Acoustic Startle Response Affected by Aging and Cholinergic Neurotransmitters

The acoustic startle response has been used to evaluate tinnitus and hyperacusis in animal models. Gap induced prepulse inhibition of the acoustic startle reflex (gap-PPI) is affected by tinnitus and loudness changes. Since tinnitus and reduced sound tolerance are commonly seen in elderly, we measured gap-PPI in Fischer 344 rats, an aging related hearing loss model, at different ages: 3-5 months, 9-12 months, and 15-17 months. The startle response was induced by three different intensity of sound: 105, 95 and 85 dB SPL. Gap-PPI was induced by different duration of silent gaps from 1 to 100 ms. When the startle was induced by 105 dB SPL sound intensity, the gap-PPI induced by 50 ms silent gap was significantly lower than those induced by 25 or 100 ms duration, showing a “notch” in the gap-PPI function. The “notch” disappeared with the reduction of startle sound, suggesting the “notch” may be related with hyper-sensitivity to loud sound. As the intensity of the stimulus decreased, the appearance of the hyperacusis-like effect decreased more quickly for the youngest group of rats. We also tested scopolamine, a muscarinic acetylcholine receptor antagonist, and mecamylamine, a nicotinic acetylcholine receptor antagonist, on the effect of gap-PPI. When scopolamine was administered, the results indicated no addition effect on the hyperacusis-like phenomenon in the two older groups. Mecamylamine, the nicotinic antagonist also showed effects on the appearance of hyperacusis on rats in different ages. The information derived from the study will be fundamental for the further research in determining the cause and treatment for hyperacusis.