INTRODUCTION G LIAL CELLS (ASTROCYTES AND MICROGLIA) play major roles in a variety of neurodegenerative and neuroin- flammatory diseases as well as in brain injury resulting from trauma, ischemia, and stroke (5, 21, 36, 45, 50). They become activated by inflammatory and/or injury signals in a wide range of CNS pathologies and perform both protective and destructive roles via their production of trophic and toxic me- diators. As one of the important physiological stimuli, hy- poxia/reoxygenation or ischemia can induce glial activation in association with neuronal injury (20, 30, 38, 45). Experi- mental studies further indicate that neuronal injury resulting from hypoxia/reoxygenation is preceded by glial activation (1, 17) and that the production by activated glia of toxic me- diators such as nitric oxide (NO) may underlie neurodegener- ative changes (4, 18). In fact, the induction of inducible NO synthase (iNOS) and ensuing NO production by activated mi- croglia under hypoxic conditions has been proposed as an im- portant cause of neuronal death following stroke and hy- poxia/ischemia (7, 9, 10, 46). However, conflicting results have been reported regarding the effects of NOS inhibitors on neuronal injury caused by hypoxia/ischemia (13, 25, 28, 43, 51). Moreover, activation of eNOS is known to play a benefi- cial role in contrast to neuronal NOS and iNOS (12, 23, 24, 37). A recent study also suggests that iNOS-derived nitric oxide, while exerting deleterious effects on the late stages of ischemic brain damage, can also be beneficial by promoting “ischemic tolerance” (8). Since both astrocytes and microglia express iNOS that may be subject to complex regulation under hypoxic conditions, it was of interest to compare and contrast their responses under controlled in vitro conditions including an analysis of the ac- tivation of intracellular signaling pathways. It is well known 911 Department of Neurology, Medical University of South Carolina, Charleston, South Carolina. *In memory of Dr. Guiwen Guo, who died January 2006. Hypoxia/Reoxygenation Differentially Modulates NF-B Activation and iNOS Expression in Astrocytes and Microglia GUIWEN GUO * and NARAYAN R. BHAT ABSTRACT Hypoxia/ischemic brain injury accompanies an inflammatory response involving an activation of glial cells. This study, using an in vitro model, investigated the signaling mechanisms mediating hypoxic responses of the two glial cell types (astrocytes and microglia) in relation to the expression of inducible nitric oxide synthase (iNOS). In cultures of rat brain microglia and astrocytes, hypoxia (8 h) followed by reoxygenation (24 h) (H/O) had little (microglia) or no (astrocytes) effect on the expression of iNOS. However, H/O elicited opposite effects on lipopolysaccharide (LPS) induction of iNOS in the two cell types: it potentiated LPS induction of iNOS in microglia but inhibited this response in astrocytes. Similar differential effects of hypoxia were ob- served on the production of tumor necrosis factor- (TNF). In contrast, there was an upregulation of hem- oxygenase-1 (HO-1), a counter-regulatory pathway, with astrocytes showing a bigger induction than mi- croglia. While hypoxic activation of mitogen-activated protein kinases (MAPKs) was similar in the two glial types, the activation pattern of NFB was clearly different: hypoxia stimulated the activation of NFB path- way and NFB-dependent transcription in microglia but not in astrocytes. Lastly, the two cell types displayed differential vulnerabilities to hypoxia-induced cell death, the astrocytes being relatively more resistant than microglia. Antioxid. Redox Signal. 8, 911–918. Forum Original Research Communication ANTIOXIDANTS & REDOX SIGNALING Volume 8, Numbers 5 & 6, 2006 © Mary Ann Liebert, Inc.