Internalization of the pro-BDNF–p75^NTR complex in cultured astrocytes. (A) Time sequence of pro-BDNF–QD–p75-GFP internalization in cultured astrocytes by TIRF imaging. White arrowheads indicate pro-BDNF–QDs (red) in close proximity to the membrane of an astrocyte transfected with p75-GFP (green). Yellow arrowheads point to reference QDs. Insets depict p75-GFP fluorescence that concentrates at the site of the QD. Bar, 5 μm. (B) Pro-BDNF–QD internalization in astrocytes transfected with p75-GFP (nine cells) or Lck-GFP (six cells). (C) Pro-BDNF–QD internalization in astrocytes from p75^NTR+/+ (22 cells) and p75^NTR−/− (11 cells) mice. (D) Immunocytochemistry showing colocalization (arrowheads) between QDs (blue) and p75^NTR (red) in astrocytes transfected with p75-GFP or Lck-GFP (green). Bar, 2 μm. Data are means ± SEM (error bars). *, P ≤ 0.05.