Pretreatment of mice with
5-fluoromethylornithine (5FMOrn), a selective inactivator of
ornithine aminotransferase, diminishes the accumulation of
ammonia in the brain after administration of
ammonium acetate, and antagonizes
ammonia-induced fatal tonic extensor convulsions. In about 50% of the treated animals the loss of the righting reflex and
coma is prevented. Presumably these effects are based on the enhancement of
urea formation by the increased liver
ornithine concentrations. However, since brain
ornithine concentrations are greatly enhanced by 5FMOrn, it is not excluded that
ornithine has direct effects on cellular events involved in
ammonia-induced seizure generation, even though 5FMOrn had no
anticonvulsant properties in a series of established animal seizure models, including N-methyl-D,
L-aspartate-induced convulsions.
NMDA receptor antagonists are capable of preventing death, but do not protect against the generation of
coma and tonic extensor convulsions in
ammonium acetate intoxicated mice. Since no evidence was found for
ammonia-induced
glutamate release from rat hippocampus, there is no convincing evidence for the idea that the tonic convulsions are mediated by
NMDA receptors.
L-Methionine-D, L-sulfoximine (MSO)-induced
seizures can be partially antagonized by pretreatment with 5FMOrn. However, the effect is considerably smaller than against
ammonia-induced convulsions, although at the time of seizure onset brain
ammonia levels of MSO-intoxicated mice were lower than in the animals receiving
ammonium acetate. This suggests that MSO-convulsions are not entirely due to the elevation of brain
ammonia concentrations, even though MSO administration mimics effects of
ammonia on cortical inhibitory neuronal interactions.(ABSTRACT TRUNCATED AT 250 WORDS)