AGE-RELATED-CHANGES IN AUDITORY BRAIN-STEM RESPONSES IN FISCHER-344 RATS - EFFECTS OF RATE AND INTENSITY

Age-related changes in auditory brainstem responses (ABR) observed in humans may reflect peripheral or centrally-occurring deficits. In clin ical studies, high stimulus repetition rates have been used to improve the identification of central auditory pathology. In the present stud y, interactions between stimulus level and repetition rate were examin ed in the Fischer 344 rat, an animal demonstrating both peripheral hea ring loss and changes in auditory brainstem neurochemistry with age. M onaural threshold and standard P;BR morphology were determined in youn g (3-6 months) and old (20-23 months) rats using clicks at 10/s, with intensity varied from 0-100 dB. The effects of increasing stimulus rep etition rate on ABR latency and morphology were evaluated at 60-100 dB using rates of 5, 10, 20, and 40/s. Old animals demonstrated elevated ABR click thresholds, reflected by shifts in the latency-intensity cu rves. With increased stimulation rates, aged rats exhibited prolonged Wave 4 and 5 latencies, especially at the highest intensities, with de graded waveform morphology. Peak amplitudes were generally reduced in old rats, irrespective of rate or stimulus level. These findings sugge st auditory processing is altered in aged animals, while the selective effects of rate increases on Waves 4 and 5 provide supporting evidenc e for possible involvement of the central auditory generators of these components.