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Melatonin MT1 and MT2 Receptors Exhibit Distinct Effects in the Modulation of Body Temperature across the Light/Dark Cycle

1
Neurobiological Psychiatry Unit, Department of Psychiatry, McGill University Health Center (MUHC), McGill University, Montreal, QC H3A1A1, Canada
2
Alan Edwards Centre for Research on Pain, McGill University, Montreal, QC H3A 0G1, Canada
3
Department of Biomolecular Sciences, University Carlo Bo, 61029 Urbino, Italy
4
Neuropsychopharmacology Unit, San Raffaele Scientific Institute and Vita-Salute University, 20132 Milan, Italy
*
Author to whom correspondence should be addressed.
These authors contributed equally to this work.
Int. J. Mol. Sci. 2019, 20(10), 2452; https://doi.org/10.3390/ijms20102452
Received: 15 April 2019 / Revised: 6 May 2019 / Accepted: 10 May 2019 / Published: 17 May 2019
(This article belongs to the Special Issue Circadian Rhythms: Molecular and Physiological Mechanisms)
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Abstract

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. View Full-Text
Keywords: melatonin; MT1 receptors; MT2 receptors; MT3 receptors; body temperature; light/dark cycle melatonin; MT1 receptors; MT2 receptors; MT3 receptors; body temperature; light/dark cycle
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López-Canul, M.; Min, S.H.; Posa, L.; De Gregorio, D.; Bedini, A.; Spadoni, G.; Gobbi, G.; Comai, S. Melatonin MT1 and MT2 Receptors Exhibit Distinct Effects in the Modulation of Body Temperature across the Light/Dark Cycle. Int. J. Mol. Sci. 2019, 20, 2452.

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