BDNF and major depressive disorder: Noteworthy, there are research findings that gene polymorphisms involved in serotonin biosynthesis, transport or signal transduction, as well as neurotrophins factors such as brain-derived neurotrophic factor (BDNF), are associated with an increased risk of depression, which may lead to structure and function differences of the amygdala (Munafo et al., 2008; Roiser et al., 2009; Scharinger et al., 2010).