基于下丘脑-垂体-甲状腺-肝脏轴探讨多氯联苯影响牙鲆变态的机制

During metamorphism of Paralichthy solivaceus, high mortality happens due to several intrinsic and extrinsic factors including environmental conditions. In the present study, effects of thyroid-disrupting chemicals on the metamorphism of Paralichthy solivaceus were investigated from the perspective of water pollution, and effects on the hypothalamic-pituitary-thyroid axis were conducted to explore the underlying mechanisms. First, the response of metamorphism period, metamorphism rate, external morphology and ecological habit was studied to quantify the influences of polychlorinated biphenyls at environmentally relevant concentrations on metamorphism of Paralichthy solivaceus. Second, thyroid hormone levels in response was determined, to prove that the adverse influences on metamorphism were caused by the thyroid-disrupting action. Finally, the effects on the hypothalamic-pituitary-thyroid axis were systematically evaluated by hormone synthesis, hormone convert and regulation of hypothalamus and pituitary, with the aim of finding out the intrinsic mechnism underlying metamorphism abnormality, delay and/or failure of Paralichthy solivaceus. A new systerm on thyroid-disrupting action of xenobiotics would be established utilizing Paralichthy solivaceus metamorphism test, and this study opened up a new perspective to the study of Paralichthy solivaceus metamorphism, namely water pollution, and provided the theoretical proofs of teleosts intensive seed cultivation.

变态期是牙鲆大量死亡的临界期，环境因素对牙鲆变态的影响是引起临界期高死亡率的重要原因之一。本项目拟从水污染角度分析甲状腺干扰物对牙鲆变态的影响，并基于下丘脑-垂体-甲状腺-肝脏轴探讨其影响机制。首先研究环境浓度多氯联苯(PCBs)暴露情况下，牙鲆由仔鱼向幼鱼变态的时间是否延迟，变态率是否改变，外部形态及生态习性是否受到干扰，明确PCBs对牙鲆变态的影响；其次，探讨PCBs对牙鲆甲状腺激素水平的影响与导致变态异常、延迟或失败的直接关系，证实PCBs对牙鲆变态的影响是由其甲状腺干扰效应引起的；并进一步从激素合成、转化、下丘脑和垂体调控作用等多个角度，系统研究PCBs对牙鲆甲状腺轴的干扰效应，查明PCBs导致牙鲆变态异常、延迟或失败的机制。本项目将运用牙鲆变态试验，构建一个系统研究外源化合物甲状腺干扰效应的新体系，开辟从水污染角度分析牙鲆变态异常的新视角，为牙鲆集约化健康苗种培育提供理论依据。

在鱼类甲状腺干扰研究中并没有公认的模式生物和系统化的指标体系。本研究以曾经广泛使用的商品化PCBs混合物Aroclor 1254为代表，研究环境相关水平的多氯联苯(PCBs)暴露(10、100、1000 ng/L)对褐牙鲆幼鱼甲状腺的干扰效应，检测了甲状腺组织学变化、血浆甲状腺激素(THs)水平(总的T3、T4 (TT3、TT4)水平和游离的T3、T4 (FT3、FT4)水平)、三种脱碘酶(IDs, ID1、ID2和ID3) mRNA表达水平，从中筛选敏感且稳定的指标；同时研究了PCBs对褐牙鲆仔鱼变态过程的影响，并从变态过程调控的关键激素—甲状腺激素的角度解释PCBs影响变态过程的机制。. 对幼鱼甲状腺干扰的研究结果表明：Aroclor1254暴露影响了褐牙鲆幼鱼的生长；Aroclor1254暴露干扰了幼鱼THs水平，暴露50天后100和1000 ng/L组血浆TT4水平显著低于对照组，1000ng/L组血浆TT3和FT3水平显著低于对照组；Aroclor1254暴露影响了幼鱼甲状腺组织结构，100和1000 ng/L暴露组甲状腺滤泡细胞高度，增生情况和胶质缺损情况均显著高于对照组；Aroclor1254干扰了IDs 尤其是ID2 mRNA的表达。对仔鱼变态过程和THs水平的影响：Aroclor 1254暴露延迟了褐牙鲆仔鱼的变态过程，对照组仔鱼在37dph所有仔鱼均完成了变态过程，100 ng/L暴露组至实验结束只有38%的仔鱼完成变态，1000 ng/L暴露组至实验结束只有21%的仔鱼完成变态；Aroclor 1254暴露抑制了THs水平的升高；Aroclor 1254暴露影响了甲状腺组织结构，暴露38天后1000 ng/L暴露组甲状腺滤泡趋于小型化，数量也明显少于对照组，并出现滤泡上皮细胞增高，增生和胶质缺损现象，说明Aroclor 1254暴露抑制了甲状腺滤泡的合成。. 本文发现环境相关浓度的PCBs暴露即对褐牙鲆幼鱼产生了明显的甲状腺干扰效应；首次发现TDCs能够影响鱼类的变态过程。