Enhanced brain expression of genes related to cell proliferation and neural differentiation is associated with cortisol receptor expression in fishes.

Stress enhances or inhibits neurogenesis in mammals and some fish species. The link between the two processes is still unclear. Most studies have been performed in very specific stressful or altered environments. Despite the known inter-individual divergence in coping abilities within populations, the relationship between the stress axis and neurogenesis has never been addressed in unstressed individuals. Here we correlate brain expression of the pcna (proliferating cell nuclear antigen) and neurod1 (neurogenic differentiation factor 1) genes, two markers of neurogenesis, with transcripts of cortisol receptors in three fish species living in very distinct environments. Within the three species, individuals with the highest expression of neurogenesis genes were also those that expressed the high levels of cortisol receptors. Based on these correlations and the hypothesis that mRNA levels are proxies of protein levels, we hypothesize that within unstressed animals, individuals sensitive to cortisol perceive a similar environment to be more stimulating, leading to increased neurogenesis. Although it is difficult to determine whether it is sensitivity to cortisol that affects neurogenesis capacities or the opposite, the proposed pathway is a potentially fruitful avenue that warrants further mechanistic experiments.

Physiological and behavioral flexibility to an acute CO2 challenge, within and between genotypes in rainbow trout.

Adaptive capacities, governing the ability of animals to cope with an environmental stressor, have been demonstrated to be strongly dependent upon genetic factors. Two isogenic lines of rainbow trout, previously described for their sensitivity and resilience to an acute confinement challenge, were used in the present study to investigate whether adaptive capacities remain consistent when fish are exposed to a different type of challenge. For this purpose, the effects of a 4-hour hypercapnia (CO2 increase) challenge at concentrations relevant in aquaculture conditions are described for the two isogenic lines. Oxygen consumption, cortisol release, group dispersion and group swimming activity were measured before, during and after the challenge. Sensitivity and resilience for each measure were extracted from temporal responses and analyzed using multivariate statistics. The two fish lines displayed significant differences in their cortisol response, translating differences in the stress axis sensitivity to the stressor. On the contrary, both lines showed, for other measures, similar temporal patterns across the study. Notable within line variability in the stress response was observed, despite identical genome between fish. The results are discussed in the context of animal robustness.

Adaptive capacities from survival to stress responses of two isogenic lines of rainbow trout fed a plant-based diet.

The composition of feed for farmed salmonids has strongly evolved during the last decades due to the substitution of fishery-derived fish oil and fishmeal by ingredients of plant origin. Little information is available regarding the effects of this transition on adaptive capacities in fish. Two rainbow trout isogenic lines, known for their divergent ability to grow on a plant-based diet (PBD), were fed for seven months from first feeding either a fully PBD or a control marine-resources diet and were compared for their growing and survival capacities over time and their behavioral and stress responses at similar sizes but different ages. Although fish displayed similar appetitive behaviour, the two lines were highly affected by the PBD translated in decreased growth and apathetic behaviour, but also stronger stress responses displayed by stronger cortisol increases and more stress-related behaviour when isolated. The two lines were found to be similarly sensitive to a PBD for the assessed stress-related parameters, but one line displayed a lower survival during the early rearing period. Overall, these results suggest that a PBD supplied to fish from the alevin stage has strong effects on physiological and behavioural parameters, with possible impairment of fish welfare, but also genome-dependent survival.