Summation of baroreflex and classically conditioned heart rate responses in dogs

Following instrumentation with pneumatic cuffs around the inferior vena cava and the descending aorta, dogs were studied either following differential classical conditioning or in a control state. The cuffs functioned to raise and lower blood pressure for the construction of baroreflex curves for heart rate. Conditioned dogs received 8 trials each day with each CS+ (tone paired with flank shock) and CS- (a different tone without shock). Curves were constructed from cuff inflations timed to coincide with the maximum conditioned heart rate response. These curves were constructed from data acquired during infusion of saline, methyl atropine, or propranolol. Comparison of these curves revealed that the CS+ shifted the curves toward higher heart rates while the CS- curve was shifted toward lower heart rates without a change in gain. The amount of shift was comparable to that of the conditioned heart rate response. This suggested that the responses were independent and additive. Neither propranolol nor atropine eliminated this separation between the curves induced by the conditioning. These observations lead us to conclude that classically conditioned stress and baroreceptor stimulation exert independent control over heart rate that are mediated by both sympathetic and parasympathetic influences.