NPY and major depressive disorder: Individuals vary in their expression of NPY, and low levels of NPY have been associated with the development of anxiety (Reichmann and Holzer, 2016), depression (Widdowson et al., 1992; Heilig et al., 2004; Mickey et al., 2011), and posttraumatic stress disorder (PTSD) (Yehuda et al., 2006).