Osmolyte Depletion and Thirst Suppression Allow Hibernators to Survive for Months without Water

Neural control of fluid homeostasis is engaged below 10°C in hibernation

Thirteen-lined ground squirrels (Ictidomys tridecemlineatus) hibernate for several months each winter without access to water,1 but the mechanisms that maintain fluid homeostasis during hibernation are poorly understood. In torpor, when body temperature (TB) reaches 4°C, squirrels decrease metabolism, slow heart rate, and reduce plasma levels of the antidiuretic hormones arginine vasopressin (AVP) and oxytocin (OXT).1 Squirrels spontaneously undergo interbout arousal (IBA) every 2 weeks, temporarily recovering an active-like metabolism and a TB of 37°C for up to 48 h.1,2 Despite the low levels of AVP and OXT during torpor, profound increases in blood pressure and heart rate during the torpor-IBA transition are not associated with massive fluid loss, suggesting the existence of a mechanism that protects against diuresis at a low TB. Here, we demonstrate that the antidiuretic hormone release pathway is activated by hypothalamic supraoptic nucleus (SON) neurons early in the torpor-arousal transition. SON neuron activity, dense-core vesicle release from the posterior pituitary, and plasma hormone levels all begin to increase before TB reaches 10°C. In vivo fiber photometry of SON neurons from hibernating squirrels, together with RNA sequencing and c-FOS immunohistochemistry, confirms that SON is electrically, transcriptionally, and translationally active to monitor blood osmolality throughout the dynamic torpor-arousal transition. Our work emphasizes the importance of the antidiuretic pathway during the torpor-arousal transition and reveals that the neurophysiological mechanism that coordinates the hormonal response to retain fluid is active at an extremely low TB, which is prohibitive for these processes in non-hibernators.

Integrated Redox-Metabolic Orchestration Sustains Life in Hibernating Ground Squirrels

Significance: The ultimate manifestations of life, birth, survival under various environmental pressures and death are based on bioenergetics. Hibernation is a unique survival strategy for many small mammals that is characterised by severe metabolic depression and transition from euthermia to hypothermia (torpor) at body temperatures close to 0°C. These manifestations of life were made possible by the remarkable "social" behavior of biomolecules during billions of years of evolution: the evolution of life with oxygen. Oxygen was necessary for energy production and the evolutionary explosion of aerobic organisms. Recent Advances: Nevertheless, reactive oxygen species, formed through oxidative metabolism, are dangerous-they can kill a cell and, on the other hand, play a plethora of fundamentally valuable roles. Therefore, the evolution of life depended on energy metabolism and redox-metabolic adaptations. The more extreme the conditions for survival are, the more sophisticated the adaptive responses of organisms become. Hibernation is a beautiful illustration of this principle. Hibernating animals use evolutionarily conserved molecular mechanisms to survive adverse environmental conditions, including reducing body temperature to ambient levels (often to ∼0°C) and severe metabolic depression. This long-built secret of life lies at the intersection of oxygen, metabolism, and bioenergetics, and hibernating organisms have learned to exploit all the underlying capacities of molecular pathways to survive. Critical Issues: Despite such drastic changes in phenotype, tissues and organs of hibernators sustain no metabolic or histological damage during hibernation or upon awakening from hibernation. This was made possible by the fascinating integration of redox-metabolic regulatory networks whose molecular mechanisms remain undisclosed to this day. Future Directions: Discovering these molecular mechanisms is not warranted only to understand hibernation in itself but to help explain complex medical conditions (hypoxia/reoxygenation, organ transplantation, diabetes, and cancer) and to even help overcome limitations associated with space travel. This is a review of integrated redox-metabolic orchestration in hibernation. Antioxid. Redox Signal. 40, 345-368.

Increased urinary creatinine during hibernation and day roosting in the Eastern bent-winged bat (Miniopterus fuliginosus) in Korea

Torpor and arousal cycles, both daily and seasonal (e.g. hibernation), are crucial for small mammals, including bats, to maintain the energy and water balance. The alternation between torpor and arousal leads to metabolic changes, leaving traceable evidence of metabolic wastes in urine. In this study we investigated urinary creatinine and acetoacetate (a ketone body) in the Eastern bent-wing bat (Miniopterus fuliginosus) in Mungyeong, South Korea. We found an increase in urinary creatinine during torpor in summer, indicating changes in renal water reabsorption rates during the active season. Although we could not confirm ketonuria in hibernating bats due to a methodological limitation caused by the small amount of urine, we verified an increase in urinary creatinine concentration during hibernation. This finding suggests that managing water stress resulting from evaporative water loss is one of key reasons for arousal during hibernation in Eastern bent-wing bats.

Thirst: neuroendocrine regulation in mammals

Animals can sense their changing internal needs and then generate specific physiological and behavioural responses in order to restore homeostasis. Water-saline homeostasis derives from balances of water and sodium intake and output (drinking and diuresis, salt appetite and natriuresis), maintaining an appropriate composition and volume of extracellular fluid. Thirst is the sensation which drives to seek and consume water, regulated in the central nervous system by both neural and chemical signals. Water and electrolyte homeostasis depends on finely tuned physiological mechanisms, mainly susceptible to plasma Na+ concentration and osmotic pressure, but also to blood volume and arterial pressure. Increases of osmotic pressure as slight as 1-2% are enough to induce thirst ("homeostatic" or cellular), by activation of specialized osmoreceptors in the circumventricular organs, outside the blood-brain barrier. Presystemic anticipatory signals (by oropharyngeal or gastrointestinal receptors) inhibit thirst when fluids are ingested, or stimulate thirst associated with food intake. Hypovolemia, arterial hypotension, Angiotensin II stimulate thirst ("hypovolemic thirst", "extracellular dehydration"). Hypervolemia, hypertension, Atrial Natriuretic Peptide inhibit thirst. Circadian rhythms of thirst are also detectable, driven by suprachiasmatic nucleus in the hypothalamus. Such homeostasis and other fundamental physiological functions (cardiocircolatory, thermoregulation, food intake) are highly interdependent.

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.

Optogenetic induction of hibernation-like state with modified human Opsin4 in mice

We recently determined that the excitatory manipulation of Qrfp-expressing neurons in the preoptic area of the hypothalamus (quiescence-inducing neurons [Q neurons]) induced a hibernation-like hypothermic/hypometabolic state (QIH) in mice. To control the QIH with a higher time resolution, we develop an optogenetic method using modified human opsin4 (OPN4; also known as melanopsin), a G protein-coupled-receptor-type blue-light photoreceptor. C-terminally truncated OPN4 (OPN4dC) stably and reproducibly induces QIH for at least 24 h by illumination with low-power light (3 μW, 473 nm laser) with high temporal resolution. The high sensitivity of OPN4dC allows us to transcranially stimulate Q neurons with blue-light-emitting diodes and non-invasively induce the QIH. OPN4dC-mediated QIH recapitulates the kinetics of the physiological changes observed in natural hibernation, revealing that Q neurons concurrently contribute to thermoregulation and cardiovascular function. This optogenetic method may facilitate identification of the neural mechanisms underlying long-term dormancy states such as sleep, daily torpor, and hibernation.

Ground squirrels

Pra et al. provide an overview of ground squirrels and the physiological adaptations these animals have evolved to contend with harsh climates.

Towards understanding the neural origins of hibernation

Hibernators thrive under harsh environmental conditions instead of initiating canonical behavioral and physiological responses to promote survival. Although the physiological changes that occur during hibernation have been comprehensively researched, the role of the nervous system in this process remains relatively underexplored. In this Review, we adopt the perspective that the nervous system plays an active, essential role in facilitating and supporting hibernation. Accumulating evidence strongly suggests that the hypothalamus enters a quiescent state in which powerful drives to thermoregulate, eat and drink are suppressed. Similarly, cardiovascular and pulmonary reflexes originating in the brainstem are altered to permit the profoundly slow heart and breathing rates observed during torpor. The mechanisms underlying these changes to the hypothalamus and brainstem are not currently known, but several neuromodulatory systems have been implicated in the induction and maintenance of hibernation. The intersection of these findings with modern neuroscience approaches, such as optogenetics and in vivo calcium imaging, has opened several exciting avenues for hibernation research.

Urinary creatinine varies with microenvironment and sex in hibernating Greater Horseshoe bats (Rhinolophus ferrumequinum) in Korea

BACKGROUND

In temperate regions many small mammals including bats hibernate during winter. During hibernation these small mammals occasionally wake up (arouse) to restore electrolyte and water balance. However, field data on water stress and concentration of bodily fluids during hibernation is scarce. Urinary creatinine concentration has long been used to calibrate urinary hormone concentration due to its close correlation with urine concentration. Therefore, by investigating urinary creatinine concentration, we can estimate bodily fluid concentration. In this study, we investigated changes in urinary creatinine from greater horseshoe bats (Rhinolophus ferrumequinum) hibernating in abandoned mineshafts in two regions in South Korea.

RESULTS

We collected 74 urine samples from hibernating greater horseshoe bats from 2018 to 2019. We found that urinary creatinine concentration was higher in February and March and then declined in April. There were also indications of a sex difference in the pattern of change in creatinine concentration over the three months. Bats in the warmer and less humid mineshaft had higher urinary creatinine concentrations than bats in the colder and more humid mineshaft.

CONCLUSIONS

These results indicate that hibernating bats face water stress as urinary concentration increases during winter and that water stress may vary depending on the microenvironment. Sex differences in behaviour during hibernation may influence arousal frequency and result in sex differences in changes in urinary creatinine concentration as hibernation progresses. Although further behavioural and endocrinal investigations are needed, our study suggests that urinary creatinine concentration can be used as a proxy to estimate the hydration status of bats and the effect of sex and environmental factors on arousal patterns during hibernation.

WITHDRAWN: Utilizing comparative models in biomedical research

The Publisher regrets that this article is an accidental duplication of an article that has already been published in Comparative Biochemistry and Physiology Part B: Biochemistry and Molecular Biology, Volume 255, 2021, 110593, https://doi.org/10.1016/j.cbpb.2021.110593. The duplicate article has therefore been withdrawn. The full Elsevier Policy on Article Withdrawal can be found at https://www.elsevier.com/about/our-business/policies/article-withdrawal.

Utilizing comparative models in biomedical research

This review serves as an introduction to a Special Issue of Comparative Biochemistry and Physiology, focused on using non-human models to study biomedical physiology. The concept of a model differs across disciplines. For example, several models are used primarily to gain an understanding of specific human pathologies and disease states, whereas other models may be focused on gaining insight into developmental or evolutionary mechanisms. It is often the case that animals initially used to gain knowledge of some unique biochemical or physiological process finds foothold in the biomedical community and becomes an established model. The choice of a particular model for biomedical research is an ongoing process and model validation must keep pace with existing and emerging technologies. While the importance of non-mammalian models, such as Caenorhabditis elegans, Drosophila melanogaster, Danio rerio and Xenopus laevis, is well known, we also seek to bring attention to emerging alternative models of both invertebrates and vertebrates, which are less established but of interest to the comparative biochemistry and physiology community.

Landscape-wide flight activity by wintering bats predictably follows pulses of warmth in the Midwestern United States

During winter hibernation, bats may become active for a variety of reasons. Such winter activity occurs at or near hibernacula, but the degree to which this activity represents long-distance travel across a wider landscape largely is unstudied. We documented patterns in landscape-wide winter activity across a west-central Indiana study site, providing some new insights into winter flight activity. We deployed acoustic recording devices in areas without any known hibernacula, each night from December through March over three consecutive winters. Twilight temperatures (1 h post-sunset) ranged from −23°C to 21°C across three winters. We recorded 4,392 call files and attributed 89% to a phonic group based on characteristic frequencies. Flight activity was recorded at all stations and during all winter months. Nightly activity mainly was a function of the temperature on that night. We recorded low-phonic bats (most likely big brown bats, Eptesicus fuscus) down to −4°C, but most activity occurred when twilight temperatures were > 0°C. Mid-phonic bat activity (most likely eastern red bats, Lasiurus borealis) occurred when temperatures were > 0°C, with most activity occurring when temperatures were > 5°C. Wind speeds > 6 m/s tended to suppress activity. The duration of inactive periods during cold spells had no effect on activity during subsequent warm nights, indicating no increasing drive for activity following long periods of inactivity. Most activity occurred within a few hours of sunset, regardless of temperature. Little pre-sunset activity was recorded in low-phonic bats, but mid-phonic bats sometimes were active in the hour before sunset. Our results suggest widespread and potentially long-distance travel by bats across our study area during warm periods, but the impetus behind this activity remains unclear.

Cellular, Molecular, and Physiological Adaptations of Hibernation: The Solution to Environmental Challenges

Thriving in times of resource scarcity requires an incredible flexibility of behavioral, physiological, cellular, and molecular functions that must change within a relatively short time. Hibernation is a collection of physiological strategies that allows animals to inhabit inhospitable environments, where they experience extreme thermal challenges and scarcity of food and water. Many different kinds of animals employ hibernation, and there is a spectrum of hibernation phenotypes. Here, we focus on obligatory mammalian hibernators to identify the unique challenges they face and the adaptations that allow hibernators to overcome them. This includes the cellular and molecular strategies used to combat low environmental and body temperatures and lack of food and water. We discuss metabolic, neuronal, and hormonal cues that regulate hibernation and how they are thought to be coordinated by internal clocks. Last, we touch on questions that are left to be addressed in the field of hibernation research. Studies from the last century and more recent work reveal that hibernation is not simply a passive reduction in body temperature and vital parameters but rather an active process seasonally regulated at the molecular, cellular, and organismal levels.

Water Balance: Abstaining from Obtaining While Retaining

Animals tightly regulate blood volume and solute concentrations. Water balance is usually achieved by a combination of managing intake and excretion but sometimes both drinking and urination are inconvenient. Hibernators have perfected internal mechanisms to maintain water balance without either.