Pharmacological Characterization of a Highly Selective and Potent Partial Agonist of the MT2 Melatonin Receptor

Background/Aims: The MT2 melatonin receptor is a potential target for treating circadian rhythm sleep disorders. This study aims to characterize the recently identified MT2 melatonin receptor agonist. Methods: The pharmacological properties of the MT2 melatonin receptor-selective agonist as exemplified by compound 1 [N-(2-[7-benzyl-1,6-dihydro-2H-indeno(5,4-b)furan-8-yl]ethyl)acetamide] were evaluated by use of cell-free binding and cell-based functional assays. Results: Competition binding assays using 2-[125I]iodomelatonin revealed rapid, reversible, and high-affinity binding of compound 1 to human, mouse, and rat MT2 melatonin receptors. cAMP, ERK1/2, and PathHunter β-arrestin recruitment assays revealed partial agonist activities. However, compound 1 induced a more intense internalization of human MT2 melatonin receptor than melatonin. Based on studies using structurally related analogs of compound 1, we further demonstrated that the extent of internalization is independent of the intrinsic efficacy of agonists. Conclusion: These findings provide novel insights into the relationship between intrinsic agonist efficacy and agonist-induced internalization and demonstrate that compound 1 could serve as a pharmacological tool for future studies to elucidate the detailed molecular mechanism of MT2 receptor internalization.