Actions of dopamine and dopaminergic drugs on cloned serotonin receptors expressed in Xenopus oocytes.

Using electrophysiological techniques, we studied interactions of dopamine and selected dopaminergic drugs with serotonin (5-hydroxytryptamine; 5-HT) receptors expressed in Xenopus oocytes by RNAs transcribed from cloned cDNAs. Oocytes showing strong expression of 5-HT1c and 5-HT2 receptors became weakly responsive to the neurotransmitter dopamine, which, like 5-HT, elicited Cl- currents through activation of the phosphatidylinositol/Ca2+ messenger pathway. The two types of 5-HT receptors showed similar sensitivity to dopamine; threshold responses were activated at concentrations as low as 1 microM. However, maximum dopamine responses were only 5-20% of maximum responses activated by 5-HT. The dopamine D1 receptor antagonist SCH 23390 was a potent agonist on 5-HT1c and 5-HT2 receptors. SCH 23390 elicited currents at concentrations as low as 1 nM, but maximum responses were again only 5-20% of those activated by 5-HT. Fenoldopam, a dopamine D1 receptor agonist, also interacted with 5-HT1c and 5-HT2 receptors, eliciting threshold responses between 10 and 20 nM. Our experiments raise the possibility that low micromolar concentrations of dopamine can cause weak activation and concomitant desensitization of serotoninergic systems in vivo and demonstrate that benzazepines can interact with 5-HT receptors at nanomolar concentrations.