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

Enkephalin-δ opioid receptor signaling partly mediates suppression of LH release during early lactation in rats

Gonadal function is often suppressed during lactation in mammals including rodents, ruminants, and primates. This suppression is thought to be mostly due to the inhibition of the tonic (pulsatile) release of gonadotropin-releasing hormone (GnRH) and consequent gonadotropin. Accumulating evidence suggests that kisspeptin neurons in the arcuate nucleus (ARC) play a critical role in the regulation of pulsatile GnRH/gonadotropin release, and kisspeptin mRNA (Kiss1) and/or kisspeptin expression in the ARC are strongly suppressed by the suckling stimuli in lactating rats. This study aimed to examine whether the central enkephalin-δ-opioid receptor (DOR) signaling mediates the suckling-induced suppression of luteinizing hormone (LH) release in lactating rats. Central administration of a selective DOR antagonist increased the mean plasma LH levels and baseline of LH pulses in ovariectomized lactating mother rats compared to vehicle-injected control dams on day 8 of lactation without affecting the number of Kiss1-expressing cells and the intensity of Kiss1 mRNA signals in the ARC. Furthermore, the suckling stimuli significantly increased the number of enkephalin mRNA (Penk)-expressing cells and the intensity of Penk mRNA signals in the ARC compared to non-lactating control rats. Collectively, these results suggest that central DOR signaling, at least in part, mediates the suppression of LH release induced by suckling stimuli in lactating rats via indirect and/or direct inhibition of ARC kisspeptin neurons.

Kisspeptin and lactational anestrus: Current understanding and future prospects

Lactational anestrus, characterized by the suppression of pulsatile gonadotropin-releasing hormone (GnRH)/luteinizing hormone (LH) release, would be a strategic adaptation to ensure survival by avoiding pregnancy during lactation in mammals. In the present article, we first provide a current understanding of the central regulation of reproduction in mammals, i.e., a fundamental role of arcuate kisspeptin neurons in mammalian reproduction by driving GnRH/LH pulses. Second, we discuss the central mechanism inhibiting arcuate Kiss1 (encoding kisspeptin) expression and GnRH/LH pulses during lactation with a focus on suckling stimulus, negative energy balance due to milk production, and the role of circulating estrogen in rats. We also discuss upper regulators that control arcuate kisspeptin neurons in rats during the early and late lactation periods based on the findings obtained by a lactating rat model. Finally, we discuss potential reproductive technology for the improvement of reproductive performance in milking cows.

Crosstalk within a brain-breast-bone axis regulates mineral and skeletal metabolism during lactation

To support the increased calcium demands for milk production during lactation, a dramatic and reversible physiological response occurs to alter bone and mineral metabolism. This coordinated process involves a brain-breast-bone axis that integrates hormonal signals that allow for adequate calcium delivery to milk yet also protects the maternal skeletal from excessive bone loss or decreases in bone quality or function. Here, we review the current knowledge on the crosstalk between the hypothalamus, mammary gland, and skeleton during lactation. We discuss the rare entity of pregnancy and lactation associated osteoporosis and consider how the physiology of bone turnover in lactation may impact the pathophysiology of postmenopausal osteoporosis. Further understanding of the regulators of bone loss during lactation, particularly in humans, may provide insights into new therapies for osteoporosis and other diseases of excess bone loss.

Circadian rhythm in hypothalamic leptin receptor (Ob-Rb) mRNA expressions and cerebrospinal fluid and circulating glucose and leptin levels in lactating rats

In lactating animals, the food consumption increases several-fold for milk supply to the pups. The present study was conducted to clarify the relationship between the hyperphagia during lactation and hypothalamic leptin receptor (Ob-Rb) mRNA expression, cerebrospinal fluid (CSF) and circulating leptin and glucose levels. Food intakes significantly higher in lactation than in non-lactation at all time points (3 points: light phase, 4 points: dark phase) of the day. However, the expression of the hypothalamic Ob-Rb mRNA showed similar circadian rhythms in both the non-lactation and lactation, with only slight differences between the two groups. CSF leptin and glucose levels were constant throughout the day in both non-lactation and lactation, and there was almost no difference between the two groups at each time point. Circulating leptin and glucose levels showed circadian rhythms only in the non-lactating period, and were lower in lactation than in non-lactation, especially in the dark phase. In conclusion, the present study provides evidence that Ob-Rb mRNA expression fluctuates in the lactation period as well as in the non-lactation period, suggesting that the expression profile of whole hypothalamic Ob-Rb may not contribute to the difference in food consumption between lactation and non-lactation, and that chronic decrease in blood glucose levels may be associated with the increase in food consumption during lactation.

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There was no difference in CSF leptin levels between lactating and non-lactating rats.

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Hypothalamic Ob-Rb mRNA expressions showed different circadian rhythms between non-lactation and lactation.

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Chronic decrease in blood glucose levels may be associated with the elevation of food consumption in lactating rats.

Central actions of insulin during pregnancy and lactation

Pregnancy and lactation are highly metabolically demanding states. Maternal glucose is a key fuel source for the growth and development of the fetus, as well as for the production of milk during lactation. Hence, the maternal body undergoes major adaptations in the systems regulating glucose homeostasis to cope with the increased demand for glucose. As part of these changes, insulin levels are elevated during pregnancy and lower in lactation. The increased insulin secretion during pregnancy plays a vital role in the periphery; however, the potential effects of increased insulin action in the brain have not been widely investigated. In this review, we consider the impact of pregnancy on brain access and brain levels of insulin. Moreover, we explore the hypothesis that pregnancy is associated with site-specific central insulin resistance that is adaptive, allowing for the increases in peripheral insulin secretion without the consequences of increased central and peripheral insulin functions, such as to stimulate glucose uptake into maternal tissues or to inhibit food intake. Conversely, the loss of central insulin actions may impair other functions, such as insulin control of the autonomic nervous system. The potential role of low insulin in facilitating adaptive responses to lactation, such as hyperphagia and suppression of reproductive function, are also discussed. We end the review with a list of key research questions requiring resolution.

Fenugreek Stimulates the Expression of Genes Involved in Milk Synthesis and Milk Flow through Modulation of Insulin/GH/IGF-1 Axis and Oxytocin Secretion

We previously demonstrated galactagogue effect of fenugreek in a rat model of lactation challenge, foreshadowing its use in women's breastfeeding management. To assess longitudinal molecular mechanisms involved in milk synthesis/secretion in dams submitted to fenugreek supplementation, inguinal mammary, pituitary glands and plasma were isolated in forty-three rats nursing large 12 pups-litters and assigned to either a control (CTL) or a fenugreek-supplemented (FEN) diet during lactation. RT-PCR were performed at days 12 and 18 of lactation (L12 and L18) and the first day of involution (Inv1) to measure the relative expression of genes related to both milk synthesis and its regulation in the mammary gland and lactogenic hormones in the pituitary gland. Plasma hormone concentrations were measured by ELISA. FEN diet induced 2- to 3-times higher fold change in relative expression of several genes related to macronutrient synthesis (Fasn, Acaca, Fabp3, B4galt1, Lalba and Csn2) and energy metabolism (Cpt1a, Acads) and in IGF-1 receptor in mammary gland, mainly at L12. Pituitary oxytocin expression and plasma insulin concentration (+77.1%) were also significantly increased. Altogether, these findings suggest fenugreek might extend duration of peak milk synthesis through modulation of the insulin/GH/IGF-1 axis and increase milk ejection by activation of oxytocin secretion.

The role of glucagon-like peptide-1 in reproduction: from physiology to therapeutic perspective

BACKGROUND

Glucagon-like peptide-1 (GLP-1) receptor agonists (GLP-1 RAs) have become firmly established in the treatment of type 2 diabetes and obesity, disorders frequently associated with diminished reproductive health. Understanding of the role of GLP-1 and GLP-1 RAs in reproduction is currently limited and largely unaddressed in clinical studies.

OBJECTIVE AND RATIONALE

The purpose of this narrative review is to provide a comprehensive overview of the role of GLP-1 in reproduction and to address a therapeutic perspective that can be derived from these findings.

SEARCH METHODS

We performed a series of PubMed database systemic searches, last updated on 1 February 2019, supplemented by the authors' knowledge and research experience in the field. A search algorithm was developed incorporating the terms glucagon-like peptide-1, GLP-1, glucagon-like peptide-1 receptor, GLP-1R, or incretins, and this was combined with terms related to reproductive health. The PICO (Population, Intervention, Comparison, Outcome) framework was used to identify interventional studies including GLP-1 RAs and dipeptidyl peptidase-4 (DPP-4) inhibitors, which prevent the degradation of endogenously released GLP-1. We identified 983 potentially relevant references. At the end of the screening process, we included 6 observational (3 preclinical and 3 human) studies, 24 interventional (9 preclinical and 15 human) studies, 4 case reports, and 1 systematic and 2 narrative reviews.

OUTCOMES

The anatomical distribution of GLP-1 receptor throughout the reproductive system and observed effects of GLP-1 in preclinical models and in a few clinical studies indicate that GLP-1 might be one of the important modulating signals connecting the reproductive and metabolic system. The outcomes show that there is mostly stimulating role of GLP-1 and its mimetics in mammalian reproduction that goes beyond mere weight reduction. In addition, GLP-1 seems to have anti-inflammatory and anti-fibrotic effects in the gonads and the endometrium affected by obesity, diabetes, and polycystic ovary syndrome (PCOS). It also seems that GLP-1 RAs and DPP-4 inhibitors can reverse polycystic ovary morphology in preclinical models and decrease serum concentrations of androgens and their bioavailability in women with PCOS. Preliminary data from interventional clinical studies suggest improved menstrual regularity as well as increased fertility rates in overweight and/or obese women with PCOS treated with GLP-1 RAs in the preconception period.

WIDER IMPLICATIONS

GLP-1 RAs and DPP-4 inhibitors show promise in the treatment of diabetes and obesity-related subfertility. Larger interventional studies are needed to establish the role of preconception intervention with GLP-1 based therapies, assessing fertility outcomes in obesity, PCOS, and diabetes-related fertility problems. The potential impact of the dose- and exposure time-response of different GLP-1 RAs need further exploration. Future research should also investigate sex-specific variability of GLP-1 on reproductive outcomes, in particular on the gonads where the observations in males are most conflicting.

The role of prolactin in co-ordinating fertility and metabolic adaptations during reproduction

Mammalian pregnancy and lactation is accompanied by a period of infertility that takes place in the midst of a sustained increase in food intake. Indeed, successful reproduction in females is dependent on co-ordination of the distinct systems that regulate reproduction and metabolism. Rather than arising from different mechanisms during pregnancy and lactation, we propose that elevations in lactogenic hormones (predominant among these being prolactin and the placental lactogens), are ideally placed to influence both of these systems at the appropriate time. We review the literature examining the impacts of lactogens on fertility and energy homeostasis in the virgin state, during pregnancy and lactation and potential long-term impacts of reproductive experience. Taken together, the literature indicates that duration and pattern of lactogen exposure is a vital factor in the ability of these hormones to alter reproduction and food intake. Transient increases in prolactin, as typically seen in healthy virgin females and males, are unable to exert lasting impacts. Importantly, both suppression of fertility and increased food intake are only observed following exposure to chronically-elevated levels of lactogens. Physiologically, the only time this pattern of lactogenic secretion is maintained in the healthy female is during pregnancy and lactation, when co-ordination between these regulatory systems emerges. This article is part of the special issue on 'Neuropeptides'.

Investigation of the potential effects of triclosan as an endocrine disruptor in female rats: Uterotrophic assay and two-generation study

Triclosan (TCS) is a phenolic compound with antimicrobial action widely used in cosmetics and other personal care products and other industry segments. Its widespread use over the decades has made TCS one of the most commonly detected compounds in wastewater and effluent worldwide already being found in human urine, plasma and milk. In this study, the (anti)estrogenicity of TCS was evaluated in the uterotrophic assay in 18-day old female Wistar rats. In a second protocol, female rats were evaluated for the reproductive effects of TCS in a two-generation reproduction toxicity study. Female rats were daily treated by gavage with TCS at the doses of 0.8, 2.4 and 8.0 mg/kg/day or corn oil (control group) over 10 weeks (F0) and over 14 weeks (F1) prior to mating and then throughout mating, gestation and lactation until weaning of F1 and F2 generation respectively. TCS had no effect on the uterus weight in the uterotrophic assay. In the two-generation study, the TCS exposure compromised female sexual behavior, decreased maternal food consumption and increased pup grooming on TCS 2.4 group. The TCS chronic exposure also decreased the perimetrium thickness of F0 females from TCS 8.0 group and growing follicle number of TCS 2.4 females from F1 generation. Despite the some specific changes detected in the two-generation study, no impairment was observed in the uterotrophic assay and other important reproductive endpoints. In a weight of evidence evaluation, the results suggest that exposure to TCS at low doses did not act as an endocrine disruptor in the female rat reproductive system.

Neuroactive steroids and diabetic complications in the nervous system

Important complications of diabetes mellitus in the nervous system are represented by diabetic peripheral neuropathy and diabetic encephalopathy. In this context, an important link is represented by neuroactive steroids (i.e., steroids coming from peripheral glands and affecting nervous functionality as well as directly synthesized in the nervous system). Indeed, diabetes does not only affect the reproductive axis and consequently the levels of sex steroid hormones, but also those of neuroactive steroids. Indeed, as will be here summarized, the levels of these neuromodulators present in the central and peripheral nervous system are affected by the pathology in a sex-dimorphic way. In addition, some of these neuroactive steroids, such as the metabolites of progesterone or testosterone, as well as pharmacological tools able to increase their levels have been demonstrated, in experimental models, to be promising protective agents against diabetic peripheral neuropathy and diabetic encephalopathy.

GLP-1R Signaling Directly Activates Arcuate Nucleus Kisspeptin Action in Brain Slices but Does not Rescue Luteinizing Hormone Inhibition in Ovariectomized Mice During Negative Energy Balance

Kisspeptin (Kiss1) neurons in the hypothalamic arcuate nucleus (ARC) are key components of the hypothalamic-pituitary-gonadal axis, as they regulate the basal pulsatile release of gonadotropin releasing hormone (GnRH). ARC Kiss1 action is dependent on energy status, and unmasking metabolic factors responsible for modulating ARC Kiss1 neurons is of great importance. One possible factor is glucagon-like peptide 1 (GLP-1), an anorexigenic neuropeptide produced by brainstem preproglucagon neurons. Because GLP fiber projections and the GLP-1 receptor (GLP-1R) are abundant in the ARC, we hypothesized that GLP-1R signaling could modulate ARC Kiss1 action. Using ovariectomized mice, we found that GLP-producing fibers come in close apposition with ARC Kiss1 neurons; these neurons also contain Glp1r mRNA. Electrophysiological recordings revealed that liraglutide (a long-acting GLP-1R agonist) increased action potential firing and caused a direct membrane depolarization of ARC Kiss1 cells in brain slices. We determined that brainstem preproglucagon mRNA is decreased after a 48-h fast in mice, a negative energy state in which ARC Kiss1 expression and downstream GnRH/luteinizing hormone (LH) release are potently suppressed. However, activation of GLP-1R signaling in fasted mice with liraglutide was not sufficient to prevent LH inhibition. Furthermore, chronic central infusions of the GLP-1R antagonist, exendin(9–39), in ad libitum–fed mice did not alter ARC Kiss1 mRNA or plasma LH. As a whole, these data identify a novel interaction of the GLP-1 system with ARC Kiss1 neurons but indicate that CNS GLP-1R signaling alone is not critical for the maintenance of LH during fasting or normal feeding.

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

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

RF-amide neuropeptides and their receptors in Mammals: Pharmacological properties, drug development and main physiological functions

RF-amide neuropeptides, with their typical Arg-Phe-NH2 signature at their carboxyl C-termini, belong to a lineage of peptides that spans almost the entire life tree. Throughout evolution, RF-amide peptides and their receptors preserved fundamental roles in reproduction and feeding, both in Vertebrates and Invertebrates. The scope of this review is to summarize the current knowledge on the RF-amide systems in Mammals from historical aspects to therapeutic opportunities. Taking advantage of the most recent findings in the field, special focus will be given on molecular and pharmacological properties of RF-amide peptides and their receptors as well as on their implication in the control of different physiological functions including feeding, reproduction and pain. Recent progress on the development of drugs that target RF-amide receptors will also be addressed.

Neuroendocrine regulation of lactation and milk production

Prolactin (PRL) released from lactotrophs of the anterior pituitary gland in response to the suckling by the offspring is the major hormonal signal responsible for stimulation of milk synthesis in the mammary glands. PRL secretion is under chronic inhibition exerted by dopamine (DA), which is released from neurons of the arcuate nucleus of the hypothalamus into the hypophyseal portal vasculature. Suckling by the young activates ascending systems that decrease the release of DA from this system, resulting in enhanced responsiveness to one or more PRL-releasing hormones, such as thyrotropin-releasing hormone. The neuropeptide oxytocin (OT), synthesized in magnocellular neurons of the hypothalamic supraoptic, paraventricular, and several accessory nuclei, is responsible for contracting the myoepithelial cells of the mammary gland to produce milk ejection. Electrophysiological recordings demonstrate that shortly before each milk ejection, the entire neurosecretory OT population fires a synchronized burst of action potentials (the milk ejection burst), resulting in release of OT from nerve terminals in the neurohypophysis. Both of these neuroendocrine systems undergo alterations in late gestation that prepare them for the secretory demands of lactation, and that reduce their responsiveness to stimuli other than suckling, especially physical stressors. The demands of milk synthesis and release produce a condition of negative energy balance in the suckled mother, and, in laboratory rodents, are accompanied by a dramatic hyperphagia. The reduction in secretion of the adipocyte hormone, leptin, a hallmark of negative energy balance, may be an important endocrine signal to hypothalamic systems that integrate lactation-associated food intake with neuroendocrine systems.

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.

Prolactin regulation of kisspeptin neurones in the mouse brain and its role in the lactation-induced suppression of kisspeptin expression

Hyperprolactinaemia is a major cause of infertility in both males and females, although the mechanism by which prolactin inhibits the reproductive axis is not clear. The aim of the present study was to test the hypothesis that elevated prolactin causes suppression of kisspeptin expression in the hypothalamus, resulting in reduced release of gonadotrophin-releasing hormone (GnRH) and consequent infertility. In oestrogen-treated ovariectomised mice, chronic prolactin-treatment prevented the rise in luteinising hormone (LH) seen in vehicle-treated mice. Kiss1 mRNA was significantly suppressed in both the rostral periventricular region of the third ventricle (RP3V) and arcuate nucleus after prolactin treatment. Exogenous prolactin treatment induced phosphorylated signal transducer and activator of transcription 5 (pSTAT5) in kisspeptin neurones, and suppression of endogenous prolactin using bromocriptine reduced levels of pSTAT5 in kisspeptin neurones, suggesting that prolactin acts directly on kisspeptin neurones. By contrast, fewer than 1% of GnRH neurones expressed pSTAT5 in either dioestrous or lactating mice. As reported previously, there was significant suppression of kisspeptin mRNA and protein in the RP3V on day 7 of lactation, although not in the arcuate nucleus. Bromocriptine treatment significantly increased Kiss1 mRNA expression in the RP3V, although not to dioestrous levels. Unilateral thelectomy, aiming to eliminate sensory inputs from nipples on one side of the body, failed to alter the reduction in the number of kisspeptin neurones observed in the RP3V. These data demonstrate that chronic prolactin administration suppressed serum LH, and reduced Kiss1 mRNA levels in both the RP3V and arcuate nucleus, consistent with the hypothesis that prolactin-induced suppression of kisspeptin secretion might mediate the inhibitory effects of prolactin on GnRH secretion. During lactation, however, the suppression of Kiss1 mRNA in the RP3V was only partially reversed by the administration of bromocriptine to block elevated levels of prolactin, suggesting that, although elevated prolactin contributes to lactational anovulation, additional non-neural factors must also contribute to the lactation-induced suppression of kisspeptin neurones.

Insulin: its role in the central control of reproduction

Insulin has long been recognized as a key regulator of energy homeostasis via its actions at the level of the brain, but in addition, plays a role in regulating neural control of reproduction. In this review, we consider and compare evidence from animal models demonstrating a role for insulin for physiological control of reproduction by effects on GnRH/LH secretion. We also review the role that insulin plays in prenatal programming of adult reproduction, and consider specific candidate neurons in the adult hypothalamus by which insulin may act to regulate reproductive function. Finally, we review clinical evidence of the role that insulin may play in adult human fertility and reproductive disorders. Overall, while insulin appears to have a significant impact on reproductive neuroendocrine function, there are many unanswered questions regarding its precise sites and mechanisms of action, and their impact on developing and adult reproductive neuroendocrine function.

Kisspeptin, c-Fos and CRFR type 2 co-expression in the hypothalamus after insulin-induced hypoglycaemia

Normal reproductive function is dependent upon availability of glucose and insulin-induced hypoglycaemia is a metabolic stressor known to disrupt the ovine oestrous cycle. We have recently shown that IIH has the ability to delay the LH surge of intact ewes. In the present study, we examined brain tissue to determine: (i) which hypothalamic regions are activated with respect to IIH and (ii) the effect of IIH on kisspeptin cell activation and CRFR type 2 immunoreactivity, all of which may be involved in disruptive mechanisms. Follicular phases were synchronized with progesterone vaginal pessaries and at 28 h after progesterone withdrawal (PW), animals received saline (n = 6) or insulin (4 IU/kg; n = 5) and were subsequently killed at 31 h after PW (i.e., 3 h after insulin administration). Peripheral hormone concentrations were evaluated, and hypothalamic sections were immunostained for either kisspeptin and c-Fos (a marker of neuronal activation) or CRFR type 2. Within 3 h of treatment, cortisol concentrations had increased whereas plasma oestradiol concentrations decreased in peripheral plasma (p < 0.05 for both). In the arcuate nucleus (ARC), insulin-treated ewes had an increased expression of c-Fos. Furthermore, the percentage of kisspeptin cells co-expressing c-Fos increased in the ARC (from 11 to 51%; p < 0.05), but there was no change in the medial pre-optic area (mPOA; 14 vs 19%). CRFR type 2 expression in the lower part of the ARC and the median eminence was not altered by insulin treatment. Thus, disruption of the LH surge after IIH in the follicular phase is not associated with decreased kisspeptin cell activation or an increase in CRFR type 2 in the ARC but may involve other cell types located in the ARC nucleus which are activated in response to IIH.

Food restriction during lactation suppresses Kiss1 mRNA expression and kisspeptin-stimulated LH release in rats

Among the numerous physiological changes that accompany lactation is the suppression of the reproductive axis. The aim of this study was to investigate a possible role for the kisspeptin system in the restoration of the hypothalamic-pituitary-gonadal axis during late lactation in rats using a food restriction model that allows manipulation of the duration of lactational anovulation. Kiss1 mRNA expression and kisspeptin-immunoreactive cell counts were examined in both food-restricted dams and ad libitum (AL)-fed dams across late lactation when LH concentrations begin to increase. In the arcuate nucleus, Kiss1 mRNA expression and kisspeptin-positive cell counts were suppressed during late lactation. In the anteroventral periventricular (AVPV), day 15 food-restricted dams had significantly lower AVPV Kiss1 mRNA expression and a decreased LH response to exogenous kisspeptin compared with the AL-fed dams. Following 5 days of ad libitum food intake, these values were restored to levels similar to those in dams that had been fed ad libitum throughout lactation. In conclusion, this study shows that delayed restoration of the reproductive axis due to food restriction is associated with a decrease in kisspeptin sensitivity and low AVPV Kiss1 mRNA in late lactation.

Fluoxetine dose and administration method differentially affect hippocampal plasticity in adult female rats

Selective serotonin reuptake inhibitor medications are one of the most common treatments for mood disorders. In humans, these medications are taken orally, usually once per day. Unfortunately, administration of antidepressant medications in rodent models is often through injection, oral gavage, or minipump implant, all relatively stressful procedures. The aim of the present study was to investigate how administration of the commonly used SSRI, fluoxetine, via a wafer cookie, compares to fluoxetine administration using an osmotic minipump, with regards to serum drug levels and hippocampal plasticity. For this experiment, adult female Sprague-Dawley rats were divided over the two administration methods: (1) cookie and (2) osmotic minipump and three fluoxetine treatment doses: 0, 5, or 10 mg/kg/day. Results show that a fluoxetine dose of 5 mg/kg/day, but not 10 mg/kg/day, results in comparable serum levels of fluoxetine and its active metabolite norfluoxetine between the two administration methods. Furthermore, minipump administration of fluoxetine resulted in higher levels of cell proliferation in the granule cell layer (GCL) at a 5 mg dose compared to a 10 mg dose. Synaptophysin expression in the GCL, but not CA3, was significantly lower after fluoxetine treatment, regardless of administration method. These data suggest that the administration method and dose of fluoxetine can differentially affect hippocampal plasticity in the adult female rat.

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.

Lactational anovulation in mice results from a selective loss of kisspeptin input to GnRH neurons

In mammals, lactation is associated with a period of infertility characterized by the loss of pulsatile secretion of GnRH and cessation of ovulatory cycles. Despite the importance of lactational infertility in determining overall fecundity of a species, the mechanisms by which the suckling stimulus suppresses GnRH secretion remain unclear. Because kisspeptin neurons are critical for fertility, the aim of this study was to test the hypothesis that reduced kisspeptin expression might mediate the lactation-induced suppression of fertility, using mouse models. In the rostral periventricular area of the third ventricle (RP3V), a progressive decrease in RP3V Kiss1 mRNA levels was observed during pregnancy culminating in a 10-fold reduction during lactation compared with diestrous controls. This was associated with approximately 60% reduction in the numbers of kisspeptin-immunoreactive neurons in the RP3V detected during lactation. Similarly, in the arcuate nucleus there was also a significant decrease in Kiss1 mRNA levels during late pregnancy and midlactation, and a notable decrease in kisspeptin fiber density during lactation. The functional characteristics of the RP3V kisspeptin input to GnRH neurons were assessed using electrophysiological approaches in an acute brain slice preparation. Although endogenous RP3V kisspeptin neurons were found to activate GnRH neurons in diestrous mice, this was never observed during lactation. This did not result from an absence of kisspeptin receptors because GnRH neurons responded normally to 100 nM exogenous kisspeptin during lactation. The kisspeptin deficit in lactating mice was selective, because GnRH neurons responded normally to RP3V gamma aminobutryic acid inputs during lactation. These data demonstrate that a selective loss of RP3V kisspeptin inputs to GnRH neurons during lactation is the likely mechanism causing lactational anovulation in the mouse.

Developmental fluoxetine exposure and prenatal stress alter sexual differentiation of the brain and reproductive behavior in male rat offspring

Depression during pregnancy and postpartum is a significant health problem and affects up to 20% of women. While selective serotonin reuptake inhibitor (SSRI) medications are the drug of choice for treatment of maternal depression, the combined effect of maternal depression and perinatal SSRI exposure on offspring development is poorly investigated. Our aim was to determine the role of exposure to fluoxetine during development on sexual behavior and sexually dimorphic brain structures in male offspring using a rodent model of maternal adversity. Sprague-Dawley rat dams were stressed during gestation and were chronically treated throughout lactation with either fluoxetine or vehicle beginning on postnatal day 1. Four groups of offspring were used: (1) Control+Vehicle, (2) Control+Fluoxetine, (3) Prenatal Stress+Vehicle, and (4) Prenatal Stress+Fluoxetine. We show here that developmental fluoxetine treatment decreases the anogenital distance in juvenile male offspring. In adult male offspring, maternal fluoxetine treatment results in a decrease in the number of intromissions, a longer latency to the first intromission, and a longer latency to the first ejaculation. Furthermore, developmental fluoxetine and/or prenatal stress decrease the area of the sexually dimorphic nucleus of the preoptic area (SDN-POA). Prenatal stress, but not exposure to developmental fluoxetine, decreases the number of tyrosine hydroxylase (TH)-positive cells in anteroventral periventricular nucleus (AVPv) and the volume of the posterior bed nucleus of the stria terminalis (pBST) in male offspring. These results provide important evidence for the long-term impact of maternal adversity and maternal fluoxetine use on the development of primary endocrinology systems in juvenile and adult male offspring.

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

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

Sex differences in the physiology of eating

Hypothalamic-pituitary-gonadal (HPG) axis function fundamentally affects the physiology of eating. We review sex differences in the physiological and pathophysiological controls of amounts eaten in rats, mice, monkeys, and humans. These controls result from interactions among genetic effects, organizational effects of reproductive hormones (i.e., permanent early developmental effects), and activational effects of these hormones (i.e., effects dependent on hormone levels). Male-female sex differences in the physiology of eating involve both organizational and activational effects of androgens and estrogens. An activational effect of estrogens decreases eating 1) during the periovulatory period of the ovarian cycle in rats, mice, monkeys, and women and 2) tonically between puberty and reproductive senescence or ovariectomy in rats and monkeys, sometimes in mice, and possibly in women. Estrogens acting on estrogen receptor-α (ERα) in the caudal medial nucleus of the solitary tract appear to mediate these effects in rats. Androgens, prolactin, and other reproductive hormones also affect eating in rats. Sex differences in eating are mediated by alterations in orosensory capacity and hedonics, gastric mechanoreception, ghrelin, CCK, glucagon-like peptide-1 (GLP-1), glucagon, insulin, amylin, apolipoprotein A-IV, fatty-acid oxidation, and leptin. The control of eating by central neurochemical signaling via serotonin, MSH, neuropeptide Y, Agouti-related peptide (AgRP), melanin-concentrating hormone, and dopamine is modulated by HPG function. Finally, sex differences in the physiology of eating may contribute to human obesity, anorexia nervosa, and binge eating. The variety and physiological importance of what has been learned so far warrant intensifying basic, translational, and clinical research on sex differences in eating.

Limits to sustained energy intake. XVII. Lactation performance in MF1 mice is not programmed by fetal number during pregnancy

Several studies have suggested that lactation performance may be programmed by the number of fetuses during pregnancy, whereas other studies indicate that processes during lactation are more important. As gestation litter size and litter size in lactation are usually strongly correlated, separating the roles of pregnancy and lactation in lactation performance is difficult. To break this link, we experimentally manipulated litter size of MF1 mice to five or 16 pups per litter by cross-fostering. Litter size and mass at birth were recorded on day 1 of lactation prior to litter size manipulation. Maternal body mass and food intake, litter size and litter mass were measured daily throughout. After weaning, the potential differential utilisation of body tissues of the mothers was investigated. Relationships between maternal mass and food intake, including asymptotic daily food intake at peak lactation, offspring traits and other maternal parameters suggested that the number of fetuses the females had carried during pregnancy had no effect on lactation performance. Litter mass increases depended only on maternal food intake, which was highly variable between individuals, but was independent of fetal litter size. The sizes of key organs and tissues like the liver and alimentary tract were not related to maximal food intake at peak lactation or to fetal litter size, but the masses of the pelage, mammary glands and retroperitoneal fat pad were. These data suggest that while growth of the mammary glands and associated structures may be initiated in gestation, and vary in relation to the number of placentas, the ultimate sizes and activities of the tissues depends primarily on factors during lactation.

Increased neurokinin B (Tac2) expression in the mouse arcuate nucleus is an early marker of pubertal onset with differential sensitivity to sex steroid-negative feedback than Kiss1

At puberty, neurokinin B (NKB) and kisspeptin (Kiss1) may help to amplify GnRH secretion, but their precise roles remain ambiguous. We tested the hypothesis that NKB and Kiss1 are induced as a function of pubertal development, independently of the prevailing sex steroid milieu. We found that levels of Kiss1 mRNA in the arcuate nucleus (ARC) are increased prior to the age of puberty in GnRH/sex steroid-deficient hpg mice, yet levels of Kiss1 mRNA in wild-type mice remained constant, suggesting that sex steroids exert a negative feedback effect on Kiss1 expression early in development and across puberty. In contrast, levels of Tac2 mRNA, encoding NKB, and its receptor (NK3R; encoded by Tacr3) increased as a function of puberty in both wild-type and hpg mice, suggesting that during development Tac2 is less sensitive to sex steroid-dependent negative feedback than Kiss1. To compare the relative responsiveness of Tac2 and Kiss1 to the negative feedback effects of gonadal steroids, we examined the effect of estradiol (E(2)) on Tac2 and Kiss1 mRNA and found that Kiss1 gene expression was more sensitive than Tac2 to E(2)-induced inhibition at both juvenile and adult ages. This differential estrogen sensitivity was tested in vivo by the administration of E(2). Low levels of E(2) significantly suppressed Kiss1 expression in the ARC, whereas Tac2 suppression required higher E(2) levels, supporting differential sensitivity to E(2). Finally, to determine whether inhibition of NKB/NK3R signaling would block the onset of puberty, we administered an NK3R antagonist to prepubertal (before postnatal d 30) females and found no effect on markers of pubertal onset in either WT or hpg mice. These results indicate that the expression of Tac2 and Tacr3 in the ARC are markers of pubertal activation but that increased NKB/NK3R signaling alone is insufficient to trigger the onset of puberty in the mouse.

Hyperprolactinemia-induced ovarian acyclicity is reversed by kisspeptin administration

Hyperprolactinemia is the most common cause of hypogonadotropic anovulation and is one of the leading causes of infertility in women aged 25-34. Hyperprolactinemia has been proposed to block ovulation through inhibition of GnRH release. Kisspeptin neurons, which express prolactin receptors, were recently identified as major regulators of GnRH neurons. To mimic the human pathology of anovulation, we continuously infused female mice with prolactin. Our studies demonstrated that hyperprolactinemia in mice induced anovulation, reduced GnRH and gonadotropin secretion, and diminished kisspeptin expression. Kisspeptin administration restored gonadotropin secretion and ovarian cyclicity, suggesting that kisspeptin neurons play a major role in hyperprolactinemic anovulation. Our studies indicate that administration of kisspeptin may serve as an alternative therapeutic approach to restore the fertility of hyperprolactinemic women who are resistant or intolerant to dopamine agonists.

Many mouths to feed: the control of food intake during lactation

Providing nutrients to their developing young is perhaps the most energetically demanding task facing female mammals. In this paper we focus primarily on studies carried out in rats to describe the changes in the maternal brain that enable the dam to meet the energetic demands of her offspring. In rats, providing milk for their litter is associated with a dramatic increase in caloric intake, a reduction in energy expenditure and changes in the pattern of energy utilization as well as storage. These behavioral and physiological adaptations result, in part, from alterations in the central pathways controlling energy balance. Differences in circulating levels of metabolic hormones such as leptin, ghrelin and insulin as well as in responsiveness to these signals between lactating and nonlactating animals, contribute to the modifications in energy balance pathways seen postpartum. Suckling stimulation from the pups both directly, and through the hormonal state that it induces in the mother, plays a key role in facilitating these adaptations.

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.

Immunofluorescent histochemical and ultrastructural studies on the innervation of kisspeptin/neurokinin B neurons to tuberoinfundibular dopaminergic neurons in the arcuate nucleus of rats

Kisspeptin is a pivotal regulator of the onset of puberty and the estrus cycle, but may also take part in pregnancy and lactation. Kisspeptin neurons and their fibers are distributed abundantly throughout the arcuate nucleus (ARC) of the hypothalamus, but the targets of the fiber projections in the ARC have not been fully investigated. The present study followed the projection of kisspeptin fibers to tuberoinfundibular dopaminergic (TIDA) neurons in the ARC, pivotal endocrine neurons that control prolactin secretion. Immunoreactive fibers of kisspeptin or neurokinin B, a peptide coexpressed in kisspeptin neurons, were abundantly found adjacent to TIDA neurons in female rats, but few were observed in male rats. The immunoreactivities of both peptides adjacent to TIDA neurons were significantly reduced in estradiol-primed ovariectomized rats. Precise 3D analysis of the attachment of kisspeptin-immunoreactive fibers to TIDA neurons was achieved using a synaptic marker that indicated synaptic connection. Finally, double-labeling immunoelectron microscopy confirmed the synaptic connections of kisspeptin-immunoreactive fibers to the cell body and fibers of TIDA neurons. These findings indicate that in female rats, kisspeptin/NKB fibers may directly affect TIDA neurons that regulate prolactin secretion, and that they are more likely to be activated during low estradiol status.

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

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

Leptin and reproductive function

Adipose tissue plays a dynamic role in whole-body energy homeostasis by acting as an endocrine organ. Collective evidence indicates a strong link between neural influences and adipocyte expression and secretion of leptin. Developmental changes in these relationships are considered important for pubertal transition in reproductive function. Leptin augments secretion of gonadotropin hormones, which are essential for initiation and maintenance of normal reproductive function, by acting centrally at the hypothalamus to regulate gonadotropin-releasing hormone (GnRH) neuronal activity and secretion. The effects of leptin on GnRH are mediated through interneuronal pathways involving neuropeptide-Y, proopiomelanocortin and kisspeptin. Increased infertility associated with diet induced obesity or central leptin resistance are likely mediated through the kisspeptin-GnRH pathway. Furthermore, Leptin regulates reproductive function by altering the sensitivity of the pituitary gland to GnRH and acting at the ovary to regulate follicular and luteal steroidogenesis. Thus leptin serves as a putative signal that links metabolic status with the reproductive axis. The intent of this review is to examine the biological role of leptin with energy metabolism, and reproduction.

Leptin is not the critical signal for kisspeptin or luteinising hormone restoration during exit from negative energy balance

Low levels of the adipocyte hormone leptin are considered to be the key signal contributing to inhibited gonadotrophin-releasing hormone (GnRH) release and reproductive acyclicity during negative energy balance. Hypoleptinaemia-induced inhibition of GnRH may be initiated with upstream inhibition of the secretagogue kisspeptin (Kiss1) because GnRH neurones do not express leptin receptors. The present study aimed to determine whether eliminating the hypoleptinaemia associated with caloric restriction (CR), by restoring leptin to normal basal levels, could reverse the suppression of the reproductive neuroendocrine axis. Fifty percent CR resulted in significant suppression of anteroventral periventricular Kiss1 mRNA, arcuate nucleus (ARH) Kiss1 and neurokinin B (NKB) mRNA levels and serum luteinising hormone (LH). Restoring leptin to normal basal levels did not restore Kiss1 or NKB mRNA or LH levels. Surprisingly, leptin did not activate expression of phosphorylated signal-transducer and activator of transcription-3 in ARC Kiss1 neurones, indicating that these neurones may not relay leptin signalling to GnRH neurones. Previous work in fasting models showing restoration of LH used a pharmacological dose of leptin. Therefore, in a 48-h fast study, replacement of leptin to pharmacological levels was compared with replacement of leptin to normal basal levels. Maintaining leptin at normal basal levels during the fast did not prevent inhibition of LH. By contrast, pharmacological levels of leptin did maintain LH at control values. These results suggest that, although leptin may be a permissive signal for reproductive function, hypoleptinaemia is unlikely to be the critical signal responsible for ARC Kiss1 and LH inhibition during negative energy balance.

Novel potential regulators of maternal adaptations during lactation: tuberoinfundibular peptide 39 and amylin

Maternal adaptations during lactation include milk synthesis and ejection, the appearance of maternal behaviours, reduced stress response, suppression of the ovarian cycle, and increased food and fluid intake. Several recently identified neuropeptides may participate in these adaptations, and we focus on two of them in the present study: tuberoinfundibular peptide of 39 residues (TIP39) and amylin. TIP39 is the ligand of the parathyroid hormone 2 receptor (PTH2 receptor) is induced in the posterior intralaminar complex of the thalamus (PIL) during lactation. TIP39 neurones in the PIL are activated in mother rats in response to pup exposure and project to preoptic, periventricular, paraventricular, arcuate and dorsomedial regions of the hypothalamus. Furthermore, an antagonist of the PTH2 receptor reduced suckling induced prolactin release. On the basis of their projections, TIP39 neurones might interact with additional neurones involved in maternal adaptations, including kisspeptin neurones participating in the control of gonadotrophin-releasing hormone function. TIP39 fibres might also interact with amylin, a peptide that we recently identified to appear in the preoptic area of rat dams. On the basis of its distribution, preoptic amylin could play a role in the control of maternal behaviours. We hypothesise that TIP39 neurones mediate the effects of suckling on different hypothalamic systems to affect maternal adaptations.

Guinea pig kisspeptin neurons are depolarized by leptin via activation of TRPC channels

Hypothalamic kisspeptin neurons are critical for driving reproductive function, but virtually nothing is known about their endogenous electrophysiological properties and the effects of leptin on their excitability. Therefore, we used the slice preparation from female guinea pigs to study the endogenous conductances and the effects of leptin on kisspeptin neurons. We targeted the arcuate kisspeptin neurons using visualized-patch whole-cell recording and identified kisspeptin neurons using immuocytochemical staining for kisspeptin or single cell RT-PCR. We also harvested dispersed arcuate neurons for analysis of expression of channel transcripts. Kisspeptin neurons exhibited a relatively negative resting membrane potential, and eighty percent of the neurons expressed a pacemaker current (h-current) and a T-type Ca(2+) current. Furthermore, the glutamate receptor agonist N-methyl D-aspartic acid depolarized and induced burst firing in kisspeptin neurons. Leptin activated an inward current that depolarized kisspeptin neurons and increased (burst) firing, but leptin hyperpolarized NPY neurons. Lanthanum, a TRPC-4,-5 channel activator, potentiated the leptin-induced inward current by 170%. The leptin-activated current reversed near -15 mV and was abrogated by the relatively selective TRPC channel blocker 2-APB. The leptin effects were also blocked by a Janus kinase inhibitor, a phosphatidylinositol 3 kinase inhibitor, and a phospholipase Cγ inhibitor. In addition, the majority of these neurons expressed TRPC1 and -5 and phospholipase Cγ1 based on single cell RT-PCR. Therefore, guinea pig kisspeptin neurons express endogenous pacemaker currents, and leptin excites these neurons via activation of TRPC channels. The leptin excitatory effects on kisspeptin neurons may be critical for governing the excitatory drive to GnRH neurons during different nutritional states.

Leptin deficiency and diet-induced obesity reduce hypothalamic kisspeptin expression in mice

The hormone leptin modulates a diverse range of biological functions, including energy homeostasis and reproduction. Leptin promotes GnRH function via an indirect action on forebrain neurons. We tested whether leptin deficiency or leptin resistance due to a high-fat diet (HFD) can regulate the potent reproductive neuropeptide kisspeptin. In mice with normalized levels of estradiol, leptin deficiency markedly reduced kisspeptin gene expression, particularly in the arcuate nucleus (ARC), and kisspeptin immunoreactive cell numbers in the rostral periventricular region of the third ventricle (RP3V). The HFD model was used to determine the effects of diet-induced obesity and central leptin resistance on kisspeptin cell number and gene expression. DBA/2J mice, which are prone to HFD-induced infertility, showed a marked decrease in kisspeptin expression in both the RP3V and ARC and cell numbers in the RP3V after HFD. This is the first evidence that kisspeptin can be regulated by HFD and/or increased body weight. Next we demonstrated that leptin does not signal (via signal transducer and activator of transcription 3 or 5, or mammalian target of rapamycin) directly on kisspeptin-expressing neurons in the RP3V. Lastly, in leptin receptor-deficient mice, neither GnRH nor kisspeptin neurons were activated during a preovulatory-like GnRH/LH surge induction regime, indicating that leptin's actions on GnRH may be upstream of kisspeptin neurons. These data provide evidence that leptin's effects on reproductive function are regulated by kisspeptin neurons in both the ARC and RP3V, although in the latter site the effects are likely to be indirect.

Identification of prolactin-sensitive GABA and kisspeptin neurons in regions of the rat hypothalamus involved in the control of fertility

High levels of circulating prolactin are known to cause infertility, but the precise mechanisms by which prolactin influences the neuroendocrine axis are yet to be determined. We used dual-label in situ hybridization to investigate whether prolactin-receptor (PRLR) mRNA is expressed in GnRH neurons. In addition, because γ-aminobutyric acidergic and kisspeptin neurons in the rostral hypothalamus are known to regulate GnRH neurons and, hence, might mediate the actions of prolactin, we investigated whether these neurons coexpress PRLR mRNA. (35)S-labeled RNA probes to detect PRLR mRNA were hybridized together with digoxigenin-labeled probes to detect either GnRH, Gad1/Gad2, or Kiss1 mRNA in the rostral hypothalamus of ovariectomized (OVX), estradiol-treated rats. Additional sets of serial sections were cut through the arcuate nucleus of OVX rats, without estradiol replacement, to examine coexpression of PRLR mRNA in the arcuate population of kisspeptin neurons. PRLR mRNA was highly expressed throughout the rostral preoptic area, particularly in periventricular regions surrounding the third ventricle, and there was a high degree of colocalization of PRLR mRNA in both Gad1/Gad2 and Kiss1 mRNA-containing cells (86 and 85.5%, respectively). In contrast, only a small number of GnRH neurons (<5%) was found to coexpress PRLR mRNA. In the arcuate nucleus of OVX rats, the majority of Kiss1 mRNA-containing cells also coexpressed PRLR mRNA. These data are consistent with the hypothesis that, in addition to a direct action on a small subpopulation of GnRH neurons, prolactin actions on GnRH neurons are predominantly mediated indirectly, through known afferent pathways.

Limits to sustained energy intake. XIII. Recent progress and future perspectives

Several theories have been proposed to explain limits on the maximum rate at which animals can ingest and expend energy. These limits are likely to be intrinsic to the animal, and potentially include the capacity of the alimentary tract to assimilate energy – the ‘central limitation’ hypothesis. Experimental evidence from lactating mice exposed to different ambient temperatures allows us to reject this and similar ideas. Two alternative ideas have been proposed. The ‘peripheral limitation’ hypothesis suggests that the maximal sustained energy intake reflects the summed demands of individual tissues, which have their own intrinsic limitations on capacity. In contrast, the ‘heat dissipation limit’ (HDL) theory suggests that animals are constrained by the maximal capacity to dissipate body heat. Abundant evidence in domesticated livestock supports the HDL theory, but data from smaller mammals are less conclusive. Here, we develop a novel framework showing how the HDL and peripheral limitations are likely to be important in all animals, but to different extents. The HDL theory makes a number of predictions – in particular that there is no fixed limit on sustained energy expenditure as a multiple of basal metabolic rate, but rather that the maximum sustained scope is positively correlated with the capacity to dissipate heat.

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

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

Characterisation of arcuate nucleus kisspeptin/neurokinin B neuronal projections and regulation during lactation in the rat

Lactation results in negative energy balance in the rat leading to decreased gonadotrophin-releasing hormone (GnRH) release and anoestrus. Inhibited GnRH release may be a result of decreased stimulatory tone from neuropeptides critical for GnRH neuronal activity, such as kisspeptin (Kiss1) and neurokinin B (NKB). The present study aimed to identify neuronal projections from the colocalised population of Kiss1/NKB cells in the arcuate nucleus (ARH) using double-label immunohistochemistry to determine where this population may directly regulate GnRH neuronal activity. Additionally, the present study further examined lactation-induced changes in the Kiss1 system that could play a role in decreased GnRH release. The colocalised ARH Kiss1/NKB fibres projected primarily to the internal zone of the median eminence (ME) where they were in close proximity to GnRH fibres; however, few Kiss1/NKB fibres from the ARH were seen at the level of GnRH neurones in the preoptic area (POA). Arcuate Kiss1/NKB peptide levels were decreased during lactation consistent with previous mRNA data. Surprisingly, anteroventral periventricular (AVPV) Kiss1 peptide levels were increased, whereas Kiss1 mRNA levels were decreased during lactation, suggesting active inhibition of peptide release. These findings indicate ARH Kiss1/NKB and AVPV Kiss1 appear to be inhibited during lactation, which may contribute to decreased GnRH release and subsequent reproductive dysfunction. Furthermore, the absence of a strong ARH Kiss1/NKB projection to the POA suggests regulation of GnRH by this population occurs primarily at the ME level via local projections.

Increased PTHrP and decreased estrogens alter bone turnover but do not reproduce the full effects of lactation on the skeleton

During lactation, calcium is mobilized from the maternal skeleton to supply the breast for milk production. This results in rapid but fully reversible bone loss. Prior studies have suggested that PTHrP, secreted from the breast, and estrogen deficiency, due to suckling-induced central hypogonadism, combine to trigger bone resorption. To determine whether this combination was sufficient to explain bone loss during lactation, we raised PTHrP levels and decreased levels of estrogens in nulliparous mice. PTHrP was infused via osmotic minipumps and estrogens were decreased either by using leuprolide, a long-acting GnRH agonist, or by surgical ovariectomy (OVX). Bone mineral density declined by 23.2 ± 1.3% in the spine and 16.8 ± 1.9% in the femur over 10 d of lactation. This was accompanied by changes in trabecular architecture and an increase in both osteoblast and osteoclast numbers. OVX and PTHrP infusion both induced a modest decline in bone mineral density over 10 d, but leuprolide treatment did not. The combination of OVX and PTHrP was more effective than either treatment alone, but there was no interaction between PTHrP and leuprolide. None of the treatments reproduced the same degree of bone loss caused by lactation. However, both forms of estrogen deficiency led to an increase in osteoclasts, whereas infusion of PTHrP increased both osteoblasts and osteoclasts. Therefore, although the combination of PTHrP and estrogen deficiency contributes to bone loss, it is insufficient to reproduce the full response of the skeleton to lactation, suggesting that other factors also regulate bone metabolism during this period.