Ligand selectivity of D ₂ dopamine receptors is modulated by changes in local dynamics produced by sodium binding

We have uncovered a significant allosteric response of the D ₂ dopamine receptor to physiologically relevant concentrations of sodium (140 mM), characterized by a sodium-enhanced binding affinity for a D ₄- selective class of agonists and antagonists. This enhancement is significantly more pronounced in a D ₂-V2.61(91)F mutant and cannot be mimicked by an equivalent concentration of the sodium replacement cation N-methyl-D-glucamine. This phenomenon was explored computationally at the molecular level by analyzing the effect of sodium binding on the dynamic properties of D ₂ receptor model constructs. Normal mode analysis identified one mode (M ₁₉), which is involved in the open/closed motions of the binding cleft as being particularly sensitive to the sodium effect. To examine the consequences for D ₂ receptor ligand recognition, one of the ligands, L-745,870 [3-{[4-(4-chlorophenyl) piperazin-1-yl]- methyl}-1H-pyrrolo[2,3-b]pyridine or CPPMA, chlorophenyl-piperazinyl methylazaindole], was docked into conformers along the M ₁₉ trajectory. Structurally and pharmacologically well established ligand-receptor interactions, including the ionic interaction with D3.32(114) and interactions between the ligand aryl moieties and V2.61(91)F, were achieved only in "open" phase conformers. The docking of (-)-raclopride [3,5-dichloro-N-(1-ethylpyrrolidin-2-ylmethyl)-2-hydroxy-6-methoxybenzamide] suggests that the same binding cleft changes in response to sodium-binding perturbation account as well for the enhancements in binding affinity for substituted benzamides in the wild-type D ₂ receptor. Our findings demonstrate how key interactions can be modulated by occupancy at an allosteric site and are consistent with a mechanism in which sodium binding enhances the affinity of selected ligands through dynamic changes that increase accessibility of substituted benzamides and 1,4-DAP ligands to the orthosteric site and accessibility of 1,4-DAPs to V2.61(91)F.