Peptide inhibitor of morphine- and β-endorphin-induced analgesia

The synthetic β-endorphine 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.