Fluorescent ligands for dopamine D 2 /D 3 receptors.

Fluorescent ligands are versatile tools for the study of G protein-coupled receptors. Depending on the fluorophore, they can be used for a range of different applications, including fluorescence microscopy and bioluminescence or fluorescence resonance energy transfer (BRET or FRET) assays. Starting from phenylpiperazines and indanylamines, privileged scaffolds for dopamine D ₂ -like receptors, we developed dansyl-labeled fluorescent ligands that are well accommodated in the binding pockets of D ₂ and D ₃ receptors. These receptors are the target proteins for the therapy for several neurologic and psychiatric disorders, including Parkinson's disease and schizophrenia. The dansyl-labeled ligands exhibit binding affinities up to 0.44 nM and 0.29 nM at D ₂ R and D ₃ R, respectively. When the dansyl label was exchanged for sterically more demanding xanthene or cyanine dyes, fluorescent ligands 10a-c retained excellent binding properties and, as expected from their indanylamine pharmacophore, acted as agonists at D ₂ R. While the Cy3B-labeled ligand 10b was used to visualize D ₂ R and D ₃ R on the surface of living cells by total internal reflection microscopy, ligand 10a comprising a rhodamine label showed excellent properties in a NanoBRET binding assay at D ₃ R.