Light microscopic and ultrastructural localization of immunoreactive substance P in the dorsal horn of monkey spinal cord.

Light- and electron-microscopic localization of substance P in the monkey spinal cord was studied by the peroxidase anti-peroxidase technique with the particular aim of examining types of interactions made by substance P-positive boutons with other neuronal elements in the dorsal horn. By light-microscopy dense labeling for immunoreactive substance P was found in laminae I, II (outer zone) and V (lateral region), consistent with findings in other mammalian species. By electron-microscopy, substance P-positive staining was mostly in unmyelinated and in some thinly myelinated small diameter fibers. Substance P-positive terminals contained both large granular vesicles (80-120 nm diameter), which were filled with reaction product, and clear round vesicles (40-60 nm). Substance P-positive large granular vesicles were sometimes observed near presynaptic sites and in contact with dense projection there. Immunoreactive substance P boutons were small to large in size (1-4 micron), formed synapses with somata and large dendrites and were the central axons of synaptic glomeruli where they were in synaptic contact with numerous small dendrites and spines. Substance P-labeled axons frequently formed synapses with dorsal horn neurons which were also postsynaptic to other types of axons. Substance P-positive profiles participated in numerous puncta adhaerentia with unlabeled cell bodies, dendrites and axons. Only rarely, some suggestive evidence was obtained indicating that axons might synapse onto substance P-containing boutons. Biochemical analysis of monkey spinal cord tissue extracts, undertaken to characterize more precisely the immunoreactive substances, indicated that only substance P and its oxide derivative were detected with the antiserum used in the immunocytochemistry. These morphological findings show that substance P is contained within a class of axon terminals, many of which have been shown previously in the monkey to originate from the dorsal root. The results suggest that modulation of substance P primary afferents terminating in the outer dorsal laminae of the monkey spinal cord occurs in part via axonal inputs onto dorsal horn neurons postsynaptic to the primary afferent.