Morphological evidence for direct interaction between arcuate nucleus neuropeptide Y (NPY) neurons and gonadotropin-releasing hormone neurons and the possible involvement of NPY Y1 receptors

Review: Gestational and postnatal nutritional effects on the neuroendocrine control of puberty and subsequent reproductive performance in heifers

Pubertal attainment is an intricate biological process that involves maturation of the reproductive neuroendocrine axis and increased pulsatile release of gonadotropin-releasing hormone (GnRH) and luteinizing hormone. Nutrition is a critical environmental factor controlling the timing of puberty attainment. Nutrient restriction during early postnatal development delays puberty, whereas increased feed intake and adiposity during this period hasten pubertal maturation by imprinting the hypothalamus. Moreover, the dam's nutrition during gestation can program the neuroendocrine system in the developing fetus and has the potential to advance or delay puberty in the offspring. Leptin, a hormone produced primarily by adipose cells, plays an important role in communicating energy status to the brain and regulating sexual maturation. Leptin's regulation of GnRH release is mediated by an upstream neuronal network since GnRH neurons do not contain the leptin receptor. Two groups of neurons located in the arcuate nucleus of the hypothalamus that express neuropeptide Y (NPY), an orexigenic peptide, and alpha melanocyte-stimulating hormone (αMSH), an anorexigenic peptide, are central elements of the neural circuitry that relay inhibitory (NPY) and excitatory (αMSH) inputs to GnRH neurons. Moreover, KNDy neurons, neurons in the arcuate nucleus that co-express kisspeptin, neurokinin B (NKB), and dynorphin, also play a role in the metabolic regulation of puberty. Our studies in beef heifers demonstrate that increased rates of BW gain during early postweaning (4-9 mo of age) result in reduced expression of NPY mRNA, increased expression of proopiomelanocortin and kisspeptin receptor mRNA, reduced NPY inhibitory inputs to GnRH neurons, and increased excitatory αMSH inputs to KNDy neurons. Finally, our most recent data demonstrate that nutrition of the cow during the last two trimesters of gestation can also induce transcriptional and structural changes in hypothalamic neurocircuitries in the heifer progeny that likely persist long-term after birth. Managerial approaches, such as supplementation of the dam during gestation (fetal programming), creep feeding, early weaning, and stair-step nutritional regimens have been developed to exploit brain plasticity and advance pubertal maturation in heifers.

Genetic variants in 5'UTR and exonic region of NPY gene alter the reproduction performance in Indian cattle breeds

Neuropeptide Y (NPY) is one of the most potent orexigenic factors which can produce diverse effects on behaviour and other physiological functions and is highly conserved in evolution. The present study was aimed to identify and associate SNPs in the 5' UTR and exon2 region of the NPY gene with reproduction and production traits in Kankrej cattle of Indian origin. Three mutations in the 5'-UTR region and one mutation in the exon2 region of the NPY gene were identified by PCR-SSCP and PCR-RFLP, respectively, followed by sequencing. Further, association studies were conducted with reproduction and production traits in Kankrej cattle. The GACCGA genotyped animals based on the 5'UTR variants indicated better dry period and calving interval, whereas with GGCCGG genotypes showed higher total lactation milk yield and 305-day milk yield in comparison to other genotypes. Also, service period and inter calving period varied significantly among the genotypes of exon2, as the GG genotyped animals had significantly longer calving interval. Other traits like age at first heat, age at first service and age at first calving were not affected by the mutations. So, the present study outlined that the bovine NPY gene may be considered to be one of the candidate gene for improvement of reproductive performance of cattle, after validation on large sample size.

Agouti-related peptide neuronal silencing overcomes delayed puberty in neonatally underfed male mice

Agouti-related peptide (AgRP) neurons are thought to indirectly regulate the activity of hypothalamic gonadotrophin-releasing hormone neurons which control fertility. AgRP neurons also drive caloric intake and are modulated by metabolically-relevant hormones, providing a link to the hypothalamic-pituitary-gonadal axis. In mice expressing Cre-dependant designer receptors (DREADDs) in AgRP neurons, we activated or silenced these neurons in vivo using the synthetic ligand clozapine-N-oxide (CNO) to observe the effect of AgRP neuron activity on timing of puberty. To validate these animals, we chronically treated both stimulatory (hM3Dq) and inhibitory (hM4Di) DREADD × AgRP-Cre mice with CNO, observing a pronounced increase and decrease of food intake, respectively, consistent with the known orexigenic effects of these neurons. RNAscope was performed to visually confirm the activation of AgRP neurons. Puberty onset was assessed in males and females. There was no effect on preputial separation in males or vaginal opening and first oestrus in females after CNO treatment from day 26 to 30 to chronically modulate AgRP neurons. Next, to determine whether the delay in puberty onset occurring in response to neonatal underfeeding could be overcome by inhibiting AgRP neuronal activity, mice were raised in large (neonatally underfed) or normal litter sizes. The delay in puberty from underfeeding was completely reversed in CNO-treated AgRP-hM4Di male mice. These data highlight the inhibitory role of AgRP neurons to delay puberty onset when undernutrition occurs during the neonatal period, at least in male mice. TRAIL REGISTRATION NUMBER: JNE-22-0081-OA.R2.

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.

Neuroendocrine mechanisms of reproductive dysfunctions in undernourished condition

It is well known that undernourished conditions disturb female reproductive functions in many species, including humans. These alterations are mainly caused by a reduction in gonadotrophin-releasing hormone (GnRH) secretion from the hypothalamus. Evidence from the literature suggests that some hypothalamic factors play pivotal roles in the coordination of reproductive functions and energy homeostasis in response to environmental cues and internal nutritional status. Generally, anorexigenic/satiety-related factors, such as leptin, alpha-melanocyte-stimulating hormone, and proopiomelanocortin, promote GnRH secretion, whereas orexigenic factors, such as neuropeptide Y, agouti-related protein, orexin, and ghrelin, attenuate GnRH secretion. Conversely, gonadotrophin-inhibitory hormone, which exerts anti-GnRH and gonadotrophic effects, promotes feeding behavior in many species. In addition, the activity of kisspeptin, which is a potent stimulator of GnRH, is reduced by undernourished conditions. Under normal nutritional conditions, these factors are coordinated to maintain both feeding behavior and reproductive functions. However, in undernourished conditions their activity levels are markedly altered to promote feeding behavior and temporarily suppress reproductive functions, in order to prioritize the survival of the individual over that of the species.

Effects of low energy availability on female reproductive function

Background

It is known that metabolic and nutritional disturbances induce reproductive dysfunction in females. The main cause of these alterations is reduced gonadotrophin‐releasing hormone (GnRH) secretion from the hypothalamus, and the underlying mechanisms have gradually been elucidated.

Methods

The present review summarizes current knowledge about the effects of nutrition/metabolism on reproductive functions, especially focusing on the GnRH regulation system.

Main findings

Various central and peripheral factors are involved in the regulation of GnRH secretion, and alterations in their activity combine to affect GnRH neurons. Satiety‐related factors, i.e., leptin, insulin, and alpha‐melanocyte‐stimulating hormone, directly and indirectly stimulate GnRH secretion, whereas orexigenic factors, i.e., neuropeptide Y, Agouti‐related protein, orexin, and ghrelin, attenuate GnRH secretion. In addition, kisspeptin, which is a potent positive regulator of GnRH, expression is reduced by metabolic and nutritional disturbances.

Conclusion

These neuroendocrine systems may be defensive mechanisms, which help organisms to survive adverse conditions by temporarily suppressing reproduction.

Β-endorphin-immunoreactive perikarya appear to receive innervation from NPY-immunoreactive fiber varicosities in the human hypothalamus

Morphological and pharmacological studies indicate that hypothalamic neuropeptide Y (NPY) and proopiomelanocortin (POMC) neurons communicate with each other in rats and regulate a variety of hypothalamic and extrahypothalamic functions. Indeed, electron microscopic studies revealed NPY-immunoreactive (NPI-IR) synapses on β-endorphin-IR neurons in the hypothalamus. However, no such connections have been reported in humans. Here, we studied the putative NPY-β-endorphin associations with high-resolution light microscopic double-label immunocytochemistry in the human hypothalamus. The majority of β-endorphin-IR perikarya appear to be innervated by abutting NPY-IR fibers in the infundibulum/median eminence, receiving more than 6 contacts (38% of the counted neurons) or three to six contacts (42% of the counted neurons). The rest of the β-endorphin-IR neurons are lightly innervated by NPY fibers (14%, one-three contacts) or do not receive any detectable NPY-IR axon varicosities (6% of the counted neurons). Since β-endorphin is cleaved from the proopiomelanocortin (POMC) precursor, the NPY-β-endorphin connections also provide the foundation for NPY-α-MSH and NPY-ACTH connections and their subsequent physiology. The close anatomical connections between NPY-IR nerve terminals and β-endorphin-IR neurons reported herein may represent functional synapses and provide the foundation for NPY-stimulated β-endorphin release. By interacting with β-endorphin, NPY may have a more widespread regulatory capacity than acting alone on different neurotransmitter systems.

Central injection of neuropeptide Y modulates sexual behavior in male rats: interaction with GnRH and kisspeptin/neurokinin B/dynorphin

AIMS

A number of studies have shown that neuropeptide Y (NPY) is considered to be one of the key regulators of hypothalamic-pituitary-gonadal (HPG) axis in the mammals. In addition, kisspeptin (encode by Kiss1 gene), neurokinin B (encode by Tac3 gene) and dynorphin (encode by Pdyn gene) (commonly known as KNDy secreting neurons) are a powerful upstream regulators of GnRH neuron in hypothalamus.

MATERIALS AND METHODS

The present study aims to investigate the effects of the intracerebroventricular (icv) injection of NPY and BIBP3226 (NPY receptor antagonist (NPYRA)) on the male sexual behavioral. Additionally, in order to see whether NPY signals can be relayed through the pathway of kisspeptin/neurokinin B/dynorphin, the gene expression of these peptides along with Gnrh1 gene in the hypothalamus were measured.

RESULTS

The icv injection of NPY decreased the latencies and increase the frequencies of sexual parameters of the male rats in a significant way. In this line, NPYRA antagonized the stimulative effects of NPY. Moreover, data from real-time quantitative PCR indicated that injection of NPY significantly increased the gene expression of Gnrh1, Kiss1 and Tac3 and decrease the Pdyn while treatment with NPYRA controlled the modulative effects of NPY on these gene expression.

CONCLUSIONS

In conclusion based on the results of this study, NPY can exert its impacts on the sexual behavior of male rats via modulation of the KNDy secreting neurons as an interneural pathway to GnRH neurons.

The effect of intracerebroventricular administration of neuropeptide Y on reproductive axis function in the male Wistar rats: Involvement of hypothalamic KiSS1/GPR54 system

Several studies have shown that neuropeptide Y (NPY) is considered to be one of the key regulators of the hypothalamic-pituitary-gonadal axis in the mammals. Also, kisspeptin is a powerful upstream regulator of gonadotropin-releasing hormone neurons in the hypothalamus. The present study aims to investigate the effects of the intracerebroventricular (ICV) injection of NPY and BIBP3226 (NPY receptor antagonist) on the reproductive axis (either hormonal or behavioral) of the male rats. Furthermore, to see whether NPY signals can be relayed through the pathway of KiSS1/GPR54, the gene expression of these peptides in the arcuate nucleus was measured. The ICV injection of NPY decreased the latencies and increased the frequencies of sexual parameters of the male rats in a significant way. Results obtained from LH and testosterone measurement showed that NPY had a significant increase in comparison with the control group. In this line, BIBP3226 antagonized the stimulative effects of NPY. Furthermore, data from real-time quantitative PCR showed that injection of NPY significantly increased the gene expression of KiSS1 and GPR54, while treatment with BIBP3226 controlled the stimulative effects of NPY on gene expression of KiSS1 and GPR54. Summing up, NPY can exert its impacts on the reproductive axis, this occurs at least partly through affecting KiSS1/GPR54 system.

Mechanism of pulsatile GnRH release in primates: Unresolved questions

The pulsatility of GnRH release is essential for reproductive function. The key events in reproductive function, such as puberty onset and ovulatory cycles, are regulated by the frequency and amplitude modulation of pulsatile GnRH release. Abnormal patterns of GnRH pulsatility are seen in association with disease states, such as polycystic ovarian syndrome and anorexia nervosa. Recent studies with physiological, track-tracing, optogenetic and electrophysiological recording experiments indicate that a group of kisspeptin neurons in the arcuate nucleus (ARC) of the hypothalamus are responsible for pulsatile GnRH release. Thus, the kisspeptin neuron in the ARC has been called the "GnRH pulse-generator." However, a few pieces of evidence do not quite fit into this concept. This article reviews some old works and discusses unresolved issues on the mechanism of GnRH pulse generation.

Obesity, Neuroinflammation, and Reproductive Function

The increasing occurrence of obesity has become a significant public health concern. Individuals with obesity have higher prevalence of heart disease, stroke, osteoarthritis, diabetes, and reproductive disorders. Reproductive problems include menstrual irregularities, pregnancy complications, and infertility due to anovulation, in women, and lower testosterone and diminished sperm count, in men. In particular, women with obesity have reduced levels of both gonadotropin hormones, and, in obese men, lower testosterone is accompanied by diminished LH. Taken together, these findings indicate central dysregulation of the hypothalamic-pituitary-gonadal axis, specifically at the level of the GnRH neuron function, which is the final brain output for the regulation of reproduction. Obesity is a state of hyperinsulinemia, hyperlipidemia, hyperleptinemia, and chronic inflammation. Herein, we review recent advances in our understanding of how these metabolic and immune changes affect hypothalamic function and regulation of GnRH neurons. In the latter part, we focus on neuroinflammation as a major consequence of obesity and discuss findings that reveal that GnRH neurons are uniquely positioned to respond to inflammatory changes.

Impact of Dietary Fats on Brain Functions

BACKGROUND

Adequate dietary intake and nutritional status have important effects on brain functions and on brain health. Energy intake and specific nutrients excess or deficiency from diet differently affect cognitive processes, emotions, behaviour, neuroendocrine functions and synaptic plasticity with possible protective or detrimental effects on neuronal physiology. Lipids, in particular, play structural and functional roles in neurons. Here the importance of dietary fats and the need to understand the brain mechanisms activated by peripheral and central metabolic sensors. Thus, the manipulation of lifestyle factors such as dietary interventions may represent a successful therapeutic approach to maintain and preserve brain health along lifespan.

METHODS

This review aims at summarizing the impact of dietary fats on brain functions.

RESULTS

Starting from fat consumption, nutrient sensing and food-related reward, the impact of gut-brain communications will be discussed in brain health and disease. A specific focus will be on the impact of fats on the molecular pathways within the hypothalamus involved in the control of reproduction via the expression and the release of Gonadotropin-Releasing Hormone. Lastly, the effects of specific lipid classes such as polyunsaturated fatty acids and of the "fattest" of all diets, commonly known as "ketogenic diets", on brain functions will also be discussed.

CONCLUSION

Despite the knowledge of the molecular mechanisms is still a work in progress, the clinical relevance of the manipulation of dietary fats is well acknowledged and such manipulations are in fact currently in use for the treatment of brain diseases.

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

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

Neuroendocrine mechanism of food intake and energy regulation in Japanese quail under differential simulated photoperiodic conditions: Involvement of hypothalamic neuropeptides, AMPK, insulin and adiponectin receptors

Neuroendocrine coordination between the reproductive and energy regulatory hypothalamic circuitries not only tightly regulates food intake and energy expenditure but also maintains the body weight and reproduction. The effect of different simulated photoperiodic conditions on food intake and neuroendocrine mechanism of energy homeostasis in Japanese quail is not investigated till date. Hence, our present study is designed to elucidate the effect of different simulated photoperiodic conditions on food consumption and neuroendocrine mechanism(s) of energy regulation in this poultry species. The alterations in hypothalamic energy balancing neuropeptides (NPY/AgRP/CART), polypeptide hormone precursor (POMC), protein kinase (AMPK-p-AMPK) as well as receptors of insulin and adiponectin [Insulin Receptor (IR), Adiponectin Receptor 1 & 2] have been investigated in photosensitive (PS), scotorefractory (SR),photorefractory (PR) and scotosensitive (SS) quail. Immunofluorescence and western blotting were used to quantify the expression of these peptides and proteins. Results showed increased food consumption and body weight gain, along with increased expression of NPY, AgRP, IR, adiponectin receptors and p-AMPK, decreased CART and POMC in the hypothalamus of photosensitive and scotorefractory quail. While, opposite findings were observed in photorefractory and scotosensitive quail. Hence, this study may suggest the hypothalamic energy channelization towards reproductive axis in photosensitive and scotorefractory quail to support the full breeding conditions, while hypothalamic energy deprivation in photorefractory and scotosensitive quail leads to reproductive quiescence.

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.

AgRP to Kiss1 neuron signaling links nutritional state and fertility

Mammalian reproductive function depends upon a neuroendocrine circuit that evokes the pulsatile release of gonadotropin hormones (luteinizing hormone and follicle-stimulating hormone) from the pituitary. This reproductive circuit is sensitive to metabolic perturbations. When challenged with starvation, insufficient energy reserves attenuate gonadotropin release, leading to infertility. The reproductive neuroendocrine circuit is well established, composed of two populations of kisspeptin-expressing neurons (located in the anteroventral periventricular hypothalamus, Kiss1^AVPV, and arcuate hypothalamus, Kiss1^ARH), which drive the pulsatile activity of gonadotropin-releasing hormone (GnRH) neurons. The reproductive axis is primarily regulated by gonadal steroid and circadian cues, but the starvation-sensitive input that inhibits this circuit during negative energy balance remains controversial. Agouti-related peptide (AgRP)-expressing neurons are activated during starvation and have been implicated in leptin-associated infertility. To test whether these neurons relay information to the reproductive circuit, we used AgRP-neuron ablation and optogenetics to explore connectivity in acute slice preparations. Stimulation of AgRP fibers revealed direct, inhibitory synaptic connections with Kiss1^ARH and Kiss1^AVPV neurons. In agreement with this finding, Kiss1^ARH neurons received less presynaptic inhibition in the absence of AgRP neurons (neonatal toxin-induced ablation). To determine whether enhancing the activity of AgRP neurons is sufficient to attenuate fertility in vivo, we artificially activated them over a sustained period and monitored fertility. Chemogenetic activation with clozapine N-oxide resulted in delayed estrous cycles and decreased fertility. These findings are consistent with the idea that, during metabolic deficiency, AgRP signaling contributes to infertility by inhibiting Kiss1 neurons.

A novel neuropeptide Y neuronal pathway linking energy state and reproductive behavior

Animals consume energy for reproduction, as well as survival. Excess or insufficient energy investment into reproduction, respectively, threatens the survival of parents or leads to the failure of reproduction. Management of energy consumption in reproduction is important, not only for the success of the process, but also for the survival of the parents. Reproductive behaviors, such as mating and parental behavior, are indispensable for achieving each event of reproduction including gametogamy, parturition, and lactation. Therefore, reproductive behavior is one of the important factors in managing energy consumption for reproduction. Orexigenic and anorexigenic molecules in the hypothalamus have been implicated in the regulation of reproductive functions. An orexigenic neuropeptide, neuropeptide Y (NPY), has been also implicated in the regulation of both reproduction and energy state of animals. In this review, we will first summarize the neuronal mechanism for regulating reproductive functions by orexigenic and anorexigenic molecules in the hypothalamus. Second, we will focus on the NPY neuronal pathways regulating reproductive behavior in the intra- and extra-hypothalamic brain areas. We will highlight the NPY neuronal pathway from the arcuate nucleus to the dorsal raphe nucleus as a novel extra-hypothalamic pathway for energy state-dependent regulation of reproductive behavior. Finally, we will propose a biological significance of the extra-hypothalamic NPY neuronal pathway, which plays an important role in the associative control of feeding and reproductive behaviors.

Prenatal testosterone exposure decreases colocalization of insulin receptors in kisspeptin/neurokinin B/dynorphin and agouti-related peptide neurons of the adult ewe

Insulin serves as a link between the metabolic and reproductive systems, communicating energy availability to the hypothalamus and enabling reproductive mechanisms. Adult Suffolk ewes prenatally exposed to testosterone (T) display an array of reproductive and metabolic dysfunctions similar to those seen in women with polycystic ovarian syndrome (PCOS), including insulin resistance. Moreover, prenatal T treatment alters neuropeptide expression in KNDy (co-expressing kisspeptin, neurokinin B/dynorphin) and agouti-related peptide (AgRP) neurons in the arcuate nucleus, two populations that play key roles in the control of reproduction and metabolism, respectively. In this study, we determined whether prenatal T treatment also altered insulin receptors in KNDy and AgRP neurons, as well as in preoptic area (POA) kisspeptin, pro-opiomelanocortin (POMC), and gonadotropin-releasing hormone (GnRH) neurons of the adult sheep brain. Immunofluorescent detection of the beta subunit of insulin receptor (IRβ) revealed that KNDy, AgRP and POMC neurons, but not GnRH or POA kisspeptin neurons, colocalize IRβ in control females. Moreover, prenatal T treatment decreased the percentage of KNDy and AgRP neurons that colocalized IRβ, consistent with reduced insulin sensitivity. Administration of the anti-androgen drug, Flutamide, during prenatal T treatment, prevented the reduction in IRβ colocalization in AgRP, but not in KNDy neurons, suggesting that these effects are programmed by androgenic and oestrogenic actions, respectively. These findings provide novel insight into the effects of prenatal T treatment on hypothalamic insulin sensitivity and raise the possibility that decreased insulin receptors, specifically within KNDy and AgRP neurons, may contribute to the PCOS-like phenotype of this animal model.

New concepts of the central control of reproduction, integrating influence of stress, metabolic state, and season

Gonadotropin releasing hormone is the primary driver of reproductive function and pulsatile GnRH secretion from the brain causes the synthesis and secretion of LH and FSH from the pituitary gland. Recent work has revealed that the secretion of GnRH is controlled at the level of the GnRH secretory terminals in the median eminence. At this level, projections of kisspeptin cells from the arcuate nucleus of the hypothalamus are seen to be closely associated with fibers and terminals of GnRH cells. Direct application of kisspeptin into the median eminence causes release of GnRH. The kisspeptin cells are activated at the time of a natural "pulse" secretion of GnRH, as reflected in the secretion of LH. This appears to be due to input to the kisspeptin cells from glutamatergic cells in the basal hypothalamus, indicating that more than 1 neural element is involved in the secretion of GnRH. Because the GnRH secretory terminals are outside the blood-brain barrier, factors such as kisspeptin may be administered systemically to cause GnRH secretion; this offers opportunities for manipulation of the reproductive axis using factors that do not cross the blood-brain barrier. In particular, kisspeptin or analogs of the same may be used to activate reproduction in the nonbreeding season of domestic animals. Another brain peptide that influences reproductive function is gonadotropin inhibitory hormone (GnIH). Work in sheep shows that this peptide acts on GnRH neuronal perikarya, but projections to the median eminence also allow secretion into the hypophysial portal blood and action of GnIH on pituitary gonadotropes. GnIH cells are upregulated in anestrus, and infusion of GnIH can block the ovulatory surge in GnRH and/or LH secretion. Metabolic status may also affect the secretion of reproduction, and this could involve action of gut peptides and leptin. Neuropeptide Y and Y-receptor ligands have a negative impact on reproduction, and Neuropeptide Y production is markedly increased in negative energy balance; this may be the cause of lowered GnRH and gonadotropin secretion in this state. There is a complex interaction between appetite-regulating peptide neurons and kisspeptin neurons that enables the former to regulate the latter both positively and negatively. In terms of how GnRH secretion is reduced during stress, recent data indicate that GnIH cells are integrally involved, with increased input to the GnRH cells. The secretion of GnIH into the portal blood is not increased during stress, so the negative effect is most likely effected at the level of GnRH neuronal cell bodies.

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

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

Coordinated seasonal regulation of metabolic and reproductive hypothalamic peptides in the desert jerboa

Jerboa (Jaculus orientalis) is a semi-desert rodent displaying strong seasonal variations in biological functions in order to survive harsh conditions. When environmental conditions become unfavorable in early autumn, it shuts down its reproductive axis, increases its body weight, and finally hibernates. In spring, the jerboa displays opposite regulations, with a reactivation of reproduction and reduction in body weight. This study investigated how genes coding for different hypothalamic peptides involved in the central control of reproduction (Rfrp and Kiss1) and energy homeostasis (Pomc, Npy, and Somatostatin) are regulated according to seasons in male jerboas captured in the wild in spring or autumn. Remarkably, a coordinated increase in the mRNA level of Rfrp in the dorso/ventromedial hypothalamus and Kiss1, Pomc, and Somatostatin in the arcuate nucleus was observed in jerboas captured in spring as compared to autumn animals. Only Npy gene expression in the arcuate nucleus displayed no significant variations between the two seasons. These variations appear in line with the jerboa's seasonal physiology, since the spring increase in Rfrp and Kiss1 expression might be related to sexual reactivation, while the spring increase in genes encoding anorexigenic peptides, POMC, and somatostatin may account for the reduced body weight reported at this time of the year. J. Comp. Neurol. 524:3717-3728, 2016. © 2016 Wiley Periodicals, Inc.

Peptides: Basic determinants of reproductive functions

Mammalian reproduction is a costly process in terms of energy consumption. The critical information regarding metabolic status is signaled to the hypothalamus mainly through peripheral peptides from the adipose tissue and gastrointestinal tract. Changes in energy stores produce fluctuations in leptin, insulin, ghrelin and glucose signals that feedback mainly to the hypothalamus to regulate metabolism and fertility. In near future, possible effects of the nutritional status on GnRH regulation can be evaluated by measuring serum or tissue levels of leptin and ghrelin in patiens suffering from infertility. The fact that leptin and ghrelin are antagonistic in their effects on GnRH neurons, their respective agonistic and antagonistic roles make them ideal candidates to use instead of GnRH agonist and antagonist. Similarly, kisspeptin expressing neurons are likely to mediate the well-established link between energy balance and reproductive functions. Exogenous kisspeptin can be used for physiological ovarian hyperstimulation for in-vitro fertilization. Moreover, kisspeptin antagonist therapy can be used for the treatment of postmenapousal women, precocious puberty, PCOS, endometriosis and uterine fibroids. In this review, we will analyze the central mechanisms involved in the integration of metabolic information and their contribution to the control of the reproductive function. Particular attention will be paid to summarize the participation of leptin, kisspeptin, ghrelin, NPY, orexin, urocortin, VIP, insulin, galanin, galanin like peptide, oxytocin, agouti gene-related peptide, and POMC neurons in this process and their possible interactions to contribute to the metabolic control of reproduction.

Seasonal plasticity in the peptide neuronal systems: potential roles of gonadotrophin-releasing hormone, gonadotrophin-inhibiting hormone, neuropeptide Y and vasoactive intestinal peptide in the regulation of the reproductive axis in subtropical Indian weaver birds

Two experiments examined the expression of gonadotrophin-releasing and inhibiting hormones (GnRH-I, GnRH-II and GnIH), neuropeptide Y (NPY) and vasoactive intestinal peptide (VIP) in subtropical Indian weaver birds, which demonstrate relative photorefractoriness. Experiment 1 measured peptide expression levels in the form of immunoreactive (-IR) cells, percentage cell area and cell optical density in the preoptic area (GnRH-I), midbrain (GnRH-II), paraventricular nucleus (GnIH), mediobasal hypothalamus [dorsomedial hypothalamus (DMH), infundibular complex (INc), NPY and VIP] and lateral septal organ (VIP) during the progressive, breeding, regressive and nonbreeding phases of the annual reproductive cycle. GnRH-I was decreased in the nonbreeding and VIP was increased in INc in the breeding and regressive states. GnRH-II and NPY levels did not differ between the testicular phases. Double-labelled immunohistochemistry (IHC) revealed a close association between the GnRH/GnIH, GnRH/NPY, GnRH/VIP and GnIH/NPY peptide systems, implicating them interacting and playing roles in the reproductive regulation in weaver birds. Experiment 2 further measured these peptide levels in the middle of day and night in weaver birds that were maintained under short days (8 : 16 h light /dark cycle; photosensitive), exposed to ten long days (16 : 8 h light /dark cycle; photostimulated) or maintained for approximately 2 years on a 16 : 8 h light /dark cycle (photorefractory). Reproductively immature testes in these groups precluded the possible effect of an enhanced gonadal feedback on the hypothalamic peptide expression. There were group differences in the GnRH-I (not GnRH-II), GnIH, NPY and VIP immunoreactivity, albeit with variations in immunoreactivity measures in the present study. These results, which are consistent with those reported in birds with relative photorefractoriness, show the distribution and possibly a complex interaction of key neuropeptides in the regulation of the annual reproductive cycle in Indian weaver birds.

Endogenous kisspeptin tone is a critical excitatory component of spontaneous GnRH activity and the GnRH response to NPY and CART

BACKGROUND/AIMS

Kisspeptin is the major excitatory regulator of gonadotropin-releasing hormone (GnRH) neurons and is responsible for basal GnRH/LH release and the GnRH/LH surge. Although it is widely assumed, based on mutations in kisspeptin and Kiss1R, that kisspeptin acts to sustain basal GnRH neuronal activity, there have been no studies to investigate whether endogenous basal kisspeptin tone plays a direct role in basal spontaneous GnRH neuronal excitability. It is also of interest to examine possible interactions between endogenous kisspeptin tone and other neuropeptides that have direct effects on GnRH neurons, such as neuropeptide Y (NPY) or cocaine- and amphetamine-regulated transcript (CART), since the activity of all these neuropeptides changes during states of negative energy balance.

METHODS

Loose cell-attached and whole-cell current patch-clamp recordings were made from GnRH-GFP neurons in hypothalamic slices from female and male rats.

RESULTS

Kisspeptin activated GnRH neurons in a concentration-dependent manner with an EC50 of 3.32 ± 0.02 nM. Surprisingly, a kisspeptin antagonist, Peptide 347, suppressed spontaneous activity in GnRH neurons, demonstrating the essential nature of the endogenous kisspeptin tone. Furthermore, inhibition of endogenous kisspeptin tone blocked the direct activation of GnRH cells that occurs in response to antagonism of NPY Y5 receptor or by CART.

CONCLUSIONS

Our electrophysiology studies suggest that basal endogenous kisspeptin tone is not only essential for spontaneous GnRH neuronal firing, but it is also required for the net excitatory effects of other neuropeptides, such as CART or NPY antagonism, on GnRH neurons. Therefore, endogenous kisspeptin tone could serve as the linchpin in GnRH activation or inhibition.

Hypophysiotropic gonadotropin-releasing hormone projections are exposed to dense plexuses of kisspeptin, neurokinin B and substance p immunoreactive fibers in the human: a study on tissues from postmenopausal women

Neuronal populations that synthesize kisspeptin (KP), neurokinin B (NKB) and substance P (SP) in the hypothalamic infundibular nucleus of humans are partly overlapping. These cells are important upstream regulators of gonadotropin-releasing hormone (GnRH) neurosecretion. Homologous neurons in laboratory animals are thought to modulate episodic GnRH secretion primarily via influencing KP receptors on the hypophysiotropic fiber projections of GnRH neurons. To explore the structural basis of this putative axo-axonal communication in humans, we analyzed the anatomical relationship of KP-immunoreactive (IR), NKB-IR and SP-IR axon plexuses with hypophysiotropic GnRH fiber projections. Immunohistochemical studies were carried out on histological samples from postmenopausal women. The neuropeptide-IR axons innervated densely the portal capillary network in the postinfundibular eminence. Subsets of the fibers formed descending tracts in the infundibular stalk, some reaching the neurohypophysis. KP-IR, NKB-IR and SP-IR plexuses intermingled, and established occasional contacts, with hypophysiotropic GnRH fibers in the postinfundibular eminence and through their lengthy course while descending within the infundibular stalk. Triple-immunofluorescent studies also revealed considerable overlap between the KP, NKB and SP signals in individual fibers, providing evidence that these peptidergic projections arise from neurons of the mediobasal hypothalamus. These neuroanatomical observations indicate that the hypophysiotropic projections of human GnRH neurons in the postinfundibular eminence and the descending GnRH tract coursing through the infundibular stalk to the neurohypophysis are exposed to neurotransmitters/neuropeptides released by dense KP-IR, NKB-IR and SP-IR fiber plexuses. Localization and characterization of axonal neuropeptide receptors will be required to clarify the putative autocrine and paracrine interactions in these anatomical regions.

Efferent projections of neuropeptide Y-expressing neurons of the dorsomedial hypothalamus in chronic hyperphagic models

The dorsomedial hypothalamus (DMH) has long been implicated in feeding behavior and thermogenesis. The DMH contains orexigenic neuropeptide Y (NPY) neurons, but the role of these neurons in the control of energy homeostasis is not well understood. NPY expression in the DMH is low under normal conditions in adult rodents but is significantly increased during chronic hyperphagic conditions such as lactation and diet-induced obesity (DIO). To understand better the role of DMH-NPY neurons, we characterized the efferent projections of DMH-NPY neurons using the anterograde tracer biotinylated dextran amine (BDA) in lactating rats and DIO mice. In both models, BDA- and NPY-colabeled fibers were limited mainly to the hypothalamus, including the paraventricular nucleus of the hypothalamus (PVH), lateral hypothalamus/perifornical area (LH/PFA), and anteroventral periventricular nucleus (AVPV). Specifically in lactating rats, BDA-and NPY-colabeled axonal swellings were in close apposition to cocaine- and amphetamine-regulated transcript (CART)-expressing neurons in the PVH and AVPV. Although the DMH neurons project to the rostral raphe pallidus (rRPa), these projections did not contain NPY immunoreactivity in either the lactating rat or the DIO mouse. Instead, the majority of BDA-labeled fibers in the rRPa were orexin positive. Furthermore, DMH-NPY projections were not observed within the nucleus of the solitary tract (NTS), another brainstem site critical for the regulation of sympathetic outflow. The present data suggest that NPY expression in the DMH during chronic hyperphagic conditions plays important roles in feeding behavior and thermogenesis by modulating neuronal functions within the hypothalamus, but not in the brainstem.

Afferent neuronal control of type-I gonadotropin releasing hormone neurons in the human

Understanding the regulation of the human menstrual cycle represents an important ultimate challenge of reproductive neuroendocrine research. However, direct translation of information from laboratory animal experiments to the human is often complicated by strikingly different and unique reproductive strategies and central regulatory mechanisms that can be present in even closely related animal species. In all mammals studied so far, type-I gonadotropin releasing hormone (GnRH) synthesizing neurons form the final common output way from the hypothalamus in the neuroendocrine control of the adenohypophysis. Under various physiological and pathological conditions, hormonal and metabolic signals either regulate GnRH neurons directly or act on upstream neuronal circuitries to influence the pattern of pulsatile GnRH secretion into the hypophysial portal circulation. Neuronal afferents to GnRH cells convey important metabolic-, stress-, sex steroid-, lactational-, and circadian signals to the reproductive axis, among other effects. This article gives an overview of the available neuroanatomical literature that described the afferent regulation of human GnRH neurons by peptidergic, monoaminergic, and amino acidergic neuronal systems. Recent studies of human genetics provided evidence that central peptidergic signaling by kisspeptins and neurokinin B (NKB) play particularly important roles in puberty onset and later, in the sex steroid-dependent feedback regulation of GnRH neurons. This review article places special emphasis on the topographic distribution, sexual dimorphism, aging-dependent neuroanatomical changes, and plastic connectivity to GnRH neurons of the critically important human hypothalamic kisspeptin and NKB systems.

Loss of NSCL-2 in gonadotropin releasing hormone neurons leads to reduction of pro-opiomelanocortin neurons in specific hypothalamic nuclei and causes visceral obesity

Regulation of sexual reproduction and energy homeostasis are closely interconnected, but only few efforts were made to explore the impact of gonadotropic neurons on metabolic processes. We have used Nscl-2 mutant mice suffering from adult onset of obesity and hypogonadotropic hypogonadism to study effects of gonadotropin releasing hormone (GnRH) neurons on neuronal circuits controlling energy balance. Inactivation of Nscl-2 in GnRH neurons but not in pro-opiomelanocortin (POMC) neurons reduced POMC neurons and increased visceral fat mass, suggesting a critical role of GnRH cells in the regulation of POMC neurons. In contrast, absence of POMC processing in the majority of Nscl-2-deficient POMC neurons had no effect on energy homeostasis. Finally, we investigated the cellular basis of the reduction of GnRH neurons in NSCL-2 mutants using a lineage tracing approach. We found that loss of Nscl-2 results in aberrant migration of GnRH neurons in Nscl-2 mutant mice causing a lineage switch of ectopically located GnRH neurons.

Cocaine- and amphetamine-regulated transcript is a potent stimulator of GnRH and kisspeptin cells and may contribute to negative energy balance-induced reproductive inhibition in females

Cocaine- and amphetamine-regulated transcript (CART) is a hypothalamic neuropeptide implicated in both metabolic and reproductive regulation, raising the possibility that CART plays a role in reproductive inhibition during negative metabolic conditions. The current study characterized CART's regulatory influence on GnRH and kisspeptin (Kiss1) cells and determined the sensitivity of different CART populations to negative energy balance. CART fibers made close appositions to 60% of GnRH cells, with the majority of the fibers (>80%) originating from the arcuate nucleus (ARH) CART/pro-opiomelanocortin population. Electrophysiological recordings in GnRH-green fluorescent protein rats demonstrated that CART postsynaptically depolarizes GnRH cells. CART fibers from the ARH were also observed in close contact with Kiss1 cells in the ARH and anteroventral periventricular nucleus (AVPV). Recordings in Kiss1-GFP mice demonstrated CART also postsynaptically depolarizes ARH Kiss1 cells, suggesting CART may act directly and indirectly, via Kiss1 populations, to stimulate GnRH neurons. CART protein and mRNA levels were analyzed in 2 models of negative energy balance: caloric restriction (CR) and lactation. Both CART mRNA levels and the number of CART-immunoreactive cells were suppressed in the ARH during CR but not during lactation. AVPV CART mRNA was suppressed during CR, but not during lactation when there was a dramatic increase in CART-immunoreactive cells. These data suggest differing regulatory signals of CART between the models. In conclusion, both morphological and electrophysiological methods identify CART as a novel and potent stimulator of Kiss1 and GnRH neurons and suppression of CART expression during negative metabolic conditions could contribute to inhibition of the reproductive axis.

Kisspeptin as a link between metabolism and reproduction: evidences from rodent and primate studies

Changes in metabolic status gate reproductive activity by still incompletely deciphered mechanisms. Many neuropeptides have been shown to be involved in restraining hypothalamic gonadotropin releasing hormone (GnRH) release under conditions of negative energy balance. Broadly, on the basis of their effect on feeding, these can be grouped as orexigenic and anorexigenic neuropeptides. Reciprocally correlated, in response to changes in systemic concentrations of metabolic hormones, the secretion of orexigenic neuropeptides increases while that of anorexigenic neuropeptides decreases during conditions of food restriction. Recently, kisspeptin signaling in hypothalamus has appeared as a pivotal regulator of the GnRH pulse generator. Kisspeptin apparently does not affect feeding, but in light of accumulating data, it has emerged as one of the major conduits in relaying body metabolic status information to GnRH neurons. The present review examines such data obtained from rodent and primate models, which suggest kisspeptin-Kiss1r signaling as a possible pathway providing a link between metabolism and reproduction.

Role of GnRH Neurons and Their Neuronal Afferents as Key Integrators between Food Intake Regulatory Signals and the Control of Reproduction

Reproductive function is regulated by a plethora of signals that integrate physiological and environmental information. Among others, metabolic factors are key components of this circuit since they inform about the propitious timing for reproduction depending on energy availability. This information is processed mainly at the hypothalamus that, in turn, modulates gonadotropin release from the pituitary and, thereby, gonadal activity. Metabolic hormones, such as leptin, insulin, and ghrelin, act as indicators of the energy status and convey this information to the reproductive axis regulating its activity. In this review, we will analyse the central mechanisms involved in the integration of this metabolic information and their contribution to the control of the reproductive function. Particular attention will be paid to summarize the participation of GnRH, Kiss1, NPY, and POMC neurons in this process and their possible interactions to contribute to the metabolic control of reproduction.

Direct regulation of GnRH neuron excitability by arcuate nucleus POMC and NPY neuron neuropeptides in female mice

Hypothalamic neuropeptide Y (NPY) and proopiomelanocortin (POMC) neurons act to sense and coordinate the brain's responses to metabolic cues. One neuronal network that is very sensitive to metabolic status is that controlling fertility. In this study, we investigated the impact of neuropeptides released by NPY and POMC neurons on the cellular excitability of GnRH neurons, the final output cells of the brain controlling fertility. The majority (∼70%) of GnRH neurons were activated by α-melanocyte-stimulating hormone, and this resulted from the direct postsynaptic activation of melanocortin receptor 3 and melanocortin receptor 4. A small population of GnRH neurons (∼15%) was excited by cocaine and amphetamine-regulated transcript or inhibited by β-endorphin. Agouti-related peptide, released by NPY neurons, was found to have variable inhibitory (∼10%) and stimulatory (∼25%) effects upon subpopulations of GnRH neurons. A variety of NPY and pancreatic polypeptide analogs was used to examine potential NPY interactions with GnRH neurons. Although porcine NPY (Y1/Y2/Y5 agonist) directly inhibited the firing of approximately 45% of GnRH neurons, [Leu(31),Pro(34)]-NPY (Y1/Y4/Y5 agonist) could excite (56%) or inhibit (19%). Experiments with further agonists indicated that Y1 receptors were responsible for suppressing GnRH neuron activity, whereas postsynaptic Y4 receptors were stimulatory. These results show that the activity of GnRH neurons is regulated in a complex manner by neuropeptides released by POMC and NPY neurons. This provides a direct route through which different metabolic cues can regulate fertility.

Neuroendocrine control of female reproductive function by the activin receptor ALK7

Activins are critical components of the signaling network that controls female reproduction. However, their roles in hypothalamus, and the specific functions of their different receptors, have not been elucidated. Here, we investigated the expression and function of the activin receptor ALK7 in the female reproductive axis using Alk7-knockout mice. ALK7 was found in subsets of SF1-expressing granulosa cells in the ovary, FSH gonadotrophs in the pituitary, and NPY-expressing neurons in the arcuate nucleus of the hypothalamus. Alk7-knockout females showed delayed onset of puberty and abnormal estrous cyclicity, had abnormal diestrous levels of FSH and LH in serum, and their ovaries showed premature depletion of follicles, oocyte degeneration, and impaired responses to exogenous gonadotropins. In the arcuate nucleus, mutant mice showed reduced expression of Npy mRNA and lower numbers of Npy-expressing neurons than wild-type controls. Alk7 knockouts showed a selective loss of arcuate NPY/AgRP innervation in the medial preoptic area, a key central regulator of reproduction. These results indicate that ALK7 is an important regulator of female reproductive function and reveal a new role for activin signaling in the control of hypothalamic gene expression and wiring. Alk7 gene variants may contribute to female reproductive disorders in humans, such as polycystic ovary syndrome.

Arcuate kisspeptin/neurokinin B/dynorphin (KNDy) neurons mediate the estrogen suppression of gonadotropin secretion and body weight

Estrogen withdrawal increases gonadotropin secretion and body weight, but the critical cell populations mediating these effects are not well understood. Recent studies have focused on a subpopulation of hypothalamic arcuate neurons that coexpress estrogen receptor α, neurokinin 3 receptor (NK(3)R), kisspeptin, neurokinin B, and dynorphin for the regulation of reproduction. To investigate the function of kisspeptin/neurokinin B/dynorphin (KNDy) neurons, a novel method was developed to ablate these cells using a selective NK(3)R agonist conjugated to the ribosome-inactivating toxin, saporin (NK(3)-SAP). Stereotaxic injections of NK(3)-SAP in the arcuate nucleus ablated KNDy neurons, as demonstrated by the near-complete loss of NK(3)R, NKB, and kisspeptin-immunoreactive (ir) neurons and depletion of the majority of arcuate dynorphin-ir neurons. Selectivity was demonstrated by the preservation of proopiomelanocortin, neuropeptide Y, and GnRH-ir elements in the arcuate nucleus and median eminence. In control rats, ovariectomy (OVX) markedly increased serum LH, FSH, and body weight, and these parameters were subsequently decreased by treatment with 17β-estradiol. KNDy neuron ablation prevented the rise in serum LH after OVX and attenuated the rise in serum FSH. KNDy neuron ablation did not completely block the suppressive effects of E(2) on gonadotropin secretion, a finding consistent with redundant pathways for estrogen negative feedback. However, regardless of estrogen status, KNDy-ablated rats had lower levels of serum gonadotropins compared with controls. Surprisingly, KNDy neuron ablation prevented the dramatic effects of OVX and 17β-estradiol (E(2)) replacement on body weight and abdominal girth. These data provide evidence that arcuate KNDy neurons are essential for tonic gonadotropin secretion, the rise in LH after removal of E(2), and the E(2) modulation of body weight.

Gonadotropin-releasing hormone 2 suppresses food intake in the zebrafish, Danio rerio

Gonadotropin-releasing hormone (GnRH) is an evolutionarily conserved neuropeptide with 10 amino acid residues, of which several structural variants exist. A molecular form known as GnRH2 ([His(5) Trp(7) Tyr(8)]GnRH, also known as chicken GnRH II) is widely distributed in vertebrates except for rodents, and has recently been implicated in the regulation of feeding behavior in goldfish. However, the influence of GnRH2 on feeding behavior in other fish has not yet been studied. In the present study, therefore, we investigated the role of GnRH2 in the regulation of feeding behavior in a zebrafish model, and examined its involvement in food intake after intracerebroventricular (ICV) administration. ICV injection of GnRH2 at 0.1 and 1 pmol/g body weight (BW) induced a marked decrease of food consumption in a dose-dependent manner during 30 min after feeding. Cumulative food intake was significantly decreased by ICV injection of GnRH2 at 1 pmol/g BW during the 30-min post-treatment observation period. The anorexigenic action of GnRH2 was completely blocked by treatment with the GnRH type I receptor antagonist Antide at 25 pmol/g BW. We also examined the effect of feeding condition on the expression level of the GnRH2 transcript in the hypothalamus. Levels of GnRH2 mRNA obtained from fish that had been provided excess food for 7 days were higher than those in fish that had been fed normally. These results suggest that, in zebrafish, GnRH2 acts as an anorexigenic factor, as is the case in goldfish.

Amino acid-dependent activation of liver estrogen receptor alpha integrates metabolic and reproductive functions via IGF-1

Throughout evolution, organisms have devised strategies to limit fertility in case of prolonged starvation. In mammals, the liver plays a central role in the orchestration of mechanisms allowing for the maintenance of energy homeostasis. We here demonstrate that dietary amino acids regulate the transcriptional activity of hepatic estrogen receptor alpha (ERα) through an mTOR-dependent mechanism. As a result of ERα activation, hepatic IGF-1 mRNA and blood IGF-1 are increased. Conversely, calorie restriction or selective ablation of ERα in the liver decrease blood IGF-1 to levels inadequate for the correct proliferation of the lumen epithelium in the uterus and the progression of the estrous cycle. We propose that the liver acts as critical mediator of energetic and reproductive functions responsible for the blockade of the estrous cycle in case of protein scarcity. Our findings may provide novel insights to understand the cause of selected forms of infertility and metabolic alterations in women after menopause.

Leptin differentially regulates NPY secretion in hypothalamic cell lines through distinct intracellular signal transduction pathways

Leptin acts as a key peripheral hormone in distinct neurons in the hypothalamus to modulate both reproductive function and energy homeostasis. The control of neuropeptide Y (NPY) secretion is an example of a process that can be differentially regulated by leptin. In order to further understand these distinct modulatory effects, we have used immortalized, neuronal hypothalamic cell lines expressing NPY, mHypoE-38 and mHypoE-46. We found that these cell lines express the endogenous leptin receptor, ObRb, and secrete detectable levels of NPY. We exposed the neurons to 100nM leptin for 1h and determined that the basal levels of NPY in the cell lines were differentially regulated: NPY secretion was inhibited in mHypoE-46 neurons, whereas NPY secretion was induced in the mHypoE-38 neurons. In order to determine the mechanisms involved in the divergent regulation of NPY release, we analyzed the activity of a number of signaling components using phospho-specific antibodies directed towards specific proteins in the MAP kinase, PI3K, and AMPK pathways, among others. We found that leptin activated a different combination of second messengers in each cell line. Importantly, we could link the regulation of NPY secretion to different signaling pathways, AMPK in the mHypoE-46 and both MAPK and PI3K in the mHypoE-38 neurons. This is the first demonstration that leptin can specifically regulate individual NPY neuron secretory responses through distinct signaling pathways.

Immortalized neurons for the study of hypothalamic function

The hypothalamus is a vital part of the central nervous system: it harbors control systems implicated in regulation of a wide range of homeostatic processes, including energy balance and reproduction. Structurally, the hypothalamus is a complex neuroendocrine tissue composed of a multitude of unique neuronal cell types that express a number of neuromodulators, including hormones, classical neurotransmitters, and specific neuropeptides that play a critical role in mediating hypothalamic function. However, neuropeptide and receptor gene expression, second messenger activation, and electrophysiological and secretory properties of these hypothalamic neurons are not yet fully defined, primarily because the heterogeneity and complex neuronal architecture of the neuroendocrine hypothalamus make such studies challenging to perform in vivo. To circumvent this problem, our research group recently generated embryonic- and adult-derived hypothalamic neuronal cell models by utilizing the novel molecular techniques of ciliary neurotrophic factor-induced neurogenesis and SV40 T antigen transfer to primary hypothalamic neuronal cell cultures. Significant research with these cell lines has demonstrated their value as a potential tool for use in molecular genetic analysis of hypothalamic neuronal function. Insights gained from hypothalamic immortalized cells used in conjunction with in vivo models will enhance our understanding of hypothalamic functions such as neurogenesis, neuronal plasticity, glucose sensing, energy homeostasis, circadian rhythms, and reproduction. This review discusses the generation and use of hypothalamic cell models to study mechanisms underlying the function of individual hypothalamic neurons and to gain a more complete understanding of the overall physiology of the hypothalamus.

Beyond Leptin: Emerging Candidates for the Integration of Metabolic and Reproductive Function during Negative Energy Balance

Reproductive status is tightly coupled to metabolic state in females, and ovarian cycling in mammals is halted when energy output exceeds energy input, a metabolic condition known as negative energy balance. This inhibition of reproductive function during negative energy balance occurs due to suppression of gonadotropin-releasing hormone (GnRH) release in the hypothalamus. The GnRH secretagogue kisspeptin is also inhibited during negative energy balance, indicating that inhibition of reproductive neuroendocrine circuits may occur upstream of GnRH itself. Understanding the metabolic signals responsible for the inhibition of reproductive pathways has been a compelling research focus for many years. A predominant theory in the field is that the status of energy balance is conveyed to reproductive neuroendocrine circuits via the adipocyte hormone leptin. Leptin is stimulatory for GnRH release and lower levels of leptin during negative energy balance are believed to result in decreased stimulatory drive for GnRH cells. However, recent evidence found that restoring leptin to physiological levels did not restore GnRH function in three different models of negative energy balance. This suggests that although leptin may be an important permissive signal for reproductive function as indicated by many years of research, factors other than leptin must critically contribute to negative energy balance-induced reproductive inhibition. This review will focus on emerging candidates for the integration of metabolic status and reproductive function during negative energy balance.

Kisspeptin directly regulates neuropeptide Y synthesis and secretion via the ERK1/2 and p38 mitogen-activated protein kinase signaling pathways in NPY-secreting hypothalamic neurons

Kisspeptin is a key component of reproduction that directly stimulates GnRH neurons. However, recent studies indicate that kisspeptin can indirectly stimulate GnRH neurons through unidentified afferent networks. Neuropeptide Y (NPY) is another key reproductive hormone that is an afferent stimulator of GnRH neurons. Herein, we report kisspeptin receptor Kiss1r mRNA expression in native NPY neurons FAC-sorted from NPY-GFP transgenic mice. Thus, we hypothesized that kisspeptin indirectly stimulates GnRH neurons through direct regulation of NPY neurons. Using hypothalamic NPY-secreting cell lines, we determined that kisspeptin stimulates NPY mRNA expression and secretion in the mHypoE-38 cells, but not the mHypoE-42 cells, using quantitative RT-PCR and enzyme immunoassays. Furthermore, agouti-related peptide, ghrelin, neurotensin, or Kiss1r mRNA expression was not changed upon exposure to kisspeptin in either cell line. These results concur with our previous work identifying the mHypoE-38 cell line as a putative reproductive NPY neuron and the mHypoE-42 cell line as a potential feeding-related NPY neuron. In the mHypoE-38 cells, kisspeptin activated the ERK1/2 and p38 MAPK kinases as shown by Western blot analysis. Moreover, inhibiting the ERK1/2 and p38 pathways with U0126 and SB239063, respectively, prevented kisspeptin induction of NPY mRNA expression and secretion. Altogether, we find that kisspeptin directly regulates NPY synthesis and secretion via the ERK1/2 and p38 MAPK pathways in a NPY-secreting cell line, and we propose NPY neurons as an afferent network by which kisspeptin indirectly stimulates GnRH secretion.

Kisspeptin directly excites anorexigenic proopiomelanocortin neurons but inhibits orexigenic neuropeptide Y cells by an indirect synaptic mechanism

The neuropeptide kisspeptin is necessary for reproduction, fertility, and puberty. Here, we show strong kisspeptin innervation of hypothalamic anorexigenic proopiomelanocortin (POMC) cells, coupled with a robust direct excitatory response by POMC neurons (n > 200) to kisspeptin, mediated by mechanisms based on activation of a sodium/calcium exchanger and possibly opening of nonselective cation channels. The excitatory actions of kisspeptin on POMC cells were corroborated with quantitative PCR data showing kisspeptin receptor GPR54 expression in the arcuate nucleus, and the attenuation of excitation by the selective kisspeptin receptor antagonist, peptide 234. In contrast, kisspeptin inhibits orexigenic neuropeptide Y (NPY) neurons through an indirect mechanism based on enhancing GABA-mediated inhibitory synaptic tone. In striking contrast, gonadotropin-inhibiting hormone (GnIH and RFRP-3) and NPY, also found in axons abutting POMC cells, inhibit POMC cells and attenuate the kisspeptin excitation by a mechanism based on opening potassium channels. Together, these data suggest that the two central peptides that regulate reproduction, kisspeptin and GnIH, exert a strong direct action on POMC neurons. POMC cells may hypothetically serve as a conditional relay station downstream of kisspeptin and GnIH to signal the availability of energy resources relevant to reproduction.

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

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

Neuropeptide Y directly inhibits neuronal activity in a subpopulation of gonadotropin-releasing hormone-1 neurons via Y1 receptors

Neuropeptide Y (NPY), a member of the pancreatic polypeptide family, is an orexigenic hormone. GnRH-1 neurons express NPY receptors. This suggests a direct link between metabolic function and reproduction. However, the effect of NPY on GnRH-1 cells has been variable, dependent on metabolic and reproductive status of the animal. This study circumvents these issues by examining the role of NPY on GnRH-1 neuronal activity in an explant model that is based on the extra-central nervous system origin of GnRH-1 neurons. These prenatal GnRH-1 neurons express many receptors found in GnRH-1 neurons in the brain and use similar transduction pathways. In addition, these GnRH-1 cells exhibit spontaneous and ligand-induced oscillations in intracellular calcium as well as pulsatile calcium-controlled GnRH-1 release. Single-cell PCR determined that prenatal GnRH-1 neurons express the G protein-coupled Y1 receptor (Y1R). To address the influence of NPY on GnRH-1 neuronal activity, calcium imaging was used to monitor individual and population dynamics. NPY treatment, mimicked with Y1R agonist, significantly decreased the number of calcium peaks per minute in GnRH-1 neurons and was prevented by a Y1R antagonist. Pertussis toxin blocked the effect of NPY on GnRH-1 neuronal activity, indicating the coupling of Y1R to inhibitory G protein. The NPY-induced inhibition was independent of the adenylate cyclase pathway but mediated by the activation of G protein-coupled inwardly rectifying potassium channels. These results indicate that at an early developmental stage, GnRH-1 neuronal activity can be directly inhibited by NPY via its Y1R.

Regulation of food intake and gonadotropin-releasing hormone/luteinizing hormone during lactation: role of insulin and leptin

Negative energy balance during lactation is reflected by low levels of insulin and leptin and is associated with chronic hyperphagia and suppressed GnRH/LH activity. We studied whether restoration of insulin and/or leptin to physiological levels would reverse the lactation-associated hyperphagia, changes in hypothalamic neuropeptide expression [increased neuropeptide Y (NPY) and agouti-related protein (AGRP) and decreased proopiomelanocortin (POMC), kisspeptin (Kiss1), and neurokinin B (NKB)] and suppression of LH. Ovariectomized lactating rats (eight pups) were treated for 48 h with sc minipumps containing saline, human insulin, or rat leptin. The arcuate nucleus (ARH) was analyzed for NPY, AGRP, POMC, Kiss1, and NKB mRNA expression; the dorsal medial hypothalamus (DMH) was analyzed for NPY mRNA. Insulin replacement reversed the increase in ARH NPY/AGRP mRNAs, partially recovered POMC, but had no effect on recovering Kiss1/NKB. Leptin replacement only affected POMC, which was fully recovered. Insulin/leptin dual replacement had similar effects as insulin replacement alone but with a slight increase in Kiss1/NKB. The lactation-induced increase in DMH NPY was unchanged after treatments. Restoration of insulin and/or leptin had no effect on food intake, body weight, serum glucose or serum LH. These results suggest that the negative energy balance of lactation is not required for the hyperphagic drive, although it is involved in the orexigenic changes in the ARH. The chronic hyperphagia of lactation is most likely sustained by the induction of NPY in the DMH. The negative energy balance also does not appear to be a necessary prerequisite for the suppression of GnRH/LH activity.

Innervation of gonadotropin-releasing hormone neurons by peptidergic neurons conveying circadian or energy balance information in the mouse

Background

Secretion of gonadotropin-releasing hormone (GnRH) produced in neurons in the basal forebrain is the primary regulator of reproductive maturation and function in mammals. Peptidergic signals relating to circadian timing and energy balance are an important influence on the reproductive axis. The aim of this study was to investigate the innervation of GnRH neurons by peptidergic neurons.

Methodology/Principal Findings

Immunohistochemistry and confocal microscopy were used to detect appositions of peptidergic fibers (NPY, β-endorphin, MCH) associated with energy balance and metabolic status in transgenic mice expressing a green fluorescent protein reporter construct in GnRH neurons. The frequency of these appositions was compared to those of vasoactive intestinal peptide (VIP), a hypothalamic neuropeptide likely to convey circadian timing information to the GnRH secretory system. The majority of GnRH neurons (73–87%) were closely apposed by fibers expressing NPY, β-endorphin, or MCH, and a significant proportion of GnRH neurons (28%) also had close contacts with VIP-ir fibers.

Conclusions/Significance

It is concluded that GnRH neurons in the mouse receive a high frequency of direct modulatory inputs from multiple hypothalamic peptide systems known to be important in conveying circadian information and signalling energy balance.

Suppression of basal spontaneous gonadotropin-releasing hormone neuronal activity during lactation: role of inhibitory effects of neuropeptide Y

Increased neuropeptide Y (NPY) activity drives the chronic hyperphagia of lactation and may contribute to the suppression of GnRH activity. The majority of GnRH neurons are contacted by NPY fibers, and GnRH cells express NPY Y5 receptor (Y5R). Therefore, NPY provides a neurocircuitry for information about food intake/energy balance to be directly transmitted to GnRH neurons. To investigate the effects of lactation on GnRH neuronal activity, hypothalamic slices were prepared from green fluorescent protein-GnRH transgenic rats. Extracellular loose-patch recordings determined basal GnRH neuronal activity from slices of ovariectomized control and lactating rats. Compared with controls, hypothalamic slices from lactating rats had double the number of quiescent GnRH neurons (14.51 +/- 2.86 vs. 7.04 +/- 2.84%) and significantly lower firing rates of active GnRH neurons (0.25 +/- 0.02 vs. 0.37 +/- 0.03 Hz). To study the NPY-postsynaptic Y5R system, whole-cell current-clamp recordings were performed in hypothalamic slices from control rats to examine NPY/Y5R antagonist effects on GnRH neuronal resting membrane potential. Under tetrodotoxin treatment, NPY hyperpolarized GnRH neurons from -56.7 +/- 1.94 to -62.1 +/- 1.83 mV; NPY's effects were blocked by Y5R antagonist. To determine whether increased endogenous NPY tone contributes to GnRH neuronal suppression during lactation, hypothalamic slices were treated with Y5R antagonist. A significantly greater percentage of GnRH cells were activated in slices from lactating rats (52%) compared with controls (28%). These results suggest that: 1) basal GnRH neuronal activity is suppressed during lactation; 2) NPY can hyperpolarize GnRH neurons via postsynaptic Y5R; and 3) increased inhibitory NPY tone during lactation is a component of the mechanisms responsible for suppression of GnRH neuronal activity.

Fasting-induced suppression of LH secretion does not require activation of ATP-sensitive potassium channels

Reproductive hormone secretions are inhibited by fasting and restored by feeding. Metabolic signals mediating these effects include fluctuations in serum glucose, insulin, and leptin. Because ATP-sensitive potassium (K(ATP)) channels mediate glucose sensing and many actions of insulin and leptin in neurons, we assessed their role in suppressing LH secretion during food restriction. Vehicle or a K(ATP) channel blocker, tolbutamide, was infused into the lateral cerebroventricle in ovariectomized mice that were either fed or fasted for 48 h. Tolbutamide infusion resulted in a twofold increase in LH concentrations in both fed and fasted mice compared with both fed and fasted vehicle-treated mice. However, tolbutamide did not reverse the suppression of LH in the majority of fasted animals. In sulfonylurea (SUR)1-null mutant (SUR1(-/-)) mice, which are deficient in K(ATP) channels, and their wild-type (WT) littermates, a 48-h fast was found to reduce serum LH concentrations in both WT and SUR(-/-) mice. The present study demonstrates that 1) blockade of K(ATP) channels elevates LH secretion regardless of energy balance and 2) acute fasting suppresses LH secretion in both SUR1(-/-) and WT mice. These findings support the hypothesis that K(ATP) channels are linked to the regulation of gonadotropin-releasing hormone (GnRH) release but are not obligatory for mediating the effects of fasting on GnRH/LH secretion. Thus it is unlikely that the modulation of K(ATP) channels either as part of the classical glucose-sensing mechanism or as a component of insulin or leptin signaling plays a major role in the suppression of GnRH and LH secretion during food restriction.

Absence of gonadotropin-releasing hormone 1 and Kiss1 activation in alpha-fetoprotein knockout mice: prenatal estrogens defeminize the potential to show preovulatory luteinizing hormone surges

Sex differences in gonadal function are driven by either cyclical (females) or tonic (males) hypothalamic GnRH1 release and, subsequently, gonadotrophin (LH and FSH) secretion from the pituitary. This sex difference seems to depend on the perinatal actions of gonadal hormones on the hypothalamus. We used alpha-fetoprotein (AFP) knockout mice (Afp(-/-)) to study the mechanisms by which estrogens affect the sexual differentiation of the GnRH1 system. Afp(-/-) mice lack the protective actions of AFP against estrogens circulating during embryonic development, leading to infertility probably due to a hypothalamic dysfunction. Therefore, we first determined whether Afp(-/-) females are capable of showing a steroid-induced preovulatory LH surge by FOS/GnRH1 immunohistochemistry and RIA of plasma LH levels. Because the KISS1/GPR54 system is a key upstream regulator of the GnRH1 system as well as being sexually dimorphic, we also analyzed whether Kisspeptin-10 neurons were activated in Afp(-/-) mice after treatment with estradiol and progesterone. We found that the GnRH1 and Kisspeptin-10 neuronal systems are defeminized in Afp(-/-) females because they did not show either steroid-induced LH surges or significant FOS/GnRH1 double labeling. Furthermore, Kisspeptin-10 immunoreactivity and neural activation, measured by the number of double-labeled FOS/Kisspeptin-10 cells, were lower in Afp(-/-) females, suggesting a down-regulation of GnRH1 function. Thus, the sex difference in the ability to show preovulatory LH surges depends on the prenatal actions of estrogens in the male hypothalamus and, thus, is lost in Afp(-/-) females because they lack AFP to protect them against the defeminizing effects of estrogens during prenatal development.

Hormonal regulation of clonal, immortalized hypothalamic neurons expressing neuropeptides involved in reproduction and feeding

The hypothalamus has been particularly difficult to study at the molecular level because of the inherent cellular heterogeneity and complexity of neuronal circuits within. We have generated a large number of immortalized, clonal cell lines through retroviral gene transfer of the oncogene SV40 T-Ag into primary murine hypothalamic neuronal cell cultures. A number of these neuronal cell lines express neuropeptides linked to the control of feeding behavior and reproduction, including neuropeptide Y (NPY) and neurotensin (NT). We review recent studies on the direct regulation of NPY gene expression by estrogen, and the leptin-mediated control of signal transduction pathways and NT transcription. These studies provide new insights into the direct control of neuropeptide synthesis by hormones and nutrients at a mechanistic level in the individual neuron, not yet possible in the whole brain. Using these novel cell models, we expect to contribute substantially to the understanding of how individual neuronal cell types control overall endocrine function, especially with regard to two of the most well-known roles of distinct peptidergic neurons; these being the control of reproduction and energy homeostasis.