Progesterone inhibits the estrogen-induced prolactin gene expression in the rat pituitary

The present study examines the inhibitory action of progesterone (P) on prolactin (PRL) gene expression in the anterior pituitary of ovariectomized, estradiol (OVX+E) treated adult rats. A single injection of P (1 mg) was administered s.c. to OVX+E treated rats, and animals were killed at 3, 6, 9, and 36 h following P administration. Northern blot analysis showed that P suppressed the E-enhanced PRL mRNA level at 3, 6, and 9 h, but not at 36 h following P administration. When the second injection of P (1 mg) was given at 30 h after the first P, it again suppressed the E-induced PRL mRNA level, indicating that the inhibitory effect of P may not last until 36 h. The inhibitory action of P was dose-dependent, and the pretreatment of RU486 (100 micrograms/rat), a P receptor antagonist at 1 h before P injection partially restored PRL mRNA level which was inhibited by P. These data indicate that P plays a crucial role in the regulation of PRL gene expression in the rat pituitary.