Substantia nigra and ventral tegmental area projections to cortex: topography and collateralization.

The substantia nigra and ventral tegmental area of the rat were examined by retrograde transport methods to determine the topography and collateralization patterns of projections to cortex and certain subcortical sites. The topographical relationships between cells and their terminal fields were confirmed and clarified by the horseradish peroxidase retrograde transport technique. The collateralization of axons was analyzed by the use of multiple fluorescent tracers. These experiments indicated that individual ventral tegmental area cells do not collateralize extensively to either subcortical or cortical terminal fields. Substantia nigra cells, however, give rise to more highly collateralized axons and single cells may simultaneously innervate different regions of cortex as well as subcortical sites. Substantia nigra cells can be divided with respect to their cortical collateralization patterns into three groups: (1) those that innervate cingulate cortices, (2) those that project to prefrontal and suprarhinal cortices, and (3) those that innervate entorhinal cortex.