Inhibitory nigral influence on cerebellar evoked responses in the rat ventromedial thalamic nucleus.

The ventral medial nucleus of the thalamus (VM) has been shown in rats and cats to constitute a common target for nigro- and cerebello-thalamic pathways. In the present study the responses of VM neurons to ipsilateral substantia nigra (SN) and contralateral cerebellar nuclei stimulation were analyzed in the rat. The typical response of VM neurons to SN stimulation consisted of a pure short-latency (1.1-3 ms), short-duration (7-17 ms) IPSP. Latencies of these responses were accorded well with the conduction velocity of the nigro-thalamic fibers as measured on the basis of antidromic activation of this pathway. A high percentage (58%) of the thalamic neurons receiving the inhibitory nigral effect were also affected by cerebellar stimulation. The cerebellar effect consisted of a short-latency depolarizing potential which could trigger an action potential. These responses were invariably blocked during the course of SN evoked inhibition. Such convergence was found with fastigial as well as interpositus/dentatus evoked responses. The demonstration of an interaction between the processing of SN and cerebellar output at thalamic level provides an important clue for the understanding of the neurophysiologic mechanisms by which SN acts on motor behavior.