PROGESTERONE INHIBITS THE ESTROGEN-INDUCED PROLACTIN GENE-EXPRESSION IN THE RAT PITUITARY

The present study examines the inhibitory action of progesterone (P) o n prolactin (PRL) gene expression in the anterior pituitary of ovariec tomized, estradiol (OVX + E) treated adult rats. A single injection of P (1 mg) was administered s.c. to OVX + E treated rats, and animals w ere killed at 3, 6, 9, and 36 h following P administration. Northern b lot 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 s econd injection of P (1 mg) was given at 30 h after the first P, it ag ain suppressed the E-induced PRL mRNA level, indicating that the inhib itory effect of P may not last until 36 h. The inhibitory action of P was dose-dependent, and the pretreatment of RU486 (100 mug/rat), a P r eceptor antagonist at 1 h before P injection partially restored PRL mR NA 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 pitui tary.