The effect of noise exposure on the aging ear

Hidden hearing loss: Fifteen years at a glance

Hearing loss affects approximately 18% of the population worldwide. Hearing difficulties in noisy environments without accompanying audiometric threshold shifts likely affect an even larger percentage of the global population. One of the potential causes of hidden hearing loss is cochlear synaptopathy, the loss of synapses between inner hair cells (IHC) and auditory nerve fibers (ANF). These synapses are the most vulnerable structures in the cochlea to noise exposure or aging. The loss of synapses causes auditory deafferentation, i.e., the loss of auditory afferent information, whose downstream effect is the loss of information that is sent to higher-order auditory processing stages. Understanding the physiological and perceptual effects of this early auditory deafferentation might inform interventions to prevent later, more severe hearing loss. In the past decade, a large body of work has been devoted to better understand hidden hearing loss, including the causes of hidden hearing loss, their corresponding impact on the auditory pathway, and the use of auditory physiological measures for clinical diagnosis of auditory deafferentation. This review synthesizes the findings from studies in humans and animals to answer some of the key questions in the field, and it points to gaps in knowledge that warrant more investigation. Specifically, recent studies suggest that some electrophysiological measures have the potential to function as indicators of hidden hearing loss in humans, but more research is needed for these measures to be included as part of a clinical test battery.

Age-dependent structural reorganization of utricular ribbon synapses

In mammals, spatial orientation is synaptically-encoded by sensory hair cells of the vestibular labyrinth. Vestibular hair cells (VHCs) harbor synaptic ribbons at their presynaptic active zones (AZs), which play a critical role in molecular scaffolding and facilitate synaptic release and vesicular replenishment. With advancing age, the prevalence of vestibular deficits increases; yet, the underlying mechanisms are not well understood and the possible accompanying morphological changes in the VHC synapses have not yet been systematically examined. We investigated the effects of maturation and aging on the ultrastructure of the ribbon-type AZs in murine utricles using various electron microscopic techniques and combined them with confocal and super-resolution light microscopy as well as metabolic imaging up to 1 year of age. In older animals, we detected predominantly in type I VHCs the formation of floating ribbon clusters, mostly consisting of newly synthesized ribbon material. Our findings suggest that VHC ribbon-type AZs undergo dramatic structural alterations upon aging.

The Relative and Combined Effects of Noise Exposure and Aging on Auditory Peripheral Neural Deafferentation: A Narrative Review

Animal studies have shown that noise exposure and aging cause a reduction in the number of synapses between low and medium spontaneous rate auditory nerve fibers and inner hair cells before outer hair cell deterioration. This noise-induced and age-related cochlear synaptopathy (CS) is hypothesized to compromise speech recognition at moderate-to-high suprathreshold levels in humans. This paper evaluates the evidence on the relative and combined effects of noise exposure and aging on CS, in both animals and humans, using histopathological and proxy measures. In animal studies, noise exposure seems to result in a higher proportion of CS (up to 70% synapse loss) compared to aging (up to 48% synapse loss). Following noise exposure, older animals, depending on their species, seem to either exhibit significant or little further synapse loss compared to their younger counterparts. In humans, temporal bone studies suggest a possible age- and noise-related auditory nerve fiber loss. Based on the animal data obtained from different species, we predict that noise exposure may accelerate age-related CS to at least some extent in humans. In animals, noise-induced and age-related CS in separation have been consistently associated with a decreased amplitude of wave 1 of the auditory brainstem response, reduced middle ear muscle reflex strength, and degraded temporal processing as demonstrated by lower amplitudes of the envelope following response. In humans, the individual effects of noise exposure and aging do not seem to translate clearly into deficits in electrophysiological, middle ear muscle reflex, and behavioral measures of CS. Moreover, the evidence on the combined effects of noise exposure and aging on peripheral neural deafferentation in humans using electrophysiological and behavioral measures is even more sparse and inconclusive. Further research is necessary to establish the individual and combined effects of CS in humans using temporal bone, objective, and behavioral measures.

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

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

Big brown bats (Eptesicus fuscus) maintain hearing sensitivity after exposure to intense band-limited noise

Thresholds to short-duration narrowband frequency-modulated (FM) sweeps were measured in six big brown bats (Eptesicus fuscus) in a two-alternative forced choice passive listening task before and after exposure to band-limited noise (lower and upper frequencies between 10 and 50 kHz, 1 h, 116-119 dB sound pressure level root mean square; sound exposure level 152 dB). At recovery time points of 2 and 5 min post-exposure, thresholds varied from -4 to +4 dB from pre-exposure threshold estimates. Thresholds after sham (control) exposures varied from -6 to +2 dB from pre-exposure estimates. The small differences in thresholds after noise and sham exposures support the hypothesis that big brown bats do not experience significant temporary threshold shifts under these experimental conditions. These results confirm earlier findings showing stability of thresholds to broadband FM sweeps at longer recovery times after exposure to broadband noise. Big brown bats may have evolved a lessened susceptibility to noise-induced hearing losses, related to the special demands of echolocation.

Echolocation behavior in big brown bats is not impaired after intense broadband noise exposures

Echolocating bats emit trains of intense ultrasonic biosonar pulses and listen to weaker echoes returning from objects in their environment. Identification and categorization of echoes are crucial for orientation and prey capture. Bats are social animals and often fly in groups in which they are exposed to their own emissions and to those from other bats, as well as to echoes from multiple surrounding objects. Sound pressure levels in these noisy conditions can exceed 110 dB, with no obvious deleterious effects on echolocation performance. Psychophysical experiments show that big brown bats (Eptesicus fuscus) do not experience temporary threshold shifts after exposure to intense broadband ultrasonic noise, but it is not known if they make fine-scale adjustments in their pulse emissions to compensate for any effects of the noise. We investigated whether big brown bats adapt the number, temporal patterning or relative amplitude of their emitted pulses while flying through an acoustically cluttered corridor after exposure to intense broadband noise (frequency range 10-100 kHz; sound exposure level 152 dB). Under these conditions, four bats made no significant changes in navigation errors or in pulse number, timing and amplitude 20 min, 24 h or 48 h after noise exposure. These data suggest that big brown bats remain able to perform difficult echolocation tasks after exposure to ecologically realistic levels of broadband noise.

Broadband noise exposure does not affect hearing sensitivity in big brown bats (Eptesicus fuscus)

In many vertebrates, exposure to intense sounds under certain stimulus conditions can induce temporary threshold shifts that reduce hearing sensitivity. Susceptibility to these hearing losses may reflect the relatively quiet environments in which most of these species have evolved. Echolocating big brown bats (Eptesicus fuscus) live in extremely intense acoustic environments in which they navigate and forage successfully, both alone and in company with other bats. We hypothesized that bats may have evolved a mechanism to minimize noise-induced hearing losses that otherwise could impair natural echolocation behaviors. The hearing sensitivity of seven big brown bats was measured in active echolocation and passive hearing tasks, before and after exposure to broadband noise spanning their audiometric range (10–100 kHz, 116 dB SPL re. 20 µPa rms, 1 h duration; sound exposure level 152 dB). Detection thresholds measured 20 min, 2 h or 24 h after exposure did not vary significantly from pre-exposure thresholds or from thresholds in control (sham exposure) conditions. These results suggest that big brown bats may be less susceptible to temporary threshold shifts than are other terrestrial mammals after exposure to similarly intense broadband sounds. These experiments provide fertile ground for future research on possible mechanisms employed by echolocating bats to minimize hearing losses while orienting effectively in noisy biological soundscapes.

Inhibition of cyclooxygenase-2 by NS398 attenuates noise-induced hearing loss in mice

Noise-induced hearing loss (NIHL) is an important occupational disorder. However, the molecular mechanisms underlying NIHL have not been fully clarified; therefore, the condition lacks effective therapeutic methods. Cyclooxygenase-2 (Cox-2) is an inducible enzyme involved in the synthesis of prostaglandins, and has been implicated in many pathophysiological events, such as oxidative stress and inflammation. In this study, we investigated the possible role of Cox-2 in the mechanisms of NIHL and the therapeutic effect of the Cox-2 inhibitor NS398 on NIHL using a mouse model. We demonstrated that Cox-2 is constitutively expressed in the mouse cochlea, and its expression could be dramatically up-regulated by high levels of noise exposure. Furthermore, we demonstrated that pre-treatment with the Cox-2 inhibitor NS398 could inhibit Cox-2 expression during noise overstimulation; and could attenuate noise-induced hearing loss and hair cell damage. Our results suggest that Cox-2 is involved in the pathogenesis of NIHL; and pharmacological inhibition of Cox-2 has considerable therapeutic potential in NIHL.

Reduction in noise-induced functional loss of the cochleae in mice with pre-existing cochlear dysfunction due to genetic interference of prestin

Various cochlear pathologies, such as acoustic trauma, ototoxicity and age-related degeneration, cause hearing loss. These pre-existing hearing losses can alter cochlear responses to subsequent acoustic overstimulation. So far, the knowledge on the impacts of pre-existing hearing loss caused by genetic alteration of cochlear genes is limited. Prestin is the motor protein expressed exclusively in outer hair cells in the mammalian cochlea. This motor protein contributes to outer hair cell motility. At present, it is not clear how the interference of prestin function affects cochlear responses to acoustic overstimulation. To address this question, a genetic model of prestin dysfunction in mice was created by inserting an internal ribosome entry site (IRES)-CreERT2-FRT-Neo-FRT cassette into the prestin locus after the stop codon. Homozygous mice exhibit a threshold elevation of auditory brainstem responses with large individual variation. These mice also display a threshold elevation and a shift of the input/output function of the distortion product otoacoustic emission, suggesting a reduction in outer hair cell function. The disruption of prestin function reduces the threshold shifts caused by exposure to a loud noise at 120 dB (sound pressure level) for 1 h. This reduction is positively correlated with the level of pre-noise cochlear dysfunction and is accompanied by a reduced change in Cdh1 expression, suggesting a reduction in molecular responses to the acoustic overstimulation. Together, these results suggest that prestin interference reduces cochlear stress responses to acoustic overstimulation.

Effect of epithelial stem cell transplantation on noise-induced hearing loss in adult mice

Noise trauma in mammals can result in damage to multiple epithelial cochlear cell types, producing permanent hearing loss. Here we investigate whether epithelial stem cell transplantation can ameliorate noise-induced hearing loss in mice. Epithelial stem/progenitor cells isolated from adult mouse tongue displayed extensive proliferation in vitro as well as positive immunolabelling for the epithelial stem cell marker p63. To examine the functional effects of cochlear transplantation of these cells, mice were exposed to noise trauma and the cells were transplanted via a lateral wall cochleostomy 2 days post-trauma. Changes in auditory function were assessed by determining auditory brainstem response (ABR) threshold shifts 4 weeks after stem cell transplantation or sham surgery. Stem/progenitor cell transplantation resulted in a significantly reduced permanent ABR threshold shift for click stimuli compared to sham-injected mice, as corroborated using two distinct analyses. Cell fate analyses revealed stem/progenitor cell survival and integration into suprastrial regions of the spiral ligament. These results suggest that transplantation of adult epithelial stem/progenitor cells can attenuate the ototoxic effects of noise trauma in a mammalian model of noise-induced hearing loss.

Frequency-dependent and longitudinal changes in noise-induced hearing loss in a bottlenose dolphin (Tursiops truncatus)

Temporary threshold shift (TTS) was measured in a bottlenose dolphin (Tursiops truncatus) after exposure to 16-s tones at 3 and 20 kHz to examine the effects of exposure frequency on the onset and growth of TTS. Thresholds were measured approximately one-half octave above the exposure frequency using a behavioral response paradigm featuring an adaptive staircase procedure. Preliminary data provide evidence of frequency-specific differences in TTS onset and growth, and increased susceptibility to auditory fatigue after exposure to 3-kHz tones compared to data obtained two years earlier.

Analysis of environmental sound levels in modern rodent housing rooms

Noise in animal housing facilities is an environmental variable that can affect hearing, behavior and physiology in mice. The authors measured sound levels in two rodent housing rooms (room 1 and room 2) during several 24-h periods. Room 1, which was subject to heavy personnel traffic, contained ventilated racks and static cages that housed large numbers of mice. Room 2 was accessed by only a few staff members, contained static cages only and housed fewer mice. In both rooms, background sound levels were usually about 80 dB, and transient noises caused sound levels to temporarily rise 30-40 dB above the baseline level; such peaks occurred frequently during work hours (8:30 AM to 4:30 PM) and infrequently during non-work hours. Noise peaks during work hours in room 1 occurred about two times as often as in room 2 (P = 0.01). Use of changing stations located in the rooms caused background noise to increase by about 10 dB. Loud noise and noise variability were attributed mainly to personnel activity. Attempts to reduce noise should concentrate on controlling sounds produced by in-room activities and experimenter traffic; this may reduce the variability of research outcomes and improve animal welfare.

Effects of sex, gonadal hormones, and augmented acoustic environments on sensorineural hearing loss and the central auditory system: insights from research on C57BL/6J mice

Mice of the C57BL/6J (B6) inbred strain exhibit genetic progressive sensorineural hearing loss and have been widely used as a model of adult-onset hearing loss and presbycusis. Males and females exhibit similar degrees of hearing loss until about 3 months of age, after which, the loss accelerates in females. This paper reviews research on how the B6 auditory system is affected by sex, gonadectomy (i.e., a reduction of gonadal hormone levels), and nightly exposure to moderately intense augmented acoustic environments (AAEs) - a low-frequency noise band (LAAE) or high-frequency band (HAAE). Several findings indicate a negative effect of ovarian hormones on the female B6 auditory system. Whereas the sex difference in high-frequency hearing loss was not significantly affected by gondadectomies, the female disadvantage in ABR thresholds at lower frequencies was erased by ovariectomy. Moreover, exposure to the LAAE or HAAE caused losses of hair cells that were more severe in intact females than in ovariectomized females or in males. Finally, intact females had more severe loss of neurons in the low-frequency region of the anterior ventral cochlear nucleus (AVCN) than other groups. In contrast, the presence of androgens had beneficial effects. Loss of hair cells and AVCN neurons after AAE exposure were more severe in orchidectomized males than in intact males. Ideas, hypotheses, and potential mechanisms concerning the findings are discussed.

Auditory efferent feedback system deficits precede age-related hearing loss: contralateral suppression of otoacoustic emissions in mice

The C57BL/6J mouse has been a useful model of presbycusis, as it displays an accelerated age-related peripheral hearing loss. The medial olivocochlear efferent feedback (MOC) system plays a role in suppressing cochlear outer hair cell (OHC) responses, particularly for background noise. Neurons of the MOC system are located in the superior olivary complex, particularly in the dorsomedial periolivary nucleus (DMPO) and in the ventral nucleus of the trapezoid body (VNTB). We previously discovered that the function of the MOC system declines with age prior to OHC degeneration, as measured by contralateral suppression (CS) of distortion product otoacoustic emissions (DPOAEs) in humans and CBA mice. The present study aimed to determine the time course of age changes in MOC function in C57s. DPOAE amplitudes and CS of DPOAEs were collected for C57s from 6 to 40 weeks of age. MOC responses were observed at 6 weeks but were gone at middle (15-30 kHz) and high (30-45 kHz) frequencies by 8 weeks. Quantitative stereological analyses of Nissl sections revealed smaller neurons in the DMPO and VNTB of young adult C57s compared with CBAs. These findings suggest that reduced neuron size may underlie part of the noteworthy rapid decline of the C57 efferent system. In conclusion, the C57 mouse has MOC function at 6 weeks, but it declines quickly, preceding the progression of peripheral age-related sensitivity deficits and hearing loss in this mouse strain.

Contralateral normal ear after mastoid surgery: evaluation by otoacoustic emissions (mastoid drilling and hearing loss)

Mastoid drilling can cause transient hearing loss in the contralateral normal hearing ear. A study was designed to evaluate hearing in the contralateral normal ear before and after the mastoid surgery in a longitudinal manner and find out the duration of this temporary hearing loss. Twenty-two patients requiring mastoid surgery in their diseased ears, having contralateral normal ear were included. Pure tone audiometry and otoacoustic emissions (OAEs) were utilized for baseline evaluation. OAEs were repeated during the immediate postoperative period and daily up to the 6th postoperative day. The amplitudes of the OAEs of contralateral normal ears were found affected immediately after surgery and progressive improvement was detected with full recovery at 72-96 h. None of the patients had permanent deterioration in OAE amplitudes. The burs used during mastoid surgery can cause temporary hearing threshold changes in the contralateral ears. This adverse effect recovers spontaneously within 72-96 h postoperatively.

Distortion product otoacoustic emissions show exceptional resistance to noise exposure in MOLF/Ei mice

Baseline distortion-product otoacoustic emissions (DPOAEs) at several primary-tone levels were compared between naive 2- to 3-month old inbred CBA/CaJ (CBA) and wild-derived MOLF/Ei (MOLF) mice. Only minor DPOAE differences were noted between the two strains and these differences were not systematic across frequency or test levels. These emission findings were consistent with earlier results on auditory brainstem response thresholds reported by others [Zheng et al., Hear. Res. 130 (1999) 94-107] thus suggesting that both CBA and MOLF strains have normal hearing. Subsequent episodes of over-exposure to a 105-dB SPL, octave-band noise centered at 10 kHz for 8 h revealed that MOLF DPOAEs were exceptionally resistant to the adverse aftereffects of excessive noise exposure as compared to CBA mice. Unlike the noise-exposure resistant inbred 129/SvEvTac strain, which has reduced baseline DPOAE levels especially at high frequencies, MOLF mice have normal DPOAEs making the interpretation of noise-exposure effects more straightforward.

Effects of prolonged exposure to an augmented acoustic environment on the auditory system of middle-aged C57BL/6J mice: cochlear and central histology and sex differences

Genetic progressive sensorineural hearing loss in mice of the C57BL/6J (B6) inbred strain begins at high frequencies during young adulthood and is severe by 12 months (middle age). Nightly treatment with an augmented acoustic environment (AAE)--12-hour periods of exposure to repetitive noise bursts of moderate intensity, begun at age 25 days--resulted in less severe hearing loss compared with control mice. Cochlear histopathological correlates of AAE treatment, assessed at 12-14 months of age, included lessened severity of progressive loss of outer hair cells in both sexes as well as small savings of spiral ganglion cells in females and inner hair cells in males. AAE effects on the number of surviving neurons (age 12-14 months) in the anterior ventral cochlear nucleus (AVCN) depended on sex. Compared with controls, the loss of AVCN neurons that typically accompanies the initial period of hearing loss (between 2 and 7 months of age) was not significantly affected by AAE treatment in females. In contrast, males treated with the AAE exhibited more severe loss of neurons in the dorsal and ventral extremes of the AVCN than male controls of the same age. AAE treatment begun at age 3-5 months resulted in significant but less severe loss of AVCN neurons in 1-year-old male mice.

Susceptibility of young adult and old rats to noise-induced hearing loss

This study was designed to test whether old rats show signs of presbyacusis and whether they would be either similarly or more or less susceptible to noise-induced hearing loss than young adult rats. Old (24 months) and young adult (3-4 months) Wistar rats were exposed to a broad-band noise of 113 dB SPL for a duration of 1 h (producing temporary threshold shifts) or 3 days (12 h noise/12 h quiet; permanent shifts). Auditory brainstem response (ABR), distortion product otoacoustic emissions (DPOAEs) and transient evoked otoacoustic emissions (TEOAEs) were measured before and after exposure. At the initial recording (before exposure), old rats demonstrated a small mean ABR threshold elevation, a reduction in amplitude of wave I (WI), a shortening of WI latency and a prolongation of the interpeak interval between WI and WIV, as compared to the young rats. The old rats also demonstrated a small DPOAE amplitude reduction and a reduction of peak-to-peak amplitude in the TEOAE response 2 ms after stimulus, but no reduction in TEOAE energy content between 2 and 4 kHz. These are signs of presbyacusis in the old rats. The noise exposures caused elevations in ABR threshold and reductions in DPOAE amplitude and TEOAE energy content that were similar in both the old and young rats. Their recovery from the noise-induced loss was also similar. Thus, the results of this study show that old and young adult rats, at least when considering clinically relevant intensities and durations of noise exposure, are equally susceptible to the effects of the exposure.

Endocochlear potentials and compound action potential recovery: functions in the C57BL/6J mouse

The C57BL/6J mouse suffers from cochlear degeneration beginning at an early age and has been used as a model of age-related hearing loss (presbyacusis). Here, the endocochlear potential (EP) and compound action potential (CAP) responses were determined in one-, four-, 12- and 24-month-old C57BL/6J mice. CAP measures included thresholds to tone pips, input/output (I/O) functions, and recovery functions to conditioning tones. EP values among the four age groups did not differ significantly (P>0.05) in either the basal or apical turns. CAP thresholds were increased significantly by 10 to 30 dB in the four-month group compared to the one-month controls at 11.3, 16, 20, and 22.6 kHz. CAP I/O functions were shallower in the four-month group compared to controls at all frequencies. In the 12- and 24-month-old mice, CAP responses were absent, despite normal EP values in these animals. Recovery functions after conditioning tones were obtained at 8, 16, 20 and 22.6 kHz; the functions had fast and slow components at all frequencies tested in both the one- and four-month-old groups. The corresponding recovery curves were identical for both age groups, even with significant threshold shifts in the older group. The two component recovery curves provide the first physiological evidence that different spontaneous rate (SR) classes of auditory neurons exist in the C57BL/6J mouse. Moreover, the unchanged recovery functions in the older group suggest that there was no loss of activity of the low-SR fiber population with age under conditions where the EP remains stable, in contrast to the gerbil model of presbyacusis where there is a loss of low-SR fiber activity and EP does decline with age.

Age and noise-induced hearing loss

The purpose of the study was to evaluate the effect of noise, age and confounders in noise-induced hearing loss (NIHL). Information about work exposure, the use of hearing protective devices, audiogram, environmental and biological factors was collected from 406 paper mill workers exposed to noise levels of 91-94 dB(A), 124 forest workers exposed to noise levels of 96-99 dB(A) and 176 shipyard workers exposed to noise levels 95-97 dB(A). In addition to noise exposure, we collected the following confounders: smoking habits, serum cholesterol, systolic or diastolic blood pressure and use of analgesics. Subjects were classified based on median values, into high- and low-risk groups. The confounders were a significant source of hearing loss (HL) in younger and elderly groups of subjects, serum cholesterol level being the most important. In risk analysis the confounders partly masked the effects of noise in the development of HL. For subjects with less than two confounders, occupational noise exposure determined the development of NIHL. As the number of confounders increased, the noise exposure was overruled by these factors in the development of HL. In analysis where the subjects were matched with pairs by age, exposure, blood pressure and serum cholesterol level, the elderly subjects were more susceptible to NIHL than younger subjects. Factors independently but causally related to age were important in the development of NIHL among workers exposed to noise levels below 98 dB(A).

Plastic changes in the central auditory system after hearing loss, restoration of function, and during learning

Traditionally the auditory system was considered a hard-wired sensory system; this view has been challenged in recent years in light of the plasticity of other sensory systems, particularly the visual and somatosensory systems. Practical experience in clinical audiology together with the use of prosthetic devices, such as cochlear implants, contributed significantly to the present view on the plasticity of the central auditory system, which was originally based on data obtained in animal experiments. The loss of auditory receptors, the hair cells, results in profound changes in the structure and function of the central auditory system, typically demonstrated by a reorganization of the projection maps in the auditory cortex. These plastic changes occur not only as a consequence of mechanical lesions of the cochlea or biochemical lesions of the hair cells by ototoxic drugs, but also as a consequence of the loss of hair cells in connection with aging or noise exposure. In light of the aging world population and the increasing amount of noise in the modern world, understanding the plasticity of the central auditory system has its practical consequences and urgency. In most of these situations, a common denominator of central plastic changes is a deterioration of inhibition in the subcortical auditory nuclei and the auditory cortex. In addition to the processes that are elicited by decreased or lost receptor function, the function of nerve cells in the adult central auditory system may dynamically change in the process of learning. A better understanding of the plastic changes in the central auditory system after sensory deafferentation, sensory stimulation, and learning may contribute significantly to improvement in the rehabilitation of damaged or lost auditory function and consequently to improved speech processing and production.

Deficiency in plasma membrane calcium ATPase isoform 2 increases susceptibility to noise-induced hearing loss in mice

Susceptibility to noise-induced hearing loss (NIHL) is poorly understood at the genetic level. Mice homozygous for a null mutation in the plasma membrane Ca2+-ATPase isoform 2 (PMCA2) gene are deaf (Kozel et al., 1998). PMCA2 is expressed on outer hair cell stereocilia (Furuta et al., 1998). Fridberger et al. (1998) observed that the outer hair cell cytoplasmic Ca2+ concentration rises following acoustic overstimulation. We hypothesized that Pmca2+/- mice may be more susceptible to NIHL. Since the auditory brainstem response (ABR) thresholds of Pmca2+/- mice vary with the presence of a modifier locus (Noben-Trauth et al., 1997), Pmca2+/- mice were outcrossed to normal hearing CAST/Ei mice. The pre-exposure ABR thresholds of the resulting Pmca2+/+ and Pmca2+/- siblings were indistinguishable. Groups of these mice were exposed to varying intensities of broadband noise, and ABR threshold shifts were calculated. Fifteen days following an 8 h, 113 dB noise exposure, the Pmca2+/- mice displayed significant (P < or = 0.0007) permanent threshold shifts at 16 and 32 kHz that were 15 or 25 dB greater than those observed in Pmca2+/+ littermates. Pmca2 may be the first gene with a known mutated protein product that confers increased susceptibility to NIHL.

Selective aspects of human pathology in high-tone hearing loss of the aging inner ear

Accompanied with aging, the thresholds for high frequency sounds may elevate and result in a progressive hearing loss described as presbycusis. Based on correlations between audiometric measures of aged patients and histologic findings garnered from postmortem examinations, four types of presbycusis have been characterized: sensory-neural, neural, strial, and conductive [Schuknecht, H.F., Gacek, M.R., 1993. Ann. Otol. Rhinol. Laryngol. 102, 1--16]. Otopathologic changes to the inner ear as a direct function of age, however, remain controversial. The focus of this investigation involves the pathological impact on remaining sensory structures in patients having sensory--neural degeneration. The current study presents seven human temporal bones extracted from patients aged 53--67 years with high-tone hearing loss and with no known history of extraordinary environmental events involving head or noise trauma, acoustic overstimulation, or ototoxicity. In previously published findings of these specimens, all but one temporal bone failed to demonstrate a meaningful correlation between audiometric measurements and loss of functional hair cell populations with secondary retrograde degeneration of nerve fibers. Using the block surface method, electron microscopic micrographs demonstrate ultrastructural changes in the cuticular plate, stereocilia, pillar cells, stria vascularis, and the spiral ligament. In all pathological specimens, the greatest incidence of degeneration was seen at the cuticular plate. Conclusively, our findings present three implications in the aging human cochlea: firstly, audiometric measures that represent a high-tone hearing loss may take various forms with respect to ultrastructural patterns of degeneration and surviving structures; secondly, the incidence of lipofuscin and lysosome granules does not correlate with the degree of hearing loss and; thirdly, as shown only in guinea pigs [Anniko, M., 1988. Scanning Microsc. 2, 1035--1041], high-tone hearing loss can be associated with deformation of the cuticular plate.

Temporary and permanent noise-induced changes in distortion product otoacoustic emissions in CBA/CaJ mice

A number of studies have shown that the ear can be protected from sound over-exposure, either by activating the cochlear efferent system, or by sound 'conditioning' in which the role of the efferent system is less certain. To study more definitively the molecular basis of deliberately induced cochlear protection from excessive sounds, it is advantageous to determine, for an inbred mouse strain, a range of noise exposure parameters that effectively alter cochlear function. As an initial step towards this goal, young CBA/CaJ mice were exposed to a 105-dB SPL octave-band noise (OBN), centered at 10 kHz, for various lengths of time consisting of 10 min, or 0.5, 1, 3, or 6 h. Distortion product otoacoustic emissions (DPOAEs) at the 2f1-f2 frequency, in response to equilevel primary tones of low to moderate levels, were used to quantify the damaging effects of these sound over-exposures on cochlear function. In addition, staining for acetylcholinesterase (AChE) activity to assess for noise-induced changes in the pattern of efferent-nerve innervation to the cochlea was also performed in a subset of mice that were exposed to the longest-lasting 6-h OBN. The 10-min OBN resulted in only temporary reductions in DPOAE levels, which recovered to pre-exposure values within 5 days. Increasing the exposure to 0.5 h resulted in permanent DPOAE losses that, for low primary-tone levels, were still present at 31 days post-exposure. Additionally, the 1-h and longer exposures caused permanent reductions in DPOAEs for all test levels, which were measurable at 31 days following exposure. Light-microscopic observations restricted to the 11-18-kHz frequency region of the organ of Corti, for a subset of mice exposed to the 6-h OBN, uncovered a significant loss of outer hair cells (OHCs). However, despite the OHC loss in this region, the AChE activity associated with the related pattern of efferent innervation remained largely intact.

Noise-induced hearing loss: the effect of melanin in the stria vascularis

Conflicting investigations regarding the potential protective effect of melanin against noise-induced sensorineural hearing loss have suggested that eumelanin and pheomelanin may have differing effects within the stria vascularis. Three strains of C57BL/6J mice, (+/+, a/a) wild-types (dark coats/black eyes), (c2j/c2j, a/a), albinos (white coats/pink eyes), and (+/+, Ay/Ay) yellow mice (yellow coats/black eyes), were subjected to five consecutive days of broad band noise exposure at 112 dB(A) SPL for 3 h/day. Cochlear function was evaluated with auditory brainstem response audiometry to pure tones immediately pre-exposure, 5-6 h postexposure, and 14 days post-exposure. No significant difference in the degree of sensorineural hearing loss induced in the three strains of mice was identified. The eumelanin and pheomelanin content of each stria vascularis and amount of protein per stria for both mouse and guinea pig (2/NCR) were determined via high performance liquid chromatography. No pheomelanin was found in the stria of yellow mice, suggesting that coat color is not an accurate predictor of strial melanin content. The melanin content per mg of strial protein was higher in mice than in guinea pigs. A species-specific difference in melanin content does not explain the absence of a protective effect in mice.

Sensorineural hearing loss after vibration: an animal model for evaluating prevention and treatment of inner ear hearing loss

Sensorineural hearing loss following a variety of acoustic trauma, including middle ear surgery, is well known. Current literature, which points to the deleterious influence of noise on the inner ear during surgery, has yet to assess the influence of vibration generated by the burr. The purpose of the study reported here was to establish an animal model that mimics drilling and can be used to explore methods of hearing loss prevention and treatment. A specially developed electromagnetic vibrator was calibrated and used in 59 guinea pigs to induce hearing loss. Both young and old guinea pigs were used. The bony external ear canal of guinea pigs were exposed to vibration or sound of varying duration and intensity. The vibration of the temporal bone and noise level in the middle ear were measured. Electrocochleography was recorded to evaluate the hearing loss. Among the young animals, 90% developed a significant threshold shift (TS > 20 dB), when vibrated with 250 Hz at an intensity of 6.2 m/s2 for 15 min. An average of 42 dB TS was observed. With 10 min exposure 63% showed a TS. The older animals vibrated for 5 min developed the same TS (mean TS 34 dB) as the young animals when vibrated for 10 min. The vibration-induced TS showed no recovery within 3 days of observation. In the contralateral ear 4 out of 5 animals showed TS > 20 dB. When exposed to sound levels exceeding the vibration-generated sound in the middle ear (119 dB at 250 Hz) only 2 out of II animals (18%) showed TS. The frequency of TS and level of TS were significantly greater in the vibrated animals than in sound-only exposed animals (p < 0.01). The degree of vibration-induced TS in the present animal model could be controlled by vibration intensity and duration. The older animals were more susceptible to vibration-induced inner-ear damage than younger animals. This model will be used in further studies to find methods for prevention and treatment of hearing loss during ear surgery.

Vulnerability to noise-induced hearing loss in 'middle-aged' and young adult mice: a dose-response approach in CBA, C57BL, and BALB inbred strains

Vulnerability of the cochlea to noise-induced permanent threshold shifts (NIPTS) was examined in young adult (1-2 months) and 'middle-aged' (5-7 months) CBA/CaJ, C57BL/6J, and BALB/cJ inbred mice. For each age and strain, a dose-response paradigm was applied, whereby groups of up to 12 animals were exposed to intense broadband noise (110 dB SPL) for varying durations. Exposure durations reliably associated with <10% and >90% probability of a criterion amount of NIPTS (determined 2 weeks post-exposure) were identified, and the minimum NIPTS exposure and the slope of the dose-response relation were then derived by numerical modeling. For all three strains, young adult mice were more susceptible to NIPTS than older adults; That is, a shorter exposure was able to cause NIPTS in the younger mice. Strain comparisons revealed that C57 mice were more susceptible than CBAs in the older age group only. At both ages examined, however, BALB mice were most susceptible to NIPTS. When animals with a similar amount of NIPTS were compared, outer hair cell loss in the cochlear base was more widespread in the younger animals. BALB mice appear particularly susceptible to noise-induced outer hair cell loss throughout life. Our data suggest that the mechanism or site of noise injury differs between young adults and older adults, and may depend on genetic background. The finding that both BALB and C57 mice, which show pronounced age-related hearing loss, are also especially vulnerable to noise supports the notion that genes associated with age-related hearing loss often act by rendering the cochlea susceptible to insults.

Prolonged exposure to an augmented acoustic environment ameliorates age-related auditory changes in C57BL/6J and DBA/2J mice

The effects of exposure to an augmented acoustic environment (AAE) on auditory function were evaluated in mouse strains that exhibit high-frequency hearing loss beginning during young adulthood (the C57BL/6J strain [C57]) or around the time of weaning/ adolescence (the DBA/2J strain [DBA]). Beginning at age 25 days, mice were exposed 12 h every night to a 70 dB SPL broad-band noise AAE. The AAE was maintained until age 14 months in C57 mice and 9 months in DBA mice. Control mice were age-matched and maintained under normal vivarium acoustic conditions. The auditory brainstem response (ABR), acoustic startle response amplitude, and prepulse inhibition (PPI) were used to assess the auditory system. Exposure to the AAE resulted in improved auditory performance in both strains (better PPI, lower ABR thresholds, bigger startle amplitudes).

Distortion product otoacoustic emissions in the CBA/J mouse model of presbycusis

CBA mice do not exhibit age-related loss of auditory sensitivity or cochlear pathology until relatively late in life. Therefore, this strain is believed to be an excellent animal model for the examination of the effects of age on the cochlea. To evaluate the effects of age on outer hair cell function, 2f1-f2 distortion product otoacoustic emissions (DPOAEs) were measured for f2 between 8 and 16 kHz in CBA/J mice between 1 and 25 months of age. CBA mice exhibited mild age-related changes in DPOAE level and detection threshold at 17 months of age, and changes of 20-40 dB by 25 months of age. The DPOAE level decreased and detection threshold increased with age in a frequency-dependent manner, starting at high frequencies and eventually extending to low frequencies. The range of frequencies in which notches were observed in the DPOAE input/output (I/O) functions extended toward lower frequencies by 17 months of age. Notches were absent in the I/O functions of 25-month-old mice. The present results for a frequency range of 8-16 kHz suggest that age has modest effects on outer hair cell function in CBA mice.

Quantitative measure of genetic differences in susceptibility to noise-induced hearing loss in two strains of mice

The CBA/CaJ (CB) and C57BL/6J (B6) inbred strains of mice were exposed for 1 h to noise intensities between 98 and 119 dB SPL. Previous studies indicated that the B6 mice exhibited permanent threshold shifts (PTS) after 1h exposure to 110 dB, whereas the CB mice did not exhibit any PTS. These differences in susceptibility to noise-induced hearing loss (NIHL) appear to be due to a gene for age-related hearing loss (AHL). The current study was designed to determine dose-response curves for NIHL over the ranges of intensities of noise that would characterize the B6 and CB inbred strains of mice. Because of the considerable differences in sensitivity to NIHL, the noise exposures for the two strains overlapped only at 110 and 113 dB. Nevertheless, the two strains exhibited two different dose-response curves, offset and with different slopes. We postulate that the B6 strain of mice exhibits a more linear increase for PTS from 98-113 dB, consistent with incremental effects on some metabolic physiological mechanism(s); the abrupt transition in NIHL between 113 and 116 dB for the CB mice is consistent with an ototraumatic structural injury.

Assessment of hearing in 80 inbred strains of mice by ABR threshold analyses

The common occurrence of hearing loss in both humans and mice, and the anatomical and functional similarities of their inner ears, attest to the potential of mice being used as models to study inherited hearing loss. A large-scale, auditory screening project is being undertaken at The Jackson Laboratory (TJL) to identify mice with inherited hearing disorders. To assess hearing sensitivity, at least five mice from each inbred strain had auditory brainstem response (ABR) thresholds determined. Thus far, we have screened 80 inbred strains of mice; 60 of them exhibited homogeneous ABR threshold values not significantly different from those of the control strain CBA/CaJ. This large database establishes a reliable reference for normal hearing mouse strains. The following 16 inbred strains exhibited significantly elevated ABR thresholds before the age of 3 months: 129/J, 129/ReJ, 129/SvJ, A/J, ALR/LtJ, ALS/LtJ, BUB/BnJ, C57BLKS/J, C57BR/cdJ, C57L/J, DBA/2J, I/LnJ, MA/MyJ, NOD/LtJ, NOR/LtJ, and SKH2/J. These hearing impaired strains may serve as models for some forms of human non-syndromic hearing loss and aid in the identification of the underlying genes.

Age- and strain-related differences in dehydrogenase activity and glycogen levels in CBA and C57 mouse cochleas

In the C57 mouse strain, loss of sensory hair cells (HCs) begins during early adulthood, starting in the base of the cochlea and progressing toward the apex as aging continues. In contrast, the CBA mouse strain exhibits no significant cochlear histopathology until relatively late in life. These strain and age differences may be related to differences in cochlear energy metabolism. To examine this possibility, we used dehydrogenase and glycogen histochemistry to evaluate the metabolic capacities of HCs and stria vascularis (SV) in cochleas of C57 and CBA mice. Reaction product density was quantified and compared as a function of strain (1.5-month-old C57 mice vs. CBA mice) and age (CBA mice, 1.5, 18 and 36 months). Young C57 mice had significantly less HC dehydrogenase activity than CBA mice of any age, lower HC glycogen levels than 18-month-old CBA mice and lower SV glycogen levels than 18- or 36-month-old CBA animals. Within the CBA strain, HC dehydrogenase activity decreased significantly between 1.5 and 18 months of age, while glycogen levels in both HCs and SV increased over the same time period. Between 18 and 36 months, HC dehydrogenase activity and SV glycogen levels remained stable. The results show that there are significant age-related changes in energy metabolism in the inner ear of CBA mice that are correlated with age-related hearing loss. Genetically determined deficits in cochlear metabolic capacity in C57 mice could be linked to the early onset of hearing loss in this strain.

Exposure to an augmented acoustic environment alters auditory function in hearing-impaired DBA/2J mice

The effects of exposure to an augmented acoustic environment (AAE) on auditory function were evaluated using DBA/2J (DBA) mice, a strain that exhibits high-frequency hearing loss beginning around the time of weaning/adolescence (between 3-4 weeks of age) and becoming severe by 2-3 months of age. Mice were exposed 12 h per night for 10 nights to a 70 dB SPL broad-band noise AAE at one of three age periods ranging from the onset of hearing loss (25-35 days of age) to more severe degrees of hearing loss (35-45 days and 45-55 days); control mice did not receive the AAE. C57BL/6J (C57) mice of the same ages provided normal-hearing. age-matched mice in both exposed and control conditions. The auditory brainstem response (ABR), acoustic startle response amplitude, and prepulse inhibition (PPI) were used to assess the auditory system. The AAE had significant effects on DBA mice, but had no effect on normal-hearing C57 mice. For the most part, AAE exposure resulted in improved auditory performance in DBA mice (better PPI, lower ABR thresholds, bigger startle amplitudes). However, the age of the mice and/or severity of hearing loss proved to be an important variable; improvement of PPI occurred only when the AAE was initiated later in the course of hearing loss (35 days of age or older); in contrast to this, beneficial effects on ABR thresholds occurred only when the AAE was initiated early in the course of hearing loss (< 45 days of age).

Effects of noise on inferior colliculus evoked potentials and cochlear anatomy in young and aged chinchillas

Like many aging humans, the aging chinchilla tends to lose high-frequency sensitivity at a faster rate than low-frequency sensitivity. This feature, combined with its excellent low-frequency hearing, makes the chinchilla attractive as an animal model for studying the relationship between noise-induced hearing loss (NIHL) and age-related hearing loss (AHL). In the present study, we examined susceptibility to noise in 15 aged (10-15 years old) and 15 young chinchillas. Two levels of noise were used, with the aim of determining whether age-related differences exist in the magnitude and rate of recovery from temporary threshold shifts produced by a moderate-level (95 dB) noise exposure, or in susceptibility to permanent threshold shifts and cochlear damage caused by a high-level (106 dB) noise exposure. Thresholds and response amplitudes at 0.5, 1, 2, 4, 8 and 16 kHz were determined from evoked potentials recorded from the inferior colliculus. Cochlear histology was performed on animals exposed to high-level noise. The results suggest that older animals are equally vulnerable to moderate-level noise, but may be slightly more vulnerable to high-level noise. For moderate-level exposures, there appears to be a simple additive relationship (in dB) between AHL and NIHL. For high-level exposures, the relationship may be more complex.

The BALB/c mouse as an animal model for progressive sensorineural hearing loss

To develop the BALB/c mouse strain as an animal model for the study of progressive sensorineural hearing loss, mice ranging in age from young adult through middle age were studied. Auditory brainstem response thresholds, histopathology [cytocochleograms for hair cells, the packing density of spiral ganglion cells (SGCs), the number of neurons and overall size of the anterior ventral cochlear nucleus (AVCN)], and behavioral paradigms (prepulse inhibition, fear-potentiated startle) were compared with previous data from C57BL/6J (C57) and DBA/2J (DBA) mouse strains. Progressive high frequency hearing loss in BALB/c mice was generally more rapid than C57 and slower than DBA (e.g. mean thresholds for 16 kHz: 10-month-old BALB/c mice = 71 dB SPL; 55-day-old DBA mice = 79 dB SPL; 12-month-old C57 mice = 50 dB SPL). Like the other strains, BALB/c exhibited a progressive loss of hair cells and SGCs that was most severe in the cochlear base and least severe in the middle turns; however, BALB/c mice had relatively more SGC loss in the apex. Unlike C57 and DBA, no loss of neurons was observed in the AVCN following cochlear pathology (although AVCN volume was reduced). Like the other strains, successful fear conditioning was obtained with a 12 kHz conditioned stimulus. Prepulse inhibition showed that middle and low frequency tones (4-12 kHz) became more salient as high frequency hearing declined. Similar results had been previously obtained with C57 and DBA mice and were interpreted as reflecting hearing-loss-induced plasticity in the central auditory system.

Distortion product otoacoustic emissions in the C57BL/6J mouse model of age-related hearing loss

One of the earliest histopathological changes associated with age-related hearing loss appears to be the disruption of outer hair cells (OHCs). To evaluate age-related changes in OHC function, distortion product otoacoustic emissions (DPOAEs) were recorded in the young and aging C57BL/6J mouse. Starting in young adulthood, the C57 mouse displays age-related elevation of auditory brainstem response thresholds, beginning in the high frequencies and progressing toward lower frequencies. The 2f1-f2 DPOAEs of mice between 2 and 20 months of age were examined for f2s between 8 and 16 kHz. In this octave region, the features of 2f1-f2 DPOAEs in the 2-month-old C57 mouse were comparable to those described for non-murine rodents in the literature in terms of optimum f2/f1 ratio, optimum primary level difference, input/output (I/O) function features and microstructure. It was determined that f2/f1 = 1.2 and L1-L2 = 20 dB were optimal stimulus parameters for investigation of the effects of age on C57 DPOAEs. Age-related changes in DPOAE I/O functions consisted of a right shift (i.e. increased DPOAE detection thresholds), disappearance of 'notches' and shallowing of the slopes after 8 months of age. As DPOAE I/O functions continued to shift to the right and DPOAE levels decreased with age, the appearance of I/O functions became complex to include regions of steep or shallow slopes and plateaus. The present results suggest that the age-related elevation of auditory thresholds in the C57 mice is associated with substantial progressive changes in OHC function.

Age-related decline of auditory function in the chinchilla (Chinchilla laniger)

The aim of this study was to examine the functional consequences of aging in the chinchilla, a rodent with a relatively long life span and a range of hearing similar to that of humans. Subjects were 21 chinchillas aged 10-15 years, and 23 young controls. Thresholds were determined from auditory evoked potentials (EVPs), and outer hair cell (OHC) functioning was assessed by measuring 2f1-f2 distortion product otoacoustic emissions (DPOAEs). Six cochleas from 11-12-year-old animals were examined for hair cell loss and gross strial pathology. The results show that the chinchilla exhibits a small but significant decline of auditory sensitivity and OHC functioning between 3 and 15 years of age, with high-frequency losses exceeding and growing more rapidly than low-frequency losses. Compared to rodents with shorter life spans, the chinchilla has a rate of loss that is more similar to that of humans, which could make it a valuable model for understanding the etiology of human presbycusis.

Aging rat vestibular ganglion: II. Quantitative electron microscopic evaluation

This laboratory has shown that age-related vestibular ganglion cell loss does not occur in the Wistar rat as it does in humans. However, in that study, intracellular changes were evident. The purpose of the present study was to quantitate some of these changes. The volume densities of mitochondria, rough endoplasmic reticulum (RER), Golgi apparatus, and aging pigment, as well as the diameter of the vestibular ganglion cells, of young (3 to 5 months) and old (24 to 31 months) female Wistar rats were determined by electron microscopy and stereological techniques. The data show a significant decrease in the volume densities of mitochondria (11.4%), Golgi apparatus (8.1%), and RER (8.9%), a significant increase in aging pigment (327%), and no change in mean profile diameter. These results suggest a decreased capacity for oxidative metabolism and protein synthesis that may reflect a decrease in the number of hair cells innervated by each ganglion cell and/or in the number of central connections. In either case, these findings suggest impaired metabolic and functional capabilities.

Quantitative measures of hair cell loss in CBA and C57BL/6 mice throughout their life spans

The CBA mouse shows little evidence of hearing loss until late in life, whereas the C57BL/6 strain develops a severe and progressive, high-frequency sensorineural hearing loss beginning around 3-6 months of age. These functional differences have been linked to genetic differences in the amount of hair cell loss as a function of age; however, a precise quantitative description of the sensory cell loss is unavailable. The present study provides mean values of inner hair cell (IHC) and outer hair cell (OHC) loss for CBA and C57BL/6 mice at 1, 3, 8, 18, and 26 months of age. CBA mice showed little evidence of hair cell loss until 18 months of age. At 26 months of age, OHC losses in the apex and base of the cochlea were approximately 65% and 50%, respectively, and IHC losses were approximately 25% and 35%. By contrast, C57BL/6 mice showed approximately a 75% OHC and a 55% IHC loss in the base of the cochlea at 3 months of age. OHC and IHC losses increased rapidly with age along a base-to-apex gradient. By 26 months of age, more than 80% of the OHCs were missing throughout the entire cochlea; however, IHC losses ranged from 100% near the base of the cochlea to approximately 20% in the apex.

The behavioral salience of tones as indicated by prepulse inhibition of the startle response: relationship to hearing loss and central neural plasticity in C57BL/6J mice

Adult C57BL/6J mice exhibit high-frequency, sensorineural hearing loss accompanied by physiological changes in the upper auditory brainstem and cortex, referred to as hearing-loss induced (HLI) plasticity: as high-frequency sensitivity declines, many neurons come to respond better to still-audible, middle-frequency tones (especially 12-16 kHz). We used prepulse inhibition (PPI) to assess the relationship between the behavioral salience of tones and HLI plasticity. The ability of a tone 'prepulse' (S1), presented 100 ms before a startle-eliciting tone (S2), to 'inhibit' startle responses was measured in normal-hearing 1-month-olds and 5-month-olds with high-frequency hearing loss. Tone bursts of 4, 8, 12, 16, and 24 kHz were used as S1s and S2s in all possible combinations. PPI was significantly improved (more inhibition) in 5-month-olds with 12 or 16 kHz S1s. This effect was not influenced by S2 frequency or the size of the startle evoked by S2-only stimuli (smaller for high-frequency S2s in older mice). The increased salience of 12-16 kHz S1s in 5-month-old C57 mice parallels changes in the central representation of tone frequency and implies a behavioral effect of HLI plasticity.

Genetics of age-related hearing loss in mice. III. Susceptibility of inbred and F1 hybrid strains to noise-induced hearing loss

Some humans and mice are genetically predisposed to age-related hearing loss (AHL), and others are variously susceptible to noise-induced hearing loss (NIHL). The inbred C57BL/6J (B6) mice exhibit AHL at an early age, whereas the inbred CBA/CaJ (CB) mice do not. The B6 mice are much more susceptible to NIHL than are the CB mice (Shone et al., 1991; Li, 1992a). The B6 mice possess an Ahl gene which maps to chromosome 10 (Erway et al., 1995). This study was designed, using these two inbred strains plus two F1 hybrid strains of mice, to begin to test the hypothesis that the Ahl genotypes may influence the susceptibility to NIHL. These strains of mice (with putative genotypes) are: inbred CB (+/+) and B6 (Ahl/Ahl); hybrid CBB6F1 (+/Ahl) and B6D2F1 (Ahl/Ahl; D2 represents inbred DBA/2J). Twenty-four mice of each of these four strains were exposed to noise (110 dB for 0, 1 or 2 h) and tested for auditory-evoked brainstem response (ABR) thresholds. The CB and CBB6F1 strains of mice did not differ significantly from each other, exhibiting mostly temporary threshold shifts. The B6 and B6D2F1 strains of mice did not differ significantly from each other, but did exhibit permanent threshold shifts. These results support the hypothesis that genetic predisposition to AHL may be revealed at a younger age by NIHL. This suggests that it may be possible to use the NIHL to distinguish segregating genotypes (+/Ahl vs. Ahl/Ahl) among backcross progeny and thereby to identify and map single genes for AHL.

Morphological changes in the anteroventral cochlear nucleus that accompany sensorineural hearing loss in DBA/2J and C57BL/6J mice

Morphological measurements were made on histological sections of the anteroventral cochlear nucleus (AVCN) in mice of the DBA/2J and C57BL/6J strains to determine the effects of sensorineural cochlear pathology on the number, packing density, and size of neurons and on AVCN volume. Both strains possess alleles that cause progressive cochlear pathology initially affecting the organ of Corti: in DBA mice, hearing loss is evident at 4 weeks of age and progresses rapidly; in C57 mice, hearing loss begins after 2 months of age and progresses more slowly. In both strains AVCN volume decreased, some loss of neurons occurred, and these changes paralleled the progression of peripheral hearing loss. Central changes were rapid in DBA mice, but the ultimate magnitude of the changes in 1-year-old mice did not differ between strains. Both strains differed from well-hearing CBA/J mice which exhibited no changes in the AVCN measures. The findings indicate that pathology of the organ of Corti in adult mice results in degenerative changes in the cochlear nucleus. The data also support earlier findings indicating that, if cochlear pathology does not begin prior to young adulthood, the age of onset and duration of sensorineural impairment have little effect on the ultimate magnitude of central effects.

Genetics of age-related hearing loss in mice. II. Strain differences and effects of caloric restriction on cochlear pathology and evoked response thresholds

The effects of genotype and diet on age-related hearing loss were evaluated using auditory brainstem response (ABR) thresholds and post-mortem cochlear histopathology in 5 inbred mouse strains, CBA/H-T6J (CH), DBA/2J (D2), C57BL/6J (B6), BALB/cByJ (BY) and WB/ReJ (WB), and their 10 F1 hybrid strains. The mice had been maintained since weaning on either a high-energy (HE) control diet or low-energy (LE) calorically restricted diet. ABR thresholds were obtained when the mice were 23 months old; the mice were allowed to age until they died from natural causes prior to obtaining the histological material. The severity of post-mortem cochlear pathology in mice maintained with the HE diet supports our earlier genetic model which postulated that B6, BY, and WB strains each possessed a different recessive allele causing age-related hearing loss, D2 mice possessed all 3 genes, and CH mice possessed none. The histopathology indicates that the genes act at the cochlear level. Dietary restriction resulted in increased longevity in a number of strains, but age-related changes in cochlear pathology were not ameliorated in any of these; indeed, in some strains long-lived LE mice exhibited severe cochlear degeneration. In strains for which longevity was not extended by caloric restriction, only B6 mice exhibited an ameliorative effect of the LE diet on cochlear pathology. ABRs in 23-month-olds indicated a slowing of age-related hearing loss in LE mice of 3 F1 hybrid strains.

Autoregulation of cochlear blood flow in young and aged mice

Autoregulation is the capacity of an organ system to maintain organ blood flow constant in response to changes in arterial blood pressure (BP). The current study was carried out to investigate the effect of age on autoregulation of cochlear blood flow (CBF) in mice. CBF was measured using a laser-Doppler flowmeter while BP was increased by angiotensin II injections and decreased by exsanguination in 2-month-old, 10-month-old and 18-month-old CBA mice. Autoregulation of CBF was significantly weaker in the 2-month-old mice when compared to the older mice. Although CBF autoregulation was weaker in the 18-month-old mice compared to the 10-month-old mice, this difference was not statistically significant. These results suggest that autoregulation changes with maturation and age. Findings are discussed in relationship to the possible development of presbycusis.

Age-related changes in cochlear vascular conductance in mice

Vascular changes contribute to age-related hearing loss but the mechanisms involved in microvascular reactivity, particularly in the aged ear, are still incompletely understood. In this study, possible age-related changes in cochlear blood flow (CBF) and vascular reactivity were studied in presbycusic mice (C57BL/6) and young, age-matched, and old controls (CBA/J) without presbycusis. Reactivity was monitored by laser Doppler flowmetry and assessed by change in cochlear vascular conductance (VC) (defined as the ratio of CBF to blood pressure) in response to round window-applied sodium nitroprusside, a vasodilating agent. Mean VC response of C57BL/6 mice differed from controls both in maximum response and in post-drug recovery time. In C57BL/6 mice, mean VC increased about 28%, in contrast to an increase of over 40% in young and age-matched CBA/J controls. A less elevated VC response, similar to that of the presbycusic mice, was shown by aged (20-21 month) controls. Also, VC response in C57BL/6 mice was sustained throughout the 60 min observation period, while response of most CBA/J controls recovered in 50 min or less. These changes suggest age-dependent, pathologically-related altered responsiveness in cochlear vascular reactivity.

Responses of inferior colliculus neurons in C57BL/6J mice with and without sensorineural hearing loss: effects of changing the azimuthal location of an unmasked pure-tone stimulus

Azimuth functions (discharge rates evoked by tone bursts as a function of stimulus azimuth) were obtained from neurons in the inferior colliculus (IC) of C57 mice aged 2, 7 and 12 months. Because of a gene that affects the cochlea, C57 mice exhibit high-frequency sensorineural hearing loss at 7 and 12 months. Azimuth functions were examined for differences that might be related to the decline in localization acuity that accompanies hearing loss in this strain. Irrespective of age group, nearly all neurons in the central area of the IC were sensitive to the azimuth of a best frequency (BF) stimulus, as revealed by azimuth functions in which firing rates varied by more than 50% from maximum to minimum at one or more intensities. The age groups were similar in many respects (e.g., there were no significant differences in the proportion of functions meeting the criterion for direction sensitivity, the proportion of neurons with direction sensitive functions over a range of intensities, azimuth function shapes, the locations or stability of 'borders' separating angles evoking high versus low discharge rates). However, in 7- and 12-month-olds: the proportion of IC neurons in which the strongest excitatory driving was evoked by ipsilateral stimulation was significantly larger; azimuth function borders were more likely to be 'reversed' (i.e., the high rates being evoked by the more ipsilateral angle); and a greater proportion of azimuth functions met the criterion for direction sensitivity only minimally. The findings suggest that binaural excitatory-inhibitory interactions are altered in IC neurons of hearing-impaired mice.

Morphology of the inferior colliculus in C57BL/6J and CBA/J mice across the life span

Basic anatomical features were evaluated in the inferior colliculus (IC) of C57BL/6J and CBA/J mice across the adult life span (1.5 to 30 months of age). C57BL/6J mice exhibit progressive age-related cochlear pathology and become severely hearing-impaired during the second year of life; CBA/J mice exhibit little hearing loss as they age. Age had little effect on the size of the IC, the size of IC neurons, or the packing density of IC neurons and there was no evidence of age-related neuron loss. However, old CBA/J mice developed numerous spongiform lesions throughout the brainstem. The absence of morphological changes in the IC of hearing-impaired C57BL/6J mice supports the hypothesis that features such as the size of neurons, survival of neurons, and volume of the neuropil are not affected by chronic sensorineural pathology in central auditory nuclei (e.g., as the IC) that do not receive direct input from primary afferent fibers. The data from both strains taken together indicate that certain basic anatomical properties of the mouse IC persist in the face of aging.

Inner ear morphology in CBA/Ca and C57BL/6J mice in relationship to noise, age and phenotype

CBA/Ca mice showing moderate hearing losses with onset late in life and C57BL/6J mice with progressive hearing losses starting when animals were young adults were exposed to a 2-7 kHz, 120 dB SPL noise band for 5 min in order to investigate morphological consequences to noise as a function of age and genotype. Permanent threshold shifts were determined by auditory brainstem responses 1 month after noise exposure at 1, 3 and 6 months of age. CBA mice had a decreasing susceptibility to noise with increasing age, while C57 mice remained equally susceptible throughout all ages tested. The threshold shifts were then analyzed in relation to morphological changes of the organ of Corti as visualized by light microscopy (LM), transmission electron microscopy (TEM) and scanning electron microscopy (SEM). CBA mice exposed at 1 month and sacrificed at 2 months had seemingly normal cochlear morphologies under LM. In these animals SEM findings demonstrated mild stereocilia damage to noise trauma, but not when mice were exposed at 3 and 6 months. There was no visible morphological aging of hair cells found during the period tested. In contrast, the C57 mice had early hair cell changes including bent and fused stereocilia, bulging of the cuticular plates, hair cell loss and swelling of afferent dendrites. These changes became more pronounced throughout the test periods with the variability of damage in this strain more evident over time. This pattern was also aggravated in all C57 age groups tested after noise exposure. Quantification with cytocochleograms demonstrated a statistically different reaction pattern to noise trauma between the two different genotypes of mice.