The pharmacological effect of morphine as a painkiller is mediated mainly via the mu opioid receptor (MOR) and is dependent on the number of MORs in the cell surface membrane. While several studies have reported that the MOR gene is regulated by various cis-and trans-acting factors, many questions remain unanswered regarding in vivo regulation. The present study shows that epigenetic silencing and activation of the MOR gene are achieved through coordinated regulation at both the histone and DNA levels. In P19 mouse embryonal carcinoma cells, expression of the MOR was greatly increased after neuronal differentiation. MOR expression could also be induced by a demethylating agent (5-aza-2-deoxycytidine) or histone deacetylase inhibitors in the P19 cells, suggesting involvement of DNA methylation and histone deacetylation for MOR gene silencing. Analysis of CpG DNA methylation revealed that the proximal promoter region was unmethylated in differentiated cells compared to its hypermethylation in undifferentiated cells. In contrast, the methylation of other regions was not changed in either cell type. Similar methylation patterns were observed in the mouse brain. In vitro methylation of the MOR promoters suppressed promoter activity in the reporter assay. Upon differentiation, the in vivo interaction of MeCP2 was reduced in the MOR promoter region, coincident with histone modifications that are relevant to active transcription. When MeCP2 was disrupted using MeCP2 small interfering RNA, the endogenous MOR gene was increased. These data suggest that DNA methylation is closely linked to the MeCP2-mediated chromatin structure of the MOR gene. Here, we propose that an epigenetic mechanism consisting of DNA methylation and chromatin modification underlies the cell stage-specific mechanism of MOR gene expression.Opioids exert their pharmacological and physiological effects through binding to their endogenous receptors. Three types of opioid receptors, mu (), delta (␦), and kappa (), all belonging to the G-protein-coupled receptor superfamily, have been cloned. Upon agonist binding, these receptors couple to G proteins and affect several signal transduction pathways thought to mediate a broad range of functions and pharmacological effects of endogenous and exogenous opioids (51). Previous studies suggested that the opioid receptor (MOR) plays a key role in mediating the major clinical effects of analgesics, such as morphine, as well as the development of tolerance and physical dependence upon prolonged administration (39). MOR is mainly expressed in the central nervous system, with densities varying greatly in different regions, which can display different functional roles (55). During mouse embryonic development, the MOR message was specifically observed as early as embryonic day 8.5 (E8.5) using the reverse transcription (RT)-PCR method (44). In contrast, MOR transcripts were detected only beginning at E12 using the radioligand binding method (70) and at E10.5 by in situ hybridization (85). Transcript levels gradually incr...