The precise localization of nigral afferents in the rat as determined by a retrograde tracing technique

Afferent innervation of the rat substantia nigra (SN) was studied by the retrograde horseradish peroxidase (HRP) method. High concentrations of HRP were deposited in discrete subregions of the SN by means of a microiontophoretic delivery system. Using this technique it was possible to demonstrate that the caudatonigral projection system is arranged topographically; All portions of the caudate-putamen except for a central medial core were found to contain HRP positive cells, indicative of retrograde transport. In the positive areas a much larger percentage of cells (30-50%) were found to participate in this projection than has previously been reported. Only medium size cells (12-20 mum) were found to contain the HRP reaction product. Other areas found to heavily innervate the SN were the globus pallidus, central nucleus of the amygdala and dorsal raphe nucleus. Areas containing fewer reactive cells but which also appear to innervate the SN included the prefrontal cortex and lateral habenula. These results emphasize the importance of striatonigral projections which recent studies have suggested contain a GABAergic link.