Effects of Parturition and Suckling on Concentrations of Pituitary Gonadotropins, Hypothalamic GnRH and Pituitary Responsiveness to GnRH in Ewes

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Abstract
Estrus was induced and ewes were bred during anestrus so that lambing would occur approximately 60 days before the end of the normal breeding season. At parturition, 18 ewes were divided into suckled (one lamb per ewe) or nonsuckled groups and were euthanized 24 hr after parturition (day 1, n=4), 11 days postpartum (n=6), or after the first ovulation in the nonsuckled group (30 to 44 days postpartum, n=8). Concentrations of progesterone were determined in serum samples collected daily to indicate when the ewes ovulated. Ovarian characteristics were observed at slaughter to confirm the occurrence of ovulation. Suckled and nonsuckled ewes resumed cycling at similar times and data were combined for analysis when significant suckling effects were not demonstrable. Concentrations of luteinizing hormone (LH), follicle-stimulating hormone (FSH) and prolactin (Prl) in the pituitaries, and gonadotropin-releasing hormone (GnRH) in the hypothalamus proper (HYP), preoptic -suprachiasmatic area (PO) and median eminence (ME) were determined. Ewes slaughtered at days 1 and 11 had less pituitary LH (P<.05) than ewes slaughtered after day 30 (74 ± 22, 217 ± 69, 1,229 ± 340 ng/mg fresh tissue, respectively). Concentrations of FSH in pituitaries tended to increase with the interval after parturition (P<.1). Serum concentrations of progesterone at slaughter were positively correlated with concentrations of LH (P<.01) and FSH (P<.01) in the pituitaries; after day 30, concentrations of progesterone in serum obtained at slaughter were greater than at day 11 (P<.05) but were not different from day 1 (2.1 ± .05, .34 ± .01, .77 ± .40 ng/ml, respectively). Concentrations of Prl in the pituitaries varied with time after parturition, with ewes slaughtered at day 11 having greater concentrations of Prl than ewes slaughtered at 1 or 30 to 44 days postpartum (P<.05). Concentrations of GnRH were 20 ± 4, 16 ± 3 and 1,683 ± 233 pg/mg fresh tissue in PO, HYP and ME, respectively; however, concentrations of GnRH did not change with time after parturition. Responsiveness of dispersed pituitary cells to GnRH (2.0 × 10−12 to 3.2 × 10−8M) was examined by measuring LH, FSH and Prl released into the media. The minimum effective dose of GnRH and doses required for half-maximal and maximal release did not differ between treatments. Pituitary cells from suckled ewes contained a greater concentration of Prl (P<.01) than pituitary cells from nonsuckled ewes (42 ± 6 vs 23 ± 6 μg/100,000 cells). Since hypothalamic concentrations of GnRH and pituitary responsiveness to GnRH did not differ, we suggest that factors controlling the synthesis of LH may be responsible for resumption of normal cyclic behavior after parturition. Copyright © 1980. American Society of Animal Science . Copyright 1980 by American Society of Animal Science