BEHAVIORAL MODULATION OF THE CARDIOVASCULAR EFFECTS OF l-NOREPINEPHRINE IN THE SQUIRREL MONKEY

¹ Departments of Psychiatry, Pharmacology and Physiology, Harvard Medical School, Boston, and New England Regional Primate Research Center, Southborough, Massachusetts

The effects of norepinephrine on mean arterial blood pressure and heart rate were studied under controlled behavioral conditions. Squirrel monkeys, prepared with chronic arterial and venous catheters, responded (pressed a key) under fixed-ratio schedules of termination of a stimulus associated with occasional electric shock or under fixed-ratio schedules of food presentation. When no behavioral scluedule was in effect, norepinephrine produced dose-dependent increases in blood pressure and decreases in heart rate. Under the fixed-ratio schedules, periods of rapid responding alternated with periods of no responding, and episodic increases in both blood pressure and heart rate occurred in phase with responding. While norepinephrine was being infused under these conditions, blood pressure was high; heart rate was correspondingly low during periods of no responding, but increased markedly during periods of responding, often returning to levels prevailing before infusion. Phenylephrine produced similar effects, whereas angiotensin had much less tendency to decrease heart rate at doses which increased blood pressure. Neostigmine, which decreased the resting heart rate, enhanced the episodic increases in heart rate with little or no change in blood pressure. Under all of these conditions, methyl atropine markedly attenuated the behaviorally induced episodic changes in heart rate. The results indicate that withdrawal of the vagally mediated bradycardia induced by the baroreceptor reflex occurs during schedule-controlled responding. Further, they suggest that heart rate modulation in the squirrel monkey under conditions of these behavioral experiments is controlled primarily by changes in vagal activity.