Multiple fluorescent ligands for dopamine receptors. I. Pharmacological characterization and receptor selectivity

We report the synthesis and pharmacological characterization of novel fluorescently labeled ligands with high affinity and specificity for D₁ and D₂ dopamine receptors. D₁-selective antagonist probes have been synthesized using (R,S)-5-(4′-aminophenyl)-8-chloro-2,3,4,5-tetrahydro-3-methyl-[1H]-3-benzazepin-7-ol, the 4′-amino derivative of the high affinity D₁-selective antagonist, SCH-23390, while D₂-selective antagonist probes were synthesized using the high affinity, D₂-selective agonist, N-(p-aminophenethyl)spiperone (NAPS). In addition, we have synthesized fluorescent probes using an amino-derivative of the high affinity, D₂-selective agonist, 2-(N-phenethyl-N-propyl)amino-5-hydroxytetralin (PPHT or N-0434). These ligands were coupled to the fluorescent moieties, fluorescein, rhodamine, coumarin, Texas red, Cascade blue, or Bodipy. This resulted in a wide variety of dopaminergic ligands which fluoresce at different wavelengths: Cascade blue and coumarin are blue fluorophores, fluorescein and Bodipy, are yellow-green, and Texas red and rhodamine are red. The interaction of these fluorescent ligands with dopamine and serotonin receptors was evaluated by examining their ability to compete for radioligand binding to D₁ and D₂ dopamine receptors and 5-HT_1A, 5-HT_1C and 5-HT₂ serotonin receptors. We report here that these novel fluorescent ligands exhibit high affinity and, in general, selectivity for either D₁ or D₂ dopamine receptors. In addition, we demonstrate that the fluorescent derivatives of PPHT retain the full agonist efficacy exhibited by the parent compound. The availability of fluorescent ligands for D₁ and D₂ dopamine receptors should enable the visualization of these receptors at the cellular and subcellular levels in the CNS as well as in following the dynamics of agonist-receptor interactions in cultured cells.