Influence of acoustic deprivation on recovery of hair cells after acoustic trauma.

The purpose of this study was to investigate how the recovery of the cochlea, after acoustic trauma, might be influenced by acoustic stimulation or deprivation. In anaesthetized adult chinchillas, both ears were simultaneously exposed to a traumatizing acoustic stimulus (2 kHz tone, at 117 dB SPL for 15 min). Probe microphones positioned in both bullae were used to ensure identical exposure to the two ears; this was important because the experiment relies on within-animal controls. Cochlear action potential thresholds across frequency (CAP audiograms) were used to verify the similarity of threshold shifts to the two ears. Immediately following, a unilateral ossiculectomy was performed which resulted in one cochlea being acoustically deprived during the recovery period, whilst the other was not. In groups of animals with recovery periods of 1, 3, 6, and 12 weeks, both the acoustically deprived and the normally stimulated cochleas were examined with scanning electron microscopy. To quantify hair cell damage, we used a damage scale based on stereociliar integrity; for each cochlea, a standard region 5.5-8.5 mm from the apex was studied in detail. We found that after acoustic trauma, hair cell damage to the cochlea which is deprived of sound during the recovery period, is significantly greater compared with that in the normally stimulated, contra-lateral cochlea. Our results suggest that mechanical activation of the inner ear acts to inhibit long-term degenerative processes, or influence repair of partially damaged hair cells.