Peptide inhibitor of morphine- and beta-endorphin-induced analgesia.

Abstract
The synthetic .beta.-endorphin analogs with the omission of the NH2-terminal [Met]enkephalin segment [.beta.-endorphin-(6-31) and .beta.-endorphin-(20-31)] inhibit morphine or .beta.-endorphin-induced analgesia in mice by the tail-flick test, whereas the synthetic NH2-terminal pentadecapeptide .beta.-endorphin-(1-15) has no inhibitory activity. Endogenous inhibiting peptides may exist in the brain which play a role in the regulation of endorphin actions.