Heart rate changes caused by chemical stimulation of the amygdaloid body

The modification of cardiovascular parameters resulting from the injection of norepinephrine (NE) or acetylcholine (ACh) into the amygdala was investigated in the chronically prepared unanesthetized restrained beagle. Blood pressure was obtained via an aortic catheter while heart rate was determined from Lead II of the ECG. Depth electrodes implanted into the hippocampus and ventromedial hypothalamus were used to record the EEG from these respective sites. NE caused a dose-related decrease in heart rate while ACh induced a tachycardia of short duration. Methyl atropine injected into the amygdala prior to ACh blocked the ACh induced tachycardia while physostigmine mimicked the cholinergic response. These responses were not associated with changes in blood pressure or respiration. It is concluded that there are two chemically coded systems within the amygdaloid body that are capable of modifying heart rate: an adrenergic system that induces bradycardia and a cholinergic system that causes tachycardia.