The role of the medial geniculate region in differential Pavlovian conditioning of bradycardia in rabbits.

The present study examined the role of the medial geniculate region (MGN) in differential Pavlovian conditioning of bradycardia and corneo-retinal potential (CRP) to acoustic stimuli in rabbits. Injections of horseradish peroxidase into the amygdala central nucleus, an area that mediates the bradycardia-conditioned response (CR), produced cell body and fiber labeling at the ventral and medial borders of the MGN. Then, bilateral electrolytic lesions were made at the medial border of the MGN or in control sites dorsal and/or rostral to the MGN. Ten days after surgery, lesioned and unoperated control animals were subjected to 7 days of differential Pavlovian conditioning. In the control lesion and unoperated groups, the CS+ consistently elicited larger bradycardia responses than the CS-. However, animals with bilateral lesions in the medial MGN did not demonstrate differential bradycardia CRs. Bradycardia response magnitude in MGN lesion animals was not affected. Evidence of CRP differential conditioning was present in each group. The present findings suggest that a region just medial to the MGN is involved in bradycardia differential conditioning in rabbits. The fact that bradycardia responses were still present after medial MGN lesions suggests that other auditory regions may also be involved in the mediation of the bradycardia CR.