Biochemical characterization and regional quantification of mu, delta and kappa opioid binding sites in rat spinal cord.

The spinal cord contains mu, delta and kappa opioid receptors which mediate the antinociceptive effects of opioid agonists administered onto the spinal cord. In this study, we characterized the binding sites for highly-selective mu, delta and kappa opioid radioligands and quantified the distribution of opioid binding sites in rat lumbosacral spinal cord using autoradiography. In sections of rat brain mounted on glass slides, the mu ligand, [3H]sufentanil, bound with high affinity with an apparent Kd of 0.46 nM. The delta ligand, [3H]DPDPE [( D-Pen2.5]-enkephalin), bound with a Kd of 4.31 nM, and the kappa-ligand, [3H]U69593, bound with a Kd of 2.27 nM. Three regions of the spinal gray were targeted for quantification of binding sites by autoradiography. The data indicate that when considered as a percentage of the total opioid binding capacity within a region, the contribution of mu sites in laminae I-II was about 90%, with delta and kappa sites 7% and 3%, respectively. In lamina V, the mu sites comprised about 70% of the total opioid sites, with delta and kappa sites comprising 28% and 2%, respectively. In the area adjacent to the central canal, mu sites contributed about 65% of the total opioid sites followed by delta sites at 33% and kappa sites at 2% of total opioid sites. These results demonstrate a differential distribution of mu, delta and kappa binding sites with respect to the organization of the spinal gray matter. The preferential occurrence of all 3 opioid binding sites in the superficial dorsal horn is noteworthy since many fine caliber primary afferent fibers mediating nociception establish synaptic contact in this region.