Unchanged [3H]MK-801 binding and increased [3H]flunitrazepam binding in turtle forebrain during anoxia

Chronic stress in Lizards: Studies on the Behavior and Benzodiazepine Receptors in Liolaemus koslowskyi and Cnemidophorus tergolaevigatus

Behavioral and physiological adaptive responses of animals facing chronic exposure to a single stressor may allow them to overcome its negative effects for future exposures to similar stressful situations. At chemical level, the GABA_A /benzodiazepine complex is considered one of the main receptor systems involved in the modulation of stress-induced responses. Here, we describe the behavioral responses of two different lizard species, Liolaemus koslowskyi and Cnemidophorus tergolaevigatus exposed to three potential chronic stressful treatments: (a) high temperature, (b) forced swimming, and (c) simulated predator. Additionally, we aimed to determine in those lizards whether the central-type benzodiazepine receptor (CBR; an allosteric modulator site of the GABA_A receptor) is related to adaptive responses to those stressful stimulations. Our results revealed that the simulated predator was the stress condition that showed the largest difference in behavioral responses between the two species, resembling previously described strategies in nature. The basal affinity of CBRs (obtained from undisturbed animals) showed differences between both species, and the simulated predator was the only stressor that altered the affinity of CBRs. L. koslowskyi CBRs showed a decreased receptor affinity, whereas C. tergolaevigatus showed an increased receptor affinity in comparison to their respective control groups. We show for the first time the effects of different types of stressors upon behavioral responses and CBR biochemical parameters in two lizard species. Our findings suggest a potential GABA/benzodiazepine role in the ability of lizards to cope with a repeated exposure to a stressful (e.g., predator) condition.

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.

Ecophysiology of neuronal metabolism in transiently oxygen-depleted environments: evidence that GABA is accumulated pre-synaptically in the cerebellum

Interactions between coral reef topography, tide cycles, and photoperiod provided selection pressure for adaptive physiological changes in sheltered hypoxic niches to be exploited by specialized tropical reef fish. The epaulette shark Hemiscyllium ocellatum withstands cyclic hypoxia in its natural environment, many hours of experimental hypoxia, and anoxia for up to 5h. It shows neuronal hypometabolism in response to 5% oxygen saturation. Northern-hemisphere hypoxia- and anoxia-tolerant vertebrates that over-winter under ice alter their inhibitory to excitatory neurotransmitter balance to forestall brain ATP depletion in the absence of oxidative phosphorylation. GABA immunochemistry, HPLC analysis and receptor binding studies in H. ocellatum cerebellum revealed a heterogeneous regional accumulation of neuronal GABA despite no change in its overall concentration, and a significant increase in GABA(A) receptor density without altered binding affinity. Increased GABA(A) receptor density would protect the cerebellum during reoxygenation when transmitter release resumes. While all hypoxia- and anoxia-tolerant teleosts examined to date respond to low oxygen levels by elevating brain GABA, the phylogenetically older epaulette shark did not, suggesting that it uses an alternative neuroprotective mechanism for energy conservation. This may reflect an inherent phylogenetic difference, or represent a novel ecophysiological adaptation to cyclic variations in the availability of oxygen.

Mechanisms of and defense against acute ammonia toxicity in the aquatic Chinese soft-shelled turtle, Pelodiscus sinensis

The objective of this study was to elucidate the mechanisms of acute ammonia toxicity in the aquatic Chinese soft-shelled turtle, Pelodiscus sinensis, and to examine how this turtle defended against a sublethal dose of NH(4)Cl injected into its peritoneal cavity. The ammonia and glutamine contents in the brains of turtles that succumbed within 3h to an intraperitoneal injection with a lethal dose (12.5 micromolg(-1) turtle) of NH(4)Cl were 21 and 4.4 micromolg(-1), respectively. Since the brain glutamine content increased to 8 micromolg(-1) at hour 6 and recovered thereafter in turtles injected with a sub-lethal dose of NH(4)Cl (7.5 micromolg(-1) turtle), it can be concluded that increased glutamine synthesis and accumulation was not the major cause of acute ammonia toxicity in P. sinensis. Indeed, the administration of l-methionine S-sulfoximine (MSO; 82 microgg(-1) turtle), a glutamine synthetase (GS) inhibitor, prior to the injection of a lethal dose of NH(4)Cl had no significant effect on the mortality rate. Although the prior administration of MSO led to an extension of the time to death, it was apparently a result of its effects on glutamate dehydrogenase and glutamate formation, instead of glutamine synthesis and accumulation, in the brain. By contrast, a prior injection with MK801 (1.6 microgg(-1) turtle), a NMDA receptor antagonist, reduced the 24h mortality of turtles injected with a lethal dose of NH(4)Cl by 50%. Thus, acute ammonia toxicity in P. sinensis was probably a result of glutamate dysfunction and the activation of NMDA receptors. NMDA receptor activation could also be exacerbated through membrane depolarization caused by the extraordinarily high level of ammonia (21 micromolg(-1) brain) in the brain of turtles that succumbed to a lethal dose of NH(4)Cl. One hour after the injection with a sub-lethal dose of NH(4)Cl, the brain of P. sinensis exhibited an extraordinarily high tolerance of ammonia (16 micromolg(-1) brain). The transient nature of ammonia accumulation indicates that P. sinensis could ameliorate ammonia toxicity through the suppression of endogenous ammonia production and/or the excretion of exogenous ammonia. Despite being ureogenic and ureotelic, only a small fraction of the exogenous ammonia was detoxified to urea. A major portion of ammonia was excreted unchanged, resulting in an apparent ammonotely in the experimental turtles. Since there were increases in total essential free amino acid contents in the brain, liver and muscle, it can be deduced that a suppression of amino acid catabolism had occurred, reducing the production of endogenous ammonia and hence alleviating the possibility of ammonia intoxication.

Effects of GABA receptor blockade on the ventilatory response to hypoxia in hypothermic newborn piglets

Hypothermic newborn piglets have a depressed ventilatory response to hypoxia, and this may be due to an increase in CNS gamma-aminobutyric acid (GABA) levels. To evaluate the effects of GABA(A) receptor blockade on the ventilatory response to hypoxia in hypothermic piglets, 31 anesthetized paralyzed mechanically ventilated newborn piglets (2-7 d) were studied at a brain temperature of 38.5 +/- 0.5 degrees C [normothermia (NT), n = 15] or 34 +/- 0.5 degrees C [hypothermia (HT), n = 16]. The central respiratory output was evaluated by measuring burst frequency and moving time average area of phrenic nerve activity. Measurements of minute phrenic output (MPO), arterial blood pressure, heart rate, oxygen consumption, and arterial blood gases were obtained at room air and during 20 min of isocapnic hypoxia [fraction of expired oxygen (FiO2) = 0.10]. After 10 min of hypoxia, a bolus injection of 20 microL of bicuculline methiodide (BM; 10 microg) or Ringer's solution was administered into the cisterna magna over a 1-min period, and the piglets remained in hypoxia for an additional 10 min. There was an initial increase of 50 +/- 6% in MPO during the first minute of hypoxia followed by a decrease to values 24 +/- 8% above baseline at 10 min in the NT group. In contrast, in the HT group, the initial increase in MPO with hypoxia was eliminated, and, at 10 min, there was a decrease to a mean value 35 +/- 4% below baseline level (NT versus HT, p < 0.03). After administration of BM, a significant increase in MPO with hypoxia was observed in both groups compared with their placebo groups (p < 0.002 in NT-BM group, p < 0.0001 in HT-BM group). However, the magnitude of the increase in MPO during hypoxia was significantly greater in the HT group after administration of BM (NT versus HT, p < 0.0001). Changes in oxygen consumption, arterial blood pressure, heart rate, pH, partial pressure of oxygen (PaO2), and base excess with hypoxia were not different between NT and HT groups before and after the administration of BM. The cardiorespiratory response to hypoxia was not modified after administration of Ringer's solution to NT and HT placebo groups. These data suggest that the depression in hypoxic ventilatory response produced by HT is in part modulated by an increased CNS GABA concentration.

Reduction of NMDA receptor activity in cerebrocortex of turtles (Chrysemys picta) during 6 wk of anoxia

Survival of brain anoxia during months of winter dormancy by the Western painted turtle, Chrysemys picta, may rely on inactivation of neuronal ion channels. During 2 h of anoxia, Ca2+ influx via the N-methyl-D-aspartate (NMDA) subtype of glutamate receptor decreases 30-40%, but it is not known if prolonged anoxic dormancy is associated with even more profound downregulation of this important channel. Because ionized Ca2+ in cerebrospinal fluid (CSF) increases five- to sixfold during prolonged anoxia, the potential for uncontrolled Ca2+ influx and neurotoxicity is increased. To study the regulation of NMDA receptor activity, we measured NMDA-mediated changes in intracellular Ca2+ (NMDA-DeltaCa2+) in turtle cerebrocortical sheets with fura 2. Turtles were kept in N2-bubbled aquariums for 2 h to 6 wk at 2-3 degrees C. NMDA-DeltaCa2+ decreased 60 +/- 14% (P < 0.05) after 2 h of anoxia and did not decrease further for 6 wk. Intracellular Ca2+ increased from 135 to 183 nM (P < 0.05) after 3 wk of anoxia and thereafter returned toward preanoxic levels. When NMDA receptor activity was assessed in artificial CSF containing the ions found in anoxic brain CSF (pH 7. 25, 69 mM lactate, 8.4 mM Ca2+, and 5.1 mM Mg2+), NMDA-DeltaCa2+ was twice control initially but was 21% less than in normoxic artificial CSF after the end of 6 wk, suggesting altered sensitivity of the NMDA receptor to ionized Ca2+ during prolonged anoxia. Regulation of NMDA receptor activity in turtle cerebrocortex during 6 wk of anoxia thus results in depression of NMDA receptor Ca2+ flux, despite a sixfold increase in ionized extracellular Ca2+.

Brain Na+/K+-ATPase activity in two anoxia tolerant vertebrates: crucian carp and freshwater turtle

The crucian carp (Carassius carassius) and freshwater turtles (Trachemys scripta) are among the very few vertebrates that can survive extended periods of anoxia. The major problem for an anoxic brain is energy deficiency. In the brain, the Na+/K+-ATPase is the single most ATP consuming enzyme, being responsible for maintaining ion gradients. We here show that the Na+/K+-ATPase activity in the turtle brain is reduced by 31% in telencephalon and by 34% in cerebellum after 24 h of anoxia. Both changes were reversed upon reoxygenation. By contrast, the Na+/K+-ATPase activities were maintained in the anoxic crucian carp brain. These results support the notion that crucian carp and turtles use divergent strategies for anoxic survival. The fall in Na+/K+-ATPase activities displayed by the turtle is likely to be related to the strong depression of brain electric and metabolic activity utilized as an anoxic survival strategy by this species.

Anoxia tolerant animals from a neurobiological perspective

This paper discusses the mechanisms for brain anoxia survival seen in crucian carp (Carassius carassius) and a few species of freshwater turtle (Chrysemys and Trachemys species). Comparisons are made with the hypoxic tolerant mammalian neonate brain. In the anoxic tolerant species the basic strategy for anoxia survival appears to be the maintenance of ion gradients, and thereby the avoidance of anoxic depolarization. Important facilitating factors involve having huge glycogen stores, increased blood supply to the brain, the suppression of electrical activity, increased release of inhibitory neuromodulators and neurotransmitters, upregulation of inhibitory neuroreceptors, the down-regulation of excitatory ion conductance and the down-regulation of Ca2+ channels. By contrast, for the mammalian neonate the most important causes of its increased hypoxia tolerance may be just simple consequences of the comparatively undifferentiated state of the brain of the newborn, with its lower energy requirements, slower decline in ATP and lower excitability levels acting to delay depolarization.