Kisspeptin signaling in the brain

The role of endocannabinoids in gonadal function and fertility along the evolutionary axis

Endocannabinoids are natural lipids able to bind to cannabinoid and vanilloid receptors. Their biological actions at the central and peripheral level are under the tight control of the proteins responsible for their synthesis, transport and degradation. In the last few years, several reports have pointed out these lipid mediators as critical signals, together with sex hormones and cytokines, in various aspects of animal and human reproduction. The identification of anandamide (AEA) and 2-arachidonoylglycerol (2-AG) in reproductive cells and tissues of invertebrates, vertebrates and mammals highlights the key role played by these endogenous compounds along the evolutionary axis. Here, we review the main actions of endocannabinoids on female and male reproductive events, and discuss the interplay between them, steroid hormones and cytokines in regulating fertility. In addition, we discuss the involvement of endocannabinoid signalling in ensuring a correct chromatin remodeling, and hence a good DNA quality, in sperm cells.

Estrogens and neuropeptides in postmenopausal women: un update

Menopause is characterized by depletion of ovarian follicles, a reduction of ovarian hormones to castrate levels and elevated levels of serum gonadotropins from the anterior pituitary gland. Although this process has significant repercussions throughout the body and affects a large proportion of our society, the neuroendocrine control mechanisms that accompany menopause are poorly understood. This review aims to examine rigorously the most accredited literature to provide an update about our current understanding of the role of the hypothalamic-pituitary axis in the onset of and transition into female reproductive senescence, focusing on the role of some specific neuropeptides in regulating the HPG axis and on their effects on several menopausal symptoms, especially referring to the cardiovascular risk, to open up new horizons for new therapeutic strategies.

Role of neurokinin B in the control of female puberty and its modulation by metabolic status

Human genetic studies have revealed that neurokinin B (NKB) and its receptor, neurokinin-3 receptor (NK3R), are essential elements for normal reproduction; however, the precise role of NKB-NK3R signaling in the initiation of puberty remains unknown. We investigated here the regulation of Tac2 and Tacr3 mRNAs (encoding NKB and NK3R, respectively) in female rats and demonstrated that their hypothalamic expression is increased along postnatal maturation. At puberty, both genes were widely expressed throughout the brain, including the lateral hypothalamic area and the arcuate nucleus (ARC)/medial basal hypothalamus, where the expression of Tacr3 increased across pubertal transition. We showed that central administration of senktide (NK3R agonist) induced luteinizing hormone (LH) secretion in prepubertal and peripubertal females. Conversely, chronic infusion of an NK3R antagonist during puberty moderately delayed the timing of vaginal opening (VO) and tended to decrease LH levels. The expression of NKB and its receptor was sensitive to changes in metabolic status during puberty, as reflected by a reduction in Tacr3 (and, to a lesser extent, Tac2) expression in the ARC after a 48 h fast. Yet, acute LH responses to senktide in pubertal females were preserved, if not augmented, under fasting conditions, suggesting sensitization of the NKB-NK3R-gonadotropin-releasing hormone signaling pathway under metabolic distress. Moreover, repeated administration of senktide to female rats with pubertal arrest due to chronic undernutrition rescued VO (in ∼50% of animals) and potently elicited LH release. Altogether, our observations suggest that NKB-NK3R signaling plays a role in pubertal maturation and that its alterations may contribute to pubertal disorders linked to metabolic stress and negative energy balance.

Sex-specific changes in the expression of kisspeptin, kisspeptin receptor, gonadotropins and gonadotropin receptors in the Senegalese sole (Solea senegalensis) during a full reproductive cycle

Kisspeptin is thought to have a major role in the control of the onset of puberty in vertebrates. However, our current understanding of its function in fish and how it integrates with other hormones is incomplete due to the high diversity of this group of animals and a still limited amount of available data. This study examined the temporal and spatial changes in expression of kisspeptin, gonadotropins and their respective receptors in the Senegalese sole during a full reproductive cycle. Kiss2 and kiss2r expression was determined by qRT-PCR in the forebrain and midbrain while expression of fshβ and lhβ was determined in the pituitary and fshr and lhr in the gonads. Plasma levels of testosterone (T), 11-ketotestosterone (11-KT) and estradiol-17β were measured by ELISA and gonadal maturation was assessed histologically. In males, kiss2 and kiss2r expression in the brain areas examined was highest towards the end of winter, just before the spawning season, which took place the following spring. This coincided with maximum levels of pituitary fshβ and lhβ, plasma T and 11-KT and the highest number of maturing fish. However, these associations were not evident in females, since the highest expression of kiss2, kiss2r and gonadotropins were observed in the fall, winter or spring, depending upon the variable and tissue considered. Taken together, these data show not only temporal and spatial, but also sex-specific differences in the expression of kisspeptin and its receptor. Thus, while expression of kiss2 in Senegalese sole males agrees with what one would expect according to its proposed role as a major regulator of the onset of reproduction, in females the situation was not so clear, since kiss2 and kiss2r expression was highest either before or during the spawning season.

Developmental increase in kisspeptin-54 release in vivo is independent of the pubertal increase in estradiol in female rhesus monkeys (Macaca mulatta)

Kisspeptin (KP) signaling has been proposed as an important regulator in the mechanism of puberty. In this study, to determine the role of KP in puberty, we assessed the in vivo release pattern of KP-54 from the basal hypothalamus/stalk-median eminence in prepubertal and pubertal ovarian-intact female rhesus monkeys. We found that there was a developmental increase in mean KP-54 release, pulse frequency, and pulse amplitude, which is parallel to the developmental changes in GnRH release that we previously reported. Moreover, a nocturnal increase in KP-54 release becomes prominent after the onset of puberty. Because the pubertal increase in GnRH release occurs independent of the pubertal increase in circulating gonadal steroids, we further examined whether ovariectomy (OVX) modifies the release pattern of KP-54. Results show that OVX in pubertal monkeys enhanced mean KP-54 release and pulse amplitude but not pulse frequency, whereas OVX did not alter the release pattern of KP-54 in prepubertal monkeys. Estradiol replacement in OVX pubertal monkeys suppressed mean KP-54 release and pulse amplitude but not pulse frequency. Estradiol replacement in OVX prepubertal monkeys did not alter the KP-54 release pattern. Collectively these results suggest that the pubertal increase in KP release occurs independent of the pubertal increase in circulating estradiol. Nevertheless, the pubertal increase in KP release is not likely responsible for the initiation of the pubertal increase in GnRH release. Rather, after puberty onset, the increase in KP release contributes to further increase GnRH release during the progression of puberty.

Assessment of epigenetic contributions to sexually-dimorphic Kiss1 expression in the anteroventral periventricular nucleus of mice

The Kiss1 gene, which encodes kisspeptin and is critical for reproduction, is sexually differentiated in the hypothalamic anteroventral periventricular (AVPV)/rostral periventricular (PeN) nuclei. Specifically, female rodents have higher AVPV/PeN Kiss1 expression than males, but how this Kiss1 sex difference is induced in early development is poorly understood. Here, we explored the contribution of epigenetic mechanisms to the establishment of the AVPV/PeN Kiss1 sex difference, focusing on histone deacetylation and DNA methylation. First, we utilized postnatal pharmacological blockade of histone deacetylation and analyzed Kiss1 expression in the AVPV/PeN. Postnatal disruption of histone deacetylase modestly increased AVPV Kiss1 cell number in both sexes but did not alter the Kiss1 sex difference. Next, we assessed whether the level of CpG methylation, which can influence transcription factor binding and gene expression, in the murine Kiss1 gene differs between males and females. We found significant sex differences in methylation at several CpG sites in the putative promoter and first intron of the Kiss1 gene in the AVPV/PeN, but not in the arcuate (which lacks adult Kiss1 sex differences), suggesting that differential methylation of the Kiss1 gene may influence sexually-dimorphic Kiss1 expression in the AVPV/PeN. Transgenic impairment of methyl CpG-binding protein-2 function did not eliminate the Kiss1 sex difference, indicating that other methylation factors are involved. Interestingly, CpG methylation in the AVPV/PeN was lower in males than females, suggesting that transcriptional repressors may contribute to the AVPV/PeN Kiss1 sex difference, a possibility supported by in silico identification of putative repressor binding sites near some of the sexually-dimorphic CpG.

A comparison of the neuroendocrine mechanisms underlying the initiation of the preovulatory LH surge in the human, Old World monkey and rodent

As recognized for decades, the role of the rodent hypothalamus in timing the LH surge is deterministic and mediated by a GnRH discharge that is generated by an obligatory interaction in the preoptic area (POA) between a threshold level of estradiol and a circadian neural signal: a view consistent with contemporary kisspeptinocentric models of the estrous cycle. In higher primates, generation of the LH surge is emancipated from control by the POA. Woman represents the exemplar of the system in higher primates, as the LH surge appears to unfold in the absence of a midcycle GnRH discharge being generated instead by facilitatory interaction between a pulsatile GnRH input to the pituitary and an action of ovarian estradiol. The neurobiology of GnRH pulse generation is only beginning to emerge but from a translational perspective this aspect of hypothalamic function is critical for understanding the human menstrual cycle and how it may be perturbed.

Kisspeptin-GPR54 signaling in mouse NO-synthesizing neurons participates in the hypothalamic control of ovulation

Reproduction is controlled in the brain by a neural network that drives the secretion of gonadotropin-releasing hormone (GnRH). Various permissive homeostatic signals must be integrated to achieve ovulation in mammals. However, the neural events controlling the timely activation of GnRH neurons are not completely understood. Here we show that kisspeptin, a potent activator of GnRH neuronal activity, directly communicates with neurons that synthesize the gaseous transmitter nitric oxide (NO) in the preoptic region to coordinate the progression of the ovarian cycle. Using a transgenic Gpr54-null IRES-LacZ knock-in mouse model, we demonstrate that neurons containing neuronal NO synthase (nNOS), which are morphologically associated with kisspeptin fibers, express the kisspeptin receptor GPR54 in the preoptic region, but not in the tuberal region of the hypothalamus. The activation of kisspeptin signaling in preoptic neurons promotes the activation of nNOS through its phosphorylation on serine 1412 via the AKT pathway and mimics the positive feedback effects of estrogens. Finally, we show that while NO release restrains the reproductive axis at stages of the ovarian cycle during which estrogens exert their inhibitory feedback, it is required for the kisspeptin-dependent preovulatory activation of GnRH neurons. Thus, interactions between kisspeptin and nNOS neurons may play a central role in regulating the hypothalamic-pituitary-gonadal axis in vivo.

Aggressive behavior and reproductive physiology in females of the social cichlid fish Cichlasoma dimerus

The South American cichlid fish Cichlasoma dimerus is a freshwater species that presents social hierarchies, a highly organized breeding activity, biparental care and a high frequency of spawning. Spawning is followed by a period of parental care (about 20 days in aquaria conditions) during which the cooperative pair takes care of the eggs, both by fanning them and by removing dead ones. The different spawning events in the reproductive period were classified as female reproductive stages which can be subdivided in four phases, according to their offspring degree of development: (1) female with prespawning activity (day 0), (2) female with eggs (day 1 after fertilization), (3) female with hatched larvae (day 3 after fertilization) and (4) female with swimming larvae (FSL, day 8 after fertilization). In Perciform species gonadotropin-releasing hormone type-3 (GnRH3) neurons are associated with the olfactory bulbs acting as a potent neuromodulator of reproductive behaviors in males. The aim of this study is to characterize the GnRH3 neuronal system in females of C. dimerus in relation with aggressive behavior and reproductive physiology during different phases of the reproductive period. Females with prespawning activity were the most aggressive ones showing GnRH-3 neurons with bigger nuclear and somatic area and higher optical density than the others. They also presented the highest levels of plasma androgen and estradiol and maximum gonadosomatic indexes. These results provide information about the regulation and functioning of hypothalamus-pituitary-gonads axis during reproduction in a species with highly organized breeding activity.

Sexually dimorphic testosterone secretion in prenatal and neonatal mice is independent of kisspeptin-Kiss1r and GnRH signaling

Kisspeptin, encoded by the Kiss1 gene, stimulates GnRH secretion and is therefore critical for sex steroid secretion at puberty and in adulthood. However, kisspeptin's role in regulating sex steroid secretion earlier in development is unexplored. In rodents, testosterone (T) levels are higher in prenatal and newborn males than females. We determined whether kisspeptin-Kiss1r and GnRH signaling plays a role in sexually dimorphic perinatal T secretion in mice. Our results demonstrate that 1) T levels in newborn males are elevated at 4 h but not 20 h after birth, but hypothalamic Kiss1 and neurokinin B (NKB) levels in males are not different between these time points (and both are lower than in females); 2) serum T levels in newborn Kiss1r knockout (KO) males are higher than in newborn females and similar to wild-type (WT) males; 3) perinatal hypothalamic progesterone receptor (Pgr) expression, which is dependent on circulating levels of gonadally produced T, is significantly higher in prenatal and newborn Kiss1r KO and WT males than similarly aged females; 4) multiple measures of testicular growth and function are not different between developing Kiss1r KO and WT mice until after postnatal d 5; and 5) GnRH neurons of newborn males do not exhibit high c-fos coexpression, and newborn hypogonadal (hpg) male mice (lacking GnRH) secrete elevated T, similar to newborn WT males. We conclude that, unlike in puberty and adulthood, elevated T secretion in prenatal and neonatal mice is independent of both kisspeptin and GnRH signaling, and the necessity of kisspeptin-Kiss1r signaling for testicular function is first apparent after d 5.

Molecular Coevolution of Neuropeptides Gonadotropin-Releasing Hormone and Kisspeptin with their Cognate G Protein-Coupled Receptors

The neuropeptides gonadotropin-releasing hormone (GnRH) and kisspeptin (KiSS), and their receptors gonadotropin-releasing hormone receptor (GnRHR) and kisspeptin receptor (KiSSR) play key roles in vertebrate reproduction. Multiple paralogous isoforms of these genes have been identified in various vertebrate species. Two rounds of genome duplication in early vertebrates likely contributed to the generation of these paralogous genes. Genome synteny and phylogenetic analyses in a variety of vertebrate species have provided insights into the evolutionary origin of and relationship between paralogous genes. The paralogous forms of these neuropeptides and their receptors have coevolved to retain high selectivity of the ligand–receptor interaction. These paralogous forms have become subfunctionalized, neofunctionalized, or dysfunctionalized during evolution. This article reviews the evolutionary mechanism of GnRH/GnRHR and KiSS/KiSSR, and the fate of the duplicated paralogs in vertebrates.

Comparative insights of the kisspeptin/kisspeptin receptor system: lessons from non-mammalian vertebrates

Kisspeptins, the peptide products of the Kiss1 gene, were initially identified in mammals as ligands of the G protein-coupled receptor 54 (GPR54; also termed Kiss1R) with ability to suppress tumor metastasis. In late 2003, the indispensable role of kisspeptins in the control of reproductive function was disclosed by the seminal observations that humans and mice carrying inactivating mutations of GPR54 displayed hypogonadotropic hypogonadism. Since then, numerous experimental studies, conducted initially in several mammalian species, have substantiated the roles of kisspeptins as essential players in the physiologic regulation of key aspects of reproductive maturation and function, including the timing of puberty onset, the dynamic control of gonadotropin secretion via stimulation of GnRH neurons, the transmission of the negative and positive feedback effects of sex steroids, the metabolic regulation of fertility and the control of reproductive function by environmental (photoperiodic) cues. Notably, while studies about kisspeptins in non-mammals appeared initially to lag behind, significant efforts have been devoted recently to define the genomic organization and functional characteristics of kiss/kisspeptins and gpr54 in different non-mammalian species, including fish, reptiles and amphibians. These analyses, which will be comprehensively revised herein, have not only substantiated the conserved, essential roles of kisspeptins in the control of reproduction, but have also disclosed intriguing evolutionary aspects of kisspeptins and their receptors. Such comparative approaches will be instrumental to fuel further studies on the molecular regulation and physiological roles of kisspeptins, thus helping to unveil the complex biology of this system as indispensable regulator of the reproductive axis in a wide diversity of animal species.

Differential sleep-wake sensitivity of gonadotropin-releasing hormone secretion to progesterone inhibition in early pubertal girls

CONTEXT

Early pubertal luteinizing hormone (LH), and by inference gonadotropin-releasing hormone (GnRH), pulse secretion is marked by high nocturnal but low daytime frequency; however, the underlying mechanisms remain unclear. Plasma concentrations of progesterone, the major regulator of GnRH frequency in women, increase in the early morning in early pubertal girls and may help slow daytime GnRH frequency.

OBJECTIVE

To evaluate the effect of progesterone on LH pulse frequency in early to mid-pubertal girls.

DESIGN

Controlled interventional study.

SETTING

General clinical research center.

PARTICIPANTS

Eighteen non-obese, non-hyperandrogenemic Tanner 1-3 girls.

INTERVENTION

Twelve-hour (19:00-07:00 h) blood sampling with or without oral progesterone administration (25-50 mg at 16:00 and 20:00 h).

MAIN OUTCOME MEASURE

LH pulse frequency.

RESULTS

Girls receiving progesterone (n = 5) exhibited lower 12-hour LH pulse frequency than controls (n = 13), but this difference was not statistically significant (average interpulse intervals 196.0 ± 61.9 and 160.4 ± 67.1 min, respectively; p = 0.2793). In contrast to controls, however, girls receiving progesterone exhibited no LH pulses during waking hours (19:00-23:00 h; estimated interpulse interval 326.0 ± 52.7 vs. 212.0 ± 120.9 min; p = 0.0376), while nighttime (23:00-07:00 h) interpulse intervals were similar (174.8 ± 62.0 vs. 167.5 ± 76.9 min, respectively; p = 0.7750).

CONCLUSIONS

Exogenous progesterone acutely suppressed daytime, but not nocturnal, LH pulse frequency in early to mid-pubertal girls, suggesting that GnRH pulse frequency is differentially regulated by progesterone depending on sleep status.

Kisspeptin inhibits high-voltage activated Ca2+ channels in GnRH neurons via multiple Ca2+ influx and release pathways

Kisspeptin plays an important role in puberty and subsequent fertility by activating its receptor, G-protein-coupled receptor 54 (GPR54), and increasing cytoplasmic free Ca(2+) concentration ([Ca(2+)](i)) and gonadotropin-releasing hormone (GnRH) secretion in GnRH neurons. Yet the mechanism by which kisspeptin increases [Ca(2+)](i) in GnRH neurons remains to be fully elucidated. In other neurons, voltage-gated Ca(2+) channel (VGCC) activity has been shown to be inversely related to [Ca(2+)](i). We used whole-cell patch-clamp recording to examine the effects of kisspeptin-10 (KP-10) on VGCC activity evoked by step depolarizations in GnRH neurons in brain slices from pubertal male GnRH-green fluorescent protein transgenic mice. Prolonged (>30 s) KP-10 application inhibited Ca(2+) currents. The GPR54 antagonist peptide 234, chelation of intracellular Ca(2+) by 1,2-bis(2-aminophenoxy)ethane N,N,N',N'-tetraacetic acid, substitution of Ba(2+) for Ca(2+), the calmodulin antagonists calmidazolium and trifluoperazine, the phospholipase C inhibitor edelfosine, the canonical transient receptor potential (TRPC) channel and inositol 1,4,5-trisphosphate receptor (IP(3)R) antagonist 2-APB, the TRPC channel antagonist BTP2 and the endoplasmic reticulum Ca(2+)-ATPase blocker cyclopiazonic acid each prevented inhibition. The IP(3)R antagonists caffeine (10 µM), heparin and intracellular 2-APB prevented inhibition to a lesser extent. The ryanodine receptor (RyR) antagonists ryanodine and dantrolene prevented inhibition, and the RyR agonist caffeine (30 mM) mimicked the effects of KP-10 on Ca(2+) currents. Our results suggest that kisspeptin induces Ca(2+) influx through TRPC channels and Ca(2+) release via IP(3)Rs and RyRs, and that this is followed by Ca(2+)/CaM-dependent inhibition of VGCCs.

Sexual differentiation of the gonadotropin surge release mechanism: a new role for the canonical NfκB signaling pathway

Sex differences in luteinizing hormone (LH) release patterns are controlled by the hypothalamus, established during the perinatal period and required for fertility. Female mammals exhibit a cyclic surge pattern of LH release, while males show a tonic release pattern. In rodents, the LH surge pattern is dictated by the anteroventral periventricular nucleus (AVPV), an estrogen receptor-rich structure that is larger and more cell-dense in females. Sex differences result from mitochondrial cell death triggered in perinatal males by estradiol derived from aromatization of testosterone. Herein we provide an historical perspective and an update describing evidence that molecules important for cell survival and cell death in the immune system also control these processes in the developing AVPV. We conclude with a new model proposing that development of the female AVPV requires constitutive activation of the Tnfα, Tnf receptor 2, NfκB and Bcl2 pathway that is blocked by induction of Tnf receptor-associated factor 2-inhibiting protein (Traip) in the male.

Kisspeptin signaling is indispensable for neurokinin B, but not glutamate, stimulation of gonadotropin secretion in mice

Kisspeptins (Kp), products of the Kiss1 gene that act via Gpr54 to potently stimulate GnRH secretion, operate as mediators of other regulatory signals of the gonadotropic axis. Mouse models of Gpr54 and/or Kiss1 inactivation have been used to address the contribution of Kp in the central control of gonadotropin secretion; yet, phenotypic and hormonal differences have been detected among the transgenic lines available. We report here a series of neuroendocrine analyses in male mice of a novel Gpr54 knockout (KO) model, generated by heterozygous crossing of a loxP-Gpr54/Protamine-Cre double mutant line. Gpr54-null males showed severe hypogonadotropic hypogonadism but retained robust responsiveness to GnRH. Gonadotropic responses to the agonist of ionotropic glutamate receptors, N-methyl-d-aspartate, were attenuated, but persisted, in Gpr54-null mice. In contrast, LH secretion after activation of metabotropic glutamate receptors was totally preserved in the absence of Gpr54 signaling. Detectable, albeit reduced, LH responses were also observed in Gpr54 KO mice after intracerebroventricular administration of galanin-like peptide or RF9, putative antagonist of neuropeptide FF receptors for the mammalian ortholog of gonadotropin-inhibiting hormone. In contrast, the stimulatory effect of senktide, agonist of neurokinin B (NKB; cotransmitter of Kiss1 neurons), was totally abrogated in Gpr54 KO males. Lack of Kp signaling also eliminated feedback LH responses to testosterone withdrawal. However, residual but sustained increases of FSH were detected in gonadectomized Gpr54 KO males, in which testosterone replacement failed to fully suppress circulating FSH levels. In sum, our study provides novel evidence for the relative importance of Kp-dependent vs. -independent actions of several key regulators of GnRH secretion, such as glutamate, galanin-like peptide, and testosterone. In addition, our data document for the first time the indispensable role of Kp signaling in mediating the stimulatory effects of NKB on LH secretion, thus supporting the hypothesis that NKB actions on GnRH neurons are indirectly mediated via its ability to regulate Kiss1 neuronal output.

Sex steroids and the control of the Kiss1 system: developmental roles and major regulatory actions

Kisspeptins, encoded by the Kiss1 gene, and their canonical receptor, GPR54 (also termed Kiss1R), are unanimously recognised as essential regulators of puberty onset and gonadotrophin secretion. These key reproductive functions stem from the capacity of kisspeptins to stimulate gonadotrophin-releasing hormone (GnRH) secretion in the hypothalamus, where discrete populations of Kiss1 neurones have been identified. In rodents, two major groups of hypothalamic Kiss1 neurones exist: one present in the arcuate nucleus (ARC) and the other located in the anteroventral periventricular area (AVPV/RP3V). In recent years, numerous signals have been identified as putative modulators of the hypothalamic Kiss1 system. Among them, the prominent role of sex steroids as being important regulators of Kiss1 neurones has been documented in different species and developmental stages, such as early brain sex differentiation, puberty, adulthood and senescence. These regulatory actions are (mainly) conducted via oestrogen receptor (ER)α, which is expressed in almost all Kiss1 neurones, and likely involve both classical and nonclassical pathways. The regulatory effects of sex steroids are nucleus-specific. Thus, sex steroids inhibit the expression of Kiss1/kisspeptin at the ARC, as a mechanism to conduct their negative-feedback actions on gonadotrophin secretion. By contrast, oestrogens enhance Kiss1 expression at the AVPV/RP3V in rodents, suggesting the involvement of this population in the positive-feedback actions of oestradiol to generate the preovulatory surge of gonadotrophins. In addition, sex steroids have been shown to act post-transcriptionally, modulating GnRH/gonadotrophin responsiveness to kisspeptin. Finally, sex steroids also regulate the expression of co-transmitters of Kiss1 neurones, such as neurokinin B, whose mRNA content in the ARC fluctuates in parallel to that of Kiss1 in response to changes in the circulating levels of sex steroids, therefore suggesting the contribution of this neuropeptide in the feedback control of gonadotrophin secretion. In sum, compelling experimental evidence obtained in different mammalian (and non-mammalian) species, including primates, demonstrates that sex steroids are essential regulators of hypothalamic Kiss1 neurones, which in turn operate as conduits for their effects on GnRH neurones. The physiological relevance of such regulatory phenomena is thoroughly discussed.

Neonatal Bisphenol A exposure alters sexually dimorphic gene expression in the postnatal rat hypothalamus

Developmental exposure to Bisphenol A (BPA), a component of polycarbonate and epoxy resins, has been purported to adversely impact reproductive function in female rodents. Because neonatal life is a critical window for the sexual dimorphic organization of the hypothalamic-pituitary-gonadal (HPG) axis, interference with this process could underlie compromised adult reproductive physiology. The goal of the present study was to determine if neonatal BPA exposure interferes with sex specific gene expression of estrogen receptor alpha (ERα), ER beta (ERβ) and kisspeptin (Kiss1) in the anterior and mediobasal hypothalamus. Long Evans (LE) neonatal rats were exposed to vehicle, 10μg estradiol benzoate (EB), 50mg/kg BPA or 50μg/kg BPA by subcutaneous injection daily from postnatal day 0 (PND 0) to PND 2. Gene expression was assessed by in situ hybridization on PNDs 4 and 10. Within the anterior hypothalamus ERα expression was augmented by BPA in PND 4 females, then fell to male-typical levels by PND 10. ERβ expression was not altered by BPA on PND 4, but significantly decreased or eliminated in both sexes by PND 10. Kiss1 expression was diminished by BPA in the anterior hypothalamus, especially in females. There were no significant impacts of BPA in the mediobasal hypothalamus. Collectively, BPA effects did not mirror those of EB. The results show that neonatal hypothalamic ER and Kiss1 expression is sensitive to BPA exposure. This disruption may alter sexually dimorphic hypothalamic organization and underlie adult reproductive deficiencies. Additionally, the discordant effects of EB and BPA indicate that BPA likely disrupts hypothalamic organization by a mechanism other than simply acting as an estrogen mimic.

The brain of teleost fish, a source, and a target of sexual steroids

Neurosteroids are defined as steroids de novo synthesized in the central nervous system. While the production of neurosteroids is well documented in mammals and amphibians, there is less information about teleosts, the largest group of fish. Teleosts have long been known for their high brain aromatase and 5α-reductase activities, but recent data now document the capacity of the fish brain to produce a large variety of sex steroids. This article aims at reviewing the available information regarding expression and/or activity of the main steroidogenic enzymes in the brain of fish. In addition, the distribution of estrogen, androgen, and progesterone nuclear receptors is documented in relation with the potential sites of production of neurosteroids. Interestingly, radial glial cells acting as neuronal progenitors, appear to be a potential source of neurosteroids, but also a target for centrally and/or peripherally produced steroids.

Social Regulation of Gene Expression in the Hypothalamic-Pituitary-Gonadal Axis

Reproduction is a critically important event in every animals' life and in all vertebrates is controlled by the brain via the hypothalamic-pituitary-gonadal (HPG) axis. In many species, this axis, and hence reproductive fitness, can be profoundly influenced by the social environment. Here, we review how the reception of information in a social context causes genomic changes at each level of the HPG axis.

Hyperalgesic activity of kisspeptin in mice

Background

Kisspeptin is a neuropeptide known for its role in the hypothalamic regulation of the reproductive axis. Following the recent description of kisspeptin and its 7-TM receptor, GPR54, in the dorsal root ganglia and dorsal horns of the spinal cord, we examined the role of kisspeptin in the regulation of pain sensitivity in mice.

Results

Immunofluorescent staining in the mouse skin showed the presence of GPR54 receptors in PGP9.5-positive sensory fibers. Intraplantar injection of kisspeptin (1 or 3 nmol/5 μl) induced a small nocifensive response in naive mice, and lowered thermal pain threshold in the hot plate test. Both intraplantar and intrathecal (0.5 or 1 nmol/3 μl) injection of kisspeptin caused hyperalgesia in the first and second phases of the formalin test, whereas the GPR54 antagonist, p234 (0.1 or 1 nmol), caused a robust analgesia. Intraplantar injection of kisspeptin combined with formalin enhanced TRPV1 phosphorylation at Ser800 at the injection site, and increased ERK1/2 phosphorylation in the ipsilateral dorsal horn as compared to naive mice and mice treated with formalin alone.

Conclusion

These data demonstrate for the first time that kisspeptin regulates pain sensitivity in rodents and suggest that peripheral GPR54 receptors could be targeted by novel drugs in the treatment of inflammatory pain.

Molecular properties of Kiss1 neurons in the arcuate nucleus of the mouse

Neurons that produce kisspeptin play a critical role in reproduction. However, understanding the molecular physiology of kisspeptin neurons has been limited by the lack of an in vivo marker for those cells. Here, we report the development of a Kiss1-CreGFP knockin mouse, wherein the endogenous Kiss1 promoter directs the expression of a Cre recombinase-enhanced green fluorescent protein (GFP) fusion protein. The pattern of GFP expression in the brain of the knockin recapitulates what has been described earlier for Kiss1 in the male and female mouse, with prominent expression in the arcuate nucleus (ARC) (in both sexes) and the anteroventral periventricular nucleus (in females). Single-cell RT-PCR showed that the Kiss1 transcript is expressed in 100% of GFP-labeled cells, and the CreGFP transcript was regulated by estradiol in the same manner as the Kiss1 gene (i.e. inhibited in the ARC and induced in the anteroventral periventricular nucleus). We used this mouse to evaluate the biophysical properties of kisspeptin (Kiss1) neurons in the ARC of the female mouse. GFP-expressing Kiss1 neurons were identified in hypothalamic slice preparations of the ARC and patch clamped. Whole-cell (and loose attached) recordings revealed that Kiss1 neurons exhibit spontaneous activity and expressed both h- (pacemaker) and T-type calcium currents, and hyperpolarization-activated cyclic nucleotide-regulated 1-4 and CaV3.1 channel subtypes (measured by single cell RT-PCR), respectively. N-methyl-D-aspartate induced bursting activity, characterized by depolarizing/hyperpolarizing oscillations. Therefore, Kiss1 neurons in the ARC share molecular and electrophysiological properties of other CNS pacemaker neurons.

Regulation of NKB pathways and their roles in the control of Kiss1 neurons in the arcuate nucleus of the male mouse

Kisspeptin (Kiss1) and neurokinin B (NKB) (encoded by the Kiss1 and Tac2 genes, respectively) are indispensable for reproduction. In the female of many species, Kiss1 neurons in the arcuate nucleus (ARC) coexpress dynorphin A and NKB. Such cells have been termed Kiss1/NKB/Dynorphin (KNDy) neurons, which are thought to mediate the negative feedback regulation of GnRH/LH secretion by 17β-estradiol. However, we have less knowledge about the molecular physiology and regulation of Kiss1/Kiss1-expressing neurons in the ARC of the male. Our work focused on the adult male mouse, where we sought evidence for coexpression of these neuropeptides in cells in the ARC, assessed the role of Kiss1 neurons in negative feedback regulation of GnRH/LH secretion by testosterone (T), and investigated the action of NKB on KNDy and GnRH neurons. Results showed that 1) the mRNA encoding Kiss1, NKB, and dynorphin are coexpressed in neurons located in the ARC; 2) Kiss1 and dynorphin A mRNA are regulated by T through estrogen and androgen receptor-dependent pathways; 3) senktide, an agonist for the NKB receptor (neurokinin 3 receptor, encoded by Tacr3), stimulates gonadotropin secretion; 4) KNDy neurons express Tacr3, whereas GnRH neurons do not; and 5) senktide activates KNDy neurons but has no discernable effect on GnRH neurons. These observations corroborate the putative role for KNDy neurons in mediating the negative feedback effects of T on GnRH/LH secretion and provide evidence that NKB released from KNDy neurons is part of an auto-feedback loop that generates the pulsatile secretion of Kiss1 and GnRH in the male.

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.

Early life exposure to endocrine-disrupting chemicals causes lifelong molecular reprogramming of the hypothalamus and premature reproductive aging

Gestational exposure to the estrogenic endocrine disruptor methoxychlor (MXC) disrupts the female reproductive system at the molecular, physiological, and behavioral levels in adulthood. The current study addressed whether perinatal exposure to endocrine disruptors re-programs expression of a suite of genes expressed in the hypothalamus that control reproductive function and related these molecular changes to premature reproductive aging. Fischer rats were exposed daily for 12 consecutive days to vehicle (dimethylsulfoxide), estradiol benzoate (EB) (1 mg/kg), and MXC (low dose, 20 μg/kg or high dose, 100 mg/kg), beginning on embryonic d 19 through postnatal d 7. The perinatally exposed females were aged to 16-17 months and monitored for reproductive senescence. After euthanasia, hypothalamic regions [preoptic area (POA) and medial basal hypothalamus] were dissected for real-time PCR of gene expression or pyrosequencing to assess DNA methylation of the Esr1 gene. Using a 48-gene PCR platform, two genes (Kiss1 and Esr1) were significantly different in the POA of endocrine-disrupting chemical-exposed rats compared with vehicle-exposed rats after Bonferroni correction. Fifteen POA genes were up-regulated by at least 50% in EB or high-dose MXC compared with vehicle. To understand the epigenetic basis of the increased Esr1 gene expression, we performed bisulfite conversion and pyrosequencing of the Esr1 promoter. EB-treated rats had significantly higher percentage of methylation at three CpG sites in the Esr1 promoter compared with control rats. Together with these molecular effects, perinatal MXC and EB altered estrous cyclicity and advanced reproductive senescence. Thus, early life exposure to endocrine disruptors has lifelong effects on neuroendocrine gene expression and DNA methylation, together with causing the advancement of reproductive senescence.

Sexually dimorphic expression of hypothalamic estrogen receptors α and β and Kiss1 in neonatal male and female rats

Release of gonadotropins in adult rodents is sex specific and dependent upon kisspeptin (Kiss1) neurons. This crucial pathway within the hypothalamic-pituitary-gonadal (HPG) axis is profoundly influenced by neonatal estrogens, which induce a male-like phenotype. Classically, estrogen activity is mediated via the estrogen receptors α and β (ERα and ERβ), but the relative roles each plays in organizing the sex-specific ontogeny of kisspeptin signaling pathways remain unresolved. Thus, the present study used in situ hybridization histochemistry (ISHH) to map the temporal and sexually dimorphic neonatal mRNA expression profiles of ERα, ERβ, and Kiss1 in the anterioventral periventricular nucleus (AVPV), medial preoptic area (MPOA), ventromedial nucleus (VMN), and arcuate nucleus (ARC), all regions critical for kisspeptin regulation of gonadotropin secretion. In general, females had higher levels of ERα, in all regions examined, a sex difference that persisted until postnatal day (PND) 19 except in the ARC. Males had significantly more ERβ expression in the AVPV at birth, but this sex difference was lost and then re-emerged on PND 19, with females having more than males. VMN ERβ levels were higher in females until PND 19. Kiss1 was not detectable until PND 11 in the anterior hypothalamus, but expression levels were equivalent at birth in the ARC. By PND 2, ARC ERα and Kiss1 levels were abundant, sexually dimorphic (higher in females), and, respectively, showed a U- and a bell-shaped pattern with age. Sex differences in ARC Kiss1 expression provide evidence that Kiss1 may play a role in the sexual dimorphic organization of the neonatal brain. These detailed profiles of neonatal Kiss1 and ERs mRNA levels will help elucidate the relative roles each plays in the sex-specific, estrogen-dependent organization of gonadotropin signaling pathways.

Neuroendocrine control by kisspeptins: role in metabolic regulation of fertility

The neurohormonal control of reproduction involves a hierarchical network of central and peripheral signals in the hypothalamic-pituitary-gonadal (HPG) axis. Development and function of this neuroendocrine system is the result of a lifelong delicate balance between endogenous regulators and environmental cues, including nutritional and metabolic factors. Kisspeptins are the peptide products of KISS1, which operate via the G-protein-coupled receptor GPR54 (also known as Kiss1R). These peptides have emerged as essential upstream regulators of neurons secreting gonadotropin-releasing hormone (GnRH), the major hypothalamic node for the stimulatory control of the HPG axis. They are potent elicitors of gonadotropin secretion in various species and physiological settings. Moreover, Kiss1 neurons in the hypothalamus participate in crucial features of reproductive maturation and function, such as brain-level sex differentiation, puberty onset and the neuroendocrine regulation of gonadotropin secretion and ovulation. Cotransmitters of Kiss1 neurons, such as neurokinin B, with roles in controlling the HPG axis have been identified by genetic, neuroanatomical and physiological studies. In addition, a putative role has been proposed for Kiss1 neurons in transmitting metabolic information to GnRH neurons, although the precise mechanisms are as yet unclear. In this Review, we present the major reproductive features of kisspeptins, especially their interplay with neurokinin B and potential roles in the metabolic control of puberty and fertility, and suggest new avenues for research.

Low-dose exposure to alkylphenols adversely affects the sexual development of Atlantic cod (Gadus morhua): acceleration of the onset of puberty and delayed seasonal gonad development in mature female cod

Produced water (PW), a by-product of the oil-production process, contains large amount of alkylphenols (APs) and other harmful oil compounds. In the last 20 years, there have been increasing concerns regarding the environmental impact of large increases in the amounts of PW released into the North Sea. We have previously shown that low levels of APs can induce disruption of the endocrine and reproductive systems of Atlantic cod (Gadus morhua). The aims of this follow-up study were to: (i) identify the lowest observable effect concentration of APs; (ii) study the effects of exposure to real PW, obtained from a North Sea oil-production platform; and (iii) study the biological mechanism of endocrine disruption in female cod. Fish were fed with feed paste containing several concentrations of four different APs (4-tert-butylphenol, 4-n-pentylphenol, 4-n-hexylphenol and 4-n-heptylphenol) or real PW for 20 weeks throughout the normal period of vitellogenesis in Atlantic cod from October to January. Male and female cod, exposed to AP and PW, were compared to unexposed fish and to fish fed paste containing 17β-oestradiol (E(2)). Approximately 60% of the females and 96% of the males in the unexposed groups were mature at the end of the experiment. Our results show that exposure to APs and E(2) have different effects depending on the developmental stage of the fish. We observed that juvenile females are advanced into puberty and maturation, while gonad development was delayed in both maturing females and males. The AP-exposed groups contained increased numbers of mature females, and significant differences between the untreated group and the AP-treated groups were seen down to a dose of 4 μg AP/kg body weight. In the high-dose AP and the E(2) exposed groups, all females matured and no juveniles were seen. These results suggest that AP-exposure can affect the timing of the onset of puberty in fish even at extremely low concentrations. Importantly, similar effects were not seen in the fish that were exposed to real PW.

Evidence that the arcuate nucleus is an important site of progesterone negative feedback in the ewe

There is now considerable evidence that dynorphin neurons mediate the negative feedback actions of progesterone to inhibit GnRH and LH pulse frequency, but the specific neurons have yet to be identified. In ewes, dynorphin neurons in the arcuate nucleus (ARC) and preoptic area (POA) are likely candidates based on colocalization with progesterone receptors. These studies tested the hypothesis that progesterone negative feedback occurs in either the ARC or POA by determining whether microimplants of progesterone into either site would inhibit LH pulse frequency (study 1) and whether microimplants of the progesterone receptor antagonist, RU486, would disrupt the inhibitory effects of peripheral progesterone (study 2). Both studies were done in ovariectomized (OVX) and estradiol-treated OVX ewes. In study 1, no inhibitory effects of progesterone were observed during treatment in either area. In study 2, microimplants of RU486 into the ARC disrupted the negative-feedback actions of peripheral progesterone treatments on LH pulse frequency in both OVX and OVX+estradiol ewes. In contrast, microimplants of RU486 into the POA had no effect on the ability of systemic progesterone to inhibit LH pulse frequency. We thus conclude that the ARC is one important site of progesterone-negative feedback in the ewe. These data, which are the first evidence on the neural sites in which progesterone inhibits GnRH pulse frequency in any species, are consistent with the hypothesis that ARC dynorphin neurons mediate this action of progesterone.

Tachykinins and the hypothalamo-pituitary-gonadal axis: An update

Tachykinins play a critical role in neuroendocrine regulation of reproduction. The best known members of the family are substance P (SP), neurokinin A and neurokinin B. Tachykinins mediate their biological actions through three G protein-coupled receptors, named NK1, NK2, and NK3. SP was suggested to play an important role in the ovulatory process in mammals and humans. Recent findings suggest a role of tachykinins in the aging of the hypothalamo-pituitary-gonadal axis. A high presence of SP was found in the sheep pars tuberalis and evidence indicates that it may have some role in the control of prolactin secretion. The presence of SP was confirmed in Leydig cells of the rat testes of animals submitted to constant light or treated with estrogens. Tachykinins were found to increase the motility of human spermatozoa. Tachykinins were also found to be present in the mouse ovary and more specifically, in the granulose cells. It is possible that tachykinins may play an important role in the ovarian function. NKB has been implicated in the steroid feedback control of GnRH release. Human mutations in the gene encoding this peptide or its receptor (TACR3) lead to a defect in the control of GnRH. A specific subset of neurons in the arcuate nucleus of the hypothalamus, colocalized three neuropeptides, kisspeptin, NKB and dynorphin. This subpopulation of neurons mediates the gonadal hormone feedback control of GnRH secretion. NKB/NK3 signaling plays a role in puberty onset and fertility in humans. This minireview summarizes the recent data about the action of tachykinins on the hypothalamo-pituitary-gonadal axis.

Circulating kisspeptin levels exhibit sexual dimorphism in adults, are increased in obese prepubertal girls and do not suffer modifications in girls with idiopathic central precocious puberty

The system KISS1-KISS1R is one of the main regulators of the hypothalamic-pituitary-gonadal axis and constitutes a link between metabolism and reproduction through its interaction with leptin. The aim of this study was to clarify the possible utility of kisspeptin as a pubertal marker and/or the possible influence of nutritional status in kisspeptin levels. To this end, we have studied kisspeptin plasma levels throughout sexual development and in prepubertal obese girls and girls affected by idiopathic central precocious puberty (CPP). Plasma kisspeptin concentrations were analyzed by RIA. An increase in kisspeptin levels was observed in adult females compared to healthy prepubertal and pubertal girls (p<0.001) and to adult males (p<0.001). Additionally, kisspeptin was increased in prepubertal obese girls compared to healthy prepubertal girls (p<0.01) and girls with idiopathic CPP (p<0.05). As revealed by the regression analysis, in prepubertal healthy and obese girls and girls with idiopathic CCP, the parameters that influenced kisspeptin levels were BMI (R(2)=0.10, p<0.05) and leptin levels (R(2)=0.14, p<0.01). In conclusion, kisspeptin levels do not seem to be a good pubertal marker. The results obtained in prepubertal and idiopathic CCP girls point to a relationship between leptin, BMI and kisspeptin at least in this group, and suggest a possible role for adipose tissue in the modulation kisspeptin synthesis.

Early metabolic programming of puberty onset: impact of changes in postnatal feeding and rearing conditions on the timing of puberty and development of the hypothalamic kisspeptin system

Kiss1 neurons have recently emerged as a putative conduit for the metabolic gating of reproduction, with leptin being a regulator of hypothalamic Kiss1 expression. Early perturbations of the nutritional status are known to predispose to different metabolic disorders later in life and to alter the timing of puberty; however, the potential underlying mechanisms remain poorly defined. Here we report how changes in the pattern of postnatal feeding affect the onset of puberty and evaluate key hormonal and neuropeptide [Kiss1/kisspeptin (Kp)] alterations linked to these early nutritional manipulations. Female rats were raised in litters of different sizes: small (four pups per dam: overfeeding), normal (12 pups per dam), and large litters (20 pups per litter: underfeeding). Postnatal overfeeding resulted in persistently increased body weight and earlier age of vaginal opening, as an external sign of puberty, together with higher levels of leptin and hypothalamic Kiss1 mRNA. Conversely, postnatal underfeeding caused a persistent reduction in body weight, lower ovarian and uterus weights, and delayed vaginal opening, changes that were paralleled by a decrease in leptin and Kiss1 mRNA levels. Kisspeptin-52 immunoreactivity (Kp-IR) in the hypothalamus displayed similar patterns, with lower numbers of Kp-IR neurons in the arcuate nucleus of postnatally underfed animals, and a trend for increased Kp-positive fibers in the periventricular area of early overfed rats. Yet, gonadotropin responses to Kp at puberty were similar in all groups, except for enhanced responsiveness to low doses of Kp-10 in postnatally underfed rats. In conclusion, our data document that the timing of puberty is sensitive to both overfeeding and subnutrition during early (postnatal) periods and suggest that alterations in hypothalamic expression of Kiss1/kisspeptin may underlie at least part of such programming phenomenon.

Transcriptional regulation of the human KiSS1 gene

Kisspeptin, the product of the KiSS1 gene, has emerged as a key component of the mechanism by which the hypothalamus controls puberty and reproductive development. It does so by stimulating the secretion of gonadotropin releasing hormone (GnRH). Little is known about the transcriptional control of the KiSS1 gene. Here we show that a set of proteins postulated to be upstream components of a hypothalamic network involved in controlling female puberty regulates KiSS1 transcriptional activity. Using RACE-PCR we determined that transcription of KiSS1 mRNA is initiated at a single transcription start site (TSS) located 153-156bp upstream of the ATG translation initiation codon. Promoter assays performed using 293 MSR cells showed that the KiSS1 promoter is activated by TTF1 and CUX1-p200, and repressed by EAP1, YY1, and CUX1-p110. EAP1 and CUX-110 were also repressive in GT1-7 cells. All four TFs are recruited in vivo to the KiSS1 promoter and are expressed in kisspeptin neurons. These results suggest that expression of the KiSS1 gene is regulated by trans-activators and repressors involved in the system-wide control of mammalian puberty.

Kisspeptin-10 is a potent stimulator of LH and increases pulse frequency in men

CONTEXT

Kisspeptins stimulate GnRH and thus gonadotropin secretion. Kisspeptin-10 is the minimal kisspeptin sequence with full intrinsic bioactivity, but it has not been studied in man.

OBJECTIVE

We investigated our hypothesis that kisspeptin-10 increases GnRH and thus LH pulse frequency.

DESIGN AND PARTICIPANTS

The dose response of kisspeptin-10 was investigated by administering iv bolus doses (0.01-3.0 μg/kg) and vehicle to healthy men. Effects on LH pulse frequency and size were determined by deconvolution analysis during infusion of kisspeptin-10 for up to 22.5 h.

RESULTS

Intravenous bolus kisspeptin-10 resulted in a rapid and dose-dependent rise in serum LH concentration, with maximal stimulation at 1 μg/kg (4.1 ± 0.4 to 12.4 ± 1.7 IU/liter at 30 min, P < 0.001, n = 6). Administration of 3 μg/kg elicited a reduced response vs. 1 μg/kg (P < 0.05). Infusion of kisspeptin-10 at 4 μg/kg · h for 22.5 h elicited an increase in LH from a mean of 5.4 ± 0.7 to 20.8 ± 4.9 IU/liter (n = 4; P < 0.05) and serum testosterone increased from 16.6 ± 2.4 to 24.0 ± 2.5 nmol/liter (P < 0.001). LH pulses were obscured at this high rate of secretion, but a lower dose infusion of kisspeptin-10 (1.5 μg/kg · h) increased mean LH from 5.2 ± 0.8 to 14.1 ± 1.7 IU/liter (n = 4; P < 0.01) and increased LH pulse frequency from 0.7 ± 0.1 to 1.0 ± 0.2 pulses/h (P < 0.05) and secretory burst mass from 3.9 ± 0.4 to 12.8 ± 2.6 IU/liter (P < 0.05).

CONCLUSIONS

Kisspeptin-10 boluses potently evoke LH secretion in men, and continuous infusion increases testosterone, LH pulse frequency, and pulse size. Kisspeptin analogues have therapeutic potential as regulators of LH and thus testosterone secretion.

Control of GnRH secretion: one step back

The reproductive system is controlled by gonadotropin releasing hormone (GnRH) secretion from the brain, which is finely modulated by a number of factors including gonadal sex steroids. GnRH cells do not express estrogen receptor α, but feedback is transmitted by neurons that are at least 'one step back' from the GnRH cells. Modulation by season, stress and nutrition are effected by neuronal pathways that converge on the GnRH cells. Kisspeptin and gonadotropin inhibitory hormone (GnIH) neurons are regulators of GnRH secretion, the former being a major conduit for transmission of sex steroid feedback. GnIH cells project to GnRH cells and may play a role in the seasonal changes in reproductive activity in sheep. GnIH also modulates the action of GnRH at the level of the pituitary gonadotrope. This review focuses on the role that kisspeptin and GnIH neurons play, as modulators that are 'one step back' from GnRH neurons.

Atrazine and breast cancer: a framework assessment of the toxicological and epidemiological evidence

The causal relationship between atrazine exposure and the occurrence of breast cancer in women was evaluated using the framework developed by Adami et al. (2011) wherein biological plausibility and epidemiological evidence were combined to conclude that a causal relationship between atrazine exposure and breast cancer is “unlikely”. Carcinogenicity studies in female Sprague-Dawley (SD) but not Fischer-344 rats indicate that high doses of atrazine caused a decreased latency and an increased incidence of combined adenocarcinoma and fibroadenoma mammary tumors. There were no effects of atrazine on any other tumor type in male or female SD or Fischer-344 rats or in three strains of mice. Seven key events that precede tumor expression in female SD rats were identified. Atrazine induces mammary tumors in aging female SD rats by suppressing the luteinizing hormone surge, thereby supporting a state of persistent estrus and prolonged exposure to endogenous estrogen and prolactin. This endocrine mode of action has low biological plausibility for women because women who undergo reproductive senescence have low rather than elevated levels of estrogen and prolactin. Four alternative modes of action (genotoxicity, estrogenicity, upregulation of aromatase gene expression or delayed mammary gland development) were considered and none could account for the tumor response in SD rats. Epidemiological studies provide no support for a causal relationship between atrazine exposure and breast cancer. This conclusion is consistent with International Agency for Research on Cancer’s classification of atrazine as “unclassifiable as to carcinogenicity” and the United States Environmental Protection Agency's classification of atrazine as “not likely to be carcinogenic.”

Gene structure analysis of kisspeptin-2 (Kiss2) in the Senegalese sole (Solea senegalensis): characterization of two splice variants of Kiss2, and novel evidence for metabolic regulation of kisspeptin signaling in non-mammalian species

Kisspeptin signaling in the brain is involved in the control of the onset of puberty in vertebrates. In this study, we present novel evidence indicating that kisspeptin may link energy balance and reproduction. For that purpose, we determined the complete gene structure of kisspeptin in a teleost fish, the Senegalese sole (Ss). In contrast to the situation evident in several fish, in this species only Kiss2 was found. Yet, two Ss Kiss2 isoforms generated by alternative splicing through intronic retention were detected: Ss Kiss2_v1, producing the functional protein, and Ss Kiss2_v2, coding for a truncated, non-functional protein. Specific qPCRs showed that the expression of these two isoforms varied differently in brain and gonads throughout maturation. In addition, and in contrast to what has been observed in mammals, fasting increased hypothalamic mRNA levels of Ss Kiss2_v1, which also caused a concomitant rise in pituitary Ss LH and Ss FSH mRNA. Together, these data indicate the impact of the nutritional status on Kiss mRNA expression as a potential regulatory mechanism for the metabolic control of reproduction in non-mammalian species, albeit with some significant differences with respect to the situation described in mammals.

Effects of kisspeptin on parameters of the HPA axis

The hypothalamo-pituitary-adrenal (HPA) and hypothalamo-pituitary-gonadal (HPG) axes have an intricate cross talk that results in the inhibition of reproductive functions during periods of chronic physiological or psychological stress. Recent studies have shown that kisspeptin neurons have projections to many non-reproductive areas of the brain including the paraventricular nucleus (PVN) of the hypothalamus, thereby providing evidence of an anatomical framework for kisspeptin to regulate the HPA axis. In this study, we tested as to whether kisspeptin modulates the HPA axis at three potential levels of regulation: (1) transcription of stress-related genes CRH, AVP, and oxytocin (OXY); (2) release of neuropeptides from PVN-derived neuronal cells via mobilization of intracellular calcium stores; and (3) in vivo regulation of the HPA axis under basal and stress-induced conditions in adult male rats. Overall, our data showed that kisspeptin did not alter basal, or stress-induced HPA axis activity (plasma corticosterone (CORT) and adrenocorticotropin hormone (ACTH)) in adult male rats and had modest, yet significant effects on CRH, AVP, and OXY gene expressions.

Projections of arcuate nucleus and rostral periventricular kisspeptin neurons in the adult female mouse brain

The important role of kisspeptin neurons in the regulation of GnRH neuron activity is now well accepted. However, the ways in which kisspeptin neurons located in the arcuate nucleus (ARN) and rostral periventricular area of the third ventricle (RP3V) control GnRH neurons are poorly understood. The present study used anterograde and retrograde tracing techniques to establish the neuronal projection patterns of kisspeptin cell populations in the female mouse brain. Anterograde tracing studies revealed that kisspeptin neurons in the ARN innervated a wide number of hypothalamic and associated limbic region nuclei, whereas RP3V kisspeptin neurons projected to a smaller number of mostly medially located hypothalamic nuclei. Retrograde tracing confirmed a major projection of RP3V kisspeptin neurons to the ARN and showed that kisspeptin neurons located in the rostral half of the ARN projected to the rostral preoptic area. Peripheral administration of Fluorogold was found to label the majority of GnRH neurons but no kisspeptin neurons. Together, these studies highlight the complexity of the brain kisspeptin neuronal system and indicate that both ARN and RP3V kisspeptin neurons participate in a variety of limbic functions. In relation to the GnRH neuronal network, these investigations demonstrate that, alongside the RP3V kisspeptin cells, rostral ARN kisspeptin neurons may also project to GnRH neuron cell bodies. However, no kisspeptin neurons innervate GnRH nerve terminals in the external layer of the median eminence. These studies provide a neuroanatomical framework for the further elucidation of the functions of the ARN and RP3V kisspeptin neuron populations.

Chronic exposure to anabolic androgenic steroids alters activity and synaptic function in neuroendocrine control regions of the female mouse

Disruption of reproductive function is a hallmark of abuse of anabolic androgenic steroids (AAS) in female subjects. To understand the central actions of AAS, patch clamp recordings were made in estrous, diestrous and AAS-treated mice from gonadotropin releasing hormone (GnRH) neurons, neurons in the medial preoptic area (mPOA) and neurons in the anteroventroperiventricular nucleus (AVPV); regions known to provide GABAergic and kisspeptin inputs to the GnRH cells. Action potential (AP) frequency was significantly higher in GnRH neurons of estrous mice than in AAS-treated or diestrous animals. No significant differences in AAS-treated, estrous or diestrous mice were evident in the amplitude or kinetics of spontaneous postsynaptic currents (sPSCs), miniature PSCs or tonic currents mediated by GABA(A) receptors or in GABA(A) receptor subunit expression in GnRH neurons. In contrast, the frequency of GABA(A) receptor-mediated sPSCs in GnRH neurons showed an inverse correlation with AP frequency across the three hormonal states. Surprisingly, AP activity in the medial preoptic area (mPOA), a likely source of GABAergic afferents to GnRH cells, did not vary in concert with the sPSCs in the GnRH neurons. Furthermore, pharmacological blockade of GABA(A) receptors did not alter the pattern in which there was lower AP frequency in GnRH neurons of AAS-treated and diestrous versus estrous mice. These data suggest that AAS do not impose their effects either directly on GnRH neurons or on putative GABAergic afferents in the mPOA. AP activity recorded from neurons in kisspeptin-rich regions of the AVPV and the expression of kisspeptin mRNA and peptide did vary coordinately with AP activity in GnRH neurons. Our data demonstrate that AAS treatment imposes a "diestrous-like" pattern of activity in GnRH neurons and suggest that this effect may arise from suppression of presynaptic kisspeptin-mediated excitatory drive arising from the AVPV. The actions of AAS on neuroendocrine regulatory circuits may contribute the disruption of reproductive function observed in steroid abuse.

Reproductive tract function and dysfunction in women

An appropriately developed and functional reproductive tract is essential for the natural production of offspring. However, abnormal development or disease, either within or external to the reproductive tract, can cause considerable morbidity and mortality. This Review describes the normal function of each section of the female reproductive tract. Some of the major conditions and diseases that cause reproductive tract dysfunction in women of reproductive age are reviewed, with a focus on endocrine-related dysfunction. As the female reproductive tract is vital for the continuation of the human species, it is crucial to have an improved understanding of how the cells and tissues of this system interact under normal conditions and to define how hormonal and genetic abnormalities cause pathophysiologic derangement of reproduction.