POTENTIATION OF OXYGEN-GLUCOSE DEPRIVATION-INDUCED NEURONAL DEATH AFTER INDUCTION OF INOS

Background and Purpose Previous studies have shown that brain ischemia and other insults can induce a marked increase in inducible nitric ox ide synthase (iNOS) expression in astrocytes and some immune cells, bu t the biological significance of this phenomenon has not been elucidat ed. The purpose of the present study was to determine whether this ind uction of astrocyte iNOS alters neuronal vulnerability to severe hypox ic insults. Methods Astrocytic iNOS was induced by exposure of murine cortical cultures to interferon gamma in combination with either inter leukin-lp or lipopolysaccharide. Cultures were exposed to combined oxy gen-glucose deprivation. The extracellular concentration of glutamate was measured by high-performance liquid chromatography. N-Methyl-D-asp artate (NMDA) receptor activity was assessed by measurement of Ca-45(2 +) influx; neuronal death was assessed by morphological examination an d quantitated by measurement of lactate dehydrogenase efflux to the ba thing medium. Results In murine neocortical cell cultures containing n eurons and astrocytes, neuronal injury induced by combined oxygen-gluc ose deprivation was not reduced by the addition of the nitric oxide sy nthase inhibitors N-G-nitro-L-arginine or L(G)-nitro-arginine methyl e ster. However, after induction of astrocyte iNOS activity with interfe ron gamma plus lipopolysaccharide or interleukin-1 beta, oxygen-glucos e deprivation-induced neuronal injury was markedly enhanced and nitric oxide synthase inhibitors became protective. This iNOS-mediated poten tiation was associated with a large increase in both extracellular glu tamate accumulation and Ca-45(2+) influx into neurons. The potentiatio n could be blocked by MK-801 but not CNQX, suggesting critical involve ment of NMDA receptor activation. Conclusions These results support th e idea that nitric oxide production mediated by induced astrocytic iNO S can potentiate NMDA receptor-mediated neuronal death consequent to h ypoxic-ischemic insults.