The development of pituitary control of hepatic steroid metabolism in the rat

In order to test the hypothesis that the hypothalamo-pituitary axis matures at around 30 days of age in the rat as regards control of hepatic steroid metabolism, an investigation has been performed whereby pituitaries from female donor rats of various ages from 7 to 56 days of age were transplanted under the kidney capsule of adult male, hypophysectomized rats. It was shown that pituitaries from donors of up to 28 days of age were without effect, but that glands from donors of 35 days of age and older caused feminization of the hepatic steroid metabolism, suggesting that the factor responsible for the feminine pattern of hepatic steroid metabolism is normally first secreted after 28 days of age. This maturation process is also reflected in the prolactin secretion from the ectopic pituitary graft. Prolactin is first measurable in serum from recipients carrying a pituitary obtained from a 28-day-old donor. Thus, release of prolactin from the implanted pituitary gland is correlated to its ability to induce feminization, but earlier experiments have shown that prolactin is not the hormone involved. In conclusion, this investigation strengthens the hypothesis that it is the hypothalamopituitary axis that matures at around 30 days of age in the rat leading to a sex difference in hepatic steroid metabolism.