Inhibition of glutamine synthetase reduces ammonia-induced astrocyte swelling in rat

Valproate-induced hyperammonemic encephalopathy

Valproate-induced hyperammonemic encephalopathy (VHE) is an unusual complication characterized by a decreasing level of consciousness, focal neurological deficits, cognitive slowing, vomiting, drowsiness, and lethargy. We have thoroughly reviewed the predisposing factors and their screening, the biochemical and physiopathological mechanisms involved, the different treatments described, and those that are being investigated. Etiopathogenesis is not completely understood, although hyperammonemia has been postulated as the main cause of the clinical syndrome. The increase in serum ammonium level is due to several mechanisms, the most important one appearing to be the inhibition of carbamoylphosphate synthetase-I, the enzyme that begins the urea cycle. Polytherapy with several drugs, such as phenobarbital and topiramate, seems to contribute to hyperammonemia. Hyperammonemia leads to an increase in the glutamine level in the brain, which produces astrocyte swelling and cerebral edema. There are several studies that suggest that treatment with supplements of carnitine can lead to an early favorable clinical response due to the probable carnitine deficiency induced by a valproate (VPA) treatment. Development of the progressive confusional syndrome, associated with an increase in seizure frequency after VPA treatment onset, obliges us to rule out VHE by screening for blood ammonium levels and the existence of urea cycle enzyme deficiency, such as ornithine carbamoyltransferase deficiency. Electroencephalography (EEG) is characterized by signs of severe encephalopathy with continuous generalized slowing, a predominance of theta and delta activity, occasional bursts of frontal intermittent rhythmic delta activity, and triphasic waves. These EEG findings, as well as clinical manifestations and hyperammonemia, tend to normalize after VPA withdrawal.

Glutamine in the mechanism of ammonia-induced astrocyte swelling

Brain edema and the subsequent increase in intracranial pressure are the major neurological complications in fulminant hepatic failure (FHF). Brain edema in FHF is predominantly "cytotoxic" due principally to astrocyte swelling. It is generally believed that ammonia plays a key role in this process, although the mechanism by which ammonia brings about such swelling is yet to be defined. It has been postulated that glutamine accumulation in astrocytes subsequent to ammonia detoxification results in increased osmotic forces leading to cell swelling. While the hypothesis is plausible and has gained support, it has never been critically tested. In this study, we examined whether a correlation exists between cellular glutamine levels and the degree of cell swelling in cultured astrocytes exposed to ammonia. Cultured astrocytes derived from rat brain cortices were exposed to ammonia (5 mM) for different time periods and cell swelling was measured. Cultures treated with ammonia for 1-3 days showed a progressive increase in astrocyte cell volume (59-127%). Parallel treatment of astrocyte cultures with ammonia showed a significant increase in cellular glutamine content (60-80%) only at 1-4 h, a time when swelling was absent, while glutamine levels were normal at 1-3 days, a time when peak cell swelling was observed. Thus no direct correlation between cell swelling and glutamine levels was detected. Additionally, acute increase in intracellular levels of glutamine by treatment with the glutaminase inhibitor 6-diazo-5-oxo-L-norleucine (DON) after ammonia exposure also did not result in swelling. On the contrary, DON treatment significantly blocked (66%) ammonia-induced astrocyte swelling at a later time point (24 h), suggesting that some process resulting from glutamine metabolism is responsible for astrocyte swelling. Additionally, ammonia-induced free radical production and induction of the mitochondrial permeability transition (MPT) were significantly blocked by treatment with DON, suggesting a key role of glutamine in the ammonia-induced free radical generation and the MPT. In summary, our findings indicate a lack of direct correlation between the extent of cell swelling and cellular levels of glutamine. While glutamine may not be acting as an osmolyte, we propose that glutamine-mediated oxidative stress and/or the MPT may be responsible for the astrocyte swelling by ammonia.

Astrocyte dysfunction in neurological disorders: a molecular perspective

Recent work on glial cell physiology has revealed that glial cells, and astrocytes in particular, are much more actively involved in brain information processing than previously thought. This finding has stimulated the view that the active brain should no longer be regarded solely as a network of neuronal contacts, but instead as a circuit of integrated, interactive neurons and glial cells. Consequently, glial cells could also have as yet unexpected roles in the diseased brain. An improved understanding of astrocyte biology and heterogeneity and the involvement of these cells in pathogenesis offers the potential for developing novel strategies to treat neurological disorders.

Late-onset ornithine transcarbamylase deficiency in male patients: prognostic factors and characteristics of plasma amino acid profile

BACKGROUND

The occurrence of male patients with ornithine transcarbamylase (OTC) deficiency during adolescence or in adulthood has now been recognized. The aim of this study was to determine the prognostic factors that affect the prognosis of life, to explore a basis for therapeutic strategy.

METHODS

In 10 patients, nine of whom carried the R40H mutation and the other one carrying the Y55D mutation in the OTC gene, 32 demographic and laboratory data were first compared between survivors and non-survivors, using the unpaired t-test. The factors with significant difference were then subjected to multiple regression analysis.

RESULTS

The factors that exhibited significant difference were: age at onset, concentration of plasma ammonium, blood pH, and concentrations of six amino acids in plasma. The multiple regression analysis then revealed concentrations of ammonium, leucine, lysine, isoleucine, phenylalanine, glutamine and proline to be significant prognostic factors. The amino acid profile in the 10 patients showed increases in glutamine, proline, lysine, valine and methionine, and decreases in serine, ornithine and arginine. There was an inverse correlation between the age at onset and the level of the residual hepatic OTC activity.

CONCLUSION

The results implied that: (i) the plasma amino acid profile was unique, in comparison to other liver diseases; (ii) the plasma concentration of each of the (mentioned above) six amino acids was a significant predictor of prognosis; and (iii) suppression of protein catabolism, as suggested by the higher concentrations in isoleucine and leucine in the non-survivors, prevention of glutamine-induced brain edema, correction of alkalosis, and supplementation with ornithine or arginine may improve the prognosis of life.

Role of astrocytes in cerebrovascular regulation

Astrocytes send processes to synapses and blood vessels, communicate with other astrocytes through gap junctions and by release of ATP, and thus are an integral component of the neurovascular unit. Electrical field stimulations in brain slices demonstrate an increase in intracellular calcium in astrocyte cell bodies transmitted to perivascular end-feet, followed by a decrease in vascular smooth muscle calcium oscillations and arteriolar dilation. The increase in astrocyte calcium after neuronal activation is mediated, in part, by activation of metabotropic glutamate receptors. Calcium signaling in vitro can also be influenced by adenosine acting on A2B receptors and by epoxyeicosatrienoic acids (EETs) shown to be synthesized in astrocytes. Prostaglandins, EETs, arachidonic acid, and potassium ions are candidate mediators of communication between astrocyte end-feet and vascular smooth muscle. In vivo evidence supports a role for cyclooxygenase-2 metabolites, EETs, adenosine, and neuronally derived nitric oxide in the coupling of increased blood flow to increased neuronal activity. Combined inhibition of the EETs, nitric oxide, and adenosine pathways indicates that signaling is not by parallel, independent pathways. Indirect pharmacological results are consistent with astrocytes acting as intermediaries in neurovascular signaling within the neurovascular unit. For specific stimuli, astrocytes are also capable of transmitting signals to pial arterioles on the brain surface for ensuring adequate inflow pressure to parenchymal feeding arterioles. Therefore, evidence from brain slices and indirect evidence in vivo with pharmacological approaches suggest that astrocytes play a pivotal role in regulating the fundamental physiological response coupling dynamic changes in cerebral blood flow to neuronal synaptic activity. Future work using in vivo imaging and genetic manipulation will be required to provide more direct evidence for a role of astrocytes in neurovascular coupling.

Unmasked adult-onset urea cycle disorders in the critical care setting

Most often, urea cycle disorders have been described as acute onset hyperammonemia in the newborn period; however, there is a growing awareness that urea cycle disorders can present at almost any age, frequently in the critical care setting. This article presents three cases of adult-onset hyperammonemia caused by inherited defects in nitrogen processing in the urea cycle, and reviews the diagnosis, management, and pathophysiology of adult-onset urea cycle disorders. Individuals who have milder molecular urea cycle defects can lead a relatively normal life until a severe environmental stress triggers a hyperammonemic crisis. Comorbid conditions such as physical trauma often delay the diagnosis of the urea cycle defect. Prompt recognition and treatment are essential in determining the outcome of these patients.

Endogenous neuro-protectants in ammonia toxicity in the central nervous system: facts and hypotheses

The paper overviews experimental evidence suggestive of the engagement of three endogenous metabolites: taurine, kynurenic acid, and glutathione (GSH) in the protection of central nervous system (CNS) cells against ammonia toxicity. Intrastriatal administration of taurine via microdialysis probe attenuates ammonia-induced accumulation of extracellular cyclic guanosine monophosphate (cGMP) resulting from over-activation of the N-methyl-D: -aspartate/nitric oxide (NMDA/NO) pathway, and this effect involves agonistic effect of taurine on the GABA-A and glycine receptors. Taurine also counteracts generation of free radicals, increased release of dopamine, and its metabolism to dihydroxyphenylacetic acid (DOPAC). Taurine reduces ammonia-induced increase of cell volume (edema) in cerebrocortical slices by a mechanism involving GABA-A receptors. Massive release of radiolabeled or endogenous taurine from CNS tissues by ammonia in vivo and in vitro is thought to promote its neuroprotective action, by making the amino acid available for interaction with cell membranes and/or by driving excess water out of the CNS cells (astrocytes) that underwent ammonia-induced swelling. Ammonia in vivo and in vitro affects in variable ways the synthesis of kynurenic acid (KYNA). Since KYNA is an endogenous NMDA receptor antagonist with a high affinity towards its glycine site, changes in its content may counter over-activation or depression of glutaminergic transmission observed at the different stages of hyperammonemia. GSH is a major antioxidant in the CNS whose synthesis is partly compartmented between neurons and astrocytes: astrocytic GSH is a source of precursors for the synthesis of neuronal GSH. Ammonia in vitro stimulates GSH synthesis in cultured astrocytes, which may compensate for increased GSH consumption (decreased GSH/GSSG ratio) in neurons.

Protein tyrosine nitration in hyperammonemia and hepatic encephalopathy

Hepatic encephalopathy is seen as a clinical manifestation of a chronic low grade cerebral edema, which is thought to trigger disturbances of astrocyte function, glioneuronal communication, and finally HE symptoms. In cultured astrocytes, hypoosmotic swelling triggers a rapid oxidative stress response, which involves the action of NADPH oxidase isoenzymes, followed by tyrosine nitration of distinct astrocytic proteins. Oxidative stress and protein tyrosine nitration (PTN) are also observed in response to ammonia, inflammatory cytokines, such as TNF-alpha or interferons, and benzodiazepines with affinity to the peripheral benzodiazepine receptor (PBR). NMDA receptor activation was identified as upstream event in protein tyrosine nitration (PTN). Cerebral PTN is also found in vivo after administration of ammonia, benzodiazepines or lipopolysaccharide and in portocaval shunted rats. PTN predominantly affects astrocytes surrounding cerebral vessels with potential impact on blood-brain-barrier permeability. Among the tyrosine-nitrated proteins, glutamine synthetase, GAPDH, extracellular signal-regulated kinase and the PBR were identified. PTN of glutamine synthetase is associated with inactivation of the enzyme. Thus, factors known to trigger hepatic encephalopathy induce oxidative/nitrosative stress on astrocytes with protein modifications through PTN. The pathobiochemical relevance of astrocytic PTN for the development of HE symptoms remains to be established.

Mechanisms of ammonia-induced astrocyte swelling

Astrocyte swelling represents the major factor responsible for the brain edema associated with fulminant hepatic failure (FHF). The edema may be of such magnitude as to increase intracranial pressure leading to brain herniation and death. Of the various agents implicated in the generation of astrocyte swelling, ammonia has had the greatest amount of experimental support. This article reviews mechanisms of ammonia neurotoxicity that contribute to astrocyte swelling. These include oxidative stress and the mitochondrial permeability transition (MPT). The involvement of glutamine in the production of cell swelling will be highlighted. Evidence will be provided that glutamine induces oxidative stress as well as the MPT, and that these events are critical in the development of astrocyte swelling in hyperammonemia.

Cytotoxic edema is responsible for raised intracranial pressure in fulminant hepatic failure: in vivo demonstration using diffusion-weighted MRI in human subjects

It is not clear whether cerebral edema in fulminant hepatic failure is predominantly vasogenic or cytotoxic, though cytotoxic edema due to astrocyte swelling is more likely. Diffusion-weighted magnetic resonance imaging can differentiate vasogenic from cytotoxic edema. We performed diffusion-weighted imaging in patients with fulminant hepatic failure to clarify the issue by measuring apparent diffusion coefficient, which quantifies movement of water molecule across cell membrane. Seven patients with fulminant hepatic failure underwent conventional and diffusion-weighted magnetic resonance imaging. Apparent diffusion coefficient was measured in four cortical areas and 12 deep white and gray matter regions in both cerebral hemispheres. Thirteen healthy subjects served as controls. The apparent diffusion coefficient values in patients and controls were compared using Wilcoxon signed rank test. Two patients who survived underwent repeat imaging using same protocol. Patients with FHF had significantly lower apparent diffusion coefficient in all cortical and deep white and gray matter regions of interest compared to controls (p < 0.001), suggesting cytotoxic cell swelling. In two survivors with repeat imaging, one showed complete resolution while the changes persisted in the other, suggesting ischemic injury. Cerebral edema in fulminant hepatic failure is predominantly due to cytotoxic edema.

Chronic and acute ammonia toxicity in mudskippers, Periophthalmodon schlosseri and Boleophthalmus boddaerti: brain ammonia and glutamine contents, and effects of methionine sulfoximine and MK801

The objective of this study was to elucidate if chronic and acute ammonia intoxication in mudskippers, Periophthalmodon schlosseri and Boleophthalmus boddaerti, were associated with high levels of ammonia and/or glutamine in their brains, and if acute ammonia intoxication could be prevented by the administration of methionine sulfoximine [MSO; an inhibitor of glutamine synthetase (GS)] or MK801 [an antagonist of N-methyl D-aspartate type glutamate (NMDA) receptors]. For P. schlosseri and B. boddaerti exposed to sublethal concentrations (100 and 8 mmol l(-1) NH4Cl, respectively, at pH 7.0) of environmental ammonia for 4 days, brain ammonia contents increased drastically during the first 24 h, and they reached 18 and 14.5 micromol g(-1), respectively, at hour 96. Simultaneously, there were increases in brain glutamine contents, but brain glutamate contents were unchanged. Because glutamine accumulated to exceptionally high levels in brains of P. schlosseri (29.8 micromol g(-1)) and B. boddaerti (12.1 micromol g(-1)) without causing death, it can be concluded that these two mudskippers could ameliorate those problems associated with glutamine synthesis and accumulation as observed in patients suffering from hyperammonemia. P. schlosseri and B. boddaerti could tolerate high doses of ammonium acetate (CH3COONH4) injected into their peritoneal cavities, with 24 h LC50 of 15.6 and 12.3 micromol g(-1) fish, respectively. After the injection with a sublethal dose of CH3COONH4 (8 micromol g(-1) fish), there were significant increases in ammonia (5.11 and 8.36 micromol g(-1), respectively) and glutamine (4.22 and 3.54 micromol g(-1), respectively) levels in their brains at hour 0.5, but these levels returned to normal at hour 24. By contrast, for P. schlosseri and B. boddaerti that succumbed within 15-50 min to a dose of CH3COONH4 (15 and 12 micromol g(-1) fish, respectively) close to the LC50 values, the ammonia contents in the brains reached much higher levels (12.8 and 14.9 micromol g(-1), respectively), while the glutamine level remained relatively low (3.93 and 2.67 micromol g(-1), respectively). Thus, glutamine synthesis and accumulation in the brain was not the major cause of death in these two mudskippers confronted with acute ammonia toxicity. Indeed, MSO, at a dosage (100 microg g(-1) fish) protective for rats, did not protect B. boddaerti against acute ammonia toxicity, although it was an inhibitor of GS activities from the brains of both mudskippers. In the case of P. schlosseri, MSO only prolonged the time to death but did not reduce the mortality rate (100%). In addition, MK801 (2 microg g(-1) fish) had no protective effect on P. schlosseri and B. boddaerti injected with a lethal dose of CH3COONH4, indicating that activation of NMDA receptors was not the major cause of death during acute ammonia intoxication. Thus, it can be concluded that there are major differences in mechanisms of chronic and acute ammonia toxicity between brains of these two mudskippers and mammalian brains.

Effect of glutamine synthetase inhibition on astrocyte swelling and altered astroglial protein expression during hyperammonemia in rats

Inhibition of glutamine synthesis reduces astrocyte swelling and associated physiological abnormalities during acute ammonium acetate infusion in anesthetized rats. We tested the hypothesis that inhibition of glutamine accumulation during more prolonged ammonium acetate infusion in unanesthetized rats reduces cortical astrocyte swelling and immunohistochemical changes in astrocytic proteins. Rats received a continuous i.v. infusion of either sodium acetate or ammonium acetate for 24 h to increase plasma ammonia from about 30-400 mumol/l. Cohorts were pretreated with vehicle or l-methionine-S-sulfoximine (MSO; 0.83 mmol/kg). MSO reduced glutamine synthetase activity by 57% and glutamine synthetase immunopositive cell number by 69%, and attenuated cortical glutamine accumulation by 71%. Hyperammonemia increased the number of swollen astrocytes in cortex and MSO reduced this increase to control values. The number of glial fibrillary acidic protein immunopositive cells in cortex was greater in hyperammonemic rats and the increase in superficial cortical layers was attenuated by MSO. Immunoreactivity for the gap junction protein connexin-43 in the neuropil, assessed by optical density, was greater in the hyperammonemic group compared with controls, but this increase was not attenuated by MSO. No changes in the optical density of GLT1 glutamate transporter immunoreactivity in cortex were detected in any group. We conclude that glutamine synthetase inhibition reduces astrocyte swelling and ameliorates some of the reactive astroglial cytoskeletal alterations seen at 24 h of hyperammonemia, but that gap junction changes in astrocytes occur independently of glutamine accumulation and swelling.

Five tropical air-breathing fishes, six different strategies to defend against ammonia toxicity on land

Most tropical fishes are ammonotelic, producing ammonia and excreting it as NH3 by diffusion across the branchial epithelia. Hence, those air-breathing tropical fishes that survive on land briefly or for an extended period would have difficulties in excreting ammonia when out of water. Ammonia is toxic, but some of these air-breathing fishes adopt special biochemical adaptations to ameliorate the toxicity of endogenous ammonia accumulating in the body. The amphibious mudskipper Periophthalmodon schlosseri, which is very active on land, reduces ammonia production by suppressing amino acid catabolism (strategy 1) during aerial exposure. It can also undergo partial amino acid catabolism, leading to the accumulation of alanine (strategy 2) to support locomotory activities on land. In this case, alanine formation is not an ammonia detoxification process but reduces the production of endogenous ammonia. The snakehead Channa asiatica, which exhibits moderate activities on land although not truly amphibious, accumulates both alanine and glutamine in the muscle, with alanine accounting for 80% of the deficit in reduction in ammonia excretion during air exposure. Unlike P. schlosseri, C. asiatica apparently cannot reduce the rates of protein and amino acid catabolism and is incapable of utilizing partial amino acid catabolism to support locomotory activities on land. Unlike alanine formation, glutamine synthesis (strategy 3) represents an ammonia detoxification mechanism that, in effect, removes the accumulating ammonia. The four-eyed sleeper Bostrichyths sinensis, which remains motionless during aerial exposure, detoxifies endogenous ammonia to glutamine for storage. The slender African lungfish Protopterus dolloi, which can aestivate on land on a mucus cocoon, has an active ornithine-urea cycle and converts endogenous ammonia to urea (strategy 4) for both storage and subsequent excretion. Production of urea and glutamine are energetically expensive and appear to be adopted by fishes that remain relatively inactive on land. The Oriental weatherloach Misgurnus anguillicaudatus, which actively burrows into soft mud during drought, manipulates the pH of the body surface to facilitate NH3 volatilization (strategy 5) and develops high ammonia tolerance at the cellular and subcellular levels (strategy 6) during aerial exposure. Hence, with regard to excretory nitrogen metabolism, modern tropical air-breathing fishes exhibit a variety of strategies to survive on land, and they represent a spectrum of specimens through which we may examine various biochemical adaptations that would have facilitated the invasion of the terrestrial habitat by fishes during evolution.

Glutamine-dependent inhibition of pial arteriolar dilation to acetylcholine with and without hyperammonemia in the rat

Glutamine has been shown to influence endothelial-dependent relaxation and nitric oxide production in vitro, possibly by limiting arginine availability, but its effects in vivo have not been well studied. Hyperammonemia is a pathophysiological condition in which glutamine is elevated and contributes to depressed CO(2) reactivity of cerebral arterioles. We tested the hypothesis that acute hyperammonemia decreases pial arteriolar dilation to acetylcholine in vivo and that this decrease could be prevented by inhibiting glutamine synthetase with L-methionine-S-sulfoximine (MSO) or by intravenous infusion of L-arginine. Pial arteriolar diameter responses to topical superfusion of acetylcholine were measured in anesthetized rats before and at 6 h of infusion of either sodium or ammonium acetate. Ammonium acetate infusion increased plasma ammonia concentration from approximately 30 to approximately 600 microM and increased cerebral glutamine concentration fourfold. Arteriolar dilation to acetylcholine was intact after infusion of sodium acetate in groups pretreated with vehicle or with MSO plus methionine, which was coadministered to prevent MSO-induced seizures. In contrast, dilation in response to acetylcholine was completely blocked in hyperammonemic groups pretreated with vehicle or methionine alone. However, MSO plus methionine administration before hyperammonemia, which maintained cerebral glutamine concentration at control values, preserved acetylcholine dilation. Intravenous infusion of L-arginine during the last 2 h of the ammonium acetate infusion partially restored dilation to acetylcholine without reducing cerebral glutamine accumulation. Superfusion of 1 or 2 mM L-glutamine through the cranial window for 1 h in the absence of hyperammonemia attenuated acetylcholine dilation but had no effect on endothelial-independent dilation to nitroprusside. We conclude that 1) hyperammonemia reduces acetylcholine-evoked dilation in cerebral arterioles, 2) this reduction depends on increased glutamine rather than ammonium ions, and 3) increasing arginine partially overcomes the inhibitory effect of glutamine.

Astrocytic swelling in cerebral ischemia as a possible cause of injury and target for therapy

In this viewpoint article, I summarize data showing that the astrocytic swelling that occurs early after the acute CNS pathologies ischemia and traumatic brain injury is damaging. We have proposed that one reason may be the release of excitatory amino acids (EAA) via volume-activated anion channels (VRACs) that are activated by such swelling. This release could be a target for therapy, which could involve blocking the astrocytic swelling or the release mechanisms. The transport mechanisms likely causing the early astrocytic swelling are therefore summarized. In terms of targeting the release mechanisms, we have found a potent inhibitor of VRACs, tamoxifen, to be strongly neuroprotective in focal ischemia with a therapeutic window of 3 h after initiation of the ischemia. The question, however, of whether neuroprotection by tamoxifen can be solely attributed to VRAC inhibition in astrocytes has yet to be resolved.

Oxidative stress in the pathogenesis of hepatic encephalopathy

The pathogenesis of hepatic encephalopathy (HE) remains elusive. While it is clear that ammonia is the likely toxin and that astrocytes are the main target of its neurotoxicity, precisely how ammonia brings about cellular injury is poorly understood. Studies over the past decade have invoked the concept of oxidative stress as a pathogenetic mechanism for ammonia neurotoxicity. This review sets out the arguments in support of this concept based on evidence derived from human observations, animal studies, and cell culture investigations. The consequences and potential therapeutic implications of oxidative stress in HE are also discussed.

Role of ammonia in reversal of glutamate-mediated Müller cell swelling in the rat retina

Glutamate is thought to participate in a variety of retinal degenerative disorders. However, when exposed to glutamate at concentrations up to 1 mM, ex vivo rat retinas typically exhibit Müller cell swelling, but not excitotoxic neuronal damage. This Müller cell swelling is reversible following glutamate washout, indicating that the glial edema is not required for glutamate-induced neuronal injury. It is unclear whether glutamate directly induces the Müller cell swelling or whether a metabolite of glutamate such as glutamine acts as an osmolyte to generate the cellular edema. To examine this issue, ex vivo rat retinas were exposed to 1 mM glutamate or 1 mM glutamine and were evaluated histologically. Glutamate was also combined with 1 mM ammonia or with methionine sulfoximine (MSO), an inhibitor of glutamine synthetase, the enzyme that catalyzes the synthesis of glutamine from glutamate and ammonia. Glutamate-mediated Müller cell swelling was blocked by co-administration of ammonia and the reversibility of Müller cell swelling was inhibited by MSO administered following glutamate exposure. Glutamine alone failed to induce Müller cell swelling. These results indicate that glutamate-mediated Müller cell swelling is unlikely to result from glutamine accumulation. Rather, conversion of glutamate to glutamine in a reaction involving ammonia helps reverse Müller cell swelling following exposure to exogenous glutamate.

Cognitive outcome in urea cycle disorders

Despite treatment, cognitive and motor deficits are common in individuals with inherited urea cycle disorders. However, the extent to which the deficits involve specific cognitive or sensorimotor domains is unknown. Furthermore, little is known about the neurochemical basis of cognitive impairment in these disorders. This paper reviews studies of cognitive and motor dysfunction in urea cycle disorders, and discusses potential venues for investigation of the underlying neural basis that may elucidate these defects. Such methods of investigation may serve as a model for studying the relationship between genes, biochemical markers, brain function, and behavior in other metabolic diseases.

Amino acid signalling and the integration of metabolism

It has become clear in recent years that amino acids are not only important as substrates for various metabolic pathways but that they can also activate a nutrient-sensitive, mTOR-mediated, signalling pathway in synergy with insulin. Leucine is the most effective amino acid in this regard. The signalling pathway is antagonised by AMP-activated protein kinase. Amino acid signalling stimulates protein synthesis and inhibits (autophagic) proteolysis. In addition, many amino acids cause an increase in cell volume. Cell swelling per se stimulates synthesis of protein, glycogen, and lipid, in part by further stimulating signalling and in part by unrelated mechanisms. Amino acids also stimulate signalling in beta-cells and stimulate beta-cell growth and proliferation. This results in increased production of insulin, which enhances the anabolic (and anti-catabolic) properties of amino acids. Finally, amino acid-dependent signalling controls the production of leptin by adipocytes, and thus contributes to the regulation of appetite.

Suppression of ammonia-induced astrocyte swelling by cyclosporin A

Brain edema is a serious complication of hepatic encephalopathy associated with fulminant hepatic failure (FHF). A major component of the edema seems to be cytotoxic, involving astrocyte swelling. Although the mechanism of brain edema in FHF is incompletely understood, it is generally believed that ammonia is involved critically in this process. Recent studies have shown that exposure of cultured astrocytes to ammonia results in the mitochondrial permeability transition (MPT), a phenomenon associated with mitochondrial failure and subsequent cellular dysfunction. The present study examined the potential role of the MPT in the astrocyte swelling associated with ammonia toxicity. Treatment of cultured astrocytes with ammonia (5 mM) caused a time-dependent increase in astrocyte cell volume (swelling), which was completely inhibited by the MPT inhibitor cyclosporin A (CsA). In this study, CsA also inhibited the ammonia-induced aquaporin 4 (AQP4) upregulation, which had been shown previously to be increased in cultured astrocytes by ammonia treatment. These findings suggest that the MPT plays a significant role in the ammonia-induced astrocyte swelling and may contribute to the brain edema associated with FHF.

Cell-selective effects of ammonia on glutamate transporter and receptor function in the mammalian brain

Increased brain ammonia concentrations are a hallmark feature of several neurological disorders including congenital urea cycle disorders, Reye's syndrome and hepatic encephalopathy (HE) associated with liver failure. Over the last decade, increasing evidence suggests that hyperammonemia leads to alterations in the glutamatergic neurotransmitter system. Studies utilizing in vivo and in vitro models of hyperammonemia reveal significant changes in brain glutamate levels, glutamate uptake and glutamate receptor function. Extracellular brain glutamate levels are consistently increased in rat models of acute liver failure. Furthermore, glutamate transport studies in both cultured neurons and astrocytes demonstrate a significant suppression in the high affinity uptake of glutamate following exposure to ammonia. Reductions in NMDA and non-NMDA glutamate receptor sites in animal models of acute liver failure suggest a compensatory decrease in receptor levels in the wake of rising extracellular levels of glutamate. Ammonia exposure also has significant effects on metabotropic glutamate receptor activation with implications, although less clear, that may relate to the brain edema and seizures associated with clinical hyperammonemic pathologies. Therapeutic measures aimed at these targets could result in effective measures for the prevention of CNS consequences in hyperammonemic syndromes.

Induction of the mitochondrial permeability transition in cultured astrocytes by glutamine

Ammonia is a toxin that has been strongly implicated in the pathogenesis of hepatic encephalopathy (HE), and astrocytes appear to be the principal target of ammonia toxicity. Glutamine, a byproduct of ammonia metabolism, has been implicated in some of the deleterious effects of ammonia on the CNS. We have recently shown that ammonia induces the mitochondrial permeability transition (MPT) in cultured astrocytes, but not in neurons. We therefore determined whether glutamine is also capable of inducing the MPT in cultured astrocytes. Astrocytes were treated with glutamine (4.5 mM) for various time periods and the MPT was assessed by changes in 2-deoxyglucose (2-DG) mitochondrial permeability, calcein fluorescence assay, and by changes in cyclosporin A (CsA)-sensitive inner mitochondrial membrane potential (deltapsi(m)) using the potentiometric dye, JC-1. Astrocytes treated with glutamine significantly increased 2-DG permeability (120%, P<0.01), decreased mitochondrial calcein fluorescence, and concomitantly dissipated the deltapsi(m). All of these effects were blocked by CsA. These data indicate that glutamine induces the MPT in cultured astrocytes. The induction of the MPT by glutamine in astrocytes, and the subsequent development of mitochondrial dysfunction, may partially explain the deleterious affects of glutamine on the CNS in the setting of hyperammonemia.

Excitotoxic mechanism of cell swelling in rat cerebral cortical slices treated acutely with ammonia

Swelling of CNS cells due to endogenous ammonia is a major cause of cerebral oedema in hyperammonaemic encephalopathies. In the present study, incubation in the presence of 5mM ammonium acetate ("ammonia") decreased steady-state distribution of [14C]inulin within incubated rat cerebrocortical minislices, indicating cell swelling. NMDA receptor antagonists, MK-801 ((+)-5-methyl-10,11-dihydro-5H-dibenzo [a,d]cycloheptene-5,10-imine maleate,10 microM) and DL-AP-5 (DL-2-amino-5-phosphonovaleric acid, 250 microM), a nitric oxide synthase inhibitor, L-nitroarginine (L-NNA, 500 microM), and an antioxidant, taurine (Tau, 10 mM), markedly attenuated the cell volume-increasing effect of ammonia. The effect of Tau (10mM) was abolished by the GABA(A) receptor antagonist bicuculline (100 microM), but was unaffected by the Tau transport inhibitor guanidynoethyl-sulfonate (GES, 500 microM). Ammonia increased the slice content of Gln, an amino acid whose excess accumulation has been implicated in hyperammonemic oedema. However, treatments that reduced the cell volume did not affect Gln content. These results indicate that ammonia-induced cell swelling is in a large degree mediated by overactivation of NMDA receptors and the ensuing generation of NO and free radicals.

Ammonia neurotoxicity: role of the mitochondrial permeability transition

Hepatic encephalopathy (HE) is an important cause of morbidity and mortality in patients with severe liver disease. Although the mechanisms responsible for HE remain elusive, ammonia is generally considered to be involved in its pathogenesis, and astrocytes are thought to be the principal target of ammonia neurotoxicity. Altered bioenergetics and oxidative stress are also thought to play a major role in this disorder. In this paper, we present data invoking the mitochondrial permeability transition (MPT) as a factor in the pathogenesis of HE/hyperammonemia. The MPT is a Ca2+-dependent, cyclosporin A (CsA) sensitive process due to the opening of a pore in the inner mitochondrial membrane that leads to a collapse of ionic gradients and ultimately to mitochondrial dysfunction. Many of the factors that facilitate the induction of the MPT are also known to be implicated in the mechanism of HE, including free radicals, Ca2+, nitric oxide, alkaline pH, and glutamine. We have recently shown that treatment of cultured astrocytes with 5 mM NH4Cl resulted in a dissipation of the mitochondrial membrane potential (delta(psi)m), which was sensitive to CsA. Similarly treated cultured neurons failed to show a loss of the delta(psi)m. Further support for the ammonia induction of the MPT was obtained by observing an increase in mitochondrial permeability to 2-deoxyglucose-6-phosphate, and a decrease in calcein fluorescence in astrocytes after ammonia treatment, both of which were also blocked by CsA. CsA was likewise capable of exerting a protective effect against hyperammonemia in mice. Taken together, our data suggest that the MPT represents an important component of the pathogenesis of HE and other hyperammonemic states.

Amino acids as regulators and components of nonproteinogenic pathways

Amino acids are not only important precursors for the synthesis of proteins and other N-containing compounds, but also participate in the regulation of major metabolic pathways. Glutamate and aspartate, for example, are components of the malate/aspartate shuttle and their concentrations control the rate of mitochondrial oxidation of glycolytic NADH. Glutamate also controls the rate of urea synthesis, not only as the precursor of ammonia and aspartate, but as substrate for synthesis of N-acetylglutamate, the essential activator of carbamoyl-phosphate synthase. This mechanism allows large variations in urea synthesis at relatively constant ammonia concentrations. Increases in intracellular amino acid concentration increase cell volume. Cell swelling per se has anabolic effects on protein, carbohydrate and lipid metabolism: enhanced synthesis of macromolecules compensates for increases in intracellular osmolarity. Mechanisms responsible for cell swelling-induced changes in pathway fluxes include changes in intracellular ion concentrations and in signal transduction. Specific amino acids (e.g., leucine) stimulate protein synthesis and inhibit (autophagic) protein degradation independent of changes in cell volume because they stimulate mTOR (mammalian target of rapamycin), a protein kinase, which is one of the components of a signal transduction pathway used by insulin. When the cellular energy state is low, stimulation of mTOR by amino acids is prevented by activation of AMP-dependent protein kinase. Amino acid-dependent signaling also promotes insulin production by beta-cells. This further adds to the anabolic properties of amino acids. It is concluded that amino acids are important regulators of major metabolic pathways.

Arginase deficiency with lethal neonatal expression: evidence for the glutamine hypothesis of cerebral edema

We describe a rare and lethal case of arginase deficiency in a 2-day-old female infant with encephalopathy and cerebral edema. The levels of glutamine and arginine but not ammonia were markedly elevated, lending support to the "glutamine hypothesis" as the mechanism of cerebral edema in urea cycle defects.

Benzodiazepine-induced protein tyrosine nitration in rat astrocytes

Recent studies indicate that ammonia and hypoosmotic astrocyte swelling can induce protein tyrosine nitration (PTN) in astrocytes with potential pathogenetic relevance for hepatic encephalopathy (HE). Because HE episodes are known to be precipitated also by sedatives, the effects of benzodiazepines on PTN in cultured rat astrocytes and rat brain in vivo were studied. In cultured rat astrocytes, diazepam, PK11195, Ro5-4864, and the benzodiazepine binding inhibitor (DBI), which acts on peripheral-type benzodiazepine receptors, induced PTN. Clonazepam, a specific ligand of the central benzodiazepine receptor, failed to induce PTN. Nanomolar concentrations of DBI and PK11195 were sufficient to increase PTN, and diazepam effects were already observed at concentrations of 1 micromol/L. Diazepam-induced PTN was insensitive to NOS inhibition and uric acid but was blunted by MK-801, BAPTA-AM, W13, and catalase, suggesting an involvement of NMDA-receptor activation, elevation of the cytosolic Ca(2+) concentration [Ca(2+)](i), and hydrogen peroxide. Diazepam induced a plateau-like increase in [Ca(2+)](i) and the generation of reactive oxygen intermediates (ROIs), which are both blunted by MK-801 and BAPTA-AM. The expression of functional N-methyl-D-aspartate (NMDA) receptors on cultured rat astrocytes was confirmed by reverse transcriptase polymerase chain reaction, Western blot analysis, immunhistochemistry, and receptor autoradiography. Astroglial PTN is also found in brains from rats challenged with diazepam, indicating the in vivo relevance of the present findings. In conclusion, production of ROIs and increased PTN by benzodiazepines may alter astrocyte function and thereby contribute to the precipitation of HE episodes.

The role of inhibitory amino acidergic neurotransmission in hepatic encephalopathy: a critical overview

Gamma-Aminobutyric acid (GABA) is the main inhibitory amino acid in the central nervous system (CNS). Experiments with animal models of HE, and with brain slices or cultured CNS cells treated with ammonia, have documented changes in GABA distribution and transport, and modulation of the responses of both the GABA(A)-benzodiazepine receptor complex and GABA(B) receptors. Although many of the data point to an enhancement of GABAergic transmission probably contributing to HE, the evidence is not unequivocal. The major weaknesses of the GABA theory are (1) in a vast majority of HE models, there were no alterations of GABA content in the brain tissue and/or extracellular space, indicating that exposure of neurons to GABA may not have been altered, (2) changes in the affinity and capacity of GABA receptor binding were either absent or qualitatively different in HE models of comparable severity and duration, and (3) no sound changes in the GABAergic system parameters were noted in clinical cases of HE. Taurine (Tau) is an amino acid that is thought to mimic GABA function because of its agonistic properties towards GABA(A) receptors, and to contribute to neuroprotection and osmoregulation. These effects require Tau redistribution between the different cell compartments and the extracellular space. Acute treatment with ammonia evokes massive release of radiolabeled or endogenous Tau from CNS tissues in vivo and in vitro, and the underlying mechanism of Tau release differs from the release evoked by depolarizing conditions or hypoosmotic treatment. Subacute or chronic HE, and also long-term treatment of cultured CNS cells in vitro with ammonia, increase spontaneous Tau "leakage" from the tissue. This is accompanied by a decreased potassium- or hypoosmolarity-induced release of Tau and often by cell swelling, indicating impaired osmoregulation. In in vivo models of HE, Tau leakage is manifested by its increased accumulation in the extrasynaptic space, which may promote inhibitory neurotransmission and/or cell membrane protection. In chronic HE in humans, decreased Tau content in CNS is thought to be one of the causes of cerebral edema. However, understanding of the impact of the changes in Tau content and transport on the pathogenic mechanisms of HE is hampered by the lack of clear-cut evidence regarding the various roles of Tau in the normal CNS.

Ammonia-induced heme oxygenase-1 expression in cultured rat astrocytes and rat brain in vivo

Ammonia is a key factor in the pathogenesis of hepatic encephalopathy (HE), which is a major complication in acute and chronic liver failure and other hyperammonemic states. The molecular mechanisms underlying ammonia neurotoxicity and the functional consequences of ammonia on gene expression in astrocytes are incompletely understood. Using cDNA array hybridization technique we identified ammonia as a trigger of heme oxygenase-1 (HO-1) mRNA levels in cultured rat astrocytes. As shown by Northern and Western blot analysis, HO-1 mRNA levels were upregulated by ammonia (0.1-5 mmol/L) after 24 h and protein expression after 72 h in astrocytes. These ammonia effects on HO-1 are probably triggered to a minor extent by ammonia-induced glutamine synthesis or by astrocyte swelling, because HO-1 expression was not inhibited by the glutamine synthetase inhibitor methionine sulfoximine (which abrogated ammonia-induced cell swelling in cultured astrocytes), and ammonia-induced HO-1 expression could only partly be mimicked by hypoosmotic astrocyte swelling. Hypoosmotic (205 mOsm/L) exposure of astrocytes led even to a decrease in HO-1 mRNA levels within 4 h, whereas hyperosmotic (405 mOsm/L) exposure increased HO-1 mRNA expression. After 24 h, hypoosmolarity slightly raised HO-1 mRNA expression. Taurine and melatonin diminished ammonia-induced HO-1 mRNA or protein expression, whereas other antioxidants (dimethylthiourea, butylated hydroxytoluene, N-acetylcysteine, and reduced glutathione) increased HO-1 mRNA levels under ammonia-free conditions. An in vivo relevance is suggested by the finding that increased HO-1 expression occurs in the brain cortex from acutely ammonia-intoxicated rats. It is concluded that ammonia-induced HO-1 expression may contribute to cerebral hyperemia in hyperammonic states.

Hypothermia for the management of intracranial hypertension in acute liver failure

Increased intracranial pressure in patients with acute liver failure (ALF) remains a major immediate cause of mortality. Several studies in animal models of ALF set the stage for the clinical application of moderate hypothermia in man. Studies in patients with ALF and increased intracranial hypertension have shown that temperatures as low as 32 degrees C are safe and effectively reduce increased intracranial pressure unresponsive to other medical therapies, and can be used as a successful bridge to liver transplantation. Data from studies in patients undergoing liver transplantation for ALF suggest that increases in intracranial pressure can be prevented during the dissection and reperfusion phases of the operation if the patients are maintained hypothermic during surgery. The present review focuses upon the clinical aspects of using hypothermia as a treatment of increased intracranial pressure in patients with ALF.

Glutamine synthetase in brain: effect of ammonia

Glutamine synthetase (GS) in brain is located mainly in astrocytes. One of the primary roles of astrocytes is to protect neurons against excitotoxicity by taking up excess ammonia and glutamate and converting it into glutamine via the enzyme GS. Changes in GS expression may reflect changes in astroglial function, which can affect neuronal functions. Hyperammonemia is an important factor responsible of hepatic encephalopathy (HE) and causes astroglial swelling. Hyperammonemia can be experimentally induced and an adaptive astroglial response to high levels of ammonia and glutamate seems to occur in long-term studies. In hyperammonemic states, astroglial cells can experience morphological changes that may alter different astrocyte functions, such as protein synthesis or neurotransmitters uptake. One of the observed changes is the increase in the GS expression in astrocytes located in glutamatergic areas. The induction of GS expression in these specific areas would balance the increased ammonia and glutamate uptake and protect against neuronal degeneration, whereas, decrease of GS expression in non-glutamatergic areas could disrupt the neuron-glial metabolic interactions as a consequence of hyperammonemia. Induction of GS has been described in astrocytes in response to the action of glutamate on active glutamate receptors. The over-stimulation of glutamate receptors may also favour nitric oxide (NO) formation by activation of NO synthase (NOS), and NO has been implicated in the pathogenesis of several CNS diseases. Hyperammonemia could induce the formation of inducible NOS in astroglial cells, with the consequent NO formation, deactivation of GS and dawn-regulation of glutamate uptake. However, in glutamatergic areas, the distribution of both glial glutamate receptors and glial glutamate transporters parallels the GS location, suggesting a functional coupling between glutamate uptake and degradation by glutamate transporters and GS to attenuate brain injury in these areas. In hyperammonemia, the astroglial cells located in proximity to blood-vessels in glutamatergic areas show increased GS protein content in their perivascular processes. Since ammonia freely crosses the blood-brain barrier (BBB) and astrocytes are responsible for maintaining the BBB, the presence of GS in the perivascular processes could produce a rapid glutamine synthesis to be released into blood. It could, therefore, prevent the entry of high amounts of ammonia from circulation to attenuate neurotoxicity. The changes in the distribution of this critical enzyme suggests that the glutamate-glutamine cycle may be differentially impaired in hyperammonemic states.

Suppression of ammonia-induced swelling by aspartate but not by ornithine in primary cultures of rat astrocytes

Cerebral edema with a rise in intracranial pressure is the hallmark of fulminant hepatic failure (FHF) and acute hyperammonemic (HA) states and is characterized by a poor survival rate. Astrocytes are the cells in brain which are swollen in these conditions. Several hypotheses have been proposed to explain the mechanism of cerebral edema in FHF and treatment strategies have evolved based on these putative mechanisms. Treatment with a mixture of ornithine and aspartate has been proven to be clinically beneficial as it reduces edema and improves the neurological status. It has been suggested that these two amino acids generate the glutamate required for the synthesis of glutamine and that they also enhance urea synthesis in surviving hepatocytes in FHF and HA. Presently, we report that of these two amino acids, only aspartate is effective in suppressing ammonia-induced swelling in primary cultures of astrocytes, while ornithine is ineffective. These results are discussed in relation to the metabolism of aspartate and ornithine in astrocytes, with an emphasis on glutamine synthesis and the malate-aspartate shuttle (MAS). We propose that the ability of aspartate to generate glutamate in the cytosol for glutamine synthesis and oxaloacetate in mitochondria to support the citric acid cycle play a role in its ability to reduce ammonia-induced swelling in astrocytes.

Disorders of glutamate metabolism

The significant role the amino acid glutamate assumes in a number of fundamental metabolic pathways is becoming better understood. As a central junction for interchange of amino nitrogen, glutamate facilitates both amino acid synthesis and degradation. In the liver, glutamate is the terminus for release of ammonia from amino acids, and the intrahepatic concentration of glutamate modulates the rate of ammonia detoxification into urea. In pancreatic beta-cells, oxidation of glutamate mediates amino acid-stimulated insulin secretion. In the central nervous system, glutamate serves as an excitatory neurotransmittor. Glutamate is also the precursor of the inhibitory neurotransmittor GABA, as well as glutamine, a potential mediator of hyperammonemic neurotoxicity. The recent identification of a novel form of congenital hyperinsulinism associated with asymptomatic hyperammonemia assigns glutamate oxidation by glutamate dehydrogenase a more important role than previously recognized in beta-cell insulin secretion and hepatic and CNS ammonia detoxification. Disruptions of glutamate metabolism have been implicated in other clinical disorders, such as pyridoxine-dependent seizures, confirming the importance of intact glutamate metabolism. This article will review glutamate metabolism and clinical disorders associated with disrupted glutamate metabolism.

Role of glutamine in cerebral nitrogen metabolism and ammonia neurotoxicity

Ammonia enters the brain by diffusion from the blood or cerebrospinal fluid, or is formed in situ from the metabolism of endogenous nitrogen-containing substances. Despite its central importance in nitrogen homeostasis, excess ammonia is toxic to the central nervous system and its concentration in the brain must be kept low. This is accomplished by the high activity of glutamine synthetase, which is localized in astrocytes and which permits efficient detoxification of incoming or endogenously generated ammonia. The location also permits the operation of an intercellular glutamine cycle. In this cycle, glutamate released from nerve terminals is taken up by astrocytes where it is converted to glutamine. Glutamine is released to the extracellular fluid to be taken up into the nerve cells, where it is converted back to glutamate by the action of glutaminase. Most extrahepatic organs lack a complete urea cycle, and for many organs, including the brain, glutamine represents a temporary storage form of waste nitrogen. As such, glutamine was long thought to be harmless to the brain. However, recent evidence suggests that excess glutamine is neurotoxic. Hyperammonemic syndromes (e.g., liver disease, inborn errors of the urea cycle, Reye's disease) consistently cause astrocyte pathology. Evidence has been presented that hyperammonemia results in increased formation of glutamine directly in astrocytes, thereby generating an osmotic stress to these cells. This osmotic stress results in impaired astrocyte function, which in turn leads to neuronal dysfunction. In this review a brief overview is presented of the role of glutamine in normal brain metabolism and in the pathogenesis of hyperammonemic syndromes.

Ammonia induces the mitochondrial permeability transition in primary cultures of rat astrocytes

Ammonia is a toxin that has been strongly implicated in the pathogenesis of hepatic encephalopathy (HE), and the astrocyte appears to be the principal target of ammonia toxicity. The specific neurochemical mechanisms underlying HE, however, remain elusive. One of the suggested mechanisms for ammonia toxicity is impaired cellular bioenergetics. Because there is evidence that the mitochondrial permeability transition (MPT) is associated with mitochondrial dysfunction, we determined whether the MPT might be involved in the bioenergetic alterations related to ammonia toxicity. Accordingly, we examined the mitochondrial membrane potential (Deltapsi(m)) in cultured astrocytes and neurons using laser-scanning confocal microscopy after loading the cells with the voltage-sensitive dye JC-1. We found that ammonia induced a dissipation of the Deltapsi(m) in a time- and concentration-dependent manner. These findings were supported by flow cytometry using the voltage-sensitive dye tetramethylrhodamine ethyl ester (TMRE). Cyclosporin A, a specific inhibitor of the MPT, completely blocked the ammonia-induced dissipation of the Deltapsi(m). We also found an increase in the mitochondrial permeability to 2-deoxyglucose in astrocytes that had been exposed to 5 mM NH(4)Cl, further supporting the concept that ammonia induces the MPT in these cells. Pretreatment with methionine sulfoximine, an inhibitor of glutamine synthetase, blocked the ammonia-induced collapse of Deltapsi(m), suggesting a role of glutamine in this process. Over a 24-hr period, ammonia had no effect on the Deltapsi(m) in cultured neurons. Collectively, our data indicate that ammonia induces the MPT in cultured astrocytes, which may be a factor in the mitochondrial dysfunction associated with HE and other hyperammonemic states.

Cerebral metabolism of ammonia and amino acids in patients with fulminant hepatic failure

BACKGROUND & AIMS

High circulating levels of ammonia have been suggested to be involved in the development of cerebral edema and herniation in fulminant hepatic failure (FHF). The aim of this study was to measure cerebral metabolism of ammonia and amino acids, with special emphasis on glutamine metabolism.

METHODS

The study consisted of patients with FHF (n = 16) or cirrhosis (n = 5), and healthy subjects (n = 8). Cerebral blood flow was measured by the 133Xe washout technique. Blood samples for determination of ammonia and amino acids were drawn simultaneously from the radial artery and the internal jugular bulb.

RESULTS

A net cerebral ammonia uptake was only found in patients with FHF (1.62 +/- 0.79 micromol x 100 g(-1) x min(-1)). The cerebral glutamine efflux was higher in patients with FHF than in the healthy subjects and cirrhotics, -6.11 +/- 5.19 vs. -1.93 +/- 1.17 and -1.50 +/- 0.29 micromol x 100 g(-1) x min(-1), respectively (P < 0.05). Patients with FHF who subsequently died of cerebral herniation (n = 6) had higher arterial ammonia concentrations, higher cerebral ammonia uptake, and higher cerebral glutamine efflux than survivors. Intervention with short-term mechanical hyperventilation in FHF reduced the net cerebral glutamine efflux, despite an unchanged net cerebral ammonia uptake.

CONCLUSIONS

Patients with FHF have an increased cerebral glutamine efflux, and short-term hyperventilation reduces this efflux. A high cerebral ammonia uptake and cerebral glutamine efflux in patients with FHF were associated with an increased risk of subsequent fatal intracranial hypertension.

Hypothermia for the management of intracranial hypertension in acute liver failure

Increased intracranial pressure in patients with acute liver failure remains a major cause of mortality. Treatment options are limited, and without urgent liver transplantation, mortality rates of up to 90% are common in those who fulfill criteria for poor prognosis. Several studies in animal models of acute liver failure set the stage for the clinical application of moderate hypothermia in humans. Few patients are treated with hypothermia for increased intracranial pressure. However, data indicate that moderate hypothermia is a safe and effective method of treatment for increased intracranial pressure that is unresponsive to other medical therapies, and that this treatment can be used as a successful bridge to liver transplantation. Recent data also suggest that increases in intracranial pressure can be prevented during the dissection and reperfusion phases of liver transplantation for acute liver failure if patients are kept hypothermic during the surgical procedure. This article focuses on the use of moderate hypothermia for the treatment of increased intracranial pressure in patients with acute liver failure.

Glutamine as a pathogenic factor in hepatic encephalopathy

Hepatic encephalopathy (HE) results from acute or chronic liver dysfunction and is associated with hyperammonemia. Ammonium ions penetrate from blood to brain, where they form glutamine (Gln) in the reaction with glutamate catalyzed by an astroglia-specific enzyme, glutamine synthetase (GS). Experimental data suggest that many manifestations of HE can be ascribed to increased Gln synthesis and accumulation in the brain. In HE resulting from acute liver failure ("fulminant hepatic failure"), the osmotic action of Gln appears to be in a large degree responsible for cerebral edema and edema-associated disturbances of cerebral blood flow and ionic homeostasis. In chronic HE not accompanied by cerebral edema, Gln contributes to impairment of cerebral energy metabolism, and its increased transport from brain to the periphery accelerates the blood-to-brain transport of aromatic amino acids, of which tryptophen (Trp) is converted to metabolites directly implicated in HE. Most of the evidence that Gln participates in pathological events has been derived from their disappearance or amelioration in HE rats in which the cerebral Gln content was reduced by treatment with a GS inhibitor, methionine sulfoximine.

Lubeluzole attenuates K(+)-evoked extracellular accumulation of taurine in the striatum of healthy rats and rats with hepatic failure

Lubeluzole is a newly designed neuroprotectant which has proved effective in the treatment of experimental stroke in rats, mainly by inhibition of the glutamate-activated NO pathway, but also by counteracting osmotic stress by a mechanism associated with the release of the osmotically active amino acid taurine (Tau). Here we show that lubeluzole administered i.p. decreases by 25% the high (50 mM) K+-evoked accumulation of Tau in striatal microdialysates of healthy rats and by 34% in rats with thioacetamide-induced hepatic failure, where the increased extracellular accumulation of Tau signifies ongoing hepatic encephalopathy. Lubeluzole does not affect the nonstimulated accumulation of Tau in either group of rats. The results indicate that lubeluzole may be effective in ameliorating ionic or osmotic stress in a range of pathological conditions involving the rise of extracellular K+, and also in decreasing the vulnerability to stress in rats with hepatic failure.

Current strategies for the management of neonatal urea cycle disorders

The treatment of newborns with urea cycle disorders has evolved over the years into a complex multidisciplinary effort. The complexity derives from the number of issues that must be addressed simultaneously. At the Urea Cycle Disorders Consensus Meeting held in Washington, D.C., a panel of physicians and other professionals with extensive experience in this field was assembled to bring some systematization to this task. This manuscript is a condensation of the collective opinion and experience of that group. The outcome of untreated or poorly treated patients with urea cycle disorders is universally bad. Although a favorable outcome is not always feasible, even with the best therapy, the methods outlined here should help treat such a patient by drawing on the experience of others who have treated patients with urea cycle disorders. This article does not purport to be the final word in treating children with these disorders. However, by establishing some common ground, new methods can be tried and compared with existing ones. In a future that holds the prospect of gene therapy "cures" for these diseases, striving for the best possible outcome in the critical newborn period is a worthy goal.

Glutamine synthetase activities in spinal white and gray matter 7 days following spinal cord injury in rats

The glial enzyme glutamine synthetase (GS) is critical for central nervous system catabolism of glutamate and glutamine production. Upregulation of GS is a hallmark of reactive astrocytosis, although such induction following spinal cord injury (SCI) has not been reported. This study's purpose was to determine if GS activity is increased following SCI. Experimental rats received a complete spinal transection at the T5 segment and control rats received a laminectomy only. GS activities were determined using an enzymatic microassay. Glutamine levels were resolved in semi-adjacent sections. At 7 days following SCI, GS activity increased an average of 170-190% in white matter and 15-25% in gray matter immediately adjacent to the transection, and 70-90% in white matter and 40-45% in gray matter from cervical and lumbar enlargements. Correlative increases in glutamine were observed also. These findings further characterize the astrocytic response to SCI, which may contribute to altered glutamine metabolism in injured spinal tissue.

Is all perfusion-weighted magnetic resonance imaging for stroke equal? The temporal evolution of multiple hemodynamic parameters after focal ischemia in rats correlated with evidence of infarction

Although perfusion-weighted imaging techniques are increasingly used to study stroke, no particular hemodynamic variable has emerged as a standard marker for accumulated ischemic damage. To better characterize the hemodynamic signature of infarction. the authors have assessed the severity and temporal evolution of ischemic hemodynamics in a middle cerebral artery occlusion model in the rat. Cerebral blood flow (CBF) and total and microvascular cerebral blood volume (CBV) changes were measured with arterial spin labeling and steady-state susceptibility contrast magnetic resonance imaging (MRI), respectively, and analyzed in regions corresponding to infarcted and spared ipsilateral tissue, based on 2,3,5-triphenyltetrazolium chloride histology sections after 24 hours ischemia. Spin echo susceptibility contrast was used to measure microvascular-weighted CBV, which had a maximum sensitivity for vessels with radii between 4 and 30 microm. Serial measurements between 1 and 3 hours after occlusion showed no change in CBF (22 +/- 20% of contralateral, mean +/- SD) or in total CBV (78 +/- 13% of contralateral) in regions destined to infarct. However, microvascular CBV progressively declined from 72 +/- 5% to 64 +/- 11% (P < 0.01) during this same period. Microvascular CBV changes with time were entirely due to decreases in subcortical infarcted zones (from 73 +/- 9% to 57 +/- 14%. P < 0.001) without changes in the cortical infarcted territory. The hemodynamic variables showed differences in magnitude and temporal response, and these changes varied based on histologic outcome and brain architecture. Such factors should be considered when designing imaging studies for human stroke.

Effects of aluminum exposure on glutamate metabolism: a possible explanation for its toxicity

The effects of aluminum (Al) exposure on glutamate metabolism were investigated to study the mechanism of Al toxicity in rat brain. In astrocytes, the glutamate-glutamine pathway prevents the accumulation of the excitatory neurotransmitter glutamate, recognized as a neuronal excitotoxin when present in excess in the extracellular space. Changes in the level of l-aspartate, l-glutamate, and its metabolite l-glutamine were investigated in various regions of rat brains following intraperitoneal injection of aluminium gluconate for 2 months. The changes observed were area- and amino-acid-specific. An increase in glutamine, but not in l-glutamate or l-aspartate, was noted in the hippocampus and neocortex of Al-treated rats. This increase in vivo was consistent with observations in vitro. Exposure of cultured astrocytes to Al chloride (200, 400, and 800 microM) specifically increased glutamine synthetase activity for the three concentrations tested. In parallel with this increase, a higher rate of disappearance of glutamate from culture medium was observed during the first 10 min of incubation for the three concentrations tested, as well as an accumulation of glutamine in the cellular extract after 30 min. These observations indicate that the astrocyte population is a potential target for Al toxic action that could mediate the pathogenesis of this metal.

Interaction of glutamine and arginine on cerebrovascular reactivity to hypercapnia

Glutamine is purported to inhibit recycling of citrulline to arginine and to limit nitric oxide release in vitro. However, vasoactive effects of glutamine have not been clearly demonstrated in vivo. During hyperammonemia, impaired cerebrovascular reactivity to CO(2) is related to glutamine accumulation. We tested the hypotheses that 1) glutamine infusion in the absence of hyperammonemia impairs cerebrovascular CO(2) reactivity and 2) arginine infusion preserves CO(2) reactivity during glutamine infusion and during hyperammonemia. Pentobarbital sodium-anesthetized rats were equipped with a closed cranial window for measuring pial arteriolar diameter. Intravenous infusion of 3 mmol. kg(-1). h(-1) of L-glutamine for 6 h produced threefold increases in plasma and cerebrospinal fluid concentrations. Dilation to hypercapnia was reduced by 45% compared with that of a time control group at 6 h but not at 3 h of glutamine infusion. Coinfusion of 2 mmol. kg(-1). h(-1) of L-arginine with glutamine maintained the hypercapnic vasodilation at the control value. Infusion of ammonium acetate at a rate known to produce threefold increases in cortical tissue glutamine concentration resulted in no significant hypercapnic vasodilation. Coinfusion of arginine with ammonium acetate maintained hypercapnic vasodilation at 60% of the control value. Arginine infusion did not augment hypercapnic vasodilation in a control group. We conclude that glutamine modulates cerebrovascular CO(2) reactivity in vivo. Glutamine probably acts by limiting arginine availability because the vascular inhibitory effect required >3 h to develop and because arginine infusion counteracted the vascular effect of both endogenously and exogenously produced increases in glutamine.

Ammonia, respiration, and longevity in nematodes: insights on metabolic regulation of life span from temporal rescaling

To better understand metabolic correlates of longevity, we used a graphical technique to compare the adult temporal patterns of several markers of metabolic activity - ammonia elimination, oxygen consumption rate, ATP levels, and (in freeze-permeabilized worms) the rate of NADPH-activated, lucigenin-mediated superoxide formation - as observed by us and others in normal and long-lived mutant Caenorhabditis elegans strains. All of these traits declined with time, and when their logarithms were plotted against time, appeared reasonably linear for most of the duration of the experiments. The profiles for ammonia output conformed well to parallel regression lines; those for the other metabolic parameters varied widely in slope as originally plotted by the authors, but much less so when replotted as logarithms against adjusted time, scaled by the reciprocal of strain longevity. This is consistent with coregulation of life span, respiration rate, ATP levels, lucigenin reactivity, but not ammonia excretion, by a physiological clock distinguishable from chronologic time. Plots, scaled appropriately for equalized slopes, highlighted y-axis intercept differences among strains. On rescaled plots, these constitute deviations from the expectation based on 'strain-specific clock' differences alone. With one exception, y-intercept effects were observed only for mutants in an insulin-like signaling pathway.

Hepatic encephalopathy: molecular mechanisms underlying the clinical syndrome

Hepatic encephalopathy (HE) and portal-systemic encephalopathy (PSE) are the terms used interchangeably to describe a complex neuropsychiatric syndrome associated with acute or chronic hepatocellular failure, increased portal systemic shunting of blood, or both. Hepatic encephalopathy complicating acute liver failure is referred to as fulminant hepatic failure (FHF). The clinical manifestations of HE or PSE range from minimal changes in personality and motor activity, to overt deterioration of intellectual function, decreased consciousness and coma, and appear to reflect primarily a variable imbalance between excitatory and inhibitory neurotransmission. Pathogenic mechanisms that may be responsible for HE have been extensively investigated using animal models of HE, or cultures of CNS cells treated with neuroactive substances that have been implicated in HE. Of the many compounds that accumulate in the circulation as a consequence of impaired liver function, ammonia is considered to play an important role in the onset of HE. Acute ammonia neurotoxicity, which may be a cause of seizures in FHF, is excitotoxic in nature, being associated with increased synaptic release of glutamate (Glu), the major excitatory neurotransmitter of the brain, and subsequent overactivation of the ionotropic Glu receptors, mainly the N-methyl-D-aspartate (NMDA) receptors. Hepatic encephalopathy complicating chronic liver failure appears to be associated with a shift in the balance between inhibitory and excitatory neurotransmission towards a net increase of inhibitory neurotransmission, as a consequence of at least two factors. The first is down-regulation of Glu receptors resulting in decreased glutamatergic tone. The down-regulation follows excessive extrasynaptic accumulation of Glu resulting from its impaired re-uptake into nerve endings and astrocytes. Liver failure inactivates the Glu transporter GLT-1 in astrocytes. The second factor is an increase in inhibitory neurotransmission by gamma-aminobutyric acid (GABA) due to (a) increased brain levels of natural benzodiazepines; (b) increased availability of GABA at GABA-A receptors, due to enhanced synaptic release of the amino acid; (c) direct interaction of modestly increased levels of ammonia with the GABA-A-benzodiazepine receptor complex; and (d) ammonia-induced up-regulation of astrocytic peripheral benzodiazepine receptors (PBZR). Brain ammonia is metabolised in astrocytes to glutamine (Gln), an osmolyte, and increased Gln accumulation in these cells may contribute to cytotoxic brain edema, which often complicates FHF. Glutamine efflux from the brain is an event that facilitates plasma-to-brain transport of aromatic amino acids. Tryptophan and tyrosine are direct precursors of the aminergic inhibitory neurotransmitters, serotonin and dopamine, respectively. Changes in serotonin and dopamine and their receptors may contribute to some of the motor manifestations of HE. Finally, oxindole, a recently discovered tryptophan metabolite with strong sedative and hypotensive properties, has been shown to accumulate in cirrhotic patients and animal models of HE.