Effect of ammonia on GABA uptake and release in cultured astrocytes

Antibiotics and Liver Cirrhosis: What the Physicians Need to Know

The liver is the primary site of drug metabolism, which can be altered by a variety of diseases affecting the liver parenchyma, especially in patients with liver cirrhosis. The use of antibiotics in patients with cirrhosis is usually a matter of concern for physicians, given the lack of practical knowledge for drug choice and eventual dose adjustments in several clinical scenarios. The aim of the current narrative review is to report, as broadly as possible, basic, and practical knowledge that any physician should have when approaching a patient with liver cirrhosis and an ongoing infection to efficiently choose the best antibiotic therapy.

Therapeutic Applications of Resveratrol in Hepatic Encephalopathy through Its Regulation of the Microbiota, Brain Edema, and Inflammation

Hepatic encephalopathy is a common complication in patients with liver cirrhosis and portosystemic shunting. Patients with hepatic encephalopathy present a variety of clinical features, including neuropsychiatric manifestations, cognitive dysfunction, impaired gut barrier function, hyperammonemia, and chronic neuroinflammation. These pathogeneses have been linked to various factors, including ammonia-induced oxidative stress, neuronal cell death, alterations in the gut microbiome, astrocyte swelling, and blood-brain barrier disruptions. Many researchers have focused on identifying novel therapeutics and prebiotics in the hope of improving the treatment of these conditions. Resveratrol is a natural polyphenic compound and is known to exert several pharmacological effects, including antioxidant, anti-inflammatory, and neuroprotective activities. Recent studies suggest that resveratrol contributes to improving the neuropathogenic effects of liver failure. Here, we review the current evidence describing resveratrol's effects in neuropathogenesis and its impact on the gut-liver axis relating to hepatic encephalopathy. We highlight the hypothesis that resveratrol exerts diverse effects in hepatic encephalopathy and suggest that these effects are likely mediated by changes to the gut microbiota, brain edema, and neuroinflammation.

The Cerebral Effect of Ammonia in Brain Aging: Blood-Brain Barrier Breakdown, Mitochondrial Dysfunction, and Neuroinflammation

Aging occurs along with multiple pathological problems in various organs. The aged brain, especially, shows a reduction in brain mass, neuronal cell death, energy dysregulation, and memory loss. Brain aging is influenced by altered metabolites both in the systemic blood circulation and the central nervous system (CNS). High levels of ammonia, a natural by-product produced in the body, have been reported as contributing to inflammatory responses, energy metabolism, and synaptic function, leading to memory function in CNS. Ammonia levels in the brain also increase as a consequence of the aging process, ultimately leading to neuropathological problems in the CNS. Although many researchers have demonstrated that the level of ammonia in the body alters with age and results in diverse pathological alterations, the definitive relationship between ammonia and the aged brain is not yet clear. Thus, we review the current body of evidence related to the roles of ammonia in the aged brain. On the basis of this, we hypothesize that the modulation of ammonia level in the CNS may be a critical clinical point to attenuate neuropathological alterations associated with aging.

Astrocyte dysfunction in Alzheimer disease

Astrocytes are glial cells that are distributed throughout the central nervous system in an arrangement optimal for chemical and physical interaction with neuronal synapses and brain blood supply vessels. Neurotransmission modulates astrocytic excitability by activating an array of cell surface receptors and transporter proteins, resulting in dynamic changes in intracellular Ca²⁺ or Na⁺ . Ionic and electrogenic astrocytic changes, in turn, drive vital cell nonautonomous effects supporting brain function, including regulation of synaptic activity, neuronal metabolism, and regional blood supply. Alzheimer disease (AD) is associated with aberrant oligomeric amyloid β generation, which leads to extensive proliferation of astrocytes with a reactive phenotype and abnormal regulation of these processes. Astrocytic morphology, Ca²⁺ responses, extracellular K⁺ removal, glutamate transport, amyloid clearance, and energy metabolism are all affected in AD, resulting in a deleterious set of effects that includes glutamate excitotoxicity, impaired synaptic plasticity, reduced carbon delivery to neurons for oxidative phosphorylation, and dysregulated linkages between neuronal energy demand and regional blood supply. This review summarizes how astrocytes are affected in AD and describes how these changes are likely to influence brain function. © 2017 Wiley Periodicals, Inc.

Optogenetic and Chemogenetic Approaches for Studying Astrocytes and Gliotransmitters

The brain consists of heterogeneous populations of neuronal and non-neuronal cells. The revelation of their connections and interactions is fundamental to understanding normal brain functions as well as abnormal changes in pathological conditions. Optogenetics and chemogenetics have been developed to allow functional manipulations both in vitro and in vivo to examine causal relationships between cellular changes and functional outcomes. These techniques are based on genetically encoded effector molecules that respond exclusively to exogenous stimuli, such as a certain wavelength of light or a synthetic ligand. Activation of effector molecules provokes diverse intracellular changes, such as an influx or efflux of ions, depolarization or hyperpolarization of membranes, and activation of intracellular signaling cascades. Optogenetics and chemogenetics have been applied mainly to the study of neuronal circuits, but their use in studying non-neuronal cells has been gradually increasing. Here we introduce recent studies that have employed optogenetics and chemogenetics to reveal the function of astrocytes and gliotransmitters.

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.

Lens glutathione homeostasis: Discrepancies and gaps in knowledge standing in the way of novel therapeutic approaches

Cataract is the major cause of blindness worldwide. The WHO has estimated around 20 million people have bilateral blindness from cataract, and that number is expected to reach 50 million in 2050. The cataract surgery is currently the main treatment approach, though often associated with complications, such as Posterior Capsule Opacification (PCO)-also known as secondary cataract. The lens is an avascular ocular structure equipped with an unusually high level of glutathione (GSH), which plays a vital role in maintaining lens transparency by regulating lenticular redox state. The lens epithelium and outer cortex are thought to be responsible for providing the majority of lens GSH via GSH de novo synthesis, assisted by a continuous supply of constituent amino acids from the aqueous humor, as well as extracellular GSH recycling from the gamma-glutamyl cycle. However, when de novo synthesis is impaired, in the presence of low GSH levels, as in the aging human lens, compensatory mechanisms exist, suggesting that the lens is able to uptake GSH from the surrounding ocular tissues. However, these uptake mechanisms, and the GSH source and its origin, are largely unknown. The lens nucleus does not have the ability to synthesize its own GSH and fully relies on transport from the outer cortex by yet unknown mechanisms. Understanding how aging reduces GSH levels, particularly in the lens nucleus, how it is associated with age-related nuclear cataract (ARNC), and how the lens compensates for GSH loss via external uptake should be a major research priority. The intent of this review, which is dedicated to the memory of David C. Beebe, is to summarize our current understanding of lens GSH homeostasis and highlight discrepancies and gaps in knowledge that stand in the way of pharmacologically minimizing the impact of declining GSH content in the prevention of age-related cataract.

The subcellular localization of GABA transporters and its implication for seizure management

The ability to modulate the synaptic GABA levels has been demonstrated by using the clinically effective and selective GAT1 inhibitor tiagabine [(R)-N-[4,4-bis(3-methyl-2-thienyl)-3-butenyl]nipecotic acid]. N-[4,4-bis(3-methyl-2-thienyl)-3-butenyl]-3-hydroxy-4-(methylamino)-4,5,6,7-tetrahydrobenzo[d]isoxazol-3-ol (EF1502) which not only inhibits GAT1 like tiagabine but also BGT1 has been shown to modulate extrasynaptic GABA levels. The simultaneous inhibition of synaptic and extrasynaptic GABA transporters using tiagabine and EF1502, respectively has been demonstrated to exert a synergistic anticonvulsant effect in several seizure models in mice. The pharmacological profile of these and similar compounds has been thoroughly investigated in in vitro systems, comparing the GAT subtype selectivity with the ability to inhibit GABA uptake in primary cultures of neurons and astrocytes. However, an exact explanation has not yet been found. In the present study, the ability of GATs to form homo and/or heterodimers was investigated as well as to which membrane micro environment the GATs reside. To investigate dimerization of GATs, fusion proteins of GATs tagged with either yellow fluorescent protein or cerulean fluorescent protein were made and fluorescence resonance energy transfer (FRET) was measured. It was found that GATs form both homo- and hetero-dimers in N2A and HEK-293 cells. Microdomain localization of GATs as investigated by detergent resistant membrane fractions after treatment of tissue with Brij-98 or Triton X-100 revealed that BGT1 and GAT1 mostly localize to non-membrane rafts independent of the detergent used. However, GAT3 localizes to membrane rafts when using Brij-98. Taken together, these results suggest that the observed hetero dimerization of GATs in the FRET study is unlikely to have functional implications since the GATs are located to very different cellular compartments and cell types.

Effects of hyperammonemia on brain energy metabolism: controversial findings in vivo and in vitro

The literature related to the effects of elevated plasma ammonia levels on brain energy metabolism is abundant, but heterogeneous in terms of the conclusions. Thus, some studies claim that ammonia has a direct, inhibitory effect on energy metabolism whereas others find no such correlation. In this review, we discuss both recent and older literature related to this controversial topic. We find that it has been consistently reported that hepatic encephalopathy and concomitant hyperammonemia lead to reduced cerebral oxygen consumption. However, this may not be directly linked to an effect of ammonia but related to the fact that hepatic encephalopathy is always associated with reduced brain activity, a condition clearly characterized by a decreased CMRO2. Whether this may be related to changes in GABAergic function remains to be elucidated.

Impairment of short term memory in rats with hepatic encephalopathy due to bile duct ligation

Hepatic encephalopathy (HE) arises from acute or chronic liver diseases and leads to cognitive deficits. Different animal models for the study of HE have demonstrated learning and memory impairment and a number of neurotransmitter systems have been proposed to be involved in this. Recently, it was described that bile duct-ligated (BDL) rats exhibited altered spatio-temporal locomotor and exploratory activities and biosynthesis of neurotransmitter GABA in brain cortices. Therefore, the aim of this study was to evaluate cognition in the same animal model. Male adult Wistar rats underwent common bile duct ligation (BDL rats) or manipulation of common bile duct without ligation (control rats). Six weeks after surgery, control and BDL rats underwent object recognition behavioral task. The BDL rats developed chronic liver failure and exhibited a decreased discrimination index for short term memory (STM) when compared to the control group. There was no difference in long term memory (LTM) as well as in total time of exploration in the training, STM and LTM sessions between the BDL and control rats. Therefore, the BDL rats demonstrated impaired STM for recognition memory, which was not due to decreased exploration.

Neurotransmitter receptor alterations in hepatic encephalopathy: a review

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

Glutamate controls tPA recycling by astrocytes, which in turn influences glutamatergic signals

Tissue-type plasminogen activator (tPA) regulates physiological processes in the brain, such as learning and memory, and plays a critical role in neuronal survival and neuroinflammation in pathological conditions. Here we demonstrate, by combining mouse in vitro and in vivo data, that tPA is an important element of the cross talk between neurons and astrocytes. The data show that tPA released by neurons is constitutively endocytosed by astrocytes via the low-density lipoprotein-related protein receptor, and is then exocytosed in a regulated manner. The exocytotic recycling of tPA by astrocytes is inhibited in the presence of extracellular glutamate. Kainate receptors of astrocytes act as sensors of extracellular glutamate and, via a signaling pathway involving protein kinase C, modulate the exocytosis of tPA. Further, by thus capturing extracellular tPA, astrocytes serve to reduce NMDA-mediated responses potentiated by tPA. Overall, this work provides the first demonstration that the neuromodulator, tPA, may also be considered as a gliotransmitter.

The inotropic effects of ammonia on isolated perfused rat hearts and the mechanisms involved

Ammonia (NH(3)) is a common exogenous gas in the atmosphere, as well as an endogenous chemical produced by amino acid catabolism and other pathways in vivo. Physiological and pathophysiological roles of NH(3) in the nervous system have been studied. Recently, endogenous NH(3) has been suggested to be a gas transmitter. However, so far the role of NH(3) in cardiovascular functions has not been reported. The present study was designed to investigate the inotropic effects of NH(3) on isolated perfused rat hearts and the possible mechanisms involved in these effects. The results showed that NH(3) had a positive inotropic effect in a concentration-dependent manner and produced a higher positive effect than NaOH and NH(4)Cl. At low concentrations, the effect of NH(3) on cardiac function was caused by NH(3) molecules; at high concentrations, the effect of NH(3) on hearts may be partly correlated with a change of pH value, but was mainly caused by NH(3) molecules. The mechanisms involved in the NH(3)-induced positive inotropic effect may be related to the ATP-sensitive K(+) (K(ATP)) channel and the nitric oxide (NO)-cyclic GMP (cGMP) signaling pathway. In addition, at a concentration of 1.5 mmol l(-1), NH(3) significantly increased the activity of creatine kinase (CK) and lactate dehydrogenase (LDH) in the coronary perfusate and decreased the activity of Na(+),K(+)-ATPase and Ca(2+),Mg(2+)-ATPase in the hearts. These results indicate that NH(3) at physiological or low concentrations may play a modulatory role in heart function, but at high concentrations had a damaging effect on isolated rat hearts.

Theories of the pathogenesis of hepatic encephalopathy

The earliest hypothesis of the pathogenesis of HE implicated ammonia, although effects of appreciable concentrations of this neurotoxin did not resemble HE. Altered eurotransmission in the brain was suggested by similarities between increased GABA-mediated inhibitory neurotransmission and HE, specifically decreased consciousness and impaired motor function. Evidence of increased GABAergic tone in models of HE has accumulated; potential mechanisms include increased synaptic availability of GABA and accumulation of natural benzodiazepine receptor ligands with agonist properties. Pathophysiological concentrations of ammonia associated with HE, have the potential of enhancing GABAergic tone by mechanisms that involve its interactions with the GABAa receptor complex.

Synthesis of neurotransmitter GABA via the neuronal tricarboxylic acid cycle is elevated in rats with liver cirrhosis consistent with a high GABAergic tone in chronic hepatic encephalopathy

Hepatic encephalopathy (HE) is a neuropsychiatric complication to liver disease. It is known that ammonia plays a role in the pathogenesis of HE and disturbances in the GABAergic system have been related to HE. Synthesis of GABA occurs by decarboxylation of glutamate formed by deamidation of astrocyte-derived glutamine. It is known that a fraction of glutamate is decarboxylated directly to GABA (referred to as the direct pathway) and that a fraction undergoes transamination with formation of alpha-ketoglutarate. The latter fraction is cycled through the neuronal tricarboxylic acid cycle, an energy-generating pathway, prior to being employed for GABA synthesis (the indirect pathway). We have previously shown that ammonia induces an elevation of the neuronal tricarboxylic acid cycle activity. Thus, the aims of the present study were to determine if increased levels of ammonia increase GABA synthesis via the indirect pathway in a rat model of HE induced by bile-duct ligation and in co-cultures of neurons and astrocytes exposed to ammonia. Employing (13) C-labeled precursors and subsequent analysis by mass spectrometry, we demonstrated that more GABA was synthesized via the indirect pathway in bile duct-ligated rats and in co-cultures subjected to elevated ammonia levels. Since the indirect pathway is associated with synthesis of vesicular GABA, this might explain the increased GABAergic tone in HE.

Ammonium-induced calcium mobilization in 1321N1 astrocytoma cells

High blood levels of ammonium/ammonia (NH(4)(+)/NH(3)) are associated with severe neurotoxicity as observed in hepatic encephalopathy (HE). Astrocytes are the main targets of ammonium toxicity, while neuronal cells are less vulnerable. In the present study, an astrocytoma cell line 1321N1 and a neuroblastoma glioma hybrid cell line NG108-15 were used as model systems for astrocytes and neuronal cells, respectively. Ammonium salts evoked a transient increase in intracellular calcium concentrations ([Ca(2+)](i)) in astrocytoma (EC(50)=6.38 mM), but not in NG108-15 cells. The ammonium-induced increase in [Ca(2+)](i) was due to an intracellular effect of NH(4)(+)/NH(3) and was independent of extracellular calcium. Acetate completely inhibited the ammonium effect. Ammonium potently reduced calcium signaling by G(q) protein-coupled receptors (H(1) and M3) expressed on the cells. Ammonium (5 mM) also significantly inhibited the proliferation of 1321N1 astrocytoma cells. While mRNA for the mammalian ammonium transporters RhBG and RhCG could not be detected in 1321N1 astrocytoma cells, both transporters were expressed in NG108-15 cells. RhBG and RhBC in brain may promote the excretion of NH(3)/NH(4)(+) from neuronal cells. Cellular uptake of NH(4)(+)/NH(3) was mainly by passive diffusion of NH(3). Human 1321N1 astrocytoma cells appear to be an excellent, easily accessible human model for studying HE, which can substitute animal studies, while NG108-15 cells may be useful for investigating the role of the recently discovered Rhesus family type ammonium transporters in neuronal cells. Our findings may contribute to the understanding of pathologic ammonium effects in different brain cells, and to the treatment of hyperammonemia.

Possible implication of ciliary neurotrophic factor (CNTF) and beta-synuclein in the ammonia effect on cultured rat astroglial cells: a study using DNA and protein microarrays

Astrocytes are considered the key cell in hepatic encephalopathy; although their precise role in the disease has not yet been determined, exposure to ammonia appears to have an important pathogenic effect. We exposed confluent cultures of rat astroglial cells to ammonia (5mM NH(4)Cl) for 1, 3, 5 and 7 days, and determined astroglial levels of actin, glial fibrillary acidic protein (GFAP), glutamine synthetase (GS), GLAST glutamate transporter, 25kDa heat-shock protein (HSP25), HSP60 and HSP70 by Western blot; the glutamine content in culture medium was measured by mass spectrometry. Significant increases were observed for GS, HSP60 and glutamine, and significant reductions for actin and GFAP. Astrocytes exposed to ammonia for 4 days were used to analyze the effect of ammonia in protein and DNA microarrays. After protein microarray data filtration by signal intensity, x-fold change and z-score, 11 proteins were selected, among which the significant increase in beta-synuclein was confirmed by Western blot. DNA microarray data filtration by intensity signal, x-fold change and p-value selected almost 600 genes. The significant increase in alpha-synuclein mRNA was confirmed by quantitative RT-PCR, but no change was observed in alpha-synuclein protein levels. A notable decrease in ciliary neurotrophic factor (CNTF) was demonstrated by Western blot after ammonia treatment, concurring with the reduction in CNTF mRNA observed in DNA microarrays. We discuss the possibility of a pathogenic role for CNTF and a protective role for beta-synuclein in experimental hyperammonemia. This study demonstrates the use of microarrays as tools to ascertain the possible implication of previously unidentified proteins in the pathogenesis of hepatic encephalopathy.

Assessment of glutamate and glutamine contribution to in vivo N-acetylaspartate quantification in human brain by (1)H-magnetic resonance spectroscopy

N-Acetylaspartate (NAA) is one of the most important metabolites detectable by brain (1)H-MRS being considered an index of neuronal integrity. At the low magnetic field used in most clinical settings beta,gamma-glutamate/glutamine (Glx) resonances are very close and partially overlap the methyl-NAA resonance interfering with NAA quantification especially at low TE and in the presence of increased Glx signals. NAA overestimation due to Glx on a set of model solutions containing NAA, glutamate, and glutamine in variable amounts was evaluated and the result tested in vivo in six healthy controls and five age- and sex-matched patients with hepatic encephalopathy (HE), the latter having an increased Glx content. A method to assess in vivo the NAA overestimation caused by Glx is proposed. A perfect match was obtained between the assessment of Glx contamination on the NAA of healthy controls and that obtained on the model solutions. However, a substantial difference in NAA overestimation was found between controls and HE patients that cannot be explained by our model. An interpretative hypothesis is provided.

Extracellular conversion of guanine-based purines to guanosine specifically enhances astrocyte glutamate uptake

Guanosine (GUO) has been shown to stimulate glutamate uptake in primary astrocyte cultures. The purpose of this study was to determine the effect and specificity of guanine- or adenine-based purines on glutamate and GABA uptake in cultured astrocytes. Stimulatory effect on glutamate uptake was observed with GUO, GMP or GTP. Simultaneous exposure with these guanine-based purines did not show an additive effect. We also investigated a possible interconversion of guanine-based purines during incubation time. Action by GTP was excluded since the hydrolysis resistant GTP analog, GMP-PNP did not stimulate glutamate uptake. Addition of an ecto-5'-nucleotidase inhibitor abolished GMP-stimulatory effect on glutamate uptake, without affecting GUO action. Taken together, these results suggest that GUO is the guanine-based purines responsible for glutamate uptake activation. In addition, the stimulatory effect on glutamate uptake was not observed with adenine-based purines. Moreover, GABA uptake was not activated by GUO. These results point to specificity in the interaction between GUO and the astrocyte glutamate uptake system.

Ammonia, the GABA neurotransmitter system, and hepatic encephalopathy

There appears to be a consensus that hepatic encephalopathy (HE) is a metabolic encephalopathy with a multifactorial pathogenesis. One of the factors considered to be important in the pathogenesis of HE is ammonia. However, the mechanisms by which ammonia contributes to the manifestations of HE remain poorly defined. Ammonia could be more definitively implicated in the pathogenesis of HE if its effects can be shown to lead to an enhancement of inhibitory neurotransmission. In this context the effects of ammonia on the GABA (gamma-aminobutyric acid) neurotransmitter system may be relevant. Ammonia, at the modestly increased concentrations that commonly occur in precoma HE (0.15 mM-0.75 mM), has been shown to increase GABA-induced chloride current in cultured neurons, probably by modifying the affinity of the GABA(A) receptor for GABA. Comparable ammonia concentrations also enhanced synergistically the binding of a GABA agonist and a benzodiazepine (BZ) agonist to the GABA(A) receptor complex, phenomena which would enhance the neuroinhibitory effects of these ligands. Also, GABA increased the potency of ammonia-induced enhancement of the binding of a BZ agonist to the GABA(A) receptor complex, and brain levels of BZ agonists are elevated in liver failure. In addition, ammonia has been shown to inhibit astrocytic uptake of GABA by 30%-50%, an effect which would increase the synaptic availability of GABA at GABA(A) receptors. Furthermore, increased ammonia concentrations upregulate the peripheral-type benzodiazepine receptor in the outer membrane of astroglial mitochondria, thereby enhancing astrocytic mitochondrial synthesis and release of neurosteroids. Some neurosteroids, for example tetrahydroprogesterone (THP) and tetrahydrodeoxycorticosterone (THDOC), are potent agonists of the GABA(A) receptor complex, on which there are specific binding sites for neurosteroids, that are distinct from those for BZs and barbiturates. Tetrahydroprogesterone and tetrahydrodeoxycorticosterone levels were found to be increased in a mouse model of acute liver failure, and, when THP or THDOC was injected into normal mice, sedation and Alzheimer type II astrocytic changes in the cortex, striatum, and hypothalmus were induced. Each of these direct or indirect effects of ammonia on the GABA neurotransmitter system has the potential of increasing inhibitory neurotransmission, and, hence, contributing to the manifestations of HE.

Direct and indirect enhancement of GABAergic neurotransmission by ammonia: implications for the pathogenesis of hyperammonemic syndromes

Experimental evidence indicates that ammonia causes neuroexcitation and seizures. This contrasts with the lethargy, confusion and other manifestations of global CNS depression commonly considered to be major components of hyperammonemic encephalopathies. Substantial data now indicates that ammonia can modulate GABAergic neurotransmission through direct and indirect mechanisms. This modulation consists of an enhancement of GABAergic neurotransmission at concentrations commonly encountered in hyperammonemic states and precedes the suppression of inhibitory neuronal function observed at higher (>1mM) ammonia concentrations. Not only is this increase in GABAergic neurotransmission consistent with the clinical picture of lethargy, ataxia and cognitive deficits associated with liver failure and congenital hyperammonemia, but it also provides a mechanism for testing new therapeutic modalities for the treatment of hyperammonemic encephalopathy.

Ammonia and Alzheimer's disease

Alzheimer's disease (AD) is the most common age-related neurodegenerative disorder. Behavioural, cognitive and memory dysfunctions are characteristic symptoms of AD. The formation of amyloid plaques is currently considered as the key event of AD. Other histological hallmarks of the disease are the formation of fibrillary tangles, astrocytosis, and loss of certain neuronal systems in cortical areas of the brain. A great number of possible aetiologic and pathogenetic factors of AD have been published in the course of the last two decades. Among the toxic factors, which have been considered to contribute to the symptoms and progression of AD, ammonia deserves special interest for the following reasons: (a) Ammonia is formed in nearly all tissues and organs of the vertebrate organism; it is the most common endogenous neurotoxic compounds. Its effects on glutamatergic and GABAergic neuronal systems, the two prevailing neuronal systems of the cortical structures, are known for many years. (b) The impairment of ammonia detoxification invariably leads to severe pathology. Several symptoms and histologic aberrations of hepatic encephalopathy (HE), of which ammonia has been recognised as a pathogenetic factor, resemble those of AD. (c) The excessive formation of ammonia in the brains of AD patients has been demonstrated, and it has been shown that some AD patients exhibit elevated blood ammonia concentrations. (d) There is evidence for the involvement of aberrant lysosomal processing of beta-amyloid precursor protein (beta-APP) in the formation of amyloid deposits. Ammonia is the most important natural modulator of lysosomal protein processing. (e) Inflammatory processes and activation of microglia are widely believed to be implicated in the pathology of AD. Ammonia is able to affect the characteristic functions of microglia, such as endocytosis, and cytokine production. Based on these facts, an ammonia hypothesis of AD has first been suggested in 1993. In the present review old and new observations are discussed, which are in support of the notion that ammonia is a factor able to produce symptoms of AD and to affect the progression of the disease.

Mechanisms of hyperammonemia

Hyperammonemia is mainly found in hepatic encephalopathy and in genetic defects of the urea cycle or other pathways of the intermediary metabolism. Clinically a difference has to be made between chronic moderate hyperammonemia and acutely increased concentrations. Pathogenetic mechanisms of ammonia toxicity to the brain are partly unraveled. In some animal models confounding variables, such as the reduced intake of food and amino acid imbalance due to liver insufficiency, do not allow to establish unequivocal causal relationships between the ammonia concentration and measured effects. In chronic moderate hyperammonemia an increased flux through the serotonin pathway is a key factor. It is caused by an increased transport of large neutral amino acids (including tryptophan) through the blood-brain barrier, accentuated by the imbalance of plasma amino acids in hepatic insufficiency. It is stimulated by D- or L-glutamine. Evidence is presented showing that a functioning gamma-glutamyl cycle (glutathione formation) is a prerequisite. In acute hyperammonemia involvement of NMDA receptors, glutamate, NO and cGMP plays an additional role. In hyperammonemic crises the increased cerebral blood flow leads to brain edema; factors discussed here are increased osmolytes in astrocytes and serotoninergic activity. Recent data indicate that axonal development is affected by ammonia and can be normalized in vitro by creatine supplementation in developing mixed brain cell aggregate cultures, thus reviving the old hypothesis of the impact of hyperammonemia on energy metabolism in the developing brain that could cause mental retardation.

Hepatic encephalopathy: a neuropsychiatric disorder involving multiple neurotransmitter systems

Evidence from both biochemical measurements and from noninvasive techniques continues to suggest that neurotransmission failure rather than primary energy failure is the major cause of hepatic encephalopathy. Major neurotransmitter systems in which abnormalities have been identified include the glutamatergic, monoaminergic and opioid systems. Further elucidation of these neurotransmitter changes could provide novel pharmacological approaches in the treatment of hepatic encephalopathy.