Pre- and postprandial effects on ghrelin signaling in the brain and on the GH/IGF-I axis in the Mozambique tilapia (Oreochromis mossambicus)

A circadian clock controls the daily function of the intestine in rainbow trout. Influence of light and food as synchronizers

Environmental factors (daily light/dark cycles, food availability, etc.) entrain endogenous oscillators in living organisms, thereby allowing them to control the rhythms of behavioral and physiological functions, such as energy homeostasis. The gastrointestinal tract (GIT) is the first site of nutrient contact upon food intake. Thus, the GIT is key in energy homeostasis. Circadian oscillators exist within the GIT of mammals, modulating the daily function of the tissue. However, little information in this respect is available for other vertebrates, such as fish. Thus, we aimed to confirm the presence of a circadian oscillator within the GIT of rainbow trout (Oncorhynchus mykiss) and its interaction with locally released hormones that participate in feeding regulation in this species. We subsequently evaluated the role of food and light in synchronizing the rhythmic functioning of the GIT. According to our results, a circadian oscillator exists throughout the GIT of rainbow trout, based on the daily rhythms of clock gene (clock1a, bmal1b, per1, cry2 and reb-ervβ-like) mRNA abundance. Light influences the function of the circadian oscillator within the GIT, but food is a key factor as a synchronizer. The feeding time and the presence and/or absence of food synchronize the rhythmic function of the GIT, as observed for GIT hormones (Ghrelin, Glp1 and Cck). Understanding the functioning of the circadian machinery in peripheral organs such as the GIT will ultimately help to improve different aspects of aquaculture, from farming strategies to welfare, among others.

Feeding frequency and dietary protein/carbohydrate ratio affect feed intake and appetite regulation-related genes expression in gilthead seabream (Sparus aurata)

To evaluate the effects of feeding frequency (FF) and dietary protein/carbohydrate (P/CH) ratios on appetite regulation of gilthead seabream, two practical diets were formulated to include high protein and low carbohydrate (P50/CH10 diet) or low protein and high carbohydrate (P40/CH20 diet) content and each diet was fed to triplicate groups of fish until visual satiation each meal at a FF of 1, 2, or 3 meals per day. Feed intake and feed conversion ratio were higher in fish fed 2 or 3 meals than 1 meal per day and in fish fed the P40/CH20 than the P50/CH10 diet. The specific growth rate was only affected by FF, being higher in fish fed 2 or 3 meals per day than 1 meal per day. Expression of the cocaine-amphetamine-related transcript, corticotropin-releasing hormone, ghrelin receptor-a (ghsr-a), leptin, and neuropeptide y in the brain, cholecystokinin (cck) in the intestine, and leptin and ghrelin in the stomach was not affected by FF or dietary P/CH ratio. This is the first time that ghrelin cells were immune-located in the stomach of gilthead seabream. Fish fed 3 meals per day presented lower cck expression in the brain than those fed twice per day and higher hepatic ghsr-b expression than those fed once per day. Fish fed P40/CH20 diet presented higher hepatic leptin expression than those fed P50/CH10 diet. In conclusion, present results indicate that feeding a P40/CH20 diet at 3 meals a day seems to decrease the satiation feeling of gilthead seabream compared to fish fed higher P/CH ratio diets or fed 1 or 2 meals a day.

Effects of Short-Term Fasting on mRNA Expression of Ghrelin and the Peptide Transporters PepT1 and 2 in Atlantic Salmon (Salmo salar)

Food intake is a vital process that supplies necessary energy and essential nutrients to the body. Information regarding luminal composition in the gastrointestinal tract (GIT) collected through mechanical and nutrient sensing mechanisms are generally conveyed, in both mammals and fish, to the hypothalamic neurocircuits. In this context, ghrelin, the only known hormone with an orexigenic action, and the intestinal peptide transporters 1 and 2, involved in absorption of dietary di- and tripeptides, exert important and also integrated roles for the nutrient uptake. Together, both are potentially involved in signaling pathways that control food intake originating from different segments of the GIT. However, little is known about the role of different paralogs and their response to fasting. Therefore, after 3 weeks of acclimatization, 12 Atlantic salmon (Salmo salar) post-smolt were fasted for 4 days to explore the gastrointestinal response in comparison with fed control (n = 12). The analysis covered morphometric (weight, length, condition factor, and wet content/weight fish %), molecular (gene expression variations), and correlation analyses. Such short-term fasting is a common and recommended practice used prior to any handling in commercial culture of the species. There were no statistical differences in length and weight but a significant lower condition factor in the fasted group. Transcriptional analysis along the gastrointestinal segments revealed a tendency of downregulation for both paralogous genes slc15a1a and slc15a1b and with significant lowered levels in the pyloric ceca for slc15a1a and in the pyloric ceca and midgut for slc15a1b. No differences were found for slc15a2a and slc15a2b (except a higher expression of the fasted group in the anterior midgut), supporting different roles for slc15 paralogs. This represents the first report on the effects of fasting on slc15a2 expressed in GIT in teleosts. Transcriptional analysis of ghrelin splicing variants (ghrl-1 and ghrl-2) showed no difference between treatments. However, correlation analysis showed that the mRNA expression for all genes (restricted to segment with the highest levels) were affected by the residual luminal content. Overall, the results show minimal effects of 4 days of induced fasting in Atlantic salmon, suggesting that more time is needed to initiate a large GIT response.

Expression of neuropeptide Y and gonadotropin-releasing hormone gene types in the brain of female Nile tilapia (Oreochromis niloticus) during mouthbrooding and food restriction

A cichlid fish, the Nile tilapia (Oreochromis niloticus), is a maternal mouthbrooder, which exhibits minimum energy expenditure and slower ovarian cycles during mouthbrooding. The objective of this study was to observe changes in the gene expression of key neuropeptides involved in the control of appetite and reproduction, including neuropeptide Y a (NPYa), reproductive neuropeptides: gonadotropin-releasing hormone (GnRH1, GnRH2 and GnRH3) and kisspeptin (Kiss2) during mouthbrooding (4- and 12-days), 12-days of food restriction and 12-days of food restriction followed by refeeding. The food restriction regime showed a significant increase in npya mRNA levels in the telencephalon. However, there were no significant alterations in npya mRNA levels during mouthbrooding. gnrh1 mRNA levels were significantly lower in mouthbrooding female as compared with females with food restriction. gnrh3 mRNA levels were also significantly lower in female with 12-days of mouthbrooding, 12-days of food restriction followed by 12-days of refeeding when compared with controls. There were no significant differences in gnrh2 and kiss2 mRNA levels between groups under different feeding regimes. No significant changes were observed in mRNA levels of receptors for peripheral metabolic signaling molecules: ghrelin (GHS-R1a and GHS-R1b) and leptin (Lep-R). These results suggested that unaffected npya mRNA levels in the telencephalon might contribute to suppression of appetite in mouthbrooding female tilapia. Furthermore, lower gnrh1 and gnrh3 mRNA levels may influence the suppression of reproductive functions such as progression of ovarian cycle and reproductive behaviours, while GnRH2 and Kiss2 may not play a significant roles in reproduction under food restriction condition.

Nutrient Regulation of Endocrine Factors Influencing Feeding and Growth in Fish

Endocrine factors regulate food intake and growth, two interlinked physiological processes critical for the proper development of organisms. Somatic growth is mainly regulated by growth hormone (GH) and insulin-like growth factors I and II (IGF-I and IGF-II) that act on target tissues, including muscle, and bones. Peptidyl hormones produced from the brain and peripheral tissues regulate feeding to meet metabolic demands. The GH-IGF system and hormones regulating appetite are regulated by both internal (indicating the metabolic status of the organism) and external (environmental) signals. Among the external signals, the most notable are diet availability and diet composition. Macronutrients and micronutrients act on several hormone-producing tissues to regulate the synthesis and secretion of appetite-regulating hormones and hormones of the GH-IGF system, eventually modulating growth and food intake. A comprehensive understanding of how nutrients regulate hormones is essential to design diet formulations that better modulate endogenous factors for the benefit of aquaculture to increase yield. This review will discuss the current knowledge on nutritional regulation of hormones modulating growth and food intake in fish.

Ghrelin stimulates growth hormone release from the pituitary via hypothalamic growth hormone-releasing hormone neurons in the cichlid, Oreochromis niloticus

Ghrelin, a gut-brain peptide hormone, is implicated in a multiplicity of biological functions, including energy homeostasis and reproduction. Neuronal systems that are involved in energy homeostasis as well as reproduction traverse the hypothalamus; however, the mechanism by which they control energy homeostasis is not fully understood. The present study analyzes the anatomical relationship of neurons expressing gonadotropin-releasing hormone (GnRH), neuropeptide Y (NPY) and growth hormone-releasing hormone (GHRH) in a cichlid, tilapia (Oreochromis niloticus). Additionally, we examine in vivo effects of ghrelin on these hypothalamic neurons and plasma growth hormone (GH) and insulin-like growth factor-1 (IGF-1) levels. Double-immunofluorescence showed neuronal fiber associations between GnRH, NPY and GHRH in the brain and pituitary. Intracerebroventricular injection of ghrelin had no effect on numbers, soma size, or optical density of GnRH and NPY neurons, whereas the number of GHRH neurons was significantly decreased in the animals injected with ghrelin when compared to controls, which may indicate administered ghrelin promoted GHRH release. Plasma GH and pituitary GH mRNA levels were significantly increased in the animals injected with ghrelin. These results suggest that central administration of ghrelin primarily act on hypothalamic GHRH neurons to stimulate GH release from the pituitary in the tilapia.

Regulation of compensatory growth by molecular mechanism in Labeo rohita juveniles under different feeding regimes

A study was carried out to assess the regulation of compensatory growth under different restriction feeding regimes in Labeo rohita juveniles by the interaction of various feed intake and growth regulating genes. A 60 day feeding trial was conducted with five treatment groups, Control (3% body weight, bw), T1 (alternate days), T2 (0.5% bw), T3 (1% bw) and T4 (2% bw) and feeding was done for first 30 days of the trial. For next 30 days, all the treatment groups were fed at a rate of 3% bw as in the control group. There was significant (p < 0.05) difference in the weight gain among the treatment groups with lowest FCR and highest PER was found in T2 group. Ghrelin gene mRNA levels were upregulated during first 30th days of the trial with highest expression levels in the T2 group. The expression levels of leptin gene mRNA were found significantly different (p < 0.05) among the treatments, which was down-regulated during initial 30 days and upregulated as the experiment progress towards 60th day. The IGF-1 mRNA expression levels were upregulated more in liver compared to the muscle tissue. The results of the study suggest that increased ghrelin levels and decreased leptin levels lead to hyperphagia during the onset of refeeding, which further triggers the compensatory growth in L. rohita. The present study describes the molecular mechanism behind the compensatory growth following a different feed restriction regime in L. rohita which is regulated due to the interaction of different energy homeostasis and growth regulating genes.

Ghrelin and Its Receptors in Gilthead Sea Bream: Nutritional Regulation

Ghrelin is involved in the regulation of growth in vertebrates through controlling different functions, such as feed intake, metabolism, intestinal activity or growth hormone (Gh) secretion. The aim of this work was to identify the sequences of preproghrelin and Ghrelin receptors (ghsrs), and to study their responses to different nutritional conditions in gilthead sea bream (Sparus aurata) juveniles. The structure and phylogeny of S. aurata preproghrelin was analyzed, and a tissue screening was performed. The effects of 21 days of fasting and 2, 5, 24 h, and 7 days of refeeding on plasma levels of Ghrelin, Gh and Igf-1, and the gene expression of preproghrelin, ghsrs and members of the Gh/Igf-1 system were determined in key tissues. preproghrelin and the receptors are well conserved, being expressed mainly in stomach, and in the pituitary and brain, respectively. Twenty-one days of fasting resulted in a decrease in growth while Ghrelin plasma levels were elevated to decrease at 5 h post-prandial when pituitary ghsrs expression was minimum. Gh in plasma increased during fasting and slowly felt upon refeeding, while plasma Igf-1 showed an inverse profile. Pituitary gh expression augmented during fasting reaching maximum levels at 1 day post-feeding while liver igf-1 expression and that of its splice variants decreased to lowest levels. Liver Gh receptors expression was down-regulated during fasting and recovered after refeeding. This study demonstrates the important role of Ghrelin during fasting, its acute down-regulation in the post-prandial stage and its interaction with pituitary Ghsrs and Gh/Igf-1 axis.

Direct actions of macronutrient components on goldfish hepatopancreas in vitro to modulate the expression of ghr-I, ghr-II, igf-I and igf-II mRNAs

In mammals and fish, somatic growth and metabolism are coordinated by the GH-IGF axis, composed of growth hormone (GH), growth hormone receptors I and II (GHR-I and GHR-II), and the insulin-like growth factors I and II (IGF-I and IGF-II). In order to determine if dietary macronutrients regulate the hepatopancreatic expression of ghr-I, ghr-II, igf-I and igf-II independently of circulating GH, organ culture experiments were conducted. Briefly, goldfish hepatopancreas sections were incubated with different doses of glucose; L-tryptophan; oleic acid; linolenic acid (LNA); eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). After two and four hours of treatment, the expression of ghr-I, ghr-II, igf-I and igf-II mRNAs was quantified. We found that glucose and L-tryptophan globally upregulate the mRNA expression of ghr-I; ghr-II; igf-I and igf-II. Duration of exposure, and unsaturation level of fatty acids differentially modulate ghr-I, ghr-II and igf-II mRNA expression. Additionally, we found that fatty acids increase the expression of igf-I depending on incubation time and fatty acid class. In conclusion, here we present evidence for GH-independent, direct effects exerted by dietary macronutrients on GHR and IGF in goldfish hepatopancreas.

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.

Effects of fasting, temperature, and photoperiod on preproghrelin mRNA expression in Chinese perch

Preproghrelin, a gut/brain peptide, plays an important role in the regulation of food intake and energy homeostasis in teleost and mammals. In the present study, we obtained the full-length preproghrelin cDNA in Chinese perch. The preproghrelin messenger RNA (mRNA) tissue expression showed that level was much higher in stomach and pituitary than in other tissues. The fasting study showed, after gastric emptying (3-6 h), short-term fasting (6-12 h) increased preproghrelin expression in the stomach. While in the pituitary, fasting reduced preproghrelin expression at 1, 3, 12, and 48 h, presenting state fluctuation of self-adjustment. The temperature study showed that the mRNA expression of preproghrelin was the highest in the brain at 26 °C and highest in the stomach at 32 °C, respectively, with different optimum temperature in these two tissues, reflecting spatiotemporal differences of regulation by central nervous system and peripheral organs. The photoperiod study showed that normal light (11 h of lightness and 13 h of darkness) led to highest preproghrelin expression, both in the brain and in the stomach, than continuous light or continuous dark, proving food intake is adapted to natural photoperiod or normal light in this study. These results all indicated that tissue-specific preproghrelin expression of Chinese perch could be significantly affected by environmental factors. Short-term fasting of 6 h after gastric emptying, 26 °C, and normal light led to higher preproghrelin expression, which indicated potential appetite increase in Chinese perch.

Interplay between the endocrine and circadian systems in fishes

The circadian system is responsible for the temporal organisation of physiological functions which, in part, involves daily cycles of hormonal activity. In this review, we analyse the interplay between the circadian and endocrine systems in fishes. We first describe the current model of fish circadian system organisation and the basis of the molecular clockwork that enables different tissues to act as internal pacemakers. This system consists of a net of central and peripherally located oscillators and can be synchronised by the light-darkness and feeding-fasting cycles. We then focus on two central neuroendocrine transducers (melatonin and orexin) and three peripheral hormones (leptin, ghrelin and cortisol), which are involved in the synchronisation of the circadian system in mammals and/or energy status signalling. We review the role of each of these as overt rhythms (i.e. outputs of the circadian system) and, for the first time, as key internal temporal messengers that act as inputs for other endogenous oscillators. Based on acute changes in clock gene expression, we describe the currently accepted model of endogenous oscillator entrainment by the light-darkness cycle and propose a new model for non-photic (endocrine) entrainment, highlighting the importance of the bidirectional cross-talking between the endocrine and circadian systems in fishes. The flexibility of the fish circadian system combined with the absence of a master clock makes these vertebrates a very attractive model for studying communication among oscillators to drive functionally coordinated outputs.

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.

Evidence that ghrelin may be associated with the food intake of gibel carp (Carassius auratus gibelio)

Ghrelin, a non-amidated peptide hormone, is a potent anorectic neuropeptide implicated in feeding regulation in mammals and non-mammalian vertebrates. However, the involvement of ghrelin in the feeding behavior of teleosts has not been well understood. To better understand the role of ghrelin in the regulation of appetite in fish, in this study, we cloned the cDNAs encoding ghrelin and investigated their mRNA distributions in gibel carp tissues. We also assessed the effects of different nutritional status on ghrelin mRNA abundance. Ghrelin mRNAs were ubiquitously expressed in ten tissues (intestine, liver, brain, mesonephron, head kidney, spleen, skin, heart, muscle, gill and pituitary gland), and relatively high expression levels were detected in the gut. Postprandial studies analysis revealed a significant postprandial decrease in ghrelin mRNA expression in the gut (1 and 3 h after the regular feeding time). In addition, ghrelin mRNA expression in the gut significantly increased at day 7 after fasting and declined sharply after refeeding, which suggested that ghrelin might be involved in the regulation of appetite in gibel carp. Overall, our result provides basis for further investigation into the regulation of feeding in gibel carp.

Ghrelin O-Acyl Transferase in Zebrafish Is an Evolutionarily Conserved Peptide Upregulated During Calorie Restriction

Ghrelin is a multifunctional orexigenic hormone with a unique acyl modification enabled by ghrelin O-acyl transferase (GOAT). Ghrelin is well-characterized in nonmammals, and GOAT sequences of several fishes are available in the GenBank. However, endogenous GOAT in non-mammals remains poorly understood. In this research, GOAT sequence comparison, tissue-specific GOAT expression, and its regulation by nutrient status and exogenous ghrelin were studied. It was found that the bioactive core of zebrafish GOAT amino acid sequence share high identity with that of mammals. GOAT mRNA was most abundant in the gut. GOAT-like immunoreactivity (i.r.) was found colocalized with ghrelin in the gastric mucosa. Food deprivation increased, and feeding decreased GOAT and preproghrelin mRNA expression in the brain and gut. GOAT and ghrelin peptides in the gut and brain showed corresponding decrease in food-deprived state. Intraperitoneal injection of acylated fish ghrelin caused a significant decrease in GOAT mRNA expression, suggesting a feedback mechanism regulating its abundance. Together, these results provide the first in-depth characterization of GOAT in a non-mammal. Our results demonstrate that endogenous GOAT expression is responsive to metabolic status and availability of acylated ghrelin, providing further evidences for GOAT in the regulation of feeding in teleosts.

Relative distribution of gastrin-, CCK-8-, NPY- and CGRP-immunoreactive cells in the digestive tract of dorado (Salminus brasiliensis)

The endocrine cells (ECs) of the gastrointestinal mucosa form the largest endocrine system in the body, not only in terms of cell numbers but also in terms of the different produced substances. Data describing the association between the relative distributions of the peptide-specific ECs in relation to feeding habits can be useful tools that enable the creation of a general expected pattern of EC distribution. We aimed to investigate the distribution of ECs immunoreactive for the peptides gastrin (GAS), cholecystokinin (CCK-8), neuropeptide Y (NPY), and calcitonin gene-related peptide (CGRP) in different segments of the digestive tract of carnivorous fish dorado (Salminus brasiliensis) by using immunohistochemistry procedures. The distribution of endocrine cells immunoreactive for gastrin (GAS), cholecystokinin (CCK-8), neuropeptide Y (NPY), and calcitonin gene-related peptide (CGRP) in digestive tract of dorado S. brasiliensis was examined by immunohistochemistry. The results describe the association between the distribution of the peptide-specific endocrine cells and feeding habits in different carnivorous fish. The largest number of endocrine cells immunoreactive for GAS, CCK-8, and CGRP were found in the pyloric stomach region and the pyloric caeca. However, NPY-immunoreactive endocrine cells were markedly restricted to the midgut. The distribution pattern of endocrine cells identified in S. brasiliensis is similar to that found in other carnivorous fishes.

Nutritional status and growth hormone regulate insulin-like growth factor binding protein (igfbp) transcripts in Mozambique tilapia

Growth in teleosts is controlled in large part by the activities of the growth hormone (Gh)/insulin-like growth factor (Igf) system. In this study, we initially identified igf-binding protein (bp)1b, -2b, -4, -5a and -6b transcripts in a tilapia EST library. In Mozambique tilapia (Oreochromis mossambicus), tissue expression profiling of igfbps revealed that igfbp1b and -2b had the highest levels of expression in liver while igfbp4, -5a and -6b were expressed at comparable levels in most other tissues. We compared changes in hepatic igfbp1b, -2b and -5a expression during catabolic conditions (28days of fasting) along with key components of the Gh/Igf system, including plasma Gh and Igf1 and hepatic gh receptor (ghr2), igf1 and igf2 expression. In parallel with elevated plasma Gh and decreased Igf1 levels, we found that hepatic igfbp1b increased substantially in fasted animals. We then tested whether systemic Gh could direct the expression of igfbps in liver. A single intraperitoneal injection of ovine Gh into hypophysectomized tilapia specifically stimulated liver igfbp2b expression along with plasma Igf1 and hepatic ghr2 levels. Our collective data suggest that hepatic endocrine signaling during fasting may involve post-translational regulation of plasma Igf1 via a shift towards the expression of igfbp1b. Thus, Igfbp1b may operate as a molecular switch to restrict Igf1 signaling in tilapia; furthermore, we provide new details regarding isoform-specific regulation of igfbp expression by Gh.

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.

The role of ghrelin in energy balance regulation in fish

Knowledge about the endocrine regulation of energy balance in fish is of interest for basic as well as aquaculture research. Ghrelin is a peptide hormone that was first identified in fish 10 years ago and has important roles in the control of food intake and metabolism. Both ghrelin and its receptor, the growth hormone secretagogue receptor (GHS-R), have been found in numerous fish species. Their tissue distributions support the idea that ghrelin has an integrative role in the regulation of energy balance at both the central nervous system level and systemic level. In tilapia and goldfish, ghrelin treatment appears to increase food intake and to stimulate lipogenesis and tissue fat deposition to promote a more positive energy status. In rainbow trout, on the other hand, ghrelin decreases food intake. Goldfish and rainbow trout are the fish species in which the mode of action of ghrelin on food intake has been most thoroughly investigated. The results from these studies indicate that ghrelin alters food intake by acting on well-known appetite signals, such as CRH, NPY and orexin, in the hypothalamus in a species-specific manner. In goldfish, sensory fibres of the vagus nerve convey the signal from gut-derived ghrelin to modulate appetite. The data also indicate that ghrelin may modulate foraging/swimming activity and the perception of food in fish. Results related to the effects of energy status, temperature, and stressors on plasma ghrelin/tissue ghrelin mRNA levels are occasionally inconsistent between short- and long-term studies, between the protein and mRNA, and between species. Recent data also imply a role of ghrelin in carbohydrate metabolism. More functional studies are required to understand the role of ghrelin and its mechanisms of action in the regulation of energy balance among fish.

Acute stress inhibits food intake and alters ghrelin signaling in the brain of tilapia (Oreochromis mossambicus)

This study investigated the effect of an acute stress on food intake and on the expression of neuropeptide Y (NPY), corticotropin-releasing hormone (CRH), and ghrelin and its receptors, growth hormone secretagogue receptors (GHSRs) in the tilapia (Oreochromis mossambicus). Food intake was significantly (P < 0.01) reduced in fish after a 30-min crowding and handling stress. In a second group of animals exposed to the same 30-min stressor, tissue samples were collected immediately after the stressor to determine changes in the neuroendocrine regulators of food intake. Although CRH and NPY are considered the major mediators of appetite during stress, both mRNA levels were unaltered in the telencephalon/pre-optic area and in the hypothalamic/optic tectum. Interestingly, there was an elevation in the ghrelin transcript (P < 0.05) in the telencephalon/pre-optic area and elevation of its functional receptor (GHSR1a-LR) (P < 0.001) in the hypothalamic/optic tectum. Elevation of GHSR-LR heteronuclear RNA (P < 0.01) in the telencephalon/pre-optic area and suppression in the hypothalamic/optic tectum (P < 0.001) suggest rapid control of the ghrelin regulatory system in response to acute stress. These results suggest that ghrelin signaling is altered during acute stress. It is not clear if these changes result in altered feeding behavior because no changes in CRH or NPY mRNA expression were observed or if ghrelin is playing a role in regulating overall metabolic changes after acute stress.

Ghrelin receptors in non-Mammalian vertebrates

The growth hormone secretagogue-receptor (GHS-R) was discovered in humans and pigs in 1996. The endogenous ligand, ghrelin, was discovered 3 years later, in 1999, and our understanding of the physiological significance of the ghrelin system in vertebrates has grown steadily since then. Although the ghrelin system in non-mammalian vertebrates is a subject of great interest, protein sequence data for the receptor in non-mammalian vertebrates has been limited until recently, and related biological information has not been well organized. In this review, we summarize current information related to the ghrelin receptor in non-mammalian vertebrates.

Cortisol treatment reduces ghrelin signaling and food intake in tilapia, Oreochromis mossambicus

It is well known that after a stressor, levels of plasma cortisol rise, inducing physiological changes within the animal that are directed toward maintaining homeostasis. Less well understood is the role of cortisol in regulating food intake in teleosts. This study investigated the effect of cortisol on food intake and regulation of the neuroendocrine appetite-stimulating hormones, neuropeptide Y (NPY) and ghrelin, in tilapia (Oreochromis mossambicus). Male and female tilapia were randomly assigned to one of the following treatments: unhandled control, vehicle-injected control, or cortisol (2 μg/g BW). Food intake was determined 24 h after injection during a 1-h feeding trial. Cortisol reduced food intake (P<0.001). An identical study was conducted to measure the effects of 24-h cortisol treatment on the endocrine regulators of food intake. Cortisol reduced stomach expression of ghrelin mRNA (P<0.05) and plasma concentrations of ghrelin (P<0.05). In the hypothalamus/optic tectum cortisol reduced levels of GHSR1a-LR (biologically active ghrelin receptor) mRNA. In the telencephalon/preoptic area cortisol significantly reduced levels of NPY and GHSR1b-LR (biologically inactive ghrelin receptor) mRNA. These findings suggest that anorexigenic actions of cortisol may be mediated via two separate pathways: (1) reducing circulating ghrelin levels as well as GHSR1a-LR expression in the hypothalamus/optic tectum and/or (2) suppressing NPY expression in the telencephalon/preoptic area.

Pre- and postprandial changes in orexigenic and anorexigenic factors in channel catfish (Ictalurus punctatus)

Ghrelin (GRLN), cocaine and amphetamine regulated transcript (CART), neuropeptide Y (NPY), and cholecystokinin (CCK) are neuropeptides involved in the regulation of appetite and feeding in vertebrates. We examined pre- and postprandial changes in the expression of plasma GHRL and mRNAs encoding GRLN, CART, NPY, and CCK in channel catfish. Fish were entrained to eat at 0900 h for 2 weeks. Fish were then sampled at 0700, 0800, and 0900 h. Remaining fish were either offered feed at 0900 h (Fed) or fasted (Unfed). Fish sampling continued at 0.5, 1, 2, and 4 h post feeding. Feeding increased abundance of whole brain CART mRNA out to 4 h with no effect observed in unfed fish. Whole brain NPY expression peaked at 0.5 h in both treatments. NPY expression then declined in fed fish but remained elevated in unfed fish. No differences in plasma or stomach GRLN expression were observed. Two separate cDNAs for CCK were identified. Brain CCKa and CCKb expression increased after feeding. These results suggest CART, NPY, and CCK play roles in the regulation of channel catfish feeding. Taken together, these results provide new insights into the neural and gastroenteric mechanisms regulating appetite in channel catfish.

Recent advances in the phylogenetic study of ghrelin

To understand fully the biology of ghrelin, it is important to know the evolutionary history of ghrelin and its receptor. Phylogenetic and comparative genomic studies of mammalian and non-mammalian vertebrates are a useful approach to that end. Ghrelin is a hormone that has apparently evaded natural selection during a long evolutionary history. Surely ghrelin plays crucial physiological roles in living animals. Phylogenetic studies reveal the nature and evolutionary history of this important signaling system.

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.

Effects of short-term starvation on ghrelin, GH-IGF system, and IGF-binding proteins in Atlantic salmon

The effects of short-time fasting on appetite, growth, and nutrient were studied in Atlantic salmon (Salmo salar) smolts. Feed deprivation did change the energy metabolism with reduced plasma protein and muscle indispensible amino acid levels. Plasma levels of ghrelin were significantly higher in starved salmon compared with fed fish after 2 days, but no differences in circulating ghrelin were found between treatments after 14 days. Two mRNA sequences for ghrelin-1 and ghrelin-2, 430 and 533 bp long, respectively, were detected. In addition, the growth hormone secretagogues-receptor like receptor (GHSR-LR) 1a and 1b were identified. Ghrelin-1 but not ghrelin-2 mRNA levels were affected by starvation in the stomach. Lower ghrelin-1 mRNA levels were detected at day 2 in starved fish compared with fed fish. The mRNA levels of GHSR-LR1a were not affected by starvation. Fasting reduced the phenotypic growth and the transcription of insulin-like growth factor (IGF)-II together with IGF-IIR, but IGF-I mRNA were not regulated in fasted salmon after 14 days. Three IGF-binding proteins (IGFBP) at 23, 32, and 43 kDa were found in salmon, and circulating 23 kDa was significantly increased after 14 days of starvation compared with fed fish, indicating increased catabolism. The levels of IGFBP-1 mRNA were significantly higher in fed and starved fish after 14 days compared to those at the start of the experiment, but no significant difference was observed between the treatments. In conclusion, we have shown that circulating ghrelin and ghrelin-1 mRNA is related to changes in energy metabolism in Atlantic salmon.

Perspectives on concordant and discordant relations between insulin-like growth factor 1 (IGF1) and growth in fishes

Many physiological processes are modulated by the endocrine system, including growth. Insulin-like growth factor 1 is one of the primary hormones involved in growth regulation in vertebrates, including fishes. Current work on IGF1 in fishes is driven both by a desire to better understand mechanisms of growth as well as to develop a reliable index of growth rate. A review of studies relating IGF1 to growth broadly reveals positive and significant relations between IGF1 and growth; however, relations found in individual studies range from no correlation to highly significant correlations. Potential sources for this variation include both biological and methodological issues and range from differences in how growth is defined (changes in length or weight), the duration of growth assessed (weeks to months) and how growth is calculated (total change, rate, percent change); yet, these methodological concerns cannot account for all the variation found. A further review of the literature reveals a number of physiological conditions and environmental factors that might influence IGF1 level and the subsequent relation of that IGF1 level to growth rate. The term concordance is introduced to categorize factors that influence IGF1 and growth in a similar fashion, such that positive and significant relations between IGF1 and growth are maintained even though the factor stimulates changes in IGF1 level. Conversely, the term discordance is introduced to categorize factors that stimulate changes in the relations between IGF1 and growth, such that IGF1 is not an efficacious index of growth for both pre and post-stimulus fish combined. IGF1 and growth relations generally remain concordant after changes in nutrition (consumption rate or diet). Differences in IGF1 level of juvenile, maturing male and maturing female fish are common and IGF1-growth relations appear discordant between these groups. Acute changes in temperature and salinity induce discordant relations between IGF1 and growth but acclimation to persistent differences in environmental condition generally result in concordant relations. Overall, by discriminating between fish of differing physiological status and discerning and categorizing differences among environments one may effectively use IGF1 as a growth index for fishes.

Ghrelin Receptor in Two Species of Anuran Amphibian, Bullfrog (Rana catesbeiana), and Japanese Tree Frog (Hyla japonica)

We have identified cDNA encoding a functional growth hormone secretagogue-receptor 1a (GHS-R1a, ghrelin receptor) in two species of anuran amphibian, the bullfrog (Rana catesbeiana), and the Japanese tree frog (Hyla japonica). Deduced receptor protein for bullfrog and Japanese tree frog (tree frog) was comprised of 374- and 371-amino acids, respectively. The two receptors shared 86% identity, and are grouped to the clade of the tetrapod homologs by phylogenetic analysis. In functional analyses, ghrelin and GHS-R1a agonists increased intracellular Ca(2+) concentration in GHS-R1a-transfected-HEK293 cell, but ligand selectivity of ghrelin with Ser(3) and Thr(3) was not observed between the two receptors. Bullfrog GHS-R1a mRNA was mainly expressed in the brain, stomach, and testis. In the brain, the gene expression was detected in the diencephalon and mesencephalon, but not in the pituitary. Tree frog GHS-R1a mRNA was predominantly expressed in the gastrointestinal tract and ovary, but not detected in the pituitary. In bullfrog stomach but not the brain, GHS-R1a mRNA expression increased after 10 days of fasting. For tree frog, GHS-R1a mRNA expression was increased in the brain, stomach and ventral skin by 10 days of fasting, and in the stomach and ventral skin by a dehydration treatment. Intracerebroventricular injection of ghrelin in dehydrated tree frog did not affect water absorption from the ventral skin. These results suggest that ghrelin is involved in energy homeostasis and possibly in osmoregulation in frogs.

Ghrelin decreases food intake in juvenile rainbow trout (Oncorhynchus mykiss) through the central anorexigenic corticotropin-releasing factor system

Ghrelin stimulates pituitary growth hormone (GH) release, and has a key role in the regulation of food intake and adiposity in vertebrates. To investigate the central effect of native rainbow trout ghrelin (rtghrelin) on food intake in rainbow trout, as well as its possible mode of action, four groups of fish received a single injection into the third brain ventricle (i.c.v. injection): (1) control group (physiological saline) (2) ghrelin-treated group (2.0 ng rtghrelin g bwt(-1)), (3) group given the corticotropin-releasing hormone receptor antagonist alpha-helical CRF 9-41 (ahCRF) (4.0 ng g bwt(-1)) and (4) group receiving the same dose of both ghrelin and ahCRF. Food intake was assessed 1h after treatment. In addition, the presence of the GHS-R (the ghrelin receptor) in the rainbow trout CNS was examined with Western blot. To investigate peripheral effects of ghrelin, rainbow trout received an intraperitoneal cholesterol-based implant with or without rtghrelin, and daily food intake was measured during 14 days. Weight and length were measured at the start and termination of the experiment and specific growth rates were calculated. Mesenteric fat stores, muscle and liver lipid content were analysed after the treatment period. Central ghrelin injections decreased food intake compared with controls, and treatment with ahCRF abolished the ghrelin-effect. Western blot analysis of the GHS-R revealed a single band at around 60 kDa in pituitary, hypothalamus, brain and stomach. Long-term peripheral ghrelin treatment decreased daily food intake compared with controls. This was reflected in a ghrelin-induced decrease in weight growth rate (p<0.06). There was no effect of ghrelin on plasma GH levels or tissue fat stores. The conclusion from this study is that the GHS-R is indicated in the CNS in rainbow trout and that ghrelin may act there as an anorexigenic hormone, through a CRF-mediated pathway. Elevated peripheral ghrelin levels also seem to lead to decreased feed intake in the longer term.

Glucose regulates ghrelin, neuropeptide Y, and the GH/IGF-I axis in the tilapia, Oreochromis mossambicus

In general, a fish's ability to clear glucose is sluggish in relation to mammals, which has lead to the idea that fish are glucose intolerant. It has been reported that circulating glucose levels do fluctuate in response to environmental challenges. Recent reports suggest that glucose may function as a metabolic signal regulating 'glucosensors' in the brain in fish, as has been reported in mammals. The current study was designed to investigate the effect of glucose on ghrelin and neuropeptide Y (NPY) signaling in the brain, and on the growth hormone/insulin-like growth factor-I (GH/IGF-I) in the tilapia, Oreochromis mossambicus. Glucose treatment significantly increased plasma and stomach mRNA levels of ghrelin. In the brain, mRNA levels of the ghrelin receptor (GRLN-R) were significantly reduced, whereas NPY mRNA levels were significantly elevated; suggesting that NPY containing neurons may be a "glucosensor" as reported in mammals. Glucose treatment resulted in changes in the GH/IGF-I axis. Liver mRNA levels of both GH receptors (GHR1 and GHR2) were significantly elevated, whereas liver IGF-I mRNA were unaltered by glucose treatment. No change in plasma or pituitary mRNA levels of GH was observed. Glucose significantly reduced plasma IGF-I levels. These data show that glucose regulates endocrine factors involved in appetite, growth, and possibly energy homeostasis, and suggests that glucose may be acting as a signal of metabolic status in fish.