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Melatonin (MLT), a neuromodulator mainly acting through two G-protein coupled receptors MT 1 and MT 2 , regulates many brain functions, including circadian rhythms, mood, pain and sleep. MLT and non-selective MT 1 /MT 2 receptor agonists are clinically used in neuropsychiatric and/or sleep disorders. However, the selective roles of the MT 1 and MT 2 receptors need to be clarified. Here, we review the role of the MT 1 receptor in neuropsychopharmacology, describe the anatomical localization of MT 1 receptors in the brain, discuss the medicinal chemistry, biochemistry and molecular aspects of the receptor, and explore the findings linking MT 1 receptors to psychiatric and neurological disorders. MT 1 receptors are localized in brain regions which regulate circadian rhythms, sleep, and mood, such as the suprachiasmatic nucleus, cortex, hippocampus, dorsal raphe nucleus and lateral hypothalamus. Their activation modulates intracellular signaling pathways also targeted by psychoactive drugs, including antidepressants and mood stabilizers. MT 1 receptor knockout mice display increased anxiety, a depressive-like phenotype, increased propensity to reward and addiction, and reduced Rapid-Eye-Movement sleep. These behavioral dysfunctions are associated with altered serotonergic and noradrenergic neurotransmissions. Several studies indicate that the MT 1 , rather than MT 2 , receptor is implicated in circadian rhythm regulation. The involvement of MT 1 receptors in Alzheimer's and Huntington diseases has also been proposed. Postmortem studies in depressed patients have further confirmed the possible involvement of MT 1 receptors in depression. Overall, there is substantial evidence indicating a role for MT 1 receptor in modulating brain function and mood. Consequently, this MLT receptor subtype deserves to be further examined as a novel target for neuropsychopharmacological drug development. [ABSTRACT FROM AUTHOR]

Abstract: The neurohormone melatonin activates two G-protein coupled receptors, MT1 and MT2. Melatonin is implicated in circadian rhythms and sleep regulation, but the role of its receptors remains to be defined. We have therefore characterized the spontaneous vigilance states in wild-type (WT) mice and in three different types of transgenic mice: mice with genetic inactivation of MT1 (MT1 −/−), MT2 (MT2 −/−) and both MT1/MT2 (MT1 −/−/MT2 −/−) receptors. Electroencephalographic (EEG) and electromyographic sleep–wake patterns were recorded across the 24-h light–dark cycle. MT1 −/−mice displayed a decrease (−37.3%) of the 24-h rapid eye movement sleep (REMS) time whereas MT2 −/−mice showed a decrease (−17.3%) of the 24-h non rapid eye movement sleep (NREMS) time and an increase in wakefulness time (14.8%). These differences were the result of changes occurring in particular during the light/inactive phase. Surprisingly, MT1 −/−/MT2 −/− mice showed only an increase (8.9%) of the time spent awake during the 24-h. These changes were correlated to a decrease of the REMS EEG theta power in MT1 −/−mice, of the NREMS EEG delta power in MT2 −/−mice, and an increase of the REMS and wakefulness EEG theta power in MT1 −/−/MT2 −/− mice. Our results show that the genetic inactivation of both MT1 and MT2 receptors produces an increase of wakefulness, likely as a result of reduced NREMS due to the lack of MT2 receptors, and reduced REMS induced by the lack of MT1 receptors. Therefore, each melatonin receptor subtype differently regulates the vigilance states: MT2 receptors mainly NREMS, whereas MT1 receptors REMS. [Copyright &y& Elsevier]

Melatonin (MLT) is a neurohormone that regulates many physiological functions including sleep, pain, thermoregulation, and circadian rhythms. MLT acts mainly through two G-protein-coupled receptors named MT1 and MT2, but also through an MLT type-3 receptor (MT3). However, the role of MLT receptor subtypes in thermoregulation is still unknown. We have thus investigated the effects of selective and non-selective MLT receptor agonists/antagonists on body temperature (Tb) in rats across the 12/12-h light–dark cycle. Rectal temperature was measured every 15 min from 4:00 a.m. to 9:30 a.m. and from 4:00 p.m. to 9:30 p.m., following subcutaneous injection of each compound at either 5:00 a.m. or 5:00 p.m. MLT (40 mg/kg) had no effect when injected at 5 a.m., whereas it decreased Tb during the light phase only when injected at 5:00 p.m. This effect was blocked by the selective MT2 receptor antagonist 4P-PDOT and the non-selective MT1/MT2 receptor antagonist, luzindole, but not by the α1/MT3 receptors antagonist prazosin. However, unlike MLT, neither the selective MT1 receptor partial agonist UCM871 (14 mg/kg) nor the selective MT2 partial agonist UCM924 (40 mg/kg) altered Tb during the light phase. In contrast, UCM871 injected at 5:00 p.m. increased Tb at the beginning of the dark phase, whereas UCM924 injected at 5:00 a.m. decreased Tb at the end of the dark phase. These effects were blocked by luzindole and 4P-PDOT, respectively. The MT3 receptor agonist GR135531 (10 mg/kg) did not affect Tb. These data suggest that the simultaneous activation of both MT1 and MT2 receptors is necessary to regulate Tb during the light phase, whereas in a complex but yet unknown manner, they regulate Tb differently during the dark phase. Overall, MT1 and MT2 receptors display complementary but also distinct roles in modulating circadian fluctuations of Tb. [ABSTRACT FROM AUTHOR]

Abstract: Melatonin (MLT) is a neurohormone known to be involved in the regulation of anxiety. Most of the physiological actions of MLT in the brain are mediated by two high-affinity G-protein-coupled receptors, denoted MT1 and MT2. However, the particular role of these receptors in anxiety remains to be defined. Here we used a novel MT2-selective partial agonist, UCM765 to evaluate the involvement of MT2 receptors in anxiety. Adult male rats were acutely injected with UCM765 (5–10–20mg/kg), MLT (20mg/kg) or diazepam (DZ, 1mg/kg). Anxiety-related behaviors were assessed in the elevated plus maze test (EPMT), novelty suppressed feeding test (NSFT) and open field test (OFT). UCM765 at the dose of 10mg/kg showed anxiolytic-like properties by increasing the time spent in the open arm of the EPMT, and by reducing the latency to eat in a novel environment in the NSFT. In the EPMT, animals treated with UCM765 (10mg/kg) or MLT (20mg/kg) spent more time in the open arms compared to vehicle-treated animals, but to a lesser extent compared to DZ (1mg/kg). In the NSFT, all treatments similarly decreased the latency to eat in a novel environment compared to vehicle. UCM765 and MLT did not affect the total time and the number of entries into the central area of the OFT, but unlike DZ, did not impair locomotion. The anxiolytic effects of UCM765 and MLT in the EPMT and the NSFT were blocked using a pre-treatment with the MT1/MT2 antagonist luzindole (10mg/kg) or the MT2 antagonist 4P-PDOT (10mg/kg). These results demonstrated, for the first time, the anxiolytic properties of UCM765 and suggest that MT2-receptors may be considered a novel target for the development of anxiolytic drugs. [Copyright &y& Elsevier]