Sistema tiroideo y osmorregulación en la dorada (Sparus aurata)

Resumen

Introduction The endocrine management of electrolyte transport seems to be critical for the acclimation to the environment in teleost fish. The mechanisms involved in hormonal control of osmoregulation has been deeply studied (Gregorio et al., 2013; McCormick, 2001; McCormick, 2013; McCormick and Bradshaw, 2006; Sakamoto et al., 1993; Takei and McCormick, 2013) establishing the role of hormones like PRL, GH/IGF-I, cortisol, AVT, IT, PTHrP, STC in these processes. However, the function of the thyroid hormones although important, remains unclear. The thyroid system is also relevant in fish energy metabolism regulation (Kushner, 1996; Ness et al., 1969; Pramfalk et al., 2011; Shameena et al., 2000; Soengas et al., 1992; Varghese and Oommen, 1999), unless its connection with the osmoregulation processes is still controversial. The gilthead seabream (Sparus aurata) is a commercially relevant euryhaline species for Mediterranean and European countries. As it can tolerate a wide range of environmental salinities, its osmoregulatory system has been studied from many different points of view (Balmaceda-Aguilera et al., 2012; Gregorio et al., 2013; Laiz-Carrion et al., 2009; Laiz-Carrion et al., 2005; Martos-Sitcha et al., 2014; Sangiao-Alvarellos et al., 2003; Vargas-Chacoff et al., 2009). That makes it an excellent model to study the implications of the thyroid system over the adaptive processes to different osmotic environments. In the present Doctoral Thesis, the interactions between environmental salinity and thyroid system under different experimental approaches were analyzed. For that reason, we have focused on the study of the most important osmoregulatory tissues in teleost fish (gills and kidney). Moreover, due to the strong energy rate involved in adaptation to different environmental salinities, and the implications of the thyroid system on metabolism, some enzymatic approaches were performed. Theoretical development Therefore, the study of those topics will be developed in the following chapters. Chapter 2 describes how environmental salinity can modify the thyroid system in S. aurata. Special relevance is given to peripheral regulation of the thyroid hormones. Chapter 3 is focused on the central hypothalamic control of the thyroid system. As it was postulated that corticotropin-releasing hormone, and not thyrotropin-releasing hormone (like in mammals), controlled the thyroid axis, we have therefore developed some molecular tools to test this hypothesis. Chapter 4 describes the effects of a hypothyroid drug, the PTU, over the acclimation to different environmental salinities in the gilthead seabream. Chapter 5 clarifies the relationships between the interwoven hypothalamus-pituitary-thyroid (HPT) and the hypothalamus-pituitary-interrenal (HPI) axes at central and peripheral levels. Chapter 6 describes the effects of T3 treatment over carbohydrate, lipid and protein metabolism in this species. Chapter 7 resumes and discusses the main results obtained in the present Doctoral Thesis. Chapter 8 reports the main conclusions and future perspectives derived from the present Doctoral Thesis. Chapter 9 resumes the conclusions derived from the present Doctoral Thesis. As extra given information, an Appendix has been added at the end of the present manuscript. It shows the epistolary exchange between the International Committee on Zoological Nomenclature and Ignacio Ruiz-Jarabo discoursing on the possibility of changing the scientific name of the gilthead seabream as it is actually wrongly written. Conclusions 1. Changes in environmental salinity evoke changes in the thyroid system of Sparus aurata as seen by its ppTRH, CRH and TSHß expression, free plasma thyroid hormones levels and deiodinases expression and activities. Thus, low salinity environments (5 and 15 ppt salinity) induced hyperthyroid conditions, while hypersaline conditions (40 and 55 ppt) induced hypothyroidism. 2. The goitrogen PTU properly induced hypothyroidism, as it affected thyroid follicles in S. aurata. Growth rates are increased in PTU-induced hypothyroid specimens, without lipid accumulation. 3. The hypothalamus-pituitary-thyroid axis is centrally controlled by CRH in S. aurata, with no data indicating TRH control. Moreover, THs exert a negative feedback over the expression of TSHß and POMCb. These data suggests the cross-relationships between the thyroid and the interrenal systems in teleost fish. 4. Hyperthyroidism induced by T3 implants increase protein anabolism that, in turn, affects osmoregulation indirectly. This occurs due to the necessity of excrete the final end products of protein catabolism (e.g. ammonium). 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