Background: Alzheimer's disease is characterized by the accumulation of neuritic plaques, containing activated microglia and beta-amyloid peptides (A beta). Fibrillar A beta can activate microglia, resulting in production of toxic and inflammatory mediators like hydrogen peroxide, nitric oxide, and cytokines. We have recently found that microglial proliferation is regulated by hydrogen peroxide derived from NADPH oxidase. Thus, in this study, we investigated whether A beta can stimulate microglial proliferation and cytokine production via activation of NADPH oxidase to produce hydrogen peroxide.|Methods: Primary mixed glial cultures were prepared from the cerebral cortices of 7-day-old Wistar rats. At confluency, microglial cells were isolated by tapping, replated, and treated either with or without A beta. Hydrogen peroxide production by cells was measured with Amplex Red and peroxidase. Microglial proliferation was assessed under a microscope 0, 24 and 48 hours after plating. TNF-alpha and IL-1 beta levels in the culture medium were assessed by ELISA.|Results: We found that 1 mu M fibrillar (but not soluble)A beta(1-40) peptide induced microglial proliferation and caused release of hydrogen peroxide, TNF-alpha and IL-1 beta from microglial cells. Proliferation was prevented by the NADPH oxidase inhibitor apocynin (10 mu M), by the hydrogen peroxide-degrading enzyme catalase (60 U/ml), and by its mimetics EUK-8 and EUK-134 (20 mu M); as well as by an antibody against TNF-alpha and by a soluble TNF receptor inhibitor. Production of TNF-alpha and IL-1 beta, measured after 24 hours of A beta treatment, was also prevented by apocynin, catalase and EUKs, but the early release (measured after 1 hour of A beta treatment)of TNF-alpha was insensitive to apocynin or catalase.|Conclusion: These results indicate that A beta(1-40)-induced microglial proliferation is mediated both by microglial release of TNF-alpha and production of hydrogen peroxide from NADPH oxidase. This suggests that TNF-alpha and NADPH oxidase, and its products, are potential targets to prevent A beta-induced inflammatory neurodegeneration.