Noradrenaline and adrenaline are high affinity agonists at dopamine D4 receptors

The activity of monoamine neurotransmitters was examined at dopamine D4 receptors. In competition binding with [3H]spiperone, noradrenaline and adrenaline exhibited a high affinity binding component (KH = 12.1 nM and 5.0 nM, respectively), similar to that of dopamine (KH = 2.6 nM), whereas serotonin (5-hydroxytryptamine, 5-HT) and histamine had low affinity (Ki > 1000 nM). Noradrenaline and adrenaline acted as agonists at dopamine D4 receptors, stimulating receptor-mediated [35S]guanylyl-gamma-thiotriphosphate ([35S]GTP gamma S) binding (EC50 = 7.8 and 5.8 microM, respectively, versus 0.1 microM for dopamine). The dopamine D4 receptor-selective ligand, 3-(4-[4-chlorophenyl]piperazin-1-yl)methyl-1H-pyrrolo[2,3b]-pyridi ne (L 745,870) and the dopaminergic antagonists, spiperone, haloperidol and clozapine, inhibited noradrenaline-stimulated [35S]GTP gamma S binding whereas alpha 1-, alpha 2- and beta-adrenoceptor antagonists did not. These results indicate that dopamine D4 receptors are activated by noradrenaline and adrenaline, although at 50-100-fold higher concentrations than dopamine.