OPRM1 and mood disorder: Although the essential role of MOR in pain, drug abuse, and mood disorders is well established, and receptor adaptations to chronic opioid use have been well studied at the cellular level (Williams et al., 2013; Cahill et al., 2016), circuit mechanisms underlying MOR function (Darcq and Kieffer, 2018) and the regulation of neuronal communication driven by MOR (Mechling et al., 2016) and MOR agonists (Nasseef et al., 2019) are poorly understood.