Abstract

Sears et al. showed that learning-related multiple unit activity (MUA) developing in the hippocampus during the classical conditioning of nictitating membrane response depends on the intactness of the cerebellar interpositus nucleus, that was confirmed by our recent study. The cerebellar interpositus nucleus is known to be an essential neural structure for this conditioning, and a place representing time-amplitude neural model of the conditioned response(CR). Lesioning the interpositus nucleus, therefore, abolishes the learning itself and as a result, the hippocampal MUA can not develop. In case the red nucleus, the principal output pathway of the interpositus nucleus is lesioned, the learning-related MUA in the interpositus nucleus can be recorded, which is possibly interpreted as the learning is established but the performance of CR is blocked. The present study was conducted to determine if the learning-related MUA could be developed in the hippocampus after lesioning the red nucleus, the target of the interpositus nucleus. Experimental rabbits were given unilateral electrolytic lesions of the red nucleus before the conditioning sessions (CS: 550msec tone, US: 100msec air puff, ISI: 450msec). Experimenal rabbits showed less than 8% CRs by the end of the 8th session, whereas the control animals showed on the average more than 80% CRs by the 4th session (F[1,10]=187.84,p<0.01). The control group showed typical increase of hippocampal MUA in the process of learning, while the group with the red nucleus lesion did not showed the increases of hippocampal MUA in the CS period(F[1,10]=10.67,p<0.05). These results suggest that the learning-related input from the interpositus nucleus via the red nucleus may play a critical role for the normal MUA development in the hippocampus.