The molecular basis of opioid receptor function.

An extensive body of pharmacological data demonstrates the existence of at least three opioid receptor subtypes mediating the diverse effects of opioids. Distinct binding and activity profiles of highly selective ligands, variable sensitivity to naloxone antagonisms, and selective protection and inactivation experiments strongly suggest that mu-, delta-, and kappa-opioid receptors represent recent discrete molecular entities. Purification and affinity labeling of receptor subunits are beginning to provide confirmation of this concept. The delta-opioid receptor affinity labeled and purified to homogeneity from NG108-15 cells comprises a glycoprotein subunit of Mr58,000 with one mol ligand bound/mol protein. Antibodies to this protein recognize native receptor in detergent solution and selectively bind to the Mr58,000 protein on immunoblots of partially purified preparations. Purification of the mu-opioid receptor from bovine striatum reveals a glycoprotein of Mr 65,000 which demonstrates opioid binding activity. Purification and affinity-labeling studies from other laboratories suggest a smaller size of Mr 58,000 for the mu-receptor however. The kappa-opioid receptor from guinea pig brain exhibits a unique mobility on sucrose density gradient centrifugation but has not been characterized in purified form. The primary structure of the opioid receptors, although unknown at present, will most likely reflect structural features of other inhibitory receptors coupled to G-proteins, with seven transmembrane helices and a large third cytoplasmic loop. Biochemical evidence clearly demonstrates the coupling of opioid receptors to Gi, accounting for opioid inhibition of adenylyl cyclase in neuronal cell culture and brain. Opioid inhibition of adenylyl cyclase has been reconstituted in IAP-treated NG108-15 cell membranes with a Gi preparation from brain. Electrophysiological evidence suggests that mu- and delta-opioid receptors can couple to a G-protein which mediates activation of inwardly rectifying potassium channels, perhaps to the same Gk mediating muscarinic potassium channel activation in heart. kappa-Opioid receptors are coupled to inhibition of voltage-dependent calcium channels in several neuronal systems. In NG108-15 cells opioid inhibition of calcium conductance is IAP sensitive and can be reconstituted with G-proteins purified from brain. Differences in the primary structure of mu-, delta-, and kappa-opioid receptors, as well as possible novel opioid receptor subtypes, will be defined by molecular cloning of recombinant DNA.(ABSTRACT TRUNCATED AT 400 WORDS)