Effects of endomorphins-1 and -2, endogenous mu-opioid receptor agonists, on spontaneous alternation performance in mice.

The effects of intracerebroventricular (i.c.v.) administration of endomorphins-1 and -2, endogenous mu-opioid receptor agonists, on the spontaneous alternation performance associated with spatial working memory were investigated in mice. Endomorphin-1 (10 and 17.5 microg) and endomorphin-2 (10 microg) produced a significant decrease in percent alternation without affecting total arm entries. beta-Funaltrexamine (5 microg) almost completely reversed the endomorphin-1 (10 microg)- and endomorphin-2 (10 microg)-induced decrease in percent alternation, although neither naltrindole (4 ng) nor nor-binaltorphimine (4 microg) produced any significant effects on alternation performance. These results suggest that endomorphins impair spatial working memory through the mediation of mu-opioid receptors.