Environmental remodelling of GABAergic and glutamatergic neurotransmission: Rise of the anoxia-tolerant turtle brain

Glutamatergic pathways in the brains of turtles: A comparative perspective among reptiles, birds, and mammals

Glutamate acts as the main excitatory neurotransmitter in the brain and plays a vital role in physiological and pathological neuronal functions. In mammals, glutamate can cause detrimental excitotoxic effects under anoxic conditions. In contrast, Trachemys scripta, a freshwater turtle, is one of the most anoxia-tolerant animals, being able to survive up to months without oxygen. Therefore, turtles have been investigated to assess the molecular mechanisms of neuroprotective strategies used by them in anoxic conditions, such as maintaining low levels of glutamate, increasing adenosine and GABA, upregulating heat shock proteins, and downregulating KATP channels. These mechanisms of anoxia tolerance of the turtle brain may be applied to finding therapeutics for human glutamatergic neurological disorders such as brain injury or cerebral stroke due to ischemia. Despite the importance of glutamate as a neurotransmitter and of the turtle as an ideal research model, the glutamatergic circuits in the turtle brain remain less described whereas they have been well studied in mammalian and avian brains. In reptiles, particularly in the turtle brain, glutamatergic neurons have been identified by examining the expression of vesicular glutamate transporters (VGLUTs). In certain areas of the brain, some ionotropic glutamate receptors (GluRs) have been immunohistochemically studied, implying that there are glutamatergic target areas. Based on the expression patterns of these glutamate-related molecules and fiber connection data of the turtle brain that is available in the literature, many candidate glutamatergic circuits could be clarified, such as the olfactory circuit, hippocampal–septal pathway, corticostriatal pathway, visual pathway, auditory pathway, and granule cell–Purkinje cell pathway. This review summarizes the probable glutamatergic pathways and the distribution of glutamatergic neurons in the pallium of the turtle brain and compares them with those of avian and mammalian brains. The integrated knowledge of glutamatergic pathways serves as the fundamental basis for further functional studies in the turtle brain, which would provide insights on physiological and pathological mechanisms of glutamate regulation as well as neural circuits in different species.

Gene expression of hypoxia-inducible factor (HIF), HIF regulators, and putative HIF targets in ventricle and telencephalon of Trachemys scripta acclimated to 21 °C or 5 °C and exposed to normoxia, anoxia or reoxygenation

In anoxia-sensitive mammals, hypoxia inducible factor (HIF) promotes cellular survival in hypoxia, but also tumorigenesis. By comparison, anoxia-tolerant vertebrates likely need to circumvent a prolonged upregulation of HIF to survive long-term anoxia, making them attractive biomedical models for investigating HIF regulation. To lend insight into the role of HIF in anoxic Trachemys scripta ventricle and telencephalon, 21 °C- and 5 °C-acclimated turtles were exposed to normoxia, anoxia (24 h at 21 °C; 24 h or 14 d at 5 °C) or anoxia + reoxygenation and the gene expression of HIF-1α (hif1a) and HIF-2α (hif2a), two regulators of HIF, and eleven putative downstream targets of HIF quantified by qPCR. Changes in gene expression with anoxia at 21 °C differentially aligned with a circumvention of HIF activity. Whereas hif1a and hif2a expression was unaffected in ventricle and telencephalon, and BCL2 interacting protein 3 gene expression reduced by 30% in telencephalon, gene expression of vascular endothelial growth factor-A increased in ventricle (4.5-fold) and telencephalon (1.5-fold), and hexokinase 1 (2-fold) and hexokinase 2 (3-fold) gene expression increased in ventricle. At 5 °C, the pattern of gene expression in ventricle or telencephalon was unaltered with oxygenation state. However, cold acclimation in normoxia induced downregulation of HIF-1α, HIF-2α, and HIF target gene expression in telencephalon. Overall, the findings lend support to the postulation that prolonged activation of HIF is counterproductive for long-term anoxia survival. Nevertheless, quantification of the effect of anoxia and acclimation temperature on HIF binding activity and regulation at the protein level are needed to provide a strong scientific framework whereby new strategies for oxygen related pathologies can be developed.

Does the ventricle limit cardiac contraction rate in the anoxic turtle (Trachemys scripta)? I. Comparison of the intrinsic contractile responses of cardiac chambers to the extracellular changes that accompany prolonged anoxia exposure

Multiple lines of evidence suggest that an inability of the ventricle to contract in coordination with the pacemaker during anoxia exposure may suppress cardiac pumping rate in anoxia-tolerant turtles. To determine under what extracellular conditions the ventricle could be the weak link that limits cardiac pumping, we compared, under various extracellular conditions, the intrinsic contractile properties of isometrically-contracting ventricular and atrial strips obtained from 21 °C- to 5 °C- acclimated turtles (Trachemys scripta) that had been exposed to either normoxia or anoxia (16 h at 21 °C; 12 days at 5 °C). We found that combined extracellular anoxia, acidosis, and hyperkalemia (AAK), severely disrupted ventricular, but not right or left atrial, excitability and contractibility of 5 °C anoxic turtles. However, combined hypercalcemia and heightened adrenergic stimulation counteracted the negative effects of AAK. We also report that the turtle heart is resilient to prolonged diastolic intervals, which would ensure that contractile force is maintained if arrhythmia were to occur during anoxia exposure. Finally, our findings reinforce that prior temperature and anoxia experiences are central to the intrinsic contractile response of the turtle myocardium to altered extracellular conditions. At 21 °C, prior anoxia exposure preconditioned the ventricle for anoxic and acidosis exposure. At 5 °C, prior anoxia exposure evoked heightened sensitivity of the ventricle to hyperkalemia, as well as all chambers to combined hypercalcemia and increased adrenergic stimulation. Overall, our findings show that the ventricle could limit cardiac pumping rate during prolonged anoxic submergence in cold-acclimated turtles if hypercalcemia and heightened adrenergic stimulation are insufficient to counteract the negative effects of combined extracellular anoxia, acidosis, and hyperkalemia.

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Turtle atria are more resilient to extracellular factors that disrupt contraction than the ventricle.

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Combined anoxia, acidosis, and hyperkalemia disrupted ventricular excitability and contractibility of 5 °C anoxic turtles.

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Heightened adrenergic stimulation counteracted the negative effects.

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The ventricle could limit cardiac pumping during anoxia at 5 °C if adrenergic stimulation is low.

Mitochondrial KATP channels stabilize intracellular Ca2+ during hypoxia in retinal horizontal cells of goldfish (Carassius auratus)

Neurons of the retina require oxygen to survive. In hypoxia, neuronal ATP production is impaired, ATP-dependent ion pumping is reduced, transmembrane ion gradients are dysregulated, and intracellular Ca2+ concentration ([Ca2+]i) increases enough to trigger excitotoxic cell death. Central neurons of the common goldfish (Carassius auratus) are hypoxia tolerant, but little is known about how goldfish retinas withstand hypoxia. To study the cellular mechanisms of hypoxia tolerance, we isolated retinal interneurons (horizontal cells; HCs), and measured [Ca2+]i with Fura-2. Goldfish HCs maintained [Ca2+]i throughout 1 h of hypoxia, whereas [Ca2+]i increased irreversibly in HCs of the hypoxia-sensitive rainbow trout (Oncorhynchus mykiss) with just 20 min of hypoxia. Our results suggest mitochondrial ATP-dependent K+ channels (mKATP) are necessary to stabilize [Ca2+]i throughout hypoxia. In goldfish HCs, [Ca2+]i increased when mKATP channels were blocked with glibenclamide or 5-hydroxydecanoic acid, whereas the mKATP channel agonist diazoxide prevented [Ca2+]i from increasing in hypoxia in trout HCs. We found that hypoxia protects against increases in [Ca2+]i in goldfish HCs via mKATP channels. Glycolytic inhibition with 2-deoxyglucose increased [Ca2+]i, which was rescued by hypoxia in a mKATP channel-dependent manner. We found no evidence of plasmalemmal KATP channels in patch-clamp experiments. Instead, we confirmed the involvement of KATP in mitochondria with TMRE imaging, as hypoxia rapidly (<5 min) depolarized mitochondria in a mKATP channel-sensitive manner. We conclude that mKATP channels initiate a neuroprotective pathway in goldfish HCs to maintain [Ca2+]i and avoid excitotoxicity in hypoxia. This model provides novel insight into the cellular mechanisms of hypoxia tolerance in the retina.

Indirect evidence that anoxia exposure and cold acclimation alter transarcolemmal Ca2+ flux in the cardiac pacemaker, right atrium and ventricle of the red-eared slider turtle (Trachemys scripta)

We indirectly assessed if altered transarcolemmal Ca²⁺ flux accompanies the decreased cardiac activity displayed by Trachemys scripta with anoxia exposure and cold acclimation. Turtles were first acclimated to 21 °C or 5 °C and held under normoxic (21N; 5N) or anoxic conditions (21A; 5A). We then compared the response of intrinsic heart rate (f_H) and maximal developed force of spontaneously contracting right atria (F_max,RA), and maximal developed force of isometrically-contracting ventricular strips (F_max,V), to Ni²⁺ (0.1-10 mM), which respectively blocks T-type Ca²⁺ channels, L-type Ca²⁺ channels and the Na⁺-Ca²⁺-exchanger at the low, intermediate and high concentrations employed. Dose-response curves were established in simulated in vivo normoxic (Sim Norm) or simulated in vivo anoxic extracellular conditions (Sim Anx; 21A and 5A preparations). Ni²⁺ decreased intrinsic f_H, F_max,RA and F_max,V of 21N tissues in a concentration-dependent manner, but the responses were blunted in 21A tissues in Sim Norm. Similarly, dose-response curves for F_max,RA and F_max,V of 5N tissues were right-shifted, whereas anoxia exposure at 5 °C did not further alter the responses. The influence of Sim Anx was acclimation temperature-, cardiac chamber- and contractile parameter-dependent. Combined, the findings suggest that: (1) reduced transarcolemmal Ca²⁺ flux in the cardiac pacemaker is a potential mechanism underlying the slowed intrinsic f_H of anoxic turtles at 21 °C, but not 5 °C, (2) a downregulation of transarcolemmal Ca²⁺ flux may aid cardiac anoxia survival at 21 °C and prime the turtle myocardium for winter anoxia and (3) confirm that altered extracellular conditions with anoxia exposure can modify turtle cardiac transarcolemmal Ca²⁺ flux.

Taurine activates glycine and GABAA receptor currents in anoxia-tolerant painted turtle pyramidal neurons

Unlike anoxia-intolerant mammals, painted turtles can survive extended periods without oxygen. This is partly accomplished by an anoxia-mediated increase in gamma-aminobutyric acid (GABA) release, which activates GABA receptors and mediates spike arrest in turtle neurons via shunting inhibition. Extracellular taurine levels also increase during anoxia; why this occurs is unknown but it is speculated that glycine and/or GABA_A/B receptors are involved. Given the general importance of inhibitory neurotransmission in the anoxia-tolerant painted turtle brain, we investigated the function of taurine as an inhibitory neuromodulator in turtle pyramidal neurons. Using whole-cell patch-clamp electrophysiological methods to record from neurons within a cortical brain sheet, we found that taurine depolarized membrane potential by ∼8 mV, increased whole-cell conductance ∼2-fold, and induced an inward current that possessed characteristics similar to GABA- and glycine-evoked currents. These effects were mitigated following glycine receptor antagonism with strychnine and GABA_A receptor antagonism with gabazine, bicuculine or picrotoxin, but were unchanged following GABA_B or glutamatergic receptor inhibition. These data indicate that a high concentration of taurine in vitro mediates its effects through both glycine and GABA_A receptors, and suggests that taurine, in addition to GABA, inhibits neuronal activity during anoxia in the turtle cortex.

Na+/K+-ATPase activity in the anoxic turtle (Trachemys scripta) brain at different acclimation temperature

Survival of prolonged anoxia requires a balance between cellular ATP demand and anaerobic ATP supply from glycolysis, especially in critical tissues such as the brain. To add insight into the ATP demand of the brain of the anoxia-tolerant red-eared slider turtle (Trachemys scripta) during prolonged periods of anoxic submergence, we quantified and compared the number of Na⁺-K⁺-ATPase units and their molecular activity in brain tissue from turtles acclimated to either 21°C or 5°C and exposed to either normoxia or anoxia (6h 21°C; 14days at 5°C). Na⁺-K⁺-ATPase activity and density per g tissue were similar at 21°C and 5°C in normoxic turtles. Likewise, anoxia exposure at 21°C did not induce any change in Na⁺-K⁺-ATPase activity or density. In contrast, prolonged anoxia at 5°C significantly reduced Na⁺-K⁺-ATPase activity by 55%, which was largely driven by a 50% reduction of the number of Na⁺-K⁺-ATPase units without a change in the activity of existing Na⁺-K⁺-ATPase pumps or α-subunit composition. These findings are consistent with the "channel arrest" hypothesis to reduce turtle brain Na⁺-K⁺-ATPase activity during prolonged, but not short-term anoxia, a change that likely helps them overwinter under low temperature, anoxic conditions.

Characterization of brevetoxin (PbTx-3) exposure in neurons of the anoxia-tolerant freshwater turtle (Trachemys scripta)

Harmful algal blooms are increasing in frequency and extent worldwide and occur nearly annually off the west coast of Florida where they affect both humans and wildlife. The dinoflagellate Karenia brevis is a key organism in Florida red tides that produces a suite of potent neurotoxins collectively referred to as the brevetoxins (PbTx). Brevetoxins bind to and open voltage gated sodium channels (VGSC), increasing cell permeability in excitable cells and depolarizing nerve and muscle tissue. Exposed animals may thus show muscular and neurological symptoms including head bobbing, muscle twitching, paralysis, and coma; large HABs can result in significant morbidity and mortality of marine life, including fish, birds, marine mammals, and sea turtles. Brevetoxicosis however is difficult to treat in endangered sea turtles as the physiological impacts have not been investigated and the magnitude and duration of brevetoxin exposure are generally unknown. In this study we used the freshwater turtle Trachemys scripta as a model organism to investigate the effects of the specific brevetoxin PbTx-3 in the turtle brain. Primary turtle neuronal cell cultures were exposed to a range of PbTx-3 concentrations to determine excitotoxicity. Agonists and antagonists of voltage-gated sodium channels and downstream targets were utilized to confirm the toxin's mode of action. We found that turtle neurons are highly resistant to PbTx-3; while cell viability decreased in a dose dependent manner across PbTx-3 concentrations of 100-2000nM, the EC₅₀ was significantly higher than has been reported in mammalian neurons. PbTx-3 exposure resulted in significant Ca²⁺ influx, which could be fully abrogated by the VGSC antagonist tetrodotoxin, NMDA receptor blocker MK-801, and tetanus toxin, indicating that the mode of action in turtle neurons is the same as in mammalian cells. As both turtle and mammalian VGSCs have a high affinity for PbTx-3, we suggest that the high resistance of the turtle neuron to PbTx-3 may be related to its ability to withstand anoxic depolarization. The ultimate goal of this work is to design treatment protocols for sea turtles exposed to red tides worldwide.