Glutamine: a Trojan horse in ammonia neurotoxicity

Short-term urea cycle inhibition in rat liver cells induced by polyethylene glycol

We illuminate the biological effect of PEG on a specific cellular pathway: the urea cycle at a molecular level.

Hyperammonaemic encephalopathy following an uncomplicated surgery

A 59-year-old woman who underwent an uncomplicated exploratory laparotomy, adhesiolysis, small bowel resection and anterolateral thigh flap had a complicated postoperative period characterised by wound dehiscence and poor nutritional intake. 29 days postoperatively, a tremor developed in her upper limbs associated with weakness. Her Glasgow Coma Scale (GCS) fell to 4 and she was transferred to the intensive care unit. The patient was reviewed by multiple specialists and multiple differentials were considered and eliminated. Eventually, investigations revealed hyperammonaemic encephalopathy, being a result of low arginine and potentially small intestinal bacterial overgrowth. Following treatment with sodium benzoate, sodium phenylbutyrate and arginine along with haemodialysis and rifaximin, GCS and hyperammonaemia rapidly improved. She was stepped down to surgical high-dependency unit, continued arginine therapy with total parenteral nutrition and percutaneous endoscopic gastrostomy feeds. She was discharged with regular follow-up from surgeons and biochemistry and continues oral arginine therapy.

Evaluation and Management of Hepatic Encephalopathy: Current Status and Future Directions

Hepatic encephalopathy is a spectrum of neurocognitive manifestations often seen in patients with liver injury or rarely in patients with portosystemic shunting without liver injury. It can be divided into minimal (covert) hepatic encephalopathy and overt hepatic encephalopathy, depending on the severity. Patients with hepatic encephalopathy have compromised clinical outcomes, decreased quality of life, and increased healthcare utilization, often resulting in a heavy financial and personal burden on caregivers. The diagnosis remains largely clinical, with the exclusion of possible other causes for the altered mental status. Current treatment strategies include nonabsorbable disaccharides and antibiotics. This review will focus on the diagnosis, management and clinical impact of hepatic encephalopathy.

Hepatic Encephalopathy: An Update on the Pathophysiology and Therapeutic Options

Hepatic encephalopathy is a spectrum of reversible neuropsychiatric abnormalities, seen in patients with liver dysfunction and/or portosystemic shunting. One of the most debilitating complications of cirrhosis, encephalopathy affects 30-45% of cirrhotics. In addition to significantly affecting the lives of patients and their caregivers, it is also associated with increased morbidity and mortality as well as significant utilization of health care resources. In this paper, we provide an overview on the pathophysiology, diagnosis, management and newer therapies of hepatic encephalopathy.

Commiphora molmol Modulates Glutamate-Nitric Oxide-cGMP and Nrf2/ARE/HO-1 Pathways and Attenuates Oxidative Stress and Hematological Alterations in Hyperammonemic Rats

Hyperammonemia is a serious complication of liver disease and may lead to encephalopathy and death. This study investigated the effects of Commiphora molmol resin on oxidative stress, inflammation, and hematological alterations in ammonium chloride- (NH₄Cl-) induced hyperammonemic rats, with an emphasis on the glutamate-NO-cGMP and Nrf2/ARE/HO-1 signaling pathways. Rats received NH₄Cl and C. molmol for 8 weeks. NH₄Cl-induced rats showed significant increase in blood ammonia, liver function markers, and tumor necrosis factor-alpha (TNF-α). Concurrent supplementation of C. molmol significantly decreased circulating ammonia, liver function markers, and TNF-α in hyperammonemic rats. C. molmol suppressed lipid peroxidation and nitric oxide and enhanced the antioxidant defenses in the liver, kidney, and cerebrum of hyperammonemic rats. C. molmol significantly upregulated Nrf2 and HO-1 and decreased glutamine and nitric oxide synthase, soluble guanylate cyclase, and Na⁺/K⁺-ATPase expression in the cerebrum of NH₄Cl-induced hyperammonemic rats. Hyperammonemia was also associated with hematological and coagulation system alterations. These alterations were reversed by C. molmol. Our findings demonstrated that C. molmol attenuates ammonia-induced liver injury, oxidative stress, inflammation, and hematological alterations. This study points to the modulatory effect of C. molmol on glutamate-NO-cGMP and Nrf2/ARE/HO-1 pathways in hyperammonemia. Therefore, C. molmol might be a promising protective agent against hyperammonemia.

Effect of Continuous Renal Replacement Therapy on Outcome in Pediatric Acute Liver Failure

OBJECTIVES

To establish the effect of continuous renal replacement therapy on outcome in pediatric acute liver failure.

DESIGN

Retrospective cohort study.

SETTING

Sixteen-bed PICU in a university-affiliated tertiary care hospital and specialist liver centre.

PATIENTS

All children (0-18 yr) admitted to PICU with pediatric acute liver failure between January 2003 and December 2013.

INTERVENTIONS

Children with pediatric acute liver failure were managed according to a set protocol. The guidelines for continuous renal replacement therapy in pediatric acute liver failure were changed in 2011 following preliminary results to indicate the earlier use of continuous renal replacement therapy for both renal dysfunction and detoxification.

MEASUREMENTS AND MAIN RESULTS

Of 165 children admitted with pediatric acute liver failure, 136 met the inclusion criteria and 45 of these received continuous renal replacement therapy prior to transplantation or recovery. Of the children managed with continuous renal replacement therapy, 26 (58%) survived: 19 were successfully bridged to liver transplantation and 7 spontaneously recovered. Cox proportional hazards regression model clearly showed reducing hyperammonemia by 48 hours after initiating continuous renal replacement therapy significantly improved survival (HR, 1.04; 95% CI, 1.013-1.073; p = 0.004). On average, for every 10% decrease in ammonia from baseline at 48 hours, the likelihood of survival increased by 50%. Time to initiate continuous renal replacement therapy from PICU admission was lower in survivors compared to nonsurvivors (HR, 0.96; 95% CI, 0.916-1.007; p = 0.095). Change in practice to initiate early and high-dose continuous renal replacement therapy led to increased survival with maximum effect being visible in the first 14 days (HR, 3; 95% CI, 1.0-10.3; p = 0.063). Among children with pediatric acute liver failure who did not receive a liver transplant, use of continuous renal replacement therapy significantly improved survival (HR, 4; 95% CI, 1.5-11.6; p = 0.006).

CONCLUSION

Continuous renal replacement therapy can be used successfully in critically ill children with pediatric acute liver failure to provide stability and bridge to transplantation. Inability to reduce ammonia by 48 hours confers poor prognosis. Continuous renal replacement therapy should be considered at an early stage to help prevent further deterioration and buy time for potential spontaneous recovery or bridge to liver transplantation.

Accumulation of lactate in the rat brain during hyperammonaemia is not associated with impaired mitochondrial respiratory capacity

In acute liver failure (ALF) cerebral oedema and high intracranial pressure (ICP) are potentially deadly complications. Astrocytes cultured in ammonia have shown mitochondrial dysfunction and in rat models of liver failure, de novo lactate production in the brain has been observed and has led to a hypothesis of compromised brain metabolism during ALF. In contrast, normal lactate levels are found in cerebral microdialysate of ALF patients and the oxygen: glucose ratio of cerebral metabolic rates remains normal. To investigate this inconsistency we studied the mitochondrial function in brain tissue with respirometry in animal models of hyperammonaemia. Wistar rats with systemic inflammation induced by lipopolysaccharide or liver insufficiency induced by 90% hepatectomy were given ammonium or sodium acetate for 120 min. A cerebral cortex homogenate was studied with respirometry and substrates of the citric acid cycle, uncouplers and inhibitors of the mitochondrial complexes were successively added to investigate the mitochondrial function in detail. In a separate dose-response experiment cortex from healthy rats was incubated for 120 min in ammonium acetate in concentrations up to 80 mM prior to respirometry. Hyperammonaemia was associated with elevated ICP and increased tissue lactate concentration. No difference between groups was found in total respiratory capacity or the function of individual mitochondrial complexes. Ammonium in concentrations of 40 and 80 mM reduced the respiratory capacity in vitro. In conclusion, acute hyperammonaemia leads to elevated ICP and cerebral lactate accumulation. We found no indications of impaired oxidative metabolism in vivo but only in vitro at extreme concentrations of ammonium.

Validation study associating glutaminase promoter variations with hepatic encephalopathy in East Asian populations

BACKGROUND AND AIM

In a recent study, microsatellite variations (GCA tandem repeats) in the promoter region of the (kidney-type) glutaminase gene were associated with the development of hepatic encephalopathy (HE) in Spanish patients with cirrhosis. The objective of this study was to validate the relation between microsatellite variations in the glutaminase promoter region and the development of overt HE in Korean patients with liver cirrhosis.

METHODS

We performed a prospective cohort study of 154 cirrhotic patients who underwent a glutaminase microsatellite study without previous overt HE history at baseline. The primary end point was the first episode of overt HE. The microsatellite length was categorized into three groups based on its repeated number, with a cutoff value of 14; 65 (42.2%), 70 (45.5%), and 19 (12.3%) patients had the short-short, short-long, and long-long alleles, respectively.

RESULTS

Over a median 3.5 years of follow-up (range = 0.1-4.4), overt HE developed in 28 patients (18.2%). The 3-year cumulative incidence of overt HE was 18.4%. Multivariate Cox model indicated that past hepatocellular carcinoma history, alcoholic etiology for cirrhosis, higher Model for End-Stage Liver Disease scores and their deterioration, and serum ammonium levels were independently associated with HE development. However, microsatellite length was not associated with the development of overt HE.

CONCLUSIONS

In Korean patients with cirrhosis, microsatellite variations in the glutaminase promoter region were not associated with development of overt HE. Thus, additional studies are needed to identify other genetic factors related to glutaminase activity in Asians with overt HE.

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

INTRODUCTION

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

Mitochondrial function in Müller cells - Does it matter?

Growing evidence suggests that mitochondrial dysfunction might play a key role in the pathogenesis of age-related neurodegenerative inner retinal diseases such as diabetic retinopathy and glaucoma. Therefore, the present review provides a perspective on the impact of functional mitochondria in the most predominant glial cells of the retina, the Müller cells. Müller cells span the entire thickness of the neuroretina and are in close proximity to retinal cells including the retinal neurons that provides visual signaling to the brain. Among multiple functions, Müller cells are responsible for the removal of neurotransmitters, buffering potassium, and providing neurons with essential metabolites. Thus, Müller cells are responsible for a stable metabolic dialogue in the inner retina and their crucial role in supporting retinal neurons is indisputable. Müller cell functions require considerable energy production and previous literature has primarily emphasized glycolysis as the main energy provider. However, recent studies highlight the need of mitochondrial ATP production to upheld Müller cell functions. Therefore, the present review aims to provide an overview of the current evidence on the impact of mitochondrial functions in Müller cells.

Nitrosative Stress-Induced Disruption of Baroreflex Neural Circuits in a Rat Model of Hepatic Encephalopathy: A DTI Study

The onset of hepatic encephalopathy (HE) in liver failure is associated with high mortality; the underlying mechanism is undecided. Here we report that in an acute liver failure model employing intraperitoneal administration of thioacetamide in Sprague-Dawley rats, diffusion weighted imaging revealed a progressive reduction in apparent diffusion coefficient in the brain stem. Diffusion tensor imaging further showed that the connectivity between nucleus tractus solitarii (NTS), the terminal site of baroreceptor afferents in brain stem and rostral ventrolateral medulla (RVLM), the origin of sympathetic innervation of blood vessels, was progressively disrupted until its disappearance, coincidental with the irreversible cessation of baroreflex-mediated sympathetic vasomotor tone signifying clinically the occurrence of brain death. In addition, superoxide, nitric oxide, peroxynitrite and ammonia levels in the NTS or RVLM were elevated, alongside swelling of astroctytes. A scavenger of peroxynitrite, but not an antioxidant, delivered intracisternally reversed all these events. We conclude that nitrosative stress because of augmented peroxynitrite related to accumulation of ammonia and swelling of astrocytes in the NTS or RVLM, leading to cytotoxic edema in the brain stem and severance of the NTS-RVLM connectivity, underpins the defunct baroreflex-mediated sympathetic vasomotor tone that accounts for the high mortality associated with HE.

Glutamate and α-ketoglutarate: key players in glioma metabolism

Glioblastoma multiforme (GBM), or grade IV astrocytoma, is the most common type of primary brain tumor. It has a devastating prognosis with a 2-year-overall survival rate of only 26 % after standard treatment, which includes surgery, radiation, and adjuvant chemotherapy with temozolomide. Also lower grade gliomas are difficult to treat, because they diffusely spread into the brain, where extensive removal of tissue is critical. Better understanding of the cancer's biology is a key for the development of more effective therapy approaches. The discovery of isocitrate dehydrogenase (IDH) mutations in leukemia and glioma drew attention to specific metabolic aberrations in IDH-mutant gliomas. In the center of the metabolic alterations is α-ketoglutarate (αKG), an intermediate metabolite in the tricarboxylic acid (TCA) cycle, and the associated amino acid glutamate (Glu). This article highlights the role of these metabolites in glioma energy and lipid production and indicates possible weak spots of IDH-mutant and IDH-wt gliomas.

Neurologic Complications in the Intensive Care Unit

Complications involving the central and peripheral nervous system are frequently encountered in critically ill patients. All components of the neuraxis can be involved including the brain, spinal cord, peripheral nerves, neuromuscular junction, and muscles. Neurologic complications adversely impact outcome and length of stay. These complications can be related to underlying critical illness, pre-existing comorbid conditions, and commonly used and life-saving procedures and medications. Familiarity with the myriad neurologic complications that occur in the intensive care unit can facilitate their timely recognition and treatment. Additionally, awareness of treatment-related neurologic complications may inform decision-making, mitigate risk, and improve outcomes.

What we know: the inflammatory basis of hepatic encephalopathy

Central Nervous System (CNS) degeneration appearing in patients with cirrhosis is responsible for cognitive and persistent motor impairments that lead to an important impact on life quality. Brain injury affects certain areas of the CNS that might affect two types of cells: neurons and astrocytes. The process leading to brain injury could be induced by portosystemic shunting accompanied by hyperammonemia and by the activation of peripheral inflammation, manifested as episodic encephalopathy. Hyperammonemia combined with a decrease on the BCA/AAA ratio induces alterations of energetic metabolism and the formation of free radicals in the CNS. This process would be stimulated by the activation of peripheral inflammatory mediators that could act on receptors of the blood brain barrier such as TLR4, activating inflammatory responses in the CNS. As a result, a persistent activation of microglia and an irreversible neuronal and astrocytic injury would be induced. A new knowledge of the mechanisms leading to brain injury in cirrhosis would develop protective strategies to correct changes of nitrogen metabolism and inflammation.

Neuroinflammation in liver disease: sessional talks from ISHEN

At the recent ISHEN ('International Symposium of Hepatic Encephalopathy & Nitrogen Metabolism') conference in London, a whole session was dedicated to our increasing awareness of the importance of inflammation in the brain - termed 'neuroinflammation', in the development of Hepatic Encephalopathy (HE) - the neurological manifestations of advanced liver disease. In this review our ISHEN speakers further discuss the content of their sessional presentations and more broadly we discuss our understanding of the role of neuroinflammation in HE pathogenesis.

Naringin regulates glutamate-nitric oxide cGMP pathway in ammonium chloride induced neurotoxicity

Naringin, plant bioflavonoid extracted mainly from grapefruit and other related citrus species. This study was designed to assess the neuroprotective effect of naringin on ammonium chloride (NH₄Cl) induced hyperammonemic rats. Experimental hyperammonemia was induced by intraperitonial injection (i.p) of NH₄Cl (100mg/kg body weight (b.w.)) thrice a week for 8 consecutive weeks. Hyperammonemic rats were treated with naringin (80mg/kg b.w.) via oral gavage. Naringin administration drastically restored the levels of blood ammonia, plasma urea, nitric oxide (NO), glutamate, glutamine, lipid peroxidation, lipid profile, activities of liver marker enzymes, antioxidant status and sodium/potassium-ATPase (Na⁺/K⁺-ATPase). In addition, naringin supplementation reverted back the pathological changes of liver, brain and kidney tissues, the expressions of Glutamine synthetase (GS), Na⁺/K⁺-ATPase, neuronal nitric oxide (nNOS) and soluble guanylate cyclase (sGC) in hyperammonemic rats. Hence, this study suggested that nargingin exhibited their protective effect against NH₄Cl induced toxicity via enhancing the activities of antioxidant enzymes and inhibiting the lipid peroxidation process. Take together, this study provides data that naingin effectively reduced neurotoxicity by attenuating hyperammonemia, suggesting that naringin act as a potential therapeutic agent to treat hyperammonemic rats.

Impact of ornithine phenylacetate (OCR-002) in lowering plasma ammonia after upper gastrointestinal bleeding in cirrhotic patients

BACKGROUND

Ornithine phenylacetate (OP) has been proven effective in lowering ammonia plasma levels in animals, and to be well tolerated in cirrhotic patients. A trial to assess OP efficacy in lowering plasma ammonia levels versus placebo in cirrhotic patients after an upper gastrointestinal bleeding was performed. The primary outcome was a decrease in venous plasma ammonia at 24 hours.

METHODS

A total of 38 consecutive cirrhotic patients were enrolled within 24 hours of an upper gastrointestinal bleed. Patients were randomized (1:1) to receive OP (10 g/day) or glucosaline for 5 days.

RESULTS

The primary outcome was not achieved. A progressive decrease in ammonia was observed in both groups, being slightly greater in the OP group, with significant differences only at 120 hours. The subanalysis according to Child-Pugh score showed a statistically significant ammonia decrease in Child-Pugh C-treated patients at 36 hours, as well as in the time-normalized area under the curve (TN-AUC) 0-120 hours in the OP group [40.16 μmol/l (37.7-42.6); median (interquartile range) (IQR)] versus placebo group [65.5 μmol/l (54-126);p = 0.036]. A decrease in plasma glutamine levels was observed in the treated group compared with the placebo group, and was associated with the appearance of phenylacetylglutamine in urine. Adverse-event frequency was similar in both groups. No differences in hepatic encephalopathy incidence were observed.

CONCLUSIONS

OP failed to significantly decrease plasma ammonia at the given doses (10 g/day). Higher doses of OP might be required in Child-Pugh A and B patients. OP appeared well tolerated.

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.

Minireview on Glutamine Synthetase Deficiency, an Ultra-Rare Inborn Error of Amino Acid Biosynthesis

Glutamine synthetase (GS) is a cytosolic enzyme that produces glutamine, the most abundant free amino acid in the human body. Glutamine is a major substrate for various metabolic pathways, and is thus an important factor for the functioning of many organs; therefore, deficiency of glutamine due to a defect in GS is incompatible with normal life. Mutations in the human GLUL gene (encoding for GS) can cause an ultra-rare recessive inborn error of metabolism—congenital glutamine synthetase deficiency. This disease was reported until now in only three unrelated patients, all of whom suffered from neonatal onset severe epileptic encephalopathy. The hallmark of GS deficiency in these patients was decreased levels of glutamine in body fluids, associated with chronic hyperammonemia. This review aims at recapitulating the clinical history of the three known patients with congenital GS deficiency and summarizes the findings from studies done along with the work-up of these patients. It is the aim of this paper to convince the reader that (i) this disorder is possibly underdiagnosed, since decreased concentrations of metabolites do not receive the attention they deserve; and (ii) early detection of GS deficiency may help to improve the outcome of patients who could be treated early with metabolites that are lacking in this condition.

Ammonium accumulation is a primary effect of 2-methylcitrate exposure in an in vitro model for brain damage in methylmalonic aciduria

Using 3D organotypic rat brain cell cultures in aggregates we recently identified 2-methylcitrate (2-MCA) as the main toxic metabolite for developing brain cells in methylmalonic aciduria. Exposure to 2-MCA triggered morphological changes and apoptosis of brain cells. This was accompanied by increased ammonium and decreased glutamine levels. However, the sequence and causal relationship between these phenomena remained unclear. To understand the sequence and time course of pathogenic events, we exposed 3D rat brain cell aggregates to different concentrations of 2-MCA (0.1, 0.33 and 1.0mM) from day in vitro (DIV) 11 to 14. Aggregates were harvested at different time points from DIV 12 to 19. We compared the effects of a single dose of 1mM 2-MCA administered on DIV 11 to the effects of repeated doses of 1mM 2-MCA. Pan-caspase inhibitors Z-VAD FMK or Q-VD-OPh were used to block apoptosis. Ammonium accumulation in the culture medium started within few hours after the first 2-MCA exposure. Morphological changes of the developing brain cells were already visible after 17h. The highest rate of cleaved caspase-3 was observed after 72h. A dose-response relationship was observed for all effects. Surprisingly, a single dose of 1mM 2-MCA was sufficient to induce all of the biochemical and morphological changes in this model. 2-MCA-induced ammonium accumulation and morphological changes were not prevented by concomitant treatment of the cultures with pan-caspase inhibitors Z-VAD FMK or Q-VD-OPh: ammonium increased rapidly after a single 1mM 2-MCA administration even after apoptosis blockade. We conclude that following exposure to 2-MCA, ammonium production in brain cell cultures is an early phenomenon, preceding cell degeneration and apoptosis, and may actually be the cause of the other changes observed. The fact that a single dose of 1mM 2-MCA is sufficient to induce deleterious effects over several days highlights the potential damaging effects of even short-lasting metabolic decompensations in children affected by methylmalonic aciduria.

Nitric oxide mediates effects of acute, not chronic, naltrexone on LPS-induced hepatic encephalopathy in cirrhotic rats

Recent studies suggest endogenous opioids and nitric oxide (NO) are involved in the pathophysiology of hepatic encephalopathy (HE). In this study, the interaction between the opioid receptor antagonist and NO was investigated on lipopolysaccharide (LPS)-induced HE in cirrhotic rats. Male rats were divided in the sham- and bile duct ligation (BDL)-operated groups. Animals were treated with saline; naltrexone (10 mg/kg, i.p.); or L-NAME (3 mg/kg, i.p.), alone or in combination with naltrexone. To induce HE, LPS (1 mg/kg, i.p.) was injected 1 h after the final drug treatment. HE scoring, hepatic histology, and plasma NO metabolites levels and mortality rate were recorded. Deteriorated level of consciousness and mortality after LPS administration significantly ameliorated following both acute and chronic treatment with naltrexone in cirrhotic rats. However, acute and chronic administration of L-NAME did not change HE scores in cirrhotic rats. The effects of acute but not chronic treatment of naltrexone on HE parameters were reversed by L-NAME. Plasma NO_x concentrations elevated in BDL rats, which were decreased after acute and chronic treatment by naltrexone or L-NAME, significantly. We suggest both acute and chronic treatment with naltrexone improved LPS-induced HE. But, only acute treatment with naltrexone may affect through NO pathway.

Neurotoxicity of Ammonia

Abnormal liver function has dramatic effects on brain functions. Hyperammonemia interferes profoundly with brain metabolism, astrocyte volume regulation, and in particular mitochondrial functions. Gene expression in the brain and excitatory and inhibitory neurotransmission circuits are also affected. Experiments with a number of pertinent animal models have revealed several potential mechanisms which could underlie the pathological phenomena occurring in hepatic encephalopathy.

Multifactorial Effects on Different Types of Brain Cells Contribute to Ammonia Toxicity

Effects of ammonia on astrocytes play a major role in hepatic encephalopathy, acute liver failure and other diseases caused by increased arterial ammonia concentrations (e.g., inborn errors of metabolism, drug or mushroom poisoning). There is a direct correlation between arterial ammonia concentration, brain ammonia level and disease severity. However, the pathophysiology of hyperammonemic diseases is disputed. One long recognized factor is that increased brain ammonia triggers its own detoxification by glutamine formation from glutamate. This is an astrocytic process due to the selective expression of the glutamine synthetase in astrocytes. A possible deleterious effect of the resulting increase in glutamine concentration has repeatedly been discussed and is supported by improvement of some pathologic effects by GS inhibition. However, this procedure also inhibits a large part of astrocytic energy metabolism and may prevent astrocytes from responding to pathogenic factors. A decrease of the already low glutamate concentration in astrocytes due to increased synthesis of glutamine inhibits the malate-aspartate shuttle and energy metabolism. A more recently described pathogenic factor is the resemblance between NH₄⁺ and K⁺ in their effects on the Na⁺,K⁺-ATPase and the Na⁺,K⁺, 2 Cl^- and water transporter NKCC1. Stimulation of the Na⁺,K⁺-ATPase driven NKCC1 in both astrocytes and endothelial cells is essential for the development of brain edema. Na⁺,K⁺-ATPase stimulation also activates production of endogenous ouabains. This leads to oxidative and nitrosative damage and sensitizes NKCC1. Administration of ouabain antagonists may accordingly have therapeutic potential in hyperammonemic diseases.

Postictal ammonia as a biomarker for electrographic convulsive seizures: A prospective study

OBJECTIVE

Transient hyperammonemia (THA) was reported to follow generalized convulsions without sufficient evidence to confirm the epileptic nature of those events. We aimed to determine if postictal THA can differentiate between different types of events as confirmed electroencephalographically using video-electroencephalography (vEEG) monitoring.

METHODS

In our prospective cohort, we screened all consented adults (>18 years) admitted to the epilepsy monitoring unit. Ammonia was checked at baseline, within 60 min of the event (for all patients) and 24 h after event (whenever possible). Patients were grouped into generalized convulsive seizures (GCS), psychogenic nonepileptic seizures with convulsions (PNES-C), or focal seizures (FS) based on vEEG. Data were analyzed using descriptive statistics and parametric/nonparametric methods.

RESULTS

Of 78 patients enrolled, 13 had GCS, 8 had FS, and 9 had PNES-C. The groups were different with regard to gender (p = 0.04) and baseline ammonia (p = 0.02), but not age. The change in ammonia postictally from baseline was significantly different among the three groups (p = 0.004). The area under the receiver operator characteristic (ROC) curve for postictal ammonia to distinguish GCS from other groups was 0.88 (95% confidence interval [CI] 0.69-0.96) suggesting ammonia to be a good test differentiating epileptic GCS from other events. An ammonia level of ≥80 μmol/L correctly classified 80% of our patients (sensitivity 53.9%, specificity 100%).

SIGNIFICANCE

Our results provide objective evidence for the association between THA and GCS seizures utilizing vEEG monitoring, and a basis for future studies to determine the role of postictal ammonia as an inexpensive diagnostic test to diagnose GCS.

Clinical aspects of urea cycle dysfunction and altered brain energy metabolism on modulation of glutamate receptors and transporters in acute and chronic hyperammonemia

In living organisms, nitrogen arise primarily as ammonia (NH3) and ammonium (NH4(+)), which is a main component of the nucleic acid pool and proteins. Although nitrogen is essential for growth and maintenance in animals, but when the nitrogenous compounds exceeds the normal range which can quickly lead to toxicity and death. Urea cycle is the common pathway for the disposal of excess nitrogen through urea biosynthesis. Hyperammonemia is a consistent finding in many neurological disorders including congenital urea cycle disorders, reye's syndrome and acute liver failure leads to deleterious effects. Hyperammonemia and liver failure results in glutamatergic neurotransmission which contributes to the alteration in the function of the glutamate-nitric oxide-cGMP pathway, modulates the important cerebral process. Even though ammonia is essential for normal functioning of the central nervous system (CNS), in particular high concentrations of ammonia exposure to the brain leads to the alterations of glutamate transport by the transporters. Several glutamate transporters have been recognized in the central nervous system and each has a unique physiological property and distribution. The loss of glutamate transporter activity in brain during acute liver failure and hyperammonemia is allied with increased extracellular brain glutamate concentrations which may be conscientious for the cerebral edema and ultimately cell death.

Molecular pathophysiology of cerebral edema

Advancements in molecular biology have led to a greater understanding of the individual proteins responsible for generating cerebral edema. In large part, the study of cerebral edema is the study of maladaptive ion transport. Following acute CNS injury, cells of the neurovascular unit, particularly brain endothelial cells and astrocytes, undergo a program of pre- and post-transcriptional changes in the activity of ion channels and transporters. These changes can result in maladaptive ion transport and the generation of abnormal osmotic forces that, ultimately, manifest as cerebral edema. This review discusses past models and current knowledge regarding the molecular and cellular pathophysiology of cerebral edema.

The Measurement of Ammonia in Human Breath and its Potential in Clinical Diagnostics

Ammonia is an important component of metabolism and is involved in many physiological processes. During normal physiology, levels of blood ammonia are between 11 and 50 µM. Elevated blood ammonia levels are associated with a variety of pathological conditions such as liver and kidney dysfunction, Reye's syndrome and a variety of inborn errors of metabolism including urea cycle disorders (UCD), organic acidaemias and hyperinsulinism/hyperammonaemia syndrome in which ammonia may reach levels in excess of 1 mM. It is highly neurotoxic and so effective measurement is critical for assessing and monitoring disease severity and treatment. Ammonia is also a potential biomarker in exercise physiology and studies of drug metabolism. Current ammonia testing is based on blood sampling, which is inconvenient and can be subject to significant analytical errors due to the quality of the sample draw, its handling and preparation for analysis. Blood ammonia is in gaseous equilibrium with the lungs. Recent research has demonstrated the potential use of breath ammonia as a non-invasive means of measuring systemic ammonia. This requires measurement of ammonia in real breath samples with associated temperature, humidity and gas characteristics at concentrations between 50 and several thousand parts per billion. This review explores the diagnostic applications of ammonia measurement and the impact that the move from blood to breath analysis could have on how these processes and diseases are studied and managed.

Carnosine Reduces Oxidative Stress and Reverses Attenuation of Righting and Postural Reflexes in Rats with Thioacetamide-Induced Liver Failure

Cerebral oxidative stress (OS) contributes to the pathogenesis of hepatic encephalopathy (HE). Existing evidence suggests that systemic administration of l-histidine (His) attenuates OS in brain of HE animal models, but the underlying mechanism is complex and not sufficiently understood. Here we tested the hypothesis that dipeptide carnosine (β-alanyl-l-histidine, Car) may be neuroprotective in thioacetamide (TAA)-induced liver failure in rats and that, being His metabolite, may mediate the well documented anti-OS activity of His. Amino acids [His or Car (100 mg/kg)] were administrated 2 h before TAA (i.p., 300 mg/kg 3× in 24 h intervals) injection into Sprague–Dawley rats. The animals were thus tested for: (i) brain prefrontal cortex and blood contents of Car and His, (ii) amount of reactive oxygen species (ROS), total antioxidant capacity (TAC), GSSG/GSH ratio and thioredoxin reductase (TRx) activity, and (iii) behavioral changes (several models were used, i.e. tests for reflexes, open field, grip test, Rotarod). Brain level of Car was reduced in TAA rats, and His administration significantly elevated Car levels in control and TAA rats. Car partly attenuated TAA-induced ROS production and reduced GSH/GSSG ratio, whereas the increase of TRx activity in TAA brain was not significantly modulated by Car. Further, Car improved TAA-affected behavioral functions in rats, as was shown by the tests of righting and postural reflexes. Collectively, the results support the hypothesis that (i) Car may be added to the list of neuroprotective compounds of therapeutic potential on HE and that (ii) Car mediates at least a portion of the OS-attenuating activity of His in the setting of TAA-induced liver failure.

The Glutamate-Glutamine Cycle in Epilepsy

Epilepsy is a complex, multifactorial disease characterized by spontaneous recurrent seizures and an increased incidence of comorbid conditions such as anxiety, depression, cognitive dysfunction, and sudden unexpected death. About 70 million people worldwide are estimated to suffer from epilepsy, and up to one-third of all people with epilepsy are expected to be refractory to current medications. Development of more effective and specific antiepileptic interventions is therefore requisite. Perturbations in the brain's glutamate-glutamine cycle, such as increased extracellular levels of glutamate, loss of astroglial glutamine synthetase, and changes in glutaminase and glutamate dehydrogenase, are frequently encountered in patients with epilepsy. Hence, manipulations of discrete glutamate-glutamine cycle components may represent novel approaches to treat the disease. The goal of his review is to discuss some of the glutamate-glutamine cycle components that are altered in epilepsy, particularly neurotransmitters and metabolites, enzymes, amino acid transporters, and glutamate receptors. We will also review approaches that potentially could be used in humans to target the glutamate-glutamine cycle. Examples of such approaches are treatment with glutamate receptor blockers, glutamate scavenging, dietary intervention, and hypothermia.

Volume-regulated anion channel--a frenemy within the brain

The volume-regulated anion channel (VRAC) is a ubiquitously expressed yet highly enigmatic member of the superfamily of chloride/anion channels. It is activated by cellular swelling and mediates regulatory cell volume decrease in a majority of vertebrate cells, including those in the central nervous system (CNS). In the brain, besides its crucial role in cellular volume regulation, VRAC is thought to play a part in cell proliferation, apoptosis, migration, and release of physiologically active molecules. Although these roles are not exclusive to the CNS, the relative significance of VRAC in the brain is amplified by several unique aspects of its physiology. One important example is the contribution of VRAC to the release of the excitatory amino acid neurotransmitters glutamate and aspartate. This latter process is thought to have impact on both normal brain functioning (such as astrocyte-neuron signaling) and neuropathology (via promoting the excitotoxic death of neuronal cells in stroke and traumatic brain injury). In spite of much work in the field, the molecular nature of VRAC remained unknown until less than 2 years ago. Two pioneer publications identified VRAC as the heterohexamer formed by the leucine-rich repeat-containing 8 (LRRC8) proteins. These findings galvanized the field and are likely to result in dramatic revisions to our understanding of the place and role of VRAC in the brain, as well as other organs and tissues. The present review briefly recapitulates critical findings in the CNS and focuses on anticipated impact on the LRRC8 discovery on further progress in neuroscience research.

In Vivo NMR Studies of the Brain with Hereditary or Acquired Metabolic Disorders

Metabolic disorders, whether hereditary or acquired, affect the brain, and abnormalities of the brain are related to cellular integrity; particularly in regard to neurons and astrocytes as well as interactions between them. Metabolic disturbances lead to alterations in cellular function as well as microscopic and macroscopic structural changes in the brain with diabetes, the most typical example of metabolic disorders, and a number of hereditary metabolic disorders. Alternatively, cellular dysfunction and degeneration of the brain lead to metabolic disturbances in hereditary neurological disorders with neurodegeneration. Nuclear magnetic resonance (NMR) techniques allow us to assess a range of pathophysiological changes of the brain in vivo. For example, magnetic resonance spectroscopy detects alterations in brain metabolism and energetics. Physiological magnetic resonance imaging (MRI) detects accompanying changes in cerebral blood flow related to neurovascular coupling. Diffusion and T1/T2-weighted MRI detect microscopic and macroscopic changes of the brain structure. This review summarizes current NMR findings of functional, physiological and biochemical alterations within a number of hereditary and acquired metabolic disorders in both animal models and humans. The global view of the impact of these metabolic disorders on the brain may be useful in identifying the unique and/or general patterns of abnormalities in the living brain related to the pathophysiology of the diseases, and identifying future fields of inquiry.

The Mitochondrial Permeability Transition Pore: Channel Formation by F-ATP Synthase, Integration in Signal Transduction, and Role in Pathophysiology

The mitochondrial permeability transition (PT) is a permeability increase of the inner mitochondrial membrane mediated by a channel, the permeability transition pore (PTP). After a brief historical introduction, we cover the key regulatory features of the PTP and provide a critical assessment of putative protein components that have been tested by genetic analysis. The discovery that under conditions of oxidative stress the F-ATP synthases of mammals, yeast, and Drosophila can be turned into Ca(2+)-dependent channels, whose electrophysiological properties match those of the corresponding PTPs, opens new perspectives to the field. We discuss structural and functional features of F-ATP synthases that may provide clues to its transition from an energy-conserving into an energy-dissipating device as well as recent advances on signal transduction to the PTP and on its role in cellular pathophysiology.

Reduced cortical thickness in patients with acute-on-chronic liver failure due to non-alcoholic etiology

Background

Acute-on-chronic liver failure (ACLF) is a form of liver disease with high short-term mortality. ACLF offers considerable potential to affect the cortical areas by significant tissue injury due to loss of neurons and other supporting cells. We measured changes in cortical thickness and metabolites profile in ACLF patients following treatment, and compared it with those of age matched healthy volunteers.

Methods

For the cortical thickness analysis we performed whole brain high resolution T1-weighted magnetic resonance imaging (MRI) on 15 ACLF and 10 healthy volunteers at 3T clinical MR scanner. Proton MR Spectroscopy (¹H MRS) was also performed to measure level of altered metabolites. Out of 15 ACLF patients 10 survived and underwent follow-up study after clinical recovery at 3 weeks. FreeSurfer program was used to quantify cortical thickness and LC- Model software was used to quantify absolute metabolites concentrations. Neuropsychological (NP) test was performed to assess the cognitive performance in follow-up ACLF patients compared to controls.

Results

Significantly reduced cortical thicknesses in multiple brain sites, and significantly decreased N-acetyl aspartate (NAA), myo-inositol (mI) and significantly increased glutamate/glutamine (glx) metabolites were observed in ACLF compared to those of controls at baseline study. Follow-up patients showed significant recovery in cortical thickness and Glx level, while NAA and mI were partially recovered compared to baseline study. When compared to controls, follow-up patients still showed reduced cortical thickness and altered metabolites level. Follow-up patients had abnormal neuropsychological (NP) scores compared to controls.

Conclusions

Neuronal loss as suggested by the reduced NAA, decreased cellular density due to increased cerebral hyperammonemia as supported by the increased glx level, and increased proinflammatory cytokines and free radicals may account for the reduced cortical thickness in ACLF patients. Presence of reduced cortical thickness, altered metabolites and abnormal NP test scores in post recovery subjects as compared to those of controls is associated with incomplete clinical recovery. The current imaging protocol can be easily implemented in clinical settings to evaluate and monitor brain tissue changes in patients with ACLF during the course of treatment.

A genetic variant in the promoter of phosphate-activated glutaminase is associated with hepatic encephalopathy

BACKGROUND

Hepatic encephalopathy (HE) is a serious complication of liver cirrhosis. Recently, a microsatellite in the promoter region of the phosphate-activated glutaminase (GLS ) gene was associated with the risk of HE. The aim of the present study was to investigate, using the critical flicker frequency (CFF) test, whether the described GLS variant increases the risk of developing HE in patients with cirrhosis.

METHODS

We recruited 158 patients (66% men; mean age 59 years, range 23-86) with liver cirrhosis. Mean model for end-stage liver disease score was 13.8 (range 5-35); 48% of patients presented with Child-Pugh score B or C. The presence and severity of HE were determined by the CFF test, with frequencies ≤39 Hz denoting cases. GLS variants were genotyped by sequencing the microsatellite in the promoter region and were classified as short, long or short-long forms (depending on the length of the macrosatellite alleles).

RESULTS

In total, 53% of patients had abnormal CFF results (i.e. ≤39 Hz; range for entire cohort 26-57). The GLS microsatellite distribution amongst patients was short form (20%), long form (32%) and short-long form (48%) and was consistent with Hardy-Weinberg equilibrium. CFF values differed significantly between groups (P = 0.043). Carriers of the GLS long microsatellite had a significantly higher risk of HE according to multivariate analyses (odds ratio 3.23, 95% confidence interval 1.46-7.13, P = 0.004).

CONCLUSION

CFF results were significantly lower amongst carriers of the GLS long microsatellite. Our findings support the role of the GLS long microsatellite in the development of HE; this could be important for identifying susceptible patients and for the prevention of this condition.

Changes in Diffusion Kurtosis Imaging and Magnetic Resonance Spectroscopy in a Direct Cranial Blast Traumatic Brain Injury (dc-bTBI) Model

Explosive blast-related injuries are one of the hallmark injuries of veterans returning from recent wars, but the effects of a blast overpressure on the brain are poorly understood. In this study, we used in vivo diffusion kurtosis imaging (DKI) and proton magnetic resonance spectroscopy (MRS) to investigate tissue microstructure and metabolic changes in a novel, direct cranial blast traumatic brain injury (dc-bTBI) rat model. Imaging was performed on rats before injury and 1, 7, 14 and 28 days after blast exposure (~517 kPa peak overpressure to the dorsum of the head). No brain parenchyma abnormalities were visible on conventional T2-weighted MRI, but microstructural and metabolic changes were observed with DKI and proton MRS, respectively. Increased mean kurtosis, which peaked at 21 days post injury, was observed in the hippocampus and the internal capsule. Concomitant increases in myo-Inositol (Ins) and Taurine (Tau) were also observed in the hippocampus, while early changes at 1 day in the Glutamine (Gln) were observed in the internal capsule, all indicating glial abnormality in these regions. Neurofunctional testing on a separate but similarly treated group of rats showed early disturbances in vestibulomotor functions (days 1–14), which were associated with imaging changes in the internal capsule. Delayed impairments in spatial memory and in rapid learning, as assessed by Morris Water Maze paradigms (days 14–19), were associated with delayed changes in the hippocampus. Significant microglial activation and neurodegeneration were observed at 28 days in the hippocampus. Overall, our findings indicate delayed neurofunctional and pathological abnormalities following dc-bTBI that are silent on conventional T2-weighted imaging, but are detectable using DKI and proton MRS.

Diagnosis and Management of Hepatic Encephalopathy in Fulminant Hepatic Failure

Hepatic encephalopathy (HE) is associated with cerebral edema (CE), increased intracranial pressure (ICP), and subsequent neurologic complications; it is the most important cause of morbidity and mortality in fulminant hepatic failure. The goal of therapy should be early diagnosis and treatment of HE with measures to reduce CE. A combination of clinical examination and diagnostic modalities can aid in prompt diagnosis. ICP monitoring and transcranial Doppler help diagnose and monitor response to treatment. Transfer to a transplant center and intensive care unit admission with airway management and reduction of CE with hypertonic saline, mannitol, hypothermia, and sedation are recommended as a bridge to liver transplantation.

Clinical and Neurologic Manifestation of Minimal Hepatic Encephalopathy and Overt Hepatic Encephalopathy

Hepatic encephalopathy (HE) shows a wide spectrum of neuropsychiatric manifestations. A combined effort with neuropsychological and psychometric evaluation has to be performed to recognize the syndrome, whereas minimal HE (MHE) is largely under-recognized. Subtle symptoms of MHE can only be diagnosed through specialized neuropsychiatric testing. Early diagnosis and treatment may drastically improve the quality of life for many cirrhotic patients. Further research to gain better insight into the pathophysiology and diagnostic accuracy of HE will help determine future management strategies.

Ammonia and Its Role in the Pathogenesis of Hepatic Encephalopathy

Hepatic encephalopathy (HE) is a commonly encountered sequela of chronic liver disease and cirrhosis with significant associated morbidity and mortality. Although ammonia is implicated in the pathogenesis of HE, the exact underlying mechanisms still remain poorly understood. Its role in the urea cycle, astrocyte swelling, and glutamine and gamma-amino-n-butyric acid systems suggests that the pathogenesis is multifaceted. Greater understanding in its underlying mechanism may offer more targeted therapeutic options in the future, and thus further research is necessary to fully understand the pathogenesis of HE.

Potential role of Plasmodium falciparum-derived ammonia in the pathogenesis of cerebral malaria

Cerebral malaria (CM) is the most severe complication associated with Plasmodium falciparum infection. The exact pathogenic mechanisms leading to the development of CM remains poorly understood while the mortality rates remain high. Several potential mechanisms including mechanical obstruction of brain microvasculature, inflammation, oxidative stress, cerebral energy defects, and hemostatic dysfunction have been suggested to play a role in CM pathogenesis. However, these proposed mechanisms, even when considered together, do not fully explain the pathogenesis and clinicopathological features of human CM. This necessitates consideration of alternative pathogenic mechanisms. P. falciparum generates substantial amounts of ammonia as a catabolic by-product, but lacks detoxification mechanisms. Whether this parasite-derived ammonia plays a pathogenic role in CM is presently unknown, despite its potential to cause localized brain ammonia elevation and subsequent neurotoxic effects. This article therefore, explores and proposes a potential role of parasite-derived ammonia in the pathogenesis and neuropathology of CM. A consideration of parasite-derived ammonia as a factor in CM pathogenesis provides plausible explanations of the various features observed in CM patients including how a largely intravascular parasite can cause neuronal dysfunction. It also provides a framework for rational development and testing of novel drugs targeting the parasite's ammonia handling.

Tranilast reduces serum IL-6 and IL-13 and protects against thioacetamide-induced acute liver injury and hepatic encephalopathy

Hepatic encephalopathy is a serious neuropsychiatric disorder usually affecting either acute or chronic hepatic failure patients. Hepatic encephalopathy was replicated in a validated rat model to assess the potential protective efficacy of tranilast against experimentally induced hepatic encephalopathy. Thioacetamide injection significantly impaired hepatic synthetic, metabolic and excretory functions with significant increase in serum NO, IL-6 and IL-13 levels and negative shift in the oxidant/antioxidant balance. Most importantly, there was a significant increase in serum ammonia levels with significant astrocytes' swelling and vacuolization; hallmarks of hepatic encephalopathy. Tranilast administration (300 mg/kg, orally) for 15 days significantly improved hepatic functions, restored oxidant/antioxidant balance, reduced serum NO, IL-6 and IL-13 levels. Meanwhile, serum ammonia significantly declined with significant reduction in astrocytes' swelling and vacuolization. Several mechanisms can be implicated in the observed hepato- and neuroprotective potentials of tranilast, such as its anti-inflammatory potential, its antioxidant potential as well as its immunomodulatory properties.