Repeated Moderate Noise Exposure in the Rat--an Early Adulthood Noise Exposure Model

Characterization of Anxiety-Like Behaviors and Neural Circuitry following Chronic Moderate Noise Exposure in Mice

BACKGROUND

Commonly encountered nontraumatic, moderate noise is increasingly implicated in anxiety; however, the neural substrates underlying this process remain unclear.

OBJECTIVES

We investigated the neural circuit mechanism through which chronic exposure to moderate-level noise causes anxiety-like behaviors.

METHODS

Mice were exposed to chronic, moderate white noise [85 decibel (dB) sound pressure level (SPL)], 4 h/d for 4 wk to induce anxiety-like behaviors, which were assessed by open field, elevated plus maze, light-dark box, and social interaction tests. Viral tracing, immunofluorescence confocal imaging, and brain slice patch-clamp recordings were used to characterize projections from auditory brain regions to the lateral amygdala. Neuronal activities were characterized by in vivo multielectrode and fiber photometry recordings in awake mice. Optogenetics and chemogenetics were used to manipulate specific neural circuitry.

RESULTS

Mice chronically (4 wk) exposed to moderate noise (85 dB SPL, 4 h/d) demonstrated greater neuronal activity in the lateral amygdala (LA), and the LA played a critical role in noise-induced anxiety-like behavior in these model mice. Viral tracing showed that the LA received monosynaptic projections from the medial geniculate body (MG) and auditory cortex (ACx). Optogenetic excitation of the MG → LA or ACx → LA circuits acutely evoked anxiety-like behaviors, whereas their chemogenetic inactivation abolished noise-induced anxiety-like behavior. Moreover, mice chronically exposed to moderate noise were more susceptible to acute stress, with more neuronal firing in the LA, even after noise withdrawal.

DISCUSSION

Mice exposed to 4 wk of moderate noise (85 dB SPL, 4 h/d) demonstrated behavioral and physiological differences compared to controls. The neural circuit mechanisms involved greater excitation from glutamatergic neurons of the MG and ACx to LA neurons under chronic, moderate noise exposure, which ultimately promoted anxiety-like behaviors. Our findings support the hypothesis that nontraumatic noise pollution is a potentially serious but unrecognized public health concern. https://doi.org/10.1289/EHP12532.

Comparing the electrophysiological effects of traumatic noise exposure between rodents

Noise-induced hearing deficits are important health problems in the industrialized world. As the underlying physiological dysfunctions are not well understood, research in suitable animal models is urgently needed. Three rodent species (Mongolian gerbil, rat and mouse) were studied to compare the temporal dynamics of noise-induced hearing loss after identical procedures of noise exposure. Auditory brainstem responses (ABRs) were measured before, during and up to eight weeks after noise exposure for threshold determination and ABR waveform analysis. Trauma induction with stepwise increasing sound pressure level was interrupted by five interspersed ABR measurements. Comparing short- and long-term dynamics underlying the following noise-induced hearing loss revealed diverging time courses between the three species. Hearing loss occurred early on during noise exposure in all three rodent species at or above trauma frequency. Initial noise level (105 dB SPL) was most effective in rats while the delayed level-increase to 115 dB SPL affected mice much stronger. Induced temporary threshold shifts in rats and mice were larger in animals with lower pre-trauma ABR thresholds. The increase in activity (gain) along the auditory pathway was derived by comparing the amplitudes of short- and long-latency ABR waveform components. Directly after trauma, significant effects were found for rats (decreasing gain) and mice (increasing gain) while gerbils revealed high individual variability in gain changes. Taken together, our comparative study revealed pronounced species-specific differences in the development of noise-induced hearing loss and the related processing along the auditory pathway.

Dose-Dependent Pattern of Cochlear Synaptic Degeneration in C57BL/6J Mice Induced by Repeated Noise Exposure

It is widely accepted that even a single acute noise exposure at moderate intensity that induces temporary threshold shift (TTS) can result in permanent loss of ribbon synapses between inner hair cells and afferents. However, effects of repeated or chronic noise exposures on the cochlear synapses especially medial olivocochlear (MOC) efferent synapses remain elusive. Based on a weeklong repeated exposure model of bandwidth noise over 2-20 kHz for 2 hours at seven intensities (88 to 106 dB SPL with 3 dB increment per gradient) on C57BL/6J mice, we attempted to explore the dose-response mechanism of prolonged noise-induced audiological dysfunction and cochlear synaptic degeneration. In our results, mice repeatedly exposed to relatively low-intensity noise (88, 91, and 94 dB SPL) showed few changes on auditory brainstem response (ABR), ribbon synapses, or MOC efferent synapses. Notably, repeated moderate-intensity noise exposures (97 and 100 dB SPL) not only caused hearing threshold shifts and the inner hair cell ribbon synaptopathy but also impaired MOC efferent synapses, which might contribute to complex patterns of damages on cochlear function and morphology. However, repeated high-intensity (103 and 106 dB SPL) noise exposures induced PTSs mainly accompanied by damages on cochlear amplifier function of outer hair cells and the inner hair cell ribbon synaptopathy, rather than the MOC efferent synaptic degeneration. Moreover, we observed a frequency-dependent vulnerability of the repeated acoustic trauma-induced cochlear synaptic degeneration. This study provides a sight into the hypothesis that noise-induced cochlear synaptic degeneration involves both afferent (ribbon synapses) and efferent (MOC terminals) pathology. The pattern of dose-dependent pathological changes induced by repeated noise exposure at various intensities provides a possible explanation for the complicated cochlear synaptic degeneration in humans. The underlying mechanisms remain to be studied in the future.

Temporary hearing threshold shift in harbor seals (Phoca vitulina) due to a one-sixth-octave noise band centered at 32 kHz

Two female harbor seals were exposed for 60 min to a continuous one-sixth-octave noise band centered at 32 kHz at sound pressure levels of 92 to 152 dB re 1 μPa, resulting in sound exposure levels (SELs) of 128 to 188 dB re 1 μPa²s. This was part of a larger project determining frequency-dependent susceptibility to temporary threshold shift (TTS) in harbor seals over their entire hearing range. After exposure, TTSs were quantified at 32, 45, and 63 kHz with a psychoacoustic technique. At 32 kHz, only small TTSs (up to 5.9 dB) were measured 1-4 min (TTS_1-4) after exposure, and recovery was within 1 h. The higher the SEL, the higher the TTS induced at 45 kHz. Below ∼176 dB re 1 μPa²s, the maximum TTS_1-4 was at 32 kHz; above ∼176 dB re 1 μPa²s, the maximum TTS_1-4 (up to 33.8 dB) was at 45 kHz. During one particular session, a seal was inadvertently exposed to an SEL of ∼191 dB re 1 μPa²s and at 45 kHz, her TTS_1-4 was >45 dB; her hearing recovered over 4 days. Harbor seals appear to be equally susceptible to TTS caused by sounds in the 2.5-32 kHz range.

Repeated Moderate Sound Exposure Causes Accumulated Trauma to Cochlear Ribbon Synapses in Mice

Repeated induction of a temporary threshold shift (TTS) may result in a permanent threshold shift (PTS) and is thought to be associated with early onset of age-related hearing loss (ARHL). The possibility that a PTS might be induced by administration of repeated TTS-inducing noise exposures (NEs) over a short period during early adulthood has not been formally investigated. We aimed to investigate possible cumulative acoustic overstimulation effects that permanently shift the auditory threshold. Young adult C57BL/6J mice were exposed twice to moderate white noise in an experimental design that minimized the effects of aging. The first exposure resulted in a reversible noise-induced hearing loss (NIHL) measured as recoverable alterations in auditory brainstem response (ABR) thresholds, waveform amplitudes, and numbers of ribbon synapses. The second NE with the same parameters caused persistent threshold shifts, wave I amplitude reductions, wave IV/I ratio enhancements, and synaptic losses, even though recovery time sufficient for a TTS had been provided. The pattern of PTS resembled NIHL since the observed impairments tonotopically followed the power spectrum of the noise insult, rather than ARHL, which distributes at higher frequencies. No significant changes were observed in the control group as the mice aged. To conclude, our results demonstrate a cumulative effect of repetitive TTS-inducing NE on hearing function and synaptic plasticity that does not cause premature ARHL, thereby providing insight into the pathophysiological mechanisms underlying NIHL and ARHL.

Temporary hearing threshold shift in harbor seals (Phoca vitulina) due to a one-sixth-octave noise band centered at 16 kHz

Temporary hearing threshold shifts (TTSs) were investigated in two adult female harbor seals after exposure for 60 min to a continuous one-sixth-octave noise band centered at 16 kHz (the fatiguing sound) at sound pressure levels of 128-149 dB re 1 μPa, resulting in sound exposure levels (SELs) of 164-185 dB re 1 μPa²s. TTSs were quantified at the center frequency of the fatiguing sound (16 kHz) and at half an octave above that frequency (22.4 kHz) by means of a psychoacoustic hearing test method. Susceptibility to TTS was similar in both animals when measured 8-12 and 12-16 min after cessation of the fatiguing sound. TTS increased with increasing SEL at both frequencies, but above an SEL of 174 dB re 1 μPa²s, TTS was greater at 22.4 kHz than at 16 kHz for the same SELs. Recovery was rapid: the greatest TTS, measured at 22.4 kHz 1-4 min after cessation of the sound, was 17 dB, but dropped to 3 dB in 1 h, and hearing recovered fully within 2 h. The affected hearing frequency should be considered when estimating ecological impacts of anthropogenic sound on seals. Between 2.5 and 16 kHz the species appears equally susceptible to TTS.

The correlation between hair and eye colour and contralateral suppression of otoacoustic emissions

Genetics and environmental factors frequently influence individual's susceptibility to hearing loss. It is postulated that melanin in the inner ear is related to individual's susceptibility to noise induced hearing loss (NIHL). General pigmentation in turn, suspected to be related to the amount of pigmentation in the inner ear. The amount of melanin in the inner ear is said to modulate the endocochlear potential and provide an otoprotective effect.

AIM

The study aimed to determine the relationship between the contralateral suppression of otoacoustic emissions (CSOAE) in individuals with brown eyes and hair, and blue eyes and blond hair, and temporary emission shift (TES) after short-term noise exposure.

SETTING AND DESIGN

The research was conducted using a quantitative research design with a quasi-experimental repeated within the subject design to compare the CSOAE in subjects with different hair and eyes colour with TES after short-term noise exposure. Quantitative research was used to determine the relationship between the measurable variables to predict occurrence.

MATERIAL AND METHOD

The hearing sensitivity of young adults was determined by using pure tone audiometry followed by CSOAE's and distortion product otoacoustic emissions (DPOAE) before listening to music for one hour individually. Pure tone audiometry and DPOAE's were repeated after music exposure to determine the amount of TES and temporary threshold shift (TTS).

STATISTICAL ANALYSIS USED

One-way ANOVA was used during the analysis of the data obtained during this research study, in addition to, two-tailed Wilcoxon Sign Rank test and Friedman's test. In all analyses, a 95% level of significance (P<0.05) was used.

RESULTS

No statistically significant difference between efferent suppression was measured by CSOAE's between the participant groups. A larger TTS at 4000 Hz and TES at 2000 Hz was evident in the blue eyes and blond hair group after short-term music exposure. Conclusion: CSOAE's were unable to predict which group of individuals were more susceptible to NIHL after short-term noise exposure.

Frequency of greatest temporary hearing threshold shift in harbor seals (Phoca vitulina) depends on fatiguing sound level

Harbor seals may suffer hearing loss due to intense sounds. After exposure for 60 min to a continuous 6.5 kHz tone at sound pressure levels of 123-159 dB re 1 µPa, resulting in sound exposure levels (SELs) of 159-195 dB re 1 μPa²s, temporary threshold shifts (TTSs) in two harbor seals were quantified at the center frequency of the fatiguing sound (6.5 kHz) and at 0.5 and 1.0 octaves above that frequency (9.2 and 13.0 kHz) by means of a psychoacoustic technique. Taking into account the different timing of post-exposure hearing tests, susceptibility to TTS was similar in both animals. The higher the SEL, the higher the TTS induced at frequencies above the fatiguing sound's center frequency. Below ∼179 dB re 1 μPa²s, the maximum TTS was at the center frequency (6.5 kHz); above ∼179 dB re 1 μPa²s, the maximum TTS was at half an octave above the center frequency (9.2 kHz). These results should be considered when interpreting previous TTS studies, and when estimating ecological impacts of anthropogenic sound on the hearing and ecology of harbor seals. Based on the results of the present study and previous studies, harbor seal hearing, in the frequency range 2.5-6.5 kHz, appears to be approximately equally susceptible to TTS.

Effects of Recreational Noise on Threshold and Suprathreshold Measures of Auditory Function

Noise exposure that causes a temporary threshold shift but no permanent threshold shift can cause degeneration of synaptic ribbons and afferent nerve fibers, with a corresponding reduction in wave I amplitude of the auditory brainstem response (ABR) in animals. This form of underlying damage, hypothesized to also occur in humans, has been termed synaptopathy , and it has been hypothesized that there will be a hidden hearing loss consisting of functional deficits at suprathreshold stimulus levels. This study assessed whether recreational noise exposure history was associated with smaller ABR wave I amplitude and poorer performance on suprathreshold auditory test measures. Noise exposure histories were collected from 26 men and 34 women with hearing thresholds ≤ 25 dB hearing loss (HL; 250 Hz to 8 kHz), and a variety of functional suprathreshold hearing tests were performed. Wave I amplitudes of click-evoked ABR were obtained at 70, 80, 90, and 99 dB (nHL) and tone-burst evoked ABR were obtained at 90 dB nHL. Speech recognition performance was measured in quiet and in competing noise, using the Words in Noise test, and the NU-6 word list in broadband noise (BBN). In addition, temporal summation to tonal stimuli was assessed in quiet and in competing BBN. To control for the effects of subclinical conventional hearing loss, distortion product otoacoustic emission amplitude, an indirect measure of outer hair cell integrity, was measured. There was no statistically significant relationship between noise exposure history scores and ABR wave I amplitude in either men or women for any of the ABR conditions. ABR wave I amplitude and noise exposure history were not reliably correlated with suprathreshold functional hearing tests. Taken together, this study found no evidence of noise-induced decreases in ABR wave I amplitude or signal processing in noise in a cohort of subjects with a history of recreational noise exposure.