Arctic Ground Squirrels Research Articles

Physiological regulation of tau phosphorylation during hibernation

The microtubule-associated protein tau is abnormally hyperphosphorylated in the brains of individuals with Alzheimer disease and other tauopathies, and is believed to play a critical role in the pathogenesis of these diseases. While the mechanisms leading to abnormal tau phosphorylation remain elusive, the recent demonstration of reversible tau phosphorylation during hibernation provides an ideal physiological model to study this critical process in vivo. In this study, Arctic ground squirrels (AGS) during hibernation were used to study mechanisms related to tau hyperphosphorylation. Our data demonstrate that tau is hyperphosphorylated at all six sites (S199, T205, S214, S262, S396, and S404) examined in hibernating AGS. Interestingly, only three of these sites (S199, S262, and S404) are dephosphorylated in aroused animals, suggesting a reversible phosphorylation at selective sites. Summer-active AGS demonstrated the lowest tau phosphorylation at all these sites. To explore the mechanisms underlying increased tau phosphorylation during hibernation, the expression level and enzyme activity of various potential tau kinases and protein phosphatases were examined. The kinetic analysis of enzyme activity at different temperatures revealed differential changes in enzyme activity with temperature decline. Specifically, increased protein kinase A activity, decreased protein phosphatase 2A activity, as well as substantial contribution from glycogen synthase kinase-3beta, likely play a key role in increased tau phosphorylation during hibernation in AGS.

Modulation of gene expression in hibernating arctic ground squirrels

We performed a broadscale screening of differential gene expression using both high-throughput bead-array technology and real-time PCR assay in brown adipose tissue, liver, heart, hypothalamus, and skeletal muscle in hibernating arctic ground squirrels, comparing animals sampled after two durations of steady-state torpor, during two stages of spontaneous arousal episodes, and in animals after they ended hibernation. Significant seasonal and torpor-arousal cycle differences of gene expression were detected in genes involved in glycolysis, fatty acid metabolism, gluconeogenesis, amino acid metabolism, molecular transport, detoxification, cardiac contractility, circadian rhythm, cell growth and apoptosis, muscle dystrophy, and RNA and protein protection. We observed, for the first time, complex modulation of gene expression during multiple stages of torpor-arousal cycles. The mRNA levels of certain metabolic genes drop significantly during the transition from late torpor to early arousal, perhaps due to the rapid turnover of mRNA transcripts resulting from the translational demands during thermogenesis in early arousal, whereas the mRNA levels of genes related to circadian rhythm, cell growth, and apoptosis rise significantly in the early or late arousal phases during torpor-arousal cycle, suggesting the resumption of circadian rhythm and cell cycle during arousal.

Simultaneous Collection of Body Temperature and Activity Data in Burrowing Mammals: a New Technique

Abstract: Integrating physiological and behavioral observations into ecological field studies of animals can provide novel insights into relationships among animal behavior, physiology, and ecology. We describe and evaluate a new technique for simultaneously collecting body temperature (Tb) and burrow use data for semi‐fossorial mammals by combining light‐sensitive radiotransmitters and implanted temperature‐sensitive dataloggers. We used this approach to collect core Tb and activity data for 9 free‐ranging arctic ground squirrels (Spermophilus parryii) in northern Alaska, USA, at approximately 5‐minute intervals for 30–90 days each to address questions related to thermoregulation, energetics, foraging, sociality, and timing of activity in natural environments.

Stable Carbon Isotope Compositions of Eastern Beringian Grasses and Sedges: Investigating Their Potential as Paleoenvironmental Indicators

ABSTRACTThe nature of vegetation cover present in Beringia during the last glaciation remains unclear. Uncertainty rests partly with the limitations of conventional paleoecological methods. A lack of sufficient taxonomic resolution most notably associated with the grasses and sedges restricts the paleoecological inferences that can be made. Stable isotope measurements of subfossil plants are frequently used to enhance paleoenvironmental reconstructions. We present an investigation of the stable carbon isotope composition (δ13C) of modern and subfossil grasses and sedges (graminoids) from Eastern Beringia. Modern grasses from wet habitats had a mean δ13C of −29.1‰ (standard deviation [SD] = 2.1‰, n = 75), while those from dry habitats had a mean of −26.9‰ (SD = 1.19, n = 27). Sedges (n = ~50) from dry, wet, marsh, and sand dune habitats had specific habitat ranges. Four modern C4 grasses had δ13C values typical of C4 plants. Analyses were also conducted using subfossil graminoid remains from several sedimentary paleoecological contexts (e.g., arctic ground squirrel nests, loess, permafrost, and paleosols) in Eastern Beringia. Results from these subfossil samples, ranging in age from >40,000 to ca. 11,000 cal. yr BP, illustrate that the δ13C of graminoid remains has altered during the past. The range of variation in the subfossil samples is within the range from modern graminoid specimens from dry and wet habitats. The results indicate that stable isotopes could contribute to a comprehensive and multiproxy reconstruction of Beringian paleoenvironments.

Arctic ground squirrels of the mammoth-steppe: paleoecology of Late Pleistocene middens (∼24 000–29 450 14C yr BP), Yukon Territory, Canada

This paper presents paleoecological analyses of 48 fossil arctic ground squirrel ( Spermophilus parryii) middens (nests and caches) recovered from ice-rich loess sediments in the Klondike region of west-central Yukon Territory. AMS radiocarbon dates and stratigraphic association of middens with Dawson tephra (∼25 300 14C yr BP), indicate these paleoecological data reflect the onset of glacial conditions of early Marine Isotope Stage (MIS) 2 and terminal MIS 3 (∼24 000–29 450 14C yr BP). Plant macrofossils include at least 60 plant taxa, including diverse graminoids ( Poa, Elymus trachycaulus, Kobresia myosuroides), steppe forbs ( Penstemon gormanii, Anemone patens var. multifida, Plantago cf. canescens), tundra forbs ( Draba spp., Bistorta vivipara), dwarf shrubs ( Salix cf. arctica, S. cf. polaris), sage ( Artemisia frigida) and rare trees ( Picea mariana). Many of these taxa identified in the middens represent the first recorded fossils for these plants in Eastern Beringia and add to our knowledge of the floristic composition of Pleistocene vegetation and biogeography in this region. Fossil beetles include typical members of the Eastern Beringian steppe–tundra fauna ( Lepidophorus lineaticollis and Connatichela artemisiae) and others suggesting predominantly dry, open habitats. Cache forage selection is suggested by some plant taxa which were particularly frequent and abundant in the middens ( Bistorta vivipara, Kobresia myosuroides, Ranunculus spp., Potentilla, Erysimum cf. cheiranthoides, Poa, Carex and Draba). Factors such as proximity of vegetation to burrows and abundance of fruits and seeds per plant were probably important in cache selection. Glacial conditions enabled arctic ground squirrels to form widespread and dense populations in regions such as the Klondike in which they are rare or absent at present. This fossil midden record supports previous hypotheses that suggest arctic ground squirrels evolved in and are well-adapted to the open, steppe–tundra vegetation, loessal soils and glacial climates of the mammoth-steppe biome.

EVALUATION OF RADIO-TRANSMITTERS ON JUVENILE RABBITS: APPLICATION TO THE SEMIFOSSORIAL PYGMY RABBIT (BRACHYLAGUS IDAHOENSIS)

Researchers doing telemetry studies involving external transmitter attachments on young small mammals often employ expandable or breakaway collars. This method has been used for arctic ground squirrels (Spermophilus parryii plesius; Byrom and Krebs 1999), snowshoe hares (Lepus americanus; O’Donoghue and Bergman 1992, Gillis and Krebs 2000), chuditch (Dasyurus geoffroii; Soderquist and Serena 2000), and others to reduce the risk of mortality if a transmitter fails or if a leg or lower jaw becomes entangled in the collar (Soderquist 1993). Expandable collars also reduce the need to repeatedly capture and refit radio-collars on growing individuals, but they may increase the likelihood that jaws or appendages of juvenile lagomorphs will become caught. One alternative for attachment of radio

Enantioseparation and Stacking of Cyanobenz[f]isoindole-Amino Acids by Reverse Polarity Capillary Electrophoresis and Sulfated β-Cyclodextrin

A capillary electrophoresis method with laser-induced fluorescence detection for the chiral separation of cyanobenz[f]isoindole (CBI) derivatives of amino acids was developed and optimized. The enantioseparations are accomplished with sulfated beta-CD (S-beta-CD) as chiral selector at low pH and reverse polarity. BGE conditions were optimized for CBI-serine and then applied to other CBI-amino acids. Baseline resolution of 13 CBI-amino acids was achieved using a single BGE formulation of 2 wt % S-beta-CD in 25 mM phosphate buffer at pH 2.00 and a voltage of -30 kV. pH is the most critical BGE parameter affecting resolution. At 2 wt % S-beta-CD, CBI-serine enantiomers are baseline-resolved at pH 2.00 but no resolution is obtained at pH 3.00. l-Glutamate, l-aspartate and d-serine are simultaneously quantified in the microdialysate of an arctic ground squirrel to illustrate the application to biological samples. Dilute solutions of the CBI-amino acids in water can be stacked by hydrodynamic injection with a 100-fold improvement in signal-to-noise ratio without loss of chiral resolution. The stacking is proposed to consist of field-amplified migration, pH-mediated stacking, and sweeping by S-beta-CD. The limit of detections for CBI-dl-serine and CBI-dl-glutamate are determined as 0.20 and 0.30 nM, respectively. The stacking method was not applicable to the high ionic strength microdialysates.

Distribution of NMDA receptor subunit NR1 in Arctic ground squirrel central nervous system

Hibernation is a natural model of neuroprotection and adult synaptic plasticity. NMDA receptors (NMDAR), which play key roles in excitotoxicity and synaptic plasticity, have not been characterized in a hibernating species. Tolerance to excitotoxicity and cognitive enhancement in Arctic ground squirrels (AGS, Spermophilus parryii) suggests that NMDAR expression may decrease in hibernation and increase upon arousal. NMDAR consist of at least one NMDAR1 (NR1) subunit, which is required for receptor function. Localization of NR1 reflects localization of the majority, if not all, NMDAR complexes. The purpose of this study, therefore, was to characterize the distribution of NR1 subunits in AGS central nervous system using immunohistochemistry. In addition, we compare NR1 expression in hippocampus of hibernating AGS (hAGS) and inter-bout euthermic AGS (ibeAGS) and assess changes in cell somata size using NR1 stained sections in three hippocampal sub-regions (CA1, CA3, and dentate gyrus). For the first time, we report that immunoreactivity of anti-NR1 is widely distributed throughout the central nervous system in AGS and is similar to other species. No differences exist in the expression and distribution of NR1 in hAGS and ibeAGS. However, we report a significant decrease in size of hippocampal CA1 and dentate gyrus NR1-expressing neuronal somata during hibernation torpor.

Cache Selection by Arctic Ground Squirrels Inhabiting Boreal-steppe Meadows of Southwest Yukon Territory, Canada

ABSTRACTWe examined food items cached by arctic ground squirrels (Spermophilus parryii) from boreal-steppe meadows of southwest Yukon Territory, Canada. Caches recovered from two sites are dominated by fruits and seeds of either northern comandra (Geocaulon lividum) or prickly rose (Rosa acicularis). These two taxa are relatively rare in the local flora at the study sites (Site 1: ≥32 available taxa, and Site 2: ≥39 available taxa), suggesting they are selectively cached as preferred items. Cache selectivity may be related to perishability, fruit size/seed abundance, and predation risk. These caches are of significantly different composition than caches from present tundra sites and Pleistocene fossil arctic ground squirrel nests and caches (middens) recovered from central Yukon. These findings suggest that although arctic ground squirrels evolved in open tundra, they can subsist on a variety of cache items and may have the ability to adapt to and select a profitable cache within a variety of boreal and tundra habitats.

Persistent Tolerance to Oxygen and Nutrient Deprivation and N-methyl-D-Aspartate in Cultured Hippocampal Slices from Hibernating Arctic Ground Squirrel

Hibernating Arctic ground squirrel (hAGS), Spermophilus parryii, survive profound decreases in cerebral perfusion during torpor and return to normal blood flow during intermittent rewarming periods without neurologic damage. Hibernating AGS tolerate traumatic brain injury in vivo, and acute hippocampal slices from hibernating animals tolerate oxygen and glucose deprivation. It remains unclear, however, if neuroprotection results from intrinsic tissue properties or from differences in response to acute trauma associated with slice preparation. The goal of this work was therefore to determine whether an intrinsic tissue tolerance persists in chronic culture of AGS hippocampal slices at 37 degrees C. A second goal was to address N-methyl-D-aspartate (NMDA) receptor involvement and channel arrest as potential mechanisms of intrinsic tissue tolerance. Baseline neuronal survival and tolerance to oxygen and nutrient deprivation (OND), an in vitro model of ischemia-reperfusion, were assessed in the CA1 region of hippocampal slices from juvenile, hAGS and interbout euthermic AGS (ibeAGS). Early in culture (insult onset at 3 h), slices from both hAGS and ibeAGS tolerate OND (4 h deprivation followed by 20 h recovery) and 500 micromol/L NMDA plus 20 mmol/L KCl. Later in culture (insult onset at 24 h), tolerance persists in slices from hAGS but not in slices from ibeAGS. Ouabain (Na(+)K(+)ATPase inhibitor) administered 24 h in culture enhances survival of slices from hAGS (assessed 24 h later). Thus, tolerance to OND in slices from hAGS is due to intrinsic tissue properties likely involving NMDA receptors and ion channel arrest.

Vegetation buried under Dawson tephra (25,300 14C years BP) and locally diverse late Pleistocene paleoenvironments of Goldbottom Creek, Yukon, Canada

Paleoecological research at Goldbottom Creek in the Klondike region of Yukon Territory (NW Canada) documents an in situ riparian grassy meadow that was buried during the winter or early spring by Dawson tephra, near the onset of Marine Isotope Stage (MIS) 2, ca. 25,300 14C years BP. Analyses of vascular plant macrofossils, bryophytes, pollen, insects and paleosols from the riparian meadow contrast with evidence for well-drained, upland steppe–tundra habitats obtained from fossil arctic ground squirrel middens within the same valley. The mesic valley bottom vegetation consisted of grasses ( Deschampsia caespitosa, Alopecurus), sedges ( Carex), horsetail ( Equisetum cf. palustre), diverse bryophytes and few forbs. Upland habitats with dry loessal soils along the valley slopes contained graminoids ( Elymus, Festuca, Kobresia myosuroides), sage ( Artemisia frigida) and diverse steppe and tundra forbs ( Phlox hoodii, Plantago cf. canescens, Anemone patens var. multifida, Bistorta vivipara, Draba). These paleoecological data highlight the effect of topographic position and moisture on substrates and their control on local-scale habitat variability. This study represents the first recorded in situ riparian surface from the unglaciated Pleistocene refugium of Beringia and provides well-documented evidence for local habitat heterogeneity and ecosystem structure within the mammoth–steppe biome. Other radiocarbon dated paleoecological data from our study sites indicate that full-glacial steppe–tundra habitats in west-central Yukon Territory were established during the later stages of the MIS 3 interstadial, by 29,000 14C years BP. The diverse data obtained through integration of multiple paleoecological methods at the site demonstrate the efficacy of interdisciplinary research in furthering our understanding of Beringian glacial environments.

Regulation of UCP1 and UCP3 in arctic ground squirrels and relation with mitochondrial proton leak

Uncoupling protein (UCP) 1 (UCP1) catalyzes a proton leak in brown adipose tissue (BAT) mitochondria that results in nonshivering thermogenesis (NST), but the extent to which UCP homologs mediate NST in other tissues is controversial. To clarify the role of UCP3 in mediating NST in a hibernating species, we measured Ucp3 expression in skeletal muscle of arctic ground squirrels in one of three activity states (not hibernating, not hibernating and fasted for 48 h, or hibernating) and housed at 5 degrees C or -10 degrees C. We then compared Ucp3 mRNA levels in skeletal muscle with Ucp1 mRNA and UCP1 protein levels in BAT in the same animals. Ucp1 mRNA and UCP1 protein levels were increased on cold exposure and decreased with fasting, with the highest UCP1 levels in thermogenic hibernators. In contrast, Ucp3 mRNA levels were not affected by temperature but were increased 10-fold during fasting and >3-fold during hibernation. UCP3 protein levels were increased nearly fivefold in skeletal muscle mitochondria isolated from fasted squirrels compared with nonhibernators, but proton leak kinetics in the presence of BSA were unchanged. Proton leak in BAT mitochondria also did not differ between fed and fasted animals but did show classical inhibition by the purine nucleotide GDP. Levels of nonesterified fatty acids were highest during hibernation, and tissue temperatures during hibernation were related to Ucp1, but not Ucp3, expression. Taken together, these results do not support a role for UCP3 as a physiologically relevant mediator of NST in muscle.

The Arctic Ground Squirrel Brain Is Resistant to Injury From Cardiac Arrest During Euthermia

Hetereothermic mammals tolerate hypoxia during euthermy and torpor, and evidence suggests this tolerance may extend beyond hypoxia to cerebral ischemia. During hibernation, CA1 hippocampal neurons endure extreme fluctuations in cerebral blood flow during transitions into and out of torpor as well as reductions in cerebral blood flow during torpor. In vitro studies likewise show evidence of ischemia tolerance in hippocampal slices harvested from euthermic ground squirrels; however, no studies have investigated tolerance in a clinically relevant model of in vivo global cerebral ischemia. The purpose of the present study was to test the hypothesis that the euthermic Arctic ground squirrel (AGS; Spermophillus parryii) is resistant to injury from asphyxial cardiac arrest (CA). Estrous-matched female rats were used as a positive control. Female euthermic AGS and rats were subjected to 8-minute CA. At the end of 7 days of reperfusion, AGS and rats were fixed for histopathological assessment. In rats subjected to CA, the number of ischemic neurons was significantly higher (P<0.001) compared with control rats in hippocampus and striatum. Cortex was mildly injured. Surprisingly, neuronal counts in AGS were not significantly different in CA and control groups in these brain regions. These data demonstrate that AGS are remarkably tolerant to global cerebral ischemia during euthermia. A better understanding of the mechanisms by which AGS tolerate severe reductions in blood flow during euthermia may provide novel neuroprotective strategies that may translate into significant improvements in human patient outcomes after CA.

Detection of differential gene expression in brown adipose tissue of hibernating arctic ground squirrels with mouse microarrays

Hibernation is an energy-saving strategy adopted by a wide range of mammals to survive highly seasonal or unpredictable environments. Arctic ground squirrels living in Alaska provide an extreme example, with 6- to 9-mo-long hibernation seasons when body temperature alternates between levels near 0 degrees C during torpor and 37 degrees C during arousal episodes. Heat production during hibernation is provided, in part, by nonshivering thermogenesis that occurs in large deposits of brown adipose tissue (BAT). BAT is active at tissue temperatures from 0 to 37 degrees C during rewarming and continuously at near 0 degrees C during torpor in subfreezing conditions. Despite its crucial role in hibernation, the global gene expression patterns in BAT during hibernation compared with the nonhibernation season remain largely unknown. We report a large-scale study of differential gene expression in BAT between winter hibernating and summer active arctic ground squirrels using mouse microarrays. Selected differentially expressed genes identified on the arrays were validated by quantitative real-time PCR using ground squirrel specific primers. Our results show that the mRNA levels of the genes involved in nearly every step of the biochemical pathway leading to nonshivering thermogenesis are significantly increased in BAT during hibernation, whereas those of genes involved in protein biosynthesis are significantly decreased compared with summer active animals in August. Surprisingly, the differentially expressed genes also include adipocyte differentiation-related protein or adipophilin (Adfp), gap junction protein 1 (Gja1), and secreted protein acidic and cysteine-rich (Sparc), which may play a role in enhancing thermogenesis at low tissue temperatures in BAT.

Decreased NR1 phosphorylation and decreased NMDAR function in hibernating Arctic ground squirrels

Heterothermic mammals such as ground squirrels tolerate ischemia and N-methyl-D-aspartate (NMDA) better than homeothermic mammals such as rats both in vivo and in vitro, and this tolerance is enhanced in the hibernating state. However, the cellular mechanisms underlying this tolerance remain unclear. NMDA receptors (NMDAR) play a key role in excitotoxicity. The purpose of the current study was therefore to test the hypothesis that NMDAR are down-regulated in hibernating Arctic ground squirrels (hAGS; Spermophilus parryii). To address this hypothesis, we used Western blot analysis to investigate NMDAR phosphorylation, an activator of NMDAR function, and internalization in naïve hippocampal tissue from hAGS, interbout euthermic AGS (ibeAGS), and rats. Furthermore, we used fura-2 calcium imaging to examine NMDAR function in cultured hippocampal slices from hAGS, ibeAGS, and rats. We report that phosphorylation of the NMDAR1 (NR1) subunit is decreased in hippocampal tissue from hAGS and that the NMDAR component of Glu-induced increase in [Ca(2+)](i) is decreased in hippocampal slices from hAGS. Moreover, the fraction of NR1 in the functional membrane pool in AGS is less than that in rats.

P.060 Acute infection of woodchucks with californian and arctic ground squirrel hepatitis virus

P.060 Acute infection of woodchucks with californian and arctic ground squirrel hepatitis virus

Seasonal effects of Selank on the behavior of hibernating animals

We studied the seasonal effects of peptide preparation Selank on orientation and exploratory activity in 36 arctic ground squirrels Citellus undulatus in the open-field and hole-board tests. Selank most significantly increased behavioral activity of hibernating animals in the spring and fall. The test peptide had no effect on locomotor activity of animals. The selective stimulatory effect of Selank on exploratory behavior of hibernating animals was season-dependent. The maximum effect was observed under conditions of seasonal depression-like state.

Home range and movements of female brown bears in southwestern Alaska

Between 1993 and 2003, 40 adult female brown bears (Ursus arctos) were radiocollared and monitored in the southwest Kuskokwim Mountains of Alaska. The 95% kernel home range size for individuals with ≥50 relocations (n = 20) ranged from 93–623 km2 (x̄ = 356 km2). Bears occupied lower elevations during July and August (x̄ = 454 m) when salmon (Oncorhynchus spp.) were available and higher elevations in September (x̄ = 520 m), presumably to feed on berries, ground squirrels (Spermophilus parryii), and caribou (Rangifer tarandus). During the denning period, bears moved to higher average elevations (x̄ = 632 m), generally remaining through June (x̄ = 570 m). Radiomarked females entered the den in mid October and exited in mid May. There was fidelity to denning areas, with an average distance between consecutive dens of 4.5 km (SD = 3.1). Females with cubs were found at higher average elevations than females without young or with yearlings. Radiomarked females were found closer to anadromous streams from 16 July–15 August, coinciding with chinook (O. tschawytscha), chum (O. keta), and sockeye salmon (O. nerka) availability. Beginning in 1998, chinook and chum salmon escapement was low in the Kuskokwim drainage. There was an inverse relationship between measures of salmon availability and bear distances to anadromous streams. This indicates that although the study area was at the fringe of salmon range, salmon abundance influenced bear seasonal movements and distribution. With reduced salmon availability, this area may support a lower bear density, and brown bear population management should be considered in salmon escapement goals.

Arousal from hibernation alters contextual learning and memory

Hibernation is a unique and highly regulated physiological state characterized by profound, albeit periodically reversible, depression in body temperature, metabolism, and consciousness. Hippocampal synapses undergo pronounced remodeling in concert with torpor and arousal. During hibernation, the number of postsynaptic densities, apical dendritic branches, and spine densities decreases substantially in the hippocampus. Upon arousal these parameters increase beyond pre-hibernation levels and peak within 2–3 h. By 24 h after arousal, dendritic parameters remain elevated but have started to subside, consistent with pruning and differentiation. The present study examined the functional consequences of these natural changes in synaptic structure. Wild-caught Arctic ground squirrels (AGS) were trained in a hippocampal-dependent contextual fear conditioning task at 3 h, 24 h, or 4 weeks after arousal (warm-adapted euthermic control group). All groups acquired the fear conditioned response similarly on the training day. During a subsequent retention test session, AGS in the 24 h group exhibited enhanced expression of contextual fear compared to the other two groups. These data suggest that the morphological and biochemical changes occurring at 24 h after arousal from hibernation affect hippocampal-dependent learning and memory. The natural change in synaptic structure during hibernation may provide a unique opportunity to assess the neural substrates underlying cognitive enhancement.

ARCTIC GROUND SQUIRRELS HANDLE STRESS WITH FINESSE

Hibernation in mammals is characterized by drastic decreases in body temperature, heart rate, respiration and other factors that greatly decrease metabolic rate and yield energy savings during the long, cold months of winter. However, hibernation is not a constant state; in such animals as ground squirrels and marmots, hibernation is regularly interrupted by periods of normal body temperature, in addition to the final arousal at winter's end. While a great deal is known about the enzymatic and cardiovascular changes that permit hibernation bouts and arousal periods, far less attention has been paid to the physiological consequences of the massive metabolic upregulation required during the return to normal body temperature and activity levels. It would seem, for example, that the increased metabolic demands of arousal could result in tissue hypoxia (low oxygenation) or, conversely, induce cellular stress from the sudden increases in blood flow and oxygen supply.Cellular stress could be induced both by inflammatory responses and by reactive oxygen species (ROS). ROS are produced by a variety of mechanisms,including oxidative phosphorylation in mitochondria. Thus, the reduced metabolism of hibernation and/or the high oxygen demands of arousal could prime the tissues for a mismatch between oxygen supply and mitochondrial activity, followed by a massive release of ROS. When normal cellular antioxidant defenses are overwhelmed, ROS interact with cellular lipids,proteins and DNA, resulting in cellular damage and death. The question, then,is whether or not hibernating animals become hypoxic during torpor or arousal,and, if so, are they able to prevent ROS damage during periods of increased blood flow and respiration?To tackle this question, Kelly Drew's group at the University of Alaska Fairbanks studied hypoxia and cellular stress in Arctic ground squirrels. To examine hypoxia, the team compared arterial oxygen levels and hemoglobin saturation in hibernating Arctic ground squirrels (torpid, cold),non-hibernating winter animals (normal body temperature) and animals that had entered an arousal period (core temperature returned to at least 34°C). They also examined the animals' brain histology for evidence of cellular damage; they compared brain levels of hypoxia inducible factor (HIF-1α)as a marker of hypoxia and the accumulation of inducible nitric oxide synthase(iNOS) as a marker of inflammation.Drew's team reports that hibernating Arctic ground squirrels show no evidence of hypoxia, cellular stress or inflammatory responses in the brain;their reduced metabolism allows these animals to avoid hypoxic stress. The enormous metabolic demands of arousal from hibernation, however, do decrease oxygen levels and lead to the accumulation of HIF-1α in the brain, but the authors found no evidence of neuronal pathology, oxidative stress or inflammation in the brain following the return to basal metabolism. By contrast, non-hibernating winter animals are apparently the most highly stressed physiologically, experiencing chronic mild hypoxia, low hemoglobin saturation and the accumulation of both HIF-1α and iNOS.So, like us, Arctic ground squirrels are apparently most stressed by real life! But their ability to avoid oxidative damage following hibernation and arousal presents an interesting natural model providing insights on how to survive periods of low oxygen and the sudden restoration of blood flow to tissues after these periods.