AbstractNotch‐1 receptor signaling pathway is involved in neuronal and glial differentiation. Its involvement in microglial functions, however, has remained elusive. This study reports the localization of Notch‐1 receptor immunoreactivity in the amoeboid microglial cells (AMC) in the postnatal rat brain. By immunofluorescence, Notch‐1 receptor was colocalized with its ligands, Jagged‐1 and Delta‐1, in the AMC in the corpus callosum and subventricular zone. Notch‐1 immunopositive cells were confirmed to be microglia labeled by OX42 and lectin. Immunoexpression of Notch‐1 receptor was progressively reduced with age. Western blot analysis showed that Notch‐1 protein level in the corpus callosum in which the AMC were heavily populated was concomitantly decreased. In postnatal rats challenged with lipopolysaccharide (LPS), Notch‐1 receptor immunofluorescence in AMC was noticeably enhanced. Furthermore, Notch‐1 protein level in the corpus callosum was increased as revealed by Western blotting analysis. In primary microglial culture treated with LPS, mRNA expression of Notch‐1 and its ligand Jagged‐1 was upregulated but that of Delta‐1 was reduced. The expression pattern of Notch‐1 and its ligands was confirmed in murine BV‐2 cells. Furthermore, Notch‐1 neutralization with its antibody reduced its protein expression. More importantly, neutralization of Notch‐1 concomitantly suppressed the mRNA expression of IL‐6, IL‐1, M‐CSF, and iNOS; TNF‐α, mRNA expression, however, was enhanced. Western blot confirmed the changes of protein level of the above except for IL‐6, which remained relatively unaltered. It is concluded that Notch‐1 signaling in the AMC and LPS‐activated microglia/BV‐2 cells modulates the expression of proinflammatory cytokines and nitric oxide. © 2008 Wiley‐Liss, Inc.