MAMILLOTHALAMIC TRACT TRANSECTION BLOCKS ANTERIOR THALAMIC TRAINING-INDUCED NEURONAL PLASTICITY AND IMPAIRS DISCRIMINATIVE AVOIDANCE-BEHAVIOR IN RABBITS

Rabbits with bilateral transecting lesions of the mamillothalamic trac t, control (tract-sparing and sham) lesions, or no lesions, and chroni c, fixed-position anterior ventral (AV) and medial dorsal (MD) thalami c and posterodorsal subicular complex unit recording electrodes were t rained to step in an activity wheel in response to a 0.5 sec tone (CS) in order to avoid a brief foot shock. The rabbits also learned to ig nore a different tone (CS-) not predictive of shock. Behavioral acquis ition was significantly retarded in rabbits with mamillothalamic tract transection compared to controls. When trained, transected rabbits fa iled to avoid the shock more often than controls. Mamillothalamic trac t transection abolished and control lesions attenuated AV thalamic dis criminative training-induced activity (i.e., development with training of greater discharges in response to the CS+ than to the CS-). Transe ction and control lesions attenuated AV thalamic excitatory training-i nduced activity (greater elicited activity during training than during unpaired tone-shock presentations before training) as well as AV thal amic ''spontaneous'' baseline unit activity. CS-elicited discharge mag nitude was reduced by control lesions and it was further reduced by tr act transecting lesions. Significant lesion-related changes were not f ound in the subicular or MD thalamic neuronal records. Mamillothalamic tract afferent information flow is thus essential for AV thalamic dis criminative training-induced activity, excitatory training-induced act ivity, tone-elicited discharges and maintenance of conditioned avoidan ce responses. The effects of the control lesions suggested that affere nts which course in parallel with and near the mamillothalamic tract m ay contribute to AV thalamic spontaneous activity and excitatory train ing-induced activity.