Naloxone Activation of μ-Opioid Receptors Mutated at a Histidine Residue Lining the Opioid Binding Cavity

Abstract

The μ-opioid receptor is the principal site of action in the brain by which morphine, other opiate drugs of abuse, and endogenous opioid peptides effect analgesia and alter mood. A member of the seven-transmembrane domain (TM) G protein-coupled receptor (GPCR) superfamily, the μ-opioid receptor modulates ion channels and second messenger effectors in an opioid agonist-dependent fashion that is reversible by the classic opiate antagonist naloxone. Mutation of a histidine residue (His297) in TM 6 afforded agonist-like G protein-coupled signal transduction mediated by naloxone and other alkaloid antagonists and enhanced the intrinsic activity of documented alkaloid partial agonists, including buprenorphine. The intrinsic activities of all opioid peptide agonists and antagonists tested were not altered at the His297 mutant receptors. Consistent with a role for the TM 6 histidine in maintaining high affinity binding sites for opioid agonists and antagonists, opioid ligand-dependent protection of this residue from a histidine-specific alkylating agent indicated that the His297 side chain is positioned in or very near the binding cavity. The TM 6 His297 mutants identify a discrete region of the receptor critical for determining whether a specific drug pharmacophore triggers receptor activation. Because many GPCRs possess a similarly positioned TM histidine residue, our findings with the μ-opioid receptor may extend to these receptors and potentially serve as a model for rational design of therapeutic GPCR partial agonists and antagonists.