Tolerance of low intracellular pH during hypercapnia by rat cortical brain slices: A 31P/1H NMR study

13C NMR metabolomic evaluation of immediate and delayed mild hypothermia in cerebrocortical slices after oxygen-glucose deprivation

BACKGROUND

Mild brain hypothermia (32°-34°C) after human neonatal asphyxia improves neurodevelopmental outcomes. Astrocytes but not neurons have pyruvate carboxylase and an acetate uptake transporter. C nuclear magnetic resonance spectroscopy of rodent brain extracts after administering [1-C]glucose and [1,2-C]acetate can distinguish metabolic differences between glia and neurons, and tricarboxylic acid cycle entry via pyruvate dehydrogenase and pyruvate carboxylase.

METHODS

Neonatal rat cerebrocortical slices receiving a C-acetate/glucose mixture underwent a 45-min asphyxia simulation via oxygen-glucose-deprivation followed by 6 h of recovery. Protocols in three groups of N=3 experiments were identical except for temperature management. The three temperature groups were: normothermia (37°C), hypothermia (32°C for 3.75 h beginning at oxygen--glucose deprivation start), and delayed hypothermia (32°C for 3.75 h, beginning 15 min after oxygen-glucose deprivation start). Multivariate analysis of nuclear magnetic resonance metabolite quantifications included principal component analyses and the L1-penalized regularized regression algorithm known as the least absolute shrinkage and selection operator.

RESULTS

The most significant metabolite difference (P<0.0056) was [2-C]glutamine's higher final/control ratio for the hypothermia group (1.75±0.12) compared with ratios for the delayed (1.12±0.12) and normothermia group (0.94±0.06), implying a higher pyruvate carboxylase/pyruvate dehydrogenase ratio for glutamine formation. Least Absolute Shrinkage and Selection Operator found the most important metabolites associated with adenosine triphosphate preservation: [3,4-C]glutamate-produced via pyruvate dehydrogenase entry, [2-C]taurine-an important osmolyte and antioxidant, and phosphocreatine. Final principal component analyses scores plots suggested separate cluster formation for the hypothermia group, but with insufficient data for statistical significance.

CONCLUSIONS

Starting mild hypothermia simultaneously with oxygen-glucose deprivation, compared with delayed starting or no hypothermia, has higher pyruvate carboxylase throughput, suggesting that better glial integrity is one important neuroprotection mechanism of earlier hypothermia.

1H nuclear magnetic resonance brain metabolomics in neonatal mice after hypoxia-ischemia distinguished normothermic recovery from mild hypothermia recoveries

BACKGROUND

Mild brain hypothermia (31-34 °C) after neonatal hypoxia-ischemia (HI) improves neurodevelopmental outcomes in human and animal neonates. Using an asphyxia model with neonatal mice treated with mild hypothermia after HI, we investigated whether (1)H nuclear magnetic resonance (NMR) metabolomics of brain extracts could suggest biomarkers and distinguish different treatments and outcome groups.

METHODS

At postnatal day 7 (P7), CD1 mice underwent right carotid artery occlusion, 30 min of HI (8% oxygen), and 3.5 h of either hypothermia (31 °C) or normothermia (37 °C). Whole brains were frozen immediately after HI, immediately after 3.5 h of hypothermia or normothermia treatments, and 24 h later. Perchloric acid extractions of 36 metabolites were quantified by 900 MHz (1)H NMR spectroscopy. Multivariate analyses included principal component analyses (PCA) and a novel regression algorithm. Histological injury was quantified after HI at 5 d.

RESULTS

PCA scores plots separated normothermia/HI animals from hypothermia/HI and control animals, but more data are required for multivariate models to be predictive. Loadings plots identified 11 significant metabolites, whereas the regression algorithm identified 6. Histological injury scores were significantly reduced by hypothermia.

CONCLUSION

Different treatment and outcome groups are identifiable by (1)H NMR metabolomics in a neonatal mouse model of mild hypothermia treatment of HI.

Outcome-related metabolomic patterns from 1H/31P NMR after mild hypothermia treatments of oxygen-glucose deprivation in a neonatal brain slice model of asphyxia

Human clinical trials using 72 hours of mild hypothermia (32°C-34°C) after neonatal asphyxia have found substantially improved neurologic outcomes. As temperature changes differently modulate numerous metabolite fluxes and concentrations, we hypothesized that (1)H/(31)P nuclear magnetic resonance (NMR) spectroscopy of intracellular metabolites can distinguish different insults, treatments, and recovery stages. Three groups of superfused neonatal rat brain slices underwent 45 minutes oxygen-glucose deprivation (OGD) and then were: treated for 3 hours with mild hypothermia (32°C) that began with OGD, or similarly treated with hypothermia after a 15-minute delay, or not treated (normothermic control group, 37°C). Hypothermia was followed by 3 hours of normothermic recovery. Slices collected at different predetermined times were processed, respectively, for 14.1 Tesla NMR analysis, enzyme-linked immunosorbent assay (ELISA) cell-death quantification, and superoxide production. Forty-nine NMR-observable metabolites underwent a multivariate analysis. Separated clustering in scores plots was found for treatment and outcome groups. Final ATP (adenosine triphosphate) levels, severely decreased at normothermia, were restored equally by immediate and delayed hypothermia. Cell death was decreased by immediate hypothermia, but was equally substantially greater with normothermia and delayed hypothermia. Potentially important biomarkers in the (1)H spectra included PCr-(1)H (phosphocreatine in the (1)H spectrum), ATP-(1)H (adenosine triphosphate in the (1)H spectrum), and ADP-(1)H (adenosine diphosphate in the (1)H spectrum). The findings suggest a potential role for metabolomic monitoring during therapeutic hypothermia.

Antioxidant effect of ethyl pyruvate in respiring neonatal cerebrocortical slices after H(2)O(2) stress

Administration of ethyl pyruvate, which is formed from pyruvate and ethanol, has been found capable of rescuing cells injured by oxidative stress. In one perspective the rescue has been postulated to be metabolic, with the resulting intracellular delivery of pyruvate seen as providing substrate for the TCA Cycle, making it possible to counteract sequela of poly(ADP-ribose)ribosylation, such as depletion of cytosolic NAD(+), glycolytic arrest, and mitochondrial deprivation of pyruvate. The rescue has also been attributed to radical scavenging via the carbonyl groups in ethyl pyruvate and pyruvate. In a previous study we exposed superfused neonatal (P7) brain slices for 60min to 2mM H(2)O(2) and found evidence for both rescue mechanisms. To see if ethyl pyruvate's actions stemmed more from being an antioxidant than from being a nutrient we conducted six new experiments using the same H(2)O(2) protocol, but with two new rescue solutions: [10mM] glucose (glc) plus one of the following: ethyl pyruvate [20mM], or the nonmetabolizable radical scavenger N-tert-butyl-alpha-phenylnitrone (PBN, 1mM). Final ATP values compared to initial, measured in 14.1T (31)P NMR spectra of PCA extracts, were the same: 0.70+/-0.08 for the former (N=3), and 0.64+/-0.08 for the latter (N=3). Quantifications of this study's (1)H NMR metabolites, also measured at 14.1T, exhibited separate clustering when pooled with data from the previous study and compared in a metabolomic multivariate analyses. Because the addition of ethyl pyruvate provided the same ATP protection as the addition of a nonmetabolizable antioxidant, antioxidant protection was its prominent protective mechanism in the chosen, high glucose protocol. Having distinct clusters in the Scores Plot of a Partial Least Squares-Discriminant Analysis suggests the feasibility of constructing statistical models that are predictive.

Effects of edaravone on N-methyl-D-aspartate (NMDA)-mediated cytochrome c release and apoptosis in neonatal rat cerebrocortical slices

N-Methyl-D-aspartate-mediated neurotoxicity is known to involve nitric oxide production and to be augmented in an environment of reactive oxygen species. We used TUNEL staining and homogenous cytosolic immunoreactivity of cytochrome c in an acute brain slice preparation to investigate the influence of edaravone (3-methyl-1-phenyl-2-pyrazolin-5-one), a free radical scavenger, on N-methyl-D-aspartate-induced apoptosis. Cerebrocortical slices were obtained from parietal lobes of 7-day-old Sprague-Dawley rats, superfused with well-oxygenated artificial cerebrospinal fluid, and metabolically recovered. Subsequent 30-min exposures to 10 microM N-methyl-D-aspartate in treated and untreated slices were followed by 4 h of recovery superfusion with oxygenated artificial cerebrospinal fluid. Outcomes were compared for three groups of slices: "the N-methyl-D-aspartate-only group"; "the edaravone treatment group", which had 20 microM edaravone present throughout and subsequent to N-methyl-D-aspartate exposure; the "control group", in which slices were superfused only with oxygenated artificial cerebrospinal fluid. At the conclusion of recovery (t = 4 h), the percentage of TUNEL-positive cells in the edaravone treatment group (7.0+/-3.3%) was significantly reduced from the percentage for the N-methyl-D-aspartate-only group (21.9+/-4.1%), and insignificantly greater than the percentage for the control group (3.4+/-2.1%). Percentages of cytochrome c positive cells at t = 1 h were significantly increased (p < 0.01) in the N-methyl-d-aspartate-only group (30.6+/-1.9%) compared to percentages for both the control group (11.4+/-2.6%) and the edaravone treatment group (15.2+/-2.1%). Edaravone's reduction in TUNEL staining and cytochrome c release provides evidence of reactive oxygen species mechanisms and antioxidant benefits in cytochrome c-mediated apoptosis during N-methyl-D-aspartate excitotoxicity.

Hypothermia and amiloride preserve energetics in a neonatal brain slice model

A period of secondary energy failure consisting of a decline in phosphocreatine/inorganic phosphate (PCr/Pi), a rise in brain lactate, and alkaline intracellular pH (pH(i)) has been described in infants with neonatal encephalopathy. Strategies that ameliorate this energy failure may be neuroprotective. We hypothesized that a neonatal rat brain slice model undergoes a progressive decline in energetics, which can be ameliorated with hypothermia or amiloride. Interleaved phosphorus ((31)P) and proton ((1)H) magnetic resonance (MR) spectra were obtained from 350 microm neonatal rat brain slices over 8 h in a bicarbonate buffer at 37 degrees C and at 32 degrees C in 7- and 14-d models. (31)P MR spectra were obtained with amiloride in a bicarbonate-free buffer at 37 degrees C in the 14-d model. Findings were similar in 7- and 14-d models. In the 14-d model, there was a Pi doublet structure corresponding to alkaline pH(i) values of 7.50 +/- 0.02 and 7.21 +/- 0.04. Compared with the stabilized baseline of 100, at 5 h PCr/Pi was 65 +/- 6.3 and lactate/NAA was 187 +/- 3 at 37 degrees C, but PCr/Pi and lactate/NAA were not significantly different from baseline at 32 degrees C. Nucleotide triphosphate (NTP)/phosphomonoester (PME) was 0.93 +/- 0.23 at 37 degrees C and 1.81 +/- 0.21 at 32 degrees C at 5 h. With amiloride exposure in the 14-d model, baseline pH(i) values were 7.25 +/- 0.09 and 6.98 +/- 0.02 and NTP/PME was 1.81 +/- 0.05; these parameters were not significantly different at 5 h. Our interpretation of these findings is that the brain slice model underwent secondary energy failure, which was delayed with hypothermia or amiloride.

PI3K inhibition in neonatal rat brain slices during and after hypoxia reduces phospho-Akt and increases cytosolic cytochrome c and apoptosis

Acute hypoxia can deplete ATP and induce mitochondrial release of cytochrome c (cyt c) to initiate or enhance apoptosis, a process delayed or overcome with sufficient ATP and phosphorylation (activation) of survival factors such as Akt (also known as Protein Kinase B). We used an ex vivo brain slice model to investigate associations between levels of phosphorylated Akt (phospho-Akt) and the extent of intrinsic pathway apoptosis. Additionally, phosphorylation (inactivation) was measured of Bad, which is known to promote mitochondrial release of cyt c. Superfused cerebrocortical slices from 7-day-old rats underwent 30-min hypoxia followed by 4-h reoxygenation. At end-hypoxia, Western blots of phospho-Akt became nearly undetectable but returned immediately during recovery and increased thereafter. Cyt c behaved oppositely, being greatest at end-hypoxia and continually decreasing during recovery. Continuous inhibition of phosphoinositide 3-kinase (PI3K) with 10 microM LY294002 suppressed post-hypoxic phospho-Akt levels, prevented post-hypoxic cytosolic cyt c reductions, and increased apoptosis evaluated by TUNEL staining and DNA fragmentation. Western blot analysis demonstrated enhanced Bad translocation from cytosol to mitochondria in the LY294002 group. Phospho-Akt/phospho-Bad double staining revealed colocalization. Parallel (31)P NMR studies showed no effects on NTP production by LY294002. The data support prominent roles for Bad phosphorylation in phospho-Akt's reduction of cyt c apoptosis, and possible apoptotic roles at mitochondrial targets of Bad.

Association between brain tissue pH and brain injury during asphyxia in piglets

BACKGROUND

Acidosis may contribute to brain injury from asphyxia, but its role is unclear. In order to evaluate the association between brain acidosis and cerebral injury, we subjected piglets to hypoxia and hypotension (HYP-HOTN) or hypoxia alone (HYP) to inflict varying amounts of brain damage. We hypothesized that piglets with a more severe brain injury would have a lower brain pH.

METHODS

Piglets had a pH microprobe inserted into the cerebral cortex. HYP animals breathed 5-8% O(2)/7% CO(2) for 30 min with mean arterial pressure (MAP) maintained at >40 mmHg. HYP-HOTN animals breathed the same gas for 30 min, but during the last 15 min, MAP was reduced to 25-30 mmHg by withdrawing blood. After 4 h of recovery, the animals were perfusion-fixed and pathology assessed. Somatosensory-evoked potentials (SEP) were also monitored.

RESULTS

HYP-HOTN piglets had more neuropathology than HYP animals. During the last 15 min of injury, brain pH in the HYP-HOTN group was significantly higher than that in HYP. However, recovery of brain pH was prolonged in the HYP-HOTN animals. The amount of time for brain pH to recover to > or =7.00 correlated very well with both the degree of neuropathology and SEP recovery. The reduction in brain pH, either absolute or relative to baseline, was not associated with the severity of damage.

CONCLUSIONS

The time needed for brain pH to recover after asphyxia, but not its severity, was associated with the amount of brain injury. Further study is warranted to determine whether immediate restoration of brain pH will reduce brain damage.

NTP and PCr responses to hypoxia by hypothermic and normothermic respiring, superfused, neonatal rat cerebrocortical slices: an NMR spectroscopy study at 14.1 Tesla

Although mechanisms of hypothermic neuroprotection during oxygen deprivation have long been investigated, further characterizations of various molecular mechanisms are appropriate. Anticipating future studies of hypothermia and hypoxia/ischemia, we investigated the extent to which our ex vivo, NMR-based, superfused brain slice model might be helpful. (Slices are approximately 350 microm thick, with 18 slices per 8 mm NMR tube.) 31P NMR spectroscopic measurements were made of hypothermia-induced changes in high energy phosphates, while simultaneously monitoring and controlling tissue temperature, using 1H NMR, the high spectroscopic resolution available at 14.1 Tesla (600 MHz for protons), and a recently published protocol. NTP and PCr concentrations in healthy, well-oxygenated slices decreased to (55 +/- 15)% and (66 +/- 30)% of their respective values at 28.0 degrees C when warmed to 38.0 degrees C, in approximate agreement with earlier in vivo studies by others. During 30 min hypoxia NTP and PCr decreased to non-observable values, regardless of temperature. After reoxygenation, NTP and PCr recoveries as percentages of respective prehypoxia values were (63% +/- 16%; 70%) +/- 5%) for hypothermic slices (28.0 degrees C), and (46% +/- 13%; 41% +/- hypothermic neuroprotection during oxygen deprivation in this model, which appears suitable for use in further studies.

Akt phosphorylation and cell survival after hypoxia-induced cytochrome c release in superfused respiring neonatal rat cerebrocortical slices

Phosphorylation of Akt before hypoxia (30 min) and during reoxygenation (4 h) was evaluated in superperfused neonatal rat cerebrocortical slices (350 microm, P7, Sprague-Dawley). Cytosolic cytochrome c intensities in Western blots, which were increased at the end of hypoxia. were decreased during reoxygenation. Western blot intensities of phosphorylated Akt (phospho-Akt), nearly undetectable at the end of hypoxia, recovered quickly during reoxygenation, in a trend opposite that for cytochrome c. At 1.5 h and 4 h after hypoxia they became larger or the same as before hypoxia. Total Akt was unchanged by hypoxia and reoxygenation. Phosphocreatine (PCr) and nucleotide triphosphates (NTP) were measured in parallel 14.1 Tesla ex vivo 31P NMR superfused brain slice studies. PCr and alpha-NTP were nearly undetectable at the end of hypoxia. Although they recovered quickly after hypoxia, they were lower than before hypoxia. Reductions in phospho-Akt during hypoxia were consistent with the general unavailability of basic high energy phosphates. Preferential phosphorylation of Akt after hypoxia suggested that substantial reductions in intracellular energy, as indicated by PCr and NTP, might be tolerated by processes important for generating phospho-Akt. Additionally, the post-hypoxia increase in phospho-Akt might have contributed to concomitant reductions in cytosolic cytochrome c.

Temperature control of respiring rat brain slices during high field NMR spectroscopy

Advancing methodologies in high field multinuclear (1)H/(31)P NMR spectroscopy continue to improve the chemical shift precision and signal amplitude sensitivity attainable in ex vivo NMR studies of respiring tissues. Thus it was straightforward in a 14.1-Tesla (600-MHz) system to use the well-known temperature dependence of the chemical shift of water protons to implement a protocol addition in our brain slice studies (350 microm thick, 15 slices per 8-mm NMR tube) of hypoxia. The protocol addition provides for accurate, continuous measurements of brain tissue temperature, a very important variable in studies of oxygen deprivation and metabolism. The basic protocol for loading and maintaining superfused respiring neonatal cerebrocortical slices, similar to protocols described previously by us and others, permitted rapid detection of glycolytic and TCA cycle activity. All brain slices have an 'injury layer' approximately 50-microm thick from mechanical shear. In our protocol the injury layer is confined to one side of the slice, and appears to contribute minimally to NMR spectra.

Cytochrome c associated apoptosis during ATP recovery after hypoxia in neonatal rat cerebrocortical slices

Cellular injury was evaluated in superfused cerebrocortical slices (350 micro m) from 7-day-old Sprague-Dawley rats exposed to 30 min hypoxia followed by 4 h of reoxygenation. At the end of hypoxia homogenous cytosolic immunoreactivity of cytochrome c increased approximately fourfold, cytochrome c intensity in western blot analyses increased more than fivefold, and whole cell and cytosolic cleaved caspase-9 underwent 50% and 100% increases, respectively. Immunostaining of sections taken 1.5 h after hypoxia showed: (i) more than a threefold increase in cleaved caspase-9; (ii) localization of cleaved caspase-9 to the interior and peripheral exterior of nuclei; and (iii) homogeneously distributed cytochrome c in the cytosol. Western blot analysis for 1.5 h after hypoxia showed that cytosolic caspase-9 returned to control values, while whole cell caspase-9 stayed approximately the same, suggesting translocation of caspase-9 to nuclei. By 4 h after hypoxia there was significant nuclear fragmentation and an increase in TUNEL positive staining. 31P/1H nuclear magnetic resonance (NMR) confirmed substantial decreases of ATP and phosphocreatine during hypoxia, with rapid but incomplete recovery being close to steady state 1 h after reoxygenation. At all time points after hypoxia the primary injury was cytochrome c associated apoptosis.

Hypercarbia and mild hypothermia, only when not combined, improve postischemic bioenergetic recovery in neonatal rat brain slices

In the immature brain, postischemic metabolism may be influenced beneficially by the effect of inducing hypercarbia or hypothermia. With use of 31P nuclear magnetic resonance spectroscopy, intracellular pH (pHi) and cellular energy metabolites in ex vivo neonatal rat cerebral cortex were measured before, during, and after substrate and oxygen deprivation in in vitro ischemia. Early postischemic hypothermia (fall in temperature -3.2 +/- 1.0 degrees C) delayed the normalization of pHi after ischemia by inducing an acid shift in pHi (P < 0.01). Postischemic hypercarbia (Krebs-Henseleit bicarbonate buffer equilibrated with 10% carbon dioxide in oxygen) and hypothermia induced separate, but potentially additive, reversible decreases in pHi, each of approximately -0.16 pH unit (P < 0.05). When these postischemic perturbations were applied in isolation, there was significant improvement of approximately 20% in the recovery of beta-ATP (P < 0.05). In combination, however, hypercarbia and hypothermia worsened recovery in ATP by approximately 20% (P < 0.05). In control tissue, which had not been exposed to ischemia, ATP content was also significantly reduced by co-administration of the two treatments (P < 0.05), an effect that persisted even after discontinuing the perturbing conditions. Therefore, in this vascular-independent neonatal preparation, early postischemic modulation of metabolism by hypercarbia or hypothermia appears to confer improved bioenergetic recovery, but only if they are not administered together.

The novel immunosuppressant SDZ-RAD protects rat brain slices from cyclosporine-induced reduction of high-energy phosphates

1. SDZ-RAD, 40-O-(2-hydroxyethyl)-rapamycin, is a novel macrolide immunosuppressant. Because of its synergistic interaction, SDZ-RAD is under clinical investigation as immunosuppressant in combination with cyclosporine after organ transplantation. Neurotoxicity is a critical side-effect of cyclosporine. 2. We studied the effect of SDZ-RAD and its combination with cyclosporine on high-energy phosphates, phosphocreatine (PCr) and nucleoside triphosphates (NTP), in brain slices using 31P-magnetic resonance spectroscopy (MRS). 3. Cyclosporine significantly reduced high-energy phosphates after 2 h in a dose-dependent manner (100 micrograms l-1: 93 +/- 3% of control (NTP), 91 +/- 3% (PCr); 500 micrograms l-1: 84 +/- 2% (NTP), 73 +/- 2 (PCr); 5000 micrograms l-1: 68 +/- 3% (NTP), 55 +/- 5% (PCr); n = 6; P < 0.02). 4. In contrast, after perfusion for 2 h, SDZ-RAD (500 micrograms l-1 and 5000 micrograms l-1) significantly increased high-energy phosphate concentrations in the brain slices (P < 0.02). Even at the lowest concentration, SDZ-RAD protected brain energy metabolism against cyclosporine toxicity: 100 micrograms l-1 SDZ-RAD + 5000 micrograms l-1 cyclosporine: 86 +/- 3% (NTP), 83 +/- 7% (PCr), n = 3, P < 0.03 compared to cyclosporine alone. 5. As evaluated using an algorithm based on Loewe isobolograms, the effects of SDZ-RAD/cyclosporine combinations on brain energy reduction were antagonistic. Both drugs were found in mitochondria using h.p.l.c-MS analysis. 6. We conclude that cyclosporine inhibits mitochondrial high-energy phosphate metabolism, which can be antagonized by SDZ-RAD.

Immunohistochemical nitrotyrosine distribution in neonatal rat cerebrocortical slices during and after hypoxia

The peroxynitrite contributions to hypoxic damage in brain slices that arise from N-methyl-D-aspartate (NMDA) receptor activation were studied by following the temporal-spatial course of nitrotyrosine (NT) formation during six conditions: hypoxia (pO(2)<5 mmHg) with or without 10 microM MK-801 treatment; with exposure to 10, 100 and 1000 microM NMDA; and no treatment (control). In each experiment, twenty 350-micrometer thick cerebrocortical slices, obtained from the parietal lobes of ten 7-day-old Sprague-Dawley rats, were metabolically recovered and allowed to respire in a well-oxygenated perfusion system. Thirty minutes exposures to hypoxia or NMDA were followed by 2 h of oxygenated reperfusion. MK-801 administration began 15 min prior to hypoxia and was discontinued during reperfusion. Anti-NT serum immunohistochemistry stains in 20-micrometer frozen sections of slices taken during oxygenated reperfusion, after hypoxia or NMDA exposure, were positive in both neurons and endothelial cells. NT-positive neurons were detected sooner after hypoxia than after NMDA exposure, suggesting that mechanisms of superoxide generation were different in both groups. After hypoxia and even more so after NMDA exposure, more intense NT-positive staining was observed in endothelial cells than in neurons. Cell damage after hypoxia was attenuated by MK-801. MK-801 decreased post-hypoxia counts of NT-stained endothelial cells by 78.5% (p<0. 001) and NT-stained neurons by 54.1% (p<0.05). Our findings suggest that NMDA receptor activation in hypoxic brain slices is associated with increased post-hypoxic peroxynitrite production that contributes to acute neuronal death and endothelial cell injury. Peroxynitrite injury to endothelial cells, caused either by increased peroxynitrite from within or from increased vulnerability to peroxynitrite from without, might play an important role in hypoxic-ischemic brain injury and NMDA-induced brain injury.

Early postischemic dantrolene-induced amelioration of poly(ADP-ribose) polymerase-related bioenergetic failure in neonatal rat brain slices

In the infant brain, ischemia-induced ionic and enzyme mechanisms may independently lead to cell death by energy depletion: resequestration of calcium mobilized from intracellular stores consumes ATP, and activated poly(ADP-ribose) polymerase (PARP) uses oxidized nicotinamide adenine dinucleotide to form polyADP-ribosyl nuclear proteins associated with DNA damage. Using 31P nuclear magnetic resonance spectroscopy, we have monitored intracellular pH and cellular energy metabolites in ex vivo neonatal rat cerebral cortex before, during, and after substrate and oxygen deprivation. In an insult that exhibited secondary energy failure and apoptosis we identified a relative 25% augmentation of high-energy phosphates at the end of recovery when the ryanodine-receptor antagonist, dantrolene, was introduced in the early (0- to 40-minute) but not late (40- to 120-minute) stage of recovery (P < 0.05). In contrast to the absence of a late dantrolene-sensitive effect, inhibition of PARP with 3-methoxybenzamide was as effective (P < 0.05) as early dantrolene, even when introduced after a 40-minute delay. The dantrolene and 3-methoxybenzamide effects on high-energy phosphates were not additive, rather the early dantrolene-sensitive effect nullified the potential 3-methoxybenzamide effect. Therefore, in this vascular-independent neonatal preparation, postischemic mobilization of calcium from intracellular stores is associated with PARP-related energy depletion. Inhibition of either of these processes confers improved postischemic bioenergetic recovery in the developing brain.

Reversible decreases in ATP and PCr concentrations in anoxic turtle brain

A hallmark of anoxia tolerance in western painted turtles is relative constancy of tissue adenylate concentrations during periods of oxygen limitation. During anoxia heart and brain intracellular compartments become more acidic and cellular energy demands are met by anaerobic glycolysis. Because changes in adenylates and pH during anoxic stress could represent important signals triggering metabolic and ion channel down-regulation we measured PCr, ATP and intracellular pH in turtle brain sheets throughout a 3-h anoxic-re-oxygenation transition with 31P NMR. Within 30 min of anoxia, PCr levels decrease 40% and remain at this level during anoxia. A different profile is observed for ATP, with a statistically significant decrease of 23% occurring gradually during 110 min of anoxic perfusion. Intracellular pH decreases significantly with the onset of anoxia, from 7.2 to 6.6 within 50 min. Upon re-oxygenation PCr, ATP and intracellular pH recover to pre-anoxic levels within 60 min. This is the first demonstration of a sustained reversible decrease in ATP levels with anoxia in turtle brain. The observed changes in pH and adenylates, and a probable concomitant increase in adenosine, may represent important metabolic signals during anoxia.

[31P]/[1H] nuclear magnetic resonance study of mitigating effects of GYKI 52466 on kainate-induced metabolic impairment in perfused rat cerebrocortical slices

PURPOSE

Kainic acid (KA) has long been used in experimental animals to induce status epilepticus (SE). A mechanistic implication of this is the association between excitotoxicity and brain damage during or after SE. We evaluated KA-induced metabolic impairment and the potential mitigating effects of GYKI 52466 [1-(4-aminophenyl)-4-methyl-7,8-methylenedioxy-5H-2,3-benzodiazepine] in superfused rat cerebral cortical slices.

METHODS

Interleaved [31P]/[1H] magnetic resonance spectroscopy (MRS) was used to assess energy metabolism, intracellular pH (pHi), N-acetyl-L-aspartate (NAA) level, and lactate (Lac) formation before, during, and after a 56-min exposure to 4 mM KA in freshly oxygenated artificial cerebrospinal fluid (oxy-ACSF).

RESULTS

In the absence of GYKI 52466 and during the KA exposure, NAA, PCr, and ATP levels were decreased to 91.1 +/- 0.8, 62.4 +/- 3.9, and 59.1 +/- 4.3% of the control, respectively; Lac was increased to 118.2 +/- 2.1 %, and pH, was reduced from 7.27 +/- 0.02 to 7.13 +/- 0.02. During 4-h recovery with KA-free ACSF, pHi rapidly and Lac gradually recovered, NAA decreased further to 85.5 +/- 0.3%, and PCr and ATP showed little recovery. Removal of Mg2+ from ACSF during KA exposure caused a more profound Lac increase (to 147.1 +/- 4.0%) during KA exposure and a further NAA decrease (to 80.4 +/- 0.5%) during reperfusion, but did not exacerbate PCr, ATP, and pHi changes. Inclusion of 100 microM GYKI 52466 during KA exposure significantly improved energy metabolism: the PCr and ATP levels were above 76.6 +/- 2.1 and 82.0 +/- 2.9% of the control, respectively, during KA exposure and recovered to 101.4 +/- 2.4 and 95.0 +/- 2.4%, respectively, during reperfusion. NAA level remained at 99.8 +/- 0.6% during exposure and decreased only slightly at a later stage of reperfusion.

CONCLUSIONS

Our finding supports the notion that KA-induced SE causes metabolic disturbance and neuronal injury mainly by overexcitation through non-N-methyl-D-aspartate (NMDA) receptor functions.

Effects of neuroprotective dose of fructose-1,6-bisphosphate on hypoxia-induced expression of c-fos and hsp70 mRNA in neonatal rat cerebrocortical slices

In situ hybridization (ISH) measurements of c-fos and hsp70 expression were made in brain slice studies of hypoxia, with or without fructose-1,6-bisphosphate (FBP) pretreatment. Each experiment used eighty 350 microns thick cerebrocortical slices, obtained from twenty 7-day old rats. Thirty minute periods of hypoxia were followed by 8 h of hyperoxic perfusion. Slices were removed at eight predetermined times, and processed for ISH and immunohistochemistry. In three of six hypoxia experiments, slices were pretreated for 60 min with 2 mM FBP, a condition known to maintain ATP level in brain slices during hypoxia. In three other hypoxia experiments slices received no pretreatment. In two control experiments slices were perfused for 11.5 h without hypoxia. In control experiments, hsp70 mRNA was barely detectable in slices at all times, although moderate c-fos mRNA expression occurred at 1 h after decapitation. Hypoxia produced a modest but statistically significant increase in c-fos mRNA and hsp70 mRNA induction 4 h following reoxygenation. At all times after hypoxia, FBP pretreatment reduced expression of c-fos and hsp70 mRNA. The absence of hsp70 mRNA in control slices suggests that intracellular protein denaturation was minimal in this preparation. In slices made hypoxic, the decrease in c-fos and hsp70 mRNA caused by FBP pretreatment suggests ameliorated progression towards injury. Immunohistochemistry showed no HSP70 protein at any time following hypoxia, with or without FBP pretreatment, presumably due to delayed HSP70 protein synthesis, or to a block in translation, as observed in vivo in other studies.

Bioenergetic recovery following ischemia in brain slices studied by 31P-NMR spectroscopy: differential age effect of depolarization mediated by endogenous nitric oxide

Proximate neurotoxic mechanisms during postischemic recovery may be influenced by stage of development and complicating factors such as cortical spreading depression or secondary brain insult. Using 31P nuclear magnetic resonance spectroscopy, we have monitored pH and cellular energy metabolites phosphocreatine (PCr) and ATP in the ex vivo rat cerebral cortex before, during, and after substrate and oxygen deprivation, which represents "in vitro ischemia." There were important developmental differences in resistance and response to an ischemic insult. Twenty-one-day-old (P21) rat cortical slices had no detectable beta-ATP or PCr at the end of a 20-min insult, while 7-day-old (P7) slices had 50 +/- 13.7% (mean +/- SD, n = 12) and 17 +/- 14.8% relative to preischemia levels, respectively. Postischemic depolarization resulted in age-dependent effects on PCr (p < 0.05): In the older tissue, depolarization significantly worsened the recovery of PCr, whereas in young tissue it ameliorated recovery. This amelioration could be prevented by inhibiting nitric oxide production with methylene blue (depolarization-methylene blue interaction, p < 0.05) and enhanced by administration of the nitric oxide donor glyceryl trinitrate (GTN; p < 0.01). However, in P21 tissue, GTN further exacerbated injury (age-GTN interaction, p < 0.01). Therefore, in this vascular-independent preparation, a neuronal or glial nitric oxide-dependent mechanism appears to confer improved postischemic bioenergetic recovery in the developing brain compared with the mature brain.

Rapid recovery of rat brain intracellular pH after cardiac arrest and resuscitation

We studied the intracellular pH in rat cerebral cortex of rats subjected to reversible total cerebral ischemia by cardiac arrest and resuscitation. Brain acidoses was more pronounced during ischemia in hyperglycemic rats (6.21 +/- 0.14) than in normoglycemic rats (6.56 +/- 0.07). Brain tissue lactate accumulated proportionally. Nevertheless, within 5 min of reperfusion, pHi in both normoglycemic and hyperglycemic groups had recovered to baseline levels, i.e. near 7.1-7.2, despite the fact that lactate concentrations were still elevated. These results demonstrate a rapid reversal of ischemic acidosis during recovery from 10 min of cardiac arrest, and suggest that acidosis, per se, may not be responsible for neuronal damage following cardiac arrest and resuscitation, even in hyperglycemic conditions.

19F NMR calcium changes, edema and histology in neonatal rat brain slices during glutamate toxicity

Respiring neonatal cerebrocortical slices (350 microns thick), loaded with the free calcium indicator 5F-BAPTA, were perfused in a 20-mm-diameter glass NMR tube with oxygenated artificial CSF, exposed to extracellular glutamate and studied at 4.7 Tesla with 19F NMR spectroscopy. 31P/1H NMR spectra, obtained concurrently, were used to assess slice integrity from determinations of intracellular pH, ATP, PCr, lactate and N-acetylaspartate. 60-min periods were induced of recoverable and nonrecoverable glutamate toxicity-defined from changes in NMR metabolites. In other NMR studies, where 5F-BAPTA was not used, metabolic toxicity was modulated by three glutamate receptor antagonists: dizocilpine, NBQX and kynurenic acid. Outcome measurements were made of edema, determined invasively in isolated slices from % swelling and water content and from histological changes in Nissl stains of slice sections. Edema was (1) detectable in all slices within minutes after onset of glutamate exposure, though never in untreated control slices, and (2) modulated differently by dizocilpine, NBQX and kynurenate. Correlations were observed between edema and NMR decreases in PCr and ATP. Nissl stains of sections from slices treated with the most protective agent, dizocilpine, showed preservation of neuronal processes. As was expected in 7-day-old rats with immature NMDA receptors, 19F NMR spectroscopy revealed only small increases in free intracellular calcium ([Ca2+]i). These occurred late during glutamate exposure and reversed early during glutamate washout. The studies demonstrate that it is possible to study correlations between repeated noninvasive NMR spectra in ensembles of brain slices and invasive measures of early cellular responses.

Modulation of glutamate-induced intracellular energy failure in neonatal cerebral cortical slices by kynurenic acid, dizocilpine, and NBQX

The severity and rapidity of acute, glutamate-induced energy failure were compared in live cerebral cortical slices. In each experiment 80 live cerebral cortical slices (350 microns thick) were obtained from neonatal Sprague-Dawley rats, suspended and perfused in a nuclear magnetic resonance (NMR) tube, and studied at 4.7 T with interleaved 31P/1H NMR spectroscopy. NMR spectra, obtained continually, were determined as 5-min averages. Slices were perfused for 60 min with artificial cerebrospinal fluid (ACSF) containing either glutamate alone or glutamate mixed with one of three glutamate-receptor antagonists: kynurenate, dizocilpine (MK-801), and 2,3-dihydroxy-6-nitro-7-sulfamoylbenzo(F)quinoxaline (NBQX). Dose-dependent decreases in high-energy phosphates were studied during glutamate exposure (0.5 to 10 mM), with and without antagonist protection. Energy recovery after glutamate exposures was measured during a 60-min washout with glutamate-free, antagonist-free ACSF. Reversible and irreversible energy failures were characterized by changes in intracellular pH, and by changes in relative concentrations of ATP, phosphocreatine (PCr), and inorganic phosphate. No changes were observed in intracellular levels of N-acetylaspartate and lactate. Some special studies were also done using R-(-)-2-amino-5-phosphonovaleric acid (100 microM) and tetrodotoxin (1 mM) to examine glutamate receptor specificity in this tissue model. Dizocilpine (150 microM) best ameliorated the energy failure caused by 2.0 mM glutamate. With dizocilpine the maximum ATP decrease was only 6 +/- 5%, instead of 35 +/- 7%.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of kainate-induced seizures on cerebral metabolism: a combined 1H and 31P NMR study in rat

The cerebral metabolic changes elicited by kainate-induced seizures in the rat were investigated by in vivo combined NMR spectroscopy of 31P and 1H. Systemic injection of kainate induced no significant changes in cerebral ATP or PCr levels during up to 90 min of continuous, generalised seizures, and the cerebral 31P spectra showed only a transient mild cerebral acidosis 30 min after kainate administration. In parallel with the changes in intracellular cerebral pH, the 1H spectra showed a significant increase in lactate, which remained elevated throughout the seizures. These findings indicate that oxidative metabolism does not completely match the increased glycolysis during seizures though the energy homeostasis is maintained. This suggests that oxidative metabolism has a limited capacity to satisfy the brain's energy needs during the kainate-induced seizures, but that the different pathways of energy production in the brain cells can overcome this limitation. Thus the brain damage associated with this experimental model of epilepsy is not due to extended major failure of the energy supply.

Influence of pH on calcium influx during hypoxia in rat cortical brain slices

BACKGROUND AND PURPOSE

Acidity of brain intracellular and extracellular fluids appears to increase brain injury from stroke, but low extracellular pH decreases the activity of N-methyl-D-aspartate receptor ion channels and decreases calcium influx into isolated neurons. To further investigate the role of acid-base balance in hypoxic brain injury, we studied the influences of intracellular and extracellular pH on calcium influx in cortical brain slices during hypoxia.

METHODS

Intracellular calcium ([Ca2+]i) and pH (pHi) were measured fluorometrically with the dyes fura-2 and biscarboxyethyl carboxyfluorescein, respectively, during two types of hypoxia: (1) slice perfusate equilibrated with N2/CO2 at pH 6.6 or 6.2 ("gaseous hypoxia") or (2) perfusate equilibrated with 95% O2/5% CO2 plus 100 mumol/L NaCN at pH 7.3, 6.6, or 6.2 ("chemical hypoxia").

RESULTS

Changes in perfusate pH under aerobic conditions did not change [Ca2+]i. However, influx of calcium caused by gaseous or chemical hypoxia increased significantly with decreasing perfusate pH. During chemical hypoxia, the elevation in [Ca2+]i at perfusate pH 6.2 was twice that at perfusate pH 7.3. Change in [Ca2+]i was correlated with perfusate pH but not pHi.

CONCLUSIONS

These results, which differ from previous studies showing acid inhibition of calcium influx in isolated neurons, suggest that low extracellular pH may exacerbate cellular injury during severe hypoxia or ischemia in the intact brain.

Modulation of edema by dizocilpine, kynurenate, and NBQX in respiring brain slices after exposure to glutamate

Brain edema caused by glutamate excitotoxicity was studied in well oxygenated neonatal cerebrocortical brain slices (350 mu thick). Slices exposed to 60 minutes of 2 mM glutamate, with or without glutamate antagonists (dizocilpine, kynurenate, or NBQX), were allowed to recover for 60 minutes. The protocol was identical to that in noninvasive multinuclear NMR spectroscopy studies (31P/1H/19F) of live slices. Percent water and swelling were determined invasively in isolated slices by wet and dry weight measurements before and after glutamate exposure. Edema was detectable within minutes in all experiments with glutamate exposures, but not in untreated control slices. Dizocilpine, kynurenate, and NBQX differently affected swelling, which correlated with PCr and ATP loss in separate NMR studies. Synaptic glutamate receptor activation appears to initiate events causing both edema and energy failure. Multiple glutamate receptor types seem to be involved. No glutamate antagonist provided greater protection against both edema and energy loss than dizocilpine. Dizocilpine might also block voltage-dependent Na+ channels, and provide protection via mechanisms other than NMDA-receptor dependent channel antagonism.

Evolving concepts about the role of acidosis in ischemic neuropathology

Cerebral ischemia is one of the most common neurological insults. Many pathological events are undoubtedly triggered by ischemia, but only recently has it become accepted that ischemic cell injury arises from a complex interaction between multiple biochemical cascades. Tissue acidosis is a well established feature of ischemic brain tissue, but its role in ischemic neuropathology is still not fully understood. Within the last few years, new evidence has challenged the historically negative view of acidosis and suggests that it may play more of a beneficial role than previously thought. This review reintroduces the concept of acidosis to ischemic brain injury and presents some new perspectives on its neuroprotective potential.