Interactive effects of morphine and nicotine on memory function depend on the central amygdala cannabinoid CB1 receptor function in rats

The present study investigated the possible involvement of the central amygdala (CeA) cannabinoid receptors type-1 (CB1Rs) in the interactive effects of morphine and nicotine on memory formation in a passive avoidance learning task. Our results showed that systemic administration of morphine (3 and 6 mg/kg, s.c.) immediately after training phase impaired memory consolidation and induced amnesia. Administration of nicotine (0.3 and 0.6 mg/kg, s.c.) before testing phase significantly restored morphine-induced amnesia, suggesting a cross state-dependent learning between morphine and nicotine. The results showed that while the administration of the lower dose of nicotine (0.1 mg/kg, s.c.) per se did not induce a significant effect on morphine-induced amnesia, intra-CeA injection of arachidonylcyclopropylamide (ACPA), a cannabinoid CB1 receptor agonist (3 and 4 ng/rat), significantly potentiated the nicotine response. Furthermore, the blockade of the CeA cannabinoid CB1 receptors by the injection of AM251 (0.75 and 1 ng/rat) reversed the potentiative effect of nicotine (0.6 mg/kg, s.c.) on morphine-induced amnesia. It should be considered that bilateral injection of the same doses of ACPA or AM251 (0.5-1 ng/rat) into the CeA by itself had no effect on morphine response in a passive avoidance learning task. Confirmed by the cubic interpolation planes, the dose-response data revealed a cross-state-dependent learning between morphine and nicotine which may be mediated by the CeA endocannabinoid system via CB1 receptors.