Receptor activation is achieved through binding of endogenous or exogenous ligands. β-endorphin, the peptide encoded by proopiomelanocortin (POMC), has high affinity and selectivity for the MOPR and is considered the endogenous MOPR ligand. In addition, a separate class of peptides has been proposed as μ-selective: the endomorphins (Zadina et al., 1997). The preproenkephalin gene encodes enkephalin, the endogenous ligands for the δ receptor, which has modest affinity for the MOPR as well (Raynor et al., 1994). A large number of opioids and non-opioid ligands exist for this receptor; however, those most commonly prescribed for their effective analgesic properties include morphine, codeine and oxycodone.
