Components of the rat conceptus that account for prolactin inhibition

Abstract. The secretions from developed concepti in the rat appear to inhibit both the rhythmic nocturnal secretion of prolactin (Prl) during the second half of pregnancy and the neurally mediated Prl release in response to suckling stimulation. In order to identify the parts of the concepti capable of inhibiting Prl secretion, the components of the concepti were systematically removed and their effect on plasma Prl levels was examined. In the first experiment, female rats which were simultaneously pregnant and lactating were tested for Prl secretion in response to suckling stimulation after one of the following treatments: 1) removal of the foetuses only, 2) removal of the foetuses and trophoblasts, 3) removal of the complete concepti. In the second experiment, pregnant rats were tested for the presence of nocturnal Prl surges after either removal of the foetuses and trophoblasts or removal of the entire concepti. The results indicate that after removal of the foetuses alone, the inhibition of Prl secretion remains. Following removal of both the foetus and the trophoblast, leaving the decidua in situ with a supplementary progesterone implant, the decidua still retain a partial capacity to inhibit Prl secretion in response to suckling stimulation, while their inhibitory effect on the nocturnal Prl surges was very minute. Degeneration of the decidua occurred following the removal of the trophoblast, when no exogenous progesterone was supplied. The deterioration of the decidua or total removal of the concepti eliminates the entire inhibition of Prl secretion, both on the nocturnal surges as well as in response to suckling stimulation. Thus, it appears that after mid-pregnancy the trophoblast cell secretes a substance that both inhibits Prl secretion and maintains the corpus luteum, whereas the decidua at the same time secretes a substance that has a partial capacity to inhibit Prl secretion in response to suckling stimulation.