br Xenobiotics and the Glucocorticoid Receptor

Xenobiotics and the Glucocorticoid Receptor
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Acknowledgements The author wishes to thank Professor Wilhelm Engström from the Swedish University of Agricultural Sciences (Department of Biomedical Sciences and Veterinary Public Health) for his proof reading of this review and the Otago Medical School (Formerly the Faculty of Medicine, University of Otago) for providing financial assistance to allow me to attend the Environnmental Mixtures Taskforce meeting for the “Halifax Project”, Halifax, Nova Scotia (2013) which captivated my interest in the interactions of nuclear receptors with xenobiotics.
Introduction Stress has been defined as a physiological cascade of events that occurs when an individual attempts to re-establish homeostatic norms in the face of a perceived threat (Schreck et al., 2001). In response to a stressor, cortisol is the primary corticosteroid secreted by the interrenal A-674563 of teleosts and plays a key role in regulating homeostatic and metabolic functions (Schreck et al., 1997; Szisch et al., 2005). One method of determining the stress response in fish is to measure changes in circulating levels of cortisol (Strange, 1980; Donaldson, 1981; Tomasso et al., 1981; Wise et al., 1993). The action of cortisol is at the cellular level by binding to intracellular glucocorticoid receptors and mineralocorticoid receptors (Prunet et al., 2006). Glucocorticoid receptors are expressed in most cells in the body and function through direct activation of gene expression or through non genomic mechanisms (Kumar and Thompson, 2005). In fishes, the development of interrenal function varies among species (Hwang et al., 1992; Barry et al., 1995a). De novo cortisol synthesis begins as early as one week after fertilization in rainbow trout (Oncorhynchus mykiss, Pillai et al., 1974) but as late as two weeks after hatch in Japanese flounder (Paralichthys olivaceus) (De Jesus et al., 1991). In many fish species including milkfish (Chanos chanos) (Hwang et al., 1992), tilapia (Orechromis mossambicus), (Hwang et al., 1992), Japanese sea bass (Lateolabrax japonicas), (Pérez et al., 1999), Asian sea bass (Lates calcarifer) (Sampath-Kumar et al., 1995), and yellow perch (Perca flavescens) (Jentoft et al., 2002), endogenous cortisol production begins soon after hatching. However, the development of a mature hypothalamic–pituitary–interrenal (HPI) axis able to produce cortisol in response to an external stressor typically occurs much later in development. For example, cortisol production as a response to stress was observed 2 weeks after hatching in turbot and rainbow trout (Stephens et al., 1997; Barry et al., 1995a, Barry et al., 1995b; Pottinger and Mosuwe, 1994), whereas it was observed only one week after hatching in yellow perch (Jentoft et al., 2002). If it is known when cortisol synthesis begins in channel catfish, it will help in understanding the physiological stress response during early development and could have implications in transporting and culturing eggs and fry. Excluding zebrafish (Danio rerio), all teleosts studied to date have two GR genes that encode different receptor protein isoforms (Ducouret et al., 1995; Takeo et al., 1996; Burry et al., 2003; Greenwood et al., 2003; Terova et al., 2005; Stolte et al., 2006; Vizzini et al., 2007; Alsop and Vijayan, 2008; Schaaf et al., 2009; Stolte et al., 2009; Li and Leatherland, 2012). Recently, two GR genes were cloned and partially characterized in juvenile (120–250 g) channel catfish (Small and Quiniou, 2017). Their results suggest channel catfish GR-1, but not GR-2 expression is significantly increased following a low-water stressor. Together, these GR expression studies show the unique regulation of the corticosteroid axis among teleosts. Previous research with catfish fry has shown that fry (<1.0 g) could elicit a cortisol response after lowering the water level and chasing the fish with a net (Peterson and Booth, 2010). However, we do not know how soon after fertilization this type of response begins. Furthermore, it is not known what roles GRs play in response to a stressor during early embryo development. This has practical applications as catfish eggs are subjected to a number of potential stressful events such as collection from spawning cans, transportation to the hatchery in trailers with low oxygen, and poor water quality during and after hatching. In addition, it has been shown in rainbow trout that early exposure to stressors can affect the stress response later in life (Auperin and Geslin, 2008). The objectives of the present study were to examine the effects of stress on cortisol and GRs during early development of channel catfish.