Introduction. To date all clinically effective antipsychotics target the D2 dopamine receptor (D2R) by competing with the neurotransmitter dopamine for the ‘orthosteric’ binding site. SB269652 was recently identified as the first negative allosteric modulator of the D2R (Silvano, Millan et al. 2010). We have progressively truncated fragments of this ligand, demonstrating that SB269652 has a ‘bitopic’ (simultaneous allosteric/orthosteric) mode of interaction with the D2R.
Aim. To gain further insight as to the specific receptor residues important for binding and function of this ligand
Methods. We selected and mutated residues in both orthosteric and putative allosteric sites of the D2R and screened these mutants in both functional (D2R mediated ERK1/2 phosphorylation) and radioligand binding assays ([3H]-spiperone)
Results. Mutation to alanine of two residues predicted to interact with the allosteric moiety of SB269652, Glu95 and Val91, caused a significant decrease in the negative cooperativity exerted by SB269652 upon dopamine (logαGlu95Ala = -0.32 ± 0.14; logαVal91Ala = -0.48 ± 0.16; n = 3, P < 0.05) as compared to the wild type receptor (logαWT = -1.20 ± 0.12). Similarly, mutation of Ser194 within the orthosteric site to alanine caused a significant decrease in negative cooperativity (logαSer194Ala = -0.52 ± 0.09, n = 3, P < 0 .05).
Discussion: These data demonstrate that SB269652 makes key interactions both within the orthosteric site and in an allosteric site at the top of transmembrane domain 2. As such this study provides validation of a bitopic mechanism of action for SB269652 and reveals the location of a novel allosteric site within the D2R.