Steroid feedback inhibition of pulsatile secretion of gonadotropin-releasing hormone in the ewe

• Published in: Biology of reproduction Vol. 36; no. 5; pp. 1207 - 1218
• Main Authors: KARSCH, F. J, CUMMINS, J. T, THOMAS, G. B, CLARKE, I. J
• Format: Journal Article
• Language: English
• Published: Madison, WI Society for the Study of Reproduction 01-06-1987
• Subjects: Anestrus; Animals; Biological and medical sciences; Estradiol - pharmacology; Feedback; Female; Fundamental and applied biological sciences. Psychology; Hormones and neuropeptides. Regulation; Hypothalamus. Hypophysis. Epiphysis. Urophysis; Luteinizing Hormone - metabolism; Ovariectomy; Ovary - physiology; Pituitary Hormone-Releasing Hormones - metabolism; Progesterone - pharmacology; Seasons; Sheep; Steroids - pharmacology; Vertebrates: endocrinology; Hypothalamohypophysoovarian axis; Peptide hormone; Gonadotropin RH; Estradiol; Vertebrata; Mammalia; Pulsatile; Feedback; Sheep; Artiodactyla; Progesterone; Sex steroid hormone; Ungulata
• ISSN: 0006-3363; 1529-7268
• DOI: 10.1095/biolreprod36.5.1207

The long-term negative feedback effects of sustained elevations in circulating estradiol and progesterone on the pulsatile
secretion of gonadotropin-releasing hormone (GnRH) and luteinizing hormone (LH) were evaluated in the ewe following ovariectomy
during the mid-late anestrous and early breeding seasons. GnRH secretion was monitored in serial samples of hypophyseal portal
blood. Steroids were administered from the time of ovariectomy by s.c. Silastic implants, which maintained plasma concentrations
of estradiol and progesterone at levels resembling those that circulate during the mid-luteal phase of the estrous cycle;
control ewes did not receive steroidal replacement. Analysis of hormonal pulse patterns in serial samples during 6-h periods
on Days 8-10 after ovariectomy disclosed discrete, concurrent pulses of GnRH in hypothalamo-hypophyseal portal blood and LH
in peripheral blood of untreated ovariectomized ewes. These pulses occurred every 97 min on the average. Treatment with either
estradiol or progesterone greatly diminished or abolished detectable pulsatile secretion of GnRH and LH, infrequent pulses
being evident in only 3 of 19 steroid-treated ewes. No major seasonal difference was observed in GnRH or LH pulse patterns
in any group of ewes. Our findings in the ovariectomized ewe provide direct support for the conclusion that the negative-feedback
effects of estradiol and progesterone on gonadotropin secretion in the ewe include an action on the brain and a consequent
inhibition of pulsatile GnRH secretion.