Termination at midpregnancy of the two daily surges of plasma prolactin initiated by mating in the rat

The pattern of plasma prolactin following uterine cervical stimulation consists of two surges each day, one nocturnal, occurring between 0100-0900 h (lights on 0600-1800 h), and one diurnal, occurring between 1500-2100 h. This pattern of prolactin continued throughout pseudopregnancy; the last surge was observed on the morning of day 11 (day 1 was taken as the first day of diestrus of pregnancy or pseudopregnancy). Prolactin levels remained low thereafter until the spontaneous proestrous surge on the afternoon of day 12, signalling the onset of a new estrous cycle. In contrast, the two daily prolactin surges did not continue throughout pregnancy, and in fact, were terminated sooner in pregnant animals than in pseudopregnant animals. The last diurnal surge was observed on day 8 while the last nocturnal surge was observed on day 10. The early termination of prolactin surges during pregnancy correlated with the increased secretion of rat placental lactogen. However, placental extracts obtained from day 11 of pregnancy and injected in large doses failed to inhibit prolactin surges in pseudopregnant animals. Prolactin surges also continued for a longer period of time in pseudopregnant rats bearing decidualized uteri than in pregnant animals. Thus, the two major components of pregnancy that differed from pseudopregnancy, that is, the presence of rat placental lactogen or decidual tissue, did not appear to account for the early termination of prolactin surges during pregnancy.