title: Assays for enhanced activity of low efficacy partial agonists at the D(2) dopamine receptor.
abstract: OBJECTIVE
  Low efficacy partial agonists at the D2 dopamine receptor may be useful for treating schizophrenia. In this report we describe a method for assessing the efficacy of these compounds based on stimulation of [35S]GTPgammaS binding.
  
  
  METHODS
  Agonist efficacy was assessed from [(35)S]GTPgammaS binding to membranes of CHO cells expressing D2 dopamine receptors in buffers with and without Na+. Effects of Na+ on receptor/G protein coupling were assessed using agonist/[3H]spiperone competition binding assays.
  
  
  RESULTS
  When [35S]GTPgammaS binding assays were performed in buffers containing Na+, some agonists (aripiprazole, AJ-76, UH-232) exhibited very low efficacy whereas other agonists exhibited measurable efficacy. When Na+ was substituted by N-methyl D-glucamine, the efficacy of all agonists increased (relative to that of dopamine) but particularly for aripiprazole, aplindore, AJ-76, (-)-3-PPP and UH-232. In ligand binding assays, substitution of Na+ by N-methyl D-glucamine increased receptor/G protein coupling for some agonists -. aplindore, dopamine and (-)-3-PPP - but for aripiprazole, AJ-76 and UH-232 there was little effect on receptor/G protein coupling.
  
  
  CONCLUSIONS
  Substitution of Na+ by NMDG increases sensitivity in [(35)S]GTPgammaS binding assays so that very low efficacy agonists were detected clearly. For some agonists the effect seems to be mediated via enhanced receptor/G protein coupling whereas for others the effect is mediated at another point in the G protein activation cycle. AJ-76, aripiprazole and UH-232 seem particularly sensitive to this change in assay conditions. This work provides a new method to discover these very low efficacy agonists.
authors.affiliation: School of Pharmacy, University of Reading, Whiteknights, Reading, Berkshire, UK.
authors.forename: H
authors.initials: H
authors.lastname: Lin
authors.forename: S G N
authors.initials: SG
authors.lastname: Saisch
authors.forename: P G
authors.initials: PG
authors.lastname: Strange
mesh_terms: D017371:8-Hydroxy-2-(di-n-propylamino)tetralin / Q000031:analogs & derivatives / Q000378:metabolism; D000818:Animals; D016466:CHO Cells; D006224:Cricetinae; D018491:Dopamine Agonists / Q000494:pharmacology*; D016244:Guanosine 5'-O-(3-Thiotriphosphate) / Q000378:metabolism; D006160:Guanosine Triphosphate / Q000494:pharmacology; D008536:Meglumine / Q000494:pharmacology; D017448:Receptors, Dopamine D2 / Q000819:agonists*; D012964:Sodium / Q000494:pharmacology
chemical_list: D018491:Dopamine Agonists; D017448:Receptors, Dopamine D2; D016244:Guanosine 5'-O-(3-Thiotriphosphate); D008536:Meglumine; D017371:8-Hydroxy-2-(di-n-propylamino)tetralin; D006160:Guanosine Triphosphate; C045971:UH 232; D012964:Sodium