Hyperammonemia is a common finding in children with
methylmalonic acidemia, an inherited
metabolic disease characterized by
mental retardation, convulsions, and accumulation of
methylmalonic acid (MMA). Although it has been suggested that MMA induces convulsions through
succinate dehydrogenase (SDH) inhibition, very little is known about the contribution of
hyperammonemia to the development of convulsions in these patients. In the present study we investigated the effects of
ammonium ions on the
convulsant action of MMA, MMA-induced inhibition of striatal
succinate dehydrogenase, and the striatal content of
thiobarbituric acid-reactive substances (
TBARS). Adult rats were injected with
ammonium acetate (1.5 mmol/kg, sc) or
sodium acetate (1.5 mmol/kg, sc), followed 5 min later by buffered MMA (3 micromol/microl) or NaCl (4.5 micromol/microl) injected into the striatum. The animals were observed in an open field for the appearance of convulsive episodes. After 30 min of behavioral evaluation, the animals were sacrificed and had their striatal
TBARS content measured.
Ammonium acetate pretreatment caused no behavioral effects per se, but potentiated MMA-induced convulsions and increased basal
TBARS content and MMA-induced
TBARS production in the striatum.
Ammonium chloride had no effect on basal
succinate dehydrogenase activity and did not alter MMA-induced inhibition of SDH in vitro. These results suggest that
ammonia potentiates MMA-induced behavioral effects through a mechanism that does not involve further
succinate dehydrogenase inhibition, but may involve facilitation of MMA-induced oxidative damage and provide evidence that
ammonia and MMA may have mutually additive toxicity.