Effects of beta-casomorphin-5 on passive avoidance response in mice.

The effects of intracerebroventricular (i.c.v.) injection of bovine beta-casomorphin-5 (beta-CM-5: Tyr-Pro-Phe-Pro-Gly), a micro-opioid agonist derived from milk beta-casein, on step-down type passive avoidance tasks were investigated in mice. Intracerebroventricular administration of a high dose (10 microg) of beta-CM-5 produced a significant decrease in step-down latency. beta-Funaltrexamine (5 microg, i.c.v.) almost completely reversed the beta-CM-5-induced shortening of step-down latency, although neither naltrindole (5 ng, i.c.v.) nor nor-binaltorphimine (5 microg, i.c.v.) had any significant influence on the effect of beta-CM-5. Meanwhile, a low dose (0.5 microg, i.c.v.) of beta-CM-5 inhibited scopolamine (1 mg/kg)-induced impairment of passive avoidance response. These results indicated that a high dose of beta-CM-5 induces amnesia, whereas a low dose ameliorates scopolamine-induced amnesia.