Kappa opioid receptors in human lumbo-sacral spinal cord

[3H]Etorphine and [3H]ethylketocyclazocine bind with high affinity (Kd between 0.25-2.0 nM) to a single class of sites in human lumbo-sacral spinal cord. Other ligands such as [3H]morphine, [3H]dihydromorphine and [3H]D-Ala2, D-Leu5-enkephalin (DADLE) did not bind to significant number of sites under our incubation conditions. Ligand selectivity pattern strongly suggests that [3H]etorphine labels kappa opioid binding sites in the human lumbo-sacral spinal cord since benzomorphans and oripavines are much more potent than mu and delta agonists. Furthermore, [3H]etorphine and [3H]ethylketocyclazocine binding is sensitive to high concentrations of DADLE suggesting that these sites are of the kappa 2 sub-type. Finally, the visualization of these sites by receptor autoradiography demonstrates that they are mainly concentrated in lamina II and III of the dorsal horn. Moderate densities of sites are present around the central canal. Thus, it is possible that kappa opioid binding sites could be involved in the control of sensory and autonomic functions in the human lumbo-sacral spinal cord.