Molecular regulation of the oxytocin receptor in peripheral organs

The oxytocin receptor belongs to the G-protein-coupled seven transmembrane receptor superfamily. Its main physiological role is regulating the contraction of uterine smooth muscle at parturition and the ejection of milk from the lactating breast. Oxytocin receptor expression is observed not only in the myometrium and mammary gland but also in the endometrium, decidua, ovary, testis, epididymis, vas deferens, thymus, heart and kidney, as well as in the brain. The expression profile shows a tissue-specific as well as a stage-specific pattern. The oxytocin receptor gene is a single-copy gene consisting of four exons and three introns, localized at 3p25-3p26(.)2 in the human chromosome. In transfection studies using a fusion construct containing the promoter region of the oxytocin receptor gene inserted in a reporter plasmid, neither proinflammatory cytokines nor oestrogen directly activate the gene. The nuclear fractions from up-regulated (term myometrium) and down-regulated (non-pregnant myometrium) tissues show differential patterns of protein binding to the 5'-flanking region, and a human homologue of chicken MafF has been cloned as a term-myometrium-specific oxytocin receptor modulator. The oxytocin receptor gene appears to be highly methylated. Methylation around intron 1 and in intron 3 might contribute to tissue-specific suppression of the gene. The oxytocin receptor is also regulated by desensitization, whose mechanism appears to involve loss of ligand-binding activity of the protein as well as suppression of the oxytocin receptor mRNA transcription. These findings taken together indicate that the oxytocin receptor is regulated in a very complicated manner, and the transcriptional regulatory elements critical for this regulation should be investigated further.