Increased brain lactate is central to the development of brain edema in rats with chronic liver disease

Covert hepatic encephalopathy: elevated total glutathione and absence of brain water content changes

Recent pathophysiological models suggest that oxidative stress and hyperammonemia lead to a mild brain oedema in hepatic encephalopathy (HE). Glutathione (GSx) is a major cellular antioxidant and known to be involved in the interception of both. The aim of this work was to study total glutathione levels in covert HE (minimal HE and HE grade 1) and to investigate their relationship with local brain water content, levels of glutamine (Gln), myo-inositol (mI), neurotransmitter levels, critical flicker frequency (CFF), and blood ammonia. Proton magnetic resonance spectroscopy ((1)H MRS) data were analysed from visual and sensorimotor cortices of thirty patients with covert HE and 16 age-matched healthy controls. Total glutathione levels (GSx/Cr) were quantified with respect to creatine. Furthermore, quantitative MRI brain water content measures were evaluated. Data were tested for links with the CFF and blood ammonia. GSx/Cr was elevated in the visual (mHE) and sensorimotor (mHE, HE 1) MRS volumes and correlated with blood ammonia levels (both P < 0.001). It was further linked to Gln/Cr and mI/Cr (P < 0.01 in visual, P < 0.001 in sensorimotor) and to GABA/Cr (P < 0.01 in visual). Visual GSx/Cr correlated with brain water content in the thalamus, nucleus caudatus, and visual cortex (P < 0.01). Brain water measures did neither show group effects nor correlations with CFF or blood ammonia. Elevated total glutathione levels in covert HE (< HE 2) correlate with blood ammonia and may be a regional-specific reaction to hyperammonemia and oxidative stress. Brain water content is locally linked to visual glutathione levels, but appears not to be associated with changes of clinical parameters. This might suggest that cerebral oedema is only marginally responsible for the symptoms of covert HE.

Impaired Gut-Liver-Brain Axis in Patients with Cirrhosis

Cirrhosis is associated with brain dysfunction known as hepatic encephalopathy (HE). The mechanisms behind HE are unclear although hyperammonemia and systemic inflammation through gut dysbiosis have been proposed. We aimed to define the individual contribution of specific gut bacterial taxa towards astrocytic and neuronal changes in brain function using multi-modal MRI in patients with cirrhosis. 187 subjects (40 controls, 147 cirrhotic; 87 with HE) underwent systemic inflammatory assessment, cognitive testing, stool microbiota analysis and brain MRI analysis. MR spectroscopy (MRS) changes of increased Glutamate/glutamine, reduced myo-inositol and choline are hyperammonemia-associated astrocytic changes, while diffusion tensor imaging (DTI) demonstrates changes in neuronal integrity and edema. Linkages between cognition, MRI parameters and gut microbiota were compared between groups. We found that HE patients had a significantly worse cognitive performance, systemic inflammation, dysbiosis and hyperammonemia compared to controls and cirrhotics without HE. Specific microbial families (autochthonous taxa negatively and Enterobacteriaceae positively) correlated with MR spectroscopy and hyperammonemia-associated astrocytic changes. On the other hand Porphyromonadaceae, were only correlated with neuronal changes on DTI without linkages with ammonia. We conclude that specific gut microbial taxa are related to neuronal and astrocytic consequences of cirrhosis-associated brain dysfunction.

Central Role of Glutamate Metabolism in the Maintenance of Nitrogen Homeostasis in Normal and Hyperammonemic Brain

Glutamate is present in the brain at an average concentration—typically 10–12 mM—far in excess of those of other amino acids. In glutamate-containing vesicles in the brain, the concentration of glutamate may even exceed 100 mM. Yet because glutamate is a major excitatory neurotransmitter, the concentration of this amino acid in the cerebral extracellular fluid must be kept low—typically µM. The remarkable gradient of glutamate in the different cerebral compartments: vesicles > cytosol/mitochondria > extracellular fluid attests to the extraordinary effectiveness of glutamate transporters and the strict control of enzymes of glutamate catabolism and synthesis in well-defined cellular and subcellular compartments in the brain. A major route for glutamate and ammonia removal is via the glutamine synthetase (glutamate ammonia ligase) reaction. Glutamate is also removed by conversion to the inhibitory neurotransmitter γ-aminobutyrate (GABA) via the action of glutamate decarboxylase. On the other hand, cerebral glutamate levels are maintained by the action of glutaminase and by various α-ketoglutarate-linked aminotransferases (especially aspartate aminotransferase and the mitochondrial and cytosolic forms of the branched-chain aminotransferases). Although the glutamate dehydrogenase reaction is freely reversible, owing to rapid removal of ammonia as glutamine amide, the direction of the glutamate dehydrogenase reaction in the brain in vivo is mainly toward glutamate catabolism rather than toward the net synthesis of glutamate, even under hyperammonemia conditions. During hyperammonemia, there is a large increase in cerebral glutamine content, but only small changes in the levels of glutamate and α-ketoglutarate. Thus, the channeling of glutamate toward glutamine during hyperammonemia results in the net synthesis of 5-carbon units. This increase in 5-carbon units is accomplished in part by the ammonia-induced stimulation of the anaplerotic enzyme pyruvate carboxylase. Here, we suggest that glutamate may constitute a buffer or bulwark against changes in cerebral amine and ammonia nitrogen. Although the glutamate transporters are briefly discussed, the major emphasis of the present review is on the enzymology contributing to the maintenance of glutamate levels under normal and hyperammonemic conditions. Emphasis will also be placed on the central role of glutamate in the glutamine-glutamate and glutamine-GABA neurotransmitter cycles between neurons and astrocytes. Finally, we provide a brief and selective discussion of neuropathology associated with altered cerebral glutamate levels.

To Break or to Brake Neuronal Network Accelerated by Ammonium Ions?

Purpose

The aim of present study was to investigate the effects of ammonium ions on in vitro neuronal network activity and to search alternative methods of acute ammonia neurotoxicity prevention.

Methods

Rat hippocampal neuronal and astrocytes co-cultures in vitro, fluorescent microscopy and perforated patch clamp were used to monitor the changes in intracellular Ca²⁺- and membrane potential produced by ammonium ions and various modulators in the cells implicated in neural networks.

Results

Low concentrations of NH₄Cl (0.1–4 mM) produce short temporal effects on network activity. Application of 5–8 mM NH₄Cl: invariably transforms diverse network firing regimen to identical burst patterns, characterized by substantial neuronal membrane depolarization at plateau phase of potential and high-amplitude Ca²⁺-oscillations; raises frequency and average for period of oscillations Ca²⁺-level in all cells implicated in network; results in the appearance of group of «run out» cells with high intracellular Ca²⁺ and steadily diminished amplitudes of oscillations; increases astrocyte Ca²⁺-signalling, characterized by the appearance of groups of cells with increased intracellular Ca²⁺-level and/or chaotic Ca²⁺-oscillations. Accelerated network activity may be suppressed by the blockade of NMDA or AMPA/kainate-receptors or by overactivation of AMPA/kainite-receptors. Ammonia still activate neuronal firing in the presence of GABA(A) receptors antagonist bicuculline, indicating that «disinhibition phenomenon» is not implicated in the mechanisms of networks acceleration. Network activity may also be slowed down by glycine, agonists of metabotropic inhibitory receptors, betaine, L-carnitine, L-arginine, etc.

Conclusions

Obtained results demonstrate that ammonium ions accelerate neuronal networks firing, implicating ionotropic glutamate receptors, having preserved the activities of group of inhibitory ionotropic and metabotropic receptors. This may mean, that ammonia neurotoxicity might be prevented by the activation of various inhibitory receptors (i.e. by the reinforcement of negative feedback control), instead of application of various enzyme inhibitors and receptor antagonists (breaking of neural, metabolic and signaling systems).

The brain in acute on chronic liver failure

Acute-on-chronic liver failure (ACLF) is a newly defined clinical entity with significant morbidity and mortality (~40-90% at 1 year dependent on need for organ support at presentation). It defines a presentation with acute severe liver injury, often with multiorgan dysfunction, on a background of previously known or unknown cirrhosis. In its severest form, it is almost indistinguishable from acute liver failure, as similarly in around 5% may rapidly progress to intracranial hypertension and cerebral oedema culminating in coma and/or death. Our understanding of such cerebral sequelae is currently limited to clinical observation, though our knowledge base is rapidly expanding since recent consensus clinical definition and guidance. Moreover, there are now animal models of ACLF and imaging modalities to better characterize events in the brain that occur with ACLF. However, as yet there has been little in the way of interventional study of this condition which are much needed. In this review we dissect existing clinical and experimental data to better characterise the manifestations of ACLF on the brain and allow for the development of targeted therapy as currently the plethora of existing interventions were designed to treat either the effects of cirrhosis or acute liver injury independently.

Elevated cerebral lactate: Implications in the pathogenesis of hepatic encephalopathy

Hepatic encephalopathy (HE), a complex neuropsychiatric syndrome, is a frequent complication of liver failure/disease. Increased concentrations of lactate are commonly observed in HE patients, in the systemic circulation, but also in the brain. Traditionally, increased cerebral lactate is considered a marker of energy failure/impairment however alterations in lactate homeostasis may also lead to a rise in brain lactate and result in neuronal dysfunction. The latter may involve the development of brain edema. This review will target the significance of increased cerebral lactate in the pathogenesis of HE.

Ammonia metabolism and hyperammonemic disorders

Human adults produce around 1000 mmol of ammonia daily. Some is reutilized in biosynthesis. The remainder is waste and neurotoxic. Eventually most is excreted in urine as urea, together with ammonia used as a buffer. In extrahepatic tissues, ammonia is incorporated into nontoxic glutamine and released into blood. Large amounts are metabolized by the kidneys and small intestine. In the intestine, this yields ammonia, which is sequestered in portal blood and transported to the liver for ureagenesis, and citrulline, which is converted to arginine by the kidneys. The amazing developments in NMR imaging and spectroscopy and molecular biology have confirmed concepts derived from early studies in animals and cell cultures. The processes involved are exquisitely tuned. When they are faulty, ammonia accumulates. Severe acute hyperammonemia causes a rapidly progressive, often fatal, encephalopathy with brain edema. Chronic milder hyperammonemia causes a neuropsychiatric illness. Survivors of severe neonatal hyperammonemia have structural brain damage. Proposed explanations for brain edema are an increase in astrocyte osmolality, generally attributed to glutamine accumulation, and cytotoxic oxidative/nitrosative damage. However, ammonia neurotoxicity is multifactorial, with disturbances also in neurotransmitters, energy production, anaplerosis, cerebral blood flow, potassium, and sodium. Around 90% of hyperammonemic patients have liver disease. Inherited defects are rare. They are being recognized increasingly in adults. Deficiencies of urea cycle enzymes, citrin, and pyruvate carboxylase demonstrate the roles of isolated pathways in ammonia metabolism. Phenylbutyrate is used routinely to treat inherited urea cycle disorders, and its use for hepatic encephalopathy is under investigation.