Ovarian and extraovarian sources of immunoreactive inhibin in the chicken: effects of dexamethasone

Intra-ovarian growth factors regulating ovarian function in avian species: a review

There is now overwhelming evidence that the avian ovary is a site of production and action of several growth factors that have also been implicated in the functioning of the mammalian ovary. Several members of the Insulin-like growth factor family (IGF), the Epidermal growth factor family (EGF), the Transforming growth factor-beta family (TGF-beta), Fibroblast growth factors (FGF), the Tumour necrosis factor-alpha (TNF-alpha), and others, have been identified either in the granulosa and/or theca compartments of ovarian follicles and in the embryonic and juvenile ovary. Some have been specifically localized to the germinal disc area containing the oocyte. The mRNAs and proteins of the growth factors, receptor proteins and binding proteins of some of the members of each group have been reported in the chicken, turkey, quail and duck. The intra-ovarian roles reported for the different growth factors include regulation of cell proliferation, steroidogenesis, follicle selection, modulation of gonadotrophin action, control of ovulation rate, cell differentiation, production of growth factors, etc. The aim of this paper is to provide a review of the current knowledge of avian ovarian growth factors and their biological activity in the ovary. The review covers the detection of the growth factor proteins, the receptor proteins, binding proteins, their spatial and temporal distribution in embryonic, juvenile and adult ovaries and their regulation. The paper also discusses their roles in each follicular compartment during follicular development. Greater emphasis is given to the major growth factors that have been studied to greater detail and others are discussed very briefly.

Developmental changes in inhibin α and inhibin/activin βA and βB mRNA levels in the gonads during post-hatch prepubertal development of male and female chickens

Dimeric inhibins and activins are barely detectable in the plasma during prepubertal development of male and female chickens. This may be misconstrued to indicate that the proteins are not produced in the gonads and have no functional significance during this period. Very few studies have actually determined the mRNA expression profile of the inhibin and activin subunits in the gonads prior to puberty in order to establish their secretion at the local level and postulate potential roles for the inhibin and activins at this developmental stage. In this study, the expression of the mRNA for the alpha-, betaA-, and betaB-subunits was determined in the ovary and testis of chickens during prepubertal development. Gene expression was determined at 3, 5, 6, 8, 10, 12, 16, and 18 weeks of age by RT-PCR. Messenger RNA level was quantified by competitive RT-PCR at 3, 6, 12, and 18 weeks of age in order to detect any changes with development, suggest potential relationship to the profile of dimeric inhibins and activins reported previously and to suggest potential paracrine and endocrine roles for them. The results show that all the inhibin/activin subunit mRNAs are expressed in the testis of the chicken throughout the period of prepubertal development up to 18 weeks of age. However, in the ovary, only the betaA- and betaB-subunits were detected at all ages whereas the alpha-subunit mRNA could only be detected just before puberty. Quantification of the mRNA levels showed variation of each subunit with age. These temporal changes suggest relationship with paracrine functional role in the ovary or the testis. Quantitative changes in expression levels also suggests that there may be some relationship between mRNA levels and the type and amount of dimeric inhibins and activins produced at any developmental stage. There are major differences between the male and female gonads in the timing of the expression of different subunits. In conclusion, the expression of the mRNA subunits in the testis and ovary suggests that inhibins and activins are being produced but may be principally involved in autocrine/paracrine function within the gonads.

Measurement of plasma corticosterone and fecal glucocorticoid metabolites in the chicken (Gallus domesticus), the great cormorant (Phalacrocorax carbo), and the goshawk (Accipiter gentilis)

A method for the non-invasive measurement of glucocorticoid metabolites in feces of chickens was established and validated. After high-performance liquid chromatography (HPLC) the presence of at least two fecal immunoreactives was demonstrated, one co-eluting with authentic corticosterone, whereas the second substance migrates close to corticosterone sulphate. We investigated the relationship between corticosterone in blood plasma obtained by a vena brachialis catheter and fecal samples in groups of five chickens after an ACTH and a dexamethasone injection to stimulate and to suppress adrenal activity. A control group received a saline injection. After ACTH plasma cortisol concentrations increased 16-fold after 1.5 h to levels between 19 and 38 ng/ml and dropped to pre-treatment levels (1.1-2.5 ng/ml) 4h after stimulation. Dexamethasone did not result in a distinct suppression of adrenocortical activity and plasma corticosterone dropped only slightly below pre-treatment levels. The concentrations in fecal metabolites corresponded to the changes in the levels of biological active hormone in plasma. Fecal peak excretion (105-295 ng/g) was obtained with a delay of approximately 4 h compared to plasma. The profile obtained after ACTH challenge reflected a broader and dampened pattern of glucocorticoid secretion and provided a more integrated measure of adrenal activity. Dexamethasone treatment did not induce a measurable decrease in fecal metabolites and concentrations fluctuated around a mean of 30.0+/-9.9 ng/g, almost identical to those obtained from the saline treatment group (29.4+/-13.9 ng/g). In a separate experiment the effect of an alternative capture method (remote-controlled injection system) was investigated in cormorants. Plasma corticosterone measurements revealed a significantly diminished stress reaction compared to traditional trapping (1.24+/-0.78 vs. 10.9+/-12.1 ng/ml). Investigations whether goshawk nestlings infected with Trichomas gallinae differ in fecal corticosterone metabolite concentrations compared to healthy subjects revealed no significant changes. However, a significant correlation was found between the glucocorticoid metabolite concentrations and the number of nestlings per nest. The demonstration that adrenal activity can be detected by the assay is a prerequisite that ecologically meaningful levels of imposed stress can be validated. Therefore, non-invasive measurements of fecal metabolites are a promising perspective to monitor stress in birds.

Effect of embryonic 19-nortestosterone treatment and surgical bursectomy on plasma concentrations of reproductive hormones, on inhibin content in adrenals and gonads and on the histological appearance of the gonads in the young chicken

Four-day-old chick embryos were hormonally treated with 19-nortestosterone in order to inhibit bursa development. At days 1, 4, 8, 15, 22, 29, and 36 of age, plasma, adrenals, and gonads from intact and hormonal treated chicks were collected. In embryonic nortestosterone treated males the appearance of a left 'ovotestis-like' gonad was observed. The occurrence of this ovotestis-like left gonad in the 19-nortestosterone treated male is probably a secondary effect of the in ovo treatment since surgically bursectomised chicks did not show the testicular morphology and histological changes as observed in 19-nortestosterone treated chicks. Additionally, both male and female hormonally or surgically treated chicks showed relatively enlarged adrenal glands. Hormonal bursectomy affected organ inhibin contents and plasma inhibin, testosterone, and FSH levels in males. Male hormonal treated chicks showed lower levels of plasma inhibin (p=0.0001), testosterone (p=0.01), and FSH (p=0.004), and a lower total testes inhibin content (p=0.0003) compared to intact chicks. However, none of these were significantly different between female intact and hormonal treated chicks, again indicating that the observed hormonal changes in males are not the result of the disappearance of the bursa but of the hormonal 19-NT treatment. The total adrenal inhibin content as well as the adrenal inhibin concentration were significantly higher in hormonally treated chicks than in intact chicks (p=0.0001), regardless of the sex.

Molecular cloning and expression of turkey inhibin-alpha and -betaA subunits

We isolated cDNA encoding turkey inhibin-alpha (tINH-alpha) and -betaA (tINH-betaA) subunits from the turkey ovary using reverse transcription-polymerase chain reaction (RT-PCR). The isolated alpha subunit and betaA subunit included the entire open reading frames encoding 329 and 424 amino acids, respectively. The amino acid sequences of mature tINH-alpha subunit and tINH-betaA subunit (12.6 and 12.9 kDa proteins, respectively), established via DNA sequence analysis, were highly conserved between the chicken and various mammals. Northern blot analysis revealed that the transcripts of tINH-alpha and tINH-betaA subunits were approximately 1.7 and 8.4 kb, respectively. In various stages of follicular development, tINH-alpha mRNA was highly expressed in small white follicles as compared to postovulatory and regressed follicles, whereas tINH-betaA mRNA was predominately expressed in preovulatory F5 follicles.

Acute pretranslational regulation of type III iodothyronine deiodinase by growth hormone and dexamethasone in chicken embryos

Both growth hormone (GH) and glucocorticoids are regulators of thyroid hormone metabolism in vertebrates. Studies on chicken embryos demonstrated that intravenous (i.v.) injection of chicken GH or glucocorticoids results in increased plasma 3,3',5-triiodothyronine (T3) concentrations, and this through a reduction of hepatic type III iodothyronine deiodinase (D3) activity. The recent cloning of chicken type I iodothyronine deiodinase (D1) and D3 offers the tools to investigate at what level (pre- or posttranslational) this downregulation of D3 occurs. Eighteen day old chicken embryos were injected with either 0.9% NaCl (control), 50 microg dexamethasone (DEX), or 20 microg cGH per animal. Plasma and tissue samples were taken 5, 10, 30, 60, 120, and 240 min post-injection. Plasma T3 and thyroxine (T4) were determined as well as in vitro hepatic D1 and D3 activities. Hepatic D1 and D3 mRNA levels were measured by both Northern analysis and competitive reverse transcription polymerase chain reaction (RT-PCR). Injection of GH or DEX resulted in a significant increase in plasma T3 when compared to controls within 30 min post-injection. This increase remained until the end of the experiment in the DEX-treated group, but not in the GH group. GH administration had no influence on plasma T4 levels, whereas DEX significantly reduced plasma T4 from 30 min onwards. Hepatic D1 activity and D1 mRNA levels showed no changes. Hepatic D3 activity, however, decreased within 10 min after DEX administration and somewhat slower after GH administration (within 30 min). Hepatic D3 activity remained low for the remainder of the experiment in the DEX-treated group, whereas D3 activity gradually returned to control levels in the GH group. This change in hepatic D3 activity was paralleled by the changes in hepatic D3 mRNA levels (r = 0.88, P = 0.0001) as confirmed by both Northern analysis and competitive RT-PCR. In conclusion, these results demonstrate that in embryonic chicken GH and DEX acutely increase plasma T3 levels by decreasing hepatic D3 activity, a decrease that seems to be regulated predominantly at the pretranslational level. These results are also an indication for the short half life (t(1/2)) of the D3 enzyme. The time lag between the effect of GH and DEX on hepatic D3 mRNA may be due to differences in the mechanism of action between both hormones, a subject that needs further investigation.

Quantification of inhibin/activin alpha and betaA subunit messenger ribonucleic acid by competitive reverse transcription-polymerase chain reaction in chicken granulosa cells during follicular development

The very sensitive quantitative competitive reverse transcription-polymerase chain reaction (RT-PCR) was used to investigate the expression of inhibin/activin subunits in the granulosa cells of developing ovarian follicles of the hen. Two competitors specific to inhibin alpha and betaA subunits were constructed. In one study, the expression of inhibin alpha and betaA genes was determined in the granulosa cells of the five largest yellow follicles (F1, F2, F3, F4/5), the small yellow follicles (SYF), and the large white follicles (LWF) of a layer strain of chickens. Competitive RT-PCR for inhibin alpha subunit revealed 10.35 +/- 2.15 pg/ microg total RNA in the LWF. The expression increased 40-fold in the SYF and remained at that level in the F4/5 but decreased markedly thereafter up to the F1 stage. Inhibin/activin betaA subunit was also detected in the LWF in low amounts and showed no significant increase until the F2 stage. The highest level was found in the F1. The pattern of the mRNA for alpha and betaA subunits in the five largest follicles (F1, F2, F3, F4/5) of a broiler breeder strain of chicken was compared with that in the layer strain. Expression of the alpha subunit was significantly higher in the three largest follicles (F1, F2, F3) of the broiler breeder hens, but only in the F2 for the betaA. The results suggest that inhibin alpha may play an important role in the recruitment and differentiation of follicles and that differences between broiler breeders and layers may have consequences at both the pituitary and ovarian levels.