Bioscientifica Proceedings

During luteal regression episodic pulses of oxytocin secretion become coupled to the release of prostaglandin F_2α (PGF_2α) following synthesis of endometrial oxytocin receptors, but in early pregnancy the inhibition of oxytocin receptor formation by the conceptus prevents the development of the pulsatile pattern of PGF_2α release needed to achieve luteolysis. Oxytocin receptors are present on the luminal epithelium in ovariectomized and anoestrous ewes, in pregnant animals throughout most of gestation (day 21 to term) and in explants of endometrial tissue cultured in vitro. These receptors can be downregulated for a brief period by progesterone (10–12 days in sheep, 12–14 days in cattle). This period of inhibition can be extended by infusion of interferon τ (IFN-τ) (which probably inhibits oxytocin receptor gene transcription) or of oxytocin into the systemic circulation (which may act further downstream, possibly at the level of translation). Oxytocin receptors also develop on the caruncular stroma and deep glands at oestrus, but these need positive upregulation and appear dependent on an oestrogenic environment. Only epithelial receptors are needed to achieve a maximal PGF_2α response to an oxytocin challenge, but the presence of oxytocin receptors does not necessarily confer responsiveness as other factors may influence intracellular coupling mechanisms and precursor availability. The duration of the luteal phase is regulated by the time of the initial post-ovulatory rise in progesterone and the duration of exposure to progesterone. However, the development of oxytocin receptors in the luminal epithelium is not directly preceded by changes in the concentration of either progesterone or its receptor, implying that an intermediary time-dependent mechanism mediates the inhibitory effect of progesterone on oxytocin receptor formation.

© 1995 Journals of Reproduction and Fertility Ltd