Two experiments were conducted to test the hypothesis that progesterone (P4) acts to induce a preovulatory rise of luteinizing hormone (LH) by initiating a true positive feedback interaction between P4 and LH. In each experiment, ten hens were stereotaxically implanted with a third ventricle cannula. Aminoglutethimide phosphate (AGP), an inhibitor of steroidogenesis, was administered (200 mg iv) to hens at 10 and 6 hr (Experiment 1) and 10 and 7 hr (Experiment 2) before an expected C1 ovulation. A 20-μg injection of P4 (n = 5) or the vehicle (n = 5) was made intraventricularly at 6 hr (Experiment 1) or 7 hr (Experiment 2) before the expected ovulation. Blood samples were taken via branchial vein cannula at regular intervals after the injection. In Experiment 1, four of five P4-treated hens ovulated, and no atretic follicles were found in any of the five P4-treated hens, including the one which did not ovulate. Ovulations were always accompanied by preovulatory LH peaks. By comparison, none of the vehicle-injected animals ovulated and in four of five hens the largest follicle was determined to be atretic. No ovulations occurred in Experiment 2 for either the P4- or vehicle-injected hens. Atresia of the largest follicle occurred in all five hens in the P4-treated group and four of five hens of the vehicle-treated group. The difference in results between Experiment 1 and Experiment 2 can be explained assuming that the steroidogenesis-inhibiting action of the initial AGP injection in Experiment 1 (at 10 hr before ovulation) had diminished prior to the second AGP treatment (at 6 h before ovulation). In both Experiments 1 and 2, intraventricular P4 injection caused a significant (P < 0.001 for Experiment 1; P < 0.03 for Experiment 2) rise in plasma P4 within 10 min after injection due to leakage of P4 from the third ventricle into the systemic circulation. Nevertheless, in the absence of any steroid feedback from the ovary (Experiment 2), the small rise in LH induced by the P4 injection was not potentiated, and no preovulatory surge of LH occurred. These results support the hypothesis that a positive feedback interaction between P4 and LH, in contrast to a stimulatory effect of P4, is necessary for the preovulatory LH surge and ovulation.
All Science Journal Classification (ASJC) codes
- Animal Science and Zoology