LESION OF THE CEREBELLAR INTERPOSITUS NUCLEUS OR THE RED NUCLEUS AFFECTS CLASSICALLY-CONDITIONED NEURONAL-ACTIVITY IN THE HIPPOCAMPUS

1. The cerebellum and the hippocampus have been known to be neural str uctures involved in classical conditioning of the nictitating membrane response in rabbits. The neuronal activities related to conditioning are observed in both structures. It is uncertain, however, whether the se conditioning-related neuronal activities are established in paralle l or hierarchically. 2. The present study was conducted to observe the effects of lesions of the cerebellar interpositus nucleus(INT) or the red nucleus(RN) on conditioned hippocampal neuronal activity. 3. Rabb its in the first experiment were trained by standard delay conditionin g and then given INT lesion by injecting the kainic acid through a can nula previously implanted. Lesions of INT abolished conditioned neuron al responses in the hippocampal CAI area, which had been established b efore lesioning, as well as behavioral conditioned responses(CRs). 4. The second experiment was to examine if conditioning-related activitie s in the hippocampus would develope after RN was lesioned with INT int act. Rabbits were first given unilateral electrolytic lesions of RN fo llowed by conditioning sessions. Besides a few CRs, they failed to sho w an increase in hippocampal CA1 activity. When training was switched to the contralateral eye, animals showed robust CRs and hippocampal re sponses immediately. However, training reswitched to the original eye, behavioral and neuronal responses disappeared again.5. These results suggest that conditioned neuronal activities in the hippocampus depend s on the cerebellum and that conditioning-related inputs from INT via RN may be critical for these conditioned neuronal response in the hipp ocampus.