Abstract
The pathophysiological function of the G-protein coupled melatonin MT1 and MT2 receptors has not yet been well-clarified. Recent advancements using selective MT1/ MT2 receptor ligands and MT1/MT2 receptor knockout mice have suggested that the activation of the MT1 receptors are mainly implicated in the regulation of rapid eye movement (REM) sleep, whereas the MT2 receptors selectively increase non-REM (NREM) sleep. Studies in mutant mice show that MT1 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 MT1 receptors is also distinct from MT2 receptors; for example, MT2 receptors are located in the reticular thalamus (NREM area), while the MT1 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 MT1-MT2 receptors ligands are not clinically recommended given their opposite roles in physiological functions, confirmed by the modest effects of melatonin or MT1/MT2 non-selective agonists when used in both preclinical and clinical studies as hypnotic drugs. In sum, MT1 and MT2 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 MT2 agonists or partial agonists might be indicated for NREM-related sleep and/or anxiety disorders, the MT1 agonists or partial agonists might be so for REM-related sleep disorders. Furthermore, MT1 but not MT2 receptors seem involved in the regulation of the circadian rhythm. Future research will help further develop MT1 and/or MT2 receptors as targets for neuropsychopharmacology drug development.
Highlights
Reviewed by: Gianluca Tosini, Morehouse School of Medicine, United States Rostislav Turecek, Academy of Sciences of the Czech Republic (ASCR), Czechia
Melatonin is an important modulator of the sleep/wake cycle by activating MT1 and MT2 receptors, even if some authors have hypothesized that melatonin can have MT1/MT2 receptorindependent hypnotic effects [102]
Melatonin receptors have been shown to be present in many brain areas/nuclei implicated in the control of the sleep/wake cycle
Summary
Reviewed by: Gianluca Tosini, Morehouse School of Medicine, United States Rostislav Turecek, Academy of Sciences of the Czech Republic (ASCR), Czechia. Used hypnotic drugs, such as benzodiazepines and thier derivates (i.e., zopiclone), act mostly on the GABAergic system, increasing SWS and decreasing REM sleep, altering the sleep architecture [2] This can result in next-day cognitive impairments and may lead to abuse. Piromelatine was shown to have both hypnotic and antinociceptive effects by electroencephalogram (EEG) recordings in an animal model of neuropathic pain, partial sciatic nerve ligation (PSL) [49] It increases NREM sleep and decreases wakefulness in PSL mice, but the effect could be blocked by preadministration of a melatonin receptor antagonist, a 5-HT1A receptor antagonist, or an opiate receptor antagonist [49], demonstrating a lack of selectivity for the melatonin receptors. The Clinicaltrials.gov database lists a study currently recruiting patients entitled “Safety and Efficacy of Piromelatine in Mild Alzheimer’s Disease Patients (ReCOGNITION),” https://clinicaltrials.gov/ct2/show/ NCT02615002, indicating that the compound will be primarily
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