Metabolic response in liver and Brockmann bodies of rainbow trout to inhibition of lipolysis; possible involvement of the hypothalamus-pituitary-interrenal (HPI) axis

Characterization and modulation of brain lipids content of rainbow trout fed with 100% plant based diet rich in omega-3 long chain polyunsaturated fatty acids DHA and EPA

Brain functions are known to be mainly modulated by adequate dietary intake. Inadequate intake as can be an excess or significant deficiency affect cognitive processes, behavior, neuroendocrine functions and synaptic plasticity with protective or harmful effects on neuronal physiology. Lipids, in particular, ω-6 and ω-3 long chain polyunsaturated fatty acids (LC-PUFAs) play structural roles and govern the different functions of the brain. Hence, the goal of this study was to characterize the whole brain fatty acid composition (precursors, enzymatic and non-enzymatic oxidation metabolites) of fish model of rainbow trout fed with three experimental plant-based diet containing distinct levels of eicosapentaenoic acid (EPA, 20:5 ω-3) and docosahexaenoic acid (DHA, 22:6 ω-3) (0% for low, 15.7% for medium and 33.4% for high, total fatty acid content) during nine weeks. Trout fed with the diet devoid of DHA and EPA showed reduced brain content of total ω-3 LC-PUFAs, with diminution of EPA and DHA. Selected enzymatic (cyclooxygenases and lipoxygenases) oxidation metabolites of arachidonic acid (AA, 20:4 ω-6) decrease in medium and high ω-3 LC-PUFAs diets. On the contrary, total selected enzymatic oxidation metabolites of DHA and EPA increased in high ω-3 LC-PUFAs diet. Total selected non-enzymatic oxidation metabolites of DHA (not detected for EPA) increased in medium and high ω-3 LC-PUFAs diets. In conclusion, this work revealed for the first time in fish model the presence of some selected enzymatic and non-enzymatic oxidation metabolites in brain and the modulation of brain lipid content by dietary DHA and EPA levels.

Stress Effects on the Mechanisms Regulating Appetite in Teleost Fish

The homeostatic regulation of food intake relies on a complex network involving peripheral and central signals that are integrated in the hypothalamus which in turn responds with the release of orexigenic or anorexigenic neuropeptides that eventually promote or inhibit appetite. Under stress conditions, the mechanisms that control food intake in fish are deregulated and the appetite signals in the brain do not operate as in control conditions resulting in changes in the expression of the appetite-related neuropeptides and usually a decreased food intake. The effect of stress on the mechanisms that regulate food intake in fish seems to be mediated in part by the corticotropin-releasing factor (CRF), an anorexigenic neuropeptide involved in the activation of the HPI axis during the physiological stress response. Furthermore, the melanocortin system is also involved in the connection between the HPI axis and the central control of appetite. The dopaminergic and serotonergic systems are activated during the stress response and they have also been related to the control of food intake. In addition, the central and peripheral mechanisms that mediate nutrient sensing capacity and hence implicated in the metabolic control of appetite are inhibited in fish under stress conditions. Finally, stress also affects peripheral endocrine signals such as leptin. In the present minireview, we summarize the knowledge achieved in recent years regarding the interaction of stress with the different mechanisms that regulate food intake in fish.

Nutrient Sensing Systems in Fish: Impact on Food Intake Regulation and Energy Homeostasis

Evidence obtained in recent years in a few species, especially rainbow trout, supports the presence in fish of nutrient sensing mechanisms. Glucosensing capacity is present in central (hypothalamus and hindbrain) and peripheral [liver, Brockmann bodies (BB, main accumulation of pancreatic endocrine cells in several fish species), and intestine] locations whereas fatty acid sensors seem to be present in hypothalamus, liver and BB. Glucose and fatty acid sensing capacities relate to food intake regulation and metabolism in fish. Hypothalamus is as a signaling integratory center in a way that detection of increased levels of nutrients result in food intake inhibition through changes in the expression of anorexigenic and orexigenic neuropeptides. Moreover, central nutrient sensing modulates functions in the periphery since they elicit changes in hepatic metabolism as well as in hormone secretion to counter-regulate changes in nutrient levels detected in the CNS. At peripheral level, the direct nutrient detection in liver has a crucial role in homeostatic control of glucose and fatty acid whereas in BB and intestine nutrient sensing is probably involved in regulation of hormone secretion from endocrine cells.

Hypothalamic fatty acid sensing in Senegalese sole (Solea senegalensis): response to long-chain saturated, monounsaturated, and polyunsaturated (n-3) fatty acids

We assessed the presence of fatty acid (FA)-sensing mechanisms in hypothalamus of Senegalese sole (Solea senegalensis) and investigated their sensitivity to FA chain length and/or level of unsaturation. Stearate (SA, saturated FA), oleate (OA, monounsaturated FA of the same chain length), α-linolenate [ALA, a n-3 polyunsaturated fatty acid (PUFA) of the same chain length], and eicosapentanoate (EPA, a n-3 PUFA of a larger chain length) were injected intraperitoneally. Parameters related to FA sensing and neuropeptide expression in the hypothalamus were assessed after 3 h and changes in accumulated food intake after 4, 24, and 48 h. Three FA sensing systems characterized in rainbow trout were also found in Senegalese sole and were activated by OA in a way similar to that previously characterized in rainbow trout and mammals. These hypothalamic FA sensing systems were also activated by ALA, differing from mammals, where n-3 PUFAs do not seem to activate FA sensors. This might suggest additional roles and highlights the importance of n-3 PUFA in fish diets, especially in marine species. The activation of FA sensing seems to be partially dependent on acyl chain length and degree of saturation, as no major changes were observed after treating fish with SA or EPA. The activation of FA sensing systems by OA and ALA, but not SA or EPA, is further reflected in the expression of hypothalamic neuropeptides involved in the control of food intake. Both OA and ALA enhanced anorexigenic capacity compatible with the activation of FA sensing systems.

Effects of insulin treatment on the response to oleate and octanoate of food intake and fatty acid-sensing systems in rainbow trout

We hypothesized that food intake and the response of fatty acid (FA)-sensing systems in hypothalamus, liver, and Brockmann bodies of rainbow trout to raised levels of oleate (OL) or octanoate (OCT) is modified by insulin treatment. To assess this hypothesis, 15 fish per group received intraperitoneally 10-mL/kg injection of saline solution alone (control), or containing insulin (2-mg bovine insulin/kg body mass), OL (300 μg/kg), OCT (300 μg/kg), insulin + OL, or insulin + OCT to be sampled 6 h later to assess parameters related to FA sensing. Our results suggest that the modulatory role of insulin on the responses of hypothalamic FA-sensing systems to changes in circulating levels of OL or OCT was of minor importance in contrast to the mammalian model. However, this is in contrast with the effects observed in another experiment assessing changes in food intake after similar treatments because insulin treatment enhanced the anorectic effects of FA alone, and the effect was especially relevant (P < 0.001) for OCT, in contrast with the mammalian model where this FA is not inducing an anorectic response. In liver and Brockmann bodies, insulin treatment enhanced the responses to OL or OCT treatment in parameters related to FA sensing. Therefore, we provide for the first time in fish, and in a non-mammalian vertebrate, evidence for the modulation of FA-sensing systems by insulin.

Feeding rainbow trout with a lipid-enriched diet: effects on fatty acid sensing, regulation of food intake, and cellular signaling pathways

Using rainbow trout fed with low fat (LF) or high-fat (HF) diets we aimed to determine if the response of food intake, mRNA abundance of hypothalamic neuropeptides involved in the metabolic regulation of food intake, and fatty acid (FA) sensing systems in hypothalamus and liver is similar to that previously observed when levels of specific FA were raised by injection. Moreover, we also aimed to determine if the phosphorylation state of intracellular energy sensor 5′-AMP-activated protein kinase (AMPK), and proteins involved in cellular signalling such as protein kinase B (Akt) and target of rapamycin (mTOR) display changes that could be related to FA-sensing and the control of food intake. The increased levels of FA in hypothalamus and liver of rainbow trout fed the HF diet only partially activated FA-sensing systems and did not elicit changes in food intake suggesting that FA-sensing response in fish to increased levels of FA is more dependent on the presence of specific FA such as oleate or octanoate rather than to the global increase in FA. We also obtained, for the first time in fish, evidence for the presence and function of energy sensors like AMPK and proteins involved in cellular signaling like mTOR and Akt in hypothalamus. These proteins in hypothalamus and liver were generally activated in fish fed the HF vs LF diet suggesting the activation of the cellular signaling pathways in response to the increased availability of FA.