The neurotrophin BDNF has been shown to play an important role not only in the hippocampal synaptic plasticity, the neurobiological basis of learning and memory, but also in emotional performance, such as depression, both of which are mediated by synaptogenesis [36], neurogenesis [37,38], changes in morphologies of cells and dendrites [39], and a modulation of the efficacy of neurotransmitter release [40]. Here, BDNF is linked to major depressive disorder.