Daily Rhythms in the Hypothalamus-Pituitary-Interrenal Axis and Acute Stress Responses in a Teleost Flatfish, Solea senegalensis

Abstract

The endocrine axis controlling the stress response displays daily rhythms in many factors such as adrenal sensitivity and cortisol secretion. These rhythms have mostly been described in mammals, whereas they are poorly understood in teleost fish, so that their impact on fish welfare in aquaculture remains unexplored. In the present research, the authors investigated the daily rhythms in the hypothalamus-pituitary-interrenal (HPI) axis in the flatfish Solea senegalensis, which has both scientific and commercial interest. In a first experiment, hypothalamic expression of corticotropin-releasing hormone (crh) and its binding protein (crhbp), both pituitary proopiomelanocortin A and B (pomca and pomcb) expression, as well as plasma cortisol, glucose, and lactate levels were analyzed throughout a 24-h cycle. All variables displayed daily rhythms (cosinor, p < .05), with acrophases varying depending on the factor analyzed: crh and cortisol peaked at the beginning of the dark phase (zeitgeber time [ZT] = 14.5 and 14.4 h, respectively), pomca and pomcb as well as glucose at the beginning of the light phase (ZT = 1.2, 2.4, and 3.4 h, respectively), and crhbp and lactate at the end of the dark phase (ZT = 22.3 and 23.0 h, respectively). In a second experiment, the influence of an acute stressor (30 s of air exposure), applied at two different time points (ZT 1 and ZT 13), was tested. The stress response differed depending on the time of day, showing higher cortisol values (96.2 ± 10.7 ng/mL) when the stressor was applied at ZT 1 than at ZT 13 (52.6 ± 11.1 ng/mL). This research describes for the first time the daily rhythms in endocrine factors of the HPI axis of the flatfish S. senegalensis, and the influence of daytime on the stress responses. A better knowledge of the chronobiology of fish provides a helpful tool for understanding the circadian physiology of the stress response, and for designing timely sound protocols to improve fish welfare in aquaculture. (Author correspondence: [email protected])

Keywords:

• Biological rhythms
• HPI axis
• Solea senegalensis
• Stress response

ACKNOWLEDGMENTS

The authors wish to thank Dr. E. Mañanós and Dr. J. Ramos from IATS-CSIC (Castellón, Spain) for kindly providing the fish used in the experiments, Jose A. Oliver for his help with the maintenance of the laboratory, and the Spanish Ministry of Defense for providing their facilities at the Naval Base of Algameca (Cartagena, Spain).

Declaration of Interest: The present research was funded by projects AGL2007-66507-C02-02, AQUAGENOMICS 28502 (Consolider-Ingenio Program) and SENECA (05690/PI/2007) granted to F.J.S.-V. by the Spanish Ministry of Science and Innovation (MICINN) and by project PO7-RNM-02843 (Proyecto de Excelencia) granted to J.M.M. by Consejería de Innovación, Ciencia y Empresa (Junta de Andalucía). J.F.L.-O. has a postdoctoral fellowship from Fundación Séneca (Murcia, Spain).

The authors report no conflict of interest. The authors alone are responsible for the content and writing of the paper.