The time course of follicle-stimulating hormone suppression by recombinant human inhibin A in the adult male rhesus monkey (Macaca mulatta)

Multiple signaling pathways convey central and peripheral signals to regulate pituitary function: Lessons from human and non-human primate models

The anterior pituitary gland is a key organ involved in the control of multiple physiological functions including growth, reproduction, metabolism and stress. These functions are controlled by five distinct hormone-producing pituitary cell types that produce growth hormone (somatotropes), prolactin (lactotropes), adrenocorticotropin (corticotropes), thyrotropin (thyrotropes) and follicle stimulating hormone/luteinizing hormone (gonadotropes). Classically, the synthesis and release of pituitary hormones was thought to be primarily regulated by central (neuroendocrine) signals. However, it is now becoming apparent that factors produced by pituitary hormone targets (endocrine and non-endocrine organs) can feedback directly to the pituitary to adjust pituitary hormone synthesis and release. Therefore, pituitary cells serve as sensors to integrate central and peripheral signals in order to fine-tune whole-body homeostasis, although it is clear that pituitary cell regulation is species-, age- and sex-dependent. The purpose of this review is to provide a comprehensive, general overview of our current knowledge of both central and peripheral regulators of pituitary cell function and associated intracellular mechanisms, focusing on human and non-human primates.

Epigenetic regulation of puberty via Zinc finger protein-mediated transcriptional repression

In primates, puberty is unleashed by increased GnRH release from the hypothalamus following an interval of juvenile quiescence. GWAS implicates Zinc finger (ZNF) genes in timing human puberty. Here we show that hypothalamic expression of several ZNFs decreased in agonadal male monkeys in association with the pubertal reactivation of gonadotropin secretion. Expression of two of these ZNFs, GATAD1 and ZNF573, also decreases in peripubertal female monkeys. However, only GATAD1 abundance increases when gonadotropin secretion is suppressed during late infancy. Targeted delivery of GATAD1 or ZNF573 to the rat hypothalamus delays puberty by impairing the transition of a transcriptional network from an immature repressive epigenetic configuration to one of activation. GATAD1 represses transcription of two key puberty-related genes, KISS1 and TAC3, directly, and reduces the activating histone mark H3K4me2 at each promoter via recruitment of histone demethylase KDM1A. We conclude that GATAD1 epitomizes a subset of ZNFs involved in epigenetic repression of primate puberty.

The NK3 Receptor Antagonist ESN364 Interrupts Pulsatile LH Secretion and Moderates Levels of Ovarian Hormones Throughout the Menstrual Cycle

Women's health disorders such as uterine fibroids and endometriosis are currently treated by GnRH modulators that effectively suppress the hypothalamic-pituitary-gonadal axis. The neurokinin-3 receptor (NK3R) is an alternative target with an important role in the modulation of this axis. In this report, we demonstrate that systemic administration of an NK3R antagonist (ESN364) prolongs the LH interpulse interval in ovarectomized ewes and significantly lowers plasma LH and FSH concentrations in castrated nonhuman primates (Macaca fascicularis). Moreover, daily oral dosing of ESN364 throughout the menstrual cycle in M fascicularis lowered plasma estradiol levels in a dose-dependent manner, although nadir levels of estradiol were maintained well above menopausal levels. Nevertheless, estradiol levels during the follicular phase were sufficiently inhibited at all doses to preclude the triggering of ovulation as evidenced by the absence of the LH surge and failure of a subsequent luteal phase rise in plasma progesterone concentrations, consistent with the absence of normal cycle changes in the uterus. Apart from the point at surge, FSH levels were not altered over the course of the menstrual cycle. These effects of ESN364 were reversible upon cessation of drug treatment. Together these data support the proposed role of neurokinin B-NK3R signaling in the control of pulsatile GnRH secretion. Furthermore, in contrast to GnRH antagonists, NK3R antagonists induce a partial suppression of estradiol and thereby offer a viable therapeutic approach to the treatment of ovarian sex hormone disorders with a mitigated risk of menopausal-like adverse events in response to long-term drug exposure.

Regulation of baboon fetal ovarian development by placental estrogen: onset of puberty is delayed in offspring deprived of estrogen in utero

Using the baboon as a model for studies of human reproductive biology, we previously showed that placental estrogen regulates fetal ovarian follicle development. In this study, offspring of baboons untreated or treated in utero with the aromatase inhibitor letrozole (estradiol reduced >95%) or letrozole and estradiol were reared to adulthood to determine whether estrogen programming of the fetal ovary impacted puberty and reproduction in adulthood. All offspring exhibited normal growth and blood pressure/chemistries. Puberty onset in untreated baboons (43.2 ± 1.4 mo) was delayed (P < 0.01) in animals of letrozole-treated mothers (49.0 ± 1.2 mo) and normal in offspring of mothers treated with letrozole and estradiol (42.7 ± 0.8 mo). During the first 2 yr postmenarche, menstrual cycles in estrogen-suppressed animals (43.2 ± 1.3 days) were longer (P < 0.05) than in untreated baboons (38.3 ± 0.5 days) or those treated with letrozole and estrogen (39.6 ± 0.8 days). Moreover, in estrogen-suppressed offspring, serum levels of estradiol were lower and follicle-stimulating hormone greater (P < 0.05) in the follicular and luteal phases, and the elevation in luteal-phase progesterone extended (P < 0.02). Thus, puberty onset was delayed and menstrual cycles prolonged and associated with altered serum hormone levels in baboon offspring that developed in an intrauterine environment in which estradiol levels were suppressed. Because puberty and follicle development, as shown previously, were normal in baboons treated in utero with letrozole and estradiol, we propose that fetal ovarian development and timely onset of puberty in the primate is programmed by fetal exposure to placental estrogen.

Adrenal androgen concentrations increase during infancy in male rhesus macaques (Macaca mulatta)

This study investigated adrenal androgens (AA), gonadotropins, and cortisol in castrated and gonad-intact male rhesus macaques from birth through infancy. Blood samples were collected longitudinally from castrated (n = 6; weekly, 1-40 wk) and intact (n = 4; every other week, 1-17 wk) males. Plasma concentrations of AA were determined by liquid chromatography-tandem mass spectrometry, and plasma concentrations of cortisol and gonadotropins were determined by RIA. Dehydroepiandrosterone sulfate (DHEAS) concentrations increased almost threefold (to 8 wk), dehydroepiandrosterone (DHEA) increased more than eightfold (to 11 wk), and androstenedione doubled (to 15 wk) in five castrated infant males and declined continuously thereafter. A sixth castrated male had markedly different temporal patterns and concentrations (many times more than 2 SDs from the cohort mean) of AA and gonadotropins from first sampling (3 wk) and was excluded from analysis. Cortisol increased over 16 wk but correlated poorly with DHEAS. Luteinizing and follicle-stimulating hormones increased to peaks at 3 and 7 wk, respectively. Testis-intact males exhibited similar profiles, but with earlier peaks of DHEAS (5 wk) and DHEA and androstenedione (7 wk). Peak concentrations of DHEAS were lower and those of DHEA and androstenedione were higher in intact than castrated infants. Testosterone was undetectable in castrated males and >0.5 ng/ml in intact males but was not correlated with DHEA or DHEAS. These are the first data documenting a transient increase in AA secretion during infancy in an Old World primate and are consistent with the previously documented time course of zona reticularis development that accompanies increases in androgen synthetic capacity of the adrenal. The rhesus is a promising model for androgen secretion from the human adrenal cortex.

Effects of oestradiol on gonadotrophin levels in normal and castrated men

CONTEXT

Testosterone inhibits gonadotrophin release in men either directly or after aromatization to oestradiol. We hypothesized that in males the androgen receptor-mediated effect of testosterone on LH release is negligible relative to that of oestradiol.

OBJECTIVE

To compare the effect of experimentally induced variations of plasma oestradiol levels on LH levels in normal (physiological testosterone levels) and castrated men (very low testosterone levels).

DESIGN

Prospective, open label, intervention.

SUBJECTS AND INTERVENTIONS

We suppressed endogenous oestradiol in 10 young men with letrozole 2.5 mg once daily. In these men and in 10 young healthy castrated men, we restored plasma oestradiol levels with oestradiol patches (first week 100 mug/day, second week 50 mug/day, third week 25 mug/day and fourth week no oestradiol patch).

MEASUREMENTS

The effect of the intervention on plasma levels of LH were monitored and compared between the groups.

RESULTS

With the intervention, the mean plasma oestradiol level in the two groups varied from supraphysiological to below the lower reference range. Levels of LH mirrored plasma oestradiol levels in both the groups, as did testosterone in the intact group. Despite similar oestradiol levels, mean levels of LH were significantly higher in the castrated group compared to the intact group for all doses of oestradiol, and supraphysiological levels of oestradiol were unable to suppress LH into the physiological range in the castrated group.

CONCLUSIONS

Physiological plasma oestradiol levels have a substantial suppressive effect on LH in men. However, low-normal testosterone levels are a prerequisite for suppression of LH into the normal range.

Estrogen promotes germ cell and seminiferous tubule development in the baboon fetal testis

The foundation for development of the male reproduction system occurs in utero, but relatively little is known about the regulation of primate fetal testis maturation. Our laboratories have shown that estrogen regulates key aspects of the physiology of pregnancy and fetal development. Therefore, in the present study, we characterized and quantified germ cells and Sertoli cells in the fetal baboon testis in late normal gestation (i.e., Day 165; term is 184 days) and in baboons administered the aromatase inhibitor letrozole throughout the second half of gestation to assess the impact of endogenous estrogen on fetal testis development. In untreated baboons, the seminiferous cords were comprised of undifferentiated (i.e., type A) spermatogonia classified by their morphology as dark (Ad) or pale (Ap), gonocytes (precursors of type A spermatogonia), unidentified cells (UI), and Sertoli cells. In letrozole-treated baboons, serum estradiol levels were decreased by 95%. The number per milligram of fetal testis (x10(4)) of Ad spermatogonia (0.42 +/- 0.11) was 45% lower (P = 0.03), and that of gonocytes (0.58 +/- 0.06) and UI (0.45 +/- 0.12) was twofold greater (P < 0.01 and P = 0.06, respectively), than in untreated baboons. Moreover, in the seminiferous cords of estrogen-deprived baboons, the basement membrane appeared fragmented, the germ cells and Sertoli cells appeared disorganized, and vacuoles were present. We conclude that endogenous estrogen promotes fetal testis development and that the changes in the germ cell population in the estrogen-deprived baboon fetus may impair spermatogenesis and fertility in adulthood.

Interrelationships between ovarian and pituitary hormones in ovulatory menstrual cycles across reproductive age

CONTEXT

Ovarian hormones regulate pituitary gonadotropin secretion across the menstrual cycle via negative and positive feedback mechanisms. The contribution of individual hormones is complex and is a continuing area of research.

OBJECTIVE

The aim of the study was to identify relationships between LH/FSH and estradiol, progesterone, inhibin A, inhibin B, and anti-Mullerian hormone (AMH) in ovulatory menstrual cycles across reproductive age.

DESIGN

Serum ovarian and pituitary hormones were studied in a group of young (<35 yr; n = 21) and older (>45 yr; n = 55) women. The slopes of the regression lines relating the ovarian and pituitary hormones were determined by multiple linear regression analysis and expressed with 95% confidence intervals for each ovarian hormone, with FSH and LH as independent variables. Both simultaneous and delayed (time lagged) relationships were examined.

RESULTS

Clear associations were evident for the lagged prediction of FSH, with significant negative associations being evident with inhibin B and AMH in the follicular phase and with estradiol, inhibin B, progesterone, and AMH in the luteal phase. For the lagged prediction of LH, significant positive and negative associations were observed with estradiol and inhibin B, respectively, in the follicular phase and a negative association with progesterone and inhibin B in the luteal phase.

CONCLUSIONS

It is concluded that in the follicular phase, inhibin B is a major feedback regulator of FSH and may also be a negative feedback regulator of LH. AMH may be indirectly involved in FSH regulation.

Effect of transient hypothyroidism during infancy on the postnatal ontogeny of luteinising hormone release in the agonadal male rhesus monkey (Macaca mulatta): implications for the timing of puberty in higher primates

The present study examined whether a transient thyroid hormone (T(4)) deficit during infancy in male monkeys would compromise the arrest of luteinising hormone (LH) secretion during the infant-juvenile transition, and/or interfere with the pubertal resurgence of LH. Animals were orchidectomized and thyroidectomized (n = 3; Tx) or sham Tx (n = 3) within 5 days of birth. T(4) replacement was initiated in two Tx monkeys at age 19 weeks to reestablish a euthyroid condition. Blood samples were drawn weekly for hormone assay. Body weight, crown-rump length, and bone age were assessed throughout the study. Within a week of Tx, plasma T(4) declined to undetectable levels and, by 6-8 weeks of age, signs of hypothyroidism were evident. Transient hypothyroidism during infancy failed to prevent either arrest of LH secretion during the infant-juvenile transition or the pubertal resurgence of LH secretion, both of which occurred at similar ages to sham Tx animals. Although body weight exhibited complete catch-up with T(4) replacement, crown-rump length and bone age did not. Thus, bone age at the time of the pubertal LH resurgence in Tx animals was less advanced than that in shams. Although Tx did not influence qualitatively the pattern of gonadotrophin secretion, LH levels during infancy and after pubertal LH resurgence were elevated in Tx monkeys. This was not associated with changes in LH pulse frequency and amplitude, but half-life (53 versus 65 min) of the slow second phase of LH clearance was greater in Tx animals. These results indicate that hypothalamic mechanisms dictating the pattern of gonadotrophin-releasing hormone release from birth to puberty are not dependent on T(4) action during infancy, and fail to support the notion that onset of puberty is causally coupled to skeletal maturation. They also indicate that LH renal clearance mechanisms may be programmed in a T(4) dependent manner during infancy.

Heat treatment induces liver receptor homolog-1 expression in monkey and rat sertoli cells

We demonstrated in this study that liver receptor homolog-1 (LRH-1) was expressed in the round spermatids in normal monkey testis, and no LRH-1 signal was observed in the Sertoli cells. After local warming (43 C) the monkey testis, however, LRH-1 expression was induced in the Sertoli cells in coincidence with activation of cytokeratin 18 (CK-18), a Sertoli cell dedifferentiated marker. Furthermore, we isolated rat primary Sertoli cells from testes at various stages of development and treated with 43 C water in vitro. The changes in LRH-1 as well as CK-18 expression were analyzed by confocal immunohistochemistry and Western blot. The results showed that LRH-1 was stage-dependently expressed in the Sertoli cells; no LRH-1-positive signal was detected in the cells obtained from the testes of adult rat on d 60 after birth when mature spermatozoa in the testis was completed. However, the mature Sertoli cells were warmed at the 43 C water bath for 15 min, and the LRH-1 signal was remarkably induced in a time-dependent manner, just like the changes of CK-18 expression in the Sertoli cells, suggesting that the heat-induced dedifferentiation of the mature Sertoli cells might be related to LRH-1 regulation. LRH-1 expression induced by the heat treatment was completely inhibited by the addition of ERK inhibitor U0126 in the culture, indicating that the heat-induced LRH-1 expression in the Sertoli cells may be regulated via ERK1/2 activation pathway. Testosterone was found to have no such effect on LRH-1 expression in the monkey and rat Sertoli cells.

Continuous human metastin 45-54 infusion desensitizes G protein-coupled receptor 54-induced gonadotropin-releasing hormone release monitored indirectly in the juvenile male Rhesus monkey (Macaca mulatta): a finding with therapeutic implications

The effect of continuous administration of the C-terminal fragment of metastin, the ligand for the G protein-coupled receptor, GPR54, on GnRH-induced LH secretion was examined in three agonadal, juvenile male monkeys whose responsiveness to GnRH was heightened by pretreatment with a chronic pulsatile iv infusion of synthetic GnRH. After bolus injection of 10 microg human (hu) metastin 45-54 (equivalent to kisspeptin 112-121), the GPR54 agonist was infused continuously at a dose of 100 microg/h and elicited a brisk LH response for approximately 3 h. This rise was then followed by a precipitous drop in LH despite continuous exposure of GPR54 to metastin 45-54. On d 4, during the final 3 h of the infusion, single boluses of hu metastin 45-54 (10 microg), N-methyl-DL-aspartic acid (NMDA) (10 mg/kg) and GnRH (0.3 microg) were administered to interrogate each element of the metastin-GPR54-GnRH-GnRH receptor cascade. Although the NMDA and GnRH boluses were able to elicit LH pulses, that of hu metastin 45-54 was not, demonstrating functional integrity of GnRH neurons (NMDA) and GnRH receptors (NMDA and GnRH) but desensitization of GPR54. The desensitization of GPR54 by continuous hu metastin 45-54 administration has therapeutic implications for a variety of conditions currently being treated by GnRH and its analogs, including restoration of fertility in patients with abnormal GnRH secretion (i.e. idiopathic hypogonadotropic hypogonadism and hypothalamic amenorrhea) and selective, reversible suppression of the pituitary-gonadal axis to achieve suppression of gonadal steroids (i.e. precocious puberty, endometriosis, uterine fibroids, and prostate cancer).

Effect of active immunization against inhibin on hormonal concentrations and semen characteristics in Shiba bucks

Active immunization against inhibin increased ovulation rate in females; in males, the effects of active immunization against inhibin on hormonal concentrations and sperm production need more investigation. To test the hypothesis that active immunization against inhibin increases FSH secretion and sperm output, the present study was undertaken to determine the effects of active immunization against inhibin on hormonal profile and sperm production in Shiba bucks. The bucks were actively immunized against inhibin alpha-subunit (immunized group, n=6) or Freund adjuvant (control group, n=5) four times, at 5-weeks intervals. Blood samples were collected twice-weekly and two successive ejaculates of semen were collected (with an artificial vagina) once-weekly. Plasma concentrations of FSH, LH and testosterone were measured by radioimmunoassay (RIA) and sperm motility characteristics were measured by computer-assisted sperm analysis (CASA). All inhibin-immunized bucks produced antibodies against inhibin. Relative to control bucks, in immunized bucks there were significant increases in plasma FSH concentrations and in sperm concentrations from 5 to 9 weeks and from 8 to 11 weeks, respectively, after primary immunization. However, plasma concentrations of LH and testosterone, semen volume, percentage of motile spermatozoa and motility parameters (straight-line velocity, curvilinear velocity and linearity index) were similar in both groups. In conclusion, active immunization against inhibin alpha-subunit increased FSH secretions and enhanced sperm production in bucks, whereas LH and testosterone concentrations, semen volume and sperm motility parameters were unaffected. Active immunization against inhibin could be used to improve fertility in Shiba bucks.

Repetitive activation of hypothalamic G protein-coupled receptor 54 with intravenous pulses of kisspeptin in the juvenile monkey (Macaca mulatta) elicits a sustained train of gonadotropin-releasing hormone discharges

The purpose of the present study was to further examine the hypothesis that activation of G protein-coupled receptor 54 (GPR54) signaling at the end of the juvenile phase of primate development is responsible for initiation of gonadarche and the onset of puberty. Accordingly, we determined whether repetitive iv administration of the GPR54 receptor agonist kisspeptin-10 (2 microg as a brief 1-min infusion once every hour for 48 h) to the juvenile male rhesus monkey would prematurely elicit sustained, pulsatile release of hypothalamic GnRH, the neuroendocrine trigger for gonadarche. GnRH release was monitored indirectly by measuring LH secretion from the in situ pituitary, the GnRH responsiveness of which had been heightened before the experiment with an intermittent iv infusion of synthetic GnRH. Agonadal animals (n = 4) were employed to eliminate any confounding and secondary effects of changing feedback signals from the testis. The first brief infusion of kisspeptin-10 evoked an LH discharge that mimicked those produced by GnRH priming, and this was followed by a train of similar LH discharges in response to hourly activation of GPR54 by repetitive kisspeptin-10 administration. Concomitant treatment with a GnRH receptor antagonist, acyline, abolished kisspeptin-10-induced LH release. Repetitive kisspeptin-10 administration also provided a GnRH-dependent signal to FSH secretion. These findings are consistent with the notion that, in primates, the transition from the juvenile (attenuated GnRH release) to pubertal (robust GnRH release) state is controlled by activation of GPR54 resulting from increased expression of hypothalamic KiSS-1 and release of kisspeptin in this region of the brain.

Gonadotropin-independent proliferation of the pale type A spermatogonia in the adult rhesus monkey (Macaca mulatta)

The goal of the present study was to examine the relative roles of testosterone (T) and FSH in the proliferation and differentiation of pale type A (Ap) spermatogonia in the rhesus monkey (Macaca mulatta). Twenty adult male monkeys were treated with daily injections of a GnRH-receptor antagonist, acyline, to suppress endogenous gonadotropin secretion during an experiment comprising three phases. Phase 1 established a chronic hypogonadotropic state marked by a profound decrease in testicular size. During phase 2, half the monkeys were implanted with T-filled capsules, and the other half received control implants. Treatment with T produced circulating T levels of approximately 15 ng/ml and normal testicular T content. At the end of phase 2, monkeys were fitted with indwelling i.v. catheters and housed in remote sampling cages for the final phase. During phase 3, five monkeys from the T- and non-T-treated groups were stimulated with recombinant human FSH. The remaining five monkeys from each group received an infusion of vehicle. On the last day of FSH or vehicle infusion, monkeys were bilaterally castrated after receiving an i.v. bolus of bromodeoxyuridine (BrdU). The BrdU labeling of Ap spermatogonia was robust in the hypogonadotropic group and was uninfluenced by treatment with T and FSH, either alone or in combination. In contrast, both T and FSH stimulated spermatogonial differentiation, and this effect was amplified by combined treatment. We conclude that marked Ap spermatogonial proliferation occurs constitutively and in a gonadotropin-independent manner and that differentiation of Ap into B spermatogonia is absolutely gonadotropin dependent and may be driven by either T or FSH.

Cloning of a novel inhibin alpha cDNA from rhesus monkey testis

BACKGROUND

Inhibins are dimeric gonadal protein hormones that negatively regulate pituitary FSH synthesis and secretion. Inhibin B is produced by testicular Sertoli cells and is the primary circulating form of inhibin in most adult male mammals. Inhibin B is comprised of the inhibin alpha subunit disulfide-linked to the inhibin/activin betaB subunit. Here we describe the cloning of the cDNAs encoding these subunits from adult rhesus monkey testis RNA.

METHODS

The subunit cDNAs were cloned by a combination of reverse transcriptase polymerase chain reaction (RT-PCR) and 5' rapid amplification of cDNA ends (RACE) RT-PCR from adult rhesus monkey testis RNA.

RESULTS

Both the inhibin alpha and betaB subunit nucleotide and predicted protein sequences are highly conserved with other mammalian species, particularly with humans. During the course of these investigations, a novel inhibin alpha mRNA isoform was also identified. This form, referred to as rhesus monkey inhibin alpha-variant 2, appears to derive from both alternative transcription initiation as well as alternative splicing. rmInhibin alpha-variant 2 is comprised of a novel 5' exon (exon 0), which is spliced in-frame with exon 2 of the conventional inhibin alpha isoforms (variant 1). Exon 1 is skipped in its entirety such that the pro-alpha and part of the alpha N regions are not included in the predicted protein. rmInhibin alpha-variant 2 is of relatively low abundance and its biological function has not yet been ascertained.

CONCLUSION

The data show that the predicted inhibin B protein is very similar between monkeys and humans. Therefore, studies in monkeys using recombinant human inhibins are likely to reflect actions of the homologous ligands. In addition, we have observed the first inhibin alpha subunit mRNA variant. It is possible that variants will be observed in other species as well and this may lead to novel insights into inhibin action.

Evidence that gonadotropin-releasing hormone (GnRH) II stimulates luteinizing hormone and follicle-stimulating hormone secretion from monkey pituitary cultures by activating the GnRH I receptor

Mammalian gonadotropin-releasing hormone (GnRH) I is the neuropeptide that regulates reproduction. In recent years, a second isoform of GnRH, GnRH II, and its highly selective type II GnRH receptor were cloned and identified in monkey brain, but its physiological function remains unknown. We sought to determine whether GnRH II stimulates LH and FSH secretion by activating specific receptors in primary pituitary cultures from male monkeys. Dispersed pituitary cells were maintained in steroid-depleted media and stimulated with GnRH I and/or GnRH II for 6 h. Cells were also treated with Antide (Bachem, King of Prussia, PA), a GnRH I antagonist, to block gonadotropin secretion. In monkey as well as rat pituitary cultures, GnRH II was a less effective stimulator of LH and FSH secretion than was GnRH I. In both cell preparations, Antide completely blocked LH and FSH release provoked by GnRH II as well as GnRH I. Furthermore, the combination of GnRH I and GnRH II was no more effective than either agonist alone. These results indicate that GnRH II stimulates FSH and LH secretion, but they also imply that this action occurs through the GnRH I receptor. The GnRH II receptors may have a unique function in the monkey brain and pituitary other than regulation of gonadotropin secretion.

Developmental regulation of follicle-stimulating hormone receptor messenger RNA expression in the baboon fetal ovary

In the adult ovary, pituitary FSH via interaction with its receptor (FSHR) is required for follicular maturation and granulosa cell development. In humans and nonhuman primates, the pool of follicles available for adult ovarian function is established in utero. However, our understanding of the ontogeny and developmental regulation of FSHR in the ovary of the primate fetus is incomplete. Our goal was to determine whether the baboon fetal ovary expresses the full-length FSHR mRNA transcript and whether levels are developmentally regulated. Fetal ovaries were obtained at mid (Day 100) and late (Day 165) gestation (term = Day 184) from untreated baboons and on Day 165 from baboons in which fetal estrogen levels were either decreased by >95% by treatment with the aromatase inhibitor CGS 20267 or restored to 30% of normal by treatment with CGS 20267 plus estradiol benzoate administered s.c. to the mother on Days 100-164. The full-length 2088-base pair FSHR mRNA transcript was expressed in ovaries of adult and fetal baboons untreated or treated with CGS 20267 or CGS 20267 and estrogen. Mean (+/-SEM) FSHR mRNA levels (ratio of FSHR mRNA:18S rRNA), quantified by reverse transcription polymerase chain reaction, were increased (P < 0.05) 2-fold between mid (0.34 +/- 0.06) and late gestation (0.76 +/- 0.07), an increase prevented (P < 0.05) in estrogen-depleted baboons (0.44 +/- 0.10) and partially restored by treatment with CGS 20267 and estrogen (0.58 +/- 0.16). We previously showed that the number of follicles/0.33 mm2 in fetal ovaries of untreated baboons in late gestation was reduced 50% by treatment with CGS 20267 and restored to normal in baboons treated with CGS 20267 and estrogen. Thus, when corrected for the number of follicles/0.33 mm2, FSHR mRNA levels were similar in baboon fetal ovaries untreated (0.010 +/- 0.001) or treated with CGS 20267 (0.009 +/- 0.002) or CGS 20267 and estrogen (0.007 +/- 0.003). We conclude that estrogen plays a major role in regulating ovarian FSHR mRNA expression in the primate fetus, and that the developmental increase in FSHR mRNA levels reflects the estrogen-dependent increase in folliculogenesis (i.e., increased number of granulosa cells and oocytes).

Neuroendocrine control of follicle-stimulating hormone (FSH) secretion: III. Is there a gonadotropin-releasing hormone-independent component of episodic FSH secretion in ovariectomized and luteal phase ewes?

Our previous studies in ovariectomized ewes have provided direct evidence that FSH secretion is comprised of basal and episodic modes. In those studies, each GnRH pulse coincided with an FSH pulse, but additional FSH pulses were noted. To determine whether non-GnRH-associated pulses of FSH represent a GnRH-independent component of FSH secretion, we determined whether episodic FSH secretion persists after blockade of GnRH action with a GnRH antagonist. Hypophyseal portal and jugular blood was collected from five ovariectomized and six luteal phase ewes at 5-min intervals for 6 h before and 6 h after a single iv injection of Nal-Glu (10 micro g/kg body weight). Hypophyseal portal LH and FSH and jugular patterns of FSH were compared with patterns of GnRH. Before Nal-Glu, in both models, there was a one-to-one concordance between GnRH and portal LH pulses, and each GnRH pulse was associated with a FSH pulse. However, additional non-GnRH-associated pulses of FSH were present. Nal-Glu administration eliminated LH but not FSH pulsatility. Nal-Glu inhibited interaction of GnRH I with GnRH type I receptor but not interaction of GnRH II with type II receptor. These studies provide the first direct evidence of the existence of an acute GnRH I-independent component of episodic FSH secretion.

Activins, inhibins, and follistatins: from endocrinology to signaling. A paradigm for the new millennium

It has been 70 years since the name inhibin was used to describe a gonadal factor that negatively regulated pituitary hormone secretion. The majority of this period was required to achieve purification and definitive characterization of inhibin, an event closely followed by identification and characterization of activin and follistatin (FS). In contrast, the last 15-20 years saw a virtual explosion of information regarding the biochemistry, physiology, and biosynthesis of these proteins, as well as identification of activin receptors, and a unique mechanism for FS action-the nearly irreversible binding and neutralization of activin. Many of these discoveries have been previously summarized; therefore, this review will cover the period from the mid 1990s to present, with particular emphasis on emerging themes and recent advances. As the field has matured, recent efforts have focused more on human studies, so the endocrinology of inhibin, activin, and FS in the human is summarized first. Another area receiving significant recent attention is local actions of activin and its regulation by both FS and inhibin. Because activin and FS are produced in many tissues, we chose to focus on a few particular examples with the most extensive experimental support, the pituitary and the developing follicle, although nonreproductive actions of activin and FS are also discussed. At the cellular level, it now seems that activin acts largely as an autocrine and/or paracrine growth factor, similar to other members of the transforming growh factor beta superfamily. As we discuss in the next section, its actions are regulated extracellularly by both inhibin and FS. In the final section, intracellular mediators and modulators of activin signaling are reviewed in detail. Many of these are shared with other transforming growh factor beta superfamily members as well as unrelated molecules, and in a number of cases, their physiological relevance to activin signal propagation remains to be elucidated. Nevertheless, taken together, recent findings suggest that it may be more appropriate to consider a new paradigm for inhibin, activin, and FS in which activin signaling is regulated extracellularly by both inhibin and FS whereas a number of intracellular proteins act to modulate cellular responses to these activin signals. It is therefore the balance between activin and all of its modulators, rather than the actions of any one component, that determines the final biological outcome. As technology and model systems become more sophisticated in the next few years, it should become possible to test this concept directly to more clearly define the role of activin, inhibin, and FS in reproductive physiology.

Neuroendocrine control of follicle-stimulating hormone (FSH) secretion: II. Is follistatin-induced suppression of FSH secretion mediated via changes in activin availability and does it involve changes in gonadotropin-releasing hormone secretion?

The objective of the present study was to determine to what extent activin participates in setting the level of FSH secretion and if this regulation includes mediation via changes in GnRH secretion. We administered follistatin, the high-affinity binding protein for activin, to five ovariectomized sheep; we reasoned that the resultant binding of follistatin to activin should lower activin bioavailability and FSH secretion. Hypophyseal portal and peripheral blood samples were collected simultaneously at 10-min intervals for 18 h to measure GnRH, LH, FSH, and both activin-free and total follistatin. Six hours into collection, each ewe received 150 microg/kg i.v. of recombinant human follistatin-288. A week later, the same ewes were subjected to a second series of blood collections of similar length (time control). The FSH levels in pituitary portal blood were approximately 8-fold higher than those in the peripheral circulation. The FSH secretory patterns changed minimally during the time-control period. In contrast, follistatin had profound suppressive effects on FSH secretion. Maximal FSH suppression after FS-288 administration occurred at 5-6 h in the pituitary portal (65% suppression) and 9-10 h in the peripheral (48% suppression) circulation. Follistatin had no effect on GnRH or LH secretory patterns. Disappearance of total follistatin (i.e., free follistatin plus activin-bound follistatin) from the circulation was slower (P < 0.05) than that of free follistatin alone, suggesting that some of the follistatin was complexed with circulating activin, thus reducing the bioavailability of activin. The slower clearance of total follistatin and the lack of follistatin effects on GnRH secretion suggest that changes in activin bioavailability dictate the level of pituitary FSH secretion and that this is a pituitary-specific effect.

The functional significance of FSH in spermatogenesis and the control of its secretion in male primates

The aim of this review is to provide an integrative analysis of the role of FSH in the control of testicular function in higher primates, including man. Attention is focused on the action of FSH during neonatal development, puberty, and adulthood. Whether FSH is the major determinant of the adult complement of Sertoli cells and whether FSH is obligatory for the initiation, maintenance, and restoration of spermatogenesis is evaluated. The mechanism whereby the circulating concentration of FSH regulates spermatogonial proliferation to dictate the sperm production rate under physiological conditions in the adult is discussed in detail. Inhibin B is the major component of the testicular negative feedback signal governing FSH beta gene expression and FSH secretion, and the evidence for this view is presented. The review concludes with the presentation of a model for the operation of the FSH-inhibin B feedback control system regulating sperm production postpubertally in monkey and man, and with speculation on issues of clinical interest.

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.

Negative feedback regulation of the secretion and actions of gonadotropin-releasing hormone in males

This minireview considers the state of knowledge regarding the interactions of testicular hormones to regulate the secretion and actions of GnRH in males, with special focus on research conducted in rams and male rhesus monkeys. In these two species, LH secretion is under the negative feedback regulation of testicular steroids that act predominantly within the central nervous system to suppress GnRH secretion. The extent to which these actions of testicular steroids result from the direct actions of testosterone or its primary metabolites, estradiol or dihydrotestosterone, is unclear. Because GnRH neurons do not contain steroid receptors, the testicular steroids must influence GnRH neurons via afferent neurons, which are largely undefined. The feedback regulation of FSH is controlled by inhibin acting directly at the pituitary gland. In male rhesus monkeys, the feedback regulation of FSH secretion is accounted for totally by the physiologically relevant form of inhibin, which appears to be inhibin B. In rams, the feedback regulation of FSH secretion involves the actions of inhibin and testosterone and interactions between these hormones, but the physiologically relevant form of inhibin has not been determined. The mechanisms of action for inhibin are not known.

Influence of the degree of stimulation of the pituitary by gonadotropin-releasing hormone on the action of inhibin and testosterone to suppress the secretion of the gonadotropins in rams

This experiment determined if the degree of stimulation of the pituitary gland by GnRH affects the suppressive actions of inhibin and testosterone on gonadotropin secretion in rams. Two groups (n = 5) of castrated adult rams underwent hypothalamopituitary disconnection and were given two i.v. injections of vehicle or 0.64 microg/kg of recombinant human inhibin A (rh-inhibin) 6 h apart when treated with i.m. injections of oil and testosterone propionate every 12 h for at least 7 days. Each treatment was administered when the rams were infused i.v. with 125 ng of GnRH every 4 h (i.e., slow-pulse frequency) and 125 ng of GnRH every hour (i.e., fast-pulse frequency). The FSH concentrations and LH pulse amplitude were lower and the LH concentrations higher during the fast GnRH pulse frequency. The GnRH pulse frequency did not influence the ability of rh-inhibin and testosterone to suppress FSH secretion. Testosterone did not affect LH secretion. Following rh-inhibin treatment, LH pulse amplitude decreased at the slow, but not at the fast, GnRH pulse frequency, and LH concentrations decreased at both GnRH pulse frequencies. We conclude that the degree of stimulation of the pituitary by GnRH does not influence the ability of inhibin or testosterone to suppress FSH secretion in rams. Inhibin may be capable of suppressing LH secretion under conditions of low GnRH.

Midcycle and luteal elevations of follicle stimulating hormone in squirrel monkeys (Saimiri boliviensis) during the estrous cycle

Follicle stimulating hormone (FSH) has fundamental importance in reproductive function, but its cyclic pattern has not previously been described in the squirrel monkey, due primarily to the lack of a suitable assay. An homologous radioimmunoassay (RIA) based on recombinant cynomolgus FSH measured changes in serum FSH relative to patterns of bioactive luteinizing hormone (LH), estradiol, and progesterone during the estrous cycle. FSH was observed to have a sharp peak during the late follicular phase coincident with the LH surge and then rose again during the luteal phase. Estradiol was low except for the midcycle rise, suggesting an inhibitory relationship. The rat granulosa cell in vitro FSH bioassay confirmed high levels of this hormone. Measurement of FSH in the squirrel monkey has found a pattern different from Old World primates in the luteal phase, which may provide insight into the reproductive mechanisms of this species.

Dynamics of the follicle-stimulating hormone (FSH)-inhibin B feedback loop and its role in regulating spermatogenesis in the adult male rhesus monkey (Macaca mulatta) as revealed by unilateral orchidectomy

The purpose of this study was to document the morphological changes in the seminiferous epithelium that underlie the compensatory testicular hypertrophy observed in response to unilateral orchidectomy (UO) in the adult rhesus monkey and to describe the concomitant response in the endocrine feedback loops controlling testicular function in this species. Adult male monkeys were implanted with indwelling venous catheters; seven animals were then subjected to UO (data are presented from six) and three to sham UO. Profiles of circulating concentrations of FSH, LH, testosterone (T), inhibin B, and pro-alpha-C were monitored in 12-h series of sequential blood samples collected before, on the day of UO (day 0), and on days 1, 2, 4, 8, 16, 32, and 42 or 43 after UO. In the UO monkeys, the remaining testis was taken on day 44. Sertoli and germ cells in the removed and remaining testes were counted and expressed either as number per testis or, in the case of the differentiated spermatogonia (B1, B2, B3, and B4), as number per cross-section of the seminiferous tubule. UO was associated with a marked increase in the number of all germ cells more mature than undifferentiated spermatogonia (Ap) in the remaining testis. Sertoli cell number, on the other hand, did not change, and it is therefore reasonable to propose that the primary locus of the spermatogenic compensation was the differentiated spermatogonia. The additional finding that the relationship between the number of Sertoli cells and total germ cells in the remaining testis became robust (r = 0.92; P < 0.01 vs. r = 0.44; P > 0.05 for the removed testis) indicated that in the monkey, spermatogenesis does not normally operate at its ceiling. The increased drive to the seminiferous tubule of the remaining testis is hypothesized to be mediated by the sustained increase in FSH secretion that was observed after UO, although a role for increased testicular T production cannot be excluded. The stimulus for increased FSH secretion was presumably provided by the abrupt, 50% decline in circulating inhibin B levels. Interestingly, inhibin B secretion by the remaining testis was not dramatically affected by UO, and therefore, the deficit in circulating levels of this hormone and thus the error signal to FSH secretion were maintained for the duration of the experiment. In contrast, the changes in circulating LH and T concentrations were only transient, and within 48 h of UO, these hormonal parameters had returned to control values. The mechanisms by which the remaining testis rapidly acquires the capacity to double T production in the face of an unchanging LH drive remains to be determined. The foregoing body of evidence suggests that sperm output by the monkey testis is regulated by the circulating concentration of FSH and that in physiological situations, FSH secretion is insufficient to stimulate spermatogenesis to its ceiling. The results also indicate that FSH secretion is controlled by a feedback system in which the feedforward arm (FSH-inhibin B) is less robust than the feedback loop (inhibin B-FSH). Thus, a decrease in the inhibin B feedback signal results in a sustained increase in FSH secretion that drives the testes toward their spermatogenic ceiling, which is presumably set by Sertoli cell number.

Partial characterization of circulating inhibin-B and pro-alphaC during development in the male rhesus monkey

Gel filtration chromatography and ELISAs for inhibin-B and pro-alphaC were used to examine the circulating forms of inhibin in the neonatal (age 2-6 weeks), juvenile (age 1-2 yr), and adult male rhesus monkey. In all samples, isoforms of inhibin-B of 26-36K and 150K were found. Both forms were significantly greater in the adult. The alpha-subunit assay detected major peaks at 45-60 and 29-31K, and a minor peak of greater than 100K. As for inhibin-B, the major forms of inhibin pro-alphaC were highest in adulthood. Inhibin-B and pro-alphaC were measurable in peripheral plasma at age 1 week, increased with the neonatal rise in plasma FSH, and then decreased but remained detectable through age 1 yr. Values in adult males were higher than at any time during the first year of life. Finally, mean values of plasma inhibin-B and pro-alphaC in five monkeys, based on multiple blood samples drawn between age 1 week and 1 yr, were rank ordered and were found to be highly positively correlated (r = 0.96), suggesting that inhibin levels in the first year of life may be a marker of Sertoli cell number, and may predict the spermatogenic capacity of the testis in adulthood.