Post-exposure treatment attenuates noise-induced hearing loss

Effect of low-level laser treatment on cochlea hair-cell recovery after acute acoustic trauma

We investigated the effect of low-level laser radiation on rescuing hair cells of the cochlea after acute acoustic trauma and hearing loss. Nine rats were exposed to noise. Starting the following day, the left ears (NL ears) of the rats were irradiated at an energy output of 100 to 165 mW/cm(2) for 60 min for 12 days in a row. The right ears (N ears) were considered as the control group. Frequency-specific hearing levels were measured before the noise exposure and also after the 1st, 3rd to 5th, 8th to 10th and 12th irradiations. After the 12th treatment, hair cells were observed using a scanning electron microscope. Compared to initial hearing levels at all frequencies, thresholds increased markedly after noise exposure. After the 12th irradiation, hearing threshold was significantly lower for the NL ears compared to the N ears. When observed using an electron microscope, the number of hair cells in the middle turn of the NL ears was significantly larger than that of the N ears. Our findings suggest that low-level laser irradiation promotes recovery of hearing thresholds after acute acoustic trauma.

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

Background

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

Material/Methods

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

Results

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

Conclusions

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

Cisplatin-induced ototoxicity is mediated by nitroxidative modification of cochlear proteins characterized by nitration of Lmo4

Tyrosine nitration is an important sequel of cellular signaling induced by reactive oxygen species. Cisplatin is an anti-neoplastic agent that damages the inner ear through reactive oxygen species and by the formation of DNA adducts. This study reveals a correlation between cisplatin-mediated hearing loss and nitroxidative modification of cochlear proteins and is the first to report nitration of Lmo4. Cisplatin induced a dose-dependent increase in hearing loss in Wistar rats. A 10-15-dB decrease in distortion product amplitude and massive loss of outer hair cells at the basal turn of the cochlea was observed 3 days post-treatment after a 16 mg/kg dose. Cisplatin induced nitration of cellular proteins within the organ of Corti, spiral ganglion, and stria vascularis, which are known targets of cisplatin ototoxicity. Nitration of a 76-kDa cochlear protein correlated with cisplatin dose. The nitrated protein was identified as Lmo4 (LIM domain only 4) by MALDI-TOF (matrix-assisted laser desorption/ionization time of flight) mass spectrometry and confirmed by reciprocal immunoprecipitation and immunoblotting. Co-localization of nitrotyrosine and Lmo4 was particularly high in outer hair cell nuclei after cisplatin treatment. Cochlear levels of Lmo4 were decreased in rats treated with cisplatin. In vitro studies supported the repression of Lmo4 in nitroxidative conditions and the induction of apoptosis upon repression of Lmo4. Inhibition of cochlear protein nitration prevented cisplatin-induced hearing loss. As Lmo4 is a transcriptional regulator that controls the choice between cell survival and cell death, these results support the hypothesis that nitration of Lmo4 influences cisplatin-induced ototoxicity.

Radix Astragali injection enhances recovery from acute acoustic trauma

CONCLUSION

The average recovery of hearing and cessation of tinnitus was significantly better after treatment with Radix Astragali (RA) than after non-treatment with RA. RA can be valuable adjuvant therapy for patients with acute acoustic trauma (AAT).

OBJECTIVES

AAT is one of the early indications for the use of RA. The reasons for administering RA to patients with AAT are based on experimental studies showing that noise exposure results in the formation of reactive oxygen species (ROS), which trigger metabolic damage to the organ of Corti. RA is a natural antioxidant. The aim of this study was to investigate the efficacy of RA in patients with AAT.

METHODS

We compared the recovery from hearing impairment and tinnitus in 40 ears treated with RA with 40 ears treated with non-RA. RA was given intravenously daily for 10 days. There were no significant differences in clinical or audiological data between RA and non-RA groups.

RESULTS

The average recovery of hearing at both high and speech frequencies was significantly better and tinnitus persisted less commonly in the RA group than in the non-RA group. Normal hearing at the end of the follow-up period was regained in 27 ears in the RA group and in 21 ears in the non-RA group (p < 0.01).

Reduced formation of oxidative stress biomarkers and migration of mononuclear phagocytes in the cochleae of chinchilla after antioxidant treatment in acute acoustic trauma

Objective. Inhibition of inflammation and free radical formation in the cochlea may be involved in antioxidant treatment in acute acoustic trauma. Procedure. Chinchilla were exposed to 105 dB sound pressure level octave band noise for 6 hours. One group of chinchilla was treated with antioxidants after noise exposure. Auditory brainstem responses, outer hair cell counts, and immunohistochemical analyses of biomarkers in the cochlea were conducted. Results. The antioxidant treatment significantly reduced hearing threshold shifts, outer hair cell loss, numbers of CD45⁺ cells, as well as 4-hydroxy-2-nonenal and nitrotyrosine formation in the cochlea. Conclusion. Antioxidant treatment may provide protection to sensory cells by inhibiting formation of reactive oxygen and nitrogen products and migration of mononuclear phagocytes in the cochlea. The present study provides further evidence of effectiveness of antioxidant treatment in reducing permanent hearing loss.

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.

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

CONCLUSIONS

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

OBJECTIVE

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

METHODS

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

RESULTS

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

Astragaloside IV attenuates impulse noise-induced trauma in guinea pig

CONCLUSION

These results suggest that the beneficial effect of astragaloside IV on impulse noise-induced hearing loss may be due to its ability to inhibit inducible nitric oxide synthase (iNOS) and prevent the formation of reactive nitrogen species (RNS).

OBJECTIVE

Astragaloside IV is the major active constituent of Astragalus membranaceus, which has been widely used for the treatment of diseases in China due to its antioxidant properties. iNOS and RNS are involved in damage induced by impulse noise trauma. The purpose of the present study was to investigate if astragaloside IV has the potential to reduce cochlear damage from impulse noise.

METHODS

Guinea pigs in the experimental group were administered astragaloside IV intragastrically. Auditory thresholds were assessed by sound-evoked auditory brainstem response (ABR) at click and tone bursts of 8, 16 and 32 kHz, 72 h before and after exposure to impulse noise. iNOS and nitrotyrosine were determined immunohistochemically. Hair cell damage was analyzed by scanning electron microscopy.

RESULTS

Astragaloside IV significantly reduced ABR deficits, reduced hair cell damage, and decreased the expression of iNOS and RNS formation.

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.

Nutrient-enhanced diet reduces noise-induced damage to the inner ear and hearing loss

Oxidative stress has been implicated broadly as a cause of cell death and neural degeneration in multiple disease conditions; however, the evidence for successful intervention with dietary antioxidant manipulations has been mixed. In this study, we investigated the potential for protection of cells in the inner ear using a dietary supplement with multiple antioxidant components, which were selected for their potential interactive effectiveness. Protection against permanent threshold shift (PTS) was observed in CBA/J mice maintained on a diet supplemented with a combination of β-carotene, vitamins C and E, and magnesium when compared with PTS in control mice maintained on a nutritionally complete control diet. Although hair cell survival was not enhanced, noise-induced loss of type II fibrocytes in the lateral wall was significantly reduced (P < 0.05), and there was a trend toward less noise-induced loss in strial cell density in animals maintained on the supplemented diet. Taken together, our data suggest that prenoise oral treatment with the high-nutrient diet can protect cells in the inner ear and reduce PTS in mice. The demonstration of functional and morphologic preservation of cells in the inner ear with oral administration of this antioxidant supplemented diet supports the possibility of translation to human patients and suggests an opportunity to evaluate antioxidant protection in mouse models of oxidative stress-related disease and pathology.

Effects of furosemide on the hearing loss induced by impulse noise

BACKGROUND

The permanent hearing loss following exposure to intense noise can be due either to mechanical structural damage (tearing) caused directly by the noise or to metabolic (biochemical) damage resulting from the elevated levels of free radicals released during transduction of the sound overstimulation. Drugs which depress active cochlear mechanics (e.g. furosemide and salicylic acid) or anti-oxidants (which counteract the free radicals) are effective in reducing the threshold shift (TS) following broadband continuous noise. This study was designed to determine whether furosemide can reduce the TS following exposure to impulse noise, similar to its action with continuous broadband noise.

METHODS

Shortly after furosemide injection, mice were exposed to simulated M16 rifle impulse noise produced by different loudspeakers and amplifiers in different exposure settings and, in other experiments, also to actual M16 rifle shots.

RESULTS

Depending on the paradigm, the simulated noises either did not produce a TS, or the TS was reduced by furosemide. The drug was not effective in reducing TS resulting from actual impulse noise.

CONCLUSION

Simulated M16 rifle impulse noise may not truly replicate the rapid rise time and very high intensity of actual rifle shots so that the TS following exposure to such noise can be reduced by these drugs. On the other hand, actual M16 impulse noise probably causes direct (frank) mechanical damage, which is not reduced by these drugs.

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

CONCLUSION

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

OBJECTIVE

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

METHODS

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

RESULTS

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

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.

Post-exposure treatment with ginsenoside compound K ameliorates auditory functional injury associated with noise-induced hearing loss in mice

Noise-induced hearing loss (NIHL) is thought to primarily involve damage to the sensory hair cells of the cochlea via mechanical and metabolic mechanisms. Unfortunately, initial studies assessing the effectiveness of post-exposure treatment after hearing loss have yielded largely disappointing results. This study explored the effects of oral treatment with Korean red ginseng (RG) and with two bioavailable ginsenoside metabolites, ginsenoside Rh1 and ginsenoside compound K (GCK), in response to NIHL in a murine model. Pharmacological treatments began 24h after noise exposure and were continued once daily for 7 days. Central auditory function was evaluated using auditory middle latency responses, and cochlear function was determined based on transient evoked otoacoustic emissions. Additionally, cochlear hair cell morphology was investigated after noise exposure. Both Korean red ginseng and compound K reduced threshold shifts, central auditory function damage, and cochlear functional and morphological deficits. In contrast, treatment with ginsenoside Rh1 did not result in recovery of NIHL in mice. These results suggest that consumption of Korean red ginseng may facilitate recovery from noise-induced hearing loss. Furthermore, one of the active constituents in ginseng is likely ginsenoside compound K.

Adenosine amine congener mitigates noise-induced cochlear injury

Hearing loss from noise exposure is a leading occupational disease, with up to 5% of the population at risk world-wide. Here, we present a novel purine-based pharmacological intervention that can ameliorate noise-induced cochlear injury. Wistar rats were exposed to narrow-band noise (8-12 kHz, 110 dB SPL, 2-24 h) to induce cochlear damage and permanent hearing loss. The selective adenosine A(1) receptor agonist, adenosine amine congener (ADAC), was administered intraperitoneally (100 microg/kg/day) at time intervals after noise exposure. Hearing thresholds were assessed using auditory brainstem responses and the hair cell loss was evaluated by quantitative histology. Free radical damage in the organ of Corti was assessed using nitrotyrosine immunohistochemistry. The treatment with ADAC after noise exposure led to a significantly greater recovery of hearing thresholds compared with controls. These results were upheld by increased survival of sensory hair cells and reduced nitrotyrosine immunoreactivity in ADAC-treated cochlea. We propose that ADAC could be a valuable treatment for noise-induced cochlear injury in instances of both acute and extended noise exposures.

In vivo protective effect of ferulic acid against noise-induced hearing loss in the guinea-pig

Ferulic acid (FA) is a phenolic compound whose neuroprotective activity was extensively studied in vitro. In this study, we provided functional in vivo evidence that FA limits noise-induced hearing loss. Guinea-pigs exposed to acoustic trauma for 1 h exhibited a significant impairment in auditory function; this injury was evident as early as 1 day from noise exposure and persisted over 21 days. Ferulic acid (150 mg/kg i.p. for 4 days) counteracted noise-induced hearing loss at days 1, 3, 7 and 21 from noise exposure. The improvement of auditory function by FA was paralleled by a significant reduction in oxidative stress, apoptosis and increase in hair cell viability in the organ of Corti. Interestingly in the guinea-pig cochleae, the neuroprotective effect of FA was functionally related not only to its scavenging ability in the peri-traumatic period but also to the up-regulation of the cytoprotective enzyme heme oxygenase-1 (HO-1); in fact, FA-induced improvement of auditory function was counteracted by the HO inhibitor zinc-protoporphyrin-IX and paralleled the time-course of HO-1 induction over 3-7 days. These results confirm the antioxidant properties of FA as free-radical scavenger and suggest a role of HO-1 as an additional mediator against noise-induced ototoxicity.

Adenosine and the auditory system

Adenosine is a signalling molecule that modulates cellular activity in the central nervous system and peripheral organs via four G protein-coupled receptors designated A₁, A_2A, A_2B, and A₃. This review surveys the literature on the role of adenosine in auditory function, particularly cochlear function and its protection from oxidative stress. The specific tissue distribution of adenosine receptors in the mammalian cochlea implicates adenosine signalling in sensory transduction and auditory neurotransmission although functional studies have demonstrated that adenosine stimulates cochlear blood flow, but does not alter the resting and sound-evoked auditory potentials. An interest in a potential otoprotective role for adenosine has recently evolved, fuelled by the capacity of A₁ adenosine receptors to prevent cochlear injury caused by acoustic trauma and ototoxic drugs. The balance between A₁ and A_2A receptors is conceived as critical for cochlear response to oxidative stress, which is an underlying mechanism of the most common inner ear pathologies (e.g. noise-induced and age-related hearing loss, drug ototoxicity). Enzymes involved in adenosine metabolism, adenosine kinase and adenosine deaminase, are also emerging as attractive targets for controlling oxidative stress in the cochlea. Other possible targets include ectonucleotidases that generate adenosine from extracellular ATP, and nucleoside transporters, which regulate adenosine concentrations on both sides of the plasma membrane. Developments of selective adenosine receptor agonists and antagonists that can cross the blood-cochlea barrier are bolstering efforts to develop therapeutic interventions aimed at ameliorating cochlear injury. Manipulations of the adenosine signalling system thus hold significant promise in the therapeutic management of oxidative stress in the cochlea.

The effect of noise on ears with a hole in the vestibule

CONCLUSION

A hole in the vestibule of the inner ear leads to augmentation of the hearing loss following noise exposure. Further research is needed to ascertain the mechanism.

OBJECTIVES

The possible effects of a hole in the wall of the inner ear at the vestibule on noise-induced hearing loss were assessed.

METHODS

The study was conducted on a total of 14 sand rats. Of these 14 animals, 10 underwent baseline auditory nerve and brainstem evoked response (ABR) threshold measurements in each ear separately (insert earphone), opening of middle ear bulla in both ears and drilling of a hole in the vestibule of one inner ear. The other ear was sham-operated. Following immediate re-assessment of ABR threshold, the 10 animals were exposed to 113 dB SPL broadband noise during 4 nights, and 3 days later ABR measurements were repeated. Four additional animals with a hole in one vestibule, not exposed to noise, served as controls.

RESULTS

Following noise exposure, ABR thresholds were elevated by 28.5 +/- 9.1 dB in the ears with the hole, and by 15.5 +/- 7.2 dB in the opposite ear without the hole (significant difference, p < 0.003). In the four control ears, ABR threshold was not elevated a week after drilling the hole.

Effects of NSAIDs on the Inner Ear: Possible Involvement in Cochlear Protection

Cyclooxygenase and lipoxygenase, two important enzymes involved in arachidonic acid metabolism, are major targets of non-steroidal anti-inflammatory drugs (NSAIDs). Recent investigations suggest that arachidonic cascades and their metabolites may be involved in maintaining inner ear functions. The excessive use of aspirin may cause tinnitus in humans and impairment of the outer hair cell functions in experimental animals. On the other hand, NSAIDs reportedly exhibit protective effects against various kinds of inner ear disorder. The present review summarizes the effects of NSAIDs on cochlear pathophysiology. NSAIDs are a useful ameliorative adjunct in the management of inner ear disorders.

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

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

Post exposure administration of A(1) adenosine receptor agonists attenuates noise-induced hearing loss

Adenosine is a constitutive cell metabolite with a putative role in protection and regeneration in many tissues. This study was undertaken to determine if adenosine signalling pathways are involved in protection against noise injury. A(1) adenosine receptor expression levels were altered in the cochlea exposed to loud sound, suggesting their involvement in the development of noise injury. Adenosine and selective adenosine receptor agonists (CCPA, CGS-21680 and Cl-IB-MECA) were applied to the round window membrane of the cochlea 6h after noise exposure. Auditory brainstem responses measured 48h after drug administration demonstrated partial recovery of hearing thresholds (up to 20dB) in the cochleae treated with adenosine (non-selective adenosine receptor agonist) or CCPA (selective A(1) adenosine receptor agonist). In contrast, the selective A(2A) adenosine receptor agonist CGS-21680 and A(3) adenosine receptor agonist Cl-IB-MECA did not protect the cochlea from hearing loss. Sound-evoked cochlear potentials in control rats exposed to ambient noise were minimally altered by local administration of the adenosine receptor agonists used in the noise study. Free radical generation in the cochlea exposed to noise was reduced by administration of adenosine and CCPA. This study pinpoints A(1) adenosine receptors as attractive targets for pharmacological interventions to reduce noise-induced cochlear injury after exposure.

Contralateral cochlear effects of ipsilateral damage: no evidence for interaural coupling

Lesion studies of the olivocochlear efferents have suggested that feedback via this neuronal pathway normally maintains an appropriate binaural balance in excitability of the two cochlear nerves (Darrow et al., 2006). If true, a decrease in cochlear nerve output from one ear, due to conductive or sensorineural hearing loss, should change cochlear nerve response in the opposite ear via modulation in olivocochlear feedback. To investigate this putative efferent-mediated interaural coupling, we measured cochlear responses repeatedly from both ears in groups of mice for several weeks before, and for up to 5weeks after, a unilateral manipulation causing either conductive or sensorineural hearing loss. Response measures included amplitude vs. level functions for distortion product otoacoustic emissions (DPOAEs) and auditory brainstem responses (ABRs), evoked at 7 log-spaced frequencies. Ipsilateral manipulations included either tympanic membrane removal or an acoustic overstimulation designed to produce a reversible or irreversible threshold shift over a restricted frequency range. None of these ipsilateral manipulations produced systematic changes in contralateral cochlear responses, either at threshold or suprathreshold levels, either in ABRs or DPOAEs. Thus, we find no evidence for compensatory contralateral changes following ipsilateral hearing loss. We did, however, find evidence for age-related increases in DPOAE amplitudes as animals mature from 6 to 12weeks and evidence for a slow apical spread of noise-induced threshold shifts, which continues for several days post-exposure.

Retinoic acid applied after noise exposure can recover the noise-induced hearing loss in mice

CONCLUSION

The early post-exposure treatment with All-trans retinoic acid (ATRA) can reduce hair cell loss and hearing deterioration in mice in which permanent threshold shift has been induced.

OBJECTIVE

One of the mechanism by which intense noise induces apoptosis of cochlea hair cells is the C-Jun NH(2)-terminal kinase (JNK) pathway. ATRA is a potent inhibitor of activator protein 1, a transcription factor of the JNK pathway. In this study we evaluated that the effect of post-exposure treatment of ATRA on noise-induced hearing loss and aimed to determine a time window for effective post-exposure treatment of ATRA.

METHODS

All mice were exposed to white noise for 3 h per day for three consecutive days and induced permanent threshold shift. The treatment groups fed with ATRA from 1 h, one day, two days, and three days after noise exposure for five days were compared with mice fed with same dosage of sesame oil. We measured the threshold shifts of hearing and survival rates of hair cells on the cytocochleogram.

RESULTS

Mice fed with ATRA beginning within two days after noise had less threshold shifts and more hair cell survivals than mice fed with sesame oil.

Water-soluble Coenzyme Q10 formulation (Q-ter) promotes outer hair cell survival in a guinea pig model of noise induced hearing loss (NIHL)

The mitochondrial respiratory chain is a powerful source of reactive oxygen species (ROS) also in noise induced hearing loss (NIHL) and anti-oxidants and free-radicals scavengers have been shown to attenuate the damage. Coenzyme Q(10) (CoQ(10)) or ubiquinone has a bioenergetic role as a component of the mithocondrial respiratory chain, it inhibits mitochondrial lipid peroxidation, inducing ATP production and it is involved in ROS removal and prevention of oxidative stress-induced apoptosis. However the therapeutic application of CoQ(10) is limited by the lack of solubility and poor bio- availability, therefore it is a challenge to improve its water solubility in order to ameliorate the efficacy in tissues and fluids. This study was conducted in a model of acoustic trauma in the guinea pig where the effectiveness of CoQ(10) was compared with a soluble formulation of CoQ(10) (multicomposite CoQ(10) Terclatrate, Q-ter) given intraperitoneally 1 h before and once daily for 3 days after pure tone noise exposure (6 kHz for 1 h at 120 dB SPL). Functional and morphological studies were carried out by measuring auditory brainstem responses, scanning electron microscopy for hair cell loss count, active caspase 3 staining and terminal deoxynucleotidyl transferase-mediated dUTP labelling assay in order to identify initial signs of apoptosis. Treatments decreased active caspase 3 expression and the number of apoptotic cells, but animals injected with Q-ter showed a greater degree of activity in preventing apoptosis and thus in improving hearing. These data confirm that solubility of Coenzyme Q(10) improves the ability of CoQ(10) in preventing oxidative injuries that result from mitochondrial dysfunction.

GFAP aggregates in the cochlear nerve increase the noise vulnerability of sensory cells in the organ of Corti in the murine model of Alexander disease

Outer hair cell (OHC) loss in the auditory sensory epithelium is a primary cause of noise-induced sensory-neural hearing loss (SNHL). To clarify the participation of glial cells in SNHL, we used an Alexander disease (AxD) mouse model. These transgenic mice harbor the AxD causal mutant of the human glial fibrillary acidic protein (GFAP) under the control of the mouse GFAP promoter. It is thought that GFAP aggregates compromise the function of astrocytes. In the auditory pathway, the formation of GFAP aggregates was observed only in GFAP-positive cells of the cochlear nerve. The presence of GFAP aggregates did not change auditory function at the threshold level. To assess the change in vulnerability to auditory excitotoxicity, both transgenic and control mice were treated with intense noise exposure. Auditory threshold shifts were assessed by auditory brainstem responses (ABR) at 1 and 4 weeks after noise exposure, and OHC damage was analyzed by quantitative histology at 4 weeks after exposure. Transgenic mice showed more severe ABR deficits and OHC damage, suggesting that cochlear nerve glial cells with GFAP aggregates play a role in noise susceptibility. Thus, we should focus more on the roles of cochlear nerve glial cells in SNHL.

Bcl-2 genes regulate noise-induced hearing loss

Proteins of the Bcl-2 family have been implicated in control of apoptotic pathways modulating neuronal cell death, including noise-induced hearing loss. In this study, we assessed the expressions of anti- and proapoptotic Bcl-2 genes, represented by Bcl-xL and Bak following noise exposures, which yielded temporary threshold shift (TTS) or permanent threshold shift (PTS). Auditory brainstem responses (ABRs) were assessed at 4, 8, and 16 kHz before exposure and on days 1, 3, 7, and 10 following exposure to 100 dB SPL, 4 kHz OBN, 1 hr (TTS) or 120 dB SPL, 4 kHz OBN, 5 hr (PTS). On day 10, subjects were euthanized. ABR thresholds increased following both exposures, fully recovered following the TTS exposure, and showed a 22.6 dB (4 kHz), 42.5 dB (8 kHz), and 44.9 dB (16 kHz) mean shift on day 10 following the PTS exposure. PTS was accompanied by outer hair cell loss progressing epically and basally from the 4-kHz region. Additional animals were euthanized for immunohistochemical assessment. BcL-xL was robustly expressed in outer hair cells following TTS exposure, whereas Bak was expressed following PTS exposure. These results indicate an important role of the Bcl-2 family proteins in regulating sensory cell survival or death following intense noise. Bcl-xL plays an essential role in prevention of sensory cell death following TTS levels of noise, and PTS exposure provokes the expression of Bak and, with that, cell death.

Antioxidant protection against acoustic trauma by coadministration of idebenone and vitamin E

Idebenone, a synthetic analogue of coenzyme Q, attenuates noise-induced hearing loss by virtue of its antioxidant properties. This study involves a guinea pig model of acoustic trauma where the effectiveness of idebenone is analyzed in comparison with Vitamin E (alpha-tocopherol) that exhibits a potent antioxidant activity in the inner ear. Idebenone and vitamin E were injected intraperitoneally 1 h before noise exposure and once daily for three days; functional and morphological studies were then carried out, respectively, by auditory brainstem responses evaluation, scanning electron microscopy and terminal deoxynucleotidyl transferase-mediated dUTP nick end labelling assay identification of missing and apoptotic cells was also performed. The results showed that the protective effects of idebenone and vitamin E were not additive implying that the two antioxidants may share competitive mechanisms.

[Protection and regeneration of sensory epithelia of the inner ear]

Dysfunctions of the inner ear such as hearing impairment due to noise exposure or presbycusis and vertigo are often caused by loss of hair cells in the sensory epithelium. There is still no specific therapy, just technical aids. Options for protecting and regenerating hair cells are explained here. The inhibition of apoptosis via caspases is presently the main target of research. They are involved in damage caused by aminoglycosides, cisplatin, or noise exposure. Bcl-2, growth factors, and oxidative stress are discussed. In regeneration the transdifferentiation of supporting cells to hair cells is explained. This can be achieved with local gene therapy using math1. Approach and media for the application are discussed, while viral vectors such as the adenovector seem the most promising in research.

Salicylic acid injection before noise exposure reduces permanent threshold shift

The permanent threshold shift (PTS) following exposure to intense noise may be due to the noise-induced excessive vibrations in the cochlea or to the generation of elevated levels of reactive oxygen species. Thus, it is possible that the resulting PTS may be reduced if the cochlear amplifier could be temporarily depressed beginningjust before the onset of the noise and continuing during the noise exposure or if antioxidant drugs were administered. These possibilities were assessed in mice by administering a single injection of salicylic acid (an antioxidant drug which also reversibly depresses the motor protein prestin of the cochlear amplifier) just before, and in other mice, just after, 3.5 h of 113-dB SPL broadband noise exposure. The PTS in the mice injected with salicylic acid just before the noise exposure was significantly smaller than that in mice exposed to the same noise without salicylic acid. The PTS in the latter was not significantly different from that in mice who received the drug just after the noise. Thus a single injection of salicylic acid, just before a noise exposure, can protect the ear from a noise-induced hearing loss.

Hyperbaric oxygenation with corticoid in experimental acoustic trauma

Among possible therapies after acute acoustic trauma, hyperbaric oxygenation (HBO) combined with corticoid was found effective in several animal studies. Such evidence was obtained for moderate 20-25 dB losses. The aim of this study was to further assess this therapy for noise-induced hearing losses greater than previously examined. Sixty-five ears from thirty-six adult guinea pigs were used. Acoustically evoked responses from intracranial electrodes chronically implanted bilaterally into the ventral cochlear nucleus were used to assess acoustic sensitivity alterations. Trauma sound was a third-octave noise-band around 8 kHz presented bilaterally at 115 dB SPL for 45 min. One control group received no treatment, one group was treated with HBO only and another with corticoid only both starting within one day post-trauma, two groups were treated with both HBO and corticoid starting for one group within one day post-trauma, and for the second group at 6 days post-trauma. Acoustic thresholds were measured between the 6th and the 16th days after acoustic trauma. Animals treated with HBO alone or corticoid alone did not differ from controls. Combined HBO and corticoid therapy provided significant protection from noise-induced loss of auditory thresholds, especially when started one day post-exposure. Hearing loss reduction induced by HBO combined with corticoid was of similar magnitude (about 10-15 dB) as in previous studies although the induced hearing loss was considerably greater (about 40 dB instead of 20-25 dB).

Free radical scavengers vitamins A, C, and E plus magnesium reduce noise trauma

Free radical formation in the cochlea plays a key role in the development of noise-induced hearing loss (NIHL). The amount, distribution, and time course of free radical formation have been defined, including a clinically significant formation of both reactive oxygen species and reactive nitrogen species 7-10 days after noise exposure. Reduction in cochlear blood flow as a result of free radical formation has also been described. Here we report that the antioxidant agents vitamins A, C, and E act in synergy with magnesium to effectively prevent noise-induced trauma. Neither the antioxidant agents nor the magnesium reliably reduced NIHL or sensory cell death with the doses we used when these agents were delivered alone. In combination, however, they were highly effective in reducing both hearing loss and cell death even with treatment initiated just 1 h before noise exposure. This study supports roles for both free radical formation and noise-induced vasoconstriction in the onset and progression of NIHL. Identification of this safe and effective antioxidant intervention that attenuates NIHL provides a compelling rationale for human trials in which free radical scavengers are used to eliminate this single major cause of acquired hearing loss.

AM-111 protects against permanent hearing loss from impulse noise trauma

The otoprotective peptide AM-111, a cell-permeable inhibitor of JNK mediated apoptosis, was tested for its efficacy as a rescue agent following impulse noise trauma. Single dose administrations of AM-111 at 1h or 4h post-impulse noise exposure (155 dB peak SPL) via systemic or local routes were evaluated with a total of 48 chinchillas. The animals received the compound either by IP injection or locally onto the round window membrane (hyaluronic acid gel formulation or osmotic mini-pump). Efficacy was determined by auditory brainstem responses (ABR) as well as cytocochleograms. Three weeks after impulse noise exposure, permanent threshold shifts (PTS) were significantly lower for AM-111 treated ears compared to controls, regardless of the drug administration route and the time point of drug delivery. Even the treatments which started 4h post-noise exposure, reduced hearing loss in the 2-8 kHz range compared to controls by up to 16-25 dB to a PTS as low as 6-17 dB, demonstrating significant protection against permanent hearing loss from impulse noise trauma. These findings suggest a key role for JNK mediated cochlear sensory cell death from oxidative stress.

Mechanisms of noise-induced hearing loss indicate multiple methods of prevention

Recent research has shown the essential role of reduced blood flow and free radical formation in the cochlea in noise-induced hearing loss (NIHL). The amount, distribution, and time course of free radical formation have been defined, including a clinically significant late formation 7-10 days following noise exposure, and one mechanism underlying noise-induced reduction in cochlear blood flow has finally been identified. These new insights have led to the formulation of new hypotheses regarding the molecular mechanisms of NIHL; and, from these, we have identified interventions that prevent NIHL, even with treatment onset delayed up to 3 days post-noise. It is essential to now assess the additive effects of agents intervening at different points in the cell death pathway to optimize treatment efficacy. Finding safe and effective interventions that attenuate NIHL will provide a compelling scientific rationale to justify human trials to eliminate this single major cause of acquired hearing loss.

Creatine and tempol attenuate noise-induced hearing loss

To define the role of free radical formation and potential energy depletion in noise induced hearing loss (NIHL), we measured the effectiveness of tempol (free radical scavenger) and creatine (enhances cellular energy storage) alone and in combination to attenuate NIHL. Guinea pigs were divided into four treatment groups: controls, 3% creatine diet (2 weeks prior to noise exposure), tempol (3 mM in drinking water 2 weeks prior to exposure), and creatine plus tempol and exposed to 120 dB SPL one-octave band noise centered at 4 kHz for 5 h. The noise-only control group showed frequency-dependent auditory threshold shifts (measured by auditory brainstem response, ABR) of up to 73 dB (16 kHz) on day 1, and up to 50 dB (8 kHz) on day 10. Creatine-treated subjects had significantly smaller ABR threshold shifts on day 1 and on day 10. Tempol alone significantly reduced ABR threshold shifts on day 10 but not on day 1. ABR shifts after combination treatment were similar to those in the creatine group. Hair cell loss on day 10 was equally attenuated by creatine and tempol alone or in combination. Our results indicate that the maintenance of ATP levels is important in attenuating both temporary and permanent NIHL, while the scavenging of free radicals provides protection from permanent NIHL.

Administration of amitriptyline attenuates noise-induced hearing loss via glial cell line-derived neurotrophic factor (GDNF) induction

Antidepressant treatments have been described to induce neurotrophic factors (NTFs) and reverse the cell loss observed in rodent stress models. Amitriptyline (AT), a tricyclic antidepressant agent, has been reported in recent studies to induce glial cell line-derived neurotrophic factor (GDNF) synthesis and release in rat C6 glioblastoma cells. GDNF has shown protection against acoustic trauma in previous studies. Therefore, we investigated whether AT could induce GDNF synthesis in the cochlea and attenuate cochlea damage against acoustic trauma. We used Hartley guinea pigs and injected AT (30 mg/kg) or saline into the peritoneum. Subjects were exposed to 117 dB SPL octave band noise centered at 4 kHz for 24 h. Noise-induced hearing loss (NIHL) was assessed with auditory brain stem response (ABR) at 4, 8 and 16 kHz measured prior to the injection, 3 days and 7 days after noise exposure. For histological assessment, we observed the sensory epithelium using a surface preparation technique and assessed the quantitative hair cell (HC) damage. We evaluated GDNF synthesis with or without intense noise exposure at 3, 12 and 24 h after the administration of AT in the cochlea using Western blot analysis. GDNF expression was shown 3 h and 12 h after the injection without noise, whereas with noise the GDNF expression lasted for 24 h. The AT-administrated group showed significantly reduced ABR threshold shift and less HC damage than the saline-administrated group. These findings suggest that the administration of AT-induced GDNF levels in the cochlea and attenuated cochlea damage from NIHL.

A protein derived from the fusion of TAT peptide and FNK, a Bcl-xL derivative, prevents cochlear hair cell death from aminoglycoside ototoxicity in vivo

We constructed a powerful artificial cytoprotective protein, FNK, from an antiapoptotic member of the BCL-2 family, Bcl-x(L). To test the efficacy of FNK in protecting cochlear hair cells (HCs) from aminoglycoside-induced cell death in vivo, we fused FNK with protein transduction domain, TAT, of the HIV/Tat protein to construct a fusion protein of TAT-FNK. We demonstrated that, after an intraperitoneal administration to guinea pigs, TAT-myc-FNK protein was diffusely distributed in the cochlea, most prominently in the HCs and supporting cells, followed by the spiral ganglion cells, 3 hr after the injection. We next demonstrated that TAT-FNK attenuated cochlear damage induced by an ototoxic combination of kanamycin sulfate (KM) and ethacrynic acid (EA) administered at 2 different dosages: 400 mg/kg KM + 50 mg/kg EA and 200 mg/kg KM + 40 mg/kg EA. TAT-FNK or vehicle was intraperitoneally injected from 3 hr before through 5 hr after inducing the ototoxic insults, 14 days after which auditory brainstem response (ABR) and HC loss were evaluated. In comparison with vehicle-administered controls, the TAT-FNK protein significantly attenuated ototoxic drug-induced ABR threshold shifts and the extent of HC death at either dosage. The TAT-FNK protein also significantly reduced the amount of cleaved poly-(ADP-ribose) polymerase-positive HCs 8 hr after the ototoxic insults compared with that in the vehicle-administered controls. These findings indicate that systemically administered TAT-FNK was successfully delivered to the guinea pig cochlea and effectively prevented apoptotic cell death of the cochlear HCs induced by KM and EA.

Inhibition of the c-Jun N-Terminal Kinase-Mediated Mitochondrial Cell Death Pathway Restores Auditory Function in Sound-Exposed Animals

We tested and characterized the therapeutic value of round window membrane-delivered (RWM) d-JNKI-1 peptide (Bonny et al., 2001) against sound trauma-induced hearing loss. Morphological characteristics of sound-damaged hair cell nuclei labeled by Hoechst staining show that apoptosis is the predominant mode of cell death after sound trauma. Analysis of the events occurring after sound trauma demonstrates that c-Jun N-terminal kinase (JNK)/stress-activated protein kinase activates a mitochondrial cell death pathway (i.e., activation of Bax, release of cytochrome c, activation of procaspases, and cleavage of fodrin). Fluorescein isothiocyanate (FITC)-conjugated d-JNKI-1 peptide applied onto an intact cochlear RWM diffuses through this membrane and penetrates cochlear tissues with the exception of the stria vascularis. A time sequence of fluorescence measurements demonstrates that FITC-labeled d-JNKI-1 remains in cochlear tissues for as long as 3 weeks. In addition to blocking JNK-mediated activation of a mitochondrial cell death pathway, RWM-delivered d-JNKI-1 prevents hair cell death and development of a permanent shift in hearing threshold that is caused by sound trauma in a dose-dependent manner (EC50 = 2.05 microM). The therapeutic window for protection of the cochlea from sound trauma with RWM delivery of d-JNKI-1 extended out to 12 h after sound exposure. These results show that the mitogen-activated protein kinase/JNK signaling pathway plays a crucial role in sound trauma-initiated hair cell death. Blocking this signaling pathway with RWM delivery of d-JNKI-1 may have significant therapeutic value as a therapeutic intervention to protect the human cochlea from the effects of sound trauma.

Effets des nuisances sonores sur l’oreille interne

The effects of noise on health depend both on individual factors and characteristics of sound exposure. In case of acoustic trauma, reversible or irreversible lesions of inner ear components are possible. Most often there is immediately an acute tinnitus and hearing loss. Audiometric tests demonstrate hearing loss on the high frequency, generally focused on 4 kHz. Immediate treatment is recommended even there is no currently indicator of the ability to restore hearing. New perspectives on treatment are directed to local treatment and/or using new procedure as antioxidative treatment. Occupational and leisure are the two conditions in which chronic exposure to noise is found. Detection and prevention of noise-induced hearing loss is easier in case of industrial workers than in case of noise exposition for musicians and other sounds and stage technicians or concert managers, and of course non-professional with exposure to amplified music.

Pharmacological rescue of noise induced hearing loss using N-acetylcysteine and acetyl-L-carnitine

Despite the use of hearing protection devices (HPDs) and engineering changes designed to improve workspaces, noise-induced hearing loss continues to be one of the most common and expensive disabilities in the US military. Many service members suffer acoustic trauma due to improper use of HPDs, sound levels exceeding the protective capacity of the HPDs, or by unexpected, injurious exposures. In these cases, there is no definitive treatment for the hearing loss. This study investigated the use of the pharmacological agents N-acetylcysteine and acetyl-L-carnitine after acoustic trauma to treat cochlear injury. N-Acetylcysteine is an antioxidant and acetyl-L-carnitine a compound that maintains mitochondrial bio-energy and integrity. N-Acetylcysteine and acetyl-L-carnitine, respectively, significantly reduced permanent threshold shifts and hair cell loss compared to saline-treated animals when given 1 and 4 h post-noise exposure. It may be possible to obtain a greater therapeutic effect using these agents in combination or at higher doses or for a longer period of time to address the secondary oxidative events occurring 7-10 days after acute noise exposure.

Protective properties of idebenone in noise-induced hearing loss in the guinea pig

Idebenone is a synthetic analogue of coenzyme Q10 with antioxidant properties. The present study investigated the antioxidant activity of idebenone in the rescue of acoustic trauma. Noise-induced hearing loss was induced by exposing guinea pigs to a continuous pure tone and idebenone was injected intraperitoneally 1 h before noise exposure and once daily for 3 days. Guinea pigs treated with idebenone showed significantly smaller auditory threshold shifts than unprotected control animals. Missing and apoptotic cells were identified with scanning electron microscopy and terminal deoxynucleotidyl transferase-mediated dUTP nick end labelling assay. Protected animals presented a lesser extent of both apoptotic activation and hair cell loss in the organ of Corti. Our results suggest an antioxidant function of idebenone in protection from noise-induced hearing loss and provide a rationale for exploring therapeutic strategies in humans.

2,3-Dihydroxybenzoic acid attenuates kanamycin-induced volume reduction in mouse utricular type I hair cells

The aminoglycoside kanamycin is a commonly used antibiotic, but unfortunately it is oto- and nephrotoxic in large doses. The negative effects are thought to be due to the formation of free radicals which is why strong antioxidants and iron chelators like 2,3-dihydroxybenzoic acid (DHB) are of great interest. This study estimates cellular quantitative changes in the utricular macula of mice following systemic treatment with kanamycin alone or in combination with DHB. The animals were injected with either saline, kanamycin or kanamycin + DHB for 15 days and perfusion fixed three weeks after last injection. Total volume of the utricle, as well as total number of hair and supporting cells, were estimated on light microscopic sections. Total volume and mean volume of hair cell types I and II and supporting cells were estimated on digital transmission electron micrographs. Total volume of the utricular macula, hair cell type I and supporting cells decreased significantly in animals injected with kanamycin but not in animals co-treated with DHB. Hair and supporting cell numbers remained unchanged in all three groups. In conclusion, the kanamycin-induced volume reduction of type I hair cells was attenuated by DHB.