Involvement of apoptosis in progression of cochlear lesion following exposure to intense noise

Toll-like receptor 4 modulates the cochlear immune response to acoustic injury

Acoustic overstimulation traumatizes the cochlea, resulting in auditory dysfunction. As a consequence of acoustic injury, the immune system in the cochlea is activated, leading to the production of inflammatory mediators and the infiltration of immune cells. However, the molecular mechanisms responsible for initiating these immune responses remain unclear. Here, we investigate the functional role of Toll-like receptor 4 (Tlr4), a cellular receptor that activates the innate immune system, in the regulation of cochlear responses to acoustic overstimulation. Using a Tlr4 knockout mouse model, we examined how Tlr4 deficiency affects sensory cell pathogenesis, auditory dysfunction and cochlear immune activity. We demonstrate that Tlr4 knockout does not affect sensory cell viability under physiological conditions, but reduces the level of sensory cell damage and cochlear dysfunction after acoustic injury. Together, these findings suggest that Tlr4 promotes sensory cell degeneration and cochlear dysfunction after acoustic injury. Acoustic injury provokes a site-dependent inflammatory response in both the organ of Corti and the tissues of the lateral wall and basilar membrane. Tlr4 deficiency affects these inflammatory responses in a site-dependent manner. In the organ of Corti, loss of Tlr4 function suppresses the production of interleukin 6 (Il6), a pro-inflammatory molecule, after acoustic injury. By contrast, the production of inflammatory mediators, including Il6, persists in the lateral wall and basilar membrane. In addition to immune molecules, Tlr4 knockout inhibits the expression of major histocompatibility complex class II, an antigen-presenting molecule, in macrophages, suggesting that Tlr4 participates in the antigen-presenting function of macrophages after acoustic trauma. Together, these results suggest that Tlr4 regulates multiple aspects of the immune response in the cochlea and contributes to cochlear pathogenesis after acoustic injury.

Inhibitors of Histone Deacetylases Attenuate Noise-Induced Hearing Loss

Loss of auditory sensory hair cells is the major pathological feature of noise-induced hearing loss (NIHL). Currently, no established clinical therapies for prevention or amelioration of NIHL are available. The absence of treatments is due to our lack of a comprehensive understanding of the molecular mechanisms underlying noise-induced damage. Our previous study indicates that epigenetic modification of histones alters hair cell survival. In this study, we investigated the effect of noise exposure on histone H3 lysine 9 acetylation (H3K9ac) in the inner ear of adult CBA/J mice and determined if inhibition of histone deacetylases by systemic administration of suberoylanilide hydroxamic acid (SAHA) could attenuate NIHL. Our results showed that H3K9ac was decreased in the nuclei of outer hair cells (OHCs) and marginal cells of the stria vascularis in the basal region after exposure to a traumatic noise paradigm known to induce permanent threshold shifts (PTS). Consistent with these results, levels of histone deacetylases 1, 2, and 3 (HDAC1, HDAC2 and HDAC3) were increased predominately in the nuclei of cochlear cells. Silencing of HDAC1, HDAC2, or HDAC3 with siRNA reduced the expression of the target HDAC in OHCs, but did not attenuate noise-induced PTS, whereas treatment with the pan-HDAC inhibitor SAHA, also named vorinostat, reduced OHC loss, and attenuated PTS. These findings suggest that histone acetylation is involved in the pathogenesis of noise-induced OHC death and hearing loss. Pharmacological targeting of histone deacetylases may afford a strategy for protection against NIHL.

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.

Therapeutic Effect of Dexamethasone for Noise-induced Hearing Loss: Systemic Versus Intratympanic Injection in Mice

OBJECTIVE

Dexamethasone is commonly used clinically to treat noise-induced hearing loss (NIHL) because the drug exerts multiple anti-inflammatory effects. In the present study, we investigated the post-noise therapeutic effects of dexamethasone given systemically or via intratympanic injection in the mouse.

ANIMALS

Twenty-four C57BL/6J mice were used. Eighteen experimental mice were exposed to 110 dB sound pressure level white noise and then divided into three groups: the noise, intraperitoneal dexamethasone injection (IP), and intratympanic dexamethasone injection (IT) groups.

METHODS

Dexamethasone (3 mg/kg/d) was injected intraperitoneally for five successive days in the IP group. Intratympanic injections were given on post-noise days 1 and 4 in the IT group. We compared hearing levels, the architecture of the organ of Corti (OC), and the microscopic appearance of the medial olivocochlear efferent terminals (MOC ETs) among the groups.

RESULTS

Both the IP and IT groups exhibited hearing recovery as revealed by auditory brainstem responses (ABRs), but recovery was not apparent in distortion product otoacoustic emissions (DPOAEs). OC degeneration as revealed by light microscopy was most extensive in the noise group and least extensive in the IP group. Scanning electron microscopy showed that the OC ultrastructure was better preserved in the IP than the IT group. Confocal microscopy showed that the ETs were shrunken in all noise-exposed groups as compared to the control group, but more shrunken in the dexamethasone-treated groups. Transmission electron microscopy showed that the MOC ET-outer hair cell (OHC) synapses were damaged in all noise-exposed groups, but the extent of degeneration was less in the IT than in the noise group.

CONCLUSION

Dexamethasone exerts reliable therapeutic effects when used to treat NIHL. It seems that the protective effects may differ by the routes of administration as the OCs were better preserved in the IP group and the ET-OHC synapses were more intact in the IT group.

Validation of Reference Genes for RT-qPCR Analysis in Noise-Induced Hearing Loss: A Study in Wistar Rat

The reverse transcriptase–quantitative polymerase chain reaction (RT–qPCR) requires adequate normalization in order to ensure accurate results. The use of reference genes is the most common method to normalize RT–qPCR assays; however, many studies have reported that the expression of frequently used reference genes is more variable than expected, depending on experimental conditions. Consequently, proper validation of the stability of reference genes is an essential step when performing new gene expression studies. Despite the fact that RT–qPCR has been widely used to elucidate molecular correlates of noise–induced hearing loss (NIHL), up to date there are no reports demonstrating validation of reference genes for the evaluation of changes in gene expression after NIHL. Therefore, in this study we evaluated the expression of some commonly used reference genes (Arbp, b–Act, b2m, CyA, Gapdh, Hprt1, Tbp, Tfrc and UbC) and examined their suitability as endogenous control genes for RT–qPCR analysis in the adult Wistar rat in response to NIHL. Four groups of rats were noise–exposed to generate permanent cochlear damage. Cochleae were collected at different time points after noise exposure and the expression level of candidate reference genes was evaluated by RT–qPCR using geNorm, NormFinder and BestKeeper software to determine expression stability. The three independent applications revealed Tbp as the most stably expressed reference gene. We also suggest a group of top–ranked reference genes that can be combined to obtain suitable reference gene pairs for the evaluation of the effects of noise on gene expression in the cochlea. These findings provide essential basis for further RT–qPCR analysis in studies of NIHL using Wistar rats as animal model.

Cellular signaling protective against noise-induced hearing loss – A role for novel intrinsic cochlear signaling involving corticotropin-releasing factor?

Hearing loss afflicts approximately 15% of the world's population, and crosses all socioeconomic boundaries. While great strides have been made in understanding the genetic components of syndromic and non-syndromic hearing loss, understanding of the mechanisms underlying noise-induced hearing loss (NIHL) have come much more slowly. NIHL is not simply a mechanism by which older individuals loose their hearing. Significantly, the incidence of NIHL is increasing, and is now involving ever younger populations. This may predict future increased occurrences of hearing loss. Current research has shown that even short-term exposures to loud sounds generating what was previously considered temporary hearing loss, actually produces an almost immediate and permanent loss of specific populations of auditory nerve fibers. Additionally, recurrent exposures to intense sound may hasten age-related hearing loss. While NIHL is a significant medical concern, to date, few compounds have delivered significant protection, arguing that new targets need to be identified. In this commentary, we will explore cellular signaling processes taking place in the cochlea believed to be involved in protection against hearing loss, and highlight new data suggestive of novel signaling not previously recognized as occurring in the cochlea, that is perhaps protective of hearing. This includes a recently described local hypothalamic-pituitary-adrenal axis (HPA)-like signaling system fully contained in the cochlea. This system may represent a local cellular stress-response system based on stress hormone release similar to the systemic HPA axis. Its discovery may hold hope for new drug therapies that can be delivered directly to the cochlea, circumventing systemic side effects.

Variation analysis of transcriptome changes reveals cochlear genes and their associated functions in cochlear susceptibility to acoustic overstimulation

Individual variation in the susceptibility of the auditory system to acoustic overstimulation has been well-documented at both the functional and structural levels. However, the molecular mechanism responsible for this variation is unclear. The current investigation was designed to examine the variation patterns of cochlear gene expression using RNA-seq data and to identify the genes with expression variation that increased following acoustic trauma. This study revealed that the constitutive expressions of cochlear genes displayed diverse levels of gene-specific variation. These variation patterns were altered by acoustic trauma; approximately one-third of the examined genes displayed marked increases in their expression variation. Bioinformatics analyses revealed that the genes that exhibited increased variation were functionally related to cell death, biomolecule metabolism, and membrane function. In contrast, the stable genes were primarily related to basic cellular processes, including protein and macromolecular syntheses and transport. There was no functional overlap between the stable and variable genes. Importantly, we demonstrated that glutamate metabolism is related to the variation in the functional response of the cochlea to acoustic overstimulation. Taken together, the results indicate that our analyses of the individual variations in transcriptome changes of cochlear genes provide important information for the identification of genes that potentially contribute to the generation of individual variation in cochlear responses to acoustic overstimulation.

[Analysis of Environmental-Stress-Related Impairments of Inner Ear]

Noise stress generated in industry is one of the environmental factors that physically affects the functions of the inner ear. Exposure to noise can cause hearing loss, resulting in serious problems in occupational and daily life. At present, however, there are very limited ways to prevent hearing impairments. The inner ear consists of the organ of Corti, vestibule and semicircular canal. Functional or morphological damage of these tissues in the inner ear caused by genetic factors, aging or environmental factors can result in hearing or balance impairments. In this review, we first introduce a deafness-related molecule found by our clinical research. Our experimental research using genetically engineered mice further demonstrated that impaired activity of the target molecule caused congenital and age-related hearing loss with neurodegeneration of spiral ganglion neurons in the inner ears. We also describe impaired balance in mice caused by exposure to low-frequency noise under experimental conditions with indoor environmental monitoring. We believe that our approaches to pursue both experimental research and fieldwork research complementarily are crucial for the development of a method for prevention of impairments of the inner ear.

Mechanisms of sensorineural cell damage, death and survival in the cochlea

The majority of acquired hearing loss, including presbycusis, is caused by irreversible damage to the sensorineural tissues of the cochlea. This article reviews the intracellular mechanisms that contribute to sensorineural damage in the cochlea, as well as the survival signaling pathways that can provide endogenous protection and tissue rescue. These data have primarily been generated in hearing loss not directly related to age. However, there is evidence that similar mechanisms operate in presbycusis. Moreover, accumulation of damage from other causes can contribute to age-related hearing loss (ARHL). Potential therapeutic interventions to balance opposing but interconnected cell damage and survival pathways, such as antioxidants, anti-apoptotics, and pro-inflammatory cytokine inhibitors, are also discussed.

Protection from noise-induced hearing loss with Src inhibitors

Noise-induced hearing loss is a major cause of acquired hearing loss around the world and pharmacological approaches to protecting the ear from noise are under investigation. Noise results in a combination of mechanical and metabolic damage pathways in the cochlea. The Src family of protein tyrosine kinases could be active in both pathways and Src inhibitors have successfully prevented noise-induced cochlear damage and hearing loss in animal models. The long-term goal is to optimize delivery methods into the cochlea to reduce invasiveness and limit side-effects before human clinical testing can be considered. At their current early stage of research investigation, Src inhibitors represent an exciting class of compounds for inclusion in a multifaceted pharmacological approach to protecting the ear from noise.

Thymoquinone treatment for inner-ear acoustic trauma in rats

OBJECTIVE

To investigate whether thymoquinone has any eliminative effects against inner-ear damage caused by acoustic trauma.

METHODS

Thirty-two male rats were divided into four groups. Group 1 was only exposed to acoustic trauma. Group 2 was given thymoquinone 24 hours before acoustic trauma and continued to receive it for 10 days after the trauma. Group 3 was only treated with thymoquinone, for 10 days. Group 4, the control group, suffered no trauma and received saline instead of thymoquinone. Groups 1 and 2 were exposed to acoustic trauma using 105 dB SPL white noise for 4 hours.

RESULTS

There was a significant decrease in distortion product otoacoustic emission values and an increase in auditory brainstem response thresholds in group 1 on days 1, 5 and 10, compared with baseline measurements. In group 2, a decrease in distortion product otoacoustic emission values and an increase in auditory brainstem response threshold were observed on day 1 after acoustic trauma, but measurements were comparable to baseline values on days 5 and 10. In group 3, thymoquinone had no detrimental effects on hearing. Similarly, the control group showed stable results.

CONCLUSION

Thymoquinone was demonstrated to be a reparative rather than preventive treatment that could be used to relieve acoustic trauma.

Impact of noise on hearing in the military

Hearing plays a vital role in the performance of a soldier and is important for speech processing. Noise-induced hearing loss is a significant impairment in the military and can affect combat performance. Military personnel are constantly exposed to high levels of noise and it is not surprising that noise induced hearing loss and tinnitus remain the second most prevalent service-connected disabilities. Much of the noise experienced by military personnel exceeds that of maximum protection achievable with double hearing protection. Unfortunately, unlike civilian personnel, military personnel have little option but to remain in noisy environments in order to complete specific tasks and missions. Use of hearing protection devices and follow-up audiological tests have become the mainstay of prevention of noise-induced hearing loss. This review focuses on sources of noise within the military, pathophysiology and management of patients with noise induced hearing loss.

Molecular profile of cochlear immunity in the resident cells of the organ of Corti

BACKGROUND

The cochlea is the sensory organ of hearing. In the cochlea, the organ of Corti houses sensory cells that are susceptible to pathological insults. While the organ of Corti lacks immune cells, it does have the capacity for immune activity. We hypothesized that resident cells in the organ of Corti were responsible for the stress-induced immune response of the organ of Corti. This study profiled the molecular composition of the immune system in the organ of Corti and examined the immune response of non-immune epithelial cells to acoustic overstimulation.

METHODS

Using high-throughput RNA-sequencing and qRT-PCR arrays, we identified immune- and inflammation-related genes in both the cochlear sensory epithelium and the organ of Corti. Using bioinformatics analyses, we cataloged the immune genes expressed. We then examined the response of these genes to acoustic overstimulation and determined how changes in immune gene expression were related to sensory cell damage.

RESULTS

The RNA-sequencing analysis reveals robust expression of immune-related genes in the cochlear sensory epithelium. The qRT-PCR array analysis confirms that many of these genes are constitutively expressed in the resident cells of the organ of Corti. Bioinformatics analyses reveal that the genes expressed are linked to the Toll-like receptor signaling pathway. We demonstrate that expression of Toll-like receptor signaling genes is predominantly from the supporting cells in the organ of Corti cells. Importantly, our data demonstrate that these Toll-like receptor pathway genes are able to respond to acoustic trauma and that their expression changes are associated with sensory cell damage.

CONCLUSION

The cochlear resident cells in the organ of Corti have immune capacity and participate in the cochlear immune response to acoustic overstimulation.

Does extracorporeal shock wave lithotripsy cause hearing impairment in children?

PURPOSE

We evaluated the possible effects of noise created by high energy shock waves on the hearing function of children treated with extracorporeal shock wave lithotripsy.

MATERIALS AND METHODS

A total of 65 children with normal hearing function were included in the study. Patients were divided into 3 groups, ie those becoming stone-free after 1 session of shock wave lithotripsy (group 1, 22 children), those requiring 3 sessions to achieve stone-free status (group 2, 21) and healthy children/controls (group 3, 22). Extracorporeal shock wave lithotripsy was applied with patients in the supine position with a 90-minute frequency and a total of 2,000 shock waves in each session (Compact Sigma, Dornier MedTech, Wessling, Germany). Second energy level was used with a maximum energy value of 58 joules per session in all patients. Hearing function and possible cochlear impairment were evaluated by transient evoked otoacoustic emissions test at 1.0, 1.4, 2.0, 2.8 and 4.0 kHz frequencies before the procedure, 2 hours later, and 1 month after completion of the first shock wave lithotripsy session in groups 1 and 2. In controls the same evaluation procedures were performed at the beginning of the study and 7 weeks later.

RESULTS

Regarding transient evoked otoacoustic emissions data, in groups 1 and 2 there was no significant alteration in values obtained after shock wave lithotripsy compared to values obtained at the beginning of the study, similar to controls.

CONCLUSIONS

A well planned shock wave lithotripsy procedure is a safe and effective treatment in children with urinary stones and causes no detectable harmful effect on hearing function.

Activated protein C rescues the cochlea from noise-induced hearing loss

Activated protein C (APC) is a serine/threonine protease and a physiological anticoagulant that exerts anti-inflammatory and anti-apoptotic effects. Although recent studies have revealed that APC has the potential to protect endothelial cells from apoptosis, the mechanisms of its cytoprotective effect are not fully understood. We examined the potential of APC to protect against noise-induced hearing loss (NIHL) and investigated phosphorylation of serine-threonine kinase (Akt) and inhibition of apoptosis as possible cytoprotective mechanisms. We administered intraperitoneal injections of APC (150, 300 U/kg) or normal saline to rats 30 min before exposure to a sound pressure level (SPL) of 126 dB and 4-kHz octave band noise for 5h. The auditory brainstem response (ABR) and distortion product otoacoustic emissions (DPOAE) were measured before and after exposure. ABR and DPOAE measurements revealed greater improvement in the APC group than in the control group 28 days after exposure. Our examination of outer hair cells (OHCs) at 28 days after noise exposure revealed a significantly higher OHC survival rate in the APC group than in the control group. Immunohistochemical analyses for cleaved-caspase 3, phospho-p38 (p-p38), TUNEL, and phospho-Akt (p-Akt) revealed strong immunoreactivities against cleaved-caspase 3, p-p38, and TUNEL in the inner ear tissues of the control group; however, these signals were decreased in the APC group. Moreover, APC significantly induced activation of p-Akt in the cochlea. These findings suggest that APC has a novel protective effect on the cochlea against NIHL that is mediated by p-Akt and the anti-apoptotic signaling pathway.

Acute and long-term effects of noise exposure on the neuronal spontaneous activity in cochlear nucleus and inferior colliculus brain slices

Noise exposure leads to an immediate hearing loss and is followed by a long-lasting permanent threshold shift, accompanied by changes of cellular properties within the central auditory pathway. Electrophysiological recordings have demonstrated an upregulation of spontaneous neuronal activity. It is still discussed if the observed effects are related to changes of peripheral input or evoked within the central auditory system. The present study should describe the intrinsic temporal patterns of single-unit activity upon noise-induced hearing loss of the dorsal and ventral cochlear nucleus (DCN and VCN) and the inferior colliculus (IC) in adult mouse brain slices. Recordings showed a slight, but significant, elevation in spontaneous firing rates in DCN and VCN immediately after noise trauma, whereas no differences were found in IC. One week postexposure, neuronal responses remained unchanged compared to controls. At 14 days after noise trauma, intrinsic long-term hyperactivity in brain slices of the DCN and the IC was detected for the first time. Therefore, increase in spontaneous activity seems to develop within the period of two weeks, but not before day 7. The results give insight into the complex temporal neurophysiological alterations after noise trauma, leading to a better understanding of central mechanisms in noise-induced hearing loss.

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.

Identification of a gene set to evaluate the potential effects of loud sounds from seismic surveys on the ears of fishes: a study with Salmo salar

Functional genomic studies were carried out on the inner ear of Atlantic salmon Salmo salar following exposure to a seismic airgun. Microarray analyses revealed 79 unique transcripts (passing background threshold), with 42 reproducibly up-regulated and 37 reproducibly down-regulated in exposed v. control fish. Regarding the potential effects on cellular energetics and cellular respiration, altered transcripts included those with roles in oxygen transport, the glycolytic pathway, the Krebs cycle and the electron transport chain. Of these, a number of transcripts encoding haemoglobins that are important in oxygen transport were up-regulated and among the most highly expressed. Up-regulation of transcripts encoding nicotinamide riboside kinase 2, which is also important in energy production and linked to nerve cell damage, points to evidence of neuronal damage in the ear following noise exposure. Transcripts related to protein modification or degradation also indicated potential damaging effects of sound on ear tissues. Notable in this regard were transcripts associated with the proteasome-ubiquitin pathway, which is involved in protein degradation, with the transcript encoding ubiquitin family domain-containing protein 1 displaying the highest response to exposure. The differential expression of transcripts observed for some immune responses could potentially be linked to the rupture of cell membranes. Meanwhile, the altered expression of transcripts for cytoskeletal proteins that contribute to the structural integrity of the inner ear could point to repair or regeneration of ear tissues including auditory hair cells. Regarding potential effects on hormones and vitamins, the protein carrier for thyroxine and retinol (vitamin A), namely transthyretin, was altered at the transcript expression level and it has been suggested from studies in mammalian systems that retinoic acid may play a role in the regeneration of damaged hair cells. The microarray experiment identified the transcript encoding growth hormone I as up-regulated by loud sound, supporting previous evidence linking growth hormone to hair cell regeneration in fishes. Quantitative (q) reverse transcription (RT) polymerase chain reaction (qRT-PCR) analyses confirmed dysregulation of some microarray-identified transcripts and in some cases revealed a high level of biological variability in the exposed group. These results support the potential utility of molecular biomarkers to evaluate the effect of seismic surveys on fishes with studies on the ears being placed in a priority category for development of exposure-response relationships. Knowledge of such relationships is necessary for addressing the question of potential size of injury zones.

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.

Non-coding RNAs in the development of sensory organs and related diseases

Genomes are transcribed well beyond the conventionally annotated protein-encoding genes and produce many thousands of regulatory non-coding RNAs (ncRNAs). In the last few years, ncRNAs, especially microRNAs and long non-coding RNA, have received increasing attention because of their implication in the function of chromatin-modifying complexes and in the regulation of transcriptional and post-transcriptional events. The morphological events and the genetic networks responsible for the development of sensory organs have been well delineated and therefore sensory organs have provided a useful scenario to address the role of ncRNAs. In this review, we summarize the current information on the importance of microRNAs and long non-coding RNAs during the development of the eye, inner ear, and olfactory system in vertebrates. We will also discuss those cases in which alteration of ncRNA expression has been linked to pathological conditions affecting these organs.

Physiological, anatomical, and behavioral changes after acoustic trauma in Drosophila melanogaster

Noise-induced hearing loss (NIHL) is a growing health issue, with costly treatment and lost quality of life. Here we establish Drosophila melanogaster as an inexpensive, flexible, and powerful genetic model system for NIHL. We exposed flies to acoustic trauma and quantified physiological and anatomical effects. Trauma significantly reduced sound-evoked potential (SEP) amplitudes and increased SEP latencies in control genotypes. SEP amplitude but not latency effects recovered after 7 d. Although trauma produced no gross morphological changes in the auditory organ (Johnston's organ), mitochondrial cross-sectional area was reduced 7 d after exposure. In nervana 3 heterozygous flies, which slightly compromise ion homeostasis, trauma had exaggerated effects on SEP amplitude and mitochondrial morphology, suggesting a key role for ion homeostasis in resistance to acoustic trauma. Thus, Drosophila exhibit acoustic trauma effects resembling those found in vertebrates, including inducing metabolic stress in sensory cells. This report of noise trauma in Drosophila is a foundation for studying molecular and genetic sequelae of NIHL.

Medical ozone therapy for the inner ear acoustic trauma

OBJECTIVES

The goal of the study was to look at the potential protective effect of ozone therapy by studying its antioxidant and vasodilatation effects against hearing loss caused by acoustic trauma.

METHODS

Thirty-two male Wistar Albino rats were divided into four groups of eight. The 1st group was exposed to acoustic trauma, the 2nd group was treated with ozone initially, and was exposed to acoustic trauma 24 h later, the 3rd group received ozone without trauma, while the 4th group was the control group. The 1st and 2nd groups were exposed to acoustic trauma with 105 dB SPL white band noise for 4h. DPOAE and ABR tests were conducted in all groups on the 1st, 5th, and 10th days after trauma.

RESULTS

In the 1st group, the effects of acoustic trauma continued on days 1, 5 and 10. The 2nd group's DPOAE and ABR results on days 5 and 10 showed significant improvement at all frequencies compared to deterioration on day 1, and the readings were comparable to baseline measurements.

CONCLUSION

Acoustic trauma is a pathology that is experienced frequently and leads to many problems in terms of health and cost. Ozone was demonstrated to be a reparative substance against acoustic trauma and, in addition, it can be supplied and applied easily.

Noise-induced cochlear inflammation

Hearing loss is the most common sensory disability with considerable social and economic implications. According to recent World Health Organization estimates, 360 million people worldwide suffer from moderate to profound hearing loss. Exposure to excessive noise is one of the major causes of sensorineural hearing loss, secondary only to age-related hearing loss (presbyacusis). Since cochlear tissues have limited abilities of repair and regeneration, this damage can be irreversible, leading to cochlear dysfunction and permanent hearing loss. Recent studies have shown that cochlear inflammation can be induced by noise exposure and contribute to the overall pathogenesis of cochlear injury and hearing loss. The cochlea is separated from the systemic circulation by the blood-labyrinth barrier, which is physiologically similar to the blood-brain barrier of the central nervous system. Because of this feature, the cochlea was originally considered an immunologically privileged organ. However, this postulate has been challenged by the evidence of an inflammatory response in the cochlea in the presence of bacterial or viral pathogens or antigens that can cause labyrinthitis. Although the main purpose of the inflammatory reaction is to protect against invading pathogens, the inflammatory response can also cause significant bystander injury to the delicate structures of the cochlea. The cochlear inflammatory response is characterised by the generation of proinflammatory mediators (cytokines, chemokines and adhesion molecules), and the recruitment of inflammatory cells (leukocytes). Here, we present an overview of the current research on cochlear inflammation, with particular emphasis on noise-induced cochlear inflammation. We also discuss treatment strategies aimed at the suppression of inflammation, which may potentially lead to mitigation of hearing loss.

RNAlater facilitates microdissection of sensory cell-enriched samples from the mouse cochlea for transcriptional analyses

Molecular analyses of cochlear pathology rely on the acquisition of high-quality cochlear samples. For small rodents, isolating sensory cell-enriched samples with well-preserved RNA integrity for transcriptional analyses poses a significant challenge. Here, we report a microdissection technique for isolating sensory cell-enriched samples from the cochlea. We found that treating the tissue with RNAlater, a RNA preservation medium, alters the physical properties of the tissue and facilitates the dissection. Unlike previous samples that have been isolated from the sensory epithelium, our samples contain defined cell populations that have a consistent ratio of sensory cells to supporting cells. Importantly, the RNA components were well preserved. With this microdissection method, we collected three types of samples: sensory cell-enriched, outer hair cell-enriched, and inner hair cell-enriched. To demonstrate the feasibility of the method, we screened multiple reference genes in the sensory cell-enriched samples and identified stable genes in noise-traumatized cochleae. The method described here balances the need for both quality and purity of sensory cells and also circumvents many limitations of the currently available techniques for collecting cochlear tissues. With our approach, the collected samples can be used in diverse downstream analyses, including qRT-PCR, microarray, and RNA sequencing.

High-throughput technologies for gene expression analyses: what we have learned for noise-induced cochlear degeneration?

Noise-induced hearing loss is a common cause of acquired hearing loss in the adult population. Acoustic overstimulation causes cochlear damage through mechanical stress to the tissue. Consequently, complex molecular changes are initiated, and these changes lead to morphological and biological alterations in the cochlea, which in turn compromise the cochlear function and cause hearing loss. In the past 10 years, there have been significant advances in our understanding of the molecular mechanisms of noise-induced hearing loss. These advances are attributed, in part, to the development of high-throughput technologies for the global analyses of molecular changes. In this review, we briefly describe the newly developed methods for investigating the molecular responses of the cochlea to acoustic trauma and the knowledge generated from these studies. We also discuss the strengths and limitations of each technique and the major challenges to investigate cochlear degeneration following acoustic injury.

Transcriptome characterization by RNA-Seq reveals the involvement of the complement components in noise-traumatized rat cochleae

Acoustic trauma, a leading cause of sensorineural hearing loss in adults, induces a complex degenerative process in the cochlea. Although previous investigations have identified multiple stress pathways, a comprehensive analysis of cochlear responses to acoustic injury is still lacking. In the current study, we used the next-generation RNA-sequencing (RNA-Seq) technique to sequence the whole transcriptome of the normal and noise-traumatized cochlear sensory epithelia (CSE). CSE tissues were collected from rat inner ears 1d after the rats were exposed to a 120-dB (sound pressure level) noise for 2 h. The RNA-Seq generated over 176 million sequence reads for the normal CSE and over 164 million reads for the noise-traumatized CSE. Alignment of these sequences with the rat Rn4 genome revealed the expression of over 17,000 gene transcripts in the CSE, over 2000 of which were exclusively expressed in either the normal or noise-traumatized CSE. Seventy-eight gene transcripts were differentially expressed (70 upregulated and 8 downregulated) after acoustic trauma. Many of the differentially expressed genes are related to the innate immune system. Further expression analyses using quantitative real time PCR confirmed the constitutive expression of multiple complement genes in the normal organ of Corti and the changes in the expression levels of the complement factor I (Cfi) and complement component 1, s subcomponent (C1s) after acoustic trauma. Moreover, protein expression analysis revealed strong expression of Cfi and C1s proteins in the organ of Corti. Importantly, these proteins exhibited expression changes following acoustic trauma. Collectively, the results of the current investigation suggest the involvement of the complement components in cochlear responses to acoustic trauma.

The miR-183/Taok1 target pair is implicated in cochlear responses to acoustic trauma

Acoustic trauma, one of the leading causes of sensorineural hearing loss, induces sensory hair cell damage in the cochlea. Identifying the molecular mechanisms involved in regulating sensory hair cell death is critical towards developing effective treatments for preventing hair cell damage. Recently, microRNAs (miRNAs) have been shown to participate in the regulatory mechanisms of inner ear development and homeostasis. However, their involvement in cochlear sensory cell degeneration following acoustic trauma is unknown. Here, we profiled the expression pattern of miRNAs in the cochlear sensory epithelium, defined miRNA responses to acoustic overstimulation, and explored potential mRNA targets of miRNAs that may be responsible for the stress responses of the cochlea. Expression analysis of miRNAs in the cochlear sensory epithelium revealed constitutive expression of 176 miRNAs, many of which have not been previously reported in cochlear tissue. Exposure to intense noise caused significant threshold shift and apoptotic activity in the cochleae. Gene expression analysis of noise-traumatized cochleae revealed time-dependent transcriptional changes in the expression of miRNAs. Target prediction analysis revealed potential target genes of the significantly downregulated miRNAs, many of which had cell death- and apoptosis-related functions. Verification of the predicted targets revealed a significant upregulation of Taok1, a target of miRNA-183. Moreover, inhibition of miR-183 with morpholino antisense oligos in cochlear organotypic cultures revealed a negative correlation between the expression levels of miR-183 and Taok1, suggesting the presence of a miR-183/Taok1 target pair. Together, miRNA profiling as well as the target analysis and validation suggest the involvement of miRNAs in the regulation of the degenerative process of the cochlea following acoustic overstimulation. The miR-183/Taok1 target pair is likely to play a role in this regulatory process.

Metalloproteinases and their associated genes contribute to the functional integrity and noise-induced damage in the cochlear sensory epithelium

Matrix metalloproteinases (MMPs) and their related gene products regulate essential cellular functions. An imbalance in MMPs has been implicated in various neurological disorders, including traumatic injuries. Here, we report a role for MMPs and their related gene products in the modulation of cochlear responses to acoustic trauma in rats. The normal cochlea was shown to be enriched in MMP enzymatic activity, and this activity was reduced in a time-dependent manner after traumatic noise injury. The analysis of gene expression by RNA sequencing and qRT-PCR revealed the differential expression of MMPs and their related genes between functionally specialized regions of the sensory epithelium. The expression of these genes was dynamically regulated between the acute and chronic phases of noise-induced hearing loss. Moreover, noise-induced expression changes in two endogenous MMP inhibitors, Timp1 and Timp2, in sensory cells were dependent on the stage of nuclear condensation, suggesting a specific role for MMP activity in sensory cell apoptosis. A short-term application of doxycycline, a broad-spectrum inhibitor of MMPs, before noise exposure reduced noise-induced hearing loss and sensory cell death. In contrast, a 7 d treatment compromised hearing sensitivity and potentiated noise-induced hearing loss. This detrimental effect of the long-term inhibition of MMPs on noise-induced hearing loss was further confirmed using targeted Mmp7 knock-out mice. Together, these observations suggest that MMPs and their related genes participate in the regulation of cochlear responses to acoustic overstimulation and that the modulation of MMP activity can serve as a novel therapeutic target for the reduction of noise-induced cochlear damage.

Evaluating D-methionine dose to attenuate oxidative stress-mediated hearing loss following overexposure to noise

Noise exposure causes an excessive reactive oxygen species (ROS) generation as an unwanted byproduct of high metabolic activity. Oxidative stress and antioxidative protective mechanisms have been therefore proposed as the most interesting issues in the development of noise-induced hearing loss. The aim of this study was to examine changes in superoxide dismutase (SOD), catalase (CAT) and the auditory brainstem response (ABR) in the cochlea of C57BL/6 mice 1, 7 and 14 days after exposure to 4 kHz octave band noise at the intensity of 110 dB SPL for 8 h. The evaluation of three d-methionine (d-met) doses (100, 200 and 400 mg/kg) has been performed in order to choose an optimal concentration displaying most effectively its antioxidant and thereby otoprotective functions. Administering d-met at the dose of 400 mg/kg resulted in a significant decrease in threshold shift (TS) independently of the evaluation time after exposure to noise. SOD activity was strongly supported by the same concentration (400 mg/kg) of d-met. This effect was seen not shortly, but 7 and 14 days after exposure to noise. CAT activity was induced only by noise and it reached the peak levels 7 days after exposure. d-Met at the doses of 200 and 400 mg/kg significantly decreased noise-induced changes in CAT activity. The findings of this study indicate that the protective effect depends on the concentration of d-met and can be fully expressed only when the drug is administered in the dose 400 mg/kg.

Low dose oxidative stress induces mitochondrial damage in hair cells

Oxidative stress has been implicated as a cause of hair cell damage after ischemia reperfusion injury, noise trauma, and ototoxic injury. Oxidative stress can induce both apoptosis or necrosis depending on the degree of exposure. To study how reactive oxygen species (ROS) interacts with hair cells, we have developed an in vitro model of oxidative stress using organ of Corti cultures exposed to physiologically relevant concentrations of hydrogen peroxide (H(2) O(2) ). Treatment of organ of Corti cultures with low concentrations of H(2) O(2) results in loss of outer hair cells in the basal turn of the explant. Higher concentrations of peroxide result in more extensive outer hair cell injury as well as loss of inner hair cells. Early outer hair cell death appears to occur though apoptosis as demonstrated by staining of activated caspase. The effect of oxidative stress on mitochondrial function is a key determinant of degree of damage. Oxidative stress results in reduction of the mitochondrial membrane potential and reduction of mitochondrial produced antioxidants. Low doses of oxidative stress induce changes in mitochondrial gene expression and induce mitochondrial DNA deletions. Recurrent oxidative stress or inhibition of mitochondrial function significantly enhanced hair cell death. This tissue culture model of oxidative hair cell injury maintains a pattern of injury similar to what is observed in vivo after oxidative injury and can be used to study the effects of ROS on hair cells over the time period of the culture.

Apoptosis Progression in the Hair Cells in the Organ of Corti of GJB2 Conditional Knockout Mice

Objectives

Apoptosis may play an important role in the mechanism underlying the GJB2 gene conditional knockout (cCx26) mice cochlear cell death. The objective of this study was to explore the the damage mode of the outer hair cells (OHCs) and its real time point of apoptosis and provide information to further explore the role of apoptosis in the happening of hearing loss in cCx26 mice.

Methods

Cochleae from mice at various developmental stages (P8, P12, and P21) were dissected out and first used to be observed under the scanning electron microscope (SEM). Basilar membranes from mice at P8, P14, P18, and P21 were stained by fluorescein isothiocyanate-conjugated phalloidin and propidium iodide (PI) and examined under confocal microscope.

Results

The loss of OHCs of cCx26 knockout mice was first set between P12 and P21 under SEM. Whole mount phalloidin and PI staining revealed that obvious apoptotic appearance of the OHCs surface morphology was observed at P18.

Conclusion

Typical apoptotic morphology was found in the OHCs in the organ of Corti of the cCx26 mice at P18. This may provide information to further study the role of apoptosis in the occurrence of hearing loss of cCx26 mice.

Cell-cell junctions: a target of acoustic overstimulation in the sensory epithelium of the cochlea

Background

Exposure to intense noise causes the excessive movement of the organ of Corti, stretching the organ and compromising sensory cell functions. We recently revealed changes in the transcriptional expression of multiple adhesion-related genes during the acute phases of cochlear damage, suggesting that the disruption of cell-cell junctions is an early event in the process of cochlear pathogenesis. However, the functional state of cell junctions in the sensory epithelium is not clear. Here, we employed graded dextran-FITC, a macromolecule tracer that is impermeable to the organ of Corti under physiological conditions, to evaluate the barrier function of cell junctions in normal and noise-traumatized cochlear sensory epithelia.

Results

Exposure to an impulse noise of 155 dB (peak sound pressure level) caused a site-specific disruption in the intercellular junctions within the sensory epithelium of the chinchilla cochlea. The most vulnerable sites were the junctions among the Hensen cells and between the Hensen and Deiters cells within the outer zone of the sensory epithelium. The junction clefts that formed in the reticular lamina were permeable to 40 and 500 but not 2,000 kDa dextran-FITC macromolecules. Moreover, this study showed that the interruption of junction integrity occurred in the reticular lamina and also in the basilar membrane, a site that had been considered to be resistant to acoustic injury. Finally, our study revealed a general spatial correlation between the site of sensory cell damage and the site of junction disruption. However, the two events lacked a strict one-to-one correlation, suggesting that the disruption of cell-cell junctions is a contributing, but not the sole, factor for initiating acute sensory cell death.

Conclusions

Impulse noise causes the functional disruption of intercellular junctions in the sensory epithelium of the chinchilla cochlea. This disruption occurs at an early phase of cochlear damage. Understanding the role of this disruption in cochlear pathogenesis will require future study.

Increased activity of mitochondrial respiratory chain complex in noise-damaged rat cochlea

CONCLUSION

Functional changes of mitochondrial respiratory chain (MRC) and hearing threshold shifts were observed after noise exposure and the pattern was different depending on the extent of the noise damage.

OBJECTIVES

The aim of this study was to reveal any functional changes of the MRC enzyme in outer hair cells after noise damage.

METHODS

Sprague-Dawley rats were used and divided into three groups including normal control group and noise exposure for 1 h and 5 h. White band noise was used with 120 dB SPL. We measured hearing threshold and activity of succinate dehydrogenase (SDH) before noise exposure, and immediately and 1, 3, 7, and 14 days after noise exposure in each group. Immunostaining using monoclonal anti-MRC complex V subunit α was performed. Transmission electron microscopy (TEM) was carried out.

RESULTS

The hearing threshold level changed depending on the exposure time in both the noise-exposed groups. In the 1 h group, hearing was improved after 1 day, and in the 5 h group, hearing was measured after 7 days. Changes in SDH activity were observed at the third day in the 1 h group and at the seventh day in the 5 h group. The expression level of MRC complex V was increased after noise exposure. TEM showed mitochondrial damage.

Astragaloside IV inhibits apoptotic cell death in the guinea pig cochlea exposed to impulse noise

CONCLUSION

The results suggest that the beneficial effect of astragaloside IV on impulse noise-induced hearing loss may be due to its ability to inhibit reactive oxygen species (ROS) and prevent apoptosis.

OBJECTIVE

Astragaloside IV is the major active constituent of Astragalus membranaceus, which has been widely used for the treatment of diseases in China for its antioxidant properties. ROS and apoptosis are involved in damage induced by impulse noise trauma. We aimed to investigate if the beneficial effects of astragaloside IV on cochlea exposed to impulse noise are associated with the inhibition of ROS and the decrease in apoptosis.

METHODS

4-Hydroxynonenal (HNE) was used as the marker of ROS. Active-caspase-3 (cas-3) served as a marker for apoptosis. 4HNE and cas-3 were determined immunohistochemically. Guinea pigs in the experimental group were administered astragaloside IV intragastrically. Auditory thresholds were assessed by sound-evoked auditory brainstem response (ABR) 72 h before and after exposure to impulse noise.

RESULTS

The results showed that astragaloside IV significantly reduced ABR deficits, and decreased the expression of ROS and cas-3.

Aircraft noise exposure affects rat behavior, plasma norepinephrine levels, and cell morphology of the temporal lobe

In order to investigate the physiological effects of airport noise exposure on organisms, in this study, we exposed Sprague-Dawley rats in soundproof chambers to previously recorded aircraft-related noise for 65 d. For comparison, we also used unexposed control rats. Noise was arranged according to aircraft flight schedules and was adjusted to its weighted equivalent continuous perceived noise levels (L(WECPN)) of 75 and 80 dB for the two experimental groups. We examined rat behaviors through an open field test and measured the concentrations of plasma norepinephrine (NE) by high performance liquid chromatography-fluorimetric detection (HPLC-FLD). We also examined the morphologies of neurons and synapses in the temporal lobe by transmission electron microscopy (TEM). Our results showed that rats exposed to airport noise of 80 dB had significantly lower line crossing number (P<0.05) and significantly longer center area duration (P<0.05) than control animals. After 29 d of airport noise exposure, the concentration of plasma NE of exposed rats was significantly higher than that of the control group (P<0.05). We also determined that the neuron and synapsis of the temporal lobe of rats showed signs of damage after aircraft noise of 80 dB exposure for 65 d. In conclusion, exposing rats to long-term aircraft noise affects their behaviors, plasma NE levels, and cell morphology of the temporal lobe.

MicroRNAs in inner ear biology and pathogenesis

MicroRNAs (miRNA) are a group of small noncoding RNAs that regulate gene expression. The discovery of these small RNAs has added a new layer of complexity to molecular biology. Every day, new advances are being made in understanding the biochemistry and genetics of miRNAs and their roles in cellular function and homeostasis. Studies indicate diverse roles for miRNAs in inner ear biology and pathogenesis. This article reviews recent developments in miRNA research in the field of inner ear biology. A brief history of miRNA discovery is discussed, and their genomics and functional roles are described. Advances in the understanding of miRNA involvement in inner ear development in the zebrafish and the mouse are presented. Finally, this review highlights the potential roles of miRNAs in genetic hearing loss, hair cell regeneration, and inner ear pathogenesis resulting from various pathological insults.

Hydrogen-rich saline alleviates experimental noise-induced hearing loss in guinea pigs

OBJECTIVE

To examine the efficiency of hydrogen-rich saline in the treatment of intensive noise-induced cochlear injury.

MATERIALS AND METHODS

Forty guinea pigs were assigned to one of four groups: HS+NOISE (i.p. injection hydrogen-rich saline), NS+NOISE (i.p. injection normal saline), NOISE ALONE (noise control), and NO TREATMENT (normal control) groups. The HS+NOISE, NS+NOISE, and NOISE ALONE groups were exposed to intensive noise (4 h at 115 dB SPL noise of 4000±100 Hz). The auditory brainstem response (ABR) was used to examine the hearing threshold in each group. Distortion product otoacoustic emission (DPOAE) was used to examine outer hair cell function. We also examined cochlear morphology to evaluate inner and outer hair cell trauma induced by noise exposure. Hydrogen-rich saline was administered twice daily for 6 days (2.5 ml/kg, i.p.) 24 h after noise exposure.

RESULTS

Baseline ABR thresholds and DPOAE values were normal in all groups at the measured frequencies (2, 4, 8, and 16 kHz) before noise exposure. The ABR threshold shift was 50-55 dB across the frequencies tested, and average DPOAE declined in the NOISE ALONE, NS+NOISE, and HS+NOISE groups 24 h after noise exposure. However, the changes in cochlear parameters were different between groups. The HS+NOISE group showed a significantly decreased ABR threshold value as compared with the NS+NOISE or NOISE ALONE group (P<0.01) on day 7. The mean DPOAE recovered to some extent in the three noise exposure groups, but at most frequencies the HS+NOISE group showed significantly increased DPOAE on day 7 as compared with the NS+NOISE group or NOISE ALONE group (P<0.01). Surface Corti organ preparations stained with succinate dehydrogenase (SDH) showed that most outer hair cells (OHCs) were still dropsical and a few were missing 7 days after noise exposure in the NS+NOISE group. Only a few OHCs were slightly dropsical in the HS+NOISE group. The numbers of missing hair cells 7 days after noise exposure were significantly greater in the NOISE ONLY and NS+NOISE groups than the HS+NOISE group (P<0.01).

CONCLUSIONS

Hydrogen-rich saline can alleviate experimental noise-induced hearing loss in guinea pigs, partially by preventing the death of cochlear hair cells after intensive noise exposure.

Protection from impulse noise-induced hearing loss with novel Src-protein tyrosine kinase inhibitors

Apoptosis is a significant mechanism of cochlear hair cell loss from noise. Molecules that inhibit apoptotic intracellular signaling reduce cochlear damage and hearing loss from noise. The current study is an extension of a previous study of the protective value of Src-protein tyrosine kinase inhibitors against noise (Harris et al., 2005). The current study tested three Src-inhibitors: the indole-based KX1-141, the biaryl-based KX2-329, and the ATP-competitive KX2-328. Each of the three drugs was delivered into the chinchillas' cochleae by allowing the solutions to diffuse across the round window membrane thirty minutes prior to exposure to impulse noise. Hearing thresholds were measured using auditory evoked responses from electrodes in the inferior colliculi. Ears treated with KX2-329 showed significantly lower threshold shifts and outer hair cell losses than the control group. The cochleae treated with KX1-141 and KX2-328 did not show statistically significant protection from the impulse noise. The finding of protection with KX2-329 demonstrates that a biaryl-based Src inhibitor has protective capacity against noise-induced hearing loss that is as good as that demonstrated by KX1-004, a Src inhibitor drug that has been studied extensively as an otoprotectant against noise, and suggests that KX2-329 could be useful for protection against noise.

Transcriptional changes in adhesion-related genes are site-specific during noise-induced cochlear pathogenesis

Cell-cell junctions and junctions between cells and extracellular matrix are essential for maintenance of the structural and functional integrity of the cochlea, and are also a major target of acoustic trauma. While morphological assessments have revealed adhesion dysfunction in noise-traumatized cochleae, the molecular mechanisms responsible for adhesion disruption are not clear. Here, we screened the transcriptional expression of 49 adhesion-related genes in normal rat cochleae and measured the expression changes in the early phases of cochlear pathogenesis after acoustic trauma. We found that genes from four adhesion families, including the immunoglobulin superfamily and the integrin, cadherin, and selectin families, are expressed in the normal cochlea. Exposure to an intense noise at 120dB sound pressure level (SPL) for 2h caused site-specific changes in expression levels in the apical and the basal sections of the sensory epithelium. Expression changes that occurred in the cochlear sensory epithelium were biphasic, with early upregulation at 2h post-noise exposure and subsequent downregulation at 1day post-exposure. Importantly, the altered expression level of seven genes (Sgce, Sell, Itga5, Itgal, Selp, Cntn1 and Col5a1) is related to the level of threshold shift of the auditory brainstem response (ABR), an index reflecting functional change in the cochlea. Notably, the genes showing expression changes exhibited diverse constitutive expression levels and belong to multiple adhesion gene families. The finding of expression changes in multiple families of adhesion genes in a temporal fashion (2h vs. 1day) and a spatial fashion (the apical and the basal sensory epithelia as well as the lateral wall tissue) suggests that acoustic overstimulation provokes a complex response in adhesion genes, which likely involves multiple adhesion-related signaling pathways.

The cochlear CRF signaling systems and their mechanisms of action in modulating cochlear sensitivity and protection against trauma

A key requirement for encoding the auditory environment is the ability to dynamically alter cochlear sensitivity. However, merely attaining a steady state of maximal sensitivity is not a viable solution since the sensory cells and ganglion cells of the cochlea are prone to damage following exposure to loud sound. Most often, such damage is via initial metabolic insult that can lead to cellular death. Thus, establishing the highest sensitivity must be balanced with protection against cellular metabolic damage that can lead to loss of hair cells and ganglion cells, resulting in loss of frequency representation. While feedback mechanisms are known to exist in the cochlea that alter sensitivity, they respond only after stimulus encoding, allowing potentially damaging sounds to impact the inner ear at times coincident with increased sensitivity. Thus, questions remain concerning the endogenous signaling systems involved in dynamic modulation of cochlear sensitivity and protection against metabolic stress. Understanding endogenous signaling systems involved in cochlear protection may lead to new strategies and therapies for prevention of cochlear damage and consequent hearing loss. We have recently discovered a novel cochlear signaling system that is molecularly equivalent to the classic hypothalamic-pituitary-adrenal (HPA) axis. This cochlear HPA-equivalent system functions to balance auditory sensitivity and susceptibility to noise-induced hearing loss, and also protects against cellular metabolic insults resulting from exposures to ototoxic drugs. We review the anatomy, physiology, and cellular signaling of this system, and compare it to similar signaling in other organs/tissues of the body.

D-methionine (D-met) significantly rescues noise-induced hearing loss: timing studies

We have previously reported rescue from noise-induced auditory brainstem response (ABR) threshold shifts with d-methionine (d-met) administration 1 h after noise exposure. The present study investigated further d-met rescue intervals at 3, 5 and 7 h post-noise exposure. Chinchillas laniger were exposed to a 6 h 105 dB sound pressure level (dB SPL) octave band noise (OBN) and then administered d-met i.p. starting 3, 5, or 7 h after noise exposure; controls received saline i.p. immediately after noise exposure. ABR assessments were performed at baseline and on post-exposure days 1 and 21. Outer hair cell (OHC) loss was measured in cochleae obtained at sacrifice 21 days post-exposure. Administration of d-met starting at any of the delay times of 3-7 h post-noise exposure significantly reduced day 21 ABR threshold shift at 2 and 4 kHz and OHC loss at all hair cell regions measured (2, 4, 6 and 8 kHz). ABR threshold shifts in the control group at 6 and 8 kHz were only 8 and 11 dB respectively allowing little opportunity to observe protection at those 2 frequencies.

Noise induced changes in the expression of p38/MAPK signaling proteins in the sensory epithelium of the inner ear

Noise exposure is a major cause of hearing loss. Classical methods of studying protein involvement have provided a basis for understanding signaling pathways that mediate hearing loss and damage repair but do not lend themselves to studying large networks of proteins that are likely to increase or decrease during noise trauma. To address this issue, antibody microarrays were used to quantify the very early changes in protein expression in three distinct regions of the chinchilla cochlea 2h after exposure to a 0.5-8 kHz band of noise for 2h at 112 dB SPL. The noise exposure caused significant functional impairment 2h post-exposure which only partially recovered. Distortion product otoacoustic emissions were abolished 2h after the exposure, but at 4 weeks post-exposure, otoacoustic emissions were present, but still greatly depressed. Cochleograms obtained 4 weeks post-exposure demonstrated significant loss of outer hair cells in the basal 60% of the cochlea corresponding to frequencies in the noise spectrum. A comparative analysis of the very early (2h post-exposure) noise-induced proteomic changes indicated that the sensory epithelium, lateral wall and modiolus differ in their biological response to noise. Bioinformatic analysis of the cochlear protein profile using "The Database for Annotation, Visualization and Integrated Discovery 2008" (DAVID - http://david.abcc. ncifcrf.gov) revealed the initiation of the cell death process in sensory epithelium and modiolus. An increase in Fas and phosphorylation of FAK and p38/MAPK in the sensory epithelium suggest that noise-induced stress signals at the cell membrane are transmitted to the nucleus by Fas and focal adhesion signaling through the p38/MAPK signaling pathway. Up-regulation of downstream nuclear proteins E2F3 and WSTF in immunoblots and microarrays along with their immunolocalization in the outer hair cells supported the pivotal role of p38/MAPK signaling in the mechanism underlying noise-induced hearing loss.

Protection against acoustic trauma by direct application of D-methionine to the inner ear

CONCLUSION

The findings from this study extend the use of the local application of D-methionine (D-met) to protect against acoustic trauma and demonstrate that D-met slowly diffuses from the perilymph.

OBJECTIVES

The objectives of the study were to determine the effect of D-met on auditory function and morphology after acoustic trauma and to measure the concentration of D-met in perilymph.

METHODS

Auditory thresholds were determine before, immediately after, and 24 h after acoustic trauma. Cochleae were analyzed using immunocytochemistry for c-Fos, TUJI, and cytochrome c. The concentration of D-met was determined from perilymph.

RESULTS

Protection against acoustic trauma (immediately and 24 h post trauma) on auditory brainstem thresholds was found at a time when the concentration of D-met in perilymph showed a fivefold increase above basal levels. The local application of D-met to the guinea pig cochlea results in elevated D-met concentrations that are maintained in the perilymph for at least 24 h.

Apoptosis in acquired and genetic hearing impairment: the programmed death of the hair cell

Apoptosis is an important physiological process. Normally, a healthy cell maintains a delicate balance between pro- and anti-apoptotic factors, allowing it to live and proliferate. It is thus not surprising that disturbance of this delicate balance may result in disease. It is a well known fact that apoptosis also contributes to several acquired forms of hearing impairment. Noise-induced hearing loss is the result of prolonged exposure to excessive noise, triggering apoptosis in terminally differentiated sensory hair cells. Moreover, hearing loss caused by the use of therapeutic drugs such as aminoglycoside antibiotics and cisplatin potentially may result in the activation of apoptosis in sensory hair cells leading to hearing loss due to the "ototoxicity" of the drugs. Finally, apoptosis is a key contributor to the development of presbycusis, age-related hearing loss. Recently, several mutations in apoptosis genes were identified as the cause of monogenic hearing impairment. These genes are TJP2, DFNA5 and MSRB3. This implies that apoptosis not only contributes to the pathology of acquired forms of hearing impairment, but also to genetic hearing impairment as well. We believe that these genes constitute a new functional class within the hearing loss field. Here, the contribution of apoptosis in the pathology of both acquired and genetic hearing impairment is reviewed.

Emerging treatments for noise-induced hearing loss

INTRODUCTION

Approximately 5% of the population worldwide suffers from industrial, military or recreational noise-induced hearing loss (NIHL) at a great economic cost and detriment to the quality of life of the affected individuals. This review discusses pharmacological strategies to attenuate NIHL that have been developed in animal models and that are now beginning to be tested in field trials.

AREAS COVERED

The review describes the epidemiology, pathology and pathophysiology of NIHL in experimental animals and humans. The underlying molecular mechanisms of damage are then discussed as a basis for therapeutic approaches to ameliorate the loss of auditory function. Finally, studies in military, industrial and recreational settings are evaluated. Literature was searched using the terms 'noise-induced hearing loss' and 'noise trauma'.

EXPERT OPINION

NIHL, in principle, can be prevented. With the current pace of development, oral drugs to protect against NIHL should be available within the next 5-10 years. Positive results from ongoing trials combined with additional laboratory tests might accelerate the time from the bench to clinical treatment.

Intracochlear perfusion of leupeptin and z-VAD-FMK: influence of antiapoptotic agents on gunshot-induced hearing loss

The therapeutic efficiency of cochlear infusion of two anti-apoptotic substances: a potent calpain inhibitor, leupeptin and a caspase inhibitor, z-VAD-FMK was evaluated in guinea pigs after a gunshot noise-induced trauma (170 dB SPL). A preliminary study showed that hair cell apoptosis appeared within 7 days of the noise trauma. For each animal, one of the cochleae was perfused directly starting 1 h after the trauma with leupeptin or z-VAD-FMK for 7 days via a mini-osmotic pump whereas the other cochlea was untreated. ABR threshold shifts were measured over a 14-day recovery period. The functional hearing study was supplemented by histological analysis. Two days after the trauma significant differences were observed between threshold shifts in the z-VAD-FMK-treated and the non-treated ears. Cochleograms showed that hair cell losses were significantly lower in z-VAD-FMK-treated ears. Regarding the leupeptin treatment, no significant difference between treated and non-treated ears was observed. This work indicates that early direct infusion of z-VAD-FMK into the cochlea accelerates hearing recovery and reduces hair cell loss after gunshot noise-induced trauma. These results suggest that the gunshot noise-induced trauma may involve the caspase pathway rather than the calpain pathway in the apoptotic process.