TLR4 and major depressive disorder: If the epithelial barrier is circumvented, bacteria [or molecules from them, such as lipopolysaccharides (LPS)] penetrating the intestinal mucosa may bind to their cognate TLRs (in particular TLR-4), driving NF-κ β activation, stimulating the expression of pro-inflammatory cytokines, including IL-1 and TNF-α, and facilitating the synthesis of arachidonic-acid derivates (Feng et al., 1995; Verstrepen et al., 2008), all of which are implicated in MDD.