OPRK1 and major depressive disorder: Kappa opioid receptors (KORs) have generated interest as a potential pharmacological target because KORs are activated by psychosocial stress (Knoll and Carlezon, 2010) and pharmacological activation of KORs induces aversion, dysphoria, and depression-like behavior in humans (Pfeiffer et al., 1986; Walsh et al., 2001) and animals (Todtenkopf et al., 2004; Carlezon et al., 2006; Land et al., 2008).