Preferential synaptic relationships between substance P-immunoreactive boutons and neurokinin 1 receptor sites in the rat spinal cord

Substance P plays an important role in the transmission of pain-related inf ormation in the dorsal horn of the spinal cord. Recent immunocytochemical s tudies have shown a mismatch between the distribution of substance P and it s receptor in the superficial laminae of the dorsal horn. Because such a mi smatch was not observed by using classical radioligand binding studies, we decided to investigate further the issue of the relationship between substa nce P and its receptor by using an antibody raised against a portion of the carboxyl terminal of the neurokinin 1 receptor and a bispecific monoclonal antibodies against substance P and horseradish peroxidase. Light microscop y revealed a good correlation between the distributions of substance P and the neurokinin 1 receptor, both being localized with highest densities in l amina I and outer lamina II of the spinal dorsal horn. An ultrastructural d ouble-labeling study, combining preembedding immunogold with enzyme-bas ed immunocytochemistry, showed that most neurokinin 1 receptor immunoreactive dendrites were apposed by substance P containing boutons, A detailed quanti tative analysis revealed that neurokinin 1 receptor immunoreactive dendrite s received more appositions and synapses from substance P immunoreactive te rminals than those not expressing the neurokinin 1 receptor. Such preferent ial innervation by substance P occurred in all superficial dorsal horn lami nae even though neurokinin 1 receptor immunoreactive dendrites were a minor ity of the total number of dendritic profiles in the above laminae, These r esults suggest that, contrary to the belief that neuropeptides act in a dif fuse manner at a considerable distance from their sites of release, substan ce P should act on profiles expressing the neurokinin 1 receptor at a short distance from its site of release.