Effect of conceptus number, hysterectomy, and progesterone on prolactin surges in rats.

Mating induces two daily surges of prolactin (PRL) secretion in the rat. Termination of the diurnal and nocturnal surges coincides with the development of the placenta and the secretion of rat placental lactogen (RPL). The number of conceptuses present was altered to induce changes in RPL levels in the blood. The presence of one intact viable conceptus is sufficient to cause termination of the diurnal PRL surge. The presence of one or zero conceptuses allowed resumption of the nocturnal surge after day 14, whereas presence of two or more conceptuses prevented the surge. Hysterectomy on day 14 allowed the resumption of the nocturnal PRL surges for 2 days and diurnal PRL surges for 3 days. Progesterone treatment stimulated an additional third nocturnal PRL surge and blunted the magnitude of the resuming diurnal surges. It is concluded that secretions from the conceptus and progesterone contribute to the regulation of prolactin during pregnancy.