The mRNA expression of cortisol axis related genes differs in Atlantic cod (Gadus morhua) categorized as high or low...

Cortisol is a major stress hormone in fish and is known, under normal or stressful conditions, to affect several physiological processes including growth and immunity. Thus, efforts have been made for several cultured finfish species, including the Atlantic cod, to determine whether fish with a high or low cortisol response to stress can be identified and selected. However, we have a limited understanding of the mechanisms that determine these two phenotypes. Thus, we measured total and free plasma cortisol levels in high and low responding cod when subjected to a 30 s handling stress, and the mRNA expression of four key genes in the glucocorticoid (i.e. cortisol) stress axis both pre- and post-stress. The cortisol data is consistent with our previous findings for cod, with high responding (HR) fish having ∼3-fold higher total and free plasma cortisol levels when compared to low responding (LR) fish. Three of the transcripts studied encode key proteins involved in steroidogenesis (StAR, P450scc and 3βHSD), and the constitutive mRNA expression of all three genes was significantly higher (∼2-fold) in the head kidney of HR fish when compared to LR cod. The other gene of interest was the glucocorticoid receptor (GR). We partly cloned and characterized a cDNA from Atlantic cod likely to be this fish's ortholog of the teleost GR1, and showed that while there was no difference in hepatic constitutive GR mRNA expression between groups, HR fish had liver GR mRNA levels that were significantly (1.8-fold) higher at 3 h post-stress as compared to LR fish. Our results suggest that the different magnitude of cortisol response between LR and HR fish is at least partially determined by the capacity of the interrenal tissue to produce steroids.