Effect of central administration of prolactin-releasing peptide on feeding in chicks

The role of central neurotransmitters in appetite regulation of broilers and layers: similarities and differences

The importance of feeding as a vital physiological function, on the one hand, and the spread of complications induced by its disorder in humans and animals, on the other hand, have led to extensive research on its regulatory factors. Unfortunately, despite many studies focused on appetite, only limited experiments have been conducted on avian, and our knowledge of this species is scant. Considering this, the purpose of this review article is to examine the role of central neurotransmitters in regulating food consumption in broilers and layers and highlight the similarities and differences between these two strains. The methodology of this review study includes a comprehensive search of relevant literature on the topic using appropriate keywords in reliable electronic databases. Based on the findings, the central effect of most neurotransmitters on the feeding of broilers and laying chickens was similar, but in some cases, such as dopamine, ghrelin, nitric oxide, and agouti-related peptide, differences were observed. Also, the lack of conducting a study on the role of some neurotransmitters in one of the bird strains made it impossible to make an exact comparison. Finally, it seems that although there are general similarities in appetite regulatory mechanisms in meat and egg-type chickens, the long-term genetic selection appropriate to breeding goals (meat or egg production) has caused differences in the effect of some neurotransmitters. Undoubtedly, conducting future studies while completing the missing links can lead to a comprehensive understanding of this process and its manipulation according to the breeding purposes of chickens.

Avian Neuropeptide Y: Beyond Feed Intake Regulation

Neuropeptide Y (NPY) is one of the most abundant and ubiquitously expressed neuropeptides in both the central and peripheral nervous systems, and its regulatory effects on feed intake and appetite- have been extensively studied in a wide variety of animals, including mammalian and non-mammalian species. Indeed, NPY has been shown to be involved in the regulation of feed intake and energy homeostasis by exerting stimulatory effects on appetite and feeding behavior in several species including chickens, rabbits, rats and mouse. More recent studies have shown that this neuropeptide and its receptors are expressed in various peripheral tissues, including the thyroid, heart, spleen, adrenal glands, white adipose tissue, muscle and bone. Although well researched centrally, studies investigating the distribution and function of peripherally expressed NPY in avian (non-mammalian vertebrates) species are very limited. Thus, peripherally expressed NPY merits more consideration and further in-depth exploration to fully elucidate its functions, especially in non-mammalian species. The aim of the current review is to provide an integrated synopsis of both centrally and peripherally expressed NPY, with a special focus on the distribution and function of the latter.

Heat Stress Responses in Birds: A Review of the Neural Components

Heat stress is one of the major environmental conditions causing significant losses in the poultry industry and having negative impacts on the world's food economy. Heat exposure causes several physiological impairments in birds, including oxidative stress, weight loss, immunosuppression, and dysregulated metabolism. Collectively, these lead not only to decreased production in the meat industry, but also decreases in the number of eggs laid by 20%, and overall loss due to mortality during housing and transit. Mitigation techniques have been discussed in depth, and include changes in air flow and dietary composition, improved building insulation, use of air cooling in livestock buildings (fogging systems, evaporation panels), and genetic alterations. Most commonly observed during heat exposure are reduced food intake and an increase in the stress response. However, very little has been explored regarding heat exposure, food intake and stress, and how the neural circuitry responsible for sensing temperatures mediate these responses. That thermoregulation, food intake, and the stress response are primarily mediated by the hypothalamus make it reasonable to assume that it is the central hub at which these systems interact and coordinately regulate downstream changes in metabolism. Thus, this review discusses the neural circuitry in birds associated with thermoregulation, food intake, and stress response at the level of the hypothalamus, with a focus on how these systems might interact in the presence of heat exposure.

Neuropeptide Y modifies a part of diencephalic catecholamine but not indolamine metabolism in chicks depending on feeding status

The role of the monoaminergic system in the feeding behavior of neonatal chicks has been reported, but the functional relationship between the metabolism of monoamines and appetite-related neuropeptides is still unclear. This study aimed to investigate the changes in catecholamine and indolamine metabolism in response to the central action of neuropeptide Y (NPY) in different feeding statuses and the underlying mechanisms. In Experiment 1, the diencephalic concentrations of amino acids and monoamines following the intracerebroventricular (ICV) injection of NPY (375 pmol/10 μl/chick), saline solution under ad libitum, and fasting conditions for 30 min were determined. Central NPY significantly decreased L-tyrosine concentration, the precursor of catecholamines under feeding condition, but not under fasting condition. Central NPY significantly increased dopamine metabolites, including 3,4-dihydroxyphenylacetic acid and homovanillic acid (HVA). The concentration of 3-methoxy-4-hydroxyphenylglycol was significantly reduced under feeding condition, but did not change under fasting condition by NPY. However, no effects of NPY on indolamine metabolism were found in either feeding status. Therefore, the mechanism of action of catecholamines with central NPY under feeding condition was elucidated in Experiment 2. Central NPY significantly attenuated diencephalic gene expression of catecholaminergic synthetic enzymes, such as tyrosine hydroxylase, L-aromatic amino acid decarboxylase, and GTP cyclohydrolase I after 30 min of feeding. In Experiment 3, co-injection of α-methyl-L-tyrosine, an inhibitor of tyrosine hydroxylase with NPY, moderately attenuated the orexigenic effect of NPY, accompanied by a significant positive correlation between food intake and HVA levels. In Experiment 4, there was a significant interaction between NPY and clorgyline, an inhibitor of monoamine oxidase A with ICV co-injection which implies that co-existence of NPY and clorgyline enhances the orexigenic effect of NPY. In conclusion, central NPY modifies a part of catecholamine metabolism, which is illustrated by the involvement of dopamine transmission and metabolism under feeding but not fasting conditions.

The anorexigenic effect of neuropeptide AF in Japanese quail, Coturnix japonica, is associated with activation of the melanocortin system

Neuropeptide AF (NPAF) decreases food and water intake in birds and food intake in mammals. In this study, the objective was to determine the effects of centrally administered NPAF on food and water intake, hypothalamic c-Fos immunoreactivity and hypothalamic mRNA abundance of appetite-regulating factors in Japanese quail (Coturnix japonica). Seven days post hatch, 6 h fasted quail were intracerebroventricularly (ICV) injected with 0 (vehicle), 4, 8, or 16 nmol of NPAF and food and water intake were measured at 30 min intervals for 180 min. In Experiment 1, chicks which received 4, 8, and 16 nmol ICV NPAF had reduced food intake for 120, 60 and 180 min following injection, respectively, and reduced water intake during the entire 180 min observation. In Experiment 2, there was increased c-Fos immunoreactivity in the paraventricular nucleus, the ventromedial nucleus of the hypothalamus, and the dorsomedial hypothalamic nucleus in NPAF-injected quail. In Experiment 3, ICV NPAF was associated with decreased corticotropin-releasing factor mRNA, and an increase in hypothalamic proopiomelanocortin and melanocortin receptor 4 mRNA. These results demonstrate that central NPAF suppresses food and water intake in quail, effects that are likely mediated via the melanocortin system in the hypothalamus.

Effect of the administration of prolactin-releasing peptide2 on feeding activity in the intertidal blenny Rhabdoblennius nitidus (Günther, 1861)

Prolactin-releasing peptide2 (PrRP2) was administered intraperitoneally to male intertidal blenny Rhabdoblennius nitidus, a species with male uniparental care of eggs, to investigate the effect on their feeding activity. A significant inhibitory effect on appetite was observed in the breeding season, but not in the nonbreeding season. These results suggest that PrRP2 and PrRP2 receptors are more active during the breeding season. The presence of a mechanism to inhibit feeding activity while parents take care of their offspring may be important for the success of parental care.

Prolactin-releasing peptide increases food intake and affects hypothalamic physiology in Japanese quail (Coturnix japonica)

Prolactin-releasing peptide (PrRP) increases food intake in birds, whereas it is a potent satiety factor in rodents and fish. The aim of this study was to determine the effects of central injection of PrRP on feeding behaviors and hypothalamic physiology in juvenile Japanese quail (Coturnix japonica). Intracerebroventricular injection of 1,692 pmol of PrRP increased food intake for the first 90 min after injection but did not affect water intake. Quail treated with PrRP displayed more food and drink pecks, less time standing but more perching, and decreased defecations. Prolactin-releasing peptide-injected quail had increased c-Fos immunoreactivity in the dorsomedial nucleus (DMN) and arcuate nucleus (ARC) of the hypothalamus. Hypothalamic neuropeptide Y receptor subtypes 2 and 5 and melanocortin receptor 4 mRNAs were greater in PrRP- than vehicle-injected quail. In the DMN, there was less corticotropin-releasing factor (CRF) mRNA and in the ARC, more CRF mRNA in PrRP- than vehicle-injected chicks. Thus, PrRP increases food intake in quail, which is associated with changes in hypothalamic CRF and neuropeptide Y receptor gene expression and c-Fos-immunolabeled cells in the ARC and DMN.

Prolactin-Releasing Peptide: Physiological and Pharmacological Properties

Prolactin-releasing peptide (PrRP) belongs to the large RF-amide neuropeptide family with a conserved Arg-Phe-amide motif at the C-terminus. PrRP plays a main role in the regulation of food intake and energy expenditure. This review focuses not only on the physiological functions of PrRP, but also on its pharmacological properties and the actions of its G-protein coupled receptor, GPR10. Special attention is paid to structure-activity relationship studies on PrRP and its analogs as well as to their effect on different physiological functions, mainly their anorexigenic and neuroprotective features and the regulation of the cardiovascular system, pain, and stress. Additionally, the therapeutic potential of this peptide and its analogs is explored.

Identification of hypothalamic genes in associating with food intake during incubation behavior in domestic chicken

The molecular mechanism underlying in the onset and maintenance of incubation behavior are not fully understood, and it is still unknown the reason why White Leghorn, a layer strain, hens never display incubation behavior. Therefore, to explore specific hypothalamic genes regulating incubation behavior, cap analysis of gene expression (CAGE) were applied to comparison between incubating Silkie and laying White Leghorn hens. In addition, mRNA expression of some differentially expressed genes (DEGs) and melanocortinergic appetite genes including agouti-related peptide (AgRP) and pro-opiomelanocortin (POMC) was also analyzed on Silkie hens under natural anorexia and starvation. The CAGE identified 217 hypothalamic DEGs in incubating Silkie hens, and that of two, transthyretin (TTR) and prolactin-releasing peptide (PrRP), suggested as appetite gene, were markedly up- and down-regulated in incubating hens, respectively. In addition, AgRP and POMC expression also increased in incubating bird. mRNA expression of TTR, PrRP, and appetite genes were not differed significantly by starvation, although TTR mRNA expression was relatively high in fasting hens. Consequently, transcriptome by CAGE identified a number of hypothalamic genes differentially expressed by incubation behavior in Silkie hens. Of these, it is suggested that TTR and PrRP may, at least in part, be related to adaptation to natural anorexia in incubating Silkie chickens.

Central NPY-Y5 sub-receptor partially functions as a mediator of NPY-induced hypothermia and affords thermotolerance in heat-exposed fasted chicks

Exposure of chicks to a high ambient temperature (HT) has previously been shown to increase neuropeptide Y (NPY) mRNA expression in the brain. Furthermore, it was found that NPY has anti‐stress functions in heat‐exposed fasted chicks. The aim of the study was to reveal the role of central administration of NPY on thermotolerance ability and the induction of heat‐shock protein (HSP) and NPY sub‐receptors (NPYSRs) in fasted chicks with the contribution of plasma metabolite changes. Six‐ or seven‐day‐old chicks were centrally injected with 0 or 375 pmol of NPY and exposed to either HT (35 ± 1°C) or control thermoneutral temperature (CT: 30 ± 1°C) for 60 min while fasted. NPY reduced body temperature under both CT and HT. NPY enhanced the brain mRNA expression of HSP‐70 and ‐90, as well as of NPYSRs‐Y5, ‐Y6, and ‐Y7, but not ‐Y1, ‐Y2, and ‐Y4, under CT and HT. A coinjection of an NPYSR‐Y5 antagonist (CGP71683) and NPY (375 pmol) attenuated the NPY‐induced hypothermia. Furthermore, central NPY decreased plasma glucose and triacylglycerol under CT and HT and kept plasma corticosterone and epinephrine lower under HT. NPY increased plasma taurine and anserine concentrations. In conclusion, brain NPYSR‐Y5 partially afforded protective thermotolerance in heat‐exposed fasted chicks. The NPY‐mediated reduction in plasma glucose and stress hormone levels and the increase in free amino acids in plasma further suggest that NPY might potentially play a role in minimizing heat stress in fasted chicks.

Central administration of neuropeptide Y differentially regulates monoamines and corticosterone in heat-exposed fed and fasted chicks

Recently, we demonstrated that brain neuropeptide Y (NPY) mRNA expression was increased in heat exposed chicks. However, the functions of brain NPY during heat stress are unknown. This study was conducted to investigate whether centrally administered NPY affects food intake, rectal temperature, monoamines, stress hormones and plasma metabolites in chicks under high ambient temperatures (HT). Five or six-day-old chicks were centrally injected with 0, 188 or 375pmol of NPY and exposed to either HT (35±1°C) or a control thermoneutral temperature (CT; 30±1°C) for 3h whilst fed or fasted. NPY increased food intake under both CT and HT. NPY reduced rectal temperature 1 and 2h after central administration under CT, but not under HT. Interestingly, NPY decreased brain serotonin and norepinephrine concentrations in fed chicks, but increased concentrations of brain dopamine and its metabolites in fasted and fed chicks, respectively. Plasma epinephrine was decreased by NPY in fed chicks, but plasma concentrations of norepinephrine and epinephrine were increased significantly by NPY in fasted-heat exposed chicks. Furthermore, NPY significantly reduced plasma corticosterone concentrations in fasted chicks. Plasma glucose and triacylglycerol were increased by NPY in fed chicks, but triacylglycerol declined in fasted NPY-injected chicks. In conclusion, brain NPY may attenuate the reduction of food intake during heat stress and the increased brain NPY might be a potential regulator of the monoamines and corticosterone to modulate stress response in heat-exposed chicks.

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.

Exogenous prolactin-releasing peptide's orexigenic effect is associated with hypothalamic neuropeptide Y in chicks

Exogenous administration of prolactin-releasing peptide (PrRP) exerts anorexigenic effects in rats while causing orexigenic effects in chicks. While the central mechanism mediating PrRP's effect on food intake in rodents is somewhat understood, in chicks information is lacking. Therefore, this study was designed to elucidate the hypothalamic mechanism of PrRP induction of hunger perception in chicks. Chicks that received intracerebroventricular (ICV) injections of PrRP dose-dependently increased their food intake with no effect on water intake or whole blood glucose concentration. The threshold of food intake stimulation was as low as 3pmol, thus as compared to other neuropeptides PrRP is exceptionally potent. The mRNA abundance of several appetite-associated neuropeptide genes was quantified and hypothalamic neuropeptide Y (NPY) mRNA was increased in PrRP-injected chicks. Therefore, the orexigenic effects of PrRP may be associated with increased NPY-ergic tone. These results provide insight into the evolutionary aspects of appetite regulation during the course of divergent evolution of mammals and birds.

Beta-cell-tropin is associated with short-term stimulation of food intake in chicks

Beta-cell-tropin, a peptide derived from adrenocorticotropic hormone, is an insulin secretagogue. When centrally injected, it increases food intake in rats, but its appetite-associated effects have not been reported in any other species. Thus, the present study was designed to evaluate the effects of central beta-cell-tropin on appetite-associated parameters in an alternative vertebrate model, the chick. Central injection of 2 or 4 nmol beta-cell-tropin increased food intake for 60 min. Whole hypothalamus was collected at 60 min post-injection, and real-time PCR performed to measure mRNA abundance of agouti-related peptide, corticotropin releasing factor, galanin, melanin concentrating hormone, neuropeptide Y, orexin, prohormone convertase 2, pro-opiomelanocortin, peroxisome proliferator-activated receptor γ, urotensin 2, and visfatin, not one of which were affected by beta-cell-tropin treatment. Results demonstrate that beta-cell-tropin is associated with short-term stimulation of food intake.

Dietary Macronutrient Composition Affects the Influence of Exogenous Prolactin-Releasing Peptide on Appetite Responses and Hypothalamic Gene Expression in Chickens

BACKGROUND

The interaction between the effects of exogenous neurotransmitters and dietary composition on appetite regulation in nonmammalian species is unclear.

OBJECTIVE

The objective of this study was to determine the effects of exogenous prolactin-releasing peptide (PrRP) and dietary macronutrient composition on food intake regulation in broiler chicks.

METHODS

Three isocaloric diets were formulated: high-carbohydrate (HC), high-fat (HF; 60% of ME from lard) and high-protein (HP) diets. In Expt. 1, 4-d-old Hubbard × Cobb-500 chicks fed 1 of the 3 diets since hatch were intracerebroventricularly injected with 0 (vehicle), 3, or 188 pmol PrRP (n = 10). Food intake was measured for 180 min. In Expt. 2, hypothalamic mRNA abundance of appetite-associated factors was measured in hypothalamus samples obtained 1 h postinjection of 0 or 188 pmol PrRP. In Expt. 3, chicks were given free access to all diets before and after intracerebroventricular injection and food intake was measured.

RESULTS

Three and 188 pmol PrRP increased (P = 0.0008 and 0.04) HP diet intake, but only 188 pmol PrRP was efficacious at increasing HC (P = 0.0011) and HF (P = 0.01) consumption compared with the vehicle. There was a diet effect on mRNA abundance of all genes (P < 0.05), with greater expression in chicks fed the HF or HP than the HC diet. Whereas neuropeptide Y (NPY) mRNA was similar between vehicle- and PrRP-injected chicks that consumed HP or HF diets, expression was greater (P < 0.05) in PrRP- than vehicle-injected chicks that consumed the HC diet. When chicks had access to all diets, 188 pmol PrRP caused preferential (P < 0.0001) intake of the HP over the HC and HF diets.

CONCLUSION

The HP diet enhanced the sensitivity of chicks to the food intake-stimulating effects of PrRP, and PrRP in turn increased preference for the HP diet. Thus, dietary macronutrient composition influences PrRP-mediated food intake, and PrRP in turn affects nutrient intake and transcriptional regulation in chicks.

A novel role for xenopsin: Stimulation of food intake

Xenopsin (XPN), an extract from frog skin, is comprised of 80 amino acids and exerts effects on the mammalian digestive tract. The purpose of the study presented here was to determine if XPN would affect food intake using chicks as models. Chicks which had been fasted for 180 min did not change food or water intake after central injection of XPN. However, ab libitum fed chicks which received 1 and 3 nmol central XPN increased food intake while water intake was not affected. When the dose was increased to 9 nmol chicks did not increase food intake but their water intake was reduced suggesting malaise. Chicks injected with XPN had increased c-Fos immunoreactivity in the lateral hypothalamus, but other hypothalamic appetite-associated nuclei were not affected. When XPN was directly injected into the lateral hypothalamus food intake was increased, suggesting a primary site of action. When the expression of appetite-associated neuropeptide mRNA was quantified chicks injected with XPN had increased proopiomelanocortin mRNA. Lastly, a comprehensive behavior analysis was performed and while XPN injected chicks had an increase in the number of feeding pecks, jumping, preening, deep rest and sitting were all decreased. Thus, we conclude that exogenous XPN functions as an orexigenic factor in chicks and its effects are mediated by the lateral hypothalamus.

Functions of two distinct "prolactin-releasing peptides" evolved from a common ancestral gene

Prolactin-releasing peptide (PrRP) is one of the RF-amide peptides and was originally identified in the bovine hypothalamus as a stimulator of prolactin (PRL) release. Independently, another RF-amide peptide was found in Japanese crucian carp and named Carassius-RFa (C-RFa), which shows high homology to PrRP and stimulates PRL secretion in teleost fish. Therefore, C-RFa has been recognized as fish PrRP. However, recent work has revealed that PrRP and C-RFa in non-mammalian vertebrates are encoded by separate genes originated through duplication of an ancestral gene. Indeed, both PrRP and C-RFa are suggested to exist in teleost, amphibian, reptile, and avian species. Therefore, we propose that non-mammalian PrRP (C-RFa) be renamed PrRP2. Despite a common evolutionary origin, PrRP2 appears to be a physiological regulator of PRL, whereas this is not a consistent role for PrRP itself. Further work revealed that the biological functions of PrRP and PrRP2 are not limited solely to PRL release, because they are also neuromodulators of several hypothalamus-pituitary axes and are involved in some brain circuits related to the regulation of food intake, stress, and cardiovascular functions. However, these actions appear to be different among vertebrates. For example, central injection of PrRP inhibits feeding behavior in rodents and teleosts, while it stimulates it in chicks. Therefore, both PrRP and PrRP2 have acquired diverse actions through evolution. In this review, we integrate the burgeoning information of structures, expression profiles, and multiple biological actions of PrRP in higher vertebrates, as well as those of PrRP2 in non-mammals.

The actions of neuropeptide SF on the hypothalamic-pituitary-adrenal axis and behavior in rats

Present experiments focused on measuring the effect of neuropeptide SF (NPSF) on the hypothalamus-pituitary-adrenal (HPA) axis and behavior. The peptide was administered in different doses (0.25, 0.5, 1, 2 μg) intracerebroventricularly to rats, and the behavior of which was then observed by telemetry and open-field test. Effect of NPSF on core temperature was also measured via telemetry. Plasma ACTH and corticosterone concentrations were measured to assess the influence of NPSF on the HPA activation. In addition, the changes in corticotrophin-releasing hormone (CRH) level in the hypothalamic paraventricular nucleus were continuously monitored by means of intracerebral microdialysis. Our results showed that NPSF augmented paraventricular CRH release and increased ACTH and corticosterone levels in the plasma. The release of corticosterone was successfully blocked by the pre-treatment of the CRH antagonist α-helical CRH9-41. Spontaneous and exploratory locomotor activity was also stimulated according to the telemetric and open-field studies. However, NPSF only tended to alter stereotyped behavior in the open-field experiments. These results demonstrate that NPSF may play a physiologic role in the regulation of such circadian functions as the activity of motor centers and the HPA axis, through the release of CRH.

Central administration of prolactin-releasing peptide shifts the utilities of metabolic fuels from carbohydrate to lipids in chicks

We have recently identified prolactin (PRL)-releasing peptides (PrRPs) and their stimulating effects on feeding behavior in chicks. To investigate further metabolic functions of PrRP, the present study was performed to clarify whether intracerebroventricular (ICV) injection of PrRP31, an active form of PrRP in chicks, affects heat production (HP), respiratory quotient (RQ) and plasma concentrations of metabolic fuels in chicks. The ICV injection of PrRP31 (94 and 375 pmol) did not affect HP but significantly lowered RQ. The change in RQ implies that PrRP31 shifted the utility of metabolic fuels in the body. This idea was confirmed by subsequent results in which ICV injection of PrRP31 significantly reduced glucose but increased non-esterified fatty acid concentrations in plasma. These shifts in blood metabolic fuels would not be through the increased plasma insulin, because the ICV injection of PrRP31 significantly decreased plasma insulin concentration. On the other hand, ICV injection of another orexigenic peptide, neuropeptide Y (NPY) also induced the insulin release and the metabolic effects were similar to those of PrRP31. Because ICV injection of PrRP31 increased NPY mRNA in the diencephalon, the NPY may mediate the metabolic functions of PrRP31. In summary, the present study suggests that central PrRP31 shifts the utilities of peripheral energy sources, which is not via hyperinsulinemia but via the diencephalon.

Isolation and characterisation of prolactin-releasing peptide in chicks and its effect on prolactin release and feeding behaviour

Prolactin (PRL)-releasing peptides (PrRP) have been identified in mammals, amphibians and fishes, and these animals have several PrRPs that consist of different numbers of amino acids such as 20, 31 and 37. In the present study, we identified the cDNA encoding chicken prepro-PrRP, which can generate putative PrRPs, and cloned and sequenced it. Sequences for the coding region suggested the occurrence of putative PrRPs of 20, 31 and 32 amino acid residues. The amino acid sequence of chicken PrRP20 showed 100%, 95% and 70% identity with those of PrRP20s from teleosts, Xenopus laevis and mammals, respectively. On the other hand, chicken PrRP31 showed approximately 90% and 52-55% homology to PrRP31s of X. laevis and mammals, respectively. Native chicken PrRPs were purified from an acid extract of chick brain by a Sep-Pak C18 cartridge (Waters Corp., Milford, MA, USA), affinity chromatography using anti-salmon PrRP serum, and reverse phase high-performance liquid chromatography (HPLC) on an ODS-120T column (TOSOH, Tokyo, Japan). The existence of chicken PrRP20 and PrRP31 in the brain was demonstrated by comparing them with the synthetic peptides using HPLC and matrix-assisted laser desorption/ionisation time-of-flight mass spectrometry. Chicken PrRP31 increased plasma PRL concentration when administered peripherally, whereas central administration decreased the concentration, suggesting that chicken PrRP31 has a distinct effect on PRL secretion between tissues in chicks. On the other hand, plasma growth hormone concentration decreased with both peripheral and central administrations of chicken PrRP31. Furthermore, central administration of chicken PrRP31 increased food intake in chicks compared to those observed in mammals and fishes. Taken together with the results indicating that chicken PrRP20 did not show endocrine and behavioural effects, we showed that chicken PrRP has a similar amino acid sequence to teleosts, Xenopus laevis and mammals, although the actions were variable among vertebrates.

Metabolic and stress-related roles of prolactin-releasing peptide

In the modern world, improvements in human health can be offset by unhealthy lifestyle factors, including the deleterious consequences of stress and obesity. For energy homeostasis, humoral factors and neural afferents from the gastrointestinal tract, in combination with long-term nutritional signals, communicate information to the brain to regulate energy intake and expenditure. Energy homeostasis and stress interact with each other, and stress affects both food intake and energy expenditure. Prolactin-releasing peptide, synthesized in discrete neuronal populations in the hypothalamus and brainstem, plays an important role in integrating these responses. This review describes how prolactin-releasing peptide neurons receive information concerning both internal metabolic states and environmental conditions, and play a key role in energy homeostasis and stress responses.

LPLRFamide causes anorexigenic effects in broiler chicks and Bobwhite quail

Although LPLRFamide was the first member of the RFamide family to be isolated from a vertebrate species, its effects on hunger and satiety-related processes are poorly documented. Thus, we intracerebroventricularly administered LPLRFamide (3.0-15.0 nmol) to both Cobb-500 (a broiler type of Gallus gallus) and Bobwhite quail (Colinus virginianus) chicks and measured their food intake. The threshold of anorexigenic response was 7.0 nmol in Cobb-500 chicks and the effect had diminished by 30 min post-injection. In Bobwhite quail all doses of LPLRFamide tested caused anorexia that remained throughout the 60 min observation period. A comprehensive behavior analysis was conducted and Cobb-500 chicks had increased food pecks early in the observation period and spent a greater amount of time in deep rest. Although food pecks were increased pecking efficiency was decreased. In Bobwhite quail, feeding pecks and the number of jumps were reduced after LPLRFamide treatment. We judged that these behaviors in both species were likely not competitive with ingestion and thus did not secondarily contribute to anorexia. These results demonstrate that LPLRFamide is associated with satiety-related processes in Cobb-500 chicks and Bobwhite quail, while threshold of responses are different.

Current knowledge of the roles of ghrelin in regulating food intake and energy balance in birds

A decade has passed since the peptide hormone ghrelin was first discovered in rat stomach. During this period, ghrelin has been identified not only in other mammals but also in fish, amphibians, reptiles and birds, and its physiological functions have been widely investigated. Avian ghrelin was first identified in chickens in 2002 and to date, the amino acid sequences of six different avian ghrelin peptides have been reported. In mammals, ghrelin is the only known gut-derived hormone to stimulate food intake when administered centrally or peripherally. In studies on chickens and quail, however, ghrelin inhibits food intake when injected centrally, while the effects on feeding behavior elicited by ghrelin injected peripherally are equivocal. This review summarizes what is currently known about the regulation of food intake and energy balance by ghrelin in birds.

The anorectic effect of neuropeptide AF is associated with satiety-related hypothalamic nuclei

Neuropeptide AF (NPAF), a member of the RFamide family, is encoded by the same gene as neuropeptide FF (NPFF), which causes short-term anorexia. However, reports on the role of NPAF on appetite-related process are lacking. Thus, i.c.v. injections of 4.0, 8.0 and 16.0 nmol NPAF were administered to chicks to observe its effect on food and water intake. Chicks treated with 8.0 and 16.0 nmol i.c.v. NPAF decreased both their food and water intake. Additionally, all doses of NPAF injected caused a similar reduction in whole blood glucose concentration 180 min after injection. In a second experiment, chicks that received i.c.v. NPAF had an increased number of c-Fos immunoreactive cells in the dorsomedial, paraventricular (magnocellular and parvicellular parts) and ventromedial nuclei. The arcuate nucleus and lateral hypothalamic area were not affected. In a third experiment, NPAF-treated chicks exhibited fewer feeding pecks and spent less time perching, whereas they spent an increased time in deep rest. Other behaviours, including exploratory pecking, escape attempts, defecations, distance moved, and time spent standing, sitting and preening, were not affected by NPAF injection. We conclude that NPAF causes anorectic effects that are associated with the hypothalamus.

Role of adrenergic alpha-2-receptors on feeding behavior in layer-type chicks

The present study was designed to investigate the role of brain adrenergic alpha-2-receptors on feeding regulation of layer-type chicks. Intracerebroventricular injection of the adrenergic alpha-2-receptor agonist, clonidine, stimulated food intake. This effect was blocked by co-injection of the alpha-2-receptor antagonist, yohimbine, demonstrating that the alpha-2-receptor is related to stimulation of feeding in layer-type chicks. Stimulation of food intake caused by neuropeptide Y and beta-endorphin was attenuated by co-injection with yohimbine. However, yohimbine did not block prolactin-releasing peptide stimulation of food intake. It is therefore likely that brain adrenergic alpha-2-receptors mediate the orexigenic effects of neuropeptide Y and beta-endorphin in layer-type chicks.

Central administration of somatostatin stimulates feeding behavior in chicks

The purpose of the present study was to determine if central administration of somatostatin influences feeding behavior in layer chicks. Five- to 7-day-old chicks that received intracerebroventricular (ICV) injections of 0.5 or 2 nmol somatostatin increased their food intake at 30 and 60 min after the injection, suggesting that central somatostatin serves as an orexigenic neuropeptide in chicks. This hypothesis was further supported since chicks ICV injected with 0.5 or 2 nmol cortistatin, which binds to somatostatin receptors, also had increased food intake at the same time. Somatostatin-associated feeding behavior was attenuated by co-administration of 20 nmol beta-funaltrexamine (an opioidergic mu-receptor antagonist) (to 31% of the orexigenic effect of somatostatin at 60 min after the injection) but not ICI-174,864 or nor-binaltorphimine (antagonists of opioidergic delta- and kappa-receptors, respectively). Co-administration of 13 nmol yohimbine, an adrenergic alpha-2 receptor antagonist, also attenuated the orexigenic effect of somatostatin (to 31% of the orexigenic effect of somatostatin at 60 min after the injection). These results suggest that somatostatin-associated feeding behavior is mediated by opioidergic mu- and adrenergic alpha-2-receptors in chicks.

Anoretic effects of neuropeptide FF are mediated via central mu and kappa subtypes of opioid receptors and receptor ligands

Recently, we demonstrated that neuropeptide FF (NPFF) causes anorexigenic effects in chicks that were associated with the hypothalamus. The present study was designed to better understand some of the central mechanisms that mediate these effects. Co-injection of NPFF and beta-funaltrexamine (FNA, a mu opioid receptor antagonist) did not suppress food intake more than when NPFF and FNA were injected alone. However, co-injection of NPFF and ICI-174,864 (ICI, a delta opioid receptor antagonist) caused a greater reduction in food intake than when NPFF and ICI were injected alone. Co-injection of NPFF and nor-binaltorphimine (BNI, a kappa opioid receptor antagonist) did not cause a greater suppression of food intake than when NPFF and BNI were injected alone. Hyperphagia induced by neuropeptide Y and beta-endorphin (both ligands of opioid receptors) was reversed by NPFF. These results suggest that NPFF-induced satiety has a relationship with mu and kappa but not delta subtypes of opioid receptors, and since NPFF does not bind opioid receptors itself NPFF-associated satiety is likely mediated by effects on opioid receptor ligands such as NPY and beta-endorphin. Thus, NPFF induced satiety may be mediated via modulation of the chick's innate opioid-associated orexigenic system.

Short-term anorexigenic effects of central neuropeptide VF are associated with hypothalamic changes in chicks

The present study was designed to measure food and water intake, changes in hypothalamic chemistry, and other behaviour modifications after central injection of neuropeptide (NP) VF in broiler type chicks. In Experiment 1, chicks responded to central NPVF with a reduction in food intake for up to 90 min post injection. Water intake was unaffected. In Experiment 2, NPVF exerted a less potent and shorter duration of attenuated food intake than did the structurally related NPFF. In Experiment 3, 16.0 nmol NPVF reversed the prolactin-releasing peptide induced orexigenic effect. In Experiment 4, central NPVF treatment was associated with decreased c-Fos immunoreactivity in the lateral hypothalamus, whereas c-Fos immunoreactivity in the dorsomedial nucleus, infundibular nucleus (homologue to the mammalian arcuate nucleus) and ventromedial nucleus was increased. In Experiment 5, behaviours unrelated to ingestion including sit, stand, deep rest and locomotion were affected by central NPVF injection. Some of these behaviours are incompatible with ingestion and may contribute to hypothalamic associated perception of satiety after central NPVF. In conclusion, NVPF is a short-term regulator of appetite and its effects are associated with hypothalamic and behaviour changes in chicks.

Prolactin-releasing peptide

Prolactin-releasing peptide (PrRP) was initially isolated from the bovine hypothalamus as an activating component that stimulated arachidonic acid release from cells stably expressing the orphan G protein-coupled receptor hGR3 (Hinuma et al. 1998) [also known as GPR10 (Marchese et al. 1995), or UHR-1 for the rat orthologue (Welch et al. 1995)]. Initially touted as a prolactin-releasing factor (therefore aptly named prolactin-releasing peptide), the perspective on the function of this peptide in the organism has been greatly expanded. Over 120 papers have been published on this subject since its initial discovery in 1998. Herein I review the state of knowledge of the PrRP system, its putative function in the organism, and implications for therapy.

Type-dependent differential expression of neuropeptide Y in chicken hypothalamus (Gallus domesticus)

Neuropeptide Y (NPY) is one of the most important orexigenic agents in central regulation of feeding behavior, body weight and energy homeostasis in domestic chickens. To examine differences in the hypothalamic NPY between layer-type and meat-type of chickens, which are two divergent kinds of the domestic chickens in feeding behavior and body weight, we detected mRNA levels of NPY in hypothalamic infundibular nucleus (IN), paraventricular nucleus (PVN) and lateral hypothalamic area (LHA) of these two types of chickens using one-step real time RT-PCR. The meat-type chicken had more food daily (about 1.7 folds) and greater body weights (about 1.5 folds) and brain weights than the layer-type chicken at the age of 14 d. In the meat-type of chicken, NPY mRNA levels of the IN and PVN were significantly greater than those of the LHA, and were not significantly different between the IN and PVN. However, in the layer-type of chicken, NPY mRNA levels were significantly greater in the IN than those in the LHA and PVN, and were not significantly different between the PVN and LHA. In all these hypothalamic regions, the layer-type of chicken had significantly higher NPY mRNA levels than the meat-type chicken did. These results suggest the expression of NPY in the hypothalamus has a type-dependent pattern in domestic chickens.

Prolactin-releasing peptide, food intake, and hydromineral balance in goldfish

A potential role for prolactin-releasing peptide (PrRP) in appetite regulation and hydromineral balance in goldfish was examined. PrRP was found to be expressed in discrete regions of the goldfish brain, in particular, the hypothalamus. Intraperitoneal (IP) or intracerebroventricular administration of PrRP had dose-dependent effects to suppress food intake in goldfish. Hypothalamic PrRP mRNA expression significantly increased after feeding, as well as after 7 days of food deprivation. Refeeding fish after 7 days food deprivation did not result in a postprandial increase in PrRP mRNA expression. These data suggest an anorexigenic role for PrRP in the short term around a scheduled meal time, but not over the longer term. IP injection of PrRP significantly increased pituitary prolactin (PRL) mRNA levels, suggesting involvement in the regulation of lactotroph activity. Acclimating goldfish to an ion-poor environment decreased serum osmolality and increased PrRP and PRL mRNA levels, providing evidence for PrRP involvement in hydromineral balance through its actions on lactotrophs. Acclimation to ion-poor water diminished the anorexigenic properties of PrRP in goldfish, indicating that a role for PrRP in goldfish satiation is counterbalanced by alternate systemic needs (i.e., osmoregulatory). This was further supported by an ability to reinstate the anorexigenic actions of PrRP in fish acclimated to ion-poor water by feeding a salt-rich diet. These studies provide evidence that PrRP is involved in regulating appetite and hydromineral balance in fish, and that the degree of involvement in either process varies according to overall systemic needs in response to environmental conditions.

Intracerebroventricular injection of neuropeptide Y modifies carbohydrate and lipid metabolism in chicks

The purpose of the present study was to investigate whether intracerebroventricular (ICV) injection of neuropeptide Y (NPY) affects heat production (HP), body temperature, and plasma concentrations of metabolic fuels in chicks. ICV injection of NPY (0, 188 or 375 pmol) did not affect HP, but significantly lowered respiratory quotient as well as the rectal temperature. These data suggest that the energy sources for HP were modified by NPY in the body. This idea was confirmed by subsequent experiments in which ICV injection of NPY significantly reduced plasma glucose and triacylglycerol concentrations but increased non-esterified fatty acid concentrations. The effect of NPY on the utilization of metabolic fuels was not associated changes in plasma catecholamine and corticosterone concentrations. In summary, the present study demonstrated that central NPY modifies peripheral carbohydrate and lipid metabolism in chicks.

Comparison of mammalian prolactin-releasing peptide and Carassius RFamide for feeding behavior and prolactin secretion in chicks

Prolactin-releasing peptide (PrRP) was named for its originally reported effects as a prolactin (PRL) secretagogue in mammals. Carassius RFamide (C-RFa) is an orthologous PRL secretagogue in fishes and a gene encoding a 20-amino acid peptide of identical sequence is present in the chicken. These facts suggest that C-RFa is a putative chicken PrRP. However, no information is available for the physiological effects of C-RFa in chickens. Therefore, in the present study, we compared the effect of intracerebroventricular (ICV) injection of C-RFa and mammalian PrRP (mPrRP) on feeding behavior and plasma PRL, growth hormone (GH), and corticosterone (CORT) concentrations. ICV injection of C-RFa did not affect feeding behavior of chicks while mPrRP was stimulatory. The injection of C-RFa also did not significantly affect plasma PRL, GH, and CORT concentrations. In contrast, ICV injection of mPrRP exerted similar effects to those reported in mammals by increasing plasma CORT and decreasing GH concentrations. Additionally, the peptide induced an unexpected inhibitory effect on plasma PRL concentrations. Overall, these data suggest that an as yet unidentified peptide that shares some functional similarities with mPrRP is present in birds, but that the physiological role of the avian 20-amino acid C-RFa peptide remains to be determined.

Gonadotropin-inhibiting hormone stimulates feeding behavior in chicks

Neuropeptides containing a C-terminal Arg-Phe-NH2 motif (RFamide peptides) are suggested to be involved in the control of feeding behavior in both invertebrates and vertebrates. Gonadotropin-inhibitory hormone (GnIH) is the first identified avian RFamide peptide that inhibits gonadotropin release from the pituitary. The GnIH precursor encodes one GnIH and its related peptides (GnIH-RP-1 and -RP-2) that shared the same C-terminal motif, Leu-Pro-Xaa-Arg-Phe-NH2 (Xaa = Leu or Gln) (LPXRFamide). GnIH neurons are localized in the paraventricular nucleus, with their fibers visible in multiple brain locations including the median eminence and brainstem. In this study, we therefore investigated the action of GnIH and its related peptides on feeding behavior. Intracerebroventricular (ICV) injection of GnIH, GnIH-RP-1 and GnIH-RP-2 significantly stimulated food intake in chicks. The chicken pentapeptide LPLRFamide, a degraded C-terminus of GnIH and GnIH-RP-1, did not stimulate feeding thereby demonstrating the importance of the N-terminus of GnIH and its related peptides for the orexigenic effect. Anti-GnIH antiserum suppressed appetite induced by fasting, but did not modify feeding under ad libitum conditions. The present study suggests that GnIH and its related peptides act as endogenous orexigenic factors in the brain of chicks.

Physiological roles of prolactin-releasing peptide

Prolactin-releasing peptide (PrRP) was first isolated from bovine hypothalamus as an orphan G-protein-coupled receptor using the strategy of reverse pharmacology. The initial studies showed that PrRP was a potent and specific prolactin-releasing factor. Morphological and physiological studies, however, indicated that PrRP may play a wide range of roles in neuroendocrinology other than prolactin release, i.e., metabolic homeostasis, stress responses, cardiovascular regulation, gonadotropin secretion, GH secretion and sleep regulation. This review will provide the current knowledge of PrRP, especially its roles in energy metabolism and stress responses.