Experimental hyperthyroidism and central mediators of stress axis and thyroid axis activity in common carp (Cyprinus carpio L.)

Abstract

The effect of experimental hyperthyroidism, realized by T 4 injection, on central mediators of the hypothalamo-pituitaryinterrenal 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 a-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 a-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 hypothalamo-pituitary-thyroidaxis functions are strongly interrelated. We suggest that the changes in plasma cortisol, thyroid hormone, and a-MSH levels reflect their concerted actions on energy metabolism.

Central and peripheral integration of interrenal and thyroid axes signals in common carp (Cyprinus carpio L.)

“…Despite indications that preoptic CRH may be involved in the regulation of the HPT axis in some fish (Kagabu et al 1998, Geven et al 2006, De Groef et al 2006), no effect of CRH on the expression of carp pituitary tshb subunit could be demonstrated in vitro. The absence of a thyrotropic action of preoptic CRH can also be inferred from experimental results obtained in vivo.…”

“…However, the thyroid-stimulating properties of ACTH and a-MSH in carp may still confer a thyrotropic action to CRH and TRH. The present study focused on TSH as the pituitary thyroid-stimulating factor in carp (Geven et al 2006), but the identification of ACTH and a-MSH as two new putative pituitary thyroid-stimulating factors re-establishes CRH and TRH as potential hypothalamic thyrotropic factors, since the release of ACTH and a-MSH in carp is stimulated by CRH and TRH (van den Burg et al 2003. The identification of more than one hypothalamic thyrotropic and pituitary thyroid-stimulating factor in carp, clearly points to an integration of multiple endocrine signals for the control of the thyroid gland activity in teleostean fishes.…”

“…In this species, TRH does not stimulate the release of TSH from cultured pituitary cells (Kagabu et al 1998). Moreover, experimental treatment with thyroxine resulted in a marked hypocortisolemia in carp, which was accompanied by an increased mRNA expression of CRHbinding protein (CRHBP) in the preoptic area and unchanged levels of CRH and prepro-TRH mRNA (Geven et al 2006). It appears that, in common carp, not TRH but CRH is controlling the activity of the thyroid gland.…”

“…Plasma levels of a-MSH are increased in hyperthyroid and stressed carp , Geven et al 2006, the exact physiological role of which still is unclear. We hypothesize that in common carp the preoptic area and the head kidney represent a central and a peripheral site respectively, for the integration of signals of the HPT and the HPI axis.…”

Effects of cortisol and thyroid hormone on peripheral outer ring deiodination and osmoregulatory parameters in the Senegalese sole (Solea senegalensis)

“…However, we cannot exclude a feedback mechanism in which cortisol reduces the activity of hypothalamic corticotropin-releasing hormone (CRH), urotensin-I (UI), and thyrotropin-releasing hormone (TRH), factors with both corticotropic and thyrotropic activities in a number of fish species (reviewed by Bernier et al (2009)). Indeed, in salmonids and eels, CRH has thyrotropic activity in vitro (Larsen et al 1998, Rousseau et al 1999, and, in carp, the interaction between thyroid hormones and the HPI axis is illustrated by the up-regulation of hypothalamic crh-binding protein gene expression after T 4 treatment (Geven et al 2006). It could well be that hypothalamic factors of the HPI axis, namely CRH, UI, TRH, are altered after cortisol treatment in S. senegalensis and then, jointly with the hepatic and renal ORD pathway, have affected plasma fT 4 concentrations.…”