INHIBITION OF INDUCIBLE NITRIC-OXIDE SYNTHASE EXPRESSION BY ENDOTHELIN IN RAT GLIAL-CELLS PREPARED FROM THE NEONATAL RAT-BRAIN

In primary cultured rat glial cells, a combination of inflammatory cyt okines such as tumor necrosis factor-alpha (TNF-alpha) and interleukin -1 beta (IL-1 beta) stimulates production of nitrite via expression of the inducible form of nitric oxide synthase (iNOS), In these cells, s imultaneous addition of endothelin (ET) decreased iNOS expression and nitrite accumulation induced by TNF-alpha/IL-1 beta. The inhibitory ef fect of ET on TNF-alpha/IL-1 beta-stimulated iNOS expression appears t o be mediated by ETB receptors, because (1) both ET-I and ET-3 inhibit ed the effects of TNF-alpha/IL-1 beta on iNOS expression and nitrite a ccumulation, (2) a selective ETB receptor agonist, Suc-[Glu(9),Ala(11, 15)]-ET-1 (8-21) (IRL1620), decreased the effects of TNF-alpha/IL-1 be ta, and (3) a selective ETB receptor antagonist, ucyl-D-1-methoxycarbo nyltryptophanyl-D-norleucine, abolished the inhibitory effects of ETs and IRL1620. Incubation of glial cells with lipopolysaccharide (LPS) c aused an increase in iNOS expression. Simultaneous addition of ET-3 de creased the effects of LPS (10 and 100 ng/ml) on iNOS expression. Furt hermore, cyclic AMP-elevating agents (dibutyryl cyclic AMP and forskol in) inhibited TNF-alpha/IL-1 beta-induced and LPS-induced iNOS express ion and nitrite accumulation. These findings suggest that ETs can decr ease TNF-alpha/IL-1 beta-induced and LPS-induced iNOS expression via E TB receptors and that cyclic AMP may be involved in this process.