Subicular lesions disrupt but do not abolish classically conditioned bradycardia in rabbits.

Rabbits and rats received horseradish peroxidase injections in the medial prefrontal cortex, and retrograde labeling was examined in the hippocampus (HC) and subicular complex (SC). Labeled cells were observed in HC and SC in the rat, but only in the SC of the rabbit. In a second experiment, separate groups of rabbits with sham, SC, or cortical control lesions were subjected to differential classical heart rate conditioning, in which 4-s, 75-db tones served as conditioned stimuli and a 3-mA paraorbital shock was the unconditioned stimulus. Although conditioned bradycardia was obtained in animals with SC lesions, it was slower to develop and was much shorter in duration than in the cortical and sham control groups. In the animals with SC lesions, the bradycardiac response was quickly replaced with tachycardia, suggesting a sympathetic bias in these animals.