Effect of Glutamine Synthesis Inhibition with Methionine Sulfoximine on the Nitric Oxide–Cyclic GMP Pathway in the Rat Striatum Treated Acutely with Ammonia: A Microdialysis Study

Abstract

Ammonia neurotoxicity is associated with overactivation of N-methyl-D-aspartate (NMDA) receptors leading to enhanced nitric oxide and cyclic GMP synthesis and to accumulation of reactive oxygen and nitrogen species. Ammonia is detoxified in the brain via synthesis of glutamine, which if accumulated in excess contributes to astrocytic swelling, mitochondrial dysfunction and cerebral edema. This study was aimed at testing the hypothesis that the activity of the NMDA/NO/cGMP pathway is controlled by the ammonia-induced production of Gln in the brain. Ammonium chloride (final concentration 5 mM), infused for 40 min to the rat striatum via a microdialysis probe, caused a significant increase in Gln (by 40%), NO oxidation products (nitrite+nitrate=NOx) (by 35%) and cGMP (by 50%) concentration in the microdialysate. A Gln synthetase inhibitor, methionine sulfoximine (MSO, 5 mM), added directly to the microdialysate, completely prevented ammonia-mediated production of Gln, and paradoxically, it increased ammonia-mediated production of NOx and cGMP by 230% and 250%, respectively. Of note, MSO given alone significantly reduced basal Gln concentration in the rat striatum, had no effect on the basal NOx concentration, and attenuated basal concentration of cGMP in the microdialysate by 50%. The results of the present study suggest that Gln, at physiological concentrations, may ameliorate excessive activation of the NO-cGMP pathway by neurotoxic concentrations of ammonia. However, in view of potential direct interference of MSO with the pathway, exogenously added Gln and less toxic modulators of Gln content and/or transport will have to be employed in further studies on the underlying mechanisms.

In vivo monitoring of extracellular 13N-glutamine derived from blood-borne 13N-ammonia in rat striatum using microdialysis with radio-LC method

“…2). A rapid and significant decline in endogenous glutamine level with MSO was also obtained (data not shown), which is consistent with recent report (Hilgier et al, 2008). These results strongly suggest that 13 N-ammonia metabolite study with this method reflects the change in biological activity of GS.…”

The Involvement of Glutamate Metabolism in the Resistance to Thermal, Nutritional, and Oxidative Stress in Trypanosoma cruzi

“…MS is a well-characterized inhibitor of glutamine synthetase in mammalians [26, 27, 37, 38] that could cause an abnormal increase of glutamate in astroglia, producing seizures and toxicity in animals [39]. However, no significant inhibitory effect was observed on the GS activity, from T. cruzi , when MS or other one analog (MSE, MSO) were tested.…”

Neurotoxicity of Ammonia

“…On the other hand, administration of ammonium chloride has been reported to reduce the expression of two NMDA receptor subunits (GluN2A and GluN2B) in the rat hippocampus [78]. Ammonium chloride infusion into the rat striatum in vivo via a microdialysis probe increases glutamine, NO oxidation products and cGMP in the microdialysate [79]. L ikewise, it activates NMDA receptors and foments the generation of hydroxyl radicals [42].…”