Biology of Opioid Peptides

Abstract

Opioid peptides are endogenous or synthetic peptides, with a spectrum of pharmacological activity similar to that of morphine and other narcotic agonist drugs. The designation opioid peptide can be made if (a) it produces morphine-like, naloxone reversible effects in several in vitro bioassay sys­ tems such as the guinea pig ileum (1), the cat nictitating membrane (2), the mouse vas deferens (3), and the neuroblastoma X glioma tissue culture preparation (4), and (b) if the peptide competes at low concentrations with opiate ligands for binding at opiate receptor binding sites (5-7). An addi­ tional, although not essential, criterion might include the production of naloxone reversible analgesia. These criteria exclude substance P which does have analgesic activity (8, 9), fragments of adrenocorticotropic hor­ mone which have very weak opiate receptor binding activity (10), and the morphine-like factor identified on the basis of morphine antibody recog­ nition (11). At present only three endogenous opiate peptides have been identified that have any claim to a physiological function. These are leucine5-enkepha­ lin (12), methionine5-enkephalin (12), and ,a-endorphin (13-16). Other frag­ ments of ,B-lipotropin such as LPH 61-69, LPH 61-76, LPH 61-77, and LPH 61-87 have opioid peptide activity (17-19) but current evidence sug­ gests that these may be artifacts of extraction or degradation products of ,B-endorphin (20). Other unidentified opioid peptides undoubtedly exist (21-24), but in the absence of structural and chemical identification it is impossible to assess their significance or biological function. The term endorphin as a generic name for opioid peptides has little to recommend