Rat Brain Slices Produce and Liberate Kynurenic Acid upon Exposure to l‐Kynurenine

Abstract

The incorporation of l-kynurenine (L-KYN) into kynurenic acid (KYNA) was examined in rat brain slices. KYNA was measured in the slices and in the incubation medium after purification by ion-exchange and HPLC chromatography. In pilot experiments, the formation of KYNA was confirmed by gas chromatography. KYNA was produced stereoselectively from l-KYN, and ∼90% of the newly synthesized KYNA was recovered from the incubation medium. Intracellular KYNA was not actively retained by the tissue and was lost from the cells upon repeated washes. Thus, regulation of the levels of extracellular KYNA appears to occur at the level of l-KYN uptake and/or kynurenine transaminase, the biosynthetic enzyme of KYNA. KYNA production from l-KYN was linear up to 4 h and reached a plateau at a l-KYN concentration of 250 γM. The process was effectively inhibited by the transaminase inhibitor aminooxyacetic acid (IC₅₀, ∼25 γM), and showed pronounced regional distribution (hippocampus > cortical areas > thalamus ± cerebellum). The conversion of l-KYN to KYNA was dependent on oxygenation and on the presence of glucose in the incubation medium. Neither deletion of Ca²⁺ or Mg²⁺ nor addition of 20 mM Mg²⁺ had any effect. However, KYNA production was significantly attenuated in the absence of Cl⁻ or in the presence of 50 mM K⁺ in the incubation medium. In Na⁺-free medium, the production of KYNA from l-KYN was increased by 30%. Experiments with ibotenate-lesioned striatal slices demonstrated a small neuronal component of the process, but indicated that the majority of KYNA in the brain is produced in and liberated from glial cells. The processes described here are suggested to underlie the role of KYNA as a neuromodulator and possible endogenous neuroprotective agent and anticonvulsant.