Abstract

Corticotropin-releasing factor (CRF), urotensin I (UI) and serotonin (5-HT) are generally recognized as key regulators of the anorexigenic stress response in vertebrates, yet the proximal effects and potential interactions of these central messengers on food intake in salmonids are not known. Moreover, no study to date in fishes has compared the appetite-suppressing effects of CRF and UI using species-specific peptides. Therefore, the objectives of this study were to (1) assess the individual effects of synthesized rainbow trout CRF (rtCRF), rtUI as well as 5-HT on food intake in rainbow trout, and (2) determine whether the CRF and serotonergic systems interact in the regulation of food intake in this species. Intracerebroventricular (icv) injections of rtCRF and rtUI both suppressed food intake in a dose-related manner but rtUI [ED50 = 17.4 ng/g body weight (BW)] was significantly more potent than rtCRF (ED50 = 105.9 ng/g BW). Co-injection of either rtCRF or rtUI with the CRF receptor antagonist α-hCRF(9–41) blocked the reduction in food intake induced by CRF-related peptides. Icv injections of 5-HT also inhibited feeding in a dose-related manner (ED50 = 14.7 ng/g BW) and these effects were blocked by the serotonergic receptor antagonist methysergide. While the anorexigenic effects of 5-HT were reversed by α-hCRF(9–41) co-injection, the appetite-suppressing effects of either rtCRF or rtUI were not affected by methysergide co-injection. These results identify CRF, UI and 5-HT as anorexigenic agents in rainbow trout, and suggest that 5-HT-induced anorexia may be at least partially mediated by CRF- and/or UI-secreting neurons.

Highlights

  • A variety of homeostatic challenges activate signaling pathways in the brain that stimulate the hypothalamic-pituitary-adrenal (HPA) stress axis in mammals and disrupt food intake regulation (Bazhan and Zelena, 2013)

  • Relative to the Control treatment, icv injection of either rainbow trout CRF (rtCRF) or rtUI inhibited food intake in a dose-dependent manner, with significant effects being observed at dosages of 5 ng/g body weight (BW) and higher

  • With the 25 and 125 ng/g BW doses, rtUI icv injections elicited a larger reduction in food intake than rtCRF, and overall rtUI (ED50 = 17.4 ng/g BW) was significantly more potent than rtCRF (ED50 = 105.9 ng/g BW) in producing anorectic effects (Figure 1B)

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Summary

Introduction

A variety of homeostatic challenges activate signaling pathways in the brain that stimulate the hypothalamic-pituitary-adrenal (HPA) stress axis in mammals and disrupt food intake regulation (Bazhan and Zelena, 2013). Intense stressors that acutely activate the HPA axis and result in a transient increase in circulating glucocorticoid levels result in decreased food intake. Acute stressors such as immobilization/restraint, social defeat and immune challenges suppress food intake in rodents (Meerlo et al, 1996; Vallès et al, 2000; Calvez et al, 2011). Acute physical, environmental, social, and immune stressors lead to appetite suppression in fish (Bernier, 2006, 2010; Leal et al, 2011). While there is a basic understanding of the neural pathways that mediate the bidirectional effects of stress on food intake in mammals (Maniam and Morris, 2012), relatively little is known about the mechanisms by which stressors alter food intake in non-mammalian species

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