Mechanisms that determine the changes in pituitary MSH activity during pseudopregnancy induced by vaginal stimulation in the rat.

Abstract

Pituitary MSH content as measured by an in vitro bioassay was found to decrease 1 h after vaginal stimulation in rats at estrus. The values remained low up to the 6th day, but regained the pre-stimulation levels on the 10th day. The early decrease in pituitary MSH was not prevented if the animals were ovariectomized 2 h before the vaginal stimulation, but failed to occur in chronic ovariectomized rats MSH-release-inhibiting factor (MSH-R-IF) was found to increase in stalk median eminence (SME) extracts 2 h after vaginal stimulation in rats in the first day of diestrus, suggesting that the induction of pseudopregnancy (PSP) blocks the release of this agent. Animals ovariectomized 2 h after vaginal stimulation showed normal values of pituitary MSH two or six days later, indicating that the ovaries are necessary for maintaining the low content of the hormone during PSP. Administration of 2 mg progesterone daily to the spayed rats restored normal values. MSH-R-IF was absent in SME extracts from rats at day six of PSP or in ovariectomized animals treated with progesterone. It is concluded that there exist two stages in PSP induced by vaginal stimulation. During the first one (neurogenic phase), the stimulus promotes a low pituitary MSH content, probably by blocking the release of MSH-R-IF. During the second stage (hormone-neural phase), progesterone secreted by the ovaries assures continuance of the phenomenon, probably by blocking MSH-R-IF synthesis.