MORPHOLOGICAL CHARACTERIZATION OF SUBSTANCE-P RECEPTOR-IMMUNOREACTIVENEURONS IN THE RAT SPINAL-CORD AND TRIGEMINAL NUCLEUS CAUDALIS

Although there is considerable evidence that primary afferent-derived substance P contributes to the transmission of nociceptive messages at the spinal cord level, the population of neurons that expresses the s ubstance P receptor, and thus are likely to respond to substance P, ha s not been completely characterized. To address this question, we used an antibody directed against the C-terminal portion of the rat substa nce P receptor to examine the cellular distribution of the receptor in spinal cord neurons. In a previous study, we reported that the substa nce P receptor decorates almost the entire dendritic and somatic surfa ce of a subpopulation of spinal cord neurons. In the present study we have taken advantage of this labeling pattern to identify morphologica lly distinct subpopulations of substance P receptor-immunoreactive neu rons throughout the rostral-caudal extent of the spinal cord. We obser ved a dense population of fusiform substance P receptor-immunoreactive neurons in lamina I at all segmental levels. Despite having the highe st concentration of substance P terminals, the substantia gelatinosa ( lamina II) contained almost no substance P receptor-immunoreactive neu rons. Several distinct populations of substance P receptor-immunoreact ive neurons were located in laminae III-V; many of these had a large, dorsally directed dendritic arbor that traversed the substantia gelati nosa to reach the marginal layer. Extensive labeling was also found in neurons of the intermediolateral cell column. In the ventral horn, we found that labeling was associated with clusters of motoneurons, nota bly those in Onuf's nucleus in the sacral spinal cord. Finally, we fou nd no evidence that primary afferent fibers express the substance P re ceptor. These results indicate that relatively few, but morphologicall y distinct, subclasses of spinal cord neurons express the substance P receptor. The majority, but not all, of these neurons are located in r egions that contain neurons that respond to noxious stimulation. (C) 1 995 Wiley-Liss, Inc.