Moenter S M, Woodfill C J, Karsch F J
Reproductive Sciences Program, University of Michigan, Ann Arbor 48109-0404.
Endocrinology. 1991 Mar;128(3):1337-44. doi: 10.1210/endo-128-3-1337.
Seasonal reproductive transitions in ewes are generated endogenously and are synchronized by annual changes in photoperiod. Previous evidence indicates that thyroidectomy prevents the transition to anestrus in ewes maintained in a fixed day length, suggesting that the thyroid is needed for endogenously generated reproductive arrest. Here we tested the hypothesis that the thyroid is required for endogenous seasonal suppression of the neuroendocrine mechanism that regulates pulsatile LH secretion. Ewes were thyroidectomized (n = 6) in summer, 6 weeks before the onset of the breeding season, or they remained thyroid intact (n = 6). They were housed in a simulated natural photoperiod until the winter solstice; thereafter, they remained on that photoperiod (10 h of light, 14 h of darkness). To monitor pulsatile LH secretion, the ewes were ovariectomized and implanted with estradiol, and LH was measured in both frequent (every 6 min for 4 h) and infrequent (twice weekly) blood samples. In this model, high LH indicates low response to estradiol negative feedback and reproductive induction; low LH signifies high response to estradiol negative feedback and reproductive arrest. LH levels (samples twice weekly) rose some 50-fold in both groups concurrently at the start of the breeding season in September. Frequent sampling in midbreeding season (autumn) revealed that both thyroidectomized and thyroid-intact ewes exhibited frequent LH pulses, with no group difference, in either the presence or absence of the estradiol implant. A marked group difference, however, emerged at the end of the breeding season. LH fell to basal values in thyroid-intact ewes (onset of low values Feb 3 +/- 8 days), whereas levels remained elevated in thyroidectomized ewes through the end of the study (April 26). At this time, thyroidectomized ewes had more frequent LH pulses than thyroid-intact ewes both in the presence and absence of estradiol. The circadian pattern of melatonin secretion and the seasonal change in PRL were found to be unaffected by thyroidectomy. These observations support the hypothesis that the thyroid is necessary for endogenous suppression of neuroendocrine mechanisms that generate LH pulses, a suppression crucial for the transition to anestrus.
母羊的季节性繁殖转换是内源性产生的，并通过光周期的年度变化实现同步。先前的证据表明，甲状腺切除术可防止处于固定日长环境中的母羊进入发情间期，这表明甲状腺是内源性产生繁殖停滞所必需的。在此，我们检验了以下假设：甲状腺对于内源性季节性抑制调节促黄体生成素（LH）脉冲式分泌的神经内分泌机制是必需的。在繁殖季节开始前6周的夏季，对6只母羊进行甲状腺切除手术（n = 6），另外6只母羊则保留完整甲状腺（n = 6）。将它们置于模拟自然光照周期的环境中直至冬至；此后，它们一直处于该光照周期（10小时光照，14小时黑暗）。为监测LH的脉冲式分泌，对母羊进行卵巢切除并植入雌二醇，同时在频繁（每6分钟一次，共4小时）和不频繁（每周两次）采集的血样中测量LH。在这个模型中，LH水平高表明对雌二醇负反馈和繁殖诱导的反应低；LH水平低则表示对雌二醇负反馈和繁殖停滞的反应高。9月繁殖季节开始时，两组母羊的LH水平（每周两次采样）同时升高了约50倍。繁殖中期（秋季）的频繁采样显示，无论有无雌二醇植入，甲状腺切除的母羊和甲状腺完整的母羊都表现出频繁的LH脉冲，两组之间无差异。然而，在繁殖季节结束时出现了明显的组间差异。甲状腺完整的母羊LH降至基础值（低值开始时间为2月3日±8天），而甲状腺切除的母羊在研究结束时（4月26日）LH水平仍保持升高。此时，无论有无雌二醇，甲状腺切除的母羊LH脉冲都比甲状腺完整的母羊更频繁。褪黑素分泌的昼夜模式和催乳素的季节性变化不受甲状腺切除术的影响。这些观察结果支持了以下假设：甲状腺对于内源性抑制产生LH脉冲的神经内分泌机制是必要的，这种抑制对于进入发情间期至关重要。
