TNF and major depressive disorder: While the molecular mechanism of TNF-α in regulating depression is unclear, researchers speculated that TNF-α might stimulate the hypothalamic-pituitary-adrenal (HPA) axis, interact with neurotransmitters (5-hydroxytryptamine, dopamine, norepinephrine and glutamate), and modulate neuroplasticity and neuroendocrine function, leading to the development of DepS [52].