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Methionine-choline-deficient ( MCD) mammals are known to accumulate liver TAG probably due to phosphatidylcholine ( PC) deficiency and thus assembly of VLDL and transport of lipids from liver to peripheral organs. To assess whether supplementation of choline could spare methionine and secure a healthy liver metabolism, by reducing the endogenous PC synthesis without interfering with lipid transport and distribution, Atlantic salmon with initial BW of 700 g were fed adequate (1.9 g Met/16 gN) or surplus methionine (2.5 g Met/16 gN) diets of which were supplemented with choline or not for a period of 19 weeks. Fish fed the low-methionine diets had reduced growth ( p = .013) due to reduced protein gain ( p = .007), while lipid gain slightly improved in fish fed the choline-supplemented diets ( p = .047). Also, feed conversion improved when fed surplus methionine ( p < .001), while choline supplementation had no impact on feed conversion. No interaction between choline and methionine on growth performance or retention existed. Phospholipid status in liver and muscle was not affected by treatments, and no liver TAG accumulation occurred at the methionine levels used. Gene expression of ApoB100 necessary for assembling VLDL or pemt necessary for endogenous PC synthesis was unaffected by treatments. Capacity of methylation ( MAT, BHMT) within the liver was not affected by treatment nor was the gene expression of enzymes in liver transsulfuration ( CBS or CDO). Methionine status within liver was unaffected by treatments, while free methionine reduced in those fish fed the low-methionine diets in muscle and plasma. Cystathionine and taurine were elevated when fed surplus methionine. Choline supplementation had no impact on sulphur amino acid metabolites in either tissue. Neither did choline supplementation improve TAG mobilization from liver to muscle. To conclude, choline does not improve endogenous phospholipid synthesis or transport of TAG from liver to muscle depot when added to diets containing 1.9 g Met/16 gN, while surplus methionine improved growth and protein retention, indicating that 1.9 g Met/16 gN is enough to support a healthy liver metabolism, but too low to support muscle protein deposition in adult salmon fed high plant protein diets for longer periods of time. [ABSTRACT FROM AUTHOR]

We previously reported that juvenile Atlantic salmon with mean initial BW 11.5 g offed a methionine deficient diet had lower weight gain due to a reduced protein accretion, while lipid gain was unaffected. Muscle of the fish fed the methionine deficient diet was depleted for sulphur amino acids, while in liver, the concentration of these metabolites was maintained within narrow limits. We speculated whether this could be due to an increased muscle proteolysis to support a prioritized liver metabolism in fish fed the low methionine diets. In this study, we assessed whether genes associated with muscle proteolysis increased under methionine deficiency. The composition of the diets was similar to those used previously containing 1.6 or 2.1 g Met/16 g N. We confirmed that the fish fed the low methionine diet gained less protein compared to fish fed the DLmethionine enriched diet (P = 0.014), but growth did not reduce significantly. Also the deficient fish maintained the concentrations of liver sulphur amino acids and reduced muscle free methionine. Several of the other free amino acids within muscle increased. Further, methylation capacity was maintained in liver but reduced in the muscle (P = 0.78 and 0.04, respectively). Gene expression of muscle IGF-1 was lower (P = 0.008) and myosin light chain 2 tended (MLC2, P = 0.06) to be reduced in fish fed low methionine diet, concurrently the activity of cathepsins B+L increased (P = 0.047) in muscle of fish fed the low methionine diet. Gene expression of the muscle-specific E3 ubiquitine ligases (Murf and MaFbx) was not affected by treatment. Thus, the lower protein gain observed in fish fed the low methionine diet may be caused by reduced protein synthesis in line with the reduced IGF-1 gene expression in the white trunk muscle. Thus, to support metabolism, the dietary protein needs to be balanced in amino acids to support metabolism in all compartments of the body and secure maximal protein gain. [ABSTRACT FROM AUTHOR]

Phosphatidylcholine is synthesized endogenously through the enzyme phosphatidylethanolamine-methyl transferase. As endogenous choline synthesis requires methyl groups from S-adenosylmethionine (SAM), the endogenous synthesis of choline may depend on SAM availability. SAM availability depends on methionine and ATP. SAM is also a precursor for cysteine and may affect taurine and glutathione concentrations. To investigate whether choline synthesis or transsulphuration is prioritized and the interactions between taurine and choline supplementation when methionine availability is low, juvenile Atlantic salmon were fed low methionine diets with and without taurine and choline supplementation. There were no differences in growth or protein accretion following treatments. Fish fed the low methionine diets did not develop a fatty liver, but choline supplementation increased the concentration of total phospholipids in liver and muscle. Taurine supplementation increased taurine concentrations in liver and muscle, but no interactions with choline were present. Liver SAM was unaffected by treatments. Two of three tanks fed the low methionine diet without choline and taurine supplementation had a higher gene expression of p38MAPK (mitogen activated phosphokinase). Choline supplementation to low methionine diets thus may have beneficial effect on the metabolic health and lipid transport to extra-hepatic tissues. Taurine supplementation had no effect on phospholipids status, but increased taurine concentration in tissues and as such may affect oxidation status when methionine availability is low. [ABSTRACT FROM AUTHOR]

The current study aimed to investigate the effect of surplus dietary arginine on polyamine and amino acid metabolism and accretion of proteins and lipids, as previous studies have demonstrated a lipid-reducing effect following surplus arginine supplementation in viscera as well as increased muscle growth. Four plant protein-based diets were given increasing concentrations of L-arginine, from 21.1 to 36.1 g kg-1 DM, and fed to quadruplicate tanks of adult Atlantic salmon (1.1 kg) for 12 weeks. No effects on growth or deposition of fat or protein were observed. Free amino acid concentrations and related metabolites were unaffected in the liver, except for urea, while concentrations in muscle and plasma reflected production of arginine metabolites. Polyamine concentrations were unaffected in liver, muscle and white adipose tissue ( WAT), as were the abundance and activity of spermidine/spermine N1-acetyltransferase ( SSAT), the rate-limiting enzyme in polyamine turnover. Gene expression demonstrated differential regulation of ornithine decarboxylase ( ODC) in liver and WAT, although overall little effects were observed on gene expression. Liver S-adenosylmethionine ( SAM) concentrations decreased with arginine supplementation. We suggest that adult Atlantic salmon have adapted to surplus arginine, and the main long-term effect appears to be increased concentrations of arginine metabolites. [ABSTRACT FROM AUTHOR]

The appetite-stimulating effect of krill hydrolysate ( KH), free amino acids and specific water-soluble low molecular N-compounds was explored in a short feeding trial with Atlantic salmon. A 100 g kg−1 fish meal positive ( MFM), a 30 g kg−1 fish meal negative control ( LFM) and six more LFM diets were produced added known attractants for fish- KH at two levels: low ( LAK) or high ( HAK), AMP, choline chloride ( CC); an amino acid mix resembling the free amino acid composition of KH ( AA) or a mix of AMP, CC and AA ( ACA). HAK, ACA and AA showed highest feed intake and significantly higher plasma phospholipids and cholesterol. The lower performing treatments ( LFM, AMP) showed higher liver lipids and hepatosomatic index. CC induced reduced liver lipids and increased plasma phospholipids and cholesterol. Appetite regulating neuropeptide gene expression analysis (q PCR) was performed in fish fed LFM, HAK and AA. Pyy showed the highest postprandial expression in LFM, whereas the expression of the anorexigenic neuropeptides cart, pomca1, pomca2 and pomca2s was low. These apparently contradictory results may be explained by initial appetite stimulation by HAK and AA, resulting in higher feed intake, followed by satiation and appetite downregulation. [ABSTRACT FROM AUTHOR]

Leucine has been identified to modulate feed intake and energy homeostasis in fish as in other vertebrates. Under allostatic conditions, energy expenditure may change, and adjustments to the processes that govern the energy homeostatic system may be necessary. We investigated the responsiveness of appetite-related neuropeptides involved in feed intake regulation in Atlantic salmon (Salmo salar) reared with high (35 g/kg leucine) or control (27.3 g/kg leucine) leucine-supplemented diets and/or under chronic stressor conditions (chasing) for eight weeks. We also analysed the response of amino acid transporters potentially involved in uptake of branched-chain amino acids (BCAA), including leucine, into areas of the brain where nutrient sensors may signal locally or to other areas involved in appetite control. At the end of the experiment, all fish were subjected to a novel-acute stressor (confinement). Our results show that fish fed with high leucine diet had a lower feed intake, growth, and hepatosomatic index (HSI) when compared to fish fed control leucine diet. In addition, increased mRNA expression of amino acid solute carrier (slc) genes in the diencephalon, and genes related to appetite control, such as proopiomelanocortin a1 (pomca1), in both the diencephalon and telencephalon, imply their involvement in leucine anorectic effect. Stress, as high leucine, reduced feed intake, growth and HSI of fish fed control or high leucine diet and antagonized the high leucine effect on the slc genes mRNA expression. An increase of neuropeptide y a1 (npya1) was observed both due to high dietary leucine and/or stress treatment which may represent a compensatory regulatory mechanism with the aim to reverse the decrease in feed intake. In summary, our results confirm an anorectic role of high dietary leucine via the activation of amino acid sensing mechanisms in the brain. Further, corticotropin-releasing hormone 1 b1 (crh1b1) and npya1 showed to play a role in the regulation of appetite in Atlantic salmon under stress conditions and/or high leucine levels. • High dietary leucine acts as anorectic factor in the control of energy balance in Atlantic salmon. • Both slc transporters and pomca1 neuropeptide are involved in the leucine anorexigenic control. • The same mechanisms do not seem to be responsible for such disruption under stressed conditions. • Instead, crf1b1 and npya1 play a role in the regulation of appetite under stressful conditions. [ABSTRACT FROM AUTHOR]

Abstract: Following feeding of a single meal to Atlantic salmon, the temporal changes in the brain mRNA expression of neuropeptide y (npy), cocaine-amphetamine regulated transcript (cart), peptide yy (pyy), two isoforms of agouti-related protein (agrp), two isoforms of cholecystokinin (cck), and four isoforms of proopiomelanocortin (pomc) were assessed by q-PCR. In the course of 24h post-feeding (hpf), several of the brain neuropeptides displayed changes in mRNA expression compared to an unfed control group, indicating that food intake and processing affect the regulation of expression of these genes in Atlantic salmon. Expression of cart, cck-l, pomc-a1 and pomc-b all increased within 3h of feeding, while most of the feed was still in the stomach, suggesting that these neuropeptides play central anorexigenic roles similar to those described in higher vertebrates, including determining meal intervals. On the other hand, the npy and agrp isoforms which have been described as playing orexigenic roles in mammals, showed an opposite response in salmon and both were elevated in the first 3h after feeding. The different isoforms of cck, agrp and pomc had different mRNA expression patterns, which indicate specific roles related to feeding regulation. The minimal effect of feeding and digestion on pyy expression in the brain indicates that PYY plays a minor role in the central control of short-term food intake in Atlantic salmon. [Copyright &y& Elsevier]

Warm water treatment, i.e. exposure to sea water at a temperature of 28–34 °C for 20–30 s, has in recent years been widely used for delousing of Atlantic salmon (Salmo salar) and rainbow trout (Oncorhynchus mykiss) in Norwegian aquaculture. High mortality and various lesions (e.g. injuries and/or bleedings in skin, fins, eyes, brain, and gills) have, however, been reported after industrial warm water treatments. The objective of this study was to reveal whether the thermal component of warm water treatment inflicts acute lesions on Atlantic salmon. The study was conducted by exposing individual, sedated Atlantic salmon post-smolts (w ¯ = 1117 ± 250 g) to sea water at a temperature of 34 °C (warm water treatment, n = 40) or 9 °C (control treatment, n = 20) for 30 s, and subsequently conducting welfare indicator scoring and histopathological examination of their skin, fins, eyes, snout, nasal pits/mucosa, palate, gills, thymus, pseudobranch, brain, heart, liver, kidney, pyloric caeca, pancreas, and spleen. The results showed that the prevalence and severity of acute lesions were not significantly different between the two treatment groups, except for higher prevalence of injuries on the caudal (p = 0.002), dorsal (p = 0.002), and right pelvic fins (p = 0.014) in the warm water treatment group. The main cause of these fin injuries may have been a strong behavioural reaction displayed by the fish when exposed to warm water. Possible consequences of fin injuries, the use of anaesthetic, and statistical limitations were discussed. It was concluded that exposure of Atlantic salmon to sea water at a temperature of 34 °C for 30 s did not lead to any statistically significant change in the prevalence of acute lesions except an increase in minor, possibly behaviour-related, fin injuries. Detection of a lower lesion prevalence than was possible in this study, but which may concern many individuals in an industrial setting, requires examination of a larger number of fish. • Warm water treatment has in recent years been widely used for delousing of Atlantic salmon and rainbow trout in Norwegian aquaculture. • The objective of this study was to reveal whether the thermal component of warm water treatment inflicts acute lesions on Atlantic salmon. • Exposure of sedated Atlantic salmon post-smolts to sea water at a temperature of 34 °C for 30 did not lead to any statistically significant change in the prevalence of acute lesions except an increase in minor fin injuries. • The main cause of these fin injuries may have been a strong behavioural reaction displayed by the fish when exposed to warm water. [ABSTRACT FROM AUTHOR]