The changing pattern of granulosa cell expression of inhibin/activin subunits and follistatin during follicle development and their differential regulation by extrinsic and intraovarian factors supports evidence from functional studies, mostly in vitro, that these proteins have important roles in folliculogenesis, oocyte maturation and corpus luteum function. Gonadal inhibins function as negative feedback hormones to regulate the synthesis and secretion of pituitary FSH, a key determinant of follicle development, but there is little supportive evidence for a peripheral endocrine role for ovary-derived activins or follistatin in this regard. However, activins and follistatin are expressed in numerous other tissues, including anterior pituitary, and they are firmly implicated as local intrapituitary regulators of FSH secretion. Intraovarian actions of granulosa cell-derived activins include the promotion of granulosa cell proliferation and upregulation of FSH receptors, P450arom, oestrogen synthesis, granulosa cell LH receptors and enhancement of oocyte maturation. Through its activin-binding role, follistatin can reverse each of these activin-induced responses. In addition to their endocrine feedback role, granulosa-derived inhibins can sensitize theca cells to LH, thereby enhancing the production of androgens, an essential requirement for follicular oestrogen synthesis. Activins can oppose this effect and suppress thecal androgen production. Granulosa cells overproduce inhibin a subunit precursor relative to betaA/betaB subunit precursors and evidence indicates that different parts of the inhibin a subunit precursor have intrinsic biological activities distinct from inhibin alphabetaA/B dimer, and serve as additional local modulators of follicle and corpus luteum function.
PG Knight and C Glister
CC Silva and PG Knight
The aim of the present study was to determine whether androgens and progesterone influence the in vitro maturation of bovine oocytes as assessed by cleavage rates and competence to form blastocysts after in vitro fertilization. Bovine cumulus-oocyte complexes were cultured (n = 20 per drop) for 22-24 h at 38.5 degrees C in TCM-199 medium supplemented with 10% oestrous cow serum, eCG (2.5 iu ml(-1)) and a range of treatments that included aromatizable (testosterone; 100 nmol l(-1)) and non-aromatizable (dihydrotestosterone; 100 nmol l(-1)) androgens, an androgen antagonist (flutamide; 36 micromol l(-1)), progesterone (300 nmol l(-1)) and a progesterone antagonist (mifeprisone, RU486; 100 nmol l(-1)). Production of inhibin A, total alpha-subunit, activin A and follistatin by each group of cumulus-oocyte complexes was also measured, since inhibin-related peptides have been implicated as modulators of oocyte maturation and their production may be influenced by steroids and anti-steroids. Both testosterone and dihydrotestosterone increased oocyte cleavage rate (25%; P < 0.01) and dihydrotestosterone also increased (24%; P < 0.05) the proportion of oocytes that reached the >/= eight-cell stage. However, neither androgen affected blastocyst yield, or the proportion of blastocysts that hatched. The stimulatory effect of dihydrotestosterone on cleavage rate was reduced by flutamide but the anti-androgen had no effect when tested alone. Treatment with testosterone, but not dihydrotestosterone, decreased (P < 0.05) endogenous follistatin and increased (P < 0.05) the activin A:follistatin ratio in maturation medium. Concentrations of inhibin A, total alpha-subunit and activin A were not affected significantly by androgen or flutamide. Addition of progesterone or the anti-progestin mifepristone to cumulus-oocyte complexes had no effect on cleavage rate. However, progesterone reduced by approximately 40% (P < 0.05) the proportions of both total oocytes and cleaved oocytes that formed blastocysts. This effect was partially reversed by mifepristone. Neither progesterone nor mifepristone affected inhibin A, activin A or follistatin production. However, total alpha-subunit concentration was significantly greater in the progesterone-treated group than in the controls (50%; P < 0.05), indicating that the negative effect of progesterone on blastocyst yield may be mediated by increased inhibin alpha-subunit expression by cumulus cells.
CC Silva, NP Groome and PG Knight
The aim of this study was to evaluate the distribution of inhibin/activin alpha, beta(A) and beta(B) subunits and follistatin in immature oocytes and in matured oocytes before and after IVF. Denuded oocytes were submitted to a whole-mount immunofluorescence procedure. Specimens were imaged and fluorescent intensities quantified by scanning laser confocal microscopy. Immunoreactivity for inhibin alpha subunit (both alpha(C) and pro-alpha regions), abundant in the ooplasm of immature oocytes, decreased after maturation (a 68% and 88% decrease, respectively; P < 0.001), but increased after IVF by 2- and 5.7-fold, respectively (P < 0.01). Intense staining for beta(A) was detected in immature oocytes (predominantly in the outer ooplasm and zona pellucida) but after maturation and fertilization it was localized mainly in the zona pellucida, perivitelline space and oolemma. Immunoreactivity for beta(A) in the ooplasm decreased by 58% after maturation (P < 0.001) but increased again by 75% after fertilization (P < 0.01). Immunoreactivity for beta(B) was localized mainly in the zona pellucida and did not change after maturation. However, immunoreactivity for beta(B) was not detected in the zona pellucida after fertilization, but remained unchanged in unfertilized oocytes. Immunoreactivity for follistatin was detected in the ooplasm and zona pellucida of immature oocytes but decreased progressively in the ooplasm after maturation (a 63% decrease; P < 0.001) and did not change after IVF. Examination of partially denuded cumulus-oocyte complexes confirmed abundant expression of alpha(C), pro-alpha, beta(A) and follistatin immunoreactivity in cumulus cells, whereas beta(B) subunit staining was weak or absent in cumulus cells, but intense in the zona pellucida. In conclusion, the present study shows that qualitative and quantitative changes in the distribution of inhibin/activin subunits and follistatin accompany oocyte maturation and fertilization. The possibility, indicated by these observations, that activin A and activin B may play distinct roles in bovine oocyte maturation and fertilization warrants further study.
TM Lovell, RT Gladwell, NP Groome and PG Knight
The aim of this study was to compare the actions and interactions of gonadotrophins (LH and FSH) and an analogue of insulin-like growth factor I (LR3-IGF-I) on the secretion of inhibin A, inhibin B and progesterone by cultured chicken granulosa cells derived from the three largest (F1--F3) follicles of the preovulatory hierarchy. Treatment with LH or FSH promoted marked dose-(P < 0.0001) and time- (P < 0.0001) dependent increases in both inhibin A and progesterone secretion, with the magnitude of response (< 15-fold compared with basal) increasing over time in culture. Concentrations of inhibin B were below the detection limit in all samples. Initially, F1 cells were more LH-responsive than were F3 cells in terms of progesterone secretion (P < 0.02) but this difference between follicles decreased over time in culture. In contrast, LH-induced inhibin A secretion tended to be highest from F3 cells, although this was not significant. Cells from F3 follicles were consistently more FSH-responsive than F1 cells in terms of both progesterone (P < 0.01) and inhibin A (P < 0.02) secretion. Initially, F1 cells were more responsive to LR3-IGF-I than were F3 cells in terms of progesterone secretion (P < 0.001) but were less responsive in terms of inhibin A secretion (P < 0.001). Again, these inter-follicle differences decreased over time in culture (not significant on day 3 of treatment). Co-treatment experiments showed that LR3-IGF-I enhanced both LH- and FSH-induced secretion of inhibin A and progesterone in a time- (P < 0.001) and follicle- (P < 0.001) dependent way. Initially, F1 cells showed highest LR3-IGF-I enhancement of LH-induced inhibin A and progesterone secretion; in contrast, F3 cells showed the highest LR3- IGF-1 enhancement of FSH-induced inhibin A and progesterone secretion. These inter-follicle differences persisted over time in the case of FSH-induced hormone responses but not in the case of LH-induced responses, even though the relative degree of LR3-IGF-I enhancement increased markedly over time. Collectively, these data support a positive role for IGF-I, presumably of thecal origin, as an amplifier of gonadotrophin action on granulosa cell inhibin A and progesterone production by preovulatory chicken follicles.
TM Lovell, RT Gladwell, NP Groome and PG Knight
Previous work has shown that activin A is expressed selectively within the theca rather than the granulosa layer of preovulatory chicken follicles. In the present study, this finding was verified and the potential paracrine actions of activin A on basal and gonadotrophin-induced secretion of inhibin A, inhibin B and progesterone by granulosa cells from the three largest preovulatory follicles (F1-F3) were investigated. Treatment with activin A (0, 0.25, 2.5 and 25 ng ml(-1)) alone increased inhibin A secretion markedly in a follicle- and time-dependent manner, with the greatest response (up to 15-fold increase; P < 0.0001) in F1 follicles after 3 days of treatment. In contrast, activin A alone had no effect on progesterone output at any time. Cells from F3 follicles were more responsive to FSH than were F1 cells in terms of both inhibin A (P < 0.02) and progesterone (P < 0.01) secretion. Furthermore, activin A greatly enhanced FSH-induced secretion of both inhibin A (up to tenfold; P < 0.0001) and progesterone (up to sixfold; P < 0.0001). In terms of LH-induced inhibin A and progesterone secretion, cells from F1, F2 and F3 follicles showed similar responses. Co-treatment with activin A enhanced LH-induced secretion of inhibin A markedly (up to ninefold; P < 0.0001) but had only a marginal effect on LH-induced progesterone secretion (up to twofold; P < 0.001). The presence of activin receptor subtypes IA, IB, IIA and IIB in cultured granulosa cells from F1, F2 and F3 follicles was demonstrated using immunocytochemistry. These findings support the hypothesis that activin A secreted by the theca layers of avian preovulatory follicles exerts a local paracrine action on granulosa cells to modulate 'basal' inhibin A secretion and to upregulate gonadotrophin-induced secretion of both inhibin A and progesterone. However, the extent to which this local role of activin A contributes to the generation of the preovulatory LH-progesterone surge remains to be established.
GM Anderson, KR Lapwood, PG Knight and TJ Parkinson
A series of experiments was conducted to examine the mechanism by which removal of the thyroid glands in seasonally suppressed rams brings about rapid testicular growth. In the first experiment, thyroidectomy at the nadir of the testicular cycle (late winter) initiated testis growth without any detectable change in the extent of spermatogenesis compared with sham-operated controls. The serum concentration of FSH, but not LH, was also markedly increased by thyroidectomy. In the second experiment, serum FSH concentration was again increased by thyroidectomy in late winter but there was no effect of thyroidectomy on LH concentration, LH pulses (measured in frequent blood samples) or testosterone concentration. Furthermore, there was no evidence of a change in central dopaminergic inhibition of GnRH, as measured by the pulsatile LH response to an i.m. injection of the dopaminergic D(2) agonist bromocriptine or antagonist sulpiride. The rapid increase in FSH concentration occurred despite a markedly increased serum inhibin A concentration in thyroidectomized rams. Therefore, the efficacy of inhibin feedback was examined by testing the FSH-suppressive effect of an inhibin preparation (5 ml charcoal-stripped bovine follicular fluid i.v.) in long-term thyroidectomized and thyroid intact castrated rams. Bovine follicular fluid suppressed FSH concentrations in control rams as expected but in marked contrast, was completely without effect in thyroidectomized animals. In castrated rams, the FSH concentration was only marginally increased by thyroidectomy, indicating that there is a major component of the mediation of the effects of thyroidectomy that is testicular in origin. It was concluded that a reduction in the ability of endogenous inhibin to inhibit FSH release at the pituitary, rather than a hypothalamic mechanism, is the primary cause of the stimulation of testis growth by thyroidectomy.
AC Evans, JD Flynn, P Duffy, PG Knight and MP Boland
The aim of this study was to examine the effect of removal of the largest follicle or all visible follicles during the first follicle wave on subsequent follicular growth, steroid, inhibin A and gonadotrophin secretion in sheep. On day 4.5 of a synchronized oestrous cycle, ewes (n = 18) were assigned to one of three groups which underwent either no treatment (control), ablation of the largest follicle (largest follicle aspirated and cauterized via laparotomy) or ablation of all follicles (all visible follicles ablated). Between day 0 and day 10 of the oestrous cycle, blood samples were collected every 8 h and ovaries were examined daily using transrectal ultrasonography. The lifespan of the second largest follicle (number of days > 3 mm in diameter) was longer (6.7 +/- 0.9 days; P < 0.05) and the maximum diameter tended to be greater (4.8 +/- 0.3 mm; P = 0.07) in ewes in which the largest follicle was ablated than in the control ewes (3.8 +/- 0.4 days; 4.2 +/- 0.3 mm). There was no difference in the day of emergence of the second follicular wave between groups (day 6.9 +/- 0.4). However, the peak of the transient increase in FSH concentrations after ablation was earlier (day 5.67 +/- 0.15; P < 0.05) in ewes in which all follicles were ablated than in control ewes (day 6.72 +/- 0.36); the timing in ewes that had only the largest follicle ablated was intermediate (day 6.11 +/- 0.28). Serum inhibin A concentrations were about three-fold lower (P < 0.05) in both follicle ablation groups than in the control group. The numbers of follicles 2-3 mm in diameter during the first 3 days of the second follicular wave were greater in 'ablated ewes' (both groups had 2.6 +/- 0.2 follicles day-1) than in control ewes (1.7 +/- 0.3 follicles day-1). It is concluded that: (i) transient increases in FSH concentrations precede the emergence of follicle waves; (ii) ablation of all follicles on day 4.5 after oestrus advanced the timing of the next peak in FSH concentrations and the numbers of small follicles associated with the development of the second follicular wave; and (iii) ablation of the largest follicle resulted in an increase in the lifespan of the second largest follicle, indicating a regulatory role of large dominant follicles over smaller subordinate follicles.
MD Fray, GE Mann, EC Bleach, PG Knight, MC Clarke and B Charleston
Bovine viral diarrhoea virus (BVDV) is a major pathogen of cattle and is responsible for considerable reproductive loss. In this study, the in vivo responses in six multiparous cows were investigated after a non-cytopathogenic BVDV challenge (strain Pe 515; 5 x 10(6) tissue culture infective dose 50) given 9 days before a synchronized ovulation. Six similar cows challenged with non-infectious culture medium served as controls. The experimental noncytopathogenic BVDV infection was followed by a viraemia and leucopenia at days 5-9 after challenge, but no other clinical signs of infection were detected. However, the BVDV infection altered endocrine function. Mean LH pulse frequency immediately before CIDR withdrawal was lower (P < or = 0.05) in the BVDV-infected (2.17 +/- 0.34 pulses per 8 h) compared with the sham-infected (4.83 +/- 1.04 pulses per 8 h) animals. At day 3 after CIDR withdrawal, plasma oestradiol concentrations remained high (P < 0.05) in the infected cows (2.19 +/- 0.51 pg ml(-1)) compared with the sham-infected controls (0.72 +/- 0.29 pg ml(-1)). However, there was no difference in the peak oestradiol concentration (BVDV: 2.31 +/- 0.29 versus sham: 2.34 +/- 0.41 pg ml(-1)). In addition, non-cytopathogenic BVDV significantly (P < 0.05) increased the duration of the interval between ovulation and onset of the postovulatory progesterone increase (values 1.0 ng ml(-1)) (BVDV: 3.0 +/- 0.26 versus sham: 4.0 +/- 0.26 days). The viral infection also significantly (P < 0.01) decreased mean plasma progesterone concentrations between day 3 and day 11 after ovulation (BVDV: 2.59 +/- 0.32 versus sham: 4.13 +/- 0.27 ng ml(-1)). These data show that non-cytopathogenic BVDV viraemias during the follicular phase can modulate the secretion of gonadotrophins and sex steroids, in particular progesterone, during a synchronized oestrous cycle. Therefore, viraemias during the follicular phase may reduce the fertility of cattle by disrupting the capacity of the ovulatory follicle to form a competent corpus luteum, thereby compromising early embryo development and maternal recognition of pregnancy.
TM Lovell, D Vanmontfort, V Bruggeman, E Decuypere, NP Groome, PG Knight and RT Gladwell
Circulating inhibin A, inhibin B, activin A, total immunoreactive inhibin alpha-subunit (ir-alpha inhibin), LH, FSH and progesterone concentrations were measured throughout the normal ovulatory cycle and after cessation of egg laying induced by feed restriction to investigate the potential involvement of inhibins and activins in the ovulatory cycle of the domestic hen. Plasma inhibin A varied significantly (P < 0.05) during the ovulatory cycle; the concentration was highest at the preovulatory LH surge and reached a nadir 10 h later, at about the time the F(2) follicle makes the transition to become the new F(1) follicle. Plasma FSH concentrations did not change significantly throughout the cycle and showed no correlation with inhibin A. Total ir-alpha inhibin concentrations were much higher than those of inhibin A at all stages of the ovulatory cycle and showed no correlation with inhibin A or FSH. Plasma concentrations of inhibin B and of activin A were below the detection limit of the assays in all plasma samples analysed. In the feed restriction study, plasma inhibin A and total ir-alpha inhibin showed little change until the last day of oviposition (day 0) after which they fell significantly (P < 0.05) and remained low to the end of the experiment (approximately 70-78% decrease relative to day -4). Conversely, plasma FSH increased after cessation of laying and was significantly higher (P < 0.05) from day 3 to the end of the study (approximately 50% increase on day 6 relative to day -4). Plasma FSH values were negatively correlated with inhibin A (r = -0.39; P < 0.005) and total ir-alpha inhibin (r = -0.36; P < 0.005). Plasma LH and progesterone also decreased (P < 0.05) during feed restriction. The decrease in LH preceded the terminal oviposition and the associated fall in inhibin A by 2 days; there was a positive correlation between LH and inhibin A (r = 0.35; P < 0.005). Taken together these findings support (i) a role for LH in promoting inhibin A secretion by preovulatory follicles and (ii) an endocrine role for inhibin A secreted by preovulatory follicles in the maintenance of tonic FSH secretion in laying hens.
TJ Parkinson, KC Smith, SE Long, JA Douthwaite, GE Mann and PG Knight
Freemartins are sterile XX/XY chimaeras that occur as a result of placental fusion between male and female fetuses during early pregnancy. Freemartins occur predominantly in cattle, although the prevalence of ovine freemartinism is increasing. In this study, the reproductive endocrinology of ovine freemartins was compared with that of normal sheep. Freemartins had significantly (P < 0.001) higher basal concentrations of LH and FSH than did normal ewes or rams, although the response of LH to GnRH (10 microg) was similar in freemartins, ewes and rams. Resting concentrations of oestradiol were similar in freemartins and ewes and were increased in both after eCG administration. Testosterone concentrations were higher in freemartins than in ewes, but were unresponsive to GnRH or eCG. Administration of 62.5 mg progesterone or 25 lg oestradiol twice a day for 3 days suppressed LH concentrations to baseline values in freemartins, ewes and rams. In ewes, 500 microg oestradiol administered twice a day caused preovulatory surges in LH concentrations, but suppressed LH in freemartins to baseline values. Thus, LH secretion can potentially be regulated in freemartins by gonadal steroids. FSH concentrations in freemartins were not suppressed by doses of inhibin that were effective in ewes and rams. Therefore, freemartins behave in part like castrated animals, as they have high basal concentrations of LH and FSH, which can be stimulated by GnRH and suppressed by gonadal steroids. Conversely, inhibin does not suppress FSH concentrations in freemartins, and freemartins have circulating concentrations of steroids intermediate between those of castrated and normal animals.