Characteristics of large neutral amino acid-induced release of preloaded L-glutamine from rat cerebral capillaries in vitro: Effects of ammonia, hepatic encephalopathy, and ?-glutamyl transpeptidase inhibitors

Exchange-mode glutamine transport across CNS cell membranes

CNS cell membranes possess four transporters capable of exchanging Lglutamine (Gln) for other amino acids: the large neutral amino acid (LNAA) transporters LAT1 and LAT2, the hybrid basic amino acid (L-arginine (Arg), L-leucine (Leu)/LNAA transporter y⁺LAT2, and the L-alanine/L-serine/L-cysteine transporter 2 (ASCT2). LAT1/LAT2 and y⁺LAT2 are present in astrocytes, neurons and the blood brain barrier (BBB) - forming cerebral vascular endothelial cells (CVEC), while the location of ASCT2 in the individual cell types is a matter of debate. In the healthy brain, contribution of the exchangers to Gln shuttling from astrocytes to neurons and thus their role in controlling the conversion of Gln to the amino acid neurotransmitters l-glutamate (Glu) and γ-aminobutyric acid (GABA) and Gln flux across the BBB appears negligible as compared to the system A and system N uniporters. Insofar, except for the contribution of LAT1 to the maintenance of Gln homeostasis in the interstitial fluid (ISF), no well-defined CNS-specific function has been established for either of the three transporters in the healthy brain. The Gln-accepting amino acid exchangers appear to gain significance under conditions of excessive brain Gln load (glutaminosis). Excess Gln efflux across the BBB enhances influx into the brain of L-tryptophan (Trp). Excess of Trp is responsible for overloading the brain with neuroactive compounds: serotonin, kynurenic acid, quinolinic acid and/or oxindole, which contribute to neurotransmission imbalance accompanying hyperammonemia. In turn, alterations of y⁺LAT2-mediated Gln/Arg exchange and Arg uptake in astrocyte, modulate astrocytic nitric oxide synthesis and oxidative/nitrosative stress in ammonia-overexposed brain. This article is part of the issue entitled 'Special Issue on Neurotransmitter Transporters'.

Citrulline uptake in rat cerebral cortex slices: modulation by Thioacetamide -Induced hepatic failure

L-citrulline (Cit) is a co-product of NO synthesis and a direct L-arginine (Arg) precursor for de novo NO synthesis. Acute liver failure (ALF) is associated with increased nitric oxide (NO) and cyclic GMP (cGMP) synthesis in the brain, indirectly implicating a role for active transport of Cit. In the present study we characterized [(3)H]Cit uptake to the cortical brain slices obtained from control rats and rats with thioacetamide (TAA)-induced ALF ("TAA slices"). In both control and TAA slices the uptake was partially Na(+)-dependent and markedly inhibited by substrates of systems L and N, including L-glutamine (Gln), which accumulates in excess in brain during ALF. Cit uptake was not affected by Arg, the y(+)/y(+)L transport system substrate, nor by amino acids taken up by systems A, xc (-)or XAG. The Vmax of the uptake in TAA slices was ~60 % higher than in control slices. Chromatographic (HPLC) analysis revealed a ~30 % increase of Cit concentration in the cerebral cortical homogenates of TAA rats. The activity of argininosuccinate synthase (ASS) and argininosuccinate lyase (ASL), the two enzymes of Cit-NO cycle catalyzing synthesis of Arg, showed an increase in TAA rats, consistent with increased ASS and ASL protein expression, by ~30 and ~20 %, respectively. The increased Cit-NO cycle activity was paralleled by increased expression of mRNA coding for inducible nitric oxide synthase (iNOS). Taken together, the results suggest a role for Cit in the activation of cerebral NO synthesis during ALF.

Roles of changes in active glutamine transport in brain edema development during hepatic encephalopathy: an emerging concept

Excessive glutamine (Gln) synthesis in ammonia-overloaded astrocytes contributes to astrocytic swelling and brain edema, the major complication of hepatic encephalopathy (HE). Much of the newly formed Gln is believed to enter mitochondria, where it is recycled to ammonia, which causes mitochondrial dysfunction (a “Trojan horse” mode of action). A portion of Gln may increase osmotic pressure in astrocytes and the interstitial space, directly and independently contributing to brain tissue swelling. Here we discuss the possibility that altered functioning of Gln transport proteins located in the cellular or mitochondrial membranes, modulates the effects of increased Gln synthesis. Accumulation of excess Gln in mitochondria involves a carrier-mediated transport which is activated by ammonia. Studies on the expression of the cell membrane N-system transporters SN1 (SNAT3) and SN2 (SNAT5), which mediate Gln efflux from astrocytes rendered HE model-dependent effects. HE lowered the expression of SN1 at the RNA and protein level in the cerebral cortex (cc) in the thioacetamide (TAA) model of HE and the effect paralleled induction of cerebral cortical edema. Neither SN1 nor SN2 expression was affected by simple hyperammonemia, which produces no cc edema. TAA-induced HE is also associated with decreased expression of mRNA coding for the system A carriers SAT1 and SAT2, which stimulate Gln influx to neurons. Taken together, changes in the expression of Gln transporters during HE appear to favor retention of Gln in astrocytes and/or the interstitial space of the brain. HE may also affect arginine (Arg)/Gln exchange across the astrocytic cell membrane due to changes in the expression of the hybrid Arg/Gln transporter y⁺LAT2. Gln export from brain across the blood–brain barrier may be stimulated by HE via its increased exchange with peripheral tryptophan.

Role of cerebral endothelial cells in the astrocyte swelling and brain edema associated with acute hepatic encephalopathy

Brain edema is an important complication of acute hepatic encephalopathy (AHE), and astrocyte swelling is largely responsible for its development. Elevated blood and brain ammonia levels have been considered as major etiological factors in this edema. In addition to ammonia, recent studies have suggested that systemic infection, inflammation (and associated cytokines (CKs)), as well as endotoxin (lipopolysaccharide (LPS)) are also involved in AHE-associated brain edema. As endothelial cells (ECs) are the first resident brain cells exposed to blood-borne "noxious agents" (i.e., ammonia, CKs, LPS) that are present in AHE, these cells may be in a critical position to react to these agents and trigger a process resulting in astrocyte swelling/brain edema. We therefore examined the effect of conditioned media (CM) from ammonia, LPS and cytokine-treated cultured brain ECs on cell swelling in cultured astrocytes. CM from ammonia-treated ECs when added to astrocytes caused significant cell swelling, and such swelling was potentiated when astrocytes were exposed to CM from ECs treated with a combination of ammonia, LPS and CKs. We also found an additive effect when astrocytes were exposed to ammonia along with CM from ammonia-treated ECs. Additionally, ECs treated with ammonia showed a significant increase in the production of oxy-radicals, nitric oxide (NO), as well as evidence of oxidative/nitrative stress and activation of the transcription factor nuclear factor kappa B (NF-κB). CM derived from ECs treated with ammonia, along with antioxidants (AOs) or the NF-κB inhibitor BAY 11-7082, when added to astrocytes resulted in a significant reduction in cell swelling, as compared to the effect of CM from ECs-treated only with ammonia. We also identified increased nuclear NF-κB expression in rat brain cortical ECs in the thioacetamide (TAA) model of AHE. These studies suggest that ECs significantly contribute to the astrocyte swelling/brain edema in AHE, likely as a consequence of oxidative/nitrative stress and activation of NF-κB.

Alterations of blood brain barrier function in hyperammonemia: an overview

Ammonia is a neurotoxin involved in the pathogenesis of neurological conditions associated with hyperammonemia, including hepatic encephalopathy, a condition associated with acute—(ALF) or chronic liver failure. This article reviews evidence that apart from directly affecting the metabolism and function of the central nervous system cells, ammonia influences the passage of different molecules across the blood brain barrier (BBB). A brief description is provided of the tight junctions, which couple adjacent cerebral capillary endothelial cells to each other to form the barrier. Ammonia modulates the transcellular passage of low-to medium-size molecules, by affecting their carriers located at the BBB. Ammonia induces interrelated aberrations of the transport of the large neutral amino acids and aromatic amino acids (AAA), whose influx is augmented by exchange with glutamine produced in the course of ammonia detoxification, and maybe also modulated by the extracellularly acting gamma-glutamyl moiety transferring enzyme, gamma-glutamyl-transpeptidase. Impaired AAA transport affects neurotransmission by altering intracerebral synthesis of catecholamines (serotonin and dopamine), and producing “false neurotransmitters” (octopamine and phenylethylamine). Ammonia also modulates BBB transport of the cationic amino acids: the nitric oxide precursor, arginine, and ornithine, which is an ammonia trap, and affects the transport of energy metabolites glucose and creatine. Moreover, ammonia acting either directly or in synergy with liver injury-derived inflammatory cytokines also evokes subtle increases of the transcellular passage of molecules of different size (BBB “leakage”), which appears to be responsible for the vasogenic component of cerebral edema associated with ALF.

Hyperammonemia increases the expression and activity of the glutamine/arginine transporter y+ LAT2 in rat cerebral cortex: implications for the nitric oxide/cGMP pathway

The pathogenesis of hepatic encephalopathy (HE) is associated with hyperammonemia (HA) and subsequent exposure of the brain to excess of ammonia. Alterations of the NO/cGMP pathway and increased glutamine (Gln) content are collectively responsible for many HE symptoms, but how the two events influence each other is not clear. Previously we had shown that Gln administered intracerebrally inhibited the NO/cGMP pathway in control rats and even more so in rats with HA, and we speculated that this effect is due to inhibition by Gln of arginine (Arg) transport (Hilgier et al., 2009). In this study we demonstrate that a 3-day HA in the ammonium acetate model increases the expression in the brain of y(+)LAT2, the heteromeric transporter which preferentially stimulates Arg efflux from the cells in exchange for Gln. The expression of the basic amino acid transporter CAT1, transporting Arg but not Gln remained unaffected by HA. Multiple parameters of Arg or Gln uptake and/or efflux and their mutual dependence were altered in the cerebral cortical slices obtained from HA rats, in a manner indicating enhanced y(+)LAT2-mediated transport. HA elevated Gln content and decreased cGMP content as measured both in the cerebral cortical tissue and microdialysates. Intracortical administration of 6-diazo-5-oxo-L-norleucine (DON), which inhibits Gln fluxes between different cells of the CNS, attenuated the HA-induced decrease of cGMP in the microdialysates of HA rats, but not of control rats. The results suggest that, reduced delivery of Arg due to enhanced y(+)LAT2-mediated exchange of extracellular Gln for intracellular Arg may contribute to the decrease of NO/cGMP pathway activity evoked in the brain by HA.

Inhibition of glutamine synthetase during ammonia exposure in rainbow trout indicates a high reserve capacity to prevent brain ammonia toxicity

Glutamine synthetase (GSase), the enzyme that catalyses the conversion of glutamate and ammonia to glutamine, is present at high levels in vertebrate brain tissue and is thought to protect the brain from elevated ammonia concentrations. We tested the hypothesis that high brain GSase activity is critical in preventing accumulation of brain ammonia and glutamate during ammonia loading in the ammonia-intolerant rainbow trout. Trout pre-injected with saline or the GSase inhibitor methionine sulfoximine (MSOX, 6 mg kg(-1)), were exposed to 0, 670 or 1000 micromol l(-1) NH(4)Cl in the water for 24 and 96 h. Brain ammonia levels were 3- to 6-fold higher in ammonia-exposed fish relative to control fish and MSOX treatment did not alter this. Brain GSase activity was unaffected by ammonia exposure, while MSOX inhibited GSase activity by approximately 75%. Brain glutamate levels were lower and glutamine levels were higher in fish exposed to ammonia relative to controls. While MSOX treatment had little impact on brain glutamate, glutamine levels were significantly reduced by 96 h. With ammonia treatment, significant changes in the concentration of multiple other brain amino acids occurred and these changes were mostly reversed or eliminated with MSOX. Overall the changes in amino acid levels suggest that multiple enzymatic pathways can supply glutamate for the production of glutamine via GSase during ammonia exposure and that alternative transaminase pathways can be recruited for ammonia detoxification. Plasma cortisol levels increased 7- to 15-fold at 24 h in response to ammonia and MSOX did not exacerbate this stress response. These findings indicate that rainbow trout possess a relatively large reserve capacity for ammonia detoxification and for preventing glutamate accumulation during hyperammonaemic conditions.

Glutamine as a mediator of ammonia neurotoxicity: A critical appraisal

Ammonia is a major neurotoxin implicated in hepatic encephalopathy (HE). Here we discuss evidence that many aspects of ammonia toxicity in HE-affected brain are mediated by glutamine (Gln), synthesized in excess from ammonia and glutamate by glutamine synthetase (GS), an astrocytic enzyme. The degree to which Gln is increased in brains of patients with HE was found to positively correlate with the grade of HE. In animals with HE, a GS inhibitor, methionine sulfoximine (MSO), reversed a spectrum of manifestations of ammonia toxicity, including brain edema and increased intracranial pressure, even though MSO itself increased brain ammonia levels. MSO inhibited, while incubation with Gln reproduced the oxidative stress and cell swelling observed in ammonia-exposed cultured astrocytes. Recent studies have shown that astrocytes swell subsequent to Gln transport into mitochondria and its degradation back to ammonia, which then generates reactive oxygen species and the mitochondrial permeability transition. This sequence of events led to the formulation of the "Trojan Horse" hypothesis. Further verification of the role of Gln in the pathogenesis of HE will have to account for: (1) modification of the effects of Gln by interaction of astrocytes with other CNS cells; and (2) direct effects of Gln on these cells. Recent studies have demonstrated a "Trojan Horse"-like effect of Gln in microglia, as well as an interference by Gln with the activation of the NMDA/NO/cGMP pathway by ammonia as measured in whole brain, a process that likely also involves neurons.

Brain edema in acute liver failure: inhibition by L-histidine

Brain edema and the associated increase in intracranial pressure are potentially lethal complications of acute liver failure (ALF). Astrocyte swelling (cytotoxic edema) represents a significant component of the brain edema in ALF, and elevated blood and brain ammonia levels have been strongly implicated in its formation. We earlier showed in cultured astrocytes that oxidative stress (OS) and the mitochondrial permeability transition (mPT) play major roles in the mechanism of ammonia-induced astrocyte swelling. Glutamine, a byproduct of ammonia metabolism, has also been shown to induce OS, the mPT, and astrocyte swelling. Such effects of glutamine were suggested to be mediated by its hydrolysis in mitochondria, potentially yielding high levels of ammonia in this organelle and leading to OS and the mPT. L-histidine, an inhibitor of mitochondrial glutamine transport, was recently shown to mitigate OS, mPT, and cell swelling in cultured astrocytes treated with ammonia. The present study examined whether L-histidine similarly abolishes OS, the mPT, and brain edema in a rat model of ALF. Treatment of rats with thioacetamide caused a significant degree of brain edema, which was associated with induction of OS and the mPT. These changes were completely abolished by L-histidine, supporting a key role of mitochondrial glutamine transport and hydrolysis in the mechanism of the brain edema associated with ALF.

Glutamine inhibits ammonia-induced accumulation of cGMP in rat striatum limiting arginine supply for NO synthesis

Brain L-glutamine (Gln) accumulation and increased activity of the NO/cGMP pathway are immediate consequences of acute exposure to ammonia. This study tested whether excess Gln may influence NO and/or cGMP synthesis. Intrastriatal administration of the glutaminase inhibitor 6-diazo-5-oxo-L-norleucine or the system A-specific Gln uptake inhibitor methylaminoisobutyrate increased microdialysate Gln concentration and reduced basal and ammonia-induced NO and cGMP accumulation. Gln applied in vivo (via microdialysis) or in vitro (to rat brain cortical slices) reduced NO and cGMP accumulation in the presence and/or absence of ammonia, but not cGMP synthesis induced by the NO donor sodium nitroprusside. Attenuation of cGMP synthesis by Gln was prevented by administration of L-arginine (Arg). The L-arginine co-substrates of y(+)LAT2 transport system, L-leucine and cyclo-leucine, mimicked the effect of exogenous Gln, suggesting that Gln limits Arg supply for NO synthesis by interfering with y+LAT2-mediated Arg uptake across the cell membrane.

Is major depressive disorder a metabolic encephalopathy?

Metabolic encephalopathy is an acute disturbance in cellular metabolism in the brain evoked by conditions of hypoxia, hypoglycaemia, oxidative stress and/or inflammation. It usually develops acutely or subacutely and is reversible if the systemic disorder is treated. If left untreated, however, metabolic encephalopathy may result in secondary structural damage to the brain. Most encephalopathies are present with neuropsychiatric symptoms, one in particular being depression. However, mood disorders are often co-morbid with cardiovascular, liver, kidney and endocrine disorders, while increasing evidence concurs that depression involves inflammatory and neurodegenerative processes. This would suggest that metabolic disturbances resembling encephalopathy may underscore the basic neuropathology of depression at a far deeper level than currently realized. Viewing depression as a form of encephalopathy, and exploiting knowledge gleaned from our understanding of the neurochemistry and treatment of metabolic encephalopathy, may assist in our understanding of the neurobiology of depression, but also in realizing new ideas in the pharmacotherapy of mood disorders.

Glutamine-induced free radical production in cultured astrocytes

Ammonia is a neurotoxin implicated in the pathogenesis of hepatic encephalopathy, Reye's syndrome, inborn errors of the urea cycle, glutaric aciduria, and other metabolic encephalopathies. Brain ammonia is predominantly metabolized to glutamine in astrocytes by glutamine synthetase. While the synthesis of glutamine has generally been viewed as the principal means of ammonia detoxification, this presumed beneficial effect has been questioned as growing evidence suggest that some of the deleterious effects of ammonia may be mediated by glutamine rather than ammonia per se. Since ammonia is known to induce the production of free radicals in cultured astrocytes, we investigated whether such production might be mediated by glutamine. Treatment of astrocytes with glutamine (4.5 mM) increased free radical production at 2-3 min (95%; P < 0.05), as well as at 1 and 3 h (42% and 49%, respectively; P < 0.05). Similarly treated cultured neurons failed to generate free radicals. Free radical production by glutamine was blocked by the antioxidants deferoxamine (40 microM) and alpha-phenyl-N-tert-butyl-nitrone (250 microM), as well as by the nitric oxide synthase inhibitor N(omega)-nitro-L-arginine methyl ester (500 microM). Free radical production was also blocked by 6-diazo-5-oxo-L-norleucine (1 mM), an inhibitor of glutaminase, suggesting that ammonia released by glutamine hydrolysis may be responsible for the generation of free radicals. Additionally, the mitochondrial permeability transition inhibitor, cyclosporin A, blocked free radical production by glutamine. The results indicate that astrocytes, but not neurons, generate free radicals following glutamine exposure. Glutamine-induced oxidative and/or nitrosative stress may represent a key mechanism in ammonia neurotoxicity.

Glutamine transport at the blood-brain and blood-cerebrospinal fluid barriers

Glutamine has multiple physiological and pathophysiological roles in the brain. Because of their position at the interface between blood and brain, the cerebral capillaries and the choroid plexuses that form the blood-brain barriers (BBB) and blood-cerebrospinal fluid (CSF) barriers, have the potential to influence brain glutamine concentrations. Despite this, there has been a paucity of data on the mechanisms and polarity of glutamine transport at these barrier tissues. In situ brain perfusion in the rat, indicates that blood to brain L-[14C]glutamine transport at the blood-brain barrier is primarily mediated by a pH-dependent, Na(+)-dependent, System N transporter, but that blood to choroid plexus transport is primarily via a pH-independent System N transporter and a Na(+)-independent carrier that is not System L. Transport studies in isolated rat choroid plexuses and primary cultures of choroid plexus epithelial cells indicate that epithelial L-[14C]glutamine transport is polarized (apical uptake>basolateral) and that uptake at the apical membrane is mediated by pH dependent System N transporters (identified as SN1 and SN2 by polymerase chain reaction) and the Na(+)-independent System L. Blood-brain barrier System N transport is markedly effected by cerebral ischemia and may be a good marker of endothelial cell dysfunction. The multiple glutamine transporters at the blood-brain and blood-CSF barriers may have role in meeting the metabolic needs of the brain and the barrier tissues themselves. However, it is likely that the main role of these transporters is removing glutamine, and thus nitrogen, from the brain.

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.

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.

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.

Effects of ammonia in vitro on endogenous taurine efflux and cell volume in rat cerebrocortical minislices: influence of inhibitors of volume-sensitive amino acid transport

Rat cerebrocortical minislices were incubated with physiological saline in the absence or presence of 5 mM ammonium acetate ("ammonia") and/or inhibitors of osmosensitive amino acid transport: 50 microM niflumic acid and 100 microM N-ethyl-maleimide for 60 min, with medium changes after 20 min and 40 min. The efflux of endogenous taurine, glutamate and glutamine was assayed by high-performance liquid chromatography, and steady-state cell volumes were monitored in the slices with the [14C]inulin method. In the absence of ammonia, niflumic acid abolished taurine efflux but did not affect glutamate or glutamine efflux at all time-points, and increased cell volume at 20 min and 60 min. N-Ethyl-maleimide increased taurine, glutamine and glutamate efflux at 20 min and 40 min, inhibited taurine and glutamine efflux at 60 min, and increased cell volume at 20 min. Ammonia strongly stimulated taurine (by 380% at 20 min), and only moderately glutamate (30% at 20 min) or glutamine efflux (76% at 20 min). Ammonia increased cell volume above the control level at all time-points. Niflumic acid inhibited, but did not abolish ammonia-dependent taurine and glutamine efflux, and did not change glutamate efflux. The effects of ammonia + niflumic acid on cell volume did not differ from the effects of each compound separately. N-Ethyl-maleimide inhibited ammonia-dependent efflux of all three amino acids except for stimulation of glutamate efflux at 20 min. N-Ethyl-maleimide + ammonia decreased the cell volumes more than did each compound separately. It is concluded that although ammonia-induced taurine efflux is accompanied by an increase in cell volume, the underlying mechanism is not simply a cell volume regulatory response normally observed in hypoosmotic stress. Increased efflux of taurine, which is an inhibitory amino acid and a cell membrane protectant, may serve to counteract the deleterious effects of increased excitatory transmission accompanying acute hyperammonemic insult.

Selective increases of extracellular brain concentrations of aromatic and branched-chain amino acids in relation to deterioration of neurological status in acute (ischemic) liver failure

Previous reports based on studies in brain tissue from humans and experimental animals suggest that aromatic amino acids (AAAs) and branched-chain amino acids (BCAA's) accumulate in brain in acute liver failure. In order to assess these changes in relation to the severity of neurological impairment and to the degree of hyperammonemia, AAAs and BCAAs were measured in vivo by cerebral microdialysis in frontal cortex of rats at various stages during the development of hepatic encephalopathy due to acute liver failure resulting from portacaval anastomosis followed by hepatic artery ligation. Extracellular brain concentrations of AAAs and of valine and leucine were elevated 2 to 4-fold following hepatic devascularization and these increases were significantly correlated to arterial ammonia concentration (r= 0.71-0.84, p<0.05). Extracellular concentrations of tyrosine paralleled the deterioration of neurological status in acute liver failure rats. In view of their role as precursors of monoamine neurotransmitters, ammonia-induced alterations of intracellular/extracellular brain concentration ratios for AAAs could account for altered neuronal excitability and contribute to the encephalopathy characteristic of acute liver failure.

The role of sulphydryl groups in efflux of taurine and GABA from cerebral cortical cells

1. Effluxes of taurine and GABA from pre-loaded cells in slices of rat cerebral cortex in isosmotic media is enhanced by the -SH reagent p-chloromercuriphenylsulphonic acid (pCMPS) (100 microM) accompanied by moderate swelling. Those effects were more pronounced with GABA than with taurine. N-ethylmaleimide (NEM) (100 microM) had only slight affects on these variables. 2. The acceleration of effluxes that occurs when medium osmolality is reduced from 315 to 265 mosmol/kg is blocked by NEM and by pCMPS, with pronounced cell swelling. 3. The inhibitory effects of these reagents on efflux is abolished when cell swelling is prevented by the addition of 25 mM sucrose to hyposmotic incubation media (with equimolar reduction in NaCl concentration). 4. Pre-exposure of slices to dithiothreitol (DTT) (100 microM) blocks the effects of NEM and pCMPS on GABA efflux in hyposmotic media, but has no effect on taurine efflux. 5. The cell membrane and cytoskeletal responses which may underlie these effects are briefly discussed.

The role of sulfhydryl groups in D-aspartate and rubidium release from neonatal rat primary astrocyte cultures

We have recently demonstrated that both methylmercury (MeHg) and mercuric chloride (MC) induce D-aspartate release from neonatal rat primary astrocyte cultures maintained in isotonic conditions. In the present study, we compare several other sulfhydryl-(-SH) selective alkylating reagents [methyl methanethiosulfonate (MMTS), N-ethylmaleimide (NEM), and iodoacetamide (IA)] in isotonic, as well as hypotonic conditions to discern the functional importance of -SH groups in [3H]D-aspartate and 86rubidium (86Rb) release from astrocytes. Treatment of astrocytes (5 min) in isotonic buffer with the hydrophobic reagent NEM (10 microM) caused a marked increase in 86Rb release but had no effect on [3H]D-aspartate release. Neither IA-, nor MMTS-treatment (both at 10 microM) induced increase in [3H]D-aspartate or 86Rb release in isotonic buffer. In hypotonic condition (-50 mM Na+), astrocytes were most sensitive to MC exposure (5 microM), exhibiting an increase in both [3H]D-aspartate and 86Rb efflux. The hydrophobic compounds MMTS and NEM, and the hydrophilic -SH modifying reagent, IA, attenuated the hypotonic-induced efflux of [3H]D-aspartate, in the absence of an effect on 86Rb release. These observations are consistent with a critical role for -SH groups both in basal (i.e. isotonic) and hypotonic-induced release of D-aspartate and Rb from astrocytes. Lack of uniformity of these effects may be attributed to site-specificity, related to the physicochemical properties of these -SH alkylating reagents.

Hyperammonemia and hepatic encephalopathy stimulate rat cerebral synaptic mitochondrial glutamate dehydrogenase activity specifically in the direction of glutamate oxidation

The effects of hepatic encephalopathy (HE) due to thioacetamide (TAA)-induced liver failure and hyperammonemia (HA) produced by repeated i.p. administration of ammonium acetate on the activity of glutamate dehydrogenase (GlDH) in the direction of glutamate (Glu) synthesis from--(GlDH-NADH) or its oxidation to alpha-ketoglutarate (alpha-KG) (GlDH-NAD), respectively, were examined in non-synaptic and synaptic mitochondria from rat cerebral hemispheres. In non-synaptic mitochondria, HE and HA stimulated the GlDH-NADH activity by, respectively, 33% and 49%, but neither condition affected the GlDH-NAD activity. In synaptic mitochondria, HE and HA decreased the GlDH-NADH activity by, respectively, 31% and 28%, but stimulated the GlDH-NAD activity by as much as 90% (HE) and 100% (HA). Kinetic assays revealed that HA increased the Vmax of the synaptic mitochondrial GLDH-NAD by 105%, without affecting the Km for Glu. The stimulation of GlDH-NAD favors the oxidation of synaptic Glu to alpha-KG, and may represent an adaptive response serving to counteract hyperammonemia-induced decrease of cerebral alpha-KG production in other metabolic pathways.