Cold arousal - A mechanism used by hibernating bats to reduce the energetic costs of disturbance

During the season of hibernation, temperate bats alternate between prolonged bouts of torpor with reduced body temperature and short arousals with a return to normothermy. Hibernating bats are sensitive to non-tactile stimuli and arouse following changes in microclimatic conditions or disturbance from other bats, potential predators, or humans. Here, we used temperature data loggers to register the skin temperature of 38 Myotis myotis bats over two winters (between January and March), during which regular visits were made to the hibernaculum. Two kinds of arousal were observed, normothermic (T_sk > 25 °C) and cold (T_sk < 15 °C). Although bats responded to the presence of a researcher by arousals of both kinds, cold arousals were more frequent (63.8%). We found that mass loss was not affected by the number of disturbances, however it was in positive relationship with the mass at the beginning of the observation and differed between sex and age categories. Furthermore normothermic bats crawling among cluster-mates initiated arousal cascades, which mainly consisted of cold arousals. We failed to detect any effect of age or sex on the number of arousals initiated by normothermic individuals. Warming by only a few degrees requires less energy than a normothermic arousal and we propose it is sufficient to activate the sensory system in order to assess the relevance of external stimuli. Our results indicate that cold arousals reflect a physiological and behavioural adaptation aimed at avoiding the energetic costs of disturbance that can lead to depletion of fat reserves.

European space agency's hibernation (torpor) strategy for deep space missions: Linking biology to engineering

Long-duration space missions to Mars will impose extreme stresses of physical and psychological nature on the crew, as well as significant logistical and technical challenges for life support and transportation. Main challenges include optimising overall mass and maintaining crew physical and mental health. These key scopes have been taken up as the baseline for a study by the European Space Agency (ESA) using its Concurrent Design Facility (CDF). It focussed on the biology of hibernation in reducing metabolism and hence stress, and its links to the infrastructure and life support. We concluded that torpor of crew members can reduce the payload with respect to oxygen, food and water but will require monitoring and artificial intelligence (AI) assisted monitoring of the crew. These studies additionally offer new potential applications for patient care on Earth. Keywords: Space flight, concurrent design facility, metabolic reduction.

Temperature alone is insufficient to understand hibernation energetics

Energy conservation has long been a focal point in hibernation research. A long-standing assumption is that ambient temperature (Ta) largely defines the rate of energy expenditure because of well-known relationships between Ta, metabolic rate and frequency of arousal from torpor. Body condition and humidity also affect energy expenditure but are usually considered secondary factors. We held tricolored bats (Perimyotis subflavus) in captivity under multiple environmental conditions to directly compare the importance of Ta, fat mass and humidity for hibernation energy expenditure. Fat mass was the best predictor of female mass loss, followed by Ta and humidity. However, males had less fat and adopted a more energetically conservative hibernation strategy. Our results demonstrate that understanding the evolution of behavior, physiology and ecology of hibernation requires disentangling the relative contributions of multiple drivers of hibernation energetics, and that Ta is not always the most important factor driving energy expenditure.

Stable suppression of skeletal muscle fructose-1,6-bisphosphatase during ground squirrel hibernation: Potential implications of reversible acetylation as a regulatory mechanism

Mammalian hibernation is a period that involves substantial metabolic change in order to promote survival in harsh conditions, with animals typically relying on non-carbohydrate fuel stores during long bouts of torpor. However, the use and maintenance of carbohydrate fuel stores remains important during periods of arousal from torpor as well as when exiting hibernation. Gluconeogenesis plays a key role in maintaining glucose stores; however, little is known about this process within the muscles of hibernating mammals. Here, we used 13-lined ground squirrels (Ictidomys tridecemlineatus) as our model for mammalian hibernation, and showed that skeletal muscle fructose-1,6-bisphosphatase (FBPase; EC 3.1.3.11), the rate-limiting enzyme for the gluconeogenic pathway, was suppressed during torpor as compared to the euthermic control. A physical assessment of partially purified FBPase via exposure to increasing concentrations of the denaturant urea indicated that FBPase from the two conditions were structurally distinct. Western blot analysis suggests that the kinetic and physical differences between euthermic and torpid FBPase may be derived from differential acetylation, whereby increased acetylation of the torpid enzyme makes FBPase more rigid and less active. This study increases our understanding of skeletal muscle carbohydrate metabolism during mammalian hibernation and sets forth a potentially novel mechanism for the regulation of FBPase during environmental stress.

Be cool to be far: Exploiting hibernation for space exploration

In mammals, torpor/hibernation is a state that is characterized by an active reduction in metabolic rate followed by a progressive decrease in body temperature. Torpor was successfully mimicked in non-hibernators by inhibiting the activity of neurons within the brainstem region of the Raphe Pallidus, or by activating the adenosine A1 receptors in the brain. This state, called synthetic torpor, may be exploited for many medical applications, and for space exploration, providing many benefits for biological adaptation to the space environment, among which an enhanced protection from cosmic rays. As regards the use of synthetic torpor in space, to fully evaluate the degree of physiological advantage provided by this state, it is strongly advisable to move from Earth-based experiments to 'in the field' tests, possibly on board the International Space Station.

Suppression of mitochondrial respiration by hydrogen sulfide in hibernating 13-lined ground squirrels

Hibernating mammals may suppress their basal metabolic rate during torpor by up to 95% to reduce energy expenditure during winter, but the underlying mechanisms remain poorly understood. Here we show that hydrogen sulfide (H₂S), a ubiquitous signaling molecule, is a powerful inhibitor of respiration of liver mitochondria isolated from torpid 13-lined ground squirrels, but has a weak effect on mitochondria isolated during summer and hibernation arousals, where metabolic rate is normal. Consistent with these in vitro effects, we find strong seasonal variations of in vivo levels of H₂S in plasma and increases of H₂S levels in the liver of squirrels during torpor compared to levels during arousal and summer. The in vivo changes of liver H₂S levels correspond with low activity of the mitochondrial H₂S oxidizing enzyme sulfide:quinone oxidoreductase (SQR) during torpor. Taken together, these results suggest that during torpor, H₂S accumulates in the liver due to a low SQR activity and contributes to inhibition of mitochondrial respiration, while during arousals and summer these effects are reversed, H₂S is degraded by active SQR and mitochondrial respiration rates increase. This study provides novel insights into mechanisms underlying mammalian hibernation, pointing to SQR as a key enzyme involved in the control of mitochondrial function.

Phase specific suppression of neutrophil function in hibernating Syrian hamster

Hibernation consists of alternating periods of reduced metabolism (torpor) with brief periods of metabolism similar to summer euthermia (arousal). The function of the innate immune system is reduced during hibernation, of which the underlying mechanisms are incompletely understood. Here, we studied neutrophil functionality during hibernation in Syrian hamsters. The inflammatory response to LPS-induced endotoxemia is inhibited in hibernation, partly mediated by reduced IL-6 production in early arousal. Furthermore, neutrophil pathogen binding, phagocytosis and oxidative burst is profoundly reduced in early arousal. Functionality of both summer and early arousal neutrophils was repressed in plasma from early arousal and mixed plasma from early arousal and summer euthermic, but restored by summer euthermic plasma, signifying that a plasma factor in early arousal inhibits TLR-recognition. Identification of the inhibiting factor may offer a target to modulate neutrophil function with relevance to (auto-)inflammatory diseases.

Glycopyrrolate does not ameliorate hypothermia associated bradycardia in healthy individuals: A randomized crossover trial

BACKGROUND

Hypothermia improves outcomes following ischemia-reperfusion injury. Shivering is common and can be mediated by agents such as dexmedetomidine. The combination of dexmedetomidine and hypothermia results in bradycardia. We hypothesized that glycopyrrolate would prevent bradycardia during dexmedetomidine-mediated hypothermia.

METHODS

We randomly assigned eight healthy subjects to premedication with a single 0.4 mg glycopyrrolate intravenous (IV) bolus, titrated glycopyrrolate (0.01 mg IV every 3 min as needed for heart rate <50), or no glycopyrrolate during three separate sessions of 3 h cooling. Following 1 mg/kg IV dexmedetomidine bolus, subjects received 20 ml/kg IV 4 °C saline and surface cooling (EM COOLS, Weinerdorf, Austria). We titrated dexmedetomidine infusion to suppress shivering but permit arousal to verbal stimuli. After 3 h of cooling, we allowed subjects to passively rewarm. We compared heart rate, core temperature, mean arterial blood pressure, perceived comfort and thermal sensation between groups using Kruskal-Wallis test and ANOVA.

RESULTS

Mean age was 27 (SD 6) years and most (N = 6, 75%) were male. Neither heart rate nor core temperature differed between the groups during maintenance of hypothermia (p > 0.05). Mean arterial blood pressure was higher in the glycopyrrolate bolus condition (p < 0.048). Thermal sensation was higher in the control condition than the glycopyrrolate bolus condition (p = 0.01). Bolus glycopyrrolate resulted in less discomfort than titrated glycopyrrolate (p = 0.04).

CONCLUSIONS

Glycopyrrolate did not prevent the bradycardic response to hypothermia and dexmedetomidine. Mean arterial blood pressure was higher in subjects receiving a bolus of glycopyrrolate before induction of hypothermia. Bolus glycopyrrolate was associated with less intense thermal sensation and less discomfort during cooling.

Pharmacological torpor prolongs rat survival in lethal normobaric hypoxia

Resistance to hypoxia is one of the most prominent features of natural hibernation and is expected to be present in the pharmacological torpor (PT) that simulates hibernation. We studied resistance to lethal hypoxia (3.5% oxygen content) in rats under PT. To initiate PT, we used the previously developed pharmacological composition (PC) which, after a single intravenous injection, can induce a daily decrease in Tb by 7 °C-8 °C at the environmental temperature of 22 °C-23 °C. Half-survival (median) time of rats in lethal hypoxia was found to increase from 5 ± 0.8 min in anesthetized control rats to 150 ± 12 min in rats injected with PC, which is a 30-fold increase. Behavioral tests after PT and hypoxia, including the traveling distance, the number of rearing and grooming episodes, revealed that animal responses are significantly restored within a week. It is assumed that the discovered unprecedented resistance of artificially torpid rats to lethal hypoxia may open up broad prospects for the therapeutic use of PT for preconditioning to various damaging factors, treatment of diseases, and extend the so-called "golden hour" for lifesaving interventions.

Polymorphism of winter phenotype in Siberian hamster: consecutive litters do not differ in photoresponsiveness but prolonged acclimation to long photoperiod inhibits winter molt

BACKGROUND

The theory of delayed life history effects assumes that phenotype of adult individual results from environmental conditions experienced at birth and as juvenile. In seasonal environments, being born late in the reproductive season affects timing of puberty, body condition, longevity, and fitness. We hypothesized that late-born individuals are more prone to respond to short photoperiod (SP) than early born ones. We used Siberian hamsters Phodopus sungorus, a model species characterized by high polymorphism of winter phenotype. We experimentally distinguished the effect of litter order (first or third) from the effect of exposure to long photoperiod (LP) before winter (3 months or 5 months) by manipulating the duration of LP acclimation in both litters. We predicted that, irrespective of the litter order, individuals exposed to long photoperiod for a short time have less time to gather energy resources and consequently are more prone to developing energy-conserving phenotypes. To assess effect of litter order, duration of acclimation to long days, and phenotype on basal cost of living we measured basal metabolic rate (BMR) of hamsters.

RESULTS

Individuals born in third litters had faster growth rates and were bigger than individuals from first litters, but these differences vanished before transfer to SP. Litter order or duration of LP acclimation had no effects on torpor use or seasonal body mass changes, but prolonged acclimation to LP inhibited winter molting both in first and third litters. Moreover, individuals that did not molt had significantly higher BMR in SP than those which molted to white fur. Although one phenotype usually predominated within a litter, littermates were often heterogeneous. We also found that over 10% of individuals presented late response to short photoperiod.

CONCLUSIONS

Our data indicate that duration of postnatal exposure to LP may define propensity to photoresponsiveness, regardless of the litter in which animal was born. Existence of littermates presenting different phenotypes suggests a prudent reproductive strategy of investing into offspring of varied phenotypes, that might be favored depending on environmental conditions. This strategy could have evolved in response to living in stochastic environment.

Markers of tissue remodeling and inflammation in the white and brown adipose tissues of a model hibernator

The thirteen-lined ground squirrel is a model fat-storing hibernator that nearly doubles its weight in the fall to fuel metabolism with triglycerides throughout the winter months. Hibernator brown and white adipose tissue (BAT, WAT) are important to study in terms of their inflammatory profile and tissue remodeling mechanisms since controlled and natural regulation of these processes could inform new pharmacological interventions that limit oxidative stress and inflammation in the adipose tissues of humans suffering from obesity, promote non-shivering thermogenesis-mediated weight loss, or prevent tissue damage in transplantable organs emerging from cold-storage. Thus, markers of inflammation like cytokines and soluble receptors and tissue remodeling proteins such as matrix metalloproteinases (MMPs) and tissue inhibitors of metalloproteinases (TIMPs) were investigated in normothermic, torpid, and arousing ground squirrels. Multiplex protein assays and western blotting revealed fewer changes in WAT compared to BAT. Pro-inflammatory IL-1α levels increased during torpor and soluble epidermal growth factor receptor protein levels increased during arousal in BAT. Given their known roles in other model systems, these proteins could regulate processes like adipogenesis, lipid catabolism, or cell motility. Decreased TIMP2 levels combined with maintained MMP2 or MMP3 protein levels suggested that BAT may avoid tissue remodeling until arousal. No changes in WAT inflammatory cytokines or soluble receptors as well as decreased MMP2 levels during torpor and arousal suggested inflammation and modification to the extracellular matrix is likely suppressed in WAT. This study emphasizes the fat-but-fit nature of the hibernating ground squirrel and the ability of its fat stores to suppress inflammation.

Behavioral thermoregulation in the fasted C57BL/6 mouse

Under relatively cool ambient temperatures and a caloric deficit, mice will undergo daily torpor - a short-term regulated reduction in metabolic rate with a concomitant drop in body temperature. Mice can alternatively achieve metabolic savings by utilizing behavioral changes, such as seeking a warmer environment. However, there is a lack of knowledge about the behavioral interaction between torpor utilization and thermotaxis. That is, if a fasted mouse is faced with a choice between a warm environment not conducive for torpor, and a cool environment that will induce torpor, which scenario will the fasting mouse choose? Here, the temperature preferences of fasted mice were studied using a temperature gradient device that allows a mouse to freely move along a gradient of temperatures. C57BL/6 mice were implanted with temperature telemeters that recorded location, core temperature (Tb), and activity concurrently over a 23-h period in the thermal gradient. When the gradient was on, mice preferred the warm end of the gradient when fed (71 ± 4% of the time) and even more so when fasted (84 ± 2%). When the gradient was on, the fasted minimum Tb was significantly higher (34.4 ± 0.3 °C) than when the gradient was off (27.7 ± 1.6 °C). Further, fasted mice lost significantly more weight when the gradient was off despite maintenance of a metabolically favorable lower minimum Tb in this condition. These results indicate that fasted mice not only prefer warm ambient temperatures when given the choice, but that it is also the pathway with more favorable metabolic outcomes in a period of reduced caloric intake.

Long-term pharmacological torpor of rats with feedback-controlled drug administration

The maintenance of pharmacological torpor and hypothermia (body temperature 28 °C - 33 °C) in rats for a week is presented. For this purpose, our laboratory has developed a device (BioFeedback-2) for the feed-back controlled multiple injections of small doses of a pharmacological composition that we created earlier. On the 7th day, the rat spontaneously come out of the pharmacological torpor, the body temperature returned to normal, and on the 8th day, the animal could consume food and water. The proposed approach for maintaining multi-day pharmacological torpor can be applied in medicine, as well as for protecting astronauts during long missions in space.

A refined method to monitor arousal from hibernation in the European hamster

Background

Hibernation is a physiological and behavioural adaptation that permits survival during periods of reduced food availability and extreme environmental temperatures. This is achieved through cycles of metabolic depression and reduced body temperature (torpor) and rewarming (arousal). Rewarming from torpor is achieved through the activation of brown adipose tissue (BAT) associated with a rapid increase in ventilation frequency. Here, we studied the rate of rewarming in the European hamster (Cricetus cricetus) by measuring both BAT temperature, core body temperature and ventilation frequency.

Results

Temperature was monitored in parallel in the BAT (IPTT tags) and peritoneal cavity (iButtons) during hibernation torpor-arousal cycling. We found that increases in brown fat temperature preceded core body temperature rises by approximately 48 min, with a maximum re-warming rate of 20.9℃*h^-1. Re-warming was accompanied by a significant increase in ventilation frequency. The rate of rewarming was slowed by the presence of a spontaneous thoracic mass in one of our animals. Core body temperature re-warming was reduced by 6.2℃*h^-1 and BAT rewarming by 12℃*h^-1. Ventilation frequency was increased by 77% during re-warming in the affected animal compared to a healthy animal. Inspection of the position and size of the mass indicated it was obstructing the lungs and heart.

Conclusions

We have used a minimally invasive method to monitor BAT temperature during arousal from hibernation illustrating BAT re-warming significantly precedes core body temperature re-warming, informing future study design on arousal from hibernation. We also showed compromised re-warming from hibernation in an animal with a mass obstructing the lungs and heart, likely leading to inefficient ventilation and circulation.

Reversible thrombocytopenia during hibernation originates from storage and release of platelets in liver sinusoids

Immobility is a risk factor for thrombosis due to low blood flow, which may result in activation of the coagulation system, recruitment of platelets and clot formation. Nevertheless, hibernating animals-who endure lengthy periods of immobility-do not show signs of thrombosis throughout or after hibernation. One of the adaptations of hemostasis in hibernators consists of a rapidly reversible reduction of the number of circulating platelets during torpor, i.e., the hibernation phase with reduction of metabolic rate, low blood flow and immobility. It is unknown whether these platelet dynamics in hibernating hamsters originate from storage and release, as suggested for ground squirrel, or from breakdown and de novo synthesis. A reduction in detaching forces due to low blood flow can induce reversible adhesion of platelets to the vessel wall, which is called margination. Here, we hypothesized that storage-and-release by margination to the vessel wall induces reversible thrombocytopenia in torpor. Therefore, we transfused labeled platelets in hibernating Syrian hamster (Mesocricetus auratus) and platelets were analyzed using flow cytometry and electron microscopy. The half-life of labeled platelets was extended from 20 to 30 h in hibernating animals compared to non-hibernating control hamsters. More than 90% of labeled platelets were cleared from the circulation during torpor, followed by emergence during arousal which supports storage-and-release to govern thrombocytopenia in torpor. Furthermore, the low number of immature platelets, plasma level of interleukin-1α and normal numbers of megakaryocytes in bone marrow make platelet synthesis or megakaryocyte rupture via interleukin-1α unlikely to account for the recovery of platelet counts upon arousal. Finally, using large-scale electron microscopy we revealed platelets to accumulate in liver sinusoids, but not in spleen or lung, during torpor. These results thus demonstrate that platelet dynamics in hibernation are caused by storage and release of platelets, most likely by margination to the vessel wall in liver sinusoids. Translating the molecular mechanisms that govern platelet retention in the liver, may be of major relevance for hemostatic management in (accidental) hypothermia and for the development of novel anti-thrombotic strategies.

Thermoregulation in hibernating mammals: The role of the "thyroid hormones system"

Hibernation is a unique evolutionary adaptation to conserve energy. During the pre-hibernation (i.e. fall) season, a progressive decline in core body temperature and further decrease in metabolism underlie a seasonal modulation in thermoregulation. The onset of hibernation requires marked changes in thermoregulatory attributes including adjustment in body temperature and tissue specific increases in thermogenic capacity. The hibernation season is characterized by a regulated suppression in thermogenesis allowing the onset of torpor interrupted by periodic activation of thermogenesis to sustain interbout arousals. Thyroid hormones are known to regulate both body temperature and metabolism, and for this reason, the hypothalamic-pituitary-thyroid axis and thyroid hormones have been investigated as modulators of thermogenesis in the phenomenon of hibernation, but the mechanisms remain poorly understood. In this review, we present an overview of what is known about the thermogenic roles of thyroid hormones in hibernating species across seasons and within the hibernating season (torpor-interbout arousal cycle). Overall, the hypothalamic-pituitary-thyroid axis and thyroid hormones play a role in the pre-hibernation season to enhance thermogenic capacity. During hibernation, thermogenesis is attenuated at the level of sympathetic premotor neurons within the raphe pallidus and by deiodinase expression in the hypothalamus. Further, as recent work highlights the direct effect of thyroid hormones within the central nervous system in activating thermogenesis, we speculate how similar mechanisms may occur in hibernating species to modulate thermogenesis across seasons and to sustain interbout arousals. However, further experiments are needed to elucidate the role of thyroid hormones in hibernation, moving towards the understanding that thyroid hormones metabolism, transport and availability within tissues may be the most telling indicator of thyroid status.

How the gut and liver hibernate

For hibernating mammals, the transition from summer active to winter hibernation seasons come with significant remodeling at cellular, organ and whole organism levels. This review summarizes and synthesizes what is known about hibernation-related remodeling in the gastrointestinal tract of the thirteen-lined ground squirrel, including intestinal and hepatic physiology and the gut microbiota. Hibernation alters intestinal epithelial, immune and cell survival pathways in ways that point to a protective phenotype in the face of prolonged fasting and major fluctuations in nutrient and oxygen delivery during torpor-arousal cycles. The prolonged fasting associated with hibernation alters lipid metabolism and systemic cholesterol dynamics, with both the gut and liver participating in these changes. Fasting also affects the gut microbiota, altering the abundance, composition and diversity of gut microbes and impacting the metabolites they produce in ways that may influence hibernation-related traits in the host. Finally, interventional studies have demonstrated that the hibernation phenotype confers resistance to experimental ischemia-reperfusion injury in both gut and liver, suggesting potential therapeutic roadmaps. We propose that the plasticity inherent to hibernation biology may contribute to this stress tolerance, and in the spirit of August Krogh, makes hibernators particularly valuable for study to identify solutions to certain problems.

Are southern African solitary mole-rats homeothermic or heterothermic under natural field conditions?

Abandoning of a stable body temperature (T_b), a phenomenon known as heterothermy, is an adaptation to cope mainly with a lack of food and water, especially in species inhabiting daily or seasonally variable environments. There is increasing evidence that African mammals avoid adverse conditions by heterothermy and eventually by entering torpor. Members of subterranean rodent family, the African mole-rats (Bathyergidae), are suitable candidates to study both phenomena, because of the diversity of their strategies in respect of maintaining stable T_b ranging from homeothermic species to a mammal with the most labile T_b, the naked mole-rat. Currently, there are field data on daily and seasonal T_b in one social species only and such information are lacking for any solitary mole-rat. In our study, we recorded yearly T_b in two solitary bathyergids, the Cape mole-rat Georychus capensis and the Cape dune mole-rat Bathyergus suillus from South Africa using intraperitoneally implanted dataloggers. Since this region is characterised by changing ecological characteristics, we expected either decreases of T_b within 24 h indicating daily torpor and/or longer-term decreases of T_b, which would indicate multiday torpor. Although we found seasonally phase shifted low amplitude daily T_b cycles, we did not find any remarkable and regular daily and/or seasonal T_b deviations, likely showing an absence of torpor in both species. Due to absence of this energy saving mechanism, we may speculate that both species could be vulnerable to ongoing global climatic change.

Physiological response to the odorant TMT in fully fed and calorically restricted laboratory mice

2,3,5-trimethyl-3-thiazoline (TMT) is a chemical compound that is extracted from red fox urine and can be used to artificially simulate the presence of a predator. The purpose of this study was to test the hypothesis that TMT would block entry into torpor in the calorically restricted C57Bl/6 mouse. We first demonstrated that TMT induced fear in the mouse. Exposure to TMT induced an acute freeze response (67.2 ± 6.7% of time), as compared to 6.7 ± 1.7% when exposed to water. Further, exposure to TMT for 30 min led to elevated circulating corticosterone levels, 377 ± 33 ng/ml, as compared to 29 ± 4 ng/ml when exposed to water. When mice were exposed to TMT during the dark or light phase, body temperature (T_b) dropped by 1.7 ± 0.9 °C and 0.7 ± 1.1 °C, respectively, over the first 110 min after exposure. To determine whether TMT influences daily torpor, mice were calorically restricted and exposed to either water or TMT. Mice were exposed 30 min before the start of torpor, determined by the bout of the previous day. Exposure to TMT significantly (p < 0.01) blunted the fall in the minimum T_b from 28.8 ± 0.3 °C (water) to 30.1 ± 0.6 °C (TMT) and significantly (p < 0.05) decreased the amount of time T_b was under 32 °C, from 431 ± 48 min (water) to 292 ± 78 min (TMT). These results establish that mice perceived the scent of TMT as a physiologically stressful stimulus and that T_b response is modestly blunted in the presence of that stressor. Our experiment highlights the intricate interplay between predation risk and energy conservation.

Thermal acclimation leads to variable muscle responses in two temperate labrid fishes

Temperature can be a key abiotic factor in fish distribution, as it affects most physiological processes. Specifically, temperature can affect locomotor capabilities, especially as species are exposed to temperatures nearing their thermal limits. In this study, we aimed to understand the effects of temperature on muscle in two labrids that occupy the Northwest Atlantic Ocean. When exposed to cold temperatures in autumn, cunner (Tautogolabrus adspersus) and tautog (Tautoga onitis) go into a state of winter dormancy. Transitions into dormancy vary slightly, where tautog will make short migrations to overwintering habitats while cunner overwinter in year-round habitats. To understand how muscle function changes with temperature, we held fish for 4 weeks at either 5 or 20°C and then ran muscle kinetic and workloop experiments at 5, 10 and 20°C. Following experiments, we used immunohistochemistry staining to identify acclimation effects on myosin isoform expression. Muscle taken from warm-acclimated cunner performed the best, whereas there were relatively few differences among the other three groups. Cunner acclimated at both temperatures downregulated the myosin heavy chain, suggesting a transition in fiber type from slow-oxidative to fast-glycolytic. This change did not amount to a detectable difference in muscle power production and kinetics. However, overall poor performance at cold temperatures could force these fishes into torpor to overwinter. Tautog, alternatively, retained myosin heavy chains, which likely increases locomotor capabilities when making short migrations to overwintering habitats.

Role of Akt signaling pathway regulation in the speckled mousebird (Colius striatus) during torpor displays tissue specific responses

Pronounced heterothermic responses are relatively rare among birds. Along with taxa such as hummingbirds and caprimulgids, the order Coliiformes (mousebirds) is known to possess the physiological capacity for torpor. During torpor, body temperature is greatly reduced and a bird becomes unresponsive to external stimuli until ambient temperatures return to more favorable conditions. Under such conditions, these birds are forced to rely only on their internal fuel storage for energy and show great reduction in metabolic rates by decreasing energy-expensive processes. This study investigated the role of the key insulin-Akt signaling kinase pathway involved in regulating energy metabolism and protein translation in the liver, kidney, heart, skeletal muscle, and brain of the speckled mousebird (Colius striatus). The degree of phosphorylation of well-conserved target residues with important regulatory function was examined in both the euthermic control and torpid birds. The results demonstrated marked differences in responses between the tissues with decreases in RPS6 S235/236 phosphorylation in the kidney (0.52 fold of euthermic) and muscle (0.29 fold of euthermic) as well as decreases in GS3K3β S9 in muscle (0.60 fold of euthermic) and GSK3α S21 (0.71 fold of euthermic) phosphorylation in kidney during torpor, suggesting a downregulation of this pathway. Interestingly, the liver demonstrated an increase in RPS6 S235/236 (2.89 fold increase) and P70S6K T412 (1.44 fold increase) phosphorylation in the torpor group suggesting that protein translation is maintained in this tissue. This study demonstrates that avian torpor is a complex phenomenon and alterations in this signaling pathway follow a tissue specific pattern.

Radioprotective effects of induced astronaut torpor and advanced propulsion systems during deep space travel

BACKGROUND

Human metabolic suppression is not a new concept, with 1950s scientific literature and movies demonstrating its potential use for deep space travel (Hock, 1960). An artificially induced state of metabolic suppression in the form of torpor would improve the amount of supplies required and therefore lessen weight and fuel required for missions to Mars and beyond (Choukèr et al., 2019). Transfer habitats for human stasis to Mars have been conceived (Bradford et al., 2018). Evidence suggests that animals, when hibernating, demonstrate relative radioprotection compared to their awake state. Experiments have also demonstrated relative radioprotection in conditions of hypothermia as well as during sleep (Bellesi et al., 2016 and Andersen et al., 2009). Circadian rhythm disrupted cells also appear to be more susceptible to radiation damage compared to those that are under a rhythmic control (Dakup et al., 2018). An induced torpor state for astronauts on deep space missions may provide a biological radioprotective state due to a decreased metabolism and hypothermic conditions. A regular enforced circadian rhythm might further limit DNA damage from radiation. The As Low As Reasonably Achievable (A.L.A.R.A.) radiation protection concept defines time, distance and shielding as ways to decrease radiation exposure. Whilst distance cannot be altered in space and shielding either passively or actively may be beneficial, time of exposure may be drastically decreased with improved propulsion systems. Whilst chemical propulsion systems have superior thrust to other systems, they lack high changes in velocity and fuel efficiency which can be achieved with nuclear or electric based propulsion systems. Radiation toxicity could be limited by reduced transit times, combined with the radioprotective effects of enforced circadian rhythms during a state of torpor or hibernation.

OBJECTIVES

1. Investigate how the circadian clock and body temperature may contribute to radioprotection during human torpor on deep space missions. 2. Estimate radiation dose received by astronauts during a transit to Mars with varying propulsion systems.

METHODS

We simulated three types of conditions to investigate the potential radioprotective effect of the circadian clock and decreased temperature on cells being exposed to radiation such that may be the case during astronaut torpor. These conditions were: - Circadian clock strength: strong vs weak. - Light exposure: dark-dark vs light-dark cycle - Body temperature: 37C vs hypothermia vs torpor. We estimated transit times for a mission to Mars from Earth utilizing chemical, nuclear and electrical propulsion systems. Transit times were generated using the General Mission Analysis Tool (GMAT) and Matlab. These times were then input into the National Aeronautics and Space Administration (NASA) Online Tool for the Assessment of Radiation In Space (OLTARIS) computer simulator to estimate doses received by an astronaut for the three propulsion methods.

RESULTS

Our simulation demonstrated an increase in radioprotection with decreasing temperature. The greatest degree of radioprotection was shown in cells that maintained a strong circadian clock during torpor. This was in contrast to relatively lower radioprotection in cells with a weak clock during normothermia. We were also able to demonstrate that if torpor weakened the circadian clock, a protective effect could be partially restored by an external drive such as lighting schedules to aid entrainment i.e.: Blue light exposure for periods of awake and no light for rest times For the propulsion simulation, estimated transit times from Earth to Mars were 258 days for chemical propulsion with 165.9mSv received, 209 days for nuclear propulsion with 134.4mSv received and 80 days for electrical propulsion with 51.4mSv received.

CONCLUSION

A state of torpor for astronauts on deep space missions may not only improve weight, fuel and storage requirements but also provide a potential biological radiation protection strategy. Moreover, maintaining a controlled circadian rhythm during torpor conditions may aid radioprotection. In the not too distant future, propulsion techniques will be improved to limit transit time and hence decrease radiation dose to astronauts. Limiting exposure time and enhancing physiological radioprotection during transit could provide superior radioprotection benefits compared with active and passive radiation shielding strategies alone.

Phagocyte activity reflects mammalian homeo- and hetero-thermic physiological states

BACKGROUND

Emergence of both viral zoonoses from bats and diseases that threaten bat populations has highlighted the necessity for greater insights into the functioning of the bat immune system. Particularly when considering hibernating temperate bat species, it is important to understand the seasonal dynamics associated with immune response. Body temperature is one of the factors that modulates immune functions and defence mechanisms against pathogenic agents in vertebrates. To better understand innate immunity mediated by phagocytes in bats, we measured respiratory burst and haematology and blood chemistry parameters in heterothermic greater mouse-eared bats (Myotis myotis) and noctules (Nyctalus noctula) and homeothermic laboratory mice (Mus musculus).

RESULTS

Bats displayed similar electrolyte levels and time-related parameters of phagocyte activity, but differed in blood profile parameters related to metabolism and red blood cell count. Greater mouse-eared bats differed from mice in all phagocyte activity parameters and had the lowest phagocytic activity overall, while noctules had the same quantitative phagocytic values as mice. Homeothermic mice were clustered separately in a high phagocyte activity group, while both heterothermic bat species were mixed in two lower phagocyte activity clusters. Stepwise regression identified glucose, white blood cell count, haemoglobin, total dissolved carbon dioxide and chloride variables as the best predictors of phagocyte activity. White blood cell counts, representing phagocyte numbers available for respiratory burst, were the best predictors of both time-related and quantitative parameters of phagocyte activity. Haemoglobin, as a proxy variable for oxygen available for uptake by phagocytes, was important for the onset of phagocytosis.

CONCLUSIONS

Our comparative data indicate that phagocyte activity reflects the physiological state and blood metabolic and cellular characteristics of homeothermic and heterothermic mammals. However, further studies elucidating trade-offs between immune defence, seasonal lifestyle physiology, hibernation behaviour, roosting ecology and geographic patterns of immunity of heterothermic bat species will be necessary. An improved understanding of bat immune responses will have positive ramifications for wildlife and conservation medicine.

Gut reaction! Neotropical nectar-feeding bats responses to direct and indirect costs of extreme environmental temperatures

One of the consequences of anthropogenic climate change is an increase in the frequency and intensity of extreme weather events. These events have caused mass mortality of different species of wildlife, including bats. In this study, we exposed two species of neotropical nectar-feeding bats that live in contrasting environmental conditions (A. geoffroyi and L. yerbabuenae) to extreme high and low temperatures while offering them diets with different energy content. This experimental approach allowed us to determine their thermal and behavioral responses, and to identify environmental conditions that impose high physiologic costs to these species. To determine how bats' responded, we monitored both changes in their body masses and skin temperatures. Both bat species responded differently, with L. yerbabuenae spending more time in normothermia at high temperatures than A. geoffroyi. While both species presented torpor, they used it differently. Torpor allowed A. geoffroyi to maintain and increase body mass at intermediate and low ambient temperatures. At the same time, L. yerbabuenae used torpor only when facing cold ambient temperatures and low-quality food. Understanding the mechanisms that allow species to face changes in their environment is essential given the current climate trends and the fact that the loss of these species could have significant negative consequences in tropical and subtropical ecosystems.

Multi-tissue profile of NFκB pathway regulation during mammalian hibernation

Hibernators have evolved effective mechanisms to overcome the challenges of torpor-arousal cycling. This study focuses on the antioxidant and inflammatory defenses under the control of the redox-sensitive and inflammatory-centered NFκB transcription factor in the thirteen-lined ground squirrel (Ictidomys tridecemlineatus), a well-established model of mammalian hibernation. While hibernators significantly depress oxygen consumption and overall metabolic rate during torpor, arousal brings with it a rapid increase in respiration that is associated with an influx of reactive oxygen species. As such, hibernators employ a variety of antioxidant defenses to combat oxidative damage. Herein, we used Luminex multiplex technology to examine the expression of key proteins in the NFκB transcriptional network, including NFκB, super-repressor IκBα, upstream activators TNFR1 and FADD, and downstream target c-Myc. Transcription factor DNA-binding ELISAs were also used to measure the relative degree of NFκB binding to DNA during hibernation. Analyses were performed across eight different tissues, cerebral cortex, brainstem, white and brown adipose tissue, heart, liver, kidney, and spleen, during euthermic control and late torpor to highlight tissue-specific NFκB mediated cytoprotective responses against oxidative stress experienced during torpor-arousal. Our findings demonstrated brain-specific NFκB activation during torpor, with elevated levels of upstream activators, inactive-phosphorylated IκBα, active-phosphorylated NFκB, and enhanced NFκB-DNA binding. Protein levels of downstream protein, c-Myc, also increased in the brain and adipose tissues during late torpor. The results show that NFκB regulation might serve a critical neuroprotective and cytoprotective role in hibernating brains and selective peripheral tissue.