Leptin indirectly regulates gonadotropin-releasing hormone neuronal function

Leptin receptor null mice with reexpression of LepR in GnRHR expressing cells display elevated FSH levels but remain in a prepubertal state

Leptin signals energy sufficiency to the reproductive hypothalamic-pituitary-gonadal (HPG) axis. Studies using genetic models have demonstrated that hypothalamic neurons are major players mediating these effects. Leptin receptor (LepR) is also expressed in the pituitary gland and in the gonads, but the physiological effects of leptin in these sites are still unclear. Female mice with selective deletion of LepR in a subset of gonadotropes show normal pubertal development but impaired fertility. Conditional deletion approaches, however, often result in redundancy or developmental adaptations, which may compromise the assessment of leptin's action in gonadotropes for pubertal maturation. To circumvent these issues, we adopted a complementary genetic approach and assessed if selective reexpression of LepR only in gonadotropes is sufficient to enable puberty and improve fertility of LepR null female mice. We initially assessed the colocalization of gonadotropin-releasing hormone receptor (GnRHR) and LepR in the HPG axis using GnRHR-IRES-Cre (GRIC) and LepR-Cre reporter (tdTomato or enhanced green fluorescent protein) mice. We found that GRIC and leptin-induced phosphorylation of STAT3 are expressed in distinct hypothalamic neurons. Whereas LepR-Cre was observed in theca cells, GRIC expression was rarely found in the ovarian parenchyma. In contrast, a subpopulation of gonadotropes expressed the LepR-Cre reporter gene (tdTomato). We then crossed the GRIC mice with the LepR null reactivable (LepR(loxTB)) mice. These mice showed an increase in FSH levels, but they remained in a prepubertal state. Together with previous findings, our data indicate that leptin-selective action in gonadotropes serves a role in adult reproductive physiology but is not sufficient to allow pubertal maturation in mice.

Leptin Responsive and GABAergic Projections to the Rostral Preoptic Area in Mice

The adipocyte-derived hormone leptin plays a critical role in the control of reproduction via signalling in hypothalamic neurones. The drivers of the hypothalamic-pituitary-gonadal axis, the gonadotrophin-releasing hormone (GnRH) neurones, do not have the receptors for leptin. Therefore, intermediate leptin responsive neurones must provide leptin-to-GnRH signalling. We investigated the populations of leptin responsive neurones that provide input to the rostral preoptic area (rPOA) where GnRH cell bodies reside. Fluorescent retrograde tracer beads (RetroBeads; Lumafluor Inc., Naples, FL, USA) were injected into the rPOA of transgenic leptin receptor enhanced green fluorescent protein (Lepr-eGFP) reporter mice. Uptake of the RetroBeads by Lepr-eGFP neurones was assessed throughout the hypothalamus. RetroBead uptake was most evident in the medial arcuate nucleus (ARC), the dorsomedial nucleus (DMN) and the ventral premammillary nucleus (PMV) of the hypothalamus. The uptake of RetroBeads specifically by Lepr-eGFP neurones was highest in the medial ARC (18% of tracer-labelled neurones Lepr-eGFP-positive). Because neurones that are both leptin responsive and GABAergic play a critical role in the regulation of fertility by leptin, we next focussed on the location of these populations. To address whether GABAergic neurones in leptin-responsive hypothalamic regions project to the rPOA, the experiment was repeated in GABA neurone reporter mice (Vgat-tdTomato). Between 10% and 45% of RetroBead-labelled neurones in the ARC were GABAergic, whereas uptake of tracer by GABAergic neurones in the DMN and PMV was very low (< 5%). These results show that both leptin responsive and GABAergic neurones from the ARC project to the region of the GnRH cell bodies. Our findings suggest that LEPR-expressing GABA neurones from the ARC may be mediators of leptin-to-GnRH signalling.

The Deep Correlation between Energy Metabolism and Reproduction: A View on the Effects of Nutrition for Women Fertility

In female mammals, mechanisms have been developed, throughout evolution, to integrate environmental, nutritional and hormonal cues in order to guarantee reproduction in favorable energetic conditions and to inhibit it in case of food scarcity. This metabolic strategy could be an advantage in nutritionally poor environments, but nowadays is affecting women's health. The unlimited availability of nutrients, in association with reduced energy expenditure, leads to alterations in many metabolic pathways and to impairments in the finely tuned inter-relation between energy metabolism and reproduction, thereby affecting female fertility. Many energetic states could influence female reproductive health being under- and over-weight, obesity and strenuous physical activity are all conditions that alter the profiles of specific hormones, such as insulin and adipokines, thus impairing women fertility. Furthermore, specific classes of nutrients might affect female fertility by acting on particular signaling pathways. Dietary fatty acids, carbohydrates, proteins and food-associated components (such as endocrine disruptors) have per se physiological activities and their unbalanced intake, both in quantitative and qualitative terms, might impair metabolic homeostasis and fertility in premenopausal women. Even though we are far from identifying a "fertility diet", lifestyle and dietary interventions might represent a promising and invaluable strategy to manage infertility in premenopausal women.

Leptin and its potential interest in assisted reproduction cycles

BACKGROUND

Leptin, an adipose hormone, has been shown to control energy homeostasis and food intake, and exert many actions on female reproductive function. Consequently, this adipokine is a pivotal factor in studies conducted on animal models and humans to decipher the mechanisms behind the infertility often observed in obese women.

METHODS

A systematic PubMed search was conducted on all articles, published up to January 2015 and related to leptin and its actions on energy balance and reproduction, using the following key words: leptin, reproduction, infertility, IVF and controlled ovarian stimulation. The available literature was reviewed in order to provide an overview of the current knowledge on the physiological roles of leptin, its involvement in female reproductive function and its potential interest as a prognostic marker in IVF cycles.

RESULTS

Animal and human studies show that leptin communicates nutritional status to the central nervous system and emerging evidence has demonstrated that leptin is involved in the control of reproductive functions by acting both directly on the ovaries and indirectly on the central nervous system. With respect to the clinical use of leptin as a biomarker in IVF cycles, a systematic review of the literature suggested its potential interest as a predictor of IVF outcome, as high serum and/or follicular fluid leptin concentrations have correlated negatively with cycle outcome. However, these preliminary results remain to be confirmed.

CONCLUSION

Leptin regulates energy balance and female reproductive function, mainly through its action on hypothalamic-pituitary-ovarian function, whose molecular and cellular aspects are progressively being deciphered. Preliminary studies evaluating leptin as a biomarker in human IVF seem promising but need further confirmation.

Congenital Hypogonadotropic Hypogonadism and Kallmann Syndrome: Past, Present, and Future

The proper development and coordination of the hypothalamic-pituitary-gonadal (HPG) axis are essential for normal reproductive competence. The key factor that regulates the function of the HPG axis is gonadotrophin-releasing hormone (GnRH). Timely release of GnRH is critical for the onset of puberty and subsequent sexual maturation. Misregulation in this system can result in delayed or absent puberty and infertility. Congenital hypogonadotropic hypogonadism (CHH) and Kallmann syndrome (KS) are genetic disorders that are rooted in a GnRH deficiency but often accompanied by a variety of non-reproductive phenotypes such as the loss of the sense of smell and defects of the skeleton, eye, ear, kidney, and heart. Recent progress in DNA sequencing technology has produced a wealth of information regarding the genetic makeup of CHH and KS patients and revealed the resilient yet complex nature of the human reproductive neuroendocrine system. Further research on the molecular basis of the disease and the diverse signal pathways involved will aid in improving the diagnosis, treatment, and management of CHH and KS patients as well as in developing more precise genetic screening and counseling regime.

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

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

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.

Effects of leptin on sperm count and morphology in Sprague-Dawley rats and their reversibility following a 6-week recovery period

Altered epididymal sperm count and morphology following leptin treatment has been reported recently. This study examined the effects of 42 days of leptin treatment on sperm count and morphology and their reversibility during a subsequent 56-day recovery period. Twelve-week-old male Sprague-Dawley rats were randomised into four leptin and four saline-treated control groups (n = 6). Intraperitoneal injections of leptin were given daily (60 μg Kg(-1) body weight) for 42 days. Controls received 0.1 ml of 0.9% saline. Leptin-treated animals and their respective age-matched controls were euthanised on either day 1, 21, 42 or 56 of recovery for collection of epididymal spermatozoa. Sperm concentration was determined using a Makler counting chamber. Spermatozoa were analysed for 8-hydroxy-2-deoxyguanosine and DNA fragmentation (Comet assay). Data were analysed using anova. Sperm concentration was significantly lower but fraction of abnormal spermatozoa, and levels of 8-hydroxy-2-deoxyguanosine were significantly higher in leptin-treated rats on day 1 of recovery. Comet assays revealed significant DNA fragmentation in leptin-treated rats. These differences were reduced by day 56 of recovery. It appears that 42 days of leptin treatment to Sprague-Dawley rats has significant adverse effects on sperm count and morphology that reverse following discontinuation of leptin treatment.

Expert consensus document: European Consensus Statement on congenital hypogonadotropic hypogonadism--pathogenesis, diagnosis and treatment

Congenital hypogonadotropic hypogonadism (CHH) is a rare disorder caused by the deficient production, secretion or action of gonadotropin-releasing hormone (GnRH), which is the master hormone regulating the reproductive axis. CHH is clinically and genetically heterogeneous, with >25 different causal genes identified to date. Clinically, the disorder is characterized by an absence of puberty and infertility. The association of CHH with a defective sense of smell (anosmia or hyposmia), which is found in ∼50% of patients with CHH is termed Kallmann syndrome and results from incomplete embryonic migration of GnRH-synthesizing neurons. CHH can be challenging to diagnose, particularly when attempting to differentiate it from constitutional delay of puberty. A timely diagnosis and treatment to induce puberty can be beneficial for sexual, bone and metabolic health, and might help minimize some of the psychological effects of CHH. In most cases, fertility can be induced using specialized treatment regimens and several predictors of outcome have been identified. Patients typically require lifelong treatment, yet ∼10-20% of patients exhibit a spontaneous recovery of reproductive function. This Consensus Statement summarizes approaches for the diagnosis and treatment of CHH and discusses important unanswered questions in the field.

Role of the adipocyte-derived hormone leptin in reproductive control

Achievement of sexual maturation and maintenance of fertility in adulthood are functions that are sensitive to the metabolic status of the organism, particularly the magnitude of fat reserves. In this sense, the adipocyte-derived hormone, leptin, plays a major role in linking metabolic cues and the control of multiple neuroendocrine axes. The hypothalamus is a key site mediating leptin actions, including those involved in the modulation of the hypothalamus-pituitary-gonads (HPG) axis at different stages of development and in different environmental conditions. In the present review, we provide an update of the role of leptin in reproduction and discuss its interactions with neurons, neurotransmitters and downstream targets of the reproductive axis, with a special emphasis on the actions of leptin in the central nervous system. We hope this review will contribute to the understanding of the mechanisms whereby metabolic signals, especially leptin, influence the reproductive neuroendocrine axis modulating its activity in different nutritional states. Special attention will be given to recent advances in the identification of key hypothalamic sites and signaling pathways relevant to leptin's action in reproductive control.

Leptin and the brain

Leptin, which comes from the Greek root leptos meaning thin, has been the focus of intense investigation since its discovery in 1994. This hormone belongs to the cytokine family and is produced by adipocytes and circulates in proportion to fat mass, thus serving as a satiety signal and informing central metabolic control centers as to the status of peripheral energy stores. However, it participates in numerous other functions both peripherally and centrally, as indicated by the wide distribution of its various receptor isoforms. Leptin is involved in brain development, most notably in development of hypothalamic centers that control metabolism, but also in other brain areas. It acts as a nutritional cue to indicate adequacy of energy stores for pubertal development and reproductive capacity. The effects of this hormone on behavior and cognition are less well studied, but it clearly is involved in specific aspects of these physiological phenomena. As obesity is a major health problem in many areas of the world, the search for pharmacological treatments to decrease appetite and increase energy expenditure is intense. Understanding the mechanisms of actions of all physiological effects of this hormone is of great interest in the pursuit of such treatment.

Insulin and Leptin Signaling Interact in the Mouse Kiss1 Neuron during the Peripubertal Period

Reproduction requires adequate energy stores for parents and offspring to survive. Kiss1 neurons, which are essential for fertility, have the potential to serve as the central sensors of metabolic factors that signal to the reproductive axis the presence of stored calories. Paradoxically, obesity is often accompanied by infertility. Despite excess circulating levels of insulin and leptin, obese individuals exhibit resistance to both metabolic factors in many neuron types. Thus, resistance to insulin or leptin in Kiss1 neurons could lead to infertility. Single deletion of the receptors for either insulin or the adipokine leptin from Kiss1 neurons does not impair adult reproductive dysfunction. However, insulin and leptin signaling pathways may interact in such a way as to obscure their individual functions. We hypothesized that in the presence of genetic or obesity-induced concurrent insulin and leptin resistance, Kiss1 neurons would be unable to maintain reproductive function. We therefore induced a chronic hyperinsulinemic and hyperleptinemic state in mice lacking insulin receptors in Kiss1 neurons through high fat feeding and examined the impact on fertility. In an additional, genetic model, we ablated both leptin and insulin signaling in Kiss1 neurons (IR/LepRKiss mice). Counter to our hypothesis, we found that the addition of leptin insensitivity did not alter the reproductive phenotype of IRKiss mice. We also found that weight gain, body composition, glucose and insulin tolerance were normal in mice of both genders. Nonetheless, leptin and insulin receptor deletion altered pubertal timing as well as LH and FSH levels in mid-puberty in a reciprocal manner. Our results confirm that Kiss1 neurons do not directly mediate the critical role that insulin and leptin play in reproduction. However, during puberty kisspeptin neurons may experience a critical window of susceptibility to the influence of metabolic factors that can modify the onset of fertility.

The involvement of gonadotropin inhibitory hormone and kisspeptin in the metabolic regulation of reproduction

Recently, kisspeptin (KP) and gonadotropin inhibitory hormone (GnIH), two counteracting neuropeptides, have been acknowledged as significant regulators of reproductive function. KP stimulates reproduction while GnIH inhibits it. These two neuropeptides seem to be pivotal for the modulation of reproductive activity in response to internal and external cues. It is well-documented that the current metabolic status of the body is closely linked to its reproductive output. However, how reproductive function is regulated by the body's energy status is less clear. Recent studies have suggested an active participation of hypothalamic KP and GnIH in the modulation of reproductive function according to available metabolic cues. Expression of KISS1, the KP encoding gene, is decreased while expression of RFRP (NPVF), the gene encoding GnIH, is increased in metabolic deficiency conditions. The lower levels of KP, as suggested by a decrease in KISS1 gene mRNA expression, during metabolic deficiency can be corrected by administration of exogenous KP, which leads to an increase in reproductive hormone levels. Likewise, administration of RF9, a GnIH receptor antagonist, can reverse the inhibitory effect of fasting on testosterone in monkeys. Together, it is likely that the integrated function of both these hypothalamic neuropeptides works as a reproductive output regulator in response to a change in metabolic status. In this review, we have summarized literature from nonprimate and primate studies that demonstrate the involvement of KP and GnIH in the metabolic regulation of reproduction.

TRAP-seq defines markers for novel populations of hypothalamic and brainstem LepRb neurons

OBJECTIVE

Leptin acts via its receptor (LepRb) on multiple subpopulations of LepRb neurons in the brain, each of which controls specific aspects of energy balance. Despite the importance of LepRb-containing neurons, the transcriptome and molecular identity of many LepRb subpopulations remain undefined due to the difficulty of studying the small fraction of total cells represented by LepRb neurons in heterogeneous brain regions. Here we sought to examine the transcriptome of LepRb neurons directly and identify markers for functionally relevant LepRb subsets.

METHODS

We isolated mRNA from mouse hypothalamic and brainstem LepRb cells by Translating Ribosome Affinity Purification (TRAP) and analyzed it by RNA-seq (TRAP-seq).

RESULTS

TRAP mRNA from LepRb cells was enriched for markers of peptidergic neurons, while TRAP-depleted mRNA from non-LepRb cells was enriched for markers of glial and immune cells. Genes encoding secreted proteins that were enriched in hypothalamic and brainstem TRAP mRNA revealed subpopulations of LepRb neurons that contained neuropeptide-encoding genes (including prodynorphin, Pdyn) not previously used as functional markers for LepRb neurons. Furthermore, Pdyn (cre) -mediated ablation of Lepr (flox) in Pdyn-expressing neurons (LepRb (Pdyn) KO mice) blunted energy expenditure to promote obesity during high-fat feeding.

CONCLUSIONS

TRAP-seq of CNS LepRb neurons defines the LepRb neuron transcriptome and reveals novel markers for previously unrecognized subpopulations of LepRb neurons.

Physiology of leptin: energy homeostasis, neuroendocrine function and metabolism

Leptin is secreted by adipose tissue and regulates energy homeostasis, neuroendocrine function, metabolism, immune function and other systems through its effects on the central nervous system and peripheral tissues. Leptin administration has been shown to restore metabolic and neuroendocrine abnormalities in individuals with leptin-deficient states, including hypothalamic amenorrhea and lipoatrophy. In contrast, obese individuals are resistant to leptin. Recombinant leptin is beneficial in patients with congenital leptin deficiency or generalized lipodystrophy. However, further research on molecular mediators of leptin resistance is needed for the development of targeted leptin sensitizing therapies for obesity and related metabolic diseases.

Leptin: A hormone linking activation of neuroendocrine axes with neuropathology

Leptin, a peptide hormone secreted by adipocytes, plays a central role in controlling appetite and weight in both rodents and humans. Basic science and clinical research suggest that this hormone not only affects the regulation of the neuroendocrine axes, but also exerts effects on the central nervous system with subsequent alterations in psychological functions. For instance, leptin suppresses cortisol secretion during stress-related activation of the adrenal axis. As psychiatric disorders like depression are associated with hypercortisolism, leptin is proposed to exert anti-depressant-like effects due to its inhibition of chronically overactive hypothalamo-pituitary-adrenal axis function. Moreover, leptin status of depressed patients could serve as a prognostic marker for therapy response. Besides its influence on neuroendocrine pathways leptin seems to have direct central effects on brain development and neuroplasticity. Low leptin levels have been shown to be associated with increased risk of developing dementia, supporting the idea of a pro-cognitive effect of leptin. These areas may have direct clinical implications and deserve to be studied further in the future.

Roles of leptin in reproduction, pregnancy and polycystic ovary syndrome: consensus knowledge and recent developments

As an essential function for perpetuation of species, reproduction, including puberty onset, is sensitive to the size of body energy stores and the metabolic state of the organism. Accordingly, impaired energy homeostasis, ranging from extreme leanness, such as in anorexia or cachexia, to morbid obesity has an impact on the timing of puberty and is often associated to fertility problems. The neuroendocrine basis for such phenomenon is the close connection between numerous metabolic hormones and nutritional cues with the various elements of the so-called hypothalamic-pituitary-gonadal (HPG) axis. Yet, despite previous fragmentary knowledge, it was only the discovery of the adipose-hormone, leptin, in 1994 what revolutionized our understanding on how metabolic and reproductive systems closely interplay and allowed the definition of the neurohormonal causes of perturbations of puberty and fertility in conditions of impaired body energy homeostasis. In this article, we aim to provide a synoptic view of the mechanisms whereby leptin engages in the regulation of different elements of the HPG axis, with special attention to its effects and mechanisms of action on the different elements of the reproductive brain and its proven direct effects in the gonads. In addition, we will summarize the state-of-the-art regarding the putative roles of leptin during gestation, including its potential function as placental hormone. Finally, comments will be made on the eventual leptin alterations in reproductive disorders, with special attention to the polycystic ovary syndrome (PCOS), a disease in which reproductive, metabolic and neuroendocrine alterations are commonly observed. All in all, we intend to provide an updated account of our knowledge on the physiological roles of leptin in the metabolic regulation of the reproductive axis and its eventual pathophysiological implications in prevalent reproductive disorders, such as PCOS.

Common features and differences of the hypothalamic-pituitary-gonadal axis in male and female

Male and female reproductive axis, comprised of hypothalamus, pituitary and gonads, present common features and differences, discussed in this review. These include the way hypothalamus regulates pituitary function, and the way pituitary, in turn, affects gonadal function. Finally, age plays an important role in axis regulation, in both genders.

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.

The role of GABA in the regulation of GnRH neurons

Gonadotropin-releasing hormone (GnRH) neurons form the final common pathway for the central regulation of reproduction. Gamma-amino butyric acid (GABA) has long been implicated as one of the major players in the regulation of GnRH neurons. Although GABA is typically an inhibitory neurotransmitter in the mature adult central nervous system, most mature GnRH neurons show the unusual characteristic of being excited by GABA. While many reports have provided much insight into the contribution of GABA to the activity of GnRH neurons, the precise physiological role of the excitatory action of GABA on GnRH neurons remains elusive. This brief review presents the current knowledge of the role of GABA signaling in GnRH neuronal activity. We also discuss the modulation of GABA signaling by neurotransmitters and neuromodulators and the functional consequence of GABAergic inputs to GnRH neurons in both the physiology and pathology of reproduction.

Connecting metabolism and reproduction: roles of central energy sensors and key molecular mediators

It is well established that pubertal activation of the reproductive axis and maintenance of fertility are critically dependent on the magnitude of body energy reserves and the metabolic state of the organism. Hence, conditions of impaired energy homeostasis often result in deregulation of puberty and reproduction, whereas gonadal dysfunction can be associated with the worsening of the metabolic profile and, eventually, changes in body weight. While much progress has taken place in our knowledge about the neuroendocrine mechanisms linking metabolism and reproduction, our understanding of how such dynamic interplay happens is still incomplete. As paradigmatic example, much has been learned in the last two decades on the reproductive roles of key metabolic hormones (such as leptin, insulin and ghrelin), their brain targets and the major transmitters and neuropeptides involved. Yet, the molecular mechanisms whereby metabolic information is translated and engages into the reproductive circuits remain largely unsolved. In this work, we will summarize recent developments in the characterization of the putative central roles of key cellular energy sensors, such as mTOR, in this phenomenon, and will relate these with other molecular mechanisms likely contributing to the brain coupling of energy balance and fertility. In doing so, we aim to provide an updated view of an area that, despite still underdeveloped, may be critically important to fully understand how reproduction and metabolism are tightly connected in health and disease.

The regulation of reproductive neuroendocrine function by insulin and insulin-like growth factor-1 (IGF-1)

The mammalian reproductive hormone axis regulates gonadal steroid hormone levels and gonadal function essential for reproduction. The neuroendocrine control of the axis integrates signals from a wide array of inputs. The regulatory pathways important for mediating these inputs have been the subject of numerous studies. One class of proteins that have been shown to mediate metabolic and growth signals to the CNS includes Insulin and IGF-1. These proteins are structurally related and can exert endocrine and growth factor like action via related receptor tyrosine kinases. The role that insulin and IGF-1 play in controlling the hypothalamus and pituitary and their role in regulating puberty and nutritional control of reproduction has been studied extensively. This review summarizes the in vitro and in vivo models that have been used to study these neuroendocrine structures and the influence of these growth factors on neuroendocrine control of reproduction.

20 years of leptin: role of leptin in human reproductive disorders

Leptin, as a key hormone in energy homeostasis, regulates neuroendocrine function, including reproduction. It has a permissive role in the initiation of puberty and maintenance of the hypothalamic-pituitary-gonadal axis. This is notable in patients with either congenital or acquired leptin deficiency from a state of chronic energy insufficiency. Hypothalamic amenorrhea is the best-studied, with clinical trials confirming a causative role of leptin in hypogonadotropic hypogonadism. Implications of leptin deficiency have also emerged in the pathophysiology of hypogonadism in type 1 diabetes. At the other end of the spectrum, hyperleptinemia may play a role in hypogonadism associated with obesity, polycystic ovarian syndrome, and type 2 diabetes. In these conditions of energy excess, mechanisms of reproductive dysfunction include central leptin resistance as well as direct effects at the gonadal level. Thus, reproductive dysfunction due to energy imbalance at both ends can be linked to leptin.

The hypothalamic ventral premammillary nucleus: A key site in leptin's regulation of reproduction

Reproduction is an energy-expensive process that relies on indicators of energy availability to adjust its proper functioning. The adipokine leptin provides one such metabolic signal, with leptin receptor-expressing neurons at sites widespread within the CNS, including regions associated with the neuroendocrine reproductive axis. One substantial population lies within the hypothalamic ventral premammillary nucleus (PMv), a region itself linked to reproductive control, which may provide a strategic site for the integration of energy availability, sensory and gonadal cues. Here we review our current understanding of leptin and PMv regulation of reproduction, including emerging details about intracellular mechanisms of leptin action at this site.

Prenatal androgen excess enhances stimulation of the GNRH pulse in pubertal female rats

In adolescent girls with polycystic ovary syndrome (PCOS), neuroendocrine derangements manifest after the onset of puberty, characterized by rapid LH pulse frequency. The early mechanism underlying the pubertal regulation of the GNRH/LH pulsatile release in adolescents with PCOS remains uncertain. To determine the effects of prenatal androgen exposure on the activation of GNRH neurons and generation of LH pulse at puberty, we administrated 5α-dihydrotestosterone to pregnant rats and observed serum LH levels and expression of hypothalamic genes in female offspring from postnatal 4 to 8 weeks. The 6-week-old prenatally androgenized (PNA) female rats exhibited an increase in LH pulse frequency. The hypothalamic expression of neurokinin B (Nkb (Tac2)) and Lepr mRNA levels in PNA rats increased remarkably before puberty and remained high during puberty, whereas elevated Kiss1 mRNA levels were detected only after the onset of puberty. Exogenous kisspeptin, NK3R agonist, and leptin triggered tonic stimulation of GNRH neurons and increased LH secretion in 6-week-old PNA rats. Leptin upregulated Kiss1 mRNA levels in the hypothalamus of pubertal PNA rats; however, pretreatment with a kisspeptin antagonist failed to suppress the elevated serum LH stimulated by leptin, indicating that the stimulatory effects of leptin may be conveyed indirectly to GNRH neurons via other neural components within the GNRH neuronal network, rather than through the kisspeptin-GPR54 pathway. These findings validate the hypotheses that NKB and leptin play an essential role in the activation of GNRH neurons and initiation of increased LH pulse frequency in PNA female rats at puberty and that kisspeptin may coordinate their stimulatory effects on LH release.

Evidence that insulin signalling in gonadotrophin-releasing hormone and kisspeptin neurones does not play an essential role in metabolic regulation of fertility in mice

Insulin in the brain plays an important role in regulating reproductive function, as demonstrated via conditional brain-specific insulin receptor (Insr) deletion (knockout). However, the specific neuronal target cells mediating the central effects of insulin on the reproductive axis remain unidentified. We first investigated whether insulin can act via direct effects on gonadotrophin-releasing hormone (GnRH) neurones. After clearly detecting Insr mRNA in an immunopurified GnRH cell fraction, we confirmed the presence of insulin receptor protein (InsR) in approximately 82% of GnRH neurones using dual-label immunohistochemistry. However, we did not observe any insulin-induced phospho-Akt (pAkt) or phospho-extracellular-signal-regulated kinase 1/2 in GnRH neurones, and therefore we investigated whether insulin signals via kisspeptin neurones to modulate GnRH release. Using dual-label immunohistochemistry, InsRs were detected only in approximately 5% of kisspeptin-immunoreactive cells. Insulin-induced pAkt was not observed in any kisspeptin-immunoreactive cells in either the rostral periventricular region of the third ventricle or arcuate nucleus in response to 200 mU of insulin treatment, although a more pharmacological dose (10 U) induced pronounced (> 20%) pAkt-kisspeptin coexpression in both regions. To confirm that insulin signalling via kisspeptin neurones does not critically modulate reproductive function, we generated kisspeptin-specific InsR knockout (KIRKO) mice and assessed multiple reproductive and metabolic parameters. No significant differences in puberty onset, oestrous cyclicity or reproductive competency were observed in the female or male KIRKO mice compared to their control littermates. However, significantly decreased fasting insulin (P < 0.05) and a nonsignificant trend towards reduced body weight were observed in male KIRKO mice. Thus, InsR signalling in kisspeptin cells is not critical for puberty onset or reproductive competency, although it may have a small metabolic effect in males.

Leptin-responsive GABAergic neurons regulate fertility through pathways that result in reduced kisspeptinergic tone

The adipocyte-derived hormone leptin plays a critical role in the central transmission of energy balance to modulate reproductive function. However, the neurocircuitry underlying this interaction remains elusive, in part due to incomplete knowledge of first-order leptin-responsive neurons. To address this gap, we explored the contribution of predominantly inhibitory (GABAergic) neurons versus excitatory (glutamatergic) neurons in the female mouse by selective ablation of the leptin receptor in each neuronal population: Vgat-Cre;Lepr(lox/lox) and Vglut2-Cre;Lepr(lox/lox) mice, respectively. Female Vgat-Cre;Lepr(lox/lox) but not Vglut2-Cre;Lepr(lox/lox) mice were obese. Vgat-Cre;Lepr(lox/lox) mice had delayed or absent vaginal opening, persistent diestrus, and atrophic reproductive tracts with absent corpora lutea. In contrast, Vglut2-Cre;Lepr(lox/lox) females exhibited reproductive maturation and function comparable to Lepr(lox/lox) control mice. Intracerebroventricular administration of kisspeptin-10 to Vgat-Cre;Lepr(lox/lox) female mice elicited robust gonadotropin responses, suggesting normal gonadotropin-releasing hormone neuronal and gonadotrope function. However, adult ovariectomized Vgat-Cre;Lepr(lox/lox) mice displayed significantly reduced levels of Kiss1 (but not Tac2) mRNA in the arcuate nucleus, and a reduced compensatory luteinizing hormone increase compared with control animals. Estradiol replacement after ovariectomy inhibited gonadotropin release to a similar extent in both groups. These animals also exhibited a compromised positive feedback response to sex steroids, as shown by significantly lower Kiss1 mRNA levels in the AVPV, compared with Lepr(lox/lox) mice. We conclude that leptin-responsive GABAergic neurons, but not glutamatergic neurons, act as metabolic sensors to regulate fertility, at least in part through modulatory effects on kisspeptin neurons.

Chemical identity of hypothalamic neurons engaged by leptin in reproductive control

The adipocyte-derived hormone leptin plays a critical role as a metabolic cue for the reproductive system. Conditions of low leptin levels observed in negative energy balance and loss-of-function mutations of leptin or leptin receptor genes are characterized by decreased fertility. In recent years, advances have been made for identifying possible hypothalamic neurons relaying leptin's neuroendocrine control of reproductive function. Studies from different laboratories have demonstrated that leptin action in the hypothalamo-pituitary-gonadal (HPG) axis is exerted via hypothalamic interneurons regulating gonadotropin-releasing hormone (GnRH) cells, oppose to direct action on GnRH neurons. Following this observation, studies focused on identifying leptin responsive interneurons. Using a Cre-loxP system to re-express or delete the leptin receptor long form (LepRb) from kisspeptin neurons, our laboratory found that leptin's action on kiss1 cells is neither required nor sufficient for leptin's role in reproductive function. Endogenous re-expression of LepRb however, in glutamatergic neurons of the ventral premammilary nucleus (PMV) or ablation of agouti-related protein (AgRP) neurons from leptin signaling-deficient mice are both sufficient to induce puberty and improve fertility. Recent studies have also shown that leptin action in first order GABAergic neurons is required for fertility. Together, these studies begin to delineate key neuronal populations involved in leptin's action in reproduction. In this review, we discuss recent advances made in the field and highlight the questions yet to be answered.

An updated view of leptin on implantation and pregnancy: a review

The hormone leptin, which is thought to be primarily produced by adipose tissue, is a polypeptide that was initially characterized by its ability to regulate food intake and energy metabolism. Leptin appears to signal the status of body energy stores to the brain, resulting in the regulation of food intake and whole-body energy expenditure. Subsequently, it was recognized as a cytokine with a wide range of peripheral actions and is involved in the regulation of a number of physiological systems including reproduction. In the fed state, leptin circulates in the plasma in proportion to body adiposity in all species studied to date. However other factors such as sex, age, body mass index (BMI), sex steroids and pregnancy may also affect leptin levels in plasma. In pregnant mice and humans, the placenta is also a major site of leptin expression. Leptin circulates in biological fluids both as free protein and in a form that is bound to the soluble isoform of its receptor or other binding proteins such as one of the immunoglobulin superfamily members Siglec-6 (OB-BP1). Although the actions of leptin in the control of reproductive function are thought to be exerted mainly via the hypothalamic-pituitary-gonadal axis, there have also been reports of local direct effects of leptin at the peripheral level, however, these data appear contradictory. Therefore, there is a need to summarize the current status of research outcomes and analyze the possible reasons for differing results and thus provide researchers with new insight in designing experiments to investigate leptin effect on reproduction. Most importantly, our recent experimental data suggesting that reproductive performance is improved by decreasing concentrations of peripheral leptin was unexpected and cannot be explained by hypotheses drawn from the experiments of excessive exogenous leptin administration to normal animals or ob/ob mice.

Reproduction Symposium: hypothalamic neuropeptides and the nutritional programming of puberty in heifers

Nutrition during the juvenile period has a major impact on timing reproductive maturity in heifers. Restricted growth delays puberty, whereas elevated BW gain advances the onset of puberty. The initiation of high-frequency episodic release of GnRH and, consequently, LH during the peripubertal period is crucial for maturation of the reproductive axis and establishment of normal estrous cycles. Nutritional signals are perceived by metabolic-sensing cells in the hypothalamus, which interact with estradiol-receptive neurons to regulate the secretory activity of GnRH neurons. The orexigenic peptide, neuropeptide Y (NPY), and the anorexigenic peptide derived from the proopiomelanocortin (POMC) gene, melanocyte-stimulating hormone α (αMSH), are believed to be major afferent pathways that transmit inhibitory (NPY) and excitatory (αMSH) inputs to GnRH neurons. The neuropeptide kisspeptin is considered a major stimulator of GnRH secretion and has been shown to mediate estradiol's effect on GnRH neuronal activity. Kisspeptin may also integrate the neuronal pathways mediating the metabolic and gonadal steroid hormone control of gonadotropin secretion. Recent studies in our laboratories indicate that functional and structural changes in the pathways involving NPY, POMC, and kisspeptin neurons occur in response to high rates of BW gain during the juvenile period in heifers. Changes include regulation of expression in NPY, POMC, and KISS1 and plasticity in the neuronal projections to GnRH neurons and within the neuronal network comprising these cells. Moreover, an intricate pattern of differential gene expression in the arcuate nucleus of the hypothalamus occurs in response to feeding high concentrate diets that promote elevated BW gain. Genes involved include those controlling feeding intake and cell metabolism, neuronal growth and remodeling, and synaptic transmission. Characterizing the cellular pathways and molecular networks involved in the mechanisms that control the timing of pubertal onset will assist in improving existing strategies and facilitate the development of novel approaches to program puberty in heifers. These include the use of diets that elevate BW gain during strategic periods of prepubertal development.

Minireview: Metabolic control of the reproductive physiology: insights from genetic mouse models

This article is part of a Special Issue Energy Balance. Over the past two decades, and in particular over the past 5-7 years, there has been a tremendous advancement in the understanding of the metabolic control of reproductive physiology. This has been in large part due to the advancement and refinement of gene targeting tools and techniques for molecular mapping. Yet despite the emergence of exciting and often times thought-provoking data through the use of new mouse models, the heavy reliance on gene targeting strategies has become fundamental in this process and thus caution must be exercised when interpreting results. This minireview article will explore the generation of new mouse models using genetic manipulation, such as viral vector delivery and the use of the Cre/loxP system, to investigate the role of circulating metabolic hormones in the coordination of reproductive physiology. In addition, we will also highlight some of the pitfalls in the use of genetic manipulation in the current paradigms. However, it has become clear that metabolic cues employ integrated and plastic neural circuits in order to modulate the neuroendocrine reproductive axis, and despite recent advances much remains to be elucidated about this circuitry.

Use of a stair-step compensatory gain nutritional regimen to program the onset of puberty in beef heifers

It was hypothesized that metabolic programming of processes underlying puberty can be shifted temporally through the use of a stair-step compensatory growth model such that puberty is optimally timed to occur at 11 to 12 mo of age. Forty crossbred beef heifers were weaned at approximately 3.5 mo of age and, after a 2-wk acclimation period, were assigned randomly to 1 of 4 nutritional groups: 1) low control (LC), restricted feed intake of a forage-based diet to promote BW gain of 0.5 kg/d until 14 mo of age, 2) high control (HC), controlled feed intake of a high-concentrate diet to promote BW gain of 1 kg/d until 14 mo of age, 3) stair-step 1 (SS-1), ad libitum feed intake of a high-concentrate diet until 6.5 mo of age followed by restricted access to a high-forage diet to promote BW gain of 0.35 kg/d until 9 mo of age, ad libitum feed intake of a high-concentrate diet until 11.5 mo of age, and restricted intake of a high-forage diet to promote BW gain of 0.35 kg/d until 14 mo of age, and 4) stair-step 2 (SS-2), reverse sequence of SS-1, beginning with restricted access to a high-forage diet. Body weight (every 2 wk) and circulating concentrations of leptin (monthly) were determined throughout the experiment. Concentrations of progesterone in blood samples collected twice weekly beginning at 8 mo of age were used to determine pubertal status. Body weight gain followed a pattern similar to that proposed in our experimental design. Circulating concentrations of leptin increased following distinct elevations in BW but decreased abruptly after feed intake restriction. Survival analysis indicated that the percentage of pubertal heifers in the LC group was lower (P < 0.05) than all other groups throughout the experiment. Although heifers in SS-1 were nutritionally restricted between 6.5 and 9 mo of age, the proportion pubertal by 12 mo of age did not differ (P = 0.36) from that of the HC group, with 80% and 70% pubertal in SS-1 and HC, respectively. In contrast, the proportion of heifers pubertal by 12 mo of age in the SS-2 group (40%) was lower (P < 0.05) than both HC and SS-1. However, by 14 mo of age, 90% of heifers in the SS-2 group had also attained puberty compared to only 40% of the LC group. In summary, these data provide evidence that changes in the nutritional and metabolic status during the early juvenile period can program the onset of puberty that occurs months later, allowing optimal timing of sexual maturation in replacement beef heifers.

Leptin-dependent neuronal NO signaling in the preoptic hypothalamus facilitates reproduction

The transition to puberty and adult fertility both require a minimum level of energy availability. The adipocyte-derived hormone leptin signals the long-term status of peripheral energy stores and serves as a key metabolic messenger to the neuroendocrine reproductive axis. Humans and mice lacking leptin or its receptor fail to complete puberty and are infertile. Restoration of leptin levels in these individuals promotes sexual maturation, which requires the pulsatile, coordinated delivery of gonadotropin-releasing hormone to the pituitary and the resulting surge of luteinizing hormone (LH); however, the neural circuits that control the leptin-mediated induction of the reproductive axis are not fully understood. Here, we found that leptin coordinated fertility by acting on neurons in the preoptic region of the hypothalamus and inducing the synthesis of the freely diffusible volume-based transmitter NO, through the activation of neuronal NO synthase (nNOS) in these neurons. The deletion of the gene encoding nNOS or its pharmacological inhibition in the preoptic region blunted the stimulatory action of exogenous leptin on LH secretion and prevented the restoration of fertility in leptin-deficient female mice by leptin treatment. Together, these data indicate that leptin plays a central role in regulating the hypothalamo-pituitary-gonadal axis in vivo through the activation of nNOS in neurons of the preoptic region.

Metabolic influences on reproduction: adiponectin attenuates GnRH neuronal activity in female mice

Metabolic dysfunctions are often linked to reproductive abnormalities. Adiponectin (ADP), a peripheral hormone secreted by white adipose tissue, is important in energy homeostasis and appetite regulation. GnRH neurons are integral components of the reproductive axis, controlling synthesis, and release of gonadotropins. This report examined whether ADP can directly act on GnRH neurons. Double-label immunofluorescence on brain sections from adult female revealed that a subpopulation of GnRH neurons express ADP receptor (AdipoR)2. GnRH/AdipoR2+ cells were distributed throughout the forebrain. To determine the influence of ADP on GnRH neuronal activity and the signal transduction pathway of AdipoR2, GnRH neurons maintained in explants were assayed using whole-cell patch clamping and calcium imaging. This mouse model system circumvents the dispersed distribution of GnRH neurons within the forebrain, making analysis of large numbers of GnRH cells possible. Single-cell PCR analysis and immunocytochemistry confirmed the presence of AdipoR2 in GnRH neurons in explants. Functional analysis revealed 20% of the total GnRH population responded to ADP, exhibiting hyperpolarization or decreased calcium oscillations. Perturbation studies revealed that ADP activates AMP kinase via the protein kinase Cζ/liver kinase B1 pathway. The modulation of GnRH neuronal activity by ADP demonstrated in this report directly links energy balance to neurons controlling reproduction.

Nutrient restriction induces failure of reproductive function and molecular changes in hypothalamus-pituitary-gonadal axis in postpubertal gilts

People on a diet to lose weight may be at risk of reproductive failure. To investigate the effects of nutrient restriction on reproductive function and the underlying mechanism, changes of reproductive traits, hormone secretions and gene expressions in hypothalamus-pituitary-gonadal axis were examined in postpubertal gilts at anestrus induced by nutrient restriction. Gilts having experienced two estrus cycles were fed a normal (CON, 2.86 kg/d) or nutrient restricted (NR, 1 kg/d) food regimens to expect anestrus. NR gilts experienced another three estrus cycles, but did not express estrus symptoms at the anticipated fourth estrus. Blood samples were collected at 5 days' interval for consecutive three times for measurement of hormone concentrations at the 23th day of the fourth estrus cycle. Individual progesterone concentrations of NR gilts from three consecutive blood samples were below 1.0 ng/mL versus 2.0 ng/mL in CON gilts, which was considered anestrus. NR gilts had impaired development of reproductive tract characterized by absence of large follicles (diameter ≥ 6 mm), decreased number of corepus lutea and atrophy of uterus and ovary tissues. Circulating concentrations of IGF-I, kisspeptin, estradiol, progesterone and leptin were significantly lower in NR gilts than that in CON gilts. Nutrient restriction down-regulated gene expressions of kiss-1, G-protein coupled protein 54, gonadotropin-releasing hormone, estrogen receptor α, progesterone receptor, leptin receptor, follicle-stimulating hormone and luteinizing hormone and insulin-like growth factor I in hypothalamus-pituitary-gonadal axis of gilts. Collectively, nutrient restriction resulted in impairment of reproductive function and changes of hormone secretions and gene expressions in hypothalamus-pituitary-gonadal axis, which shed light on the underlying mechanism by which nutrient restriction influenced reproductive function.

Evidence that hypothalamic RFamide related peptide-3 neurones are not leptin-responsive in mice and rats

Leptin, a permissive hormonal regulator of fertility, provides information about the body's energy reserves to the hypothalamic gonadotrophin-releasing hormone (GnRH) neuronal system that drives reproduction. Leptin does not directly act on GnRH neurones, and the neuronal pathways that it uses remain unclear. RFamide-related peptide-3 (RFRP-3) neurones project to GnRH neurones and primarily inhibit their activity. We tested whether leptin could act via RFRP-3 neurones to potentially modulate GnRH activity. First, the effects of leptin deficiency or high-fat diet-induced obesity on RFRP-3 cell numbers and gene expression were assessed in male and female mice. There was no significant difference in Rfrp mRNA levels or RFRP-3-immunoreactive cell counts in wild-type versus leptin-deficient ob/ob animals, or in low-fat versus high-fat diet fed wild-type mice. Second, the presence of leptin-induced signalling in RFRP-3 neurones was examined in male and female wild-type mice and rats. Dual label immunohistochemistry revealed leptin-induced phosphorylated signal transducer and activator of transcription-3 in close proximity to RFRP-3 neurones, although there was very little (2-13%) colocalisation and no significant differences between vehicle and leptin-treated animals. Furthermore, we were unable to detect leptin receptor mRNA in a semi-purified RFRP-3 cell preparation. Because GABA neurones form critical leptin-responsive GnRH inputs, we also determined whether RFRP-3 and GABA cells were colocalised. No such colocalisation was detected. These results support the concept that leptin has little or no effects on RFRP-3 neurones, and that these neurones are unlikely to be an important neuronal pathway for the metabolic regulation of fertility by leptin.

Effect of restricted feeding on nocturnality and daily leptin rhythms in OVLT in aged male Wistar rats

Circadian system has direct relevance to the problems of modern lifestyle, shift workers, jet lag etc. To understand non-photic regulation of biological clock, the effects of restricted feeding (RF) on locomotor activity and daily leptin immunoreactivity (ir) rhythms in three age groups [3, 12 and 24 months (m)] of male Wistar rats maintained in light:dark (LD) 12:12 h conditions were studied. Leptin-ir was examined in the suprachiasmatic nucleus (SCN), the medial preoptic area (MPOA) and organum vasculosum of the lamina terminalis (OVLT). Reversal of feeding time due to restricted food availability during daytime resulted in switching of the animals from nocturnality to diurnality with significant increase in day time activity and decrease in night time activity. The RF resulted in % diurnality of approximately 32, 29 and 73 from % nocturnality of 82, 92 and 89 in control rats of 3, 12 and 24 m age, respectively. The increase in such switching from nocturnality to diurnality with restricted feeding was found to be robust in 24 m rats. The OVLT region showed daily leptin-ir rhythms with leptin-ir maximum at ZT-0 in all the three age groups. However leptin-ir levels were minimum at ZT-12 in 3 and 12 m though at ZT-18 in 24 m. In addition the mean leptin-ir levels decreased with increase in food intake and body weight significantly in RF aged rats. Thus we report here differential effects of food entrained regulation in switching nocturnality to diurnality and daily leptin-ir rhythms in OVLT in aged rats.

Cross-talk between reproduction and energy homeostasis: central impact of estrogens, leptin and kisspeptin signaling

The central nervous system receives hormonal cues (e.g., estrogens and leptin, among others) that influence reproduction and energy homeostasis. 17β-estradiol (E2) is known to regulate gonadotropin-releasing hormone (GnRH) secretion via classical steroid signaling and rapid non-classical membrane-initiated signaling. Because GnRH neurons are void of leptin receptors, the actions of leptin on these neurons must be indirect. Although it is clear that the arcuate nucleus of the hypothalamus is the primary site of overlap between these two systems, it is still unclear which neural network(s) participate in the cross-talk of E2 and leptin, two hormones essential for reproductive function and metabolism. Herein we review the progress made in understanding the interactions between reproduction and energy homeostasis by focusing on the advances made to understand the cellular signaling of E2 and leptin on three neural networks: kisspeptin, pro-opiomelanocortin (POMC) and neuropeptide Y (NPY). Although critical in mediating the actions of E2 and leptin, considerable work still remains to uncover how these neural networks interact in vivo.

Kisspeptin and energy balance in reproduction

Kisspeptin is vital for the neuroendocrine regulation of GNRH secretion. Kisspeptin neurons are now recognized as a central pathway responsible for conveying key homeostatic information to GNRH neurons. This pathway is likely to mediate the well-established link between energy balance and reproductive function. Thus, in states of severely altered energy balance (either negative or positive), fertility is compromised, as isKiss1expression in the arcuate nucleus. A number of metabolic modulators have been proposed as regulators of kisspeptin neurons including leptin, ghrelin, pro-opiomelanocortin (POMC), and neuropeptide Y (NPY). Whether these regulate kisspeptin neurons directly or indirectly will be discussed. Moreover, whether the stimulatory role of leptin on reproduction is mediated by kisspeptin directly will be questioned. Furthermore, in addition to being expressed in GNRH neurons, the kisspeptin receptor (Kiss1r) is also expressed in other areas of the brain, as well as in the periphery, suggesting alternative roles for kisspeptin signaling outside of reproduction. Interestingly, kisspeptin neurons are anatomically linked to, and can directly excite, anorexigenic POMC neurons and indirectly inhibit orexigenic NPY neurons. Thus, kisspeptin may have a direct role in regulating energy balance. Although data fromKiss1rknockout and WT mice found no differences in body weight, recent data indicate that kisspeptin may still play a role in food intake and glucose homeostasis. Thus, in addition to regulating reproduction, and mediating the effect of energy balance on reproductive function, kisspeptin signaling may also be a direct regulator of metabolism.

Thyroid hormone and leptin in the testis

Leptin is primarily expressed in white adipose tissue; however, it is expressed in the hypothalamus and reproductive tissues as well. Leptin acts by activating the leptin receptors (Ob-Rs). Additionally, the regulation of several neuroendocrine and reproductive functions, including the inhibition of glucocorticoids and enhancement of thyroxine and sex hormone concentrations in human beings and mice are leptin functions. It has been suggested that thyroid hormones (TH) could directly regulate leptin expression. Additionally, hypothyroidism compromises the intracellular integration of leptin signaling specifically in the arcuate nucleus. Two TH receptor isoforms are expressed in the testis, TRa and TRb, with TRa being the predominant one that is present in all stages of development. The effects of TH involve the proliferation and differentiation of Sertoli and Leydig cells during development, spermatogenesis, and steroidogenesis. In this context, TH disorders are associated with sexual dysfunction. An endocrine and/or direct paracrine effect of leptin on the gonads inhibits testosterone production in Leydig cells. Further studies are necessary to clarify the effects of both hormones in the testis during hypothyroidism. The goal of this review is to highlight the current knowledge regarding leptin and TH in the testis.

Serum leptin levels in polycystic ovary syndrome and its relationship with metabolic and hormonal profile in pakistani females

The study aimed to investigate the levels of serum leptin in PCOS females and to correlate it with metabolic and hormonal parameters. Sixty-two PCOS and ninety normal cycling (NC) females with matched age and body mass index (BMI) were recruited for this cross-sectional study. Serum leptin, FSH, LH, E2, free testosterone, progesterone, thyroid profile, and FBG levels were measured. The mean leptin levels in PCOS and NC were not significantly different (45.56 ng/mL ± 1.49 vs 41.78 ± 1.31 ng/mL, P > 0.05); however, leptin levels showed a strong correlation with BMI in PCOS and NC group (r = 0.77, P < 0.0001; r = 0.82, P < 0.0001, resp.). High E2 levels in NC had a significant correlation with leptin whereas FBG correlated with leptin in PCOS (r = 0.51, P = 0.005). TSH had a substantial correlation (r = 0.49, P < 0.005; r = 0.69, P < 0.005) in PCOS and NC, respectively. There was no significant difference found in circulating leptin concentration between PCOS and NC subjects. Leptin levels in PCOS were related with metabolic impairments manifested by disturbance in FBG levels and impairment of reproductive functions in terms of reduced E2 secretion.

Examination of the influence of leptin and acute metabolic challenge on RFRP-3 neurons of mice in development and adulthood

BACKGROUND

The neuropeptide RFamide-related peptide-3 (RFRP-3; mammalian ortholog to gonadotropin-inhibiting hormone) can inhibit luteinizing hormone (LH) release and increases feeding, but the regulation and development of RFRP-3 neurons remains poorly characterized, especially in mice.

METHODS AND RESULTS

We first confirmed that peripheral injections of murine RFRP-3 peptide could markedly suppress LH secretion in adult mice, as in other species. Second, given RFRP-3's reported orexigenic properties, we performed double-label in situ hybridization for metabolic genes in Rfrp neurons of mice. While Rfrp neurons did not readily coexpress neuropeptide Y, thyrotropin-releasing hormone, or MC4R, a small subset of Rfrp neurons did express the leptin receptor in both sexes. Surprisingly, we identified no changes in Rfrp expression or neuronal activation in adult mice after acute fasting. However, we determined that Rfrp mRNA levels in the dorsal-medial nucleus were significantly reduced in adult obese (Ob) mice of both sexes. Given the lower Rfrp levels observed in adult Ob mice, we asked whether leptin might also regulate RFRP-3 neuron development. Rfrp gene expression changed markedly over juvenile development, correlating with the timing of the juvenile 'leptin surge' known to govern hypothalamic feeding circuit development. However, the dramatic developmental changes in juvenile Rfrp expression did not appear to be leptin driven, as the pattern and timing of Rfrp neuron development were unaltered in Ob juveniles.

CONCLUSION

Leptin status modulates RFRP-3 expression in adulthood, but is not required for normal development of the RFRP-3 system. Leptin's regulation of adult RFRP-3 neurons likely occurs primarily via indirect signaling, and may be secondary to obesity, as only a small subset of RFRP-3 neurons express the long form of the leptin receptor (LepRb).

Energy metabolism and fertility: a balance preserved for female health

In female animals, energy metabolism and fertility are tightly connected, and reciprocally regulated. However, the relative contributions of metabolic and reproductive pathways have changed over the course of evolution. In oviparous animals, metabolic factors take precedence over fertility, enabling egg production to be inhibited in a nutritionally poor environment. By contrast, in placental mammals, the opposite occurs: the need to feed a developing embryo and neonate forces metabolic pathways to adapt to these reproductive needs. This physiological necessity explains why in female mammals alterations of gonadal activity, including age-dependent cessation of ovarian functions, are associated with a disruption of metabolic homeostasis and consequent inflammatory reactions that trigger the onset of metabolic, cardiovascular, skeletal and neural pathologies. This Review discusses how metabolic homeostasis and reproductive functions interact to optimize female fertility and explains the pathogenic mechanisms underlying the disordered energy metabolism associated with human ovarian dysfunction owing to menopause, polycystic ovary syndrome and Turner syndrome. Finally, this article highlights how hormone replacement therapy might aid the restoration of metabolic homeostasis in women with ovarian dysfunction.

Leptin signaling in GABA neurons, but not glutamate neurons, is required for reproductive function

The adipocyte-derived hormone leptin acts in the brain to modulate the central driver of fertility: the gonadotropin releasing hormone (GnRH) neuronal system. This effect is indirect, as GnRH neurons do not express leptin receptors (LEPRs). Here we test whether GABAergic or glutamatergic neurons provide the intermediate pathway between the site of leptin action and the GnRH neurons. Leptin receptors were deleted from GABA and glutamate neurons using Cre-Lox transgenics, and the downstream effects on puberty onset and reproduction were examined. Both mouse lines displayed the expected increase in body weight and region-specific loss of leptin signaling in the hypothalamus. The GABA neuron-specific LEPR knock-out females and males showed significantly delayed puberty onset. Adult fertility observations revealed that these knock-out animals have decreased fecundity. In contrast, glutamate neuron-specific LEPR knock-out mice displayed normal fertility. Assessment of the estrogenic hypothalamic-pituitary-gonadal axis regulation in females showed that leptin action on GABA neurons is not necessary for estradiol-mediated suppression of tonic luteinizing hormone secretion (an indirect measure of GnRH neuron activity) but is required for regulation of a full preovulatory-like luteinizing hormone surge. In conclusion, leptin signaling in GABAergic (but not glutamatergic neurons) plays a critical role in the timing of puberty onset and is involved in fertility regulation throughout adulthood in both sexes. These results form an important step in explaining the role of central leptin signaling in the reproductive system. Limiting the leptin-to-GnRH mediators to GABAergic cells will enable future research to focus on a few specific types of neurons.

Agouti-related peptide plays a critical role in leptin's effects on female puberty and reproduction

Deficient leptin signaling causes infertility via reduced activity of GnRH neurons, causing a hypogonadal state in both rodents and humans. Because GnRH neurons do not express leptin receptors, leptin's effect on GnRH neurons must be indirect. Neurons within the hypothalamic arcuate nucleus that coexpress AGRP and NPY are considered to be important intermediate neurons involved in leptin regulation of GnRH neurons. Previously, we reported that the absence of AGRP and haploinsufficiency of MC4R in leptin receptor mutant (Lepr(db/db)) females result in restoration of fertility and lactation despite the persistence of obesity and insulin resistance. The overarching hypothesis in the present study is that the absence or reduction of leptin's inhibition of AGRP/NPY neurons leads to suppression of GnRH release in cases of leptin signaling deficiency. Since TAC2 (NKB)-TAC3R signaling plays a role in puberty maturation and is modulated by metabolic status, the other aim of this study is to test whether TAC2/NKB neurons in ARC regulated by melanocortinergic signals herein affect leptin's action on puberty and reproduction. Our data showed that AGRP deficiency in Lepr(db/db) females restores normal timing of vaginal opening and estrous cycling, although uterine weight gain and mammary gland development are morphologically delayed. Nonetheless, Agrp(-/-) Lepr(db/db) females are fertile and sustain adequate nutrition of pups with lactation to weaning age. AGRP deficiency results in advanced vaginal opening in wild-type female mice. The postpubertal increase in hypothalamic TAC2 mRNA was not observed in Lepr(db/db) females, whereas AGRP deficiency restored it in Lepr(db/db) females. Additionally, MC4R activation with MTII induced FOS expression in TAC2 neurons, supporting the concept of melanocortinergic regulation of TAC2 neurons. These studies suggest that AGRP imposes an inhibitory effect on puberty and that TAC2 neurons may transmit melanocortinergic inhibition of GnRH neurons.

Evaluation of the hypothalamic-pituitary-gonadal axis in eugonadal men with type 2 diabetes mellitus

Men with type 2 diabetes mellitus (DM2) have lower testosterone levels and a higher prevalence of hypogonadism. It still remains unclear the mechanism by which there is a relationship between hypogonadism and DM2. The objective was to evaluate the hypothalamic-pituitary-gonadal axis at different levels in eugonadal patients with DM2. Fourteen patients with DM2 (DM2 group) and 15 subjects without DM2 (normal glucose tolerance test) as control group (CG) were included. We assessed: (i) fasting glucose, insulin, Homeostasis Model Assessment (HOMA); (ii) luteinizing hormone (LH) pulsatility through blood collections every 10 min for 4 h; (iii) gonadotropin-releasing hormone (GnRH) test: basal LH and 30, 60 and 90 min after 100 μg of i.v. GnRH; (iv) human chorionic gonadotropin (hCG) test: basal total testosterone (TT), bioavailable testosterone (BT), free testosterone (FT), estradiol (E2), bioavailable E2 (BE2) and sex hormone-binding globulin (SHBG) and 72 h post 5000 IU of i.m. hCG. There were no differences in age, body mass index and waist circumference between groups. Glucose was higher in the DM2 group vs. CG: 131.1 ± 25.5 vs. 99.1 ± 13.6 mg/dL, p = 0.0005. There were no difference in basal insulin, HOMA, TT, BT, FT, E2, BE2, SHBG and LH levels between groups. The DM2 group had lower LH pulse frequency vs. CG: 0.8 ± 0.8 vs. 1.5 ± 0.5 pulses, p = 0.009. Differences in LH pulse amplitude were not found. A negative correlation was found between the number of LH pulses and glucose, r: -0.39, p = 0.03. There were no differences in the response of LH to GnRH between groups nor in the response of sexual steroids and SHBG to hCG. Patients with DM2 showed lower hypothalamic pulse frequency without changes in the pituitary response to GnRH nor testicular response to hCG. Glucose levels negatively correlated with the number of LH pulses which suggests a negative effect of hyperglycaemia in the hypothalamic secretion of GnRH.