Biol Reprod Keystone Symposia Conference on Frontiers in Reproductive Biology & Regulation of Fertility.
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Biology of Reproduction, Vol 21, 219-227, Copyright © 1979 by Society for the Study of Reproduction

Stimulatory Feedback Action of Estradiol in Intact and Ovariectomized Lactating Rats

R. J. COPPINGS 1, and S. M. McCANN 1

1 Department of Physiology, University of Texas Health Science Center at Dallas; Southwestern Medical School, Dallas, Texas 75235


The ability of lactating rats to secrete surges of LH in response to exogenous estradiol-17beta (E2beta) was examined. Lactating rats with varying numbers of pups (0-16) were submitted to 1 of 3 treatments: 1) ovariectomy on the day of parturition; 2) ovariectomy plus E2beta administration; or 3) E2beta administration. Ovariectomized (ovx) and ovx E2beta treated virgin females (VF) served as additional controls. Estradiol-17beta was given via capsules implanted s.c. on the morning of Day 10 of lactation. Blood was obtained from the jugular veins of etherized rats during the morning and at 1700 hon Days 11-14 of lactation.

The elevation of plasma LH and FSH normally observed following ovariectomy was attenuated in lactating females in a manner proportional to the number of suckling young. Implantation of ovx and intact lactating rats with E2beta-filled Silastic capsules induced an LH surge on the afternoon of the following day in all animals. Suckling very large litters (16 pups) resulted in a significant reduction in the magnitude of the LH surge. Surge release of LH persisted on Days 12, 13 and 14 in ovx E2beta treated rats with 0 or 2 pups, while a progressive inhibition was noted in rats with larger litters. No LH surges were noted in intact E2beta treated lactating rats on Days 12-14. Plasma FSH was elevated at 1700 h on Day 11 in all E2beta treated rats. Inconsistent increases in FSH were seen on Days 12-14 in ovx E2beta treated rats but no further increases were observed after Day 11 in intact E2beta treated animals. Average values of plasma prolactin in samples obtained at decapitation on Day 15 were greatest in rats nursing 8 or more pups. Large reductions (up to 90%) in pituitary concentrations of LH and FSH were apparent in animals with the larger litters. In a second experiment, the increment in plasma LH 20 min after injection of 100 ng of LHRH on Day 11 of lactation was similar in E2beta treated rats with 2 or 12 pups; however, the response of rats with 12 pups was significantly less than that of rats with 2 pups on Day 13.

The following conclusions were drawn from these experiments: 1) suckling inhibits in a manner proportional to litter size the increase in LH and FSH which follows ovx; 2) the feedback mechanism by which E2beta induces an LH surge is intact in lactating rats; 3) the presence of the ovaries and/or suckling of a large number of young, however, can inhibit the daily expression of this mechanism; 4) the inhibitory effects of lactation on gonadotropin release are associated with reduced pituitary LH and FSH; 5) moreover, reduced responsiveness of the pituitary to LHRH may occur following an initial surge release of LH in those animals with large litters.

 Submitted on September 8, 1978
Accepted on March 29, 1979




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A. Fukushima, P. Yin, M. Ishida, N. Sugiyama, and J. Arita
Pup removal suppresses estrogen-induced surges of LH secretion and activation of GnRH neurons in lactating rats.
J. Endocrinol., October 1, 2006; 191(1): 339 - 348.
[Abstract] [Full Text] [PDF]


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Copyright © 1979 by the Society for the Study of Reproduction.