Definition of estrogen receptor pathway critical for estrogen positive feedback to gonadotropin-releasing hormone neurons and fertility

Kisspeptin acts directly and indirectly to increase gonadotropin-releasing hormone neuron activity and its effects are modulated by estradiol

GnRH neurons play a pivotal role in the central regulation of fertility. Kisspeptin greatly increases GnRH/LH release and GnRH neuron firing activity and may be involved in estradiol feedback, but the neurobiological mechanisms for these actions are unknown. G protein-coupled receptor 54, the receptor for kisspeptin, is expressed by GnRH neurons as well as other hypothalamic neurons, suggesting both direct and indirect effects are possible. To investigate this and determine whether kisspeptin activation of GnRH neurons is estradiol sensitive, we recorded the firing rate of GnRH neurons in brain slices from adult female mice that were ovariectomized (OVX) and either treated with estradiol (E) capsules (OVX+E) or left without further treatment. Kisspeptin increased GnRH neuronal activity in a dose-dependent manner in cells from both OVX and OVX+E mice, and estradiol significantly potentiated the response. To begin to distinguish direct from indirect actions of kisspeptin, fast synaptic transmission mediated by ionotropic gamma-aminobutyric acid and glutamate receptors was pharmacologically blocked (blockade). Blockade reduced GnRH response to kisspeptin in OVX+E but not in OVX mice. Actions of kisspeptin were also assessed using whole-cell voltage- and current-clamp recording in slices from OVX animals. Kisspeptin application depolarized GnRH neurons in current-clamp and generated inward current in voltage-clamp recordings, even after blocking action potential-dependent neural communication, consistent with a direct effect. Blockers of potassium channels abolished the inward current. Together our data indicate that kisspeptin activates GnRH neurons via both direct and transsynaptic mechanisms and that transsynaptic mechanisms are either enabled and/or potentiated by estradiol.

Opposite roles of estrogen receptor (ER)-alpha and ERbeta in the modulation of luteinizing hormone responses to kisspeptin in the female rat: implications for the generation of the preovulatory surge

Ovulation is triggered by the preovulatory rise of gonadotropins, which is in turn elicited by the preceding increase in circulating estrogen. Kisspeptins, ligands of G protein-coupled receptor 54 encoded by the KiSS-1 gene, have emerged as potent stimulators of GnRH/LH secretion, and KiSS-1 neurons at the anteroventral periventricular nucleus have been shown to be involved in the generation of preovulatory LH surge, estrogen being a potent elicitor of KiSS-1 gene expression selectively at the anteroventral periventricular nucleus. Whether, in addition to transcriptional effects, estrogen influences other aspects of kisspeptin-induced GnRH/LH release in the female remains unexplored. We provide herein evidence for the specific roles of estrogen receptor (ER)-alpha and ERbeta in the modulation of LH responses to kisspeptin and the generation of the preovulatory surge. Selective blockade of ERalpha in cyclic females blunted LH responses to kisspeptin, eliminated the endogenous preovulatory rise of LH, and blocked ovulation. In contrast, antagonism of ERbeta failed to cause major changes in terms of LH surge and ovulatory rate but significantly augmented acute LH responses to kisspeptin. Notably, defective LH secretion and ovulation after ERalpha blockade were not observed after GnRH stimulation, which elicited maximal acute (<2 h) LH responses regardless of ERalpha/ERbeta signaling. In addition, net LH secretion in response to kisspeptin was decreased by ovariectomy and increased after selective activation of ERalpha but not ERbeta. Altogether, our data document the prominent positive role of ERalpha in the regulation of GnRH/LH responsiveness to kisspeptin and, thereby, ovulation. In addition, our results disclose the putative function of ERbeta as negative modifier of GnRH/LH response to kisspeptin, a phenomenon that might contribute to partially restraining LH secretion at certain physiological states.

Kisspeptin signalling in the brain: Steroid regulation in the rodent and ewe

The Kiss1 gene encodes a family of peptides called kisspeptins, which are the natural ligands for the receptor GPR54. In humans and mice, inactivating mutations of GPR54 results in hypogonadotropic hypogonadism, indicating that kisspeptins play a vital role in the regulation of GnRH secretion. In many species, centrally administered kisspeptins stimulate gonadotrophin secretion in a GnRH-dependant manner. Moreover, virtually all GnRH neurons coexpress GPR54. In the hypothalamus, the vast majority of kisspeptin producing cells also express sex steroid receptors, particularly estrogen receptor alpha. Thus, sex steroids are able to directly regulate the expression of Kiss1 mRNA, implicating kisspeptins as the 'missing link' between sex steroid feedback and GnRH secretion. Kiss1-expressing cells are localised to various regions of the forebrain in rodents, primates and sheep. In the arcuate nucleus (ARC) of the rodent and the ewe, sex steroids inhibit the expression of Kiss1 mRNA, suggesting that the kisspeptin secreting neurons here are the conduit for the negative feedback regulation of GnRH secretion. However, in the rodent anteroventral periventricular nucleus (AVPV), sex steroids induce the expression of Kiss1, implying that these kisspeptin neurons play a role in the positive feedback regulation of GnRH secretion. In sheep, there are no Kiss1 neurons in the AVPV and Kiss1 mRNA expression in the ARC is stimulated immediately prior to the preovulatory GnRH/luteinising hormone surge. Thus, kisspeptin neurons in the ARC of the ewe appear well placed to play a role in the negative and positive feedback regulation of GnRH exerted by sex steroids.

Estradiol and the developing brain

Estradiol is the most potent and ubiquitous member of a class of steroid hormones called estrogens. Fetuses and newborns are exposed to estradiol derived from their mother, their own gonads, and synthesized locally in their brains. Receptors for estradiol are nuclear transcription factors that regulate gene expression but also have actions at the membrane, including activation of signal transduction pathways. The developing brain expresses high levels of receptors for estradiol. The actions of estradiol on developing brain are generally permanent and range from establishment of sex differences to pervasive trophic and neuroprotective effects. Cellular end points mediated by estradiol include the following: 1) apoptosis, with estradiol preventing it in some regions but promoting it in others; 2) synaptogenesis, again estradiol promotes in some regions and inhibits in others; and 3) morphometry of neurons and astrocytes. Estradiol also impacts cellular physiology by modulating calcium handling, immediate-early-gene expression, and kinase activity. The specific mechanisms of estradiol action permanently impacting the brain are regionally specific and often involve neuronal/glial cross-talk. The introduction of endocrine disrupting compounds into the environment that mimic or alter the actions of estradiol has generated considerable concern, and the developing brain is a particularly sensitive target. Prostaglandins, glutamate, GABA, granulin, and focal adhesion kinase are among the signaling molecules co-opted by estradiol to differentiate male from female brains, but much remains to be learned. Only by understanding completely the mechanisms and impact of estradiol action on the developing brain can we also understand when these processes go awry.

Male sex hormones exacerbate lung function impairment after bleomycin-induced pulmonary fibrosis

The roles of sex hormones as modulators of lung function and disease have received significant attention as differential sex responses to various lung insults have been recently reported. The present study used a bleomycin-induced pulmonary fibrosis model in C57BL/6 mice to examine potential sex differences in physiological and pathological outcomes. Endpoints measured included invasive lung function assessment, immunological response, lung collagen deposition, and a quantitative histological analysis of pulmonary fibrosis. Male mice had significantly higher basal static lung compliance than female mice (P < 0.05) and a more pronounced decline in static compliance after bleomycin administration when expressed as overall change or percentage of baseline change (P < 0.05). In contrast, there were no significant differences between the sexes in immune cell infiltration into the lung or in total lung collagen content after bleomycin. Total lung histopathology scores measured using the Ashcroft method did not differ between the sexes, while a quantitative histopathology scoring system designed to determine where within the lung the fibrosis occurred indicated a tendency toward more fibrosis immediately adjacent to airways in bleomycin-treated male versus female mice. Furthermore, castrated male mice exhibited a female-like response to bleomycin while female mice given exogenous androgen exhibited a male-like response. These data indicate that androgens play an exacerbating role in decreased lung function after bleomycin administration, and traditional measures of fibrosis may miss critical differences in lung function between the sexes. Sex differences should be carefully considered when designing and interpreting experimental models of pulmonary fibrosis in mice.

The alpha-herpesviruses: molecular pathfinders in nervous system circuits

Several neuroinvasive viruses can be used to study the mammalian nervous system. In particular, infection by pseudorabies virus (PRV), an alpha-herpesvirus with broad host range, reveals chains of functionally connected neurons in the nervous systems of a variety of mammals. The specificity of PRV trans-neuronal spread has been established in several systems. One attenuated strain, PRV-Bartha, causes a reduced inflammatory response and also spreads only from infected post- to pre-synaptic neurons. We review the basics of PRV tracing and then discuss new developments and novel approaches that have enabled a more detailed understanding of the architecture of the nervous system. As questions and techniques evolve in the field of neuroscience, advances in PRV tracing will certainly follow.

Activation of neuronal nitric oxide release inhibits spontaneous firing in adult gonadotropin-releasing hormone neurons: a possible local synchronizing signal

The activation of nitric oxide (NO) signaling pathways in hypothalamic neurons plays a key role in the control of GnRH secretion that is central to reproductive function. It is unknown whether NO directly modulates the firing behavior of GnRH neurons in the preoptic region of the mature brain. Using patch-clamp recordings from GnRH neurons expressing green fluorescent protein in adult mice brain slices, we demonstrate that the NO precursor, L-arginine (Arg), or the NO donor, diethylamine/NO, induced a robust and reversible reduction in the spontaneous firing activity of GnRH neurons, including bursting activity. The effects of L-Arg were prevented by the NO synthase inhibitor N omega-nitro-L-Arg methyl ester hydrochloride. Histochemical studies revealing a close anatomical relationship between neurons producing NO and GnRH perikarya, together with the loss of the L-Arg-mediated inhibition of GnRH neuronal activity via the selective blockade of neuronal NO synthase, suggested that the primary source of local NO production in the mouse preoptic region was neuronal. Synaptic transmission uncoupling did not alter the effect of NO, suggesting that NO affects the firing pattern of GnRH neurons by acting at a postsynaptic site. We also show that the NO-mediated changes in membrane properties in the GnRH neurons require soluble guanylyl cyclase activity and may involve potassium conductance. By revealing that NO is a direct modulator of GnRH neuronal activity, our results introduce the intriguing possibility that this gaseous neurotransmitter may be used by the sexual brain to modulate burst firing patterns. It may set into phase the bursting activity of GnRH neurons at key stages of reproductive physiology.

Gonadotropin-releasing hormone neuron requirements for puberty, ovulation, and fertility

The absolute requirement for reproduction implies that the hypothalamo-pituitary-gonadal axis, controlling fertility, is an evolutionary robust mechanism. The GnRH neurons of the hypothalamus represent the key cell type within the body dictating fertility. However, the level of functional redundancy within the GnRH neuron population is unknown. As a result of a fortuitous transgene insertion event, GNR23 mice exhibit a marked allele-dependent reduction in GnRH neuron number within their brain. Wild-type mice have approximately 600 GnRH neurons, compared with approximately 200 (34%) and approximately 70 (12%) in GNR23(+/-) and GNR23(-/-) mice, respectively. Using these mice, we examined the minimal GnRH neuron requirements for fertility. Male GNR23(-/-) mice exhibited normal fertility. In contrast, female GNR23(-/-) mice were markedly subfertile, failing to produce normal litters, have estrous cycles, or ovulate. The failure of ovulation resulted from an inability of the few existing GnRH neurons to generate the LH surge. This was not the case, however, for the first cycle at puberty that appeared normal. Together, these observations demonstrate that 12% of the GnRH neuron population is sufficient for pulsatile gonadotropin secretion and puberty onset, whereas between 12 and 34% are required for cyclical control in adult female mice. This indicates that substantial redundancy exists within the GnRH neuronal population and suggests that the great majority of GnRH neurons must be dysfunctional before fertility is affected.

Kisspeptins: a multifunctional peptide system with a role in reproduction, cancer and the cardiovascular system

The pairing of the kisspeptins (KP) with the KISS1 (GPR54) receptor has received growing attention since the description of the receptor as a molecular switch for puberty. The role of KP and its receptor, GPR54, in puberty is the most exciting finding made in the field of reproductive biology since the discovery of Gonadotropin Releasing Hormone (GnRH) in 1970s. A significant body of evidence across several species now suggests that KISS1 (GPR54) activation is a critical point in the commencement of puberty, although further investigation is required to characterize the interaction between KP and GnRH cascade. Given such pivotal roles of kisspeptins and GPR54 as gatekeepers of reproductive function, and the proven ability of sex steroids to physiologically regulate this system, it is plausible that environmental compounds with ability to interfere oestrogen and/or androgen signaling (agonists or antagonists) may target the hypothalamic kiss-1/GPR54 system, thereby inducing functional alterations of the hypothalamic-pituitary-gonadal axis. Synthetic agonists targeting KISS1 (GPR54) may represent novel therapeutic agents for the treatment of hypogonadotrophic hypogonadism in some affected individuals. The diverse multifunctional nature of the KP is beginning to unravel. The unexpected role of these peptides in puberty has raised a number of important questions that remain to be answered.

New frontiers in kisspeptin/GPR54 physiology as fundamental gatekeepers of reproductive function

Identification, in late 2003, of inactivating mutations of the G protein-coupled receptor GPR54 as causative factor for absence of puberty and hypogonadotropic hypogonadism in humans and mice was a major breakthrough in modern Neuroendocrinology, and drew considerable interest on the characterization of the roles of this receptor and its ligands (kisspeptins, encoded by the KiSS-1 gene) in the physiological control of essential facets of reproduction. After 3 years of intense research activity, kisspeptins are universally recognized as essential activators of the gonadotropic axis, with key roles in puberty onset and the control of gonadotropin secretion. While these fundamental functions are now well settled, novel aspects of kisspeptin/GPR54 physiology have emerged, including their involvement in the neuroendocrine control of ovulation and the metabolic gating of reproductive function. In addition, the 'comparative endocrinology' of this system has begun to be explored recently. These facets of kisspeptin/GPR54 function, as fundamental gatekeepers of reproduction, will be comprehensively reviewed herein.

Pituitary gonadotroph estrogen receptor-alpha is necessary for fertility in females

Estrogens play a central role in regulating female reproduction throughout the reproductive axis, and the pituitary is one of the major targets of estrogen action. We hypothesized that estrogen receptor alpha (ERalpha) mediates estrogen action in the pituitary gonadotroph. To test this hypothesis, we generated a mouse line with a selective ERalpha deletion in the gonadotropin alpha-subunit (alphaGSU)-expressing pituitary cells (pituitary-specific ERalpha knockout; ERalpha(flox/flox) alphaGSU(cre)). Although the ERalpha(flox/flox) alphaGSU(cre) female mice maintain a basal level of serum LH and FSH and their ovulatory capacity is comparable to that in controls, they do not display regular estrous cycles and are infertile, indicating a potential disorder in regulating LH and/or FSH secretion. The ERalpha(flox/flox) alphaGSU(cre) female mice express equivalent levels of LHbeta and alphaGSU mRNA compared with wild-type mice as determined by microarray analysis. Taken together, these findings indicate that pituitary gonadotroph ERalpha carries out the effects of estrogens with regard to estrous cyclicity and ultimately fertility.

NMDA receptor subunit NR2b: effects on LH release and GnRH gene expression in young and middle-aged female rats, with modulation by estradiol

BACKGROUND/AIMS

The loss of reproductive capacity during aging involves changes in the neural regulation of the hypothalamic gonadotropin-releasing hormone (GnRH) neurons controlling reproduction. This neuronal circuitry includes glutamate receptors on GnRH neurons. Previously, we reported an increase in the expression of the NR2b subunit protein of the NMDA receptor on GnRH neurons in middle-aged compared to young female rats. Here, we examined the functional implications of the NR2b subunit on the onset of reproductive aging, using an NR2b-specific antagonist ifenprodil.

METHODS

Young (3-5 months) and middle-aged (10-13 months) female rats were ovariectomized (OVX), 17beta-estradiol (E2) or vehicle (cholesterol) treated, and implanted with a jugular catheter. Serial blood sampling was undertaken every 10 min for 4 h, with ifenprodil (10 mg/kg) or vehicle injected (i.p.) after 1 h of baseline sampling. The pulsatile release of pituitary LH and levels of GnRH mRNA in hypothalamus were quantified as indices of the reproductive axis.

RESULTS

Our results showed effects of ifenprodil on both endpoints. In OVX rats given cholesterol, neither age nor ifenprodil had any effects on LH release. In E2-treated rats, aging was associated with significant decreases in pulsatile LH release. Additionally, ifenprodil stimulated parameters of pulsatile LH release in both young and middle-aged animals. Ifenprodil had few effects on GnRH mRNA; the only significant effect of ifenprodil was found in the middle-aged, cholesterol group.

CONCLUSION

Together, these findings support a role for the NR2b subunit of the NMDAR in GnRH/LH regulation. Because most of these effects were exhibited on pituitary LH release in the absence of a concomitant change in GnRH gene expression, it is likely that NMDA receptors containing the NR2b subunit play a role in GnRH-induced LH release, independent of de novo GnRH gene expression.

Definition of brainstem afferents to gonadotropin-releasing hormone neurons in the mouse using conditional viral tract tracing

Brainstem monoamines have long been considered to play a role in regulating the activity of GnRH neurons, although their neuroanatomical relationship with these cells has remained unclear. Using a Cre-dependent pseudorabies virus (Ba2001) technique that permits retrograde tracing selectively from GnRH neurons in the mouse, we have examined the organization of brainstem inputs to rostral preoptic area (rPOA) GnRH neurons. Two days after injection of Ba2001 into the rPOA of adult female GnRH-Cre transgenic mice, five to nine GnRH neurons located immediately adjacent to the injection site were found to express green fluorescent protein (GFP), the marker of virus infection, with no GFP expression anywhere else in the brain. In mice killed 24 h later (3 d after injection), GFP-expressing cells were identified (in order of density) in the raphe nuclei, periaqueductal grey, locus coeruleus, nucleus tractus solitarius, and area postrema. This time course is compatible with these neurons representing primary afferent inputs to the GnRH neurons. Four and 6 d after Ba2001 injection, GFP-expressing cells were found in additional brain regions. Dual-label immunofluorescence experiments in 3-d postinjection mice demonstrated that 100% of GFP-expressing neurons in the raphe were positive for tryptophan hydroxylase, whereas 100% and approximately 50% of GFP neurons in the locus coeruleus and nucleus tractus solitarius, respectively, expressed tyrosine hydroxylase. These observations demonstrate that rPOA GnRH neurons receive direct projections from brainstem A2 and A6 noradrenergic neurons and that, surprisingly, the largest afferent input from the brainstem originates from raphe serotonin neurons in the mouse.

Suppression of kisspeptin expression and gonadotropic axis sensitivity following exposure to inhibitory day lengths in female Siberian hamsters

To avoid breeding during unsuitable environmental or physiological circumstances, the reproductive axis adjusts its output in response to fluctuating internal and external conditions. The ability of the reproductive system to alter its activity appropriately in response to these cues has been well established. However, the means by which reproductively relevant cues are interpreted, integrated and relayed to the reproductive axis remain less well specified. The neuropeptide kisspeptin has been shown to be a potent positive stimulator of the hypothalamo-pituitary-gonadal (HPG) axis, suggesting a possible neural locus for the interpretation/integration of these cues. Because a failure to inhibit reproduction during winter would be maladaptive for short-lived female rodents, female Siberian hamsters (Phodopus sungorus) housed in long and short days were examined. In long "summer" photoperiods, kisspeptin is highly expressed in the anteroventral periventricular nucleus (AVPV), with low expression in the arcuate nucleus (Arc). A striking reversal in this pattern is observed in animals held in short, "winter" photoperiods, with negligible kisspeptin expression in the AVPV and marked staining in the Arc. Although all studies to date suggest that both populations act to stimulate the reproductive axis, these contrasting expression patterns of AVPV and Arc kisspeptin point to disparate roles for these two cell populations. Additionally, we found that the stimulatory actions of exogenous kisspeptin are blocked by acyline, a gonadotropin-releasing hormone (GnRH) receptor antagonist, suggesting an action of kisspeptin on the GnRH system rather than pituitary gonadotropes. Finally, females held in short day lengths exhibit a reduced response to exogenous kisspeptin treatment relative to long-day animals. Together, these findings indicate a role for kisspeptin in the AVPV and Arc as an upstream integration center for reproductively relevant stimuli and point to a dual mechanism of reproductive inhibition in which kisspeptin expression is reduced concomitant with reduced sensitivity of the HPG axis to this peptide.

The role of kisspeptin-GPR54 signaling in the tonic regulation and surge release of gonadotropin-releasing hormone/luteinizing hormone

The Kiss1 gene codes for kisspeptin, which binds to GPR54, a G-protein-coupled receptor. Kisspeptin and GPR54 are expressed in discrete regions of the forebrain, and they have been implicated in the neuroendocrine regulation of reproduction. Kiss1-expressing neurons are thought to regulate the secretion of gonadotropin-releasing hormone (GnRH) and thus coordinate the estrous cycle in rodents; however, the precise role of kisspeptin-GPR54 signaling in the regulation of gonadotropin secretion is unknown. In this study, we used female mice with deletions in the GPR54 gene [GPR54 knock-outs (KOs)] to test the hypothesis that kisspeptin-GPR54 signaling provides the drive necessary for tonic GnRH/luteinizing hormone (LH) release. We predicted that tonic GnRH/LH secretion would be disrupted in GPR54 KOs and that such animals would be incapable of showing a compensatory rise in LH secretion after ovariectomy. As predicted, we found that GPR54 KO mice do not exhibit a postovariectomy rise in LH, suggesting that tonic GnRH secretion is disrupted in the absence of kisspeptin-GPR54 signaling. We also postulated that kisspeptin-GPR54 signaling is critical for the generation of the estradiol (E)-induced GnRH/LH surge and thus E should be incapable of inducing an LH surge in the absence of GPR54. However, we found that E induced Fos expression in GnRH neurons and produced a GnRH-dependent LH surge in GPR54 KOs. Thus, in mice, kisspeptin-GPR54 signaling is required for the tonic stimulation of GnRH/LH secretion but is not required for generating the E-induced GnRH/LH surge.

Genetic dissection of estrogen receptor signaling in vivo

The multiple actions of estrogen in mammalian physiology are brought about, on a molecular level, by several signaling pathways, and mediated by at least two receptors-estrogen receptor (ER) alpha and beta. Analysis of knock-out mice devoid of either or both receptor isoforms revealed the essential function of estrogen receptor alpha in female reproduction, as ERalpha deficiency leads to a complex endocrine phenotype, severe disturbances in several reproductive organs, and infertility. This reflects the many actions of estrogen in female reproductive endocrinology. To carry the understanding of estrogen action to a cellular resolution, modern genetic technologies can be employed, including artificial chromosome-based transgenesis and conditional gene targeting. The combination of these techniques yields mouse models that lack ERalpha in specific cell types of the body. Using cell-type-specific ERalpha mutants, it could be shown that ERa in neurons is essential for the luteinizing hormone (LH) surge that triggers ovulation. Studies using ERalpha and ERbeta-selective agonists reveal that ERalpha activation is sufficient to induce an ovulatory hormonal stimulus. Thus, genetic analysis and selective pharmacological tools can complement each other in the molecular and cellular dissection of hormone receptor function in vivo.

Prolactin regulation of gonadotropin-releasing hormone neurons to suppress luteinizing hormone secretion in mice

Hyperprolactinemia causes infertility, but the mechanisms involved are not known. The present study aimed to determine whether and how prolactin may influence LH secretion in the adult female mouse. Using ovariectomized, estrogen-treated (OVX+E) mice, we found that 7 d of intracerebroventricular prolactin potently suppressed serum LH levels (P < 0.05). To examine whether this central action of prolactin may involve the GnRH neurons, the effects of acute and chronic prolactin on cAMP response element-binding protein phosphorylation (pCREB) in GnRH neurons were examined using dual-label immunocytochemistry. In diestrous and OVX+E mice, a single sc injection of ovine prolactin resulted in a significant (P < 0.05) doubling of the number of GnRH neurons expressing pCREB. OVX+E mice treated with five injections of ovine prolactin over 48 h showed a 4-fold increase in the number of GnRH neurons with pCREB. To determine whether GnRH neurons might be regulated directly by prolactin, we examined prolactin receptor (PRL-R) mRNA expression in green fluorescent protein-tagged GnRH neurons by single-cell RT-PCR. As a positive control, PRL-R mRNA was measured in arcuate dopaminergic neurons obtained from green fluorescent protein-tagged tyrosine hydroxylase neurons. Three of 23 GnRH neurons (13%) were identified to express PRL-R transcripts, whereas nine of 11 arcuate dopaminergic neurons (82%) were found to coexpress PRL-R mRNA. These data demonstrate that prolactin suppresses LH levels in the mouse, as it does in other species, and indicate that it acts centrally to regulate intracellular signaling within GnRH neurons. This is likely to occur, at least in part, through the direct regulation of a subpopulation of GnRH neurons.

GnRH and GnRH receptors in metazoa: a historical, comparative, and evolutive perspective

About 50years after Harris's first demonstration of its existence, GnRH has strongly stimulated the interest and imagination of scientists, resulting in a high number of studies in an increasing number of species. For the endocrinologist, GnRH, via its actions on the synthesis and release of pituitary gonadotrophins, is first an essential hormone for the initiation and maintenance of the reproductive axis, but recent data suggest that GnRH emerged in animals lacking a pituitary. In this context, this review intends to explore the current status of knowledge on GnRH and GnRH receptors in metazoa in order to see if it is possible to draw an evolutive scenario according to which GnRH actions progressively evolved from the control of simple basic functions in early metazoa to an indirect mean of controlling gonadal activity in vertebrates through a sophisticated network of finely tuned neurons developing in a rather fascinating way. This review also intends to provide an evolutive scenario based on the recent advances of whole genome sequencing possibly explaining the number of GnRH and GnRH receptor variants according to the 2R and 3R theories accompanied by gene losses.

Coupling of neuronal nitric oxide synthase to NMDA receptors via postsynaptic density-95 depends on estrogen and contributes to the central control of adult female reproduction

Considerable research has been devoted to the understanding of how nitric oxide (NO) influences brain function. Few studies, however, have addressed how its production is physiologically regulated. Here, we report that protein-protein interactions between neuronal NO synthase (nNOS) and glutamate NMDA receptors via the scaffolding protein postsynaptic density-95 (PSD-95) in the hypothalamic preoptic region of adult female rats is sensitive to cyclic estrogen fluctuation. Coimmunoprecipitation experiments were used to assess the physical association between nNOS and NMDA receptor NR2B subunit in the preoptic region of the hypothalamus. We found that nNOS strongly interacts with NR2B at the onset of the preovulatory surge at proestrus (when estrogen levels are highest) compared with basal-stage diestrous rats. Consistently, estrogen treatment of gonadectomized female rats also increases nNOS/NR2B complex formation. Moreover, endogenous fluctuations in estrogen levels during the estrous cycle coincide with changes in the physical association of nNOS to PSD-95 and the magnitude of NO release in the preoptic region. Finally, temporary and local in vivo suppression of PSD-95 synthesis by using antisense oligodeoxynucleotides leads to inhibition of nNOS activity in the preoptic region and disrupted estrous cyclicity, a process requiring coordinated activation of neurons containing gonadotropin-releasing hormone (the neuropeptide controlling reproductive function). In conclusion, our findings identify a novel steroid-mediated molecular mechanism that enables the adult mammalian brain to control NO release under physiological conditions.

Defining the gonadotrophin-releasing hormone neuronal network: transgenic approaches to understanding neurocircuitry

The gonadotrophin-releasing hormone (GnRH) neurones are the final downstream effector neurones driving the central regulation of reproductive function and fertility in all mammalian species. Although it is abundantly clear that successful fertility relies upon the communication of a variety signals regarding internal and external cues to the GnRH neuronal population, how this is achieved remains poorly understood. A range of technical limitations has posed significant hurdles to defining, with any certainty, the complexities of the synaptic neuronal network regulating GnRH neurones. However, recent advances in transgenic technology have opened up new avenues to reconsider questions aimed at understanding this critical network. This article addresses some of the latest advances that use transgenic mouse models as tools to understand the neuronal circuitry underpinning the regulation of the GnRH neurones. By incorporating standard morphological and viral tract tracing techniques with innovative transgenic tools, recent studies have uncovered previously unappreciated qualities of the GnRH neurone, including extensive dendritic lengths, numerous somal and dendritic spines and plasticity over pubertal development, along with beginning to define the primary and higher-order afferents that make up the GnRH neuronal network.

Estrogen receptor beta in the brain: from form to function

Estrogens have numerous effects on the brain, both in adulthood and during development. These actions of estrogen are mediated by two distinct estrogen receptor (ER) systems, ER alpha (ERalpha) and ER beta (ERbeta). In brain, ERalpha plays a critical role in regulating reproductive neuroendocrine function and behavior, however, a definitive role for ERbeta in any neurobiological function has been slow in forthcoming. Clues to the function of ERbeta in the central nervous system can be gleaned from the neuroanatomical distribution of ERbeta and the phenotypes of neurons that express ERbeta. ERbeta immunoreactivity has been found in populations of GnRH, CRH, vasopressin, oxytocin and prolactin containing neurons in the hypothalamus. Utilizing subtype-selective estrogen receptor agonists can help determine the roles for ERbeta in non-reproductive behaviors in rat models. ERbeta-selective agonists exert potent anxiolytic activity when animals were tested in a number of behavioral paradigms. Consistent with this, ERbeta-selective agonists also inhibited the ACTH and corticosterone response to stress. In contrast, ERalpha selective agonists were found to be anxiogenic and correspondingly increased the hormonal stress response. Taken together, our studies implicate ERbeta as an important modulator of some non-reproductive neurobiological systems. The molecular and neuroanatomical targets of estrogen that are mediated by ERbeta remain to be determined. A number of splice variants of ERbeta mRNA have been reported in brain tissue. Imaging of eGFP labeled chimeric receptor proteins transfected into cell lines shows that ERbeta splice variation can alter trafficking patterns and function. The originally described ERbeta (herein termed ERbeta1) is characterized by possessing a high affinity for estradiol. Similar to ERalpha, it is localized in the nucleus and is trafficked to nuclear sites termed "hyperspeckles" following ligand binding. In contrast, ERbeta2 contains an 18 amino acid insert within the ligand-binding domain and as a result can be best described as a low affinity form of ERbeta. A delta3 (delta3) variant of ERbeta has a deletion of the 3rd exon (coding for the second half of the DNA-binding domain) and as a result does not bind an estrogen response element in DNA. delta3 variants are trafficked to a unique low abundance and larger nuclear site following ligand binding. A delta4 (delta4) variant lacks exon 4 and as a result is localized to the cytoplasm. The amount of individual splice variant mRNAs varies depending upon brain region. Examination of neuropeptide promoter regulation by ERbeta splice variants demonstrates that ERbeta functions as a constitutively active transcription factor. Moreover, it appears that splice variation of ERbeta alters its ability to regulate transcription in a promoter-dependent and ligand-dependent fashion.

Estrogen positive feedback to gonadotropin-releasing hormone (GnRH) neurons in the rodent: the case for the rostral periventricular area of the third ventricle (RP3V)

Increasing levels of circulating estradiol during the follicular phase of the ovarian cycle act on the brain to trigger a sudden and massive release of gonadotropin-releasing hormone (GnRH) that evokes the pituitary luteinizing hormone surge responsible for ovulation in mammals. The mechanisms through which estrogen is able to exert this potent "positive feedback" influence upon the GnRH neurons are beginning to be unravelled. Recent studies utilizing mouse models with global and cell-specific deletions of the different estrogen receptors (ERs) have shown that estrogen positive feedback is likely to use an indirect pathway involving the modulation of ERalpha-expressing neurons that project to GnRH neurons. Conventional tract tracing studies in rats, and experiments involving conditional pseudorabies virus tract tracing from GnRH neurons in the transgenic mouse, indicate that the dominant populations of ERalpha-expressing neuronal afferents to GnRH neurons reside in the anteroventral periventricular, median preoptic and periventricular preoptic nuclei. Together these estrogen-sensitive afferents to GnRH neurons form a periventricular continuum that can be referred to as rostral periventricular area of the third ventricle (RP3V) neurons. The neurochemical identity of some RP3V neurons has been determined and there is mounting evidence for important roles of glutamate, GABA, kisspeptin and neurotensin-expressing RP3V neurons in estrogen positive feedback. The definition of the key cluster of estrogen-sensitive neurons responsible for activating the GnRH neurons to evoke the GnRH surge (and ovulation) should be of substantial value to on-going efforts to understand the molecular and cellular basis of the estrogen positive feedback mechanism.

Kisspeptins: a multifunctional peptide system with a role in reproduction, cancer and the cardiovascular system

Orphan G-protein-coupled receptors that have recently been paired with their cognate ligand are an often untapped resource for novel drug development. The KISS1 receptor (previously designated GPR54) has been paired with biologically active cleavage peptides of the KiSS-1 gene product, the kisspeptins (KP). The focus of this review is the emerging pharmacology and physiology of the KP. Genetic linkage analysis in humans revealed that mutations in KISS1 (GPR54, AXOR12 or hOT7T175) result in idiopathic hypogonadotrophic hypogonadism and knockout mouse studies confirmed this finding. Identification of KISS1 (GPR54) as a molecular switch for puberty subsequently led to the discovery that KP activate the GnRH cascade. Prior to the role of KISS1 (GPR54) in puberty being described, KP had been shown to be inhibitors of tumour metastasis across a range of cancers. Subsequently the mechanism of this inhibition has been suggested to be via altered cell motility and adhesiveness. PCR detected highest expression of KP and KISS1 (GPR54) in placenta, and changes in KP levels throughout pregnancy and expression in trophoblasts suggests a role in placentation. Placentation and metastasis are invasive processes that require angiogenesis. Investigation of KISS1 (GPR54) and KP in vasculature revealed discrete localisation of KISS1 (GPR54) to blood vessels prone to atherosclerosis and a potent vasoconstrictor action. A role for KP has also been shown in whole body homeostasis. KP are multifunctional peptides and further investigation is required to fully elucidate the complex pathways regulated by these peptides and how these pathways integrate in the whole body system.

Nonclassical estrogen receptor alpha signaling mediates negative feedback in the female mouse reproductive axis

Ovarian estrogen exerts both positive and negative feedback control over luteinizing hormone (LH) secretion during the ovulatory cycle. Estrogen receptor (ER) alpha but not ERbeta knockout mice lack estrogen feedback. Thus, estrogen feedback appears to be primarily mediated by ERalpha. However, it is now recognized that, in addition to binding to estrogen response elements (EREs) in DNA to alter target gene transcription, ERalpha signals through ERE-independent or nonclassical pathways, and the relative contributions of these pathways in conveying estrogen feedback remain unknown. Previously we created a knockin mouse model expressing a mutant form of ERalpha (AA) with ablated ERE-dependent but intact ERE-independent activity. Breeding this allele onto the ERalpha-null (-/-) background, we examine the ability of ERE-independent ERalpha signaling pathways to convey estrogen feedback regulation of the female hypothalamic-pituitary axis in vivo. ERalpha-/AA exhibited 69.9% lower serum LH levels compared with ERalpha-/- mice. Additionally, like wild type, ERalpha-/AA mice exhibited elevated LH after ovariectomy (OVX). Furthermore, the post-OVX rise in serum LH was significantly suppressed by estrogen treatment in OVX ERalpha-/AA mice. However, unlike wild type, both ERalpha-/AA and ERalpha-/- mice failed to exhibit estrous cyclicity, spontaneous ovulation, or an afternoon LH surge response to estrogen. These results indicate that ERE-independent ERalpha signaling is sufficient to convey a major portion of estrogen's negative feedback actions, whereas positive feedback and spontaneous ovulatory cyclicity require ERE-dependent ERalpha signaling.

Sexual differentiation of Kiss1 gene expression in the brain of the rat

The Kiss1 gene codes for kisspeptins, which have been implicated in the neuroendocrine regulation of reproduction. In the brain, Kiss1 mRNA-expressing neurons are located in the arcuate (ARC) and anteroventral periventricular (AVPV) nuclei. Kiss1 neurons in the AVPV appear to play a role in generating the preovulatory GnRH/LH surge, which occurs only in females and is organized perinatally by gonadal steroids. Because Kiss1 is involved in the sexually dimorphic GnRH/LH surge, we hypothesized that Kiss1 expression is sexually differentiated, with females having more Kiss1 neurons than either males or neonatally androgenized females. To test this, male and female rats were neonatally treated with androgen or vehicle; then, as adults, they were left intact or gonadectomized and implanted with capsules containing sex steroids or nothing. Kiss1 mRNA levels in the AVPV and ARC were determined by in situ hybridization. Normal females expressed significantly more Kiss1 mRNA in the AVPV than normal males, even under identical adult hormonal conditions. This Kiss1 sex difference was organized perinatally, as demonstrated by the observation that neonatally androgenized females displayed a male-like pattern of adulthood Kiss1 expression in the AVPV. In contrast, there was neither a sex difference nor an influence of neonatal treatment on Kiss1 expression in the ARC. Using double-labeling techniques, we determined that the sexually differentiated Kiss1 neurons in the AVPV are distinct from the sexually differentiated population of tyrosine hydroxylase (dopaminergic) neurons in this region. Our findings suggest that sex differences in kisspeptin signaling from the AVPV subserve the cellular mechanisms controlling the sexually differentiated GnRH/LH surge.

Estradiol induces diurnal shifts in GABA transmission to gonadotropin-releasing hormone neurons to provide a neural signal for ovulation

Ovulation is initiated by a surge of gonadotropin-releasing hormone (GnRH) secretion by the brain. GnRH is normally under negative feedback control by ovarian steroids. During sustained exposure to estradiol in the late follicular phase of the reproductive cycle, however, the feedback action of this steroid switches to positive, inducing the surge. Here, we used an established ovariectomized, estradiol-treated (OVX+E) mouse model exhibiting daily surges to investigate the neurobiological mechanisms underlying this switch. Specifically, we examined changes in GABA transmission to GnRH neurons, which can be excited by GABA(A) receptor activation. Spontaneous GABAergic postsynaptic currents (PSCs) were recorded in GnRH neurons from OVX+E and OVX mice in coronal and sagittal slices. There were no diurnal changes in PSC frequency in cells from OVX mice in either slice orientation. In OVX+E cells in both orientations, PSC frequency was low during negative feedback but increased at surge onset. During the surge peak, this increase subsided in coronal slices but persisted in sagittal slices. Comparison of PSCs before and during tetrodotoxin (TTX) treatment showed TTX decreased PSC frequency in OVX+E cells in sagittal slices, but not coronal slices. This indicates estradiol acts on multiple GABAergic afferent populations to increase transmission through both activity-dependent and -independent mechanisms. Estradiol also increased PSC amplitude during the surge. Estradiol and the diurnal cycle thus interact to induce shifts in both GABA transmission and postsynaptic response that would produce appropriate changes in GnRH neuron firing activity and hormone release.

Cell type-specific expression of a genetically encoded calcium indicator reveals intrinsic calcium oscillations in adult gonadotropin-releasing hormone neurons

The gonadotropin-releasing hormone (GnRH) neurons exhibit a unique pattern of episodic activity to control fertility in all mammals. To enable the measurement of intracellular calcium concentration ([Ca2+]i) in adult GnRH neurons in situ, we generated transgenic mice in which the genetically encodable calcium indicator ratiometric Pericam was expressed by approximately 95% of GnRH neurons. Real-time monitoring of [Ca2+]i within adult male GnRH neurons in the acute brain slice revealed that approximately 70% of GnRH neurons exhibited spontaneous, 10-15 s duration [Ca2+]i transients with a mean frequency of 7 per hour. The remaining 30% of GnRH neurons did not exhibit calcium transients nor did a population of non-GnRH cells located within the lateral septum that express Pericam. Pharmacological studies using antagonists to the inositol-1,4,5-trisphosphate receptor (InsP3R) and several calcium channels, demonstrated that [Ca2+]i transients in GnRH neurons were generated by an InsP3R-dependent store-release mechanism and were independent of plasma membrane ligand- or voltage-gated calcium channels. Interestingly, the abolition of action potential-mediated transmission with tetrodotoxin reduced the number of [Ca2+]i transients in GnRH neurons by 50% (p < 0.05), suggesting a modulatory role for synaptic inputs on [Ca2+]i transient frequency. Using a novel transgenic strategy that enables [Ca2+]i to be examined in a specific neuronal phenotype in situ, we provide evidence for spontaneous [Ca2+]i fluctuations in adult GnRH neurons. This represents the initial description of spontaneous [Ca2+]i transients in mature neurons and shows that they arise from an InsP3R-generating mechanism that is further modulated by synaptic inputs.

Detection and localization of an estrogen receptor beta splice variant protein (ERβ2) in the adult female rat forebrain and midbrain regions

Estrogens regulate neural processes such as neuronal development, reproductive behavior, and hormone secretion, and signal through estrogen receptor (ER) alpha and ERbeta (here called ERbeta1). Recent studies have found variations in ERalpha and ERbeta1 mRNA splicing in rodents and humans. Functional reporter gene assays suggest that these splicing variations alter ER-mediated transcriptional regulation. Estrogen receptor beta 2 (ERbeta2), an ERbeta1 splice variant containing an 18 amino acid (AA) insert in the ligand binding domain, binds estradiol with approximately 10-fold lower affinity than ERbeta1, suggesting that it may serve as a low-affinity ER. Moreover, ERbeta2 reportedly acts in a dominant-negative fashion when heterodimerized with ERbeta1 or ERalpha. To explore the function of ERbeta2 in brain, an antiserum (TwobetaER.1) targeting the 18 AA insert was developed and characterized. Western blot analysis and transient expression of ERbeta2 in cell lines demonstrated that TwobetaER.1 recognizes ERbeta2. In the adult female rat brain, ERbeta2 immunoreactivity is localized in the cell nucleus and is expressed with a distribution similar to that of ERbeta1 mRNA. ERbeta2 immunoreactive cell numbers were high in, for example, piriform cortex, paraventricular nucleus, supraoptic nucleus, arcuate nucleus, and hippocampal CA regions, whereas it was low in the dentate gyrus. Moreover, ERbeta2 is coexpressed in gonadotropin-releasing hormone and oxytocin neurons. These studies demonstrate ERbeta splice variant proteins in brain and support the hypothesis that ER signaling diversity depends not only on ligand or coregulatory proteins, but also on regional and phenotypic selectivity of ER splice variant proteins.

Postnatal development of kisspeptin neurons in mouse hypothalamus; sexual dimorphism and projections to gonadotropin-releasing hormone neurons

The neuropeptide kisspeptin has recently been implicated as having a critical role in the activation of the GnRH neurons to bring about puberty. We examined here the postnatal development of kisspeptin neuronal populations and their projections to GnRH neurons in the mouse. Three populations of kisspeptin neurons located in the 1) anteroventral periventricular nucleus (AVPV) and the preoptic periventricular nucleus (PeN), 2) dorsomedial hypothalamus, and 3) arcuate nucleus were identified using an antisera raised against mouse kisspeptin-10. A marked 10-fold (P<0.01), female-dominant sex difference in the numbers of kisspeptin neurons existed in the AVPV/PeN but not elsewhere. Kisspeptin neurons in the AVPV/PeN of both sexes displayed a similar pattern of postnatal development with no cells detected at postnatal day (P) 10, followed by increases from P25 to reach adult levels by puberty onset (P<0.01; P31 females and P45 males). This pattern was not found in the dorsomedial hypothalamus or arcuate nucleus. Dual immunofluorescence experiments demonstrated close appositions between kisspeptin fibers and GnRH neuron cell bodies that were first apparent at P25 and increased across postnatal development in both sexes. These studies demonstrate kisspeptin peptide expression in the mouse hypothalamus and reveal the postnatal development of a sexually dimorphic continuum of kisspeptin neurons within the AVPV and PeN. This periventricular population of kisspeptin neurons reaches adult-like proportions at the time of puberty onset and is the likely source of the kisspeptin inputs to GnRH neurons.