PDYN and major depressive disorder: Dynorphin, an endogenous member of the opioid neuropeptide family (Goldstein et al., 1979), is thought to mediate negative emotional states associated with stress, depression, and drug use withdrawal (Koob and Le Moal, 2008; Bruchas et al., 2010; Knoll and Carlezon, 2010; Hang et al., 2015).