The cerebello-tectal projection arising from the interpositus nucleus was investigated electrophysiologically to test the hypothesis that the deep cerebellar nuclei constitute a source of tonic excitation in the superior colliculus. A total of 117 spontaneously active collicular neurons were recorded during GABA microinjection into 26 interpositus sites, where tonic single-cell deep cerebellar activity was also simultaneously recorded. GABA injection always led to suppression of interpositus activity, while in the colliculus a clear pattern of results emerged. 58% of superior colliculus cells showed no response to suppression of interpositus activity, 35% showed a frequency decrease and 7% showed a frequency increase. The majority of these responsive cells were found in a laterally located sheet of cells mainly restricted to the intermediate white layer, in close register with the known cells of origin of the predorsal bundle and completely overlapping the terminals of the nigrotectal pathway originating in dorsolateral substantia nigra pars reticulata. The implications of these results for cooperative theories of head movement control involving the superior colliculus, cerebellum and precerebellar nuclei are discussed.

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.