Differential regulation of MeCP2 and PP1 in passive or voluntary administration of cocaine or food

Cocaine exposure induces changes in the expression of numerous genes, in part through epigenetic modifications. We have initially shown that cocaine increases the expression of the chromatin remodeling protein methyl-CpG binding protein 2 (MeCP2) and characterized the protein phosphatase-1C beta (PP1C beta) gene, as repressed by passive i.p. cocaine injections through a Mecp2-mediated mechanism involving de novo DNA methylation. Both proteins being involved in learning and memory processes, we investigated whether voluntary cocaine administration would similarly affect their expression using an operant self-administration paradigm. Passive and voluntary i.v. cocaine intake was found to induce Mecp2 and to repress PP1C beta in the prefrontal cortex and the caudate putamen. This observation is consistent with the role of Mecp2 acting as a transcriptional repressor of PP1C beta and shows that passive intake was sufficient to alter their expression. Surprisingly, striking differences were observed under the same conditions in food-restricted rats tested for food pellet delivery. In the prefrontal cortex and throughout the striatum, both proteins were induced by food operant conditioning, but remained unaffected by passive food delivery. Although cocaine and food activate a common reward circuit, changes observed in the expression of other genes such as reelin and GAD67 provide new insights into molecular mechanisms differentiating neuroadaptations triggered by each reinforcer. The identification of hitherto unknown genes differentially regulated by drugs of abuse and a natural reinforcer should improve our understanding of how two rewarding stimuli differ in their ability to drive behavior.