Low-frequency 'conditioning' provides long-term protection from noise-induced threshold shifts in chinchillas

Studies have shown that loss of auditory sensitivity caused by exposure to high-level acoustic stimuli can be significantly reduced by pre-exposing the subject to moderate-level acoustic stimuli. Although the protective effects of such 'conditioning' exposures have been well documented, very little is known about the persistence of conditioning-induced protection, or about the biological mechanisms underlying it. In the present study, the persistence of conditioning-induced protection was examined in chinchillas by imposing either a 30- or 60-day recovery period between conditioning (10 days of exposure to 0.5 kHz noise at 90 or 95 dB, 6 h/day) and high-level (0.5 kHz noise at 106 dB for 48 h) exposures. Comparisons of threshold shifts between conditioned animals and control animals exposed only to high-level noise indicated that conditioning provided significant protection from noise-induced threshold shifts for at least 2 months. Conditioned animals sustained outer hair cell losses similar to controls, ranging from 15 to 30% in the apical half of the cochlea. The results suggest that low-frequency conditioning can trigger long-lasting changes in cochlear homeostasis rather than temporary changes in physiology or reductions in susceptibility to hair cell loss in chinchillas.