DELAYED DEVELOPMENT OF CONDITIONED HEART-RATE RESPONSES TO AUDITORY-STIMULI IN THE RAT

This study examined heart rate responses to an auditory conditioned st imulus during development in the rat. The unconditioned stimulus (US) in all experiments was an acoustic startle stimulus. The conditioned s timulus elicited a monophasic bradycardia in animals 21 days of age an d older, but no response in younger animals. These results are strikin gly similar to an earlier study that used shock as the US (Campbell gr Ampuero, 1985). Further, the conditioned cardiac response in adults w as found to be mediated entirely through activation of the parasympath etic division of the autonomic nervous system (ANS) even though the US unconditionally activates the sympathetic system at that age. The del ayed development of several different forms of conditioned fear is dis cussed. (C) 1995 John Wiley & Sons, Inc.