The High Affinity Dopamine D2 Receptor Agonist MCL-536: A New Tool for Studying Dopaminergic Contribution to Neurological Disorders

The dopamine D 2 receptor exists in two different states, D 2 high and D 2low ; the former is the functional form of the D 2 receptor and associates with intracellular G-proteins. The D 2 agonist [ 3 H]MCL-536 has high affinity for the D 2 receptor ( K d 0.8 nM) and potently displaces the binding of ( R -(-)- N - n -propylnorapomorphine (NPA; K i 0.16 nM) and raclopride ( K i 0.9 nM) in competition binding assays. Here, we further characterize [ 3 H]MCL-536. [ 3 H]MCL-536 was metabolically stable, with about 75% of the compound remaining intact after 1 h incubation with human liver microsomes. Blood-brain barrier penetration in rats was good, attaining at 15 min a % injected dose per gram of wet tissue (%ID/g) of 0.28 in males versus 0.42 in females in the striatum. Specific uptake ratios ([%ID/g striatum]/[%ID/g cerebellum]) were stable in males during the first 60 min and in females up to 15-30 min. The D 2 -rich striatum exhibited the highest uptake and slowest washout compared to D 2 -poor cortex or cerebellum. In peripheral organs, uptake peaked at 15 min but declined to baseline at 60 min, indicating good clearance from the body. In vitro autoradiography on transaxial and coronal brain sections showed specific binding of [ 3 H]MCL-536, which was abolished by preincubation with D 2 /D 3 ligands sulpiride, NPA, and raclopride and in the presence of the stable GTP analogue guanylylimidodiphosphate. In amphetamine-sensitized animals, striatal binding was higher than in controls, indicating specificity for the D 2 high receptor state. [ 3 H]MCL-536's unique properties make it a valuable tool for research on neurological disorders involving the dopaminergic system like Parkinson's disease or schizophrenia.