Allosteric Modulation of the Mu‐Opioid Receptor: Probe Dependence and Role of Na + Ions

Abstract

The mu-opioid receptor (MOPr), a G protein-coupled receptor (GPCR), is the site of action of opioid analgesics like morphine and oxycodone. All clinically used MOPr ligands bind to the orthosteric site on MOPr, and compete with the endogenous opioid peptides for binding. We recently discovered the first positive allosteric modulators of MOPr (mu-PAMs), with BMS-986122 representing the prototype. We propose that mu-PAMs could represent a novel method for pain management. Here we characterize the effects of BMS-986122 in C6 rat glioma cell membranes expressing MOPr using radioligand binding and G protein activation as measured by GTPγ35S binding to determine orthosteric agonist affinity, potency, and maximal response. BMS-986122 had no effect on antagonist binding to MOPr. In contrast, BMS-986122 enhanced the affinity and/or efficacy of several orthosteric agonists depending on their efficacy. BMS-986122 enhanced the affinity and potency of high efficacy ligands while having no effect on maximal activity. For lower efficacy agonists, BMS-986122 did not alter ligand affinity, but caused an increase in maximal G protein activation. Na+ ions are known to act as negative modulators of MOPr, reducing the affinity and efficacy of orthosteric agonists. We found that there was a strong negative correlation between the effect of Na+ ions and of BMS-986122 on orthosteric ligand affinity. We also show that the presence of BMS-986122 allosterically inhibits the ability of Na+ to alter receptor state. These findings suggest that BMS-986122 has PAM activity at MOPr by allosterically disrupting the ability of Na+ to stabilize the inactive receptor state. This may represent a common mechanism for allostery at Class A GPCRs that are sensitive to Na+ ions. Supported by T32 DA007267 and DA035316.