Changes in plasma catecholamines and behavior of rats during the anticipation of footshock

A chronic catheter was inserted into the ventral caudal artery of male Sprague-Dawley rats to allow for sampling of blood and measurement of blood pressure and heart rate in conscious animals without handling. The day after surgery, one group of rats was transferred individually from the home cage to a shock chamber and after 5 min received 60 footshocks (2.5 mA, 0.4 sec in duration, at 5-sec intervals). This procedure was repeated two additional times during the same day. Control animals were handled in an identical manner but were not shocked. Previous experience with footshock had no effect on basal plasma levels of norepinephrine (NE) and epinephrine (EPI) or on resting blood pressure and heart rate as measured 2 days after surgery. When transferred to the shock chamber, previously shocked rats had greater increases in plasma NE and EPI and heart rate. In addition, previously shocked rats were less active and defecated more frequently than did control rats. However, there were no differences in the responses of previously shocked and control rats to 5 min of intermittent footshock. Results of this study demonstrate an activation of the sympatho-adrenal medullary system and attendant changes in the cardiovascular system and behavior of rats during the anticipation of footshocks. This suggests that the functioning of sympathetic nervous system and the adrenal medulla provides a sensitive measure of arousal and fear in rats.