Endocrine communication between conceptus and mother: placental lactogen stimulation of maternal behavior.

The possible role of the conceptus in stimulating the onset of maternal behavior through its secretion of placental lactogens and their passage into the brain was investigated in female rats. In the first study, significant mitogenic activity in the Nb2 lymphoma cell bioassay was detected in cerebrospinal fluid (CSF) samples collected by push-pull perfusion from rats on days 12-21 of pregnancy, coincident with the establishment of placental function. In contrast, mitogenic activity was absent from CSF in lactating and gonadectomized, virgin females. In a second study the mitogenic activity in day 12 pregnant samples was neutralized 71% with antibodies to rat placental lactogen-I (rPL-I) and > 90% with a combination of antibodies to rPL-I plus rPL-II. In contrast, activity on day 21 of pregnancy, 1 day prepartum, was reduced by antibodies to rPL-II (> 85%), but not by antibodies to rPL-I, indicating that the predominant lactogen in the CSF prepartum is rPL-II. The behavioral actions of placental secretions were assessed in the third experiment by infusing recombinant rPL-I and purified rPL-II directly into the medial preoptic area of the brain of steroid-primed, nulliparous rats. Latencies to respond maternally to foster young were significantly reduced in rPL-I- and rPL-II-treated rats (2- to 3-day latencies) when compared with latencies in control females (5- to 6-day latencies). Thus, the conceptus through its secretion of rPLs which apparently gain access to the CSF helps to prime the pregnant female's brain to respond maternally at the end of gestation. This endocrine communication between the developing conceptus and pregnant female appears to be an important part of the biological system which helps to establish successful maternal care.