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

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 MT and MT, but also through an MLT type-3 receptor (MT). 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 (T) 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 T during the light phase only when injected at 5:00 p.m. This effect was blocked by the selective MT receptor antagonist 4P-PDOT and the non-selective MT/MT receptor antagonist, luzindole, but not by the α/MT receptors antagonist prazosin. However, unlike MLT, neither the selective MT receptor partial agonist UCM871 (14 mg/kg) nor the selective MT partial agonist UCM924 (40 mg/kg) altered T during the light phase. In contrast, UCM871 injected at 5:00 p.m. increased T at the beginning of the dark phase, whereas UCM924 injected at 5:00 a.m. decreased T at the end of the dark phase. These effects were blocked by luzindole and 4P-PDOT, respectively. The MT receptor agonist GR135531 (10 mg/kg) did not affect T. These data suggest that the simultaneous activation of both MT and MT receptors is necessary to regulate T during the light phase, whereas in a complex but yet unknown manner, they regulate T differently during the dark phase. Overall, MT and MT receptors display complementary but also distinct roles in modulating circadian fluctuations of T.