Differential coupling of μ-, δ-, and κ-opioid receptors to Gα₁₆- mediated stimulation of phospholipase C

The μ-opioid receptor has recently been shown to stimulate phosphoinositide-specific phospholipase C via the pertussis toxin-sensitive G₁₆ protein. Given the promiscuous nature of G₁₆ and the high degree of resemblance of signaling properties of the three opioid receptors, both δ- and κ-opioid receptors are likely to activate G₁₆. Interactions of δ- and κ-opioid receptors with G₁₆ were examined by coexpressing the opioid receptors and Gα₁₆ in COS-7 cells. The δ-selective agonist [D-Pen²,D- Pen⁵] enkephalin potently stimulated the formation of inositol phosphates in cells coexpressing the δ-opioid receptor and Gα₁₆. The δ-opioid receptor-mediated stimulation of phospholipase C was absolutely dependent on the coexpression of Gα₁₆ and exhibited appropriate ligand selectivity and dose dependency. Similar transfection studies revealed only weak stimulation by the μ-opioid receptor, whereas the κ-opioid receptor produced moderate phospholipase C activity. Gα₁₆ thus appeared to interact differentially with the three opioid receptors. Radioligand binding assays indicate that the μ-opioid receptor was expressed at a ewer level than those of the δ-and κ- opioid receptors. To examine if differential coupling to Gα₁₆ is prevalent, a panel of G(s)- or G(l)-coupled receptors was coexpressed with Gα₁₆ in COS-7 cells and assayed for agonist-induced stimulation of phospholipase C. Activation of α₂- and β₂-adrenergic, dopamine D₁ and D₂, adenosine A₁, somatostatin-1 and -2, C5a, formyl peptide, and luteinizing hormone receptors all resulted in stimulation of phospholipase C, with maximal stimulations ranging from 1.5- to almost 17-fold. These findings suggest that the promiscuous Gα₁₆ can in fact discriminate among different receptors and that such preferential interaction might in part be due to the abundance of receptors.