The brain in experimental portal-systemic encephalopathy. I. Morphological changes in three animal models

Acute Liver Failure Induces Glial Reactivity, Oxidative Stress and Impairs Brain Energy Metabolism in Rats

Acute liver failure (ALF) implies a severe and rapid liver dysfunction that leads to impaired liver metabolism and hepatic encephalopathy (HE). Recent studies have suggested that several brain alterations such as astrocytic dysfunction and energy metabolism impairment may synergistically interact, playing a role in the development of HE. The purpose of the present study is to investigate early alterations in redox status, energy metabolism and astrocytic reactivity of rats submitted to ALF. Adult male Wistar rats were submitted either to subtotal hepatectomy (92% of liver mass) or sham operation to induce ALF. Twenty-four hours after the surgery, animals with ALF presented higher plasmatic levels of ammonia, lactate, ALT and AST and lower levels of glucose than the animals in the sham group. Animals with ALF presented several astrocytic morphological alterations indicating astrocytic reactivity. The ALF group also presented higher mitochondrial oxygen consumption, higher enzymatic activity and higher ATP levels in the brain (frontoparietal cortex). Moreover, ALF induced an increase in glutamate oxidation concomitant with a decrease in glucose and lactate oxidation. The increase in brain energy metabolism caused by astrocytic reactivity resulted in augmented levels of reactive oxygen species (ROS) and Poly [ADP-ribose] polymerase 1 (PARP1) and a decreased activity of the enzymes superoxide dismutase and glutathione peroxidase (GSH-Px). These findings suggest that in the early stages of ALF the brain presents a hypermetabolic state, oxidative stress and astrocytic reactivity, which could be in part sustained by an increase in mitochondrial oxidation of glutamate.

Rats with minimal hepatic encephalopathy due to portacaval shunt show differential increase of translocator protein (18 kDa) binding in different brain areas, which is not affected by chronic MAP-kinase p38 inhibition

Neuroinflammation plays a main role in neurological deficits in rats with minimal hepatic encephalopathy (MHE) due to portacaval shunt (PCS). Treating PCS rats with SB239063, an inhibitor of MAP-kinase-p38, reduces microglial activation and brain inflammatory markers and restores cognitive and motor function. The translocator protein-(18-kDa) (TSPO) is considered a biomarker of neuroinflammation. TSPO is increased in brain of PCS rats and of cirrhotic patients that died in hepatic coma. Rats with MHE show strong microglial activation in cerebellum and milder in other areas when assessed by MHC-II immunohistochemistry. This work aims were assessing: 1) whether binding of TSPO ligands is selectively increased in cerebellum in PCS rats; 2) whether treatment with SB239063 reduces binding of TSPO ligands in PCS rats; 3) which cell type (microglia, astrocytes) increases TSPO expression. Quantitative autoradiography was used to assess TSPO-selective (3)H-(R)-PK11195 binding to different brain areas. TSPO expression increased differentially in PCS rats, reaching mild expression in striatum or thalamus and very high levels in cerebellum. TSPO was expressed in astrocytes and microglia. Treatment with SB239063 did not reduces (3)[H]-PK11195 binding in PCS rats. SB239063 reduces microglial activation and levels of inflammatory markers, but not binding of TSPO ligands. This indicates that SB239063-induced neuroinflammation reduction in PCS rats is not mediated by effects on TSPO. Also, enhanced TSPO expression is not always associated with cognitive or motor deficits. If enhanced TSPO expression plays a role in mechanisms leading to neurological alterations in MHE, SB239063 would interfere these mechanisms at a later step.

Role of astrocytes in manganese mediated neurotoxicity

Astrocytes are responsible for numerous aspects of metabolic support, nutrition, control of the ion and neurotransmitter environment in central nervous system (CNS). Failure by astrocytes to support essential neuronal metabolic requirements plays a fundamental role in the pathogenesis of brain injury and the ensuing neuronal death. Astrocyte-neuron interactions play a central role in brain homeostasis, in particular via neurotransmitter recycling functions. Disruption of the glutamine (Gln)/glutamate (Glu) -γ-aminobutyric acid (GABA) cycle (GGC) between astrocytes and neurons contributes to changes in Glu-ergic and/or GABA-ergic transmission, and is associated with several neuropathological conditions, including manganese (Mn) toxicity. In this review, we discuss recent advances in support of the important roles for astrocytes in normal as well as neuropathological conditions primarily those caused by exposure to Mn.

Oxidative and nitrosative stress in ammonia neurotoxicity

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

Alterations of blood brain barrier function in hyperammonemia: an overview

Ammonia is a neurotoxin involved in the pathogenesis of neurological conditions associated with hyperammonemia, including hepatic encephalopathy, a condition associated with acute—(ALF) or chronic liver failure. This article reviews evidence that apart from directly affecting the metabolism and function of the central nervous system cells, ammonia influences the passage of different molecules across the blood brain barrier (BBB). A brief description is provided of the tight junctions, which couple adjacent cerebral capillary endothelial cells to each other to form the barrier. Ammonia modulates the transcellular passage of low-to medium-size molecules, by affecting their carriers located at the BBB. Ammonia induces interrelated aberrations of the transport of the large neutral amino acids and aromatic amino acids (AAA), whose influx is augmented by exchange with glutamine produced in the course of ammonia detoxification, and maybe also modulated by the extracellularly acting gamma-glutamyl moiety transferring enzyme, gamma-glutamyl-transpeptidase. Impaired AAA transport affects neurotransmission by altering intracerebral synthesis of catecholamines (serotonin and dopamine), and producing “false neurotransmitters” (octopamine and phenylethylamine). Ammonia also modulates BBB transport of the cationic amino acids: the nitric oxide precursor, arginine, and ornithine, which is an ammonia trap, and affects the transport of energy metabolites glucose and creatine. Moreover, ammonia acting either directly or in synergy with liver injury-derived inflammatory cytokines also evokes subtle increases of the transcellular passage of molecules of different size (BBB “leakage”), which appears to be responsible for the vasogenic component of cerebral edema associated with ALF.

Role of astrocytes in brain function and disease

Astrocytes assume multiple roles in maintaining an optimally suited milieu for neuronal function. Select astrocytic functions include the maintenance of redox potential, the production of trophic factors, the regulation of neurotransmitter and ion concentrations, and the removal of toxins and debris from the cerebrospinal fluid (CSF). Impairments in these and other functions, as well as physiological reactions of astrocytes to injury, can trigger or exacerbate neuronal dysfunction. This review addresses select metabolic interactions between neurons and astrocytes and emphasizes the role of astrocytes in mediating and amplifying the progression of several neurodegenerative disorders, such as Parkinson's disease (PD), hepatic encephalopathy (HE), hyperammonemia (HA), Alzheimer's disease (AD), and ischemia.

Hepatic encephalopathy: pathophysiology and emerging therapies

Hepatic encephalopathy is characterized by neuropsychiatric abnormalities in patients with liver failure. Severe hepatic encephalopathy is an indication for liver transplantation as it portends poor outcome. Treatment of hepatic encephalopathy involves correction of precipitating factors such as sepsis, gastrointestinal bleeding, medications, and electrolyte imbalance. Effective therapies include lactulose and antibiotics such as neomycin, metronidazole, and rifaximin.

Portal-systemic shunt encephalopathy presenting with diffuse cerebral white matter lesion: an autopsy case

We report herein an autopsy case of portal-systemic encephalopathy (PSE) presenting with diffuse tissue rarefaction in the cerebral deep white matter. Clinically, the patient showed recurrent episodes of unconsciousness, abnormal behavior and urinary incontinence, as well as flapping tremor. Cognitive impairment and peripheral neuropathy developed following recurrent episodes. Although conventional arterial portography revealed a small portal-systemic collateral vessel of a left gastro-renal venous shunt, abdominal CT and liver biopsy showed no evidence of liver cirrhosis and serum ammonia level showed a mild increase. T2-weighted MRI demonstrated symmetrical signal hyperintensities in the deep white matter. Neuropathological findings showed Alzheimer type II astrocytes in the deep layers of the cerebral cortices and severe tissue rarefaction with no or slight reactive astrocytosis in the subcortical and deep white matter. These white matter changes have been reported infrequently in patients with PSE. The present case suggests that chronic PSE without liver cirrhosis may develop diffuse white matter lesions.

Functional abnormalities of the motor tract in the rat after portocaval anastomosis and after carbon tetrachloride induction of cirrhosis

INTRODUCTION

Hepatic encephalopathy is a neurologic syndrome secondary to liver failure that causes cognitive and motor abnormalities. Impairment in the function of the first neuron of the motor tract (corticospinal tract) has been demonstrated in patients with cirrhosis and minimal hepatic encephalopathy.

AIM

Investigate the function of the first neuron of the motor tract in experimental models of minimal hepatic encephalopathy.

MATERIAL AND METHODS

Rats with portocaval anastomosis (n = 8) and rats with carbon tetrachloride induced cirrhosis (n = 11) underwent neurophysiological recording under light anesthesia with propofol. Motor evoked potentials were elicited applying a transcranial electric pulse and were recorded in the tibialis anterior muscle. The effect of the dose of anesthesia was assessed in a group of normal rats (n = 10).

RESULTS

Rats with portocaval anastomosis exhibited a decrease in motor evoked potentials amplitude following surgery (67 +/- 11 to 41 +/- 16%, P < 0.001). Cirrhotic rats exhibited an increase in motor evoked potentials latency after the appearance of ascites (4.65 +/- 0.43 to 5.15 +/- 0.67 ms., P = 0.04). Increasing doses of propofol produced a decrease in the amplitude and an increase in the latency of motor evoked potentials.

CONCLUSION

It is possible to reproduce functional abnormalities of the central motor tract in rats with portocaval anastomosis and carbon tetrachloride induced cirrhosis. The development of motor abnormalities in experimental models of minimal hepatic encephalopathy offers the possibility to investigate the mechanisms involved in the pathogenesis of hepatic encephalopathy and test therapeutic strategies.

The inflammatory bases of hepatic encephalopathy

A hypothesis about the inflammatory etiopathogeny mediated by astroglia of hepatic encephalopathy is being proposed. Three evolutive phases are considered in chronic hepatic encephalopathy: an immediate or nervous phase with ischemia-reperfusion, which is associated with reperfusion injury, edema and oxidative stress; an intermediate or immune phase with microglia hyperactivity, which produces cytotoxic cytokines and chemokines and is involved in enzyme hyperproduction and phagocytosis; and a late or endocrine phase, in which neuroglial remodeling, with an alteration of angiogenesis and neurogenesis, stands out. The increasingly complex trophic meaning that the metabolic alterations have in the successive phases making up this chronic inflammation could explain the metabolic regression produced in acute and acute-on-chronic hepatic encephalopathy. In these two types of hepatic encephalopathy, characterized by edema, neuronal nutrition by diffusion would guarantee an appropriate support of substrates, in accordance with the reduced metabolic needs of the cerebral tissue.

Brain edema and inflammatory activation in bile duct ligated rats with diet-induced hyperammonemia: A model of hepatic encephalopathy in cirrhosis

Studies of the pathogenesis of hepatic encephalopathy are hampered by the lack of a satisfactory animal model. We examined the neurological features of rats after bile duct ligation fed a hyperammonemic diet (BDL+HD). Six groups were studied: sham, sham pair-fed, hyperammonemic, bile duct ligation (BDL), BDL pair fed, and BDL+HD. The BDL+HD rats were made hyperammonemic via an ammonia-containing diet that began 2 weeks after operation. One week later, the animals were sacrificed. BDL+HD rats displayed an increased level of cerebral ammonia and neuroanatomical characteristics of hepatic encephalopathy (HE), including the presence of type II Alzheimer astrocytes. Both BDL and BDL+HD rats showed activation of the inflammatory system. BDL+HD rats showed an increased amount of brain glutamine, a decreased amount of brain myo-inositol, and a significant increase in the level of brain water. In coordination tests, BDL+HD rats showed severe impairment of motor activity and performance as opposed to BDL rats, whose results seemed only mildly affected. In conclusion, the BDL+HD rats displayed similar neuroanatomical and neurochemical characteristics to human HE in liver cirrhosis. Brain edema and inflammatory activation can be detected under these circumstances.

Animal models in the study of episodic hepatic encephalopathy in cirrhosis

The availability of an animal model is crucial in studying the pathophysiological mechanisms of disease and to test possible therapies. Now, there are several models for the study of liver diseases, but there still remains a lack of a satisfactory animal model of chronic liver disease with hepatic encephalopathy (HE) and abnormalities in nitrogen metabolism, as seen in humans. In rats, two models of chronic HE are widely used: rats after portacaval anastomosis (PCA) and rats with chronic hyperammonemia. The first one mimics the situation induced in cirrhosis by collateral circulation, and has the problem of the absence of hepatocellular injury. The model of hyperammonemia is useful to study the effect of ammonia as a brain toxic substance, but also lacks liver failure. Bile-duct ligation has been used to induce cirrhosis and could also be a model of HE, probably with the addition of a precipitant factor. An ideal model of HE in chronic liver disease must have liver cirrhosis and a precipitant factor of HE; it must also show neuropathological characteristic findings of HE, neurochemical alterations in the main pathways impaired in these complications of cirrhosis, and low-grade brain edema.

Deficits in cortico-striatal synaptic plasticity and behavioral habituation in rats with portacaval anastomosis

Hepatic encephalopathy is characterized by disturbances of motor and cognitive functions involving the basal ganglia. So far no standards for assessment of neuropsychiatric abnormalities (disorders of sleep, mood, anxiety and personality) in subclinical hepatic encephalopathy have been defined. Using an animal model of mild (subclinical) hepatic encephalopathy we investigated now striatum-related behaviors and cortico-striatal synaptic plasticity in rats 2 months after introduction of a portacaval shunt and sham-operated matched controls. In a novel open field portacaval shunt rats displayed less locomotor activity; unlike controls they also showed no habituation to the field and no recall of the field environment after 24 h, indicative of cognitive deficit. The elevated-plus maze test indicated no differences in fear/anxiety in the portacaval shunt animals. Tetanic stimulation of cortical afferents in magnesium-free solution evoked an N-methyl-D-aspartate-dependent long-term potentiation in sham-operated animals. In portacaval shunt animals long-term potentiation was significantly impaired. Histamine, a potent modulator of cortico-striatal transmission, induced a larger long-term depression of field potentials in control compared with portacaval shunt rats. In conclusion, a combination of electrophysiological and behavioral approaches has revealed functional changes in cortico-striatal transmission. These data are relevant for understanding the mechanisms of motor and cognitive dysfunctions in hepatic encephalopathy patients and for the development of precise psychometric tests, evaluating cognitive deficits in subclinical hepatic encephalopathy.

Glutamine synthetase in brain: effect of ammonia

Glutamine synthetase (GS) in brain is located mainly in astrocytes. One of the primary roles of astrocytes is to protect neurons against excitotoxicity by taking up excess ammonia and glutamate and converting it into glutamine via the enzyme GS. Changes in GS expression may reflect changes in astroglial function, which can affect neuronal functions. Hyperammonemia is an important factor responsible of hepatic encephalopathy (HE) and causes astroglial swelling. Hyperammonemia can be experimentally induced and an adaptive astroglial response to high levels of ammonia and glutamate seems to occur in long-term studies. In hyperammonemic states, astroglial cells can experience morphological changes that may alter different astrocyte functions, such as protein synthesis or neurotransmitters uptake. One of the observed changes is the increase in the GS expression in astrocytes located in glutamatergic areas. The induction of GS expression in these specific areas would balance the increased ammonia and glutamate uptake and protect against neuronal degeneration, whereas, decrease of GS expression in non-glutamatergic areas could disrupt the neuron-glial metabolic interactions as a consequence of hyperammonemia. Induction of GS has been described in astrocytes in response to the action of glutamate on active glutamate receptors. The over-stimulation of glutamate receptors may also favour nitric oxide (NO) formation by activation of NO synthase (NOS), and NO has been implicated in the pathogenesis of several CNS diseases. Hyperammonemia could induce the formation of inducible NOS in astroglial cells, with the consequent NO formation, deactivation of GS and dawn-regulation of glutamate uptake. However, in glutamatergic areas, the distribution of both glial glutamate receptors and glial glutamate transporters parallels the GS location, suggesting a functional coupling between glutamate uptake and degradation by glutamate transporters and GS to attenuate brain injury in these areas. In hyperammonemia, the astroglial cells located in proximity to blood-vessels in glutamatergic areas show increased GS protein content in their perivascular processes. Since ammonia freely crosses the blood-brain barrier (BBB) and astrocytes are responsible for maintaining the BBB, the presence of GS in the perivascular processes could produce a rapid glutamine synthesis to be released into blood. It could, therefore, prevent the entry of high amounts of ammonia from circulation to attenuate neurotoxicity. The changes in the distribution of this critical enzyme suggests that the glutamate-glutamine cycle may be differentially impaired in hyperammonemic states.

Loss of expression of glial fibrillary acidic protein in acute hyperammonemia

Glial fibrillary acid protein (GFAP) is a major component of the glial filament network and alterations in expression of this protein in cultured astrocytes have been reported in response to acute ammonia exposure in vitro. In order to determine the effects of acute hyperammonemia in vivo on GFAP expression, brain extracts from rats with acute liver failure due to hepatic devascularization (portacaval anastomosis followed 24h later by hepatic artery ligation, HAL) were analyzed for GFAP mRNA using reverse transcription-polymerase chain reaction (RT-PCR) and appropriate oligonucleotide primers. GFAP protein was assayed by immunoblotting using a polyclonal antibody. Hepatic devascularization resulted in a significant 55-68% decrease (P<0.01) of GFAP mRNA and a concomitant loss of GFAP protein at precoma and coma stages of encephalopathy when brain water content was significantly increased and brain ammonia concentrations were in the millimolar range (1-5mM). Expression of a second glial filament protein S-100beta was unaffected by acute hyperammonemia. These findings suggest a role for GFAP in cell volume regulation and that loss of GFAP expression could contribute to the pathogenesis of brain edema in acute hyperammonemic syndromes.

Thiophene, a sulfur-containing heterocyclic hydrocarbon, causes widespread neuronal degeneration in rats

Thiophene is a sulfur-containing heterocyclic hydrocarbon that has been detected in a number of environmental sources as various derivatives. Previous studies with rats have shown that thiophene induces selective degeneration of granule cells in the cerebellum, as observed with methyl mercury. To study the neurotoxicity of thiophene, Wistar rats received daily intramuscular injections of 0.2 mL thiophene for 3 days. Ataxia and convulsions were noted in all animals within 24 h after the final dose. Histologically, multiple foci of necrosis were observed in the cerebellum, predominantly in the granular layer. Neuronal damage was also found in the cerebral cortex, inferior colliculus and inferior olive. These findings suggest that thiophene causes widespread neuronal degeneration in rats and that the regional distribution of brain lesions induced by thiophene is different from that caused by methyl mercury poisoning.

Azoxymethane-induced fulminant hepatic failure in C57BL/6J mice: characterization of a new animal model

Without transplantation, approximately 50-90% of all patients with fulminant hepatic failure (FHF) die. This poor outcome is due in part to the absence of an appropriate animal model, which would allow for a greater understanding of the pathophysiology of this syndrome. Given the reports of liver injury in humans and livestock fed cycad palm nuts on the island of Guam, we hypothesized that the active ingredient azoxymethane (AOM) could cause FHF. We therefore evaluated AOM in C57BL/6J mice. Histologically, we observed microvesicular steatosis 2 h, sinusoidal dilatation 4 h, and centrilobular necrosis 20 h after AOM administration, and transmission electron microscopy showed that this agent causes mitochondrial injury. FHF was associated with all four stages of encephalopathy, as well as by a prodromal period of decreased eating and drinking lasting approximately 15 h before the development of stage I encephalopathy (i.e., loss of scatter reflex). Late encephalopathy was associated with increased arterial ammonia, decreased serum glucose, and evidence of brain edema (astrocyte swelling). We show that AOM-induced FHF is highly reproducible, without evidence of lot-to-lot variability, and is dose dependent. These findings therefore suggest that AOM is an excellent agent for the study of FHF, as well as indicate that Guamanian FHF may be due to AOM found in unwashed cycad palm nuts.

Modulation of AMPA receptor subunits GluR1 and GluR2/3 in the rat cerebellum in an experimental hepatic encephalopathy model

The immunohistochemical expression and distribution of the AMPA-selective receptor subunits GluR1 and GluR2/3 were investigated in the rat cerebellum following portocaval anastomosis (PCA) at 1 and 6 months. With respect to controls, GluR1 and GluR2/3 immunoreactivities increased over 1 to 6 months following PCA, although immunolabelling patterns for both antibodies were different at the two analysed times. GluR1 immunoreactivity was expressed by Bergmann glial cells, which showed immunoreactive glial processes crossing the molecular layer at 6 months following PCA. The GluR2/3 subunit was expressed by Purkinje neurons and moderately expressed by neurons of the granule cell layer. Immunoreactivity for GluR2/3 was detectable in cell bodies and dendrites of Purkinje cells in young control cerebella, whereas GluR2/3 immunoreactivity was scarce 1 month post PCA. However, despite a lack of immunoreactivity in the Purkinje somata and main processes of adult control rats, GluR2/3 immunoreactivity was strongly enhanced in Purkinje neurons following long-term PCA. These findings suggest that the localization of the GluR2/3 subunit in Purkinje cells undergoes an alteration and/or reorganization as a consequence of long-term PCA. The combination of enhanced GluR immunoreactivity in long-term PCA, both in Bergmann glial cells and in Purkinje neurons, suggests some degree of neuro-glial interaction, possibly through glutamate receptors, in this type of encephalopathy.

Distinctive pattern of Bergmann glial pathology in human hepatic encephalopathy

Alzheimer type II astrocytosis is the pathological hallmark of hepatic encephalopathy. These astrocytes undergo a characteristic morphological change and, in addition, lose immunoreactivity for glial fibrillary acidic protein (GFAP). However, a previous study in the portacaval shunted rat, a model of hepatic encephalopathy, revealed increased rather than decreased GFAP immunoreactivity in Bergmann glia, a specialized group of cerebellar astrocytes. In the present study, sections of cerebellar vermis from 15 cirrhotic patients with hepatic encephalopathy and varying degrees of Alzheimer type II astrocytosis were stained using antisera to GFAP. The Bergmann glial cells did not show altered GFAP immunoreactivity compared to controls. In addition, the degree of GFAP immunoreactivity was not correlated with the degree of Alzheimer type II change nor related to the aetiology of the liver disease. These results suggest a differential response of Bergmann glia in human hepatic encephalopathy.

Ammonia stimulates glutamine uptake to the cerebral non-synaptic mitochondria of the rat

The uptake of [3H]glutamine (GLN) to non-synaptic mitochondria isolated from rat cerebral hemispheres was measured in the absence or presence of 3 mM ammonium ion (ammonium chloride; ammonia). Ammonia increased Vmax of the saturable component of GLN uptake by > 20%, without affecting K(m), but did not change a non-saturable component of GLN transport representing diffusion or uptake mediated by a very low affinity carrier. Since GLN is an idiogenic osmole, its increased uptake may contribute to the swelling of astrocytic mitochondria and, subsequently, to a decrease in cerebral energy metabolism usually associated with acute hyperammonemic states. The result is consistent with the recent view that GLN accumulating in the brain in hyperammonemic conditions contributes to ammonia neurotoxicity.

Inhibition of glutamine synthetase reduces ammonia-induced astrocyte swelling in rat

Astrocyte hypertrophy and swelling occur in a variety of pathophysiological conditions, including diseases associated with hyperammonemia. Ammonia is rapidly incorporated into glutamine by glutamine synthetase localized in astrocytes. We tested the hypotheses that (1) 6 h of hyperammonemia (500-600 microM) is adequate for producing astrocyte enlargement, and (2) astrocyte enlargement is attenuated by inhibition of glutamine synthetase with methionine sulfoximine. Pentobarbital-anesthetized rats received an intravenous infusion of either sodium or ammonium acetate after intraperitoneal pretreatment with vehicle, methionine sulfoximine (0.8 mmol/kg) or buthionine sulfoximine (4 mmol/kg), an analogue that does not inhibit glutamine synthetase. Hyperammonemia produced enlarged cortical astrocytes characterized by (1) decreased electron density of cytoplasmic matrix in perikaryon, processes and perivascular endfeet, (2) increased circumference of nuclear membrane, (3) increased numbers of mitochondria and rough and smooth endoplasmic reticulum in perikarya and large processes, and (4) less compact bundles of intermediate filaments. Pretreatment with methionine sulfoximine, but not buthionine sulfoximine, attenuated the decrease in cytoplasmic density and the increase in nuclear circumference; most perivascular endfeet remained as dense as occurred with sodium acetate infusion. However, increased numbers of organelles in expanded perikarya and large processes occurred after methionine sulfoximine treatment with and without ammonium acetate infusion. In separate groups of rats, hyperammonemia produced an increase in cortical tissue water content which was inhibited by methionine sulfoximine, but not buthionine sulfoximine. We conclude that clinically-relevant levels of hyperammonemia can cause astrocyte enlargement within 6 h in vivo characterized by both watery cytoplasm and increased organelles indicative of a cellular metabolic stress and altered astrocyte function. The watery cytoplasm component of astrocyte enlargement depends on glutamine synthesis rather than on ammonium ions per se, and is possibly caused by the osmotic effect accumulated glutamine.

Hyperammonemia induces transient GFAP immunoreactivity changes in goldfish spinal cord (Carassius auratus L.)

In this study we have demonstrated that high ammonia concentration in tank water induces changes in the glial fibrillary acidic protein (GFAP) of ependymal cells and radial astrocytes in the goldfish spinal cord. Hyperammonemia was induced by elevating the ammonia concentration in the tank water to 0.88 mM using ammonium chloride; ammonia in control water was less than 0.1 mM. Immunohistochemical methods were used for GFAP and vimentin, and levels were measured at 4, 8, 16, 30, 60, 90 and 120 days. GFAP quantification was made by means of a digital analysis system. The GFAP immunoreactivity was significantly lower at 30 and 60 days of treatment and at 90 days it had returned to control levels. However, no changes in vimentin immunoreactivity were appreciated in any case. GFAP loss was general and was not selective in any specific spinal cord region. To explain this transient generalized loss of GFAP and its posterior recuperation, a possible relation between glutamine synthetase distribution and GFAP changes is discussed.

Different response of astrocytes and Bergmann glial cells to portacaval shunt: an immunohistochemical study in the rat cerebellum

The present study was performed in order to follow the response of rat cerebellum astroglial cells (Bergmann glial cells and astrocytes) to long-term portacaval shunt (PCS), by means of glial fibrillary acidic protein (GFAP) and vimentin immunoreactivities. Bergmann glia accumulated GFAP in response to PCS, whereas astrocytes decreased GFAP immunoreactivity when compared to control rats. The increase of GFAP occurs in cells located in the cerebellar layer where glutamate is mainly released. Since the vimentin content remained unaltered in response to PCS, when compared to control rats, it can be concluded that only the GFAP filaments are affected by PCS. Nevertheless, GFAP immunoreactivity presents regional differences in the cerebellar astroglial population, and the factors responsible for these variations are still unknown.

Heterogeneous astroglial response in the rat spinal cord to long-term portacaval shunt: an immunohistochemical study

Glial fibrillary acidic protein (GFAP) immunoreactivity has been used to study the astroglial response in the rat spinal cord to long-term portacaval shunt (PCS). The astroglial response in PCS rats is heterogeneous. In general, astrocytes show a loss of GFAP immunoreactivity, as well as shrinking and pyknosis in their nuclei; however, while GFAP reactivity was unchanged in the periependymal region, it was strongly increased in the dorsolateral region of the spinal cord (lateral spinal nucleus, dorsal root entry zone, and the most dorsal region of the dorsal horn). Three possibilities are postulated to explain how astrocytes, in the periependymal and dorsolateral regions, can support the effects of PCS: a) astrocytes related to glutamatergic pathways ought to possess a more efficient ammonia uptake and detoxification system, b) long-term PCS can activate nociceptive pathways (substancePergic fibers), and c) astrocytes located in periependymal and dorsolateral regions can be exposed to lower concentrations of ammonia because of its diffusion into the cerebro-spinal fluid close to these regions.

Acute hepatotoxicity with resultant pulmonary and cerebral embolism in guinea pigs given tunicamycin

The hepatotoxicity of tunicamycin was studied in 8 to 10-week-old guinea pigs. Acute hepatic damage was produced consistently in guinea pigs given a single dose of 400 micrograms/kg of tunicamycin and observed at intervals up to 72 h post-injection. Significant elevations occurred in serum levels of liver enzymes and ammonia, while concentrations of serum proteins were lowered. A periportal pattern of hepatocellular damage, with death of many hepatocytes, was consistently observed by 72 h. Severe vacuolation of hepatocytes resulted from lipid accumulation and dilatation of cisternae of rough endoplasmic reticulum, and bile ductule hyperplasia was also observed. Swollen hepatocyte cytoplasm protruded into many hepatic blood vessels, and detached portions of hepatocytes produced emboli in pulmonary and cerebral capillaries, thus contributing to capillary occlusion in the brains of treated guinea pigs.

Quantitation and karyometry of cerebral neuroglia and endothelial cells in liver cirrhosis and in the hepatosplenic schistosomiasis mansoni

A morphological, karyometric, and quantitative study of cerebral neuroglia and endothelial cells of blood capillaries was done in cirrhotic and in hepatosplenic schistosomotic human autopsied cases. Cluster analysis applied to them revealed three subgroups (cirrhosis and schistosomiasis polar groups and one intermediate). The comparison of these three groups with a control revealed increased numbers of astrocytes, oligodendrocytes and endothelial cells, but no nuclear enlargement in the schistosomiasis group; the cirrhosis group exhibited a pronounced nuclear enlargement of both astrocyte and oligodendrocytes but no increase in cell numbers. The intermediate group, which encompasses the majority of pathological cases, is heterogeneous but on average behave as the cirrhosis group in that nuclear enlargement, but no increase in cell numbers, was noted. Such changes could represent a response of the nervous system to the metabolic disturbances present in hepatic and/or portal-systemic encephalopathy. There was a positive correlation between glial and endothelial cell numbers in cerebral cortex, suggesting a functional relationship between the glial cells and the capillary bed. This study points out the importance of clustering the cases, because the physiopathological status of individuals belonging to the same nosological condition can be different. Comparisons considering this aspect should be useful in understanding the progression of the pathological process.

Amino acid imbalance following portal diversion in the rat. The relevance of nutrition and of hepatic function

End-to-end portacaval transposition has previously been shown to produce less hepatocellular dysfunction than end-to-side portacaval shunt in the rat. Liver weight is also significantly reduced after portacaval shunt compared to portacaval transposition and these differences are not abolished by pair-feeding. Histological evidence of CNS damage is also reduced in transposed rats compared to shunted animals. This study examines the amino acid and hormone changes in these models. The characteristic amino acid changes of chronic liver disease (decreased branched-chain and elevated aromatic amino acids) are reproduced in portacaval shunt rats, but not in portacaval transposition. The differences between these groups in the branched-chain amino acids, but not those in the aromatic amino acids, are reduced by pair-feeding. Insulin and glucagon are elevated to a similar extent in both groups. These findings add further support to a role for peripheral amino acid imbalance in the pathogenesis of portal-systemic encephalopathy. Normal liver function, maintained by replacement of portal inflow with systemic blood, appears to minimize both CNS damage and amino acid changes.

The impact of time after portacaval shunt in the rat on behavior, brain serotonin, and brain and muscle histology

We investigated open field behavior, cellular fluorescence of brain serotonin and serotonin metabolism in different regions of the central nervous system (CNS), and brain and muscle histology in rats 3 weeks or 6 months after surgical end-to-side portacaval shunt (PCS). The results revealed a similar disturbance of the CNS serotonin at 3 weeks and 6 months after PCS in the rat. Progressive neurohistological changes were present between 3 weeks and 6 months after PCS. The open field behavioral impairment appeared, however, to diminish with time. There was no evidence from muscle biopsies that lesions in the muscles or in the peripheral nerves contributed to the motor disturbance.

Electron microscopic evaluation of brain edema in rabbits with galactosamine-induced fulminant hepatic failure: ultrastructure and integrity of the blood-brain barrier

Brain edema is a major complication of fulminant hepatic failure and is responsible for death in a large percentage of patients. We previously demonstrated the progressive accumulation of water in grey matter areas of the brain in the rabbit with galactosamine-induced fulminant hepatic failure. We now report the electron microscopic morphology of the brain in the same model of acute hepatic failure following the intravenous injection of horseradish peroxidase, an intravascular tracer which forms an electron-dense reaction product. Rabbits with both mild and severe encephalopathy had normal blood pressures and blood gases at the time of study. Fixation of brain tissue was obtained by whole-body perfusion. Marked swelling of the cytoplasm, perineuronal and perivascular processes of astrocytes were noted in cortical gray, but not white, matter areas; the other cellular components of the brain had normal morphology. Capillary endothelial cells were normal, and there was no evidence of horseradish peroxidase in endothelial cell vesicles, basement membranes or the brain parenchyma, suggesting that the blood-brain barrier was impermeable to large molecules. Histologic evidence of brain edema is seen in this model, with swelling of astrocytes as the primary manifestation of the accumulation of water. Damage to astrocytes or inhibition of their function may contribute to the pathogenesis of hepatic encephalopathy in this model.

Morphometric and densitometric investigations of protoplasmic astrocytes and neurons in human hepatic encephalopathy

Morphometric and densitometric evaluations were made of nuclei of astrocytes and nerve cells of 49 cases with chronic liver diseases and of 9 control cases. The data measured from Nissl-stained specimens of putamen were compared with clinical degrees of encephalopathy and with blood ammonia levels. The parameters measured included nuclear area and optical density of nuclei. The nuclear area of astrocytes (AAREA), on average, was found to grow significantly along with aggravation of encephalopathy, that growth being from 39 micron 2 in the control group to about 60 micron 2 in cases of severe encephalopathy. Furthermore the proportion of astrocyte nuclei with an area above 70 micron 2 (ALRG) and the proportion of the optical light area of the total nuclear area (AHOLE) rises. Mean compactness (ACEXT) and mean extinction of astrocyte nuclei (AMEXT) dropped along with growing severity of encephalopathy. Mean blood ammonia levels rose from 42 mumol/l in cases with no microscopically detectable signs of encephalopathy to about six times higher values in cases with severe encephalopathy. Optical density of astrocyte nuclei was negatively correlated and mean nuclear area positively correlated to blood ammonia levels. No characteristic morphometric and densitometric changes of nerve cell nuclei were recordable from the putamen.

The brain in experimental portal-systemic encephalopathy. II. Water and electrolyte changes

The water content and amounts of sodium, potassium and chloride were measured in the brains of normal rats, rats with PCA, normal rats fed ammoniated cationic exchange resin, and rats with PCA fed the resin. Plasma electrolytes and ammonia levels were also measured, and sodium and chloride spaces were calculated. Rats with PCA showed increased water content, sodium space and chloride space in the brainstem compared to controls. Rats with PCA fed ammoniated resin showed increased chloride content and Na+:K+ ratio in the brainstem, and an increased chloride space in the brainstem. In these rats the chloride spaces in the cerebrum and cerebellum exceeded the sodium spaces. It is concluded that high circulating ammonia levels can in vivo produce ionic shifts which may interfere with nervous function. It is also concluded that increased cytoplasmic osmolarity produced by ammonium ion-induced stimulation of (Na+ + Ka+) ATPase may result in the appearance of swollen astrocytes in conventional electron micrographs.