Progesterone modulation of gonadotropin secretion by dispersed rat pituitary cells in culture. IV. Follicle-stimulating hormone synthesis and release

Regulation of follicle-stimulating hormone secretion by the interactions of activin-A, dexamethasone and testosterone in anterior pituitary cell cultures of male rats

This study was designed to evaluate the effects of glucocorticoids and gonadal steroids on the expression of inhibin/activin subunits and follistatin of the anterior pituitary and test the hypothesis that resulting changes in the local activin/inhibin/follistatin tone contribute to steroid effects on follicle stimulating hormone (FSH) production from gonadotropes. In primary cell cultures of male rat anterior pituitaries, dexamethasone (DEX) or testosterone (T) stimulated FSH secretion and FSHbeta mRNA and their effects were additive with activin-A. Follistatin (FS288) and inhibin-A antagonized the rise in FSH secretion both in the absence and presence of exogenous activin-A. Despite the similarity in their action on FSH production, DEX and T had opposite effects on follistatin mRNA levels. Follistatin mRNA levels of cultured rat anterior pituitary cells were elevated upon the addition of DEX but attenuated by T. On the other hand, both DEX and T suppressed inhibin/activin betaB mRNA levels while only DEX affected betaA mRNA. In these cells, activin-A stimulated follistatin and inhibin/activin betaB mRNA levels but had no effect on betaA. Together, DEX and activin-A caused a further increase in follistatin mRNA levels while T attenuated the effect of activin-A alone. Both steroids attenuated the effect of activin-A on betaB mRNA accumulation. These results support the possibility that DEX and T, possibly acting on different subsets of anterior pituitary cells, use distinct mechanisms to modify the local activin/inhibin/follistatin circuitry and thereby upregulate FSH production from the anterior pituitary gonadotropes.

Follistatin suppresses steroid-enhanced follicle-stimulating hormone release in vitro in rats

Previous in vitro and in vivo studies from our laboratory showed that progesterone (P(4)), 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 P(4), 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, P(4), or T in the absence or presence of FS. Act, B, P(4), 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 P(4)-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 P(4) did not fully overcome the FS-induced suppression of FSH release. The effects of P(4), B, and Act in the presence and absence of estradiol on steady-state mRNA levels of FSHbeta, Actbeta(B), and FS were determined in primary pituitary cell cultures from metestrous female rats by reverse transcription-polymerase chain reaction. Whereas Act, P(4), B increased FSHbeta mRNA levels, only Act raised the level of FS mRNA, and neither steroid increased Actbeta(B) mRNA. The results support the hypothesis that endogenous Act is a common mediator of the action of P(4), 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 P(4), B, and T is mediated, in part, by Act and involves modulation of a tightly regulated Act/FS autocrine-paracrine loop.

Mechanisms controlling the function and life span of the corpus luteum

The primary function of the corpus luteum is secretion of the hormone progesterone, which is required for maintenance of normal pregnancy in mammals. The corpus luteum develops from residual follicular granulosal and thecal cells after ovulation. Luteinizing hormone (LH) from the anterior pituitary is important for normal development and function of the corpus luteum in most mammals, although growth hormone, prolactin, and estradiol also play a role in several species. The mature corpus luteum is composed of at least two steroidogenic cell types based on morphological and biochemical criteria and on the follicular source of origin. Small luteal cells appear to be of thecal cell origin and respond to LH with increased secretion of progesterone. LH directly stimulates the secretion of progesterone from small luteal cells via activation of the protein kinase A second messenger pathway. Large luteal cells are of granulosal cell origin and contain receptors for PGF(2alpha) and appear to mediate the luteolytic actions of this hormone. If pregnancy does not occur, the corpus luteum must regress to allow follicular growth and ovulation and the reproductive cycle begins again. Luteal regression is initiated by PGF(2alpha) of uterine origin in most subprimate species. The role played by PGF(2alpha) in primates remains controversial. In primates, if PGF(2alpha) plays a role in luteolysis, it appears to be of ovarian origin. The antisteroidogenic effects of PGF(2alpha) appear to be mediated by the protein kinase C second messenger pathway, whereas loss of luteal cells appears to follow an influx of calcium, activation of endonucleases, and an apoptotic form of cell death. If the female becomes pregnant, continued secretion of progesterone from the corpus luteum is required to provide an appropriate uterine environment for maintenance of pregnancy. The mechanisms whereby the pregnant uterus signals the corpus luteum that a conceptus is present varies from secretion of a chorionic gonadotropin (primates and equids), to secretion of an antiluteolytic factor (domestic ruminants), and to a neuroendocrine reflex arc that modifies the secretory patterns of hormones from the anterior pituitary (most rodents).

Gonadotropin secretion in infantile rats exposed to ethanol in utero

Previous studies have shown that fetal alcohol exposure (FAE) alters reproductive function in both male and female rats. In females, FAE delays the onset of puberty, reduces a preovulatory-like LH surge, and results in an early onset of acyclicity. In males exposed to ethanol in utero, the perinatal surge of testosterone is reduced. During the infantile period of the female rat, there is a dramatic increase in plasma follicle-stimulating hormone (FSH), which is thought to play a role in initiating ovarian activity and perhaps the onset of puberty. In this study, we determined the effects of FAE on the patterns of gonadotropin secretion during the infantile period [postnatal days (PND) 8-21] in both male and female rats. Timed pregnant dams were fed a liquid diet containing 35% ethanol-derived calories during the final week of gestation. Control dams were fed either an isocaloric diet with sucrose substituted for ethanol (pair fed, PF) or laboratory chow (chow fed, CF). Male and female pups were sacrificed on PND 8, 10, 12, 15, 18, and 21, and trunk blood was collected. In males, LH levels decreased to a nadir on PND 18, and this decrease was blunted by FAE (p < 0.05). In contrast, FSH secretion was not altered by FAE. In females, plasma FSH levels were greater than males, and increased to peak on PND 12. This FSH peak was significantly delayed in FAE females (p < 0.02). There was no age-related change in LH levels in FAE females, and LH levels were not altered by FAE. The delayed peak of FSH secretion by FAE correlates with the delay in puberty previously seen in females. To investigate this further, we examined the possibility that the delay in the peak of serum FSH in FAE females is due to a reduced number of FSH-producing gonadotrophs. FSH-containing gonadotrophs were identified by immunocytochemistry. Cell counts of FSH-immunoreactive cells in pituitaries from PND 8, 15, and 21 control-fed and FAE female rats showed developmental increases in the number of FSH gonadotrophs per unit area (p < 0.001), but no treatment differences were observed. Overall, these data show that fetal alcohol exposure can alter gonadotropin secretion in infantile life in male and female rats. Importantly, the delay in FSH secretion in females may ultimately play a role in the delay in puberty observed in the FAE female rat.

Inhibition of ovulation by progestin analogs (agonists vs antagonists): preliminary evidence for different sites and mechanisms of actions

Continuous administration of the antiprogesterone RU486 inhibits ovulation in women and in monkeys; in this regard RU486 may act as a progestin agonist rather than as an antagonist. We compared the site(s) and mechanism(s) of RU486-induced ovulation inhibition with those of levonorgestrel (LNG). Six regularly menstruating cynomolgus monkeys each received placebo, RU486 (1 mg/kg/d) or LNG (2 g/kg/d) i.m. between days (cd) 2-22 of three separate menstrual cycles. Serum levels of estradiol (E2), progesterone (P4), androstenedione, LH and FSH were analyzed by RIAs in daily blood samples. Basal and GnRH-stimulated (1 and 50 g of GnRH i.v. 2 h apart) secretion of LH and FSH was assessed using serial blood samples collected for 12 h on cd 10. Mean cycle length was prolonged by RU486 and LNG treatments from 32 d to 70 d and 52 d, respectively (p < 0.02). Ovulation was inhibited in five of the six primates during RU486, and in all six during LNG treatment. During RU486 treatment, serum E2 levels were similar to those of the control cycle; despite peaks of E2 secretion, no LH peaks were seen. In contrast, E2 concentrations were profoundly suppressed during LNG treatment (p < 0.005). The reduction in serum E2 was accompanied by lower levels of androstenedione, and suppressed ratio of E2/androstenedione (p < 0.02) suggesting both reduced synthesis and aromatization of androgen precursors during administration of LNG. Consequently, LNG treatment was associated with higher levels of serum FSH and LH (p < 0.001; 1-way ANOVA). Similarly, as during the luteal phase of the menstrual cycle, the amplitude of basal LH-pulses was increased during LNG treatment (p < 0.05), whereas RU486 treatment did not affect basal LH secretion. The GnRH-stimulated release of LH was similar during the placebo, RU486 and LNG cycles; enhanced release of FSH was seen during administration of LNG. Thus, in the present model system, RU486 seems to inhibit ovulation mainly at the level of hypothalamus, possibly by interfering with the steroidal positive feedback signals from the ovary. However, LNG inhibits ovulation differently, most likely via direct progesterone-like effects on folliculogenesis and the hypothalamus. The pituitary does not appear to be the major site of action(s) of RU486 or LNG. Thus, the differential mechanisms of ovulation inhibition by RU486 and LNG seem to result from lesser intraovarian impact of RU486 as well as dissimilar influences on tonic gonadotropin secretory levels. We conclude that when inhibiting ovulation, RU486 does not act as a progestin agonist, but rather, functions through a hypothalamic mechanism(s), which might be unique to RU486 as a progesterone antagonist.

Feedback regulation of gonadotropins by androgens in rats: is 5 alpha-reduction involved?

The action of testosterone (T) on the sex accessory organs, such as ventral prostate (VP) and seminal vesicles (SV) is amplified by its 5 alpha-reduction to dihydrotestosterone (DHT). This does not happen in the case of muscle (levator ani, LA) which contains little or no 5 alpha-reductase activity. It has been suggested that the regulation of gonadotropins by T may also be mediated by its 5 alpha-reduced metabolites. We investigated this question by utilizing two types of androgens: (1) T and 17 alpha-methyl-testosterone (17MT), whose potency increases following 5 alpha-reduction; and (2) 19-nortestosterone (NT) and 17 alpha-methyl-19-nortestosterone (17MNT) whose potency decreases following 5 alpha-reduction. Castrated rats were used to investigate the ability of these androgens to stimulate VP, and SV (androgenic action) and LA growth (anabolic action) and to suppress the post-castration rise in LH levels. In addition, modification of these actions by a 5 alpha-reductase inhibitor (5 alpha-RI) was studied. Compared to T, NT was approximately 5 times less potent in stimulating VP and SV. By contrast, it was twice as potent as T in stimulating LA growth. Similarly, 17MNT was 5 times less androgenic but twice as anabolic as 17MT. The antigonadotropic potency of both the 19-nor compounds was 2-3 times greater than that of their respective 19-methylated parent compounds. The similarity in their anabolic and antigonadotropic potency suggested that 5 alpha-reduction is not a factor in their antigonadotropic action. This was confirmed by the use of the 5 alpha-RI. Treatment of rats receiving the androgens with 5 alpha-RI showed that it decreases the androgenic activity of T and 17MT while it increases the androgenic activity of NT and 17 MNT. In all cases the anabolic activity and the antigonadotropic potency remained unchanged. It is concluded that the regulation of pituitary gonadotropin secretion by T does not depend upon its 5 alpha-reduction to DHT.