BDNF and major depressive disorder: The depression ameliorating effects of BoNT/A was associated with increased expression levels of 5‐hydroxytryptamine and brain‐derived neurotrophic factor (BDNF) in mice brain and BDNF/extracellular signal regulated kinase(ERK)/cyclic AMP response element (CRE)‐binding protein (CREB) pathways (Li et al., 2019).