Exogenous melatonin administration modifies cutaneous vasoconstrictor response to whole body skin cooling in humans

  Humans and other diurnal species experience a fall in internal temperature ( T int ) at night, accompanied by increased melatonin and altered thermoregulatory control of skin blood flow (SkBF). Also, exogenous melatonin induces a fall in T int , an increase in distal skin temperatures and altered control of the cutaneous active vasodilator system, suggesting an effect of melatonin on the control of SkBF. To test whether exogenous melatonin also affects the more tonically active vasoconstrictor system in glabrous and nonglabrous skin during cooling, healthy males (n = 9) underwent afternoon sessions of whole body skin temperature ( T sk ) cooling (water‐perfused suits) after oral melatonin (Mel; 3 mg) or placebo (Cont). Cutaneous vascular conductance (CVC) was calculated from SkBF (laser Doppler flowmetry) and non‐invasive blood pressure. Baseline T int was lower in Mel than in Cont ( P  <   0.01). During progressive reduction of T sk from 35°C to 32°C, forearm CVC was first significantly reduced at T sk of 34.33 ± 0.01°C ( P  < 0.05) in Cont. In contrast, CVC in Mel was not significantly reduced until T sk reached 33.33 ± 0.01°C ( P  < 0.01). The decrease in forearm CVC in Mel was significantly less than in Cont at T sk of 32.66 ± 0.01°C and lower ( P  < 0.05). In Mel, palmar CVC was significantly higher than in Cont above T sk of 33.33 ± 0.01°C, but not below. Thus exogenous melatonin blunts reflex vasoconstriction in nonglabrous skin and shifts vasoconstrictor system control to lower T int . It provokes vasodilation in glabrous skin but does not suppress the sensitivity to falling T sk . These findings suggest that by affecting the vasoconstrictor system, melatonin has a causal role in the nocturnal changes in body temperature and its control.