Dopaminergic nigrotectal projection in the rat.

After injecting a fluorescent tracer (Fluoro-gold) into the rat superior colliculus, retrogradely labeled neurons in the rostral, ventrolateral portions of the substantia nigra pars reticulata were also immunohistochemically labeled with tyrosine hydroxylase antisera. 1-Methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) pretreatment of the medial forebrain bundle resulted in the disappearance of the nigral neurons double-labeled with the two markers. The existence of a dopaminergic nigrotectal projection susceptible to MPTP might provide a morphological substrate for abnormal saccadic eye movements in parkinsonism.