Neuropeptide Y: A hypothalamic brake restraining the onset of puberty in primates

Associations of Obesity with Linear Growth and Puberty

BACKGROUND

The prevalence of obesity in childhood has increased dramatically in recent decades with increased risk of developing cardiometabolic and other comorbidities. Childhood adiposity may also influence processes of growth and puberty.

SUMMARY

Growth patterns of obesity during childhood have been shown to be associated with increased linear growth in early childhood, leading to accelerated epiphyseal growth plate (EGP) maturation. Several hormones secreted by the adipose tissue may affect linear growth in the context of obesity, both via the growth hormone IGF-1 axis and via a direct effect on the EGP. The observation that children with obesity tend to mature earlier than lean children has led to the assumption that the degree of body fatness may trigger the neuroendocrine events that lead to pubertal onset. The most probable link between obesity and puberty is leptin and its interaction with the kisspeptin system, which is an important regulator of puberty. However, peripheral action of adipose tissue could also be involved in changes in the onset of puberty. In addition, nutritional factors, epigenetics, and endocrine-disrupting chemicals are potential mediators linking pubertal onset to obesity. In this review, we focused on interactions of obesity with linear growth and pubertal processes, based on basic research and clinical data in humans.

KEY MESSAGE

Children with obesity are subject to accelerated linear growth with risk of impaired adult height and early puberty, with its psychological consequences. The data highlight another important objective in combatting childhood obesity, for the prevention of abnormal growth and pubertal patterns.

The mechanism underlying the pubertal increase in pulsatile GnRH release in primates

In primates, the gonatotropin-releasing hormone (GnRH) neurosecretory system, consisting of GnRH, kisspeptin, and neurokinin B neurons, is active during the neonatal/early infantile period. During the late infantile period, however, activity of the GnRH neurosecretory system becomes minimal as a result of gonadal steroid independent central inhibition, and this suppressed GnRH neurosecretory state continues throughout the prepubertal period. At the initiation of puberty, the GnRH neurosecretory system becomes active again because of the decrease in central inhibition. During the progress of puberty, kisspeptin and neurokinin B signaling to GnRH neurons further increases, resulting in the release of gonadotropins and subsequent gonadal maturation, and hence puberty. This review further discusses potential substrates of central inhibition and subsequent pubertal modification of the GnRH neurosecretory system by the pubertal increase in steroid hormones, which ensures the regulation of adult reproductive function.

Genetic factors in precocious puberty

Pubertal onset is known to result from reactivation of the hypothalamic-pituitary-gonadal (HPG) axis, which is controlled by complex interactions of genetic and nongenetic factors. Most cases of precocious puberty (PP) are diagnosed as central PP (CPP), defined as premature activation of the HPG axis. The cause of CPP in most girls is not identifiable and, thus, referred to as idiopathic CPP (ICPP), whereas boys are more likely to have an organic lesion in the brain. ICPP has a genetic background, as supported by studies showing that maternal age at menarche is associated with pubertal timing in their offspring. A gain of expression in the kisspeptin gene (KISS1), gain-of-function mutation in the kisspeptin receptor gene (KISS1R), loss-of-function mutation in makorin ring finger protein 3 (MKRN3), and loss-of-function mutations in the delta-like homolog 1 gene (DLK1) have been associated with ICPP. Other genes, such as gamma-aminobutyric acid receptor subunit alpha-1 (GABRA1), lin-28 homolog B (LIN28B), neuropeptide Y (NPYR), tachykinin 3 (TAC3), and tachykinin receptor 3 (TACR3), have been implicated in the progression of ICPP, although their relationships require elucidation. Environmental and socioeconomic factors may also be correlated with ICPP. In the progression of CPP, epigenetic factors such as DNA methylation, histone posttranslational modifications, and noncoding ribonucleic acids may mediate the relationship between genetic and environmental factors. CPP is correlated with short- and long-term adverse health outcomes, which forms the rationale for research focusing on understanding its genetic and nongenetic factors.

Inverse age-related changes between hypothalamic NPY and KISS1 gene expression during pubertal initiation in male rhesus monkey

The neuroendocrine mechanism underlying the sinusoidal wave nature of gonadotropin-releasing hormone pulse generator activity from infantile to adult age still needs to be meticulously defined. Direct inhibition of kisspeptin neurons by neuropeptide Y (NPY) and close intimacy between the two rekindle the importance of these two neuropeptides controlling reproductive axis activity. Thus, the present study was undertaken to decipher simultaneous fluctuations and to profile correlative changes in the relative expression of KISS1, NPY, and their receptor genes from the mediobasal hypothalamus of infant (n = 3), juvenile, pre-pubertal, and adult (n = 4 in each stage) male rhesus monkey (Macaca mulatta) by RT-qPCR. Significant elevation (p < 0.05-0.01) in KISS1 and KISS1R and low (p < 0.05) expression in NPY and NPY1R mRNA in the adult group as compared to the pre-pubertal group was observed. Moreover, significantly high (p < 0.05) expression of NPY and NPY1R mRNA with non-significant (p> 0.05) decline in KISS1 and KISS1R in pre-pubertal animals in comparison to infants describe inverse correlative age-associated changes during pubertal development. Current findings imply that NPY may contribute as a neurobiological brake for the dormancy of kisspeptin neurons before pubertal onset, while dwindling of this brake is likely to occasion kisspeptin dependent hypothalamic-pituitary-gonadal axis activation at puberty. These findings may help in the development of clinical and therapeutic strategies to regulate fertility in humans.

How alcohol affects insulin-like growth factor-1's influences on the onset of puberty: A critical review

Alcohol (ALC) is capable of delaying signs associated with pubertal development in laboratory animals, as well as in humans. The normal onset of puberty results from a timely increase in gonadotropin‐releasing hormone (GnRH) secretion, which is associated with a gradual decline in prepubertal inhibitory influences, and the establishment of excitatory inputs that increase GnRH release, which together drive pubertal development. In recent years, insulin‐like growth factor‐1 (IGF‐1) has emerged as a pivotal contributor to prepubertal GnRH secretion and pubertal development, whose critical actions are interfered with by ALC abuse. Here we review the neuroendocrine research demonstrating the important role that IGF‐1 plays in pubertal development, and describe the detrimental effects and mechanisms of action of ALC on the onset and progression of pubertal maturation.

Epigenetics of the developing and aging brain: Mechanisms that regulate onset and outcomes of brain reorganization

Brain development is a life-long process that encompasses several critical periods of transition, during which significant cognitive changes occur. Embryonic development, puberty, and reproductive senescence are all periods of transition that are hypersensitive to environmental factors. Rather than isolated episodes, each transition builds upon the last and is influenced by consequential changes that occur in the transition before it. Epigenetic marks, such as DNA methylation and histone modifications, provide mechanisms by which early events can influence development, cognition, and health outcomes. For example, parental environment influences imprinting patterns in gamete cells, which ultimately impacts gene expression in the embryo which may result in hypersensitivity to poor maternal nutrition during pregnancy, raising the risks for cognitive impairment later in life. This review explores how epigenetics induce and regulate critical periods, and also discusses how early environmental interactions prime a system towards a particular health outcome and influence susceptibility to disease or cognitive impairment throughout life.

Neurobiology of puberty and its disorders

Puberty, which in humans is considered to include both gonadarche and adrenarche, is the period of becoming capable of reproducing sexually and is recognized by maturation of the gonads and development of secondary sex characteristics. Gonadarche referring to growth and maturation of the gonads is fundamental to puberty since it encompasses increased gonadal steroid secretion and initiation of gametogenesis resulting from enhanced pituitary gonadotropin secretion, triggered in turn by robust pulsatile GnRH release from the hypothalamus. This chapter reviews the development of GnRH pulsatility from before birth until the onset of puberty. In humans, GnRH pulse generation is restrained during childhood and juvenile development. This prepubertal hiatus in hypothalamic activity is considered to result from a neurobiological brake imposed upon the GnRH pulse generator resident in the infundibular nucleus. Reactivation of the GnRH pulse generator initiates pubertal development. Current understanding of the genetics and physiology of the brake will be discussed, as will hypotheses proposed to account for timing the resurgence in pulsatile GnRH and initiation of puberty. The chapter ends with a discussion of disorders associated with precocious or delayed puberty with a focus on those with etiologies attributed to aberrant GnRH neuron anatomy or function. A pediatric approach to patients with pubertal disorders is provided and contemporary treatments for both precocious and delayed puberty outlined.

Neuroendocrine mechanisms of puberty in non-human primates

An increase in pulsatile release of gonadotropin releasing hormone (GnRH) initiates puberty in mammalian species. While mutations in KISS1 and TAC3 and their receptors, KISS1R and NK3R, respectively, result in the absence or abnormal timing of puberty, the neurocircuitry and precise role of kisspeptin and neurokinin B (NKB) in regulation of the GnRH neurosecretory system in primate puberty remain elusive. This review discusses how kisspeptin and NKB signaling contributes to the pubertal increase in GnRH release in non-human primates and how remodeling of the NKB and kisspeptin signaling circuitry controlling GnRH neurons takes place during the progress of puberty. Importantly, the pubertal remodeling of kisspeptin and NKB signaling ensures efficient functions of the GnRH neurosecretory system that regulates sex-specific reproduction in primates.

The neurobiological mechanism underlying hypothalamic GnRH pulse generation: the role of kisspeptin neurons in the arcuate nucleus

This review recounts the origins and development of the concept of the hypothalamic gonadotropin-releasing hormone (GnRH) pulse generator. It starts in the late 1960s when striking rhythmic episodes of luteinizing hormone secretion, as reflected by circulating concentrations of this gonadotropin, were first observed in monkeys and ends in the present day. It is currently an exciting time witnessing the application, primarily to the mouse, of contemporary neurobiological approaches to delineate the mechanisms whereby Kiss1/NKB/Dyn (KNDy) neurons in the arcuate nucleus of the hypothalamus generate and time the pulsatile output of kisspeptin from their terminals in the median eminence that in turn dictates intermittent GnRH release and entry of this decapeptide into the primary plexus of the hypophysial portal circulation. The review concludes with an examination of questions that remain to be addressed.

The neurobiological mechanism underlying hypothalamic GnRH pulse generation: the role of kisspeptin neurons in the arcuate nucleus

This review recounts the origins and development of the concept of the hypothalamic gonadotropin-releasing hormone (GnRH) pulse generator. It starts in the late 1960s when striking rhythmic episodes of luteinizing hormone secretion, as reflected by circulating concentrations of this gonadotropin, were first observed in monkeys and ends in the present day. It is currently an exciting time witnessing the application, primarily to the mouse, of contemporary neurobiological approaches to delineate the mechanisms whereby Kiss1/NKB/Dyn (KNDy) neurons in the arcuate nucleus of the hypothalamus generate and time the pulsatile output of kisspeptin from their terminals in the median eminence that in turn dictates intermittent GnRH release and entry of this decapeptide into the primary plexus of the hypophysial portal circulation. The review concludes with an examination of questions that remain to be addressed.

Physiology of puberty in boys and girls and pathological disorders affecting its onset

Puberty is a physiological event involving the attainment of reproductive capability and complete development of sexual and physical organs. Changing from childhood to adulthood is a complex process and is tightly controlled by interconnection pathways at the level of the hypothalamus which can be influenced by environmental, psychosocial, and endocrine factors. Although various mechanisms underlying the onset of normal puberty have been investigated in humans and animals, the exact molecular mechanisms thereof remain unclear. The aim of this review is to summarize the current state of knowledge and provide a synoptic overview about the physiology of puberty in adolescent boys and girls, and describe pathological disorders affecting its onset.

The Emerging Role of Chromatin Remodeling Factors in Female Pubertal Development

To attain sexual competence, all mammalian species go through puberty, a maturational period during which body growth and development of secondary sexual characteristics occur. Puberty begins when the diurnal pulsatile gonadotropin-releasing hormone (GnRH) release from the hypothalamus increases for a prolonged period of time, driving the adenohypophysis to increase the pulsatile release of luteinizing hormone with diurnal periodicity. Increased pubertal GnRH secretion does not appear to be driven by inherent changes in GnRH neuronal activity; rather, it is induced by changes in transsynaptic and glial inputs to GnRH neurons. We now know that these changes involve a reduction in inhibitory transsynaptic inputs combined with increased transsynaptic and glial excitatory inputs to the GnRH neuronal network. Although the pubertal process is known to have a strong genetic component, during the last several years, epigenetics has been implicated as a significant regulatory mechanism through which GnRH release is first repressed before puberty and is involved later on during the increase in GnRH secretion that brings about the pubertal process. According to this concept, a central target of epigenetic regulation is the transcriptional machinery of neurons implicated in stimulating GnRH release. Here, we will briefly review the hormonal changes associated with the advent of female puberty and the role that excitatory transsynaptic inputs have in this process. In addition, we will examine the 3 major groups of epigenetic modifying enzymes expressed in the neuroendocrine hypothalamus, which was recently shown to be involved in pubertal development and progression.

Central Mechanism Controlling Pubertal Onset in Mammals: A Triggering Role of Kisspeptin

Pubertal onset is thought to be timed by an increase in pulsatile gonadotropin-releasing hormone (GnRH)/gonadotropin secretion in mammals. The underlying mechanism of pubertal onset in mammals is still an open question. Evidence accumulated in the last 15 years suggests that kisspeptin/neurokinin B/dynorphin A (KNDy) neurons in the hypothalamic arcuate nucleus play a key role in pubertal onset by triggering pulsatile GnRH/gonadotropin secretin in mammals. Specifically, KNDy neurons are now considered a part of GnRH pulse generator, in which neurokinin B facilitates and dynorphin A inhibits, the synchronized discharge of KNDy neurons in autocrine and/or paracrine manners. Kisspeptin serves as a potent secretagogue of GnRH secretion and thus its release is fundamental to pubertal increase in GnRH/gonadotropin secretion in mammals. Proposed mechanisms inhibiting Kiss1 (kisspeptin gene) expression during childhood to juvenile varies from species to species: we envisage that negative feedback action of estrogen plays a key role in the inhibition of Kiss1 expression in KNDy neurons in rodents and sheep, whereas estrogen-independent inhibition of kisspeptin secretion by γ-amino butyric acid or neuropeptide Y are suggested to be responsible for the pre-pubertal suppression of GnRH/gonadotropin secretion in primates. Taken together, the timing of pubertal onset is postulated to be controlled by upstream regulators for kisspeptin biosynthesis and secretion in mammals.

Putative Effects of Obesity on Linear Growth and Puberty

Childhood obesity is a major public health problem that has grown to epidemic proportions throughout the world. Obesity is influenced by genetic and environmental factors. The nutritional status plays an important role in growth and body weight regulation. Excess adiposity during childhood can affect the process of growth and puberty. Obese children are frequently tall for their age, with accelerated epiphyseal growth plate maturation despite low growth hormone levels. Several regulatory hormones may affect the process of linear growth in the constellation of obesity, as high levels of insulin and leptin are observed in obese children. Leptin can act as a skeletal growth factor, with a direct effect on skeletal growth centers. The finding that overweight children, especially girls, tend to mature earlier than lean children has led to the hypothesis that the degree of body fatness may trigger the neuroendocrine events that lead to the onset of puberty. Leptin receptors have been identified in the hypothalamus, as well as in gonadotrope cells, ovarian follicular cells, and Leydig cells. The increased leptin and androgen levels seen in obese children may be implicated in their earlier onset of puberty and accelerated pubertal growth. This review is focused on the interaction between childhood obesity and growth and pubertal processes.
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Changes in the Responsiveness of the Hypothalamic-Pituitary-Gonadal Axis to Kisspeptin-10 Administration during Pubertal Transition in Boys

In human, no studies are available regarding changes in kisspeptin1 receptor (KISS1R) sensitivity during pubertal transition. In this study, healthy boys were classified into 5 Tanner stages of puberty (n = 5/stage). Human kisspeptin-10 was administered to boys at each Tanner stage and to adult men (n = 5) as an IV bolus for comparison. Serial blood samples were collected for 30 min pre- and 120 min post-kisspeptin injection periods at 30 min interval for measuring plasma LH and testosterone levels. There was insignificant effect of kisspeptin on LH and testosterone levels in boys of Tanner stages I-III. At Tanner stage IV, the effect of kisspeptin on plasma LH was insignificant. However, a paired t-test on a log-transformed data showed a significant (P < 0.05) increase in mean peak post-kisspeptin testosterone level. In Tanner stage V, a significant (P < 0.05) increase was observed in mean post-kisspeptin peak LH level as compared to the mean basal LH value. Post-kisspeptin plasma testosterone levels were also significantly (P < 0.05) increased as compared to the pre-kisspeptin level in Tanner stage V. Our data suggest that sensitivity of KISS1R on GnRH neurons with reference to LH stimulation in boys develops during the later part of puberty reaching to adult level at Tanner stage V. This trial is registered with WHO International Clinical Trial Registration ID NCT03286517.

Arcuate nucleus neuropeptide coexpression and connections to gonadotrophin-releasing hormone neurones in the female rhesus macaque

The underlying hypothalamic neurocircuitry by which metabolism and feeding regulates reproductive function has been well-studied in the rodent; however, recent data have demonstrated significant neuroanatomical differences in the human brain. The present study had three objectives, centred on arcuate nucleus neuropeptides regulating feeding and reproduction: (i) to characterise coexpression patterns in the female nonhuman primate; (ii) to establish whether these neuronal populations make potential contacts with gonadotophin-releasing hormone (GnRH) neurones; and (iii) to determine whether these contacts differ between the low and high GnRH-releasing states of pre-puberty and adulthood, respectively. Female nonhuman primates have several coexpression patterns of hypothalamic neuropeptides that differ from those reported in rodents. Cocaine- and amphetamine-regulated transcript (CART) is not coexpressed with pro-opiomelanocortin but instead with neuropeptide Y (NPY). CART is also expressed in a subpopulation of kisspeptin cells in the nonhuman primate, similar to observations in humans but diverging from findings in rodents. Very few GnRH-expressing neurones received close appositions from double-labelled kisspeptin/CART fibres; however, both single-labelled kisspeptin and CART fibres were in frequent apposition with GnRH neurones, with no differences between prepubertal and adult animals. NPY/agouti-related peptide (AgRP) coexpressing fibres contacted significantly more GnRH neurones in prepubertal animals than adults, consistent with increased NPY and AgRP mRNA observed in prepubertal animals. The findings of the present study detail significant differences in arcuate nucleus neuropeptide coexpression in the monkey compared to the rodent and are consistent with the hypothesis that arcuate nucleus NPY/AgRP neurones play an inhibitory role in controlling GnRH neuronal regulation in the prepubertal primate.

Control of the onset of puberty

PURPOSE OF REVIEW

The mechanism of puberty initiation remains an enigma, despite extensive research in the field. Pulsatile pituitary gonadotropin secretion under the guidance of hypothalamic gonadotropin-releasing hormone (GnRH) constitutes a sine qua non for pubertal onset. In turn, the secretion of GnRH in the human hypothalamus is regulated by kisspeptin and its receptor as well as by permissive or opposing signals mediated by neurokinin B and dynorphin acting on their respective receptors. These three supra-GnRH regulators compose the Kisspeptin, Neurokinin B and Dynorhin neurons (KNDy) system, a key player in pubertal onset and progression.

RECENT FINDINGS

The recent discovery that makorin ring finger protein 3 is also involved in puberty initiation provided further insights into the regulation of the KNDy pathway. In fact, the inhibitory (γ-amino butyric acid, neuropeptide Y, and RFamide-related peptide-3) and stimulatory signals (glutamate) acting upstream of KNDy called into question the role of makorin ring finger protein 3 as the gatekeeper of puberty. Meanwhile, the findings that 'neuroestradiol' produced locally and endocrine disruptors from the environment may influence GnRH secretion is intriguing. Finally, epigenetic mechanisms have been implicated in pubertal onset through recently discovered mechanisms.

SUMMARY

The exact molecular machinery underlying puberty initiation in humans is under intensive investigation. In this review, we summarize research evidence in the field, while emphasizing the areas of uncertainty and underlining the impact of current information on the evolving theory regarding this fascinating phenomenon.

Neuroestradiol in the Stalk Median Eminence of Female Rhesus Macaques Decreases in Association With Puberty Onset

In primates, despite the fact that GnRH neurons are mature at birth, a gonadal steroid independent central inhibition restrains the initiation of puberty. The neural substrates responsible for this central inhibition, however, are unclear. In this study, we tested the hypothesis that neuroestradiol release in the hypothalamus decreases prior to the pubertal increase in GnRH release. We found that in female monkeys at the prepubertal stage, when GnRH release was low, estradiol (E2) levels in the stalk-median eminence of the hypothalamus were higher than those in older, early pubertal females in which nocturnal GnRH release begins to increase. Furthermore, estrone (E1) levels were higher in the stalk-median eminence of prepubertal and early pubertal monkeys compared with midpubertal monkeys, which have the highest GnRH release. The elevated E2 and E1 levels at the prepubertal stage are likely hypothalamic in origin because circulating E2 and E1 levels in prepubertal and early pubertal monkeys were much lower than those in midpubertal monkeys. Heightened synthesis and release of neuroestradiol during the prepubertal period and subsequent reduction at puberty onset indicate possible roles for neuroestradiol in central inhibition of GnRH release. The mechanism governing the reduction in neuroestradiol synthesis at puberty onset remains to be determined.

The Distribution of Substance P and Kisspeptin in the Mediobasal Hypothalamus of the Male Rhesus Monkey and a Comparison of Intravenous Administration of These Peptides to Release GnRH as Reflected by LH Secretion

Substance P (SP) was recently reported to be expressed in human kisspeptin/neurokinin B/dynorphin (KNDy) neurons and to enhance KNDy neuron excitability in the mouse hypothalamus. We therefore examined (1) interactions of SP and kisspeptin in the mediobasal hypothalamus of adult male rhesus monkeys using immunofluorescence, and (2) the ability of SP to induce LH release in GnRH-primed, agonadal juvenile male monkeys. SP cell bodies were observed only occasionally in the arcuate nucleus (Arc), but more frequently dorsal to the Arc in the region of the premammillary nucleus. Castration resulted in an increase in the number of SP cell bodies in the Arc but not in the other regions. SP fibers innervated the Arc, where they were found in close apposition with kisspeptin perikarya in the periphery of this nucleus. Beaded SP axons projected to the median eminence, where they terminated in the external layer and intermingled with beaded kisspeptin axons. Colocalization of the two peptides, however, was not observed. Although close apposition between SP fibers and kisspeptin neurons suggest a role for SP in modulating GnRH pulse generator activity, i.v. injections of SP failed to elicit release of GnRH (as reflected by LH) in the juvenile monkey. Although the finding of structural interactions between SP and kisspeptin neurons is consistent with the notion that this tachykinin may be involved in regulating pulsatile GnRH release, the apparent absence of expression of SP in KNDy neurons suggests that this peptide is unlikely to be a fundamental component of the primate GnRH pulse generator.

Neuroendocrine control of the onset of puberty

This chapter is based on the Geoffrey Harris Memorial Lecture presented at the 8th International Congress of Neuroendocrinology, which was held in Sydney, August 2014. It provides the development of our understanding of the neuroendocrine control of puberty since Harris proposed in his 1955 monograph (Harris, 1955) that "a major factor responsible for puberty is an increased rate of release of pituitary gonadotrophin" and posited "that a neural (hypothalamic) stimulus, via the hypophysial portal vessels, may be involved." Emphasis is placed on the neurobiological mechanisms governing puberty in highly evolved primates, although an attempt is made to reverse translate a model for the timing of puberty in man and monkey to non-primate species.

Targeted resequencing of the pericentromere of chromosome 2 linked to constitutional delay of growth and puberty

Constitutional delay of growth and puberty (CDGP) is the most common cause of pubertal delay. CDGP is defined as the proportion of the normal population who experience pubertal onset at least 2 SD later than the population mean, representing 2.3% of all adolescents. While adolescents with CDGP spontaneously enter puberty, they are at risk for short stature, decreased bone mineral density, and psychosocial problems. Genetic factors contribute heavily to the timing of puberty, but the vast majority of CDGP cases remain biologically unexplained, and there is no definitive test to distinguish CDGP from pathological absence of puberty during adolescence. Recently, we published a study identifying significant linkage between a locus at the pericentromeric region of chromosome 2 (chr 2) and CDGP in Finnish families. To investigate this region for causal variation, we sequenced chr 2 between the genomic coordinates of 79-124 Mb (genome build GRCh37) in the proband and affected parent of the 13 families contributing most to this linkage signal. One gene, DNAH6, harbored 6 protein-altering low-frequency variants (< 6% in the Finnish population) in 10 of the CDGP probands. We sequenced an additional 135 unrelated Finnish CDGP subjects and utilized the unique Sequencing Initiative Suomi (SISu) population reference exome set to show that while 5 of these variants were present in the CDGP set, they were also present in the Finnish population at similar frequencies. Additional variants in the targeted region could not be prioritized for follow-up, possibly due to gaps in sequencing coverage or lack of functional knowledge of non-genic genomic regions. Thus, despite having a well-characterized sample collection from a genetically homogeneous population with a large population-based reference sequence dataset, we were unable to pinpoint variation in the linked region predisposing delayed puberty. This study highlights the difficulties of detecting genetic variants under linkage regions for complex traits and suggests that advancements in annotation of gene function and regulatory regions of the genome will be critical for solving the genetic background of complex phenotypes like CDGP.

The gonadal function in obese adolescents: review

This review deals with the relationship between obesity in male adolescents and gonadal function. The article is structured in two main paragraphs; the first one is about population studies that have assessed puberty timing and its mode of onset in relation with body weight to evaluate if and how the latter can influence the gonadal function in this phase of life. These studies analyze issues such as increased BMI and early onset of male puberty, gender differences, secular trend toward early onset of puberty in males, effects of a different body composition on male puberty and consequences of a different stage of childhood obesity on the onset of male puberty. The second paragraph examines the possible mechanisms through which, obesity may alter the timing of puberty in young males, including the role of SHBG, leptin, insulin resistance, ghrelin, GH-IGF-1 axis, AR polymorphisms, primary testicular dysfunction, retinol binding protein 4 (RBP-4) and liver function abnormalities. However, despite the numerous studies in the literature, the etiology of gonadal disfunction in obese adolescents on puberty remains uncertain.

Reciprocal changes in leptin and NPY during nutritional acceleration of puberty in heifers

Feeding a high-concentrate diet to heifers during the juvenile period, resulting in increased body weight (BW) gain and adiposity, leads to early-onset puberty. In this study, we tested the hypothesis that the increase in GnRH/LH release during nutritional acceleration of puberty is accompanied by reciprocal changes in circulating leptin and central release of neuropeptide Y (NPY). The heifers were weaned at 3.5 months of age and fed to gain either 0.5 (Low-gain; LG) or 1.0 kg/day (High-gain; HG) for 30 weeks. A subgroup of heifers was fitted surgically with third ventricle guide cannulas and was subjected to intensive cerebrospinal fluid (CSF) and blood sampling at 8 and 9 months of age. Mean BW was greater in HG than in LG heifers at week 6 of the experiment and remained greater thereafter. Starting at 9 months of age, the percentage of pubertal HG heifers was greater than that of LG heifers, although a replicate effect was observed. During the 6-h period in which CSF and blood were collected simultaneously, all LH pulses coincided with or shortly followed a GnRH pulse. At 8 months of age, the frequency of LH pulses was greater in the HG than in the LG group. Beginning at 6 months of age, concentrations of leptin were greater in HG than in LG heifers. At 9 months of age, concentrations of NPY in the CSF were lesser in HG heifers. These observations indicate that increased BW gain during juvenile development accelerates puberty in heifers, coincident with reciprocal changes in circulating concentrations of leptin and hypothalamic NPY release.

Effects of estradiol on kisspeptin neurons during puberty

The activation of the gonadotropin-releasing hormone (GnRH) neurons from a state of relative quiescence is critical for initiating puberty in mammals. Kisspeptin and its G-protein coupled receptor Gpr54 are essential for puberty, with disruption to either resulting in failed puberty in humans and mice. Robust data from several species indicate that Kiss1 mRNA and/or kisspeptin peptide expression within the hypothalamus increases during pubertal development. Kisspeptin fiber innervation of GnRH neurons and kisspeptin release within the hypothalamus also increase during pubertal development, indicating that there is increased kisspeptinergic drive to GnRH neurons during pubertal development. It is becoming increasingly apparent that gonadal steroids play important roles in the regulation of kisspeptin expression during pubertal development, and in particular, estradiol signaling through estrogen receptor alpha appears to be necessary for these changes to occur. This review focuses on the role that estradiol plays in the regulation of kisspeptin expression during pubertal development.

Epigenetic control of gonadotropin releasing hormone neurons

Epigenetic modifications to the genome, including DNA methylation and histone modifications, occur in response to external stimuli. Reproductive function is highly sensitive to environmental conditions including season, diet, hormonal changes, and exposure to chemical contaminants. GnRH neurons, which play a key role in reproduction, are particularly sensitive to various environmental stimuli. We recently reported that the rhesus monkey GnRH gene exhibits distinct epigenetic changes during embryonic development. More recently, we further found that a similar epigenetic phenomenon occurs across puberty. In this article we highlight recent findings, including those of afferent inputs, to describe the epigenetic control of GnRH circuit development as a link between the environment and reproductive function.

Kisspeptin and puberty in mammals

Since the discovery of the G-protein coupled receptor 54 (kisspeptin receptor) and its ligand, kisspeptin, our understanding of the neurobiological mechanisms that govern the pituitary-gonadal axis has evolved dramatically. In this chapter, we have reviewed progress regarding the relationship between kisspeptin and puberty, and have proposed a novel hypothesis for the role of kisspeptin signaling in the onset of this crucial developmental event. According to this hypothesis, although kisspeptin neurons in the arcuate nucleus (ARC) are critical for puberty, this is simply because these cells are an integral component of the hypothalamic GnRH pulse generating mechanism that drives intermittent release of the decapeptide, as an increase in GnRH is obligatory for the onset of puberty. In our model, ARC kisspeptin neurons play no "regulatory" role in controlling the timing of puberty. Rather, as a component of the neural network responsible for GnRH pulse generation, they subserve upstream regulatory mechanisms that are responsible for the timing of puberty.

Tonic control of kisspeptin release in prepubertal monkeys: implications to the mechanism of puberty onset

Previously we have shown that a reduction in γ-amino butyric acid (GABA) inhibition is critical for the mechanism initiating puberty onset because chronic infusion of the GABA(A) receptor antagonist, bicuculline, significantly increased GnRH release and accelerated the timing of menarche and first ovulation in female rhesus monkeys. Because previous studies in our laboratory indicate that in prepubertal female monkeys, kisspeptin release in the medial basal hypothalamus is low, whereas kisspeptin-10 can stimulate GnRH release, we hypothesized that a low level of kisspeptin release prior to puberty onset is due to tonic GABA inhibition. To test this hypothesis we examined the effects of bicuculline infusion on kisspeptin release using a microdialysis method. We found that bicuculline at 1 μM dramatically stimulates kisspeptin release in the medial basal hypothalamus of prepubertal monkeys but had little effect on kisspeptin release in midpubertal monkeys. We further examined whether bicuculline-induced GnRH release is blocked by the presence of the kisspeptin antagonist, peptide 234. We found that inhibition of kisspeptin signaling blocked the bicuculline-induced stimulation of GnRH release, suggesting that kisspeptin neurons may relay inhibitory GABA signals to GnRH neurons. This implies that a reduction in tonic GABA inhibition of GnRH release is, at least in part, mediated through kisspeptin neurons.

Mechanisms affecting neuroendocrine and epigenetic regulation of body weight and onset of puberty: potential implications in the child born small for gestational age (SGA)

Signaling peptides produced in peripheral tissues such as gut, adipose tissue, and pancreas communicate with brain centers, such as hypothalamus and hindbrain to manage energy homeostasis. These regulatory mechanisms of energy intake and storage have evolved during long periods of hunger in the evolution of man to protect the species from extinction. It is now clear that these circuitries are influenced by prenatal and postnatal environmental factors including endocrine disruptive chemicals. Hypothalamic appetite regulatory systems develop and mature in utero and early infancy, and involve signaling pathways that are important also for the regulation of puberty onset. Recent studies in humans and animals have shown that metabolic pathways involved in regulation of growth, body weight gain and sexual maturation are largely affected by epigenetic programming that can impact both current and future generations. In particular, intrauterine and early infantile developmental phases of high plasticity are susceptible to factors that affect metabolic programming that therefore, affect metabolic function throughout life. In children born small for gestational age, poor nutritional conditions during gestation can modify metabolic systems to adapt to expectations of chronic undernutrition. These children are potentially poorly equipped to cope with energy-dense diets and are possibly programmed to store as much energy as possible, leading to later obesity, metabolic syndrome, disturbed regulation of normal puberty and early onset of cardiovascular disease. Most cases of disturbed energy balance are likely a result of a combination of genetics, epigenetics and environment. This review will discuss potential mechanisms linking intrauterine growth retardation with changes in growth, energy homeostasis and sexual maturation.

Neuroendocrine pathways mediating nutritional acceleration of puberty: insights from ruminant models

The pubertal process is characterized by an activation of physiological events within the hypothalamic-adenohypophyseal-gonadal axis which culminate in reproductive competence. Excessive weight gain and adiposity during the juvenile period is associated with accelerated onset of puberty in females. The mechanisms and pathways by which excess energy balance advances puberty are unclear, but appear to involve an early escape from estradiol negative feedback and early initiation of high-frequency episodic gonadotropin-releasing hormone (GnRH) secretion. Hypothalamic neurons, particularly neuropeptide Y and proopiomelanocortin neurons are likely important components of the pathway sensing and transmitting metabolic information to the control of GnRH secretion. Kisspeptin neurons may also have a role as effector neurons integrating metabolic and gonadal steroid feedback effects on GnRH secretion at the time of puberty. Recent studies indicate that leptin-responsive neurons within the ventral premammillary nucleus play a critical role in pubertal progression and challenge the relevance of kisspeptin neurons in this process. Nevertheless, the nutritional control of puberty is likely to involve an integration of major sensor and effector pathways that interact with modulatory circuitries for a fine control of GnRH neuron function. In this review, observations made in ruminant species are emphasized for a comparative perspective.

Determinants of menarche

Menarche is a milestone in a woman's life as it denotes the start of reproductive capacity. Aim of this report is to review the recent developments and the current knowledge in the neuroendocrinology of pubertal onset and the factors, genetic and environmental, that influence menarcheal age. We also review the implications of early or late menarcheal age on a young woman's life.

Insulin and NPY pathways and the control of GnRH function and puberty onset

Energy balance exerts a critical influence on reproductive function. Leptin and insulin are among the metabolic factors signaling the nutritional status of an individual to the hypothalamus, and their role in the overall modulation of the activity of GnRH neurons is increasingly recognized. As such, they participate to a more generalized phenomenon: the signaling of peripheral metabolic changes to the central nervous system. The physiological importance that the interactions occurring between peripheral metabolic factors and the central nervous system bear for the control of food intake is increasingly recognized. The central mechanisms implicated are the focus of attention of very many research groups worldwide. We review here the experimental data that suggest that similar mechanisms are at play for the metabolic control of the neuroendocrine reproductive function. It is appearing that metabolic signals are integrated at the levels of first-order neurons equipped with the proper receptors, ant that these neurons send their signals towards hypothalamic GnRH neurons which constitute the integrative element of this network.

Social subordination and polymorphisms in the gene encoding the serotonin transporter enhance estradiol inhibition of luteinizing hormone secretion in female rhesus monkeys

Psychosocial factors, particularly social stress, may compromise reproduction. However, some individuals may be more susceptible to socially induced infertility. The present study used group-housed, adult, ovariectomized rhesus monkeys to test the hypothesis that exposure to psychosocial stress, imposed by social subordination, would enhance estradiol (E2)-negative feedback inhibition of LH. Because polymorphisms in the gene encoding the serotonin transporter (SLC6A4) may contribute to individual differences in response to adverse environments, we determined whether subordinate females with the short-promoter-length allele (s-variant) would show greater suppression of LH. Subordinate females, particularly those with the s-variant SLC6A4 genotype, received significantly higher rates of noncontact aggression from more dominant cage mates and had consistently lower body weights. Serum LH was not influenced by social status in the absence of E2. In contrast, subordinate females were hypersensitive to E2-negative feedback inhibition of LH. Furthermore, serum LH in subordinate females with s-variant SLC6A4 genotype was maximally suppressed by Day 4 of treatment, whereas nadir concentrations were not reached until later in treatment in other females. Finally, pharmacological elevation of serum cortisol potentiated E2-negative feedback inhibition in all females. The current data suggest that infertility induced by psychosocial stressors may be mediated by hypersensitivity to E2-negative feedback and that polymorphisms in the SLC6A4 gene may contribute to differences in reproductive compromise in response to chronic stress.

Effect of transient hypothyroidism during infancy on the postnatal ontogeny of luteinising hormone release in the agonadal male rhesus monkey (Macaca mulatta): implications for the timing of puberty in higher primates

The present study examined whether a transient thyroid hormone (T(4)) deficit during infancy in male monkeys would compromise the arrest of luteinising hormone (LH) secretion during the infant-juvenile transition, and/or interfere with the pubertal resurgence of LH. Animals were orchidectomized and thyroidectomized (n = 3; Tx) or sham Tx (n = 3) within 5 days of birth. T(4) replacement was initiated in two Tx monkeys at age 19 weeks to reestablish a euthyroid condition. Blood samples were drawn weekly for hormone assay. Body weight, crown-rump length, and bone age were assessed throughout the study. Within a week of Tx, plasma T(4) declined to undetectable levels and, by 6-8 weeks of age, signs of hypothyroidism were evident. Transient hypothyroidism during infancy failed to prevent either arrest of LH secretion during the infant-juvenile transition or the pubertal resurgence of LH secretion, both of which occurred at similar ages to sham Tx animals. Although body weight exhibited complete catch-up with T(4) replacement, crown-rump length and bone age did not. Thus, bone age at the time of the pubertal LH resurgence in Tx animals was less advanced than that in shams. Although Tx did not influence qualitatively the pattern of gonadotrophin secretion, LH levels during infancy and after pubertal LH resurgence were elevated in Tx monkeys. This was not associated with changes in LH pulse frequency and amplitude, but half-life (53 versus 65 min) of the slow second phase of LH clearance was greater in Tx animals. These results indicate that hypothalamic mechanisms dictating the pattern of gonadotrophin-releasing hormone release from birth to puberty are not dependent on T(4) action during infancy, and fail to support the notion that onset of puberty is causally coupled to skeletal maturation. They also indicate that LH renal clearance mechanisms may be programmed in a T(4) dependent manner during infancy.

Pubertal development and menarche

Puberty is the developmental process that culminates in reproductive capability and is the result of a complex series of molecular and physiological events. The release of gonadotropin-releasing hormone from specialized neurons of the hypothalamus begins the hormonal cascade that causes gonadal activation and the physical changes of puberty. Several factors have been proposed to influence the activation of the hypothalamus to trigger puberty, but the involved pathways have not been fully elucidated. The recent observations that the age of pubertal onset may be lowering in American girls calls attention to the lack of knowledge of modulating factors that affect the pubertal process. Genes necessary for puberty have been found by studying persons who do not achieve puberty; such studies have provided insights into the pathways necessary for pubertal development. A multidisciplinary focus is required to elucidate the complex mechanisms involved in the initiation and progression of puberty.

Hypothalamic control of the pituitary-gonadal axis in higher primates: key advances over the last two decades

This review provides a brief historical background to the foundation of primate reproductive neuroendocrinology that was laid by Ernst Knobil during the late 1960s and early 1970s. This is followed by a discussion of studies conducted over the last two decades that I view as having contributed to the current understanding of the field of primate reproductive neuroendocrinology. The review concludes with a short summary of key questions that remain to be addressed.

Gene-environment interactions, not neonatal growth hormone deficiency, time puberty in female rhesus monkeys

The factors that influence the timing of puberty and the onset of adult fertility are poorly understood. While focus on the juvenile period has provided insights into how growth-related cues affect pubertal timing, growth velocity during infancy that is sustained into the juvenile period may be important. On the other hand, social factors, specifically exposure to psychosocial stressors, can delay sexual maturation, possibly by altering growth velocities during development. Using female rhesus monkeys, the present study used a prospective analysis to determine how neonatal growth hormone (GH) inhibition with a sandostatin analog or suppression of the pituitary-gonadal axis with a GnRH analog affected growth and sexual maturation. A separate retrospective analysis was done assessing the effects of social dominance status during development on pubertal timing. Because a specific polymorphism in the gene encoding the serotonin (5HT) reuptake transporter increases vulnerability to psychosocial stressors, females were also genotyped and were then classified as socially dominant, having both alleles for the long promoter variant or having at least one allele for the short promoter variant, or as socially subordinate, having the long variant or having the short variant. Neonatal treatments were not balanced for social status or genotype, so analyses were performed separately. Although the neonatal treatments reduced GH secretion postnatally and through the juvenile period, neither growth nor sexual maturation was affected. In contrast, the retrospective analysis showed sexual maturation was delayed significantly in subordinate females carrying at least one allele of the short promoter variant in the gene encoding the 5HT reuptake transporter, and this delay was associated with reduced GH and leptin secretion during the juvenile phase but not with differences in growth velocities from birth. These data suggest that decreased neonatal GH secretion does not adversely affect sexual maturation, but that polymorphisms in the gene encoding the 5HT transporter modulate the adverse consequences of social subordination on the timing of puberty in female rhesus monkeys.

The genetic basis for the timing of human puberty

Puberty is a complex, coordinated biological process with multiple levels of regulation. Epidemiological observations suggest that the timing of pubertal events is a heritable trait, although environmental factors can modulate such genetic influence. The study of pathological states of early and late puberty has provided valuable insight into those genes that regulate gonadotrophin-releasing hormone (GnRH) activity. The development of pulsatile release of GnRH secretion mediated through kisspeptin-1 activation of G-protein coupled receptor-54 appears to be a central event at the onset and during progression of puberty. Stimulating and restraining influences (e.g. in the form of glutamatergic and GABAergic neuronal inputs) are likely to influence the timing of this process. The study of extreme variants of 'normality', such as constitutional delay of growth and puberty and early puberty, may lead to the recognition of additional genes and pathways that can modulate both the timing of pubertal onset and its tempo.

An increase in in vivo release of LHRH and precocious puberty by posterior hypothalamic lesions in female rhesus monkeys (Macaca mulatta)

We have previously shown that a decrease in gamma-aminobutyric acid (GABA) tone and a subsequent increase in glutamatergic tone occur in association with the pubertal increase in luteinizing hormone releasing hormone (LHRH) release in primates. To further determine the causal relationship between developmental changes in GABA and glutamate levels and the pubertal increase in LHRH release, we examined monkeys with precocious puberty induced by lesions in the posterior hypothalamus (PH). Six prepubertal female rhesus monkeys (17.4 +/- 0.1 mo of age) received lesions in the PH, three prepubertal females (17.5 +/- 0.1 mo) received sham lesions, and two females received no treatments. LHRH, GABA, and glutamate levels in the stalk-median eminence before and after lesions were assessed over two 6-h periods (0600-1200 and 1800-2400) using push-pull perfusion. Monkeys with PH lesions exhibited external signs of precocious puberty, including significantly earlier menarche in PH lesion animals (18.8 +/- 0.2 mo) than in sham/controls (25.5 +/- 0.9 mo, P<0.001). Moreover, PH lesion animals had elevated LHRH levels and higher evening glutamate levels after lesions, whereas LHRH changes did not occur in sham/controls until later. Changes in GABA release were not discernible, since evening GABA levels already deceased at 18-20 mo of age in both groups and morning levels remained at the prepubertal levels. The age of first ovulation in both groups did not differ. Collectively, PH lesions may not be a good tool to investigate the mechanism of puberty, and, taking into account the recent findings on the role of kisspeptins, the mechanism of the puberty onset in primates is more complex than we initially anticipated.

Reproductive ageing in women

The traditional view in respect to female reproduction is that the number of oocytes at birth is fixed and continuously declines towards the point when no more oocytes are available after menopause. In this review we briefly discuss the embryonic development of female germ cells and ovarian follicles. The ontogeny of the hypothalamic-pituitary-gonadal axis is then discussed, with a focus on pubertal transition and normal ovulatory menstrual cycles during female adult life. Biochemical markers of menopausal transition are briefly examined. We also examine the effects of age on female fertility, the contribution of chromosomal abnormalities of the oocyte to the observed decline in female fertility with age and the possible biological basis for the occurrence of such abnormalities. Finally, we consider the effects of maternal age on obstetric complications and perinatal outcome. New data that have the potential to revolutionize our understanding of mammalian oogenesis and follicular formation, and of the female reproductive ageing process, are also briefly considered.

Molecular analysis of the neuropeptide Y1 receptor gene in human idiopathic gonadotropin-dependent precocious puberty and isolated hypogonadotropic hypogonadism

OBJECTIVE

To investigate the role of mutations or polymorphisms in the NPY-Y1R gene in human idiopathic central pubertal disorders.

DESIGN

Molecular studies.

SETTING

University hospital.

PATIENT(S)

Thirty-three patients with gonadotropin-dependent precocious puberty, 22 with hypogonadotropic hypogonadism, and 50 controls.

INTERVENTION(S)

Genomic DNA extraction, NPY-Y1R gene sequence analysis, cell-surface expression, and functional activity of an identified receptor variant.

MAIN OUTCOME MEASURE(S)

Results of sequencing, cell-surface receptor expression, and receptor function.

RESULT(S)

A heterozygous substitution of lysine (K) by threonine (T) at position 374 in the carboxyl terminal region of NPY-Y1R was identified in a girl with familial GDPP. Her mother, who had pubertal developmental at appropriate age, carried the same genetic variant. Introduction of the K374T variant into an expression vector containing the human NPY-Y1R complementary DNA led to a partial reduction in cell-surface expression of NPY-Y1R in transiently transfected HEK293 cells. This mutation did not lead to a significant reduction in NPY-stimulated activity of the receptor in this heterologous expression system. No other allelic variants of the NPY-Y1R gene were identified in patients or controls.

CONCLUSION(S)

We have identified an inherited heterozygous variant of the NPY-Y1R gene in a girl with precocious puberty; however, this most likely did not contribute to her phenotype. Mutations of the highly conserved NPY-Y1R gene do not appear to represent a frequent mechanism underlying human idiopathic central pubertal disorders.

The role of KiSS-1 in the regulation of puberty in higher primates

Puberty in higher primates is triggered by resurgence in the pulsatile secretion of hypothalamic GnRH after a hiatus in the robust release of this hypophysiotropic signal during childhood and juvenile development. Interestingly, the prepubertal decline in GnRH release is not associated with a marked reduction in the expression of either the gene that codes for GnRH (GnRH-1) or the decapeptide itself, and the network of GnRH neurons in the hypothalamus of the juvenile may by activated prematurely and with surprising ease by intermittent neurochemical stimulation with N-methyl-d-aspartate (NMDA), a glutamate receptor agonist. KiSS-1, a gene that encodes for kisspeptin-121, which is proteolytically cleaved to a 54 amino acid peptide, metastin, was initially studied in the context of tumor suppression. In 2003, however, inactivating mutations in the metastin receptor, GPR54, were reported to be associated with hypogonadotropic hypogonadism and absent puberty in man. Subsequent studies in the rhesus monkey have shown that GPR54 and KiSS-1 are expressed in the mediobasal hypothalamus (MBH), KiSS-1 expression in the MBH increases at the time of the pubertal resurgence in GnRH release and pulsatile, but not continuous, i.v. administration of metastin 45-54 in the juvenile male monkey elicits sustained GnRH release precociously. The significance of these findings in the context of the initiation of the onset of puberty is discussed.

Prenatal development of hypothalamic neuropeptide systems in the nonhuman primate

In the rodent, arcuate nucleus of the hypothalamus (ARH)-derived neuropeptide Y (NPY) and proopiomelanocortin (POMC) neurons have efferent projections throughout the hypothalamus that do not fully mature until the second and third postnatal weeks. Since this process is likely completed by birth in primates we characterized the ontogeny of NPY and melanocortin systems in the fetal Japanese macaque during the late second (G100), early third (G130) and late third trimesters (G170). NPY mRNA was expressed in the ARH, paraventricular nucleus (PVH), and dorsomedial nucleus of the hypothalamus (DMH) as early as G100. ARH-derived NPY projections to the PVH were initiated at G100 but were limited and variable; however, there was a modest increase in density and number by G130. ARH-NPY/agouti-related peptide (AgRP) fiber projections to efferent target sites were completely developed by G170, but the density continued to increase in the postnatal period. In contrast to NPY/AgRP projections, alphaMSH fibers were minimal at G100 and G130 but were moderate at G170. This study also revealed several significant species differences between rodent and the nonhuman primate (NHP). There were few NPY/catecholamine projections to the PVH and ARH prior to birth, while projections were increased in the adult. A substantial proportion of the catecholamine fibers did not coexpress NPY. In addition, cocaine and amphetamine-related transcript (CART) and alpha-melanocyte stimulating hormone (alphaMSH) were not colocalized in fibers or cell bodies. As a consequence of the prenatal development of these neuropeptide systems in the NHP, the maternal environment may critically influence these circuits. Additionally, because differences exist in the neuroanatomy of NPY and melanocortin circuitry the regulation of these systems may be different in primates than in rodents.

Physiology and gene regulation of the brain NPY Y1 receptor

Neuropeptide Y (NPY) is one of the most prominent and abundant neuropeptides in the mammalian brain where it interacts with a family of G-protein coupled receptors, including the Y(1) receptor subtype (Y(1)R). NPY-Y(1)R signalling plays a prominent role in the regulation of several behavioural and physiological functions including feeding behaviour and energy balance, sexual hormone secretion, stress response, emotional behaviour, neuronal excitability and ethanol drinking. Y(1)R expression is regulated by neuronal activity and peripheral hormones. The Y(1)R gene has been isolated from rodents and humans and it contains multiple regulatory elements that may participate in the regulation of its expression. Y(1)R expression in the hypothalamus is modulated by changes in energetic balance induced by a wide variety of conditions (fasting, pregnancy, hyperglycaemic challenge, hypophagia, diet induced obesity). Estrogens up-regulate responsiveness to NPY to stimulate preovulatory GnRH and gonadotropin surges by increasing Y(1)R gene expression both in the hypothalamus and the pituitary. Y(1)R expression is modulated by different kinds of brain insults, such as stress and seizure activity, and alteration in its expression may contribute to antidepressant action. Chronic modulation of GABA(A) receptor function by benzodiazepines or neuroactive steroids also affects Y(1)R expression in the amygdala, suggesting that a functional interaction between the GABA(A) receptor and Y(1)R mediated signalling may contribute to the regulation of emotional behaviour. In this paper, we review the state of the art concerning Y(1)R function and gene expression, including our personal contribution to many of the subjects mentioned above.

The male monkey as a model for the study of the neurobiology of puberty onset in man

This chapter is based on the material that was presented in the Symposium titled "Puberty in mechanistic perspective: animal models" at Sixth International Conference on the Control of the Onset of Puberty held in Evian, May 2005.

Control of GnRH neuronal activity by metabolic factors: the role of leptin and insulin

Energy balance exerts a critical influence on reproductive function. Leptin and insulin are among the metabolic factors signaling the nutritional status of an individual to the hypothalamus, and their role in the overall modulation of the activity of GnRH neurons is increasingly recognized. The experiments described here were designed to further investigate the central mechanisms of action of these two hormones and the precise hypothalamic pathways implicated in their effects on the reproductive axis. NPY neurons represent a primary target of leptin actions within the hypothalamus We used mice lacking the NPY Y1 receptor (Y1-/- mice) to investigate the physiological importance of the hypothalamic NPY neuronal system and its downstream pathways involving Y1 in the reproductive effects of leptin. Results point to a crucial role for the NPY Y1 receptor in the control of the onset of puberty and the maintenance of reproductive functions by leptin. A striking finding of these experiments was the observation that juvenile Y1-/- mice submitted to food restriction can proceed through puberty like normally fed animals, demonstrating that the absence of Y1 impairs the perception of decreasing energy stores by the gonadotrope axis. Next, we used parallel in vivo and in vitro experiments to delineate the role of insulin in the stimulation and maintenance of the activity of the neuroendocrine reproductive axis. First, we observed that the increase in circulating insulin levels achieved during hyperinsulinemic clamp studies in normal male mice was associated with a significant rise in LH secretion. This effect of insulin is likely mediated at the hypothalamic level, as insulin stimulates the secretion and the expression of GnRH by hypothalamic neurons in culture. Using primary neuronal cultures as well as a novel GnRH neuronal cell line obtained by conditional immortalization of adult rat hypothalamic neurons, we have recently demonstrated that this effect of insulin on GnRH gene expression is probably mediated directly at the level of GnRH neurons, and involves the stimulation of the MAP kinase Erk1/2 pathway. Taken together, these results provide new insights into the mechanisms involved in the regulation of GnRH neuronal activity by metabolic factors.