Hypolocomotion in rats with chronic liver failure is due to increased glutamate and activation of metabotropic glutamate receptors in substantia nigra

Neuroinflammation alters GABAergic neurotransmission in hyperammonemia and hepatic encephalopathy, leading to motor incoordination. Mechanisms and therapeutic implications

Enhanced GABAergic neurotransmission contributes to impairment of motor coordination and gait and of cognitive function in different pathologies, including hyperammonemia and hepatic encephalopathy. Neuroinflammation is a main contributor to enhancement of GABAergic neurotransmission through increased activation of different pathways. For example, enhanced activation of the TNFα-TNFR1-NF-κB-glutaminase-GAT3 pathway and the TNFα-TNFR1-S1PR2-CCL2-BDNF-TrkB pathway in cerebellum of hyperammonemic rats enhances GABAergic neurotransmission. This is mediated by mechanisms affecting GABA synthesizing enzymes GAD67 and GAD65, total and extracellular GABA levels, membrane expression of GABAA receptor subunits, of GABA transporters GAT1 and GAT three and of chloride co-transporters. Reducing neuroinflammation reverses these changes, normalizes GABAergic neurotransmission and restores motor coordination. There is an interplay between GABAergic neurotransmission and neuroinflammation, which modulate each other and altogether modulate motor coordination and cognitive function. In this way, neuroinflammation may be also reduced by reducing GABAergic neurotransmission, which may also improve cognitive and motor function in pathologies associated to neuroinflammation and enhanced GABAergic neurotransmission such as hyperammonemia, hepatic encephalopathy or Parkinson's disease. This provides therapeutic targets that may be modulated to improve cognitive and motor function and other alterations such as fatigue in a wide range of pathologies. As a proof of concept it has been shown that antagonists of GABAA receptors such as bicuculline reduces neuroinflammation and improves cognitive and motor function impairment in rat models of hyperammonemia and hepatic encephalopathy. Antagonists of GABAA receptors are not ideal therapeutic tools because they can induce secondary effects. As a more effective treatment to reduce GABAergic neurotransmission new compounds modulating it by other mechanisms are being developed. Golexanolone reduces GABAergic neurotransmission by reducing the potentiation of GABAA receptor activation by neurosteroids such as allopregnanolone. Golexanolone reduces neuroinflammation and GABAergic neurotransmission in animal models of hyperammonemia, hepatic encephalopathy and cholestasis and this is associated with improvement of fatigue, cognitive impairment and motor incoordination. This type of compounds may be useful therapeutic tools to improve cognitive and motor function in different pathologies associated with neuroinflammation and increased GABAergic neurotransmission.

Intravenous MSC-Treatment Improves Impaired Brain Functions in the R6/2 Mouse Model of Huntington's Disease via Recovered Hepatic Pathological Changes

Huntington's disease (HD), a congenital neurodegenerative disorder, extends its pathological damages beyond the nervous system. The systematic manifestation of HD has been extensively described in numerous studies, including dysfunction in peripheral organs and peripheral inflammation. Gut dysbiosis and the gut-liver-brain axis have garnered greater emphasis in neurodegenerative research, and increased plasma levels of pro-inflammatory cytokines have been identified in HD patients and various in vivo models, correlating with disease progression. In the present study, we investigated hepatic pathological markers in the liver of R6/2 mice which convey exon 1 of the human mutant huntingtin gene. Furthermore, we evaluated the impact of intravenously administered Mesenchymal Stromal Cells (MSCs) on the liver enzymes, changes in hepatic inflammatory markers, as well as brain pathology and behavioral deficits in R6/2 mice. Our results revealed altered enzyme expression and increased levels of inflammatory mediators in the liver of R6/2 mice, which were significantly attenuated in the MSC-treated R6/2 mice. Remarkably, neuronal pathology and altered motor activities in the MSC-treated R6/2 mice were significantly ameliorated, despite the absence of MSCs in the postmortem brain. Our data highlight the importance of hepatic pathological changes in HD, providing a potential therapeutic approach. Moreover, the data open new perspectives for the search in blood biomarkers correlating with liver pathology in HD.

Cognitive Functions, Neurotransmitter Alterations, and Hippocampal Microstructural Changes in Mice Caused by Feeding on Western Diet

Metabolic Dysfunction Associated Steatotic Liver Disease (MASLD) is the most common chronic liver disease in Western countries. It is becoming increasingly evident that peripheral organ-centered inflammatory diseases, including liver diseases, are linked with brain dysfunctions. Therefore, this study aims to unravel the effect of MASLD on brain histology, cognitive functions, and neurotransmitters. For this purpose, mice fed for 48 weeks on standard (SD) or Western diet (WD) were evaluated by behavioral tests, followed by sacrifice and analysis of the liver-brain axis including histopathology, immunohistochemistry, and biochemical analyses. Histological analysis of the liver showed features of Metabolic Dysfunction-Associated Steatohepatitis (MASH) in the WD-fed mice including lipid droplet accumulation, inflammation, and fibrosis. This was accompanied by an elevation of transaminase and alkaline phosphatase activities, increase in inflammatory cytokine and bile acid concentrations, as well as altered amino acid concentrations in the blood. Interestingly, compromised blood capillary morphology coupled with astrogliosis and microgliosis were observed in brain hippocampus of the WD mice, indicating neuroinflammation or a disrupted neurovascular unit. Moreover, attention was impaired in WD-fed mice along with the observations of impaired motor activity and balance, enhanced anxiety, and stereotyped head-twitch response (HTR) behaviors. Analysis of neurotransmitters and modulators including dopamine, serotonin, GABA, glutamate, and acetylcholine showed region-specific dysregulation in the brain of the WD-fed mice. In conclusion, the induction of MASH in mice is accompanied by the alteration of cellular morphology and neurotransmitter expression in the brain, associated with compromised cognitive functions.

Effects of oxidative stress on hepatic encephalopathy pathogenesis in mice

Oxidative stress plays a crucial role in the pathogenesis of hepatic encephalopathy (HE), but the mechanism remains unclear. GABAergic neurons in substantia nigra pars reticulata (SNr) contribute to the motor deficit of HE. The present study aims to investigate the effects of oxidative stress on HE in male mice. The results validate the existence of oxidative stress in both liver and SNr across two murine models of HE induced by thioacetamide (TAA) and bile duct ligation (BDL). Systemic mitochondria-targeted antioxidative drug mitoquinone (Mito-Q) rescues mitochondrial dysfunction and oxidative injury in SNr, so as to restore the locomotor impairment in TAA and BDL mice. Furthermore, the GAD2-expressing SNr population (SNr^GAD2) is activated by HE. Both overexpression of mitochondrial uncoupling protein 2 (UCP2) targeted to SNr^GAD2 and SNr^GAD2-targeted chemogenetic inhibition targeted to SNr^GAD2 rescue mitochondrial dysfunction in TAA-induced HE. These results define the key role of oxidative stress in the pathogenesis of HE.

J-difference GABA-edited MRS reveals altered cerebello-thalamo-cortical metabolism in patients with hepatic encephalopathy

Hepatic encephalopathy (HE) is a common neurological manifestation of liver cirrhosis and is characterized by an increase of ammonia in the brain accompanied by a disrupted neurotransmitter balance, including the GABAergic and glutamatergic systems. The aim of this study is to investigate metabolic abnormalities in the cerebello-thalamo-cortical system of HE patients using GABA-edited MRS and links between metabolite levels, disease severity, critical flicker frequency (CFF), motor performance scores, and blood ammonia levels. GABA-edited MRS was performed in 35 participants (16 controls, 19 HE patients) on a clinical 3 T MRI system. MRS voxels were placed in the right cerebellum, left thalamus, and left motor cortex. Levels of GABA+ and of other metabolites of interest (glutamine, glutamate, myo-inositol, glutathione, total choline, total NAA, and total creatine) were assessed. Group differences in metabolite levels and associations with clinical metrics were tested. GABA+ levels were significantly increased in the cerebellum of patients with HE. GABA+ levels in the motor cortex were significantly decreased in HE patients, and correlated with the CFF (r = 0.73; p < .05) and motor performance scores (r = -0.65; p < .05). Well-established HE-typical metabolite patterns (increased glutamine, decreased myo-inositol and total choline) were confirmed in all three regions and were closely linked to clinical metrics. In summary, our findings provide further evidence for alterations in the GABAergic system in the cerebellum and motor cortex in HE. These changes were accompanied by characteristic patterns of osmolytes and oxidative stress markers in the cerebello-thalamo-cortical system. These metabolic disturbances are a likely contributor to HE motor symptoms in HE. In patients with hepatic encephalopathy, GABA+ levels in the cerebello-thalamo-cortical loop are significantly increased in the cerebellum and significantly decreased in the motor cortex. GABA+ levels in the motor cortex strongly correlate with critical flicker frequency (CFF) and motor performance score (pegboard test tPEG), but not blood ammonia levels (NH3).

Peroxisome proliferator-activated receptor-γ doesn't modify altered electrophysiological properties of the CA1 pyramidal neurons in a rat model of hepatic cirrhosis

Regarding the low quality of life due to the cognitive complications in the patients with hepatic cirrhosis (HC), the goal of this study was to examine the possible neuroprotective effect of pioglitazone (PIO) on the electrophysiological alterations of hippocampus, a major area of cognition, in the experimental model of bile duct ligation (BDL). We used adult male Wistar rats in the present study to perform BDL or sham surgery. Pioglitazone was administered in BDL rats two weeks after the surgery for the next continuous four weeks. The effects of pioglitazone on BDL-induced electrophysiological alterations of the CA1 pyramidal neurons in the hippocampus were evaluated by whole-cell patch clamp recordings. Our findings demonstrated that chronic administration of PIO could not reverse the electrophysiological changes in the CA1 pyramidal neurons of the hippocampus in BDL rats but could improve the hepatic dysfunction.Together, the results of this study suggest that PIO administration cannot counteract altered intrinsic properties of the hippocampal neurons which has been shown recently as an involved mechanism of the cognitive impairments in hepatic encephalopathy (HE).

Cholestasis alters brain lipid and bile acid composition and compromises motor function in neonatal piglets

Infants with neonatal cholestasis are prone to neurodevelopmental deficits, however, the underlying pathogenesis is unclear. Lipid malabsorption and accumulation of potentially neurotoxic molecules in the blood such as bile acids are important yet relatively unexplored pathways. Here, we developed a translational piglet model to understand how the molecular bile acid and lipid composition of the brain is affected by this disease and relates to motor function. Piglets (8-days old) had bile duct ligation or sham surgery and were fed a formula diet for 3 weeks. Alongside sensory-motor deficits observed in bile duct-ligated animals, we found a shift toward a more hydrophilic and conjugated bile acid profile in the brain. Additionally, comprehensive lipidomics of the cerebellum revealed a decrease in total lipids including phosphatidylinositols and phosphatidylserines and increases in lysophospholipid species. This was paralleled by elevated cerebellar expression of genes related to inflammation and tissue damage albeit without significant impact on the brain transcriptome. This study offers new insights into the developing brain's molecular response to neonatal cholestasis indicating that bile acids and lipids may contribute in mediating motor deficits.

The meningeal lymphatic vasculature in neuroinflammation

The lymphatic vasculature is a unidirectional network of lymphatic endothelial cells, whose main role is to maintain fluid homeostasis along with the absorption of dietary fat in the gastrointestinal organs and management and coordination of immune cell trafficking into lymph nodes during homeostasis and under inflammatory conditions. In homeostatic conditions, immune cells, such as dendritic cells, macrophages, or T cells can enter into the lymphatic vasculature and move easily through the lymph reaching secondary lymph nodes where immune cell activation or peripheral tolerance can be modulated. However, under inflammatory conditions such as pathogen infection, increased permeabilization of lymphatic vessels allows faster immune cell migration into inflamed tissues following a chemokine gradient, facilitating pathogen clearance and the resolution of inflammation. Interestingly, since the re-discovery of lymphatic vasculature in the central nervous system, known as the meningeal lymphatic vasculature, the role of these lymphatics as a key player in several neurological disorders has been described, with emphasis on the neurodegenerative process. Alternatively, less has been discussed about meningeal lymphatics and its role in neuroinflammation. In this review, we discuss current knowledge about the anatomy and function of the meningeal lymphatic vasculature and specifically analyze its contribution to different neuroinflammatory processes, highlighting the potential therapeutic target of meningeal lymphatic vasculature in these pathological conditions.

Altered motor cortical plasticity in patients with hepatic encephalopathy: A paired associative stimulation study

OBJECTIVE

Hepatic encephalopathy (HE) is a potentially reversible brain dysfunction caused by liver failure. Altered synaptic plasticity is supposed to play a major role in the pathophysiology of HE. Here, we used paired associative stimulation with an inter-stimulus interval of 25 ms (PAS25), a transcranial magnetic stimulation (TMS) protocol, to test synaptic plasticity of the motor cortex in patients with manifest HE.

METHODS

23 HE-patients and 23 healthy controls were enrolled in the study. Motor evoked potential (MEP) amplitudes were assessed as measure for cortical excitability. Time courses of MEP amplitude changes after the PAS25 intervention were compared between both groups.

RESULTS

MEP-amplitudes increased after PAS25 in the control group, indicating PAS25-induced synaptic plasticity in healthy controls, as expected. In contrast, MEP-amplitudes within the HE group did not change and were lower than in the control group, indicating no induction of plasticity.

CONCLUSIONS

Our study revealed reduced synaptic plasticity of the primary motor cortex in HE.

SIGNIFICANCE

Reduced synaptic plasticity in HE provides a link between pathological changes on the molecular level and early clinical symptoms of the disease. This decrease may be caused by disturbances in the glutamatergic neurotransmission due to the known hyperammonemia in HE patients.

The Association between Hepatic Encephalopathy and Diabetic Encephalopathy: The Brain-Liver Axis

Hepatic encephalopathy (HE) is one of the main consequences of liver disease and is observed in severe liver failure and cirrhosis. Recent studies have provided significant evidence that HE shows several neurological symptoms including depressive mood, cognitive dysfunction, impaired circadian rhythm, and attention deficits as well as motor disturbance. Liver disease is also a risk factor for the development of diabetes mellitus. Diabetic encephalopathy (DE) is characterized by cognitive dysfunction and motor impairment. Recent research investigated the relationship between metabolic changes and the pathogenesis of neurological disease, indicating the importance between metabolic organs and the brain. Given that a diverse number of metabolites and changes in the brain contribute to neurologic dysfunction, HE and DE are emerging types of neurologic disease. Here, we review significant evidence of the association between HE and DE, and summarise the common risk factors. This review may provide promising therapeutic information and help to design a future metabolic organ-related study in relation to HE and DE.

Hepatoprotective effect of ultrasonicated ginseng berry extract on a rat mild bile duct ligation model

Background

The Panax ginseng berry extract (GBE) is well known to have an antidiabetic effect. The aim of this study is to evaluate and investigate the protective effect of ultrasonication-processed P. ginseng berry extract (UGBE) compared with GBE on liver fibrosis induced by mild bile duct ligation (MBDL) model in rats. After ultrasonication process, the composition ratio of ginsenoside in GBE was changed. The component ratio of ginsenosides Rh1, Rh4, Rg2, Rg3, Rk1, Rk3, and F4 in the extract was elevated.

Methods

In this study, the protective effect of the newly developed UGBE was evaluated on hepatotoxicity and neuronal damage in MBDL model. Silymarin (150 mg/kg) was used for positive control. UGBE (100 mg/kg, 250 mg/kg, 500 mg/kg), GBE (250 mg/kg), and silymarin (150 mg/kg) were orally administered for 6 weeks after MBDL surgery.

Results

The MBDL surgery induced severe hepatotoxicity that leads to liver inflammation in rats. Also, the serum ammonia level was increased by MBDL surgery. However, the liver dysfunction of MBDL surgery–operated rats was attenuated by UGBE treatment via myeloid differentiation factor 88-dependent Toll-like receptor 4 signaling pathways.

Conclusion

UGBE has a protective effect on liver fibrosis induced by MBDL in rats through inhibition of the TLR4 signaling pathway in liver.

Hepatic encephalopathy changes mitochondrial dynamics and autophagy in the substantia nigra

Hepatic encephalopathy (HE) has been reported in more than 40% of patients with cirrhosis in clinical practice. HE changes mitochondrial dysfunction. Mitochondrial dynamics and autophagy are important for maintaining and removing damaged mitochondria. We used molecular biology and morphology methods to evaluate changes in mitochondrial dynamics and autophagy of the substantia nigra (SN) and prefrontal cortex (PFC) in HE. In this study, we observed that HE increased mitochondrial dynamics and autophagy in the SN, which was not seen in the PFC. HE stimulated dynamin-related protein 1 (DRP1) transformation from the cytosolic to the mitochondria within SN cells, which increased mitochondrial fission and the number of mitochondria. The fusion protein L-OPA1 (long isoforms of OPA1) was increased in the SN of HE mice. HE also increased the levels of autophagy proteins PINK1/PARKIN and P62/LC3-B in the SN, which can selectively remove damaged mitochondria and cell, respectively. Additionally, we used electron microscopy to directly observe changes in mitochondrial morphology in the SN of HE mice and found the number of mitochondria was increased. However, there were no significant changes in the fission, fusion or autophagy proteins in PFC-purified mitochondrial proteins in HE mice. The number of mitochondria also did not show alterations in the PFC of HE mice compared with that in a sham group. These results illustrate that mitochondria can protect themselves by changing the dynamics and autophagy in the SN of HE mice. Changes in the mitochondrial dynamics and autophagy related to HE can help repair damaged mitochondria and provide a further understanding of the mechanisms of hepatic encephalopathy.

Targeted upregulation of uncoupling protein 2 within the basal ganglia output structure ameliorates dyskinesia after severe liver failure

Impaired motor function, due to the dysfunction of the basal ganglia, is the most common syndrome of hepatic encephalopathy (HE), and its etiology remains poorly understood. Neural oxidative stress is shown to be the major cellular defects contributing to HE pathogenesis. Mitochondrial uncoupling protein 2 (UCP2) has been implicated in neuroprotection in several neurological disorders. We explored the neuroprotective role of UCP2 within the substantia nigra pars reticulate (SNr), the output structure of the basal ganglia, in HE. The toxin thioacetamide (TAA) induced HE mice showed hypolocomotion, which was associated with decreased ATP levels and loss of antioxidant substances SOD and GSH within the SNr. Stable overexpression of UCP2 via AAV-UCP2 under the control of the UCP2 promoter in bilateral SNr preserved local ATP level, increased antioxidant substances, and ameliorated locomotion defects after severe liver failure. Contrary to UCP2 overexpression, targeted knockdown of UCP2 within bilateral SNr via AAV-UCP2 shRNA exacerbated the impaired mitochondrial dysfunction and hypokinesia in HE mice. The modulatory effects of UCP2 was due to mediation of K⁺-Cl^- cotransporter-2 (KCC2) expression on GABAergic neurons of SNr. Taken together, our results demonstrate that UCP2 exerts a neural protective role at the subcortical level by increasing the resistance of neurons to oxidative stress, which may offer a novel therapeutic target for the treatment of motor dysfunction diseases.

Characterization of the CA1 pyramidal neurons in rat model of hepatic cirrhosis: insights into their electrophysiological properties

Although the key contributors of altering neurological function in hepatic encephalopathy are relatively well known, the electrophysiological mechanism of CA1 damage, a key vulnerable area during hyperammonemia, have not yet been defined. Therefore, here we focus on the electrophysiological mechanisms of cognitive impairments following bile duct ligation (BDL). We performed patch-clamp recordings from the CA1 pyramidal neurons in hippocampus of male Wistar rats, which underwent sham or BDL surgery. A striking electrophysiological change of hippocampal neurons in experimental model of BDL was observed in the present study. Spontaneous firing frequency and rate of action potential (AP) rebound was decreased and afterhyperpolarization amplitude (AHP) was increased significantly in hippocampal cells of BDL animals compared to sham group. Together, the results suggest that altered intrinsic properties of the hippocampal neurons may contribute to the cognitive abnormalities during hepatic encephalopathy (HE), highlighting the electrophysiological mechanisms for providing new treatments against HE.

Modulation of GABAA receptors by neurosteroids. A new concept to improve cognitive and motor alterations in hepatic encephalopathy

Hepatic encephalopathy (HE) is a complex neuropsychiatric syndrome affecting patients with liver diseases, mainly those with liver cirrhosis. The mildest form of HE is minimal HE (MHE), with mild cognitive impairment, attention deficit, psychomotor slowing and impaired visuo-motor and bimanual coordination. MHE may progress to clinical HE with worsening of the neurological alterations which may lead to reduced consciousness and, in the worse cases, may progress to coma and death. HE affects several million people in the world and is a serious health, social and economic problem. There are no specific treatments for the neurological alterations in HE. The mechanisms underlying the cognitive and motor alterations in HE are beginning to be clarified in animal models. These studies have allowed to design and test in animal models of HE new therapeutic approaches which have successfully restored cognitive and motor function in rats with HE. In this article we review the evidences showing that.

Hyperammonemia in gene-targeted mice lacking functional hepatic glutamine synthetase

Urea cycle defects and acute or chronic liver failure are linked to systemic hyperammonemia and often result in cerebral dysfunction and encephalopathy. Although an important role of the liver in ammonia metabolism is widely accepted, the role of ammonia metabolizing pathways in the liver for maintenance of whole-body ammonia homeostasis in vivo remains ill-defined. Here, we show by generation of liver-specific Gln synthetase (GS)-deficient mice that GS in the liver is critically involved in systemic ammonia homeostasis in vivo. Hepatic deletion of GS triggered systemic hyperammonemia, which was associated with cerebral oxidative stress as indicated by increased levels of oxidized RNA and enhanced protein Tyr nitration. Liver-specific GS-deficient mice showed increased locomotion, impaired fear memory, and a slightly reduced life span. In conclusion, the present observations highlight the importance of hepatic GS for maintenance of ammonia homeostasis and establish the liver-specific GS KO mouse as a model with which to study effects of chronic hyperammonemia.

Cerebral effects of ammonia in liver disease: current hypotheses

Hyperammonemia is necessary for development of the cerebral complications to liver disease including hepatic encephalopathy and cerebral edema but the mechanisms are unclear. Ammonia is taken up by the brain in proportion to its arterial concentration. The flux into the brain is most likely by both diffusion of NH3 and mediated transport of NH4 (+) . Astrocytic detoxification of ammonia involves formation of glutamine at concentrations high enough to produce cellular edema, but compensatory mechanisms reduce this effect. Glutamine can be taken up by astrocytic mitochondria and initiate the mitochondrial permeability transition but the clinical relevance is uncertain. Elevated astrocytic glutamine interferes with neurotransmission. Thus, animal studies show enhanced glutamatergic neurotransmission via the NMDA receptor which may be related to the acute cerebral complications to liver failure, while impairment of the NMDA activated glutamate-NO-cGMP pathway could relate to the behavioural changes seen in hepatic encephalopathy. Elevated glutamine also increases GABA-ergic tone, an effect which is aggravated by mitochondrial production of neurosteroids; this may relate to decreased neurotransmission and precipitation of encephalopathy by GABA targeting drugs. Hyperammonemia may compromise cerebral energy metabolism as elevated cerebral lactate is generally reported. Hypoxia is unlikely since cerebral oxygen:glucose utilisation and lactate:pyruvate ratio are both normal in clinical studies. Ammonia inhibits α-ketoglutaratedehydrogenase in isolated mitochondria, but the clinical relevance is dubious due to the observed normal cerebral oxygen:glucose utilization. Recent studies suggest that ammonia stimulates glycolysis in excess of TCA cycle activity, a hypothesis that may warrant further testing, in being in accordance with the limited clinical observations.

Interplay between glutamatergic and GABAergic neurotransmission alterations in cognitive and motor impairment in minimal hepatic encephalopathy

The cognitive and motor alterations in hepatic encephalopathy (HE) are the final result of altered neurotransmission and communication between neurons in neuronal networks and circuits. Different neurotransmitter systems cooperate to modulate cognitive and motor function, with a main role for glutamatergic and GABAergic neurotransmission in different brain areas and neuronal circuits. There is an interplay between glutamatergic and GABAergic neurotransmission alterations in cognitive and motor impairment in HE. This interplay may occur: (a) in different brain areas involved in specific neuronal circuits; (b) in the same brain area through cross-modulation of glutamatergic and GABAergic neurotransmission. We will summarize some examples of the (1) interplay between glutamatergic and GABAergic neurotransmission alterations in different areas in the basal ganglia-thalamus-cortex circuit in the motor alterations in minimal hepatic encephalopathy (MHE); (2) interplay between glutamatergic and GABAergic neurotransmission alterations in cerebellum in the impairment of cognitive function in MHE through altered function of the glutamate-nitric oxide-cGMP pathway. We will also comment the therapeutic implications of the above studies and the utility of modulators of glutamate and GABA receptors to restore cognitive and motor function in rats with hyperammonemia and hepatic encephalopathy.

Cerebral oedema is not responsible for motor or cognitive deficits in rats with hepatic encephalopathy

BACKGROUND & AIMS

Low-grade cytotoxic oedema is considered a main contributor to the neurological (motor and cognitive) alterations in patients with hepatic encephalopathy (HE). This assumption is mainly based on studies with cultured astrocytes treated with very large ammonia concentrations or with animal models of acute liver failure with strong HE. However, the possible contribution of cerebral oedema (vasogenic or cytotoxic) to cognitive or motor alterations in chronic mild HE has not been demonstrated. The aim of this work was to assess whether cerebral oedema contributes to cognitive and/or motor alterations in rats with chronic mild HE.

METHODS

Motor activity and coordination and different types of learning and memory were assessed in rats with porta-caval shunts (PCS). Brain oedema was assessed by gravimetry in cerebellum and cortex and apparent diffusion coefficient (ADC) by magnetic resonance in 16 areas.

RESULTS

Four weeks after surgery, PCS rats show reduced motor activity and coordination, impaired ability to learn a conditional discrimination task in the Y maze and reduced spatial memory in the Morris water maze. PCS rats did not show increased brain water content at 4 or 10 weeks or changes in ADC at 4 weeks. At 10 weeks, increased ADC in some areas is compatible with vasogenic but not cytotoxic oedema.

CONCLUSION

Cerebral oedema is not involved in motor and cognitive alterations in rats (and likely in humans) with mild HE. Proper understanding of the mechanisms responsible for the neurological alterations in HE is necessary to design efficient treatments.

Hepatic encephalopathy: effects of liver failure on brain function

Liver failure affects brain function, leading to neurological and psychiatric alterations; such alterations are referred to as hepatic encephalopathy (HE). Early diagnosis of minimal HE reveals an unexpectedly high incidence of mild cognitive impairment and psychomotor slowing in patients with liver cirrhosis - conditions that have serious health, social and economic consequences. The mechanisms responsible for the neurological alterations in HE are beginning to emerge. New therapeutic strategies acting on specific targets in the brain (phosphodiesterase 5, type A GABA receptors, cyclooxygenase and mitogen-activated protein kinase p38) have been shown to restore cognitive and motor function in animal models of chronic HE, and NMDA receptor antagonists have been shown to increase survival in acute liver failure. This article reviews the latest studies aimed at understanding how liver failure affects brain function and potential ways to ameliorate these effects.

Synergistic effects between CA1 mu opioid and dopamine D1-like receptors in impaired passive avoidance performance induced by hepatic encephalopathy in mice

BACKGROUND AND AIM

Numerous investigations have indicated that hepatic encephalopathy (HE) alters the levels of various neurotransmitters. However, comprehensive data regarding the effects of CA1 opioidergic and dopaminergic (DAergic) systems on HE-induced amnesia are still lacking.

METHODS

Following intra-dorsal hippocampal (CA1) injection of mu opioid and dopamine D1- and D2-like receptors antagonists in male mice, one-trial step-down and hole-board paradigms were used to assess memory and exploratory behaviors, respectively.

RESULTS

Our data demonstrated that HE impairs memory 24 days after bile duct ligation (BDL). Furthermore, while the higher dose of DA D1-like receptor antagonist (SCH23390, 0.5 μg/mouse) induced amnesia and anxiogenic-like behaviors, mu receptor antagonist (naloxone: 0.0125, 0.025 and 0.05 μg/mouse) and DA D2-like receptor antagonist (sulpiride: 0.0625, 0.125 and 0.25 μg/mouse) by themselves, could not exert an effect on memory performance in passive avoidance task. On the other hand, pre-test injection of all drugs reversed the HE-induced amnesia 24 days after BDL, while having no effect on exploratory behaviors. Pre-test co-administration of the subthreshold dose SCH23390 (0.25 μg/mouse) and sulpiride (0.0625 μg/mouse) or naloxone (0.0125 μg/mouse) could likewise reverse the BDL-induced amnesia. However, when the subthreshold sulpiride plus naloxone were co-administered, BDL-induced amnesia was not blocked.

CONCLUSIONS

Memory performance is impaired 24 days post BDL and CA1 mu opioid and DA D1-like receptors antagonist synergistic effects are likely involved in this phenomenon.

Chronic hyperammonemia, glutamatergic neurotransmission and neurological alterations

This mini-review focus on our studies on alterations in glutamatergic neurotransmission and their role in neurological alterations in rat models of chronic hyperammonemia and hepatic encephalopathy (HE). Hyperammonemia impairs the glutamate-nitric oxide (NO)-cGMP pathway in cerebellum, which is responsible for reduced learning ability. We studied the underlying mechanisms and designed treatments to restore the pathway and learning. This was achieved by treatment with: phosphodiesterase 5 inhibitors, cGMP, anti-inflammatories (ibuprofen), p38 inhibitors or GABAA receptor antagonists (bicuculline). Hyperammonemia alters signal transduction associated to metabotropic glutamate receptors (mGluRs). Hypokinesia in hyperammonemia and HE is due to increased extracellular glutamate and mGluR1 activation in substantia nigra; blocking this receptor restores motor activity. The motor responses to mGluRs activation in nucleus accumbens (NAcc) are altered in hyperammonemia and HE, with reduced dopamine and increased glutamate release. This leads to activation of different neuronal circuits and enhanced motor responses. These studies show that altered responses to activation of NMDA receptors and mGluRs play essential roles in cognitive and motor alterations in hyperammonemia and HE and provide new treatments restoring cognitive and motor function.

Abnormal chloride homeostasis in the substancia nigra pars reticulata contributes to locomotor deficiency in a model of acute liver injury

BACKGROUND

Altered chloride homeostasis has been thought to be a risk factor for several brain disorders, while less attention has been paid to its role in liver disease. We aimed to analyze the involvement and possible mechanisms of altered chloride homeostasis of GABAergic neurons within the substantia nigra pars reticulata (SNr) in the motor deficit observed in a model of encephalopathy caused by acute liver failure, by using glutamic acid decarboxylase 67 - green fluorescent protein knock-in transgenic mice.

METHODS

Alterations in intracellular chloride concentration in GABAergic neurons within the SNr and changes in the expression of two dominant chloride homeostasis-regulating genes, KCC2 and NKCC1, were evaluated in mice with hypolocomotion due to hepatic encephalopathy (HE). The effects of pharmacological blockade and/or activation of KCC2 and NKCC1 functions with their specific inhibitors and/or activators on the motor activity were assessed.

RESULTS

In our mouse model of acute liver injury, chloride imaging indicated an increase in local intracellular chloride concentration in SNr GABAergic neurons. In addition, the mRNA and protein levels of KCC2 were reduced, particularly on neuronal cell membranes; in contrast, NKCC1 expression remained unaffected. Furthermore, blockage of KCC2 reduced motor activity in the normal mice and led to a further deteriorated hypolocomotion in HE mice. Blockade of NKCC1 was not able to normalize motor activity in mice with liver failure.

CONCLUSION

Our data suggest that altered chloride homeostasis is likely involved in the pathophysiology of hypolocomotion following HE. Drugs aimed at restoring normal chloride homeostasis would be a potential treatment for hepatic failure.

Hyperammonemia alters the modulation by different neurosteroids of the glutamate-nitric oxide-cyclic GMP pathway through NMDA- GABAA - or sigma receptors in cerebellum in vivo

Several neurosteroids modulate the glutamate-nitric oxide (NO)-cGMP pathway in cerebellum through modulation of NMDA- GABAA - or sigma receptors. Hyperammonemia alters the concentration of several neurosteroids and impairs the glutamate-NO-cGMP pathway, leading to impaired learning ability. This work aimed to assess whether chronic hyperammonemia alters the modulation by different neurosteroids of GABAA, NMDA, and/or sigma receptors and of the glutamate-NO-cGMP pathway in cerebellum. Neurosteroids were administered through microdialysis probes, and extracellular cGMP and citrulline were measured. Then NMDA was administered to assess the effects on the glutamate-NO-cGMP pathway activation. Hyperammonemia completely modifies the effects of pregnanolone and pregnenolone. Pregnanolone acts as a GABAA receptor agonist in controls, but as an NMDA receptor antagonist in hyperammonemic rats. Pregnenolone does not induce any effect in controls, but acts as a sigma receptor agonist in hyperammonemic rats. Hyperammonemia potentiates the actions of tetrahydrodeoxy-corticosterone (THDOC) as a GABAA receptor agonist, allopregnanolone as an NMDA receptor antagonist, and pregnenolone sulfate as an NMDA receptor activation enhancer. Neurosteroids that reduce the pathway (pregnanolone, THDOC, allopregnanolone, DHEAS) may contribute to cognitive impairment in hyperammonemia and hepatic encephalopathy. Pregnenolone would impair cognitive function in hyperammonemia. Neurosteroids that restore the pathway in hyperammonemia (pregnenolone sulfate) could restore cognitive function in hyperammonemia and encephalopathy.

Pathogenesis of hepatic encephalopathy

Hepatic encephalopathy can be a serious complication of acute liver failure and chronic liver diseases, predominantly liver cirrhosis. Hyperammonemia plays the most important role in the pathogenesis of hepatic encephalopathy. The brain-blood barrier disturbances, changes in neurotransmission, neuroinflammation, oxidative stress, GABA-ergic or benzodiazepine pathway abnormalities, manganese neurotoxicity, brain energetic disturbances, and brain blood flow abnormalities are considered to be involved in the development of hepatic encephalopathy. The influence of small intestine bacterial overgrowth (SIBO) on the induction of minimal hepatic encephalopathy is recently emphasized. The aim of this paper is to present the current views on the pathogenesis of hepatic encephalopathy.

Impairment of the organization of locomotor and exploratory behaviors in bile duct-ligated rats

Hepatic encephalopathy (HE) arises from acute or chronic liver diseases and leads to several problems, including motor impairment. Animal models of chronic liver disease have extensively investigated the mechanisms of this disease. Impairment of locomotor activity has been described in different rat models. However, these studies are controversial and the majority has primarily analyzed activity parameters. Therefore, the aim of the present study was to evaluate locomotor and exploratory behavior in bile duct-ligated (BDL) rats to explore the spatial and temporal structure of behavior. Adult female Wistar rats underwent common bile duct ligation (BDL rats) or the manipulation of common bile duct without ligation (control rats). Six weeks after surgery, control and BDL rats underwent open-field, plus-maze and foot-fault behavioral tasks. The BDL rats developed chronic liver failure and exhibited a decrease in total distance traveled, increased total immobility time, smaller number of rearings, longer periods in the home base area and decreased percentage of time in the center zone of the arena, when compared to the control rats. Moreover, the performance of the BDL rats was not different from the control rats for the elevated plus-maze and foot-fault tasks. Therefore, the BDL rats demonstrated disturbed spontaneous locomotor and exploratory activities as a consequence of altered spatio-temporal organization of behavior.

Exploratory investigation on nitro- and phospho-proteome cerebellum changes in hyperammonemia and hepatic encephalopathy rat models

Hepatic encephalopathy (HE) is a neurological disease associated with hepatic dysfunction. Current knowledge suggests that hyperammonemia, related to liver failure, is a main factor contributing to the cerebral alterations in HE and that hyperammonemia might impair signal transduction associated with post-translational modification of proteins such as tyrosine-nitration and phosphorylation. However, the molecular bases of the HE remain unclear and very little is known about the occurrence of post-translational modification on in vivo proteins. In this exploratory study we look for evidence of post-translation modifications of proteins in the cerebellum of experimental HE rat models using a proteomic approach. For the first time we showed that hyperammonemia without liver failure (HA rats) and experimental HE with liver failure due to portacaval shunt (PCS rats) lead to a reduced protein nitration in rat cerebellum, where the undernitrated proteins were involved in energy metabolism and cytoskeleton remodelling. Moreover we showed that tyrosine nitration loss of these proteins was not necessarily associated to a change in their phosphorylation state as result of the disease. Interestingly the rat cerebellum phosphoproteome was mainly perturbed in PCS rats, whereas HA rats did not shown appreciable changes in their phosphoprotein profile. Since the protein nitration level decreased similarly in the cerebellum of both HA and PCS rats, this implies that the two disease models share common effects but also present some differential signalling effects in the cerebellum of the same animals. This study highlights the interest for studying the concerted action of multiple signalling pathways in HE development.

Patients with minimal hepatic encephalopathy show impaired mismatch negativity correlating with reduced performance in attention tests

Attention deficit is an early event in the cognitive impairment of patients with minimal hepatic encephalopathy (MHE). The underlying mechanisms remain unclear. Mismatch negativity (MMN) is an auditory event-related potential that reflects an attentional trigger. Patients with schizophrenia show impaired attention and cognitive function, which are reflected in altered MMN. We hypothesized that patients with MHE, similarly to those with schizophrenia, should show MMN alterations related with attention deficits. The aims of this work were to assess whether (1) MMN is altered in cirrhotic patients with MHE, compared to those without MHE, (2) MMN changes in parallel with performance in attention tests and/or MHE in a longitudinal study, and (3) MMN predicts performance in attention tests and/or in the Psychometric Hepatic Encephalopathy Score (PHES). We performed MMN analysis as well as attention and coordination tests in 34 control subjects and in 37 patients with liver cirrhosis without MHE and 23 with MHE. Patients with MHE show reduced performance in selective and sustained attention tests and in visuomotor and bimanual coordination tests. The MMN wave area was reduced in patients with MHE, but not in those without MHE. In the longitudinal study, MMN area improved in parallel with performance in attention tests and PHES in 4 patients and worsened in parallel in another 4. Logistic regression analyses showed that MMN area predicts performance in attention tests and in PHES, but not in other tests or critical flicker frequency. Receiver operating characteristic curve analyses showed that MMN area predicts attention deficits in the number connection tests A and B, Stroop tasks, and MHE, with sensitivities of 75%-90% and specificities of 76%-83%.

CONCLUSION

MMN area is useful to diagnose attention deficits and MHE in patients with liver cirrhosis.

Differential modulation of the glutamate-nitric oxide-cyclic GMP pathway by distinct neurosteroids in cerebellum in vivo

The glutamate-nitric oxide (NO)-cGMP pathway mediates many responses to activation of N-methyl-d-aspartate (NMDA) receptors, including modulation of some types of learning and memory. The glutamate-NO-cGMP pathway is modulated by GABAergic neurotransmission. Activation of GABA(A) receptors reduces the function of the pathway. Several neurosteroids modulate the activity of GABA(A) and/or NMDA receptors, suggesting that they could modulate the function of the glutamate-NO-cGMP pathway. The aim of this work was to assess, by in vivo microdialysis, the effects of several neurosteroids with different effects on GABA(A) and NMDA receptors on the function of the glutamate-NO-cGMP pathway in cerebellum in vivo. To assess the effects of the neurosteroids on the glutamate-NO-cGMP pathway, they were administered through the microdialysis probe before administration of NMDA and the effects on NMDA-induced increase in extracellular cGMP were analyzed. We also assessed the effects of the neurosteroids on basal levels of extracellular cGMP. To assess the effects of neurosteroids on nitric oxide synthase (NOS) activity and on NMDA-induced activation of NOS, we also measured the effects of the neurosteroids on extracellular citrulline. Pregnanolone and tetrahydrodeoxy-corticosterone (THDOC) behave as agonists of GABA(A) receptors and completely block NMDA-induced increase in cGMP. Pregnanolone but not THDOC also reduced basal levels of extracellular cGMP. Pregnenolone did not affect extracellular cGMP or its increase by NMDA administration. Pregnenolone sulfate increased basal extracellular cGMP and potentiated NMDA-induced increase in cGMP, behaving as an enhancer of NMDA receptors activation. Allopregnanolone and dehydroepiandrosterone sulphate behave as antagonists of NMDA receptors, increasing basal cGMP and blocking completely NMDA-induced increase in cGMP. Dehydroepiandrosterone sulphate seems to do this by activating sigma receptors. These data support the concept that, at physiological concentrations, different neurosteroids may rapidly modulate, in different ways and by different mechanisms, the function of the glutamate-NO-cGMP pathway and, likely, some forms of learning and memory modulated by this pathway.

Manganese accumulation in the CNS and associated pathologies

Manganese (Mn) is an essential metal for life. It is a key constituent of clue enzymes in the central nervous system, contributing to antioxidant defenses, energetic metabolism, ammonia detoxification, among other important functions. Until now, Mn transport mechanisms are partially understood; however, it is known that it shares some mechanisms of transport with iron. CNS is susceptible to Mn toxicity because it possesses mechanisms that allow Mn entry and favor its accumulation. Cases of occupational Mn exposure have been extensively reported in the literature; however, there are other ways of exposure, such as long-term parental nutrition and liver failure. Manganism and hepatic encephalopathy are the most common pathologies associated with the effects of Mn exposure. Both pathologies are associated with motor and psychiatric disturbances, related in turn to mechanisms of damage such as oxidative stress and neurotransmitters alterations, the dopaminergic system being one of the most affected. Although manganism and Parkinson's disease share some characteristics, they differ in many aspects that are discussed here. The mechanisms for Mn transport and its participation in manganism and hepatic encephalopathy are also considered in this review. It is necessary to find an effective therapeutic strategy to decrease Mn levels in exposed individuals and to treat Mn long term effects. In the case of patients with chronic liver failure it would be worthwhile to test a low-Mn diet in order to ameliorate symptoms of hepatic encephalopathy possibly related to Mn accumulation.

Locomotor impairment and cerebrocortical oxidative stress in portal vein ligated rats in vivo

BACKGROUND & AIMS

Oxidative/nitrosative stress plays an important role in the pathogenesis of hepatic encephalopathy and ammonia toxicity. The present study was undertaken in order to investigate the impact of portal vein ligation on cerebrocortical oxidative stress and its relation to locomotor activity.

METHODS

Cerebral protein tyrosine nitration, RNA oxidation, locomotor activity, and microglia activation were studied in rats that underwent portal vein ligation (PVL).

RESULTS

Two weeks after PVL, increased levels of protein tyrosine nitration and RNA oxidation were found in the brain. PVL rats exhibited hyperammonemia and reduced locomotor behaviour, but displayed no signs of microglia activation or upregulation of the mRNAs for interleukin-1ß and tumor necrosis factor-α. PVL also had no effect on astrocytic glutamate transporter or inducible nitric-oxide synthase expression. Only cerebral Il-6 mRNA levels were increased. Daily administration of indomethacin prevented PVL-induced protein tyrosine nitration, RNA oxidation, Il-6 mRNA increase, and the impairment of locomotor activity, but did not prevent PVL-induced hyperammonemia.

CONCLUSIONS

The data suggest that PVL triggers oxidative/nitrosative stress in the brain without activation of microglia and neuroinflammation. Prevention of protein tyrosine nitration and RNA oxidation by indomethacin also prevents the disturbances in locomotor activity pointing to a relevance of oxidative stress in the pathophysiology of HE.

Treatment with sildenafil prevents impairment of learning in rats born to pre-eclamptic mothers

Pre-eclampsia is an important hypertensive pregnancy disorder and a main cause of maternal and fetal morbidity and mortality. Children born from mothers with pre-eclampsia may present cognitive deficits. The mechanisms leading to this cognitive impairment remain unclear and no treatments to improve it have been tested. Pre-eclampsia is associated with impaired regulation of the nitric oxide-3'-5'guanosine monophosphate cyclic (cGMP) pathway, which modulates some cognitive functions. We hypothesized that alterations in the NO-cGMP pathway would be involved in the mechanisms leading to cognitive impairment in rats born to pre-eclamptic mothers and that treatment with sildenafil, an inhibitor of the phosphodiesterase that degrades cGMP, could restore their cognitive function. To test these hypotheses, we used an animal model of pre-eclampsia in rats: pregnant rats treated with l-nitro-arginine methyl ester, an inhibitor of nitric oxide synthase. Using this model, we assessed: (1) whether rats born to pre-eclamptic mothers show reduced learning ability and/or altered motor activity or coordination when they are 2 months-old; (2) whether cognitive impairment is associated with reduced function of the glutamate-NO-cGMP pathway in brain in vivo; and (3) whether treatment of the mothers with sildenafil prevents this cognitive and motor alterations. The results reported show that the ability to learn a conditional discrimination task in a Y maze is reduced in rats born to pre-eclamptic mothers. This impairment was associated with reduced function of the glutamate-NO-cGMP pathway in brain in vivo, as assessed by microdialysis in freely moving rats. Treatment with sildenafil restores the function of this pathway and learning ability.

Extrapyramidal signs predict the development of overt hepatic encephalopathy in patients with liver cirrhosis

BACKGROUND/AIMS

The long-term evolution of cirrhotic patients with extrapyramidal signs has not yet been studied. We have investigated the influence of extrapyramidal signs on the prognosis, evolution, and quality of life of patients with liver cirrhosis.

METHODS

Forty-six patients with cirrhosis were followed up and 18 of them were reevaluated, a mean of 45 months later. Cognitive impairment was measured with psychometric tests (Trail-Making Test part A, Grooved-Pegboard, Block-Design, Oral Symbol Digit and Stroop Test). Extrapyramidal signs were evaluated using the Unified Parkinson's Disease Rating Scale (UPDRS). Health-related quality of life was measured using the SF-36 scale and the Chronic Liver Disease Questionnaire.

RESULTS

Eleven of the 46 patients who were followed up developed overt hepatic encephalopathy (HE) during the follow-up. The presence of extrapyramidal signs was the unique factor that predicted overt HE and patients with basal higher score in the part 3 of the UPDRS developed overt HE more frequently [hazard ratio=1.29; 95% confidence interval (1.04-1.60) P=0.023]. In the 18 reevaluated patients, there was an increase in the score of the UPDRS part 3 after follow-up. There was a worsening in the score values of the block design and the Number Connection Test. In health-related quality of life scales, patients scored better in the area of mental health of the SF-36 scale. Patients with extrapyramidal signs persisted with worse scores in several items of the SF-36 scale and the Chronic Liver Disease Questionnaire.

CONCLUSION

The presence of extrapyramidal signs in patients with liver cirrhosis predicts the development of overt HE. These signs increased along the follow-up, and remain a bad influence on quality of life.

Effect of the class I metabotropic glutamate receptor antagonist AIDA on certain behaviours in rats with experimental chronic hyperammonemia

PURPOSE

This study examines possible interactions between behavioral effects and mGluR1 (class I metabotropic glutamate receptor) by injecting AIDA [(RS)-1-aminoindan-1,5-dicarboxylic acid] in rats with experimental chronic hyperammonemia (chHA).

MATERIAL/METHODS

The effects of mGluR1 antagonist on some behaviors were tested in control groups of rats and in rats with chHA. Experimental chHA was induced by intraperitoneal injection of ammonium acetate (12 mmol/kg) for five consecutive days. We used the following behavioural tests: the open field test, the passive avoidance test and the elevated "plus" maze.

RESULTS

In control rats AIDA administered intracerebroventricularly (i.c.v.) at the dose 100 nmol decreased the number of crossings and bar approaches in the open field test and impaired acquisition and recall in the passive avoidance situation. ChHA significantly inhibited locomotor and exploratory activity and profoundly impaired acquisition and recall processes in the passive avoidance test and significantly increased acute stress responses. AIDA increased locomotor activity in chHA rats (especially number of crossed fields and rearings) and produced anxiety enhancement in rats with chHA. AIDA used in rats with chHA significantly improved acquisition and retrieval processes.

CONCLUSIONS

The obtained results suggest that AIDA, the antagonist of mGluR1, had beneficial effects on learning and memory in rats with experimental chronic hyperammonemia.

Ammonia metabolism, the brain and fatigue; revisiting the link

This review addresses the ammonia fatigue theory in light of new evidence from exercise and disease studies and aims to provide a view of the role of ammonia during exercise. Hyperammonemia is a condition common to pathological liver disorders and intense or exhausting exercise. In pathology, hyperammonemia is linked to impairment of normal brain function and the onset of the neurological condition, hepatic encephalopathy. Elevated blood ammonia concentrations arise due to a diminished capacity for removal via the liver and lead to increased exposure of organs, such as the brain, to the toxic effects of ammonia. High levels of brain ammonia can lead to deleterious alterations in astrocyte morphology, cerebral energy metabolism and neurotransmission, which may in turn impact on the functioning of important signalling pathways within the neuron. Such changes are believed to contribute to the disturbances in neuropsychological function, in particular the learning, memory, and motor control deficits observed in animal models of liver disease and also patients with cirrhosis. Hyperammonemia in exercise occurs as a result of an increased production by contracting muscle, through adenosine monophosphate (AMP) deamination (the purine nucleotide cycle) and branched chain amino acid (BCAA) deamination prior to oxidation. Plasma concentrations of ammonia during exercise often achieve or exceed those measured in liver disease patients, resulting in increased cerebral uptake. In this article we propose that exercise-induced hyperammonemia may lead to concomitant disturbances in brain function, potentially through similar mechanisms underpinning pathology, which may impact on performance as fatigue or reduced function, especially during extreme exercise.

Glutamine synthetase activity and glutamate uptake in hippocampus and frontal cortex in portal hypertensive rats

AIM: To study glutamine synthetase (GS) activity and glutamate uptake in the hippocampus and frontal cortex (FC) from rats with prehepatic portal vein hypertension.

METHODS: Male Wistar rats were divided into sham-operated group and a portal hypertension (PH) group with a regulated stricture of the portal vein. Animals were sacrificed by decapitation 14 d after portal vein stricture. GS activity was determined in the hippocampus and FC. Specific uptake of radiolabeled L-glutamate was studied using synaptosome-enriched fractions that were freshly prepared from both brain areas.

RESULTS: We observed that the activity of GS increased in the hippocampus of PH rats, as compared to control animals, and decreased in the FC. A significant decrease in glutamate uptake was found in both brain areas, and was more marked in the hippocampus. The decrease in glutamate uptake might have been caused by a deficient transport function, significantly and persistent increase in this excitatory neurotransmitter activity.

CONCLUSION: The presence of moderate ammonia blood levels may add to the toxicity of excitotoxic glutamate in the brain, which causes alterations in brain function. Portal vein stricture that causes portal hypertension modifies the normal function in some brain regions.

Neuroinflammation contributes to hypokinesia in rats with hepatic encephalopathy: ibuprofen restores its motor activity

Patients with hepatic encephalopathy show altered motor function, psychomotor slowing, and hypokinesia, which are reproduced in rats with portacaval shunts (PCS). Increased extracellular glutamate in substantia nigra pars reticulata (SNr) is responsible for hypokinesia in PCS rats. The mechanisms by which liver failure leads to increased extracellular glutamate in SNr remain unclear. Inflammation seems to act synergistically with hyperammonemia to induce neurological alterations in hepatic encephalopathy. It is therefore possible that inflammation-associated alterations may contribute to motor alterations in hepatic encephalopathy. The aim of this work was to assess whether treatment with an antiinflammatory, ibuprofen, is able to normalize extracellular glutamate in SNr and/or to improve hypokinesia in PCS rats. The amounts of the glutamate transporters GLT-1 and EAAC-1 are reduced by 26% and 32%, respectively, in SNr of PCS rats. This reduction is associated with a tenfold increase in extracellular glutamate in SNr and a reduction in motor activity. Chronic treatment with 30 mg/kg ibuprofen completely normalizes the amount of GLT-1 and EAAC-1 and significantly reduces (by 53%) extracellular glutamate in SNr of PCS rats. Moreover, ibuprofen, at 15 or 30 (but not at 5) mg/kg/day, completely eliminates hypokinesia, restoring normal motor activity. This supports the idea that inflammation is a main contributor to the induction of hypokinesia in hepatic encephalopathy. Moreover, these data point to the possible therapeutic utility of decreasing inflammation, by safe procedures, in the treatment of the motor deficits in patients with hepatic encephalopathy.

Hyperammonemia increases GABAergic tone in the cerebellum but decreases it in the rat cortex

BACKGROUND & AIMS

GABAA receptors modulate the function of the glutamate-nitric oxide-guanosine 3',5'-cyclic monophosphate (cGMP) pathway, which is reduced in cerebellum in hyperammonemic rats. It has been proposed that hyperammonemia-induced increases in gamma-aminobutyric acid "(GABAergic) tone" contribute to the pathogenesis of hepatic encephalopathy (HE), although this has not been assessed in vivo in animal models. We studied whether chronic hyperammonemia in rats increases GABAergic tone in the cerebellum and/or cerebral cortex and whether this increase contributes to cognitive impairment.

METHODS

We blocked GABAA receptors of rats with bicuculline and analyzed the function of this pathway in cerebellum and effects on learning ability.

RESULTS

Hyperammonemia increased GABAergic tone in cerebellum but decreased it in the cerebral cortex of rats. Increased GABAergic tone in the cerebellum of rats with hyperammonemia could have been caused by increases in extracellular GABA; tetrahydrodeoxy-corticosterone (a neurosteroid that enhances GABAA receptor activation); or amounts of the alpha1, alpha6, and gamma2 subunits of GABAA receptors. The decrease in GABAergic tone observed in the cortex could have resulted from the reduced amount of GABAA receptors delta and gamma2 subunits or increased levels of pregnanolone (5-fold), which selectively reduces activation of GABAA receptors that contain alpha4 subunits (widely expressed in cortex but not in cerebellum). Treatment with bicuculline normalized GABAergic tone and restored the increase in cGMP that was induced by activation of N-methyl-D-aspartate receptors and learning ability in hyperammonemic rats.

CONCLUSIONS

Increased GABAergic tone in the cerebellum contributes to cognitive impairment in hyperammonemic rats.

Glutamatergic and gabaergic neurotransmission and neuronal circuits in hepatic encephalopathy

Patients with hepatic encephalopathy (HE) may present different neurological alterations including impaired cognitive function and altered motor activity and coordination. HE may lead to coma and death. Many of these neurological alterations are the consequence of altered neurotransmission. Hyperammonemia is a main contributor to the alterations in neurotransmission and in neurological functions in HE. Both glutamatergic and GABAergic neurotransmission are altered in animal models of HE. We review some of these alterations, especially those alterations in glutamatergic neurotransmission responsible for some specific neurological alterations in hyperammonemia and HE: the role 1) of excessive NMDA receptors activation in death induced by acute hyperammonemia; 2) of impaired function of the glutamate-nitric oxide-cGMP pathway, associated to NMDA receptors, in cognitive impairment in chronic HE; 3) of increased extracellular glutamate and activation of metabotropic glutamate receptors in substantia nigra in hypokinesia in chronic HE. The therapeutic implications are discussed. We also review the alterations in the function of the neuronal circuits between basal ganglia-thalamus-cortex modulating motor activity and the role of sequential alterations in glutamatergic and GABAergic neurotransmission in these alterations. HE would be a consequence of altered neuronal communication due to alterations in general neurotransmission involving different neurotransmitter systems in different neurons.

Mechanisms of cognitive alterations in hyperammonemia and hepatic encephalopathy: therapeutical implications

Patients with liver diseases (e.g. cirrhosis) may present hepatic encephalopathy (HE), an alteration in cerebral function which is a consequence of previous failure of liver function. Patients with minimal or clinical HE present different levels of cognitive impairment. Hyperammonemia is considered a main contributor to the neurological alterations in HE. Animal models of chronic HE (e.g. rats with portacaval shunts) or of "pure" hyperammonemia also show impaired cognitive function. The studies summarized here show that the impairment of some types of cognitive function in chronic HE is due to the impaired function of the glutamate-nitric oxide-cGMP pathway in brain. Both hyperammonemia and neuroinflammation contribute to the impairment of the pathway and of cognitive function. Treatment of rats with chronic HE or hyperammonemia with inhibitors of phosphodiesterase 5 restores the function of the glutamate-nitric oxide-cGMP pathway and cGMP levels in brain as well as the ability to learn a Y maze conditional discrimination task. The same beneficial effects may be obtained by treating the rats chronically with an anti-inflammatory, ibuprofen. As the function of this pathway is also altered in brain of patients died in HE, this alteration would also contribute to cognitive impairment in patients with HE. Increasing cGMP by using inhibitors of phosphodiesterase 5 (PDE-5) or anti-inflammatories (under safe conditions) would be therefore a new therapeutic approach to improve learning and memory performance in individuals with minimal or clinical HE.

Spatial memory alterations in three models of hepatic encephalopathy

A behavioural evaluation was carried out on three chronic models of hepatic encephalopathy: two models of type B HE, portacaval shunt (PCS) and portal hypertension (PH) and one of type C HE with cirrhosis and portal hypertension from thioacetamide intoxication (TAA). The tasks selected cover a wide range of behaviours related to: locomotion (rotarod-accelerod test), anxiety (open field and elevated plus maze) and memory (Morris water maze). The results indicate that neither locomotor activity nor anxiety was affected in our models, in comparison with their respective controls. However, this is not the case for the mnesic tasks. Hence, the PCS and TAA groups displayed a severe alteration in spatial reference memory and cannot correctly perform the Morris maze task, while this alteration is less severe in the PH group. On the contrary, the PH group revealed a deficit in spatial working memory, like the TAA group, but this does not occur in subjects with PCS. These results reveal a double dissociation in spatial reference memory and spatial working memory between the PCS and PH groups, which would be of great interest to study about cerebral causes and substrates of the alterations accompanying HE.

Contribution of altered signal transduction associated to glutamate receptors in brain to the neurological alterations of hepatic encephalopathy

Patients with liver disease may present hepatic enceph-alopathy (HE), a complex neuropsychiatric syndrome covering a wide range of neurological alterations, including cognitive and motor disturbances. HE reduces the quality of life of the patients and is associated with poor prognosis. In the worse cases HE may lead to coma or death.

The mechanisms leading to HE which are not well known are being studied using animal models. The neurological alterations in HE are a consequence of impaired cerebral function mainly due to alterations in neurotransmission. We review here some studies indicating that alterations in neurotransmission associated to different types of glutamate receptors are responsible for some of the cognitive and motor alterations present in HE.

These studies show that the function of the signal transduction pathway glutamate-nitric oxide-cGMP associated to the NMDA type of glutamate receptors is impaired in brain in vivo in HE animal models as well as in brain of patients died of HE. Activation of NMDA receptors in brain activates this pathway and increases cGMP. In animal models of HE this increase in cGMP induced by activation of NMDA receptors is reduced, which is responsible for the impairment in learning ability in these animal models. Increasing cGMP by pharmacological means restores learning ability in rats with HE and may be a new therapeutic approach to improve cognitive function in patients with HE. However, it is necessary to previously assess the possible secondary effects.

Patients with HE may present psychomotor slowing, hypokinesia and bradykinesia. Animal models of HE also show hypolocomotion. It has been shown in rats with HE that hypolocomotion is due to excessive activation of metabotropic glutamate receptors (mGluRs) in substantia nigra pars reticulata. Blocking mGluR1 in this brain area normalizes motor activity in the rats, suggesting that a similar treatment for patients with HE could be useful to treat psychomotor slowing and hypokinesia. However, the possible secondary effects of mGluR1 antagonists should be previously evaluated.

These studies are setting the basis for designing therapeutic procedures to specifically treat the individual neurological alterations in patients with HE.