Regulation of kynurenic acid synthesis in C6 glioma cells

Studies with brain slices have provided evidence that synthesis of kynurenic acid (KYNA) from kynurenine (KYN), which occurs in astrocytes, is modulated by changes in the ionic composition of the medium and the presence of depolarizing agents or the excitatory amino acid glutamate (Glu). The present study analyzed the effects of changes in incubation medium on KYNA synthesis in cultured C6 glioma cells. The synthesis was not affected by omission of Na(+) and raising K(+) concentration to 50 mM, conditions that in brain slices stimulate or inhibit KYNA formation, respectively. KYNA synthesis in C6 cells was inhibited by the absence of Ca(2+), which contrasts with its Ca(2+) independence in brain slices. Also, lack of Mg(2+) and addition of a chloride channel blocker, 4-acetamido-4'-isothiocyanatostilbene-2,2'-disulfonate (SITS), did not affect the synthesis. KYNA synthesis in C6 cells was dose dependently inhibited by Glu. The inhibitory effect of Glu was not affected by GDPbetaS, an antagonist of metabotropic Glu receptors, the receptor class prevailing in C6 cells, suggesting that Glu acted intracellularly. NH(4)Cl and veratridine decreased KYNA production, mirroring the effects noted in brain slices. KYNA synthesis was strongly reduced in the presence of leucine (Leu), and the uptake of [(14)C]Leu was inhibited by the KYNA precursor KYN, which points to Leu as a potential endogenous modulator of KYNA formation in CNS cells.