Furthermore, it has been reported that AASs reduce hippocampal levels of neurotrophic growth factors (Tirassa et al., 1997), including BDNF (Krishnan and Nestler, 2008), with a consequent decline in neurogenesis (Tirassa et al., 1997) and hippocampal volume, a phenomenon that might be involved in the pathogenesis of depression (Krishnan and Nestler, 2008). This evidence concerns the gene BDNF and major depressive disorder.