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Department of Animal Physiology, Faculty of Science, Radboud University Nijmegen, Toernooiveld 1, 6525 ED Nijmegen, The Netherlands
(Requests for offprints should be addressed to P H M Klaren; Email: p.klaren{at}science.ru.nl)
(E J W Geven and F Verkaar contributed equally to this work)
The effect of experimental hyperthyroidism, realized by T4 injection, on central mediators of the hypothalamopituitaryinterrenal axis (HPI-axis) in common carp (Cyprinus carpio L.) was studied. Our results show that hyperthyroidism evokes a marked 3.2-fold reduction in basal plasma cortisol levels. Corticotropin-releasing hormone-binding protein (CRH-BP) mRNA levels in the hypothalamus, measured by real-time quantitative PCR, were significantly elevated by 40%, but CRH, urotensin-I, prepro-TRH, prohormone convertase-1 (PC1), and POMC mRNA levels were unchanged. In the pituitary pars distalis, PC1, CRH receptor-1, and POMC mRNA levels were unaffected, as was ACTH content. Plasma
-MSH concentrations were significantly elevated by 30% in hyperthyroid fish, and this was reflected in PC1 and POMC mRNA levels in pituitary pars intermedia that were increased 1.5- and 2.4-fold respectively. The
-MSH content of the pars intermedia was unchanged. Hyperthyroidism has profound effects on the basal levels of a central mediator, i.e., CRH-BP, of HPI-axis function in unstressed carp in vivo, and we conclude that HPI- and hypothalamopituitarythyroid-axis functions are strongly interrelated. We suggest that the changes in plasma cortisol, thyroid hormone, and
-MSH levels reflect their concerted actions on energy metabolism.
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