Plasma concentrations of immunoreactive inhibin and gonadotropins following removal of ovarian follicles in the domestic hen

Relationship between cyclic follicle recruitment and ovulation in the hen ovary

Determining whether follicle recruitment in the domestic hen is functionally linked to ovulation would inform investigations on the exact time of cyclic recruitment and subsequently, the earliest cellular event mediating recruitment. The objective of the present studies was to determine if the absence of ovulation results in the failure of cyclic recruitment. 3 groups of Hy-Line W-36 hens were studied: no-ovulation, first-ovulation, or late-ovulation within the sequence. Time and occurrence of oviposition and ovulation were documented by video recording and cloacal palpation. To determine the presence of a recently recruited follicle, ovaries were collected during the last hour of photoperiod, and follicles weighing 280 to 900 mg, with yellow yolk incorporated, and diameter of 9 to 12 mm, were considered recently recruited. To compare the amount of yolk uptake in recruited follicles, hens were fed Sudan IV dye 6 h before ovary collection. Percent dyed yolk was quantified from cross-sections of the recruited follicles. To confirm follicle viability, granulosa cell (GC) mitotic activity was assessed via flow cytometry. Initial results indicated the presence of a recently recruited follicle in 10 of 11, 7 of 10, and 9 of 10, no-ovulation, first-ovulation, and late-ovulation ovaries, respectively, with no significant differences in weight (543 ± 62 mg, 456 ± 71 mg, 620 ± 75 mg, respectively). There were no significant differences in yolk incorporation among the most recently recruited follicles with 19.0 ± 2.0%, 18.0 ± 3.7%, and 19.8 ± 3.2% dyed yolk in no-ovulation, first-ovulation, and late-ovulation ovaries, respectively. Finally, there were no significant differences in % GC in the S/G2-M phase of mitosis among recruited follicles (19.0 ± 5.0%, 22.0 ± 4.2%, 15.67 ± 1.0%, in no-ovulation, first-ovulation, and late-ovulation ovaries, respectively), confirming viability of all recruited follicles. We conclude that cyclic recruitment occurs independently of ovulation and propose that the order and timing of cyclic recruitment is predetermined at an earlier stage of follicle development.

The characteristics of oviposition and hormonal and gene regulation of ovarian follicle development in Magang geese

BACKGROUND

Egg laying in Magang geese is characterized by extended interruption between clutches and lowing laying rate. Both the ovarian follicular development and ovulation characteristics, and the associated endocrine and molecular regulatory mechanisms involved are poorly understood, but could be important for guiding development of molecule aided selection of egg laying performances in geese. This study, therefore, recorded egg-laying characteristics of Magang geese, and the endocrine and molecular regulatory mechanisms of ovarian follicular development, maturation, and ovulation in Magang geese.

METHODS

Oviposition, ovarian follicle development, and reproductive hormone and gene expression profiles were observed in a small flock of Magang geese.

RESULTS

Greater than 73% of eggs were laid during the day. The average oviposition interval was 46.8 h (36-55 h). It took approximately 18 days for large white follicles to develop into mature F1 follicles; follicular growth was exponential. LHR expression levels increased from the small to the large mature follicles, but FSHR expression decreased in the granulosa and thecal layers. As the follicles matured, inhibin alpha and inhibin betaA expression increased in the granulosa layer. Activin IR, activin IIRA, activin IIRB, and beta-glycan expressions also increased as the follicles increased in size, but were more abundantly expressed in the thecal than in the granulosa layers. During the oviposition cycle, plasma concentrations of gonadal hormones decreased rapidly, whereas the level of PGFM peaked around ovulation. The profiles of activin, inhibin, follistatin, estradiol, and progesterone leading to ovulation were characterized.

CONCLUSIONS

The molecular and endocrine mechanisms that regulate follicular development in Magang geese are similar to those in chickens. Moreover, gonadotropin regulation and interaction between activin, inhibin, and follistatin secretion may govern 3-stage maturation in the final preovulatory follicles in Magang geese. The rapid rebound of post-ovulatory secretions of inhibin and follistatin may inhibit recruitment of new SYF recruitment once a sequence of eggs is started, and may limit the egg clutch size to no more than the number of LYFs present before the first sequence egg.

Follicle dynamics and its relation with plasma concentrations of progesterone, luteinizing hormone and estradiol during the egg-laying cycle in ostriches

The aims of this study were (i) to describe the changes in the volume of large ovarian follicles (diameter >3 cm) during the 48 h egg laying cycle in farmed ostriches, and (ii) to quantify factors affecting the volume of the largest measured follicle and the plasma concentrations of progesterone (P(4)) and estradiol-17beta (E(2)beta). In eight egg-producing birds, which all ovulated during the study period, transcutaneous ultrasound scanning and blood sampling was performed at 3 h intervals. The average volume of the total number of visualized large follicles (V(total)), the largest measured follicle (V(F1)), the second largest follicle (V(F2)) and of all follicles smaller than F2 (V(F3-Fn)) were each higher before than after oviposition. V(total), V(F2) and V(F3-Fn) nearly doubled in the 24-h period before oviposition, while V(F1) remained at an equal, rather high level until oviposition. Immediately after oviposition V(total), as well as the volume of the other follicle categories, decreased within 6 h, i.e. around the moment of ovulation. By performing statistical analysis on the basis of linear mixed-effects modelling, we quantified that: (i) V(F1) was 13.2% higher before than after oviposition and increased with 6.5% when LH increased with 1 ng/ml; (ii) P(4) levels were 93.2% higher before than after oviposition and increased with 43.1% for every 3 h closer to oviposition; when LH and E(2)beta levels and V(F1) increased with 1 ng/ml, 10 pg/ml and 10 ml, respectively, P(4) increased with 116.6%, 50% and 6.1%; and (iii) E(2)beta levels were 35.6% higher before than after oviposition, increased with 2.7% for every 3 h closer to oviposition and increased with 14.6% when LH increased with 1 ng/ml. It is concluded that during the egg-laying cycle in ostriches: (i) follicular mass, as estimated by the volume of visualized follicles larger than 3 cm, increases before and decreases after ovulation, and (ii) follicular dynamics and its accompanying endocrine plasma hormone profiles during the egg-laying cycle in ostriches follow a pattern similar to that in chickens.

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.

Update on neuroendocrine regulation and medical intervention of reproduction in birds

In avian species, reproductive disorders and undesirable behaviors commonly reflect abnormalities in the neuroendocrine regulation of the reproductive system. Current treatment options are often disappointing, show no long-lasting effect, or have significant side effects. A possible reason for our lack of success is a dearth of knowledge of the underlying neuroendocrine, behavioral, and autonomous physiology of the reproductive processes. Tremendous progress has been made in the last few years in our understanding of the neuroendocrine control of reproduction in birds. Advantage should be taken of these experimentally derived data to develop appropriate and safe treatment protocols for avian patients suffering from reproductive disorders.

Pituitary progesterone receptor expression and plasma gonadotrophin concentrations in the reproductively dysfunctional mutant restricted ovulator chicken

Female mutant restricted ovulator (RO) chickens of the White Leghorn strain carry a naturally occurring single nucleotide mutation in the very low density lipoprotein receptor (VLDLR) gene. Due to this mutation, RO hens fail to express a functional VLDLR protein on the oocyte membrane, which results in an impaired uptake of circulating yolk precursor macromolecules. Mutant RO hens subsequently develop hyperlipidemia and generally fail to lay eggs due to follicular atresia. Since RO hens also reportedly have three-fold higher basal plasma estrogen concentrations, combined with four-fold lower levels of circulating progesterone as compared to wild-type (WT) hens, we hypothesized that RO hens would have an increased abundance of pituitary progesterone receptor (PR) mRNA and PR isoforms A and B as well as alterations in circulating gonadotrophin levels. Quantitative PCR assays revealed significantly greater (P<or=0.05) pituitary PR mRNA abundance in RO hens as compared to WT hens. Similarly, pituitary PR isoforms A and B quantities were significantly greater (P<or=0.05) in the RO hens compared to WT hens. In addition, mutant RO hens had significantly greater plasma concentrations of luteinizing hormone, follicle stimulating hormone, estrone, and estradiol, but lower circulating progesterone levels. Collectively, elevated circulating estrogen and/or decreased progesterone levels may have contributed to the upregulation of PR mRNA and PR isoforms A and B in the RO hen pituitary gland. Lastly, in order to gain a more complete understanding of why RO hens are reproductively dysfunctional, a model is proposed that links humoral and ovarian factors to observed and putative changes in the hypothalamic-pituitary axis.

Changes in reproductive neuroendocrine mRNAs with decreasing ovarian function in ageing hens

Egg production declines with advancing age in the domestic chicken and this is particularly pronounced in breeding stocks of meat type hens (broiler breeders). The objective of this study was to establish whether declining egg production with reproductive ageing in broiler breeders is correlated with plasma LH and FSH, and with mRNAs encoding hypothalamic gonadotrophin-releasing hormone-I (GnRH-I), gonadotrophin inhibitory hormone (GnIH), and gonadotrophin subunits. Comparisons were made between hens at the peak of egg laying (young: 30 weeks) and at the end of a laying year (old: 60 weeks). Old hens were subdivided into laying and out-of-lay groups. Plasma LH and FSH were lower in old than in young laying hens. Compared with old laying hens, old out-of-lay hens had significantly increased plasma FSH but not plasma LH. There were no differences in total hypothalamic GnRH-I and GnIH mRNAs between young and old hens. In old laying hens, the decrease in plasma LH was correlated with decreased gonadotrophin alpha-subunit but not LHbeta mRNAs. The decrease in plasma FSH was not associated with a change in FSHbeta mRNA. In old out-of-lay hens, the increase in plasma FSH was correlated with increased FSHbeta mRNA, while unchanged plasma LH was associated with increased LHbeta mRNA. A regression analysis of all plasma gonadotrophin and gonadotrophin subunit mRNA data collected from the study demonstrated that plasma LH is correlated with alpha-subunit but not LHbeta mRNAs, while plasma FSH is correlated with FSHbeta but not alpha-subunit mRNAs. It is concluded that the decrease in the rate of lay in ageing broiler breeders is not correlated with decreased GnRH-I mRNA nor with increased GnIH mRNA, but it is related to a decrease in alpha-subunit mRNA which may account for the associated reduction in plasma LH but not FSH.

Developmental changes of plasma inhibin, gonadotropins, steroid hormones, and thyroid hormones in male and female Shao ducks

Plasma samples from developing male and female Shao ducks were assayed for immunoreactive (ir-) inhibin, follicle-stimulating hormone (FSH), luteinizing hormone (LH), steroid hormones, and thyroid hormones. In the male, plasma ir-inhibin significantly increased between 75 and 155 days posthatch, and then decreased slightly at day 165. Plasma FSH of male ducks decreased from day 35 to day 55, followed by progressive elevation until day 95. Plasma FSH of male ducks fell significantly at days 135 and 165, while plasma ir-inhibin rose to high level. In female ducks, plasma ir-inhibin remained low until the start of lay, and thereafter significantly increased at day 135. Plasma FSH fluctuated before day 95 and significantly rose at day 115, and decreased thereafter. In males, plasma LH did not vary significantly before day 135, however, plasma testosterone significantly increased from day 95 onwards. No changes in plasma LH were observed during development of female ducks. Plasma estradiol-17beta gradually increased reaching a peak level at day 135. Plasma progesterone did not vary significantly before day 135 and thereafter significantly increased. Both sexes showed a similar pattern in changes of plasma thyroid hormones during sexual development. There was a significant increase in plasma thyroxine (T4) at day 95, thereafter decreased. Plasma triiodothyronine (T3) was at high level at the earlier stage of development and significantly decreased at day 55. Significant increase in plasma T3 in male and female ducks was observed at 135 and 115 days, respectively. In conclusion, these results demonstrated that the rise in inhibin is correlated with age of sexual maturity in the female while the rise in inhibin significantly precedes sexual maturity in the male. There was a progressive increase in plasma steroid hormones towards sexual maturity, and there was no sex difference in the time course of thyroid hormones.

Pattern of secretion of immunoreactive inhibin/activin subunits by avian granulosa cells

The messenger RNA expression for the inhibin/activin subunits in the granulosa layer of avian follicles of different developmental stages has previously been reported. In the present study, we examined the pattern of secretion of these protein subunits from cultured granulosa cells (GC) of avian follicles of defined maturity. Laying hens were euthanized and the F1, F2, F3, F4, small yellow follicles (SYF; 6-10 mm) and large white follicles (LWF; 3-5 mm) were removed. GC were isolated from the follicles, plated by size at a density of 6.25 x 10(5)cells per well (3 wells per follicle size) and cultured for 48 h in medium 199 with 5% FBS, antibiotics, and 1.0 microg/ml bovine insulin. After 48 h, the cultures were terminated and the media were saved (n = 6 replications). Proteins were precipitated from media, reconstituted for electrophoresis (SDS-PAGE), and analyzed by Western blot. Progesterone was also measured in the medium. For detection of the inhibin alpha-subunit, a rabbit antibody against the chicken inhibin alpha-subunit (1-26 aa) was used. The betaA-subunit was detected with rabbit anti-betaA-subunit (81-113 aa) and the betaB-subunit was detected with rabbit anti-betaB-subunit (80-112 aa). Under reduced conditions, GC from the larger follicle sizes (F1-F4) secreted the most (p < 0.05) immunoreactive inhibin alpha-subunit compared to smaller follicle sizes. Under non-reduced conditions, a band at approximately 32 kDa was detected by both the alpha-subunit antibody and by the betaA-subunit antibody in media from GC of the F1-F4 follicles, suggesting secretion of intact inhibin A. Immunoreactive alpha-subunit and betaB-subunit were detected under reduced conditions in media from the GC of the SYF, suggesting that this follicle population may secrete intact inhibin B. In addition, under non-reduced conditions, cells from the SYF secreted the greatest amount of intact inhibin B (p < 0.05) at a size of approximately 32 kDa. Cells from the LWF expressed low levels of all inhibin subunits. Progesterone concentration in the media from the F1 follicle was greatest and was decreased in media from smaller follicles. It is suggested that the largest follicles in the hierarchy are the source of most circulating intact inhibin A while the small follicles are the source of intact inhibin B.

Messenger RNA and Protein Expression Analysis of Betaglycan in the Pituitary and Ovary of the Domestic Hen1

Betaglycan was originally characterized as the type III receptor for TGFbeta, yet recent research has indicated that betaglycan can serve as an accessory receptor for inhibin. To understand better the action of inhibin in avian follicular development, we have investigated the expression of betaglycan in the pituitary gland and ovary of the hen. In experiments 1 and 2, betaglycan mRNA was detected at 6 kilobases (kb) by Northern blot analysis (n = 5) in chicken pituitary, granulosa, and theca layers and whole ovary. Expression of betaglycan was greatest in the pituitary gland in experiment 1 and greater in the granulosa layer of small yellow follicles (SYF) compared with the granulosa layer of larger follicles. In experiment 2, betaglycan mRNA was more abundantly expressed in the theca layer compared with the granulosa layer for all follicle sizes, although there was no significant difference in betaglycan expression in the theca layer among follicle sizes. In experiment 3, immunohistochemical analysis revealed betaglycan protein in the anterior pituitary as well as in the ovary (n = 4) and SYF (n = 4). Colocalization studies revealed a high abundance of cells within the anterior pituitary expressing both betaglycan and FSH (n = 4). Betaglycan protein was found in the granulosa layer; however, markedly enhanced staining was observed in the theca layer of ovarian follicles. Our results provide evidence for expression of betaglycan mRNA and protein colocalization with FSH in the anterior pituitary, consistent with known inhibin effects. Ovarian localization of betaglycan, particularly in the theca layer, suggests a paracrine role for inhibin in the hen.

Lack of Efficacy of Injectable Ketamine with Xylazine or Diazepam for Anesthesia in Chickens

Compared to other laboratory animals, little is known about the use of anesthetics in birds, potentially resulting in the use of improper dosing regimens. The authors compared two commonly used ketamine combinations with isoflurane and concluded that the injectable doses were ineffective for induction of surgical anesthesia in chickens.

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.

Developmental endocrinology of the reproductive axis in the chicken embryo

In mammals, the phenotype of the homogametic sex develops in the (relative) absence of steroids and the phenotype of the heterogametic sex is imposed by the early action of steroids. In contrast, the heterogametic sex in avian species is the female and the presence of estrogens and their receptors plays a crucial role in female sexual differentiation. The time- and sex-dependent expression of enzymes involved in steroidogenesis which determine the ratio of androgens/estrogens produced by the gonads has been extensively investigated during the last 5-6 years. These results all show that the lack of estrogen synthesis in the male appears to be due to the extremely low levels of 17beta-hydroxysteroid dehydrogenase and P450aromatase expression. In females, extensive expression of the aromatase gene (around day 5-6 of incubation), leading to estrogen synthesis, and specific expression of the estrogen receptor-mRNA in the left gonad results in the development of a functional left ovary. Other sex differences can be found in the expression of the inhibin subunit genes in gonads of chicken embryos and in circulating concentrations of inhibin, follicle stimulating hormone (FSH) and steroids. Sex reversal attempts have been made by varying incubation temperatures, by using anti-estrogens, androgens, aromatase inhibitors and synthetic steroids. In ovo administration of a sex steroid hormone or an inhibitor of endogenous sex steroid synthesis can cause phenotypical sex reversal. All these experiments show that the development of gonads in birds is very sensitive to changes in the embryonic hormonal environment, sometimes resulting in changes of postnatal reproduction and even growth.

Active immunization of broiler breeder hens with a recombinant chicken inhibin fusion protein enhances egg lay

We have demonstrated that inoculation of female Coturnix with an inhibin-based immunogen (MBP-cINA521) accelerated puberty and enhanced hen-day egg production (HDEP). Herein, MBP-cINA521, a fusion protein, which consists of the bacterial maltose binding protein (MBP) and a fragment of the infinity-subunit of chicken inhibin (cINA521), was tested for its ability to enhance production performance in broiler breeders. Pullets (Arbor Acres Classic Females; n = 60 birds/treatment group) were given (subcutaneously) 0, 1, 3, or 5 mg of MBP-cINA521 in Freund's complete adjuvant at 20 wk and 4 d of age. Booster immunizations (one-half of the primary dosages) were given at 23 wk of age. The vehicle for controls (CON; no booster) and MBP-cINA521-boosted birds was Freund's incomplete adjuvant. Blood samples were obtained at the end of the trial to assess immunological response to the antigen with a titer ELISA. The onset of puberty was assessed by calculation of the average ages at first lay (FIRST) and at 50% egg production (FIFTY). Cumulative percentage HDEP was determined weekly throughout the laying period (40 wk). Egg weight (EWT) and specific gravity (SG) assessments were made periodically during the trial. Body weight gain (BWG) and mortality (MORT) data were also collected. Significant injection treatment differences (P < 0.01) in inhibin antibody titers were detected according to the following order: low dose = intermediate dose > high dose > CON. A dose of MBP-cINA521 capable of accelerating puberty and increasing overall egg lay was identified. FIRST and FIFTY responses were decreased (P < 0.05) in birds given the intermediate dose (3.0 mg) of MBP-cINA521 when compared to the CON. FIFTY responses were also lower (P < 0.05) than CON responses in those birds given the highest dose (5.0 mg) of the inhibin antigen. Cumulative HDEP was also higher (P < 0.05), beginning at 3 wk of lay and weekly thereafter (P < 0.05, for the remaining 40 wk), in birds given the intermediate immunogen dosage when compared to the CON. By Week 40, an average increase of 9.5% HDEP was realized in birds given 3.0 mg of MBP-cINA521. MORT rates were similar in the CON and in the two lowest MBP-cINA521 treatment groups but were higher (P < 0.05) in those birds given 5.0 mg of the antigen. EWT, SG, and BWG measurements were unaffected by treatment with the inhibin vaccine. In agreement with our findings in quail, immunoneutralization of inhibin enhanced production performance in breeder hens.

Effects of active immunization with inhibin alpha subunit on reproductive characteristics of turkey hens

The hypothesis for the present study is that the active immunization of female turkeys with inhibin (INH) would neutralize endogenous INH, and increase levels of circulating follicle stimulating hormone (FSH) and the number of preovulatory follicles, and subsequently enhance egg production. Two experiments were conducted with female turkeys in their first (30 wk of age) and second (62 wk of age) laying cycles. Treatment groups included control turkeys immunized with keyhole limpet hemocyanine (KLH) and experimental turkeys immunized with recombinant turkey inhibin alpha conjugated to KLH (rtINH), vasoactive intestinal peptide (VIP) conjugated to KLH or rtINH+VIP. Egg production increased (P < 0.05) in VIP and rtINH+VIP immunized birds, but not in rtINH immunized hens in comparison with a control group. A similar number of ovarian follicles, arranged in the follicular hierarchy of laying hens, was observed in all experimental groups. However, there was a larger number of nongraded yellow follicles in rtINH-immunized (62.5%) and rtINH+VIP-immunized (73.5%) groups compared with that of controls, suggesting overstimulation by FSH. Anterior pituitary FSH beta subunit, LH beta subunit, and prolactin (PRL) mRNA contents were determined by Northern blot analysis and reverse transcriptase-polymerase chain reaction (RT-PCR) in laying hens at the end of the experimental period. Hens immunized with rtINH showed increased FSH beta subunit mRNA content, but no change in the content of LH beta subunit or PRL mRNA. Hens immunized with VIP or rtINH+VIP had significant increases in both pituitary LH beta subunit and FSH beta subunit mRNA contents, accompanied by a decline in PRL mRNA abundance. The magnitude of the increase in FSH beta subunit to INH immunoneutralization was greater in first-cycle hens than in second-cycle hens. These data suggest that active immunization of female turkeys with INH neutralizes endogenous INH and increases both circulating FSH and the number of preovulatory follicles. However, no significant increase in egg production was observed in INH-immunized hens. The data confirm previous reports that VIP immunoneutralization increases egg production in turkey hens and shows for the first time that it also increases FSH beta subunit and LH beta subunit gene expression.

Preovulatory follicles in the ovary as the source of circulating inhibin in the duck

Inhibin secretion in the adult female duck was investigated. The bovine inhibin radioimmunoassay (RIA) system and human enzyme-linked immunosorbent assay (ELISA) of inhibin A and inhibin B were first validated for use in the duck. In both RIA and ELISA, the dilution curves of plasma and homogenate of the first largest follicle (F1) were parallel to each standard curve, indicating that plasma and the F1 follicle contained immunoreactive (ir) and dimeric inhibins. Short-term food deprivation caused follicular atresia in the ovary and significantly depressed the plasma concentration of ir-inhibin. Positive immunostaining for inhibin alpha-, betaA-, and betaB-subunits was clearly detected in the granulosa cells of the four largest preovulatory follicles. Immunolocalization of these three inhibin subunits was also weakly seen in the interna theca cells of these follicles. These results demonstrate that inhibin alpha-, betaA-, and betaB-subunit proteins are colocalized in the granulosa cell and theca cell of the four largest preovulatory follicles in the duck ovary. The present results, therefore, indicate that the four largest follicles in the ovary are the main source of circulating inhibin in the female duck.

Follicle-stimulating hormone regulation of inhibin alpha- and beta(B)-subunit and follistatin messenger ribonucleic acid in cultured avian granulosa cells

FSH regulation of inhibin alpha-, beta(B)-subunit and follistatin mRNA was investigated in cultured chicken granulosa cells, which were isolated and pooled according to size from the F(4) + F(5) follicles, small yellow follicles (SYF), and large white follicles (LWF). In experiment 1 (four replicate experiments), granulosa cells were cultured, and the effect of FSH (50 ng/ml) on the growth of cells from the different follicles was examined at 24 and 48 h of culture. Cell viability was >95% for all of the granulosa cell cultures at 24 and 48 h. At 24 h, the number of granulosa cells in both the FSH-treated and the untreated cultures for all follicle types was numerically greater than the number of cells originally plated. At 48 h, FSH-treated cultures for all follicle types had twice (P: < 0. 05) the number of cells as the untreated cultures. In experiment 2 (three replicate experiments), FSH increased expression of the mRNA for inhibin alpha-subunit in LWF granulosa cells at 4 and 24 h to detectable levels and increased inhibin alpha-subunit protein accumulation to detectable levels by 24 h in granulosa cells from the LWF. FSH also increased (P: < 0.05) mRNA levels for the inhibin alpha-subunit at 4 and 24 h in SYF granulosa cells and at 24 h in F(4) + F(5) granulosa cells. The effects of FSH on follistatin and ss(B)-subunit were variable with respect to follicle development and culture duration. These results suggest that FSH plays an important role in stimulating the production of mRNA and protein for the inhibin alpha-subunit in small prehierarchical follicles.

Insulin-like growth factors in the regulation of avian ovarian functions

In the past three decades, overwhelming evidence has accumulated to show that insulin-like growth factor (IGF)-I and -II, their receptors and binding proteins (IGFBP) (the IGF system), have major roles to play in the regulation of ovarian function in mammals. Although studies in birds did not start until 5-6 years ago, the limited information thus far available suggests that the IGFs act as autocrine/paracrine regulators of follicular growth and differentiation, just as observed in mammals. The genes for IGF-I and -II, type-I IGF receptor, IGFBP-2, and IGFBP-5 are expressed in both granulosa and theca cells of the chicken ovary. The mechanisms by which the IGF system controls ovarian function in the avian species are complex and involve interactions with the gonadotrophins (LH and FSH), growth hormone, and even other growth factors. Effects are different between strains and nutritional status.

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.

Expression and regulation of mRNA for inhibin/activin alpha- and betaA-subunits in the granulosa layer of the two largest preovulatory follicles during the hen ovulatory cycle

In order to understand the role of inhibin and activin in regulating follicular development in the hen, the steady-state mRNA levels of inhibin/activin alpha- and betaA-subunits in the granulosa layer of the largest (F1) and second largest (F2) follicles of the hen were investigated at 4-hr intervals throughout the ovulatory cycle. In addition, because it was hypothesized that luteinizing hormone (LH) regulated betaA-subunit expression, the effect of in vivo administration of ovine LH (oLH) on the expression of these subunits during the early- and mid-ovulatory cycle was examined. Northern blot analysis, using 32P-labeled cDNA probes of chicken inhibin/activin alpha- and betaA-subunits and glyceraldehyde-3-phosphate dehydrogenase (GAPDH, internal control), revealed that in the F1 follicle, the relative level of betaA-mRNA (n = 3) was low at 23.5 hr and increased (P < 0.05) at 19.5, 15.5, and 11.5 hr before the next predicted ovulation. It then decreased (P < 0.05) at 7.5 hr and was further reduced at 3.5 and 0.5 hr prior to ovulation. In the F2 follicle, betaA-mRNA was maintained at a basal level throughout the sampling period except for a brief increase (P < 0.05) at 0.5 hr before ovulation. In contrast to the betaA-subunit, inhibin alpha-mRNA was abundantly expressed with no significant variations throughout the ovulatory cycle in either the F1 or the F2 follicle. When oLH was injected at 18 hr before ovulation, 200 but not 100 or 50 microg/kg (n = 3 hens per dose) significantly (P < 0.05) reduced the betaA-mRNA level in the F1 follicle by 2 hr after injection compared to the control (saline). The experiment was repeated at 12 hr before ovulation and both 100 and 200 but not 50 microg/kg oLH significantly (P < 0.05) reduced the expression of betaA-subunit mRNA with no significant difference between 100 and 200 microg/kg oLH. In contrast to the betaA-subunit, inhibin alpha-subunit mRNA was abundantly expressed and not affected by oLH treatment. Our data indicate that the expression of inhibin/activin betaA- but not alpha-subunit mRNA is developmentally regulated in the granulosa layer of the two largest follicles during the hen ovulatory cycle. In addition, LH may participate, directly or indirectly, in negative regulation of the betaA-subunit.

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

The present study investigates whether besides the ovary, extragonadal sources contribute to the total amount of immunoreactive inhibin in the plasma of the domestic hen. A comparison of the inhibin content of different organ shows that, expressed per milligram of tissue, the adrenal ranks second only to the ovarian granulosa layer. To explore the contribution of the adrenals to plasma inhibin, dexamethasone (100 micrograms/kg BW) was injected i.v. into intact, ovariectomized, and sham-operated hens. Control animals of each experimental group were injected with saline (0.9% (w/v) NaCI). Dexamethasone significantly (P < 0.05) decreased plasma inhibin concentrations in the three groups. The suppressive effect of dexamethasone in intact hens, however, was caused by a direct effect of this synthetic glucocorticoid on the gonads. Indeed, dexamethasone decreased the production of inhibin by granulosa cells in vitro and also lowered the immunoreactive inhibin concentration in ovariectomized animals. The decreased plasma inhibin concentration in ovariectomized animals is probably due to a direct effect of dexamethasone on the adrenals. Adrenal cells produced immunoreactive inhibin in vitro. The inhibin secretion by adrenal cells was significantly (P < 0.05) depressed by dexamethasone. In conclusion, the ovary is the major source of plasma immunoreactive inhibin in the laying hen. The presence of substantial amounts of immunoreactive inhibin in the adrenal, the secretion of inhibin by cultured adrenal cells, and the decreased immunoreactive inhibin in ovariectomized animals treated with dexamethasone indicate that the adrenal is a likely source of extragonadal inhibin. The nature and the role of this adrenal inhibin remain to be investigated.

Developmental changes in immunoreactive inhibin and FSH in plasma of chickens from hatch to sexual maturity

1. The relationship between immunoreactive inhibin and follicle-stimulating hormone (FSH) was studied in male and female chickens from hatch to sexual maturity. Plasma inhibin was estimated by a heterologous radioimmunoassay validated for use in the chicken. FSH was measured by a recently developed homologous radioimmunoassay. 2. In a cross-sectional study, blood samples and gonads were collected from chickens of both sexes at 1, 3, 5, 7, 14, 21 and 28 d after hatching and subsequently at 14-day intervals until 182 d of age. 3. In the female, plasma progesterone concentration (P4) progressively increased during sexual development. The plasma luteinising hormone (LH) concentration rose during the first week after hatching, and fluctuated thereafter, with troughs at 6 and 14 weeks and peaks at weeks 10 and 18. The plasma inhibin and FSH concentrations remained low until the start of puberty and increased simultaneously thereafter. However, from week 18 on, plasma inhibin continued to rise while plasma FSH fell. Hence, FSH and inhibin were positively correlated before puberty, but developed a negative correlation during sexual maturation. 4. In the male, plasma testosterone and LH concentrations increased 38- and 3.7-fold respectively over the period studied. Inhibin and FSH followed similar time courses and were consequently positively correlated. 5. These results suggest sex differences in the role of inhibin in regulating FSH secretion during development. The FSH-inhibin feedback loop may become operational at the onset of sexual maturity in the hens. In male chickens, the similar pattern of inhibin and FSH secretion suggests that inhibin secretion is driven by FSH.