Functional interaction between morphine and central amygdala cannabinoid CB1 receptors in the acquisition and expression of conditioned place preference

The present study was done to determine whether cannabinoid CB1 receptors of the central amygdala (CeA) are implicated in morphine-induced place preference. Using a 3-day schedule of conditioning, it was found that subcutaneous (s.c.) administration of morphine (2, 4 and 6 mg/kg) caused a significant dose-dependent conditioned place preference (CPP) in male Wistar rats. Intra-CeA microinjection of the cannabinoid CB1 receptor agonist arachidonylcyclopropylamide (ACPA; 0.5, 2.5 and 5 ng/rat) dose-dependently potentiated the morphine (2 mg/kg)-induced CPP. Furthermore, the administration of ACPA (5 ng/rat, intra-CeA) alone induced a significant CPP. It should be considered that the higher dose of ACPA (5 ng/rat, intra-CeA) in combination with morphine decreased locomotor activity on the testing phase. On the other hand, intra-CeA microinjection of the cannabinoid CB1 receptor antagonist AM251 (120 ng/rat) alone induced a significant conditioned place aversion (CPA). Moreover, intra-CeA microinjection of AM251 (90 and 120 ng/rat) inhibited the morphine-induced place preference with a significant interaction. Intra-CeA microinjection of AM251 reversed the effect of ACPA on morphine response. Interestingly, microinjection of ACPA (2.5 and 5 ng/rat) or AM251 (60–120 ng/rat) into the CeA increased or decreased the expression of morphine (6 mg/kg)-induced place preference respectively. These observations provide evidence that cannabinoid CB1 receptors of the CeA are involved in mediating reward and these receptors are also implicated in the acquisition and expression of morphine-induced CPP.

Research highlights
► Morphine caused a conditioned place preference.
► Activation of the central amygdala CB1 receptors potentiated the morphine effect.
► Inhibition of the central amygdala CB1 receptors decreased the morphine response.
► Influence of CB1 receptor agonist on morphine response reversed by its antagonist.
► Activation or inhibition of the receptors altered the expression of morphine effect.