Robinson J E, Kaynard A H, Karsch F J
Neuroendocrinology. 1986;43(6):635-40. doi: 10.1159/000124593.
The diurnal secretion of melatonin from the pineal gland transduces information about day length to the reproductive axis of many seasonal breeders including the ewe. In the sheep the target for melatonin is thought to be neural, such that the hormone acts through the GnRH pulse generator to produce seasonal alterations in the frequency of pulsatile LH secretion. These effects on the pulse generation mechanism take approximately 50 days to become evident. It is possible that melatonin also exerts direct effects at the level of the pituitary gland to alter responsiveness to GnRH. Such effects have been noted in other species. The site of action of melatonin to regulate pulsatile LH secretion was assessed in the ewe by determining whether the animal's endogenous melatonin acutely modifies pituitary responsiveness to sustained pulsatile administration of GnRH. Using an animal model in which endogenous GnRH was blocked, pituitary responsiveness to hourly pulses of exogenous GnRH was assessed under conditions of both high (dark period) and low (light period) melatonin. No evidence for acute effects of melatonin on pituitary response to GnRH was found. In another experiment, the amplitude and frequency of endogenously generated LH pulses in ovariectomized ewes was found not to change during the 24-hour light/dark cycle. These data lead to the conclusion that melatonin does not act at the pituitary gland to produce acute effects on LH secretion. Rather, our findings are consistent with the hypothesis that the action of melatonin, in this short-day breeder is long term, and is directed towards the neural elements of the hypothalamic pulse-generating mechanism.
松果体分泌褪黑素呈现昼夜节律，该节律将白昼时长信息传递给包括母羊在内的许多季节性繁殖动物的生殖轴。在绵羊中，褪黑素的作用靶点被认为是神经组织，即这种激素通过促性腺激素释放激素（GnRH）脉冲发生器发挥作用，从而使促黄体生成素（LH）脉冲分泌频率产生季节性变化。这些对脉冲生成机制的影响大约需要50天才能显现出来。褪黑素也可能在垂体水平发挥直接作用，改变对GnRH的反应性。在其他物种中也观察到了这种作用。通过确定动物内源性褪黑素是否能急性改变垂体对持续脉冲式给予GnRH的反应性，来评估褪黑素调节LH脉冲分泌的作用位点。使用一种阻断内源性GnRH的动物模型，在高（黑暗期）、低（光照期）褪黑素条件下评估垂体对外源性GnRH每小时脉冲的反应性。未发现褪黑素对垂体对GnRH反应有急性影响的证据。在另一项实验中，发现去卵巢母羊内源性LH脉冲的幅度和频率在24小时昼夜周期中没有变化。这些数据得出结论，褪黑素不会在垂体水平发挥作用对LH分泌产生急性影响。相反，我们的研究结果与以下假设一致：在这种短日照繁殖动物中，褪黑素的作用是长期的，且针对下丘脑脉冲生成机制的神经元件。
