Collateral projection from the substantia nigra to the striatum and superior colliculus in the rat.

Our retrograde fluorescent double labeling study demonstrated the existence of divergent collateral projections from the substantia nigra to the striatum and superior colliculus in the rat. These bifurcating projection neurons were located predominantly in the ventrolateral portions of the substantia nigra pars reticulata at its rostral level, where they formed a narrow band along the boundary between the substantia nigra and cerebral peduncle. Such specific projection cells were also seen in the substantia nigra pars lateralis. However, nigral neurons did not give off axonal branches to the striatum and ventromedial thalamic nucleus. The new nigral cell population proposed here might constitute a neuroanatomical substrate for abnormal saccadic eye movements clinically manifested by many parkinsonian patients.