In situ hybridizations for BDNF and trkB mRNA revealed that virtually all CSNs express trkB, whereas only half of them express BDNF. As revealed with 5-HT immunocytochemistry, BDNF infusions into the neocortex of intact (non-PCA-lesioned) rats caused a substantial increase in 5-HT axon density in a 3 mm diameter region surrounding the cannula tip.