Susceptibility to noise exposure during postnatal development in rats.

Susceptibility to noise-induced hearing loss was studied during maturation in 20 female pigmented rats (strain Long-Evans). Young rats, 3, 4, 5 and 6-7 weeks old, were exposed for 1 h to a broad-band noise with an intensity of 120 dB SPL. The thresholds and amplitudes of middle latency responses (MLR) recorded from electrodes implanted on the surface of the auditory cortex were analyzed before and after noise exposure. The results were compared with data from our previous study, in which the effects of broad-band noise exposure on MLR were investigated in adult rats [Syka, J. and Rybalko, N. (2000) Hear. Res. 139, 59-68]. The hearing thresholds of 3-7 week old rats before noise exposure were within the normal adult range. Noise exposure in young rats produced an adult-like pattern with an elevation of hearing thresholds. One-two weeks post-exposure a recovery of MLR thresholds was observed, though full recovery only occurred in the low frequency range. Recovery of hearing thresholds in the high frequency range depended on the age of the animal at the time of exposure. In all animals aged less that 6-7 weeks, exposure resulted in a permanent threshold shift in the range of 4-32 kHz. The mean values of permanent threshold shifts at 16 kHz (the frequency of maximal hearing loss) were 53.0+/-4.5, 47.6+/-9.6, 37.5+/-7.5 and 27+/-10 dB for rats exposed at 3, 4, 5 and 6-7 weeks of age, respectively. Similar to adult rats, young rats exposed to noise exhibited an enhancement of MLR amplitudes. This amplitude enhancement was more pronounced in the high frequency range. In several rats exposed at 3-5 weeks of age, the recovery period to normal amplitudes was substantially prolonged and lasted 4-8 weeks in comparison with 1-2 weeks in adult rats. These results demonstrate a greater susceptibility to noise exposure in rats during the first 5 postnatal weeks.