Effects of ammonia and allopurinol on rat hippocampal NMDA receptors

Abstract

Ammonia is considered to be the main agent responsible for hepatic encephalopathy which progressively leads to altered mental status. N-methyl-D-aspartate (NMDA) is an ionotropic glutamate receptor, which is involved in synaptogenesis, memory and neurotoxicity. The aim of this study was to investigate the effects of ammonia intoxication and allopurinol, a xanthine oxidase (XO) inhibitor, on NMDA receptor subunits, NR2A and NR2B, in the hippocampus of rats. Thirty-six male rats were divided into three groups (n = 12/group) as follows: (1)control group (phosphate buffered saline (PBS) solution); (2)ammonia group (ammonium acetate, 2.5 mmol/kg), (3)ammonia + allopurinol group (ammonium acetate, 2.5 mmol/kg, allopurinol, 50 mg/kg). Each rat received intraperitoneal injection for 28 days. Western Blotting technique was used for detecting NR2A and NR2B expressions. Both NR2A and NR2B subunit expressions decreased 27 and 11%, respectively, in ammonia group with respect to the control group. Ammonium acetate decreased significantly in NR2A subunit expressions in the hippocampus (p < 0.01). Administration of ammonia + allopurinol caused statistically significant increases in NR2A subunit expressions compared to the ammonia group (p < 0.001). The down-regulation of NMDA receptors caused by ammonium acetate suggest that these receptors may play role in the process of hepatic encephalopathy and using allopurinol may have some protective effects in ammonia toxicity.

The effect of hyperammonemia on myostatin and myogenic regulatory factor gene expression in broiler embryos

“…A total of 200 mg (50 mg per dose) of ammonium acetate was delivered into the amniotic sac, via injection of 0.2 mL (250 mg/mL) every 12 hours, over 2 days. The ammonium acetate dosage and frequency of administration was empirically derived, using non-toxic intraparitoneal dosages from murine models (2.5 mmol/kg) and intraparitoneal LD 50 toxicity observed in 4-week-old chicks as guidelines for an initial low (2 mmol/kg) and high (10 mmol/kg) amount of ammonium acetate to be administered (Wilson et al , 1968; Yonden et al , 2010). The average body weight (12g) of ten ED15 embryos was used to calculate dosages for all injections.…”

Neurotoxicity of Ammonia

“…The NMDA receptor antagonist dizocilpine (MK-801) blocks the ammonia-induced generation of reactive oxygen and nitrogen species in astrocytes [77]. 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].…”

The modulatory effects of allopurinol on N‐methyl d‐aspartate receptors in the central nervous system

“…2 The impairment of glutamatergic pathways may prevent some of the toxic effects of ammonia on the overactivation of NMDA receptors but also reduces normal physiological functions, potentially contributing to the clinical symptoms displayed in chronic hyperammonemia. 3 Although Yoden et al 1 pointed out that the main effect of allopurinol in the context of chronic hyperammonemia is by acting as an antioxidant, the indirect contribution of other neurotransmitters or neuromodulators to these findings could not be fully ruled out. This might be especially important when interpreting allopurinol effects on chronic hyperammonemia.…”

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We read with great interest the elegant manuscript by Yonden et al, 1 showing that allopurinol is capable of normalizing N-methyl D-aspartate (NMDA) pathway functions altered by ammonia either by activation of NMDA receptors or by enhancing the expression of NMDA receptor subunits, particularly the NR2A subunit. The authors argue that this modulatory effect may be due to allopurinol's antioxidant effect by inhibiting the xanthine oxidase (XO) pathway and by reducing the generation of free radicals.

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“…However, we propose here a supplementary explanation for their results. 1 Chronic hyperammonemia leads to a depression of excitatory neurotransmission by the impairment of NMDA receptor function and by the inhibition of NMDA receptor-mediated signal transduction pathways, possibly as an adaptative response to the increased extracellular glutamate levels found in hepatic encephalopathy. 2 The impairment of glutamatergic pathways may prevent some of the toxic effects of ammonia on the overactivation of NMDA receptors but also reduces normal physiological functions, potentially contributing to the clinical symptoms displayed in chronic hyperammonemia.…”