| Summary: | Several strategies to reduce stress among fish in aquaculture are currently under development. A biologically conserved feature of the stress response in vertebrates is the role of the brain serotonergic (5-hydroxytryptamine, 5-HT) signalling system in controlling the endocrine response to stress, primarily production of the steroid hormone cortisol through the hypothalamus-pituitary-adrenal axis (HPA, or HPI [interrenal] in fishes). The precursor of the monoamine neurotransmitter 5-HT is the amino acid tryptophan (TRP). Plasma levels of TRP directly affect 5-HT production in the brain. Dietary TRP treatment has previously been shown to inhibit the cortisol response to stress in both fishes and mammals. Altering monoaminergic neurotransmission and stress responsiveness may have enduring effects on neural plasticity, but it is not known whether the effect of TRP remain also after exogenous supplementation has been terminated. In the current study, three groups of juvenile Atlantic salmon (Salmo salar) were tagged and acclimated during 10 weeks. Thereafter, the fish were treated with three different diets (TRP 1x = standard commercial feed, and TRP 2x and 3x = 2 and 3 times standard level of TRP) during 1 week, whereupon standard food were given until the end of the experiment. In order to investigate the long-term effect of TRP on the stress response, fish were subjected to an acute stressor during 1 hour at two different occasions, 1 and 3 weeks after TRP treatment, and blood samples were obtained to analyse plasma levels of the fish corticosteroid hormone cortisol after stress. Fish fed with TRP3 showed a decrease of plasma cortisol levels during both occasions, specifically as compared to TRP 2x at sampling 1 and both TRP 2x and TRP 1x and sampling 2. Notably, fish fed with TRP 2 showed a significant elevation of plasma cortisol at first sampling, however, this effect was abated after 3 weeks. These results demonstrate that neuroendocrine effects of dietary TRP are both time- and dose-dependent. I hypothesize that long term effects of TRP on cortisol levels, are mediated by the fish brain serotonergic system. Chronic stress in fish cause brain damages, loss of appetite, impaired growth and muscle wasting, brain effects, immunosuppression, decreased reproduction and mortality. Consequently, dietary TRP should be further evaluated in aquaculture production in order to maintain animal welfare, and limit economic losses. Several strategies to reduce stress among fish in aquaculture are currently under development. A biologically conserved feature of the stress response in vertebrates is the role of the brain serotonergic (5-hydroxytryptamine, 5-HT) signalling system in controlling the endocrine response to stress, primarily production of the steroid hormone cortisol through the hypothalamus-pituitary-adrenal axis (HPA, or HPI [interrenal] in fishes). The precursor of the monoamine neurotransmitter 5-HT is the amino acid tryptophan (TRP). Plasma levels of TRP directly affect 5-HT production in the brain. Dietary TRP treatment has previously been shown to inhibit the cortisol response to stress in both fishes and mammals. Altering monoaminergic neurotransmission and stress responsiveness may have enduring effects on neural plasticity, but it is not known whether the effect of TRP remain also after exogenous supplementation has been terminated. In the current study, three groups of juvenile Atlantic salmon (Salmo salar) were tagged and acclimated during 10 weeks. Thereafter, the fish were treated with three different diets (TRP 1x = standard commercial feed, and TRP 2x and 3x = 2 and 3 times standard level of TRP) during 1 week, whereupon standard food were given until the end of the experiment. In order to investigate the long-term effect of TRP on the stress response, fish were subjected to an acute stressor during 1 hour at two different occasions, 1 and 3 weeks after TRP treatment, and blood samples were obtained to analyse plasma levels of the fish corticosteroid hormone cortisol after stress. Fish fed with TRP3 showed a decrease of plasma cortisol levels during both occasions, specifically as compared to TRP 2x at sampling 1 and both TRP 2x and TRP 1x and sampling 2. Notably, fish fed with TRP 2 showed a significant elevation of plasma cortisol at first sampling, however, this effect was abated after 3 weeks. These results demonstrate that neuroendocrine effects of dietary TRP are both time- and dose-dependent. I hypothesize that long term effects of TRP on cortisol levels, are mediated by the fish brain serotonergic system. Chronic stress in fish cause brain damages, loss of appetite, impaired growth and muscle wasting, brain effects, immunosuppression, decreased reproduction and mortality. Consequently, dietary TRP should be further evaluated in aquaculture production in order to maintain animal welfare, and limit economic losses. DTU and UMB
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