• MOR knockout mice had disrupted BPN-induced reduction in latency in the NIH test (m) [175]• Mouse model of the OPRM1 A118G polymorphism had disrupted BPN-induced reduction in latency in NIH test (m) [176]• OPRM1 A118G polymorphism associated with suicidal ideation (h) [177] and more severe depression (h) [178]. Here, OPRM1 is linked to major depressive disorder.