The Effects of Luteinizing Hormone Releasing Hormone (LH-RH) in Pregnant Rats. I. Postnidatory Effects

Gestation in rats has been divided into periods to determine when pregnancy can be terminated by LH-RH. After nidation, peptide administration (500 μg b.i.d.) on Day 9, 10 or 11 terminated pregnancy while minimal or no effects were demonstrable from Days 6 to 8 or from Days 13 to 16. Injection for more than one day was also effective if administration included one of Days 9–11. Also, injections on Days 7 and 8 were partially effective. On day 10, the ED50 for b.i.d. administration was 150 μg whereas for a single injection, it was 550 μg. LH-RH treatment on Day 9 was followed by a measurable fall in circulating progesterone on Days 10 and 11. Pregnancy termination by LH-RH from Days 7–12 was reversed by simultaneous administration of large doses of progesterone; the effective progesterone doses were comparable to those required to maintain pregnancy in animals spayed on Day 7 or 8. Addition of estradiol-17β had no potentiating effect on progesterone, while alone it appeared to retard embryonic resorption slightly. These data suggest that the postnidatory antifertility effect of LH-RH is mediated via functional luteolysis.