Interleukin-1 beta, inducible nitric oxide synthase, and nuclear factor-kappa B are induced in morphologically distinct microglia after rat hippocampal lipopolysaccharide/interferon-gamma injection

In a number of pathological states of the brain, the activation of the indu cible nitric oxide synthase (iNOS) plays a major role. Interleukin (IL)-1 b eta is believed to be an essential factor in the induction of iNOS, However , little is known about the cascade of events culminating in iNOS expressio n in vivo. To identify the morphological as well as temporal relationship o f lipopolysaccharide (LPS)/interferon-gamma (IFN-gamma) -induced microglial iNOS- and IL-1 beta expression, a mixture of LPS and IFN-gamma was injecte d into the rat hippocampus. IL-1 beta immunoreactivity was detected as earl y as 3 hr following surgery in ramified microglia in the lesioned hippocamp us and in distal cortical layers adjacent to the pia mater. By 12 hr postin jection, IL-1 beta immunoreactive, ramified microglia with swollen processe s were widely distributed throughout hippocampal and neocortical areas, and staining was observed up to 48 hr after treatment, In contrast, iNOS immun ostaining was seen in activated amoeboid microglia/macrophages in the ipsil ateral hippocampus and around blood vessels but not earlier than 12 hr post -surgery. The temporal pattern of iNOS and IL-1 beta expression corresponde d to newly induced transcriptional activity as revealed by RT-PCR, Activati on of NF-kappa B was restricted to brain regions in which IL-1 beta was exp ressed and was detected both in microglia and astrocytes. A number of LPS/I FN-gamma-stimulated, IL-1 beta-expressing microglia exhibited co-staining f or activated NP-kappa B. The finding that IL-1 beta precedes iNOS expressio n is consistant with a role of IL-1 beta in the intercellular signaling eve nts leading to microglial iNOS-induction. Colocalization of IL-1 beta and N F-kappa B suggests an association between IL-1 beta and NF-kappa B inductio n, (C) 1999 Wiley-Liss,Inc.