Previously, we identified a minimum core promoter of the mouse ␦-opioid receptor (DOR) gene. The DOR promoter contains an E-box that binds upstream stimulatory factor and is crucial for the DOR promoter activity in NS20Y cells, a mouse neuronal cell line that constitutively expresses DOR. In the present study, we further analyzed the DOR promoter in NS20Y cells and have demonstrated that transcription factor Ets-1 binds to an Ets-1-binding site overlapping the E-box and trans-activates the DOR promoter by synergizing with upstream stimulatory factor in specific DNA binding. In addition, the Ets-1 DNA-binding domain is sufficient to play the functional role of Ets-1 in trans-activating the DOR promoter. Furthermore, through in vivo cross-linking assays and Northern blot analyses, we have demonstrated that Ets-1 binds to the DOR promoter in the neonatal mouse brain and that overexpressed Ets-1 can significantly enhance the expression of DOR mRNA in primary neonatal mouse neuronal cells. Collectively, our data suggest that Ets-1 functions as a trans-activator of the DOR promoter in the neonatal mouse brain and thus may contribute to the development of the mouse brain DOR system.Opioids are the preferred clinical analgesics for severe pain. Three major types of opioid receptors (ORs), 1 , ␦, and , have been cloned and shown to belong to the G protein-coupled receptor superfamily (1). Although all three ORs mediate opioid-induced analgesia, each receptor type exhibits a distinct pharmacological profile as well as a unique pattern of temporal and spatial expression (2-3).The localization of the ␦-opioid receptor (DOR) generally matches the pharmacological action sites of ␦-opioids (2). In addition, it has been reported that the expression levels of DOR can determine the DOR agonist's activities (4). Thus, understanding the molecular mechanism underlying the spatiotemporal expression of DOR may raise the possibility of maximizing the pharmacological benefits of ␦-opioids by manipulation of the DOR expression levels.DOR is mainly confined to the central nervous system. There is a strong correlation between the spatiotemporal presence of DOR mRNA and ␦-opioid-binding sites (5). In addition, levels of DOR mRNA as well as ␦-opioid-binding sites can be regulated by various agents in some neuronal cell lines. For example, nerve growth factor (6), ethanol (7), or retinoic acid (8) can up-regulate DOR mRNA and ␦-opioid-binding sites. On the other hand, activation of the protein kinase A pathway by forskolin or cyclic AMP analogues results in down-regulation of DOR mRNA and ␦-opioid-binding sites (9). All of these studies suggest that the expression of DOR is subjected to several regulation mechanisms at the transcriptional level. Therefore, study of the transcriptional regulation of DOR gene will provide insights into the spatiotemporal expression of DOR.Previously, we identified a minimum core promoter of the mouse DOR gene. We found that an E box and a GC box in the DOR promoter are crucial for the DOR promoter activity in N...