Activation of type II adenylyl cyclase by the cloned μ-opioid receptor: Coupling to multiple G proteins

Opioid receptors are multifunctional receptors that utilize G proteins for signal transduction. The cloned δ-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 μ-opioid receptor to stimulate type II adenylyl cyclase was examined. Coexpression of the μ-opioid receptor with type II adenylyl cyclase in human embryonic kidney 293 cells allowed the p-selective agonist, [D-Ala²,N-Me-Phe⁴,Gly⁵-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 μ-opioid receptor and various G protein α 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 α subunit of G(z), whereas those of G(q), G₁₂, or G₁₃ inhibited the opioid response. When pertussis toxin-sensitive G protein α subunits were tested under similar conditions, all three forms of α(i) and both forms of α₀ were able to enhance the opioid response to various extents. Enhancement of type II adenylyl cyclase responses by the co- expression of α subunits reflects a functional coupling between α subunits and the μ-opioid receptor, because such potentiations were not observed with the constitutively activated a subunit mutants. These results indicate that the μ-opioid receptor can couple to G(i1-3), G(o1-2), and G(z), but not to G(s), G(q), G₁₂, G₁₃, or G(t).