Leptin increases energy expenditure of a marsupial by inhibition of daily torpor

Energy and time optimization during exit from torpor in vertebrate endotherms

Torpor is used in small sized birds and mammals as an energy conservation trait. Considerable effort has been put towards elucidating the mechanisms underlying its entry and maintenance, but little attention has been paid regarding the exit. Firstly, we demonstrate that the arousal phase has a stereotyped dynamic: there is a sharp increase in metabolic rate followed by an increase in body temperature and, then, a damped oscillation in body temperature and metabolism. Moreover, the metabolic peak is around two-fold greater than the corresponding euthermic resting metabolic rate. We then hypothesized that either time or energy could be crucial variables to this event and constructed a model from a collection of first principles of physiology, control engineering and thermodynamics. From the model, we show that the stereotyped pattern of the arousal is a solution to save both time and energy. We extended the analysis to the scaling of the use of torpor by endotherms and show that variables related to the control system of body temperature emerge as relevant to the arousal dynamics. In this sense, the stereotyped dynamics of the arousal phase necessitates a certain profile of these variables which is not maintained as body size increases.

A thermodynamic-based approach to model the entry into metabolic depression by mammals and birds

For decades, there was an intense debate in relation to the mechanism behind the entry into metabolic depression (EMD) of mammals and birds. The fulcrum of the argument was whether the depression of metabolic rate ([Formula: see text]) was caused by the drop in body temperature, the so-called "Q₁₀ effect", or whether it was caused by a metabolic downregulation. One present-day model of this process is a qualitative (textual) description: the initial step of EDM would be a downregulation in [Formula: see text] from the value maintaining euthermia at a given ambient temperature to the basal metabolic rate of the animal and, then, Q₁₀ effect would take over and drop [Formula: see text] to its lower levels. Despite widely accepted, this qualitative description still misses a theoretical analysis. Here, we transpose the descriptive model to a formal quantitative one and analyze it under necessary thermodynamic conditions of a system. We, then, compare the results of the formal model to empirical data of EMD by mammals and birds. The comparisons indicate that the metabolic evolution in the course of the entry phase does not follow the descriptive model. Instead, as proposed by alternate models, EMD is a downregulated process as a whole until a new equilibrium T_b is attained.

Turn it off and on again: characteristics and control of torpor

Torpor is a hypothermic, hypoactive, hypometabolic state entered into by a wide range of animals in response to environmental challenge. This review summarises the current understanding of torpor. We start by describing the characteristics of the wide-ranging physiological adaptations associated with torpor. Next follows a discussion of thermoregulation, control of food intake and energy expenditure, and the interactions of sleep and thermoregulation, with particular emphasis on how those processes pertain to torpor. We move on to review the evidence for the systems that control torpor entry, including both the efferent circulating factors that signal the need for torpor, and the central processes that orchestrate it. Finally, we consider how the putative circuits responsible for torpor induction integrate with the established understanding of thermoregulation under non-torpid conditions and highlight important areas of uncertainty for future studies.

Turn it off and on again: characteristics and control of torpor

Torpor is a hypothermic, hypoactive, hypometabolic state entered into by a wide range of animals in response to environmental challenge. This review summarises the current understanding of torpor. We start by describing the characteristics of the wide-ranging physiological adaptations associated with torpor. Next follows a discussion of thermoregulation, control of food intake and energy expenditure, and the interactions of sleep and thermoregulation, with particular emphasis on how those processes pertain to torpor. We move on to take a critical view of the evidence for the systems that control torpor entry, including both the efferent circulating factors that signal the need for torpor, and the central processes that orchestrate it. Finally, we consider how the putative circuits responsible for torpor induction integrate with the established understanding of thermoregulation under non-torpid conditions and highlight important areas of uncertainty for future studies.

Hibernating astronauts-science or fiction?

For long-duration manned space missions to Mars and beyond, reduction of astronaut metabolism by torpor, the metabolic state during hibernation of animals, would be a game changer: Water and food intake could be reduced by up to 75% and thus reducing payload of the spacecraft. Metabolic rate reduction in natural torpor is linked to profound changes in biochemical processes, i.e., shift from glycolysis to lipolysis and ketone utilization, intensive but reversible alterations in organs like the brain and kidney, and in heart rate control via Ca²⁺. This state would prevent degenerative processes due to organ disuse and increase resistance against radiation defects. Neuro-endocrine factors have been identified as main targets to induce torpor although the exact mechanisms are not known yet. The widespread occurrence of torpor in mammals and examples of human hypometabolic states support the idea of human torpor and its beneficial applications in medicine and space exploration.

Preoptic leptin signaling modulates energy balance independent of body temperature regulation

The adipokine leptin acts on the brain to regulate energy balance but specific functions in many brain areas remain poorly understood. Among these, the preoptic area (POA) is well known to regulate core body temperature by controlling brown fat thermogenesis, and we have previously shown that glutamatergic, long-form leptin receptor (Lepr)-expressing neurons in the POA are stimulated by warm ambient temperature and suppress energy expenditure and food intake. Here we further investigate the role of POA leptin signaling in body weight regulation and its relationship to body temperature regulation in mice. We show that POA Lepr signaling modulates energy expenditure in response to internal energy state, and thus contributes to body weight homeostasis. However, POA leptin signaling is not involved in ambient temperature-dependent metabolic adaptations. Our study reveals a novel cell population through which leptin regulates body weight.

Energy Homeostasis in Monotremes

In 1803, the French anatomist Étienne Geoffroy Saint-Hilaire decided that the newly described echidna and platypus should be placed in a separate order, the monotremes, intermediate between reptiles and mammals. The first physiological observations showed monotremes had low body temperatures and metabolic rates, and the consensus was that they were at a stage of physiological development intermediate between "higher mammals" and "lower vertebrates." Subsequent studies demonstrated that platypuses and echidnas are capable of close thermoregulation in the cold although less so under hot conditions. Because the short-beaked echidna Tachyglossus aculeatus, may show very large daily variations in body temperature, as well as seasonal hibernation, it has been suggested that it may provide a useful model of protoendotherm physiology. Such analysis is complicated by the very significant differences in thermal relations between echidnas from different climates. In all areas female echidnas regulate Tb within 1°C during egg incubation. The lactation period is considered to be the most energetically expensive time for most female mammals but lactating echidnas showed no measurable difference in field metabolic rate from non-lactating females, while the lactation period is more than 200 days for Kangaroo Island echidnas but only 150 days in Tasmania. In areas with mild winters echidnas show reduced activity and shallow torpor in autumn and early winter, but in areas with cold winters echidnas enter true hibernation with Tb falling as low as 4.5°C. Monotremes do not possess brown adipose tissue and maximum rates of rewarming from hibernation in echidnas were only half those of marmots of the same mass. Although echidnas show very large seasonal variations in fat stores associated with hibernation there is no relationship between plasma leptin and adiposity. Leptin levels are lowest during post-reproductive fattening, supporting suggestions that in evolutionary terms the anorectic effects of leptin preceded the adiposity signal. BMR of platypuses is twice that of echidnas although maximum metabolism is similar. High levels of thyroid hormones in platypuses may be driving metabolism limited by low body temperature. Monotremes show a mosaic of plesiomorphic and derived features but can still inform our understanding of the evolution of endothermy.

The effects of graded levels of calorie restriction: III. Impact of short term calorie and protein restriction on mean daily body temperature and torpor use in the C57BL/6 mouse

A commonly observed response in mammals to calorie restriction (CR) is reduced body temperature (T_b). We explored how the T_b of male C57BL/6 mice responded to graded CR (10 to 40%), compared to the response to equivalent levels of protein restriction (PR) over 3 months. Under CR there was a dynamic change in daily T_b over the first 30–35 days, which stabilized thereafter until day 70 after which a further decline was noted. The time to reach stability was dependent on restriction level. Body mass negatively correlated with T_b under ad libitum feeding and positively correlated under CR. The average T_b over the last 20 days was significantly related to the levels of body fat, structural tissue, leptin and insulin-like growth factor-1. Some mice, particularly those under higher levels of CR, showed periods of daily torpor later in the restriction period. None of the changes in T_b under CR were recapitulated by equivalent levels of PR. We conclude that changes in T_b under CR are a response only to the shortfall in calorie intake. The linear relationship between average T_b and the level of restriction supports the idea that T_b changes are an integral aspect of the lifespan effect.

Torpor and hypothermia: reversed hysteresis of metabolic rate and body temperature

Regulated torpor and unregulated hypothermia are both characterized by substantially reduced body temperature (Tb) and metabolic rate (MR), but they differ physiologically. Although the remarkable, medically interesting adaptations accompanying torpor (e.g., tolerance for cold and ischemia, absence of reperfusion injury, and disuse atrophy) often do not apply to hypothermia in homeothermic species such as humans, the terms "torpor" and "hypothermia" are often used interchangeably in the literature. To determine how these states differ functionally and to provide a reliable diagnostic tool for differentiating between these two physiologically distinct states, we examined the interrelations between Tb and MR in a mammal (Sminthopsis macroura) undergoing a bout of torpor with those of the hypothermic response of a similar-sized juvenile rat (Rattus norvegicus). Our data show that under similar thermal conditions, 1) cooling rates differ substantially (approximately fivefold) between the two states; 2) minimum MR is approximately sevenfold higher during hypothermia than during torpor despite a similar Tb; 3) rapid, endogenously fuelled rewarming occurs in torpor but not hypothermia; and 4) the hysteresis between Tb and MR during warming and cooling proceeds in opposite directions in torpor and hypothermia. We thus demonstrate clear diagnostic physiological differences between these two states that can be used experimentally to confirm whether torpor or hypothermia has occurred. Furthermore, the data can clarify the results of studies investigating the ability of physiological or pharmacological agents to induce torpor. Consequently, we recommend using the terms "torpor" and "hypothermia" in ways that are consistent with the underlying regulatory differences between these two physiological states.

Glutamate release mediates leptin action on energy expenditure

Restricting energy expenditure is an adaptive response to food shortage. Despite being insulated with massive amount of fat tissues, leptin-deficient mice lose the ability to maintain their body temperature and develop deep hypothermia, which can be suppressed by exogenous leptin, suggesting an important role for leptin in energy expenditure regulation. However, the mechanism underlying the leptin action is not clear. We generated mice with disruption of glutamate release from leptin receptor-expressing neurons by deleting vesicular glutamate transporter 2 in these neurons, and found that these mice developed mild obesity purely due to reduced energy expenditure, exhibited bouts of rapidly reduced energy expenditure, body temperature and locomotion. In addition, these mice exhibited lower energy expenditure and body temperature in response to fasting and were defective in leptin-mediated thermogenic action in brown adipose tissues. Taken together, our results identify a role for glutamate release in mediating leptin action on energy expenditure.

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.

Does leptin signal adiposity in the egg-laying mammal, Tachyglossus aculeatus?

Leptin is a peptide hormone best known for its role in feedback regulation of adiposity in eutherian mammals. Normally an increase in adipose tissue mass leads to an increase in circulating leptin which increases energy expenditure and limits food intake, but in hibernating eutherian mammals this relationship may change to allow prehibernatory fattening. The echidna (Tachyglossus aculeatus) is a monotreme mammal which accumulates significant fat reserves before entering hibernation, and mates immediately at the end of hibernation. We hypothesised that echidnas would show a strong relationship between body mass and plasma leptin for most of the year which would change during the pre-hibernatory period. We measured plasma leptin and body mass in free-ranging echidnas over several reproductive and hibernation cycles. There were significant seasonal variations in plasma leptin in both sexes, with the highest levels occurring in hibernation and in mating females. The lowest levels were found in males when they were foraging maximally after the reproductive period. We used mass%, body mass at the time of sampling as a percentage of long term mean mass, as a proxy for adiposity. There was a weak negative relationship between mass% and plasma leptin, from which we infer a weak negative relationship between adiposity and plasma leptin as has been found in reptiles and birds, rather than the strong positive relationship found in other mammals.

Thermoregulation: an orphan receptor finds its way in the cold

The hypometabolic state of torpor is a widely utilized and well-orchestrated response to food shortage. A new study shows that the melatonin-related orphan receptor GPR50 plays an important function in metabolic regulation for entry into torpor.

Effects of ambient temperature on adaptive thermogenesis during maintenance of reduced body weight in mice

We showed previously that, at ambient room temperature (22°C), mice maintained at 20% below their initial body weight by calorie restriction expend energy at a rate below that which can be accounted for by the decrease of fat and fat-free mass. Food-restricted rodents may become torpid at subthermoneutral temperatures, a possible confounding factor when using mice as human models in obesity research. We examined the bioenergetic, hormonal, and behavioral responses to maintenance of a 20% body weight reduction in singly housed C57BL/6J +/+ and Lep(ob) mice housed at both 22°C and 30°C. Weight-reduced high-fat-fed diet mice (HFD-WR) showed similar quantitative reductions in energy expenditure-adjusted for body mass and composition-at both 22°C and 30°C: -1.4 kcal/24 h and -1.6 kcal/24 h below predicted, respectively, and neither group entered torpor. In contrast, weight-reduced Lep(ob) mice (OB-WR) housed at 22°C became torpid in the late lights-off period (0200-0500) but did not when housed at 30°C. These studies indicate that mice with an intact leptin axis display similar decreases in "absolute" energy expenditure in response to weight reduction at both 22°C and 30°C ambient temperature. More importantly, the "percent" decrease in total energy expenditure observed in the HFD-WR compared with AL mice is much greater at 30°C (-19%) than at 22°C (-10%). Basal energy expenditure demands are ∼45% lower in mice housed at 30°C vs. 22°C, since the mice housed at thermoneutrality do not allocate extra energy for heat production. The higher total energy expenditure of mice housed at 22°C due to these increased thermogenic demands may mask physiologically relevant changes in energy expenditure showing that ambient temperature must be carefully considered when quantifying energy metabolism in both rodents and humans.

Co-localization of TRHR1 and LepRb receptors on neurons in the hindbrain of the rat

We have reported a highly cooperative interaction between leptin and thyrotropin releasing hormone (TRH) in the hindbrain to generate thermogenic responses (Hermann et al., 2006) (Rogers et al., 2009). Identifying the locus in the hindbrain where leptin and TRH act synergistically to increase thermogenesis will be necessary before we can determine the mechanism(s) by which this interaction occurs. Here, we performed heat-induced epitope recovery techniques and in situ hybridization to determine if neurons or afferent fibers in the hindbrain possess both TRH type 1 receptor and long-form leptin receptor [TRHR1; LepRb, respectively]. LepRb receptors were highly expressed in the solitary nucleus [NST], dorsal motor nucleus of the vagus [DMN] and catecholaminergic neurons of the ventrolateral medulla [VLM]. All neurons that contained LepRb also contained TRHR1. Fibers in the NST and the raphe pallidus [RP] and obscurrus [RO] that possess LepRb receptors were phenotypically identified as glutamatergic type 2 fibers (vglut2). Fibers in the NST and RP that possess TRHR1 receptors were phenotypically identified as serotonergic [i.e., immunopositive for the serotonin transporter; SERT]. Co-localization of LepRb and TRHR1 was not observed on individual fibers in the hindbrain but these two fiber types co-mingle in these nuclei. These anatomical arrangements may provide a basis for the synergy between leptin and TRH to increase thermogenesis.

Leptin "gates" thermogenic action of thyrotropin-releasing hormone in the hindbrain

Leptin, acting as a measure of metabolic fuel availability, exerts a powerful permissive influence on neurogenic thermogenesis. During starvation and an absence of leptin, animals cannot produce thermogenic reactions to cold stress. However, thermogenesis is rescued by restoring leptin. We have previously observed (Hermann, G.E., Barnes, M.J., Rogers, R.C., 2006. Leptin and thyrotropin-releasing hormone: cooperative action in the hindbrain to activate brown adipose thermogenesis. Brain Res. 1117, 118-124.) a highly cooperative interaction between leptin and thyrotropin-releasing hormone [TRH] to activate hindbrain generated thermogenic responses. Specifically, exposure to both leptin and TRH elicited a 3.5 degrees C increase in brown adipose tissue [BAT] thermogenesis, while leptin alone did not evoke any change, and TRH alone caused only approximately 1 degrees C increase. The present study shows that the leptin-TRH synergy in controlling brown adipose [BAT] thermogenesis is order-specific and dependent on the feeding status of the animal. That is, fourth ventricular [4V] application of leptin to the food-deprived animal, before TRH injection, yields a substantial increase in BAT; while the reverse order yields a significantly smaller effect. If the animal were fed within minutes of anesthesia, then exogenous leptin was not necessary for TRH to yield a large increase in BAT temperature. The leptin-TRH synergy was uncoupled by pretreatment with the phosphoinositol-tris phosphate kinase [PI3K] inhibitor, wortmannin and the Src-SH2 antagonist, PP2. The TRH transduction mechanism utilizes phospholipase C [PLC] potently regulated by the SH2 site. Previous work in culture systems suggests that the product of PI3K activity [PIP3] potently upregulates PLC by activating the SH2 domain of the PLC complex. Perhaps leptin "gates" the thermogenic action of TRH in the hindbrain by invoking this same mechanism.

A single bout of torpor in mice protects memory processes

Memory consolidation is the process by which new and labile information is stabilized as long-term memory. Consolidation of spatial memories is thought to involve the transfer of information from the hippocampus to cortical regions. While the hypometabolic and hypothermic state of torpor dramatically changes hippocampal connectivity, little work has considered the functional consequences of these changes. The present study examines the role of a single bout of shallow torpor in the process of memory consolidation in mice. Adult female C57Bl/6NHSD mice were trained on the Morris Water Maze (MWM) task. Immediately following acquisition, the mice were exposed to one of four experimental manipulations for 24 h: fasted at an ambient temperature of 19 degrees C, fasted at 29 degrees C, allowed free access to food at 19 degrees C, or allowed free access to food at 29 degrees C. Mice fasted at 19 degrees C entered a bout of torpor as assessed by core body temperature while none of the mice in the other conditions did so. Spatial biases were then assessed with a probe trial in the MWM. During the probe trial, mice that had entered torpor and mice that were fed at 29 degrees C spent twice as much time in the prior target platform location than mice that were fed at 19 degrees C and those that were fasted at 29 degrees C. These findings demonstrate that, while food restriction or cool ambient temperature independently disrupt memory processes, together they cause physiological changes including the induction of a state of torpor that result in functional preservation of the memory process.

Norepinephrine controls both torpor initiation and emergence via distinct mechanisms in the mouse

Some mammals, including laboratory mice, enter torpor in response to food deprivation, and leptin can attenuate these bouts of torpor. We previously showed that dopamine β-hydroxylase knockout (Dbh −/−) mice, which lack norepinephrine (NE), do not reduce circulating leptin upon fasting nor do they enter torpor. To test whether the onset of torpor in mice during a fast requires a NE-mediated reduction in circulating leptin, double mutant mice deficient in both leptin (ob/ob) and DBH (DBL MUT) were generated. Upon fasting, control and ob/ob mice entered torpor as assessed by telemetric core T_b acquisition. While fasting failed to induce torpor in Dbh −/− mice, leptin deficiency bypassed the requirement for NE, as DBL MUT mice readily entered torpor upon fasting. These data indicate that sympathetic activation of white fat and suppression of leptin is required for the onset of torpor in the mouse. Emergence from torpor was severely retarded in DBL MUT mice, revealing a novel, leptin-independent role for NE in torpor recovery. This phenotype was mimicked by administration of a β₃ adrenergic receptor antagonist to control mice during a torpor bout. Hence, NE signaling via β₃ adrenergic receptors presumably in brown fat is the first neurotransmitter-receptor system identified that is required for normal recovery from torpor.

Effect of food availability and leptin on the physiology and hypothalamic gene expression of the golden spiny mouse: a desert rodent that does not hoard food

Food availability and quality in desert habitats are spatially and temporally unpredictable, and animals face periods of food shortage. The golden spiny mouse ( Acomys russatus) is an omnivorous desert rodent that does not hoard food, requiring it to withstand such periods by physiological means alone. In response to food restriction, plasma leptin concentrations, core body temperature, and energy expenditure of the spiny mouse decrease significantly after 24 h, and most spiny mice are able to maintain their body mass to ∼85% of ad libitum for a prolonged period of time. Both 1-day food deprivation and long-term food restriction had a significant effect on body mass and plasma leptin concentrations, which decreased significantly with a high correlation, as well as on the orexigenic agouti-related protein, which increased significantly as a result of the 24-h food deprivation; and on neuropeptide Y (NPY), in which the increase was more pronounced under long-term food restriction. Food restriction and food deprivation had no effect, however, on the anorexigenic pro-opiomelanocortin and cocaine and amphetamine-related transcript. Leptin administration to food-restricted spiny mice did not affect food intake or the rate of decrease in body mass, indicating that it cannot overcome the drive to eat when food is scarce. However, it did result in a significant decrease in NPY levels, and the spiny mice spent less time at low body temperatures compared with PBS-treated golden spiny mice. These results show that in food-restricted golden spiny mice, leptin affects thermogenesis, but not food consumption, and suggest that the thermoregulatory effects of leptin are mediated by NPY.

The pharmacology and molecular mechanisms underlying temperature regulation and torpor

The ability to enter a hypometabolic state upon restriction of caloric intake is pivotal for animal survival: balancing the energy budget in endotherms can be a real struggle when food is not available and/or the demand for heat production to maintain homeothermy becomes excessive. Bouts of torpor, characterized by metabolic rates well below a basal metabolic rate and core body temperatures that may be just a few degrees above the ambient temperature, are utilized among many organisms across the animal kingdom, including those that could be described as typical laboratory animals, like the mouse or hamster. Daily heterotherms, which are the focus of this commentary, enter shallow torpor bouts and do so usually under acute food shortage conditions and a relatively cool environment. Due to their small size, the body temperature of these animals is very responsive to food deprivation, pharmacological inhibition of metabolic rate, and cardiovascular depressants. This commentary examines recent developments concerning the neuroendocrine mechanisms in place that may mediate fasting-induced torpor in daily heterotherms. Further this commentary highlights pharmacological induction of hypothermia in small mammals.

NPY Y1 receptor antagonist prevents NPY-induced torporlike hypothermia in cold-acclimated Siberian hamsters

Siberian hamsters ( Phodopus sungorus) undergo bouts of daily torpor during which body temperature decreases by as much as 20°C and provides a significant savings in energy expenditure. Natural torpor in this species is normally triggered by winterlike photoperiods and low ambient temperatures. Intracerebroventricular injection of neuropeptide Y (NPY) reliably induces torporlike hypothermia that resembles natural torpor. NPY-induced torporlike hypothermia is also produced by intracerebroventricular injections of an NPY Y1 receptor agonist but not by injections of an NPY Y5 receptor agonist. In this research, groups of cold-acclimated Siberian hamsters were either coinjected with a Y1 receptor antagonist (1229U91) and NPY or were coinjected with a Y5 receptor antagonist ( CGP71683 ) and NPY in counterbalanced designs. Paired vehicle + NPY induced torporlike hypothermia in 92% of the hamsters, whereas coinjection of Y1 antagonist + NPY induced torporlike hypothermia in 4% of the hamsters. In contrast, paired injections of vehicle + NPY and Y5 antagonist + NPY induced torporlike hypothermia in 100% and 91% of the hamsters, respectively. Although Y5 antagonist treatment alone had no effect on body temperature, Y1 antagonist injections produced hyperthermia compared with controls. Both Y1 antagonist and Y5 antagonist injections significantly reduced food ingestion 24 h after treatment. We conclude that activation of NPY 1 receptors is both sufficient and necessary for NPY-induced torporlike hypothermia.

Monosodium glutamate-induced arcuate nucleus damage affects both natural torpor and 2DG-induced torpor-like hypothermia in Siberian hamsters

Siberian hamsters ( Phodopus sungorus) have the ability to express daily torpor and decrease their body temperature to ∼15°C, providing a significant savings in energy expenditure. Daily torpor in hamsters is cued by winterlike photoperiods and occurs coincident with the annual nadirs in body fat reserves and chronic leptin concentrations. To better understand the neural mechanisms underlying torpor, Siberian hamster pups were postnatally treated with saline or MSG to ablate arcuate nucleus neurons that likely possess leptin receptors. Body temperature was studied telemetrically in cold-acclimated (10°C) male and female hamsters moved to a winterlike photoperiod (10:14-h light-dark cycle) ( experiments 1 and 2) or that remained in a summerlike photoperiod (14:10-h light-dark cycle) ( experiment 3). In experiment 1, even though other photoperiodic responses persisted, MSG-induced arcuate nucleus ablations prevented the photoperiod-dependent torpor observed in saline-treated Siberian hamsters. MSG-treated hamsters tended to possess greater fat reserves. To determine whether reductions in body fat would increase frequency of photoperiod-induced torpor after MSG treatment, hamsters underwent 2 wk of food restriction (70% of ad libitum) in experiment 2. Although food restriction did increase the frequency of torpor in both MSG- and saline-treated hamsters, it failed to normalize the proportion of MSG-treated hamsters undergoing photoperiod-dependent torpor. In experiment 3, postnatal MSG treatments reduced the proportion of hamsters entering 2DG-induced torpor-like hypothermia by ∼50% compared with saline-treated hamsters (38 vs. 72%). In those MSG-treated hamsters that did become hypothermic, their minimum temperature during hypothermia was significantly greater than comparable saline-treated hamsters. We conclude that 1) arcuate nucleus mechanisms mediate photoperiod-induced torpor, 2) food-restriction-induced torpor may also be reduced by MSG treatments, and 3) arcuate nucleus neurons make an important, albeit partial, contribution to 2DG-induced torpor-like hypothermia.

ICV NPY Y1 receptor agonist but not Y5 agonist induces torpor-like hypothermia in cold-acclimated Siberian hamsters

The reduced metabolism derived from daily torpor enables numerous small mammals, including Siberian hamsters, to survive periods of energetic challenge. Little is known of the neural mechanisms underlying the initiation and expression of torpor. Hypothalamic neuropeptide Y (NPY) contributes to surviving energetic challenges by both increasing food ingestion and reducing metabolic expenditure. Intracerebroventricular injections of NPY in cold-acclimated Siberian hamsters induce torpor-like hypothermia comparable to natural torpor. Multiple NPY receptor subtypes have been identified, and the Y1 receptor and Y5 receptor both contribute to the orexigenic effect of NPY. The purpose of this research was to compare and contrast the effects of Y1 receptor activation by a specific Y1 agonist ([d-Arg25]-NPY) or Y5 receptor activation by a specific Y5 agonist ([d-Trp34]-NPY) on body temperature and subsequent food intake in cold-acclimated Siberian hamsters. Intracerebroventricular injections of Y1 agonist produced torporlike hypothermia closely resembling that induced by intracerebroventricular NPY. The intracerebroventricular Y5 agonist infrequently produced hypothermia reaching criterion for torpor and that failed to resemble either NPY-induced or natural torpor. Combined injections of Y1 and Y5 agonists resulted in hypothermia comparable to Y5 agonist treatments alone, negating the mimicry of NPY treatment seen with Y1 agonist alone. Prior treatment with Y1 agonist or Y5 agonist surprisingly had lingering effects on NPY-induced torpor expression, Y1 agonist enhanced and Y5 agonist inhibited the effect of NPY. The ability of NPY to induce torporlike hypothermia, especially its initiation, most likely involves activation of the NPY Y1 receptor subtype.

AMP does not induce torpor

Torpor, a state characterized by a well-orchestrated reduction of metabolic rate and body temperature (T(b)), is employed for energetic savings by organisms throughout the animal kingdom. The nucleotide AMP has recently been purported to be a primary regulator of torpor in mice, as circulating AMP is elevated in the fasted state, and administration of AMP causes severe hypothermia. However, we have found that the characteristics and parameters of the hypothermia induced by AMP were dissimilar to those of fasting-induced torpor bouts in mice. Although administration of AMP induced hypothermia (minimum T(b) = 25.2 +/- 0.6 degrees C) similar to the depth of fasting-induced torpor (24.9 +/- 1.5 degrees C), ADP and ATP were equally effective in lowering T(b) (minimum T(b): 24.8 +/- 0.9 degrees C and 24.0 +/- 0.5 degrees C, respectively). The maximum rate of T(b) fall into hypothermia was significantly faster with injection of adenine nucleotides (AMP: -0.24 +/- 0.03; ADP: -0.24 +/- 0.02; ATP: -0.25 +/- 0.03 degrees C/min) than during fasting-induced torpor (-0.13 +/- 0.02 degrees C/min). Heart rate decreased from 755 +/- 15 to 268 +/- 17 beats per minute (bpm) within 1 min of AMP administration, unlike that observed during torpor (from 646 +/- 21 to 294 +/- 19 bpm over 35 min). Finally, the hypothermic effect of AMP was blunted with preadministration of an adenosine receptor blocker, suggesting that AMP action on T(b) is mediated via the adenosine receptor. These data suggest that injection of adenine nucleotides into mice induces a reversible hypothermic state that is unrelated to fasting-induced torpor.

Peripheral ghrelin deepens torpor bouts in mice through the arcuate nucleus neuropeptide Y signaling pathway

Many small mammals have the ability to enter torpor, characterized by a controlled drop in body temperature (Tb). We hypothesized that ghrelin would modulate torpor bouts, because torpor is induced by fasting in mice coincident with elevated circulating ghrelin. Female National Institutes of Health (NIH) Swiss mice were implanted with a Tb telemeter and housed at an ambient temperature (Ta) of 18 degrees C. On fasting, all mice entered a bout of torpor (minimum Tb: 23.8+/-2.0 degrees C). Peripheral ghrelin administration (100 microg) during fasting significantly deepened the bout of torpor (Tb minimum: 19.4+/-0.5 degrees C). When the arcuate nucleus (ARC) of the hypothalamus, a ghrelin receptor-rich region of the brain, was chemically ablated with monosodium glutamate (MSG), fasted mice failed to enter torpor (minimum Tb=31.6+/-0.6 degrees C). Furthermore, ghrelin administration had no effect on the Tb minimum of ARC-ablated mice (31.8+/-0.8 degrees C). Two major pathways that regulate food intake reside in the ARC, the anorexigenic alpha-melanocyte stimulating hormone (alpha-MSH) pathway and the orexigenic neuropeptide Y (NPY) signaling pathway. Both Ay mice, which have the alpha-MSH pathway blocked, and Npy-/-mice exhibited shallow, aborted torpor bouts in response to fasting (Tb minimum: 29.1+/-0.6 degrees C and 29.9+/-1.2 degrees C, respectively). Ghrelin deepened torpor in Ay mice (Tb minimum: 22.8+/-1.3 degrees C), but had no effect in Npy-/-mice (Tb minimum: 29.5+/-0.8 degrees C). Collectively, these data suggest that ghrelin's actions on torpor are mediated via NPY neurons within the ARC.

Defending body mass during food restriction inAcomys russatus: a desert rodent that does not store food

Golden spiny mice, which inhabit rocky deserts and do not store food, must therefore employ physiological means to cope with periods of food shortage. Here we studied the physiological means used by golden spiny mice for conserving energy during food restriction and refeeding and the mechanism by which food consumption may influence thermoregulatory mechanisms and metabolic rate. As comparison, we studied the response to food restriction of another rocky desert rodent, Wagner’s gerbil, which accumulates large seed caches. Ten out of 12 food-restricted spiny mice (resistant) were able to defend their body mass after an initial decrease, as opposed to Wagner’s gerbils ( n = 6). Two of the spiny mice (nonresistant) kept losing weight, and their food restriction was halted. In four resistant and two nonresistant spiny mice, we measured heart rate, body temperature, and oxygen consumption during food restriction. The resistant spiny mice significantly ( P < 0.05) reduced energy expenditure and entered daily torpor. The nonresistant spiny mice did not reduce their energy expenditure. The gerbils’ response to food restriction was similar to that of the nonresistant spiny mice. Resistant spiny mice leptin levels dropped significantly ( n = 6, P < 0.05) after 24 h of food restriction, and continued to decrease throughout food restriction, as did body fat. During refeeding, although the golden spiny mice gained fat, leptin levels were not correlated with body mass ( r2= 0.014). It is possible that this low correlation allows them to continue eating and accumulate fat when food is plentiful.

The full expression of fasting-induced torpor requires beta 3-adrenergic receptor signaling

Torpor, a controlled rapid drop in metabolic rate and body temperature (Tb), is a hypometabolic adaptation to stressful environmental conditions, which occurs in many small mammals, marsupials, and birds. To date, signaling pathways required for torpor have not been identified. We examined the role of the sympathetic nervous system (SNS) in mediating the torpor adaptation to fasting by telemetrically monitoring the Tb of dopamine beta-hydroxylase knock-out (Dbh-/-) mice, which lack the ability to produce the SNS transmitters, norepinephrine (NE), and epinephrine. Control (Dbh+/-) mice readily reduced serum leptin levels and entered torpor after a fast in a cool environment. In contrast, Dbh-/- mice failed to reduce serum leptin and enter torpor under fasting conditions, whereas restoration of peripheral but not central NE lowered serum leptin levels and rescued the torpor response. Torpor was expressed in fasted Dbh-/- mice immediately after administration of either the nonselective beta-adrenergic receptor agonist isoproterenol or the beta3-adrenergic receptor (AR)-specific agonist CL 316243 [disodium (RR)-5-[2-[[2-(3-chlorophenyl)-2-hydroxyethyl]-amino]propyl]-1,3-benzodioxazole-2,2-dicarboxylate], but not after administration of beta1, beta2, or alpha1 agonists. Importantly, the beta3-specific antagonist SR 59230A [3-(2-ethylphenoxy)-1-[(1,S)-1,2,3,4-tetrahydronapth-1-ylamino]-2S-2-propanol oxalate] severely blunted fasting-induced torpor in control mice, whereas other AR antagonists were ineffective. These results define a critical role of peripheral SNS activity at beta3-AR-containing tissues in the torpor adaptation to limited energy availability and cool ambient temperature.

Annual lipid cycles in hibernators: integration of physiology and behavior

Mammalian hibernation is a temporary suspension of euthermia allowing endotherms to undergo reversible hypothermia and generate a marked savings in energy expenditure. In most fat-storing hibernator species, seasonal changes in food intake, triacylglycerol deposition, metabolism, and reproductive development are controlled by a circannual clock. In ground-dwelling sciurid rodents (ground squirrels and marmots), for example, energy intake increases during a summer body mass gain phase, and toward the end of this phase metabolic rate also begins to decrease, resulting in a profound increase in lipid deposition as fat. Increased activity of lipogenic hormones and enzymes correspond with this increase. The hibernation mass loss phase begins after the body mass peak in the fall and ends in spring. During this phase, stored lipids are slowly utilized in a programmed manner by undergoing deep torpor or hibernation during which the hypothalamic setpoint for body temperature is typically reduced to just above 0 degrees C. Throughout the hibernation season, bouts of deep torpor are punctuated by periodic arousals in which brown adipose tissue thermogenesis plays a critical role. Lipid oxidation nearly exclusively fuels deep torpor and most of the rewarming process. The fatty acid composition of stored lipids can affect the depth and duration of deep torpor, and saturated fatty acids may be preferentially used during hibernation, whereas polyunsaturated fatty acids may be preferentially retained. Female and underweight male hibernators terminate hibernation in spring when aboveground food becomes available; in contrast, heavier males with sufficient lipid reserves spontaneously terminate hibernation several weeks before females and independent of food availability. Mating occurs shortly after emergence from hibernation, and the lipid cycle begins again with the completion of reproduction. Lipid deposition and mobilization, temperature regulation, reproduction, and circannual timing are intimately interdependent. The unique manner in which they are controlled during the annual cycle, especially lipid reserves, makes hibernators valuable and promising models for research into the mechanisms underlying these processes in all mammals.

Exogenously administered leptin leads to weight loss and increased physical activity in the marsupial Sminthopsis crassicaudata

The adipose tissue derived cytokine leptin, modifies energy balance via effects on both food intake and energy expenditure. It is not clear, however, whether the component of energy expenditure accounted for by voluntary (nonexercise) physical activity is increased in response to leptin. The aim of this study was to investigate the effect of exogenously administered leptin on physical activity in the marsupial Sminthopsis crassicaudata. Body weight, tail width and food intake, were measured daily and physical activity was measured hourly in normal lean S. crassicaudata (n=8) with ad libitum access to standard laboratory diet. After 5 days baseline the animals were divided into two equal groups (n=4), and either human recombinant leptin (2.5 mg/kg) or placebo was administered twice daily intraperitoneally. Approximately 81% of the total daily activity during the baseline period occurred during the nocturnal phase. After 9 days of leptin administration, there were significant decreases in body weight (P<0.001) and fat content (P<0.01), which were not accompanied by a decrease in total energy intake. Overall daily physical activity increased (P=0.028); this effect was confined to the dark phase (P=0.033). We conclude that in lean S. crassicaudata the exogenous administration of human recombinant leptin results in a decrease in adiposity which occurs in the absence of a measurable effect on food intake and is associated with an increase in non-exercise physical activity at least over the duration of this study.

Reduced leptin concentrations are permissive for display of torpor in Siberian hamsters

A photoperiod with a short photophase induces a winterlike phenotype in Siberian hamsters that includes a progressive decrease in food intake and body mass and reproductive organ regression, as well as reversible hypothermia in the form of short-duration torpor. Torpor substantially reduces energy utilization and is not initiated until body mass, fat stores, and serum leptin concentrations are at their nadir. Because photoperiod-dependent torpor is delayed until fat reserves are lowest, leptin concentrations may be a permissive factor for torpor onset. This conjecture was tested by implanting osmotic minipumps into Siberian hamsters manifesting spontaneous torpor; the animals received a constant release of leptin or vehicle for 14 days. Exogenous leptin treatment eliminated torpor in a significant proportion of treated hamsters, whereas treatment with the vehicle did not. Similarly, endogenous serum leptin concentrations were markedly reduced in all animals undergoing daily torpor. Although simply reducing leptin concentrations below a threshold value is not sufficient for torpor initiation, reduced leptin concentrations nevertheless appear necessary for its occurrence. It is proposed that drastically reduced leptin concentrations provide a “starvation signal” to an as yet unidentified central mechanism mediating torpor initiation.

Metabolic rate and body temperature reduction during hibernation and daily torpor

Although it is well established that during periods of torpor heterothermic mammals and birds can reduce metabolic rates (MR) substantially, the mechanisms causing the reduction of MR remain a controversial subject. The comparative analysis provided here suggests that MR reduction depends on patterns of torpor used, the state of torpor, and body mass. Daily heterotherms, which are species that enter daily torpor exclusively, appear to rely mostly on the fall of body temperature (Tb) for MR reduction, perhaps with the exception of very small species and at high torpor Tb, where some metabolic inhibition may be used. In contrast, hibernators (species capable of prolonged torpor bouts) rely extensively on metabolic inhibition, in addition to Tb effects, to reduce MR to a fraction of that observed in daily heterotherms. In small hibernators, metabolic inhibition and the large fall of Tb are employed to maximize energy conservation, whereas in large hibernators, metabolic inhibition appears to be employed to facilitate MR and Tb reduction at torpor onset. Over the ambient temperature (Ta) range where torpid heterotherms are thermo-conforming, the Tb-Ta differential is more or less constant despite a decline of MR with Ta; however, in thermo-regulating torpid individuals, the Tb-Ta differential is maintained by a proportional increase of MR as during normothermia, albeit at a lower Tb. Thermal conductance in most torpid thermo-regulating individuals is similar to that in normothermic individuals despite the substantially lower MR in the former. However, conductance is low when deeply torpid animals are thermo-conforming probably because of peripheral vasoconstriction.

Effects of temperature acclimation on maximum heat production, thermal tolerance, and torpor in a marsupial

Marsupials, unlike placental mammals, are believed to be unable to increase heat production and thermal performance after cold-acclimation. It has been suggested that this may be because marsupials lack functional brown fat, a thermogenic tissue, which proliferates during cold-acclimation in many placentals. However, arid zone marsupials have to cope with unpredictable, short-term and occasionally extreme changes in environmental conditions, and thus they would benefit from an appropriate physiological response. We therefore investigated whether a sequential two to four week acclimation in Sminthopsis macroura (body mass approx. 25 g) to both cold (16 degrees C) and warm (26 degrees C) ambient temperatures affects the thermal physiology of the species. Cold-acclimated S. macroura were able to significantly increase maximum heat production (by 27%) and could maintain a constant body temperature at significantly lower effective ambient temperatures (about 9 degrees C lower) than when warm-acclimated. Moreover, metabolic rates during torpor were increased following cold-acclimation in comparison to warm-acclimation. Our study shows that, despite the lack of functional brown fat, short-term acclimation can have significant effects on thermoenergetics of marsupials. It is likely that the rapid response in S. macroura reflects an adaptation to the unpredictability of the climate in their habitat.

Effects of fasting on thermoregulatory processes and the daily oscillations in rats

To investigate the mechanism involved in the reduction of body core temperature (T(core)) during fasting in rats, which is selective in the light phase, we measured T(core), surface temperature, and oxygen consumption rate in fed control animals and in fasted animals on day 3 of fasting and day 4 of recovery at an ambient temperature (T(a)) of 23 degrees C by biotelemetry, infrared thermography, and indirect calorimetry, respectively. On the fasting day, 1) T(core) in the light phase decreased (P < 0.05) from the control; however, T(core) in the dark phase was unchanged, 2) tail temperature fell from the control (P < 0.05, from 30.7 +/- 0.1 to 23.9 +/- 0.1 degrees C in the dark phase and from 29.4 +/- 0.1 to 25.2 +/- 0.2 degrees C in the light phase), 3) oxygen consumption rate decreased from the control (P < 0.05, from 24.37 +/- 1.06 to 16.24 +/- 0.69 ml. min(-1). kg body wt(-0.75) in the dark phase and from 18.91 +/- 0.64 to 14.00 +/- 0.41 ml. min(-1). kg body wt(-0.75) in the light phase). All these values returned to the control levels on the recovery day. The results suggest that, in the fasting condition, T(core) in the dark phase was maintained by suppression of the heat loss mechanism, despite the reduction of metabolic heat production. In contrast, the response was weakened in the light phase, decreasing T(core) greatly. Moreover, the change in the regulation of tail blood flow was a likely mechanism to suppress heat loss.

MSG lesions decrease body mass of suckling-age rats by attenuating circadian decreases of energy expenditure

Suckling-age rats display endogenous circadian rhythmicity of metabolic rate (MR) with energy-saving, torpor-like decreases, which are sympathetically controlled and suppressed by leptin treatment. We investigated whether neonatal monosodium glutamate (MSG) treatment, known to cause arcuate nucleus damage and adult-age obesity, alters energy balance in the first two postnatal weeks. Continuously recorded MR and core temperatures (T(c)) show that MSG treatment disinhibits the periodic, sympathetically controlled, energy-saving drops of T(c) and MR. Increased energy expenditure thus explains reduced body fat at normal lean body mass found in MSG-treated pups artificially nourished identically to controls. In MSG-treated mother-reared pups, lean body mass is additionally reduced, suggesting that MSG also reduces suckling. Plasma leptin levels are similar in controls and MSG-treated pups but higher per unit of fat mass in the latter. We conclude that the postweaning development of MSG obesity and depressed thermogenesis are preceded by an early phase of increased energy expenditure with decreased fat deposition during suckling age and hypothesize cell damage in the arcuate nucleus to be involved in both.

Accelerated substrate cycling: a new energy-wasting role for leptin in vivo

Simultaneous lipolysis and reesterification form the triacylglycerol/fatty acid (TAG/FA) cycle, a substrate cycle commonly used for thermogenesis. Its rate was measured in vivo by indirect calorimetry and continuous infusion of [2-(3)H]glycerol and [1-(14)C]palmitate, after injection of leptin or vehicle saline in rabbits. Leptin stimulated in vivo lipolysis from 9.66 +/- 0.62 to 14.78 +/- 0.93 micromol x kg(-1) x min(-1), the rate of appearance of FA from 20.69 +/- 2.14 to 29.03 +/- 3.03 micromol x kg(-1) x min(-1), and TAG/FA cycling from 24.82 +/- 1.73 to 37.09 +/- 2.49 micromol FA x kg(-1) x min(-1). This large increase in total cycling was caused by an 85% rise in primary cycling (reesterification without transit in the circulation) and accounted for 14% of the difference in metabolic rate between the controls and the leptin-treated animals. This study shows that leptin causes a strong activation of TAG/FA cycling, lipolysis, and FA oxidation, shifting fuel preference from carbohydrates to lipids. Therefore, the acceleration of substrate cycling is a new mechanism triggered by leptin to increase metabolic rate, besides the known induction of uncoupling proteins.

Torpor in mice is induced by both leptin-dependent and -independent mechanisms

We tested the effect of chronic leptin treatment on fasting-induced torpor in leptin-deficient A-ZIP/F-1 and ob/ob mice. A-ZIP/F-1 mice have virtually no white adipose tissue and low leptin levels, whereas ob/ob mice have an abundance of fat but no leptin. These two models allowed us to examine the roles of adipose tissue and leptin in the regulation of entry into torpor. Torpor is a short-term hibernation-like state that allows conservation of metabolic fuels. We first characterized the A-ZIP/F-1 animals, which have a 10-fold reduction in total body triglyceride stores. Upon fasting, A-ZIP/F-1 mice develop a lower metabolic rate and decreased plasma glucose, insulin, and triglyceride levels, with no increase in free fatty acids or beta-hydroxybutyrate. Unlike control mice, by 24 hr of fasting, they have nearly exhausted their triglycerides and are catabolizing protein. To conserve energy supplies during fasting, A-ZIP/F-1 (but not control) mice entered deep torpor, with a minimum core body temperature of 24 degrees C, 2 degrees C above ambient. In ob/ob mice, fasting-induced torpor was completely reversed by leptin treatment. In contrast, neither leptin nor thyroid hormone prevented torpor in A-ZIP/F-1 mice. These data suggest that there are at least two signals for entry into torpor in mice, a low leptin level and another signal that is independent of leptin and thyroid hormone levels. Studying rodent torpor provides insight into human torpor-like states such as near drowning in cold water and induced hypothermia for surgery.