Changes in brain metabolism during hyperammonemia and acute liver failure: results of a comparative 1H-NMR spectroscopy and biochemical investigation

Dietary protein load affects the energy and nitrogen balance requiring liver glutamate dehydrogenase to maintain physical activity

Provision of amino acids to the liver is instrumental for gluconeogenesis while it requires safe disposal of the amino group. The mitochondrial enzyme glutamate dehydrogenase (GDH) is central for hepatic ammonia detoxification by deaminating excessive amino acids toward ureagenesis and preventing hyperammonemia. The present study investigated the early adaptive responses to changes in dietary protein intake in control mice and liver-specific GDH KO mice (Hep-Glud1-/-). Mice were fed chow diets with a wide coverage of protein contents; i.e., suboptimal 10%, standard 20%, over optimal 30%, and high 45% protein diets; switched every 4 days. Metabolic adaptations of the mice were assessed in calorimetric chambers before tissue collection and analyses. Hep-Glud1-/- mice exhibited impaired alanine induced gluconeogenesis and constitutive hyperammonemia. The expression and activity of GDH in liver lysates were not significantly changed by the different diets. However, applying an in situ redox-sensitive assay on cryopreserved tissue sections revealed higher hepatic GDH activity in mice fed the high-protein diets. On the same section series, immunohistochemistry provided corresponding mapping of the GDH expression. Cosinor analysis from calorimetric chambers showed that the circadian rhythm of food intake and energy expenditure was altered in Hep-Glud1-/- mice. In control mice, energy expenditure shifted from carbohydrate to amino acid oxidation when diet was switched to high protein content. This shift was impaired in Hep-Glud1-/- mice and consequently the spontaneous physical activity was markedly reduced in GDH KO mice. These data highlight the central role of liver GDH in the energy balance adaptation to dietary proteins.

Residual Amino Acid Imbalance in Rats during Recovery from Acute Thioacetamide-Induced Hepatic Encephalopathy Indicates Incomplete Healing

The delayed consequences of the influence of hepatic encephalopathy (HE) on the metabolism of animals have not been studied enough. We have previously shown that the development of acute HE under the influence of the thioacetamide (TAA) toxin is accompanied by pathological changes in the liver, an imbalance in CoA and acetyl CoA, as well as a number of metabolites of the TCA cycle. This paper discusses the change in the balance of amino acids (AAs) and related metabolites, as well as the activity of glutamine transaminase (GTK) and ω-amidase enzymes in the vital organs of animals 6 days after a single exposure to TAA. The balance of the main AAs in blood plasma, liver, kidney, and brain samples of control (n = 3) and TAA-induced groups (n = 13) of rats that received the toxin at doses of 200, 400, and 600 mg/kg was considered. Despite the apparent physiological recovery of the rats at the time of sampling, a residual imbalance in AA and associated enzymes persisted. The data obtained give an idea of the metabolic trends in the body of rats after their physiological recovery from TAA exposure and may be useful for prognostic purposes when choosing the necessary therapeutic agents.

Emerging findings of glutamate-glutamine imbalance in the medial prefrontal cortex in attention deficit/hyperactivity disorder: systematic review and meta-analysis of spectroscopy studies

One of the main challenges in investigating the neurobiology of ADHD is our limited capacity to study its neurochemistry in vivo. Magnetic resonance spectroscopy (MRS) estimates metabolite concentrations within the brain, but approaches and findings have been heterogeneous. To assess differences in brain metabolites between patients with ADHD and healthy controls, we searched 12 databases screening for MRS studies. Studies were divided into 'children and adolescents' and 'adults' and meta-analyses were performed for each brain region with more than five studies. The quality of studies was assessed by the Newcastle-Ottawa Scale. Thirty-three studies met our eligibility criteria, including 874 patients with ADHD. Primary analyses revealed that the right medial frontal area of children with ADHD presented higher concentrations of a composite of glutamate and glutamine (p = 0.02, SMD = 0.53). Glutamate might be implicated in pruning and neurodegenerative processes as an excitotoxin, while glutamine excess might signal a glutamate depletion that could hinder neurotrophic activity. Both neuro metabolites could be implicated in the differential cortical thinning observed in patients with ADHD across all ages. Notably, more homogeneous designs and reporting guidelines are the key factors to determine how suitable MRS is for research and, perhaps, for clinical psychiatry. Results of this meta-analysis provided an overall map of the brain regions evaluated so far, addressed the role of glutamatergic metabolites in the pathophysiology of ADHD, and pointed to new perspectives for consistent use of the tool in the field.

Increased susceptibility to quinolinic acid-induced seizures and long-term changes in brain oscillations in an animal model of glutaric acidemia type I

Glutaric acidemia type I (GA-I) is an inborn error of metabolism of lysine, hydroxylysine, and tryptophan, caused by glutaryl-CoA-dehydrogenase (GCDH) deficiency, characterized by the buildup of toxic organic acids predominantly in the brain. After acute catabolic states, patients usually develop striatal degeneration, but the mechanisms behind this damage are still unknown. Quinolinic acid (QA), a metabolite of the kynurenine pathway, increases especially during infections/inflammatory processes, and could act synergically with organic acids, contributing to the neurological features of GA-I. The aim of this study was to investigate whether QA increases seizure susceptibility and modifies brain oscillation patterns in an animal model of GA-I, the Gcdh^-/- mice taking high-lysine diet (Gcdh^-/- -Lys). Therefore, the characteristics of QA-induced seizures and changes in brain oscillatory patterns were evaluated by video-electroencephalography (EEG) analysis recorded in Gcdh^-/- -Lys, Gcdh^+/+ -Lys, and Gcdh^-/- -N (normal diet) animals. We found that the number of seizures per animal was similar for all groups receiving QA, Gcdh^-/- -Lys-QA, Gcdh^+/+ -Lys-QA, and Gcdh^-/- -N-QA. However, severe seizures were observed in the majority of Gcdh^-/- -Lys-QA mice (82%), and only in 25% of Gcdh^+/+ -Lys-QA and 44% of Gcdh^-/- -N-QA mice. All Gcdh^-/- -Lys animals developed spontaneous recurrent seizures (SRS), but Gcdh^-/- -Lys-QA animals had increased number of SRS, higher mortality rate, and significant predominance of lower frequency oscillations on EEG. Our results suggest that QA plays an important role in the neurological features of GA-I, as Gcdh^-/- -Lys mice exhibit increased susceptibility to intrastriatal QA-induced seizures and long-term changes in brain oscillations.

Glutamate Dehydrogenase Is Important for Ammonia Fixation and Amino Acid Homeostasis in Brain During Hyperammonemia

Impaired liver function may lead to hyperammonemia and risk for hepatic encephalopathy. In brain, detoxification of ammonia is mediated mainly by glutamine synthetase (GS) in astrocytes. This requires a continuous de novo synthesis of glutamate, likely involving the action of both pyruvate carboxylase (PC) and glutamate dehydrogenase (GDH). An increased PC activity upon ammonia exposure and the importance of PC activity for glutamine synthesis has previously been demonstrated while the importance of GDH for generation of glutamate as precursor for glutamine synthesis has received little attention. We therefore investigated the functional importance of GDH for brain metabolism during hyperammonemia. To this end, brain slices were acutely isolated from transgenic CNS-specific GDH null or litter mate control mice and incubated in aCSF containing [U-¹³C]glucose in the absence or presence of 1 or 5 mM ammonia. In another set of experiments, brain slices were incubated in aCSF containing 1 or 5 mM ¹⁵N-labeled NH₄Cl and 5 mM unlabeled glucose. Tissue extracts were analyzed for isotopic labeling in metabolites and for total amounts of amino acids. As a novel finding, we reveal a central importance of GDH function for cerebral ammonia fixation and as a prerequisite for de novo synthesis of glutamate and glutamine during hyperammonemia. Moreover, we demonstrated an important role of the concerted action of GDH and alanine aminotransferase in hyperammonemia; the products alanine and α-ketoglutarate serve as an ammonia sink and as a substrate for ammonia fixation via GDH, respectively. The role of this mechanism in human hyperammonemic states remains to be studied.

In vivo N-15 MRS study of glutamate metabolism in the rat brain

In vivo ¹⁵N MRS has made a unique contribution to kinetic studies of the individual pathways that control glutamate flux in the rat brain. This review covers the following topics: (1) the advantages and limitations of in vivo ¹⁵N MRS and its indirect detection through coupled ¹H; (2) kinetic methods; (3) major findings from our and other laboratories in the areas: (a) the uptake of the neurotransmitter glutamate from the extracellular fluid into glia; (b) the metabolism of glutamate to glutamine; (c) glutamine transport to the extracellular fluid; (d) hydrolysis of neuronal glutamine to glutamate; and (e) contribution of transamination from leucine to replenish the glutamate nitrogen. In vivo glutamine synthetase activities measured at several levels of hyperammonemia showed that this enzyme becomes saturated at blood ammonia concentration >0.9 μmol/g, and causes the elevation of brain ammonia. Implications of the results for the cause of hyperammonemic encephalopathy are discussed. Leucine provides >25% of glutamate nitrogen. An intriguing possibility that supplementing leucine may restore cognitive function after brain injury is discussed. Finally, some characteristics of ¹⁵N MRS that may facilitate the future application of this technique to the study of the human brain at 4 or 7 T are described.

Elevated cerebral lactate: Implications in the pathogenesis of hepatic encephalopathy

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

In vivo studies of brain metabolism in animal models of Hepatic Encephalopathy using ¹H Magnetic Resonance Spectroscopy

Hepatic encephalopathy (HE) is a common and severe neuropsychiatric complication present in acute and chronic liver disease. The unique advantages of high field (1)H MRS provide a method for assessing pathogenic mechanism, diagnosis and monitoring of HE, as well as for treatment assessment or recovery after liver transplantation, in a reproducible and reliable non-invasive way. The purpose of the present review is to present some new features of in vivo proton Magnetic Resonance Spectroscopy ((1)H MRS) at high magnetic fields combined with some basic requirements for reliable metabolic profiling. Finally, in vivo applications of (1)H MRS in different HE animal models are presented.

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.

The role of glutamine synthetase and glutamate dehydrogenase in cerebral ammonia homeostasis

In the brain, glutamine synthetase (GS), which is located predominantly in astrocytes, is largely responsible for the removal of both blood-derived and metabolically generated ammonia. Thus, studies with [(13)N]ammonia have shown that about 25 % of blood-derived ammonia is removed in a single pass through the rat brain and that this ammonia is incorporated primarily into glutamine (amide) in astrocytes. Major pathways for cerebral ammonia generation include the glutaminase reaction and the glutamate dehydrogenase (GDH) reaction. The equilibrium position of the GDH-catalyzed reaction in vitro favors reductive amination of α-ketoglutarate at pH 7.4. Nevertheless, only a small amount of label derived from [(13)N]ammonia in rat brain is incorporated into glutamate and the α-amine of glutamine in vivo. Most likely the cerebral GDH reaction is drawn normally in the direction of glutamate oxidation (ammonia production) by rapid removal of ammonia as glutamine. Linkage of glutamate/α-ketoglutarate-utilizing aminotransferases with the GDH reaction channels excess amino acid nitrogen toward ammonia for glutamine synthesis. At high ammonia levels and/or when GS is inhibited the GDH reaction coupled with glutamate/α-ketoglutarate-linked aminotransferases may, however, promote the flow of ammonia nitrogen toward synthesis of amino acids. Preliminary evidence suggests an important role for the purine nucleotide cycle (PNC) as an additional source of ammonia in neurons (Net reaction: L-Aspartate + GTP + H(2)O → Fumarate + GDP + P(i) + NH(3)) and in the beat cycle of ependyma cilia. The link of the PNC to aminotransferases and GDH/GS and its role in cerebral nitrogen metabolism under both normal and pathological (e.g. hyperammonemic encephalopathy) conditions should be a productive area for future research.

Reprint of: Neuroinflammation in acute liver failure: mechanisms and novel therapeutic targets

It is increasingly evident that neuroinflammatory mechanisms are implicated in the pathogenesis of the central nervous system (CNS) complications (intracranial hypertension, brain herniation) of acute liver failure (ALF). Neuroinflammation in ALF is characterized by microglial activation and arterio-venous difference studies as well as studies of gene expression confirm local brain production and release of proinflammatory cytokines including TNF-α and the interleukins IL-1β and IL-6. Although the precise nature of the glial cell responsible for brain cytokine synthesis is not yet established, evidence to date supports a role for both astrocytes and microglia. The neuroinflammatory response in ALF progresses in parallel with the progression of hepatic encephalopathy (HE) and with the severity of brain edema (astrocyte swelling). Mechanisms responsible for the relaying of signals from the failing liver to the brain include transduction of systemic proinflammatory signals as well as the effects of increased brain lactate leading to increased release of cytokines from both astrocytes and microglia. There is evidence in support of a synergistic effect of proinflammatory cytokines and ammonia in the pathogenesis of HE and brain edema in ALF. Therapeutic implications of the findings of a neuroinflammatory response in ALF are multiple. Removal of both ammonia and proinflammatory cytokines is possible using antibiotics or albumen dialysis. Mild hypothermia reduces brain ammonia transfer, brain lactate production, microglial activation and proinflammatory cytokine production resulting in reduced brain edema and intracranial pressure in ALF. N-Acetylcysteine acts as both an antioxidant and anti-inflammatory agent at both peripheral and central sites of action independently resulting in slowing of HE progression and prevention of brain edema. Novel treatments that directly target the neuroinflammatory response in ALF include the use of etanercept, a TNF-α neutralizing molecule and minocycline, an agent with potent inhibitory actions on microglial activation that are independent of its antimicrobial properties; both agents have been shown to be effective in reducing neuroinflammation and in preventing the CNS complications of ALF. Translation of these findings to the clinic has the potential to provide rational targeted approaches to the prevention and treatment of these complications in the near future.

Pathogenesis of hepatic encephalopathy and brain edema in acute liver failure: role of glutamine redefined

Acute liver failure (ALF) is characterized neuropathologically by cytotoxic brain edema and biochemically by increased brain ammonia and its detoxification product, glutamine. The osmotic actions of increased glutamine synthesis in astrocytes are considered to be causally related to brain edema and its complications (intracranial hypertension, brain herniation) in ALF. However studies using multinuclear (1)H- and (13)C-NMR spectroscopy demonstrate that neither brain glutamine concentrations per se nor brain glutamine synthesis rates correlate with encephalopathy grade or the presence of brain edema in ALF. An alternative mechanism is now proposed whereby the newly synthesized glutamine is trapped within the astrocyte as a consequence of down-regulation of its high affinity glutamine transporter SNAT5 in ALF. Restricted transfer out of the cell rather than increased synthesis within the cell could potentially explain the cell swelling/brain edema in ALF. Moreover, the restricted transfer of glutamine from the astrocyte to the adjacent glutamatergic nerve terminal (where glutamine serves as immediate precursor for the releasable/transmitter pool of glutamate) could result in decreased excitatory transmission and excessive neuroinhibition that is characteristic of encephalopathy in ALF. Paradoxically, in spite of renewed interest in arterial ammonia as a predictor of raised intracranial pressure and brain herniation in ALF, ammonia-lowering agents aimed at reduction of ammonia production in the gut have so far been shown to be of limited value in the prevention of these cerebral consequences. Mild hypothermia, shown to prevent brain edema and intracranial hypertension in both experimental and human ALF, does so independent of effects on brain glutamine synthesis; whether or not hypothermia restores expression levels of SNAT5 in ALF awaits further studies. While inhibitors of brain glutamine synthesis such as methionine sulfoximine, have been proposed for the prevention of brain edema in ALF, potential adverse effects have so far limited their applicability.

The role of glutamate dehydrogenase in mammalian ammonia metabolism

Glutamate dehydrogenase (GDH) catalyzes the reversible inter-conversion of glutamate to α-ketoglutarate and ammonia. High levels of GDH activity is found in mammalian liver, kidney, brain, and pancreas. In the liver, GDH reaction appears to be close-to-equilibrium, providing the appropriate ratio of ammonia and amino acids for urea synthesis in periportal hepatocytes. In addition, GDH produces glutamate for glutamine synthesis in a small rim of pericentral hepatocytes. Hence, hepatic GDH can be either a source for ammonia or an ammonia scavenger. In the kidney, GDH function produces ammonia from glutamate to control acidosis. In the human, the presence of two differentially regulated isoforms (hGDH1 and hGDH2) suggests a complex role for GDH in ammonia homeostasis. Whereas hGDH1 is sensitive to GTP inhibition, hGDH2 has dissociated its function from GTP control. Furthermore, hGDH2 shows a lower optimal pH than hGDH1. The hGDH2 enzyme is selectively expressed in human astrocytes and Sertoli cells, probably facilitating metabolic recycling processes essential for their supportive role. Here, we report that hGDH2 is also expressed in the epithelial cells lining the convoluted tubules of the renal cortex. As hGDH2 functions more efficiently under acidotic conditions without the operation of the GTP energy switch, its presence in the kidney may increase the efficacy of the organ to maintain acid base equilibrium.

Multi-dimensional MR spectroscopy: towards a better understanding of hepatic encephalopathy

Hepatic encephalopathy (HE) is normally diagnosed by neuropsychological (NP) tests. The goals of this study were to quantify cerebral metabolites, separate glutamate (Glu) from glutamine (Gln) in patients with minimal hepatic encephalopathy (MHE) as well as healthy subjects using the prior-knowledge fitting (ProFit) algorithm on data acquired by two-dimensional (2D) localized correlated spectroscopy (L-COSY) on two different MR scanners, and to correlate the metabolite changes with neuropsychological (NP) tests. We studied 14 MHE patients and 18 healthy controls using a GE 1.5 T Signa MR scanner. Another group of 16 MHE patients and 18 healthy controls were studied using a Siemens 1.5 T Avanto MR scanner. The following parameters were used for L-COSY: TR/TE = 2 s/30 ms, 3 × 3 × 3 cm(3) voxel size, 96 Δt(1) increments with 8 averages per Δt(1). Using the ProFit algorithm, we were able to differentiate Gln from Glu on the GE 1.5 T data in the medial frontal white/gray matter. The ratios of myo-inositol (mI), Glu, total choline, scyllo-inositol (sI), phosphoethanolamine (PE), and total N-acetyl aspartate (NAA) showed statistically significant decline in HE patients compared to healthy controls, while the ratio of Gln was significantly increased. Similar trend was seen in the ProFit quantified Siemens 1.5 T data in the frontal and occipito-parietal white/gray regions. Among the NP domain scores, motor function, cognitive speed, executive function and the global scores showed significant differences. Excellent correlations between various NP domains and metabolite ratios were also observed. ProFit based cerebral metabolite quantitation enhances the understanding and basis of the current hypothesis of MHE.

Neuroinflammation in acute liver failure: mechanisms and novel therapeutic targets

It is increasingly evident that neuroinflammatory mechanisms are implicated in the pathogenesis of the central nervous system (CNS) complications (intracranial hypertension, brain herniation) of acute liver failure (ALF). Neuroinflammation in ALF is characterized by microglial activation and arterio-venous difference studies as well as studies of gene expression confirm local brain production and release of proinflammatory cytokines including TNF-α and the interleukins IL-1β and IL-6. Although the precise nature of the glial cell responsible for brain cytokine synthesis is not yet established, evidence to date supports a role for both astrocytes and microglia. The neuroinflammatory response in ALF progresses in parallel with the progression of hepatic encephalopathy (HE) and with the severity of brain edema (astrocyte swelling). Mechanisms responsible for the relaying of signals from the failing liver to the brain include transduction of systemic proinflammatory signals as well as the effects of increased brain lactate leading to increased release of cytokines from both astrocytes and microglia. There is evidence in support of a synergistic effect of proinflammatory cytokines and ammonia in the pathogenesis of HE and brain edema in ALF. Therapeutic implications of the findings of a neuroinflammatory response in ALF are multiple. Removal of both ammonia and proinflammatory cytokines is possible using antibiotics or albumen dialysis. Mild hypothermia reduces brain ammonia transfer, brain lactate production, microglial activation and proinflammatory cytokine production resulting in reduced brain edema and intracranial pressure in ALF. N-Acetylcysteine acts as both an antioxidant and anti-inflammatory agent at both peripheral and central sites of action independently resulting in slowing of HE progression and prevention of brain edema. Novel treatments that directly target the neuroinflammatory response in ALF include the use of etanercept, a TNF-α neutralizing molecule and minocycline, an agent with potent inhibitory actions on microglial activation that are independent of its antimicrobial properties; both agents have been shown to be effective in reducing neuroinflammation and in preventing the CNS complications of ALF. Translation of these findings to the clinic has the potential to provide rational targeted approaches to the prevention and treatment of these complications in the near future.

Primary somatosensory cortex in chronic low back pain - a H-MRS study

The goal of this study was to investigate whether certain metabolites, specific to neurons, glial cells, and the neuronal-glial neurotransmission system, in the primary somatosensory cortex (SSC), are altered and correlated with clinical characteristics of pain in patients with chronic low back pain (LBP). Eleven LBP patients and eleven age-matched healthy controls were included. N-acetylaspartate (NAA), choline (Cho), myo-inositol (mI), and glutamine/glutamate (Glx) were measured with proton magnetic resonance spectroscopy (¹H-MRS) in left and right SSC. Differences in metabolite concentrations relative to those of controls were evaluated as well as analyses of metabolite correlations within and between SSCs. Relationships between metabolite concentrations and pain characteristics were also evaluated. We found decreased NAA in the left SSC (P = 0.001) and decreased Cho (P = 0.04) along with lower correlations between all metabolites in right SSC (P = 0.007) in LBP compared to controls. In addition, we found higher and significant correlations between left and right mI (P < 0.001 in LBP vs P = 0.1 in controls) and between left mI and right Cho (P = 0.048 vs P = 0.6). Left and right NAA levels were negatively correlated with pain duration (P = 0.04 and P = 0.02 respectively) while right Glx was positively correlated with pain severity (P = 0.04). Our preliminary results demonstrated significant altered neuronal-glial interactions in SSC, with left neural alterations related to pain duration and right neuronal-glial alterations to pain severity. Thus, the ¹H-MRS approach proposed here can be used to quantify relevant cerebral metabolite changes in chronic pain, and consequently increase our knowledge of the factors leading from these changes to clinical outcomes.

Correlation of findings in advanced MRI techniques with global severity scales in patients with Parkinson disease

RATIONALE AND OBJECTIVES

This work is aimed at determining whether magnetic resonance spectroscopy (MRS) and diffusion tensor imaging (DTI) may correlate with disease severity in a series of Parkinson disease (PD) patients.

MATERIALS AND METHODS

We recruited a consecutive sample of 39 PD patients in several stages of the disease according to Hoehn and Yahr scale. There were 22 men, and the mean age was 74.5 years (SD 7.5). Disease severity was measured with the Unified Parkinson Disease Rating Scale (UPDRS). All of them underwent ¹H MRS in basal ganglia and the anterior cingulate area, as well as DTI in bilateral substantia nigra. Correlation was made between radiological findings and UPDRS.

RESULTS

We found significant negative correlation between UPDRS scores and the Glx (glutamate+glutamine) levels in the right (r = -0.35; P = .03) and the left (r = -0.44; P = .006) lentiform nucleus; as well as with glutamate (r = -0.43; P = .008), the Glx/Cr ratio in the right (r = -0.41; P = .01), and in the left lentiform nucleus (r = -0.36; P = .02). We also found positive correlation between UPDRS scores and DTI in right rostral substantia nigra (r = 0.36; P = .02). Glx was increased in lentiform nucleus and fractional anisotropy was reduced in the rostral SN of subjects with PD in early stages.

CONCLUSIONS

The results are consistent with the view that more than half the dopaminergic neurons in the nigrostriatal projection are lost before the onset of PD.

Localized 1H-NMR spectroscopy in patients with fibromyalgia: a controlled study of changes in cerebral glutamate/glutamine, inositol, choline, and N-acetylaspartate

INTRODUCTION

The purpose of this study was to investigate whether single-voxel (SV) proton magnetic resonance spectroscopy (MRS), diffusion-weighted imaging (DWI), and diffusion tensor imaging (DTI) detected differences between fibromyalgia (FM) patients and healthy controls. We also searched for correlations between neuroimaging abnormalities and neuropsychological variables.

METHODS

Ten patients with FM and 10 gender- and age-matched control subjects were studied. A neuropsychological examination, DWI, DTI, and proton MRS were performed on the brain areas known to be associated with pain processing.

RESULTS

Compared with healthy controls, FM patients had significantly higher levels of glutamate + glutamine (Glx) (mean ± SD, 10.71 ± 0.50 arbitrary institutional units versus 9.89 ± 1.04; P = 0.049) and higher glutamate + glutamine/creatine (Glx/Cr) ratios (1.90 ± 0.12 versus 1.72 ± 0.23; P = 0.034) in the posterior gyrus. Myoinositol (Ins) levels of the right and left hippocampi were significantly lower in FM patients (4.49 ± 0.74 versus 5.17 ± 0.62; P = 0.008 and 4.91 ± 0.85 versus 6.09 ± 0.78; P = 0.004, respectively). In FM patients, decreased myoinositol/creatine (Ins/Cr) ratios were found in the left sensorimotor area (P = 0.05) and the left hippocampus (P = 0.002) and lower levels of choline (P = 0.019) and N-acetyl aspartate + N-acetyl aspartyl glutamate (NAA + NAG) (P = 0.034) in the left hippocampus. Significant correlations between depression, pain, and global function and the posterior gyrus Glx levels and Glx/Cr ratios were observed.

CONCLUSIONS

Glx within the posterior gyrus could be a pathologic factor in FM. Hippocampal dysfunction may be partially responsible for the depressive symptoms of FM. Additional studies with larger samples are required to confirm these preliminary data.

Increase brain lactate in hepatic encephalopathy: cause or consequence?

Hepatic encephalopathy (HE) is a complex neuropsychiatric syndrome which develops as a result of liver failure or disease. Increased concentrations of brain lactate (microdialysate, cerebrospinal fluid, tissue) are commonly measured in patients with HE induced by either acute or chronic liver failure. Whether an increase in brain lactate is a cause or a consequence of HE remains undetermined. A rise in cerebral lactate may occur due to (1) blood-borne lactate (hyperlactataemia) crossing the blood-brain barrier, (2) increased glycolysis due to energy failure or impairment and (3) increased lactate production/release or decreased lactate utilization/uptake. This review explores the different reasons for lactate accumulation in the brain during liver failure and describes the possible roles of lactate in the pathogenesis of HE.

Detoxification of ammonia in mouse cortical GABAergic cell cultures increases neuronal oxidative metabolism and reveals an emerging role for release of glucose-derived alanine

Cerebral hyperammonemia is believed to play a pivotal role in the development of hepatic encephalopathy (HE), a debilitating condition arising due to acute or chronic liver disease. In the brain, ammonia is thought to be detoxified via the activity of glutamine synthetase, an astrocytic enzyme. Moreover, it has been suggested that cerebral tricarboxylic acid (TCA) cycle metabolism is inhibited and glycolysis enhanced during hyperammonemia. The aim of this study was to characterize the ammonia-detoxifying mechanisms as well as the effects of ammonia on energy-generating metabolic pathways in a mouse neuronal-astrocytic co-culture model of the GABAergic system. We found that 5 mM ammonium chloride affected energy metabolism by increasing the neuronal TCA cycle activity and switching the astrocytic TCA cycle toward synthesis of substrate for glutamine synthesis. Furthermore, ammonia exposure enhanced the synthesis and release of alanine. Collectively, our results demonstrate that (1) formation of glutamine is seminal for detoxification of ammonia; (2) neuronal oxidative metabolism is increased in the presence of ammonia; and (3) synthesis and release of alanine is likely to be important for ammonia detoxification as a supplement to formation of glutamine.

In vivo evidence for alcohol-induced neurochemical changes in rat brain without protracted withdrawal, pronounced thiamine deficiency, or severe liver damage

Magnetic resonance spectroscopy (MRS) studies in human alcoholics report decreases in N-acetylaspartate (NAA) and choline-containing (Cho) compounds. Whether alterations in brain metabolite levels are attributable to alcohol per se or to physiological effects of protracted withdrawal or impaired nutritional or liver status remains unclear. Longitudinal effects of alcohol on brain metabolites measured in basal ganglia with single-voxel MRS were investigated in sibling pairs of wild-type Wistar rats, with one rat per pair exposed to escalating doses of vaporized alcohol, the other to vapor chamber air. MRS was conducted before alcohol exposure and twice during exposure. After 16 weeks of alcohol exposure, rats achieved average blood alcohol levels (BALs) of approximately 293 mg per 100 ml and had higher Cho and a trend for higher glutamine+glutamate (Glx) than controls. After 24 weeks of alcohol exposure, BALs rose to approximately 445 mg per 100 ml, and alcohol-exposed rats had higher Cho, Glx, and glutamate than controls. Thiamine and thiamine monophosphate levels were significantly lower in the alcohol than the control group but did not reach levels low enough to be considered clinically relevant. Histologically, livers of alcohol-exposed rats exhibited greater steatosis and lower glycogenosis than controls, but were not cirrhotic. This study demonstrates a specific pattern of neurobiochemical changes suggesting excessive membrane turnover or inflammation, indicated by high Cho, and alterations to glutamate homeostasis in the rat brain in response to extended vaporized alcohol exposure. Thus, we provide novel in vivo evidence for alcohol exposure as causing changes in brain chemistry in the absence of protracted withdrawal, pronounced thiamine deficiency, or severe liver damage.

Acute and chronic liver insufficiency

The term “liver insufficiency” denotes a break down in the functions of the liver. The syndrome of functional liver failure covers a wide spectrum of clinical, biochemical and neurophysiological changes. In principle, liver insufficiency can occur without previous liver damage as well as with already existing liver disease. It is characterized by a deterioration in the synthesizing, regulatory and detoxifying function of the liver. This final stage of liver disease terminates in hepatic coma.

New concepts in the mechanism of ammonia-induced astrocyte swelling

It is generally accepted that astrocyte swelling forms the major anatomic substrate of the edema associated with acute liver failure (ALF) and that ammonia represents a major etiological factor in its causation. The mechanisms leading to such swelling, however, remain elusive. Recent studies have invoked the role of oxidative stress in the mechanism of hepatic encephalopathy (HE), as well as in the brain edema related to ALF. This article summarizes the evidence for oxidative stress as a major pathogenetic factor in HE/ALF and discusses mechanisms that are triggered by oxidative stress, including the induction of the mitochondrial permeability transition (MPT) and activation of signaling kinases. We propose that a cascade of events initiated by ammonia-induced oxidative stress results in cell volume dysregulation leading to cell swelling/brain edema. Blockade of this cascade may provide novel therapies for the brain edema associated with ALF.

Nuclear magnetic resonance studies of energy metabolism and glutamine shunt in hepatic encephalopathy and hyperammonemia

Hepatic encephalopathy (HE) in both acute and chronic liver failure is more likely a reversible functional disease rather than an irreversible pathological lesion of brain cells. Metabolic alterations underlie many of the mechanisms leading to HE. This paper summarizes in vivo and ex vivo (1)H-, (13)C-, and (15)N-nuclear magnetic resonance (NMR) spectroscopy data on patients and experimental models of HE. In vivo NMR spectroscopy provides a unique opportunity to study metabolic changes noninvasively in the brain in vivo, and to quantify various metabolites in localized brain areas, and ex vivo NMR permits the high-resolution measurement of metabolites and the identification of different metabolic pathways. In vivo and ex vivo (1)H-NMR investigations consistently reveal severalfold increases in brain glutamine and concomitant decreases in myo-inositol, an important osmolyte in astrocytes. An osmotic disturbance in these cells has long been suggested to be responsible for astrocyte swelling and brain edema. However, ex vivo (13)C-NMR studies have challenged the convention that glutamine accumulation is the major cause of brain edema in acute HE. They rather indicate a limited anaplerotic flux and capacity of astrocytes to detoxify ammonia by glutamine synthesis and emphasize distortions of energy and neurotransmitter metabolism. However, recent (15)N-NMR investigations have demonstrated that glutamine fluxes between neurons and astrocytes are affected by ammonia. Further NMR studies may provide novel insights into the relationship between brain edema and/or astrocyte pathology and changes in inter- and intracellular glutamine homeostasis, which may secondarily alter brain energy metabolism.

1H NMR spectroscopy for the prediction of therapeutic outcome in patients with fulminant hepatic failure

A high-resolution (1)H NMR study of serum and urine of fulminant hepatic failure patients (n = 22) [surviving (n = 12) and non-surviving (n = 10)] is reported. Glutamine in serum and urine glutamine:creatinine ratio were higher in non-surviving patients compared with surviving patients [serum glutamine, 3.08 (1.68-7.11) vs 0.56 (0.34-0.99) mM, median and range; p = 0.0001 and urine glutamine:creatinine ratio, 1.72 (0.24-7.76) vs 0.39 (0.1-0.84), p = 0.1], and urine urea:creatinine ratio was higher in surviving patients compared with non-surviving patients [10.83 (0.2-22.6) vs 2.09 (0.96-4.0), p = 0.002]. On the other hand, no significant differences were found in the conventionally employed clinical parameters such as serum alanylaminotransferase, aspartylaminotransferase and bilirubin except prothrombin time (p = 0.02). The difference in serum glutamine and urine urea was significant in the two categories of patients and distinctly different values of serum glutamine for both the categories of patients correctly predicted the outcome. These results promise immense potential for NMR spectroscopy in rapidly deciding on the need for advanced therapeutic intervention such as artificial liver support or emergency liver transplantation in FHF.

The brain glutamate system in liver failure

Liver failure results in significant alterations of the brain glutamate system. Ammonia and the astrocyte play major roles in such alterations, which affect several components of the brain glutamate system, namely its synthesis, intercellular transport (uptake and release), and function. In addition to the neurological symptoms of hepatic encephalopathy, modified glutamatergic regulation may contribute to other cerebral complications of liver failure, such as brain edema, intracranial hypertension and changes in cerebral blood flow. A better understanding of the cause and precise nature of the alterations of the brain glutamate system in liver failure could lead to new therapeutic avenues for the cerebral complications of liver disease.

Oxidative stress and mitogen-activated protein kinase phosphorylation mediate ammonia-induced cell swelling and glutamate uptake inhibition in cultured astrocytes

Hepatic encephalopathy (HE) is a major neurological complication in patients with severe liver failure. Elevated levels of ammonia have been strongly implicated as a factor in HE, and astrocytes appear to be the primary target of its neurotoxicity. Mechanisms mediating key aspects of ammonia-induced astrocyte dysfunction such as cell swelling and inhibition of glutamate uptake are not clear. We demonstrated previously that cultured astrocytes exposed to ammonia increase free radical production. We now show that treatment with antioxidants significantly prevents ammonia-induced astrocyte swelling as well as glutamate uptake inhibition. Because one consequence of oxidative stress is the phosphorylation of mitogen-activated protein kinases (MAPKs), we investigated whether phosphorylation of MAPKs may mediate astrocyte dysfunction. Primary cultured astrocytes exposed to 5 mm NH4Cl for different time periods (1-72 h) significantly increased phosphorylation of extracellular signal-regulated kinase 1/2 (ERK1/2), p38(MAPK), and c-Jun N-terminal kinase (JNK) 1/2/3, which was inhibited by appropriate MAPK inhibitors 1, 4-diamino-2, 3-dicyano-1, 4-bis (2-aminophenylthio) butadiene (UO126; for ERK1/2), trans-1-(4-hydroxyclyclohexyl)-4-(4-fluorophenyl)-5-(2-methoxypyrimidin-4-yl)imidazole (SB 239063; for p38(MAPK)), and anthra[1,9-cd]pyrazol-6(2H)-one (SP600125; for JNK1/2/3), as well as by antioxidants. Kinase inhibitors partially or completely prevented astrocyte swelling. Although SB239063 and SP600125 significantly reversed glutamate uptake inhibition and ammonia-induced decline in glutamate-aspartate transporter protein levels, UO126 did not, indicating a differential effect of these kinases in ammonia-induced astrocyte swelling and glutamate transport impairment. These studies strongly suggest the involvement of oxidative stress and phosphorylation of MAPKs in the mechanism of ammonia-induced astrocyte dysfunction associated with ammonia neurotoxicity.

Selectively increased expression of the astrocytic/endothelial glucose transporter protein GLUT1 in acute liver failure

Acute liver failure (ALF) is consistently accompanied by alterations in brain energy metabolites and recent nuclear magnetic resonance (NMR) studies suggest disturbances in brain oxidative metabolism in experimental ALF. Glucose transport across the blood-brain barrier is essential to sustain brain energy metabolism and is accomplished by the facilitative glucose transporter GLUT1. To investigate alterations in brain glucose uptake in acute liver failure further, GLUT1 expression and [14C]-2-deoxy-D-glucose uptake were measured in the brains of rats with hepatic devascularization. RT-PCR and Western blot analyses showed significant increases in steady-state levels of GLUT1 mRNA and protein in frontal cortex as early as 6 h following hepatic devascularization, (prior to the onset of brain edema and encephalopathy) which remained elevated at coma stages of encephalopathy. Expression of the astrocytic (45-kDa) and endothelial (55-kDa) forms of GLUT1 was increased as a result of hepatic devascularization. Exposure of cultured astrocytes to pathophysiologically relevant concentrations of ammonia resulted in increased GLUT1 expression, suggesting that elevated ammonia levels are responsible for GLUT1 upregulation in ALF. Increased GLUT1 expression in ALF was selective, since expression of the neuronal glucose transporter GLUT3 and other glucose-regulated proteins (GRP-78 and GRP-94) was unaltered. [14C]-2-deoxy-D-glucose autoradiography revealed increases in cerebral glucose uptake following the induction of GLUT1 in ALF. These results suggest that ammonia-induced increases of GLUT1 expression resulting in increased cerebral glucose uptake occur in ALF and could contribute to the pathophysiological mechanisms responsible for the neurological complications of this condition.

Mechanisms of ammonia-induced astrocyte swelling

Astrocyte swelling represents the major factor responsible for the brain edema associated with fulminant hepatic failure (FHF). The edema may be of such magnitude as to increase intracranial pressure leading to brain herniation and death. Of the various agents implicated in the generation of astrocyte swelling, ammonia has had the greatest amount of experimental support. This article reviews mechanisms of ammonia neurotoxicity that contribute to astrocyte swelling. These include oxidative stress and the mitochondrial permeability transition (MPT). The involvement of glutamine in the production of cell swelling will be highlighted. Evidence will be provided that glutamine induces oxidative stress as well as the MPT, and that these events are critical in the development of astrocyte swelling in hyperammonemia.

Low myo-inositol and high glutamine levels in brain are associated with neuropsychological deterioration after induced hyperammonemia

The neuropsychological effect of hyperammonemia is variable. This study tests the hypothesis that the effect of ammonia on the neuropsychological function in patients with cirrhosis is determined by the ability of the brain to buffer ammonia-induced increase in glutamine within the astrocyte by losing osmolytes like myo-inositol (mI) and not by the magnitude of the induced hyperammonemia. Fourteen cirrhotic patients with no evidence of overt hepatic encephalopathy were given a 75-g amino acid (aa) solution mimicking the hemoglobin molecule to induce hyperammonemia. Measurement of a battery of neuropsychological function tests including immediate memory, ammonia, aa, and short-echo time proton magnetic resonance spectroscopy were performed before and 4 h after administration of the aa solution. Eight patients showed deterioration in the Immediate Memory Test at 4 h. Demographic factors, severity of liver disease, change in plasma ammonia, and aa profiles after the aa solution were similar in those that showed a deterioration compared with those who did not. In patients who showed deterioration in the memory test, the mI-to-creatine ratio (mI/Cr) was significantly lower at baseline than those that did not deteriorate. In contrast, the glutamate/glutamine-to-Cr ratio was significantly greater in the patients that deteriorated. The observation that deterioration in the memory test scores was greater in those with lower mI/Cr supports the hypothesis that the neuropsychological effects of induced hyperammonemia is determined by the capacity of the brain to handle ammonia-induced increase in glutamine.

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.

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.

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.

Selective increase of brain lactate synthesis in experimental acute liver failure: results of a [H-C] nuclear magnetic resonance study

Acute liver failure (ALF) results in alterations of energy metabolites and of glucose-derived amino acid neurotransmitters in brain. However, the dynamics of changes in glucose metabolism remain unclear. The present study was undertaken using (1)H and (13)C nuclear magnetic resonance (NMR) spectroscopy to determine the rates of incorporation of glucose into amino acids and lactate via cell-specific pathways in relation to the severity of encephalopathy and brain edema in rats with ALF because of hepatic devascularization. Early (precoma) stages of encephalopathy were accompanied by significant 2- to 4.5-fold (P <.001) increases of total brain glutamine and lactate concentrations. More severe (coma) stages of encephalopathy and brain edema led to a further significant increase in brain lactate but no such increase in glutamine. Furthermore, (13)C isotopomer analysis showed a selective increase of de novo synthesis of lactate from [1-(13)C]glucose resulting in 2.5-fold increased fractional (13)C enrichments in lactate at coma stages. [2-(13)C]glutamine, synthesized through the astrocytic enzyme pyruvate carboxylase, increased 10-fold at precoma stages but showed no further increase at coma stages of encephalopathy. (13)C-label incorporation into [4-(13)C]glutamate, synthesized mainly through neuronal pyruvate dehydrogenase, was selectively reduced at coma stages, whereas brain GABA synthesis was unchanged at all time points. In conclusion, increased brain lactate synthesis and impaired glucose oxidative pathways rather than intracellular glutamine accumulation are the major cause of brain edema in ALF. Future NMR spectroscopic studies using stable isotopes and real-time measurements of metabolic rates could be valuable in the elucidation of the cerebral metabolic consequences of ALF in humans.

1H magnetic resonance in the study of hepatic encephalopathy in humans

1H magnetic resonance (1H MR) studies of the brain in patients with liver diseases have shown several abnormalities that may be relevant for the pathogenesis of hepatic encephalopathy. 1H magnetic resonance imaging shows a typical pallidal hyperintensity on T1-weighted images. This abnormality appears to be secondary to the accumulation of manganese in basal ganglia because of portal-systemic shunting. No direct correlation between the magnitude of pallidal hyperintensity and the grade of hepatic encephalopathy has been found, but some studies have related pallidal hyperintensity to parkinsonism. 1H magnetic resonance spectroscopy shows relative to creatine an increase in glutamine/glutamate (Glx) signal and a decrease of choline containing compounds (Cho) and myo-inositol. Abnormalities in the Glx signal have been interpreted as an increase in brain glutamine secondary to the metabolism of ammonia in astrocytes. Disturbances of Cho and myo-inositol have been interpreted as a compensatory response to the increase in intracellular osmolality caused by the accumulation of glutamine in astrocytes. In addition, magnetization transfer imaging shows signs compatible with low-grade cerebral edema. Altogether, 1H MR studies suggest the accumulation of manganese and the development of osmotic abnormalities in the brain of patients with cirrhosis. These abnormalities appear to participate in some of the neurological manifestations of hepatic encephalopathy.

Abnormal brain energy metabolism shown by in vivo phosphorus magnetic resonance spectroscopy in patients with chronic liver disease

We used phosphorus magnetic resonance spectroscopy (31P-MRS) to assess in vivo the brain bioenergetics of 28 patients with liver cirrhosis. Seven had clinical hepatic encephalopathy (HE), nine hepatocellular carcinoma. 31P-MRS was performed by the DRESS localisation technique on occipital lobes. Brain phosphocreatine was significantly reduced in patients with or without overt HE, and inorganic phosphate was increased in both groups of patients. The cytosolic phosphorylation potential (PP), the relative rate of oxidative metabolism and the regulatory [ADP] were all abnormal. Brain PP was inversely correlated with serum ammonia concentration only in patients without liver cancer. The degree of bioenergetic failure was significantly higher in the presence of overt encephalopathy. We conclude that patients with liver cirrhosis had a derangement of brain energy metabolism, and that 31P-MRS offers a non-invasive method for investigating the underlying mechanisms of HE, with relevant implications in the identification and management of this condition.

Follow-up by one- and two-dimensional NMR of plasma from pigs with ischemia-induced acute liver failure treated with a bioartificial liver

Hepatic encephalopathy may occur following acute hepatic failure (AHF), which results in the release of toxic compounds from the injured liver. These compounds, which induce cerebral edema, are not well characterized, yet. The aim of this study was to evaluate the potential interest of NMR spectroscopy in the follow-up of different plasma compounds in pigs with ischemia-induced fulminant hepatic failure treated or not with a bioartificial liver (BAL), which has been previously shown to improve the neurological status of the animals. Qualitative analysis of pig plasma was achieved by one-dimensional-(1)H CPMG, two-dimensional homonuclear (1)H-(1)H TOCSY CPMG and heteronuclear (1)H-(13)C HSQC sequences. Semi-quantitative analysis of selected plasma metabolites along the disease evolution was carried out on pigs with ischemia-induced AHF treated with the BAL containing alginate beads with or without hepatocytes. A quantitative longitudinal follow-up was performed on characteristic metabolites via a one-dimensional CPMG sequence, including choline, glutamine, N-acetyl-glucosamine (NAG), pyruvate and trimethylamine-N-oxide (TMAO). The concentrations of choline and TMAO increased from the beginning to the end in animals treated with the BAL containing alginate beads without hepatocytes. Treatment of pigs with BAL containing hepatocytes resulted in an improvement of survival, the plasma concentrations of choline and TMAO being decreased in three out of five animals. Thus, NMR spectroscopy is a useful approach for the identification of toxic compounds which are involved in hepatic encephalopathy associated with AHF. These compounds can be cleared by a BAL resulting in the improvement of survival and neurological parameters of the animals.

Cerebral energy metabolism in hepatic encephalopathy and hyperammonemia

Hepatic encephalopathy (HE) is an important cause of morbidity and mortality in patients with severe liver disease. Although the molecular basis for the neurological disorder in HE remains elusive, elevated ammonia and its chief metabolite glutamine are believed to be important factors responsible for altered cerebral functions, including multiple neurotransmitter system(s) failure, altered bioenergetics, and more recently oxidative stress. Accumulated evidence suggests that direct interference of ammonia at several points in cerebral energy metabolism, including glycolysis, TCA cycle, and the electron transport chain, could lead to energy depletion. Additionally, recent studies from our laboratory have invoked the possibility that ammonia and glutamine may induce the mitochondrial permeability transition in astrocytes, a process capable of causing mitochondrial dysfunction. Altered mitochondrial metabolism appears to be an important mechanism responsible for the cerebral abnormalities associated with HE and other hyperammonemic states.

NMR spectroscopic study on the metabolic fate of [3-(13)C]alanine in astrocytes, neurons, and cocultures: implications for glia-neuron interactions in neurotransmitter metabolism

Nuclear magnetic resonance (NMR) spectroscopy and biochemical assays were used to study the fate of [3-(13)C]alanine in astrocytes, neurons, and cocultures. (1)H- and (13)C-NMR analysis of the media demonstrated a high and comparable uptake of [3-(13)C]alanine by the cells. Thereafter, alanine is transaminated predominantly to [3-(13)C]pyruvate, from which the (13)C-label undergoes different metabolic pathways in astrocytes and neurons: Lactate is almost exclusively synthesized in astrocytes, while in neurons and cocultures labeled neurotransmitter amino acids are formed, i.e., glutamate and gamma-aminobutyric acid (GABA). A considerable contribution of the anaplerotic pathway is observed in cocultures, as concluded from the ratio (C-2-C-3)/C-4 of labeled glutamine. Analysis of the multiplet pattern of glutamate isotopomers indicates carbon scrambling through the TCA cycle and the use of alanine also as energy substrate in neurons. In cocultures, astrocyte-deduced lactate and unlabeled exogenous carbon substrates contribute to glutamate synthesis and dilute the [2-(13)C]acetyl-CoA pool by 30%. The coupling of neuronal activity with shuttling of tricarboxylic acid (TCA) cycle-derived metabolites between astrocytes and neurons is concluded from the use of [4-(13)C]-monolabeled glutamate leaving the first TCA cycle turn already for glutamine and GABA synthesis, as well as from the labeling pattern of extracellular glutamine. Further evidence of a metabolic interaction between astrocytes and neurons is obtained, as alanine serves as a carbon and nitrogen carrier through the synthesis and regulated release of lactate from astrocytes for use by neurons. Complementary to the glutamine-glutamate cycle in the brain, a lactate-alanine shuttle between astrocytes and neurons would account for the nitrogen exchange of the glutamatergic neurotransmitter cycle in mammalian brain.

Proton magnetic resonance spectroscopy (1H-MRS) findings for the brain in patients with liver cirrhosis reflect the hepatic functional reserve

OBJECTIVE

Proton magnetic resonance spectroscopy (1H-MRS) has been used to assess the metabolic changes in the brain in patients with liver cirrhosis. Decreased myo-inositol and increased glutamine levels were noted to be the most sensitive spectroscopic markers for cirrhotic patients with hepatic encephalopathy (HE). The purpose of this study was to assess how the abnormalities seen on the 1H-MRS of the brain in patients with liver cirrhosis are related to clinical and laboratory parameters.

METHODS

In a prospective study, localized 1H-MRS was performed in the basal ganglia and parietal white matter regions in liver cirrhosis patients with (n = 48) and without (n = 52) HE and chronic hepatitis (CH) (n = 15), and in normal controls (n = 20).

RESULTS

Among cirrhotic patients, the myo-inositol levels were significantly lower (p < 0.01) and the glutamine levels were higher (p < 0.05) for patients with HE than for those without HE. The myo-inositol and glutamine levels, respectively, were inversely (r = -0.50; p < 0.001) and linearly (r = 0.50; p < 0.001) related to the Child-Pugh score. However, by subgroup analysis of Child-Pugh class C patients, there were no significant differences in the myo-inositol and glutamine levels between cirrhotic patients with (n = 40) and without HE (n = 24). A follow-up study of eight cirrhotic patients with HE showed no significant differences in the myo-inositol and glutamine levels after clinical improvement of HE.

CONCLUSIONS

The abnormalities seen on the 1H-MRS of the brain of patients with liver cirrhosis are not likely to reflect the severity of HE or acute alteration in the level of consciousness. Rather, we believe they represent the chronic metabolic derangement of the brain associated with hepatic functional reserve.

Astroglial dysfunction in hepatic encephalopathy

While the pathogenesis of hepatic encephalopathy (HE) remains elusive, there is considerable evidence pointing to a key role of ammonia-induced dysfunction of astrocytes in this condition. Deficits in the ability of astrocytes to take up glutamate from the extracellular space may lead to abnormal glutamatergic neurotransmission. Furthermore, excessive stimulation of neuronal and glial glutamate receptors by elevated extracellular levels of glutamate may lead to excitotoxicity and greater glial dysfunction. Ammonia also causes upregulation of astroglial peripheral-type benzodiazepine receptors (PBRs) which is associated with increased production of neurosteroids. These neurosteroids have potent positive modulatory effects on the neuronal GABA(A) receptor which, combined with an ammonia-induced astroglial defect in GABA uptake, may result in enhanced GABAergic tone. Brain edema, associated with fulminant hepatic failure, may also result from astroglial abnormalities as the edema appears to be principally caused by swelling of these cells. Increased amounts of glutamine in astrocytes resulting from elevated brain ammonia levels may be a factor in this swelling. Other osmolytes such as glutathione may also be involved. Glial swelling may also result from NH4+ - and K+ -mediated membrane depolarization as well as by the actions of PBR agonists and neurosteroids. These findings show that an ammonia-induced gliopathy is a major factor in the pathogenesis of HE.

New aspects on etiology, biochemistry, and therapy of portal systemic encephalopathy: a critical survey

There is scientific agreement that portal systemic encephalopathy (PSE) is caused morphologically by portal systemic shunts and biochemically by constituents of the portal venous blood. Ammonium has a key role in the pathogenesis of PSE. Direct correlations with the degree of PSE have been established exclusively with glutamine, i.e. the terminal product of the peripheral detoxification of ammonium. In PSE, ammonium is probably responsible for damage to astrocytic and neuronal cells. Ammonium's toxic effect is due to the intracerebral glutamine synthesis. After several metabolic steps, which will be discussed in detail, brain cell damage is caused directly or indirectly (exitotoxically) by energy deficiency. Hyperammonemia and PSE are each well defined though different forms of disturbance. Therefore, ammonium is not the sole decisive factor in the pathogenesis of PSE. We performed a detailed and critical analysis of all studies on amino acid therapy of PSE, especially those that were randomized and controlled. This analysis revealed a close and direct correlation between qualitative and quantitative dosages of amino acids on one hand, and parallel improvements of amino acid imbalance (essentially associated with PSE) and degree of PSE on the other. A close and direct dose/efficacy correlation must be assumed. Disturbed plasmatic amino acid homeostasis and cerebral monoaminergic neurotransmission are probably important pathogenic factors of PSE. A fundamental cofactor in the efficacy of each adequate amino acid therapy might be a substantial decrease of endogenous ammonium production. Physiologic benzodiazepines may also have an important function in the pathogenesis of PSE: not so, however, the glutamate-ergic and GABA-ergic neurotransmission, which are disturbed principally in PSE. In close correlation to pathogenesis, established and proposed therapies of PSE are critically discussed.

The effects of ammonia and portal-systemic shunting on brain metabolism, neurotransmission and intracranial hypertension in hyperammonaemia-induced encephalopathy

BACKGROUND/AIMS

The pathogenetic factors contributing to encephalopathy in portacaval shunted rats with hyperammonaemia were studied.

METHODS

Hyperammonaemia was induced by ammonium-acetate infusions in portacaval shunted rats (2.8 mmol.kg bw-1.h-1; AI-portacaval shunted rats) and in sham-portacaval shunted rats (6.5 mmol.kg bw-1.h-1; AI-NORM rats). Severity of encephalopathy was quantified by clinical grading and EEG spectral analysis. Changes in brain metabolites were assessed by amino acid analysis of brain cortex homogenates, whereas changes in amino acids with neurotransmitter activity were assessed in cerebrospinal fluid; brain water content was measured by subtracting dry from wet brain weights and intracranial pressure was measured by a pressure transducer connected to a cisterna magna cannula.

RESULTS

Although similar increased blood and brain ammonia concentrations were obtained in both experimental groups, only AI-portacaval shunted rats developed encephalopathy, associated with a significant increase in intracranial pressure. Other significant differences were: higher concentrations of brain glutamine and aromatic amino acids, higher concentrations of cerebrospinal fluid glutamine, aromatic amino acids, glutamate and aspartate in AI-portacaval shunted rats than in AI-NORM rats.

CONCLUSIONS

These results indicate that hyperammonaemia alone dose not induce encephalopathy, whereas portal-systemic shunting adds an essential contribution to the pathogenesis of encephalopathy. It is hypothesised that the larger increase in brain glutamine in AI-portacaval shunted rats than in AI-NORM rats is responsible for increased brain concentrations of aromatic amino acids, for cell swelling and for extracellular release of glutamate and aspartate. This might promote encephalopathy. If cell swelling is not restricted, intracranial hypertension will develop.

Methionine sulfoximine, a glutamine synthetase inhibitor, attenuates increased extracellular potassium activity during acute hyperammonemia

Hyperammonemia causes glutamine accumulation and astrocyte swelling. Inhibition of glutamine synthesis reduces ammonia-induced edema formation and watery swelling in astrocyte processes. Ordinarily, astrocytes tightly control extracellular K+ activity [K+]e. We tested the hypothesis that acute hyperammonemia interferes with this tight regulation such that [K+]e increases and that inhibition of glutamine synthetase reduces this increase in [K+]e. Ion-sensitive microelectrodes were used to measure [K+]e in parietal cortex continuously over a 6-h period in anesthetized rats. After i.v. sodium acetate infusion in eight control rats, plasma ammonia concentration was 33 +/- 26 mumol/L (+/- SD) and [K+]e remained stable at 4.3 +/- 1.6 mmol/L. During ammonium acetate infusion in nine rats, plasma ammonia increased to 594 +/- 124 mumol/L at 2 h and to 628 +/- 135 mumol/L at 6 h. There was a gradual increase in [K+]e from 3.9 +/- 0.7 to 6.8 +/- 2.7 mmol/L at 2 h and 11.8 +/- 6.7 mmol/L at 6 h. In eight rats, L-methionine-D,L-sulfoximine (150 mg/kg) was infused 3 h before ammonium acetate infusion to inhibit glutamine synthetase. At 2 and 6 h of ammonium acetate infusion, plasma ammonia concentration was 727 +/- 228 and 845 +/- 326 mumol/L, and [K+]e was 4.5 +/- 1.9 and 6.1 +/- 3.8 mmol/L, respectively. The [K+]e value at 6 h was significantly less than that obtained with ammonium acetate infusion alone but was not different from that obtained with sodium acetate infusion. We conclude that acute hyperammonemia impairs astrocytic control of [K+]e and that this impairment is linked to glutamine accumulation rather than ammonium ions per se.