Fluorescent Tools for the Imaging of Dopamine D₂-Like Receptors

The family of dopamine D-2-like receptors represents an interesting target for a variety of neurological diseases, e. g. Parkinson's disease (PD), addiction, or schizophrenia. In this study we describe the synthesis of a new set of fluorescent ligands as tools for visualization of dopamine D-2-like receptors. Pharmacological characterization in radioligand binding studies identified UR-MN212 (20) as a high-affinity ligand for D-2-like receptors (pK(i) (D2longR)=8.24, pK(i) (D3R)=8.58, pK(i) (D4R)=7.78) with decent selectivity towards D-1-like receptors. Compound 20 is a neutral antagonist in a G(o1) activation assay at the D2longR, D3R, and D4R, which is an important feature for studies using whole cells. The neutral antagonist 20, equipped with a 5-TAMRA dye, displayed rapid association to the D2longR in binding studies using confocal microscopy demonstrating its suitability for fluorescence microscopy. Furthermore, in molecular brightness studies, the ligand's binding affinity could be determined in a single-digit nanomolar range that was in good agreement with radioligand binding data. Therefore, the fluorescent compound can be used for quantitative characterization of native D-2-like receptors in a broad variety of experimental setups.