Comparative study on kynurenic acid production in the rat striatum by tryptophan enantiomers: an in vivo microdialysis study.

Kynurenic acid (KYNA), an endogenous antagonist of N-methyl-D-aspartate and α(7) nicotinic acetylcholine receptors, is one of the L-tryptophan (Trp) metabolites. To compare the level of KYNA produced in the striatum of rats after independent administration of L-Trp and D-Trp, rats were intraperitoneally administered L-Trp and/or D-Trp (100 mg/kg), and a microdialysis (MD) probe was implanted in the striatum. The KYNA level in the MD samples was determined using the column-switching high-performance liquid chromatography system. KYNA levels in the MD samples increased by approximately twofold in rats that were administered D-Trp or L-Trp; this result suggests that just as L-Trp, D-Trp was also metabolized to KYNA in the striatum. Additionally, 30 min before the administration of D-Trp, rats were administered 3-methyl pyrazole-5-carboxylic acid (MPC) (50 mg/kg), which is a specific inhibitor of D-amino acid oxidase (DAAO). Pretreatment with MPC suppressed striatal KYNA production; this result suggests that DAAO, encoded by one of the susceptible genes for schizophrenia, may contribute to the production of KYNA from D-Trp in the striatum of rats.