysis for dietary factors affecting HI in noise-exposed groups, retinol (OR: 1.356; 95% CI: 1.068-1.722), niacin (OR: 1.5; 95% CI: 1.022-2.201), and carbohydrates (OR: 0.692; 95% CI: 0.486-0.985) showed a significant association with NIHL. Age was identified as the only factor significantly affecting NIHL. When the dietary factors of the noise-exposed group were analyzed, high intake of niacin and retinol and low intake of carbohydrates appeared to reduce the risk of hearing loss.

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.

Age-Related Hearing Loss: The Link between Inflammaging, Immunosenescence, and Gut Dysbiosis

This article provides a theoretical overview of the association between age-related hearing loss (ARHL), immune system ageing (immunosenescence), and chronic inflammation. ARHL, or presbyacusis, is the most common sensory disability that significantly reduces the quality of life and has a high economic impact. This disorder is linked to genetic risk factors but is also influenced by a lifelong cumulative effect of environmental stressors, such as noise, otological diseases, or ototoxic drugs. Age-related hearing loss and other age-related disorders share common mechanisms which often converge on low-grade chronic inflammation known as “inflammaging”. Various stimuli can sustain inflammaging, including pathogens, cell debris, nutrients, and gut microbiota. As a result of ageing, the immune system can become defective, leading to the accumulation of unresolved inflammatory processes in the body. Gut microbiota plays a central role in inflammaging because it can release inflammatory mediators and crosstalk with other organ systems. A proinflammatory gut environment associated with ageing could result in a leaky gut and the translocation of bacterial metabolites and inflammatory mediators to distant organs via the systemic circulation. Here, we postulate that inflammaging, as a result of immunosenescence and gut dysbiosis, accelerates age-related cochlear degeneration, contributing to the development of ARHL. Age-dependent gut dysbiosis was included as a hypothetical link that should receive more attention in future studies.

Nitrative Stress and Auditory Dysfunction

Nitrative stress is increasingly recognized as a critical mediator of apoptotic cell death in many pathological conditions. The accumulation of nitric oxide along with superoxide radicals leads to the generation of peroxynitrite that can eventually result in the nitration of susceptible proteins. Nitrotyrosine is widely used as a biomarker of nitrative stress and indicates oxidative damage to proteins. Ototoxic insults, such as exposure to noise and ototoxic drugs, enhance the generation of 3-nitrotyrosine in different cell types in the cochlea. Nitrated proteins can disrupt critical signaling pathways and eventually lead to apoptosis and loss of sensory receptor cells in the cochlea. Accumulating evidence shows that selective targeting of nitrative stress attenuates cellular damage. Anti-nitrative compounds, such as peroxynitrite decomposition catalysts and inducible nitric oxide synthase inhibitors, prevent nitrative stress-mediated auditory damage. However, the role of nitrative stress in acquired hearing loss and its potential significance as a promising interventional target is yet to be fully characterized. This review provides an overview of nitrative stress mechanisms, the induction of nitrative stress in the auditory tissue after ototoxic insults, and the therapeutic value of targeting nitrative stress for mitigating auditory dysfunction.

Cochlear Immune Response in Presbyacusis: a Focus on Dysregulation of Macrophage Activity

Age-related hearing loss, or presbyacusis, is a prominent chronic degenerative disorder that affects many older people. Based on presbyacusis pathology, the degeneration occurs in both sensory and non-sensory cells, along with changes in the cochlear microenvironment. The progression of age-related neurodegenerative diseases is associated with an altered microenvironment that reflects chronic inflammatory signaling. Under these conditions, resident and recruited immune cells, such as microglia/macrophages, have aberrant activity that contributes to chronic neuroinflammation and neural cell degeneration. Recently, researchers identified and characterized macrophages in human cochleae (including those from older donors). Along with the age-related changes in cochlear macrophages in animal models, these studies revealed that macrophages, an underappreciated group of immune cells, may play a critical role in maintaining the functional integrity of the cochlea. Although several studies deciphered the molecular mechanisms that regulate microglia/macrophage dysfunction in multiple neurodegenerative diseases, limited studies have assessed the mechanisms underlying macrophage dysfunction in aged cochleae. In this review, we highlight the age-related changes in cochlear macrophage activities in mouse and human temporal bones. We focus on how complement dysregulation and the nucleotide-binding oligomerization domain-like receptor family pyrin domain containing 3 inflammasome could affect macrophage activity in the aged peripheral auditory system. By understanding the molecular mechanisms that underlie these regulatory systems, we may uncover therapeutic strategies to treat presbyacusis and other forms of sensorineural hearing loss.

Cochlear Inflammaging in Relation to Ion Channels and Mitochondrial Functions

The slow accumulation of inflammatory biomarker levels in the body-also known as inflammaging-has been linked to a myriad of age-related diseases. Some of these include neurodegenerative conditions such as Parkinson's disease, obesity, type II diabetes, cardiovascular disease, and many others. Though a direct correlation has not been established, research connecting age-related hearing loss (ARHL)-the number one communication disorder and one of the most prevalent neurodegenerative diseases of our aged population-and inflammaging has gained interest. Research, thus far, has found that inflammatory markers, such as IL-6 and white blood cells, are associated with ARHL in humans and animals. Moreover, studies investigating ion channels and mitochondrial involvement have shown promising relationships between their functions and inflammaging in the cochlea. In this review, we summarize key findings in inflammaging within the auditory system, the involvement of ion channels and mitochondrial functions, and lastly discuss potential treatment options focusing on controlling inflammation as we age.

The role of oxidative stress in the susceptibility of noise-impaired cochleae to synaptic loss induced by intracochlear electrical stimulation

Intracochlear electrical stimulation (ES) generated by cochlear implants (CIs) is used to activate auditory nerves to restore hearing perception in deaf subjects and those with residual hearing who use electroacoustic stimulation (EAS) technology. Approximately 1/3 of EAS recipients experience loss of residual hearing a few months after ES activation, but the underlying mechanism is unknown. Clinical evidence indicates that the loss is related to the previous history of noise-induced hearing loss (NIHL). In this report, we investigated the impact of intracochlear ES on oxidative stress levels and synaptic counts in inner hair cells (IHCs) of the apical, middle and basal regions of guinea pigs with normal hearing (NH) and NIHL. Our results demonstrated that intracochlear ES with an intensity of 6 dB above the thresholds of electrically evoked compound action potentials (ECAPs) could induce the elevation of oxidative stress levels, resulting in a loss of IHC synapses near the electrodes in the basal and middle regions of the NH cochleae. Furthermore, the apical region of cochleae with NIHL were more susceptible to synaptic loss induced by relatively low-intensity ES than that of NH cochleae, resulting from the additional elevation of oxidative stress levels and the reduced antioxidant capability throughout the whole cochlea.

The Role of Nrf2 in Hearing Loss

Hearing loss is a major unresolved problem in the world, which has brought a heavy burden to society, economy, and families. Hair cell damage and loss mediated by oxidative stress are considered to be important causes of hearing loss. The nuclear factor erythroid 2–related factor 2 (Nrf2) is a major regulator of antioxidant capacity and is involved in the occurrence and development of a series of toxic and chronic diseases associated with oxidative stress. In recent years, studies on the correlation between hearing loss and Nrf2 target have continuously broadened our knowledge, and Nrf2 has become a new strategic target for the development and reuse of hearing protection drugs. This review summarized the correlation of Nrf2 in various types of hearing loss, and the role of drugs in hearing protection through Nrf2 from the literature.

Apelin-13 prevents apoptosis in the cochlear tissue of noise-exposed rat via Sirt-1 regulation

Noise-induced hearing loss (NIHL) is the second most common cause of acquired hearing loss. Acoustic trauma can cause oxidative damage in the cochlear hair cells (HCs) through apoptotic pathways. Apelin is a newly discovered neuropeptide with neuroprotective effects against the oxidative stress in neurodegenerative disorder. We investigated the preventive effects of apelin-13 on the cochlear HCs and spiral ganglion neurons (SGNs) against acoustic trauma via Sirtuin-1 (Sirt-1) regulation in rats. Animals were assigned to control, control + apelin-13 (50 or 100 μg/kg, ip), and noise exposure groups without any treatment or were administered apelin-13 (50 or 100 μg/kg, ip) and EX-527 (an inhibitor of Sirt-1) prior to each noise session. In the noise groups, 110 dB white noise was applied for 6 h per 5 days. Pre- and post-exposure distortion product otoacoustic emissions (DPOAE) and cochlear superoxide dismutase (SOD) activity were assessed. Western blot evaluated the cochlear protein expressions of Sirt-1, cleaved-caspase-3, Bax, and Bcl-2. Cell apoptosis was detected through TUNEL staining. Immunofluorescence was used to examine expression of HCs and SGNs specific protein. DPOAE level were significantly improved in the noise exposure group receiving 100 μg/kg apelin-13. At high doses, apelin augmented SOD levels in the rat cochlea subjected to noise. Apelin 100 markedly increased Sirt-1, and decreased cleaved- caspase-3 expression as well as Bax/Bcl-2 ratio in the cochlea tissue of noise-exposed rats. These findings suggest the promising therapeutic potential of apelin-13 for the prevention of noise-induced injury to cochlea and hearing loss.

Treatment With Calcineurin Inhibitor FK506 Attenuates Noise-Induced Hearing Loss

Attenuation of noise-induced hair cell loss and noise-induced hearing loss (NIHL) by treatment with FK506 (tacrolimus), a calcineurin (CaN/PP2B) inhibitor used clinically as an immunosuppressant, has been previously reported, but the downstream mechanisms of FK506-attenuated NIHL remain unknown. Here we showed that CaN immunolabeling in outer hair cells (OHCs) and nuclear factor of activated T-cells isoform c4 (NFATc4/NFAT3) in OHC nuclei are significantly increased after moderate noise exposure in adult CBA/J mice. Consequently, treatment with FK506 significantly reduces moderate-noise-induced loss of OHCs and NIHL. Furthermore, induction of reactive oxygen species (ROS) by moderate noise was significantly diminished by treatment with FK506. In agreement with our previous finding that autophagy marker microtubule-associated protein light chain 3B (LC3B) does not change in OHCs under conditions of moderate-noise-induced permanent threshold shifts, treatment with FK506 increases LC3B immunolabeling in OHCs after exposure to moderate noise. Additionally, prevention of NIHL by treatment with FK506 was partially abolished by pretreatment with LC3B small interfering RNA. Taken together, these results indicate that attenuation of moderate-noise-induced OHC loss and hearing loss by FK506 treatment occurs not only via inhibition of CaN activity but also through inhibition of ROS and activation of autophagy.

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.

RNA-seq Profiling and Co-expression Network Analysis of Long Noncoding RNAs and mRNAs Reveal Novel Pathogenesis of Noise-induced Hidden Hearing Loss

Noise-induced hidden hearing loss (NIHHL), one of the family of conditions described as noise-induced hearing loss (NIHL), is characterized by synaptopathy following moderate noise exposure that causes only temporary threshold elevation. Long noncoding RNAs (lncRNAs) mediate several essential regulatory functions in a wide range of biological processes and diseases, but their roles in NIHHL remain largely unknown. In order to determine the potential roles of these lncRNAs in the pathogenesis of NIHHL, we first evaluated their expression in NIHHL mice model and mapped possible regulatory functions and targets using RNA-sequencing (RNA-seq). In total, we identified 133 lncRNAs and 522 mRNAs that were significantly dysregulated in the NIHHL model. Gene Ontology (GO) showed that these lncRNAs were involved in multiple cell components and systems including synapses and the nervous and sensory systems. In addition, a lncRNA-mRNA network was constructed to identify core regulatory lncRNAs and transcription factors. KEGG analysis was also used to identify the potential pathways being affected in NIHHL. These analyses allowed us to identify the guanine nucleotide binding protein alpha stimulating (GNAS) gene as a key transcription factor and the adrenergic signaling pathway as a key pathway in the regulation of NIHHL pathogenesis. Our study is the first, to our knowledge, to isolate a lncRNA mediated regulatory pathway associated with NIHHL pathogenesis; these observations may provide fresh insight into the pathogenesis of NIHHL and may pave the way for therapeutic intervention in the future.

The rat animal model for noise-induced hearing loss

Rats make excellent models for the study of medical, biological, genetic, and behavioral phenomena given their adaptability, robustness, survivability, and intelligence. The rat's general anatomy and physiology of the auditory system is similar to that observed in humans, and this has led to their use for investigating the effect of noise overexposure on the mammalian auditory system. The current paper provides a review of the rat model for studying noise-induced hearing loss and highlights advancements that have been made using the rat, particularly as these pertain to noise dose and the hazardous effects of different experimental noise types. In addition to the traditional loss of auditory function following acoustic trauma, recent findings have indicated the rat as a useful model in observing alterations in neuronal processing within the central nervous system following noise injury. Furthermore, the rat provides a second animal model when investigating noise-induced cochlear synaptopathy, as studies examining this in the rat model resemble the general patterns observed in mice. Together, these findings demonstrate the relevance of this animal model for furthering the authors' understanding of the effects of noise on structural, anatomical, physiological, and perceptual aspects of hearing.

Use of the guinea pig in studies on the development and prevention of acquired sensorineural hearing loss, with an emphasis on noise

Guinea pigs have been used in diverse studies to better understand acquired hearing loss induced by noise and ototoxic drugs. The guinea pig has its best hearing at slightly higher frequencies relative to humans, but its hearing is more similar to humans than the rat or mouse. Like other rodents, it is more vulnerable to noise injury than the human or nonhuman primate models. There is a wealth of information on auditory function and vulnerability of the inner ear to diverse insults in the guinea pig. With respect to the assessment of potential otoprotective agents, guinea pigs are also docile animals that are relatively easy to dose via systemic injections or gavage. Of interest, the cochlea and the round window are easily accessible, notably for direct cochlear therapy, as in the chinchilla, making the guinea pig a most relevant and suitable model for hearing. This article reviews the use of the guinea pig in basic auditory research, provides detailed discussion of its use in studies on noise injury and other injuries leading to acquired sensorineural hearing loss, and lists some therapeutics assessed in these laboratory animal models to prevent acquired sensorineural hearing loss.

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.

Targeting Inflammatory Processes Mediated by TRPVI and TNF-α for Treating Noise-Induced Hearing Loss

Noise trauma is the most common cause of hearing loss in adults. There are no known FDA approved drugs for prevention or rescue of noise-induced hearing loss (NIHL). In this study, we provide evidence that implicates stress signaling molecules (TRPV1, NOX3, and TNF-α) in NIHL. Furthermore, we provide evidence that inhibiting any one of these moieties can prevent and treat NIHL when administered within a window period. Hearing loss induced by loud noise is associated with the generation of reactive oxygen species (ROS), increased calcium (Ca²⁺) in the endolymph and hair cells, and increased inflammation in the cochlea. Increased (Ca²⁺) and ROS activity persists for several days after traumatic noise exposure (NE). Chronic increases in (Ca²⁺) and ROS have been shown to increase inflammation and apoptosis in various tissue. However, the precise role of Ca²⁺ up-regulation and the resulting inflammation causing a positive feedback loop in the noise-exposed cochlea to generate sustained toxic amounts of Ca²⁺ are unknown. Here we show cochlear TRPV1 dysregulation is a key step in NIHL, and that inflammatory TNF-α cytokine-mediated potentiation of TRPV1 induced Ca²⁺ entry is an essential mechanism of NIHL. In the Wistar rat model, noise produces an acute (within 48 h) and a chronic (within 21 days) increase in cochlear gene expression of TRPV1, NADPH oxidase 3 (NOX3) and pro-inflammatory mediators such as tumor necrosis factor-α (TNF-α) and cyclooxygenase-2 (COX2). Additionally, we also show that H₂O₂ (100 μM) produces a robust increase in Ca²⁺ entry in cell cultures which is enhanced by TNF-α via the TRPV1 channel and which involves ERK1/2 phosphorylation. Mitigation of NIHL could be achieved by using capsaicin (TRPV1 agonist that rapidly desensitizes TRPV1. This mechanism is used in the treatment of pain in diabetic peripheral neuropathy) pretreatment or by inhibition of TNF-α with Etanercept (ETA), administered up to 7 days prior to NE or within 24 h of noise. Our results demonstrate the importance of the synergistic interaction between TNF-α and TRPV1 in the cochlea and suggest that these are important therapeutic targets for treating NIHL.

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.

An Oral Combination of Vitamins A, C, E, and Mg++ Improves Auditory Thresholds in Age-Related Hearing Loss

The increasing rate of age-related hearing loss (ARHL), with its subsequent reduction in quality of life and increase in health care costs, requires new therapeutic strategies to reduce and delay its impact. The goal of this study was to determine if ARHL could be reduced in a rat model by administering a combination of antioxidant vitamins A, C, and E acting as free radical scavengers along with Mg⁺⁺, a known powerful cochlear vasodilator (ACEMg). Toward this goal, young adult, 3 month-old Wistar rats were divided into two groups: one was fed with a diet composed of regular chow (“normal diet,” ND); the other received a diet based on chow enriched in ACEMg (“enhanced diet,” ED). The ED feeding began 10 days before the noise stimulation. Auditory brainstem recordings (ABR) were performed at 0.5, 1, 2, 4, 8, 16, and 32 kHz at 3, 6–8, and 12–14 months of age. No differences were observed at 3 months of age, in both ND and ED animals. At 6–8 and 12–14 months of age there were significant increases in auditory thresholds and a reduction in the wave amplitudes at all frequencies tested, compatible with progressive development of ARHL. However, at 6–8 months threshold shifts in ED rats were significantly lower in low and medium frequencies, and wave amplitudes were significantly larger at all frequencies when compared to ND rats. In the oldest animals, differences in the threshold shift persisted, as well as in the amplitude of the wave II, suggesting a protective effect of ACEMg on auditory function during aging. These findings indicate that oral ACEMg may provide an effective adjuvant therapeutic intervention for the treatment of ARHL, delaying the progression of hearing impairment associated with age.

Inner Ear Hair Cell Protection in Mammals against the Noise-Induced Cochlear Damage

Inner ear hair cells are mechanosensory receptors that perceive mechanical sound and help to decode the sound in order to understand spoken language. Exposure to intense noise may result in the damage to the inner ear hair cells, causing noise-induced hearing loss (NIHL). Particularly, the outer hair cells are the first and the most affected cells in NIHL. After acoustic trauma, hair cells lose their structural integrity and initiate a self-deterioration process due to the oxidative stress. The activation of different cellular death pathways leads to complete hair cell death. This review specifically presents the current understanding of the mechanism exists behind the loss of inner ear hair cell in the auditory portion after noise-induced trauma. The article also explains the recent hair cell protection strategies to prevent the damage and restore hearing function in mammals.

Cochlear Damage Caused by the Striking Noise of Titanium Head Golf Driver

Objectives.

To investigate how mouse cochleae are affected by the striking noise of titanium head golf driver.

Methods.

Thirty-two BALB/c mice (20–22 g) with normal hearing were used. The impact acoustic stimulus generated by the striking of titanium golf driver head was centered around 4.5 kHz with 120.5 dB sound pressure level. The recorded impact noise was provided to mice in two ways with the same exposure time of 288 seconds: 1,440 repetitions and an impact duration of 0.2 seconds for 2 hours (repetitive noise) or serially connected impact noise for 288 seconds (continuous noise). Auditory brainstem responses were measured at baseline, day 7, and day 14 after exposure. The mice were then sacrificed for histology.

Results.

Both groups showed statistically significant threshold shifts immediately after noise exposure. Mice in the continuous exposure group, except for those exposed to 32 kHz noise, recovered from threshold shifts 1–2 weeks after noise exposure. However, in the repetitive exposure group, threshold shifts remained for 2 weeks after exposure. The repetitive exposure group had greater hair cell damage than did the continuous exposure group. Structural changes in the stria vascularis were observed in the repetitive exposure group.

Conclusion.

Overexposure to impact noise caused by hitting of titanium head golf driver may be hazardous to the cochlea, and repetitive exposure may induce greater damage than continuous exposure.

Protection for medication-induced hearing loss: the state of the science

OBJECTIVE

This review will summarise the current state of development of pharmaceutical interventions (prevention or treatment) for medication-induced ototoxicity.

DESIGN

Currently published literature was reviewed using PubMed and ClinicalTrials.gov to summarise the current state of the science. Details on the stage of development in the market pipeline are provided, along with evidence for clinical safety and efficacy reported.

STUDY SAMPLE

This review includes reports from 44 articles and clinical trial reports regarding agents in clinical or preclinical trials, having reached approved Investigational New Drug status with the Federal Drug Administration.

RESULTS

Vitamins and antioxidants are the most common agents currently evaluated for drug-induced ototoxicity intervention by targeting the oxidative stress pathway that leads to cochlear cell death and hearing loss. However, other strategies, including steroid treatment and reduction of ototoxic properties of the primary drugs, are discussed.

CONCLUSIONS

Retention of hearing during and after a life threatening illness is a major quality-of-life issue for patients receiving ototoxic drugs and their families. The agents discussed herein, while not mature enough at this point, offer great promise towards that goal. This review will provide a knowledge base for hearing providers to inquiries about such options from patients and interdisciplinary care teams alike.

Research into occupational noise induced hearing loss in South African large-scale mines: Access denied?

Background: The South African mining industry is frequently criticized for its poor health and safety record and high numbers of fatalities, thereby prompting researchers to conduct research on challenges faced by this industry. Therefore, this study aimed to investigate the feasibility of conducting audiological research into occupational noise induced hearing loss (ONIHL) within the South African mining sector. Specific objectives involved determining ease of identifying focal persons in charge of hearing conservation programmes (HCP); establishing response time and rate of identified focal person for interviews; and exploring focal person’s willingness to share information regarding HCP. Methods: A qualitative research strategy comprising online desk research and interviews was undertaken through purposive sampling to recruit participants. Data were collected through checklists, logbooks and interviews. Deductive thematic analysis was used to analyse data. Results: Gaining access into the mining sector is negatively impacted by the following factors: firstly, contact details of focal person are not always listed on the websites. Secondly, prolonged response rate between initial contact and the time in which the participants respond. Lastly, unwillingness to share information regarding the management of ONIHL and progress made in HCP at various mines. Conclusions: Findings contribute evidence regarding possible barriers to effective and successful implementation of application of best practice in HCPs; guided by evidence that is contextually relevant. Challenges with accessing mines for research purposes by external and independent researchers arguably impacts on the available evidence produced by mines, due to a possible unavoidable conflict of interest. Success of HCPs depend on objective evidence regardless of whether it paints the mining industry in positive or negative light. It is only when this evidence is critically engaged with, that real and effective solutions can be deliberated and implemented. Clear, transparent, and open lines of communication between various stakeholders are key to achieving this.

Oxidative Stress and Inflammation Induced by Environmental and Psychological Stressors: A Biomarker Perspective

SIGNIFICANCE

The environment can elicit biological responses such as oxidative stress (OS) and inflammation as a consequence of chemical, physical, or psychological changes. As population studies are essential for establishing these environment-organism interactions, biomarkers of OS or inflammation are critical in formulating mechanistic hypotheses. Recent Advances: By using examples of stress induced by various mechanisms, we focus on the biomarkers that have been used to assess OS and inflammation in these conditions. We discuss the difference between biomarkers that are the result of a chemical reaction (such as lipid peroxides or oxidized proteins that are a result of the reaction of molecules with reactive oxygen species) and those that represent the biological response to stress, such as the transcription factor NRF2 or inflammation and inflammatory cytokines.

CRITICAL ISSUES

The high-throughput and holistic approaches to biomarker discovery used extensively in large-scale molecular epidemiological exposome are also discussed in the context of human exposure to environmental stressors.

FUTURE DIRECTIONS

We propose to consider the role of biomarkers as signs and to distinguish between signs that are just indicators of biological processes and proxies that one can interact with and modify the disease process. Antioxid. Redox Signal. 28, 852-872.

An Antioxidant Screen Identifies Candidates for Protection of Cochlear Hair Cells from Gentamicin Toxicity

Reactive oxygen species are important elements in ototoxic damage to hair cells (HCs), appearing early in the damage process. Higher levels of natural antioxidants are positively correlated with resistance to ototoxins and many studies have shown that exogenous antioxidants can protect HCs from damage. While a very wide variety of antioxidants with different characteristics and intracellular targets exist, most ototoxicity studies have focused upon one or a few well-characterized compounds. Relatively little research has attempted to determine the comparative efficacy of large variety of different antioxidants. This has been in part due to the lack of translation between cell culture and in vivo measures of efficacy. To circumvent this limitation, we used an in vitro assay based on micro-explants from the basal and middle turns of the neonatal mouse organ of Corti to screen a commercial redox library of diverse antioxidant compounds for their ability to protect mammalian HCs from a high dose of the ototoxic antibiotic gentamicin. The library included several antioxidants that have previously been studied as potential treatments for HC damage, as well as many antioxidants that have never been applied to ototoxicity. The micro-explants were treated with 200 μM gentamicin alone, gentamicin plus one of three dosages of a redox compound, the highest dosage of compound alone, or were untreated. HC counts were determined before the gentamicin insult and at 1, 2, and 3 days afterward to evaluate the HC survival. From a total of 81 antioxidant compounds, 13 exhibited significant protection of HCs. These included members of a variety of antioxidant classes with several novel antioxidants, not previously tested on HCs, appearing to alleviate the damaging gentamicin effect. Some compounds previously shown to be protective of HCs were correspondingly protective in this in vitro screen, while others were not. Finally, one of the three pro-oxidant compounds included in the library as well as six antioxidants exhibited evidence of toxicity in the absence of gentamicin. The results demonstrate the wide variability in the ability of antioxidants to protect HCs from high-dose gentamicin damage, and identify promising candidate leads for further study as potential drug targets.

HIGHLIGHTS

• A medium-throughput assay based on micro-explants of the organ of Corti was developed to screen mammalian cochlear hair cells for protection from damage by ototoxins.• Eighty one antioxidants and 3 pro-oxidants were evaluated for hair cell protection from high-dose gentamicin.• Thirteen antioxidants were significantly protective, while 6 proved to be damaging.• The use of a common assay permitted an evaluation of the relative capacity of different antioxidants for the protection of hair cells.

Furosemide Administered before Noise Exposure can Protect the Ear

Objectives

We assessed the effect of furosemide administration on noise-induced hearing loss. This drug reversibly elevates the auditory threshold by inducing a temporary reduction of the endocochlear potential and thereby suppresses the cochlear amplifier and active cochlear mechanics.

Methods

Mice were given a single injection of furosemide 30 minutes before exposure to 113 dB sound pressure level broadband noise. Control animals received saline solution. Furosemide was administered in other mice after the noise exposure. Auditory threshold shifts were assessed by recording auditory nerve brain stem evoked response (ABR) thresholds to broadband clicks.

Results

The mean ABR threshold in the group injected with furosemide and exposed to temporary threshold shift (TTS)-producing noise was elevated by 20.4 ± 12.3 dB, and that in the saline control group was elevated by 35.4 ± 18.3 dB (p < 0.02). The mean threshold elevations in the group injected with furosemide and exposed to permanent threshold shift (PTS)-producing noise and in the PTS saline control group were 15.0 ± 10.3 dB and 27.0 ± 12.7 dB, respectively (p < 0.01). Similar results were obtained when the PTS was assessed with an 8-kHz tone burst ABR. There was no significant difference in the PTS between mice given a single injection of furosemide and those given saline solution after the noise; this finding shows that furosemide is not acting as an antioxidant.

Conclusions

It appears that reversible hearing threshold elevation as a result of furosemide administration before noise exposure can reduce the TTS and PTS. This finding provides insight into the mechanism of noise-induced hearing loss.

Impulse Noise: Theoretical Solutions to the Quandary of Cochlear Protection

Workers in industries with impact noise, as well as soldiers exposed to supersonic blasts from armament and explosive devices, appear to be more at risk for hearing loss than are their counterparts exposed to continuous noise. Alternative considerations for hearing protection are dictated because of a disproportionately increased biophysical response in comparison to continuous noise. Impulse noise is a significant and distinct problem that requires a new strategy for hearing protection. A review of current clinical and occupational literature suggests that impulse noise may be more damaging than continuous sound. Statistical measurements such as kurtosis hold promise for the quantitative prediction of hearing loss. As sound energy to the cell increases, the mechanism of cochlear damage shifts from biochemical injury to mechanical injury. Outer hair cells appear to be more sensitive than inner hair cells to impulse noise because of their energy requirements, which lead to increased production of reactive oxygen and nitrogen species and self-destruction by apoptosis. Hearing protective devices currently in use for impulse noise include hunters' hearing devices, active noise-reduction headsets, and various in-ear plugs, including nonlinear reacting inserts. Existing equipment is hampered by the materials used and by present-day electronic technology. Antioxidants administered before sound exposure show promise in mitigating hearing loss in industrial and combat situations. New materials with improved damping, reflective, and absorption characteristics are required. Hearing protective devices that allow passage of ambient sound while blocking harmful noise might improve the compliance and safety of those exposed. Sensing devices that instantaneously and selectively hyperpolarize outer hair cells are discussed as alternate protection.

Emerging therapeutic interventions against noise-induced hearing loss

INTRODUCTION

Noise-induced hearing loss (NIHL) due to industrial, military, and recreational noise exposure is a major, but also potentially preventable cause of acquired hearing loss. For the United States it is estimated that 26 million people (15% of the population) between the ages of 20 and 69 have a high-frequency NIHL at a detriment to the quality of life of the affected individuals and great economic cost to society. Areas covered: This review outlines the pathology and pathophysiology of hearing loss as seen in humans and animal models. Results from molecular studies are presented that have provided the basis for therapeutic strategies successfully applied to animals. Several compounds emerging from these studies (mostly antioxidants) are now being tested in field trials. Expert opinion: Although no clinically applicable intervention has been approved yet, recent trials are encouraging. In order to maximize protective therapies, future work needs to apply stringent criteria for noise exposure and outcome parameters. Attention needs to be paid not only to permanent NIHL due to death of sensory cells but also to temporary effects that may show delayed consequences. Existing results combined with the search for efficacious new therapies should establish a viable treatment within a decade.

Pre-conditioning with near infrared photobiomodulation reduces inflammatory cytokines and markers of oxidative stress in cochlear hair cells

Hearing loss is a serious occupational health problem worldwide. Noise, aminoglycoside antibiotics and chemotherapeutic drugs induce hearing loss through changes in metabolic functions resulting in sensory cell death in the cochlea. Metabolic sequelae from noise exposure increase production of nitric oxide (NO) and Reactive Oxygen Species (ROS) contributing to higher levels of oxidative stress beyond the physiologic threshold levels of intracellular repair. Photobiomodulation (PBM) therapy is a light treatment involving endogenous chromophores commonly used to reduce inflammation and promote tissue repair. Near infrared light (NIR) from Light Emitting Diodes (LED) at 810 nm wavelength were used as a biochemical modulator of cytokine response in cultured HEI-OC1 auditory cells placed under oxidative stress. Results reported here show that NIR PBM at 810 nm, 30 mW/cm² , 100 seconds, 1.0 J, 3 J/cm² altered mitochondrial metabolism and oxidative stress response for up to 24 hours post treatment. We report a decrease of inflammatory cytokines and stress levels resulting from NIR applied to HEI-OC1 auditory cells before treatment with gentamicin or lipopolysaccharide. These results show that cells pretreated with NIR exhibit reduction of proinflammatory markers that correlate with inhibition of mitochondrial superoxide, ROS and NO in response to continuous oxidative stress challenges. Non-invasive biomolecular down regulation of proinflammatory intracellular metabolic pathways and suppression of oxidative stress via NIR may have the potential to develop novel therapeutic approaches to address noise exposure and ototoxic compounds associated with hearing loss.

Targeting nitrative stress for attenuating cisplatin-induced downregulation of cochlear LIM domain only 4 and ototoxicity

Cisplatin-induced ototoxicity remains a primary dose-limiting adverse effect of this highly effective anticancer drug. The clinical utility of cisplatin could be enhanced if the signaling pathways that regulate the toxic side-effects are delineated. In previous studies, we reported cisplatin-induced nitration of cochlear proteins and provided the first evidence for nitration and downregulation of cochlear LIM domain only 4 (LMO4) in cisplatin ototoxicity. Here, we extend these findings to define the critical role of nitrative stress in cisplatin-induced downregulation of LMO4 and its consequent ototoxic effects in UBOC1 cell cultures derived from sensory epithelial cells of the inner ear and in CBA/J mice. Cisplatin treatment increased the levels of nitrotyrosine and active caspase 3 in UBOC1 cells, which was detected by immunocytochemical and flow cytometry analysis, respectively. The cisplatin-induced nitrative stress and apoptosis were attenuated by co-treatment with SRI110, a peroxynitrite decomposition catalyst (PNDC), which also attenuated the cisplatin-induced downregulation of LMO4 in a dose-dependent manner. Furthermore, transient overexpression of LMO4 in UBOC1 cells prevented cisplatin-induced cytotoxicity while repression of LMO4 exacerbated cisplatin-induced cell death, indicating a direct link between LMO4 protein levels and cisplatin ototoxicity. Finally, auditory brainstem responses (ABR) recorded from CBA/J mice indicated that co-treatment with SRI110 mitigated cisplatin-induced hearing loss. Together, these results suggest that cisplatin-induced nitrative stress leads to a decrease in the levels of LMO4, downregulation of LMO4 is a critical determinant in cisplatin-induced ototoxicity, and targeting peroxynitrite could be a promising strategy for mitigating cisplatin-induced hearing loss.

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Cisplatin-induced nitrative stress leads to a decrease in the levels of LMO4.

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Downregulation of LMO4 is a critical factor in cisplatin-induced ototoxicity.

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SRI110 appears to be a promising candidate for preventing cisplatin ototoxicity.

ACEMg Diet Supplement Modifies Progression of Hereditary Deafness

Dietary supplements consisting of beta-carotene (precursor to vitamin A), vitamins C and E and the mineral magnesium (ACEMg) can be beneficial for reducing hearing loss due to aminoglycosides and overstimulation. This regimen also slowed progression of deafness for a boy with GJB2 (CONNEXIN 26) mutations. To assess the potential for treating GJB2 and other forms of hereditary hearing loss with ACEMg, we tested the influence of ACEMg on the cochlea and hearing of mouse models for two human mutations: GJB2, the leading cause of childhood deafness, and DIAPH3, a cause of auditory neuropathy. One group of mice modeling GJB2 (Gjb2-CKO) received ACEMg diet starting shortly after they were weaned (4 weeks) until 16 weeks of age. Another group of Gjb2-CKO mice received ACEMg in utero and after weaning. The ACEMg diet was given to mice modeling DIAPH3 (Diap3-Tg) after weaning (4 weeks) until 12 weeks of age. Control groups received food pellets without the ACEMg supplement. Hearing thresholds measured by auditory brainstem response were significantly better for Gjb2-CKO mice fed ACEMg than for the control diet group. In contrast, Diap3-Tg mice displayed worse thresholds than controls. These results indicate that ACEMg supplementation can influence the progression of genetic hearing loss.

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.

Slowing the progression of age-related hearing loss: Rationale and study design of the ASPIRIN in HEARING, retinal vessels imaging and neurocognition in older generations (ASPREE-HEARING) trial

BACKGROUND

Age-related hearing loss (ARHL) is a leading cause of disability in the elderly. Low-grade inflammation and microvessel pathology may be responsible for initiating or exacerbating some of the hearing loss associated with aging. A growing body of evidence demonstrates an association of hearing loss with cognitive decline. A shared etiological pathway may include a role of inflammation, alongside vascular determinants. The ASPREE-HEARING study aims to determine whether low-dose aspirin decreases the progression of ARHL, and if so, whether this decrease in progression is also associated with retinal microvascular changes and/or greater preservation of cognitive function.

DESIGN AND METHODS

A three year double-blind, randomized controlled trial of oral 100mg enteric-coated aspirin or matching placebo, enrolling 1262 Australians aged ≥70years with normal cognitive function and no overt cardiovascular disease. The primary outcome is the change in mean pure tone average hearing threshold (decibels) in the better ear, over a 3-year period. Secondary outcomes consist of changes in retinal microvascular indicators, and changes in cognitive function. Participants are recruited from a larger trial, ASPirin in Reducing Events in the Elderly (ASPREE), which is designed to assess whether daily low dose aspirin will extend disability-free life.

DISCUSSION

ASPREE-HEARING will determine whether aspirin slows development or progression of ARHL, and will interrogate the relationship between inflammatory and microvascular mechanisms that may underlie the effects of aspirin on ARHL. This study will improve understanding of the patterns of comorbidity with, and the relationships between, aging and ARHL, alongside modeling the impacts of ARHL.

Synergistic effects of free radical scavengers and cochlear vasodilators: a new otoprotective strategy for age-related hearing loss

The growing increase in age-related hearing loss (ARHL), with its dramatic reduction in quality of life and significant increase in health care costs, is a catalyst to develop new therapeutic strategies to prevent or reduce this aging-associated condition. In this regard, there is extensive evidence that excessive free radical formation along with diminished cochlear blood flow are essential factors involved in mechanisms of other stress-related hearing loss, such as that associated with noise or ototoxic drug exposure. The emerging view is that both play key roles in ARHL pathogenesis. Therapeutic targeting of excessive free radical formation and cochlear blood flow regulation may be a useful strategy to prevent onset of ARHL. Supporting this idea, micronutrient-based therapies, in particular those combining antioxidants and vasodilators like magnesium (Mg²⁺), have proven effective in reducing the impact of noise and ototoxic drugs in the inner ear, therefore improving auditory function. In this review, the synergistic effects of combinations of antioxidant free radicals scavengers and cochlear vasodilators will be discussed as a feasible therapeutic approach for the treatment of ARHL.

Local mechanisms for loud sound-enhanced aminoglycoside entry into outer hair cells

Loud sound exposure exacerbates aminoglycoside ototoxicity, increasing the risk of permanent hearing loss and degrading the quality of life in affected individuals. We previously reported that loud sound exposure induces temporary threshold shifts (TTS) and enhances uptake of aminoglycosides, like gentamicin, by cochlear outer hair cells (OHCs). Here, we explore mechanisms by which loud sound exposure and TTS could increase aminoglycoside uptake by OHCs that may underlie this form of ototoxic synergy. Mice were exposed to loud sound levels to induce TTS, and received fluorescently-tagged gentamicin (GTTR) for 30 min prior to fixation. The degree of TTS was assessed by comparing auditory brainstem responses (ABRs) before and after loud sound exposure. The number of tip links, which gate the GTTR-permeant mechanoelectrical transducer (MET) channels, was determined in OHC bundles, with or without exposure to loud sound, using scanning electron microscopy. We found wide-band noise (WBN) levels that induce TTS also enhance OHC uptake of GTTR compared to OHCs in control cochleae. In cochlear regions with TTS, the increase in OHC uptake of GTTR was significantly greater than in adjacent pillar cells. In control mice, we identified stereociliary tip links at ~50% of potential positions in OHC bundles. However, the number of OHC tip links was significantly reduced in mice that received WBN at levels capable of inducing TTS. These data suggest that GTTR uptake by OHCs during TTS occurs by increased permeation of surviving, mechanically-gated MET channels, and/or non-MET aminoglycoside-permeant channels activated following loud sound exposure. Loss of tip links would hyperpolarize hair cells and potentially increase drug uptake via aminoglycoside-permeant channels expressed by hair cells. The effect of TTS on aminoglycoside-permeant channel kinetics will shed new light on the mechanisms of loud sound-enhanced aminoglycoside uptake, and consequently on ototoxic synergy.

Adenosine amine congener as a cochlear rescue agent

We have previously shown that adenosine amine congener (ADAC), a selective A₁ adenosine receptor agonist, can ameliorate noise- and cisplatin-induced cochlear injury. Here we demonstrate the dose-dependent rescue effects of ADAC on noise-induced cochlear injury in a rat model and establish the time window for treatment. Methods. ADAC (25–300 μg/kg) was administered intraperitoneally to Wistar rats (8–10 weeks old) at intervals (6–72 hours) after exposure to traumatic noise (8–16 kHz, 110 dB sound pressure level, 2 hours). Hearing sensitivity was assessed using auditory brainstem responses (ABR) before and 12 days after noise exposure. Pharmacokinetic studies investigated ADAC concentrations in plasma after systemic (intravenous) administration. Results. ADAC was most effective in the first 24 hours after noise exposure at doses >50 μg/kg, providing up to 21 dB protection (averaged across 8–28 kHz). Pharmacokinetic studies demonstrated a short (5 min) half-life of ADAC in plasma after intravenous administration without detection of degradation products. Conclusion. Our data show that ADAC mitigates noise-induced hearing loss in a dose- and time-dependent manner, but further studies are required to establish its translation as a clinical otological treatment.

Associations between dietary quality, noise, and hearing: data from the National Health and Nutrition Examination Survey, 1999-2002

OBJECTIVE

A statistically significant relationship between dietary nutrient intake and threshold sensitivity at higher frequencies has been reported, but evidence conflicts across studies. Here, the potential interaction between noise and diet in their association to hearing was examined.

DESIGN

This cross-sectional analysis was based on Healthy Eating Index data and audiological threshold pure-tone averages for low (0.5 to 2 kHz) and high (3 to 8 kHz) frequencies.

STUDY SAMPLE

Data were drawn from the National Health and Nutrition Examination Survey, 1999-2002.

RESULTS

Controlling for age, sex, race/ethnicity, education, diabetes, hypertension, and smoking we found statistically significant relationships between dietary quality and high-frequency threshold sensitivity as well as noise exposure and high-frequency thresholds. In addition, there was a statistically significant interaction between dietary quality and reported noise exposure with respect to high-frequency threshold sensitivity in participants, where greater reported noise exposure and poorer diet were associated with poorer hearing (p's < 0.05).

CONCLUSIONS

The current findings support an association between healthier eating and better hearing at higher frequencies; the strength of this relationship varied as a function of participant noise history, with the most robust relationship in those that reported military service or firearm use.

Hydrogen-saturated saline protects intensive narrow band noise-induced hearing loss in guinea pigs through an antioxidant effect

The purpose of the current study was to evaluate hydrogen-saturated saline protecting intensive narrow band noise-induced hearing loss. Guinea pigs were divided into three groups: hydrogen-saturated saline; normal saline; and control. For saline administration, the guinea pigs were given daily abdominal injections (1 ml/100 g) 3 days before and 1 h before narrow band noise exposure (2.5–3.5 kHz 130 dB SPL, 1 h). The guinea pigs in the control group received no treatment. The hearing function was assessed by the auditory brainstem response (ABR) and distortion product otoacoustic emission (DPOAE) recording. The changes of free radicals in the cochlea before noise exposure, and immediately and 7 days after noise exposure were also examined. By Scanning electron microscopy and succinate dehydrogenase staining, we found that pre-treatment with hydrogen-saturated saline significantly reduced noise-induced hair cell damage and hearing loss. We also found that the malondialdehyde, lipid peroxidation, and hydroxyl levels were significantly lower in the hydrogen-saturated saline group after noise trauma, indicating that hydrogen-saturated saline can decrease the amount of harmful free radicals caused by noise trauma. Our findings suggest that hydrogen-saturated saline is effective in preventing intensive narrow band noise-induced hearing loss through the antioxidant effect.

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.

Noise-induced hearing loss in the 21st century: A research and translational update

Millions of people worldwide are exposed to harmful levels of noise daily in their work and leisure environment. This makes noise-induced hearing loss (NIHL) a major occupational health risk globally. NIHL is the second most common form of acquired hearing loss after age-related hearing loss and is itself a major contributing factor to presbycusis. Temporary threshold shifts, once thought to be relatively harmless and recoverable, are now known to cause permanent cochlear injury leading to permanent loss of hearing sensitivity. This article reviews the current understanding of the cellular and molecular pathophysiology of NIHL with latest findings from animal models. Therapeutic approaches to protect against or to mitigate NIHL are discussed based on their proposed action against these known mechanisms of cochlear injury. Successes in identifying genes that predispose individuals to NIHL by candidate gene association studies are discussed with matched gene knockout animal models. This links to exciting developments in experimental gene therapy to replace and regenerate lost hair cells and post-noise otoprotective therapies currently being investigated in clinical trials. The aim is to provide new insights into current and projected future strategies to manage NIHL; bench to bedside treatment is foreseeable in the next 5 to 10 years.

Noise-induced changes in expression levels of NADPH oxidases in the cochlea

NADPH oxidases are enzymes that transport electrons across the plasma membrane and generate superoxide radical from molecular oxygen. The current study investigated the expression and distribution of NOX/DUOX members of the NADPH oxidase family (NOX1-5 and DUOX1-2) in the rat cochlea and their regulation in response to noise. Wistar rats (8-10 weeks) were exposed for 24 h to band noise (8-12 kHz) at moderate (100 dB) or traumatic (110 dB) sound pressure levels (SPL). Animals exposed to ambient noise (45-55 dB SPL) served as controls. Immunohistochemistry demonstrated predominant expression of all NOX/DUOX isoforms in the sensory and supporting cells of the organ of Corti, with very limited immunoexpression in the lateral wall tissues and spiral ganglion neurons. Noise exposure induced up-regulation of NOX1 and DUOX2 in the cochlea, whereas NOX3 was down-regulated. A significant reduction in the intensity of NOX3 immunolabeling was observed in the inner sulcus region of the cochlea after exposure to noise. Post-exposure inhibition of NADPH oxidases by Diphenyleneiodonium (DPI), a broadly selective NADPH oxidase inhibitor, mitigated noise-induced hearing loss.

CONCLUSION

Noise-induced up-regulation of NOX1 and DUOX2 could be linked to cochlear injury. In contrast, down-regulation of NOX3 may represent an endogenous protective mechanism to reduce oxidative stress in the noise-exposed cochlea. Inhibition of NADPH oxidases is potentially a novel pathway for therapeutic management of noise-induced hearing loss.

Prophylactic and therapeutic functions of drug combinations against noise-induced hearing loss

Noise is the most common occupational and environmental hazard. Noise-induced hearing loss (NIHL) is the second most common form of sensorineural hearing deficit, after age-related hearing loss (presbycusis). Although promising approaches have been identified for reducing NIHL, currently there are no effective medications to prevent NIHL. Development of an efficacious treatment has been hampered by the complex array of cellular and molecular pathways involved in NIHL. We turned this difficulty into an advantage by asking whether NIHL could be effectively prevented by targeting multiple signaling pathways with a combination of drugs already approved by U.S. Food and Drug Administration (FDA). We previously found that antiepileptic drugs blocking T-type calcium channels had both prophylactic and therapeutic effects for NIHL. NIHL can also be reduced by an up-regulation of glucocorticoid (GC) signaling pathways. Based on these findings, we tested a combination therapy for NIHL that included ethosuximide and zonisamide (anticonvulsants) and dexamethasone and methylprednisolone (synthetic GCs) in mice under exposure conditions typically associated with dramatic permanent threshold shifts (PTS). We first examined possible prophylactic effects for each drug when administered alone 2 h before noise, and calculated the median effective dose (ED50). We then tested for synergistic effects of two-drug combinations (anticonvulsant + GC), and identified combinations with the strongest synergy against NIHL, based on a previously established combination index (CI) metric. We repeated similar tests to determine their therapeutic effects when administered the same drugs 24 h after the noise exposure. Our study shows the feasibility of developing pharmacological intervention in multiple pathways, and discovering drug combinations with optimal synergistic effects in preventing permanent NIHL.

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.

Traumatic noise activates Rho-family GTPases through transient cellular energy depletion

Small GTPases mediate transmembrane signaling and regulate the actin cytoskeleton in eukaryotic cells. Here, we characterize the auditory pathology of adult male CBA/J mice exposed to traumatic noise (2-20 kHz; 106 dB; 2 h). Loss of outer hair cells was evident 1 h after noise exposure in the basal region of the cochlea and spread apically with time, leading to permanent threshold shifts of 35, 60, and 65 dB at 8, 16, and 32 kHz. Several biochemical and molecular changes correlated temporally with the loss of cells. Immediately after exposure, the concentration of ATP decreased in cochlear tissue and reached a minimum after 1 h while the immunofluorescent signal for p-AMPKα significantly increased in sensory hair cells at that time. Levels of active Rac1 increased, whereas those of active RhoA decreased significantly 1 h after noise attaining a plateau at 1-3 h; the formation of a RhoA-p140mDia complex was consistent with an activation of Rho GTPase pathways. Also at 1-3 h after exposure, the caspase-independent cell death marker, Endo G, translocated to the nuclei of outer hair cells. Finally, experiments with the inner ear HEI-OC1 cell line demonstrated that the energy-depleting agent oligomycin enhanced both Rac1 activity and cell death. The sum of the results suggests that traumatic noise induces transient cellular ATP depletion and activates Rho GTPase pathways, leading to death of outer hair cells in the cochlea.