beta-Endorphin-induced analgesia is inhibited by synthetic analogs of beta-endorphin.

Abstract
Competitive antagonism of human .beta.-endorphin (.beta.h-EP)-induced analgesia by synthetic .beta.h-EP analogs with high in vitro opiate receptor binding to in vivo analgesic potency ratio has been demonstrated. A parallel shift of the dose-response curve for analgesia [in rats] to the right was observed when either .beta.h-EP or [Trp27]-.beta.h-EP was coinjected with various doses of [Gln8,Gly31]-.beta.h-EP-Gly-Gly-NH2, [Arg9,19,24,28,29]-.beta.h-EP or [Cys11,26,Phe27,Gly31]-.beta.h-EP. It was estimated that the most potent antagonist, [Gln8, Gly31]-.beta.h-EP-Gly-Gly-NH2, is at least 200 times more potent than naloxone.