Circadian and age-related modulation of thermoreception and temperature regulation: mechanisms and functional implications

At older ages, the circadian rhythm of body temperature shows a decreased amplitude, an advanced phase, and decreased stability. The present review evaluates to what extent these changes may result from age-related deficiencies at several levels of the thermoregulatory system, including thermoreception, thermogenesis and conservation, heat loss, and central regulation. Whereas some changes are related to the aging process per se, others appear to be secondary to other factors, for which the risk increases with aging, notably a decreased level of fitness and physical activity. Moreover, functional implications of the body temperature rhythm are discussed. For example, the relation between circadian rhythm and thermoregulation has hardly been investigated, while evidence showed that sleep quality is dependent on both aspects. It is proposed that the circadian rhythm in temperature in homeotherms should not be regarded as a leftover of ectothermy in early evolution, but appears to be of functional significance for physiology from the level of molecules to cognition. A new view on the functional significance of the circadian rhythm in peripheral vasodilation and the consequent out-of-phase rhythms in skin and core temperature is presented. It is unlikely that the strong, daily occurring, peripheral vasodilation primarily represents heat loss in response to a lowering of set point, since behavioral measures are simultaneously taken in order to prevent heat loss. Several indications rather point towards a supportive role in immunological host defense mechanisms. Given the functional significance of the temperature rhythm, research should focus on the feasibility and effectiveness of methods that can in principle be applied in order to enhance the weakened circadian temperature rhythm in the elderly.     

Ambient temperature related sleep changes in rats neonatally treated with capsaicin

Ambient temperature related sleep changes in rats neonatally treated with capsaicin. PHYSIOL BEHAV 00(0) 000-000, 2004. The study was conducted on adult male Wistar rats, neonatally treated with capsaicin to destroy the peripheral warm receptors. The sleep-wakefulness was recorded for 5 h at an ambient temperature (T(amb)) of 18, 24, 30 and 33 degrees C on different days. The rectal temperatures (T(r)) of the rats were studied on exposure to 6 and 37 degrees C for 2 h to assess their thermoregulatory ability. The changes in the behavioral thermoregulation were assessed by noting the thermal preference of rats when they were placed in an environmental chamber with 3 interconnected compartments maintained at 24, 27 and 30 degrees C. Slow wave (SWS) and rapid eye movement (REM) sleep were decreased at 18 degrees C and increased at 30 degrees C, in control rats. There was a decrease in REM sleep and no change in SWS when T(amb) was raised from 30 to 33 degrees C. However, in neonatally capsaicin treated rats, sleep was increased even at 33 degrees C, though there was no significant change in sleep when T(amb) was increased from 18 to 24 degrees C. Capsaicin treated rats showed thermoregulatory deficiency at 37 degrees C but the thermal preference was unaltered in these rats. The results suggest that the central warm receptors can produce alteration in sleep at different T(amb), even in absence of peripheral warm receptors. The behavioral thermoregulation was unaffected in these rats, though their ability to defend the body temperature in warm environment was affected.     

Work performance breathing normoxic nitrogen or helium gas mixtures

Summary Twelve subjects completed a progressive treadmill test to maximal aerobic capacity while breathing air or a 79% helium — 21% oxygen gas mixture (HeO₂). Metabolic and thermoregulatory responses to work while breathing the two mixtures were compared at rest, 30–40%, 60–70%, and 85–95% of maximal performance, and at maximal effort. Ventilation, ventilatory equivalent, and respiratory rates were increased and oxygen uptakes decreased by breathing HeO₂ when the level of work exceeded 85–95% of maximum. Heat loss through the respiratory tract was greater breathing HeO₂. The reduction in maximal oxygen uptake is probably due to a reduction in the oxygen cost of breathing a less dense gas. It was not related to a lower body temperature and probably not to O₂ transport or circulatory limitation. HeO₂ breathing had no effect on maximal mechanical work capacity.The nature and purpose of the study and the risks involved were explained verbally and given on a written form to each subject prior to his or her voluntary consent to participate. The protocol and procedures for this study have been approved by the Committee on Activities Involving Human Subjects, of the University of California, Santa Barbara, California, USA     

Thermal tolerance following artificially induced polycythaemia

Polycythaemia has been shown to improve physical performance, possibly due to increased arterial oxygen transport. Enhanced thermoregulatory function may also accompany this manipulation, since a greater proportion of the cardiac output becomes available for heat dissipation. We further examined this possibility in five trained men, who participated in three-phase heat stress trials (20 min rest, 20 min cycling at 30% peak power W_peak and 20 min at 45% W_peak at 38.3 (SEM 0.7)°C [relative humidity 41.4 (SEM 2.9)%]. Trials were performed during normocythaemia (control) and polycythaemia, obtained by reinfusion of autologous red blood cells and resulting in significant elevation of arterial oxygen transport. During the polycythaemic trials, the subjects demonstrated diminished thermal strain, as evidenced by a significant reduction in cardiac frequency (f _c: 12 beats · min^–1 lower throughout the test;P < 0.05), and reduced auditory canal temperatures (T _ae) during the latter 20-min phase (P < 0.05). Forearm sweat onset was more rapid (363.0 compared to 1083.0 s;P < 0.05), and forearm sweat rate (. _msw) sensitivity was elevated from 1.80 to 2.91 · mg · cm^–2 · min^–1 · °C^–1 (P < 0.05). Foreheadm _sw was depressed during the final 20 min, while forearmm _sw was greater during all test phases, averaging 0.94 and 1.20 mg · cm^–2 · min^–1, respectively, over the 60 min. Skin blood flows for the upper back, upper arm and forearm were reduced (P < 0.05). Polycythaemia enhanced thermoregulation, through an elevation in forearm sweat sensitivity and.m _sw, but not via increased cutaneous blood flow. These modifications occurred simultaneously with decreases inf _c andT _ae, resulting in greater thermal tolerance.     

Impaired temperature regulation in rats after anosmia induced peripherally or centrally

Bilateral olfactory bulbectomy or destruction of the olfactory epithelium of rats resulted in elevated body temperature in room temperature, and lowered water/food ratio in 30 degrees ambient temperature. The results suggest the involvement of the olfactory system in the thermoregulation.     

Acute thermoregulatory effects of unilateral electrolytic lesions of the medial and lateral preoptic area in rats

Unilateral anodal lesions of the medial or lateral preoptic area (POA) in unanesthetized rats had opposite thermoregulatory effects immediately after the lesions were made. Lesions of the medial POA evoked hyperthermias and accompanying cold defense responses, including vasoconstriction of the tail, increased oxygen consumption, shivering, and heat conservation postures. The hyperthermias had latencies of 0–30 minutes and reached maximum values within 120 minutes postlesion. They were independent of ambient temperature and dissociable from the hyperactivity often seen after such lesions. Damage to the lateroventral POA elicited acute falls in body temperature, as well as vasodilation of the tail, decreased oxygen consumption, inhibition of shivering in cool environments, and prone body extension. Unilateral cathodal lesions throughout the POA yielded only hyperthermia. These results suggest a possible anatomical segregation of heat and cold defense functions within the anterior basal forebrain.     

Reduced responsiveness of locus coeruleus neurons to cutaneous thermal stimuli in capsaicin-treated rats

Recent electrophysiological experiments have shown that brain norepinephrine (NE) neurons in the locus coeruleus (LC) are activated by cutaneous thermal stimuli of both non-noxious and noxious character. In the present study the LC neuronal response to thermal stimuli was used to evaluate cutaneous thermal sensitivity in capsaicin-treated rats, a treatment that is described to cause impaired thermoregulation. Capsaicin treatment, of neonates as well as of adult rats, caused a reduced responsiveness of brain LC neurons to thermal stimuli. The results suggest that a reduction in peripheral thermal afferent transmission may be one mechanism underlying the capsaicin-induced thermoregulatory dysfunction.     

Direct measurement of the thermal responses of nude resting men in dry environments

Summary Two nude resting men were exposed for two-hour periods to each of 25 dry environments, with air temperatures ranging between 12.8° C and 49.1° C and wind speeds between 0.67 m/sec and 4.94 m/sec. The mean radiant temperature of the surroundings was kept equal to the air temperature. Rates of radiant and convective heat exchange were measured directly, separately and continuously. The men had reached a thermal steady state after 105 min in the warm environments, but not in the cold environments. Graphs are presented to show the effect of ambient temperature and wind speed on the radiation and convection rates attained after 105 min, as well as on metabolic rate, sweat evaporation rate, rectal temperature and mean skin temperature. These graphs revealed some important aspects of the behaviour of man's thermal control system. In particular the physiological conductance increased with increasing ambient temperature and then saturated at an ambient temperature near 35° C. This saturation resulted in a constant difference between rectal temperature and mean skin temperature irrespective of the environmental conditions.Published with the permission of the Chamber of Mines of South Africa.     

Effects of adenosine 3′, 5′-cyclic monophosphate on thermoregulation in both rats and rabbits

Summary The effects of intraventricular administration of dibutyryl adenosine 3, 5-cyclic monophosphate (db cyclic AMP) on the thermoregulatory responses of unanesthetized rats and rabbits to different ambient temperatures (T_a) were assessed. Administration of db cyclic AMP (10–60 mM) produced dose-dependent hypothermia in both rats and rabbits at T_a 2–22 °C. The hypothermia in response to db cyclic AMP was due to decreased metabolic heat production and cutaneous vasodilatation. There was no change in respiratory evaporative heat loss. In contrast, in the heat (30–32 °C), db cyclic AMP administration produced dose-dependent hyperthermia in these animals. The hyperthermia was due to increased metabolism (due to muscular shivering) and decreased heat losses. The reduction in heat losses was shown by a decrease in both cutaneous circulation and respiratory evaporative heat loss. The data demonstrate that the thermoregulatory responses induced by central administration of db cyclic AMP are T_a-dependent.     

Thermoregulatory responses to ice slurry ingestion during low and moderate intensity exercises with restrictive heat loss

ObjectivesWe investigated the thermoregulatory responses to ice slurry ingestion during low- and moderate-intensity exercises with restrictive heat loss.DesignRandomised, counterbalanced, cross-over design.MethodsFollowing a familiarisation trial, ten physically active males exercised on a motorised treadmill at low-intensity (L; 40% VO_2max) or moderate-intensity (M; 70% VO_2max) for 75-min, in four randomised, counterbalanced trials. Throughout the exercise bout, participants donned a raincoat to restrict heat loss. Participants ingested 2 g kg^?1 body mass of ambient water (L + AMB and M + AMB trials) or ice slurry (L + ICE and M + ICE trials) at 15-min intervals during exercise in environmental conditions of T_db, 25.1 ± 0.6 °C and RH, 63 ± 5%. Heart rate (HR), gastrointestinal temperature (T_gi), mean weighted skin temperature (T_sk), estimated sweat loss, ratings of perceived exertion (RPE) and thermal sensation (RTS) were recorded.ResultsCompared to L + AMB, participants completed L + ICE trials with lower ΔT_gi (0.8 ± 0.3°C vs 0.6 ± 0.2 °C; p = 0.03), mean RPE (10 ± 1 vs 9 ± 1; p = 0.03) and estimated sweat loss (0.91 ± 0.2 L vs 0.78 ± 0.27 L; p = 0.04). Contrastingly, T_gi (p = 0.22), T_sk (p = 0.37), HR (p = 0.31), RPE (p = 0.38) and sweat loss (p = 0.17) were similar between M + AMB and M + ICE trials. RTS was similar during both low-intensity (4.9 ± 0.5 vs 4.7 ± 0.3; p = 0.10) and moderate-intensity exercise (5.3 ± 0.47 vs 5.0 ± 0.4; p = 0.09).ConclusionsPer-cooling using ice slurry ingestion marginally reduced thermal strain during low-intensity but not during moderate-intensity exercise. Ice slurry may be an effective and practical heat mitigation strategy during low-intensity exercise such as in occupational and military settings, but a greater volume should be considered to ensure its efficacy.