US Environmental Protection Agency, Office of Research and Development, National Health and Environmental Effects Research Laboratory, Mid-Continent Ecology Division, 6201 Congdon Boulevard, Duluth, MN 55804, USA.
Gen Comp Endocrinol. 2011 Sep 15;173(3):428-37. doi: 10.1016/j.ygcen.2011.06.020. Epub 2011 Jul 22.
We used ex vivo and in vivo experiments with Xenopus laevis tadpoles to examine the hypothesis that the set-point for negative feedback on pituitary thyroid-stimulating hormone (TSH) synthesis and secretion by thyroid hormones (THs) increases as metamorphosis progresses to allow for the previously documented concomitant increase in serum TH concentrations and pituitary TSH mRNA expression during this transformative process. First, pituitaries from climactic tadpoles were cultured for up to 96 h to characterize the ability of pituitary explants to synthesize and secrete TSHβ in the absence of hypothalamic and circulating hormones. Next, pituitary explants from tadpoles NF stages 54-66 were exposed to physiologically-relevant concentrations of THs to determine whether stage-specific differences exist in pituitary sensitivity to negative feedback by THs. Finally, in vivo exposures of tadpoles to THs were conducted to confirm the results of the ex vivo experiments. When pituitaries from climactic tadpoles were removed from the influence of endogenous hormones, TSHβ mRNA expression increased late or not at all whereas the rate of TSHβ secreted into media increased dramatically, suggesting that TSH secretion, but not TSH mRNA expression, is under the negative regulation of an endogenous signal during the climactic stages of metamorphosis. Pituitaries from pre- and prometamorphic tadpoles were more sensitive to TH-induced inhibition of TSHβ mRNA expression and secretion than pituitaries from climactic tadpoles. The observed decrease in sensitivity of pituitary TSHβ mRNA expression to negative feedback by THs from premetamorphosis to metamorphic climax was confirmed by in vivo experiments in which tadpoles were reared in water containing THs. Based on the results of this study, a model is proposed to explain the seemingly paradoxical, concurrent rise in serum TH concentrations and pituitary TSH mRNA expression during metamorphosis in larval anurans.
我们使用 Xenopus laevis 蝌蚪的离体和体内实验来检验以下假说：甲状腺激素（THs）对垂体促甲状腺激素（TSH）合成和分泌的负反馈设定点随着变态发育的进展而增加，以允许在这个转型过程中同时记录到血清 TH 浓度和垂体 TSH mRNA 表达的增加。首先，对处于变态高潮期的蝌蚪的垂体进行培养，最长可达 96 小时，以表征垂体离体组织在没有下丘脑和循环激素的情况下合成和分泌 TSHβ 的能力。接下来，用生理相关浓度的 THs 处理 NF 期 54-66 的蝌蚪的垂体离体组织，以确定垂体对 TH 负反馈的敏感性是否存在阶段特异性差异。最后，对蝌蚪进行体内 TH 暴露实验，以确认离体实验的结果。当去除处于变态高潮期的蝌蚪的内源激素的影响时，TSHβmRNA 表达增加较晚或根本不增加，而 TSHβ分泌到培养基中的速率则急剧增加，这表明 TSH 分泌而不是 TSH mRNA 表达受到内源性信号的负调控在变态高潮期。与处于变态高潮期的蝌蚪的垂体相比，处于预变态和前变态期的蝌蚪的垂体对 TH 诱导的 TSHβmRNA 表达和分泌的抑制更为敏感。通过在含有 TH 的水中饲养蝌蚪的体内实验，证实了从预变态到变态高潮期，垂体 TSHβmRNA 表达对 TH 负反馈的敏感性降低。根据这项研究的结果，提出了一个模型来解释在幼蛙的变态过程中血清 TH 浓度和垂体 TSH mRNA 表达同时升高的看似矛盾的现象。
