Circadian expression of Mel1a and PL-II genes in placenta: effects of melatonin on the PL-II gene expression in the rat placenta.

In the mammal, melatonin regulates the seasonal and/or circadian rhythm of PRL levels. Since several members of the PRL gene family are expressed during late pregnancy, we investigated the relationship between the expression of placental lactogen (PL)-II-one member of the PRL family- and melatonin, as well as the placental expression of one of the receptors for melatonin, melatonin receptor 1a (Mel(1a())). Herein we provide the first demonstration that Mel(1a) is not only expressed in the rat placenta, but that it is spatially and temporally regulated throughout late pregnancy. In situ hybridization and Northern blot analyses show that Mel(1a) mRNA is localized in the rat placenta on gestational day 19, and is mainly restricted to the spongiotrophoblast and trophoblast giant cells. Interestingly, the junctional zone of the placenta at this time showed the strongest gene expression when the tissue was obtained at 16:00 h (daytime) and showed the least expression when it was obtained at 04:00 h (night-time). In contrast, the labyrinth zone showed the strongest expression in tissue obtained at night and showed the least expression in tissue obtained during the day. PL-II gene expression also exhibited a circadian rhythm but the direction of the fluctuation was exactly opposite to that of the Mel(1a) gene, such that at night the junctional zone had the strongest expression, while the labyrinth zone had the weakest. In vitro treatment of placental tissue with an melatonin agonist, chloromelatonin, greatly decreased PL-II mRNA levels. That Mel(1a) plays a regulatory role in the expression of PL-II in the late-pregnancy rat placenta is strongly suggested by the pattern of its own spatial and temporal expression.