Morphological characterization of substance P receptor-immunoreactive neurons 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 substance P receptor, and thus are likely to respond to substance P, has not been completely characterized. To address this question, we used an antibody directed against the C-terminal portion of the rat substance P receptor to examine the cellular distribution of the receptor in spinal cord neurons. In a previous study, we reported that the substance P receptor decorates almost the entire dendritic and somatic surface of a subpopulation of spinal cord neurons. In the present study we have taken advantage of this labeling pattern to identify morphologically distinct subpopulations of substance P receptor-immunoreactive neurons throughout the rostral-caudal extent of the spinal cord. We observed a dense population of fusiform substance P receptor-immunoreactive neurons in lamina I at all segmental levels. Despite having the highest concentration of substance P terminals, the substantia gelatinosa (lamina II) contained almost no substance P receptor-immunoreactive neurons. Several distinct populations of substance P receptor-immunoreactive neurons were located in laminae III-V; many of these had a large, dorsally directed dendritic arbor that traversed the substantia gelatinosa 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, notably those in Onuf's nucleus in the sacral spinal cord. Finally, we found no evidence that primary afferent fibers express the substance P receptor. These results indicate that relatively few, but morphologically distinct, subclasses of spinal cord neurons express the substance P receptor. The majority, but not all, of these neurons are located in regions that contain neurons that respond to noxious stimulation.