Mechanisms of inverse agonist action at D2 dopamine receptors.
Mechanisms of inverse agonist action at the D2(short) dopamine receptor have been examined. Discrimination of G-protein-coupled and -uncoupled forms of the receptor by inverse agonists was examined in competition ligand-binding studies versus the agonist [3H]NPA at a concentration labelling both G-protein-coupled and -uncoupled receptors. Competition of inverse agonists versus [3H]NPA gave data that were fitted best by a two-binding site model in the absence of GTP but by a one-binding site model in the presence of GTP. K(i) values were derived from the competition data for binding of the inverse agonists to G-protein-uncoupled and -coupled receptors. K(coupled) and K(uncoupled) were statistically different for the set of compounds tested (ANOVA) but the individual values were different in a post hoc test only for (+)-butaclamol. These observations were supported by simulations of these competition experiments according to the extended ternary complex model. Inverse agonist efficacy of the ligands was assessed from their ability to reduce agonist-independent [35S]GTP gamma S binding to varying degrees in concentration-response curves. Inverse agonism by (+)-butaclamol and spiperone occurred at higher potency when GDP was added to assays, whereas the potency of (-)-sulpiride was unaffected. These data show that some inverse agonists ((+)-butaclamol, spiperone) achieve inverse agonism by stabilising the uncoupled form of the receptor at the expense of the coupled form. For other compounds tested, we were unable to define the mechanism.
Roberts
David J
DJ
School of Animal and Microbial Sciences, University of Reading, Whiteknights, Reading RG6 6AJ.
Strange
Philip G
PG
D000818:Animals; D001058:Apomorphine / Q000031:analogs & derivatives* / Q000378:metabolism; D016466:CHO Cells; D006224:Cricetinae; D003412:Cricetulus; D065127:Dopamine D2 Receptor Antagonists; D016244:Guanosine 5'-O-(3-Thiotriphosphate) / Q000378:metabolism; D006153:Guanosine Diphosphate / Q000494:pharmacology; D008024:Ligands; D011485:Protein Binding; D011869:Radioligand Assay; D017448:Receptors, Dopamine D2 / Q000819:agonists* / Q000378:metabolism*; D014316:Tritium
D065127:Dopamine D2 Receptor Antagonists; D008024:Ligands; D017448:Receptors, Dopamine D2; D014316:Tritium; D006153:Guanosine Diphosphate; D016244:Guanosine 5'-O-(3-Thiotriphosphate); C009074:N-n-propylnorapomorphine; D001058:Apomorphine
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