Molecular Neurobiology of Acute Liver Failure

Acute liver failure results in encephalopathy and brain edema that is characterized by astrocytic cell swelling. Molecular biological techniques have led to the identification of alterations in expression of several genes coding for key astrocytic proteins in acute liver failure. Such proteins include amino acid transporters, structural proteins, the endothelial cell glucose transporter GLUT-1, the mitochondrial “peripheral-type” benzodiazepine receptor, and the water channel protein aquaporin IV. Magnetic resonance spectroscopic studies reveal increased brain lactate concentrations that are positively correlated with severity of encephalopathy and brain edema in acute liver failure, suggesting a deficit of cellular oxidative capacity and impending brain energy failure. Mild hypothermia prevents brain edema in acute liver failure, and mechanisms responsible for this beneficial effect include reduced blood-brain ammonia transfer as well as normalization of astrocytic amino acid transport and brain energy metabolism. Further elucidation of the molecular mechanisms responsible for brain edema and encephalopathy in acute liver failure will undoubtedly lead to novel treatment strategies for these complications.

#

KEYWORDS

Acute liver failure - brain edema - gene expression - ammonia - brain energy metabolism - astrocytes - transporters - osmoregulation - hypothermia

Acute liver failure (ALF) invariably results in severe central nervous system dysfunction that includes encephalopathy progressing to coma and seizures. Brain edema resulting in increased intracranial pressure and brain herniation is the major cause of mortality in ALF. Neuropathological investigation of brain capillaries from patients who died in ALF reveals marked swelling of astrocytes (Fig. [1]), suggesting a mainly cytotoxic mechanism.[1] Studies in experimental animals with ALF resulting from the injection of toxins or from hepatic devascularization likewise show swelling of the cytoplasm and perineuronal and perivascular processes of astrocytes that are most prominent in gray matter structures,[2] leading to the suggestion that damage to astrocytes or inhibition of their function, or both, contributes to the pathogenesis of encephalopathy and brain edema in ALF. Although there is little convincing evidence for gross disruption of the blood-brain barrier in either experimental or human ALF, there is evidence of alterations of endothelial cell morphology[1] and function,[3] [4] suggesting that blood-brain barrier transport of key central nervous system substrates may be impaired.

The precise mechanisms responsible for the encephalopathy and brain edema in ALF have not been completely elucidated. However, studies in experimental animal models together with modern spectroscopic techniques in patients with ALF continue to provide important clues to the pathogenesis of these complications.

#

BRAIN METABOLISM IN FULMINANT HEPATIC FAILURE

Brain metabolism is seriously compromised in ALF. Disturbances of metabolism include increased brain uptake and metabolism of ammonia and amino acids, increased glycolytic flux, lactate accumulation, and alterations in the expression of genes coding for key astrocytic proteins.[5]

#

Ammonia

Hyperammonemia together with increased cerebrospinal fluid and brain ammonia concentrations is a consistent finding in experimental ALF resulting from hepatectomy,[6] hepatic devascularization,[7] [8] or toxic liver injury.[9] [10] Moreover, a significant positive correlation has been reported between arterial ammonia concentrations and the incidence of brain herniation due to raised intracranial pressure in patients with ALF.[11]

There is a discordance between circulating and brain concentrations of ammonia in ALF. Whereas arterial ammonia concentrations rarely exceed the 0.5 to 0.7 mM range, corresponding brain ammonia may attain concentrations as high as 5 mM.[7] These findings support the suggestion that brain to blood concentration ratios in ALF may be as high as 8 (normal ratio, 1 to 2)[12] and provide an explanation for the poor correlation between neurological status and circulating ammonia concentrations in ALF.[13] The most plausible explanation for the disproportionately high concentrations of ammonia in ALF relates to increased ammonia uptake by the brain.[14] A recent study also shows increased brain ammonia uptake in patients with ALF.[15]

Because the brain does not express several of the enzymes of the urea cycle, ammonia removal by the brain relies almost exclusively on glutamine synthesis. The enzyme responsible, glutamine synthetase (GS), has a predominantly astrocytic localization (Fig. [2]). Consequently, it is the astrocyte that is responsible for ammonia removal in brain. Not surprisingly, increased cerebrospinal fluid and brain concentrations of glutamine are a consistent finding in both experimental[7] [8] [10] and human[16] [17] [18] ALF, confirming that the brain is exposed to increased concentrations of ammonia. It has been proposed that increased brain glutamine concentrations are causally related to encephalopathy and brain edema in ALF. However, correlations between brain glutamine content and neurological status in experimental animal models of ALF are poor.[7] [8] Moreover, a recent study using ¹H/¹³C magnetic resonance spectroscopy confirmed that, although astrocytic synthesis of glutamine was increased in experimental ALF, the magnitude of the increased synthesis was not significantly correlated with either encephalopathy grade or the presence of brain edema.[19] On the other hand, it has been proposed that the increase of brain glutamine in ALF may result (at least in part) from decreased glutamine hydrolysis in brain as a consequence of inhibition of glutaminase by its product (ammonia).[20]

Increased brain glutamine concentrations are accompanied by concomitant reductions of glutamate in brain tissue from patients with ALF,[16] as well as in the brains of experimental animals with ALF due to hepatectomy,[6] hepatic devascularization,[7] [8] or thioacetamide-induced liver injury.[21] However, whether the loss of brain glutamate reflects its loss from the metabolic pool or from other pools associated with stimulating the activity of glutamate or γ-aminobutyric acid (GABA) neurons has not been established. It was suggested that a reduction in availability of glutamate could result in a malfunction of the malate-aspartate shuttle, a mechanism responsible for the transfer of reducing equivalents.[6] On the other hand, a loss of glutamate from the astrocytic pool would be expected to limit the capacity of GS to remove ammonia. Interestingly, exposure of both cultured astrocytes and cultured neurons to ammonia results in a significant decrease in cellular glutamate content.[22] Taken together, these findings indicate that loss of brain glutamate observed in ALF results from reductions in more than one pool.

In contrast to glutamate, brain GABA concentrations are not significantly altered in most experimental models[6] [7] [8] [19] or in human[16] ALF.

#

Lactate, Energy Metabolism

Brain concentrations of high-energy phosphates (phosphocreatine, adenosine triphosphate) measured biochemically are not altered in experimental ALF.[6] [8] Similar negative findings have been reported using ¹H or ³¹P nuclear magnetic resonance spectroscopy.[23] [24] On the other hand, there is convincing evidence to suggest that brain glucose metabolismis significantly altered in ALF. Brain lactate concentrations are increased in a wide range of experimental animal models of ALF,[6] [8] [10] [24] [25]as well as in extracellular brain fluid from patients with ALF.[26] In this latter study, increased extracellular brain lactate concentrations were found to precede surges in high intracranial pressure. Findings of a concomitant increase of brain alanine in experimental ALF suggest that the lactate accumulation results from decreased oxidation of pyruvate.[8] [19] Deterioration of neurological status assessed either by clinical evaluation[8] [19] or by electroencephalogram spectral analysis[24] in ALF is significantly correlated with increases of brain lactate. Studies using ;s1H/;s1;s3C nuclear magnetic resonance spectroscopy demonstrate unequivocally that the increase of brain lactate in experimental ALF is the consequence of increased de novo synthesis from glucose.[19]

Increased brain lactate has been shown to be associated with worsening of intracranial hypertension and a poor neurological outcome in dogs with ALF,[27] suggesting that increased production of lactate is implicated in the pathogenesis of brain edema. Consistent with this possibility is the report of significant cell swelling after exposure of astrocytes in culture to high levels of lactate.[28]

The cause of the lactate accumulation in the brain in ALF is unknown. However, it has been proposed that ammonia plays a significant role. In support of this, in vitro studies reveal that ammonia inhibits the rate-limiting tricarboxylic acid enzyme α-ketoglutarate dehydrogenase[29] and concomitantly stimulates the glycolytic enzyme phosphofructokinase.[30] Mild hypothermia effectively prevents brain edema in experimental ALF and significantly attenuates brain lactate accumulation.[25]

#

ASTROCYTIC FUNCTION IN FULMINANT HEPATIC FAILURE

Astrocytes play a key role in the maintenance of central nervous system function by virtue of their interactions with other neural cells (neurons, endothelial cells, other glia). Such interactions have both morphological features (example: astrocytic endfeet on the vascular endothelium) and biochemical characteristics (example: the role of the astrocyte in the uptake of neuronally released glutamate at the glutamatergic synapse). Other important roles of astrocytes include K⁺ spatial buffering, removal of toxins (such as ammonia), supply of substrates for cerebral energy homeostasis, and osmoregulation. An impressive body of new evidence demonstrates that ALF results in alterations of many of these functions of the astrocyte as a result of changes in expression of genes coding for astrocytic proteins with important roles in brain function (Table [1]).

#

Astrocytic Structural Proteins

Glial fibrillary acidic protein (GFAP) is an intracytoplasmic filamentous protein that constitutes the principal component of astrocytic intermediate filaments.[31] GFAP is implicated in the modulation of astrocyte motility and morphology by providing structural stability to astrocytic processes. Experimental ALF resulting from hepatic devascularization results in decreased expression of GFAP mRNA and protein that is significantly correlated with increased brain ammonia and water content.[32] These findings led to the suggestion that the loss of GFAP was implicated in the pathogenesis of brain edema in ALF. Consistent with this possibility are reports of a loss of GFAP expression in cultured astrocytes exposed to millimolar concentrations of ammonia sufficient to cause significant cell swelling.[33] Other reports that support this possibility include those using GFAP knockout mice in which release of the osmoeffective amino acid taurine following hypotonic stress is significantly modified.[34] Loss of GFAP expression in the brain in experimental ALF is a selective phenomenon; expression of a second protein constituent of glial filaments, S100β was found to be unaltered,[32] and it was proposed that this selective loss of GFAP could result in alterations of viscoelastic properties of the cell and, in this way, cause a loss of regulation of cell volume, contributing to the pathogenesis of brain edema.

#

Astrocytic Transporters

A major function of perineuronal astrocytes relates to the maintenance of the extracellular milieu of the brain. For this purpose, the astrocyte is equipped with high-affinity uptake systems (transporters) for a wide range of ions, neuroactive compounds, and metabolic fuels. Increasing evidence suggests that ALF results in alterations in expression and function of a number of these transporters.

The removal of neuronally released glutamate (the major excitatory neurotransmitter of mammalian brain) from the synaptic cleft is achieved primarily by high-capacity, high-affinity transporters localized on the perineural astrocyte (Fig. [2]). Excitatory amino acid transporter-2 (EAAT-2) is a cloned and characterized glutamate transporter localized on astrocytes of mammalian forebrain, and “knockout” of the EAAT-2 gene in mice results in increased extracellular brain glutamate concentrations, seizures, and brain edema,[35] all of which are features of ALF.[36] [37] Experimental ALF resulting from hepatic devascularization in the rat results in a loss of expression of EAAT-2 mRNA and protein (Fig. [3]),[38] a concomitant reduction in high-affinity glutamate transport in the brain,[39] and increased extracellular brain concentrations of glutamate.[36] These changes are significantly correlated with arterial[36] and brain[7] ammonia concentrations and with brain water accumulation in this model of ALF.

Because uptake of glutamate by the astrocyte is (in part) necessary as a substrate for GS, the primary ammonia-metabolizing enzyme of the brain, which is localized in astrocytes, reduced expression of EAAT-2 could seriously compromise the brain's ability to detoxify ammonia (see Fig. [2]). Increased extracellular brain glutamate as a consequence of a deficit of EAAT-2 could also be implicated in the pathogenesis of brain edema in ALF given the reports that exposure of astrocytes to glutamate leads to significant cell swelling.[40]

Extracellular concentrations of the neuroactive amino acid glycine are regulated by high-affinity glycine transporters, one of which, GLYT-1, is expressed by astrocytes.[41] Consequently, GLYT-1 plays a key role in the regulation of glycine concentrations at synapses of glycinergic neurons (in brainstem and spinal cord) as well as in cerebral cortex, where glycine functions as a positive allosteric modulator of glutamate (N-methyl-D-aspartate [NMDA]) receptors (see Fig. [2]). Coma resulting from hepatic devascularization in the rat results in a significant loss of GLYT-1 mRNA expression[42] and a concomitant increase in extracellular concentrations of glycine[36] in cerebral cortex.

Hepatic devascularization also results in reductions in the density of sites for serotonin[43] and noradrenaline[44] transporters. However, in contrast to EAAT-2 and GLYT-1, the loss of these monoamine transporters occurs via posttranslational mechanisms. Expression of the astrocytic GABA-transporter (GAT-2) is unaltered in ALF.

In contrast to the findings of a loss of activity or function, or both, of astrocytic transporters for neuroactive amino acids and monoamines (earlier), expression of the glucose transporter GLUT-1 is increased in the brain in experimental ALF.[4] GLUT-1 is a recently cloned member of a family of glucose transporters localized on astrocytes and perivascular endothelial cells and plays a key role in the movement of glucose into these cells and, consequently, across the blood-brain barrier. Increased GLUT-1 mRNA and protein expression occur in parallel to increased brain lactate synthesis[19] and the development of brain edema in experimental ALF. Expression of the neuronal glucose transporter, GLUT-3 is not modified in ALF, again underscoring the selective vulnerability of the astrocyte. It has been suggested that increased GLUT-1 expression is part of a compensatory mechanism leading to increased glycolytic flux and lactate synthesis occurring in the brain in ALF (see Fig. [2]). It has been proposed that GLUT-1 plays a role in the movement of water into the cell.[45] Consequently, increases in its expression could relate to the phenomenon of brain edema in ALF. Altered expression of the gene coding for the astrocytic water channel protein aquaporin IV in experimental ALF has also been reported.[46]

#

The “Peripheral-Type” (Astrocytic) Benzodiazepine Receptor

In spite of its name, the “peripheral-type” benzodiazepine receptor (PTBR) is not a component of the neuronal GABA receptor complex in the brain. Rather, it is localized on the outer mitochondrial membrane of astrocytes. Its function is primarily to modulate uptake of cholesterol. Increased expression of PTBR mRNA[5] and of PTBR binding sites[47] has been reported in experimental ALF. In the case of toxic liver injury because of thioacetamide ingestion, increased PTBR binding sites were accompanied by an increase in synthesis of pregnenolone, the parent compound of a class of compounds known as “neurosteroids,” some of which have potent excitatory or inhibitory properties. Increases in the synthesis of these steroids in the brain after PTBR activation could potentially play a role in the pathogenesis of hepatic encephalopathy in ALF. Further studies are currently ongoing.

#

Osmoregulation

Osmoregulation is a major function of the astrocyte. Osmoeffective substances, which are synthesized, stored, or released by astrocytes, include glutamine, taurine, and myoinositol. As discussed in the section on ammonia earlier, increased brain glutamine is a consistent finding in human and experimental ALF. Administration of the GS inhibitor methionine sulfoximine to experimental animals given ammonium salts in either the absence[48] or the presence[49] of chronic liver failure results in a significant reduction in brain edema. Similar studies in ALF have not been performed so far.

Taurine is both an inhibitory amino acid neurotransmitter and an osmoeffector. Brain concentrations of taurine are decreased in experimental ALF because of thioacetamide hepatotoxicity[10] but are inconsistent after hepatic devascularization.[7] [8] [9] [23] Significant increases of taurine have been reported in the cerebrospinal fluid of rats with experimental ALF,[8] but no significant correlation between taurine concentrations in brain extracellular space and severity of encephalopathy or brain edema was apparent in these animals.[23] [35] Taken together, these findings do not support a major contribution of brain taurine movement in relation to brain edema in ALF.

Myoinositol has not been extensively studied in relation to brain edema in ALF. Studies in patients with ALF using spectroscopic techniques provide findings that are equivocal with both decreases[18] and no changes[17] of brain myoinositol concentrations.

#

THERAPEUTIC IMPLICATIONS

Despite the consistent findings of hyperammonemia and increased brain ammonia concentrations in ALF, strategies aimed at lowering the gut production of ammonia (lactulose, antibiotics) have not generally been found to be beneficial. On the other hand, studies in experimental ALF demonstrate a lowering of blood ammonia accompanied by a delay in onset of encephalopahy and prevention of brain edema following the administration of L-ornithine L-aspartate (L-OA).[50] The mechanisms of the ammonia-lowering action of this agent involve the stimulation of residual hepatic urea synthesis and more importantly (in the case of ALF) the stimulation of ammonia removal by the muscle. So far, there have been no clinical studies of the effectiveness of L-OA in patients with ALF.

#

Hypothermia

Hypothermia extends survival time and prevents brain edema in animals with acute hyperammonemia with[51] [52] or without[53] liver failure. The mechanisms responsible for the beneficial effects of hypothermia in experimental ALF include reduction of blood-brain ammonia transfer,[51] normalization of extracellular brain glutamate concentrations indicative of an improvement in astrocytic transporter function,[51] and decreased lactate synthesis consistent with improved energy status in brain.[25] Pilot studies in patients with ALF reveal a significant beneficial effect of mild hypothermia that results in decreased intracranial pressure and decreased brain ammonia uptake.[14]

#

Neuropharmacology

There is evidence to suggest that compounds with central (GABA-related) benzodiazepine receptor agonist properties are present in brain tissue from both patients[54] and experimental animals[55] with ALF. However, the significance of these findings (in particular with reference to the ingestion of pharmaceutical benzodiazepines) has been questioned.[56] Administration of the benzodiazepine receptor antagonist flumazenil in ALF has, likewise, resulted in conflicting results with both beneficial effects[57] and no effects[58] being reported. Clinical experience in humans suggests that although flumazenil may be beneficial in the improvement of neurological status in a subgroup of patients, it has no significant effects on brain edema.[57] It may therefore have a therapeutic “niche” for the reversal of encephalopathy in ALF patients in whom there is a suspected ingestion of pharmaceutical benzodiazepines.

Although there is substantial evidence to suggest that the glutamatergic neurotransmitter system is implicated in the pathogenesis of the central nervous system complications of ALF, few studies have attempted to manipulate this system, partly because of the inherent toxic properties of glutamate receptor antagonists. However, in one study in experimental ALF due to hepatic devascularization, the weak glutamate (NMDA) receptor antagonist memantine was found to improve neurological status.[59]

As the precise molecular mechanisms responsible for the pathogenesis of brain edema and encephalopahy in ALF become clearer, the identification of new therapeutic targets such as improvement of cellular pyruvate oxidation, reduction of blood-brain ammonia transfer, improvement of astrocytic transporter function, and neurosteroid antagonists may provide new opportunities for the management of the central nervous system complications of this disorder.

#

ABBREVIATIONS

ALFacute liver failure

EAAT-2excitatory amino acid transporter-2

GABAγ-aminobutyric acid

GAT-2GABA transporter-2

GFAPglial fibrillary acidic protein

GLUT-1endothelial cell glucose transporter-1

GLUT-3neuronal glucose transporter-3

GLYT-1glycine transporter-1

GSglutamine synthetase

K⁺potassium

L-OAL-ornithine L-aspartate

mRNAmessenger RNA

NMDAN-methyl-D-aspartate

PTBRperipheral-type benzodiazepine receptor

*Figure 1* Electron micrograph of cerebral cortex from a patient who died in acute liver failure. Perivascular astrocytes (A) are markedly swollen and vacuolar. Endoplasmic reticulum is dilated (arrow) and mitochondria (M) are also swollen (original magnification × 6000). From Kato et al,[1] with permission.

*Figure 2* Key proteins involved in astrocytic function. Glutamate, the major excitatory neurotransmitter of mammalian brain, is released from neurons and inactivated by transport into perineuronal astrocytes. EAAT-2 is the major astrocytic glutamate transporter in the forebrain. Glutamine synthetase is the enzyme responsible for removal of blood-borne ammonia. GLYT-1 is a high-affinity transporter for the inhibitory amino acid neurotransmitter, glycine. Expression of EAAT-2 and GLYT-1 is decreased in the brain in ALF, which leads to increased extracellular concentrations of glutamate and glycine and a limit on the capacity of GS to remove ammonia.

*Figure 3* Expression of the astrocytic glutamate transporter EAAT-2 is decreased in experimental ALF. (A) EAAT-2 mRNA is decreased in the brain in ALF compared with control (CTL). (B) Loss of EAAT-2 protein in ALF compared with CTL. (C) Reduced binding sites for the EAAT-2 ligand ³H-D-aspartate (³H-D-Asp) compared with CTL.

#

REFERENCES

1 Kato M, Hughes R D, Keays R T, Williams R. Electron microscopic study of brain capillaries in cerebral edema from fulminant hepatic failure. Hepatology . 1992; 15 1060-1066

PubMed Search in Google Scholar
2 Traber P G, DalCanto M, Ganger D R, Blei A T. Electron microscopic evaluation of brain edema in rabbits with galactosamine-induced fulminant hepatic failure: ultrastructure and integrity of the blood-brain barrier. Hepatology . 1987; 7 1273-1277

PubMed Search in Google Scholar
3 Potvin M, Morrison H F, Hinchey E J. et al . Cerebral abnormalities in hepatectomized rats with acute hepatic coma. Lab Invest . 1984; 50 560-564

PubMed Search in Google Scholar
4 Desjardins P, Michalak A, Therrien G. et al . Increased expression of the astrocytic/endothelial cell glucose transporter (and water channel) protein GLUT-1 in relation to brain glucose metabolism and edema in acute liver failure (Abst 254). Hepatology . 2001; 34 237

PubMed Search in Google Scholar
5 Desjardins P, Bélanger M, Butterworth R F. Alterations in expression of genes coding for key astrocytic proteins in acute liver failure. J Neurosci Res . 2001; 66 967-971

Crossref PubMed Search in Google Scholar
6 Holmin T, Agardh C D, Alinder G. et al . The influence of total hepatectomy on cerebral energy state, ammonia-related amino acids of the brain and plasma amino acids in the rat. Eur J Clin Invest . 1983; 13 215-220

PubMed Search in Google Scholar
7 Swain M, Butterworth R F, Blei A T. Ammonia and related amino acids in the pathogenesis of brain edema in acute ischemic liver failure in rats. Hepatology . 1992; 15 449-453

PubMed Search in Google Scholar
8 Mans A M, DeJoseph M R, Hawkins R A. Metabolic abnormalities and grade of encephalopathy in acute hepatic failure. J Neurochem . 1994; 63 1829-1838

PubMed Search in Google Scholar
9 Swain M S, Bergeron M, Audet R. et al . Monitoring of neurotransmitter amino acids by means of an indwelling cisterna magna catheter: a comparison of two rodent models of fulminant liver failure. Hepatology . 1992; 16 1028-1035

PubMed Search in Google Scholar
10 Peeling J, Shoemaker L, Gauthier T. et al . Cerebral metabolic and histological effects of thioacetamide-induced liver failure. Am J Physiol . 1993; 265 G572-G578

PubMed Search in Google Scholar
11 Clemmesen J O, Larsen F S, Kondrup J. et al . Cerebral herniation in patients with acute liver failure is correlated with arterial ammonia concentration. Hepatology . 1999; 29 648-653

Crossref PubMed Search in Google Scholar
12 Cooper A J, Plum F. Biochemistry and physiology of brain ammonia. Physiol Rev . 1987; 67 440-519

PubMed Search in Google Scholar
13 Butterworth R F. Pathophysiology of hepatic encephalopathy: a new look at ammonia. Metab Brain Dis . 2002; 17 221-227

Crossref PubMed Search in Google Scholar
14 Dejong C HC, Kampman M T, Deutz N EP, Soeters P B. Cerebral cortex ammonia and glutamine metabolism during liver insufficiency-induced hyperammonemia in the rat. J Neurochem . 1992; 59 1071-1079

PubMed Search in Google Scholar
15 Jalan R, Dalmink S WM, Lee O. et al . Moderate hypothermia for uncontrolled intracranial hypertension in acute liver failure. Lancet . 1999; 354 1164-1168

Crossref PubMed Search in Google Scholar
16 Record C O, Buxton B, Chase R A. et al . Plasma and brain amino acids in fulminant hepatic failure and their relationship to hepatic encephalopathy. Eur J Clin Invest . 1976; 6 387-394

Crossref PubMed Search in Google Scholar
17 McConnell J R, Antonson D L, Ong C S. et al . Proton spectroscopy of brain glutamine in acute liver failure. Hepatology . 1995; 22 69-74

Crossref PubMed Search in Google Scholar
18 Gupta R K, Saraswat V A, Poptani H. et al . Magnetic resonance imaging and localized in vivo proton spectroscopy in patients with fulminant hepatic failure. Am J Gastroenterol . 1993; 88 670-674

PubMed Search in Google Scholar
19 Zwingmann C, Chatauret N, Leibfritz D, Butterworth R F. Selective increase of brain lactate synthesis in experimental acute liver failure: results of a [¹H-¹³C] NMR study. Hepatology . 2003; 37 420-428

Crossref PubMed Search in Google Scholar
20 Tyce G M, Ogg J, Owen Jr A C. Metabolism of acetate to amino acids in brains of rats after complete hepatectomy. J Neurochem . 1981; 36 640-650

PubMed Search in Google Scholar
21 Zimmermann C, Ferenci P, Pifl C. et al . Hepatic encephalopathy in thioacetamide-induced acute liver failure in rats: characterization of an improved model and study of amino acid-ergic neurotransmission. Hepatology . 1989; 9 594-601

PubMed Search in Google Scholar
22 Hertz L, Murthy C R, Lai J C, Fitzpatrick S M, Cooper A J. Some metabolic effects of ammonia on astrocytes and neurons in primary cultures. Neurochem Pathol . 1987; 6 97-129

PubMed Search in Google Scholar
23 Bosman D K, Deutz N EP, Maas M AW. et al . Amino acid release from cerebral cortex in experimental acute liver failure, studied by in vivo cerebral cortex microdialysis. J Neurochem . 1992; 59 591-599

PubMed Search in Google Scholar
24 Deutz N EP, DeGraaf A A, De Haan G J. et al .In vivo brain 1H-NMR spectroscopy (IH-NMRS) during acute hepatic encephalopathy (HE). In: Soeters PB Wilson JHP Meijer AJ Holm E eds. Advances in Ammonia Metabolism and Hepatic Encephalopathy Amsterdam: Excerpta Medica 1988: 439-446

Search in Google Scholar
25 Chatauret N, Rose C, Therrien G, Butterworth R F. Mild hypothermia prevents cerebral edema and CSF lactate accumulation in acute liver failure. Metab Brain Dis . 2001; 16 95-102

Crossref PubMed Search in Google Scholar
26 Tofteng F, Jorgensen L, Hansen B A. et al . Cerebral microdialysis in patients with fulminant hepatic failure, Hepatology . 2002; 36 1333-1340

Crossref PubMed Search in Google Scholar
27 Nyberg S L, Cerra F B, Gruetter R. Brain lactate by magnetic resonance spectroscopy during fulminant hepatic failure in the dog. Liver Transpl Surg . 1998; 4 158-165

Crossref PubMed Search in Google Scholar
28 Staub F, Baethmann A, Peters J. et al . Effects of lactacidosis on glial cell volume and viability. J Cereb Blood Flow Metab . 1990; 10 866-876

PubMed Search in Google Scholar
29 Lai J CK, Cooper A JL. α-ketoglutarate dehydrogenase complex: kinetic properties, regional distribution and effects of inhibitors. J Neurochem . 1986; 47 1376-1386

Crossref PubMed Search in Google Scholar
30 Lowry O H, Passonneau J V. Kinetic evidence for multiple binding sites on phosphofructokinase. J Biol Chem . 1966; 241 2268-2279

PubMed Search in Google Scholar
31 Eng L F, Ghirnikar R S, Lee Y L. Glial fibrillary acidic protein: GFAP-31 years (1969-2000). Neurochem Res . 2000; 25 1439-1451

Search in Google Scholar
32 Bélanger M, Desjardins P, Chatauret N, Butterworth R F. Loss of expression of glial fibrillary acidic protein in acute hyperammonemia. Neurochem Int . 2002; 41 155-160

Crossref PubMed Search in Google Scholar
33 Norenberg M D, Neary J T, Norenberg L O, McCarthy M. Ammonia induced decreased in glial fibrillary acidic protein in cultured astrocytes. J Neuropathol Exp Neurol . 1990; 49 399-405

PubMed Search in Google Scholar
34 Ding M, Eliasson C, Betsholtz C. et al . Altered taurine release following hypotonic stress in astrocytes from mice deficient for GFAP and vimentin. Mol Brain Res . 1998; 62 77-81

Crossref PubMed Search in Google Scholar
35 Tanaka K, Watase K, Manabe T. et al . Epilepsy and exacerbation of brain injury in mice lacking the glutamate transporter GLT-1. Science . 1997; 276 1699-1702

Crossref PubMed Search in Google Scholar
36 Michalak A, Rose C, Butterworth J, Butterworth R F. Neuroactive amino acids and glutamate (NMDA) receptors in frontal cortex of rats with experimental acute liver failure. Hepatology . 1996; 24 908-913

PubMed Search in Google Scholar
37 Ellis A J, Wendon J A, Williams R. Subclinical seizure activity and prophylactic phenytoin infusion in acute liver failure: a controlled clinical trial. Hepatology . 2000; 32 536-541

Crossref PubMed Search in Google Scholar
38 Knecht K, Michalak A, Rose C. et al . Decreased glutamate transporter (GLT-1) expression in frontal cortex of rats with acute liver failure. Neurosci Lett . 1997; 229 201-203

Crossref PubMed Search in Google Scholar
39 Butterworth R F. Glutamate transporters in hyperammonemia. Neurochem Int . 2002; 41 81-85

PubMed Search in Google Scholar
40 Van Harreveld A, Fifkova E. Light- and electron-microscopic changes in central nervous tissue after electrophoretic injection of glutamate. Exp Mol Pathol . 1971; 15 61-81

Crossref PubMed Search in Google Scholar
41 Zafra F, Aragon C, Olivares L. et al . Glycine transporters are differentially expressed among CNS cells. J Neurosci . 1995; 15 3952-3969

PubMed Search in Google Scholar
42 Zwingmann C, Desjardins P, Hazell A. et al . Reduced expression of astrocytic glycine transporter (GLYT-1) in acute liver failure. Metab Brain Dis . 2002; 17 263-273

Crossref PubMed Search in Google Scholar
43 Michalak A, Chatauret N, Butterworth R F. Evidence for a serotonin transporter deficit in experimental acute liver failure. Neurochem Int . 2001; 38 163-168

PubMed Search in Google Scholar
44 Michalak A, Rose C, Butterworth R F. Loss of noradrenaline transporter sites in frontal cortex of rats with acute (ischemic) liver failure. Neurochem Int . 2001; 38 25-30

Crossref PubMed Search in Google Scholar
45 Fischbarq J, Kuang K Y, Vera J C. et al . Glucose transporters serve as water channels. Proc Natl Acad Sci USA . 1990; 87 3244-3247

PubMed Search in Google Scholar
46 Margulies J E, Thompson R C, Demetriou A A. Aquaporin-4 water channel is up-regulated in the brain in fulminant hepatic failure (Abst). Hepatology . 1999; 30 395A

Crossref PubMed Search in Google Scholar
47 Itzhak Y, Roig-Cantisano A, Dombro R S. et al . Acute liver failure and hyperammonemia increase peripheral-type receptor binding and pregnenolone synthesis in mouse brain. Brain Res . 1995; 705 345-348

Crossref PubMed Search in Google Scholar
48 Takahashi H, Koehler R C, Brusilow S W, Traystman R J. Inhibition of brain glutamine accumulation prevents cerebral edema in hyperammonemic rats. Am J Physiol . 1991; 261 H825-H829

PubMed Search in Google Scholar
49 Blei A T, Olafsson S, Therrien G, Butterworth R F. Ammonia-induced brain edema and intracranial hypertension in rats after portacaval anastomosis. Hepatology . 1994; 19 1437-1444

Crossref PubMed Search in Google Scholar
50 Rose C, Michalak A, Rama Rao V K. et al . L-ornithine L-aspartate lowers plasma and cerebrospinal fluid ammonia and prevents brain edema in rats with acute liver failure. Hepatology . 1999; 30 636-640

Crossref PubMed Search in Google Scholar
51 Rose C, Michalak A, Pannunzio M. et al . Mild hypothermia delays the onset of coma and prevents brain edema and extracellular brain glutamate accumulation in rats with acute liver failure. Hepatology . 2000; 3 872-877

PubMed Search in Google Scholar
52 Traber P, DalCanto M, Ganger D, Blei A T. Effect of body temperature on brain edema and encephalopathy in the rat after hepatic devascularization. Gastroenterology . 1989; 96 885-891

PubMed Search in Google Scholar
53 Schenker S, Warren K S. Effect of temperature variation on toxicity and metabolism of ammonia in mice. J Lab Clin Med . 1962; 60 291-301

PubMed Search in Google Scholar
54 Basile A S, Hughes R D, Harrison P M. et al . Elevated brain concentrations of 1,4-benzodiazepines in fulminant hepatic failure. N Engl J Med . 1991; 325 473-478

PubMed Search in Google Scholar
55 Basile A S, Pannell L, Jaouni T. et al . Brain concentrations of benzodiazepines are elevated in an animal model of hepatic encephalopathy. Proc Natl Acad Sci USA . 1990; 87 5263-5267

PubMed Search in Google Scholar
56 Widler P, Fisch H U, Schoch P. et al . Increased benzodiazepine-like activity is neither necessary nor sufficient to explain acute hepatic encephalopthy in the thioacetamide-treated rat. Hepatology . 1993; 18 1459-1464

Crossref PubMed Search in Google Scholar
57 Ferenci P, Grimm G. Treatment of hepatic encephalopathy with the benzodiazepine antagonist flumazenil. In: Butterworth RF, Pomier Layrargues G, eds. Hepatic Encephalopathy: Pathophysiology and Treatment Clifton, NJ 1989: 597-612

Search in Google Scholar
58 Van der Rijt C C, de Knegt J R, Schalm S W. et al . Flumazenil does not improve hepatic encephalopathy associated with acute ischemic liver failure in the rabbit. Metab Brain Dis 1990; 5 131-141

PubMed Search in Google Scholar
59 Vogels B A, Maas M A, Daalhuisen J. et al . Memantine, a non-competitive NMDA receptor antagonist improves hyperammonemia-induced encephalopathy and acute hepatic encephalopathy in rats. Hepatology . 1997; 25 820-827

Crossref PubMed Search in Google Scholar

#

REFERENCES

1 Kato M, Hughes R D, Keays R T, Williams R. Electron microscopic study of brain capillaries in cerebral edema from fulminant hepatic failure. Hepatology . 1992; 15 1060-1066

PubMed Search in Google Scholar
2 Traber P G, DalCanto M, Ganger D R, Blei A T. Electron microscopic evaluation of brain edema in rabbits with galactosamine-induced fulminant hepatic failure: ultrastructure and integrity of the blood-brain barrier. Hepatology . 1987; 7 1273-1277

PubMed Search in Google Scholar
3 Potvin M, Morrison H F, Hinchey E J. et al . Cerebral abnormalities in hepatectomized rats with acute hepatic coma. Lab Invest . 1984; 50 560-564

PubMed Search in Google Scholar
4 Desjardins P, Michalak A, Therrien G. et al . Increased expression of the astrocytic/endothelial cell glucose transporter (and water channel) protein GLUT-1 in relation to brain glucose metabolism and edema in acute liver failure (Abst 254). Hepatology . 2001; 34 237

PubMed Search in Google Scholar
5 Desjardins P, Bélanger M, Butterworth R F. Alterations in expression of genes coding for key astrocytic proteins in acute liver failure. J Neurosci Res . 2001; 66 967-971

Crossref PubMed Search in Google Scholar
6 Holmin T, Agardh C D, Alinder G. et al . The influence of total hepatectomy on cerebral energy state, ammonia-related amino acids of the brain and plasma amino acids in the rat. Eur J Clin Invest . 1983; 13 215-220

PubMed Search in Google Scholar
7 Swain M, Butterworth R F, Blei A T. Ammonia and related amino acids in the pathogenesis of brain edema in acute ischemic liver failure in rats. Hepatology . 1992; 15 449-453

PubMed Search in Google Scholar
8 Mans A M, DeJoseph M R, Hawkins R A. Metabolic abnormalities and grade of encephalopathy in acute hepatic failure. J Neurochem . 1994; 63 1829-1838

PubMed Search in Google Scholar
9 Swain M S, Bergeron M, Audet R. et al . Monitoring of neurotransmitter amino acids by means of an indwelling cisterna magna catheter: a comparison of two rodent models of fulminant liver failure. Hepatology . 1992; 16 1028-1035

PubMed Search in Google Scholar
10 Peeling J, Shoemaker L, Gauthier T. et al . Cerebral metabolic and histological effects of thioacetamide-induced liver failure. Am J Physiol . 1993; 265 G572-G578

PubMed Search in Google Scholar
11 Clemmesen J O, Larsen F S, Kondrup J. et al . Cerebral herniation in patients with acute liver failure is correlated with arterial ammonia concentration. Hepatology . 1999; 29 648-653

Crossref PubMed Search in Google Scholar
12 Cooper A J, Plum F. Biochemistry and physiology of brain ammonia. Physiol Rev . 1987; 67 440-519

PubMed Search in Google Scholar
13 Butterworth R F. Pathophysiology of hepatic encephalopathy: a new look at ammonia. Metab Brain Dis . 2002; 17 221-227

Crossref PubMed Search in Google Scholar
14 Dejong C HC, Kampman M T, Deutz N EP, Soeters P B. Cerebral cortex ammonia and glutamine metabolism during liver insufficiency-induced hyperammonemia in the rat. J Neurochem . 1992; 59 1071-1079

PubMed Search in Google Scholar
15 Jalan R, Dalmink S WM, Lee O. et al . Moderate hypothermia for uncontrolled intracranial hypertension in acute liver failure. Lancet . 1999; 354 1164-1168

Crossref PubMed Search in Google Scholar
16 Record C O, Buxton B, Chase R A. et al . Plasma and brain amino acids in fulminant hepatic failure and their relationship to hepatic encephalopathy. Eur J Clin Invest . 1976; 6 387-394

Crossref PubMed Search in Google Scholar
17 McConnell J R, Antonson D L, Ong C S. et al . Proton spectroscopy of brain glutamine in acute liver failure. Hepatology . 1995; 22 69-74

Crossref PubMed Search in Google Scholar
18 Gupta R K, Saraswat V A, Poptani H. et al . Magnetic resonance imaging and localized in vivo proton spectroscopy in patients with fulminant hepatic failure. Am J Gastroenterol . 1993; 88 670-674

PubMed Search in Google Scholar
19 Zwingmann C, Chatauret N, Leibfritz D, Butterworth R F. Selective increase of brain lactate synthesis in experimental acute liver failure: results of a [¹H-¹³C] NMR study. Hepatology . 2003; 37 420-428

Crossref PubMed Search in Google Scholar
20 Tyce G M, Ogg J, Owen Jr A C. Metabolism of acetate to amino acids in brains of rats after complete hepatectomy. J Neurochem . 1981; 36 640-650

PubMed Search in Google Scholar
21 Zimmermann C, Ferenci P, Pifl C. et al . Hepatic encephalopathy in thioacetamide-induced acute liver failure in rats: characterization of an improved model and study of amino acid-ergic neurotransmission. Hepatology . 1989; 9 594-601

PubMed Search in Google Scholar
22 Hertz L, Murthy C R, Lai J C, Fitzpatrick S M, Cooper A J. Some metabolic effects of ammonia on astrocytes and neurons in primary cultures. Neurochem Pathol . 1987; 6 97-129

PubMed Search in Google Scholar
23 Bosman D K, Deutz N EP, Maas M AW. et al . Amino acid release from cerebral cortex in experimental acute liver failure, studied by in vivo cerebral cortex microdialysis. J Neurochem . 1992; 59 591-599

PubMed Search in Google Scholar
24 Deutz N EP, DeGraaf A A, De Haan G J. et al .In vivo brain 1H-NMR spectroscopy (IH-NMRS) during acute hepatic encephalopathy (HE). In: Soeters PB Wilson JHP Meijer AJ Holm E eds. Advances in Ammonia Metabolism and Hepatic Encephalopathy Amsterdam: Excerpta Medica 1988: 439-446

Search in Google Scholar
25 Chatauret N, Rose C, Therrien G, Butterworth R F. Mild hypothermia prevents cerebral edema and CSF lactate accumulation in acute liver failure. Metab Brain Dis . 2001; 16 95-102

Crossref PubMed Search in Google Scholar
26 Tofteng F, Jorgensen L, Hansen B A. et al . Cerebral microdialysis in patients with fulminant hepatic failure, Hepatology . 2002; 36 1333-1340

Crossref PubMed Search in Google Scholar
27 Nyberg S L, Cerra F B, Gruetter R. Brain lactate by magnetic resonance spectroscopy during fulminant hepatic failure in the dog. Liver Transpl Surg . 1998; 4 158-165

Crossref PubMed Search in Google Scholar
28 Staub F, Baethmann A, Peters J. et al . Effects of lactacidosis on glial cell volume and viability. J Cereb Blood Flow Metab . 1990; 10 866-876

PubMed Search in Google Scholar
29 Lai J CK, Cooper A JL. α-ketoglutarate dehydrogenase complex: kinetic properties, regional distribution and effects of inhibitors. J Neurochem . 1986; 47 1376-1386

Crossref PubMed Search in Google Scholar
30 Lowry O H, Passonneau J V. Kinetic evidence for multiple binding sites on phosphofructokinase. J Biol Chem . 1966; 241 2268-2279

PubMed Search in Google Scholar
31 Eng L F, Ghirnikar R S, Lee Y L. Glial fibrillary acidic protein: GFAP-31 years (1969-2000). Neurochem Res . 2000; 25 1439-1451

Search in Google Scholar
32 Bélanger M, Desjardins P, Chatauret N, Butterworth R F. Loss of expression of glial fibrillary acidic protein in acute hyperammonemia. Neurochem Int . 2002; 41 155-160

Crossref PubMed Search in Google Scholar
33 Norenberg M D, Neary J T, Norenberg L O, McCarthy M. Ammonia induced decreased in glial fibrillary acidic protein in cultured astrocytes. J Neuropathol Exp Neurol . 1990; 49 399-405

PubMed Search in Google Scholar
34 Ding M, Eliasson C, Betsholtz C. et al . Altered taurine release following hypotonic stress in astrocytes from mice deficient for GFAP and vimentin. Mol Brain Res . 1998; 62 77-81

Crossref PubMed Search in Google Scholar
35 Tanaka K, Watase K, Manabe T. et al . Epilepsy and exacerbation of brain injury in mice lacking the glutamate transporter GLT-1. Science . 1997; 276 1699-1702

Crossref PubMed Search in Google Scholar
36 Michalak A, Rose C, Butterworth J, Butterworth R F. Neuroactive amino acids and glutamate (NMDA) receptors in frontal cortex of rats with experimental acute liver failure. Hepatology . 1996; 24 908-913

PubMed Search in Google Scholar
37 Ellis A J, Wendon J A, Williams R. Subclinical seizure activity and prophylactic phenytoin infusion in acute liver failure: a controlled clinical trial. Hepatology . 2000; 32 536-541

Crossref PubMed Search in Google Scholar
38 Knecht K, Michalak A, Rose C. et al . Decreased glutamate transporter (GLT-1) expression in frontal cortex of rats with acute liver failure. Neurosci Lett . 1997; 229 201-203

Crossref PubMed Search in Google Scholar
39 Butterworth R F. Glutamate transporters in hyperammonemia. Neurochem Int . 2002; 41 81-85

PubMed Search in Google Scholar
40 Van Harreveld A, Fifkova E. Light- and electron-microscopic changes in central nervous tissue after electrophoretic injection of glutamate. Exp Mol Pathol . 1971; 15 61-81

Crossref PubMed Search in Google Scholar
41 Zafra F, Aragon C, Olivares L. et al . Glycine transporters are differentially expressed among CNS cells. J Neurosci . 1995; 15 3952-3969

PubMed Search in Google Scholar
42 Zwingmann C, Desjardins P, Hazell A. et al . Reduced expression of astrocytic glycine transporter (GLYT-1) in acute liver failure. Metab Brain Dis . 2002; 17 263-273

Crossref PubMed Search in Google Scholar
43 Michalak A, Chatauret N, Butterworth R F. Evidence for a serotonin transporter deficit in experimental acute liver failure. Neurochem Int . 2001; 38 163-168

PubMed Search in Google Scholar
44 Michalak A, Rose C, Butterworth R F. Loss of noradrenaline transporter sites in frontal cortex of rats with acute (ischemic) liver failure. Neurochem Int . 2001; 38 25-30

Crossref PubMed Search in Google Scholar
45 Fischbarq J, Kuang K Y, Vera J C. et al . Glucose transporters serve as water channels. Proc Natl Acad Sci USA . 1990; 87 3244-3247

PubMed Search in Google Scholar
46 Margulies J E, Thompson R C, Demetriou A A. Aquaporin-4 water channel is up-regulated in the brain in fulminant hepatic failure (Abst). Hepatology . 1999; 30 395A

Crossref PubMed Search in Google Scholar
47 Itzhak Y, Roig-Cantisano A, Dombro R S. et al . Acute liver failure and hyperammonemia increase peripheral-type receptor binding and pregnenolone synthesis in mouse brain. Brain Res . 1995; 705 345-348

Crossref PubMed Search in Google Scholar
48 Takahashi H, Koehler R C, Brusilow S W, Traystman R J. Inhibition of brain glutamine accumulation prevents cerebral edema in hyperammonemic rats. Am J Physiol . 1991; 261 H825-H829

PubMed Search in Google Scholar
49 Blei A T, Olafsson S, Therrien G, Butterworth R F. Ammonia-induced brain edema and intracranial hypertension in rats after portacaval anastomosis. Hepatology . 1994; 19 1437-1444

Crossref PubMed Search in Google Scholar
50 Rose C, Michalak A, Rama Rao V K. et al . L-ornithine L-aspartate lowers plasma and cerebrospinal fluid ammonia and prevents brain edema in rats with acute liver failure. Hepatology . 1999; 30 636-640

Crossref PubMed Search in Google Scholar
51 Rose C, Michalak A, Pannunzio M. et al . Mild hypothermia delays the onset of coma and prevents brain edema and extracellular brain glutamate accumulation in rats with acute liver failure. Hepatology . 2000; 3 872-877

PubMed Search in Google Scholar
52 Traber P, DalCanto M, Ganger D, Blei A T. Effect of body temperature on brain edema and encephalopathy in the rat after hepatic devascularization. Gastroenterology . 1989; 96 885-891

PubMed Search in Google Scholar
53 Schenker S, Warren K S. Effect of temperature variation on toxicity and metabolism of ammonia in mice. J Lab Clin Med . 1962; 60 291-301

PubMed Search in Google Scholar
54 Basile A S, Hughes R D, Harrison P M. et al . Elevated brain concentrations of 1,4-benzodiazepines in fulminant hepatic failure. N Engl J Med . 1991; 325 473-478

PubMed Search in Google Scholar
55 Basile A S, Pannell L, Jaouni T. et al . Brain concentrations of benzodiazepines are elevated in an animal model of hepatic encephalopathy. Proc Natl Acad Sci USA . 1990; 87 5263-5267

PubMed Search in Google Scholar
56 Widler P, Fisch H U, Schoch P. et al . Increased benzodiazepine-like activity is neither necessary nor sufficient to explain acute hepatic encephalopthy in the thioacetamide-treated rat. Hepatology . 1993; 18 1459-1464

Crossref PubMed Search in Google Scholar
57 Ferenci P, Grimm G. Treatment of hepatic encephalopathy with the benzodiazepine antagonist flumazenil. In: Butterworth RF, Pomier Layrargues G, eds. Hepatic Encephalopathy: Pathophysiology and Treatment Clifton, NJ 1989: 597-612

Search in Google Scholar
58 Van der Rijt C C, de Knegt J R, Schalm S W. et al . Flumazenil does not improve hepatic encephalopathy associated with acute ischemic liver failure in the rabbit. Metab Brain Dis 1990; 5 131-141

PubMed Search in Google Scholar
59 Vogels B A, Maas M A, Daalhuisen J. et al . Memantine, a non-competitive NMDA receptor antagonist improves hyperammonemia-induced encephalopathy and acute hepatic encephalopathy in rats. Hepatology . 1997; 25 820-827

Crossref PubMed Search in Google Scholar

Permissions and Reprints