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Melatonin activates two brain G-protein coupled receptors, MT₁ and MT₂, whose differential roles in the sleep–wake cycle remain to be defined. The novel MT₂ receptor partial agonist, N-{2-[(3-methoxyphenyl) phenylamino] ethyl} acetamide (UCM765), is here shown to selectively promote non-rapid eye movement sleep (NREMS) in rats and mice. The enhancement of NREMS by UCM765 is nullified by the pharmacological blockade or genetic deletion of MT₂ receptors. MT₂, but not MT₁, knock-out mice show a decrease in NREMS compared to the wild strain. Immunohistochemical labeling reveals that MT₂ receptors are localized in sleep-related brain regions, and notably the reticular thalamic nucleus (Rt). Microinfusion of UCM765 in the Rt promotes NREMS, and its systemic administration induces an increase in firing and rhythmic burst activity of Rt neurons, which is blocked by the MT₂ antagonist 4-phenyl-2 …