Effects of noise exposure on development of tinnitus and hyperacusis: Prevalence rates 12 months after exposure in middle-aged rats.

Fischer Brown Norway (FBN) rats (n = 233) were unilaterally exposed to 12 different combinations of noise intensity, duration, and spectrum, while 46 rats served as sham-exposed controls. Rats were behaviorally tested for tinnitus and hyperacusis using gap-induced inhibition of the acoustic startle reflex (Gap) and prepulse inhibition (PPI) using 60-dB SPL before noise-exposure and at regular intervals for 12 mo. 12-mo after noise exposure the middle-aged rats were then tested again for tinnitus and hyperacusis before collecting Auditory Brainstem Response (ABR) thresholds. Collapsing across all noise exposure conditions a significant tinnitus-like deficit in responding to silent gaps was observed, with the most likely tinnitus pitch around 16 kHz. Rates of tinnitus 12-mo after noise exposure were greatest in groups receiving the four least intense noise doses (110-dB for 30, 60 and 120 min, and 116-dB for 30 min), while some of the greatest rates of hyperacusis occurred in groups receiving more intense or longer exposures. The results suggest that rates for developing tinnitus in animal models may not be easily predicted based upon noise exposure dose, but that low-to-moderate noise exposures may result in the greatest likelihood for producing tinnitus.