Collateralization of monoamine neurons: mesotelencephalic dopamine projections to caudate, septum, and frontal cortex.

Axonal collateralization of substantia nigra (SN) and ventral tegmental area (VTA) neurons was studied in the rat with the multiple retrograde labeling technique using fluorescent dyes. Injections of nuclear yellow, granular blue, and propidium iodide were made into the caudate-putamen, septum, and frontal cortex and cells were found to be single, double, and triple labeled in the SN-VTA. Cells in the VTA were predominantly single labeled with the three tracers and cells in the medial SN were predominantly double labeled with the tracers. These results suggest that mesotelencephalic neurons in the SN-VTA can be subdivided into two subpopulations on the basis of relative degree of collateralization. The SN axons appear to be highly collateralized to anatomically and functionally distinct telencephalic areas and the VTA axons appear to have a more singular one-to-one type of projection to the same forebrain regions.