Specific inhibition of kynurenate synthesis enhances extracellular dopamine levels in the rodent striatum

Abstract

Fluctuations in the endogenous levels of kynurenic acid (KYNA), a potent alpha7 nicotinic and NMDA receptor antagonist, affect extracellular dopamine (DA) concentrations in the rat brain. Moreover, reductions in KYNA levels increase the vulnerability of striatal neurons to NMDA receptor-mediated excitotoxic insults. We now assessed the role of a key KYNA-synthesizing enzyme, kynurenine aminotransferase II (KAT II), in these processes in the rodent striatum, using KAT II KO mice-which have reduced KYNA levels-and the selective KAT II inhibitor (S)-4-(ethylsulfonyl)benzoylalanine (S-ESBA) as tools. S-ESBA (applied by reverse dialysis) raised extracellular DA levels in the striatum of KYNA-deficient mice threefold and caused a much larger, 15-fold increase in wild-type mice. In the rat striatum, S-ESBA produced a 35% reduction in extracellular KYNA, which was accompanied by a 270% increase in extracellular DA. The latter effect was abolished by co-infusion of 100 nM KYNA. Intrastriatal S-ESBA pre-treatment augmented the size of a striatal quinolinate lesion by 370%, and this potentiation was prevented by co-infusion of KYNA. In separate animals, acute inhibition of KAT II reduced the de novo synthesis of KYNA during an early excitotoxic insult without enhancing the formation of the related neurotoxic metabolites 3-hydroxykynurenine and quinolinate. Taken together, these results provide further support for the concept that KAT II is a critical determinant of functionally relevant KYNA fluctuations in the rodent striatum.

Fig. 1

Effect of S-ESBA on extracellular KYNA (A) and DA (B) levels in the striatum of 21 day-old wild-type (open circles) and KAT II knockout (solid circles) mice. S-ESBA (10 mM) was perfused by reverse dialysis for 2 h (bar), and KYNA and DA were measured in the same dialysates. Data are the mean±S.E.M. (n=6 per group). * P<0.05 vs. baseline, ^# P<0.05 vs. wild-type animals (two-way ANOVA with Bonferroni's post hoc test).

Fig. 2

(A) Effect of 1 mM S-ESBA on the extracellular levels of KYNA (open circles) and DA (solid circles) in the rat striatum. S-ESBA was applied by reverse dialysis for 2 h (bar), and KYNA and DA were measured in the same dialysates. Data are expressed as % of baseline values and are the mean±S.E.M. (n=6). ^# and * P<0.05 vs. the respective baseline (two-way ANOVA with Bonferroni's post hoc test). (B) Co-perfusion of KYNA (100 nM) prevents the S-ESBA-induced increase in extracellular DA in the rat striatum. Both compounds were administered for 2 h by reverse dialysis (bar). Data are expressed as % of baseline values and are the mean±S.E.M. (n=5).

Fig. 3

Effect of S-ESBA (E; 1 mM) on QUIN-induced neurodegeneration in the rat striatum. S-ESBA was infused intrastriatally 1 h before QUIN (Q; 20 mM) or QUIN+KYNA [Q (20 mM)+K (10 µM)]. Control animals were pre-treated with PBS (S). Rats were sacrificed 4 days after the QUIN injection, and the lesion volume was quantified as described in the text. Data are the mean±S.E.M. (n=7–10 per group). * P<0.05 (one-way ANOVA with Bonferroni's post hoc test). n.s.: Not significant.

Fig. 4

Effect S-ESBA (E; 1 mM) pre-treatment on the de novo synthesis of ³H-KYNA, ³H-3-HK and ³H-QUIN following an intrastriatal infusion of QUIN (20 mM) and ³H-kynurenine in the rat striatum. S-ESBA was infused intrastriatally 1 h before QUIN and ³H-kynurenine, as described in the text. Contralateral striata (C) were pre-treated with PBS. Data are expressed as % of recovered radioactivity and are the mean±S.E.M. (n=8). * P<0.05 vs. contralateral controls (paired t-test).