A zebrafish model of hyperammonemia.

Hyperammonemia is the principal consequence of urea cycle defects and liver failure, and the exposure of the brain to elevated ammonia concentrations leads to a wide range of neuro-cognitive deficits, intellectual disabilities, coma and death. Current treatments focus almost exclusively on either reducing ammonia levels through the activation of alternative pathways for ammonia disposal or on liver transplantation. Ammonia is toxic to most fish and its pathophysiology appears to be similar to that in mammals. Since hyperammonemia can be induced in fish simply by immersing them in water with elevated concentration of ammonia, we sought to develop a zebrafish (Danio rerio) model of hyperammonemia. When exposed to 3mM ammonium acetate (NH4Ac), 50% of 4-day old (dpf) fish died within 3hours and 4mM NH4Ac was 100% lethal. We used 4dpf zebrafish exposed to 4mM NH4Ac to test whether the glutamine synthetase inhibitor methionine sulfoximine (MSO) and/or NMDA receptor antagonists MK-801, memantine and ketamine, which are known to protect the mammalian brain from hyperammonemia, prolong survival of hyperammonemic fish. MSO, MK-801, memantine and ketamine all prolonged the lives of the ammonia-treated fish. Treatment with the combination of MSO and an NMDA receptor antagonist was more effective than either drug alone. These results suggest that zebrafish can be used to screen for ammonia-neuroprotective agents. If successful, drugs that are discovered in this screen could complement current treatment approaches to improve the outcome of patients with hyperammonemia.

Inhibition of glutamine synthetase activity prevents memory consolidation.

Methionine sulfoximine, a specific inhibitor of the exclusively glial enzyme glutamine synthetase, was shown, at a concentration of 3.5-4.5 mM, to prevent consolidation of memory for a passive avoidance task in day-old chicks. Provided the drug was administered 5-20 min before the learning task, significant retention loss was observed from the normal time of onset of the second of three postulated stages in the memory formation sequence but the drug had to be administered considerably earlier. The amnestic effect of methionine sulfoximine was successfully counteracted by L-glutamine (10 mM) and monosodium glutamate (4 mM), and also by a cocktail of alpha-ketoglutarate (5 mM) and alanine (5 mM). This effect of methionine sulfoximine is attributed to its blockade of the production of glutamine via the glutamate-glutamine cycle, leading to a reduced capacity of neurons to replenish their transmitter glutamate.

Correlation between carbohydrate and catecholamine level impairments in methionine sulfoximine epileptogenic rat brain.

This work shows that the convulsant methionine sulfoximine induces an increase in glucose and glycogen levels and a parallel decrease in norepinephrine and dopamine levels in rat brain. Among the epileptogenic agents, methionine sulfoximine is known to have a glycogenic property in the central nervous system. The aim of this work is to look for the neurochemical mechanism underlying this property. For this, catecholamines, glucose, and glycogen were measured at the same time in different areas of the brain in rats submitted to methionine sulfoximine. The convulsant induced an increase in glucose and glycogen levels as previously described and a decrease in dopamine and norepinephrine levels in all the areas of the rat brain. These changes were roughly dose dependent. When L-dihydroxyphenylalanine and benserazide (a decarboxylase inhibitor) were administered with methionine sulfoximine, the latter failed to induce seizures in rat up to 8 h after dosing. Moreover, the glucose and glycogen amounts did not increase. In all these experiments, there was an obvious evidence of parallelism between seizures, increase in carbohydrate levels, and decrease in catecholamine levels. These results allow to conclude that the glycogenic property of methionine sulfoximine in the central nervous system probably results from its ability to decrease norepinephrine and dopamine levels. Because the effect of the convulsant on the catecholamine levels persisted for long, it is normal that glucose and glycogen levels increased during preconvulsive, convulsive and postconvulsive period. Methionine sulfoximine is probably glycogenic in rat brain because it decreases catecholamine levels for a long time.

Effects of fasting and glutathione depletors on the GSH-dependent enzyme system in the gastrointestinal mucosa of the rat.

In rats, the glutathione content of the gastrointestinal mucosa amounted to 50-60% of that of the liver. The GSH-S-transferase activity towards an aryl substrate (CDNB) was low in the stomach, colon and rectum, i.e. 5% of hepatic activity. In the small intestine there was a typical decline of activity from proximal to distal segments. GSH-Peroxidase was much lower in the intestinal mucosa as compared to the stomach and liver, whereas the GSSG-reductase was 2-3 times higher in the gastrointestinal tract in comparison to the liver. Fasting for 24 h significantly decreased the GSH content, GSH-aryltransferase and GSSG-reductase activities in the liver but not in the intestine, where even higher GSH concentrations were found in the proximal segments. L-Buthionine-sulfoximine, an inhibitor of the GSH-synthesis, caused a marked decrease of the GSH levels in the liver, stomach, proximal small intestine, colon and rectum and a concomitant decline in GSSG-reductase activity. Among the GSH-depleting agents, paracetamol exerted the strongest effect, whereas 1,1-dichloroethylene and phorone only decreased the GSH content in the liver and stomach.

Suppression of methionine sulfoximine seizures by intranigral gamma-vinyl GABA injection.

Studies were undertaken to determine whether enhancement of GABAergic transmission within the substantia nigra could inhibit seizures caused by the novel, long latency convulsant, L-methionine, sulfoximine (MSO; 200 mg/kg i.p.). Bilateral injections of gamma-vinyl GABA (10 micrograms/side) into several brain sites resulted in varying degrees of protection against MSO-induced seizures. However, significant protection was afforded only when gamma-vinyl GABA was infused into the nigra. The protective effect was reduced when injections were made at sites 2 mm dorsal to the nigra, and was further diminished when the drug was injected into the striatum. These results further support the hypothesis that elevation of cerebral GABA levels is protective against a range of experimentally induced seizures, particularly when the substantia nigra is the target of such manipulations.

The effect of diazepam on brain levels of S-adenosyl-L-methionine and S-adenosyl-L-homocysteine: possible correlation with protection from methionine sulfoximine seizures.

Administration of the long latency convulsant, L-methionine-d,1-sulfoximine (MSO) results in an increase in brain methylation flux. We determined the effects of the anticonvulsant, diazepam (DZ) on MSO seizures and on brain levels of S-adenosyl-L-methionine (AdoMet) and S-adenosyl-L-homocysteine (AdoHcy) to indicate possible alterations in the brain methylation pathway. We report a dose related inhibition of MSO seizures by DZ. In addition, DZ significantly increased brain levels of AdoMet and AdoHcy and reversed the MSO-induced decreases in AdoMet and AdoHcy. DZ also blocked the MSO induced increase in the methylation index (AdoMet/AdoHcy). The data indicates an inhibition of MSO induced increases in brain methylation by DZ. Possible mechanisms for the effect of DZ on the cerebral methylation pathway are discussed.

Intranigral methionine sulfoximine induces turning behaviour in the rat.

Unilateral infusion of methionine sulfoximine (MSO) into the substantia nigra of the rat induced contralateral rotations appearing progressively after a delay of about 100 min. The rotations were maximally antagonized by picrotoxin and bicuculline intraperitoneally injected at approximately 50-80 min after the intranigral infusion of MSO. Contraversive turning behaviour was absent after intranigral infusion of MSO in unilaterally 6-hydroxydopamine-lesioned rats.