Abstract

The effects of i.c.v. injection of the mu-selective opioid receptor agonist DAMGO and the effects of its combination with the endogenous kappa-opioid receptor agonist dynorphin A-(1-13) on memory processes were examined in mice, using spontaneous alternation performance associated with working memory in a Y-maze. DAMGO (10 and/or 30 ng) impaired spontaneous alternation performance and increased total arm entries, which are considered to reflect locomotor activity. beta-Funaltrexamine (5 micrograms, i.c.v.), a mu-selective opioid receptor antagonist, almost completely antagonized the impairment of alternation performance induced by DAMGO (10 ng). Physostigmine (0.1 mg/kg, i.p.), a cholinesterase inhibitor, improved the DAMGO (10 ng)-induced impairment of alternation performance. Dynorphin A-(1-13) (1, 3 and 10 micrograms, i.c.v.) alone was without significant effects on alternation performance. On the other hand, dynorphin A-(1-13) (3 and 10 micrograms) significantly improved the impairment of spontaneous alternation performance induced by DAMGO (10 ng). The effects of dynorphin A-(1-13) (3 micrograms) on the DAMGO-induced impairment of spontaneous alternation were almost completely reversed by pretreatment with nor-binaltorphimine (4 micrograms, i.c.v.), a kappa-selective opioid receptor antagonist. The present results demonstrate that DAMGO impairs alternation performance by activating mu-opioid receptors, whereas dynorphin A-(1-13) attenuates the DAMGO-induced impairment of alternation performance through the mediation of kappa-opioid receptors. These findings suggest that mu- and kappa-opioid systems are fully involved in memory function and have opposite effects on spontaneous alternation performance as it is reflected by working memory in mice.