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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.
