BDNF and major depressive disorder: Consistent with the clinical findings for BDNF expression in depression, in rodents, different stress paradigms that can induce depressive-like behaviors, such as chronic stress, social defeat, or exposure to pre- and perinatal stress, also affect BDNF mRNA and protein levels in various brain regions, including the hippocampus and the prefrontal cortex (Krishnan et al., 2007; Yu and Chen, 2011; Stepanichev et al., 2014; Zaletel et al., 2017; Wei et al., 2018).