Differential expression of endocannabinoid system-related genes in the dorsal hippocampus following expression and reinstatement of morphine conditioned place preference in mice

The endocannabinoid signaling plays a critical role in mediating rewarding effects to morphine. The relative stability for the expression and reinstatement of morphine conditioned place preference (CPP) suggests the involvement of differential neuroadaptations in learned associations between environmental cues and morphine. Changes in gene expression in hippocampus through the endogenous cannabinoid system (eCB) may accompany and mediate the development of such neuroadaptations to repeated morphine stimulation. To test this possibility, we systematically compared the expression of eCB-related genes in the dorsal hippocampus following the expression, extinction, and reinstatement of morphine CPP using quantitative RT-PCR analyses. We found that expression of morphine CPP was associated with significant increases in mRNA expression for the primary clearance routes for anandamide (AEA) and 2-AG (fatty acid amide hydrolase [FAAH] and monoacylglycerol lipase [MAGL], respectively), but with reductions in cannabinoid 1 receptors (CB1R) and CB2R in dorsal hippocampus following the expression of CPP. However, our results indicated that decreased in MAGL and increased CB1R mRNA levels were accompanied with morphine CPP reinstatement. No significant changes in mRNA expression for enzymes involved in AEA and 2-AG biosynthesis (N-acylphosphatidylethanolamine phospholipase D [NAPEPLD] and diacylglycerol lipase-α/β [DAGLα/β], respectively) were found in all conditions. These results suggest that differential regulation of the synthesis and/or degradation of the eCB system contribute to the expression and reinstatement of morphine CPP.