Terminal Arousal Research Articles

Non-shivering thermogenesis is differentially regulated during the hibernation season in Arctic ground squirrels.

Arctic ground squirrels are small mammals that experience physiological extremes during the hibernation season. Body temperature rises from 1°C to 40°C during interbout arousal and requires tight thermoregulation to maintain rheostasis. Tissues from wild-caught Arctic ground squirrels were sampled over 9months to assess the expression of proteins key to thermogenic regulation. Animals were sacrificed while aroused, and the extensor digitorum longus, diaphragm, brown adipose tissue, and white adipose tissue were probed using Western blots to assess protein expression and blood was sampled for metabolite analysis. Significant seasonal expression patterns emerged showing differential regulation. Contrary to our prediction, white adipose tissue showed no expression of uncoupling protein 1, but utilization of uncoupling protein 1 peaked in brown adipose tissue during the winter months and began to taper after terminal arousal in the spring. The opposite was true for muscular non-shivering thermogenesis. Sarco/endoplasmic reticulum calcium ATPase 1a and 2a expressions were depressed during the late hibernation season and rebounded after terminal arousal in diaphragm tissues, but only SERCA2a was differentially expressed in the extensor digitorum longus. The uncoupler, sarcolipin, was only detected in diaphragm samples and had a decreased expression during hibernation. The differential timing of these non-shivering pathways indicated distinct functions in maintaining thermogenesis which may depend on burrow temperature, availability of endogenous resources, and other seasonal activity demands on these tissues. These results could be impacted by fiber type makeup of the muscles collected, the body weight of the animal, and the date of entrance or exit from hibernation.

Turning Down the Heat: Role of Sarcolpin in Thermoregulation of Hibernating Arctic Ground Squirrels

Hibernation in mammals is a whole‐body phenotype that involves profound reductions in metabolism and core body temperature resulting in a state of torpor. Some mammalian hibernators, such as ground squirrels, have the ability to reverse this state of hypometabolism by rewarming periodically (~every 1–4 weeks) to normothermic levels. The predominant sources of thermogenesis during these arousal periods have been shown to be brown adipose tissue (BAT) and skeletal muscle shivering. Recently, skeletal muscle non‐shivering thermogenesis has been speculated to be a significant source of heat production, which may occur via sarcolipin (SLN) uncoupling of sarcoendoplasmic reticulum (SR) calcium ATPase (SERCA)‐mediated calcium pumping. During torpor, the thermogenic contribution of BAT and skeletal muscle need to be reduced in order for hibernating mammals to maintain hypometabolic conditions. We hypothesized that skeletal muscle non‐shivering thermogenesis is a significant source of heat production and, in hibernating mammals, must be reduced during torpor. Skeletal muscle samples were taken from 2 species of ground squirrels (13‐lined and Arctic), in and out of torpor conditions, to assess fluctuations in protein expression. Preliminary results show that SLN expression is decreased during periods of torpor and significantly increase after terminal arousal from hibernation in the spring in both ground squirrel species. Furthermore, SERCA expression significantly increased during torpor in Arctic ground squirrels, which fell to normothermic levels post arousal, suggesting an enhancement of calcium handling during torpor. Next, Arctic ground squirrels were implanted with peristaltic pumps subcutaneously which delivered inhibitors of BAT‐uncoupling (SR59230A) or skeletal muscle‐uncoupling (dantrolene) during hibernation to quantify the effect of reducing specific thermogenic mechanisms directly on rewarming and metabolic rate during interbout arousals. To measure metabolic rate changes in AGS, VO2 was monitored during rewarming using respirometry. Results show that suppressing skeletal muscle uncoupling; using dantrolene (RyR inhibitor that decreases cytosolic calcium) caused a dose dependent reduction in metabolic rate during AGS rewarming. Inhibition of BAT thermogenesis resulted in faster rates of metabolism increase during IBAs. These data suggest that skeletal muscle nonshivering thermogenesis could be a significant regulator of core body temperature and basal metabolic rate, which need to be suppressed during hibernation to maintain torpor.Support or Funding InformationThirteen lined ground squirrel data funded by NIH Grant 1RC2HL101625‐01, U.S. Army Medical Research and Materiel Command contract W81XWH‐11‐0409, and the University of Minnesota McKnight Presidential Endowment. Arctic Ground Squirrel data supported by a UAF INBRE IDeA pilot award from NIH NIGMS grant number P20GM103395.This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Skeletal muscle mass and composition during mammalian hibernation.

Hibernation is characterized by prolonged periods of inactivity with concomitantly low nutrient intake, conditions that would typically result in muscle atrophy combined with a loss of oxidative fibers. Yet, hibernators consistently emerge from winter with very little atrophy, frequently accompanied by a slight shift in fiber ratios to more oxidative fiber types. Preservation of muscle morphology is combined with down-regulation of glycolytic pathways and increased reliance on lipid metabolism instead. Furthermore, while rates of protein synthesis are reduced during hibernation, balance is maintained by correspondingly low rates of protein degradation. Proposed mechanisms include a number of signaling pathways and transcription factors that lead to increased oxidative fiber expression, enhanced protein synthesis and reduced protein degradation, ultimately resulting in minimal loss of skeletal muscle protein and oxidative capacity. The functional significance of these outcomes is maintenance of skeletal muscle strength and fatigue resistance, which enables hibernating animals to resume active behaviors such as predator avoidance, foraging and mating immediately following terminal arousal in the spring.

Brain energy metabolism and neurotransmission at near‐freezing temperatures: in vivo1H MRS study of a hibernating mammal

The brain of a hibernating mammal withstands physiological extremes that would result in cerebral damage and death in a non-hibernating species such as humans. To examine the possibility that this neuroprotection results from alterations in cerebral metabolism, we used in vivo(1)H NMR spectroscopy at high field (9.4 T) to measure the concentration of 18 metabolites (neurochemical profile) in the brain of 13-lined ground squirrels (Spermophilus tridecemlineatus) before, during, and after hibernation. Resolved in vivo(1)H NMR spectra were obtained even at low temperature in torpid hibernators ( approximately 7 degrees C). The phosphocreatine-to-creatine ratio was increased during torpor (+143%) indicating energy storage, and remained increased to a lesser extent during interbout arousal (IBA) (+83%). The total gamma-aminobutyric acid concentration was increased during torpor (+135%) and quickly returned to baseline during IBA. Glutamine (Gln) was decreased (-54%) during torpor but quickly returned to normal levels during IBA and after terminal arousal in the spring. Glutamate (Glu) was also decreased during torpor (-17%), but remained decreased during IBA (-20% compared with fall), and returned to normal level in the spring. Our observation that Glu and Gln levels are depressed in the brain of hibernators suggests that the balance between anaplerosis and loss of Glu and Gln (because of glutamatergic neurotransmission or other mechanisms) is altered in hibernation.

REVERSE MOHAWK CUT IS WAY TOO COOL

While a thick fur coat may be a chic way for fashionable ladies to keep warm, it is a vital necessity for mammals overwintering in cold environments. But what if your body temperature during hibernation is near freezing anyway?Does a fur coat confer any advantages? This question was recently addressed by the laboratories of Alexander Kauffman, Matthew Paul and Irving Zucker.Small mammals such as rodents escape the rigors of winter by hibernating:reducing metabolism and numerous physiological functions and allowing body temperature to drop to within a degree or two of ambient temperature. The metabolic rate of hibernators may drop to less than 6% of normothermic values,conferring considerable energy savings and allowing the animal to survive the winter, utilizing stored body fat for fuel. Such hibernators, however, do not remain torpid throughout the winter but instead arouse periodically for approximately 24 h before returning to a hypometabolic state. The function of these periodic arousals is as yet unknown, although they are presumed to be critical events. The energy costs of such arousals are immense – up to 70% of a hibernating mammal's winter energy expenditure is consumed by these arousal events. This study examined the role of furry insulation on the energetics of hibernation in golden-mantled ground squirrels.The team kept the hibernating squirrels in a cold chamber(5±1°C) and gave the animals a reverse Mohawk haircut, shaving and depilating the rodents during an arousal event and removing the hair from the entire dorsal surface except the head and paws. The recovered animals were also fitted with intraperitoneal temperature transmitters, and returned to the cold chamber where they re-entered the torpid state. Control animals were handled similarly but less than 10% of their pelage was removed from the ventral surface (which remained insulated while the animals hibernated, curled up in a ball).Despite the lack of insulating fur, the authors found no differences between the shaved and control squirrels in the mean length of torpid bouts,the mean duration of the normothermic intervals or the food intake during these arousal periods. Nor were there differences in minimum body temperature between shaved and furry hibernators. However, significant effects of shaving were seen during arousal episodes. The shaved animals re-warmed more slowly and utilized far greater fat stores than did animals with intact pelages. Thus furless animals lost body mass more quickly; although both control and shaved animals had the same body mass upon their final arousal, this resulted only because shaved animals terminated hibernation at an average of 3.7 weeks ahead of the control group. During the first 3 weeks after the terminal arousal,body mass in shaved squirrels also increased more slowly than in intact animals, despite the fact that the food intake in shaved squirrels was significantly greater in the same period. This presumably reflects the greater energy expenditure required to remain warm when someone has stolen your fur coat!It appears, then, that insulation is not critical to hibernators when their body temperature is close to ambient, but a nice fur coat is important during metabolically expensive arousal periods to decrease heat loss and reduce overall energy expenditures. Without these energy savings, ground squirrels`woke up' nearly a month earlier than their insulated counterparts, a potentially fatal consequence should food stores be limited and spring not yet sprung.

Increased heat loss affects hibernation in golden-mantled ground squirrels.

During hibernation at ambient temperatures (T(a)) above 0 degrees C, rodents typically maintain body temperature (T(b)) approximately 1 degrees C above T(a), reduce metabolic rate, and suspend or substantially reduce many physiological functions. We tested the extent to which the presence of an insulative pelage affects hibernation. T(b) was recorded telemetrically in golden-mantled ground squirrels (Spermophilus lateralis) housed at a T(a) of 5 degrees C; food intake and body mass were measured at regular intervals throughout the hibernation season and after the terminal arousal. Animals were subjected to complete removal of the dorsal fur or a control procedure after they had been in hibernation for 3-4 wk. Shaved squirrels continued to hibernate with little or no change in minimum T(b), bout duration, duration of periodic normothermic bouts, and food intake during normothermia. Rates of rewarming from torpor were, however, significantly slower in shaved squirrels, and rates of body mass loss were significantly higher, indicating increased depletion of white adipose energy stores. An insulative pelage evidently conserves energy over the course of the hibernation season by decreasing body heat loss and reducing energy expenditure during periodic arousals from torpor and subsequent intervals of normothermia. This prolongs the hibernation season by several weeks, thereby eliminating the debilitating consequences associated with premature emergence from hibernation.

Hibernation reduces pancreatic amylase levels in ground squirrels

Pancreatic enzyme levels in mammals are influenced by food intake and dietary composition. In this study, we examined the activity and expression of pancreatic amylase in a hibernating mammal, a natural model for long-term fasting. Pancreatic tissues were obtained from summer-active 13-lined ground squirrels and hibernating squirrels that had not eaten for at least 6 weeks. Amylase specific activity was reduced by ∼50% in the torpid hibernators compared with summer squirrels, and immunoblot analysis revealed that amylase protein expression was reduced by ∼40% in the hibernators. Similar reductions in amylase specific activity were observed in interbout euthermic hibernators. These results support a strong influence of food intake on pancreatic enzyme expression in hibernating mammals. The maintenance of basal levels of this key digestive enzyme at ∼50% of summer values despite the extended winter fast likely facilitates the rapid resumption of digestive function after terminal arousal in the spring.

Energy expenditure and testosterone in free-living male yellow-pine chipmunks.

The onset of mating in yellow-pine chipmunks (Tamias amoenus) follows emergence from a prolonged period of energy conservation during hibernation. Energy expenditures are greatly accelerated to meet the demands of the reproductive season. When emerging from hibernation, typical male chipmunks (breeders) have enlarged testes and a high level of plasma testosterone (T). However, certain males that do not participate in reproduction (nonbreeders) maintain small testes and low plasma T levels and emerge several weeks later than the breeders. The timing of the terminal arousal from hibernation and onset of mating are associated with increased plasma T levels. Experimental elevation of T levels in T. amoenus outside the mating season has been associated with a decrease in body mass, further suggesting an effect of T on energy balance. To test this hypothesis, we measured daily energy expenditure (DEE) in free-living, nonbreeding male chipmunks in the presence and absence of a T-implant. We also measured DEE in breeding males when endogenous T levels were high. DEE of the nonbreeders was not affected by our manipulation of plasma T, and the DEE of breeding males did not differ from that of nonbreeders. We conclude that energy expenditure on a daily basis in male yellow-pine chipmunks is not influenced by levels of T. However, on a seasonal basis, the earlier emergence from hibernation by breeding males, which appears to be influenced by T, represents an overall seasonal energy expenditure that exceeds that of nonbreeding males.

Gonadectomy in the spring reinstates hibernation in male golden-mantled ground squirrels.

We tested the hypothesis that continued secretion of gonadal steroids is necessary to suppress hibernation in male golden-mantled ground squirrels in the weeks after the terminal arousal in spring. Juvenile and adult males were gonadectomized or sham gonadectomized 1 wk after the terminal arousal; 64% of castrated and none of the shamcastrated animals resumed hibernation. Latency to resumption of torpor was 9 +/- 2 days from the time of castration, and squirrels underwent 4.3 +/- 0.9 bouts before permanently regaining euthermia. Among squirrels that resumed hibernation, bout duration was significantly shorter and torpor was shallower after castration. Castration as late as 3 wk after the terminal arousal reinstated hibernation. We suggest that the terminal arousal of male squirrels in the spring is provoked by a steroid-independent mechanism similar to that operating earlier in the hibernation season; abandonment of hibernation is contingent on concomitant sustained increases in androgen secretion during the first few weeks of euthermia.

Accelerated reproductive development in juvenile male ground squirrels fed a high-fat diet.

Male golden-mantled ground squirrels held at 23 degrees C were fed high-fat (HF) or standard (chow) diets. In December, ambient temperature was reduced to 6 degrees C, food was removed, and frequency and duration of torpor bouts were monitored continuously by radiotelemetry. Reproductive condition and body composition were assessed upon terminal arousal in the spring. Juvenile males fed the HF diet weighed more than chow-fed controls before and throughout the hibernation season and had significantly greater lipid masses at terminal arousal. Testes masses and plasma testosterone concentrations were substantially higher in HF than in chow-fed juveniles. The accelerated reproductive development of fatter squirrels was not contingent upon increases in the total number of days spent in torpor, number of torpor bouts, or the average duration of each arousal from torpor. Access to the HF diet had no effect on body mass, adiposity, or reproductive status of adult male ground squirrels in spring. Threshold levels of white adipose tissue and associated differences in availability of metabolic fuels may be permissive for testicular growth during the hibernation season. Juveniles exceed this threshold only when fed the HF diet.

Plasma androgen and gonadotropin levels during hibernation and testicular maturation in golden-mantled ground squirrels.

The 4-5-mo hibernation season of golden-mantled ground squirrels consists of extended torpor bouts interspersed with brief, periodic intervals of normothermic arousal. Plasma levels of testosterone (T), luteinizing hormone (LH), and follicle-stimulating hormone (FSH) and degree of scrotal pigmentation were measured in torpid and aroused male ground squirrels throughout a season of hibernation and in active animals after the termination of torpor. T was basal in torpid animals; beginning 3 weeks before torpor ended, T was elevated in normothermic males during the first half of periodic arousals but returned to basal levels before animals reentered torpor. After the last (terminal) arousal from torpor, T levels were moderately elevated for 4 wk and maximal for the next 6 wk before they returned to basal values. LH patterns were similar to those of T; however, levels of T and LH were positively correlated only in aroused or posthibernation males. FSH levels remained constant and low during most of the heterothermic season but increased in several torpid males within 3 days of terminal arousal. FSH levels peaked 2 wk after the end of heterothermy. Scrotal pigmentation developed over the first 4 wk after terminal arousal. Maturation of reproductive function occurs during the 4 wk after termination of heterothermy, but elevated levels of T during arousals and variable levels of FSH in the last days of torpor suggest that activation or increased sensitivity of the hypothalamic-pituitary-gonadal axis is important in the termination of heterothermy in ground squirrels.

Annual plasma testosterone and thyroxine cycles in relation to hibernation in the edible dormouse Glis glis

Twelve male edible dormice, captured in the autumn of 1979, were studied for 2 years under natural lighting in an animal enclosure with permanently open windows, at Montpellier, in Southern France (latitude 43°50′N). Ambient temperature fluctuated annually between +4 and 27°, and the photoperiod between 10L:14D and 16L: 8D. Another group of 26 animals, captured in the autumn of 1980, was observed under the same conditions for 1 year. Body weight and plasma testosterone and thyroxine concentrations were measured at monthly intervals under standardized conditions. Hibernation generally extended from October to February but lasted till May in a few cases, and the critical temperature for dormancy varied between 13 and 16° in November and 23° in May. Body weight fluctuated annually according to a biphasic pattern, with two maxima, respectively, in spring and autumn (prehibemation), and two minima, in summer and winter. However, plasma testosterone and thyroxine concentrations displayed clearly monophasic annual cycles, closely correlated, both to each other and to the ambient temperature and photoperiod. Both the testosterone and thyroxine cycles culminated in June-July. Interestingly, the annual ascending phase for both hormonal cycles never preceded the terminal arousal of the animal. Further, whatever the season, 30° was clearly the critical minimal body temperature required for a seasonal increase to occur in plasma testosterone. These annual cycles are discussed in the light of the data acquired recently for hibernators other than the dormouse.

Thyroidal iodine and thyroglobulin in hibernating and active Richardson's ground squirrels Spermophilus richardsoni

1. 1. Thyroidal iodine, protein and thyroglobulin (TG) were investigated in a hibernator, Spermophilus richardsoni, sampled in the field and from laboratory-held winter colonies. 2. 2. In field animals, thyroidal iodine and protein contents were similar at onset of hibernation and at terminal arousal. 3. 3. Iodine content was increased in laboratory-held animals, a function of alimentary supply, but no differences were observed between hibernators and non-hibernators. 4. 4. Density sucrose gradients showed that approx. 20% of the TG was present as the 12S precursor sub-unit. 5. 5. No variations in the iodine content, nor the iodoamino acid composition of the TG occurred as a function of hibernation.

Long-term effects of pinealectomy on the annual cycle of golden-mantled ground squirrels,Spermophilus lateralis

Pinealectomies and radical sham pinealectomies were performed on adult golden-mantled ground squirrels (Spermophilus lateralis) approximately one month prior to the beginning of the normal winter hibernation season. The cyclic components of the overwintering period and subsequent active season (the rhythms of homeothermy-heterothermy, body mass and testicular function) of the first year were not disrupted in either of the operated groups as compared with the intact animals. In the second year, the initiation of the hibernatory period occurred as usual in all intact, pinealectomized, and sham operated animals, but the pinealectomized group failed to show the progressive increase in the length of heterothermic bouts that was characteristic of the intact and sham-pinealectomized squirrels. The loss of body mass also was accelerated in the pinealectomized squirrels with respect to the intact and sham groups and appeared to be associated with terminal arousal which occurred approximately six weeks earlier in the second year after pinealectomy. Additionally, male squirrels showed a correspondingly early onset of testicular development as compared to the intact and sham operated males.

Effects of bulbectomy on hibernation, food intake and body weight in the european hamster, Cricetus cricetus

Bulbectomy was performed on European male wild hamsters in autumn, habitual phase of weight decline and entrance into hibernation. Total bulbectomy suppresses both nest building behavior and hibernation, and is followed by an immediate increase in food intake and obesity. The body weight curve of totally bulbectomized animals is in opposition of phase with that of the controls. Incomplete bulbectomy suppresses nest building behavior but hibernation is present with a diminution of periodic arousals. Hyperphagia starts in spring after the terminal arousal; it is similar to that of the total bulbectomized animals. Anosmic animals do not differ from the control animals.

Changes in plasma magnesium concentration during hibernation in the golden-mantled ground squirrel ( Citellus lateralis)

1. 1. Plasma magnesium concentrations of active (37°C) and hibernating (3°C) ground squirrels ( Citellus lateralis) have been compared over the whole hibernation period. 2. 2. In general, hibernating animals have a significantly higher plasma magnesium concentration than active animals. The increase is higher in females than males, but there is no difference between sexes in active animals. 3. 3. Over the whole hibernation period females have a declining plasma magnesium concentration eventually reaching the active level, which may be responsible for the terminal arousal. The hibernating behaviour of the animals has been related to the changes in plasma magnesium. 4. 4. The evidence suggests that the changes in plasma magnesium going from a hibernating to an aroused state, and vice versa, are active physiological processes.