UPTAKE OF KYNURENINE INTO RAT-BRAIN SLICES

Abstract: The transport of [3H]kynurenine ([3H]KYN) into slices from rat tissue was examined in vitro. Brain accumulated KYN seven to eight times more effectively than any of several peripheral organs. Of all the organs tested, only the brain exhibited a sodium‐dependent component of the uptake process. After an incubation period of 1 h, sodium‐dependent transport amounted to 60% of total uptake. Both processes were abolished by prior sonication of the tissue and significantly inhibited by inclusion of metabolic blockers in the incubation medium. Time resolution showed that the sodium‐independent uptake occurred rapidly and reached saturation within 30 min. In contrast, sodium‐dependent transport was linear for at least 2 h of incubation. Brain regional analysis revealed a sevenfold difference between the areas of highest (cortex) and lowest (cerebellum) uptake. With the exception of cerebellar tissue, the ratio between sodium‐dependent and sodium‐independent processes was consistent among brain regions. Kinetic analyses were performed on striatal slices and revealed a Km of 927 μM and a Vmax of 18 nmol/h/mg of protein for the sodium‐dependent process, and a Km of 3.8 mM and a Vmax of 38 nmol/10 min/mg of protein for the sodium‐independent transport. The transporters were equally amenable to inhibition by KYN and tryptophan, indicating that KYN entry into the cell may be mediated by neutral amino acid uptake sites. No strict stereoselectivity existed, but L enantiomers were clearly more active than the D forms. Studies in ibotenate‐lesioned striata showed a 70%decrease in the sodium‐dependent process and a concomitant 60%increase in the sodium‐independent transport, suggesting differential cellular associations of the two uptake sites. Cellular uptake of KYN may constitute a critical step in the disposition of KYN metabolites such as quinolinic acid and kynurenic acid in the brain. Copyright © 1990, Wiley Blackwell. All rights reserved