Distinct inactive conformations of the dopamine D2 and D3 receptors correspond to different extents of inverse agonism.

Abstract

By analyzing and simulating inactive conformations of the highly homologous dopamine D₂ and D₃ receptors (D₂R and D₃R), we find that eticlopride binds D₂R in a pose very similar to that in the D₃R/eticlopride structure but incompatible with the D₂R/risperidone structure. In addition, risperidone occupies a sub-pocket near the Na⁺ binding site, whereas eticlopride does not. Based on these findings and our experimental results, we propose that the divergent receptor conformations stabilized by Na⁺-sensitive eticlopride and Na⁺-insensitive risperidone correspond to different degrees of inverse agonism. Moreover, our simulations reveal that the extracellular loops are highly dynamic, with spontaneous transitions of extracellular loop 2 from the helical conformation in the D₂R/risperidone structure to an extended conformation similar to that in the D₃R/eticlopride structure. Our results reveal previously unappreciated diversity and dynamics in the inactive conformations of D₂R. These findings are critical for rational drug discovery, as limiting a virtual screen to a single conformation will miss relevant ligands.