The activated microglia produce ROS. Hydroethidine (2 mg/kg), which can be converted to ethidium by ROS, was injected to the experimental animals intraperitoneally. After 30 min, brains were harvested and sections of SN were prepared for hydroethidine histochemistry to detect superoxide. (A) Representative ethidium fluorescence of the contralateral (Cont) and ipsilateral (Ipsi) SN at 7, 14, and 28 dpl following unilateral MFB transection. The number of ethidium fluorescent spots increases to peak level at 7 dpl, and then decreases gradually by 28 dpl. (B) Immunofluorescent images of TH, OX6, ethidium (ET), and their merged image in the ipsilateral SN at 14 dpl. Many OX6 immunoreactive microglia are co-localized with ethidium (arrows). Scale bars represent 50 μm.