Effects of intense auditory stimulation: hearing losses and inner ear changes in the chinchilla

Changes in audiometric threshold and frequency selectivity correlate with cochlear histopathology in macaque monkeys with permanent noise-induced hearing loss

Exposure to loud noise causes damage to the inner ear, including but not limited to outer and inner hair cells (OHCs and IHCs) and IHC ribbon synapses. This cochlear damage impairs auditory processing and increases audiometric thresholds (noise-induced hearing loss, NIHL). However, the exact relationship between the perceptual consequences of NIHL and its underlying cochlear pathology are poorly understood. This study used a nonhuman primate model of NIHL to relate changes in frequency selectivity and audiometric thresholds to indices of cochlear histopathology. Three macaques (one Macaca mulatta and two Macaca radiata) were trained to detect tones in quiet and in noises that were spectrally notched around the tone frequency. Audiograms were derived from tone thresholds in quiet; perceptual auditory filters were derived from tone thresholds in notched-noise maskers using the rounded-exponential fit. Data were obtained before and after a four-hour exposure to a 50-Hz noise centered at 2 kHz at 141 or 146 dB SPL. Noise exposure caused permanent audiometric threshold shifts and broadening of auditory filters at and above 2 kHz, with greater changes observed for the 146-dB-exposed monkeys. The normalized bandwidth of the perceptual auditory filters was strongly correlated with audiometric threshold at each tone frequency. While changes in audiometric threshold and perceptual auditory filter widths were primarily determined by the extent of OHC survival, additional variability was explained by including interactions among OHC, IHC, and ribbon synapse survival. This is the first study to provide within-subject comparisons of auditory filter bandwidths in an animal model of NIHL and correlate these NIHL-related perceptual changes with cochlear histopathology. These results expand the foundations for ongoing investigations of the neural correlates of NIHL-related perceptual changes.

The chinchilla animal model for hearing science and noise-induced hearing loss

The chinchilla animal model for noise-induced hearing loss has an extensive history spanning more than 50 years. Many behavioral, anatomical, and physiological characteristics of the chinchilla make it a valuable animal model for hearing science. These include similarities with human hearing frequency and intensity sensitivity, the ability to be trained behaviorally with acoustic stimuli relevant to human hearing, a docile nature that allows many physiological measures to be made in an awake state, physiological robustness that allows for data to be collected from all levels of the auditory system, and the ability to model various types of conductive and sensorineural hearing losses that mimic pathologies observed in humans. Given these attributes, chinchillas have been used repeatedly to study anatomical, physiological, and behavioral effects of continuous and impulse noise exposures that produce either temporary or permanent threshold shifts. Based on the mechanistic insights from noise-exposure studies, chinchillas have also been used in pre-clinical drug studies for the prevention and rescue of noise-induced hearing loss. This review paper highlights the role of the chinchilla model in hearing science, its important contributions, and its advantages and limitations.

The use of nonhuman primates in studies of noise injury and treatment

Exposure to prolonged or high intensity noise increases the risk for permanent hearing impairment. Over several decades, researchers characterized the nature of harmful noise exposures and worked to establish guidelines for effective protection. Recent laboratory studies, primarily conducted in rodent models, indicate that the auditory system may be more vulnerable to noise-induced hearing loss (NIHL) than previously thought, driving renewed inquiries into the harmful effects of noise in humans. To bridge the translational gaps between rodents and humans, nonhuman primates (NHPs) may serve as key animal models. The phylogenetic proximity of NHPs to humans underlies tremendous similarity in many features of the auditory system (genomic, anatomical, physiological, behavioral), all of which are important considerations in the assessment and treatment of NIHL. This review summarizes the literature pertaining to NHPs as models of hearing and noise-induced hearing loss, discusses factors relevant to the translation of diagnostics and therapeutics from animals to humans, and concludes with some of the practical considerations involved in conducting NHP research.

Genetic Effects on Sensorineural Hearing Loss and Evidence-based Treatment for Sensorineural Hearing Loss

In this article, the mechanism of inheritance behind inherited hearing loss and genetic susceptibility in noise-induced hearing loss are reviewed. Conventional treatments for sensorineural hearing loss (SNHL), i.e. hearing aid and cochlear implant, are effective for some cases, but not without limitations. For example, they provide little benefit for patients of profound SNHL or neural hearing loss, especially when the hearing loss is in poor dynamic range and with low frequency resolution. We emphasize the most recent evidence-based treatment in this field, which includes gene therapy and allotransplantation of stem cells. Their promising results have shown that they might be options of treatment for profound SNHL and neural hearing loss. Although some treatments are still at the experimental stage, it is helpful to be aware of the novel therapies and endeavour to explore the feasibility of their clinical application.

Gentamicin differentially alters cellular metabolism of cochlear hair cells as revealed by NAD(P)H fluorescence lifetime imaging

Aminoglycoside antibiotics are implicated as culprits of hearing loss in more than 120,000 individuals annually. Research has shown that the sensory cells, but not supporting cells, of the cochlea are readily damaged and/or lost after use of such antibiotics. High-frequency outer hair cells (OHCs) show a greater sensitivity to antibiotics than high- and low-frequency inner hair cells (IHCs). We hypothesize that variations in mitochondrial metabolism account for differences in susceptibility. Fluorescence lifetime microscopy was used to quantify changes in NAD(P)H in sensory and supporting cells from explanted murine cochleae exposed to mitochondrial uncouplers, inhibitors, and an ototoxic antibiotic, gentamicin (GM). Changes in metabolic state resulted in a redistribution of NAD(P)H between subcellular fluorescence lifetime pools. Supporting cells had a significantly longer lifetime than sensory cells. Pretreatment with GM increased NAD(P)H intensity in high-frequency sensory cells, as well as the NAD(P)H lifetime within IHCs. GM specifically increased NAD(P)H concentration in high-frequency OHCs, but not in IHCs or pillar cells. Variations in NAD(P)H intensity in response to mitochondrial toxins and GM were greatest in high-frequency OHCs. These results demonstrate that GM rapidly alters mitochondrial metabolism, differentially modulates cell metabolism, and provides evidence that GM-induced changes in metabolism are significant and greatest in high-frequency OHCs.

The prevention of noise induced hearing loss in children

Increasingly, our acoustic environment is filled with amplified sound sources (e.g., MP3 players, video game stations, and sports/entertainment venues). There is serious concern and also some controversy about the risks of acoustic trauma in children. This overview provides some basic information on the physiological mechanisms that lead to noise induced hearing loss, a survey of various studies that, on balance, indicates that there is cause for concern, and finally a discussion on measures that can help to prevent noise induced hearing loss in children. This paper is designed for public health and other healthcare professions (ENT, audiologists, family doctors, and pediatricians) who should understand the risks of noise induced hearing loss and its prevention.

The design and screening of drugs to prevent acquired sensorineural hearing loss

INTRODUCTION

Sensorineural hearing loss affects a high percentage of the population. Ototoxicity is a serious and pervasive problem in patients treated with cisplatin. Strategies to ameliorate ototoxicity without compromising on antitumor activity of treatments are urgently needed. Similar problems occur with aminoglycoside antibiotic therapy for infections. Noise-induced hearing loss affects a large number of people. The use of ear protection is not always possible or effective. The prevention of hearing loss with drug therapy would have a huge impact in reducing the number of people with hearing loss from these major causes.

AREAS COVERED

This review discusses significant research findings dealing with the use of protective agents against hearing loss caused by cisplatin, aminoglycoside antibiotics and noise trauma. The efficacy in animal studies and the application of these protective agents in clinical trials that are ongoing are presented.

EXPERT OPINION

The reader will gain new insights into current and projected future strategies to prevent sensorineural hearing loss from cisplatin chemotherapy, aminoglycoside antibiotic therapy and noise exposure. The future appears to offer numerous agents to prevent hearing loss caused by cisplatin, aminoglycoside antibiotics and noise. Novel delivery systems will provide ways to guide these protective agents to the desired target areas in the inner ear and circumvent problems with therapeutic interference of antitumor and antibiotics agents as well as minimize undesired side effects.

Relation of focal hair-cell lesions to noise-exposure parameters from a 4- or a 0.5-kHz octave band of noise

In a previous study, we examined the relation between total energy in a noise exposure and the percentage losses of outer (OHC) and inner (IHC) hair cells in the basal and apical halves of 607 chinchilla cochleae [Harding, G.W., Bohne, B.A., 2004a. Noise-induced hair-cell loss and total exposure energy: analysis of a large data set. J. Acoust. Soc. Am. 115, 2207-2220]. The animals had been exposed continuously to either a 4-kHz octave band of noise (OBN) at 47-108 dB SPL for 0.5h-36 d, or a 0.5-kHz OBN at 65-128 dB SPL for 3.5h-433 d. Interrupted exposures were also employed with both OBNs. Post-exposure recovery times ranged from 0 to 913 days. Cluster analysis was used to separate the data into three magnitudes of damage. The data were also separated into recovery times of 0 days (acute) and >0 days (chronic) and the apical and basal halves of the organ of Corti (OC). A substantial part of these hair-cell losses occurred in focal lesions (i.e., >or=50% loss of IHCs, OHCs or both over a distance of >or=0.03 mm). This aspect of the damage from noise was not included in the previous analysis. The present analysis describes, within the same three clusters, the apex-to-base distribution of 1820 focal lesions found in 468 of 660 (71%) noise-exposed cochleae. In these cochleae, OC length in mm was converted to percent distance from the apex. The lesion data were analyzed for location in percent distance from the apex and size (mm) of the lesions. In 55 of 140 (39%) non-noise-exposed, control OCs, there were 186 focal hair-cell lesions, the characteristics of which were also determined. Focal lesions with hair-cell loss >or=50% involved predominantly OHCs, IHCs only, or both OHCs and IHCs (i.e., combined OHC-IHC lesions). The predominantly OHC and combined lesions were pooled together for the analysis. The distributions of lesion location (in percent distance from the apex), weighted by lesion size (in percent of OC length) were tallied in 2%-distance bins. In controls, focal lesions were uniformly distributed from apex to base and 70% of them were pure IHC lesions. In cochleae exposed to the 4-kHz OBN, lesions were distributed throughout the basal half of the OC. In cochleae exposed to the 0.5-kHz OBN, lesions occurred in both halves of the OC. With continuous exposures, 74% of the lesions were predominantly OHC or combined lesions. With interrupted exposures, 52% of the lesions were OHC or combined lesions. Lesion size was generally larger in the chronic compared to acute cochleae with similar exposures. There was a minimum total energy at which focal lesions began to appear and slightly higher energies resulted in nearly all exposed cochleae having focal lesions.

Metabolic imaging of the organ of corti--a window on cochlea bioenergetics

Hair cell loss is a major cause of sensorineural hearing loss. We have developed a method to examine metabolic events in hair cells in response to stimuli known to cause hair cell loss, such as acoustic trauma and aminoglycoside administration. The method employs two-photon excitation of the metabolic intermediate, reduced nicotinamide adenine dinucleotide (NADH), in hair cell mitochondria in an explanted mouse cochlea. Using this method, we show evidence that the aminoglycoside gentamicin selectively affects the level of mitochondrial NADH in outer hair cells, but not inner hair cells, within minutes of administration.

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.

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.

Brainstem auditory-evoked response in the rat. Normative studies, with observations concerning the effects of ossicular disruption

Six young adult Sprague-Dawley rats were unilaterally cochleotomized, Brain-stem auditory-evoked responses (BAERs) to clicks and to 1-, 2-, 4-, 8- and 16-kHz tone bursts were obtained. In addition, response thresholds were estimated before and after ossicular disruption in the noncochleotomized ear of 4 animals. With increasing tone burst frequency, there was a decrease in BAER peak latencies as well as a decrease in threshold. With increasing click and tone burst intensity, there was a decrease in peak latencies and an increase in peak amplitudes. BAER peak latency/intensity functions to click stimuli ranged from -.013 to -.018 ms/dB. With increasing tone burst frequency there was a decrease in the slope of the latency/intensity function. Following ossicular disruption, BAER thresholds to clicks were elevated by an average of 49 dB. Threshold shifts to tone burst stimuli were smallest for 1- and 2-kHz tone bursts (35-36 dB) and increased with increasing frequency up to a maximum of 65 dB for 16-kHz tone bursts.

Impulse noise and continuous noise of equivalent frequency spectrum and total sound energy

It has been proposed that impulse noise and continuous noise affect the inner ear differently and investigations have found impulse noise to be harmful to both the inner hair cells and the outer hair cells. Scanning electron microscopy and non-standard methods for statistical analysis have facilitated the evaluation of different types of morphological changes after exposure to various kinds of noise. Morphological differences were compared in groups of guinea pigs exposed to either impulse noise or continuous noise of equivalent duration, spectral content and energy. Functionally, the groups also showed similar threshold elevations. In order to separate the two groups, subtle hair cell changes were recorded and evaluated either alone, in combination with each other or with hair cell loss. It was found that both the inner hair cells and the outer hair cells were affected differently by impulse noise than by continuous noise even though the auditory thresholds were similar.

Overview of mechanical damage to the inner ear: noise as a tool to probe cochlear function

The majority of experiments causing mechanical damage to the cochlea involve the use of sound pressure waves to cause overstimulation. This presentation is an overview of the research during the past years on the structural damage produced by noise. The effect of noise on the cochlea depends on the type of noise exposure-impulse or continuous. Experiments have been conducted to determine the effect of increasing intensity, the effect of increasing duration, and the effect of equal energy presented over varying periods of time. The initial mechanism of damage, the progression of damage over time, and the ability of hair cells to recover are discussed. Noise has been used as a tool to probe cochlear function by selectively damaging regions along the length of the sensory epithelium and by selectively damaging one of the two types of hair cells. Results obtained from these types of experiments have given us information on cochlear mechanics, as well as of stereocilia micromechanics and transduction. Information on susceptibility of hair cells to noise confirms previous results, suggesting the presence of structural and metabolic gradients both longitudinally and radially within the sensory epithelium. Moreover, noise lesions have been used to map the afferent innervation pattern to the cochlear nucleus, and noise studies show correlation of hair cell damage with efferent innervation pattern.

Age-dependent effects of a pure tone trauma in the chick basilar papilla: evidence for a development of the tonotopic organization

58 chicks or chick embryos were continuously exposed during 12 h on either embryonic day 18 or 20 or on post-hatching days 1, 10, 20 or 30 to 1.5 kHz pure tone at an intensity of 125 dB SPL. After a 20- or 30-day survival time, audiograms were recorded and then the basilar papillae were prepared for scanning electron microscopy. The frequency of maximum threshold elevation was seen at about 4 kHz when the chicks were exposed to the traumatic tone at embryonic day 18. It was shifted toward lower frequencies when the exposure was done at later stages. This shifting ended when the animals were exposed one day after hatching. After this stage, the maximum threshold elevation stabilized about one octave above the frequency of the traumatic tone. The position of maximum anatomical damage was located at 29.14% of the total length of the basilar papilla measured from the base when the exposure was done at embryonic day 20. It was shifted to 37% when the chicks were exposed one day after hatching or later. These results are in good agreement with recent hypotheses on development of the place principle. This development change seems to end at post-natal day 1 which also corresponds to the end of the anatomical and functional maturation of the basilar papilla.

Correlation of audiometric data with changes in cochlear hair cell stereocilia resulting from impulse noise trauma

In a previous experiment, after chinchillas had been exposed to impulse noise trauma, plastic-embedded surface preparations of the organ of Corti were examined with the light microscope. A consistent relationship between cochlear hair cell loss and hearing loss was not found (Hamernik et al., 1980). In the present study, four cochleas from that experiment were sectioned and examined with the transmission electron microscope to determine if their were consistent patterns of damage to the sensory cells at the ultrastructural level that would more closely correlate with the audiometric data. Alterations of the outer hair cell stereocilia were found when threshold was elevated 15 to 30 dB. The membranes of the stereocilia appeared loose and wrinkled and the stereocilia were no longer erect. In some cases, predominantly in the first row of outer hair cells, stereocilia were missing and in other cases, stereocilia were fused. Within these giant stereocilia, the rootlets of the individual stereocilia had disintegrated. Other alterations in sensory cell ultrastructure, though present, had no consistent pattern and could not be related to changes in hearing thresholds. Only the changes in the outer hair cell stereocilia appeared to correlate with hearing loss and the degree of damage was reflected in the amount of threshold elevation.

Reduced temporary and permanent hearing losses with multiple tone exposures

Acoustic overstimulation of the guinea pig cochlea with a 16 kHz pure tone induces a loss in threshold sensitivity that can be either temporary or permanent depending on the duration of the trauma. When a second tone of lower frequency (10, 5 or 2 kHz) is presented to the same cochlea simultaneously with the first tone, then the resultant threshold loss produced by the 16 kHz tone is significantly less. This applies to both temporary and permanent threshold losses. The reduced threshold loss disappears as the intensity of the second tone decreases. This type of nonlinear cochlea behaviour is similar to other acoustically evoked nonlinearities generally grouped under the term, two-tone suppression or inhibition. The results disagree with the "equal energy hypothesis' as a method to establish damage risk criteria in noise-induced hearing loss.

Total energy and critical intensity concepts in noise damage

Groups of chinchillas were given a series of noise exposures of approximately equal energy ranging from 22 minutes at 120 dB SPL to 150 days at 82 dB. For all exposures involving levels of 112 dB or less, the same average permanent hearing losses (15-20 dB) and degree of outer hair cell destruction (8-10%) resulted, thus confirming the validity of the total energy principle for assessing the hazard associated with single continuous exposures at moderate levels. The 22-minute, 120-dB exposure, however, produced a 60-dB hearing loss and massive hair cell destruction (70-80%), indicating that some critical level had been exceeded, thus producing acoustic trauma. Further histological study suggests that the massive destruction is a result of breaks in the organ of Corti, produced by severe mechanical stress, that permit the mixture of endolymph with perilymph, thus creating a hostile environment for the hair cells.

Ultrastructural changes to the cochlea resulting from impulse noise

Following impulse noise trauma to chinchillas, observation of plastic-embedded surface preparations of the organ of Corti showed no consistent relationship between cochlear hair cell loss and permanent hearing loss (Hamernik et al. 1980). In some animals there was a loss of hearing when hair cells were present. The cochleas from that experiment were examined with transmission electron microscopy to determine at the ultrastructural level if there was damage to the sensory cells that would explain the change in threshold sensitivity. Ultrastructural changes in cochlear hair cells include an increase in lysosomes, multivesicular bodies, vacuolization of subsurface cisternae, and proliferation of Hensen bodies. These changes are observed in all experimented animals. Alterations to the ultrastructure of the stereocilia vary from animal to animal and on the outer hair cells, the changes include loosening of the stereocilia membranes, loss of stiffness, fusion of the stereocilia and disintegration of the rootlets. These changes are observed only in animals that have a permanent threshold shift after noise trauma.

Use of the CO2 laser in otology

There are conflicting reports of the effect on the inner ear of the use of a carbon dioxide (CO2) laser. In four cats, we made middle ear lesions with a CO2 laser. Subsequent evaluation by inverted-phase and scanning electron microscopy revealed no inner ear pathology attributable to the CO2 laser. Apparently, appropriate use of the CO2 laser does not harm the inner ear.

Surgical anatomy of the temporal bone in the chinchilla

The chinchilla is of value in otological research for many reasons, including the surgical accessibility of the majority of structures within its temporal bone. This paper describes the anatomy of the chinchilla's temporal bone, and four surgical approaches to the labyrinth and ossicular chain, three through the bulla and the other via the external canal. No one approach reveals all the temporal bone structures, and each route is therefore more suited to some surgical procedures than others. The cochlea is particularly accessible for microsurgical procedures because it projects into the labyrinthine part of the bulla and because its bony capsule is thin. Surgery in the posterior cranial fossae is both hazardous and difficult; the hazard is bleeding from the venous sinuses which run within the occipital and temporal bones, and the difficulty is the limited access due to the intervening cerebellum and the closeness of the brain stem.

The effect of high level sound on hearing sensitivity, cochlear sensorineuroepithelium and vasculature of the chinchilla

Ten young chinchilla were tested for hearing and then exposed for 8 hours to a 110 dB (SPL) broad-band noise. Post-stimulation recovery was assessed daily until permanent threshold shifts were obtained. Cochlear tissues were prepared in order to allow viewing of sensory cells as well as the vascular supply of the cochlea. Findings included inconsistent displacement of the vestibular membrane, poor injection of a contrast medium into vessels, moderate outer hair cell loss and displaced inner hair cell nuclei. Small basal turn cell damage was accompanied by greater than expected losses for high frequency pure tones.