BDNF and major depressive disorder: In the hippocampus of the animal model of depression, LG upregulated the concentrations of 5-hydroxytryptamine (serotonin; 5-HT) and norepinephrine (NE) and acted on the PI3K/Akt/mTOR-mediated BDNF/TrkB pathway in the hippocampus [151,169].