Selective autonomic blockade of conditioned and unconditioned heart rate changes in rabbits

Autonomic blockades were used to examine relationships between heart rate conditioned responses (CRs) and unconditioned responses (URs) during differential classical conditioning in rabbits. Electrical stimulation of septal region or hypothalamus through chronically implanted electrodes was the unconditioned stimulus (US); conditioned stimuli (CSs) were intracranial stimulation of left and right lateral geniculate nuclei. Cholinergic blockade by atropine methylnitrate abolished bradycardia URs and CRs; whereas, beta-adrenergic blockade by propranolol did not diminish these responses. This indicates that autonomic mediation of the bradycardia responses consisted of increases in vagal tone. Alpha-adrenergic blockade by phentolamine abolished bradycardia URs, but not CRs, indicating that different central mechanisms mediated these responses. In a second experiment, one group of rabbits was injected with saline and another group was injected with phentolamine before each of six acquisition sessions. Both groups developed bradycardia CRs although phentolamine abolished the blood pressure URs and converted the cardiodecelerative UR to cardioacceleration.