Cerebral energy metabolism in cyanide encephalopathy

The Untargeted Metabolomics Reveals Differences in Energy Metabolism in Patients with Different Subtypes of Ischemic Stroke

AIMS

Ischemic stroke (IS) is the most common subtype of stroke. The risk factors and pathogenesis of IS are complex and varied due to different subtypes. Therefore, we used metabolomics technology to investigate the biomarkers and potential pathophysiological mechanisms of different subtypes of IS.

METHODS

We included 126 IS patients and divided them into two groups based on the TOAST classification: large-artery atherosclerosis (LAA) group (n = 87) and small-vessel occlusion (SVO) group (n = 39). Plasma metabolomics analysis was performed using liquid chromatography-high-resolution mass spectrometry (LC-HRMS) to identify metabolic profiles in LAA and SVO subtype IS patients and to determine metabolic differences between patients with the two subtypes of IS.

RESULTS

We identified 26 differential metabolites between LAA and SVO subtype IS. A multiple prediction model based on the plasm metabolites had good predictive ability for IS subtyping (AUC = 0.822, accuracy = 77.8%), with 12,13-DHOME being the most important differential metabolite in the model. The differential metabolic pathways between the two subtypes of IS patients included tricarboxylic acid (TCA) cycle, alanine, aspartate and glutamate metabolism, and pyruvate metabolism, mainly focused on energy metabolism.

CONCLUSION

12,13-DHOME emerged as the primary discriminatory metabolite between LAA and SVO subtypes of IS. In LAA subtype IS patients, energy metabolism, encompassing pyruvate metabolism and the TCA cycle, exhibited lower activity levels when compared to patients with the SVO subtype IS. The utilization of targeted metabolomics holds the potential to improve diagnostic accuracy for distinguishing stroke subtypes.

The two faces of cyanide: an environmental toxin and a potential novel mammalian gasotransmitter

Cyanide is traditionally viewed as a cytotoxic agent, with its primary mode of action being the inhibition of mitochondrial Complex IV (cytochrome c oxidase). However, recent studies demonstrate that the effect of cyanide on Complex IV in various mammalian cells is biphasic: in lower concentrations (nanomolar to low micromolar) cyanide stimulates Complex IV activity, increases ATP production and accelerates cell proliferation, while at higher concentrations (high micromolar to low millimolar) it produces the previously known ('classic') toxic effects. The first part of the article describes the cytotoxic actions of cyanide in the context of environmental toxicology, and highlights pathophysiological conditions (e.g., cystic fibrosis with Pseudomonas colonization) where bacterially produced cyanide exerts deleterious effects to the host. The second part of the article summarizes the mammalian sources of cyanide production and overviews the emerging concept that mammalian cells may produce cyanide, in low concentrations, to serve biological regulatory roles. Cyanide fulfills many of the general criteria as a 'classical' mammalian gasotransmitter and shares some common features with the current members of this class: nitric oxide, carbon monoxide, and hydrogen sulfide.

Development of sodium tetrathionate as a cyanide and methanethiol antidote

CONTEXT

Hydrogen cyanide and methanethiol are two toxic gases that inhibit mitochondrial cytochrome c oxidase. Cyanide is generated in structural fires and methanethiol is released by decaying organic matter. Current treatments for cyanide exposure do not lend themselves to treatment in the field and no treatment exists for methanethiol poisoning. Sodium tetrathionate (tetrathionate), a product of thiosulfate oxidation, could potentially serve as a cyanide antidote, and, based on its chemical structure, we hypothesized it could react with methanethiol.

RESULTS

We show that tetrathionate, unlike thiosulfate, reacts directly with cyanide in vitro under physiological conditions, and based on rabbit studies where we monitor cyanide poisoning in real-time, tetrathionate likely reacts directly with cyanide in vivo. We found that tetrathionate administered by intramuscular injection rescues >80% of juvenile, young adult, and old adult mice from exposure to inhaled hydrogen cyanide gas that is >80% lethal. Tetrathionate also rescued young adult rabbits from intravenously administered sodium cyanide. Tetrathionate was reasonably well-tolerated by mice and rats, yielding a therapeutic index of ∼5 in juvenile and young adult mice, and ∼3.3 in old adult mice; it was non-mutagenic in Chinese Hamster ovary cells and by the Ames bacterial test. We found by gas chromatography-mass spectrometry that both tetrathionate and thiosulfate react with methanethiol to generate dimethyldisulfide, but that tetrathionate was much more effective than thiosulfate at recovering intracellular ATP in COS-7 cells and rescuing mice from a lethal exposure to methanethiol gas.

CONCLUSION

We conclude that tetrathionate has the potential to be an effective antidote against cyanide and methanethiol poisoning.

Cyclosporin A ameliorates cerebral oxidative metabolism and infarct size in the endothelin-1 rat model of transient cerebral ischaemia

Cerebral microdialysis can be used to detect mitochondrial dysfunction, a potential target of neuroprotective treatment. Cyclosporin A (CsA) is a mitochondrial stabiliser that in a recent clinical stroke trial showed protective potential in patients with successful recanalisation. To investigate specific metabolic effects of CsA during reperfusion, and hypothesising that microdialysis values can be used as a proxy outcome measure, we assessed the temporal patterns of cerebral energy substrates related to oxidative metabolism in a model of transient focal ischaemia. Transient ischaemia was induced by intracerebral microinjection of endothelin-1 (150 pmol/15 µL) through stereotaxically implanted guide cannulas in awake, freely moving rats. This was immediately followed by an intravenous injection of CsA (NeuroSTAT; 15 mg/kg) or placebo solution during continuous microdialysis monitoring. After reperfusion, the lactate/pyruvate ratio (LPR) was significantly lower in the CsA group vs placebo (n = 17, 60.6 ± 24.3%, p = 0.013). Total and striatal infarct volumes (mm³) were reduced in the treatment group (n = 31, 61.8 ± 6.0 vs 80.6 ± 6.7, p = 0.047 and 29.9 ± 3.5 vs 41.5 ± 3.9, p = 0.033). CsA treatment thus ameliorated cerebral reperfusion metabolism and infarct size. Cerebral microdialysis may be useful in evaluating putative neuroprotectants in ischaemic stroke.

Biochemical indications of cerebral ischaemia and mitochondrial dysfunction in severe brain trauma analysed with regard to type of lesion

BACKGROUND

The study focuses on three questions related to the clinical usefulness of microdialysis in severe brain trauma: (1) How frequently is disturbed cerebral energy metabolism observed in various types of lesions? (2) How often does the biochemical pattern indicate cerebral ischaemia and mitochondrial dysfunction? (3) How do these patterns relate to mortality?

METHOD

The study includes 213 consecutive patients with severe brain trauma (342 intracerebral microdialysis catheters). The patients were classified into four groups according to the type of lesion: extradural haematoma (EDH), acute subdural haematoma (SDH), cerebral haemorrhagic contusion (CHC) and no mass lesion (NML). Altogether about 150,000 biochemical analyses were performed during the initial 96 h after trauma.

RESULTS

Compromised aerobic metabolism occurred during 38 % of the study period. The biochemical pattern indicating mitochondrial dysfunction was more common than that of ischaemia. In EDH and NML aerobic metabolism was generally close to normal. In SDH or CHC it was often severely compromised. Mortality was increased in SDH with impaired aerobic metabolism, while CHC did not exhibit a similar relation.

CONCLUSIONS

Compromised energy metabolism is most frequent in patients with SDH and CHC (32 % and 49 % of the study period, respectively). The biochemical pattern of mitochondrial dysfunction is more common than that of ischaemia (32 % and 6 % of the study period, respectively). A correlation between mortality and biochemical data is obtained provided the microdialysis catheter is placed in an area where energy metabolism reflects tissue outcome in a large part of the brain.

Regulation of Glutamate Transporter Function

Normal neuronal function and neuronal survival require that brain extracellular glutamate concentrations be maintained at low micromolar levels. This is accomplished by a family of Na+-dependent glutamate transporters. These transporters are expressed on both glia and neurons, but uptake by glia seems to predominate. Several transporter subtypes have been identified that differ in anatomical distribution, cell type of expression, and electrophysiological properties. Activity of the transporters can be influenced by changes in the uptake driving forces (thermodynamic forces) and by phosphorylation and other modulations that alter their kinetic properties. An understanding of the modulatory mech anisms and signal transduction systems that govern glutamate transport is now beginning to take shape. NEUROSCIENTIST 5:280-282, 1999

Mechanisms underlying attenuation of apoptosis of cortical neurons in the hypoxic brain by flavonoids from the stems and leaves of Scutellaria baicalensis Georgi

Flavonoids from the stems and leaves of Scutellaria baicalensis Georgi, an antioxidant, markedly improve memory impairments and neuronal injuries. In the present study, primary cortical neurons of rats were exposed to potassium cyanide to establish a model of in vitro neural cell apoptosis. Inhibition of apoptosis by flavonoids from the stems and leaves of Scutellaria baicalensis Georgi at concentrations of 18.98, 37.36, and 75.92 μg/mL was detected using this model. These flavonoids dramatically increased cell survival, inhibited cell apoptosis and excessive production of malondialdehyde, and increased the activities of superoxide dismutase, glutathione peroxidase, and Na(+)-K(+)-ATPase in primary cortical neurons exposed to potassium cyanide. The flavonoids from the stems and leaves of Scutellaria baicalensis Georgi were originally found to have a polyhydric structure and to protect against cerebral hypoxia in in vitro and in vivo models, including hypoxia induced by potassium cyanide or cerebral ischemia. The present study suggests that flavonoids from the stems and leaves of Scutellaria baicalensis Georgi exert neuroprotective effects via modulation of oxidative stress, such as malondialdehyde, superoxide dismutase, glutathione peroxidase and Na(+)-K(+)-ATPase disorders induced by potassium cyanide.

Bedside diagnosis of mitochondrial dysfunction in aneurysmal subarachnoid hemorrhage

OBJECTIVES

Aneurysmal subarachnoid hemorrhage (SAH) is frequently associated with delayed neurological deterioration (DND). Several studies have shown that DND is not always related to vasospasm and ischemia. Experimental and clinical studies have recently documented that it is possible to diagnose and separate cerebral ischemia and mitochondrial dysfunction bedside. The study explores whether cerebral biochemical variables in SAH patients most frequently exhibit a pattern indicating ischemia or mitochondrial dysfunction.

METHODS

In 55 patients with severe SAH, intracerebral microdialysis was performed during neurocritical care with bedside analysis and display of glucose, pyruvate, lactate, glutamate, and glycerol. The biochemical patterns observed were compared to those previously described in animal studies of induced mitochondrial dysfunction as well as the pattern obtained in patients with recirculated cerebral infarcts.

RESULTS

In 29 patients, the biochemical pattern indicated mitochondrial dysfunction while 10 patients showed a pattern of cerebral ischemia, six of which also exhibited periods of mitochondrial dysfunction. Mitochondrial dysfunction was observed during 5162 h. An ischemic pattern was obtained during 688 h. Four of the patients (40%) with biochemical signs of ischemia died at the neurosurgical department as compared with three patients (10%) in the group of mitochondrial dysfunction.

CONCLUSIONS

The study documents that mitochondrial dysfunction is a common cause of disturbed cerebral energy metabolism in patients with SAH. Mitochondrial dysfunction may increase tissue sensitivity to secondary adverse events such as vasospasm and decreased cerebral blood flow. The separation of ischemia and mitochondrial dysfunction bedside by utilizing microdialysis offers a possibility to evaluate new therapies.

Mitochondrial dysfunction induced by nuclear poly(ADP-ribose) polymerase-1: a treatable cause of cell death in stroke

Many drugs targeting excitotoxic cell death have demonstrated robust neuroprotective effects in animal models of cerebral ischemia. However, these neuroprotective effects have almost universally required drug administration at relatively short time intervals after ischemia onset. This finding has translated to clinical trial results; interventions targeting excitotoxicity have had no demonstrable efficacy when initiated hours after ischemia onset, but beneficial effects have been reported with more rapid initiation. Consequently, there continues to be a need for interventions with efficacy at later time points after ischemia. Here, we focus on mitochondrial dysfunction as both a relatively late event in ischemic neuronal death and a recognized cause of delayed neuronal death. Activation of poly(ADP-ribose) polymerase-1 (PARP-1) is a primary cause of mitochondrial depolarization and subsequent mitochondria-triggered cell death in ischemia reperfusion. PARP-1 consumes cytosolic NAD(+), thereby blocking both glycolytic ATP production and delivery of glucose carbon to mitochondria for oxidative metabolism. However, ketone bodies such as pyruvate, beta- and gamma-hydroxybutyrate, and 1,4-butanediol can fuel mitochondrial metabolism in cells with depleted cytosolic NAD(+) as long as the mitochondria remain functional. Ketone bodies have repeatedly been shown to be highly effective in preventing cell death in animal models of ischemia, but a rigorous study of the time window of opportunity for this approach remains to be performed.

Cerebral energy metabolism during mitochondrial dysfunction induced by cyanide in piglets

BACKGROUND

Mitochondrial dysfunction is an important factor contributing to tissue damage in both severe traumatic brain injury and ischemic stroke. This experimental study explores the possibility to diagnose the condition bedside by utilising intracerebral microdialysis and analysis of chemical variables related to energy metabolism.

METHODS

Mitochondrial dysfunction was induced in piglets and evaluated by monitoring brain tissue oxygen tension (PbtO2 ) and cerebral levels of glucose, lactate, pyruvate, glutamate, and glycerol bilaterally. The biochemical variables were obtained by microdialysis and immediate enzymatic analysis. Mitochondrial function was blocked by unilateral infusion of NaCN/KCN (0.5 mol/L) through the microdialysis catheter (N = 5). As a reference, NaCl (0.5 mol/L) was infused by intracerebral microdialysis in one group of animals (N = 3).

RESULTS

PbtO2 increased during cyanide infusion and returned to baseline afterwards. The lactate/pyruvate (LP) ratio increased significantly following cyanide infusion because of a marked increase in lactate level while pyruvate remained within normal limits. Glutamate and glycerol increased after cyanide infusion indicating insufficient energy metabolism and degradation of cellular membranes, respectively.

CONCLUSION

Mitochondrial dysfunction is characterised by an increased LP ratio signifying a shift in cytoplasmatic redox state at normal or elevated PbtO2 . The condition is biochemically characterised by a marked increase in cerebral lactate with a normal or elevated pyruvate level. The metabolic pattern is different from cerebral ischemia, which is characterised by simultaneous decreases in intracerebral pyruvate and PbtO2 . The study supports the hypothesis that cerebral ischemia and mitochondrial dysfunction may be identified and separated at the bedside by utilising intracerebral microdialysis.

Value of lactic acidosis in the assessment of the severity of acute cyanide poisoning

OBJECTIVE

To test the hypothesis that plasma lactate concentrations could be of confirmatory value in patients with histories consistent with acute pure cyanide poisoning because immediate laboratory confirmation of suspected cyanide poisoning is rarely possible and because clinicians must rapidly decide whether to administer specific antidotes, which may have severe side effects.

DESIGN

Retrospective clinical study.

SETTING

An intensive care unit in a university-affiliated teaching hospital.

PATIENTS

All acute cyanide-poisoned patients admitted to our intensive care unit, excluding fire victims, from 1988 to 1999.

INTERVENTIONS

None.

MEASUREMENTS AND MAIN RESULTS

Eleven patients were studied. Before antidotal treatment, the median plasma lactate concentration was 168 mg/dL, the median blood cyanide concentration was 4.2 mg/L. Using Spearman's test, there was a significant correlation between plasma lactate and blood cyanide concentrations ( =.74, =.017). Before antidotal treatment, plasma lactate concentration correlated positively with anion gap and inversely with systolic blood pressure, spontaneous respiratory rate, and arterial pH. During the course of cyanide poisonings, a plasma lactate concentration of >or=72 mg/d/L (8 mmol/L) was sensitive (94%) and moderately specific (70%) for a toxic blood cyanide concentration (>or=1.0 mg/L). The specificity was substantially improved in patients not receiving catecholamines (85%).

CONCLUSIONS

The immediate and serial measurement of plasma lactate concentrations is useful in assessing the severity of cyanide poisoning.

Induced mitochondrial failure in the feline brain: implications for understanding acute post-traumatic metabolic events

OBJECTIVE

Recently, evidence has become available implicating mitochondrial failure as a crucial factor in the pathogenesis of acute brain damage following severe traumatic brain injury (TBI). However, it remains unclear how mitochondrial dysfunction affects cerebral metabolism. Therefore the aim of the study was to evaluate the impact of 'isolated' mitochondrial failure on local cerebral metabolism.

METHODS

Cerebral mitochondrial metabolism was blocked by local microdialysis perfusion with cyanide in seven cats. Local brain tissue oxygen tension (p(tiO(2))), carbon dioxide tension (p(tiCO(2))) and pH, as well as extracellular cerebral fluid, glucose, lactate, pyruvate and glutamate were monitored, using a Neurotrend sensor and microdialysis, respectively. Tissue oxygen consumption was measured in a microrespirometric system, and ultrastructural changes evaluated via electron microscopy.

RESULTS

Brain tissue oxygen tension increased from a baseline of 31+/-9 mmHg to 84+/-30 mmHg after 60 min of cyanide perfusion (P<0.05), concomitant a decrease in oxygen consumption from 14.45+/-3.91 microl/h/mg to 10.83+/-1.74 microl/h/mg (P<0.05). Brain tissue pH was decreased after 60 min of cyanide perfusion (6.83+/-0.16) compared to baseline (7.07+/-0.39) (P<0.05), whereas p(tiCO(2)) did not show significant changes. Lactate massively increased from a baseline of 599+/-270 micromol/l to 2609+/-1188 micromol/l immediately after cyanide perfusion (P<0.05). The lactate:glucose ratio increased from 0.79+/-0.15 before cyanide perfusion to 6.40+/-1.44 at 40 min after cyanide perfusion (P<0.05), while no significant changes in the lactate:pyruvate ratio could be observed. Glutamate increased from a baseline of 11.6+/-7.2 micromol/l to 61.4+/-44.7 micromol/l after cyanide perfusion (P<0.05).

CONCLUSION

The results of this study show that 'isolated' cerebral mitochondrial failure initiates changes in cerebral substrates and biochemistry, which are very similar to most of the changes seen after severe human head injury, except for the early fall in p(tiO(2)), further indicating a crucial involvement of mitochondrial impairment in the development of brain damage after TBI.

Regional potassium distribution in the brain in forensic relevant types of intoxication preliminary morphometric evaluation using a histochemical method

A histochemical-morphometric method was used to measure potassium (K+) levels in gray and white matter of rats following sublethal intoxication with 11 different neurotoxic compounds of high forensic significance. Six rats were each given a single substance applied intraperitoneally, the same dosage being given to two animals each. The animals were subsequently killed, the brains immediately frozen, and cryosections cut. K+ levels were evaluated morphometrically. A drop in K+ levels was used as the criterion for cytotoxic edema. Application of ethanol, atropine, carbromal, carbon monoxide, morphine or triethyltin led to a rise in K+ levels in the gray matter and a simultaneous decline in the white matter. By contrast, administration of amitriptyline, glycerin, potassium cyanide, parathion or phenobarbital initiated an increase in K+ levels in both gray and white matter. A cytotoxic edema could thus be reliably excluded in these intoxications. Although the study design allows no statistical analysis, these conclusions are supported by the marked differences in K+ levels in gray and white matter induced by the different toxicants.

CNS energy metabolism as related to function

Large amounts of energy are required to maintain the signaling activities of CNS cells. Because of the fine-grained heterogeneity of brain and the rapid changes in energy demand, it has been difficult to monitor rates of energy generation and consumption at the cellular level and even more difficult at the subcellular level. Mechanisms to facilitate energy transfer within cells include the juxtaposition of sites of generation with sites of consumption and the transfer of approximately P by the creatine kinase/creatine phosphate and the adenylate kinase systems. There is evidence that glycolysis is separated from oxidative metabolism at some sites with lactate becoming an important substrate. Carbonic anhydrase may play a role in buffering activity-induced increases in lactic acid. Relatively little energy is used for 'vegetative' processes. The great majority is used for signaling processes, particularly Na(+) transport. The brain has very small energy reserves, and the margin of safety between the energy that can be generated and the energy required for maximum activity is also small. It seems probable that the supply of energy may impose a limit on the activity of a neuron under normal conditions. A number of mechanisms have evolved to reduce activity when energy levels are diminished.

Acute effects of acetyl-L-carnitine on sodium cyanide-induced behavioral and biochemical deficits

In the present study we investigated the effects of acute treatment with acetyl-L-carnitine (50 mg/kg, i.v. 90 min before the sodium cyanide injection) on a sodium cyanide-induced behavioral deficit in the Morris water escape task. In a first experiment the spatial discrimination performance of the rats was found to be dose-dependently impaired after an i.c.v. injection of sodium cyanide (2.5 and 5.0 microg). Acute treatment with acetyl-L-carnitine was found to increase the behavioral deficit after sodium cyanide. These findings were replicated in a second experiment. Based on these results it can be argued that an acute administration of acetyl-L-carnitine appears to potentiate a sodium cyanide-induced behavioral deficit. An additional in vitro experiment with rat brain synaptosomes showed clear effects of administered sodium cyanide on the energy-dependent incorporation of inositol into phosphoinositides and on the ATP concentration. In vitro acetyl-L-carnitine administration had no effect on the sodium cyanide-induced energy depletion. The negative behavioral findings are in contrast with our previously found protective effect of chronic treatment with acetyl-L-carnitine (via drinking water) on the sodium cyanide-induced behavioral deficit. Since chronic acetyl-L-carnitine treatment has no effect on the phosphoinositide metabolism it was suggested that acetyl-L-carnitine may act via the formation of an ATP-independent reservoir of activated acyl groups. Thus, fatty acids as acylated derivatives can be used for reacylation processes during an acute period of energy depletion. However, we have no clear explanation for the discrepancy in behavioral results between the chronic vs acute treatment of acetyl-L-carnitine at present. Further research is needed to characterize the mechanism of action of acetyl-L-carnitine in relation to sodium cyanide.

The effects in vivo of hypoxia on brain injury

To separately analyze the hypoxic component of hypoxic-ischemic encephalopathy, rats were prepared such that their paO2 was maintained at 20 mmHg while maintaining systemic arterial pressures. During the 20-min experiment, brain oxygen concentration and extracellular amino acid concentrations were monitored. At sacrifice, the brains were studied for morphologic evidence of injury by immunocytochemical staining for the non-constitutive stress protein HSP-72 or neuronal death by acid fuchsin staining. Oxygenated rats subjected to global ischemia were prepared for comparison. In these experiment, hypoxia resulted in no increase in extracellular glutamate concentration, and no morphologic injury was detected. Thus, hypoxia without ischemia is well tolerated by brain.

Acidosis causes failure of astrocyte glutamate uptake during hypoxia

Failure of glutamate uptake during ischemia can lead to neurotoxic accumulations of glutamate in brain extracellular space. Hypoxia and acidosis are metabolic consequences of ischemia that may individually or in combination impair glutamate uptake. We used primary rat astrocyte cultures to study the effects of acidosis, chemical hypoxia, and the combination of acidosis plus chemical hypoxia on glutamate uptake. Chemical hypoxia alone reduced uptake by 35-45%. Reduction in pH from 7.4 to 5.8 also caused a significant but incomplete inhibition of glutamate uptake, and this effect was more pronounced in medium buffered with CO2/bicarbonate. However, the combination of chemical hypoxia plus acidosis reduced glutamate uptake to below 10% of controls. Astrocyte ATP levels, like glutamate uptake, were significantly reduced by chemical hypoxia and further reduced by the combination of hypoxia plus acidosis. Acidosis under normoxic conditions had no significant effect on astrocyte ATP levels. These results suggest two mechanisms by which acidosis may contribute to failure of astrocyte glutamate uptake during ischemia: Acidosis may act in concert with hypoxia to cause ATP depletion, and acidosis may also have direct effects on glutamate transporters unrelated to effects on cellular ATP levels. pH effects on glutamate uptake may be an important factor affecting neuronal survival during incomplete ischemia.

Is there an energy conservation "system" in brain that protects against the consequences of energy depletion?

A poorly understood marked decrease (circa 50% of control) in local cerebral glucose utilization is caused by sublethal doses of NaCN. The decrease is global, occurring in essentially all brain regions and is entirely reversible within hours, leaving no obvious pathology. This event is not unique to NaCN in so far as a strikingly similar pattern of decreased glucose utilization occurs with some other toxins. Nor can it be attributed to a direct action of NaCN since local application by microdialysis to the striatum produces a global depression. These results imply that some widely distributed "system" or substance is involved. We speculate the existence of a "system" possibly related to the reticular activating system that senses a fall in energy production and acts globally to make cells quiescent and thus would give some protection from excitotoxic driven damage.

Glucose can fuel glutamate uptake in ischemic brain

Astrocytes in culture can maintain glutamate uptake during hypoxia if glucose is available. To determine whether this capacity is shared by brain in situ, extracellular glutamate levels were measured in ischemic brain under conditions of continued glucose delivery. Microdialysis probes were placed bilaterally in caudate nuclei of rats and perfused with artificial cerebrospinal fluid (CSF) containing either 30 or 0 mM glucose. Global cerebral ischemia was induced by cardiac arrest. Dialysate collected from probes not perfused with glucose showed a 50-fold increase in glutamate levels over the 60 min following cardiac arrest. Addition of glucose to the perfusate reduced the glutamate rise to < 20% of the levels attained in the glucose-free probes. The glucose effect was negated by the addition of 0.5 mM of the glutamate uptake blocker threo-beta-hydroxyaspartate to the artificial CSF. These results show that oxygen is not required to maintain efficient uptake of extracellular glutamate in brain and suggest that elevations in extracellular glutamate levels during ischemia result from metabolic perturbations other than hypoxia.

Behavioural and biochemical effects of acute central metabolic inhibition: effects of acetyl-l-carnitine

In the present study we evaluated a new method to assess the behavioural and biochemical effects of a brief period of acute hypoxia in the brain. In this method, cyanide is injected into the lateral ventricles. Spatial navigation performance in a Morris task was found to be impaired 1 and 5 min after an i.c.v. injection of 5.0 micrograms cyanide but not after 2.5 micrograms cyanide. Increased rate of phosphatidic acid formation, reflecting increased phospholipase C activity, were observed after injection of 5.0 micrograms cyanide, indicating that energy-dependent phosphoinositide metabolism was affected. Chronic treatment with acetyl-l-carnitine attenuated the cyanide-induced behavioural deficit, but had no effect on energy-dependent phosphoinositide metabolism. The results suggest that, in this model, acetyl-l-carnitine may act via free fatty acid metabolism, by increasing the reservoir of activated acyl groups which are involved in the reacylation of membrane phospholipids.

The effects of cyanide on brain mitochondrial cytochrome oxidase and respiratory activities

Brain mitochondrial cytochrome oxidase and respiratory activities were compared after in vivo and in vitro exposure to cyanide. For the in vivo studies, mice were exposed to a non-lethal (4 mg kg-1) or lethal (20 mg kg-1) dose of KCN. From these mice, purified brain mitochondria were prepared and cytochrome oxidase and respiratory activities measured. Results of these experiments revealed greater inhibition of cytochrome oxidase activity following a lethal (20 mg kg-1) than a non-lethal (4 mg kg-1) KCN dose (57 and 45% inhibition, respectively). Respiration states 3 and 4 of brain mitochondria prepared from mice that received 4 mg kg-1 KCN were inhibited by 15 and 20%, respectively. In mice that received a lethal 20 mg kg-1 KCN dose, respiration states 3 and 4 were each inhibited by ca. 30% (P < 0.05). In vitro, mitochondrial cytochrome oxidase activity was inhibited in a concentration-dependent fashion at cyanide concentrations of 10(-6)-10(-2) M. A biphasic inhibition of ADP-stimulated (state 3) respiration was observed. Cyanide concentrations of 10(-6)-10(-4) M produced only a 25% inhibition of respiration state 3, whereas 10(-3) M produced 80% inhibition. Because this dramatic inhibition only occurred at cyanide concentrations that caused > 50% inhibition of mitochondrial cytochrome oxidase activity, these findings suggest that a large proportion of cytochrome oxidase activity may be functional reserve and that cyanide poisoning likely involves other mechanisms in addition to inhibition of cytochrome oxidase.

Creatine kinase-catalyzed reaction rate in the cyanide-poisoned mouse brain

Brain creatine kinase (CK)-catalyzed phosphorus flux from phosphocreatine (PC) to ATP was measured in vivo in young adult mice made reversibly hypoxic by injection of cyanide. Phosphorus spectra and saturation transfer measurements of CK-catalyzed flux were acquired using a high-field (8.45 T) nuclear magnetic resonance (NMR) spectrometer. After low cyanide doses (1-3 mg/kg of body weight), there were no measurable changes in brain pH or in concentrations of PC, the nucleoside triphosphates (including ATP), and Pi. The CK-catalyzed phosphorus flux increased about 75% after the low cyanide dose. Higher doses (4-6 mg/kg) produced a transient 30-40% decrease in PC concentration, doubling of Pi, and a 0.2 unit decrease in pH. The CK-catalyzed phosphorus flux decreased 50-80% after the higher cyanide doses. This decrease in phosphorus flux was present long after reactant concentrations returned to precyanide values. It is proposed that the increase in brain CK-catalyzed phosphorus flux with the lower cyanide doses is due to an increase in ADP concentration. The large, prolonged decrease in CK-catalyzed reaction rate in the moderately poisoned brain may be due to loss of activity of the mitochondrial CK isoform.

Astrocyte glutamate uptake during chemical hypoxia in vitro

Glutamate uptake was measured in primary rat cortical astrocyte cultures exposed to sodium azide, 2,4-dinitrophenol, or antimycin A to assess the ability of astrocytes to function under hypoxic conditions. Uptake was maintained at 54-63% of control values despite maximal inhibition of oxidative ATP production. In contrast, the glycolytic inhibitor sodium fluoride (20 mM) reduced glutamate uptake by more than 95% when glucose was the only available substrate. These data suggest that glutamate uptake is largely maintained during hypoxia provided glucose remains available. Astrocyte glutamate uptake may aid neuronal survival during conditions such as incomplete ischemia where oxygen but not glucose is depleted.

Selective effects of cyanide (100 μM) on the excitatory amino acid-induced elevation of intracellular calcium levels in neuronal culture

The effect of low concentrations of cyanide on the excitatory amino acid-induced elevations of intracellular calcium levels ([Ca2+]i) was studied in cerebellar granule cells using ratio fluorometry with fura-2. Glutamate, kainate, N-methyl-D-aspartate (NMDA), quisqualate (50 microM, each) and membrane depolarization by 40 mM KCl caused elevations of [Ca2+]i which were 10-, 10-, 3-, 2.3-, 10-fold over baseline levels, respectively. Cyanide, 100 microM, greatly augmented the increases in [Ca2+]i induced by glutamate, kainate and NMDA but not those induced by quisqualate or KCl. In the absence of these excitatory amino acids, cyanide had no significant effect in concentrations up to 400 microM. Elevations of [Ca2+]i induced by quisqualate and KCl were not significantly augmented by higher concentrations of cyanide (400 microM). Selective antagonists could block the effect of cyanide+the respective agonist; however, the calcium channel blockers, lanthanum and diltiazem lowered both NMDA- and kainate-induced elevations of [Ca2+]i, yet neither blocked increases in calcium when 100 microM cyanide was added. Collectively, these data support an interaction of cyanide with the excitatory amino acid receptor.

Cyanide selectively augments kainate- but not NMDA-induced release of glutamate and taurine

The effect of cyanide on the kainate-, quisqualate- and N-methyl-D-aspartate (NMDA)-induced release of several amino acids from cerebellar granule neurons was studied. Cyanide, 100 microM, augmented the kainate- and quisqualate-induced release of glutamate and taurine in neurons but had no effect on the NMDA-induced release of these excitatory amino acids. In addition to the interaction with the above excitatory amino acids, cyanide had effects on several amino acids independent of excitatory amino acid stimulation; cyanide treatment resulted in a significant elevation over saline controls of arginine and taurine, but not alanine, aspartate+asparagine or glycine. With the exception of taurine, this pattern was not apparent in cells treated with any of the above excitatory amino acid.