In the present study, effects of intracerebroventricular (i.c.v.) injections of μ-opioid receptor agonist and antagonist on lithium state-dependency were investigated. For memory assessment, a one-trial step-down inhibitory avoidance task was used in adult male NMRI mice. Intraperitoneal (i.p.) administration of lithium (10 mg/kg) after training impaired memory when retrieval was tested 24 h later. The memory impairment was reversed by pre-test administration of the same dose of lithium, suggesting state-dependency induced by lithium. In addition, i.c.v. administration of both lithium (2 and 4 μg/mouse, i.c.v.) and morphine (3 and 6 μg/mouse, i.c.v.) before the test reversed memory impairment induced by post-training lithium (10 mg/kg, i.p.). On the other hand, pre-test administration of naloxone (1 and 2 mg/kg) which had no effects alone on inhibitory avoidance response, prevented the improving effects of both morphine (3 μg/mouse, i.c.v.) and lithium (2 μg/mouse, i.c.v.) on memory retrieval. The results suggest that the μ-opioid receptors in the central nervous system may be involved in the retrieval of lithium state-dependent learning.
