Hypothalamo-Hypophyseal Sensitivity to Hormones in the Hen. I. Plasma Concentrations of LH, Progesterone, and Testosterone in Response to Central Injections of Progesterone and R50201

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Abstract
Experiments were conducted to elucidate further the site of action and mechanism(s) by which progesterone (P4) induces ovulation in the domestic hen. Injection of 500 µg P4, i.m., at a time 14 to 12 h prior to the C1 ovulation, induced premature ovulation in five of five hens and caused preovulatory surges of luteinizing hormone (LH) and testosterone (T) which peaked within 120 and 150 min after injection, respectively. Plasma P4 continued to rise but remained within physiological concentrations throughout the 240 min sampling interval. Injections into the third ventricle of 5 and 20 µg P4 in the seven hens that were injected and subsequently had blood samples taken induced premature ovulation in three of seven and six of seven hens, respectively, and initiated preovulatory surges of LH and T comparable to those induced by peripheral administration of P4. Concentrations of plasma P4 30 min after injection (t30) were significantly increased over preinjection (t0) levels in the 20 µg treatment group, but were significantly lower at t60, t90, and t120 min than values at the same intervals in the peripherally injected group. In the four hens injected with 5µ g P4, the one hen injected with 20 µg P4, and all seven vehicle-injected hens that ovulated within the normally expected interval, concentrations of LH and T were not significantly altered over the sampling interval. Plasma P4 in the single hen injected with 20 µg P4 that did not ovulate prematurely was increased at t60 min over concentrations at t60 min in vehicle-injected hens, indicating a small amount of leakage of P4 into the peripheral circulatory system. Injection of 20 µg R5020 (a synthetic progestin, specific for progestin receptors) induced premature ovulation in four of five hens, while the fifth hen failed to ovulate for 12 days after the day of injection. Plasma concentrations of LH and T in the four hens that ovulated prematurely in response to R5020 were similar to those in hens injected centrally or peripherally with P4, while plasma P4 was not significantly different at any time interval from values in hens injected centrally with 20 µg P4. In the single hen in which ovulation was blocked, concentrations of LH, P4, and T were greater than t0 values at t60 min, then fell below concentrations found in vehicle-injected hens. We conclude that P4 acts to induce ovulation at the level of the hypothalamus, with a possible indirect role at the pituitary, via P4 receptors.