Viguié C, Thibault J, Thiéry J C, Tillet Y, Malpaux B
Institut National de la Recherche Agronomique, Physiologie de la Reproduction des Mammiferes Domestiques, Laboratoire de Neuroendocrinologie Sexuelle, Nouzilly, France.
Endocrinology. 1997 Jan;138(1):499-506. doi: 10.1210/endo.138.1.4865.
In the ewe, photoperiod modulates LH and PRL secretion as well as median eminence (ME) dopaminergic activity. The studies reported here were designed to characterize the functional significance of this photoperiodic modulation of ME dopaminergic neuron activity in relation to the regulation of LH and PRL secretion. The aim of the first experiment was to assess whether photoperiodic changes in hypothalamic dopaminergic activity were temporally linked to changes in either PRL or LH secretion. The purpose of the second experiment was to determine whether melatonin mimicked the effects of photoperiod on ME dopaminergic activity. In the first experiment, LH and PRL secretion, hypothalamic tyrosine hydroxylase (TH) activity, and catecholamine contents were determined in ovariectomized estradiol-treated ewes either during long days (LD; control group) or after 5, 25, and 76 short days (SD). SD were associated with a stimulation of LH secretion and a decrease in ME TH activity, which were both expressed only in the 76 SD group. In contrast, the SD-induced inhibition of PRL secretion was already maximal in the 25 SD group. In the second experiment, LH secretion and hypothalamic dopaminergic activity were studied in ovariectomized estradiol-treated ewes kept in LD and then treated for 0 (control), 25, or 77 days with melatonin implants producing a SD-like effect on LH secretion. Melatonin induced a decrease in PRL secretion (observed after 25 days of treatment), as well as a stimulation of LH secretion and a decrease in ME TH activity and dopamine content (observed only after 77 days of treatment). In conclusion, the decrease in ME dopaminergic activity associated with SD exposure or the SD-like effect of melatonin appears unrelated to the regulation of PRL secretion. The SD-like effect of melatonin on ME dopaminergic activity suggests that melatonin mediates the effect of SD on this activity. The regulation of ME dopaminergic activity can thus be considered a probable step in the photoperiodic regulation of LH secretion.
在母羊中，光周期调节促黄体生成素（LH）和催乳素（PRL）的分泌以及正中隆起（ME）的多巴胺能活性。本文报道的研究旨在阐明ME多巴胺能神经元活性的这种光周期调节与LH和PRL分泌调节相关的功能意义。第一个实验的目的是评估下丘脑多巴胺能活性的光周期变化是否与PRL或LH分泌的变化存在时间上的关联。第二个实验的目的是确定褪黑素是否模拟光周期对ME多巴胺能活性的影响。在第一个实验中，对去卵巢并经雌二醇处理的母羊，在长日照（LD；对照组）期间或5、25和76个短日照（SD）后，测定LH和PRL分泌、下丘脑酪氨酸羟化酶（TH）活性以及儿茶酚胺含量。短日照与LH分泌的刺激以及ME TH活性的降低相关，这两种变化仅在76个短日照组中出现。相反，短日照诱导的PRL分泌抑制在25个短日照组中已达到最大值。在第二个实验中，对去卵巢并经雌二醇处理的母羊进行研究，这些母羊先处于长日照，然后分别用对LH分泌产生类似短日照效应的褪黑素植入物处理0（对照）、25或77天。褪黑素诱导PRL分泌减少（在处理25天后观察到），以及LH分泌的刺激和ME TH活性及多巴胺含量的降低（仅在处理77天后观察到）。总之，与短日照暴露或褪黑素的类似短日照效应相关的ME多巴胺能活性降低似乎与PRL分泌的调节无关。褪黑素对ME多巴胺能活性的类似短日照效应表明，褪黑素介导了短日照对该活性的影响。因此，ME多巴胺能活性的调节可被视为LH分泌光周期调节中的一个可能步骤。
