EŠects of b-Casomorphin-5 on Passive Avoidance Response in Mice

The eŠects of intracerebroventricular (i.c.v.) injection of bovine b-casomorphin-5 (b-CM-5: Tyr-Pro-PhePro-Gly), a m-opioid agonist derived from milk bcasein, on step-down type passive avoidance tasks were investigated in mice. Intracerebroventricular administration of a high dose (10 mg) of b-CM-5 produced a signiˆcant decrease in step-down latency. b-Funaltrexamine (5 mg, i.c.v.) almost completely reversed the b-CM5-induced shortening of step-down latency, although neither naltrindole (5 ng, i.c.v.) nor nor-binaltorphimine (5 mg, i.c.v.) had any signiˆcant in‰uence on the eŠect of b-CM-5. Meanwhile, a low dose (0.5 mg, i.c.v.) of b-CM-5 inhibited scopolamine (1 mgWkg)-induced impairment of passive avoidance response. These results indicated that a high dose of b-CM-5 induces amnesia, whereas a low dose ameliorates scopolamine-induced amnesia.