Despite previous studies in knock-in mouse models and heterologous expression systems, the precise molecular and cellular consequences of MOR N40D have remained unclear, primarily due to species-specific and context-specific mechanisms in the modulation of MOR signaling. For instance, overexpression systems have suggested that the D40 allele confers a “gain-of-function” effect by causing an increased potency for DAMGO and other MOR agonists 9, 13, 14, 37. Similarly, human studies (heterologous expression systems) and rodent models suggest that the D40 allele carriers exhibit altered reward to nicotine 38, alcohol 39–48, and heroin 49. However, subsequent studies have reported that the D40 allele is associated with reduced mRNA and protein expression in multiple brain regions of knock-in mice 12, 15 along with reduced antinociceptive responses to morphine 10, providing support for a “loss-of-function” phenotype. These contradictory results in the literature strongly suggest not only species-specific but also context-specific mechanisms in the modulation of MOR signaling. For example, overexpression studies were primarily done in cell models in order to elucidate differences in signaling pathways relevant to the human condition. However, it is also possible
