mt1 receptor-mediated antiproliferative effects of melatonin on the rat uterine antimesometrial stromal cells

It has been shown that melatonin regulates uterine function. Our previous studies have demonstrated the presence of melatonin receptors in the rat uterine endometrium, indicating that melatonin may act directly on the uterus. In the present study, the histological localization of the rat uterine melatonin binding was revealed by autoradiography and the molecular subtyping was studied by in situ hybridization in the stromal cells. The signal transduction process and effects of melatonin on stromal cell proliferation was also investigated. Our autoradiograms showed that 2[125I]iodomelatonin binding sites were localized in the antimesometrial endometrial stroma. In situ hybridization with specific mt1 receptor cDNA probe in the primary culture of antimesometrial stromal cells demonstrated the expression of mt1 receptor mRNAs. Melatonin dose-dependently inhibited forskolin-stimulated cAMP accumulation, which was reversed by pertussis toxin. This indicates that the rat uterine melatonin receptors are negatively coupled to adenylate cyclase via pertussis toxin sensitive Gi protein. Melatonin also inhibited the incorporation of [3H]thymidine in the rat uterine antimesometrial stromal cells, showing that melatonin has an anti-proliferative effect on the uterus. Our results suggest that melatonin may act directly on the mt1 melatonin receptors in the rat uterine antimesometrial stromal cells to inhibit their proliferation. Its action may be mediated through a pertussis toxin-sensitive adenylate cyclase coupled Gi-protein. Mol. Reprod. Dev. 61: 192–199, 2002. © 2002 Wiley-Liss, Inc.