Assays for enhanced activity of low efficacy partial agonists at the D(2) dopamine receptor.
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.
Lin
H
H
School of Pharmacy, University of Reading, Whiteknights, Reading, Berkshire, UK.
Saisch
S G N
SG
Strange
P G
PG
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
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