Comparative Biochemistry and Physiology Part A: Molecular & Integrative Physiology
This article is a part of the special issue on aquacultureModulation of stress and innate immune response by corticosteroids in pacu (Piaractus mesopotamicus)☆
Introduction
To allow aquaculture production to continue to increase, the industry and researchers have to deal with a range of biotic and abiotic challenges such as climate changes (Sae-Lim et al., 2017), sustainable intensification (Little et al., 2018), disease outbreaks (Murray and Peeler, 2005; Perez-Sanchez et al., 2018), the replacement of animal protein in fish meal (Lazzarotto et al., 2018) and water pollution. However, at some points, these challenges will converge on aspects related to stressors, such as environmental factors (temperature and oxygen), social interaction, and nutrition, among others. The losses caused directly and indirectly by stress in aquaculture are currently quite complex to measure (Davis, 2010) and very likely are inestimable. Long-term stress (chronic) is known to impair appetite, growth performance, and immune responses and may cause disease and mortality (Nardocci et al., 2014), with these two last factors eventually leading to financial losses. Thus, stress is one of the most important concerns to address to raise productivity in intensive and sustainable production, and the reduction of losses requires knowledge about stressors, such as the appropriate identification and indicators of stress, stress levels and fitness outcomes.
During a stress response, two hormonal systems are activated: firstly, catecholamines are released (adrenaline and noradrenaline), and secondly, corticosteroids, mainly cortisol, which is considered the most important stress indicator in fish, are secreted. Briefly, the hypothalamus releases corticotrophin-releasing hormone (CRF) to the pituitary, promoting the synthesis of proopiomelanocortin (POMC) and release of the adrenocorticotropic hormone (ACTH), which, in turn, activates corticosteroid biosynthesis (mainly cortisol) in specific cells in the head kidney (interrenal tissue). To regulate its own synthesis, cortisol has three main effects: inhibition of the synthesis and release of CRF (1) and POMC (2) through negative feed-back and control of the abundance of glucocorticoid receptors (GRs) (3), and it is the main mediator of stress responses [for review see Schreck and Tort, 2016].
In addition to the system discussed above, the immune system is regulated by a complex bidirectional neuroendocrine-immune path (Fast et al., 2008), mediated by cytokines and hormones (Engelsma et al., 2002), and cortisol is a key modulator (Tort, 2011). High concentrations of cortisol affect cytokine production by immune cells (Verburg-Van Kemenade et al., 2009), induce apoptosis in B cells (Weyts et al., 1998a; Weyts et al., 1998b), and inhibit several immune responses in fish, including phagocytosis, respiratory activity of leukocytes (Esteban et al., 2004) and mitogenesis of lymphocytes (Harris and Bird, 2000). However, some evidence suggests that short-term stress may have positive effects on immune function, and it also may involve cortisol and catecholamines (Dhabhar and McEwen, 1997; Dhabhar, 2000, Dhabhar, 2009; Tort, 2011). Further, the mechanisms of how corticosteroids act in teleost fish seem more complex than previously hypothesized (Mommsen et al., 1999; Padgett and Glaser, 2003), since fish possess three corticosteroid receptors, and cortisol is not the only corticosteroid involved in corticosteroid-mediated physiological processes (Mathieu et al., 2014).
Synthetic corticosteroids such as dexamethasone (DXM) and hydrocortisone (HC) are widely therapeutically used in mammals, mainly to treat harmful overreactions of the immune system such as in the cure of inflammatory, autoimmune and allergic disease (Rice et al., 2017). Their clinical effect is due to their ability to mimic natural corticosteroids such as cortisol and both DXM and HC can bind effectively to GRs and trigger a cellular response (Salas-Leiton et al., 2012). Hence, the use of these potent synthetic corticosteroids is a valuable tool/model to investigate the mechanism and evaluate the effects of corticosteroids on stress and immune response in fish (Lovy et al., 2008).
Pacu (Piaractus mesopotamicus) is an important species for aquaculture in South America and its production is increasing constantly (Valladao et al., 2018). However, this species under intensive farming is vulnerable to disease outbreaks indirectly provoked by stress such as the furunculosis caused by the opportunist bacteria Aeromonas hydrophila (Carraschi et al., 2012; Carraschi et al., 2014). Thus, deeper knowledge of the effects of cortisol on innate immune responses is crucial to understanding how corticosteroids may increase susceptibility to a disease after a stressful condition (Salas-Leiton et al., 2012). Hence, the aims of this study were to investigate the effects of long-term exposure to corticosteroids (7 days) and short-term stress (transport for 4 h) on the stress and innate immune response of pacu. Transport is a common procedure used in aquaculture which can lead to immunosuppression (Zanuzzo et al., 2017). In addition, we investigated how fish post-treated with dietary corticosteroids and transported (i.e. immune-compromised) respond to a potential immunogen by inoculation with heat-killed A. hydrophila.
Section snippets
Animal welfare statement
The experimental procedures were approved by the Institutional Animal Care Committee of Sao Paulo State University (CEUA, Protocol# 002111/12) and performed in accordance with the guidelines of the Brazilian Council on Animal Care (Colégio Brasileiro de Experimentação Animal, COBEA).
Experimental animals
We used a total of 180 (82.5 ± 3.4 g) juvenile fish obtained from the Centro de Aquicultura of UNESP. The fish were allowed to grow in earthen ponds for 3 months before being transported to the Laboratory of Fish
Plasma cortisol and glucose levels
Except at recovery sampling, plasma cortisol levels of fish fed with DXM or HC were significantly lower than the control (P < .01; Fig. 1A). At the initial condition, the levels were noticeably (approx. 15-fold) lower compared to control values. Further, immediately after the transport, the cortisol levels increased in all groups compared to their respective initial values (control, P = .014; DXM, P = .006 and HC, P < .001). Fish from the control group inoculated with PBS had higher cortisol
Discussion
Understanding stress and the innate immune response in the face of a stressful procedure is important for the aquaculture industry. Thus, here we investigated the effects of the dietary administration of exogenous corticosteroids (7 days; long-term exposure) and transport (4 h; short-term stress) on stress and innate immune responses in pacu. The corticosteroid treatments for 7 days reduced the cortisol resting level by negative feedback on the HPI-axis. Otherwise, the transport increased the
Acknowledgements
This study was financed in part by the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - Brasil (CAPES) - Finance Code 001.
Declarations of interest
None.
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This article is part of a special issue entitled: Aquaculture, edited by: Dr. Tillmann Benfey, Dr. Inna Sokolova and Dr. Mike Hedrick