Agonist-specific voltage sensitivity at the dopamine D2S receptor--molecular determinants and relevance to therapeutic ligands.

Voltage sensitivity has been demonstrated for some GPCRs. At the dopamine D(2S) receptor, this voltage sensitivity is agonist-specific; some agonists, including dopamine, exhibit decreased potency at depolarized potentials, whereas others are not significantly affected. In the present study, we examined some of the receptor-agonist interactions contributing to these differences, and investigated how dopamine D(2S) receptor voltage sensitivity affects clinically used dopamine agonists. GIRK channel activation in voltage-clamped Xenopus oocytes was used as readout of receptor activation. Structurally distinct agonists and complementary site-directed mutagenesis of the receptor's binding site were used to investigate the role of agonist-receptor interactions. We also confirmed that the depolarization-induced decrease of dopamine potency in GIRK activation is correlated by decreased binding of radiolabeled dopamine, and by decreased potency in G protein activation. In the mutagenesis experiments, a conserved serine residue as well as the conserved aspartate in the receptor's binding site were found to be important for voltage sensitive potency of dopamine. Furthermore, the voltage sensitivity of the receptor had distinct effects on different therapeutic D(2) agonists. Depolarization decreased the potency of several compounds, whereas for others, efficacy was reduced. For some agonists, both potency and efficacy were diminished, whereas for others still, neither parameter was significantly altered. The present work identifies some of the ligand-receptor interactions which determine agonist-specific effects of voltage at the dopamine D(2S) receptor. The observed differences between therapeutic agonists might be clinically relevant, and make them potential tools for investigating the roles of dopamine D(2) receptor voltage sensitivity in native tissue.