ACTIVATION OF TYPE-II ADENYLYL-CYCLASE BY THE CLONED MU-OPIOID RECEPTOR - COUPLING TO MULTIPLE G-PROTEINS

Opioid receptors are multifunctional receptors that utilize G proteins for signal transduction. The cloned delta-opioid receptor has been shown recently to stimulate phospholipase C, as well as to inhibit or stimulate different isoforms of adenylyl cyclase. By using transient transfection studies, the ability of the cloned mu-opioid receptor to stimulate type II adenylyl cyclase was examined. Coexpression of the mu-opioid receptor with type II adenylyl cyclase in human embryonic kidney 293 cells allowed the mu-selective agonist, [D-Ala(2),N-Me-Phe(4),Gly(5)-ol]enkephalin, to stimulate cyclic AMP accumulation in a dose-dependent manner. The opioid-induced stimulation of type II adenylyl cyclase was mediated via pertussis toxin-sensitive G(i) proteins, because it was abolished completely by the toxin. Possible coupling between the mu-opioid receptor and various G protein cr subunits was examined in the type II adenylyl cyclase system. The opioid-induced response became pertussis toxin-insensitive and was enhanced significantly upon co-expression with the alpha subunit of G(z), whereas those of G(q), G(12), or G(13) inhibited the opioid response. When pertussis toxin-sensitive G protein a subunits were tested under similar conditions, all three forms of alpha(i) and, both forms of alpha(o) were able to enhance the opioid response to various extents. Enhancement of type II adenylyl cyclase responses by the co-expression of alpha subunits reflects a functional coupling between alpha subunits and the mu-opioid receptor, because such potentiations were not observed with the constitutively activated alpha subunit mutants. These results indicate that the mu-opioid receptor can couple to G(i1-3), G(o1-2), and G(z), but not to G(s), G(q), G(12), G(13), or G(t).