Gradual inhibition of inducible nitric oxide synthase but not of interleukin-1β production in rat microglial cells of endotoxin- treated mixed glial cell cultures

In cultures of purified microglial cells and astrocytes from newborn rats, the immunocytochemical localization of interleukin-1β (IL- 1β) and inducible nitric oxide synthase (iNOS) using recently developed antibodies, as well as the release of IL-1β and nitric oxide (NO), was studied following exposure of the cells to endotoxin [lipopolysaccharide (LPS)]. In the absence of LPS, IL-1β- and iNOS-immunoreactive microglial cells and IL- 1β or NO release were not observed, whereas in the presence of the endotoxin, the production of NO and IL-1β by microglial cells dramatically exceeded their synthesis and release by astrocytes. Interestingly, microglial cells cultured for 4-8 days in the presence of astrocytes appeared to lose their ability to produce iNOS, whereas the release of IL- 1β remained unaltered. Moreover, endotoxin-stimulated microglial cells appeared to regain their ability to synthesize iNOS following their separation from astrocytes. These data show that microglia are primarily responsible for NO and IL-1β production in mixed glial cell cultures upon endotoxin stimulation. Moreover, in the presence of astrocytes the induction of iNOS, but not that of IL-1β in microglial cells is gradually inhibited.