http://utsc.utoronto.ca/%7Ebritto/publications/amtox.pdf

NH4+ toxicity in higher plants: a critical review

Mitochondria play a central role in the survival and death of neurons. The detailed bioenergetic mechanisms by which isolated mitochondria generate ATP, sequester Ca2+, generate reactive oxygen species, and undergo Ca2+-dependent permeabilization of their inner membrane are currently being applied to the function of mitochondria in situ within neurons under physiological and pathophysiological conditions. Here we review the functional bioenergetics of isolated mitochondria, with emphasis on the chemiosmotic proton circuit and the application (and occasional misapplication) of these principles to intact neurons. Mitochondria play an integral role in both necrotic and apoptotic neuronal cell death, and the bioenergetic principles underlying current studies are reviewed.

https://www.physiology.org/doi/pdf/10.1152/physrev.2000.80.1.315

Mitochondria and Neuronal Survival

A stereotaxic atlas of the rat brain

Vasodilation and increased blood flow are characteristic early vascular responses to acute hyperglycemia and tissue hypoxia. In hypoxic tissues these vascular changes are linked to metabolic imbalances associated with impaired oxidation of NADH to NAD+ and the resulting increased ratio of NADH/NAD+. In hyperglycemic tissues these vascular changes also are linked to an increased ratio of NADH/NAD+, in this case because of an increased rate of reduction of NAD+ to NADH. Several lines of evidence support the likelihood that the increased cytosolic ratio of free NADH/NAD+ caused by hyperglycemia, referred to as pseudohypoxia because tissue partial pressure oxygen is normal, is a characteristic feature of poorly controlled diabetes that mimics the effects of true hypoxia on vascular and neural function and plays an important role in the pathogenesis of diabetic complications. These effects of hypoxia and hyperglycemia-induced pseudohypoxia on vascular and neural function are mediated by a branching cascade of imbalances in lipid metabolism, increased production of superoxide anion, and possibly increased nitric oxide formation.

Hyperglycemic Pseudohypoxia and Diabetic Complications

Regulation of the Calmodulin-stimulated Protein Phosphatase, Calcineurin

Injection of large doses of ammonia into rats leads to depletion of brain ATP. However, the molecular mechanism leading to ATP depletion is not clear. The aim of the present work was to assess whether ammonium-induced depletion of ATP is mediated by activation of the NMDA receptor. It is shown that injection of MK-801, an antagonist of the NMDA receptor, prevented ammonia-induced ATP depletion but did not prevent changes in glutamine, glutamate, glycogen, glucose, and ketone bodies. Ammonia injection increased Na+,K(+)-ATPase activity by 76%. This increase was also prevented by previous injection of MK-801. The molecular mechanism leading to activation of the ATPase was further studied. Na+,K(+)-ATPase activity in samples from ammonia-injected rats was normalized by "in vitro" incubation with phorbol 12-myristate 13-acetate, an activator of protein kinase C. The results obtained suggest that ammonia-induced ATP depletion is mediated by activation of the NMDA receptor, which results in decreased protein kinase C-mediated phosphorylation of Na+,K(+)-ATPase and, therefore, increased activity of the ATPase and increased consumption of ATP.

Brain ATP depletion induced by acute ammonia intoxication in rats is mediated by activation of the NMDA receptor and Na+,K(+)-ATPase.

https://febs.onlinelibrary.wiley.com/doi/pdf/10.1016/0014-5793%2892%2980404-5

Acute ammonia toxicity is mediated by the NMDA type of glutamate receptors

The aim of this work was to assess whether ammonia concentrations similar to the increase found in the brain of hyperammonemic rats (100 μm), impair N-methyl-d-aspartate (NMDA) receptor-mediated signal transduction. We first measured glutamate neurotoxicity, which in these neurons is mediated by activation of NMDA receptors, as an initial parameter reflecting activation of NMDA receptor-mediated pathways. Long-term treatment of cultured neurons with ammonia prevents glutamate-induced neuronal death. The EC50 was 20 μm, and at 100 μm the protection was complete. The induction of the protective effect was not immediate, but took several hours.



  Treatment with 100 μm ammonia did not prevent a glutamate- or NMDA-induced rise of intracellular calcium. Ammonia impaired the glutamate–nitric oxide–cGMP (3′,5′-cyclic guanosine monophosphate) pathway in a dose- and time-dependent manner. Glutamate-induced formation of cGMP was reduced by 42%, while activation of nitric oxide synthase was not affected. Ammonia reduced by 31% cGMP formation induced by S-nitroso-N-acetyl-penicillamine (SNAP), a NO-generating agent, confirming that the interference occurs at the level of guanylate cyclase activation by nitric oxide.



  To assess whether chronic moderate hyperammonemia in vivo also impairs the glutamate–nitric oxide–cGMP pathway, we determined by in vivo brain microdialysis in freely moving rats the formation of cGMP induced by NMDA. In hyperammonemic rats, the formation of cGMP induced by NMDA and SNAP was reduced by ca. 60 and 41%, respectively, indicating that chronic hyperammonemia in the animal in vivo also impairs the glutamate–nitric oxide–cGMP pathway. Impairment of this pathway can contribute to the neurological alterations found in hyperammonemia and hepatic encephalopathy.

Chronic hyperammonemia impairs the glutamate-nitric oxide-cyclic GMP pathway in cerebellar neurons in culture and in the rat in vivo.

We proposed that acute ammonia toxicity is mediated by activation of NMDA receptors. To confirm this hypothesis we have tested whether different NMDA receptor antagonists, acting on different sites of NMDA receptors, prevent death of mice induced by injection of 14 mmol/Kg of ammonium acetate, a dose that induces death of 95% of mice. MK-801, phencyclidine and ketamine, which block the ion channel of NMDA receptors, prevent death of at least 75% of mice. CPP, AP-5, CGS 19755, and CGP 40116, competitive antagonists acting on the binding site for NMDA, also prevent death of at least 75% of mice. Butanol, ethanol and methanol which block NMDA receptors, also prevent death of mice. There is an excellent correlation between the EC50 for preventing ammonia-induced death and the IC50 for inhibiting NMDA-induced currents. Acute ammonia toxicity is not prevented by antagonists of kainate/AMPA receptors, of muscarinic or nicotinic acetylcholine receptors or of GABA receptors. Inhibitors of nitric oxide synthase afford partial protection against ammonia toxicity while inhibitors of calcineurin, of glutamine synthetase or antioxidants did not prevent ammonia-induced death of mice. These results strongly support the idea that acute ammonia toxicity is mediated by activation of NMDA receptors.

NMDA receptor antagonists prevent acute ammonia toxicity in mice.

Nicotine prevents glutamate-induced proteolysis of the microtubule-associated protein MAP-2 and glutamate neurotoxicity in primary cultures of cerebellar neurons.

María-Dolores Miñana

Papers

Brain ATP depletion induced by acute ammonia intoxication in rats is mediated by activation of the NMDA receptor and Na+,K(+)-ATPase.

Acute ammonia toxicity is mediated by the NMDA type of glutamate receptors

Chronic hyperammonemia impairs the glutamate-nitric oxide-cyclic GMP pathway in cerebellar neurons in culture and in the rat in vivo.

NMDA receptor antagonists prevent acute ammonia toxicity in mice.

Nicotine prevents glutamate-induced proteolysis of the microtubule-associated protein MAP-2 and glutamate neurotoxicity in primary cultures of cerebellar neurons.