ABSTRACT3 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 M), 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 G␣ i3 and phosphorylation of extracellular-signal-regulated kinase (ERK)1/2, when concentrations of dopamine were increased by 10-fold, from 1 M to 10 M, SB269,652 only submaximally inhibited dopamine-induced stimulation of G␣ i3 . SB269,652 (up to 10 M) 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 G␣ i3 and G␣ 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 ␤-arrestin2 to D 2 receptors. Finally, Schild analysis using G␣ i3 assays, and studies of radioligand association and dissociation kinetics, supported allosteric actions of SB269,652 at D 3 and D 2 receptors.

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