Abstract

PIP: Specific binding of luteinizing hormone-releasing hormone (LHRH) to the anterior pituitary gland during the estrous cycle was investigated in the rat. The amount of iodine-125-LHRH bound was 150 fmole/,g protein at proestrus, 20 fmole/mg at estrus, and 16 fmole/mg at metestrus. Specific binding of LHRH to the pituitary was not detectable in rats ovariectomized during the afternoon of diestrous 1 or morning of estrus but it was restored by treatment with estradiol-17-beta along or with progesterone. Peripheral plasma LHRH levels were 5 pg/ml at diestrous and increased to 20 pg/ml at proestrus (p greater than .05). Plasma LH levels were 16.7 ng/m1 at 4-5 p.m. of proestrus, 4 ng/m1 at 1-2 p.m. of preoestrus and 1-2 ng/m1 at other times. Preovulatory LH surge was not observed in rats ovariectomized at diestrus or sacrificed at the expected time of proestrus, however, estradiol treatment elevated LH levels and progesterone augmented this effect. These results suggest that in the rat the ovulatory LH surge at proestrus involves the hypothalmic release of LHRH as well as increased LHRH binding in the pituitary gland, both of which may be stimualted by the prevolulatory rise in estrogen and/or progesterone.