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_2long R)=8.24, pK_i (D₃ R)=8.58, pK_i (D₄ R)=7.78) with decent selectivity towards D₁ -like receptors. Compound 20 is a neutral antagonist in a G_o1 activation assay at the D_2long R, D₃ R, and D₄ R, 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_2long R 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.