Effect of 3-nitropropionic acid on kynurenine aminotransferase in the rat brain

Activation of excitatory amino acid receptors by endogenous excitotoxins results in degenerative changes characteristic of neurodegenerative brain diseases such as Huntington's disease. Excitatory amino acid receptors are present in the highest concentration in the striatum, the hippocampal region, and the temporal lobe. The most potent, naturally occurring excitatory amino acid receptor antagonist is kynurenic acid (KYNA) which acts preferentially on N-methyl-D-aspartate (NMDA) receptors. KYNA is produced from L-kynurenine, by the action of the enzymes kynurenine aminotransferases (KAT I and KAT II). Several inhibitors of mitochondrial energy metabolism result in an indirect excitotoxic neuronal degeneration. We examined whether systemic administration of the mitochondrial toxin 3-nitroproprionic acid (3-NP), an irreversible inhibitor of succinate dehydrogenase, which also acts by an indirect excitotoxic mechanism, would produce alterations in the immunohistochemical pattern of KAT I. Our present investigations demonstrate that after 15 days of administration of 3-NP, an inhibitor of mitochondrial Complex II, the most severe depletion of KAT I occurred in the striatum; less severe depletion occurred in other brain areas investigated, following a striatum > hippocampus > temporal cortex gradient. The alterations induced by 15 days of 3-NP treatment were less conspicuous in 6-week-old (young) animals than in 3-month-old adults. In these adult animals, 3-NP induced necrotic cores in the striatum, characterized by destruction of neuronal and glial elements, similar to that seen in the histologic and neurochemical features of Huntington's disease. It appears that immunohistochemical depletion of KAT after administration of 3-NP to adult animals may contribute to the pathological processes that characterize Huntington's disease.