Stimulatory effect of PYY-(3-36) on gonadotropin secretion is potentiated in fasted rats

Gut hormones and reproduction (Hormones intestinalis et reproduction)

Reproduction and metabolism are intricately linked. Gut hormones play key roles in the regulation of body weight and glucose homeostasis, factors that influence the functioning of the hypothalamic-pituitary-gonadal axis and reproductive outcomes. Data from rodent models suggest gut hormones may have direct stimulatory effects on reproductive hormone release. However, the effects of gut hormones on reproductive function in humans is more complex, with possible involvement of direct (e.g. via gut hormone receptor agonism) as well as indirect (e.g. via weight reduction in people with obesity) mechanisms. The use of gut hormone receptor agonists has become an integral part of the management of metabolic diseases (including obesity and type 2 diabetes), with additional indications for their use on the horizon. Future work may identify specific roles for gut hormones receptor agonists in the treatment of reproductive co-morbidities that are increasingly being recognised in people with metabolic diseases.

Effects of Peptide YY on the Hypothalamic-Pituitary-Gonadal Axis in Healthy Men

CONTEXT

Central and peripheral administration of peptide YY (PYY) has potent anorectic effects, and PYY analogs are under development as antiobesity treatments. Recent animal data suggest PYY may also influence the reproductive axis; however the effects of PYY on the human reproductive system are unknown.

OBJECTIVE

To investigate the effects of PYY administration on the reproductive axis in healthy young men.

DESIGN

Single-blind, randomized, placebo-controlled crossover study.

SETTING

Clinical Research Facility, Imperial College Healthcare NHS Trust.

PARTICIPANTS

Eighteen healthy eugonadal men (mean age 24.1 ± 0.9 years, mean body mass index 22.2 ± 0.4 kg/m2).

INTERVENTION

Eight-hour intravenous infusion of 0.4 pmol/kg/min PYY3-36 or rate-matched vehicle infusion.

MAIN OUTCOME MEASURES

Number of luteinizing hormone (LH) pulses, LH, follicle stimulating hormone (FSH), and testosterone levels.

RESULTS

The number of LH pulses (mean number of LH pulses/8 hours: PYY 4.4 ± 0.3 vs vehicle 4.4 ± 0.4, P > .99), LH area under the curve (AUC) (PYY 1503 ± 79 IU.min/L vs vehicle 1574 ± 86 IU.min/L, P = .36), FSH AUC (PYY 1158 ± 513 IU.min/L vs vehicle 1199 ± 476 IU.min/L, P = .49) and testosterone AUC (PYY 10 485 ± 684 IU.min/L vs vehicle 11 133 ± 803 IU.min/L, P = .24) were similar during PYY and vehicle infusions.

CONCLUSIONS

Acute intravenous infusion of 0.4 pmol/kg/min PYY does not affect the reproductive axis in healthy men.

Neuroendocrine integration of nutritional signals on reproduction

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

Anorexia Nervosa and Its Associated Endocrinopathy in Young People

Anorexia nervosa (AN) is a condition of severe undernutrition associated with adaptive changes in many endocrine axes. These changes include hypogonadotropic hypogonadism, acquired growth hormone resistance with low insulin-like growth factor 1 (IGF-1) levels, hypercortisolemia, altered secretion of adipokines and appetite-regulating hormones, and low bone mineral density (BMD). Bone health is impaired subsequent to a low body mass index, decreased lean mass, and the endocrine changes described above. In addition to low areal BMD, AN is characterized by a decrease in volumetric BMD, changes in bone geometry, and reductions in strength estimates, leading to an increased risk for fracture. Weight restoration is essential for restoration of normal endocrine function; however, hypercortisolemia, high peptide YY levels, and ghrelin dynamics may not completely normalize. In some patients, hypogonadotropic hypogonadism persists despite weight restoration. Weight gain and menstrual recovery are critical for improving bone health in AN; however, residual deficits may persist. Physiologic estrogen replacement using transdermal, but not oral, estrogen increases bone accrual in adolescents with AN, while bisphosphonates improve BMD in adults. Recombinant human IGF-1 and teriparatide have been used in a few studies as bone anabolic therapies. More data are necessary to determine the optimal therapeutic strategies for low BMD in AN.

Neuroendocrine mechanisms in athletes

Athletic activity may be associated with alterations in various neuroendocrine axes depending on the state of energy availability. In addition, genetic factors and an underlying predilection for polycystic ovarian syndrome (PCOS) may predispose some athletes to develop functional hypothalamic amenorrhea earlier than other athletes. In conditions of low energy availability associated with athletic activity, changes that occur in various neuroendocrine axes are primarily adaptive, and aim to either conserve energy for the most essential functions, or allow the body to draw on its reserves to meet energy needs. These hormonal changes, however, then lead to changes in body composition and bone metabolism. Impaired bone accrual in younger athletes and low bone density in older athletes constitutes the major pathologic consequence of neuroendocrine changes associated with low energy availability. The female athlete triad of low energy availability, menstrual dysfunction, and low bone density is prevalent in certain kinds of sports and activities, particularly endurance sports, gymnastics, and ballet. It is essential to screen for this condition in athletes at every preparticipation physical and during office visits, and to put in place an effective treatment team to manage the triad early, in order to optimize outcomes.

The relationship between gut and adipose hormones, and reproduction

BACKGROUND

Reproductive function is tightly regulated by nutritional status. Indeed, it has been well described that undernutrition or obesity can lead to subfertility or infertility in humans. The common regulatory pathways which control energy homeostasis and reproductive function have, to date, been poorly understood due to limited studies or inconclusive data. However, gut hormones and adipose tissue hormones have recently emerged as potential regulators of both energy homeostasis and reproductive function.

METHODS

A PubMed search was performed using keywords related to gut and adipose hormones and associated with keywords related to reproduction.

RESULTS

Currently available evidence that gut (ghrelin, obestatin, insulin, peptide YY, glucagon-like peptide-1, glucose-dependent insulinotropic peptide, oxyntomodulin, cholecystokinin) and adipose hormones (leptin, adiponectin, resistin, omentin, chemerin) interact with the reproductive axis is presented. The extent, site and direction of their effects on the reproductive axis are variable and also vary depending on species, sex and pubertal stage.

CONCLUSIONS

Gut and adipose hormones interact with the reproductive axis as well as with each other. While leptin and insulin have stimulatory effects and ghrelin has inhibitory effects on hypothalamic GnRH secretion, there is increasing evidence for their roles in other sites of the reproductive axis as well as evidence for the roles of other gut and adipose hormones in the complex interplay between nutrition and reproduction. As our understanding improves, so will our ability to identify and design novel therapeutic options for reproductive disorders and accompanying metabolic disorders.

Characterization of the reproductive effects of the Vgf-derived peptide TLQP-21 in female rats: in vivo and in vitro studies

BACKGROUND

VGF (non-acronymic), a protein expressed in the hypothalamus and pituitary, is involved in the control of metabolism and body weight homeostasis. Different active peptide fragments are generated from VGF, including TLQP-21. Previous studies of our group reported that this molecule participates also in the regulation of reproductive function in male rats, with predominant stimulatory effects.

METHODS

We report herein a series of studies on the reproductive effects of TLQP-21 in female rats, as evaluated by a combination of in vivo and in vitro analyses.

RESULTS

TLQP-21 modestly increased serum LH levels after systemic administration and directly stimulated pituitary LH and FSH secretion in prepubertal female rats, while acute central injection of TLQP-21 was unable to modify LH secretion at this age. Repeated central administration of TLQP-21 during the pubertal transition (between PND-28 and -35) to female rats fed ad libitum advanced the timing of vaginal opening and increased the percentage of animals with signs of ovulation. Moreover, an analogous treatment slightly enhanced ovarian maturation in pubertal female rats subjected to chronic undernutrition, but was unable to rescue the delay of vaginal opening induced by food deprivation. In addition, TLQP-21 oppositely modified LH secretion in adult female rats depending on the stage of the ovarian cycle: it stimulated LH secretion when injected in the morning of diestrus and decreased the magnitude of the preovulatory LH (but not FSH) surge when injected in the afternoon of proestrus.

CONCLUSIONS

Our data are the first to document the potential involvement of TLQP-21 in the control of reproductive function in female rats.

Metabolic signals in human puberty: effects of over and undernutrition

Puberty in mammals is associated with important physical and psychological changes due to the increase in sex steroids and growth hormone (GH). Indeed, an increase in growth velocity and the attainment of sexual maturity for future reproductive function are the hallmark changes during this stage of life. Both growth and reproduction consume high levels of energy, requiring suitable energy stores to face these physiological functions. During the last two decades our knowledge concerning how peptides produced in the digestive tract (in charge of energy intake) and in adipose tissue (in charge of energy storage) provide information regarding metabolic status to the central nervous system (CNS) has increased dramatically. Moreover, these peptides have been shown to play an important role in modulating the gonadotropic axis with their absence or an imbalance in their secretion being able to disturb pubertal onset or progression. In this article we will review the current knowledge concerning the role played by leptin, the key adipokine in energy homeostasis, and ghrelin, the only orexigenic and growth-promoting peptide produced by the digestive tract, on sexual development. The normal evolutionary pattern of these peripherally produced metabolic signals throughout human puberty will be summarized. The effect of two opposite situations of chronic malnutrition, obesity and anorexia, on these signals and how they influence the course of puberty will also be discussed. Finally, we will briefly mention other peptides derived from the digestive tract (such as PYY) that may be involved in the regulatory link between energy homeostasis and sexual development.

Ghrelin and peptide YY (PYY) profiles in gastrointestinal tissues and the circulation of the rat during pregnancy and lactation

Plasma and tissue profiles of gastrointestinal hormones ghrelin and peptide YY (PYY) were investigated in different female rat reproductive states. Neither plasma nor tissue ghrelin concentrations were suppressed during pregnancy despite elevated leptin. The highest concentrations of stomach ghrelin were measured in late pregnancy. PYY concentrations in plasma, descending colon and rectum tissues were increased (P<0.001) throughout pregnancy and lactation. PYY peaked at day 5 of lactation in plasma, as well as descending colon and rectum tissues (proestrus vs day 5 of lactation: 25+/-3.0 pmol/l vs 55+/-8.0 pmol/l; 85+/-4.5 pmol/g wwt vs 418+/-45.0 pmol/g wwt; 23+/-3.0 pmol/g wwt vs 78+/-9.1 pmol/g wwt). This PYY peak was temporally associated with the luteinizing hormone peak on day 1 of lactation. Following weaning, dam adiposity and plasma leptin increased whereas ghrelin stomach peptide decreased. Relative PYY concentrations in the tissues of the gut varied in the different states suggesting regional alterations taking place in the colon. The ascending colon produced the highest concentrations in non-pregnant rats, the descending colon the highest concentrations during lactation with the pregnant rats and the dams postweaning in a transition state between. It is unclear what role the increased PYY in various tissues observed has during pregnancy and lactation as it would be expected to be reduced in these states of greatly increased appetite. PYY may have an influence on maternal dietary adaptation, intestinal hypertrophy and weight gain during pregnancy and lactation although it is still unclear precisely how it acts.

Expression of peptide YY in human brain and pituitary tissues

Expression of peptide YY (PYY) in the human brain and pituitary tissues was studied by radioimmunoassay, immunocytochemistry, and reverse transcription polymerase chain reaction. The polyclonal antibody raised against human PYY(1-36) in a rabbit was used in the assay, which showed 100% cross-reactivity with PYY(3-36) and no significant cross-reactivity with other peptides including neuropeptide Y and pancreatic polypeptide. The highest concentration of immunoreactive PYY was found in the hypothalamus (0.44+/-0.060 pmol/g of wet weight, mean +/- SEM, n=8), followed by the pituitary (0.41+/-0.16 pmol/g of wet weight, n=3). Reverse-phase high performance liquid chromatography of tissue extracts of human rectum and cortical brain showed a peak eluted in the position of authentic PYY(1-36) and PYY(3-36). Immunocytochemistry showed positive immunostaining for PYY in neurons of the paraventricular, arcuate, and supraoptic nuclei of the human hypothalamus. Moreover, reverse transcription polymerase chain reaction analysis showed expression of mRNA for PYY in human brain and pituitary tissues. The present study has shown for the first time expression of PYY in the human brain and pituitary tissues, suggesting a role for PYY as a neurotransmitter, in the neuroendocrine physiology, such as regulation of appetite and energy expenditure and modulation of pituitary hormone secretion.

Metabolic fuel and clinical implications for female reproduction

Reproduction is a physiologically costly process that consumes significant amounts of energy. The physiological mechanisms controlling energy balance are closely linked to fertility. This close relationship ensures that pregnancy and lactation occur only in favourable conditions with respect to energy. The primary metabolic cue that modulates reproduction is the availability of oxidizable fuel. An organism's metabolic status is transmitted to the brain through metabolic fuel detectors. There are many of these detectors at both the peripheral (e.g., leptin, insulin, ghrelin) and central (e.g., neuropeptide Y, melanocortin, orexins) levels. When oxidizable fuel is scarce, the detectors function to inhibit the release of gonadotropin-releasing hormone and luteinizing hormone, thereby altering steroidogenesis, reproductive cyclicity, and sexual behaviour. Infertility can also result when resources are abundant but food intake fails to compensate for increased energy demands. Examples of these conditions in women include anorexia nervosa and exercise-induced amenorrhea. Infertility associated with obesity appears to be less related to an effect of oxidizable fuel on the hypothalamic-pituitary-ovarian axis. Impaired insulin sensitivity may play a role in the etiology of these conditions, but their specific etiology remains unresolved. Research into the metabolic regulation of reproductive function has implications for elucidating mechanisms of impaired pubertal development, nutritional amenorrhea, and obesity-related infertility. A better understanding of these etiologies has far-reaching implications for the prevention and management of reproductive dysfunction and its associated comorbidities.

Selective role of neuropeptide Y receptor subtype Y2 in the control of gonadotropin secretion in the rat

Different signals with key roles in energy homeostasis regulate the reproductive axis. These include neuropeptide Y and polypeptide YY(3-36), whose type Y(2) receptor is the most abundant of this family in the brain. We evaluated herein the putative roles of Y(2) receptors in the control of gonadotropin secretion by means of central administration of PYY(13-36) (agonist of Y(2) receptors) and BIIE 0246 (antagonist of Y(2) receptors) to intact and orchidectomized male rats. In addition, the ability of PYY(13-36) to elicit GnRH and gonadotropin secretion in vitro and the impact of fasting on LH responses to PYY(13-36) in vivo were also monitored. Central administration of PYY(13-36) significantly decreased the circulating levels of both gonadotropins, an effect that was observed in prepubertal and adult rats. Yet a dual action of Y(2) receptors in the control of male gonadotropic axis was evidenced as their activation induced 1) stimulation of gonadotropin responses to GnRH at the pituitary but 2) inhibition of GnRH secretion at the hypothalamus. Antagonization of Y(2) receptors failed to modify basal LH secretion in intact males either after being fed ad libitum or after being fasted. In contrast, their central blockade in orchidectomized rats evoked a significant increase in circulating LH and FSH level, suggesting the constitutive activation of Y(2) receptor in such stimulated conditions. In summary, our data evidence a complex mode of action of Y(2) receptors in the control of gonadotropic axis, with stimulatory and inhibitory actions at different levels of the system that are sensitive to the gonadal status.

Fasting inhibits the growth and reproductive axes via distinct Y2 and Y4 receptor-mediated pathways

Neuropeptide Y, a neuropeptide abundantly expressed in the brain, has been implicated in the regulation of the hypothalamo-pituitary-somatotropic axis and the hypothalamo-pituitary-gonadotropic axis. Elevated hypothalamic neuropeptide Y expression, such as that occurs during fasting, is known to inhibit both of these axes. However, it is not known which Y receptor(s) mediate these effects. Here we demonstrate, using Y receptor knockout mice, that Y2 and Y4 receptors are separately involved in the regulation of these axes. Fasting-induced inhibition of hypothalamic GHRH mRNA expression and reduction of circulating IGF-I levels were observed in wild-type and Y4(-/-) mice but not Y2(-/-) or Y2(-/-)Y4(-/-) mice. In contrast, fasting-induced reduction of GnRH expression in the medial preoptic area and testis testosterone content were abolished in the absence of Y4 receptors. Colocalization of Y2 receptors and GHRH in the arcuate nucleus (Arc) suggests that GHRH mRNA expression in this region might be directly regulated by Y2 receptors. Indeed, hypothalamic-specific deletion of Y2 receptors in conditional knockout mice prevented the fasting-induced reduction in Arc GHRH mRNA expression. On the other hand, fasting-induced decrease in GnRH mRNA expression in the medial preoptic area is more likely indirectly influenced by Y4 receptors because no Y4 receptors could be detected on GnRH neurons in this region. Together these data show that fasting inhibits the somatotropic axis via direct action on Y2 receptors in the Arc and indirectly inhibits the gonadotropic axis via Y4 receptors.

Novel signals for the integration of energy balance and reproduction

Although the close link between body weight and fertility has been known for eons, only recently have the peripheral signals and neuroendocrine networks responsible for such a phenomenon begun to be identified. A key event in this field was the cloning of the adipocyte-derived hormone leptin, which has been demonstrated as a pivotal regulator for the integration of energy homeostasis and reproduction. In addition, other metabolic hormones, such as insulin, contribute to this physiological integration. Moreover, compelling experimental evidence implicates hormonal products of the gastrointestinal tract as adjuncts in the complex coordination and regulation of body weight and reproduction. Here, we review recent studies evaluating the reproductive effects and sites of action of ghrelin and PYY3-36, two hormonal signals of gastrointestinal origin involved in the control food intake and energy balance. In addition, we summarize the potential contribution of kisspeptin, the recently characterized gatekeeper of the GnRH system encoded by Kiss1 gene, to integrating reproductive function and energy status. Evidence suggests that besides having direct gonadal effects, ghrelin may participate in the regulation of gonadotropin secretion and it may influence the timing of puberty. Likewise, PYY3-36 modulates GnRH and gonadotropin release. In addition, the hypothalamic KiSS-1 system is sensitive to nutritional status, and its diminished expression during states of negative energy balance might contribute to the suppression of reproductive function in such conditions. We propose that the peripheral hormones, ghrelin and PYY3-36, and the central neuropeptide, kisspeptin, are 'novel' players in the neuroendocrine networks that integrate energy balance and reproduction.