In vivo induction of inducible nitric oxide synthase by microinjection with interferon-γ and lipopolysaccharide in rat hippocampus

To clarify whether the inducible nitric oxide synthase (iNOS) protein can be induced in in vivo brain, we examined the influence of direct intrahippocampal injection with interferon-γ (IFN-γ) plus lipopolysaccharide (LPS) in the rat. In the area surrounding the microinjection site, NOS activity (NO₂ accumulation) was enhanced 24 h after injection with IFN-γ plus LPS. Although the level of 160-kDa nNOS protein was not changed, the 130-kDa iNOS protein was induced 12 h after the injection. On the other hand, iNOS mRNA could be detected at 6 and 12 h but not at 24 h. iNOS immunoreactivity was observed in CD11b-immunopositive microglia in close proximity to the injection site, but the immunoreactivity was not colocalized with glial fibrillary acidic protein-immunopositive astrocytes. Although CD11b-immunopositive microglia were of the ramified type even after injection with vehicle after 24 h, injection with IFN-γ plus LPS caused numerous microglia to change to the ameboid type and to express major histocompatibility complex (MHC) class II antigens. In some of these ameboidal microglia, iNOS immunoreactivity was observed. These results suggest that intrahippocampal injection with IFN-γ plus LPS induced iNOS mRNA after 6 h and iNOS protein after 12 h in some of the ameboidal microglia that expressed MHC class II antigens in in vivo rat brain. © 1996 Wiley-Liss, Inc.