Evaluating the protective role of the olivocochlear bundle against acoustic overexposure in rats by using Fos immunohistochemistry

Efferent inhibition on the cochlea is suggested as a possible function of t he olivocochlear bundle (OCB). Substantial evidence supports the finding th at the OCB may protect the inner ear from noise-induced damage. However, th ere is relatively less known about the effects of noise on the central audi tory transmission compared to the effects on the periphery. In the present animal study, two experimental paradigms were designed to analyze the influ ence of OCB lesion on the central auditory transmission following acoustic overexposure. In order to evaluate the animal's auditory function, its hear ing threshold and the tone-evoked Fos expression shown in auditory nuclei w ere examined. Fos is a protein product of proto-oncogene c-fos. Via appropr iate acoustic stimulation, Fos expression reveals the activated neuronal el ements along the ascending auditory pathway. Thus, in experiment 1, no expo sure sound was introduced and therefore no significant differences were sho wn in hearing thresholds and Fos expression among all rats, regardless of t he status of their OCB. This result indicates that, without acoustic overex posure, OCB lesion caused no significant effect on brainstem auditory trans mission. In contrast, in experiment 2, rats were exposed to continuous 8 kH z tones at 85 dB sound pressure level (SPL). A significantly increasing thr eshold was observed in rats with OCB lesion following an exposure period of 5 or 10 days. In addition, Fos expression was invisible first in rats with OCB lesion following 5-day exposure and almost no Fos expression could be examined in all rats after 10-day exposure. Taken together, the present dat a demonstrate that damaging the OCB renders an animal more easily vulnerabl e to acoustic damage than that of rat with intact OCB, and then reduces its cochlear activities, which eventually leads to increasing difficulty to in duce tone-evoked Fos expression along the ascending auditory pathway. (C) 2 000 Elsevier Science B.V. All rights reserved.