Involvement of retinoic acid-induced peroxiredoxin 6 expression in recovery of noise-induced temporary hearing threshold shifts

All-trans retinoic acid (ATRA) is reported to reduce hair cell loss and hearing deterioration caused by noise-induced hearing loss (NIHL). The present study investigates the involvement of peroxiredoxin 6 (Prdx 6) in ATRA-mediated protection of temporary threshold shift of hearing. Mice fed with ATRA before or after exposure to white noise showed a faster recovery than untreated controls within 1 week, with a concomitant increase of cochlear Prdx 6 expression. Treatment of mouse auditory cells with ATRA induced Prdx 6 expression. A putative retinoic acid (RA)-response element (RARE) was identified in a murine Prdx 6 promoter region. Prdx 6 promoter activities were elevated in wild-type reporter plasmid-transfected cells, whereas no significant change in activity was in those with RARE-disrupted mutant reporter. RA receptor α (RARα) functions as a transactivator of Prdx 6 gene expression. These findings suggest that ATRA-induced Prdx 6 expression may be associated with rapid recovery from temporary NIHL.

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.

Noise-induced cochlear neuropathy is selective for fibers with low spontaneous rates

Acoustic overexposure can cause a permanent loss of auditory nerve fibers without destroying cochlear sensory cells, despite complete recovery of cochlear thresholds (Kujawa and Liberman 2009), as measured by gross neural potentials such as the auditory brainstem response (ABR). To address this nominal paradox, we recorded responses from single auditory nerve fibers in guinea pigs exposed to this type of neuropathic noise (4- to 8-kHz octave band at 106 dB SPL for 2 h). Two weeks postexposure, ABR thresholds had recovered to normal, while suprathreshold ABR amplitudes were reduced. Both thresholds and amplitudes of distortion-product otoacoustic emissions fully recovered, suggesting recovery of hair cell function. Loss of up to 30% of auditory-nerve synapses on inner hair cells was confirmed by confocal analysis of the cochlear sensory epithelium immunostained for pre- and postsynaptic markers. In single fiber recordings, at 2 wk postexposure, frequency tuning, dynamic range, postonset adaptation, first-spike latency and its variance, and other basic properties of auditory nerve response were all completely normal in the remaining fibers. The only physiological abnormality was a change in population statistics suggesting a selective loss of fibers with low- and medium-spontaneous rates. Selective loss of these high-threshold fibers would explain how ABR thresholds can recover despite such significant noise-induced neuropathy. A selective loss of high-threshold fibers may contribute to the problems of hearing in noisy environments that characterize the aging auditory system.

Review of interventions to increase hearing protective device use in youth who live or work on farms

AIMS AND OBJECTIVES

To identify and compare hearing protection interventions for youth working and living on farms.

BACKGROUND

Noise-induced hearing loss represents a significant risk factor for injury, disability and death in agricultural populations due to an inability to receive sensory information that can indicate dangerous situations. Despite the availability of hearing protection devices, rates of utilisation remain low, while rates of noise-induced hearing loss are high within the agricultural youth population.

DESIGN

Comprehensive review of the literature.

METHOD

Electronic database searches were conducted to identify research studies of hearing protection interventions for youth living or working on farms.

RESULTS

Three intervention studies were identified as meeting the review criteria, with additional reanalysis studies based on the original intervention studies. The intervention studies demonstrated increased use of hearing protection devices among youth who received the interventions compared to the control groups although audiometry testing results were not improved as a result of the intervention.

CONCLUSIONS

The findings of this review highlight the need for additional research into the effectiveness of hearing protection device use as a noise-induced hearing loss preventative measure in youth who live or work on farms and the creation of effective interventions to increase hearing protection device use and decrease Noise-induced hearing loss risk.

RELEVANCE TO CLINICAL PRACTICE

It is recommended that nurses be educated in the areas of health promotion relevant to farming communities to be able to effectively meet their rural clients' needs in relation to noise-induced hearing loss. Working with youth, families, and schools, as well as developing community partnerships, can facilitate the dissemination of hearing protection promotion.

Association between hearing loss and saccular dysfunction in older individuals

OBJECTIVE

1) Describe the association between hearing loss and dysfunction of each of the 5 vestibular end-organs--the horizontal, superior, and posterior semicircular canals; saccule; and utricle--in older individuals. 2) Evaluate whether hearing loss and vestibular end-organ deficits share any risk factors.

STUDY DESIGN

Cross-sectional study.

SETTING

Academic medical center.

PATIENTS

Fifty-one individuals age 70 years or older.

INTERVENTIONS

Audiometry, head-thrust dynamic visual acuity (htDVA), sound-evoked cervical vestibular-evoked myogenic potential (cVEMP), and tap-evoked ocular VEMP (oVEMP).

MAIN OUTCOME MEASURES

Audiometric pure-tone averages (PTA), htDVA LogMAR scores as a measure of semicircular canal function in each canal plane, and cVEMP and oVEMP amplitudes as a measure of saccular and utricular function, respectively.

RESULTS

We observed a significant correlation between hearing loss at high frequencies and reduced cVEMP amplitudes (or reduced saccular function; r = -0.37, p < 0.0001) in subjects age 70 years or older. In contrast, hearing loss was not associated with oVEMP amplitudes (or utricular function), or htDVA LogMAR scores (or semicircular canal function) in any of the canal planes. Age and noise exposure were significantly associated with measures of both cochlear and saccular dysfunction.

CONCLUSION

The concomitant decline in the cochlear and saccular function associated with aging may reflect their common embryologic origin in the pars inferior of the labyrinth. Noise exposure seems to be related to both saccular and cochlear dysfunction. These findings suggest a potential benefit of screening individuals with presbycusis-particularly those with significant noise exposure history-for saccular dysfunction, which may contribute to fall risk in the elderly.

Noise-induced hearing loss and associated factors among vector control workers in a Malaysian state

This study aims to determine the prevalence and associated factors of noise-induced hearing loss (NIHL) among vector control workers in the state of Negeri Sembilan, Malaysia. This was an analytical cross-sectional study conducted on 181 vector control workers who were working in district health offices in a state in Malaysia. Data were collected using a self-administered questionnaire and audiometry. Prevalence of NIHL was 26% among this group of workers. NIHL was significantly associated with the age-group of 40 years and older, length of service of 10 or more years, current occupational noise exposure, listening to loud music, history of firearms use, and history of mumps/measles infection. Following logistic regression, age of more than 40 years and noise exposure in current occupation were associated with NIHL with an odds ratio of 3.45 (95% confidence interval = 1.68-7.07) and 6.87 (95% confidence interval = 1.54-30.69), respectively, among this group of vector control workers.

Permanent hearing loss among professional spice grinders in an urban community in southwest Nigeria

This study set out to determine the pattern and predictors of noise-induced hearing loss (NIHL) among small-scale and self-employed chili pepper grinders in Lagos, Southwest Nigeria. Audiological evaluation was conducted for all participants after noise level measurement. Of 136 studied, 85 (62.5%) were confirmed with slight-to-moderate NIHL. Mean age was 40.2 years, mean years spent as grinders was 9.3 years and mean hours spent daily at work was 13.3 hours. The mean age of those with NIHL was significantly higher than those without. Spending over 10 years in commercial grinding and working ≤12 hours daily were predictive of NIHL. Questionnaire-based screening using symptoms of NIHL was associated with a sensitivity of 44.7%, specificity of 62.7%, and positive predictive value of 66.7%. In conclusion, pepper grinding is associated with high/excessive noise levels and NIHL. Hearing conservation program incorporating engineering modification of locally fabricated grinders is warranted in this and similar populations in developing countries.

Measurement of impulse peak insertion loss for four hearing protection devices in field conditions

OBJECTIVE

In 2009, the U.S. Environmental Protection Agency (EPA) proposed an impulse noise reduction rating (NRR) for hearing protection devices based upon the impulse peak insertion loss (IPIL) methods in the ANSI S12.42-2010 standard. This study tests the ANSI S12.42 methods with a range of hearing protection devices measured in field conditions.

DESIGN

The method utilizes an acoustic test fixture and three ranges for impulse levels: 130-134, 148-152, and 166-170 dB peak SPL. For this study, four different models of hearing protectors were tested: Bilsom 707 Impact II electronic earmuff, E·A·R Pod Express, E·A·R Combat Arms version 4, and the Etymotic Research, Inc. Electronic BlastPLG™ EB1.

STUDY SAMPLE

Five samples of each protector were fitted on the fixture or inserted in the fixture's ear canal five times for each impulse level. Impulses were generated by a 0.223 caliber rifle.

RESULTS

The average IPILs increased with peak pressure and ranged between 20 and 38 dB. For some protectors, significant differences were observed across protector examples of the same model, and across insertions.

CONCLUSIONS

The EPA's proposed methods provide consistent and reproducible results. The proposed impulse NRR rating should utilize the minimum and maximum protection percentiles as determined by the ANSI S12.42-2010 methods.

Awarding and promoting excellence in hearing loss prevention

OBJECTIVE

To describe the rationale and creation of a national award to recognize and promote hearing loss prevention.

DESIGN

In 2007, the National Institute for Occupational Safety and Health partnered with the National Hearing Conservation Association to create the Safe-in-Sound Excellence in Hearing Loss Prevention Award™ ( www.safeinsound.us ). The objectives of this initiative were to recognize organizations that document measurable achievements and to share leading edge information to a broader community.

RESULTS

An expert committee developed specific and explicit award evaluation criteria of excellence in hearing loss prevention for organizations in different industrial sectors. The general approach toward award criteria was to incorporate current 'best practices' and familiar benchmarks of hearing loss prevention programs. This approach was reviewed publicly. In addition, mechanisms were identified to measure the impact of the award itself. Interest in the award was recorded through the monitoring of the visitor traffic registered by the award web site and is increasing yearly. Specific values and strategies common across award winners are presented.

CONCLUSION

The Safe-in-Sound Award™ has obtained high quality field data; identified practical solutions, disseminated successful strategies to minimize the risk of hearing loss, generated new partnerships, and shared practical solutions with others in the field.

A connection between the Efferent Auditory System and Noise-Induced Tinnitus Generation. Reduced contralateral suppression of TEOAEs in patients with noise-induced tinnitus

BACKGROUND

Subjective tinnitus is an auditory perception that is not caused by external stimulation, its source being anywhere in the auditory system. Furthermore, evidence exists that exposure to noise alters cochlear micromechanics, either directly or through complex feed-back mechanisms, involving the medial olivocochlear efferent system. The aim of this study was to assess the role of the efferent auditory system in noise-induced tinnitus generation.

MATERIAL/METHODS

Contralateral sound-activated suppression of TEOAEs was performed in a group of 28 subjects with noise-induced tinnitus (NIT) versus a group of 35 subjects with normal hearing and tinnitus, without any history of exposure to intense occupational or recreational noise (idiopathic tinnitus-IT). Thirty healthy, normally hearing volunteers were used as controls for the efferent suppression test.

RESULTS

Suppression of the TEOAE amplitude less than 1 dB SPL was considered abnormal, giving a false positive rate of 6.7%. Eighteen out of 28 (64.3%) patients of the NIT group and 9 out of 35 (25.7%) patients of the IT group showed abnormal suppression values, which were significantly different from the controls' (p<0.0001 and p<0.045, respectively).

CONCLUSIONS

The abnormal activity of the efferent auditory system in NIT cases might indicate that either the activity of the efferent fibers innervating the outer hair cells (OHCs) is impaired or that the damaged OHCs themselves respond abnormally to the efferent stimulation.

Different cellular and genetic basis of noise-related endocochlear potential reduction in CBA/J and BALB/cJ mice

The acute and permanent effects of noise exposure on the endocochlear potential (EP) and cochlear lateral wall were evaluated in BALB/cJ (BALB) inbred mice, and compared with CBA/J (CBA) and C57BL/6 (B6) mice. Two-hour exposure to broadband noise (4-45 kHz) at 110 dB SPL leads to a approximately 50 mV reduction in the EP in BALB and CBA, but not B6. EP reduction in BALB and CBA is reliably associated with characteristic acute cellular pathology in stria vascularis and spiral ligament. By 8 weeks after exposure, the EP in CBA mice has returned to normal. In BALBs, however, the EP remains depressed by an average approximately 10 mV, so that permanent EP reduction contributes to permanent threshold shifts in these mice. We recently showed that the CBA noise phenotype in part reflects the influence of a large effect quantitative trait locus on Chr. 18, termed Nirep (Ohlemiller et al., Hear Res 260:47-53, 2010b). While CBA "EP susceptibility" alleles are dominant to those in B6, examination of (B6 × BALB) F1 hybrid mice and (F1 × BALB) N2 backcross mice revealed that noise-related EP reduction and associated cell pathology in BALBs are inherited in an autosomal recessive manner, and are dependent on multiple genes. Moreover, while N2 mice formed from B6 and CBA retain strong correspondence between acute EP reduction, ligament pathology, and strial pathology, N2s formed from B6 and BALB include subsets that dissociate pathology of ligament and stria. We conclude that the genes and cascades that govern the very similar EP susceptibility phenotypes in BALB and CBA mice need not be the same. BALBs appear to carry alleles that promote more pronounced long term effects of noise on the lateral wall. Separate loci in BALBs may preferentially impact stria versus ligament.

The more the worse: the grade of noise-induced hearing loss associates with the severity of tinnitus

Tinnitus disturbs lives and negatively affects the quality of life of about 2% of the adult world population. Research has shown that the main cause of tinnitus is hearing loss. To analyze a possible association of the degree of hearing loss with the severity of tinnitus, we have performed a retrospective study using admission data on 531 patients suffering from chronic tinnitus. We have found that 83% of our tinnitus patients had a high frequency hearing loss corresponding to a noise-induced hearing loss (NIHL). There was a significant correlation between the mean hearing loss and the tinnitus loudness (p < 0.0001). Interestingly, patients suffering from decompensated chronic tinnitus had a greater degree of hearing loss than the patients with compensated form of tinnitus. In addition, we demonstrate that the degree of hearing loss positively correlates with the two subscales ("intrusiveness" and "auditory perceptional difficulties") of the Tinnitus Questionnaire. Our retrospective study provides indirect evidence supporting the hypothesis that the degree of noise-induced hearing loss influences the severity of tinnitus.

The role of glucocorticoids for spiral ganglion neuron survival

Glucocorticoids, which are steroidal stress hormones, have a broad array of biological functions. Synthetic glucocorticoids are frequently used therapeutically for many pathologic conditions, including diseases of the inner ear; however, their exact functions in the cochlea are not completely understood. Recent work has clearly demonstrated the presence of glucocorticoid signaling pathways in the cochlea and elucidated their protective roles against noise-induced hearing loss. Furthermore, indirect evidence suggests the involvement of glucocorticoids in age-related loss of spiral ganglion neurons and extensive studies in the central nervous system demonstrate profound effects of glucocorticoids on neuronal functions. With the advancement of recent pharmacologic and genetic tools, the role of these pathways in the survival of spiral ganglion neurons after noise exposure and during aging should be revealed.

Knowledge, attitude and practice of sawmill workers towards noise-induced hearing loss in kota bharu, kelantan

The purpose of this study was to determine the sawmill workers' knowledge, attitude and practice (KAP) in relation to noise-induced hearing loss (NIHL). A cross-sectional study was conducted involving 83 workers from 3 factories in Kota Bharu, Kelantan. Questionnaires were distributed to obtain the socio-demography, knowledge, attitude and practice level in relation to noise-induced hearing loss (NIHL). The weak areas identified in the knowledge section were treatment aspects (15.5%), signs and symptoms of NIHL (20.2%) and risk factors (31%). As for attitude; the prevention aspects were the lowest (25.3%), followed by risk taking attitude (26.2%), and causes of hearing loss (42.1%). Overall, the practice was not encouraging at all. It is important to have an education program to raise workers' awareness and to improve their attitude and practices towards noise-induced hearing loss.

Occupational noise-induced hearing loss in India

Noise is the insidious of all industrial pollutants, involving every industry and causing severe hearing loss in every country in the world. Exposure to excessive noise is the major avoidable cause of permanent hearing impairment. Worldwide, 16% of the disabling hearing loss in adults is attributed to occupational noise, ranging from 7 to 21% in the various subregions. The estimated cost of noise to developed countries ranges from 0.2 to 2% of the gross domestic product (GDP). Noise-induced hearing loss (NIHL) is bilateral and symmetrical, usually affecting the higher frequencies (3k, 4k or 6k Hz) and then spreading to the lower frequencies (0.5k, 1k or 2k Hz). Other major health effects are lack of concentration, irritation, fatigue, headache, sleep disturbances, etc. The major industries responsible for excessive noise and exposing workers to hazardous levels of noise are textile, printing, saw mills, mining, etc. Hearing protectors should be used when engineering controls and work practices are not feasible for reducing noise exposure to safe levels. Earmuffs, ear plugs and ear canal caps are the main types of hearing protectors. In India, NIHL has been a compensable disease since 1948. It is only in 1996 that the first case got compensation. Awareness should be created among workers about the harmful effects of noise on hearing and other body systems by implementing compulsory education and training programs. There are very few published studies of NIHL in India. More extensive studies are needed to know the exact prevalence of NIHL among the various industries in India.

Noise-induced hearing loss and hearing aids requirement

Subjective disturbances, due to hearing loss, are auditory disability and handicap which can be evaluated with a questionnaire. The present study refers to a population of industrial workers affected by noise-induced hearing loss. Aim of the study is to identify the minimal level of hearing loss over which the patient felt changes in his quality of life, and the average auditory threshold at which the patient considered the application of a hearing aid useful or necessary. The sample comprised 180 males with noise-induced hearing loss. Each subject received a questionnaire designed for this study. Data show a correlation between disability, handicap and the degree of noise-induced hearing loss. The most relevant problems in noise-induced hearing loss are correlated with disability rather than handicap. 35 dB can be considered as the level above which these devices can be suggested to patients. Hearing aids can become a therapeutic instrument even in the presence of a low degree of hearing loss.

Loud music listening

Over the past four decades, there has been increasing interest in the effects of music listening on hearing. The purpose of this paper is to review published studies that detail the noise levels, the potential effects (e.g. noise-induced hearing loss), and the perceptions of those affected by music exposure in occupational and non-occupational settings. The review employed Medline, PubMed, PsychINFO, and the World Wide Web to find relevant studies in the scientific literature. Considered in this review are 43 studies concerning the currently most significant occupational sources of high-intensity music: rock and pop music playing and employment at music venues, as well as the most significant sources of non-occupational high-intensity music: concerts, dicotheques (clubs), and personal music players. Although all of the activities listed above have the potential for hearing damage, the most serious threat to hearing comes from prolonged exposures to amplified live music (concerts). The review concludes that more research is needed to clarify the hearing loss risks of music exposure from personal music players and that current scientific literature clearly recognizes an unmet hearing health need for more education regarding the risks of loud music exposure and the benefits of wearing hearing protection, for more hearing protection use by those at risk, and for more regulations limiting music intensity levels at music entertainment venues.

A systematic review of the interventions to promote the wearing of hearing protection

CONTEXT AND OBJECTIVE

Noise-induced hearing loss can only be prevented by eliminating or lowering noise exposure levels. When the source of the noise cannot be eliminated, workers have to rely on hearing protection equipment. The aim here was to summarize the evidence for the effectiveness of interventions to enhance the wearing of hearing protection among workers exposed to noise in the workplace.

DATA SOURCE

Studies with random assignment were identified by an electronic search of the medical literature up to 2005. Data were double-entered into the Review Manager software, version 4.2.5.

DATA SYNTHESIS

Two studies were found. A computer-based intervention tailored to individual workers risks and lasting 30 minutes was not found to be more effective than a video providing general information for workers. A second randomized controlled trial evaluated the effect of a four-year school-based hearing loss prevention program among schoolchildren working on their parents farms. The intervention group was twice as likely to wear some kind of hearing protection as was the control group (which received only minimal intervention). REVIEWERS CONCLUSIONS: The limited evidence does not show whether tailored interventions are more or less effective than general interventions among workers, 80% of whom already use hearing protection. Long-lasting school-based interventions may increase the use of hearing protection substantially. Better interventions to enhance the use of hearing protection need to be developed and evaluated in order to increase the prevention of noise-induced hearing loss among workers.

Pilot survey of subway and bus stop noise levels

Excessive noise exposure is a serious global urban health problem, adversely affecting millions of people. One often cited source of urban noise is mass transit, particularly subway systems. As a first step in determining risk within this context, we recently conducted an environmental survey of noise levels of the New York City transit system. Over 90 noise measurements were made using a sound level meter. Average and maximum noise levels were measured on subway platforms, and maximum levels were measured inside subway cars and at several bus stops for comparison purposes. The average noise level measured on the subway platforms was 86 +/- 4 dBA (decibel-A weighting). Maximum levels of 106, 112, and 89 dBA were measured on subway platforms, inside subway cars, and at bus stops, respectively. These results indicate that noise levels in subway and bus stop environments have the potential to exceed recommended exposure guidelines from the World Health Organization (WHO) and U.S. Environmental Protection Agency (EPA), given sufficient exposure duration. Risk reduction strategies following the standard hierarchy of control measures should be applied, where feasible, to reduce subway noise exposure.

Current aspects of hearing loss from occupational and leisure noise

Hearing loss from occupational and leisure noise numbers amongst the most frequent causes of an acquired sensorineural hearing loss. Here we present a review of up-to-date findings on the pathophysiology of acoustic injury to the inner ear, with special attention being paid to its molecular-biological and genetic aspects. Epidemiological aspects shall also be dealt with, as shall the roles of lacking recovery from occupational noise due to additional exposure by leisure noise and the combined exposure of noise and chemicals. Based on the epidemiological and pathophysiological findings and against the background of published animal-experimental, pre-clinical and clinical findings, the various approaches for prevention, protection and therapeutic intervention with acoustic trauma are discussed. Pharmacological strategies involving anti-oxidative, anti-excitotoxic and anti-apoptotic substances as well as non-pharmacological strategies like "sound conditioning" are given attention. Furthermore, systemic and local substance application as well as the therapy of acute acoustic trauma and chronic hearing problems (including modern therapy forms for comorbidities such as tinnitus) shall be delved into.

Protection from acoustic trauma is not a primary function of the medial olivocochlear efferent system

The medial olivocochlear (MOC) efferent system is an important component of an active mechanical outer hair cell system in mammals. An extensive neurophysiological literature demonstrates that the MOC system attenuates the response of the cochlea to sound by reducing the gain of the outer hair cell mechanical response to stimulation. Despite a growing understanding of MOC physiology, the biological role of the MOC system in mammalian audition remains uncertain. Some evidence suggests that the MOC system functions in a protective role by acting to reduce receptor damage during intense acoustic exposure. For the MOC system to have evolved as a protective mechanism, however, the inner ears of mammals must be exposed to potentially damaging sources of noise that can elicit MOC-mediated protective effects under natural conditions. In this review, we evaluate the possibility that the MOC system evolved to protect the inner ear from naturally occurring environmental noise. Our survey of nonanthropogenic noise levels shows that while sustained sources of broadband noise are found in nearly all natural acoustic environments, frequency-averaged ambient noise levels in these environments rarely exceed 70 dB SPL. Similarly, sources reporting ambient noise spectra in natural acoustic environments suggest that noise levels within narrow frequency bands are typically low in intensity (<40 dB SPL). Only in rare instances (e.g., during frog choruses) are ambient noise levels sustained at moderately high intensities (~70-90 dB SPL). By contrast, all experiments in which an MOC-mediated protective effect was demonstrated used much higher sound intensities to traumatize the cochlea (100-150 dB SPL). This substantial difference between natural ambient noise levels and the experimental conditions necessary to evoke MOC-mediated protection suggests that even the noisiest natural acoustic environments are not sufficiently intense to have selected for the evolution of the MOC system as a protective mechanism. Furthermore, although relatively intense noise environments do exist in nature, they are insufficiently distributed to account for the widespread distribution of the MOC system in mammals. The paucity of high-intensity noise and the near ubiquity of low-level noise in natural environments supports the hypothesis that the MOC system evolved as a mechanism for "unmasking" biologically significant acoustic stimuli by reducing the response of the cochlea to simultaneous low-level noise. This suggested role enjoys widespread experimental support.

A peptide inhibitor of c-Jun N-terminal kinase protects against both aminoglycoside and acoustic trauma-induced auditory hair cell death and hearing loss

Hearing loss can be caused by a variety of insults, including acoustic trauma and exposure to ototoxins, that principally effect the viability of sensory hair cells via the MAP kinase (MAPK) cell death signaling pathway that incorporates c-Jun N-terminal kinase (JNK). We evaluated the otoprotective efficacy of D-JNKI-1, a cell permeable peptide that blocks the MAPK-JNK signal pathway. The experimental studies included organ cultures of neonatal mouse cochlea exposed to an ototoxic drug and cochleae of adult guinea pigs that were exposed to either an ototoxic drug or acoustic trauma. Results obtained from the organ of Corti explants demonstrated that the MAPK-JNK signal pathway is associated with injury and that blocking of this signal pathway prevented apoptosis in areas of aminoglycoside damage. Treatment of the neomycin-exposed organ of Corti explants with D-JNKI-1 completely prevented hair cell death initiated by this ototoxin. Results from in vivo studies showed that direct application of D-JNKI-1 into the scala tympani of the guinea pig cochlea prevented nearly all hair cell death and permanent hearing loss induced by neomycin ototoxicity. Local delivery of D-JNKI-1 also prevented acoustic trauma-induced permanent hearing loss in a dose-dependent manner. These results indicate that the MAPK-JNK signal pathway is involved in both ototoxicity and acoustic trauma-induced hair cell loss and permanent hearing loss. Blocking this signal pathway with D-JNKI-1 is of potential therapeutic value for long-term protection of both the morphological integrity and physiological function of the organ of Corti during times of oxidative stress.

Efferent protection from acoustic injury is mediated via alpha9 nicotinic acetylcholine receptors on outer hair cells

Exposure to intense sound can damage the mechanosensors of the inner ear and their afferent innervation. These neurosensory elements are innervated by a sound-activated feedback pathway, the olivocochlear efferent system. One major component of this system is cholinergic, and known cholinergic effects are mediated by the alpha9/alpha10 nicotinic acetylcholine receptor (nAChR) complex. Here, we show that overexpression of alpha9 nAChR in the outer hair cells of bacterial artificial chromosome transgenic mice significantly reduces acoustic injury from exposures causing either temporary or permanent damage, without changing pre-exposure cochlear sensitivity to low- or moderate-level sound. These data demonstrate that efferent protection is mediated via the alpha9 nAChR in the outer hair cells and provide direct evidence for a protective role, in vivo, of a member of the nAChR family.

Hearing loss associated with the modifier of deaf waddler (mdfw) locus corresponds with age-related hearing loss in 12 inbred strains of mice

The modifier of deaf waddler (mdfw) and age-related hearing loss (Ahl) loci were both discovered as inbred strain polymorphisms that affect hearing loss in mice. Both loci map to the same position on chromosome (Chr) 10. The mdfw locus interacts epistatically with the deaf waddler (dfw) mutation on Chr 6, and the Ahl locus is a major contributor to AHL in several inbred strains. To investigate the possibility of allelism, we examined the correspondence of mdfw and Ahl phenotypes among 12 inbred mouse strains. The effects of strain-specific mdfw alleles on hearing loss were assessed in dfw2J/+ F1 hybrids produced from mating BALB-dfw2J/+ mice with mice from each of 12 inbred strains. F1 hybrids were then assessed for hearing by auditory-evoked brainstem response threshold analysis and classified as dfw2J/+ or +/+ by polymerase chain reaction typing. Heterozygosity for dfw2J accelerated hearing loss in F1 hybrids derived from all strains tested, except those produced with the B6.CAST + Ahl congenic strain. dfw2J/+ F1 hybrids derived from parental strains 129P1/ReJ, A/J, BUB/BnJ, C57BR/cdJ, DBA/2J, NOD/LtJ and SKH2/J exhibited a severe hearing loss by 12 weeks of age. Those derived from strains 129T2/SvEmsJ, C3H/HeJ, CBA/CaJ and NON/LtJ exhibited only a slight to intermediate hearing loss at that age. The hearing loss associated with these strain-specific mdfw alleles corresponds with previously determined Ahl allele effects, providing additional evidence that mdfw and Ahl are manifestations of the same gene. A functional relationship therefore may exist between the Ca2+ transporting activity of the dfw gene (Atp2b2) and AHL.