Effect of des-Tyr1-gamma-endorphin and des-enkephalin-gamma-endorphin on active and passive avoidance behavior of rats; a dose-response relationship study.

The potency of two beta-endorphin fragments, des-Tyr1-gamma-endorphin (DT gamma E, beta E-(2-17) and des-enkephalin-gamma-endorphin (DE gamma E-(6-17) was compared on extinction of pole-jumping avoidance behavior and on retention of a one-trial step-through passive avoidance procedure. Both peptides facilitated the extinction of pole-jumping avoidance behavior and attenuated passive avoidance behavior. The gamma-type endorphin exhibited an inverted U-shaped dose-response curve on passive avoidance behavior but not on extinction of pole-jumping avoidance behavior. DE gamma E appeared to be approximately three times more potent than DT gamma E on extinction of pole-jumping avoidance behavior but one hundred times more potent on passive avoidance behavior. It is suggested that DE gamma E rather than DT gamma E represents the endogenous neuroleptic-like neuropeptide derived from beta-endorphin.