Phencyclidine and sigma receptors in rat spinal cord: Binding characterization and quantitative autoradiography

These experiments were designed to compare phencyclidine (PCP) and s̀ (sigma) receptor binding sites in the rat spinal cord by using receptor binding and autoradiographic techniques. Binding sites for ³H‐TCP (³H‐1‐[1‐(2‐thienyl)cyclohexy]piperidine), a PCP receptor agonist, and (+)³H‐3‐PPP (³H‐(+)‐3‐(3‐hydroxyphenyl)‐N‐(1‐propyl)piperidine), a s̀ receptor agonist, in the rat spinal cord were shown to represent two populations of recognition sites. Inhibition studies revealed that ligands with high affinity for the PCP receptor (MK‐801 and PCP) were potent competitors at ³H‐TCP binding sites whereas the putative s̀ receptor ligands (±)pentazocine and haloperidol were potent competitors at (+)³H‐3‐PPP binding sites. The autoradiographic distribution of ³H‐TCP and (+)³H‐3‐PPP binding sites in adjacent sections of rat spinal cord demonstrated the occurrence of two distinct populations of binding sites. ³H‐TCP binding sites were localized primarily in laminae I and II in cervical and thoracic spinal segments. Binding sites in lamina I decreased in density along a rostral to caudal gradient in the spinal cord. The highest density of (+)³H‐3‐PPP binding sites was found in the ventral horn (lamina VIII and IX) and over perikarya in dorsal root ganglia. Significantly elevated densities of (+)³H‐3‐PPP binding sites were also found in lamina X within thoracic and lumbar segments and in the intermediolateral cell column. The results of the present study show that PCP and s̀ receptor binding sites are differentially localized in the rat spinal cord and suggest that separate binding sites exist for PCP and s̀ agonists.