Anoxia and adenosine induce increased cerebral blood flow rate in crucian carp

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.

Hypoxia acclimation alters reactive oxygen species homeostasis and oxidative status in estuarine killifish (Fundulus heteroclitus)

Hypoxia is common in aquatic environments, and exposure to hypoxia followed by re-oxygenation is often believed to induce oxidative stress. However, there have been relatively few studies of reactive oxygen species (ROS) homeostasis and oxidative status in fish that experience natural hypoxia-re-oxygenation cycles. We examined how exposure to acute hypoxia (2 kPa O2) and subsequent re-oxygenation (to 20 kPa O2) affects redox status, oxidative damage and anti-oxidant defenses in estuarine killifish (Fundulus heteroclitus), and whether these effects were ameliorated or potentiated by prolonged (28 days) acclimation to either constant hypoxia or intermittent cycles of nocturnal hypoxia (12 h:12 h normoxia:hypoxia). Acute hypoxia and re-oxygenation led to some modest and transient changes in redox status, increases in oxidized glutathione, depletion of scavenging capacity and oxidative damage to lipids in skeletal muscle. The liver had greater scavenging capacity, total glutathione concentrations and activities of anti-oxidant enzymes (catalase, glutathione peroxidase) than muscle, and generally experienced less variation in glutathiones and lipid peroxidation. Unexpectedly, acclimation to constant hypoxia or intermittent hypoxia led to a more oxidizing redox status (muscle and liver) and it increased oxidized glutathione (muscle). However, hypoxia-acclimated fish exhibited little to no oxidative damage (as reflected by lipid peroxidation and aconitase activity), in association with improvements in scavenging capacity and catalase activity in muscle. We conclude that hypoxia acclimation leads to adjustments in ROS homeostasis and oxidative status that do not reflect oxidative stress, but may instead be part of the suite of responses that killifish use to cope with chronic hypoxia.

The effect of short-term hypoxic exposure on metabolic gene expression

The long-term effect of hypoxia is to decrease both the production and use of ATP and thus decrease the reliance on mitochondrial oxidative energy production. Yet, recent studies include more immediate affects of hypoxia on gene expression and these data suggest the maintenance of mitochondrial function. To better understand the short-term physiological response to hypoxia, we quantified metabolic mRNA expression in the heart ventricles and livers of the teleost fish Fundulus grandis exposed to partial oxygen pressure of 2.8 kPa (-13.5% air saturation).Twenty-eight individuals from a single population were exposed to hypoxia for 0, 4, 8, 12, 24, 48, and 96  hr. Liver and cardiac tissues were sampled from the same individuals at 0-48  hr. At 96  hr, only cardiac tissue was assayed. Gene expression was significantly different (ANOVA, P < 0.05) for 17 of 226 metabolic genes (7.5%) in cardiac tissue and for 20 of 256 (7.8%) metabolic genes in hepatic tissue. For the two tissues examined in this study, the maximum response occurred at different times. For cardiac tissue, using Dunnett's post hoc test, most of these significant differences occurred at 96  hr of exposure. For liver, all but one significant difference occurred at 4  hr. Surprisingly, too many (relative to random expectations) of the genes with significant increase in mRNA are involved in the oxidative phosphorylation pathway: 44% of the significant genes at 96  hr in the heart and 33% of the significant genes at 4  hr in the liver are involved in the oxidative phosphorylation pathway. These data indicate that there are tissue-specific differences in the timing of the response to hypoxia, yet both cardiac and hepatic tissues have increases in mRNA that code for enzyme in the oxidative phosphorylation pathway. If these changes in mRNA produce a similar change in protein, then these data suggest that the initial response to hypoxia involves an increase in the oxidative pathway potentially as a mechanism to maintain ATP production.

The strengths of in vivo magnetic resonance imaging (MRI) to study environmental adaptational physiology in fish

Adaptational physiology studies how animals cope with their environment, even if this environment is subject to permanent fluctuations such as tidal or seasonal variations. Aquatic organisms are generally more prone to be exposed to osmotic, hypoxic and temperature challenges than terrestrial animals. Some of these challenges are more restraining in an aquatic environment. To date, very few studies have used in vivo magnetic resonance imaging (MRI) to uncover the physiological mechanisms that respond to or compensate for these challenges. This paper provides an overview of what has been accomplished thus far by using MRI to study the environmental physiology of fish. It introduces the reader to the use of small teleost fish such as carp (12 cm, 60 g) and eelpout (25 cm, 50 g) as models for such research and to provide new perceptions into the applicability of MRI tools based on new insights into the nature of MRI contrast. Representative MRI studies have made contributions to the identification of the lack of cell volume repair in stenohaline fish during osmotic stress. They have studied the underlying physiological mechanisms of brain anoxia tolerance in fish and have qualified the role of the cardio-circulatory system in setting thermal tolerance windows of fish.

Vertebrate brains at the pilot light

While the brains of most vertebrates are unable to tolerate more than a few minutes of anoxia, some freshwater turtles (Trachemys and Chrysemys), crucian carp (Carassius carassius) and frogs (Rana pipens and Rana temporaria) can survive anoxia for hours to months. Obviously, anoxia tolerance has evolved separately several times and this is also reflected in the divergent strategies these animals utilize to survive without oxygen. The turtles and crucian carp defend their brain ATP levels and avoid a loss of ion homeostasis by reducing ATP use. In the turtles, the early release of adenosine and the activation of K(ATP) channels, a progressive release of GABA and a drastic reduction in electric activity and ion fluxes send the brain into a comatose like state. The crucian carp displays a more modest depression of ATP use, probably achieved through a moderated release of GABA and adenosine, allowing the animal to maintain physical activity in anoxia. The anoxic frog, on the other hand, seems to rely on mechanisms that greatly retard the anoxia induced fall in ATP levels and loss of ion homeostasis, so that the brain can be saved as long as the anoxia is limited to a few hours. The sequence of events characterizing the anoxic frog brain is similar to that of failing anoxic mammalian brain, although over a greatly extended time frame, allowing the frog to die slowly in anoxia, rather than survive. By contrast the only factor that limits anoxic survival in turtles and crucian carp may be the final depletion of their glycogen reserves.

Hypoxic survival strategies in two fishes: extreme anoxia tolerance in the North European crucian carp and natural hypoxic preconditioning in a coral-reef shark

Especially in aquatic habitats, hypoxia can be an important evolutionary driving force resulting in both convergent and divergent physiological strategies for hypoxic survival. Examining adaptations to anoxic/hypoxic survival in hypoxia-tolerant animals may offer fresh ideas for the treatment of hypoxia-related diseases. Here, we summarise our present knowledge of two fishes that have evolved to survive hypoxia under very different circumstances.

The crucian carp (Carassius carassius) is of particular interest because of its extreme anoxia tolerance. During the long North European winter, it survives for months in completely oxygen-deprived freshwater habitats. The crucian carp also tolerates a few days of anoxia at room temperature and, unlike anoxia-tolerant freshwater turtles, it is still physically active in anoxia. Moreover, the crucian carp does not appear to reduce neuronal ion permeability during anoxia and may primarily rely on more subtle neuromodulatory mechanisms for anoxic metabolic depression.

The epaulette shark (Hemiscyllium ocellatum) is a tropical marine vertebrate. It lives on shallow reef platforms that repeatedly become cut off from the ocean during periods of low tides. During nocturnal low tides, the water [O2] can fall by 80% due to respiration of the coral and associated organisms. Since the tides become lower and lower over a period of a few days, the hypoxic exposure during subsequent low tides will become progressively longer and more severe. Thus, this shark is under a natural hypoxic preconditioning regimen. Interestingly, hypoxic preconditioning lowers its metabolic rate and its critical PO2. Moreover, repeated anoxia appears to stimulate metabolic depression in an adenosine-dependent way.

Anoxia tolerant brains

While medical science has struggled to find ways to counteract anoxic brain damage with limited success, evolution has repeatedly solved this problem. The best-studied examples of anoxia-tolerant vertebrates are the crucian carp and some North American Freshwater turtles. These can survive anoxia for days to months, depending of temperature. Both animals successfully fight any major fall in brain ATP levels, but the strategies they use to accomplish this are quite divergent. The anoxic turtle suppresses brain activity to such a degree that it becomes virtually comatose. The underlying mechanisms involve closing down ion conductances and releasing GABA and adenosine. By contrast, the crucian carp remains active in anoxia, although it suppresses selected brain functions, and avoids lactate self-poisoning by producing an exotic anaerobic end-product. These animals provide unique models for studying anoxic survival mechanisms both on a molecular and physiological level.

α-Adrenergic regulation of systemic peripheral resistance and blood flow distribution in the turtleTrachemys scriptaduring anoxic submergence at 5°C and 21°C

Anoxic exposure in the anoxia-tolerant freshwater turtle is attended by substantial decreases in heart rate and blood flows, but systemic blood pressure (Psys) only decreases marginally due to an increase in systemic peripheral resistance (Rsys). Here,we investigate the role of the α-adrenergic system in modulating Rsys during anoxia at 5°C and 21°C in the turtle Trachemys scripta, and also describe how anoxia affects relative systemic blood flow distribution(%Q̇sys) and absolute tissue blood flows. Turtles were instrumented with an arterial cannula for measurement of Psys and ultrasonic flow probes on major systemic blood vessels for determination of systemic cardiac output(Q̇sys). α-Adrenergic tone was assessed from vascular injections of α-adrenergic agonists and antagonists (phenylephrine and phentolamine, respectively) during normoxia and following either 6 h (21°C) or 12 days (5°C) of anoxic submergence. Coloured microspheres, injected through a left atrial cannula during normoxia and anoxia, as well as after α-adrenergic stimulation and blockade during anoxia at both temperatures, were used to determine relative and absolute tissue blood flows.

Anoxia was associated with an increased Rsys and functional α-adrenergic vasoactivity at both acclimation temperatures. However, while anoxia at 21°C was associated with a high systemicα-adrenergic tone, the progressive increase of Rsysat 5°C was not mediated by α-adrenergic control. A redistribution of blood flow away from ancillary vascular beds towards more vital circulations occurred with anoxia at both acclimation temperatures.%Q̇sys and absolute blood flow were reduced to the digestive and urogenital tissues (approximately 2- to 15-fold), while %Q̇sys and absolute blood flows to the heart and brain were maintained at normoxic levels. The importance of liver and muscle glycogen stores in fueling anaerobic metabolism were indicated by increases in%Q̇sys to the muscle at 21°C (1.3-fold) and liver at 5°C (1.7-fold). As well, the crucial importance of the turtle shell as a buffer reserve during anoxic submergence was indicated by 40-50% of Q̇sys being directed towards the shell during anoxia at both 5°C and 21°C. α-Adrenergic stimulation and blockade during anoxia caused few changes in%Q̇sys and absolute tissue blood flow. However, there was evidence of α-adrenergic vasoactivity contributing to blood flow regulation to the liver and shell during anoxic submergence at 5°C.

Surviving anoxia with the brain turned on

Crucian carp is one of few vertebrates that tolerate anoxia. It maintains brain ATP during anoxia partially by reducing ATP consumption. However, unlike turtles, which become comatose during anoxia, this fish remains physically active. This striking difference in anoxic survival strategy is reflected all the way down to the cellular level.

Does anoxia induce cell swelling in carp brains? In vivo MRI measurements in crucian carp and common carp

Although both common and crucian carp survived 2 h of anoxia at 18 degrees C, the response of their brains to anoxia was quite different and indicative of the fact that the crucian carp is anoxia tolerant while the common carp is not. Using in vivo T(2) and diffusion-weighted magnetic resonance imaging (MRI), we studied anoxia induced changes in brain volume, free water content (T(2)), and water homeostasis (water diffusion coefficient). The anoxic crucian carp showed no signs of brain swelling or changes in brain water homeostasis even after 24 h except for the optic lobes, where cellular edema was indicated. The entire common carp brain suffered from cellular edema, net water gain, and a volume increase (by 6.5%) that proceeded during 100 min normoxic recovery (by 10%). The common carp recovered from this insult, proving that the changes were reversible and suggesting that the oversized brain cavity allows brain swelling during energy deficiency without a resultant increase in intracranial pressure and global ischemia. It is tempting to suggest that this is a function of the large brain cavity seen in many ectothermic vertebrates.