Ammonia-mediated LTP inhibition: effects of NMDA receptor antagonists and L-carnitine

Is NMDA-Receptor-Mediated Oxidative Stress in Mitochondria of Peripheral Tissues the Essential Factor in the Pathogenesis of Hepatic Encephalopathy?

Background: Hepatic encephalopathy (HE) is a neuropsychiatric syndrome of increased ammonia-mediated brain dysfunction caused by impaired hepatic detoxification or when the blood bypasses the liver. Ammonia-activated signal transduction pathways of hyperactivated NMDA receptors (NMDAR) are shown to trigger a cascade of pathological reactions in the brain, leading to oxidative stress. NMDARs outside the brain are widely distributed in peripheral tissues, including the liver, heart, pancreas, and erythrocytes. To determine the contribution of these receptors to ammonia-induced oxidative stress in peripheral tissues, it is relevant to investigate if there are any ammonia-related changes in antioxidant enzymes and free radical formation and whether blockade of NMDARs prevents these changes. Methods: Hyperammonemia was induced in rats by ammonium acetate injection. Oxidative stress was measured as changes in antioxidant enzyme activities and O2•− and H2O2 production by mitochondria isolated from the tissues and cells mentioned above. The effects of the NMDAR antagonist MK-801 on oxidative stress markers and on tissue ammonia levels were evaluated. Results: Increased ammonia levels in erythrocytes and mitochondria isolated from the liver, pancreas, and heart of hyperammonemic rats are shown to cause tissue-specific oxidative stress, which is prevented completely (or partially in erythrocyte) by MK-801. Conclusions: These results support the view that the pathogenesis of HE is multifactorial and that ammonia-induced multiorgan oxidative stress-mediated by activation of NMDAR is an integral part of the disease and, therefore, the toxic effects of ammonia in НЕ may be more global than initially expected.

Neurotoxicity of Ammonia

Abnormal liver function has dramatic effects on brain functions. Hyperammonemia interferes profoundly with brain metabolism, astrocyte volume regulation, and in particular mitochondrial functions. Gene expression in the brain and excitatory and inhibitory neurotransmission circuits are also affected. Experiments with a number of pertinent animal models have revealed several potential mechanisms which could underlie the pathological phenomena occurring in hepatic encephalopathy.

Role of neurosteroids in hepatic encephalopathy

Hepatic encephalopathy (HE) is a neuropsychiatric manifestation of chronic or acute liver disease. Neurosteroids are synthesized from cholesterol and its precursors by glial cells, oligodendrocytes and neurons in the brain. The mechanisms by which neurosteroids affect brain function may involve both genetic and non-genetic effects. On one hand, neurosteroids bind and modulate different types of neuronal membrane receptors, including gamma-amino butyric acid-A receptor (GABA-A), N-methyl-D-aspartic acid receptor (NMDA), 5-hydroxytryptamine 3 (5-HT3) and opioid receptors which have been showed to be involved in HE. On the other hand, some neurosteroids bind to intracellular receptors through which they also regulate gene expression. Of note, neurosteroids play a role in the pathogenesis of HE through inhibiting long-term potentiation. Neurosteroids might provide a new avenue for HE treatment.

Acute and chronic effects of ethanol on learning-related synaptic plasticity

Alcoholism is associated with acute and long-term cognitive dysfunction including memory impairment, resulting in substantial disability and cost to society. Thus, understanding how ethanol impairs cognition is essential for developing treatment strategies to dampen its adverse impact. Memory processing is thought to involve persistent, use-dependent changes in synaptic transmission, and ethanol alters the activity of multiple signaling molecules involved in synaptic processing, including modulation of the glutamate and gamma-aminobutyric acid (GABA) transmitter systems that mediate most fast excitatory and inhibitory transmission in the brain. Effects on glutamate and GABA receptors contribute to ethanol-induced changes in long-term potentiation (LTP) and long-term depression (LTD), forms of synaptic plasticity thought to underlie memory acquisition. In this paper, we review the effects of ethanol on learning-related forms of synaptic plasticity with emphasis on changes observed in the hippocampus, a brain region that is critical for encoding contextual and episodic memories. We also include studies in other brain regions as they pertain to altered cognitive and mental function. Comparison of effects in the hippocampus to other brain regions is instructive for understanding the complexities of ethanol's acute and long-term pharmacological consequences.

Metaplastic LTP inhibition after LTD induction in CA1 hippocampal slices involves NMDA Receptor-mediated Neurosteroidogenesis

Long-term depression (LTD) induced by low-frequency electrical stimulation (LFS) in the CA1 region of the hippocampus is a form of synaptic plasticity thought to contribute to learning and memory and to the pathophysiology of neuropsychiatric disorders. In naïve hippocampal slices from juvenile rats, we previously found that LTD induction can impair subsequent induction of long-term potentiation (LTP) via a form of N-methyl-d-aspartate receptor (NMDAR)-dependent metaplasticity, and have recently observed that pharmacologically induced NMDAR-dependent LTP inhibition involves 5α-reduced neurosteroids that augment the actions of γ-aminobutyric acid (GABA). In this study, we found that both LFS-induced LTD and subsequent inhibition of LTP induction involve neurosteroid synthesis via NMDAR activation. Furthermore, the timing of 5α-reductase inhibition relative to LFS can dissociate effects on LTD and metaplastic LTP inhibition. These findings indicate that 5α-reduced neurosteroids play an important role in synaptic plasticity and synaptic modulation in the hippocampus.

Ammonia toxicity to the brain

Hyperammonemia can be caused by various acquired or inherited disorders such as urea cycle defects. The brain is much more susceptible to the deleterious effects of ammonium in childhood than in adulthood. Hyperammonemia provokes irreversible damage to the developing central nervous system: cortical atrophy, ventricular enlargement and demyelination lead to cognitive impairment, seizures and cerebral palsy. The mechanisms leading to these severe brain lesions are still not well understood, but recent studies show that ammonium exposure alters several amino acid pathways and neurotransmitter systems, cerebral energy metabolism, nitric oxide synthesis, oxidative stress and signal transduction pathways. All in all, at the cellular level, these are associated with alterations in neuronal differentiation and patterns of cell death. Recent advances in imaging techniques are increasing our understanding of these processes through detailed in vivo longitudinal analysis of neurobiochemical changes associated with hyperammonemia. Further, several potential neuroprotective strategies have been put forward recently, including the use of NMDA receptor antagonists, nitric oxide inhibitors, creatine, acetyl-L-carnitine, CNTF or inhibitors of MAPKs and glutamine synthetase. Magnetic resonance imaging and spectroscopy will ultimately be a powerful tool to measure the effects of these neuroprotective approaches.

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.

Ammonia inhibits long-term potentiation via neurosteroid synthesis in hippocampal pyramidal neurons

Neurosteroids are a class of endogenous steroids synthesized in the brain that are believed to be involved in the pathogenesis of neuropsychiatric disorders and memory impairment. Ammonia impairs long-term potentiation (LTP), a synaptic model of learning, in the hippocampus, a brain region involved in memory acquisition. Although mechanisms underlying ammonia-mediated LTP inhibition are not fully understood, we previously found that the activation of N-methyl-d-aspartate receptors (NMDARs) is important. Based on this, we hypothesize that metabolic stressors, including hyperammonemia, promote untimely NMDAR activation and result in neural adaptations that include the synthesis of allopregnanolone (alloP) and other GABA-potentiating neurosteroids that dampen neuronal activity and impair LTP and memory formation. Using an antibody against 5α-reduced neurosteroids, we found that 100 μM ammonia acutely enhanced neurosteroid immunostaining in pyramidal neurons in the CA1 region of rat hippocampal slices. The enhanced staining was blocked by finasteride, a selective inhibitor of 5α-reductase, a key enzyme required for alloP synthesis. Finasteride also overcame LTP inhibition by 100 μM ammonia, as did picrotoxin, an inhibitor of GABA-A receptors. These results indicate that GABA-enhancing neurosteroids, synthesized locally within pyramidal neurons, contribute significantly to ammonia-mediated synaptic dysfunction. These results suggest that the manipulation of neurosteroid synthesis could provide a strategy to improve cognitive function in individuals with hyperammonemia.

Region-specific causal mechanism in the effects of ammonia on cerebral glucose metabolism in the rat brain

Ammonia, which is considered to be the main agent responsible for hepatic encephalopathy, inhibits oxidative glucose metabolism in the brain. However, the effects of ammonia on cerebral glucose metabolism in different brain regions remains unclear. To clarify this issue, we added ammonia directly to fresh rat brain slices and measured its effects on glucose metabolism. Dynamic positron autoradiography with [(18)F]2-fluoro-2-deoxy-D-glucose and 2-(4-iodophenyl)-3-(4-nitrophenyl)-5-(2,4-disulfophenyl)-2H-tetrazolium (WST-1) colorimetric assay revealed that ammonia significantly increased the cerebral glucose metabolic rate and depressed mitochondrial function, as compared to the unloaded control in each of the brain regions examined (cerebral cortex, striatum, and cerebellum), reflecting increased glycolysis that compensates for the decrease in aerobic metabolism. Pre-treatment with (+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine hydrogen maleate (MK-801), a N-methyl-D-aspartate (NMDA) receptor antagonist, significantly attenuated these changes induced by ammonia in cerebellum, but not in cerebral cortex or striatum. The addition of ammonia induced an increase in cyclic guanosine monophosphate (cGMP) levels in cerebellum, but not in cerebral cortex or striatum, reflecting the activation of the NMDA receptor-nitric oxide-cGMP pathway. These results suggested that NMDA receptor activation is responsible for the impairment of glucose metabolism induced by ammonia specifically in cerebellum.

Hepatic encephalopathy: a review of its pathophysiology and treatment

Hepatic encephalopathy (HE) is a broad spectrum of neuropsychiatric manifestations usually affecting individuals with end-stage liver disease. The presence of HE is a poor prognostic sign, with 1-year mortality rates of almost 60%. There is much debate about the underlying mechanisms that result in this syndrome; however, elevated plasma and central nervous system ammonia levels are considered key factors in its pathogenesis. Initial evaluation of the patient presenting with overt HE should include a careful search for predisposing factors, including underlying infection, gastrointestinal (GI) bleeding, electrolyte disturbances, hepatocellular carcinoma, dehydration, hypotension, and excessive use of benzodiazepines, psychoactive drugs, or alcohol. The mainstay of treatment for many years has been nonabsorbable disaccharides, particularly lactulose. Alternative treatments, which usually are second line in patients who do not respond to lactulose, include zinc, antibiotics (neomycin, metronidazole, and rifaximin), ornithine aspartate, sodium benzoate, probiotics, and surgical intervention. Accepted treatments for HE are associated with significant unpleasant side effects, including diarrhea, renal failure, neuropathy, and other GI disturbance. Newer therapies are still in development, and most are awaiting human trials in order to confirm their benefit. These include manganese chelators, L-carnitine, N-methyl-d-aspartate receptor antagonists, blood purification dialysis system, and an intravenous combination of sodium benzoate and phenylacetate.

Ammonia-induced deficit in corticostriatal long-term depression and its amelioration by zaprinast

Hyperammonemia is a major pathophysiological factor in encephalopathies associated with acute and chronic liver failure. On mouse brain slice preparations, we analyzed the effects of ammonia on the characteristics of corticostriatal long-term depression (LTD) induced by electrical stimulation of cortical input or pharmacological activation of metabotropic glutamate receptors. Long exposure of neostriatal slices to ammonium chloride impaired the induction and/or expression of all studied forms of LTD. This impairment was reversed by the phosphodiesterase inhibitor zaprinast implying lowered cGMP signaling in LTD suppression. Polyphenols from green tea rescued short-term corticostriatal plasticity, but failed to prevent the ammonia-induced deficit of LTD. Zaprinast counteracts the ammonia-induced impairment of long-term corticostriatal plasticity and may thus improve fine motor skills and procedural learning in hepatic encephalopathy.

NMDA receptors and metaplasticity: mechanisms and possible roles in neuropsychiatric disorders

N-methyl-D-aspartate receptors (NMDARs) are key components of neural signaling, playing roles in synaptic transmission and in the synaptic plasticity thought to underlie learning and memory. NMDAR activation can also have neurotoxic consequences contributing to several forms of neurodegeneration. Additionally, NMDARs can modulate neuronal function and regulate the ability of synapses to undergo synaptic plasticity. Evidence gathered over the past 20 years strongly supports the idea that untimely activation of NMDARs impairs the induction of long-term potentiation (LTP) by a form of metaplasticity. This metaplasticity can be triggered by multiple stimuli including physiological receptor activation, and metabolic and behavioral stressors. These latter findings raise the possibility that NMDARs contribute to cognitive dysfunction associated with neuropsychiatric disorders. This paper examines NMDAR metaplasticity and its potential role in cognition. Recent studies using NMDAR antagonists for therapeutic purposes also raise the possibility that metaplasticity may contribute to clinical effects of certain drugs.

Neuroprotective effects of pyruvate following NMDA-mediated excitotoxic insults in hippocampal slices

The activation of N-methyl-D-aspartate (NMDA) receptors and subsequent release of nitric oxide (NO) are likely contributors to the delayed neuronal damage that accompanies ischemia and other neurodegenerative conditions. NMDA receptor antagonists and inhibitors of NO synthesis, however, are of limited benefit when administered following excitotoxic events, suggesting the importance of determining downstream events that result in neuronal degeneration. Inhibition of glyceraldehyde-3-phosphate-dehydrogenase (GAPDH), a key glycolytic enzyme, which may result in glycolytic impairment, is one of the biological targets of NO. This suggests that alternative energy substrates may prevent neuronal damage. Using rat hippocampal slices from juvenile rats, we examined the role of glycolytic impairment in NMDA-mediated excitotoxicity and whether pyruvate, an end product of glycolysis, prevents the excitotoxic neuronal injury. We observed that administration of NMDA acutely depresses ATP levels and result in a slowly developing inhibition of GAPDH. Unlike NMDA receptor antagonists or NO inhibitors, exogenously applied pyruvate is effective in restoring ATP levels and preventing delayed neuronal degeneration and synaptic deterioration when administered in the period following NMDA receptor activation. This raises the possibility that treatment with agents that maintain cellular energy function can prevent delayed excitotoxicity.

Current concepts in the pathogenesis of urea cycle disorders

The common feature of urea cycle diseases (UCD) is a defect in ammonium elimination in liver, leading to hyperammonemia. This excess of circulating ammonium eventually reaches the central nervous system, where the main toxic effects of ammonium occur. These are reversible or irreversible, depending on the age of onset as well as the duration and the level of ammonium exposure. The brain is much more susceptible to the deleterious effects of ammonium during development than in adulthood, and surviving UCD patients may develop cortical and basal ganglia hypodensities, cortical atrophy, white matter atrophy or hypomyelination and ventricular dilatation. While for a long time, the mechanisms leading to these irreversible effects of ammonium exposure on the brain remained poorly understood, these last few years have brought new data showing in particular that ammonium exposure alters several amino acid pathways and neurotransmitter systems, cerebral energy, nitric oxide synthesis, axonal and dendritic growth, signal transduction pathways, as well as K(+) and water channels. All these effects of ammonium on CNS may eventually lead to energy deficit, oxidative stress and cell death. Recent work also proposed neuroprotective strategies, such as the use of NMDA receptor antagonists, nitric oxide inhibitors, creatine and acetyl-l-carnitine, to counteract the toxic effects of ammonium. Better understanding the pathophysiology of ammonium toxicity to the brain under UCD will allow the development of new strategies for neuroprotection.

Neurologic damage and neurocognitive dysfunction in urea cycle disorders

Although the survival of patients who have urea cycle disorders has improved with the use of modalities such as alternative pathway therapy and hemodialysis, neurologic outcome is suboptimal. Patients often manifest with a variety of neurologic abnormalities, including cerebral edema, seizures, cognitive impairment, and psychiatric illness. Current hypotheses of the pathogenesis underlying brain dysfunction in these patients have focused on several lines of investigation, including the role of glutamine in causing cerebral edema, mitochondrial dysfunction leading to energy failure and the production of free radicals, and altered neurotransmitter metabolism. Advances in understanding the pathogenetic mechanisms underlying brain impairment in urea cycle disorders may lead to the development of therapies designed to interfere with the molecular cascade that ultimately leads to cerebral edema and other brain pathological findings.

Neurological implications of urea cycle disorders

The urea cycle disorders constitute a group of rare congenital disorders caused by a deficiency of the enzymes or transport proteins required to remove ammonia from the body. Via a series of biochemical steps, nitrogen, the waste product of protein metabolism, is removed from the blood and converted into urea. A consequence of these disorders is hyperammonaemia, resulting in central nervous system dysfunction with mental status changes, brain oedema, seizures, coma, and potentially death. Both acute and chronic hyperammonaemia result in alterations of neurotransmitter systems. In acute hyperammonaemia, activation of the NMDA receptor leads to excitotoxic cell death, changes in energy metabolism and alterations in protein expression of the astrocyte that affect volume regulation and contribute to oedema. Neuropathological evaluation demonstrates alterations in the astrocyte morphology. Imaging studies, in particular (1)H MRS, can reveal markers of impaired metabolism such as elevations of glutamine and reduction of myoinositol. In contrast, chronic hyperammonaemia leads to adaptive responses in the NMDA receptor and impairments in the glutamate-nitric oxide-cGMP pathway, leading to alterations in cognition and learning. Therapy of acute hyperammonaemia has relied on ammonia-lowering agents but in recent years there has been considerable interest in neuroprotective strategies. Recent studies have suggested restoration of learning abilities by pharmacological manipulation of brain cGMP with phosphodiesterase inhibitors. Thus, both strategies are intriguing areas for potential investigation in human urea cycle disorders.

Hyperammonemia-induced toxicity for the developing central nervous system

In pediatric patients, hyperammonemia can be caused by various acquired or inherited disorders such as urea cycle deficiencies or organic acidemias. The brain is much more susceptible to the deleterious effects of ammonium during development than in adulthood. Hyperammonemia can provoke irreversible damages to the developing central nervous system that lead to cortical atrophy, ventricular enlargement and demyelination, responsible for cognitive impairment, seizures and cerebral palsy. Until recently, the mechanisms leading to these irreversible cerebral damages were poorly understood. Using experimental models allowing the analysis of the neurotoxic effects of ammonium on the developing brain, these last years have seen the emergence of new clues showing that ammonium exposure alters several amino acid pathways and neurotransmitter systems, as well as cerebral energy metabolism, nitric oxide synthesis, oxidative stress, mitochondrial permeability transition and signal transduction pathways. Those alterations may explain neuronal loss and impairment of axonal and dendritic growth observed in the different models of congenital hyperammonemia. Some neuroprotective strategies such as the potential use of NMDA receptor antagonists, nitric oxide inhibitors, creatine and acetyl-l-carnitine have been suggested to counteract these toxic effects. Unraveling the molecular mechanisms involved in the chain of events leading to neuronal dysfunction under hyperammonemia may be useful to develop new potential strategies for neuroprotection.

Taurine rescues hippocampal long-term potentiation from ammonia-induced impairment

Hyperammonemia, a major pathophysiological factor in hepatic encephalopathy, impairs long-term potentiation (LTP) of synaptic transmission, a cellular model of learning and memory, in the hippocampus. We have now studied the protective action of taurine on this paradigm by analyzing LTP characteristics in mouse hippocampal slices treated with ammonium chloride (1 mM) in the presence of taurine (1 mM), an ubiquitous osmolyte, antioxidant, and neuromodulator, as well as other substances with such properties. Ammonia-treated slices displayed a significant impairment of LTP maintenance. Taurine and the mitochondrial enhancer l-carnitine, but not the antioxidants (ascorbate, carnosine, and the novel compound GVS-111) or the osmolyte betaine prevented this impairment. The protective effect of taurine was preserved under the blockade of inhibitory GABA(A) and glycine receptors. It is suggested that taurine may rescue the mechanisms of hippocampal synaptic plasticity by improving mitochondrial function under hyperammonemic conditions.