Two distinct strio-nigral substance P pathways in the rat: an experimental immunohistochemical study.

Re-examination of the strio-nigral substance P (SP) tract by means of an experimental immunohistochemical method in the rats demonstrated the presence of two distinct pathways from the nucleus caudatus putamen (CP) to the substantia nigra (SN). Destruction of the posterior portion of the CP resulted in the disappearance of SP-positive fibers in the SN pars lateralis but not in the SN pars compacta or reticulata. On the other hand, destruction of the ventrolateral portion of the anterior portion of the CP caused the disappearance of SP-positive fibers in the SN pars compacta and pars reticulata but not in the SN pars lateralis. In addition, destruction of the dorsal portion of the anterior portion of the CP, where 3-6 cell islets of SP-positive cells are located, failed to decrease SP-positive fibers in any of the subdivisions of the SN. These findings strongly suggest that SP-positive neurons in the posterior portion of the CP project to the SN pars lateralis (posterior strio-nigral SP tract), SP-positive cells in the lateroventral part of the anterior portion of the CP extend to the SN pars compacta and pars reticulata, but SP-positive cells in the dorsal part of the anterior portion of the CP do not innervate the SN.