Changes in progesterone receptor binding of preoptic hypothalamus during an ovulatory cycle of the hen

By the use of radioligand assays, cytosolic and nuclear fractions of hypothalamic tissue involving preoptic area (HPOA) of the hen was found to contain a specific [17 alpha-methyl-3H]promegestone ([3H]-R5020; a synthetic progesterone) binding component having properties of receptor for progesterone. The equilibrium dissociation constant (Kd) was different neither between cytosolic and nuclear fractions nor between the fractions of laying and nonlaying hens. The maximum binding capacity (Bmax) per milligram of tissue of both fractions was greater in laying hens than in nonlaying hens. During the ovulatory cycle of laying hens, the specific [3H]-R5020 binding in cytosolic fraction showed a decrease, with an increase in the nuclear fraction from 21 to 18 h and from 6 to 3 h before ovulation. Such a change was not observed in nonlaying hens during a 24-h day. The results suggest that progesterone receptors are present in the preoptic hypothalamus and may be related to the incidence of ovulation in the hen.