TNF and major depressive disorder: Exposure to LPS significantly increased the expression of proinflammatory cytokines IL-1β, IL-6, TNF-α, and inflammatory mediators including inducible- and neuronal nitric oxide synthases (iNOS and nNOS), cyclooxygenase-2 (COX-2), and COX-2 mRNA expression via activation of nuclear factor kappa B (NF-κB) in the hippocampus of an animal model of depression (17).