Selective loss of N-methyl-D-aspartate-sensitive L-[3H]glutamate binding sites in rat brain following portacaval anastomosis

Effects of CA1 glutamatergic systems upon memory impairments in cholestatic rats

BACKGROUND

Bile duct ligation (BDL) is shown to induce cholestasis-related liver function impairments as well as consequent cognitive dysfunctions (i.e. impaired learning and memory formation). Glutamatergic neurotransmission plays an important role in hippocampal modulation of learning and memory function. The present study aimed to investigate the possible involvement of dorsal hippocampal (CA1) glutamatergic systems upon cholestasis-induced amnesia.

METHOD

Cholestasis was induced in male Wistar rats through double-ligation of the main bile duct (at two points) and transection of the interposed segment. Step-through passive avoidance test was employed to examine rats' learning and memory function. All drugs were injected into CA1 region of the hippocampus.

RESULTS

our results indicated a decrease in memory retrieval following cholestasis (11, 17 and 24 days post BDL). Only subthreshold doses of N-methyl-d-aspartate (NMDA; 0.125 and 0.25 μg/μl) but not its effective dose (0.5 μg/μl), restored the cholestasis-induced amnesia in step-through passive avoidance test, 11, 17 and 24 days post BDL. This effect was blocked by the subthreshold dose of D-[1]-2-amino-7-phosphonoheptanoic acid (D-AP7, NMDA receptor antagonist; 0.0625 μg/μl, intra-CA1) at 0.125 μg/μl and 0.25 μg/μl doses of NMDA. Moreover, our data revealed that only effective doses of D-AP7 (0.125 and 0.25 μg/μl, intra-CA1) potentiate memory impairments in 11 days after BDL. It was noted that none of applied drugs/doses exerted an effect on memory acquisition and locomotors activity, 10 and 12 days post laparotomy, respectively.

CONCLUSION

Our findings suggest the potential involvement of CA1 glutamatergic system(s) in cholestasis-induced memory deficits.

Ammonia upregulates kynurenine aminotransferase II mRNA expression in rat brain: a role for astrocytic NMDA receptors?

Kynurenine aminotransferase II (KAT-II) is the astrocytic enzyme catalyzing the synthesis of kynurenic acid (KYNA), an endogenous inhibitor of the α7-nicotinic receptor and the NMDA receptor (NMDAr). A previous study demonstrated an increase of KYNA synthesis in the brain of rats with thioacetamide (TAA)-induced acute liver failure. Here we show that TAA administration increases KAT-II expression in the rat cerebral cortex and the effect is mimicked in cerebral cortical astrocytes in culture treated with high (5 mM) concentration of ammonia. KAT-II expression in control and TAA-treated rats was increased by NMDAr antagonist memantine, and the effects of TAA and memantine appeared additive. In astrocytes, the NMDAr antagonist MK-801 raised KAT-II expression as well, while NMDA added alone had no effect. Glutamate decreased KAT-II mRNA level, which was attenuated by MK-801. The results suggest that stimulation of KAT-II expression during hepatic encephalopathy may be associated with a partial inactivation of astrocytic NMDAr by ammonia.

Synaptic plasticity in hepatic encephalopathy - a molecular perspective

Hepatic encephalopathy (HE)(1) is a common neuropsychiatric complication of both acute and chronic liver disease. Clinical symptoms may include motor disturbances and cognitive dysfunction. Available animal models of HE mimic the deficits in cognitive performance including the impaired ability to learn and memorize information. This review explores the question how HE might affect cognitive functions at molecular levels. Both acute and chronic models of HE constrain the plasticity of glutamatergic neurotransmission. Thus, long-lasting activity-dependent changes in synaptic efficiency, known as long-term potentiation (LTP) and long-term depression (LTD) are significantly impeded. We discuss molecules and signal transduction pathways of LTP and LTD that are targeted by experimental HE, with a focus on ionotropic glutamate receptors of the AMPA-subtype. Finally, a novel strategy of functional proteomic analysis is presented, which, if applied differentially, may provide molecular insight into disease-related dysfunction of membrane protein complexes, i.e. disturbed ionotropic glutamate receptor signaling in HE.

Neurotransmitter receptor alterations in hepatic encephalopathy: a review

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

[Expression of NMDA receptor subunits in rat prefrontal cortex with CCL4-induced hepatic damage after a treatment with Rosmarinus officinalis L]

INTRODUCTION

In cirrhosis some toxic substances accumulate in brain and modify the expression of several neuronal receptors. Thus, the use of medicinal plants such as Rosmarinus officinalis L. has been proposed in several pathologies due to its hepatoprotective, antioxidant and neuroprotective activity. In this study we evaluated the expression of the subunits NR1, NR2A and NR2B of the glutamate receptor in rat prefrontal cortex in a model of hepatic damage induced with carbon tetrachloride after a treatment with Rosmarinus officinalis L.

METHODS

We used a total of 24 male Wistar rats weighing 80-90 g. body weight. We formed three study groups: control group (C) without a treatment, carbon tetrachloride group (CC14), and CC14 group plus Rosmarinus officinalis L (CCl4+ROM; 1.5 g/kg of extract orally).

RESULTS

The expression of the NR1, NR2A and NR2B subunits in cirrhotic animals increased compared to the control group, however treatment with Rosmarinus officinalis L. was able to reduce this expression to normal levels compared with CC14 and CCl4+ROM groups. These results could be due to an improvement in hepatic function.

CONCLUSION

Treatment with extract of Rosmarinus officinalis L. in cirrhotic animals modifies the expression of subunits of the NMDA receptor due to an improvement in hepatocellular function in the presence of antioxidant compounds and flavonoids.

Hyperammonemia increases the expression and activity of the glutamine/arginine transporter y+ LAT2 in rat cerebral cortex: implications for the nitric oxide/cGMP pathway

The pathogenesis of hepatic encephalopathy (HE) is associated with hyperammonemia (HA) and subsequent exposure of the brain to excess of ammonia. Alterations of the NO/cGMP pathway and increased glutamine (Gln) content are collectively responsible for many HE symptoms, but how the two events influence each other is not clear. Previously we had shown that Gln administered intracerebrally inhibited the NO/cGMP pathway in control rats and even more so in rats with HA, and we speculated that this effect is due to inhibition by Gln of arginine (Arg) transport (Hilgier et al., 2009). In this study we demonstrate that a 3-day HA in the ammonium acetate model increases the expression in the brain of y(+)LAT2, the heteromeric transporter which preferentially stimulates Arg efflux from the cells in exchange for Gln. The expression of the basic amino acid transporter CAT1, transporting Arg but not Gln remained unaffected by HA. Multiple parameters of Arg or Gln uptake and/or efflux and their mutual dependence were altered in the cerebral cortical slices obtained from HA rats, in a manner indicating enhanced y(+)LAT2-mediated transport. HA elevated Gln content and decreased cGMP content as measured both in the cerebral cortical tissue and microdialysates. Intracortical administration of 6-diazo-5-oxo-L-norleucine (DON), which inhibits Gln fluxes between different cells of the CNS, attenuated the HA-induced decrease of cGMP in the microdialysates of HA rats, but not of control rats. The results suggest that, reduced delivery of Arg due to enhanced y(+)LAT2-mediated exchange of extracellular Gln for intracellular Arg may contribute to the decrease of NO/cGMP pathway activity evoked in the brain by HA.

Metal toxicity, liver disease and neurodegeneration

Hepatocerebral disorders are serious neuropsychiatric conditions that result from liver failure. These disorders are characterized neuropathologically by varying degrees of neuronal cell death in basal ganglia, cerebellum, and spinal cord, and include clinical entities such as Wilson's Disease, post-shunt myelopathy, hepatic encephalopathy, and acquired non-Wilsonian hepatocerebral degeneration. Morphologic changes to astrocytes (Alzheimer type II astrocytosis) are a major feature of hepatocerebral disorders. Neurological symptoms include Parkinsonism, cognitive dysfunction, and ataxia. Pathophysiologic mechanisms responsible for cerebral dysfunction and neuronal cell death in hepatocerebral disorders include ammonia toxicity and neurotoxic effects of metals such as copper, manganese, and iron. Molecular mechanisms of neurotoxicity include oxidative/nitrosative stress, glutamate (NMDA)-receptor-mediated excitotoxicity, and neuroinflammatory mechanisms. However, neuronal cell death in hepatocerebral disorders is limited by adaptive mechanisms that may include NMDA-receptor down-regulation, the synthesis of neuroprotective steroids and hypothermia. Management and treatment of hepatocerebral disorders include chelation therapy (Wilson's Disease), the use of ammonia-lowering agents (lactulose, antibiotics, ornithine aspartate) and liver transplantation.

Induction of NOS and nitrotyrosine expression in the rat striatum following experimental hepatic encephalopathy

Hepatic encephalopathy (HE) is a neurologic disease associated with hepatic dysfunction. Astroglial and neuronal alterations have been described in the basal ganglia in HE. Our study was performed to determine whether such alterations are mediated by nitric oxide (NO), by using an experimental model of HE (portacaval anastomosis [PCA]). The expression of the NO synthases (nNOS and iNOS) and the production of nitrotyrosine (NT) were evaluated in the striatum of rats exposed to PCA for 1 and 6 months. The expression of nNOS in the striatal neurons of PCA rats was increased compared to controls. nNOS expression was also detectable in astrocytes after 6 months of exposure to PCA. Whereas astroglial cells in the normal striatum showed no iNOS expression, iNOS was expressed in the astrocytes of PCA brains, mainly in perivascular processes at 6 months PCA exposure (demonstrated by colocalization with GFAP). The increased expression of both the nNOS and iNOS isoforms in PCA rats might indicate a critical role for NO in the pathomechanism of HE. To study the potential cell damage caused by NO, the deposition of NT in PCA-rats was analysed. Nitrotyrosine was detected in neurons although it was mainly seen in the astrocytes of PCA brains, in which double immunolabelling showed NT to be colocalized with GFAP. Thus, the present study shows the induction of iNOS and NT in astrocytes, which increases with the duration of PCA exposure. This suggests that the induced astroglial production of NO during PCA might be one of the main factors contributing to HE.

The expression of NMDA receptor subunit mRNA in human chronic alcoholics

Ethanol is a modulator at the N-methyl-d-aspartate class of glutamate receptors in the brain. In animal studies the receptor adapts to sustained ethanol exposure through altered expression of the subunits that make up the receptor complex. We used real-time RT-PCR normalized to GAPDH to assay NR1, NR2A, and NR2B subunit mRNA in superior frontal and primary motor cortex tissue obtained at autopsy from chronic alcoholics with and without co-morbid cirrhosis of the liver, and from matched controls. The expression of all three subunits was significantly lower in both areas of cirrhotic alcoholics than in the corresponding areas in both controls and alcoholics without co-morbid disease, who did not differ significantly from each other. The decrease was area-dependent when cases were partitioned by the 5-HTTLPR allele. Thus, polymorphisms in one gene can have a significant effect on the expression of a second, unrelated, gene. The expression of the N-methyl-d-aspartate glutamate receptor complex is under multifactorial control.

Associative learning deficit in two experimental models of hepatic encephalopathy

People with hepatic insufficiency can develop hepatic encephalopathy (HE), a complex neuropsychological syndrome covering a wide range of neurological and cognitive and motor alterations. The cognitive deficits include disturbances in intellectual functions such as memory and learning. In spite of its high prevalence in western societies, the causes of HE have not yet been clearly established. For this reason, experimental models of HE are used to study this condition. In this work, two experimental models were used, one Type B HE (portacaval shunt) and the other Type C HE (cirrhosis by intoxication with thioacetamide), to evaluate its effect on two tasks of associative learning: two-way active avoidance and step-through passive avoidance. The results show an impediment both in acquisition and retention of active avoidance in both models of HE. However, in passive avoidance, only the rats with portacaval shunt presented a memory deficit for the aversive event. In our opinion, these results can be explained by alterations in the neurotransmission system presented by animals with hepatic insufficiency, which are mainly caused by a rise in cerebral histamine and a dysfunction of the glutamatergic system.

Chronic exposure to glufosinate-ammonium induces spatial memory impairments, hippocampal MRI modifications and glutamine synthetase activation in mice

Glufosinate-ammonium (GLA), the active compound of a worldwide-used herbicide, acts by inhibiting the plant glutamine synthetase (GS) leading to a lethal accumulation of ammonia. GS plays a pivotal role in the mammalian brain where it allows neurotransmitter glutamate recycling within astroglia. Clinical studies report that an acute GLA ingestion induces convulsions and memory impairment in humans. Toxicological studies performed at doses used for herbicidal activity showed that GLA is probably harmless at short or medium range periods. However, effects of low doses of GLA on chronically exposed subjects are not known. In our study, C57BL/6J mice were treated during 10 weeks three times a week with 2.5, 5 and 10mg/kg of GLA. Effects of this chronic treatment were assessed at behavioral, structural and metabolic levels by using tests of spatial memory, locomotor activity and anxiety, hippocampal magnetic resonance imaging (MRI) texture analysis, and hippocampal GS activity assay, respectively. Chronic GLA treatments have effects neither on anxiety nor on locomotor activity of mice but at 5 and 10mg/kg induce (1) mild memory impairments, (2) a modification of hippocampal texture and (3) a significant increase in hippocampal GS activity. It is suggested that these modifications may be causally linked one to another. Since glutamate is the main neurotransmitter in hippocampus where it plays a crucial role in spatial memory, hippocampal MRI texture and spatial memory alterations might be the consequences of hippocampal glutamate homeostasis modification revealed by increased GS activity in hippocampus. The present study provides the first data that show cerebral alterations after chronic exposure to GLA.

Neuronal cell death in hepatic encephalopathy

It is generally assumed that neuronal cell death is minimal in liver failure and is insufficient to account for the neuropsychiatric symptoms characteristic of hepatic encephalopathy. However, contrary to this assumption, neuronal cell damage and death are well documented in liver failure patients, taking the form of several distinct clinical entities namely acquired (non-Wilsonian) hepatocerebral degeneration, cirrhosis-related Parkinsonism, post-shunt myelopathy and cerebellar degeneration. In addition, there is evidence to suggest that liver failure contributes to the severity of neuronal loss in Wernicke's encephalopathy. The long-standing nature of the thalamic and cerebellar lesions, over 80% of which are missed by routine clinical evaluation, together with the probability that they are nutritional in origin, underscores the need for careful nutritional management (adequate dietary protein, Vitamin B(1)) in liver failure patients. Mechanisms identified with the potential to cause neuronal cell death in liver failure include NMDA receptor-mediated excitotoxicity, lactic acidosis, oxidative/nitrosative stress and the presence of pro-inflammatory cytokines. The extent of neuronal damage in liver failure may be attenuated by compensatory mechanisms that include down-regulation of NMDA receptors, hypothermia and the presence of neuroprotective steroids such as allopregnanolone. These findings suggest that some of the purported "sequelae" of liver transplantation (gait ataxia, memory loss, confusion) could reflect preexisting neuropathology.

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

BACKGROUND/AIMS

Patients with hepatic encephalopathy show altered motor function, psychomotor slowing and hypokinesia. The underlying mechanisms remain unclear. This work's aims were: (1) to analyse in rats with chronic liver failure due to portacaval shunt (PCS) the neurochemical alterations in the basal ganglia-thalamus-cortex circuits; (2) to correlate these alterations with those in motor function and (3) to normalize motor activity of PCS rats by pharmacological means.

METHODS

Extracellular neurotransmitters levels were analysed by in vivo brain microdialysis. Motor activity was determined by counting crossings in open field.

RESULTS

Extracellular glutamate is increased in substantia nigra pars reticulata (SNr) of PCS rats. Blocking metabotropic receptor 1 (mGluR1) in SNr normalizes motor activity in PCS rats. In ventro-medial thalamus of PCS rats GABA is increased and it is normalized by blocking mGluR1 in SNr. Blocking mGluR1 in SNr increases and mGluR1 activation reduces glutamate in motor cortex and motor activity.

CONCLUSIONS

Increased extracellular glutamate and activation of mGluR1 in SNr are responsible for reduced motor activity in rats with chronic liver failure. Blocking mGluR1 in SNr normalizes motor activity in PCS rats, suggesting that, under appropriate conditions, similar treatments could be useful to treat the psychomotor slowing and hypokinesia in patients with hepatic encephalopathy.

The brain glutamate system in liver failure

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

Limited capacity for ammonia removal by brain in chronic liver failure: potential role of nitric oxide

Chronic liver failure leads to hyperammonemia and consequently increased brain ammonia concentrations, resulting in hepatic encephalopathy. When the liver fails to regulate ammonia concentrations, the brain, devoid of a urea cycle, relies solely on the amidation of glutamate to glutamine through glutamine synthetase, to efficiently clear ammonia. Surprisingly, under hyperammonemic conditions, the brain is not capable of increasing its capacity to remove ammonia, which even decreases in some regions of the brain. This non-induction of glutamine synthetase in astrocytes could result from possible limiting substrates or cofactors for the enzyme, or an indirect effect of ammonia on glutamine synthetase expression. In addition, there is evidence that nitration of the enzyme resulting from exposure to nitric oxide could also be implicated. The present review summarizes these possible factors involved in limiting the increase in capacity of glutamine synthetase in brain, in chronic liver failure.

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.

Kynurenic acid synthesis in cerebral cortical slices of rats with progressing symptoms of thioacetamide-induced hepatic encephalopathy

Increased ammonia is a major pathogenic factor in hepatic encephalopathy (HE), a neurologic syndrome associated with glutamatergic dysfunction. Previous studies have shown that in rat cerebral cortical slices or a glia-derived cell line, acute treatment with ammonia in vitro and in vivo inhibits the production of a broad-spectrum antagonist of excitatory amino acid receptors, kynurenic acid (KYNA). The present study analyzed KYNA synthesis in cerebral cortical slices obtained from rats with progressing HE symptoms accompanying acute liver failure induced by one, two, or three intraperitoneal administrations of thioacetamide (TAA) at 24-hr intervals. KYNA synthesis was found decreased to 83% of control 24 hr after one administration of TAA and unaffected after two TAA injections, when moderate hyperammonemia was associated by metabolic and bioelectric activation of the central nervous system, but was not accompanied by typical HE symptoms. KYNA synthesis was elevated to 155% of control after three TAA administrations, a period in which the rats showed advanced HE symptoms including stupor or coma. KYNA synthesis at the advanced HE stage was inhibited by glutamate in a degree comparable to that observed in control slices. The elevation of KYNA synthesis was associated with increased activity of a kynurenine aminotransferase (KAT) isomer, KAT-II. KYNA synthesis did not differ from control 21 days after the third TAA administration when HE symptoms receded. The results suggest that alterations of KYNA synthesis may contribute to the imbalance between neural excitation and inhibition at different stages of HE.

Value of regional cerebral blood flow in the evaluation of chronic liver disease and subclinical hepatic encephalopathy

AIMS

Regional changes in cerebral blood flow in patients with chronic hepatitis, cirrhosis and subclinical hepatic encephalopathy were investigated in the present study using single photon emission computed tomography (SPECT).

METHODS

Twenty patients with cirrhosis, 11 patients with chronic hepatitis, and nine healthy controls were included in the study. Cerebral SPECT were obtained for all patients. The percentages of cerebral blood flow of 14 regions to the cerebellar blood flow were determined. Only the patients with cirrhosis underwent psychometric evaluation: visual evoked potentials (VEP) measurements and electroencephalogram (EEG) recordings along with blood levels of albumin, bilirubin, and ammonia were measured and prothrombin time was determined in cirrhotic patients. These patients were classified according to the Child-Pugh classification.

RESULTS

Among cirrhotic patients, six had abnormal results in VEP studies, 11 in psychometric tests and with six in EEG evaluation. Any abnormality in psychometric tests and/or VEP studies is taken as the main criterion; subclinical hepatic encephalopathy was detected in 12 of 20 patients. According to SPECT results in patients with subclinical encephalopathy, a statistically significant decrease in cerebral blood flow in right thalamus and nearly significant decrease in left thalamus were observed. Regional blood flow was significantly higher in the frontal lobes of patients with cirrhosis when compared with healthy controls. Similarly, cerebral blood flow in frontal and cingulate regions was significantly higher in patients with chronic hepatitis than in healthy controls. There was no relationship between cerebral blood flow and blood levels of ammonia or Child-Pugh score, in cirrhotic patients.

CONCLUSION

Significant changes in cerebral blood flow may be present in chronic liver diseases and the authors suggest that the measurement of changes in cerebral blood flow might be useful in detecting subclinical hepatic encephalopathy.

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

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

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

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

Postoperative pain relief after hepatic resection in cirrhotic patients: the efficacy of a single small dose of ketamine plus morphine epidurally

In cirrhotic patients undergoing hepatic surgery, postoperative analgesia remains a challenge. In this study, we evaluated the efficacy of a single dose of morphine combined with small-dose ketamine given epidurally for postoperative pain relief. One-hundred-four classification "Child A" cirrhotic patients were randomly assigned to two groups: 1) (MKG, n = 54): epidural morphine (3.5-5 mg) plus ketamine (20/30 mg); and 2) epidural morphine (3.5/5 mg) (MG, n = 50). The level of analgesia, side effects, psychomimetic and neurological disorders, additional analgesic needs, and overall quality of the analgesia were recorded. The mean duration of analgesia was longer in the MKG group (27.2 +/- 8 h versus 16.4 +/- 10 h; P < 0.05). In the MKG group, the visual analog scale (VAS) score began to be significantly lower from 14 h at rest and 12 h on coughing until the end of the study. The need for additional analgesia was also smaller in the MKG group (P < 0.05): at 24 h, only 10% of patients in the MKG group needed complementary analgesia, whereas in the MG group it was 100% (P = 0.003). Side effects were similar in both groups. Psychomimetic side effects and neurological disorders were not detected. These results suggest that postoperative analgesia provided by a single dose of epidural morphine with small-dose ketamine is effective in cirrhotic Child's A patients having major upper abdominal surgery.

Effects of hyperammonemia and liver failure on glutamatergic neurotransmission

Glutamate is the main excitatory neurotransmitter in mammals. Glutamatergic neurotransmission involves several steps, beginning with release of glutamate from the presynaptic neuron. Glutamate in the extracellular space activates glutamate receptors present in the synaptic membranes, leading to activation of signal transduction pathways associated with these receptors. To avoid continuous activation of glutamate receptors, glutamate is removed from the synaptic cleft by specific glutamate transporters located mainly on astrocytes. All these steps are tightly modulated under physiological conditions, and alterations of any of the above steps may result in impairment of glutamatergic neurotransmission, leading to neurological alterations. There are studies in the literature reporting alterations in all these steps in hyperammonemia and/or hepatic failure. Glutamatergic neurotransmission modulates important cerebral processes. Some of these processes are altered in patients with liver disease and hepatic encephalopathy, who show altered sleep-wake patterns, neuromuscular coordination, and decreased intellectual capacity. The alterations in glutamatergic neurotransmission may be responsible for some of these neurological alterations found in hepatic encephalopathy. The effects of hyperammonemia and liver failure on different steps of glutamatergic neurotransmission including alterations of glutamate concentration in the extracellular fluid in brain, transport and transporters of glutamate, the content and function of different types of glutamate receptors and signal transduction pathways. Alterations induced by hyperammonemia and liver failure on the glutamate-nitric oxide-cGMP pathway in brain may result in changes in long-term potetiation and learning ability.

Region selective alterations of soluble guanylate cyclase content and modulation in brain of cirrhotic patients

Modulation of soluble guanylate cyclase (sGC) by nitric oxide (NO) is altered in brain from experimental animals with hyperammonemia with or without liver failure. The aim of this work was to assess the content and modulation of sGC in brain in chronic liver failure in humans. Expression of the alpha-1, alpha-2, and beta-1 subunits of sGC was measured by immunoblotting in autopsied frontal cortex and cerebellum from cirrhotic patients and controls. The contents of alpha-1 and alpha-2 subunits of guanylate cyclase was increased both in cortex and cerebellum, whereas the beta-1 subunit was not affected. Addition of the NO-generating agent S-nitroso-N-acetyl-penicillamine (SNAP) to homogenates of frontal cortex from controls increased the activity of sGC 87-fold, whereas, in homogenates from cirrhotic patients, the increase was significantly higher (183-fold). In contrast, in cerebellum, activation of guanylate cyclase by NO was significantly lower in patients (156-fold) than in controls (248-fold). A similar regional difference was found in rats with portacaval anastomosis. In conclusion, these findings show that the NO-guanylate cyclase signal transduction pathway is strongly altered in brain in patients with chronic liver failure and that the effects are different in different brain areas. Given that activation of sGC by NO in brain is involved in the modulation of important cerebral processes such as intercellular communication, learning and memory, and the sleep-wake cycle, these changes could be implicated in the pathogenesis of hepatic encephalopathy in these patients.

Effects of ammonia and hepatic failure on the net efflux of endogenous glutamate, aspartate and taurine from rat cerebrocortical slices: modulation by elevated K+ concentrations

Cerebrocortical minislices derived from control rats ("control slices") and from rats with thioacetamide (TAA)-induced hepatic failure showing moderate hyperammonemia and symptoms of hepatic encephalopathy (HE) ("HE slices"), were incubated with physiological saline in the absence or presence of 5 mM ammonium acetate ("ammonia"), at potassium ion (K+) concentrations ranging from 5 to 15 mM. The efflux of endogenous aspartate (Asp), glutamate (Glu) and taurine (Tau) to the incubation medium was assayed by HPLC. At 5 mM K+, perfusion of control slices with ammonia did not affect Glu and slightly depressed Asp efflux. Raising K+ concentrations in the incubation medium to 7.5 led to inhibition of Glu and Asp efflux by ammonia and the inhibitory effect was further potentiated at 10 mM K+. The inhibition was also significant at 15 mM K+. This suggests that, depression of excitatory neurotransmission associated with acute hyperammonemia is more pronounced under conditions of intense neuronal activity than in the resting state. HE moderately increased the efflux of Glu and Asp, and the stimulatory effect of HE on Glu and Asp efflux showed virtually no variation upon changing K+ concentration up to 15 mM. Ammonia strongly, and HE moderately, increased Tau efflux at 5 mM K+. However, both the ammonia- and HE-dependent Tau efflux decreased with increasing K+ concentration in the medium and was no longer significant at 10 mM concentration, indicating that intense neuronal activity obliterates the neuroprotective functions of this amino acid triggered by hyperammonemia.

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.

The "peripheral-type" benzodiazepine (omega 3) receptor in hyperammonemic disorders

Increased levels of brain ammonia occur in both congenital and acquired hyperammonemic syndromes including hepatic encephalopathy, fulminant hepatic failure, Reye's syndrome and congenital urea cycle disorders. In addition to its effect on neurotransmission and energy metabolism, ammonia modulates the expression of various genes including the astrocytic "peripheral-type" benzodiazepine (or omega 3) receptor (PTBR). Increased expression of the isoquinoline carboxamide binding protein (IBP), one of the components of the PTBR complex, is observed in brain and peripheral tissues following chronic liver failure as well as in cultured astrocytes exposed to ammonia. Increased densities of binding sites for the PTBR ligand [3H]-PK11195 are also observed in these conditions as well as in brains of animals with acute liver failure, congenital urea cycle disorders and in patients who died in hepatic coma. The precise role of PTBR in brain function has not yet fully elucidated, but among other functions, PTBR mediates the transport of cholesterol across the mitochondrial membrane and thus plays a key role in the biosynthesis of neurosteroids some of which modulate major neurotransmitter systems such as the gamma-aminobutyric acid (GABA(A)) and glutamate (N-methyl-D-aspartate (NMDA)) receptors. Activation of PTBR in chronic and acute hyperammonemia results in increased synthesis of neurosteroids which could lead to an imbalance between excitatory and inhibitory neurotransmission in the CNS. Preliminary reports suggest that positron emission tomography (PET) studies using [11C]-PK11195 may be useful for the assessment of the neurological consequences of chronic liver failure.

Neurobiology of ammonia

Hyperammonemia resulting from inherited urea cycle enzyme deficiencies or liver failure results in severe central nervous system dysfunction including brain edema, convulsions and coma. Neuropathologic evaluation in these disorders reveals characteristic alterations of astrocyte morphology ranging from cell swelling (acute hyperammonemia) to Alzheimer Type II astrocytosis (chronic hyperammonemia). Having no effective urea cycle, brain relies on glutamine synthesis for the removal of excess ammonia and the enzyme responsible, glutamine synthetase, has a predominantly astrocytic localization. Accumulation of ammonia in brain results in a redistribution of cerebral blood flow and metabolism from cortical to sub-cortical structures. In addition to changes in astrocyte morphology, increased brain ammonia concentrations result in alterations in expression of key astrocyte proteins including glial fibrillary acidic protein, glutamate and glycine transporters and "peripheral-type" (mitochondrial) benzodiazepine receptors. Such changes result in alterations of astrocytic volume and increased extracellular concentrations of excitatory and inhibitory substances. In addition, the ammonium ion has direct effects on excitatory-inhibitory transmission via distinct mechanisms involving cellular chloride extrusion and postsynaptic receptor function. Acute ammonia exposure leads to activation of NMDA receptors and their signal transduction pathways. Chronic hyperammonemia also results in increased concentrations of neuroactive L-tryptophan metabolites including serotonin and quinolinic acid. Therapy in hyperammonemic syndromes continues to rely on ammonia-lowering strategies via peripheral mechanisms (reduction of ammonia production in the gastrointestinal tract, increased ammonia removal by muscle).

Mechanisms of hyperammonemia

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

GeneChip analysis shows altered mRNA expression of transcripts of neurotransmitter and signal transduction pathways in the cerebral cortex of portacaval shunted rats

Identifying the gene expression changes induced by hepatic encephalopathy (HE) leads to a better understanding of the molecular mechanisms of HE-induced neurological dysfunction. Using GeneChip and real-time PCR, the present study evaluated the gene expression profile of rat cerebral cortex at 4 weeks after portacaval shunting. Among 1,263 transcripts represented on the chip, mRNA levels of 31 transcripts were altered (greater than twofold; 16 increased and 15 decreased) in the portacaval shunted (PCS) rat compared to sham control. Changes observed by GeneChip analysis were confirmed for 20 transcripts (8 increased, 7 decreased, and 5 unchanged in PCS rat brain) by real-time PCR. Neurotransmitter receptors, transporters, and members of the second messenger signal transduction are the major groups of genes altered in PCS rat brain. Of importance was that the increased heme oxygenase-1 and decreased Cu,Zn-superoxide dismutase expression observed raise the possibility of oxidative stress playing a pathogenic role in chronic HE.

Glutamate transporter and receptor function in disorders of ammonia metabolism

Disorders of ammonia metabolism including urea cycle enzymopathies, Reye Syndrome, and liver failure are associated with brain edema and severe neurological impairment. Excess blood-borne ammonia crosses the blood-brain barrier by diffusion as NH(3) where it interacts with various cellular processes involved in neurotransmission and brain energy metabolism. Ammonia exerts a potent effect on glutamate (AMPA) receptor-mediated neurotransmission. Ammonia also inhibits high affinity transport of glutamate by an action on astrocytic glutamate transporter expression, an action which results in increased extracellular concentrations of glutamate. Acute hyperammonemia directly activates the NMDA subclass of glutamate receptors resulting in increased intracellular Ca(2+) and increased synthesis of nitric oxide and cGMP. Chronic hyperammonemia, on the other hand, results in a loss of NMDA receptor sites. Activation of NMDA receptors in acute ammonia toxicity results in depletion of ATP in brain. Neuropathologic studies in experimental animals with congenital urea cycle disorders and severe hyperammonemia reveal evidence of neuronal cell death which is excitotoxic in nature. These findings suggest that overactivation of NMDA receptors is a significant feature of acute hyperammonemic syndromes and that antagonists of these receptors or of their signal transduction pathway enzymes such as nNOS could be beneficial in the treatment of the central nervous system manifestations (encephalopathy, brain edema) which are characteristic of hyperammonemic disorders.

Altered content and modulation of soluble guanylate cyclase in the cerebellum of rats with portacaval anastomosis

It is shown that the glutamate-NO-cGMP pathway is impaired in cerebellum of rats with portacaval anastomosis in vivo as assessed by in vivo brain microdialysis in freely moving rats. NMDA-induced increase in extracellular cGMP in the cerebellum was significantly reduced (by 27%) in rats with portacaval anastomosis. Activation of soluble guanylate cyclase by the NO-generating agent S-nitroso-N-acetyl-penicillamine and by the NO-independent activator YC-1 was also significantly reduced (by 35-40%), indicating that portacaval anastomosis leads to remarkable alterations in the modulation of guanylate cyclase in cerebellum. Moreover, the content of soluble guanylate cyclase was increased ca. two-fold in the cerebellum of rats with portacaval anastomosis. Activation of soluble guanylate cyclase by NO was higher in lymphocytes isolated from rats with portacaval anastomosis (3.3-fold) than in lymphocytes from control rats (2.1-fold). The results reported show that the content and modulation of soluble guanylate cyclase are altered in brain of rats with hepatic failure, resulting in altered function of the glutamate-NO-cGMP pathway in the rat in vivo. This may lead to alterations in cerebral processes such as intercellular communication, circadian rhythms, including the sleep-waking cycle, long-term potentiation, and some forms of learning and memory.

Brain tryptophan/serotonin perturbations in metabolic encephalopathy and the hazards involved in the use of psychoactive drugs

Several combined pathogenetic factors such as hyperammonemia, different brain tryptophan metabolic disturbances and serotonin physiological/pharmacological alterations not yet defined in all details, will often give rise to the clinical neuropsychiatric condition known as hepatic encephalopathy (HE). Indeed, to this the probable exposure to novel potent CNS-monoamine acting drugs today may put such patients at certain risk for other pharmacodynamic (PD) responses than usually are expected from these "safe" drugs. Moreover, with a compromised liver function in HE, also pharmacokinetic (PK) features for the drugs are likely changed in these patients. Thus, the ultimate clinical outcome by this probable but unknown PD/PK-deviation for such psychoactive drugs when given to HE-patients needs further clarification. Accordingly, delineation of both PD- and PK-effects in experimental HE should shed light on this issue of relevance for monoamine-active drug safety as well as on some further details in the complex tryptophan/monoamine-related pathophysiology that comes into play in HE.

Activation of N-methyl-D-aspartate receptors in rat brain in vivo following acute ammonia intoxication: characterization by in vivo brain microdialysis

Ammonia is considered the main agent responsible for the neurological alterations in hepatic encephalopathy. It was suggested that ammonia toxicity is mediated by activation of N-methyl-D-aspartate (NMDA) receptors. The aim of this work was to assess, by in vivo brain microdialysis in freely moving rats, whether acute ammonia intoxication leads to activation of NMDA receptors in the cerebellum of the rat in vivo. We measured the effects of ammonia intoxication on the neuronal glutamate-nitric oxide-cyclic guanosine monophosphate (cGMP) pathway, by measuring the ammonia-induced increase of extracellular cGMP. Ammonia intoxication increases extracellular cGMP, and this increase is prevented by (5R,10S)-5-methyl-10,11-dihydro-5H-dibenzo[a, d]cyclohepten-5,10-imine hydrogen maleate (MK-801). There is a good correlation between the increase in cGMP and the seriousness of the neurological symptoms elicited by different doses of ammonia. Ammonia doses inducing coma did not affect extracellular glutamate, while doses leading to death increased it by 349%. The time courses of ammonia-induced increases in extracellular ammonia, cGMP, and glutamate indicate that NMDA receptor activation occurs before the increase in extracellular glutamate. Ammonia-induced increase in glutamate is prevented by MK-801. These results indicate that ammonia intoxication leads to activation of NMDA receptors in the animal in vivo, and that this activation is not caused by increased extracellular glutamate. The possible underlying mechanism is discussed.

Regional cerebral blood flow during mechanical hyperventilation in patients with fulminant hepatic failure

Hyperventilation is frequently used to prevent or postpone the development of cerebral edema and intracranial hypertension in patients with fulminant hepatic failure (FHF). The influence of such therapy on regional cerebral blood flow (rCBF) remains, however, unknown. In this study the CBF-distribution pattern was determined within the first 12 hours after development of hepatic encephalopathy (HE) stage 4 before and during hyperventilation. Ten consecutive patients (median age 48 [range 33-57] years) with FHF and 9 healthy controls (median age 54 [24-58] years) had rCBF determined by single photon emission computed tomography (SPECT) using intravenous injection of 133Xenon. For determination of high resolution CBF pattern, the patients were also studied with 99mTc-hexa-methylpropyleneamine oxime (HMPAO) in the hyperventilation condition. There was no significant difference in the rCBF distribution pattern during normoventilation as compared with hyperventilation. The anterior to posterior (AP) ratio was significantly lower in patients as compared with healthy controls. After hepatic recovery and disappearance of HE, 3 patients had restored normal rCBF distribution pattern as compared with healthy controls. We conclude that in sedated patients with FHF, a relatively lower rCBF is found in the frontal regions and in the basal ganglia as compared with posterior regions. This rCBF-distribution pattern was not aggravated during hyperventilation. It is speculated that this change of rCBF in patients with FHF may render the frontal brain regions more susceptible to hypoxia. The relative frontal rCBF decrease was shown to be reversible with hepatic recovery and alleviation of HE.

Hepatic encephalopathy: molecular mechanisms underlying the clinical syndrome

Hepatic encephalopathy (HE) and portal-systemic encephalopathy (PSE) are the terms used interchangeably to describe a complex neuropsychiatric syndrome associated with acute or chronic hepatocellular failure, increased portal systemic shunting of blood, or both. Hepatic encephalopathy complicating acute liver failure is referred to as fulminant hepatic failure (FHF). The clinical manifestations of HE or PSE range from minimal changes in personality and motor activity, to overt deterioration of intellectual function, decreased consciousness and coma, and appear to reflect primarily a variable imbalance between excitatory and inhibitory neurotransmission. Pathogenic mechanisms that may be responsible for HE have been extensively investigated using animal models of HE, or cultures of CNS cells treated with neuroactive substances that have been implicated in HE. Of the many compounds that accumulate in the circulation as a consequence of impaired liver function, ammonia is considered to play an important role in the onset of HE. Acute ammonia neurotoxicity, which may be a cause of seizures in FHF, is excitotoxic in nature, being associated with increased synaptic release of glutamate (Glu), the major excitatory neurotransmitter of the brain, and subsequent overactivation of the ionotropic Glu receptors, mainly the N-methyl-D-aspartate (NMDA) receptors. Hepatic encephalopathy complicating chronic liver failure appears to be associated with a shift in the balance between inhibitory and excitatory neurotransmission towards a net increase of inhibitory neurotransmission, as a consequence of at least two factors. The first is down-regulation of Glu receptors resulting in decreased glutamatergic tone. The down-regulation follows excessive extrasynaptic accumulation of Glu resulting from its impaired re-uptake into nerve endings and astrocytes. Liver failure inactivates the Glu transporter GLT-1 in astrocytes. The second factor is an increase in inhibitory neurotransmission by gamma-aminobutyric acid (GABA) due to (a) increased brain levels of natural benzodiazepines; (b) increased availability of GABA at GABA-A receptors, due to enhanced synaptic release of the amino acid; (c) direct interaction of modestly increased levels of ammonia with the GABA-A-benzodiazepine receptor complex; and (d) ammonia-induced up-regulation of astrocytic peripheral benzodiazepine receptors (PBZR). Brain ammonia is metabolised in astrocytes to glutamine (Gln), an osmolyte, and increased Gln accumulation in these cells may contribute to cytotoxic brain edema, which often complicates FHF. Glutamine efflux from the brain is an event that facilitates plasma-to-brain transport of aromatic amino acids. Tryptophan and tyrosine are direct precursors of the aminergic inhibitory neurotransmitters, serotonin and dopamine, respectively. Changes in serotonin and dopamine and their receptors may contribute to some of the motor manifestations of HE. Finally, oxindole, a recently discovered tryptophan metabolite with strong sedative and hypotensive properties, has been shown to accumulate in cirrhotic patients and animal models of HE.

N-methyl-D-aspartate-evoked changes in the striatal extracellular levels of dopamine and its metabolites in vivo in rats with acute hepatic encephalopathy

Acute hepatic encephalopathy (HE) is associated with disturbances in motor functions, but the underlying mechanisms remain obscure. Considerable experimental evidence suggests that motor activity is modulated by striatal dopamine neurons whose discharge is under glutamatergic control, mostly through activation of N-methyl-D-aspartate (NMDA) receptors. In this study we used intrastriatal microdialysis to compare the effects of infusion of 10 mM NMDA or 50 mM KCl as a general release stimulus, on the extracellular levels of endogenous dopamine (DA) and its metabolites dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) in control rats and in rats with acute HE induced by repeated administration of thioacetamide. The basal levels of DA and DOPAC were not significantly altered by HE, while the HVA level was reduced. HE did not significantly affect the NMDA- or KCl-evoked increase in extracellular DA. Infusion of NMDA or KCl led to a decrease in extracellular DOPAC, and HE did not modulate these effects. However, HE attenuated the NMDA- but not the KCl-induced reduction in extracellular HVA. The results point to the impairment of modulation of striatal DA discharge and metabolism by glutamate acting at NMDA receptors, contributing to the motor disturbances in HE.

Nutritional support in hepatic encephalopathy

Hepatic encephalopathy (HE) is a syndrome of global cerebral dysfunction resulting from underlying liver disease or portal-systemic shunting. HE can present as one of four syndromes, depending on the rapidity of onset of hepatic failure and the presence or absence of preexisting liver disease. The precise pathogenesis is unknown but likely involves impaired hepatic detoxification of ammonia as well as alterations in brain transport and metabolism of amino acids and amines. The etiology of malnutrition in hepatic failure is multifactorial. Nutritional deficits may be clinically manifest as marasmus or kwashiorkor, or both. Nutritional support in HE is directed toward reducing morbidity related to underlying malnutrition and concurrent disease. However, reaching nutritional goals is often complicated by protein and carbohydrate intolerance. The use of protein restriction in HE is controversial. Modified formulas that are supplemented in branched chain amino acids may be of value in patients who exhibit protein intolerance with standard feeding solutions or in patients who present with advanced degrees of encephalopathy.

Reduction in the MK-801 binding sites of the NMDA sub-type of glutamate receptor in a mouse model of congenital hyperammonemia: prevention by acetyl-L-carnitine

Our earlier studies on the pharmacotherapeutic effects of acetyl-L-carnitine (ALCAR), in sparse-fur (spf) mutant mice with X linked ornithine transcarbamylase deficiency, have shown a restoration of cerebral ATP, depleted by congenital hyperammonemia and hyperglutaminemia. The reduced cortical glutamate and increased quinolinate may cause a down-regulation of the N-methyl-D-aspartate (NMDA) receptors, observed by us in adult spf mice. We have now studied the kinetics of [3H]-MK-801 binding to NMDA receptors in spf mice of different ages to see the effect of chronic hyperammonemia on the glutamate neurotransmission. We have also studied the Ca2+-dependent and independent (4-aminopyridine (AP) and veratridine-mediated) release of glutamate and the uptake of [3H]-glutamate in synaptosomes isolated from mutant spf mice and normal CD-1 controls. All these studies were done with and without ALCAR treatment (4 mmol/kg wt i.p. daily for 2 weeks), to see if its effect on ATP repletion could correct the glutamate neurotransmitter abnormalities. Our results indicate a normal MK-801 binding in 12-day-old spf mice but a significant reduction immediately after weaning (21 day), continuing into the adult stage. The Ca2+-independent release of endogenous glutamate from synaptosomes was significantly elevated at 35 days, while the uptake of glutamate into synaptosomes was significantly reduced in spf mice. ALCAR treatment significantly enhanced the MK-801 binding, neutralized the increased glutamate release and restored the glutamate uptake into synaptosomes of spf mice. These studies point out that: (a) the developmental abnormalities of the NMDA sub-type of glutamate receptor in spf mice could be due to the effect of sustained hyperammonemia, causing a persistent release of excess glutamate and inhibition of the ATP-dependent glutamate transport, (b) the modulatory effects of ALCAR on the NMDA binding sites could be through a repletion of ATP, required by the transporters to efficiently remove extracellular glutamate.

Alterations of neurotransmitter-related gene expression in human and experimental portal-systemic encephalopathy

Portal-systemic encephalopathy (PSE) is a serious neuropsychiatric condition that results from chronic liver failure and portal-systemic shunting of venous blood. PSE is particularly prevalent following treatment of portal hypertension or ascites by the TIPS procedure. Recent studies both in autopsied brain tissue from PSE patients as well as in experimental animal models of PSE reveal that chronic liver failure results in altered expression of several genes coding for proteins having key roles in the control of neuronal excitability. Such alterations include increased expression of monoamine oxidase (MAO-A isoform), the "peripheral-type" benzodiazepine receptor (PTBR) as well as constitutive, neuronal nitric oxide synthase (nNOS). Such changes result in altered protein expression and in increased degradation of monoamine neurotransmitters, increased synthesis of neurosteroids with inhibitory properties and increased production of nitric oxide (respectively) in brain in chronic liver failure. In the case of PTBR and nNOS, increases in expression result from exposure to ammonia and/or manganese, two neurotoxic agents shown previously to be increased in brain in chronic liver failure. Further elucidation of the consequences of neurotransmitter-related gene expression could identify new pathophysiologic mechanisms and result in new approaches to the prevention of PSE in chronic liver disease in humans.

Differential inhibition by hyperammonemia of the electron transport chain enzymes in synaptosomes and non-synaptic mitochondria in ornithine transcarbamylase-deficient spf-mice: restoration by acetyl-L-carnitine

Sparse-fur (spf) mouse is the ideal animal model to study the neuropathology of congenital ornithine transcarbamylase (OTC) deficiency. Our current hypothesis implies that an ammonia-induced depletion of energy metabolism in the spf mouse, could be due to a reduction in the activities of the enzymes of the electron transport chain and a treatment with acetyl-L-carnitine could normalize this abnormality. We also hypothesized that there might be a differential degree of inhibition in synaptosomal and non-synaptic mitochondria, for the enzymes of the electron transport chain, caused by congenital hyperammonemia. We have therefore measured the activities of NADH-cytochrome C oxidoreductase, succinate cytochrome C oxidoreductase and cytochrome C oxidase in synaptosomes and non-synaptic mitochondria, isolated from spf mice and CD-1 controls with and without acetyl-L-carnitine treatment. Our results indicate a significant reduction (19-34%) in the activities of these complexes in synaptosomes in untreated spf mice, whereas in non-synaptic mitochondria, there was a tendency for the activities to decrease. Acetyl-L-carnitine treatment enhanced these activities (15-64%) for all the three enzyme complexes and its effect was more prominent on succinate cytochrome C oxidoreductase activity (64%). These studies point out that: (a) ammonia-induced disturbances in the energy metabolism could be more pronounced in neuronal mitochondria, and (b) the effect of acetyl-L-carnitine on the restoration of cerebral ATP in hyperammonemia could be through an enhancement of the activities of various electron transport chain enzymes.

Portacaval anastomosis results in more widespread alterations of cerebral metabolism in old versus young adult rats: implications for post-shunt encephalopathy

Treatment of portal hypertension by portal decompressive surgery or transjugular intrahepatic portosystemic stent shunt (TIPS) results in new or worsening episodes of portal-systemic encephalopathy, particularly in older patients. As part of a series of studies to elucidate the pathophysiologic mechanisms responsible for the age-related increased portal-systemic encephalopathy following shunt surgery, local cerebral glucose utilization, a measure of regional brain functional activity, was assessed using the 14C-2-deoxyglucose autoradiographic technique in 2 month-old (young adult) and 24 month-old (old adult) rats following end-to-side portacaval anastomosis. Cerebral glucose utilization was decreased by 22% (p<0.05) in frontal cortex of 2 month-old rats following portacaval anastomosis. More widespread alterations of glucose utilization, involving frontal and frontoparietal cortices, as well as thalamic structures were observed in the brains of 24 month-old rats following portacaval anastomosis despite blood ammonia concentrations of a comparable magnitude. Decreased cerebral glucose utilization in frontal and frontoparietal cortex of old adult rats following portacaval anastomosis probably results from decreased cerebral energy requirements as a consequence of neurotransmitter-related dysfunction. The greater susceptibility of aging brain to the deleterious effects of portacaval anastomosis is consistent with the higher incidence of encephalopathy in older cirrhotic patients following portacaval anastomosis or TIPS.

Effects of hyperammonaemia on brain function

Neuropsychiatric symptoms of hyperammonaemia include alterations of mood and personality, cognitive impairment, ataxia, convulsions and coma. The nature and severity of CNS dysfunction depend upon the aetiology and degree of hyperammonaemia, its acuteness of onset and the age of the patient. Neuropathological studies reveal Alzheimer type II astrocytosis in the adult hyperammonaemic patient, whereas hyperammonaemia in the infant resulting from congenital urea cycle disorders or Reye syndrome is accompanied by cerebral atrophy, neuronal loss and cerebral oedema. Several electrophysiological and biochemical mechanisms have been proposed to explain the deleterious effects of ammonia on CNS function. Such mechanisms include direct effects of the ammonium ion on excitatory and inhibitory neurotransmission and a deficit in cerebral energy metabolism due to ammonia-induced inhibition of alpha-ketoglutarate dehydrogenase. In addition, ammonia has been shown to interfere with normal processes of uptake, storage and release of various neurotransmitters. Ammonia disrupts monoamine storage, inhibits the high-affinity uptake of glutamate by both astrocytic and neuronal elements and activates 'peripheral-type' benzodiazepine receptors leading to the potential synthesis of neuroactive steroids in brain. On the basis of these actions, it has been proposed that ammonia disrupts neuron-astrocyte trafficking of amino acids and monoamines in brain. The increased formation of brain glutamine in hyperammonaemic syndromes could be responsible for the phenomenon of brain oedema in these disorders. Therapies aimed at either decreasing ammonia production in the gastrointestinal tract or increasing ammonia removal by liver or skeletal muscle are the mainstay in the prevention and treatment of the CNS consequences of hyperammonaemia. New therapeutic approaches aimed at correction of the neurotransmitter and cerebral energy deficits in these syndromes could hold promise for the future.

Selective alterations of cerebrospinal fluid amino acids in dogs with congenital portosystemic shunts

Numerous studies suggest that modifications in concentrations of both excitatory and inhibitory amino acids are implicated in the pathophysiology of portal-systemic encephalopathy (PSE), a neuropsychiatric disorder associated with chronic liver disease in humans. In this study, amino acid levels were measured by High Performance Liquid Chromatography (HPLC) in Cerebrospinal Fluid (CSF) of 10 dogs (age range: 3 mo.- 3 yr 4 mo.) exhibiting a congenital portal-systemic shunt, either intra or extra-hepatic, and 8 age-matched control dogs who showed no signs of hepatic or neurologic disorders. Dogs with congenital shunts manifested signs of encephalopathy such as disorientation, head pressing, vocalization, depression, seizures and coma. CSF from dogs with congenital shunts contained significantly increased amounts of glutamate (2 to 3-fold increase, p<0.01), glutamine (6-fold increase, p<0.05) and aromatic amino acids (phenylalanine, tyrosine and tryptophan) compared to CSF of control dogs. Concentrations of GABA and branched chain amino acids (valine, leucine, isoleucine) were within normal limits. Modifications of brain glutamate (an excitatory amino acid) as well as tryptophan (the precursor of serotonin) could contribute to the neurological syndrome characteristic of congenital PSE in dogs.

Effects of thioacetamide-induced hepatic failure on the N-methyl-D-aspartate receptor complex in the rat cerebral cortex, striatum, and hippocampus. Binding of different ligands and expression of receptor subunit mRNAs

Hepatic encephalopathy (HE) is characterized by symptoms pointing at disturbances in glutamatergic neurotransmission in the brain, particularly in the striatum. The binding parameters of ligands specific for different recognition sites in the N-methyl-D-aspartate (NMDA) receptor complex and the distribution of the receptor subunit mRNAs (NR1, NR2A-D) were assessed in rats with acute HE induced with a hepatotoxin, thioacetamide (TAA). The binding of: 1. L-[3H]glutamate (NMDA-displaceable); 2. [3H]dizocilpine and N-(1-[2-thienyl]-cyclohexyl) [3H]piperidine ([3H]TCP); and 3. The coactivator site agonist [3H]glycine was assayed in purified membranes of the cerebral cortex, hippocampus, and striatum. In HE rats, Bmax of NMDA-displaceable glutamate binding was increased in the cerebral cortex and hippocampus, but slightly decreased in the striatum. In this region, the binding affinity was also slightly increased. In HE, Bmax of [3H]dizocilpine binding was unchanged in the striatum and cerebral cortex, but substantially decreased in the hippocampus. Pretreatment with phorbol ester enhanced the binding of dizocilpine more in HE than in control rats. Bmax of [3H]TCP binding was decreased in the cerebral cortex and striatum, but increased in the hippocampus. The different responses of these two phencyclidine site antagonists to HE may be indicative of a conformational change within the ion channel and/or the presence of microdomains reacting differently to extrinsic factors. HE did not affect glycine binding, but potentiated the maximal stimulation of [3H]dizocilpine binding by glycine in the cerebral cortex. The results emphasize the brain region and domain specificity of the responses of the NMDA receptor complex to HE.