Bicuculline-induced circling from the rat superior colliculus is blocked by GABA microinjection into the deep cerebellar nuclei.

In a recent electrophysiological experiment, we showed the deep cerebellar nuclei to be a major source of excitatory input to the superior colliculus. Furthermore, target neurons in the colliculus were found, in every case, to receive convergent tonic inhibitory input from the substantia nigra pars reticulata. In the present study, we investigated these effects in the awake rat. We asked whether circling behaviour, induced by unilateral injection of a GABA antagonist into the lateral colliculus, could be suppressed by concurrent cerebellar inactivation. Rats were chronically implanted with bilateral guide cannulae located above the superior colliculus and deep cerebellar nuclei. Bicuculline methiodide (25 pmol) was microinjected unilaterally into intermediate layers of the colliculus at increasing depths until an optimal contralateral circling response was elicited. This behaviour was taken as the "baseline response" and was the first of three treatments. The second was an identical manipulation of the colliculus with a concurrent 200-nl microinjection of 1 M GABA into the contralateral deep cerebellar nuclei. The third was a repeat of BIC alone into the colliculus or, if rotation had been suppressed by more than 50% on test 2, the treatment was collicular BIC plus deep cerebellar saline. This latter treatment was used as a control for possible non-pharmacological injection effects. The effect of cerebellar GABA at 26 sites (17 within cerebellar nuclei and 9 outside) on BIC-induced rotation at 15 collicular sites was studied in ten animals. Only GABA injections at sites that fell within the cerebellar nuclei significantly reduced turning (P < 0.0001). A full behavioural analysis showed that this was a specific suppression of turning, not the result of general motor impairment. These results provide clear behavioral evidence that opposing, convergent influences from the basal ganglia and cerebellum interact in the lateral superior colliculus to control head and body movements. They furthermore suggest that the tonic deep cerebellar excitation of the superior colliculus could be the driving force in the expression of rotation induced by manipulations of the basal ganglia.