Conditioned bradycardia in the rabbit: effects of knife cuts and ibotenic acid lesions in the lateral hypothalamus.

New Zealand albino rabbits received either parasagittal or coronal knife cuts in the lateral hypothalamus (LH). Other animals received ibotenic acid lesions of LH. Pavlovian conditioning was studied in these animals and compared with that of sham operated, vehicle, or unoperated control animals. Tones served as conditioned stimuli (CSs) and periorbital electric shocks as unconditioned stimuli (USs). Coronal knife cuts that interrupted fibers passing through LH abolished the bradycardia elicited by these contingencies, as well as the cardiac component of the orienting reflex (OR), which also consisted of bradycardia. Parasagittal knife cuts medial to the temporal lobe but lateral to the major nuclei of the hypothalamus also completely abolished conditioned bradycardia, but had no effect on the OR, compared to sham and unoperated control animals. These lesions also had no effect on concomitantly occurring eyeblink conditioning. Ibotenic acid lesions of LH had no effect on conditioned bradycardia but diminished the magnitude of the cardiac OR. Control experiments suggest that the damage produced by these manipulations had no effect on either sensitivity to the CS or US and produced no general motoric difficulties. These data thus suggest that structures rostral to the hypothalamus, probably in the amygdala and/or agranular prefrontal cortex, mediate the bradycardia associated with classical conditioning contingencies, but that hypothalamic mechanisms may mediate the OR.