Gonadotropin-releasing hormone II stimulates female sexual behavior in marmoset monkeys

Aromatase Inhibition Eliminates Sexual Receptivity Without Enhancing Weight Gain in Ovariectomized Marmoset Monkeys

Context

Ovarian estradiol supports female sexual behavior and metabolic function. While ovariectomy (OVX) in rodents abolishes sexual behavior and enables obesity, OVX in nonhuman primates decreases, but does not abolish, sexual behavior, and inconsistently alters weight gain.

Objective

We hypothesize that extra-ovarian estradiol provides key support for both functions, and to test this idea, we employed aromatase inhibition to eliminate extra-ovarian estradiol biosynthesis and diet-induced obesity to enhance weight gain.

Methods

Thirteen adult female marmosets were OVX and received (1) estradiol-containing capsules and daily oral treatments of vehicle (E2; n = 5); empty capsules and daily oral treatments of either (2) vehicle (VEH, 1 mL/kg, n = 4), or (3) letrozole (LET, 1 mg/kg, n = 4).

Results

After 7 months, we observed robust sexual receptivity in E2, intermediate frequencies in VEH, and virtually none in LET females (P = .04). By contrast, few rejections of male mounts were observed in E2, intermediate frequencies in VEH, and high frequencies in LET females (P = .04). Receptive head turns were consistently observed in E2, but not in VEH and LET females. LET females, alone, exhibited robust aggressive rejection of males. VEH and LET females demonstrated increased % body weight gain (P = .01). Relative estradiol levels in peripheral serum were E2 >>> VEH > LET, while those in hypothalamus ranked E2 = VEH > LET, confirming inhibition of local hypothalamic estradiol synthesis by letrozole.

Conclusion

Our findings provide the first evidence for extra-ovarian estradiol contributing to female sexual behavior in a nonhuman primate, and prompt speculation that extra-ovarian estradiol, and in particular neuroestrogens, may similarly regulate sexual motivation in other primates, including humans.

Neuropeptidergic control of neurosteroids biosynthesis

Neurosteroids are steroids synthesized within the central nervous system either from cholesterol or by metabolic reactions of circulating steroid hormone precursors. It has been suggested that neurosteroids exert pleiotropic activities within the central nervous system, such as organization and activation of the central nervous system and behavioral regulation. It is also increasingly becoming clear that neuropeptides exert pleiotropic activities within the central nervous system, such as modulation of neuronal functions and regulation of behavior, besides traditional neuroendocrinological functions. It was hypothesized that some of the physiological functions of neuropeptides acting within the central nervous system may be through the regulation of neurosteroids biosynthesis. Various neuropeptides reviewed in this study possibly regulate neurosteroids biosynthesis by controlling the activities of enzymes that catalyze the production of neurosteroids. It is now required to thoroughly investigate the neuropeptidergic control mechanisms of neurosteroids biosynthesis to characterize the physiological significance of this new neuroendocrinological phenomenon.

Sexual incentive motivation, sexual behavior, and general arousal: Do rats and humans tell the same story?

Sexual incentive stimuli activate sexual motivation and heighten the level of general arousal. The sexual motive may induce the individual to approach the incentive, and eventually to initiate sexual acts. Both approach and the ensuing copulatory interaction further enhance general arousal. We present data from rodents and humans in support of these assertions. We then suggest that orgasm is experienced when the combined level of excitation surpasses a threshold. In order to analyze the neurobiological bases of sexual motivation, we employ the concept of a central motive state. We then discuss the mechanisms involved in the long- and short-term control of that state as well as those mediating the momentaneous actions of sexual incentive stimuli. This leads to an analysis of the neurobiology behind the interindividual differences in responsivity of the sexual central motive state. Knowledge is still fragmentary, and many contradictory observations have been made. Nevertheless, we conclude that the basic mechanisms of sexual motivation and the role of general arousal are similar in rodents and humans.

Role of Habenula in Social and Reproductive Behaviors in Fish: Comparison With Mammals

Social behaviors such as mating, parenting, fighting, and avoiding are essential functions as a communication tool in social animals, and are critical for the survival of individuals and species. Social behaviors are controlled by a complex circuitry that comprises several key social brain regions, which is called the social behavior network (SBN). The SBN further integrates social information with external and internal factors to select appropriate behavioral responses to social circumstances, called social decision-making. The social decision-making network (SDMN) and SBN are structurally, neurochemically and functionally conserved in vertebrates. The social decision-making process is also closely influenced by emotional assessment. The habenula has recently been recognized as a crucial center for emotion-associated adaptation behaviors. Here we review the potential role of the habenula in social function with a special emphasis on fish studies. Further, based on evolutional, molecular, morphological, and behavioral perspectives, we discuss the crucial role of the habenula in the vertebrate SDMN.

Comparative analyses of the Pan lineage reveal selection on gene pathways associated with diet and sociality in bonobos

Chimpanzees (Pan troglodytes) and bonobos (Pan paniscus) diverged into distinct species approximately 1.7 million years ago when the ancestors of modern-day bonobo populations were separated by the Congo River. This geographic boundary separates the two species today and the associated ecological factors, including resource distribution and feeding competition, have likely shaped the divergent social behavior of both species. The most striking behavioral differences pertain to between group interactions in which chimpanzees behave aggressively towards unfamiliar conspecifics, while bonobos display remarkable tolerance. Several hypotheses attempt to explain how different patterns of social behavior have come to exist in the two species, some with specific genetic predictions, likening the evolution of bonobos to a process of domestication. Here, we utilize 73 ape genomes and apply linkage haplotype homozygosity and structure informed allele frequency differentiation methods to identify positively selected regions in bonobos since their split from a common pan ancestor to better understand the environment and processes that resulted in the behavioral differences observed today. We find novel evidence of selection in genetic regions that aid in starch digestion (AMY2) along with support for two genetic predictions related to self-domestication processes hypothesized to have occurred in the bonobo. We also find evidence for selection on neuroendocrine pathways associated with social behavior including the oxytocin, serotonin, and gonadotropin releasing hormone pathways.

Morphological Evidence for Functional Crosstalk Between Multiple GnRH Systems in the Male Tilapia, Oreochromis niloticus

Gonadotropin-releasing hormone (GnRH) is a reproductive neuropeptide, which controls vertebrate reproduction. In most vertebrates, there are more than two GnRH orthologs in the brain. In cichlid fish, the Nile tilapia (Oreochromis niloticus), GnRH1 is the primary hypophysiotropic hormone, while GnRH2 and GnRH3 are non-hypophysiotropic but neuromodulatory in function. Hypophysiotropic GnRH neurons are thought to inter-communicate, while it remains unknown if hypophysiotropic and non-hypophysiotropic GnRH systems communicate with each other. In the present study, we examined interrelationship between three GnRH types using specific antibodies raised against their respective GnRH associated peptide (GAP) sequence. Double-immunofluorescence labeling coupled with confocal microscopy revealed that in sexually mature males, GnRH-GAP1-immunoreactive (-ir) processes are in proximities of GnRH-GAP3-ir cell somata in the terminal nerve, while GnRH-GAP1-ir cell somata were also accompanied by GnRH-GAP3-ir processes in the preoptic area. However, such interaction was not seen in immature males. Further, there was no interaction between GnRH-GAP2 and GnRH-GAP1 or GnRH-GAP3 neurons. Single cell gene expression analysis revealed co-expression of multiple GnRH receptor genes (gnrhr1 and gnrhr2) in three GnRH-GAP cell types. In mature males, high levels of gnrhr2 mRNA were expressed in GnRH-GAP1-ir cells. In immature males, gnrhr1 and gnrhr2 mRNAs are highly expressed in GnRH-GAP3-ir cells. These results suggest heterologous interactions between the three GnRH-GAP cell types and their potential functional interaction during different reproductive stages.

Identification of sex differences in zebrafish (Danio rerio) brains during early sexual differentiation and masculinization using 17α-methyltestoterone

Sexual behavior in teleost fish is highly plastic. It can be attributed to the relatively few sex differences found in adult brain transcriptomes. Environmental and hormonal factors can influence sex-specific behavior. Androgen treatment stimulates behavioral masculinization. Sex dimorphic gene expression in developing teleost brains and the molecular basis for androgen-induced behavioral masculinization are poorly understood. In this study, juvenile zebrafish (Danio rerio) were treated with 100 ng/L of 17 alpha-methyltestosterone (MT) during sexual development from 20 days post fertilization to 40 days and 60 days post fertilization. We compared brain gene expression patterns in MT-treated zebrafish with control males and females using RNA-Seq to shed light on the dynamic changes in brain gene expression during sexual development and how androgens affect brain gene expression leading to behavior masculinization. We found modest differences in gene expression between juvenile male and female zebrafish brains. Brain aromatase (cyp19a1b), prostaglandin 3a synthase (ptges3a), and prostaglandin reductase 1 (ptgr1) were among the genes with sexually dimorphic expression patterns. MT treatment significantly altered gene expression relative to both male and female brains. Fewer differences were found among MT-treated brains and male brains compared to female brains, particularly at 60 dpf. MT treatment upregulated the expression of hydroxysteroid 11-beta dehydrogenase 2 (hsd11b2), deiodinase, iodothyronine, type II (dio2), and gonadotrophin releasing hormones (GnRH) 2 and 3 (gnrh2 and gnrh3) suggesting local synthesis of 11-ketotestosterone, triiodothyronine, and GnRHs in zebrafish brains which are influenced by androgens. Androgen, estrogen, prostaglandin, thyroid hormone, and GnRH signaling pathways likely interact to modulate teleost sexual behavior.

Adverse Effects of Aromatase Inhibition on the Brain and Behavior in a Nonhuman Primate

Breast cancer patients using aromatase inhibitors (AIs) as an adjuvant therapy often report side effects, including hot flashes, mood changes, and cognitive impairment. Despite long-term use in humans, little is known about the effects of continuous AI administration on the brain and cognition. We used a primate model of human cognitive aging, the common marmoset, to examine the effects of a 4-week daily administration of the AI letrozole (20 μg, p.o.) on cognition, anxiety, thermoregulation, brain estrogen content, and hippocampal pyramidal cell physiology. Letrozole treatment was administered to both male and female marmosets and reduced peripheral levels of estradiol (E2), but unexpectedly increased E2 levels in the hippocampus. Spatial working memory and intrinsic excitability of hippocampal neurons were negatively affected by the treatment possibly due to increased hippocampal E2. While no changes in hypothalamic E2 were observed, thermoregulation was disrupted by letrozole in females only, indicating some impact on hypothalamic activity. These findings suggest adverse effects of AIs on the primate brain and call for new therapies that effectively prevent breast cancer recurrence while minimizing side effects that further compromise quality of life.SIGNIFICANCE STATEMENT Aromatase inhibitors (AIs) are used as an adjuvant therapy for estrogen-receptor-positive breast cancer and are associated with side effects, including hot flashes, depression/anxiety, and memory deficits severe enough for many women to discontinue this life-saving treatment. AIs are also used by men, yet sex differences in the reported side effects have not been systematically studied. We show that AI-treated male and female marmosets exhibit behavioral changes consistent with these CNS symptoms, as well as elevated hippocampal estradiol and compromised hippocampal physiology. These findings illustrate the need for (1) a greater understanding of the precise mechanisms by which AIs impact brain function and (2) the development of new treatment approaches for breast cancer patients that minimize adverse effects on the brain.

Comparative and Evolutionary Aspects of Gonadotropin-Inhibitory Hormone and FMRFamide-Like Peptide Systems

Gonadotropin-inhibitory hormone (GnIH) is a hypothalamic neuropeptide that was found in the brain of Japanese quail when investigating the existence of RFamide peptides in birds. GnIH was named because it decreased gonadotropin release from cultured anterior pituitary, which was located in the hypothalamo-hypophysial system. GnIH and GnIH precursor gene related peptides have a characteristic C-terminal LPXRFamide (X = L or Q) motif that is conserved in jawed vertebrates. Orthologous peptides to GnIH are also named RFamide related peptide or LPXRFamide peptide from their structure. A G-protein coupled receptor GPR147 is the primary receptor for GnIH. Similarity-based clustering of neuropeptide precursors in metazoan species indicates that GnIH precursor of vertebrates is evolutionarily related to FMRFamide precursor of mollusk and nematode. FMRFamide peptide is the first RFamide peptide that was identified from the ganglia of the venus clam. In order to infer the evolutionary history of the GnIH-GnIH receptor system we investigate the structural similarities between GnIH and its receptor and well-studied nematode Caenorhabditis elegans (C. elegans) FMRFamide-like peptides (FLPs) and their receptors. We also compare the functions of FLPs of nematode with GnIH of chordates. A multiple sequence alignment and phylogenetic analyses of GnIH, neuropeptide FF (NPFF), a paralogous peptide of GnIH, and FLP precursors have shown that GnIH and NPFF precursors belong to different clades and some FLP precursors have structural similarities to either precursor. The peptide coding regions of FLP precursors in the same clade align well with those of GnIH or NPFF precursors. Alignment of GnIH (LPXRFa) peptides of chordates and FLPs of C. elegans grouped the peptides into five groups according to the last C-terminal amino acid sequences, which were MRFa, LRFa, VRFa, IRFa, and PQRFa. Phylogenetic analysis of receptors suggested that GPR147 has evolutionary relationships with FLP receptors, which regulate reproduction, aggression, locomotion, and feeding. GnIH and some FLPs mediate the effect of stress on reproduction and behavior, which may also be a conserved property of these peptide systems. Future studies are needed to investigate the mechanism of how neuropeptide precursor genes are mutated to evolve new neuropeptides and their inheritance.

Intrinsic links among sex, emotion, and reproduction

Species survival is dependent on successful reproduction. This begins with a desire to mate, followed by selection of a partner, copulation and in monogamous mammals including humans, requires emotions and behaviours necessary to maintain partner bonds for the benefit of rearing young. Hormones are integral to all of these stages and not only mediate physiological and endocrine processes involved in reproduction, but also act as neuromodulators within limbic brain centres to facilitate the expression of innate emotions and behaviours required for reproduction. A significant body of work is unravelling the roles of several key hormones in the modulation of mood states and sexual behaviours; however, a full understanding of the integration of these intrinsic links among sexual and emotional brain circuits still eludes us. This review summarises the evidence to date and postulates future directions to identify potential psycho-neuroendocrine frameworks linking sexual and emotional brain processes with reproduction.

RFamide-related Peptide-3 and the Trade-off between Reproductive and Ingestive Behavior

Ingestive and sex behaviors are important for individual survival and reproductive success, but when environmental energy availability is limited, individuals of many different species make a trade-off, forfeiting sex for ingestive behavior. For example, food-deprived female Syrian hamsters (Mesocricetus auratus) forego vaginal scent marking and lordosis (sex behaviors) in favor of foraging, hoarding, and eating food (ingestive behavior). Reproductive processes tend to be energetically costly, and individual survival requires homeostasis in metabolic energy. Thus, during energetic challenges, the chances of survival are enhanced by decreasing the energy expended on reproductive processes. The entire hypothalamic-pituitary-gonadal (HPG) system is inhibited by severe energetic challenges, but comparatively little is known about the effects of mild energetic challenges. We hypothesized that (1) a trade-off is made between sex and ingestive behavior even when the level of food restriction is insufficient to inhibit the HPG system; (2) mild energetic challenges force a trade-off between appetitive ingestive and sex behaviors, but not consummatory versions of the same behaviors; and (3) the trade-off is orchestrated by ovarian steroid modulation of RFamide-related peptide 3 (RFRP-3). In other species, RFRP-3, an ortholog of avian gonadotropin-inhibitory hormone, is implicated in control of behavior in response to energetic challenges and stressful stimuli. In support of our three hypotheses, there is a "dose-response" effect of food restriction and re-feeding on the activation of RFRP-3-immunoreactive cells in the dorsomedial hypothalamus and on appetitive behaviors (food hoarding and sexual motivation), but not on consummatory behaviors (food intake and lordosis), with no significant effect on circulating levels of estradiol or progesterone. The effect of food restriction on the activation of RFRP-3 cells is modulated at the time of estrus in gonadally-intact females and in ovariectomized females treated with progesterone alone or with estradiol plus progesterone. Intracerebral treatment with RFRP-3 results in significant decreases in sexual motivation and results in significant but small increases in food hoarding in hamsters fed ad libitum. These and other results are consistent with the idea that ovarian steroids and RFRP-3 are part of a system that orchestrates trade-offs in appetitive behaviors in environments where energy availability fluctuates.

Characterization of Gnrh/Gnih elements in the olfacto-retinal system and ovary during zebrafish ovarian maturation

Gonadotropin releasing hormone (GnRH) is one of the key players of brain-pituitary-gonad axis, exerting overall control over vertebrate reproduction. In zebrafish, two variants were characterized and named as Gnrh2 and Gnrh3. In this species, Gnrh3, the hypohysiotropic form, is expressed by neurons of the olfactory-retinal system, where it is related with food detection, intra/interspecific recognition, visual acuity and retinal processing modulation. Previous studies have reported the presence of Gnrh receptors in the zebrafish retina, but not yet in the zebrafish olfactory epithelium. The current study analyzed the presence of gnrh2 and gnrh3, their receptors (gnrhr 1,2,3 and 4) and gnih (gonadotropin inhibitory hormone) transcripts, as well as the Gnrh3 protein in the olfactory epithelium (OE), olfactory bulb (OB), retina and ovary during zebrafish ovarian maturation. We found an increase of gnrh receptors transcripts in the OE at the final stages of ovarian maturation. In the OE, Gnrh3 protein was detected in the olfactory receptor neurons cilia and in the olfactory nerve fibers. Interestingly, in the OB, we found an inverse expression pattern between gnih and gnrh3. In the retina, gnrhr4 mRNA was found in the nuclei of amacrine, bipolar, and ganglion cells next to Gnrh3 positive fibers. In the ovary, gnrh3, gnrhr2 and gnrhr4 transcripts were found in perinucleolar oocytes, while gnih in oocytes at the cortical alveolus stage. Our results suggested that Gnrh/Gnih elements are involved in the neuromodulation of the sensorial system particularly at the final stages of maturation, playing also a paracrine role in the ovary.

Effects of pinealectomy on the neuroendocrine reproductive system and locomotor activity in male European sea bass, Dicentrarchus labrax

The seasonally changing photoperiod controls the timing of reproduction in most fish species, however, the transduction of this photoperiodic information to the reproductive axis is still unclear. This study explored the potential role of two candidate neuropeptide systems, gonadotropin-inhibitory hormone (Gnih) and kisspeptin, as mediators between the pineal organ (a principle transducer of photoperiodic information) and reproductive axis in male European sea bass, Dicentrarchus labrax. Two seven-day experiments of pinealectomy (Px) were performed, in March (end of reproductive season) and August (resting season). Effects of Px and season on the brain expression of gnih (sbgnih) and its receptor (sbgnihr), kisspeptins (kiss1, kiss2) and their receptors (kissr2, kissr3) and gonadotropin-releasing hormone (gnrh1, gnrh2, gnrh3) and the main brain receptor (gnrhr-II-2b) genes, plasma melatonin levels and locomotor activity rhythms were examined. Results showed that Px reduced night-time plasma melatonin levels. Gene expression analyses demonstrated a sensitivity of the Gnih system to Px in March, with a reduction in sbgnih in the mid-hindbrain, a region with bilateral connections to the pineal organ. In August, kiss2 levels increased in Px animals but not in controls. Significant differences in expression were observed for diencephalic sbgnih, sbgnihr, kissr3 and tegmental gnrh2 between seasons. Recordings of locomotor activity following surgery revealed a change from light-synchronised to free-running rhythmic behavior. Altogether, the Gnih and Kiss2 sensitivity to Px and seasonal differences observed for Gnih and its receptor, Gnrh2, and the receptor for Kiss2 (Kissr3), suggested they could be mediators involved in the relay between environment and seasonal reproduction.

Conservation of Three-Dimensional Helix-Loop-Helix Structure through the Vertebrate Lineage Reopens the Cold Case of Gonadotropin-Releasing Hormone-Associated Peptide

GnRH-associated peptide (GAP) is the C-terminal portion of the gonadotropin-releasing hormone (GnRH) preprohormone. Although it was reported in mammals that GAP may act as a prolactin-inhibiting factor and can be co-secreted with GnRH into the hypophyseal portal blood, GAP has been practically out of the research circuit for about 20 years. Comparative studies highlighted the low conservation of GAP primary amino acid sequences among vertebrates, contributing to consider that this peptide only participates in the folding or carrying process of GnRH. Considering that the three-dimensional (3D) structure of a protein may define its function, the aim of this study was to evaluate if GAP sequences and 3D structures are conserved in the vertebrate lineage. GAP sequences from various vertebrates were retrieved from databases. Analysis of primary amino acid sequence identity and similarity, molecular phylogeny, and prediction of 3D structures were performed. Amino acid sequence comparison and phylogeny analyses confirmed the large variation of GAP sequences throughout vertebrate radiation. In contrast, prediction of the 3D structure revealed a striking conservation of the 3D structure of GAP1 (GAP associated with the hypophysiotropic type 1 GnRH), despite low amino acid sequence conservation. This GAP1 peptide presented a typical helix-loop-helix (HLH) structure in all the vertebrate species analyzed. This HLH structure could also be predicted for GAP2 in some but not all vertebrate species and in none of the GAP3 analyzed. These results allowed us to infer that selective pressures have maintained GAP1 HLH structure throughout the vertebrate lineage. The conservation of the HLH motif, known to confer biological activity to various proteins, suggests that GAP1 peptides may exert some hypophysiotropic biological functions across vertebrate radiation.

Expression and Role of Gonadotropin-Releasing Hormone 2 and Its Receptor in Mammals

Gonadotropin-releasing hormone 1 (GnRH1) and its receptor (GnRHR1) drive mammalian reproduction via regulation of the gonadotropins. Yet, a second form of GnRH (GnRH2) and its receptor (GnRHR2) also exist in mammals. GnRH2 has been completely conserved throughout 500 million years of evolution, signifying high selection pressure and a critical biological role. However, the GnRH2 gene is absent (e.g., rat) or inactivated (e.g., cow and sheep) in some species but retained in others (e.g., human, horse, and pig). Likewise, many species (e.g., human, chimpanzee, cow, and sheep) retain the GnRHR2 gene but lack the appropriate coding sequence to produce a full-length protein due to gene coding errors; although production of GnRHR2 in humans remains controversial. Certain mammals lack the GnRHR2 gene (e.g., mouse) or most exons entirely (e.g., rat). In contrast, old world monkeys, musk shrews, and pigs maintain the coding sequence required to produce a functional GnRHR2. Like GnRHR1, GnRHR2 is a 7-transmembrane, G protein-coupled receptor that interacts with G_αq/11 to mediate cell signaling. However, GnRHR2 retains a cytoplasmic tail and is only 40% homologous to GnRHR1. A role for GnRH2 and its receptor in mammals has been elusive, likely because common laboratory models lack both the ligand and receptor. Uniquely, both GnRH2 and GnRHR2 are ubiquitously expressed; transcript levels are abundant in peripheral tissues and scarcely found in regions of the brain associated with gonadotropin secretion, suggesting a divergent role from GnRH1/GnRHR1. Indeed, GnRH2 and its receptor are not physiological modulators of gonadotropin secretion in mammals. Instead, GnRH2 and GnRHR2 coordinate the interaction between nutritional status and sexual behavior in the female brain. Within peripheral tissues, GnRH2 and its receptor are novel regulators of reproductive organs. GnRH2 and GnRHR2 directly stimulate steroidogenesis within the porcine testis. In the female, GnRH2 and its receptor may help mediate placental function, implantation, and ovarian steroidogenesis. Furthermore, both the GnRH2 and GnRHR2 genes are expressed in human reproductive tumors and represent emerging targets for cancer treatment. Thus, GnRH2 and GnRHR2 have diverse functions in mammals which remain largely unexplored.

Molecular, cellular, morphological, physiological and behavioral aspects of gonadotropin-inhibitory hormone

Gonadotropin-inhibitory hormone (GnIH) is a hypothalamic neuropeptide that was isolated from the brains of Japanese quail in 2000, which inhibited luteinizing hormone release from the anterior pituitary gland. Here, we summarize the following fifteen years of researches that investigated on the mechanism of GnIH actions at molecular, cellular, morphological, physiological, and behavioral levels. The unique molecular structure of GnIH peptide is in its LPXRFamide (X=L or Q) motif at its C-terminal. The primary receptor for GnIH is GPR147. The cell signaling pathway triggered by GnIH is initiated by inhibiting adenylate cyclase and decreasing cAMP production in the target cell. GnIH neurons regulate not only gonadotropin synthesis and release in the pituitary, but also regulate various neurons in the brain, such as GnRH1, GnRH2, dopamine, POMC, NPY, orexin, MCH, CRH, oxytocin, and kisspeptin neurons. GnIH and GPR147 are also expressed in gonads and they may regulate steroidogenesis and germ cell maturation in an autocrine/paracrine manner. GnIH regulates reproductive development and activity. In female mammals, GnIH may regulate estrous or menstrual cycle. GnIH is also involved in the regulation of seasonal reproduction, but GnIH may finely tune reproductive activities in the breeding seasons. It is involved in stress responses not only in the brain but also in gonads. GnIH may inhibit male socio-sexual behavior by stimulating the activity of cytochrome P450 aromatase in the brain and stimulates feeding behavior by modulating the activities of hypothalamic and central amygdala neurons.

Reproductive Neuroendocrine Pathways of Social Behavior

Social behaviors are key components of reproduction, because they are essential for successful fertilization. Social behaviors, such as courtship, mating, and aggression, are strongly associated with sex steroids, such as testosterone, estradiol, and progesterone. Secretion of sex steroids from the gonads is regulated by the hypothalamus-pituitary-gonadal (HPG) axis in vertebrates. Gonadotropin-releasing hormone (GnRH) is a pivotal hypothalamic neuropeptide that stimulates gonadotropin release from the pituitary. In recent years, the role of neuropeptides containing the C-terminal Arg-Phe-NH2 (RFamide peptides) has been emphasized in vertebrate reproduction. In particular, two key RFamide peptides, kisspeptin and gonadotropin-inhibitory hormone (GnIH), emerged as critical accelerator and suppressor of gonadotropin secretion. Kisspeptin stimulates GnRH release by directly acting on GnRH neurons, whereas GnIH inhibits gonadotropin release by inhibiting kisspeptin, GnRH neurons, or pituitary gonadotropes. These neuropeptides can regulate social behavior by regulating the HPG axis. However, distribution of neuronal fibers of GnRH, kisspeptin, and GnIH neurons is not limited within the hypothalamus, and the existence of extrahypothalamic neuronal fibers suggests direct control of social behavior within the brain. It has traditionally been shown that central administration of GnRH can stimulate female sexual behavior in rats. Recently, it was shown that Kiss1, one of the paralogs of kisspeptin peptide family, regulates fear responses in zebrafish and GnIH inhibits sociosexual behavior in birds. Here, we highlight recent findings regarding the role of GnRH, kisspeptin, and GnIH in the regulation of social behaviors in fish, birds, and mammals and discuss their importance in future biological and biomedical research.

Flibanserin-Stimulated Partner Grooming Reflects Brain Metabolism Changes in Female Marmosets

INTRODUCTION

Female sexual interest and arousal disorder is personally distressing for women. To better understand the mechanism of the candidate therapeutic, flibanserin, we determined its effects on an index of brain glucose metabolism.

AIM

We hypothesized that chronic treatment with flibanserin would alter metabolism in brain regions associated with serotonergic function and female sexual behavior.

METHODS

In a crossover design, eight adult female common marmosets (Calithrix jacchus) received daily flibanserin or vehicle. After 7-12 weeks of treatment, the glucose metabolism radiotracer [(18) F]fluorodeoxyglucose (FDG) was administered to each female immediately prior to 30 minutes of interaction with her male pairmate, after which females were anesthetized and imaged by positron emission tomography. Whole-brain normalized images were analyzed with anatomically defined regions of interest. Whole-brain voxelwise mapping was used to explore treatment effects. Correlations were examined between alterations in metabolism and pairmate social grooming.

MAIN OUTCOME MEASURES

Changes in metabolism associated with flibanserin were determined for dorsal raphe, medial prefrontal cortex (mPFC), medial preoptic area of hypothalamus (mPOA), ventromedial nucleus of hypothalamus, and field cornu ammonis 1 (CA1) of the hippocampus.

RESULTS

In response to chronic flibanserin, metabolism in mPOA declined, and this reduction correlated with increases in pairmate grooming. A cluster of voxels in frontal cortico-limbic regions exhibited reduced metabolism in response to flibanserin and overlapped with a voxel cluster in which reductions in metabolism correlated with increases in pairmate grooming. Finally, reductions in mPOA metabolism correlated with increases in metabolism in a cluster of voxels in somatosensory cortex.

CONCLUSIONS

Taken together, these results suggest that flibanserin-induced reductions in female mPOA neural activity increase intimate affiliative behavior with male pairmates.

LH-Independent Testosterone Secretion Is Mediated by the Interaction Between GNRH2 and Its Receptor Within Porcine Testes

Unlike classic gonadotropin-releasing hormone 1 (GNRH1), the second mammalian isoform (GNRH2) is an ineffective stimulant of gonadotropin release. Species that produce GNRH2 may not maintain a functional GNRH2 receptor (GNRHR2) due to coding errors. A full-length GNRHR2 gene has been identified in swine, but its role in reproduction requires further elucidation. Our objective was to examine the role of GNRH2 and GNRHR2 in testicular function of boars. We discovered that GNRH2 levels were higher in the testis than in the anterior pituitary gland or hypothalamus, corresponding to greater GNRHR2 abundance in the testis versus the anterior pituitary gland. Moreover, GNRH2 immunostaining was most prevalent within seminiferous tubules, whereas GNRHR2 was detected in high abundance on Leydig cells. GNRH2 pretreatment of testis explant cultures elicited testosterone secretion similar to that of human chorionic gonadotropin stimulation. Treatment of mature boars with GNRH2 elevated testosterone levels similar to those of GNRH1-treated males, despite minimal GNRH2-induced release of luteinizing hormone (LH). When pretreated with a GNRHR1 antagonist (SB-75), subsequent GNRH2 treatment stimulated low levels of testosterone secretion despite a pattern of LH release similar to that in the previous trial, suggesting that SB-75 inhibited testicular GNRHR2s. Given that pigs lack testicular GNRHR1, these data may indicate that GNRH2 and its receptor are involved in autocrine or paracrine regulation of testosterone secretion. Notably, our data are the first to suggest a biological function of a novel GNRH2-GNRHR2 system in the testes of swine.

Evolution of gonadotropin-inhibitory hormone receptor and its ligand

Gonadotropin-inhibitory hormone (GnIH) is a neuropeptide inhibitor of gonadotropin secretion, which was first identified in the Japanese quail hypothalamus. GnIH peptides share a C-terminal LPXRFamide (X=L or Q) motif in most vertebrates. The receptor for GnIH (GnIHR) is the seven-transmembrane G protein-coupled receptor 147 (GPR147) that inhibits cAMP production. GPR147 is also named neuropeptide FF (NPFF) receptor 1 (NPFFR1), because it also binds NPFF that has a C-terminal PQRFamide motif. To understand the evolutionary history of the GnIH system in the animal kingdom, we searched for receptors structurally similar to GnIHR in the genome of six mammals (human, mouse, rat, cattle, cat, and rabbit), five birds (pigeon, chicken, turkey, budgerigar, and zebra finch), one reptile (green anole), one amphibian (Western clawed flog), six fishes (zebrafish, Nile tilapia, Fugu, coelacanth, spotted gar, and lamprey), one hemichordate (acorn worm), one echinoderm (purple sea urchin), one mollusk (California sea hare), seven insects (pea aphid, African malaria mosquito, honey bee, buff-tailed bumblebee, fruit fly, jewel wasp, and red flour beetle), one cnidarian (hydra), and constructed phylogenetic trees by neighbor joining (NJ) and maximum likelihood (ML) methods. A multiple sequence alignment of the receptors showed highly conserved seven-transmembrane domains as well as disulfide bridge sites between the first and second extracellular loops, including the receptor of hydra. Both NJ and ML analyses grouped the receptors of vertebrates into NPFFR1 and NPFFR2 (GPR74), and the receptors of insects into the receptor for SIFamide peptides that share a C-terminal YRKPPFNGSIFamide motif. Although human, quail and zebrafish GnIHR (NPFFR1) were most structurally similar to SIFamide receptor of fruit fly in the Famide peptide (FMRFamide, neuropeptide F, short neuropeptide F, drosulfakinin, myosuppressin, SIFamide) receptor families, the amino acid sequences and the peptide coding regions of GnIH precursors were most similar to FMRFamide precursor of fruit fly in the precursors of Famide peptide families. Chromosome synteny analysis of the precursor genes of human, quail and zebrafish GnIH and fruit fly Famide peptides further identified conserved synteny in vertebrate GnIH and fruit fly FMRFa precursor genes as well as other Famide peptide precursor genes. These results suggest that GnIH and its receptor pair and SIFamide and its receptor pair may have diverged and co-evolved independently in vertebrates and insects, respectively, from their ancestral Famide peptide and its receptor pair, during diversification and evolution of deuterostomian and protostomian species.

Hedgehog-PKA signaling and gnrh3 regulate the development of zebrafish gnrh3 neurons

GnRH neurons secrete GnRH that controls the development of the reproduction system. Despite many studies, the signals controlling the development of GnRH neurons from its progenitors have not been fully established. To understand the development of GnRH neurons, we examined the development of gnrh3-expressing cells using a transgenic zebrafish line that expresses green fluorescent protein (GFP) and LacZ driven by the gnrh3 promoter. GFP and LacZ expression recapitulated that of gnrh3 in the olfactory region, olfactory bulb and telencephalon. Depletion of gnrh3 by morpholinos led to a reduction of GFP- and gnrh3-expressing cells, while over-expression of gnrh3 mRNA increased the number of these cells. This result indicates a positive feed-forward regulation of gnrh3 cells by gnrh3. The gnrh3 cells were absent in embryos that lack Hedgehog signaling, but their numbers were increased in embryos overexpressing shhb. We manipulated the amounts of kinase that antagonizes the Hedgehog signaling pathway, protein kinase A (PKA), by treating embryos with PKA activator forskolin or by injecting mRNAs encoding its constitutively active catalytic subunit (PKA*) and dominant negative regulatory subunit (PKI) into zebrafish embryos. PKA* misexpression or forskolin treatment decreased GFP cell numbers, while PKI misexpression led to ectopic production of GFP cells. Our data indicate that the Hedgehog-PKA pathway participates in the development of gnrh3-expressing neurons during embryogenesis.

Oestradiol modulation of cognition in adult female marmosets (Callithrix jacchus)

The common marmoset (Callithrix jacchus) provides many advantages over traditional rodent and macaque species as a model for human ageing and may be very useful for studying the effects of sex steroids on cognitive and brain ageing. We present the first study examining the effects of oestrogens on cognitive function in female marmosets. Adult monkeys (3-5 years of age) were trained to a specific learning criterion on a battery of cognitive tasks preoperatively (object discrimination, delayed response with increasing delays and detour reaching with opaque box) and were tested on different versions of these tasks (object reversals, delayed response with randomised delays and detour reaching with clear box) after ovariectomy and simultaneous implantation with 17β-oestradiol (E2 ) (n = 6) or blank (n = 6) Silastic capsules. Acquisition of a delayed matching-to-position task with a 1-s delay was also administered after completion of these tests. E2 -treated monkeys were significantly impaired on the second reversal and showed an increase in perseverative responding from reversals 1-3. Their performance also tended to be worse than that of control monkeys on the delayed response task. Performance acquisition on the delayed matching-to-position tended to be better in E2 -treated relative to control monkeys, although the group difference did not reach statistical significance. No effect of treatment was detected for detour reaching or affiliative behaviours. Overall, the findings indicate that E2 compromises performance on prefrontally-mediated tasks. The suggestion that E2 may improve acquisition on tasks dependent on the hippocampus will require further validation. These results are discussed in the context of dopaminergic and serotonergic signalling. We conclude that the marmoset is a useful new primate model for examining the effects of oestrogens on cognitive function.

Comprehensive analysis of GnRH2 neuronal projections in zebrafish

The presence and conservation of GnRH2 across vertebrate species suggest important biological roles. However, the function of GnRH2 remains unclear. A good research model for GnRH2 functional studies is still lacking largely due to the absence of GnRH2 in the widely used mouse model. Hence, we used the zebrafish, for which powerful genetic tools are available, and developed a transgenic (Tg) line expressing enhanced green fluorescence protein (eGFP). The high sensitivity of eGFP, which can diffuse throughout the neuron, enables us to document the complete projectome of GnRH2 neurons at different developmental stages. Fine projection structures were observed without sacrificing the fish. Crossed with the GnRH3:tdTomato Tg line, the GnRH2:eGFP Tg line provides us with an opportunity to visualize the entire GnRH system simultaneously in one organism. This work will provide a framework to understand the function of the highly-conserved GnRH2 system.

Central and direct regulation of testicular activity by gonadotropin-inhibitory hormone and its receptor

Gonadotropin-inhibitory hormone (GnIH) was first identified in Japanese quail to be an inhibitor of gonadotropin synthesis and release. GnIH peptides have since been identified in all vertebrates, and all share an LPXRFamide (X = L or Q) motif at their C-termini. The receptor for GnIH is the G protein-coupled receptor 147 (GPR147), which inhibits cAMP signaling. Cell bodies of GnIH neurons are located in the paraventricular nucleus (PVN) in birds and the dorsomedial hypothalamic area (DMH) in most mammals. GnIH neurons in the PVN or DMH project to the median eminence to control anterior pituitary function via GPR147 expressed in gonadotropes. Further, GnIH inhibits gonadotropin-releasing hormone (GnRH)-induced gonadotropin subunit gene transcription by inhibiting the adenylate cyclase/cAMP/PKA-dependent ERK pathway in an immortalized mouse gonadotrope cell line (LβT2 cells). GnIH neurons also project to GnRH neurons that express GPR147 in the preoptic area (POA) in birds and mammals. Accordingly, GnIH can inhibit gonadotropin synthesis and release by decreasing the activity of GnRH neurons as well as by directly inhibiting pituitary gonadotrope activity. GnIH and GPR147 can thus centrally suppress testosterone secretion and spermatogenesis by acting in the hypothalamic-pituitary-gonadal axis. GnIH and GPR147 are also expressed in the testis of birds and mammals, possibly acting in an autocrine/paracrine manner to suppress testosterone secretion and spermatogenesis. GnIH expression is also regulated by melatonin, stress, and social environment in birds and mammals. Accordingly, the GnIH-GPR147 system may play a role in transducing physical and social environmental information to regulate optimal testicular activity in birds and mammals. This review discusses central and direct inhibitory effects of GnIH and GPR147 on testosterone secretion and spermatogenesis in birds and mammals.

Gonadotropin-inhibitory hormone (GnIH), GnIH receptor and cell signaling

Gonadotropin-inhibitory hormone (GnIH) is an inhibitor of gonadotropin synthesis and release, which was originally identified in the hypothalamus of the Japanese quail (Coturnix japonica). The GnIH precursor polypeptide encodes one GnIH and two GnIH related peptides (GnIH-RP-1 and GnIH-RP-2) in birds that share the same C-terminal LPXRFamide (X=L or Q) motif. The receptor for GnIH is thought to be the G protein-coupled receptor 147 (GPR147) which has been shown to couple predominantly through the Gαi protein to inhibit cAMP production. The crude membrane fraction of COS-7 cells transfected with GPR147 cDNA specifically bound GnIH and GnIH-RPs in a concentration-dependent manner. Scatchard plot analysis of the binding showed that GPR147 possessed a single class of high-affinity binding sites. GnIH neurons project to the median eminence to control anterior pituitary function and GPR147 is expressed in the gonadotropes. GnIH neurons also project to gonadotropin-releasing hormone (GnRH)-I and GnRH-II neurons, and GnRH-I and GnRH-II neurons express GPR147. Thus, GnIH may inhibit gonadotropin synthesis and release by decreasing the activity of GnRH-I neurons as well as directly inhibiting the effects of GnRH on gonadotropes. GnIH may also partially inhibit reproductive behaviors by inhibiting GnRH-II neurons. GnIH and GPR147 are also expressed in the gonads, possibly acting in an autocrine/paracrine manner. The cell signaling process of GPR147 was extensively studied using LβT2 cells, a mouse gonadotrope cell line. In this cell line, mouse GnIH inhibits GnRH-induced gonadotropin subunit, LHβ, FSHβ, and common α, gene transcriptions by inhibiting adenylate cyclase/cAMP/PKA dependent ERK pathway. This review summarizes the functions of GnIH, GnIH receptor and its cell signaling processes in birds and discusses related findings in mammals.

Neuropeptides and central control of sexual behaviour from the past to the present: a review

Of the numerous neuropeptides identified in the central nervous system, only a few are involved in the control of sexual behaviour. Among these, the most studied are oxytocin, adrenocorticotropin, α-melanocyte stimulating hormone and opioid peptides. While opioid peptides inhibit sexual performance, the others facilitate sexual behaviour in most of the species studied so far (rats, mice, monkeys and humans). However, evidence for a sexual role of gonadotropin-releasing hormone, corticotropin releasing factor, neuropeptide Y, galanin and galanin-like peptide, cholecystokinin, substance P, vasoactive intestinal peptide, vasopressin, angiotensin II, hypocretins/orexins and VGF-derived peptides are also available. Corticotropin releasing factor, neuropeptide Y, cholecystokinin, vasopressin and angiotensin II inhibit, while substance P, vasoactive intestinal peptide, hypocretins/orexins and some VGF-derived peptide facilitate sexual behaviour. Neuropeptides influence sexual behaviour by acting mainly in the hypothalamic nuclei (i.e., lateral hypothalamus, paraventricular nucleus, ventromedial nucleus, arcuate nucleus), in the medial preoptic area and in the spinal cord. However, it is often unclear whether neuropeptides influence the anticipatory phase (sexual arousal and/or motivation) or the consummatory phase (performance) of sexual behaviour, except in a few cases (e.g., opioid peptides and oxytocin). Unfortunately, scarce information has been added in the last 15 years on the neural mechanisms by which neuropeptides influence sexual behaviour, most studied neuropeptides apart. This may be due to a decreased interest of researchers on neuropeptides and sexual behaviour or on sexual behaviour in general. Such a decrease may be related to the discovery of orally effective, locally acting type V phosphodiesterase inhibitors for the therapy of erectile dysfunction.

Brain region-specific transcriptomic markers of serotonin-1A receptor agonist action mediating sexual rejection and aggression in female marmoset monkeys

INTRODUCTION

In a marmoset model of hypoactive female sexual function, we have shown that repeated administration of the serotonin (5-HT)-1A agonist R-(+)-8-hydroxy-2-(di-N-propylamino)tetralin (8-OH-DPAT) inhibits sexual receptivity in female marmoset monkeys and increases aggression toward the male pairmate.

AIM

The aims of this study are to investigate gene expression changes induced by 8-OH-DPAT in laser-microdissected brain areas that regulate female sexual function and to identify genes, functional gene classes, and pathways associated with 8-OH-DPAT-mediated inhibition of female sexual receptivity.

METHODS

Gene expression was measured in the medial prefrontal cortex (mPFC), medial preoptic area (mPOA), cornu ammonis-1 (CA1) area of the hippocampus (CA1), and dorsal raphé nucleus (DRN) of four 8-OH-DPAT-treated (0.1 mg/kg; daily administration for 16 weeks) and four vehicle-treated female marmosets using a marmoset-specific microarray (European Marmoset Microarray [EUMAMA]) and validated by real-time quantitative polymerase chain reaction (RTqPCR). Enriched functional gene classes were determined. In a parallel candidate gene approach, the expression of serotonergic candidate genes, i.e., the 5-HT1A, 5-HT2A, and 5-HT7 receptors and the 5-HT transporter (5-HTT), was measured by RTqPCR.

MAIN OUTCOME MEASURES

The main outcome is the differential expression of genes between 8-OH-DPAT- and vehicle-treated marmosets.

RESULTS

8-OH-DPAT affected the gene classes important to neural development (mPFC, mPOA, and DRN), neurotransmission (mPOA), energy production (mPFC and mPOA), learning and memory (CA1), and intracellular signal transduction (DRN). Oxytocin (OXT) in the mPOA and 5-HTT in the DRN were strongly increased by 8-OH-DPAT. 5-HT1A tended to increase in the mPFC, while 5-HT7 was decreased in the CA1.

CONCLUSIONS

Brain region-specific alterations of gene expression regulating neural circuitries, energy demands, and learning processes are associated with 8-OH-DPAT-induced decrease in female sexual receptivity and increase in pairmate aggression. The role of OXT in the serotonergic regulation of female sexual behavior and partner interactions warrants attention in future studies.

Hormonal predictors of sexual motivation in natural menstrual cycles

Little is known regarding which hormonal signals may best predict within- and between-women variance in sexual motivation among naturally cycling women. To address this, we collected daily saliva samples across 1-2 menstrual cycles from a sample of young women; assayed samples for estradiol, progesterone, and testosterone; and also collected daily diary reports of women's sexual behavior and subjective sexual desire. With respect to within-cycle, day-to-day fluctuations in subjective desire, we found evidence for positive effects of estradiol and negative effects of progesterone. Desire exhibited a mid-cycle peak, similar to previous findings; measured progesterone concentrations statistically mediated the fall in desire from mid-cycle to the luteal phase, but no combination of hormone measures substantially mediated the follicular phase rise in desire, which suggests that other signals may be implicated in this effect. Hormonal predictors of within-cycle fluctuations in sexual behavior generally reached only trend levels of statistical significance, though the patterns again suggested positive effects of estradiol and negative effects of progesterone. Between-women and within-women, between-cycle effects of hormone concentrations were generally absent, although statistical power was more limited at these higher levels of analysis. There were no significant effects of testosterone concentrations when controlling for the effects of estradiol and progesterone, which raises questions regarding the importance of this hormone for the regulation of sexual motivation in natural cycles. Our study is among the first to identify hormonal predictors of within-cycle fluctuations in sexual motivation, and thus adds novel evidence regarding the endocrine correlates of human sexuality.

Chronic systemic administration of serotonergic ligands flibanserin and 8-OH-DPAT enhance HPA axis responses to restraint in female marmosets

BACKGROUND

Flibanserin, a novel serotonin (5-HT)(1A) agonist and 5-HT(2A) antagonist, has been shown to increase sexual desire and reduce distress in women with Hypoactive Sexual Desire Disorder (HSDD). In marmoset monkeys, flibanserin has demonstrated pro-social effects on male-female pairmates, while the classic 5-HT(1A) agonist 8-OH-DPAT suppresses female sexual behavior and increases aggressive interactions between pairmates. Activation of 5-HT(1A) and 5-HT(2A) receptors is known to stimulate the hypothalamic-pituitary-adrenal (HPA) axis. This study aims to characterize the effects of repeated flibanserin and 8-OH-DPAT administration on the marmoset HPA axis and to elucidate endocrine correlates of altered marmoset pair behavior.

METHODS

Adrenocorticotropic hormone (ACTH) and cortisol were examined at baseline and during 5-HT(1A) agonist and restraint challenges in 8 female marmoset monkeys receiving daily flibanserin (15mg/kg) and an additional 8 female marmosets receiving 8-OH-DPAT (0.1mg/kg) for 15-16weeks. Corresponding vehicle treatments were administered in a counterbalanced, within-subject design. All females were housed in stable male-female pairs. Treatment-induced changes in ACTH and cortisol levels were correlated with previously assessed marmoset pair behavior.

RESULTS

While morning basal cortisol levels and HPA responses to a 5-HT(1A) agonist challenge were not altered by chronic flibanserin or 8-OH-DPAT, both treatments increased the responsiveness of the marmoset HPA axis to restraint. Enhanced ACTH responses to restraint correlated with reduced sexual receptivity and increased aggression in 8-OH-DPAT-, but not in flibanserin-treated female marmosets.

CONCLUSIONS

Unaltered HPA responses to a 5-HT(1A) agonist challenge after chronic flibanserin and 8-OH-DPAT treatments indicate little or no de-sensitization of the HPA axis to repeated 5-HT(1A) manipulation. Chronic 8-OH-DPAT, but not flibanserin, leads to aggravated ACTH responses to stress that may contribute to anti-sexual and anti-social behavior between 8-OH-DPAT-treated females and their male pairmates. Despite similar flibanserin and 8-OH-DPAT induced ACTH responses to restraint stress, flibanserin-treated females show unchanged cortisol profiles. This is possibly due to flibanserin's regional selectivity in 5-HT(1A) activation and concurrent 5-HT(2A) inhibition. The contrasting restraint-related cortisol responses emulate contrasting behavioral phenotypes of diminished pair-bond of 8-OH-DPAT-treated females compared to the more affiliative pair-bond of flibanserin-treated females.

The marmoset monkey: a multi-purpose preclinical and translational model of human biology and disease

The development of biologic molecules (monoclonal antibodies, cytokines, soluble receptors) as specific therapeutics for human disease creates a need for animal models in which safety and efficacy can be tested. Models in lower animal species are precluded when the reagents fail to recognize their targets, which is often the case in rats and mice. In this Feature article we will highlight the common marmoset, a small-bodied nonhuman primate (NHP), as a useful model in biomedical and preclinical translational research.

Flibanserin and 8-OH-DPAT implicate serotonin in association between female marmoset monkey sexual behavior and changes in pair-bond quality

INTRODUCTION

Psychopathological origins of personally distressing, hypoactive sexual desire disorder (HSDD) in women are unknown, but are generally attributed to an inhibitory neural regulator, serotonin (5-HT). Flibanserin, a 5-HT(1A) agonist and 5-HT(2A) antagonist, shows promise as a treatment for HSDD.

AIM

To test the hypothesis that female marmoset sexual behavior is enhanced by flibanserin and diminished by 8-OH-DPAT, in order to evaluate the efficacy of serotonergic modulation of female sexual behavior in a pairmate social setting comparable to humans.

METHODS

Sexual and social behavior were examined in eight female marmoset monkeys receiving daily flibanserin (15 mg/kg), 8-OH-DPAT (0.1 mg/kg), or corresponding vehicle for 15-16 weeks in a counterbalanced, within-subject design, while housed in long-term, stable male-female pairs.

MAIN OUTCOME MEASURES

Marmoset pairmate interactions, including sexual and social behavior, were scored during weeks 5-6 of daily flibanserin, 8-OH-DPAT or vehicle treatment. 24-hour pharmacokinetic profiles of the drugs and their metabolites, as well as drug-induced acute symptoms of the 5-HT behavioral syndrome were also assessed.

RESULTS

Two-way analysis of variance reveals that flibanserin-treated females attract more male sexual interest (P=0.020) and trigger increased grooming (P=0.001) between partners. In contrast, 8-OH-DPAT-treated females show increased rejection of male sexual advances (P=0.024), a tendency for decreased male sexual interest (P=0.080), and increased aggression with their male pairmates (P=0.049).

CONCLUSIONS

While 8-OH-DPAT-treated female marmosets display decreased sexual receptivity and increased aggressive interactions with their male pairmates, flibanserin-treated female marmosets demonstrate increased affiliative behavior with their male pairmates. Such pro-affiliation attributes may underlie flibanserin's effectiveness in treating HSDD in women.

Positron emission tomography assessment of 8-OH-DPAT-mediated changes in an index of cerebral glucose metabolism in female marmosets

As part of a larger experiment investigating serotonergic regulation of female marmoset sexual behavior, this study was designed to (1) advance methods for PET imaging of common marmoset monkey brain, (2) measure normalized FDG uptake as an index of local cerebral metabolic rates for glucose, and (3) study changes induced in this index of cerebral glucose metabolism by chronic treatment of female marmosets with a serotonin 1A receptor (5-HT(1A)) agonist. We hypothesized that chronic treatment with the 5-HT(1A) agonist 8-OH-DPAT would alter the glucose metabolism index in dorsal raphe (DR), medial prefrontal cortex (mPFC), medial preoptic area of hypothalamus (mPOA), ventromedial nucleus of hypothalamus (VMH), and field CA1 of hippocampus. Eight adult ovariectomized female common marmosets (Callithrix jacchus) were studied with and without estradiol replacement. In a crossover design, each subject was treated daily with 8-OH-DPAT (0.1mg/kg SC daily) or saline. After 42-49 days of treatment, the glucose metabolism radiotracer FDG was administered to each female immediately prior to 30 min of interaction with her male pairmate, after which the subject was anesthetized and imaged by PET. Whole brain normalized PET images were analyzed with anatomically defined regions of interest (ROI). Whole brain voxelwise mapping was also used to explore treatment effects and correlations between alterations in the glucose metabolism index and pairmate interactions. The rank order of normalized FDG uptake was VMH/mPOA>DR>mPFC/CA1 in both conditions. 8-OH-DPAT did not induce alterations in the glucose metabolism index in ROIs. Voxelwise mapping showed a significant reduction in normalized FDG uptake in response to 8-OH-DPAT in a cluster in medial occipital cortex as well as a significant correlation between increased rejection of mount attempts and reduced normalized FDG uptake in an overlapping cluster. In conclusion, PET imaging has been used to measure FDG uptake relative to whole brain in marmoset monkeys. Voxelwise mapping shows that 8-OH-DPAT reduces this index of glucose metabolism in medial occipital cortex, consistent with alterations in female sexual behavior.

Differential roles of GnRH-I and GnRH-ii neurons in the control of the primate reproductive axis

In vertebrates, gonadotropin-releasing hormone (GnRH) represents the primary neuroendocrine link between the brain and the reproductive axis, and in some species up to three different forms of GnRH have been detected. Until recently, it had been assumed that humans and non-human primates only express one form (GnRH-I), but it is now clear they also express a second form (GnRH-II). GnRH-II, like GnRH-I, is highly effective at stimulating gonadotropin release, both in vitro and in vivo, but the neurons that produce GnRH-II are completely distinct from those producing GnRH-I. Moreover, GnRH-II and GnRH-I producing neurons respond very differently to estradiol; specifically, estradiol stimulates GnRH-II gene expression in the former and inhibit GnRH-I gene expression in the latter. Consequently, the negative feedback action of estradiol may be mediated exclusively by the subpopulation of GnRH neurons that express GnRH-I, while the positive feedback action may be mediated exclusively by the subpopulation that expresses GnRH-II. Taken together, these findings raise the possibility that two completely different GnRH neuronal systems participate in the control of primate reproductive physiology. The primary role of GnRH-I neurons is likely to be focused on the maintenance and modulation of tonic pulsatile LH release, whereas the primary role of GnRH-II neurons is likely to be focused on the generation of the preovulatory LH surge. This functional segregation of the primate neuroendocrine reproductive axis lends itself for novel targeted approaches to fertility control and for treatment of human reproductive disorders.

Gonadotropin-inhibitory hormone action in the brain and pituitary

Gonadotropin-inhibitory hormone (GnIH) was first identified in the Japanese quail as a hypothalamic neuropeptide inhibitor of gonadotropin secretion. Subsequent studies have shown that GnIH is present in the brains of birds including songbirds, and mammals including humans. The identified avian and mammalian GnIH peptides universally possess an LPXRFamide (X = L or Q) motif at their C-termini. Mammalian GnIH peptides are also designated as RFamide-related peptides from their structures. The receptor for GnIH is the G protein-coupled receptor 147 (GPR147), which is thought to be coupled to G(αi) protein. Cell bodies of GnIH neurons are located in the paraventricular nucleus (PVN) in birds and the dorsomedial hypothalamic area (DMH) in mammals. GnIH neurons in the PVN or DMH project to the median eminence to control anterior pituitary function. GPR147 is expressed in the gonadotropes and GnIH suppresses synthesis and release of gonadotropins. It was further shown in immortalized mouse gonadotrope cell line (LβT2 cells) that GnIH inhibits gonadotropin-releasing hormone (GnRH) induced gonadotropin subunit gene transcriptions by inhibiting adenylate cyclase/cAMP/PKA-dependent ERK pathway. GnIH neurons also project to GnRH neurons in the preoptic area, and GnRH neurons express GPR147 in birds and mammals. Accordingly, GnIH may inhibit gonadotropin synthesis and release by decreasing the activity of GnRH neurons as well as directly acting on the gonadotropes. GnIH also inhibits reproductive behavior possibly by acting within the brain. GnIH expression is regulated by a nocturnal hormone melatonin and stress in birds and mammals. Accordingly, GnIH may play a role in translating environmental information to inhibit reproductive physiology and behavior of birds and mammals. Finally, GnIH has therapeutic potential in the treatment of reproductive cycle and hormone-dependent diseases, such as precocious puberty, endometriosis, uterine fibroids, and prostatic and breast cancers.

Inhibition of maternal behaviour by central infusion of corticotrophin-releasing hormone in marmoset monkeys

Stress can inhibit maternal behaviour and increase rates of child abuse in humans and other animals; however, the neuroendocrine mechanisms are not known. To determine whether corticotrophin-releasing hormone (CRH) plays a role in stress-induced disruption of maternal behaviour in primates, we characterised the effects of acute i.c.v. infusions of CRH on maternal and abusive behaviour in common marmoset monkeys (Callithrix jacchus). Nulliparous females were implanted with indwelling i.c.v. guide cannulae before conception. Between 18 and 58 days after the birth of her first infants, each female underwent a series of i.c.v. infusions of human CRH (0, 2, 8 and 25 μg) in 8 μl of artificial cerebrospinal fluid. In the 70 min after infusion, marmosets were tested with one of their infants, first in their home cage and, subsequently, in an unfamiliar cage in which the infant was confined in a transparent box on the cage floor. In the home cage, the highest dose of CRH significantly reduced the amount of time that mothers spent carrying their infants, as compared to vehicle alone, although it did not reliably affect aggression toward the infant or other behaviours. In the confined-infant test, the highest dose of CRH significantly reduced the amount of time that mothers spent on the cage floor, increased mothers' vocalisation rates, and tended to reduce their activity levels and time spent in proximity to their infant. Twenty-five micrograms of CRH also elicited significant elevations in plasma adrenocorticotrophic hormone and cortisol concentrations compared to vehicle. These results indicate that i.c.v.-administered CRH reduces maternal behaviour in marmoset mothers, in both familiar and unfamiliar environments, but does not increase infant abuse.

Recent studies of gonadotropin-inhibitory hormone (GnIH) in the mammalian hypothalamus, pituitary and gonads

The hypothalamo-pituitary-gonadal (HPG) axis integrates internal and external cues via a balance of stimulatory and inhibitory neurochemical systems to time reproductive activity. The cumulative output of these positive and negative modulators drives secretion of gonadotropin-releasing hormone (GnRH), a neuropeptide that causes pituitary gonadotropin synthesis and secretion. Ten years ago, Tsutsui and colleagues discovered a peptide in quail hypothalamus that is capable of inhibiting gonadotropin secretion in cultured quail pituitary cells. Later studies by a variety of researchers examined the presence and functional role for the mammalian ortholog of GnIH. To date, GnIH exhibits a similar distribution and functional role in all mammals investigated, including humans. This overview summarizes the role of GnIH in modulation of mammalian reproductive physiology and suggests avenues for further study by those interested in the neuroendocrine control of reproductive physiology and sexual behavior.

The zebrafish as a model system for forebrain GnRH neuronal development

Development and function of the forebrain gonadotropin-releasing hormone (GnRH) neuronal system has long been the focus of study in various vertebrate species. This system is crucial for reproduction and an important model for studying tangential neuronal migration. In addition, the finding that multiple forms of GnRH exist in the CNS as well as in non-CNS tissues, coupled with the fact that GnRH fibers project to many CNS regions, implies that GnRH has a variety of functions in addition to its classic reproductive role. The study of the GnRH system and its functions is, however, limited by available model systems and methodologies. The transgenic (Tg) GnRH3:EGFP zebrafish line, in which GnRH3 neurons express EGFP, allows in vivo study of the GnRH3 system in the context of the entire animal. Coupling the use of this line with the attributes and molecular tools available in zebrafish has expanded our ability to study the forebrain GnRH system. Herein, we discuss the use of the Tg(GnRH3:EGFP) zebrafish line as a model for studying forebrain GnRH neurons, both in developing larvae and in sexually mature animals. We also discuss the potential use of this line to study regulation of GnRH3 system development.

Signaling by G-protein-coupled receptor (GPCR): studies on the GnRH receptor

Gonadotropin-releasing hormone (GnRH) is the first key hormone of reproduction. GnRH analogs are extensively used in in vitro fertilization, and treatment of sex hormone-dependent cancers, due to their ability to bring about 'chemical castration'. The interaction of GnRH with its cognate type I receptor (GnRHR) in pituitary gonadotropes results in the activation of Gq/G(11), phospholipase Cbeta (PLCbetaI), PLA(2), and PLD. Sequential activation of the phospholipases generates the second messengers inositol 1, 4, 5-trisphosphate (IP(3)), diacylglycerol (DAG), and arachidonic acid (AA), which are required for Ca(2+) mobilization, the activation of various protein kinase C isoforms (PKCs), and the production of prostaglandin (PG) and other metabolites of AA, respectively. PKC isoforms are the major mediators of the downstream activation of a number of mitogen-activated protein kinase (MAPK) cascades by GnRH, namely: extracellular signal-regulated kinase (ERK), jun-N-terminal kinase (JNK), and p38MAPK. The activated MAPKs phosphorylate both cytosolic and nuclear proteins to initiate the transcriptional activation of the gonadotropin subunit genes and the GnRHR. While Ca(2+) mobilization has been found to initiate rapid gonadotropin secretion, Ca(2+), together with various PKC isoforms, MAPKs and AA metabolites also serve as key nodes, in the GnRH-stimulated signaling network that enables the gonadotropes to decode GnRH pulse frequencies and translating that into differential gonadotropin synthesis and release. Even though pulsatility of GnRH is recognized as a major determinant for differential gonadotropin subunit gene expression and gonadotropin secretion very little is yet known about the signaling circuits governing GnRH action at the 'Systems Biology' level. Direct apoptotic and metastatic effects of GnRH analogs in gonadal steroid-dependent cancers expressing the GnRHR also seem to be mediated by the activation of the PKC/MAPK pathways. However, the mechanisms dictating life (pituitary) vs. death (cancer) decisions made by the same GnRHR remain elusive. Understanding these molecular mechanisms triggered by the GnRHR through biochemical and 'Systems Biology' approaches would provide the basis for the construction of the dynamic connectivity maps, which operate in the various cell types (endocrine, cancer, and immune system) targeted by GnRH. The connectivity maps will open a new vista for exploring the direct effects of GnRH analogs in tumors and the design of novel combined therapies for fertility control, reproductive disorders and cancers.

Interactions between gonadotropin-releasing hormone (GnRH) and orexin in the regulation of feeding and reproduction in goldfish (Carassius auratus)

Links between energy homeostasis and reproduction have been demonstrated in vertebrates. As a general rule, abundant food resources favor reproduction whereas low food availability induces an inhibition of reproductive processes. In both mammals and fish, gonadotropin-releasing hormone (GnRH) and orexin (OX) are hypothalamic neuropeptides that play critical roles in the regulation of sexual behavior and appetite, respectively. In order to assess possible interactions between orexin and GnRH in the control of feeding and reproduction in goldfish, we examined the effects of chicken GnRH (cGnRH-II) intracerebroventricular (ICV) injection on feeding behavior and OX brain mRNA expression as well as the effects of orexin ICV injections on spawning behavior and cGnRH-II brain mRNA expression. Treatment with cGnRH-II at doses that stimulate spawning (0.5 ng/g or 1 ng/g) resulted in a decrease in both food intake and hypothalamic orexin mRNA expression. Treatment with orexin A at doses that stimulate feeding (10 ng/g) induced an inhibition of spawning behavior and a decrease in cGnRH-II expression in the hypothalamus and optic tectum-thalamus. Our results suggest that the anorexigenic actions of cGnRH-II in goldfish might be in part mediated by OX and that orexin inhibits reproductive behavior in part via the inhibition of the GnRH system. Our data suggest the existence of a coordinated control of feeding and reproduction by the orexin and GnRH systems in goldfish.

Structural and functional evolution of gonadotropin-releasing hormone in vertebrates

The neuropeptide gonadotropin-releasing hormone (GnRH) has a central role in the neural control of vertebrate reproduction. This review describes an overview of what is currently known about GnRH in vertebrates in the context of its structural and functional evolution. A large body of evidence has demonstrated the existence of three paralogous genes for GnRH (GnRH1, GnRH2 and GnRH3) in the vertebrate lineage. They are most probably the products of whole-genome duplications that occurred early in vertebrate evolution. Although GnRH3 has been identified only in teleosts, comparative genomic analyses indicated that GnRH3 has not arisen from a teleost-specific genome duplication, but has been derived from an earlier genome duplication in an ancestral vertebrate, followed by its loss in the tetrapod lineage. A loss of other paralogous genes has also occurred independently in different vertebrate lineages, leading to species-specific differences in the organization of the GnRH system. In addition to the GnRH3 gene, the GnRH2 gene has been deleted or silenced in certain mammalian species, while some teleosts seem to have lost the GnRH1 or GnRH3 gene. The duplicated GnRH genes have undergone subfunctionalization during the evolution of vertebrates; GnRH1 has become the major stimulator of gonadotropins and probably other pituitary hormones as well, whereas GnRH2 and GnRH3 would have functioned as neuromodulators, affecting reproductive behaviour. Conversely, in cases where a paralogous gene for GnRH has been lost, one of the remaining paralogues appears to have adopted its role.

The control of reproductive physiology and behavior by gonadotropin-inhibitory hormone

Gonadotropin-releasing hormone (GnRH) controls the reproductive physiology and behavior of vertebrates by stimulating synthesis and release of gonadotropin from the pituitary gland. In 2000, another hypothalamic neuropeptide, gonadotropin-inhibitory hormone (GnIH), was discovered in quail and found to be an inhibiting factor for gonadotropin release. GnIH homologs are present in the brains of vertebrates, including birds, mammals, amphibians, and fish. These peptides, categorized as RF amide-related peptides (RFRPs), possess a characteristic LPXRF-amide (X = L or Q) motif at their C-termini. GnIH/RFRP precursor mRNA encodes a polypeptide that is possibly cleaved into three mature peptides in birds and two in mammals. The names of these peptides are GnIH, GnIH-related peptide-1 (GnIH-RP-1) and GnIH-RP-2 in birds, and RFRP-1 and RFRP-3 in mammals. GnIH/RFRP is synthesized in neurons of the paraventricular nucleus of the hypothalamus in birds and the dorsomedial hypothalamic area in mammals. GnIH neurons project to the median eminence, thus providing a functional neuroanatomical infrastructure to regulate anterior pituitary function. In quail, GnIH inhibits gonadal activity by decreasing synthesis and release of gonadotropin. The widespread distribution of GnIH/RFRP immunoreactive fibers in all animals tested suggests various actions within the brain. In accordance, GnIH/RFRP receptor mRNA is also expressed widely in the brain and the pituitary. GnIH/RFRP immunoreactive axon terminals are in probable contact with GnRH neurons in birds and mammals, and we recently demonstrated expression of GnIH receptor mRNA in GnRH-I and GnRH-II neurons in European starlings. Thus, GnIH/RFRP may also inhibit gonadotropin synthesis and release by inhibiting GnRH neurons in addition to having direct actions on the pituitary gland. Intracerebroventricular administration of GnIH/RFRP further inhibits reproductive behaviors in songbirds and rodents, possibly via direct actions on the GnRH system. The expression of GnIH/RFRP is regulated by melatonin which is an internal indicator of day length in vertebrates. Stress stimuli also regulate the expression of GnIH/RFRP in songbirds and rodents. Accordingly, GnIH/RFRP may serve as a transducer of environmental information and social interactions into endogenous physiology and behavior of the animal. Recently, it was shown that GnIH/RFRP and its receptor are also expressed in the gonads of birds, rodents and primates. In sum, the existing data suggest that GnIH/RFRP is an important mediator of reproductive function acting at the level of the brain, pituitary, and the gonad in birds and mammals.

Gonadotropin-releasing hormone II: a multi-purpose neuropeptide

Close to 30 forms of gonadotropin releasing hormone (GnRH) and at least five GnRH receptors have been identified in a wide variety of vertebrates and some invertebrates. One form, now called GnRH II, has the broadest distribution and the most ancient and conserved phylogeny. The distribution of the neurons that produce this peptide are completely nonoverlapping with any other GnRH forms. Fibers that project from these neurons overlap with GnRH I cells and/or fibers in a few regions, but are primarily divergent. The musk shrew (Suncus murinus) continues to be the most tractable mammalian species to use for studies of the function of GnRH II. The brain of the musk shrew has two GnRH genes (I and II), two GnRH receptors (types-1 and -2), and two different behaviors can be influenced by central infusion of GnRH II, but not by GnRH I; receptivity and feeding. Here, we summarize research on the musk shrew relative to the behavioral functions of GnRH II. First, female musk shrews are continually sexually receptive by virtue of their lack of an ovarian and/or behavioral estrus cycle. This feature of their reproductive ecology may be related to their semi-tropical distribution and their breeding season is highly dependent on changes in the availability of food. When food is not abundant, females stop mating, but brief bouts of feeding reinstate reproductive behavior. Likewise, intake of food is related to GnRH II mRNA and peptide content in the brain; after mild food restriction both decline. When GnRH II is infused centrally, at times when its content is low, it can both enhance receptivity and inhibit food intake. Simultaneous administration of a type-1 antagonist does not change the effect of GnRH II and use of an analog (135-18) that is a specific GnRH II agonist as well as a type-1 antagonist has the same effect as the endogenous GnRH II peptide. We propose that GnRH II plays a critical role by orchestrating the coordination of reproduction with the availability of nutritional support for these activities. Humans are bombarded with copious nutritional opportunities and at present obesity is a larger threat to health in many parts of the world than is under nutrition. It is our hope that understanding neuropeptides such as GnRH II that regulate food intake can ultimately lead to products that may curb appetite and thus decrease obesity and related risks to health.

Early development of forebrain gonadotrophin-releasing hormone (GnRH) neurones and the role of GnRH as an autocrine migration factor

Normal migration of the gonadotrophin-releasing hormone (GnRH) neurones during early development, from the olfactory region to the hypothalamus, is crucial for reproductive development in all vertebrates. The establishment of the GnRH system includes tangential migration of GnRH perikarya as well as extension of GnRH fibres to various areas of the central nervous system (CNS). The exact spatio-temporal nature of this process, as well as the factors governing it, are not fully understood. We studied the development of the GnRH system and the effects of GnRH knockdown using a newly developed GnRH3:EGFP transgenic zebrafish line. We found that enhanced green fluorescent protein is specifically and robustly expressed in GnRH3 neurones and fibres. GnRH3 fibres in zebrafish began to extend as early as 26 h post-fertilisation and by 4-5 days post-fertilisation had developed into an extensive network reaching the optic tract, telencephalon, hypothalamus, midbrain tegmentum and hindbrain. GnRH3 fibres also innervated the retina and projected into the trunk via the spinal cord. GnRH3 perikarya were observed migrating along their own fibres from the olfactory region to the preoptic area (POA) via the terminal nerve ganglion and the ventral telencephalon. GnRH3 cells were also observed in the trigeminal ganglion. The establishment of the GnRH3 fibre network was disrupted by morpholino-modified antisense oligonucleotides directed against GnRH3 causing abnormal fibre development and pathfinding, as well as anomalous GnRH3 perikarya localisation. These findings support the hypothesis that GnRH3 neurones migrate from the olfactory region to the POA and caudal hypothalamus. Novel data regarding the early development of the GnRH3 fibre network in the CNS and beyond are described. Moreover we show, in vivo, that GnRH3 is an important factor regulating GnRH3 fibre pathfinding and neurone localisation in an autocrine fashion.

Gonadotropin-inhibitory hormone neurons interact directly with gonadotropin-releasing hormone-I and -II neurons in European starling brain

Gonadotropin-inhibitory hormone (GnIH) is a hypothalamic dodecapeptide (SIKPSAYLPLRF-NH(2)) that directly inhibits gonadotropin synthesis and release from quail pituitary. The action of GnIH is mediated by a novel G-protein coupled receptor. This gonadotropin-inhibitory system may be widespread in vertebrates, at least birds and mammals. In these higher vertebrates, histological evidence suggests contact of GnIH immunoreactive axon terminals with GnRH neurons, thus indicating direct regulation of GnRH neuronal activity by GnIH. In this study we investigated the interaction of GnIH and GnRH-I and -II neurons in European starling (Sturnus vulgaris) brain. Cloned starling GnIH precursor cDNA encoded three peptides that possess characteristic LPXRF-amide (X = L or Q) motifs at the C termini. Starling GnIH was further identified as SIKPFANLPLRF-NH(2) by mass spectrometry combined with immunoaffinity purification. GnIH neurons, identified by in situ hybridization and immunocytochemistry (ICC), were clustered in the hypothalamic paraventricular nucleus. GnIH immunoreactive fiber terminals were present in the external layer of the median eminence in addition to the preoptic area and midbrain, where GnRH-I and GnRH-II neuronal cell bodies exist, respectively. GnIH axon terminals on GnRH-I and -II neurons were shown by GnIH and GnRH double-label ICC. Furthermore, the expression of starling GnIH receptor mRNA was identified in both GnRH-I and GnRH-II neurons by in situ hybridization combined with GnRH ICC. The cellular localization of GnIH receptor has not previously been identified in any vertebrate brain. Thus, GnIH may regulate reproduction of vertebrates by directly modulating GnRH-I and GnRH-II neuronal activity, in addition to influencing the pituitary gland.