Inhibitory control by substantia nigra of generalized epilepsy in the cat

Previous investigations have shown that the basal ganglia may exert a regulating influence on cortical epilepsy. Stimulation of the caudate nucleus enhances cortical penicillin (PCN) spikes. Stimulation of globus pallidus internus reduces cortical spike frequency. Since the substantia nigra pars reticulata (SNpr) seems to have an inhibitory action on the ventro-anterior (VA) and ventro-lateral (VL) thalamic nuclei and thalamic neurones send an excitatory influence to the cortex, we undertook an investigation to study nigral influence on cortical epilepsy induced by PCN. Experiments were conducted on encéphale isolé cats in which steady interictal activity was induced by means of parenteral PCN administration (feline generalized PCN epilepsy). Variations occurring in cortical PCN spikes following activation of either pars compacta (SNpc) or SNpr were analyzed. Electrical stimulation of SNpc reduced spike frequency and amplitude in 19% of the total number of stimulations; SNpr stimulation significantly inhibited cortical spikes, especially in the precruciate gyrus, in 80% of cases. The experimental findings constitute an electrophysiological feature of the control exerted by SNpr on the thalamo-cortical re-exciting loop. A putative preferential role of SNpr in the regulation of abnormal phenomena involving the neocortex is emphasized.