Partial agonist properties of the antipsychotics SSR181507, aripiprazole and bifeprunox at dopamine D2 receptors: G protein activation and prolactin release.

Dopamine D2 receptor antagonists induce hyperprolactinemia depending on the extent of D2 receptor blockade. We compared the effects of the new antipsychotic agents SSR181507 ((3-exo)-8-benzoyl-N-[[(2 s)7-chloro-2,3-dihydro-1,4-benzodioxin-1-yl]methyl]-8-azabicyclo[3.2.1]octane-3-methanamine monohydrochloride), bifeprunox (DU127090: 1-(2-Oxo-benzoxazolin-7-yl)-4-(3-biphenyl)methylpiperazinemesylate) and SLV313 (1-(2,3-dihydro-benzo[1,4]dioxin-5-yl)-4-[5-(4-fluorophenyl)-pyridin-3-ylmethyl]-piperazine) with those of aripiprazole (7-{4-[4-(2,3-dichlorophenyl)-1-piperazinyl]-butyloxy)-3,4-dihydro-2(1 H)-quinolinone), clozapine and haloperidol, on functional measures of dopamine D2 receptor activity in vitro and in vivo: [35S]-GTPgammaS binding to membranes from Sf9 insect cells expressing human dopamine D2 Long (hD2 L) receptors, and serum prolactin levels in the rat. All compounds antagonized apomorphine-induced G protein activation at dopamine hD2 L receptors. Antagonist potencies of aripiprazole, bifeprunox and SLV313 were similar to haloperidol (pK(b) = 9.12), whereas SSR181507 (8.16) and clozapine (7.35) were less potent. Haloperidol, SLV313 and clozapine were silent antagonists but SSR181507, bifeprunox and aripiprazole stimulated [35S]-GTPgammaS binding by 17.5%, 26.3% and 25.6%, respectively, relative to 100 microM apomorphine (Emax = 100%). pEC50s were: SSR181507, 8.08; bifeprunox, 8.97; aripiprazole, 8.56. These effects were antagonized by raclopride. Following oral administration in vivo, the drugs increased prolactin release to different extents. SLV313 and haloperidol potently (ED50 0.12 and 0.22 mg/kg p.o., respectively) stimulated prolactin release up to 86 and 83 ng/ml. Aripiprazole potently (ED50 0.66 mg/kg p.o.) but partially (32 ng/ml) induced prolactin release. SSR181507 (ED50 4.9 mg/kg p.o.) also partially (23 ng/ml) enhanced prolactin release. Bifeprunox only weakly increased prolactin at high doses (13 ng/ml at 40 mg/kg) and clozapine only affected prolactin at the highest dose tested (41 ng/ml at 40 mg/kg). Prolactin levels of the corresponding vehicle-treated animals were <4.3 ng/ml. These data show that (1) SSR181507, aripiprazole and bifeprunox, but not SLV313, are partial agonists at dopamine hD2 L receptors in vitro; (2) SSR181507, bifeprunox and aripiprazole exhibit reduced prolactin release in vivo compared with drugs that are neutral antagonists at dopamine D2 receptors.