The µ-opioid receptor (oprm1) is the principal site of action in the brain by which morphine, other opiate drugs of abuse, and endogenous opioid peptides effect analgesia and alter mood. Opioid receptors belong to the rhodopsin family of G protein-coupled receptors (GPCRs). The three types of opioid receptors (µ, d, and ?) have been shown to associate with each other in a homotypic or heterotypic fashion when expressed in heterologous cells. A member of the seven-transmembrane domain (™) G protein-coupled receptor (GPCR) superfamily, the µ-opioid receptor modulates ion channels and second messenger effectors in an opioid agonist-dependent fashion that is reversible by the classic opiate antagonist naloxone.