Hypoxia-tolerant neonatal CA1 neurons: relationship of survival to evoked glutamate release and glutamate receptor-mediated calcium changes in hippocampal slices

Severe hypoxia exposure inhibits larval brain development but does not affect the capacity to mount a cortisol stress response in zebrafish

Fish nursery habitats are increasingly hypoxic and the brain is recognized as highly hypoxia-sensitive, yet there is a lack of information on the effects of hypoxia on the development and function of the larval fish brain. Here, we tested the hypothesis that by inhibiting brain development, larval exposure to severe hypoxia has persistent functional effects on the cortisol stress response in zebrafish (Danio rerio). Exposing 5 days post-fertilization (dpf) larvae to 10% dissolved O2 (DO) for 16 h only marginally reduced survival, but it decreased forebrain neural proliferation by 55%, and reduced the expression of neurod1, gfap, and mbpa, markers of determined neurons, glia, and oligodendrocytes, respectively. The 5 dpf hypoxic exposure also elicited transient increases in whole body cortisol and in crf, uts1, and hsd20b2 expression, key regulators of the endocrine stress response. Hypoxia exposure at 5 dpf also inhibited the cortisol stress response to hypoxia in 10 dpf larvae and increased hypoxia tolerance. However, 10% DO exposure at 5 dpf for 16h did not affect the cortisol stress response to a novel stressor in 10 dpf larvae or the cortisol stress response to hypoxia in adult fish. Therefore, while larval exposure to severe hypoxia can inhibit brain development, it also increases hypoxia tolerance. These effects may transiently reduce the impact of hypoxia on the cortisol stress response but not its functional capacity to respond to novel stressors. We conclude that the larval cortisol stress response in zebrafish has a high capacity to cope with severe hypoxia-induced neurogenic impairment.

Age-related differences in hypoxia-associated genes and cytokine profile in male Wistar rats

Hypoxia tolerance of the organism depends on many factors, including age. High newborn organisms tolerance and high level of oxidative stress throughout aging were demonstrated by many studies. However, there is lack of investigations reflecting the expression of key hypoxia-inducible factor HIF in different age organisms in correlation to levels of pro-inflammatory and anti-inflammatory cytokines. Liver is a sensitive to hypoxia organ, and is an important organ in providing an acute reaction to infections – it synthesizes acute inflammation phase proteins, in particular, C-reactive protein. The aim of study was to determine relationship between age-related tolerance to hypoxia and HIF-1 and PHD2 (prolyl hydroxylase domain protein) expression levels in the liver and the production of cytokines in the spleen in newborn, prepubertal and adult Wistar rats. Newborn rats are characterized by high mRNA Hif-1α expression level in the liver, accompanied by a low content of HIF-1 protein and high level of PHD2. The growth in HIF-1α protein level throughout age is accompanied by the growth of pro-inflammatory cytokines level. Prepubertal animals are the least hypoxia resistant and their HIF-1α mRNA expression level was higher than in adult animals. The PHD2 activity in prepubertal animals was significantly reduced in comparison to newborn rats, and the HIF-1α protein level did not change. Further studies require the identification of additional mechanisms, determining the regulation of the HIF-1α level in prepubertal animals.

Subcellular Energetics and Metabolism: A Cross-Species Framework

Although it is generally believed that oxidative phosphorylation and adequate oxygenation are essential for life, human development occurs in a profoundly hypoxic environment and "normal" levels of oxygen during embryogenesis are even harmful. The ability of embryos not only to survive but also to thrive in such an environment is made possible by adaptations related to metabolic pathways. Similarly, cancerous cells are able not only to survive but also to grow and spread in environments that would typically be fatal for healthy adult cells. Many biological states, both normal and pathological, share underlying similarities related to metabolism, the electron transport chain, and reactive species. The purpose of Part I of this review is to review the similarities among embryogenesis, mammalian adaptions to hypoxia (primarily driven by hypoxia-inducible factor-1), ischemia-reperfusion injury (and its relationship with reactive oxygen species), hibernation, diving animals, cancer, and sepsis, with a particular focus on the common characteristics that allow cells and organisms to survive in these states.

Susceptibility to oxygen-glucose deprivation is reduced in acute hippocampal slices from euthermic Syrian golden hamsters relative to slices from Sprague-Dawley rats

Hibernation in mammals is characterised by a marked decrease in body temperature and a dramatic suppression of metabolism. In addition, despite experiencing a reduced cardiac output that would normally cause profound cerebral ischaemia, hibernating animals display robust neuroprotection. However, whether the reduced susceptibility to neural injury displayed by hibernators is attributable to an innate factor, or to the physiologic changes that accompany hibernation, remains uncertain. To help clarify the nature of the ischaemic tolerance displayed by hibernators, the current study examined hippocampal slices from rodents not capable of hibernation (rat) and rodents that could undergo hibernation (hamsters), but were active immediately prior to slice preparation. Slices from each species were subjected to oxygen-glucose deprivation (OGD; a commonly used in vitro model of ischaemia), and their viability examined after a recovery period. Although OGD reduced plasma membrane integrity in each species, rat-derived slices displayed a nearly threefold greater degree of effect. In addition, only slices harvested from rats showed reductions in synaptic mitochondrial function. While the improved ischaemic tolerance displayed by euthermic hamster brain slices maintained at a physiological temperature suggests an intrinsic, protection-related variable, the synaptic level of the GluN1 subunit (which is required to form functional NMDA receptors) was not found to be different between the two species. Further work is needed to improve understanding of the molecular mechanisms underlying the intrinsic injury tolerance of hibernator brain, which should help provide inspiration for new approaches to neuroprotection.

Precise spatial and temporal control of oxygen within in vitro brain slices via microfluidic gas channels

The acute brain slice preparation is an excellent model for studying the details of how neurons and neuronal tissue respond to a variety of different physiological conditions. But open slice chambers ideal for electrophysiological and imaging access have not allowed the precise spatiotemporal control of oxygen in a way that might realistically model stroke conditions. To address this problem, we have developed a microfluidic add-on to a commercially available perfusion chamber that diffuses oxygen throughout a thin membrane and directly to the brain slice. A microchannel enables rapid and efficient control of oxygen and can be modified to allow different regions of the slice to experience different oxygen conditions. Using this novel device, we show that we can obtain a stable and homogeneous oxygen environment throughout the brain slice and rapidly alter the oxygen tension in a hippocampal slice. We also show that we can impose different oxygen tensions on different regions of the slice preparation and measure two independent responses, which is not easily obtainable with current techniques.

Effect of hypobaric hypoxia on the P2X receptors of pyramidal cells in the immature rat hippocampus CA1 sub-field

PRIMARY OBJECTIVE

This study was designed to evaluate the effect of hypobaric hypoxia (HH) on the function and expression of P2X receptors in rat hippocampus CA1 pyramidal cells.

RESEARCH DESIGN

The functional changes of P2X receptors were investigated through the cell HH model and the expressional alterations of P2X receptors were observed through the animal HH model.

METHODS AND PROCEDURE

P2X receptors mediated currents were recorded from the freshly dissociated CA1 pyramidal cells of 7-day-old SD rats by whole cell patch clamp recording. The expression and distribution of P2X receptors were observed through immunohistochemistry and western blot at HH 3-day and 7-day.

MAIN OUTCOMES AND RESULTS

In acute HH conditions, the amplitudes of ATP evoked peak currents were decreased compared to control. The immunohistochemistry and western blot results reflected there was no change in P2X receptors expression after 3 days HH injury, while P2X receptors expression was up-regulated in response to 7 days HH injury.

CONCLUSIONS

These findings supported the possibility that the function of P2X receptors was sensitive to HH damage and long-term function decrease should result in the expression increase of P2X receptors.

Hypoxia associated NMDA receptor 2 subunit composition: developmental comparison between the hypoxia-tolerant subterranean mole-rat, Spalax, and the hypoxia-sensitive rat

Vertebrate brains are sensitive to oxygen depletion, which may lead to cell death. Hypoxia sensitivity originates from the high intrinsic rate of ATP consumption of brain tissue, accompanied by the release of glutamate, leading to the opening of ionotropic glutamate receptors, such as N-methyl-D-aspartate (NMDA) receptors (NMDARs). The relative expression levels of the four NMDAR-2 (NR2) subunits change during mammalian development with higher levels of units NR2B and NR2D observed during early development and correlated with hypoxic tolerance during embryonic and neonatal stages of development. Higher levels of NR2D are also abundant in brains of hypoxia tolerant species such as the crucian carp. The subterranean mole-rat, Spalax spends its life underground in sealed burrows and has developed a wide range of adaptations to this special niche including hypoxia-tolerance. In this study, we compared the in vivo mRNA expression of NR2 subunits in the brains of embryonic, neonatal and adult Spalax and rat. Our results demonstrate that under normoxic conditions, mRNA levels of NR2D are higher in Spalax than in rat at all developmental stages studied and are similar to levels in neonatal rat and in other hypoxia/anoxia tolerant species. Furthermore, under hypoxia Spalax NR2D mRNA levels increase while no response was observed in rat. Similarly, hypoxia induces an increase in mRNA levels of Spalax NR2A, claimed to promote neuronal survival. We suggest that indeed the proportional combinations of NMDAR-2 subunits contribute to the ability of the Spalax brain to cope with hypoxic environments.

Enhanced hypoxic preconditioning by isoflurane: signaling gene expression and requirement of intracellular Ca2+ and inositol triphosphate receptors

Neurons preconditioned with non-injurious hypoxia or the anesthetic isoflurane express different genes but are equally protected against severe hypoxia/ischemia. We hypothesized that neuroprotection would be augmented when preconditioning with isoflurane and hypoxic preconditioning are combined. We also tested if preconditioning requires intracellular Ca(2+) and the inositol triphosphate receptor, and if gene expression is similar in single agent and combined preconditioning. Hippocampal slice cultures prepared from 9 day old rats were preconditioned with hypoxia (95% N(2), 5% CO(2) for 15 min, HPC), 1% isoflurane for 15 min (APC) or their combination (CPC) for 15 min. A day later cultures were deprived of O(2) and glucose (OGD) to produce neuronal injury. Cell death was assessed 48 h after OGD. mRNA encoding 119 signal transduction genes was quantified with cDNA micro arrays. Intracellular Ca(2+) in CA1 region was measured with fura-2 during preconditioning. The cell-permeable Ca(2+) buffer BAPTA-AM, the IP(3) receptor antagonist Xestospongin C and RNA silencing were used to investigate preconditioning mechanisms. CPC decreased CA1, CA3 and dentate region death by 64-86% following OGD, more than HPC or APC alone (P<0.01). Gene expression following CPC was an amalgam of gene expression in HPC and APC, with simultaneous increases in growth/development and survival/apoptosis regulation genes. Intracellular Ca(2+) chelation and RNA silencing of IP(3) receptors prevented preconditioning neuroprotection and gene responses. We conclude that combined isoflurane-hypoxia preconditioning augments neuroprotection compared to single agents in immature rat hippocampal slice cultures. The mechanism involves genes for growth, development, apoptosis regulation and cell survival as well as IP(3) receptors and intracellular Ca(2+).

Differential NMDA receptor-dependent calcium loading and mitochondrial dysfunction in CA1 vs. CA3 hippocampal neurons

Hippocampal CA1 pyramidal neurons are selectively vulnerable to ischemia, while adjacent CA3 neurons are relatively resistant. Although glutamate receptor-mediated mitochondrial Ca(2+) overload and dysfunction is a major component of ischemia-induced neuronal death, no direct relationship between selective neuronal vulnerability and mitochondrial dysfunction has been demonstrated in intact brain preparations. Here, we show that in organotypic slice cultures NMDA induces much larger Ca(2+) elevations in vulnerable CA1 neurons than in resistant CA3. Consequently, CA1 mitochondria exhibit stronger calcium accumulation, more extensive swelling and damage, stronger depolarization of their membrane potential, and a significant increase in ROS generation. NMDA-induced Ca(2+) and ROS elevations were abolished in Ca(2+)-free medium or by NMDAR antagonists, but not by zinc chelation. We conclude that Ca(2)(+) overload-dependent mitochondrial dysfunction is a determining factor in the selective vulnerability of CA1 neurons.

Synthesis of Lipoamino acids and their activity against cerebral ischemic injury

A series of lipoamino acids were synthesized and their neuroprotective effect against brain ischemia induced by oxygen-glucose deprivation (OGD) on rat cerebral slices was evaluated. Among these compounds, N-stearoyl-l-tyrosine (4), N-stearoyl-l-serine (5) and N-stearoyl-L-threonine (6) exhibited good neuroprotective activity. We found that the neuroprotective activity of lipoamino acids depended on the acyl group, the presence of a free carboxylic function and a free hydroxyl group at the branched chain of the amino acids. The results also showed that 5 was the most active compound, protecting rat brain slices against OGD as well as hydrogen peroxide (H₂O₂) insult at the range of 1–10 M.

Brain development and susceptibility to damage; ion levels and movements

Responses of immature brains to physiological and pathological stimuli often differ from those in the adult. Because CNS function critically depends on ion movements, this chapter evaluates ion levels and gradients during ontogeny and their alterations in response to adverse conditions. Total brain Na(+) and Cl(-) content decreases during development, but K(+) content rises, reflecting shrinkage of the extracellular and increase in the intracellular water spaces and a reduction in total brain water volume. Unexpectedly, [K(+)](i) seems to fall during the first postnatal week, which should reduce [K(+)](i)/ [K(+)](e) and result in a lower V(m), consistent with experimental observations. Neuronal [Cl(-)](i) is high during early postnatal development, hence the opening of Cl(-) conduction pathways may lead to plasma membrane depolarization. Equivalent loss of K(+)(i) into a relatively large extracellular space leads to a smaller increase in [K(+)](e) in immature animals, while the larger reservoir of Ca(2+)(e) may result in a greater [Ca(2+)](i) rise. In vivo and in vitro studies show that compared with adult, developing brains are more resistant to hypoxic/ischemic ion leakage: increases in [K(+)](e) and decreases in [Ca(2+)](e) are slower and smaller, consistent with the known low level of energy utilization and better maintenance of [ATP]. Severe hypoxia/ischemia may, however, lead to large Ca(2+)(i) overload. Rises in [K(+)](e) during epileptogenesis in vivo are smaller and take longer to manifest themselves in immature brains, although the rate of K(+) clearance is slower. By contrast, in vitro studies suggest the existence of a period of enhanced vulnerability sometime during the developmental period. This chapter concludes that there is a great need for more information on ion changes during ontogeny and poses the question whether the rat is the most appropriate model for investigation of mechanisms of pathological changes in human neonates.

Expression of signal transduction genes differs after hypoxic or isoflurane preconditioning of rat hippocampal slice cultures

BACKGROUND

Preconditioning neurons with noninjurious hypoxia (hypoxic preconditioning, HPC) or the anesthetic isoflurane (APC) induces tolerance of severe ischemic stress. The mechanisms of both types of preconditioning in the hippocampus require moderate increases in intracellular Ca and activation of protein kinase signaling. The authors hypothesized that the expression of signal transduction genes would be similar after APC and HPC.

METHODS

Hippocampal slice cultures prepared from 9-day-old rats were preconditioned with hypoxia (5 min of 95% nitrogen/5% carbon dioxide) or 1% isoflurane in air/5% carbon dioxide for 1 h. A day later, cultures were subjected to 10 min oxygen and glucose deprivation (simulated ischemia). Intracellular Ca, measured in CA1 neurons at the completion of preconditioning, and cell death in CA1, CA3, and dentate regions was assessed 48 h after simulated ischemia. Message RNA encoding 119 signal transduction genes was quantified with rat complimentary DNA microarrays from pre-oxygen-glucose deprivation samples.

RESULTS

Both APC and HPC increased intracellular Ca approximately 50 nm and decreased CA1, CA3, and dentate neuron death by about 50% after simulated ischemia. Many signaling genes were increased after preconditioning, with hypoxia increasing more apoptosis/survival genes (8 of 10) than isoflurane (0 of 10). In contrast, isoflurane increased more cell cycle/development/growth genes than did hypoxia (8 of 14 genes, vs. 1 of 14).

CONCLUSIONS

Despite sharing similar upstream signaling and neuroprotective outcomes, the genomic response to APC and HPC is different. Increased expression of antiapoptosis genes after HPC and cell development genes after APC has implications both for neuroprotection and long-term effects of anesthetics.

Oxygen and glucose deprivation in an organotypic hippocampal slice model of the developing rat brain: the effects on N-methyl-D-aspartate subunit composition

BACKGROUND

Organotypic hippocampal slices (OHS) are commonly used to screen for neuroprotective effects of pharmacological agents relevant to pediatric brain injury. The importance of donor rat pup age and N-methyl-D-aspartate (NMDA) receptor subunit composition have not been addressed. In this study, we evaluated the age-dependent effect of oxygen-glucose deprivation (OGD) in the developing rat brain and determined whether OGD modulates the NMDA receptor subunit composition.

METHODS

OHS were prepared from rat pups on postnatal days (PND) 4, 7, 14, and 21 and cultured 7 days in vitro. The slices were exposed to OGD for durations of 5-60 min. After 24 and 72 h, OHS survival and NMDA subunit composition were assessed.

RESULTS

Cell death was evident in OHS prepared from PND 14 and 21 rat pups (P < 0.001) with OGD durations of 5 and 10 min, respectively. In OHS prepared from PND7 rat pups, neurodegeneration was not evident until 20 min OGD (P < 0.001). Exposure to OGD in OHS prepared from PND4 and PND7 rat pups was associated with a transition in the NMDA receptor subunit composition from NR2B predominant to NR2A predominant subunit composition.

CONCLUSIONS

This in vitro neonatal rat pup investigation using OHS supports both an age and an NMDA receptor subunit composition-dependent relationship between OGD and neuronal cell death.

Inositol 1,4,5-triphosphate receptors and NAD(P)H mediate Ca2+ signaling required for hypoxic preconditioning of hippocampal neurons

Exposure of neurons to a non-lethal hypoxic stress greatly reduces cell death during subsequent severe ischemia (hypoxic preconditioning, HPC). In organotypic cultures of rat hippocampus, we demonstrate that HPC requires inositol triphosphate (IP3) receptor-dependent Ca2+ release from the endoplasmic reticulum (ER) triggered by increased cytosolic NAD(P)H. Ca2+ chelation with intracellular BAPTA, ER Ca2+ store depletion with thapsigargin, IP3 receptor block with xestospongin, and RNA interference against subtype 1 of the IP3 receptor all blunted the moderate increases in [Ca2+](i) (50-100 nM) required for tolerance induction. Increases in [Ca2+](i) during HPC and neuroprotection following HPC were not prevented with NMDA receptor block or by removing Ca2+ from the bathing medium. Increased NAD(P)H fluorescence in CA1 neurons during hypoxia and demonstration that NADH manipulation increases [Ca2+](i) in an IP3R-dependent manner revealed a primary role of cellular redox state in liberation of Ca2+ from the ER. Blockade of IP3Rs and intracellular Ca2+ chelation prevented phosphorylation of known HPC signaling targets, including MAPK p42/44 (ERK), protein kinase B (Akt) and CREB. We conclude that the endoplasmic reticulum, acting via redox/NADH-dependent intracellular Ca2+ store release, is an important mediator of the neuroprotective response to hypoxic stress.

Superoxide (·O2−) Production in CA1 Neurons of Rat Hippocampal Slices Exposed to Graded Levels of Oxygen

Neuronal signaling, plasticity, and pathologies in CA1 hippocampal neurons are all intimately related to the redox environment and, thus tissue oxygenation. This study tests the hypothesis that hyperoxic superfusate (95% O2) causes a time-dependent increase in superoxide anion (·O2−) production in CA1 neurons in slices, which will decrease as oxygen concentration is decreased. Hippocampal slices (400 μm) from weaned rats were incubated with the fluorescent probe dihydroethidium (DHE), which detects intracellular ·O2− production. Slices were loaded for 30 min using 10 μM DHE and maintained using one-sided superfusion or continuously loaded using 2.5 μM DHE and maintained using two-sided superfusion (36°C). Continuous loading of DHE and two-sided superfusion gave the highest temporal resolution measurements of ·O2− production, which was estimated by the increase in fluorescence intensity units (FIUs) per minute (FIU/min ± SE) over 4 h. Superoxide production (2.5 μM DHE, 2-sided superfusion) was greatest in 95% O2 (6.6 ± 0.4 FIU/min) and decreased significantly during co-exposure with antioxidants (100 μM melatonin, 25 μM MnTMPyP) and lower levels of O2 (60, 40, and 20% O2 at 5.3 ± 0.3, 3.3 ± 0.1, and 1.6 ± 0.2 FIU/min, respectively). CA1 cell death after 4 h (ethidium homodimer-1 staining) was greatest in 95% O2 and lowest in 40 and 20% O2. CA1 neurons generated evoked action potentials in 20% O2 for >4 h, indicating viability at lower levels of oxygenation. We conclude that ·O2− production and cell death in CA1 neurons increases in response to increasing oxygen concentration product (= PO2 × time). Additionally, lower levels of oxygen (20–40%) and antioxidants should be considered to minimize superoxide-induced oxidative stress in brain slices.

Neonatal Seizures: Gaps Between the Laboratory and the Clinic

Seizures in neonates (NBs) remain the most frequent neurological problem in the nursery. Considerable debate about their consequences exists between data and deductions reached through animal experimentations and those obtained through clinical investigations. The main conflicting issues are whether seizures in NBs can plant the roots for epileptogenesis and cause long-term deficits. The purpose of this chapter is to evaluate both laboratory and clinical results.

METHODS

Clinical data will be presented, including a 20-year-long cohort of NBs. This will be followed by the main seminal discoveries obtained in neonatal models. The phenomenon of transient or persistent dysmaturity following NB seizures will be discussed in relation to etiological factors.

RESULTS

The findings and deductions from animal models support the notions that epileptogenesis and cognitive deficits result from NB seizures. These conclusions contrast with clinical investigations maintaining that NB seizures, per se, are symptomatic markers of preexisting or of ongoing morbidities. The reasons for contrasting views will be discussed. Suggestions will be advanced for more animal models whose seizures are consistent with the etiologies and the phenotypes of human NB seizures.

Drosophila dMRP4 regulates responsiveness to O2 deprivation and development under hypoxia

For most vertebrates, oxygen is a prerequisite for survival. Although we have previously shown that Drosophila melanogaster is hypoxia tolerant, how this species senses O(2) deprivation and how it survives oxygen-limiting conditions are as yet poorly understood. We began to address this question by testing for anoxic responsiveness in Drosophila adult flies following overexpression of existing EP lines. In this screen, we identified Drosophila CG14709 gene as a homolog of the human multidrug resistance protein 4 (MRP4/ABCC4) that is tightly regulated to oxygen. Ubiquitous expression of dMRP4 in adult flies resulted in increased sensitivity to anoxia as they had longer recovery time from anoxic stupor. When exposed to 4% oxygen chronically (throughout its lifespan), constitutive expression of dMRP4 in larvae caused larval lethality due to growth arrest. Mutations of dMRP4 led to a hypersensitive response to acute anoxia in adult flies but had less impact on larval survival under chronic hypoxia compared with dMRP4 overexpression. Selective expression of this gene in neurons, but not in glia or muscles, mirrored the same phenotype as the ubiquitous one. Thus, our data suggest novel roles for MRP in vivo: 1) dMRP4 regulates the sensitivity to acute or chronic O(2) deprivation, and 2) dMRP expression in neurons is sufficient to induce the sensitivity to O(2) in the whole organism.

GABAergic protection of hippocampal pyramidal neurons against glutamate insult: deficit in young animals compared to adults

Hypoxia-ischemia (HI) injury in neonatal animals leads to selective regional loss of neurons including CA1 and CA3 pyramidal neurons of the hippocampus as well as nonlethal pathologies. Glutamate-receptor over-activation and Ca2+ influx are involved in these neonatal changes. We examined glutamate-evoked Ca2+ responses in neonatal (PN 7-13) and young adult (PN 21-27) CA1 pyramidal neurons in acute slices from rats. In neonates, transient exposure to glutamate produced large Ca2+ increases throughout neurons, including distal dendrites (primary Ca2+ responses). Repeated exposures resulted in sustained Ca2+ increases in apical dendrites (secondary Ca2+ responses) that were independent of continued glutamate exposure. These responses propagated and invaded the soma, resulting in irrecoverably high Ca2+ elevations. In neurons from adults, identical glutamate exposure evoked primary Ca2+ responses only in somata and proximal apical dendrites. Repeated glutamate exposures in the adult neurons also led to secondary Ca2+ responses, but they arose in the peri-somatic region and then spread outward to distal apical dendrites, again without recovery. More stimuli were required to initiate secondary responses in neurons from adult versus neonates. Block of GABAA receptors in adults caused the primary and secondary responses to revert to the spatial pattern seen in the neonates and greatly increased their vulnerability to glutamate. These findings suggest that neurodegenerative secondary Ca2+ events may be important determinants of susceptibility to HI injury in the developing CNS and that immature CA1 neurons may be more susceptible to excitotoxic injury due at least in part to insufficient development of GABAergic inputs to their dendrites.

Adaptive responses of vertebrate neurons to anoxia--matching supply to demand

Oxygen depleted environments are relatively common on earth and represent both a challenge and an opportunity to organisms that survive there. A commonly observed survival strategy to this kind of stress is a lowering of metabolic rate or metabolic depression. Whether metabolic rate is at a normal or a depressed level the supply of ATP (glycolysis and oxidative phosphorylation) must match the cellular demand for ATP (protein synthesis and ion pumping), a condition that must of course be met for long-term survival in hypoxic and anoxic environments. Underlying a decrease in metabolic rate is a corresponding decrease in both ATP supply and ATP demand pathways setting a new lower level for ATP turnover. Both sides of this equation can be actively regulated by second messenger pathways but it is less clear if they are regulated differentially or even sequentially with the onset of anoxia. The vertebrate brain is extremely sensitive to low oxygen levels yet some species can survive in oxygen depleted environments for extended periods and offer a working model of brain survival without oxygen. Hypoxia tolerant vertebrate brain will be the primary focus of this review; however, we will draw upon research involving hypoxia/ischemia tolerance mechanisms in liver and heart to offer clues to how brain can tolerate anoxia. The issue of regulating ATP supply or demand pathways will also be addressed with a focus on ion channel arrest being a significant mechanism to reduce ATP demand and therefore metabolic rate. Furthermore, mitochondria are ideally situated to serve as cellular oxygen sensors and mediator of protective mechanisms such as ion channel arrest. Therefore, we will also describe a mitochondria based mechanism of ion channel arrest involving ATP-sensitive mitochondrial K(+) channels, cytosolic calcium and reaction oxygen species concentrations.

Neuroprotective effect of the stearic acid against oxidative stress via phosphatidylinositol 3-kinase pathway

Stearic acid is a long-chain saturated fatty acid consisting of 18 carbon atoms without double bonds. In the present study, we reported the neuroprotective effects and mechanism of stearic acid on cortical or hippocampal slices insulted by oxygen-glucose deprivation, NMDA or hydrogen peroxide (H(2)O(2)) in vitro. Different types of models of brain slice injury in vitro were developed by 10 min of oxygen/glucose deprivation, 0.5 mM NMDA or 2 mM H(2)O(2), respectively. After 30 min of preincubation with stearic acid (3-30 microM), cortical or hippocampal slices were subjected to oxygen-glucose deprivation, NMDA or H(2)O(2). Then the tissue activities were evaluated by using the 2,3,5-triphenyltetrazolium chloride (TTC) method. Population spikes were recorded in randomly selected hippocampal slices. Stearic acid (3-30 microM) dose-dependently protected brain slices from oxygen-glucose deprivation, NMDA and H(2)O(2) insults. Its neuroprotective effect against H(2)O(2) insults can be completely blocked by wortmannin (inhibitor of PI3K) and partially blocked by H7 (inhibitor of PKC) or genistein (inhibitor of TPK). Treatment of cortical or hippocampal slices with 30 microM stearic acid resulted in a significant increase in PI3K activity at 5, 10, 30 and 60 min. These observations reveal that stearic acid can protect cortical or hippocampal slices against injury induced by oxygen-glucose deprivation, NMDA or H(2)O(2), and its neuroprotective effects are via phosphatidylinositol 3-kinase dependent mechanism.

Development of the respiratory response to hypoxia in the isolated brainstem of the bullfrog Rana catesbeiana

The aim of this study was to examine the effects of cellular hypoxia, and the contribution of anaerobic metabolism, on respiratory activity in bullfrogs at different stages of development. Respiratory-related neural activity was recorded from cranial nerve rootlets in isolated brainstem preparations from pre-metamorphic (Taylor-Kollros (T-K) stages VIII-XVI) and postmetamorphic tadpoles (T-K stages XXIV-XXV) and adults. Changes in fictive gill/lung activity in brainstems from pre-metamorphic tadpoles and lung activity in postmetamorphic tadpoles and adults were examined during superfusion with control (98% O(2)/2% CO(2)) or hypoxic (98% N(2)/2% CO(2)) artificial cerebrospinal fluid (aCSF). Iodoacetate (IAA; 100 micromol l(-1)) was used in conjunction with hypoxic aCSF to inhibit glycolysis. Gill burst frequency in pre-metamorphic brainstems did not change over a 3 h exposure to hypoxia and fictive lung burst frequency slowed significantly, but only after 3 h hypoxia. Blockade of glycolysis with IAA during hypoxia significantly reduced the time respiratory activity could be maintained in pre-metamorphic, but not in adult, brainstems. In brainstems from post-metamorphic tadpoles and adults, lung burst frequency became significantly more episodic within 5-15 min hypoxic exposure, but respiratory neural activity was subsequently abolished in every preparation. The cessation of fictive breathing was restored to control levels upon reoxygenation. Neither tadpole nor adult brainstems exhibited changes in neural bursts resembling 'gasping' that is observed in mammalian brainstems exposed to severe hypoxia. There was also a significant increase in the frequency of 'non-respiratory' bursts in hypoxic postmetamorphic and adult brainstems, but not in pre-metamorphic brainstems. These results indicate that pre-metamorphic tadpoles are capable of maintaining respiratory activity for 3 h or more during severe hypoxia and rely to a great extent upon anaerobic metabolism to maintain respiratory motor output. Upon metamorphosis, however, hypoxia results in significant changes in respiratory frequency and pattern, including increased lung burst episodes, non-ventilatory bursts and a reversible cessation of respiratory activity. Adults have little or no ability to maintain respiratory activity through glycolysis but, instead, stop respiratory activity until oxygen is available. This 'switch' in the respiratory response to hypoxia coincides morphologically with the loss of gills and obligate air-breathing in the postmetamorphic frog. We hypothesize that the cessation of respiratory activity in post-metamorphic tadpoles and adults is an adaptive, energy-saving response to low oxygen.

Intracellular calcium and survival of tadpole forebrain cells in anoxia

The frog brain survives hypoxia with a slow loss of energy charge and ion homeostasis. Because hypoxic death in most neurons is associated with increases in intracellular calcium ([Ca2+]i), we examined the relationship between [Ca2+]i and survival of a mixed population of isolated cells from the forebrain of North American bullfrog Rana catesbeiana tadpoles. Forebrain cells from stage V-XV tadpoles were isolated by enzymatic digestion and loaded with one of three different calcium indicators (Fura-2, Fura 2-FF and BTC) to provide estimates of [Ca2+]i accurate at low and high[Ca2+]i. Propidium iodide (PI) fluorescence was used as an indicator of cell viability. Cells were exposed to anoxia (100%N2) and measurements of [Ca2+]i and cell survival made from 1 h to 18 h. Intracellular [Ca2+] increased significantly after 3-6 h anoxia (P&lt;0.05), regardless of the type of Ca2+ indicator used; however, there were substantial differences in the measurements of [Ca2+]i with the different indicators, reflecting their varying affinities for Ca2+. Resting[Ca2+]i was approximately 50 nmol l-1 and increased to about 9-30 μmol l-1 after 4-6 h anoxia. The significant increase in [Ca2+]i during anoxia was not associated with significant increases in cell death, with 85-95% survival over this time period. Cells exposed to anoxia for 18 h, or those made anoxic for 4-6 and reoxygenated for 12 h to 16 h, had survival rates greater than 70%,but survival was significantly less than normoxic controls. These results indicate that large increases in [Ca2+]i are not necessarily associated with hypoxic cell death in vertebrate brain cells.

Mild hypothermia, but not propofol, is neuroprotective in organotypic hippocampal cultures

The neuroprotective potency of anesthetics such as propofol compared to mild hypothermia remains undefined. Therefore, we determined whether propofol at two clinically relevant concentrations is as effective as mild hypothermia in preventing delayed neuron death in hippocampal slice cultures (HSC). Survival of neurons was assessed 2 and 3 days after 1 h oxygen and glucose deprivation (OGD) either at 37 degrees C (with or without 10 or 100 microM propofol) or at an average temperature of 35 degrees C during OGD (mild hypothermia). Cell death in CA1, CA3, and dentate neurons in each slice was measured with propidium iodide fluorescence. Mild hypothermia eliminated death in CA1, CA3, and dentate neurons but propofol protected dentate neurons only at a concentration of 10 microM; the more ischemia vulnerable CA1 and CA3 neurons were not protected by either 10 microM or 100 microM propofol. In slice cultures, the toxicity of 100 muM N-methyl-D-aspartate (NMDA), 500 microM glutamate, and 20 microM alpha-amino-5-methyl-4-isoxazole propionic acid (AMPA) was not reduced by 100 microM propofol. Because propofol neuroprotection may involve gamma-aminobutyric acid (GABA)-mediated indirect inhibition of glutamate receptors (GluRs), the effects of propofol on GluR activity (calcium influx induced by GluR agonists) were studied in CA1 neurons in HSC, in isolated CA1 neurons, and in cortical brain slices. Propofol (100 and 200 microM, approximate burst suppression concentrations) decreased glutamate-mediated [Ca2+]i increases (Delta[Ca2+]i) responses by 25%-35% in isolated CA1 neurons and reduced glutamate and NMDA Delta[Ca2+]i in acute and cultured hippocampal slices by 35%-50%. In both CA1 neurons and cortical slices, blocking GABAA receptors with picrotoxin reduced the inhibition of GluRs substantially. We conclude that mild hypothermia, but not propofol, protects CA1 and CA3 neurons in hippocampal slice cultures subjected to oxygen and glucose deprivation. Propofol was not neuroprotective at concentrations that reduce glutamate and NMDA receptor responses in cortical and hippocampal neurons.

Clinical perspectives: neuroprotection lessons from hypoxia-tolerant organisms

An effective treatment for brain ischemia is a pressing medical need. Research on brain ischemia has largely focused on understanding the mechanisms of neuron death as a way of identifying targets for therapy. An attractive alternative approach is to identify the survival strategies of hypoxia-tolerant neurons. The adaptation of vertebrate neurons to hypoxia occurs in at least three major ways: (1) as a constitutive property of neurons in anoxia-tolerant turtles and fish, (2) as a property of intra-uterine and early post-natal mammalian development, and (3) as part of a slower, chronic process, as in acclimitization to high altitude. Research on hypoxia-tolerant neurons has already revised several earlier concepts, including the role of calcium in cell death and survival, and the value of N-methyl-d-aspartate (NMDA) receptor antagonism. A broad and fundamental understanding of how neurons adapt to hypoxia is likely to help guide efforts to find new treatments for brain hypoxia and ischemia.

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.

Hypoxia increases calcium flux through cortical neuron glutamate receptors via protein kinase C

The effects of 30 s to 10 min hypoxia (PO2-10 mmHg) on glutamate receptor activity were studied in murine cortical neurons. Receptor activity was assessed as a rise in intracellular calcium concentration ([Ca2+]i) following a 10 s application of 1 mm glutamate or 100 micro mN-methy-d-aspartate (NMDA) in the presence of 0.1 mm Mg2+ and 10 micro m glycine. Change in [Ca2+]i elicited by glutamate increased 26% (n = 192, p < 0.001) and that to NMDA by 74% (n = 9, p < 0.01) during a 100-s period of hypoxia. After 10 min hypoxia, responses to glutamate were 62% smaller than those in normoxia, with increased basal intracellular [Ca2+]i predicting reduced receptor activity. When neurons were exposed to NMDA after 10 min of hypoxia, [Ca2+]i increases were 12% smaller than after 100 s hypoxia, but still 53% larger than in oxygenated neurons (n = 9, p = 0.01). Neurons expressed relatively similar amounts of NR2A, -B, -C, and -D subunits. The phosphorylation of NMDA NR1 subunits increased during hypoxia. Pre-treatment of neurons with a protein kinase C (PKC) inhibitor (chelerythrine, 10 micro m) prevented increases in N-methy-d-aspartate receptor (NMDAR) activity during hypoxia and reduced the phosphorylation of NR1 subunits. These results suggest that enhancement of glutamate receptor activity during the first minutes of hypoxia is mediated by phosphorylation of NMDARs by PKC and that other mechanisms, possibly involving intracellular calcium, limit glutamate receptor-mediated calcium influx during longer periods of hypoxia.

Moderate increases in intracellular calcium activate neuroprotective signals in hippocampal neurons

Although large increases in neuronal intracellular calcium concentrations ([Ca(2+)](i)) are lethal, moderate increases in [Ca(2+)](i) of 50-200 nM may induce immediate or long-term tolerance of ischemia or other stresses. In neurons in rat hippocampal slice cultures, we determined the relationship between [Ca(2+)](i), cell death, and Ca(2+)-dependent neuroprotective signals before and after a 45 min period of oxygen and glucose deprivation (OGD). Thirty minutes before OGD, [Ca(2+)](i) was increased in CA1 neurons by 40-200 nM with 1 nM-1 microM of a Ca(2+)-selective ionophore (calcimycin or ionomycin-"Ca(2+) preconditioning"). Ca(2+) preconditioning greatly reduced cell death in CA1, CA3 and dentate during the following 7 days, even though [Ca(2+)](i) was similar (approximately 2 microM) in preconditioned and control neurons 1 h after the OGD. When pre-OGD [Ca(2+)](i) was lowered to 25 nM (10 nM ionophore in Ca(2+)-free medium) or increased to 8 microM (10 microM ionophore), more than 90% of neurons died. Increased levels of the anti-apoptotic protein protein kinase B (Akt) and the MAP kinase ERK (p42/44) were present in preconditioned slices after OGD. Reducing Ca(2+) influx, inhibiting calmodulin, and preventing Akt or MAP kinase p42/44 upregulation prevented Ca(2+) preconditioning, supporting a specific role for Ca(2+) in the neuroprotective process. Further, in continuously oxygenated cultured hippocampal/cortical neurons, preconditioning for 30 min with 10 nM ionomycin reduced cell death following a 4 microM increase in [Ca(2+)](i) elicited by 1 microM ionomycin. Thus, a zone of moderately increased [Ca(2+)](i) before a potentially lethal insult promotes cell survival, uncoupling subsequent large increases in [Ca(2+)](i) from initiating cell death processes.

Oxygen sensitivity of NMDA receptors: relationship to NR2 subunit composition and hypoxia tolerance of neonatal neurons

Neonatal rats survive and avoid brain injury during periods of anoxia 25 times longer than adults. We hypothesized that oxygen activates and hypoxia suppresses NMDA receptor (NMDAR) responses in neonatal rat neurons, explaining the innate hypoxia tolerance of these cells. In CA1 neurons isolated from neonatal rat hippocampus (mean postnatal age [P] 5.8 days), hypoxia (PO(2) 10 mm Hg) reduced NMDA receptor-channel open-time percentage and NMDA-induced increase in [Ca(2+)](i) (NMDA DeltaCa(2+)) by 38 and 68% (P<0.01), respectively. In P20 neurons the reductions were not significant. In P3-10 CA1 neurons within intact hippocampal slices, hypoxia reduced NMDA DeltaCa(2+) by 52% (P=0.002) and decreased NMDA-induced death by 45% (P=0.004). Phalloidin, a microtubule stabilizer, prevented hypoxia-induced inhibition of NMDA DeltaCa(2+) in P3-10 neurons. To test whether NMDARs prevalent in neonates (NR1 plus NR2B or NR2D subunits) are inhibited by hypoxia compared with those in mature neurons (NR2A and NR2C), we expressed these receptors in Xenopus oocytes. Compared with responses in 21% O(2), hypoxia (PO(2) 17 mm Hg) reduced currents from neonatal type NR1/NR2D receptors by 25%, increased currents from NR1/NR2C by 18%, and had no effect on NR1/NR2A or NR1/NR2B. Modulation of NMDARs by hypoxia may play an important role in the hypoxia tolerance of the mammalian neonate. In addition, oxygen sensing by NMDARs could play a significant role in postnatal brain development.

Isoflurane decreases AMPA-induced dark cell degeneration and edematous damage of Purkinje neurons in the rat cerebellar slices

This study was designed to investigate whether isoflurane, a commonly used volatile anesthetic with neuroprotective property, reduces alpha-amino-3-hydroxy-5-methyl-4-isoxazol propionic acid (AMPA)-induced neurotoxicity in a concentration- and time (when isoflurane was applied in relation to the AMPA exposure)-dependent manner. Cerebellar slices from postnatal 10-14-day-old rats were exposed to 30 microM AMPA for 30 min followed by a 120-min AMPA-free recovery period at 37 degrees C. This protocol resulted in dark cell degeneration (DCD) in the majority of Purkinje neurons (60.8+/-6.9%). Fewer Purkinje neurons (31.6+/-5.2%) had edematous damage (ED) characters. Application of isoflurane (1, 2, or 3%) during both the AMPA exposure and recovery periods significantly increased the percentage of morphologically normal Purkinje neurons (neurons without DCD or ED changes) but the effects were apparently not dose-dependent. Isoflurane (3%) applied before, during or after the AMPA exposure period also significantly increased the percentage of morphologically normal Purkinje neurons. These isoflurane-induced increases in the percentage of morphologically normal Purkinje neurons were mainly due to fewer cells with DCD changes. Isoflurane decreased AMPA-induced ED significantly only when isoflurane (1 or 2%) was present during both the AMPA exposure and recovery periods. Isoflurane applied before, during or after the AMPA exposure period did not significantly affect the percentage of cells with ED changes. These results suggest that isoflurane time-dependently but not concentration-dependently reduce AMPA-induced neurotoxicity. These effects may be one mechanism for the isoflurane-induced neuroprotection demonstrated in previous studies.

Adaptive responses of vertebrate neurons to hypoxia

The damage caused to mammalian neurons during ischaemic events in the brain(e.g. following a stroke), is an area of major interest to neuroscientists. The neurons of hypoxia-tolerant vertebrates offer unique models for identifying new strategies to enhance the survival of hypoxia-vulnerable neurons. In this review, we describe recent advances in our understanding of how hypoxia-tolerant neurons detect decreases in oxygen and create signals that have immediate and long-term effects on cell function and survival. Sensing and adapting to low oxygen tension involves numerous modalities with different times of activation and effect. Sensors include membrane proteins such as ionotropic ion channels, membrane or cytosolic heme proteins,mitochondrial proteins and/or oxygen sensitive transcription factors such as HIF-1α and NFκB. Signaling molecules involved in O2sensing include mitogen-activated protein kinases, ions such as Ca2+ and metabolites such as adenosine. These signals act rapidly to reduce the conductance of ion channels (ion flux arrest) and production of energy (metabolic arrest), and slowly to activate specific genes. The ability to construct an energy budget, illustrating which physiological processes are depressed during both long-term and acute metabolic suppression in hypoxia-tolerant neurons, would be of significant value in devising new strategies for neuroprotection. Additionally it is not known how metabolism is regulated at `pilot-light' levels at which energy-producing and energy-consuming processes are balanced. The regulation of organelle and cell fate during long-term hypoxia is almost completely unexplored, and whether programmed cell death and regeneration of lost neurons occur following protracted dormancy is also of considerable interest.

Neuroprotection and intracellular Ca2+ modulation with fructose-1,6-bisphosphate during in vitro hypoxia-ischemia involves phospholipase C-dependent signaling

The neuroprotectant fructose-1,6-bisphosphate (FBP) preserves cellular [ATP] and prevents catastrophic increases in [Ca2+]i during hypoxia. Because FBP does not enter neurons or glia, the mechanism of protection is not clear. In this study, we show that FBP's capacity to protect neurons and stabilize [Ca2+]i during hypoxia derives from signaling by a phospholipase-C-intracellular Ca2+-protein kinases pathway, rather than Ca2+ chelation or glutamate receptor inhibition. FBP reduced [Ca2+]i changes in hypoxic hippocampal neurons, regardless of [Ca2+]e, and preserved cellular integrity as measured by trypan blue or propidium iodide exclusion and [ATP]. FBP also prevented hypoxia-induced increases in [Ca2+]i when glucose was absent and when [Ca2+]e was increased to negate Ca2+ chelation by FBP. These protective effects were observed equally in postnatal day 2 (P2) and P16 neurons. Inhibiting glycolysis with iodoacetate eliminated the protective effects of FBP in P16 neurons. FBP did not alter Ca2+ influx stimulated by brief applications of NMDA or glutamate during normoxia or hypoxia, but did reduce the increase in [Ca2+]i produced by 10 min of glutamate exposure during hypoxia. Because FBP increases basal [Ca2+]i and stimulates membrane lipid hydrolysis, we tested whether FBP's protective action was dependent on phospholipase C signaling. The phospholipase C inhibitor U73122 prevented FBP-induced increases in [Ca2+]i and eliminated FBP's ability to stabilize [Ca2+]i and increase survival during anoxia. Similarly, FBP's protection was eliminated in the presence of the mitogen/extracellular signal protein kinase (MEK) inhibitor U0126. We conclude that FBP may produce neuroprotection via activation of neuroprotective signaling pathways that modulate Ca2+ homeostasis.

Effects of neuroprotective cocktails on hippocampal neuron death in an in vitro model of cerebral ischemia

Cocktails of neuroprotectants acting at different parts of the ischemic injury cascade may have advantages over single agents. This study investigated, singly and in combination, the neuroprotective efficacy of an energy substrate (3.5 mM fructose 1,6-bisphosphate, FBP), an antagonist of NMDA receptors (1 and 10 microM MK-801), a free-radical scavenger (100 microM ascorbate), an adenosine A1 receptor agonist (10 microM 2-chloroadenosine), and an inhibitor of neurotransmission (2% isoflurane). These agents were evaluated for their ability to prevent loss and morphologic damage of CA1 neurons in rat hippocampal slices when these agents were administered during 30 minutes in vitro ischemia (combined oxygen/glucose deprivation at 37 degrees C) followed by 5 hours of recovery. Ten microM MK-801, alone or in combination with the other compounds, prevented loss of CA1 neurons and preserved their histologic appearance. Isoflurane, which prevents glutamate receptor-dependent cell death in this model, was also protective. Protection against neuron loss was also found when a subtherapeutic concentration of MK-801 (1 microM) was combined with 2-chloroadenosine (which indirectly causes NMDA receptor suppression), but not FBP or ascorbate. The authors conclude that in this model, the strategy of antagonizing NMDA receptors appears more protective than fructose-1,6-bisphosphate, 2-chloroadenosine or ascorbate.

The effect of CNTF on glutamate-induced increases in intracellular free Ca2+ in hippocampal neurons

Ciliary neurotrophic factor (CNTF) acts through the JAK/STAT signal transduction pathway. However, the rapid action of CNTF cannot readily be explained by reference to this pathway. Using the fluorophore, Fura 2-AM, and fluorescence imaging, the effect of CNTF on glutamate-induced increases in hippocampal intraneuronal free Ca2+ ([Ca2+]i) was investigated. Glutamate induces a rapid increase in [Ca2+]i. Incubation of hippocampal neurons with CNTF for 5 min inhibited the glutamate-induced increase in [Ca2+]i. In the absence of glutamate, CNTF had no effect on [Ca2+]i. Pertussis toxin (PTX), a G-protein antagonist, partially blocked the effect of CNTF. This suggests that CNTF may act via an alternative signal transduction pathway besides the generic JAK/STAT pathway.

Hypoxia-induced silencing of NMDA receptors in turtle neurons

Hypoxia-induced suppression of NMDA receptors (NMDARs) in western painted turtle (Chrysemys picta) cortical neurons may be critical for surviving months of anoxic dormancy. We report that NMDARs are silenced by at least three different mechanisms operating at different times during anoxia. In pyramidal neurons from cerebrocortex, 1-8 min anoxia suppressed NMDAR activity (Ca(2+) influx and open probability) by 50-60%. This rapid decrease in receptor activity was controlled by activation of phosphatase 1 or 2A but was not associated with an increase in [Ca(2+)](i). However, during 2 hr of anoxia, [Ca(2+)](i) in cerebrocortical neurons increased by 35%, and suppression of NMDARs was predicted by the increase of [Ca(2+)](i) and controlled by calmodulin. An additional mechanism of NMDAR silencing, reversible removal of receptors from the cell membrane, was found in cerebrocortex of turtles remaining anoxic at 3 degrees C for 3-21 d. When suppression of NMDARs was prevented with phosphatase inhibitors, tolerance of anoxia was lost. Silencing of NMDARs is thus critical to the remarkable ability of C. picta to tolerate life without oxygen.

Ca(2+)- and metabolism-related changes of mitochondrial potential in voltage-clamped CA1 pyramidal neurons in situ

In hippocampal slices from rats, dialysis with rhodamine-123 (Rh-123) and/or fura-2 via the patch electrode allowed monitoring of mitochondrial potential (DeltaPsi) changes and intracellular Ca(2+) ([Ca(2+)](i)) of CA1 pyramidal neurons. Plasmalemmal depolarization to 0 mV caused a mean [Ca(2+)](i) rise of 300 nM and increased Rh-123 fluorescence signal (RFS) by </=50% of control. The evoked RFS, indicating depolarization of DeltaPsi, and the [Ca(2+)](i) transient were abolished by Ca(2+)-free superfusate or exposure of Ni(2+)/Cd(2+). Simultaneous measurements of RFS and [Ca(2+)](i) showed that the kinetics of both the Ca(2+) rise and recovery were considerably faster than those of the DeltaPsi depolarization. The plasmalemmal Ca(2+)/H(+) pump blocker eosin-B potentiated the peak of the depolarization-induced RFS and delayed recovery of both the RFS and [Ca(2+)](i) transient. Thus the DeltaPsi depolarization due to plasmalemmal depolarization is related to mitochondrial Ca(2+) sequestration secondary to Ca(2+) influx through voltage-gated Ca(2+) channels. CN(-) elevated [Ca(2+)](i) by <50 nM but increased RFS by 221% as a result of extensive depolarization of DeltaPsi. Oligomycin decreased RFS by 52% without affecting [Ca(2+)](i). In the presence of oligomycin, CN(-) and p-trifluoromethoxy-phenylhydrazone (FCCP) elevated [Ca(2+)](i) by <50 nM and increased RFS by 285 and 290%, respectively. Accordingly, the metabolism-related DeltaPsi changes are independent of [Ca(2+)](i). Imaging techniques revealed that evoked [Ca(2+)](i) rises are distributed uniformly over the soma and primary dendrites, whereas corresponding changes in RFS occur more localized in subregions within the soma. The results show that microfluorometric measurement of the relation between mitochondrial function and intracellular Ca(2+) is feasible in whole cell recorded mammalian neurons in situ.

Modulation of N-type Ca2+ channels by intracellular pH in chick sensory neurons

Both physiological and pathological neuronal events, many of which elevate intracellular [Ca2+], can produce changes in intracellular pH of between 0.15 and 0.5 U, between pH 7.4 and 6.8. N-type Ca2+ channels, which are intimately involved in exocytosis and other excitable cell processes, are sensitive to intracellular pH changes. However, the pH range over which N-type Ca2+ channels are sensitive, and the sensitivity of N-type Ca2+ channels to small changes in intracellular pH, are unknown. We studied the influence of intracellular pH changes on N-type calcium channel currents in dorsal root ganglion neurons, acutely isolated from 14-day-old chick embryos. Intracellular pH was monitored in patch-clamp recordings with the fluorescent dye, BCECF, and manipulated in both the acidic and basic direction by extracellular application of NH4+ in the presence and absence of intracellular NH4+. Changes in intracellular pH between 6.6 and 7.5 produced a graded change in Ca2+ current magnitude with no apparent shift in activation potential. Intracellular acidification from pH 7.3 to 7.0 reversibly inhibited Ca2+ currents by 40%. Acidification from pH 7.3 to pH 6.6 reversibly inhibited Ca2+ currents by 65%. Alkalinization from pH 7.3 to 7.5 potentiated Ca2+ currents by approximately 40%. Channels were sensitive to pHi changes with high intracellular concentrations of the Ca2+ chelator, bis-(o-aminophenoxy)-N,N,N',N'-tetraacetic acid, which indicates that the effects of pHi did not involve a Ca2+-dependent mechanism. These data indicate that N-type Ca2+ channel currents are extremely sensitive to small changes in pHi in the range produced by both physiological and pathological events. Furthermore, these data suggest that modulation of N-type Ca2+ channels by pHi may play an important role in physiological processes that produce small changes in pHi and a protective role in pathological mechanisms that produce larger changes in pHi.

Energy metabolism and NAD-NADH redox state in brain slices in response to glutamate exposure and ischemia

A comparative study of the effects of excitotoxic levels of glutamate with ischemia on the cerebral energy metabolism and [NAD]/[NADH] ratio was carried out in adult rat brain slices. Glutamate moderately decreased the high energy phosphates and intracellular pH whereas ischemia showed a pronounced decrease in the high energy phosphates and intracellular pH. The [NAD]/[NADH] ratio increased continuously during glutamate exposure whereas an initial reduction and subsequent oxidation occurred during ischemia. Uptake of glutamate prevailed throughout the glutamate exposure to brain slices signifying favorable glial energy levels while efflux occurred during ischemia indicating complete neuronal and glial depolarization. A net synthesis of glutamate was also observed during ischemia. A small but significant increase in lactate may be a result of increased glycolysis during glutamate exposure, on the other hand a large increase in lactate during ischemia suggests a total failure of oxidative metabolism. Our results show that glutamate exposure to brain slices causes a mild energetic stress and an increase in [NAD]/[NADH] ratio whereas predominant inhibition of phosphate metabolites and dual effect on NAD/NADH redox state was observed during ischemia. It is suggested that the NAD/NADH redox state together with phosphate metabolites and intracellular pH of the periinfarct region could provide vital evidence about the possible involvement of glutamate.