GONADOTROPIN RELEASING MECHANISM VIA RECEPTORS OF PITUITARY CELLS IN POULTRY

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(1) Gonadotropin-releasing hormone receptors having a high affinity and low capacity have been found in the hen pituitary cells. The equilibrium dissociation constant of the receptor and the maximum number of binding sites per milligram of membrane protein were greater in laying hens than in nonlaying hens. A single i.m. injection of progesterone, estradiol-17beta, or 5alpha-dihydrotestosterone into nonlaying hens caused an increase in Kd and Bmax values. During an ovulatory cycle of the laying hen, Kd and Bmax values decreased from 16 to 14 h and 8 to 6 h before ovulation. A single i.v. injection of chicken LHRH-1 caused a decrease in the Kd and Bmax values within 2 min after the injection. It is suggested that the hen pituitary gonadotropin-releasing hormone receptor bindings are affected by ovarian steroid hormones and that their changes during the ovulatory cycle may relate to the release of a gonadotropin from the pituitary. (2) The amount of progesterone receptor in the cytosolic fraction of the pituitary cells of the hen was found to increase when incubated with estradiol-17beta for 1-4 h, but not when incubated with the presence of cycloheximide, actinomycin-D, or alpha-amanitin. It is suggested that the cytosolic progesterone receptor binding of the hen pituitary cells was enhanced by the action of estrogen through a protein synthesizing pathway. (3) The cellular content of Luteinizing hormone was found to increase when the pituitary cells were preincubated with estradiol-17beta for 2 h and then incubated with progesterone for 4 h. The increase in the cellular LH in response to progesterone was dose dependent. Protein synthesis inhibitors (cycloheximide, actinomycin-D, and alpha-amanitin) blocked the response of the cells to progesterone. It is suggested that LH production in estrogen-primed pituitary cells of the hen was stimulated by progesterone through a protein-synthesizing pathway.