Cardiovascular changes during daily torpor in the laboratory mouse

Feeding behavior modifies the circadian variation in RR and QT intervals by distinct mechanisms in mice

Rhythmic feeding behavior is critical for regulating phase and amplitude in the ≈24-h variation of heart rate (RR intervals), ventricular repolarization (QT intervals), and core body temperature in mice. We hypothesized changes in cardiac electrophysiology associated with feeding behavior were secondary to changes in core body temperature. Telemetry was used to record electrocardiograms and core body temperature in mice during ad libitum-fed conditions and after inverting normal feeding behavior by restricting food access to the light cycle. Light cycle-restricted feeding modified the phase and amplitude of 24-h rhythms in RR and QT intervals, and core body temperature to realign with the new feeding time. Changes in core body temperature alone could not account for changes in phase and amplitude in the ≈24-h variation of the RR intervals. Heart rate variability analysis and inhibiting β-adrenergic and muscarinic receptors suggested that changes in the phase and amplitude of 24-h rhythms in RR intervals were secondary to changes in autonomic signaling. In contrast, changes in QT intervals closely mirrored changes in core body temperature. Studies at thermoneutrality confirmed that the daily variation in QT interval, but not RR interval, primarily reflected daily changes in core body temperature (even in ad libitum-fed conditions). Correcting the QT interval for differences in core body temperature helped unmask QT interval prolongation after starting light cycle-restricted feeding and in a mouse model of long QT syndrome. We conclude feeding behavior alters autonomic signaling and core body temperature to regulate phase and amplitude in RR and QT intervals, respectively.NEW & NOTEWORTHY We used time-restricted feeding and thermoneutrality to demonstrate that different mechanisms regulate the 24-h rhythms in heart rate and ventricular repolarization. The daily rhythm in heart rate reflects changes in autonomic input, whereas daily rhythms in ventricular repolarization reflect changes in core body temperature. This novel finding has major implications for understanding 24-h rhythms in mouse cardiac electrophysiology, arrhythmia susceptibility in transgenic mouse models, and interpretability of cardiac electrophysiological data acquired in thermoneutrality.

Methods to estimate body temperature and energy expenditure dynamics in fed and fasted laboratory mice: effects of sleep deprivation and light exposure

Monitoring body temperature and energy expenditure in freely-moving laboratory mice remains a powerful methodology used widely across a variety of disciplines-including circadian biology, sleep research, metabolic phenotyping, and the study of body temperature regulation. Some of the most pronounced changes in body temperature are observed when small heterothermic species reduce their body temperature during daily torpor. Daily torpor is an energy saving strategy characterized by dramatic reductions in body temperature employed by mice and other species when challenged to meet energetic demands. Typical measurements used to describe daily torpor are the measurement of core body temperature and energy expenditure. These approaches can have drawbacks and developing alternatives for these techniques provides options that can be beneficial both from an animal-welfare and study-complexity perspective. First, this paper presents and assesses a method to estimate core body temperature based on measurements of subcutaneous body temperature, and second, a separate approach to better estimate energy expenditure during daily torpor based on core body temperature. Third, the effects of light exposure during the habitual dark phase and sleep deprivation during the light period on body temperature dynamics were tested preliminary in fed and fasted mice. Together, the here-published approaches and datasets can be used in the future to assess body temperature and metabolism in freely-moving laboratory mice.

Feeding Behavior Modifies the Circadian Variation in RR and QT intervals by Distinct Mechanisms in Mice

Rhythmic feeding behavior is critical for regulating the phase and amplitude in the ≍24-hour variation of the heart rate (RR intervals), ventricular repolarization (QT intervals), and core body temperature in mice. We hypothesized the changes in cardiac electrophysiology associated with feeding behavior were secondary to changes in core body temperature. Telemetry was used to record electrocardiograms and core body temperature in mice during ad libitum-fed conditions and after inverting normal feeding behavior by restricting food access to the light cycle. Light cycle-restricted feeding quickly modified the phase and amplitude of the 24-hour rhythms in RR intervals, QT intervals, and core body temperature to realign with the new feeding time. Heart rate variability analysis and inhibiting β-adrenergic and muscarinic receptors suggested that the changes in the phase and amplitude of the 24-hour rhythms in RR intervals were secondary to changes in autonomic signaling. In contrast, the changes in the QT intervals closely mirrored changes in core body temperature. Studies at thermoneutrality confirmed the daily variation in the QT interval, but not the RR interval, and reflected daily changes in core body temperature (even in ad libitum-fed conditions). Correcting the QT interval for differences in core body temperature helped to unmask QT interval prolongation after starting light cycle-restricted feeding and in a mouse model of long QT syndrome. We conclude feeding behavior alters autonomic signaling and core body temperature to regulate the phase and amplitude in RR and QT intervals, respectively.

A torpor-like state (TLS) in mice slows blood epigenetic aging and prolongs healthspan

Torpor and hibernation are extreme physiological adaptations of homeotherms associated with pro-longevity effects. Yet the underlying mechanisms of how torpor affects aging, and whether hypothermic and hypometabolic states can be induced to slow aging and increase health span, remain unknown. We demonstrate that the activity of a spatially defined neuronal population in the avMLPA, which has previously been identified as a torpor-regulating brain region, is sufficient to induce a torpor like state (TLS) in mice. Prolonged induction of TLS slows epigenetic aging across multiple tissues and improves health span. We isolate the effects of decreased metabolic rate, long-term caloric restriction, and decreased core body temperature (Tb) on blood epigenetic aging and find that the pro-longevity effect of torpor-like states is mediated by decreased Tb. Taken together, our findings provide novel mechanistic insight into the pro-longevity effects of torpor and hibernation and support the growing body of evidence that Tb is an important mediator of aging processes.

Activation of oxytocinergic neurons enhances torpor in mice

Mus musculus enters a torpid state in response to caloric restriction in sub-thermoneutral ambient temperatures. This torpid state is characterized by an adaptive and controlled decrease in metabolic rate, heart rate, body temperature, and activity. Previous research has identified the paraventricular nucleus (PVN) within the hypothalamus, a region containing oxytocin neurons, as a location that is active during torpor onset. We hypothesized that oxytocin neurons within the PVN are part of this neural circuit and that activation of oxytocin neurons would deepen and lengthen torpor bouts. We report that activation of oxytocin neurons alone is not sufficient to induce a torpor-like state in the fed mouse, with no significant difference in body temperature or heart rate upon activation of oxytocin neurons. However, we found that activation of oxytocin neurons prior to the onset of daily torpor both deepens and lengthens the subsequent bout, with a 1.7 ± 0.4 °C lower body temperature and a 135 ± 32 min increase in length. We therefore conclude that oxytocin neurons are involved in the neural circuitry controlling daily torpor in the mouse.

HCN channels sense temperature and determine heart rate responses to heat

Heart rate increases with heat, [1-3] constituting a fundamental physiological relationship in vertebrates. Each normal heartbeat is initiated by an action potential generated in a sinoatrial nodal pacemaker cell. Pacemaker cells are enriched with hyperpolarization activated cyclic nucleotide-gated ion channels (HCN) that deliver cell membrane depolarizing inward current that triggers action potentials. HCN channel current increases due to cAMP binding, a mechanism coupling adrenergic tone to physiological 'fight or flight' heart rate acceleration. However, the mechanism(s) for heart rate response to thermal energy is unknown. We used thermodynamical and homology computational modeling, site-directed mutagenesis and mouse models to identify a concise motif on the S4-S5 linker of the cardiac pacemaker HCN4 channels (M407/Y409) that determines HCN4 current (If) and cardiac pacemaker cell responses to heat. This motif is required for heat sensing in cardiac pacemaker cells and in isolated hearts. In contrast, the cyclic nucleotide binding domain is not required for heat induced HCN4 current increases. However, a loss of function M407/Y409 motif mutation prevented normal heat and cAMP responses, suggesting that heat sensing machinery is essential for operating the cAMP allosteric pathway and is central to HCN4 modulation. The M407/Y409 motif is conserved across all HCN family members suggesting that HCN channels participate broadly in coupling heat to changes in cell membrane excitability.

Mechanosensation of the heart and gut elicits hypometabolism and vigilance in mice

Interoception broadly refers to awareness of one's internal milieu. Vagal sensory afferents monitor the internal milieu and maintain homeostasis by engaging brain circuits that alter physiology and behavior. While the importance of the body-to-brain communication that underlies interoception is implicit, the vagal afferents and corresponding brain circuits that shape perception of the viscera are largely unknown. Here, we use mice to parse neural circuits subserving interoception of the heart and gut. We determine vagal sensory afferents expressing the oxytocin receptor, hereafter referred to as NDGOxtr, send projections to the aortic arch or stomach and duodenum with molecular and structural features indicative of mechanosensation. Chemogenetic excitation of NDGOxtr significantly decreases food and water consumption, and remarkably, produces a torpor-like phenotype characterized by reductions in cardiac output, body temperature, and energy expenditure. Chemogenetic excitation of NDGOxtr also creates patterns of brain activity associated with augmented hypothalamic-pituitary-adrenal axis activity and behavioral indices of vigilance. Recurrent excitation of NDGOxtr suppresses food intake and lowers body mass, indicating that mechanosensation of the heart and gut can exert enduring effects on energy balance. These findings suggest that the sensation of vascular stretch and gastrointestinal distention may have profound effects on whole body metabolism and mental health.

What to consider for ECG in mice-with special emphasis on telemetry

Genetically or surgically altered mice are commonly used as models of human cardiovascular diseases. Electrocardiography (ECG) is the gold standard to assess cardiac electrophysiology as well as to identify cardiac phenotypes and responses to pharmacological and surgical interventions. A variety of methods are used for mouse ECG acquisition under diverse conditions, making it difficult to compare different results. Non-invasive techniques allow only short-term data acquisition and are prone to stress or anesthesia related changes in cardiac activity. Telemetry offers continuous long-term acquisition of ECG data in conscious freely moving mice in their home cage environment. Additionally, it allows acquiring data 24/7 during different activities, can be combined with different challenges and most telemetry systems collect additional physiological parameters simultaneously. However, telemetry transmitters require surgical implantation, the equipment for data acquisition is relatively expensive and analysis of the vast number of ECG data is challenging and time-consuming. This review highlights the limits of non-invasive methods with respect to telemetry. In particular, primary screening using non-invasive methods can give a first hint; however, subtle cardiac phenotypes might be masked or compensated due to anesthesia and stress during these procedures. In addition, we detail the key differences between the mouse and human ECG. It is crucial to consider these differences when analyzing ECG data in order to properly translate the insights gained from murine models to human conditions.

Sex-Dependent Impairment of Endothelium-Dependent Relaxation in Aorta of Mice with Overexpression of Hyaluronan in Tunica Media

Diabetic macroangiopathy is characterized by increased extracellular matrix deposition, including excessive hyaluronan accumulation, vessel thickening and stiffness, and endothelial dysfunction in large arteries. We hypothesized that the overexpression of hyaluronan in the tunica media also led to endothelial cell (EC) dysfunction. To address this hypothesis, we investigated the following in the aortas of mice with excessive hyaluronan accumulation in the tunica media (HAS-2) and wild-type mice: EC dysfunction via myograph studies, nitric oxide (NO) bioavailability via diaminofluorescence, superoxide formation via dihydroethidium fluorescence, and the distances between ECs via stereological methods. EC dysfunction, characterized by blunted relaxations in response to acetylcholine and decreased NO bioavailability, was found in the aortas of male HAS-2 mice, while it was unaltered in the aortas of female HAS-2 mice. Superoxide levels increased and extracellular superoxide dismutase (ecSOD) expression decreased in the aortas of male and female HAS-2 mice. The EC-EC distances and LDL receptor expression were markedly increased in the HAS-2 aortas of male mice. Our findings suggest hyaluronan increases oxidative stress in the vascular wall and that together with increased EC distance, it is associated with a sex-specific decrease in NO levels and endothelial dysfunction in the aorta of male HAS-2 transgenic mice.

Opportunities and barriers to translating the hibernation phenotype for neurocritical care

Targeted temperature management (TTM) is standard of care for neonatal hypoxic ischemic encephalopathy (HIE). Prevention of fever, not excluding cooling core body temperature to 33°C, is standard of care for brain injury post cardiac arrest. Although TTM is beneficial, HIE and cardiac arrest still carry significant risk of death and severe disability. Mammalian hibernation is a gold standard of neuroprotective metabolic suppression, that if better understood might make TTM more accessible, improve efficacy of TTM and identify adjunctive therapies to protect and regenerate neurons after hypoxic ischemia brain injury. Hibernating species tolerate cerebral ischemia/reperfusion better than humans and better than other models of cerebral ischemia tolerance. Such tolerance limits risk of transitions into and out of hibernation torpor and suggests that a barrier to translate hibernation torpor may be human vulnerability to these transitions. At the same time, understanding how hibernating mammals protect their brains is an opportunity to identify adjunctive therapies for TTM. Here we summarize what is known about the hemodynamics of hibernation and how the hibernating brain resists injury to identify opportunities to translate these mechanisms for neurocritical care.

Cold stress during room temperature housing alters skeletal response to simulated microgravity (hindlimb unloading) in growing female C57BL6 mice

Laboratory mice are typically housed at temperatures below the thermoneutral zone for the species, resulting in cold stress and premature cancellous bone loss. Furthermore, mice are more dependent upon non-shivering thermogenesis to maintain body temperature during spaceflight, suggesting that microgravity-induced bone loss may be due, in part, to altered thermogenesis. Consequently, we assessed whether housing mice at room temperature modifies the skeletal response to simulated microgravity. This possibility was tested using the hindlimb unloading (HLU) model to mechanically unload femora. Humeri were also assessed as they remain weight bearing during HLU. Six-week-old female C57BL6 (B6) mice were housed at room temperature (22 °C) or near thermoneutral (32 °C) and HLU for 2 weeks. Compared to baseline, HLU resulted in cortical bone loss in femur, but the magnitude of reduction was greater in mice housed at 22 °C. Cancellous osteopenia in distal femur (metaphysis and epiphysis) was noted in HLU mice housed at both temperatures. However, bone loss occurred at 22 °C, whereas the bone deficit at 32 °C was due to failure to accrue bone. HLU resulted in cortical and cancellous bone deficits (compared to baseline) in humeri of mice housed at 22 °C. In contrast, fewer osteopenic changes were detected in mice housed at 32 °C. These findings support the hypothesis that environmental temperature alters the skeletal response to HLU in growing female mice in a bone compartment-specific manner. Taken together, species differences in thermoregulation should be taken into consideration when interpreting the skeletal response to simulated microgravity.

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.

A cardiac-null mutation of Prdm16 causes hypotension in mice with cardiac hypertrophy via increased nitric oxide synthase 1

Hypertension or hypotension prevails as a comorbidity in patients with heart failure (HF). Although blood pressure (BP) is an important factor in managing the mortality of HF, the molecular mechanisms of changes in BP have not been clearly understood in cases of HF. We and others have demonstrated that a loss in PRDM16 causes hypertrophic cardiomyopathy, leading to HF. We aimed to determine whether BP is altered in mice that experience cardiac loss of Prdm16 and identify the underlying mechanism of BP-associated changes. BP decreased significantly only in female mice with a cardiac-null mutation of Prdm16 compared with controls, by an invasive protocol under anesthesia and by telemetric method during conscious, unrestrained status. Mice with a cardiac loss of Prdm16 had higher heart-to-body weight ratios and upregulated atrial natriuretic peptide, suggesting cardiac hypertrophy. Plasma aldosterone-to-renin activity ratios and plasma sodium levels decreased in Prdm16-deficient mice versus control. By RNA-seq and in subsequent functional analyses, Prdm16-null hearts were enriched in factors that regulate BP, including Adra1a, Nos1, Nppa, and Nppb. The inhibition of nitric oxide synthase 1 (NOS1) reverted the decrease in BP in cardiac-specific Prdm16 knockout mice. Mice with cardiac deficiency of Prdm16 present with hypotension and cardiac hypertrophy. Further, our findings suggest that the increased expression of NOS1 causes hypotension in mice with a cardiac-null mutation of Prdm16. These results provide novel insights into the molecular mechanisms of hypotension in subjects with HF and contribute to our understanding of how hypotension develops in patients with HF.

Effect of season and diet on heart rate and blood pressure in female red deer (Cervus elaphus) anaesthetised with medetomidine-tiletamine-zolazepam

Temperate zone ungulates like red deer (Cervus elaphus) show pronounced seasonal acclimatisation. Hypometabolism during winter is associated with cardiovascular changes, including a reduction in heart rate (fH) and temporal peripheral vasoconstriction. How anaesthesia with vasoactive substances such as medetomidine affect the seasonally acclimatised cardiovascular system is not yet known. We anaesthetised eleven healthy female red deer with medetomidine (0.1 mg/kg) and tiletamine/zolazepam (3 mg/kg) twice in winter (ad libitum and restricted feed) and in summer (ad libitum and restricted feed), with a two-week washout-period in-between, to test for the effect of season, food availability and supplementation with omega-3 or omega-6 polyunsaturated fatty acid (PUFA) on fH and arterial blood pressure (ABP) during anaesthesia. Six animals received pellets enriched with omega-6 fatty acids (FA), and five animals with omega-3 FA. Anaesthesia significantly decreased fH in summer but not in winter and ABP was lower in winter (p < 0.05). The combination of omega-6 FA enriched pellets and food restriction resulted in a lower fH and higher ABP during anaesthesia with more pronounced changes in winter (p < 0.001). Our results demonstrate that season, food availability and type of PUFA supplementation in red deer affect the cardiovascular system during anaesthesia.

Neurons in the Dorsomedial Hypothalamus Promote, Prolong, and Deepen Torpor in the Mouse

Torpor is a naturally occurring, hypometabolic, hypothermic state engaged by a wide range of animals in response to imbalance between the supply and demand for nutrients. Recent work has identified some of the key neuronal populations involved in daily torpor induction in mice, in particular, projections from the preoptic area of the hypothalamus to the dorsomedial hypothalamus (DMH). The DMH plays a role in thermoregulation, control of energy expenditure, and circadian rhythms, making it well positioned to contribute to the expression of torpor. We used activity-dependent genetic TRAPing techniques to target DMH neurons that were active during natural torpor bouts in female mice. Chemogenetic reactivation of torpor-TRAPed DMH neurons in calorie-restricted mice promoted torpor, resulting in longer and deeper torpor bouts. Chemogenetic inhibition of torpor-TRAPed DMH neurons did not block torpor entry, suggesting a modulatory role for the DMH in the control of torpor. This work adds to the evidence that the preoptic area of the hypothalamus and the DMH form part of a circuit within the mouse hypothalamus that controls entry into daily torpor.SIGNIFICANCE STATEMENT Daily heterotherms, such as mice, use torpor to cope with environments in which the supply of metabolic fuel is not sufficient for the maintenance of normothermia. Daily torpor involves reductions in body temperature, as well as active suppression of heart rate and metabolism. How the CNS controls this profound deviation from normal homeostasis is not known, but a projection from the preoptic area to the dorsomedial hypothalamus has recently been implicated. We demonstrate that the dorsomedial hypothalamus contains neurons that are active during torpor. Activity in these neurons promotes torpor entry and maintenance, but their activation alone does not appear to be sufficient for torpor entry.

Erectile Dysfunction and Altered Contribution of KCa1.1 and KCa2.3 Channels in the Penile Tissue of Type-2 Diabetic db/db Mice

BACKGROUND

Activation of endothelial small conductance calcium-activated K+ channels (KCa2.3) and intermediate conductance calcium-activated K+ channels (KCa3.1) leads to vascular relaxation. We found endothelial KCa2.3 down-regulation in the corpus cavernosum diminishes erectile function.

AIM

We hypothesized that in type-2 diabetic mice, the function of KCa2.3 and KCa1.1 channels is impaired in erectile tissue.

METHODS

Erectile function was measured, and corpus cavernosum strips were mounted for functional studies and processed for qPCR and immunoblotting.

OUTCOMES

Effects of type 2 diabetes on erectile function, expression and function of calcium-activated potassium channels.

RESULTS

In anesthetized diabetic db/db mice, erectile function was markedly decreased compared to non-diabetic heterozygous db/+ mice, and the impairment was even more pronounced compared to normal C57BL/6 mice. qPCR revealed KCa2.3 and KCa1.1α channel expressions were upregulated in corpus cavernosum from db/db mice. Immunoblotting showed down-regulation of KCa2.3 channels in the corpus cavernosum from db/db mice. Acetylcholine relaxations were impaired while relaxations induced by the nitric oxide, donor SNP were unaltered in corpus cavernosum from db/db compared to C57BL/6 and db/+ mice. Apamin, a blocker of KCa2 channels, inhibited acetylcholine relaxation in corpus cavernosum from all experimental groups. In the presence of apamin, acetylcholine relaxation was markedly decreased in corpus cavernosum from db/db vs C57BL/6 and db/+ mice. An opener of KCa2 and KCa3.1 channels, NS309, potentiated acetylcholine relaxations in corpus cavernosum from db/+ and db/db mice. Iberiotoxin, a blocker of KCa1.1 channels, inhibited acetylcholine relaxation in corpus cavernosum from db/+ mice, while there was no effect in tissue from db/db mice.

CLINICAL TRANSLATION

Erectile function in diabetic db/db mice was severely affected compared to heterozygous and control mice, findings suggesting the non-diabetic db/+ and diabetic db/db mice for translational purpose can be used for drug testing on, respectively, moderate and severe erectile dysfunction. The altered expressions and impaired acetylcholine relaxation in the presence of apamin compared to C57BL/6 mice may suggest decreased KCa1.1 channel function may underpin impaired endothelium-dependent relaxation and erectile dysfunction in diabetic db/db mice.

STRENGTHS & LIMITATIONS

The present study provides a mouse model for type 2 diabetes to test moderate and severe erectile dysfunction drugs. Decreased KCa1.1 channel function contributes to erectile dysfunction, and it is a limitation that it is not supported by electrophysiological measurements.

CONCLUSION

Our results suggest that the contribution of iberiotoxin-sensitive KCa1.1 channels to relaxation is reduced in the corpus cavernosum, while apamin-sensitive KCa2.3 channels appear upregulated. The impaired KCa1.1 channel function may contribute to the impaired erectile function in diabetic db/db mice.

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.

Bezafibrate Rescues Mitochondrial Encephalopathy in Mice via Induction of Daily Torpor and Hypometabolic State

Leigh syndrome (LS) is one of the most common mitochondrial encephalopathy diseases in infants. To date, there is still an absence of effective therapy. Bezafibrate (BEZ), a pan-peroxisome proliferator-activated receptor (PPAR) agonist, ameliorates the phenotype of the mouse model of mitochondrial disease via an unclear mechanism. Here, we applied it to Ndufs4 knockout (KO) mice, a widely used LS animal model, to observe the therapeutic effects and metabolic changes associated with BEZ treatment to explore the therapeutic strategies for mitochondrial diseases. Administration of BEZ significantly enhances survival and attenuates disease progression in Ndufs4 KO mice. Decreased oxidative stress and stunted growth were also observed. As a PPAR agonist, we did not find mitochondrial biogenesis or enhanced metabolism upon BEZ treatment. On the contrary, mice with dietary BEZ showed daily torpor bouts and lower metabolic rates. We speculate that activating energy-saving metabolism in mice may be associated with the therapeutic effects of BEZ, but the exact mechanism of action requires further study.

Leptin and environmental temperature as determinants of bone marrow adiposity in female mice

Bone marrow adipose tissue (BMAT) levels are higher in distal femur metaphysis of female mice housed at thermoneutral (32°C) than in mice housed at 22°C, as are abdominal white adipose tissue (WAT) mass, and serum leptin levels. We performed two experiments to explore the role of increased leptin in temperature-enhanced accrual of BMAT. First, we supplemented 6-week-old female C57BL/6J (B6) mice with leptin for 2 weeks at 10 µg/d using a subcutaneously implanted osmotic pump. Controls consisted of ad libitum (ad lib) fed mice and mice pair fed to match food intake of leptin-supplemented mice. The mice were maintained at 32°C for the duration of treatment. At necropsy, serum leptin in leptin-supplemented mice did not differ from ad lib mice, suggesting suppression of endogenous leptin production. In support, Ucp1 expression in BAT, percent body fat, and abdominal WAT mass were lower in leptin-supplemented mice. Leptin-supplemented mice also had lower BMAT and higher bone formation in distal femur metaphysis compared to the ad lib group, changes not replicated by pair-feeding. In the second experiment, BMAT response was evaluated in 6-week-old female B6 wild type (WT), leptin-deficient ob/ob and leptin-treated (0.3 μg/d) ob/ob mice housed at 32°C for the 2-week duration of the treatment. Compared to mice sacrificed at baseline (22°C), BMAT increased in ob/ob mice as well as WT mice, indicating a leptin independent response to increased temperature. However, infusion of ob/ob mice with leptin, at a dose rate having negligible effects on either energy metabolism or serum leptin levels, attenuated the increase in BMAT. In summary, increased housing temperature and increased leptin have independent but opposing effects on BMAT in mice.

Changes in breathing pattern during severe hypothermia and autoresuscitation from hypothermic respiratory arrest in anesthetized mice

Some evidence suggests that both hypothermia and anesthesia can exert similar effects on metabolism and ventilation. This study examined the synergistic effects of anesthesia and hypothermia on ventilation in spontaneously breathing adult mice under three different conditions, that is, (1) pentobarbital group (n = 7) in which mice were anesthetized with intraperitoneal pentobarbital of 80 mg/kg, (2) sevoflurane-continued group (n = 7) in which mice were anesthetized with 1 MAC sevoflurane, and (3) sevoflurane-discontinued group (n = 7) in which sevoflurane was discontinued at a body temperature below 22˚C. We cooled mice in each group until breathing ceased and followed this with artificial rewarming while measuring changes in respiratory variables and heart rate. We found that the body temperature at which respiration arrested is much lower in the sevoflurane-discontinued group (13.8 ± 2.0˚C) than that in the sevoflurane-continued group (16.7 ± 1.2˚C) and the pentobarbital group (17.0 ± 1.4˚C). Upon rewarming, all animals in all three groups spontaneously recovered from respiratory arrest. There was a considerable difference in breathing patterns between sevoflurane-anesthetized mice and pentobarbital-anesthetized mice during progressive hypothermia in terms of changes in tidal volume and respiratory frequency. The changes in the respiratory pattern during rewarming are nearly mirrored images of the changes observed during cooling in all three groups. These observations indicate that adult mice are capable of autoresuscitation from hypothermic respiratory arrest and that anesthesia and hypothermia exert synergistic effects on the occurrence of respiratory arrest while the type of anesthetic affects the breathing pattern that occurs during progressive hypothermia leading to respiratory arrest.

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.

Impact of endogenous melatonin on rhythmic behaviors, reproduction, and survival revealed in melatonin-proficient C57BL/6J congenic mice

The hormone melatonin is synthesized from serotonin by two enzymatic reactions (AANAT and ASMT/HIOMT) in the pineal gland following a circadian rhythm with low levels during the day and high levels at night. The robust nightly peak of melatonin secretion is an output signal of the circadian clock to the whole organism. However, so far the regulatory roles of endogenous melatonin in mammalian biological rhythms and physiology processes are poorly understood. Here, we establish congenic mouse lines (>N10 generations) that are proficient or deficient in melatonin synthesis (AH+/+ or AH-/- mice, respectively) on the C57BL/6J genetic background by crossing melatonin-proficient MSM/Ms with C57BL/6J. AH+/+ mice displayed robust nightly peak of melatonin secretion and had significantly higher levels of pineal and plasma melatonin vs AH-/- mice. Using this mice model, we investigated the role of endogenous melatonin in regulating multiple biological rhythms, physiological processes, and rhythmic behaviors. In the melatonin-proficient (AH+/+) mice, the rate of re-entrainment of wheel-running activity was accelerated following a 6-hour phase advance of dark onset when comparted with AH-/- mice, suggesting a role of endogenous melatonin in facilitating clock adjustment. Further in the AH+/+ mice, there was a significant decrease in body weight, gonadal weight and reproductive performance, and a significant increase in daily torpor (a hypothermic and hypometabolic state lasting only hours during adverse conditions). Endogenous melatonin, however, had no effect in the modulation of the diurnal rhythm of 2-[¹²⁵ I]-iodomelatonin receptor expression in the SCN, free-running wheel behavior in constant darkness, life span, spontaneous homecage behaviors, and various types of social-emotional behaviors. The findings also shed light on the role of endogenous melatonin in mice domestication and provide new insights into melatonin's action in reducing energy expenditure during a food shortage. In summary, the congenic mice model generated in this study offers a significant advantage toward understanding of the role of endogenous melatonin in regulating melatonin receptor-mediated rhythm behaviors and physiological functions.

Evidence for increased thermogenesis in female C57BL/6J mice housed aboard the international space station

Sixteen-week-old female C57BL/6J mice were sacrificed aboard the International Space Station after 37 days of flight (RR-1 mission) and frozen carcasses returned to Earth. RNA was isolated from interscapular brown adipose tissue (BAT) and gonadal white adipose tissue (WAT). Spaceflight resulted in differential expression of genes in BAT consistent with increased non-shivering thermogenesis and differential expression of genes in WAT consistent with increased glucose uptake and metabolism, adipogenesis, and β-oxidation.

Pirfenidone Is a Vasodilator: Involvement of KV7 Channels in the Effect on Endothelium-Dependent Vasodilatation in Type-2 Diabetic Mice

Endothelial cell dysfunction and fibrosis are associated with worsening of the prognosis in patients with cardiovascular disease. Pirfenidone has a direct antifibrotic effect, but vasodilatation may also contribute to the effects of pirfenidone. Therefore, in a first study we investigated the mechanisms involved in the relaxant effect of pirfenidone in rat intrapulmonary arteries and coronary arteries from normal mice. Then in a second study, we investigated whether pirfenidone restores endothelial function in the aorta and mesenteric arteries from diabetic animals. From 16–18-week old normal male C57BL/6 mice and normoglycemic (db/db+), and type 2 diabetic (db/db) male and female mice, arteries were mounted in microvascular isometric myographs for functional studies, and immunoblotting was performed. In rat pulmonary arteries and mouse coronary arteries, pirfenidone induced relaxations, which were inhibited in preparations without endothelium. In mouse coronary arteries, pirfenidone relaxation was inhibited in the presence of a nitric oxide (NO) synthase inhibitor, N^G-nitro-l-arginine (L-NOARG), a blocker of large-conductance calcium-activated potassium channels (BK_Ca), iberiotoxin, and a blocker of K_V7 channels, XE991. Patch clamp studies in vascular smooth muscle revealed pirfenidone increased iberiotoxin-sensitive current. In the aorta and mesenteric small arteries from diabetic db/db mice relaxations induced by the endothelium-dependent vasodilator, acetylcholine, were markedly reduced compared to db/db + mice. Pirfenidone enhanced the relaxations induced by acetylcholine in the aorta from diabetic male and female db/db mice. An opener of K_V7 channels, flupirtine, had the same effect as pirfenidone. XE991 reduced the effect of pirfenidone and flupirtine and further reduced acetylcholine relaxations in the aorta. In the presence of iberiotoxin, pirfenidone still increased acetylcholine relaxation in aorta from db/db mice. Immunoblotting for K_V7.4, K_V7.5, and BK_Ca channel subunits were unaltered in aorta from db/db mice. Pirfenidone failed to improve acetylcholine relaxation in mesenteric arteries, and neither changed acetylcholine-induced transient decreases in blood pressure in db/db+ and db/db mice. In conclusion, pirfenidone vasodilates pulmonary and coronary arteries. In coronary arteries from normal mice, pirfenidone induces NO-dependent vasodilatation involving BK_Ca and K_V7 channels. Pirfenidone improves endothelium-dependent vasodilatation in aorta from diabetic animals by a mechanism involving voltage-gated K_V7 channels, a mechanism that may contribute to the antifibrotic effect of pirfenidone.

In vivo cardiopulmonary impact of skeletal M3Dq DREADD expression: a pilot study

The muscarinic M₃ receptor (M₃R) is implicated in cardiopulmonary control and many other peripheral physiologic functions. Previous observations report mortality in mice expressing a Gq-linked designer G-protein coupled receptor (Dq) selectively in striated muscle, while M₃Dq DREADD (Designer Receptor Exclusively Activated by Designer Drug), selectively expressed in skeletal muscle (SKM) impacts glucose metabolism. We investigated whether activation of SKM M₃Dq impacts cardiopulmonary function. Heart rate (HR), body temperature (Tb) and locomotor activity (ACT) were measured in 4 conscious, chronically instrumented M₃Dq DREADD mice and 4 wildtype controls. Circadian values of HR, BT and ACT were not different between genotypes (p > 0.05). Activation of the M₃Dq DREADD by clozapine N-oxide (CNO; 0.1 mg/kg) resulted in: a significant drop in heart rate, 2 h after injection, compared with a time-matched baseline control period from the same animals (460 ± 28 vs. 532 ± 6, p < 0.05), significantly lower ACT compared to the baseline control (p < 0.05) and reduced pulmonary minute ventilation compared to pre-CNO control (p < 0.05). M₃Dq DREADD activation did not cause bronchoconstriction (separate protocol), however, there was a concomitant reduction in HR, Tb and ventilation, accompanied by cardiac arrhythmias. We speculate that reductions in Tb, HR and ventilation reflect a mechanistic link between SKM Gq signaling and the metabolic responses associated with the initiation of torpor. Supported by the Canadian Institutes of Health Research (CIHR MOP-81211).

Micro- and Macroenvironmental Conditions and Stability of Terrestrial Models

Environmental variables can have profound effects on the biological responses of research animals and the outcomes of experiments dependent on them. Some of these influences are both predictable and unpredictable in effect, many are challenging to standardize, and all are influenced by the planning and conduct of experiments and the design and operation of the vivarium. Others are not yet known. Within the immediate environment where the research animal resides, in the vivarium and in transit, the most notable of these factors are ambient temperature, relative humidity, gaseous pollutant by-products of animal metabolism and physiology, dust and particulates, barometric pressure, electromagnetic fields, and illumination. Ambient temperatures in the animal housing environment, in particular those experienced by rodents below the thermoneutral zone, may introduce degrees of stress and thermoregulatory compensative responses that may complicate or invalidate study measurements across a broad array of disciplines. Other factors may have more subtle and specific effects. It is incumbent on scientists designing and executing experiments and staff responsible for animal husbandry to be aware of, understand, measure, systematically record, control, and account for the impact of these factors on sensitive animal model systems to ensure the quality and reproducibility of scientific studies.

Metabolic adaptation to calorie restriction

Calorie restriction (CR) enhances health span (the length of time that an organism remains healthy) and increases longevity across species. In mice, these beneficial effects are partly mediated by the lowering of core body temperature that occurs during CR. Conversely, the favorable effects of CR on health span are mitigated by elevating ambient temperature to thermoneutrality (30°C), a condition in which hypothermia is blunted. In this study, we compared the global metabolic response to CR of mice housed at 22°C (the standard housing temperature) or at 30°C and found that thermoneutrality reverted 39 and 78% of total systemic or hypothalamic metabolic variations caused by CR, respectively. Systemic changes included pathways that control fuel use and energy expenditure during CR. Cognitive computing-assisted analysis of these metabolomics results helped to prioritize potential active metabolites that modulated the hypothermic response to CR. Last, we demonstrated with pharmacological approaches that nitric oxide (NO) produced through the citrulline-NO pathway promotes CR-triggered hypothermia and that leucine enkephalin directly controls core body temperature when exogenously injected into the hypothalamus. Because thermoneutrality counteracts CR-enhanced health span, the multiple metabolites and pathways altered by thermoneutrality may represent targets for mimicking CR-associated effects.

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.

Endothelial Dysfunction and Passive Changes in the Aorta and Coronary Arteries of Diabetic db/db Mice

Endothelial cell dysfunction and vessel stiffening are associated with a worsened prognosis in diabetic patients with cardiovascular diseases. The present study hypothesized that sex impacts endothelial dysfunction and structural changes in arteries from diabetic mice. In diabetic (db/db) and normoglycaemic (db/db+) mice, the mechanical properties were investigated in pressurized isolated left anterior descending coronary arteries and aorta segments that were subjected to tensile testing. Functional studies were performed on wire-mounted vascular segments. The male and female db/db mice were hyperglycaemic and had markedly increased body weight. In isolated aorta segments without the contribution of smooth muscle cells, load to rupture, viscoelasticity, and collagen content were decreased suggesting larger distensibility of the arterial wall in both male and female db/db mice. In male db/db aorta segments with smooth muscle cell contribution, lumen diameter was smaller and the passive stretch-tension curve was leftward-shifted, while they were unaltered in female db/db aorta segments versus control db/db+ mice. In contrast to female db/db mice, coronary arteries from male db/db mice had altered stress-strain relationships and increased distensibility. Transthoracic echocardiography revealed a dilated left ventricle with unaltered cardiac output, while aortic flow velocity was decreased in male db/db mice. Impairment of acetylcholine relaxation was aggravated in aorta from female db/db compared to control and male db/db mice, while impairment of sodium nitroprusside relaxations was only observed in aorta from male db/db mice. The remodeling in the coronary arteries and aorta suggests an adaptation of the arterial wall to the reduced flow velocity with sex-specific differences in the passive properties of aorta and coronary arteries. The findings of less distensible arteries and more pronounced endothelial dysfunction in female compared to male diabetic mice may have implications for the observed higher incidence of macrovascular complications in diabetic women.

Neurons that regulate mouse torpor

The advent of endothermy, which is achieved through the continuous homeostatic regulation of body temperature and metabolism^1,2, is a defining feature of mammalian and avian evolution. However, when challenged by food deprivation or harsh environmental conditions, many mammalian species initiate adaptive energy-conserving survival strategies-including torpor and hibernation-during which their body temperature decreases far below its homeostatic set-point^3-5. How homeothermic mammals initiate and regulate these hypothermic states remains largely unknown. Here we show that entry into mouse torpor, a fasting-induced state with a greatly decreased metabolic rate and a body temperature as low as 20 °C⁶, is regulated by neurons in the medial and lateral preoptic area of the hypothalamus. We show that restimulation of neurons that were activated during a previous bout of torpor is sufficient to initiate the key features of torpor, even in mice that are not calorically restricted. Among these neurons we identify a population of glutamatergic Adcyap1-positive cells, the activity of which accurately determines when mice naturally initiate and exit torpor, and the inhibition of which disrupts the natural process of torpor entry, maintenance and arousal. Taken together, our results reveal a specific neuronal population in the mouse hypothalamus that serves as a core regulator of torpor. This work forms a basis for the future exploration of mechanisms and circuitry that regulate extreme hypothermic and hypometabolic states, and enables genetic access to monitor, initiate, manipulate and study these ancient adaptations of homeotherm biology.

White mouse pups can use torpor for energy conservation

White mice are ubiquitous laboratory animals and have been extensively studied. To reveal potential undiscovered traits, we tested the hypothesis that during development, when heat loss in mouse pups is high, they can use daily torpor for energy conservation. We determined at what age individual mouse pups are able to defend their body temperature at room temperature (ambient temperature, T_a = 20 °C) and whether they could use torpor from that time. Initially at 5/6 days (body mass, BM ~ 3 g), still naked mice cooled rapidly. In contrast, at ~ 14 days (BM ~ 6 g), they could maintain a high, constant body temperature and, therefore, had reached competent endothermy. These mouse pups at ~ 20% of adult BM were able to enter into and arouse from torpor as determined via the rate of oxygen consumption; this was the case for both individuals that were exposed to a cooling regime as well as those that were not. During torpor, metabolism fell by up to > 90% and torpor lasted for up to 12 h. As mice grew, torpor was still used but was less pronounced. Our study shows that although the physiology of laboratory mice has been widely examined, their functional capabilities have still not been fully revealed, which has implications for biomedicine. Our and other developmental data suggest that because torpor is so efficient in conserving energy, it is likely to be used during the growth phase by diverse mammals and birds to survive energetic and thermal challenges.

Seasonal decrease in thermogenesis and increase in vasoconstriction explain seasonal response to N6 -cyclohexyladenosine-induced hibernation in the Arctic ground squirrel (Urocitellus parryii)

Hibernation is a seasonal phenomenon characterized by a drop in metabolic rate and body temperature. Adenosine A₁ receptor agonists promote hibernation in different mammalian species, and the understanding of the mechanism inducing hibernation will inform clinical strategies to manipulate metabolic demand that are fundamental to conditions such as obesity, metabolic syndrome, and therapeutic hypothermia. Adenosine A₁ receptor agonist-induced hibernation in Arctic ground squirrels is regulated by an endogenous circannual (seasonal) rhythm. This study aims to identify the neuronal mechanism underlying the seasonal difference in response to the adenosine A₁ receptor agonist. Arctic ground squirrels were implanted with body temperature transmitters and housed at constant ambient temperature (2°C) and light cycle (4L:20D). We administered CHA (N⁶ -cyclohexyladenosine), an adenosine A₁ receptor agonist in euthermic-summer phenotype and euthermic-winter phenotype and used cFos and phenotypic immunoreactivity to identify cell groups affected by season and treatment. We observed lower core and subcutaneous temperature in winter animals and CHA produced a hibernation-like response in winter, but not in summer. cFos-ir was greater in the median preoptic nucleus and the raphe pallidus in summer after CHA. CHA administration also resulted in enhanced cFos-ir in the nucleus tractus solitarius and decreased cFos-ir in the tuberomammillary nucleus in both seasons. In winter, cFos-ir was greater in the supraoptic nucleus and lower in the raphe pallidus than in summer. The seasonal decrease in the thermogenic response to CHA and the seasonal increase in vasoconstriction, assessed by subcutaneous temperature, reflect the endogenous seasonal modulation of the thermoregulatory systems necessary for CHA-induced hibernation. Cover Image for this issue: doi: 10.1111/jnc.14528.

Effect of Leptin Deficiency on the Skeletal Response to Hindlimb Unloading in Adult Male Mice

Based on body weight, morbidly obese leptin-deficient ob/ob mice have less bone than expected, suggesting that leptin plays a role in the skeletal response to weight bearing. To evaluate this possibility, we compared the skeletal response of wild type (WT) and ob/ob mice to hindlimb unloading (HU). Mice were individually housed at 32 °C (thermoneutral) from 4 weeks of age (rapidly growing) to 16 weeks of age (approaching skeletal maturity). Mice were then randomized into one of 4 groups (n = 10/group): (1) WT control, (2) WT HU, (3) ob/ob control, and (4) ob/ob HU and the results analyzed by 2-way ANOVA. ob/ob mice pair-fed to WT mice had normal cancellous bone volume fraction (BV/TV) in distal femur, lower femur length and total bone area, mineral content (BMC) and density (BMD), and higher cancellous bone volume fraction in lumbar vertebra (LV). HU resulted in lower BMC and BMD in total femur, and lower BV/TV in distal femur and LV in both genotypes. Cancellous bone loss in femur in both genotypes was associated with increases in osteoclast-lined bone perimeter. In summary, leptin deficiency did not attenuate HU-induced osteopenia in male mice, suggesting that leptin is not required for bone loss induced by unweighting.

Is Adenosine Action Common Ground for NREM Sleep, Torpor, and Other Hypometabolic States?

This review compares two states that lower energy expenditure: non-rapid eye movement (NREM) sleep and torpor. Knowledge on mechanisms common to these states, and particularly on the role of adenosine in NREM sleep, may ultimately open the possibility of inducing a synthetic torpor-like state in humans for medical applications and long-term space travel. To achieve this goal, it will be important, in perspective, to extend the study to other hypometabolic states, which, unlike torpor, can also be experienced by humans.

Changes in blood glucose as a function of body temperature in laboratory mice: implications for daily torpor

Many small mammals, such as the laboratory mouse, utilize the hypometabolic state of torpor in response to caloric restriction. The signals that relay the lack of fuel to initiate a bout of torpor are not known. Because the mouse will only enter a torpid state when calorically challenged, it may be that one of the inputs for initiation into a bout of torpor is the lack of the primary fuel (glucose) used to power brain metabolism in the mouse. Using glucose telemetry in mice, we tested the hypotheses that 1) circulating glucose (GLC), core body temperature (T_b), and activity are significantly interrelated; and 2) that the level of GLC at the onset of torpor differs from both GLC during arousal from torpor and during feeding when there is no torpor. To test these hypotheses, six C57Bl/6J mice were implanted with glucose telemeters and exposed to different feeding conditions (ad libitum, fasting, limited food intake, and refeeding) to create different levels of GLC and T_b. We found a strong positive and linear correlation between GLC and T_b during ad libitum feeding. Furthermore, mice that were calorically restricted entered torpor bouts readily. GLC was low during torpor entry but did not drop precipitously as T_b did at the onset of a torpor bout. GLC significantly increased during arousal from torpor, indicating the presence of endogenous glucose production. While low GLC itself was not predictive of a bout of torpor, hyperactivity and low GLC preceded the onset of torpor, suggesting that this may be involved in triggering torpor.

Individual differences in torpor expression in adult mice are related to relative birth mass

Daily torpor is a physiological adaptation in small mammals and birds, characterised by drastic reductions in metabolism and body temperature. Energy-constraining conditions, such as cold and starvation, are known to cause the expression of daily torpor. However, the reason for high degrees of inter- and intra-individual variation in torpor expression (TE) in similar situations is not clear. As littermates of altricial animals are exposed to an uneven allocation of maternal resources from conception to weaning, we tested whether early nutritional experiences have long-term effects on TE in adults. We used full-sibling littermates of laboratory mice that as adults were starved overnight to induce torpor. We measured body mass from birth until adulthood as an indicator of nutritional status, and calculated the relative body mass (RBM) as an indicator of the difference in nutritional status within a litter. After maturation, we subjected mice to five repeated torpor induction trials involving 24 h of fasting and 5 days of recovery. Half of the female mice displayed great individual variation in TE whereas male mice rarely exhibited daily torpor. In females, RBM at birth influenced TE, irrespective of body mass in adulthood; thus, female mice born with low RBMs displayed high TE in adulthood. In conclusion, we provide evidence that TE in mice differs among littermates, and that this variation is linked closely to heterogeneous nutritional experiences during the fetal period.

No effect of season on the electrocardiogram of long-eared bats (Nyctophilus gouldi) during torpor

Heterothermic animals regularly undergo profound alterations of cardiac function associated with torpor. These animals have specialised tissues capable of withstanding fluctuations in body temperature > 30 °C without adverse effects. In particular, the hearts of heterotherms are able to resist fibrillation and discontinuity of the cardiac conduction system common in homeotherms during hypothermia. To investigate the patterns of cardiac conduction in small insectivorous bats which enter torpor year round, I simultaneously measured ECG and subcutaneous temperature (T_sub) of 21 Nyctophilus gouldi (11 g) during torpor at a range of ambient temperatures (T_a 1-28 °C). During torpor cardiac conduction slowed in a temperature dependent manner, primarily via prolongation along the atrioventricular pathway (PR interval). A close coupling of depolarisation and repolarisation was retained in torpid bats, with no isoelectric ST segment visible until animals reached T_sub <6 °C. There was little change in ventricular repolarisation (JT interval) with decreasing T_sub, or between rest and torpor at mild T_a. Bats retained a more rapid rate of ventricular conduction and repolarisation during torpor relative to other hibernators. Throughout all recordings across seasons (> 2500 h), there was no difference in ECG morphology or heart rate during torpor, and no manifestations of significant conduction blocks or ventricular tachyarrhythmias were observed. My results demonstrate the capacity of bat hearts to withstand extreme fluctuations in rate and temperature throughout the year without detrimental arrhythmogenesis. I suggest that this conduction reserve may be related to flight and the daily extremes in metabolism experienced by these animals, and warrants further investigation of cardiac electrophysiology in other flying hibernators.

Involvement of orexin neurons in fasting- and central adenosine-induced hypothermia

We examined whether orexin neurons might play a protective role against fasting- and adenosine-induced hypothermia. We first measured body temperature (BT) in orexin neuron-ablated (ORX-AB) mice and wild-type (WT) controls during 24 hours of fasting. As expected, the magnitude of BT drop and the length of time suffering from hypothermia were greater in ORX-AB mice than in WT mice. Orexin neurons were active just before onset of hypothermia and during the recovery period as revealed by calcium imaging in vivo using G-CaMP. We next examined adenosine-induced hypothermia via an intracerebroventricular administration of an adenosine A1 receptor agonist, N6-cyclohexyladenosine (CHA), which induced hypothermia in both ORX-AB and WT mice. The dose of CHA required to initiate a hypothermic response in ORX-AB mice was more than 10 times larger than the dose for WT mice. Once hypothermia was established, the recovery was seemingly slower in ORX-AB mice. Activation of orexin neurons during the recovery phase was confirmed by immunohistochemistry for c-Fos. We propose that orexin neurons play dual roles (enhancer in the induction phase and compensator during the recovery phase) in adenosine-induced hypothermia and a protective/compensatory role in fasting-induced hypothermia.

Heart rate reveals torpor at high body temperatures in lowland tropical free-tailed bats

Reduction in metabolic rate and body temperature is a common strategy for small endotherms to save energy. The daily reduction in metabolic rate and heterothermy, or torpor, is particularly pronounced in regions with a large variation in daily ambient temperature. This applies most strongly in temperate bat species (order Chiroptera), but it is less clear how tropical bats save energy if ambient temperatures remain high. However, many subtropical and tropical species use some daily heterothermy on cool days. We recorded the heart rate and the body temperature of free-ranging Pallas' mastiff bats (Molossus molossus) in Gamboa, Panamá, and showed that these individuals have low field metabolic rates across a wide range of body temperatures that conform to high ambient temperature. Importantly, low metabolic rates in controlled respirometry trials were best predicted by heart rate, and not body temperature. Molossus molossus enter torpor-like states characterized by low metabolic rate and heart rates at body temperatures of 32°C, and thermoconform across a range of temperatures. Flexible metabolic strategies may be far more common in tropical endotherms than currently known.

Protein restriction does not affect body temperature pattern in female mice

Daily torpor is a physiological adaptation in mammals and birds characterized by a controlled reduction of metabolic rate and body temperature during the resting phase of circadian rhythms. In laboratory mice, daily torpor is induced by dietary caloric restriction. However, it is not known which nutrients are related to daily torpor expression. To determine whether dietary protein is a key factor in inducing daily torpor in mice, we fed mice a protein-restricted (PR) diet that included only one-quarter of the amount of protein but the same caloric level as a control (C) diet. We assigned six non-pregnant female ICR mice to each group and recorded their body weights and core body temperatures for 4 weeks. Body weights in the C group increased, but those in the PR group remained steady or decreased. Mice in both groups did not show daily torpor, but most mice in a food-restricted group (n=6) supplied with 80% of the calories given to the C group exhibited decreased body weights and frequently displayed daily torpor. This suggests that protein restriction is not a trigger of daily torpor; torpid animals can conserve their internal energy, but torpor may not play a significant role in conserving internal protein. Thus, opportunistic daily torpor in mice may function in energy conservation rather than protein saving.

The mouse thermoregulatory system: Its impact on translating biomedical data to humans

The laboratory mouse has become the predominant test species in biomedical research. The number of papers that translate or extrapolate data from mouse to human has grown exponentially since the year 2000. There are many physiological and anatomical factors to consider in the process of extrapolating data from one species to another. Body temperature is, of course, a critical determinant in extrapolation because it has a direct impact on metabolism, cardiovascular function, drug efficacy, pharmacokinetics of toxins and drugs, and many other effects. While most would consider the thermoregulatory system of mice to be sufficiently stable and predictable as to not be a cause for concern, the thermal physiology of mice does in fact present unique challenges to the biomedical researcher. A variable and unstable core temperature, high metabolic rate, preference for warm temperatures, large surface area: body mass ratio, and high rate of thermal conductance, are some of the key factors of mice that can affect the interpretation and translation of data to humans. It is the intent of this brief review to enlighten researchers studying interspecies translation of biomedical data on the salient facets of the mouse thermal physiology and show how extrapolation in fields such as physiology, psychology, nutrition, pharmacology, toxicology, and pathology.

Specialist-generalist model of body temperature regulation can be applied at the intraspecific level

According to theoretical predictions, endothermic homeotherms can be classified as either thermal specialists or thermal generalists. In high cost environments, thermal specialists are supposed to be more prone to using facultative heterothermy than generalists. We tested this hypothesis at the intraspecific level using male laboratory mice (C57BL/cmdb) fasted under different thermal conditions (20 and 10°C) and for different time periods (12-48 h). We predicted that variability of body temperature (T_b) and time spent with T_b below normothermy would increase with the increase of environmental demands (duration of fasting and cold). To verify the above prediction, we measured T_b and energy expenditure of fasted mice. We did not record torpor bouts but we found that variations in T_b and time spent in hypothermia increased with environmental demands. In response to fasting, mice also decreased their energy expenditure. Moreover, animals that showed more precise thermoregulation when fed had more variable T_b when fasted. We postulate that the prediction of the thermoregulatory generalist-specialist trade-off can be applied at the intraspecific level, offering a valid tool for identifying mechanistic explanations of the differences in animal responses to variations in energy supply.

Bone Marrow Adipose Tissue Deficiency Increases Disuse-Induced Bone Loss in Male Mice

Bone marrow adipose tissue (MAT) is negatively associated with bone mass. Since osteoblasts and adipocytes are derived from the same precursor cells, adipocyte differentiation may occur at the expense of osteoblast differentiation. We used MAT-deficient Kit^W/W−v (MAT-) mice to determine if absence of MAT reduced bone loss in hindlimb-unloaded (HU) mice. Male MAT- and wild-type (WT) mice were randomly assigned to a baseline, control or HU group (n = 10 mice/group) within each genotype and HU groups unloaded for 2 weeks. Femurs were evaluated using micro-computed tomography, histomorphometry and targeted gene profiling. MAT- mice had a greater reduction in bone volume fraction after HU than did WT mice. HU MAT- mice had elevated cancellous bone formation and resorption compared to other treatment groups as well as a unique profile of differentially expressed genes. Adoptive transfer of WT bone marrow-derived hematopoietic stem cells reconstituted c-kit but not MAT in Kit^W/W−v mice. The MAT- WT → Kit^W/W−v mice lost cancellous bone following 2 weeks of HU. In summary, results from this study suggest that MAT deficiency was not protective, and was associated with exaggerated disuse-induced cancellous bone loss.

Central activation of the A1 adenosine receptor in fed mice recapitulates only some of the attributes of daily torpor

Mice enter bouts of daily torpor, drastically reducing metabolic rate, core body temperature (T _b), and heart rate (HR), in response to reduced caloric intake. Because central adenosine activation has been shown to induce a torpor-like state in the arctic ground squirrel, and blocking the adenosine-1 (A₁) receptor prevents daily torpor, we hypothesized that central activation of the A₁ adenosine receptors would induce a bout of natural torpor in mice. To test the hypothesis, mice were subjected to four different hypothermia bouts: natural torpor, forced hypothermia (FH), isoflurane-anesthesia, and an intracerebroventricular injection of the selective A₁ receptor agonist N^6-cyclohexyladenosine (CHA). All conditions induced profound hypothermia. T _b fell more rapidly in the FH, isoflurane-anesthesia, and CHA conditions compared to torpor, while mice treated with CHA recovered at half the rate of torpid mice. FH, isoflurane-anesthesia, and CHA-treated mice exhibited a diminished drop in HR during entry into hypothermia as compared to torpor. Mice in all conditions except CHA shivered while recovering from hypothermia, and only FH mice shivered substantially while entering hypothermia. Circulating lactate during the hypothermic bouts was not significantly different between the CHA and torpor conditions, both of which had lower than baseline lactate levels. Arrhythmias were largely absent in the FH and isoflurane-anesthesia conditions, while skipped beats were observed in natural torpor and periodic extended (>1 s) HR pauses in the CHA condition. Lastly, the hypothermic bouts showed distinct patterns of gene expression, with torpor characterized by elevated hepatic and cardiac Txnip expression and all other hypothermic states characterized by elevated c-Fos and Egr-1 expression. We conclude that CHA-induced hypothermia and natural torpor are largely different physiological states.

From Gutenberg to Open Science: An Unfulfilled Odyssey

Preclinical Research With the almost global availability of the Internet comes the expectation of universal accessibility to knowledge, including scientific knowledge-particularly that generated by public funding. Currently this is not the case. In this Commentary we discuss access to this knowledge, the politics that govern peer review and publication, and the role of this knowledge as a public good in medicine.  Gutenberg's invention of the printing press in 1440 opened an avenue for the distribution of scholarly information to the entire world. The scientific literature first appeared in 1665 with Le Journal des Sçavans followed in the same year by Philosophical Transactions. Today there are more than 5000 scientific publishing companies, 25,000 journals and 1.5 million articles published/year generating revenue of $25 billion USD.  The European Union and the Organization for Economic Cooperation and Development have argued for open access (OA) to scientific data for all publicly funded research by 2020 with a similar initiative in the USA via the Fair Access to Science and Technology Research Act (FASTR). However, OA to published science is but one step in this odyssey. If the products of science are not openly available then it can be argued that the norms of science as defined by Merton including "universalism" and "communalism" have yet to be accomplished. Nowhere is this more apparent than in the delivery of medicines to the poor and for rare diseases, the attempts to privatize human genetic information and, not least, dealing with the challenges of antibiotic resistance and new disease pandemics exacerbated by climate change. Drug Dev Res 78 : 3-23, 2017.   © 2016 Wiley Periodicals, Inc.

Heart rate dynamics in a marsupial hibernator

The eastern pygmy-possum (Cercartetus nanus) is a small marsupial that can express spontaneous short bouts of torpor, as well as multi-day bouts of deep hibernation. To examine heart rate (HR) control at various stages of torpor in a marsupial hibernator, and to see whether HR variability differs from deep placental hibernators, we used radiotelemetry to measure ECG and Tb while measuring the rate of O2 consumption and ventilation. The HR and rate of O2 consumption during euthermia was at its minimum (321±34 bpm, 0.705±0.048 ml O2/g*h) at an ambient temperature (Ta) of 31°C. HR had an inverse linear relationship with Ta to a maximum of 630±19 bpm at a Ta of 20°C. During entry into torpor at an Ta of 20°C, HR slowed primarily as a result of episodic periods of cardiac activity where electrical activity of the heart occurred in groups of 3 or 4 heart beats. When Tb was stable at 24°C in these torpor bouts, the episodic nature of HR had disappeared (i.e. no asystoles) with a rate of 34±3 bpm. For multi-day bouts of deep torpor, Ta was lowered to 6.6±0.8°C. During these deep bouts of torpor, Tb reached a minimum of 8.0±1.0°C, with a minimum HR of 8 bpm and a minimum rate of O2 consumption of 0.029±0.07 ml O2/g*h. Shivering bouts occurred in deep torpor about every 8 minutes, during which ventilation occurred, and HR was elevated to 40 bpm. The duration of the QRS complex increased from 12ms during euthermia to 69 ms at a Tb of 8°C. These findings demonstrate the dynamic functioning range of heart rate to be about 600 bpm (∼80 fold), one of the largest known ranges in mammals. Our study shows that despite a separation of ∼160 million years the control and function of the cardiac system seems indistinguishable in marsupial and placental hibernating mammals.

Hypometabolism during Daily Torpor in Mice is Dominated by Reduction in the Sensitivity of the Thermoregulatory System

Some mammals enter a hypometabolic state either daily torpor (minutes to hours in length) or hibernation (days to weeks), when reducing metabolism would benefit survival. Hibernators demonstrate deep torpor by reducing both the sensitivity (H) and the theoretical set-point temperature (T_R) of the thermogenesis system, resulting in extreme hypothermia close to ambient temperature. However, these properties during daily torpor remain poorly understood due to the very short steady state of the hypometabolism and the large variation among species and individuals. To overcome these difficulties in observing and evaluating daily torpor, we developed a novel torpor-detection algorithm based on Bayesian estimation of the basal metabolism of individual mice. Applying this robust method, we evaluated fasting induced torpor in various ambient temperatures (T_As) and found that H decreased 91.5% during daily torpor while T_R only decreased 3.79 °C in mice. These results indicate that thermogenesis during daily torpor shares a common property of sensitivity reduction with hibernation while it is distinct from hibernation by not lowering T_R. Moreover, our findings support that mice are suitable model animals to investigate the regulation of the heat production during active hypometabolism, thus suggesting further study of mice may provide clues to regulating hypometabolism in mammals.