Serotonergic characteristics of rainbow trout divergent in stress responsiveness.

Juvenile rainbow trout divergent in their cortisol response to confinement stress (HR: high responsive or LR: low responsive fish) were exposed to either 1 or 3 h of confinement stress. Untreated fish served as control. After the exposure blood and brain samples were collected. From the blood samples, the levels of cortisol and catecholamines were determined, while the brain serotonergic and monoamineoxidase (MAO) activity was determined in four different brain areas (brain stem, hypothalamus, telencephalon and optic tectum). Our results show that the LR fish responds to handling stress with a higher increase in plasma epinephrine compared to HR fish. Our results also show that confinement stress leads to a larger increase in the serotonergic activity in the brain stem and telencephalon in LR fish compared to HR fish. These results support the hypothesis that stress coping strategies similar to those described in mammals also exists in fish. Further, our results have shown that the MAO activity increases in optic tectum and hypothalamus of rainbow trout during confinement stress, while it remains unchanged or decreases in brain stem and telencephalon. Moreover, the MAO activity does not differ between the two selection lines. This indicates that MAO participates actively in the stress response without contributing to the differences in stress coping strategies.