Cortisol-mediated molecular responses during salinity exposure in fish

Through effective mechanisms of ion regulation, teleost fish are able to sustain osmotic and ionic homeostasis allowing them to inhabit both hypertonic seawater (SW) and hypotonic fresh water (FW) environments. Cortisol has been shown to be a key hormone facilitating the SW acclimation process by playing a dominant role in the activation of ion transporting proteins, especially those involved in the gill sodium and chloride regulation. In addition to its stimulatory effect on ion regulation, cortisol also plays a predominant role in energy metabolism, which is essential for coping with the increased energy demand associated with seawater acclimation. While cortisol has been clearly linked with SW adaptation, its mode of action in mediating physiological responses remains unclear. This study investigated the impact of salinity exposure, and the modulatory effect of cortisol treatment, on the expression of genes involved in cortisol signaling and osmoregulation in two species of teleosts, the rainbow trout (Oncorhynchus mykiss) and Mozambique tilapia (Oreochromis mossambicus). Specifically, tissue-specific transcriptional responses of the corticosteroid receptors (CRs) and osmotic stress transcription factor (OSTF1) were examined in trout gill filaments and hepatocytes in vitro. Additionally, Na+K. +-ATPase activity and alpha-isoform expression was measured in the gill filaments to further understand the role of cortisol in regulating ion transporters upon salinity exposure. Investigation of these molecular responses to cortisol in the gill and liver of seawater exposed fish in vivo was also carried out using the euryhaline tilapia.;In vivo, either SW exposure or cortisol treatment significantly elevated gill OSTF1 gene expression in tilapia. Also, OSTF1 response in SW was further potentiated by cortisol. Gill GR mRNA, but not liver or brain, was significantly upregulated in SW tilapia, but this response was not modified by cortisol treatment. The metabolic and sodium pump response seen in SW tilapia treated with cortisol suggests a metabolic depression in the gill, the significance of this response, however, remains to be elucidated.;Overall, the upregulation of OSTF1 mRNA levels with cortisol points to a key role for corticosteroid signaling in the molecular responses to hyperosmotic stress in fish. While the function of OSTF1 in fish remains unclear, the results from this thesis leads to the hypothesis that this transcription factor is a key molecular link mediating the action(s) of cortisol on ionic and osmotic homeostasis in SW teleostean fishes.;The results demonstrate for the first time that salinity and cortisol treatment independently upregulate OSTF1 gene expression in rainbow trout gills and hepatocytes. Additionally, OSTF1 response to salinity was further enhanced by cortisol, clearly indicating a permissive role for this steroid in modulating OSTF1 gene expression. Salinity also affected CR isoform transcript abundance in trout gills and hepatocytes suggesting that cortisol signaling may be CR isoform-and/or tissue-pecific. Cortisol appears to modulate mineralocorticoid (MR) and glucocorticoid receptor 2 (GR2) transcript levels in trout gills, but not in the liver. A significant decrease in Na+K +-ATPase activity was observed when isolated gill filaments were exposed to hyperosmotic shock (NaCl 250 and 500 mM) and this corresponded with a significant reduction in Na+K+-ATPase alpha1a and alpha1b mRNA abundance. While cortisol treatment had no significant effect on Na +K+-ATPase alpha-isoform mRNA abundance, this steroid did significantly increase the sodium pump activity in trout gills filaments. Together, in trout tissues salinity-mediated CR, OSTF and sodium pump expression is modulated by cortisol signaling.