Brain energy metabolism and neurotransmission at near-freezing temperatures: in vivo (1)H MRS study of a hibernating mammal

The future of artificial hibernation medicine: protection of nerves and organs after spinal cord injury

Spinal cord injury is a serious disease of the central nervous system involving irreversible nerve injury and various organ system injuries. At present, no effective clinical treatment exists. As one of the artificial hibernation techniques, mild hypothermia has preliminarily confirmed its clinical effect on spinal cord injury. However, its technical defects and barriers, along with serious clinical side effects, restrict its clinical application for spinal cord injury. Artificial hibernation is a future-oriented disruptive technology for human life support. It involves endogenous hibernation inducers and hibernation-related central neuromodulation that activate particular neurons, reduce the central constant temperature setting point, disrupt the normal constant body temperature, make the body "adapt" to the external cold environment, and reduce the physiological resistance to cold stimulation. Thus, studying the artificial hibernation mechanism may help develop new treatment strategies more suitable for clinical use than the cooling method of mild hypothermia technology. This review introduces artificial hibernation technologies, including mild hypothermia technology, hibernation inducers, and hibernation-related central neuromodulation technology. It summarizes the relevant research on hypothermia and hibernation for organ and nerve protection. These studies show that artificial hibernation technologies have therapeutic significance on nerve injury after spinal cord injury through inflammatory inhibition, immunosuppression, oxidative defense, and possible central protection. It also promotes the repair and protection of respiratory and digestive, cardiovascular, locomotor, urinary, and endocrine systems. This review provides new insights for the clinical treatment of nerve and multiple organ protection after spinal cord injury thanks to artificial hibernation. At present, artificial hibernation technology is not mature, and research faces various challenges. Nevertheless, the effort is worthwhile for the future development of medicine.

Mass spectrometry of the white adipose metabolome in a hibernating mammal reveals seasonal changes in alternate fuels and carnitine derivatives

Mammalian hibernators undergo substantial changes in metabolic function throughout the seasonal hibernation cycle. We report here the polar metabolomic profile of white adipose tissue isolated from active and hibernating thirteen-lined ground squirrels (Ictidomys tridecemlineatus). Polar compounds in white adipose tissue were extracted from five groups representing different timepoints throughout the seasonal activity-torpor cycle and analyzed using hydrophilic interaction liquid chromatography-mass spectrometry in both the positive and negative ion modes. A total of 224 compounds out of 660 features detected after curation were annotated. Unsupervised clustering using principal component analysis revealed discrete clusters representing the different seasonal timepoints throughout hibernation. One-way analysis of variance and feature intensity heatmaps revealed metabolites that varied in abundance between active and torpid timepoints. Pathway analysis compared against the KEGG database demonstrated enrichment of amino acid metabolism, purine metabolism, glycerophospholipid metabolism, and coenzyme A biosynthetic pathways among our identified compounds. Numerous carnitine derivatives and a ketone that serves as an alternate fuel source, beta-hydroxybutyrate (BHB), were among molecules found to be elevated during torpor. Elevated levels of the BHB-carnitine conjugate during torpor suggests the synthesis of beta-hydroxybutyrate in white adipose mitochondria, which may contribute directly to elevated levels of circulating BHB during hibernation.

Integrative transcriptomic and metabolic analyses of the mammalian hibernating brain identifies a key role for succinate dehydrogenase in ischemic tolerance

Ischemic stroke results in a loss of tissue homeostasis and integrity, the underlying pathobiology of which stems primarily from the depletion of cellular energy stores and perturbation of available metabolites ¹ . Hibernation in thirteen-lined ground squirrels (TLGS), Ictidomys tridecemlineatus , provides a natural model of ischemic tolerance as these mammals undergo prolonged periods of critically low cerebral blood flow without evidence of central nervous system (CNS) damage ² . Studying the complex interplay of genes and metabolites that unfolds during hibernation may provide novel insights into key regulators of cellular homeostasis during brain ischemia. Herein, we interrogated the molecular profiles of TLGS brains at different time points within the hibernation cycle via RNA sequencing coupled with untargeted metabolomics. We demonstrate that hibernation in TLGS leads to major changes in the expression of genes involved in oxidative phosphorylation and this is correlated with an accumulation of the tricarboxylic acid (TCA) cycle intermediates citrate, cis-aconitate, and α-ketoglutarate-αKG. Integration of the gene expression and metabolomics datasets led to the identification of succinate dehydrogenase (SDH) as the critical enzyme during hibernation, uncovering a break in the TCA cycle at that level. Accordingly, the SDH inhibitor dimethyl malonate (DMM) was able to rescue the effects of hypoxia on human neuronal cells in vitro and in mice subjected to permanent ischemic stroke in vivo . Our findings indicate that studying the regulation of the controlled metabolic depression that occurs in hibernating mammals may lead to novel therapeutic approaches capable of increasing ischemic tolerance in the CNS.

Female and male C57BL/6J offspring exposed to maternal obesogenic diet develop altered hypothalamic energy metabolism in adulthood

Maternal obesity is exceedingly common and strongly linked to offspring obesity and metabolic disease. Hypothalamic function is critical to obesity development. Hypothalamic mechanisms causing obesity following exposure to maternal obesity have not been elucidated. Therefore, we studied a cohort of C57BL/6J dams, treated with a control or high-fat-high-sugar diet, and their adult offspring to explore potential hypothalamic mechanisms to explain the link between maternal and offspring obesity. Dams treated with obesogenic diet were heavier with mild insulin resistance, which is reflective of the most common metabolic disease in pregnancy. Adult offspring exposed to maternal obesogenic diet had no change in body weight but significant increase in fat mass, decreased glucose tolerance, decreased insulin sensitivity, elevated plasma leptin, and elevated plasma thyroid-stimulating hormone. In addition, offspring exposed to maternal obesity had decreased energy intake and activity without change in basal metabolic rate. Hypothalamic neurochemical profile and transcriptome demonstrated decreased neuronal activity and inhibition of oxidative phosphorylation. Collectively, these results indicate that maternal obesity without diabetes is associated with adiposity and decreased hypothalamic energy production in offspring. We hypothesize that altered hypothalamic function significantly contributes to obesity development. Future studies focused on neuroprotective strategies aimed to improve hypothalamic function may decrease obesity development.NEW & NOTEWORTHY Offspring exposed to maternal diet-induced obesity demonstrate a phenotype consistent with energy excess. Contrary to previous studies, the observed energy phenotype was not associated with hyperphagia or decreased basal metabolic rate but rather decreased hypothalamic neuronal activity and energy production. This was supported by neurochemical changes in the hypothalamus as well as inhibition of hypothalamic oxidative phosphorylation pathway. These results highlight the potential for neuroprotective interventions in the prevention of obesity with fetal origins.

Human torpor: translating insights from nature into manned deep space expedition

During a long-duration manned spaceflight mission, such as flying to Mars and beyond, all crew members will spend a long period in an independent spacecraft with closed-loop bioregenerative life-support systems. Saving resources and reducing medical risks, particularly in mental heath, are key technology gaps hampering human expedition into deep space. In the 1960s, several scientists proposed that an induced state of suppressed metabolism in humans, which mimics 'hibernation', could be an ideal solution to cope with many issues during spaceflight. In recent years, with the introduction of specific methods, it is becoming more feasible to induce an artificial hibernation-like state (synthetic torpor) in non-hibernating species. Natural torpor is a fascinating, yet enigmatic, physiological process in which metabolic rate (MR), body core temperature (T_b ) and behavioural activity are reduced to save energy during harsh seasonal conditions. It employs a complex central neural network to orchestrate a homeostatic state of hypometabolism, hypothermia and hypoactivity in response to environmental challenges. The anatomical and functional connections within the central nervous system (CNS) lie at the heart of controlling synthetic torpor. Although progress has been made, the precise mechanisms underlying the active regulation of the torpor-arousal transition, and their profound influence on neural function and behaviour, which are critical concerns for safe and reversible human torpor, remain poorly understood. In this review, we place particular emphasis on elaborating the central nervous mechanism orchestrating the torpor-arousal transition in both non-flying hibernating mammals and non-hibernating species, and aim to provide translational insights into long-duration manned spaceflight. In addition, identifying difficulties and challenges ahead will underscore important concerns in engineering synthetic torpor in humans. We believe that synthetic torpor may not be the only option for manned long-duration spaceflight, but it is the most achievable solution in the foreseeable future. Translating the available knowledge from natural torpor research will not only benefit manned spaceflight, but also many clinical settings attempting to manipulate energy metabolism and neurobehavioural functions.

The Effects of Early-Life Iron Deficiency on Brain Energy Metabolism

Iron deficiency (ID) is one of the most prevalent nutritional deficiencies in the world. Iron deficiency in the late fetal and newborn period causes abnormal cognitive performance and emotional regulation, which can persist into adulthood despite iron repletion. Potential mechanisms contributing to these impairments include deficits in brain energy metabolism, neurotransmission, and myelination. Here, we comprehensively review the existing data that demonstrate diminished brain energetic capacity as a mechanistic driver of impaired neurobehavioral development due to early-life (fetal-neonatal) ID. We further discuss a novel hypothesis that permanent metabolic reprogramming, which occurs during the period of ID, leads to chronically impaired neuronal energetics and mitochondrial capacity in adulthood, thus limiting adult neuroplasticity and neurobehavioral function. We conclude that early-life ID impairs energy metabolism in a brain region- and age-dependent manner, with particularly strong evidence for hippocampal neurons. Additional studies, focusing on other brain regions and cell types, are needed.

Molecular interactions underpinning the phenotype of hibernation in mammals

Mammals maintain a constant warm body temperature, facilitating a wide variety of metabolic reactions. Mammals that hibernate have the ability to slow their metabolism, which in turn reduces their body temperature and leads to a state of hypothermic torpor. For this metabolic rate reduction to occur on a whole-body scale, molecular interactions that change the physiology of cells, tissues and organs are required, resulting in a major departure from normal mammalian homeostasis. The aim of this Review is to cover recent advances in the molecular biology of mammalian hibernation, including the role of small molecules, seasonal changes in gene expression, cold-inducible RNA-binding proteins, the somatosensory system and emerging information on hibernating primates. To underscore the importance of differential gene expression across the hibernation cycle, mRNA levels for 14,261 ground squirrel genes during periods of activity and torpor are made available for several tissues via an interactive transcriptome browser. This Review also addresses recent findings on molecular interactions responsible for multi-day survival of near-freezing body temperatures, single-digit heart rates and a slowed metabolism that greatly reduces oxygen consumption. A better understanding of how natural hibernators survive these physiological extremes is beginning to lead to innovations in human medicine.

Neonatal hyperglycemia alters the neurochemical profile, dendritic arborization and gene expression in the developing rat hippocampus

Hyperglycemia (blood glucose concentration >150 mg/dL) is common in extremely low gestational age newborns (ELGANs; birth at <28 week gestation). Hyperglycemia increases the risk of brain injury in the neonatal period. The long-term effects are not well understood. In adult rats, hyperglycemia alters hippocampal energy metabolism. The effects of hyperglycemia on the developing hippocampus were studied in rat pups. In Experiment 1, recurrent hyperglycemia of graded severity (moderate hyperglycemia (moderate-HG), mean blood glucose 214.6 ± 11.6 mg/dL; severe hyperglycemia (severe-HG), 338.9 ± 21.7 mg/dL; control, 137.7 ± 2.6 mg/dL) was induced from postnatal day (P) 3 to P12. On P30, the hippocampal neurochemical profile was determined using in vivo ¹ H MR spectroscopy. Dendritic arborization in the hippocampal CA1 region was determined using microtubule-associated protein (MAP)-2 immunohistochemistry. In Experiment 2, continuous hyperglycemia (mean blood glucose 275.3 ± 25.8 mg/dL; control, 142.3 ± 2.6 mg/dL) was induced from P2 to P6 by injecting streptozotocin (STZ) on P2. The mRNA expression of glycogen synthase 1 (Gys1), lactate dehydrogenase (Ldh), glucose transporters 1 (Glut1) and 3 (Glut3) and monocarboxylate transporters 1 (Mct1), 2 (Mct2) and 4 (Mct4) in the hippocampus was determined on P6. In Experiment 1, MRS demonstrated lower lactate concentration and glutamate/glutamine (Glu/Gln) ratio in the severe-HG group, compared with the control group (p < 0.05). Phosphocreatine/creatine ratio was higher in both hyperglycemia groups (p < 0.05). MAP-2 histochemistry demonstrated longer apical segment length, indicating abnormal synaptic efficacy in both hyperglycemia groups (p < 0.05). Experiment 2 showed lower Glut1, Gys1 and Mct4 expression and higher Mct1 expression in the hyperglycemia group, relative to the control group (p < 0.05). These results suggest that hyperglycemia alters substrate transport, lactate homeostasis, dendritogenesis and Glu-Gln cycling in the developing hippocampus. Abnormal neurochemical profile and dendritic structure due to hyperglycemia may partially explain the long-term hippocampus-mediated cognitive deficits in human ELGANs.

Enhanced oxidative capacity of ground squirrel brain mitochondria during hibernation

During hibernation, thirteen-lined ground squirrels (Ictidomys tridecemlineatus) regularly cycle between bouts of torpor and interbout arousal (IBA). Most of the brain is electrically quiescent during torpor but regains activity quickly upon arousal to IBA, resulting in extreme oscillations in energy demand during hibernation. We predicted increased functional capacity of brain mitochondria during hibernation compared with spring to accommodate the variable energy demands of hibernation. To address this hypothesis, we examined mitochondrial bioenergetics in the ground squirrel brain across three time points: spring (SP), torpor (TOR), and IBA. Respiration rates of isolated brain mitochondria through complex I of the electron transport chain were more than twofold higher in TOR and IBA than in SP (P < 0.05). We also found a 10% increase in membrane potential between hibernation and spring (P < 0.05), and that proton leak was lower in TOR and IBA than in SP. Finally, there was a 30% increase in calcium loading in SP brain mitochondria compared with TOR and IBA (P < 0.01). To analyze brain mitochondrial abundance between spring and hibernation, we measured the ratio of copy number in a mitochondrial gene (ND1) vs. a nuclear gene (B2M) in frozen cerebral cortex samples. No significant differences were observed in DNA copies between SP and IBA. These data show that brain mitochondrial bioenergetics are not static across the year and suggest that brain mitochondria function more effectively during the hibernation season, allowing for rapid production of energy to meet demand when extreme physiological changes are occurring.

Metabolic Flexibility: Hibernation, Torpor, and Estivation

Many environmental conditions can constrain the ability of animals to obtain sufficient food energy, or transform that food energy into useful chemical forms. To survive extended periods under such conditions animals must suppress metabolic rate to conserve energy, water, or oxygen. Amongst small endotherms, this metabolic suppression is accompanied by and, in some cases, facilitated by a decrease in core body temperature-hibernation or daily torpor-though significant metabolic suppression can be achieved even with only modest cooling. Within some ectotherms, winter metabolic suppression exceeds the passive effects of cooling. During dry seasons, estivating ectotherms can reduce metabolism without changes in body temperature, conserving energy reserves, and reducing gas exchange and its inevitable loss of water vapor. This overview explores the similarities and differences of metabolic suppression among these states within adult animals (excluding developmental diapause), and integrates levels of organization from the whole animal to the genome, where possible. Several similarities among these states are highlighted, including patterns and regulation of metabolic balance, fuel use, and mitochondrial metabolism. Differences among models are also apparent, particularly in whether the metabolic suppression is intrinsic to the tissue or depends on the whole-animal response. While in these hypometabolic states, tissues from many animals are tolerant of hypoxia/anoxia, ischemia/reperfusion, and disuse. These natural models may, therefore, serve as valuable and instructive models for biomedical research.

The use of magnetic resonance spectroscopy as a tool for the measurement of bi-hemispheric transcranial electric stimulation effects on primary motor cortex metabolism

Transcranial direct current stimulation (tDCS) is a neuromodulation technique that has been increasingly used over the past decade in the treatment of neurological and psychiatric disorders such as stroke and depression. Yet, the mechanisms underlying its ability to modulate brain excitability to improve clinical symptoms remains poorly understood. To help improve this understanding, proton magnetic resonance spectroscopy ((1)H-MRS) can be used as it allows the in vivo quantification of brain metabolites such as γ-aminobutyric acid (GABA) and glutamate in a region-specific manner. In fact, a recent study demonstrated that (1)H-MRS is indeed a powerful means to better understand the effects of tDCS on neurotransmitter concentration. This article aims to describe the complete protocol for combining tDCS (NeuroConn MR compatible stimulator) with (1)H-MRS at 3 T using a MEGA-PRESS sequence. We will describe the impact of a protocol that has shown great promise for the treatment of motor dysfunctions after stroke, which consists of bilateral stimulation of primary motor cortices. Methodological factors to consider and possible modifications to the protocol are also discussed.

Critical roles of mitochondria in brain activities of torpid Myotis ricketti bats revealed by a proteomic approach

Bats are the only mammals that fly and hibernate. Little is known about their overall metabolism in the brain during hibernation. In this study, brain proteins of torpid and active Myotis ricketti bats were fractionated and compared using a proteomic approach. Results showed that 21% (23 proteins) of identified proteins with significant expression changes were associated with amino acid metabolism and proteostasis. The expression levels of proteins involved in energy metabolism (15 proteins), cytoskeletal structure (18 proteins), and stress response (13 proteins) were also significantly altered in torpid bats. Over 30% (34 proteins) of differentially expressed proteins were associated with mitochondrial functions. Various post-translational modifications (PTMs) on PDHB, DLD, and ARG1 were detected, suggesting that bats use PTMs to regulate protein functions during torpor. Antioxidation and stress responses in torpid bats were similar to those of hibernated squirrels, suggesting a common strategy adopted by small hibernators against brain dysfunction. Since many amino acids that metabolize in mitochondria modulate neuronal transmissions, results of this study reveal pivotal roles of mitochondria in neural communication, metabolic regulation, and brain cell survival during bat hibernation. This article is part of a Special Issue entitled: Proteomics of non-model organisms.

BIOLOGICAL SIGNIFICANCE

This study reveals the mechanisms used by bats to regulate brain activities during torpor. These mechanisms include post-translational modifications and differential expression of proteins involved in mitochondrial electron transport, anaerobic glycolysis, TCA cycle efflux, cytoskeletal plasticity, amino acid metabolism, vesicle structure, antioxidation defense, stress response, and proteostasis. Our study provides insights in metabolic regulation of flying mammals during torpor and common strategies used by small hibernators in response to hibernation. This article is part of a Special Issue entitled: Proteomics of non-model organisms.

Circannual transitions in gene expression: lessons from seasonal adaptations

Circannual timing is important for the coordination of seasonal activities, particularly promoting the survival of individuals in adverse conditions through adaptive physiological and behavioral changes. This includes optimizing the survival of offspring by coordinating reproductive efforts at appropriate times. Thus, timing is very important for overall fitness. In this chapter, we provide several examples of circannually timed events, including mammalian hibernation, discussing the physiological changes that accompany these events, and some of the known genes and pathways underlying these changes. We then describe five candidate systems that are potentially involved in circannual timing. Finally, we discuss several recent advances in molecular biology and animal husbandry that have made the use of nonmodel organisms for research more feasible, which will hopefully promote and encourage further advancement in the knowledge of circannual timing.

Inhibition of NMDA-type glutamate receptors induces arousal from torpor in hibernating arctic ground squirrels (Urocitellus parryii)

Hibernation is an adaptation to overcome periods of resource limitation often associated with extreme climatic conditions. The hibernation season consists of prolonged bouts of torpor that are interrupted by brief interbout arousals. Physiological mechanisms regulating spontaneous arousals are poorly understood, but may be related to a need for gluconeogenesis or elimination of metabolic wastes. Glutamate is derived from glutamine through the glutamate-glutamine cycle and from glucose via the pyruvate carboxylase pathway when nitrogen balance favors formation of glutamine. This study tests the hypothesis that activation of NMDA-type glutamate receptors (NMDAR) maintains torpor in arctic ground squirrel (arctic ground squirrel (AGS); Urocitellus parryii). Administration of NMDAR antagonists MK-801 (5 mg/kg, i.p.) that crosses the blood-brain barrier and AP5 (5 mg/kg, i.p.) that does not cross the blood-brain barrier induced arousal in AGS. Central administration of MK-801 (0.2, 2, 20 or 200 μg; icv) to hibernating AGS failed to induce arousal. Results suggest that activation of NMDAR at a peripheral or circumventricular site is necessary to maintain prolonged torpor and that a decrease in glutamate at these sites may contribute to spontaneous arousal in AGS.

Neuroprotection: lessons from hibernators

Mammals that hibernate experience extreme metabolic states and body temperatures as they transition between euthermia, a state resembling typical warm blooded mammals, and prolonged torpor, a state of suspended animation where the brain receives as low as 10% of normal cerebral blood flow. Transitions into and out of torpor are more physiologically challenging than the extreme metabolic suppression and cold body temperatures of torpor per se. Mammals that hibernate show unprecedented capacities to tolerate cerebral ischemia, a decrease in blood flow to the brain caused by stroke, cardiac arrest or brain trauma. While cerebral ischemia often leads to death or disability in humans and most other mammals, hibernating mammals suffer no ill effects when blood flow to the brain is dramatically decreased during torpor or experimentally induced during euthermia. These animals, as adults, also display rapid and pronounced synaptic flexibility where synapses retract during torpor and rapidly re-emerge upon arousal. A variety of coordinated adaptations contribute to tolerance of cerebral ischemia in these animals. In this review we discuss adaptations in heterothermic mammals that may suggest novel therapeutic targets and strategies to protect the human brain against cerebral ischemic damage and neurodegenerative disease.

Metabolic cycles in a circannual hibernator

Hibernation as manifested in ground squirrels is arguably the most plastic and extreme of physiological phenotypes in mammals. Homeostasis is challenged by prolonged fasting accompanied by heterothermy, yet must be facilitated for survival. We performed LC and GC-MS metabolomic profiling of plasma samples taken reproducibly during seven natural stages of the hibernator's year, three in summer and four in winter (each n ≥ 5), employing a nontargeted approach to define the metabolite shifts associated with the phenotype. We quantified 231 named metabolites; 106 of these altered significantly, demarcating a cycle within a cycle where torpor-arousal cycles recur during the winter portion of the seasonal cycle. A number of robust hibernation biomarkers that alter with season and winter stage are identified, including specific free fatty acids, antioxidants, and previously unpublished modified amino acids that are likely to be associated with the fasting state. The major pattern in metabolite levels is one of either depletion or accrual during torpor, followed by reversal to an apparent homeostatic level by interbout arousal. This finding provides new data that strongly support the predictions of a long-standing hypothesis that periodic arousals are necessary to restore metabolic homeostasis.

Elevation of brain serotonin during dying

Death and dying are central events in the live of an organism, but neurobiological changes during this process are still rarely understood. Extracellular levels of serotonin, one of the phylogenetically oldest neurotransmitters, were measured continuously during dying. Serotonin levels increased threefold, while the EEG recorded simultaneously went down to a zero-line of no activity. This could be caused by the neuroprotective activity of brain serotonergic system, which subjectively makes dying easier due to the mood enhancing function of this neurotransmitter.

Small-volume d-β-hydroxybutyrate solution infusion increases survivability of lethal hemorrhagic shock in rats

A small-volume therapeutic approach based on the biochemistry of hibernating mammals was evaluated to test the hypothesis that passive hypothermia and systemic administration of d-β-hydroxybutyrate (d-BHB) plus melatonin will increase survival of animals subjected to hemorrhagic shock ([HS] 60% blood loss). Anesthetized Sprague-Dawley male rats (320 ± 23 g) underwent controlled loss of 60% blood volume. Rats were instrumented to measure mean arterial pressure, body temperature (Tb), and heart rate. A passive decrease in rat Tb in response to HS significantly increased survival over animals maintained at 37°C (n = 5-6). Infusion of 4 M d-BHB, at a volume of only 5.5% of the total blood removed, significantly prolonged survival to a mean of 3 h compared with 90 min using equal osmolar 4 M NaCl (n = 6). In experiments where the shed blood was returned after 1 h of 60% blood loss, 4% fluid replacement with 4 M d-BHB plus 43 mM melatonin significantly prolonged survival up to 10 days after blood return compared with 4 M NaCl plus 43 mM melatonin and other control solutions (n = 10). We conclude that a slow decrease in animal Tb resulting from 60% blood loss, combined with infusion of 4 M d-BHB plus 43 mM melatonin, was beneficial for long-term survival after return of shed blood. This HS therapy is designed as a portable low-volume solution for further evaluation in a large-animal model and is ultimately intended for use in HS patients by first responders.

Lungfish aestivating activities are locked in distinct encephalic γ-aminobutyric acid type A receptor α subunits

Ammonia in dipnoans plays a crucial role on neuronal homeostasis, especially for those brain areas that maintain torpor and awakening states in equilibrium. In the present study, specific α subunits of the major neuroreceptor inhibitory complex (GABA(A) R), which predominated during some phases of aestivation of the lungfish Protopterus annectens, turned out to be key adaptive factors of this species. From the isolation, for the first time, of the encoding sequence for GABA(A) R α₁, α₄ , and α₅ subunits in Protopterus annectens, qPCR and in situ hybridization levels of α₄ transcript in thalamic (P < 0.001) and mesencephalic (P < 0.01) areas proved to be significantly higher during long aestivating maintenance states. Very evident α₅ mRNA levels were detected in diencephalon during short inductive aestivating states, whereas an α₄ /α₁ turnover characterized the arousal state. Contextually, the recovery of physiological activities appeared to be tightly related to an evident up-regulation of α₁ transcripts in telencephalic and cerebellar sites. Surprisingly, TUNEL and amino cupric silver methods corroborated apoptotic and neurodegenerative cellular events, respectively, above all in telencephalon and cerebellum of lungfish exposed to long maintenance aestivating conditions. Overall, these results tend to underlie a novel GABAergic-related ON/OFF molecular switch operating during aestivation of the lungfish, which might have a bearing on sleeping disorders.

Region-specific changes in ascorbate concentration during rat brain development quantified by in vivo (1)H NMR spectroscopy

Ascorbate (vitamin C, Asc) was quantified in vivo using short-TE (1)H NMR spectra from a previously published study on regional and developmental changes in the neurochemical profile of the rat brain (Tkac I, Rao R, Georgieff MK, Gruetter R. Magn Reson Med. 2003; 50: 24-32). Asc concentration was quantified on postnatal days P7-P28 from three regions that are of interest in the study of neurocognitive development, i.e. the hippocampus, striatum and cerebral cortex. The previously measured (1)H NMR spectra were re-analyzed using LCModel with the Asc spectrum included in the basis set. The Asc concentration was consistently quantified from all 110 re-analyzed spectra with an estimated fitting error of 7% (i.e. the average Cramer-Rao lower bound). The sensitivity of Asc quantification was sufficiently high to detect regional and developmental changes in Asc concentration. The concentration of Asc was highest on P7, and decreased with age in all three brain regions (p < 0.001) in agreement with previous in vitro studies. At P10 and older postnatal ages, an inhomogeneous distribution of Asc among brain regions was detected. In addition to facilitating the quantification of this important antioxidant concentration, the inclusion of the Asc spectrum in the LCModel basis set improved the quantification accuracy of other brain metabolite concentrations in the neurochemical profile.

Global analysis of circulating metabolites in hibernating ground squirrels

Hibernation in mammals involves major alterations in nutrition and metabolism that would be expected to affect levels of circulating molecules. To gain insight into these changes we conducted a non-targeted LC-MS based metabolomic analysis of plasma using hibernating ground squirrels in late torpor (LT, T(b)~5 °C) or during an interbout arousal period (IBA, T(b)~5 °C) and non-hibernating squirrels in spring (T(b)~37 °C). Several metabolites varied and allowed differentiation between hibernators and spring squirrels, and between torpid and euthermic squirrels. Methionine and the short-chain carnitine esters of propionate and butyryate/isobutyrate were reduced in LT compared with the euthermic groups. Pantothenic acid and several lysophosphatidylcholines were elevated in LT relative to the euthermic groups, whereas lysophosphatidylethanolamines were elevated during IBA compared to LT and spring animals. Two regulatory lipids varied among the groups: sphingosine 1-phosphate was lower in LT vs. euthermic groups, whereas cholesterol sulfate was elevated in IBA compared to spring squirrels. Levels of long-chain fatty acids (LCFA) and total NEFA tended to be elevated in hibernators relative to spring squirrels. Three long-chain acylcarnitines were reduced in LT relative to IBA; free carnitine was also lower in LT vs. IBA. Our results identified several biochemical changes not previously observed in the seasonal hibernation cycle, including some that may provide insight into the metabolic limitations of mammalian torpor.

Glutamine is required for persistent epileptiform activity in the disinhibited neocortical brain slice

The neurotransmitter glutamate is recycled through an astrocytic-neuronal glutamate-glutamine cycle in which synaptic glutamate is taken up by astrocytes, metabolized to glutamine, and transferred to neurons for conversion back to glutamate and subsequent release. The extent to which neuronal glutamate release is dependent upon this pathway remains unclear. Here we provide electrophysiological and biochemical evidence that in acutely disinhibited rat neocortical slices, robust release of glutamate during sustained epileptiform activity requires that neurons be provided a continuous source of glutamine. We demonstrate that the uptake of glutamine into neurons for synthesis of glutamate destined for synaptic release is not strongly dependent on the system A transporters, but requires another unidentified glutamine transporter or transporters. Finally, we find that the attenuation of network activity through inhibition of neuronal glutamine transport is associated with reduced frequency and amplitude of spontaneous events detected at the single-cell level. These results indicate that availability of glutamine influences neuronal release of glutamate during periods of intense network activity.

Deep thiopental anesthesia alters steady-state glucose homeostasis but not the neurochemical profile of rat cortex

Barbiturates are regularly used as an anesthetic for animal experimentation and clinical procedures and are frequently provided with solubilizing compounds, such as ethanol and propylene glycol, which have been reported to affect brain function and, in the case of (1)H NMR experiments, originate undesired resonances in spectra affecting the quantification. As an alternative, thiopental can be administrated without any solubilizing agents. The aim of the study was to investigate the effect of deep thiopental anesthesia on the neurochemical profile consisting of 19 metabolites and on glucose transport kinetics in vivo in rat cortex compared with alpha-chloralose using localized (1)H NMR spectroscopy. Thiopental was devoid of effects on the neurochemical profile, except for the elevated glucose at a given plasma glucose level resulting from thiopental-induced depression of glucose consumption at isoelectrical condition. Over the entire range of plasma glucose levels, steady-state glucose concentrations were increased on average by 48% +/- 8%, implying that an effect of deep thiopental anesthesia on the transport rate relative to cerebral glucose consumption ratio was increased by 47% +/- 8% compared with light alpha-chloralose-anesthetized rats. We conclude that the thiopental-induced isoelectrical condition in rat cortex significantly affected glucose contents by depressing brain metabolism, which remained substantial at isoelectricity.

In vivo proton MRS to quantify anesthetic effects of pentobarbital on cerebral metabolism and brain activity in rat

To quantitatively investigate the effects of pentobarbital anesthesia on brain activity, brain metabolite concentrations and cerebral metabolic rate of glucose, in vivo proton MR spectra, and electroencephalography were measured in the rat brain with various doses of pentobarbital. The results show that (1) the resonances attributed to propylene glycol, a solvent in pentobarbital injection solution, can be robustly detected and quantified in the brain; (2) the concentration of most brain metabolites remained constant under the isoelectric state (silent electroencephalography) with a high dose of pentobarbital compared to mild isoflurane anesthesia condition, except for a reduction of 61% in the brain glucose level, which was associated with a 37% decrease in cerebral metabolic rate of glucose, suggesting a significant amount of "housekeeping" energy for maintaining brain cellular integrity under the isoelectric state; and (3) electroencephalography and cerebral metabolic activities were tightly coupled to the pentobarbital anesthesia depth and they can be indirectly quantified by the propylene glycol resonance signal at 1.13 ppm. This study indicates that in vivo proton MR spectroscopy can be used to measure changes in cerebral metabolite concentrations and cerebral metabolic rate of glucose under varied pentobarbital anesthesia states; moreover, the propylene glycol signal provides a sensitive biomarker for quantitatively monitoring these changes and anesthesia depth noninvasively.

Hypothesis: Muscular glutamine deficiency in sepsis--a necessary step for a hibernation-like state?

Glutamine depletion in skeletal muscle of severely ill patients is an outstanding metabolic marker related to acute skeletal muscle wasting. To date it is unclear why intracellular glutamine concentrations are lowered in skeletal muscle to such an extent when simultaneously muscular glutamine synthesis and release are stimulated. This essay introduces a hypothesis that intracellular glutamine deficiency is part of a metabolic program maintaining cell integrity. This program seems to resemble short-term hibernation, which can be observed in various mammalian species during periods of starvation. Interestingly, even in septic patients who do not survive, there are no signs of apoptosis or necrosis in affected organs. Therefore, in severe illness evolutionarily conserved energy saving programs may be switched on for protecting the organs in a mode reminiscent of hibernation. This would explain the low energy expenditure as described for septic patients and the limited success of nutrition in avoiding skeletal muscle atrophy in sepsis.

Seasonal protein changes support rapid energy production in hibernator brainstem

During the torpor phase of mammalian hibernation when core body temperature is near 4 degrees C, the autonomic system continues to maintain respiration, blood pressure and heartbeat despite drastic reductions in brain activity. In addition, the hibernator's neuronal tissues enter into a protected state in which the potential for ischemia-reperfusion injury is markedly minimized. Evolutionary adaptations for continued function and neuroprotection throughout cycles of torpor and euthermia in winter are predicted to manifest themselves partly in changes in the brainstem proteome. Here, we compare the soluble brainstem protein complement from six summer active ground squirrels and six in the early torpor (ET) phase of hibernation. Thirteen percent of the approximately 1,500 quantifiable 2D gel spots alter significantly from summer to ET; the proteins identified in these differing spots are known to play roles in energy homeostasis via the tricarboxylic acid cycle (8 proteins), cytoarchitecture and cell motility (14 proteins), anabolic protein processes (13 proteins), redox control (11 proteins) and numerous other categories including protein catabolism, oxidative phosphorylation, signal transduction, glycolysis, intracellular protein trafficking and antiapoptotic function. These protein changes represent, at least in part, the molecular bases for restructuring of cells in the brainstem, a shift away from glucose as the primary fuel source for brain in the winter, and the generation of a streamlined mechanism capable of efficient and rapid energy production and utilization during the torpor and arousal cycles of hibernation.

Torpor induction in mammals: recent discoveries fueling new ideas

When faced with a harsh climate or inadequate food, some mammals enter a state of suspended animation known as torpor. A major goal of torpor research is to determine mechanisms that integrate environmental cues, gene expression and metabolism to produce periods of torpor lasting from hours to weeks. Recent discoveries spanning the Metazoa suggest that sirtuins, the mammalian circadian clock, fibroblast growth factor 21 (FGF21) and lipids are involved in torpor induction. For example, sirtuins link cellular energy status to the mammalian circadian clock, oxidative stress and metabolic fuel selection. In this review, we discuss how these recent discoveries form a new hypothesis linking changes in the physical environment with changes in the expression of genes that regulate torpor induction.

In vivo neurochemical profiling of rat brain by 1H-[13C] NMR spectroscopy: cerebral energetics and glutamatergic/GABAergic neurotransmission

The quantification of excitatory and inhibitory neurotransmission and the associated energy metabolism is crucial for a proper understanding of brain function. Although the detection of glutamatergic neurotransmission in vivo by (13)C NMR spectroscopy is now relatively routine, the detection of GABAergic neurotransmission in vivo has remained elusive because of the low GABA concentration and spectral overlap. Using (1)H-[(13)C] NMR spectroscopy at high magnetic field in combination with robust spectral modeling and the use of different substrates, [U-(13)C(6)]-glucose and [2-(13)C]-acetate, it is shown that GABAergic, as well as glutamatergic neurotransmitter fluxes can be detected non-invasively in rat brain in vivo.

Hibernation induces pentobarbital insensitivity in medulla but not cortex

The 13-lined ground squirrel (Ictidomys tridecemlineatus), a hibernating species, is a natural model of physiological adoption to an extreme environment. During torpor, body temperature drops to 0-4 degrees C, and the cortex is electrically silent, yet the brain stem continues to regulate cardiorespiratory function. The mechanisms underlying selective inhibition in the brain during torpor are not known. To test whether altered GABAergic function is involved in regional and seasonal differences in neuronal activity, cortical and medullary slices from summer-active (SA) and interbout aroused (IBA) squirrels were placed in a standard in vitro recording chamber. Silicon multichannel electrodes were placed in cortex, ventral respiratory column (VRC), and nucleus tractus solitarius (NTS) to record spontaneous neuronal activity. In slices from IBA squirrels, bath-applied pentobarbital sodium (300 microM) nearly abolished cortical neuronal activity, but VRC and NTS neuronal activity was unaltered. In contrast, pentobarbital sodium (300 microM) nearly abolished all spontaneous cortical, VRC, and NTS neuronal activity in slices from SA squirrels. Muscimol (20 microM; GABA(A) receptor agonist) abolished all neuronal activity in cortical and medullary slices from both IBA and SA squirrels, thereby demonstrating the presence of functional GABA(A) receptors. Pretreatment of cortical slices from IBA squirrels with bicuculline (100 microM; GABA(A) receptor antagonist) blocked pentobarbital-dependent inhibition of spontaneous neuronal activity. We hypothesize that GABA(A) receptors undergo a seasonal modification in subunit composition, such that cardiorespiratory neurons are uniquely unaffected by surges of an endogenous positive allosteric modulator.

Analysis of the hibernation cycle using LC-MS-based metabolomics in ground squirrel liver

A hallmark of hibernation in mammals is metabolic flexibility, which is typified by reversible bouts of metabolic depression (torpor) and the seasonal shift from predominantly carbohydrate to lipid metabolism from summer to winter. To provide new insight into the control and consequences of hibernation, we used LC/MS-based metabolomics to measure differences in small molecules in ground squirrel liver in five activity states: summer, entering torpor, late torpor, arousing from torpor, and interbout arousal. There were significant alterations both seasonally and within torpor-arousal cycles in enzyme cofactor metabolism, amino acid catabolism, and purine and pyrimidine metabolism, with observed metabolites reduced during torpor and increased upon arousal. Multiple lipids also changed, including 1-oleoyllysophosphatidylcholine, cholesterol sulfate, and sphingosine, which tended to be lowest during torpor, and hexadecanedioic acid, which accumulated during a torpor bout. The results reveal the dramatic alterations that occur in several classes of metabolites, highlighting the value of metabolomic analyses in deciphering the hibernation phenotype.

Adaptive mechanisms regulate preferred utilization of ketones in the heart and brain of a hibernating mammal during arousal from torpor

Hibernating mammals use reduced metabolism, hypothermia, and stored fat to survive up to 5 or 6 mo without feeding. We found serum levels of the fat-derived ketone, D-beta-hydroxybutyrate (BHB), are highest during deep torpor and exist in a reciprocal relationship with glucose throughout the hibernation season in the thirteen-lined ground squirrel (Spermophilus tridecemlineatus). Ketone transporter monocarboxylic acid transporter 1 (MCT1) is upregulated at the blood-brain barrier, as animals enter hibernation. Uptake and metabolism of 13C-labeled BHB and glucose were measured by high-resolution NMR in both brain and heart at several different body temperatures ranging from 7 to 38 degrees C. We show that BHB and glucose enter the heart and brain under conditions of depressed body temperature and heart rate but that their utilization as a fuel is highly selective. During arousal from torpor, glucose enters the brain over a wide range of body temperatures, but metabolism is minimal, as only low levels of labeled metabolites are detected. This is in contrast to BHB, which not only enters the brain but is also metabolized via the tricarboxylic acid (TCA) cycle. A similar situation is seen in the heart as both glucose and BHB are transported into the organ, but only 13C from BHB enters the TCA cycle. This finding suggests that fuel selection is controlled at the level of individual metabolic pathways and that seasonally induced adaptive mechanisms give rise to the strategic utilization of BHB during hibernation.

Mammalian cerebral metabolism and amino acid neurotransmission during hibernation

This report demonstrates that during the torpor phase of hibernation, hamsters utilize (14)C and (13)C glucose in torpor-specific brain metabolic pathways. Microdialysis of (14)C glucose into the striatum rapidly induced a steady state labeling of extracellular fluid (ECF) lactate and labeling of tissue GABA, glutamate, glutamine, and alanine in ipsilateral and contralateral striata. The same tissue metabolites were labeled in cortex, hypothalamus, and brainstem after microdialysis of (14)C lactate into the lateral ventricle. Serine, aspartate, glycine, taurine, tyrosine, and methionine were not synthesized from glucose or lactate during torpor. ECF levels of amino and organic acids were low and unchanging during torpor and increased late during arousal to cenothermia. Labeled intracellular (14)C GABA and glutamate were not communicated to the striatal ECF or ventricular space during torpor. (13)C NMR demonstrated rapid formation of lactate and functional tricarboxylic acid cycles in GABAergic and glutamatergic neurons, and enrichment of glutamine and alanine after i.v. (13)C glucose. Large changes in tissue levels of amino acids occur prior to or during entrance into torpor but not during torpor. It is proposed that cerebral intracellular dehydration, the enlargement of ECF and the biochemistries associated with brain water homeostasis may have a role in regulating hibernation.

Quantitative analysis of liver metabolites in three stages of the circannual hibernation cycle in 13-lined ground squirrels by NMR

Thirteen-lined ground squirrels and other circannual hibernators undergo profound physiological changes on an annual basis, transitioning from summer homeothermy [body temperature (Tb) ∼37°C] to winter heterothermy (Tbcycling between 0°C and 37°C). We hypothesize that these physiological changes are reflected in biochemical changes that provide mechanistic insights into, and biomarkers for, hibernation states. Here we report the results of an NMR-based metabolomics analysis of liver extracts from ground squirrels in three distinct physiological states of circannual hibernation: summer active (SA), late torpor (LT), and reentering torpor (Ent) after one of the euthermic arousals. Of the 43 identified and quantified metabolites, 36 differed among these three states and fell into two patterns of variation: 1) SA differed from both of the two winter states; or 2) the two winter states differed from each other, but one of the two was not different from SA. Concentrations of hepatic glucose, lactate, alanine, succinate, β-hydroxybutyrate, glutamine, and betaine were identified as robust hepatic biomarkers that together distinguish among animals in these three states of the circannual hibernation rhythm. These data are consistent with a proposed two-switch model of hibernation, in which setting the summer-winter switch to winter enables expression of a distinct torpor-arousal switch. The summer-winter switch is characterized by the metabolites associated with the well-known switch from carbohydrate to lipid fuel utilization during hibernation. The torpor-arousal switch is characterized by the accumulation of metabolites of nitrogen (glutamine) and phospholipid (betaine) catabolism in LT with the capacity to act as protective osmolytes.