OPRM1 and major depressive disorder: Several subsequent studies have evaluated the antidepressant effects of the MOR partial agonist, KOR antagonist, buprenorphine in MDD (Emrich et al., 1982, Mongan and Callaway, 1990, Bodkin et al., 1995, Nyhuis et al., 2008, Karp et al., 2014).