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Effects of Estradiol-17β, Naloxone and Gonadotropin Releasing Hormone on Postpartum Secretion of Luteinizing Hormone in Fall-Lambing Ewes^1,2,3,

Purdue University^,6, West Lafayette, IN 47907

Abstract

The interaction among exogenous estradiol-17β, naloxone and gonadotropin releasing hormone (GnRH) in the control of luteinizing hormone (LH) secretion was studied in intact postpartum ewes nursing their offspring. One-half of 30 fall-lambing ewes were implanted subcutaneously with an estradiol-17β containing Silastic capsule between postpartum d 1 and 12 which doubled their serum concentrations of estradiol (16.0 ± .1 vs 8.4 ± .1 pg/ml). Blood samples were collected from implanted and non-implanted ewes at 15-min intervals for 5 h on d 3, 8, 13, 20 and 28 postpartum. Pre-injection samples were collected for 1 h, and ewes were injected with saline, naloxone (NAL; 1 mg/kg) or GnRH (100 µg/ewe). When averaged across all days and implant groups, serum LH in the three post-NAL samples was higher (P<.05) than in the three pre-NAL samples (3.6 ± 1.2 vs .6 ± .2 ng/ml). Post-GnRH concentrations of serum LH were lower (P<.05) in estradiol-implanted ewes than in non-implanted ewes on d 8 and 13, but there were no differences in any LH characteristics on d 20 and 28 after implant removal on d 12. In non-implanted ewes, serum LH responses to GnRH increased (P<.05) eightfold from d 3 (3.8 ± 1.4 ng/ml) to d 8 (31.6 ± 1.4 ng/ml), remained elevated through d 20, but declined by d 28 (10.8 ± 1.4 ng/ml). In estradiol-implanted ewes, serum LH response to GnRH on d 8 was identical to that on d 3 and increased slightly on d 13, although remaining lower than in non-implanted ewes. Post-NAL increase in serum LH tended to be reduced by estradiol treatment on d 8 and 13, which was consistent with reduced LH response to GnRH on these days. In summary, transient treatment with estradiol-17β between d 1 to 12 after parturition briefly depressed serum LH responses to GnRH and perhaps NAL. However, there were no long-lasting inhibitory or stimulatory effects on serum LH or postpartum interval due to estradiol implants following their removal. Because NAL transiently increased serum LH, endogenous opioid peptides are implicated as inhibitors of LH release in fall-lambing postpartum ewes.

¹ Journal paper no. 10863, Purdue Univ., Agr. Exp. Sta.

² Presented in part at the 78th Annu. Meet, of the Amer. Soc. of Anim. Sci., Manhattan, KS, Abstr. #437.

³ The authors thank Doris Blair for her technical assistance with the RIA; G. D. Niswender, Colorado State Univ., for supplying antisera to progesterone and LH; N. R. Mason, Eli Lilly and Co., Indianapolis, IN for providing antiserum to estradiol-17β and the Natl. Pituitary Agency for supplying iodination grade LH.

⁴ Address reprint requests to this author.

⁵ Present address: Dept. of Anim. Sci., Univ. of Wyoming, Laramie 82071.

⁶ Dept. of Anim. Sci.