OPRM1 and major depressive disorder: Clinical reports have described the effectiveness of MOR agonists, including oxycodone, tramadol, oxymorphone, and buprenorphine, as well as β-endorphin, in patients suffering from depression (Darko et al., 1992; Bodkin et al., 1995; Stoll and Rueter, 1999; Shapira and DeGraw, 2001).