LEP and major depressive disorder: Leptin deficiency during development also leads to altered cognitive performance and neurological disorders such as major depressive disorder (MDD), while leptin replacement alleviates these symptoms and improves cognitive development (Antonijevic et al., 1998; Kraus et al., 2002; Westling et al., 2004; Esel et al., 2005; Lu et al., 2006; Paz-Filho et al., 2008; Signore et al., 2008; Yamada et al., 2011; Guo et al., 2012).