Production of proinflammatory cytokines such as tumor necrosis factor (TNF)-α and interleukin (IL)-1β by peripheral immune cells and central nervous system (CNS)-resident microglia has been proposed as a biochemical link between pain and depression (Fasick et al., 2015, Fiore and Austin, 2016, Laumet et al., 2017, Walker et al., 2014, Zhou et al., 2015). This evidence concerns the gene TNF and major depressive disorder.