BDNF and major depressive disorder: Secondly, many studies had shown that the mechanism of KXS in the treatment of depression was to improve the activity of neurotransmitters such as 5-HT [26] and acetylcholine [27], repair the secretion decline of neurotrophic factors such as BDNF [28] and NGF [29] or their related signaling pathway damage, reduce the level of central nervous inflammatory factors such as inhibiting the increase of IL-1β, IL-6, and TNF-α [30], and increase the activity of superoxide dismutase to reduce lipid peroxidation, protecting hippocampal nerve cells.