LEP and major depressive disorder: Leptin can also increase the expression of BDNF mRNA, activate BDNF-expressing neurons (Komori et al., 2006; Liao et al., 2012), activate the PI3K-AKT-mTOR pathway to regulate the growth of neurons and regulate stress-induced depression and antidepressant response (Fazolini et al., 2015; Gui et al., 2018) while reversing the dysfunction in the HPA axis.