Autoradiographic localization and characterization of spinal cord substance P binding sites: high densities in sensory, autonomic, phrenic, and Onuf's motor nuclei.

The presence of the neurotransmitter or neuromodulator, substance P (SP), in the spinal cord implies that a discrete localization of SP receptors also occurs. To map the distribution of and to characterize SP binding sites in the spinal cord, light microscopic autoradiography was used. SP binding sites occurred in the dorsal horn, intermediolateral cell column (IML) and lamina X-region. In the ventral horn, the phrenic, Onuf's and sacral ventromedial motor nuclei were densely labeled. Other regions of the ventral horn were moderately labeled for SP binding sites. The localization of binding sites parallels the regional distribution of SP-containing nerve fibers in the spinal cord. A close correlation between the binding sites for SP and the presence of cholinesterase-stained neurons occurred, and suggest that the SP receptors are located on or proximal to cholinergic neurons. The density of the binding sites in the dorsal horn was highest in the sacral section, followed by the lumbar, thoracic and cervical section. In the lamina X region, however, the density was highest in the thoracic followed by the sacral, lumbar and cervical sections. The high density binding of 125I-Bolton-Hunter reagent labeled SP was inhibited, in a dose-dependent manner, by unlabeled SP. Quantification of the dose-dependent inhibition binding, using computer densitometry, showed differences in the inhibition curves for the cervical lamina X-region and the IML as compared with the other loci containing high density binding sites. The differential sensitivity of the SP receptors to unlabeled SP suggests that there are heterogeneous receptors for SP in the spinal cord, which may be relevant to the role of SP in different spinal cord functions. The binding to specific motor nuclei in the ventral horn also suggest that SP may play a role in the function of specialized striated muscles.