Progesterone receptors in the oviduct of a viviparous snake (Nerodia): correlations with ovarian function and plasma steroid levels.

The physiological significance of progesterone during gestation in viviparous reptiles is not understood, although there is strong evidence for luteal synthesis and secretion of this steroid. In a previous study we characterized an oviduct progesterone receptor in the viviparous snake, Nerodia, and in this study we describe changes in receptor during the reproductive cycle. Correlation of the amount of receptor with changes in plasma estradiol 17 beta and progesterone and the effect of injected hormones suggest that the receptor may be regulated by these steroids as in mammals. Thus, the basis exists for target organ responses to progesterone and provides additional evidence for the potential role of progesterone in the early hormonal adaptations in the evolution of viviparity.