Expression of Chicken Thyroid-Stimulating Hormone β-Subunit Messenger Ribonucleic Acid during Embryonic and Neonatal Development*

Abstract

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.