Biology of Reproduction, Vol 23, 910-917, Copyright © 1980 by Society for the Study of Reproduction

Hypothalamo-Hypophyseal Sensitivity to Hormones in the Hen. I. Plasma Concentrations of LH, Progesterone, and Testosterone in Response to Central Injections of Progesterone and R5020

¹ Department of Poultry and Avian Sciences, Cornell University, Ithaca, New York 14853

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 5 µg 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.

Note:
ACKNOWLEDGMENTS We wish to thank Ms. S. M. Quirk, R. D. Karaka, and V. Eisner for expert technical assistance; Dr. F. J. Cunningham for LH assay materials; Dr. W. R. Butler for anti-rabbit globulin; Dr. P. Concannon for the gift of R5020; Dr. H. P. Behrman, Merck Institute for Therapeutic Research, for the gift of P₄ antiserum; and Ms. P. A. Johnson for her critical review of the manuscript.

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