The selective deletion of BDNF in the hippocampus does not induce an increase in depression-like behavior per se but attenuates the responses of the antidepressant (Adachi et al., 2008), suggesting that the downregulated endogenous BDNF-TrkB signaling in the hippocampus of CFSS rats may not contribute to CFSS-induced anxiety/depression-like behaviors but to the susceptibility to mood disorders (Adachi et al., 2008). This evidence concerns the gene BDNF and major depressive disorder.