The effect of Nestorone on gonadotropic cells in pituitary of rats

The implant containing Nestorone is a promising long-acting contraceptive especially suitable for lactating women. In this study, two experiments were designed to observe the effect of Nestorone on the gonadotropic cells in pituitary of rats for analyzing its antiovulation mechanism. In the first experiment, the ED₅₀ of Nestorone on inhibiting ovulation was found to be 1.32 mg/kg. The serum luteinizing hormone (LH) levels were significantly lower 60 h after being treated with Nestorone at 8:30–9:00 a.m. on Day 2 (D2) of estrus. Image analysis showed that the average size of the LH cells in groups treated with Nestorone at 2 or 4 mg/kg was larger than that of the control. In the group treated with 4 mg/kg, most of gonadotropic cells were regular round in shape. And, abundant granules in cytoplasm were found in those cells, which indicated that the LH stored in cells was not released. In the second experiment, the rats were treated with Nestorone at 5 mg/kg at 11:30–12:00 a.m. on D2 of estrus. The normal or higher expression of LHβ mRNA in pituitary suggested that the synthesis of LH was not inhibited by the treatment with Nestorone. The expression of PR mRNA in pituitary was significantly lower than that of the control at 33 h after treatment. This might be a direct effect of Nestorone, since there were no differences in the serum E₂ and P₄ levels between the treated and the control group. It is concluded that Nestorone prevents ovulation through inhibition of LH secretion and it has no effect on synthesis of LH. Progesterone receptors in pituitary might be involved in this process, but further study is needed to gain more evidence.