PROLACTIN-SYNTHESIZING AND PROLACTIN-RELEASING ACTIVITY OF FETAL AND EARLY POSTNATAL RAT PITUITARIES - IN-VIVO AND IN-VITRO STUDIES USING RIA, REVERSE HEMOLYTIC PLAQUE-ASSAY AND IMMUNOCYTOCHEMISTRY

In vivo and in vitro prolactin (PRL)-synthesizing and PRL releasing ac tivity of fetal (days 12-22) and early postnatal (days 1-10 after birt h) rat pituitaries were studied by means of radioimmunoassay (RIA), re verse hemolytic plaque assay and immunocytochemistry. Using RIA, PRL c ould first be detected, both in the pituitary and in the serum, on day 17 of fetal development. From this day on, pituitary PRL gradually in creased, the rise was particularly marked during the postnatal period and became depressed for the first 10 days of postnatal life. On fetal day 18, 12-15% of monodispersed pituitary cells displayed PRL immunop ositivity, but only 3-5% of PRL-positive cells were plaque-forming, i. e. released PRL. By the end of gestation 19-25% and on postnatal day 1 0 42-45% of all pituitary cells were PRL cells and 31-35 and 15-17% of PRL-positive cells, respectively released PRL. Both pre- and postnata l PRL cells in monolayers were insensitive to TRH treatment. Pituitary primordia immunocytochemically and radioimmunologically negative for PRL (13- to 14-day-old fetal) when placed in serum-free organ culture were able to synthesize and release PRL. Fetal pituitary exhibited a h ighly regular increasing pattern of daily PRL release during a 7-day-c ulture period. Data obtained both in vivo and in vitro did not exhibit any sex differences. The present findings are consistent with all tho se observations suggesting an early emergence of fetal rat pituitary l actotrophs. The in vitro results support the concept that Rathke's pou ch cells have a substantial degree of independence from extrapituitary regulatory actions in the expression and further progression of speci fic functions. This study is the first demonstrating PRL release of in dividual fetal lactotrophs from day 18 of intrauterine life onwards an d the ontogenetic change in the percentage of functionally active and inactive subpopulations of PRL cells. Further, the observations are in line with the assumption that the dopaminergic inhibition of PRL rele ase may start in early postnatal life.