DIFFERENTIAL COUPLING OF MU-OPIOID, DELTA-OPIOID, AND KAPPA-OPIOID RECEPTORS TO G-ALPHA(16)-MEDIATED STIMULATION OF PHOSPHOLIPASE-C

Te mu-opioid receptor has recently been shown to stimulate phosphoinos itide-specific phospholipase C via the pertussis toxin-sensitive G(16) protein. Given the promiscuous nature of G(16) and the high degree of resemblance of signaling properties of the three opioid receptors, bo th delta- and kappa-opioid receptors are likely to activate G(16). Int eractions of delta- and kappa-opioid receptors with G(16) were examine d by coexpressing the opioid receptors and G alpha(16) in COS-7 cells. The delta-selective agonist [D-Pen(2),D-Pen(5)]enkephalin potently st imulated the formation of inositol phosphates in cells coexpressing th e delta-opioid receptor and G alpha(16). The delta-opioid receptor-med iated stimulation of phospholipase C was absolutely dependent on the c oexpression of G alpha(16) and exhibited appropriate ligand selectivit y and dose dependency. Similar transfection studies revealed only weak stimulation by the mu-opioid receptor, whereas the kappa-opioid recep tor produced moderate phospholipase C activity. G alpha(16) thus appea red to interact differentially with the three opioid receptors. Radiol igand binding assays indicate that the mu-opioid receptor was expresse d at a lower level than those of the delta- and kappa-opioid receptors . To examine if differential coupling to G alpha(16) is prevalent, a p anel of G(s)- or G(i)-coupled receptors was coexpressed with G alpha(1 6) in COS-7 cells and assayed for agonist-induced stimulation of phosp holipase C. Activation of alpha(2)- and beta(2)-adrenergic, dopamine D -1 and D-2, adenosine A(1), somatostatin-l and -2, C5a, formyl peptide , and luteinizing hormone receptors all resulted in stimulation of pho spholipase C, with maximal stimulations ranging from 1.5- to almost 17 -fold. These findings suggest that the promiscuous G alpha(16) can in fact discriminate among different receptors and that such preferential interaction might in part be due to the abundance of receptors.