Quantification of growth hormone receptor extra- and intracellular domain gene expression in chicken liver by quantitative competitive RT-PCR

Should I Lay or Should I Grow: Photoperiodic Versus Metabolic Cues in Chickens

While photoperiod has been generally accepted as the primary if not the exclusive cue to stimulate reproduction in photoperiodic breeders such as the laying hen, current knowledge suggests that metabolism, and/or body composition can also play an influential role to control the hypothalamic-pituitary gonadal (HPG)-axis. This review thus intends to first describe how photoperiodic and metabolic cues can impact the HPG axis, then explore and propose potential common pathways and mechanisms through which both cues could be integrated. Photostimulation refers to a perceived increase in day-length resulting in the stimulation of the HPG. While photoreceptors are present in the retina of the eye and the pineal gland, it is the deep brain photoreceptors (DBPs) located in the hypothalamus that have been identified as the potential mediators of photostimulation, including melanopsin (OPN4), neuropsin (OPN5), and vertebrate-ancient opsin (VA-Opsin). Here, we present the current state of knowledge surrounding these DBPs, along with their individual and relative importance and, their possible downstream mechanisms of action to initiate the activation of the HPG axis. On the metabolic side, specific attention is placed on the hypothalamic integration of appetite control with the stimulatory (Gonadotropin Releasing Hormone; GnRH) and inhibitory (Gonadotropin Inhibitory Hormone; GnIH) neuropeptides involved in the control of the HPG axis. Specifically, the impact of orexigenic peptides agouti-related peptide (AgRP), and neuropeptide Y (NPY), as well as the anorexigenic peptides pro-opiomelanocortin (POMC), and cocaine-and amphetamine regulated transcript (CART) is reviewed. Furthermore, beyond hypothalamic control, several metabolic factors involved in the control of body weight and composition are also presented as possible modulators of reproduction at all three levels of the HPG axis. These include peroxisome proliferator-activated receptor gamma (PPAR-γ) for its impact in liver metabolism during the switch from growth to reproduction, adiponectin as a potential modulator of ovarian development and follicular maturation, as well as growth hormone (GH), and leptin (LEP).

Response of the chicken ovary to GH treatment during a pause in laying induced by fasting

This study was undertaken to examine the effect of GH treatment during a pause in laying on (1) ovarian follicle formation, growth (folliculogenesis), and atresia; (2) follicle cell proliferation and apoptosis; and (3) mRNA expression of selected yolk-specific proteins in the chicken liver. A pause in egg laying was induced by food deprivation for 5 d, followed by feeding every other day, and then feeding daily from day 10 onward. Birds were divided into 3 groups: control (n = 18) fed ad libitum, subjected to a pause in laying (n = 18), and subjected to a pause in laying and injected every day with 200 μg/kg BW of chicken GH (chGH; n = 18). The liver, ovarian stroma, and follicles were isolated from the hens of each group on days 6 (ovary regression), 13 (ovary recrudescence), and 17 or 20 (ovary rejuvenated) of the experiment. The results showed that injection of chGH during fasting (1) increased the number of follicles <1 mm and proliferating cell nuclear antigen (PCNA)-positive (proliferating) cells in these follicles; (2) attenuated the expression of PCNA and survivin mRNA in the white follicles and the activity of caspases 3, 8, and 9 in the stroma and white follicles; (3) intensified the atresia of yellow hierarchical follicles; and (4) deepened the effect of starvation on egg yolk gene expression concomitantly with considerably increased IGF-1 transcription levels in the liver (P < 0.05 to P < 0.001). Prolongation of chGH injections into the refeeding period did not exert pronounced effects on the examined parameters. In summary, the results provide evidence that GH promotes the formation and development of prehierarchical follicles in the hen ovary during a pause in laying by regulating cell proliferation and apoptosis. Alterations in cell proliferation- and apoptosis-related gene expression or enzyme activity in ovarian follicles as well as the expression of egg yolk proteins in the liver after chGH treatment strongly suggest that this hormone is involved in determining the rate of regression and rejuvenation of the chicken ovary during a pause in laying.

Growth hormone production and role in the reproductive system of female chicken

The expression and role of growth hormone (GH) in the reproductive system of mammals is rather well established. In birds the limited information thus far available suggests that GH is an endocrine or paracrine/autocrine regulator of ovarian and oviductal functions too. GH and its receptors are expressed in all compartments of the ovary and oviduct and change accordingly to physiological state. The intra-ovarian role of GH likely includes the regulation of steroidogenesis, cell proliferation and apoptosis, the modulation of LH action and the synthesis of IGFs (insulin-like growth factors). In the oviduct, GH is also involved in the regulation of oviduct-specific protein expression. The present study provides a review of current knowledge on the presence and action of GH in the female reproduction, in which it is likely that act in endocrine, autocrine or paracrine mechanisms.

Growth hormone and growth?

Pituitary GH is obligatory for normal growth in mammals, but the importance of pituitary GH in avian growth is less certain. In birds, pituitary GH is biologically active and has growth promoting actions in the tibia-test bioassay. Its importance in normal growth is indicated by the growth suppression following the surgical removal of the pituitary gland or after the immunoneutralization of endogenous pituitary GH. The partial restoration of growth in some studies with GH-treated hypophysectomized birds also suggests GH dependency in avian growth, as does the dwarfism that occurs in some strains with GHR dysfunctions. Circulating GH concentrations are also correlated with body weight gain, being high in young, rapidly growing birds and low in slower growing older birds. Nevertheless, despite these observations, there is an extensive literature that concludes pituitary GH is not important in avian growth. This is based on numerous studies with hypophysectomized and intact birds that show only slight, transitory or absent growth responses to exogenous GH-treatment. Moreover, while circulating GH levels correlate with weight gain in young birds, this may merely reflect changes in the control of pituitary GH secretion during aging, as numerous studies involving experimental alterations in growth rate fail to show positive correlations between plasma GH concentrations and the alterations in growth rate. Furthermore, growth is known to occur in the absence of pituitary GH, as most embryonic development occurs prior to the ontogenetic appearance of pituitary somatotrophs and the appearance of GH in embryonic circulation. Early embryonic growth is also independent of the endocrine actions of pituitary GH, since removal of the presumptive pituitary gland does not impair early growth. Embryonic growth does, however, occur in the presence of extrapituitary GH, which is produced by most tissues and has autocrine or paracrine roles that locally promote growth and development. The role of GH in avian growth is therefore still unclear.

Effects ofad libitumfeeding on performance of different strains of broiler breeders

(1) Tolerance to ad libitum feeding was compared in three genotypes of broiler breeder hens: a standard broiler breeder fed ad libitum (SA) or restricted (SR), a slow growing 'label' broiler breeder (L) and an experimental dwarf heavy broiler breeder (E). Two similar experiments were conducted in two distinct research centres. (2) Feed intake and body weight were measured every 3 weeks from hatch to 40 to 49 weeks of age. Egg production and egg abnormalities were recorded. The number of yellow follicles in ovaries was counted at the age of 32 weeks. (3) Body weight was stabilised at 2.2, 3.7 and 5.4 kg after 24 weeks of age in L, E and SA hens, respectively. Growth of the SR hens was similar to that of L up to 20 weeks and stabilised at a similar level to that of E hens after 30 weeks of age. (4) Sexual maturity was delayed by 6 weeks in restricted breeders compared to ad libitum fed hens that started to lay at 20 weeks. SA hens had low egg production and a high proportion of defective eggs, which was largely compensated for by feed restriction. However, productivity of SR hens remained lower than that of L breeders. (5) Compared to the low viability and reproductive fitness observed with SA hens, the E dwarf broiler breeder tolerated ad libitum feeding and had better egg production, fewer egg abnormalities and yellow follicles per ovary and a higher egg production. However, laying rate was still lower than that of the SR and L groups. Energy conversion (kJ/g egg) from 32 to 40 weeks of age was much higher in the SA group than in the other three groups. 6. The feasibility of feeding a dwarf broiler breeder ad libitum calls for further research on implications of specific IGF and GH-receptor expression at the level of the ovary in dw chickens.

The chicken pituitary-specific transcription factor PIT-1 is involved in the hypothalamic regulation of pituitary hormones

Pit-1 is a pituitary-specific POU-domain DNA binding factor, which binds to and trans-activates promoters of growth hormone- (GH), prolactin- (PRL) and thyroid stimulating hormone-beta- (TSHbeta) encoding genes. Thyrotropin-releasing hormone (TRH) is located in the hypothalamus and stimulates TSH, GH and PRL release from the pituitary gland. In the present study, we successfully used the cell aggregate culture system for chicken pituitary cells to study the effect of TRH administration on the ggPit-l* (chicken Pit-1), GH and TSHbeta mRNA expression in vitro. In pituitary cell aggregates of 11-day-old male broiler chicks the ggPit-l * mRNA expression was significantly increased following TRH administration, indicating that the stimulatory effects of TRH on several pituitary hormones are mediated via its effect on the ggPit-l* gene expression. Therefore, a semiquantitative RT-PCR method was used to detect possible changes in GH and TSHbeta mRNA levels. TRH affected both the GH and TSHbeta mRNA levels. The results of this in vitro study reveal that ggPit-1 * has a role in mediating the stimulatory effects of TRH on pituitary hormones like GH and TSHbeta in the chicken pituitary.

Thyroid hormone availability and activity in avian species: a review

The intracellular thyroid hormone (TH) availability is influenced by different metabolic pathways. Some of the changes in intracellular TH availability can be linked to changes in local deiodination and sulfation capacities. The secretion of the chicken thyroid consists predominantly of thyroxine (T4). TH receptors (TRs) preferentially bind 3,5,3'-triiodothyronine (T3). Therefore, the metabolism of T4 secreted by the thyroid gland in peripheral tissues, resulting in the production and degradation of receptor-active T3, plays a major role in thyroid function. Food restriction in growing chickens increases hepatic type III deiodinase (D3) levels but decreases growth hormone (GH)-dependent variables such as plasma insulin-like growth factor-I (IGF-I) and T3 concentrations. Refeeding restores hepatic D3 and plasma T3 to control levels within a few hours. It can be concluded that the tissue and time dependent regulation of the balance between TH activating and inactivating enzymes plays an essential role in the control of local T3 availability and hence in TH activity. Two separate genes encode multiple TR isoforms, i.e. TRalpha and TRbeta. These TRs consist of a DNA-binding domain, a ligand-binding domain, a hinge region and an amino-terminal (A/B) domain. TRs mediate their effects on transcription by binding as homodimers or heterodimers to the TH response elements (TREs). Also, unliganded TRs can bind to TREs and may so modulate transcription of target genes.

BMPs and BMPRs in chicken ovary and effects of BMP-4 and -7 on granulosa cell proliferation and progesterone production in vitro

Bone morphogenetic proteins (BMPs) and their receptors (BMPRs) are now known to have important roles in mammalian ovarian folliculogenesis. This study determined the expression of the mRNA encoding for BMPs and their receptors in the chicken ovary and explored possible roles for them. The expression of the mRNA for BMP-2, -4, -6, -7, and BMPR-IA, -IB, and -II was determined and quantified by a semiquantitative RT-PCR. The mRNAs for all the BMPs and receptors determined were present in both the granulosa (G) and theca (T) cells of the F1, F2, and F3 follicles. All BMP mRNAs increased in G cells with follicular development, whereas only BMP-7 mRNA had this trend in the T cells. BMP-2, -4, and -6 mRNAs in T were similar between follicles. BMPR-IA mRNA was similar in F2G and F3G but lower in F1G. BMPR-IB mRNA was similar in G of all follicles, and BMPR-II mRNA increased with development. In the T, each receptor subtype showed equal distribution between follicles. mRNA levels for BMPR-IB and -II were higher in G than in T, suggesting that the G is a major target for BMPs. BMP-4 and -7 stimulated basal, IGF-I-, and gonadotropin-stimulated progesterone production by cultured G cells, with differential responses between cells from the F1 and F3/4. This suggests involvement in follicular differentiation. BMP-4 and -7 reversed the inhibitory effects of transforming growth factor (TGF)-alpha on basal and gonadotropin-stimulated G cell progesterone production, with greater effect in the F1 than in the F3/4. This effect suggests an important role for BMPs interacting with TGF-alpha in modulating the effects of gonadotropins and IGF-I on follicular differentiation. Finally, BMP-7 stimulated G cell proliferation, but BMP-4 inhibited TGF-alpha+ IGF-I- and/or FSH-stimulated G cell proliferation, suggesting a role in the control of follicular growth during development. These effects of BMP-4 and -7 on the G cell function showed relationships with the expression levels of the BMPs and the BMPR-II.

Pre- and postprandial changes in plasma hormone and metabolite levels and hepatic deiodinase activities in meal-fed broiler chickens

The present study aimed to study the effects of food deprivation and subsequent postprandial changes in plasma somatotrophic and thyrotrophic hormone levels and focused on the inter-relationships between these hormonal axes and representative metabolites of the intermediary metabolism of meal-fed broiler chickens. Male broiler chickens (2 weeks old) were fed a meal of 40-45 g/bird per d for two consecutive weeks (food-restricted (FR) treatment). The daily allowance was consumed in about 30 min. At 4 weeks of age, FR chickens were killed at several time intervals (ten per sampling time) in relation to the daily food allowance: before feeding (about 23.5 h of food deprivation), and at 10, 20, 30 (end of feeding), 40, 50, 60, 90, 120 and 200 min after initiation of feeding. Birds fed ad libitum served as controls (ad-libitum (AL) treatment). Liver tissue was collected for deiodinase type I and type III activity measurements and blood samples for analysis of growth hormone (GH), insulin-like growth factor (IGF)-I, thyroxine (T4), 3,3',5-triiodothyronine (T3), glucose, non-esterified fatty acids (NEFA), uric acid, triacylglycerol (TG) and lactate levels. Food deprivation caused a shift from lipogenesis to lipolysis and increased fatty acid turnover, a reduction in protein anabolism and reduced metabolic rate. Food intake was followed immediately by a pronounced increase in metabolic rate, initially mainly based on anaerobic mechanisms. Refeeding gradually reversed the fasting-induced alterations in plasma hormone and metabolite levels, but the time course of changes differed between metabolites, which clearly preceded those of the hormones investigated. The order of responsiveness after food provision were glucose>uric acid>or=NEFA>lactate>TG for the plasma metabolites and for hormones. Based on these different postprandial time courses, several functional relationships are proposed. Glucose is believed to be the primary trigger for the normalisation of the effects of fasting on these plasma variables by restoring hepatic GH receptor capacity, as well as decreasing deiodinase type III activity.