AUTONOMIC MEDIATION OF UNCONDITIONED AND CONDITIONED HEART-RATE RESPONSES IN THE 16-DAY-OLD RAT

In three experiments, we examined the cardiac response of 16 day-old r ats to a 10-sec olfactory stimulus before and after aversive Pavlovian conditioning. The unlearned (orienting) response to the novel stimulu s was bradycardia, and the conditioned response was a brief period of bradycardia early in the conditioned stimulus, followed by pronounced tachycardia. After selective blockade of the parasympathetic and sympa thetic systems with atropine or atenolol, the deceleratory orienting r esponse was found to be the result of an increase in vagal discharge, accompanied by a cooccurring decrease in sympathetic stimulation of th e heart. The initial deceleratory component of the conditioned respons e was produced by an increase in parasympathetic stimulation of the he art, and the acceleratory component was generated by an increase in sy mpathetic activity. These results are discussed in terms of their sign ificance for the neural and autonomic origins of the phasic changes in heart rate that are elicited by the olfactory stimulus during differe nt stages of conditioning.