BDNF and major depressive disorder: The levels of pro-inflammatory factors (IL-1β, IL-6, and TNF-α) and factors involved in BDNF-TrkB signaling (mBDNF, proBDNF, TrkB, and tPA) in the peripheral blood of MDD and healthy controls were also examined to investigate the function of inflammation and BDNF signaling in MDD patients.