Behavioral and Neuroendocrine Correlates of Selection for Stress Responsiveness in Rainbow Trout--a Review

In rainbow trout the magnitude of the cortisol response to stress shows both consistency over time and a moderate to high degree of heritability, and high responding (HR) and low responding (LR) lines of rainbow trout have been generated by individual selection for consistently high or low post-stress cortisol values. Using 2nd and 3rd generation fish, we tested the hypothesis that differential stress responsiveness is associated with behavioral alterations in the HR-LR trout model. LR fish showed a tendency to become socially dominant, a rapid recovery of food intake after transfer to a novel environment, and a reduced locomotor response in a territorial intrusion test. Furthermore, stress induced elevation of brain stem and optic tectum concentrations of the monoamine neurotransmitters serotonin, dopamine, and norepinephrine and their metabolites suggests that both synthesis and metabolism of these transmitters were elevated after stress to a larger degree in HR than in LR trout. A divergent pattern was seen in the hypothalamus, where LR fish displayed elevated levels of 5-hydroxyindoleacetic acid (a serotonin metabolite) and 3-methoxy-4-hydroxyphenylglycol (a norepinephrine metabolite). Thus, selection for a single trait, cortisol responsiveness, in rainbow trout is associated with concurrent changes in both behavior and central signaling systems. The apparent parallel to genetically determined stress coping styles in mammals, and the existence of similar trait associations in unselected populations of rainbow trout, suggests an evolutionarily conserved correlation between multiple traits. Continuing studies on the HR and LR trout lines are aimed at providing the physiological and genetic basis for new marker-assisted selection strategies in the rapidly developing finfish aquaculture industry, as well as increased knowledge of the function and evolution of central neuroendocrine signaling systems.