Dynamic changes in hair cell stereocilia and cochlear transduction after noise exposure

The structures of cochlear transduction include stereocilia at the apical surface of hair cells and their connection to the tectorial membrane. The transduction site is one of the loci for noise-induced cochlear damage. Although stereocilia are susceptible to noise, it has been found that in the inner ears of avians, this fragile structure is largely self-repairing and is associated with recovery of hearing sensitivity after noise exposure, as observed in the difference between the temporal threshold shift (TTS) and the permanent threshold shift (PTS). In the mammalian cochleae, however, threshold shifts measured in the auditory brainstem responses (ABR) did not parallel the chronological changes in the stereocilia on hair cells. It is unclear how the morphological recovery of the stereocilia on the mammalian hair cells is correlated with the changes in cochlear transduction that can be assessed by measuring receptor potential. In the present study, guinea pigs were exposed to a broadband noise of 110 dB SPL for 2 h. Auditory sensitivity was evaluated using ABR and cochlear transduction was assessed using cochlear microphonics (CM). Stereocilia morphology was quantified at different time points after the noise and compared with the control. The noise produced a TTS of 55.69 ± 14.13 dB in frequency-averaged ABR thresholds. The threshold shift was reduced to 9.58 ± 11.75 dB SPL 1 month later with virtually no loss of hair cells. Damage to the stereocilia immediately after noise exposure was found to be associated with depression of CM amplitude. Virtually no abnormal stereocilia were observed 1 month after the noise in association with a fully recovered CM.     

变压器噪声暴露对SD大鼠听力及应激状态的影响研究

目的:探究短时间内低声级强度低频的变压器噪声暴露对SD大鼠听力及应激状态方面的影响。方法:选取90只SPF级健康无听力障碍的(雌雄各半)SD大鼠作为实验对象,随机分为实验A、B组和对照C组,A、B组分别给予声级上限为65 dB SPL、60 dB SPL(频谱范围:100~800 Hz)的变压器噪声,噪声暴露时程为8周,每日噪声给予时间为22点至次日8点,C组在相同条件下饲养,不给予噪声暴露。噪声暴露结束后,通过DPOAE(畸变耳声发射)、ABR(听性脑干反应)检测、耳蜗铺片及毛细胞计数对SD大鼠听力学状况进行评估;通过血清中促肾上腺皮质激素(ACTH)、血清皮质醇(CORT)对SD大鼠的应激状态进行评估。结果:在变压器噪声暴露的8周内,各组大鼠生长状况良好,体重均呈正常生理性增长,组间无明显差异(P0.05);在变压器噪声暴露8周后,对A、B、C三组大鼠的听力学指标进行两两比较,组间均无明显差异(P0.05),对大鼠血清中促肾上腺皮质激素(ACTH)、血清皮质醇(CORT)的含量进行三组间比较,组间差异均无统计学意义(P0.05)。结论:连续暴露于声压级上限65/60 dB SPL,频谱范围为100~800 Hz的变压器噪声下8周(10小时/天)对SD大鼠听力未产生明显影响,未引发SD大鼠应激状态。     

噪声短时暴露所致的耳蜗电位改变

本实验观察115dB(SPL)白噪声暴露20min对豚鼠耳蜗直流电位(EP),复合听神经动作电位(CAP),微音器电位(CM)的影响。发现此种噪声暴露确可提高源于血管纹的正EP(P-EP),说明有血管纹功能的代偿性增强;而负EP(N-EP)变化不大。AP及CM输入-输出函数的变化说明噪声首先影响外毛细胞的主动运动功能。EP与耳蜗电图的对照分析表明,血管纹功能的改变确能影响噪声性听损伤的发展。     

强噪声暴露后听觉脑干电反应及皮层电反应的变化

为了观察噪声作用豚鼠后,在暂时性或移和永久性阈移期间,听觉末梢和听觉各级中枢电反庆的变化规律,我们进行了６２ｄ的测定。听神经电反应振幅减小２９％（Ｐ＜０．０５）,耳蜗核电反应振幅减小２８％（Ｐ＜０．０５）,上橄榄核电反应反而增加２１％（Ｐ＜０．０５）,中脑下丘的电反应振幅进而增大３７％（Ｐ＜０．０５）,听觉皮层电反应振幅却意外地增加１３１％（Ｐ＜０．００１）。这说明强噪声（１２５ｄＢ １５０ｍｉｎ     

稳态白噪声对豚鼠耳蜗电位影响的研究

本工作利用面神经管慢性埋植电极在清醒动物上对噪声暴露前后听神经动作电位(AP)和微音器电位(CM) 进行了研究,结果表明:当改变测试声强和频率时CM应起始点会发生移位(CM-shift); 123dBSPL稳态白噪声暴露1小时后.AP各参数(幅值、潜伏期和阈值)均发生明显变化.而CM各参数(幅值、CM-shift和检测阈)变化则基本未达显著性.说明AP对产损伤的敏感性要高于CM另外发现噪声暴露后AP的N_2峰的恢复比N_1峰快、CM在恢复过程中会出现幅值加大现象.     

Effect of c-myc on the ultrastructural structure of cochleae in guinea pigs with noise induced hearing loss

Noise over-stimulation may induce hair cells loss and hearing deficit. The c-myc oncogene is a major regulator for cell proliferation, growth, and apoptosis. However, the role of this gene in the mammalian cochlea is still unclear. The study was designed to firstly investigate its function under noise condition, from the aspect of cochlear ultrastructural changes. We had established the adenoviral vector of c-myc gene and delivered the adenovirus suspension into the scala tympani of guinea pigs 4 days before noise exposure. The empty adenoviral vectors were injected as control. Then, all subjects were exposed to 4-kHz octave-band noise at 110 dB SPL for 8 h/day, 3 days consecutively. Auditory thresholds were assessed by auditory brainstem response, prior to and 7 days following noise exposure. On the seventh days after noise exposure, the cochlear sensory epithelia surface was observed microscopically and the cochleae were taken to study the ultrastructural changes. The results indicated that auditory threshold shift after noise exposure was higher in the ears treated with Ad.EGFP than that treated with Ad.c-myc-EGFP. Stereocilia loss and the disarrangement of outer hair cells were observed, with greater changes found in the Ad.EGFP group. Also, the ultrastructure changes were severe in the Ad.EGFP group, but not obvious in the Ad.c-myc-EGFP group. Therefore, c-myc gene might play an unexpected role in hearing functional and morphological protection from acoustic trauma.     

Adenosine amine congener mitigates noise-induced cochlear injury

Hearing loss from noise exposure is a leading occupational disease, with up to 5% of the population at risk world-wide. Here, we present a novel purine-based pharmacological intervention that can ameliorate noise-induced cochlear injury. Wistar rats were exposed to narrow-band noise (8–12 kHz, 110 dB SPL, 2–24 h) to induce cochlear damage and permanent hearing loss. The selective adenosine A₁ receptor agonist, adenosine amine congener (ADAC), was administered intraperitoneally (100 μg/kg/day) at time intervals after noise exposure. Hearing thresholds were assessed using auditory brainstem responses and the hair cell loss was evaluated by quantitative histology. Free radical damage in the organ of Corti was assessed using nitrotyrosine immunohistochemistry. The treatment with ADAC after noise exposure led to a significantly greater recovery of hearing thresholds compared with controls. These results were upheld by increased survival of sensory hair cells and reduced nitrotyrosine immunoreactivity in ADAC-treated cochlea. We propose that ADAC could be a valuable treatment for noise-induced cochlear injury in instances of both acute and extended noise exposures.     

Hidden hearing loss is associated with loss of ribbon synapses of cochlea inner hair cells

The present study aimed to observe the changes in the cochlea ribbon synapses after repeated exposure to moderate-to-high intensity noise. Guinea pigs received 95 dB SPL white noise exposure 4 h a day for consecutive 7 days (we regarded it a medium-term and moderate-intensity noise, or MTMI noise). Animals were divided into four groups: Control, 1DPN (1-day post noise), 1WPN (1-week post noise), and 1MPN (1-month post noise). Auditory function analysis by auditory brainstem response (ABR) and compound action potential (CAP) recordings, as well as ribbon synapse morphological analyses by immunohistochemistry (Ctbp2 and PSD95 staining) were performed 1 day, 1 week, and 1 month after noise exposure. After MTMI noise exposure, the amplitudes of ABR I and III waves were suppressed. The CAP threshold was elevated, and CAP amplitude was reduced in the 1DPN group. No apparent changes in hair cell shape, arrangement, or number were observed, but the number of ribbon synapse was reduced. The 1WPN and 1MPN groups showed that part of ABR and CAP changes recovered, as well as the synapse number. The defects in cochlea auditory function and synapse changes were observed mainly in the high-frequency region. Together, repeated exposure in MTMI noise can cause hidden hearing loss (HHL), which is partially reversible after leaving the noise environment; and MTMI noise-induced HHL is associated with inner hair cell ribbon synapses.     

Acoustic overstimulation activates 5'-AMP-activated protein kinase through a temporary decrease in ATP level in the cochlear spiral ligament prior to permanent hearing loss in mice

Inner ear disorders are known to be elicited by mitochondrial dysfunction, which decreases the ATP level in the inner ear. 5′-AMP-activated protein kinase (AMPK) is a serine/threonine kinase activated by metabolic stress and by an increase in the AMP/ATP ratio. To elucidate the involvement of AMPK-derived signals in noise-induced hearing loss, we investigated whether in vivo acoustic overstimulation would activate AMPK in the cochlea of mice. Std-ddY mice were exposed to 8 kHz octave band noise at a 90-, 110- or 120-dB sound pressure level (SPL) for 2 h. Exposure to the noise at 110 or 120 dB SPL produced outer hair cell death in the organ of Corti and permanent hearing loss. Exposure to the noise at 120-dB SPL elevated the level of the phospho-AMPK α-subunit (p-AMPKα), without affecting the protein level of this subunit, immediately and at 12-h post-exposure in the lateral wall structures including the spiral ligament and stria vascularis. In the hair cells and spiral ganglion cells, no marked change in the level of p-AMPKα was observed at any time post-exposure. The level of phospho-c-Jun N-terminal kinase (p-JNK) was increased in the lateral wall structures at 2- to 4-h post-exposure at 120 dB SPL. Noise exposure significantly, but temporarily, decreased the ATP level in the spiral ligament, in an SPL-dependent manner at 110 dB and above. Likewise, elevation of p-AMPKα and p-JNK levels was also observed in the lateral wall structures post-exposure to noise at an SPL of 110 dB and above. Taken together, our data suggest that AMPK and JNK were activated by ATP depletion in the cochlear spiral ligament prior to permanent hearing loss induced by in vivo acoustic overstimulation.     

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)-CreER^T2-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.     

Contrasting mechanisms for hidden hearing loss: Synaptopathy vs myelin defects

Hidden hearing loss (HHL) is an auditory neuropathy characterized by normal hearing thresholds but reduced amplitudes of the sound-evoked auditory nerve compound action potential (CAP). In animal models, HHL can be caused by moderate noise exposure or aging, which induces loss of inner hair cell (IHC) synapses. In contrast, recent evidence has shown that transient loss of cochlear Schwann cells also causes permanent auditory deficits in mice with similarities to HHL. Histological analysis of the cochlea after auditory nerve remyelination showed a permanent disruption of the myelination patterns at the heminode of type I spiral ganglion neuron (SGN) peripheral terminals, suggesting that this defect could be contributing to HHL. To shed light on the mechanisms of different HHL scenarios observed in animals and to test their impact on type I SGN activity, we constructed a reduced biophysical model for a population of SGN peripheral axons whose activity is driven by a well-accepted model of cochlear sound processing. We found that the amplitudes of simulated sound-evoked SGN CAPs are lower and have greater latencies when heminodes are disorganized, i.e. they occur at different distances from the hair cell rather than at the same distance as in the normal cochlea. These results confirm that disruption of heminode positions causes desynchronization of SGN spikes leading to a loss of temporal resolution and reduction of the sound-evoked SGN CAP. Another mechanism resulting in HHL is loss of IHC synapses, i.e., synaptopathy. For comparison, we simulated synaptopathy by removing high threshold IHC-SGN synapses and found that the amplitude of simulated sound-evoked SGN CAPs decreases while latencies remain unchanged, as has been observed in noise exposed animals. Thus, model results illuminate diverse disruptions caused by synaptopathy and demyelination on neural activity in auditory processing that contribute to HHL as observed in animal models and that can contribute to perceptual deficits induced by nerve damage in humans.     

Fast and slow effects of medial olivocochlear efferent activity in humans

Background

The medial olivocochlear (MOC) pathway modulates basilar membrane motion and auditory nerve activity on both a fast (10–100 ms) and a slow (10–100 s) time scale in guinea pigs. The slow MOC modulation of cochlear activity is postulated to aide in protection against acoustic trauma. However in humans, the existence and functional roles of slow MOC effects remain unexplored.

Methodology/Principal Findings

By employing contralateral noise at moderate to high levels (68 and 83 dB SPL) as an MOC reflex elicitor, and spontaneous otoacoustic emissions (SOAEs) as a non-invasive probe of the cochlea, we demonstrated MOC modulation of human cochlear output both on a fast and a slow time scale, analogous to the fast and slow MOC efferent effects observed on basilar membrane vibration and auditory nerve activity in guinea pigs. The magnitude of slow effects was minimal compared with that of fast effects. Consistent with basilar membrane and auditory nerve activity data, SOAE level was reduced by both fast and slow MOC effects, whereas SOAE frequency was elevated by fast and reduced by slow MOC effects. The magnitudes of fast and slow effects on SOAE level were positively correlated.

Conclusions/Significance

Contralateral noise up to 83 dB SPL elicited minimal yet significant changes in both SOAE level and frequency on a slow time scale, consistent with a high threshold or small magnitude of slow MOC effects in humans.     

TEMPORARY THRESHOLD SHIFTS AFTER NOISE EXPOSURE IN THE BOTTLENOSE DOLPHIN (TURSIOPS TRUNCATUS) MEASURED USING EVOKED AUDITORY POTENTIALS

The time course of recovery from temporary threshold shift (TTS) was measured in a bottlenose dolphin, Tursiops truncatus , using an evoked-potential procedure. The envelope-following response (EFR), which is a rhythmic train of auditory brainstem responses (ABR) to sinusoidally amplitude-modulated tones, was used as an indicator of the sound reception by the animal. Variation of the intensity of the stimulus allowed us to measure the animal's hearing via EFR thresholds. During each session, following an initial measure of threshold, the trained animal voluntary positioned itself within a hoop 1 m underwater while a 160 dB re 1 μPa noise of a 4–11 kHz bandwidth was presented for 30 min. After the noise exposure, thresholds were measured again at delays of 5, 10, 15, 25, 45, and 105 min. Measurements were made at test frequencies of 8, 11.2, 16, 22.5, and 32 kHz. The maximum TTS occurred 5 min after exposure and rapidly recovered with a rate of around 1.5 dB per doubling of time. TTS occurred at test frequencies from 8 to 16 kHz, with the maximum at 16 kHz. TTS was negligible at 22.5 kHz and absent at 32 kHz.     

提高外淋巴钙浓度对耳蜗电位的影响

本实验以人工外淋巴灌流方式,提高豚鼠耳蜗外淋巴液钙离子浓度（［Ｃａ２＋］ＰＬ）,观察蜗内直流电位（ＥＰ）和耳蜗电图（ＥＣｏｃｈＧ）的变化,ＥＣｏｃｈＧ包括听神经复合动作电位（ＣＡＰ）、耳蜗微音电位（ＣＭ）。结果可见：高钙灌流明显抑制ＣＡＰ幅值,延长同一声强下（９０ｄＢＳＰＬ）Ｎ１－峰潜伏期,但不改变ＣＭ的幅值及总和ＥＰ（Ｇ－ＥＰ）。高钙灌流降低了ＥＰ对噪声的给－撤声反应（ＥＰ－ＯＮ,ＥＰ－ＯＦＦ）和缺氧所得到的最大负ＥＰ（Ｎ－ＥＰ）绝对值。本文分析了外淋巴高钙影响耳蜗电位的可能机制。     

Noise‐induced cochlear F‐actin depolymerization is mediated via ROCK2/p‐ERM signaling

Our previous work has suggested that traumatic noise activates Rho‐GTPase pathways in cochlear outer hair cells (OHCs), resulting in cell death and noise‐induced hearing loss (NIHL). In this study, we investigated Rho effectors, Rho‐associated kinases (ROCKs), and the targets of ROCKs, the ezrin‐radixin‐moesin (ERM) proteins, in the regulation of the cochlear actin cytoskeleton using adult CBA/J mice under conditions of noise‐induced temporary threshold shift (TTS) and permanent threshold shift (PTS) hearing loss, which result in changes to the F/G‐actin ratio. The levels of cochlear ROCK2 and p‐ERM decreased 1 h after either TTS‐ or PTS‐noise exposure. In contrast, ROCK2 and p‐ERM in OHCs decreased only after PTS‐, not after TTS‐noise exposure. Treatment with lysophosphatidic acid, an activator of the Rho pathway, resulted in significant reversal of the F/G‐actin ratio changes caused by noise exposure and attenuated OHC death and NIHL. Conversely, the down‐regulation of ROCK2 by pretreatment with ROCK2 siRNA reduced the expression of ROCK2 and p‐ERM in OHCs, exacerbated TTS to PTS, and worsened OHC loss. Additionally, pretreatment with siRNA against radixin, an ERM protein, aggravated TTS to PTS. Our results indicate that a ROCK2‐mediated ERM‐phosphorylation signaling cascade modulates noise‐induced hair cell loss and NIHL by targeting the cytoskeleton.

     

Chronic exposure to low frequency noise at moderate levels causes impaired balance in mice

We are routinely exposed to low frequency noise (LFN; below 0.5 kHz) at moderate levels of 60-70 dB sound pressure level (SPL) generated from various sources in occupational and daily environments. LFN has been reported to affect balance in humans. However, there is limited information about the influence of chronic exposure to LFN at moderate levels for balance. In this study, we investigated whether chronic exposure to LFN at a moderate level of 70 dB SPL affects the vestibule, which is one of the organs responsible for balance in mice. Wild-type ICR mice were exposed for 1 month to LFN (0.1 kHz) and high frequency noise (HFN; 16 kHz) at 70 dB SPL at a distance of approximately 10-20 cm. Behavior analyses including rotarod, beam-crossing and footprint analyses showed impairments of balance in LFN-exposed mice but not in non-exposed mice or HFN-exposed mice. Immunohistochemical analysis showed a decreased number of vestibular hair cells and increased levels of oxidative stress in LFN-exposed mice compared to those in non-exposed mice. Our results suggest that chronic exposure to LFN at moderate levels causes impaired balance involving morphological impairments of the vestibule with enhanced levels of oxidative stress. Thus, the results of this study indicate the importance of considering the risk of chronic exposure to LFN at a moderate level for imbalance.     

Auditory evoked responses in awake rabbits after exposure to high intensity noise impulses

Impulse noise effects were tested in chronic experiments on 8 awake rabbits. Alterations of cochlear potentials and evoked responses from the inferior collinulus and the medial geniculate body were studied. The rabbits were subsequently exposed to 10 noise impulses of 144 dB SPL, then (after recovery) to 10 impulses of 153 and 164 dB SPL. After exposure the amplitudes of all potentials were reduced. Time of restitution depended on the intensity of the noise, the restitution failed after exposure to 164 dB SPL impulses. Time lapses of the amplitude-reduction and restitution process were comparable for both structures of the auditory pathway. The peak latencies were prolonged significantly in only two of the rabbits after this impulse intensity. Impulses of 164 dB SPL were followed by irreversible changes of all evoked responses.     

Reg3b在大鼠耳蜗的分布及噪声刺激后的表达变化

目的：探讨Reg3b在大鼠耳蜗中的分布情况及在噪声刺激前后的表达变化,为治疗噪声性聋提供新思路。方法：30只健康成年SD大鼠,分为噪声暴露组和正常对照组,利用110dBSPL宽频稳态白噪声对噪声组进行噪声暴露,通过免疫组织荧光技术,观察Reg3b在正常及噪声刺激后成年sD大鼠耳蜗内的分布情况。采用实时定量PCR技术（Realtime-PCR）方法检测大鼠接受噪声刺激前后Reg3b在耳蜗内的表达变化。结果：免疫组织荧光技术提示,Reg3b在噪声暴露后主要表达于大鼠耳蜗的内毛细胞、外毛细胞,以及螺旋神经节处,而正常大鼠耳蜗中Reg3b表达不明显或呈阴性表达。与噪声刺激前相比,噪声刺激后,Reg3b在mRNA水平表达较噪声前明显提高。结论：Reg3b在耳蜗内的分布及在噪声刺激后的表达显著升高提示其在噪声诱导的细胞死亡及对抗噪声损伤方面具有一定作用,可能成为治疗感音神经性聋的新靶点。     

Noise improves transfer of near-threshold, phase-locked activity of the cochlear nerve: evidence for stochastic resonance?

 Stochastic resonance can be described as improved detection of weak periodic stimuli by a dynamic nonlinear system, resulting from the simultaneous presentation of a restricted dynamic range of low-intensity noise. This property has been reported in simple physical and biological activities. The present study describes data consistent with the interpretation that stochastic resonance can be observed in the response of cochlear neurons. These experiments utilized low levels (−5 to 25 dB SPL) of stimuli and noise (5 to 30 dB SPL). Stimuli consisted of simultaneously presented 8 kHz (F ₁) and 8.8 kHz (F ₂) tone bursts, which generated an 800 Hz F ₂–F ₁ cochlear nerve envelope ensemble response in the gerbil. The mean response threshold was approximately −3 dB SPL. Simultaneous presentation of a low-intensity wideband noise increased the amplitude of this response. This was observed with tonal stimuli having intensities of 0–5 dB SPL; responses to stimulus levels >10 dB were attenuated by noise. Response amplitude was increased by noise levels of 10–15 dB; the amplitude was unaffected by lower levels of noise, and decreased in the presence of higher noise levels. These properties are compatible with those of stochastic resonance. Accepted: 11 March 1999