Maintenance Of Torpor Research Articles

Ghrelin is essential for lowering blood pressure during torpor.

Daily torpor is an active hypothermic phenomenon that is observed in some mammals and birds during fasting. A decrease in blood pressure has also been observed in torpor; however, there remains a lack of knowledge of the underlying mechanism. We have previously reported that ghrelin, an orexigenic hormone, has a hypothermic effect and is essential for the induction and maintenance of torpor. It is also known that the ghrelin secretion is enhanced during fasting and that ghrelin receptors are distributed in the cardiovascular system. Therefore, this study was conducted to test the hypothesis that ghrelin is actively involved in the regulation of blood pressure during torpor induction. Male wild-type and ghrelin gene-deficient mice were generated by homologous recombination as previously reported. Mice, 10 weeks old, were included in this study and housed five per cage. The mice were maintained on a 12-h light/dark cycle (lights on from 7:00 to 19:00) with access to food and water ad libitum. The continuous measurement of blood pressure using a telemetry system showed that induction of torpor by fasting did not decrease blood pressure in ghrelin gene-deficient mice. The analysis of heart rate variability revealed that sympathetic nerve activity was predominant in ghrelin-deficient mice during fasting. Furthermore, these features were cancelled by administration of a ghrelin receptor agonist and were comparable to those in wild-type mice. In this study, we showed that blood pressure was elevated in ghrl-/- mice and that the blood pressure rhythm was abnormal. Furthermore, we showed that the ghrelin gene deficiency does not cause sufficient blood pressure reduction upon entry into the torpor, and that the administration of the ghrelin receptor agonist, GHRP-6, causes blood pressure reduction associated with torpor. Thus, we have shown for the first time that the active role of ghrelin is essential for active blood pressure reduction associated with torpor, and that this action is mediated by the inhibition of sympathetic nerve activity by ghrelin.

Mitochondrial respiration in rats during hypothermia resulting from central drug administration.

The ability to induce a hypothermia resembling that of natural torpor would be greatly beneficial in medical and non-medical fields. At present, two procedures based on central nervous pharmacological manipulation have been shown to be effective in bringing core body temperature well below 30°C in the rat, a non-hibernator: the first, based on the inhibition of a key relay in the central thermoregulatory pathway, the other, based on the activation of central adenosine A1 receptors. Although the role of mitochondria in the activation and maintenance of torpor has been extensively studied, no data are available for centrally induced hypothermia in non-hibernators. Thus, in the present work the respiration rate of mitochondria in the liver and in the kidney of rats following the aforementioned hypothermia-inducing treatments was studied. Moreover, to have an internal control, the same parameters were assessed in a well-consolidated model, i.e., mice during fasting-induced torpor. Our results show that state 3 respiration rate, which significantly decreased in the liver of mice, was unchanged in rats. An increase of state 4 respiration rate was observed in both species, although it was not statistically significant in rats under central adenosine stimulation. Also, a significant decrease of the respiratory control ratio was detected in both species. Finally, no effects were detected in kidney mitochondria in both species. Overall, in these hypothermic conditions liver mitochondria of rats remained active and apparently ready to be re-activated to produce energy and warm up the cells. These findings can be interpreted as encouraging in view of the finalization of a translational approach to humans.

Specific shifts in the endocannabinoid system in hibernating brown bears

In small hibernators, global downregulation of the endocannabinoid system (ECS), which is involved in modulating neuronal signaling, feeding behavior, energy metabolism, and circannual rhythms, has been reported to possibly drive physiological adaptation to the hibernating state. In hibernating brown bears (Ursus arctos), we hypothesized that beyond an overall suppression of the ECS, seasonal shift in endocannabinoids compounds could be linked to bear’s peculiar features that include hibernation without arousal episodes and capacity to react to external disturbance. We explored circulating lipids in serum and the ECS in plasma and metabolically active tissues in free-ranging subadult Scandinavian brown bears when both active and hibernating. In winter bear serum, in addition to a 2-fold increase in total fatty acid concentration, we found significant changes in relative proportions of circulating fatty acids, such as a 2-fold increase in docosahexaenoic acid C22:6 n-3 and a decrease in arachidonic acid C20:4 n-6. In adipose and muscle tissues of hibernating bears, we found significant lower concentrations of 2-arachidonoylglycerol (2-AG), a major ligand of cannabinoid receptors 1 (CB1) and 2 (CB2). Lower mRNA level for genes encoding CB1 and CB2 were also found in winter muscle and adipose tissue, respectively. The observed reduction in ECS tone may promote fatty acid mobilization from body fat stores, and favor carbohydrate metabolism in skeletal muscle of hibernating bears. Additionally, high circulating level of the endocannabinoid-like compound N-oleoylethanolamide (OEA) in winter could favor lipolysis and fatty acid oxidation in peripheral tissues. We also speculated on a role of OEA in the conservation of an anorexigenic signal and in the maintenance of torpor during hibernation, while sustaining the capacity of bears to sense stimuli from the environment.

Physiological Differences Between Torpor and Forced Hypothermia in Mice

Clinical advantages of forced hypothermia (FH) are mitigated by homeostatic mechanisms invoked to prevent core cooling. Torpor, a physiological state used by mice as a survival strategy, lowers body temperature (Tb) without eliciting such mechanisms. It was hypothesized that torpor is different from FH at multiple levels, including autonomic balance, aerobic/anaerobic metabolism, and hypoxia related gene expression in heart and liver. To test this hypothesis, female C57Bl/6 mice were implanted with ECG/ Tb telemeters and subjected to both conditions‐ FH and naturally induced torpor‐ in a random crossover design. During cooling and at a Tb of 30°C, mice showed a heart rate (HR) of 185 ± 18 bpm during torpor, and a HR of 484 ± 20 bpm during forced hypothermia. Analysis of hysteresis curves of Tb vs HR showed parasympathetic influence during entrance and maintenance of torpor, which shifted to sympathetic influence during recovery. Mice undergoing FH showed consistent and maximal sympathetic activity. ECG tracings showed evidence of substantial shivering during rewarming from both states, but showed vigorous shivering during cooling only in FH. Circulating lactate levels were significantly elevated in FH relative to torpor, (3.9 ± 0.9 vs 0.6 ± 0.2 mM, respectively), suggesting an increased reliance on anaerobic metabolism during forced hypothermia. Steady state mRNA levels in the heart and liver from torpid mice, as assessed by qRTPCR arrays (80 different genes), showed a unique pattern of expression compared to euthermic and forced hypothermic mice. In conclusion, data in this study support the hypothesis that forced and natural hypothermia only have low Tb in common, and have vastly different sympathetic, shivering, anaerobic, and hepatic/cardiac gene expression profiles.

RAISING THE ‘DEAD’ – REPERFUSION FROM TORPOR

> Kelly Drew discusses Paul Chatfield and Charles Lyman's classic paper ‘Circulatory changes during the process of arousal in the hibernating hamster’, published in the American Journal of Physiology in 1950. Over 60 years ago, Paul Chatfield and Charles Lyman described fundamental

Determination In Vivo of Newly Synthesized Gene Expression in Hamsters During Phases of the Hibernation Cycle

This study measured in vivo synthesis of total RNA and protein from cortex, cerebellum and midbrain/brainstem and 6 major organs from Syrian hamsters (Mesocricetus auratus) during (a) 33 h of torpor (body temperature 5-6 degrees C); (b) 90 min of the early arousal; (c) 90 min of the middle arousal; (d) 90 min in cold adapted cenothermic (CEN) hamsters of the same circannual period. Appropriate physiological parameters were used to confirm the phase of the hibernation cycle during infusion and incorporation of [3H]-uridine and [14C]-leucine. In torpor, RNA synthesis was 5-25% of CEN levels depending upon tissue. In brain and heart mRNA was not preferentially synthesized. Protein was synthesized at low, tissue specific levels during torpor. Initiation of arousal and the warming of anterior organs via non-shivering thermogenesis during the early arousal occurred without measurable synthesis of RNA or proteins. Tissue specific levels of RNA and protein synthesis occurred later after shivering thermogenesis had been recruited and was strongly influenced by thermal gradients in the body. In the middle arousal phase, protein synthesis is most active in the brain despite modest synthesis of RNA and mRNA. The majority of molecular processing required for the induction and maintenance of torpor and the arousal from torpor up until the onset of shivering thermogenesis occurs during the cenothermic period before the hamster initiates the hibernation cycle.

Diurnal rhythms of 5-HT 1A and 5-HT 2 receptor binding in euthermic and torpor prone deermice, Peromyscus maniculatus

Deermice display both spontaneous and induced daily torpor bouts, attaining minimum body temperatures of 15–20°C. There is evidence that brain serotonin may be involved in the initiation and/or maintenance of torpor. Inhibition of serotonin [5-hydroxytryptamine (5-HT)] synthesis markedly reduces the duration and depth of torpor. Because a certain percentage of deermice will not enter torpor under any circumstances, we were able to compare 5-HT receptor subtypes in deermice that readily enter into torpor (TP) and in non-torpor prone (NTP) animals. Deermice were trapped in the wild and subjected to food rationing and low ambient temperature and then sacrificed either in a normothermic or torpid state at 11:00 p.m. or 11:00 a.m. Whole brain was assayed for 5-HT 1A and 5-HT 2 receptor differences using [ 3H]8-OH-DPAT and [ 3H]ketanserin, respectively. The B max values for 5-HT 1A receptors were significantly greater in both TP and NTP animals sacrificed at 11:00 p.m. compared to animals sacrificed at 11:00 a.m. In contrast, the density of 5-HT 2 receptors was significantly greater in animals sacrificed at 11:00 a.m. compared to animals sacrificed at 11:00 p.m. This is consistent with the opposing functions of these receptors in the regulation of temperature and sleep. The affinity ( K d) of each receptor was unchanged. A comparison of TP and NTP animals sacrificed at the same time of day revealed no significant differences in either B max or in K d values, indicating that differences in 5-HT 1A and 5-HT 2 receptors may not explain the heterogeneity of deermice in their ability to enter torpor. However, the diurnal fluctuation in 5-HT receptors described here may be involved in the serotonergic regulation of hormone rhythmicity and the onset of torpor in deermice.