The relationship between noise-induced hearing loss and hair cell loss in rats

Noise-induced hearing loss (NIHL) and hair cell loss are known to show only a moderate correlation. One reason for this is that NIHL may reflect not only the sum of dead hair cells, but also the sum of impaired but still living hair cells. This report compares hair cell loss in different cochlear regions in rats with noise-induced compound action potential (CAP) threshold elevation at corresponding frequencies. CAP threshold elevation and hair cell loss were determined 4 weeks after noise exposure. In the apical turn (<35% from the apex) there was no hair cell loss even when a 60 dB CAP threshold elevation was induced. In the region of 40-60% from the apex in the middle turn, significant hair cell loss was not observed until CAP threshold elevation exceeded about 40-50 dB. This critical level decreased towards the basal turn. In the basal turn, outer hair cell (OHC) loss was observed in almost all of the noise-exposed rats, even in some cases without detectable NIHL, but inner hair cell (IHC) loss was still not observed until 50 dB threshold elevation. In the region of 75-90% from the apex related to the highest frequencies tested in this study (30-40 kHz), a linear NIHL/OHC loss relationship was observed. The results of this paper suggest that the high frequency hair cells in rat cochlea may die relatively rapidly after injury, leading to a linear relation between NIHL and hair cell loss, but that the low frequency hair cells may survive without auditory function.