TLR4 and major depressive disorder: Brain-derived neurotrophic factor (BDNF)-tropomyosin receptor kinase B (TrkB) pathway, mitogen-activated protein kinase (MAPK) pathway, glycogen synthase kinase-3 (GSK-3) pathway, toll-like receptor 4 (TLR4) pathway, etc. are involved in the pathological process of depression.