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

The Neuroendocrine Regulation of Reproductive Behavior and Emotional Control by Kisspeptin

Reproductive success and ultimately species survival at a population level is contingent on a plethora of neuroendocrine signals working in concert to regulate gonadal function and reproductive behavior. Among these, the neuropeptide kisspeptin (encoded by the KISS1/Kiss1 gene) has emerged as the master regulator of the hypothalamic-pituitary-gonadal axis. Besides the hypothalamus, both kisspeptin and its cognate receptor are extensively expressed throughout cortico-limbic brain structures in rodents and humans, which are regions traditionally implicated in behavioral and emotional responses. Thus, there exists a neuroanatomical framework through which kisspeptin can integrate reproductive behavior and emotional regulation with the reproductive axis. Accordingly, this sets the scene for recent findings derived from an assortment of species, including humans, unveiling kisspeptin as an important gatekeeper of reproductive behavior and emotional control. Herein, we summarize the major preclinical animal and human experimental evidence identifying kisspeptin as a key neuromodulator of reproductive behavior and emotional state. Such findings have laid the foundations for clinical applications of kisspeptin-based therapies for patients with related reproductive and psychosexual disorders.

Comprehensive chemoanatomical mapping, and the gonadal regulation, of seven kisspeptin neuronal populations in the mouse brain

Kisspeptinergic signaling is well-established as crucial for the regulation of reproduction, but its potential broader role in brain function is less understood. This study investigates the distribution and chemotyping of kisspeptin-expressing neurons within the mouse brain. RNAscope single, dual, and multiplex in situ hybridization methods were used to assess kisspeptin mRNA (Kiss1) expression and its co-expression with other neuropeptides, excitatory and inhibitory neurotransmitter markers, and sex steroid receptors in wild-type intact and gonadectomized young adult mice. Seven distinct kisspeptin neuronal chemotypes were characterized, including two novel kisspeptin-expressing groups described for the first time, that is, the Kiss1 population in the ventral premammillary nucleus and the nucleus of the solitary tract. Kiss1 mRNA was also observed to localize in both somatic and dendritic compartments of hypothalamic neurons. High androgen receptor expression and changes in medial amygdala and septo-hypothalamic Kiss1 expression following GDX in males, but not in females, suggest a role for androgen receptors in regulating kisspeptin signaling. This study provides a detailed chemoanatomical map of kisspeptin-expressing neurons, highlighting their potential functional diversity. The discovery of a new kisspeptin-expressing group and gonadectomy-induced changes in Kiss1 expression patterns suggest broader roles for kisspeptin in brain functions beyond those of reproduction.

Involvement of nuclear receptor corepressor 2 (NCOR2) in estrogen-induced repression of arcuate Kiss1 expression in female rats

Hypothalamic arcuate (ARC) kisspeptin neurons are considered the gonadotropin-releasing hormone pulse generator in rats. In virgin rats, the expression of the ARC kisspeptin gene (Kiss1) is repressed by proestrous levels of estradiol-17β (high E2) but not by diestrous levels of E2 (low E2). In lactating rats, ARC Kiss1 expression is repressed by low E2 during late lactation. This study aimed to investigate whether nuclear receptor corepressor 2 (NCOR2, encoded by Ncor2), an estrogen receptor α corepressor, is involved in the estrogen-induced repression of ARC Kiss1 expression in rats. Double in situ hybridization for Kiss1 and Ncor2 revealed that approximately 80% of ARC Kiss1-expressing cells co-expressed Ncor2 in ovariectomized (OVX) + low E2 virgin rats, while approximately 90% of ARC Kiss1-expressing cells co-expressed Ncor2 in OVX + low E2 lactating rats. To further examine the role of Ncor2, we studied the effects of Kiss1-dependent Ncor2 knockdown on ARC Kiss1 expression and luteinizing hormone (LH) pulses. An adeno-associated virus vector carrying Cre-activated short hairpin RNA (shRNA) for Ncor2 was administered to the ARC in two Kiss1-Cre rat models: OVX + high E2 Kiss1-Cre virgin rats and OVX + low E2 Kiss1-Cre lactating rats. Ncor2-shRNA treatment significantly increased the number of ARC Kiss1-expressing cells and the intensity of Kiss1 signals in OVX + high E2 virgin rats but failed to fully restore low E2-induced Kiss1 repression in lactating rats. The Ncor2-shRNA treatment failed to affect LH pulses in both models. These findings suggest that NCOR2 in ARC kisspeptin neurons mediates high E2-induced repression of ARC Kiss1 expression in virgin rats.

Reduction in minipubertal gonadotropin levels alters reproductive lifespan and ovarian follicular loss in female mice

STUDY QUESTION

What is the effect of attenuating the physiological hypergonadotropic activity encountered at minipuberty on female reproductive function in a mouse model?

SUMMARY ANSWER

Decreasing the surge of gonadotropins at minipuberty extended reproductive lifespan, coinciding with alterations in neuroendocrine and ovarian aging.

WHAT IS KNOWN ALREADY

Minipuberty is characterized by the tremendous activation of the gonadotrope axis, as evidenced by elevated levels of gonadotropins regulating folliculogenesis and the synthesis of ovarian hormones, but its role in fertility remains unclear.

STUDY DESIGN, SIZE, DURATION

To determine the link between gonadotrope axis activity at minipuberty and reproductive parameters, we used a pharmacological approach to suppress gonadotropin levels in Swiss mice by injecting daily a GnRH receptor antagonist (GnRHR) (Ganirelix, 10 µg/mouse) or its vehicle between 10 and 16 postnatal days, to cover the entire duration of minipuberty. We analyzed the onset of puberty and estrous cyclicity as well as fertility in young (3-5 months) and middle-aged (11 months) mice from control (CTR) and antagonist-treated groups (n = 17-20 mice/age and treatment group). Ovaries and brains were collected, fixed, and sectioned (for histology, follicle count, and immunohistochemistry) or frozen (for analysis of follicular markers, aging, and inflammation) from adult females, and blood was collected by cardiac puncture for hormonal assays (n = 3-8 mice/age and treatment group).

PARTICIPANTS/MATERIALS, SETTING, METHODS

To analyze the initiation of puberty, we monitored vaginal opening and performed vaginal smears in CTR and antagonist-treated mice. We studied estrous cyclicity on vaginal smears at the beginning of reproductive life. Mice were mated several times with males to assess fertility rates, delay of conception, and litter size. To evaluate ovarian function, we counted follicles at different stages and corpora lutea, and we determined the relative intra-ovarian abundance of key follicular markers by real-time RT-PCR, as well as the levels of circulating anti-Müllerian hormone (AMH) and progesterone by ELISA and GC-MS, respectively. We also analyzed features of ovarian aging and inflammation by histology and by measuring the relative intra-ovarian abundance of some markers using real-time RT-PCR. To determine the impact on neuroendocrine determinants related to the CTR of reproduction, we analyzed circulating gonadotropin levels using Luminex assays as well as kisspeptin and GnRH immunoreactivity in the hypothalamus by immunohistochemistry.

MAIN RESULTS AND THE ROLE OF CHANCE

Our results show that the treatment had no impact on the initiation of puberty, estrous cyclicity, or fertility at the beginning of reproductive life. However, it increased reproductive lifespan, as shown by the higher percentage of antagonist-treated females than CTRs still fertile at 11 months of age (33% versus 6%; P = 0.0471). There were no significant differences in the number of kisspeptin and GnRH neurons, nor in the density of kisspeptin- and GnRH-immunoreactive neurons in the hypothalamic areas involved in reproduction between the two groups of mice studied at either 4 or 11 months. In addition, basal levels of FSH were comparable between the two groups at 4 and 11 months, but not those of LH at 11 months which were much lower in females treated with antagonist than in their age-matched CTRs (237 ± 59.6 pg/ml in antagonist-treated females versus 1027 ± 226.3 pg/ml in CTRs, P = 0.0069). Importantly, at this age, antagonist-treated mice had basal LH levels comparable to young mice (e.g. in 4-month-old CTRs: 294 ± 71.75 pg/ml, P > 0.05). Despite their prolonged reproductive lifespan and delayed neuroendocrine aging, antagonist-treated mice exhibited earlier depletion of their follicles, as shown by lower numbers of primordial, primary, and preantral follicles associated with lower circulating AMH levels and relative intra-ovarian abundance of Amh transcripts than CTR mice. However, they exhibited comparable completion of folliculogenesis, as suggested by the numbers of antral follicles and corpora lutea, relative intra-ovarian abundance of Cyp19a1, Inhba, and Inhbb transcripts, and circulating progesterone levels that all remained similar to those of the CTR group. These observed alterations in ovarian function were not associated with increased ovarian aging or inflammation.

LARGE-SCALE DATA

None.

LIMITATIONS, REASONS FOR CAUTION

This study was carried out on mice, which is a validated research model. However, human research is needed for further validation.

WIDER IMPLICATIONS OF THE FINDINGS

This study, which is the first to investigate the physiological role of minipuberty on reproductive parameters, supports the idea that suppressing the high postnatal levels of gonadotropins may have long-term effects on female fertility by extending the duration of reproductive life. Perturbations in gonadotropin levels during this period of life, such as those observed in infants born prematurely, may thus have profound consequences on late reproductive functions.

STUDY FUNDING/COMPETING INTEREST(S)

This research was conducted with the financial support of ANR AAPG2020 (ReproFUN), CNRS, Inserm, Université Paris Cité, and Sorbonne Université. The authors declare that they have no conflicts of interest.

Kisspeptin and neurokinin B: roles in reproductive health

Kisspeptin and neurokinin B (NKB) play a key role in several physiological processes including in puberty, adult reproductive function including the menstrual cycle, as well as mediating the symptoms of menopause. Infundibular kisspeptin neurons, which coexpress NKB, regulate the activity of gonadotropin-releasing hormone (GnRH) neurons and thus the physiological pulsatile secretion of GnRH from the hypothalamus. Outside of their hypothalamic reproductive roles, these peptides are implicated in several physiological functions including sexual behavior and attraction, placental function, and bone health. Over the last two decades, research findings have considerably enhanced our understanding of the physiological regulation of the hypothalamic-pituitary-gonadal (HPG) axis and identified potential therapeutic applications. For example, recognition of the role of kisspeptin as the natural inductor of ovulation has led to research investigating its use as a safer, more physiological trigger of oocyte maturation in in vitro fertilization (IVF) treatment. Moreover, the key role of NKB in the pathophysiology of menopausal hot flashes has led to the development of pharmacological antagonism of this pathway. Indeed, fezolinetant, a neurokinin 3 receptor antagonist, has recently received Food and Drug Administration (FDA) approval for clinical use to treat menopausal vasomotor symptoms. Here, we discuss the roles of kisspeptin and NKB in human physiology, including in the regulation of puberty, menstrual cyclicity, reproductive behavior, pregnancy, menopause, and bone homeostasis. We describe how perturbations of these key physiological processes can result in disease states and consider how kisspeptin and NKB could be exploited diagnostically as well as therapeutically to treat reproductive disorders.

Regulatory Involvement of Kisspeptin in Energy Balance and Reproduction

The hypothalamic-pituitary-gonadal axis, which regulates steroidogenesis and germ cell formation, closely regulates the reproduction process. Nonetheless, other chemical mediators, such as kisspeptin, influence this axis. Kisspeptin is a hypothalamic neuropeptide that modulates the function of this axis and also plays a central role in energy balance. The present study reviews the impact and associated mechanisms of kisspeptin on male and female reproduction based on available evidence in the literature. Kisspeptin and its neurons exert anorexigenic activity, thus maintaining adequate energy balance for optimal reproductive function. Also, they stimulate the release of GnRH, resulting in the optimal performance of gonadal physiological processes viz. production of steroid sex hormones and germ cells. However, studies linking kisspeptin to reproduction are yet scanty. Hence, studies exploring the upstream and downstream signaling pathways activated by kisspeptin concerning reproduction in an attempt to better understand the associated mechanisms of the regulatory activities of kisspeptin on reproduction are recommended. In addition, potential factors that may modulate kisspeptin activities may be useful in the management of infertility and perhaps, in the development of contraceptives for those who do not intend to achieve conception.

GnRH and Cognition

GnRH is traditionally recognized as the central regulator of reproduction through its pulsatile secretion, which governs the hypothalamic-pituitary-gonadal axis. However, recent evidence has highlighted its broader role in brain development and function, including in cognitive and higher intellectual processes. GnRH production follows distinct phases, from its early activation during minipuberty-the first postnatal activation of GnRH neurons during the infantile period-, its reactivation and stabilization starting at puberty, and its eventual decline with age and the loss of gonadal steroid feedback. This evolution depends on the establishment, maturation and activation of GnRH neurons, a complex process regulated by the cellular and molecular environment of these neurons, including multiple neuronal and glial types as well as a minipubertal "switch" in gene expression, the perturbation of which may have long-term or delayed consequences for both reproductive and cognitive function. The cognitive role of GnRH may be related to its recently revealed involvement in maintaining myelination and synaptic plasticity, whereas disruptions in its finely tuned rhythmic secretion, either age-related or pathological, are associated with cognitive decline and neurodegenerative disorders. Restoring physiological GnRH levels and pulsatility can reverse age-related cognitive decline and improve sensory functions even in adulthood, suggesting a mobilization of the "cognitive reserve" in both animal models and human patients. This review highlights recent advances in our understanding of the GnRH system and the therapeutic potential of pulsatile GnRH therapy to mitigate age-related cognitive decline and neurodegenerative processes.

Estrogen-Regulated Lateral Septal Kisspeptin Neurons Abundantly Project to GnRH Neurons and the Hypothalamic Supramammillary Nucleus

While hypothalamic kisspeptin (KP) neurons play well-established roles in the estrogen-dependent regulation of reproduction, little is known about extrahypothalamic KP-producing (KPLS) neurons of the lateral septum. As established previously, Kiss1 expression in this region is low and regulated by estrogen receptor- and GABAB receptor-dependent mechanisms. Our present experiments on Kiss1-Cre/ZsGreen knock-in mice revealed that transgene expression in the LS begins at Postnatal Day (P)33-36 in females and P40-45 in males and is stimulated by estrogen receptor signaling. Fluorescent cell numbers continue to increase in adulthood and are higher in females. Viral tracing uncovered that the bulk of KPLS fibers joins the medial forebrain bundle and terminates in the hypothalamic supramammillary nucleus. Smaller subsets innervate the medial amygdala or project to other limbic structures. One-quarter of gonadotropin-releasing hormone (GnRH)-immunoreactive perikarya in the preoptic area and their dendrites receive appositions from KPLS axons. OVX adult Kiss1-Cre/ZsGreen mice treated for 4 d with 17β-estradiol or vehicle were used for RNA sequencing studies of laser-microdissected KPLS neurons. The transcriptome included markers of GABAergic and neuropeptidergic (Penk, Cartpt, Vgf) cotransmission and 571 estrogen-regulated transcripts. Estrogen treatment upregulated the acetylcholine receptor transcript Chrm2 and, in slice electrophysiology experiments, caused enhanced muscarinic inhibition of KPLS neurons. Finally, we provided immunohistochemical evidence for homologous neurons in the postmortem human brain, suggesting that KPLS neurons may contribute to evolutionarily conserved regulatory mechanisms. Future studies will need to investigate the putative roles of KPLS neurons in the estrogen-dependent control of GnRH neurons and/or various hypothalamic/limbic functions.

Nutritionally responsive PMv DAT neurons are dynamically regulated during pubertal transition

Pubertal development is tightly regulated by energy balance. The crosstalk between metabolism and reproduction is orchestrated by complex neural networks and leptin action in the hypothalamus plays a critical role. The ventral premammillary nucleus (PMv) leptin receptor (LepRb) neurons act as an essential relay for leptin action on reproduction. Here, we show that mouse PMv cells expressing the dopamine transporter (DAT) gene, Slc6a3 (PMvDAT) form a novel subpopulation of LepRb neurons. Virtually all PMvDAT neurons expressed Lepr mRNA and responded to acute leptin treatment. Electrophysiological recordings from DATCRE;tdTomato mice showed that PMvDAT cells in prepubertal females have a hyperpolarized resting membrane potential compared to diestrous females. Slc6a3 mRNA expression in the PMv was higher in prepubertal than in adult females. In prepubertal females Slc6a3 mRNA expression was higher in overnourished females from small size litters than in controls. Prepubertal Lep ob females showed decreased PMv Slc6a3 mRNA expression, that recovered to control levels after 3 days of leptin injections. Using a tracer adenoassociated virus in the PMv of adult DATCre;Kiss1hrGFP females, we observed PMvDAT projections in the anteroventral periventricular and periventricular nucleus (AVPV/PeN), surrounding Kiss1hrGFP neurons, a population critical for sexual maturation and positive estrogen feedback in females. The DATCRE;tdTomato projections to the AVPV were denser in adult than in prepubertal females. In adults, they surrounded tyrosine hydroxylase neurons. Overall, these findings suggest that the DAT expressing PMvLepRb subpopulation play a role in leptin regulation of sexual maturation via actions on AVPV kisspeptin/tyrosine hydroxylase neurons.

Multiple effects of kisspeptin on neuroendocrine, reproduction, and metabolism in polycystic ovary syndrome

Polycystic ovary syndrome (PCOS) is a highly prevalent and heterogeneous disease characterized by a combination of reproductive and endocrine abnormalities, often associated with metabolic and mental health disorders. The etiology and pathogenesis of PCOS remain unclear, but recent research has increasingly focused on the upstream mechanisms underlying its development. Among these, kisspeptin (KISS) signaling has emerged as a pivotal component in the regulation of the hypothalamic-pituitary-gonadal axis, with significant roles in reproductive function, energy regulation, and metabolism. Women with PCOS commonly exhibit disruptions in gonadotropin secretion, including elevated luteinizing hormone (LH) levels, imbalanced LH/follicle-stimulating hormone (FSH) ratios, and increased androgen levels, all of which are usually parallel with abnormal KISS signaling. Furthermore, alterations in the KISS/KISS1R system within the central and circulatory systems, as well as peripheral tissues, have been implicated in the development of PCOS. These changes affect multiple pathophysiological domains, including reproductive function, energy regulation, metabolic homeostasis, inflammatory response, and emotional disorders, and are further influenced by lifestyle and environmental factors. This review aims to comprehensively summarize the existing experimental and clinical evidence supporting these roles of KISS in PCOS, with the goal of establishing a foundation for future research and potential clinical applications.

Developmental expression patterns of gonadal hormone receptors in arcuate kisspeptin and GABA neurons of the postnatal female mouse

The arcuate nucleus of the hypothalamus (ARC) is central in the neuronal regulation of fertility and reproduction through translating gonadal steroid hormone cues into the GnRH signaling pathway in the brain. Evidence suggests that circulating gonadal steroids play an important role in modulating female reproduction via kisspeptin and γ-aminobutyric acid (GABA) neurons in the ARC in both development and adulthood. However, the temporal onset of these ARC neurons' sensitivity to gonadal steroids is unknown. Using RNAscope® in situ hybridization, we localized androgen receptor (Ar), estrogen receptor alpha (Esr1), and progesterone receptor (Pgr) expression in ARC kisspeptin or GABA neurons of female mice at postnatal day (P)4, P8, P12, P20, and P60. A probe that binds to kiss1 mRNA or vGat mRNA was used to produce signal in kisspeptin or GABA neurons, respectively. In adult, we identified that the vast majority of kisspeptin neurons coexpressed Esr1 (95%) and Pgr (93%), while a smaller proportion coexpressed Ar (66%). Similar proportions of Ar- or Esr1-positive kisspeptin neurons were seen from P4, suggesting that kisspeptin neurons develop adult-like sensitivity to androgen and estrogen in early postnatal life. In contrast, the proportion of Pgr-positive kisspeptin cells in early life was significantly lower than in adulthood, suggesting that progesterone sensitivity develops over time in the ARC kisspeptin population. ARC GABA neurons also colocalized with Ar (70%), Esr1 (64%), or Pgr (85%) in adulthood. GABA neurons continuously expressed Esr1 or Pgr from the postnatal stages to adulthood, while the proportion of Ar-positive GABA neurons gradually increased from P4 (24%) to P20 (59%). These results suggest that while ARC GABA neurons can respond to circulating estrogen and progesterone from early postnatal ages, this same population may become more sensitive to androgens during later postnatal life. Our findings identified the expression patterns of Ar, Esr1, and Pgr by ARC kisspeptin and GABA neurons during early postnatal life. These data provide the understanding for the hormone sensitivity of these populations during early postnatal life, the critical time for the formation and regulation of female reproductive physiology.Esr1 (95%) and Pgr (93%), while a smaller proportion coexpressed Ar (66%). Similar proportions of Ar- or Esr1-positive kisspeptin neurons were seen from P4, suggesting that kisspeptin neurons develop adult-like sensitivity to androgen and estrogen in early postnatal life. In contrast, the proportion of Pgr-positive kisspeptin cells in early life was significantly lower than in adulthood, suggesting that progesterone sensitivity develops over time in the ARC kisspeptin population. ARC GABA neurons also colocalized with Ar (70%), Esr1 (64%), or Pgr (85%) in adulthood. GABA neurons continuously expressed Esr1 or Pgr from the postnatal stages to adulthood, while the proportion of Ar-positive GABA neurons gradually increased from P4 (24%) to P20 (59%). These results suggest that while ARC GABA neurons can respond to circulating estrogen and progesterone from early postnatal ages, this same population may become more sensitive to androgens during later postnatal life. Our findings identified the expression patterns of Ar, Esr1, and Pgr by ARC kisspeptin and GABA neurons during early postnatal life. These data provide the understanding for the hormone sensitivity of these populations during early postnatal life, the critical time for the formation and regulation of female reproductive physiology.

AVPV Kiss1 neuron-specific knockdown of purinergic P2X2 receptor suppresses LH surge and ovulation in Kiss1-Cre rats

Ovulation disorders are a major cause of low pregnancy rates and infertility in humans and livestock. Kisspeptin neurons located in the anteroventral periventricular nucleus (AVPV) are responsible for the generation of the gonadotropin-releasing hormone (GnRH)/luteinizing hormone (LH) surge and the consequent ovulation in female rodents. The present study aimed to examine whether purinergic neurons are direct upstream stimulators of AVPV kisspeptin neurons that trigger the GnRH/LH surge and consequent ovulation in Kiss1-Cre rats. We specifically knocked down the mRNA expression of the P2rx2 purinergic receptor in AVPV kisspeptin neurons by administering an adeno-associated virus (AAV) vector containing Cre-dependent P2rx2 short hairpin RNA (shRNA) into the AVPV region of ovariectomized (OVX) Kiss1-Cre rats treated with a proestrus level of estradiol-17β (OVX + high E2) or ovary-intact Kiss1-Cre rats. The E2-induced afternoon LH surge was significantly suppressed by AVPV kisspeptin neuron-specific knockdown of P2rx2 in OVX + high E2 Kiss1-Cre rats compared with scrambled shRNA-treated control OVX + high E2 Kiss1-Cre rats. Furthermore, the specific knockdown of P2rx2 in AVPV kisspeptin neurons largely disrupted the estrous cycle, spontaneous LH surge, and ovulation in ovary-intact Kiss1-Cre rats. These findings suggest that purinergic neurons directly stimulate AVPV kisspeptin neurons via P2X2 receptors (P2RX2) to induce the GnRH/LH surge and consequent ovulation in female rats.

Comprehensive chemotyping, and the gonadal regulation, of seven kisspeptinergic neuronal populations in the mouse brain

BACKGROUND

Kisspeptinergic signaling is well-established as crucial for regulation of reproduction, but its potential broader role in brain function is less understood. This study investigates the distribution and chemotyping of kisspeptin-expressing neurons within the mouse brain.

METHODS

RNAscope singleplex, duplex and multiplex in situ hybridization methods were used to assess kisspeptin mRNA (Kiss1) expression and its co-expression with other neuropeptides, excitatory and inhibitory neurotransmitter markers, and sex steroid receptors in intact and gonadectomized young adult mice.

RESULTS

Seven distinct kisspeptin neuronal chemotypes were characterized, including within two novel Kiss1-expressing groups described here for the first time: the ventral premammillary nucleus, and the nucleus of the solitary tract. Kiss1 mRNA was also localized in the soma, and within the dendritic compartment, of hypothalamic neurons. Altered Kiss1 expression following gonadectomy suggests a previously unappreciated role for androgen receptors in regulating kisspeptin signaling.

CONCLUSION

This study provides a detailed chemoanatomical map of kisspeptin-expressing neurons in the brain, highlighting their potential functional diversity. The discovery of new kisspeptin-expressing neuronal populations, and gonadectomy-induced changes in Kiss1 expression patterns, provide a basis for further exploration of non-endocrine roles for kisspeptin in brain function.

Molecular and functional mapping of the neuroendocrine hypothalamus: a new era begins

BACKGROUND

Recent advances in neuroscience tools for single-cell molecular profiling of brain neurons have revealed an enormous spectrum of neuronal subpopulations within the neuroendocrine hypothalamus, highlighting the remarkable molecular and cellular heterogeneity of this brain area.

RATIONALE

Neuronal diversity in the hypothalamus reflects the high functional plasticity of this brain area, where multiple neuronal populations flexibly integrate a variety of physiological outputs, including energy balance, stress and fertility, through crosstalk mechanisms with peripheral hormones. Intrinsic functional heterogeneity is also observed within classically 'defined' subpopulations of neuroendocrine neurons, including subtypes with distinct neurochemical signatures, spatial organisation and responsiveness to hormonal cues.

AIM

The aim of this review is to critically evaluate past and current research on the functional diversity of hypothalamic neuroendocrine neurons and their plasticity. It focuses on how this neuronal plasticity in this brain area relates to metabolic control, feeding regulation and interactions with stress and fertility-related neural circuits.

CONCLUSION

Our analysis provides an original framework for improving our understanding of the hypothalamic regulation of hormone function and the development of neuroendocrine diseases.

Selective depletion of kisspeptin neurons in the hypothalamic arcuate nucleus in early juvenile life reduces pubertal LH secretion and delays puberty onset in mice

Puberty is the critical developmental transition to reproductive capability driven by the activation of gonadotropin-releasing hormone (GnRH) neurons. The complex neural mechanisms underlying pubertal activation of GnRH secretion still remain unknown, yet likely include kisspeptin neurons. However, kisspeptin neurons reside in several hypothalamic areas and the specific kisspeptin population timing pubertal onset remains undetermined. To investigate this, we strategically capitalized on the differential ontological expression of the Kiss1 gene in different hypothalamic nuclei to selectively ablate just arcuate kisspeptin neurons (aka KNDy neurons) during the early juvenile period, well before puberty, while sparing RP3V kisspeptin neurons. Both male and female transgenic mice with a majority of their KNDy neurons ablated (KNDyABL) by diphtheria toxin treatment in juvenile life demonstrated significantly delayed puberty onset and lower peripubertal LH secretion than controls. In adulthood, KNDyABL mice demonstrated normal in vivo LH pulse frequency with lower basal and peak LH levels, suggesting that only a small subset of KNDy neurons is sufficient for normal GnRH pulse timing but more KNDy cells are needed to secrete normal LH concentrations. Unlike prior KNDy ablation studies in rats, there was no alteration in the occurrence or magnitude of estradiol-induced LH surges in KNDyABL female mice, indicating that a complete KNDy neuronal population is not essential for normal LH surge generation. This study teases apart the contributions of different kisspeptin neural populations to the control of puberty onset, demonstrating that a majority of KNDy neurons in the arcuate nucleus are necessary for the proper timing of puberty in both sexes.

Kisspeptin in functional hypothalamic amenorrhea: Pathophysiology and therapeutic potential

Functional hypothalamic amenorrhea (FHA) is one of the most common causes of secondary amenorrhea, resulting in anovulation and infertility, and is a low estrogen state that increases the risk of cardiovascular disease and impairs bone health. FHA is characterized by acquired suppression of physiological pulsatile gonadotropin-releasing hormone (GnRH) release by the hypothalamus in the absence of an identifiable structural cause, resulting in a functional hypogonadotropic hypogonadism. FHA results from either decreased energy intake and/or excessive exercise, leading to low energy availability and weight loss-often in combination with psychological stress on top of a background of genetic susceptibility. The hypothalamic neuropeptide kisspeptin is a key component of the GnRH pulse generator, tightly regulating pulsatile GnRH secretion and the downstream reproductive axis. Here, we review the physiological regulation of pulsatile GnRH secretion by hypothalamic kisspeptin neurons and how their activity is modulated by signals of energy status to affect reproductive function. We explore endocrine factors contributing to the suppression of GnRH pulsatility in the pathophysiology of FHA and how hypothalamic kisspeptin neurons likely represent a final common pathway through which these factors affect GnRH pulse generation. Finally, we discuss the therapeutic potential of kisspeptin as a novel treatment for women with FHA.

Interactions between kisspeptin and bone: Cellular mechanisms, clinical evidence, and future potential

The neuropeptide kisspeptin and its cognate receptor have been extensively studied in reproductive physiology, with diverse and well-established functions, including as an upstream regulator of pubertal onset, reproductive hormone secretion, and sexual behavior. Besides classical reproduction, both kisspeptin and its receptor are extensively expressed in bone-resorbing osteoclasts and bone-forming osteoblasts, which putatively permits direct bone effects. Accordingly, this sets the scene for recent compelling findings derived from in vitro experiments through to in vivo and clinical studies revealing prominent regulatory interactions for kisspeptin signaling in bone metabolism, as well as certain oncological aspects of bone metabolism. Herein, we comprehensively examine the experimental evidence obtained to date supporting the interaction between kisspeptin and bone. A comprehensive understanding of this emerging facet of kisspeptin biology is fundamental to exploiting the future therapeutic potential of kisspeptin-based medicines as a novel strategy for treating bone-related disorders.

Role of Membrane Estrogen Receptor Alpha on the Positive Feedback of Estrogens on Kisspeptin and GnRH Neurons

Estrogens act through nuclear and membrane-initiated signaling. Estrogen receptor alpha (ERα) is critical for reproduction, but the relative contribution of its nuclear and membrane signaling to the central regulation of reproduction is unclear. To address this question, two complementary approaches were used: estetrol (E4) a natural estrogen acting as an agonist of nuclear ERs, but as an antagonist of their membrane fraction, and the C451A-ERα mouse lacking mERα. E4 dose- dependently blocks ovulation in female rats, but the central mechanism underlying this effect is unknown. To determine whether E4 acts centrally to control ovulation, its effect was tested on the positive feedback of estradiol (E2) on neural circuits underlying luteinizing hormone (LH) secretion. In ovariectomized females chronically exposed to a low dose of E2, estradiol benzoate (EB) alone or combined with progesterone (P) induced an increase in the number of kisspeptin (Kp) and gonadotropin-releasing hormone (GnRH) neurons coexpressing Fos, a marker of neuronal activation. E4 blocked these effects of EB, but not when combined to P. These results indicate that E4 blocked the central induction of the positive feedback in the absence of P, suggesting an antagonistic effect of E4 on mERα in the brain as shown in peripheral tissues. In parallel, as opposed to wild-type females, C451A-ERα females did not show the activation of Kp and GnRH neurons in response to EB unless they are treated with P. Together these effects support a role for membrane-initiated estrogen signaling in the activation of the circuit mediating the LH surge.

Participation of kisspeptin, progesterone, and GnRH receptors on lordosis behavior induced by kisspeptin

The neuropeptide kisspeptin (Kiss) is crucial in regulating the hypothalamic-pituitary-gonadal axis. It is produced by two main groups of neurons in the hypothalamus: the rostral periventricular region around the third ventricle and the arcuate nucleus. Kiss is the peptide product of the KiSS-1 gene and serves as the endogenous agonist for the GPR54 receptor. The Kiss/GPR54 system functions as a critical regulator of the reproductive system. Thus, we examined the effect of intracerebroventricular administration of 3 μg of Kiss to the right lateral ventricle of ovariectomized rats primed with a dose of 5 μg subcutaneous (sc) of estradiol benzoate (EB). Kiss treatment increased the lordosis quotient at all times tested. However, the lordosis reflex score was comparatively lower yet still significant compared to the control group. To investigate receptor specificity and downstream mechanisms on lordosis, we infused 10 μg of GPR54 receptor antagonist, Kiss-234, 5 μg of the progestin receptor antagonist, RU486, or 3 μg of antide, a gonadotropin-releasing hormone-1 (GnRH-1) receptor antagonist, to the right lateral ventricle 30 min before an infusion of 3 μg of Kiss. Results demonstrated a significant reduction in the facilitation of lordosis behavior by Kiss at 60 and 120 min when Kiss-234, RU486, or antide were administered. These findings suggest that Kiss stimulates lordosis expression by activating GPR54 receptors on GnRH neurons and that Kiss/GPR54 system is an essential intermediary by which progesterone activates GnRH.

Do Sex and Gender Have Separate Identities?

The largely binary nature of biological sex and its conflation with the socially constructed concept of gender has created much strife in the last few years. The notion of gender identity and its differences and similarities with sex have fostered much scientific and legal confusion and disagreement. Settling the debate can have significant repercussions for science, medicine, legislation, and people's lives. The present review addresses this debate though different levels of analysis (i.e., genetic, anatomical, physiological, behavioral, and sociocultural), and their implications and interactions. We propose a rationale where both perspectives coexist, where diversity is the default, establishing a delimitation to the conflation between sex and gender, while acknowledging their interaction. Whereas sex in humans and other mammals is a biological reality that is largely binary and based on genes, chromosomes, anatomy, and physiology, gender is a sociocultural construct that is often, but not always, concordant with a person' sex, and can span a multitude of expressions.

Nitric oxide mediated kisspeptin regulation of steroidogenesis and gametogenesis in the catfish, Clarias batrachus

Nitric oxide (NO) is a gaseous molecule that regulates various reproductive functions. It is a well-recognized regulator of GnRH-FSH/LH-sex steroid secretion in vertebrates including fish. Kisspeptin is a recently discovered neuropeptide which also regulates GnRH secretion. Nitrergic and kisspeptin neurons are reported in close physical contact in the mammalian brain suggesting their interactive role in the release of GnRH. The existence of kisspeptin and NOS is also demonstrated in vertebrate gonads, but information on their reciprocal relation in gonads, if any, is obscure. Therefore, attempts were made to evaluate the functional reciprocal relation between nitric oxide and kisspeptin in the catfish gonads, if any, by administering the nitric oxide synthase (NOS) inhibitor, L-NAME {N(G)-nitro-L-arginine methyl ester}, which reduces NO production, and kisspeptin agonist (KP-10) and assessing their impacts on the expressions of kisspeptin1, different NOS isoforms, NO and steroid production in the gonadal tissue. The results revealed that L-NAME suppressed the expression of kiss1 in gonads of the catfish establishing the role of NO in kisspeptin expression. However, KP-10 increased the expression of all the isoforms of NOSs (iNOS, eNOS, nNOS) and concurrently NO and steroids in the ovary and testis. In vitro studies also indicate that kisspeptin stimulates the production of NO and estradiol and testosterone levels in the gonadal explants and medium. Thus, in vivo results clearly suggest a reciprocal interaction between kisspeptin and NO to regulate the gonadal activity of the catfish. The in vitro findings further substantiate our contention regarding the interactive role of kisspeptin and NO in gonadal steroidogenesis.

Kisspeptin control of hypothalamus-pituitary-ovarian functions

The discovery of Kisspeptin (Kiss) has opened a new direction in research on neuroendocrine control of reproduction in vertebrates. Belonging to the RF amide family of peptides, Kiss and its cognate receptor Gpr54 (Kissr) have a long and complex evolutionary history. Multiple forms of Kiss and Kissr are identified in non-mammalian vertebrates, with the exception of birds, and monotreme mammals. However, only a single form of the ligand (KISS1/Kiss1) and receptor (KISS1R/Kiss1r) is retained in higher mammals. Kiss1 is distributed in the hypothalamus-pituitary-gonadal (HPG) axis and its primary function is to stimulate gonadotropin-releasing hormone (GnRH) secretion. Kiss1 neurons are distributed in the rostral periventricular area of the third ventricle (RP3V) and arcuate/infundibular nucleus (ARN/IFN). The ARN/IFN is considered the GnRH pulse generator controlled by steroid negative feedback, and the RP3V neurons is concerned with GnRH surge induced by steroid positive feedback in females. The Kiss1-Kiss1r signaling is important in all aspects of reproduction: puberty onset, maintenance of adult gonadal functions and reproductive aging, and hence assumes therapeutic potentials in the treatment of reproductive dysfunctions and induction of artificial reproduction. This chapter reviews involvement of Kiss1 in the control of the HPG axis functions in female mammals.

Male animal sterilization: history, current practices, and potential methods for replacing castration

Sterilization and castration have been synonyms for thousands of years. Making an animal sterile meant to render them incapable of producing offspring. Castration or the physical removal of the testes was discovered to be the most simple but reliable method for managing reproduction and sexual behavior in the male. Today, there continues to be global utilization of castration in domestic animals. More than six hundred million pigs are castrated every year, and surgical removal of testes in dogs and cats is a routine practice in veterinary medicine. However, modern biological research has extended the meaning of sterilization to include methods that spare testis removal and involve a variety of options, from chemical castration and immunocastration to various methods of vasectomy. This review begins with the history of sterilization, showing a direct link between its practice in man and animals. Then, it traces the evolution of concepts for inducing sterility, where research has overlapped with basic studies of reproductive hormones and the discovery of testicular toxicants, some of which serve as sterilizing agents in rodent pests. Finally, the most recent efforts to use the immune system and gene editing to block hormonal stimulation of testis function are discussed. As we respond to the crisis of animal overpopulation and strive for better animal welfare, these novel methods provide optimism for replacing surgical castration in some species.

Brain RFamide Neuropeptides in Stress-Related Psychopathologies

The RFamide peptide family is a group of proteins that share a common C-terminal arginine-phenylalanine-amide motif. To date, the family comprises five groups in mammals: neuropeptide FF, LPXRFamides/RFamide-related peptides, prolactin releasing peptide, QRFP, and kisspeptins. Different RFamide peptides have their own cognate receptors and are produced by different cell populations, although they all can also bind to neuropeptide FF receptors with different affinities. RFamide peptides function in the brain as neuropeptides regulating key aspects of homeostasis such as energy balance, reproduction, and cardiovascular function. Furthermore, they are involved in the organization of the stress response including modulation of pain. Considering the interaction between stress and various parameters of homeostasis, the role of RFamide peptides may be critical in the development of stress-related neuropathologies. This review will therefore focus on the role of RFamide peptides as possible key hubs in stress and stress-related psychopathologies. The neurotransmitter coexpression profile of RFamide-producing cells is also discussed, highlighting its potential functional significance. The development of novel pharmaceutical agents for the treatment of stress-related disorders is an ongoing need. Thus, the importance of RFamide research is underlined by the emergence of peptidergic and G-protein coupled receptor-based therapeutic targets in the pharmaceutical industry.

Melatonin, modulation of hypothalamic activity, and reproduction

Light is the most reliable environmental cue allowing animals to breed successfully when conditions are optimal. In seasonal breeders, photoperiod (length of daylight) information is sensed by the eyes and transmitted to the suprachiasmatic nucleus, the master clock region located in the hypothalamus. This structure has a 24-h firing rhythm involving a cycle of clock protein synthesis and degradation, and provides the timing to synchronize the synthesis and release of melatonin, the chemical signal that transduces the photoperiod information. The enzyme arylalkylamine N-acetyltransferase, responsible for melatonin synthesis in the pineal gland, is modulated by environmental light. Melatonin is secreted during the dark hours of the night to blood circulation and cerebrospinal fluid conveying photoperiod information to other tissues. Melatonin exerts its action by binding to specific membrane receptors MT1 and MT2, and can modulate several pathways including neurotransmitters, and hormones like kisspeptin, the gonadotropin-inhibitory hormone, and thyroid hormones, all of them impacting on gonadotropin-releasing hormone (GnRH) secretion. Then, GnRH will modulate in turn the reproductive axis. In conclusion, acting as a transducer of photoperiod information, this hormone exerts precisely timed activation of different pathways that modulate seasonal breeding ensuring optimal conditions for reproduction.

Distribution of the kisspeptin system and its relation with gonadotropin-releasing hormone in the hypothalamus

Kisspeptin (KISS1), originally catalogued as metastin because of its capacity as a metastasis suppressor in human melanoma and breast cancer, is now recognized as the major puberty gatekeeper and gonadotropin-releasing hormone (GnRH) neuroendocrine system modulator. It is a member of the family of RFamide-related peptides that also includes the neuropeptide FF group, the gonadotropin-inhibitory hormone, the prolactin-releasing peptide, and the 26RFa peptides. The KISS1 precursor peptide is processed into a family of peptides known as kisspeptins. Its expression has been described in the hypothalamus as well as in the whole reproductive axis and several extra reproductive tissues of mammals as well as fish and amphibians, but not in birds. KISS1 plays an essential role as a regulator of the reproductive axis by inducing the synthesis and release of GnRH, acting through specific receptors. The study of the kisspeptin system and its relation with reproduction in wild and non-classical laboratory species is extremely useful to understand and become aware of the role of KISS1 in the wide variety of possible different reproductive strategies. In this chapter, KISS1 involvement in non-classical laboratory rodents, fishes, and birds is discussed.

The association between leptin, adiponectin levels and the ovarian reserve in women of reproductive age

Background

Reproduction ability requires a certain amount of body fat that is necessary for ovulation, menstruation and pregnancy. Fat tissue represents an endocrine organ with high metabolic activity as it produces adipokines such as leptin and adiponectin. Our aim is to examine potential associations between women of reproductive age's ovarian reserves and their levels of leptin and adiponectin.

Method

74 women between 19 and 40 years of age consented to take part. Based on the patterns of their ovarian reserves, the women were divided into three main groups: women with adequate ovarian reserves (AOR - Group A, n=30), women with polycystic ovary syndrome (PCOS - Group B, n=31) and women with depleted ovarian reserves (DOR - Group C, n=13). Among these groups, several biochemical and demographic parameters were statistically compared.

Results

Compared to the other two groups, women with DOR had statistically higher age and follicle stimulation hormone (FSH) levels. For estradiol (E2) and thyroid-stimulating hormone (TSH), no statistically significant difference was seen between the groups. In addition, women with PCOS had higher body mass index (BMI), luteinizing hormone (LH), total testosterone (TT), 17 hydroxyprogesterone (17-OHP), dehydroepiandrosterone (DHEA), anti-Mullerian hormone (AMH), and antral follicle count (AFC) than the other two groups. In line with expectations, women with DOR also had lower levels of AMH and AFC than the other two groups. Women with PCOS had higher leptin levels than the other two groups, but there was no statistically significant difference. Women with PCOS had lower levels of adiponectin than the other groups, however the difference was not statistically significant.

Conclusion

The way we classified women in our study according to their ovarian reserves is completely consistent with what has been published internationally. The ovarian reserve in women of reproductive age is not strongly correlated with leptin and adiponectin levels. For safe conclusions, more research including a greater number of samples is required.

Early adversity causes sex-specific deficits in perforant pathway connectivity and contextual memory in adolescent mice

BACKGROUND

Early life adversity impairs hippocampal development and function across diverse species. While initial evidence indicated potential variations between males and females, further research is required to validate these observations and better understand the underlying mechanisms contributing to these sex differences. Furthermore, most of the preclinical work in rodents was performed in adult males, with only few studies examining sex differences during adolescence when such differences appear more pronounced. To address these concerns, we investigated the impact of limited bedding (LB), a mouse model of early adversity, on hippocampal development in prepubescent and adolescent male and female mice.

METHODS

RNA sequencing, confocal microscopy, and electron microscopy were used to evaluate the impact of LB and sex on hippocampal development in prepubescent postnatal day 17 (P17) mice. Additional studies were conducted on adolescent mice aged P29-36, which included contextual fear conditioning, retrograde tracing, and ex vivo diffusion magnetic resonance imaging (dMRI).

RESULTS

More severe deficits in axonal innervation and myelination were found in the perforant pathway of prepubescent and adolescent LB males compared to LB female littermates. These sex differences were due to a failure of reelin-positive neurons located in the lateral entorhinal cortex (LEC) to innervate the dorsal hippocampus via the perforant pathway in males, but not LB females, and were strongly correlated with deficits in contextual fear conditioning.

CONCLUSIONS

LB impairs the capacity of reelin-positive cells located in the LEC to project and innervate the dorsal hippocampus in LB males but not female LB littermates. Given the critical role that these projections play in supporting normal hippocampal function, a failure to establish proper connectivity between the LEC and the dorsal hippocampus provides a compelling and novel mechanism to explain the more severe deficits in myelination and contextual freezing found in adolescent LB males.

Stress during pubertal development affects female sociosexual behavior in mice

Puberty is a crucial phase for the development of female sexual behavior. Growing evidence suggests that stress during this period may interfere with the development of sexual behavior. However, the neural circuits involved in this alteration remain elusive. Here, we demonstrated in mice that pubertal stress permanently disrupted sexual performance without affecting sexual preference. This was associated with a reduced expression and activation of neuronal nitric oxide synthase (nNOS) in the ventrolateral part of the ventromedial hypothalamus (VMHvl). Fiber photometry revealed that VMHvl nNOS neurons are strongly responsive to male olfactory cues with this activation being substantially reduced in pubertally stressed females. Finally, treatment with a NO donor partially restored sexual performance in pubertally stressed females. This study provides insights into the involvement of VMHvl nNOS in the processing of olfactory cues important for the expression of female sexual behavior. In addition, exposure to stress during puberty disrupts the integration of male olfactory cues leading to reduced sexual behavior.

Kisspeptin a potential therapeutic target in treatment of both metabolic and reproductive dysfunction

Kisspeptins (KPs) are proteins that were first recognized to have antimetastatic action. Later, the critical role of this peptide in the regulation of reproduction was proved. In recent years, evidence has been accumulated supporting a role for KPs in regulating metabolic processes in a sexual dimorphic manner. It has been proposed that KPs regulate metabolism both indirectly via gonadal hormones and/or directly via the kisspeptin receptor in the brain, brown adipose tissue, and pancreas. The aim of the review is to provide both experimental and clinical evidence indicating that KPs are peptides linking metabolism and reproduction. We propose that KPs could be used as a potential target to treat both metabolic and reproductive abnormalities. Thus, we focus on the consequences of disruptions in KPs and their receptors in metabolic conditions such as diabetes, undernutrition, obesity, and reproductive disorders (hypogonadotropic hypogonadism and polycystic ovary syndrome). Data from both animal models and human subjects indicate that alterations in KPs in the case of metabolic imbalance lead also to disruptions in reproductive functions. Changes both in the hypothalamic and peripheral KP systems in animal models of the aforementioned disorders are discussed. Finally, an overview of current clinical studies involving KP in fertility and metabolism show fewer studies on metabolism (15%) and only one to date on both. Presented data indicate a dynamic and emerging field of KP studies as possible therapeutic targets in treatments of both reproductive and metabolic dysfunctions.

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.

The Emerging Therapeutic Potential of Kisspeptin and Neurokinin B

Kisspeptin (KP) and neurokinin B (NKB) are neuropeptides that govern the reproductive endocrine axis through regulating hypothalamic gonadotropin-releasing hormone (GnRH) neuronal activity and pulsatile GnRH secretion. Their critical role in reproductive health was first identified after inactivating variants in genes encoding for KP or NKB signaling were shown to result in congenital hypogonadotropic hypogonadism and a failure of pubertal development. Over the past 2 decades since their discovery, a wealth of evidence from both basic and translational research has laid the foundation for potential therapeutic applications. Beyond KP's function in the hypothalamus, it is also expressed in the placenta, liver, pancreas, adipose tissue, bone, and limbic regions, giving rise to several avenues of research for use in the diagnosis and treatment of pregnancy, metabolic, liver, bone, and behavioral disorders. The role played by NKB in stimulating the hypothalamic thermoregulatory center to mediate menopausal hot flashes has led to the development of medications that antagonize its action as a novel nonsteroidal therapeutic agent for this indication. Furthermore, the ability of NKB antagonism to partially suppress (but not abolish) the reproductive endocrine axis has supported its potential use for the treatment of various reproductive disorders including polycystic ovary syndrome, uterine fibroids, and endometriosis. This review will provide a comprehensive up-to-date overview of the preclinical and clinical data that have paved the way for the development of diagnostic and therapeutic applications of KP and NKB.

Arcuate Nucleus of the Hypothalamus: Anatomy, Physiology, and Diseases

The hypothalamus is part of the diencephalon and has several nuclei, one of which is the arcuate nucleus. The arcuate nucleus of hypothalamus (ARH) consists of neuroendocrine neurons and centrally-projecting neurons. The ARH is the center where the homeostasis of nutrition/metabolism and reproduction are maintained. As such, dysfunction of the ARH can lead to disorders of nutrition/metabolism and reproduction. Here, we review various types of neurons in the ARH and several genetic disorders caused by mutations in the ARH.

CRISPR-Cas9 knockdown of ESR1 in preoptic GABA-kisspeptin neurons suppresses the preovulatory surge and estrous cycles in female mice

Evidence suggests that estradiol-sensing preoptic area GABA neurons are involved in the preovulatory surge mechanism necessary for ovulation. In vivo CRISPR-Cas9 editing was used to achieve a 60-70% knockdown in estrogen receptor alpha (ESR1) expression by GABA neurons located within the regions of the rostral periventricular area of the third ventricle (RP3V) and medial preoptic nuclei (MPN) in adult female mice. Mice exhibited variable reproductive phenotypes with the only significant finding being mice with bilateral ESR1 deletion in RP3V GABA neurons having reduced cFos expression in gonadotropin-releasing hormone (GnRH) neurons at the time of the surge. One sub-population of RP3V GABA neurons expresses kisspeptin. Re-grouping ESR1-edited mice on the basis of their RP3V kisspeptin expression revealed a highly consistent phenotype; mice with a near-complete loss of kisspeptin immunoreactivity displayed constant estrus and failed to exhibit surge activation but retained pulsatile luteinizing hormone (LH) secretion. These observations demonstrate that ESR1-expressing GABA-kisspeptin neurons in the RP3V are essential for the murine preovulatory LH surge mechanism.

Sex and interspecies differences in ESR2-expressing cell distributions in mouse and rat brains

BACKGROUND

ESR2, a nuclear estrogen receptor also known as estrogen receptor β, is expressed in the brain and contributes to the actions of estrogen in various physiological phenomena. However, its expression profiles in the brain have long been debated because of difficulties in detecting ESR2-expressing cells. In the present study, we aimed to determine the distribution of ESR2 in rodent brains, as well as its sex and interspecies differences, using immunohistochemical detection with a well-validated anti-ESR2 antibody (PPZ0506).

METHODS

To determine the expression profiles of ESR2 protein in rodent brains, whole brain sections from mice and rats of both sexes were subjected to immunostaining for ESR2. In addition, to evaluate the effects of circulating estrogen on ESR2 expression profiles, ovariectomized female mice and rats were treated with low or high doses of estrogen, and the resulting numbers of ESR2-immunopositive cells were analyzed. Welch's t-test was used for comparisons between two groups for sex differences, and one-way analysis of variance followed by the Tukey-Kramer test were used for comparisons among multiple groups with different estrogen treatments.

RESULTS

ESR2-immunopositive cells were observed in several subregions of mouse and rat brains, including the preoptic area, extended amygdala, hypothalamus, mesencephalon, and cerebral cortex. Their distribution profiles exhibited sex and interspecies differences. In addition, low-dose estrogen treatment in ovariectomized female mice and rats tended to increase the numbers of ESR2-immunopositive cells, whereas high-dose estrogen treatment tended to decrease these numbers.

CONCLUSIONS

Immunohistochemistry using the well-validated PPZ0506 antibody revealed a more localized expression of ESR2 protein in rodent brains than has previously been reported. Furthermore, there were marked sex and interspecies differences in its distribution. Our histological analyses also revealed estrogen-dependent changes in ESR2 expression levels in female brains. These findings will be helpful for understanding the ESR2-mediated actions of estrogen in the brain.

Kisspeptin modulation of nonapeptide and cytochrome P450 aromatase mRNA expression in the brain and ovary of the catfish Heteropneustes fossilis: in vivo and in vitro studies

In Heteropneustes fossilis, kisspeptins (Kiss) and nonapeptides (NPs; vasotocin, Vt; isotocin, Itb; Val8-isotocin, Ita) stimulate the hypothalamus-pituitary-gonadal (HPG) axis, and estrogen feedback modulates the expression of these systems. In this study, functional interactions among these regulatory systems were demonstrated in the brain and ovary at the mRNA expression level. Human KISS1 (hKISS1) and H. fossilis Kiss2 (HfKiss2) produced biphasic effects on brain and ovarian vt, itb and ita expression at 24 h post injection: low and median doses produced inhibition, no change or mild stimulation, and the highest dose consistently stimulated the mRNA levels. The Kiss peptides produced an upregulation of NP mRNA expression at 24 h incubation of brain and ovarian slices by increasing the concentration of hKISS1 and HfKiss2. The kiss peptides stimulated brain cyp19a1b and ovary cyp19a1a expression, both in vivo and in vitro. Peptide234, a Kiss1 receptor antagonist, inhibited basal mRNA expression of the NPs, cyp19a1b and cyp19a1a, which was prevented by the Kiss peptides, both in vivo and in vitro. In all the experiments, HfKiss2 was more effective than hKISS1 in modulating mRNA expression. The results suggest that the NP and E2 systems are functional targets of Kiss peptides and interact with each other.

Maternal cafeteria diet influences kisspeptin (Kiss1), kisspeptin receptor(Gpr54), and sirtuin (Sirt1) genes, hormonal and metabolic profiles, and reproductive functions in rat offspring in a sex-specific manner†

Kisspeptin (KP, encoded by Kiss1, binding to the Gpr54 receptor) is a neuropeptide conveying information on the metabolic status to the hypothalamic-pituitary-gonadal axis. KP acts together with dynorphin A (encoded by Pdyn) and neurokinin B (encoded by Tac2) to regulate reproduction. KP is crucial for the onset of puberty and is under the control of sirtuin (encoded by Sirt1). We hypothesize that the maternal cafeteria (CAF) diet has adverse effects on the offspring's hormonal, metabolic, and reproductive functions due to sex-specific alterations in the expression of Kiss1, Gpr54, Pdyn, Tac2, and Sirt1 in the hypothalamus, and Kiss1, Gpr54, and Sirt1 in the liver. Rats were fed a CAF diet before pregnancy, during pregnancy, and during lactation. The vaginal opening was monitored. Offspring were sacrificed in three age points: PND 30, PND 35, and PND 60 (females) and PND 40, PND 45, and PND 60 (males). Their metabolic and hormonal status was assessed. mRNA for Kiss1, Gpr54, Pdyn, Tac2, and Sirt1 were measured by real-time PCR in the hypothalamus and/or livers. We found that CAF offspring had lower weight and altered body composition; increased cholesterol and triglyceride levels, sex-specific changes in glucose and insulin levels; sex-dependent changes in Sirt1/Kiss1 mRNA ratio in the hypothalamus; sex-specific alterations in Kiss1 and Sirt1 mRNA in the liver with more diversity in males; and a delayed puberty onset in females. We concluded that the mother's CAF diet leads to sex-specific alterations in metabolic and reproductive outcomes via Kiss1/Gpr54 and Sirt1 systems in offspring.

Postnatal Development and Maintenance of Functional Pituitary Gonadotrophs Is Dependent on PI4-Kinase A

Postnatal development of functional pituitary gonadotrophs is necessary for maturation of the hypothalamic-pituitary-gonadal axis, puberty, and reproduction. Here we examined the role of PI4-kinase A, which catalyzes the biosynthesis of PI4P in mouse reproduction by knocking out this enzyme in cells expressing the gonadotropin-releasing hormone (GnRH) receptor. Knockout (KO) mice were infertile, reflecting underdeveloped gonads and reproductive tracts and lack of puberty. The number and distribution of hypothalamic GnRH neurons and Gnrh1 expression in postnatal KOs were not affected, whereas Kiss1/kisspeptin expression was increased. KO of PI4-kinase A also did not alter embryonic establishment and neonatal development and function of the gonadotroph population. However, during the postnatal period, there was a progressive loss of expression of gonadotroph-specific genes, including Fshb, Lhb, and Gnrhr, accompanied by low gonadotropin synthesis. The postnatal gonadotroph population also progressively declined, reaching approximately one-third of that observed in controls at 3 months of age. In these residual gonadotrophs, GnRH-dependent calcium signaling and calcium-dependent membrane potential changes were lost, but intracellular administration of inositol-14,5-trisphosphate rescued this signaling. These results indicate a key role for PI4-kinase A in the postnatal development and maintenance of a functional gonadotroph population.

Sex difference in developmental changes in visualized Kiss1 neurons in newly generated Kiss1-Cre rats

Hypothalamic kisspeptin neurons are master regulators of mammalian reproduction via direct stimulation of gonadotropin-releasing hormone and consequent gonadotropin release. Here, we generated novel Kiss1 (kisspeptin gene)-Cre rats and investigated the developmental changes and sex differences in visualized Kiss1 neurons of Kiss1-Cre-activated tdTomato reporter rats. First, we validated Kiss1-Cre rats by generating Kiss1-expressing cell-specific Kiss1 knockout (Kiss1-KpKO) rats, which were obtained by crossing the current Kiss1-Cre rats with Kiss1-floxed rats. The resulting male Kiss1-KpKO rats lacked Kiss1 expression in the brain and exhibited hypogonadotropic hypogonadism, similar to the hypogonadal phenotype of global Kiss1 KO rats. Histological analysis of Kiss1 neurons in Kiss1-Cre-activated tdTomato reporter rats revealed that tdTomato signals in the anteroventral periventricular nucleus (AVPV) and arcuate nucleus (ARC) were not affected by estrogen, and that tdTomato signals in the ARC, AVPV, and medial amygdala (MeA) were sexually dimorphic. Notably, neonatal AVPV tdTomato signals were detected only in males, but a larger number of tdTomato-expressing cells were detected in the AVPV and ARC, and a smaller number of cells in the MeA was detected in females than in males at postpuberty. These findings suggest that Kiss1-visualized rats can be used to examine the effect of estrogen feedback mechanisms on Kiss1 expression in the AVPV and ARC. Moreover, the Kiss1-Cre and Kiss1-visualized rats could be valuable tools for further detailed analyses of sexual differentiation in the brain and the physiological role of kisspeptin neurons across the brain in rats.

Serotonin stimulates female preoptic area kisspeptin neurons via activation of type 2 serotonin receptors in mice

Background

The neuroendocrine control of ovulation is orchestrated by neuronal circuits that ultimately drive the release of gonadotropin-releasing hormone (GnRH) from the hypothalamus to trigger the preovulatory surge in luteinizing hormone (LH) secretion. While estrogen feedback signals are determinant in triggering activation of GnRH neurons, through stimulation of afferent kisspeptin neurons in the rostral periventricular area of the third ventricle (RP3VKISS1 neurons), many neuropeptidergic and classical neurotransmitter systems have been shown to regulate the LH surge. Among these, several lines of evidence indicate that the monoamine neurotransmitter serotonin (5-HT) has an excitatory, permissive, influence over the generation of the surge, via activation of type 2 5-HT (5-HT2) receptors. The mechanisms through which this occurs, however, are not well understood. We hypothesized that 5-HT exerts its influence on the surge by stimulating RP3VKISS1 neurons in a 5-HT2 receptor-dependent manner.

Methods

We tested this using kisspeptin neuron-specific calcium imaging and electrophysiology in brain slices obtained from male and female mice.

Results

We show that exogenous 5-HT reversibly increases the activity of the majority of RP3VKISS1 neurons. This effect is more prominent in females than in males, is likely mediated directly at RP3VKISS1 neurons and requires activation of 5-HT2 receptors. The functional impact of 5-HT on RP3VKISS1 neurons, however, does not significantly vary during the estrous cycle.

Conclusion

Taken together, these data suggest that 5-HT2 receptor-mediated stimulation of RP3VKISS1 neuron activity might be involved in mediating the influence of 5-HT on the preovulatory LH surge.

Differential regulation of Kiss1 gene expression by oestradiol in the hypothalamus of the female Damaraland mole-rat, an induced ovulator

Kisspeptin, a product of the Kiss1 gene is considered a potent stimulator of gonadotropin release, by interacting with its receptor, the G protein-coupled receptor 54. Kiss1 neurons are known to mediate the positive and negative feedback effects of oestradiol on GnRH neurons that control the pulsatile and surge secretion of GnRH. While in spontaneously ovulating mammals the GnRH/LH surge is initiated by a rise in ovarian oestradiol secreted from maturing follicles, in induced ovulators, the primary trigger is the mating stimulus. Damaraland mole rats (Fukomys damarensis) are cooperatively breeding, subterranean rodents that exhibit induced ovulation. We have previously described in this species the distribution and differential expression pattern of Kiss1-expressing neurons in the hypothalamus of males and females. Here we examine whether oestradiol (E2) regulates the hypothalamic Kiss1 expression in a similar way as described for spontaneously ovulating rodent species. By means of in situ hybridisation, we measured Kiss1 mRNA among groups of ovary-intact, ovariectomized (OVX) and OVX females treated with E2 (OVX + E2). In the arcuate nucleus (ARC), Kiss1 expression increased after ovariectomy and decreased with E2 treatment. In the preoptic region, Kiss1 expression after gonadectomy was similar to the level of wild-caught gonad-intact controls, but was dramatically upregulated with E2 treatment. The data suggest that, similar to other species, Kiss1 neurons in the ARC, which are inhibited by E2, play a role in the negative feedback control on GnRH release. The exact role of the Kiss1 neuron population in the preoptic region, which is stimulated by E2, remains to be determined.

KNDy neurones and GnRH/LH pulse generation: Current understanding and future aspects

Uncovering the central mechanism underlying mammalian reproduction is warranted to develop new therapeutic approaches for reproductive disorders in humans and domestic animals. The present study focused on the role of arcuate kisspeptin neurones (also known as KNDy neurones) as an intrinsic gonadotropin-releasing hormone (GnRH) pulse generator, which plays a fundamental role in mammalian reproduction via the stimulation of pituitary gonadotropin synthesis and release and thereby in gametogenesis and steroidogenesis in the gonads of mammals. We also discuss the mechanism that inhibits pulsatile GnRH/gonadotropin release under a negative energy balance, considering that reproductive disorders often occur during malnutrition in humans and livestock.

Is There an Association Between Circulating Kisspeptin Levels and Ovarian Reserve in Women of Reproductive Age?

BACKGROUND/AIM

To investigate the possible association of kisspeptin levels with the ovarian reserves of women of reproductive age.

PATIENTS AND METHODS

Eighty women aged 19-40 participated after signing an informed consent. Of these, 74 were finally included as in 6 women the blood samples were considered inappropriate due to hemolysis. They were divided into three main groups according to their ovarian reserve patterns: women with adequate ovarian reserves (Group A - AOR) (n=30), women with increased ovarian reserves (Group B - PCOS) (n=31), and women with diminished ovarian reserves (Group C - DOR) (n=13).

RESULTS

Women with diminished ovarian reserves had statistically significantly increased age and FSH compared to the other two groups. No statistically significant difference was found between the three groups for estradiol and thyroid stimulating hormone. Moreover, body mass index, luteinizing hormone, total testosterone, 17-hydroxyprogesterone, dehydroepiandrosterone, anti-Mullerian hormone (AMH), and antral follicle count (AFC) were increased in group B compared to the other two groups. AMH and AFC were decreased in women with diminished ovarian reserves compared to the other two groups, as expected. The comparison of kisspeptin levels between the three groups showed that kisspeptin levels were increased in women with diminished ovarian reserves, compared to the other two groups, but without a statistically significant difference. However, kisspeptin levels in group C were statistically significantly higher than those in group A.

CONCLUSION

There are no strong indications that kisspeptin levels are associated with the ovarian reserve in women of reproductive age.

Continuous acceleration of neural activity of the GnRH pulse generator during chronic peripheral infusion of neurokinin 3 receptor agonist in goats

Secretion of pulsatile gonadotropin-releasing hormone (GnRH) is essential for reproduction. Kisspeptin neurons in the arcuate nucleus (ARC), which coexpress neurokinin B (NKB) and its receptor (NK3R), are believed to be components of the GnRH pulse generator that regulates pulsatile GnRH secretion. We examined the effects of peripheral infusion of senktide, an NK3R selective agonist, on GnRH pulse generator activity by monitoring multiple unit activity (MUA) in the goat ARC. Previous studies have shown that characteristic increases in MUA (MUA volleys) reflect GnRH pulse generator activity. Senktide was infused intravenously or intravaginally for 2 h while recording MUA. Both infusions significantly increased the MUA volley frequency compared with the control. These results demonstrate that peripherally administered senktide acts centrally to sustainably accelerate the neural activity of the GnRH pulse generator throughout the infusion period. This suggests the possibility of practical applications of NK3R agonists for improving reproductive activity in farm animals.

The sex-dependent and enduring impact of pubertal stress on health and disease

Illness is often predicated long before the manifestation of its symptoms. Exposure to stressful experiences particularly during critical periods of development, such as puberty and adolescence, can induce various physical and mental illnesses. Puberty is a critical period of maturation for neuroendocrine systems, such as the hypothalamic-pituitary-gonadal (HPG) and hypothalamic-pituitary-adrenal (HPA) axes. Exposure to adverse experiences during puberty can impede normal brain reorganizing and remodelling and result in enduring consequences on brain functioning and behaviour. Stress responsivity differs between the sexes during the pubertal period. This sex difference is partly due to differences in circulating sex hormones between males and females, impacting stress and immune responses differently. The effects of stress during puberty on physical and mental health remains under-examined. The purpose of this review is to summarize the most recent findings pertaining to age and sex differences in HPA axis, HPG axis, and immune system development, and describe how disruption in the functioning of these systems can propagate disease. Lastly, we delve into the notable neuroimmune contributions, sex differences, and the mediating role of the gut microbiome on stress and health outcomes. Understanding the enduring consequences of adverse experiences during puberty on physical and mental health will allow a greater proficiency in treating and preventing stress-related diseases early in development.

Kisspeptin neuron projections to oxytocin neurons are not necessary for parturition in the mouse

Oxytocin is synthesized by hypothalamic supraoptic nucleus (SON) and paraventricular nucleus (PVN) neurons and is released from the posterior pituitary gland to trigger uterine contractions during parturition. In rats, oxytocin neuron innervation by periventricular nucleus (PeN) kisspeptin neurons increases over pregnancy and intra-SON kisspeptin administration excites oxytocin neurons only in late pregnancy. To test the hypothesis that kisspeptin neurons excite oxytocin neurons to trigger uterine contractions during birth in C57/B6J mice, double-label immunohistochemistry for kisspeptin and oxytocin first confirmed that kisspeptin neurons project to the SON and PVN. Furthermore, kisspeptin fibers expressed synaptophysin and formed close appositions with oxytocin neurons in the mouse SON and PVN before and during pregnancy. Stereotaxic viral delivery of caspase-3 into the AVPV/PeN of Kiss-Cre mice before mating reduced kisspeptin expression in the AVPV, PeN, SON and PVN by > 90% but did not affect the duration of pregnancy or the timing of delivery of each pup during parturition. Therefore, it appears that AVPV/PeN kisspeptin neuron projections to oxytocin neurons are not necessary for parturition in the mouse.

Defining potential targets of prenatal androgen excess: Expression analysis of androgen receptor on hypothalamic neurons in the fetal female mouse brain

Polycystic ovary syndrome (PCOS) is a female endocrine disorder that is associated with prenatal exposure to excess androgens. In prenatally androgenized (PNA) mice that model PCOS, GABAergic neural transmission to and innervation of GnRH neurons is increased. Evidence suggests that elevated GABAergic innervation originates in the arcuate nucleus (ARC). We hypothesized that GABA-GnRH circuit abnormalities are a direct consequence of PNA, resulting from DHT binding to androgen receptor (AR) in the prenatal brain. However, whether prenatal ARC neurons express AR at the time of PNA treatment is presently unknown. We used RNAScope in situ hybridization to localize AR mRNA (Ar)-expressing cells in healthy gestational day (GD) 17.5 female mouse brains and to assess coexpression levels in specific neuronal phenotypes. Our study revealed that less than 10% of ARC GABA cells expressed Ar. In contrast, we found that ARC kisspeptin neurons, critical regulators of GnRH neurons, were highly colocalized with Ar. Approximately 75% of ARC Kiss1-expressing cells also expressed Ar at GD17.5, suggesting that ARC kisspeptin neurons are potential targets of PNA. Investigating other neuronal populations in the ARC we found that ~50% of pro-opiomelanocortin (Pomc) cells, 22% of tyrosine hydroxylase (Th) cells, 8% of agouti-related protein (Agrp) cells and 8% of somatostatin (Sst) cells express Ar. Lastly, RNAscope in coronal sections showed Ar expression in the medial preoptic area (mPOA), and the ventral part of the lateral septum (vLS). These Ar-expressing regions were highly GABAergic, and 22% of GABA cells in the mPOA and 25% of GABA cells in the vLS also expressed Ar. Our findings identify specific neuronal phenotypes in the ARC, mPOA, and vLS that are androgen sensitive in late gestation. PNA-induced functional changes in these neurons may be related to the development of impaired central mechanisms associated with PCOS-like features.

Review: Unveiling the effect of beta-nerve growth factor on the reproductive function in llamas and cows

The actions of the beta-nerve growth factor (β-NGF) on the neuroendocrine and reproductive system have challenged classical views on the control of reproductive function. After endometrial absorption, β-NGF triggers ovulation and promotes the development of functional corpora lutea in camelids. In this article, we review evidence showing that, in camelids, β-NGF exerts its actions by acting in both the hypothalamus and the ovary. In the hypothalamus, β-NGF may induce gonadotropin-releasing hormone (GnRH) release by interacting with neurons or glial cells expressing receptors for β-NGF. The LH surge occurs under the influence of ovarian estradiol and requires the release of GnRH into the portal vessels to reach the pituitary gland. In the ovary, β-NGF may be promoting the differentiation of follicular to luteal cells by modifying the steroidogenic profile of ovarian follicular cells in both camelids and ruminants. Although the mechanisms for these actions are largely undetermined, we aim to offer an update on the current understanding of the effects of β-NGF controlling reproductive function in camelids and ruminants.

DNA methylation and demethylation shape sexual differentiation of neurochemical phenotype

Some of the best-studied neural sex differences depend on differential cell death in males and females, but other sex differences persist even if cell death is prevented. These include sex differences in neurochemical phenotype (i.e., stable patterns of gene expression). Work in our laboratory over the last several years has tested the hypothesis that sex differences in DNA methylation early in life underlie sexual differentiation of neuronal phenotype. We have shown that 1) expression of enzymes that place or remove DNA methylation marks is greatest during the first week of life in the mouse brain and overlaps with the perinatal critical period of sexual differentiation; 2) a transient inhibition of DNA methylation during neonatal life abolishes several sex differences in cell phenotype in the mouse hypothalamus; 3) both DNA methylation and de-methylation contribute to the development of neural sex differences; and 4) the effects of DNA methylation and de-methylation are brain region- and cell type-specific.

Hypothalamic kisspeptin neurons as potential mediators of estradiol negative and positive feedback

Gonadal steroid feedback regulates the brain's patterned secretion of gonadotropin-releasing hormone (GnRH). Negative feedback, which occurs in males and during the majority of the female cycle, modulates the amplitude and frequency of GnRH pulses. Positive feedback occurs in females when high estradiol induces a surge pattern of GnRH release. These two forms of feedback and their corresponding patterns of GnRH secretion are thought to be mediated by kisspeptin-expressing neurons in two hypothalamic areas: the arcuate nucleus and the anteroventral periventricular area. In this review, we present evidence for this theory and remaining questions to be addressed.