Neena B. Schwartz

Neuroendocrine Consequences of Prenatal Androgen Exposure in the Female Rat: Absence of Luteinizing Hormone Surges, Suppression of Progesterone Receptor Gene Expression, and Acceleration of the Gonadotropin-Releasing Hormone Pulse Generator

Abstract Preovulatory GnRH and LH surges depend on activation of estrogen (E2)-inducible progesterone receptors (PGRs) in the preoptic area (POA). Surges do not occur in males, or in perinatally androgenized females. We sought to determine whether prenatal androgen exposure suppresses basal or E2-induced Pgr mRNA expression or E2-induced LH surges (or both) in adulthood, and whether any such effects may be mediated by androgen receptor activation. We also assessed whether prenatal androgens alter subsequent GnRH pulsatility. Pregnant rats received testosterone or vehicle daily on Embryonic Days 16–19. POA-hypothalamic tissues were obtained in adulthood for PgrA and PgrB (PgrA+B) mRNA analysis. Females that had prenatal exposure to testosterone (pT) displayed reduced PgrA+B mRNA levels (P < 0.01) compared with those that had prenatal exposure to vehicle (pV). Additional pregnant animals were treated with vehicle or testosterone, or with 5α-dihydrotestosterone (DHT). In adult ovariectomized offspring, estradiol benzoate produced a 2-fold increase (P < 0.05) in PgrA+B expression in the POA of pV females, but not in pT females or those that had prenatal exposure to DHT (pDHT). Prenatal testosterone and DHT exposure also prevented estradiol benzoate-induced LH surges observed in pV rats. Blood sampling of ovariectomized rats revealed increased LH pulse frequency in pDHT versus pV females (P < 0.05). Our findings support the hypothesis that prenatal androgen receptor activation can contribute to the permanent defeminization of the GnRH neurosecretory system, rendering it incapable of initiating GnRH surges, while accelerating basal GnRH pulse generator activity in adulthood. We propose that the effects of prenatal androgen receptor activation on GnRH neurosecretion are mediated in part via permanent impairment of E2-induced PgrA+B gene expression in the POA.

Progesterone Receptor A and B Messenger Ribonucleic Acid Levels in the Anterior Pituitary of Rats Are Regulated by Estrogen

Abstract In target tissues of most mammalian and avian species, progesterone receptors (PR) are expressed as structurally related, but functionally distinct, isoforms A and B, and they are regulated by estrogen (E) as well as by their cognate ligand, progesterone (P4). The objectives of the present work were to identify mRNA expression for the A and B isoforms of PR in the anterior pituitary of the rat, to examine its regulation by gonadal steroids, and to compare this regulation with that in the primary target organ, the uterus. Messenger RNAs for the PR isoforms, determined by two separate reverse transcription-polymerase chain reaction protocols, one that detects PR A and PR B equally and the other specific for PR B, were identified in anterior pituitary of female and male rats. In anterior pituitary of cycling female rats, steady-state mRNA levels for both PR A+B and PR B were highest at 0900 h on proestrus, declined rapidly to nadir values at 0900 h on metestrus (PR A+B) or 0900 h on estrus (PR B), and remained below proestrous values through 2100 h on diestrus. Administration of E to intact proestrous female rats caused significant increases in mRNA for both PR A+B and PR B on estrus and metestrus. Blockade of P4 action by administration of the antiprogestins RU-486 and ZK-98299 on proestrus had no effect on PR mRNA levels on the morning of estrus. Ovariectomy two and ten days after surgery markedly reduced mRNA levels for both PR A+B and PR B. Whereas treatment of 10-day-ovariectomized rats with E led to marked induction of mRNA for PR A+B and PR B two days later, treatment with P4 one day after treatment had no effect on basal or E-stimulated PR mRNA. Regulation of PR mRNA expression in the pituitary differed from that in the uterus, in which P4 treatment of ovariectomized rats antagonized the E-induced rise in mRNA for PR B, and antiprogestins increased mRNA for both isoforms. In addition to induction of PR mRNA in the pituitary of female rats by E in vivo, we also demonstrated induction by E in primary culture of anterior pituitary cells in vitro. We conclude that in the anterior pituitary of female rats, both the A and B isoforms of PR are expressed and regulated by E.

Differential Suppression of Follicle-Stimulating Hormone and Luteinizing Hormone Secretion in vivo by a Gonadotropin-Releasing Hormone Antagonist

Because of some indication that FSH secretion is less dependent than LH secretion on GnRH in vivo, we performed experiments to examine the effects of a GnRH antagonist (antag) on LH and FSH secretion. We first showed that pituitary cells superfused with GnRH showed a similar pattern of suppressed secretion of both LH and FSH in response to addition of antag. In contrast, antag administration to ovariectomized rats had differing effects on LH and FSH secretion. Serum LH was suppressed in a dose-dependent fashion by 2 h (20-50% of control values). Recovery from the lower doses of antag was seen by 12 h, but the two highest doses maintained serum LH levels at 10% of control values for 72 h. In contrast, the effect on serum FSH was not manifested until 12 h. FSH was maximally decreased only to 40-60% of control values. The two highest doses maintained this effect for 72 h. These results reinforce previous suggestions that FSH secretion in vivo may occur independently of acute changes in GnRH secretion, and may have an GnRH-independent component.

STEROID CONTROL OF GONADOTROPIN RELEASE

SUMMARY Several studies have led to the concept that the length of the rat estrous cycle is dependent on the timing of estrogen (E) secretion. According to this hypothesis E secretion during the 5-day cycle is delayed. However, we have found LH and E levels in both 4 and 5 day cycles are identical. Therefore, even though E is critical for a cyclic gonadotropin release to occur it does not appear to be the factor, which under normal circumstances, determines cycle length. During the day of proestrus, progesterone (P) increase coincided, but did not precede the gonadotropin(s) surge in our study thus eliminating P as the normal physiological trigger of gonadotropin release. If P was the determining factor, one would expect to find 5-day P levels higher than 4-day cycle levels during the earlier stages of the cycle. Approximately 48 h following ovulation, on the day of diestrus, we found that the 5-day cycle did indeed have higher P levels. We propose that the regulation of P secretion during the early stages of the cycle controls the timing of the normal periodic gonadotropin release.

Characterization of the FSH-Suppressing Activity in Follicular Fluid

Publisher Summary This chapter presents the characterization of the FSH-suppressing activity in follicular fluid. In every species tested, removal of the ovaries eventually leads to an elevation in serum levels of both LH and FSH. Circulating estradiol levels are maintained in these ovariectomized animals by adrenal secretion of estradiol. Moreover, following unilateral ovariectomy, serum FSH rises in the face of the unchanged estradiol levels. Serum LH does not show a reliable rise until 3 or 4 days postbilateral ovariectomy. These acute FSH changes in response to the alterations in ovarian feedback in the rat argue against the hypothesis that estradiol is the sole negative feedback signal for FSH. In perimenopausal women, serum FSH begins to rise before estradiol levels fall, while serum LH rises only after steroid levels decline. Folliculostatin is effective in suppressing serum FSH on each day of the estrous cycle in intact animals. Follicular fluid from many mammalian species suppresses FSH secretion significantly and selectively when injected intraperitoneally or intravenously in intact, bilaterally ovariectomized or unilaterally ovariectomized recipients. The degree of FSH suppression is proportional to the dose administered whether the ovaries are present in the recipient.

Steroid control of gonadotropin release

Abstract Several studies have led to the concept that the length of the rat estrous cycle is dependent on the timing of estrogen (E) secretion. According to this hypothesis E secretion during the 5-day cycle is delayed. However, we have found LH and E levels in both 4 and 5 day cycles are identical. Therefore, even though E is critical for a cyclic gonadotropin release to occur it does not appear to be the factor, which under normal circumstances, determines cycle length. During the day of proestrus, progesterone (P) increase coincided, but did not precede the gonadotropin(s) surge in our study thus eliminating P as the normal physiological trigger of gonadotropin release. If P was the determining factor, one would expect to find 5-day P levels higher than 4-day cycle levels during the earlier stages of the cycle. Approximately 48 h following ovulation, on the day of diestrus, we found that the 5-day cycle did indeed have higher P levels. We propose that the regulation of P secretion during the early stages of the cycle controls the timing of the normal periodic gonadotropin release.

Inhibin at 90: From Discovery to Clinical Application, a Historical Review

When it was initially discovered in 1923, inhibin was characterized as a hypophysiotropic hormone that acts on pituitary cells to regulate pituitary hormone secretion. Ninety years later, what we know about inhibin stretches far beyond its well-established capacity to inhibit activin signaling and suppress pituitary FSH production. Inhibin is one of the major reproductive hormones involved in the regulation of folliculogenesis and steroidogenesis. Although the physiological role of inhibin as an activin antagonist in other organ systems is not as well defined as it is in the pituitary-gonadal axis, inhibin also modulates biological processes in other organs through paracrine, autocrine, and/or endocrine mechanisms. Inhibin and components of its signaling pathway are expressed in many organs. Diagnostically, inhibin is used for prenatal screening of Down syndrome as part of the quadruple test and as a biochemical marker in the assessment of ovarian reserve. In this review, we provide a comprehensive summary of our current understanding of the biological role of inhibin, its relationship with activin, its signaling mechanisms, and its potential value as a diagnostic marker for reproductive function and pregnancy-associated conditions.

Follistatin Suppresses Steroid-Enhanced Follicle-Stimulating Hormone Release In Vitro in Rats

Abstract Previous in vitro and in vivo studies from our laboratory showed that progesterone (P4), corticosterone (B), and testosterone (T) increase intracellular content and release of FSH in the anterior pituitary. Activin (Act) and inhibin (Inh) are structurally related proteins with antagonistic actions, as Act stimulates and Inh inhibits FSH secretion from the anterior pituitary. Together with follistatin (FS), a protein that bioneutralizes Act, they form an autocrine-paracrine loop in the anterior pituitary that tightly regulates FSH secretion. The objective of the present study was to test the hypothesis that P4, B, and T modulate this autocrine-paracrine loop to favor increased FSH secretion. If Act were to mediate steroid-induced FSH release, FS would be expected to block these effects. To test this interaction, cell cultures were prepared from anterior pituitaries of male and female rats, and treated with Act, B, P4, or T in the absence or presence of FS. Act, B, P4, and T increased FSH release; FS suppressed both basal and Act- and steroid-stimulated FSH release to approximately 50% below basal levels. Cell cultures from anterior pituitary of female rats were used to compare the interaction of incremental concentrations of FS on dose-related Act- and P4-stimulated FSH release. With increasing concentrations of Act, the FS-induced suppression of FSH release was attenuated and eventually abolished; in contrast, maximally stimulatory concentrations of P4 did not fully overcome the FS-induced suppression of FSH release. The effects of P4, B, and Act in the presence and absence of estradiol on steady-state mRNA levels of FSHβ, ActβB, and FS were determined in primary pituitary cell cultures from metestrous female rats by reverse transcription-polymerase chain reaction. Whereas Act, P4, B increased FSHβ mRNA levels, only Act raised the level of FS mRNA, and neither steroid increased ActβB mRNA. The results support the hypothesis that endogenous Act is a common mediator of the action of P4, B, and T in the rat primary anterior pituitary cell culture. We conclude that the stimulation of FSH release and intracellular content in the gonadotroph by P4, B, and T is mediated, in part, by Act and involves modulation of a tightly regulated Act/FS autocrine-paracrine loop.

Steroid feedback regulation of luteinizing hormone and follicle‐stimulating hormone secretion rates in male and female rats

The long‐accepted dogma of feedback control of luteinizing hormone (LH) and follicle‐stimulating hormone (FSH) secretion by gonadal steroids is reconsidered in this article in light of recent investigations in the rat, using radioimmunoassay measurements of both the steroids and the gonadotropins. The negative feedback control of gonadotropin secretion in the female rat has been studied by many approaches; in every case, LH and FSH appear to respond differently to either removal or administration of gonadal steroids. Serum FSH increases rapidly after removal of one or both ovaries, but its secretion cannot be completely suppressed by any combination of exogenous gonadal steroids. LH secretion shows a delayed increase after ovariectomy or hemiovariectomy, but serum levels are readily suppressed by estradiol administration. Furthermore, the delayed increase in LH after ovariectomy corresponds more closely to the delayed drop in estradiol levels than does the rapid increase in FSH. Serum FSH secretion in the...

Differential effect of glucocorticoids on synthesis and secretion of Luteinizing Hormone (LH) and Follicle Stimulating Hormone (FSH)

Our previous work has suggested that glucocorticoid pretreatment suppresses the enhanced responsiveness to GnRH seen in serum LH 12 h after castration. By contrast, serum FSH continues to show the castration-induced hypersensitivity to GnRH. Our attempts to replicate this LH suppression in static pituitary culture in vitro were not successful. This suggested to us the possibility that corticoids in vivo might be preventing castration-induced increases in pituitary GnRH receptor levels. We tested this at 24 h post-castration and, in fact, corticoids did not suppress the increase in GnRH receptors. In addition to the aforementioned effects of corticoids, we have seen that cortisol reverses the castration-induced drop in pituitary FSH content. It does this for 7 days post-castration, even though it no longer has an effect in suppressing serum LH. Thus, our accumulated data reveal that glucocorticoids have a differential effect on LH and FSH synthesis and secretion. Further studies are needed to clarify the site(s) of action of glucocorticoids in gonadotropin secretion and synthesis. Glucocorticoids may well prove to be a key in unlocking the mystery of the mechanism of differential control of regulation of LH and FSH.