Insulin Controls Clock Gene Expression in the Liver of Goldfish Probably via Pi3k/Akt Pathway

The liver circadian clock plays a pivotal role in driving metabolic rhythms, being primarily entrained by the feeding schedule, although the underlying mechanisms remain elusive. This study aimed to investigate the potential role of insulin as an intake signal mediating liver entrainment in fish. To achieve this, the expression of clock genes, which form the molecular basis of endogenous oscillators, was analyzed in goldfish liver explants treated with insulin. The presence of insulin directly increased the abundance of per1a and per2 transcripts in the liver. The dependency of protein translation for such insulin effects was evaluated using cycloheximide, which revealed that intermediate protein translation is seemingly unnecessary for the observed insulin actions. Furthermore, the putative interaction between insulin and glucocorticoid signaling in the liver was examined, with the results suggesting that both hormones exert their effects by independent mechanisms. Finally, to investigate the specific pathways involved in the insulin effects, inhibitors targeting PI3K/AKT and MEK/ERK were employed. Notably, inhibition of PI3K/AKT pathway prevented the induction of per genes by insulin, supporting its involvement in this process. Together, these findings suggest a role of insulin in fish as a key element of the multifactorial system that entrains the liver clock to the feeding schedule.

Sex dimorphism of glucosensing parameters and appetite-regulating peptides in the hypothalamus of rainbow trout broodstocks

Rainbow trout (Oncorhynchus mykiss) is traditionally considered as a poor user of digestible carbohydrates harbouring persistent postprandial hyperglycaemia and decreased growth performances when fed a diet containing more than 20% of digestible carbohydrates. While this glucose-intolerant phenotype is well-described in juveniles, evidence points to a particular regulation of glucose metabolism in rainbow trout broodstrocks. By detecting changes in glucose levels and triggering a specific metabolic response, the hypothalamus plays a key role in the regulation of peripheral glucose metabolism. Therefore, our objective was to assess, for the first time in fish, the short-term consequences in hypothalamus, the glucose sensing and feed intake regulation of feeding mature female and male, and neomale rainbow trout with a diet containing either no or a 33% carbohydrate. The hypothalamic glucosensing capacity was assessed through mRNA levels of glucosensing related-genes and feed intake regulation through appetite-regulating peptides. Our data indicate that a brief period of carbohydrate intake (5 meals at 8 °C) did not induce specific changes in glucosensing capacity and appetite-regulating peptides in the hypothalamus of rainbow trout broodstock. Our results did however demonstrate, for the first time in fish, the existence of sex dimorphism of glucosensing-related genes and appetite-regulating peptides.

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.

Central serotonin participates in the anorexigenic effect of GLP-1 in rainbow trout (Oncorhynchus mykiss)

The incretin, glucagon-like peptide-1 (GLP-1) is a major player in the gut-brain axis regulation of energy balance and in fish it seems to exert a negative influence on food intake. In this study, we investigated the role of the brain serotonergic system in the effects promoted by a peripheral GLP-1 injection on food intake in rainbow trout (Oncorhynchus mykiss). For this, in a first experiment the incretin was intraperitoneally injected (100 ng/g body weight) alone or in combination with a 5HT_2C receptor antagonist (SB 242084, 1 µg/g body weight) and food intake was measured 30, 90, and 180 min later. In a second experiment, we studied the effect of these treatments on mRNA abundance of hypothalamic neuropeptides that control food intake. In addition, the effect of GLP-1 on serotonin metabolism was assessed in hindbrain and hypothalamus. Our results show that GLP-1 induced a significant food intake inhibition, which agreed with the increased expression of anorexigenic neuropeptides pomc and cart in the hypothalamus. Furthermore, GLP-1 stimulated the synthesis of serotonin in the hypothalamus, which might be indicative of a higher use of the neurotransmitter. The effects of GLP-1 on food intake were partially reversed when a serotonin receptor antagonist, SB 242084, was previously administered to trout. This antagonist also reversed the stimulatory effect of the hormone in hypothalamic pomca1 mRNA abundance. We conclude that hypothalamic serotonergic pathways are essential for mediating the effects of GLP-1 on food intake in rainbow trout. In addition, the 5HT2C receptor subtype seems to have a prominent role in the inhibition of food intake induced by GLP-1 in this species.

The gut–brain axis in vertebrates: implications for food intake regulation

The gut and brain are constantly communicating and influencing each other through neural, endocrine and immune signals in an interaction referred to as the gut–brain axis. Within this communication system, the gastrointestinal tract, including the gut microbiota, sends information on energy status to the brain, which, after integrating these and other inputs, transmits feedback to the gastrointestinal tract. This allows the regulation of food intake and other physiological processes occurring in the gastrointestinal tract, including motility, secretion, digestion and absorption. Although extensive literature is available on the mechanisms governing the communication between the gut and the brain in mammals, studies on this axis in other vertebrates are scarce and often limited to a single species, which may not be representative for obtaining conclusions for an entire group. This Review aims to compile the available information on the gut–brain axis in birds, reptiles, amphibians and fish, with a special focus on its involvement in food intake regulation and, to a lesser extent, in digestive processes. Additionally, we will identify gaps of knowledge that need to be filled in order to better understand the functioning and physiological significance of such an axis in non-mammalian vertebrates.

Central regulation of food intake in fish: an evolutionary perspective

Evidence indicates that central regulation of food intake is well conserved along the vertebrate lineage, at least between teleost fish and mammals. However, several differences arise in the comparison between both groups. In this review, we describe similarities and differences between teleost fish and mammals on an evolutionary perspective. We focussed on the existing knowledge of specific fish features conditioning food intake, anatomical homologies and analogies between both groups as well as the main signalling pathways of neuroendocrine and metabolic nature involved in the homeostatic and hedonic central regulation of food intake.

Glucosensing capacity of rainbow trout telencephalon

To assess the hypothesis of glucosensing systems present in fish telencephalon, we first demonstrated in rainbow trout, by in situ hybridisation, the presence of glucokinase (GK). Then, we assessed the response of glucosensing markers in rainbow trout telencephalon 6 hours after i.c.v. treatment with glucose or 2-deoxyglucose (inducing glucoprivation). We evaluated the response of parameters related to the mechanisms dependent on GK, liver X receptor (LXR), mitochondrial activity, sweet taste receptor and sodium-glucose linked transporter 1 (SGLT-1). We also assessed mRNA abundance of neuropeptides involved in the metabolic control of food intake (agouti-related protein, neuropeptide Y, pro-opiomelanocortin, and cocaine- and amphetamine-related transcript), as well as the abundance and phosphorylation status of proteins possibly involved in linking glucosensing with neuropeptide expression, such as protein kinase B (AkT), AMP-activated protein kinase (AMPK), mechanistic target of rapamycin and cAMP response element-binding protein (CREB). The responses obtained support the presence in the telencephalon of a glucosensing mechanism based on GK and maybe one based on LXR, although they do not support the presence of mechanisms dependent on mitochondrial activity and SGLT-1. The mechanism based on sweet taste receptor responded to glucose but in a converse way to that characterised previously in the hypothalamus. In general, systems responded only to glucose but not to glucoprivation. Neuropeptides did not respond to glucose or glucoprivation. By contrast, the presence of glucose activates Akt and inhibits AMPK, CREB and forkhead box01. This is the first study in any vertebrate species in which the response to glucose of putative glucosensing mechanisms is demonstrated in the telencephalon. Their role might relate to processes other than homeostatic control of food intake, such as the hedonic and reward system.

Differential effects of exposure to parasites and bacteria on stress response in turbot Scophthalmus maximus simultaneously stressed by low water depth

The stress response of turbot Scophthalmus maximus was evaluated in fish maintained 8 days under different water depths, normal (NWD, 30 cm depth, total water volume 40 l) or low (LWD, 5 cm depth, total water volume 10 l), in the additional presence of infection-infestation of two pathogens of this species. This was caused by intraperitoneal injection of sublethal doses of the bacterium Aeromonas salmonicida subsp. salmonicida or the parasite Philasterides dicentrarchi (Ciliophora:Scuticociliatida). The LWD conditions were stressful for fish, causing increased levels of cortisol in plasma, decreased levels of glycogen in liver and nicotinamide adenine dinucleotide phosphate (NADP) and increased activities of G6Pase and GSase. The presence of bacteria or parasites in fish under NWD resulted in increased cortisol levels in plasma whereas in liver, changes were of minor importance including decreased levels of lactate and GSase activity. The simultaneous presence of bacteria and parasites in fish under NWD resulted a sharp increase in the levels of cortisol in plasma and decreased levels of glucose. Decreased levels of glycogen and lactate and activities of GSase and glutathione reductase (GR), as well as increased activities of glucose-6-phosphate dehydrogenase (G6PDH), 6-phosphogluconate dehydrogenase (6PGDH) and levels of nicotinamide adenine dinucleotide phosphate (NADPH) occurred in the same fish in liver. Finally, the presence of pathogens in S. maximus under stressful conditions elicited by LWD resulted in synergistic actions of both type of stressors in cortisol levels. In liver, the presence of bacteria or parasites induced a synergistic action on several variables such as decreased activities of G6Pase and GSase as well as increased levels of NADP and NADPH and increased activities of GPase, G6PDH and 6PGDH.

Evolutionary history of glucose-6-phosphatase encoding genes in vertebrate lineages: towards a better understanding of the functions of multiple duplicates

Background

Glucose-6-phosphate (G6pc) is a key enzyme involved in the regulation of the glucose homeostasis. The present study aims at revisiting and clarifying the evolutionary history of g6pc genes in vertebrates.

Results

g6pc duplications happened by successive rounds of whole genome duplication that occurred during vertebrate evolution. g6pc duplicated before or around Osteichthyes/Chondrichthyes radiation, giving rise to g6pca and g6pcb as a consequence of the second vertebrate whole genome duplication. g6pca was lost after this duplication in Sarcopterygii whereas both g6pca and g6pcb then duplicated as a consequence of the teleost-specific whole genome duplication. One g6pca duplicate was lost after this duplication in teleosts. Similarly one g6pcb2 duplicate was lost at least in the ancestor of percomorpha. The analysis of the evolution of spatial expression patterns of g6pc genes in vertebrates showed that all g6pc were mainly expressed in intestine and liver whereas teleost-specific g6pcb2 genes were mainly and surprisingly expressed in brain and heart. g6pcb2b, one gene previously hypothesised to be involved in the glucose intolerant phenotype in trout, was unexpectedly up-regulated (as it was in liver) by carbohydrates in trout telencephalon without showing significant changes in other brain regions. This up-regulation is in striking contrast with expected glucosensing mechanisms suggesting that its positive response to glucose relates to specific unknown processes in this brain area.

Conclusions

Our results suggested that the fixation and the divergence of g6pc duplicated genes during vertebrates’ evolution may lead to adaptive novelty and probably to the emergence of novel phenotypes related to glucose homeostasis.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-017-3727-1) contains supplementary material, which is available to authorized users.

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.

Gradation of the stress response in rainbow trout exposed to stressors of different severity: the role of brain serotonergic and dopaminergic systems

After an intense acute stressor, fish develop a metabolic and behavioural response that usually lasts for several hours. Brain monoaminergic systems, particularly the serotonergic system, appear to play a key role in the central regulation of the stress response. However, the influence of stressor severity on brain monoaminergic systems and on the induced stress responses is yet poorly understood. We hypothesise that serotonergic system could have a direct role in the integration of sensory information during stressor exposure and in the organisation of the subsequent integrated stress response. According to our hypothesis, a low stressor intensity would induce a low response of brain serotonergic system and therefore stress responses of low magnitude and duration. To test this hypothesis, we exposed fish to handling disturbance for 5 s, 15 s or 3 min. We sampled fish at 0 (controls), 3, 15, 45 and 240 min after the start of the stress protocol. Brain levels of serotonin, dopamine and their respective main oxidative metabolites were quantified, along with plasma levels of stress markers (catecholamines, cortisol, glucose and lactate). Regarding stress markers, the 5-s and 15-s stress protocols induced similar and relatively low elevations in all parameters assessed. As expected, the 3-min protocol induced responses of a higher intensity and duration in all plasma parameters. Interestingly, the alterations of brain monoaminergic systems did not follow the same trend. The three stress protocols induced increases in the serotonergic activity in all brain regions analysed (hypothalamus, telencephalon and medulla oblongata), independently of the duration of the handling disturbance, whereas the effects on the dopaminergic system were minor and brain region-dependent. These data suggest that the brain serotonergic system, although likely involved in the recognition of the stressor stimuli, is not the only actor determining the magnitude and duration of the acute stress response in trout.

Arginine vasotocin treatment induces a stress response and exerts a potent anorexigenic effect in rainbow trout, Oncorhynchus mykiss

The peptide arginine vasotocin (AVT), homologous to mammalian arginine vasopressin, is involved in many aspects of fish physiology, such as osmoregulation, regulation of biological rhythms, reproduction, metabolism or responses to stress, and the modulation of social behaviours. Because a decrease in appetite is a general response to stress in fish and other vertebrates, we investigated the role of AVT as a possible food intake regulator in fish. We used i.c.v. injections for central administration of AVT to rainbow trout (Oncorhynchus mykiss). In a first experiment, we evaluated the temporal response of food intake after AVT treatment. In a second experiment, we investigated the effects of central AVT administration on the response of typical stress markers (plasma cortisol, glucose and lactate), as well as brain serotonergic, noradrenergic and dopaminergic activity. In addition, the mRNA levels of genes involved in food intake regulation [neuropetide Y, pro-opiomelanocortin (POMC), cocaine- and amphetamine-regulated transcript (CART) and corticotrophin-releasing factor (CRF)] and in CRF- (CRF-binding protein) and AVT-signalling (pro-VT and AVT receptor), were also assessed after AVT treatment. Our results showed that AVT is a potent anorexigenic factor in fish. Increases of plasma cortisol and glucose after AVT treatment strongly suggest that AVT administration induced a stress response and that AVT action was mediated by hypothalamic-pituitary-interrenal axis activation, which was also supported by the increase of the serotonergic activity in trout telencephalon and hypothalamus. The increased hypothalamic levels of POMC and CART suggest that these peptides might have a role in the anorexigenic action of AVT, whereas the involvement of CRF signalling is unclear.

Oral administration of melatonin counteracts several of the effects of chronic stress in rainbow trout

To assess a possible antistress role of melatonin in fish, we orally administered melatonin to rainbow trout for 10 d and then kept the fish under normal or high stocking density conditions during the last 4 d. Food intake; biochemical parameters in plasma (cortisol, glucose, and lactate concentrations); liver (glucose and glycogen concentrations, and glycogen synthase activity); enzyme activities of amylase, lipase, and protease in foregut and midgut; and content of the hypothalamic neurotransmitters dopamine and serotonin, as well as their oxidized metabolites, 3,4-dihydroxyphenylacetic acid and 5-hydroxy-3-indoleacetic acid, were evaluated under those conditions. High stocking density conditions alone induced changes indicative of stress conditions in plasma cortisol concentrations, liver glycogenolytic potential, the activities of some digestive enzymes, and the 3,4-dihydroxyphenylacetic acid-to-dopamine and 5-hydroxy-3-indoleacetic acid-to-serotonin ratios in the hypothalamus. Melatonin treatment in nonstressed fish induced an increase in liver glycogenolytic potential, increased the activity of some digestive enzymes, and enhanced serotoninergic and dopaminergic metabolism in hypothalamus. The presence of melatonin in stressed fish resulted in a significant interaction with cortisol concentrations in plasma, glycogen content, and glycogen synthase activity in liver and dopaminergic and serotoninergic metabolism in the hypothalamus. In general, the presence of melatonin mitigated several of the effects induced by stress, supporting an antistress role for melatonin in rainbow trout.

Ghrelin effects on central glucosensing and energy homeostasis-related peptides in rainbow trout

Although the role of ghrelin (GHRL) on fish appetite regulation had been widely studied in past years, its involvement in the regulation of glucose metabolism had been little explored. In the present study we hypothesize that GHRL may have a role in the regulation of glucose homeostasis in fish. Therefore, we carried out different experimental approaches in rainbow trout to assess brain glucosensing potential and glucose metabolism in response to GHRL treatment. We found that after either systemic or central GHRL administration to trout deprived of food, glycemia remained unaffected, whereas (in clear contrast with the mammalian model) a consistent activation of the main glucosensing markers (glucose transporter 2, glucokinase, and ATP-sensitive inward rectified K+ channel) was noticed in both hypothalamus and hindbrain. Some of these results were further confirmed by in vitro incubations of hypothalamus and hindbrain in the presence of GHRL. Despite the lack of changes in glycemia, we suggest that the changes elicited by GHRL on the glucosensing system are direct and could be related to a helper action of this hormone when glucose arrived in the postprandial phase. Moreover, we also studied the effect of GHRL treatment on the expression of several food intake-related neuropeptides, such as neuropeptide Y, cocaine- and amphetamine-regulated transcript, pro-opiomelanocortin, and corticotropin-releasing factor. We observed an important variability in the effects of GHRL attributable either to the route of GHRL administration or to the brain regions assessed, which could help explain the contradictory results described in fish literature about GHRL role in food intake control.

Evidence for a gut-brain axis used by glucagon-like peptide-1 to elicit hyperglycaemia in fish

In mammals, glucagon-like peptide-1 (GLP-1) produces changes in glucose and energy homeostasis through a gut-pancreas-brain axis. In fish, the effects of GLP-1 are opposed to those described in other vertebrates, such as stimulation of hyperglycaemia and the lack of an effect of incretin. In the present study conducted in a teleost fish such as the rainbow trout, we present evidence of a gut-brain axis used by GLP-1 to exert its actions on glucose and energy homeostasis. We have assessed the effects of GLP-1 on glucose metabolism in the liver as well as the glucose-sensing potential in the hypothalamus and hindbrain. We confirm that peripheral GLP-1 administration elicits sustained hyperglycaemia, whereas, for the first time in a vertebrate species, we report that central GLP-1 treatment increases plasma glucose levels. We have observed (using capsaicin) that at least part of the action of GLP-1 on glucose homeostasis was mediated by vagal and splanchnic afferents. GLP-1 has a direct effect in parameters involved in glucose sensing in the hindbrain, whereas, in the hypothalamus, changes occurred indirectly through hyperglycaemia. Moreover, in the hindbrain, GLP-1 altered the expression of peptides involved in the control of food intake. We have elaborated a model for the actions of GLP-1 in fish in which this peptide uses a mammalian-like ancestral gut-brain axis to elicit the regulation of glucose homeostasis in different manner than the model described in mammals. Finally, it is worth noting that the hyperglycaemia induced by this peptide and the lack of incretin function could be related to the glucose intolerance observed in carnivorous teleost fish species such as the rainbow trout.

Osmoregulatory action of 17β-estradiol in the gilthead sea breamSparus auratus

The osmoregulatory action of 17beta-estradiol (E2) was examined in the euryhaline teleost Sparus auratas. In a first set of experiments, fish were injected once with vegetable oil containing E2 (1, 2 and 5 microg/g body weight), transferred 12h after injection from sea water (SW, 38 ppt salinity) to hypersaline water (HSW, 55 ppt) or to brackish water (BW, 5 ppt salinity) and sampled 12h later (i.e. 24 h post-injection). In a second experiment, fish were injected intraperitoneally with coconut oil alone or containing E2 (10 microg/g body weight) and sampled after 5 days. In the same experiment, after 5 days of treatment, fish of each group were transferred to HSW, BW and SW and sampled 4 days later (9 days post-implant). Gill Na+,K+ -ATPase activity, plasma E2 levels, plasma osmolality, and plasma levels of ions (sodium and calcium), glucose, lactate, protein, triglyceride, and hepatosomatic index were examined. Transfer from SW to HSW produced no significant effects on any parameters assessed. E2 treatment did not affect any parameter. Transfer from SW to BW resulted in a significant decrease in plasma osmolality and plasma sodium but did not affect gill Na+,K+ -ATPase activity. A single dose of E2 attenuated the decrease in these parameters after transfer from SW to BW, but was without effect on gill Na+,K+ -ATPase activity. An implant of E2 (10 microg/g body weight) for 5 days significantly increased plasma calcium, hepatosomatic index, plasma metabolic parameters, and gill Na+,K+ -ATPase activity. In coconut oil-implanted (sham) fish, transfer from SW to HSW or BW during 4 days significantly elevated gill Na+,K+ -ATPase. Gill Na+,K+ -ATPase activity remained unaltered after transfer of E2-treated fish to HSW or BW. However, in E2-treated fish transferred from SW to SW (9 days in SW after E2-implant), gill Na+,K+ -ATPase activity decreased with respect to HSW- or BW-transferred fish. Shams transferred to HSW showed increased levels of lactate, protein, and trygliceride in plasma, while those transferred to BW only displayed increased trygliceride levels. E2-treated fish transferred to HSW showed higher protein levels without any change in other plasmatic parameters, while those transferred to BW displayed elevated plasma glucose levels but decreased osmolality and protein levels. These results substantiate a chronic stimulatory action of E2 on gill Na+,K+ -ATPase activity in the euryhaline teleost Sparus auratas.

Brain glucose and insulin: effects on food intake and brain biogenic amines of rainbow trout

The effects of central (intracerebroventricular, 9 microg fish(-1)) and peripheral (intraperitoneal, 4 mg kg(-1)) administration of bovine insulin, as well as the effect of hyperglycemia (oral administration of 1 g glucose fish(-1)) and brain glucodeprivation (intracerebroventricular administration of 2-deoxy-D-glucose) on food intake and levels of brain (telencephalon, preoptic area, and hypothalamus) biogenic amines (serotonin, dopamine, noradrenaline and their metabolites 5-hydroxyindoleacetic acid, and dihydroxyphenylacetic acid) were assessed on rainbow trout ( Oncorhynchus mykiss). Treatment with insulin inhibited food intake after 26 or 52 h of administration, central or peripheral, respectively. This effect was still apparent after 74 h of central treatment. When assessing changes in the levels of biogenic amines after 26 h of central insulin administration, there was a significant increase in the levels of 5-hydroxyindoleacetic acid, and in the ratio of dihydroxyphenylacetic acid/dopamine of insulin-treated fish, in telencephalon and hypothalamus, respectively. These results suggest that peripherally administered insulin is involved in a feedback regulatory loop with food intake and body weight. Moreover, at least part of the effects of insulin could be mediated by hypothalamic dopaminergic activity. The strong hyperglycemia induced by oral administration of glucose did not induce significant changes either on food intake (control versus treated), or in brain levels of biogenic amines. The intracerebroventricular administration of 2-deoxy-D-glucose induced an increase in food intake without altering plasma glucose levels, suggesting that fish brain possesses a control system for detecting hypoglycemia in plasma and therefore keep brain glucose levels high enough for brain function.

Brain serotonin and the control of food intake in rainbow trout (Oncorhynchus mykiss): effects of changes in plasma glucose levels

The levels of 5-hydroxytryptamine and its main metabolite 5-hydroxyindoleacetic acid were assessed in two brain regions, hypothalamus and telencephalon, of rainbow trout (Oncorhynchus mykiss) submitted to increases or decreases in plasma glucose levels through different experimental approaches. Thus, intraperitoneal glucose treatment (500 mg kg(-1)) increased 5-hydroxytryptamine telencephalic levels. Long-term food deprivation up to 3 weeks significantly increased hypothalamic (2 weeks and 3 weeks) and telencephalic (1 week, 2 weeks, and 3 weeks) levels of 5-hydroxyindoleacetic acid, whereas the ratio 5-hydroxyindoleacetic acid/5-hydroxytryptamine significantly increased throughout the food-deprivation period assessed. Intraperitoneal treatment with bovine insulin (4 mg kg(-1)) decreased the 5-hydroxyindoleacetic acid/5-hydroxytryptamine ratio in hypothalamus after 1 h. Intraperitoneal administration of fenfluramine (3 mg kg(-1)) caused a depression in food intake coincident with a significant decrease of the hypothalamic 5-hydroxyindoleacetic acid/5-hydroxytryptamine ratio. These data are discussed in the context of the involvement of serotonergic system in the control of food intake in rainbow trout.

Glucagon effects on brain carbohydrate and ketone body metabolism of rainbow trout

The levels of glycogen in brain, lactate and acetoacetate in brain and plasma, glucose in plasma and the activities of brain key enzymes of glycogen metabolism (glycogen phosphorylase, GPase, glycogen synthetase, GSase), gluconeogenesis (fructose 1,6-bisphosphatase, FBPase), and glycolysis (6-phosphofructo 1-kinase, PFK) were evaluated in rainbow trout, Oncorhynchus mykiss, from 0.5 to 3 hr after intraperitoneal injection of 1 ml/kg(-1) body weight of saline alone (controls) or containing bovine glucagon at three different doses: 10, 50, and 100 ng/g(-1) body weight. The results obtained demonstrate, for the first time in a teleost fish, the existence of changes in brain carbohydrate and ketone body metabolism following peripheral glucagon treatment. A clear stimulation of brain glycogenolytic potential was observed after glucagon treatment, as judged by the time- and dose-dependent changes observed in brain glycogen levels (up to 88% decrease), and GPase (up to 30% increase) and GSase (up to 42% decrease) activities. In addition, clear time- and dose-dependent increased and decreased levels were observed in brain of glucagon-treated rainbow trout for lactate (up to 60% increase) and acetoacetate (up to 67% decrease), respectively. In contrast, no significant changes were observed after glucagon treatment in those parameters related to glycolytic/gluconeogenic capacity of rainbow trout brain. Altogether, these in vivo results suggest that glucagon may play a role (direct or indirect) in the regulation of carbohydrate and ketone body metabolism in brain of rainbow trout.

Uptake of 3-O-methyl-D-[U-14C]glucose into brain of rainbow trout: possible effects of melatonin

The influx of glucose into the brain and plasma glucose disappearance were estimated in rainbow trout (Oncorhynchus mykiss) intravenously injected (1 ml x kg(-1) body weight) with a single dose (15 microCi x kg(-1) body weight) of 3-O-methyl-D-[U-14C]glucose ([U-14C]-3-OMG) at different times (2-160 min), and after intravenous injection at 15 min of increased doses (10-60 microCi x kg(-1) body weight) of [U-14C]-3-OMG. Brain and plasma radiotracer concentrations were measured, and several kinetic parameters were calculated. The apparent brain glucose influx showed a maximum after 15-20 min of injection then decreased to a plateau after 80 min. Brain distribution space of 3-OMG increased from 2 min to 20 min reaching equilibrium from that time onwards at a value of 0.14 ml x g(-1). The unidirectional clearance of glucose from blood to brain (k1) and the fractional clearance of glucose from brain to blood (k2) were estimated to be 0.093 m x min(-1) x g(-1), and 0.867 min(-1), respectively. A linear increase was observed in brain and plasma radiotracer concentrations when increased doses of [U-(14)C]3-OMG were used. All these findings support a facilitative transport of glucose through the blood-brain barrier of rainbow trout with characteristics similar to those observed in mammals. The injection of different doses of melatonin (0.25-1.0 mg x kg(-1)) significantly increased brain glucose influx suggesting a possible role for melatonin in the regulation of glucose transport into the brain.

Acute effects of L-tryptophan on tryptophan hydroxylation rate in brain regions (hypothalamus and medulla) of rainbow trout (Oncorhynchus mykiss)

Levels of 5-hydroxytryptophan (5-HTP) in brain regions (hypopthalamus and medulla) of rainbow trout were analysed by HPLC-EC 0, 10, 30, and 40 min after intraperitoneal administration of different doses of L-tryptophan (Trp) (0, 12.5, and 25 mg. kg(-1) body weight) in fish treated with 3-hydroxybenzylhydrazine (NSD1015; 75 mg. kg(-1)). The results show that, in control fish, 5-HTP levels in hypothalamus (58.03 +/- 6.36 pg. mg(-1) brain tissue) were significantly higher than those observed in medulla (28.04 +/- 4.32 pg. mg(-1) brain tissue). Basal tryptophan hydroxylation rates (after 0 mg. kg(-1) Trp administration) were 0.42 +/- 0.07 pg 5-HTP. mg(-1). min(-1), and 0.63 +/- 0.24 pg 5HTP. mg(-1). min(-1), for hypothalamus and medulla respectively. On the other hand, the results demonstrate that L-tryptophan administration induced significant increases in the rate of tryptophan hydroxylation, both in hypothalamus and medulla. These findings indicate that, in a way similar to that observed in mammals, brain tryptophan hydroxylase is unsaturated by its substrate (tryptophan) under normal physiological conditions. J. Exp. Zool. 286:131-135, 2000.

Transport and metabolism of glucose in isolated enterocytes of the black bullhead ictalurus melas: effects of diet and hormones

The uptake and metabolism of glucose were assessed in enterocytes isolated from black bullhead Ictalurus melas. The objective of this study was to examine the effects of diet and hormone treatment on glucose transport and metabolism, so the enterocyte was the most appropriate preparation. Glucose transport was estimated using specific inhibitors:glucose uptake measured in the presence of phlorizin presumably represents transport at the basolateral membrane, whereas glucose uptake in the presence of cytochalasin B presumably represents transport at the brush border. Feeding bullheads a standard diet resulted in maximum enterocyte rates of glucose uptake of 438.2+/-35.5 nmol mg-1 cells h-1 for transport in the presence of cytochalasin B and 427.0+/-49.7 nmol mg-1 cells h-1(means +/- s.e.m., N=12) for transport in the presence of phlorizin. These values represent 50 % of the total 3-O-methylglucose transported. The rate of transport in the presence of cytochalasin B was increased in bullheads fed a high-carbohydrate diet. Incubating bullhead enterocytes with glucagon or glucagon-like peptide-1 (GLP) at 10(-8 )mol l-1 and with dexamethasone or isoproterenol at 10(-6 )mol l-1 significantly increased the rate of brush-border transport, but not the apparent affinity constant (Kt). Activation was dependent on hormone concentration. In contrast, insulin was without effect on transport rates, nor did it counteract activation by glucagon-family peptides. CO2 production rates from d-[14C]glucose indicated that glucose metabolism was not limited by transport rates in the enterocytes. Glucagon and GLP decreased maximal oxidation rates, whereas dexamethasone, isoproterenol and insulin did not alter these rates. The activities of enterocyte hexokinase exceeded the rate of glucose oxidation but not the rate of transport of glucose, at least at maximum activities,implicating this enzyme as one component of the strategy to ensure that glucose is maximally available to the blood of this species.

Glucose, lactate, and beta-hydroxybutyrate utilization by rainbow trout brain: changes during food deprivation

In order to evaluate the normal (fed conditions) substrate utilization rates of rainbow trout (Oncorhynchus mykiss) brain, CO2 production from glucose, lactate, and beta-hydroxybutyrate was tested in pooled brains. Oxidation rates, as well as the capacity for metabolism of carbohydrate and ketone bodies, were also evaluated in brain of rainbow trout that were food-deprived for 14 d. Under normal (fed) conditions, rainbow trout brain oxidized glucose and lactate at rates higher than those described for mammals; oxidation rates of beta-hydroxybutyrate were lower in rainbow trout brain than those observed for lactate and glucose, and also lower than those described for mammals. Under food-deprivation conditions, glucose and lactate oxidation rates decreased in brains, suggesting the existence of brain metabolic depression, and beta-hydroxybutyrate oxidation rates sharply increased, suggesting increased utilization of ketone bodies.

Effects of an acute exposure to lindane (gamma-hexachlorocyclohexane) on brain and liver carbohydrate metabolism of rainbow trout

The capacity of carbohydrate and ketone bodies metabolism in brain and liver was evaluated in rainbow trout control and exposed to lindane (0.05 mg.L-1) for 12 hr. The results obtained demonstrate the existence of changes in several parameters of brain carbohydrate metabolism due to lindane treatment. Thus, increased plasma glucose levels are reflected in brain by the mobilization of glycogen stores and increased lactate levels probably reflecting an increased anaerobic use. Also, ketone bodies appear to be used under this stressful condition. In liver, the main results obtained suggest that glycogen stores are being mobilized to be probably used as glucose in pathways other than glycolysis.

Food deprivation and refeeding in Atlantic salmon,Salmo salar: effects on brain and liver carbohydrate and ketone bodies metabolism

The capacity of carbohydrate and ketone bodies metabolism in brain and liver was evaluated in fed and food-deprived Atlantic salmon (Salmo salar) in a time period covering from 1 to 7 days (Experiment I), and in Atlantic salmon food deprived for 6 weeks, and food deprived for 4 weeks and refed for 2 weeks (Experiment II). The results obtained demonstrate for the first time in a teleost the existence of changes in brain metabolism due to food deprivation. Thus, decreased glucose levels in plasma are reflected in the brain by an increased mobilization of glycogen reserves, and by a decreased glycolytic capacity. Also, ketone bodies appear to increase their importance as a metabolic fuel from day 7 of food deprivation onwards. A possible increase in the gluconeogenic potential in brain simultaneously is not discarded. All these metabolic changes are reversed under refeeding conditions.

Kidney ATPase response in seawater-transferred rainbow trout (Oncorhynchus mykiss). Effect of salinity and fish size

Two sizes of domesticated rainbow trout (Oncorhynchus mykiss, 40 +/- 0.67 and 180 +/- 3.9 g) were directly transferred to brackish water (9 ppt) and seawater (28 ppt). Kidney Na(+)-K(+)-ATPase and Mg(2+)-ATPase activities were measured in fresh water, and after long-term seawater adaptation (up to 21 days). Renal Na(+)-K(+)-ATPase activity increased after saltwater loading in small trout, while large trout displayed an unmodified ATPase activity. The smallest trout showed a low but progressive increase in renal Mg(2+)-ATPase activity after the transfer to both salinities. However, ATPase activity remained unchanged or significantly decreased in large trout after the transfer to seawater or brackish water, respectively.

Post-feeding carbohydrate and ketone bodies metabolism in brain and liver of Atlantic salmon

A study of several pathways of carbohydrate and ketone bodies metabolism was carried out in Atlantic salmon (Salmo salar) to assess the basal metabolism of the brain, and the possible existence of post-feeding changes in brain and liver metabolism. The main results obtained in brain of Atlantic salmon indicate a use of exogenous glucose as a main fuel source since important hexokinase activities were noticed, and brain glycogen levels were usually very low. Several post-feeding changes were observed in brain including an apparent decrease in glycolytic potential, as well as a decreased use of ketone bodies. In contrast, no major post-feeding changes were detected in liver metabolism. A role for ketone bodies as a metabolic fuel in brain of Atlantic salmon is supported by both the high levels of acetoacetate found in brain, and the presence of an active beta-hydroxybutyrate dehydrogenase.

HCO3(-)-ATPase and Ca2+ dependent ATPase activities in the gills of the rainbow trout after the transfer to brackishwater and seawater

The effect of seawater and brackishwater exposure on gill HCO3(-)-ATPase and Ca2+ dependent ATPase activity in rainbow trout (Oncorhynchus mykiss) was investigated at different periods of time. HCO3(-)-ATPase activity decreased after the transfer to either brackishwater or seawater. Ca2+ dependent ATPase activity decreased during the initial period (1 to 4 days) in both salinities and recovered freshwater values from the 7th day onwards. No effect from fish size was detected in both parameters after saltwater transfer. The results are discussed in terms of salinity and long-term saltwater adaptation.

Direct transfer of rainbow trout to seawater induces several changes in kidney carbohydrate metabolism

The levels of glycogen and glucose, and the activities of several key enzymes of glycogenolysis, glycolysis, gluconeogenesis and the pentose phosphate shunt were assessed in kidneys of rainbow trout (Oncorhynchus mykiss) of two sizes (80 and 140 g) after transfer to seawater (28 p.p.t.) during 7 days. The results indicated changes, mainly size-independent, in kidney carbohydrate metabolism during transfer of rainbow trout to seawater. An enhanced glycogenolysis and a concomitant increase in gluconeogenic enzyme activity were clearly observed in kidneys of both sizes of animals during transfer to seawater. Changes are suggested to be related to the known role of kidney as a glucose producer tissue thus satisfying, at least in part, the high energetic requirements of the osmoregulatory work performed by other tissues using glucose as fuel, such as the gills, during adaptation to seawater.