Examination of guinea pig luteinizing hormone-releasing hormone gene reveals a unique decapeptide and existence of two transcripts in the brain

GABAergic and Glutamatergic Phenotypes of Neurons Expressing Calcium-Binding Proteins in the Preoptic Area of the Guinea Pig

The mammalian preoptic area (POA) has large populations of calbindin (CB), calretinin (CR) and parvalbumin (PV) neurons, but phenotypes of these cells are unknown. Therefore, the question is whether neurons expressing CB, CR, and/or PV are GABAergic or glutamatergic. Double-immunofluorescence staining followed by epifluorescence and confocal microscopy was used to determine the coexpression patterns of CB, CR and PV expressing neurons with vesicular GABA transporters (VGAT) as specific markers of GABAergic neurons and vesicular glutamate transporters (VGLUT 2) as specific markers of glutamatergic neurons. The guinea pig was adopted as, like humans, it has a reproductive cycle with a true luteal phase and a long gestation period. The results demonstrated that in the guinea pig POA of both sexes, ~80% of CB+ and ~90% of CR+ neurons coexpress VGAT; however, one-fifth of CB+ neurons and one-third of CR+ cells coexpress VGLUT. About two-thirds of PV+ neurons express VGAT, and similar proportion of them coexpress VGLUT. Thus, many CB+, CR+ and PV+ neurons may be exclusively GABAergic (VGAT-expressing cells) or glutamatergic (VGLUT-expressing cells); however, at least a small fraction of CR+ cells and at least one-third of PV+ cells are likely neurons with a dual GABA/glutamate phenotype that may coexpress both transporters.

Neuropeptidergic and Neuroendocrine Systems Underlying Eusociality and the Concomitant Social Regulation of Reproduction in Naked Mole-Rats: A Comparative Approach

The African mole-rat family (Bathyergidae) includes the first mammalian species identified as eusocial: naked mole-rats. Comparative studies of eusocial and solitary mole-rat species have identified differences in neuropeptidergic systems that may underlie the phenomenon of eusociality. These differences are found in the oxytocin, vasopressin and corticotrophin-releasing factor (CRF) systems within the nucleus accumbens, amygdala, bed nucleus of the stria terminalis and lateral septal nucleus. As a corollary of their eusociality, most naked mole-rats remain pre-pubertal throughout life because of the presence of the colony's only reproductive female, the queen. To elucidate the neuroendocrine mechanisms that mediate this social regulation of reproduction, research on the hypothalamo-pituitary-gonadal axis in naked mole-rats has identified differences between the many individuals that are reproductively suppressed and the few that are reproductively mature: the queen and her male consorts. These differences involve gonadal steroids, gonadotrophin-releasing hormone-1 (GnRH-1), kisspeptin, gonadotrophin-inhibitory hormone/RFamide-related peptide-3 (GnIH/RFRP-3) and prolactin. The comparative findings in eusocial and solitary mole-rat species are assessed with reference to a broad range of studies on other mammals.

Origin and Evolution of the Neuroendocrine Control of Reproduction in Vertebrates, With Special Focus on Genome and Gene Duplications

In humans, as in the other mammals, the neuroendocrine control of reproduction is ensured by the brain-pituitary gonadotropic axis. Multiple internal and environmental cues are integrated via brain neuronal networks, ultimately leading to the modulation of the activity of gonadotropin-releasing hormone (GnRH) neurons. The decapeptide GnRH is released into the hypothalamic-hypophysial portal blood system and stimulates the production of pituitary glycoprotein hormones, the two gonadotropins luteinizing hormone and follicle-stimulating hormone. A novel actor, the neuropeptide kisspeptin, acting upstream of GnRH, has attracted increasing attention in recent years. Other neuropeptides, such as gonadotropin-inhibiting hormone/RF-amide related peptide, and other members of the RF-amide peptide superfamily, as well as various nonpeptidic neuromediators such as dopamine and serotonin also provide a large panel of stimulatory or inhibitory regulators. This paper addresses the origin and evolution of the vertebrate gonadotropic axis. Brain-pituitary neuroendocrine axes are typical of vertebrates, the pituitary gland, mediator and amplifier of brain control on peripheral organs, being a vertebrate innovation. The paper reviews, from molecular and functional perspectives, the evolution across vertebrate radiation of some key actors of the vertebrate neuroendocrine control of reproduction and traces back their origin along the vertebrate lineage and in other metazoa before the emergence of vertebrates. A focus is given on how gene duplications, resulting from either local events or from whole genome duplication events, and followed by paralogous gene loss or conservation, might have shaped the evolutionary scenarios of current families of key actors of the gonadotropic axis.

Gonadotropin-releasing hormone analog therapeutics

Dysregulation at any level of the hypothalamic-pituitary-gonadal (HPG) axis results in, or aggravates, a number of hormone-dependent diseases such as delayed or precocious puberty, infertility, prostatic and ovarian cancer, benign prostatic hyperplasia, polycystic ovarian syndrome, endometriosis, uterine fibroids, lean body mass, as well as metabolism and cognitive impairment. As gonadotropin-releasing-hormone (GnRH) is an essential regulator of the HPG axis, agonist and antagonist analogs are efficacious in the treatment of these conditions. GnRH analogs also play an important role in assisted reproductive therapies. This review highlights the current and future therapeutic potential of GnRH analogs and upstream regulators of GnRH secretion.

Hypothalamic-pituitary-adrenal and hypothalamic-pituitary-gonadal axes: sex differences in regulation of stress responsivity

Gonadal hormones play a key role in the establishment, activation, and regulation of the hypothalamic-pituitary-adrenal (HPA) axis. By influencing the response and sensitivity to releasing factors, neurotransmitters, and hormones, gonadal steroids help orchestrate the gain of the HPA axis to fine-tune the levels of stress hormones in the general circulation. From early life to adulthood, gonadal steroids can differentially affect the HPA axis, resulting in sex differences in the responsivity of this axis. The HPA axis influences many physiological functions making an organism's response to changes in the environment appropriate for its reproductive status. Although the acute HPA response to stressors is a beneficial response, constant activation of this circuitry by chronic or traumatic stressful episodes may lead to a dysregulation of the HPA axis and cause pathology. Compared to males, female mice and rats show a more robust HPA axis response, as a result of circulating estradiol levels which elevate stress hormone levels during non-threatening situations, and during and after stressors. Fluctuating levels of gonadal steroids in females across the estrous cycle are a major factor contributing to sex differences in the robustness of HPA activity in females compared to males. Moreover, gonadal steroids may also contribute to epigenetic and organizational influences on the HPA axis even before puberty. Correspondingly, crosstalk between the hypothalamic-pituitary-gonadal (HPG) and HPA axes could lead to abnormalities of stress responses. In humans, a dysregulated stress response is one of the most common symptoms seen across many neuropsychiatric disorders, and as a result, such interactions may exacerbate peripheral pathologies. In this review, we discuss the HPA and HPG axes and review how gonadal steroids interact with the HPA axis to regulate the stress circuitry during all stages in life.

Sequence analysis, tissue distribution and molecular physiology of the GnRH preprogonadotrophin in the South American plains vizcacha (Lagostomus maximus)

Gonadotropin-releasing hormone (GnRH) is the regulator of the hypothalamic-hypophyseal-gonadal (HHG) axis. GnRH and GAP (GnRH-associated protein) are both encoded by a single preprohormone. Different variants of GnRH have been described. In most mammals, GnRH is secreted in a pulsatile manner that stimulates the release of follicle-stimulating hormone (FSH) and luteinizing hormone (LH). The South-American plains vizcacha, Lagostomus maximus, is a rodent with peculiar reproductive features including natural poly-ovulation up to 800 oocytes per estrous cycle, pre-ovulatory follicle formation throughout pregnancy and an ovulatory process which takes place at mid-gestation and adds a considerable number of secondary corpora lutea. Such features should occur under a special modulation of the HHG axis, guided by GnRH. The aim of this study was to sequence hypothalamic GnRH preprogonadotrophin mRNA in the vizcacha, to compare it with evolutionarily related species and to identify its expression, distribution and pulsatile pattern of secretion. The GnRH1variant was detected and showed the highest homology with that of chinchilla, its closest evolutionarily related species. Two isoforms of transcripts were identified, carrying the same coding sequence, but different 5' untranslated regions. This suggests a sensitive equilibrium between RNA stability and translational efficiency. A predominant hypothalamic localization and a pulsatile secretion pattern of one pulse of GnRH every hour were found. The lower homology found for GAP, also among evolutionarily related species, depicts a potentially different bioactivity.

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.

Humoral immune responses against gonadotropin releasing hormone elicited by immunization with phage-peptide constructs obtained via phage display

Phage display is based on genetic engineering of phage coat proteins resulting in fusion peptides displayed on the surface of phage particles. The technology is widely used for generation of phages with novel characteristics for numerous applications in biomedicine and far beyond. The focus of this study was on development of phage-peptide constructs that stimulate production of antibodies against gonadotropin releasing hormone (GnRH). Phage-peptide constructs that elicit production of neutralizing GnRH antibodies can be used for anti-fertility and anti-cancer applications. Phage-GnRH constructs were generated via selection from a phage display library using several types of GnRH antibodies as selection targets. Such phage constructs were characterized for sequence similarities to GnRH peptide and frequency of their occurrence in the selection rounds. Five of the constructs with suitable characteristics were tested in mice as a single dose 5×10(11) virions (vir) vaccine and were found to be able to stimulate production of GnRH-specific antibodies, but not to suppress testosterone (indirect indicator of GnRH antibody neutralizing properties). Next, one of the constructs was tested at a higher dose of 2×10(12) vir per mouse in combination with a poly(lactide-co-glycolide) (PLGA)-based adjuvant. This resulted in multifold increase in GnRH antibody production and significant reduction of serum testosterone, indicating that antibodies produced in response to the phage-GnRH immunization possess neutralizing properties. To achieve optimal immune responses for desired applications, phage-GnRH constructs can be modified with respect to flanking sequences of GnRH-like peptides displayed on phage. Anticipated therapeutic effects also might be attained using optimized phage doses, a combination of several constructs in a single treatment, or application of adjuvants and advanced phage delivery systems.

Molecular cloning, sequencing, and distribution of feline GnRH receptor (GnRHR) and resequencing of canine GnRHR

GnRH receptors play vital roles in mammalian reproduction via regulation of gonadotropin secretion, which is essential for gametogenesis and production of gonadal steroids. GnRH receptors for more than 20 mammalian species have been sequenced, including human, mouse, and dog. This study reports the molecular cloning and sequencing of GnRH receptor (GnRHR) cDNA from the pituitary gland of the domestic cat, an important species in biomedical research. Feline GnRHR cDNA is composed of 981 nucleotides and encodes a 327 amino acid protein. Unlike the majority of mammalian species sequenced so far, but similar to canine GnRHR, feline GnRHR protein lacks asparagine in position three of the extracellular domain of the protein. At the amino acid level, feline GnRHR exhibits 95.1% identity with canine, 93.8% with human, and 88.9% with mouse GnRHR. Comparative sequence analysis of GnRHRs for multiple mammalian species led to resequencing of canine GnRHR, which differed from that previously published by a single base change that translates to a different amino acid in position 193. This single base change was confirmed in dogs of multiple breeds. Reverse transcriptase PCR analysis of GnRHR messenger RNA in different tissues from four normal cats indicated the presence of amplicons of varying lengths, including full-length as well as shortened GnRHR amplicons, pointing to the existence of truncated GnRHR transcripts in the domestic cat. This study is the first insight into molecular composition and expression of feline GnRHR and promotes better understanding of receptor organization, and distribution in various tissues of this species.

Central pathways integrating metabolism and reproduction in teleosts

Energy balance plays an important role in the control of reproduction. However, the cellular and molecular mechanisms connecting the two systems are not well understood especially in teleosts. The hypothalamus plays a crucial role in the regulation of both energy balance and reproduction, and contains a number of neuropeptides, including gonadotropin-releasing hormone (GnRH), orexin, neuropeptide-Y, ghrelin, pituitary adenylate cyclase-activating polypeptide, α-melanocyte stimulating hormone, melanin-concentrating hormone, cholecystokinin, 26RFamide, nesfatin, kisspeptin, and gonadotropin-inhibitory hormone. These neuropeptides are involved in the control of energy balance and reproduction either directly or indirectly. On the other hand, synthesis and release of these hypothalamic neuropeptides are regulated by metabolic signals from the gut and the adipose tissue. Furthermore, neurons producing these neuropeptides interact with each other, providing neuronal basis of the link between energy balance and reproduction. This review summarizes the advances made in our understanding of the physiological roles of the hypothalamic neuropeptides in energy balance and reproduction in teleosts, and discusses how they interact with GnRH, kisspeptin, and pituitary gonadotropins to control reproduction in teleosts.

Current and future applications of GnRH, kisspeptin and neurokinin B analogues

Reproductive hormones affect all stages of life from gamete production, fertilization, fetal development and parturition, neonatal development and puberty through to adulthood and senescence. The reproductive hormone cascade has, therefore, been the target for the development of numerous drugs that modulate its activity at many levels. As the central regulator of the cascade, gonadotropin-releasing hormone (GnRH) agonists and antagonists have found extensive applications in treating a wide range of hormone-dependent diseases, such as precocious puberty, prostate cancer, benign prostatic hyperplasia, endometriosis and uterine fibroids, as well as being an essential component of in vitro fertilization protocols. The neuroendocrine peptides that regulate GnRH neurons, kisspeptin and neurokinin B, have also been identified as therapeutic targets, and novel agonists and antagonists are being developed as modulators of the cascade upstream of GnRH. Here, we review the development and applications of analogues of the major neuroendocrine peptide regulators of the reproductive hormone cascade: GnRH, kisspeptin and neurokinin B.

Review: regulatory mechanisms of gonadotropin-inhibitory hormone (GnIH) synthesis and release in photoperiodic animals

Gonadotropin-inhibitory hormone (GnIH) is a novel hypothalamic neuropeptide that was discovered in quail as an inhibitory factor for gonadotropin release. GnIH inhibits gonadotropin synthesis and release in birds through actions on gonadotropin-releasing hormone (GnRH) neurons and gonadotropes, mediated via the GnIH receptor (GnIH-R), GPR147. Subsequently, GnIH was identified in mammals and other vertebrates. As in birds, mammalian GnIH inhibits gonadotropin secretion, indicating a conserved role for this neuropeptide in the control of the hypothalamic-pituitary-gonadal (HPG) axis across species. Identification of the regulatory mechanisms governing GnIH expression and release is important in understanding the physiological role of the GnIH system. A nocturnal hormone, melatonin, appears to act directly on GnIH neurons through its receptor to induce expression and release of GnIH in quail, a photoperiodic bird. Recently, a similar, but opposite, action of melatonin on the inhibition of expression of mammalian GnIH was shown in hamsters and sheep, photoperiodic mammals. These results in photoperiodic animals demonstrate that GnIH expression is photoperiodically modulated via a melatonin-dependent process. Recent findings indicate that GnIH may be a mediator of stress-induced reproductive disruption in birds and mammals, pointing to a broad role for this neuropeptide in assessing physiological state and modifying reproductive effort accordingly. This paper summarizes the advances made in our knowledge regarding the regulation of GnIH synthesis and release in photoperiodic birds and mammals. This paper also discusses the neuroendocrine integration of environmental signals, such as photoperiods and stress, and internal signals, such as GnIH, melatonin, and glucocorticoids, to control avian and mammalian reproduction.

Gonadotropin-inhibitory hormone (GnIH): discovery, progress and prospect

A hypothalamic neuropeptide, gonadotropin-releasing hormone (GnRH), is the primary factor regulating gonadotropin secretion. An inhibitory hypothalamic neuropeptide for gonadotropin secretion was, until recently, unknown, although gonadal sex steroids and inhibin can modulate gonadotropin secretion. Findings from the last decade, however, indicate that GnRH is not the sole hypothalamic regulatory neuropeptide of vertebrate reproduction, with gonadotropin-inhibitory hormone (GnIH) playing a key role in the inhibition of reproduction. GnIH was originally identified in birds and subsequently in mammals and other vertebrates. GnIH acts on the pituitary and on GnRH neurons in the hypothalamus via a novel G protein-coupled receptor (GPR147). GnIH decreases gonadotropin synthesis and release, inhibiting gonadal development and maintenance. Such a down-regulation of the hypothalamo-pituitary-gonadal (HPG) axis may be conserved across vertebrates. Recent evidence further indicates that GnIH operates at the level of the gonads as an autocrine/paracrine regulator of steroidogenesis and gametogenesis. More recent evidence suggests that GnIH also acts both upstream of the GnRH system and at the level of the gonads to appropriately regulate reproductive activity across the seasons and during times of stress. The discovery of GnIH has fundamentally changed our understanding of hypothalamic control of reproduction. This review summarizes the discovery, progress and prospect of GnIH, a key regulator of vertebrate reproduction.

The contribution of lower vertebrate animal models in human reproduction research

Many advances have been carried out on the estrogens, GnRH and endocannabinoid system that have impact in the reproductive field. Indeed, estrogens, the generally accepted female hormones, have performed an unsuspected role in male sexual functions thanks to studies on non-mammalian vertebrates. Similarly, these animal models have provided important contributions to the identification of several GnRH ligand and receptor variants and their possible involvement in sexual behavior and gonadal function regulation. Moreover, the use of non-mammalian animal models has contributed to a better comprehension about the endocannabinoid system action in several mammalian reproductive events. We wish to highlight here how non-mammalian vertebrate animal model research contributes to advancements with implications on human health as well as providing a phylogenetic perspective on the evolution of reproductive systems in vertebrates.

Characterization and putative role of a type I gonadotropin-releasing hormone in the cephalochordate amphioxus

GnRH, originally isolated from mammalian hypothalamus, is a key player in the control of vertebrate reproduction. Employing reverse-phase chromatography, we purified a peptide of relative molecular mass of 1182.60 Da from the cephalochordate amphioxus Branchiostoma lanceolatum. We found that its amino acid sequence (pGlu-His-Trp-Ser-Tyr-Gly-Leu-Arg-Pro-Gly-NH(2)) was identical to that of mammalian GnRH. The highest concentrations (4.04 +/- 0.3 microg/g tissue), localized in the anterior part of the body, occurred in November, a time when amphioxus gonads prepare for the seasonal spawning. Furthermore, the biological activity of amphioxus GnRH was investigated by examining its capability to elicit LH release from the rodent pituitary gland. The origins of GnRH can be traced back to the origins of chordates. The seasonal variations of amphioxus GnRH also suggest an ancient role of this peptide in the control of reproduction in chordates, even before the evolution of a proper pituitary gland.

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.

Role of gonadotropin-releasing hormone II in the mammalian nervous system

Gonadotropin-releasing hormone (GnRH) is a small neuropeptide of which there are multiple structural variants. The first variant identified in mammals, GnRH I, controls the release of pituitary gonadotropins. More recently, a second isoform, GnRH II, first isolated in the bird, was identified in the mammalian brain and periphery. Although it is unlikely to be a primary regulator of gonadotropin release, GnRH II appears to have a wide array of physiological and behavioral functions. GnRH II-containing fibers are present in several nuclei known to regulate reproduction and/or feeding, and its concentration in several of these areas fluctuates in response to changes in food availability, and thus energetic status. In musk shrews, GnRH II acts as a permissive regulator of female reproductive behavior based on energy status, as well as an inhibitor of short-term food intake. In this regard, GnRH II is similar to leptin, neuropeptide Y and several other neurotransmitters that regulate both feeding and reproduction. At least two GnRH receptors are present in the mammalian brain, and increasing evidence suggests that the behavioral effects of GnRH II are mediated by receptor subtypes distinct from the type-1 GnRH receptor (which mediates GnRH I action); the most probable candidate is the type-2 GnRH receptor. GnRH II also regulates the density and/or activity of calcium and potassium channels in the nervous systems of amphibians and fish, a function that may also exist in mammalian neurons. It is likely that the highly conserved GnRH II system has been co-opted over evolutionary time to possess multiple regulatory functions in a broad range of neurobiological aspects.

Cloning and characterization of a functional type II gonadotropin-releasing hormone receptor with a lengthy carboxy-terminal tail from an ancestral vertebrate, the sea lamprey

A full-length transcript encoding a functional type II GnRH receptor was cloned from the pituitary of the sea lamprey, Petromyzon marinus. The current study is the first to identify a pituitary GnRH receptor transcript in an agnathan, which is the oldest vertebrate lineage. The cloned receptor retains the conserved structural features and amino acid motifs of other known GnRH receptors and notably includes a C-terminal intracellular tail of approximately 120 amino acids, the longest C-terminal tail of any vertebrate GnRH receptor identified to date. The lamprey GnRH receptor was shown to activate the inositol phosphate (IP) signaling system; stimulation with either lamprey GnRH-I or lamprey GnRH-III led to dose-dependent responses in transiently transfected COS7 cells. Furthermore, analyses of serially truncated lamprey GnRH receptor mutants indicate perturbations of the C-terminal tail disrupts IP accumulation, however, the tailless lamprey GnRH receptor was not only functional but was also capable of stimulating IP levels equal to wild type. Expression of the receptor transcript was demonstrated in the pituitary and testes using RT-PCR, whereas in situ hybridization showed expression and localization of the transcript in the proximal pars distalis of the pituitary. The phylogenetic placement and structural and functional features of this GnRH receptor suggest that it is representative of an ancestral GnRH receptor. In addition to having an important role in lamprey reproductive processes, the extensive C-terminal tail of this lamprey GnRH receptor may have great significance for understanding the evolutionary change of this vital structural feature within the GnRH receptor family.

Nonmammalian gonadotropin-releasing hormone molecules in the brain of promoter transgenic rats

Mammalian gonadotropin-releasing hormone (GnRH1) and nonmammalian immunoreactive GnRH subtypes were examined in transgenic rats carrying an enhanced GFP (EGFP) reporter gene driven by a rat GnRH1 promoter. Double-label immunocytochemistry was performed on EGFP(+)/GnRH1 brain sections by using antisera against GnRH1, GnRH2 (chicken II), GnRH3 (salmon), or seabream GnRH. EGFP(+)/GnRH1 neurons were in the septal-preoptic hypothalamus but not in the midbrain, consistent with GnRH1-immunopositive neurons in WT rats. Apparent coexpression of EGFP(+)/GnRH1 with other GnRH subtypes was observed. All EGFP(+) neurons in the septal-preoptic hypothalamus were GnRH1-immunopositive. However, only approximately 80% of GnRH1-immunopositive neurons were EGFP(+), which awaits further elucidation. GnRH subtypes-immunopositive fibers and EGFP(+)/GnRH1 fibers were conspicuous in the organum vasculosum of the lamina terminalis, median eminence, and surrounding the ependymal walls of the third ventricle and the aqueduct in the midbrain. These results demonstrate that the expression of the EGFP-GnRH1 transgene is restricted to the bona fide GnRH1 population and provide clear morphological evidence supporting the existence of GnRH1 neuronal subpopulations in the septal-preoptic hypothalamus, which might be driven by different segments of the GnRH promoter. This genetic construct permits analyses of promoter usage in GnRH neurons, and our histochemical approaches open questions about functional relations among isoforms of this peptide, which regulates reproductive physiology in its behavioral and endocrine aspects.

Expression and distribution of octopus gonadotropin-releasing hormone in the central nervous system and peripheral organs of the octopus (Octopus vulgaris) by in situ hybridization and immunohistochemistry

We recently purified a peptide with structural features similar to vertebrate gonadotropin-releasing hormone (GnRH) from the brain of Octopus vulgaris, cloned a cDNA encoding the precursor protein, and named it oct-GnRH. In the current study, we investigated the expression and distribution of oct-GnRH throughout the central nervous system (CNS) and peripheral organs of Octopus by in situ hybridization on the basis of the cDNA sequence and by immunohistochemistry using a specific antiserum against oct-GnRH. Oct-GnRH mRNA-expressing cell bodies were located in 10 of 19 lobes in the supraesophageal and subesophageal parts of the CNS. Several oct-GnRH-like immunoreactive fibers were seen in all the neuropils of the CNS lobes. The sites of oct-GnRH mRNA expression and the mature peptide distribution were consistent with each other as judged by in situ hybridization and immunohistochemistry. In addition, many immunoreactive fibers were distributed in peripheral organs such as the heart, the oviduct, and the oviducal gland. Modulatory effects of oct-GnRH on the contractions of the heart and the oviduct were demonstrated. The results suggested that, in the context of reproduction, oct-GnRH is a key peptide in the subpedunculate lobe and/or posterior olfactory lobe-optic gland-gonadal axis, an octopus analogue of the hypothalamo-hypophysial-gonadal axis. It may also act as a modulatory factor in controlling higher brain functions such as feeding, memory, movement, maturation, and autonomic functions

Gonadotropin-releasing hormone receptors

GnRH and its analogs are used extensively for the treatment of hormone-dependent diseases and assisted reproductive techniques. They also have potential as novel contraceptives in men and women. A thorough delineation of the molecular mechanisms involved in ligand binding, receptor activation, and intracellular signal transduction is kernel to understanding disease processes and the development of specific interventions. Twenty-three structural variants of GnRH have been identified in protochordates and vertebrates. In many vertebrates, three GnRHs and three cognate receptors have been identified with distinct distributions and functions. In man, the hypothalamic GnRH regulates gonadotropin secretion through the pituitary GnRH type I receptor via activation of G(q). In-depth studies have identified amino acid residues in both the ligand and receptor involved in binding, receptor activation, and translation into intracellular signal transduction. Although the predominant coupling of the type I GnRH receptor in the gonadotrope is through productive G(q) stimulation, signal transduction can occur via other G proteins and potentially by G protein-independent means. The eventual selection of intracellular signaling may be specifically directed by variations in ligand structure. A second form of GnRH, GnRH II, conserved in all higher vertebrates, including man, is present in extrahypothalamic brain and many reproductive tissues. Its cognate receptor has been cloned from various vertebrate species, including New and Old World primates. The human gene homolog of this receptor, however, has a frame-shift and stop codon, and it appears that GnRH II signaling occurs through the type I GnRH receptor. There has been considerable plasticity in the use of different GnRHs, receptors, and signaling pathways for diverse functions. Delineation of the structural elements in GnRH and the receptor, which facilitate differential signaling, will contribute to the development of novel interventive GnRH analogs.

Three gonadotropin-releasing hormone neuronal groups with special reference to teleosts

Gonadotropin-releasing hormone (GnRH) is a decapeptide, which has been isolated from the hypothalamus as a releasing hormone of gonadotropins from the pituitary. However, subsequent morphological studies have demonstrated the presence of multiple GnRH neuronal groups outside the hypothalamus and preoptic area. In most vertebrate lineages studied to date, GnRH neuronal groups are present along the terminal nerve and in the midbrain tegmentum, in addition to a population in the preoptico-hypothalamic areas. The presence of GnRH fibers in extrahypothalamic areas has also been demonstrated, indicating a significance for GnRH neurons in functions other than those that are purely hypophysiotropic. Among vertebrate lineages, GnRH neurons have been most extensively studied in teleost fish through morphological, electrophysiological, behavioral and molecular approaches. To date, studies on differential roles of GnRH neuronal groups have been mostly restricted to teleosts. In the present review, the anatomy and functions of each GnRH neuronal group are reconsidered, based mainly on knowledge from teleosts. Recent findings in teleosts indicate that the preoptico-hypothalamic GnRH neurons are hypophysiotropic and that GnRH neurons of the terminal nerve and midbrain tegmentum regulate neural activities in various regions, including extrahypothalamic areas. The latter populations presumably serve as neuromodulatory systems to control aspects of neural functions such as reproductive behavior. Similar functional differentiation may be generalized to other vertebrate lineages as well.

Stimulation of luteinizing hormone secretion by N-methyl-D,L-aspartic acid in the adult male guinea-pig: incomplete blockade by gonadotropin-releasing hormone receptor antagonism

Stimulation of luteinizing hormone (LH) secretion by N-methyl-D,L-aspartic acid (NMA), reported for several mammalian species, is generally accepted to be mediated through stimulation of hypothalamic gonadotropin-releasing hormone (GnRH) release. In view of a previously reported unexpected inhibitory action of NMA on GnRH release from hypothalamic explant of intact male guinea-pigs, the aim of the present study was to assess the in vivo effects of NMA in the adult male guinea-pig. In the gonadally intact male, NMA (5 mg/animal) elicited a robust LH secretion, which was blocked by the N-methyl-D-asparte-receptor antagonist DL-2-amino-5-phosphonovaleric acid (AP-5,12 mg/animal). In the castrated male, NMA elicited only a marginal and inconsistent LH secretion. Cetrorelix (CET), a GnRH receptor antagonist, administered intracardiacally 1 min or 45 min preceding bolus injection of NMA significantly reduced the LH response to NMA in the intact male. Surprisingly, following GnRH receptor blockade with CET, there still was a substantial residual serum LH response to NMA, while CET completely abolished the serum LH response to high dose (1 microg or 10 microg) guinea-pig GnRH (gpGnRH). These results indicate that NMA stimulates LH secretion in the gonadally intact male guinea-pig in vivo and that this effect is mediated in part through gpGnRH-independent mechanisms.

Action of gonadotropin-releasing hormone II on the baboon ovary

The content, binding affinity, and bioactivity of chicken II GnRH (GnRH II) and a stable analogue of GnRH II (GnRH II analogue) in the baboon ovary were studied. Although mammalian GnRH is rapidly degraded by baboon ovarian extracts, we designed a GnRH II analogue that is stable to ovarian enzymatic degradation. This analogue binds to the ovarian membranes with high affinity (41 +/- 3 nM), having 20-fold the affinity of a potent mammalian GnRH analogue. The bioactivity of GnRH II and this GnRH II analogue on the regulation of ovarian progesterone release was compared with that for a potent mammalian GnRH analogue using a baboon granulosa cell culture system. Both GnRH II and GnRH II analogue produced significant inhibition of progesterone release from the granulosa cells (P < 0.03 and P < 0.005, respectively), with a greater reduction observed using the GnRH II analogue. After 24 h in culture, this GnRH II analogue produced a 59% +/- 5% inhibition of progesterone with a concentration as low as 1 nM. Maximal inhibition of 75% +/- 1% was attained with 10 nM GnRH II analogue. The endogenous GnRH II content in the baboon ovary was 5-14 pmoles/g protein. The release of endogenous GnRH II from granulosa cells was observed throughout the 48 h in culture. These studies demonstrated the presence of high enzymatic activity for the degradation of mammalian GnRH in the ovary, whereas this GnRH II analogue was stable. High-affinity binding sites for this GnRH II analogue were also found. GnRH II and this GnRH II analogue can regulate progesterone production from baboon granulosa cells, suggesting that GnRH II is a potent regulator of ovarian function.

Evolutionary aspects of cellular communication in the vertebrate hypothalamo-hypophysio-gonadal axis

This review emphasizes the comparative approach for developing insight into knowledge related to cellular communications occurring in the hypothalamus-pituitary-gonadal axis. Indeed, research on adaptive phenomena leads to evolutionary tracks. Thus, going through recent results, we suggest that pheromonal communication precedes local communication which, in turn, precedes communication via the blood stream. Furthermore, the use of different routes of communication by a certain mediator leads to a conceptual change related to what hormones are. Nevertheless, endocrine communication should leave out of consideration the source (glandular or not) of mediator. Finally, we point out that the use of lower vertebrate animal models is fundamental to understanding general physiological mechanisms. In fact, different anatomical organization permits access to tissues not readily approachable in mammals.

Gonadotropin-releasing hormone (GnRH): from fish to mammalian brains

This work deals with a family of neuropeptides, gonadotropin-releasing hormone (GnRH), that play a key role in the development and maintenance of reproductive function in vertebrates. 2. Until now, a total of 16 GnRH structural variants have been isolated and characterized from vertebrate and protochordate nervous tissue. All vertebrate species already investigated have at least two GnRH forms coexisting in the central nervous system. However, it is now well accepted that three forms of GnRH in early and late evolved bony fishes are present. 3. In these cases, cGnRH-II is expressed by midbrain neurons, a species-specific GnRH is present mainly in the preoptic area and the hypothalamus, and sGnRH is localized in the terminal nerve ganglion (TNG). In this context it is possible to think that three GnRH forms and three GnRH receptor (GnRH-R) subtypes are expressed in the central nervous system of a given species. 4. Then it is possible to propose three different GnRH lineages expressed by distinct brain areas in vertebrates: (1) the conserved cGnRH-II or mesencephalic lineage; or (2) the hypothalamic or "releasing" lineage whose primary structure has diverged by point mutations (mGnRH and its orthologous forms: hrGnRH, wfGnRH, cfGnRH, sbGnRH, and pjGnRH); and (3) the telencephalic sGnRH form. Also different GnRH nomenclatures are discussed.

Spatio-temporal expression of gonadotropin-releasing hormone receptor subtypes in gonadotropes, somatotropes and lactotropes in the cichlid fish

The description of two or more forms of gonadotropin-releasing hormone (GnRH) in most vertebrates suggests multiple roles for this family of peptide hormones. In order to verify these functions, we analysed the anatomical location, time of initial expression and ontogenic changes in three distinct GnRH receptors (GnRH-Rs) in developing and sexually mature tilapia, using antisera raised against the extracellular loop three of the receptor, which is a determinant in ligand-selectivity and receptor coupling to signalling pathways. In all age groups, including males and females, using in situ hybridization and double-label immunological methods, GnRH-R type IA was colocalized in cells containing luteinizing hormone (LH) beta-subunit in the pituitary. GnRH-R type IB was visualized in prolactin cells and LH cells. The type III GnRH-R was expressed in growth hormone cells. On day 8 after fertilization, GnRH-R type III was first seen in growth hormone cells and, subsequently, on day 15, GnRH-Rs type IA and type IB were first seen in LH and prolactin cells, respectively. On day 25, the receptor occupied area per pituitary and the staining intensity of GnRH-R type IA increased significantly, consistent with the hypothesis that differentiation of GnRH neurones and their inputs to the pituitary coincide precisely with gonadal sex differentiation and steroidogenesis in tilapia. The differential distribution of GnRH-Rs in the pituitary provides the first clear evidence that the three native GnRH variants in tilapia have cognate receptors, each capable of regulating different pituitary endocrine cells.

Three forms of gonadotropin-releasing hormone, including a novel form, in a basal salmonid, Coregonus clupeaformis

Multiple forms of GnRH within individual brains may have different functions. However, some vertebrates such as salmonids continue to reproduce even though they have lost or do not express 1 of the 3 forms of GnRH found in most other teleosts. We examined a basal salmonid, lake whitefish, to determine the mechanism by which a reduction in the number of GnRH forms occurs. We identified for the first time 3 distinct GnRHs in a salmonid. One form is novel and is designated whitefish GnRH. The primary structure is pGlu-His-Trp-Ser-Tyr-Gly-Met-Asn-Pro-Gly-NH(2). HPLC and RIA were used for purification followed by Edman degradation for sequence determination. Mass spectroscopy was used to confirm the sequence and amidation of the peptide. The other 2 forms, salmon GnRH and chicken GnRH-II, are identical to the 2 forms found in salmon, which evolved later than whitefish. Synthetic whitefish GnRH is biologically active, as it increased mRNA expression of growth hormone and the alpha-subunit for LH and thyroid-stimulating hormone in dispersed fish pituitary cells. Our data support the hypothesis that the ancestral salmonid had a third GnRH form when the genome doubled (tetraploidization), but the third form was lost later in some salmonids due to chromosomal rearrangements. We suggest that the salmon GnRH form compensated for the loss of the third form.

Guinea pig GnRH: localization and physiological activity reveal that it, not mammalian GnRH, is the major neuroendocrine form in guinea pigs

The isolation of GnRH cDNA from guinea pig hypothalamus predicted a novel form of GnRH with two unique amino acid substitutions relative to all known forms of this essential decapeptide. The predicted substitution at amino acid 2 in guinea pig (gp) GnRH was particularly intriguing because of the proposed importance of position 2 for binding and activation of the GnRH receptor. In the present study, gpGnRH was synthesized, and a specific antibody was generated and used to assess translation of the gpGnRH transcript. The localization of intensely labeled gpGnRH-positive cell bodies and processes in tissue sections through the preoptic area and hypothalamus argue that gpGnRH is the major neuroendocrine form of GnRH in guinea pigs. Guinea pig GnRH stimulated LH release in guinea pigs and increased LH output from guinea pig pituitary fragments, thus demonstrating biological activity in this species. In contrast, gpGnRH demonstrated little ability to stimulate LH release in rats, a species known to possess the highly conserved mammalian GnRH receptor. These findings suggest that: (1) the amino acid substitutions in gpGnRH impede binding to and/or activation of the mammalian GnRH receptor, and (2) the unique amino acid substitutions in gpGnRH are accompanied by changes in the guinea pig GnRH receptor.

Two different messenger RNAs for salmon gonadotropin-releasing hormone are expressed in rainbow trout (Oncorhynchus mykiss) brain

Two different precursor genes encoding the decapeptide salmon GnRH (sGnRH) are present in most salmonid species. In rainbow trout, a precedent Southern blot study revealed the existence of two different sGnRH genes and, recently, two different genes and their complementary DNAs that encode the identical peptide sGnRH were isolated from ovary and testis. Our study confirms the existence of two different mRNAs encoding sGnRH (sGnRH mRNA-I and sGnRH mRNA-II) in the brain of rainbow trout and, for the first time, full-length complementary DNA sequences are given. Central and peripheral distributions of the two messengers are described and seem to indicate different regulation of their expression. sGnRH mRNA-I is found essentially in the olfactory bulbs and telencephalon, whereas sGnRH mRNA-II is more widely expressed in the brain. Our observations allow speculation on the respective roles of two genes encoding the same decapeptide.

Gonadotropin-releasing hormones (GnRHs) in a primitive teleost, the arowana: phylogenetic evidence that three paralogous lineages of GnRH occurred prior to the emergence of teleosts

Multiple molecular forms of gonadotropin-releasing hormone (GnRH) are present in a single vertebrate species. To extend the knowledge on GnRH evolution and the number of GnRH forms in one organism, GnRH cDNAs have been isolated and characterized from one of the most primitive teleosts, the arowana Scleropages jardini. This species had two molecular forms of GnRH: salmon-type GnRH (sGnRH) and chicken-II-type GnRH (cGnRH-II). Sequence comparison between the prepro-GnRHs of the arowana and those of other teleosts indicated that sGnRH represented a paralogue separate from any other forms of GnRH. Consistently, subsequent phylogenetic analysis showed that known forms of GnRH in teleosts fell into three paralogous lineages: sGnRH alone on one lineage, cGnRH-II on another, and many other forms on the other. These results suggest that an ancestral GnRH gene duplicated twice prior to the emergence of teleosts and, therefore, that teleosts, and probably also tetrapods, would possess three paralogous forms of GnRH in individual brains.

Presence of luteinizing hormone-releasing hormone fragments in the rhesus monkey forebrain

Previously, we have shown that two types of luteinizing hormone-releasing hormone (LHRH) -like neurons, "early" and "late" cells, were discernible in the forebrain of rhesus monkey fetuses by using antiserum GF-6, which cross-reacts with several forms of LHRH. The "late" cells that arose from the olfactory placode of monkey fetuses at embryonic days (E) 32-E36, are bona fide LHRH neurons. The "early" cells were found in the forebrain at E32-E34 and settled in the extrahypothalamic area. The molecular form of LHRH in "early" cells differs from "late" cells, because "early" cells were not immunopositive with any specific antisera against known forms of LHRH. In this study, we investigated the molecular form of LHRH in the "early" cells in the nasal regions and brains of 13 monkey fetuses at E35 to E78. In situ hybridization studies suggested that both "early" and "late" LHRH cells expressed mammalian LHRH mRNA. Furthermore, "early" cells predominantly contain LHRH1-5-like peptide and its cleavage enzyme, metalloendopeptidase E.C.3.4.24.15 (EP24.15), which cleaves LHRH at the Tyr5-Gly6 position. This conclusion was based on immunocytochemical labeling with various antisera, including those against LHRH1-5, LHRH4-10, or EP24.15, and on preabsorption tests. Therefore, in primates, a group of neurons containing mammalian LHRH mRNA arises at an early embryonic stage before the migration of bona fide LHRH neurons, and is ultimately distributed in the extrahypothalamic region. These extrahypothalamic neurons contain LHRH fragments, rather than fully mature mammalian LHRH. The origin and function of these neurons remain to be determined.

Primary structure of a novel gonadotropin-releasing hormone in the brain of a teleost, Pejerrey

The neuropeptide GnRH is the major regulator of reproduction in vertebrates acting as a first signal from the hypothalamus to pituitary gonadotropes. Three GnRH molecular variants were detected in the brain of a fish, pejerrey (Odontesthes bonariensis), using chromatographic and immunological methods. The present study shows that one form is identical to chicken GnRH-II (sequence analysis and mass spectrometry) and the second one is immunologically and chromatographically similar to salmon GnRH. The third form was proven to be a novel form of GnRH by isolating the peptide from the brain and determining its primary structure by chemical sequencing and mass spectrometry. The sequence of the novel pejerrey GnRH is pGlu-His-Trp-Ser-Phe-Gly-Leu-Ser-Pro-Gly-NH(2), which is different from the known forms of the vertebrate and protochordate GnRH family. The new form of GnRH is biologically active in releasing gonadotropin and GH from pituitary cells in an in vitro assay.

Cloning and characterization of cDNAs encoding the GnRH1 and GnRH2 precursors from bullfrog (Rana catesbeiana)

We have isolated the cDNAs encoding the GnRH1 and GnRH2 precursors, respectively, from bullfrog (Rana catesbeiana) brain. The first cDNA consists of 648 bp and contains an open-reading frame of 270 nucleotides, encoding the bullfrog GnRH1 precursor. The second cDNA consists of 1053 bp and contains an open-reading frame of 255 nucleotides, encoding the bullfrog GnRH2 precursor. Both types of bullfrog GnRH precursor have a similar molecular architecture as observed in other GnRH precursors, consisting of a signal peptide, followed by the GnRH decapeptide, a conserved carboxy-terminal amidation and proteolytical processing site, and a GnRH-associated peptide (GAP). In addition, we have identified a third cDNA, containing 24 additional nucleotides in its GAP-coding region. Genomic PCR and sequence analysis confirmed that this cDNA represents an alternative splice variant of the bullfrog GnRH2-precursor pre-mRNA. The bullfrog GnRH1 precursor exhibits 60% and less than 40% amino acid identity to its Xenopus and mammalian counterparts, respectively, whereas the bullfrog GnRH2 precursor displays 50% to 60% amino acid identity to that of its nonmammalian counterparts, but shares only 25% amino acid identity with its mammalian counterparts. Northern blot analysis revealed a single GnRH1-precursor mRNA species of approximately 0.75 kilobases, expressed in bullfrog forebrain, and a single GnRH2-precursor mRNA species of approximately 1.1 kilobases, expressed in bullfrog midbrain/hindbrain. Furthermore, both bullfrog GnRH-precursor mRNAs exhibited a differential spatiotemporal expression pattern. Genomic Southern blot analysis indicated that both bullfrog GnRH genes are present as single copy genes. This is the first report on the molecular cloning of a GnRH2-precursor cDNA from an amphibian species. In addition, we present data showing that alternative splicing is utilized to generate different GnRH2-precursor mRNAs. J. Exp. Zool. 289:190-201, 2001.

Action of chicken II GnRH on the human placenta

The stability, receptor binding, bioactivity, and production of chicken II GnRH and its analogs in the human placenta were studied. Both chicken II and mammalian GnRH are rapidly degraded by placental enzymes, yet the chicken II isoform is six times more stable. Analogs of chicken II GnRH were synthesized, and their stability in the presence of placental enzymes was tested. The D-Arg(6)-chicken II GnRH-aza-Gly(10)-NH(2) analog was found to be resistant to enzymatic degradation. The placental receptor binding activity of the chicken II GnRH analogs and chicken II GnRH was compared with that of mammalian GnRH and its analogs. Because D-Arg(6)-chicken II GnRH-aza-Gly(10)-NH(2) had the highest affinity for the placental receptor and was stable in placental extracts, the bioactivity of this analog on the regulation of placental human CG (hCG) release was compared with that for mammalian and chicken II GnRH using a human term placental explant culture system. This chicken II GnRH analog effected a stimulation of hCG at the lowest concentration studied (250 nM). With extended exposure and/or higher concentrations of this chicken II GnRH analog, the release of hCG from human placental explants was inhibited. Using a placental explant perifusion system and a highly specific RIA for chicken II GnRH, the pulsatile release of chicken II GnRH from the early human placenta was demonstrated. These studies are the first to demonstrate bioactivity of a second form of GnRH in a human tissue and to identify the pulsatile release of chicken II GnRH from a human tissue. These data led us to propose that chicken II GnRH and its synthetic analogs may be potent ligands for hormone regulation during pregnancy.

Three distinct types of GnRH receptor characterized in the bullfrog

It has been proposed recently that two types of GnRH receptors (GnRHR) exist in a particular species. Here we present data demonstrating that at least three types of GnRHR are expressed in a single diploid species, the bullfrog. Three different cDNAs, encoding distinct types of bullfrog GnRHR (bfGnRHR-1, bfGnRHR-2, and bfGnRHR-3), were isolated from pituitary and hindbrain of the bullfrog. BfGnRHR-1 mRNA was expressed predominantly in pituitary, whereas bfGnRHR-2 and -3 mRNAs were expressed in brain. The bfGnRHR-1, bfGnRHR-2, and bfGnRHR-3 proteins have an amino acid identity of approximately 30% to approximately 35% with mammalian GnRHRs and approximately 40% to approximately 50% with nonmammalian GnRHRs. Interestingly, bfGnRHR-2 has an 85% amino acid homology with Xenopus GnRHR. Less than 53% amino acid identity was observed among the three bfGnRHRs. All isolated cDNAs encode functional receptors because their transient expression in COS-7 cells resulted in a ligand-dependent increase in inositol phosphate production. Notably, all three receptors exhibited a differential ligand selectivity. For all receptors, cGnRH-II has a higher potency than mGnRH. In addition, salmon GnRH also has a strikingly high potency to stimulate all three receptors. In conclusion, we demonstrated the presence of three GnRHRs in the bullfrog. Their expression in pituitary and brain suggests that bfGnRHRs play an important role in the regulation of reproductive functions in the bullfrog.

Three distinct types of GnRH receptor characterized in the bullfrog

It has been proposed recently that two types of GnRH receptors (GnRHR) exist in a particular species. Here we present data demonstrating that at least three types of GnRHR are expressed in a single diploid species, the bullfrog. Three different cDNAs, encoding distinct types of bullfrog GnRHR (bfGnRHR-1, bfGnRHR-2, and bfGnRHR-3), were isolated from pituitary and hindbrain of the bullfrog. BfGnRHR-1 mRNA was expressed predominantly in pituitary, whereas bfGnRHR-2 and -3 mRNAs were expressed in brain. The bfGnRHR-1, bfGnRHR-2, and bfGnRHR-3 proteins have an amino acid identity of approximately 30% to approximately 35% with mammalian GnRHRs and approximately 40% to approximately 50% with nonmammalian GnRHRs. Interestingly, bfGnRHR-2 has an 85% amino acid homology with Xenopus GnRHR. Less than 53% amino acid identity was observed among the three bfGnRHRs. All isolated cDNAs encode functional receptors because their transient expression in COS-7 cells resulted in a ligand-dependent increase in inositol phosphate production. Notably, all three receptors exhibited a differential ligand selectivity. For all receptors, cGnRH-II has a higher potency than mGnRH. In addition, salmon GnRH also has a strikingly high potency to stimulate all three receptors. In conclusion, we demonstrated the presence of three GnRHRs in the bullfrog. Their expression in pituitary and brain suggests that bfGnRHRs play an important role in the regulation of reproductive functions in the bullfrog.

Differential expression of three different prepro-GnRH (gonadotrophin-releasing hormone) messengers in the brain of the european sea bass (Dicentrarchus labrax)

The expression sites of three prepro-gonadotrophin-releasing hormones (GnRHs), corresponding to seabream GnRH (sbGnRH: Ser(8)-mGnRH, mammalian GnRH), salmon GnRH (sGnRH: Trp(7)Leu(8)-mGnRH), and chicken GnRH-II (cGnRH-II: His(5)Trp(7)Tyr(8)-mGnRH) forms were studied in the brain of a perciform fish, the European sea bass (Dicentrarchus labrax) by means of in situ hybridization. The riboprobes used in this study correspond to the three GnRH-associated peptide (GAP)-coding regions of the prepro-GnRH cDNAs cloned from the same species (salmon GAP: sGAP; seabream GAP: sbGAP; chicken GAP-II: cIIGAP), which show little oligonucleotide sequence identity (sGAP versus sbGAP: 42%; cIIGAP versus sbGAP: 36%; sGAP versus cIIGAP: 41%). Adjacent paraffin sections (6 mm) throughout the entire brain were treated in parallel with each of the three anti-sense probes and the corresponding sense probes, demonstrating the high specificity of the hybridization signal. The results showed that both sGAP and sbGAP mRNAs had a broader expression in the olfactory bulbs, ventral telencephalon, and preoptic region, whereas cIIGAP mRNA expression was confined to large cells of the nucleus of the medial longitudinal fascicle. In the olfactory bulbs, both the signal intensity and the number of positive cells were higher with the sGAP probe, whereas sbGAP mRNA-expressing cells were more numerous and intensely stained in the preoptic region. Additional isolated sbGAP-positive cells were detected in the ventrolateral hypothalamus. These results demonstrate a clear overlapping of sGAP- and sbGAP-expressing cells in the forebrain of the European sea bass, in contrast to previous reports in other perciforms showing a clear segregation of these two cell populations.

A novel form of gonadotropin-releasing hormone in the medaka, Oryzias latipes

The present study has identified three molecular forms of gonadotropin-releasing hormone (GnRH) in the brain of a teleost, the medaka, by isolation of their cDNAs. This species has a novel GnRH, which is here named medaka-type GnRH (mdGnRH), in addition to two characterized forms, chicken-II-type GnRH (cGnRH-II) and salmon-type GnRH (sGnRH). Phylogenetic analysis showed that mdGnRH is a medaka homolog of and seabream-type GnRH (sbGnRH) and mammalian-type GnRH (mGnRH) in other species, and suggested that all vertebrates have three distinct GnRHs. Furthermore, in situ hybridization revealed that the mdGnRH gene is expressed only in neurons clustered within the preoptic area as sbGnRH and mGnRH genes in other species are, while the genes for cGnRH-II and sGnRH are only in the midbrain tegmentum and nucleus olfactoretinalis, respectively. This result suggested that mdGnRH is a hypophysiotropic factor and the other two forms are involved in other physiological events as neuromodulators or neurotransmitters.

Molecular cloning and tissue-specific expression of a gonadotropin-releasing hormone receptor in the Japanese eel

Gonadotropin-releasing hormone (GnRH) is a key regulatory neuropeptide involved in the control of reproduction in vertebrates. In the Japanese eel, one of the most primitive teleost species, two molecular forms of GnRH, mammalian-type GnRH and chicken-II-type GnRH (cGnRH-II), have been identified. This study has isolated a full-length cDNA for a GnRH receptor from the pituitary of the eel. The 3233-bp cDNA encodes a 380-amino acid protein which contains seven hydrophobic transmembrane domains and N- and C-terminal regions. The exon/intron organization of the open reading frame of the eel GnRH receptor gene was also determined. The open reading frame consists of three exons and two introns. The exon-intron splice site is similar to that of the GnRH receptor genes of mammals reported so far. Expression of the eel GnRH receptor was detected in various parts of the brain, pituitary, eye, olfactory epithelium, and testis. This result suggests that GnRH has local functions in these tissues in addition to its actions on gonadotropin synthesis and release in the pituitary. This tissue-specific expression pattern is similar to that of the eel cGnRH-II. Furthermore, the present eel receptor shows very high amino acid identity with the catfish and goldfish GnRH receptors, which are highly selective for the cGnRH-II. These results suggest that the cGnRH-II acts through binding to the present receptor in the eel.

Gonadotropin-releasing hormone (GnRH) variants in a lizard brain: is mammalian GnRH being expressed?

In reptiles as in other vertebrates, multiple forms of gonadotropin-releasing hormone (GnRH) within a single brain have been identified. In this group the following GnRH molecular variants have been characterized either by direct or indirect methods: chicken GnRH I (cGnRH-I), chicken GnRH II (cGnRH-II), salmon GnRH (sGnRH) and several unidentified GnRH-like forms. In the present study GnRH variants were investigated in brain extracts of the lizard Tupinambis teguixin (= T. merinae) by combining high-performance liquid chromatography (RP-HPLC) followed by radioimmunoassays (RIA). Two peaks showing GnRH immunoreactivity with the elution position of synthetic mammalian GnRH (mGnRH) and cGnRH-II were detected. Both peaks were further analyzed with different radioimmunoassay systems specific for mGnRH, cGnRH-I, and cGnRH-II. Pooled fractions corresponding to the first eluting peak showed no crossreactivity when analyzed with a cGnRH-I specific assay and logit-log displacement curves were not significantly different from those of synthetic mGnRH with homologous RIA systems. The second peak showed immunological characteristics of cGnRH-II when analyzed with a specific antiserum. The first ir-GnRH peak was selected for further RP-HPLC purification showing similar chromatographic behavior as mGnRH synthetic standard. We demonstrated the absence of cGnRH-I in this lizard using well-characterized antisera.

Biological and immunological characterization of multiple GnRH in an opisthobranch mollusk, Aplysia californica

Gonadotropin-releasing hormone (GnRH) is a neurohormone central to the regulation of reproductive functions in vertebrates. Recently, several studies have reported the presence of GnRH immunoreactivity (IR) in a number of mollusks, suggesting that the distribution of GnRH may not be restricted to Phylum Chordata. In the present study, we extend our investigations to an opisthobranch mollusk, Aplysia californica, to characterize the source, chemical nature, and biological activity of molluscan GnRH-related molecules. Specific radioimmunoassays (RIAs) of various tissue extracts of Aplysia revealed that only ovotestis, hemocytes, and hemolymph contained significant amounts of GnRH that crossreacts with antisera raised against tunicate-I (tI) and mammalian (m) GnRH. Further RIAs and extractions revealed that the GnRH-IR in the hemolymph is biochemically and immunologically distinct from the GnRH-IR in the hemocytes and ovotestis. Using reverse-phase high-performance liquid chromatography coupled with RIAs, the GnRH-IR in the hemolymph was resolved into two major peaks. The first peak eluted earlier than most known forms of vertebrate GnRH, and the later peak coeluted with m, lamprey I, chicken II, and tI-GnRH. However, both peaks were broad and may contain a heterogeneous mixture of GnRH-IR. Immunocytochemical study showed that tI-GnRH-IR was present in the connective sheath surrounding the central nervous system, with a strong presence in what appeared to be vascular space, again suggesting the close association between Aplysia GnRH-IR and circulation. Finally, treatment of the neuroendocrine bag cells with chicken II GnRH significantly decreased the duration of the afterdischarge (AD, a characteristic pattern of electrical firing in bag cell neurons) and the number of action potentials fired during an AD, indicating the presence of a corresponding GnRH receptor in the Aplysia central nervous system. Overall, the results demonstrated the presence of multiple forms of GnRH-IR that crossreact with tI- and mGnRH antisera in A. californica and the ability of a vertebrate GnRH to alter Aplysia neural activity. Together, these data suggest that GnRH may be a factor released by the ovotestis and hemocytes into the circulation to alter neural functions. GnRH-IR produced by the latter may serve as a novel mediator of the neural and immune functions in Aplysia.