New insights into the mechanism and actions of growth hormone (GH) in poultry.

Despite well documented anabolic effects of GH in mammals, a clear demonstration of such responses in domestic poultry is lacking. Recently, comprehensive dose-response studies of GH have been conducted in broilers during late post-hatch development (8 to 9 weeks of age). GH reduced feed intake (FI) and body weight gain in a dose-dependent manner, whereas birds pair-fed to the level of voluntary FI of GH-infused birds did not differ from controls. The reduction in voluntary FI may involve centrally mediated mechanisms, as hypothalamic neuropeptide Y protein and mRNA were reduced with GH, coincident with the maximal depression in FI. Growth of breast muscle was also reduced in a dose-dependent manner. Circulating IGF-I was not enhanced by GH, despite evidence that early events in the GH signaling pathway were intact. A GH dose-dependent increase in circulating 3,3',5-triiodothyronine(T3) paralleled decreases in hepatic 5D-III monodeiodinase activity, whereas 5'D-I activity was not altered. This confirms that a marked hyperthyroid response to GH occurs in late posthatch chickens, resulting from a decrease in the degradative pathway of T3 metabolism. This secondary hyperthyroidism would account for the decreased skeletal muscle mass (52) and lack of enhanced IGF-I (53) in GH-treated birds. Based upon these studies, it is now evident that GH does in fact have significant effects in poultry, but metabolic responses may confound the anabolic potential of the hormone.     

Biology of leptin in the pig

The recently discovered protein, leptin, which is secreted by fat cells in response to changes in body weight or energy, has been implicated in regulation of feed intake, energy expenditure and the neuroendocrine axis in rodents and humans. Leptin was first identified as the gene product found deficient in the obese ob/ob mouse. Administration of leptin to ob/ob mice led to improved reproduction as well as reduced feed intake and weight loss. The porcine leptin receptor has been cloned and is a member of the class 1 cytokine family of receptors. Leptin has been implicated in the regulation of immune function and the anorexia associated with disease. The leptin receptor is localized in the brain and pituitary of the pig. The leptin response to acute inflammation is uncoupled from anorexia and is differentially regulated among swine genotypes. In vitro studies demonstrated that the leptin gene is expressed by porcine preadipocytes and leptin gene expression is highly dependent on dexamethasone induced preadipocyte differentiation. Hormonally driven preadipocyte recruitment and subsequent fat cell size may regulate leptin gene expression in the pig. Expression of CCAAT-enhancer binding protein (C/EBP) mediates insulin dependent preadipocyte leptin gene expression during lipid accretion. In contrast, insulin independent leptin gene expression may be maintained by C/EBP auto-activation and phosphorylation/dephosphorylation. Adipogenic hormones may increase adipose tissue leptin gene expression in the fetus indirectly by inducing preadipocyte recruitment and subsequent differentiation. Central administration of leptin to pigs suppressed feed intake and stimulated growth hormone (GH) secretion. Serum leptin concentrations increased with age and estradiol-induced leptin mRNA expression in fat was age and weight dependent in prepuberal gilts. This occurred at the time of expected puberty in intact contemporaries and was associated with greater LH secretion. Further work demonstrated that leptin acts directly on pituitary cells to enhance LH and GH secretion, and brain tissue to stimulate gonadotropin releasing hormone secretion. Thus, development of nutritional schemes and (or) gene therapy to manipulate leptin secretion will lead to practical methods of controlling appetite, growth and reproduction in farm animals, thereby increasing efficiency of lean meat production.     

Peripheral leptin effect on food intake in young chickens is influenced by age and strain

The acute effect of leptin on the regulation of food intake was investigated in layer and broiler chickens. In an initial study, we observed that a single intraperitoneal injection of recombinant chicken leptin (1 mg/kg BW) dramatically reduced (38%) food intake in 56-day-old layer chickens, more moderately reduced (15%) food intake in 9-day-old layer chicks, and had no significant effect in 9-day-old broiler chicks. In a subsequent study, body weight and plasma concentrations of leptin were measured weekly in layer and broiler chicks from day 1 to 35 of age and brain leptin receptor and neuropeptide Y (NPY) mRNA expression were analyzed at 1, 9, and 35 days of age. At day 1 of age, peripheral concentrations of leptin were significantly greater in layer than broiler chicks. Subsequently, despite increases in body weight and differences in growth rates between layer and broiler chicks from day 8 to day 35 of age, peripheral concentrations of leptin were constant and similar in both genotypes. Leptin receptor and NPY mRNA were expressed in brain from day 1 in chicks of both genotypes and increased significantly to day 35 of age. These observations provide evidence that the inhibitory effect of leptin on the regulation of food intake in growing chicks is an age dependent process. Furthermore, acquisition of the anorectic effect of leptin is likely to be associated with greater expression of the leptin receptor and NPY mRNAs than to changes in blood levels of leptin. Finally, this study provides evidence that chickens selected for high growth rates may be less sensitive or responsive to peripheral concentrations of leptin than chickens with low growth rates (layers), suggesting that the faster growth of broiler chicks may be related to a lessened responsiveness to anorexigenic factors.     

Effects of dietary essential oil components on growth performance,digestive enzymes and lipid metabolism in female broiler chickens

1. The present experiment was conducted to describe the effects of thymol, cinnamaldehyde and a commercial preparation of essential oil components (CRINA Poultry), in female broilers. Feed and water were provided for ad libitum consumption. 2. Feed intake, weight gain and feed:gain ratio were not different among the treatments. Water intake was significantly lowered by cinnamaldehyde. Relative liver weight (g/100 g of body weight) was highest in birds given thymol, but this was seen only at the age of 21 d and not at 40 d. Patterns of digestive enzymes in pancreatic tissue were similar for the 4 treatments. 3. Amylase activity in intestinal digesta was highest in chickens given CRINA Poultry for 21 d, but the effect had disappeared after 40 d. Ileal digestibility coefficients for starch and protein were high and identical for all treatments. 4. Fatty acid composition of diet was reflected in that of adipose tissue. Plasma lipid concentrations were not changed by any dietary treatment. 5. Thus, the present results show no effect of essential oil constituents on growth performance in female broiler chickens, but it cannot be excluded that positive effects would have been observed under less hygienic environmental conditions or when using a less digestible diet.     

The ontogeny of leptin mRNA expression in growing broilers and its relationship to metabolic body weight.

The polypeptide hormone leptin is produced by both adipose tissue and the liver and has been shown to induce satiety in chickens. In this study we have investigated the developmental regulation of leptin mRNA expression in growing broiler chickens. Leptin expression generally increases in all tissues from 1-12 weeks of age. In the subcutaneous fat depot there is an apparent pattern of increased leptin mRNA expression occurring at 2, 6, and 10 weeks post-hatch. This pattern was not evident in the other tissues surveyed and may relate to the cycle of loading and unloading of adipocytes with lipid. No consistent gender differences in leptin expression patterns were detected in the tissues surveyed, as is often observed in mammals. Positive correlations between metabolic body weight and adipose leptin expression levels were observed. Leptin expression by the liver was highly correlated with metabolic body weight from 1-6 weeks of age, and uncorrelated from 6-12 weeks of age. This pattern of increasing liver leptin expression with increasing body weight during the early rapid growth phase of the bird may be due to limited fat storage during this period, which is followed by rapid body fat accumulation from 6-12 weeks. The characterization and tissue specific distribution of leptin mRNA expression in the growing broiler indicate similar patterns of leptin production to that of growing mammals. Leptin may be involved in lipid flux through the adipocyte as well as the shift in lipid metabolism to increased storage during pre-puberty.     

Chicken leptin: properties and actions

Chicken leptin cDNA shows a high homology to mammalian homologous, with an expression localized in the liver and adipose tissue. It is noteworthy, that the hepatic expression is most likely associated with the primary role that this organ plays in lipogenic activity in avian species. As in mammals, chicken leptin expression is regulated by hormonal and nutritional status. This regulation is tissue-specific and with a high sensitivity in the liver compared to adipose tissue. The blood leptin levels are regulated by the nutritional state with high levels in the fed state compared to the fasted state. The recombinant chicken leptin markedly inhibits food intake as reported in mammals, suggesting the presence of an hypothalamic leptin receptor. The chicken leptin receptor has been identified and all functional motifs are highly conserved compared to mammalian homologous. Chicken leptin receptor is expressed in the hypothalamus but also in other tissues such as pancreas, where leptin inhibits insulin secretion and thus may have a key role in regulating nutrient utilization in this species.     

Abdominal adipose tissue from broiler chickens selected for body weight or for food efficiency differ in in vitro lipolytic sensitivity to glucagon and to chicken growth hormone, but not to dibutyryl cAMP.

1. Differences in responses to lipolytic agents have been investigated in vitro in abdominal adipose tissue from lines of broiler chickens selected for body weight (GL, a 'fat' line) or for food efficiency (FC, a 'lean' line). 2. Dibutyryl cyclic adenosine monophosphate stimulated in vitro lipolysis, as measured by the glycerol release, by adipose tissue from GL or from FC chickens to the same extent. 3. Glucagon stimulated glycerol release from adipose tissue from FC chickens, but not from GL chickens. 4. Adipose tissue from GL chickens was much more sensitive to chicken growth hormone (GH) compared to FC chickens. 5. It is concluded that the selection criteria applied influenced the number of adipose GH and glucagon receptors, the number of adipose GH receptors being lower and of glucagon receptors being higher in FC chickens compared to GL chickens.     

Expression of genes for interleukins, neuropeptides, growth hormone receptor, and leptin receptor in adipose tissue from growing broiler chickens

In this study, total RNA was collected from abdominal adipose tissue samples obtained from 10 broiler chickens at 3, 4, 5, and 6 wk of age and prepared for quantitative real-time PCR analysis. Quantitative real-time PCR analysis was used to examine the influence of age on the expression of the adipose tissue genes for IL-1β, -6, -10, -15, -18; brain-derived neurotropic factor; ciliary neurotropic factor; interferon γ, neuropeptide Y receptor Y1; neuropeptide Y; nucleobindin 2; growth hormone receptor; leptin receptor; and visfatin. Between 3 and 6 wk of age, leptin receptor expression decreased (P = 0.013) with age, whereas expression of IL-15 (P = 0.015) and growth hormone receptor (P = 0.002) increased. Furthermore, IL-18 (P < 0.001) and visfatin (P = 0.007) expression increased between 4 and 6 wk of age. This is a unique exhibition of age-related changes in cytokine gene expression in chicken adipose tissue. Future studies are needed to elucidate the role of adipose tissue cytokines in growth and, possibly, disease resistance.     

Leptin in ruminants. Gene expression in adipose tissue and mammary gland, and regulation of plasma concentration

This paper reviews data on leptin gene expression in adipose tissue (AT) and mammary gland of adult ruminants, as well as on plasma leptin variations, according to genetic, physiological, nutritional and environmental factors. AT leptin mRNA level was higher in sheep and goat subcutaneous than visceral tissues, and the opposite was observed in cattle; it was higher in fat than in lean selection line in sheep; it was decreased by undernutrition and increased by refeeding in cattle and sheep, and not changed by adding soybeans to the diet of lactating goats; it was increased by injection of NPY to sheep, and by GH treatment of growing sheep and cattle. Insulin and glucocorticoids in vitro increased AT leptin mRNA in cattle, and leptin production in sheep. Long daylength increased AT lipogenic activities and leptin mRNA, as well as plasma leptin in sheep. Mammary tissue leptin mRNA level was high during early pregnancy and was lower but still expressed during late pregnancy and lactation in sheep. Leptin was present in sheep mammary adipocytes, epithelial and myoepithelial cells during early pregnancy, late pregnancy and lactation, respectively. Plasma leptin in cattle and sheep was first studied thanks to a commercial “multi-species” kit. It was positively related to body fatness and energy balance or feeding level, and decreased by β-agonist injection. The recent development of specific RIA for ruminant leptin enabled more quantitative study of changes in plasma leptin concentration, which were explained for 35–50% by body fatness and for 15–20% by feeding level. The response of plasma leptin to meal intake was related positively to glycemia, and negatively to plasma 3-hydroxybutyrate. The putative physiological roles of changes in leptin gene expression are discussed in relation with published data on leptin receptors in several body tissues, and on in vivo or in vitro effects of leptin treatment.     

Corticosterone administration and dietary glucose supplementation enhance fat accumulation in broiler chickens

1. The effects of exogenous corticosterone administration and glucose supplementation on energy intake, lipid metabolism and fat deposition of broiler chickens were investigated. 2. A total of 144 three-d-old male chickens were randomly assigned to one of the following 4 treatments for 7 d: a low energy diet (10.9 MJ ME/kg, 200 g/kg CP) with or without corticosterone (30 mg/kg diet) and drinking water supplemented with glucose (80 g/l) or saccharine (2 g/l, control). 3. Body weight (BW) gain and breast and thigh muscle yields (% body mass) were all significantly decreased by corticosterone treatment. The relative cumulative feed intake (RCFI) and relative ME intake (RMEI), rather than the feed (FI) or ME intake (MEI) were increased by corticosterone administration. Both feed efficiency (FE) and caloric efficiency (CE) were decreased by corticosterone administration. Corticosterone administration had no obvious effect on water consumption. 4. Glucose supplementation had no influence on BW gain and breast and thigh muscle yield (as % of body mass). FI or RCFI was decreased while MEI or RMEI was increased by glucose supplementation. FE was improved by glucose treatment, whereas CE was reduced. 5. Liver weight and abdominal, cervical and thigh fat deposits were all significantly increased by either corticosterone or glucose treatment. 6. Plasma concentrations of glucose, urate, triglyceride, non-esterified fatty acids (NEFA), very low density lipoprotein and insulin were all significantly increased by corticosterone treatment. Glucose supplementation had no obvious influence on any of the measured plasma parameters except for NEFA, which were significantly increased. 7. Lipoprotein lipase activities in either cervical or abdominal adipose tissues, rather than in thigh fat tissue, were significantly elevated by either glucose or corticosterone treatment.     

Effect of Escherichia coli endotoxin on tissue lipoprotein lipase activities in chickens

1. Four-week-old broiler chickens were injected intravenously with from 0.01 to 1 mg of E. coli endotoxin/kg body weight or with saline. 2. At all doses used endotoxin markedly depressed food intake and lipoprotein lipase activities in muscle and adipose tissue within 8 h. Heart lipoprotein lipase activity was significantly depressed only at doses of 0.1 mg endotoxin/kg body weight or greater. 3. Treatment of birds with 0.3 mg endotoxin/kg body weight reduced post-heparin lipoprotein lipase activity to 0.13 of that in control birds in 8 h. 4. Endotoxin generally depressed plasma very-low-density lipoprotein concentration. Plasma non-esterified fatty acid concentration was significantly elevated only in birds given 1 mg endotoxin/kg body weight. 5. Fatty acid synthetase activity in the liver of endotoxin-treated birds was significantly lower than in control birds 16 h after administration of endoxin, but not after 8 h. 6. These results show that tissue lipoprotein lipase activity in birds is very responsive to E. coli endotoxin, as in mammals. Hypertriglyceridaemia occurs only occasionally in endotoxin-treated chickens, most probably because of the particularly close relationship between food intake and hepatic lipoprotein synthesis in birds.     

Effect of dexamethasone on hypothalamic expression of appetite-related genes in chickens under different diet and feeding conditions

Background: Glucocorticoids(GCs) are involved in the control of appetite in birds and mammals. The effect of GCs on feed intake in birds depends on their dietary energy level. But the regulation mechanism of GCs on appetite is still unclear in chickens facing to different energy level. An experiment was conducted to investigate the effect of dexamethasone(DEX) on hypothalamic expression of appetite-related peptides in chickens fed high/low fat diet and under fasting/feeding condition.Results: An interaction between DEX injection and dietary energy level was found on hypothalamic corticotropinreleasing hormone(CRH) gene expression in fasted chickens(P 0.05). The chickens, given a DEX injection and a low fat diet treatment, had the highest CRH m RNA levels than any of the fasted chickens given treatments(P 0.05).Under fasting conditions, the DEX treatment significantly increased hypothalamic neuropeptide Y(NPY) and GC receptors m RNA levels(P 0.05). Under re-feeding conditions, DEX treatment significantly decreased hypothalamic expression levels of NPY and agouti-related peptide(Ag RP) but significantly increased the level of hypothalamic CRH expression(P 0.05).Conclusion: A regulatory network formed by NPY, Ag RP and CRH is associated with the appetite-control by GCs.The result suggests that the regulation of GCs on orexigenic neuropeptides expression is dependent at least partially on dietary energy level and feeding state.     

Long form leptin receptor mRNA expression in the brain, pituitary, and other tissues in the pig

Much effort has focused recently on understanding the role of leptin, the obese gene product secreted by adipocytes, in regulating growth and reproduction in rodents, humans and domestic animals. We previously demonstrated that leptin inhibited feed intake and stimulated growth hormone (GH) and luteinizing hormone (LH) secretion in the pig. This study was conducted to determine the location of long form leptin receptor (Ob-Rl) mRNA in various tissues of the pig. The leptin receptor has several splice variants in the human and mouse, but Ob-Rl is the major form capable of signal transduction. The Ob-Rl is expressed primarily in the hypothalamus of the human and rodents, but has been located in other tissues as well. In the present study, a partial porcine Ob-Rl cDNA, cloned in our laboratory and specific to the intracellular domain, was used to evaluate the Ob-Rl mRNA expression by RT-PCR in the brain and other tissues in three 105 d-old prepuberal gilts and in a 50 d-old fetus. In 105 d-old gilts, Ob-Rl mRNA was expressed in the hypothalamus, cerebral cortex, amygdala, thalamus, cerebellum, area postrema and anterior pituitary. In addition, Ob-Rl mRNA was expressed in ovary, uterine body, liver, kidney, pancreas, adrenal gland, heart, spleen, lung, intestine, bone marrow, muscle and adipose tissue. However, expression was absent in the thyroid, thymus, superior vena cava, aorta, spinal cord, uterine horn and oviduct. In the 50 d-old fetus, Ob-Rl mRNA was expressed in brain, intestine, muscle, fat, heart, liver and umbilical cord. These results support the idea that leptin might play a role in regulating numerous physiological functions.     

Early pregnancy alters the metabolic responses to restricted nutrition in sheep

This study investigated whether a 27-day period of nutrition at half-maintenance during early pregnancy (up to Day 14) could alter maternal endocrine responses. Forty-six ewes were fed all or half of their maintenance requirements and slaughtered on Day 14 of the oestrous cycle or pregnancy. We used real time RT-PCR to study gene expression of growth hormone receptor (GHR) and leptin in adipose tissue and GHR, GHR1A and of the insulin-like growth factor I (IGF-I) in the liver. Blood profiles of metabolites and metabolic hormones were also determined. Throughout the experiment, underfed animals presented lower body weight and body condition, greater plasma concentrations of non-esterified fatty acids (NEFA), and lower plasma concentrations of leptin, compared to adequately fed animals. Undernutrition affected the patterns of gene expression in adipose and hepatic tissues, and the responses differed between pregnant and non-pregnant ewes. In adequately fed ewes, pregnancy up-regulated leptin mRNA expression in adipose tissue, a response that was impaired in underfed ewes. The hepatic expression of IGF-I mRNA was increased by pregnancy in underfed animals while no effect was observed in adequately fed ewes. It remains to be determined whether the changes in the endocrine milieu are paralleled by modifications in uterine gene expression that could alter the environment of the embryo during early pregnancy.     

Effects of an intravenous injection of NPY on leptin and NPY-Y1 receptor mRNA expression in ovine adipose tissue

Neuropeptide Y (NPY) is highly expressed in hypothalami of undernourished and genetically obese animals, and is a potent regulator of food intake and reproduction. Leptin, a protein expressed by adipocytes, has been reported to reduce hypothalamic NPY expression. We recently detected (by ribonuclease protection assay [RPA]) expression of the NPY receptor subtype Y1 (but not Y2) mRNA in adipose tissue. Based on these observations we hypothesized that NPY-Y1 receptors in adipose may represent a peripheral mechanism by which NPY can regulate leptin expression in a direct and rapid manner. To test this hypothesis, adipose samples were biopsied from the tailhead region of 48 ± 3 kg wether lambs immediately before and 30 min after a single intravenous injection of 50 μg porcine NPY (“treated” animals, n = 5), or vehicle (“control” animals, n = 4). Injection of NPY resulted in an increase in expression (P = 0.013; as measured by RPA) of both leptin and NPY-Y1 mRNA. In treated animals, negative correlations were found between response in leptin expression and body weight (r = −0.82, P = 0.092), and between leptin response and initial leptin mRNA levels (r = −0.81, P = 0.097). These data provide evidence of a peripheral mechanism by which NPY may regulate adipocyte expression of both leptin and NPY-Y1 receptor mRNA.