Figure 1.

Microglia are resistant to nitric oxide inhibition of indoleamine 2, 3 dioxygenase. Primary mouse microglia were primed with IFNγ (1 ng/ml) and triggered with LPS (10 ng/ml) in the presence or absence of iNOS inhibitor L-NIL (30 μM) for 4 h and 24 h, respectively. (A) Co-stimulation with IFNγ + LPS strongly enhanced iNOS mRNA (Ct cycle for IFNγ + LPS treatment was 19 ± 0.4) compared to constitutive iNOS mRNA in primary microglia incubated in medium alone (Ct cycle was 30 ± 0.5). L-NIL had no effect on expression of iNOS transcripts (Ct cycle for IFNγ + LPS treatment with L-NIL was 19 ± 0.2). (B) IFNγ + LPS significantly increased nitrate accumulation 24 h later, and this accumulation of nitrite was blocked by L-NIL. (C) No IDO mRNA was detected in microglia incubated for 4 h in medium (>40 amplification cycles). However, IFNγ + LPS increased expression of steady-state IDO transcripts by over a 1,000-fold, and this increase in IDO mRNA was not affected by L-NIL (Ct cycles for IFNγ + LPS treatment were 29 ± 0.7 in the absence of L-NIL and 29 ± 0.3 in the presence of L-NIL). (D) IDO enzymatic activity in microglia was increased by the combination of IFNγ + LPS, and this increase was not affected by inhibition of iNOS. ** p < 0.01 compared to control medium. Each bar represents the mean ± SEM of results from 3 separate experiments.