The origin, distribution and synaptic relationships of substance P axons in rat spinal cord

Abstract Substance P positive (SP + ) immunoperoxidase reaction product has been localized in light and electron microscopic preparations of rat lumbar spinal cord using an immunocytochemical method. SP + reaction product was found to be highly concentrated in dorsal horn laminae I, II, portions of III, Lissauer's tract, s small nucleus in the dorsal part of the lateral funiculus, and in a longitudinal bundle of fibers just ventral to the central canal. Moderate accumulations of SP + reaction product were observed in portions of laminae III‐V, lamina X and in a narrow zone bordering fasciculus gracilis which expanded in its ventral aspect to include nucleus cornucommissuralis dorsalis and nucleus dorsalis. Remaining portions of the spinal gray matter exhibited extremely sparse staining. Light microscopic observations indicated that SP + product was concentrated in fine axons and collaterals, which displayed densely‐stained varicosities and terminal puncta. These varicosities and puncta were closely associated with neurons and blood vessels. Dorsal rhizotomy produced a marked reduction in the number of SP + fibers in Lissauer's tract, dorsal horn laminae I‐III and the nucleus of the dorsolateral funiculus. However, dorsal root fibers were not the only source of SP + structures in these regions since some SP + fibers remained following dorsal root resections. In addition, this residual population of SP + fibers did not appear to be altered by a combination of dorsal rhizotomy and ipsilateral transverse hemisection of the spinal cord. In conjunction with the results of other investigators, this finding suggests that the SP + fibers which remain after dorsal rhizotomy are derived from local circuit interneurons of the spinal cord. Electron microscopic observations revealed that certain axons and axon terminals contained concentrations of SP + product which were associated with large, granular vesicles, while lesser amounts were in the terminal cytoplasm and associated with the exterior surfaces of small, agranular synaptic vesicles. SP + terminals formed axodendritic, axosomatic and axoaxonal synapses, but the deposition of SP + product often was not concentrated precisely at the “active sites” of these synaptic junctions. Some SP + axons also made rudimentary contacts with astrocytic processes including those surrounding blood vessels. Since many of the SP + terminals are similar in several respects to neuroendocrine terminals, it is possible that substance P may be released at several different types of non‐synaptic sites as well as at conventional synaptic junctions. Thus our findings suggest that substance P could participate in a variety of neural functions ranging from those which are limited to synaptic junctions to those which are more generally distributed via an involvement of axonal plexuses as well as the glial, vascular and ventricular systems.