Brian M. Barnes

Hibernation in Black Bears: Independence of Metabolic Suppression from Body Temperature

Hibernating black bears suppress their metabolic rate to 25% of normal, but only slightly reduce their body temperature. Black bears hibernate for 5 to 7 months a year and, during this time, do not eat, drink, urinate, or defecate. We measured metabolic rate and body temperature in hibernating black bears and found that they suppress metabolism to 25% of basal rates while regulating body temperature from 30° to 36°C, in multiday cycles. Heart rates were reduced from 55 to as few as 9 beats per minute, with profound sinus arrhythmia. After returning to normal body temperature and emerging from dens, bears maintained a reduced metabolic rate for up to 3 weeks. The pronounced reduction and delayed recovery of metabolic rate in hibernating bears suggest that the majority of metabolic suppression during hibernation is independent of lowered body temperature.

MOLECULAR AND METABOLIC ASPECTS OF MAMMALIAN HIBERNATION

713eat seeds from cones and are toolarge to use the subnivian space; forthese animals, winter can be a longseason without foraging opportuni-ties, and they have therefore evolvedthe ability to pass winter by while ina torpid state of lethargy.A highly regulated sequence ofphysiological events beginningmonths in advance of winter coordi-nates entrance into the suspendedstate of animation known as hiber-nation. However, with the exceptionof bears, which become only moder-ately hypothermic, no hibernatingmammal remains deeply hypother-mic for longer than several weeks.Instead, for reasons that are not yetknown, they expend significantamounts of energy to periodicallyrewarm back to normal body tem-peratures for less than a day beforerecooling.The seasonal changes in body tem-perature that occur in an arctic groundsquirrel (

Annual Cycle of Body Composition and Hibernation in Free-Living Arctic Ground Squirrels

We monitored a natural population of arctic ground squirrels ( Spermophilus parryii kennicottii ) on the North Slope of Alaska for seasonal changes in body mass and composition and dates of immergence into and emergence from hibernation. Yearlings and adult females were at the lowest body mass of their active season at emergence in spring. Their mean body mass did not increase for 1 month after emergence and peaked in July (adult females) and August (yearlings). Body mass of adult males was near the highest of the active season when they emerged from hibernation and decreased by 21% over the subsequent 10-day mating season. Juveniles gained body mass during their active season, except for significant losses associated with dispersal. During hibernation, females lost >30% of their body mass, but adult males emerged in spring without significant decreases in body mass, fat, or lean. Yearling and nonreproductive males were significantly lower in fat but not lean mass at emergence than immergence, and females were significantly lower in fat and lean mass. Arctic ground squirrels entered hibernation over a >1 -month interval beginning in early August; females entered before males, and adults of each sex immerged before juveniles. Reproductive males emerged before females, and fatter females emerged significantly earlier than leaner females. Vaginal estrus was maximal at 3 days post-emergence. Nonreproductive males emerged last from hibernation. Mean ± SE days in hibernation was 240.1 ± 12.1 for adult females (69% of the year), 235.8 ± 10.3 for juvenile females, 230.3 ± 4.2 for nonreproductive males, 220.3 ± 12.5 for adult males, and 214.7 ± 6.5 for juvenile males. Timing of immergence into and emergence from hibernation for arctic ground squirrels did not differ significantly from sciurid populations in temperate latitudes.

Warming up for sleep? - ground squirrels sleep during arousals from hibernation

Hypothermia during mammalian hibernation is periodically interrupted by arousals to euthermy, the function of which is unknown. We report that arctic ground squirrels (Spermophilus parryii) consistently sleep during these arousals, and that their EEG shows the decrease in slow wave activity (delta power) that is characteristic of a declining requirement for sleep. These results are consistent with the novel hypothesis that the need for sleep slowly accumulates during torpor, and that returning to euthermy is periodically required to allow sleep. Sleep thus seems to be energetically expensive for a hibernating mammal, and cannot be considered solely a strategy for saving energy.

Central nervous system regulation of mammalian hibernation: implications for metabolic suppression and ischemia tolerance

Torpor during hibernation defines the nadir of mammalian metabolism where whole animal rates of metabolism are decreased to as low as 2% of basal metabolic rate. This capacity to decrease profoundly the metabolic demand of organs and tissues has the potential to translate into novel therapies for the treatment of ischemia associated with stroke, cardiac arrest or trauma where delivery of oxygen and nutrients fails to meet demand. If metabolic demand could be arrested in a regulated way, cell and tissue injury could be attenuated. Metabolic suppression achieved during hibernation is regulated, in part, by the central nervous system through indirect and possibly direct means. In this study, we review recent evidence for mechanisms of central nervous system control of torpor in hibernating rodents including evidence of a permissive, hibernation protein complex, a role for A1 adenosine receptors, mu opiate receptors, glutamate and thyrotropin‐releasing hormone. Central sites for regulation of torpor include the hippocampus, hypothalamus and nuclei of the autonomic nervous system. In addition, we discuss evidence that hibernation phenotypes can be translated to non‐hibernating species by H2S and 3‐iodothyronamine with the caveat that the hypothermia, bradycardia, and metabolic suppression induced by these compounds may or may not be identical to mechanisms employed in true hibernation.

Differential regulation of uncoupling protein gene homologues in multiple tissues of hibernating ground squirrels

Nonshivering thermogenesis in brown adipose tissue (BAT) provides heat through activation of a mitochondrial uncoupling protein (UCP1), which causes futile electron transport cycles without the production of ATP. Recent discovery of two molecular homologues, UCP2, expressed in multiple tissues, and UCP3, expressed in muscle, has resulted in investigation of their roles in thermoregulatory physiology and energy balance. To determine the expression pattern of Ucp homologues in hibernating mammals, we compared relative mRNA levels of Ucp1, -2, and -3 in BAT, white adipose tissue (WAT), and skeletal muscle of arctic ground squirrels (Spermophilus parryii) hibernating at different ambient and body temperatures, with levels determined in tissues from ground squirrels not in hibernation. Here we report significant increases in mRNA levels for Ucp2 in WAT (1. 6-fold) and Ucp3 in skeletal muscle (3-fold) during hibernation. These results indicate the potential for a role of UCP2 and UCP3 in thermal homeostasis during hibernation and indicate that parallel mechanisms and multiple tissues could be important for nonshivering thermoregulation in mammals.Nonshivering thermogenesis in brown adipose tissue (BAT) provides heat through activation of a mitochondrial uncoupling protein (UCP1), which causes futile electron transport cycles without the production of ATP. Recent discovery of two molecular homologues, UCP2, expressed in multiple tissues, and UCP3, expressed in muscle, has resulted in investigation of their roles in thermoregulatory physiology and energy balance. To determine the expression pattern of Ucp homologues in hibernating mammals, we compared relative mRNA levels of Ucp1, -2, and -3 in BAT, white adipose tissue (WAT), and skeletal muscle of arctic ground squirrels ( Spermophilus parryii) hibernating at different ambient and body temperatures, with levels determined in tissues from ground squirrels not in hibernation. Here we report significant increases in mRNA levels for Ucp2 in WAT (1.6-fold) and Ucp3 in skeletal muscle (3-fold) during hibernation. These results indicate the potential for a role of UCP2 and UCP3 in thermal homeostasis during hibernation and indicate that parallel mechanisms and multiple tissues could be important for nonshivering thermoregulation in mammals.

Temperatures of Hibernacula and Changes in Body Composition of Arctic Ground Squirrels over Winter

Soil temperatures near hibernacula of free-living arctic ground squirrels (Spermophilus parryii) were recorded over 3 winters (October-April 1993-1996) at Toolik Lake, Alaska. Means and minima of soil temperature at 20 burrows averaged -8.9 and -18.8°C, respectively. Soil temperatures were 5 months, which represented the duration of winter that hibernating arctic ground squirrels were actively thermogenic. Individual burrows did not differ significantly in mean soil temperature over 3 years, but significant differences in mean and minimum soil temperatures were observed among burrows. Sites of burrows with shrubby vegetation accumulated more snow and had significantly higher soil temperatures over winter than windswept sites in non-shrubby vegetation. Female ground squirrels hibernated in burrows that had significantly higher mean and minimum soil temperatures than burrows of males, and adults hibernated in burrows with significantly higher soil temperatures than burrows of juveniles. Although ground squirrels occupying colder burrows were predicted to lose more body mass during hibernation than those in warmer burrows, changes in body, fat, and lean masses over winter were not correlated with soil temperature for any sex or age. Relationships between change in body composition of hibernating arctic ground squirrels and temperatures of their hibernacula may be confounded by use of food caches, differing thermal conductance of nests, or differences in individuals energetics of hibernating not related to the gradient between body and soil temperatures.

mRNA stability and polysome loss in hibernating Arctic ground squirrels (Spermophilus parryii).

ABSTRACT All small mammalian hibernators periodically rewarm from torpor to high, euthermic body temperatures for brief intervals throughout the hibernating season. The functional significance of these arousal episodes is unknown, but one suggestion is that rewarming may be related to replacement of gene products lost during torpor due to degradation of mRNA. To assess the stability of mRNA as a function of the hibernation state, we examined the poly(A) tail lengths of liver mRNA from arctic ground squirrels sacrificed during four hibernation states (early and late during a torpor bout and early and late following arousal from torpor) and from active ground squirrels sacrificed in the summer. Poly(A) tail lengths were not altered during torpor, suggesting either that mRNA is stabilized or that transcription continues during torpor. In mRNA isolated from torpid ground squirrels, we observed a pattern of 12 poly(A) residues at greater densities approximately every 27 nucleotides along the poly(A) tail, which is a pattern consistent with binding of poly(A)-binding protein. The intensity of this pattern was significantly reduced following arousal from torpor and undetectable in mRNA obtained from summer ground squirrels. Analyses of polysome profiles revealed a significant reduction in polyribosomes in torpid animals, indicating that translation is depressed during torpor.

Shotgun Proteomics Analysis of Hibernating Arctic Ground Squirrels

Mammalian hibernation involves complex mechanisms of metabolic reprogramming and tissue protection. Previous gene expression studies of hibernation have mainly focused on changes at the mRNA level. Large scale proteomics studies on hibernation have lagged behind largely because of the lack of an adequate protein database specific for hibernating species. We constructed a ground squirrel protein database for protein identification and used a label-free shotgun proteomics approach to analyze protein expression throughout the torpor-arousal cycle during hibernation in arctic ground squirrels (Urocitellus parryii). We identified more than 3,000 unique proteins from livers of arctic ground squirrels. Among them, 517 proteins showed significant differential expression comparing animals sampled after at least 8 days of continuous torpor (late torpid), within 5 h of a spontaneous arousal episode (early aroused), and 1–2 months after hibernation had ended (non-hibernating). Consistent with changes at the mRNA level shown in a previous study on the same tissue samples, proteins involved in glycolysis and fatty acid synthesis were significantly underexpressed at the protein level in both late torpid and early aroused animals compared with non-hibernating animals, whereas proteins involved in fatty acid catabolism were significantly overexpressed. On the other hand, when we compared late torpid and early aroused animals, there were discrepancies between mRNA and protein levels for a large number of genes. Proteins involved in protein translation and degradation, mRNA processing, and oxidative phosphorylation were significantly overexpressed in early aroused animals compared with late torpid animals, whereas no significant changes at the mRNA levels between these stages had been observed. Our results suggest that there is substantial post-transcriptional regulation of proteins during torpor-arousal cycles of hibernation.

Body temperature patterns before, during, and after semi-natural hibernation in the European ground squirrel.

Abstract. Ground squirrels undergo extreme body temperature fluctuations during hibernation. The effect of low body temperatures on the mammalian circadian system is still under debate. Using implanted temperature loggers, we recorded body temperature patterns in European ground squirrels kept in an enclosure under natural conditions. Although hibernation onset was delayed, hibernation end corresponded closely to that measured in a field population. Circadian body temperature fluctuations were not detected during deep torpor, but indications of circadian timing of arousal episodes at higher temperatures were found at the beginning and end of hibernation. One male exhibited synchronised arousals to a relatively constant phase of the day throughout hibernation. All animals first entered torpor in the afternoon. Daily body temperature fluctuations were decreased or distorted during the first days after hibernation. We hypothesise that hibernation may affect the circadian system by either decreasing the expression of the circadian oscillator, or by decreasing the amplitude of the circadian oscillator itself, possibly due to gradual, temperature dependent, internal desynchronisation. The latter mechanism may be beneficial because it might facilitate post-hibernation re-entrainment rates.