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The effects of oestradiol and progesterone on LH-subunit mRNA levels were investigated in ovariectomized rats. Four weeks after ovariectomy, rats were implanted with silicone elastomer capsules containing oestradiol and/or injected daily with progesterone in oil (5 mg/rat) for 8 days. The levels of pituitary mRNA encoding and LH-β were determined using direct hybridization with specific [³²P]cDNA probes. After oestradiol implantation in ovariectomized rats, both and LH-β mRNA decreased with time, with maximum inhibition after 6–8 days of treatment. Progesterone injected alone did not show any effect on and LH-β mRNA. Cytosolic progesterone receptors, determined using [³H]methyl-17-progesterone as ligand, were undectable in control ovariectomized rats. In contrast, 2 days after oestradiol implantation, the number of receptors increased to 287·5 ± 35·4 (S.E.M.) fmol/pituitary and reached a plateau of 400 ± 21·8 fmol/pituitary after 4 days. The effects of progesterone were therefore examined by first implanting ovariectomized rats with oestradiol to induce progesterone receptors and then injecting progesterone daily for a further period of 6 days. As a result of this treatment, progesterone induced a decrease in the pituitary gland contents of both and LH-β mRNAs, and LH release was significantly greater than that observed in the group receiving oestradiol alone. Moreover, the mRNA levels in the animals treated with oestradiol plus progesterone were lower after 8 days of treatment than those observed in ovariectomized rats treated with a tenfold higher dose of oestradiol alone.

These data demonstrate that progesterone, together with oestradiol, is capable of negatively regulating the mRNAs encoding subunits in vivo, provided that progesterone receptors are present in the pituitary gland.

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