Daily torpor in the Djungarian hamster (Phodopus sungorus): photoperiodic regulation, characteristics and circadian organization

Pronounced differences in heart rate and metabolism distinguish daily torpor and short-term hibernation in two bat species

Torpor, and its differential expression, is essential to the survival of many mammals and birds. Physiological characteristics of torpor appear to vary between those species that express strict daily heterothermy and those capable of multiday hibernation, but comparisons are complicated by the temperature-dependence of variables. Previous reviews have compared these different torpor strategies by measuring the depth and duration of torpor in multiple species. However, direct comparison of multiple physiological parameters under similar thermal conditions are lacking. Here, we quantified three physiological variables; body temperature, metabolic rate (MR) and heart rate (HR) of two small heterothermic bats (daily heterotherm Syconycteris australis, and hibernator Nyctophilus gouldi) under comparable thermal conditions and torpor bout durations. When normothermic and resting both MR and HR were similar for the two species. However, during torpor the minimum HR was more than fivefold higher, and minimum MR was 6.5-fold higher for the daily heterotherm than for the hibernator at the same subcutaneous T_b (16 ± 0.5 °C). The data show that the degree of heterothermy defined using T_b is not necessarily a precise proxy for physiological capacity during torpor in these bats and is unlikely to reveal accurate energy budgets. Our study provides evidence supporting a distinction between daily torpor in a daily heterotherm and short term torpor in a hibernator, at least within the Chiroptera with regard to these physiological variables. This exists even when individuals display the same degree of T_b reduction, which has clear implications for the modelling of their energy expenditure.

Hypothesis and Theory: A Two-Process Model of Torpor-Arousal Regulation in Hibernators

Hibernating mammals drastically lower their metabolic rate (MR) and body temperature (Tb) for up to several weeks, but regularly rewarm and stay euthermic for brief periods. It has been hypothesized that the necessity for rewarming is due to the accumulation or depletion of metabolites, or the accrual of cellular damage that can be eliminated only in the euthermic state. Recent evidence for significant inverse relationships between the duration of torpor bouts (TBD) and MR in torpor strongly supports this hypothesis. We developed a new mathematical model that simulates hibernation patterns. The model involves an hourglass process H (Hibernation) representing the depletion/accumulation of a crucial enzyme/metabolite, and a threshold process Hthr. Arousal, modelled as a logistic process, is initiated once the exponentially declining process H reaches Hthr. We show that this model can predict several phenomena observed in hibernating mammals, namely the linear relationship between TMR and TBD, effects of ambient temperature on TBD, the modulation of torpor depth and duration within the hibernation season, (if process Hthr undergoes seasonal changes). The model does not need but allows for circadian cycles in the threshold T, which lead to arousals occurring predominantly at certain circadian phases, another phenomenon that has been observed in certain hibernators. It does not however, require circadian rhythms in Tb or MR during torpor. We argue that a two-process regulation of torpor-arousal cycles has several adaptive advantages, such as an easy adjustment of TBD to environmental conditions as well as to energy reserves and, for species that continue to forage, entrainment to the light-dark cycle.

Body Temperature and Activity Adaptation of Short Photoperiod-Exposed Djungarian Hamsters (Phodopus sungorus): Timing, Traits, and Torpor

To survive the Siberian winter, Djungarian hamsters (Phodopus sungorus) adjust their behavior, morphology, and physiology to maintain energy balance. The reduction of body mass and the improvement of fur insulation are followed by the expression of spontaneous daily torpor, a state of reduced metabolism during the resting phase to save additional energy. Since these complex changes require time, the upcoming winter is anticipated via decreasing photoperiod. Yet, the extent of adaptation and torpor use is highly individual. In this study, adaptation was triggered by an artificially changed light regime under laboratory conditions with 20°C ambient temperature and food and water ad libitum. Two approaches analyzed data on weekly measured body mass and fur index as well as continuously recorded core body temperature and activity during: (1) the torpor period of 60 hamsters and (2) the entire adaptation period of 11 hamsters, aiming to identify parameters allowing (1) a better prediction of torpor expression in individuals during the torpor period as well as (2) an early estimation of the adaptation extent and torpor proneness. In approach 1, 46 torpor-expressing hamsters had a median torpor incidence of 0.3, covering the spectrum from no torpor to torpor every day within one representative week. Torpor use reduced the body temperature during both photo- and scotophase. Torpor was never expressed by 14 hamsters. They could be identified by a high, constant body temperature during the torpor period and a low body mass loss during adaptation to a short photoperiod. Already in the first week of short photoperiod, approach 2 revealed that the hamsters extended their activity over the prolonged scotophase, yet with reduced scotophase activity and body temperature. Over the entire adaptation period, scotophase activity and body temperature of the scoto- and photophases were further reduced, later accompanied by a body mass decline and winter fur development. Torpor was expressed by those hamsters with the most pronounced adaptations. These results provide insights into the preconditions and proximate stimuli of torpor expression. This knowledge will improve experimental planning and sampling for neuroendocrine and molecular research on torpor regulation and has the potential to facilitate acute torpor forecasting to eventually unravel torpor regulation processes.

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

BACKGROUND

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

RESULTS

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

CONCLUSIONS

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

Impaired function of the suprachiasmatic nucleus rescues the loss of body temperature homeostasis caused by time-restricted feeding

The suprachiasmatic nucleus (SCN) is the master circadian pacemaker that drives body temperature rhythm. Time-restricted feeding (TRF) has potential as a preventative or therapeutic approach against many diseases. The potential side effects of TRF remain unknown. Here we show that a 4-hour TRF stimulus in mice can severely impair body temperature homeostasis and can result in lethality. Nearly half of the mice died at 21 °C, and all mice died at 18 °C during 4-hour TRF. Moreover, this effect was modulated by the circadian clock and was associated with severe hypothermia due to loss of body temperature homeostasis, which is different from "torpor", an adaptive response under food deprivation. Disrupting the circadian clock by the SCN lesions or a non-invasive method (constant light) which disrupts circadian clock rescued lethality during TRF. Analysis of circadian gene expression in the dorsomedial hypothalamus (DMH) demonstrated that TRF reprograms rhythmic transcriptome in DMH and suppresses expression of genes, such as Ccr5 and Calcrl, which are involved in thermoregulation. We demonstrate a side effect of 4-hour TRF on the homeostasis of body temperature and a rescue function by impairing the SCN function. Altogether, our results suggested that constructing a circadian arrhythmicity may have a beneficial effect on the host response to an acute stress.

Individual variation of daily torpor and body mass change during winter in the large Japanese field mouse (Apodemus speciosus)

Daily torpor is a strategy used by some overwintering small endotherms to aid in energy conservation. However, the pattern of torpor varies among individuals within species and populations, even under the same environmental conditions, with significant implications for survival rate and reproductive success. Body mass is one factor that may influence this variation, especially in some small mammals that accumulate fat stores prior to overwintering. However, to our knowledge there has been no previous study examining the detailed relationships between torpor expression and body mass change in small mammals that hoard food as an energy resource during winter. The large Japanese field mouse, Apodemus speciosus, whose winter survival strategy depends on food caches instead of fat stores, displays daily torpor under artificial winter conditions (short-day photoperiod and cold). The present study clarifies the characteristics and patterns of daily torpor and body mass change in this species in the laboratory. Although expression of daily torpor was facilitated progressively as in other species, the observed patterns of torpor expression and body mass change showed considerable individual variation. Moreover, there was no obvious correlation between body mass and daily torpor expression. Therefore, it is suggested that in A. speciosus body mass may not contribute to individual variation of daily torpor during winter. Daily torpor during winter may be adjusted by not only mechanisms common to other small mammals, but also species-specific factors relating to the external or internal reserves of energy in small mammals.

Clocks and meals keep mice from being cool

Daily torpor is used by small mammals to reduce daily energy expenditure in response to energetic challenges. Optimizing the timing of daily torpor allows mammals to maximize its energetic benefits and, accordingly, torpor typically occurs in the late night and early morning in most species. However, the regulatory mechanisms underlying such temporal regulation have not been elucidated. Direct control by the circadian clock and indirect control through the timing of food intake have both been suggested as possible mechanisms. Here, feeding cycles outside of the circadian range and brain-specific mutations of circadian clock genes (Vgat-Cre⁺ CK1δ^fl/fl ε^fl/+ ; Vgat-Cre⁺ Bmal1^fl/fl ) were used to separate the roles of the circadian clock and food timing in controlling the timing of daily torpor in mice. These experiments revealed that the timing of daily torpor is transiently inhibited by feeding, while the circadian clock is the major determinant of the timing of torpor. Torpor never occurred during the early part of the circadian active phase, but was preferentially initiated late in the subjective night. Food intake disrupted torpor in the first 4-6 h after feeding by preventing or interrupting torpor bouts. Following interruption, re-initiation of torpor was unlikely until after the next circadian active phase. Overall, these results demonstrate that feeding transiently inhibits torpor while the central circadian clock gates the timing of daily torpor in response to energetic challenges by restricting the initiation of torpor to a specific circadian phase.

Hypothalamic control systems show differential gene expression during spontaneous daily torpor and fasting-induced torpor in the Djungarian hamster (Phodopus sungorus)

Djungarian hamsters are able to use spontaneous daily torpor (SDT) during the winter season as well as fasting-induced torpor (FIT) at any time of the year to cope with energetically challenging environmental conditions. Torpor is a state of severely reduced metabolism with a pronounced decrease in body temperature, which enables animals to decrease their individual energy requirements. Despite sharing common characteristics, such as reduced body mass before first torpor expression and depressed metabolism and body temperature during the torpid state, FIT and SDT differ in several physiological properties including torpor bout duration, minimal body temperature, fuel utilization and circadian organization. It remains unclear, whether SDT and FIT reflect the same phenomenon or two different physiological states. The hypothalamus has been suggested to play a key role in regulating energy balance and torpor. To uncover differences in molecular control mechanisms of torpor expression, we set out to investigate hypothalamic gene expression profiles of genes related to orexigenic (Agrp/Npy), circadian clock (Bmal1/Per1) and thyroid hormone (Dio2/Mct8) systems of animals undergoing SDT and FIT during different torpor stages. Orexigenic genes were mainly regulated during FIT and remained largely unaffected by SDT. Expression patterns of clock genes showed disturbed circadian clock rhythmicity in animals undergoing FIT, but not in animals undergoing SDT. During both, SDT and FIT, decreased Dio2 expression was detected, indicating reduced hypothalamic T3 availability in both types of torpor. Taken together, our results provide evidence that SDT and FIT also differ in certain central control mechanisms and support the observation that animals undergoing SDT are in energetical balance, whereas animals undergoing FIT display a negative energy balance. This should be carefully taken into account when interpreting data in torpor research, especially from animal models of fasting-induced hypometabolism such as mice.

Transcriptome Analysis of Hypothalamic Gene Expression during Daily Torpor in Djungarian Hamsters (Phodopus sungorus)

Animals living at high or temperate latitudes are challenged by extensive changes in environmental conditions over seasons. Djungarian hamsters (Phodopus sungorus) are able to cope with extremely cold ambient temperatures and food scarcity in winter by expressing spontaneous daily torpor. Daily torpor is a circadian controlled voluntary reduction of metabolism that can reduce energy expenditure by up to 65% when used frequently. In the past decades it has become more and more apparent, that the hypothalamus is likely to play a key role in regulating induction and maintenance of daily torpor, but the molecular signals, which lead to the initiation of daily torpor, are still unknown. Here we present the first transcriptomic study of hypothalamic gene expression patterns in Djungarian hamsters during torpor entrance. Based on Illumina sequencing we were able to identify a total number of 284 differentially expressed genes, whereby 181 genes were up- and 103 genes down regulated during torpor entrance. The 20 most up regulated group contained eight genes coding for structure proteins, including five collagen genes, dnha2 and myo15a, as well as the procoagulation factor vwf. In a proximate approach we investigated these genes by quantitative real-time PCR (qPCR) analysis over the circadian cycle in torpid and normothermic animals at times of torpor entrance, mid torpor, arousal and post-torpor. These qPCR data confirmed up regulation of dnah2, myo15a, and vwf during torpor entrance, but a decreased mRNA level for all other investigated time points. This suggests that gene expression of structure genes as well as the procoagulation factor are specifically initiated during the early state of torpor and provides evidence for protective molecular adaptions in the hypothalamus of Djungarian hamsters including changes in structure, transport of biomolecules and coagulation.

Feeding Schedule Controls Circadian Timing of Daily Torpor in SCN-Ablated Siberian Hamsters

Timing of daily torpor was assessed in suprachiasmatic nucleus-ablated (SCNx) and sham-ablated Siberian hamsters fed restricted amounts of food each day either in the light or dark phase of a 14:10 light-dark cycle. Eighty-five percent of sham-ablated and 45% of SCNx hamsters displayed a preferred hour for torpor onset. In each group, time of torpor onset was not random but occurred at a mean hour that differed significantly from chance. Time of food presentation almost completely accounted for the timing of torpor onset in SCNx animals and significantly affected timing of this behavior in intact hamsters. These results suggest that the circadian pacemaker in the SCN controls the time of torpor onset indirectly by affecting timing of food intake, rather than by, or in addition to, direct neural and humoral outputs to relevant target tissues.

Fasting-induced daily torpor in desert hamsters (Phodopus roborovskii)

Daily torpor is frequently expressed in small rodents when facing energetically unfavorable ambient conditions. Desert hamsters (Phodopus roborovskii, ~20g) appear to be an exception as they have been described as homeothermic. However, we hypothesized that they can use torpor because we observed reversible decreases of body temperature (Tb) in fasted hamsters. To test this hypothesis we (i) randomly exposed fasted summer-acclimated hamsters to ambient temperatures (Tas) ranging from 5 to 30°C or (ii) supplied them with different rations of food at Ta 23°C. All desert hamsters showed heterothermy with the lowest mean Tb of 31.4±1.9°C (minimum, 29.0°C) and 31.8±2.0°C (minimum, 29.0°C) when fasted at Ta of 23°C and 19°C, respectively. Below Ta 19°C, the lowest Tb and metabolic rate increased and the proportion of hamsters using heterothermy declined. At Ta 5°C, nearly all hamsters remained normothermic by increasing heat production, suggesting that the heterothermy only occurs in moderately cold conditions, perhaps to avoid freezing at extremely low Tas. During heterothermy, Tbs below 31°C with metabolic rates below 25% of those during normothermia were detected in four individuals at Ta of 19°C and 23°C. Consequently, by definition, our observations confirm that fasted desert hamsters are capable of shallow daily torpor. The negative correlation between the lowest Tbs and amount of food supply shows that heterothermy was mainly triggered by food shortage. Our data indicate that summer-acclimated desert hamsters can express fasting-induced shallow daily torpor, which may be of significance for energy conservation and survival in the wild.

Leptin regulates energy intake but fails to facilitate hibernation in fattening Daurian ground squirrels (Spermophilus dauricus)

Body fat storage before hibernation affects the timing of immergence in Daurian ground squirrels (Spermophilus dauricus). Leptin is an adipose signal and plays vital role in energy homeostasis mainly by action in brain. To test the hypothesis that leptin plays a role in facilitating the process of hibernation, squirrels were administrated with recombinant murine leptin (1μg/day) through intracerebroventricular (ICV) injection for 12 days during fattening. From day 7 to 12, animals were moved into a cold room (5±1°C) with constant darkness which functioned as hibernaculum. Energy intake, body mass and core body temperature (Tb) were continuously monitored throughout the course of experiment. Resting metabolic rate (RMR) was measured under both warm and cold conditions. At the end of leptin administration, we measured the serum concentration of hormones related to energy regulation, mRNA expression of hypothalamic neuropeptides and uncoupling protein 1 (UCP1) levels in brown adipose tissue (BAT). Our results showed that during leptin administration, the cumulative food intake and increase of body mass were suppressed while Tb and RMR were unaltered. The proportion of torpid squirrels was not different between two groups. At the end of leptin administration, the expressions of hypothalamic neuropeptide Y and agouti gene-related protein were suppressed. There were no differences in UCP1 mRNA expression or protein content in BAT between groups. Our data suggest that leptin can affect energy intake via hypothalamic neuropeptides, but is not involved in the initiation of hibernation in fattening Daurian ground squirrels.

The Chemistry of Cold: Mechanisms of Torpor Regulation in the Siberian Hamster

Siberian hamsters use spontaneous daily torpor, a state of hypometabolism and hypothermia, to save energy during winter. Multiple neuroendocrine signals set the scene for spontaneous torpor to occur, and several brain areas have been identified as potential sites for torpor regulation. Here, we summarize the known mechanisms of a fascinating physiological state in the Siberian hamster.

Hibernation in the pygmy slow loris (Nycticebus pygmaeus): multiday torpor in primates is not restricted to Madagascar

Hibernation and short daily torpor are states of energy conservation with reduced metabolism and body temperature. Both hibernation, also called multiday torpor, and daily torpor are common among mammals and occur in at least 11 orders. Within the primates, there is a peculiar situation, because to date torpor has been almost exclusively reported for Malagasy lemurs. The single exception is the African lesser bushbaby, which is capable of daily torpor, but uses it only under extremely adverse conditions. For true hibernation, the geographical restriction was absolute. No primate outside of Madagascar was previously known to hibernate. Since hibernation is commonly viewed as an ancient, plesiomorphic trait, theoretically this could mean that hibernation as an overwintering strategy was lost in all other primates in mainland Africa, Asia, and the Americas. However, we hypothesized that a good candidate species for the use of hibernation, outside of Madagascar should be the pygmy slow loris (Nycticebus pygmaeus), a small primate inhabiting tropical forests. Here, we show that pygmy slow lorises exposed to natural climatic conditions in northern Vietnam during winter indeed undergo torpor lasting up to 63 h, that is, hibernation. Thus, hibernation has been retained in at least one primate outside of Madagascar.

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.

Spontaneous daily torpor and fasting-induced torpor in Djungarian hamsters are characterized by distinct patterns of metabolic rate

The Djungarian hamster is a rodent species that expresses both spontaneous daily torpor (SDT) when acclimated to winter conditions as well as fasting-induced torpor (FIT) during summer. In an earlier report we argued that these two thermoregulatory phenomena differ in several parameters. In the present study, we further complete this comparison by showing that metabolic rate patterns differ between both SDT and FIT. SDT bouts were significantly longer and deeper compared to FIT bouts. Additionally, respiratory quotient measures support the view that SDT is entered from a state of energetic balance while FIT appears to be an emergency shutdown of energy demanding thermogenesis due to a shortage of energy sources. In a second experiment, we also confirm that brief periods of food restriction during the hamsters' torpor season increase the frequency of SDT, but do not affect its depth or duration. Although winter-acclimated animals could flexibly alter torpor frequency in order to stay in energetic balance, we also found evidence for torpor expression patterns that resembled FIT, rather than SDT. Consequently, if energetic challenges cannot be compensated with increased SDT expression any longer, the hamsters seem to be driven in a negative energy balance resulting in FIT as a last resort.

Photorefractoriness and energy availability interact to permit facultative timing of spring breeding

In seasonally breeding mammals, vernal reproductive development is not directly triggered by increases in day length, rather, an endogenous program of photorefractoriness to short winter days initiates spontaneous development in advance of spring. The transition to the reproductive phenotype is energetically demanding. How food availability in late winter and early spring impacts the onset and expression of photorefractoriness is not known. In this study, male Siberian hamsters were born into a simulated natural photoperiod, and at the winter solstice, they were subjected to a restricted feeding protocol in which a daily food ration was provided in an amount equal to ad libitum (AL) intake during the weeks preceding the solstice. Over the next several months, AL-fed control hamsters exhibited spontaneous recrudescence or spontaneous development. In contrast, vernal reproductive development was abolished in most food-rationed hamsters. In food-rationed hamsters that did exhibit recrudescence, conspicuous delays in the onset of gonadal development and decreases in the magnitude of growth were evident. In all hamsters, the termination of food rationing triggered rapid gonadal development. The data indicate that late winter/early spring increases in environmental food availability are required for the normal manifestation of photorefractoriness-induced reproductive development and suggest that a function of photorefractoriness may be merely to disinhibit the reproductive axis from photoperiodic suppression. Vernal gonadal development or recrudescence appears to be strongly affected by proximate energy availability.

Social thermoregulation and torpor in the Siberian hamster

Social thermoregulation and huddling bring about energy benefits to animals sharing a nest because of the smaller surface-to-volume ratio of a huddle and the higher local temperature in the nest. We tested whether living in groups and huddling affect daily torpor, metabolic rate and seasonal changes in the body mass of a small heterothermic rodent, the Siberian hamster (Phodopus sungorus), housed under semi-natural conditions both singly and in groups of four litter-mates. We predicted that in hamsters housed in groups: (1) synchronized torpor bouts would be longer and deeper than non-synchronized ones but shallower than in solitary hamsters, (2) seasonal variations in metabolic rate would be lower than in solitary hamsters, and (3) the winter decrease in body mass would be smaller in grouped than in singly housed hamsters. We found that group housing led to a smaller decrease in body mass in winter, and affected the length and depth of daily torpor. In group-living hamsters more than 50% of all torpor episodes were synchronized and torpid animals were often found in huddles formed of all cage-mates. The longest and deepest torpor bouts in groups were recorded when all animals in a group entered torpor simultaneously. Although the minimum body temperature during torpor was higher, torpor duration was slightly longer than in solitary hamsters. We did not record significant differences in the body mass-adjusted rate of oxygen consumption between solitary and grouped animals, either in the cold or at the lower critical temperature. We conclude that social thermoregulation enables maintenance of a larger body mass, and thus a larger body fat content, which can ensure better body condition at the beginning of the reproductive season.

Daily Torpor Alters Multiple Gene Expression in the Suprachiasmatic Nucleus and Pineal Gland of the Djungarian Hamster (Phodopus sungorus)

Circadian rhythms are still expressed in animals that display daily torpor, implying a temperature compensation of the pacemaker. Nevertheless, it remains unclear how the clock works in hypothermic states and whether torpor itself, as a temperature pulse, affects the circadian system. To reveal changes in the clockwork during torpor, we compared clock gene and neuropeptide expression by in situ hybridization in the suprachiasmatic nucleus (SCN) and pineal gland of normothermic and torpid Djungarian hamsters (Phodopus sungorus). Animals from light-dark (LD) 8ratio16 were sacrificed at 8 time points throughout 24 h. To investigate the effect of a previous torpor episode on the clock, we sacrificed a group of normothermic hamsters 1 day after torpor. In normothermic animals, Per1 peaked at zeitgeber time (ZT)4; whereas, Bmal1 reached maximal expression between ZT16 and ZT19. AVP mRNA in the SCN showed highest levels at ZT7. On the day of torpor, the levels of all mRNAs investigated, except for AVP mRNA, were increased during the torpor bout. Moreover, the Bmal1 rhythm was advanced. On the day after the hypothermia, Bmal1 and AVP rhythms showed severely depressed amplitude. Those distinct amplitude changes of Bmal1 and AVP on the day after a torpor episode expression suggests that torpor affects the circadian system, probably by altered translational processes that might lead to a modified protein feedback on gene expression. In the pineal gland, an important clock output, Aanat expression, peaked between ZT16 and ZT22 in normothermic animals. Aanat levels were significantly advanced on the day of hypothermia, an effect which was still visible 1 day afterward. In summary, this study showed that daily torpor affects the phase and amplitude of rhythmic clock gene and clock-controlled gene expression in the SCN. Furthermore, the rhythmic gene expression in a peripheral oscillator, the pineal gland, is also affected.

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.

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.

Environmental conditions, rather than season, determine torpor use and temperature selection in large mouse-eared bats (Myotis myotis)

We tested whether food availability, thermal environment and time of year affect torpor use and temperature selection in the large mouse-eared bat (Myotis myotis) in summer and winter. Food-deprived bats were torpid longer than bats offered food ad libitum. Bats placed in a gradient of low (0 degrees C-25 degrees C) ambient temperatures (T(a)) spent more time in torpor than bats in a gradient of high (7 degrees C-43 degrees C) T(a)'s. However, we did not observe seasonal variations in the use of torpor. Moreover, even when food deprived in winter, bats never entered prolonged torpor at T(a)'s characteristic of their natural hibernation. Instead, bats preferred shallow torpor at relatively high T(a), but they always maintained a difference between body and ambient temperatures of less than 2 degrees C. Calculations based on respirometric measurements of metabolic rate showed that food deprived bats spent less energy per unit of time in torpor than fed individuals, even when they entered torpor at higher T(a)'s. We conclude that T(a) likely serves as a signal of food availability and daily torpor is apparently an adaptation to unpredictable changes in food availability, such as its decrease in summer or its increase in winter. Thus, we interpret hibernation to be a second step in the evolution of heterothermy in bats, which allows survival in seasonal environments.

Juvenile Siberian Hamsters Display Torpor and Modified Locomotor Activity and Body Temperature Rhythms in Response to Reduced Food Availability

Siberian hamsters as young as 16 and 28 d displayed torpor in response to treatment with 2,500 mg/kg 2-deoxy-D-glucose and reduced food availability, respectively. In addition, most food-restricted hamsters displayed increased locomotor activity and elevated body temperatures in the 3 h immediately preceding daily food delivery. This anticipatory activity disappeared within a few days of reimposition of ad lib. feeding. Torpor first appeared spontaneously at approximately 13 wk of age in hamsters fed ad lib. and maintained in short day lengths. The onset of this "spontaneous" torpor was unaffected by the hamsters' history of food restriction before age 2 mo. Siberian hamsters born late in the breeding season can conserve energy by undergoing torpor immediately after weaning when they contend with food shortages and concurrent energetic challenges imposed by growth requirements and low ambient temperatures.

Daily hypothermia in captive grey mouse lemurs (Microcebus murinus): effects of photoperiod and food restriction

The grey mouse lemur (Microcebus murinus) is a small nocturnal primate exhibiting daily torpor. In constant ambient temperature (22-24 degrees C), body temperature (Tb) and locomotor activity were monitored by telemetry in animals exposed to short (SP: 10 h light/day) or long (LP: 14 light/day) photoperiods. They were first fed ad libitum for 8 days and then subjected to 80% restricted feeding for 8 more days. During ad libitum feeding, locomotor activity was significantly lower in SP-exposed animals than in LP-exposed animals. Whatever the photoperiod, animals entered daily hypothermia within the first hours following the light onset. Depth of daily hypothermia increased irregularly under SP exposure, whereas minimal daily Tb was constantly above 35 degrees C under LP exposure. After the transfer from long photoperiod to short photoperiod corresponding to the induction of seasonal fattening, locomotor activity and depth of controlled daily hypothermia did not change significantly. In contrast, food restriction led to a significant increase in locomotor activity and in frequency of daily torpor (Tb<33 degrees C) and body temperature reached minimum values averaging 25 degrees C. However, SP-exposed animals exhibited lower minimal daily Tb and higher torpor duration than LP exposed animals. Therefore, daily torpor appears as a rapid response to food restriction occurring whatever the photoperiod, although enhanced by short photoperiod.

Hibernation: when good clocks go cold

Hibernating animals have been a successful model system for elucidating fundamental properties of many physiological systems. Over the past 50 years, a diverse literature has emerged on the role of the circadian system in control and expression of winter torpor in several orders of birds and mammals. This body of research has also provided insights to circadian function in non-hibernating species. The aim of this review is to examine how this work applies to questions of general interest to chronobiologists, such as temperature compensation, the 2-oscillator model of entrainment, and suprachiasmatic nucleus (SCN) function. Convergent lines of evidence suggest a role for the SCN in timing daily torpor and controlling several parameters of hibernation. In addition to its role as a circadian pacemaker, the SCN may serve a noncircadian function in hibernators related to maintenance of energy balance.

Daily hypothermia and torpor in a tropical primate: synchronization by 24-h light-dark cycle

To study the temporal organization of daily hypothermia and torpor in a nocturnal Malagasy primate, the gray mouse lemur, body temperature (T(b)) and locomotor activity were recorded using telemetry on 39 males held in 24-h light-dark cycles of different photoperiods. Under free-running condition, the circadian T(b) and locomotor activity rhythms had a period shorter than 24 h. Circadian daily hypothermia started by a rapid drop in T(b) (0.24 degrees C/10 min) at the end of subjective night (13 h 25 +/- 20 min) and was characterized by minimal T(b) values 3 h 20 +/- 5 min later. Spontaneous arousal from daily hypothermia occurred at a fixed time (6 h 05 +/- 15 min, n = 7) after the beginning of subjective day. In animals exposed to 24-h light-dark cycles with night duration varying from 10 to 14 h, locomotor activity was strictly restricted to dark time, but the temporal organization of daily hypothermia was not modified, although changes in amplitude of T(b) rhythm were observed. Daily hypothermia was directly induced by light and lasted 5 h 10 +/- 10 min, with minimal T(b) values 3 h 30 +/- 30 min (n = 28) after lights on, on condition that nighttime did not exceed the duration of subjective night. However, in animals exposed to 24-h light-dark cycles with night duration varying from 10 to 5 h, the limit of induction of daily hypothermia by light was ~9 h after the beginning of night. Finally, under short days (14:10-h light-dark cycle), long bouts (6 h 50 +/- 40 min) of actual torpor (minimum T(b) 27.6 +/- 0.9 degrees C) were observed and would involve mechanisms depending on physiological changes induced by short day exposure.

Torpor characteristics and energy requirements of furless Siberian hamsters

After approximately 10 wk of exposure to decreasing day lengths, Siberian hamsters (Phodopus sungorus) begin to display spontaneous torpor bouts several times each week. Torpor is associated with reduced daily energy expenditure and lower food consumption and ameliorates the thermoregulatory challenges of winter. We tested the extent to which the energy savings conferred by daily torpor depend on the presence of an insulative pelage. Female hamsters were housed in a winter day length (8L:16D) at 5 degrees C; daily food intake and torpor characteristics were recorded for 5 wk in shaved (furless) or normal hamsters. Torpor-bout incidence decreased by 62% in furless hamsters, but the duration of individual bouts and the minimum body temperature attained during torpor were unaffected by loss of pelage. Body temperature declined more rapidly during entry into torpor and increased more slowly during arousal from torpor in furless than in control hamsters. Energy savings per torpor bout, assessed by the amount of food consumed on days that included a torpor bout, was substantially greater in normal than in furless hamsters (16.0% vs. 3.3%); this difference likely reflects the increased cost of thermoregulation during torpor, as well as the increased caloric expenditure incurred by furless hamsters during arousal from torpor. An insulative pelage may be a prerequisite for the energetic benefits derived from heterothermy in this species.

Metabolic adjustments during daily torpor in the Djungarian hamster

Djungarian hamsters (Phodopus sungorus) acclimated to a short photoperiod (8:16-h light-dark cycle) display spontaneous daily torpor with ad libitum food availability. The time course of body temperature (Tb), metabolic rate, respiratory quotient (RQ), and substrate and enzyme changes was measured during entrance into torpor and in deep torpor. RQ, blood glucose, and serum lipids are high during the first hours of torpor but then gradually decline, suggesting that glucose is the primary fuel during the first hours of torpor, with a gradual change to lipid utilization. No major changes in enzyme activities were observed during torpor except for inactivation of the pyruvate dehydrogenase (PDH) complex in liver, brown adipose tissue, and heart muscle. PDH inactivation closely correlates with the reduction of total metabolic rate, whereas in brain, kidney, diaphragm, and skeletal muscle, PDH activity was maintained at the initial level. These findings suggest inhibition of carbohydrate oxidation in heart, brown adipose tissue, and liver during entrance into daily torpor.

Hypoglycemia and torpor in Siberian hamsters

We tested whether reduced blood glucose concentrations are necessary and sufficient for initiation of torpor in Siberian hamsters. During spontaneous torpor bouts, body temperature (Tb) decreases from the euthermic value of 37 to <31 degrees C. Among hamsters that displayed torpor during maintenance in a short-day length (10 h light/day) at an air temperature of 15 degrees C, blood glucose concentrations decreased significantly by 28% as Tb fell from 37 to <31 degrees C and increased during rewarming so that by the time Tb first was >36 degrees C, glucose concentrations had returned to the value preceding torpor. Hamsters did not display torpor when maintained in a long-day length (16 h light/day) and injected with a range of insulin doses (1-50 U/kg body mass), some of which resulted in sustained, pronounced hypoglycemia. We conclude that changes in blood glucose concentrations may be a consequence rather than a cause of the torpid state and question whether induction of torpor by 2-deoxy-D-glucose is due to its general glucoprivic actions.

Regional differences in testosterone effects on vasopressin receptors and on vasopressin immunoreactivity in intact and castrated Siberian hamsters

Vasopressin binding sites were detected in the brain of the Siberian hamster, using [3H]vasopressin and a 125I-labelled linear vasopressin antagonist specific for V1 vasopressin receptors. In the ventromedial and premammillary nuclei, the density of the binding was lower in the females than in the males. The effect of castration and of testosterone replacement was assessed in males. Two distinct effects were observed. Orchidectomy diminished significantly the vasopressin binding in the ventromedial nucleus, an effect which was prevented by implantation of a mini-pump releasing testosterone. On the contrary, in the premammillary nucleus no significant differences were noticed following castration and testosterone treatment. In addition, vasopressin immunoreactivity was examined in males, in females and in castrated males. No sex differences were evident. However, in the bed nucleus of the stria terminalis and the lateral septal nucleus, castration decreased vasopressin immunoreactivity in either sex. This effect of castration was prevented by testosterone. Vasopressin immunoreactivity was detected neither in the ventromedial nor in the premammillary hypothalamic nuclei. Our observations suggest that, in adult Siberian hamster premammillary nucleus, the expression of vasopressin receptors is not controlled by gonadal steroids but is sex related and could be induced during fetal or early postnatal life.

Effects of temperature, steroids and castration on daily torpor in the Djungarian hamster (Phodopus sungorus)

1. In Phodopus sungorus the frequency and characteristics of daily torpor were not affected by a decrease in temperature. The seasonal cycle of daily torpor is essentially under photoperiodic control. 2. Testicular regression is a necessary condition for daily torpor occurrence. Testosterone and 5 alpha-dihydrotestosterone administration totally inhibited daily torpor in hamsters which were exposed to short days. The temporal inhibition of torpor by steroids had no effect on the seasonal time measuring mechanism governing daily torpor. 3. Castration influenced certain aspects of the daily torpor display. Castrated hamsters showed a delay in terminating torpor season lending further support to the hypothesis that recrudescence of the testis plays a role in terminating the daily torpor season. Castration, when performed before or on the day of transfer to short photoperiod led to an increase in the frequency of torpor bouts and reduce SP exposure duration which is necessary for the daily torpor season beginning.