Central lipoprivation-induced suppression of luteinizing hormone pulses is mediated by paraventricular catecholaminergic inputs in female rats

Kobayashi Award 2019: The neuroendocrine regulation of the mammalian reproduction

Mammalian reproductive function is a complex system of many players orchestrated by the hypothalamus-pituitary-gonadal (HPG) axis. The hypothalamic gonadotropin-releasing hormone (GnRH) and the consequent pituitary gonadotropin release show two modes of secretory patterns, namely the surge and pulse modes. The surge mode is triggered by the positive feedback action of estrogen secreted from the mature ovarian follicle to induce ovulation in females of most mammalian species. The pulse mode of GnRH release is required for stimulating tonic gonadotropin secretion to drive folliculogenesis, spermatogenesis and steroidogenesis and is negatively fine-tuned by the sex steroids. Accumulating evidence suggests that hypothalamic kisspeptin neurons are the master regulator for animal reproduction to govern the HPG axis. Specifically, kisspeptin neurons located in the anterior hypothalamus, such as the anteroventral periventricular nucleus (AVPV) in rodents and preoptic nucleus (POA) in ruminants, primates and others, and the neurons located in the arcuate nucleus (ARC) in posterior hypothalamus in most mammals are considered to play a key role in generating the surge and pulse modes of GnRH release, respectively. The present article focuses on the role of AVPV (or POA) kisspeptin neurons as a center for GnRH surge generation and of the ARC kisspeptin neurons as a center for GnRH pulse generation to mediate estrogen positive and negative feedback mechanisms, respectively, and discusses how the estrogen epigenetically regulates kisspeptin gene expression in these two populations of neurons. This article also provides the mechanism how malnutrition and lactation suppress GnRH/gonadotropin pulses through an inhibition of the ARC kisspeptin neurons. Further, the article discusses the programming effect of estrogen on kisspeptin neurons in the developmental brain to uncover the mechanism underlying the sex difference in GnRH/gonadotropin release as well as an irreversible infertility induced by supra-physiological estrogen exposure in rodent models.

AMP-activated protein kinase activation reduces the transcriptional activity of the murine luteinizing hormone β-subunit gene

Malnutrition is one of the factors that induces reproductive disorders. However, the underlying biological processes are unclear. AMP-activated protein kinase (AMPK) is an enzyme that plays crucial role as a cellular energy sensor. In the present study, we examined the effects of AMPK activation on the transcription of the murine gonadotropin subunit genes Cga, Lhb, and Fshb, and the gonadotropin-releasing hormone receptor Gnrh-r. Real-time PCR and transcription assay using LβT2 cells demonstrated that 5-amino-imidazole carboxamide riboside (AICAR), a cell-permeable AMP analog, repressed the expression of Lhb. Next, we examined deletion mutants of the upstream region of Lhb and found that the upstream regulatory region of Lhb (-2527 to -2198 b) was responsible for the repression by AICAR. Furthermore, putative transcription factors (SP1, STAT5a, and TEF) that might mediate transcriptional control of the Lhb repression induced by AICAR were identified. In addition, it was confirmed that both AICAR and a competitive inhibitor of glucose metabolism, 2-deoxy-D-glucose, induced AMPK phosphorylation in LβT2 cells. Therefore, the upstream region of Lhb is one of the target sites for glucoprivation inducing AMPK activation. In addition, AMPK plays a role in repressing Lhb expression through the distal -2527 to -2198 b region.

Ad libitum feeding triggers puberty onset associated with increases in arcuate Kiss1 and Pdyn expression in growth-retarded rats

Increasing evidence shows that puberty onset is largely dependent on body weight rather than chronological age. To investigate the mechanism involved in the energetic control of puberty onset, the present study examined effects of chronic food restriction during the prepubertal period and the resumption of ad libitum feeding for 24 and 48 h on estrous cyclicity, Kiss1 (kisspeptin gene), Tac3 (neurokinin B gene) and Pdyn (dynorphin A gene) expression in the hypothalamus, luteinizing hormone (LH) secretion and follicular development in female rats. When animals weighed 75 g, they were subjected to a restricted feeding to retard growth to 70–80 g by 49 days of age. Then, animals were subjected to ad libitum feeding or remained food-restricted. The growth-retarded rats did not show puberty onset associated with suppression of both Kiss1 and Pdyn expression in the arcuate nucleus (ARC). 24-h ad libitum feeding increased tonic LH secretion and the number of Graafian and non-Graafian tertiary follicles with an increase in the numbers of ARC Kiss1- and Pdyn-expressing cells. 48-h ad libitum feeding induced the vaginal proestrus and a surge-like LH increase with an increase in Kiss1-expressing cells in the anteroventral periventricular nucleus (AVPV). These results suggest that the negative energy balance causes pubertal failure with suppression of ARC Kiss1 and Pdyn expression and then subsequent gonadotropin secretion and ovarian function, while the positive energetic cues trigger puberty onset via an increase in ARC Kiss1 and Pdyn expression and thus gonadotropin secretion and follicular development in female rats.

Long-chain unsaturated fatty acids reduce the transcriptional activity of the rat follicle-stimulating hormone β-subunit gene

Here, we assessed the effects of long-chain fatty acids (LCFAs) and the LCFA receptor agonist GW9508 on the transcription of the gonadotropin subunit genes Cga, Lhb and Fshb because LCFA receptor GPR120 was observed in mouse gonadotropes in our recent study. A transcription assay using LβT2 cells demonstrated that LCFAs, oleic acid, α-linolenic acid, docosahexaenoic acid and palmitate, repressed the expression of Cga, Lhb, and Fshb at concentrations between 50 and 100 µM. On the other hand, treatment with 10 µM unsaturated LCFAs, oleic acid, α-linolenic acid and docosahexaenoic acid, repressed only Fshb expression, while the same dose of a saturated LCFA, palmitate, had no effect on the expression of gonadotropin subunit genes. Furthermore, GW9508 did not affect promoter activity. Next, we examined deletion mutants of the upstream region of Fshb and found that the upstream regulatory region (-2824 to -2343 bp) of Fshb was responsible for the notable repression by 10 µM unsaturated LCFAs. Our results suggest that the upstream region of Fshb is susceptible to unsaturated LCFAs. In addition, unsaturated LCFAs play a role in repressing Fshb expression through the distal -2824 to -2343 bp region, which might be independent of the LCFA receptor GPR120 pathway.

Regulation of gonadotropin secretion by monitoring energy availability

Nutrition is a principal environmental factor influencing fertility in animals. Energy deficit causes amenorrhea, delayed puberty, and suppression of copulatory behaviors by inhibiting gonadal activity. When gonadal activity is impaired by malnutrition, the signals originating from an undernourished state are ultimately conveyed to the gonadotropin-releasing hormone (GnRH) pulse generator, leading to suppressed secretion of GnRH and luteinizing hormone (LH). The mechanism responsible for energetic control of gonadotropin release is believed to involve metabolic signals, sensing mechanisms, and neuroendocrine pathways. The availabilities of blood-borne energy substrates such as glucose, fatty acids, and ketone bodies, which fluctuate in parallel with changes in nutritional status, act as metabolic signals that regulate the GnRH pulse generator activity and GnRH/LH release. As components of the specific sensing system, the ependymocytes lining the cerebroventricular wall in the lower brainstem integrate the information derived from metabolic signals to control gonadotropin release. One of the pathways responsible for the energetic control of gonadal activity consists of noradrenergic neurons from the solitary tract nucleus in the lower brainstem, projecting to the paraventricular nucleus of the hypothalamus. Further studies are needed to elucidate the mechanisms underlying energetic control of reproductive function.

When do we eat? Ingestive behavior, survival, and reproductive success

The neuroendocrinology of ingestive behavior is a topic central to human health, particularly in light of the prevalence of obesity, eating disorders, and diabetes. The study of food intake in laboratory rats and mice has yielded some useful hypotheses, but there are still many gaps in our knowledge. Ingestive behavior is more complex than the consummatory act of eating, and decisions about when and how much to eat usually take place in the context of potential mating partners, competitors, predators, and environmental fluctuations that are not present in the laboratory. We emphasize appetitive behaviors, actions that bring animals in contact with a goal object, precede consummatory behaviors, and provide a window into motivation. Appetitive ingestive behaviors are under the control of neural circuits and neuropeptide systems that control appetitive sex behaviors and differ from those that control consummatory ingestive behaviors. Decreases in the availability of oxidizable metabolic fuels enhance the stimulatory effects of peripheral hormones on appetitive ingestive behavior and the inhibitory effects on appetitive sex behavior, putting a new twist on the notion of leptin, insulin, and ghrelin "resistance." The ratio of hormone concentrations to the availability of oxidizable metabolic fuels may generate a critical signal that schedules conflicting behaviors, e.g., mate searching vs. foraging, food hoarding vs. courtship, and fat accumulation vs. parental care. In species representing every vertebrate taxa and even in some invertebrates, many putative "satiety" or "hunger" hormones function to schedule ingestive behavior in order to optimize reproductive success in environments where energy availability fluctuates.

Scientific and Regulatory Policy Committee (SRPC) Paper

Hormonally mediated effects on the female reproductive system may manifest as pathologic changes of endocrine-responsive organs and altered reproductive function. Identification of these effects requires proper assessment, which may include investigative studies to profile female reproductive hormones. Here, we briefly describe normal hormonal patterns across the estrous or menstrual cycle and provide general guidance on measuring female reproductive hormones and characterizing hormonal disturbances in nonclinical toxicity studies. Although species used in standard toxicity studies share basic features of reproductive endocrinology, there are important species differences that affect both study design and interpretation of results. Diagnosing female reproductive hormone disturbances can be complicated by many factors, including estrous/menstrual cyclicity, diurnal variation, and age- and stress-related factors. Thus, female reproductive hormonal measurements should not generally be included in first-tier toxicity studies of standard design with groups of unsynchronized intact female animals. Rather, appropriately designed and statistically powered investigative studies are recommended in order to properly identify ovarian and/or pituitary hormone changes and bridge these effects to mechanistic evaluations and safety assessments. This article is intended to provide general considerations and approaches for these types of targeted studies.

Kisspeptins and Reproduction: Physiological Roles and Regulatory Mechanisms

Procreation is essential for survival of species. Not surprisingly, complex neuronal networks have evolved to mediate the diverse internal and external environmental inputs that regulate reproduction in vertebrates. Ultimately, these regulatory factors impinge, directly or indirectly, on a final common pathway, the neurons producing the gonadotropin-releasing hormone (GnRH), which stimulates pituitary gonadotropin secretion and thereby gonadal function. Compelling evidence, accumulated in the last few years, has revealed that kisspeptins, a family of neuropeptides encoded by the Kiss1 gene and produced mainly by neuronal clusters at discrete hypothalamic nuclei, are pivotal upstream regulators of GnRH neurons. As such, kisspeptins have emerged as important gatekeepers of key aspects of reproductive maturation and function, from sexual differentiation of the brain and puberty onset to adult regulation of gonadotropin secretion and the metabolic control of fertility. This review aims to provide a comprehensive account of the state-of-the-art in the field of kisspeptin physiology by covering in-depth the consensus knowledge on the major molecular features, biological effects, and mechanisms of action of kisspeptins in mammals and, to a lesser extent, in nonmammalian vertebrates. This review will also address unsolved and contentious issues to set the scene for future research challenges in the area. By doing so, we aim to endow the reader with a critical and updated view of the physiological roles and potential translational relevance of kisspeptins in the integral control of reproductive function.

Sense and nonsense in metabolic control of reproduction

An exciting synergistic interaction occurs among researchers working at the interface of reproductive biology and energy homeostasis. Reproductive biologists benefit from the theories, experimental designs, and methodologies used by experts on energy homeostasis while they bring context and meaning to the study of energy homeostasis. There is a growing recognition that identification of candidate genes for obesity is little more than meaningless reductionism unless those genes and their expression are placed in a developmental, environmental, and evolutionary context. Reproductive biology provides this context because metabolic energy is the most important factor that controls reproductive success and gonadal hormones affect energy intake, storage, and expenditure. Reproductive hormone secretion changes during development, and reproductive success is key to evolutionary adaptation, the process that most likely molded the mechanisms that control energy balance. It is likely that by viewing energy intake, storage, and expenditure in the context of reproductive success, we will gain insight into human obesity, eating disorders, diabetes, and other pathologies related to fuel homeostasis. This review emphasizes the metabolic hypothesis: a sensory system monitors the availability of oxidizable metabolic fuels and orchestrates behavioral motivation to optimize reproductive success in environments where energy availability fluctuates or is unpredictable.

Mesenteric infusion of a volatile fatty acid prevents body weight loss and transiently restores luteinising hormone pulse frequency in ovariectomised, food-restricted ewes

Pulsatile luteinising hormone (LH) secretion is suppressed by food restriction and rapidly restored by return to ad lib. feeding concomitant with an increase in the oxidation of free fatty acids, although there is no increase in plasma leptin concentrations or body fat content in ovariectomised ewes. The ingestion of food may stimulate LH secretion by increasing availability of oxidisable metabolic substrates. Ruminal digestion is characterised by the production of volatile fatty acids and, of these, propionate is the major gluconeogenic substrate, and both glucose and propionate are oxidisable in a variety of tissues. To examine whether increases in mesenteric propionate concentrations are sufficient for restoration of pulsatile LH secretion during a period of food restriction, adult, food-restricted, hypogonadotrophic, ovariectomised ewes received mesenteric vein infusions of 5 μmol/min/kg body weight (BW) propionate or saline, whereas normal weight, ad lib.-fed ewes received mesenteric infusions of saline for 10 days. Blood samples were taken every 10 min for 5 h before the start of the 10-day infusion period, and continued throughout the first 5 h of infusion on the afternoon of day 1, and in the morning on days 2, 7 and 10. Propionate-infused, food-restricted and ad lib.-fed, saline-infused ewes showed a significantly higher LH pulse frequency compared to that of food-restricted-saline-infused ewes on postinfusion days 1 and 2 but not on days 7 and 10, and only the saline-infused, food-restricted group lost a significant amount of body weight. These results indicate that the reproductive system can respond acutely to infusion of metabolic fuels such as propionate, although a sustained recovery of pulsatile LH secretion requires more than an increase in this single metabolic substrate.

Involvement of brain ketone bodies and the noradrenergic pathway in diabetic hyperphagia in rats

Uncontrolled type 1 diabetes leads to hyperphagia and severe ketosis. This study was conducted to test the hypothesis that ketone bodies act on the hindbrain as a starvation signal to induce diabetic hyperphagia. Injection of an inhibitor of monocarboxylate transporter 1, a ketone body transporter, into the fourth ventricle normalized the increase in food intake in streptozotocin (STZ)-induced diabetic rats. Blockade of catecholamine synthesis in the hypothalamic paraventricular nucleus (PVN) also restored food intake to normal levels in diabetic animals. On the other hand, hindbrain injection of the ketone body induced feeding, hyperglycemia, and fatty acid mobilization via increased sympathetic activity and also norepinephrine release in the PVN. This result provides evidence that hyperphagia in STZ-induced type 1 diabetes is signaled by a ketone body sensed in the hindbrain, and mediated by noradrenergic inputs to the PVN.

PI3K: An Attractive Candidate for the Central Integration of Metabolism and Reproduction

In neurons, as in a variety of other cell types, the enzyme phosphatidylinositol-3-kinase (PI3K) is a key intermediate that is common to the signaling pathways of a number of peripheral metabolic cues, including insulin and leptin, which are well known to regulate both metabolic and reproductive functions. This review article will explore the possibility that PI3K is a key integrator of metabolic and neural signals regulating gonadotropin releasing hormone (GnRH)/luteinizing hormone (LH) release and explore the hypothesis that this enzyme is pivotal in many disorders where gonadotropin release is at risk. Although the mechanisms mediating the influence of metabolism and nutrition on fertility are currently unclear, the strong association between metabolic disorders and infertility is undeniable. For example, women suffering from anorectic disorders experience amenorrhea as a consequence of malnutrition-induced impairment of LH release, and at the other extreme, obesity is also commonly co-morbid with menstrual dysfunction and infertility. Impaired hypothalamic insulin and leptin receptor signaling is thought to be at the core of reproductive disorders associated with metabolic dysfunction. While low levels of leptin and insulin characterize states of negative energy balance, prolonged nutrient excess is associated with insulin and leptin resistance. Metabolic models known to alter GnRH/LH release such as diabetes, diet-induced obesity, and caloric restriction are also accompanied by impairment of PI3K signaling in insulin and leptin sensitive tissues including the hypothalamus. However, a clear link between this signaling pathway and the control of GnRH release by peripheral metabolic cues has not been established. Investigating the role of the signaling pathways shared by metabolic cues that are critical for a normal reproductive state can help identify possible targets in the treatment of metabolic and reproductive disorders such as polycystic ovarian syndrome.

Kisspeptins: bridging energy homeostasis and reproduction

Body energy reserves and metabolic state are relevant modifiers of puberty onset and fertility; forms of metabolic stress ranging from persistent energy insufficiency to morbid obesity are frequently linked to reproductive disorders. The mechanisms for such a close connection between energy balance and reproduction have been the subject of considerable attention; however, our understanding of the neurobiological basis for this phenomenon is still incomplete. In mid 1990s, the adipose-hormone, leptin, was proven as an essential signal for transmitting metabolic information onto the centers governing puberty and reproduction; yet, the ultimate mode of action of leptin on GnRH neurons has remained contentious for years. More recently, kisspeptins, a family of neuropeptides encoded by the Kiss1 gene, have emerged as conduits for the metabolic regulation of reproduction and putative effectors of leptin actions on GnRH neurons. This review recapitulates the experimental evidence obtained to date, mostly in laboratory rodents, supporting the function of kisspeptins in bridging energy balance and reproduction, with special emphasis on recent developments in this field, such as the recognition of mTOR (mammalian target of rapamycin) and Crtc1 (Creb1-regulated transcription coactivator-1) as putative mediators for leptin regulation of Kiss1 expression, as well as the identification of other potential metabolic modulators of kisspeptin signaling, such as ghrelin, neuropeptide Y (NPY) and melanin-concentrating hormone (MCH).

Androgen receptor antagonism and an insulin sensitizer block the advancement of vaginal opening by high-fat diet in mice

Reduced hypothalamic sensitivity to steroid negative feedback may contribute to the onset of puberty. In high fat-fed rodents, the timing of vaginal opening (VO) is advanced, suggesting that puberty begins earlier. Because obesity can increase androgens, which interfere with normal steroid feedback in adult females, we hypothesized that androgens reduce hypothalamic sensitivity to negative feedback during puberty and that blocking androgen action would prevent advanced VO in high fat-fed mice. Age at VO was examined in mice fed high-fat or low-fat diets from weaning and treated with the androgen receptor antagonist flutamide or vehicle (controls). VO was advanced in high-fat vs. low-fat controls, and flutamide blocked this advancement. VO was also delayed in low fat-fed flutamide-treated females, suggesting involvement of androgens in the timing of normal puberty. We next investigated if high-fat diet-induced insulin resistance contributes to early VO, as elevated insulin can stimulate androgen production. VO was examined in mice on either diet treated with the insulin sensitizer metformin. Metformin blocked high-fat advancement of VO but did not alter the timing of VO in low fat-fed mice. Insulin was elevated in high fat-fed females that had undergone VO compared with age-matched low fat-fed or metformin-treated animals on either diet that had not undergone VO. Together, these data suggest a model in which metabolic changes induced by high-fat diet, including transient increased circulating insulin, act in part by increasing androgen action to influence the timing of puberty in females.

Influence of ovarian hormones on development of ingestive responding to alterations in fatty acid oxidation in female rats

Adult male rats have been demonstrated to increase food intake in response to administration of drugs that interfere with oxidation of fatty acids (e.g. methyl palmoxirate and mercaptoacetate [MA]), effects that are larger in animals maintained on a high-fat diet. In contrast, while administration of MA has been reported to stimulate food intake in pre-pubertal female rats, food intake is not stimulated by MA in adult female rats. Instead, administration of MA to adult females results in changes in reproductive behavior and physiology. The present experiments were designed to examine the effects of administration of MA on food intake in adult female rats. The results demonstrated that, as previously reported, food intake was stimulated by MA in adult male rats on low-fat and high-fat diets, but food intake was not stimulated by MA in gonadally-intact adult female rats on either low-fat or high-fat diet. Further, MA did not stimulate food intake in female rats ovariectomized as adults. However, when females were ovariectomized prior to the onset of puberty (postnatal day 25-28), food intake was stimulated by administration of MA in adulthood. Finally, cyclic injections of 17-beta-estradiol benzoate given to females ovariectomized prior to the onset of puberty abolished the stimulatory effects of MA on food intake in adult females. Taken together, the data suggest that exposure to estrogens during the time of puberty in female rats can persistently alter adult ingestive responding to signals related to changes in energy utilization.