Paradoxical autonomic responses to aversive stimuli in the developing rat.

The goals of this research were to determine (a) the change in heart rate elicited by aversive auditory stimuli in the laboratory rat at different ages and (b) the autonomic origins of those changes at each age. The results of the first 2 experiments showed that aversive white noise 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 showed 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 the noise in periadolescent rats was mediated by parasympathetic withdrawal of the heart, and (c) the noise-induced increase in heart rate in adult rats was primarily mediated by sympathetic activation of the heart.