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In teleostean fishes the hypothalamic–pituitary–thyroid axis (HPT axis) and the hypothalamic–pituitary–interrenal axis (HPI axis) regulate the release of thyroid hormones (THs) and cortisol respectively. Since many actions of both hormones are involved in the regulation of metabolic processes, communication between both signal pathways can be anticipated. In this study, we describe central and peripheral sites for direct interaction between mediators of both neuroendocrine axes in the common carp (Cyprinus carpio). Despite suggestions in the literature that CRH is thyrotropic in some fish; we were not able to establish stimulatory effects of CRH on the expression of the pituitary TSHβ subunit gene. In preoptic area tissue incubated with 10−7 M thyroxine (T4) a 2.9-fold increase in the expression of CRH-binding protein (CRHBP) was observed. Thus, T4 could reduce the bioavailable hypothalamic crh via the up regulation of crhbp expression and hence down regulate the HPI axis. At the peripheral level, cortisol (10−6 M), ACTH (10−7 M), and α-MSH (10−7 M) stimulate the release of T4 from kidney and head kidney fragments, which contain all functional thyroid follicles in carp, by two- to fourfold. The substantiation of three pituitary thyrotropic factors, viz. TSH, ACTH, and α-MSH, in common carp, allows for an integration of central thyrotropic signals. Clearly, two sites for interaction between the HPT axis, the HPI axis, and α-MSH are present in common carp. These interactions may be key to the proper regulation of general metabolism in this fish.
Department of Animal Physiology, Departamento de Biología, Instituto de Zoología, Institute for Water and Wetland Research, Faculty of Science, Radboud University Nijmegen, Heyendaalseweg 135, 6525 AJ Nijmegen, The Netherlands
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Department of Animal Physiology, Departamento de Biología, Instituto de Zoología, Institute for Water and Wetland Research, Faculty of Science, Radboud University Nijmegen, Heyendaalseweg 135, 6525 AJ Nijmegen, The Netherlands
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The thyroid gland in fish mainly secretes the thyroid prohormone 3,5,3′,5′-tetraiodothyronine (T4), and extrathyroidal outer ring deiodination (ORD) of the prohormone to 3,5,3′-triiodothyronine (T3) is pivotal in thyroid hormone economy. Despite its importance in thyroid hormone metabolism, factors that regulate ORD are still largely unresolved in fish. In addition, the osmoregulatory role of T3 is still a controversial issue in teleosts. In this study, we investigated the regulation of the ORD pathway by cortisol and T3 in different organs (liver, kidney, and gills) of Solea senegalensis and the involvement of T3 in the control of branchial and renal Na+, K+-ATPase activity, a prime determinant of the hydromineral balance in teleosts. Animals were treated with i.p. slow-release coconut oil implants containing cortisol or T3. Hepatic and renal ORD activities were up-regulated in cortisol-injected animals. T3-treated fish showed a prominent decrease in plasma-free T4 levels, whereas ORD activities did not change significantly. Branchial and renal Na+, K+-ATPase activities were virtually unaffected by T3, but were transiently up-regulated by cortisol. We conclude that cortisol regulates local T3 bioavailability in S. senegalensis via ORD in an organ-specific manner. Unlike T3, cortisol appears to be directly implicated in the up-regulation of branchial and renal Na+, K+-ATPase activities.
