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The effect of common husbandry conditions (crowding, social environment, water quality, handling, and background color) on the cortisol stress response in adult zebrafish, Danio rerio, was investigated to check the usefulness of zebrafish as a model organism in aquaculture research. In addition, a noninvasive methodology for assessing stress was evaluated. Zebrafish showed a fast cortisol response with high values at 30 min that returned to basal levels within 2 h of poststress. There was a significant positive correlation between trunk cortisol concentrations and the free water cortisol rate (r(2)=0.829-0.850, p<0.001), indicating that measurement of the water-borne cortisol release rate may serve as a noninvasive and reliable stress indicator at the population level. Crowding resulted in 13- to 21-fold greater mean trunk cortisol concentrations compared with controls. However, even at low stocking density (2-5 fish/L), the maintenance cost was higher than the one at higher densities (10 fish/L) due to the formation of dominance hierarchies. The background color affected trunk cortisol concentrations, with fish exposed to brighter backgrounds (green and white) showing 3- to 8-fold greater mean trunk cortisol concentrations than fish exposed to a black background or transparent aquaria. Fish exposed to high stocking densities for 2 h or 5 days had similar high mean trunk cortisol levels, indicating that exposure of fish for the period of 2 h to a specific stressor may represent a chronic situation in zebrafish. It is concluded that adult laboratory zebrafish had a preference for a transparent or black background aquarium, at a number of 10 individuals per 2 L of available water volume, to express their normal behavior and avoid increased cortisol stress reaction.

Zebrafish, Danio rerio, is an emerging model organism in stress and neurobehavioral studies. In nature, the species forms shoals, yet when kept in pairs it exhibits an agonistic and anxiety-like behavior that leads to the establishment of dominant-subordinate relationships. Fluoxetine, a selective serotonin reuptake inhibitor, is used as an anxiolytic tool to alter aggressive behavior in several vertebrates and as an antidepressant drug in humans. Pairs of male zebrafish were held overnight to develop dominant—subordinate behavior, either treated or non-treated for 2 h with fluoxetine (5 mg L−1), and allowed to interact once more for 1 h. Behavior was recorded both prior and after fluoxetine administration. At the end of the experiment, trunk and brain samples were also taken for cortisol determination and mRNA expression studies, respectively. Fluoxetine treatment significantly affected zebrafish behavior and the expression levels of several genes, by decreasing offensive aggression in dominants and by eliminating freezing in the subordinates. There was no statistically significant difference in whole-trunk cortisol concentrations between dominant and subordinate fish, while fluoxetine treatment resulted in higher (P = 0.004) cortisol concentrations in both groups. There were statistically significant differences between dominant and subordinate fish in brain mRNA expression levels of genes involved in stress axis (gr, mr), neural activity (bdnf, c-fos), and the serotonergic system (htr2b, slc6a4b). The significant decrease in the offensive and defensive aggression following fluoxetine treatment was concomitant with a reversed pattern in c-fos expression levels. Overall, an acute administration of a selective serotonin reuptake inhibitor alters aggressive behavior in male zebrafish in association with changes in the neuroendocrine mediators of coping styles.

The cortisol stress response and the molecular programming of the corticoid axis were characterized for the first time during early ontogeny in a Mediterranean marine teleost, the European sea bass (Dicentrarchus labrax). Sea bass embryos, pre-larvae and larvae at specific points of development were exposed to acute stressors and the temporal patterns of cortisol whole body concentrations and the expression of genes involved in corticosteroid biosynthesis, degradation and signaling were determined. Expression of genes (gr1, gr2, mr, crf) involved into the corticoid response regulation combined with histological data indicated that, although a cortisol stress response is evident for the first time around first feeding, a pattern becomes established in larvae at flexion until the formation of all fins. Moreover, mRNA transcript levels of 11β-hydroxylase and 11β-hsd2 showed a strong correlation with the whole body cortisol concentrations. Concluding, our data reveal the presence of an adaptive mechanism in European sea bass at early ontogeny enabling to cope with external stressful stimuli and provide a better insight into the onset and regulation of the stress response in this species.

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