Fig. 4. The dose–response profile for BDNF (0.4–36 μg/d) to stimulate 5-HT axon sprouting in cortex (protocol, Fig.1D, 2 week BDNF infusions were started 4 d after PCA). A, C, E, G, 5-HT-immunoreactive axons in cortex (dark-field photomicrographs). B, D, F, H, BDNF diffusion as determined by BDNF immunocytochemistry in adjacent sections (bright-field photomicrographs). A, B, BDNF, 0.4 μg/d; C, D, BDNF, 1 μg/d; E, F,BDNF, 4 μg/d; G, H, 36 μg/d of BDNF. The effects of vehicle infusion and the 12 μg/d dose of BDNF on 5-HT axons (using this same treatment protocol) are shown in Figure 2, Fand E, respectively. Scale bar, 1 mm.