In vitro leptin treatment of rainbow trout hypothalamus and hindbrain affects glucosensing and gene expression of neuropeptides involved in food intake regulation

The feedback regulation of carbohydrates intake on food intake and appetite in grass carp (Ctenopharyngodon idella)

Improving carbohydrate utilization can contribute to sustainability of aquaculture. In order to explore the feedback mechanism of glucose homeostasis in fish, one control diet (25% carbohydrate and 40% protein), one relatively high carbohydrate diet named HG (42% carbohydrate and 40% protein), and one high dietary carbohydrate coupled with relatively low protein diet named HGP (42% carbohydrate and 25% protein) were fed to grass carp for 40 days. After the feeding trial, HG group impeded the food intake and growth performance of fish compared with the other two groups. Meanwhile, the serum glucose and insulin level were both significantly elevated under the condition of high carbohydrates intake when compared HG with control group. However, although no significant difference was observed in peripheral glucose or insulin between HG and HGP groups, fish fed with HGP diet increased growth performance and food intake compared with the HG group. Gene expression data indicated that fish selectively regulated the expressions of the cerebral anorexigenic genes (cart and pomc) to adapt to the HG and HGP intake. Therefore, the HGP diet with high carbohydrate and low protein was more suitable for grass carp feeding and growth when compared with the other two diets, possibly because the diet composition was closer to the natural food of this fish. In addition, the serum leptin level was highly consistent with changes in food intake and anorexigenic gene expressions when comparing the three experimental diets, indicating that leptin might be the key to mediate the feedback regulation of carbohydrates intake on food intake and appetite in fish.

Leptin signalling in teleost fish with emphasis in food intake regulation

Leptin, the product of the obese (ob or Lep) gene, was first cloned in teleost fish in 2005, more than a decade after its identification in mammals. This was because bony fish and mammalian leptins share a very low amino acid sequence identity, which suggests different functionality of the leptin system in fish compared to that of mammals. Indeed, major differences are evident between the mammalian and fish leptin system. Thus, for instance, mammalian leptin is synthesized and released by the adipose tissue in response to the amount of fat depots, while several tissues (mainly the liver) are the main sources of leptin in fish, whose determining factors of production are still unclear. In mammals, the main physiological role for leptin is its involvement in the maintenance of energy balance by decreasing food intake and increasing energy expenditure, although a wide variety of actions have been attributed to this hormone (e.g., regulation of lipid and carbohydrate metabolism, reproduction and immune functions). In fish, available literature also points towards a multifunctional nature for leptin, although knowledge on its functions is limited. In this review, we offer an overview of teleostean leptin structure and mechanism of action, and discuss the available knowledge on the role of this hormone in food intake regulation in teleost fish, aiming to provide a comparative overview between the functioning of the teleostean and mammalian leptin systems.

In vitro insulin treatment reverses changes elicited by nutrients in cellular metabolic processes that regulate food intake in fish

This research assessed the direct effects of insulin on nutrient-sensing mechanisms in the brain of rainbow trout (Oncorhynchus mykiss) using an in vitro approach. Cultured hypothalamus and hindbrain were exposed to 1 µmol l-1 insulin for 3 h, and signals involved in appetite regulation and nutrient-sensing mechanisms were measured. Additionally, the involvement of the phosphatidylinositide 3-kinase (PI3K)/protein kinase B (Akt) signaling pathway in the actions of insulin was studied by using the inhibitor wortmannin. Treatment with insulin alone did not elicit many changes in the appetite regulators and nutrient-sensing-related genes and enzymes tested in the hypothalamus and hindbrain. However, we found that, when insulin and nutrients were added together, insulin reversed most of the effects exerted by nutrients alone, suggesting that insulin changes responsiveness to nutrients at the central level. Effects reversed by insulin included expression levels of genes related to the sensing of both glucose (slc2a2, slc5a1, gck, pck1, pklr, g6pcb, gys1, tas1r3 and nr1h3 in the hindbrain, and slc2a2, pklr and pck1 in the hypothalamus) and fatty acid (cd36 in the hindbrain, and cd36 and acly in the hypothalamus). Nutrient-induced changes in the activity of Acly and Cpt-1 in the hindbrain and of Pepck, Acly, Fas and Hoad in the hypothalamus were also reversed by insulin. Most of the insulin effects disappeared in the presence of wortmannin, suggesting the PI3K/Akt pathway is a mediator of the effects of insulin reported here. This study adds new information to our knowledge of the mechanisms regulating nutrient sensing in fish.

First evidence on the role of palmitoylethanolamide in energy homeostasis in fish

The objective of this study was to investigate the role of palmitoylethanolamide (PEA) in the regulation of energy homeostasis in goldfish (Carassius auratus). We examined the effects of acute or chronic intraperitoneal treatment with PEA (20 μg·g^-1 body weight) on parameters related to food intake and its regulatory mechanisms, locomotor activity, glucose and lipid metabolism, and the possible involvement of transcription factors and clock genes on metabolic changes in the liver. Acute PEA treatment induced a decrease in food intake at 6 and 8 h post-injection, comparable to that observed in mammals. This PEA anorectic effect in goldfish could be mediated through interactions with leptin and NPY, as PEA increased hepatic expression of leptin aI and reduced hypothalamic expression of npy. The PEA chronic treatment reduced weight gain, growth rate, and locomotor activity. The rise in glycolytic potential together with the increased potential of glucose to be transported into liver suggests an enhanced use of glucose in the liver after PEA treatment. In addition, part of glucose may be exported to be used in other tissues. The activity of fatty acid synthase (FAS) increased after chronic PEA treatment, suggesting an increase in the hepatic lipogenic capacity, in contrast with the mammalian model. Such lipogenic increment could be linked with the PEA-induction of REV-ERBα and BMAL1 found after the chronic treatment. As a whole, the present study shows the actions of PEA in several compartments related to energy homeostasis and feeding behavior, supporting a regulatory role for this N-acylethanolamine in fish.

Responses of glucosensing system to glucose in Japanese flounder Paralichthys olivaceus fed diets with different carbohydrate content

A 10-week feeding trial was conducted to investigate the response of glucosensing system to glucose in Japanese flounder Paralichthys olivaceus (initial body weight: 7.14 ± 0.10 g) fed diets with different carbohydrate content. Two experimental diets were designed as carbohydrate free (CF) and suitable carbohydrate (SC) supplementation, respectively. The dietary carbohydrate contents were 0.93% and 15.6%, respectively. After a 10-week feeding trial, a glucose tolerance test (GTT) was performed. Results showed that after the last meal in the feeding trial, the blood glucose of fish fed with diet CF peaked at 3 h (4.64 ± 0.29 mM), the duration of hyperglycemia was about 5 h (1-6 h). The blood glucose in SC group peaked at 9 h (3.28 ± 0.66 mM), and the duration of hyperglycemia was approximately 6 h (6-12 h). After GTT, blood glucose reached the first peak at 6 h both in the two groups, and the duration of hyperglycemia was obvious 24 h. During the 3-12 h after injection, blood glucose level in SC group was significantly higher than that in CF group. However, blood glucose level in group SC was significantly lower than that in group CF at 24 h. The blood glucose level decreased to half of the peak at 10.97 h after injection of glucose in SC group and at 27.26 h in CF group. The 6-24 h clearance ability in SC group (6.57 ± 1.68%/h) was significantly higher than that in CF group (2.81 ± 1.11%/h). Compared with CF diet, SC diet significantly increase the expression of glucosensing-related genes including glucose facilitative transporter type 2, glucokinase, inward rectifier K⁺ channel pore type 6.2, sulfonylurea receptor, carnitine palmitoyltransferase 1b, hydroxyacyl-CoA dehydrogenase, cytochrome c oxidase subunit 4, mitochondrial uncoupling protein 2a, liver X receptor, sodium/glucose co-transporter 1, a heterodimer of type 1 receptor subunits depending on T1R2 + T1R3 in liver and intestine. Meanwhile, activities of glucokinase, pyruvate kinase and glycogen synthase in liver, and hepatic glycogen content were also increased. In conclusion, glucosensing systems in Japanese flounder are responsive to dietary carbohydrate levels, especially the suitable dietary carbohydrate level, at which the glucose tolerance capacity of Japanese flounder was improved.

Sensing Glucose in the Central Melanocortin Circuits of Rainbow Trout: A Morphological Study

In mammals, glucosensing markers reside in brain areas known to play an important role in the control of food intake. The best characterized glucosensing mechanism is that dependent on glucokinase (GK) whose activation by increased levels of glucose leads in specific hypothalamic neurons to decreased or increased activity, ultimately leading to decreased food intake. In fish, evidence obtained in recent years suggested the presence of GK-like immunoreactive cells in different brain areas related to food intake control. However, it has not been established yet whether or not those neuronal populations having glucosensing capacity are the same that express the neuropeptides involved in the metabolic control of food intake. Therefore, we assessed through dual fluorescent in situ hybridization the possible expression of GK in the melanocortinergic neurons expressing proopiomelanocortin (POMC) or agouti-related protein (AGRP). POMC and AGRP expression localized exclusively in the rostral hypothalamus, in the ventral pole of the lateral tuberal nucleus, the homolog of the mammalian arcuate nucleus. Hypothalamic GK expression confined to the ependymal cells coating the ventral pole of the third ventricle but some expression level occurred in the AGRP neurons. GK expression seems to be absent in the hypothalamic POMC neurons. These results suggest that AGRP neurons might sense glucose directly through a mechanism involving GK. In contrast, POMC neurons would not directly respond to glucose through GK and would require presynaptic inputs to sense glucose. Ependymal cells could play a critical role relying glucose metabolic information to the central circuitry regulating food intake in fish, especially in POMC neurons.

Nesfatin-1 Regulates Feeding, Glucosensing and Lipid Metabolism in Rainbow Trout

Nesfatin-1 is an 82 amino acid peptide that has been involved in a wide variety of physiological functions in both mammals and fish. This study aimed to elucidate the role of nesfatin-1 on rainbow trout food intake, and its putative effects on glucose and fatty acid sensing systems. Intracerebroventricular administration of 25 ng/g nesfatin-1 resulted in a significant inhibition of appetite, likely mediated by the activation of central POMC and CART. Nesfatin-1 stimulated the glucosensing machinery (changes in sglt1, g6pase, gsase, and gnat3 mRNA expression) in the hindbrain and hypothalamus. Central fatty acid sensing mechanisms were unaltered by nesfatin-1, but this peptide altered the expression of mRNAs encoding factors regulating lipid metabolism (fat/cd36, acly, mcd, fas, lpl, pparα, and pparγ), suggesting that nesfatin-1 promotes lipid accumulation in neurons. In the liver, intracerebroventricular nesfatin-1 treatment resulted in decreased capacity for glucose use and lipogenesis, and increased the potential of fatty acid oxidation. Altogether, the present results demonstrate that nesfatin-1 is involved in the homeostatic regulation of food intake and metabolism in fish.

Hypothalamic Integration of Metabolic, Endocrine, and Circadian Signals in Fish: Involvement in the Control of Food Intake

The regulation of food intake in fish is a complex process carried out through several different mechanisms in the central nervous system (CNS) with hypothalamus being the main regulatory center. As in mammals, a complex hypothalamic circuit including two populations of neurons: one co-expressing neuropeptide Y (NPY) and Agouti-related peptide (AgRP) and the second one population co-expressing pro-opiomelanocortin (POMC) and cocaine- and amphetamine-regulated transcript (CART) is involved in the integration of information relating to food intake control. The production and release of these peptides control food intake, and the production results from the integration of information of different nature such as levels of nutrients and hormones as well as circadian signals. The present review summarizes the knowledge and recent findings about the presence and functioning of these mechanisms in fish and their differences vs. the known mammalian model.

Expression and distribution of leptin and its receptors in the digestive tract of DIO (diet-induced obese) zebrafish

The expression and localization of leptin (A and B) and its receptor family in control and diet-induced obese (DIO) adult male zebrafish gut, after 5-weeks overfeeding, administering Artemia nauplii, as fat-rich food, were investigated. Recently, the obese adult zebrafish was considered an experimental model with pathophysiological pathways similar to mammalian obesity. Currently, there are no reports about leptin in fish obesity, or in a state of altered energy balance. By qRT-PCR, leptin A and leptin B expression levels were significantly higher in DIO zebrafish gut than in the control group (CTRL), and the lowest levels of leptin receptor mRNA appeared in DIO zebrafish gut. The presence of leptin and its receptor proteins in the intestinal tract was detected by western blot analysis in both control and DIO zebrafish. By single immunohistochemical staining, leptin and leptin receptor immunoreactive endocrine cells were identified in the intestinal tract either in DIO or control zebrafish. Moreover, leptin immunopositive enteric nervous system elements were observed in both groups. By double immunohistochemical staining, leptin and its receptor were colocalized especially in DIO zebrafish. Thus, our study represents a starting point in the investigation of a possible involvement of leptin in control of energy homeostasis in control and DIO zebrafish.

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.

Changes in the levels and phosphorylation status of Akt, AMPK, CREB, and FoxO1 in hypothalamus of rainbow trout under conditions of enhanced glucosensing activity

There is no available information in fish about mechanisms linking glucosensing activation and changes in the expression of brain neuropeptides controlling food intake. Therefore, we assessed in rainbow trout hypothalamus the effects of raised levels of glucose on the levels and phosphorylation status of two transcription factors, FoxO1 and CREB, possibly involved in linking those processes. Moreover, we also aimed to assess the changes in the levels and phosphorylation status of two proteins possibly involved in the modulation of these transcription factors such as Akt and AMPK. Therefore, we evaluated in pools of hypothalamus incubated for 3h and 6h at 15 °C in modified Hanks’ medium containing 2, 4, or 8 mM D-glucose the response of parameters related to glucosensing mechanisms, neuropeptide expression, and levels and phosphorylation status of proteins of interest. The activation of hypothalamic glucosensing systems and the concomitant enhanced anorectic potential occurred in parallel with activation of Akt and inhibition of AMPK. The changes in these proteins would relate to neuropeptide expression through changes in the levels and phosphorylation status of transcription factors under their control, such as CREB and FoxO1, which displayed inhibitory (CREB) or activatory (FoxO1) responses to increased glucose.

Assessing the Functional Role of Leptin in Energy Homeostasis and the Stress Response in Vertebrates

Leptin is a pleiotropic hormone that plays a critical role in regulating appetite, energy metabolism, growth, stress, and immune function across vertebrate groups. In mammals, it has been classically described as an adipostat, relaying information regarding energy status to the brain. While retaining poor sequence conservation with mammalian leptins, teleostean leptins elicit a number of similar regulatory properties, although current evidence suggests that it does not function as an adipostat in this group of vertebrates. Teleostean leptin also exhibits functionally divergent properties, however, possibly playing a role in glucoregulation similar to what is observed in lizards. Further, leptin has been recently implicated as a mediator of immune function and the endocrine stress response in teleosts. Here, we provide a review of leptin physiology in vertebrates, with a particular focus on its actions and regulatory properties in the context of stress and the regulation of energy homeostasis.

Effect of long-term fasting and a subsequent meal on mRNA abundances of hypothalamic appetite regulators, central and peripheral leptin expression and plasma leptin levels in rainbow trout

Knowledge about neuroendocrine mechanisms regulating appetite in fish, including the role of leptin, is inconclusive. We investigated leptin mRNA abundance in various tissues, plasma leptin levels and the hypothalamic gene expression of putative orexigenic (neuropeptide Y and agouti-regulated peptide) and anorexigenic (melanocortin receptor, proopiomelanocortins (POMCs), cocaine- and amphetamine-regulated transcript and corticotropin-releasing factor) neuropeptides in relation to feeding status in rainbow trout (Oncorhynchus mykiss). Blood and tissues were first (Day 1) sampled from trout that had been fed or fasted for 4 months and the day after (Day 2) from fasted fish after they had been given a large meal, and their continuously fed counterparts. The fasted fish ate vigorously when they were presented a meal. There were no differences between fed, fasted and re-fed fish in hypothalamic neuropeptide transcript levels, except for pomca1 and pomcb, which were higher in fasted fish than in fed fish at Day 1, and which, for pomcb, decreased to the level in fed fish after the meal at Day 2. Plasma leptin levels did not differ between fasted, re-fed and fed fish. A higher leptina1 transcript level was seen in the belly flap of fasted fish than in fed fish, even after re-feeding on Day 2. The data do not reveal causative roles of the investigated brain neuropeptides, or leptin, in appetite regulation. It is suggested that the elevated pomc transcript levels provide a satiety signal that reduces energy expenditure during prolonged fasting. The increase in belly flap leptin transcript with fasting, which did not decrease upon re-feeding, indicates a tissue-specific role of leptin in long-term regulation of energy homeostasis.

JAK family members: Molecular cloning, expression profiles and their roles in leptin influencing lipid metabolism in Synechogobius hasta

Janus kinase (JAK) is a family of non-receptor tyrosine kinases that participate in transducing cytokine signals from the external environment to the nucleus in various biological processes. Currently, information about their genes structure and evolutionary history has been extensively studied in mammals as well as in several fish species. By contrast, limited reports have addressed potential role of diverse JAK in signaling responses to leptin in fish. In this study, we identified and characterized five JAK members of Synechogobius hasta. Compared to mammals, more members of the JAK family were found in S. hasta, which provided evidence that the JAK family members had arisen by the whole genome duplications during vertebrate evolution. For protein structure, all of these members possessed similar domains compared with those of mammals. Their mRNAs were expressed in a wide range of tissues, but at the different levels. Incubation in vitro of freshly isolated hepatocytes of S. hasta with different concentrations of recombinant human leptin decreased the intracellular triglyceride content and lipogenic genes expression, and increased mRNA expression of several JAK and lipolytic genes. AG490, a specific inhibitor of JAK, reversed leptin-induced effects on TG content and JAK2a, JAK2b, hormone-sensitive lipase (HSL2) and acetyl-CoA carboxylase (ACCa), indicating that the JAK2a/b may have mediated the actions of leptin on lipid metabolism at transcriptional level.

Effects of random food deprivation and refeeding on energy metabolism, behavior and hypothalamic neuropeptide expression in Apodemus chevrieri

Maintaining adaptive control of behavior and physiology is the main strategy used by animals in responding to changes of food resources. To investigate the effects of random food deprivation (FD) and refeeding on energy metabolism and behavior in Apodemus chevrieri, we acclimated adult males to FD for 4weeks, then refed them ad libitum for 4weeks (FD-Re group). During the period of FD, animals were fed ad libitum for 4 randomly assigned days each week, and deprived of food the other 3days. A control group was fed ad libitum for 8weeks. At 4 and 8weeks we measured body mass, thermogenesis, serum leptin levels, body composition, gastrointestinal tract morphology, behavior and hypothalamic neuropeptide expression. At 4weeks, food intake, gastrointestinal mass, neuropeptide Y (NPY) and agouti-related protein (AgRP) mRNA expressions increased and thermogenesis, leptin levels, pro-opiomelanocortin (POMC) and cocaine- and amphetamine-regulated transcript (CART) expressions decreased in FD compared with controls. FD also showed more resting behavior and less activity than the controls on ad libitum day. There were no differences between FD-Re and controls at 8weeks, indicating significant plasticity. These results suggested that animals can compensate for unpredictable reduction in food availability by increasing food intake and reducing energy expended through thermogenesis and activity. Leptin levels, NPY, AgRP, POMC, and CART mRNA levels may also regulate energy metabolism. Significant plasticity in energy metabolism and behavior was shown by A. chevrieri over a short timescale, allowing them to adapt to food shortages in nutritionally unpredictable environments.

Energy stores, lipid mobilization and leptin endocrinology of rainbow trout

The physiological role of leptin in fish is not fully elucidated. In the present study, the involvement of the leptin system in lipid deposition and mobilization in rainbow trout during feeding and 1, 2 and 4 weeks of fasting was investigated in two lines of rainbow trout with different muscle and visceral adiposity: a fat line (FL) with high total energy reserves, high muscle adiposity, but low visceral adiposity and a lean line (LL) with lower total energy reserves and lower muscle adiposity, but higher visceral adiposity. During 4 weeks of fasting, muscle lipids decreased by 63 % in the FL fish, while no such energy mobilization from muscle occurred in the LL fish. On the other hand, lipid stores in liver and visceral adipose tissue was utilized to a similar extent by the two fish lines during fasting. Under normal feeding conditions, plasma leptin levels were higher in the LL than the FL fish, suggesting a possible contribution of visceral adipocytes to plasma leptin levels. Plasma leptin-binding protein levels did not differ between the lines and were not affected by fasting. After 4 weeks of fasting, the long leptin receptor and the leptin-binding protein isoforms 1 and 3 muscle expression increased in the LL fish, as well as hepatic expression of leptin A1 and the two binding protein isoforms. These responses were not seen in the FL fish. The data suggest that the Lep system in rainbow trout is involved in regulation of energy stores and their mobilization.

Acute anorexigenic action of leptin in rainbow trout is mediated by the hypothalamic Pi3k pathway

Leptin (Lep) is an anorexigenic hormone and regulates appetite-related neuropeptides in mammals. A number of neuropeptides have also been linked to appetite regulation in teleost fish, but Lep signaling activation and effects on appetite-regulating neurons are poorly elucidated in early vertebrates. This study uses cellular, tissue and organismal approaches to elucidate the acute, central Lep action in rainbow trout. The results demonstrate that Lep activates phosphorylation of protein kinase B (Akt) and signal transducer and activator of transcription 3 in rainbow trout hypothalamus-derived cells, and that the phosphatidylinositol-3-kinase (Pi3k) inhibitor LY294002 can suppress the Lep-induced Akt phosphorylation. Intracerebroventricular (ICV) Lep administration strongly suppresses food intake at the doses of 0.05 and 0.5 µg Lep fish(-1) At low dose, Lep stimulates hypothalamic transcription of anorexigenic cocaine- and amphetamine-regulated transcript (Cart) and orexigenic neuropeptide Y. At high dose, Lep stimulates hypothalamic transcription of anorexigenic proopiomelanocortin (Pomc) A1, A2, and B, while coinjection with LY294002 reverses this upregulation. The data suggest that the anorexigenic action of Lep in rainbow trout is mediated through stimulation of the anorexigenic neuropeptides Pomc and Cart. Furthermore, ICV Lep treatment increases phosphor-Akt-immunoreactive cells in the nucleus lateralis tuberis, periventricular zone along infundibulum, and lateral recess surrounded by nucleus anterior tuberis, while LY294002 inhibits this effect. Lep receptor-immunoreactive cells are also predominant in these regions. These results demonstrate that Lep activates the Pi3k-Akt pathway in the lateral tuberal hypothalamus of rainbow trout for acute appetite regulation, indicating the conservation of anorexigenic Lep action in the mediobasal hypothalamus.

Isolation and Expression Analysis of STAT Members from Synechogobius hasta and Their Roles in Leptin Affecting Lipid Metabolism

Signal transducers and activators of transcription proteins (STATs) act as important mediators in multiple biological processes induced by a large number of cytokines. In the present study, full-length cDNA sequences of seven STAT members, including some splicing variants different from those in mammals, were obtained from Synechogobius hasta. The phylogenetic analysis revealed that the seven STAT members were derived from paralogous genes that might have arisen by whole genome duplication (WGD) events during vertebrate evolution. All of these members share similar domain structure compared with those of mammals, and were widely expressed across the tested tissues (brain, gill, heart, intestine, liver, muscle and spleen), but at variable levels. Incubation in vitro of recombinant human leptin changed the intracellular triglyceride (TG) content and mRNA levels of several STATs members, as well as expressions and activities of genes involved in lipid metabolism. Furthermore, Tyrphostin B42 (AG490), a specific inhibitor of the Janus Kinase 2(JAK2)-STAT pathway, partially reversed leptin-induced change on STAT3 and its two spliced isoforms expression, as well as expressions and activities of genes involved in lipid metabolism. As a consequence, the decrease of TG content was also reversed. Thus, our study suggests that STAT3 is the requisite for the leptin signal and the activation of the STAT3 member may account for the leptin-induced changes in lipid metabolism in S. hasta.

Elevated plasma leptin levels of fasted rainbow trout decrease rapidly in response to feed intake

Leptin has an anorexigenic effect in fish, indicating a role in regulation of growth and energy homeostasis. The study aimed to further clarify the physiological role of leptin in rainbow trout, specifically its short-term response to feed intake after a period of fasting. Utilizing a salmonid leptin radioimmunoassay, the study demonstrates differences in plasma leptin levels in fishes with different nutritional status and at the onset of feeding. Some of the fasted fish were clearly in a state of anorexia, and did not initiate feeding during the 72h refeeding period. For those fish that did initiate feeding, both previously fed and fasted, plasma leptin levels rapidly decreased during the first 24h in correlation with increased amount of food reaching the gastrointestinal tract, while non-feeding individuals retained a high plasma leptin levels. The data indicate that the leptin-induced anorexic state is broken after onset of feeding and that the regulatory mechanisms leading to decreased plasma leptin levels are linked to nutrient levels.

Seasonal changes in body mass, serum leptin levels and hypothalamic neuropeptide gene expression in male Eothenomys olitor

The present study examined seasonal changes in body mass and energy metabolism in the Chaotung vole (Eothenomys olitor) and the physiological mechanisms underpinning these changes. Seasonal changes in the following parameters were measured in male E. olitor, body mass, food intake, thermogenesis, enzyme activity, masses of tissues and organs, hormone concentrations and expression of hypothalamic arcuate nucleus energy balance genes including neuropeptide Y (NPY), agouti-related protein (AgRP), pro-opiomelanocortin (POMC), and cocaine- and amphetamine-regulated transcript (CART). Body mass was constant over the year, but the masses of tissues and organs differed significantly between seasons. There were significant changes in body fat mass and serum leptin levels over the four seasons. E. olitor showed significant seasonal changes in food intake and thermogenesis, uncoupling protein 1 (UCP1) content, enzyme activity, and serum tri-iodothyronine (T3) and thyroxine (T4) levels. Moreover, mRNA expression in the hypothalamus showed significant seasonal changes. All of our results suggested that E. olitor had constant body mass over the year, which was inconsistent with the prediction of the 'set-point' hypothesis. However, body fat mass and serum leptin levels were significantly different among the four seasons, providing support for the 'set-point' hypothesis. The changes in leptin, NPY, AgRP, POMC, and CART mRNA levels may play a role in the regulation of energy intake in E. olitor. Furthermore, the role of leptin and hypothalamic neuropeptide gene in the regulation of energy metabolism and body mass may be different in animals that are acclimated to different seasons.

Effects of recombinant human leptin administration on hepatic lipid metabolism in yellow catfish Pelteobagrus fulvidraco: in vivo and in vitro studies

The present study was conducted to investigate the effects and mechanism of leptin influencing lipid metabolism in yellow catfish Pelteobagrus fulvidraco. To this end, hepatic lipid (in vivo experiment) and intracellular triglyceride (TG) (in vitro experiment) content, the activities and/or expression level of several enzymes (CPT-1, 6PGD, G6PD, FAS, ME and ICDH) as well as the mRNA expression of transcription factors (PPARα, PPARγ and SREBP-1) involved in lipid metabolism were determined. Using the primary hepatocytes of yellow catfish, specific inhibitors AG490 (JAK-STAT inhibitor) and wortmannin (IRS-PI3K inhibitor) were used to explore the signaling pathways of leptin effects on lipid metabolism. Intraperitoneal injection of recombinant human leptin (rt-hLEP) significantly reduced hepatic lipid content, activities of lipogenic enzymes (6PGD, G6PD, ME, ICDH and FAS) as well as mRNA levels of 6PGD, G6PD, FAS, PPARγ and SREBP-1 genes, but up-regulated activity and mRNA level of CPT-1 and PPARα. Using primary hepatocytes, rt-hLEP incubation also reduced intracellular TG content, mRNA levels of G6PD and PPARγ genes, but enhanced mRNA levels of PPARα, CPT-1 and SREBP-1. Leptin-induced effects could partially be reversed by specific inhibitors AG490, suggesting that JAK-STAT signaling pathways played important roles in the process of leptin-induced changes in lipid metabolism. Wortmannin significantly suppressed the decrease of TG content induced by leptin, reflecting that IRS-PI3K was involved in the leptin-mediate changes as well. To our knowledge, the present study provides, for the first time, evidence that rt-hLEP can increase lipolysis and reduce lipogenesis at the both enzymatic and molecular levels in fish with the combination of in vivo with in vitro studies, which serves to increase our understanding into the roles and mechanisms of leptin regulating lipid metabolism in fish.

Hormone-sensitive lipase in yellow catfish Pelteobagrus fulvidraco: molecular characterization, mRNA tissue expression and transcriptional regulation by leptin in vivo and in vitro

Hormone-sensitive lipase (hsl) plays a pivotal role in regulation of lipolysis in mammals, but information is very scarce about its gene structure and function in fish. In this study, two distinct hsl cDNAs, designated hsl1 and hsl2, were firstly isolated and characterized from yellow catfish Pelteobagrus fulvidraco. The validated cDNAs encoding for hsl1 and hsl2 were 2739 and 2629bp in length, encoding peptides of 679 and 813 amino acid residues, respectively, and shared 57.7% amino acid identity. The phylogenetic analysis revealed that hsl1 and hsl2 derived from paralogous genes that might have arisen during a teleost-specific genome duplication event. Both hsl mRNAs were expressed in a wide range of tissues, but the abundance of each hsl mRNA showed the tissue- and developmental stage-dependent expression patterns. Intraperitoneal injection in vivo and incubation in vitro of recombinant human leptin (rb-hLEP) stimulated the mRNA expression of hsl2, but not hsl1, in the liver and hepatocytes of P. fulvidraco, respectively, suggesting that two hsl isoforms might serve different roles in lipid metabolism. To our knowledge, for the first time, the present study provides evidence that two hsl mRNAs are differentially expressed with and among tissues during different developmental stages and also differentially regulated by leptin both in vivo and in vitro, which serves to increase our understanding on hsl physiological function in fish.

Contribution of glucose- and fatty acid sensing systems to the regulation of food intake in fish. A review

Food intake in fish is a complex process regulated through many different factors including abundance of energy and nutrients. In recent years, evidence have been obtained in several fishes, mainly in rainbow trout, regarding the presence and functioning in brain areas of metabolic sensors informing about changes in the levels of nutrients like glucose and fatty acids. The activity of these sensors relate to the control of food intake through changes in the expression of anorexigenic and orexigenic neuropeptides. The present review will provide a picture of the main results obtained to date in these studies, as well as perspectives for future research in the field.

Leptin signaling in the rainbow trout central nervous system is modulated by a truncated leptin receptor isoform

Central leptin (Lep) signaling is important in control of appetite and energy balance in mammals, but information on Lep signaling and physiological roles in early vertebrates is still lacking. To elucidate fish Lep signaling activation and modulation, a long-form Lep receptor (LepRL) and a truncated LepR (LepRT) are functionally characterized from rainbow trout. The receptors generated in alternatively splicing events have identical extracellular and transmembrane domains but differ in the intracellular sequence, both in length and identity. Gene transfection experiments show that LepRL is expressed as a 125-kDa protein in rainbow trout hepatoma cell line RTH-149, whereas LepRT is 100 kDa; both receptors specifically bind Lep. Homogenous Lep induces tyrosine phosphorylation of Janus kinase 2 and signal transducer and activation of transcription 3 in LepRL-expressing RTH-149 cells. This response is diminished in cells coexpressing LepRL and LepRT, suggesting that the LepRT which lacks these kinase-associated motifs competes with the LepRL for Lep availability, thus attenuating the Lep signal. Both receptor genes are highly expressed in the central nervous system. The mRNA levels of LepRT in hypothalamus, but not LepRL, change postprandially, with decreased transcription at 2 hours postfeeding and then elevated at 8 hours, concomitant with changes in proopiomelanocortin-A1 transcription. However, both receptors have no change in mRNA levels during 3 weeks of fasting. These data indicate that LepRT transcription is more likely a mechanism for modulating Lep effects on short-term feed intake than in regulating energy balance in the long term. In vitro and physiological characterization of LepR isoforms indicates divergent Lep signaling modulation patterns among vertebrates with different life histories and metabolic profiles.

CART in the brain of vertebrates: circuits, functions and evolution

Cocaine- and amphetamine-regulated transcript peptide (CART) with its wide distribution in the brain of mammals has been the focus of considerable research in recent years. Last two decades have witnessed a steady rise in the information on the genes that encode this neuropeptide and regulation of its transcription and translation. CART is highly enriched in the hypothalamic nuclei and its relevance to energy homeostasis and neuroendocrine control has been understood in great details. However, the occurrence of this peptide in a range of diverse circuitries for sensory, motor, vegetative, limbic and higher cortical areas has been confounding. Evidence that CART peptide may have role in addiction, pain, reward, learning and memory, cognition, sleep, reproduction and development, modulation of behavior and regulation of autonomic nervous system are accumulating, but an integration has been missing. A steady stream of papers has been pointing at the therapeutic potentials of CART. The current review is an attempt at piecing together the fragments of available information, and seeks meaning out of the CART elements in their anatomical niche. We try to put together the CART containing neuronal circuitries that have been conclusively demonstrated as well as those which have been proposed, but need confirmation. With a view to finding out the evolutionary antecedents, we visit the CART systems in sub-mammalian vertebrates and seek the answer why the system is shaped the way it is. We enquire into the conservation of the CART system and appreciate its functional diversity across the phyla.

Central pathways integrating metabolism and reproduction in teleosts

Energy balance plays an important role in the control of reproduction. However, the cellular and molecular mechanisms connecting the two systems are not well understood especially in teleosts. The hypothalamus plays a crucial role in the regulation of both energy balance and reproduction, and contains a number of neuropeptides, including gonadotropin-releasing hormone (GnRH), orexin, neuropeptide-Y, ghrelin, pituitary adenylate cyclase-activating polypeptide, α-melanocyte stimulating hormone, melanin-concentrating hormone, cholecystokinin, 26RFamide, nesfatin, kisspeptin, and gonadotropin-inhibitory hormone. These neuropeptides are involved in the control of energy balance and reproduction either directly or indirectly. On the other hand, synthesis and release of these hypothalamic neuropeptides are regulated by metabolic signals from the gut and the adipose tissue. Furthermore, neurons producing these neuropeptides interact with each other, providing neuronal basis of the link between energy balance and reproduction. This review summarizes the advances made in our understanding of the physiological roles of the hypothalamic neuropeptides in energy balance and reproduction in teleosts, and discusses how they interact with GnRH, kisspeptin, and pituitary gonadotropins to control reproduction in teleosts.

Genes involved in fatty acid metabolism: molecular characterization and hypothalamic mRNA response to energy status and neuropeptide Y treatment in the orange-spotted grouper Epinephelus coioides

As in mammals, fatty acid (FA) metabolism plays diverse and vital roles in regulating food intake in fish. Multiple lines of evidence suggest that the effect of FA metabolism on food intake is linked to changes in the level of neuropeptide Y (NPY) in the hypothalamus of the rainbow trout. In mammals, the evidence suggests that FA metabolism regulates feeding via hypothalamic NPY. NPY is therefore considered an important factor that mediates the modulation of food intake by FA metabolism in vertebrates. The stimulatory effect of NPY on food intake is well known. However, to the best of our knowledge, the effect of NPY on FA metabolism in the hypothalamus has not been examined. In this study, we cloned the cDNA of four key enzymes involved in FA metabolism and assessed the effect of energy status and NPY on their mRNA expression in the hypothalamus of grouper. The full-length cDNAs of UCP2 and CPT1a and the partial coding sequence (CDS) of ACC1 and FAS were isolated from the grouper hypothalamus. These genes are expressed in the hypothalamus and during the organogenetic stage of embryogenesis. A feeding rhythm study showed that the hypothalamic expression level of NPY and CPT1a was highly correlated with feeding rhythm. Long-term fasting was found to significantly induce the hypothalamic mRNA expression of NPY, CPT1a and UCP2. An in vitro study demonstrated that NPY strongly stimulated CPT1a and UCP2 mRNA expression in a time- and dose-dependent manner. Collectively, these results suggest that these four genes related to FA metabolism may play a role in regulating food intake in grouper and, that NPY modulates FA metabolism in the grouper hypothalamus. This study showed, for the first time in vertebrates, the effect of NPY on the gene expression of FA metabolism-related enzymes.

Molecular cloning, characterization and expression profiles of multiple leptin genes and a leptin receptor gene in orange-spotted grouper (Epinephelus coioides)

Leptin plays key roles in body weight regulation, energy metabolism, food intake, reproduction and immunity in mammals. However, its function in teleosts is still unclear. In the present study, two leptin genes (gLepA and gLepB) and one leptin receptor gene (gLepR) were cloned and characterized in orange-spotted grouper (Epinephelus coioides). The cDNAs of gLepA and gLepB were 671 bp and 684 bp in length, encoding for proteins of 161 amino acid (aa) and 158 aa, respectively. The three-dimensional (3D) structures modeling of gLepA and gLepB showed strong conservation of tertiary structure with that of other vertebrates. The total length of gLepR cDNA was 4242 bp, encoding a protein of 1169 aa which contained all functionally important domains conserved among vertebrate LEPR. Tissue distribution analysis showed that gLepA was highly expressed in cerebellum, liver and ovary, while gLepB mRNA abundantly in the brain regions, as well as in the ovary with some extend. The gLepR was mainly expressed in kidney, head kidney and most of brain regions. Analysis of expression profiles of gLep and gLepR genes during the embryonic stages showed that high expression of gLepR was observed in the brain vesicle stage, while neither gLepA nor gLepB mRNA was detected during different embryonic stages. Finally, fasting and refeeding experiments were carried out to investigate the possible function of leptin genes in food intake and energy metabolism, and the results showed that a significant increase of gLepA expression in the liver was induced by food deprivation in both short-term (7 days) and long-term (3 weeks) fasting and gLepA mRNA upregulation was eliminated after refeeding, while gLepB wasn't detected in the liver of grouper during fasting. No significant differences in hypothalamic leptin and leptin receptor expression were found during short-term fasting and refeeding. Hepatic expression of gLepA mRNA increased significantly 9h after a single meal. These results suggested gLepA, other than gLepB, functioned in the regulation of energy metabolism and food intake in this Perciform fish.

Melatonin treatment alters glucosensing capacity and mRNA expression levels of peptides related to food intake control in rainbow trout hypothalamus

As demonstrated in previous studies, the functioning of brain glucosensing systems in rainbow trout is altered under stress conditions in a way that they are unable to respond properly to changes in glucose levels. Melatonin has been postulated as necessary for homeostatic control of energy metabolism in several vertebrate groups, and in fish it has been suggested as an anti-stress molecule. To evaluate the possible effects of melatonin on glucosensing, we have incubated hypothalamus and hindbrains of rainbow trout at different glucose concentrations in the presence of increased doses (0.01, 1, and 100nM) of melatonin assessing whether or not the responses to changes in glucose levels of parameters related to glucosensing (glucose, glycogen and glucose 6-phosphate levels, activities of GK, GSase and PK, and mRNA content of GK, GLUT2, Kir6.x-like, and SUR-like) are modified in the presence of melatonin. While no effects of melatonin were observed in hindbrain, in hypothalamus melatonin treatment up-regulated glucosensing parameters, especially under hypo- and normo-glycaemic conditions. The effects of melatonin in hypothalamus occurred apparently through MT(1) receptors since most effects were counteracted by the presence of luzindole but not by the presence of 4-P-PDOT. Moreover, melatonin treatment induced in hypothalamus increased mRNA expression levels of NPY and decreased mRNA levels of POMC, CART, and CRF. A role of the hormone in daily re-adjustment of hypothalamic glucosensor machinery is discussed.

Plasma leptin and growth hormone levels in the fine flounder (Paralichthys adspersus) increase gradually during fasting and decline rapidly after refeeding

In fish, recent studies have indicated an anorexigenic role of leptin and thus its possible involvement in regulation of energy balance and growth. In the present study, the effects of fasting and refeeding periods on plasma leptin levels were studied in the fine flounder, a flatfish with remarkably slow growth. To further assess the endocrine status of the fish during periods of catabolism and anabolism, plasma growth hormone (GH) levels were also analyzed. Under normal feeding condition, plasma leptin and GH levels remained stable and relatively high in comparison with other teleost species. For the three separate groups of fish, fasted for 2, 3, and 4 weeks, respectively, plasma leptin levels increase gradually, becoming significantly elevated after 3 weeks, and reaching highest levels after 4-week fasting. Plasma GH levels were significantly elevated after 2-week fasting. At the onset of refeeding, following a single meal, leptin levels decline rapidly to lower than initial levels within 2 h, irrespective of the length of fasting. Plasma GH also decline, the decrease being significant after 4, 24 and 2 h for the 2, 3 and 4-week fasted groups, respectively. This study shows that plasma leptin levels in the fine flounder are strongly linked to nutritional status and suggests that leptin secretion is regulated by fast-acting mechanisms. Elevated leptin levels in fasted fish may contribute to a passive survival strategy of species which experience natural food shortage periods by lowering appetite and limiting physical foraging activity.

Glucose metabolism in fish: a review

Teleost fishes represent a highly diverse group consisting of more than 20,000 species living across all aquatic environments. This group has significant economical, societal and environmental impacts, yet research efforts have concentrated primarily on salmonid and cyprinid species. This review examines carbohydrate/glucose metabolism and its regulation in these model species including the role of hormones and diet. Over the past decade, molecular tools have been used to address some of the downstream components of these processes and these are incorporated to better understand the roles played by carbohydrates and their regulatory paths. Glucose metabolism remains a contentious area as many fish species are traditionally considered glucose intolerant and, therefore, one might expect that the use and storage of glucose would be considered of minor importance. However, the actual picture is not so clear since the apparent intolerance of fish to carbohydrates is not evident in herbivorous and omnivorous species and even in carnivorous species, glucose is important for specific tissues and/or for specific activities. Thus, our aim is to up-date carbohydrate metabolism in fish, placing it to the context of these new experimental tools and its relationship to dietary intake. Finally, we suggest that new research directions ultimately will lead to a better understanding of these processes.

Macronutrient-induced differences in food intake relate with hepatic oxidative metabolism and hypothalamic regulatory neuropeptides in rainbow trout (Oncorhynchus mykiss)

This study examines how dietary macronutrient-induced changes in voluntary food intake (FI) relate to changes in markers of hepatic oxidative metabolism and in the expression of FI regulatory neuropeptides in a teleost model, the rainbow trout. Rainbow trout were fed for 6weeks with one of four iso-energetic diets (2×2 factorial design), containing either a high (HP, ~500 g·kg(-1) DM) or a low (LP, ~250 g·kg(-1) DM) protein level (PL) with, at each PL, fat (diets HP-F and LP-F) being substituted by an iso-energetic amount of gelatinized corn starch (diets HP-St and LP-St) as non-protein energy source (ES). Irrespective of the dietary PL, FI (g·kg(-0.8)·d(-1)) and digestible energy intake (DEI, kJ·kg(-0.8)·d(-1)) were significantly (P<0.05) reduced by the iso-energetic replacement of fat by starch as non-protein ES. Interestingly, trout fed these St-diets had higher gene expression of markers of hepatic oxidative phosphorylation (OxPhos), i.e., ubiquinol-cytochrome c reductase subunit 2 (UCR2) and cytochrome oxidase subunit 4 (COX4) and of aerobic oxidative capacity (CS, citrate synthase), which paralleled glucokinase (GK) transcription. This positive relation suggests that glucose phosphorylation and markers of mitochondrial OxPhos are linked at the hepatic level and possibly triggered the observed reduction in FI. Moreover, trout displaying the reduced FI had higher cocaine amphetamine regulator transcript (CART) mRNA in hypothalamus, whereas neuropeptide Y (NPY) mRNA did not follow the macronutrient-induced changes in FI. Further studies are needed to unravel the mechanisms by which diet-induced changes in hepatic metabolism inform central feeding centers involved in the regulation of FI in fish.

CRF treatment induces a readjustment in glucosensing capacity in the hypothalamus and hindbrain of rainbow trout

Stress conditions induced in rainbow trout a readjustment in the glucosensing response of the hypothalamus and hindbrain such that those sensors did not respond properly to changes in glucose levels, as demonstrated in previous studies. To evaluate the hypothesis that corticotropin-releasing factor (CRF) could be involved in that response, we have incubated the hypothalamus and hindbrain of rainbow trout at different glucose concentrations in the presence of different concentrations of CRF. Under those conditions, we evaluated whether parameters related to glucosensing [the levels of glucose, glycogen and glucose 6-phosphate, the activities of glucokinase (GK), glycogen synthase (GSase) and pyruvate kinase (PK), and mRNA abundance of transcripts for GK, Glut2, Kir.6-like and sulfonylurea receptor (SUR)-like] are modified in the presence of CRF in a way comparable to that observed under stress conditions. We obtained evidence allowing us to suggest that CRF could be involved in the interaction between stress and glucosensing as CRF treatment of the hypothalamus and hindbrain in vitro induced a readjustment in glucosensing parameters similar to that previously observed under stress conditions in vivo. We had also previously demonstrated that stress elicits alterations in food intake in parallel with the readjustment of glucosensing systems. Here, we provide evidence that the mRNA abundance of several of the neuropeptides involved in the regulation of food intake, such as neuropeptide Y (NPY) or cocaine and amphetamine-regulated transcript (CART), is affected by CRF treatment, in such a way that their expression does not respond to changes in glucose levels in the same way as controls, allowing us to suggest that the food intake response that is integrated by changes in those peptides and known to be reduced by stress could be also mediated by CRF action in glucosensing areas.

Glucosensing and glucose homeostasis: from fish to mammals

This review is focused on two topics related to glucose in vertebrates. In a first section devoted to glucose homeostasis we describe how glucose levels fluctuate and are regulated in different classes of vertebrates. The detection of these fluctuations is essential for homeostasis and for other physiological processes such as regulation of food intake. The capacity of that detection is known as glucosensing, and the different mechanisms through which it occurs are known as glucosensors. Different glucosensor mechanisms have been demonstrated in different tissues and organs of rodents and humans whereas the information obtained for other vertebrates is scarce. In the second section of the review we describe the present knowledge regarding glucosensor mechanisms in different groups of vertebrates, with special emphasis in fish.

Time-dependent effects of leptin on food intake and locomotor activity in goldfish

The present study investigates the possible circadian dependence of leptin effects on food intake, locomotor activity, glycemia and plasma cortisol levels in goldfish (Carassius auratus). Fish were maintained under 12L:12D photoperiod and subjected to two different feeding schedules, one group fed during photophase (10:00) and the other one during scotophase (22:00). Leptin or saline were intraperitoneally injected at two different times (10:00 or 22:00), coincident or not with the meal time. To eliminate the entraining effect of the light/dark cycle, goldfish maintained under 24h light (LL) were fed and leptin-injected at 10:00. A reduction in food intake and locomotor activity and an increase in glycemia were found in goldfish fed and leptin-injected at 10:00. No significant changes in circulating cortisol were observed. Those effects were not observed when leptin was administered during the scotophase, regardless the feeding schedule; neither in fish maintained under LL, suggesting that a day/night cycle would be necessary to observe the actions of leptin administered during the photophase. Changes in locomotor activity and glycemia were only observed in goldfish when leptin was injected at daytime, coincident with the feeding schedule, suggesting that these leptin actions could be dependent on the feeding time as zeitgeber. In view of these results it appears that the circadian dependence of leptin actions in goldfish can be determined by the combination of both zeitgebers, light/dark cycle and food. Our results point out the relevance of the administration time when investigating regulatory functions of hormones.