Nicotinylalanine Increases the Formation of Kynurenic Acid in the Brain and Antagonizes Convulsions

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Abstract
Kynurenic acid (KYNA) was quantified in the extracellular spaces of the rat hippocampus using microdialysis and HPLC (fluorimetric detection) to study the possible role of this tryptophan metabolite in the modulation of the function of the N‐methyl‐D‐aspartate (NMDA) receptor. Addition of probenecid (1 mM), which is an inhibitor of the organic acid transport system, to the Ringer's solution perfusing the dialysis probe increased the KYNA concentration in the dialysate from 10.4 ± 0.9 to 48 ± 6 nM. Addition of 2 mM aminooxyacetic acid, a nonspecific inhibitor of KYNA synthesis, reduced this concentration by 50%. These data suggest that KYNA is continuously synthesized in the rat hippocampus. Nicotinylalanine (NAL), 200–400 mg/kg i.p., an analogue of kynurenine that is able to direct the flow of tryptophan metabolites toward the synthesis of KYNA, significantly increased the KYNA concentration in the hippocampal dialysate and significantly potentiated the effect of tryptophan on the accumulation of KYNA in the brain and other organs. This increase resulted in pharmacological actions compatible with an antagonism of the NMDA receptors. In fact, NAL antagonized sound‐induced seizures and prevented death in DBA/2 mice. Pretreatment of the mice with D‐serine (100 μg intracerebroventricularly), a glycine agonist and a competitive antagonist of KYNA, completely prevented the anticonvulsive action of NAL. These data suggest that changes in the extracellular concentration of KYNA in the brain are associated with a modulation of NMDA receptor function.