INDUCIBLE NITRIC-OXIDE SYNTHASE EXPRESSION IN ACTIVATED RAT MICROGLIAL CULTURES IS DOWN-REGULATED BY EXOGENOUS PROSTAGLANDIN E(2) AND BY CYCLOOXYGENASE INHIBITORS

Prostaglandins and nitric oxide (NO) are among the numerous substances released by activated microglial cells, the brain resident macrophage s, and they mediate several important microglial functions. We have pr eviously shown that cyclooxygenase-2 (COX-2) and inducible NO synthase (iNOS), the two key enzymes in prostaglandin and NO synthesis, respec tively, are rapidly co-induced in rat neonatal microglial cultures act ivated by bacterial endotoxin (lipopolysaccharide [LPS]) and that COX- 2 expression appears to be under the negative control of endogenous as well as exogenous NO. In this study we show that exogenous prostaglan din E(2) (PGE(2)), which is known to increase cyclic adenosine monopho sphate (cAMP) levels in microglial cells, downregulates LPS-induced iN OS expression in a dose-dependent manner. The involvement of cAMP in t he PGE(2)-dependent inhibition of iNOS is supported by several pieces of evidence. First, iNOS expression was also inhibited by agents such as isoproterenol and forskolin, which cause an elevation of cAMP level s, and by dibutyryl cAMP (dbcAMP), a cAMP stable analogue. Second, the inhibitory effect of PGE(2) was mimicked by 11-deoxy-16,16-dm PGE(2), a selective agonist at the PGE(2) receptor subtype EP2, coupled to th e activation of adenylyl cyclase, but not by sulprostone, a potent ago nist at receptor subtypes EP3 and EP1, associated with an inhibition o f adenylyl cyclase activity and intracellular Ca2+ elevation, respecti vely. Third, the inhibitory effect of PGE(2) an NO synthesis was block ed by SQ 22,536, a specific inhibitor of adenylyl cyclase. Interesting ly, the abrogation of endogenous prostanoid production by several COX inhibitors caused a reduction of iNOS expression, suggesting a positiv e modulatory effect of endogenous prostanoids of iNOS expression, as o pposed to the inhibitory effect of exogenous PGE(2). (C) 1991 Wiley-Li ss, Inc.