Assays for enhanced activity of low efficacy partial agonists at the D 2 dopamine receptor

Abstract

Background and purpose: Low efficacy partial agonists at the D 2 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]GTPγS binding. Experimental approach: Agonist efficacy was assessed from [ 35S]GTPγS binding to membranes of CHO cells expressing D 2 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. Key results: When [ 35S]GTPγS 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 and implications: Substitution of Na + by NMDG increases sensitivity in [ 35S]GTPγS 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. © 2006 Nature Publishing Group All rights reserved.