CONDITIONED FEAR-INDUCED TACHYCARDIA IN THE RAT - VAGAL INVOLVEMENT

The effects of conditioned fear on gross activity, heart rate, PQ inte rval, noradrenaline and adrenaline were studied in freely moving rats. Subcutaneous (s.c.) injections of atropine methyl nitrate (0.5 mg/kg) during rest resulted in a significant shortening of the PQ interval, indicating that the PQ interval can be used as a measure of vagal acti vity. Conditioned fear was induced by 10-min forced exposure to a cage in which the rat had previously experienced footshocks (5 X 0.5 mA x 3 s). In non-shocked controls, an increase in gross activity was found and a pronounced tachycardia, without changes in PQ interval. Conditi oned fear rats showed immobility behaviour, associated with a less pro nounced tachycardia and an increase in PQ interval. Noradrenaline was similarly increased in both groups, whereas adrenaline was increased i n conditioned fear rats only. To further evaluate the role of the vagu s, rats were exposed to conditioned fear after pre-treatment with atro pine methyl nitrate (0.5 mg/kg, s.c.). Again, immobility was observed with a concomitant tachycardia, but without an increase in Po interval . These results indicate that the autonomic nervous system is differen tially involved in heart rate regulation in conditioned fear rats and in non-shocked controls: in non-shocked controls a predominant sympath etic nervous system activation results in an increase in heart rate, w hereas in conditioned fear rats the tachycardiac response is attenuate d by a simultaneous activation of sympathetic nervous system and paras ympathetic nervous system. (C) 1998 Elsevier Science B.V. All rights r eserved.