Fluorescent tools for the imaging of dopamine D₂-like receptors

The family of dopamine D₂-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 D₂-like receptors. Pharmacological characterization in radioligand binding studies identified UR-MN212 (20) as a high-affinity ligand for D₂-like receptors (pK_i (D_2longR)=8.24, pK_i (D₃R)=8.58, pK_i (D₄R)=7.78) with decent selectivity towards D₁-like receptors. Compound 20 is a neutral antagonist in a G_o1 activation assay at the D_2longR, 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 D_2longR 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 D₂-like receptors in a broad variety of experimental setups.