Intense noise-induced apoptosis in hair cells of guinea pig cochleae

Cells can die by two distinct pathways: apoptosis and necrosis. To explore whether intense noise can induce hair cell (HC) death via the apoptotic pathway, we systematically examined morphological changes in guinea pig cochlear HC nuclei stained with Hoechst 33342, a fluorescent dye specifically labelling the nuclear DNA. A narrow band noise centred at 4 kHz with levels at 110 dB, 115 dB or 120 dB (SPL) was applied for 4 h and the exposed cochleae were collected at various intervals (3 h, 3 or 14 days) after the noise exposure. Auditory function was monitored by measuring thresholds of auditory brain stem responses. In the noise-damaged cochleae, there were two major types of nuclear changes, nuclear condensation appeared as karyorrhexis or karyopyknosis and nuclear swelling. Karyorrhexis and karyopyknosis predominately appeared in the severely damaged cochlear region in the animals exposed to 120 dB noise and examined 3 h after the noise exposure. In contrast, swelling of nuclei occurred in all of the noise- exposed cochleae, and was the feature change in the animals exposed to 110 and 115 dB noise. This pathological change persisted at least for 14 days after the noise exposure. The typical changes of karyorrhexis and karyopyknosis noted in the animals exposed to 120 dB noise were morphologically similar to those nuclear changes described in previous studies for apoptosis, suggesting that the apoptotic process may be involved in intense noise-induced HC death.