Blocking NMDA receptors prevents the oxidative stress induced by acute ammonia intoxication

Impaired Enzymatic Antioxidant Defense in Erythrocytes of Rats with Ammonia-Induced Encephalopathy: Role of NMDA Receptors

Hepatic encephalopathy (HE), a neuropsychiatric disorder developing in patients with severe hepatic dysfunction, has been known for more than a century. However, pathogenetic mechanisms of cerebral dysfunction associated with liver disease are still poorly understood. There is a consensus that the primary cause of HE is accumulation of ammonia in the brain as a result of impaired liver detoxification capacity or the portosystemic shunt. Current evidence suggests that ammonia toxicity is mediated by hyperactivation of glutamate receptors, mainly N-methyl-D-aspartate receptors (NMDARs), and affects brain aerobic metabolism, which provides energy for multiple specific functions and neuronal viability. Recent reports on the presence of functional NMDARs in erythrocytes and the data on the deviations of blood parameters from their normal ranges indicate impaired hemodynamics and reduced oxygen-carrying capacity of erythrocytes in most patients with HE, thus suggesting a relationship between erythrocyte damage and cerebral dysfunction. In order to understand how hyperammonemia (HA)-induced disturbances in the energy metabolism in the brain (which needs a constant supply of large amounts of oxygen in the blood) lead to encephalopathy, it is necessary to reveal ammonia-induced impairments in the energy metabolism and antioxidant defense system of erythrocytes and to explore a potential role of ammonia in reduced brain oxygenation. To identify the said missing link, the activities of antioxidant enzymes and concentrations of reduced glutathione (GSH), oxidized glutathione (GSSG), and H2O2 were measured in the erythrocytes of rats with HA that were injected with the noncompetitive NMDAR antagonist MK-801. We found that in rats with HA, ammonia was accumulated in erythrocytes (cells lacking ammonia removal enzymes), which made them more susceptible to the prooxidant environment created during oxidative stress. This effect was completely or partially inhibited by MK-801. The data obtained might help to identify the risk factors in cognitive disorders and facilitate prediction of unfavorable outcomes of hypoperfusion in patients with a blood elevated ammonia concentration.

Cellular Pathogenesis of Hepatic Encephalopathy: An Update

Hepatic encephalopathy (HE) is a neuropsychiatric syndrome derived from metabolic disorders due to various liver failures. Clinically, HE is characterized by hyperammonemia, EEG abnormalities, and different degrees of disturbance in sensory, motor, and cognitive functions. The molecular mechanism of HE has not been fully elucidated, although it is generally accepted that HE occurs under the influence of miscellaneous factors, especially the synergistic effect of toxin accumulation and severe metabolism disturbance. This review summarizes the recently discovered cellular mechanisms involved in the pathogenesis of HE. Among the existing hypotheses, ammonia poisoning and the subsequent oxidative/nitrosative stress remain the mainstream theories, and reducing blood ammonia is thus the main strategy for the treatment of HE. Other pathological mechanisms mainly include manganese toxicity, autophagy inhibition, mitochondrial damage, inflammation, and senescence, proposing new avenues for future therapeutic interventions.

Overview of oxidative stress findings in hepatic encephalopathy: From cellular and ammonium-based animal models to human data

Oxidative stress is a natural phenomenon in the body. Under physiological conditions intracellular reactive oxygen species (ROS) are normal components of signal transduction cascades, and their levels are maintained by a complex antioxidants systems participating in the in-vivo redox homeostasis. Increased oxidative stress is present in several chronic diseases and interferes with phagocytic and nervous cell functions, causing an up-regulation of cytokines and inflammation. Hepatic encephalopathy (HE) occurs in both acute liver failure (ALF) and chronic liver disease. Increased blood and brain ammonium has been considered as an important factor in pathogenesis of HE and has been associated with inflammation, neurotoxicity, and oxidative stress. The relationship between ROS and the pathophysiology of HE is still poorly understood. Therefore, sensing ROS production for a better understanding of the relationship between oxidative stress and functional outcome in HE pathophysiology is critical for determining the disease mechanisms, as well as to improve the management of patients. This review is emphasizing the important role of oxidative stress in HE development and documents the changes occurring as a consequence of oxidative stress augmentation based on cellular and ammonium-based animal models to human data.

Is NMDA-Receptor-Mediated Oxidative Stress in Mitochondria of Peripheral Tissues the Essential Factor in the Pathogenesis of Hepatic Encephalopathy?

Background: Hepatic encephalopathy (HE) is a neuropsychiatric syndrome of increased ammonia-mediated brain dysfunction caused by impaired hepatic detoxification or when the blood bypasses the liver. Ammonia-activated signal transduction pathways of hyperactivated NMDA receptors (NMDAR) are shown to trigger a cascade of pathological reactions in the brain, leading to oxidative stress. NMDARs outside the brain are widely distributed in peripheral tissues, including the liver, heart, pancreas, and erythrocytes. To determine the contribution of these receptors to ammonia-induced oxidative stress in peripheral tissues, it is relevant to investigate if there are any ammonia-related changes in antioxidant enzymes and free radical formation and whether blockade of NMDARs prevents these changes. Methods: Hyperammonemia was induced in rats by ammonium acetate injection. Oxidative stress was measured as changes in antioxidant enzyme activities and O2•− and H2O2 production by mitochondria isolated from the tissues and cells mentioned above. The effects of the NMDAR antagonist MK-801 on oxidative stress markers and on tissue ammonia levels were evaluated. Results: Increased ammonia levels in erythrocytes and mitochondria isolated from the liver, pancreas, and heart of hyperammonemic rats are shown to cause tissue-specific oxidative stress, which is prevented completely (or partially in erythrocyte) by MK-801. Conclusions: These results support the view that the pathogenesis of HE is multifactorial and that ammonia-induced multiorgan oxidative stress-mediated by activation of NMDAR is an integral part of the disease and, therefore, the toxic effects of ammonia in НЕ may be more global than initially expected.

NMDA Receptor Mediates the Anticonvulsant Effect of Hydroalcoholic Extract of Artemisia persica in PTZ-Induced Seizure in Mice

It is necessary to seek more effective sources to design new drug against epilepsy. This study aimed to evaluate the effect of hydroalcoholic extract of Artemisia persica on pentylenetetrazole- (PTZ-) induced seizure in male mice by investigating the possible role of the NMDA receptor and antioxidative stress effect. The phenolic profile of A. persica extract was determined by HPLC-DAD analysis. Mice were treated with normal saline or A. persica extract or pentobarbital or a subeffective dose of extract plus ketamine (NMDA receptor antagonist) and/or effective dose of extract plus NMDA. PTZ (90 mg/kg) was injected intravenously for induction of seizure. The seizure threshold was measured. Then mice were euthanized and the antioxidant capacity and the level of malondialdehyde (MDA) of the prefrontal cortex and serum were measured. The gene expression of NMDA receptor subunits (Nr2a and Nr2b) was determined by real-time PCR. Findings showed that A. persica extract increased the seizure threshold, increased antioxidant capacity, and decreased MDA levels in the serum and brain samples. A. persica extract reduced the expression of NMDA receptor subunits. The result showed that ketamine potentiated the effect of the subeffective dose of extract. HPLC analysis showed that quercetin had the highest flavonoid content and also caffeic acid had the highest content of the phenolic acids. A. persica extract probably via NMDA receptor exerts anticonvulsant properties.

Pathomechanisms in hepatic encephalopathy

Hepatic encephalopathy (HE) is a frequent neuropsychiatric complication in patients with acute or chronic liver failure. Symptoms of HE in particular include disturbances of sensory and motor functions and cognition. HE is triggered by heterogeneous factors such as ammonia being a main toxin, benzodiazepines, proinflammatory cytokines and hyponatremia. HE in patients with liver cirrhosis is triggered by a low-grade cerebral edema and cerebral oxidative/nitrosative stress which bring about a number of functionally relevant alterations including posttranslational protein modifications, oxidation of RNA, gene expression changes and senescence. These alterations are suggested to impair astrocyte/neuronal functions and communication. On the system level, a global slowing of oscillatory brain activity and networks can be observed paralleling behavioral perceptual and motor impairments. Moreover, these changes are related to increased cerebral ammonia, alterations in neurometabolite and neurotransmitter concentrations and cortical excitability in HE patients.

Exogenous inositol ameliorates the effects of acute ammonia toxicity on intestinal oxidative status, immune response, apoptosis, and tight junction barriers of great blue-spotted mudskippers (Boleophthalmus pectinirostris)

Ammonia toxicity can disrupt the intestinal health of aquatic animals. It is important to find substances that alleviate these adverse effects. The present study explored the possible protective role of myo-inositol (MI) in ammonia-induced toxicity in the fish intestine. Great blue-spotted mudskippers (Boleophthalmus pectinirostris) accumulated in artificial seawater (15‰ salinity, n = 600) were randomly selected and intraperitoneally injected with NaCl (0.68%) or MI (2.5 mg/g fish in 0.68% NaCl) then exposed to artificial seawater alone (NaCl and MI group) or seawater containing 57.025 mmol/L ammonium chloride (NH₃ and NH₃ + MI group). After a 24-h experiment, it showed that ammonia exposure down-regulated the mRNA expression levels of intestinal barrier function proteins (Zo-1, Ocln, Cldn-5, Cldn-12, and Cldn-15) and anti-inflammatory cytokines (Tgf-β and Il-10) while the acute ammonia stress up-regulated the apoptosis genes (p53, Bax, Caspase-3, and Caspase-9) and pro-inflammatory cytokines (Tnf-α and Il-1β). Furthermore, ammonia challenge also induced oxidative stress, as the malondialdehyde and the protein carbonyl contents were increased. In addition, ammonia stress down-regulated the antioxidant enzymes (Cu/Zn-Sod, Cat, Gpx, and Gst) activities as well as their gene transcription levels. The administration of the exogenous myo-inositol greatly ameliorated the ammonia-induced changes in redox capacity, immune response, apoptosis, inflammation, and tight junction barrier function to levels similar to those of the NaCl group. Furthermore, fish injected with MI alone showed no significant changes compared with the NaCl group. Taken together, pretreatment with myo-inositol had no obvious side-effects and effectively protected the mudskippers' intestine from the toxicity caused by acute ammonia stress.

Disturbance of the Glutamate-Glutamine Cycle, Secondary to Hepatic Damage, Compromises Memory Function

Glutamate fulfils many vital functions both at a peripheral level and in the central nervous system (CNS). However, hyperammonemia and hepatic failure induce alterations in glutamatergic neurotransmission, which may be the main cause of hepatic encephalopathy (HE), an imbalance which may explain damage to both learning and memory. Cognitive and motor alterations in hyperammonemia may be caused by a deregulation of the glutamate-glutamine cycle, particularly in astrocytes, due to the blocking of the glutamate excitatory amino-acid transporters 1 and 2 (EAAT1, EAAT2). Excess extracellular glutamate triggers mechanisms involving astrocyte-mediated inflammation, including the release of Ca2+-dependent glutamate from astrocytes, the appearance of excitotoxicity, the formation of reactive oxygen species (ROS), and cell damage. Glutamate re-uptake not only prevents excitotoxicity, but also acts as a vital component in synaptic plasticity and function. The present review outlines the evidence of the relationship between hepatic damage, such as that occurring in HE and hyperammonemia, and changes in glutamine synthetase function, which increase glutamate concentrations in the CNS. These conditions produce dysfunction in neuronal communication. The present review also includes data indicating that hyperammonemia is related to the release of a high level of pro-inflammatory factors, such as interleukin-6, by astrocytes. This neuroinflammatory condition alters the function of the membrane receptors, such as N-methyl-D-aspartate (NMDA), (α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid) AMPA, and γ-aminobutyric acid (GABA), thus affecting learning and spatial memory. Data indicates that learning and spatial memory, as well as discriminatory or other information acquisition processes in the CNS, are damaged by the appearance of hyperammonemia and, moreover, are associated with a reduction in the production of cyclic guanosine monophosphate (cGMP). Therefore, increased levels of pharmacologically controlled cGMP may be used as a therapeutic tool for improving learning and memory in patients with HE, hyperammonemia, cerebral oedema, or reduced intellectual capacity.

Ammonia toxicity in the yellow catfish (Pelteobagrus fulvidraco): The mechanistic insight from physiological detoxification to poisoning

Ammonia is toxic to fishes. Different fish have different defense strategies against ammonia, so the mechanism of ammonia poisoning is different. In this study, yellow catfish were exposed to three levels of ammonia (0, 5.70 and 57.00 mg L^-1) for 96 h. The results showed that ammonia poisoning could lead to free amino acid imbalance (ornithine and citrulline contents declined; arginine content elevated), urea cycle enzymes deficiency (carbamyl phosphate synthetase and arginase contents declined), oxidative stress (superoxide dismutase, catalase and glutathione peroxidase activities declined), immunosuppression (lysozyme activity, 50% hemolytic complement and total immunoglobulin contents and phagocytic index declined) and cytokines release (TNF, IL 1 and IL 8 contents elevated). In addition, ammonia poisoning could induce up-regulation of antioxidant enzymes (Cu/Zn-SOD, Mn-SOD, CAT and GPx), cytokines (TNFα, IL 1 and IL 8) and apoptosis (p53, Bax, cytochrome c, Caspase 3 and Caspase 9) genes transcription. This study suggesting that the urea cycle and glutamine synthesis both were involved in the ammonia detoxification of yellow catfish, and the immunosuppression, inflammation and apoptotic induced by ammonia poisoning in yellow catfish are related to oxidative stress.

Acute ammonia poisoning in dolly varden char (Salvelinus malma) and effect of methionine sulfoximine

Ammonia is toxic to most bony fishes. However, little information is available on the toxicology mechanisms induced by ammonia and the means to mitigate the effects by various fishes. In this study, four groups of experiments were designed and carried out to test the response of dolly varden char to ammonia toxicity and their mitigation through methionine sulfoximine (MSO). NaCl group was injected with NaCl, NH₃ group was injected with ammonium acetate, NH₃+MSO group was injected with ammonium acetate and MSO, MSO group was injected with MSO. Results showed that ammonia toxicity could lead to blood deterioration (elevation in white blood cell and blood ammonia), free amino acid imbalance (elevation in glutamine, glutamate, arginine and ornithine, coupled with reduction of citrulline and aspartate), ammonia metabolism enzyme activity inhibition (reduction in carbamyl phosphate synthetase, ornithine transcarbamylase and arginase), oxidative stress (reduction in superoxide dismutase, catalase and glutathione peroxidase) and immunosuppression (reduction in lysozyme, 50% hemolytic complement, total immunoglobulin and phagocytic index), but the MSO can eliminate fatal effect of oxidative damage. In addition, ammonia poisoning could induce down-regulation of antioxidant enzymes coding genes (SOD, CAT and GPx) and up-regulation of inflammatory cytokine genes (TNFα, IL-1β and IL-8) transcription, suggesting that immunosuppression and inflammation may relate to oxidative stress in fish.

Systemic Oxidative Stress Markers in Cirrhotic Patients with Hepatic Encephalopathy: Possible Connections with Systemic Ammoniemia

Background and objectives: Oxidative stress shows evidence of dysregulation in cirrhotic patients with hepatic encephalopathy (HE), although there are still controversies regarding the connections between oxidative stress and ammonia in these patients. The aim of this study was to evaluate the oxidative stress implication in overt HE pathogenesis of cirrhotic patients. Materials and Methods: We performed a prospective case-control study, which included 40 patients divided into two groups: group A consisted of 20 cirrhotic patients with HE and increased systemic ammoniemia, and group B consisted of 20 cirrhotic patients with HE and normal systemic ammoniemia. The control group consisted of 21 healthy subjects matched by age and sex. The activity of superoxide dismutase (SOD), glutathione peroxidase (GPx), malondialdehyde (MDA) levels (lipid peroxidation marker), and ammoniemia were evaluated. Results: We found a significant decrease in SOD and GPx activity and also a significant increase of MDA levels in cirrhotic patients with HE as compared to the healthy age-matched control group (1.35 ± 0.08 vs. 0.90 ± 0.08 U/mL, p = 0.002; 0.093 ± 0.06 vs. 0.006 ± 0.008 U/mL, p = 0.001; and 35.94 ± 1.37 vs. 68.90 ± 5.68 nmols/mL, p = 0.0001, respectively). Additionally, we found significant correlations between the main oxidative stress markers and the levels of systemic ammonia (r = 0.452, p = 0.005). Patients from group A had a significant increase of MDA as compared with those from group B (76.93 ± 5.48 vs. 50.06 ± 5.60 nmols/mL, p = 0.019). Also, there was a compensatory increase in the activity of both antioxidant enzymes (SOD and GPx) in patients with increased systemic ammoniemia (group A), as compared to HE patients from group B. Conclusions: Our results demonstrated a significant decrease in antioxidants enzymes activities (SOD and GPx), as well as a significant increase in MDA concentrations, adding new data regarding the influence of oxidative stress in HE pathogenesis in cirrhotic patients.

Cerebral edema and liver disease: Classic perspectives and contemporary hypotheses on mechanism

Liver disease is a growing public health concern. Hepatic encephalopathy, the syndrome of brain dysfunction secondary to liver disease, is a frequent complication of both acute and chronic liver disease and cerebral edema (CE) is a key feature. While altered ammonia metabolism is a key contributor to hepatic encephalopathy and CE in liver disease, there is a growing appreciation that additional mechanisms contribute to CE. In this review we will begin by presenting three classic perspectives that form a foundation for a discussion of CE in liver disease: 1) CE is unique to acute liver failure, 2) CE in liver disease is only cytotoxic, and 3) CE in liver disease is primarily an osmotically mediated consequence of ammonia and glutamine metabolism. We will present each classic perspective along with more recent observations that call in to question that classic perspective. After highlighting these areas of debate, we will explore the leading contemporary mechanisms hypothesized to contribute to CE during liver disease.

Astragaloside IV Alleviates Ammonia-Induced Apoptosis and Oxidative Stress in Bovine Mammary Epithelial Cells

Ammonia is one of the major toxic components of metabolites in blood and tissues of high-producing dairy cows and could affect the health of bovine mammary glands. Bovine mammary epithelial cells are sensitive to oxidative stress induced by intensive cell metabolism. In our previous study, we found that ammonia could induce oxidative stress, apoptosis and inflammatory responses in bovine mammary epithelial cells. In the present study, the cytoprotective effects of astragaloside IV against ammonia in vitro were explored. The results demonstrated that pretreatment of MAC-T cells with astragaloside IV could potently suppress the increase in the level of intracellular reactive oxygen species (ROS) and the rate of cell apoptosis, inhibit the ammonia-induced inflammatory responses, and rescue the decrease of cell viability. Astragaloside IV prevented ammonia-induced endoplasmic reticulum stress. Astragaloside IV also significantly suppressed the levels of BAX, caspase 3 and p53 phosphorylation in ammonia-induced MAC-T cells. Nuclear factor erythroid 2-related factor 2(Nrf2) was essential for cytoprotective effects of astragaloside IV in MAC-T cells, as knockdown of Nrf2 dramatically abolished the prosurvival effects of astragaloside IV on treated cells. Furthermore, the PI3K/AKT and ERK/MAPK pathways were responsible for the induction of Nrf2 by astragaloside IV. In conclusion, astragaloside IV played a beneficial role against ammonia-induced damage of MAC-T cells. This provides a cue for future study to use astragaloside IV as a protective and curative agent against ammonia exposure of mammary glands in dairy cows.

Air-breathing and excretory nitrogen metabolism in fishes

During water-land transition, ancient fishes acquired the ability to breathe air, but air-breathing engendered problems in nitrogenous waste excretion. Nitrogen is a fundamental component of amino acids, proteins, and nucleic acids, and the degradation of these nitrogen-containing compounds releases ammonia. Ammonia is toxic and must be removed. Fishes in water excrete ammonia as the major nitrogenous waste through gills, but gills of air-breathing fishes are modified for air-breathing or largely replaced by air-breathing organs. Notably, fishes emerged from water can no longer excrete ammonia effectively because of a lack of water to flush the gills. Hence, ancient fishes that participated in water-land transition must have developed means to deal with ammonia toxicity. Extant air-breathing fishes, particularly amphibious ones, can serve as models to examine adaptations which might have facilitated the emergence of ancient fishes from water. Some of these fishes can actively emerge from water and display complex behaviors on land, while a few can burrow into mud and survive for years during drought. Many of them are equipped with mechanisms to ameliorate ammonia toxicity during emersion. In this review, the mechanisms adopted by air-breathing fishes to deal with ammonia toxicity during emersion were organized into seven disparate strategies. In addition, eight extant air-breathing fishes with distinctive terrestrial behaviors and peculiar natural habitats were selected to describe in detail how these seven strategies could be adopted in disparate combinations to ameliorate ammonia toxicity during emersion.

Effects of acute ammonia toxicity on oxidative stress, immune response and apoptosis of juvenile yellow catfish Pelteobagrus fulvidraco and the mitigation of exogenous taurine

Ammonia can easily form in intensive culture systems due to ammonification of uneaten food and animal excretion, which usually brings detrimental health effects to fish. However, little information is available on the mechanisms of the detrimental effects of ammonia stress and mitigate means in fish. In this study, the four experimental groups were carried out to test the response of yellow catfish to ammonia toxicity and their mitigation through taurine: group 1 was injected with NaCl, group 2 was injected with ammonium acetate, group 3 was injected with ammonium acetate and taurine, and group 4 was injected taurine. The results showed that ammonia poisoning could induce ammonia, glutamine, glutamate and malondialdehyde accumulation, and subsequently lead to blood deterioration (red blood cell, hemoglobin and serum biochemical index reduced), oxidative stress (superoxide dismutase and catalase activities declined) and immunosuppression (lysozyme, 50% hemolytic complement, total immunoglobulin, phagocytic index and respiratory burst reduced), but the exogenous taurine could mitigate the adverse effect of ammonia poisoning. In addition, ammonia poisoning could induce up-regulation of antioxidant enzymes (Cu/Zn-SOD, CAT, GPx and GR), inflammatory cytokines (TNF, IL-1 and IL-8) and apoptosis (p53, Bax, caspase 3 and caspase 9) genes transcription, suggesting that cell apoptotic and inflammation may relate to oxidative stress. This result will be helpful to understand the mechanism of aquatic toxicology induced by ammonia in fish.

Hypothesis: role for ammonia neutralization in the prevention and reversal of heart failure

Ammonia plays a central role in the life and death of all living organisms and has been studied for over 100 yr. Ammonia is necessary for growth and development, but it is toxic in excess, and, as a result, differing methods of ammonia neutralization have evolved. After physiological and pathological stress to the heart, tissue ammonia levels rise. Local ammonia neutralization may be inadequate, and excess ammonia may exert its toxic effects. Phenylbutyrate (PBA), which is Federal Drug Administration approved for the treatment of elevated blood ammonia in urea cycle disorders, provides an accessory pathway for ammonia excretion. Recently, PBA has also been found to prevent specific cardiomyopathies. The central theme presents the hypothesis that stress to the myocardium from a variety of environmental sources causes injury, cell death, necrosis, and ammonia production. Ammonia, if not neutralized, exerts downstream toxic effects. Here, data are presented showing that neutralization with PBA alone and PBA combined with angiotensin-converting enzyme inhibition prevent and reverse pathophysiology associated with specific cardiomyopathies. NEW & NOTEWORTHY Ammonia produced after myocardial injury is hypothesized to be an upstream stress contributing to the pathophysiology of heart failure, effects that may be attenuated by a documented ammonia-reducing treatment. Reversal of heart failure can be achieved using an angiotensin-converting enzyme inhibitor combined with an ammonia-reducing treatment.

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.

Growth, blood health, antioxidant status, immune response and resistance to Aeromonas hydrophila of juvenile yellow catfish exposed to di-2-ethylhexyl phthalate (DEHP)

Triplicate groups of juvenile yellow catfish Pelteobagrus fulvidraco were exposed to three levels of DEHP (0, 0.1 and 0.5mgL^-1) for 56days. Fish survival (100%) was not affected by different levels of ambient DEHP. Final body weight, weight gain, specific growth rate and feed intake of fish exposed to 0.5mgL^-1 DEHP were the highest. On the contrary, hepatosomatic index of fish exposed to 0.1 and 0.5mgL^-1 DEHP were the lowest. Serum total protein, glutamic-pyruvic transaminase, glutamic-oxaloacetic transaminase, glucose and triglycerides increased with the increasing concentrations of DEHP exposure. Superoxide dismutase and glutathione peroxidase activities of fish exposed to 0.5mgL^-1 DEHP were the lowest, but malondialdehyde contents of fish exposed to 0.1 and 0.5mgL^-1 DEHP were higher than that of control fish. Phagocytic indices of the control group were the highest. After being intraperitoneally injected with Aeromonas hydrophila, fish in the control group had the highest expression of toll like receptor 5, and the expression of myeloid differentiation factor 88 of fish exposed to 0.5mgL^-1 DEHP was the lowest. This study indicates that DEHP exerts its toxic effects by interfering with hepatic metabolism, inducing ROS generation and malondialdehyde accumulation, leading to blood deterioration and immunosuppression.

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.

Acute ammonia toxicity in crucian carp Carassius auratus and effects of taurine on hyperammonemia

The four experimental groups were carried out to test the response of crucian carp Carassius auratus to ammonia toxicity and taurine: group 1 was injected with NaCl, group 2 was injected with ammonium acetate, group 3 was injected with ammonium acetate and taurine, and group 4 was injected with taurine. Fish in group 2 had the highest ammonia and glutamine contents, and the lowest glutamate content in liver and brain. Serum superoxide dismutase (SOD), glutathione (GSH) activities, red cell count (RBC), white cell count (WBC), lysozyme (LYZ) activity, complement C3 content of fish in group 2 reflected the lowest, but malondialdehyde content was the highest. Importantly, serum SOD and GSH activites, RBC, WBC, and LYZ activity, C3, C4 and total immunoglobulin contents of fish in group 3 were significantly higher than those of fish in group 2. This study indicates that ammonia exerts its toxic effects by interfering with amino acid transport, inducing ROS generation, leading to malondialdehyde accumulation and immunosuppression of crucian carp. The exogenous taurine could mitigate the adverse effect of high ammonia level on fish physiological disorder.

Asymmetric Dimethylarginine and Hepatic Encephalopathy: Cause, Effect or Association?

The methylated derivative of l-arginine, asymmetric dimethylarginine (ADMA) is synthesized in different mammalian tissues including the brain. ADMA acts as an endogenous, nonselective, competitive inhibitor of all three isoforms of nitric oxide synthase (NOS) and may limit l-arginine supply from the plasma to the enzyme via reducing its transport by cationic amino acid transporters. Hepatic encephalopathy (HE) is a relatively frequently diagnosed complex neuropsychiatric syndrome associated with acute or chronic liver failure, characterized by symptoms linked with impaired brain function leading to neurological disabilities. The l-arginine—nitric oxide (NO) pathway is crucially involved in the pathomechanism of HE via modulating important cerebral processes that are thought to contribute to the major HE symptoms. Specifically, activation of this pathway in acute HE leads to an increase in NO production and free radical formation, thus, contributing to astrocytic swelling and cerebral edema. Moreover, the NO-cGMP pathway seems to be involved in cerebral blood flow (CBF) regulation, altered in HE. For this reason, depressed NO-cGMP signaling accompanying chronic HE and ensuing cGMP deficit contributes to the cognitive and motor failure. However, it should be remembered that ADMA, a relatively little known element limiting NO synthesis in HE, may also influence the NO-cGMP pathway regulation. In this review, we will discuss the contribution of ADMA to the regulation of the NO-cGMP pathway in the brain, correlation of ADMA level with CBF and cognitive alterations observed during HE progression in patients and/or animal models of HE.

Relationship between oxidative stress and brain swelling in goldfish (Carassius auratus) exposed to high environmental ammonia

Buildups of ammonia can cause potentially fatal brain swelling in mammals, but such swelling is reversible in the anoxia- and ammonia-tolerant goldfish (Carassius auratus). We investigated brain swelling and its possible relationship to oxidative stress in the brain and liver of goldfish acutely exposed to high external ammonia (HEA; 5 mmol/l NH₄Cl) at two different acclimation temperatures (14°C, 4°C). Exposure to HEA at 14°C for 72h resulted in increased internal ammonia and glutamine concentrations in the brain, and it caused cellular oxidative damage in the brain and liver. However, oxidative damage was most pronounced in brain, in which there was a twofold increase in thiobarbituric acid-reactive substances, a threefold increase in protein carbonylation, and a 20% increase in water volume (indicative of brain swelling). Increased activities of catalase, glutathione peroxidase, and glutathione reductase in the brain suggested that goldfish upregulate their antioxidant capacity to partially offset oxidative stress during hyperammonemia at 14°C. Notably, acclimation to colder (4°C) water completely attenuated the oxidative stress response to HEA in both tissues, and there was no change in brain water volume despite similar increases in internal ammonia. We suggest that ammonia-induced oxidative stress may be responsible for the swelling of goldfish brain during HEA, but further studies are needed to establish a mechanistic link between reactive oxygen species production and brain swelling. Nevertheless, a high capacity to withstand oxidative stress in response to variations in internal ammonia likely explains why goldfish are more resilient to this stressor than most other vertebrates.

The protective effects of taurine on acute ammonia toxicity in grass carp Ctenopharynodon idellus

The four experimental groups were carried out to test the response of grass carp Ctenopharyngodon idella to ammonia toxicity and taurine: group 1 was injected with NaCl, group 2 was injected with ammonium acetate, group 3 was injected with ammonium acetate and taurine, and group 4 was injected taurine. Fish in group 2 had the highest ammonia content in the liver and brain, and alanine, arginine, glutamine, glutamate and glycine contents in liver. Brain alanine and glutamate of fish in group 2 were significantly higher than those of fish in group 1. Malondialdehyde content of fish in group 2 was the highest, but superoxide dismutase and glutathione activities were the lowest. Although fish in group 2 had the lowest red cell count and hemoglobin, the highest alkaline phosphatase, complement C3, C4 and total immunoglobulin contents appeared in this group. In addition, superoxide dismutase and glutathione activities, red cell count and hemoglobin of fish in group 3 were significantly higher than those of fish in group 2, but malondialdehyde content is the opposite. This study indicates that ammonia exerts its toxic effects by interfering with amino acid transport, inducing reactive oxygen species generation and malondialdehyde accumulation, leading to blood deterioration and over-activation of immune response. The exogenous taurine could mitigate the adverse effect of high ammonia level on fish physiological disorder.

Ammonia as a Potential Neurotoxic Factor in Alzheimer's Disease

Ammonia is known to be a potent neurotoxin that causes severe negative effects on the central nervous system. Excessive ammonia levels have been detected in the brain of patients with neurological disorders such as Alzheimer disease (AD). Therefore, ammonia could be a factor contributing to the progression of AD. In this review, we provide an introduction to the toxicity of ammonia and putative ammonia transport proteins. We also hypothesize how ammonia may be linked to AD. Additionally, we discuss the evidence that support the hypothesis that ammonia is a key factor contributing to AD progression. Lastly, we summarize the old and new experimental evidence that focuses on energy metabolism, mitochondrial function, inflammatory responses, excitatory glutamatergic, and GABAergic neurotransmission, and memory in support of our ammonia-related hypotheses of AD.

The trout farm effect on Dinocras megacephala (Plecoptera: Perlidae) larvae: Antioxidative defense

Trout production represents a major agricultural activity in Serbia. Organic compounds are drained into the environment, usually without previous sedimentation, thus affecting the stream biota. Considering that biological monitoring is commonly based on aquatic macroinvertebrates as target organisms, the authors used larvae of Dinocras megacephala to estimate trout farm effects on the mass of the larvae and their antioxidative defense in pollution stress conditions. Four locations were chosen along the channel of the Raška River, 2 upstream (L1, L2) and 2 downstream (L3, L4) from the trout farm outlet. Basic physical and chemical water parameters were measured. Superoxide dismutase (SOD) and catalase (CAT) activity changes were analyzed to determine the level of oxidative stress caused by the increase of organic compounds originating from the trout farm. Dissolved oxygen concentration decreased from the upstream to downstream locations. Furthermore, the concentration of ionized ammonia was almost 10 times higher at the downstream locations than at the upstream locations. Larval mass, as well as CAT activity, was significantly higher at L3 compared with the other 3 locations. Activity of SOD was significantly higher at L3 than at L1. The results indicate that higher concentrations of organic compounds from the trout farm induce clear changes in the status of the antioxidant defense of D. megacephala larvae. Environ Toxicol Chem 2016;35:1775-1782. © 2015 SETAC.

Ammonia toxicity induces glutamine accumulation, oxidative stress and immunosuppression in juvenile yellow catfish Pelteobagrus fulvidraco

A study was carried to test the response of yellow catfish for 28 days under two ammonia concentrations. Weight gain of fish exposure to high and low ammonia abruptly increased at day 3. There were no significant changes in fish physiological indexes and immune responses at different times during 28-day exposure to low ammonia. Fish physiological indexes and immune responses in the treatment of high ammonia were lower than those of fish in the treatment of low ammonia. When fish were exposed to high ammonia, the ammonia concentration in the brain increased by 19-fold on day 1. By comparison, liver ammonia concentration reached its highest level much earlier at hour 12. In spite of a significant increase in brain and liver glutamine concentration, there was no significant change in glutamate level throughout the 28-day period. The total superoxide dismutase (SOD), glutathione peroxidase (GPX) and glutathione reductase (GR) activities in the brain gradually decreased from hour 0 to day 28. Liver SOD, GPX and GR activities reached the highest levels at hour 12, and then gradually decreased. Thiobarbituric acid reactive substance brain and liver content gradually increased throughout the 28-day period. Lysozyme, acid phosphatase and alkaline phosphatase activities in the liver reached exceptionally low levels after day 14. This study indicated that glutamine accumulation in the brain was not the major cause of ammonia poisoning, the toxic reactive oxygen species is not fully counter acted by the antioxidant enzymes and immunosuppression is a process of gradual accumulation of immunosuppressive factors.

Dexmedetomidine Protects against Transient Global Cerebral Ischemia/Reperfusion Induced Oxidative Stress and Inflammation in Diabetic Rats

BACKGROUND

Transient global cerebral ischemia/reperfusion (I/R) is a major perioperative complication, and diabetes increases the response of oxidative stress and inflammation induced by I/R. The objective of this study was to determine the protective effect of dexmedetomidine against transient global cerebral ischemia/reperfusion induced oxidative stress and inflammation in diabetic rats.

METHODS

Sixty-four rats were assigned into four experimental groups: normoglycemia, normoglycemia + dexmedetomidine, hyperglycemia, and hyperglycemia + dexmedetomidine and all subsequent neurological examinations were evaluated by a blinded observer. Damage to the brain was histologically assessed using the TUNEL staining method while western blotting was used to investigate changes in the expression levels of apoptosis-related proteins as well as the microglia marker, ionized calcium-binding adapter molecule 1 (Iba1). Water content in the brain was also analyzed. In addition, hippocampal concentrations of malondialdehyde (MDA) and Nox2 (a member of the Nox family of NADPH oxidases), and the activity of superoxide dismutase and catalase were analyzed. Finally, changes in serum concentrations of tumor necrosis factor-α (TNF-α) and interleukin (IL)-6 were detected.

RESULTS

Results showed that diabetes increased brain water content, the number of apoptotic neurons, early neurological deficit scores, oxidative stress (MDA and Nox2) and inflammation (pro-inflammatory cytokines including TNF-α and IL-6) levels following transient global I/R injury, but that these symptoms were attenuated following administration of dexmedetomidine.

CONCLUSIONS

These findings suggest that dexmedetomidine can significantly alleviate damage resulting from I/R, and this mechanism may be related to a reduction in both oxidative stress and inflammation which is normally associated with I/R.

Therapeutic potential of alpha-ketoglutarate against acetaminophen-induced hepatotoxicity in rats

Objective:

Alpha-ketoglutarate (α-KG) is a cellular intermediary metabolite of Krebs cycle, involved in energy metabolism, amino acid synthesis, and nitrogen transport. It is available over-the-counter and marketed as a nutritional supplement. There is a growing body of evidence to suggest that dietary α-KG has the potential to maintain cellular redox status and thus can protect various oxidative stress induced disease states. The aim of the present study was to investigate the hepatoprotective role of α-KG in acetaminophen (APAP) induced toxicity in rats.

Materials and Methods:

Animals were divided into three groups of six animals each. Group I (Vehicle control): Normal Saline, Group II (APAP): A single intraperitoneal injection of 0.6 g/kg, Group III (APAP + α-KG): APAP as in Group II with α-KG treatment at a dose of 2 g/kg, orally for 5 days. Then the levels of alanine aminotransferase (ALT), aspartate aminotransferase (AST), and alkaline phosphatase (ALP) with oxidative stress markers including malondialdehyde (MDA), reduced glutathione (GSH), superoxide dismutase (SOD), catalase (CAT), and histopathology were analyzed.

Results:

The results indicate that APAP caused significant elevations in ALT, AST, ALP, and MDA levels, while GSH, SOD, and CAT were significantly depleted while co-administration of α-KG showed a significant (P < 0.05) reduction in the severity of these damages. Histologically, the liver showed inflammation and necrosis after APAP treatment, which were significantly restored with co-administration of α-KG.

Conclusion:

These results indicate the possible therapeutic potential of α-KG in protecting liver damage by APAP in rats.

Pannexin1 as a novel cerebral target in pathogenesis of hepatic encephalopathy

Hepatic encephalopathy (HE) represents a nervous system disorder caused due to liver dysfunction. HE is broadly classified as acute/overt and moderate-minimal HE. Since HE syndrome severely affects quality of life of the patients and it may be life threatening, it is important to develop effective therapeutic strategy against HE. Mainly ammonia neurotoxicity is considered accountable for HE. Increased level of ammonia in the brain activates glutamate-NMDA (N-methyl-D-aspartate) receptor (NMDAR) pathway leading to Ca(2+) influx, energy deficit and oxidative stress in the post synaptic neurons. Moreover, NMDAR blockage has been found to be a poor therapeutic option, as this neurotransmitter receptor plays important role in maintaining normal neurophysiology of the brain. Thus, searching new molecular players in HE pathogenesis is of current concern. There is an evolving concept about roles of the trans-membrane channels in the pathogenesis of a number of neurological complications. Pannexin1 (Panx1) is one of them and has been described to be implicated in stroke, epilepsy and ischemia. Importantly, the pathogenesis of these complications relates to some extent with NMDAR over activation. Thus, it is speculated that HE pathogenesis might also involve Panx1. Indeed, some recent observations in the animal models of HE provide support to this argument. Since opening of Panx1 channel is mostly associated with the neuronal dysfunctions, down regulation of this channel could serve as a relevant therapeutic strategy without producing any serious side effects. In the review article an attempt has been made to summarize the current information on implication of Panx1 in the brain disorders and its prospects for being examined as pharmacological target in HE pathogenesis.

A zebrafish model of hyperammonemia

Hyperammonemia is the principal consequence of urea cycle defects and liver failure, and the exposure of the brain to elevated ammonia concentrations leads to a wide range of neuro-cognitive deficits, intellectual disabilities, coma and death. Current treatments focus almost exclusively on either reducing ammonia levels through the activation of alternative pathways for ammonia disposal or on liver transplantation. Ammonia is toxic to most fish and its pathophysiology appears to be similar to that in mammals. Since hyperammonemia can be induced in fish simply by immersing them in water with elevated concentration of ammonia, we sought to develop a zebrafish (Danio rerio) model of hyperammonemia. When exposed to 3mM ammonium acetate (NH4Ac), 50% of 4-day old (dpf) fish died within 3hours and 4mM NH4Ac was 100% lethal. We used 4dpf zebrafish exposed to 4mM NH4Ac to test whether the glutamine synthetase inhibitor methionine sulfoximine (MSO) and/or NMDA receptor antagonists MK-801, memantine and ketamine, which are known to protect the mammalian brain from hyperammonemia, prolong survival of hyperammonemic fish. MSO, MK-801, memantine and ketamine all prolonged the lives of the ammonia-treated fish. Treatment with the combination of MSO and an NMDA receptor antagonist was more effective than either drug alone. These results suggest that zebrafish can be used to screen for ammonia-neuroprotective agents. If successful, drugs that are discovered in this screen could complement current treatment approaches to improve the outcome of patients with hyperammonemia.

Growth, oxidative stress responses, and gene transcription of juvenile bighead carp (Hypophthalmichthys nobilis) under chronic-term exposure of ammonia

Ammonia toxicity has become a universal problem for aquatic animals, especially fish. The purpose of the present study was to assess the chronic toxicity of ammonia to the juvenile bighead carp (Hypophthalmichthys nobilis). The authors measured the responses of growth performance (specific growth rate, condition factor, body weight, and body length), oxidative stress, and related gene transcription of juvenile bighead carp exposed to solutions with different concentrations of un-ionized ammonia (UIA; 0 mg L(-1) , 0.053 mg L(-1) , 0.106 mg L(-1) , 0.159 mg L(-1) , and 0.212 mg L(-1) ). The results showed that UIA had no effect on growth performance, glutathione content, or glutathione S-transferase gene transcription, but superoxide dismutase (SOD) activity was significantly elevated. In addition, different concentrations of UIA produced different degrees of damage to juvenile bighead carp: compared with control, lower UIA levels significantly decreased gene transcription of catalase (CAT) and increased malondialdehyde (MDA) levels; higher UIA concentration (0.212 mg L(-1) ) significantly increased gene transcription of the antioxidant enzymes CAT and SOD and reduced MDA levels. The data clearly demonstrate that chronic exposure of UIA at lower concentrations can result in some degree of impairment of antioxidative function, and chronic exposure at higher concentrations can enhance damage to juvenile bighead carp by modulating antioxidant enzyme activities and gene transcription.

Concise review of current concepts on nomenclature and pathophysiology of hepatic encephalopathy

Hepatic encephalopathy is a neuropsychiatric complication of liver cirrhosis the symptoms of which may vary from imperceptible to severe, invaliding, and even lethal. Minimal hepatic encephalopathy is also important because of its tendency to impair patients' cognitive functions and quality of life. The polyetiological pathogenesis of hepatic encephalopathy is intensively studied. A general consensus exists that not only excess of ammonia but also inflammatory, oxidative, and other processes are significant in the development of hepatic encephalopathy.

Anti-oxidative defences are modulated differentially in three freshwater teleosts in response to ammonia-induced oxidative stress

Oxidative stress and the antioxidant response induced by high environmental ammonia (HEA) were investigated in the liver and gills of three freshwater teleosts differing in their sensitivities to ammonia. The highly ammonia-sensitive salmonid Oncorhynchus mykiss (rainbow trout), the less ammonia sensitive cyprinid Cyprinus carpio (common carp) and the highly ammonia-resistant cyprinid Carassius auratus (goldfish) were exposed to 1 mM ammonia (as NH₄HCO₃) for 0 h (control), 3 h, 12 h, 24 h, 48 h, 84 h and 180 h. Results show that HEA exposure increased ammonia accumulation significantly in the liver of all the three fish species from 24 h–48 h onwards which was associated with an increment in oxidative stress, evidenced by elevation of xanthine oxidase activity and levels of hydrogen peroxide (H₂O₂) and malondialdehyde (MDA). Unlike in trout, H₂O₂ and MDA accumulation in carp and goldfish liver was restored to control levels (84 h–180 h); which was accompanied by a concomitant increase in superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase activity and reduced ascorbate content. Many of these defence parameters remained unaffected in trout liver, while components of the glutathione redox cycle (reduced glutathione, glutathione peroxidase and glutathione reductase) enhanced to a greater extent. The present findings suggest that trout rely mainly on glutathione dependent defensive mechanism while carp utilize SOD, CAT and ascorbate as anti-oxidative sentinels. Hepatic cells of goldfish appear to utilize each of these protective systems, and showed more effective anti-oxidative compensatory responses towards HEA than carp, while trout were least effective. The present work also indicates that HEA exposure resulted in a relatively mild oxidative stress in the gills of all three species. This probably explains the almost complete lack of anti-oxidative responses in branchial tissue. This research suggests that oxidative stress, as well as the antioxidant potential clearly differ between salmonid and cyprinid species.

Excretory nitrogen metabolism and defence against ammonia toxicity in air-breathing fishes

With the development of air-breathing capabilities, some fishes can emerge from water, make excursions onto land or even burrow into mud during droughts. Air-breathing fishes have modified gill morphology and morphometry and accessory breathing organs, which would tend to reduce branchial ammonia excretion. As ammonia is toxic, air-breathing fishes, especially amphibious ones, are equipped with various strategies to ameliorate ammonia toxicity during emersion or ammonia exposure. These strategies can be categorized into (1) enhancement of ammonia excretion and reduction of ammonia entry, (2) conversion of ammonia to a less toxic product for accumulation and subsequent excretion, (3) reduction of ammonia production and avoidance of ammonia accumulation and (4) tolerance of ammonia at cellular and tissue levels. Active ammonia excretion, operating in conjunction with lowering of ambient pH and reduction in branchial and cutaneous NH₃ permeability, is theoretically the most effective strategy to maintain low internal ammonia concentrations. NH₃ volatilization involves the alkalization of certain epithelial surfaces and requires mechanisms to prevent NH₃ back flux. Urea synthesis is an energy-intensive process and hence uncommon among air-breathing teleosts. Aestivating African lungfishes detoxify ammonia to urea and the accumulated urea is excreted following arousal. Reduction in ammonia production is achieved in some air-breathing fishes through suppression of amino acid catabolism and proteolysis, or through partial amino acid catabolism leading to alanine formation. Others can slow down ammonia accumulation through increased glutamine synthesis in the liver and muscle. Yet, some others develop high tolerance of ammonia at cellular and tissue levels, including tissues in the brain. In summary, the responses of air-breathing fishes to ameliorate ammonia toxicity are many and varied, determined by the behaviour of the species and the nature of the environment in which it lives.

Properties and expression of Na+/K+-ATPase α-subunit isoforms in the brain of the swamp eel, Monopterus albus, which has unusually high brain ammonia tolerance

The swamp eel, Monopterus albus, can survive in high concentrations of ammonia (>75 mmol l(-1)) and accumulate ammonia to high concentrations in its brain (4.5 µmol g(-1)). Na(+)/K(+)-ATPase (Nka) is an essential transporter in brain cells, and since NH4(+) can substitute for K(+) to activate Nka, we hypothesized that the brain of M. albus expressed multiple forms of Nka α-subunits, some of which might have high K(+) specificity. Thus, this study aimed to clone and sequence the nka α-subunits from the brain of M. albus, and to determine the effects of ammonia exposure on their mRNA expression and overall protein abundance. The effectiveness of NH4(+) to activate brain Nka from M. albus and Mus musculus was also examined by comparing their Na(+)/K(+)-ATPase and Na(+)/NH4(+)-ATPase activities over a range of K(+)/NH4(+) concentrations. The full length cDNA coding sequences of three nkaα (nkaα1, nkaα3a and nkaα3b) were identified in the brain of M. albus, but nkaα2 expression was undetectable. Exposure to 50 mmol l(-1) NH4Cl for 1 day or 6 days resulted in significant decreases in the mRNA expression of nkaα1, nkaα3a and nkaα3b. The overall Nka protein abundance also decreased significantly after 6 days of ammonia exposure. For M. albus, brain Na(+)/NH4(+)-ATPase activities were significantly lower than the Na(+)/K(+)-ATPase activities assayed at various NH4(+)/K(+) concentrations. Furthermore, the effectiveness of NH4(+) to activate Nka from the brain of M. albus was significantly lower than that from the brain of M. musculus, which is ammonia-sensitive. Hence, the (1) lack of nkaα2 expression, (2) high K(+) specificity of K(+) binding sites of Nkaα1, Nkaα3a and Nkaα3b, and (3) down-regulation of mRNA expression of all three nkaα isoforms and the overall Nka protein abundance in response to ammonia exposure might be some of the contributing factors to the high brain ammonia tolerance in M. albus.

Pathophysiology of cerebral oedema in acute liver failure

Cerebral oedema is a devastating consequence of acute liver failure (ALF) and may be associated with the development of intracranial hypertension and death. In ALF, some patients may develop cerebral oedema and increased intracranial pressure but progression to life-threatening intracranial hypertension is less frequent than previously described, complicating less than one third of cases who have proceeded to coma since the advent of improved clinical care. The rapid onset of encephalopathy may be dramatic with the development of asterixis, delirium, seizures and coma. Cytotoxic and vasogenic oedema mechanisms have been implicated with a preponderance of experimental data favouring a cytotoxic mechanism. Astrocyte swelling is the most consistent neuropathological finding in humans with ALF and ammonia plays a definitive role in the development of cytotoxic brain oedema. The mechanism(s) by which ammonia induces astrocyte swelling remains unclear but glutamine accumulation within astrocytes has led to the osmolyte hypothesis. Current evidence also supports an alternate ‘Trojan horse’ hypothesis, with glutamine as a carrier of ammonia into mitochondria, where its accumulation results in oxidative stress, energy failure and ultimately astrocyte swelling. Although a complete breakdown of the blood-brain barrier is not evident in human ALF, increased permeation to water and other small molecules such as ammonia has been demonstrated resulting from subtle alterations in the protein composition of paracellular tight junctions. At present, there is no fully efficacious therapy for cerebral oedema other than liver transplantation and this reflects our incomplete knowledge of the precise mechanisms underlying this process which remain largely unknown.

High brain ammonia tolerance and down-regulation of Na+:K+:2Cl(-) Cotransporter 1b mRNA and protein expression in the brain of the Swamp Eel, Monopterus albus, exposed to environmental ammonia or terrestrial conditions

Na(+):K(+):2Cl(-) cotransporter 1 (NKCC1) has been implicated in mediating ischemia-, trauma- or ammonia-induced astrocyte swelling/brain edema in mammals. This study aimed to determine the effects of ammonia or terrestrial exposure on ammonia concentrations in the plasma and brain, and the mRNA expression and protein abundance of nkcc/Nkcc in the brain, of the swamp eel Monopterusalbus. Ammonia exposure led to a greater increase in the ammonia concentration in the brain of M. albus than terrestrial exposure. The brain ammonia concentration of M. albus reached 4.5 µmol g(-1) and 2.7 µmol g(-1) after 6 days of exposure to 50 mmol l(-1) NH4Cl and terrestrial conditions, respectively. The full cDNA coding sequence of nkcc1b from M. albus brain comprised 3276 bp and coded for 1092 amino acids with an estimated molecular mass of 119.6 kDa. A molecular characterization indicated that it could be activated through phosphorylation and/or glycosylation by osmotic and/or oxidative stresses. Ammonia exposure for 1 day or 6 days led to significant decreases in the nkcc1b mRNA expression and Nkcc1b protein abundance in the brain of M. albus. In comparison, a significant decrease in nkcc1b mRNA expression was observed in the brain of M. albus only after 6 days of terrestrial exposure, but both 1 day and 6 days of terrestrial exposure resulted in significant decreases in the protein abundance of Nkcc1b. These results are novel because it has been established in mammals that ammonia up-regulates NKCC1 expression in astrocytes and NKCC1 plays an important role in ammonia-induced astrocyte swelling and brain edema. By contrast, our results indicate for the first time that M. albus is able to down-regulate the mRNA and protein expression of nkcc1b/Nkcc1b in the brain when confronted with ammonia toxicity, which could be one of the contributing factors to its extraordinarily high brain ammonia tolerance.

Ammonia toxicity to the brain

Hyperammonemia can be caused by various acquired or inherited disorders such as urea cycle defects. The brain is much more susceptible to the deleterious effects of ammonium in childhood than in adulthood. Hyperammonemia provokes irreversible damage to the developing central nervous system: cortical atrophy, ventricular enlargement and demyelination lead to cognitive impairment, seizures and cerebral palsy. The mechanisms leading to these severe brain lesions are still not well understood, but recent studies show that ammonium exposure alters several amino acid pathways and neurotransmitter systems, cerebral energy metabolism, nitric oxide synthesis, oxidative stress and signal transduction pathways. All in all, at the cellular level, these are associated with alterations in neuronal differentiation and patterns of cell death. Recent advances in imaging techniques are increasing our understanding of these processes through detailed in vivo longitudinal analysis of neurobiochemical changes associated with hyperammonemia. Further, several potential neuroprotective strategies have been put forward recently, including the use of NMDA receptor antagonists, nitric oxide inhibitors, creatine, acetyl-L-carnitine, CNTF or inhibitors of MAPKs and glutamine synthetase. Magnetic resonance imaging and spectroscopy will ultimately be a powerful tool to measure the effects of these neuroprotective approaches.

Inflammation and hepatic encephalopathy

Hepatic encephalopathy (HE) is a neuropsychiatric syndrome associated with both acute and chronic liver dysfunction, spanning a spectrum that ranges from mild neuropsychological disturbances to coma. The central role of ammonia in the pathogenesis of HE remains incontrovertible however, there is a robust evidence base indicating the important role of inflammation in exacerbating the neurological effects of HE. Inflammation can arise directly within the brain itself as a result of deranged nitrogen and energy homeostasis, with resultant neuronal, astrocyte and microglial dysfunction. Inflammation may also originate in the peripheral circulation and exert effects on the brain indirectly, via the release of pro-inflammatory mediators which directly signal to the brain via the vagus nerve. This review summarises the data that demonstrate the synergistic relationship of inflammation and ammonia that culminates in the manifestation of HE. Sterile inflammation arising from the inflamed or necrotic liver, circulating endotoxin arising from the gut (bacterial translocation) inducing immune dysfunction, and superimposed sepsis will be comprehensively discussed. Finally, this review will provide an overview of the existing and novel treatments on the horizon which can target the inflammatory response, and how they might translate into clinical practise as therapies in the prophylaxis and treatment of HE.

The influence of sorafenib on hepatic encephalopathy and the mechanistic survey in cirrhotic rats

BACKGROUND

Sorafenib, a multikinase inhibitor that inhibits angiogenesis and carcinogenesis, has been used for patients with advanced hepatocellular carcinoma. However, sporadic cases have been reported with the development of hepatic encephalopathy (HE) after sorafenib treatment, mostly in those with cirrhosis. Liver function impairment, portal-systemic collaterals and brain oxidative stress participate in the pathogenesis of HE. The study therefore aimed to investigate the potential influences of sorafenib on HE and the relevant risk factors in cirrhotic rats.

METHODS

Liver cirrhosis was induced in Spraque-Dawley rats with common bile duct ligation (CBDL). CBDL rats received sorafenib 1 mg/kg/day or distilled water (DW) via oral gavage since the 15th day post surgery for 2 weeks. On the 28th day, after motor activities measurements, mean arterial pressure, portal pressure and heart rate were checked. Thereafter, cerebral cortex and cerebellum were dissected for oxidative stress study and blood was collected for liver biochemistry survey.

RESULTS

Sorafenib significantly reduced portal pressure (22%) and collateral shunting degree (15%) in cirrhotic rats. Alanine transaminase, aspartate transaminase, total bilirubin and ammonia were similar in sorafenib- and DW-treated groups. Motor activities were not significantly altered by sorafenib. In cerebrum, the oxidant and antioxidant substances activities were not significantly different between the two groups, whereas they were divergent in cerebellum and hippocampus.

CONCLUSION

By surveying three main aspects involved in the pathogenesis of HE, this study demonstrates that sorafenib does not increase the risk of HE in cirrhotic rats.

Oxidative and nitrosative stress in ammonia neurotoxicity

Increased ammonia accumulation in the brain due to liver dysfunction is a major contributor to the pathogenesis of hepatic encephalopathy (HE). Fatal outcome of rapidly progressing (acute) HE is mainly related to cytotoxic brain edema associated with astrocytic swelling. An increase of brain ammonia in experimental animals or treatment of cultured astrocytes with ammonia generates reactive oxygen and nitrogen species in the target tissues, leading to oxidative/nitrosative stress (ONS). In cultured astrocytes, ammonia-induced ONS is invariably associated with the increase of the astrocytic cell volume. Interrelated mechanisms underlying this response include increased nitric oxide (NO) synthesis which is partly coupled to the activation of NMDA receptors and increased generation of reactive oxygen species by NADPH oxidase. ONS and astrocytic swelling are further augmented by excessive synthesis of glutamine (Gln) which impairs mitochondrial function following its accumulation in there and degradation back to ammonia ("the Trojan horse" hypothesis). Ammonia also induces ONS in other cell types of the CNS: neurons, microglia and the brain capillary endothelial cells (BCEC). ONS in microglia contributes to the central inflammatory response, while its metabolic and pathophysiological consequences in the BCEC evolve to the vasogenic brain edema associated with HE. Ammonia-induced ONS results in the oxidation of mRNA and nitration/nitrosylation of proteins which impact intracellular metabolism and potentiate the neurotoxic effects. Simultaneously, ammonia facilitates the antioxidant response of the brain, by activating astrocytic transport and export of glutathione, in this way increasing the availability of precursors of neuronal glutathione synthesis.

Expression pattern of potential biomarker genes related to growth, ion regulation and stress in response to ammonia exposure, food deprivation and exercise in common carp (Cyprinus carpio)

Waterborne ammonia has become a persistent pollutant of aquatic habitats. During certain periods (e.g. winter), food deprivation may occur simultaneously in natural water. Additionally, under such stressful circumstances, fish may be enforced to swim at a high speed in order to catch prey, avoid predators and so on. Consequently, fish need to cope with all these stressors by altering physiological processes which in turn are controlled by their genes. In this present study, toxicogenomic analyses using real time PCR was used to characterize expression patterns of potential biomarker genes controlling growth, ion regulation and stress responses in common carp subjected to elevated ammonia (1 mg/L; Flemish water quality guideline for surface water) following periods of feeding (2% body weight) and fasting (unfed for 7 days prior to sampling). Both feeding groups of fish were exposed to high environment ammonia (HEA) for 0 h (control), 3h, 12h, 1 day, 4 days, 10 days, 21 days and 28 days, and were sampled after performing swimming at different speeds (routine versus exhaustive). Results show that the activity and expression of Na(+)/K(+)-ATPase, an important branchial ion regulatory enzyme, was increased after 4-10 days of exposure. Effect of HEA was also evident on expression patterns of other ion-regulatory hormone and receptor genes; prolactin and cortisol receptor mRNA level(s) were down-regulated and up-regulated respectively after 4, 10 and 21 days. Starvation and exhaustive swimming, the additional challenges in present study significantly further enhanced the HEA effect on the expression of these two genes. mRNA transcript of growth regulating hormone and receptor genes such as Insulin-like growth factor I, growth hormone receptor, and the thyroid hormone receptor were reduced in response to HEA and the effect of ammonia was exacerbated in starved fish, with levels that were remarkably reduced compared to fed exposed fish. However, the expression of the growth hormone gene itself was up-regulated under the same conditions. Expression of somatolactin remained unaltered. Stress representative genes, cytochrome oxidase subunit 1 showed an up-regulation in response to HEA and starvation while the mRNA level of heat shock protein 70 was increased in response to all the three stressors. The expression kinetics of the studied genes could permit to develop a "molecular biomarker system" to identify the underlying physiological processes and impact of these stressors before effects at population level occur.

Hepatic encephalopathy: An approach to its multiple pathophysiological features

Hepatic encephalopathy (HE) is a neuropsychiatric complex syndrome, ranging from subtle behavioral abnormalities to deep coma and death. Hepatic encephalopathy emerges as the major complication of acute or chronic liver failure. Multiplicity of factors are involved in its pathophysiology, such as central and neuromuscular neurotransmission disorder, alterations in sleep patterns and cognition, changes in energy metabolism leading to cell injury, an oxidative/nitrosative state and a neuroinflammatory condition. Moreover, in acute HE, a condition of imminent threat of death is present due to a deleterious astrocyte swelling. In chronic HE, changes in calcium signaling, mitochondrial membrane potential and long term potential expression, N-methyl-D-aspartate-cGMP and peripheral benzodiazepine receptors alterations, and changes in the mRNA and protein expression and redistribution in the cerebral blood flow can be observed. The main molecule indicated as responsible for all these changes in HE is ammonia. There is no doubt that ammonia, a neurotoxic molecule, triggers or at least facilitates most of these changes. Ammonia plasma levels are increased two- to three-fold in patients with mild to moderate cirrhotic HE and up to ten-fold in patients with acute liver failure. Hepatic and inter-organ trafficking of ammonia and its metabolite, glutamine (GLN), lead to hyperammonemic conditions. Removal of hepatic ammonia is a differentiated work that includes the hepatocyte, through the urea cycle, converting ammonia into GLN via glutamine synthetase. Under pathological conditions, such as liver damage or liver blood by-pass, the ammonia plasma level starts to rise and the risk of HE developing is high. Knowledge of the pathophysiology of HE is rapidly expanding and identification of focally localized triggers has led the development of new possibilities for HE to be considered. This editorial will focus on issues where, to the best of our knowledge, more research is needed in order to clarify, at least partially, controversial topics.

Moderate grade hyperammonemia induced concordant activation of antioxidant enzymes is associated with prevention of oxidative stress in the brain slices

Acute hyperammonemia (HA) induced oxidative stress in the brain is considered to play critical roles in the neuropathology of end stage hepatic encephalopathy (HE). Moderate grade HA led minimal/moderate type HE is more common in the patients with chronic liver failure. However, implication of oxygen free radical ([Formula: see text]) based oxidative mechanisms remain to be defined during moderate grade HA. This article describes profiles of all the antioxidant enzymes Vis a Vis status of oxidative stress/damage in the brain slices exposed to 0.1-1 mM ammonia, reported to exist in the brain of animals with chronic liver failure and in liver cirrhotic patients. Superoxide dismutase catalyzes the first step of antioxidant mechanism and, with concerted activity of catalase, neutralizes [Formula: see text] produced in the cells. Both these enzymes remained unchanged up to 0.2-0.3 mM ammonia, however, with significant increments (P < 0.01-0.001) in the brain slices exposed to 0.5-1 mM ammonia. This was consistent with the similar pattern of production of reactive oxygen species in the brain slices. However, level of lipid peroxidation remained unchanged throughout the ammonia treatment. Synchronized activities of glutathione peroxidase and glutathione reductase regulate the level of glutathione to maintain reducing equivalents in the cells. The activities of both these enzymes also increased significantly in the brain slices exposed to 0.5-1 mM ammonia with concomitant increments in GSH/GSSG ratio and in the levels of total and protein bound thiol. The findings suggest resistance of brain cells from ammonia induced oxidative damage during moderate grade HA due to concordant activations of antioxidant enzymes.

Antioxidant Potential of Momordica Charantia in Ammonium Chloride-Induced Hyperammonemic Rats

The present study was aimed to investigate the antioxidant potential of Momordica charantia fruit extract (MCE) in ammonium chloride-induced (AC) hyperammonemic rats. Experimental hyperammonemia was induced in adult male Wistar rats (180-200 g) by intraperitoneal injections of ammonium chloride (100 mg kg(-1) body weight) thrice a week. The effect of oral administration (thrice a week for 8 consecutive weeks) of MCE (300 mg kg(-1) body weight) on blood ammonia, plasma urea, serum liver marker enzymes and oxidative stress biomarkers in normal and experimental animals was analyzed. Hyperammonemic rats showed a significant increase in the activities of thiobarbituric acid reactive substances, hydroperoxides and liver markers (alanine transaminase, aspartate transaminase and alkaline phosphatase), and the levels of glutathione peroxidase, superoxide dismutase, catalase and reduced glutathione were decreased in the liver and brain tissues. Treatment with MCE normalized the above-mentioned changes in hyperammonemic rats by reversing the oxidant-antioxidant imbalance during AC-induced hyperammonemia, and offered protection against hyperammonemia. Our results indicate that MCE exerting the antioxidant potentials and maintaining the cellular integrity of the liver tissue could offer protection against AC-induced hyperammonemia. However, the exact underlying mechanism is yet to be investigated, and examination of the efficacy of the active constituents of the M. charantia on hyperammonemia is desirable.

Activation of neuronal nitric oxide synthase in cerebellum of chronic hepatic encephalopathy rats is associated with up-regulation of NADPH-producing pathway

Cerebellum-associated functions get affected during mild hepatic encephalopathy (MHE) in patients with chronic liver failure (CLF). Involvement of nitrosative and antioxidant factors in the pathogenesis of chronic hepatic encephalopathy is an evolving concept and needs to be defined in a true CLF animal model. This article describes profiles of NADPH-dependent neuronal nitric oxide synthase (nNOS) and those of glutathione peroxidase and glutathione reductase (GR) vis-a-vis regulation of NADPH-producing pathway in the cerebellum of CLF rats induced by administration of thioacetamide (100 mg kg⁻¹ b.w., i.p.) up to 10 days and confirming MHE on Morris water maze tests. Significant increases in the expression of nNOS protein and nitric oxide (NOx) level coincided with a similar increment in NADPH-diaphorase activity in the cerebellum of CLF rats. Glutathione peroxidase and GR utilize NADPH to regenerate reduced glutathione (GSH) in the cells. Both these enzymes and GSH level were found to be static and thus suggested efficient turnover of GSH in the cerebellum of MHE rats. Relative levels of glucose-6-phosphate dehydrogenase (G6PD) vs. phosphofructokinase 2 (PFK2) determine the rate of pentose phosphate pathway (PPP) responsible to synthesize NADPH. The cerebellum of CLF rats showed overactivation of G6PD with a significant decline in the expression of PFK2 and thus suggested activation of PPP in the cerebellum during MHE. It is concluded that concordant activations of PPP and nNOS in cerebellum of MHE rats could be associated with the implication of NOx in the pathogenesis of MHE.