IL6 and major depressive disorder: Several studies based on animals have found elevated concentrations of pro-inflammatory cytokines—IL-6, IL-1β, and TNF-α (interleukin-6, interleukin-1β, and tumor necrosis factor-α, respectively)—as well as anti-inflammatory cytokines in both the central and peripheral nervous systems in various rodents’ stress paradigms, namely the chronic mild stress (CMS) [23,24,25,26], social isolation [27,28], learned helplessness [29], olfactory bulbectomy (OB) [30], and social defeat [31,32] models of depression.