Evidence that Melatonin Acts in the Pituitary Gland through a Dopamine‐independent Mechanism to Mediate Effects of Daylength on the Secretion of Prolactin in the Ram
Evidence that Melatonin Acts in the Pituitary Gland through a Dopamine‐independent Mechanism to Mediate Effects of Daylength on the Secretion of Prolactin in the Ram
复制标题
有证据表明褪黑素通过不依赖多巴胺的机制在脑垂体中发挥作用，调节日长对公羊催乳素分泌的影响
DOI：
10.1111/j.1365-2826.1995.tb00802.x
复制
发表时间：
1995
影响因子：
3.2
通讯作者：
Iain J. Clarke
中科院分区：
文献类型：
--
作者：
G. A. Lincoln;Iain J. Clarke
先前的一项研究提供的证据表明，褪黑激素在脑下垂体中发挥作用，介导日长对绵羊催乳素分泌的影响。这是基于以下观察：下丘脑垂体断开 (HPD) 的索伊公羊的外周血催乳素浓度变化显示出正常模式，以响应光周期的变化（长日照下浓度比短日照高 10 倍），以及对外源性褪黑激素（施用持续释放的褪黑激素植入物后迅速下降）。本研究的目的是确定多巴胺 (DA) 是否可能参与介导褪黑激素对催乳素分泌的影响。 HPD 组 (n = 7) 和对照 Soay 公羊 (n = 8) 分别用媒介物（对照，2.0 ml 0.1 M 酒石酸/盐水，皮下注射）、溴隐亭（DA 激动剂，皮下注射 0.06 mg/kg）或舒必利（DA 拮抗剂，皮下注射 0.6 mg/kg）治疗，并在接下来的 4 小时内测量急性催乳素反应。治疗分别在短日照（8L：16天，低催乳素）、长日照（16L：8天，高催乳素）和长日照（有褪黑激素持续释放植入物的情况下）进行（低催乳素）。还测量了催乳素对TRH（1.25μg/kg iv）的反应。溴隐亭在短日照和长日照下均导致 HPD 和对照公羊的血浆催乳素浓度降低。舒必利在任何情况下都对 HPD 公羊没有影响，但在短日照、长日照和长日照+褪黑激素下引起对照公羊催乳素血浆浓度非常显着的增加。 TRH 导致 HPD 公羊在长日照和短日照下血浆催乳素浓度急剧增加，尽管与对照组相比，反应显着降低，特别是在长日照 + 褪黑激素下。总体而言，HPD 公羊对 DA 激动剂的抑制反应表明，离体垂体中存在与功能性催乳素相关的 DA D2 受体。然而，对 DA 拮抗剂完全没有反应表明 HPD 公羊中不存在调节催乳素分泌的内源 DA 机制。结论是褪黑素直接作用于垂体，通过不依赖 DA 的机制介导光周期的影响。
A previous study provided evidence that melatonin acts in the pituitary gland to mediate the effects of daylength on the secretion of prolactin in sheep. This was based on the observation that hypothalamo‐pituitary disconnected (HPD) Soay rams showed normal patterns in the changes in the peripheral blood concentrations of prolactin in response to alterations in photoperiod (10‐fold higher concentrations under long than short days), and in response to exogenous melatonin (rapid decline following the administration of a constant‐release implant of melatonin). The purpose of this study was to establish whether dopamine (DA) might be involved in mediating the effects of melatonin on the secretion of prolactin. Groups of HPD (n = 7) and control Soay rams (n = 8) were treated with vehicle (control, 2.0 ml 0.1 M tartaric acid/saline sc), bromocriptine (DA agonist, 0.06 mg/kg sc) or sulpiride (DA antagonist, 0.6 mg/kg sc), and the acute prolactin responses were measured over the next 4 h. Treatments were carried out under short days (8L:16D, low prolactin), long days (16L:8D, high prolactin), and under long days in the presence of a constant‐release implant of melatonin (low prolactin). The prolactin response to TRH (1.25μg/kg iv) was also measured. Bromocriptine caused a decrease in the plasma concentrations of prolactin in both HPD and control rams under short and long days. Sulpiride had no effect in the HPD rams on any occasion, but caused a very marked increase in the plasma concentrations of prolactin in the control rams under short days, long days, and under long days + melatonin. TRH caused an acute increase in the plasma concentrations of prolactin in the HPD rams under both long and short days although the responses were notably reduced compared with the controls especially under long days + melatonin. Overall, the inhibitory response to the DA agonist in HPD rams indicates the presence of DA D2 receptors linked to functional lactotrophs in the isolated pituitary gland. However, the total lack of a response to the DA antagonist indicates the absence of endogenous DA mechanisms regulating the secretion of prolactin in the HPD rams. The conclusion is that melatonin acts directly on the pituitary gland to mediate effects of photoperiod through a DA‐independent mechanism.