PARADOXICAL AUTONOMIC RESPONSES TO AVERSIVE STIMULI IN THE DEVELOPINGRAT

The goals of this research were to determine (a) the change in heart r ate elicited by aversive auditory stimuli in the laboratory rat at dif ferent ages and (b) the autonomic origins of those changes at each age . The results of the first 2 experiments showed that aversive white no ise stimuli elicited cardiac deceleration in preweanling (16-day-old) rats and cardiac acceleration in weanling (23-day-old), periadolescent (30-day-old), and adult (60-day-old) rats. Subsequent experiments sho wed that (a) the decrease in heart rate elicited by the noise stimulus in preweanling rats was mediated by parasympathetic activation of the heart, (b) the stimulus-elicited increase in heart rate elicited by t he noise in periadolescent rats was mediated by parasympathetic withdr awal of the heart, and (c) the noise-induced increase in heart rate in adult rats was primarily mediated by sympathetic activation of the he art.