The μ-opioid receptor is the site of action of β-endorphin, morphine, and methadone (Kreek et al, 2005). A common variant in the OPRM1 gene, A118G, encodes an asparagine to aspartic acid substitution. This variant was associated with vulnerability to develop heroin addiction(Bart et al, 2004), and was shown to alter the function of the hypothalamic-pituitary-adrenal axis [reviewed in(Kreek, 2006; Kreek et al, 2005)].
