Opposing excitatory and inhibitory influences from the cerebellum and basal ganglia converge on the superior colliculus: an electrophysiological investigation in the rat.

We recently showed (Westby et al., Eur. J. Neurosci., 5, 1378-1388, 1993) that the cerebellar interpositus nucleus is a source of excitatory drive for a population of spontaneously active neurons in the lateral intermediate layers of the contralateral superior colliculus. Anatomical and physiological studies have shown that this region of the colliculus contains cells of origin of the crossed descending tectoreticulospinal tract and receives GABAergic input from the ipsilateral basal ganglia. In the present study we tested the hypothesis that the same neurons receiving excitatory drive from the cerebellum also receive tonic inhibitory input from the substantia nigra pars reticulata. From a sample of 73 spontaneously active collicular cells we found that in 53% the firing rate was suppressed by GABA microinjection into the contralateral deep cerebellar nuclei; a further 15% showed a frequency increase. Of the collicular cells identified as receiving excitatory cerebellar input, 85% were found to be disinhibited by nigral GABA microinjection. The remainder were all inhibited by nigral GABA. These data show that the main excitatory influence from the cerebellum and the main inhibitory influence from the substantia nigra converge on at least one population of spontaneously active cells in the lateral intermediate layers of the superior colliculus. This finding is discussed in relation to the possible function of these spontaneous cells in movement control and nociception.