Microglia are resident brain macrophages that become activated and proliferate 
following brain damage or stimulation by immune mediators, such as IL-1beta or 
TNF-alpha. We investigated the mechanisms by which microglial proliferation is 
regulated in primary cultures of rat glia. We found that basal proliferation of 
microglia was stimulated by proinflammatory cytokines IL-1beta or TNF-alpha, and 
this proliferation was completely inhibited by catalase, implicating hydrogen 
peroxide as a mediator of proliferation. In addition, inhibitors of NADPH 
oxidase (diphenylene iodonium or apocynin) also prevented microglia 
proliferation, suggesting that this may be the source of hydrogen peroxide. 
IL-1beta and TNF-alpha rapidly stimulated the rate of hydrogen peroxide produced 
by isolated microglia, and this was inhibited by diphenylene iodonium, implying 
that the cytokines were acting directly on microglia to stimulate the NADPH 
oxidase. Low concentrations of PMA or arachidonic acid (known activators of 
NADPH oxidase) or xanthine/xanthine oxidase or glucose oxidase (generating 
hydrogen peroxide) also increased microglia proliferation and this was blocked 
by catalase, showing that NADPH oxidase activation or hydrogen peroxide was 
sufficient to stimulate microglia proliferation. In contrast to microglia, the 
proliferation of astrocytes was unaffected by the presence of catalase. In 
conclusion, these findings indicate that microglial proliferation in response to 
IL-1beta or TNF-alpha is mediated by hydrogen peroxide from NADPH oxidase.