INTERFERON-GAMMA AND NITRIC-OXIDE DOWN-REGULATE LIPOPOLYSACCHARIDE-INDUCED PROSTANOID PRODUCTION IN CULTURED RAT MICROGLIAL CELLS BY INHIBITING CYCLOOXYGENASE-2 EXPRESSION

We have used purified microglial cultures obtained from neonatal rat b rains to study some aspects of the regulation of prostanoid production in these cells. We found that nitric oxide, which is released into th e culture medium along with prostanoids when the cells are exposed to lipopolysaccharide (1-100 ng/ml), can down-regulate prostanoid biosynt hesis, Indeed, the abrogation of endogenous nitric oxide production, o btained by depleting the medium of the precursor L-arginine or by bloc king the activity of nitric oxide synthase by the specific inhibitor N G-monomethyl-L-arginine, led to a remarkable increase in lipopolysacch aride-induced prostanoid release, Moreover, the nitric oxide-generatin g compound 3-morpholinosydnonimine caused a substantial reduction of p rostanoid formation, in the absence of endogenous nitric oxide, sugges ting that both endogenous and exogenous nitric oxide may inhibit the i nduced prostanoid production. We also found that interferon-gamma pote ntiated the effect of lipopolysaccharide on nitrite accumulation as pr eviously described by others and depressed the lipopolysaccharide-evok ed production of prostaglandin E(2), prostaglandin D-2, and thromboxan e. It is interesting that the inhibitory effect of interferon-gamma on prostanoid production did not appear to depend on the potentiation of NO release, as it was present also when the endogenous synthesis of n itric oxide was abrogated, Additional experiments showed that interfer on-gamma and nitric oxide act mainly by down-regulating the lipopolysa ccharide-induced enzymatic activity and expression (analyzed by wester n blot) of cyclooxygenase-2. Our data indicate that microglial prostan oid biosynthesis induced by proinflammatory stimuli, such as lipopolys accharide, is tightly regulated by nitric oxide. Interferon-gamma appe ars to affect the balance between these local mediators by favoring ni tric oxide production and inhibiting the prostanoid cascade and may th us contribute to the modulation of inflammation, local immune reactivi ty, and neuronal damage.