BDNF and major depressive disorder: In our previous studies, an imbalance in the expression of BDNF in the hippocampus or prefrontal cortex and its related adaptive change in synaptic plasticity, including decreased expression of Syt-1, synapsin-1, and PSD95 has been associated with impaired learning and memory or depression-like behaviors in animal models with depression [59], nonalcoholic fatty liver disease (NAFLD) [60] or AD [38].