Pre-exposure of rat anterior pituitary cells to somatostatin enhances subsequent growth hormone secretion.

The precise roles of GH-releasing factor (GRF) and somatostatin (SRIF) in the orchestration of pulsatile GH secretion have not yet been fully determined. We examined the interactions of rat GRF and SRIF in the concentration ranges present in rat hypophysial-portal blood, on the secretion of GH from dispersed male rat anterior pituitary cells in monolayer culture. The effects of exposing cells to GRF and/or SRIF (0.01-1.nmol/l) for 1 h were compared with the effects of preincubation of cells with SRIF before experimental incubations. As anticipated, the stimulatory effects of 0.1-1 nmol GRF/1 were abolished by concurrent incubation with SRIF at an equimolar concentration, although SRIF, at these concentrations, did not significantly inhibit basal GH secretion. Conversely, pre-exposure to 0.1 nmol SRIF/1 for 30 or 60 min, resulted in an increase in GH secretion during a subsequent 60-min incubation period, both in the absence or in the presence of GRF (0.01-1 nmol/l). Pretreatment with GRF caused increased responsivity to GRF rather than significant sensitization of the GH response to GRF. These observations demonstrate actions of SRIF, at low and probably physiological concentrations, which are more complex than those of a pure inhibitor of GH secretion. Pre-exposure of the pituitary to SRIF enhances subsequent GH secretion, suggesting that SRIF may play an additional physiological role in amplifying the GRF signal.     

Effect of thyrotropin-releasing hormone and dopamine on the in vitro secretion of prolactin by the bullfrog pituitary gland

Pituitary glands of bullfrogs (Rana catesbeiana) were incubated in medium containing thyrotropin-releasing hormone (TRH) and/or dopamine in order to see their effects on the release and synthesis of prolactin, which was measured by a homologous radioimmunoassay. Prolactin synthesis was measured by monitoring the incorporation of [3H]leucine into prolactin. TRH (0.1-10 ng/ml) stimulated the release of immunoassayable prolactin and newly synthesized [3H]prolactin into the medium in a dose-dependent manner; however, it was ineffective in increasing total prolactin (medium plus pituitary) and the incorporation of [3H]leucine into the total prolactin during the experimental period (20 hr). The TRH-induced elevation of prolactin release was suppressed by the addition of dopamine (5 X 10(-7) M) to the medium.     

Normal and adenomatous human pituitaries secrete thyrotropin-releasing hormone in vitro: modulation by dopamine, haloperidol, and somatostatin

TRH is present in human normal pituitaries and in pituitary adenomas. In this study we demonstrated that the same tissues can release TRH in vitro. Fragments from seven normal pituitaries (10-15 mg/syringe) and dispersed cells from eight prolactinomas, four GH-secreting and two nonsecreting adenomas (1-3 x 10(6) cells/syringe) were perifused using a Krebs-Ringer culture medium. After 1 h of equilibration the perifusion medium was collected every 2 min (1 mL/fraction) for 3 h. TRH, PRL, and GH were measured by RIA under basal conditions and in the presence of 10(-10) to 10(-6) mol/L dopamine (DA), alone or concomitant with haloperidol, or in the presence of 10(-10) or 10(-6) mol/L somatostatin. Both normal pituitary fragments and pituitary adenomatous cells (from all types of adenomas studied) spontaneously released TRH in vitro. TRH was detected in the perifusion medium either immediately after the end of the equilibration period or 30-60 min later. The molecular identity of TRH was assessed by high pressure liquid chromatography. There was no difference in the profile and the rate of TRH secretion between normal and tumoral tissues, and no correlation was found between the level of TRH release and that of PRL or GH secretion. DA stimulated TRH release from normal pituitaries and from PRL- and GH-secreting adenomas at doses as low as 10(-10) mol/L. A concomitant decrease in PRL and GH release was observed from adenomatous cells and in one case of normal tissue. Haloperidol (10(-7) mol/L) antagonized the effect of 10(-8) mol/L DA on both TRH and PRL secretion in normal pituitary and in prolactinomas. DA had no effect on TRH release from two nonsecreting tumors. The amounts of TRH released during 1 h of perifusion were 60-1640 pg/2 mg wet wt tissue in normal pituitaries and 54-2174 pg/10(6) cells in adenomas; these values were very high compared to those precedently reported within the tissues. These results indicate that pituitary cells can release TRH in vitro and suggest that TRH might be synthesized in situ. We suggest that TRH could act on pituitary hormone secretion and/or cell proliferation via a paracrine and/or an autocrine mechanism.     

Substance P stimulates luteinizing hormone secretion from anterior pituitary cells in culture.

Substance P (SP) is present in the anterior pituitary gland (AP), and its concentration there is regulated by the hormonal status of the animal. The observation that SP is releasable from hemipituitaries in a K(+)-stimulated, Ca(2+)-dependent manner and the demonstration of SP-binding sites in the AP have led to the suggestion that SP participates in a paracrine or autocrine manner in the regulation of AP function. Contradictory reports of the effects of SP on the secretion of AP hormones, particularly LH, led us to address the question of whether SP can act directly on the AP to effect LH secretion. We found that SP (100 nM) can stimulate LH release (300-400% of control values) in short term cultures of AP cells and that this effect varies as a function of the age and sex of the animal. There was no significant effect of SP on the release of LH from AP cells of male and female prepubertal rats (20-30 days). During the peripubertal period (30-35 days), a sharp increase in the response to SP occurred in both sexes. This responsiveness was dose dependent and persisted at all ages studied in AP cells from the female rat. In contrast, the responsiveness of AP cells from male rats that developed during the peripubertal period diminished during maturation and was absent after 60 days of age. When adult female rats were exposed to androgens for 6 weeks in vivo and tested for the ability of SP to stimulate the LH secretion, the response was significantly diminished. These studies support the speculation that SP has a functional role in the secretion of LH.     

The effects of bromocriptine, thyrotropin-releasing hormone, and gonadotropin-releasing hormone on hormone secretion by gonadotropin-secreting pituitary adenomas in vivo and in vitro

The characteristics and dynamics of hormone secretion in vivo and in vitro were investigated in six patients with gonadotropin-secreting pituitary adenomas. All six tumors secreted and contained FSH and different combinations of LH, beta-LH, and alpha-subunit. In addition, immunohistochemical examination of the pituitary tumor tissue showed staining with both LH and FSH in three and either LH or FSH in the other three tumors. TRH and GnRH stimulated hormone secretion in vivo and in vitro, and they also increased the hormone content of the cultured tumor cells. Bromocriptine significantly inhibited hormone release and reduced the hormone content of the tumor cells. In vivo, 2.5 mg bromocriptine significantly suppressed plasma hormone levels; the inhibiting effect on alpha-subunit concentrations was in general more marked than that on LH and FSH. We conclude that hormone release by gonadotropin-secreting pituitary adenomas can be stimulated by TRH and GnRH and inhibited by bromocriptine. Most of these tumors synthesize FSH, but there is a wide variation in the production of LH, beta-LH, and alpha-subunits. The sensitivity of hormone release to bromocriptine suggests that chronic therapy with this drug might have a beneficial effect on pituitary tumor size.     

Failure of bromocriptine, dopamine, and thyrotropin-releasing hormone to affect prolactin secretion by human decidual tissue in vitro.

To determine whether the secretion of PRL by human decidual tissue in vitro is influenced by factors which inhibit or stimulate pituitary PRL secretion, explants of decidual tissue were incubated in media containing bromocriptine, dopamine, or TRH at concentrations known to affect pituitary PRL secretion in vitro. The quantities of PRL secreted by the explants exposed to these factors were compared with amounts secreted by explants incubated in control medium. Bromocriptine in concentrations ranging from 1.5 x 10(-10) to 1.5 x 10(-7) M did not inhibit PRL secretion over a 3-day period and dopamine in concentrations ranging from 5 x 10(-5)-10(-9) M did not inhibit PRL secretion over a 4-h period. TRH in concentrations ranging from 10(-9)-10(-3) M did not stimulate PRL secretion. These results suggest that the mechanism of PRL secretion by decidual tissue in vitro is different, at least in part, from the mechanism of pituitary PRL secretion.     

Biphasic action of forskolin on growth hormone and prolactin secretion by rat anterior pituitary cells in vitro.

To assess the role of cAMP-mediated signal transduction processes in mediation of secretagogue-stimulated GH release, we examined the dose-related effects of the diterpene adenylate cyclase activator forskolin (FSK) in primary monolayer cultures of rat adenohypophyseal cells. In cell cultures prepared from both immature (12 days old) and adult (6 weeks to 4 months old) male or female rats, the dose-related stimulation of GH release by FSK was biphasic. With increasing FSK concentrations from 0.03-3.16 microM, GH release increased progressively to maximal values of 442 +/- 19% and 303 +/- 10% of basal release in cells from immature and adult rats, respectively. FSK concentrations above 3.16 microM induced progressively diminished GH responses, with net inhibition to below basal release evident at 100 microM FSK. FSK stimulated PRL release to a lesser degree than it did GH release; the PRL response to FSK was also biphasic. When maximal stimulatory concentrations (Emax) of FSK and GH-releasing factor (GRF; 10 nM) were added in combination, the GH response was significantly less than the individual response to either secretagogue alone. In response to FSK alone, GRF alone, and FSK plus GRF, GH release was 478 +/- 7%, 583 +/- 11%, and 244 +/- 5%; 278 +/- 4%, 283 +/- 3%, and 175 +/- 2%; and 299 +/- 12%, 351 +/- 5%, and 191 +/- 17% of basal release in cells from 12-day-old, adult male, and adult female rats, respectively (P less than 0.01 for all responses to combined addition vs. the individual responses). Submaximal stimulatory concentrations of GRF added in combination with submaximal FSK elicited partially additive GH responses; the GH response to Emax GRF, on the other hand, was inhibited in a dose-related manner by all concentrations of FSK that by themselves were stimulatory. The GH responses were also suppressed when Emax FSK was added to cultured cells of 12-day-old rats in combination with Emax cholera toxin (2.5 ng/ml) or prostaglandin E2 (10 microM), agents whose actions, like that of GRF, involve adenylate cyclase activation. In contrast, FSK did not suppress but in most cases augmented the maximal GH responses to secretagogues whose action is independent of adenylate cyclase activation: (Bu)2cAMP (0.5 mM), TRH (100 nM), phorbol myristate acetate (50 nM), the Ca2+ ionophore A23187 (250 microM), and the dihydropyridine Ca2+ channel agonist BAY K8644 (10 microM). Indeed, combined addition of FSK with the latter two agents resulted in synergistic stimulation.(ABSTRACT TRUNCATED AT 400 WORDS)     

Bradykinin parallels thyrotropin-releasing hormone actions on prolactin release from rat anterior pituitary cells

Bradykinin (BK), a nonapeptide, originally discovered in blood, is also present in neurons and fibers of the hypothalamus. We tested the putative releasing factor properties of BK on prolactin (PRL) release from anterior pituitary cells in vitro. BK stimulated the release of PRL in a dose-dependent manner, the threshold concentration being in the range. 0.1-1.0 nM. The release of PRL induced by BK at 1 nM concentration was about 2-fold, delayed and sustained over many minutes. Higher concentrations of BK stimulated PRL release in two phases. The shape of the BK-induced PRL release was superficially similar to that induced by thyrotropin-releasing hormone (TRH). 10 nM BK and 10 nM TRH induced about a 4-fold increase in PRL release within 5 min, followed by a gradual recovery to basal secretion. These results indicate that this peptide can act directly at the anterior pituitary gland to release PRL. Phorbol ester also promoted PRL release over the range of 1-10 nM, but the time course of the release was somewhat different.     

Influence of coexisting hypothalamic messengers on growth hormone secretion from rat anterior pituitary cells in vitro

An increasing number of messengers have recently been found to coexist with growth hormone (GH)-releasing factor (GRF) in hypothalamic neurons. In view of a possible cosecretion of these substances with GRF into the portal circulation, the effect of synthetic rat hypothalamic GRF(1-43) alone, or together with dopamine (DA), L-dopa, gamma-aminobutyric acid (GABA), neurotensin (NT) or galanin (GAL) on GH release was investigated by using dispersed rat anterior pituitary cells in monolayer culture. GRF in concentrations of 10(-16)-10(-7) M stimulated GH release from somatotrophs in a dose-related manner. DA (10(-5) M), L-dopa (10(-8) and 10(-5) M) and GABA (10(-9) and 10(-5) M) did not affect basal GH release, whereas DA, but not L-dopa or GABA, significantly suppressed GRF-induced GH secretion. However, the inhibitory effect of DA on GRF-stimulated GH secretion was not observed in the presence of somatostatin (10(-6) M). NT (10(-6) M) and GAL (10(-6) M) did not change basal GH release. GAL, but not NT, inhibited GRF-stimulated GH release, but the addition of NT abolished the inhibitory actions of both GAL and DA. These results indicate that substances, probably coreleased with GRF from the same nerve endings, interact in the regulation of GH secretion at the pituitary level.     

Luteinizing hormone secretion from wild-type and progesterone receptor knockout mouse anterior pituitary cells.

The progesterone receptor (PR) has a central role in the hypothalamo-pituitary events culminating in the preovulatory LH surge, and mice with genetically ablated PR provide a model for dissecting cellular pathways subserving this role. The aims of this study were to determine 1) whether the GnRH self-priming response and acute progesterone augmentation of secretagogue-stimulated LH secretion are present in cultured wild-type (WT) mouse pituitary cells, and 2) whether the PR is essential for self-priming by comparing the responses in PR knockout (PRKO) cells. Pituitary cells from ovariectomized WT or PRKO mice cultured +/- 17beta-estradiol (E(2)) for 3 days were challenged with hourly pulses of 1 nM GnRH or 54 mM K(+). A background of E(2) had no effect on the initial LH secretory response for either WT or PRKO cells. However, for subsequent GnRH pulses, E(2) was permissive for the GnRH self-priming response in WT cells. PRKO cells exhibited a blunted GnRH self-priming response. Exposure to progesterone for 90 min before secretagogue stimulation resulted in a modest (1.5-fold) augmentation of the LH response to GnRH but not K(+) pulses in WT cells; progesterone had no effect in PRKO cells. Unlike in the rat, the PR antagonists RU486 or ZK98299 failed to prevent potentiation of LH secretory responses to multiple GnRH pulses in WT cells. Although RU486 blocked progesterone augmentation of the initial GnRH pulse, it was ineffective in blocking progesterone's action after multiple GnRH pulses. In WT cells, 8- bromo-cAMP (8-Br-cAMP) was able to substitute for the GnRH priming pulse; 8-Br-cAMP also augmented GnRH-stimulated secretion in PRKO cells but less effectively. 8-Br-cAMP augmented K(+)-stimulated LH secretion in WT and PRKO cells equally. These results suggest that, although mouse gonadotropes show GnRH self-priming, they have adapted strategies different than rat cells for amplifying the GnRH signal as shown by the residual self-priming in PRKO cells, the modest or absent augmentation by acute progesterone of GnRH- or K(+)-stimulated secretion in WT cells, and the reduced ability of PR antagonists to interfere with GnRH self-priming and progesterone augmentation. We speculate that the adaptations could involve, at least in part, differences in the ratio of PR isoforms.     

Effects of dopamine and somatostatin on pulsatile pituitary glycoprotein secretion.

The hypothalamic factors dopamine (DA) and somatostatin (SRIH) inhibit pituitary glycoprotein secretion, but little is known regarding the effects of these factors on glycoprotein pulses. To address this question, 12 healthy volunteers underwent frequent blood sampling over 12 h at baseline and during 12-h infusions of DA and/or SRIH. TSH, LH, FSH, and alpha-subunit (alpha) levels were measured in all samples, and hormone pulses were located by Cluster analysis. Both DA and SRIH suppressed TSH pulse amplitude by 70%, while SRIH decreased TSH pulse frequency as well. Both infusions decreased LH pulse amplitude by 30-35%, but had no effect on pulse frequency. In contrast, neither infusion significantly altered FSH pulse parameters, although mean FSH levels declined 15%. DA had no effect on pulsatile alpha secretion, while SRIH decreased alpha pulse frequency. Serum thyroid hormone levels declined during both infusions, but there were no major changes in serum sex steroid levels. Thus, the hypothalamic inhibitory factors DA and SRIH had divergent effects on glycoprotein hormone pulses. The major effects on pulse amplitude, rather than frequency, imply that these factors do not play major roles in the generation of glycoprotein pulses, although SRIH may directly affect the TSH and alpha pulse generators.     

Dihydropyridine-sensitive calcium channel activity related to prolactin, growth hormone, and luteinizing hormone release from anterior pituitary cells in culture: interactions with somatostatin, dopamine, and estrogens

In the present work, we determined the activity of voltage-dependent dihydropyridine (DHP)-sensitive Ca2+ channels related to PRL, GH, and LH secretion in primary cultures of pituitary cells from male or female rats. We investigated their modulation by 17 beta-estradiol (E2) and their involvement in dopamine (DA) and somatostatin (SRIF) inhibition of PRL and GH release. BAY-K-8644 (BAYK), a DHP agonist which increases the opening time of already activated channels, stimulated PRL and GH secretion in a dose-dependent manner. The effect was more pronounced on PRL than on GH release. BAYK-evoked hormone secretion was further amplified by simultaneous application of K+ (30 or 56 mM) to the cell cultures; in parallel, BAYK-induced 45Ca uptake by the cells was potentiated in the presence of depolarizing stimuli. In contrast, BAYK was unable to stimulate LH secretion from male pituitary cells, but it potentiated LHRH- as well as K+-induced LH release; it had only a weak effect on LH secretion from female cell cultures. Basal and BAYK-induced pituitary hormone release were blocked by the Ca2+ channel antagonist nitrendipine. Under no condition did BAYK affect the hydrolysis of phosphoinositides or cAMP formation. Pretreatment of female pituitary cell cultures with E2 (10(-9) M) for 72 h enhanced LH and PRL responses to BAYK, but was ineffective on GH secretion. DA (10(-7) M) inhibited basal and BAYK-induced PRL release from male or female pituitary cells treated or not treated with E2 (10(-9) M). SRIF (10(-9) and 10(-8) M) reversed BAYK-evoked GH release to the same extent in cell cultures derived from male or female animals. It was ineffective on BAYK-induced PRL secretion in the absence of E2, but antagonized it after E2 pretreatment. The effect was dependent upon the time of steroid treatment and was specific, since 17 alpha-estradiol was inactive. In addition, DA and SRIF decreased the 45Ca uptake induced by the calcium agonist. These data demonstrate that DHP-sensitive voltage-dependent calcium channels of the L type present on different pituitary cells are not equally susceptible to BAYK activation under steady state basal conditions, indicating that their spontaneous activity and/or distribution vary according to the cell type; their activity is modulated by sex steroids. In addition, these data suggest that Ca2+ channels represent a possible site of DA and SRIF inhibition of PRL and GH release, respectively, by gating calcium entry into the corresponding cells.     

Tumor necrosis factor-alpha inhibits growth hormone secretion from cultured anterior pituitary cells

Anabolic proteins such as pituitary GH enhance the function of several immune cell types. The converse could also exist, that is communication from the immune cells to GH-producing somatotrophs. To test this hypothesis, tumor necrosis factor-alpha (TNF-alpha), a product of activated macrophages, was exposed to cultured rat pituitary cells, and GH release was monitored. TNF alpha inhibited basal and GH-releasing hormone-stimulated GH accumulation, with IC50 values of 170 U/ml (5.2 ng/ml) and 50 U/ml (1.5 ng/ml), respectively. This inhibition was first measured after 6 h of TNF alpha treatment, continued for at least 3 days, and was reversible. A number of measurements (e.g. trypan blue exclusion, chromium release, and GH cell content) yielded no signs of cytotoxicity to explain the inhibition. We conclude that TNF alpha can reduce basal and stimulated pituitary GH release in vitro.     

Activin-A modulates growth hormone secretion from cultures of rat anterior pituitary cells

Activins, initially identified as FSH-releasing proteins, have now been shown to exert effects on other cell types of the anterior pituitary, including the somatotrophs. In the present study the inhibitory action of activin-A (beta A beta A) on GH secretion was characterized using primary cultures of rat anterior pituitary cells. Activin-A suppressed basal GH secretion for up to 72 h (the longest time tested). Immediately after the treatment period with activin-A, when the cells were thoroughly washed and further incubated with or without rat GH-releasing factor (rGRF), basal and stimulated GH secretion were partially inhibited as well. In parallel, activin-A pretreatment diminished rGRF-stimulated cAMP accumulation. The effects of activin-A were time- and concentration-dependent, with half-maximal inhibition occurring in the range of 20-30 pM activin-A. A minimum pretreatment time of 3 h was required for maximal effect, and when rGRF and activin-A were added simultaneously, no inhibition was evident. Secretory responses of activin-A-pretreated cells to rGRF were influenced by glucocorticoids. When cells were cultured in the presence of the synthetic glucocorticoid dexamethasone, pretreatment (72 h) with activin-A attenuated rGRF-stimulated GH secretion only during short (1-h), but not longer (3-h), exposure periods to the neuropeptide. In the absence of dexamethasone, rGRF-stimulated GH secretion was inhibited at all incubation times tested (up to 3 h). A 3-h exposure to the protein factor did not alter total (cellular plus secreted) immunoreactive GH levels, suggesting that the inhibition of secretion with the shorter treatment was not secondary to attenuated GH biosynthesis. However, longer (72-h) treatment with activin-A decreased total GH levels, indicating lower GH biosynthetic rates, as previously shown. Somatostatin is recognized as the primary negative modulator of GH secretion. Activin-A and SRIF inhibited GH secretion additively, suggesting distinct mechanisms of action for each. GH secretion in response to other secretagogues, such as 12-O-tetradecanoyl-phorbol-13-acetate, forskolin, cholera toxin, and 8-bromo-cAMP, was also suppressed after activin-A pretreatment. The presence of the RNA synthesis inhibitor actinomycin-D completely blocked the inhibitory effect of a 3-h activin-A pretreatment on subsequent rGRF-stimulated GH secretion. Pertussis toxin was only partially effective in preventing the inhibition by activin-A. The results of this study indicate that activin-A plays a crucial role as a modulator of somatotropic function, inhibiting GH secretion at the level of the secretory process and secondary to the inhibition of GH biosynthesis.     

Effects of dopamine and morphine on immunoreactive somatostatin and LH-releasing hormone secretion from hypothalamic fragments in vitro

Dopamine and morphine modulate GH and LH release, probably at a hypothalamic locus. To investigate this in more detail, we studied the influence of these substances on somatostatin and LH-releasing hormone (LHRH) release from rat hypothalamic fragments in vitro. Hypothalamic fragments were incubated in Earle's medium. After 60 min of preincubation, medium from two 20-min incubations was collected and somatostatin and LHRH levels measured by radioimmunoassay. Dopamine (10 nmol/1-0.1 mmol/l) induced a progressive increase (r = 0.41; P less than 0.01) in basal somatostatin levels. K+ (30 mmol/l)-induced somatostatin release was also increased (r = 0.54; P less than 0.01) by increasing doses of dopamine. Metoclopramide (10 mumol/l) blocked the dopamine (1 mumol/l)-induced increase in somatostatin release. No significant relationship between dopamine and LHRH was found either basally or after K+ (30 mmol/l) stimulation. Basal somatostatin was negatively correlated (r = -0.63; P less than 0.01) with morphine concentrations. No significant correlation was found after K+ (30 mmol/l) depolarization. Basal LHRH release was not influenced by morphine, while K+ (30 mmol/l)-induced release was significantly lower than controls only at a concentration of 10 nmol/l. These results suggest that dopamine and morphine act at a hypothalamic level to modulate GH release through alterations in somatostatin secretion. Dopamine and morphine have no consistent effect on hypothalamic LHRH release.     

Galanin is an estrogen-inducible, secretory product of the rat anterior pituitary.

Galanin is a peptide widely distributed throughout vertebrate central and peripheral nervous systems. Although its precise physiologic role is unknown, it can stimulate the pituitary secretion of prolactin and growth hormone. We examined the control of rat galanin (rGal) gene expression in the anterior pituitary using RNA blot and in situ hybridization analyses and using specific RIA. Pituitaries of normal male and ovariectomized female rats contained little detectable rGal mRNA. Treatment of these animals with 17 beta-estradiol increased pituitary rGal mRNA up to 4000-fold. These increases depended on time and dose of estrogen administration and correlated with up to 50-fold increases in pituitary galanin-like immunoreactivity. Galanin-like immunoreactivity was detectable in the plasma of estrogen-treated animals. Pituitary levels of rGal mRNA in female rats varied greater than 30-fold during the estrous cycle, with a peak on estrus and a nadir on diestrus. Estrogen-induced rGal gene expression was also observed in transplantable MtTW15 prolactin- and growth hormone-containing tumors but not in neuronal tissues expressing this gene. These data demonstrate that rGal is a secreted product of rat anterior pituitary cells, where its gene expression is strongly affected by physiologic levels of circulating estrogen.