Different effects of peripheral and central ghrelin on regulation of food intake in the Japanese quail

Circulating profile of the appetite-regulating hormone ghrelin during moult-fast and chick provisioning in southern rockhopper penguins (Eudyptes chrysocome chrysocome)

A multitude of animal species undergo prolonged fasting events at regularly occurring life history stages. During such periods of food deprivation, individuals need to suppress their appetite. The satiety signalling gut hormone ghrelin has received much attention in this context in studies looking at mammalian systems. In wild birds, however, knowledge on the ghrelin system and its role during extended fasts is still scarce. In this study, we collected plasma samples for measurements of circulating ghrelin concentrations from adult southern rockhopper penguins (Eudyptes chrysocome chrysocome) during the three to four week-long moult-fast that they repeat annually to replace their feathers. We further sampled chicks before and after feeding bouts and non-moulting adults. Circulating ghrelin levels did not differ significantly between fed and unfed chicks but chicks had significantly lower plasma ghrelin levels compared to adults. Furthermore, penguins in late moult (i.e. individuals at the end of the prolonged fasting bout) had higher ghrelin levels compared to non-moulting adults. Our results show elevated levels of circulating ghrelin during moult and generally lower levels of ghrelin in chicks than in adults regardless of feeding state. Given the scarcity or absence of knowledge on the function of ghrelin in seabirds and in fasting birds in general, our results add greatly to our understanding of the avian ghrelin system.

Daily feeding frequency affects feed intake and body weight management of growing layers

The objective of this study was to investigate the effect of feeding behavior on feed intake and body weight in growing layers and the underlying mechanisms, thereby providing a scientific foundation for optimal feeding practices in growing layers' management. A total of 144 Hy-line brown growing layers of 10 wk old and similar body weight, were divided into 3 treatment groups with different feeding frequency and equal cumulative daily feeding amount: the once-a-day feeding group (F1) was fed at 9:00 am every day, with feeding amount of 150 g/layer; the twice-a-day feeding group (F2) were fed at 9:00 am and 13:00 pm every day, with each feeding amount of 75 g/layer; the 4 times-a-day feeding group (F4) were fed at 9:00 am, 11:00 am, 13:00 pm, and 15:00 pm every day, with each feeding amount of 37.5 g/layer. Pre-experiment lasted for 1 wk and formal experiment lasted for 8 wk. The results indicated that the daily feed intake and body weight were decreased (P < 0.05) while feed conversion ratio was not affected (P > 0.05) as daily feeding times increased. The glandular stomach proportion was significantly increased in twice-a-day feeding group, while liver proportion and ileum length were significantly increased in 4 times-feeding group (P < 0.05). Additionally, 4 times-feeding daily resulted in a significant elevation of blood glucose levels, which may have suppressed feed intake (P < 0.05). In 4 times-feeding group, the plasma triglyceride levels increased as feeding times, accompanied by a notable up-regulation in the mRNA level of appetite-suppressing gene, hypothalamic pro-opiomelanocortin (POMC) and glandular stomach ghrelin. This modulation effectively suppressed the subsequent feed intake and body weight. Therefore, 4 times feeding daily is recommended in growing layers' management, because it reduced the feed cost without affecting the feed conversion efficiency.

Ultrastructural study of neuronal cells and localization of ghrelin-like peptide and its receptor in the ganglia of the golden apple snail (Pomacea canaliculata)

Pomacea canaliculata is an invasive snail species causing major problems in agriculture. The snail biology was then investigated. The main objective of the present study was to investigate the nervous system of the snail. The nervous system comprises pairs of cerebral, buccal, pedal, pleural, parietal ganglia and an unpaired visceral ganglion. Most neurons were concentrated at the periphery of the ganglia. The neurons were classified into four types: NR1, NR2, NR3, and NR4. The percentages of the NR3 and NR4 in the pleural and pedal ganglia were significantly higher than those of other ganglia. Ultrastructural study revealed that nuclei of all neuronal types exhibited mostly euchromatins. Many organelles including ribosomes and endoplasmic reticulum were found in their cytoplasm. However, various mitochondria were found in the NR2 and NR3. The immunohistochemistry revealed immunoreactivity of ghrelin-like peptide in the neurons of the cerebral, pleural and pedal ganglia. However, immunoreactivity of GHS-R1a-like peptide existed only in the neurons of the pleural and pedal ganglia. The present study is the first to demonstrate the existence of ghrelin-like peptide and its receptor in P. canaliculata nervous system.

Effects of ghrelin on food caching behaviour and body composition in black-capped chickadees (Poecile atricapillus)

Several metabolic hormones signal an organism's energy balance to the brain and modulate feeding behaviours accordingly. These metabolic signals may also regulate other behaviour related to energy balance, such as food caching or hoarding. Ghrelin is one such hormone, but it appears to exert different effects on appetite and fat levels in birds and mammals. Ghrelin treatment inhibits food intake and decreases fat stores in some bird species, but these effects may differ between acylated and unacylated (des-acyl) forms of ghrelin. The effect of ghrelin on food caching in birds has been examined in only one study, that found both leptin and unacylated ghrelin reduced food caching and mass gain in coal tits (Periparus ater). We expanded on this to test how both forms of ghrelin affect food caching and body composition in black-capped chickadees (Poecile atricapillus). We injected each bird with acylated ghrelin, unacylated ghrelin, and a saline control and then measured food caching every 20 min for two hours post-injection. We also measured body mass fat levels the day before, and after treatment using quantitative magnetic resonance (QMR). Contrary to prior work, we found no effects of either form of ghrelin on food caching, or body or fat mass. Future work is required to determine if the difference between our results and those of the prior study stems from species differences in response to ghrelin and/or in the motivation to cache food, or ghrelin effects being modulated by energy reserves.

Low-Protein Diets Differentially Regulate Energy Balance during Thermoneutral and Heat Stress in Cobb Broiler Chicken (Gallus domesticus)

The objective was to assess whether low-protein (LP) diets regulate food intake (FI) and thermogenesis differently during thermoneutral (TN) and heat stress (HS) conditions. Two-hundred-day-old male broiler chicks were weight-matched and assigned to 36 pens with 5-6 chicks/pen. After 2 weeks of acclimation, birds were subjected into four groups (9 pens/group) including (1) a normal-protein diet under TN (ambient temperature), (2) an LP diet under TN, (3) a normal-protein diet under HS (35 °C for 7 h/day), and (4) an LP diet under HS, for 4 weeks. During HS, but not TN, LP tended to decrease FI, which might be associated with a lower mRNA abundance of duodenal ghrelin and higher GIP during HS. The LP group had a higher thermal radiation than NP under TN, but during HS, the LP group had a lower thermal radiation than NP. This was linked with higher a transcript of muscle β1AR and AMPKα1 during TN, but not HS. Further, LP increased the gene expression of COX IV during TN but reduced COX IV and the sirtuin 1 abundance during HS. The dietary protein content differentially impacted plasma metabolome during TN and HS with divergent changes in amino acids such as tyrosine and tryptophan. Compared to NP, LP had increased abundances of p_Tenericutes, c_Mollicutes, c_Mollicutes_RF9, and f_tachnospiraceae under HS. Overall, LP diets may mitigate the negative outcome of heat stress on the survivability of birds by reducing FI and heat production. The differential effect of an LP diet on energy balance during TN and HS is likely regulated by gut and skeletal muscle and alterations in plasma metabolites and cecal microbiota.

Update on Feeding Regulation by Ghrelin in Birds: Focused on Brain Network

Ghrelin is known to be a feeding stimulatory hormone in mammals, but in birds, in contrast to mammals, the feeding behavior is regulated in inhibitory manners. This is because the neuropeptides associated with the regulation in the brain are different from those in mammals, i.e., it has been shown that, in chickens, a corticotropin-releasing hormone family peptide, urocortin, which is a feeding-inhibitory peptide, is mainly involved in the inhibitory mechanism. However, feeding is also regulated by various neurotransmitters in the brain, and recently, their interaction with the mechanisms underlying feeding inhibition by ghrelin in birds has been intensively studied and clarified. This review summarizes these findings.

Applications of Enteroendocrine Cells (EECs) Hormone: Applicability on Feed Intake and Nutrient Absorption in Chickens

This review focuses on the role of hormones derived from enteroendocrine cells (EECs) on appetite and nutrient absorption in chickens. In response to nutrient intake, EECs release hormones that act on many organs and body systems, including the brain, gallbladder, and pancreas. Gut hormones released from EECs play a critical role in the regulation of feed intake and the absorption of nutrients such as glucose, protein, and fat following feed ingestion. We could hypothesize that EECs are essential for the regulation of appetite and nutrient absorption because the malfunction of EECs causes severe diarrhea and digestion problems. The importance of EEC hormones has been recognized, and many studies have been carried out to elucidate their mechanisms for many years in other species. However, there is a lack of research on the regulation of appetite and nutrient absorption by EEC hormones in chickens. This review suggests the potential significance of EEC hormones on growth and health in chickens under stress conditions induced by diseases and high temperature, etc., by providing in-depth knowledge of EEC hormones and mechanisms on how these hormones regulate appetite and nutrient absorption in other species.

Ghrelin, not corticosterone, is associated with transitioning of phenotypic states in a migratory Galliform

In both captive and free-living birds, the emergence of the migratory phenotype is signalled by rapid and marked increases in food intake and fuelling, as well as changes in amount of nocturnality or migratory restlessness. The metabolic hormone corticosterone and, as more recently suggested, the gut-derived hormone ghrelin have been suggested to play a role in mediating such phenomenal phenotypic flexibility given that they both regulate fuel metabolism and locomotion across vertebrate taxa. Here, using the Common quail (Coturnix coturnix) as our study species, we induced autumn migration followed by a non-migratory wintering phase through controlled changes in daylight. We thus compared plasma corticosterone and ghrelin concentrations between the two sampling phases and assessed whether these hormones might reflect the migratory state. While we found no differences in plasma corticosterone between the two sampling phases and no link of this hormone with changes in body mass, levels of food intake or migratory restlessness, the migratory birds had substantially higher levels of plasma ghrelin relative to the non-migratory birds. Furthermore, while ghrelin did not correlate with the gain in body mass over the entire pre-migratory fuelling phase (over an average of nine weeks preceding blood sampling), plasma ghrelin did positively correlate with the gain in body mass observed during the final fattening stages (over an average of three weeks preceding blood sampling). Again, variation in plasma ghrelin also reflected the amount of body mass depleted over both the long- and short-time frame as birds returned to their non-migratory baseline - lower levels of plasma ghrelin consistently correlated with larger losses in body mass. Thus, while our data do not highlight a role of the hormone corticosterone in sustaining pre-migratory fattening as shown in other bird species, they do add evidence for a potential role of ghrelin in mediating migratory behaviour and further suggest that this hormone might be important in regulating the transitioning of migratory states, possibly by promoting fuel mobilisation and usage.

Chicken LEAP2 Level Substantially Changes with Feed Intake and May Be Regulated by CDX4 in Small Intestine

Ghrelin O-acyltransferase (GOAT), ghrelin, and GHSR have been reported to play important roles that influence feed intake in mammals. LEAP2, an endogenous antagonist of GHSR, plays an important role in the regulation of feed intake. However, chicken ghrelin has also been reported to have an inhibitory effect on feed intake. The role of the GOAT-Ghrelin-GHSR-LEAP2 axis in chicken-feed intake remains unclear. Therefore, it is necessary to systematically evaluate the changes in the tissue expression levels of these genes under different energy states. In this study, broiler chicks in different energy states were subjected to starvation and feeding, and relevant gene expression levels were measured using quantitative real-time PCR. Different energy states significantly modulated the expression levels of LEAP2 and GHSR but did not significantly affect the expression levels of GOAT and ghrelin. A high expression level of LEAP2 was detected in the liver and the whole small intestine. Compared to the fed group, the fasted chicks showed significantly reduced LEAP2 expression levels in the liver and the small intestine; 2 h after being refed, the LEAP2 expression of the fasted chicks returned to the level of the fed group. Transcription factor prediction and results of a dual luciferase assay indicated that the transcription factor CDX4 binds to the LEAP2 promoter region and positively regulates its expression. High expression levels of GHSR were detected in the hypothalamus and pituitary. Moreover, we detected GHSR highly expressed in the jejunum-this finding has not been previously reported. Thus, GHSR may regulate intestinal motility, and this aspect needs further investigation. In conclusion, this study revealed the function of chicken LEAP2 as a potential feed-intake regulator and identified the potential mechanism governing its intestine-specific expression. Our study lays the foundations for future studies on avian feed-intake regulation.

Effect of chronic heat stress on gastrointestinal histology and expression of feed intake-regulatory hormones in broiler chickens

Heat stress (HS) dramatically impairs the growth performance of broiler chickens, mainly as a consequence of reduced feed intake due to the loss of appetite. This study was aimed at evaluating the alterations induced by chronic HS conditions on the morphological and morphometric features of the gastrointestinal (GI) tract and on the expression of some enteroendocrine cells (EECs) involved in the regulation of feed intake in chickens. Three hundred male chickens (Ross 308) were divided into two experimental groups and raised either in thermoneutral environment for the whole fattening period (0-41 days) (TNT group) or subjected to chronic HS conditions (30 °C for 24 h/day) from 35 to 41 days (HS group). Samples of proventriculus, duodenum, jejunum and cecum were collected from 24 broilers (12/group). Haematoxylin-eosin was used for the morphometric evaluations, while immunohistochemistry was applied for the evaluation of EECs expressing ghrelin (GHR), cholecystokinin (CCK), neuropeptide Y (NPY), glucagon-like peptide-1 (GLP-1), and serotonin (5-HT). In the proventriculus, HS reduced total wall thickness and mucous layer height (P ≤ 0.01) as well as mean diameter, circumference, and area of the compound tubular glands (P ≤ 0.001) with respect to TNT. The small intestine of HS birds was characterised by decreased villous height and total thickness (duodenum, P ≤ 0.01; jejunum, P ≤ 0.001), whereas crypt depth and width were reduced only in the jejunum (P ≤ 0.01). HS had negligible effects on the morphological aspects of the cecum. In the proventriculus, an increase in GHR and NPY EECs was observed in response to HS (P ≤ 0.001). Similarly, the small intestine villi of the HS group showed greater GLP-1 (P ≤ 0.05), 5-HT (P ≤ 0.001) and CCK (P ≤ 0.01) EECs. Moreover, the expression of 5-HT EECs was higher in the duodenal (P ≤ 0.01) and jejunal (P ≤ 0.01) crypts of HS birds, whereas GLP-1 and CCK EECs increased only in jejunal crypts (P ≤ 0.05). Finally, 5-HT EEC expression was increased in the cecum of HS group (P ≤ 0.01). In conclusion, these outcomes demonstrate that chronic HS induces morphometric alterations not only in the small intestine but also in a key organ such as the proventriculus. Furthermore, HS conditions affect the presence and distribution of EECs, suggesting that some GI peptides and biogenic amine may be implicated in the regulation of appetite and voluntary feed intake in heat-stressed broiler chickens.

Opioid receptor μ, not δ and κ modulate food intake induced by ghrelin in laying chickens

Evidence from animal studies suggests that opioidergic system and gherlin have a regulatory role in food intake, but their interaction(s) have not been studied in laying chickens. So in this study, 4 experiments (each included 4 groups) were designed. The first experiment was performed to evaluate the effect of ghrelin on the cumulative food intake. Experiments 2 to 4 were designed to investigate the possibility of μ, δ, or κ opioid receptors mediating ghrelin-induced hypophagia. All drugs were injected intracerebroventricularly (ICV) at 5 days of age. The results of this study showed that the ICV injection of 1.5 nmol ghrelin did not affect cumulative food intake. But ICV injection of ghrelin with doses of 3 and 6 nmol significantly reduced the cumulative food intake (p<0.05). However, co-injection of ghrelin with NTI, and nor-BNI did not show a significant change in decreased food intake compared to ghrelin. Also, opioid μ receptor gene expression significantly increased (p<0.05), but δ and κ opioid receptors gene expression did not significantly change. These results indicated the opioidergic system is involved in developing ghrelin-induced hypophagic effects in laying chickens. Accordingly, this effect of ghrelin to modify the nutritional behavior is possibly mediated by opioid μ receptor.

Ghrelin-Ghrelin receptor (GSHR) pathway via endocannabinoid signal affects the expression of NPY to promote the food intake of Siberian sturgeon (Acipenser baerii)

Previous data suggested that activation of endocannabinoid receptor 1 (CB1) was necessary for the orexigenic effect of Ghrelin in rodents, but the information is limited in teleosts. To investigate the feeding regulation pathway of Ghrelin and CB1 in Siberian sturgeon (Acipenser baerii), this study first identified the Ghrelin (345 bp, complete coding sequence) and Ghrelin receptor (GHSR, 500 bp, partial coding sequence) sequences, and then detected their tissue distribution patterns, which showed that Ghrelin is mainly distribution in peripheral tissues, while GSHR is mainly in different brain divisions. Besides, the qPCR before and after feeding showed that the mRNA expressions of Ghrelin and GHSR were inhibited after feeding in telencephalon, diencephalon and mesencephalon. Subsequently, the food intake and appetite factor expressions were measured by i.c.v. co-injection of Ghrelin and GSHR antagonist. The results showed that Ghrelin promoted the food intake of Siberian sturgeon, which was reversed by its receptor antagonist. Besides, i.c.v. injection of Ghrelin decreased telencephalon CART expression while increased NPY expression in the three brain regions. In addition, to further explore the relationship of Ghrelin and CB1 signal regulating feeding, the co-injection of Ghrelin and CB1 antagonists was performed. The results showed that AM6545 (CB1 peripheral restricted antagonist) failed to affect the orexigenic effect of Ghrelin and the expression pattern of NPY mRNA in the telencephalon. While in the diencephalon, the increase of food intake and NPY mRNA expression induced by Ghrelin was completely reversed by Rimonabant (CB1 global antagonist). These results indicate Ghrelin-GSHR pathway promotes the food intake of Siberian sturgeon by inducing the expression of NPY in the diencephalon, and the stimulating effect will be reversed by cannabinoid receptor antagonism. This study provides a foundation for understanding the pathways Ghrelin and CB1 signals in appetite regulation of the teleost.

Systemic infusion of exogenous ghrelin in male broiler chickens (Gallus gallus domesticus). The effect of pulse frequency, doses, and ghrelin forms on feed intake, average daily gain, corticosterone, and growth hormone concentrations

There is limited information on the effect of exogenous ghrelin infusion on feed intake (FI) in chickens. Therefore, male broilers were used in 3 factorial experiments to determine the relationships between doses (0, 1, or 4 nM; Dose), frequency (once every two h; 2 h), once every 4th h (4 h) or continuous infusion, and ghrelin forms including acylated-ghrelin (AG) and desacylated-ghrelin (DAG) on FI, ADG, and concentrations of corticosterone and Growth Hormone (GH). Treatments were delivered via a jugular cannula, using programmable pumps for 11 consecutive days. FI and ADG were recorded, and plasma was collected. Data were analyzed using a factorial design. In Experiment 1 the effect of AG pulse frequency and doses were evaluated. There was a linear decrease in FI (P = 0.002) and a linear increase in corticosterone (P = 0.033) and GH (P = 0.011) concentrations when AG was infused. However, ADG decreased with doses (P = 0.011) only when AG was given at 2 h. In Experiment 2 the effect of ghrelin forms and doses given at 2 h was evaluated. There was a linear decrease in FI when AG was infused and a linear increase in FI when DAG was infused (P < 0.05). Birds infused with DAG gained more weight than those infused with AG. There was a linear increase in corticosterone and GH concentrations only when AG was infused (P < 0.01). In Experiment 3 the effect of continuous infusion of 2 doses (0 and 1 nM) of AG and DAG were evaluated. There was a linear decrease in FI and ADG when AG (P < 0.001) was infused and a linear increase in FI and ADG when DAG was infused (P < 0.05). There was an increase in corticosterone concentrations only when AG was infused (P = 0.022). However, GH concentrations were not affected by treatments. We concluded that AG and DAG pulse frequency and doses had a differential effect on FI, ADG, corticosterone, and GH concentrations in broiler chickens.

Experimental ghrelin administration affects migratory behaviour in a songbird

Twice a year, billions of birds take on drastic physiological and behavioural changes to migrate between breeding and wintering areas. On migration, most passerine birds regularly stop over along the way to rest and refuel. Endogenous energy stores are not only the indispensable fuel to complete long distance flights, but are also important peripheral signals that once integrated in the brain modulate crucial behavioural decisions, such as the decision to resume migration after a stopover. A network of hormones signals metabolic fuel availability to the brain in vertebrates, including the recently discovered gut-hormone ghrelin. Here, we show that ghrelin takes part in the control of migratory behaviour during spring migration in a wild migratory passerine. We manipulated blood concentrations of ghrelin of 53 yellow-rumped warblers (Setophaga coronata coronata) caught during stopover and automatically radio-tracked their migratory behaviour following release. We found that injections of acylated and unacylated ghrelin rapidly induced movements away from the release site, indicating that the ghrelin system acts centrally to mediate stopover departure decisions. The effects of the hormone manipulation declined within 8 h following release, and did not affect the overall rate of migration. These results provide experimental evidence for a pivotal role of ghrelin in the modulation of behavioural decisions during migration. In addition, this study offers insights into the regulatory functions of metabolic hormones in the dialogue between gut and brain in birds.

Ghrelin is a signal to facilitate the utilization of fatty acids and save glucose by the liver, skeletal muscle, and adipose tissues in chicks

Ghrelin, classically known as a central appetite-stimulating hormone, has recently been recognized to play an important role in peripheral tissue energy metabolism. In chicken, contrary to mammal, ghrelin acts as an anorexia signal, increased by fasting and further elevated after refed. In the present study, the effect of ghrelin on glucose/lipid utilization by peripheral tissues was investigated. Injection of exogenous acyl ghrelin reduced plasma triglyceride and glucose levels of chickens at both fasting and fed status. In the in vitro cultured chicken primary hepatocytes, adipocytes, and myoblasts, ghrelin suppressed glucose uptake, stimulated fatty acids uptake and oxidation, and decreased TG content. In hepatocyte, ghrelin increased the activities of LPL and HL, and upregulated the expression levels of gene ACC, CPT1, and PPARα. Ghrelin treatment markedly increased the protein level of p-ACC, PPARγ, PGC1α, and CPT1 in hepatocytes, adipocytes and myoblasts. Inhibition of AMPK activity by Compound C had no influence on glucose uptake by hepatocyte, adipocyte, and myoblast, but further amplified the stimulated fatty acid uptake of adipocyte by ghrelin. The present result demonstrates that ghrelin facilitates the uptake and oxidation of fatty acid and cut down the utilization of glucose by the liver, muscle, and adipose tissues. The result suggests that ghrelin functions as a signal of fatty acid oxidation. The study provides a vital framework for understanding the intrinsic role of ghrelin as a crucial factor in the concerted regulation of metabolic substrate of hepatocytes, adipocytes, and myoblasts.

Effect of Cyclic Heat Stress on Feeding-Related Hypothalamic Neuropeptides of Three Broiler Populations and Their Ancestor Jungle Fowl

Heat stress (HS) has been increasingly jeopardizing the sustainability of the poultry production. Moreover, modern high-performing chickens are far less able to withstand HS than their predecessors due to higher growth rate and metabolic rates. Performance losses caused by HS are mainly ascribed to decreases in feed consumption. Since feed intake is tightly controlled by the hypothalamic centers of hunger and satiety, we sought to determine the effect of chronic cyclic HS on the expression of feeding-related hypothalamic neuropeptides (FRHN) in unselected chickens (i.e., the ancestor junglefowl-JF) and three broiler lines from diverse stages of genetic selection (i.e., the slow growing ACRB, the moderate growing 95RN, and the fast growing MRB). From 29 to 56 days, birds (n = 150 birds for each population) were subjected to either thermoneutral (TN, 25°C) or cyclic heat stress (HS, 36°C, 0900-1,800 h) conditions. Molecular data were analyzed by two-way ANOVA with interaction between the main factors, namely environmental temperature and line. The expression of major FHRN, like neuropeptide Y, agouti-related peptide, proopiomelanocortin, and cocaine and amphetamine regulated transcript remained unchanged. However, melanocortin receptor 1 exhibited a line-dependent decreasing trend from JF to MRB under both TN and HS (p = 0.09), adiponectin expression showed a distinct trend toward significance with 95RB exhibiting the highest mRNA level irrespective of the environmental temperature (p = 0.08), and JF had a greater mRNA abundance of visfatin than ACRB under TN (p < 0.05). The hypothalamic integration of circadian information, acclimation to long-lasting HS exposure, stable hypothalamic pathways unaffected by evolution and genetic selection, focus on mRNA abundances, and use of the entire hypothalamus masking gene expression in specific hypothalamic nuclei are all possible explanations for the lack of variations observed in this study. In conclusion, this is the first assessment of the impacts of heat stress on feeding-related hypothalamic neuropeptides of chicken, with a valuable and informative comparison between the ancestor junglefowl and three differently performing broiler lines.

Capromorelin, a ghrelin receptor agonist, increases feed intake and body weight gain in broiler chickens (Gallus gallus domesticus)

Ghrelin is a hormone that induces orexigenic effects in mammals. However, in avian species, there is scant and conflictive results on the effect of ghrelin on feed intake (FI). Therefore, we evaluated the effect of a ghrelin receptor agonist (capromorelin) on FI, ADG, water intake (WI), animal behavior and concentrations of ghrelin, glucose, growth hormone (GH) and insulin in broiler chickens. One-day-old male broilers were reared as recommended by the industry. At 4 wk of age (experimental day 0; D0), birds were blocked by weight and randomly assigned to 3 treatments in 2 identical trials. Control birds received a vehicle control solution containing 0 mg/kgBW/d of capromorelin. Birds in treatments 2 and 3 received capromorelin at target doses of 6 or 12 mg/kgBW/d of capromorelin (n = 27). FI and WI were measured 3 times a day at 0700 h (Period 1; P1), 1200 h (P2) and 1700 h (P3), while BW was recorded daily. Blood samples were collected on D-1 and D5. Bird behavior (pecking, sitting and standing) was evaluated for 9 h on D2. Data were analyzed using a randomized complete block design with repeated measures over time. Orthogonal polynomial contrasts were used to determine linear and quadratic effects of increasing levels of capromorelin. Polynomial contrasts showed that capromorelin doses linearly increased FI (P = 0.002) and ADG (P = 0.019). There were no treatment, day or treatment x d interactions on glucose, ghrelin and GH concentrations. However, there was a treatment x d interaction (P = 0.041) on insulin concentrations. Concentrations of insulin were higher on D5 for the 0 and 12 mg/kgBW/d treatments as compared with D-1. Polynomial contrasts showed that capromorelin doses linearly increased number of pecks/h (P = 0.018). Per hour FI and WI was higher during P1 (i.e., 0700-1200) as compared to P2 and P3 (P < 0.001). Our observations suggest that capromorelin linearly increases feed intake; thus, the same effect of that reported in mammalian 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.

Ghrelin serves as a signal of energy utilization and is involved in maintaining energy homeostasis in broilers

Ghrelin, one of the most important appetite regulating peptides, is involved in the regulation of energy homeostasis. The anorexia effect of ghrelin in chickens is contrary to that of ghrelin in mammals. In the present study, the effects of feeding status and dietary energy level on plasma total ghrelin levels and expression were studied in broilers. The gene expression of ghrelin and its receptor GHS-R1a were measured in the hypothalamus, proventriculus, duodenum, liver, and abdominal fat pad. The results showed that ghrelin mRNA and GHS-R1a mRNA are moderately expressed in liver and abdominal fat. Ghrelin secretion was increased by fasting and refeeding. The gene expression of ghrelin and GHS-R1a in the hypothalamus, proventriculus, liver, and abdominal fat pad were changed by feeding status and dietary energy level. The results suggest that ghrelin is a signal of energy utilization in chickens. The abundant expression of ghrelin and GHS-R1a in liver and abdominal fat pad may be associated with energy balance.

Exendin-4 antagonizes the metabolic action of acylated ghrelinergic signaling in the hypothalamic paraventricular nucleus

In the current study we investigated the interaction of hypothalamic paraventricular nucleus (PVN) glucagon-like peptide-1 (GLP-1) and ghrelin signaling in the control of metabolic function. We first demonstrated that acylated ghrelin injected directly into the PVN reliably altered the respiratory exchange ratio (RER) of adult male Sprague Dawley rats. All testing was carried out during the initial 2 h of the nocturnal cycle using an indirect open circuit calorimeter. Results indicated that acylated ghrelin induced a robust increase in RER representing a shift toward enhanced carbohydrate oxidation and reduced lipid utilization. In contrast, treatment with comparable dosing of des-acyl ghrelin failed to significantly impact metabolic activity. In separate groups of rats we subsequently investigated the ability of exendin-4 (Ex-4), a GLP-1 analogue, to alter acylated ghrelin's metabolic effects. Rodents were treated with either systemic or direct PVN Ex-4 followed by acyl ghrelin microinjection. While our results showed that both systemic and PVN administration of Ex-4 significantly reduced RER, importantly, Ex-4 pretreatment itself reliably inhibited the impact of ghrelin on RER. Overall, these findings provide increasingly compelling evidence that GLP-1 and ghrelin signaling interact in the neural control of metabolic function within the PVN.

Regulation of Gastrointestinal Motility by Motilin and Ghrelin in Vertebrates

The energy balance of vertebrates is regulated by the difference in energy input and energy expenditure. Generally, most vertebrates obtain their energy from nutrients of foods through the gastrointestinal (GI) tract. Therefore, food intake and following food digestion, including motility of the GI tract, secretion and absorption, are crucial physiological events for energy homeostasis. GI motility changes depending on feeding, and GI motility is divided into fasting (interdigestive) and postprandial (digestive) contraction patterns. GI motility is controlled by contractility of smooth muscles of the GI tract, extrinsic and intrinsic neurons (motor and sensory) and some hormones. In mammals, ghrelin (GHRL) and motilin (MLN) stimulate appetite and GI motility and contribute to the regulation of energy homeostasis. GHRL and MLN are produced in the mucosal layer of the stomach and upper small intestine, respectively. GHRL is a multifunctional peptide and is involved in glucose metabolism, endocrine/exocrine functions and cardiovascular and reproductive functions, in addition to feeding and GI motility in mammals. On the other hand, the action of MLN is restricted and species such as rodentia, including mice and rats, lack MLN peptide and its receptor. From a phylogenetic point of view, GHRL and its receptor GHS-R1a have been identified in various vertebrates, and their structural features and various physiological functions have been revealed. On the other hand, MLN or MLN-like peptide (MLN-LP) and its receptors have been found only in some fish, birds and mammals. Here, we review the actions of GHRL and MLN with a focus on contractility of the GI tract of species from fish to mammals.

Peripherally injected ghrelin and leptin reduce food hoarding and mass gain in the coal tit (Periparus ater)

In birds little is known about the hormonal signals that communicate nutritional state to the brain and regulate appetitive behaviours. In mammals, the peptide hormones ghrelin and leptin elevate and inhibit consumption and food hoarding, respectively. But in birds, administration of both ghrelin and leptin inhibit food consumption. The role of these hormones in the regulation of food hoarding in avian species has not been examined. To investigate this, we injected wild caught coal tits (Periparus ater) with leptin, high-dose ghrelin, low-dose ghrelin and a saline control in the laboratory. We then measured food hoarding and mass gain, as a proxy of food consumption, every 20 min for 2 h post-injection. Both high-dose ghrelin and leptin injections significantly reduced hoarding and mass gain compared with controls. Our results provide the first evidence that hoarding behaviour can be reduced by both leptin and ghrelin in a wild bird. These findings add to evidence that the hormonal control of food consumption and hoarding in avian species differs from that in mammals. Food hoarding and consumptive behaviours consistently show the same response to peripheral signals of nutritional state, suggesting that the hormonal regulation of food hoarding has evolved from the consumption regulatory system.

Immunohistochemical Characterization of PepT1 and Ghrelin in Gastrointestinal Tract of Zebrafish: Effects of Spirulina Vegetarian Diet on the Neuroendocrine System Cells After Alimentary Stress

Gastrointestinal function in vertebrates is influenced by stressors, such as fasting and refeeding, different types of diet and hormonal factors. The aim of this paper was to analyze the effect of a Spirulina (Arthrospira platensis) diet, a microalga known for its nutraceutical properties, on the gastrointestinal tract of zebrafish (Danio rerio) regarding expression of oligopeptide transporter 1 (PepT1) and ghrelin (GHR). Food deprivation and refeeding was investigated to elucidate expression of PepT1 and GHR at a gastrointestinal level and the zebrafish compensatory mechanism. PepT1 is responsible for absorbing di- and tripeptides through a brush border membrane of intestinal mucosa. GHR is a brain-gut peptide in fish and mammals, stimulating growth hormone secretion and regulating appetite. Samples were taken after 2 and 5 days of specimen fasting, and 2 and 5 days of refeeding with Sera Spirulina tabs, in which the major constituent is Spirulina sp. (50.2% protein). Morphological and immunohistochemical analysis of PepT1 and GHR were carried out. Control specimen intestinal tract showed normal morphology of the digestive tract. Fasting caused fold structural changes and intestinal lumen constriction. Immunohistochemical analysis showed a PepT1 level reduction after fasting and an increase after refeeding, reaching very high levels after 5 days, compared to controls. GHR levels increased after food deprivation and gradually decreased after refeeding. Increased expression of PepT1 in refeeding fish suggests a compensatory physiological mechanism, as does the increase in GHR levels in fasting fish followed by a reduction after refeeding. A compensatory mechanism may be induced by fasting and refeeding and by a higher protein Spirulina diet. The microalga, for its nutraceutical properties, is an excellent candidate for animal breeding and human diet.

Cues, corticosterone and departure decisions in a partial migrant

Most migrating birds make multiple stopovers to fuel and/or rest. The decision to resume migration from stopover is based on various cues, such as time within the season and wind conditions. There are hints that the strength of these departure cues shapes corticosterone level, which in its turn appears to regulate the timing of departure. We here provide results that very strongly indicate that indeed departure cues jointly shape corticosterone level of migrants at stopover. We compared corticosterone level between migrating and sedentary common blackbirds (Turdus merula) sampled simultaneously at the same location during autumn migration. As expected, in migrating individuals corticosterone level was positively associated with time within the season and with current wind conditions. The latter was only apparent in adult birds and not in 1st year migrants, thus matching the observation that 1st year autumnal migrants are less wind selective than adults. In contrast to the migrants, in sedentary blackbirds these "cues" did not explain variation in corticosterone level. Furthermore, stopover departure seemed more likely and to occur earlier in the night in migrants with high corticosterone level. Our unique comparative study thus supports the newly developed concept that corticosterone mediates between departure cues and stopover departure timing in avian migrants.

Ghrelin plasma concentration does not covary with energy demand in adult laying hens

The peptide hormone ghrelin is suggested to be involved in food intake regulation in young growing chicken. Whether ghrelin is involved in the regulation of energetic balance associated with laying performance in adult laying hens was studied by use of 4 chicken lines that differ in laying performance and phylogeny (4 lines; 16 hens per line). As housing conditions are also known to affect energy demand, half of the hens per line were housed in single cages and the other half of hens were maintained in a floor housing system. Plasma samples were collected at 17 to 19, 33 to 35, 49 to 51, and 72 wk of age and analyzed with a chicken ghrelin ELISA Kit. From caged hens, individual food consumption and laying performance additionally was recorded. Due to its function in growth and its relationship with ghrelin, also GH plasma concentrations were analyzed. Ghrelin concentrations did not differ between the 4 lines at any of the test periods (all P > 0.05). Ghrelin was negatively related to food consumption only in the growing period of the high-performing lines (both P < 0.0001). During this phase, floor-housed hens showed greater ghrelin concentrations compared with caged hens (P < 0.0001). Our results suggest that in adult layers ghrelin is not involved in regulating energy intake related to laying performance but rather seems to be related to body growth and housing condition before start of lay, the latter possibly due to differences in hens' behavioral activity.

Ghrelin enhances food intake and carbohydrate oxidation in a nitric oxide dependent manner

In the present study we sought to investigate interactions between hypothalamic nitric oxide (NO) and ghrelin signaling on food intake and energy substrate utilization as measured by the respiratory exchange ratio (RER). Guide cannulae were unilaterally implanted in either the arcuate (ArcN) or paraventricular (PVN) nuclei of male Sprague-Dawley rats. Animals were pretreated with subcutaneous (2.5-10mg/kg/ml) or central (0-100pmol) N-nitro-l-Arginine methyl ester (l-NAME) followed by 50pmol of ghrelin administered into either the ArcN or PVN. Both l-NAME and ghrelin were microinjected at the onset of the active cycle and food intake and RER were assessed 2h postinjection. RER was measured as the ratio of the volume of carbon dioxide expelled relative to the volume of oxygen consumed (VCO₂/VO₂) using an open-circuit indirect calorimeter. Our results demonstrated that peripheral and central l-NAME pretreatment dose-dependently attenuated ghrelin induced increases in food intake and RER in either the ArcN or PVN. In fact the 100pmol dose largely reversed the metabolic effects of ghrelin in both anatomical regions. These findings suggest that ghrelin enhancement of food intake and carbohydrate oxidation in the rat ArcN and PVN is NO-dependent.

Gut Hormones and Regulation of Food Intake in Birds

Gut hormones act as appetite regulatory hormones in mammals. For example, the hunger hormone ghrelin, which is released from the stomach before food intake, stimulates appetite. In contrast, satiety hormones such as cholecystokinin, glucagon-like peptide-1, and peptide YY, which are released from the intestines after food intake, suppress appetite. The effects of these peptides on food intake have been shown to be similar in both mammals and fishes. However, evidence suggests that the physiological roles of these gut hormones may be different between birds and other vertebrates. This review summarizes the current information on the roles of gut hormones in the regulation of food intake in birds, especially in chickens.

Ghrelin affects stopover decisions and food intake in a long-distance migrant

Billions of birds migrate long distances to either reach breeding areas or to spend the winter at more benign places. On migration, most passerines frequently stop over to rest and replenish their fuel reserves. To date, we know little regarding how they decide that they are ready to continue their journey. What physiological signals tell a bird's brain that its fuel reserves are sufficient to resume migration? A network of hormones regulates food intake and body mass in vertebrates, including the recently discovered peptide hormone, ghrelin. Here, we show that ghrelin reflects body condition and influences migratory behavior of wild birds. We measured ghrelin levels of wild garden warblers (Sylvia borin) captured at a stopover site. Further, we manipulated blood concentrations of ghrelin to test its effects on food intake and migratory restlessness. We found that acylated ghrelin concentrations of garden warblers with larger fat scores were higher than those of birds without fat stores. Further, injections of unacylated ghrelin decreased food intake and increased migratory restlessness. These results represent experimental evidence that appetite-regulating hormones control migratory behavior. Our study lays a milestone in migration physiology because it provides the missing link between ecologically dependent factors such as condition and timing of migration. In addition, it offers insights in the regulation of the hormonal system controlling food intake and energy stores in vertebrates, whose disruption causes eating disorders and obesity.

Neuropeptide Control of Feeding Behavior in Birds and Its Difference with Mammals

Feeding is an essential behavior for animals to sustain their lives. Over the past several decades, many neuropeptides that regulate feeding behavior have been identified in vertebrates. These neuropeptides are called “feeding regulatory neuropeptides.” There have been numerous studies on the role of feeding regulatory neuropeptides in vertebrates including birds. Some feeding regulatory neuropeptides show different effects on feeding behavior between birds and other vertebrates, particularly mammals. The difference is marked with orexigenic neuropeptides. For example, melanin-concentrating hormone, orexin, and motilin, which are regarded as orexigenic neuropeptides in mammals, have no effect on feeding behavior in birds. Furthermore, ghrelin and growth hormone-releasing hormone, which are also known as orexigenic neuropeptides in mammals, suppress feeding behavior in birds. Thus, it is likely that the feeding regulatory mechanism has changed during the evolution of vertebrates. This review summarizes the recent knowledge of peptidergic feeding regulatory factors in birds and discusses the difference in their action between birds and other vertebrates.

Is really endogenous ghrelin a hunger signal in chickens? Association of GHSR SNPs with increase appetite, growth traits, expression and serum level of GHRL, and GH

Chicken growth hormone secretagogue receptor (GHSR) is a receptor for ghrelin (GHRL), a peptide hormone produced by chicken proventriculus, which stimulates growth hormone (GH) release and food intake. The purpose of this study was to search for single nucleotide polymorphisms (SNPs) in exon 2 of GHSR gene and to analyze their effect on the appetite, growth traits and expression levels of GHSR, GHRL, and GH genes as well as serum levels of GH and GHRL in Mandara chicken. Two adjacent SNPs, A239G and G244A, were detected in exon 2 of GHSR gene. G244A SNP was non-synonymous mutation and led to replacement of lysine amino acid (aa) by arginine aa, while A239G SNP was synonymous mutation. The combined genotypes of A239G and G244A SNPs produced three haplotypes; GG/GG, GG/AG, AG/AG, which associated significantly (P<0.05) with growth traits (body weight, average daily gain, shank length, keel length, chest circumference) at age from >4 to 16w. Chickens with the homozygous GG/GG haplotype showed higher growth performance than other chickens. The two SNPs were also correlated with mRNA levels of GHSR and GH (in pituitary gland), and GHRL (in proventriculus and hypothalamus) as well as with serum level of GH and GHRL. Also, chickens with GG/GG haplotype showed higher mRNA and serum levels. This is the first study to demonstrate that SNPs in GHSR can increase appetite, growth traits, expression and level of GHRL, suggesting a hunger signal role for endogenous GHRL.

Comparison of the effects of human and chicken ghrelin on chicken ovarian hormone release

The aim of the present experiments was to examine the species-specific and cell-specific effects of ghrelin on chicken ovarian hormone release. For this purpose, we compared the effects of chicken and human ghrelin on the release of estradiol (E), testosterone (T), progesterone (P) and arginine-vasotocin (AVT) by cultured fragments of chicken ovarian follicles and on the release of T and AVT by cultured ovarian granulosa cells. In cultured chicken ovarian fragments, both human and chicken ghrelin promoted E release. T output was stimulated by chicken ghrelin but not by human ghrelin. No effect of either human or chicken ghrelin on P release was observed. Human ghrelin promoted but chicken ghrelin suppressed AVT release by chicken ovarian fragments. In cultured ovarian granulosa cells, human ghrelin inhibited while chicken ghrelin stimulated T release. Both human and chicken ghrelin suppressed AVT output by chicken granulosa cells. These data confirm the involvement of ghrelin in the control of ovarian secretory activity and demonstrate that the effect of ghrelin is species-specific. The similarity of avian ghrelin on avian ovarian granulosa cells and ovarian fragments (containing both granulosa and theca cells) suggests that ghrelin can influence chicken ovarian hormones primarily by acting on granulosa cells.

Localization of ghrelin-like peptide in the gastrointestinal tract of the golden apple snail (Pomacea canaliculata) and changing of its concentration during fasting

Ghrelin is an endogenous hormone detected in the gastrointestinal tracts (GI) of various species. In the present study, ghrelin-like peptide (ghrelin-LP) was identified in the GI tract of the golden apple snail, Pomacea canaliculata. Using immunohistochemistry, the result revealed an immunoreactivity (-ir) of ghrelin-LP in regions of the GI tract. The ghrelin-LP-ir was observed in both opened-type and closed-type cells of the esophagus, stomach, and small and large intestines. The highest density of ghrelin-LP immunoreactive cells was found in the esophagus and the least density was detected in the stomach. The highest percentages of the opened-type and closed-type cells were present in the esophagus and small intestine, respectively. In immunoblotting, the molecular weight of ghrelin-LP was related to the human ghrelin peptide (∼13kDa). Moreover, the concentration of ghrelin-LP was significantly higher in snails that were fasted for 24h compared with fed snails. The concentration decreased after refeeding. The present study could be useful for understanding the physiological role of ghrelin-LP in mollusk species.

The effect of peripheral administration of ghrelin on the performance of growing geese

Abstract. The purpose of the present study is to investigate the effect of intraperitoneal (IP) injection of ghrelin on goose performance. Hence, forty-eight 28-day-old geese were assigned into three treatments which lasted 40 days. The first intact group included no injection; that is, treatment 1 was characterized by G0; treatment 2, given to the second intact group, was characterized by G50, 50 ng kg−1 ghrelin body weight (BW); and treatment 3, given to the third intact group, was characterized by G100, 100 ng kg−1 ghrelin BW. Ghrelin was injected at the outset of the experimental rearing period (28-day-old birds). Blood samples were taken at two different times: (1) 12 h after the injection and (2) at the end of the rearing period. The effects of the injections were examined and evaluated during two rearing periods (28–48 days old and 48–68 days old). In the second treatment (G50), ghrelin injection caused an increase in the feed intake during the growing period but not the finishing period. Body weight gain and feed conversion ratio (FCR) of the growing period and finishing period did not change following any ghrelin treatments (P > 0.05). Injection of ghrelin at G100 increased breast muscle (pectoral) weight. The results of the present study indicate that ghrelin has a significant impact on feeding regulation and muscle growth at a certain period in geese. Nevertheless, it should be noted that ghrelin may have different effects on feeding of avian species.

Tissue distribution and developmental changes of ghrelin and GOAT expression in broiler chickens during embryogenesis

Ghrelin plays important roles, such as regulating growth hormone release and energy metabolism, but little is known about its developmental changes in the proventriculi of chicken embryos. This study was designed to elucidate the distributions and developmental changes of ghrelin and ghrelin-O-acyltransferase (GOAT) expression in broiler embryos using qRT-PCR and immunohistochemistry. Our results demonstrated the following: (1) on E18, ghrelin and GOAT are ubiquitously expressed in every tissue examined. The expression level of ghrelin mRNA was the highest in the proventriculus, reaching a level that was 50-fold higher than that in the hypothalamus, while GOAT mRNA expression was low in the proventriculus and it was only 67.6% as high as that of hypothalamus; (2) ghrelin and GOAT mRNA expression were detected in the proventriculus on E9, but only at 1.9% and 1.7% of the level expressed on E18, respectively, and their expression levels increased rapidly from E18 to E21. There was similar developmental pattern in the ghrelin and GOAT mRNA expression; and (3) ghrelin-immunopositive cells were first detected in the proventriculus on E15, were located only in the compound tubular glands of the proventriculus, and were of the closed-cell type. The density of ghrelin-immunopositive cells increased significantly from E15 to E21. These results suggest that ghrelin may be an important regulating factor that plays a vital role during the development of chicken embryos.

Ghrelin-induced hypophagia is mediated by the β2 adrenergic receptor in chicken

The purpose of this study was to examine the effects of intracerebroventricular injection of metoprolol (a β1 adrenergic receptor antagonist), ICI 118,551 (a β2 adrenergic receptor antagonist), and SR 59230R (a β3 adrenergic receptor antagonist) on ghrelin-induced food and water intake by 3-h food-deprived (FD3) cockerels. The chickens were randomly allocated to 4 treatment groups with 8 replicates in each group. A cannula was surgically implanted into the lateral ventricle of the brain. In experiment 1, chickens received the β1 adrenergic receptor antagonist (24 nmol) before injection of the ghrelin (0.6 nmol). In experiment 2, chickens received the β2 adrenergic receptor antagonist (5 nmol) before injection of the ghrelin (0.6 nmol). In experiment 3, birds were injected with ghrelin (0.6 nmol) after the β3 adrenergic receptor antagonist (20 nmol). Cumulative food and water intake were recorded 3-h post injection and analyzed by two-way analysis of variance. According to the results, ghrelin injection reduced food and water intake by broiler cockerels (p≤0.05). The effect of ghrelin on food intake was significantly attenuated by pretreatment with the β2 receptor antagonist (p≤0.05). Furthermore, the β2 receptor antagonist had no effect on water intake induced by ghrelin. Also, pretreatment with the β1 and β3 receptors antagonists had no effect on ghrelin-induced food and water intake. These results suggest that the effect of ghrelin on cumulative food intake by cockerels is mediated via β2 adrenergic receptors.

Corticosterone predicts nocturnal restlessness in a long-distance migrant

The decision made by migrating birds to stop refueling and to depart from stopover depends on cues from innate rhythms, intrinsic factors such as fuel reserves, and extrinsic factors such as weather conditions. The physiological mechanism behind this decision, however, is largely unexplored. The transition from refueling to flight involves an increase in both locomotion and energetic demands. Because, at baseline levels, corticosterone stimulates locomotion and is involved in the mobilization of energy, this hormone could encourage departure of migrants. We collected field data on baseline corticosterone, migratory restlessness, and actual departure in northern wheatears at stopover. Additionally, in refueling long-term captive conspecifics, we measured migratory restlessness while simultaneously collecting droppings to determine glucocorticoid metabolite (GCM) levels. We found that migratory restlessness at stopover was positively correlated with corticosterone level. Similarly, in refueling long-term captive birds, migratory restlessness was positively correlated with concurrently measured GCM levels in droppings. To our best knowledge, our study is the first to simultaneously measure a hormonal signal and migratory restlessness. In conclusion, our results are consistent with the hypothesis that, by increasing locomotor activity, baseline corticosterone is involved in the regulation of departure of migrants at stopover. Future studies could reveal how corticosterone is up-regulated in migrants that are ready to depart.

Corticosterone, food intake and refueling in a long-distance migrant

Elevated baseline corticosterone levels function to mobilize energy in predictable life-history stages, such as bird migration. At the same time, baseline corticosterone has a permissive effect on the accumulation of fat stores (fueling) needed for migratory flight. Most migrants alternate flight bouts with stopovers, during which they replenish the fuel used during the preceding flight (refueling). The role of corticosterone in refueling is currently unclear. In a fasting-re-feeding experiment on northern wheatears (Oenanthe oenanthe) in autumn we found that baseline total and free corticosterone levels were negatively related with both food intake and the rate of fuel deposition after fasting. This confirms our earlier findings in wild conspecifics in spring and indicates that corticosterone does not stimulate stopover refueling. Whether the negative relationship between baseline corticosterone level and fuel deposition rate is causal is questionable, because within-individual comparison of corticosterone metabolite levels in droppings did not reveal differences between refueling and control periods. In other words, corticosterone does not appear to be down-regulated during refueling, which would be expected if it directly hampers refueling. We discuss possible correlates of corticosterone level that may explain the negative association between corticosterone and stopover refueling. Additionally, we found that fasting decreases total corticosterone level, which contrasts with previous studies. We propose that the difference is due to the other studies being conducted outside of the migration life-history stage, and provide a possible explanation for the decrease in corticosterone during fasting in migrating birds.

Central injection of urocortin-3 but not corticotrophin-releasing hormone influences the ghrelin/GHS-R1a system of the proventriculus and brain in chicks

Ghrelin, the endogenous ligand for growth hormone secretagogue receptor 1a (GHS-R1a), stimulates food intake in mammals centrally and peripherally. In contrast, central injection of ghrelin inhibits feeding in neonatal chicks (Gallus gallus), which is thought to be mediated by the corticotrophin-releasing hormone (CRH) system, indicating that the mechanisms underlying ghrelin's action are different in chicks and mammals. However, the interaction between the ghrelin system and the CRH system has not been fully clarified in chicks. In the present study, we examined the effect of intracerebroventricular (ICV) injection of CRH and urocortin-3 (UCN-3), a CRH family peptide and an endogenous ligand for the CRH type-2 receptor (CRH-R2), on synthesis and secretion of ghrelin in chicks. Intracerebroventricular injection of UCN-3 but not CRH increased plasma ghrelin concentration (P < 0.05), diencephalic mRNA expression of ghrelin, and GHS-R1a (P < 0.05) and tended to decrease ghrelin (P = 0.08) and GHS-R1a (P = 0.10) mRNA expression in the proventriculus. Moreover, ICV injection of UCN-3 tended to increase diencephalic mRNA expression of CRH-R2 (P = 0.08) and CRH had no effect on it. In addition, ICV injection of CRH but not UCN-3 increased plasma corticosterone concentration (P < 0.05) and decreased the diencephalic mRNA expression of CRH-R1 (P < 0.05). These results clearly indicate that the roles of the CRH system for the ghrelin system are divided. The present study suggests that UCN-3 is mainly involved in the ghrelin system in chicks perhaps through the CRH-R2.

Contributions of endocrinology to the migration life history of birds

Migration is a key life cycle stage in nearly 2000 species of birds and is a greatly appreciated phenomenon in both cultural and academic arenas. Despite a long research tradition concerning many aspects of migration, investigations of hormonal contributions to migratory physiology and behavior are more limited and represent a comparatively young research field. We review advances in our understanding of the hormonal mechanisms of migration with particular emphasis on the sub-stages of the migration life history: development, departure, flight and arrival. These sub-stages vary widely in their behavioral, ecological and physiological contexts and, as such, should be given appropriate individual consideration.

Update on ghrelin biology in birds

Ghrelin is a peptide found in the mucosal layer of the rat stomach that exhibits growth hormone-releasing and appetite-stimulating activities. Since the discovery of ghrelin in chicken in 2002, information on its structure, distribution, function, and receptors has been accumulated, mainly in poultry. Here, we summarize the following findings since 2008 in birds: (1) central ghrelin acts as an anorexigenic neuropeptide, but the effect of peripheral ghrelin differs depending on the chicken strain and light conditions the birds are kept in; (2) central ghrelin inhibits not only food intake but also water drinking, and it may be mediated by urocortin, a member of the corticotropin-releasing factor family; (3) peripheral ghrelin acts as an anti-lipogenic factor in broiler chickens but not in rats; (4) the enzyme involved in ghrelin acylation (ghrelin-O-acyltransferase [GOAT]) has been identified in chickens; (5) dietary lipids are used for ghrelin acylation; (6) des-acyl ghrelin administered alone or with ghrelin does not affect feeding behavior; (7) the existence and physiological function of obestatin must now be carefully examined in birds; (8) other than the growth hormone secretagogue receptors (GHS) R1a and 1b, GHS-R variants not found in mammals have been found in chicken and Japanese quail; and finally (9) little is known about the involvement of the ghrelin system in wild birds and in avian-specific behavior such as brooding and migration.

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.