dizocilpine-maleate and Hyperammonemia

It was established in experiments on cell cultures of neurons and astrocytes that ammonium ions at concentrations of 4-8 mM cause hyperexcitation of the neuronal network, as a result of which there is a disturbance of calcium homeostasis, which can lead to the death of neurons. In the present study, we investigated the effect of toxic doses of ammonium (8 mM NH

Topics: Animals; Astrocytes; Cells, Cultured; Dizocilpine Maleate; Excitatory Amino Acid Antagonists; Glutamic Acid; Hyperammonemia; N-Methylaspartate; Neurons; Rats, Sprague-Dawley; Receptors, AMPA; Receptors, N-Methyl-D-Aspartate

The roles of high- and low-affinity AMPA receptors in modulating extracellular glutamate in the cerebellum remain unclear. Altered glutamatergic neurotransmission is involved in neurological alterations in hyperammonemia, which differently affects high- and low-affinity AMPA receptors. The aims were to assess by in vivo microdialysis (a) the effects of high- and low-affinity AMPA receptor activation on extracellular glutamate in the cerebellum; (b) whether chronic hyperammonemia alters extracellular glutamate modulation by high- and/or low-affinity AMPA receptors; and (c) the contribution of NMDA receptors and EAAC1 transporter to AMPA-induced changes in extracellular glutamate. In control rats, high affinity receptor activation does not affect extracellular glutamate but increases glutamate if NMDA receptors are blocked. Low affinity AMPA receptor activation increases transiently extracellular glutamate followed by reduction below basal levels and return to basal values. The reduction is associated with transient increased membrane expression of EAAC1 and is prevented by blocking NMDA receptors. Blocking NMDA receptors with MK-801 induces a transient increase in extracellular glutamate which is associated with reduced membrane expression of EAAC1 followed by increased membrane expression of the glutamate transporter GLT-1. Chronic hyperammonemia does not affect responses to activation of low affinity AMPA receptors. Activation of high affinity AMPA receptors increases extracellular glutamate in hyperammonemic rats by an NMDA receptor-dependent mechanism. In conclusion, these results show that there is a tightly controlled interplay between AMPA and NMDA receptors and an EAAC1 transporter in controlling extracellular glutamate. Hyperammonemia alters high- but not low-affinity AMPA receptors.

Topics: alpha-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic Acid; Animals; Cerebellum; Disease Models, Animal; Dizocilpine Maleate; Dose-Response Relationship, Drug; Excitatory Amino Acid Antagonists; Excitatory Amino Acid Transporter 2; Excitatory Amino Acid Transporter 3; Extracellular Fluid; Glutamic Acid; Hyperammonemia; Male; Microdialysis; Rats; Rats, Wistar; Receptors, AMPA; Receptors, N-Methyl-D-Aspartate; Time Factors

Treatment of patients with acute liver failure (ALF) is unsatisfactory and mortality remains unacceptably high. Blocking NMDA receptors delays or prevents death of rats with ALF. The underlying mechanisms remain unclear. Clarifying these mechanisms will help to design more efficient treatments to increase patient's survival. The aim of this work was to shed light on the mechanisms by which blocking NMDA receptors delays rat's death in ALF. ALF was induced by galactosamine injection. NMDA receptors were blocked by continuous MK-801 administration. Edema and cerebral blood flow were assessed by magnetic resonance. The time course of ammonia levels in brain, muscle, blood, and urine; of glutamine, lactate, and water content in brain; of glomerular filtration rate and kidney damage; and of hepatic encephalopathy (HE) and intracranial pressure was assessed. ALF reduces kidney glomerular filtration rate (GFR) as reflected by reduced inulin clearance. GFR reduction is due to both reduced renal perfusion and kidney tubular damage as reflected by increased Kim-1 in urine and histological analysis. Blocking NMDA receptors delays kidney damage, allowing transient increased GFR and ammonia elimination which delays hyperammonemia and associated changes in brain. Blocking NMDA receptors does not prevent cerebral edema or blood-brain barrier permeability but reduces or prevents changes in cerebral blood flow and brain lactate. The data show that dual protective effects of MK-801 in kidney and brain delay cerebral alterations, HE, intracranial pressure increase and death. NMDA receptors antagonists may increase survival of patients with ALF by providing additional time for liver transplantation or regeneration.

Topics: Animals; Blood-Brain Barrier; Body Temperature; Brain; Brain Edema; Cerebrovascular Circulation; Disease Progression; Dizocilpine Maleate; Drug Evaluation, Preclinical; Excitatory Amino Acid Antagonists; Galactosamine; Glomerular Filtration Rate; Hepatic Encephalopathy; Hyperammonemia; Intracranial Hypertension; Inulin; Kidney; Lactates; Liver Failure; Liver Regeneration; Male; Rats; Rats, Wistar; Receptors, N-Methyl-D-Aspartate; Time Factors

Reduced function of the glutamate--nitric oxide (NO)--cGMP pathway is responsible for some cognitive alterations in rats with hyperammonemia and hepatic encephalopathy. Hyperammonemia impairs the pathway in cerebellum by increasing neuronal nitric oxide synthase (nNOS) phosphorylation in Ser847 by calcium-calmodulin-dependent protein kinase II (CaMKII), reducing nNOS activity, and by reducing nNOS amount in synaptic membranes, which reduces its activation following NMDA receptors activation. The reason for increased CaMKII activity in hyperammonemia remains unknown. We hypothesized that it would be as a result of increased tonic activation of NMDA receptors. The aims of this work were to assess: (i) whether tonic NMDA activation receptors is increased in cerebellum in chronic hyperammonemia in vivo; and (ii) whether this tonic activation is responsible for increased CaMKII activity and reduced activity of nNOS and of the glutamate--NO--cGMP pathway. Blocking NMDA receptors with MK-801 increases cGMP and NO metabolites in cerebellum in vivo and in slices from hyperammonemic rats. This is because of reduced phosphorylation and activity of CaMKII, leading to normalization of nNOS phosphorylation and activity. MK-801 also increases nNOS in synaptic membranes and reduces it in cytosol. This indicates that hyperammonemia increases tonic activation of NMDA receptors leading to reduced activity of nNOS and of the glutamate--NO--cGMP pathway.

Topics: Animals; Calcium-Calmodulin-Dependent Protein Kinase Type 2; Cerebellum; Cyclic GMP; Disease Models, Animal; Dizocilpine Maleate; Excitatory Amino Acid Antagonists; Glutamic Acid; Hyperammonemia; In Vitro Techniques; Male; Microdialysis; Nitrates; Nitric Oxide Synthase Type I; Nitrites; Phosphorylation; Rats; Rats, Wistar; Receptors, N-Methyl-D-Aspartate; Serine; Statistics, Nonparametric; Subcellular Fractions; Threonine

In hyperammonemia, a decrease in brain ATP can be a result of adenine nucleotide catabolism. Xanthine dehydrogenase (XD) and xanthine oxidase (XO) are the end steps in the purine catabolic pathway and directly involved in depletion of the adenylate pool in the cell. Besides, XD can easily be converted to XO to produce reactive oxygen species in the cell. In this study, the effects of acute ammonia intoxication in vivo on brain adenine nucleotide pool and xanthine and hypoxanthine, the end degradation products of adenine nucleotides, during the conversion of XD to XO were studied. Injection of rats with ammonium acetate was shown to lead to the dramatic decrease in the ATP level, adenine nucleotide pool size and adenylate energy charge and to the great increase in hypoxanthine and xanthine 11 min after the lethal dose indicating rapid degradation of adenylates. Conversion of XD to XO in hyperammonemic rat brain was evidenced by elevated XO/XD activity ratio. Injection of MK-801, a NMDA receptor blocker, prevented ammonia-induced catabolism of adenine nucleotides and conversion of XD to XO suggesting that in vivo these processes are mediated by activation of NMDA receptors. The in vitro dose-dependent effects of sodium nitroprusside, a NO donor, on XD and XO activities are indicative of the direct modification of the enzymes by nitric oxide. This is the first report evidencing the increase in brain xanthine and hypoxanthine levels and adenine nucleotide breakdown in acute ammonia intoxication and NMDA receptor-mediated prevention of these alterations.

Topics: Acetates; Animals; Brain; Dizocilpine Maleate; Dose-Response Relationship, Drug; Excitatory Amino Acid Antagonists; Hyperammonemia; Male; Nitric Oxide; Nitric Oxide Donors; Nitroprusside; Purines; Rats; Rats, Wistar; Receptors, N-Methyl-D-Aspartate; Xanthine Dehydrogenase; Xanthine Oxidase

Developing procedures to delay the mechanisms of acute liver failure-induced death would increase patients' survival by allowing time for liver regeneration or to receive a liver for transplantation. Hyperammonemia is a main contributor to brain herniation and mortality in acute liver failure (ALF). Acute ammonia intoxication in rats leads to N-methyl-D-aspartate (NMDA) receptor activation in brain. Blocking these receptors prevents ammonia-induced death. Ammonia-induced activation of NMDA receptors could contribute to ALF-induced death. If this were the case, blocking NMDA receptors could prevent or delay ALF-induced death. The aim of this work was to assess 1) whether ALF leads to NMDA receptors activation in brain in vivo and 2) whether blocking NMDA receptors prevents or delays ALF-induced death of rats. It is shown, by in vivo brain microdialysis, that galactosamine-induced ALF leads to NMDA receptors activation in brain. Blocking NMDA receptors by continuous administration of MK-801 or memantine through miniosmotic pumps affords significant protection against ALF-induced death, increasing the survival time approximately twofold. Also, when liver injury is not 100% lethal (1.5 g/kg galactosamine), blocking NMDA receptors increases the survival rate from 23 to 62%. This supports that blocking NMDA receptors could have therapeutic utility to improve survival of patients with ALF.

Topics: Ammonia; Animals; Brain; Disease Models, Animal; Disease Progression; Dizocilpine Maleate; Excitatory Amino Acid Antagonists; Galactosamine; Hepatic Encephalopathy; Hyperammonemia; Infusion Pumps, Implantable; Liver Failure, Acute; Male; Memantine; Microdialysis; Rats; Rats, Wistar; Receptors, N-Methyl-D-Aspartate; Severity of Illness Index; Time Factors