Endogenous inhibition of prolactin secretion in pregnant lactating rats.

The purpose of the present study was to test whether rat placental lactogen, proposed to be inhibitory for endogenous rhythmic PRL secretion during pregnancy, is also inhibitory for a neurally mediated PRL response to exogenous stimulation, i.e. suckling. Pregnant lactating rats were used as the model in this study. Blood samples for PRL were taken via a chronic atrial cannula before and after 30 min of suckling stimulation in the following groups of females: 1) bearing conceptuses of different degrees of development; 2) after removal of conceptuses; and 3) after injection of placental extract immediately before exposure to suckling stimulation. It was found that 1) the PRL response to suckling diminished in conjunction with placental development and the known concomitant rise in rat placental lactogen; 2) removal of the conceptuses permits elevation of PRL after suckling stimulation; and 3) injection of placental extract obtained on days 11-12 of pregnancy failed to block the elevation of PRL in response to suckling. It was concluded that 1) lactation can be maintained even in the presence of very low levels of PRL, provided developed placentae are present, and 2) the developed placentae are capable of inhibiting the acute PRL release in response to exogenous stimulation as well as endogenous rhythmic PRL secretion.