Ida Beitnes Johansen defended her thesis on March 18th 2011

Ane Gro Siri Skjelfjord

Stress responsiveness in salmonids: Effects on genes, organs, and organisms

Summary from the thesis: PhD thesis 2011:06

Stress is an inescapable burden for fish like for all other animals. For individuals that frequently encounter stressful situations, repeated or prolonged physiological stress responses, can potentially compromise the physiological and psychological basis of that organism's health and welfare. In particular, stress has been shown to adversely affect important biological systems such as the immune system, the central nervous system (CNS) and the cardiovascular system (CVS). The impact of stress largely depends on the inherent stress coping style of the affected individual. Stress coping style can be defined by the set of behavioral and physiological responses to stress that is consistently employed by one individual across unrelated and temporally separated situations. One such physiological stress response, which is highly heritable, yet subject to great individual variation, is the cortisol response. Cortisol, an interrenal/adrenal steroid stress hormone, is responsible for most stress¬related diseases in man as well as in fish.
High heritability of the cortisol response has allowed for the generation of two strains of rainbow trout (Oncorhynchus mykiss) that differ consistently in stress-induced cortisol production and consequently in morphology, endocrinology, behavior and cognition. Low¬responding (LR) fish were initially selected for low post-stress cortisol levels and subsequently display proactive behaviors and endocrine profiles. High-responding (HR) fish, on the other hand, were selected for high post-stress cortisol levels and display reactive behaviors and endocrine profiles.

The phenomenon of contrasting stress coping styles is attracting considerable scientific and public interest. Particularly, the recognition of individual variation in disease vulnerability has encouraged scientists to try to elucidate the biology behind stress coping. In this context, the LR-HR model serves as an excellent model to study the proximate mechanisms behind genetically linked behavioral and physiological trait characteristics. In this work we aimed at investigating CNS plasticity and cardiac remodeling by integrating studies at the molecular (genes), physiological-anatomical (organ) and behavioral (organism) levels in the context of divergent stress coping styles. In particular, we investigated putative CNS mechanisms controlling behavior and memory retention (i.e. mRNA expression of genes involved in the cortisol response, postembryonic neurogenesis and neuronal plasticity) in stressed and non-stressed LR and HR rainbow trout. Further, the association between cortisol responsiveness and cardiac morphology (i.e. size, composition and collagen depositions) and gene expression (i.e. mRNA expression of specific markers of cardiac remodeling) in LR and HR rainbow trout was explored. Finally, we investigated if a trait correlation between cortisol responsiveness and heart size also existed in wild-type European brown trout (Salmo trutta).

We discovered that the expression of central cortisol receptors was affected by heritable variation in stress coping (i.e. LR vs. HR) and downregulated by stress in a brain region-specific manner. We also found that the expression of genes involved in CNS plasticity was affected by heritable variation in stress coping and also differentially affected by short - and long-term stress. Lastly, we show that high cortisol responsiveness was associated with cardiac remodeling (i.e. growth) in HR fish and in high cortisol-responding wild-type brown trout. Further, the cardiac growth in HR fish appeared to be caused mainly by hypertrophic growth of the compact myocardium. This growth was accompanied by focal collagen depositions and a high expression of genes involved in hypertrophy, development of fibrosis and the cortisol response. The latter indicates that cortisol is directly mediating the cardiac remodeling in HR trout.
Combined, the results showing that the expression of central cortisol receptors and genes involved in CNS plasticity differed between LR and HR trout, can contribute to increased knowledge about the proximate mechanisms behind genetically linked physiological and behavioral trait characteristics. Also, the strong association between cortisol responsiveness and cardiac remodeling in two salmonid species suggests that cortisol might be a general causative factor in salmonid cardiac disease. The presence of this anatomical¬physiological trait correlation in a wild population of salmonids, also suggests an ecological¬evolutionary role for individually variable heart function.

Published: 09.06.11
Updated: 10.06.11
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