Allometries of the durations of torpid and euthermic intervals during mammalian hibernation: A test of the theory of metabolic control of the timing of changes in body temperature

Summary The durations of the intervals of torpor and euthermia during mammalian hibernation were found to be dependent on body mass. These relationships support the concept that the timing of body temperature changes is controlled by some metabolic process. Data were obtained from species spanning nearly three orders of magnitude in size, that were able to hibernate for over six months without food at 5°C. The timing of body temperature changes was determined from the records of copper-constantan thermocouples placed directly underneath each animal. Because all species underwent seasonal changes in their patterns of hibernation, animals were compared in midwinter when the duration of euthermic intervals was short and relatively constant and when the duration of torpid intervals was at its longest. Large hibernators remained euthermic longer than small hibernators (Fig. 2). This was true among and within species. The duration of euthermic intervals increased with mass at the same rate (mass^0.38) that mass-specific rates of euthermic metabolism decrease, suggesting that hibernators remain at high body temperatures until a fixed amount of metabolism has been completed. These data are consistent with the theory that each interval of euthermia is necessary to restore some metabolic imbalance that developed during the previous bout of torpor. In addition, small species remained torpid for longer intervals, than large species (Fig. 3). The absolute differences between different-sized species were large, but, on a proportional basis, they were comparatively slight. Mass-specific rates of metabolism during torpor also appear to be much less dependent on body mass than those during euthermia, but the precision of these metabolic measurements is insufficient for them to provide a conclusive test of the metabolic theory. Finally, small species with high mass-specific rates of euthermic metabolism are under tighter energetic constraints during dormancy than large species. The data presented here show that, in midwinter, small species compensate both by spending less time at high body temperatures following each arousal episode and by arousing less frequently, although the former is far more important energetically than the latter.     

Pineal melatonin rhythms in female Turkish hamsters: effects of photoperiod and hibernation

Daily rhythms of pineal and serum melatonin content were characterized for adult female Turkish hamsters (Mesocricetus brandti) exposed to long days (16L:8D, 22 degrees C) or after transfer to short days (10L:14D, 22 degrees C). The nocturnal peak of pineal melatonin content was found to be approximately 3 b greater in duration on short than on long days. Changes in levels of serum melatonin closely paralleled those of pineal melatonin. Thus, an effect of photoperiod on synthesis and secretion of pineal melatonin was demonstrated. In a separate experiment, female hamsters were induced to hibernate by exposure to a short-day, cold environment (10L:14D, 6 degrees C). During the 4 to 5-mo hibernation season, Turkish hamsters are known to display 4 to 8-day hours of torpor (body temperature = 7-9 degrees C) alternating with 1 to 3-day intervals of euthermia (body temperature = 35-37 degrees C). Little evidence of nocturnal synthesis or secretion of pineal melatonin was detected in females sampled during torpor. However, animals sampled during the first day after arousal from a torpor bout displayed melatonin rhythms no different in phase or amplitude from those seen in females held at 22 degrees C. Thus, despite the absence of pineal melatonin output during torpor, the pineal gland of hibernating Turkish hamsters produces an appropriately phased, rhythmic melatonin signal during intervals of euthermia.     

Daily torpor and hibernation in birds and mammals

Many birds and mammals drastically reduce their energy expenditure during times of cold exposure, food shortage, or drought, by temporarily abandoning euthermia, i.e. the maintenance of high body temperatures. Traditionally, two different types of heterothermy, i.e. hypometabolic states associated with low body temperature (torpor), have been distinguished: daily torpor, which lasts less than 24 h and is accompanied by continued foraging, versus hibernation, with torpor bouts lasting consecutive days to several weeks in animals that usually do not forage but rely on energy stores, either food caches or body energy reserves. This classification of torpor types has been challenged, suggesting that these phenotypes may merely represent extremes in a continuum of traits. Here, we investigate whether variables of torpor in 214 species (43 birds and 171 mammals) form a continuum or a bimodal distribution. We use Gaussian‐mixture cluster analysis as well as phylogenetically informed regressions to quantitatively assess the distinction between hibernation and daily torpor and to evaluate the impact of body mass and geographical distribution of species on torpor traits. Cluster analysis clearly confirmed the classical distinction between daily torpor and hibernation. Overall, heterothermic endotherms tend to be small; hibernators are significantly heavier than daily heterotherms and also are distributed at higher average latitudes (～35°) than daily heterotherms (～25°). Variables of torpor for an average 30 g heterotherm differed significantly between daily heterotherms and hibernators. Average maximum torpor bout duration was >30‐fold longer, and mean torpor bout duration >25‐fold longer in hibernators. Mean minimum body temperature differed by ～13°C, and the mean minimum torpor metabolic rate was ～35% of the basal metabolic rate (BMR) in daily heterotherms but only 6% of BMR in hibernators. Consequently, our analysis strongly supports the view that hibernators and daily heterotherms are functionally distinct groups that probably have been subject to disruptive selection. Arguably, the primary physiological difference between daily torpor and hibernation, which leads to a variety of derived further distinct characteristics, is the temporal control of entry into and arousal from torpor, which is governed by the circadian clock in daily heterotherms, but apparently not in hibernators.     

The role of succinate dehydrogenase and oxaloacetate in metabolic suppression during hibernation and arousal

Hibernation elicits a major reduction in whole-animal O₂ consumption that corresponds with active suppression of liver mitochondrial electron transport capacity at, or downstream of, succinate dehydrogenase (SDH). During arousal from the torpor phase of hibernation this suppression is reversed and metabolic rates rise dramatically. In this study, we used the 13-lined ground squirrel (Ictidomys tridecemlineatus) to assess isolated liver mitochondrial respiration during the torpor phase of hibernation and various stages of arousal to elucidate a potential role of SDH in metabolic suppression. State 3 and state 4 respiration rates were seven- and threefold lower in torpor compared with the summer-active and interbout euthermic states. Respiration rates increased during arousal so that when body temperature reached 30°C in late arousal, state 3 and state 4 respiration were 3.3- and 1.8-fold greater than during torpor, respectively. SDH activity was 72% higher in interbout euthermia than in torpor. Pre-incubating with isocitrate [to alleviate oxaloacetate (OAA) inhibition] increased state 3 respiration rate during torpor by 91%, but this rate was still fourfold lower than that measured in interbout euthermia. Isocitrate pre-incubation also eliminated differences in SDH activity among hibernation bout stages. OAA concentration correlated negatively with both respiration rates and SDH activity. These data suggest that OAA reversibly inhibits SDH in torpor, but cannot fully account for the drastic metabolic suppression observed during this hibernation phase.     

The temporal organization of daily torpor and hibernation: circadian and circannual rhythms

Mammals and birds have evolved the ability to maintain a high and constant body temperature T_b over a wide range of ambient temperatures T_a using endogenous heat production. In many, especially small endotherms, cost for thermoregulatory heat production can exceed available energy; to overcome these energetic bottlenecks, they enter a state of torpor (a regulated reduction of T_b and metabolic rate). Since the occurrence of torpor in many species is a seasonal event and occurs at certain times of the day, we review whether circadian and circannual rhythms, important in the timing of biological events in active animals, also play an important role during torpor when T_b is reduced substantially and may even fall below 0°C. The two distinct patterns of torpor, hibernation (prolonged torpor) and daily torpor, differ substantially in their interaction with the circadian system. Daily torpor appears to be integrated into the normal circadian rhythm of activity and rest, although torpor is not restricted only to the normal rest phase of an animal. In contrast, hibernation can last for several days or even weeks, although torpor never spans the entire hibernation season, but is interrupted by periodic arousals and brief normothermic periods. Clearly, a day is no longer divided in activity and rest, and at first glance the role of the circadian system appears negligible. However, in several hibernators, arousals not only follow a regular pattern consistent with a circadian rhythm, but also are entrainable by external stimuli such as photoperiod and T_a. The extent of the interaction between the circadian and circannual system and hibernation varies among species. Biological rhythms of hibernators for which food availability appears to be predictable seasonally and that hibernate in deep and sealed burrows show little sensitivity to external stimuli during hibernation and hence little entrainability of arousal events. In contrast, opportunistic hibernators, which some times use arousals for foraging and hibernate in open and accessible hibernacula, are susceptible to external zeitgebers. In opportunistic hibernators, the circadian system plays a major role in maintaining synchrony between the normal day-night cycle and occasional foraging. Although the daily routine of activity and rest is abandoned during hibernation, the circadian system appears to remain functional, and there is little evidence it is significantly affected by low T_b. (Chronobiology International, 17(2), 103-128, 2000)     

Physiological ecology of cheirogaleid primates: variation in hibernation and torpor

Torpor (i.e. the reduction of body temperature and metabolic rate for less than 24 h) and hibernation (i.e. torpor phases longer than 24 h) are among the most extreme adaptations to seasonality in primate habitats. Although widespread among mammals, this form of extreme thermoregulation is rare among primates and is reported only for species of the cheirogaleid family. Understanding their physiological ecology is crucial for many aspects of cheirogaleid socioecology like their social organization and their mating systems. This paper first provides an overview of published information on hibernation and torpor and identifies a patchy distribution for the occurrence of hibernation across genera, species and populations. Based on a review of published studies from the wild and from captivity, we then propose a possible explanation for variation in hibernation behavior among Microcebus species and populations. Accordingly, the amount of energy that can be saved during torpor early in the lean dry season, which is determined by the minimum ambient temperature will be decisive. Only where temperatures are low, early dry season torpor bouts will be long enough to save enough energy to build up fat reserves for longer bouts of hibernation. Finally, we summarize information on the causal factors for the occurrence of hibernation by analyzing sex differences within populations. Further physiological studies on other cheirogaleid species are needed to identify the phylogenetic origin of hibernation in primates.     

The role of polyunsaturated fatty acids in the expression of torpor by mammals: a review

Heterothermic mammals increase the proportion of polyunsaturated fatty acids (PUFA) in their body fats prior to entering torpor. Because PUFA have low melting points, it is thought that they play an important role in maintaining the fluidity of depot fats and membrane phospholipids at low body temperatures. However, PUFA are more prone to autoxidation when exposed to reactive oxygen species (ROS) during torpor and during the periodic arousals that characterize hibernation. A lack of PUFA or an excess of PUFA may constrain the use of torpor by heterothermic mammals. We performed a mixed model meta-analysis of 17 controlled-feeding studies to test the effect of dietary PUFA on the depth and expression of torpor by daily heterotherms and hibernators. We also reviewed the literature on the PUFA content of the diet and depot fats of heterothermic mammals to address two principal topics: (1) Do low dietary levels of PUFA reduce the expression of torpor under laboratory conditions and, if so, are free-ranging animals constrained by a lack of PUFA? (2) Do high dietary levels of PUFA result in a reduction in the use, depth, and duration of torpor and, if so, do free-ranging animals seek to optimize rather than maximize PUFA intake? Low-PUFA diets consistently increase the lower setpoint for body temperature and minimum metabolic rate for both hibernators and daily heterotherms. Above the lower setpoint, low-PUFA diets usually increase body temperature and metabolic rate and decrease the duration of torpor bouts and this effect is similar for hibernators and daily heterotherms. Free-ranging rodent hibernators have dietary PUFA intakes that are far higher than those of the low-PUFA diets offered in controlled-feeding experiments, so these hibernators may never experience the constraints associated with a lack of PUFA. Diets of free-ranging insectivorous bats and echidnas have PUFA levels that are less than half as high as those offered in experimental low-PUFA diets, yet they exhibit deep and extended bouts of torpor. We argue that alternate mechanisms exist for maintaining the fluidity of body fats and that high-PUFA intake may not be a prerequisite for deep and extended bouts of torpor. Four studies indicate that animals that were fed high-PUFA diets are reluctant to enter torpor and show shallower and shorter torpor bouts. Although authors attribute this response to autoxidation, these animals did not have a higher PUFA content in their depot fats than animals where PUFA was shown to enhance torpor. We suggest that these contradictory results indicate inter-specific or inter-individual variation in the ability to control ROS and limit autoxidation of PUFA. High dietary levels of PUFA will constrain the expression of torpor only when the oxidative challenge exceeds the capacity of the antioxidant defence system. Studies of diet selection indicate that insectivorous species with low dietary PUFA levels seek to maximize PUFA intake. However, herbivorous species that have access to plants and plant parts of high-PUFA content do not appear to maximize PUFA intake. These data suggest that animals attempt to optimize rather than maximize PUFA intake. The effect of PUFA should be viewed in the light of a cost-benefit trade-off, where the benefit of high-PUFA intake is an easier access to low body temperatures and the cost is increased risk of autoxidation.     

Cytoskeletal Regulation Dominates Temperature-Sensitive Proteomic Changes of Hibernation in Forebrain of 13-Lined Ground Squirrels

13-lined ground squirrels, Ictidomys tridecemlineatus, are obligate hibernators that transition annually between summer homeothermy and winter heterothermy – wherein they exploit episodic torpor bouts. Despite cerebral ischemia during torpor and rapid reperfusion during arousal, hibernator brains resist damage and the animals emerge neurologically intact each spring. We hypothesized that protein changes in the brain underlie winter neuroprotection. To identify candidate proteins, we applied a sensitive 2D gel electrophoresis method to quantify protein differences among forebrain extracts prepared from ground squirrels in two summer, four winter and fall transition states. Proteins that differed among groups were identified using LC-MS/MS. Only 84 protein spots varied significantly among the defined states of hibernation. Protein changes in the forebrain proteome fell largely into two reciprocal patterns with a strong body temperature dependence. The importance of body temperature was tested in animals from the fall; these fall animals use torpor sporadically with body temperatures mirroring ambient temperatures between 4 and 21°C as they navigate the transition between summer homeothermy and winter heterothermy. Unlike cold-torpid fall ground squirrels, warm-torpid individuals strongly resembled the homeotherms, indicating that the changes observed in torpid hibernators are defined by body temperature, not torpor per se. Metabolic enzymes were largely unchanged despite varied metabolic activity across annual and torpor-arousal cycles. Instead, the majority of the observed changes were cytoskeletal proteins and their regulators. While cytoskeletal structural proteins tended to differ seasonally, i.e., between summer homeothermy and winter heterothermy, their regulatory proteins were more strongly affected by body temperature. Changes in the abundance of various isoforms of the microtubule assembly and disassembly regulatory proteins dihydropyrimidinase-related protein and stathmin suggested mechanisms for rapid cytoskeletal reorganization on return to euthermy during torpor-arousal cycles.     

Remodeling mitochondrial membranes during arousal from hibernation

During arousal from hibernation, body temperature (T(b)) increases by ～30°C and liver mitochondrial respiration increases threefold in as little as 2 h. We analyzed liver mitochondria purified from ground squirrels (Ictidomys tridecemlineatus) to see whether membrane phospholipids were remodeled during spontaneous arousal. Cardiolipin content did not change among animals in torpor (T ～ 5°C), the early phase of arousal (T ～ 15°C), late arousal (T ～ 30°C), interbout euthermia (T ～ 37°C), and summer-active animals (T ～ 37°C) that do not hibernate. Phosphatidylcholine content increased in late arousal relative to interbout euthermia, while phosphatidylethanolamine decreased. Phospholipid monounsaturated fatty acids (MUFAs) did not change throughout arousal, but polyunsaturated fatty acids (PUFAs) and MUFA/PUFA decreased and increased, respectively. In the fatty acid conjugates of phospholipids, neither unsaturation index nor n-3/n-6 differed. Few changes in individual fatty acids were noted, but palmitoleic acid (16:1, n-7) was higher in interbout euthermia and summer. Although 16:1 accounted for less than 1.5% of phospholipid fatty acids, it correlated strongly and positively with succinate-fueled state 3 mitochondrial respiration. No other phospholipid characteristic measured here correlated with mitochondrial respiration. These data show that mitochondrial membranes are remodeled rapidly during arousal, but the contribution to reversible suppression of mitochondrial respiration remains unclear.     

Oscillatory pattern in oxygen consumption of Hummingbirds

Mammals and birds offer the most conspicuous example of homeothermic endothermy, a metabolic feature that implies maintenance of a constant body temperature along broad ranges of ambient temperature. The concept of homeothermic endothermy has been developed in close association with the terms thermoneutral zone and basal metabolic rate. These two metabolic parameters, however, are not easily estimated in micro-endotherms, a difficulty that might emerge from intrinsic aspects of endothermy in minute animals. To address this issue, we used empirical work derived from theoretical considerations. Our theoretical analysis is based on a model of body temperature control by shifts in metabolic rate, and assumes that micro-endotherms lose heat very quickly due to body size, and exhibit a remarkable capacity to rapidly increase metabolic output. We found that these two metabolic traits can lead to non-equilibrium metabolic rate and body temperature. We then measured metabolic rate and body temperature during euthermia in two species of hummingbirds, and analyzed data using the χ² periodogram statistic and a power spectral analysis. We found long-range correlation in both oxygen consumption and body temperature during euthermia, a finding that suggests non-random 1/f oscillations. A similar pattern was not found in the rat, a much larger endotherm. Hummingbirds, then, do not appear to maintain steady-state metabolic conditions during euthermia. If, as we suggest, this pattern applies to micro-endotherms in general, the traditional concepts of thermoneutral zone and basal rate of metabolism might not apply to these animals.     

Seasonal protein changes support rapid energy production in hibernator brainstem

During the torpor phase of mammalian hibernation when core body temperature is near 4°C, the autonomic system continues to maintain respiration, blood pressure and heartbeat despite drastic reductions in brain activity. In addition, the hibernator’s neuronal tissues enter into a protected state in which the potential for ischemia–reperfusion injury is markedly minimized. Evolutionary adaptations for continued function and neuroprotection throughout cycles of torpor and euthermia in winter are predicted to manifest themselves partly in changes in the brainstem proteome. Here, we compare the soluble brainstem protein complement from six summer active ground squirrels and six in the early torpor (ET) phase of hibernation. Thirteen percent of the ~1,500 quantifiable 2D gel spots alter significantly from summer to ET; the proteins identified in these differing spots are known to play roles in energy homeostasis via the tricarboxylic acid cycle (8 proteins), cytoarchitecture and cell motility (14 proteins), anabolic protein processes (13 proteins), redox control (11 proteins) and numerous other categories including protein catabolism, oxidative phosphorylation, signal transduction, glycolysis, intracellular protein trafficking and antiapoptotic function. These protein changes represent, at least in part, the molecular bases for restructuring of cells in the brainstem, a shift away from glucose as the primary fuel source for brain in the winter, and the generation of a streamlined mechanism capable of efficient and rapid energy production and utilization during the torpor and arousal cycles of hibernation.     

THE TEMPORAL ORGANIZATION OF DAILY TORPOR AND HIBERNATION: CIRCADIAN AND CIRCANNUAL RHYTHMS

Mammals and birds have evolved the ability to maintain a high and constant body temperature T_b over a wide range of ambient temperatures T_a using endogenous heat production. In many, especially small endotherms, cost for thermoregulatory heat production can exceed available energy; to overcome these energetic bottlenecks, they enter a state of torpor (a regulated reduction of T_b and metabolic rate). Since the occurrence of torpor in many species is a seasonal event and occurs at certain times of the day, we review whether circadian and circannual rhythms, important in the timing of biological events in active animals, also play an important role during torpor when T_b is reduced substantially and may even fall below 0°C. The two distinct patterns of torpor, hibernation (prolonged torpor) and daily torpor, differ substantially in their interaction with the circadian system. Daily torpor appears to be integrated into the normal circadian rhythm of activity and rest, although torpor is not restricted only to the normal rest phase of an animal. In contrast, hibernation can last for several days or even weeks, although torpor never spans the entire hibernation season, but is interrupted by periodic arousals and brief normothermic periods. Clearly, a day is no longer divided in activity and rest, and at first glance the role of the circadian system appears negligible. However, in several hibernators, arousals not only follow a regular pattern consistent with a circadian rhythm, but also are entrainable by external stimuli such as photoperiod and T_a. The extent of the interaction between the circadian and circannual system and hibernation varies among species. Biological rhythms of hibernators for which food availability appears to be predictable seasonally and that hibernate in deep and sealed burrows show little sensitivity to external stimuli during hibernation and hence little entrainability of arousal events. In contrast, opportunistic hibernators, which some times use arousals for foraging and hibernate in open and accessible hibernacula, are susceptible to external zeitgebers. In opportunistic hibernators, the circadian system plays a major role in maintaining synchrony between the normal day-night cycle and occasional foraging. Although the daily routine of activity and rest is abandoned during hibernation, the circadian system appears to remain functional, and there is little evidence it is significantly affected by low T_b. (Chronobiology International, 17(2), 103–128, 2000)     

Regulated hypothermia in the desert shrew

Summary Desert shrews (Notiosorex crawfordi; 4 g) enter into daily bouts of very shallow torpor, when restricted in their food intake. These bouts, though interrupted and uneven, last throughout that portion of the day the animals' cages are lighted. Body temperature is apparently regulated by fine adjustments of metabolic heat production in hypothermic as well as euthermic desert shrews. Thus, these animals seem to have two temperature thresholds for thermoregulation or body temperature rheostat settings. One is near 38 °C while the other, near 28 °C is likely used exclusively when energy supplies are low. The coefficient of heat transfer is the same at both body temperatures. Power saved by hypothermic animals at air temperatures between 20 and 25 °C amounts to about 96 mW. This is half of the metabolic power output of euthermic shrews at 20 °C and 80% at 25 °C. These results suggest a compromise between the energy savings of a deep torpor and the unimpaired functioning of euthermia.Supported by NSF: DEB 75-18576 and NIH: AM 05738     

Defining torpor in free-ranging bats: experimental evaluation of external temperature-sensitive radiotransmitters and the concept of active temperature

A variety of definitions involving body temperature (Tb), metabolic rate and behavior have been used to define torpor in mammals and birds. This problem is confounded in some studies of free-ranging animals that employ only skin temperature (Tsk), a measure that approximates but may not precisely reflect Tb. We assess the accuracy of Tsk in the context of a recent definition for torpor called active temperature. We compared the active temperatures of individual big brown bats (Eptesicus fuscus), which aggregate in cavities, with solitary, foliage-roosting hoary bats (Lasiurus cinereus). In captive big brown bats, we compared Tsk and core Tb at a range of ambient temperatures for clustered and solitary roosting animals, compared Tsk and Tb during arousal from torpor, and quantified the effect of flight on warming from torpor. Hoary bats had significantly lower active temperatures than big brown bats despite having the same normothermic Tsk. Tsk was significantly lower than Tb during normothermia but often greater than Tb during torpor. Flight increased the rate of warming from torpor. This effect was more pronounced for Tsk than Tb. This suggests that bats could rely on heat generated by flight muscles to complete the final stages of arousal. Using active temperature to define torpor may underestimate torpor due to ambient cooling of external transmitters or animals leaving roosts while still torpid. Conversely, active temperature may also overestimate shallow torpor use if it is recorded during active arousal when shivering and non-shivering thermogenesis warm external transmitters. Our findings illuminate the need for laboratory studies that quantify the relationship between metabolic rate and Tsk over a range of ambient temperatures.     

Reversible depression of oxygen consumption in isolated liver mitochondria during hibernation

The biochemical mechanisms by which hibernators cool as they enter torpor are not fully understood. In order to examine whether rates of substrate oxidation vary as a function of hibernation, liver mitochondria were isolated from telemetered ground squirrels (Spermophilus lateralis) in five phases of their annual hibernation cycle: summer active, and torpid, interbout aroused, entrance, and arousing hibernators. Rates of state 3 and state 4 respiration were measured in vitro at 25 degrees C. Relative to mitochondria from summer-active animals, rates of state 3 respiration were significantly depressed in mitochondria from torpid animals yet fully restored during interbout arousals. These findings indicate that a depression of ADP-dependent respiration in liver mitochondria occurs during torpor and is reversed during the interbout arousals to euthermia. Because this inhibition was determined to be temporally independent of entrance and arousal, it is unlikely that active suppression of state 3 respiration causes entrance into torpor by facilitating metabolic depression. In contrast to the observed depression of state 3 respiration in torpid animals, state 4 respiration did not differ significantly among any of the five groups, suggesting that alterations in proton leak are not contributing appreciably to downregulation of respiration in hibernation.     

Metabolic Hormone FGF21 Is Induced in Ground Squirrels during Hibernation but Its Overexpression Is Not Sufficient to Cause Torpor

Hibernation is a natural adaptation that allows certain mammals to survive physiological extremes that are lethal to humans. Near freezing body temperatures, heart rates of 3–10 beats per minute, absence of food consumption, and depressed metabolism are characteristic of hibernation torpor bouts that are periodically interrupted by brief interbout arousals (IBAs). The molecular basis of torpor induction is unknown, however starved mice overexpressing the metabolic hormone fibroblast growth factor 21 (FGF21) promote fat utilization, reduce body temperature, and readily enter torpor–all hallmarks of mammalian hibernation. In this study we cloned FGF21 from the naturally hibernating thirteen-lined ground squirrel (Ictidomys tridecemlineatus) and found that levels of FGF21 mRNA in liver and FGF21 protein in serum are elevated during hibernation torpor bouts and significantly elevated during IBAs compared to summer active animals. The effects of artificially elevating circulating FGF21 concentrations 50 to 100-fold via adenoviral-mediated overexpression were examined at three different times of the year. This is the first time that a transgenic approach has been used in a natural hibernator to examine mechanistic aspects of hibernation. Surgically implanted transmitters measured various metrics of the hibernation phenotype over a 7-day period including changes in motor activity, heart rate and core body temperature. In April fed-state animals, FGF21 overexpression decreased blood insulin and free fatty acid concentrations, effects similar to those seen in obese mice. However, elevated FGF21 concentrations did not cause torpor in these fed-state animals nor did they cause torpor or affect metabolic parameters in fasted-state animals in March/April, August or October. We conclude that FGF21 is strongly regulated during torpor and IBA but that its overexpression is not sufficient to cause torpor in naturally hibernating ground squirrels.     

Invited review: molecular adaptations in mammalian hibernators: unique adaptations or generalized responses?

Hibernators are unique among mammals in their ability to attain, withstand, and reverse low body temperatures. Hibernators repeatedly cycle between body temperatures near zero during torpor and 37 degrees C during euthermy. How do these mammals maintain cardiac function, cell integrity, blood fluidity, and energetic balance during their prolonged periods at low body temperature and avoid damage when they rewarm? Hibernation is often considered an example of a unique adaptation for low-temperature function in mammals. Although such adaptation is apparent at the level of whole animal physiology, it is surprisingly difficult to demonstrate clear examples of adaptations at the cellular and biochemical levels that improve function in the cold and are unique to hibernators. Instead of adaptation for improved function in the cold, the key molecular adaptations of hibernation may be to exploit the cold to depress most aspects of biochemical function and then rewarm without damage to restore optimal function of all systems. These capabilities are likely due to novel regulation of biochemical pathways shared by all mammals, including humans.     

光照黑暗循环条件下达乌尔黄鼠的冬眠模式和能量消耗

为研究冬眠季节的光照条件对贮脂类冬眠动物入眠的影响,在达乌尔黄鼠腹腔埋植体温记录元件iButton,在体重高峰后的下降阶段置于5℃和12L:12D的光照条件下,观察测定其冬眠模式和能量消耗。达乌尔黄鼠冬眠模式出现深冬眠型、少冬眠型和不冬眠型,蛰眠阵包括深冬眠阵、短冬眠阵和日眠阵。不同冬眠阵中最低体温、冬眠阵的持续时间、阵间产热的持续时间、冷却速率和复温速率差异显著;阵间产热的最高体温基本相同。平均每日能量消耗在日眠阵中最高、短冬眠阵中居中、深冬眠阵中最低。入眠时间多集中于黑暗时相,觉醒时间多集中于光照时相。本实验结果提示,在冬眠季节施加光照黑暗循环条件可减少达乌尔黄鼠冬眠的时间,升高阵间最低体温,缩短冬眠阵与阵间产热的持续时间,降低复温速率;增加冬眠期间能量消耗。入眠与觉醒受光照条件影响,具有明显的光暗节律。     

Kidney proteome changes provide evidence for a dynamic metabolism and regional redistribution of plasma proteins during torpor-arousal cycles of hibernation

Hibernating ground squirrels maintain homeostasis despite extreme physiological challenges. In winter, these circannual hibernators fast for months while cycling between prolonged periods of low blood flow and body temperature, known as torpor, and short interbout arousals (IBA), where more typical mammalian parameters are rapidly restored. Here we examined the kidney proteome for changes that support the dramatically different physiological demands of the hibernator's year. We identified proteins in 150 two-dimensional gel spots that altered by at least 1.5-fold using liquid chromatography and tandem mass spectrometry. These data successfully classified individuals by physiological state and revealed three dynamic patterns of relative protein abundance that dominated the hibernating kidney: 1) a large group of proteins generally involved with capturing and storing energy were most abundant in summer; 2) a select subset of these also increased during each arousal from torpor; and 3) 14 spots increased in torpor and early arousal were enriched for plasma proteins that enter cells via the endocytic pathway. Immunohistochemistry identified α(2)-macroglobulin and albumin in kidney blood vessels during late torpor and early arousal; both exhibited regional heterogeneity consistent with highly localized control of blood flow in the glomeruli. Furthermore, albumin, but not α(2)-macroglobulin, was detected in the proximal tubules during torpor and early arousal but not in IBA or summer animals. Taken together, our findings indicate that normal glomerular filtration barriers remain intact throughout torpor-arousal cycles but endocytosis, and hence renal function, is compromised at low body temperature during torpor and then recovers with rewarming during arousal.     

Hormonal changes and energy substrate availability during the hibernation cycle of Syrian hamsters

Animals have to adapt to seasonal variations in food resources and temperature. Hibernation is one of the most efficient means used by animals to cope with harsh winter conditions, wherein survival is achieved through a significant decrease in energy expenditure. The hibernation period is constituted by a succession of torpor bouts (hypometabolism and decrease in body temperature) and periodic arousals (eumetabolism and euthermia). Some species feed during these periodic arousals, and thus show different metabolic adaptations to fat-storing species that fast throughout the hibernation period. Our study aims to define these metabolic adaptations, including hormone (insulin, glucagon, leptin, adiponectin, GLP-1, GiP) and metabolite (glucose, free fatty acids, triglycerides, urea) profiles together with body composition adjustments. Syrian hamsters were exposed to varied photoperiod and temperature conditions mimicking different phases of the hibernation cycle: a long photoperiod at 20 °C (LP20 group), a short photoperiod at 20 °C (SP20 group), and a short photoperiod at 8 °C (SP8). SP8 animals were sampled either at the beginning of a torpor bout (Torpor group) or at the beginning of a periodic arousal (Arousal group). We show that fat store mobilization in hamsters during torpor bouts is associated with decreased circulating levels of glucagon, insulin, leptin, and an increase in adiponectin. Refeeding during periodic arousals results in a decreased free fatty acid plasma concentration and an increase in glycemia and plasma incretin concentrations. Reduced incretin and increased adiponectin levels are therefore in accordance with the changes in nutrient availability and feeding behavior observed during the hibernation cycle of Syrian hamsters.