Noise-induced hearing loss in normotensive and spontaneously hypertensive rats

Hearing loss was investigated in normotensive (N) and spontaneously hypertensive (SH) rats after prolonged exposure to a simulated industrial noise environment. Exposure was initiated in a group of young rats (3 months old) and continued up to 18 months of age, and in a group of old rats (15 months old) and followed up to 18 months of age. Hearing thresholds were determined behaviorally with a conditioned suppression technique before and after 1, 2, 3 and, for some groups, also after 15 months of exposure. A frequency-modulated noise band sweeping from 3 to 30 kHz at a frequency of 0.5 Hz was presented 10 hours daily at a level of 100 dB Leq(lin). The results showed that young N and SH rats not exposed to noise had identical hearing thresholds. Noise exposure induced a significantly greater hearing loss in SH rats than in N rats; SH animals were more susceptible to noise than were young ones; no difference was seen between males and females. The histology of the inner ears of the rats was examined by light microscopy after the end of the experiment. The young noise-exposed N rats showed no abnormal loss of hair cells in spite of the fact that they sustained hearing losses of 30-40 dB. The SH rats showed a significantly greater loss of hair cells than did the N rats. The stereocilia were found to be fused on a large number of inner hair cells in the basal turns of both N and SH animals. It was concluded that SH and N rats constitute an interesting model for the investigation of biological mechanisms behind individual differences in susceptibility to noise-induced hearing loss.