Quantitative analysis of apoptotic and necrotic outer hair cells after exposure to different levels of continuous noise.

We have reported that by 2 days after noise exposure the size of cochlear lesion was expanding by outer hair cells (OHCs) dying either by apoptosis or necrosis. The current study was designed to compare the prevalence of the two cell death pathways as a function of time after exposure to noises of different levels. Chinchillas were exposed to a narrow band noise at either 104 or 108 dB SPL for 1 h. At three time points (1, 4 and 30 days) after the noise exposure, the numbers of missing, apoptotic and necrotic OHCs in the cochleas were identified and documented with a combination of TUNEL, caspase-3 and propidium iodide labeling. The subjects exposed to the 108-dB noise showed significantly more apoptotic OHCs than necrotic OHCs in the cochleas examined at days 1 and 4 after the noise exposure. By day 30, apoptotic and necrotic pathologies continued, although in small quantity, with no significant difference in quantity between two types of cell death. The subjects exposed to the 104-dB noise showed a significant difference in the numbers of apoptotic and necrotic OHCs at day 1 after the noise exposure, whereas the difference became statistically insignificant at day 4 and day 30 after the noise exposure. The results of the study indicate that the early expansion of cochlear lesion is attributed primarily to apoptosis, whereas the later stage of lesion expansion is likely the result of an equal contribution from apoptosis and necrosis.