Abstract
A single subcutaneous injection of progesterone (5mg/rat), chlormadinone acetate (CMA, 17α-acetoxy-6-chloro-pregna-4, 6-diene-3, 20-dione; 5mg/rat), medroxyprogesterone acetate (MAP, 17α-acetoxy-6α-methyl-pregn-4-ene-3, 20-dione; 0.5mg/rat) or norethindrone (17α-ethyny1-17β-hydroxyestr-4-en-3-one; 0.5mg/rat) into 4-day cyclic rats at 11 a. m. on the 2nd day of diestrus inhibited spontaneous ovulation on the day corresponding to the following estrus. Also, oral administration of CMA or MAP inhibited ovulation completely at the dosage of 5 mg per rat, whereas norethindrone given in the similar manner did not exhibit the same effect. No inhibition of ovulation was observed in the rats to which norethynodrel (17α-ethyny1-17β-hydroxyestr-5 (10)-en-3-one) had been given either by subcutaneous or by oral route at the dosages up to 10 mg per rat. Of the other steroids tested, androgen was the only one that inhibited spontaneous ovulation. Progesterone, MAP, CMA and norethindrone, administered atthe dosage of 10 mg per rat 50 hrs. after the initial progesterone treatment, restored ovulation in 24hrs., which had been expected to be delayed with 3 mg of progesterone given at 11 a. m. on the 2nd day of diestrus. These results indicate that synthetic progestins as well as progesterone have a biphasic action, inhibitory and facilitatory, on the neural mechanism (s) regulating the release of luteinizing hormone in the rat.
