protective effect of conditioning on noise-induced hearing loss is frequency-dependent

We compared the extent of temporary threshold shift [TTS] and hair cell loss following high level 4 kHz noise exposure with those preconditioned with moderate level 1 and 4 kHz octave band noise. Fifteen Male albino guinea pigs [300- 350 g in weight] were randomly allocated into three groups: those exposed to 4 kHz octave band noise at 102 dB SPL [group 1, n=5]; those conditioned with 1 kHz octave band noise at 85 dB SPL, 6 hours per day for 5 days, then exposed to noise [group 2, n=5]; those conditioned with 4 kHz octave band noise at 85 dB SPL, then exposed to noise [group 3, n=5]. An hour and one week after noise exposure, threshold shifts were evaluated by auditory-evoked brainstem response [ABR] and then animals were euthanized for histological evaluation. We found that TTS and cochlear damage caused by noise exposure were significantly reduced by 1 kHz and 4 kHz conditioning [P<0.001]. We also showed that 4 kHz protocol attenuates noise- induced TTS but no significant TTS reduction occurred by 1 kHz conditioning. Both protocol protected noise-induced cochlear damage. We concluded that lower tone conditioning could not protect against higher tone temporary noise-induced hearing loss, thus conditioning is a local acting and frequency-dependent phenomenon

[Effect of sound conditioning on click auditory brainstem response threshold shifts in guinea pigs]

Sound conditioning is exposure to a non-traumatic, moderate level of sound which increases inner ear resistance against further severe noise. In this study, we aimed to survey the effect of sound conditioning on auditory brainstem response [ABR] threshold shifts using click stimulus, and the effect of the frequency of conditioning on hearing protection. Fifteen guinea pigs were randomly divided into 3 groups. Two conditioned groups were exposed to 1 kHz, and 4 kHz octave band noise at 85 dB SPL, 6 hours per day for 5 days, respectively. On the sixth day, the animals were exposed to 4 kHz octave band noise at 105 dB SPL, for 4 hours. The control group was exposed to intense noise, 4 kHz at 105 Db SPL for 4 hours [without conditioning]. After exposure, ABR thresholds using click were recorded an hour, and 7 days after noise exposure. The results of the ABR with click stimulus showed less thresold shifts in conditioned groups than control [p 0.05]. Electrophysiological data of our study showed that sound conditioning has a protective effect against subsequent intensive noise exposure, and the frequency of conditioning does not have significant effect on ABR threshold shifts when using click stimulus