The expression of eight candidate genes, with roles implicated in the stress response and associated behaviour, was quantified in the brains of lines of rainbow trout, Oncorhynchus mykiss Walbaum, that were selectively bred for divergent cortisol responses to stress. These lines represent a low responding (LR) and high responding (HR) line that are known to differ in their stress physiology and behavioural phenotype with LR always dominating HR. To understand whether there is differential gene expression in the brains of these lines fish were held individually and then sampled for brain gene expression of eight candidate genes (AANAT, calcineurin, CRF, urotensin I, glucocorticoid receptor II, mineralocorticoid receptor, melatonin receptor 1a, and melanocortin 2 receptor) to obtain a better understanding of the gene expression profile between HR and LR. Seven genes were expressed at a significantly higher level in the low-responding line compared to the high-responding line, and four genes, AANAT, CRF, UI and CaN, were positively correlated with baseline plasma cortisol concentrations. Only AANAT was not significantly differentially expressed between the two stress lines (due to high variation among individuals). Hence there is clear evidence of a tangible relationship between gene expression and stress physiology in these animals prior to stress
