Quantification and regulation of thyroid stimulating hormone (TSH) and TSH messenger RNA in salmon
Larsen, Donald A., 1963-
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Thyroid activity is regulated by the hypothalamic-pituitary-thyroid axis. Using a recently developed radioimmunoassay (RIA) for coho salmon (Oncorhynchus kisutch) thyroid stimulating hormone (TSH), an in vitro pituitary cell culture technique was employed to examine regulation of TSH secretion by corticotropin-releasing hormone (CRH) family peptides (ovine CRH, carp urotensin I, and frog sauvagine) as well as thyrotropin-releasing hormone, salmon growth hormone-releasing hormone, and salmon gonadotropin-releasing hormone. The results of these experiments demonstrated the potency of a CRH-like peptide in the hypothalamic regulation of TSH in fish similar to that which has been observed by others in premetamorphic amphibians.In order to study salmon TSH transcription, an RNase protection assay (RPA) was designed for quantification of steady state levels of salmon TSH $\beta$ subunit mRNA expression. This assay, along with a similar RPA previously designed for coho salmon total $\alpha$-subunit mRNA, was used to examine the effects of feeding T3 and methimazole (a thyroid inhibitor) on TSH subunit gene expression. These experiments confirmed that, as in mammals, TSH $\alpha$ and $\beta$ subunit expression in teleosts may be differentially regulated by negative feedback from the thyroid hormones.Thyroid activity increases dramatically during salmonid smoltification. We quantified, and examined feedback relations between, pituitary TSH $\beta$ mRNA (by RPA) and pituitary and plasma TSH and plasma thyroid hormone levels (by RIA) during smoltification of coho salmon. Pituitary TSH $\beta$ mRNA changed dramatically, from high levels in winter to low levels in the spring, while pituitary and plasma TSH levels showed a small, but statistically non-significant change. In contrast with TSH protein levels, large increases in plasma T4 and modest increases in plasma T3 were observed. Regression analyses showed a significant positive relationship between plasma T4 and T3, and a negative relationship between plasma T3 and pituitary TSH $\beta$ mRNA. All other relations were not significant. These data are supportive of a peripheral (i.e., change in T4-T3 conversion, tissue sensitivity and clearance) rather than central regulatory pathway for thyroid activity in fish.
- Fisheries