Regardless of the mechanisms responsible for the observed changes, since microglia are the main cells expressing pro-inflammatory cytokines in the brain (Hanisch, 2002; Hinkerohe et al., 2005) and the expression of Iba1 is enhanced in both brain areas of adult rats, microglia are a good candidate for the first cells contributing to an abnormal brain neuronal-immune dialog leading to the development of the behavioral disturbances that are observed in depression. This evidence concerns the gene AIF1 and major depressive disorder.