Gonadotropin-releasing hormone neurons in the rhesus macaque are not immunoreactive for the estrogen receptor

Neuroendocrine control of gonadotropin-releasing hormone: Pulsatile and surge modes of secretion

The concept that different systems control episodic and surge secretion of gonadotropin-releasing hormone (GnRH) was well established by the time that GnRH was identified and formed the framework for studies of the physiological roles of GnRH, and later kisspeptin. Here, we focus on recent studies identifying the neural mechanisms underlying these two modes of secretion, with an emphasis on their core components. There is now compelling data that kisspeptin neurons in the arcuate nucleus that also contain neurokinin B (NKB) and dynorphin (i.e., KNDy cells) and their projections to GnRH dendrons constitute the GnRH pulse generator in mice and rats. There is also strong evidence for a similar role for KNDy neurons in sheep and goats, and weaker data in monkeys and humans. However, whether KNDy neurons act on GnRH dendrons and/or GnRH soma and dendrites that are found in the mediobasal hypothalamus (MBH) of these species remains unclear. The core components of the GnRH/luteinising hormone surge consist of an endocrine signal that initiates the process and a neural trigger that drives GnRH secretion during the surge. In all spontaneous ovulators, the core endocrine signal is a rise in estradiol secretion from the maturing follicle(s), with the site of estrogen positive feedback being the rostral periventricular kisspeptin neurons in rodents and neurons in the MBH of sheep and primates. There is considerable species variations in the neural trigger, with three major classes. First, in reflex ovulators, this trigger is initiated by coitus and carried to the hypothalamus by neural or vascular pathways. Second, in rodents, there is a time of day signal that originates in the suprachiasmatic nucleus and activates rostral periventricular kisspeptin neurons and GnRH soma and dendrites. Finally, in sheep nitric oxide-producing neurons in the ventromedial nucleus, KNDy neurons and rostral kisspeptin neurons all appear to participate in driving GnRH release during the surge.

Kisspeptin and neurokinin B expression in the human hypothalamus: Relation to reproduction and gender identity

Gonadotropin-releasing hormone (GnRH) neurons in the hypothalamus are at the core of reproductive functioning. GnRH released into the median eminence regulates the secretion of the gonadotropins from the anterior pituitary, which in turn activates gametogenesis and steroid synthesis by the gonads. The GnRH system displays functional sex differences: GnRH is secreted in pulses at a constant frequency in men, whereas in women, pulse frequency varies over the menstrual cycle. In both sexes, GnRH release is regulated by sex steroid hormones, acting at the level of the hypothalamus and the anterior pituitary in a classic feedback loop. Because GnRH neurons do not express sex steroid receptors, hormone effects on GnRH release are presumed to be mediated indirectly through other steroid-sensitive neuronal systems, which then converge onto GnRH cell bodies and/or terminals. Human genetic studies demonstrated that kisspeptin (KP) as well as neurokinin B (NKB) signaling are both potent regulators of GNRH secretion. In humans, postmortem studies using immunohistochemistry have shown that women have higher KP and NKB expression in the infundibular nucleus than men. Sex differences in KP expression are present throughout life, which is from the infant/prepubertal into the elderly period, whereas sex differences in NKB expression do not emerge until adulthood. KP and NKB are often coexpressed together with dynorphin by the same population of neurons, also known as KDNy neurons in other species. Indeed, significant coexpression between KP and NKB but not with Dynorphin has been observed thereby challenging the KDNy concept in humans. Female-typical expression of both KP and NKB were observed in the infundibular nucleus of trans women (male sex assigned at birth and female gender identity). Taken together, sex differences in KP and NKB expression most likely reflect organizational actions of sex steroid hormones on the developing brain but they also remain sensitive to circulating sex steroids in adulthood. The female-dominant sex difference in infundibular KP and NKB expression suggests that this brain region is most likely involved in both the negative and positive feedback actions of estrogens on GnRH secretion. Finally, the sex-reversal observed in KP and NKB expression in trans women might reflect, at least partially, an atypical sexual differentiation of the brain.

Prenatal Testosterone Exposure Alters GABAergic Synaptic Inputs to GnRH and KNDy Neurons in a Sheep Model of Polycystic Ovarian Syndrome

Prenatal testosterone (T)-treated female sheep display reproductive deficits similar to women with polycystic ovarian syndrome (PCOS), including an increase in LH pulse frequency due to actions of the central GnRH pulse generator. In this study, we used multiple-label immunocytochemistry to investigate the possibility of changes in the γ-aminobutyric acid (GABA) neurotransmitter system at two key components of the GnRH pulse generator in prenatal T-treated sheep: kisspeptin/neurokinin B/dynorphin (KNDy) neurons of the arcuate nucleus, and GnRH neurons in the preoptic area (POA) and mediobasal hypothalamus (MBH). We observed a significant decrease and increase, respectively, in the number of GABAergic synapses onto POA and MBH GnRH neurons in prenatal T-treated ewes; additionally, there was a significant increase in the number of GABAergic inputs onto KNDy neurons. To determine the actions of GABA on GnRH and KNDy neurons, we examined colocalization with the chloride transporters NKCC1 and KCC2, which indicate stimulatory or inhibitory activation of neurons by GABA, respectively. Most GnRH neurons in both POA and MBH colocalized NKCC1 cotransporter whereas none contained the KCC2 cotransporter. Most KNDy neurons colocalized either NKCC1 or KCC2, and 28% of the KNDy population contained NKCC1 alone. Therefore, we suggest that, as in the mouse, GABA in the sheep is stimulatory to GnRH neurons, as well as to a subset of KNDy neurons. Increased numbers of stimulatory GABAergic inputs to both MBH GnRH and KNDy neurons in prenatal T-treated animals may contribute to alterations in steroid feedback control and increased GnRH/LH pulse frequency seen in this animal model of PCOS.

Effects of Age and Estradiol on Gene Expression in the Rhesus Macaque Hypothalamus

BACKGROUND

The hypothalamus plays a key role in mediating the effects of estrogen on many physiological functions, including reproduction, metabolism, and thermoregulation. We have previously observed marked estrogen-dependent gene expression changes within the hypothalamus of rhesus macaques during aging, especially in the KNDy neurons of the arcuate-median eminence (ARC-ME) that produce kisspeptin, neurokinin B, and dynorphin A. Little is known, however, about the mechanisms involved in mediating the feedback from estrogen onto these neurons.

METHODS

We used quantitative real-time PCR to profile age- and estrogen-dependent gene expression changes in the rhesus macaque hypothalamus. Our focus was on genes that encode steroid receptors (ESR1, ESR2, PGR, and AR) and on enzymes that contribute to the local synthesis of 17β-estradiol (E2; STS, HSD3B1/2, HSD17B5, and CYP19A). In addition, we used RT(2) Profiler™ PCR Arrays to profile a larger set of genes that are integral to hypothalamic function.

RESULTS

KISS1, KISS1R, TAC3, and NPY2R mRNA levels increased in surgically menopausal (ovariectomized) old females relative to age-matched ovariectomized animals that received E2 hormone therapy. In contrast, PGR, HSD17B, GNRH2, SLC6A3, KISS1, TAC3, and NPY2R mRNA levels increased after E2 supplementation.

CONCLUSION

The rhesus macaque ARC-ME expresses many genes that are responsive to changes in circulating estrogen levels, even during old age, and these may contribute to causing the normal and pathophysiological changes that occur during menopause.

G-protein coupled estrogen receptor, estrogen receptor α, and progesterone receptor immunohistochemistry in the hypothalamus of aging female rhesus macaques given long-term estradiol treatment

Steroid hormone receptors are widely and heterogeneously expressed in the brain, and are regulated by age and gonadal hormones. Our goal was to quantify effects of aging, long-term estradiol (E2 ) treatment, and their interactions, on expression of G protein-coupled estrogen receptor (GPER), estrogen receptor α (ERα) and progesterone receptor (PR) immunoreactivity in two hypothalamic regions, the arcuate (ARC) and the periventricular area (PERI) of rhesus monkeys as a model of menopause and hormone replacement. Ovariectomized (OVX) rhesus macaques were young (∼ 11 years) or aged (∼ 25 years), given oil (vehicle) or E2 every 3 weeks for 2 years. Immunohistochemistry and stereologic analysis of ERα, PR, and GPER was performed. More effects were detected for GPER than the other two receptors. Specifically, GPER cell density in the ARC and PERI, and the percent of GPER-immunoreactive cells in the PERI, were greater in aged than in young monkeys. In addition, we mapped the qualitative distribution of GPER in the monkey hypothalamus and nearby regions. For ERα, E2 treated monkeys tended to have higher cell density than vehicle monkeys in the ARC. The percent of PR density in the PERI tended to be higher in E2 than vehicle monkeys of both ages. This study shows that the aged hypothalamus maintains expression of hormone receptors with age, and that long-term cyclic E2 treatment has few effects on their expression, although GPER was affected more than ERα or PR. This result is surprising in light of evidence for E2 regulation of the receptors studied here, and differences may be due to the selected regions, long-term nature of E2 treatment, among other possibilities.

GnRH neurons of young and aged female rhesus monkeys co-express GPER but are unaffected by long-term hormone replacement

Menopause is caused by changes in the function of the hypothalamic-pituitary-gonadal axis that controls reproduction. Hypophysiotropic gonadotropin-releasing hormone (GnRH) neurons in the hypothalamus orchestrate the activity of this axis and are regulated by hormonal feedback loops. The mechanisms by which GnRH responds to the primary regulatory sex steroid hormone, estradiol (E2), are still poorly understood in the context of menopause. Our goal was to determine whether the G protein-coupled estrogen receptor (GPER) is co-expressed in adult primate GnRH neurons and whether this changes with aging and/or E2 treatment. We used immunofluorescence double-labeling to characterize the co-expression of GPER in GnRH perikarya and terminals in the hypothalamus. Young and aged rhesus macaques were ovariectomized and given long-term (~2-year) hormone treatments (E2, E2 + progesterone, or vehicle) selected to mimic currently prescribed hormone replacement therapies used for the alleviation of menopausal symptoms in women. We found that about half of GnRH perikarya co-expressed GPER, while only about 12% of GnRH processes and terminals in the median eminence (ME) were double-labeled. Additionally, many GPER-labeled processes were in direct contact with GnRH neurons, often wrapped around the perikarya and processes and in close proximity in the ME. These results extend prior work by showing robust co-localization of GPER in GnRH in a clinically relevant model, and they support the possibility that GPER-mediated E2 regulation of GnRH occurs both in the soma and terminals in nonhuman primates.

Sex differences in the neurokinin B system in the human infundibular nucleus

CONTEXT

The recent report that loss-of-function mutations in either the gene encoding neurokinin B (NKB) or its receptor (NK3R) produce gonadotropin deficiencies in humans strongly points to NKB as a key regulator of GnRH release.

OBJECTIVES

We used NKB immunohistochemistry on postmortem human brain tissue to determine: 1) whether the human NKB system in the infundibular nucleus (INF) is sexually dimorphic; 2) at what stage in development the infundibular NKB system would diverge between men and women; 3) whether this putative structural difference is reversed in male-to-female (MtF) transsexual people; and 4) whether menopause is accompanied by changes in infundibular NKB immunoreactivity.

METHODS

NKB immunohistochemical staining was performed on postmortem hypothalamus material of both sexes from the infant/pubertal period into the elderly period and from MtF transsexuals.

RESULTS

Quantitative analysis demonstrated that the human NKB system exhibits a robust female-dominant sexual dimorphism in the INF. During the first years after birth, both sexes displayed a moderate and equivalent level of NKB immunoreactivity in the INF. The adult features emerged progressively around puberty until adulthood, where the female-dominant sex difference appeared and continued into old age. In MtF transsexuals, a female-typical NKB immunoreactivity was observed. Finally, in postmenopausal women, there was a significant increase in NKB immunoreactivity compared to premenopausal women.

CONCLUSION

Our results indicate that certain sex differences do not emerge until adulthood when activated by sex steroid hormones and the likely involvement of the human infundibular NKB system in the negative and positive feedback of estrogen on GnRH secretion.

Estrogenic regulation of the GnRH neuron

Reproductive function is regulated by the secretion of luteinizing hormone (LH) and follicle-stimulating hormone from the pituitary and the steroid hormones from the gonads. The dynamic changes in the levels of the reproductive hormones regulate secondary sex characteristics, gametogenesis, cellular function, and behavior. Hypothalamic GnRH neurons, with cell bodies located in the basal hypothalamus, represent the final common pathway for neuronally derived signals to the pituitary. As such, they serve as integrators of a dizzying array of signals including sensory inputs mediating information about circadian, seasonal, behavioral, pheromonal, and emotional cues. Additionally, information about peripheral physiological function may also be included in the integrative signal to the GnRH neuron. These signals may communicate information about metabolic status, disease, or infection. Gonadal steroid hormones arguably exert the most important effects on GnRH neuronal function. In both males and females, the gonadal steroid hormones exert negative feedback regulation on axis activity at both the level of the pituitary and the hypothalamus. These negative feedback loops regulate homeostasis of steroid hormone levels. In females, a cyclic reversal of estrogen feedback produces a positive feedback loop at both the hypothalamic and pituitary levels. Central positive feedback results in a dramatic increase in GnRH secretion (Moenter et al., 1992; Xia et al., 1992; Clarke, 1993; Sisk et al., 2001). This is coupled with an increase in pituitary sensitivity to GnRH (Savoy-Moore et al., 1980; Turzillo et al., 1995), which produces the massive surge in secretion of LH that triggers ovulation. While feedback regulation of the axis in males is in part mediated by estrogen receptors (ER), there is not a clear consensus as to the relative role of ER versus AR signaling in males (Lindzey et al., 1998; Wersinger et al., 1999). Therefore, this review will focus on estrogenic signaling in the female.

Differential roles of GnRH-I and GnRH-ii neurons in the control of the primate reproductive axis

In vertebrates, gonadotropin-releasing hormone (GnRH) represents the primary neuroendocrine link between the brain and the reproductive axis, and in some species up to three different forms of GnRH have been detected. Until recently, it had been assumed that humans and non-human primates only express one form (GnRH-I), but it is now clear they also express a second form (GnRH-II). GnRH-II, like GnRH-I, is highly effective at stimulating gonadotropin release, both in vitro and in vivo, but the neurons that produce GnRH-II are completely distinct from those producing GnRH-I. Moreover, GnRH-II and GnRH-I producing neurons respond very differently to estradiol; specifically, estradiol stimulates GnRH-II gene expression in the former and inhibit GnRH-I gene expression in the latter. Consequently, the negative feedback action of estradiol may be mediated exclusively by the subpopulation of GnRH neurons that express GnRH-I, while the positive feedback action may be mediated exclusively by the subpopulation that expresses GnRH-II. Taken together, these findings raise the possibility that two completely different GnRH neuronal systems participate in the control of primate reproductive physiology. The primary role of GnRH-I neurons is likely to be focused on the maintenance and modulation of tonic pulsatile LH release, whereas the primary role of GnRH-II neurons is likely to be focused on the generation of the preovulatory LH surge. This functional segregation of the primate neuroendocrine reproductive axis lends itself for novel targeted approaches to fertility control and for treatment of human reproductive disorders.

Neurokinin B signalling in the human reproductive axis

Recent human genetic studies have established that neurokinin B (NKB) signalling via the neurokinin 3 receptor (NK3R) is required for normal developmental activation of pulsatile GnRH secretion from the hypothalamus. As increasing numbers of patients with loss-of-function mutations have been described, evidence has emerged that peripheral NKB is not necessary for normal pregnancy despite high placental expression and high plasma levels of NKB in late gestation. Nevertheless many key questions about the role of NKB in the function of the GnRH pulse generator remain to be answered. Differences in requirement for NKB/NK3R for hypothalamic-pituitary-gonadal (HPG) maturation amongst different species, and their varied responses to stimulation with NKB represent a challenge for higher resolution studies. Neuroanatomical investigation has, however, identified key "KNDy" (Kisspeptin, Neurokinin B, Dynorphin) arcuate neurones that are conserved amongst different species and that are intimately connected both to each other and to the GnRH nerve termini. Several lines of evidence suggest that these may be the core of the GnRH pulse generator, and with experimental tools now in place in humans, monkeys and other experimental animals to pursue the function of these interconnected neurones and the functional hierarchy of their neuroendocrine inputs, understanding of the enigmatic GnRH pulse generator may at last be within reach.

Immortalized neurons for the study of hypothalamic function

The hypothalamus is a vital part of the central nervous system: it harbors control systems implicated in regulation of a wide range of homeostatic processes, including energy balance and reproduction. Structurally, the hypothalamus is a complex neuroendocrine tissue composed of a multitude of unique neuronal cell types that express a number of neuromodulators, including hormones, classical neurotransmitters, and specific neuropeptides that play a critical role in mediating hypothalamic function. However, neuropeptide and receptor gene expression, second messenger activation, and electrophysiological and secretory properties of these hypothalamic neurons are not yet fully defined, primarily because the heterogeneity and complex neuronal architecture of the neuroendocrine hypothalamus make such studies challenging to perform in vivo. To circumvent this problem, our research group recently generated embryonic- and adult-derived hypothalamic neuronal cell models by utilizing the novel molecular techniques of ciliary neurotrophic factor-induced neurogenesis and SV40 T antigen transfer to primary hypothalamic neuronal cell cultures. Significant research with these cell lines has demonstrated their value as a potential tool for use in molecular genetic analysis of hypothalamic neuronal function. Insights gained from hypothalamic immortalized cells used in conjunction with in vivo models will enhance our understanding of hypothalamic functions such as neurogenesis, neuronal plasticity, glucose sensing, energy homeostasis, circadian rhythms, and reproduction. This review discusses the generation and use of hypothalamic cell models to study mechanisms underlying the function of individual hypothalamic neurons and to gain a more complete understanding of the overall physiology of the hypothalamus.

Neurokinin B signaling in puberty: human and animal studies

Recent reports of humans who have normosmic idiopathic hypogonadotropic hypogonadism due to TAC3 or TACR3 (encoding neurokinin B and its receptor, NK3R, respectively) mutations provided compelling evidence for the involvement of neurokinin B (NKB) signaling in puberty. This apparently stimulated the field to understand the exact mechanism through which NKB signaling exerts its effects. With the important findings from these recent studies a sketch of GnRH pulse generator has emerged in which NKB signaling appears to play a key role. In this communication, NKB involvement in puberty is reviewed from the perspective of the fundamental question of "what controls puberty?"

Neurokinin 3 receptor immunoreactivity in the septal region, preoptic area and hypothalamus of the female sheep: colocalisation in neurokinin B cells of the arcuate nucleus but not in gonadotrophin-releasing hormone neurones

Recent evidence has implicated neurokinin B (NKB) in the complex neuronal network mediating the effects of gonadal steroids on the regulation of gonadotrophin-releasing hormone (GnRH) secretion. Because the neurokinin 3 receptor (NK3R) is considered to mediate the effects of NKB at the cellular level, we determined the distribution of immunoreactive NK3R in the septal region, preoptic area (POA) and hypothalamus of the ewe. NK3R cells and/or fibres were found in areas including the bed nucleus of the stria terminalis, POA, anterior hypothalamic and perifornical areas, dopaminergic A15 region, dorsomedial and lateral hypothalamus, arcuate nucleus (ARC) and the ventral premammillary nucleus. We also used dual-label immunocytochemistry to determine whether a neuroanatomical basis for direct modulation of GnRH neurones by NKB was evident. No GnRH neurones at any rostral-caudal level were observed to contain NK3R immunoreactivity, although GnRH neurones and fibres were in proximity to NK3R-containing fibres. Because NKB fibres formed close contacts with NKB neurones in the ARC, we determined whether these NKB neurones also contained immunoreactive NK3R. In luteal-phase ewes, 64% +/- 11 of NKB neurones colocalised NK3R. In summary, NK3R is distributed in areas of the sheep POA and hypothalamus known to be involved in the control of reproductive neuroendocrine function. Colocalisation of NK3R in NKB neurones of the ARC suggests a potential mechanism for the autoregulation of this subpopulation; however, the lack of NK3R in GnRH neurones suggests that the actions of NKB on GnRH neurosecretory activity in the ewe are mediated indirectly via other neurones and/or neuropeptides.

Hypothalamic cell lines to investigate neuroendocrine control mechanisms

The hypothalamus is the control center for most physiological processes; yet has been difficult to study due to the inherent heterogeneity of this brain region. For this reason, researchers have turned towards cell models. Primary hypothalamic cultures are difficult to maintain, are heterogeneous neuronal and glial cell populations and often contain a minimal number of viable peptide-secreting neurons. In contrast, immortalized, clonal cell lines represent an unlimited, homogeneous population of neurons that can be manipulated using a number of elegant molecular techniques. Cell line studies and in vivo experimentation are complementary and together provide a powerful tool to drive scientific discovery. This review focuses on three key neuroendocrine systems: energy homeostasis, reproduction, and circadian rhythms; and the use of hypothalamic cell lines to dissect the complex pathways utilized by individual neurons in these systems.

Role of astrocytes in reproduction and neuroprotection

Hypothalamic astrocytes secrete TGF-beta and 3 alpha,5 alpha-tetrahydro progesterone (3 alpha,5 alpha-THP) in culture. When the astrocyte-conditioned medium (ACM) was incubated with the hypothalamic cell line GT1-7, it resulted in the secretion of GnRH. Immunoneutralization with TGF-beta antibody or ultra-filteration with a 10 kDa cut off filter resulted in attenuation of the GnRH releasing ability of ACM, indicating that TGF-beta was a major factor involved with GnRH release. Treatment with estrogens increases TGF-beta secretion. These observations indicate a significant role of astrocytes in GnRH secretion. Serum-deprivation results in the death of GT1-7 neurons in culture and addition of ACM or TGF-beta to the culture, attenuates cell death. The mechanism of protection from cell death appears to involve phosphorylation of MKK4, JNK, c-Jun(Ser63), and enhancement of AP-1 binding. Co-administration of JNK inhibitors, but not MEK inhibitors attenuated ACM or TGF-beta-induced c-Jun(Ser63) phosphorylation and their neuroprotective effects. These studies suggest that astrocytes can protect neurons, at least in part, by the release of TGF-beta and activation of a c-Jun/AP-1 protective pathway.

Estrogens in males: what have we learned in the last 10 years?

This review focuses on the role of estrogen in men, mainly in male reproduction. The continuing increase in data obtained, and recent discoveries in this area will enable a better understanding of male physiology; these, in turn, will have important clinical implications.

Oestrogen receptor beta-immunoreactive neurones in the ovine hypothalamus: distribution and colocalisation with gonadotropin-releasing hormone

Oestrogen powerfully affects the secretion of gonadotropin-releasing hormone (GnRH) from the brain in all species investigated, including sheep. Until recently, it was hypothesised that such regulation occurs indirectly because few or no GnRH neurones were found to express oestrogen receptor (ER) alpha. The discovery of a second oestrogen receptor, ERbeta, and its subsequent localisation in numerous GnRH neurones in the rat, led to a reconsideration of this hypothesis. However, colocalisation of immunoreactive ERbeta protein in GnRH neurones has only been demonstrated in the rat, raising the possibility that such putative direct regulation of GnRH neurones by oestrogen may be peculiar to this species. We have previously shown that steroid receptors in the sheep brain are acutely sensitive to fixation and the full complement of immunoreactive cells can only be visualised after antigen retrieval. The aims of this study were therefore to map immunocytochemically the distribution of ERbeta neurones in the ewe brain, and to determine which proportion of GnRH neurones express ERbeta. Brain sections (20 microm) from four ewes killed in anestrus were subjected to high temperature antigen retrieval and immunocytochemistry. Numerous ERbeta-immunoreactive cells were located throughout the hypothalamus and, following dual-label immunocytochemistry, over 50% of the GnRH neurones were found to express immunoreactive ERbeta. The functional significance of these ERbeta-expressing GnRH neurones in the ovine brain remains to be determined.

In vivo and in vitro sex steroids stimulate seabream gonadotropin-releasing hormone content and release in the protandrous black porgy, Acanthopagrus schlegeli

The objective of the present study was to investigate the regulation of seabream gonadotropin-releasing hormone (sbGnRH) release using in vivo and in vitro approaches in the protandrous black porgy, Acanthopagrus schlegeli. Estradiol-17beta (E2), testosterone (T), and 11-ketotestosterone (11-KT) were found to significantly stimulate the increase of sbGnRH levels in pituitary of black porgy after 5-96 h of injection. An in vitro culture system using dispersed brain neurons was also developed to investigate the effects of various steroids on sbGnRH release. Different doses (10(-6) - 10(-12) M) of E2, T, 11-KT, and cortisol were applied during 6 h experiment. KCl stimulated sbGnRH release at a dose- and time-dependent manner. The concentration of sbGnRH increased 2-fold in the highest dose of KCl treatment compared to the control. Treatments with E2, T, 11-KT and cortisol significantly stimulated the release of sbGnRH from the cultured brain neurons. The concentration of sbGnRH in medium was increased by 2-, 1.9-, 2.1-, and 4.9-fold when treated with E2, T, 11-KT, and cortisol, respectively, as compared to the respective control. Cholesterol did not have any stimulatory effects in the release of sbGnRH. The results showed that sex steroids and cortisol had direct effect on brain neuronal cells stimulating the release of sbGnRH.

Direct and indirect regulation of gonadotropin-releasing hormone neurons by estradiol

Estrogen signaling to GnRH neurons is critical for coordinating the preovulatory surge release of LH with follicular maturation. Until recently it was thought that estrogen signaled GnRH neurons only indirectly through numerous afferent systems. This minireview presents new evidence indicating that GnRH neurons are directly regulated by estradiol (E2), primarily through estrogen receptor (ER)-beta, and indirectly through E2-sensitive neurons in the anteroventral periventricular (AVPV) region. The data described suggest that E2 generally represses GnRH gene expression but that this repression is transiently overcome by indirect E2-dependent signals relayed by AVPV neurons. We also present evidence that the AVPV neurons responsible for relaying E2 signals to GnRH neurons are multifunctional gamma aminobutyric acid-ergic/glutamatergic/neuropeptidergic neurons.

Estrogen Modulation of G-Protein-Coupled Receptor Activation of Potassium Channels in the Central Nervous System

Estrogen rapidly alters the excitability of hypothalamic neurons that are involved in regulating numerous homeostatic functions including reproduction, stress responses, feeding, and motivated behaviors. Neurosecretory neurons, such as gonadotropin-releasing hormone (GnRH) and dopamine neurons, and local circuitry neurons, such as pro-opiomelanocortin (POMC) and gamma-aminobutyric acid (GABA) neurons, are among those involved. We have identified membrane-initiated, rapid-signaling pathways through which 17beta-estradiol (E(2)) alters synaptic responses in these neurons using whole-cell patch recording in hypothalamic slices from ovariectomized female guinea pigs. E(2) rapidly uncouples micro -opioid and GABA(B) receptors from G-protein-gated inwardly rectifying K(+) (GIRK) channels in POMC and dopamine neurons as manifested by a reduction in the potency of micro -opioid and GABA(B) receptor agonists to activate these channels. These effects are mimicked by the selective E(2) receptor modulators raloxifene and 4OH-tamoxifen, the membrane impermeable E(2)-bovine serum albumin (BSA), but not by 17alpha-estradiol. Furthermore, the anti-estrogen ICI 182,780 antagonizes these rapid effects of E(2). Inhibitors of phospholipase C, protein kinase C, and protein kinase A block the actions of E(2), indicating that the E(2) receptor is G-protein-coupled to activation of this cascade. Conversely, estrogen enhances the efficacy of alpha1-adrenergic receptor agonists to inhibit apamin-sensitive small-conductance, Ca(2+)-activated K(+) (SK) currents in preoptic GABAergic neurons; it does so in both a rapid and sustained fashion. Finally, we observed a direct, steroid-induced hyperpolarization of GnRH neurons. These findings indicate that E(2) can modulate K(+) channels in hypothalamic (POMC, dopamine, GABA, GnRH) neurons that are involved in regulating numerous homeostatic functions through multiple intracellular signaling pathways.

Estradiol modulates acetylcholine-induced Ca2+ signals in LHRH-releasing GT1-7 cells through a membrane binding site

Estrogen regulation of the female reproductive axis involves the rapid inhibition (< 30 min) of luteinizing hormone-releasing hormone (LHRH) secretion from hypothalamic neurons. This fast time-course suggests interactions with potential plasma membrane binding sites that could result in short-term effects on LHRH neurons. Because LHRH release is calcium dependent, we have studied the acute effects of 17beta-estradiol (E2) and estradiol-peroxidase (E-HRP) on the elevations of intracellular calcium ([Ca2+]i) induced by acetylcholine (ACh) in LHRH-producing GT1-7 cells. Exposure to ACh (1-100 micro m) induced transient increases of [Ca2+]i, whereas pretreatment with E2 or E-HRP (10 nm) for 2 min reduced this response by 50-60%. The effect was specific for E2 as neither 17alpha-estradiol (1 micro m) nor the synthetic antiestrogens ICI182 780 (1 micro m) or tamoxifen (1 micro m) elicited any change on the ACh-induced Ca2+ signal. Both the latency of the effect and the response to the membrane impermeant conjugate suggested a membrane-mediated mechanism. Such membrane binding sites for E2 in GT1-7 cells were demonstrated by visualizing the binding of E-HRP and estradiol-BSA-fluorescein isothiocyanate (E-BSA-FITC) conjugates. Competition studies showed that E-HRP binding was blocked by preincubation with E2, but not with 17alpha-E2, ICI182 780, tamoxifen or progesterone, indicating that the plasma membrane binding site is highly specific for E2 and exhibits a pharmacological profile different from classical estrogen receptors. We conclude that ACh-induced increase in [Ca2+]i in GT1-7 cells is modulated acutely by physiological E2 concentrations in a manner which is compatible with the existence of an estrogen-specific membrane binding site.

Rapid signaling of estrogen in hypothalamic neurons involves a novel G-protein-coupled estrogen receptor that activates protein kinase C

Classically, 17beta-estradiol (E2) is thought to control homeostatic functions such as reproduction, stress responses, feeding, sleep cycles, temperature regulation, and motivated behaviors through transcriptional events. Although it is increasingly evident that E2 can also rapidly activate kinase pathways to have multiple downstream actions in CNS neurons, the receptor(s) and the signal transduction pathways involved have not been identified. We discovered that E2 can alter mu-opioid and GABA neurotransmission rapidly through nontranscriptional events in hypothalamic GABA, proopiomelanocortin (POMC), and dopamine neurons. Therefore, we examined the effects of E2 in these neurons using whole-cell recording techniques in ovariectomized female guinea pigs. E2 reduced rapidly the potency of the GABAB receptor agonist baclofen to activate G-protein-coupled, inwardly rectifying K+ channels in hypothalamic neurons. These effects were mimicked by the membrane impermeant E2-BSA and selective estrogen receptor modulators, including a new diphenylacrylamide compound, STX, that does not bind to intracellular estrogen receptors alpha or beta, suggesting that E2 acts through a unique membrane receptor. We characterized the coupling of this estrogen receptor to a Galpha(q)-mediated activation of phospholipase C, leading to the upregulation of protein kinase Cdelta and protein kinase A activity in these neurons. Moreover, using single-cell reverse transcription-PCR, we identified the critical transcripts, PKCdelta and its downstream target adenylyl cyclase VII, for rapid, novel signaling of E2 in GABA, POMC, and dopamine neurons. Therefore, this unique Gq-coupled estrogen receptor may be involved in rapid signaling in hypothalamic neurons that are critical for normal homeostatic functions.

Astrocytes and brain function: implications for reproduction

Recent evidence suggests that astrocytes have important neuroregulatory functions in addition to their classic functions of support and segregation of neurons. These newly revealed functions include regulation of neuron communication, neurosecretion, and synaptic plasticity. Although these actions occur throughout the brain, this review will focus on astrocyte-neuron interactions in the hypothalamus, particularly with respect to their potential contribution to the regulation of gonadotropin-releasing hormone (GnRH) secretion and reproduction. Hypothalamic astrocytes have been documented to release a variety of neuroactive factors, including transforming growth factors-alpha and -beta, insulin-like growth factor-1, prostaglandin E2, and the neurosteroid, 3 alpha-hydroxy-5 alpha-pregnane-20-one. Each of these factors has been shown to stimulate GnRH release, and receptors for each factor have been documented on GnRH neurons. Astrocytes have also been implicated in the regulation of synaptic plasticity in key areas of the hypothalamus that control GnRH release, an effect achieved by extension and retraction of glial processes (i.e., glial ensheathment). Through this mechanism, the number of synapses on GnRH neurons and GnRH regulatory neurons can potentially be modulated, thereby influencing the activation state of GnRH neurons. The steroid hormone 17beta-estradiol, which triggers the GnRH and luteinizing hormone surge, has been shown to induce the astrocyte-regulated changes in hypothalamic synaptic plasticity, as well as enhance formation and release of the astrocyte neuroactive factors, thereby providing another potential mechanistic layer for astrocyte regulation of GnRH release. As a whole, these studies provide new insights into the diversity of astrocytes and their potential role in reproductive neuroendocrine function.

Rapid effects of estrogen on G protein-coupled receptor activation of potassium channels in the central nervous system (CNS)

Estrogen rapidly alters the excitability of hypothalamic neurons that are involved in regulating numerous homeostatic functions including reproduction, stress responses, feeding and motivated behaviors. Some of the neurons include neurosecretory neurons such as gonadotropin-releasing hormone (GnRH) and dopamine neurons, and local circuitry neurons such as proopiomelanocortin (POMC) and gamma-aminobutyric acid (GABA) neurons. We have elucidated several non-genomic pathways through which the steroid alters synaptic responses in these hypothalamic neurons. We have examined the modulation by estrogen of the coupling of various receptor systems to inwardly-rectifying and small-conductance, Ca(2+)-activated K(+) (SK) channels using intracellular sharp-electrode and whole-cell recording techniques in hypothalamic slices from ovariectomized female guinea pigs. Estrogen rapidly uncouples mu-opioid receptors from G protein-gated inwardly-rectifying K(+) (GIRK) channels in POMC neurons and GABA(B) receptors from GIRK channels in dopamine neurons as manifested by a reduction in the potency of mu-opioid and GABA(B) receptor agonists to hyperpolarize their respective cells. This effect is blocked by inhibitors of protein kinase A (PKA) and protein kinase C (PKC). In addition, after 24h following steroid administration in vivo, the GABA(B)/GIRK channel uncoupling observed in GABAergic neurons of the preoptic area is associated with reduced agonist efficacy. Conversely, estrogen enhances the efficacy of alpha(1)-adrenergic receptor agonists to inhibit apamin-sensitive SK currents in these preoptic GABAergic neurons, and does so in both a rapid and sustained fashion. Finally, we observed a direct, steroid-induced hyperpolarization of GnRH neurons. These findings indicate a richly complex yet coordinated steroid modulation of K(+) channel activity in hypothalamic (POMC, dopamine, GABA, GnRH) neurons that are involved in regulating numerous homeostatic functions.

Estrogen modulation of K(+) channel activity in hypothalamic neurons involved in the control of the reproductive axis

Here we report on the progress we have made in elucidating the mechanisms through which estrogen alters synaptic responses in hypothalamic neurons. We examined the modulation by estrogen of the coupling of various receptor systems to inwardly rectifying and small conductance, Ca(2+)-activated K(+) (SK) channels. We used intracellular sharp-electrode and whole-cell recordings in hypothalamic slices from ovariectomized female guinea pigs. Estrogen rapidly uncouples mu-opioid receptors from G protein-gated inwardly rectifying K(+) (GIRK) channels in beta-endorphin neurons, manifest by a reduction in the potency of mu-opioid receptor agonists to hyperpolarize these cells. This effect is blocked by inhibitors of protein kinase A and protein kinase C. Estrogen also uncouples gamma-aminobutyric acid (GABA)(B) receptors from the same population of GIRK channels coupled to mu-opioid receptors. At 24 h after steroid administration, the GABA(B)/GIRK channel uncoupling observed in GABAergic neurons of the preoptic area (POA) is associated with reduced agonist efficacy. Conversely, estrogen enhances the efficacy of alpha(1)-adrenergic receptor agonists to inhibit apamin-sensitive SK currents in these POA GABAergic neurons, and does so in both a rapid and sustained fashion. Finally, we observed a direct, steroid-induced hyperpolarization of both arcuate and POA neurons, among which gonadotropin-releasing hormone (GnRH) neurons are particularly sensitive. These findings indicate a richly complex yet coordinated steroid modulation of K(+) channel activity that serves to control the excitability of hypothalamic neurons involved in regulating the reproductive axis.

Oestradiol microimplants in the ventromedial preoptic area inhibit secretion of luteinizing hormone via dopamine neurones in anoestrous ewes

Oestradiol exerts a season-specific negative feedback effect on the GnRH/LH neurosecretory system of the Suffolk ewe. This neuroendocrine suppression is mediated in part by dopamine A15 neurones, but these neurones do not possess the oestrogen receptor. Based on indirect evidence, we hypothesized that oestrogen receptor-containing neurones in the ventromedial preoptic area (vmPOA) may be the initial step in a neuronal system whereby oestradiol suppresses GnRH secretion during the non-breeding season. To test this, three experiments were conducted using ovariectomized ewes receiving either empty or oestradiol-containing bilateral microimplants directed at the vmPOA or s.c. subcutaneous oestradiol-containing implants. In the first experiment, LH pulse frequency was measured on days 0, 1, 7 and 14 of treatment during seasonal anoestrus. In vmPOA oestradiol and s.c. oestradiol groups only, LH pulse frequency was suppressed on days 7 and 14, with maximal suppression evident by day 7. In the second experiment, this protocol was repeated during the breeding season, with LH pulses examined on days 0 and 7; LH pulse frequency did not change in any group. The third experiment tested if the effect of vmPOA oestradiol during anoestrus could be overcome by an injection of the dopamine-D2 receptor antagonist (-)-sulpiride. The vmPOA microimplants and s.c. oestradiol implants again suppressed LH pulse frequency and this was reversed by sulpiride in vmPOA oestradiol ewes. We conclude that oestradiol acts on cells in the vmPOA to stimulate a system involving dopamine neurones that inhibits GnRH/LH pulsatility in the anoestrous ewe.

Congenital estrogen deficiency: in search of the estrogen role in human male reproduction

Recently, a remarkable progress has been made in our understanding about the role of sex steroids in male physiology. In this paper, we consider the clinical aspects of congenital estrogen deficiency - notably, estrogen resistance and aromatase deficiency - in men and we discuss both well-established and supposed estrogen roles in the human male reproductive function. These topics include the role of estrogens in the control of gonadotropin secretion, in male fertility determination and psychosexual behavior. Briefly, estrogens play a pivotal role in the control of serum gonadotropin concentrations in the human male. Furthermore, a possible role of estrogens on both human male fertility and sexuality has also been suggested by recent studies, even though the available data are far from being conclusive. Conversely, for what concern fertility and sexual behavior, a well-established effect of estrogens has been provided by recent studies on male rodents, which show impaired sexual behavior and fertility as a consequence of estrogen defect.

Estrogen biphasically modifies hypothalamic GABAergic function concomitantly with negative and positive control of luteinizing hormone release

The principal role of estrogen is its control of the female ovulatory cycle via negative and positive feedback on gonadotropin secretion. However, a detailed, cohesive picture of how the steroid specifically regulates the excitability of hypothalamic neurons involved in the central control of gonadotropin secretion is still emerging. Here, we used an ovariectomized female guinea pig model to test the hypothesis that estrogen acts on GABAergic neurons in the preoptic area (POA) to elicit a biphasic profile of luteinizing hormone (LH) secretion. Intracellular electrophysiological recordings revealed that estradiol benzoate (EB; 25 microgram, s.c.) decreased the hyperpolarizing response of GABAergic neurons to the GABA(B) receptor agonist baclofen 24 hr after treatment. This effect of GABA(B) receptor stimulation in unidentified POA neurons was still depressed 42 hr after EB administration. By the use of a ribonuclease protection assay, however, EB reduced glutamic acid decarboxylase mRNA expression 42 hr but not 24 hr after its administration. Thus, estrogen attenuated the autoinhibition of GABAergic POA neurons during the initial LH suppressive (i.e., negative feedback) phase and subsequently reduced GABAergic function during the LH surge (i.e., positive feedback). These studies demonstrate that the effects of estrogen on hypothalamic GABAergic neurons coincide with the inhibitory and stimulatory actions, respectively, of the steroid on LH secretion. Furthermore, the data provide novel insights into the mechanism by which estrogen regulates hypothalamic GABAergic neurons, which are critical for the biphasic modulation of LH release observed over the course of the female ovulatory cycle.

Reactive astrocytes express estrogen receptors in the injured primate brain

Previous studies have suggested that estrogen may regulate the expression of genes related to the inflammatory response within the nervous system, particularly within glia. In the present study, we examined whether injury induces estrogen sensitivity in reactive glia in the primate brain. Three adult Macaca fascicularis (cynomolgous) monkeys received unilateral fimbria fornix transections followed by chronic intracranial cannula implants through which a vehicle solution was infused intracerebroventricularly for a 4-week period. Astrocytes adjacent to areas of parenchymal disruption caused either by the lesion or by the instrumentation procedure became reactive, as evidenced by cellular hypertrophy and up-regulation of glial fibrillary acidic protein (GFAP) immunolabeling. Of note, specific estrogen receptor-alpha immunolabeling also was induced adjacent to injured regions, and this labeling strictly colocalized with GFAP immunoreactivity upon double fluorescent confocal immunolabeling. Induction of estrogen receptor immunoreactivity in reactive astrocytes occurred in all monkeys examined, whereas nonreactive glia distant from disrupted regions did not exhibit estrogen receptor labeling. Thus, expression of estrogen receptors is up-regulated in reactive astrocytes of the primate brain, potentially allowing estrogen to modulate aspects of the central nervous system's inflammatory response to injury.

Estrogen receptors and aromatase activity in the hypothalamus of the female frog, Rana esculenta. Fluctuations throughout the reproductive cycle

It is well known that certain actions of androgen are mediated through in situ aromatization to estrogen in neural target tissues. This study was undertaken to investigate androgen utilization in the hypothalamus of the female frog, Rana esculenta, through a quantification of estrogen receptors and aromatase activity during the reproductive cycle. 3H-estradiol-binding molecules were present in both the cytosol and the nuclear extract of the hypothalamus. These molecules bound specifically 3H-estradiol with high affinity (Kd 10(-10) M) and low capacity (cytosol: 1.2+/-0.4 fmol/mg protein; nuclear extract: 7.9+/-0.6 fmol/mg protein). Aromatase activity was detected in the microsomal fraction of the hypothalamus using a sensitive in vitro radiometric assay. Both aromatase activity and nuclear estrogen receptor binding fluctuated in synchrony throughout the reproductive cycle. Western blot analysis of aromatase protein revealed one immunoreactive band with a molecular weight of approximately 56 kDa. In contrast to aromatase enzyme activity, the relative levels of aromatase protein changed little during the reproductive cycle suggesting that post-translational mechanisms may be involved in regulating estrogen synthesis in the frog brain. A possible role for estrogens in the modulation of the reproductive behavior in this species is suggested.

Aromatase inhibition in the human male reveals a hypothalamic site of estrogen feedback

The preponderance of evidence states that, in adult men, estradiol (E2) inhibits LH secretion by decreasing pulse amplitude and responsiveness to GnRH consistent with a pituitary site of action. However, this conclusion is based on studies that employed pharmacologic doses of sex steroids, used nonselective aromatase inhibitors, and/or were performed in normal (NL) men, a model in which endogenous counterregulatory adaptations to physiologic perturbations confound interpretation of the results. In addition, studies in which estrogen antagonists were administered to NL men demonstrated an increase in LH pulse frequency, suggesting a potential additional hypothalamic site of E2 feedback. To reconcile these conflicting data, we used a selective aromatase inhibitor, anastrozole, to examine the impact of E2 suppression on the hypothalamic-pituitary axis in the male. Parallel studies of NL men and men with idiopathic hypogonadotropic hypogonadism (IHH), whose pituitary-gonadal axis had been normalized with long-term GnRH therapy, were performed to permit precise localization of the site of E2 feedback. In this so-called tandem model, a hypothalamic site of action of sex steroids can thus be inferred whenever there is a difference in the gonadotropin responses of NL and IHH men to alterations in their sex steroid milieu. A selective GnRH antagonist was also used to provide a semiquantitative estimate of endogenous GnRH secretion before and after E2 suppression. Fourteen NL men and seven IHH men were studied. In Exp 1, nine NL and seven IHH men received anastrozole (10 mg/day po x 7 days). Blood samples were drawn daily between 0800 and 1000 h in the NL men and immediately before a GnRH bolus dose in the IHH men. In Exp 2, blood was drawn (every 10 min x 12 h) from nine NL men at baseline and on day 7 of anastrozole. In a subset of five NL men, 5 microg/kg of the Nal-Glu GnRH antagonist was administered on completion of frequent blood sampling, then sampling continued every 20 min for a further 8 h. Anastrozole suppressed E2 equivalently in the NL (136 +/- 10 to 52 +/-2 pmol/L, P < 0.005) and IHH men (118 +/- 23 to 60 +/- 5 pmol/L, P < 0.005). Testosterone levels rose significantly (P < 0.005), with a mean increase of 53 +/- 6% in NL vs. 56 +/- 7% in IHH men. Despite these similar changes in sex steroids, the increase in gonadotropins was greater in NL than in IHH men (100 +/- 9 vs. 58 +/- 6% for LH, P = 0.07; and 85 +/- 6 vs. 41 +/- 4% for FSH, P < 0.002). Frequent sampling studies in the NL men demonstrated that this rise in mean LH levels, after aromatase blockade, reflected an increase in both LH pulse frequency (10.2 +/- 0.9 to 14.0 +/- 1.0 pulses/24 h, P < 0.05) and pulse amplitude (5.7 +/- 0.7 to 8.4 +/- 0.7 IU/L, P < 0.001). Percent LH inhibition after acute GnRH receptor blockade was similar at baseline and after E2 suppression (69.2 +/- 2.4 vs. 70 +/- 1.9%), suggesting that there was no change in the quantity of endogenous GnRH secreted. From these data, we conclude that in the human male, estrogen has dual sites of negative feedback, acting at the hypothalamus to decrease GnRH pulse frequency and at the pituitary to decrease responsiveness to GnRH.

Neuroendocrine regulation of GnRH release in induced ovulators

GnRH is the key neuropeptide controlling reproductive function in all vertebrate species. Two different neuroendocrine mechanisms have evolved among female mammals to regulate the mediobasal hypothalamic (MBH) release of GnRH leading to the preovulatory secretion of LH by the anterior pituitary gland. In females of spontaneously ovulating species, including rats, mice, guinea pigs, sheep, monkeys, and women, ovarian steroids secreted by maturing ovarian follicles induce a pulsatile pattern of GnRH release in the median eminence that, in turn, stimulates a preovulatory LH surge. In females of induced ovulating species, including rabbits, ferrets, cats, and camels, the preovulatory release of GnRH, and the resultant preovulatory LH surge, is induced by the receipt of genital somatosensory stimuli during mating. Induced ovulators generally do not show "spontaneous" steroid-induced LH surges during their reproductive cycles, suggesting that the positive feedback actions of steroid hormones on GnRH release are reduced or absent in these species. By contrast, mating-induced preovulatory surges occasionally occur in some spontaneously ovulating species. Most research in the field of GnRH neurobiology has been performed using spontaneous ovulators including rat, guinea pig, sheep, and rhesus monkey. This review summarizes the literature concerning the neuroendocrine mechanisms controlling GnRH biosynthesis and release in females of several induced ovulating species, and whenever possible it contrasts the results with those obtained for spontaneously ovulating species. It also considers the adaptive, evolutionary benefits and disadvantages of each type of ovulatory control mechanism. In females of induced ovulating species estradiol acts in the brain to induce aspects of proceptive and receptive sexual behavior. The primary mechanism involved in the preovulatory release of GnRH among induced ovulators involves the activation of midbrain and brainstem noradrenergic neurons in response to genital-somatosensory signals generated by receipt of an intromission from a male during mating. These noradrenergic neurons project to the MBH and, when activated, promote the release of GnRH from nerve terminals in the median eminence. In contrast to spontaneous ovulators, there is little evidence that endogenous opioid peptides normally inhibit MBH GnRH release among induced ovulators. Instead, the neural signals that induce a preovulatory LH surge in these species seem to be primarily excitatory. A complete understanding of the neuroendocrine control of ovulation will only be achieved in the future by comparative studies of several animal model systems in which mating-induced as well as spontaneous, hormonally stimulated activation of GnRH neurons drives the preovulatory LH surge.

Gonadotropin-releasing hormone (GnRH) partially reverses the inhibitory effect of 2,3,7,8-tetrachlorodibenzo-p-dioxin on ovulation in the immature gonadotropin-treated rat

Several studies have shown that 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) has inhibitory effects on ovulation. This action may be the result of either direct effect(s) of TCDD on ovarian function or via altered secretion of pituitary luteinizing hormone (LH) and follicle stimulating hormone (FSH) which regulate ovarian follicular development and ovulation. To further evaluate the effects of TCDD on pituitary gonadotropins and their regulation, the potential role of gonadotropin-releasing hormone (GnRH) was investigated in the current study. Immature (23-day-old) female Sprague-Dawley rats were dosed with TCDD (32 microg/kg) in corn oil or vehicle alone. Equine chorionic gonadotropin (eCG) was injected subcutaneously (5 IU, sc) 24 h later to induce follicular development. Immediately prior to the expected time of the LH/FSH surges, 54 h after eCG injection, half of TCDD- or corn oil-treated rats were injected with GnRH (2 microg/rat, sc). Blood and ovaries were collected at 54, 56, 58, 60 and 72 h after eCG. Serum concentrations of 17beta-estradiol (E(2)), progesterone (P(4)), LH, and FSH were determined by radioimmunoassay. An indication of ovulation rate was assessed at 72 h after injection of eCG by irrigating the ova from oviducts. TCDD reduced the number of ova in the oviducts by 70-80% (2-3 ova/rat) and this was confirmed by the number of corpora lutea. GnRH partially restored ovulation (6-7 ova/rat) in TCDD-treated rats without reversing its effect on ovarian weight reduction. In controls, the LH and FSH surges at 58 h after eCG were significantly reduced at that time in TCDD-treated rats. However, in rats treated with TCDD and GnRH, a huge LH/FSH surges occurred at 56 h after eCG injection. GnRH alone enhanced E(2) and P(4) serum levels at 56-58 h after eCG injection. In rats treated with both TCDD and GnRH, E(2) secretion was significantly lower at 58, 60, and 72 h when compared with GnRH alone, whereas serum P(4) was only decreased at 72 h after eCG injection. The results indicate that exogenous GnRH induces LH and FSH surges in TCDD-treated rats, but only partially restores the inhibitory effects of TCDD on ovulation.

The role of glutamate and nitric oxide in the reproductive neuroendocrine system

The preovulatory surge of gonadotropin releasing hormone (GnRH) is essential for mammalian reproduction. Recent work has implicated the neurotransmitters glutamate and nitric oxide as having a key role in this process. Large concentrations of glutamate are found in several hypothalamic nuclei known to be important for GnRH release and glutamate receptors are also located in these key hypothalamic nuclei. Administration of glutamate agonists stimulate GnRH and LH release, while glutamate receptor antagonists attenuate the steroid-induced and preovulatory LH surge. Glutamate has also been implicated in the critical processes of puberty, hormone pulsatility, and sexual behavior. Glutamate is believed to elicit many of these effects by activating the release of the gaseous neurotransmitter, nitric oxide (NO). NO potently stimulates GnRH by activating a heme containing enzyme, guanylate cyclase, which in turn leads to increased production of cGMP and GnRH release. Recent work has focused on identifying anchoring and (or) clustering proteins that target glutamate receptors to the synapse and couple the glutamate-NO neurotransmission system. The present review will discuss these new findings, as well as the role of glutamate and nitric oxide in important mammalian reproductive events, with a focus on the hypothalamic control of preovulatory GnRH release.

Key words: glutamate, nitric oxide, GnRH, postsynaptic density, hypothalamus.

Estrogen directly respresses gonadotropin-releasing hormone (GnRH) gene expression in estrogen receptor-alpha (ERalpha)- and ERbeta-expressing GT1-7 GnRH neurons

Estrogen has wide-ranging and complex effects on the reproductive axis, which are often difficult to interpret from in vivo studies. Estrogen negatively regulates tonic GnRH synthesis and also plays a pivotal role in the positive regulation of GnRH necessary for the preovulatory surge. To dissect the mechanisms by which these divergent effects occur, we attempted to observe the direct action of estrogen on the regulation of GnRH messenger RNA (mRNA) levels using the well characterized, GnRH-secreting, hypothalamic cell line, GT1-7. Using RT-PCR, we first investigated estrogen receptor transcript expression in GT1-7 neurons. We found that the GT1-7 cells express both estrogen receptor-alpha (ERalpha) and the recently described ERbeta mRNAs. We also detected the presence of both receptor subtypes in the GT1-7 neurons by Western blot analysis using specific ER antibodies. By Northern blot analysis of total GT1-7 RNA, we found that 17beta-estradiol (1 nM) down-regulates GnRH mRNA levels to approximately 55% of basal levels over a 48-h time course. This effect appears to occur specifically through an ER-mediated mechanism, as ICI 182,780, a complete ER antagonist, blocks the repression of GnRH mRNA levels by estradiol. The recently reported ERalpha-specific agonist/ERbeta-specific antagonist 2,2-bis-(p-hydroxyphenyl-1,1,1-trichloroethane (HPTE), a methoxychlor metabolite, also down-regulated GnRH gene expression. The repression of GnRH mRNA levels appears to occur at the transcriptional level, as simian virus 40 T antigen mRNA expression, which is under the control of 2.3 kb of the rat GnRH 5'-regulatory region, mimics the down-regulation of GnRH after treatment with estradiol. As the rat GnRH regulatory region in GT1-7 neurons does not appear to harbor a classic estrogen response element, the mechanism involved in the repression of GnRH has yet to be determined. These results suggest that estradiol directly regulates GnRH gene expression at the level of the GnRH neuron and may exert its neuroendocrine control through direct interaction with specific receptors expressed in these cells.

Estrogen receptor immunoreactivity is present in the majority of central histaminergic neurons: evidence for a new neuroendocrine pathway associated with luteinizing hormone-releasing hormone-synthesizing neurons in rats and humans

The central regulation of the preovulatory LH surge requires a complex sequence of interactions between neuronal systems that impinge on LH-releasing hormone (LHRH)-synthesizing neurons. The reported absence of estrogen receptors (ERs) in LHRH neurons indicates that estrogen-receptive neurons that are afferent to LHRH neurons are involved in mediating the effects of this steroid. We now present evidence indicating that central histaminergic neurons, exclusively located in the tuberomammillary complex of the caudal diencephalon, serve as an important relay in this system. Evaluation of this system revealed that 76% of histamine-synthesising neurons display ERalpha-immunoreactivity in their nucleus; furthermore histaminergic axons exhibit axo-dendritic and axo-somatic appositions onto LHRH neurons in both the rodent and the human brain. Our in vivo studies show that the intracerebroventricular administration of the histamine-1 (H1) receptor antagonist, mepyramine, but not the H2 receptor antagonist, ranitidine, can block the LH surge in ovariectomized estrogen-treated rats. These data are consistent with the hypothesis that the positive feedback effect of estrogen in the induction of the LH surge involves estrogen-receptive histamine-containing neurons in the tuberomammillary nucleus that relay the steroid signal to LHRH neurons via H1 receptors.

Estrogen regulates gonadotropin-releasing hormone in the nervus terminalis of Xenopus laevis

The nervus terminalis or terminal nerve (TN) is a neuronal plexus found in the nasal cavity and rostral forebrain of most vertebrates. The hormone gonadotropin-releasing hormone (GnRH) is found in a population of TN neurons as well as hypothalamic neurons which regulate pituitary secretion of the gonadotropins. The GnRH-containing neurons of the TN appear to represent a rostral continuation of the hypothalamic population since they both originate from the olfactory placode and are frequently anatomically continuous. Previous studies have shown that the hypothalamic GnRH neurons are regulated by circulating estrogen levels. Ovariectomy decreases while estrogen administration increases GnRH content in these neurons. It is not known whether the GnRH-containing TN neurons are also regulated in a similar manner. This study demonstrates that ovariectomy and estrogen readministration alters GnRH-immunoreactive (ir) levels in the TN of female Xenopus laevis in a manner similar to that seen in the hypothalamus. One week after ovariectomy, the density of TN GnRH-ir fibers in the olfactory bulb region (one site of TN termination) is significantly decreased. In contrast, a significant increase in GnRH-ir TN fiber density is observed following estrogen readministration to ovariectomized frogs. These findings demonstrate that estrogen regulates GnRH metabolism in neurons of the TN.

Gonadotropin-releasing hormone genes: phylogeny, structure, and functions

Gonadotropin-releasing hormone (GnRH, previously called leutinizing hormone-releasing hormone, LHRH) is the final common signaling molecule used by the brain to regulate reproduction in all vertebrates. Recently, genes encoding two other GnRH forms have been discovered. Here we present a phylogenetic analysis that shows that the GnRH genes fall naturally into three distinct branches, each of which shares not only a molecular signature but also characteristic expression sites in the brain. The GnRH genes appear to have arisen through gene duplication from a single ancestral GnRH whose origin predates vertebrates. Several lines of data support this suggestion, including the fact that all three genes share an identical exonic structure. The existence of three distinct GnRH families suggests a new, natural nomenclature for the genes, and in addition, we present a logical proposal for naming the peptide sequences. The two recently discovered GnRH genes are unusual because they encode decapeptides that are identical in all the species in which they have been found. The control of gene expression also differs among the three gene families as might be expected since they have had separate evolutionary trajectories for perhaps 500 million years.

Evidence for oestrogen receptor alpha-immunoreactivity in gonadotrophin-releasing hormone-expressing neurones

Having used the cingulate cortex to demonstrate the validity of our methods for detecting hitherto unrecognized oestrogen receptor alpha (ERalpha)-immunoreactive neurones, we have now employed immunoprecipitation and double-label immunohistochemistry to investigate whether the ERalpha protein is present in gonadotrophin-releasing hormone (GnRH)-containing cells. The immortalized GnRH cell line GT1-7 and GnRH neurones within the rat preoptic area were found to possess ERalpha-immunoreactivity (ERalpha-IR). These observations indicate that oestrogen may regulate the synthesis and release of GnRH by direct actions on GnRH neurones.

Estrogen modulation of neuropeptides: somatostatin, neurotensin and substance P, in the ventrolateral and arcuate nuclei of the female guinea pig

In the guinea pig, steroid target cells reside in the ventrolateral hypothalamic nucleus (VLH), an important site in the mediation of female receptive behavior, and in the arcuate nucleus (AR), a structure essential for stimulation effects of ovarian hormones on gonadotropin secretion. However, the mechanisms by which these steroid-dependent reproductive neuroendocrine processes occur are only partially understood. Estrogen is known to affect the hypothalamus content of certain neuropeptides. In the present study, we investigated the effects of estradiol benzoate (EB) on immunoreactivity of neurons containing one of three following neuropeptides: somatostatin (SOM), neurotensin (NT) and substance P (SP) in VLH and AR. The number of immunoreactive (IR)-neurons was quantified in anatomically matched sections through VLH and AR of ovariectomized (OVX), OVX + EB and OVX + oil-treated guinea pigs. Analysis of variance revealed that the number of SOM-IR and SP-IR neurons significantly increased in all regions of VLH of OVX + EB-treated guinea pigs as compared to OVX or OVX + oil-treated animals (P < 0.01) but showed no EB effect on the number of NT-IR neurons. Although the number of SOM-IR and NT-IR neurons slightly increased following treatment with EB in AR, analysis of variance revealed no significant change. The present results provide additional information relevant to possible involvement of these neuropeptides in facilitation of female typical sexual behavior.

Genomic structure and expression sites of three gonadotropin-releasing hormone genes in one species

In the teleost fish, Haplochromis burtoni, gonadotropin-releasing hormone (GnRH) peptide has been localized to three distinct regions in the brain. Each GnRH population is associated with expression of a distinct cDNA as previously described. Here we report the complete genomic sequences encoding these three forms and compare their structural organization, putative regulatory elements, and expression patterns in the body. All three genes share a common structure of four exons: the first exon encodes the 5' untranslated region; the second exon encodes the signal sequence, GnRH decapeptide, and the 5' end of the GnRH-associated peptide (GAP); the third exon consists entirely of GAP coding sequence; and the fourth exon encodes the 3' end of GAP and the 3' untranslated region. Each of the three GnRH genes has been shown previously to have a distinct spatial expression pattern in the brain, and here we use reverse transcription and cDNA amplification to demonstrate that each gene is expressed in the body. The gene encoding the releasing form, ¿Ser8¿GnRH, is expressed in the heart, liver, spleen, kidney, and testis, as well as in the preoptic area. The ¿His5Trp7Tyr8¿GnRH gene is expressed in the testis as well as in the midbrain. The ¿Trp7Leu8¿GnRH gene is expressed in the testis and the terminal nerve area. We examined the 500 bp upstream of exon 1 in all three H. burtoni genes and identified putative binding sites for glucocorticoid receptor, androgen receptor, and progesterone receptor, as well as the transcription factors Ap-1 and Sp-1. The genomic sequence encoding the terminal nerve form of GnRH (i.e., ¿Trp7Leu8¿GnRH) in H. burtoni is remarkably similar to that encoding the presumed releasing form of GnRH in salmonids, especially in the 3' intergenic region. Taken together with phylogenetic and mRNA localization data in salmonids, these data suggest that the gene encoding the releasing form of GnRH in salmonids may not yet be described.

Modulating mechanisms of neuroendocrine cell activity: the LHRH pulse generator

1. Luteinizing hormone-releasing hormone (LHRH), synthesized in specialized neurons in the hypothalamus, is the prime regulator of reproduction. In its absence, reproductive development is arrested and disorders of LHRH secretion result in several reproductive dysfunctions. 2. The LHRH neuronal network plays a paramount role in the regulatory loop controlling gonadal homeostasis. LHRH input to the pituitary gland maintains gonadotropin secretion, which, in turn, is responsible for gonadal trophism. Steroidal and peptidergic hormones from the gonad close the regulatory system by establishing negative (male and females) and positive (females) feedback loops. 3. Interestingly, LHRH input to the pituitary is intermittent rather than continuous. In fact, continuous exposure to LHRH results in paradoxical hypogonadism. Several studies in animals have provided direct evidence for episodic secretion of LHRH into the hypophyseal portal system. However, the nature of the system(s) responsible for the generation of the LHRH pulsatile profile is not currently known. The recent observation that immortalized LHRH neurons secrete LHRH in a pulsatile manner suggests that the pulse generating mechanism resides within the LHRH neuronal network. 4. In this overview, we compile several lines of evidence supporting this notion and put this characteristic of LHRH neurons in perspective with gonadal influences both internal and external to the LHRH neuronal network. Some recent data regarding the site of action of gonadal steroids on the LHRH neuronal system, the functional significance of galanin colocalization with LHRH, and the role of nitric oxide in the pulse generating mechanism are also discussed.

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.