Glial-gonadotrophin hormone (GnRH) neurone interactions in the median eminence and the control of GnRH secretion

Identification of puberty related miRNAs in the hypothalamus of female mice

BACKGROUND AND AIMS

Puberty is a crucial developmental stage marked by the transition from childhood to adulthood, organized by complex hormonal signaling within the neuroendocrine system. The hypothalamus, a central region in this system, regulates pubertal functions through the hypothalamic-pituitary-gonadal (HPG) axis. Gonadotropin-releasing hormone (GnRH) neurons, essential in puberty control, release GnRH in a pulsatile manner, initiating the production of sex hormones. Major influence in pubertal timing has been attributed to genetic predisposition, environmental factors, and nutritional status. MicroRNAs (miRNAs), small non-coding RNA molecules, have emerged as key regulators in various cellular processes by either repressing genes or activating them by inhibiting their repressors. The present study aims to investigate the involvement of miRNAs in the control of puberty.

METHODS

Small RNA sequencing was used to identify and compare the total population of miRNAs in the hypothalamus of female mice before, during and after puberty. Bioinformatic analysis was applied to analyse the expression profile of miRNAs with altered levels followed by pathway enrichment analysis.

RESULTS

Expression levels of several miRNAs were found up- or down-regulated from pre-pubertal to pubertal stage. Furthermore, monitoring the levels of these miRNAs at the post-pubertal stage revealed four expression patterns, in which pathway analysis displayed the associations of these miRNAs with developmental processes, cell cycle regulation, metabolic biosynthesis and epigenetic regulation.

CONCLUSION

The findings of the present study improve our understanding of the molecular pathways underlying puberty and stress the significance of miRNAs in fine-tuning gene expression within the hypothalamus during this critical developmental stage.

Altered GnRH neuron-glia networks close to interface of polycystic ovary syndrome: Molecular mechanism and clinical perspectives

Polycystic ovary syndrome (PCOS) has been noticed as a neuroendocrine syndrome manifested by reproductive hormone dysregulation involving increased luteinizing hormone (LH) pulse frequency and an increased LH to follicle-stimulating hormone ratio, yet theory is just beginning to be established. Neuroglia located in the arcuate nucleus and median eminence (ARC-ME) that are close to gonadotropin-releasing hormone (GnRH) axon terminals, comprise the blood-brain barrier and fenestrated vessels implying their putative roles in the modulation of the abnormal GnRH pulse in PCOS. This review outlines the disturbances of neuron-glia networks that underlie hypothetically the deregulation of GnRH-LH release and impaired sex hormone negative feedback in PCOS. We then discuss chronic and low-grade inflammatory status together with gut dysbiosis and how the detriments may intrude the hypothalamus by virtue of violating interfaces between the brain and periphery, which might contribute to the etiology of the impaired neural circuits in the ARC-ME to induce PCOS.

Insulin-like growth factor-1 and cognitive health: Exploring cellular, preclinical, and clinical dimensions

Age and insulin-like growth factor-1 (IGF-1) have an inverse association with cognitive decline and dementia. IGF-1 is known to have important pleiotropic functions beginning in neurodevelopment and extending into adulthood such as neurogenesis. At the cellular level, IGF-1 has pleiotropic signaling mechanisms through the IGF-1 receptor on neurons and neuroglia to attenuate inflammation, promote myelination, maintain astrocytic functions for homeostatic balances, and neuronal synaptogenesis. In preclinical rodent models of aging and transgenic models of IGF-1, increased IGF-1 improves cognition in a variety of behavioral paradigms along with reducing IGF-1 via knockout models being able to induce cognitive impairment. At the clinical levels, most studies highlight that increased levels of IGF-1 are associated with better cognition. This review provides a comprehensive and up-to-date evaluation of the association between IGF-1 and cognition at the cellular signaling levels, preclinical, and clinical levels.

Densitometric analysis of GnRH and IBA1 immunocytochemistry in the basal ventromedial hypothalamus of the ewe

Gonadotropin releasing hormone (GnRH) synthesis and secretion regulates seasonal fertility. In the brain, the distribution of GnRH-positive neurons is diffuse, hindering efforts to monitor variations in its cellular and tissue levels. Here, we aim at assessing GnRH immunoreactivity in nuclei responsible for seasonal fertility regulation (SFR) within the posterior, anterior, and preoptic areas of the basal hypothalamus during estrous in ewes. We detected reaction products in the ventromedial basal hypothalamus in neurons, nerve fibers, non-neuronal immunoreactive bodies, and diffuse interstitial areas. Immunoreactivity correlated with the distribution of the main SFR nuclei in the arcuate, retrochiasmatic, periventricular, medial preoptic, supraoptic, and preoptic areas. By independent component analysis density segmentation and by interferential contrast, we identified GnRH non-neuronal positive bodies as microglial cells encapsulated within a dense halo of reaction products. These GnRH-positive microglial cells were distributed in patches and rows throughout the basal ventromedial hypothalamus, suggesting their role in paracrine or juxtacrine signaling. Moreover, as shown by ionized calcium-binding adaptor molecule 1 (IBA1) immunocytochemistry, the distribution of GnRH reaction products overlapped with the microglial dense reactive zones. Therefore, our findings support the assertion that a combined densitometric analysis of GnRH and IBA1 immunocytochemistry enables activity mapping for monitoring seasonal changes following experimental interventions.

Kisspeptin signaling in astrocytes modulates the reproductive axis

Reproduction is safeguarded by multiple, often cooperative, regulatory networks. Kisspeptin signaling, via KISS1R, plays a fundamental role in reproductive control, primarily by regulation of hypothalamic GnRH neurons. We disclose herein a pathway for direct kisspeptin actions in astrocytes that contributes to central reproductive modulation. Protein-protein interaction and ontology analyses of hypothalamic proteomic profiles after kisspeptin stimulation revealed that glial/astrocyte markers are regulated by kisspeptin in mice. This glial-kisspeptin pathway was validated by the demonstrated expression of Kiss1r in mouse astrocytes in vivo and astrocyte cultures from humans, rats, and mice, where kisspeptin activated canonical intracellular signaling-pathways. Cellular coexpression of Kiss1r with the astrocyte markers GFAP and S100-β occurred in different brain regions, with higher percentage in Kiss1- and GnRH-enriched areas. Conditional ablation of Kiss1r in GFAP-positive cells in the G-KiR-KO mouse altered gene expression of key factors in PGE2 synthesis in astrocytes and perturbed astrocyte-GnRH neuronal appositions, as well as LH responses to kisspeptin and LH pulsatility, as surrogate marker of GnRH secretion. G-KiR-KO mice also displayed changes in reproductive responses to metabolic stress induced by high-fat diet, affecting female pubertal onset, estrous cyclicity, and LH-secretory profiles. Our data unveil a nonneuronal pathway for kisspeptin actions in astrocytes, which cooperates in fine-tuning the reproductive axis and its responses to metabolic stress.

Metabolic hormones are integral regulators of female reproductive health and function

The female reproductive system is strongly influenced by nutrition and energy balance. It is well known that food restriction or energy depletion can induce suppression of reproductive processes, while overnutrition is associated with reproductive dysfunction. However, the intricate mechanisms through which nutritional inputs and metabolic health are integrated into the coordination of reproduction are still being defined. In this review, we describe evidence for essential contributions by hormones that are responsive to food intake or fuel stores. Key metabolic hormones-including insulin, the incretins (glucose-dependent insulinotropic polypeptide and glucagon-like peptide-1), growth hormone, ghrelin, leptin, and adiponectin-signal throughout the hypothalamic-pituitary-gonadal axis to support or suppress reproduction. We synthesize current knowledge on how these multifaceted hormones interact with the brain, pituitary, and ovaries to regulate functioning of the female reproductive system, incorporating in vitro and in vivo data from animal models and humans. Metabolic hormones are involved in orchestrating reproductive processes in healthy states, but some also play a significant role in the pathophysiology or treatment strategies of female reproductive disorders. Further understanding of the complex interrelationships between metabolic health and female reproductive function has important implications for improving women's health overall.

SEMA6A drives GnRH neuron-dependent puberty onset by tuning median eminence vascular permeability

Innervation of the hypothalamic median eminence by Gonadotropin-Releasing Hormone (GnRH) neurons is vital to ensure puberty onset and successful reproduction. However, the molecular and cellular mechanisms underlying median eminence development and pubertal timing are incompletely understood. Here we show that Semaphorin-6A is strongly expressed by median eminence-resident oligodendrocytes positioned adjacent to GnRH neuron projections and fenestrated capillaries, and that Semaphorin-6A is required for GnRH neuron innervation and puberty onset. In vitro and in vivo experiments reveal an unexpected function for Semaphorin-6A, via its receptor Plexin-A2, in the control of median eminence vascular permeability to maintain neuroendocrine homeostasis. To support the significance of these findings in humans, we identify patients with delayed puberty carrying a novel pathogenic variant of SEMA6A. In all, our data reveal a role for Semaphorin-6A in regulating GnRH neuron patterning by tuning the median eminence vascular barrier and thereby controlling puberty onset.

Investigation of the Effects of Metallic Nanoparticles on Fertility Outcomes and Endocrine Modification of the Hypothalamic-Pituitary-Gonadal Axis

Nanotechnology is a very disruptive twenty-first-century revolution that will allow social and economic welfare to increase although it also involves a significant human exposure to nanoparticles. The aim of the present study was to contribute to the elucidation on whether metallic nanoparticles have a potential to induce fertility impairments. Regulatory studies that observed official OECD guidelines 415, 416 and 422 have failed to detect any fertility alterations caused by nanoparticle exposure. However, the scientific literature provides evidence that some nanoparticles may cause gonad impairments although the actual impact on fertility remains uncertain. This aim of the present study is to revisit the previously published RNAseq studies by analyzing the effects of several nanoparticles on the transcriptome of T98G human glioblastoma cells given that glial cells are known to play a pivotal role in the regulation of gonadotropin releasing hormone neurons. We found evidence that nanoparticles impair the gonadotropin releasing hormone receptor pathway and several related biological process like, among others, the cellular response to follicular stimulating hormone, cellular response to gonadotropin stimulus, cellular response to hormone stimulus, response to steroid hormone, ovulation cycle and response to estradiol. We propose that nanoparticles interfere with the ability of glial cells to regulate gonadotropin-releasing hormone neurons and, subsequently, the hypothalamic-pituitary-gonadal axis, potentially leading to fertility impairments. To our knowledge, this is the first proposal of a mode of action based on endocrine disruption for explaining the possible effects of nanoparticles on fertility. Whether these finding can be extended to other types of nanoparticles requires further investigation.

Adult Neurogenesis in the Mammalian Hypothalamus: Impact of Newly Generated Neurons on Hypothalamic Function

In mammals, adult neurogenesis was first demonstrated in the subventricular zone of the lateral ventricle (SVZ) and the dentate gyrus of the hippocampal formation. Further research showed that adult neurogenesis persists in other brain structures, such as the cerebral cortex, piriform cortex, striatum, amygdala, and hypothalamus. However, the origin of newly generated cells in these structures is not clear. Accumulating evidence indicates that newly generated neurons in the striatum or amygdala are derived from the SVZ, while in the adult hypothalamus, the proliferation of progenitor cells occurs in the ependymal cells lining the third ventricle, which give rise to new neurons. The heterogeneous cellular organization of the ependymal layer of the hypothalamus leads to different conclusions regarding the type of hypothalamic progenitor cells. In addition, adult hypothalamic neurogenesis occurs at low levels. Based on comparative and functional approaches, we synthesize the knowledge of newly generated cells in the adult hypothalamus. The aim of this review is to provide new insights on adult neurogenesis in the mammalian hypothalamus, with particular attention given to marsupial species. We highlight the number of adult-born neurons in various hypothalamic nuclei, debating whether their low number has an impact on hypothalamic function.

Neuro-pharmacological reinstatement of ovulation and associated neurobiology in a macaque model of functional hypothalamic amenorrhoea

STUDY QUESTION

What is the underlying neuropathology in a cynomolgus macaque model of functional hypothalamic amenorrhoea (FHA) and can it be normalized to restore ovulation?

SUMMARY ANSWER

Anovulatory monkeys exhibited increased hypothalamic norepinephrine (NE), kisspeptin and gonadotropin-releasing hormone (GnRH) in the early follicular phase, but administration of the NE reuptake inhibitor (NRI), reboxetine (REB), restored ovulation during stress and normalized NE, kisspeptin and GnRH.

WHAT IS KNOWN ALREADY

Female cynomolgus macaques, like women, show individual reproductive sensitivity to modest psychosocial and metabolic stress. During stress, resilient females ovulate through two menstrual cycles whereas stress-sensitive (SS) macaques immediately cease ovulation. On Day 5 of a non-stressed menstrual cycle, resilient macaques have less NE synthesizing enzyme [dopamine β-hydroxylase (DBH)], kisspeptin and GnRH innervation of the medial basal hypothalamus but more endogenous serotonin than SS macaques. Stress increased DBH/NE, kisspeptin and GnRH but did not alter serotonin.

STUDY DESIGN, SIZE, DURATION

In a longitudinal design, 27 adult (7-13 years) female cynomolgus macaques (Macaca fascicularis) with three different levels of sensitivity to stress were monitored with daily vaginal swabs and frequent serum progesterone (P) measurements. Three 90-day experimental periods called 'Cycle Sets' were monitored. A Cycle Set consisted of one ovulatory menstrual cycle without stress, and two cycles, or 60 days, with modest stress. Each Cycle Set was followed by a rest period. During a Cycle Set, individuals were either untreated (placebo) or administered escitalopram (CIT) or REB. Ultimately, half of each sensitivity group was euthanized during stress with CIT or REB treatment and the hypothalamus was obtained. Neurobiological endpoints were compared between CIT and REB treatment groups in stress resilient and SS monkeys.

PARTICIPANTS/MATERIALS, SETTING, METHODS

The monkeys were housed at the University of Pittsburgh primate facility for the duration of the experiments. Upon euthanasia, their brains and serum samples were shipped to the Oregon National Primate Research Center. The hypothalamus was examined with immunohistochemistry for the expression of DBH (a marker for NE axons), kisspeptin and GnRH. P was measured in the serum samples by radioimmunoassay.

MAIN RESULTS AND THE ROLE OF CHANCE

Daily administration of REB restored ovulation in 9 of 10 SS animals during stress. Of note, REB significantly increased P secretion during stress in the most sensitive group (P = 0.032), which indicates ovulation. CIT lacked efficacy. REB significantly reduced DBH/NE, kisspeptin and GnRH axon density in the hypothalamus relative to CIT treatment (P = 0.003. 0.018 and 0.0001, respectively) on Day 5 of the menstrual cycle in resilient and sensitive groups.

LARGE SCALE DATA

N/A.

LIMITATIONS, REASONS FOR CAUTION

The US FDA has not approved REB for human use, although it is used in Europe for the treatment of depression/anxiety as EdronaxTR. Whether REB could be useful for the treatment of FHA in women has not been determined.

WIDER IMPLICATIONS FOR THE FINDINGS

The use of an NRI to treat FHA is a novel approach and the potential reinstatement of ovulation could be straightforward compared to current treatment protocols. The underlying neurobiology provides a compelling case for treating the origin of the pathology, i.e. elevated NE, rather than circumventing the hypothalamus altogether with gonadotropins, which have associated risks such as hyperstimulation syndrome or multiple births.

STUDY FUNDING/COMPETING INTEREST(S)

Portions of this study were supported by NIH grant HD062864 to C.L.B., NIH grant HD62618 to J.L.C. and C.L.B. and 1P51 OD011092 for the operation of the Oregon National Primate Research Center. There were no competing interests.

Conditional Fgfr1 Deletion in GnRH Neurons Leads to Minor Disruptions in the Reproductive Axis of Male and Female Mice

In humans and mice, inactivating mutations in fibroblast growth factor receptor 1 (Fgfr1) lead to gonadotropin-releasing hormone (GnRH) deficiency and a host of downstream reproductive disorders. It was unclear if Fgfr1 signaling directly upon GnRH neurons critically drove the establishment of a functional GnRH system. To answer this question, we generated a mouse model with a conditional deletion of Fgfr1 in GnRH neurons using the Cre/loxP approach. These mice, called Fgfr1cKO mice, were examined along with control mice for their pubertal onset and a host of reproductive axis functions. Our results showed that Fgfr1cKO mice harbored no detectable defects in the GnRH system and pubertal onset, suffered only subtle changes in the pituitary function, but exhibited significantly disrupted testicular and ovarian morphology at 25 days of age, indicating impaired gametogenesis at a young age. However, these disruptions were transient and became undetectable in older mice. Our results suggest that Fgfr1 signaling directly on GnRH neurons supports, to some extent, the reproductive axis function in the period leading to the early phase of puberty, but is not critically required for pubertal onset or reproductive maintenance in sexually mature animals.

Role of insulin in the neuroendocrine control of reproduction

Infertility associated with insulin resistance is characterised by abnormal hormone secretion by the hypothalamus, pituitary gland and gonads. These endocrine tissues can maintain insulin sensitivity even when tissues such as the muscle and liver become insulin-resistant, resulting in excessive insulin stimulation as hyperinsulinaemia develops. Experiments conducted to determine the role of neuronal insulin signalling in fertility were unable to recapitulate early findings of hypogonadotrophic hypogonadism in mice lacking insulin receptors throughout the brain. Rather, it was eventually shown that astrocytes critically mediate the effects of insulin on puberty timing and adult reproductive function. However, specific roles for neurones and gonadotrophs have been revealed under conditions of hyperinsulinaemia and by ablation of insulin and leptin receptors. The collective picture is one of multiple insulin-responsive inputs to gonadotrophin releasing hormone neurones, with astrocytes being the most important player.

IGF-1 Influences Gonadotropin-Releasing Hormone Regulation of Puberty

The pubertal process is initiated as a result of complex neuroendocrine interactions within the preoptic and hypothalamic regions of the brain. These interactions ultimately result in a timely increase in the secretion of gonadotropin-releasing hormone (GnRH). Researchers for years have believed that this increase is due to a diminished inhibitory tone which has applied a prepubertal brake on GnRH secretion, as well as to the gradual development of excitatory inputs driving the increased release of the peptide. Over the years, insulin-like growth factor-1 (IGF-1) has emerged as a prime candidate for playing an important role in the onset of puberty. This review will first present initial research demonstrating that IGF-1 increases in circulation as puberty approaches, is able to induce the release of prepubertal GnRH, and can advance the timing of puberty. More recent findings depict an early action of IGF-1 to activate a pathway that releases the inhibitory brake on prepubertal GnRH secretion provided by dynorphin, as well as demonstrating that IGF-1 can also act later in the process to regulate the synthesis and release of kisspeptin, a potent stimulator of GnRH at puberty.

Multimodal hypothalamo-hypophysial communication in the vertebrates

The vertebrate pituitary is arguably one of the most complex endocrine glands from the evolutionary, anatomical and functional perspectives. The pituitary plays a master role in endocrine physiology for the control of growth, metabolism, reproduction, water balance, and the stress response, among many other key processes. The synthesis and secretion of pituitary hormones are under the control of neurohormones produced by the hypothalamus. Under this conceptual framework, the communication between the hypophysiotropic brain and the pituitary gland is at the foundation of our understanding of endocrinology. The anatomy of the connections between the hypothalamus and the pituitary gland has been described in different vertebrate classes, revealing diverse modes of communication together with varying degrees of complexity. In this context, the evolution and variation in the neuronal, neurohemal, endocrine and paracrine modes will be reviewed in light of recent discoveries, and a re-evaluation of earlier observations. There appears to be three main hypothalamo-pituitary communication systems: 1. Diffusion, best exemplified by the agnathans; 2. Direct innervation of the adenohypophysis, which is most developed in teleost fish, and 3. The median eminence/portal blood vessel system, most conspicuously developed in tetrapods, showing also considerable variation between classes. Upon this basic classification, there exists various combinations possible, giving rise to taxon and species-specific, multimodal control over major physiological processes. Intrapituitary paracrine regulation and communication between folliculostellate cells and endocrine cells are additional processes of major importance. Thus, a more complex evolutionary picture of hypothalamo-hypophysial communication is emerging. There is currently little direct evidence to suggest which neuroendocrine genes may control the evolution of one communication system versus another. However, studies at the developmental and intergenerational timescales implicate several genes in the angiogenesis and axonal guidance pathways that may be important.

New insights of the role of β-NGF in the ovulation mechanism of induced ovulating species

The type of stimuli triggering GnRH secretion has been used to classify mammalian species into two categories: spontaneous or induced ovulators. In the former, ovarian steroids produced by a mature follicle elicit the release of GnRH from the hypothalamus, but in the latter, GnRH secretion requires coital stimulation. However, the mechanism responsible for eliciting the preovulatory LH surge in induced ovulators is still not well understood and seems to vary among species. The main goal of this review is to offer new information regarding the mechanism that regulates coitus-induced ovulation. Analysis of several studies documenting the discovery of β-NGF in seminal plasma and its role in the control of ovulation in the llama and rabbit will be described. We also propose a working hypothesis regarding the sites of action of β-NGF in the llama hypothalamus. Finally, we described the presence of β-NGF in the semen of species categorized as spontaneous ovulators, mainly cattle, and its potential role in ovarian function. The discovery of this seminal molecule and its ovulatory effect in induced ovulators challenges previous concepts about the neuroendocrinology of reflex ovulation and has provided a new opportunity to examine the mechanism(s) involved in the cascade of events leading to ovulation. The presence of the factor in the semen of induced as well as spontaneous ovulators highlights the importance of understanding its signaling pathways and mechanism of action and may have broad implications in mammalian fertility.

Delayed Puberty-Phenotypic Diversity, Molecular Genetic Mechanisms, and Recent Discoveries

This review presents a comprehensive discussion of the clinical condition of delayed puberty, a common presentation to the pediatric endocrinologist, which may present both diagnostic and prognostic challenges. Our understanding of the genetic control of pubertal timing has advanced thanks to active investigation in this field over the last two decades, but it remains in large part a fascinating and mysterious conundrum. The phenotype of delayed puberty is associated with adult health risks and common etiologies, and there is evidence for polygenic control of pubertal timing in the general population, sex-specificity, and epigenetic modulation. Moreover, much has been learned from comprehension of monogenic and digenic etiologies of pubertal delay and associated disorders and, in recent years, knowledge of oligogenic inheritance in conditions of GnRH deficiency. Recently there have been several novel discoveries in the field of self-limited delayed puberty, encompassing exciting developments linking this condition to both GnRH neuronal biology and metabolism and body mass. These data together highlight the fascinating heterogeneity of disorders underlying this phenotype and point to areas of future research where impactful developments can be made.

Ablating astrocyte insulin receptors leads to delayed puberty and hypogonadism in mice

Insulin resistance and obesity are associated with reduced gonadotropin-releasing hormone (GnRH) release and infertility. Mice that lack insulin receptors (IRs) throughout development in both neuronal and non-neuronal brain cells are known to exhibit subfertility due to hypogonadotropic hypogonadism. However, attempts to recapitulate this phenotype by targeting specific neurons have failed. To determine whether astrocytic insulin sensing plays a role in the regulation of fertility, we generated mice lacking IRs in astrocytes (astrocyte-specific insulin receptor deletion [IRKOGFAP] mice). IRKOGFAP males and females showed a delay in balanopreputial separation or vaginal opening and first estrous, respectively. In adulthood, IRKOGFAP female mice also exhibited longer, irregular estrus cycles, decreased pregnancy rates, and reduced litter sizes. IRKOGFAP mice show normal sexual behavior but hypothalamic-pituitary-gonadotropin (HPG) axis dysregulation, likely explaining their low fecundity. Histological examination of testes and ovaries showed impaired spermatogenesis and ovarian follicle maturation. Finally, reduced prostaglandin E synthase 2 (PGES2) levels were found in astrocytes isolated from these mice, suggesting a mechanism for low GnRH/luteinizing hormone (LH) secretion. These findings demonstrate that insulin sensing by astrocytes is indispensable for the function of the reproductive axis. Additional work is needed to elucidate the role of astrocytes in the maturation of hypothalamic reproductive circuits.

Tanycytes: A rich morphological history to underpin future molecular and physiological investigations

Tanycytes are located at the base of the brain and retain characteristics from their developmental origins, such as radial glial cells, throughout their life span. With transport mechanisms and modulation of tight junction proteins, tanycytes form a bridge connecting the cerebrospinal fluid with the external limiting basement membrane. They also retain the powers of self-renewal and can differentiate to generate neurones and glia. Similar to radial glia, they are a heterogeneous family with distinct phenotypes. Although the four subtypes so far distinguished display distinct characteristics, further research is likely to reveal new subtypes. In this review, we have re-visited the work of the pioneers in the field, revealing forgotten work that is waiting to inspire new research with today's cutting-edge technologies. We have conducted a systematic ultrastructural study of α-tanycytes that resulted in a wealth of new information, generating numerous questions for future study. We also consider median eminence pituicytes, a closely-related cell type to tanycytes, and attempt to relate pituicyte fine morphology to molecular and functional mechanism. Our rationale was that future research should be guided by a better understanding of the early pioneering work in the field, which may currently be overlooked when interpreting newer data or designing new investigations.

Genes underlying delayed puberty

The genetic control of pubertal timing has been a field of active investigation for the last decade, but remains a fascinating and mysterious conundrum. Self-limited delayed puberty (DP), also known as constitutional delay of growth and puberty, represents the extreme end of normal pubertal timing, and is the commonest cause of DP in both boys and girls. Familial self-limited DP has a clear genetic basis. It is a highly heritable condition, which often segregates in an autosomal dominant pattern (with or without complete penetrance) in the majority of families. However, the underlying neuroendocrine pathophysiology and genetic regulation has been largely unknown. Very recently novel gene discoveries from next generation sequencing studies have provided insights into the genetic mutations that lead to familial DP. Further understanding has come from sequencing genes known to cause GnRH deficiency, next generation sequencing studies in patients with early puberty, and from large-scale genome wide association studies in the general population. Results of these studies suggest that the genetic basis of DP is likely to be highly heterogeneous. Abnormalities of GnRH neuronal development, function, and its downstream pathways, metabolic and energy homeostatic derangements, and transcriptional regulation of the hypothalamic-pituitary-gonadal axis may all lead to DP. This variety of different pathogenic mechanisms affecting the release of the puberty 'brake' may take place in several age windows between fetal life and puberty.

Cell adhesion and matricellular support by astrocytes of the tripartite synapse

Astrocytes contribute to the formation, function, and plasticity of synapses. Their processes enwrap the neuronal components of the tripartite synapse, and due to this close interaction they are perfectly positioned to modulate neuronal communication. The interaction between astrocytes and synapses is facilitated by cell adhesion molecules and matricellular proteins, which have been implicated in the formation and functioning of tripartite synapses. The importance of such neuron-astrocyte integration at the synapse is underscored by the emerging role of astrocyte dysfunction in synaptic pathologies such as autism and schizophrenia. Here we review astrocyte-expressed cell adhesion molecules and matricellular molecules that play a role in integration of neurons and astrocytes within the tripartite synapse.

Clarifying the Ghrelin System's Ability to Regulate Feeding Behaviours Despite Enigmatic Spatial Separation of the GHSR and Its Endogenous Ligand

Ghrelin is a hormone predominantly produced in and secreted from the stomach. Ghrelin is involved in many physiological processes including feeding, the stress response, and in modulating learning, memory and motivational processes. Ghrelin does this by binding to its receptor, the growth hormone secretagogue receptor (GHSR), a receptor found in relatively high concentrations in hypothalamic and mesolimbic brain regions. While the feeding and metabolic effects of ghrelin can be explained by the effects of this hormone on regions of the brain that have a more permeable blood brain barrier (BBB), ghrelin produced within the periphery demonstrates a limited ability to reach extrahypothalamic regions where GHSRs are expressed. Therefore, one of the most pressing unanswered questions plaguing ghrelin research is how GHSRs, distributed in brain regions protected by the BBB, are activated despite ghrelin’s predominant peripheral production and poor ability to transverse the BBB. This manuscript will describe how peripheral ghrelin activates central GHSRs to encourage feeding, and how central ghrelin synthesis and ghrelin independent activation of GHSRs may also contribute to the modulation of feeding behaviours.

Quantitative assessment of fibroblast growth factor receptor 1 expression in neurons and glia

Background

Fibroblast growth factors (FGFs) and their receptors (FGFRs) have numerous functions in the developing and adult central nervous system (CNS). For example, the FGFR1 receptor is important for proliferation and fate specification of radial glial cells in the cortex and hippocampus, oligodendrocyte proliferation and regeneration, midline glia morphology and soma translocation, Bergmann glia morphology, and cerebellar morphogenesis. In addition, FGFR1 signaling in astrocytes is required for postnatal maturation of interneurons expressing parvalbumin (PV). FGFR1 is implicated in synapse formation in the hippocampus, and alterations in the expression of Fgfr1 and its ligand, Fgf2 accompany major depression. Understanding which cell types express Fgfr1 during development may elucidate its roles in normal development of the brain as well as illuminate possible causes of certain neuropsychiatric disorders.

Methods

Here, we used a BAC transgenic reporter line to trace Fgfr1 expression in the developing postnatal murine CNS. The specific transgenic line employed was created by the GENSAT project, tgFGFR1-EGFPGP338Gsat, and includes a gene encoding enhanced green fluorescent protein (EGFP) under the regulation of the Fgfr1 promoter, to trace Fgfr1 expression in the developing CNS. Unbiased stereological counts were performed for several cell types in the cortex and hippocampus.

Results

This model reveals that Fgfr1 is primarily expressed in glial cells, in both astrocytes and oligodendrocytes, along with some neurons. Dual labeling experiments indicate that the proportion of GFP+ (Fgfr1+) cells that are also GFAP+ increases from postnatal day 7 (P7) to 1 month, illuminating dynamic changes in Fgfr1 expression during postnatal development of the cortex. In postnatal neurogenic areas, GFP expression was also observed in SOX2, doublecortin (DCX), and brain lipid-binding protein (BLBP) expressing cells. Fgfr1 is also highly expressed in DCX positive cells of the dentate gyrus (DG), but not in the rostral migratory stream. Fgfr1 driven GFP was also observed in tanycytes and GFAP+ cells of the hypothalamus, as well as in Bergmann glia and astrocytes of the cerebellum.

Conclusions

The tgFGFR1-EGFPGP338Gsat mouse model expresses GFP that is congruent with known functions of FGFR1, including hippocampal development, glial cell development, and stem cell proliferation. Understanding which cell types express Fgfr1 may elucidate its role in neuropsychiatric disorders and brain development.

Neuroendocrine integration of nutritional signals on reproduction

Reproductive function in mammals is energetically costly and therefore tightly regulated by nutritional status. To enable this integration of metabolic and reproductive function, information regarding peripheral nutritional status must be relayed centrally to the gonadotropin-releasing hormone (GNRH) neurons that drive reproductive function. The metabolically relevant hormones leptin, insulin and ghrelin have been identified as key mediators of this 'metabolic control of fertility'. However, the neural circuitry through which they act to exert their control over GNRH drive remains incompletely understood. With the advent of Cre-LoxP technology, it has become possible to perform targeted gene-deletion and gene-rescue experiments and thus test the functional requirement and sufficiency, respectively, of discrete hormone-neuron signaling pathways in the metabolic control of reproductive function. This review discusses the findings from these investigations, and attempts to put them in context with what is known from clinical situations and wild-type animal models. What emerges from this discussion is clear evidence that the integration of nutritional signals on reproduction is complex and highly redundant, and therefore, surprisingly difficult to perturb. Consequently, the deletion of individual hormone-neuron signaling pathways often fails to cause reproductive phenotypes, despite strong evidence that the targeted pathway plays a role under normal physiological conditions. Although transgenic studies rarely reveal a critical role for discrete signaling pathways, they nevertheless prove to be a good strategy for identifying whether a targeted pathway is absolutely required, critically involved, sufficient or dispensable in the metabolic control of fertility.

Structural Reconstruction of the Perivascular Space in the Adult Mouse Neurohypophysis During an Osmotic Stimulation

Oxytocin (OXT) and arginine vasopressin (AVP) neuropeptides in the neurohypophysis (NH) control lactation and body fluid homeostasis, respectively. Hypothalamic neurosecretory neurones project their axons from the supraoptic and paraventricular nuclei to the NH to make contact with the vascular surface and release OXT and AVP. The neurohypophysial vascular structure is unique because it has a wide perivascular space between the inner and outer basement membranes. However, the significance of this unique vascular structure remains unclear; therefore, we aimed to determine the functional significance of the perivascular space and its activity-dependent changes during salt loading in adult mice. The results obtained revealed that pericytes were the main resident cells and defined the profile of the perivascular space. Moreover, pericytes sometimes extended their cellular processes or 'perivascular protrusions' into neurohypophysial parenchyma between axonal terminals. The vascular permeability of low-molecular-weight (LMW) molecules was higher at perivascular protrusions than at the smooth vascular surface. Axonal terminals containing OXT and AVP were more likely to localise at perivascular protrusions than at the smooth vascular surface. Chronic salt loading with 2% NaCl significantly induced prominent changes in the shape of pericytes and also increased the number of perivascular protrusions and the surface area of the perivascular space together with elevations in the vascular permeability of LMW molecules. Collectively, these results indicate that the perivascular space of the NH acts as the main diffusion route for OXT and AVP and, in addition, changes in the shape of pericytes and perivascular reconstruction occur in response to an increased demand for neuropeptide release.

Advances in Understanding of Structural Reorganization in the Hypothalamic Neurosecretory System

The hypothalamic neurosecretory system synthesizes neuropeptides in hypothalamic nuclei and releases them from axonal terminals into the circulation in the neurohypophysis (NH) and median eminence (ME). This system plays a crucial role in regulating body fluid homeostasis and social behaviors as well as reproduction, growth, metabolism, and stress responses, and activity-dependent structural reorganization has been reported. Current knowledge on dynamic structural reorganization in the NH and ME, in which the axonal terminals of neurosecretory neurons directly contact the basement membrane (BM) of a fenestrated vasculature, is discussed herein. Glial cells, pituicytes in the NH and tanycytes in the ME, engulf axonal terminals and interpose their cellular processes between axonal terminals and the BM when hormonal demands are low. Increasing demands for neurosecretion result in the retraction of the cellular processes of glial cells from axonal terminals and the BM, permitting increased neurovascular contact. The shape conversion of pituicytes and tanycytes is mediated by neurotransmitters and sex steroid hormones, respectively. The NH and ME have a rough vascular BM profile of wide perivascular spaces and specialized extension structures called "perivascular protrusions." Perivascular protrusions, the insides of which are occupied by the cellular processes of vascular mural cells pericytes, contribute to increasing neurovascular contact and, thus, the efficient diffusion of hypothalamic neuropeptides. A chronic physiological stimulation has been shown to increase perivascular protrusions via the shape conversion of pericytes and the profile of the vascular surface. Continuous angiogenesis occurs in the NH and ME of healthy normal adult rodents depending on the signaling of vascular endothelial growth factor (VEGF). The inhibition of VEGF signaling suppresses the proliferation of endothelial cells (ECs) and promotes their apoptosis, which results in decreases in the population of ECs and axonal terminals. Pituicytes and tanycytes are continuously replaced by the proliferation and differentiation of stem/progenitor cells, which may be regulated by matching those of ECs and axonal terminals. In conclusion, structural reorganization in the NH and ME is caused by the activity-dependent shape conversion of glial cells and vascular mural cells as well as the proliferation of endothelial and glial cells by angiogenesis and gliogenesis, respectively.

ST8SIA2 promotes oligodendrocyte differentiation and the integrity of myelin and axons

ST8SIA2 is a polysialyltransferase that attaches polysialic acid to the glycoproteins NCAM1 and CADM1. Polysialylation is involved in brain development and plasticity. ST8SIA2 is a schizophrenia candidate gene, and St8sia2^−/− mice exhibit schizophrenia‐like behavior. We sought to identify new pathological consequences of ST8SIA2 deficiency. Our proteomic analysis suggested myelin impairment in St8sia2^−/− mice. Histological and immune staining together with Western blot revealed that the onset of myelination was not delayed in St8sia2^−/− mice, but the content of myelin was lower. Ultrastructure analysis of the corpus callosum showed thinner myelin sheaths, smaller and irregularly shaped axons, and white matter lesions in adult St8sia2^−/− mice. Then we evaluated oligodendrocyte differentiation in vivo and in vitro. Fewer OLIG2+ cells in the cortex and corpus callosum, together with the higher percentage of undifferentiated oligodenroglia in St8sia2^−/− mice suggested an impairment in oligodendrocyte generation. Experiment on primary cultures of oligodendrocyte precursor cells (OPCs) confirmed a cell‐autonomous effect of ST8SIA2 in oligodendroglia, and demonstrated that OPC to oligodendrocyte transition is inhibited in St8sia2^−/− mice. Concluding, ST8SIA2‐mediated polysialylation influences on oligodendrocyte differentiation, and oligodendrocyte deficits in St8sia2 mice are a possible cause of the demyelination and degeneration of axons, resembling nerve fiber alterations in schizophrenia. GLIA 2016;65:34–49

Diencephalic and septal structures containing the avian vasotocin receptor (V1aR) involved in the regulation of food intake in chickens, Gallus gallus

Recently, it was found that the avian central vasotocin receptor (V1aR) is associated with the regulation of food intake. To identify V1aR-containing brain structures regulating food intake, a selective V1aR antagonist SR-49059 that induced food intake was administrated intracerebroventricularly in male chickens followed by detection of brain structures using FOS immunoreactivity. Particularly, the hypothalamic core region of the paraventricular nucleus, lateral hypothalamic area, dorsomedial hypothalamic nucleus, a subnucleus of the central extended amygdalar complex [dorsolateral bed nucleus of the stria terminalis], medial septal nucleus and caudal brainstem [nucleus of the solitary tract] showed significantly increased FOS-ir cells. On the other hand, the supraoptic nucleus of the preoptic area and the nucleus of the hippocampal commissure of the septum showed suppressed FOS immunoreactivity in the V1aR antagonist treatment group. Further investigation revealed that neuronal activity of arginine vasotocin (AVT-ir) magnocellular neurons in the supraoptic nucleus, preoptic periventricular nucleus, paraventricular nucleus and ventral periventricular hypothalamic nucleus and most likely corticotropin releasing hormone (CRH-ir) neurons in the nucleus of the hippocampal commissure were reduced following the antagonist treatment. Dual immunofluorescence labeling results showed that perikarya of AVT-ir magnocellular neurons in the preoptic area and hypothalamus were colabeled with V1aR. Within the nucleus of the hippocampal commissure, CRH-ir neurons were shown in close contact with V1aR-ir glial cells. Results of the present study suggest that the V1aR plays a role in the regulation of food intake by modulating neurons that synthesize and release anorectic neuropeptides in the avian brain.

The role of astrocytes in the hypothalamic response and adaptation to metabolic signals

The hypothalamus is crucial in the regulation of homeostatic functions in mammals, with the disruption of hypothalamic circuits contributing to chronic conditions such as obesity, diabetes mellitus, hypertension, and infertility. Metabolic signals and hormonal inputs drive functional and morphological changes in the hypothalamus in attempt to maintain metabolic homeostasis. However, the dramatic increase in the incidence of obesity and its secondary complications, such as type 2 diabetes, have evidenced the need to better understand how this system functions and how it can go awry. Growing evidence points to a critical role of astrocytes in orchestrating the hypothalamic response to metabolic cues by participating in processes of synaptic transmission, synaptic plasticity and nutrient sensing. These glial cells express receptors for important metabolic signals, such as the anorexigenic hormone leptin, and determine the type and quantity of nutrients reaching their neighboring neurons. Understanding the mechanisms by which astrocytes participate in hypothalamic adaptations to changes in dietary and metabolic signals is fundamental for understanding the neuroendocrine control of metabolism and key in the search for adequate treatments of metabolic diseases.

Manganese protects against the effects of alcohol on hypothalamic puberty-related hormones

AIMS

Since manganese (Mn) is capable of stimulating the hypothalamic-pituitary unit and advancing female puberty, we assessed the possibility that this element might overcome some of the detrimental effects of prepubertal alcohol (ALC) exposure on the hypothalamic control of pituitary function.

MAIN METHODS

Rats received either saline or Mn (10mg/kg) daily by gastric gavage from day 12 to day 31. After weaning, all rats were provided Lab Chow diet ad libitum until day 27 when they began receiving either the Bio Serv control or ALC diet regime. On day 31, the medial basal hypothalamus (MBH) was collected to assess luteinizing hormone-releasing hormone (LHRH) and cyclooxygenase 2 (COX2) protein levels. Release of prostaglandin-E2 (PGE2), LHRH and serum luteinizing hormone (LH) were also assessed. Other animals were not terminated on day 31, but remained in study to assess timing of puberty.

KEY FINDINGS

Short-term ALC exposure caused elevated hypothalamic LHRH content, suggesting an inhibition in peptide release, resulting in a decrease in LH. Both actions of ALC were reversed by Mn supplementation. COX2 synthesis, as well as PGE2 and LHRH release were suppressed by ALC exposure, but Mn supplementation caused an increase in COX2 synthesis and subsequent PGE2 and LHRH release in the presence of ALC. Mn supplementation also ameliorated the action of ALC to delay puberty.

SIGNIFICANCE

These results suggest that low level Mn supplementation acts to protect the hypothalamus from some of the detrimental effects of ALC on puberty-related hormones.

GnRH Episodic Secretion Is Altered by Pharmacological Blockade of Gap Junctions: Possible Involvement of Glial Cells

Episodic release of GnRH is essential for reproductive function. In vitro studies have established that this episodic release is an endogenous property of GnRH neurons and that GnRH secretory pulses are associated with synchronization of GnRH neuron activity. The cellular mechanisms by which GnRH neurons synchronize remain largely unknown. There is no clear evidence of physical coupling of GnRH neurons through gap junctions to explain episodic synchronization. However, coupling of glial cells through gap junctions has been shown to regulate neuron activity in their microenvironment. The present study investigated whether glial cell communication through gap junctions plays a role in GnRH neuron activity and secretion in the mouse. Our findings show that Glial Fibrillary Acidic Protein-expressing glial cells located in the median eminence in close vicinity to GnRH fibers expressed Gja1 encoding connexin-43. To study the impact of glial-gap junction coupling on GnRH neuron activity, an in vitro model of primary cultures from mouse embryo nasal placodes was used. In this model, GnRH neurons possess a glial microenvironment and were able to release GnRH in an episodic manner. Our findings show that in vitro glial cells forming the microenvironment of GnRH neurons expressed connexin-43 and displayed functional gap junctions. Pharmacological blockade of the gap junctions with 50 μM 18-α-glycyrrhetinic acid decreased GnRH secretion by reducing pulse frequency and amplitude, suppressed neuronal synchronization and drastically reduced spontaneous electrical activity, all these effects were reversed upon 18-α-glycyrrhetinic acid washout.

Age and Long-Term Hormone Treatment Effects on the Ultrastructural Morphology of the Median Eminence of Female Rhesus Macaques

The median eminence (ME) of the hypothalamus comprises the hypothalamic nerve terminals, glia (especially tanycytes) and the portal capillary vasculature that transports hypothalamic neurohormones to the anterior pituitary gland. The ultrastructure of the ME is dynamically regulated by hormones and undergoes organizational changes during development and reproductive cycles in adult females, but relatively little is known about the ME during aging, especially in nonhuman primates. Therefore, we used a novel transmission scanning electron microscopy technique to examine the cytoarchitecture of the ME of young and aged female rhesus macaques in a preclinical monkey model of menopausal hormone treatments. Rhesus macaques were ovariectomized and treated for 2 years with vehicle, estradiol (E2), or estradiol + progesterone (E2 + P4). While the overall cytoarchitecture of the ME underwent relatively few changes with age and hormones, changes to some features of neural and glial components near the portal capillaries were observed. Specifically, large neuroterminal size was greater in aged compared to young adult animals, an effect that was mitigated or reversed by E2 alone but not by E2 + P4 treatment. Overall glial size and the density and tissue fraction of the largest subset of glia were greater in aged monkeys, and in some cases reversed by E2 treatment. Mitochondrial size was decreased by E2, but not E2 + P4, only in aged macaques. These results contrast substantially with work in rodents, suggesting that the ME of aging macaques is less vulnerable to age-related disorganization, and that the effects of E2 on monkeys' ME are age specific.

Insulin Does Not Target CamkIIα Neurones to Critically Regulate the Neuroendocrine Reproductive Axis in Mice

Insulin signalling in the brain plays an important role in the central regulation of energy homeostasis and fertility, such that mice exhibiting widespread deletion of insulin receptors (InsR) throughout the brain and peripheral nervous system display diet sensitive obesity and hypothalamic hypogonadism. However, the specific cell types mediating the central effects of insulin on fertility remain largely unidentified. To date, the targeted deletion of InsR from individual neuronal populations implicated in the metabolic control of fertility has failed to recapitulate the hypogonadic and subfertile phenotype observed in brain-specific InsR knockout mice. Because insulin and leptin share similar roles as centrally-acting metabolic regulators of fertility, we used the Cre-loxP system to generate mice with a selective inactivation of the Insr gene from the same widespread neuronal population previously shown to mediate the central effects of leptin on fertility by crossing Insr-flox mice with calcium/calmodulin-dependent protein kinase type IIα (CamkIIα)-Cre mice. Multiple reproductive and metabolic parameters were then compared between male and female Insr-flox/Cre-positive (CamK-IRKO) and Insr-flox/Cre-negative control mice. Consistent with brain-specific InsR knockout mice, CamK-IRKO mice exhibited a mild but significant obesogenic phenotype. Unexpectedly, CamK-IRKO mice exhibited normal reproductive maturation and function compared to controls. No differences in the age of puberty onset, oestrous cyclicity or fecundity were observed between CamK-IRKO and control mice. We conclude that the central effects of insulin on the neuroendocrine reproductive axis are not critically mediated via the same neuronal populations targeted by leptin.

Alcohol alters hypothalamic glial-neuronal communications involved in the neuroendocrine control of puberty: In vivo and in vitro assessments

The onset of puberty is the result of the increased secretion of hypothalamic luteinizing hormone-releasing hormone (LHRH). The pubertal process can be altered by substances that can affect the prepubertal secretion of this peptide. Alcohol is one such substance known to diminish LHRH secretion and delay the initiation of puberty. The increased secretion of LHRH that normally occurs at the time of puberty is due to a decrease of inhibitory tone that prevails prior to the onset of puberty, as well as an enhanced development of excitatory inputs to the LHRH secretory system. Additionally, it has become increasingly clear that glial-neuronal communications are important for pubertal development because they play an integral role in facilitating the pubertal rise in LHRH secretion. Thus, in recent years attempts have been made to identify specific glial-derived components that contribute to the development of coordinated communication networks between glia and LHRH cell bodies, as well as their nerve terminals. Transforming growth factor-α and transforming growth factor-β1 are two such glial substances that have received attention in this regard. This review summarizes the use of multiple neuroendocrine research techniques employed to assess these glial-neuronal communication pathways involved in regulating prepubertal LHRH secretion and the effects that alcohol can have on their respective functions.

Architecture of GnRH-Gonadotrope-Vasculature Reveals a Dual Mode of Gonadotropin Regulation in Fish

The function and components of the hypothalamic-pituitary axis are conserved among vertebrates; however, in fish, a neuroglandular mode of delivery (direct contact between axons and endocrine cells) was considered dominant, whereas in tetrapods hypothalamic signals are relayed to their targets via the hypophysial portal blood system (neurovascular delivery mode). By using a transgenic zebrafish model we studied the functional and anatomical aspects of gonadotrope regulation thus revisiting the existing model. FSH cells were found to be situated close to the vasculature whereas the compact organization of LH cells prevented direct contact of all cells with the circulation. GnRH3 fibers formed multiple boutons upon reaching the pituitary, but most of these structures were located in the neurohypophysis rather than adjacent to gonadotropes. A close association was observed between FSH cells and GnRH3 boutons, but only a fifth of the LH cells were in direct contact with GnRH3 axons, suggesting that FSH cells are more directly regulated than LH cells. GnRH3 fibers closely followed the vasculature in the neurohypophysis and formed numerous boutons along these tracts. These vessels were found to be permeable to relatively large molecules, suggesting the uptake of GnRH3 peptides. Our findings have important implications regarding the differential regulation of LH and FSH and contradict the accepted notion that fish pituitary cells are mostly regulated directly by hypothalamic fibers. Instead, we provide evidence that zebrafish apply a dual mode of gonadotrope regulation by GnRH3 that combines both neuroglandular and neurovascular components.

Hypothalamic tanycytes-masters and servants of metabolic, neuroendocrine, and neurogenic functions

There is a resurgent interest in tanycytes, a radial glial-like cell population occupying the floor and ventro-lateral walls of the third ventricle (3V). Tanycytes reside in close proximity to hypothalamic neuronal nuclei that regulate appetite and energy expenditure, with a subset sending projections into these nuclei. Moreover, tanycytes are exposed to 3V cerebrospinal fluid and have privileged access to plasma metabolites and hormones, through fenestrated capillaries. Indeed, some tanycytes act as conduits for trafficking of these molecules into the brain parenchyma. Tanycytes can also act as neural stem/progenitor cells, supplying the postnatal and adult hypothalamus with new neurons. Collectively, these findings suggest that tanycytes regulate and integrate important trophic and metabolic processes and possibly endow functional malleability to neuronal circuits of the hypothalamus. Hence, manipulation of tanycyte biology could provide a valuable tool for modulating hypothalamic functions such as energy uptake and expenditure in order to tackle prevalent eating disorders such as obesity and anorexia.

New aspects in fenestrated capillary and tissue dynamics in the sensory circumventricular organs of adult brains

The blood-brain barrier (BBB) generally consists of endothelial tight junction barriers that prevent the free entry of blood-derived substances, thereby maintaining the extracellular environment of the brain. However, the circumventricular organs (CVOs), which are located along the midlines of the brain ventricles, lack these endothelial barriers and have fenestrated capillaries; therefore, they have a number of essential functions, including the transduction of information between the blood circulation and brain. Previous studies have demonstrated the extensive contribution of the CVOs to body fluid and thermal homeostasis, energy balance, the chemoreception of blood-derived substances, and neuroinflammation. In this review, recent advances have been discussed in fenestrated capillary characterization and dynamic tissue reconstruction accompanied by angiogenesis and neurogliogenesis in the sensory CVOs of adult brains. The sensory CVOs, including the organum vasculosum of the lamina terminalis (OVLT), subfornical organ (SFO), and area postrema (AP), have size-selective and heterogeneous vascular permeabilities. Astrocyte-/tanycyte-like neural stem cells (NSCs) sense blood- and cerebrospinal fluid-derived information through the transient receptor potential vanilloid 1, a mechanical/osmotic receptor, Toll-like receptor 4, a lipopolysaccharide receptor, and Nax, a Na-sensing Na channel. They also express tight junction proteins and densely and tightly surround mature neurons to protect them from blood-derived neurotoxic substances, indicating that the NSCs of the CVOs perform BBB functions while maintaining the capacity to differentiate into new neurons and glial cells. In addition to neurogliogenesis, the density of fenestrated capillaries is regulated by angiogenesis, which is accompanied by the active proliferation and sprouting of endothelial cells. Vascular endothelial growth factor (VEGF) signaling may be involved in angiogenesis and neurogliogenesis, both of which affect vascular permeability. Thus, recent findings advocate novel concepts for the CVOs, which have the dynamic features of vascular and parenchymal tissues.

Effects of GnRH immunization on the reproductive axis and thymulin

The bidirectional regulation of thymulin in the reproductive-endocrine function of the hypothalamic-pituitary-gonadal (HPG) axis of rats immunized against GnRH remains largely unclear. We explored the alterations in hormones in the HPG axis in immunized rats to dissect the repressive effect of immunization on thymulin, and to clarify the interrelation of reproductive hormones and thymulin in vivo. The results showed that, in the first 2 weeks of booster immunization, thymulin was repressed when reproductive hormones were severely reduced. The self-feedback regulation of thymulin was then stimulated in later immune stages: the rising circulating thymulin upregulated LH and FSH, including GnRH in the hypothalamus, although the levels of those hormones were still significantly lower than in the control groups. In astrocytes, thymulin produced a feedback effect in regulated GnRH neurons. However, in the arcuate nucleus (Arc) and the median eminence (ME), the mediator of astrocytes and other glial cells were also directly affected by reproductive hormones. Thus, in immunized rats, the expression of glial fibrillary acidic protein was distinctly stimulated in the Arc and ME. This study demonstrated that thymulin was downregulated by immunization against GnRH in early stage. Subsequently, the self-feedback regulation was provoked by low circulating thymulin. Thereafter, rising thymulin levels promoted pituitary gonadotropins levels, while acting directly on GnRH neurons, which was mediated by astrocytes in a region-dependent manner in the hypothalamus.

Pathology or normal variant: what constitutes a delay in puberty?

Puberty is a complex maturation process that begins during fetal life and persists until the acquisition of reproduction function. The fundamental event that activates puberty occurs in the hypothalamus. A complex neuron network stimulates GnRH secretion, which stimulates pituitary gonadotropin secretion and then gonadal steroid secretion. Pubertal delay is defined as the presentation of clinical signs of puberty 2-2.5 SD later than in the normal population. Three major groups of etiopathogeneses are described: (1) hypogonadotropic hypogonadism, (2) hypergonadotropic hypogonadism, and (3) constitutional delay of puberty (CDP) - the most common cause of delayed puberty in boys. The differential diagnosis between CDP and isolated hypogonadotropic hypogonadism remains difficult. Mechanisms of pubertal timing are now better understood and genetic or epigenetic causes can explain some pubertal delays. However, there are still unexplained mechanisms. Treatment of delayed puberty is necessary to ensure full pubertal development for the adolescent and in case of hypogonadism, to restore fertility. Finally, precocious diagnosis of hypogonadism is primordial but can be difficult during childhood and in cases of partial hypogonadism. The study of genetic pubertal diseases or of different animal models could help to discover new diagnostic or therapeutic tools.

Expression of Vesicular Glutamate Transporter 2 (vGluT2) on Large Dense-Core Vesicles within GnRH Neuroterminals of Aging Female Rats

The pulsatile release of GnRH is crucial for normal reproductive physiology across the life cycle, a process that is regulated by hypothalamic neurotransmitters. GnRH terminals co-express the vesicular glutamate transporter 2 (vGluT2) as a marker of a glutamatergic phenotype. The current study sought to elucidate the relationship between glutamate and GnRH nerve terminals in the median eminence—the site of GnRH release into the portal capillary vasculature. We also determined whether this co-expression may change during reproductive senescence, and if steroid hormones, which affect responsiveness of GnRH neurons to glutamate, may alter the co-expression pattern. Female Sprague-Dawley rats were ovariectomized at young adult, middle-aged and old ages (~4, 11, and 22 months, respectively) and treated four weeks later with sequential vehicle + vehicle (VEH + VEH), estradiol + vehicle (E₂ + VEH), or estradiol + progesterone (E₂+P₄). Rats were perfused 24 hours after the second hormone treatment. Confocal microscopy was used to determine colocalization of GnRH and vGluT2 immunofluorescence in the median eminence. Post-embedding immunogold labeling of GnRH and vGluT2, and a serial electron microscopy (EM) technique were used to determine the cellular interaction between GnRH terminals and glutamate signaling. Confocal analysis showed that GnRH and vGluT2 immunofluorescent puncta were extensively colocalized in the median eminence and that their density declined with age but was unaffected by short-term hormone treatment. EM results showed that vGluT2 immunoreactivity was extensively associated with large dense-core vesicles, suggesting a unique glutamatergic signaling pathway in GnRH terminals. Our results provide novel subcellular information about the intimate relationship between GnRH terminals and glutamate in the median eminence.

Alcohol alters insulin-like growth factor-1-induced transforming growth factor β1 synthesis in the medial basal hypothalamus of the prepubertal female rat

BACKGROUND

Insulin-like growth factor-1 (IGF-1) and transforming growth factor β1 (TGFβ1) are produced in hypothalamic astrocytes and facilitate luteinizing hormone-releasing hormone (LHRH) secretion. IGF-1 stimulates release by acting directly on the LHRH nerve terminals and both peptides act indirectly through specific plastic changes on glial/tanycyte processes that further support LHRH secretion. Because the relationship between these growth factors in the hypothalamus is not known, we assessed the ability of IGF-1 to induce TGFβ1 synthesis and release and the actions of alcohol (ALC) on this mechanism prior to the onset of puberty.

METHODS

Hypothalamic astrocytes were exposed to medium only, medium plus IGF-1 (200 ng/ml), or medium plus IGF-1 with 50 mM ALC. After 18 hours, media were collected and assayed for TGFβ1. For the in vivo experiment, prepubertal female rats were administered either ALC (3 g/kg) or water via gastric gavage at 07:30 hours and at 11:30 hours. At 09:00 hours, saline or IGF-1 was administered into the third ventricle. Rats were killed at 15:00 hours and the medial basal hypothalamus (MBH) was collected for assessment of TGFβ1, IGF-1 receptor (IGF-1R), and Akt.

RESULTS

IGF-1 induced TGFβ1 release (p < 0.01) from hypothalamic astrocytes in culture, an action blocked by ALC. In vivo, IGF-1 administration caused an increase in TGFβ1 protein compared with controls (p < 0.05), an action blocked by ALC as well as a phosphatidylinositol 3 kinase/Akt inhibitor. IGF-1 stimulation also increased both total (p< 0.01) and phosphorylated (p)-IGF-1R (p < 0.05) protein levels, and phosphorylated (p)-Akt levels (p < 0.01), which were also blocked by ALC.

CONCLUSIONS

This study shows that ALC blocks IGF-1 actions to stimulate synthesis and release of hypothalamic TGFβ1, total and p-IGF-1R, and p-Akt levels further demonstrating the inhibitory actions of ALC on puberty-related events associated with hypothalamic LHRH release.

VEGF-dependent and PDGF-dependent dynamic neurovascular reconstruction in the neurohypophysis of adult mice

Hypothalamo-neurohypophysial system (HNS) releases arginine vasopressin (AVP) and oxytocin (OXT) from axonal terminals of the neurohypophysis (NH) into blood circulation for controlling body fluid homeostasis and lactation. Chronic osmotic and suckling stimulations have been shown to cause neurovascular and neuroglial reconstruction in the NH of adult mammals and no study has been reported for vascular dynamics. The aim of this study was to elucidate the occurrence of continuous angiogenesis and growth factor-dependent neurovascular reconstruction in the NH of adult mice. Active proliferation of endothelial cells and oligodendrocyte progenitor cells (OPCs) was observed using the immunohistochemistry of bromodeoxyuridine and Ki-67. Vascular endothelial growth factor A (VEGFA) and VEGF receptor 2 (VEGFR2 (KDR)) were highly expressed at pituicytes and endothelial cells respectively. Moreover, prominent expression of platelet-derived growth factor B (PDGFB) and PDGF receptor beta was observed at OXT-containing axonal terminals and pericytes respectively. Administration of the selective tyrosine kinase inhibitor AZD2171 for VEGFRs and STI571 for PDGFRs significantly decreased proliferation of endothelial cells and OPCs. Moreover, AZD2171 treatment decreased vascular density by facilitating apoptosis of endothelial cells and the withdrawal of its treatment led to remarkable rebound proliferation of endothelial cells, so that vascular density rapidly returned to normal levels. AZD2171 decreased the density of both AVP- and OXT-containing axonal terminals, whereas STI571 selectively decreased the density of AVP-containing ones. Thus, this study demonstrates that the signaling pathways of VEGF and PDGF are crucial mediators for determining proliferation of endothelial cells and OPCs and the density of AVP- and OXT-containing axonal terminals in the HNS.

Elevated hypothalamic aromatization at the onset of precocious puberty in transgenic female mice hypersecreting human chorionic gonadotropin: effect of androgens

Transgenic female mice overexpressing the α- and β- subunits of human chorionic gonadotropin (hCGαβ+) exhibited precocious puberty, as evidenced by early vaginal opening. Chronically elevated hCG in 21-day-old hCGαβ+ females stimulated gonadal androgen production, which exerted negative feedback over the endogenous gonadotropin synthesis, and activated the hypothalamic GnRH pulsatility and gene expression. Transgenic females also exhibited elevated hypothalamic aromatization in the preoptic area (POA), which is the sexually-differentiated area that controls the LH surge in adulthood. Ovariectomy at 14 days of age was unable to rescue this phenotype. However, the blockade of androgen action by flutamide from postnatal day 6 onwards reduced the aromatase levels in the POA of hCGαβ+ females. Our results suggest that early exposure of females to androgen action during a critical period between postnatal days 6-14 induces sex-specific organizational changes of the brain, which affect the aromatase expression in the POA at the onset of precocious puberty.

Actions and interactions of alcohol and transforming growth factor β1 on prepubertal hypothalamic gonadotropin-releasing hormone

BACKGROUND

Alcohol (ALC) diminishes gonadotropin-releasing hormone (GnRH) secretion and delays puberty. Glial transforming growth factor β1 (TGFβ1) plays a role in glial-neuronal communications facilitating prepubertal GnRH secretion. We assessed the effects of acute ALC administration on TGFβ1-induced GnRH gene expression in the brain preoptic area (POA) and release of the peptide from the medial basal hypothalamus (MBH). Furthermore, we assessed actions and interactions of TGFβ1 and ALC on an adhesion/signaling gene family involved in glial-neuronal communications.

METHODS

Prepubertal female rats were administered ALC or water via gastric gavage at 7:30 am. At 9:00 am, saline or TGFβ1 (100 ng/3 μl) was administered into the third ventricle. At 3:00 pm, the POA was removed and frozen for gene expression analysis and repeated for protein assessments. In another experiment, the MBH was removed from ALC-free rats. After equilibration, tissues were incubated in Locke's medium only or medium containing TGFβ1 with or without 50 mM ALC for measurement of GnRH peptide released in vitro.

RESULTS

TGFβ1 induced GnRH gene expression in the POA, and this effect was blocked by ALC. We also described the presence and responsiveness of the TGFβ1 receptor in the POA and showed that acute ALC exposure not only altered the TGFβ1-induced increase in TGFβ-R1 protein expression but also the activation of receptor-associated proteins, Smad2 and Smad3, key downstream components of the TGFβ1 signaling pathway. Assessment of an adhesion/signaling family consisting of glial receptor protein tyrosine phosphatase beta and neuronal contactin-associated protein-1 (Caspr1) and contactin showed that the neuronal components were induced by TGFβ1 and that ALC blocked these effects. Finally, TGFβ1 was shown to induce release of the GnRH peptide in vitro, an action that was blocked by ALC.

CONCLUSIONS

We have demonstrated glial-derived TGFβ1 induces GnRH gene expression in the POA and stimulates release of the peptide from the MBH, actions necessary for driving the pubertal process. Importantly, ALC acted at both brain regions to block stimulatory effects of TGFβ1. Furthermore, ALC altered neuronal components of an adhesion/signaling family previously shown to be expressed on GnRH neurons and implicated in glial-GnRH neuronal communications. These results further demonstrate detrimental effects of ALC at puberty.

GnRH neurons of young and aged female rhesus monkeys co-express GPER but are unaffected by long-term hormone replacement

Menopause is caused by changes in the function of the hypothalamic-pituitary-gonadal axis that controls reproduction. Hypophysiotropic gonadotropin-releasing hormone (GnRH) neurons in the hypothalamus orchestrate the activity of this axis and are regulated by hormonal feedback loops. The mechanisms by which GnRH responds to the primary regulatory sex steroid hormone, estradiol (E2), are still poorly understood in the context of menopause. Our goal was to determine whether the G protein-coupled estrogen receptor (GPER) is co-expressed in adult primate GnRH neurons and whether this changes with aging and/or E2 treatment. We used immunofluorescence double-labeling to characterize the co-expression of GPER in GnRH perikarya and terminals in the hypothalamus. Young and aged rhesus macaques were ovariectomized and given long-term (~2-year) hormone treatments (E2, E2 + progesterone, or vehicle) selected to mimic currently prescribed hormone replacement therapies used for the alleviation of menopausal symptoms in women. We found that about half of GnRH perikarya co-expressed GPER, while only about 12% of GnRH processes and terminals in the median eminence (ME) were double-labeled. Additionally, many GPER-labeled processes were in direct contact with GnRH neurons, often wrapped around the perikarya and processes and in close proximity in the ME. These results extend prior work by showing robust co-localization of GPER in GnRH in a clinically relevant model, and they support the possibility that GPER-mediated E2 regulation of GnRH occurs both in the soma and terminals in nonhuman primates.

A system biology approach to identify regulatory pathways underlying the neuroendocrine control of female puberty in rats and nonhuman primates

This article is part of a Special Issue "Puberty and Adolescence". Puberty is a major developmental milestone controlled by the interaction of genetic factors and environmental cues of mostly metabolic and circadian nature. An increased pulsatile release of the decapeptide gonadotropin releasing hormone (GnRH) from hypothalamic neurosecretory neurons is required for both the initiation and progression of the pubertal process. This increase is brought about by coordinated changes that occur in neuronal and glial networks associated with GnRH neurons. These changes ultimately result in increased neuronal and glial stimulatory inputs to the GnRH neuronal network and a reduction of transsynaptic inhibitory influences. While some of the major players controlling pubertal GnRH secretion have been identified using gene-centric approaches, much less is known about the system-wide control of the overall process. Because the pubertal activation of GnRH release involves a diversity of cellular phenotypes, and a myriad of intracellular and cell-to-cell signaling molecules, it appears that the overall process is controlled by a highly coordinated and interactive regulatory system involving hundreds, if not thousands, of gene products. In this article we will discuss emerging evidence suggesting that these genes are arranged as functionally connected networks organized, both internally and across sub-networks, in a hierarchical fashion. According to this concept, the core of these networks is composed of transcriptional regulators that, by directing expression of downstream subordinate genes, provide both stability and coordination to the cellular networks involved in initiating the pubertal process. The integrative response of these gene networks to external inputs is postulated to be coordinated by epigenetic mechanisms.

Accessibility of low-molecular-mass molecules to the median eminence and arcuate hypothalamic nucleus of adult mouse

Blood-derived molecules are able to access to the median eminence (ME) and arcuate hypothalamic nucleus (Arc) due to the lack of the blood-brain barrier. In the present study, we examined the accessibility of low-molecular-mass (LMM) molecules into parenchyma in the ME and Arc of adult mice by administration of Dextran 3000 (Dex3k), Dex10k, Evans blue (EB) and fluorescein isothiocyanate (FITC). In the external zone of the ME, the fluorescence of Dex3k, EB and FITC tracers generated an intensity gradient from fenestrated capillary, but that of Dex10k was detected only between the inner and outer basement membrane of pericapillary space. The fluorescence of FITC in the external zone of the ME was closely associated with axonal terminals and surrounded by cellular processes of tanycytes-like cells and astrocytes. In the ependymal/internal zone of the ME and Arc, the fluorescence of all LMM tracers was seen at tanycytes-like cells and neurons. The fluorescence of EB and FITC in these regions was not detected when brains were fixed during or before the administration of tracers. The inhomogeneity of accessibility for fluorescent tracers depended on routes for tracer administration. Thus, the present study indicates that the accessibility of LMM blood-derived molecules to parenchyma depends on fenestration of the capillary in the external zone of the ME and active transport of ependymal cells in the ependymal/internal zone of the ME and Arc.