Physiological role of metastin/kisspeptin in regulating gonadotropin-releasing hormone (GnRH) secretion in female rats

Neonatal Aromatase Inhibition Blocked Defeminization of AVPV Kiss1 Neurons and LH Surge-Generating System in Male Rats

The neuroendocrine system that controls the preovulatory surge of gonadotropin-releasing hormone (GnRH)/luteinizing hormone (LH), which triggers ovulation in female mammals, is sexually differentiated in rodents. A transient increase in circulating testosterone levels in male rats within a few hours of birth is primarily responsible for the defeminization of anteroventral periventricular nucleus (AVPV) kisspeptin neurons, which are critical regulators of the GnRH/LH surge. The present study aimed to determine whether neonatal estradiol-17β (E2) converted from testosterone by aromatase primarily causes the defeminization of AVPV kisspeptin neurons and the surge of GnRH/LH in male rodents. The results of the present study showed that the neonatal administration of letrozole (LET), a nonsteroidal aromatase inhibitor, within 2 hours of birth rescued AVPV Kiss1 expression and the LH surge in adult male rats, while the neonatal administration of testosterone propionate (TP) irreversibly attenuated AVPV Kiss1 expression and the LH surge in adult female rats. Furthermore, the neonatal LET-treated Kiss1-Cre-activated tdTomato reporter males exhibited a comparable number of AVPV Kiss1-Cre-activated tdTomato-expressing cells to that of vehicle-treated female rats, while neonatal TP-treated females showed fewer AVPV Kiss1-Cre-activated tdTomato-expressing cells than vehicle-treated females. Moreover, neonatal TP administration significantly decreased the number of arcuate Kiss1-expressing and Kiss1-Cre-activated tdTomato-positive cells and suppressed LH pulses in adult gonadectomized female rats; however, neonatal LET administration failed to affect them. These results suggest that E2 converted from neonatal testosterone is primarily responsible for the defeminization of AVPV kisspeptin neurons and the subsequent GnRH/LH surge generation in male rats.

Hypothalamic Kisspeptin Neurons: Integral Elements of the GnRH System

Highly sophisticated and synchronized interactions of various cells and hormonal signals are required to make organisms competent for reproduction. GnRH neurons act as a common pathway for multiple cues for the onset of puberty and attaining reproductive function. GnRH is not directly receptive to most of the signals required for the GnRH secretion during the various phases of the ovarian cycle. Kisspeptin neurons of the hypothalamus convey these signals required for the synchronized release of the GnRH. The steroid-sensitive anteroventral periventricular nucleus (AVPV) kisspeptin and arcuate nucleus (ARC) KNDy neurons convey steroid feedback during the reproductive cycle necessary for GnRH surge and pulse, respectively. AVPV region kisspeptin neurons also communicate with nNOS synthesizing neurons and suprachiasmatic nucleus (SCN) neurons to coordinate the process of the ovarian cycle. Neurokinin B (NKB) and dynorphin play roles in the GnRH pulse stimulation and inhibition, respectively. The loss of NKB and kisspeptin function results in the development of neuroendocrine disorders such as hypogonadotropic hypogonadism (HH) and infertility. Ca²⁺ signaling is essential for GnRH pulse generation, which is propagated through gap junctions between astrocytes-KNDy and KNDy-KNDy neurons. Impaired functioning of KNDy neurons could develop the characteristics associated with polycystic ovarian syndrome (PCOS) in rodents. Kisspeptin-increased synthesis led to excessive secretion of the LH associated with PCOS. This review provides the latest insights and understanding into the role of the KNDy and AVPV/POA kisspeptin neurons in GnRH secretion and PCOS.

Reduction of arcuate kappa-opioid receptor-expressing cells increased luteinizing hormone pulse frequency in female rats

The brain mechanism responsible for the pulsatile secretion of gonadotropin-releasing hormone (GnRH) is important for maintaining reproductive function in mammals. Accumulating evidence suggests that kisspeptin/neurokinin B/dynorphin A (KNDy) neurons in the hypothalamic arcuate nucleus (ARC) play a critical role in the regulation of pulsatile GnRH and subsequent gonadotropin secretion. Dynorphin A (Dyn) and its receptor, kappa-opioid receptor (KOR, encoded by Oprk1), have been shown to be involved in the suppression of pulsatile GnRH/luteinizing hormone (LH) release. On the other hand, it is still unclear whether the inhibitory Dyn signaling affects KNDy neurons or KOR-expressing non-KNDy cells in the ARC or other brain regions. We therefore aimed to clarify the role of ARC-specific Dyn-KOR signaling in the regulation of pulsatile GnRH/LH release by the ARC specific cell deletion of KOR-expressing cells using Dyn-conjugated-saporin (Dyn-SAP). Estrogen-primed ovariectomized female rats were administered Dyn-SAP to the ARC. In situ hybridization of Oprk1 showed that ARC Dyn-SAP administration significantly decreased the number of Oprk1-expressing cells in the ARC, but not in the ventromedial hypothalamic nucleus and paraventricular nucleus. The frequency of LH pulses significantly increased in animals bearing the ARC Dyn-SAP administration. The number of Kiss1-expressing cells in the ARC was not affected by ARC Dyn-SAP treatment. Dyn-KOR signaling within the ARC seems to mediate the suppression of the frequency of pulsatile GnRH/LH release, and ARC non-KNDy KOR neurons may be involved in the mechanism modulating GnRH/LH pulse generation.

Expression of type one cannabinoid receptor in different subpopulation of kisspeptin neurons and kisspeptin afferents to GnRH neurons in female mice

The endocannabinoids have been shown to target the afferents of hypothalamic neurons via cannabinoid 1 receptor (CB1) and thereby to influence their excitability at various physiological and/or pathological processes. Kisspeptin (KP) neurons form afferents of multiple neuroendocrine cells and influence their activity via signaling through a variation of co-expressed classical neurotransmitters and neuropeptides. The differential potency of endocannabinoids to influence the release of classical transmitters or neuropeptides, and the ovarian cycle-dependent functioning of the endocannabinoid signaling in the gonadotropin-releasing hormone (GnRH) neurons initiated us to study whether (a) the different subpopulations of KP neurons express CB1 mRNAs, (b) the expression is influenced by estrogen, and (c) CB1-immunoreactivity is present in the KP afferents to GnRH neurons. The aim of the study was to investigate the site- and cell-specific expression of CB1 in female mice using multiple labeling in situ hybridization and immunofluorescent histochemical techniques. The results support that CB1 mRNAs are expressed by both the GABAergic and glutamatergic subpopulations of KP neurons, the receptor protein is detectable in two-thirds of the KP afferents to GnRH neurons, and the expression of CB1 mRNA shows an estrogen-dependency. The applied estrogen-treatment, known to induce proestrus, reduced the level of CB1 transcripts in the rostral periventricular area of the third ventricle and arcuate nucleus, and differently influenced its co-localization with vesicular GABA transporter or vesicular glutamate transporter-2 in KP neurons. This indicates a gonadal cycle-dependent role of endocannabinoid signaling in the neuronal circuits involving KP neurons.

Circulating kisspeptin and anti-müllerian hormone levels, and insulin resistance in women with polycystic ovary syndrome: A systematic review, meta-analysis, and meta-regression

OBJECTIVE

This systematic review and meta-analysis aimed to summarize the available evidence regarding circulating kisspeptin and anti-müllerian hormone (AMH) and the homeostasis model assessment of insulin resistance (HOMA-IR) index in adolescents and women with and without polycystic ovary syndrome (PCOS).

METHOD

We performed a comprehensive literature search in Medline, Embase, Cochrane, Scopus, and Web of Science for studies evaluating circulating kisspeptin levels in women with and without PCOS published until September 24th, 2020. Co-primary outcomes were the HOMA-IR index and AMH. The quality of included studies was assessed using the Newcastle-Ottawa Scale. Random-effects models were used to estimate outcomes, and effects reported as mean difference (MD) or standardized MD (SMD) and their 95 % confidence interval (CI). The systematic review and meta-analysis was registered in the International Prospective Register of Systematic Reviews (PROSPERO) as number CRD42020205030.

RESULTS

We evaluated 18 studies including, 1282 PCOS cases and 977 controls. Participants with PCOS were younger (MD = -2.38 years, 95 %CI -4.32 to -0.44), with higher BMI (MD = 1.16, 95 % CI 0.54-1.78), waist-to-hip ratio (MD = 0.04, 95 %CI 0.02 to 0.05), circulating kisspeptin (SMD = 1.15, 95 %CI 0.68-1.62), luteinizing hormone (SMD = 1.29, 95 %CI 0.76-1.83), AMH (SMD = 0.97, 95 %CI 0.60-1,34), total testosterone (SMD = 2.48, 95 %CI 1.73-3.23), free testosterone (SMD = 1.37, 95 %CI 0.56-2.17), and dehydroepiandrosterone sulphate (SMD = 0.72, 95 %CI 0.32-1.13) levels, and Ferriman-Gallwey score (SMD = 5.08, 95 %CI 2.76-7.39), and lower sex hormone-binding globulin level (SMD = -1.34, 95 %CI -2.15 to -0.52). Besides, participants with PCOS had higher HOMA-IR index (SMD = 0.76, 95 %CI 0.35-1.17), and circulating insulin (SMD = 0.75, 95 %CI 0.30-1.19), leptin (SMD = 2.82, 95 %CI 1.35-4.29), and triglycerides (SMD = 2.15, 95 %CI 1.08-3.23) levels than participants without the syndrome. The meta-regression did not identify significant factors influencing circulating kisspeptin.

CONCLUSION

Patients with PCOS showed higher kisspeptin, LH, insulin, AMH, and androgen levels and HOMA-IR index, and lower sex hormone-binding globulin levels than those without the syndrome.

Clock control of mammalian reproductive cycles: Looking beyond the pre-ovulatory surge of gonadotropins

Several aspects of the physiology and behavior of organisms are expressed rhythmically with a 24-h periodicity and hence called circadian rhythms. Such rhythms are thought to be an adaptive response that allows to anticipate cyclic events in the environment. In mammals, the circadian system is a hierarchically organized net of endogenous oscillators driven by the hypothalamic suprachiasmatic nucleus (SCN). This system is synchronized by the environment throughout afferent pathways and in turn it organizes the activity of tissues by means of humoral secretions and neuronal projections. It has been shown that reproductive cycles are regulated by the circadian system. In rodents, the lesion of the SCN results on alterations of the estrous cycle, sexual behavior, tonic and phasic secretion of gonadotropin releasing hormone (GnRH)/gonadotropins and in the failure of ovulation. Most of the studies regarding the circadian control of reproduction, in particular of ovulation, have only focused on the participation of the SCN in the triggering of the proestrus surge of gonadotropins. Here we review aspects of the evolution and organization of the circadian system with particular focus on its relationship with the reproductive cycle of laboratory rodents. Experimental evidence of circadian control of neuroendocrine events indispensable for ovulation that occur prior to proestrus are discussed. In order to offer a working model of the circadian regulation of reproduction, its participation on aspects ranging from gamete production, neuroendocrine regulation, sexual behavior, mating coordination, pregnancy and deliver of the product should be assessed experimentally.

A neural circadian signal essential for ovulation is generated in the suprachiasmatic nucleus during each stage of the oestrous cycle

NEW FINDINGS

What is the central question of this study? Is the suprachiasmatic nucleus the structure that generates the neural circadian signals that occur during every stage of the oestrous cycle, not only pro-oestrus, and are these signals essential for proper regulation of ovulation? What is the main finding and its importance? Transient inhibition of Na⁺ -dependent action potentials in the suprachiasmatic nucleus by tetrodotoxin microinjection at 14.00 h inhibits ovulation irrespective of the stage of the oestrous cycle at which the procedure is performed. Microinjection of saline solution into the suprachiasmatic nucleus has a disruptive effect on ovulation that depends on the stage of the cycle at which it is administered.

ABSTRACT

Reproduction is a highly timed process that depends on both the reproductive and circadian systems. The core oscillator of the latter resides at the suprachiasmatic nuclei (SCN) and it is pivotal for the regulation of the pro-oestrus pre-ovulatory surge of gonadotropins in females. There is evidence to suggest that this system may be involved in the regulation of neuroendocrine events that are essential for ovulation and that occur prior to pro-oestrus. We explored this possibility by transiently inactivating the SCN. Female rats were implanted with guide cannulas aimed at the SCN. After recovery of the oestrous cycle, animals were injected with tetrodotoxin (TTX), artificial cerebrospinal fluid (ACSF) or saline solution while freely moving. Injections were performed at 14.00 h of each stage of the oestrous cycle. Animals were killed on the next predicted oestrus day, the number of ova shed was counted and intact rats at oestrus stage were used as absolute control. ACSF did not modify ovulation. Saline solution blocked ovulation in oestrus- and dioestrus-injected rats. Irrespectively of the stage of the oestrous cycle, TTX blocked ovulation. These results lead us to suggest that a neural circadian signal, pivotal for triggering the gonadotropin pre-ovulatory surge, arises from the SCN during the critical window of pro-oestrus. We also suggest that a similar signal, needed for the regulation of other events that are indispensable for proper regulation of ovulation, is also generated in this nucleus during the other stages of the cycle at a similar time.

Effects of Low Energy Availability on Reproductive Functions and Their Underlying Neuroendocrine Mechanisms

It is known that metabolic disturbances suppress reproductive functions in females. The mechanisms underlying metabolic and nutritional effects on reproductive functions have been established based on a large body of clinical and experimental data. From the 1980s to 1990s, it was revealed that disrupted gonadotropin-releasing hormone (GnRH) secretion is the main cause of reproductive impairments in metabolic and nutritional disorders. From the late 1990s to early 2000s, it was demonstrated that, in addition to their primary functions, some appetite- or metabolism-regulating factors affect GnRH secretion. Furthermore, in the early 2000s, kisspeptin, which is a potent positive regulator of GnRH secretion, was newly discovered, and it has been revealed that kisspeptin integrates the effects of metabolic status on GnRH neurons. Recent studies have shown that kisspeptin mediates at least some of the effects of appetite- and metabolism-regulating factors on GnRH neurons. Thus, kisspeptin might be a useful clinical target for treatments aimed at restoring reproductive functions in individuals with metabolic or nutritional disturbances, such as those who exercise excessively, experience marked weight loss, or suffer from eating disorders. This paper presents a review of what is currently known about the effects of metabolic status on reproductive functions and their underlying mechanisms by summarizing the available evidence.

Hormonal and nutritional regulation of postnatal hypothalamic development

The hypothalamus is a key centre for regulation of vital physiological functions, such as appetite, stress responsiveness and reproduction. Development of the different hypothalamic nuclei and its major neuronal populations begins prenatally in both altricial and precocial species, with the fine tuning of neuronal connectivity and attainment of adult function established postnatally and maintained throughout adult life. The perinatal period is highly susceptible to environmental insults that, by disrupting critical developmental processes, can set the tone for the establishment of adult functionality. Here, we review the most recent knowledge regarding the major postnatal milestones in the development of metabolic, stress and reproductive hypothalamic circuitries, in the rodent, with a particular focus on perinatal programming of these circuitries by hormonal and nutritional influences. We also review the evidence for the continuous development of the hypothalamus in the adult brain, through changes in neurogenesis, synaptogenesis and epigenetic modifications. This degree of plasticity has encouraging implications for the ability of the hypothalamus to at least partially reverse the effects of perinatal mal-programming.

Co-expression of the calcitonin receptor gene in the hypothalamic kisspeptin neurons in female rats

Purpose

Hypothalamic kisspeptin neurons are considered to play a critical role in regulating mammalian reproduction and integrating humoral and neuronal inputs that control gonadotropin‐releasing hormone (GnRH)/gonadotropin release. The present study aimed to investigate the upstream regulator candidates for kisspeptin neurons.

Methods

Visualized kisspeptin neurons that were taken from the arcuate nucleus (ARC) of Kiss1‐tdTomato rats were subjected to next‐generation sequencing (NGS) analysis. In situ hybridization (ISH) for the calcitonin receptor gene (Calcr) was performed throughout the whole forebrain of ovariectomized wild‐type female rats that had been implanted with a negative feedback level of estrogen, because the Calcr expression was evident in the ARC kisspeptin neurons from the NGS analysis. Then, a double ISH was performed for the Calcr and kisspeptin gene (Kiss1) in the brain regions, containing either the anteroventral periventricular nucleus (AVPV) or ARC of the female rats.

Results

The NGS analysis revealed that the Calcr was highly expressed in the ARC kisspeptin neurons. It was found that the Calcr was co‐expressed in 12% and 22% of the Kiss1‐expressing cells in the ARC and AVPV, respectively.

Conclusion

The present study suggests that calcitonin receptor signaling could be involved in the regulation of reproductive function through the direct control of the ARC and/or AVPV kisspeptin neurons, and then GnRH/gonadotropin release.

Impact of Triclosan on Female Reproduction through Reducing Thyroid Hormones to Suppress Hypothalamic Kisspeptin Neurons in Mice

Triclosan (TCS), a broad-spectrum antimicrobial agent, is widely used in clinical settings and various personal care products. The aim of this study was to evaluate the influence of TCS on reproductive endocrine and function. Here, we show that the exposure of adult female mice to 10 or 100 mg/kg/day TCS caused prolongation of diestrus, and decreases in antral follicles and corpora lutea within 2 weeks. TCS mice showed decreases in the levels of serum luteinizing hormone (LH), follicle-stimulating hormone (FSH) and progesterone, and gonadotrophin-releasing hormone (GnRH) mRNA with the lack of LH surge and elevation of prolactin (PRL). TCS mice had lower kisspeptin immunoreactivity and kiss1 mRNA in anteroventral periventricular nucleus (AVPV) and arcuate nucleus (ARC). Moreover, the estrogen (E2)-enhanced AVPV-kisspeptin expression was reduced in TCS mice. In addition, the serum thyroid hormones (triiodothyronine (T3) and thyroxine (T4)) in TCS mice were reduced with increases in levels of thyroid stimulating hormone (TSH) and thyroid releasing hormone (TRH). In TCS mice, the treatment with Levothyroxine (L-T4) corrected the increases in PRL, TSH and TRH; the administration of L-T4 or type-2 dopamine receptors agonist quinpirole inhibiting PRL release could rescue the decline of kisspeptin expression in AVPV and ARC; the treatment with L-T4, quinpirole or the GPR45 agonist kisspeptin-10 recovered the levels of serum LH and FSH and progesterone, and GnRH mRNA. Furthermore, TCS mice treated with L-T4 or quinpirole resumed regular estrous cycling, follicular development and ovulation. Together, these results indicate that exposing adult female mice to TCS (≥10 mg/kg) reduces thyroid hormones causing hyperprolactinemia that then suppresses hypothalamic kisspeptin expression, leading to deficits in reproductive endocrine and function.

Linking Stress and Infertility: A Novel Role for Ghrelin

Infertility affects a remarkable one in four couples in developing countries. Psychological stress is a ubiquitous facet of life, and although stress affects us all at some point, prolonged or unmanageable stress may become harmful for some individuals, negatively impacting on their health, including fertility. For instance, women who struggle to conceive are twice as likely to suffer from emotional distress than fertile women. Assisted reproductive technology treatments place an additional physical, emotional, and financial burden of stress, particularly on women, who are often exposed to invasive techniques associated with treatment. Stress-reduction interventions can reduce negative affect and in some cases to improve in vitro fertilization outcomes. Although it has been well-established that stress negatively affects fertility in animal models, human research remains inconsistent due to individual differences and methodological flaws. Attempts to isolate single causal links between stress and infertility have not yet been successful due to their multifaceted etiologies. In this review, we will discuss the current literature in the field of stress-induced reproductive dysfunction based on animal and human models, and introduce a recently unexplored link between stress and infertility, the gut-derived hormone, ghrelin. We also present evidence from recent seminal studies demonstrating that ghrelin has a principal role in the stress response and reward processing, as well as in regulating reproductive function, and that these roles are tightly interlinked. Collectively, these data support the hypothesis that stress may negatively impact upon fertility at least in part by stimulating a dysregulation in ghrelin signaling.

An administration of TAK-683 at a minimally effective dose for luteinizing hormone stimulation under the absence of the ovary induces luteinizing hormone surge in ovary-intact goats

The present study aimed to evaluate hormonal responses and their association with the TAK-683 blood concentrations in goats administered TAK-683 at a low dose, which had been previously determined as the minimally effective dose for luteinizing hormone (LH) stimulation in ovariectomized goats. In Experiment 1, 5 µg of TAK-683 treatment had no significant stimulatory effect on LH secretion in ovariectomized Shiba goats (n = 4). In Experiment 2, cycling goats received the treatment of prostaglandin F_2α and progesterone-releasing controlled internal drug releasing (CIDR) to induce the follicular phase, then they were treated with 5 µg of TAK-683 (hour 0) intravenously (n = 4, IV) or subcutaneously (n = 3, SC) or with vehicle intravenously (n = 4, control) at 12 h after CIDR removal. Blood samples were collected at 10-min (–2–6 h), 2-h (6–24 h), or 6-h (24–48 h) intervals. Ovarian ultrasonographic images were assessed daily to confirm ovulation after the treatment. A surge-like release of LH was immediately observed after injection in all animals in the IV (peak time: 4.2 ± 0.6 h, peak concentration: 73.3 ± 27.5 ng/ml) and SC (peak time: 4.6 ± 0.4 h, peak concentration: 62.6 ± 23.2 ng/ml) groups, but not in the control group. Ovulation was detected within 3 days after TAK-683 injection in all animals in the IV and SC groups, and the interval period from TAK-683 administration to ovulation in the IV group was significantly (P < 0.05) shorter than that of the control group. No significant changes were observed between the IV and SC groups in terms of luteal diameter and blood progesterone levels after ovulation. The present findings suggest that the involvement of one or more ovarian factor(s) is indispensable for a TAK-683-induced LH surge leading to ovulation in goats.

Hypothalamic-pituitary GnRH/LH axis activity is affected by salsolinol in sheep during lactation: Effects of intracerebroventricular infusions of salsolinol and its antagonizing analogue

The aim of the study was to test the hypothesis that salsolinol, a derivative of dopamine, is involved in the regulation of hypothalamic-pituitary gonadotropic (GnRH/LH) axis activity in lactating sheep. In the first experiment performed on sheep during the fifth week of lactation, a structural analogue of salsolinol (1-MeDIQ) was infused into the third brain ventricle (IIIv) to antagonize its action within the central nervous system (CNS). A push-pull perfusion of the infundibular nucleus/median eminence was performed simultaneously, and blood samples were collected from the jugular vein. In the second experiment, sheep received infusions of salsolinol into the IIIv, 48 hours after the weaning of their 8-week-old lambs. Blood samples were collected during the experimental periods, and the anterior pituitary (AP) tissue was dissected immediately after the end of the experiment. Perfusate GnRH concentration (experiment 1), plasma LH concentration (experiments 1 and 2), and relative LHβ mRNA levels in the AP tissue (experiment 2) were assayed. Blocking of salsolinol action in the CNS of lactating sheep caused a significant (P < 0.001) decrease in the perfusate GnRH concentrations in comparison with controls. Treatment with 1-MEDIQ also significantly decreased (P < 0.001) the LH concentration in the blood plasma. In turn, salsolinol infused 48 hours after lamb weaning significantly (P < 0.001) increased plasma LH concentration, reflected in the significant (P < 0.05) increase in the amplitude of LH pulses in the treated sheep as compared to the control animals. There was no significant difference in the relative levels of LHβ-subunit mRNA in the AP between control and salsolinol-infused sheep. The results lead to a conclusion that salsolinol affects the secretory activity of the GnRH/LH axis in sheep during lactation. Whether salsolinol infused into the IIIv evokes this stimulatory effect by itself or by modulation of other regulatory systems needs to be clarified.

Immunohistochemical characterization of the arcuate kisspeptin/neurokinin B/dynorphin (KNDy) and preoptic kisspeptin neuronal populations in the hypothalamus during the estrous cycle in heifers

Elucidating the physiological mechanisms that control reproduction is an obvious strategy for improving the fertility of cattle and developing new agents to control reproductive functions. The present study aimed to identify kisspeptin neurons in the bovine hypothalamus, clarifying that a central mechanism is also present in the cattle brain, as kisspeptin is known to play an important role in the stimulation of gonadotropin-releasing hormone (GnRH)/gonadotropin secretion in other mammals. To characterize kisspeptin neurons in the bovine hypothalamus, the co-localizations of kisspeptin and neurokinin B (NKB) or kisspeptin and dynorphin A (Dyn) were examined. Hypothalamic tissue was collected from Japanese Black or Japanese Black × Holstein crossbred cows during the follicular and luteal phases. Brain sections, including the arcuate nucleus (ARC) and the preoptic area (POA), were dual immunostained with kisspeptin and either NKB or Dyn. In the ARC, both NKB and Dyn were co-localized in kisspeptin neurons during both the follicular and luteal phases, demonstrating the presence of kisspeptin/NKB/Dyn-containing neurons, referred to as KNDy neurons, in cows. In the POA, no co-localization of kisspeptin with either NKB or Dyn was detected. Kisspeptin expression in the follicular phase was higher than that in the luteal phase, suggesting that kisspeptin expression in the POA is positively controlled by estrogen in cows. The kisspeptin neuronal populations in the ARC and POA likely play important roles in regulating the GnRH pulse and surge, respectively, in cows.

Mood, Food, and Fertility: Adaptations of the Maternal Brain

Successfully rearing young places multiple demands on the mammalian female. These are met by a wide array of alterations in maternal physiology and behavior that are coordinated with the needs of the developing young, and include adaptations in neuroendocrine systems not directly involved in maternal behavior or lactation. In this article, attenuations in the behavioral and neuroendocrine responses to stressors, the alterations in metabolic pathways facilitating both increased food intake and conservation of energy, and the changes in fertility that occur postpartum are described. The mechanisms underlying these processes as well as the factors that contribute to them and the relative contributions of these stimuli at different times postpartum are also reviewed. The induction and maintenance of the adaptations observed in the postpartum maternal brain are dependent on mother-young interaction and, in most cases, on suckling stimulation and its consequences for the hormonal profile of the mother. The peptide hormone prolactin acting on receptors within the brain makes a major contribution to changes in metabolic pathways, suppression of fertility and the attenuation of the neuroendocrine response to stress during lactation. Oxytocin is also released, both into the circulation and in some hypothalamic nuclei, in response to suckling stimulation and this hormone has been implicated in the decrease in anxiety behavior seen in the early postpartum period. The relative importance of these hormones changes across lactation and it is becoming increasingly clear that many of the adaptations to motherhood reviewed here reflect the outcome of multiple influences. © 2016 American Physiological Society. Compr Physiol 6:1493-1518, 2016.

RF-amide neuropeptides and their receptors in Mammals: Pharmacological properties, drug development and main physiological functions

RF-amide neuropeptides, with their typical Arg-Phe-NH2 signature at their carboxyl C-termini, belong to a lineage of peptides that spans almost the entire life tree. Throughout evolution, RF-amide peptides and their receptors preserved fundamental roles in reproduction and feeding, both in Vertebrates and Invertebrates. The scope of this review is to summarize the current knowledge on the RF-amide systems in Mammals from historical aspects to therapeutic opportunities. Taking advantage of the most recent findings in the field, special focus will be given on molecular and pharmacological properties of RF-amide peptides and their receptors as well as on their implication in the control of different physiological functions including feeding, reproduction and pain. Recent progress on the development of drugs that target RF-amide receptors will also be addressed.

Expression of ESR1 in Glutamatergic and GABAergic Neurons Is Essential for Normal Puberty Onset, Estrogen Feedback, and Fertility in Female Mice

Circulating estradiol exerts a profound influence on the activity of the gonadotropin-releasing hormone (GnRH) neuronal network controlling fertility. Using genetic strategies enabling neuron-specific deletion of estrogen receptor α (Esr1), we examine here whether estradiol-modulated GABA and glutamate transmission are critical for the functioning of the GnRH neuron network in the female mouse. Using Vgat- and Vglut2-ires-Cre knock-in mice and ESR1 immunohistochemistry, we demonstrate that subpopulations of GABA and glutamate neurons throughout the limbic forebrain express ESR1, with ESR1-GABAergic neurons being more widespread and numerous than ESR1-glutamatergic neurons. We crossed Vgat- and Vglut2-ires-Cre mice with an Esr1(lox/lox) line to generate animals with GABA-neuron-specific or glutamate-neuron-specific deletion of Esr1. Vgat-ires-Cre;Esr1(lox/lox) mice were infertile, with abnormal estrous cycles, and exhibited a complete failure of the estrogen positive feedback mechanism responsible for the preovulatory GnRH surge. However, puberty onset and estrogen negative feedback were normal. Vglut2-ires-Cre;Esr1(lox/lox) mice were also infertile but displayed a wider range of deficits, including advanced puberty onset, abnormal negative feedback, and abolished positive feedback. Whereas <25% of preoptic kisspeptin neurons expressed Cre in Vgat- and Vglut2-ires-Cre lines, ∼70% of arcuate kisspeptin neurons were targeted in Vglut2-ires-Cre;Esr1(lox/lox) mice, possibly contributing to their advanced puberty phenotype. These observations show that, unexpectedly, ESR1-GABA neurons are only essential for the positive feedback mechanism. In contrast, we reveal the key importance of ESR1 in glutamatergic neurons for multiple estrogen feedback loops within the GnRH neuronal network required for fertility in the female mouse.

Increased Litter Size and Suckling Intensity Stimulate mRNA of RFamide-related Peptide in Rats

Background

RFamide-related peptide-3 (RFRP-3) inhibits gonadotropin releasing hormone (GnRH) and luteinizing hormone (LH) secretion in rats. This study evaluates the effects of litter size and suckling intensity on RFRP mRNA expression in the dorsomedial hypothalamic nucleus (DMH) of rats.

Materials and Methods

A total of 32 pregnant and 4 non-lactating ovariectomized (control group) Sprague-Dawley rats were used in this experimental study. Lactating rats were allotted to 8 equal groups. In 3 groups, the litter size was adjusted to 5, 10, or 15 pups upon parturition. Dams were allowed to suckle their pups continuously until 8 days postpartum. In the other 3 groups, the litter size was adjusted to 5 pups following birth. These pups were separated from the dams for 6 hours on day 8 postpartum, after which the pups were allowed to suckle for 2.5, 5, or 7.5 minutes prior to killing the dams. In 2 groups, lactating rats with 10 and 15 pups were separated from their pups for 6 hours on day 8 postpartum. In these groups, the pups were allowed to suckle their dams for 5 minutes before the dams were killed. All rats were killed on day 8 postpartum and the DMH was removed from each rat. We evaluated RFRP mRNA expression using realtime polymerase chain reaction (PCR).

Results

The expression of RFRP mRNA in the DMH increased with increased litter size and suckling intensity compared to the controls. The effect of suckling intensity on the expression of RFRP mRNA was more pronounced compared to the litter size.

Conclusion

Increased litter size and suckling intensity stimulated RFRP mRNA expression in the DMH which might contribute to lactation anestrus in rats.

Oestrogen-induced activation of preoptic kisspeptin neurones may be involved in the luteinising hormone surge in male and female Japanese monkeys

The oestrogen-induced luteinising hormone (LH) surge is evident in male primates, including humans, whereas male rodents never show the LH surge, even when treated with a preovulatory level of oestrogen. This suggests that the central mechanism governing reproductive hormones in primates is different from that in rodents. The present study aimed to investigate whether male Japanese monkeys conserve a brain mechanism mediating the oestrogen-induced LH surge via activation of kisspeptin neurones. Adult male and female Japanese monkeys were gonadectomised and then were treated with oestradiol-17β for 2 weeks followed by a bolus injection of oestradiol benzoate. Both male and female monkeys showed an oestrogen-induced LH surge. In gonadectomised monkeys sacrificed just before the anticipated time of the LH surge, oestrogen treatment significantly increased the number of KISS1-expressing cells in the preoptic area (POA) and enhanced the expression of c-fos in POA KISS1-positive cells of males and females. The oestrogen treatment failed to induce c-fos expression in the arcuate nucleus (ARC) kisspeptin neurones in both sexes just prior to LH surge onset. Thus, kisspeptin neurones in the POA but not in the ARC might be involved in the positive-feedback action of oestrogen that induces LH surge in male Japanese monkeys, as well as female monkeys. The present results indicate that oestrogen-induced activation of POA kisspeptin neurones may contribute to the LH surge generation in both sexes. The conservation of the LH surge generating system found in adult male primates, unlike rodents, could be a result of the capability of oestrogen to induce POA kisspeptin expression and activation.

Lactational anovulation in mice results from a selective loss of kisspeptin input to GnRH neurons

In mammals, lactation is associated with a period of infertility characterized by the loss of pulsatile secretion of GnRH and cessation of ovulatory cycles. Despite the importance of lactational infertility in determining overall fecundity of a species, the mechanisms by which the suckling stimulus suppresses GnRH secretion remain unclear. Because kisspeptin neurons are critical for fertility, the aim of this study was to test the hypothesis that reduced kisspeptin expression might mediate the lactation-induced suppression of fertility, using mouse models. In the rostral periventricular area of the third ventricle (RP3V), a progressive decrease in RP3V Kiss1 mRNA levels was observed during pregnancy culminating in a 10-fold reduction during lactation compared with diestrous controls. This was associated with approximately 60% reduction in the numbers of kisspeptin-immunoreactive neurons in the RP3V detected during lactation. Similarly, in the arcuate nucleus there was also a significant decrease in Kiss1 mRNA levels during late pregnancy and midlactation, and a notable decrease in kisspeptin fiber density during lactation. The functional characteristics of the RP3V kisspeptin input to GnRH neurons were assessed using electrophysiological approaches in an acute brain slice preparation. Although endogenous RP3V kisspeptin neurons were found to activate GnRH neurons in diestrous mice, this was never observed during lactation. This did not result from an absence of kisspeptin receptors because GnRH neurons responded normally to 100 nM exogenous kisspeptin during lactation. The kisspeptin deficit in lactating mice was selective, because GnRH neurons responded normally to RP3V gamma aminobutryic acid inputs during lactation. These data demonstrate that a selective loss of RP3V kisspeptin inputs to GnRH neurons during lactation is the likely mechanism causing lactational anovulation in the mouse.

Kisspeptin-gpr54 signaling at the GnRH neuron is necessary for negative feedback regulation of luteinizing hormone secretion in female mice

Kisspeptin-Gpr54 signaling is critical for regulating the activity of gonadotropin-releasing hormone (GnRH) neurons in mammals. Previous studies have shown that the negative feedback mechanism is disrupted in global Gpr54-null mutants. The present investigation aimed to determine (1) if a lack of cyclical estrogen exposure of the GnRH neuronal network in the life-long hypogonadotropic Gpr54-null mice contributed to their failed negative feedback mechanism and (2) the cellular location of disrupted kisspeptin-Gpr54 signaling. Plasma luteinizing hormone (LH) concentrations were determined in individual adult female mice when intact, following ovariectomy (OVX) and in response to an acute injection of 17β-estradiol (E2). Control mice exhibited a characteristic rise in LH after OVX that was suppressed by acute E2. Global Gpr54-null mice failed to exhibit any post-OVX increase in LH or response to E2. Adult female global Gpr54-null mice given a cyclical regimen of estradiol for three cycles prior to OVX also failed to exhibit any post-OVX increase in LH or response to E2. To address whether Gpr54 signaling at the GnRH neuron itself was necessary for the failed response to OVX in global Gpr54-null animals, adult female mice with a GnRH neuron-selective deletion of Gpr54 were examined. These mice also failed to exhibit any post-OVX increase in LH or response to E2. These experiments demonstrate defective negative feedback in global Gpr54-null mice that cannot be attributed to a lack of prior exposure of the GnRH neuronal network to cyclical estradiol. The absence of negative feedback in GnRH neuron-selective Gpr54-null mice demonstrates the necessity of direct kisspeptin signaling at the GnRH neuron for this mechanism to occur.

Estrogen receptor beta: tissue distribution and the still largely enigmatic physiological function

In 1996, the molecular biology of E2 had to be reevaluated: in an effort to identify novel nuclear receptors or unknown isoforms of existing receptors Kuiper and colleague described the expression of a novel subtype of the estrogen receptor (ER) in rat prostate and ovary. Upon this pioneering discovery the already known ER was renamed ERα while the newly described ER was termed ERβ. In this review an attempt is made to summarize the current knowledge regarding the expression and function of ERβ in selected reproductive and non-reproductive organs under physiological conditions. The data suggest that ERβ may be considered as a dominant-negative regulator of ERα modulating transcriptional responses to estrogens. The ratio of ER α vs. β. within a cell may determine the cell sensitivity to estrogens and its biological responses to the hormone.

CONCLUSION

It is not the ligand, it is the multiplicity of receptors which determines the plethora of estrogen actions. This article is part of a Special Issue entitled 'Phytoestrogens'.

A role for endocannabinoids in acute stress-induced suppression of the hypothalamic-pituitary-gonadal axis in male rats

OBJECTIVE

Stress is known to be an inhibitor of the reproductive hypothalamic-pituitary-gonadal (HPG) axis. However, the neural and molecular connections between stress and reproduction are not yet understood. It is well established that in both humans and rodents, kisspeptin (encoded by the kiss1 gene) is a strong stimulator of the HPG axis. In the present study we hypothesized that endocannabinoids, an important neuromodulatory system in the brain, can act on the HPG axis at the level of kiss1 expression to inhibit reproductive function under stress.

METHODS

Adult male Wistar rats were unilaterally implanted with an intracerebroventricular cannula. Afterwards, the animals were exposed to immobilization stress, with or without the presence of the cannabinoid CB1 receptor antagonist AM251 (1 µg/rat). Blood samples were collected through a retro-orbital plexus puncture before and after stress. Five hours after the stress, brain tissue was collected for reverse transcriptase-quantitative polymerase chain reaction measurements of kiss1 mRNA.

RESULTS

Immobilization stress (1 hour) resulted in a decrease in the serum luteinizing hormone concentration. Additionally, kiss1 gene expression was decreased in key hypothalamic nuclei that regulate gonadotrophin secretion, the medial preoptic area (mPOA), and to some extent the arcuate nucleus (ARC). A single central administration of AM251 was effective in blocking these inhibitory responses.

CONCLUSION

These findings suggest that endocannabinoids mediate, at least in part, immobilization stress-induced inhibition of the reproductive system. Our data suggest that the connection between immobilization stress and the HPG axis is kiss1 expression in the mPOA rather than the ARC.

Molecular evolution of kiss2 genes and peptides in vertebrates

The kiss1 peptide (kisspeptin), a product of the kiss1 gene, is one of the key neuropeptides regulating vertebrate reproduction. In 2009, we identified a paralogous gene of kiss1 in the brain of amphibians and named it kiss2. Currently, the presence of the kiss2 gene and the kiss2 peptide is still obscure in amniotes compared with that in other vertebrates. Therefore, we performed genome database analyses in primates and reptiles to investigate the molecular evolution of the kiss2 gene in vertebrates. Because the mature kiss2 peptide has been identified only in amphibians, we further performed immunoaffinity purification and mass spectrometry to identify the mature endogenous kiss2 peptide in the brains of salmon and turtle that possessed the kiss2 gene. Here we provide the first evidence for the presence of a kiss2-like gene in the genome database of primates including humans. Synthetic amidated human KISS2 peptide activated human GPR54 expressed in COS7 cells, but nonamidated KISS2 peptide was inactive. The endogenous amidated kiss2 peptide may not be produced in primates because of the lack of an amidation signal in the precursor polypeptide. The kiss2-like gene may be nonfunctional in crocodilians because of premature stop codons. We identified the mature amidated kiss2 peptide in turtles and fish and analyzed the localization of kiss2 peptide mRNA expression in fish. The present study suggests that the kiss2 gene may have mutated in primates and crocodilians and been lost in birds during the course of evolution. In contrast, the kiss2 gene and mature kiss2 peptide are present in turtles and fish.

Kisspeptin and GnRH pulse generation

The reproductive neuropeptide gonadotropin-releasing hormone (GnRH) has two modes of secretion. Besides the surge mode, which induces ovulation in females, the pulse mode of GnRH release is essential to cause various reproductive events in both sexes, such as spermatogenesis, follicular development, and sex steroid synthesis. Some environmental cues control gonadal activities through modulating GnRH pulse frequency. Researchers have looked for the anatomical location of the mechanism generating GnRH pulses, the GnRH pulse generator, in the brain, because an artificial manipulation of GnRH pulse frequency is of therapeutic importance to stimulate or suppress gonadal activity. Discoveries of kisspeptin and, consequently, KNDy (kisspeptin/neurokinin B/dynorphin) neurons in the hypothalamus have provided a clue to the possible location of the GnRH pulse generator. Our analyses of hypothalamic multiple-unit activity revealed that KNDy neurons located in the hypothalamic arcuate nucleus might play a central role in the generation of GnRH pulses in goats, and perhaps other mammalian species. This chapter further discusses the possible mechanisms for GnRH pulse generation.

Modulation of body temperature and LH secretion by hypothalamic KNDy (kisspeptin, neurokinin B and dynorphin) neurons: a novel hypothesis on the mechanism of hot flushes

Despite affecting millions of individuals, the etiology of hot flushes remains unknown. Here we review the physiology of hot flushes, CNS pathways regulating heat-dissipation effectors, and effects of estrogen on thermoregulation in animal models. Based on the marked changes in hypothalamic kisspeptin, neurokinin B and dynorphin (KNDy) neurons in postmenopausal women, we hypothesize that KNDy neurons play a role in the mechanism of flushes. In the rat, KNDy neurons project to preoptic thermoregulatory areas that express the neurokinin 3 receptor (NK3R), the primary receptor for NKB. Furthermore, activation of NK₃R in the median preoptic nucleus, part of the heat-defense pathway, reduces body temperature. Finally, ablation of KNDy neurons reduces cutaneous vasodilatation and partially blocks the effects of estrogen on thermoregulation. These data suggest that arcuate KNDy neurons relay estrogen signals to preoptic structures regulating heat-dissipation effectors, supporting the hypothesis that KNDy neurons participate in the generation of flushes.

Collapsin response mediator protein 4 affects the number of tyrosine hydroxylase-immunoreactive neurons in the sexually dimorphic nucleus in female mice

In the sexually dimorphic anteroventral periventricular nucleus (AVPV) of the hypothalamus, females have a greater number of tyrosine hydroxylase-immunoreactive (TH-ir) and kisspeptin-immunoreactive (kisspeptin-ir) neurons than males. In this study, we used proteomics analysis and gene-deficient mice to identify proteins that regulate the number of TH-ir and kisspeptin-ir neurons in the AVPV. Analysis of protein expressions in the rat AVPV on postnatal day 1 (PD1; the early phase of sex differentiation) using two-dimensional fluorescence difference gel electrophoresis followed by MALDI-TOF-MS identified collapsin response mediator protein 4 (CRMP4) as a protein exhibiting sexually dimorphic expression. Interestingly, this sexually differential expressions of CRMP4 protein and mRNA in the AVPV was not detected on PD6. Prenatal testosterone exposure canceled the sexual difference in the expression of Crmp4 mRNA in the rat AVPV. Next, we used CRMP4-knockout (CRMP4-KO) mice to determine the in vivo function of CRMP4 in the AVPV. Crmp4 knockout did not change the number of kisspeptin-ir neurons in the adult AVPV in either sex. However, the number of TH-ir neurons was increased in the AVPV of adult female CRMP4-KO mice as compared with the adult female wild-type mice. During development, no significant difference in the number of TH-ir neurons was detected between sexes or genotypes on embryonic day 15, but a female-specific increase in TH-ir neurons was observed in CRMP4-KO mice on PD1, when the sex difference was not yet apparent in wild-type mice. These results indicate that CRMP4 regulates the number of TH-ir cell number in the female AVPV.

Molecular cloning and identification of the transcriptional regulatory domain of the goat neurokinin B gene TAC3

Neurokinin B (NKB), encoded by TAC3, is thought to be an important accelerator of pulsatile gonadotropin-releasing hormone release. This study aimed to clarify the transcriptional regulatory mechanism of goat TAC3. First, we determined the full-length mRNA sequence of goat TAC3 from the hypothalamus to be 820 b, including a 381 b coding region, with the putative transcription start site located 143-b upstream of the start codon. The deduced amino acid sequence of NKB, which is produced from preproNKB, was completely conserved among goat, cattle, and human. Next, we cloned 5'-upstream region of goat TAC3 up to 3400 b from the translation initiation site, and this region was highly homologous with cattle TAC3 (89%). We used this goat TAC3 5'-upstream region to perform luciferase assays. We created a luciferase reporter vector containing DNA constructs from -2706, -1837, -834, -335, or -197 to +166 bp (the putative transcription start site was designated as +1) of goat TAC3 and these were transiently transfected into mouse hypothalamus-derived N7 cells and human neuroblastoma-derived SK-N-AS cells. The luciferase activity gradually increased with the deletion of the 5'-upstream region, suggesting that the transcriptional suppressive region is located between -2706 and -336 bp and that the core promoter exists downstream of -197 bp. Estradiol treatment did not lead to significant suppression of luciferase activity of any constructs, suggesting the existence of other factor(s) that regulate goat TAC3 transcription.

Kisspeptin as a master player in the central control of reproduction in mammals: an overview of kisspeptin research in domestic animals

The hypothalamo-pituitary-gonadal (HPG) axis is the regulatory system for reproduction in mammals. Because secretion of gonadotropin-releasing hormone (GnRH) into the portal vessels is the final step at which the brain controls gonadal activities, the GnRH neuronal system had been thought to be central to the HPG axis. A newly discovered neural peptide, kisspeptin, has opened a new era in reproductive neuroendocrinology. As shown in a variety of mammals, kisspeptin is a potent endogenous secretagogue of GnRH, and the kisspeptin neuronal system governs both the pulsatile GnRH secretion that drives folliculogenesis, spermatogenesis and steroidogenesis, and the GnRH surge that triggers ovulation in females. The kisspeptin neuronal system is therefore considered a master player in the central control of mammalian reproduction, and kisspeptin and related substances could therefore be valuable for the development of novel strategies for the management of fertility in farm animals. To this end, the present review aimed to summarize the current research on kisspeptin signaling with a focus on domestic animals such as sheep, goats, cattle, pigs and horses.

Characterization of the receptor binding residues of kisspeptins by positional scanning using peptide photoaffinity probes

Kisspeptins, endogenous peptide ligands for GPR54, play an important role in GnRH secretion. Since in vivo administration of kisspeptins induces increased plasma LH levels, GPR54 agonists hold promise as therapeutic agents for the treatment of hormonal secretion diseases. To facilitate the design of novel potent GPR54 ligands, residues in kisspeptins that involve in the interaction with GPR54 were investigated by kisspeptin-based photoaffinity probes. Herein, we report the design and synthesis of novel kisspeptin-based photoaffinity probes, and the application to crosslinking experiments for GPR54-expressing cells.

dynorphin-kappa opioid receptor signaling partly mediates estrogen negative feedback effect on LH pulses in female rats

Accumulating evidence suggests that the arcuate nucleus (ARC) kisspeptin/neurokinin B (NKB)/dynorphin (KNDy) neurons play a role in estrogen negative feedback action on pulsatile gonadotropin-releasing hormone (GnRH)/luteinizing hormone (LH) release. The present study aimed to determine if dynorphin (Dyn) is involved in estrogen negative feedback on pulsatile GnRH/LH release. The effect of the injection of nor-binaltorphimine (nor-BNI), a kappa-opioid receptor (KOR) antagonist, into the third cerebroventricle (3V) on LH pulses was determined in ovariectomized (OVX) adult female rats with/without replacement of negative feedback levels of estradiol (low E2). The mean LH concentrations and baseline levels of LH secretion in nor-BNI-injected, low E2-treated rats were significantly higher compared with vehicle-treated controls. On the other hand, the nor-BNI treatment failed to affect any LH pulse parameters in OVX rats without low E2 treatment. These results suggest that Dyn is involved in the estrogen negative feedback regulation of pulsatile GnRH/LH release. The low E2 treatment had no significant effect on the numbers of ARC Pdyn (Dyn gene)-,Kiss1- and Tac2 (NKB gene)-expressing cells. The treatment also did not affect mRNA levels of Pdyn and Oprk1 (KOR gene) in the ARC-median eminence region, but significantly increased the ARC kisspeptin immunoreactivity. These findings suggest that the negative feedback level of estrogen suppresses kisspeptin release from the ARC KNDy neurons through an unknown mechanism without affecting the Dyn and KOR expressions in the ARC. Taken together, the present result suggests that Dyn-KOR signaling is a part of estrogen negative feedback action on GnRH/LH pulses by reducing the kisspeptin release in female rats.

Regulation of gonadotropins by corticotropin-releasing factor and urocortin

While stress activates the hypothalamic-pituitary-adrenal (HPA) axis, it suppresses the hypothalamic-pituitary-gonadal (HPG) axis. Corticotropin-releasing factor (CRF) is a major regulatory peptide in the HPA axis during stress. Urocortin 1 (Ucn1), a member of the CRF family of peptides, has a variety of physiological functions and both CRF and Ucn1 contribute to the stress response via G protein-coupled seven transmembrane receptors. Ucn2 and Ucn3, which belong to a separate paralogous lineage from CRF, are highly selective for the CRF type 2 receptor (CRF(2) receptor). The HPA and HPG axes interact with each other, and gonadal function and reproduction are suppressed in response to various stressors. In this review, we focus on the regulation of gonadotropins by CRF and Ucn2 in pituitary gonadotrophs and of gonadotropin-releasing hormone (GnRH) via CRF receptors in the hypothalamus. In corticotrophs, stress-induced increases in CRF stimulate Ucn2 production, which leads to the inhibition of gonadotropin secretion via the CRF(2) receptor in the pituitary. GnRH in the hypothalamus is regulated by a variety of stress conditions. CRF is also involved in the suppression of the HPG axis, especially the GnRH pulse generator, via CRF receptors in the hypothalamus. Thus, complicated regulation of GnRH in the hypothalamus and gonadotropins in the pituitary via CRF receptors contributes to stress responses and adaptation of gonadal functions.

Organizational and activational effects of sex steroids on kisspeptin neuron development

Kisspeptin, encoded by the Kiss1 gene, is a neuropeptide required for puberty and adult reproductive function. Understanding the regulation and development of the kisspeptin system provides valuable knowledge about the physiology of puberty and adult fertility, and may provide insights into human pubertal or reproductive disorders. Recent studies, particularly in rodent models, have assessed how kisspeptin neurons develop and how hormonal and non-hormonal factors regulate this developmental process. Exposure to sex steroids (testosterone and estradiol) during critical periods of development can induce organizational (permanent) effects on kisspeptin neuron development, with respect to both sexually dimorphic and non-sexually dimorphic aspects of kisspeptin biology. In addition, sex steroids can also impart activational (temporary) effects on kisspeptin neurons and Kiss1 gene expression at various times during neonatal and peripubertal development, as they do in adulthood. Here, we discuss the current knowledge--and in some cases, lack thereof--of the influence of hormones and other factors on kisspeptin neuronal development.

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.

Influence of ERβ selective agonism during the neonatal period on the sexual differentiation of the rat hypothalamic-pituitary-gonadal (HPG) axis

Background

It is well established that sexual differentiation of the rodent hypothalamic-pituitary-gonadal (HPG) axis is principally orchestrated by estrogen during the perinatal period. Here we sought to better characterize the mechanistic role the beta form of the estrogen receptor (ERβ) plays in this process.

Methods

To achieve this, we exposed neonatal female rats to three doses (0.5, 1 and 2 mg/kg) of the ERβ selective agonist diarylpropionitrile (DPN) using estradiol benzoate (EB) as a positive control. Measures included day of vaginal opening, estrous cycle quality, GnRH and Fos co-localization following ovariectomy and hormone priming, circulating luteinizing hormone (LH) levels and quantification of hypothalamic kisspeptin immunoreactivity. A second set of females was then neonatally exposed to DPN, the ERα agonist propyl-pyrazole-triol (PPT), DPN+PPT, or EB to compare the impact of ERα and ERβ selective agonism on kisspeptin gene expression in pre- and post-pubescent females.

Results

All three DPN doses significantly advanced the day of vaginal opening and induced premature anestrus. GnRH and Fos co-labeling, a marker of GnRH activation, following ovariectomy and hormone priming was reduced by approximately half at all doses; the magnitude of which was not as large as with EB or what we have previously observed with the ERα agonist PPT. LH levels were also correspondingly lower, compared to control females. No impact of DPN was observed on the density of kisspeptin immunoreactive (-ir) fibers or cell bodies in the arcuate (ARC) nucleus, and kisspeptin-ir was only significantly reduced by the middle (1 mg/kg) DPN dose in the preoptic region. The second experiment revealed that EB, PPT and the combination of DPN+PPT significantly abrogated preoptic Kiss1 expression at both ages but ARC expression was only reduced by EB.

Conclusion

Our results indicate that selective agonism of ERβ is not sufficient to completely achieve male-typical HPG organization observed with EB or an ERα agonist.

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.

Kisspeptin neurons mediate reflex ovulation in the musk shrew (Suncus murinus)

The present study investigated whether kisspeptin-G protein-coupled receptor 54 (GPR54) signaling plays a role in mediating mating-induced ovulation in the musk shrew (Suncus murinus), a reflex ovulator. For this purpose, we cloned suncus Kiss1 and Gpr54 cDNA from the hypothalamus and found that suncus kisspeptin (sKp) consists of 29 amino acid residues (sKp-29). Injection of exogenous sKp-29 mimicked the mating stimulus to induce follicular maturation and ovulation. Administration of several kisspeptins and GPR54 agonists also induced presumed ovulation in a dose-dependent manner, and Gpr54 mRNA was distributed in the hypothalamus, showing that kisspeptins induce ovulation through binding to GPR54. The sKp-29-induced ovulation was blocked completely by pretreatment with a gonadotropin-releasing hormone (GnRH) antagonist, suggesting that kisspeptin activates GnRH neurons to induce ovulation in the musk shrew. In addition, in situ hybridization revealed that Kiss1-expressing cells are located in the medial preoptic area (POA) and arcuate nucleus in the musk shrew hypothalamus. The number of Kiss1-expressing cells in the POA or arcuate nucleus was up-regulated or down-regulated by estradiol, suggesting that kisspeptin neurons in these regions were the targets of the estrogen feedback action. Finally, mating stimulus largely induced c-Fos expression in Kiss1-positive cells in the POA, indicating that the mating stimulus activates POA kisspeptin neurons to induce ovulation. Taken together, these results indicate that kisspeptin-GPR54 signaling plays a role in the induction of ovulation in the musk shrew, a reflex ovulator, as it does in spontaneous ovulators.

Neonatal exposure to genistein adversely impacts the ontogeny of hypothalamic kisspeptin signaling pathways and ovarian development in the peripubertal female rat

Neonatal exposure to estrogenic endocrine disrupting compounds (EDCs) can advance pubertal onset and induce premature anestrous in female rats. It was recently discovered that hypothalamic kisspeptin (KISS) signaling pathways are sexually dimorphic and regulate both the timing of pubertal onset and estrous cyclicity. Thus we hypothesized that disrupted sex specific ontogeny of KISS signaling pathways might be a mechanism underlying these EDC effects. We first established the sex specific development of KISS gene expression, cell number and neural fiber density across peripuberty in the anteroventral periventricular nucleus (AVPV) and arcuate nucleus (ARC), hypothesizing that the sexually dimorphic aspects of KISS signaling would be most vulnerable to EDCs. We next exposed female rats to the phytoestrogen genistein (GEN, 1 or 10 mg/kg bw), estradiol benzoate (EB, 10 μg), or vehicle from post natal day (P) 0-3 via subcutaneous (sc) injection. Animals were sacrificed on either P21, 24, 28, or 33 (n=5-14 per group at each age). Vaginal opening was significantly advanced by EB and the higher dose of GEN compared to control animals and was accompanied by lower numbers of KISS immunoreactive fibers in the AVPV and ARC. Ovarian morphology was also assessed in all age groups for the presence of multiple oocyte follicles (MOFs). The number of MOFs decreased over time in each group, and none were observed in control animals by P24. MOFs were still present, however, in the EB and 10 mg/kg GEN groups beyond P24 indicating a disruption in the timing of ovarian development.

Increase of anteroventral periventricular kisspeptin neurons and generation of E2-induced LH-surge system in male rats exposed perinatally to environmental dose of bisphenol-A

Perinatal exposure to environmental levels of bisphenol-A (BPA) impairs sexually dimorphic behaviors in rodents. Kisspeptin neurons in anteroventral periventricular nucleus (AVPV), which plays an important role in the activation of GnRH neurons and the initiation of LH-surge, have been suggested to be sexual dimorphism in rats. This study focused on exploring the influence of a perinatal exposure to an environmental dose of BPA on the development and maturation of male AVPV kisspeptin neurons and hypothalamus-pituitary-gonadal axis. Female rats were injected sc with 2 μg BPA/kg·d from gestation d 10 through lactation d 7. Anatomical and functional changes in AVPV kisspeptin neurons and hypothalamus-pituitary-gonadal axis were examined in prepubertal, pubertal, and adult male rats exposed perinatally to BPA (BPA-rats). Here, we show that in postnatal d (PND)30/50/90 BPA-rats, the number of AVPV kisspeptin-immunoreactive cells was persistently increased in comparison with age-matched control male rats. The number of GnRH-immunoreactive cells in PND30 BPA-rats declined approximately 40% compared with control male rats, whereas that in PND50/90 BPA-rats was increased in a G protein-coupled receptor 54-dependent manner. Estradiol could induce a stable LH-surge in PND90 BPA-rats and control female rats, which was sensitive to the G protein-coupled receptor 54 inhibitor. In PND30/50 BPA-rats, plasma level of LH was higher, but the level of testosterone was lower than control male rats. These findings provide evidence that perinatal exposure to an environmental dose of BPA causes a sustained increase in AVPV kisspeptin neurons in male rats, leading to the generation of estradiol-induced LH-surge system.

Involvement of neurokinin receptors in the control of pulsatile luteinizing hormone secretion in rats

It has recently been shown that neurokinin B, a tachykinin, is associated with GnRH pulse generation in sheep and goats. The aim of the present study was to clarify the role of tachykinin receptors in the control of LH secretion in rats. To this end, we evaluated the effect of CS-003, an antagonist for all three neurokinin receptors (NK1, NK2 and NK3 receptors), on pulsatile LH secretion in both sexes of rats with different routes of administration. Both oral and third ventricular administration of CS-003 suppressed LH secretion in both sexes of gonadectomized animals. Furthermore, intact male rats with oral administration of CS-003 showed decreased serum testosterone levels, which might be due to suppressed LH secretion. None of the three subtype-specific neurokinin receptor antagonists showed a significant effect on LH secretion in ovariectomized rats when each antagonist was singly administered. The present results suggest that neurokinins play a role in the control of pulsatile GnRH/LH secretion via multiple neurokinin receptors in both male and female rats.

Cross-Talk between Metabolism and Reproduction: The Role of POMC and SF1 Neurons

Energy homeostasis and reproduction require tight coordination, but the mechanisms underlying their interaction are not fully understood. Two sets of hypothalamic neurons, namely pro-opiomelanocortin (POMC) neurons in the arcuate nucleus and steroidogenic factor-1 (SF1) neurons in the ventromedial hypothalamic nucleus, are emerging as critical nodes where metabolic and reproductive signals communicate. This view is supported by recent genetic studies showing that disruption of metabolic signals (e.g., leptin and insulin) or reproductive signals (e.g., estradiol) in these neurons leads to impaired regulation of both energy homeostasis and fertility. In this review, we will examine the potential mechanisms of neuronal communication between POMC, SF1, and gonadotropin-releasing hormone neurons in the regulation of metabolism and reproduction.

Fasting reduces the kiss1 mRNA levels in the caudal hypothalamus of gonadally intact adult female rats

Kisspeptin, which is the product of the kiss1 gene and its receptor kiss1r, have emerged as the essential gatekeepers of reproduction. The present study used gonadally intact female rats to evaluate fasting-induced suppression of the KiSS-1 system of anteroventral periventricular nucleus (AVPV) and arcuate nucleus (ARC) under normal physiological conditions. Starting on the day of estrous, one group of rats was subjected to 72 h of food deprivation, while the other group of rats was able to continue feeding ad libitum. The length of the estrous cycle was significantly longer in the food-deprived rats as compared to the feeding rats. At the end of the 72-h food deprivation period, all of the food-deprived rats were at the diestrous phase, with their serum concentrations of LH and leptin significantly lower than that observed in the feeding rats. In addition, as compared to the feeding rats, the expression levels of kiss1 mRNA were significantly lower in the food-deprived rats in the posterior hypothalamic block, which contained the ARC, but not in the anterior hypothalamic block, which contain the AVPV. However, both the kiss1r mRNA expression levels in the anterior and posterior hypothalamic blocks and the neurokinin B and neurokinin 3 receptor mRNA expression levels in the posterior hypothalamic block were not significantly different between the feeding and food-deprived rats. Thus, lower kiss1 mRNA levels in the ARC appear to be responsible for the fasting-induced inhibition of gonadotrophin secretion and subsequent prolongation of the estrous cycle.

Effects of brief exposure of male pheromone on multiple-unit activity at close proximity to kisspeptin neurons in the goat arcuate nucleus

Exposure of females to the male pheromone induces pulsatile release of gonadotropin-releasing hormone (GnRH) in goats. Recently, kisspeptin neurons in the arcuate nucleus (ARC) have been suggested to represent the proximate source of the GnRH pulse generator. In this study, we examined the effects of the pheromone on multiple-unit activity (MUA) in female goats fitted with recording electrodes aimed at the ARC kisspeptin neurons. In all eight goats, periodic bursts in MUA (MUA volleys), which were considered to be electrophysiological manifestations of the GnRH pulse generator, were observed. The mean intervolley interval (T) during the control period was calculated in each goat that was then exposed to the male pheromone for 1 sec at timings of 1/4 T, 1/2 T or 3/4 T after one regularly occurring MUA volley. An instantaneous rise in MUA was observed immediately after the exposure regardless of timing. Exposure at a timing of 3/4 T resulted in an MUA volley within 60 sec following the instantaneous rise in all goats. In contrast, an MUA volley was induced in only 2 goats by exposure at 1/2 T, while exposure at 1/4 T failed to induce an MUA volley in any goats. These results suggest that transmission of the pheromone signal to the ARC, represented by an instantaneous rise, activates the GnRH pulse generator. Moreover, the timing-dependent pheromone action in inducing an MUA volley indicates that the GnRH pulse generator has a refractory period for the pheromone signal after the burst.

The neuroendocrine basis of lactation-induced suppression of GnRH: role of kisspeptin and leptin

Lactation is an important physiological model of the integration of energy balance and reproduction, as it involves activation of potent appetitive neuropeptide systems coupled to a profound inhibition of pulsatile GnRH/LH secretion. There are multiple systems that contribute to the chronic hyperphagia of lactation: 1) suppression of the metabolic hormones, leptin and insulin, 2) activation of hypothalamic orexigenic neuropeptide systems NPY, AGRP, orexin (OX) and melanin concentrating hormone (MCH), 3) special induction of NPY expression in the dorsomedial hypothalamus, and 4) suppression of anorexigenic systems POMC and CART. These changes ensure adequate energy intake to meet the metabolic needs of milk production. There is significant overlap in all of the systems that regulate food intake with the regulation of GnRH, suggesting there could be several redundant factors acting to suppress GnRH/LH during lactation. In addition to an overall increase in inhibitory tone acting directly on GnRH cell bodies that is brought about by increases in orexigenic systems, there are also effects at the ARH to disrupt Kiss1/neurokinin B/dynorphin neuronal function through inhibition of Kiss1 and NKB. These changes could lead to an increase in inhibitory auto-regulation of the Kiss1 neurons and a possible disruption of pulsatile GnRH release. While the low levels of leptin and insulin contribute to the changes in ARH appetitive systems, they do not appear to contribute to the suppression of ARH Kiss1 or NKB. The inhibition of Kiss1 may be the key factor in the suppression of GnRH during lactation, although the mechanisms responsible for its inhibition are unknown.

Corticotrophin-releasing factor and stress-induced inhibition of the gonadotrophin-releasing hormone pulse generator in the female

It is well established that stress activates the hypothalamo-pituitary-adrenal (HPA) axis and suppresses the hypothalamo-pituitary-gonadal (HPG) axis. A large literature dealing with various stressors that regulate gonadotrophin-releasing hormone (GnRH) secretion in a variety of species (including nonhuman primates, sheep, and rats) provides evidence that stress modulates GnRH secretion by activating the corticotrophin-releasing factor (CRF) system and sympathoadrenal pathways, as well as the limbic brain. Different stressors may suppress the HPG axis by activating or inhibiting various pathways in the CNS. In addition to CRF being the principal hypophysiotropic factor driving the HPA axis, it is a potent inhibitor of the GnRH pulse generator. The suppression of the GnRH pulse generator by a variety of stressful stimuli can be blocked by CRF antagonists, suggesting a pivotal role for endogenous CRF. The differential roles for CRF receptor type 1 (CRF-R1) and CRF-R2 in stress-induced suppression of the GnRH pulse generator add to the complexity of CRF regulation of the HPG axis. Although the precise sites and mechanisms of action remain to be elucidated, noradrenergic and gamma-amino-butyric acid (GABA) neurones are implicated in the system's regulation, and opioids and kisspeptin in the medial preoptic area (mPOA) and hypothalamic arcuate nucleus (ARC) may operate downstream of the CRF neuronal system.