Regulation of divergent cortisol responsiveness in European sea bass, Dicentrarchus labrax L.

RESEARCH ARTICLE Regulation of divergent cortisol responsiveness in European sea bass, Dicentrarchus labrax L. Athanasios Samaras*, Michail Pavlidis Department of Biology, University of Crete, Heraklion, Crete, Greece * a1111111111 a1111111111 a1111111111 a1111111111 a1111111111 OPEN ACCESS Citation: Samaras A, Pavlidis M (2018) Regulation of divergent cortisol responsiveness in European sea bass, Dicentrarchus labrax L.. PLoS ONE 13 (8): e0202195. https://doi.org/10.1371/journal. pone.0202195 Editor: Tzong-Yueh Chen, National Cheng Kung University, TAIWAN Received: January 11, 2018 Accepted: July 30, 2018 Published: August 10, 2018 Copyright: © 2018 Samaras, Pavlidis. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Data Availability Statement: All relevant data are within the paper and its Supporting Information files. Funding: This study has been partially funded by the Greek Ministry of Rural Development and Food, Operational Programme “EPAL 2007-2013” under grant agreement no 185359. A. Samaras was supported by a postgraduate scholarship of the Onassis Foundation, Greece. The authors would also like to thank the Special Account for Research Funds of University of Crete for the financial support to cover the publication costs. The funders Abstract Mechanisms regulating differences in cortisol responsiveness between low (LR) and high response (HR) individuals have been poorly studied. In this context, we aimed to study key regulatory processes in cortisol dynamics at the head kidneys of LR and HR European sea bass. To do so, resting plasma cortisol and ACTH concentrations were quantified in these fish. Additionally, the head kidneys of these individuals were superfused through an in vitro superfusion system and stimulated with the same amount of ACTH to assess their cortisol biosynthetic capacity. Moreover, the expression of important genes in cortisol regulation was assessed. Results showed that LR fish had lower resting cortisol concentrations than HR, although no differences existed in the circulating levels of ACTH. Additionally, the biosynthetic capacity of HR was higher than that of LR fish when in vitro stimulated with ACTH. At the molecular level, a statistically significant 3.4-fold higher expression of the ACTH receptor, mc2r, and a 2.3-fold, though not significant, higher expression of 11β-hydroxylase (cyp11b1), an enzyme involved in cortisol biosynthesis, was observed in the HR fish. Finally, a statistically significant 1.3-fold lower expression of 11β-hydroxysteroid dehydrogenase 2 (hsd11b2), an enzyme involved in cortisol inactivation, was observed in HR when compared to LR fish. Therefore, it was for the first time indicated that cortisol dynamics can also be regulated at the post-production level in the head kidney. Collectively, our results highlight the crucial role of the interrenal tissue in the regulation of differences in cortisol response between LR and HR sea bass individuals. Introduction Cortisol is considered the major stress hormone in marine teleost fish, regulating many metabolic actions for the redistribution of energy towards survival mechanisms and away from growth and reproduction [1–2]. In mammals, cortisol is synthetized at the adrenal gland. Fish, however, do not have a discrete adrenal gland, and cortisol is produced at the interrenal cells, which are distributed in the head kidney [3]. Cortisol production, secretion and actions are under complex control, regulated by the Hypothalamus–Pituitary–Interrenal (HPI) axis (see [3–4] for review). PLOS ONE | https://doi.org/10.1371/journal.pone.0202195 August 10, 2018 1 / 13 Regulation of cortisol responsiveness in sea bass had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Competing interests: The authors have declared that no competing interests exist. Briefly, the nucleus preopticus (NPO) at the hypothalamus produces corticotropin-releasing hormone (CRH) that stimulates the pituitary pars distalis to produce the adrenocorticotropic hormone, ACTH. This hormone subsequently stimulates the interrenal cells at the head kidney to produce cortisol [5–6]. ACTH acts by binding to an ACTH-specific receptor, namely the melanocortin 2 receptor (MC2R) [7–9]. The expression of the mc2r gene is crucial for the regulation of cortisol biosynthesis from the interrenal cells [9]. Once ACTH binds to MC2R at the interrenal tissue, a series of enzymatic reactions takes place to produce cortisol. The final step of this enzymatic reactions is the conversion of 11-deoxycortisol to cortisol, catalyzed by 11bhydroxylase (CYP11B1) [10]. Moreover, cortisol can be inactivated to cortisone by 11b-hydroxysteroid dehydrogenase 2 (HSD11B2) [10–11]. Finally, cortisol is released into the circulation, where it stimulates various target-tissues regulating their actions. These actions are mediated by cortisol receptors, which in most teleost species are two glucocorticoid (GRs) and one mineralocorticoid receptors (MR) [3; 12]. Divergence in cortisol responsiveness between individuals of the same species has been long described in fish [13–16] and other vertebrates [17–18]. Subsequently, animals that consistently show low (LR) or high (HR) cortisol circulating levels after exposure to stressors have been characterized [13–16]. These phenotypes have been associated with differences in performance, such as growth [19–21], and hepatic metabolic processes [16,22], as well as behavioral differences [23–24]. Still, however, the underlying mechanisms regulating this individual cortisol responsiveness have not been extensively described. The role of the hypothalamus in the regulation of individual responses is still ambiguous. Specifically, no differences were found in resting crh mRNA expression between LR and HR individuals of rainbow trout, Oncorhynchus mykiss [25]. Additionally, the concentration of the pituitary-derived ACTH was also not significantly different between unstressed or stressed LR and HR trout [26]. On the contrary, the interrenal tissue can play a crucial role in the regulation of cortisol responsiveness between LR and HR fish [26]. In rainbow trout, higher cortisol production in HR compared to LR fish has been observed after stimulation with ACTH [26]. Moreover, higher expression of mc2r in the head kidney of these fish has been reported [27]. Additionally, an upregulation of mRNA expression of genes encoding for enzymes regulating cortisol synthesis (StAR, P450scc, 3βHSD) has been observed in HR individuals of Atlantic cod, Gadus morhua, [15]. Recently, individual divergent cortisol responsiveness has been described in European sea bass, and LR and HR individuals have been characterized [16]. These fish, apart from differences in their post-stress p (...truncated)