In intensive farming conditions fish encounter stress situations, which often increase susceptibility to disease. Consequently, feeds should not only fulfil the nutrient requirements for growth, but also strengthen the immune system and stress resistance. Several studies have evaluated peptides and protein hydrolysates (PH) activity as nutraceuticals. The dietary inclusion of PH has been shown to improve growth, antioxidant activity and fish immunity. Hydrolysis can give rise to bioactive peptides believed to be more effective than the whole protein in terms of disease resistance. Low molecular weight peptides (< 3kDa) are described as having antimicrobial, immune-stimulating, anti-inflammatory and antioxidant properties. The present work aims to assess the effects of different PH on stress resilience, immune response and oxidative stress of seabass (Dicentrarchus labrax) juveniles prior and after acute stress.
A practical commercial-like diet was used as control (CTR), whereas 3 other diets were formulated based on CTR to contain a 3% inclusion level of a short chain purified bioactive peptide in diet BPP, 3% shrimp PH in diet SPH and 3% feather meal PH in diet FMH. Diets were randomly assigned to triplicate groups of 120 fish (IBW approx. 2 g) that were hand fed to satiation 4 times a day for 21 days. After 3 weeks of feeding, fish were subjected to an acute stressful event (i.e., water volume in experimental tanks was reduced to 1.7% of the initial volume for 30 minutes). Intestine samples were taken for gene expression, immune parameters and oxidative stress analysis, immediately prior to stress and one day following acute stress. Caudal fin samples were also taken for cortisol levels measurement, one hour after stress.
Dietary treatments induced mild effects on the innate immune and antioxidant functions in the gut. However, transcriptomic data showed cd4, cd8β, il1β and tnfα genes down-regulation with significant differences arising one day after the stress stimulus. Integrating gene responses into a multivariate analysis (PCA-DA), it becomes clearer that acute stress induced a general down-regulation of immune genes in seabass gut that was independent from the dietary treatments. Moreover, after the stress stimulus a dietary effect is perceivable, fish fed BPP and FMH diets clustered together and were different from CTR. Differences at this stage pointed to higher tnfα and cd8β expression than CTR. In conclusion, BPP and FMH dietary treatments seem to counteract more effectively the immunosuppression in seabass gut caused by acute stress. This, could be of advantage to improve immunocompetence during acute stressful events.
This work is part of project NOSTRESS_047122 supported by Portugal and the European Union through FEDER, LISBOA 2020, NORTE 2020 and CRESC Algarve 2020, in the framework of Portugal 2020.