Gregory C C, Dean C E, Porter T E
Department of Poultry Science, Texas A&M University, College Station 77843-2472, USA.
Endocrinology. 1998 Feb;139(2):474-8. doi: 10.1210/endo.139.2.5756.
The importance of thyroid hormone from embryonic through neonatal life has been documented in both avian and mammalian species. However, the regulation of thyroid hormone production during this period is not completely understood. The objective of this study was to characterize expression of chicken TSHbeta messenger RNA (mRNA) compared with that of thyroid hormones and GH in embryonic and neonatal chickens. Total pituitary RNA was extracted on embryonic days (e-) 11, 13, 15, 17, and 19 and neonatal days (d-) 1, 3, 6, 9, and 12 and subjected to ribonuclease protection assays (RPA) for chicken TSHbeta mRNA. TSHbeta mRNA levels increased through e-19, with e-19 levels being greater than those at all other embryonic ages (P < 0.05). Levels decreased markedly on d-1, then slowly increased to d-6 and stayed elevated through d-12. RIAs were performed for T4, T3, and GH at the same ages. Serum T4 levels increased slowly from less than 1.0 ng/ml on e-11 to a peak of 6.6 ng/ml on d-1 (P < 0.05). After the peak on d-1, posthatch T4 levels stabilized between 3.5-4.5 ng/ml through d-12 (P < 0.05). T3 concentrations were less than 0.25 ng/ml on e-11, increased dramatically between e-19 and d-1 (P < 0.05), and remained high throughout the rest of the experiment, with a concentration of 3.25 ng/ml on d-6 (P < 0.05). GH levels for e-11 through e-17 were below the sensitivity of the GH RIA. On e-19, the GH level was 3 ng/ml and continued to increase through d-12 to a level of 130 ng/ml. As thyroid hormone levels were preceded by maximal TSHbeta mRNA levels on e-19, we next determined whether TSHbeta gene expression on e-19 was under TRH and T3 regulation. E-19 anterior pituitary cells were cultured in serum-free medium with either TRH (10[-8]) or T3 (10[-8]) for 20-24 h. Treatment with T3 significantly decreased levels of TSHbeta mRNA (P < 0.05). However, TRH did not produce a significant increase in TSHbeta mRNA, although TRH did increase TSHbeta mRNA by 60%, on the average, in this study. Therefore, these results indicate that an increase in pituitary TSH production probably regulates thyroid hormone levels during late embryonic development and that negative feedback inhibition of TSH production by thyroid hormones also exists at this critical developmental stage.
甲状腺激素在胚胎期至新生期的重要性已在鸟类和哺乳动物中得到证实。然而，这一时期甲状腺激素产生的调节机制尚未完全明确。本研究的目的是比较胚胎期和新生期鸡的促甲状腺激素β（TSHβ）信使核糖核酸（mRNA）与甲状腺激素及生长激素（GH）的表达特征。在胚胎第11、13、15、17和19天以及新生第1、3、6、9和12天提取垂体总RNA，进行鸡TSHβ mRNA的核糖核酸酶保护分析（RPA）。TSHβ mRNA水平在胚胎第19天前持续升高，胚胎第19天的水平高于其他所有胚胎期（P<0.05）。新生第1天水平显著下降，然后缓慢上升至第6天，并在第12天前保持升高。在相同年龄段进行甲状腺素（T4）、三碘甲状腺原氨酸（T3）和GH的放射免疫分析（RIA）。血清T4水平从胚胎第11天的低于1.0 ng/ml缓慢上升至新生第1天的峰值6.6 ng/ml（P<0.05）。在新生第1天达到峰值后，出壳后T4水平在第12天前稳定在3.5 - 4.5 ng/ml之间（P<0.05）。胚胎第11天T3浓度低于0.25 ng/ml，在胚胎第19天至新生第1天之间显著升高（P<0.05），并在实验剩余时间内保持较高水平，新生第6天浓度为3.25 ng/ml（P<0.05）。胚胎第11天至第17天的GH水平低于GH RIA的检测灵敏度。胚胎第19天，GH水平为3 ng/ml，并在新生第12天前持续上升至130 ng/ml。由于甲状腺激素水平在胚胎第19天出现TSHβ mRNA水平峰值之后达到高峰，我们接下来确定胚胎第19天的TSHβ基因表达是否受促甲状腺激素释放激素（TRH）和T3的调节。将胚胎第19天的垂体前叶细胞在无血清培养基中分别用TRH（10[-8]）或T3（10[-8]）处理20 - 24小时。用T3处理显著降低了TSHβ mRNA水平（P<0.05）。然而，TRH虽使TSHβ mRNA平均增加了60%，但未使其显著升高。因此，这些结果表明，垂体TSH分泌增加可能在胚胎发育后期调节甲状腺激素水平，并且在这个关键的发育阶段也存在甲状腺激素对TSH分泌的负反馈抑制。
