Anoxia tolerant animals from a neurobiological perspective

The dynamic transcriptomic response of the goldfish brain under chronic hypoxia

Oxygen is essential to fuel aerobic metabolism. Some species evolved mechanisms to tolerate periods of severe hypoxia and even anoxia in their environment. Among them, goldfish (Carassius auratus) are unique, in that they do not enter a comatose state under severely hypoxic conditions. There is thus significant interest in the field of comparative physiology to uncover the mechanistic basis underlying hypoxia tolerance in goldfish, with a particular focus on the brain. Taking advantage of the recently published and annotated goldfish genome, we profile the transcriptomic response of the goldfish brain under normoxic (21 kPa oxygen saturation) and, following gradual reduction, constant hypoxic conditions after 1 and 4 weeks (2.1 kPa oxygen saturation). In addition to analyzing differentially expressed protein-coding genes and enriched pathways, we also profile differentially expressed microRNAs (miRs). Using in silico approaches, we identify possible miR-mRNA relationships. Differentially expressed transcripts compared to normoxia were either common to both timepoints of hypoxia exposure (n = 174 mRNAs; n = 6 miRs), or exclusive to 1-week (n = 441 mRNAs; n = 23 miRs) or 4-week hypoxia exposure (n = 491 mRNAs; n = 34 miRs). Under chronic hypoxia, an increasing number of transcripts, including those of paralogous genes, was downregulated over time, suggesting a decrease in transcription. GO-terms related to the vascular system, oxidative stress, stress signalling, oxidoreductase activity, nucleotide- and intermediary metabolism, and mRNA posttranscriptional regulation were found to be enriched under chronic hypoxia. Known 'hypoxamiRs', such as miR-210-3p/5p, and miRs such as miR-29b-3p likely contribute to posttranscriptional regulation of these pathways under chronic hypoxia in the goldfish brain.

The effect of cytochrome c on Na,K-ATPase

Na,K-ATPase is a crucial enzyme responsible for maintaining Na+, K+-gradients across the cell membrane, which is essential for numerous physiological processes within various organs and tissues. Due to its significance in cellular physiology, inhibiting Na,K-ATPase can have profound physiological consequences. This characteristic makes it a target for various pharmacological applications, and drugs that modulate the pump's activity are thus used in the treatment of several medical conditions. Cytochrome c (Cytc) is a protein with dual functions in the cell. In the mitochondria, it is essential for ATP synthesis and energy production. However, in response to apoptotic stimuli, it is released into the cytosol, where it triggers programmed cell death through the intrinsic apoptosis pathway. Aside from its role in canonical intrinsic apoptosis, Cytc also plays additional roles. For instance, Cytc participates in certain non-apoptotic functions -those which are less well-understood in comparison to its role in apoptosis. Within this in vitro study, we have shown the impact of Cytc on Na,K-ATPase for the first time. Cytc has a biphasic action on Na,K-ATPase, with activation at low concentrations (0.06 ng/ml; 6 ng/ml) and inhibition at high concentration (120 ng/ml). Cytc moreover displays isoform/subunit specificity and regulates the Na+ form of the enzyme, while having no effect on the activity or kinetic parameters of the K+-dependent form of the enzyme. Changing the affinity of p-chloromercuribenzoic acid (PCMB) by Cytc is therefore both a required and sufficient condition for confirming that PCMB and Cytc share the same target, namely the thiol groups of cysteine in Na,K-ATPase.

Surviving without oxygen involves major tissue specific changes in the proteome of crucian carp (Carassius carassius)

The crucian carp (Carassius carassius) can survive complete oxygen depletion (anoxia) for several months at low temperatures, making it an excellent model for studying molecular adaptations to anoxia. Still, little is known about how its global proteome responds to anoxia and reoxygenation. By applying mass spectrometry-based proteome analyses on brain, heart and liver tissue from crucian carp exposed to normoxia, five days anoxia, and reoxygenation, we found major changes in particularly cardiac and hepatic protein levels in response to anoxia and reoxygenation. These included tissue-specific differences in mitochondrial proteins involved in aerobic respiration and mitochondrial membrane integrity. Enzymes in the electron transport system (ETS) decreased in heart and increased massively in liver during anoxia and reoxygenation but did not change in the brain. Importantly, the data support a special role for the liver in succinate handling upon reoxygenation, as suggested by a drastic increase of components of the ETS and uncoupling protein 2, which could allow for succinate metabolism without excessive formation of reactive oxygen species (ROS). Also during reoxygenation, the levels of proteins involved in the cristae junction organization of the mitochondria changed in the heart, possibly functioning to suppress ROS formation. Furthermore, proteins involved in immune (complement) system activation changed in the anoxic heart compared to normoxic controls. The results emphasize that responses to anoxia are highly tissue-specific and related to organ function.

Effect of acute hypoxia on the brain energy metabolism of the scorpionfish Scorpaena porcus Linnaeus, 1758: the pattern of oxidoreductase activity and adenylate system

The activity of oxidoreductases, malate dehydrogenase and lactate dehydrogenase (MDH, 1.1.1.37; LDH, 1.1.1.27), as well as parameters of adenylate system-[ATP], [ADP], [AMP], total adenylate pool (AP), and adenylate energy charge (AEC) in medulla oblongata (MB) and forebrain, midbrain, and diencephalon (FDMB)-were studied in the scorpionfish under acute hypoxia (0.9-1.2 mg O₂·L^-1, 90 min). A higher MDH activity level was observed in MB and FDMB, as compared to LDH (p < 0.05). At the same time, MB showed a higher adenylate content and increased AP (p < 0.05). AEC did not exceed ~ 0.7 (vs. the maximum of this index ~ 0.9-1.0) in the brain of the scorpionfish indicating adaptation of the tissue energy status to hypoxia. A rapid decrease in MDH activity (p < 0.05) was observed in MB under acute hypoxia. These changes were accompanied by insignificant LDH activation. A pronounced LDH activation (p < 0.05), a decrease in MDH activity, and the highest AP raise (p < 0.05) were observed in FDMB, suggesting activation of glycolysis and simultaneous decrease in the rate of ATP consumption. MB and FDMB demonstrated the ability to a relative retention of AEC during hypoxia. The unidirectional metabolic adaptation was based on the intensification of glycolysis, a decrease of ATP consumption, and a subsequent increase in adenylate concentration that allowed the scorpionfish brain structures to maintain the energy status under acute hypoxia.

A Step further-The Role of Trigeminocardiac Reflex in Therapeutic Implications: Hypothesis, Evidence, and Experimental Models

The trigeminocardiac reflex (TCR) is a well-recognized brainstem reflex that represents a unique interaction between the brain and the heart through the Vth and Xth cranial nerves and brainstem nuclei. The TCR has mainly been reported as an intraoperative phenomenon causing cardiovascular changes during skull-base surgeries. However, it is now appreciated that the TCR is implicated during non-neurosurgical procedures and in nonsurgical conditions, and its complex reflex pathways have been explored as potential therapeutic options in various neurological and cardiovascular diseases. This narrative review presents an in-depth overview of hypothetical and experimental models of the TCR phenomenon in relation to the Vth and Xth cranial nerves. In addition, primitive interactions between these 2 cranial nerves and their significance are highlighted. Finally, therapeutic models of the complex interactions of the TCR and areas for further research will be considered.

[Surviving the Lack: Natural Adaptations in Newborns]

Newborns are equipped with a number of natural adaptation mechanisms preventing them from impaired energy supply, despite their elevated (size-related) metabolic rate. These include the diving response known from aquatic mammals, which - being composed of apnea, bradycardia, and vasoconstriction - ensures an economical use of O₂ reserves and results in a subsequent influx of lactate out of peripheral tissues. From a metabolic point of view, mammalian fetuses behave "like an organ of the mother" and thus exhibit a hibernation-like deviation from the overall metabolic size relationship that adapts them to the limited intrauterine O₂/substrate availability. In case of lacking supply, they can reduce their energy demands even further by foregoing growth, with the placenta acting as a gatekeeper. Postnatal hypoxia does not only result in the suppression of non-shivering thermogenesis, but also in a hypoxic hypometabolism that otherwise has only been known from poikilothermic animals. After prolonged apnea, gasps do occur that maintain a rudimentary heart action through short elevations in pO₂ (autoresuscitation). Overall, these mechanisms postpone a critical O₂ deficit and thereby provide a "resistance" rather than a "tolerance" to hypoxia. As they are based on an (active) reduction in energy demand, they are not easy to distinguish from the (passive) breakdown of metabolism resulting from hypoxia.

Antioxidant capacity and anoxia-tolerance in Austrofundulus limnaeus embryos

Embryos of Austrofundulus limnaeus can tolerate extreme environmental stresses by entering into a state of metabolic and developmental arrest known as diapause. Oxidative stress is ubiquitous in aerobic organisms and the unique biology and ecology of A. limnaeus likely results in frequent and repeated exposures to oxidative stress during development. Antioxidant capacity of A. limnaeus was explored during development by measuring antioxidant capacity due to small molecules and several enzymatic antioxidant systems. Diapause II embryos can survive for several days in 1% hydrogen peroxide without indications of negative effects. Surprisingly, both small and large molecule antioxidant systems are highest during early development and may be due to maternal provisioning. Antioxidant capacity is largely invested in small molecules during early development and in enzymatic systems during late development. The switch in antioxidant mechanisms and decline in small molecule antioxidants during development correlates with the loss of extreme anoxia tolerance.

Resveratrol Preconditioning Induces Genomic and Metabolic Adaptations within the Long-Term Window of Cerebral Ischemic Tolerance Leading to Bioenergetic Efficiency

Neuroprotective agents administered post-cerebral ischemia have failed so far in the clinic to promote significant recovery. Thus, numerous efforts were redirected toward prophylactic approaches such as preconditioning as an alternative therapeutic strategy. Our laboratory has revealed a novel long-term window of cerebral ischemic tolerance mediated by resveratrol preconditioning (RPC) that lasts for 2 weeks in mice. To identify its mediators, we conducted an RNA-seq experiment on the cortex of mice 2 weeks post-RPC, which revealed 136 differentially expressed genes. The majority of genes (116/136) were downregulated upon RPC and clustered into biological processes involved in transcription, synaptic signaling, and neurotransmission. The downregulation in these processes was reminiscent of metabolic depression, an adaptation used by hibernating animals to survive severe ischemic states by downregulating energy-consuming pathways. Thus, to assess metabolism, we used a neuronal-astrocytic co-culture model and measured the cellular respiration rate at the long-term window post-RPC. Remarkably, we observed an increase in glycolysis and mitochondrial respiration efficiency upon RPC. We also observed an increase in the expression of genes involved in pyruvate uptake, TCA cycle, and oxidative phosphorylation, all of which indicated an increased reliance on energy-producing pathways. We then revealed that these nuclear and mitochondrial adaptations, which reduce the reliance on energy-consuming pathways and increase the reliance on energy-producing pathways, are epigenetically coupled through acetyl-CoA metabolism and ultimately increase baseline ATP levels. This increase in ATP would then allow the brain, a highly metabolic organ, to endure prolonged durations of energy deprivation encountered during cerebral ischemia.

Small Non-coding RNA Expression and Vertebrate Anoxia Tolerance

Background: Extreme anoxia tolerance requires a metabolic depression whose modulation could involve small non-coding RNAs (small ncRNAs), which are specific, rapid, and reversible regulators of gene expression. A previous study of small ncRNA expression in embryos of the annual killifish Austrofundulus limnaeus, the most anoxia-tolerant vertebrate known, revealed a specific expression pattern of small ncRNAs that could play important roles in anoxia tolerance. Here, we conduct a comparative study on the presence and expression of small ncRNAs in the most anoxia-tolerant representatives of several major vertebrate lineages, to investigate the evolution of and mechanisms supporting extreme anoxia tolerance. The epaulette shark (Hemiscyllium ocellatum), crucian carp (Carassius carassius), western painted turtle (Chrysemys picta bellii), and leopard frog (Rana pipiens) were exposed to anoxia and recovery, and small ncRNAs were sequenced from the brain (one of the most anoxia-sensitive tissues) prior to, during, and following exposure to anoxia. Results: Small ncRNA profiles were broadly conserved among species under normoxic conditions, and these expression patterns were largely conserved during exposure to anoxia. In contrast, differentially expressed genes are mostly unique to each species, suggesting that each species may have evolved distinct small ncRNA expression patterns in response to anoxia. Mitochondria-derived small ncRNAs (mitosRNAs) which have a robust response to anoxia in A. limnaeus embryos, were identified in the other anoxia tolerant vertebrates here but did not display a similarly robust response to anoxia. Conclusion: These findings support an overall stabilization of the small ncRNA transcriptome during exposure to anoxic insults, but also suggest that multiple small ncRNA expression pathways may support anoxia tolerance, as no conserved small ncRNA response was identified among the anoxia-tolerant vertebrates studied. This may reflect divergent strategies to achieve the same endpoint: anoxia tolerance. However, it may also indicate that there are multiple cellular pathways that can trigger the same cellular and physiological survival processes, including hypometabolism.

Translational regulation in the anoxic turtle, Trachemys scripta elegans

The red-eared slider turtle (Trachemys scripta elegans), has developed remarkable adaptive mechanisms for coping with decreased oxygen availability during winter when lakes and ponds become covered with ice. Strategies for enduring anoxia tolerance include an increase in fermentable fuel reserves to support anaerobic glycolysis, the buffering of end products to minimize acidosis, altered expression in crucial survival genes, and strong metabolic rate suppression to minimize ATP-expensive metabolic processes such as protein synthesis. The mammalian target of rapamycin (mTOR) is at the center of the insulin-signaling pathway that regulates protein translation. The present study analyzed the responses of the mTOR signaling pathway to 5 (5H) or 20 h (20H) of anoxic submergence in liver and skeletal muscle of T. scripta elegans with a particular focus on regulatory changes in the phosphorylation states of targets. The data showed that phosphorylation of multiple mTOR targets was suppressed in skeletal muscle, but activated in the liver. Phosphorylated mTOR^Ser2448 showed no change in skeletal muscle but had increased by approximately 4.5-fold in the liver after 20H of anoxia. The phosphorylation states of upstream positive regulators of mTOR (p-PDK-1^Ser241, p-AKT^Ser473, and protein levels of GβL), the relative levels of dephosphorylated active PTEN, as well as phosphorylation state of negative regulators (TSC2^Thr1462, p-PRAS40^Thr246) were generally found to be differentially regulated in skeletal muscle and in liver. Downstream targets of mTOR (p-p70 S6K^Thr389, p-S6^Ser235, PABP, p-4E-BP1^Thr37/46, and p-eIF4E^Ser209) were generally unchanged in skeletal muscle but upregulated in most targets in liver. These findings indicate that protein synthesis is enhanced in the liver and suggests an increase in the synthesis of crucial proteins required for anoxic survival.

Diel cyclic hypoxia alters plasma lipid dynamics and impairs reproduction in goldfish (Carassius auratus)

Diel cyclic hypoxia occurs with varying frequency and duration in freshwater habitats, yet little is known about its effects on reproduction of freshwater fishes. The present study shows that long-term exposure of goldfish (Carassius auratus) to cyclic hypoxia (0.8 ± 0.2 mg/l dissolved oxygen) for 9 h or more, per day, altered plasma lipid and sex steroid profiles, which in turn directly or indirectly suppressed ovarian growth and viable spermatozoa production. Hypoxia decreased total cholesterol and high density lipoprotein (HDL p < 0.05) and elevated triglycerides (TG; p < 0.05) in both sexes. Plasma steroid concentrations particularly of 17α-hydroxyprogesterone (17-HP), estradiol (E2), testosterone (T) in females, and T and 11-ketotestosterone (11-KT) in males were attenuated under diel hypoxic conditions. Intriguingly, both diel and continuous hypoxia elevated plasma E2 and vitellogenin levels in males. However, neither lipid nor steroid profiles recorded any variation in a dose-dependent manner in response to diel hypoxia. The reduced GSI, decreased number of tertiary oocytes, and motile spermatozoa in hypoxic fish clearly indicate suppression of gametogenesis. Thereby, prolonged diel cyclic hypoxia may affect valuable fishery resources and fish population structure by impairing reproductive performances and inducing estrogenic effects in males.

Cysteine residues 244 and 458-459 within the catalytic subunit of Na,K-ATPase control the enzyme's hydrolytic and signaling function under hypoxic conditions

Our previous findings suggested that reversible thiol modifications of cysteine residues within the actuator (AD) and nucleotide binding domain (NBD) of the Na,K-ATPase may represent a powerful regulatory mechanism conveying redox- and oxygen-sensitivity of this multifunctional enzyme. S-glutathionylation of Cys244 in the AD and Cys 454-458-459 in the NBD inhibited the enzyme and protected cysteines' thiol groups from irreversible oxidation under hypoxic conditions. In this study mutagenesis approach was used to assess the role these cysteines play in regulation of the Na,K-ATPase hydrolytic and signaling functions. Several constructs of mouse α1 subunit of the Na,K-ATPase were produced in which Cys244, Cys 454-458-459 or Cys 244-454-458-459 were replaced by alanine. These constructs were expressed in human HEK293 cells. Non-transfected cells and those expressing murine α1 subunit were exposed to hypoxia or treated with oxidized glutathione (GSSG). Both conditions induced inhibition of the wild type Na,K-ATPase. Enzymes containing mutated mouse α1 lacking Cys244 or all four cysteines (Cys 244-454-458-459) were insensitive to hypoxia. Inhibitory effect of GSSG was observed for wild type murine Na,K-ATPase, but was less pronounced in Cys454-458-459Ala mutant and completely absent in the Cys244Ala and Cys 244-454-458-459Ala mutants. In cells, expressing wild type enzyme, ouabain induced activation of Src and Erk kinases under normoxic conditions, whereas under hypoxic conditions this effect was inversed. Cys454-458-459Ala substitution abolished Src kinase activation in response to ouabain treatment, uncoupled Src from Erk signaling, and interfered with O2-sensitivity of Na,K-ATPase signaling function. Moreover, modeling predicted that S-glutathionylation of Cys 458 and 459 should prevent inhibitory binding of Src to NBD. Our data indicate for the first time that cysteine residues within the AD and NBD influence hydrolytic as well as receptor function of the Na,K-ATPase and alter responses of the enzyme to hypoxia or upon treatment with cardiotonic steroids.

"Oxygen Sensing" by Na,K-ATPase: These Miraculous Thiols

Control over the Na,K-ATPase function plays a central role in adaptation of the organisms to hypoxic and anoxic conditions. As the enzyme itself does not possess O2 binding sites its "oxygen-sensitivity" is mediated by a variety of redox-sensitive modifications including S-glutathionylation, S-nitrosylation, and redox-sensitive phosphorylation. This is an overview of the current knowledge on the plethora of molecular mechanisms tuning the activity of the ATP-consuming Na,K-ATPase to the cellular metabolic activity. Recent findings suggest that oxygen-derived free radicals and H2O2, NO, and oxidized glutathione are the signaling messengers that make the Na,K-ATPase "oxygen-sensitive." This very ancient signaling pathway targeting thiols of all three subunits of the Na,K-ATPase as well as redox-sensitive kinases sustains the enzyme activity at the "optimal" level avoiding terminal ATP depletion and maintaining the transmembrane ion gradients in cells of anoxia-tolerant species. We acknowledge the complexity of the underlying processes as we characterize the sources of reactive oxygen and nitrogen species production in hypoxic cells, and identify their targets, the reactive thiol groups which, upon modification, impact the enzyme activity. Structured accordingly, this review presents a summary on (i) the sources of free radical production in hypoxic cells, (ii) localization of regulatory thiols within the Na,K-ATPase and the role reversible thiol modifications play in responses of the enzyme to a variety of stimuli (hypoxia, receptors' activation) (iii) redox-sensitive regulatory phosphorylation, and (iv) the role of fine modulation of the Na,K-ATPase function in survival success under hypoxic conditions. The co-authors attempted to cover all the contradictions and standing hypotheses in the field and propose the possible future developments in this dynamic area of research, the importance of which is hard to overestimate. Better understanding of the processes underlying successful adaptation strategies will make it possible to harness them and use for treatment of patients with stroke and myocardial infarction, sleep apnoea and high altitude pulmonary oedema, and those undergoing surgical interventions associated with the interruption of blood perfusion.

Fisheries conservation on the high seas: linking conservation physiology and fisheries ecology for the management of large pelagic fishes

Populations of tunas, billfishes and pelagic sharks are fished at or over capacity in many regions of the world. They are captured by directed commercial and recreational fisheries (the latter of which often promote catch and release) or as incidental catch or bycatch in commercial fisheries. Population assessments of pelagic fishes typically incorporate catch-per-unit-effort time-series data from commercial and recreational fisheries; however, there have been notable changes in target species, areas fished and depth-specific gear deployments over the years that may have affected catchability. Some regional fisheries management organizations take into account the effects of time- and area-specific changes in the behaviours of fish and fishers, as well as fishing gear, to standardize catch-per-unit-effort indices and refine population estimates. However, estimates of changes in stock size over time may be very sensitive to underlying assumptions of the effects of oceanographic conditions and prey distribution on the horizontal and vertical movement patterns and distribution of pelagic fishes. Effective management and successful conservation of pelagic fishes requires a mechanistic understanding of their physiological and behavioural responses to environmental variability, potential for interaction with commercial and recreational fishing gear, and the capture process. The interdisciplinary field of conservation physiology can provide insights into pelagic fish demography and ecology (including environmental relationships and interspecific interactions) by uniting the complementary expertise and skills of fish physiologists and fisheries scientists. The iterative testing by one discipline of hypotheses generated by the other can span the fundamental-applied science continuum, leading to the development of robust insights supporting informed management. The resulting species-specific understanding of physiological abilities and tolerances can help to improve stock assessments, develop effective bycatch-reduction strategies, predict rates of post-release mortality, and forecast the population effects of environmental change. In this synthesis, we review several examples of these interdisciplinary collaborations that currently benefit pelagic fisheries management.

Control of carbohydrate metabolism in an anoxia-tolerant nervous system

Anoxia-tolerant animal models are crucial to understand protective mechanisms during low oxygen excursions. As glycogen is the main fermentable fuel supporting energy production during oxygen tension reduction, understanding glycogen metabolism can provide important insights about processes involved in anoxia survival. In this report we studied carbohydrate metabolism regulation in the central nervous system (CNS) of an anoxia-tolerant land snail during experimental anoxia exposure and subsequent reoxygenation. Glucose uptake, glycogen synthesis from glucose, and the key enzymes of glycogen metabolism, glycogen synthase (GS) and glycogen phosphorylase (GP), were analyzed. When exposed to anoxia, the nervous ganglia of the snail achieved a sustained glucose uptake and glycogen synthesis levels, which seems important to maintain neural homeostasis. However, the activities of GS and GP were reduced, indicating a possible metabolic depression in the CNS. During the aerobic recovery period, the enzyme activities returned to basal values. The possible strategies used by Megalobulimus abbreviatus CNS to survive anoxia are discussed.

Oxygen-conserving reflexes of the brain: the current molecular knowledge

The trigemino-cardiac reflex (TCR) may be classified as a sub-phenomenon in the group of the so-called 'oxygen-conserving reflexes'. Within seconds after the initiation of such a reflex, there is neither a powerful and differentiated activation of the sympathetic system with subsequent elevation in regional cerebral blood flow (CBF) with no changes in the cerebral metabolic rate of oxygen (CMRO(2)) or in the cerebral metabolic rate of glucose (CMRglc). Such an increase in regional CBF without a change of CMRO(2) or CMRglc provides the brain with oxygen rapidly and efficiently and gives substantial evidence that the TCR is an oxygen-conserving reflex. This system, which mediates reflex protection projects via currently undefined pathways from the rostral ventrolateral medulla oblongata to the upper brainstem and/or thalamus which finally engage a small population of neurons in the cortex. This cortical centre appears to be dedicated to reflexively transduce a neuronal signal into cerebral vasodilatation and synchronization of electrocortical activity. Sympathetic excitation is mediated by cortical-spinal projection to spinal pre-ganglionic sympathetic neurons whereas bradycardia is mediated via projections to cardiovagal motor medullary neurons. The integrated reflex response serves to redistribute blood from viscera to brain in response to a challenge to cerebral metabolism, but seems also to initiate a preconditioning mechanism. Better and more detailed knowledge of the cascades, transmitters and molecules engaged in such endogenous (neuro) protection may provide new insights into novel therapeutic options for a range of disorders characterized by neuronal death and into cortical organization of the brain.

The injured nervous system: a Darwinian perspective

Much of the permanent damage that occurs in response to nervous system damage (trauma, infection, ischemia, etc.) is mediated by endogenous secondary processes that can contribute to cell death and tissue damage (excitotoxicity, oxidative damage and inflammation). For humans to evolve mechanisms to minimize secondary pathophysiological events following CNS injuries, selection must occur for individuals who survive such insults. Two major factors limit the selection for beneficial responses to CNS insults: for many CNS disease states the principal risk factor is advanced, post-reproductive age and virtually all severe CNS traumas are fatal in the absence of modern medical intervention. An alternative hypothesis for the persistence of apparently maladaptive responses to CNS damage is that the secondary exacerbation of damage is the result of unavoidable evolutionary constraints. That is, the nervous system could not function under normal conditions if the mechanisms that caused secondary damage (e.g., excitotoxicity) in response to injury were decreased or eliminated. However, some vertebrate species normally inhabit environments (e.g., hypoxia in underground burrows) that could potentially damage their nervous systems. Yet, neuroprotective mechanisms have evolved in these animals indicating that natural selection can occur for traits that protect animals from nervous system damage. Many of the secondary processes and regeneration-inhibitory factors that exacerbate injuries likely persist because they have been adaptive over evolutionary time in the healthy nervous system. Therefore, it remains important that researchers consider the role of the processes in the healthy or developing nervous system to understand how they become dysregulated following injury.

Physiological effects of hyperchloraemia and acidosis

The advent of balanced solutions for i.v. fluid resuscitation and replacement is imminent and will affect any specialty involved in fluid management. Part of the background to their introduction has focused on the non-physiological nature of 'normal' saline solution and the developing science about the potential problems of hyperchloraemic acidosis. This review assesses the physiological significance of hyperchloraemic acidosis and of acidosis in general. It aims to differentiate the effects of the causes of acidosis from the physiological consequences of acidosis. It is intended to provide an assessment of the importance of hyperchloraemic acidosis and thereby the likely benefits of balanced solutions.

Extreme anoxia tolerance in embryos of the annual killifishAustrofundulus limnaeus: insights from a metabolomics analysis

The annual killifish Austrofundulus limnaeus survives in ephemeral pond habitats by producing drought-tolerant diapausing embryos. These embryos probably experience oxygen deprivation as part of their normal developmental environment. We assessed the anoxia tolerance of A. limnaeus embryos across the duration of embryonic development. Embryos develop a substantial tolerance to anoxia during early development, which peaks during diapause II. This extreme tolerance of anoxia is retained during the first 4 days of post-diapause II development and is then lost. Metabolism during anoxia appears to be supported mainly by production of lactate, with alanine and succinate production contributing to a lesser degree. Anoxic embryos also accumulate large quantities of γ-aminobutyrate (GABA), a potential protector of neural function. It appears that the suite of characters associated with normal development and entry into diapause II in this species prepares the embryos for long-term survival in anoxia even while the embryos are exposed to aerobic conditions. This is the first report of such extreme anoxia tolerance in a vertebrate embryo, and introduces a new model for the study of anoxia tolerance in vertebrates.

Pharmacological activation of mGlu2/3 metabotropic glutamate receptors protects retinal neurons against anoxic damage in the goldfish Carassius auratus

We examined the expression of mGlu2/3 metabotropic glutamate receptors in the retina of the goldfish Carassius auratus. mGlu2/3 receptors were expressed in all retinal layers internal to the photoreceptor layer, particularly in the outer and inner nuclear layers. Although the goldfish brain is able to tolerate prolonged periods of anoxia, we examined whether anoxia could induce retinal damage. Three hours of anoxia induced in the retina the development of apoptotic cell death, as assessed 48 h later by TUNEL staining. TUNEL-positive cells were particularly found in the inner and outer nuclear layers, and were also present in the ganglion cell layer. Pharmacological activation of mGlu2/3 receptors by systemic injection of LY379268 (0.5 mg/kg, i.p., 15 min before the onset of anoxia) substantially protected retinas against anoxia-induced cell death. In contrast, systemic injection of the mGlu2/3 receptor antagonist, LY341495 (1 mg/kg, i.p., 15 min before the onset of anoxia), significantly amplified cell death. Finally, as mGlu2/3 receptors are implicated in the control of extracellular glutamate concentrations, we examined the stimulation of glutamate release in isolated goldfish retinas. Depolarizing medium containing 30 mM KCl led to a significant increase in glutamate release, which was substantially reduced by LY379268. We conclude that activation of mGlu2/3 receptors may provide a major defensive mechanism against ischemic/anoxic retinal damage.

The arctic ground squirrel brain is resistant to injury from cardiac arrest during euthermia

BACKGROUND AND PURPOSE

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).

METHODS

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.

RESULTS

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.

CONCLUSIONS

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.

Astrocytic contributions to bioenergetics of cerebral ischemia

Astrocytes are multifunctional cells that interact with neurons and other astrocytes in signaling and metabolic functions, and their resistance to pathophysiological conditions can help restrict loss of tissue after an ischemic event provided adequate nutrients are supplied to support their requirements. Astrocytes have substantial oxidative capacity and mechanisms to upregulate glycolytic capability when respiration is impaired. An astrocytic enzyme that synthesizes a powerful activator of glycolysis is not present in neurons, endowing astrocytes with the ability to sustain ATP production under restrictive conditions. The monocarboxylic acid transporter (MCT) isoforms predominating in astrocytes are optimized to facilitate very large increases in lactate flux as lactate concentration increases within (1-3 mM) and above (>3 mM) the normal range. In sharp contrast, the major neuronal MCT serves as a barrier to increased transmembrane transport as lactate rises above 1 mM, restricting both entry and efflux. Lactate can serve as fuel during recovery from ischemia but direct evidence that lactate is oxidized by neurons (vs. astrocytes) to maintain synaptic function is lacking. Astrocytes have critical roles in regulation of ionic homeostasis and control of extracellular glutamate levels, and spreading depression associated with ischemia places high demands on energy supplies in astrocytes and contributes to metabolic exhaustion and demise. Disruption of Ca2+ homeostasis, generation of oxygen free radicals and nitric oxide, and mitochondrial depolarization contribute to astrocyte death during and after a metabolic insult. Novel pharmaceutical agents targeted to astrocytes and hyperoxic therapy that restores penumbral oxygen level during energy failure might improve postischemic outcome.

Carbohydrate metabolism in the central nervous system of the megalobulimus oblongus snail during anoxia exposure and post-anoxia recovery

The effects of anoxic exposure and the post-anoxia aerobic recovery period on carbohydrate metabolism in the central nervous system (CNS) of the land snail Megalobulimus oblongus, an anoxia-tolerant land gastropod, were studied. The snails were exposed to anoxia for periods of 1.5, 3, 6, 12, 18, or 24 hr. In order to study the post-anoxia recovery phase, snails exposed to a 3-hr period of anoxia were returned to aerobic conditions for 1.5, 3, 6, or 15 hr. Glycogen and glucose concentrations in the CNS, hemolymph glucose concentration, and glycogen phosphorylase (active form, GPa) activity in the CNS were analyzed. Anoxia does not significantly affect the concentration of CNS glucose but induces hyperglycemia and a reduction of CNS GPa activity. The glycogen concentration was decreased at 12 hr of anoxia; however, by 18 and 24 hr in anoxia, the glycogen content was not significantly different from basal control values. During the post-anoxia period, the reduction in GPa activity and the increased hemolymph glucose concentration induced by anoxia returned to control values. These results suggest that the CNS of M. oblongus may use hemolymph glucose to fulfill the metabolic demands during anoxia. However, the hypothesis of tissue metabolic arrest cannot be excluded.

Axon function persists during anoxia in mammalian white matter

Axon function in the CNS has been reported to fail rapidly during anoxia, implying that there is no anaerobic capacity. This phenomenon was reassessed in rodent white matter using mouse or rat optic nerve. Axon function was semiquantitatively measured as area under the compound action potential. Mouse optic nerves exposed to anoxia (30-180 minutes) or cyanide (30-60 minutes) at 37 degrees C exhibited significant persistent function that was abolished by removing glucose. Reduction in compound action potential area increased with anoxia duration reaching a maximum of about 70% after 90 min. Rat optic nerves exposed to anoxia, in contrast to mouse optic nerves, showed rapid and complete loss of function. When artificial CSF glucose was increased from 10 to 30 mmol/L, rat optic nerves responded to anoxia in a similar manner to mouse optic nerves in 10-mmol/L glucose. The authors conclude that white matter is resistant to anoxia with a subset of axons able to subsist exclusively on anaerobically derived energy. Because the rat optic nerve is about twice the diameter of the mouse optic nerve, glucose diffusion into the rat optic nerve was inadequate during anoxia when artificial CSF glucose was 10 mmol/L but became adequate when artificial CSF glucose was 30 mmol/L. These observations have implications for white matter energy metabolism and susceptibility to injury during focal ischemia.

Group II metabotropic glutamate receptors regulate the vulnerability to hypoxic brain damage

We examined the expression of metabotropic glutamate (mGlu) receptors in species of fish that differ for their vulnerability to anoxic brain damage. Although expression of mGlu1a and mGlu5 receptors was similar in the brain of all species examined, expression of mGlu2/3 receptors was substantially higher in the brain of anoxia-tolerant species (i.e., the carp Carassius carassius and the goldfish Carassius auratus) than in the brain of species that are highly vulnerable to anoxic damage, such as the trouts Salmo trutta and Oncorhynchus mykiss. This difference was confirmed by measuring the mGlu2/3 receptor-mediated inhibition of forskolin-stimulated cAMP formation in slices prepared from the telencephalon of C. auratus and S. trutta. We exposed the goldfish C. auratus to water deprived of oxygen for 4 hr for the induction of hypoxic brain damage. Although the goldfish survived this treatment, the occurrence of apoptotic cell death could be demonstrated by terminal deoxynucleotidyl transferase-mediated biotinylated UTP nick end labeling staining and by the assessment of caspase-3 activity in different brain region. The extent of cell death was highest in the medulla oblongata, followed by the optic tectum, cerebellum, and hypothalamus. No cell death was found in the telencephalon. This regional pattern of hypoxic damage was inversely related to the expression of mGlu2/3 receptors, which was lowest in the medulla oblongata and highest in the telencephalon. Treatment of the goldfish with the brain permeant mGlu2/3 receptor antagonist LY341495 (1 mg/kg, i.p.) amplified anoxic damage throughout the brain and enabled the induction of cell death by anoxia in the telencephalon. In contrast, treatment of the goldfish with the mGlu2/3 receptor agonist LY379268 (0.5 or 1 mg/kg, i.p.) was highly protective against anoxic brain damage. Finally, exposure to the antagonist LY341495 (0.5 microm) greatly amplified the release of glutamate induced by hypoxia in slices prepared from the medulla oblongata and the telencephalon of the goldfish. We conclude that expression of mGlu2/3 receptors provides a major defensive mechanism against brain damage in anoxia-tolerant species.

Distribution and expression of A1 adenosine receptors, adenosine deaminase and adenosine deaminase-binding protein (CD26) in goldfish brain

The expression patterns of adenosine A(1) receptors (A(1)Rs), adenosine deaminase (ADA) and ADA binding protein (CD26) were studied in goldfish brain using mammalian monoclonal antibody against A(1)R and polyclonal antibodies against ADA and CD26. Western blot analysis revealed the presence of a band of 35 kDa for A(1)R in membrane preparations and a band of 43 kDa for ADA in both cytosol and membranes. Immunohistochemistry on goldfish brain slices showed that A(1) receptors were present in several neuronal cell bodies diffused in the telencephalon, cerebellum, optic tectum. In the rhombencephalon, large and medium sized neurons of the raphe nucleus showed a strong immunopositivity. A(1)R immunoreactivity was also present in the glial cells of the rhombencephalon and optic tectum. An analogous distribution was observed for ADA immunoreactivity. Tests for the presence of CD26 gave positive labelling in several populations of neurons in the rhombencephalon as well as in the radial glia of optic tectum, where immunostaining for ADA and A(1)R was observed. In goldfish astrocyte cultures the immunohistochemical staining of A(1)R, ADA and CD26, performed on the same cell population, displayed a complete overlapping distribution of the three antibodies. The parallel immunopositivity, at least in some discrete neuronal areas, for A(1)Rs, ADA and CD26 led us to hypothesize that a co-localization among A(1)R, ecto-ADA and CD26 also exists in the neurons of goldfish since it has been established to exist in the neurons of mammals. Moreover, we have demonstrated for the first time, that A(1)R, ecto-ADA and CD26 co-localization is present on the astroglial component of the goldfish brain. This raises the possibility that a similar situation is also shown in the glia of the mammalian brain.

PACAP inhibits anoxia-induced changes in physiological responses in horizontal cells in the turtle retina

Pituitary adenylate cyclase activating polypeptide (PACAP) has neurotrophic and neuroprotective effects against various cytotoxic agents in vitro, and ischemia in vivo. Anoxia tolerance is most highly developed in some species of turtles. Recently, we have demonstrated high levels of PACAP38 in the turtle brain, exceeding those in corresponding rat and human brain areas by 10- to 100-fold. The aim of the present study was to investigate with electrophysiological methods the protective effects of PACAP in anoxia-induced neuronal damage of turtle retinal horizontal cells. Adult turtles (Pseudemys scripta elegans) were used for the experiments. After decapitation, half of the isolated eyecup slices were placed into a non-oxygenated Ringer solution, the other half into 0.165 microM PACAP solution. Intracellular recordings were obtained from horizontal cells 18, 22, 42 and 46 h after removal of the eyes. The amplitudes of light responses with the exception of the 0-h measurement, were larger at all time-points in PACAP-incubated slices than in control retinal slices. After both 18 and 22 h, the response amplitudes of PACAP-treated cells exceeded those taken from control horizontal cells by 1.2-fold. At later times, this difference became larger than 2-fold. In summary, the present results provide evidence that PACAP has neuroprotective effects on the anoxic retinal cells in the turtle.

Pituitary adenylate cyclase activating polypeptide is highly abundant in the nervous system of anoxia-tolerant turtle, Pseudemys scripta elegans

The levels of the pituitary adenylate cyclase activating polypeptide (PACAP) were measured in the central nervous system and in peripheral organs of the anoxia-tolerant freshwater turtle, Pseudemys scripta elegans by radioimmunoassay. The concentration of PACAP38 was strikingly high in the central nervous system and lower but considerable immunoreactivity was detected in the peripheral organs. Levels of PACAP38 in the turtle brain exceed those measured in rat and human brain areas by 10-100-fold. Based on these exceptionally high levels of PACAP and the known neuroprotective role of the peptide, it can be suggested that PACAP38 plays a role in the extraordinary resistance of the turtle brain from anoxia-induced neuronal damage.

Predominant expression of group-II metabotropic glutamate receptors in the goldfish brain

Group-II metabotropic glutamate (mGlu) receptors (mGlu2/3 receptors) were highly expressed in various regions (telencephalon, optic tectum, and cerebellum, but not vagal lobe) of the goldfish brain. In the goldfish telencephalon, expression of mGlu2/3 receptors was even higher than in the rat cerebral cortex. In contrast, mGlu5 receptors showed low levels of expression in all goldfish brain regions, whereas mGlu1a receptors were only expressed in the goldfish cerebellum. Pharmacological activation of group-II mGlu receptors with the selective agonists, 2R,4R-4-aminopyrrolidine-2, 4-dicarboxylic acid and (2S,2'R,3'R)-2-(2,3-dicarboxycyclopropyl) glycine, reduced the evoked release of glutamate from goldfish brain synaptosomes, whereas agonists of group-I and -III mGlu receptors (3, 5-dihydroxyphenylglycine and L-2-amino-4-phosphonobutanoate) were inactive. The predominance of group-II over group-I mGlu receptors in the goldfish brain may provide a natural defense against excitotoxic neuronal death and contribute to the unusually high resistance of goldfish against hypoxic brain damage.

Neonatal tolerance to hypoxia: a comparative-physiological approach

Newborn mammals exhibit a number of physiological reactions which differ from normal adult physiology and are often regarded as signs of immaturity. However, when looked upon from a comparative point of view, it becomes obvious that some of these 'physiological peculiarities' bear striking similarity to adaptation mechanisms known from hypoxia-tolerant animals and may thus contribute to the well-established, yet poorly understood, phenomenon of neonatal hypoxia tolerance. As the mammalian fetus lives at oxygen partial pressures corresponding to 8000 m altitude, the first line of perinatal hypoxia defense consists of long-term adaptations to limited intrauterine oxygen supply: (1) improved O2 transport by fetal acclimatization to high altitude, (2) reduced metabolic rate by hibernation-like deviation from metabolic size allometry, (3) diminished cerebral vulnerability by functional analogies to diving turtle brain, and (4) enhanced metabolic flexibility by optional repartitioning of energy supply from growth to maintenance metabolism. In the case of birth asphyxia, these background mechanisms are complemented by short-term responses to acute oxygen lack: (1) reduction of body temperature as in natural torpor, (2) reduction of heart rate and redistribution of circulation as in diving mammals, (3) reduction of respiration rate typical of 'hypoxic hypometabolism', and (4) reduction of blood pH according to the concept of 'acidotic torpidity'. Although anaerobic metabolism is improved in neonatal mammals by increased glycogen stores, reduced metabolic demands, and sustained wash-out of acid metabolites, neonatal hypoxia tolerance seems to be primarily based on the ability to maintain tissue aerobiosis as long as possible. This is even reflected by isoenzyme patterns which do not consistently favour anaerobic glycolysis and, thus, are reminiscent of the 'lactate paradox' found in high altitude adaptation. Altogether, from a biological point of view, the perinatal period appears as a source of adaptive mechanisms that can be refound, in varying combinations, in many survival strategies. From a clinical point of view, the interplay of long- and short-term mechanisms offers a novel approach to estimation of the newborn's ability to withstand temporary oxygen lack. However, most of these mechanisms are not unambiguous and, above all, not unlimited in their protective effect so that they do not release obstetricians or neonatologists from their obligation to counteract fetal or neonatal hypoxia without delay.

Ionic mechanisms underlying depolarizing responses of an identified insect motor neuron to short periods of hypoxia

Hypoxia can dramatically disrupt neural processing because energy-dependent homeostatic mechanisms are necessary to support normal neuronal function. In a human context, the long-term effects of such disruption may become all too apparent after a "stroke," in which blood-flow to part of the brain is compromised. We used an insect preparation to investigate the effects of hypoxia on neuron membrane properties. The preparation is particularly suitable for such studies because insects respond rapidly to hypoxia, but can recover when they are restored to normoxic conditions, whereas many of their neurons are large, identifiable, and robust. Experiments were performed on the "fast" coxal depressor motoneuron (Df) of cockroach (Periplaneta americana). Five-minute periods of hypoxia caused reversible multiphasic depolarizations (10-25 mV; n = 88), consisting of an initial transient depolarization followed by a partial repolarization and then a slower phase of further depolarization. During the initial depolarizing phase, spontaneous plateau potentials normally occurred, and inhibitory postsynaptic potential frequency increased considerably; 2-3 min after the onset of hypoxia all electrical activity ceased and membrane resistance was depressed. On reoxygenation, the membrane potential began to repolarize almost immediately, becoming briefly more negative than the normal resting potential. All phases of the hypoxia response declined with repeated periods of hypoxia. Blockade of ATP-dependent Na/K pump by 30 microM ouabain suppressed only the initial transient depolarization and the reoxygenation-induced hyperpolarization. Reduction of aerobic metabolism between hypoxic periods (produced by bubbling air through the chamber instead of oxygen) had a similar effect to that of ouabain. Although the depolarization seen during hypoxia was not reduced by tetrodotoxin (TTX; 2 microM), lowering extracellular Na+ concentration or addition of 500 microM Cd2+ greatly reduced all phases of the hypoxia-induced response, suggesting that Na influx occurs through a TTX-insensitive Cd2+-sensitive channel. Exposure to 20 mM tetraethylammonium and 1 mM 3,4-diaminopyridine increased the amplitude of the hypoxia-induced depolarization, suggesting that activation of K channels may normally limit the amplitude of the hypoxia response. In conclusion we suggest that the slow hypoxia-induced depolarization on motoneuron Df is mainly carried by a TTX-resistant, Cd2+-sensitive sodium influx. Ca2+ entry may also make a direct or indirect contribution to the hypoxia response. The fast transient depolarization appears to result from block of the Na/K pump, whereas the reoxygenation-induced hyperpolarization is largely caused by its subsequent reactivation.

Intracellular Ca2+ during metabolic activation of KATP channels in spontaneously active dorsal vagal neurons in medullary slices

Intracellular Ca2+ ([Ca2+]i) and membrane properties were measured in fura-2 dialysed dorsal vagal neurons (DVN) spontaneously active at a frequency of 0.5-5 Hz. [Ca2+]i increased by about 30 nm upon rising spike frequency by more than 200% due to 20-50 pA current pulses or 10 micrometer serotonin. It fell by 30 nm upon block of spiking by current-injection, tetrodotoxin or Ni2+ and also during hyperpolarization due to gamma-aminobutyric acid or opening of adenosine triphosphate (ATP) -sensitive K+ (KATP) channels with diazoxide. KATP channel-mediated hyperpolarizations during anoxia or cyanide produced an initial [Ca2+]i decrease which reversed into a secondary Ca2+ rise by less than 100 nm. Similar moderate rises of [Ca2+]i were observed during block of aerobic metabolism under voltage-clamp as well as in intact cells, loaded with fura-2 AM. The magnitude of the metabolism-related [Ca2+]i transients did not correlate with the amplitude of the KATP channel-mediated outward current. [Ca2+]i did not change during diazoxide-induced or spontaneous activation of KATP outward current observed in 10% of cells after establishing whole-cell recording. Increasing [Ca2+]i with cyclopiazonic acid did not activate KATP channels. [Ca2+]i was not affected upon block of outward current with sulphonylureas, but these KATP channel blockers were effective to reverse inhibition of spike discharge and, thus, the initial [Ca2+]i fall upon spontaneous or diazoxide-, anoxia- and cyanide-induced KATP channel activation. A sulphonylurea-sensitive hyperpolarization and [Ca2+]i fall was also revealed in the early phase of iodoacetate-induced metabolic arrest, whereas after about 20 min, occurrence of a progressive depolarization led to an irreversible rise of [Ca2+]i to more than 1 micrometer. The results indicate that KATP channel activity in DVN is not affected by physiological changes of intracellular Ca2+ and the lack of a major perturbance of Ca2+ homeostasis contributes to their high tolerance to anoxia.

ATP-sensitive K+ channels in cardiac muscle from cold-acclimated goldfish: characterization and altered response to ATP

ATP-sensitive potassium channels (K(ATP)) play an important, if incompletely defined, role in myocardial function in mammals. With the discovery that K(ATP) channels are also present at high densities in the hearts of vertebrate ectotherms, speculation arises as to their function during periods of cold-acclimation and depressed ATP synthesis. We used single-channel and intracellular recording techniques to examine the possibility that channel activity would be altered in cardiac muscle from goldfish (Carassius auratus) acclimated at 7+/-1 degrees C relative to control (21+/-1 degrees C). As previously observed in mammals, K(ATP) channels in isolated ventricular myocytes were inwardly rectified with slope conductances of 63 pS. However, channel mean open-time and overall open-state probability (Po) were significantly increased in cells from the cold-acclimated animals. In addition, K(ATP) channels in cells from fish acclimated at 7 degrees were nearly insensitive to the inhibitory effects of 2 mM ATP, whether studied at 7 or at 21 degrees C. Transmembrane action potential duration (APD) in hearts of cold-acclimated fish studied at 21 degrees was significantly shorter than that observed in hearts of warm-acclimated fish at the same temperature; this difference was eliminated by the K(ATP) channel antagonist glibenclamide (5 microM). These data suggest that K(ATP) channels in the hearts of cold-acclimated animals are more active and less sensitive to ATP-inhibition than those in warm-acclimated fish, possibly reflecting a functional adaptation to promote tolerance of low temperatures in this species.