PROGESTERONE NEGATIVE FEEDBACK ON PROLACTIN SECRETION - IMPORTANCE OF THE BRAIN CONTROL AND OF ESTRADIOL

Studies were carried out to investigate the negative feedback effect of progesterone on the secretion of prolactin (Prl). Ovariectomized adult rats were treated with either 50 μg of estradiol, 25 mg of progesterone, a combination of both or the oil diluent. 74 hr after treatment, progesterone given alone did not modify pituitary weight, Prl synthesis or the serum levels of this hormone, while estradiol alone increased these parameters. When progesterone was injected simultaneously with estradiol, serum Prl levels were reduced, but the other parameters remained unchanged. In rats with electrolytic lesion of the median eminence (ME), pituitary synthesis and serum levels of Prl were increased, and treatment with estradiol or progesterone singly or in combination was unable to modify them. Furtheremore, several doses of progesterone administered at different times before sampling also did not change the high Prl titers found in ME-destroyed rats. Finally, the effect of different concentrations of progesterone on the synthesis of Prl by incubated pituitaries was studied; only the highest and probably non-physiological concentration was able to reduce Prl synthesis. Since progesterone action of Prl levels in serum is observed only in rats with normal brain-pituitary relationship and when high levels of estrogens are present, it is postulated that the negative feedback effect of progesterone on Prl release by the pituitary is in part at the level of the brain, blocking the stimulatory effect of estrogen on Prl secretion.