Figure 2.

Different hypotheses for depression converge together and are interconnected. As seen in the figure, microglia can cause depression through different molecular pathways. These mechanistic pathways are sometimes interrelated making the whole mechanistic link between microglial action and depression complex. Red boxes indicate “neuroinflammatory pathway,” gray boxes indicate “altered neurotrophin levels pathway,” and “impaired hippocampal neurogenesis pathway,” blue boxes indicate “altered brain tryptophan metabolism pathway,” green boxes indicate “stimulation of peripheral immune system pathway,” brown boxes indicate “psychological/chronic stress and reduced immunity pathway,” and purple boxes indicate “Inflammasome pathway.” Black boxes are part of more than one pathways. IFN, interferon; TNF, tumor necrosis factor; IL, interleukin; P2X7, two-transmembrane ATP-gated ionotropic purinoreceptor; CVD, cardiovascular disease; LPS, lipopolysaccharide; PAMPs, pathogen-associated molecular patterns; DAMPs, damage-associated molecular patterns; CNS, central nervous system; TBI, traumatic brain injury; IDO, indoleamine 2, 3-dioxygenase enzyme; KP, kynurenine pathway.