Neurokinin B signaling in the female rat: a novel link between stress and reproduction

Genome sequencing projects reveal new insights into the mammalian Gonadotropin-releasing Hormone II system

The type II gonadotropin-releasing hormone (GnRH-II) was first discovered in chicken (Gallus gallus) brain and then shown to be present in many vertebrates. Indeed, its structure is conserved unchanged throughout vertebrate evolution from teleost fish through to mammals suggesting a crucial function. Yet the functional significance has been largely unexplored. Studies in comparative endocrinology show that the GnRH-II system is differentially functional in mammalian species. Intact GnRH-II neuropeptide and receptor genes (GnRH2 and GnRH receptor 2 GnRHR2) occur in marmoset monkeys (Callithrix jacchus), musk shrews (Suncus murinus) and pigs (Sus scrofa). However, one or other or both of these genes are inactivated in other species, where mutations or remnants affecting GnRH2 neuropeptide and/or type II GnRHR exons are retained in conserved genomic loci. New data from DNA sequencing projects facilitate extensive analysis of species-specific variation in these genes. Here, we describe GnRH2 and GnRHR2 genes spanning a collection of 21 taxonomic orders, encompassing around 140 species from Primates, Scandentia, Eulipotyphla, Rodentia, Lagomorpha, Artiodactyla, Carnivora, Perissodactyls, Pholidota, Chiroptera, Afrotheria, Xenarthra and Marsupialia. Intact coding exons for both GnRH2 and GnRHR2 occur in monkeys, tree shrews, shrews, moles, hedgehogs, several rodents (degu, kangaroo-rat, pocket mouse), pig, pecarry and warthog, camels and alpaca, bears, Weddell seal, hyena, elephant, aardvark and marsupials. Inactivating mutations affecting GnRH2 and GnRHR2, some located at conserved sites within exons, occur in species of primates, most rodents, lagomorphs, bovidae, cetaceans, felidae, canidae and other carnivora, pangolins, most bats, armadillo, brushtail and echidna. A functional GnRH-II system appears retained within several taxonomic families of mammals, but intact retention does not extend to whole taxonomic orders. Defining how endogenous GnRH-II neuropeptide operates in different mammals may afford functional insight into its actions in the brain, especially as, unlike the type I GnRH system, it is expressed in the mid brain and not the hypothalamus.

NK3R signalling in the posterodorsal medial amygdala is involved in stress-induced suppression of pulsatile LH secretion in female mice

Psychosocial stress negatively impacts reproductive function by inhibiting pulsatile luteinizing hormone (LH) secretion. The posterodorsal medial amygdala (MePD) is responsible in part for processing stress and modulating the reproductive axis. Activation of the neurokinin 3 receptor (NK3R) suppresses the gonadotropin-releasing hormone (GnRH) pulse generator, under hypoestrogenic conditions, and NK3R activity in the amygdala has been documented to play a role in stress and anxiety. We investigate whether NK3R activation in the MePD is involved in mediating the inhibitory effect of psychosocial stress on LH pulsatility in ovariectomised female mice. First, we administered senktide, an NK3R agonist, into the MePD and monitored the effect on pulsatile LH secretion. We then delivered SB222200, a selective NK3R antagonist, intra-MePD in the presence of predator odour, 2,4,5-trimethylthiazole (TMT) and examined the effect on LH pulses. Senktide administration into the MePD dose-dependently suppresses pulsatile LH secretion. Moreover, NK3R signalling in the MePD mediates TMT-induced suppression of the GnRH pulse generator, which we verified using a mathematical model. The model verifies our experimental findings: (i) predator odour exposure inhibits LH pulses, (ii) activation of NK3R in the MePD inhibits LH pulses and (iii) NK3R antagonism in the MePD blocks stressor-induced inhibition of LH pulse frequency in the absence of ovarian steroids. These results demonstrate for the first time that NK3R neurons in the MePD mediate psychosocial stress-induced suppression of the GnRH pulse generator.

Early life adversity accelerates hypothalamic drive of pubertal timing in female rats with associated enhanced acoustic startle

Early life adversity in the form of childhood maltreatment in humans or as modeled by maternal separation (MS) in rodents is often associated with an earlier emergence of puberty in females. Earlier pubertal initiation is an example of accelerated biological aging and predicts later risk for anxiety in women, especially in populations exposed to early life trauma. Here we investigated external pubertal markers as well as hypothalamic gene expression of pubertal regulators kisspeptin and gonadotropin-releasing hormone, to determine a biological substrate for MS-induced accelerated puberty. We further investigated a mechanism by which developmental stress might regulate pubertal timing. As kisspeptin and gonadotropin-releasing hormone secretion are typically inhibited by corticotropin releasing hormone at its receptor CRH-R1, we hypothesized that MS induces a downregulation of Crhr1 gene transcription in a cell-specific manner. Finally, we explored the association between pubertal timing and anxiety-like behavior in an acoustic startle paradigm, to drive future preclinical research linking accelerated puberty and anxiety. We replicated previous findings that MS leads to earlier puberty in females but not males, and found expression of kisspeptin and gonadotropin-releasing hormone mRNA to be prematurely increased in MS females. RNAscope confirmed increased expression of these genes, and further revealed that kisspeptin-expressing neurons in females were less likely to express Crhr1 after MS. Early puberty was associated with higher acoustic startle magnitude in females. Taken together, these findings indicate precocial maturation of central pubertal timing mechanisms after MS, as well as a potential role of CRH-R1 in these effects and an association with a translational measure of anxiety.

Impact of stress on male fertility: role of gonadotropin inhibitory hormone

Studies have implicated oxidative stress-sensitive signaling in the pathogenesis of stress-induced male infertility. However, apart from oxidative stress, gonadotropin inhibitory hormone (GnIH) plays a major role. The present study provides a detailed review of the role of GnIH in stress-induced male infertility. Available evidence-based data revealed that GnIH enhances the release of corticosteroids by activating the hypothalamic-pituitary-adrenal axis. GnIH also mediates the inhibition of the conversion of thyroxine (T4) to triiodothyronine (T3) by suppressing the hypothalamic-pituitary-thyroidal axis. In addition, GnIH inhibits gonadotropin-releasing hormone (GnRH), thus suppressing the hypothalamic-pituitary-testicular axis, and by extension testosterone biosynthesis. More so, GnIH inhibits kisspeptin release. These events distort testicular histoarchitecture, impair testicular and adrenal steroidogenesis, lower spermatogenesis, and deteriorate sperm quality and function. In conclusion, GnIH, via multiple mechanisms, plays a key role in stress-induced male infertility. Suppression of GnIH under stressful conditions may thus be a beneficial prophylactic and/or therapeutic strategy.

The impact of inflammatory stress on hypothalamic kisspeptin neurons: Mechanisms underlying inflammation-associated infertility in humans and domestic animals

Inflammatory diseases attenuate reproductive functions in humans and domestic animals. Lipopolysaccharide (LPS), an endotoxin released by bacteria, is known to disrupt female reproductive functions in various inflammatory diseases. LPS administration has been used to elucidate the impact of pathophysiological activation of the immune system on reproduction. Hypothalamic kisspeptin neurons are the master regulators of mammalian reproduction, mediating direct stimulation of hypothalamic gonadotropin-releasing hormone (GnRH) release and consequent release of gonadotropins, such as luteinizing hormone (LH) and follicle-stimulating hormone from the pituitary. The discovery of kisspeptin neurons in the mammalian hypothalamus has drastically advanced our understanding of how inflammatory stress causes reproductive dysfunction in both humans and domestic animals. Inflammation-induced ovarian dysfunction could be caused, at least partly, by aberrant GnRH and LH secretion, which is regulated by kisspeptin signaling. In this review, we focus on the effects of LPS on hypothalamic kisspeptin neurons to outline the impact of inflammatory stress on neuroendocrine regulation of mammalian reproductive systems. First, we summarize the attenuation of female reproduction by LPS during inflammation and the effects of LPS on ovarian and pituitary function. Second, we outline the inhibitory effects of LPS on pulsatile- and surge-mode GnRH/LH release. Third, we discuss the LPS-responsive hypothalamic-pituitary-adrenal axis and hypothalamic neural systems in terms of the cytokine-mediated pathway and the possible direct action of LPS via its hypothalamic receptors. This article describes the impact of LPS on hypothalamic kisspeptin neurons and the possible mechanisms underlying LPS-mediated disruption of LH pulses/surge via kisspeptin neurons.

Intrauterine LPS inhibited arcuate Kiss1 expression, LH pulses, and ovarian function in rats

In brief

Uterine inflammatory diseases are a major cause of infertility in humans and domestic animals. The current findings that intrauterine lipopolysaccharide is absorbed in systemic circulation and attenuates ovarian cyclic activities could provide a basis for developing novel treatments to improve fertility.

Abstract

Uterine inflammatory diseases are a major cause of infertility in humans and domestic animals. Circulating lipopolysaccharide (LPS), a bacterial endotoxin causing uterine inflammation, reportedly downregulates the hypothalamic-pituitary-gonadal axis to mediate ovarian dysfunction. In contrast, the mechanism whereby intrauterine LPS affects ovarian function has not been fully clarified. This study aimed to elucidate whether uterine exposure to LPS downregulates hypothalamic kisspeptin gene (Kiss1) expression, gonadotropin release, and ovarian function. Uterine inflammation was induced by intrauterine LPS administration to ovary-intact and ovariectomized female rats. As a result, plasma LPS concentrations were substantially higher in control rats until 48 h post injection, and the estrous cyclicity was disrupted with a prolonged diestrous phase. Three days post injection, the number of Graafian follicles and plasma estradiol concentration were reduced in LPS-treated rats, while numbers of Kiss1-expressing cells in the anteroventral periventricular nucleus and arcuate nucleus (ARC) were comparable in ovary-intact rats. Four days post injection, ovulation rate and plasma progesterone levels reduced significantly while gene expression of interleukin1β and tumor necrosis factor α was upregulated in the ovaries of LPS-treated rats that failed to ovulate. Furthermore, the number of Kiss1-expressing cells in the ARC and pulsatile luteinizing hormone (LH) release were significantly reduced in ovariectomized rats 24 h post injection. In conclusion, these results indicate that intrauterine LPS is absorbed in systemic circulation and attenuates ovarian function. This detrimental effect might be caused, at least partly, by the inhibition of ARC Kiss1 expression and LH pulses along with an induction of ovarian inflammatory response.

In vivo imaging of the GnRH pulse generator reveals a temporal order of neuronal activation and synchronization during each pulse

A hypothalamic pulse generator located in the arcuate nucleus controls episodic release of gonadotropin-releasing hormone (GnRH) and luteinizing hormone (LH) and is essential for reproduction. Recent evidence suggests this generator is composed of arcuate "KNDy" cells, the abbreviation based on coexpression of kisspeptin, neurokinin B, and dynorphin. However, direct visual evidence of KNDy neuron activity at a single-cell level during a pulse is lacking. Here, we use in vivo calcium imaging in freely moving female mice to show that individual KNDy neurons are synchronously activated in an episodic manner, and these synchronized episodes always precede LH pulses. Furthermore, synchronization among KNDy cells occurs in a temporal order, with some subsets of KNDy cells serving as "leaders" and others as "followers" during each synchronized episode. These results reveal an unsuspected temporal organization of activation and synchronization within the GnRH pulse generator, suggesting that different subsets of KNDy neurons are activated at pulse onset than afterward during maintenance and eventual termination of each pulse. Further studies to distinguish KNDy "leader" from "follower" cells is likely to have important clinical significance, since regulation of pulsatile GnRH secretion is essential for normal reproduction and disrupted in pathological conditions such as polycystic ovary syndrome and hypothalamic amenorrhea.

Neuroendocrine regulation of pubertal suppression in the naked mole-rat: What we know and what comes next

Puberty is a key developmental milestone that marks an individual's maturation in several ways including, but not limited to, reproductive maturation, changes in behaviors and neural organization. The timing at which puberty occurs is variable both within individuals of the same species and between species. These variations can be aligned with ecological cues that delay or suppress puberty. Naked mole-rats are colony-living rodents where reproduction is restricted to a few animals; all other animals are pubertally-suppressed. Animals removed from suppressive colony cues can reproductively mature, presenting the unique opportunity to study adult-onset puberty. Recently, we found that RFRP-3 administration sustains pubertal delay in naked mole-rats removed from colony. In this review, we explore what is known about regulators that control puberty onset, the role of stress/social status in pubertal timing, the status of knowledge of pubertal suppression in naked mole-rats and what comes next.

Expression of genes for Kisspeptin (KISS1), Neurokinin B (TAC3), Prodynorphin (PDYN), and gonadotropin inhibitory hormone (RFRP) across natural puberty in ewes

Expression of particular genes in hypothami of ewes was measured across the natural pubertal transition by in situ hybridization. The ewes were allocated to three groups (n = 4); prepubertal, postpubertal and postpubertally gonadectomized (GDX). Prepubertal sheep were euthanized at 20 weeks of age and postpubertal animals at 32 weeks. GDX sheep were also euthanized at 32 weeks, 1 week after surgery. Expression of KISS1, TAC3, PDYN in the arcuate nucleus (ARC), RFRP in the dorsomedial hypothalamus and GNRH1 in the preoptic area was quantified on a cellular basis. KISS1R expression by GNRH1 cells was quantified by double-label in situ hybridization. Across puberty, detectable KISS1 cell number increased in the caudal ARC and whilst PDYN cell numbers were low, numbers increased in the rostral ARC. TAC3 expression did not change but RFRP expression/cell was reduced across puberty. There was no change across puberty in the number of GNRH1 cells that expressed the kisspeptin receptor (KISS1R). GDX shortly after puberty did not increase expression of any of the genes of interest. We conclude that KISS1 expression in the ARC increases during puberty in ewes and this may be a causative factor in the pubertal activation of the reproductive axis. A reduction in expression of RFRP may be a factor in the onset of puberty, removing negative tone on GNRH1 cells. The lack of changes in expression of genes following GDX suggest that the effects of gonadal hormones may differ in young and mature animals.

Dynamic Hormone Control of Stress and Fertility

Neuroendocrine axes display a remarkable diversity of dynamic signaling processes relaying information between the brain, endocrine glands, and peripheral target tissues. These dynamic processes include oscillations, elastic responses to perturbations, and plastic long term changes observed from the cellular to the systems level. While small transient dynamic changes can be considered physiological, larger and longer disruptions are common in pathological scenarios involving more than one neuroendocrine axes, suggesting that a robust control of hormone dynamics would require the coordination of multiple neuroendocrine clocks. The idea of apparently different axes being in fact exquisitely intertwined through neuroendocrine signals can be investigated in the regulation of stress and fertility. The stress response and the reproductive cycle are controlled by the Hypothalamic-Pituitary-Adrenal (HPA) axis and the Hypothalamic-Pituitary-Gonadal (HPG) axis, respectively. Despite the evidence surrounding the effects of stress on fertility, as well as of the reproductive cycle on stress hormone dynamics, there is a limited understanding on how perturbations in one neuroendocrine axis propagate to the other. We hypothesize that the links between stress and fertility can be better understood by considering the HPA and HPG axes as coupled systems. In this manuscript, we investigate neuroendocrine rhythms associated to the stress response and reproduction by mathematically modeling the HPA and HPG axes as a network of interlocked oscillators. We postulate a network architecture based on physiological data and use the model to predict responses to stress perturbations under different hormonal contexts: normal physiological, gonadectomy, hormone replacement with estradiol or corticosterone (CORT), and high excess CORT (hiCORT) similar to hypercortisolism in humans. We validate our model predictions against experiments in rodents, and show how the dynamic responses of these endocrine axes are consistent with our postulated network architecture. Importantly, our model also predicts the conditions that ensure robustness of fertility to stress perturbations, and how chronodisruptions in glucocorticoid hormones can affect the reproductive axis' ability to withstand stress. This insight is key to understand how chronodisruption leads to disease, and to design interventions to restore normal rhythmicity and health.

The reproductive stress hypothesis

In this paper, we propose the reproductive stress hypothesis that describes the pregnant females response to reproductive events based upon the activation of the hypothalamic–pituitary–adrenal axis and sympathetic adrenomedullary system. The main components of the reproductive stress hypothesis can be summarized as follows: (1) events unique to reproduction including empathema, pregnancy, parturition and lactation cause non-specific responses in females, called active reproductive stress; (2) the fetus is a special stressor for pregnant females where endocrine hormones, including corticotropin-releasing hormones and fetal glucocorticoids secreted by the fetus and placenta, enter the maternal circulatory system, leading to another stress response referred to as passive reproductive stress and (3) response to uterine tension and intrauterine infection is the third type of stress, called fetal intrauterine stress. Appropriate reproductive stress is a crucial prerequisite in normal reproductive processes. By contrast, excessive or inappropriate reproductive stress may result in dysfunctions of the reproductive system, such as compromised immune function, leading to susceptibility to disease. The novel insights of the reproductive stress hypothesis have important implications for deciphering the pathogenesis of certain diseases in pregnant animals, including humans, which in turn may be applied to preventing and treating their occurrence.

Kisspeptin/Neurokinin B/Dynorphin (KNDy) cells as integrators of diverse internal and external cues: evidence from viral-based monosynaptic tract-tracing in mice

Neurons in the hypothalamic arcuate nucleus (ARC) that co-express kisspeptin, neurokinin B and dynorphin (KNDy cells) are essential for mammalian reproduction as key regulators of gonadotropin-releasing hormone (GnRH) secretion. Although multiple endogenous and exogenous signals act indirectly via KNDy neurons to regulate GnRH, the identity of upstream neurons that provide synaptic input to this subpopulation is unclear. We used rabies-mediated tract-tracing in transgenic Kiss1-Cre mice combined with whole-brain optical clearing and multiple-label immunofluorescence to create a comprehensive and quantitative brain-wide map of neurons providing monosynaptic input to KNDy cells, as well as identify the estrogen receptor content and peptidergic phenotype of afferents. Over 90% of monosynaptic input to KNDy neurons originated from hypothalamic nuclei in both male and female mice. The greatest input arose from non-KNDy ARC neurons, including proopiomelanocortin-expressing cells. Significant female-dominant sex differences in afferent input were detected from estrogen-sensitive hypothalamic nuclei critical for reproductive endocrine function and sexual behavior in mice, indicating KNDy cells may provide a unique site for the coordination of sex-specific behavior and gonadotropin release. These data provide key insight into the structural framework underlying the ability of KNDy neurons to integrate endogenous and environmental signals important for the regulation of reproductive function.

Exploring the involvement of Tac2 in the mouse hippocampal stress response through gene networking

Tachykinin 2 (Tac2) is expressed in a number of areas throughout the brain, including the hippocampus. However, knowledge about its function has been only well explored in the hypothalamus in the context of reproductive health. In this study, we identified and validated increased hippocampal Tac2 mRNA expression in response to chronic mild stress in mice. Expression quantitative trait locus (eQTL) analysis showed Tac2 is cis-regulated in the hippocampus. Using a systems genetics approach, we constructed a Tac2 co-expression network to better understand the relationship between Tac2 and the hippocampal stress response. Our network identified 69 total genes associated with Tac2, several of which encode major neuropeptides involved in hippocampal stress signaling as well as critical genes for producing neural plasticity, indicating that Tac2 is involved in these processes. Pathway analysis for the member of Tac2 gene network revealed a strong connection between Tac2 and neuroactive ligand-receptor interaction, calcium signaling pathway, as well as cardiac muscle contraction. In addition, we also identified 46 stress-related phenotypes, specifically fear conditioning response, that were significantly correlated with Tac2 expression. Our results provide evidence for Tac2 as a strong candidate gene who likely plays a role in hippocampal stress processing and neural plasticity.

KNDy Cells Revisited

In the past decade since kisspeptin/neurokinin B/dynorphin (KNDy) cells were first identified in the mammalian hypothalamus, a plethora of new research has emerged adding insights into the role of this neuronal population in reproductive neuroendocrine function, including the basis for GnRH pulse generation and the mechanisms underlying the steroid feedback control of GnRH secretion. In this mini-review, we provide an update of evidence regarding the roles of KNDy peptides and their postsynaptic receptors in producing episodic GnRH release and assess the relative contribution of KNDy neurons to the "GnRH pulse generator." In addition, we examine recent work investigating the role of KNDy neurons as mediators of steroid hormone negative feedback and review evidence for their involvement in the preovulatory GnRH/LH surge, taking into account species differences that exist among rodents, ruminants, and primates. Finally, we summarize emerging roles of KNDy neurons in other aspects of reproductive function and in nonreproductive functions and discuss critical unresolved questions in our understanding of KNDy neurobiology.

Evidence That Dynorphin Acts Upon KNDy and GnRH Neurons During GnRH Pulse Termination in the Ewe

A subpopulation of neurons located within the arcuate nucleus, colocalizing kisspeptin, neurokinin B, and dynorphin (Dyn; termed KNDy neurons), represents key mediators of pulsatile GnRH secretion. The KNDy model of GnRH pulse generation proposes that Dyn terminates each pulse. However, it is unknown where and when during a pulse that Dyn is released to inhibit GnRH secretion. Dyn acts via the κ opioid receptor (KOR), and KOR is present in KNDy and GnRH neurons in sheep. KOR, similar to other G protein-coupled receptors, are internalized after exposure to ligand, and thus internalization can be used as a marker of endogenous Dyn release. Thus, we hypothesized that KOR will be internalized at pulse termination in both KNDy and GnRH neurons. To test this hypothesis, GnRH pulses were induced in gonad-intact anestrous ewes by injection of neurokinin B (NKB) into the third ventricle and animals were euthanized at times of either pulse onset or termination. NKB injections produced increased internalization of KOR within KNDy neurons during both pulse onset and termination. In contrast, KOR internalization into GnRH neurons was seen only during pulse termination, and only in GnRH neurons within the mediobasal hypothalamus (MBH). Overall, our results indicate that Dyn is released onto KNDy cells at the time of pulse onset, and continues to be released during the duration of the pulse. In contrast, Dyn is released onto MBH GnRH neurons only at pulse termination and thus actions of Dyn upon KNDy and GnRH cell bodies may be critical for pulse termination.

Sex- and brain region-specific patterns of gene expression associated with socially-mediated puberty in a eusocial mammal

The social environment can alter pubertal timing through neuroendocrine mechanisms that are not fully understood; it is thought that stress hormones (e.g., glucocorticoids or corticotropin-releasing hormone) influence the hypothalamic-pituitary-gonadal axis to inhibit puberty. Here, we use the eusocial naked mole-rat, a unique species in which social interactions in a colony (i.e. dominance of a breeding female) suppress puberty in subordinate animals. Removing subordinate naked mole-rats from this social context initiates puberty, allowing for experimental control of pubertal timing. The present study quantified gene expression for reproduction- and stress-relevant genes acting upstream of gonadotropin-releasing hormone in brain regions with reproductive and social functions in pre-pubertal, post-pubertal, and opposite sex-paired animals (which are in various stages of pubertal transition). Results indicate sex differences in patterns of neural gene expression. Known functions of genes in brain suggest stress as a key contributing factor in regulating male pubertal delay. Network analysis implicates neurokinin B (Tac3) in the arcuate nucleus of the hypothalamus as a key node in this pathway. Results also suggest an unappreciated role for the nucleus accumbens in regulating puberty.

Evidence That Endogenous Somatostatin Inhibits Episodic, but Not Surge, Secretion of LH in Female Sheep

Two modes of gonadotropin-releasing hormone (GnRH) and luteinizing hormone (LH) secretion are necessary for female fertility: surge and episodic secretion. However, the neural systems that regulate these GnRH secretion patterns are still under investigation. The neuropeptide somatostatin (SST) inhibits episodic LH secretion in humans and sheep, and several lines of evidence suggest SST may regulate secretion during the LH surge. In this study, we examined whether SST alters the LH surge in ewes by administering a SST receptor (SSTR) 2 agonist (octreotide) or antagonist [CYN154806 (CYN)] into the third ventricle during an estrogen-induced LH surge and whether endogenous SST alters episodic LH secretion. Neither octreotide nor CYN altered the amplitude or timing of the LH surge. Administration of CYN to intact ewes during the breeding season or anestrus increased LH secretion and increased c-Fos in a subset GnRH and kisspeptin cells during anestrus. To determine if these stimulatory effects are steroid dependent or independent, we administered CYN to ovariectomized ewes. This SSTR2 antagonist increased LH pulse frequency in ovariectomized ewes during anestrus but not during the breeding season. This study provides evidence that endogenous SST contributes to the control of LH secretion. The results demonstrate that SST, acting through SSTR2, inhibits episodic LH secretion, likely acting in the mediobasal hypothalamus, but action at this receptor does not alter surge secretion. Additionally, these data provide evidence that SST contributes to the steroid-independent suppression of LH pulse frequency during anestrus.

Kisspeptin innervation of the hypothalamic paraventricular nucleus: sexual dimorphism and effect of estrous cycle in female mice

The hypothalamic paraventricular nucleus (PVN) is the major autonomic output area of the hypothalamus and a critical regulatory center for energy homeostasis. The organism's energetic balance is very important for both the regular onset of puberty and regulation of fertility. Several studies have suggested a relationship among neural circuits controlling food intake, energy homeostasis and the kisspeptin peptide. The kisspeptin system is clustered in two main groups of cell bodies [the anterior ventral periventricular region (AVPV) and the arcuate nucleus (ARC)] projecting mainly to gonadotropin-releasing hormone (GnRH) neurons and to a few other locations, including the PVN. In the present study, we investigated the distribution of the kisspeptin fibers within the PVN of adult CD1 mice. We observed a significant sexual dimorphism for AVPV and ARC, as well as for the PVN innervation. Kisspeptin fibers showed a different density within the PVN, being denser in the medial part than in the lateral one; moreover, in female, the density changed, according to different phases of the estrous cycle (the highest density being in estrus phase). The presence of a profound effect of estrous cycle on the kisspeptin immunoreactivity in AVPV (with a higher signal in estrus) and ARC, and the strong co-localization between kisspeptin and NkB only in ARC and not in PVN suggested that the majority of the kisspeptin fibers found in the PVN might arise directly from AVPV.

Do Substance P and Neurokinin A Play Important Roles in the Control of LH Secretion in Ewes?

There is now general agreement that neurokinin B (NKB) acts via neurokinin-3-receptor (NK3R) to stimulate secretion of GnRH and LH in several species, including rats, mice, sheep, and humans. However, the roles of two other tachykinins, substance P (SP) and neurokinin A, which act primarily via NK1R and NK2R, respectively, are less clear. In rodents, these signaling pathways can stimulate LH release and substitute for NKB signaling; in humans, SP is colocalized with kisspeptin and NKB in the mediobasal hypothalamus. In this study, we examined the possible role of these tachykinins in control of the reproductive axis in sheep. Immunohistochemistry was used to describe the expression of SP and NK1R in the ovine diencephalon and determine whether these proteins are colocalized in kisspeptin or GnRH neurons. SP-containing cell bodies were largely confined to the arcuate nucleus, but NK1R-immunoreactivity was more widespread. However, there was very low coexpression of SP or NK1R in kisspeptin cells and none in GnRH neurons. We next determined the minimal effective dose of these three tachykinins that would stimulate LH secretion when administered into the third ventricle of ovary-intact anestrous sheep. A much lower dose of NKB (0.2 nmol) than of neurokinin A (2 nmol) or SP (10 nmol) consistently stimulated LH secretion. Moreover, the relative potency of these three neuropeptides parallels the relative selectivity of NK3R. Based on these anatomical and pharmacological data, we conclude that NKB-NK3R signaling is the primary pathway for the control of GnRH secretion by tachykinins in ewes.

Does Dynorphin Play a Role in the Onset of Puberty in Female Sheep?

Puberty onset involves increased gonadotrophin-release (GnRH) release as a result of decreased sensitivity to oestrogen (E₂ )-negative feedback. Because GnRH neurones lack E₂ receptor α, this pathway must contain interneurones. One likely candidate is KNDy neurones (kisspeptin, neurokinin B, dynorphin). The overarching hypothesis of the present study was that the prepubertal hiatus in luteinising hormone (LH) release involves reduced kisspeptin and/or heightened dynorphin input. We first tested the specific hypothesis that E₂ would reduce kisspeptin-immunopositive cell numbers and increase dynorphin-immunopositive cell numbers. We found that kisspeptin cell numbers were higher in ovariectomised (OVX) lambs than OVX lambs treated with E₂ (OVX+ E₂ ) or those left ovary-intact. Very few arcuate dynorphin cells were identified in any group. Next, we hypothesised that central blockade of κ-opioid receptor (KOR) would increase LH secretion at a prepubertal (6 months) but not postpubertal (10 months) age. Luteinising hormone pulse frequency and mean LH increased during infusion of a KOR antagonist, norbinaltorphimine, in OVX + E₂ lambs at the prepubertal age but not in the same lambs at the postpubertal age. We next hypothesised that E₂ would increase KOR expression in GnRH neurones or alter synaptic input to KNDy neurones in prepubertal ewes. Oestrogen treatment decreased the percentage of GnRH neurones coexpressing KOR (approximately 68%) compared to OVX alone (approximately 78%). No significant differences in synaptic contacts per cell between OVX and OVX + E₂ groups were observed. Although these initial data are consistent with dynorphin inhibiting pulsatile LH release prepubertally, additional work will be necessary to define the source and mechanisms of this inhibition.

Sexual Motivation in the Female and Its Opposition by Stress

A well worked-out motivational system in laboratory animals produces estrogen-dependent female sex behavior. Here, we review (a) the logical definition of sexual motivation and (b) the basic neuronal and molecular mechanisms that allow the behavior to occur. Importantly, reproductive mechanisms in the female can be inhibited by stress. This is interesting because, in terms of the specificity of neuroendocrine dynamics in space and time, the two families of phenomena, sex and stress, are the opposite of each other. We cover papers that document stress effects on the underlying processes of reproductive endocrinology in the female. Not all of the mechanisms for such inhibition have been clearly laid out. Finally, as a current topic of investigation, this system offers several avenues for new investigation which we briefly characterize.

Endogenous opiates and behavior: 2014

This paper is the thirty-seventh consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2014 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior (endogenous opioids and receptors), and the roles of these opioid peptides and receptors in pain and analgesia (pain and analgesia); stress and social status (human studies); tolerance and dependence (opioid mediation of other analgesic responses); learning and memory (stress and social status); eating and drinking (stress-induced analgesia); alcohol and drugs of abuse (emotional responses in opioid-mediated behaviors); sexual activity and hormones, pregnancy, development and endocrinology (opioid involvement in stress response regulation); mental illness and mood (tolerance and dependence); seizures and neurologic disorders (learning and memory); electrical-related activity and neurophysiology (opiates and conditioned place preferences (CPP)); general activity and locomotion (eating and drinking); gastrointestinal, renal and hepatic functions (alcohol and drugs of abuse); cardiovascular responses (opiates and ethanol); respiration and thermoregulation (opiates and THC); and immunological responses (opiates and stimulants). This paper is the thirty-seventh consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2014 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior (endogenous opioids and receptors), and the roles of these opioid peptides and receptors in pain and analgesia (pain and analgesia); stress and social status (human studies); tolerance and dependence (opioid mediation of other analgesic responses); learning and memory (stress and social status); eating and drinking (stress-induced analgesia); alcohol and drugs of abuse (emotional responses in opioid-mediated behaviors); sexual activity and hormones, pregnancy, development and endocrinology (opioid involvement in stress response regulation); mental illness and mood (tolerance and dependence); seizures and neurologic disorders (learning and memory); electrical-related activity and neurophysiology (opiates and conditioned place preferences (CPP)); general activity and locomotion (eating and drinking); gastrointestinal, renal and hepatic functions (alcohol and drugs of abuse); cardiovascular responses (opiates and ethanol); respiration and thermoregulation (opiates and THC); and immunological responses (opiates and stimulants).

Glucocorticoid Regulation of Reproduction

It is well accepted that stress, measured by increased glucocorticoid secretion, leads to profound reproductive dysfunction. In times of stress, glucocorticoids activate many parts of the fight or flight response, mobilizing energy and enhancing survival, while inhibiting metabolic processes that are not necessary for survival in the moment. This includes reproduction, an energetically costly procedure that is very finely regulated. In the short term, this is meant to be beneficial, so that the organism does not waste precious energy needed for survival. However, long-term inhibition can lead to persistent reproductive dysfunction, even if no longer stressed. This response is mediated by the increased levels of circulating glucocorticoids, which orchestrate complex inhibition of the entire reproductive axis. Stress and glucocorticoids exhibits both central and peripheral inhibition of the reproductive hormonal axis. While this has long been recognized as an issue, understanding the complex signaling mechanism behind this inhibition remains somewhat of a mystery. What makes this especially difficult is attempting to differentiate the many parts of both of these hormonal axes, and new neuropeptide discoveries in the last decade in the reproductive field have added even more complexity to an already complicated system. Glucocorticoids (GCs) and other hormones within the hypothalamic-pituitary-adrenal (HPA) axis (as well as contributors in the sympathetic system) can modulate the hypothalamic-pituitary-gonadal (HPG) axis at all levels-GCs can inhibit release of GnRH from the hypothalamus, inhibit gonadotropin synthesis and release in the pituitary, and inhibit testosterone synthesis and release from the gonads, while also influencing gametogenesis and sexual behavior. This chapter is not an exhaustive review of all the known literature, however is aimed at giving a brief look at both the central and peripheral effects of glucocorticoids on the reproductive function.

The role of neurokinin B signalling in reproductive neuroendocrinology

The KNDy neuropeptides, kisspeptin, neurokinin B (NKB) and dynorphin A (Dyn), have been implicated in regulating pulsatile luteinising hormone (LH) secretion. Studies of the interactions between KNDy signalling systems, however, are currently few. Although the stimulatory effect of kisspeptin and the inhibitory effect of Dyn on the gonadotropin-releasing hormone pulse generator are widely accepted, the effects of NKB in rodents are variable and sometimes controversial. Literature describing increased LH secretion in response to NKB receptor agonism predominates and is in line with human physiology, as well as the pathophysiology of pubertal failure associated with disruption of NKB signalling. However, the robust suppression of the LH pulse, induced by the same treatment under hypoestrogenic conditions, may hold clues as to the mechanisms of reproductive inhibition under pathological conditions. This review discusses the recent evidence for this paradox and outlines a revised working model incorporating the mechanisms by which KNDy neuropeptides modulate the reproductive axis.