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Vol. 52, Issue 6, 983-992, 1997
Cellular Neurobiology (C.K.S.) and
Molecular Neurobiology (C.E.S.,
C.L.B., B.K.S., L.D.H., C.J.B., G.R.U.) Branches, Intramural Research
Program, National Institute on Drug Abuse, Baltimore, Maryland 21224
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.
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