Effects of Estrogen and Progesterone on Serum and Pituitary Prolactin Levels in Ovariectomized Rats

Serum and anterior pituitary (AP) prolactin levels from mature ovariectomized rats were assayed after injection with daily doses of 0.1–500 ng estradiol benzoate (EB) or 0.5–10 mg progesterone for 6 days. Doses of 0.1, 1 or 5 μg EB increased serum prolactin concentration 2-, 3- and 10-fold, respectively, over ovariectomized controls, and doses of 10–500 μg EB evoked smaller increases in serum and AP prolactin levels. AP prolactin concentration and content were increased 3- to 4-fold. Doses of 0.5, 1 or 4 mg progesterone partially inhibited the increase in serum prolactin levels evoked by 1 μg EB, but only the inhibition by 4 mg progesterone was significant. A daily dose of 4 mg progesterone injected alone appeared to have no effect on serum and AP prolactin levels, but a 10 mg dose significantly increased prolactin secretion. These results indicate that low doses of estrogen are more effective than high doses for increasing AP and serum prolactin levels, and provide no evidence that large doses of estroge...