Functional activation by central monoamines of human dopamine D 4 receptor polymorphic variants coupled to GIRK channels in Xenopus oocytes

Abstract

We studied the functional activation of different polymorphic variants of the human dopamine D 4 receptors by the three major central monoamines, dopamine, noradrenaline and serotonin. Dopamine D 4 receptors carrying two (D4.2), four (D4.4) or seven (D4.7) repeats within the third intracellular domain were co-expressed with G protein-regulated inwardly rectifying potassium channels (GIRK1) in frog oocytes. All the dopamine D 4 receptor variants coupled to oocyte G i/o proteins and modulated co-expressed GIRK1 channels. Monoamine-induced responses were detected as increases in voltage-clamp recorded GIRK1 currents. Dopamine, noradrenaline as well as serotonin stimulated dopamine D 4 receptors. Dose-response analysis showed that dopamine and noradrenaline are full agonists whereas serotonin acted as partial agonist. Dopamine was 5-fold more potent on D4.2 and D4.7 (EC 50 = 1 nM) than on D4.4 (EC 50 = 5 nM) suggesting that the actions of dopamine and therapeutic drugs on dopamine D 4 receptors might vary among individuals depending on their repertoire of expressed alleles. In contrast, noradrenaline and serotonin did not discriminate among dopamine D 4 receptor variants (EC 50 NA = 50 nM, EC 50 5-HT = 1.5 μM). All monoamine effects were blocked by the specific dopaminergic D 4 antagonist (S)-(−)-4-[4-[2-(Isochroman-1-yl)ethyl]piperazin-1-yl]benzenesulfonamide (PNU101387). Sequence analyses of dopamine D 4 receptors and related monoamine receptors revealed that dopamine D 4 receptors have most aminoacidic residues necessary for binding of dopamine, noradrenaline and serotonin. Our data indicate that dopamine D 4 receptors can be pharmacologically stimulated by any the three major central monoamines.