DEVELOPMENTAL AND PHARMACOLOGICAL ANALYSIS OF THE CARDIAC RESPONSE TOAN ACOUSTIC STARTLE STIMULUS

The purpose of this research was to study the cardiac response of prew eanling and adult rats to 10 presentations of an acoustic startle stim ulus (0 ms rise time, 100 ms, 130 dB, white noise stimulus). The first presentation of the startle stimulus produced a decrease in heart rat e (HR) at both ages. With continued stimulus presentations, the respon se shifted to tachycardia in the adults but remained bradycardia in th e preweanlings. Pharmacological analysis revealed that the startle sti mulus activated only the parasympathetic system in the preweanling rat s on all 10 trials. In contrast, the startle stimulus produced coactiv ation of the parasympathetic and sympathetic systems in the adults on the first trial, with the parasympathetic system predominating, and so lely sympathetic activation on later trials. These results are discuss ed in terms of current psychophysiological models of (a) the cardiac r esponse to startle stimuli and (b) autonomic space.