Activities of neuronal and astrocytic marker enzymes in autopsied brain tissue from patients with hepatic encephalopathy

Modulation of brain energy metabolism in hepatic encephalopathy: impact of glucose metabolic dysfunction

Cerebral function is linked to a high level of metabolic activity and relies on glucose as its primary energy source. Glucose aids in the maintenance of physiological brain activities; as a result, a disruption in metabolism has a significant impact on brain function, launching a chain of events that leads to neuronal death. This metabolic insufficiency has been observed in a variety of brain diseases and neuroexcitotoxicity disorders, including hepatic encephalopathy. It is a significant neurological complication that develops in people with liver disease, ranging from asymptomatic abnormalities to coma. Hyperammonemia is the main neurotoxic villain in the development of hepatic encephalopathy and induces a wide range of complications in the brain. The neurotoxic effects of ammonia on brain function are thought to be mediated by impaired glucose metabolism. Accordingly, in this review, we provide an understanding of deranged brain energy metabolism, emphasizing the role of glucose metabolic dysfunction in the pathogenesis of hepatic encephalopathy. We also highlighted the differential metabolic profiles of brain cells and the status of metabolic cooperation between them. The major metabolic pathways that have been explored are glycolysis, glycogen metabolism, lactate metabolism, the pentose phosphate pathway, and the Krebs cycle. Furthermore, the lack of efficacy in current hepatic encephalopathy treatment methods highlights the need to investigate potential therapeutic targets for hepatic encephalopathy, with regulating deficient bioenergetics being a viable alternative in this case. This review also demonstrates the importance of the development of glucose metabolism-focused disease diagnostics and treatments, which are now being pursued for many ailments.

Hepatic Encephalopathy: From Metabolic to Neurodegenerative

Hepatic encephalopathy (HE) is a neuropsychiatric syndrome of both acute and chronic liver disease. As a metabolic disorder, HE is considered to be reversible and therefore is expected to resolve following the replacement of the diseased liver with a healthy liver. However, persisting neurological complications are observed in up to 47% of transplanted patients. Several retrospective studies have shown that patients with a history of HE, particularly overt-HE, had persistent neurological complications even after liver transplantation (LT). These enduring neurological conditions significantly affect patient's quality of life and continue to add to the economic burden of chronic liver disease on health care systems. This review discusses the journey of the brain through the progression of liver disease, entering the invasive surgical procedure of LT and the conditions associated with the post-transplant period. In particular, it will discuss the vulnerability of the HE brain to peri-operative factors and post-LT conditions which may explain non-resolved neurological impairment following LT. In addition, the review will provide evidence; (i) supporting overt-HE impacts on neurological complications post-LT; (ii) that overt-HE leads to permanent neuronal injury and (iii) the pathophysiological role of ammonia toxicity on astrocyte and neuronal injury/damage. Together, these findings will provide new insights on the underlying mechanisms leading to neurological complications post-LT.

N-acetyl cysteine treatment preserves mitochondrial indices of functionality in the brain of hyperammonemic mice

AIM OF THE STUDY

Acute or chronic live failure could result in hyperammonemia and hepatic encephalopathy (HE). HE is a clinical complication characterized by severe cognitive dysfunction and coma. The ammonium ion (NH4 +) is the most suspected toxic molecule involved in the pathogenesis of HE. NH4 + is a neurotoxic agent. Different mechanisms, including oxidative/nitrosative stress, inflammatory response, excitotoxicity, and mitochondrial impairment, are proposed for NH4 +-induced neurotoxicity. N-acetyl cysteine (NAC) is a well-known thiol-reductant and antioxidant agent. Several investigations also mentioned the positive effects of NAC on mitochondrial function. In the current study, the effect of NAC treatment on brain mitochondrial indices and energy status was investigated in an animal model of HE.

MATERIAL AND METHODS

Acetaminophen (APAP)-induced acute liver failure was induced by a single dose of the drug (800 mg/kg, i.p.) to C57BL/6J mice. Plasma and brain levels of NH4 + were measured. Then, brain mitochondria were isolated, and several indices, including mitochondrial depolarization, ATP level, lipid peroxidation, glutathione content, mitochondrial permeabilization, and dehydrogenase activity, were assessed.

RESULTS

A significant increase in plasma and brain NH4 + was evident in APAP-treated animals. Moreover, mitochondrial indices of functionality were impaired, and mitochondrial oxidative stress biomarkers were significantly increased in APAP-treated mice. It was found that NAC treatment (100, 200, and 400 mg/kg, i.p.) significantly mitigated mitochondrial impairment in the brain of APAP-treated animals.

CONCLUSIONS

These data suggest the effects of NAC on brain mitochondrial function and energy status as a pivotal mechanism involved in its neuroprotective properties during HE.

Portacaval shunting causes differential mitochondrial superoxide production in brain regions

The portacaval shunting (PCS) prevents portal hypertension and recurrent bleeding of esophageal varices. On the other hand, it can induce chronic hyperammonemia and is considered to be the best model of mild hepatic encephalopathy (HE). Pathogenic mechanisms of HE and dysfunction of the brain in hyperammonemia are not fully elucidated, but it was originally suggested that the pathogenetic defect causes destruction of antioxidant defense which leads to an increase in the production of reactive oxygen species (ROS) and the occurrence of oxidative stress. In order to gain insight into the pathogenic mechanisms of HE in the brain tissue, we investigated the effects of PCS in rats on free radicals production and activity levels of antioxidant and prooxidant enzymes in mitochondria isolated from different brain areas. We found that O₂^·- production, activities of Mn-superoxide dismutase (Mn-SOD), glutathione peroxidase (GPx), glutathione reductase (GR), glutathione transferase (GT), nitric oxide synthase (NOS), and levels of carbonylated proteins differed between the four brain regions both in the amount and response to PCS. In PCS rats, Mn-SOD activity in the cerebellum was significantly decreased, and remained unchanged in the neocortex, hippocampus and striatum compared with that in sham-operated animals. Among the four brain regions in control rats, the levels of the carbonyl groups in mitochondrial proteins were maximal in the cerebellum. 4 weeks after PCS, the content of carbonylated proteins were higher only in mitochondria of this brain region. Under control conditions, O₂^·- production by submitochondrial particles in the cerebellum was significantly higher than in other brain regions, but was significantly increased in each brain region from PCS animals. Indeed, the production of O₂^·- by submitochondrial particles correlated with mitochondrial ammonia levels in the four brain regions of control and PCS-animals. These findings are the first to suggest that in vivo levels of ammonia in the brain directly affect the rate of mitochondrial O₂^·- production.

Hyperammonemia compromises glutamate metabolism and reduces BDNF in the rat hippocampus

Ammonia is putatively the major toxin associated with hepatic encephalopathy (HE), a neuropsychiatric manifestation that results in cognitive impairment, poor concentration and psychomotor alterations. The hippocampus, a brain region involved in cognitive impairment and depressive behavior, has been studied less than neocortical regions. Herein, we investigated hippocampal astrocyte parameters in a hyperammonemic model without hepatic lesion and in acute hippocampal slices exposed to ammonia. We also measured hippocampal BDNF, a neurotrophin commonly related to synaptic plasticity and cognitive deficit, and peripheral S100B protein, used as a marker for brain damage. Hyperammonemia directly impaired astrocyte function, inducing a decrease in glutamate uptake and in the activity of glutamine synthetase, in turn altering the glutamine-glutamate cycle, glutamatergic neurotransmission and ammonia detoxification itself. Hippocampal BDNF was reduced in hyperammonemic rats via a mechanism that may involve astrocyte production, since the same effect was observed in astrocyte cultures exposed to ammonia. Ammonia induced a significant increase in S100B secretion in cultured astrocytes; however, no significant changes were observed in the serum or in cerebrospinal fluid. Data demonstrating hippocampal vulnerability to ammonia toxicity, particularly due to reduced glutamate uptake activity and BDNF content, contribute to our understanding of the neuropsychiatric alterations in HE.

Therapeutic effects of methionine sulfoximine in multiple diseases include and extend beyond inhibition of glutamine synthetase

INTRODUCTION

Methionine sulfoximine (MSO), a well-characterized inhibitor of glutamine synthetase, displays significant therapeutic benefits in animal models for several human diseases. This amino acid might therefore be a viable candidate for drug development to treat diseases for which there are few effective therapies. Areas covered: We describe the effects of MSO on brain swelling occurring in overt hepatic encephalopathy resulting from liver failure, the effects of MSO on excitotoxic damage involved in amyotrophic lateral sclerosis (ALS) or resulting from stroke, and the effects of MSO on a model for an inflammatory immune response involved in a range of diseases. We conclude that these results imply the existence of another therapeutic target for MSO in addition to glutamine synthetase. Expert opinion: We summarize the various diseases for which MSO treatment might be a candidate for drug development. We discuss why MSO has limited enthusiasm in the scientific and medical communities for use in humans, with a rebuttal to those negative opinions. And we conclude that MSO should be considered a candidate drug to treat brain swelling involved in overt hepatic encephalopathy and diseases involving an inflammatory immune response.

Neurosteroids in hepatic encephalopathy: Novel insights and new therapeutic opportunities

Hepatic encephalopathy (HE) is a serious neuropsychiatric disorder resulting from liver failure. Symptoms of HE include mild cognitive impairment, stupor and coma. Morphological changes to neuroglia (both astrocytes and microglia) occur in HE consisting of cytotoxic brain edema (astrocyte swelling) in acute liver failure and Alzheimer type-2 astrocytosis in cirrhosis. Visual-evoked responses in animals with liver failure and HE manifest striking similarities to those in animals treated with agonists of the GABA-A receptor complex. Neurosteroids are synthesized in brain following activation of translocator protein (TSPO), a mitochondrial neuroglial cholesterol-transporter protein. TSPO sites are activated in both animal models of HE as well as in autopsied brain tissue from HE patients. Activation of TSPO sites results in increased cholesterol transport into the mitochondrion followed by stimulation of a metabolic pathway culminating in the synthesis of allopregnanolone (ALLO) and tetrahydrodeoxycorticosterone (THDOC), neurosteroids with potent positive allosteric modulatory action on the GABA-A receptor complex. Concentrations of ALLO and THDOC in brain tissue from mice with HE resulting from toxic liver injury are sufficient to induce sedation in animals of the same species and significant increases in concentrations of ALLO have been reported in autopsied brain tissue from cirrhotic patients with HE leading to the proposal that "increased GABAergic tone" in HE results from that increased brain concentrations of this neurosteroid. Agents with the potential to decrease neurosteroid synthesis and/or prevent their modulatory actions on the GABA-A receptor complex may provide novel approaches to the management and treatment of HE. Such agents include indomethacin, benzodiazepine receptor inverse agonists and a novel series of compounds known as GABA-A receptor-modulating steroid antagonists (GAMSA).

Glutamine Synthetase: Role in Neurological Disorders

Glutamine synthetase (GS) is an ATP-dependent enzyme found in most species that synthesizes glutamine from glutamate and ammonia. In brain, GS is exclusively located in astrocytes where it serves to maintain the glutamate-glutamine cycle, as well as nitrogen metabolism. Changes in the activity of GS, as well as its gene expression, along with excitotoxicity, have been identified in a number of neurological conditions. The literature describing alterations in the activation and gene expression of GS, as well as its involvement in different neurological disorders, however, is incomplete. This review summarizes changes in GS gene expression/activity and its potential contribution to the pathogenesis of several neurological disorders, including hepatic encephalopathy, ischemia, epilepsy, Alzheimer's disease, amyotrophic lateral sclerosis, traumatic brain injury, Parkinson's disease, and astroglial neoplasms. This review also explores the possibility of targeting GS in the therapy of these conditions.

Pathophysiology of brain dysfunction in hyperammonemic syndromes: The many faces of glutamine

Ineffective hepatic clearance of excess ammonia in the form of urea, as occurs in urea cycle enzymopathies (UCDs) and in liver failure, leads to increases in circulating and tissue concentrations of glutamine and a positive correlation between brain glutamine and the severity of neurological symptoms. Studies using 1H/13C Nuclear Magnetic Resonance (NMR) spectroscopy reveal increased de novo synthesis of glutamine in the brain in acute liver failure (ALF) but increases of synthesis rates per se do not correlate with either the severity of encephalopathy or brain edema. Skeletal muscle becomes primarily responsible for removal of excess ammonia in liver failure and in UCDs, an adaptation that results from a post-translational induction of the glutamine synthetase (GS) gene. The importance of muscle in ammonia removal in hyperammonemia accounts for the resurgence of interest in maintaining adequate dietary protein and the use of agents aimed at the stimulation of muscle GS. Alternative or additional metabolic and regulatory pathways that impact on brain glutamine homeostasis in hyperammonemia include (i) glutamine deamination by the two isoforms of glutaminase, (ii) glutamine transamination leading to the production of the putative neurotoxin alpha-ketoglutaramate and (iii) alterations of high affinity astrocytic glutamine transporters (SNATs). Findings of reduced expression of the glutamine transporter SNAT-5 (responsible for glutamine clearance from the astrocyte) in ALF raise the possibility of "glutamine trapping" within these cells. Such a trapping mechanism could contribute to cytotoxic brain edema and to the imbalance between excitatory and inhibitory neurotransmission in this disorder.

CNS proteomes in alcohol and drug abuse and dependence

Drugs of abuse, including alcohol, can induce dependency formation and/or brain damage in brain regions important for cognition. 'High-throughput' approaches, such as cDNA microarray and proteomics, allow the analysis of global expression profiles of genes and proteins. These technologies have recently been applied to human brain tissue from patients with psychiatric illnesses, including substance abuse/dependence and appropriate animal models to help understand the causes and secondary effects of these complex disorders. Although these types of studies have been limited in number and by proteomics techniques that are still in their infancy, several interesting hypotheses have been proposed. Focusing on CNS proteomics, we aim to review and update current knowledge in this rapidly advancing area.

Astrocytes and glutamate homoeostasis in Alzheimer's disease: a decrease in glutamine synthetase, but not in glutamate transporter-1, in the prefrontal cortex

Astrocytes control tissue equilibrium and hence define the homoeostasis and function of the CNS (central nervous system). Being principal homoeostatic cells, astroglia are fundamental for various forms of neuropathology, including AD (Alzheimer's disease). AD is a progressive neurodegenerative disorder characterized by the loss of cognitive functions due to specific lesions in mnesic-associated regions, including the mPFC (medial prefrontal cortex). Here, we analyzed the expression of GS (glutamine synthetase) and GLT-1 (glutamate transporter-1) in astrocytes in the mPFC during the progression of AD in a triple-transgenic mouse model (3xTg-AD). GS is an astrocyte-specific enzyme, responsible for the intracellular conversion of glutamate into glutamine, whereas the removal of glutamate from the extracellular space is accomplished mainly by astroglia-specific GLT-1. We found a significant decrease in the numerical density (Nv, cells/mm³) of GS-positive astrocytes from early to middle ages (1–9 months; at the age of 1 month by 17%, 6 months by 27% and 9 months by 27% when compared with control animals) in parallel with a reduced expression of GS (determined by Western blots), which started at the age of 6 months and was sustained up to 12 months of age. We did not, however, find any changes in the expression of GLT-1, which implies an intact glutamate uptake mechanism. Our results indicate that the decrease in GS expression may underlie a gradual decline in the vital astrocyte-dependent glutamate–glutamine conversion pathway, which in turn may compromise glutamate homoeostasis, leading towards failures in synaptic connectivity with deficient cognition and memory.

Neurotransmitter receptor alterations in hepatic encephalopathy: a review

Hepatic encephalopathy (HE), a complex neuropsychiatric syndrome with symptoms ranging from subtle neuropsychiatric and motor disturbances to deep coma and death, is thought to be a clinical manifestation of a low-grade cerebral oedema associated with an altered neuron-astrocyte crosstalk and exacerbated by hyperammonemia and oxidative stress. These events are tightly coupled with alterations in neurotransmission, either in a causal or a causative manner, resulting in a net increase of inhibitory neurotransmission. Therefore, research focussed mainly on the potential role of γ-aminobutyric acid-(GABA) or glutamate-mediated neurotransmission in the pathophysiology of HE, though roles for other neurotransmitters (e.g. serotonin, dopamine, adenosine and histamine) or for neurosteroids or endogenous benzodiazepines have also been suggested. Therefore, we here review HE-related alterations in neurotransmission, focussing on changes in the levels of classical neurotransmitters and the neuromodulator adenosine, variations in the activity and/or concentrations of key enzymes involved in their metabolism, as well as in the densities of their receptors.

Kynurenines: from the perspective of major psychiatric disorders

Psychiatric disorders are documented to be associated with a mild pro-inflammatory state. Pro-inflammatory mediators could activate the tryptophan breakdown and kynurenine pathway with a shift toward the neurotoxic arm where excitotoxic N-methyl-D-aspartate receptor agonist quinolinic acid is formed. An unbalanced metabolism in terms of neuroprotective and neurotoxic effects, such as reduced kynurenic acid to kynurenine ratio, has been demonstrated in the major psychiatric disorders such as unipolar depression, bipolar manic-depressive disorder and schizophrenia, and in drug-induced neuropsychiatric side effects such as interferon-α treated patients. The changes in serum or plasma are shown to be associated with central changes such as in the cerebrospinal fluid and certain brain areas. While currently available antidepressants and mood stabilizers could not efficiently improve these neurochemical changes within the same period that could induce clinical improvement, some antipsychotic treatments could reverse certain metabolic imbalances. Some of these changes were tested also in animal models. In this review the role of this unbalanced kynurenine metabolism through interactions with other neurochemicals is discussed as a major contributing pathophysiological mechanism in psychiatric disorders. Moreover, the biomarker role of kynurenine metabolites and future therapeutic opportunities are also discussed.

Brain energy metabolism and mitochondrial dysfunction in acute and chronic hepatic encephalopathy

One proposed mechanism for acute and chronic hepatic encephalopathy (HE) is a disturbance in cerebral energy metabolism. It also reviews the current status of this mechanism in both acute and chronic HE, as well as in other hyperammonemic disorders. It also reviews abnormalities in glycolysis, lactate metabolism, citric acid cycle, and oxidative phosphorylation as well as associated energy impairment. Additionally, the role of mitochondrial permeability transition (mPT), a recently established factor in the pathogenesis of HE and hyperammonemia, is emphasized. Energy failure appears to be an important pathogenetic component of both acute and chronic HE and a potential target for therapy.

Serum glutamine synthetase has no value as a diagnostic biomarker for Alzheimer's disease

In order to test whether serum glutamine synthetase (GS) is of potential diagnostic value for Alzheimer's disease (AD), we set up a study to compare serum GS concentrations between AD patients and control subjects. The study population (n = 165) consisted of AD patients (n = 94) and age-matched (n = 41) and age-unmatched (n = 30) control subjects. Serum GS analysis was performed by means of ELISA. No significant differences in serum GS levels were found between the AD group and age-matched controls. Age correlated positively with serum GS concentrations in AD patients and control subjects. This study suggests that serum GS levels have no diagnostic value for AD.

Mechanisms of and defense against acute ammonia toxicity in the aquatic Chinese soft-shelled turtle, Pelodiscus sinensis

The objective of this study was to elucidate the mechanisms of acute ammonia toxicity in the aquatic Chinese soft-shelled turtle, Pelodiscus sinensis, and to examine how this turtle defended against a sublethal dose of NH(4)Cl injected into its peritoneal cavity. The ammonia and glutamine contents in the brains of turtles that succumbed within 3h to an intraperitoneal injection with a lethal dose (12.5 micromolg(-1) turtle) of NH(4)Cl were 21 and 4.4 micromolg(-1), respectively. Since the brain glutamine content increased to 8 micromolg(-1) at hour 6 and recovered thereafter in turtles injected with a sub-lethal dose of NH(4)Cl (7.5 micromolg(-1) turtle), it can be concluded that increased glutamine synthesis and accumulation was not the major cause of acute ammonia toxicity in P. sinensis. Indeed, the administration of l-methionine S-sulfoximine (MSO; 82 microgg(-1) turtle), a glutamine synthetase (GS) inhibitor, prior to the injection of a lethal dose of NH(4)Cl had no significant effect on the mortality rate. Although the prior administration of MSO led to an extension of the time to death, it was apparently a result of its effects on glutamate dehydrogenase and glutamate formation, instead of glutamine synthesis and accumulation, in the brain. By contrast, a prior injection with MK801 (1.6 microgg(-1) turtle), a NMDA receptor antagonist, reduced the 24h mortality of turtles injected with a lethal dose of NH(4)Cl by 50%. Thus, acute ammonia toxicity in P. sinensis was probably a result of glutamate dysfunction and the activation of NMDA receptors. NMDA receptor activation could also be exacerbated through membrane depolarization caused by the extraordinarily high level of ammonia (21 micromolg(-1) brain) in the brain of turtles that succumbed to a lethal dose of NH(4)Cl. One hour after the injection with a sub-lethal dose of NH(4)Cl, the brain of P. sinensis exhibited an extraordinarily high tolerance of ammonia (16 micromolg(-1) brain). The transient nature of ammonia accumulation indicates that P. sinensis could ameliorate ammonia toxicity through the suppression of endogenous ammonia production and/or the excretion of exogenous ammonia. Despite being ureogenic and ureotelic, only a small fraction of the exogenous ammonia was detoxified to urea. A major portion of ammonia was excreted unchanged, resulting in an apparent ammonotely in the experimental turtles. Since there were increases in total essential free amino acid contents in the brain, liver and muscle, it can be deduced that a suppression of amino acid catabolism had occurred, reducing the production of endogenous ammonia and hence alleviating the possibility of ammonia intoxication.

Neuronal cell death in hepatic encephalopathy

It is generally assumed that neuronal cell death is minimal in liver failure and is insufficient to account for the neuropsychiatric symptoms characteristic of hepatic encephalopathy. However, contrary to this assumption, neuronal cell damage and death are well documented in liver failure patients, taking the form of several distinct clinical entities namely acquired (non-Wilsonian) hepatocerebral degeneration, cirrhosis-related Parkinsonism, post-shunt myelopathy and cerebellar degeneration. In addition, there is evidence to suggest that liver failure contributes to the severity of neuronal loss in Wernicke's encephalopathy. The long-standing nature of the thalamic and cerebellar lesions, over 80% of which are missed by routine clinical evaluation, together with the probability that they are nutritional in origin, underscores the need for careful nutritional management (adequate dietary protein, Vitamin B(1)) in liver failure patients. Mechanisms identified with the potential to cause neuronal cell death in liver failure include NMDA receptor-mediated excitotoxicity, lactic acidosis, oxidative/nitrosative stress and the presence of pro-inflammatory cytokines. The extent of neuronal damage in liver failure may be attenuated by compensatory mechanisms that include down-regulation of NMDA receptors, hypothermia and the presence of neuroprotective steroids such as allopregnanolone. These findings suggest that some of the purported "sequelae" of liver transplantation (gait ataxia, memory loss, confusion) could reflect preexisting neuropathology.

Differential protein expression profiles in the hippocampus of human alcoholics

Mild to severe cognitive impairments are frequently observed symptoms in chronic alcoholics. Decline of cognitive function significantly affects patients' recovery process and prognosis. The hippocampal region is sensitive to the effects of alcohol and it has been suggested that alcohol-induced hippocampal damage and/or changes in neuronal circuitry play an important role in generating these symptoms. Although various hypotheses have been proposed, molecular mechanisms underlying these alterations in the hippocampus are largely unknown. In the present study, we employed a 2DE-based proteomics approach to compare the protein expression profiles of the hippocampus in human alcoholic and healthy control brains. In the alcoholic hippocampus, 20 protein spots were found to be differentially regulated, 2 increased and 18 decreased. Seventeen proteins were identified using mass spectroscopy and were subcategorized into three energy metabolic proteins, six protein metabolic proteins, four signalling proteins, two oxidative stress-related proteins, one vesicle trafficking protein and one cytoskeletal protein. Some of these proteins have been previously implicated in alcohol-induced brain pathology. Based upon the results, several hypotheses were generated to explain the mechanisms underlying possible functional and/or structural alterations induced by chronic alcohol use in this brain region.

Spatio-temporal decomposition of the electroencephalogram in patients with cirrhosis

BACKGROUND/AIMS

Slowing of the electroencephalogram (EEG) is a recognised feature of hepatic encephalopathy but its diagnostic sensitivity is indeterminate. Recent advances in EEG analysis should provide better quantifiable/more informative data. The aim of this study was to isolate and determine the scalp distribution of the posterior basic rhythm, in patients with cirrhosis, using a technique for spatio-temporal decomposition (SEDACA) of the EEG.

METHODS

One hundred and ten patients with cirrhosis, classified, using clinical and psychometric criteria, as neuropsychiatrically unimpaired or as having minimal/overt hepatic encephalopathy were studied. Eyes-closed, awake EEGs were obtained and subjected to standard spectral analysis and spatio-temporal decomposition. Control data were obtained from 26 reference EEGs.

RESULTS

The error in the estimate of the SEDACA-derived mean dominant frequency was lower than for the standard EEG derivation (P<0.00001). The SEDACA-derived spectral estimates correlated better with neuropsychiatric status and allowed differentiation of the patients with minimal hepatic encephalopathy from the reference population. The SEDACA-derived spatial information showed an anteriorization of the posterior basic rhythm, which became more prominent as the degree of neuropsychiatric impairment increased (P=0.00052).

CONCLUSIONS

Analysis of the EEG utilising SEDACA provides significantly more diagnostic information on the neuropsychiatric status of patients with cirrhosis than obtained conventionally.

Pathophysiology of hepatic encephalopathy: a new look at GABA from the molecular standpoint

Hepatic encephalopathy (HE) is a neuropsychiatric disorder associated with either acute or chronic liver failure. More than two decades ago, the role of altered GABAergic neurotransmission was proposed following evidence of "increased GABAergic tone" in HE. Increased GABAergic tone was based on several observations: (i) Similarity of visual evoked response potential patterns between rabbits with galactosamine-induced fulminant hepatic failure and animals treated with various allosteric agonists of the GABA receptor complex (GRC). (ii) Spontaneous activities of isolated Purkinje neurons from rabbits with galactosamine-induced fulminant hepatic failure are more depressed by GRC modulator compounds compared to normal animals. (iii) Flumazenil, a high selective benzodiazepine antagonist at the GRC, ameliorates behavioral symptoms and EEG activity in some HE patients. Pathophysiological mechanisms put forward to explain increased GABAergic tone in HE include (1) increase in brain GABA content due to increased brain GABA uptake through altered permeability of the blood brain barrier, (2) alteration of the integrity of constituents of the GRC, and (3) increase of endogenous GRC modulators such as benzodiazepines (and more recently neurosteroids) with potent agonist properties at the GRC. Studies performed subsequently excluded alterations of either GABA content or GRC integrity in favor of increased brain concentrations of endogenous agonists. While the role of endogenous benzodiazepines remains controversial, the presence of neurosteroids with GABA agonist properties affords a plausible explanation for increased GABAergic tone in HE.

Role of manganese accumulation in increased brain glutamine of the cirrhotic rat

Cirrhosis promotes increases of both manganese and glutamine in brain. Manganese is a modulator and glutamine is the product of glutamine synthetase. This work studies the relationship between manganese and glutamine synthetase in a model of cirrhosis in the rat. We administered manganese (1 g/L) in the drinking water of sham-operated and bile-duct obstructed rats. We evaluated the manganese and glutamine accumulation and the glutamine synthetase activity in frontal cortex, striatum, and pallidum after 2, 4, and 6 weeks of biliary obstruction or sham surgery. Cirrhotic rats receiving manganese increased their brain content of metal about 400%-600% after 4 weeks of treatment (P < .05) and also remarkably accumulated glutamine through time in the three regions studied (P < .05 at week 6). Interestingly, bile-duct obstructed rats treated with manganese showed no effect on glutamine synthetase activity. Results from this study suggest that manganese induces increases of brain glutamine independently of its synthesis.

Glutamine synthetase in brain: effect of ammonia

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

Neurobiology of ammonia

Hyperammonemia resulting from inherited urea cycle enzyme deficiencies or liver failure results in severe central nervous system dysfunction including brain edema, convulsions and coma. Neuropathologic evaluation in these disorders reveals characteristic alterations of astrocyte morphology ranging from cell swelling (acute hyperammonemia) to Alzheimer Type II astrocytosis (chronic hyperammonemia). Having no effective urea cycle, brain relies on glutamine synthesis for the removal of excess ammonia and the enzyme responsible, glutamine synthetase, has a predominantly astrocytic localization. Accumulation of ammonia in brain results in a redistribution of cerebral blood flow and metabolism from cortical to sub-cortical structures. In addition to changes in astrocyte morphology, increased brain ammonia concentrations result in alterations in expression of key astrocyte proteins including glial fibrillary acidic protein, glutamate and glycine transporters and "peripheral-type" (mitochondrial) benzodiazepine receptors. Such changes result in alterations of astrocytic volume and increased extracellular concentrations of excitatory and inhibitory substances. In addition, the ammonium ion has direct effects on excitatory-inhibitory transmission via distinct mechanisms involving cellular chloride extrusion and postsynaptic receptor function. Acute ammonia exposure leads to activation of NMDA receptors and their signal transduction pathways. Chronic hyperammonemia also results in increased concentrations of neuroactive L-tryptophan metabolites including serotonin and quinolinic acid. Therapy in hyperammonemic syndromes continues to rely on ammonia-lowering strategies via peripheral mechanisms (reduction of ammonia production in the gastrointestinal tract, increased ammonia removal by muscle).

The astrocytic ("peripheral-type") benzodiazepine receptor: role in the pathogenesis of portal-systemic encephalopathy

An increasing body of evidence supports the notion that activation of astrocytic (peripheral-type) benzodiazepine receptors contributes to the pathogenesis of the central nervous system symptoms which are characteristic of portal-systemic encephalopathy (PSE). Binding site densities for the PTBR ligand [3H-PK11195] are increased in autopsied brain tissue from PSE patients as well as in the brains of animals with experimental chronic liver failure. In the case of the animal studies, increased PTBR sites resulted from increased PTBR gene expression. Exposure of cultured astrocytes to ammonia or manganese (two neurotoxic agents which under normal circumstances are removed by the hepatobiliary system and which are found to accumulate in brain in PSE) results in increased densities of [3H-PK11195] binding sites. Activation of PTBR is known to result in increased cholesterol uptake and increased synthesis in brain of neurosteroids some of which have potent positive allosteric modulator properties on the GABA-A receptor system. Accumulation of such substances in the brain in chronic liver failure could explain the neural inhibition characteristics of PSE.

Effect of portacaval anastomosis on glutamine synthetase protein and gene expression in brain, liver and skeletal muscle

The effects of chronic liver insufficiency resulting from end-to-side portacaval anastomosis (PCA) on glutamine synthetase (GS) activities, protein and gene expression were studied in brain, liver and skeletal muscle of male adult rats. Four weeks following PCA, activities of GS in cerebral cortex and cerebellum were reduced by 32% and 37% (p<0.05) respectively whereas GS activities in muscle were increased by 52% (p<0.05). GS activities in liver were decreased by up to 90% (p<0.01), a finding which undoubtedly reflects the loss of GS-rich perivenous hepatocytes following portal-systemic shunting. Immunoblotting techniques revealed no change in GS protein content of brain regions or muscle but a significant loss in liver of PCA rats. GS mRNA determined by semi-quantitative RT-PCR was also significantly decreased in the livers of PCA rats compared to sham-operated controls. These findings demonstrate that PCA results in a loss of GS gene expression in the liver and that brain does not show a compensatory induction of enzyme activity, rendering it particularly sensitive to increases in ammonia in chronic liver failure. The finding of a post-translational increase of GS in muscle following portacaval shunting suggests that, in chronic liver failure, muscle becomes the major organ responsible for the removal of excess blood-borne ammonia.

Alterations of neurotransmitter-related gene expression in human and experimental portal-systemic encephalopathy

Portal-systemic encephalopathy (PSE) is a serious neuropsychiatric condition that results from chronic liver failure and portal-systemic shunting of venous blood. PSE is particularly prevalent following treatment of portal hypertension or ascites by the TIPS procedure. Recent studies both in autopsied brain tissue from PSE patients as well as in experimental animal models of PSE reveal that chronic liver failure results in altered expression of several genes coding for proteins having key roles in the control of neuronal excitability. Such alterations include increased expression of monoamine oxidase (MAO-A isoform), the "peripheral-type" benzodiazepine receptor (PTBR) as well as constitutive, neuronal nitric oxide synthase (nNOS). Such changes result in altered protein expression and in increased degradation of monoamine neurotransmitters, increased synthesis of neurosteroids with inhibitory properties and increased production of nitric oxide (respectively) in brain in chronic liver failure. In the case of PTBR and nNOS, increases in expression result from exposure to ammonia and/or manganese, two neurotoxic agents shown previously to be increased in brain in chronic liver failure. Further elucidation of the consequences of neurotransmitter-related gene expression could identify new pathophysiologic mechanisms and result in new approaches to the prevention of PSE in chronic liver disease in humans.

Long-term changes in glial fibrillary acidic protein and glutamine synthetase immunoreactivities in the supraoptic nucleus of portacaval shunted rats

The present study was undertaken to ascertain whether, and to what extent, glial fibrillary acidic protein (GFAP) and glutamine synthetase (GS) expressions in the supraoptic nucleus (SON) could be modulated after one month and six months of portacaval shunting (PCS) in rats. GFAP and GS immunoreactivities were significantly higher in PCS rats than in control rats at one and six months. The increased GFAP and GS immunoreactivities observed in the SON astrocytes were directly related to the duration of PCS. In PCS rats, the number and length of both GFAP and GS immunopositive astroglial processes increased not only in the hypothalamic nucleus but in the perinuclear zone, where glutamatergic pathways have been described, whereas GFAP and GS expressions decreased in the ventral glial lamina. Since GS is one of the glutamate metabolizing enzymes and the SON is one of the areas of glutamatergic activity, our results show that astrocytes respond differentially to glutamate toxicity. This suggests that overexpression of GFAP and GS immunoreactivities could be associated with glutamatergic neurotransmission disorders.

Neuroactive amino acids in hepatic encephalopathy

There is abundant evidence to suggest that alterations of excitatory and inhibitory amino acids play a significant role in the pathogenesis of hepatic encephalopathy (HE) in both acute and chronic liver diseases. Brain glutamate concentrations are reduced in patients who died in hepatic coma as well as in experimental HE, astrocytic reuptake of glutamate is compromised in liver failure and postsynaptic glutamate receptors (both NMDA and non-NMDA subclasses) are concomitantly reduced in density. Recent studies in experimental acute liver failure suggest reduced capacity of the astrocytic glutamate transporter in this condition. Together, this data suggests that neuron-astrocytic trafficking of glutamate is impared in HE. Other significant alterations of neuroactive amino acids in HE include a loss of taurine from brain cells to extracellular space, a phenomenon which could relate both to HE and to brain edema in acute liver failure. Increased concentrations of benzodiazepine-like compounds have been reported in human and experimental HE. Clinical trials with the benzodiazepine antagonist flumazenil reveal a beneficial effect in some patients with HE; the mechanism responsible for this effect, however, remains to be determined.

Thiamine phosphatases in human brain: regional alterations in patients with alcoholic cirrhosis

Activities of thiamine monophosphatase (TMPase) and thiamine diphosphatase (TDPase) were measured in homogenates of brain tissue obtained at autopsy from eight alcoholic cirrhotic patients who died in hepatic coma and nine controls matched for age and for postmortem delay interval and free from neurological or psychiatric disorders, hepatic disease, or other conditions of grossly impaired nutritional status. Enzyme activities were measured by standard spectrophotometric techniques. Both TMPase and TDPase were distributed unevenly in brain with highest activities being recorded in temporal cortex. Regional correlations between TMPase and TDPase, however, were poor. TDPase activities in brain tissue from alcoholic cirrhotic patients were significantly increased in 5 of 6 brain regions, by 26 to 153% (p < 0.05). TMPase activities in alcoholic cirrhotics, on the other hand, were unchanged in all brain regions, with the exception of caudate nucleus where they were increased by 70% (p < 0.05). These findings add to the substantial body of evidence suggesting that alcoholic liver disease is associated with abnormal thiamine status and with altered thiamine neurochemistry. Increased TDP degradation resulting from increased activities of TDPase could contribute to the pathophysiological mechanisms involved in alcohol-related brain dysfunction.

Pathophysiology of alcoholic brain damage: synergistic effects of ethanol, thiamine deficiency and alcoholic liver disease

Chronic alcoholism results in brain damage and dysfunction leading to a constellation of neuropsychiatric symptoms including cognitive dysfunction, the Wernicke-Korsakoff Syndrome, alcoholic cerebellar degeneration and alcoholic dementia. That these clinically-defined entities result from independent pathophysiologic mechanisms is unlikely. Alcohol and its metabolite acetaldehyde are directly neurotoxic. Alcoholics are thiamine deficient as a result of poor diet, gastrointestinal disorders and liver disease. In addition, both alcohol and acetaldehyde have direct toxic effects on thiamine-related enzymes in liver and brain. Alcoholics frequently develop severe liver disease and liver disease per se results in altered thiamine homeostasis, in cognitive dysfunction and in neuropathologic damage to astrocytes. The latter may result in the loss of neuron-astrocytic trafficking of neuroactive amino acids and thiamine esters, essential to CNS function. The present review article proposes mechanisms whereby the effects of alcohol, thiamine deficiency and liver disease combine synergistically to contribute to the phenomena of cognitive dysfunction and "alcoholic brain damage".

Alterations of [3H]8-OH-DPAT and [3H]ketanserin binding sites in autopsied brain tissue from cirrhotic patients with hepatic encephalopathy

Affinities and densities of binding sites for the 5HT1A receptor ligand [3H]8-hydroxy(di-n-propylamino)tetralin ([3H]8-OH-DPAT) and the 5HT2 receptor ligand [3H]ketanserin were measured using a rapid filtration assay in crude membrane preparations from frontal cortex and hippocampus of nine cirrhotic patients who died in hepatic encephalopathy and from an equal number of age-matched subjects free from hepatic, neurological or psychiatric disorders. Binding site densities (Bmax) obtained by Scatchard analysis of saturation binding isotherms for [3H]8-OH-DPAT were decreased in frontal cortex (by 56%, P < 0.05) and hippocampus (by 30%, P < 0.05). [3H]ketanserin binding sites were concomitantly increased (by 55%, P < 0.05) in hippocampus of cirrhotic patients. Ligand binding affinities were within normal ranges in all cases. Previous reports have described the accumulation of the 5HT metabolite 5-hydroxyindoleacetic acid and increased activities of the 5HT-metabolizing enzyme MAOA in this same material from patients with hepatic encephalopathy. Taken together, these findings suggest that alterations of serotoninergic function in brain could be responsible for some of the neuropsychiatric symptoms of hepatic encephalopathy observed in humans with chronic liver disease.

Cerebellar glutamate metabolizing enzymes in spinocerebellar ataxia type I

We measured the levels of three glutamate metabolizing enzymes, namely, glutamate dehydrogenase (GDH), aspartate aminotransferase (AAT), and glutamine synthetase (GS) in cerebellar and occipital cortices of nine patients with dominantly-inherited olivopontocerebellar atrophy (OPCA; spinocerebellar ataxia type I). As compared with the controls, mean GDH activities in cerebellar cortex of the OPCA patients were normal whereas levels of AAT (-17%) and the glial enzyme GS (-27%) were significantly reduced. No statistically significant changes were observed in occipital cortex, a morphologically unaffected brain area. We suggest that the decreased GS levels could reflect impaired capacity of astrocytes to metabolize glutamate which might contribute to the degenerative processes in OPCA cerebellum.

Region-selective reductions in activities of glutamine synthetase in rat brain following portacaval anastomosis

Portacaval anastomosis in the rat results in liver atrophy, sustained hyperammonemia and mild encephalopathy. Previous studies have demonstrated region-selective alterations of glutamine and other ammonia-related amino acids in brain following portacaval anastomosis. Ammonia removal by brain relies on glutamine synthesis and the enzyme responsible, glutamine synthetase, has an almost exclusively astrocytic localization. Glutamine synthetase activities were measured using a radioenzymatic assay in homogenates of seven brain regions of rats four weeks after end-to-side portacaval anastomosis. Enzyme activities were significantly reduced in hippocampus (by 25%, p < 0.01), in cerebellum (by 29%, p < 0.01) and in cerebral cortex (by 14%, p < 0.05). Enzyme activities in other brain regions were within normal limits. Region-selective reductions of glutamine synthetase following portacaval anastomosis could result in disruption of neuron-glial metabolic interactions and in a deficit of glutamatergic synaptic regulation. Similar mechanisms could be implicated in the pathogenesis of hepatic encephalopathy accompanying chronic liver disease in humans.

Astrocytes and the entry of circulating ammonia into the brain: effect of fluoroacetate

Chronic hyperammonemia is known to lead to pathological forms of astrocytes. To test the influence of these changes on the neurotoxicity of ammonia, the glial metabolic poison fluoroacetate (FA) was applied locally, through microdialysis to the hippocampal dentate gyrus. The penetration of ammonia into the brain following the i.p. injection of 7.8 mmol/kg NH4 acetate was evaluated by measuring the ammonia and glutamine content of the microdialysate. Field EPSPs (fEPSPs) evoked by perforant path stimulation were recorded 1.5 mm from the microdialysis probe. When 20 mM FA was perfused, NH4 acetate injection increased the ammonia efflux by 300% and decreased fEPSPs by 40%, but glutamine concentration remained low. With no FA in the microdialysate, NH4 acetate treatment increased the efflux of ammonia by only 60%, did not affect fEPSPs but doubled glutamine efflux. Arterial ammonia content, as measured by microdialysis in the common carotid, increased 4-5 fold following i.p. administration of NH4 acetate, while arterial glutamine was not elevated. Systemically administered FA did not affect either of these changes significantly, but slightly reduced arterial pH. These observations indicate that FA applied by microdialysis acted locally on astrocytes and therefore impaired astrocytic function contributes to the development of hepatic encephalopathy by facilitating the entry of ammonia into the brain. Inhibition of excitatory synaptic transmission by elevated brain ammonia may underlay CNS depression in hepatic encephalopathy.

Increased activities of MAOA and MAOB in autopsied brain tissue from cirrhotic patients with hepatic encephalopathy

Using radioenzymatic assays, activities of MAOA and MAOB were measured in autopsied brain tissue from cirrhotic patients who died in hepatic coma and in material from an equal number of age-matched subjects who were free from hepatic, neurological or psychiatric disorders. Activities of both MAOA and MAOB were significantly increased in frontal cortex and caudate nucleus, two brain regions shown previously to be the site of functional and morphological alterations of astrocytes and increased concentrations of the acid metabolites of dopamine and serotonin. These findings suggest that increased monoamine metabolism and subsequent modifications of monoaminergic synaptic function could contribute to the pathogenesis of hepatic encephalopathy.

Amino acid release from cerebral cortex in experimental acute liver failure, studied by in vivo cerebral cortex microdialysis

Both increased gamma-aminobutyric acid (GABA)-ergic and decreased glutamatergic neurotransmission have been suggested relative to the pathophysiology of hepatic encephalopathy. This proposed disturbance in neurotransmitter balance, however, is based mainly on brain tissue analysis. Because the approach of whole tissue analysis is of limited value with regard to in vivo neurotransmission, we have studied the extracellular concentrations in the cerebral cortex of several neuroactive amino acids by application of the in vivo microdialysis technique. During acute hepatic encephalopathy induced in rats by complete liver ischemia, increased extracellular concentrations of the neuroactive amino acids glutamate, taurine, and glycine were observed, whereas extracellular concentrations of aspartate and GABA were unaltered and glutamine decreased. It is therefore suggested that hepatic encephalopathy is associated with glycine potentiated glutamate neurotoxicity rather than with a shortage of the neurotransmitter glutamate. In addition, increased extracellular concentration of taurine might contribute to the disturbed neurotransmitter balance. The observation of decreasing glutamine concentrations, after an initial increase, points to a possible astrocytic dysfunction involved in the pathophysiology of hepatic encephalopathy.

Effect of portacaval anastomosis on glutamine synthetase activities in liver, brain, and skeletal muscle

Glutamine synthetase is responsible for the ATP-dependent amidation of glutamate to glutamine. In liver the enzyme is highly localized in perivenous hepatocytes; in brain the enzyme is localized in astrocytes. Portacaval anastomosis resulted in liver atrophy, hyperammonemia, and up to 90% loss of glutamine synthetase activity in liver homogenates. This effect, which appears to be irreversible, probably reflects the selective loss of perivenous hepatocytes following portacaval anastomosis. Glutamine synthetase activities in brain were unaffected by portacaval anastomosis of up to 12 weeks' duration. Enzyme activities in homogenates of skeletal muscle, on the other hand, were significantly increased at one and four weeks after shunt surgery. These effects were not the result of decreased food intake in shunted animals. These findings suggest fundamentally different regulatory mechanisms for glutamine synthetase in these tissues. Skeletal muscle may thus provide an important alternative site for ammonia detoxification after portal-systemic shunting.

Effect of portacaval anastomosis on electrically stimulated release of glutamate from rat hippocampal slices

To evaluate the effects of chronic liver failure on release of the excitatory transmitter glutamate, electrically stimulated Ca2(+)-dependent and Ca2(+)-independent release of glutamate in the absence or presence of NH4+ was studied in superfused slices of hippocampus from portacaval-shunted or sham-operated rats 4 weeks after surgery. Spontaneous and stimulation-evoked release of glutamate was higher in shunted rats in the presence of normal or low Ca2+ concentrations, and this release was depressed by 5 mM ammonium chloride. These findings suggest that portacaval shunting results in increased levels of extracellular glutamate in brain, probably due to a decreased reuptake of glutamate into perineuronal astrocytes, shown in previous studies to undergo neuropathological changes following portacaval shunting. Changes in the inactivation of transmitter glutamate could be responsible, at least in part, for the neurological dysfunction resulting from sustained hyperammonemia and portal-systemic shunting resulting from chronic liver failure.

Thiamine-dependent enzyme changes in temporal cortex of patients with Alzheimer's disease

Activities of thiamine-dependent enzymes [pyruvate dehydrogenase (PDHC), alpha-ketoglutarate dehydrogenase (alpha KGDH), and transketolase (TK)] were measured in autopsied samples of temporal cortex from six patients with Alzheimer's disease and from eight age-matched control subjects who were free from neurological or psychiatric diseases. Times from death to freezing of dissected material at -70 degrees C were matched. Significant decreases in PDHC (decreased by 70%; P less than 0.01), alpha KGDH (decreased by 70%; p less than 0.01), and TK (decreased by 52%; P less than 0.01) were observed in brain tissue from patients with Alzheimer's disease. In contrast, activities of glutamate dehydrogenase were within normal limits. These findings suggest a possible role for alterations of brain thiamine metabolism or utilization in Alzheimer's disease.

Selective loss of N-methyl-D-aspartate-sensitive L-[3H]glutamate binding sites in rat brain following portacaval anastomosis

Excitatory amino acids have been implicated in the pathogenesis of hepatic encephalopathy. In the present study, kainate, quisqualate and N-methyl-D-aspartate (NMDA) subclasses of L-glutamate receptors were measured in adult rat brain by quantitative receptor autoradiography following surgical construction of an end-to-side portacaval anastomosis (PCA). PCA resulted in sustained hyperammonemia and decreased binding of L-glutamate to the NMDA receptor when compared to sham-operated controls. Decreases in binding ranged from 17 to 39% in several regions of cerebral cortex, hippocampus, striatum, and thalamus. Binding to quisqualate and kainate receptor subtypes was not altered. PCA leads to astrocytic changes in brain but does not result in any measurable loss of neuronal integrity. It is therefore proposed that decreased glutamate binding to the NMDA receptor following PCA results from increased extracellular glutamate caused by decreased reuptake into perineuronal astrocytes and a compensatory down-regulation of these receptors. Such changes could be of pathophysiological significance in hepatic encephalopathy.

Increased densities of peripheral-type benzodiazepine receptors in brain autopsy samples from cirrhotic patients with hepatic encephalopathy

Peripheral-type benzodiazepine receptors were evaluated using the specific ligand [3H]-PK 11195 in brain homogenates from nine cirrhotic patients who died in hepatic coma and from an equal number of age-matched control subjects. Histopathological studies showed evidence of severe Alzheimer type II astrocytosis in the brains of all cirrhotic patients. Saturation-binding assays revealed a single saturable binding site for [3H]-PK 11195 in brain, with affinities in the 2- to 3-nmol/L range. Diazepam was found to be a relatively potent inhibitor of 3H-PK 11195 binding (IC50 = 253 nmol/L), whereas the central benzodiazepine antagonist Ro 15-1788 displaced 3H-PK 11195 binding with low affinity (IC50 greater than 40 mumols/L). Densities of [3H]-PK 11195 binding sites were found to be increased by 48% (p less than 0.01) and 25% (p less than 0.05) in frontal cortex and caudate nuclei, respectively, from cirrhotic patients. Densities of [3H]-PK 11195 binding sites in frontal cortex from two nonencephalopathic cirrhotic patients were not significantly different from control values. No concomitant changes of affinities of these binding sites were observed. Because it has been suggested that peripheral-type benzodiazepine receptors may be localized on mitochondrial membranes and may therefore be involved in cerebral oxidative metabolism, the alterations observed in this study could be of pathophysiological significance in hepatic encephalopathy.

Affinities and densities of high-affinity [3H]muscimol (GABA-A) binding sites and of central benzodiazepine receptors are unchanged in autopsied brain tissue from cirrhotic patients with hepatic encephalopathy

The integrity of GABA-A receptors and of central benzodiazepine receptors was evaluated in membrane preparations from prefrontal cortex and caudate nuclei obtained at autopsy from nine cirrhotic patients who died in hepatic coma and an equal number of age-matched control subjects. Histopathological studies revealed Alzheimer Type II astrocytosis in all cases in the cirrhotic group; controls were free from neurological, psychiatric or hepatic diseases. Binding to GABA-A receptors was studied using [3H]muscimol as radioligand. The integrity of central benzodiazepine receptors was evaluated using [3H]flunitrazepam and [3H]Ro15-1788. Data from saturation binding assays was analyzed by Scatchard plot. No modifications of either affinities (Kd) or densities (Bmax) of [3H]muscimol of central benzodiazepine binding sites were observed. These findings do not support recent suggestions that alterations of either high-affinity GABA or benzodiazepine receptors play a significant role in the pathogenesis of hepatic encephalopathy.