Gradation of the stress response in rainbow trout exposed to stressors of different severity: the role of brain serotonergic and dopaminergic systems

After an intense acute stressor, fish develop a metabolic and behavioural response that usually lasts for several hours. Brain monoaminergic systems, particularly the serotonergic system, appear to play a key role in the central regulation of the stress response. However, the influence of stressor severity on brain monoaminergic systems and on the induced stress responses is yet poorly understood. We hypothesise that serotonergic system could have a direct role in the integration of sensory information during stressor exposure and in the organisation of the subsequent integrated stress response. According to our hypothesis, a low stressor intensity would induce a low response of brain serotonergic system and therefore stress responses of low magnitude and duration. To test this hypothesis, we exposed fish to handling disturbance for 5 s, 15 s or 3 min. We sampled fish at 0 (controls), 3, 15, 45 and 240 min after the start of the stress protocol. Brain levels of serotonin, dopamine and their respective main oxidative metabolites were quantified, along with plasma levels of stress markers (catecholamines, cortisol, glucose and lactate). Regarding stress markers, the 5-s and 15-s stress protocols induced similar and relatively low elevations in all parameters assessed. As expected, the 3-min protocol induced responses of a higher intensity and duration in all plasma parameters. Interestingly, the alterations of brain monoaminergic systems did not follow the same trend. The three stress protocols induced increases in the serotonergic activity in all brain regions analysed (hypothalamus, telencephalon and medulla oblongata), independently of the duration of the handling disturbance, whereas the effects on the dopaminergic system were minor and brain region-dependent. These data suggest that the brain serotonergic system, although likely involved in the recognition of the stressor stimuli, is not the only actor determining the magnitude and duration of the acute stress response in trout.