Apoptotic mechanisms after repeated noise trauma in the mouse medial geniculate body and primary auditory cortex

A correlation between noise-induced apoptosis and cell loss has previously been shown after a single noise exposure in the cochlear nucleus, inferior colliculus, medial geniculate body (MGB) and primary auditory cortex (AI). However, repeated noise exposure is the most common situation in humans and a major risk factor for the induction of noise-induced hearing loss (NIHL). The present investigation measured cell death pathways using terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) in the dorsal, medial and ventral MGB (dMGB, mMGB and vMGB) and six layers of the AI (AI-1 to AI-6) in mice (NMRI strain) after a second noise exposure (double-exposure group). Therefore, a single noise exposure group has been investigated 7 (7-day-group-single) or 14 days (14-day-group-single) after noise exposure (3 h, 5-20 kHz, 115 dB SPL peak-to-peak). The double-exposure group received the same noise trauma for a second time 7 days after the initial exposure and was either TUNEL-stained immediately (7-day-group-double) or 1 week later (14-day-group-double) and data were compared to the corresponding single-trauma group as well as to an unexposed control group. It was shown that TUNEL increased immediately after the second noise exposure in AI-3 and stayed upregulated in the 14-day-group-double. A significant increase in TUNEL was also seen in the 14-day-group-double in vMGB, mMGB and AI-1. The present results show for the first time the influence of a repeated noise trauma on cell death mechanisms in thalamic and cortical structures and might contribute to the understanding of pathophysiological findings and psychoacoustic phenomena accompanying NIHL.