A morphological and electrophysiological study of nigrotectal pathway in the rat.

After horseradish peroxidase injections in the superior colliculus of the rat, labelled cells were found in the pars reticulata of the substantia nigra. Nigrotectal cells are organized in a band beginning in the ventromedial area of the rostral part of pars reticulata, while at caudal levels they occupy a more lateral position, closely packed near the cerebral peduncle. In another series of experiments, the effects of substantia nigra electrical stimulation on collicular unitary activity was studied in anaesthetized rats. Electrical stimulation of the substantia nigra decreased the spontaneous and pharmacologically induced activity of superior colliculus neurons. Inhibition took place with short latencies (1-4 msec). Inhibited cells were localized in deep layers of the superior colliculus. In addition, long latency activation and inhibition were also obtained. It is concluded that nigrotectal pathway is mainly inhibitory in character.