Effects of bilateral lesions of the cerebellar interpositus nucleus on the conditioned forelimb flexion reflex in mice

Three groups of mice, unoperated controls, sham and lesioned, were submitted to an associative conditioning of forelimb flexion reflex (FFR). Light and tone constituted the conditioned stimulus (CS) paired with a forelimb electric shock, the unconditioned stimulus (UCS). The first two groups were able to acquire an appropriate conditioned response. In the third group, each animal received a bilateral lesion of the cerebellar interpositus nucleus (IN). The subjects of this group were unable to acquire the conditioning. When bilateral lesions of the IN were done after the acquisition, no effect of the lesions could be detected during retention test sessions 10 days after surgery, by comparison with sham controls. It is therefore concluded that the cerebellar interpositus nucleus is an essential part of the circuit for the acquisition of associative conditioning of the forelimb flexion response in mice, but not for the retention of this task. Moreover, no direct sensorimotor effect of the lesion on performance itself could be evoked.