Modification of internal temperature regulation for cutaneous vasodilation and sweating by bright light exposure at night

Seasonal variation of temperature regulation: do thermoregulatory responses "spring" forward and "fall" back?

Seasonal variations in day length and light intensity can affect the circadian rhythm as well as some characteristics of temperature regulation. We investigated characteristics of autonomic (ATR), behavioural (BTR) and nocturnal (NTR) temperature regulation during spring and autumn. Eleven participants underwent experiments in both seasons. To assess ATR, participants performed a 30-min bout of submaximal upright exercise on a cycle ergometer, followed by 100 min of water immersion (28 °C). Thresholds for the onset of shivering and sweating and vasomotor response were measured. BTR was assessed using a water-perfused suit, with participants regulating the water-perfused suit temperature (Twps) within a range, considered as thermally comfortable. The Twps changed in a saw-tooth manner from 10 to 50 °C; by depressing a switch, the direction of the Twps changed, and this limit defined the thermal comfort zone (TCZ) for each participant. A 24-h proximal (calf)-distal (toe) skin temperature gradient (∆Tc-t) was measured to assess NTR. Initiation of vasomotor tone, shivering and sweating was similar between trials. Width of the TCZ was 8.1 °C in spring and 8.6 °C in autumn (p = 0.1), with similar upper and lower regulated temperatures. ∆Tc-t exhibited a typical circadian rhythm with no difference between seasons. Minor changes in skin temperature and oxygen consumption (p ˂ 0.05) between the seasons may indicate a degree of seasonal adaptation over the course of winter and summer, which persisted in spring and autumn. Other factors, such as country, race, sex and age could however modify the outcome of the study.

Effect of postprandial thermogenesis on the cutaneous vasodilatory response during exercise

To examine the effect of postprandial thermogenesis on the cutaneous vasodilatory response, 10 healthy male subjects exercised for 30 min on a cycle ergometer at 50% of peak oxygen uptake, with and without food intake. Mean skin temperature, mean body temperature (Tb), heart rate, oxygen uptake, carbon dioxide elimination, and respiratory quotient were all significantly higher at baseline in the session with food intake than in the session without food intake. To evaluate the cutaneous vasodilatory response, relative laser Doppler flowmetry values were plotted against esophageal temperature (Tes) and Tb. Regression analysis revealed that the [Formula: see text] threshold for cutaneous vasodilation tended to be higher with food intake than without it, but there were no significant differences in the sensitivity. To clarify the effect of postprandial thermogenesis on the threshold for cutaneous vasodilation, the between-session difference in the Tes threshold and the Tb threshold were plotted against the between-session difference in baseline Tes and baseline Tb, respectively. Linear regression analysis of the resultant plot showed significant positive linear relationships (Tes: r = 0.85, P < 0.01; Tb: r = 0.67, P < 0.05). These results suggest that postprandial thermogenesis increases baseline body temperature, which raises the body temperature threshold for cutaneous vasodilation during exercise.

Effects of the melatonin MT-1/MT-2 agonist ramelteon on daytime body temperature and sleep

STUDY OBJECTIVES

A reduction in core temperature and an increase in the distal-proximal skin gradient (DPG) are reported to be associated with shorter sleep onset latencies (SOL) and better sleep quality. Ramelteon is a melatonin MT-1/MT-2 agonist approved for the treatment of insomnia. At night, ramelteon has been reported to shorten SOL. In the present study we tested the hypothesis that ramelteon would reduce core temperature, increase the DPG, as well as shorten SOL, reduce wakefulness after sleep onset (WASO), and increase total sleep time (TST) during a daytime sleep opportunity.

DESIGN

Randomized, double-blind, placebo-controlled, cross-over design. Eight mg ramelteon or placebo was administered 2 h prior to a 4-h daytime sleep opportunity.

SETTING

Sleep and chronobiology laboratory.

PARTICIPANTS

Fourteen healthy adults (5 females), aged (23.2 +/- 4.2 y).

MEASUREMENTS AND RESULTS

Primary outcome measures included core body temperature, the DPG and sleep physiology (minutes of total sleep time [TST], wake after sleep onset [WASO], and SOL). We also assessed as secondary outcomes, proximal and distal skin temperatures, sleep staging and subjective TST. Repeated measures ANOVA revealed ramelteon significantly reduced core temperature and increased the DPG (both P < 0.05). Furthermore, ramelteon reduced WASO and increased TST, and stages 1 and 2 sleep (all P < 0.05). The change in the DPG was negatively correlated with SOL in the ramelteon condition.

CONCLUSIONS

Ramelteon improved daytime sleep, perhaps mechanistically in part by reducing core temperature and modulating skin temperature. These findings suggest that ramelteon may have promise for the treatment of insomnia associated with circadian misalignment due to circadian sleep disorders.

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 degrees C to 32 degrees C, forearm CVC was first significantly reduced at T(sk) of 34.33 +/- 0.01 degrees C (P < 0.05) in Cont. In contrast, CVC in Mel was not significantly reduced until T(sk) reached 33.33 +/- 0.01 degrees 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 degrees C and lower (P < 0.05). In Mel, palmar CVC was significantly higher than in Cont above T(sk) of 33.33 +/- 0.01 degrees 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.

Modification of cutaneous vasodilator response to heat stress by daytime exogenous melatonin administration

In humans, the nocturnal fall in internal temperature is associated with increased endogenous melatonin and with a shift in the thermoregulatory control of skin blood flow (SkBF), suggesting a role for melatonin in the control of SkBF. The purpose of this study was to test whether daytime exogenous melatonin would shift control of SkBF to lower internal temperatures during heat stress, as is seen at night. Healthy male subjects ( n = 8) underwent body heating with melatonin administration (Mel) or without (control), in random order at least 1 wk apart. SkBF was monitored at sites pretreated with bretylium to block vasoconstrictor nerve function and at untreated sites. Cutaneous vascular conductance, calculated from SkBF and arterial pressure, sweating rate (SR), and heart rate (HR) were monitored. Skin temperature was elevated to 38°C for 35–50 min. Baseline esophageal temperature (Tes) was lower in Mel than in control ( P < 0.01). The Tes threshold for cutaneous vasodilation and the slope of cutaneous vascular conductance with respect to Tes were also lower in Mel at both untreated and bretylium-treated sites ( P < 0.05). The Tes threshold for the onset of sweating and the Tes for a standard HR were reduced in Mel. The slope of the relationship of HR, but not SR, to Tes was lower in Mel ( P < 0.05). These findings suggest that melatonin affects the thermoregulatory control of SkBF during hyperthermia via the cutaneous active vasodilator system. Because control of SR and HR are also modified, a central action of melatonin is suggested.