To identify OPs in situ, 15-Km coronal brain sections were immunostained for NG2 and PDGF>R (12, 19) . Primary antibodies used were rabbit polyclonal antiNG2 antibody (gift from Dr. William Stallcup, La Jolla, CA) and rat monoclonal antiPDGF>R antibody (APA5; Pharmingen, San Diego, CA). Donkey antirabbit IgG F(ab') 2 conjugated with Cy3 (Jackson Immunoresearch, West Grove, PA) was used to detect NG2, whereas the PDGF>R was detected with biotinylated donkey antirat IgG F(ab') 2 (Jackson Immunoresearch) followed by coumarin tyramide amplification (New England Nuclear, Boston, MA).