Ghrelin in the summer flounder: Immunolocalization to the gastric glands and action on plasma cortisol levels

Molecular characterization and distribution of motilin and motilin receptor in the Japanese medaka Oryzias latipes

Motilin (MLN) is a peptide hormone originally isolated from the mucosa of the porcine intestine. Its orthologs have been identified in various vertebrates. Although MLN regulates gastrointestinal motility in tetrapods from amphibians to mammals, recent studies indicate that MLN is not involved in the regulation of isolated intestinal motility in zebrafish, at least in vitro. To determine the unknown function of MLN in teleosts, we examined the expression of MLN and the MLN receptor (MLNR) at the cellular level in Japanese medaka (Oryzias latipes). Quantitative PCR revealed that mln mRNA was limitedly expressed in the gut, whereas mlnr mRNA was not detected in the gut but was expressed in the brain and kidney. By in situ hybridization and immunohistochemistry, mlnr mRNA was detected in the dopaminergic neurons of the area postrema in the brain and the noradrenaline-producing cells in the interrenal gland of the kidney. Furthermore, we observed efferent projections of mlnr-expressing dopaminergic neurons in the lobus vagi (XL) and nucleus motorius nervi vagi (NXm) of the medulla oblongata by establishing a transgenic medaka expressing the enhanced green fluorescence protein driven by the mlnr promoter. The expression of dopamine receptor mRNAs in the XL and cholinergic neurons in NXm was confirmed by in situ hybridization. These results indicate novel sites of MLN activity other than the gastrointestinal tract. MLN may exert central and peripheral actions through the regulation of catecholamine release in medaka.

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.

Brain transcriptome profile after CRISPR-induced ghrelin mutations in zebrafish

Ghrelin (GRL) is a gut-brain hormone with a role in a wide variety of physiological functions in mammals and fish, which points out the ghrelinergic system as a key element for the appropriate biological functioning of the organism. However, many aspects of the multifunctional nature of GRL remain to be better explored, especially in fish. In this study, we used the CRISPR/Cas9 genome editing technique to generate F0 zebrafish in which the expression of grl is compromised. Then, we employed high-throughput mRNA sequencing (RNA-seq) to explore changes in the brain transcriptome landscape associated with the silencing of grl. The CRISPR/Cas9 technique successfully edited the genome of F0 zebrafish resulting in individuals with considerably lower levels of GRL mRNAs and protein and ghrelin O-acyl transferase (goat) mRNAs in the brain, intestine, and liver compared to wild-type (WT) zebrafish. Analysis of brain transcriptome revealed a total of 1360 differentially expressed genes (DEGs) between the grl knockdown (KD) and WT zebrafish, with 664 up- and 696 downregulated DEGs in the KD group. Functional enrichment analysis revealed that DEGs are highly enriched for terms related to morphogenesis, metabolism (especially of lipids), entrainment of circadian clocks, oxygen transport, apoptosis, and response to stimulus. The present study offers valuable information on the central genes and pathways implicated in functions of GRL, and points out the possible involvement of this peptide in some novel functions in fish, such as apoptosis and oxygen transport.

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.

Crosstalking between the "gut-brain" hormone ghrelin and the circadian system in the goldfish. Effects on clock gene expression and food anticipatory activity

Ghrelin is a potent orexigenic signal mainly synthesized in the stomach and foregut of vertebrates. Recent studies in rodents point out that ghrelin could also act as an input for the circadian system and/or as an output of peripheral food-entrainable oscillators, being involved in the food anticipatory activity (FAA). In this study we pursue the possible interaction of ghrelin with the circadian system in a teleost, the goldfish (Carassius auratus). First, we analyzed if ghrelin is able to modulate the core clock functioning by regulating clock gene expression in fish under a light/dark cycle 12L:12D and fed at 10 am. As expected the acute intraperitoneal (IP) injection of goldfish ghrelin (gGRL[1-19], 44 pmol/g bw) induced the expression of hypothalamic orexin. Moreover, ghrelin also induced (∼ 2-fold) some Per clock genes in hypothalamus and liver. This effect was partially counteracted in liver by the ghrelin antagonist ([D-Lys(3)]-GHRP-6, 100 pmol/g bw). Second, we investigated if ghrelin is involved in daily FAA rhythms. With this aim locomotor activity was studied in response to IP injections (5-10 days) of gGRL[1-19] and [D-Lys(3)]-GHRP-6 at the doses above indicated. Ghrelin and saline injected fish showed similar 24h activity patterns. However, ghrelin antagonist treatment abolished the FAA in schedule fed fish under 24h light, suggesting the involvement of the endogenous ghrelin system in this pre-feeding activity. Altogether these results suggest that ghrelin could be acting as an input for the entrainment of the food-entrainable oscillators in the circadian organization of goldfish.

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.

What is the general action of ghrelin for vertebrates? - comparisons of ghrelin's effects across vertebrates

Ten years and more passed since ghrelin was discovered. Various physiological actions of ghrelin have been documented in both mammalian and nonmammalian vertebrates. Do these actions have any commonality? In this review, we focused on several effects of ghrelin, and compared the effect across vertebrates. We would like to discuss possible general function of ghrelin in vertebrates.

Ghrelin effect on nutritional indices, midgut and fat body of Lymantria dispar L. (Lymantriidae)

Ghrelin is a 28-amino acid peptide that has significant effects on appetite and growth in humans and animals. The aim of this study was to examine 4th instar larvae of the pest insect Lymantria dispar L. after ghrelin treatment. Parameters included changes in nutritional indices (efficiency of conversion of ingested food, efficiency of conversion of digested food, approximate digestibility); midgut and fat body mass; total proteases, trypsin and leucine aminopeptidase activities in the midgut; number, height and width of columnar and goblet cells and their nuclei in the midgut epithelium and detection of ghrelin-like immunoreactivity in the midgut tissue. Four subpicomolar injections of ghrelin (0.3pmol) or physiological saline (control) were applied every 24h. The nutritional indices were higher in the ghrelin treated than in the control group. Ghrelin treatment was also associated with elevation of midgut mass, induced digestive enzyme activities, increased fat body mass and morphometric changes in columnar and goblet cells. This is the first report of the presence of ghrelin-like hormone in endocrine cells of an insect midgut. Such information provides additional evidence for application of this relatively simple model system in the future studies of the mechanisms underlying of digestion and energy balance in more complex organisms.

Nesfatin-1 suppresses energy intake, co-localises ghrelin in the brain and gut, and alters ghrelin, cholecystokinin and orexin mRNA expression in goldfish

Nesfatin-1 is a novel anorectic peptide encoded in the precursor protein nucleobindin-2 (NUCB2). We recently reported the presence and appetite suppressing effects of nesfatin-1 in goldfish. Nesfatin-1 has been co-localised with ghrelin in the stomach of rats. Whether nesfatin-1 influences other appetite regulatory peptides in goldfish remains unclear. The main objectives of the present study were to investigate whether nesfatin-1 co-localises ghrelin in goldfish, and to test whether exogenous nesfatin-1 influences endogenous ghrelin, cholecystokinin (CCK) and orexin A (OXA). We found co-localisation of nesfatin-1-like and ghrelin-like immunoreactivity in the enteroendocrine cells of the goldfish anterior intestine (J-loop). Furthermore, co-localisation of ghrelin and nesfatin-1 was also observed in the posterior nucleus lateralis tuberis of the goldfish hypothalamus, a brain region implicated in the regulation of food intake. These findings suggest a functional relationship between ghrelin and nesfatin-1 in goldfish. In support of this, i.c.v. administration of goldfish (gf) nesfatin-1 [25 ng/g body weight (BW)], suppressed food intake and the expression of mRNAs encoding preproghrelin, ghrelin receptor (GHS-R 1a-1), CCK and NUCB2 in the forebrain of fed fish, as well as ghrelin and NUCB2 mRNA in the hypothalamus of unfed fish, both at 1 h post-injection. Nesfatin-1 stimulated hypothalamic CCK mRNA expression at 30 min post-injection in fed fish, and inhibited OXA mRNA in the unfed fish hypothalamus 1 h post-injection. Similarly, i.c.v. injections of gfghrelin (1 ng/g BW), although stimulating food intake, suppressed NUCB2 and preproghrelin mRNAs, but not ghrelin receptor mRNA expression in the forebrain. It is also evident that exogenous ghrelin and nesfatin-1 mRNAs encoding these peptides. Our novel results indicate interactions between nesfatin-1 and ghrelin, CCK and orexin, and show that nesfatin-1 acts on other appetite regulatory peptides in a time- and feeding status-dependent, as well as tissue-specific, manner in goldfish.

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.