OPRK1 and major depressive disorder: In support of this, buprenorphine, a partial MOR agonist/ KOR antagonist, has been shown to have antidepressant efficacy in MDD in observational studies (Emrich et al., 1982; Walsh et al., 1994, 1995; Bodkin et al., 1995; Cowan, 2003; Nyhuis et al., 2008; Karp et al., 2014) and to reduce depressive symptoms in heroin addicted patients who were depressed at intake (Kosten et al., 1990).