Endomorphins 1 and 2, endogenous mu-opioid receptor agonists, impair passive avoidance learning in mice

The effects of intracerebroventricular administration of endomorphin-1 and endomorphin-2, endogenous mu -opioid receptor agonists, on passive avoidanc e learning associated with long-term memory were investigated in mice. Endo morphin-1 (10 and 17.5 mug) and endomorphin-2 (17.5 mug) produced a signifi cant decrease in step-down latency in a passive avoidance learning task. be ta -Funaltrexamine (5 mug) almost completely reversed the endomorphin-1 (17 .5 mug)- and endomorphin-2 (17.5 mug)-induced shortening of step-down laten cy, although neither naltrindole (4 ng) nor nor-binaltorphimine (4 mug) pro duced any significant effects on the effects of endomorphins 1 and 2. These results suggest that endomorphins 1 and 2 impair long-term memory through the mediation of mu -opioid receptors in the brain. (C) 2001 Elsevier Scien ce B.V. All rights reserved.