Archive for the ‘Hormone’ Category


Puberty in Female Sheep: DISCUSSION(6)

Sep 1, 2013 Author: Walter Mcneil | Filed under: Hormone

DISCUSSION(6)

As expected and in agreement with earlier studies, progesterone treatment delayed the onset of LH surge in all groups when compared with the timing observed in sheep from part 1 of experiment 2, where estradiol positive feedback was tested in the absence of progesterone priming. Progesterone pretreatment protects against the detrimental effects on the LH surge system of prolonged estradiol exposure in cystic cows and sheep. Furthermore, progesterone increases the sensitivity of the surge system to estradiol feedback effects. Thus, we expected progesterone pretreatment to normalize the positive feedback effects of estradiol in the prenatally androgenized females. To a certain extent, this prediction was true; the D60-90 androgenized animals did not manifest any delay in surge generation as compared with the controls. However, progesterone treatment failed to normalize the timing and magnitude of the gonadotropin surges in the D30-90 group. One possible explanation for this discrepancy is that the LH stores may be depleted in the D30-90 prenatally an-drogenized group because of the hypergonadotropic tendency of these animals. Alternatively, the magnitude of the LH and FSH surges may not be a function of estradiol sensitivity. Further studies are necessary to resolve these issues.

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  • Puberty in Female Sheep: DISCUSSION(5)

    Aug 26, 2013 Author: Walter Mcneil | Filed under: Hormone

    Contrary to our findings of the existence of a surge system in the D30-90 ovary-intact sheep (albeit with a latency), a majority of the androgenized adult females studied long term by other investigators had no estradiol positive feedback response, although the majority of animals showed progestogenic cycles. In those studies, induction of surges was monitored only for 25 or 36 h after estradiol treatment. In contrast, in our studies the positive feedback effects of estradiol were monitored for periods up to 72 h, and surge induction was clearly evident in all but one animal. The animal that did not show a definable surge had an increase in LH but did not meet the surge criteria. It is therefore possible that earlier studies may have missed the induction of surges because of shorter monitoring periods.

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  • Puberty in Female Sheep: DISCUSSION(4)

    Aug 21, 2013 Author: Walter Mcneil | Filed under: Hormone

    DISCUSSION(4)

    Effect of Prenatal Androgenization on Estradiol Positive Feedback

    Based on results of earlier studies using the ovx+E model, we predicted that the gonadotropin surge system would be intact in the D60-90 androgenized animals but absent in the D30-90 animals. In contrast to the findings in ovx+E model, the D30-90 ovary-intact animals had an intact surge system like that of the D60-90 group, although the magnitude of the LH and FSH surges generated was markedly reduced. Both groups of androgenized animals showed delayed responses to the positive feedback actions of estradiol (evidenced by the delayed onset of the LH and FSH surges), with the delay being more pronounced in the D30-90 than in the D60-90 females. Although the number of animals in the D30-90 group was small, the delayed induction of the surge is consistent with results from other studies of D60-90 animals where sufficiently large numbers of animals were available for testing. Together, these findings provide clear evidence in support of the disruptive effects of prenatal androgenization on the positive feedback actions of estradiol in ovary-intact animals. This impairment in estradiol positive feedback, manifested as delayed and low-magnitude LH and FSH surges, may culminate in ovulatory defects. Disruptive effects of prenatal exposure to testosterone on the reproductive capability of ewes to ovulate or to exhibit estrous cyclicity have been reported previously.

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  • Puberty in Female Sheep: DISCUSSION(3)

    Aug 15, 2013 Author: Walter Mcneil | Filed under: Hormone

    There are at least two possible explanations for the observed differences in the ovx+E and ovary-intact models. First, such differences may relate to the nature and time of estradiol exposure and the subsequent organizational effects of estradiol. In the ovariectomized model, females are constantly exposed to exogenous estradiol during the postnatal period. The ovary-intact model used in this study is arguably different because these animals are not exposed to constant estradiol input but rather to changing levels of estradiol, which wax and wane with the follicular waves. The presence of continuous circulating estradiol during postnatal growth may have contributed to the masculinization/defeminization from tonic LH secretion during the prepubertal period along with defeminization of the LH surge system in the ovx+E model.

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  • Puberty in Female Sheep: DISCUSSION(2)

    Aug 9, 2013 Author: Walter Mcneil | Filed under: Hormone

    DISCUSSION(2)

    Although the differences were not statistically significant, the androgenized animals tended to have higher LH and FSH levels and LH:FSH ratios than did controls. A possibility that remains to be explored is whether these animals develop hypergonadotropism as they advance reproductively. Fabre-Nys and Venier determined that follicular phase levels of LH are higher in prenatally androgenized than in control animals. This observation, in conjunction with the multifollicular ovarian morphology and anovulatory condition that these sheep develop with advancing age, suggests that prenatally an-drogenized sheep can be used as a cost-effective model for understanding the pathophysiology of polycystic ovarian disease, the major cause of infertility in women of reproductive age. These women manifest, hypergonadotropism, polycystic ovaries, hyperandrogenism, and anovulation.

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  • Puberty in Female Sheep: DISCUSSION(1)

    Aug 3, 2013 Author: Walter Mcneil | Filed under: Hormone

    This study was conducted to determine whether 1) prenatal exposure to androgens would advance the pubertal increase in gonadotropins in ovary-intact female sheep, as predicted by the early pubertal rise in LH in the ovx+E model, 2) initiation of reproductive (progesterone) cycles parallels the gonadotropin increases, and 3) prenatally androgenized animals have a disrupted gonadotropin surge system.

    Effect of Prenatal Androgenization on Tonic Gonadotropin Secretion

    In intact sheep, during the prepubertal period GnRH/LH secretion is highly sensitive to the inhibitory effects of gonadal steroids. As such, circulating gonadotropin concentrations remain low during this period. As the animal approaches puberty, there is an escape from the inhibitory effect of steroids, and pubertal increase in gonadotropin secretion occurs around 1-2 wk before first ovulation. This increase in pubertal increase in LH was evident only in studies where changes in circulating LH were monitored frequently. In the ovx+E model, the increase in circulating LH was evident at 27 wk of age in control females, at around 10 wk of age in male lambs, and at 10 wk of age in the prenatally androgenized animals, as determined by measurement of LH in biweekly samples.

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  • Puberty in Female Sheep: RESULTS(3)

    Jul 27, 2013 Author: Walter Mcneil | Filed under: Hormone

    RESULTS(3)

    FSH surge. In general, the FSH responses to surge-inducing levels of estradiol were qualitatively similar to those of LH, although there were some subtle differences. In part 1 of the experiment when the competency of the surge system was tested in the absence of progesterone priming, the D60-90 ewes started their FSH surges 6 h later (P < 0.001), exhibited surge peak 7 h later (P < 0.01), and ended their surge 8 h later (P < 0.001) than did the controls (Figs. 6 and 7). D30-90 animals showed a greater delay in the time of onset, peak, and end of FSH surges relative to the D60-90 animals and the controls (Figs. 6 and 7). The small number of animals in this group precluded statistical analysis. In part 2 of the experiment (with progesterone priming), the time of onset of FSH surges in the D60-90 ewes was comparable to that of controls, although there was a delay in the time to peak (P < 0.001, Fig. 6) and end (P < 0.05, Fig. 7) of the FSH surges. One of the D30-90 androgenized ewes that did not produce an LH surge also failed to produce an FSH surge in both parts of the experiment.

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  • Puberty in Female Sheep: RESULTS(2)

    Jul 23, 2013 Author: Walter Mcneil | Filed under: Hormone

    There was a longer delay in the timing of the onset of the LH surges in the D30-90 androgenized ewes (27 ¬Ī 1.0 h) compared with the D60-90 androgenized females, although the small number of animals in the D30-90 group did not permit a valid statistical comparison. The duration of LH surges in the D30-90 animals was longer than that in the control group but similar to that in the D60-90 group. One of the D30-90 androgenized ewes did not show a definable surge. The magnitude of the LH surge in the two D30-90 ewes that produced surges was highly reduced (one third of the control and D60-90 group values) (Fig. 5).

    In part 2 of the experiment (progesterone pretreatment), the beginning time, ending time, duration, and magnitude of the LH surge were not different between the control and D60-90 groups (Fig. 5). Although the LH surges started at the same time in the D60-90 and control animals, the LH surge peak was achieved later in the D60-90 group compared with the control group (P < 0.005). The interval between the onset and the peak of LH surge in the D60-90 group was approximately 2.5 h longer (P < 0.001) than that in the controls (Fig. 4). In the D30-90 androgenized group, one of the ewes (same ewe that did not show any surge in part 1 of the experiment) showed no LH surge. The other two ewes showed delay in the onset of the LH surge as compared with the controls and the D60-90 androgenized group. As in part 1 of the experiment, the magnitude of the LH surge was reduced in the D30-90 androgenized ewes, but the D60-90 androgenized ewes did not differ from the controls (Figs. 4 and 5).

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