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Dysregulation of the GC system permitting excessive inflammation in the presence of elevated GCs occurs in depression, and is thought to be due to impaired GC signalling. We propose FKBP5, which acts to reduce GR activation, has a role in GC dysregulation. Glucocorticoid induced leucine zipper (GILZ) is a GC-inducible gene that is used as a readout of GR activation. The current objective was to examine the ability of IFN-α, IL-6, TNF-α, and IL-1β to impact GC sensitivity in microglia. These cytokines were chosen due to their implication in depression. The ability of these cytokines to alter mRNA expression of GR, FKBP5 and GILZ under basal conditions and in response to dexamethasone (DEX) in BV2 microglia was examined. Both IL-6 and IFN-α attenuated DEX-induced increases of GILZ, indicating the presence of GC resistance. IFN-α increased basal expression of FKBP5 and consistent with its negative feedback function attenuated DEX-induced FKBP5 expression. IL-6 did not alter basal expression of GR, FKBP5 or GILZ, but did attenuate the DEX-induced increases of FKBP5 and GILZ. Immunofluorescent staining shows that IFN-αand IL-6 reduce the degree of DEX-induced GR translocation and that IFN-α increased the co-localisation of GR and FKBP5. Taken together, IFN-α and IL-6 induce GC resistance in microglia and we hypothesise that IFN-α-induced increases in FKBP5 result in GR being held in the cytosol thereby preventing GC signalling.

The kynurenine pathway (KP) is the main pathway by which tryptophan is metabolized. Its rate limiting enzyme indolamine 2, 3-dioxygenase (IDO) catalyses the production of kynurenine, which in turn is metabolised by either kynurenine 3-hydroxylase (KMO) which leads to the production of quinolinic acid (neurotoxic), or by kynurenine aminotransferase (KAT II) which yields kynurenic acid (neuroprotective). These enzymes are localised in microglia and astrocytes respectively. The present studies demonstrated that IFN-γ induces expression of IDO, KMO and kynase along with the production of neurotoxic molecules from BV-2 microglia. This was determined using a cell culture system whereby microglia were activated with IFN-γ and this conditioned medium was then placed on primary cortical neurons. Inhibition of the kynurenine pathway with 1-(D)-methyl-tryptophan (IDO inhibitor) and Ro 61-8048 (KMO inhibitor) completely attenuated this microglial-mediated neurotoxicity, as did the NMDA antagonist MK-801. Furthermore the anti-depressant fluoxetine partially attenuated this neurotoxicity and IFN-γ induced increases in IDO and kynase mRNA expression. Inhibition of classical neurotoxic factors released from microglia i.e. iNOS and the enzyme NADPH oxidase had no effect on this microglial mediated neurotoxicity, and neither did the anti-inflammatory agent dexamethasone. These results propose IFN-γ activated microglia induce neurotoxicity by activating the KP, and this can be attenuated using fluoxetine, suggesting a possible role for anti-depressants in ameliorating neuronal damage and in influencing the KP balance.