OPRK1 and major depressive disorder: KOR antagonists are capable of overcoming the pro-depression and anxiogenic effects of chronic or acute stressors (Carr et al., 2010; Browne et al., 2018), and a number of KOR antagonists are already in clinical trials for the treatment of psychiatric disorders (Lowe et al., 2014; Buda et al., 2015; Chavkin and Martinez, 2015).