Circadian control of kisspeptin and a gated GnRH response mediate the preovulatory luteinizing hormone surge

Cyanidin-3-O-glucoside protects against cadmium-induced dysfunction of sex hormone secretion via the regulation of hypothalamus-pituitary-gonadal axis in male pubertal mice

Cadmium (Cd) has been generally recognized as an endocrine-disrupting chemical for its toxic effects on the hypothalamus-pituitary-gonadal (HPG) axis accompanied by dysfunction in sex hormone secretion. Particularly, exposure to Cd during puberty versus post-puberty exhibits differing age-dependent effects that require further examination. This study sought to determine if cyanidin-3-O-glucoside (C3G), a typical anthocyanin with neuroprotective bioactivity, could protect against Cd-induced sex hormone-disorder in Pubertal male mice. C3G treatment reversed the disruption of hormone levels and increased Gnrh1 gene expression in the hypothalamus. In addition, the levels of gonadotropins, including luteinizing hormone (LH) and follicle stimulating hormone (FSH), were reversed by C3G. Interestingly, C3G improved the expression of LH and FSH receptor in the testis in mice exposed to Cd. Furthermore, C3G activated the signaling pathway related to the synthesis of testosterone processing. In conclusion, C3G protected against Cd-induced dysfunction of sex hormone secretion through the regulation of the HPG axis in male mice during puberty. The results of this study suggest that consumption of anthocyanins can be protective against metal-induced male reproductive dysfunction.

The Contribution of the Circadian Gene Bmal1 to Female Fertility and the Generation of the Preovulatory Luteinizing Hormone Surge

In rodents, the preovulatory LH surge is temporally gated, but the timing cue is unknown. Estrogen primes neurons in the anteroventral periventricular nucleus (AVPV) to secrete kisspeptin, which potently activates GnRH neurons to release GnRH, eliciting a surge of LH to induce ovulation. Deletion of the circadian clock gene Bmal1 results in infertility. Previous studies have found that Bmal1 knockout (KO) females do not display an LH surge at any time of day. We sought to determine whether neuroendocrine disruption contributes to the absence of the LH surge. Because Kiss1 expression in the AVPV is critical for regulating ovulation, we hypothesized that this population is disrupted in Bmal1 KO females. However, we found an appropriate rise in AVPV Kiss1 and Fos mRNA at the time of lights out in ovariectomized estrogen-treated animals, despite the absence of a measureable increase in LH. Furthermore, Bmal1 KO females have significantly increased LH response to kiss-10 administration, although the LH response to GnRH was unchanged. We then created Kiss1- and GnRH-specific Bmal1 KO mice to examine whether Bmal1 expression is necessary within either kisspeptin or GnRH neurons. We detected no significant differences in any measured reproductive parameter. Our results indicate that disruption of the hypothalamic regulation of fertility in the Bmal1 KO females is not dependent on endogenous clocks within either the GnRH or kisspeptin neurons.

The suprachiasmatic nucleus; a responsive clock regulating homeostasis by daily changing the setpoints of physiological parameters

The suprachiasmatic nucleus (SCN) is responsible for determining circadian variations in physiological setpoints. The SCN achieves such control through projections to different target structures within and outside the hypothalamus. Thus the SCN prepares the physiology of the body every 24 h via hormones and autonomic nervous system (ANS), to coming changes in behavior. Resulting rhythms in hormones and ANS activity transmit a precise message to selective organs, adapting their sensitivity to coming hormones, metabolites or other essentials. Thus the SCN as autonomous clock gives rhythm to physiological processes. However when the body is challenged by infections, low or high temperature, food shortage or excess: physiological setpoints need to be changed. For example, under fasting conditions, setpoints for body temperature and glucose levels are lowered at the beginning of the sleep (inactive) phase. However, starting the active phase, a normal increase in glucose and temperature levels take place to support activities associated with the acquisition of food. Thus, the SCN adjusts physiological setpoints in agreement with time of the day and according to challenges faced by the body. The SCN is enabled to do this by receiving extensive input from brain areas involved in sensing the condition of the body. Therefore, when the body receives stimuli contradicting normal physiology, such as eating or activity during the inactive period, this information reaches the SCN, adapting its output to correct this disbalance. As consequence frequent violations of the SCN message, such as by shift work or night eating, will result in development of disease.

Kisspeptin Neurons in the Arcuate Nucleus of the Hypothalamus Orchestrate Circadian Rhythms and Metabolism

Successful reproduction in female mammals is precisely timed and must be able to withstand the metabolic demand of pregnancy and lactation. We show that kisspeptin-expressing neurons in the arcuate hypothalamus (Kiss1^ARH) of female mice control the daily timing of food intake, along with the circadian regulation of locomotor activity, sleep, and core body temperature. Toxin-induced silencing of Kiss1^ARH neurons shifts wakefulness and food consumption to the light phase and induces weight gain. Toxin-silenced mice are less physically active and have attenuated temperature rhythms. Because the rhythm of the master clock in the suprachiasmatic nucleus (SCN) appears to be intact, we hypothesize that Kiss1^ARH neurons signal to neurons downstream of the master clock to modulate the output of the SCN. We conclude that, in addition to their well-established role in regulating fertility, Kiss1^ARH neurons are a critical component of the hypothalamic circadian oscillator network that times overt rhythms of physiology and behavior.

Functional Implications of RFRP-3 in the Central Control of Daily and Seasonal Rhythms in Reproduction

Adaptation of reproductive activity to environmental changes is essential for breeding success and offspring survival. In mammals, the reproductive system displays regular cycles of activation and inactivation which are synchronized with seasonal and/or daily rhythms in environmental factors, notably light intensity and duration. Thus, most species adapt their breeding activity along the year to ensure that birth and weaning of the offspring occur at a time when resources are optimal. Additionally, female reproductive activity is highest at the beginning of the active phase during the period of full oocyte maturation, in order to improve breeding success. In reproductive physiology, it is therefore fundamental to delineate how geophysical signals are integrated in the hypothalamo-pituitary-gonadal axis, notably by the neurons expressing gonadotropin releasing hormone (GnRH). Several neurochemicals have been reported to regulate GnRH neuronal activity, but recently two hypothalamic neuropeptides belonging to the superfamily of (Arg)(Phe)-amide peptides, RFRP-3 and kisspeptin, have emerged as critical for the integration of environmental cues within the reproductive axis. The goal of this review is to survey the current understanding of the role played by RFRP-3 in the temporal regulation of reproduction, and consider how its effect might combine with that of kisspeptin to improve the synchronization of reproduction to environmental challenges.

Circadian Regulation of the Brain and Behavior: A Neuroendocrine Perspective

Neuroendocrine systems are key regulators of brain and body functions, providing an important nexus between internal states and the external world, which then modulates appropriate behavioral outputs. Circadian (daily) rhythms are endogenously generated rhythms of approximately 24 h that help to synchronize internal physiological processes and behavioral states to the external environmental light-dark cycle. Given the importance of timing (hours, days, annual) in many different neuroendocrine axes, understanding how the circadian timing system regulates neuroendocrine function is particularly critical. Similarly, neuroendocrine signals can significantly affect circadian timing, and understanding these mechanisms can provide insights into general concepts of neuroendocrine regulation of brain circuits and behavior. This chapter will review the circadian timing system and its control of two key neuroendocrine systems: the hypothalamic-pituitary-gonadal (HPG) axis and the hypothalamic-pituitary-adrenal (HPA) axis. It will also discuss how outputs from these axes feedback to affect the circadian clock. Given that disruption of circadian timing is a central component of many mental and physical health conditions and that neuroendocrine function is similarly implicated in many of the same conditions, understanding these links will help illuminate potentially shared causality and perhaps lead to a better understanding of how to manipulate these systems when they begin to malfunction.

Emerging insights into hypothalamic-pituitary-gonadal axis regulation and interaction with stress signalling

Reproduction and fertility are regulated via hormones of the hypothalamic-pituitary-gonadal (HPG) axis. Control of this reproductive axis occurs at all levels, including the brain and pituitary, and allows for the promotion or inhibition of gonadal sex steroid secretion and function. In addition to guiding proper gonadal development and function, gonadal sex steroids also act in negative- and positive-feedback loops to regulate reproductive circuitry in the brain, including kisspeptin neurones, thereby modulating overall HPG axis status. Additional regulation is also provided by sex steroids made within the brain, including neuroprogestins. Furthermore, because reproduction and survival need to be coordinated and balanced, the HPG axis is able to modulate (and be modulated by) stress hormone signalling, including cortiscosterone, from the hypothalamic-pituitary-adrenal (HPA) axis. This review covers recent data related to the neural, hormonal and stress regulation of the HPG axis and emerging interactions between the HPG and HPA axes, focusing on actions at the level of the brain and pituitary.

Circadian disruption of food availability significantly reduces reproductive success in mice

Circadian disruptions impair reproductive health in human populations and in animal models. We tested the hypothesis that mistimed food, a common disruptive feature of shift work, impairs reproductive success in mice. Male and female mPer2^Luc mice on a C57BL/6 background were fed during the light or dark phase in two experiments. Food-induced internal misalignment of the liver clock was verified by in vivo bioluminescence in anesthetized mice in both experiments. In Experiment 1, food-restricted pairs were monitored for litters for 18 weeks. In the light-fed group, birth of the first litter was significantly delayed, and total reproductive output was significantly reduced by 38%. In Experiment 2, estrous cycling was monitored for 3 weeks, and then after pairing, copulatory plugs, pregnancy, litter sizes, and uterine implantation sites were measured. Fewer light-fed females birthed litters (25% versus 73%). This was attributable to a difference in behavior as mating success was significantly reduced in light-fed mice: 42% were observed with a copulatory plug compared to 82% for dark-fed mice. The proportion of mice displaying uterine implantation sites was the same as the proportion observed with copulatory plugs, suggesting no deficit in initiating pregnancy after mating. Estrous cycling and pregnancy maintenance did not differ between the groups. We conclude that mistimed feeding inhibits reproduction in mice by reducing successful mating behavior.

The 3rd World Conference on Kisspeptin, "Kisspeptin 2017: Brain and Beyond":Unresolved questions, challenges and future directions for the field

The 3rd World Conference on Kisspeptin, "Kisspeptin 2017: Brain and Beyond" was held March 30-31 at the Rosen Centre Hotel in Orlando, Florida, providing an international forum for multidisciplinary scientists to meet and share cutting-edge research on kisspeptin biology and its relevance to human health and disease. The meeting built upon previous world conferences focused on the role of kisspeptin and associated peptides in the control of gonadotropin-releasing hormone (GnRH) secretion and reproduction. Based on recent discoveries, the scope of this meeting was expanded to include functions of kisspeptin and related peptides in other physiological systems including energy homeostasis, pregnancy, ovarian and uterine function, and thermoregulation. In addition, discussions addressed the translation of basic knowledge of kisspeptin biology to the treatment of disease, with the goal of seeking consensus about the best approaches to improve human health. The two-day meeting featured a non-traditional structure, with each day starting with poster sessions followed by lunch discussions and facilitated large-group sessions with short presentations to maximize the exchange of new, unpublished data. Topics were identified by a survey prior to the meeting, and focused on major unresolved questions, important controversies, and future directions in the field. Finally, career development activities provided mentoring for trainees and junior investigators, and networking opportunities for those individuals with established researchers in the field. Overall, the meeting was rated as a success by attendees and covered a wide range of lively and provocative discussion topics on the changing nature of the field of "kisspeptinology" and its future. This article is protected by copyright. All rights reserved.

The Role of Kiss1 Neurons As Integrators of Endocrine, Metabolic, and Environmental Factors in the Hypothalamic-Pituitary-Gonadal Axis

Kisspeptin-GPR54 signaling in the hypothalamus is required for reproduction and fertility in mammals. Kiss1 neurons are key regulators of gonadotropin-releasing hormone (GnRH) release and modulation of the hypothalamic-pituitary-gonadal (HPG) axis. Arcuate Kiss1 neurons project to GnRH nerve terminals in the median eminence, orchestrating the pulsatile secretion of luteinizing hormone (LH) through the intricate interaction between GnRH pulse frequency and the pituitary gonadotrophs. Arcuate Kiss1 neurons, also known as KNDy neurons in rodents and ruminants because of their co-expression of neurokinin B and dynorphin represent an ideal hub to receive afferent inputs from other brain regions in response to physiological and environmental changes, which can regulate the HPG axis. This review will focus on studies performed primarily in rodent and ruminant species to explore potential afferent inputs to Kiss1 neurons with emphasis on the arcuate region but also considering the rostral periventricular region of the third ventricle (RP3V). Specifically, we will discuss how these inputs can be modulated by hormonal, metabolic, and environmental factors to control gonadotropin secretion and fertility. We also summarize the methods and techniques that can be used to study functional inputs into Kiss1 neurons.

Identification of differential genomic DNA Methylation in the hypothalamus of pubertal rat using reduced representation Bisulfite sequencing

BACKGROUND

There are many variables affecting the onset of puberty in animals, including genetic, nutritional, and environmental factors. Recent studies suggest that epigenetic regulation, especially DNA methylation, plays a majorrole in the regulation of puberty. However, there have been no reports on DNA methylation of the pubertal genome.

METHODS

We investigated DNA methylation in the female rat hypothalamus at prepuberty and puberty using reduced representation bisulfite sequencing technology. The identified genes and signaling pathways exhibiting changes to DNA methylation in pubertal rats were determined by Gene Ontogeny and Kyoto Encyclopedia of Genes and Genomes analysis.

RESULTS

The distribution of the three types of methylated C bases in promoter and CpG island (CGI) regions in the hypothalamus was as follows: 87.79% CG, 3.05% CHG, 9.16% CHH for promoters, and 88.35% CG, 3.21% CHG, 88.35% CHH for CGI in prepubertal rats; and 90.78% CG, 2.13% CHG, 7.09% CHH for promoters, and 88.59% CG, 88.59% CHG, 8.35% CHH for CGI in pubertal animals. CG showed the highest percentage of methylation, and was the highest methylation state in CGI. Compared to prepubertal hyoyhalamus samples, we identified ten genes with altered methylation in promoter regions in the pubertal hypothalamus samples, and 43 genes with altered methylation in the CGI. Changes in DNA methylation were found in gonadotropin-releasing hormone signaling pathways, and the oocyte meiosis pathway.

CONCLUSION

Our results demonstrate changes in DNA methylation occur in female rats from prepuberty to puberty suggestng DNA methylation may play a crucial role in the regulation of puberty onset. This study provides essential information for future studies on the role of epigenetics in the regulation of puberty.

Sex Differences in Steroid Receptor Coexpression and Circadian-Timed Activation of Kisspeptin and RFRP-3 Neurons May Contribute to the Sexually Dimorphic Basis of the LH Surge

In rodents, the ovulation-inducing luteinizing hormone (LH) surge is sexually dimorphic, occurring only in females, but the reasons for this sex difference are unclear. Two neuropeptides, kisspeptin and RFamide-related peptide 3 (RFRP-3), are hypothesized to regulate the gonadotropin-releasing hormone (GnRH)/LH surge. In females, both of these systems show circadian changes coincident with the LH surge, but whether males show similar temporal changes under comparable hormonal conditions is unknown. Here, we evaluated circadian time (CT)-dependent changes in gene expression and neuronal activation of Kiss1 and Rfrp neurons of female and male mice given identical LH surge-inducing estrogen regimens. As expected, females, but not males, displayed a late afternoon LH surge and GnRH neuronal activation. Kiss1 expression in the anteroventral periventricular nucleus (AVPV) was temporally increased in females in the late afternoon, whereas males demonstrated no temporal changes in AVPV Kiss1 expression. Likewise, neuronal activation of AVPV Kiss1 neurons was dramatically elevated in the late afternoon in females but was low at all circadian times in males. Estrogen receptor α levels in AVPV Kiss1 neurons were sexually dimorphic, being higher in females than males. AVPV progesterone receptor levels were also higher in females than males. Hypothalamic Rfrp messenger RNA levels showed no CT-dependent changes in either sex. However, Rfrp neuronal activation was temporally diminished in the afternoon/evening in females but not males. Collectively, the identified sex differences in absolute and CT-dependent AVPV Kiss1 levels, AVPV sex steroid receptor levels, and circadian-timed changes in neuronal activation of both Kiss1 and Rfrp neurons suggest that multiple sexually dimorphic processes in the brain may underlie proper LH surge generation.

Daily rhythms count for female fertility

Female ovulation depends on a surge in circulating luteinizing hormone (LH) which occurs at the end of the resting period and requests high circulating estradiol. This fine tuning involves both an estradiol feedback as an indicator of oocyte maturation, and the master circadian clock of the suprachiasmatic nuclei as an indicator of the time of the day. This review describes the mechanisms through which daily time cues are conveyed to reproductive hypothalamic neurons to time the pre-ovulatory surge. In female rodents, neurotransmitters released by the suprachiasmatic nuclei activate the stimulatory kisspeptin neurons and reduce the inhibitory RFRP neurons precisely at the end of the afternoon of proestrus to allow a full surge in LH secretion. From these findings, the impact of circadian disruptions (during shift or night work) on female reproductive performance and fertility should now being investigated in both animal models and humans.

Ontogeny of clock and KiSS-1 metastasis-suppressor (Kiss1) gene expression in the prepubertal mouse hypothalamus

Kisspeptin is crucial for the generation of the circadian-gated preovulatory gonadotrophin-releasing hormone (GnRH)-LH surge in female rodents, with expression in the anteroventral periventricular nucleus (AVPV) peaking in the late afternoon of pro-oestrus. Given kisspeptin expression is established before puberty, the aim of the present study was to investigate kisspeptin and clock gene rhythms during the neonatal period. Anterior and posterior hypothalami were collected from C57BL/6J mice on Postnatal Days (P) 5, 15 and 25, at six time points across 24h, for analysis of gene expression by reverse transcription-quantitative polymerase chain reaction. Expression of aryl hydrocarbon receptor nuclear translocator-like gene (Bmal1) and nuclear receptor subfamily 1, group D, member 2 (Rev-erbα) in the anterior hypothalamus (containing the suprachiasmatic nucleus) was not rhythmic at P5 or P15, but Bmal1 expression exhibited rhythmicity in P25 females, whereas Rev-erbα expression was rhythmic in P25 males. KiSS-1 metastasis-suppressor (Kiss1) expression did not exhibit time-of-day variation in the anterior (containing the AVPV) or posterior (containing the arcuate nucleus) hypothalami in female and male mice at P5, P15 or P25. The data indicate that the kisspeptin circadian peak in expression observed in the AVPV of pro-oestrous females does not manifest at P5, P15 or P25, likely due to inadequate oestrogenic stimuli, as well as incomplete development of clock gene rhythmicity before puberty.

Food restriction-induced changes in motivation differ with stages of the estrous cycle and are closely linked to RFamide-related peptide-3 but not kisspeptin in Syrian hamsters

We tested the hypothesis that the effects of food restriction on behavioral motivation are mediated by one or both of the RFamide peptides, RFamide-related peptide-3 (RFRP-3) and kisspeptin (Kp) in female Syrian hamsters (Mesocricetus auratus). Female hamsters fed ad libitum and given a choice between food and adult male hamsters are highly motivated to visit males instead of food on all four days of the estrous cycle, but after 8days of mild food restriction (75% of ad libitum intake) they shift their preference toward food every day of the estrous cycle until the day of estrus, when they shift their preference back toward the males. In support of a role for RFRP-3 in these behavioral changes, the preference for food and the activation of RFRP-3-immunoreactive (Ir) cells in the dorsomedial hypothalamus (DMH) showed the same estrous cycle pattern in food-restricted females, but no association was observed between behavior and the activation of Kp cells in the hypothalamic arcuate nucleus or preoptic area. Next, we tested the hypothesis that food-restriction-induced activation of RFRP-3-Ir cells is modulated by high levels of ovarian steroids at the time of estrus. In support of this idea, on nonestrous days, mild food restriction increased activation of RFRP-3-Ir cells, but failed to do so on the day of estrus even though this level of food restriction did not significantly decrease circulating concentrations of estradiol or progesterone. Furthermore, in ovariectomized females, food-restriction-induced increases in activation of RFRP-3-Ir cells were blocked by systemic treatment with progesterone alone, estradiol plus progesterone, but not estradiol alone. Central infusion with RFRP-3 in ad libitum-fed females significantly decreased sexual motivation and produced significant increases in 90-minute food hoarding, in support of the hypothesis that elevated central levels of RFRP-3 are sufficient to create the shift in behavioral motivation in females fed ad libitum. Together, these results are consistent with the hypothesis that high levels of ingestive motivation are promoted during the nonfertile phase of the estrous cycle by elevated activation of RFRP-3-Ir cells, and RFRP-3-Ir cellular activation is modulated by ovarian steroids around the time of estrus, thereby diverting attention away from food and increasing sexual motivation.

The role of kisspeptin and RFRP in the circadian control of female reproduction

In female mammals, reproduction shows ovarian and daily rhythms ensuring that the timing of the greatest fertility coincides with maximal activity and arousal. The ovarian cycle, which lasts from a few days to a few weeks, depends on the rhythm of follicle maturation and ovarian hormone production, whereas the daily cycle depends on a network of circadian clocks of which the main one is located in the suprachiasmatic nuclei (SCN). In the last ten years, major progress has been made in the understanding of the neuronal mechanisms governing mammalian reproduction with the finding that two hypothalamic Arg-Phe-amide peptides, kisspeptin (Kp) and RFRP, regulate GnRH neurons. In this review we discuss the pivotal role of Kp and RFRP neurons at the interface between the SCN clock signal and GnRH neurons to properly time gonadotropin-induced ovulation. We also report recent findings indicating that these neurons may be part of the multi-oscillatory circadian system that times female fertility. Finally, we will discuss recent investigations indicating a role, and putative therapeutic use, of these neuropeptides in human reproduction.

Vasoactive Intestinal Peptide Excites GnRH Neurons in Male and Female Mice

A variety of external and internal factors modulate the activity of GnRH neurons to control fertility in mammals. A direct, vasoactive intestinal peptide (VIP)-mediated input to GnRH neurons originating from the suprachiasmatic nucleus is thought to relay circadian information within this network. In the present study, we examined the effects of VIP on GnRH neuron activity in male and female mice at different stages of the estrous cycle. We carried out cell-attached recordings in slices from GnRH-green fluorescent protein mice and calcium imaging in slices from a mouse line expressing the genetically encoded calcium indicator GCaMP3 selectively in GnRH neurons. We show that 50%-80% of GnRH neurons increase their firing rate in response to bath-applied VIP (1nM-1000nM) in both male and female mice and that this is accompanied by a robust increase in intracellular calcium concentrations. This effect is mediated directly at the GnRH neuron likely through activation of high-affinity VIP receptors. Because suprachiasmatic nucleus-derived timing cues trigger the preovulatory surge only on the afternoon of proestrus in female mice, we examined the effects of VIP during the estrous cycle at different times of day. VIP responsiveness in GnRH neurons did not vary significantly in diestrous and proestrous mice before or around the time of the expected preovulatory surge. These results indicate that the majority of GnRH neurons in male and female mice express functional VIP receptors and that the effects of VIP on GnRH neurons do not alter across the estrous cycle.

Control of puberty onset and fertility by gonadotropin-releasing hormone neurons

The gonadotropin-releasing hormone (GnRH) neuronal network generates pulse and surge modes of gonadotropin secretion critical for puberty and fertility. The arcuate nucleus kisspeptin neurons that innervate the projections of GnRH neurons in and around their neurosecretory zone are key components of the pulse generator in all mammals. By contrast, kisspeptin neurons located in the preoptic area project to GnRH neuron cell bodies and proximal dendrites and are involved in surge generation in female rodents (and possibly other species). The hypothalamic-pituitary-gonadal axis develops embryonically but, apart from short periods of activation immediately after birth, remains suppressed through a combination of gonadal and non-gonadal mechanisms. At puberty onset, the pulse generator reactivates, probably owing to progressive stimulatory influences on GnRH neurons from glial and neurotransmitter signalling, and the re-emergence of stimulatory arcuate kisspeptin input. In females, the development of pulsatile gonadotropin secretion enables final maturation of the surge generator that ultimately triggers the first ovulation. Representation of the GnRH neuronal network as a series of interlocking functional modules could help conceptualization of its functioning in different species. Insights into pulse and surge generation are expected to aid development of therapeutic strategies ameliorating pubertal disorders and infertility in the clinic.

Diurnal regulation of hypothalamic kisspeptin is disrupted during mouse pregnancy

Kisspeptin, the neuropeptide product of the Kiss1 gene, is critical in driving the hypothalamic-pituitary-gonadal (HPG) axis. Kisspeptin neurons in the anteroventral periventricular nucleus (AVPV) and arcuate nucleus (Arc) of the hypothalamus mediate differential effects, with the Arc regulating negative feedback of sex steroids and the AVPV regulating positive feedback, vital for the preovulatory surge and gated under circadian control. We aimed to characterize hypothalamic Kiss1 and Kiss1r mRNA expression in nonpregnant and pregnant mice, and investigate potential circadian regulation. Anterior and posterior hypothalami were collected from C57BL/6J mice at diestrus, proestrus, and days 6, 10, 14, and 18 of pregnancy, at six time points across 24h, for real-time PCR analysis of gene expression. Analysis confirmed that Kiss1 mRNA expression in the AVPV increased at ZT13 during proestrus, with a luteinizing hormone surge observed thereafter. No diurnal regulation was seen at diestrus or at any stage of pregnancy. Anterior hypothalamic Avp mRNA expression exhibited no diurnal variation, but Avpr1a peaked at 12:00h during proestrus, possibly reflecting the circadian input from the suprachiasmatic nucleus to AVPV Kiss1 neurons. Rfrp (Npvf) expression in the posterior hypothalamus did not demonstrate diurnal variation at any stage. Clock genes Bmal1 and Rev-erbα were strongly diurnal, but there was little change between diestrus/proestrus and pregnancy. Our data indicate the absence of the circadian input to Kiss1 in pregnancy, despite high gestational estradiol levels and normal clock gene expression, and may suggest a disruption of a kisspeptin-specific diurnal rhythm that operates in the nonpregnant state.

Aggressive interactions are associated with reductions in RFamide-related peptide, but not kisspeptin, neuronal activation in mice

Aggressive interactions lead to changes in both future behavior and circulating testosterone (T) concentrations in animals across taxa. The specific neural circuitry and neurochemical systems by which these encounters alter neuroendocrine functioning are not well understood. Neurons expressing the inhibitory and stimulatory neuropeptides, RFamide-related peptide (RFRP) and kisspeptin, respectively, project to neural loci regulating aggression in addition to neuroendocrine cells controlling sex steroid production. Given these connections to both the reproductive axis and aggression circuitry, RFRP and kisspeptin are in unique positions to mediate post-encounter changes in both T and behavior. The present study examined the activational state of RFRP and kisspeptin neurons of male C57BL/6 mice following an aggressive encounter. Both winners and losers exhibited reduced RFRP/FOS co-localization relative to handling stress controls. Social exposure controls did not display reduced RFRP neuronal activation, indicating that this effect is due to aggressive interaction specifically rather than social interaction generally. RFRP neuronal activation positively correlated with latencies to display several offensive behaviors within winners. These effects were not observed in the anteroventral periventricular (AVPV) nucleus kisspeptin cell population. Together, these findings point to potential neuromodulatory role for RFRP in aggressive behavior and in disinhibiting the reproductive axis to facilitate an increase in T in response to social challenge.

A "Timed" Kiss Is Essential for Reproduction: Lessons from Mammalian Studies

Reproduction is associated with the circadian system, primarily as a result of the connectivity between the biological clock in the suprachiasmatic nucleus (SCN) and reproduction-regulating brain regions, such as preoptic area (POA), anteroventral periventricular nucleus (AVPV), and arcuate nucleus (ARC). Networking of the central pacemaker to these hypothalamic brain regions is partly represented by close fiber appositions to specialized neurons, such as kisspeptin and gonadotropin-releasing hormone (GnRH) neurons; accounting for rhythmic release of gonadotropins and sex steroids. Numerous studies have attempted to dissect the neurochemical properties of GnRH neurons, which possess intrinsic oscillatory features through the presence of clock genes to regulate the pulsatile and circadian secretion. However, less attention has been given to kisspeptin, the upstream regulator of GnRH and a potent mediator of reproductive functions including puberty. Kisspeptin exerts its stimulatory effects on GnRH secretion via its cognate Kiss-1R receptor that is co-expressed on GnRH neurons. Emerging studies have found that kisspeptin neurons oscillate on a circadian basis and that these neurons also express clock genes that are thought to regulate its rhythmic activities. Based on the fiber networks between the SCN and reproductive nuclei such as the POA, AVPV, and ARC, it is suggested that interactions among the central biological clock and reproductive neurons ensure optimal reproductive functionality. Within this neuronal circuitry, kisspeptin neuronal system is likely to "time" reproduction in a long term during development and aging, in a medium term to regulate circadian or estrus cycle, and in a short term to regulate pulsatile GnRH secretion.

The Increase in Signaling by Kisspeptin Neurons in the Preoptic Area and Associated Changes in Clock Gene Expression That Trigger the LH Surge in Female Rats Are Dependent on the Facilitatory Action of a Noradrenaline Input

In rodents, kisspeptin neurons in the rostral periventricular area of the third ventricle (RP3V) of the preoptic area are considered to provide a major stimulatory input to the GnRH neuronal network that is responsible for triggering the preovulatory LH surge. Noradrenaline (NA) is one of the main modulators of GnRH release, and NA fibers are found in close apposition to kisspeptin neurons in the RP3V. Our objective was to interrogate the role of NA signaling in the kisspeptin control of GnRH secretion during the estradiol induced LH surge in ovariectomized rats, using prazosin, an α1-adrenergic receptor antagonist. In control rats, the estradiol-induced LH surge at 17 hours was associated with a significant increase in GnRH and kisspeptin content in the median eminence with the increase in kisspeptin preceding that of GnRH and LH. Prazosin, administered 5 and 3 hours prior to the predicted time of the LH surge truncated the LH surge and abolished the rise in GnRH and kisspeptin in the median eminence. In the preoptic area, prazosin blocked the increases in Kiss1 gene expression and kisspeptin content in association with a disruption in the expression of the clock genes, Per1 and Bmal1. Together these findings demonstrate for the first time that NA modulates kisspeptin synthesis in the RP3V through the activation of α1-adrenergic receptors prior to the initiation of the LH surge and indicate a potential role of α1-adrenergic signaling in the circadian-controlled pathway timing of the preovulatory LH surge.

Photoperiodic Modulation of Circadian Clock and Reproductive Axis Gene Expression in the Pre-Pubertal European Sea Bass Brain

The acquisition of reproductive competence requires the activation of the brain-pituitary-gonad (BPG) axis, which in most vertebrates, including fishes, is initiated by changes in photoperiod. In the European sea bass long-term exposure to continuous light (LL) alters the rhythm of reproductive hormones, delays spermatogenesis and reduces the incidence of precocious males. In contrast, an early shift from long to short photoperiod (AP) accelerates spermatogenesis. However, how photoperiod affects key genes in the brain to trigger the onset of puberty is still largely unknown. Here, we investigated if the integration of the light stimulus by clock proteins is sufficient to activate key genes that trigger the BPG axis in the European sea bass. We found that the clock genes clock, npas2, bmal1 and the BPG genes gnrh, kiss and kissr share conserved transcription factor frameworks in their promoters, suggesting co-regulation. Other gene promoters of the BGP axis were also predicted to be co-regulated by the same frameworks. Co-regulation was confirmed through gene expression analysis of brains from males exposed to LL or AP photoperiod compared to natural conditions: LL fish had suppressed gnrh1, kiss2, galr1b and esr1, while AP fish had stimulated npas2, gnrh1, gnrh2, kiss2, kiss1rb and galr1b compared to NP. It is concluded that fish exposed to different photoperiods present significant expression differences in some clock and reproductive axis related genes well before the first detectable endocrine and morphological responses of the BPG axis.

Rapid modulation of hypothalamic Kiss1 levels by the suckling stimulus in the lactating rat

In mammals, lactation suppresses GnRH/LH secretion resulting in transient infertility. In rats, GnRH/LH secretion is rescued within 18-48 h after pup separation (PS) and rapidly re-suppressed by subsequent re-exposure of pups. To elucidate the mechanisms underlying these rapid modulations, changes in the expression of kisspeptin, a stimulator of GnRH secretion, in several lactating conditions (normal-lactating; 4-h PS; 18-h PS; 4-h PS +1-h re-exposure of pups; non-lactating) were examined using in situ hybridization. PS for 4 h or 18 h increased Kiss1 expressing neurons in both the anteroventral periventricular nucleus (AVPV) and the arcuate nucleus (ARC), and subsequent exposure of pups re-suppressed Kiss1 in the AVPV. A change in Kiss1 expression was observed prior to the reported time of the change in GnRH/LH, indicating that the change in GnRH/LH results from changes in kisspeptin. We further examined the mechanisms underlying the rapid modulation of Kiss1. We first investigated the possible involvement of ascending sensory input during the suckling stimulus. Injection of the anterograde tracer to the subparafascicular parvocellular nucleus (SPFpc) in the midbrain, which relays the suckling stimulus, revealed direct neuronal connections between the SPFpc and kisspeptin neurons in both the AVPV and ARC. We also examined the possible involvement of prolactin (PRL). Administration of PRL for 1 h suppressed Kiss1 expression in the AVPV but not in the ARC. These results indicate that suckling stimulus rapidly modulates Kiss1 expression directly via neuronal connections and indirectly through serum PRL, resulting in modulation in GnRH/LH secretion.

Evidence for a Putative Circadian Kiss-Clock in the Hypothalamic AVPV in Female Mice

The kisspeptin (Kp) neurons in the anteroventral periventricular nucleus (AVPV) are essential for the preovulatory LH surge, which is gated by circulating estradiol (E2) and the time of day. We investigated whether AVPV Kp neurons in intact female mice may be the site in which both E2 and daily signals are integrated and whether these neurons may host a circadian oscillator involved in the timed LH surge. In the afternoon of proestrous day, Kp immunoreactivity displayed a marked and transient decrease 2 hours before the LH surge. In contrast, Kp content was stable throughout the day of diestrus, when LH levels are constantly low. AVPV Kp neurons expressed the clock protein period 1 (PER1) with a daily rhythm that is phase delayed compared with the PER1 rhythm measured in the main clock of the suprachiasmatic nuclei (SCN). PER1 rhythm in the AVPV, but not in the SCN, exhibited a significant phase delay of 2.8 hours in diestrus as compared with proestrus. Isolated Kp-expressing AVPV explants from PER2::LUCIFERASE mice displayed sustained circadian oscillations of bioluminescence with a circadian period (23.2 h) significantly shorter than that of SCN explants (24.5 h). Furthermore, in AVPV explants incubated with E2 (10 nM to 1 μM), the circadian period was lengthened by 1 hour, whereas the SCN clock remained unaltered. In conclusion, these findings indicate that AVPV Kp neurons display an E2-dependent daily rhythm, which may possibly be driven by an intrinsic circadian clock acting in combination with the SCN timing signal.

Estrogen permits vasopressin signaling in preoptic kisspeptin neurons in the female mouse

The cellular mechanisms governing the impact of the central circadian clock on neuronal networks are incompletely understood. We examine here the influence of the suprachiasmatic nucleus output neuropeptide arginine-vasopressin (AVP) on the activity of preoptic area kisspeptin neurons. These cells integrate circadian and hormonal signals within the neuronal network that regulates fertility in females. Electrophysiological recordings in brain slices from kisspeptin-GFP mice showed that AVP dose-dependently increased the firing rate of most kisspeptin neurons. These actions were mediated directly at the kisspeptin neuron. Experiments in mice expressing the calcium indicator GCaMP3 in kisspeptin neurons enabled simultaneous monitoring of intracellular calcium concentrations ([Ca(2+)]i) in multiple cells and revealed that AVP increased [Ca(2+)]i in >80% of diestrous kisspeptin neurons via a mechanism involving voltage-gated calcium channels. We next examined whether AVP signaling in kisspeptin neurons was time and ovarian cycle dependent. AVP exerted the same effects on diestrous and proestrous days of the ovarian cycle, whether hours before [zeitgeber time 4 (ZT4)-ZT6] or just before (ZT10) the expected time of the proestrous preovulatory luteinizing hormone surge. Remarkably, however, AVP signaling was critically dependent on circulating ovarian steroids as AVP no longer excited preoptic kisspeptin neurons in ovariectomized mice, an effect that was fully restored by estradiol treatment. Together, these studies show that AVP exerts a potent and direct stimulatory influence upon the electrical activity and [Ca(2+)]i of most preoptic kisspeptin neurons. Unexpectedly, estrogen is found to permit circadian AVP signaling at preoptic kisspeptin neurons rather than dynamically modulate its activity throughout the estrous cycle.

Does salt have a permissive role in the induction of puberty?

Puberty is starting earlier than ever before and there are serious physiological and sociological implications as a result of this development. Current research has focused on the potential role of high caloric, and commensurate high adiposity, contributions to early puberty. However, girls with normal BMI also appear to be initiating puberty earlier. Westernized diets, in addition to being high in fat and sugar, are also high in salt. To date, no research has investigated a link between elevated salt and the reproductive axis. We hypothesize that a high salt diet can result in an earlier onset of puberty through three mechanisms that are not mutually exclusive. (1) High salt activates neurokinin B, a hormone that is involved in both the reproductive axis and salt regulation, and this induces kisspeptin release and ultimate activation of the reproductive axis. (2) Vasopressin released in response to high salt acts on vasopressin receptors expressed on kisspeptin neurons in the anteroventral periventricular nucleus, thereby stimulating gonadotropin releasing hormone and subsequently luteinizing hormone secretion. (3) Salt induces metabolic changes that affect the reproductive axis. Specifically, salt acts indirectly to modulate adiposity, ties in with the obesity epidemic, and further compounds the pathologic effects of obesity. Our overall hypothesis offers an additional cause behind the induction of puberty and provides testable postulates to determine the mechanism of potential salt-mediated affects on puberty.

Relative Importance of the Arcuate and Anteroventral Periventricular Kisspeptin Neurons in Control of Puberty and Reproductive Function in Female Rats

Kisspeptin plays a critical role in pubertal timing and reproductive function. In rodents, kisspeptin perikarya within the hypothalamic arcuate (ARC) and anteroventral periventricular (AVPV) nuclei are thought to be involved in LH pulse and surge generation, respectively. Using bilateral microinjections of recombinant adeno-associated virus encoding kisspeptin antisense into the ARC or AVPV of female rats at postnatal day 10, we investigated the relative importance of these two kisspeptin populations in the control of pubertal timing, estrous cyclicity, and LH surge and pulse generation. A 37% knockdown of kisspeptin in the AVPV resulted in a significant delay in vaginal opening and first vaginal estrous, abnormal estrous cyclicity, and reduction in the occurrence of spontaneous LH surges, although these retained normal amplitude. This AVPV knockdown had no effect on LH pulse frequency, measured after ovariectomy. A 32% reduction of kisspeptin in the ARC had no effect on the onset of puberty but resulted in abnormal estrous cyclicity and decreased LH pulse frequency. Additionally, the knockdown of kisspeptin in the ARC decreased the amplitude but not the incidence of LH surges. These results might suggest that the role of AVPV kisspeptin in the control of pubertal timing is particularly sensitive to perturbation. In accordance with our previous studies, ARC kisspeptin signaling was critical for normal pulsatile LH secretion in female rats. Despite the widely reported role of AVPV kisspeptin neurons in LH surge generation, this study suggests that both AVPV and ARC populations are essential for normal LH surges and estrous cyclicity.

Circadian Control of the Female Reproductive Axis Through Gated Responsiveness of the RFRP-3 System to VIP Signaling

Throughout most of the ovulatory cycle, estrogen negative feedback restrains the GnRH neuronal system. Just before ovulation, however, estrogen negative feedback is removed to permit stimulation of the preovulatory GnRH/LH surge (positive feedback) by the circadian clock in the suprachiasmatic nucleus (SCN). The mammalian ortholog of avian gonadotropin-inhibitory hormone, RFamide-related peptide 3 (RFRP-3), participates in the circadian-timed removal of estrogen negative feedback to permit the LH surge. The present study examined the specific neurochemical means by which the SCN controls RFRP-3 activity and explored whether the RFRP-3 system exhibits time-dependent responsiveness to SCN signaling to precisely time the LH surge. We found that RFRP-3 cells in female Syrian hamsters (Mesocricetus auratus) receive close appositions from SCN-derived vasopressin-ergic and vasoactive intestinal peptide (VIP)-ergic terminal fibers. Central VIP administration markedly suppressed RFRP-3 cellular activity in the evening, but not the morning, relative to saline controls, whereas vasopressin was without effect at either time point. Double-label in situ hybridization for Rfrp-3 and the VIP receptors VPAC1 and VPAC2 revealed that the majority of RFRP-3 cells do not coexpress either receptor in Syrian hamsters or mice, suggesting that SCN VIP-ergic signaling inhibits RFRP-3 cells indirectly. The timing of this VIP-mediated disinhibition is further coordinated via temporally gated responsiveness of RFRP-3 cells to circadian signaling. Together, these findings reveal a novel circadian hierarchy of control coordinating the preovulatory LH surge and ovulation.

Positive, but not negative feedback actions of estradiol in adult female mice require estrogen receptor α in kisspeptin neurons

Hypothalamic kisspeptin (Kiss1) neurons express estrogen receptor α (ERα) and exert control over GnRH/LH secretion in female rodents. It has been proposed that estradiol (E2) activation of ERα in kisspeptin neurons in the arcuate nucleus (ARC) suppresses GnRH/LH secretion (negative feedback), whereas E2 activation of ERα in kisspeptin neurons in the anteroventral periventricular nucleus (AVPV) mediates the release of preovulatory GnRH/LH surges (positive feedback). To test these hypotheses, we generated mice bearing kisspeptin cell-specific deletion of ERα (KERαKO) and treated them with E2 regimens that evoke either negative or positive feedback actions on GnRH/LH secretion. Using negative feedback regimens, as expected, E2 effectively suppressed LH levels in ovariectomized (OVX) wild-type (WT) mice to the levels seen in ovary-intact mice. Surprisingly, however, despite the fact that E2 regulation of Kiss1 mRNA expression was abrogated in both the ARC and AVPV of KERαKO mice, E2 also effectively decreased LH levels in OVX KERαKO mice to the levels seen in ovary-intact mice. Conversely, using a positive feedback regimen, E2 stimulated LH surges in WT mice, but had no effect in KERαKO mice. These experiments clearly demonstrate that ERα in kisspeptin neurons is required for the positive, but not negative feedback actions of E2 on GnRH/LH secretion in adult female mice. It remains to be determined whether the failure of KERαKO mice to exhibit GnRH/LH surges reflects the role of ERα in the development of kisspeptin neurons, in the active signaling processes leading to the release of GnRH/LH surges, or both.

GABAergic transmission to kisspeptin neurons is differentially regulated by time of day and estradiol in female mice

Gonadotropin-releasing hormone (GnRH) secretion is regulated by estradiol feedback. This feedback switches from negative to positive in females; this switch depends on time of day in many species. Estradiol feedback is likely conveyed via afferents. Kisspeptin neurons of the arcuate nucleus and anteroventral-periventricular region (AVPV) may differentially regulate GnRH neurons during negative and positive feedback, respectively. We tested estradiol and time of day regulation of GABAergic transmission and postsynaptic response to GABA in these two populations using transgenic mice with GFP-identified kisspeptin neurons. Ovariectomized (OVX) mice treated or not with estradiol (E) were studied in the AM (negative feedback) or PM (positive feedback). GABAA receptor reversal potential was unaffected by time of day or estradiol. GABA depolarized the membrane potential of arcuate neurons from OVX+E mice; this response was blunted in cells from OVX mice. GABA hyperpolarized AVPV kisspeptin neurons, except in the OVX PM group in which GABA did not alter membrane potential attributable to a PM hyperpolarization of baseline membrane potential. In both kisspeptin neuron populations from OVX mice, the frequency of GABAergic spontaneous postsynaptic currents was increased in the PM; this increase was blunted by estradiol. In arcuate, but not AVPV, kisspeptin neurons, estradiol reduced miniature postsynaptic current amplitude independent of time of day. Using nonstationary fluctuation analysis and diazepam to manipulate GABAA receptor apparent affinity, the decrease in arcuate miniature postsynaptic current amplitude was attributed to decreased number of receptors bound by GABA. Time of day and estradiol feedback thus both target presynaptic and postsynaptic mechanisms to differentially regulate kisspeptin neurons via GABAergic transmission.

Network-mediated encoding of circadian time: the suprachiasmatic nucleus (SCN) from genes to neurons to circuits, and back

The transcriptional architecture of intracellular circadian clocks is similar across phyla, but in mammals interneuronal mechanisms confer a higher level of circadian integration. The suprachiasmatic nucleus (SCN) is a unique model to study these mechanisms, as it operates as a ∼24 h clock not only in the living animal, but also when isolated in culture. This "clock in a dish" can be used to address fundamental questions, such as how intraneuronal mechanisms are translated by SCN neurons into circuit-level emergent properties and how the circuit decodes, and responds to, light input. This review addresses recent developments in understanding the relationship between electrical activity, [Ca(2+)]i, and intracellular clocks. Furthermore, optogenetic and chemogenetic approaches to investigate the distinct roles of neurons and glial cells in circuit encoding of circadian time will be discussed, as well as the epigenetic and circuit-level mechanisms that enable the SCN to translate light input into coherent daily rhythms.

A Multi-Oscillatory Circadian System Times Female Reproduction

Rhythms in female reproduction are critical to insure that timing of ovulation coincides with oocyte maturation and optimal sexual arousal. This fine tuning of female reproduction involves both the estradiol feedback as an indicator of oocyte maturation, and the master circadian clock of the suprachiasmatic nuclei (SCN) as an indicator of the time of the day. Herein, we are providing an overview of the state of knowledge regarding the differential inhibitory and stimulatory effects of estradiol at different stages of the reproductive axis, and the mechanisms through which the two main neurotransmitters of the SCN, arginine vasopressin, and vasoactive intestinal peptide, convey daily time cues to the reproductive axis. In addition, we will report the most recent findings on the putative functions of peripheral clocks located throughout the reproductive axis [kisspeptin (Kp) neurons, gonadotropin-releasing hormone neurons, gonadotropic cells, the ovary, and the uterus]. This review will point to the critical position of the Kp neurons of the anteroventral periventricular nucleus, which integrate both the stimulatory estradiol signal, and the daily arginine vasopressinergic signal, while displaying a circadian clock. Finally, given the critical role of the light/dark cycle in the synchronization of female reproduction, we will discuss the impact of circadian disruptions observed during shift-work conditions on female reproductive performance and fertility in both animal model and humans.

Circadian Insights into Motivated Behavior

For an organism to be successful in an evolutionary sense, it and its offspring must survive. Such survival depends on satisfying a number of needs that are driven by motivated behaviors, such as eating, sleeping, and mating. An individual can usually only pursue one motivated behavior at a time. The circadian system provides temporal structure to the organism's 24 hour day, partitioning specific behaviors to particular times of the day. The circadian system also allows anticipation of opportunities to engage in motivated behaviors that occur at predictable times of the day. Such anticipation enhances fitness by ensuring that the organism is physiologically ready to make use of a time-limited resource as soon as it becomes available. This could include activation of the sympathetic nervous system to transition from sleep to wake, or to engage in mating, or to activate of the parasympathetic nervous system to facilitate transitions to sleep, or to prepare the body to digest a meal. In addition to enabling temporal partitioning of motivated behaviors, the circadian system may also regulate the amplitude of the drive state motivating the behavior. For example, the circadian clock modulates not only when it is time to eat, but also how hungry we are. In this chapter we explore the physiology of our circadian clock and its involvement in a number of motivated behaviors such as sleeping, eating, exercise, sexual behavior, and maternal behavior. We also examine ways in which dysfunction of circadian timing can contribute to disease states, particularly in psychiatric conditions that include adherent motivational states.

Electrical properties of kisspeptin neurons and their regulation of GnRH neurons

Kisspeptin neurons are critical components of the neuronal network controlling the activity of the gonadotropin-releasing hormone (GnRH) neurons. A variety of genetically-manipulated mouse models have recently facilitated the study of the electrical activity of the two principal kisspeptin neuron populations located in the rostral periventricular area of the third ventricle (RP3V) and arcuate nucleus (ARN) in acute brain slices. We discuss here the mechanisms and pathways through which kisspeptin neurons regulate GnRH neuron activity. We then examine the different kisspeptin-green fluorescent protein mouse models being used for kisspeptin electrophysiology and the data obtained to date for RP3V and ARN kisspeptin neurons. In light of these new observations on the spontaneous firing rates, intrinsic membrane properties, and neurotransmitter regulation of kisspeptin neurons, we speculate on the physiological roles of the different kisspeptin populations.

Heterozygous deletion of ventral anterior homeobox (vax1) causes subfertility in mice

The known genetic causes of idiopathic hypogonadotropic hypogonadism (IHH) are often associated with the loss of GnRH neurons, leading to the disruption of the hypothalamic pituitary gonadal axis and subfertility. The majority of IHH cases have unknown origins and likely arise from compound mutations in more than one gene. Here we identify the homeodomain transcription factor ventral anterior homeobox1 (Vax1) as a potential genetic contributor to polygenic IHH. Although otherwise healthy, male and female Vax1 heterozygous (HET) mice are subfertile, indicating dosage sensitivity for the Vax1 allele. Although Vax1 mRNA is expressed in the pituitary, hypothalamus, and testis, we did not detect Vax1 mRNA in the sperm, ovary, or isolated pituitary gonadotropes. Whereas Vax1 HET females produced normal numbers of superovulated oocytes, corpora lutea numbers were reduced along with a slight increase in circulating basal LH and estrogen. The subfertility originated in the hypothalamus in which kisspeptin and GnRH transcripts were altered along with a substantial reduction of GnRH neuron number. Although the pituitary responded normally to a GnRH challenge, diestrus females had reduced LHβ and FSHβ in diestrus. Furthermore, Vax1 HET males had reduced GnRH mRNA and neuron numbers, whereas the pituitary had normal transcript levels and response to GnRH. Interestingly, the Vax1 HET males had an 88% reduction of motile sperm. Taken together, our data suggest that Vax1 HET subfertility originates in the hypothalamus by disrupting the hypothalamic-pituitary-gonadal axis. In addition, male subfertility may also be due to an unknown effect of Vax1 in the testis.

Disrupted reproduction, estrous cycle, and circadian rhythms in female mice deficient in vasoactive intestinal peptide

The female reproductive cycle is gated by the circadian timing system and may be vulnerable to disruptions in the circadian system. Prior work suggests that vasoactive intestinal peptide (VIP)-expressing neurons in the suprachiasmatic nucleus (SCN) are one pathway by which the circadian clock can influence the estrous cycle, but the impact of the loss of this peptide on reproduction has not been assessed. In the present study, we first examine the impact of the genetic loss of the neuropeptide VIP on the reproductive success of female mice. Significantly, mutant females produce about half the offspring of their wild-type sisters even when mated to the same males. We also find that VIP-deficient females exhibit a disrupted estrous cycle; that is, ovulation occurs less frequently and results in the release of fewer oocytes compared with controls. Circadian rhythms of wheel-running activity are disrupted in the female mutant mice, as is the spontaneous electrical activity of dorsal SCN neurons. On a molecular level, the VIP-deficient SCN tissue exhibits lower amplitude oscillations with altered phase relationships between the SCN and peripheral oscillators as measured by PER2-driven bioluminescence. The simplest explanation of our data is that the loss of VIP results in a weakened SCN oscillator, which reduces the synchronization of the female circadian system. These results clarify one of the mechanisms by which disruption of the circadian system reduces female reproductive success.

Diurnal and circadian oscillations in expression of kisspeptin, kisspeptin receptor and gonadotrophin-releasing hormone 2 genes in the grass puffer, a semilunar-synchronised spawner

In seasonally breeding animals, the circadian and photoperiodic regulation of neuroendocrine system is important for precisely-timed reproduction. Kisspeptin, encoded by the Kiss1 gene, acts as a principal positive regulator of the reproductive axis by stimulating gonadotrophin-releasing hormone (GnRH) neurone activity in vertebrates. However, the precise mechanisms underlying the cyclic regulation of the kisspeptin neuroendocrine system remain largely unknown. The grass puffer, Takifugu niphobles, exhibits a unique spawning rhythm: spawning occurs 1.5-2 h before high tide on the day of spring tide every 2 weeks, and the spawning rhythm is connected to circadian and lunar-/tide-related clock mechanisms. The grass puffer has only one kisspeptin gene (kiss2), which is expressed in a single neural population in the preoptic area (POA), and has one kisspeptin receptor gene (kiss2r), which is expressed in the POA and the nucleus dorsomedialis thalami. Both kiss2 and kiss2r show diurnal variations in expression levels, with a peak at Zeitgeber time (ZT) 6 (middle of day time) under the light/dark conditions. They also show circadian expression with a peak at circadian time 15 (beginning of subjective night-time) under constant darkness. The synchronous and diurnal oscillations of kiss2 and kiss2r expression suggest that the action of Kiss2 in the diencephalon is highly dependent on time. Moreover, midbrain GnRH2 gene (gnrh2) but not GnRH1 or GnRH3 genes show a unique semidiurnal oscillation with two peaks at ZT6 and ZT18 within a day. The cyclic expression of kiss2, kiss2r and gnrh2 may be important in the control of the precisely-timed diurnal and semilunar spawning rhythm of the grass puffer, possibly through the circadian clock and melatonin, which may transmit the photoperiodic information of daylight and moonlight to the reproductive neuroendocrine centre in the hypothalamus.

Vasoactive intestinal peptide modulation of the steroid-induced LH surge involves kisspeptin signaling in young but not in middle-aged female rats

Age-related LH surge dysfunction in middle-aged rats is characterized, in part, by reduced responsiveness to estradiol (E2)-positive feedback and reduced hypothalamic kisspeptin neurotransmission. Vasoactive intestinal peptide (VIP) neurons in the suprachiasmatic nucleus project to hypothalamic regions that house kisspeptin neurons. Additionally, middle-age females express less VIP mRNA in the suprachiasmatic nucleus on the day of the LH surge and intracerebroventricular (icv) VIP infusion restores LH surges. We tested the hypothesis that icv infusion of VIP modulates the LH surge through effects on the kisspeptin and RFamide-related peptide-3 (RFRP-3; an estradiol-regulated inhibitor of GnRH neurons) neurotransmitter systems. Brains were collected for in situ hybridization analyses from ovariectomized and ovarian hormone-primed young and middle-aged females infused with VIP or saline. The percentage of GnRH and Kiss1 cells coexpressing cfos and total Kiss1 mRNA were reduced in saline-infused middle-aged compared with young females. In young females, VIP reduced the percentage of GnRH and Kiss1 cells coexpressing cfos, suggesting that increased VIP signaling in young females adversely affected the function of Kiss1 and GnRH neurons. In middle-aged females, VIP increased the percentage of GnRH but not Kiss1 neurons coexpressing cfos, suggesting VIP affects LH release in middle-aged females through kisspeptin-independent effects on GnRH neurons. Neither reproductive age nor VIP affected Rfrp cell number, Rfrp mRNA levels per cell, or coexpression of cfos in Rfrp cells. These data suggest that VIP differentially affects activation of GnRH and kisspeptin neurons of female rats in an age-dependent manner.

Cellular mechanisms and integrative timing of neuroendocrine control of GnRH secretion by kisspeptin

The hypothalamus integrates endogenous and exogenous inputs to control the pituitary-gonadal axis. The ultimate hypothalamic influence on reproductive activity is mediated through timely secretion of GnRH in the portal blood, which modulates the release of gonadotropins from the pituitary. In this context neurons expressing the RF-amide neuropeptide kisspeptin present required features to fulfill the role of the long sought-after hypothalamic integrative centre governing the stimulation of GnRH neurons. Here we focus on the intracellular signaling pathways triggered by kisspeptin through its cognate receptor KISS1R and on the potential role of proteins interacting with this receptor. We then review evidence implicating both kisspeptin and RFRP3--another RF-amide neuropeptide--in the temporal orchestration of both the pre-ovulatory LH surge in female rodents and the organization of seasonal breeding in photoperiodic species.

Central circadian control of female reproductive function

Over the past two decades, it has become clear just how much of our physiology is under the control of the suprachiasmatic nucleus (SCN) and the cell-intrinsic molecular clock that ticks with a periodicity of approximately 24 h. The SCN prepares our digestive system for meals, our adrenal axis for the stress of waking up in the morning, and the genes expressed in our muscles when we prepare to exercise. Long before molecular studies of genes such as Clock, Bmal1, and the Per homologs were possible, it was obvious that female reproductive function was under strict circadian control at every level of the hypothalamic-pituitary-gonadal axis, and in the establishment and successful maintenance of pregnancy. This review highlights our current understanding of the role that the SCN plays in regulating female reproductive physiology, with a special emphasis on the advances made possible through the use of circadian mutant mice.

The endocrinology of the menstrual cycle

The ovulatory menstrual cycle is the result of the integrated action of the hypothalamus, pituitary, ovary, and endometrium. Like a metronome, the hypothalamus sets the beat for the menstrual cycle by the pulsatile release of gonadotropin-releasing hormone (GnRH). GnRH pulses occur every 1-1.5 h in the follicular phase of the cycle and every 2-4 h in the luteal phase of the cycle. Pulsatile GnRH secretion stimulates the pituitary gland to secrete luteinizing hormone (LH) and follicle stimulating hormone (FSH). The pituitary gland translates the tempo set by the hypothalamus into a signal, LH and FSH secretion, that can be understood by the ovarian follicle. The ovarian follicle is composed of three key cells: theca cells, granulosa cells, and the oocyte. In the ovarian follicle, LH stimulates theca cells to produce androstenedione. In granulosa cells from small antral follicles, FSH stimulates the synthesis of aromatase (Cyp19) which catalyzes the conversion of theca-derived androstenedione to estradiol. A critical concentration of estradiol, produced from a large dominant antral follicle, causes positive feedback in the hypothalamus, likely through the kisspeptin system, resulting in an increase in GnRH secretion and an LH surge. The LH surge causes the initiation of the process of ovulation. After ovulation, the follicle is transformed into the corpus luteum, which is stimulated by LH or chorionic gonadotropin (hCG) should pregnancy occur to secrete progesterone. Progesterone prepares the endometrium for implantation of the conceptus. Estradiol stimulates the endometrium to proliferate. Estradiol and progesterone cause the endometrium to become differentiated to a secretory epithelium. During the mid-luteal phase of the cycle, when progesterone production is at its peak, the secretory endometrium is optimally prepared for the implantation of an embryo. A diagrammatic representation of the intricate interactions involved in coordinating the menstrual cycle is provided in Fig. 1.

Sex differences in circadian timing systems: implications for disease

Virtually every eukaryotic cell has an endogenous circadian clock and a biological sex. These cell-based clocks have been conceptualized as oscillators whose phase can be reset by internal signals such as hormones, and external cues such as light. The present review highlights the inter-relationship between circadian clocks and sex differences. In mammals, the suprachiasmatic nucleus (SCN) serves as a master clock synchronizing the phase of clocks throughout the body. Gonadal steroid receptors are expressed in almost every site that receives direct SCN input. Here we review sex differences in the circadian timing system in the hypothalamic-pituitary-gonadal axis (HPG), the hypothalamic-adrenal-pituitary (HPA) axis, and sleep-arousal systems. We also point to ways in which disruption of circadian rhythms within these systems differs in the sexes and is associated with dysfunction and disease. Understanding sex differentiated circadian timing systems can lead to improved treatment strategies for these conditions.

Comparing label-free quantitative peptidomics approaches to characterize diurnal variation of peptides in the rat suprachiasmatic nucleus

Mammalian circadian rhythm is maintained by the suprachiasmatic nucleus (SCN) via an intricate set of neuropeptides and other signaling molecules. In this work, peptidomic analyses from two times of day were examined to characterize variation in SCN peptides using three different label-free quantitation approaches: spectral count, spectra index and SIEVE. Of the 448 identified peptides, 207 peptides were analyzed by two label-free methods, spectral count and spectral index. There were 24 peptides with significant (adjusted p-value < 0.01) differential peptide abundances between daytime and nighttime, including multiple peptides derived from secretogranin II, cocaine and amphetamine regulated transcript, and proprotein convertase subtilisin/kexin type 1 inhibitor. Interestingly, more peptides were analyzable and had significantly different abundances between the two time points using the spectral count and spectral index methods than with a prior analysis using the SIEVE method with the same data. The results of this study reveal the importance of using the appropriate data analysis approaches for label-free relative quantitation of peptides. The detection of significant changes in so rich a set of neuropeptides reflects the dynamic nature of the SCN and the number of influences such as feeding behavior on circadian rhythm. Using spectral count and spectral index, peptide level changes are correlated to time of day, suggesting their key role in circadian function.