Webster J R, Moenter S M, Woodfill C J, Karsch F J
Reproductive Sciences Program, University of Michigan, Ann Arbor 48109-0404.
Endocrinology. 1991 Jul;129(1):176-83. doi: 10.1210/endo-129-1-176.
A series of four experiments was conducted to examine the previously described phenomenon that the thyroid gland is required for the breeding season to end in female sheep. Exp 1 tested the hypothesis that the thyroid is required because of its secretion of T4. Ewes thyroidectomized (THX) in the anestrous season either received T4 replacement or no further treatment; seasonal reproductive shifts were compared to those of thyroid-intact controls. All ewes were housed in conditions in which the end of the breeding season results from the expression of an endogenous annual rhythm. The ewes were ovariectomized and given constant-release estradiol (E) implants; reproductive state was assessed from serum LH levels to monitor seasonal shifts in response to E negative feedback. THX did not alter onset of the breeding season (LH rise) but, in the absence of T4 replacement, blocked its end (LH fall). T4 replacement reversed this effect of THX. Exp 2 tested the hypothesis that the thyroid is required only until the onset of the breeding season for reproductive activity to end at its normal time. THX after the breeding season began, however, sustained the elevation in LH, suggesting that the thyroid is required after reproductive onset for the breeding season to end. Exp 3 tested the hypothesis that THX causes a widespread disruption of steroid feedback responses. No effect of THX, however, was observed with respect to either the ability of an E rise to elicit the LH surge or the ability of E, or progesterone, to suppress LH secretion in the breeding season. Of the steroid feedback responses tested, THX altered only the seasonal shift in potency of E negative feedback. Exp 4 examined circulating T4 in thyroid-intact ewes over a 2-yr period. An annual cycle of serum T4 was detected; values reached a peak in winter (late breeding season) and a nadir in summer (late anestrus). Our findings support the concept that the thyroid gland plays a fundamental role in seasonal reproduction in the ewe. In this regard, secretion of T4 after the onset of reproductive activity is required for an endogenously generated change in the neuroendocrine axis that leads to an intensified E negative feedback and an end to the breeding season.
进行了一系列四项实验，以研究先前描述的现象，即雌性绵羊繁殖季节结束需要甲状腺。实验1检验了以下假设：需要甲状腺是因为其分泌甲状腺素（T4）。在乏情季节进行甲状腺切除（THX）的母羊要么接受T4替代治疗，要么不再接受进一步治疗；将季节性繁殖变化与甲状腺完整的对照组进行比较。所有母羊都饲养在繁殖季节结束是由内源性年度节律表达导致的条件下。母羊进行卵巢切除并植入缓释雌二醇（E）；通过血清促黄体生成素（LH）水平评估繁殖状态，以监测对E负反馈的季节性变化。THX并未改变繁殖季节的开始（LH升高），但在没有T4替代治疗的情况下，阻断了繁殖季节的结束（LH下降）。T4替代治疗逆转了THX的这种作用。实验2检验了以下假设：甲状腺仅在繁殖季节开始前是必需的，以便繁殖活动能在正常时间结束。然而，在繁殖季节开始后进行THX，会使LH持续升高，这表明繁殖开始后甲状腺对于繁殖季节结束是必需的。实验3检验了以下假设：THX会导致类固醇反馈反应的广泛破坏。然而，在繁殖季节，无论是E升高引发LH激增的能力，还是E或孕酮抑制LH分泌的能力，均未观察到THX有任何影响。在所测试的类固醇反馈反应中，THX仅改变了E负反馈效力的季节性变化。实验4在两年时间内检测了甲状腺完整的母羊体内循环T4的情况。检测到血清T4的年度周期；其值在冬季（繁殖季节后期）达到峰值，在夏季（乏情后期）达到最低点。我们的研究结果支持这样的概念，即甲状腺在母羊的季节性繁殖中起基本作用。在这方面，繁殖活动开始后T4的分泌对于神经内分泌轴内源性产生的变化是必需的，这种变化会导致E负反馈增强以及繁殖季节结束。
