Quinolinic acid formation in immune-activated mice: studies with ( m-nitrobenzoyl)-alanine ( mNBA) and 3,4-dimethoxy-[- N-4-(-3-nitrophenyl) thiazol-2yl]-benzenesulfonamide (Ro 61-8048), two potent and selective inhibitors of kynurenine hydroxylase

Abstract

The role of kynurenine hydroxylase activity in the neo-formation of the excitotoxin quinolinic acid (QUIN) has been studied in mice by using ( m-nitrobenzoyl)-alanine ( mNBA) and 3,4-dimethoxy-[- N-4-(-3-nitrophenyl) thiazol-2yl]-benzenesulfonamide (Ro 61-8048), two potent and selective inhibitors of this enzyme. Immune-stimulation with pokeweed mitogen (PWM, 200 μg i.v., 12 h) induced a robust increase in kynurenine (KYN) and its metabolites kynurenic acid (KYNA) and QUIN in blood and brain. When incubated in a medium containing KYN but not tryptophan, spleen, lung and liver (but not brain) slices accumulated a measurable amount of QUIN in the supernatant. Slices obtained from PWM treated animals had a ten-fold increase in QUIN accumulation in spleen, no changes in lung and a 40% decrease in liver, suggesting that the spleen contributes to the increased QUIN levels found in the blood and brain of immune-stimulated mice. Large doses of kynurenine hydroxylase inhibitors increased KYN and KYNA, but unexpectedly did not decrease QUIN content in control blood and brain. When tested in organ slices obtained from either controls or immune-stimulated animals, mNBA (1–1000 μM) and Ro 61-8048 (0.1–100 μM) strongly reduced QUIN neo-formation, suggesting that, in vitro, kynurenine hydroxylase activity is required for QUIN neosynthesis. Indeed, after repeated doses of mNBA or Ro 61-8048, QUIN content in blood and brain of immune-stimulated animals significantly decreased. Our results suggest that, under basal conditions, sufficient QUIN synthesis may occur through kynurenine hydroxylase-independent pathways. In immune-stimulated animals, however, kynurenine hydroxylase inhibitors significantly reduce blood and brain accumulation of QUIN.