Mediodorsal thalamic lesions impair differential Pavlovian heart rate conditioning

Rabbits with bilateral ibotenic acid or vehicle injections into the mediodorsal nucleus of the thalamus (MD) received differential Pavlovian heart rate (HR) conditioning in which tones of different frequencies served as conditioned stimuli (CSs) and a paraorbital electric shock train was the unconditioned stimulus (US). Rabbits with ibotenic acid lesions of MD demonstrated impaired differential HR conditioning, due primarily to enhanced responding to the non-reinforced CS-. The magnitude of the HR conditioned response (CR) was somewhat enhanced in lesioned animals compared to vehicle control animals, as was the HR component of the orienting response to non-reinforced tones. These results are consistent with a role for MD in some aspect of association formation, possibly through its mediation of the late-occurring sympathetic component of the conditioned HR response. These data also demonstrate that MD does not participate in the vagal drive that produces Pavlovian conditioned bradycardia.