LEP and major depressive disorder: In humans, an elevated leptin level has been associated with autism (Ashwood et al., 2008; Blardi et al., 2010; Raghavan et al., 2018) and Rett syndrome (Blardi et al., 2007, 2009), whereas a decrease in leptin is linked to schizophrenia and depression (Kraus et al., 2001; Atmaca et al., 2003).