The nigro-tectal pathway. An electrophysiological reinvestigation in the rat

Controversial data have been reported concerning the effect induced by stimulation of substantia nigra (SN) on the cells of the ipsilateral superior colliculus (SC) in the rat. Stimulation applied within SN activates not only the nigral neurons but also fibers passing through and in the vicinity of SN, especially those of cerebral peduncle (Ped) and optic tract (OT). The effect of SN stimulation has therefore been reinvestigated on two types of preparation: firstly in animals whose Ped--OT fibers were transected rostral to SN; and secondly in rats whose nigral cells destroyed by kainic acid injection. In normal animals, SN stimulation induced both inhibitory and excitatory responses in SC cells. After transection of Ped--OT fibers, SN stimulation no longer induced excitatory but only inhibitory effects. This observation clearly demonstrates that the nigro-tectal pathway is inhibitory. The excitatory responses observed in normal rats are related to activation of Ped--OT fibers at the level of SN. Indeed these excitations induced by SN stimulation still persist in kainic acid lesioned animals. Moreover, stimulation of Ped--OT fibers applied rostral to SN also produce these excitatory effects on SC cells. On the other hand, Ped--OT stimulation not only induces excitations but also inhibitory responses. These latter effects can still be evoked by SN stimulation after kainic acid induced loss of nigral cells. In conclusion the SC receives both excitatory and inhibitory input from Ped--OT fibers and only inhibitory input from nigral cells. This inhibitory nigral control is mainly exerted on cells of intermediate and deep collicular layers.