Effects of excess prolactin from pituitary grafts on the reproductive functions in the rat

Abstract

The effects of prolactin on the estrous cycle, reproduction and lactation in the rat were studied by transplanting single neonatal, single adult and triple neonatal pituitary grafts to the kidneys of the intact rats. Plasma prolactin concentrations were found to be elevated to about twice those of normal pseudopregnancy, in rats with single adult (55.86 ± 7.1 ng/ml) and multiple neonatal grafts (53.02 ± 8.47 ng/ml). Elevation of progesterone was not evident from traumatization of the uterus. The effect of the additional prolactin from the grafts was to induce consecutive pseudopregnancies in the estrous cycles, with frequent absences of estrous smears between pseudopregnancies. This luteotrophic effect was observed more frequently in rats with single adult (78.57 %) or multiple neonatal grafts (78.78 %) than in rats with single neonatal grafts (46.15 %). The ovaries of the treated rats had numerous corpora lutea, indistinguishable from those of normal pseudopregnant rats. The mean pseudopregnancy lengths of rats with single adult (12.4 days) and multiple neonatal grafts (12.5 days) were longer than those of rats with single neonatal grafts (11.4 days) or those induced by stress of operations in the controls (10.8 days). No inhibitory actions of excess prolactin on the estrous behavior and ovulation (either on its occurrence or the number of ova shed) were observed. Graft prolactin had no effect on the duration of pregnancy. Lactation, as judged by both litter growth and milk yield, was normal in the grafted rats. Pituitary grafting stimulated body weight gains in rats with single adult or multiple neonatal grafts, but had no effects on the ovarian and adrenal weights. The conclusions are as follows: 1) Prolactin is involved in the regulation of luteal function during pseudopregnancy but does not play a major luteotrophic role after the twelfth day of pregnancy, 2) the failure of vaginal cornification is due to the effect of progesterone, 3) the inhibition of estrous behavior and ovulation by prolactin is mainly secondary to its stimulation of progesterone secretion, 4) excess prolactin does not impair the synchronization of estrus and ovulation, tubal transport of ova or implantations, and 5) any negative feedback action of prolactin on in situ pituitary prolactin secretion can be overridden by the direct lactogenic effect of the excess prolactin from the grafts, to maintain normal lactation.