Rat placental lactogen-I abolishes nocturnal prolactin surges in the pregnant rat

The twice-daily surges of prolactin (PRL) present during the first half of pregnancy abruptly terminate at midpregnancy concurrent with the appearance of high levels of placental lactogen-I (PL-I) in the blood. This study addressed the role PL-I and other pituitary or placental hormones have in terminating PRL surges in pregnant rats. Implantation of rat PL-I (rPL-I) or ovine PRL into the arcuate-median eminence area of the hypothalamus of day 7 pregnant rats totally eliminated nocturnal PRL surges on days 8 and 9. To assess the specificity of the inhibitory effects of hormones from the PRL-growth hormone (GH) family, rat growth hormone (rGH), human growth hormone (hGH), and rat prolactin-like protein-A (PLP-A) were tested. Only the lactogenic hormone, hGH, had any effect. Since lactogenic hormones may inhibit PRL by stimulation of dopamine synthesis and release into the hypophysial portal blood vessels leading to the anterior pituitary, the effect of these hormones on tyrosine hydroxylase (TH), the rate-limiting enzyme for the synthesis of dopamine activity, was determined. In pregnant rats, both ovine prolactin (oPRL) and hGH significantly increased (64%) TH activity, whereas rPL-I was less effective. In ovariectomized, bromocriptine-treated rats, both rPL-I and oPRL increased TH activity 207 and 151%, respectively. This supports the concept that termination of PRL surges at midpregnancy are owing to secretion of placental lactogens (PLs) from the placenta. However, the mechanism for the inhibition cannot be entirely attributed to an increase in tuberoinfundibular dopaminergic neuronal activity.