Differential Function of Melatonin MT(1) and MT(2) Receptors in REM and NREM Sleep

The pathophysiological function of the G-protein coupled melatonin MT(1) and MT(2) receptors has not yet been well-clarified. Recent advancements using selective MT(1)/ MT(2) receptor ligands and MT(1)/MT(2) receptor knockout mice have suggested that the activation of the MT(1) receptors are mainly implicated in the regulation of rapid eye movement (REM) sleep, whereas the MT(2) receptors selectively increase non-REM (NREM) sleep. Studies in mutant mice show that MT(1) knockout mice have an increase in NREM sleep and a decrease in REM sleep, while MT2 knockout mice a decrease in NREM sleep. The localization of MT(1) receptors is also distinct from MT2 receptors; for example, MT(2) receptors are located in the reticular thalamus (NREM area), while the MT(1) receptors in the Locus Coeruleus and lateral hypothalamus (REM areas). Altogether, these findings suggest that these two receptors not only have a very specialized function in sleep, but that they may also modulate opposing effects. These data also suggest that mixed MT(1)-MT(2) receptors ligands are not clinically recommended given their opposite roles in physiological functions, confirmed by the modest effects of melatonin or MT(1)/MT(2) non-selective agonists when used in both preclinical and clinical studies as hypnotic drugs. In sum, MT(1) and MT(2) receptors have specific roles in the modulation of sleep, and consequently, selective ligands with agonist, antagonist, or partial agonist properties could have therapeutic potential for sleep; while the MT(2) agonists or partial agonists might be indicated for NREM-related sleep and/or anxiety disorders, the MT(1) agonists or partial agonists might be so for REM-related sleep disorders. Furthermore, MT(1) but not MT(2) receptors seem involved in the regulation of the circadian rhythm. Future research will help further develop MT(1) and/or MT(2) receptors as targets for neuropsychopharmacology drug development.