Critical role of arcuate nucleus kisspeptin and Kiss1R in regulation of the ovine luteinizing hormone surge

Hypothalamic kisspeptin (Kiss), neurokinin B (NKB), and dynorphin-containing (KNDy) neurons in the arcuate nucleus (ARC) have consistently been shown to be the central generator of gonadotropin-releasing hormone (GnRH) and corresponding luteinizing hormone (LH) pulses in mammals and possibly contribute to surge secretion as well. Additionally, recent evidence from experiments in sheep suggests that ARC Kiss1R-containing neurons play an important role in regulating the timing and amplitude of LH pulses. In this study, we examined the functional role of ARC KNDy and Kiss1R-containing neurons in ovine LH surge secretion via injection of saporin-ligand conjugates (SAP) to ablate these neural populations. NKB-SAP injections significantly reduced the percentage of ARC Kiss1 (~65% decrease) cells compared to control animals, and a surge-like increase of LH was prevented in ewes with the greatest degree of Kiss1 cell ablation. Kiss-SAP injections had no effect on Kiss1 cell percentage or ARC Kiss1R cell number compared to controls, the latter perhaps due to Kiss1R suppression in control animals from elevated estradiol concentrations during the LH surge. However, Kiss-SAP injections consistently and robustly decreased LH surge amplitude, with 80% of Kiss-SAP-treated ewes failing to generate a surge. While the exact identity of these ARC Kiss1R neurons has yet to be fully elucidated, they likely act downstream or in concert with KNDy neurons and possibly integrate other surge-centric signaling pathways to generate the ovine LH surge. These results support the conclusion that KNDy neurons contribute significantly to the ovine LH surge, while ARC Kiss1R neurons appear to be necessary for a functional surge to occur in sheep.

Morphological and functional evidence for sexual dimorphism in neurokinin B signalling in the retrochiasmatic area of sheep

Neurokinin B (NKB) is critical for fertility in humans and stimulates gonadotrophin-releasing hormone/luteinising hormone (LH) secretion in several species, including sheep. There is increasing evidence that the actions of NKB in the retrochiasmatic area (RCh) contribute to the induction of the preovulatory LH surge in sheep. In the present study, we determined whether there are sex differences in the response to RCh administration of senktide, an agonist to the NKB receptor (neurokinin receptor-3 [NK3R]), and in NKB and NK3R expression in the RCh of sheep. To normalise endogenous hormone concentrations, animals were gonadectomised and given implants to mimic the pattern of ovarian steroids seen in the oestrous cycle. In females, senktide microimplants in the RCh produced an increase in LH concentrations that lasted for at least 8 hours after the start of treatment, whereas a much shorter increment (approximately 2 hours) was seen in males. We next collected tissue from gonadectomised lambs 18 hours after the insertion of oestradiol implants that produce an LH surge in female, but not male, sheep for immunohistochemical analysis of NKB and NK3R expression. As expected, there were more NKB-containing neurones in the arcuate nucleus of females than males. Interestingly, there was a similar sexual dimorphism in NK3R-containing neurones in the RCh, NKB-containing close contacts onto these RCh NK3R neurones, and overall NKB-positive fibres in this region. These data demonstrate that there are both functional and morphological sex differences in NKB-NK3R signalling in the RCh and raise the possibility that this dimorphism contributes to the sex-dependent ability of oestradiol to induce an LH surge in female sheep.

Evidence that Nitric Oxide Is Critical for LH Surge Generation in Female Sheep

Elevated and sustained estradiol concentrations cause a gonadotropin-releasing hormone (GnRH) and luteinizing hormone (LH) surge that is necessary for ovulation. In sheep, several different neural systems have been implicated in this stimulatory action of estradiol and this study focused on somatostatin (SST) neurons in the ventral lateral region of the ventral medial nucleus (vlVMN) which express c-Fos during the surge. First, we determined if increased activity of SST neurons could be related to elevated GnRH secretion by assessing SST synapses onto GnRH neurons and neurons coexpressing kisspeptin, neurokinin B, dynorphin (KNDy). We found that the percentage of preoptic area GnRH neurons that receive SST input increased during the surge compared with other phases of the cycle. However, since SST is generally inhibitory, and pharmacological manipulation of SST signaling did not alter the LH surge in sheep, we hypothesized that nitric oxide (NO) was also produced by these neurons to account for their activation during the surge. In support of this hypothesis we found that (1) the majority of SST cells in the vlVMN (>80%) contained neuronal nitric oxide synthase (nNOS); (2) the expression of c-Fos in dual-labeled SST-nNOS cells, but not in single-labeled cells, increased during the surge compared with other phases of the cycle; and (3) intracerebroventricular (ICV) infusion of the nitric oxide synthase inhibitor, N(G)-nitro-L-arginine methyl ester, completely blocked the estrogen-induced LH surge. These data support the hypothesis that the population of SST-nNOS cells in the vlVMN are a source of NO that is critical for the LH surge, and we propose that they are an important site of estradiol positive feedback in sheep.

Role of kisspeptin neurons as a GnRH surge generator: Comparative aspects in rodents and non-rodent mammals

Ovulation is an essential phenomenon for reproduction in mammalian females along with follicular growth. It is well established that gonadal function is controlled by the neuroendocrine system called the hypothalamus-pituitary-gonadal (HPG) axis. Gonadotropin-releasing hormone (GnRH) neurons, localized in the hypothalamus, had been considered to be the head in governing the HPG axis for a long time until the discovery of kisspeptin. In females, induction of ovulation and folliculogenesis has been linked to a surge mode and pulse mode of GnRH releases, respectively. The mechanisms of how the two modes of GnRH are differently regulated had long remained elusive. The discovery of kisspeptin neurons, distributed in two hypothalamic nuclei, such as the arcuate nucleus in the caudal hypothalamus and preoptic area or the anteroventral periventricular nucleus in the rostral hypothalamic regions, and analyses of the detailed functions of kisspeptin neurons have led marked progress on the understanding of different mechanisms regulating GnRH surges (ovulation) and GnRH pulses (folliculogenesis). The present review will focus on the role of kisspeptin neurons as the GnRH surge generator, including the sexual differentiation of the surge generation system and factors that regulate the surge generator. Comparative aspects between mammalian species are especially focused on.

Polycystic ovary syndrome: Understanding the role of the brain

Polycystic ovary syndrome (PCOS) is a prevalent endocrine disorder and the leading cause of anovulatory infertility. Characterised by hyperandrogenism, menstrual dysfunction and polycystic ovaries, PCOS is a broad-spectrum disorder unlikely to stem from a single common origin. Although commonly considered an ovarian disease, the brain is now a prime suspect in both the ontogeny and pathology of PCOS. We discuss here the neuroendocrine impairments present in PCOS that implicate involvement of the brain and review evidence gained from pre-clinical models of the syndrome about the specific brain circuitry involved. In particular, we focus on the impact that developmental androgen excess and adult hyperandrogenemia have in programming and regulating brain circuits important in the central regulation of fertility. The studies discussed here provide compelling support for the importance of the brain in PCOS ontogeny and pathophysiology and highlight the need for a better understanding of the underlying mechanisms involved.

KNDy neurone activation prior to the LH surge of the ewe is disrupted by LPS

In the ewe, steroid hormones act on the hypothalamic arcuate nucleus (ARC) to initiate the GnRH/LH surge. Within the ARC, steroid signal transduction may be mediated by estrogen receptive dopamine-, β-endorphin- or neuropeptide Y (NPY)-expressing cells, as well as those co-localising kisspeptin, neurokinin B (NKB) and dynorphin (termed KNDy). We investigated the time during the follicular phase when these cells become activated (i.e., co-localise c-Fos) relative to the timing of the LH surge onset and may therefore be involved in the surge generating mechanism. Furthermore, we aimed to elucidate whether these activation patterns are altered after lipopolysaccharide (LPS) administration, which is known to inhibit the LH surge. Follicular phases of ewes were synchronised by progesterone withdrawal and blood samples were collected every 2 h. Hypothalamic tissue was retrieved at various times during the follicular phase with or without the administration of LPS (100 ng/kg). The percentage of activated dopamine cells decreased before the onset of sexual behaviour, whereas activation of β-endorphin decreased and NPY activation tended to increase during the LH surge. These patterns were not disturbed by LPS administration. Maximal co-expression of c-Fos in dynorphin immunoreactive neurons was observed earlier during the follicular phase, compared to kisspeptin and NKB, which were maximally activated during the surge. This indicates a distinct role for ARC dynorphin in the LH surge generation mechanism. Acute LPS decreased the percentage of activated dynorphin and kisspeptin immunoreactive cells. Thus, in the ovary-intact ewe, KNDy neurones are activated prior to the LH surge onset and this pattern is inhibited by the administration of LPS.

The luteinising hormone surge-generating system is functional in male goats as in females: involvement of kisspeptin neurones in the medial preoptic area

A luteinising hormone (LH) surge is fundamental to the induction of ovulation in mammalian females. The administration of a preovulatory level of oestrogen evokes an LH surge in ovariectomised females, whereas the response to oestrogen in castrated males differs among species; namely, the LH surge-generating system is sexually differentiated in some species (e.g. rodents and sheep) but not in others (e.g. primates). In the present study, we aimed to determine whether there is a functional LH surge-generating system in male goats, and whether hypothalamic kisspeptin neurones in male goats are involved in the regulation of surge-like LH secretion. By i.v. infusion of oestradiol (E2; 6 μg/h) for 16 h, a surge-like LH increase occurred in both castrated male and ovariectomised female goats, although the mean peak LH concentration was lower and the mean peak of the LH surge was later in males compared to females. Dual staining with KISS1 in situ hybridisation and c-Fos immunohistochemistry revealed that E2 treatment significantly increased c-Fos expression in the medial preoptic area (mPOA) KISS1 cells in castrated males, as well as ovariectomised females. By contrast, dual-labelled cells were scarcely detected in the arcuate nucleus (ARC) after E2 treatment in both sexes. These data suggest that kisspeptin neurones in the mPOA, but not those in the ARC, are involved in the induction of surge-like LH secretion in both male and female goats. In summary, our data show that the mechanism that initiates the LH surge in response to oestrogen, the mPOA kisspeptin neurones, is functional in male goats. Thus, sexual differentiation of the LH surge-generating system would not be applicable to goats.

Co-expression of c-Fos with oestradiol receptor α or somatostatin in the arcuate nucleus, ventromedial nucleus and medial preoptic area in the follicular phase of intact ewes: alteration after insulin-induced hypoglycaemia

The aim of this study was to investigate how acute insulin-induced hypoglycaemia (IIH) alters the activity of cells containing oestradiol receptor α (ERα) or somatostatin (SST) in the arcuate nucleus (ARC) and ventromedial nucleus (VMN), and ERα cells in the medial preoptic area (mPOA) of intact ewes. Follicular phases were synchronized with progesterone vaginal pessaries. Control animals were killed at 0 h or 31 h (n = 5 and 6, respectively) after progesterone withdrawal (PW; time zero). At 28 h, five other animals received insulin (INS; 4 iu/kg) and were subsequently killed at 31 h. Hypothalamic sections were immunostained for ERα or SST each with c-Fos, a marker of neuronal transcriptional activation. Insulin did not alter the percentage of activated ERα cells in the ARC; however, it appeared visually that two insulin-treated animals (INS responders, with no LH surge) had an increase in the VMN (from 32 to 78%) and a decrease in the mPOA (from 40 to 12%) compared to no increase in the two INS non-responders (with an LH surge). The percentage of activated SST cells in the ARC was greater in all four insulin-treated animals (from 10 to 60%), whereas it was visually estimated that activated SST cells in the VMN increased only in the two insulin responders (from 10 to 70%). From these results, we suggest that IIH stimulates SST activation in the ARC as part of the glucose-sensing mechanism but ERα activation is unaffected in this region. We present evidence to support a hypothesis that disruption of the GnRH/LH surge may occur in insulin responders via a mechanism that involves, at least in part, SST cell activation in the VMN along with decreased ERα cell activation in the mPOA.

Activation of cells containing estrogen receptor alpha or somatostatin in the medial preoptic area, arcuate nucleus, and ventromedial nucleus of intact ewes during the follicular phase, and alteration after lipopolysaccharide

Cells in the medial preoptic area (mPOA), arcuate nucleus (ARC), and ventromedial nucleus (VMN) that possess estrogen receptor alpha (ER alpha) mediate estradiol feedback to regulate endocrine and behavioral events during the estrous cycle. A percentage of ER alpha cells located in the ARC and VMN express somatostatin (SST) and are activated in response to estradiol. The aims of the present study were to investigate the location of c-Fos, a marker for activation, in cells containing ER alpha or SST at various times during the follicular phase and to determine whether lipopolysaccharide (LPS) administration, which leads to disruption of the luteinizing hormone (LH) surge, is accompanied by altered ER alpha and/or SST activation patterns. Follicular phases were synchronized with progesterone vaginal pessaries, and control animals were killed at 0, 16, 31, and 40 h (n = 4-6/group) after progesterone withdrawal (PW [time 0]). At 28 h, other animals received LPS (100 ng/kg) and were subsequently killed at 31 h or 40 h (n = 5/group). Hypothalamic sections were immunostained for c-Fos and ER alpha or SST. LH surges occurred only in control ewes with onset at 36.7 ± 1.3 h after PW; these animals had a marked increase in the percentage of ER alpha cells that colocalized c-Fos (%ER alpha/c-Fos) in the ARC and mPOA from 31 h after PW and throughout the LH surge. In the VMN, %ER alpha/c-Fos was higher in animals that expressed sexual behavior than in those that did not. SST cell activation in the ARC and VMN was greater during the LH surge than in other stages in the follicular phase. At 31 or 40 h after PW (i.e., 3 or 12 h after treatment, respectively), LPS decreased %ER alpha/c-Fos in the ARC and the mPOA, but there was no change in the VMN compared to that in controls. The %SST/c-Fos increased in the VMN at 31 h after PW (i.e., 3 h after LPS) with no change in the ARC compared to controls. These results indicate that there is a distinct temporal pattern of ER alpha cell activation in the hypothalamus during the follicular phase, which begins in the ARC and mPOA at least 6-7 h before the LH surge onset and extends to the VMN after the onset of sexual behavior and LH surge. Furthermore, during the surge, some of these ER alpha-activated cells may be SST-secreting cells. This pattern is markedly altered by LPS administered during the late follicular phase, indicating that the disruptive effects of this stressor are mediated by suppressing ER alpha cell activation at the level of the mPOA and ARC and enhancing SST cell activation in the VMN, leading to the attenuation of the LH surge.

Nonhuman primate models of polycystic ovary syndrome

With close genomic and phenotypic similarity to humans, nonhuman primate models provide comprehensive epigenetic mimics of polycystic ovary syndrome (PCOS), suggesting early life targeting for prevention. Fetal exposure to testosterone (T), of all nonhuman primate emulations, provides the closest PCOS-like phenotypes, with early-to-mid gestation T-exposed female rhesus monkeys exhibiting adult reproductive, endocrinological and metabolic dysfunctional traits that are co-pathologies of PCOS. Late gestational T exposure, while inducing adult ovarian hyperandrogenism and menstrual abnormalities, has less dysfunctional metabolic accompaniment. Fetal exposures to dihydrotestosterone (DHT) or diethylstilbestrol (DES) suggest androgenic and estrogenic aspects of fetal programming. Neonatal exposure to T produces no PCOS-like outcome, while continuous T treatment of juvenile females causes precocious weight gain and early menarche (high T), or high LH and weight gain (moderate T). Acute T exposure of adult females generates polyfollicular ovaries, while chronic T exposure induces subtle menstrual irregularities without metabolic dysfunction.

Developmental programming: postnatal steroids complete prenatal steroid actions to differentially organize the GnRH surge mechanism and reproductive behavior in female sheep

In female sheep, estradiol (E2) stimulates the preovulatory GnRH/LH surge and receptive behavior, whereas progesterone blocks these effects. Prenatal exposure to testosterone disrupts both the positive feedback action of E2 and sexual behavior although the mechanisms remain unknown. The current study tested the hypothesis that both prenatal and postnatal steroids are required to organize the surge and sex differences in reproductive behavior. Our approach was to characterize the LH surge and mating behavior in prenatally untreated (Control) and testosterone-treated (T) female sheep subsequently exposed to one of three postnatal steroid manipulations: endogenous E2, excess E2 from a chronic implant, or no E2 due to neonatal ovariectomy (OVX). All females were then perfused at the time of the expected surge and brains processed for estrogen receptor and Fos immunoreactivity. None of the T females exposed postnatally to E2 exhibited an E2-induced LH surge, but a surge was produced in five of six T/OVX and all Control females. No surges were produced when progesterone was administered concomitantly with E2. All Control females were mounted by males, but significantly fewer T females were mounted by a male, including the T/OVX females that exhibited LH surges. The percentage of estrogen receptor neurons containing Fos was significantly influenced in a brain region-, developmental stage-, and steroid-specific fashion by testosterone and E2 treatments. These findings support the hypothesis that the feedback controls of the GnRH surge are sensitive to programming by prenatal and postnatal steroids in a precocial species.

Developmental programming: prenatal androgen exposure alters the gonadotroph population of the ovine pituitary gland

In utero exposure of the female foetus to androgens during development disrupts the reproductive axis and results in hypersecretion of luteinising hormone (LH) (but not follicle-stimulating hormone) in postnatal life. Abnormalities in the neural circuits controlling hypothalamic gonadotrophin-releasing hormone have been documented; however, androgens could also programme abnormalities in the pituitary gland. Ovine foetuses were exposed to either testosterone propionate or the non-aromatisable androgen dihydro-testosterone from days 30-90 of gestation (term 147 days) and the effects on the functional morphology of the pituitary were determined. Exogenous testosterone propionate exposure resulted in pituitary glands in adult male and female sheep that were 40% heavier than controls. Because this effect was not observed in the dihydro-testosterone-exposed animals, these actions are mediated via the oestrogen receptor (ER). No significant differences were apparent in 90- or 140-day foetuses. There was no difference between control and androgen-exposed animals in the density of LHβ or ERα immunoreactive cells in the pituitary although the density of follicle-stimulating hormone-β immunoreactive cells was lower in the testosterone-treated animals. The percentage of cells co-localising LHβ and ERα was lower in the testosterone-treated ewes and this may, in part, explain a reduced ability to respond to steroid feedback. Thus, enlargement of the pituitary gland, coupled with a reduced sensitivity to oestrogen negative-feedback, may contribute to the hyper-secretion of LH observed in animals that have been exposed to excess androgens during foetal life.

Developmental programming: reproductive endocrinopathies in the adult female sheep after prenatal testosterone treatment are reflected in altered ontogeny of GnRH afferents

The GnRH system represents a useful model of long-term neural plasticity. An unexplored facet of this plasticity relates to the ontogeny of GnRH neural afferents during critical periods when the hypothalamic-pituitary-gonadal axis is highly susceptible to perturbation by sex steroids. Sheep treated with testosterone (T) in utero exhibit profound reproductive neuroendocrine dysfunctions during their lifespan. The current study tested the hypothesis that these changes are associated with alterations in the normal ontogeny of GnRH afferents and glial associations. Adult pregnant sheep (n=50) were treated with vehicle [control (CONT)] or T daily from gestational day (GD)30 to GD90. CONT and T fetuses (n=4-6/treatment per age group) were removed by cesarean section on GD90 and GD140 and the brains frozen at -80°C. Brains were also collected from CONT and T females at 20-23 wk (prepubertal), 10 months (normal onset of puberty and oligo-anovulation), and 21 months (oligo-anovulation in T females). Tissue was analyzed for GnRH immunoreactivity (ir), total GnRH afferents (Synapsin-I ir), glutamate [vesicular glutamate transporter-2 (VGLUT2)-ir], and γ-aminobutyric acid [GABA, vesicular GABA transporter (VGAT)-ir] afferents and glial associations (glial fibrillary acidic protein-ir) with GnRH neurons using optical sectioning techniques. The results revealed that: 1) GnRH soma size was slightly reduced by T, 2) the total (Synapsin-I) GnRH afferents onto both somas and dendrites increased significantly with age and was reduced by T, 3) numbers of both VGAT and VGLUT inputs increased significantly with age and were also reduced by T, and 4) glial associations with GnRH neurons were reduced (<10%) by T. Together, these findings reveal a previously unknown developmental plasticity in the GnRH system of the sheep. The altered developmental trajectory of GnRH afferents after T reinforces the notion that prenatal programming plays an important role in the normal development of the reproductive neuroendocrine axis.