BDNF and major depressive disorder: Interestingly, these responses were not observed when CS exposure occurred in late postnatal life (near puberty in mice), suggesting that a critical period of susceptibility to CS exposure exists in the prenatal and early postnatal period of brain development in mice, which results in a downregulation in BDNF/TrkB signaling in the hippocampus and enhances depression-like behaviors later in life.