Xiang-Yang Zheng

The Role of Antioxidants in Protection from Impulse Noise

ABSTRACT: The hearing loss from exposure to noise and ototoxic drugs share a number of audiological and pathological similarities. Recent research has shown that reactive oxygen species (ROS) may be a common factor in both noise‐ and drug‐induced hearing loss. This review describes three experiments that point to ROS as a causative factor in both noise‐ and drug‐induced hearing loss and antioxidants as a protective agent. In the first experiment, the ears of chinchillas were treated with R‐N6‐phenylisopropyladenoisine (R‐PIA) and exposed to 150‐dB impulse noise. The treated ears developed substantially less permanent hearing loss (PTS) and hair cell loss than the untreated ears. One interpretation of this experiment is that R‐PIA increases the availability of glutathione (GSH). In the second experiment, the role of GSH was specifically examined. The ears of chinchillas were treated with glutathione monoethylester (GEE), a pro‐GSH drug that has been shown to readily cross cell membranes and increase GSH levels. The GEE‐treated ears had significantly less PTS and hair‐cell loss than the nontreated ear. Previous research has shown that moderate levels of noise exposure can increase a subjects resistance to noise, and also increase the availability of antioxidant enzymes in the cochlea. In the third experiment, chinchillas were given a series of “toughening” exposures (i.e., 6 h of a 0.5‐kHz OB noise at 95 dB for 10 days). After the series of “toughening” exposures, the subjects were treated with carboplatin, a drug that causes massive inner‐hair‐cell lesions in the chinchilla. The animals receiving the 10‐day toughening exposure developed less PTS and hair‐cell loss than the control animals.

Evidence that inner hair cells are the major source of cochlear summating potentials.

The role of the inner hair cells (IHCs) in generating the cochlear summating potentials (SP) was assessed by measuring SP, cochlear nerve action potentials (CAP), cochlear microphonics (CM) and 2f1-f2 distortion product otoacoustic emissions (DPOAEs) in 15 chinchillas with either acute chemical de-afferentation, accomplished by applying kainic acid to the round window, or surgical de-afferentation and basal IHC loss, which developed within two months after sectioning the auditory nerve. In the auditory nerve sectioned ears, type I ganglion cells disappeared whereas most, if not all, type II ganglion cells were still present. Histological analysis of surface preparations and sections through the modiolus verified the de-afferentation in both models and showed a large IHC loss at the base of the cochlea in the ears with the auditory nerve sectioned while other structures of the cochlea remained intact. Unoperated (left) ears of 9 animals served as controls. CM and DPOAEs were normal in all ears whereas the CAP was substantially depressed in de-afferented ears. Comparisons among the SP input-output functions suggest that (1) the IHCs are the major generator of SP recorded from the round window in chinchilla, in particular at low to moderate stimulus levels, (2) the SP recorded from the round window largely reflects the responses from hair cells at the base of the cochlea, and (3) kainic acid results in an increase of SP amplitude to high-level stimuli whereas the SP to low- to moderate-level stimuli remains in the normal range.

Recovery of structure and function of inner ear afferent synapses following kainic acid excitotoxicity

The present study was conducted to examine the re-establishment of IHC/VIII nerve synapses following kainic acid (KA) excitotoxicity and to discern if the re-organized afferent could render not only a normal auditory threshold but also a normal supra-threshold function. KA (60 mM) applied to the intact round window membrane in chinchilla destroyed postsynaptic endings of the auditory nerve, depressed the input-output (I/O) functions of auditory evoked potentials (EVP) and produced an average loss of sensitivity of over 80 dB at 4, 8, and 16 kHz, with less substantial losses (40-60 dB) at lower frequencies. However, there was no significant difference in 2f1-f2 distortion-product otoacoustic emissions (DPOAE) before and after the application of KA. The nerve endings went through a sequence of swelling, degeneration and recovery over a 3-5 day period at higher frequency. Auditory sensitivity and supra-threshold response returned accordingly. In contrast, complete recovery at lower frequencies (1 and 2 kHz) required more than 5 days. The results provide strong evidence that (1) excitotoxically damaged cochlear afferent neurons can recover and render both a normal EVP threshold and EVP I/O function and (2) afferent innervation to IHCs is not necessary for DPOAE generation.

Effects of kainic acid on the cochlear potentials and distortion product otoacoustic emissions in chinchilla

In absence of acoustic stimulation, the auditory nerve generates electrical noise with a spectral peak between 300 and 3000 Hz (Dolan et al., 1990). This electrical noise is eliminated when the dendrites of auditory nerve fibers are damaged by kainic acid (KA). We hypothesized that the KA-induced damage to the afferent dendrites might alter cochlear micromechanics or modify outer hair cell (OHC) electromotility. The KA-induced decrease in spontaneous electrical noise from the auditory nerve could conceivably reduce the spontaneous sounds recorded in the ear canal and the postulated change in cochlear micromechanics might alter distortion product otoacoustic emissions (DPOAE). To evaluate these hypotheses, we applied KA to the round window of the cochlea. KA reduced the spontaneous electrical noise recorded from the round window and significantly reduced the amplitude of the compound action potential (CAP) to tone bursts at 2, 4 and 8 kHz. KA caused only a slight reduction in the amplitude of the cochlear microphonic (CM) recorded from the round window: however, it had no effect on the spontaneous acoustic noise in the car canal or on 2 f1-f2 DPOAEs. These results suggest that the KA-induced reduction of electrical noise from the auditory nerve has no measurable effect on OHC electromotility as reflected in spontaneous otoacoustic emissions and that damage to the afferent dendrites has no effect on cochlear micromechanics as reflected in DPOAEs.

Cochlear de-efferentation and impulse noise-induced acoustic trauma in the chinchilla.

The olivocochlear bundle (OCB) has been shown to protect the ear from acoustic trauma induced by continuous noise or tones. The present study examines the OCBs role in the ears response to impulse noise (150 dB pSPL, 100 impulses, 50 s total exposure duration). Successful section of the OCB was achieved through a posterior parafloccular fossa approach for the right ears of six out of 15 adult chinchillas. The left ears from the same animals served as efferent-innervated controls. Measurements of inferior colliculus evoked potentials (ICPs) showed that the de-efferented ears incurred similar temporary and permanent threshold shifts as the control ears. Twenty days after noise exposure, depressed ICP amplitudes had virtually recovered to pre-values in the control ears whereas those in the de-efferented ears remained significantly depressed. Greater loss of inner hair cells was seen in the de-efferented ears than in the control ears. Both control and de-efferented ears incurred large loss of outer hair cells, with no statistically significant differences between groups. The current data are intriguing, yielding tentative evidence to suggest that inner hair cells of de-efferented ears are more susceptible to impulse noise than those in efferented control ears. In contrast, outer hair cell vulnerability to impulse noise appears to be unaffected by de-efferentation.

Vestibular destruction by slow infusion of gentamicin into semicircular canals.

Intratympanic or round window application of gentamicin is often used to alleviate disabling vertigo arising from unilateral Menieres disease; however, treatment is often accompanied by hearing loss because the drug initially enters the cochlea before diffusing to the vestibular system. In order to enhance vestibular damage and reduce the risk of hearing loss, gentamicin was infused directly into the vestibular system. An osmotic pump containing 50, 100, 200 or 400 microg/ml of gentamicin was infused into the superior semicircular canal of the chinchilla for 7 days. Afterwards, vestibular damage was evaluated by measuring the decline in hair cell density in the utricle, saccule and superior semicircular canals. Auditory damage was assessed with distortion product otoacoustic emissions (DPOAE) and outer hair cell (OHC) and inner hair cell (IHC) loss. Infusion with the two lowest gentamicin concentrations resulted in significant hair cell loss and reduced duration of the nystagmus response, but had little or no effect on OHC or DPOAE. Higher doses of gentamicin damaged cochlear hair cells and reduced the DPOAE. In conclusion, slow infusion of a low dose of gentamicin into the semicircular canals mainly damages the vestibular hair cells and inactivates the nystagmus response without damaging cochlear hair cells or DPOAE.

Cochlear microphonics and otoacoustic emissions in chronically de-efferented chinchilla

The effects of eliminating the olivocochlear bundle (OCB) on cochlear electromechanical properties were examined by measuring cochlear microphonics (CM) and distortion product otoacoustic emissions (DPOAEs) in chronically de-efferented chinchillas. The OCB fibers to the right ears were successfully sectioned in six out of 15 adult chinchillas via a posterior paraflocular fossa approach. At the end of the experiment, these ears were histologically verified as being deprived of both lateral and medial OCB fibers. The opposite (left) ears from the animals served as controls. Following de-efferentation, changes of the inter-modulation distortion components (2f(1)-f(2), f(2)-f(1), 3f(1)-2f(2), 3f(2)-2f(1)) varied, depending on the frequencies and levels of the stimuli. DPOAE amplitudes to low-level stimuli were within the 95% confidence intervals around mean DPOAE amplitudes of the control ears at all the frequencies (1-8 kHz). At high stimulus levels, DPOAE amplitudes increased by 5-20 dB at 1 and 2 kHz while remaining in the normal range at 4 and 8 kHz. In contrast, the CM input/output functions to stimuli from 1 to 8 kHz were significantly reduced by approximately 40-50% at all input levels. The results suggest that the OCB may play a role in modulating electrical properties of the outer hair cells and in reducing the magnitude of cochlear distortion to high-level stimuli.

Conditioning-induced protection from impulse noise in female and male chinchillas

Sound conditioning (pre-exposure to a moderate-level acoustic stimulus) can induce resistance to hearing loss from a subsequent traumatic exposure. Most sound conditioning experiments have utilized long-duration tones and noise at levels below 110 dB SPL as traumatic stimuli. It is important to know if sound conditioning can also provide protection from brief, high-level stimuli such as impulses produced by gunfire, and whether there are differences between females and males in the response of the ear to noise. In the present study, chinchillas were exposed to 95 dB SPL octave band noise centered at 0.5 kHz for 6 h/day for 5 days. After 5 days of recovery, they were exposed to simulated M16 rifle fire at a level of 150 dB peak SPL. Animals that were sound conditioned showed less hearing loss and smaller hair cell lesions than controls. Females showed significantly less hearing loss than males at low frequencies, but more hearing loss at 16 kHz. Cochleograms showed slightly less hair cell loss in females than in males. The results show that significant protection from impulse noise can be achieved with a 5-day conditioning regimen, and that there are consistent differences between female and male chinchillas in the response of the cochlea to impulse noise.

Faster recovery in central than in peripheral auditory system following a reversible cochlear deafferentation

Included among the exciting findings in auditory neuroscience are (i) central plasticity after peripheral injury and (ii) regeneration of auditory nerve fibres following excitotoxic damage. The present study extends our understanding of auditory system plasticity by examining changes in peripheral and central physiology as the cochlea recovers from temporary deafferentation due to excitotoxicity. Application of kainic acid (60 mM) to the round window membrane substantially depressed responses from both auditory nerve and brain stem (inferior colliculus), without affecting distortion-product otoacoustic emissions from the inner ear. The auditory nerve input/output functions recovered over a 30-day period whereas recovery of brainstem response amplitudes occurred within five days. In contrast to amplitudes, thresholds at both peripheral and central levels recovered simultaneously, within five days after kainic acid application. The results indicate that (i) cochlear afferent neurons can recover after excitotoxic damage; (ii) response threshold itself, either central or peripheral, is not sufficient to assess the integrity of the auditory periphery; (iii) the central auditory system can recover more rapidly than the periphery; and (iv) the system can maintain its function in the normal range as peripheral function continues to improve.