Functional activation by central monoamines of human dopamine D4 receptor polymorphic variants coupled to GIRK channels in Xenopus oocytes

We studied the functional activation of different polymorphic variants of the human dopamine D4 receptors by the three major central monoamines, dopamine, noradrenaline and serotonin. Dopamine D4 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 D4 receptor variants coupled to oocyte Gi/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 D4 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 (EC50 = 1 nM) than on D4.4 (EC50 = 5 nM) suggesting that the actions of dopamine and therapeutic drugs on dopamine D4 receptors might vary among individuals depending on their repertoire of expressed alleles. In contrast, noradrenaline and serotonin did not discriminate among dopamine D4 receptor variants (EC50 NA = 50 nM, EC50 5-HT = 1.5 μM). All monoamine effects were blocked by the specific dopaminergic D4 antagonist (S)-(−)-4-[4-[2-(Isochroman-1-yl)ethyl]piperazin-1-yl]benzenesulfonamide (PNU101387). Sequence analyses of dopamine D4 receptors and related monoamine receptors revealed that dopamine D4 receptors have most aminoacidic residues necessary for binding of dopamine, noradrenaline and serotonin. Our data indicate that dopamine D4 receptors can be pharmacologically stimulated by any the three major central monoamines.