Gonadotrophin releasing hormone receptor regulation in relationship to gonadotrophin secretion

Ovariectomy induces hyperalgesia accompanied by upregulated estrogen receptor α and protein kinase B in the rat spinal cord

Hormone supplementation is one of the common therapies for menopause-related disorders. Among different tools, the ovariectomy (OVX) rodents are widely accepted as an appropriate menopausal pain model. Our previous study has showed that OVX produces a significant pain facilitation in both acute pain and tonic pain, however, the underlying mechanisms remain unclear. In this study, we examined the effects of OVX treatment and estradiol (E2) supplementation on formalin-induced nociceptive responses, and explored the associated spinal mechanisms. Female Sprague-Dawley rats underwent bilateral OVX, and E2 supplementation was given subcutaneously from the 5th week after surgery (30 μg/day for 7 days). Our results showed that formalin-induced nociceptive behaviors did not differ between diestrus and proestrus stages of the estrous in intact rats. However, OVX exacerbated formalin-evoked inflammatory pain, especially in the late phase at 4-5 weeks but not 2 weeks post-surgery. E2 supplementation significantly reversed the OVX-triggered hyperalgesia. Double immunofluorescence staining revealed that both ERα and ERβ in the spinal dorsal horn were co-labeled with the neuronal markers, but not with markers of astrocytes or microglia. The spinal ERα (but not ERβ) expression significantly increased in the OVX group, which was reversed by E2 supplementation. Moreover, the OVX individuals showed an increased protein kinase B (AKT) level in lumbar spinal cord, and E2 supplementation diminished the AKT expression in OVX rats. Finally, intrathecal injection Wortmannin, an inhibitor for AKT signaling, effectively reduced the nociceptive behaviors in the late phase and the number of c-fos positive cells. Together, our findings indicate that E2 supplementation alleviates the OVX-induced hyperalgesia, which might be involved in spinal ERα and AKT mechanisms.

Maximal expression of Foxl2 in pituitary gonadotropes requires ovarian hormones

Gonadotropin-releasing hormone (GnRH) and activin regulate synthesis of FSH and ultimately fertility. Recent in vivo studies cast SMAD4 and FOXL2 as master transcriptional mediators of activin signaling that act together and independently of GnRH to regulate Fshb gene expression and female fertility. Ovarian hormones regulate GnRH and its receptor (GNRHR) through negative and positive feedback loops. In contrast, the role of ovarian hormones in regulating activin, activin receptors, and components of the activin signaling pathway, including SMAD4 and FOXL2, remains understudied. The widespread distribution of activin and many of its signaling intermediates complicates analysis of the effects of ovarian hormones on their synthesis in gonadotropes, one of five pituitary cell types. We circumvented this complication by using a transgenic model that allows isolation of polyribosomes selectively from gonadotropes of intact females and ovariectomized females treated with or without a GnRH antagonist. This paradigm allows assessment of ovarian hormonal feedback and distinguishes responses that are either independent or dependent on GnRH. Surprisingly, our results indicate that Foxl2 levels in gonadotropes decline significantly in the absence of ovarian input and independently of GnRH. Expression of the genes encoding other members of the activin signaling pathway are unaffected by loss of ovarian hormonal feedback, highlighting their selective effect on Foxl2. Expression of Gnrhr, a known target of FOXL2, also declines upon ovariectomy consistent with reduced expression of Foxl2 and loss of ovarian hormones. In contrast, Fshb mRNA increases dramatically post-ovariectomy due to increased compensatory input from GnRH. Together these data suggest that ovarian hormones regulate expression of Foxl2 thereby expanding the number of genes controlled by the hypothalamic-pituitary-gonadal axis that ultimately dictate reproductive fitness.

Conditional knockout of the androgen receptor in gonadotropes reveals crucial roles for androgen in gonadotropin synthesis and surge in female mice

Polycystic ovary syndrome is the major cause of infertility in reproductive aged women. Polycystic ovary syndrome is associated with high circulating levels of androgens and impaired metabolic function. The goal of this study was to understand how androgen signaling via the androgen receptor (AR) affects reproductive function. We knocked out the AR gene specifically in pituitary gonadotropes (PitARKO) to explore the role of androgen on the development of reproductive function in female mice. There was no difference in the age of puberty between control and PitARKO littermates, which was assessed by the ages of vaginal opening and first estrus. Cyclicity and fertility were also studied, and there was no significant difference between control and PitARKO mice. We observed a significant decrease in basal FSH serum and mRNA levels with no corresponding change in LH serum and mRNA levels. Although the numbers of litters born to control and PitARKO females were the same, the litter size was significantly smaller for PitARKO mice. The LH and FSH responses to ovariectomy was altered with reduced LH/FSH hormone and mRNA levels in PitARKO females. This reduction may be due to reduced expression of activin A/B and gnrhr. The preovulatory surge levels of LH and FSH were dramatically lower in PitARKO mice. The number of corpora lutea was decreased whereas the number of antral follicles was similar between control and PitARKO mice. Overall the pituitary AR contributes to the elaboration of the LH surge and normal reproductive function by regulating LH/FSH expression and secretion.

GnRH-Induced Ca(2+) Signaling Patterns and Gonadotropin Secretion in Pituitary Gonadotrophs. Functional Adaptations to Both Ordinary and Extraordinary Physiological Demands

PITUITARY GONADOTROPHS ARE A SMALL FRACTION OF THE ANTERIOR PITUITARY POPULATION, YET THEY SYNTHESIZE GONADOTROPINS: luteinizing (LH) and follicle-stimulating (FSH), essential for gametogenesis and steroidogenesis. LH is secreted via a regulated pathway while FSH release is mostly constitutive and controlled by synthesis. Although gonadotrophs fire action potentials spontaneously, the intracellular Ca(2+) rises produced do not influence secretion, which is mainly driven by Gonadotropin-Releasing Hormone (GnRH), a decapeptide synthesized in the hypothalamus and released in a pulsatile manner into the hypophyseal portal circulation. GnRH binding to G-protein-coupled receptors triggers Ca(2+) mobilization from InsP3-sensitive intracellular pools, generating the global Ca(2+) elevations necessary for secretion. Ca(2+) signaling responses to increasing (GnRH) vary in stereotyped fashion from subthreshold to baseline spiking (oscillatory), to biphasic (spike-oscillatory or spike-plateau). This progression varies somewhat in gonadotrophs from different species and biological preparations. Both baseline spiking and biphasic GnRH-induced Ca(2+) signals control LH/FSH synthesis and exocytosis. Estradiol and testosterone regulate gonadotropin secretion through feedback mechanisms, while FSH synthesis and release are influenced by activin, inhibin, and follistatin. Adaptation to physiological events like the estrous cycle, involves changes in GnRH sensitivity and LH/FSH synthesis: in proestrus, estradiol feedback regulation abruptly changes from negative to positive, causing the pre-ovulatory LH surge. Similarly, when testosterone levels drop after orquiectomy the lack of negative feedback on pituitary and hypothalamus boosts both GnRH and LH secretion, gonadotrophs GnRH sensitivity increases, and Ca(2+) signaling patterns change. In addition, gonadotrophs proliferate and grow. These plastic changes denote a more vigorous functional adaptation in response to an extraordinary functional demand.

Castration-induced modifications of GnRH-elicited [Ca2+](i) signaling patterns in male mouse pituitary gonadotrophs in situ: studies in the acute pituitary slice preparation

Gonadotropin-releasing hormone (GnRH) binds to pituitary gonadotroph receptors and initiates [Ca(2+)](i) signals and gonadotropin secretion. Here, we recorded GnRH-induced Ca(2+) signals in acute pituitary slices from both intact and castrated male mice 15 and 45 days after orchiectomy (GnX). Cells responding with "noncanonical" sequences of Ca(2+) signaling to increasing GnRH concentrations ([GnRH]; oscillatory responses at a given [GnRH] and transient responses at both lower and higher concentrations) were augmented significantly in the castrated mice. Also, 15 days after GnX the number and size of gonadotrophs were augmented, confirming earlier anatomical studies. Hypertrophied gonadotrophs after 15 days after GnX tended to display GnRH-induced Ca(2+) responses of greater amplitude. Furthermore, median effective dose (ED50) for GnRH decreased from 0.17 nM (control) to ~0.07 nM after GnX, suggesting increased GnRH responsiveness of the gonadotroph population. The progression of Ca(2+) response patterns reported in control male rat gonadotrophs (oscillations declining and spike-plateau responses dominating at increasing [GnRH]) was less conspicuous in mouse gonadotrophs in situ. Also, GnX-induced alterations in rat gonadotrophs (persistence of Ca(2+) oscillations even at [GnRH] >100 nM) were not mirrored by mouse gonadotrophs in situ. Contrary to observations in intact and 15-day castrated mice, after 45 days of GnX the hump component diminished and oscillations were augmented with increasing [GnRH], but Ca(2+) response patterns of gonadotrophs in situ remained virtually unchanged in response to [GnRH]s >1 nM, suggesting dose discrimination failure at high [GnRH]s. This study underscores the notion that GnRH responsiveness and the effects of testosterone deficiency may not be equal in pituitary gonadotrophs across species.

Transcriptional regulation of the GnRH receptor gene by glucocorticoids

Expression of the gonadotropin-releasing hormone (GnRH) receptor gene is stimulated by dexamethasone in GnRH-deficient rodents. In this study we identify a 1226 bp sequence at the 5'-flanking region of the mouse GnRH-R gene that confers dexamethasone responsiveness when expressed in host cells. Further, a glucocorticoid antagonist blocks transcriptional activity of the mGnRHR promoter. Progressive 5'-deletion of the mGnRHR promoter localized the response sequence between the -331/-255 region. Analysis of this region revealed binding sites for the AP-1 transcription factor. Mutation in AP-1 modified the functional activity of the mGnRHR promoter following GnRH agonist or dexamethasone-stimulation. Using an electrophoretic mobility shift assay, a protein complex is shown to bind to the AP-1 site. These results suggest that AP-1 proteins and glucocorticoid receptor regulate transcription of the GnRH-R promoter in a heterologous system.

Human gonadotropin-releasing hormone receptor gene transcription: up-regulation by 3',5'-cyclic adenosine monophosphate/protein kinase A pathway

Transient transfection of mouse gonadotrope-derived (alphaT3-1) cells with a 2297 bp human GnRHR promoter-luciferase construct (p2300-LucF) showed a dose- and time-dependent increase in the human gonodotropin-releasing hormone receptor (GnRHR) promoter activity after forskolin treatment. An average of 4.8-fold increase in promoter activity was observed after 12 h of 10 microM forskolin treatment. This effect was mimicked by administration of cholera toxin, cAMP analog or pituitary adenylate cyclase activating polypeptide 38 (PACAP). A specific adenylate cyclase (AC) inhibitor (ACI) or protein kinase A (PKA) inhibitor (PKAI) pretreatment reversed the forskolin- and PACAP-induced increase in the human GnRHR promoter activity. These results not only confirm the stimulatory effect of Cyclic adenosine monophosphate (cAMP) in human GnRHR promoter activation, but also suggest that hormones or neurotransmitters that activate adenylate cyclase in pituitary gonadotropes may increase the expression of human GnRHR gene in transcriptional level. Progressive 5' deletion assays identified a 412 bp fragment (-577 to 167) in the human GnRHR 5'-flanking region that is essential in maintaining the basal responsiveness to cAMP. Mutagenesis coupled with functional studies have identified two putative AP-1/CREB binding sites, namely hGR-AP/CRE-1 and hGR-AP/CRE-2 that participated in mediating the cAMP-stimulatory effect. Mutation of the putative hGR-AP/CRE-1 and hGR-CRE-2 resulted in a 38 and 32% decrease in the forskolin-induced stimulation. However, mutation of both binding sites did not completely abolish the cAMP-stimulatory effect, suggesting that multiple transcription factor binding sites were involved in full response in cAMP stimulation. The binding of CREB to these motifs was confirmed by gel mobility shift assay and antibody supershift assay.

The expression, regulation and signal transduction pathways of the mammalian gonadotropin-releasing hormone receptor

Normal mammalian sexual maturation and reproductive functions require the integration and precise coordination of hormones at the hypothalamic, pituitary, and gonadal levels. Hypothalamic gonadotropin-releasing hormone (GnRH) is a key regulator in this system; after binding to its receptor (GnRHR), it stimulates de novo synthesis and release of gonadotropins in anterior pituitary gonadotropes. Since the isolation of the GnRHR cDNA, the expression of GnRHR mRNA has been detected not only in the pituitary, but also in extrapituitary tissues, including the ovary and placenta. It has been shown that change in GnRHR mRNA is one of the mechanisms for regulating the expression of the GnRHR. To help understand the molecular mechanism(s) involved in transcriptional regulation of the GnRHR gene, the 5' flanking region of the GnRHR gene has recently been isolated. Initial characterization studies have identified several DNA regions in the GnRHR 5' flanking region which are responsible for both basal expression and GnRH-mediated homologous regulation of this gene in pituitary cells. The mammalian GnRHR lacks a C-terminus and possesses a relatively short third intracellular loop; both features are important in desensitization of many others G-protein coupled receptors (GPCRs), Homologous desensitization of GnRHR has been shown to be regulated by various serine-threonine protein kinases including protein kinase A (PKA) and protein kinase C (PKC), as well as by G-protein coupled receptor kinases (GRKs). Furthermore, GnRHR was demonstrated to couple with multiple G proteins (Gq/11, Gs, and Gi), and to activate cascades that involved the PKC, PKA, and mitogen-activator protein kinases. These results suggest the diversity of GnRHR-G protein coupling and signal transduction systems. The identification of second form of GnRH (GnRH-II) in mammals adds to the complexity of the GnRH-GnRHR system. This review summaries our recent progress in understanding the regulation of GnRHR gene expression and the GnRHR signal transduction pathways.Key words: gonadotropin-releasing hormone receptor, transcriptional regulation, desensitization, signal transduction.

Transcriptional down-regulation of human gonadotropin-releasing hormone (GnRH) receptor gene by GnRH: role of protein kinase C and activating protein 1

Clinical applications of GnRH agonists (GnRHa) are based primarily on the decrease in gonadotropin release after down-regulation of the GnRH receptor (GnRHR) by continuous GnRHa administration. However, the molecular mechanisms underlying the transcriptional regulation of the human GnRHR gene after prolonged GnRH treatment remain poorly understood. In the present study GnRHa-mediated regulation of human GnRHR gene transcription was studied by transiently transfecting the mouse gonadotrope-derived (alphaT3-1) cells with a 2297-bp human GnRHR promoter-luciferase construct (p2300-LucF). A dose- and time-dependent decrease in human GnRHR promoter activity was observed after GnRHa treatment. An average 71% decrease in promoter activity was observed after 24-h treatment with 0.1 microM GnRHa, which was blocked by cotreatment of the GnRH antagonist, antide. This effect was mimicked by phorbol 12-myristate 13-acetate (TPA) administration. In addition, the GnRHa- and TPA-mediated decrease in the human GnRHR promoter activity was reversed by a specific protein kinase C (PKC) inhibitor, GF109203X, or depletion of PKC by TPA pretreatment. These findings indicate that the activation of the PKC pathway is important in regulating the human GnRHR gene expression. By progressive 5'-deletion studies, we have identified a 248-bp DNA fragment (-1018 to -771, relative to the translation start site) at the 5'-flanking region of the human GnRHR gene that is responsible for the GnRHa-mediated down-regulation of human GnRHR promoter activity. Analysis of this sequence reveals the existence of two putative activating protein-1 (AP-1) sites with 87% homology to the consensus sequence (5'-TGA(G/C)T(C/A)A-3'), located at -1000 to -994 (5'-TTAGACA-3', in complementary orientation) and -943 to 937 (5'-TGAATAA-3'). Using competitive gel mobility shift assays, AP-1 binding was observed within this 248-bp region. Site-directed mutation of the putative AP-1-binding site located at -1000 to -994 abolished the GnRHa-induced inhibition. Further competitive GMSA and supershift experiments confirmed the identity of AP-1 binding in this region. By the use of Western blot analysis, a significant increase in c-Jun (100%; P < 0.05) and c-Fos (50%; P < 0.05) protein levels was observed after GnRHa treatment in alphaT3-1 cells. In addition, our data suggested that a change in AP-1 composition, particularly c-Fos, was important in mediating GnRHa-induced inhibition of human GnRHR gene expression. We conclude that activation of the PKC pathway by GnRH is important in controlling human GnRHR gene expression. In addition, the putative AP-1-binding site located at -1000 to -994 of the human GnRHR5'-flanking region has been functionally identified to be involved in mediating this down-regulatory effect.

Responsiveness of the ovine gonadotropin-releasing hormone receptor gene to estradiol and gonadotropin-releasing hormone is not detectable in vitro but is revealed in transgenic mice

Although the ability of estradiol to enhance pituitary sensitivity to GnRH is established, the underlying mechanism(s) remain undefined. Herein, we find that approximately 9,100 bp of 5' flanking region from the ovine GnRH receptor (oGnRHR) gene is devoid of transcriptional activity in gonadotrope-derived cell lines and is not responsive to either estradiol or GnRH. In stark contrast, this same 9,100 bp promoter fragment directed tissue-specific expression of luciferase in multiple lines of transgenic mice. To test for hormonal regulation of the 9,100-bp promoter, ovariectomized transgenic females were treated with a GnRH antiserum alone or in combination with estradiol. Treatment with antiserum alone reduced pituitary expression of luciferase by 80%. Pituitary expression of luciferase in animals receiving both antiserum and estradiol was approximately 50-fold higher than animals receiving antiserum alone. The estradiol response of the -9,100-bp promoter was equally demonstrable in males. In addition, a GnRH analog (D-Ala-6-GnRH) that does not cross-react with the GnRH antiserum restored pituitary expression of luciferase in males passively immunized against GnRH to levels not different from castrate controls. Finally, treatment with both estradiol and D-Ala-6-GnRH increased pituitary expression of luciferase to a level greater than the sum of the individual treatments suggesting synergistic activation of the transgene by these two hormones. Thus, despite the complete absence of transcriptional activity and hormonal responsiveness in vitro, 9,100 bp of proximal promoter from the oGnRHR gene is capable of directing tissue-specific expression and is robustly responsive to both GnRH and estradiol in transgenic mice. To begin to refine the functional boundaries of the critical cis-acting elements, we next constructed transgenic mice harboring a transgene consisting of 2,700 bp of 5' flanking region from the oGnRHR gene fused to luciferase. As with the -9,100 bp promoter, expression of luciferase in the -2,700 lines was primarily confined to the pituitary gland, brain and testes. Furthermore, the passive immunization-hormonal replacement paradigms described above revealed both GnRH and estradiol responsiveness of the -2,700-bp promoter. Thus, 2,700 bp of proximal promoter from the oGnRHR gene is sufficient for tissue-specific expression as well as GnRH and estradiol responsiveness. Given the inability to recapitulate estradiol regulation of GnRHR gene expression in vitro, transgenic mice may represent one of the few viable avenues for ultimately defining the molecular mechanisms underlying estradiol regulation of GnRHR gene expression.

Noradrenergic neurotoxin suppresses gonadotropin-releasing hormone (GnRH) and GnRH receptor gene expression in ovariectomized and steroid-treated rats

The present study was designed to investigate whether noradrenergic neurotransmission regulates the gene expression of gonadotropin-releasing hormone (GnRH) in the preoptic area and GnRH receptor in the pituitary. To this end, N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine (DSP4, 50 mg/kg), an intraperitoneal (i.p.) injection of selective noradrenergic neurotoxin, was administered 1 h before progesterone (1 mg) treatment in ovariectomized and estradiol-treated prepubertal rats. Treatment with DSP4 effectively blocked the progesterone-induced increase in hypothalamic noradrenaline content, but not dopamine content, indicating that DSP4 selectively inhibits noradrenergic neurotransmission. DSP4 significantly blocked progesterone-induced increase in serum luteinizing hormone (LH) concentrations as well as GnRH release from hypothalamic fragments incubated in vitro. DSP4 concomitantly down-regulated GnRH mRNA levels in the preoptic area, as determined by competitive reverse transcription-polymerase chain reaction. DSP4 also clearly down-regulated progesterone-induced GnRH receptor mRNA levels in the pituitary, whereas it failed to alter LHbeta mRNA levels. In summary, blockade of noradrenergic neurotransmission with DSP4 resulted in profound reductions of hypothalamic GnRH and pituitary GnRH receptor gene expression.

Structural organization and characterization of the promoter region of the rat gonadotropin-releasing hormone receptor gene

The gene encoding the rat gonadotropin-releasing hormone (GnRH) receptor was isolated, and its structural organization and promoter region were characterized. The gene was found to consist of three exons that encode the receptor protein, and spanned about 20 kb. Of two genomic clones analyzed, one contained the 5'-untranslated region and the first exon, and the other contained the second and third exons. The sizes of the first, second, and third exons are 625, 217, and 1476 nt, respectively. The first intron is at least 12 kb in length and is located between nucleotides 522 and 523 of the cDNA reading frame, in the middle of the fourth transmembrane domain. The second intron is about 2.5 kb and is also located in the reading frame between nucleotides 739 and 740, separating the fifth and sixth transmembrane domains. Genomic blots in combination with cloning and sequencing suggested that a single GnRH receptor gene is present in the rat genome. Primer extension indicated that the transcription start site is located 103 nt upstream of the translational start codon. A putative TATA box is positioned 23 nt in front of the transcription initiation site. The 1.8 kb 5' flanking sequence contains an SF-1 site, an AP-1 site, CCAAT sequences, a Pit-1 binding site, and a potential CRE-like sequence. To evaluate promoter activity, the 1.8 kb and two 5' deleted fragments of 1.2 and 0.6 kb were fused to the luciferase reporter gene and transiently expressed in immortalized pituitary gonadotrophs (alphaT3-1 cells) and hypothalamic neurons (GT1-7 cells), and in nonpituitary (COS-7) cells. Luciferase gene expression was significantly increased by all three fragments in pituitary and hypothalamic cells, but not in COS-7 cells. The promoter activity of the 1.2 kb fragment was higher than that of the other fragments. Forskolin and cAMP analogs increased luciferase gene expression in both alphaT3-1 and GT1-7 cells, but activation of protein kinase C by phorbol myristate acetate had no effect. These studies indicate that positive and negative regulatory elements are present within the 1.8 kb 5' flanking sequence of the GnRH receptor. Knowledge of the genomic organization and analysis of the promoter region of the rat GnRH receptor gene will facilitate the elucidation of its transcriptional control in pituitary gonadotrophs and hypothalamic neurons.

Hormonal and neurotransmitter regulation of GnRH gene expression and related reproductive behaviors

Gonadotropin-releasing hormone (GnRH), having a highly conserved structure across mammalian species, plays a pivotal role in the control of the neuroendocrine events and the inherent sexual behaviors essential for reproductive function. Recent advances in molecular genetic technology have contributed greatly to the investigation of several aspects of GnRH physiology, particularly steroid hormone and neurotransmitter regulation of GnRH gene expression. Behavioral studies have focused on the actions of GnRH in steroid-sensitive brain regions to understand better its role in the facilitation of mating behavior. To date, however, there are no published reports which directly correlate GnRH gene expression and reproductive behavior. The intent of this article is to review the current understanding of the way in which changes in GnRH gene expression, and modifications of GnRH neuronal activity, may ultimately influence reproductive behavior.

A model for intracellular calcium signaling and the coordinate regulation of hormone biosynthesis, receptors and secretion

A two-state model for the stimulus-induced nongraded response of a single cell is formulated. Individual metestrus gonadotropes stimulated with LHRH operate as a simple switch: either on or off. At a given concentration of stimulus some gonadotropes switch on, while others do not switch on, secretion. The probability of a gonadotrope being in the secretory state is enhanced with each increment of LHRH concentration. Individual gonadotropes in a secretory state are envisioned to decrease their number of LHRH receptors and to switch off LH biosynthesis. On the other hand, individual gonadotropes that are not in a secretory state are thought to increase their number of LHRH receptors and to switch on LH biosynthesis. The group of individuals in the population that have thresholds falling in the range of a given stimulus initiate secretion. And, the group of individuals in the population that have thresholds that fall above the range of a given stimulus do not initiate secretion. More remarkable is evidence that the cells that are protected from hormone secretion nevertheless respond with a set of intracellular signals and this provides a new perspective of how they switch on hormone biosynthesis and up-regulate the LHRH receptors. These changes are envisioned to reduce the threshold of an individual cell and accordingly to enhance the probability that the cell responds in the secretory state with the next stimulus. This scheme would appear to lead to automatic cycles of secretion and biosynthesis since an individual cell can occupy only one of two states at any time and occupancy of either state promotes change to the other. This may provide a solution to the problem of how an endocrine gland might reconcile differences in the time-course of hormone secretion which occurs rapidly and hormone biosynthesis that requires a longer period of time. Parenthetically, the model may also be adapted to the case where the vast majority of individuals in the population are generally subthreshold in relation to the physiological stimulus: such an adaption leads to interesting ways of viewing the mammalian reproductive cycle and the regulation of the preovulatory LH surge. A two-state model of the internal Ca2+ store is outlined here to stimulate thought on how the intracellular signals of each binary state may switch a variety of cellular responses either on or off. The model provides a new perspective on the coordinate regulation of hormone biosynthesis, receptors, and secretion that may be useful in the final reconciliation of population studies with insights about individual cells.

Altered regulation of pituitary gonadotropin-releasing hormone (GnRH) receptor number and pituitary responsiveness to GnRH in 2,3,7,8-tetrachlorodibenzo-p-dioxin-treated male rats

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) increases the potency of androgens as feedback inhibitors of luteinizing hormone (LH) secretion. Our objectives were to determine if this increase is due to pituitary or hypothalamic dysfunction (or both), and to investigate the mechanism by which TCDD produces this effect. Seven days after dosing, TCDD inhibited the compensatory increases in (i) pituitary gonadotropin-releasing hormone (GnRH) receptor number, (ii) LH secretory responsiveness of the pituitary to GnRH, and (iii) plasma LH concentrations which should have occurred in response to TCDD-induced decreases in plasma testosterone concentrations. TCDD did not inhibit these compensatory responses in the absence of testicular hormones, while treatment of castrated rats with testosterone restored the ability of TCDD to prevent these increases. These findings demonstrate that TCDD alters the androgenic regulation of pituitary GnRH receptor number and pituitary responsiveness to GnRH stimulation. The pituitary is therefore a target organ for TCDD; whether a hypothalamic defect is also involved in the altered regulation of LH secretion was not resolved. The compensatory increases in pituitary GnRH receptor number and plasma LH concentration elicited by low plasma testosterone concentrations were inhibited by similar doses of TCDD (ED50 20 micrograms TCDD/kg for both responses). We concluded that TCDD increases the potency of androgens as feedback inhibitors of LH secretion by increasing their potency as regulators of both pituitary GnRH receptor number and GnRH responsiveness. This is the first demonstration that TCDD treatment (i) affects pituitary responsiveness to a hormone secreted by a peripheral organ (testosterone), and (ii) alters the regulation of pituitary responsiveness to a hypothalamic hormone (GnRH).