Induction of indolamine 2,3-dioxygenase and kynurenine 3-monooxygenase in rat brain following a systemic inflammatory challenge: A role for IFN-γ?

Inflammation-mediated dysregulation of the kynurenine pathway has been implicated as a contributor to a number of major brain disorders. Consequently, we examined the impact of a systemic inflammatory challenge on kynurenine pathway enzyme expression in rat brain. Indoleamine 2,3-dioxygenase (IDO) expression was induced in cortex and hippocampus following systemic lipopolysaccharide (LPS) administration. Whilst IDO expression was paralleled by increased circulating interferon (IFN)-γ concentrations, IFN-γ expression in the brain was only modestly altered following LPS administration. In contrast, induction of IDO was associated with increased central tumour necrosis factor (TNF)-α and interleukin (IL)-6 expression. Similarly, in cultured glial cells LPS-induced IDO expression was accompanied by increased TNF-α and IL-6 expression, whereas IFN-γ was not detectable. These findings indicate that IFN-γ is not required for LPS-induced IDO expression in brain. A robust increase in kynurenine-3-monooxygenase (KMO) expression was observed in rat brain 24 h post LPS, without any change in kynurenine aminotransferase II (KAT II) expression. In addition, we report that constitutive expression of KAT II is approximately 8-fold higher than KMO in cortex and 20-fold higher in hippocampus. Similarly, in glial cells constitutive expression of KAT II was approximately 16-fold higher than KMO, and expression of KMO but not KAT II was induced by LPS. These data are the first to demonstrate that a systemic inflammatory challenge stimulates KMO expression in brain; a situation that is likely to favour kynurenine metabolism in a neurotoxic direction. However, our observation that expression of KAT II is much higher than KMO in rat brain is likely to counteract potential neurotoxicity that could arise from KMO induction following an acute inflammation.