Dopamine D2 receptor partial agonists display differential or contrasting characteristics in membrane and cell-based assays of dopamine D2 receptor signaling

Clinical evidence suggests that dopamine D2 receptor partial agonists must have a sufficiently low intrinsic activity to be effective as antipsychotics. Here, we used dopamine D2 receptor signaling assays to compare the in vitro functional characteristics of the antipsychotic aripiprazole with other dopamine D2 receptor partial agonists (7-{3-[4-(2,3-dimethylphenyl)-piperazinyl]propoxy}-2(1H)-quinolinone [OPC-4392], (−)-3-(3-hydroxy-phenyl)-N-n-propylpiperidine [(−)3-PPP] and (+)terguride) and dopamine D2 receptor antagonists. Aripiprazole and OPC-4392 were inactive in a guanosine-5′-O-(3-[35S]thio)-triphosphate ([35S]GTPγS) binding assay using Chinese Hamster Ovary (CHO) cell membranes expressing cloned human dopamine D2Long (hD2L) receptors, whereas (−)3-PPP and (+)terguride displayed low intrinsic activity. Aripiprazole also had no effect on [35S]GTPγS binding to CHO-hD2L cells, while OPC-4392, (−)3-PPP and (+)terguride were partial agonists. In contrast, aripiprazole, OPC-4392, (−)3-PPP, and (+)terguride were inactive in a [35S]GTPγS binding assay using rat striatal membranes. However, at a more downstream level of CHO-hD2L cell signalling, these drugs all behaved as dopamine hD2L receptor partial agonists, with aripiprazole displaying an intrinsic activity 2 to 3-fold lower (inhibition of forskolin-induced adenosine 3′,5′-cyclic monophosphate accumulation) and almost half as high (enhancement of adenosine triphosphate-stimulated [3H]arachidonic acid release) as OPC-4392, (−)3-PPP and (+)terguride. Dopamine activity was blocked in each case by (−)raclopride, which was inactive on its own in every assay, as were the antipsychotics haloperidol, olanzapine, ziprasidone and clozapine. Together, these data, whilst preclinical in nature, are consistent with clinical evidence suggesting the favorable antipsychotic profile of aripiprazole, compared with the other clinically ineffective partial agonists, is dependent on its low intrinsic activity at dopamine D2 receptors. This study also highlights the limitations of using [35S]GTPγS binding assays to identify dopamine D2 receptor partial agonists.