Ammonia and hepatic encephalopathy: the more things change, the more they remain the same

Ammonia is thought to be central in the pathogenesis of hepatic encephalopathy and has been of importance to generations dating back to the early Egyptians. Hippocrates 2500 years ago described 'encephalopathy' simply translated as 'inside head suffering.' Over 1500 papers have been written on hepatic encephalopathy since 1966, but only a minority of these actually refer to the original observation of hepatic encephalopathy and the link with ammonia made by Marcel Nencki and Ivan Pavlov in 1893 with very little acknowledgement being made to the early landmark studies which described the importance of the muscle and kidneys in maintaining ammonia homeostasis as well as the liver and gut. Furthermore, infection was recognized as being an important modulator of brain function by the ancient Greek physicians and philosophers. This review focuses upon the original experiments of Nencki and Pavlov and describes how they fit into what we understand about the pathophysiology and treatment of hepatic encephalopathy today.

Dehydroascorbic acid normalizes several markers of oxidative stress and inflammation in acute hyperglycemic focal cerebral ischemia in the rat

We investigated the effect of dehydroascorbic acid (DHA), the oxidized form of vitamin C which is a superoxide scavenger, on manganese superoxide dismutase (MnSOD), copper-zinc SOD (CuZnSOD), cyclooxygenase-2 (COX-2) and interleukin-1beta (IL-1beta) expression in a rat model of focal cerebral ischemia under normo- and hyperglycemic conditions. Edema formation was also assessed. MnSOD, CuZnSOD, COX-2 and IL-1beta mRNA and protein expression were studied 3 h post-ischemia. No changes were observed in MnSOD and CuZnSOD mRNA expression among the groups. COX-2 and IL-1beta mRNA expression were upregulated by ischemia but were not influenced by the glycemic state. At the protein level, hyperglycemic cerebral ischemia increased MnSOD and CuZnSOD [Bémeur, C., Ste-Marie, L., Desjardins, P., Butterworth, R.F., Vachon, L., Montgomery, J., Hazell, A.S., 2004a. Expression of superoxide dismutase in hyperglycemic focal cerebral ischemia in the rat. Neurochem. Int. 45, 1167-1174] and IL-1beta expression compared to normoglycemic ischemia. COX-2 protein expression was also significantly higher following hyperglycemic ischemia compared to hyperglycemic shams. DHA administration did not change the pattern of COX-2 or IL-1beta mRNA expression, but normalized the increased protein expression following hyperglycemic ischemia. DHA administration also normalized MnSOD and CuZnSOD protein expression to the levels observed in normoglycemic ischemic animals. Edema formation was significantly reduced by DHA administration in hyperglycemic ischemic animals. The DHA-induced post-transcriptional normalization of MnSOD, CuZnSOD, COX-2 and IL-1beta levels and the decreased edema formation suggest that hyperglycemia accelerates superoxide formation and the inflammatory response, thus contributing to early damage in hyperglycemic stroke and strategies to scavenge superoxide should be an important therapeutic avenue.

Mild hypothermia prevents brain edema and attenuates up-regulation of the astrocytic benzodiazepine receptor in experimental acute liver failure

BACKGROUND/AIMS

Mild hypothermia has proven useful in the clinical management of patients with acute liver failure. Acute liver failure in experimental animals results in alterations in the expression of genes coding for astrocytic proteins including the "peripheral-type" (astrocytic) benzodiazepine receptor (PTBR), a mitochondrial complex associated with neurosteroid synthesis. To gain further insight into the mechanisms whereby hypothermia attenuates the neurological complications of acute liver failure, we investigated PTBR expression in the brains of hepatic devascularized rats under normothermic (37 degrees C) and hypothermic (35 degrees C) conditions.

METHODS

PTBR mRNA was measured using semi-quantitative RT-PCR in cerebral cortical extracts and densities of PTBR sites were measured by quantitative receptor autoradiagraphy. Brain pregnenolone content was measured by radioimmunoassay.

RESULTS

At coma stages of encephalopathy, animals with acute liver failure manifested a significant increase of PTBR mRNA levels. Brain pregnenolone content and [(3)H]PK 11195 binding site densities were concomitantly increased. Mild hypothermia prevented brain edema and significantly attenuated the increased receptor expression and pregnenolone content.

CONCLUSIONS

These findings suggest that an attenuation of PTBR up-regulation resulting in the prevention of increased brain neurosteroid content represents one of the mechanisms by which mild hypothermia exerts its protective effects in ALF.

The biosynthesis of ascorbate protects isolated rat hepatocytes from cumene hydroperoxide-mediated oxidative stress

Most animals synthesize ascorbate. It is an essential enzymatic cofactor for the synthesis of a variety of biological molecules and also a powerful antioxidant. There is, however, little direct evidence supporting an antioxidant role for endogenously produced ascorbate. Recently, we demonstrated that incubation of rat hepatocytes with 1-bromoheptane or phorone simultaneously depleted glutathione (GSH) and triggered rapid ascorbate synthesis. The present study investigates the hypothesis that endogenous ascorbate synthesis can confer protection against oxidative stress. Rat and guinea pig hepatocytes were depleted of GSH with 1-bromoheptane and subsequently treated with the oxidative stressor cumene hydroperoxide (CHP) in the presence or absence of the ascorbate synthesis inhibitor sorbinil. In rat hepatocytes, ascorbate content increased linearly (from 15.1 to 35.8 nmol/10(6) cells) over a 105-min incubation. Prior depletion of GSH increased CHP-induced cellular reactive oxygen species (ROS) production, lipid peroxidation, and cell death in rat and guinea pig hepatocytes. Inhibiting ascorbate synthesis, however, further elevated ROS production (2-fold), lipid peroxidation (1.5-fold), and cell death (2-fold) in rat hepatocytes only. This is the first time that endogenous ascorbate synthesis has been shown to decrease cellular susceptibility to oxidative stress. Protection by endogenously produced ascorbate may therefore need to be addressed when extrapolating data to humans from experiments using rodents capable of synthesizing ascorbate.

Expression of superoxide dismutase in hyperglycemic focal cerebral ischemia in the rat

This study investigated the possibility that hyperglycemia induces early expression of various superoxide dismutases (SOD) and nitric oxide synthases (NOS) following focal cerebral ischemia in the rat. MnSOD, CuZnSOD, nNOS and eNOS mRNA and protein expression were examined 3 h after permanent middle cerebral artery occlusion under acute hyperglycemic or normoglycemic conditions. 2,3,5-triphenyltetrazolium chloride (TTC) treatment post-mortem revealed a significant area at risk of infarction following ischemia in hyperglycemic compared to normoglycemic rats. Although no changes in MnSOD, CuZnSOD, nNOS and eNOS mRNA expression were detected, Western blots of ischemic cortex revealed an increase in MnSOD and CuZnSOD protein expression in hyperglycemic compared to normoglycemic rats. Pre-treatment of hyperglycemic rats with the NOS inhibitors L-nitroarginine methyl ester (L-NAME) and 7-nitroindazole (7-NI) or dehydroascorbic acid (DHA), a superoxide scavenger, significantly reduced the TTC delineated zone. The hyperglycemia-induced post-transcriptional upregulation of MnSOD and CuZnSOD levels suggest a response to increased superoxide production which, in the presence of increased nitric oxide production, may play a major role in the increased risk of damage following hyperglycemic stroke.

Increased brain concentrations of endogenous (non-benzodiazepine) GABA-A receptor ligands in human hepatic encephalopathy

It has been suggested that alterations of GABAergic neurotransmission are implicated in the pathophysiology of hepatic encephalopathy (HE). Increased concentrations of endogenous benzodiazepines with positive allosteric modulatory properties at the GABA-A receptor complex were proposed as a pathophysiological mechanism to explain increased GABAergic tone in HE. However, results of controlled trials with benzodiazepine receptor antagonists have yielded equivocal results and increases in benzodiazepine levels in body fluids of cirrhotic patients were suggested to be largely accounted for by previous pharmaceutical benzodiazepine intake. In the present study the issue of benzodiazepine receptor ligands in brains of cirrhotic patients, and their contribution to alterations of GABA-A receptor complex in HE are addressed. "Benzodiazepine-like" ligands were present in trace amounts in autopsied brain tissue from control subjects (0.2 +/- 0.2 ng/g tissue), and from cirrhotic patients not previously exposed to benzodiazepine medication (0.8 +/- 0.4 ng/g tissue). In contrast, these ligands accumulate in brain extracts from cirrhotic patients previously exposed to benzodiazepines by up to 200-fold (161.5 +/- 93.2 DE ng/g tissue). Brain extracts from cirrhotic patients increased the binding of the GABA-A receptor agonist [3H]muscimol. This increase was minimal with brain extracts from controls (6.8 +/- 2.8%), but was significant with brain extracts from cirrhotic patients without (29.4 +/- 2.7%), or with (55.1 +/- 7.6%) previous exposure to benzodiazepines. Addition of flumazenil, a selective benzodiazepine receptor antagonist did not significantly modify the increase of [3H]muscimol binding by brain extracts from patients without prior exposure to benzodiazepines and only partially inhibited the increase of [3H]muscimol binding in presence of brain extracts from cirrhotic patients previously exposed to benzodiazepines. These findings suggest the presence of nonbenzodiazepine substances (possibly neurosteroids) with positive allosteric modulatory properties at the GABA-A receptor complex in brain in hepatic encephalopathy.

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

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

Primary cultures of rat astrocytes respond to thiamine deficiency-induced swelling by downregulating aquaporin-4 levels

Imaging studies indicate that cerebral edema is an important consequence of Wernicke's encephalopathy (WE), a disorder caused by thiamine deficiency (TD). We have investigated this problem using a recently developed in vitro astrocyte model of TD. Measurement of cell volume using the 3-O-methylglucose uptake method revealed a dose-dependent swelling of astrocytes during exposure to TD conditions. Time course studies indicated a progressive volume increase up to a maximum of 93% above controls after 4 days of treatment. This swelling then partially resolved, and remained stable for up to 10 days following commencement of TD treatment. Measurement of aquaporin-4 (AQP-4) levels showed a 44% loss after 10 days and a temporal profile consistent with an important role for this water channel protein in astrocyte cell volume changes during TD. Our findings of astrocyte swelling in TD are consistent with previous reports of focal brain edema in cases of WE, and indicate that AQP-4 may be an important target for ameliorating some of the clinical problems associated with this disorder.

Increased brain endothelial nitric oxide synthase expression in thiamine deficiency: relationship to selective vulnerability

Thiamine deficiency results in selective neuronal cell death in thalamic structures. Previous studies provide evidence for a role implicating nitric oxide (NO) in the pathogenesis of cell death due to thiamine deficiency. In order to ascertain the origin of increased NO in the thiamine deficient brain, expression of endothelial nitric oxide synthase isoform (eNOS), was measured in the medial thalamus and in the inferior colliculus and compared to the frontal cortex (a spared region) of rats in which thiamine deficiency was induced through a feeding protocol of thiamine-deficient diet concomitant with daily administration of pyrithiamine, a central thiamine antagonist. eNOS mRNA and protein expression were significantly increased as a function of the severity of neurological impairment and the degree of neuronal cell loss in the medial thalamus and in the inferior colliculus. These findings suggest that the vascular endothelium is a major site of NO production in the brain in thiamine deficiency and that eNOS-derived NO could account for the selective damage to the thalamic structures that are observed in this particular disorder.

Selective alterations of brain osmolytes in acute liver failure: protective effect of mild hypothermia

The principal cause of mortality in patients with acute liver failure (ALF) is brain herniation resulting from intracranial hypertension caused by a progressive increase of brain water. In the present study, ex vivo high-resolution 1H-NMR spectroscopy was used to investigate the effects of ALF, with or without superimposed hypothermia, on brain organic osmolyte concentrations in relation to the severity of encephalopathy and brain edema in rats with ALF due to hepatic devascularization. In normothermic ALF rats, glutamine concentrations in frontal cortex increased more than fourfold at precoma stages, i.e. prior to the onset of severe encephalopathy, but showed no further increase at coma stages. In parallel with glutamine accumulation, the brain organic osmolytes myo-inositol and taurine were significantly decreased in frontal cortex to 63% and 67% of control values, respectively, at precoma stages (p<0.01), and to 58% and 67%, respectively, at coma stages of encephalopathy (p<0.01). Hypothermia, which prevented brain edema and encephalopathy in ALF rats, significantly attenuated the depletion of myo-inositol and taurine. Brain glutamine concentrations, on the other hand, did not respond to hypothermia. These findings demonstrate that experimental ALF results in selective changes in brain organic osmolytes as a function of the degree of encephalopathy which are associated with brain edema, and provides a further rationale for the continued use of hypothermia in the management of this condition.

Increased brain serotonin turnover correlates with the degree of shunting and hyperammonemia in rats following variable portal vein stenosis

BACKGROUND/AIMS

Hepatic encephalopathy (HE) is a serious neuropsychiatric complication of chronic liver disease. Brain monoamines have been implicated in the pathogenesis of HE. We examined the relationship between monoamine dysfunction and the degree of portal-systemic shunting (PSS) in rats with varying degrees of PSS.

METHODS

Concentrations of catecholamines, serotonin, histamine, precursors and metabolites in frontal cortex of rats with varying degrees of PSS (9-99.8%) were measured by HPLC.

RESULTS

The concentrations of the serotonin precursor, tryptophan, and its metabolite, 5-HIAA were increased up to 4-fold in brains of rats with various degrees of PSS and were significantly correlated with the degree of shunting and with arterial ammonia levels. Brain levels of histamine, its precursor, l-histidine, and metabolite, tele-methylhistamine were significantly increased only following total shunting. Concentrations of catecholamines and their metabolites were not significantly correlated with degree of PSS or hyperammonemia.

CONCLUSIONS

Given the established role of the serotonin system in the regulation of sleep, circadian rhythmicity and locomotion these findings suggest that selective alterations of this system could be implicated in the pathogenesis of HE. Therapeutic approaches aimed at the normalization of serotonin turnover could be beneficial in the prevention and treatment of early neuropsychiatric symptoms of HE.

Manganese Distribution in the Brain and Neurobehavioral Changes Following Inhalation Exposure of Rats to Three Chemical Forms of Manganese

The central nervous system is an important target for manganese (Mn) intoxication in humans; it may cause neurological symptoms similar to Parkinson's disease. Manganese compounds emitted from the tailpipe of vehicles using methylcyclopentadienyl manganese tricarbonyl (MMT) are primarily Mn phosphate, Mn sulfate, and Mn phosphate/sulfate mixture. The purpose of this study is to compare the patterns of Mn distribution in various brain regions (olfactory bulb, frontal parietal cortex, globus pallidus, striatum and cerebellum) and other tissues (lung, liver, kidney, testis) and the neurobehavioral damage following inhalation exposure of rats to three Mn species. Rats (n=15 rats per Mn species) were exposed 6 h per day, 5 days per week for 13 consecutive weeks to metallic Mn, Mn phosphate or Mn phosphate/sulfate mixture at about 3000 microgm(-3) and compared to controls. At the end of the exposure period, spontaneous motor activity was measured for 36 h using a computerized autotrack system. Mn in tissues was determined by instrumental neutron activation analysis (INAA). The Mn concentrations in the brain were significantly higher in rats exposed to Mn phosphate and Mn phosphate/sulfate mixture than in control rats or rats exposed to metallic Mn. Exposure to Mn phosphate/sulfate mixture caused a decrease in the total ambulatory count related to locomotor activity. Our results confirm that Mn species and solubility have an influence on the brain distribution of Mn in rats.

Role of circulating neurotoxins in the pathogenesis of hepatic encephalopathy: potential for improvement following their removal by liver assist devices

Both acute and chronic liver failure result in impaired cerebral function known as hepatic encephalopathy (HE). Evidence suggests that HE is the consequence of the accumulation in brain of neurotoxic and/or neuroactive substance including ammonia, manganese, aromatic amino acids, mercaptans, phenols, short-chain fatty acids, bilirubin and a variety of neuroactive medications prescribed as sedatives to patients with liver failure. Brain ammonia concentrations may attain levels in excess of 2 mm, concentrations which are known to adversely affect both excitatory and inhibitory neurotransmission as well as brain energy metabolism. Manganese exerts toxic effects on dopaminergic neurones. Prevention and treatment of HE continues to rely heavily on the reduction of circulating ammonia either by reduction of gut production using lactulose or antibiotics or by increasing its metabolism using L-ornithine-L-aspartate. No specific therapies have so far been designed to reduce circulating concentrations of other toxins. Liver assist devices offer a potential new approach to the reduction of circulating neurotoxins generated in liver failure. In this regard, the Molecular Adsorbents Recirculating System (MARS) appears to offer distinct advantages over hepatocyte-based systems.

Brain lactate synthesis in thiamine deficiency: A re-evaluation using1H-13C nuclear magnetic resonance spectroscopy

Region-selective accumulation of brain lactate occurs in TD; however, the mechanisms responsible have not been elucidated fully. (1)H and (13)C nuclear magnetic resonance (NMR) spectroscopy were therefore used to investigate de novo lactate synthesis from [1-(13)C]glucose in vulnerable (medial thalamus) and nonvulnerable (frontal cortex) brain regions of rats made thiamine deficient by administration of the central thiamine antagonist pyrithiamine. De novo synthesis of lactate was increased in the medial thalamus to 148% and 226% of pair-fed control values at presymptomatic and symptomatic stages of thiamine deficiency, respectively, whereas no such changes were observed in the frontal cortex. Administration of a glucose load selectively worsened the changes in medial thalamus. Pyruvate recycling and peripherally derived lactate did not contribute significantly to the lactate increase within the thiamine-deficient brain. Increases in immunolabeling of the lactate dehydrogenase isoenzymes (LDH1 and LDH5) were observed in the medial thalamus of thiamine-deficient animals. Metabolic impairment due to thiamine deficiency thus results in increased glycolysis, increased LDH immunolabeling of neurons and astrocytes and increased de novo synthesis of lactate in brain regions vulnerable to thiamine deficiency. These results are consistent with the notion that focal lactate accumulation participates in the worsening of neurologic symptoms in thiamine-deficient patients.

Pathogenesis of hepatic encephalopathy: update on molecular mechanisms

Hepatic encephalopathy (HE) is a major neuropsychiatric complication of acute and chronic liver failure. Neuropathologically, HE in chronic liver failure is characterized by astrocytic (rather than neuronal) changes known as Alzheimer type II astrocytosis and in altered expression of key astrocytic proteins. Magnetic resonance imaging in cirrhotic patients reveals bilateral signal hyperintensities in globus pallidus on T1-weighted imaging, which appear to result from manganese deposition. Proton (1H) magnetic resonance spectroscopy shows an increase in glutamine resonance in brain, a finding that confirms previous biochemical studies and is consistent with increased uptake of ammonia by the brain (glutamine synthesis). Recent molecular biological studies show an increased expression of several genes coding for neurotransmitter-related proteins in chronic liver failure. Such genes include those for monoamine oxidase (MAO-A isoform), nitric oxide synthase (nNOS isoform) and the peripheral-type benzodiazepine receptor. Activation of these systems may lead to alterations of monoamine and amino acid neurotransmitter function and changes in cerebral blood flow in chronic liver failure.

Brain edema in acute liver failure

Brain edema and consequent increase in intracranial pressure is a major complication of acute liver failure (ALF) and is a major cause of death in this condition. Rapid accumulation of ammonia in brain has been implicated in the pathogenesis of brain edema in ALF. Increased brain ammonia may cause brain swelling via the osmotic effects of an increase in astrocytic glutamine concentration or by inhibition of glutamate removal from brain extracellular space. Acute liver failure results in altered expression of several genes in the brain, some of which code for proteins involved in central nervous system function such as the glutamate transporter GLT-1, the astrocytic structural protein, glial fibrillary acidic protein, and the water channel protein, aquaporin IV. Loss of expression of GLT-1 results in increased extracellular brain glutamate. Therapeutic measures currently used to prevent and treat brain edema in acute liver failure include mannitol; strategies aimed at lowering of gut ammonia production are generally ineffective. Studies in experimental animals suggest that mild hypothermia or the use of L-ornithine-L-aspartate may be useful in the prevention of brain edema in these patients.

Normal coupling of brain benzodiazepine and neurosteroid modulatory sites on the GABA-A receptor complex in human hepatic encephalopathy

To assess the possible implication of the allosteric coupling of different modulatory sites at the GABA-A receptor complex in hepatic encephalopathy (HE), we investigated in autopsied frontal cortex of six cirrhotic patients and six appropriately-matched controls, the modulatory effects of the benzodiazepine site agonist flunitrazepam on the binding of [3H]muscimol and the effect of the neurosteroid site agonist allopregnanolone (5alpha-pregnan-3alpha-ol-20-one) on the binding of [3H]muscimol and [3H]flunitrazepam. There were no significant differences in either the magnitude E(max): 11.5+/-1.1% (controls) versus 10.2+/-2.2% (HE patients) or the efficacy EC(50): 20.2+/-5.5 nM (controls) versus 17.7+/-6.2 nM (HE patients) of flunitrazepam modulation of [3H]muscimol binding. Allopregnanolone also showed modulation of both sites to a comparable extent in brain tissue from cirrhotic patients and controls E(max): [3H]muscimol, 15.1+/-2.8% (controls) versus 13.8+/-1.9% (HE patients); [3H]flunitrazepam, 17.9+/-2.3% (controls) versus 19.1+/-2.3% (HE patients), EC(50): [3H]muscimol, 386.5+/-25.8 nM (controls) versus 373.8+/-13.1 nM (HE patients); [3H]flunitrazepam, 49.8+/-22.9 nM (controls) versus 55.5+/-14.0 nM (HE patients). These findings demonstrate unequivocally that the GABA-A sites and their benzodiazepine and neurosteroid modulatory sites manifest normal allosteric coupling in brain in human HE. Therefore, if increased "GABAergic tone" is implicated in the pathophysiology of HE, this must be the consequence of increased brain concentrations of endogenous benzodiazepine and/or neurosteroid ligands for components of the GABA-A receptor complex rather than alterations of the receptor proteins themselves.

Increased concentrations of histamine and its metabolite, tele-methylhistamine and down-regulation of histamine H3 receptor sites in autopsied brain tissue from cirrhotic patients who died in hepatic coma

BACKGROUND/AIMS

Hepatic encephalopathy (HE) is a serious neuropsychiatric complication of chronic liver disease. To determine whether changes in the central histaminergic system are a feature of human HE, we studied histamine, tele-methylhistamine, and presynaptic autoregulatory H(3) receptors in cerebral cortex and caudate-putamen obtained at autopsy from six cirrhotic patients and six appropriately matched controls.

METHODS

Histamine was assayed by HPLC; tele-methylhistamine by GC-MS. H(3) receptors were studied by in vitro receptor binding using [3H]R-alpha-methylhistamine as ligand.

RESULTS

In HE patients, there was a significant fourfold increase of histamine in caudate-putamen and a significant increase in all cortical regions studied. tele-Methyhistamine was also increased and the densities of histamine H(3) receptor sites were significantly decreased in patient material.

CONCLUSIONS

These findings are consistent with activation of the histaminergic system in HE. Given that histamine participates in the regulation of arousal and circadian rhythmicity, they indicate that induction of central histamine mechanisms may contribute to the development of neuropsychiatric symptoms, such as sleep disturbances and altered circadian rhythms in chronic HE and suggest that pharmacological manipulation of the histaminergic system could be beneficial in the treatment of HE in chronic liver failure.

Cell-selective effects of ammonia on glutamate transporter and receptor function in the mammalian brain

Increased brain ammonia concentrations are a hallmark feature of several neurological disorders including congenital urea cycle disorders, Reye's syndrome and hepatic encephalopathy (HE) associated with liver failure. Over the last decade, increasing evidence suggests that hyperammonemia leads to alterations in the glutamatergic neurotransmitter system. Studies utilizing in vivo and in vitro models of hyperammonemia reveal significant changes in brain glutamate levels, glutamate uptake and glutamate receptor function. Extracellular brain glutamate levels are consistently increased in rat models of acute liver failure. Furthermore, glutamate transport studies in both cultured neurons and astrocytes demonstrate a significant suppression in the high affinity uptake of glutamate following exposure to ammonia. Reductions in NMDA and non-NMDA glutamate receptor sites in animal models of acute liver failure suggest a compensatory decrease in receptor levels in the wake of rising extracellular levels of glutamate. Ammonia exposure also has significant effects on metabotropic glutamate receptor activation with implications, although less clear, that may relate to the brain edema and seizures associated with clinical hyperammonemic pathologies. Therapeutic measures aimed at these targets could result in effective measures for the prevention of CNS consequences in hyperammonemic syndromes.

Effects of hypothermia on brain glucose metabolism in acute liver failure: a H/C-nuclear magnetic resonance study

BACKGROUND & AIMS

Mild hypothermia has a protective effect on brain edema and encephalopathy in both experimental and human acute liver failure. The goals of the present study were to examine the effects of mild hypothermia (35 degrees C) on brain metabolic pathways using combined (1)H and (13)C-Nuclear Magnetic Resonance (NMR) spectroscopy, a technique which allows the study not only of metabolite concentrations but also their de novo synthesis via cell-specific pathways in the brain.

METHODS

(1)H and (13)C NMR spectroscopy using [1-(13)C] glucose was performed on extracts of frontal cortex obtained from groups of rats with acute liver failure induced by hepatic devascularization whose body temperature was maintained either at 37 degrees C (normothermic) or 35 degrees C (hypothermic), and appropriate sham-operated controls.

RESULTS

At coma stages of encephalopathy in the normothermic acute liver failure animals, glutamine concentrations in frontal cortex increased 3.5-fold compared to sham-operated controls (P < 0.001). Comparable increases of brain glutamine were observed in hypothermic animals despite the absence of severe encephalopathy (coma). Brain glutamate and aspartate concentrations were respectively decreased to 60.9% +/- 7.7% and 42.2% +/- 5.9% (P < 0.01) in normothermic animals with acute liver failure compared to control and were restored to normal values by mild hypothermia. Concentrations of lactate and alanine in frontal cortex were increased to 169.2% +/- 15.6% and 267.3% +/- 34.0% (P < 0.01) respectively in normothermic rats compared to controls. Furthermore, de novo synthesis of lactate and alanine increased to 446.5% +/- 48.7% and 707.9% +/- 65.7% (P < 0.001), of control respectively, resulting in increased fractional (13)C-enrichments in these cytosolic metabolites. Again, these changes of lactate and alanine concentrations were prevented by mild hypothermia.

CONCLUSIONS

Mild hypothermia (35 degrees C) prevents the encephalopathy and brain edema resulting from hepatic devascularization, selectively normalizes lactate and alanine synthesis from glucose, and prevents the impairment of oxidative metabolism associated with this model of ALF, but has no significant effect on brain glutamine. These findings suggest that a deficit in brain glucose metabolism rather than glutamine accumulation is the major cause of the cerebral complications of acute liver failure.

Molecular neurobiology of acute liver failure

Acute liver failure results in encephalopathy and brain edema that is characterized by astrocytic cell swelling. Molecular biological techniques have led to the identification of alterations in expression of several genes coding for key astrocytic proteins in acute liver failure. Such proteins include amino acid transporters, structural proteins, the endothelial cell glucose transporter GLUT-1, the mitochondrial "peripheral-type" benzodiazepine receptor, and the water channel protein aquaporin IV. Magnetic resonance spectroscopic studies reveal increased brain lactate concentrations that are positively correlated with severity of encephalopathy and brain edema in acute liver failure, suggesting a deficit of cellular oxidative capacity and impending brain energy failure. Mild hypothermia prevents brain edema in acute liver failure, and mechanisms responsible for this beneficial effect include reduced blood-brain ammonia transfer as well as normalization of astrocytic amino acid transport and brain energy metabolism. Further elucidation of the molecular mechanisms responsible for brain edema and encephalopathy in acute liver failure will undoubtedly lead to novel treatment strategies for these complications.

Effects of ammonia on high affinity glutamate uptake and glutamate transporter EAAT3 expression in cultured rat cerebellar granule cells

Increased levels of extracellular glutamate are a consistent feature of hepatic encephalopathy (HE) associated with liver failure and other hyperammonemic pathologies. Reduction of glutamate uptake has been described in ammonia-exposed cultured astrocytes, synaptosomes, and in animal models of hyperammonemia. In the present study, we examine the effects of pathophysiological concentrations of ammonia on D-aspartate (a non-metabolizable analog of glutamate) uptake by cultured rat cerebellar granule neurons. Exposure of these cells to ammonia resulted in time-dependent (24% reduction at 24h and 60% reduction at 5 days, P<0.001) and dose-dependent (21, 37, and 57% reduction at 1, 2.5, and 5mM for 5 days, P<0.01) suppression of D-aspartate uptake. Kinetic analyses revealed significant decreases in the velocity of uptake (V(max)) (37% decrease at 2.5mM NH(4)Cl, P<0.05 and 52% decrease at 5mM NH(4)Cl, P<0.001) as well as significant reductions in K(m) values (25% reduction at 2.5mM NH(4)Cl, P<0.05 and 45% reduction at 5mM NH(4)Cl, P<0.001). Western blotting, on the other hand, showed no significant changes in the neuronal glutamate transporter EAAC1/EAAT3 protein, the only glutamate transporter currently known to be expressed by these cells. In addition, 1H combined with 13C-NMR spectroscopy studies using the stable isotope [1-13C]-glucose demonstrated a significant increase in intracellular glutamate levels derived from the oxidative metabolism of glucose, rather than from the deamidation of exogenous glutamine in cultured granule neurons exposed to ammonia. The present study provides evidence that the effects of ammonia on glutamate uptake are not solely an astrocytic phenomenon and that unlike the astrocytic glutamate transporter counterpart, EAAT3 protein expression in cultured cerebellar granule cells is not down-regulated when exposed to ammonia. Decrease of glutamate uptake in these cellular preparations may afford an additional regulatory mechanism aimed at controlling intracellular levels of glutamate and ultimately the releasable pool of glutamate in neurons.

Induction of astrocytic cyclooxygenase-2 in epileptic patients with hippocampal sclerosis

Induction of cyclooxygenase-2 (COX-2) has been described in a wide range of neurological diseases including animal models of epilepsy. The present study was undertaken to assess COX-2 expression in hippocampal biopsies from patients with therapy-refractive temporal lobe epilepsy (TLE). For this purpose, hippocampal CA1 subfield was dissected from epileptic patients with (n=5) or without (n=2) hippocampal sclerosis (HS). COX-2 expression was investigated using immunohistochemistry and semi-quantitative RT-PCR. COX-2 immunoreactivity in TLE patient material in the absence of HS was restricted to a few neurons of the hippocampus. In the presence of HS, on the other hand, a significant induction of astrocytic COX-2 immunoreactivity associated with a concomitant increase in the steady-state level of COX-2 mRNA was observed in the CA1 subfield. These findings suggest that induction of astrocytic COX-2 is implicated in the pathogenesis of HS in TLE and is consistent with the previous findings of increased concentrations of prostaglandins in the cerebrospinal fluid of these patients.