The tetrahydroisoquinoline derivative SB269,652 is an allosteric antagonist at dopamine D3 and D2 receptors

In view of the therapeutic importance of dopamine D(3) and D(2) receptors, there remains considerable interest in novel ligands. Herein, we show that the tetrahydroisoquinoline 1H-indole-2-carboxylic acid {4-[2-(cyano-3,4-dihydro-1H-isoquinolin-2-yl)-ethyl]-cyclohexyl}-amide (SB269,652) behaves as an atypical, allosteric antagonist at D(3) and D(2) receptors. Accordingly, SB269,652 potently (low nanomolar range) abolished specific binding of [(3)H]nemanopride and [(3)H]spiperone to Chinese hamster ovary-transfected D(3) receptors when radioligands were used at 0.2 and 0.5 nM, respectively. However, even at high concentrations (5 μM), SB269,652 only submaximally inhibited the specific binding of these radioligands when they were employed at 10-fold higher concentrations. By analogy, although SB269,652 potently blocked D(3) receptor-mediated activation of Gα(i3) and phosphorylation of extracellular-signal-regulated kinase (ERK)1/2, when concentrations of dopamine were increased by 10-fold, from 1 μM to 10 μM, SB269,652 only submaximally inhibited dopamine-induced stimulation of Gα(i3). SB269,652 (up to 10 μM) only weakly and partially (by approximately 20-30%) inhibited radioligand binding to D(2) receptors. Likewise, SB269,652 only submaximally suppressed D(2) receptor-mediated stimulation of Gα(i3) and Gα(qi5) (detected with the aequorin assay) and phosphorylation of ERK1/2 and Akt. Furthermore, SB269,652 only partially (35%) inhibited the dopamine-induced recruitment of β-arrestin2 to D(2) receptors. Finally, Schild analysis using Gα(i3) assays, and studies of radioligand association and dissociation kinetics, supported allosteric actions of SB269,652 at D(3) and D(2) receptors.