Neuroactive steroids and fatigue severity in patients with primary biliary cirrhosis and hepatitis C

Fatigue is one of the most common non-specific symptoms associated with several disease states including liver diseases. Recently, it was reported that levels of progesterone metabolites such as allopregnanolone (3alpha,5alpha-tetrahydroprogesterone; 3alpha,5alpha-THP) and isopregnanolone (3beta,5alpha-THP) were increased in plasma of patients with chronic fatigue syndrome. We hypothesize that THP metabolites might be associated with fatigue commonly observed in chronic liver diseases. We evaluated fatigue scores and plasma levels of five progesterone metabolites in 16 patients with primary biliary cirrhosis (PBC), 12 patients with chronic hepatitis C (CHC) and 11 age-matched controls. The fatigue impact scale (FIS) ratio was significantly increased (P < 0.01) in patients with PBC and CHC compared to controls. Plasma levels of 3alpha,5alpha-THP and pregnanolone (3alpha,5beta-THP) were significantly increased in PBC and CHC patients. The other progesterone metabolites, i.e. 3beta,5alpha-THP, 3beta,5beta-THP and 3alpha,5alpha-tetrahydrodeoxycorticosterone were either undetectable or detected only in some patients. Plasma levels of 3alpha,5alpha-THP and 3alpha,5beta-THP were found to be significantly higher in patients with fatigue (P < 0.05), while those of patients without fatigue were not significantly different from controls. Both 3alpha,5alpha-THP and 3alpha,5beta-THP are positive allosteric modulators of the gamma-aminobutyric acid type A (GABA-A) receptor and readily cross the blood-brain barrier. The present preliminary findings suggest that increased inhibition through GABA-A receptors due to the accumulation of neuroinhibitory steroids may represent an important pathophysiological mechanism of fatigue in chronic liver diseases.

Selective increase of neuronal cyclooxygenase-2 (COX-2) expression in vulnerable brain regions of rats with experimental Wernicke's encephalopathy: effect of nimesulide

Thiamine deficiency (TD) in both humans and experimental animals results in severe mitochondrial dysfunction and leads to selective neuronal cell death in diencephalic and cerebellar structures. We have investigated cyclooxygenase-2 (COX-2) expression in vulnerable (medial thalamus, inferior colliculus) and spared (frontal cortex) regions of rats with thiamine deficiency. Expression of COX-2 mRNA was selectively increased (twofold, p < 0.001) in vulnerable regions at symptomatic stages of encephalopathy (14 days) of TD compared to pair-fed controls or presymptomatic (days 12) rats. Induction of COX-2 expression was accompanied by a significant increase (two- to threefold, p < 0.001) in prostanglandin E2 (PGE2) synthesis in vulnerable regions at symptomatic stages of TD. COX-2 immunolabeling revealed a neuronal localization and COX-2 immunoreactive neurons were significantly increased at symptomatic stages of encephalopathy. Administration of nimesulide, a highly specific COX-2 inhibitor, significantly reduced PGE-2 levels in vulnerable regions but, rather than being neuroprotective, precipitated encephalopathy and exacerbated neuronal cell death due to TD. These findings suggest that newly synthesized prostanoids exert a neuroprotective role in TD.

Reduced plasma dehydroepiandrosterone sulfate levels are significantly correlated with fatigue severity in patients with primary biliary cirrhosis

Fatigue is a common debilitating complication of primary biliary cirrhosis (PBC), the pathophysiologic mechanism of which is poorly understood. Recently, the neuroactive steroid dehydroepinadrosterone sulfate (DHEAS) was reported to be implicated in Chronic Fatigue Syndrome in the absence of liver disease. The present study was undertaken to analyse fatigue scores and their relationship with disease severity and circulating levels of DHEAS as well as its precursors DHEA and pregnenolone in PBC patients with (n=15) or without fatigue (n=10) compared to control subjects (n=11). Fatigue was assessed using the fatigue impact scale (FIS) including cognitive, physical and psychosocial subclasses. Steroids were measured by radioimmunoassay or gas chromatography/mass spectrometry. Plasma concentrations of DHEAS were significantly reduced in PBC patients with fatigue as compared to controls, while those of its precursors DHEA and pregnenolone remained within the control range. Plasma levels of DHEAS in PBC patients were significantly correlated with fatigue severity as reflected by total FIS scores including total (rp=-0.42; p=0.018), as well as the cognitive (rp=-0.37; p=0.03), physical (rp=-0.48; p=0.006) and psychosocial (rp=-0.35; p=0.04) subclasses of fatigue scores. No correlation of fatigue scores was observed with indices of liver function. These findings suggest that reduced levels of the neurosteroid DHEAS may contribute to fatigue in patients with PBC; substitutive therapy using DHEAS or its precursor DHEA could be beneficial in the management of fatigue in patients with low levels of DHEAS.

Glucose loading precipitates focal lactic acidosis in the vulnerable medial thalamus of thiamine-deficient rats

Glucose loading in thiamine-deficient patients is known to precipitate Wernicke's Encephalopathy; however, the mechanisms responsible have not been fully elucidated. Lactate accumulation occurs in brains of thiamine-deficient rats. In order to determine whether glucose loading in thiamine-deficient rats causes selective lactic acidosis in vulnerable brain structures, cerebral pH was measured autoradiographically using 14-labeled 5,5-dimethyloxazolidine-2, 4-dione ([(14)C]DMO) in the medial thalamus, a vulnerable brain region, versus cerebral cortex, a brain region that is spared in thiamine deficiency. Following administration of a glucose load, regional lactate levels and de novo lactate synthesis measured by (1)H-(13)C-NMR spectroscopy, increased significantly to 21.86 +/- 3.04 mumol/g (wet weight) in the medial thalamus (p < 0.001) and pH in this brain region was decreased significantly from 7.08 +/- 0.04 to 6.87 +/- 0.05 (p < 0.001). No such changes were observed in cerebral cortex following a glucose load. These results demonstrate that the increased production and accumulation of brain lactate result in acidosis following glucose loading in thiamine deficiency. Alterations of brain pH could contribute to the pathogenesis of thalamic neuronal damage and consequent cerebral dysfunction in Wernicke's Encephalopathy.

Mechanisms of brain edema in acute liver failure and impact of novel therapeutic interventions

Continued elucidation of the mechanisms of brain edema in acute liver failure (ALF) has established ammonia and the astrocyte as major players in its pathogenesis. The metabolism of ammonia to glutamine appears to be a requisite, and is followed by an osmotic disturbance in the brain, mitochondrial dysfunction with oxidative/nitrosative stress, and alterations of brain glucose metabolism. Cerebral blood flow (CBF) is also altered in ALF and strongly influence the development of brain edema and intracranial hypertension. Additional factors such as systemic inflammation, alterations of the brain extracellular concentration of amino acids and neurotransmitters, and others have been identified and may contribute to the cerebral alterations of ALF. Such pathophysiologic insights are reflected in the various clinical trials of novel therapeutic interventions using ammonia-lowering agents, N-acetylcysteine, hypertonic saline, indomethacin, high-volume plasmapheresis, bio-artificial liver assist devices, albumin dialysis and mild hypothermia.

Thiamin deficiency and brain disorders

Thiamin plays a key role in the maintenance of brain function. Thiamin diphosphate is cofactor for several enzymes involved in glucose metabolism whereas thiamin triphosphate has distinct properties at the neuronal membrane. Thiamin metabolism in the brain is compartmented between neurons and neighbouring glial cells. Thiamin deficiency is commonly encountered in severe malnutrition associated with chronic alcoholism, HIV–AIDS and gastrointestinal disease where it frequently results in Wernicke's encephalopathy (the Wernicke–Korsakoff syndrome). Wernicke's encephalopathy is severely underdiagnosed according to clinical criteria in both alcoholic and HIV–AIDS patients. Magnetic resonance imaging reveals bilateral ventricular enlargement, mammillary body atrophy and cerebellar degeneration indicative of selective neuronal loss that is characteristic of Wernicke's encephalopathy. Several mechanisms have been proposed to explain this selective loss of neurons including a cerebral energy deficit resulting from reductions in activity of thiamin diphosphate-dependent enzymes, oxidative stress and N-methyl-D-aspartate receptor-mediated excitotoxicity. Both microglia and perivascular endothelial cells are sources of NO and oxidative stress in thiamin deficiency. Decreased activities of thiamin diphosphate-dependent enzymes (in particular α-ketoglutarate dehydrogenase) have also been reported in neurodegenerative diseases such as Alzheimer's and Parkinson's diseases independent of patient malnutrition. In these cases, decreased activities result from direct toxic actions of oxidative stress and β-amyloid produced as part of the neuronal cell death cascade in these disorders.

The neurosteroid system: an emerging therapeutic target for hepatic encephalopathy

Both acute and chronic liver failure induce cerebral complications known as hepatic encephalopathy (HE) and thought to selectively involve brain astrocytes. Alterations of astrocytic-neuronal cross talk occurs affecting brain function. In acute liver failure, astrocyte undergo swelling, which results in increased intracranial pressure and may lead to brain herniation. In chronic liver failure, Alzheimer-type II astrocytosis is a characteristic change. Neurosteroids (NS) synthesized in the brain mainly by astrocytes independent of peripheral steroidal sources (adrenals and gonads) are suggested to play a role in HE. NS bind and modulate different types of membrane receptors. Effects on the gamma amino butyric acid (GABA)-A receptor complex are the most extensively studied. For example, the NS tetrahydroprogesterone (allopregnanolone), and tetrahydrodeoxycorticosterone (THDOC) are potent positive allosteric modulators of GABA-A receptors. As a consequence of modulation of these receptors, NS are well-known to modulate inhibitory neurotransmission in the central nervous system. Some NS bind to intracellular receptors, and in this way may also regulate gene expression. In HE, it has been well documented that neurotransmission and gene expression alterations occur during the progression of the disease. This review summarizes findings of relevance for the involvement of NS in human and experimental HE.

The neurosteroid system: implication in the pathophysiology of hepatic encephalopathy

Hepatic encephalopathy (HE) is a serious cerebral complication of both acute and chronic liver failure. In acute liver failure, astrocytes undergo swelling which results in increased intracranial pressure and may lead to brain herniation and death. In chronic liver failure, Alzheimer-type II astrocytosis is the characteristic neuropathologic finding. Patients with liver failure manifest severe alterations of their quality of life including sleep disorders as well as memory, learning, and locomotor abnormalities. Neurosteroids (NS) are synthesized in the brain mainly by astrocytes independent of peripheral steroidal sources (adrenals and gonads) and are suggested to play a role in the pathogenesis of HE. NS bind and modulate different types of neural receptors; effects on the gamma amino butyric acid (GABA)-A receptor complex are the most extensively studied. For example, the NS tetrahydroprogesterone (allopregnanolone), and tetrahydrodeoxycorticosterone (THDOC) are potent positive allosteric modulators of the GABA-A receptor. As a consequence of modulation of these receptors, NS stimulate inhibitory neurotransmission in the CNS, and neuroinhibitory changes including "increased GABA-ergic tone" have been suggested as pathophysiological mechanisms in HE. Moreover, some NS bind to intracellular receptors through which they also regulate gene expression, and there is substantial evidence confirming that expression of genes coding for key astrocytic and neuronal proteins are altered in HE. This review summarizes findings consistent with the involvement of NS in human and experimental HE.

Mild hypothermia attenuates liver injury and improves survival in mice with acetaminophen toxicity

BACKGROUND & AIMS

Body temperature may critically affect mechanisms of liver injury in acetaminophen (APAP) hepatotoxicity. In addition, mild hypothermia is used to treat intracranial hypertension in human liver failure without detailed information on its effects on the injured liver itself. Therefore, we investigated the effects of body temperature on the progression of APAP-induced liver injury in mice.

METHODS

Male C57BL6 mice treated with saline or APAP (300 mg/kg intraperitoneally) were maintained at normothermia (35.5-37.5 degrees C) by external warming or were allowed to develop mild hypothermia (32.0-35.0 degrees C) after 2 hours from APAP administration.

RESULTS

Mild hypothermia resulted in improved survival after APAP intoxication. Liver damage was reduced, as assessed histologically and by plasma alanine aminotransferase levels. Early effects of hypothermia included a reduction of hepatic congestion and improved recovery of glycogen stores. At later time points (8-12 hours), APAP-treated mice that were maintained at normothermia manifested increased hepatocyte apoptosis, as assessed by terminal deoxynucleotidyl transferase-mediated deoxyuridine triphosphate nick-end labeling staining and cleavage of poly(adenosine diphosphate-ribose) polymerase. Mild hypothermia did not affect the formation of APAP-protein adducts or the depletion of glutathione, nor did it abrogate hepatocyte DNA synthesis.

CONCLUSIONS

Mild hypothermia improved survival and attenuated liver injury and apoptosis in APAP-treated mice by reducing hepatic congestion and improving glycogen recovery without affecting hepatic regeneration. Results of the study underscore the need for a strict control of body temperature in animal models of liver failure and suggest that the benefits of mild hypothermia in liver failure may extend beyond those related to reduced cerebral complications.

The brain glutamate system in liver failure

Liver failure results in significant alterations of the brain glutamate system. Ammonia and the astrocyte play major roles in such alterations, which affect several components of the brain glutamate system, namely its synthesis, intercellular transport (uptake and release), and function. In addition to the neurological symptoms of hepatic encephalopathy, modified glutamatergic regulation may contribute to other cerebral complications of liver failure, such as brain edema, intracranial hypertension and changes in cerebral blood flow. A better understanding of the cause and precise nature of the alterations of the brain glutamate system in liver failure could lead to new therapeutic avenues for the cerebral complications of liver disease.

Direct molecular and spectroscopic evidence for increased ammonia removal capacity of skeletal muscle in acute liver failure

BACKGROUND/AIMS

It has been proposed that, in acute liver failure, skeletal muscle adapts to become the principle organ responsible for removal of blood-borne ammonia by increasing glutamine synthesis, a reaction that is catalyzed by the cytosolic ATP-dependent enzyme glutamine synthetase. To address this issue, glutamine synthetase expression and activities were measured in skeletal muscle of rats with acute liver failure resulting from hepatic devascularization.

METHODS

Glutamine synthetase protein and gene expression were investigated using immunoblotting and semi-quantitative RT-PCR analysis. Glutamine synthetase activity and glutamine de novo synthesis were measured using, respectively, a standard enzymatic assay and [13C]-nuclear magnetic resonance spectroscopy.

RESULTS

Glutamine synthetase protein (but not gene) expression and enzyme activities were significantly up-regulated leading to increased de novo synthesis of glutamine and increased skeletal muscle capacity for ammonia removal in acute liver failure. In contrast to skeletal muscle, expression and activities of glutamine synthetase in the brain were significantly decreased.

CONCLUSIONS

These findings demonstrate that skeletal muscle adapts, through a rapid induction of glutamine synthetase, to increase its capacity for removal of blood-borne ammonia in acute liver failure. Maintenance of muscle mass together with the development of agents with the capacity to stimulate muscle glutamine synthetase could provide effective ammonia-lowering strategies in this disorder.

An interaction between benzodiazepines and neuroactive steroids at GABA A receptors in cultured hippocampal neurons

Neurosteroids are modulators of several receptors and ion channels and are implicated in the pathophysiology of several neuropsychiatric diseases including hepatic encephalopathy (HE). The neurosteroid, allopregnanolone, a positive allosteric modulator of GABA(A) receptors, accumulates in the brains of HE patients where it can potentiate GABA(A) receptor-mediated responses. Attenuation of the effects of neurosteroids on GABA-ergic neurotransmission is therefore of interest for the management of HE. In the present study, we determined the effect of the benzodiazepine partial inverse agonist, Ro15-4513, and the benzodiazepine antagonist, flumazenil on modulation of the GABA(A) mediated chloride currents by allopregnanolone and on spontaneous synaptic activity in cultured hippocampal neurons using the patch-clamp technique. Allopregnanolone (0.03-0.3 microM), dose-dependently potentiated GABA-induced currents, an action significantly reduced by Ro15-4513 (10 microM). In contrast, flumazenil (10 microM) had no effect on the ability of allopregnanolone to potentiate GABA(A) currents but it blocked the effects of Ro15-4513. The frequency of spontaneous synaptic activity was significantly reduced in the presence of allopregnanolone (0.1 microM) from 1.5+/-0.7 to 0.1+/-0.04Hz. This action was partially reversed by Ro15-4513 (10 microM) but was not significantly influenced by flumazenil (10 microM). These findings suggest that the beneficial affects of Ro15-4513 in experimental HE result from attenuation of the effects of neurosteroids at GABA(A) receptors. Our results may provide a rational basis for the use of benzodiazepine inverse agonists in the management and treatment of hepatic encephalopathy in patients with liver failure.

Neurobiological characterization of an azoxymethane mouse model of acute liver failure

Molecular biological approaches continue to lead to the identification of alterations in expression of genes coding for key central nervous system proteins involved in water homeostasis, energy metabolism and neurotransmitter regulation in acute liver failure (ALF). However, studies aimed at elucidating the pathophysiological consequences of these changes in gene expression are impeded by the lack of a suitable mouse model of ALF. A previous report described hepatic pathology characteristic of ALF resulting from the administration of azoxymethane (AOM) in mice [Matkowskyj, K.A., Marrero, J.A., Carroll, R.E., Danilkovich, A.V., Green, R.M., Benya, R.V., 1999. Azoxymethane-induced fulminant hepatic failure in C57BL/6J mice: characterization of a new animal model. Am. J. Physiol. 277, G455-G462]. In a series of experiments to further assess this treatment as an effective model of ALF, the effects of administration of AOM to male C57BL mice on hepatic and cerebral function were studied. With maintenance of body temperature at 37 degrees C and control of hypoglycemia, mice developed signs of encephalopathy (decreased locomotor activity followed by loss of righting and corneal reflexes) within 16 h of AOM treatment. AOM-treated mice were hyperammonemic, developed spontaneous hypothermia and brain edema. Brain ammonia concentrations were increased to 0.98+/-0.12 mM at coma stages of encephalopathy. Brain amino acid profiles determined by HPLC were typical of ALF in other species including humans. Mild hypothermia (35 degrees C) led to significant attenuation of brain edema, ammonia, and amino acid changes. These findings demonstrate that AOM treatment affords a simple, reproducible mouse model of ALF which may be suitable for the study of the effects of gene manipulation on the cerebral complications of ALF.

Selectively increased expression of the astrocytic/endothelial glucose transporter protein GLUT1 in acute liver failure

Acute liver failure (ALF) is consistently accompanied by alterations in brain energy metabolites and recent nuclear magnetic resonance (NMR) studies suggest disturbances in brain oxidative metabolism in experimental ALF. Glucose transport across the blood-brain barrier is essential to sustain brain energy metabolism and is accomplished by the facilitative glucose transporter GLUT1. To investigate alterations in brain glucose uptake in acute liver failure further, GLUT1 expression and [14C]-2-deoxy-D-glucose uptake were measured in the brains of rats with hepatic devascularization. RT-PCR and Western blot analyses showed significant increases in steady-state levels of GLUT1 mRNA and protein in frontal cortex as early as 6 h following hepatic devascularization, (prior to the onset of brain edema and encephalopathy) which remained elevated at coma stages of encephalopathy. Expression of the astrocytic (45-kDa) and endothelial (55-kDa) forms of GLUT1 was increased as a result of hepatic devascularization. Exposure of cultured astrocytes to pathophysiologically relevant concentrations of ammonia resulted in increased GLUT1 expression, suggesting that elevated ammonia levels are responsible for GLUT1 upregulation in ALF. Increased GLUT1 expression in ALF was selective, since expression of the neuronal glucose transporter GLUT3 and other glucose-regulated proteins (GRP-78 and GRP-94) was unaltered. [14C]-2-deoxy-D-glucose autoradiography revealed increases in cerebral glucose uptake following the induction of GLUT1 in ALF. These results suggest that ammonia-induced increases of GLUT1 expression resulting in increased cerebral glucose uptake occur in ALF and could contribute to the pathophysiological mechanisms responsible for the neurological complications of this condition.

Increased levels of pregnenolone and its neuroactive metabolite allopregnanolone in autopsied brain tissue from cirrhotic patients who died in hepatic coma

It has been suggested that neurosteroids with agonist properties at the central GABA-A receptor are implicated in the pathogenesis of hepatic encephalopathy (HE) in chronic liver disease. In order to address this issue, gas chromatography/mass spectrometry was used to measure the neurosteroids pregnenolone, allopregnanolone, and tetrahydrodeoxycorticosterone (THDOC) in postmortem brain tissue from controls, cirrhotic patients who died without HE, a patient who died in uremic coma, and cirrhotic patients who died in hepatic coma. Exposure of rat cerebral cortical membranes to brain extracts from hepatic coma patients resulted in a 53% (p < 0.001) increase in binding of [3H]muscimol, a GABA-A receptor ligand. Subsequent GC/MS analysis showed that concentrations of the GABA-A receptor agonist neurosteroid allopregnanolone were significantly increased in brain tissue from hepatic coma patients compared to patients without HE or controls (p < 0.001). Brain allopregnanolone concentrations were significantly correlated with the magnitude of induction of [3H]muscimol binding (r2 = 0.82, p < 0.0001). Concentrations of allopregnanolone comparable to those observed in hepatic coma brains are pathophysiologically relevant. Concentrations of the neurosteroid precursor pregnenolone were also increased in brain tissue from hepatic coma patients, while those of a second neurosteroid THDOC were below the levels of detection in all groups. Brain concentrations of benzodiazepine receptor ligands estimated by radioreceptor assay were not significantly increased in cirrhotic patients with or without hepatic coma. These findings suggest that increased levels of allopregnanolone rather than "endogenous benzodiazepines" offer a cogent explanation for the phenomenon of "increased GABAergic tone" previously proposed in HE.

Role of mitochondrial dysfunction and oxidative stress in the pathogenesis of selective neuronal loss in Wernicke's encephalopathy

Thiamine deficiency results in Wernicke's encephalopathy and is commonly encountered in chronic alcoholism, gastrointestinal diseases, and HIV AIDS. The earliest metabolic consequence of thiamine deficiency is a selective loss in activity of the thiamine diphosphate-dependent enzyme alpha-ketoglutarate dehydrogenase (alpha-KGDH), a rate-limiting tricarboxylic acid cycle enzyme. Thiamine deficiency is characterized neuropathologically by selective neuronal cell death in the thalamus, pons, and cerebellum. The cause of this region-selective neuronal loss is unknown, but mechanisms involving cellular energy failure, focal lactic acidosis, and NMDA receptor-mediated excitotoxicity have classically been implicated. More recently, evidence supports a role for oxidative stress. Evidence includes increased endothelial nitric oxide synthase, nitrotyrosine deposition, microglial activation, and lipid peroxidation. Reactive oxygen species production results in decreased expression of astrocytic glutamate transporters and decreased activities of alpha-KGDH, resulting in an amplification of cell death mechanisms in thiamine deficiency.

Effect of albumin dialysis on intracranial pressure increase in pigs with acute liver failure: a randomized study

BACKGROUND

Increased intracranial pressure (ICP) worsens the outcome of acute liver failure (ALF). This study investigates the underlying pathophysiological mechanisms and evaluates the therapeutic effect of albumin dialysis in ALF with use of the Molecular Adsorbents Recirculating System without hemofiltration/dialysis (modified, M-MARS).

METHODS

Pigs were randomized into three groups: sham, ALF, and ALF + M-MARS. ALF was induced by hepatic devascularization (time = 0). M-MARS began at time = 2 and ended with the experiment at time = 6. ICP, arterial ammonia, brain water, cerebral blood flow (CBF), and plasma inflammatory markers were measured.

RESULTS

ICP and arterial ammonia increased significantly over 6 hrs in the ALF group, in comparison with the sham group. M-MARS attenuated (did not normalize) the increased ICP in the ALF group, whereas arterial ammonia was unaltered by M-MARS. Brain water in the frontal cortex (grey matter) and in the subcortical white matter at 6 hrs was significantly higher in the ALF group than in the sham group. M-MARS prevented a rise in water content, but only in white matter. CBF and inflammatory mediators remained unchanged in all groups.

CONCLUSION

The initial development of cerebral edema and increased ICP occurs independently of CBF changes in this noninflammatory model of ALF. Factor(s) other than or in addition to hyperammonemia are important, however, and may be more amenable to alteration by albumin dialysis.

Keeping cool in acute liver failure: rationale for the use of mild hypothermia

Encephalopathy, brain edema and intracranial hypertension are neurological complications responsible for substantial morbidity/mortality in patients with acute liver failure (ALF), where, aside from liver transplantation, there is currently a paucity of effective therapies. Mirroring its cerebro-protective effects in other clinical conditions, the induction of mild hypothermia may provide a potential therapeutic approach to the management of ALF. A solid mechanistic rationale for the use of mild hypothermia is provided by clinical and experimental studies showing its beneficial effects in relation to many of the key factors that determine the development of brain edema and intracranial hypertension in ALF, namely the delivery of ammonia to the brain, the disturbances of brain organic osmolytes and brain extracellular amino acids, cerebro-vascular haemodynamics, brain glucose metabolism, inflammation, subclinical seizure activity and alterations of gene expression. Initial uncontrolled clinical studies of mild hypothermia in patients with ALF suggest that it is an effective, feasible and safe approach. Randomized controlled clinical trials are now needed to adequately assess its efficacy, safety, clinical impact on global outcomes and to provide the guidelines for its use in ALF.

Acute liver failure: a critical appraisal of available animal models

The availability of adequate experimental models of acute liver failure (ALF) is of prime importance to provide a better understanding of this condition and allow the development and testing of new therapeutic approaches for patients with ALF. However, the numerous etiologies and complications of ALF contribute to the complexity of this condition and render the development of an ideal experimental model of ALF more difficult than expected. Instead, a number of different models that may be used for the study of specific aspects of ALF have been developed. The most common approaches used to induce ALFin experimental animals are surgical procedures, toxic liver injury,or a combination of both. Despite the high prevalence of viral hepatitis worldwide, very few satisfactory viral models of ALF are available. Established and newly developed models of ALF are reviewed.

Role of endogenous benzodiazepine ligands and their GABA-A--associated receptors in hepatic encephalopathy

Benzodiazepine receptor ligands are suggested to play a role in the pathogenesis of hepatic encephalopathy (HE). Accumulation of these ligands in brain was suggested to explain in part the notion of"increased GABAergic tone," the rational for which arose initially from reports of a beneficial effect of the selective benzodiazepine antagonist flumazenil in HE patients. It was suggested on the basis of the effect of flumazenil in human HE that liver failure may result in alterations of the density and/or affinity of the benzodiazepine-associated GABA-A receptor site. Subsequent controlled-clinical trials showed that fumazenil had a transient beneficial effect in only a subpopulation of HE patients. In contrast to the antagonists, partial inverse agonists of the benzodiazepine receptor have unequivocal beneficial effects on behavioral and electro-physiological performance in all experimental models of HE studied so far. Benzodiazepine-associated GABA-A receptors have consistently been demonstrated to be unaltered in both human and experimental HE. Contrary to initial reports, the so-called "endogenous benzodiazepines" do not appear to be significantly related to the pathogenesis of HE. On the other hand, nonbenzodiazepine GABA-A receptor complex modulators, such as neuro-steroids, recently identified in brain in human and experimental HE, may provide a new mechanistic basis for this disorder and lead to novel treatments for human HE.

Differential effects of ammonia on the benzodiazepine modulatory site on the GABA-A receptor complex of human brain

Ammonia is a key factor in the pathogenesis of encephalopathies associated with liver failure. A direct effect of ammonia on GABAergic neurotransmission was proposed as a mechanism that may explain its neurotoxic effect on the basis of electrophysiological and biochemical studies performed in animal models of liver failure. In the present study, we investigated using a radiometric assay the effect of ammonia on the binding of GABA-A receptor ligands to membranes from normal human brains. Ammonium tartrate significantly decreased the maximal binding of [3H]flunitrazepam to well-washed frontal cortical membranes (366+/-63 fmol/mg protein in absence of ammonia versus 294.1+/-51 fmol/mg protein in presence of 2 mM ammonia; p<0.05). The efficacy of the effects of ammonia was within the millimolar range (IC50=4.8 mM). This effect was not seen in cerebellum or hippocampus. Ammonia exposure decreased the maximal binding of [3H]flumazenil (284.9+/-24.2 fmol/mg protein in absence of ammonia versus 146.4+/-15.6 fmol/mg protein in presence of 2 mM ammonia; p<0.01). This effect was seen with a greater potency (Imax=32.4%) and a lower IC50 (0.1 mM). Inhibition of [3H]flumazenil binding was significant in all brain regions. The apparent ammonia-induced decrease of [3H]flunitrazepam and [3H]flumazenil binding was due to a decrease in the binding affinities of these ligands for the benzodiazepine site. In contrast, ammonium tartrate exposure did not cause significant changes to the binding of [3H]muscimol in any brain region. These findings demonstrate that ammonia interacts negatively with components of the benzodiazepine-associated site at the GABA-A receptor complex in human brain in contrast to previous reports in the rat, and thus, does not support the notion that ammonia directly activates the GABA-A receptor complex resulting in increased GABAergic neurotransmission in human hepatic encephalopathy. These findings also suggest that positron emission tomography studies in cirrhotic patients using [11C]flumazenil may be underestimating GABA-A receptor sites depending upon the degree of hyperammonemia of the patient.