Peptide Inhibitor of Morphine- and β -Endorphin-Induced Analgesia

The synthetic β -endorphin analogs with the omission of the NH2-terminal [Met]enkephalin segment [β -endorphin-(6-31) and β -endorphin-(20-31)] are shown to inhibit morphine- or β -endorphin-induced analgesia in mice by the tail-flick test, whereas the synthetic NH2-terminal pentadecapeptide β -endorphin-(1-15) has no inhibitory activity. This study raises the possibility that endogenous inhibiting peptides exist in the brain which play a role in the regulation of endorphin actions.