Abstract
In previous studies the cerebellar interpositus (IP) nucleus, but not the hippocampus, was shown to be necessary both for initial learning and retention and for long-term retention of the standard delay eyeblink conditioned response (CR). However, in the trace eyeblink CR procedure, the hippocampus is also necessary for initial learning and retention, but not for long-term retention. Here the authors evaluate the role of the IP nucleus in both initial learning and retention, and in long-term retention of the trace eyeblink CR, using muscimol infusion to reversibly inactivate the IP nucleus. For the short-term study, there were two subgroups, the first sequentially passed through acquisition, inactivation, and reacquisition phases, whereas the second subgroup went through inactivation, acquisition, and inactivation phases. For the long-term study, the rabbits acquired the CR and then rested for a month. Next, they were distributed into two subgroups: with or without retention training, and finally went through inactivation and reacquisition phases. The results showed that the prelearning IP nucleus inactivation prevented the acquisition of the trace CR, whereas the postlearning inactivation reversibly abolished the expression of both the short- and long-term CR.