title: Ligands raise the constraint that limits constitutive activation in G protein-coupled opioid receptors.
abstract: Using a cell-free bioluminescence resonance energy transfer strategy we compared the levels of spontaneous and ligand-induced receptor-G protein coupling in δ (DOP) and μ (MOP) opioid receptors. In this assay GDP can suppress spontaneous coupling, thus allowing its quantification. The level of constitutive activity was 4-5 times greater at the DOP than at the MOP receptor. A series of opioid analogues with a common peptidomimetic scaffold displayed remarkable inversions of efficacy in the two receptors. Agonists that enhanced coupling above the low intrinsic level of the MOP receptor were inverse agonists in reducing the greater level of constitutive coupling of the DOP receptor. Yet the intrinsic activities of such ligands are identical when scaled over the GDP base line of both receptors. This pattern is in conflict with the predictions of the ternary complex model and the "two state" extensions. According to this theory, the order of spontaneous and ligand-induced coupling cannot be reversed if a shift of the equilibrium between active and inactive forms raises constitutive activation in one receptor type. We propose that constitutive activation results from a lessened intrinsic barrier that restrains spontaneous coupling. Any ligand, regardless of its efficacy, must enhance this constraint to stabilize the ligand-bound complexed form.
authors.affiliation: Department of Pharmacology, Istituto Superiore di Sanità, 00161 Rome, Italy.
authors.forename: Vanessa
authors.initials: V
authors.lastname: Vezzi
authors.forename: H Ongun
authors.initials: HO
authors.lastname: Onaran
authors.forename: Paola
authors.initials: P
authors.lastname: Molinari
authors.forename: Remo
authors.initials: R
authors.lastname: Guerrini
authors.forename: Gianfranco
authors.initials: G
authors.lastname: Balboni
authors.forename: Girolamo
authors.initials: G
authors.lastname: Calò
authors.forename: Tommaso
authors.initials: T
authors.lastname: Costa
mesh_terms: D019390:Arrestins / Q000378:metabolism; D045744:Cell Line, Tumor; D003198:Computer Simulation; D054314:Drug Inverse Agonism; D019204:GTP-Binding Proteins / Q000378:metabolism; D006153:Guanosine Diphosphate / Q000378:metabolism; D006801:Humans; D008024:Ligands; D008954:Models, Biological; D011812:Quinuclidines / Q000737:chemistry / Q000494:pharmacology; D017465:Receptors, Opioid, delta / Q000819:agonists / Q000378:metabolism*; D017450:Receptors, Opioid, mu / Q000378:metabolism*; D014443:Tyrosine / Q000031:analogs & derivatives / Q000737:chemistry / Q000494:pharmacology; D000071557:beta-Arrestins
chemical_list: D019390:Arrestins; D008024:Ligands; D011812:Quinuclidines; D017465:Receptors, Opioid, delta; D017450:Receptors, Opioid, mu; D000071557:beta-Arrestins; C076121:2',6'-dimethyltyrosine; D006153:Guanosine Diphosphate; D014443:Tyrosine; D019204:GTP-Binding Proteins
keywords: Bioluminescence Resonance Energy Transfer (BRET); Constitutive Receptor Activation; Cooperativity; G Protein-coupled Receptors (GPCR); G Proteins; Inverse Agonism; Opiate Opioid