BDNF and major depressive disorder: It is well-known that hippocampal BDNF regulates hippocampal neurogenesis and synaptic plasticity, and depression is accompanied with not only abnormal behaviors but also dysfunction in these two physiological processes (Leal et al., 2014; Serafini et al., 2014; Liu and Nusslock, 2018; Berger et al., 2020; Tartt et al., 2022).