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

Abstract

The potent opioid peptide .beta.-endorphin is found in the brain and pituitary with 2 related fragments, .beta.-endorphin-(1-27) and .beta.-endorphin-(1-26). The fragments retain substantial opioid-receptor binding activity but are virtually inactive analgesically. .beta.-Endorphin-(1-27) inhibits .beta.-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 .beta.-endorphin in the presence of .beta.-endorphin-(1-27). Its potency is 4-5 times greater than that of the opiate antagonist naloxone. .beta.-Endorphin-(1-26) does not antagonize the antinociceptive action of etorphine or .beta.-endorphin in doses up to 500 pmol/animal.