β-endorphin-(1-27) is a naturally occurring antagonist to etorphine-induced analgesia

The potent opioid peptide β-endorphin is found in the brain and pituitary with two related fragments, β-endorphin-(1-27) and β-endorphin-(1-26). The fragments, retain substantial opioid-receptor binding activity but are virtually inactive analgesically. β-Endorphin-(1-27) inhibits β-endorphin-induced and etorphine-induced analgesia when coinjected intracerebroventricularly into mice. Antagonism by competition at the same site(s) is suggested from parallel shifts of the dose-response curves of etorphine or β-endorphin in the presence of β-endorphin-(1-27). Its potency is 4-5 times greater than that of the opiate antagonist naloxone. β-Endorphin-(1-26) does not antagonize the antinociceptive action of etorphine or β-endorphin in doses up to 500 pmol per animal