Increasing evidence has suggested an important role for environmental toxins 
such as pesticides in the pathogenesis of Parkinson's disease (PD). Chronic 
exposure to rotenone, a common herbicide, reproduces features of Parkinsonism in 
rats. Mechanistically, rotenone-induced dopaminergic neurodegeneration has been 
associated with both its inhibition of neuronal mitochondrial complex I and the 
enhancement of activated microglia. Our previous studies with NADPH oxidase 
inhibitors, diphenylene iodonium and apocynin, suggested that NADPH 
oxidase-derived superoxide might be a major factor in mediating the 
microglia-enhanced rotenone neurotoxicity. However, because of the relatively 
low specificity of these inhibitors, the exact source of superoxide induced by 
rotenone remains to be further determined. In this study, using primary 
mesencephalic cultures from NADPH oxidase--null (gp91phox-/-) or wild-type 
(gp91phox+/+) mice, we demonstrated a critical role for microglial NADPH oxidase 
in mediating microglia-enhanced rotenone neurotoxicity. In neuron--glia 
cultures, dopaminergic neurons from gp91phox-/- mice were more resistant to 
rotenone neurotoxicity than those from gp91phox+/+ mice. However, in 
neuron-enriched cultures, the neurotoxicity of rotenone was not different 
between the two types of mice. More importantly, the addition of microglia 
prepared from gp91phox+/+ mice but not from gp91phox-/- mice to neuron-enriched 
cultures markedly increased rotenone-induced degeneration of dopaminergic 
neurons. Furthermore, apocynin attenuated rotenone neurotoxicity only in the 
presence of microglia from gp91phox+/+ mice. These results indicated that the 
greatly enhanced neurotoxicity of rotenone was attributed to the release of 
NADPH oxidase-derived superoxide from activated microglia. This study also 
suggested that microglial NADPH oxidase may be a promising target for PD 
treatment.