Quack leptin

A Bird's-Eye Overview of Leptin and Female Reproduction -with Mammalian Comparisons

Leptin, a key regulator of reproductive physiology, influences various processes in vertebrates, including oocyte proliferation, embryogenesis, the onset of puberty, ovarian function, and follicle development. In mammals, leptin affects steroidogenesis, folliculogenesis, and hormonal regulation through the hypothalamic-pituitary-gonadal axis. Instead, in avian species, leptin-controlled mechanisms are poorly understood, because birds do not produce leptin in adipocytes. In birds, leptin is expressed in the brain, pituitary glands, and gonads, where it enhances ovarian function and egg-laying performance, particularly during feed deprivation. In this review, we discuss and summarize the recently discovered role of leptin in regulating ovarian function during different life stages in birds and compare it with its function in mammals.

A genome-wide association analysis for body weight at 35 days measured on 137,343 broiler chickens

BACKGROUND

Body weight (BW) is an economically important trait in the broiler (meat-type chickens) industry. Under the assumption of polygenicity, a "large" number of genes with "small" effects is expected to control BW. To detect such effects, a large sample size is required in genome-wide association studies (GWAS). Our objective was to conduct a GWAS for BW measured at 35 days of age with a large sample size.

METHODS

The GWAS included 137,343 broilers spanning 15 pedigree generations and 392,295 imputed single nucleotide polymorphisms (SNPs). A false discovery rate of 1% was adopted to account for multiple testing when declaring significant SNPs. A Bayesian ridge regression model was implemented, using AlphaBayes, to estimate the contribution to the total genetic variance of each region harbouring significant SNPs (1 Mb up/downstream) and the combined regions harbouring non-significant SNPs.

RESULTS

GWAS revealed 25 genomic regions harbouring 96 significant SNPs on 13 Gallus gallus autosomes (GGA1 to 4, 8, 10 to 15, 19 and 27), with the strongest associations on GGA4 at 65.67-66.31 Mb (Galgal4 assembly). The association of these regions points to several strong candidate genes including: (i) growth factors (GGA1, 4, 8, 13 and 14); (ii) leptin receptor overlapping transcript (LEPROT)/leptin receptor (LEPR) locus (GGA8), and the STAT3/STAT5B locus (GGA27), in connection with the JAK/STAT signalling pathway; (iii) T-box gene (TBX3/TBX5) on GGA15 and CHST11 (GGA1), which are both related to heart/skeleton development); and (iv) PLAG1 (GGA2). Combined together, these 25 genomic regions explained ~ 30% of the total genetic variance. The region harbouring significant SNPs that explained the largest portion of the total genetic variance (4.37%) was on GGA4 (~ 65.67-66.31 Mb).

CONCLUSIONS

To the best of our knowledge, this is the largest GWAS that has been conducted for BW in chicken to date. In spite of the identified regions, which showed a strong association with BW, the high proportion of genetic variance attributed to regions harbouring non-significant SNPs supports the hypothesis that the genetic architecture of BW35 is polygenic and complex. Our results also suggest that a large sample size will be required for future GWAS of BW35.

Impairment of testes development in Yangzhou ganders by augmentation of leptin receptor signaling

The aim of this study was to determine the cellular and molecular mechanisms of leptin (LEP) and the leptin receptor (LEPR) in testicular development of prepubertal ganders. In an in vivo animal experiment, active immunization against LEPR severely depressed prepubertal testicular development by significantly reducing testicular weights at 200 and 227 days of age. The number of elongated spermatids in the seminiferous tubules was also significantly decreased by immunization with LEPR at ages of 200 and 227 days. Inhibition of testicular development by LEPR immunization was associated with decreases in LHR, StAR, 3β-HSD, CYP11A1, CYP17A1, and PRLR mRNA expression levels in testicular tissue, which resulted in a significant decrease in testosterone synthesis. In the in vitro experiments, the addition of LEP combined with anti-LEPR antibodies strengthened LEPR signal transduction, and inhibited significantly testosterone production in cultured Leydig cells isolated from prepubertal gander testes. The mRNA expression of LHR, StAR, 3β-HSD, CYP11A1, CYP17A1 also decreased significantly after treatment with LEP combined with anti-LEPR antibodies in cultured Leydig cells. These results suggest that anti-LEPR antibodies strengthen LEPR signaling transduction in the presence of LEP, and immunization against LEPR inhibited testes development and testosterone secretion in prepubertal ganders.

Exogenous leptin promotes reproductive behavior during aphagia in red-sided garter snakes (Thamnophis sirtalis parietalis)

Despite the established dichotomy between investment in either reproduction or self-maintenance, a hormonal mechanism that influences an organism's decision to prioritize these behaviors remains elusive. The protein hormone leptin is a likely candidate because it is secreted from adipocytes in proportion to the amount of stored fat in numerous species. Although the majority of studies suggest that leptin stimulates reproduction, the actions of leptin can be context-dependent. Leptin increases sexual behavior in fed individuals, but inhibits sexual behavior in food-restricted individuals. We investigated if exogenous leptin influences sexual behavior in red-sided garter snakes (Thamnophis sirtalis parietalis) experiencing a predictable bout of aphagia during the mating season. We tested two doses of recombinant murine leptin injected for three days. Males were subjected to three mating trials, one on each day of injections, while females were subjected to one mating trial on the last day of injections. Leptin affects male and female snakes similarly by increasing both appetitive (i.e., mating behavior score) and consummatory (i.e., number of copulations, proportion of individuals copulated) sex behavior. We found no evidence to suggest that leptin influenced latency to copulate or duration of copulation. Because leptin promotes reproductive behavior in non-feeding garter snakes, these findings do not align with research on food-restricted mammals. Further investigations into how leptin affects sexual behavior in snakes exposed to food-restriction manipulations would clarify if the role of leptin is evolutionarily divergent.

β2 adrenergic receptors and leptin interplay to decrease food intake in chicken

1. The present study was designed to examine the effects of intracerebroventricular (ICV) injection of different α and [Formula: see text] adrenergic receptor antagonists on leptin-induced hypophagia in broiler chickens.2. The study consisted of six experiments. In all experiments, chickens were deprived of feed for 3 h prior to the ICV injections and thereafter were returned immediately to the individual cages and cumulative feed intake, based on the percentage of body weight, was measured at 30, 60 and 120 min post-injection.3. In experiment 1, leptin (2.5, 5 or 10 µg) were injected in birds. In experiment 2, groups received either control solution, prazosin (10 nmol), leptin (10 µg) or a co-injection of prazosin (10 nmol) and leptin (10 µg). The other experiments were conducted as experiment 2, but instead of prazosine (10 nmol), yohimbine (13 nmol) was used in experiment 3, metoprolol (24 nmol) in experiment 4, ICI 118,551 (5 nmol) in experiment 5 and SR 59230R (5 nmol) in experiment 6 were injected either in a group or in combination with leptin (10 µg).4. The results of this study revealed a dose-dependent hypophagic effect of leptin and, in experiment 5, ICV co-injection of ICI118, 551 (5 nmol) and leptin (10 µg) significantly attenuated this effect (P˂0.5). These results suggest that the hypophagic effect of leptin is probably mediated by β2 adrenergic receptors in chickens.

Avian Leptin: Bird's-Eye View of the Evolution of Vertebrate Energy-Balance Control

Discovery of the satiety hormone leptin in 1994 and its characterization in mammals provided a key tool to deciphering the complex mechanism governing adipose tissue regulation of appetite and energy expenditure. Surprisingly, despite the perfectly logical notion of an energy-storing tissue announcing the amount of fat stores using leptin signaling, alternate mechanisms were chosen in bird evolution. This conclusion emerged based on the recent discovery and characterization of genuine avian leptin - after it had been assumed missing by some, and erroneously identified by others. Critical evaluation of the past and present indications of the role of leptin in Aves provides a new perspective on the evolution of energy-balance control in vertebrates; proposing a regulation strategy alternative to the adipostat mechanism.

Sequence properties of certain GC rich avian genes, their origins and absence from genome assemblies: case studies

Background

More and more eukaryotic genomes are sequenced and assembled, most of them presented as a complete model in which missing chromosomal regions are filled by Ns and where a few chromosomes may be lacking. Avian genomes often contain sequences with high GC content, which has been hypothesized to be at the origin of many missing sequences in these genomes. We investigated features of these missing sequences to discover why some may not have been integrated into genomic libraries and/or sequenced.

Results

The sequences of five red jungle fowl cDNA models with high GC content were used as queries to search publicly available datasets of Illumina and Pacbio sequencing reads. These were used to reconstruct the leptin, TNFα, MRPL52, PCP2 and PET100 genes, all of which are absent from the red jungle fowl genome model. These gene sequences displayed elevated GC contents, had intron sizes that were sometimes larger than non-avian orthologues, and had non-coding regions that contained numerous tandem and inverted repeat sequences with motifs able to assemble into stable G-quadruplexes and intrastrand dyadic structures. Our results suggest that Illumina technology was unable to sequence the non-coding regions of these genes. On the other hand, PacBio technology was able to sequence these regions, but with dramatically lower efficiency than would typically be expected.

Conclusions

High GC content was not the principal reason why numerous GC-rich regions of avian genomes are missing from genome assembly models. Instead, it is the presence of tandem repeats containing motifs capable of assembling into very stable secondary structures that is likely responsible.

Avian Expression Patterns and Genomic Mapping Implicate Leptin in Digestion and TNF in Immunity, Suggesting That Their Interacting Adipokine Role Has Been Acquired Only in Mammals

In mammals, leptin and tumor-necrosis factor (TNF) are prominent interacting adipokines mediating appetite control and insulin sensitivity. While TNF pleiotropically functions in immune defense and cell survival, leptin is largely confined to signaling energy stores in adipocytes. Knowledge about the function of avian leptin and TNF is limited and they are absent or lowly expressed in adipose, respectively. Employing radiation-hybrid mapping and FISH-TSA, we mapped TNF and its syntenic genes to chicken chromosome 16 within the major histocompatibility complex (MHC) region. This mapping position suggests that avian TNF has a role in regulating immune response. To test its possible interaction with leptin within the immune system and beyond, we compared the transcription patterns of TNF, leptin and their cognate receptors obtained by meta-analysis of GenBank RNA-seq data. While expression of leptin and its receptor (LEPR) were detected in the brain and digestive tract, TNF and its receptor mRNAs were primarily found in viral-infected and LPS-treated leukocytes. We confirmed leptin expression in the duodenum by immunohistochemistry staining. Altogether, we suggest that whereas leptin and TNF interact as adipokines in mammals, in birds, they have distinct roles. Thus, the interaction between leptin and TNF may be unique 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.

Comparative omics and feeding manipulations in chicken indicate a shift of the endocrine role of visceral fat towards reproduction

Background

The mammalian adipose tissue plays a central role in energy-balance control, whereas the avian visceral fat hardly expresses leptin, the key adipokine in mammals. Therefore, to assess the endocrine role of adipose tissue in birds, we compared the transcriptome and proteome between two metabolically different types of chickens, broilers and layers, bred towards efficient meat and egg production, respectively.

Results

Broilers and layer hens, grown up to sexual maturation under free-feeding conditions, differed 4.0-fold in weight and 1.6-fold in ovarian-follicle counts, yet the relative accumulation of visceral fat was comparable. RNA-seq and mass-spectrometry (MS) analyses of visceral fat revealed differentially expressed genes between broilers and layers, 1106 at the mRNA level (FDR ≤ 0.05), and 203 at the protein level (P ≤ 0.05). In broilers, Ingenuity Pathway Analysis revealed activation of the PTEN-pathway, and in layers increased response to external signals. The expression pattern of genes encoding fat-secreted proteins in broilers and layers was characterized in the RNA-seq and MS data, as well as by qPCR on visceral fat under free feeding and 24 h-feed deprivation. This characterization was expanded using available RNA-seq data of tissues from red junglefowl, and of visceral fat from broilers of different types. These comparisons revealed expression of new adipokines and secreted proteins (LCAT, LECT2, SERPINE2, SFTP1, ZP1, ZP3, APOV1, VTG1 and VTG2) at the mRNA and/or protein levels, with dynamic gene expression patterns in the selected chicken lines (except for ZP1; FDR/P ≤ 0.05) and feed deprivation (NAMPT, SFTPA1 and ZP3) (P ≤ 0.05). In contrast, some of the most prominent adipokines in mammals, leptin, TNF, IFNG, and IL6 were expressed at a low level (FPKM/RPKM< 1) and did not show differential mRNA expression neither between broiler and layer lines nor between fed vs. feed-deprived chickens.

Conclusions

Our study revealed that RNA and protein expression in visceral fat changes with selective breeding, suggesting endocrine roles of visceral fat in the selected phenotypes. In comparison to gene expression in visceral fat of mammals, our findings points to a more direct cross talk of the chicken visceral fat with the reproductive system and lower involvement in the regulation of appetite, inflammation and insulin resistance.

Electronic supplementary material

The online version of this article (10.1186/s12864-018-4675-0) contains supplementary material, which is available to authorized users.

Mapping of leptin and its syntenic genes to chicken chromosome 1p

Background

Misidentification of the chicken leptin gene has hampered research of leptin signaling in this species for almost two decades. Recently, the genuine leptin gene with a GC-rich (~70%) repetitive-sequence content was identified in the chicken genome but without indicating its genomic position. This suggests that such GC-rich sequences are difficult to sequence and therefore substantial regions are missing from the current chicken genome assembly.

Results

A radiation hybrid panel of chicken-hamster Wg3hCl2 cells was used to map the genome location of the chicken leptin gene. Contrary to our expectations, based on comparative genome mapping and sequence characteristics, the chicken leptin was not located on a microchromosome, which are known to contain GC-rich and repetitive regions, but at the distal tip of the largest chromosome (1p). Following conserved synteny with other vertebrates, we also mapped five additional genes to this genomic region (ARF5, SND1, LRRC4, RBM28, and FLNC), bridging the genomic gap in the current Galgal5 build for this chromosome region. All of the short scaffolds containing these genes were found to consist of GC-rich (54 to 65%) sequences comparing to the average GC-content of 40% on chromosome 1. In this syntenic group, the RNA-binding protein 28 (RBM28) was in closest proximity to leptin. We deduced the full-length of the RBM28 cDNA sequence and profiled its expression patterns detecting a negative correlation (R = − 0.7) between the expression of leptin and of RBM28 across tissues that expressed at least one of the genes above the average level. This observation suggested a local regulatory interaction between these genes. In adipose tissues, we observed a significant increase in RBM28 mRNA expression in breeds with lean phenotypes.

Conclusion

Mapping chicken leptin together with a cluster of five syntenic genes provided the final proof for its identification as the true chicken ortholog. The high GC-content observed for the chicken leptin syntenic group suggests that other similar clusters of genes in GC-rich genomic regions are missing from the current genome assembly (Galgal5), which should be resolved in future assemblies of the chicken genome.

Electronic supplementary material

The online version of this article (doi:10.1186/s12863-017-0543-1) contains supplementary material, which is available to authorized users.

Central injection of a synthetic chicken partial leptin peptide does not affect food intake in chicks

Leptin is an adipose tissue-derived hormone in mammals that plays an important role in whole body energy balance via its inhibitory effects on food intake mediated through the hypothalamus. Chicken leptin has a low sequence homology to mammalian leptin and its role in appetite regulation is not reported; hence the objective of this study was to determine effects of central injection of chicken leptin on food and water intake and associated behaviors in chicks. Chicks were intracerebroventricularly injected with 0 (vehicle), 0.3, 1.0, or 3.0 nmol of a synthetic chicken leptin partial peptide and food and water intake were monitored. There were no effects observed and a second experiment was conducted to evaluate food and water intake at higher doses; after injection of 0, 2.5, 5.0, or 10.0 nmol leptin. Again, there were no effects on food or water intake. In the third experiment, behaviors were analyzed during the first 30 min post-injection of vehicle or 10 nmol leptin. At 5 min post-injection, vehicle-injected chicks spent more time sitting than leptin-injected chicks. A wide dose range was evaluated however, the absence of an effect on food intake or behavior suggests that the chicken leptin peptide that was tested does not mediate effects on appetite in the brain and that chicken leptin likely has a different physiological role in birds than in mammals.

Regulation of Agouti-Related Protein and Pro-Opiomelanocortin Gene Expression in the Avian Arcuate Nucleus

The arcuate nucleus is generally conserved across vertebrate taxa in its neuroanatomy and neuropeptide expression. Gene expression of agouti-related protein (AGRP), neuropeptide Y (NPY), pro-opiomelanocortin (POMC), and cocaine- and amphetamine-regulated transcript (CART) has been established in the arcuate nucleus of several bird species and co-localization demonstrated for AGRP and NPY. The proteins encoded by these genes exert comparable effects on food intake in birds after central administration to those seen in other vertebrates, with AGRP and NPY being orexigenic and CART and α-melanocyte-stimulating hormone anorexigenic. We have focused on the measurement of arcuate nucleus AGRP and POMC expression in several avian models in relation to the regulation of energy balance, incubation, stress, and growth. AGRP mRNA and POMC mRNA are, respectively, up- and downregulated after energy deprivation and restriction. This suggests that coordinated changes in the activity of AGRP and POMC neurons help to drive the homeostatic response to replace depleted energy stores in birds as in other vertebrates. While AGRP and POMC expression are generally positively and negatively correlated with food intake, respectively, we review here situations in some avian models in which AGRP gene expression is dissociated from the level of food intake and may have an influence on growth independent of changes in appetite. This suggests the possibility that the central melanocortin system exerts more pleiotropic functions in birds. While the neuroanatomical arrangement of AGRP and POMC neurons and the sensitivity of their activity to nutritional state appear generally conserved with other vertebrates, detailed knowledge is lacking of the key nutritional feedback signals acting on the avian arcuate nucleus and there appear to be significant differences between birds and mammals. In particular, recently identified avian leptin genes show differences between bird species in their tissue expression patterns and appear less closely linked in their expression to nutritional state. It is presently uncertain how the regulation of the central melanocortin system in birds is brought about in the situation of the apparently reduced importance of leptin and ghrelin compared to mammals.

In ovo leptin administration modulates glucocorticoid receptor mRNA expression specifically in the hypothalamus of broiler chickens

The glucocorticoid receptor (GR) is well documented to play a crucial role in the central control of energy homeostasis in mammals. However, the distribution and function of the GR in the chicken brain are less clear. Leptin is a key hormone regulating energy homeostasis in mammals, yet its action in the chicken is still under debate. In this study, the distribution of GR mRNA in the chicken brain and the effects of in ovo administration of leptin and its antagonist on early post-hatch growth and GR mRNA expression in different hypothalamic nuclei were investigated via in situ hybridization (ISH) and quantitative PCR. GR mRNA was widely expressed in the chicken brain, mainly in the corpus striatum, nucleus rotundus, dorsolateral nucleus, nucleus ovoidalis, nucleus reticularis superior and the hippocampus (Hp) and in the preoptic area of the hypothalamus. High doses of leptin (5.0μg) significantly promoted post-hatch growth, resulting in a significant high body weight increased by 24.64% at day (D) 21 of life. Meanwhile, hypothalamic expression of GR mRNA in the LL and HL groups was down-regulated significantly by 7.02% and 13.65% respectively (P<0.05). ISH revealed region-specific changes: GR mRNA was found to be significantly decreased (P<0.05) in the paraventricular nucleus, periventricular nucleus and ventromedial nucleus but not in the Hp, infundibular nucleus or lateral hypothalamus of D21 broiler chickens. The leptin antagonist was able to reverse the effect of leptin on the growth rate and hypothalamic GR mRNA expression. These results provide evidence that in ovo administration of leptin influences early post-hatch growth and the hypothalamic expression of GR mRNA in broiler chickens.

Identification of the Long-Sought Leptin in Chicken and Duck: Expression Pattern of the Highly GC-Rich Avian leptin Fits an Autocrine/Paracrine Rather Than Endocrine Function

More than 20 years after characterization of the key regulator of mammalian energy balance, leptin, we identified the leptin (LEP) genes of chicken (Gallus gallus) and duck (Anas platyrhynchos). The extreme guanine-cytosine content (∼70%), the location in a genomic region with low-complexity repetitive and palindromic sequence elements, the relatively low sequence conservation, and low level of expression have hampered the identification of these genes until now. In vitro-expressed chicken and duck leptins specifically activated signaling through the chicken leptin receptor in cell culture. In situ hybridization demonstrated expression of LEP mRNA in granular and Purkinje cells of the cerebellum, anterior pituitary, and in embryonic limb buds, somites, and branchial arches, suggesting roles in adult brain control of energy balance and during embryonic development. The expression patterns of LEP and the leptin receptor (LEPR) were explored in chicken, duck, and quail (Coturnix japonica) using RNA-sequencing experiments available in the Short Read Archive and by quantitative RT-PCR. In adipose tissue, LEP and LEPR were scarcely transcribed, and the expression level was not correlated to adiposity. Our identification of the leptin genes in chicken and duck genomes resolves a long lasting controversy regarding the existence of leptin genes in these species. This identification was confirmed by sequence and structural similarity, conserved exon-intron boundaries, detection in numerous genomic, and transcriptomic datasets and characterization by PCR, quantitative RT-PCR, in situ hybridization, and bioassays. Our results point to an autocrine/paracrine mode of action for bird leptin instead of being a circulating hormone as in mammals.

Discovery and functional characterization of leptin and its receptors in Japanese quail (Coturnix japonica)

Leptin is an important endocrine regulation factor of food intake and energy homeostasis in mammals; however, the existence of a poultry leptin gene (LEP) is still debated. Here, for the first time, we report the cloning of a partial exon 3 sequence of LEP (qLEP) and four different leptin receptor splicing variants, including a long receptor (qLEPRl) and three soluble receptors (qLEPR-a, qLEPR-b and qLEPR-c) in Japanese quail (Coturnix japonica). The qLEP gene had high GC content (64%), which is similar to other reported avian leptin genes. The encoded qLEP protein possessed the conserved pair of cysteine residues that are required to form a lasso knot for full biological activity, but shared relatively low identities with LEPs of other vertebrates. The translated qLEPRl protein contained 1143 amino acids and shared high amino acid sequence identity with a chicken homolog (89% identity). qLEPRl also contained all the motifs, domains, and basic tyrosine residues that are conserved in the LEPRl proteins of other vertebrates. qRT-PCR analysis showed that LEP and the four LEPR variants were expressed extensively in all tissues examined; the expression levels of LEP were relatively high in hypothalamus, skeletal muscle, and pancreas, while the expression levels of the LEPRs were highest in the pituitary. Compared with the expression levels of juvenile qLEP and total qLEPR (including all LEPR variants), the expression levels of mature qLEP and total qLEPR were up-regulated in the hypothalamus and pituitary, and down-regulated in the ovary. The expressions of LEP/LEPR increased when fasting and decreased when refeeding in the brain and peripheral tissues of juvenile quail, which suggested that the LEP/LEPR system modulated food intake and energy expenditure, although, unlike in mammals, LEP may actually act to inhibit food intake during fasting, at least in juvenile quail. The results indicate that qLEP and qLEPR have unique expression patterns and that the encoded proteins play important roles in the regulation of reproduction and energy status in Japanese quail.

Regulation of the avian central melanocortin system and the role of leptin

The avian central melanocortin system is well conserved between birds and mammals in terms of the component genes, the localisation of their expression in the hypothalamic arcuate nucleus, the effects on feeding behaviour of their encoded peptides and the sensitivity of agouti-related protein (AGRP) and pro-opiomelanocortin (POMC) gene expression to changes in energy status. Our recent research has demonstrated that AGRP gene expression precisely differentiates between broiler breeder hens with different histories of chronic food restriction and refeeding. We have also shown that the sensitivity of AGRP gene expression to loss of energy stores is maintained even when food intake has been voluntarily reduced in chickens during incubation and in response to a stressor. However, the similarity between birds and mammals does not appear to extend to the way AGRP and POMC gene expression are regulated. In particular, the preliminary evidence from the discovery of the first avian leptin (LEP) genes suggests that LEP is more pleiotropic in birds and may not even be involved in regulating energy balance. Similarly, ghrelin exerts inhibitory, rather than stimulatory, effects on food intake. The fact that the importance of these prominent long-term regulators of AGRP and POMC expression in mammals appears diminished in birds suggests that the balance of regulatory inputs in birds may have shifted to more short-term influences such as the tone of cholecystokinin (CCK) signalling. This is likely to be related to the different metabolic fuelling required to support flight.

Hypothalamic Agouti-Related Peptide mRNA is Elevated During Natural and Stress-Induced Anorexia

As part of their natural lives, animals can undergo periods of voluntarily reduced food intake and body weight (i.e. animal anorexias) that are beneficial for survival or breeding, such as during territorial behaviour, hibernation, migration and incubation of eggs. For incubation, a change in the defended level of body weight or 'sliding set point' appears to be involved, although the neural mechanisms reponsible for this are unknown. We investigated how neuropeptide gene expression in the arcuate nucleus of the domestic chicken responded to a 60-70% voluntary reduction in food intake measured both after incubation and after an environmental stressor involving transfer to unfamiliar housing. We hypothesised that gene expression would not change in these circumstances because the reduced food intake and body weight represented a defended level in birds with free access to food. Unexpectedly, we observed increased gene expression of the orexigenic peptide agouti-related peptide (AgRP) in both incubating and transferred animals compared to controls. Also pro-opiomelanocortin (POMC) mRNA was higher in incubating hens and significantly increased 6 days after exposure to the stressor. Conversely expression of neuropeptide Y and cocaine- and amphetamine-regulated transcript gene was unchanged in both experimental situations. We conclude that AgRP expression remains sensitive to the level of energy stores during natural anorexias, which is of adaptive advantage, although its normal orexigenic effects are over-ridden by inhibitory signals. In the case of stress-induced anorexia, increased POMC may contribute to this inhibitory role, whereas, for incubation, reduced feeding may also be associated with increased expression in the hypothalamus of the anorexigenic peptide vasoactive intestinal peptide.

Characterization of the Two CART Genes (CART1 and CART2) in Chickens (Gallus gallus)

Cocaine- and amphetamine-regulated transcript (CART) peptide is implicated in the control of avian energy balance, however, the structure and expression of CART gene(s) remains largely unknown in birds. Here, we cloned and characterized two CART genes (named cCART1 and cCART2) in chickens. The cloned cCART1 is predicted to generate two bioactive peptides, cCART1(42-89) and cCART1(49-89), which share high amino acid sequence identity (94-98%) with their mammalian counterparts, while the novel cCART2 may produce a bioactive peptide cCART2(51-91) with 59% identity to cCART1. Interestingly, quantitative RT-PCR revealed that cCART1 is predominantly expressed in the anterior pituitary and less abundantly in the hypothalamus. In accordance with this finding, cCART1 peptide was easily detected in the anterior pituitary by Western blot, and its secretion from chick pituitaries incubated in vitro was enhanced by ionomycin and forskolin treatment, indicating that cCART1 is a novel peptide hormone produced by the anterior pituitary. Moreover, cCART1 mRNA expression in both the pituitary and hypothalamus is down-regulated by 48-h fasting, suggesting its expression is affected by energy status. Unlike cCART1, cCART2 is only weakly expressed in most tissues examined by RT-PCR, implying a less significant role of cCART2 in chickens. As in chickens, 2 or more CART genes, likely generated by gene and genome duplication event(s), were also identified in other non-mammalian vertebrate species including coelacanth. Collectively, the identification and characterization of CART genes in birds helps to uncover the roles of CART peptide(s) in vertebrates and provides clues to the evolutionary history of vertebrate CART genes.