p75^NTR–clathrin-mediated internalization of pro-BDNF in astrocytes. (A) Colocalization between GFAP, pro-BDNF and p75^NTR, clathrin, or EEA1 immunoreactivity in astrocytes 10 min after slice exposure to TBS. Colocalization signal is shown at the site of astrocytic contact with a neuron (box and inset 1) and astrocytic processes (box and inset 2). Bar, 10 μm. (B) Pro-BDNF/GFAP colocalization in astrocytes of control (five slices and 22 cells) or TBS slices (five slices and 18 cells) in p75^NTR+/+ and p75^NTR−/− mice. (C) Pro-BDNF/GFAP colocalization in astrocytes of control (six slices and nine cells) or TBS slices in the absence (four slices and 12 cells) or presence of K252a (four slices and nine cells), MDC (three slices and 11 cells), or D15 (three slices and 11 cells). (D) Western blot showing surface expression of p75^NTR, TrkB, or TrkB-t from control astrocytes or astrocytes exposed to BDNF (mix). TrkB and TrkB-t expression from cultured neurons is shown for comparison. The right panel shows ELISA measurement of BDNF concentration in astrocytes. Data are means ± SEM (error bars). *, P ≤ 0.05.