MT₁ and MT₂ melatonin receptors are expressed throughout the body and regulate an immense diversity of physiological processes. While the melatonin receptor subtypes may work in concert to regulate various chronobiotic and homeostatic responses, the distinct roles of MT₁ and MT₂ spur the interest to develop subtype-specific pharmacological agents. A series of substituted isoquinolinones were synthesized and their binding affinities and functional activities towards human melatonin MT₁ and MT₂ receptors were evaluated. Structure-activity relationship analysis revealed that substituted isoquinolinones bearing a 3-methoxylbenzyloxyl group conferred effective binding and selectivity toward the MT2 receptor. These ligands represent valuable tools for delineating potential structural d...[ Read more ]

MT₁ and MT₂ melatonin receptors are expressed throughout the body and regulate an immense diversity of physiological processes. While the melatonin receptor subtypes may work in concert to regulate various chronobiotic and homeostatic responses, the distinct roles of MT₁ and MT₂ spur the interest to develop subtype-specific pharmacological agents. A series of substituted isoquinolinones were synthesized and their binding affinities and functional activities towards human melatonin MT₁ and MT₂ receptors were evaluated. Structure-activity relationship analysis revealed that substituted isoquinolinones bearing a 3-methoxylbenzyloxyl group conferred effective binding and selectivity toward the MT2 receptor. These ligands represent valuable tools for delineating potential structural determinants that underlie the molecular properties of the MT₁ and MT₂ receptors. Mutagenesis studies have been used to develop an understanding of the molecular basis of ligand selectivity of melatonin receptor subtypes with the aim of developing models of the ligand binding site. Conserved residues on both MT₁ and MT₂ receptors, including those had been reported to have essential interactions with melatonin, were subjected to Alanine substitution. Point-mutated receptors were examined for Ca²⁺ mobilization upon MT₂-selective isoquinolinone stimulation in transfected cells and their expression was confirmed by radioligand binding assays. Structural interpretation and molecular docking suggested that the isoquinolinones and melatonin utilize different subsets of residues for receptor activation, and identify residues on TM7 contributing to subtype selectivity.