Presence and biological activity of a GnRH-like factor in the nervous system of Helisoma trivolvis

Identification and localization of a gonadotropin-releasing hormone-related neuropeptide in Biomphalaria, an intermediate host for schistosomiasis

Freshwater snails of the genus Biomphalaria serve as obligatory hosts for the digenetic trematode Schistosoma mansoni, the causative agent for the most widespread form of intestinal schistosomiasis. Within Biomphalaria, S. mansoni larvae multiply and transform into the cercariae form that can infect humans. Trematode development and proliferation is thought to be facilitated by modifications of host behavior and physiological processes, including a reduction of reproduction known as "parasitic castration." As neuropeptides participate in the control of reproduction across phylogeny, a neural transcriptomics approach was undertaken to identify peptides that could regulate Biomphalaria reproductive physiology. The present study identified a transcript in Biomphalaria alexandrina that encodes a peptide belonging to the gonadotropin-releasing hormone (GnRH) superfamily. The precursor and the predicted mature peptide, pQIHFTPDWGNN-NH₂ (designated Biom-GnRH), share features with peptides identified in other molluscan species, including panpulmonates, opisthobranchs, and cephalopods. An antibody generated against Biom-GnRH labeled neurons in the cerebral, pedal, and visceral ganglia of Biomphalaria glabrata. GnRH-like immunoreactive fiber systems projected to all central ganglia. In the periphery, immunoreactive material was detected in the ovotestis, oviduct, albumen gland, and nidamental gland. As these structures serve crucial roles in the production, transport, nourishment, and encapsulation of eggs, disruption of the GnRH system of Biomphalaria could contribute to reduced reproductive activity in infected snails.

Neuropeptides predicted from the transcriptome analysis of the gray garden slug Deroceras reticulatum

The gray garden slug, Deroceras reticulatum (Gastropoda: Pulmonata), is one of the most common terrestrial molluscs. Research for this slug has focused mainly on its ecology, biology, and management due to the severe damage it causes on a wide range of vegetables and field crops. However, little is known about neuropeptides and hormonal signalings. This study, therefore, aimed to establish the transcriptome of D. reticulatum and to identify a comprehensive repertoire of neuropeptides in this slug. Illumina high-throughput sequencing of the whole body transcriptome of D. reticulatum generated a total of 5.9 billion raw paired-end reads. De novo assembly by Trinity resulted in 143,575 transcripts and further filtration selected 120,553 unigenes. Gene Ontology (GO) terms were assigned to 30,588 unigenes, composed of biological processes (36.9%), cellular components (30.2%) and molecular functions (32.9%). Functional annotation by BLASTx revealed 39,987 unigenes with hits, which were further categorized into important functional groups based on sequence abundance. Neuropeptides, ion channels, ribosomal proteins, G protein-coupled receptors, detoxification, immunity and cytoskeleton-related sequences were dominant among the transcripts. BLAST searches and PCR amplification were used to identify 65 putative neuropeptide precursor genes from the D. reticulatum transcriptome, which include achatin, AKH, allatostatin A, B and C, allatotropin, APGWamide, CCAP, cerebrin, conopressin, cysteine-knot protein hormones (bursicon alpha/beta and GPA2/GPB5), elevenin, FCAP, FFamide, FVamide (enterin, fulicin, MIP and PRQFVamide), GGNG, GnRH, insulin, NdWFamide, NKY, PKYMDT, PRXamide (myomodulin, pleurin and sCAP), RFamide (CCK/SK, FMRFamide, FxRIamide, LFRFamide, luqin and NPF), and tachykinin. Over 330 putative peptides were encoded by these precursors. Comparative analysis among different molluscan species clearly revealed that, while D. reticulatum neuropeptide sequences are conserved in Mollusca, there are also some unique features distinct from other members of this species. This is the first transcriptome-wide report of neuropeptides in terrestrial slugs. Our results provide comprehensive transcriptome data of the gray garden slug, with a more detailed focus on the rich repertoire of putative neuropeptide sequences, laying the foundation for molecular studies in this terrestrial slug pest.

Characterisation of Reproduction-Associated Genes and Peptides in the Pest Land Snail, Theba pisana

Increased understanding of the molecular components involved in reproduction may assist in understanding the evolutionary adaptations used by animals, including hermaphrodites, to produce offspring and retain a continuation of their lineage. In this study, we focus on the Mediterranean snail, Theba pisana, a hermaphroditic land snail that has become a highly invasive pest species within agricultural areas throughout the world. Our analysis of T. pisana CNS tissue has revealed gene transcripts encoding molluscan reproduction-associated proteins including APGWamide, gonadotropin-releasing hormone (GnRH) and an egg-laying hormone (ELH). ELH isoform 1 (ELH1) is known to be a potent reproductive peptide hormone involved in ovulation and egg-laying in some aquatic molluscs. Two other non-CNS ELH isoforms were also present in T. pisana (Tpi-ELH2 and Tpi-ELH3) within the snail dart sac and mucous glands. Bioactivity of a synthetic ELH1 on sexually mature T. pisana was confirmed through bioassay, with snails showing ELH1-induced egg-laying behaviours, including soil burrowing and oviposition. In summary, this study presents a detailed molecular analysis of reproductive neuropeptide genes in a land snail and provides a foundation for understanding ELH function.

Immunohistochemical detection of GnRH-like peptides in the neural ganglia and testis of Haliotis asinina

Gonadotropin releasing hormone (GnRH) is a peptide that is conserved in both vertebrate and invertebrate species. In this study, we have demonstrated the distribution pattern of two isoforms of GnRH-like peptides in the neural ganglia and testis of reproductively mature male abalone, H. asinina, by immunohistochemistry and whole mount immunofluorescence. We found octopus (oct) GnRH and tunicate-I (t) GnRH-I immunoreactivities (ir) in type 1 neurosecretory cells (NS1) and they were expressed mostly within the ventral horn of the cerebral ganglion, whereas in pleuropedal ganglia they were localized primarily in the dorsal horn. Furthermore, tGnRH-I-ir were strongly detected in fibers at the caudal part of the cerebral ganglia and both ventral and dorsal horns of the pleuropedal ganglia. In the testis, only octGnRH-ir was found primarily in the granulated cell and central capillaries within the trabeculae. These results suggest that multiple GnRH-like peptides are present in the neural ganglia which could be the principal source of their production, whereas GnRH may also be synthesized locally in the testis and act as the paracrine control of testicular maturation.

Molluscan GnRH associated with reproduction

Gonadotropin-releasing hormone (GnRH) is a neuropeptide that has an essential role in the neural regulation of vertebrate reproduction. Over the past two decades there has been increasing evidence strongly indicating that members of the GnRH superfamily, which includes GnRH, adipokinetic hormone (AKH), corazonin (Crz) and adipokinetic hormone/corazonin-related peptides (ACP), are almost ubiquitous amongst bilateral animals. Moreover GnRH possibly has origins in even more ancient, non-bilateral ancestors. Current knowledge about molluscan GnRH has been accumulated regarding immunological identification, physiological function and sequence analysis. In the present review we summarized a current status of molluscan GnRH research and focus on its role in the reproduction of the molluscs. In cephalopods and gastropods the presence of a GnRH-like peptide was detected with heterologous antibodies and the identified GnRH was suggested to be involved with behavior and reproduction. Reproductive roles for GnRH have been confirmed in both bivalve and cephalopod molluscs. These findings will provide useful insights into the evolution of reproductive endocrinology.

Gonadotropin-releasing hormone in protostomes: insights from functional studies on Aplysia californica

Several protostomian molecules that structurally resemble chordate gonadotropin-releasing hormone (GnRH) have been identified through cloning, biochemical purification or data mining. These molecules share considerable sequence and structural similarities with chordate GnRH, leading to the current belief that protostomian and chordate forms of GnRH share a common ancestor. However, the physiological significance of these protostomian GnRH-like molecules remains poorly understood. This knowledge gap hampers our understanding of how GnRH has evolved functionally over time. This review provides a summary of our recent functional characterization of a GnRH-like molecule (ap-GnRH) in a gastropod mollusk, Aplysia californica, and presents preliminary proof for a cognate ap-GnRH receptor (ap-GnRHR). Our data reveal that ap-GnRH is a general neural regulator capable of exerting diverse central and motor effects, but plays little or no role in reproductive activation. This notion is supported by the abundance of a putative ap-GnRHR transcript in the central nervous system and the foot. Comparing these results to the available functional data from a cephalopod mollusk, Octopus vulgaris, we surmise that protostomian GnRH-like molecules are likely to assume a wide range of physiological roles, and reproductive activation is not an evolutionarily conserved role of these molecules. Future functional studies using suitable protostomian models are required to identify functional changes in protostomian GnRH-like molecules that accompany major taxa-level transitions.

Mollusc gonadotropin-releasing hormone directly regulates gonadal functions: a primitive endocrine system controlling reproduction

Gonadotropin-releasing hormone (GnRH) is central to the control of vertebrate reproductive cycles and since GnRH orthologs are also present in invertebrates, it is likely that the common ancestor of bilateral animals possessed a GnRH-like peptide. In order to understand the evolutionary and comparative biology of GnRH peptides we cloned the cDNA transcripts of prepro GnRH-like peptides from two species of bivalve molluscs, the Yesso scallop Patinopecten yessoensis and the Pacific oyster Crassostrea gigas. We compared their deduced uncleaved and mature amino acid sequences with those from other invertebrates and vertebrates, and determined their sites of expression and biological activity. The two molluscan GnRH sequences increased the number of known protostome GnRHs to six different forms, indicating the current classification of protostome GnRHs requires further revision. In both molluscs, RT-PCR analysis showed that the genes were highly expressed in nervous tissue with lower levels present in peripheral tissues including the gonads, while immunocytochemistry, using anti-octopus GnRH-like peptide, demonstrated the presence of GnRH-like peptide in neural tissue. Putative scallop GnRH-like peptide stimulated spermatogonial cell division in cultured scallop testis, but the scallop GnRH-like peptide did not stimulate LH release from cultured quail pituitary cells. This is the first report of the cloning of bivalve GnRH-like peptide genes and of molluscan GnRH-like peptides that are biologically active in molluscs, but not in a vertebrate.

A gonadotropin-releasing hormone-like molecule modulates the activity of diverse central neurons in a gastropod mollusk, aplysia californica

In vertebrates, gonadotropin-releasing hormone (GnRH) is a crucial decapeptide that activates the hypothalamic-pituitary-gonadal axis to ensure successful reproduction. Recently, a GnRH-like molecule has been isolated from a gastropod mollusk, Aplysia californica. This GnRH (ap-GnRH) is deduced to be an undecapeptide, and its function remains to be explored. Our previous study demonstrated that ap-GnRH did not stimulate a range of reproductive parameters. Instead, it affected acute behavioral and locomotive changes unrelated to reproduction. In this study, we used electrophysiology and retrograde tracing to further explore the central role of ap-GnRH. Sharp-electrode intracellular recordings revealed that ap-GnRH had diverse effects on central neurons that ranged from excitatory, inhibitory, to the alteration of membrane potential. Unexpectedly, extracellular recordings revealed that ap-GnRH suppressed the onset of electrical afterdischarge in bag cell neurons, suggesting an inhibitory effect on female reproduction. Lastly, using immunocytochemistry coupled with nickel backfill, we demonstrated that some ap-GnRH neurons projected to efferent nerves known to innervate the foot and parapodia, suggesting ap-GnRH may directly modulate the motor output of these peripheral tissues. Overall, our results suggested that in A. californica, ap-GnRH more likely functioned as a central modulator of complex behavior and motor regulation rather than as a conventional reproductive stimulator.

The existence of gonadotropin-releasing hormone-like peptides in the neural ganglia and ovary of the abalone, Haliotis asinina L

Gonadotropin-releasing hormone (GnRH) is a neuropeptide that is conserved in both vertebrate and invertebrate species. In this study, we have demonstrated the presence and distribution of two isoforms of GnRH-like peptides in neural ganglia and ovary of reproductively mature female abalone, Haliotis asinina, using immunohistochemistry. We found significant immunoreactivities (ir) of anti-lamprey(l) GnRH-III and anti-tunicate(t) GnRH, but with variation of labeling intensity by each anti-GnRH type. lGnRH-III-ir was detected in numerous type 1 neurosecretory cells (NS1) throughout the cerebral and pleuropedal ganglia, whereas tGnRH-I-ir was detected in only a few NS1 cells in the dorsal region of cerebral and pleuropedal ganglia. In addition, a small number of type 2 neurosecretory cells (NS2) in cerebral ganglion showed lGnRH-III-ir. Long nerve fibers in the neuropil of ventral regions of the cerebral and pluropedal ganglia showed strong tGnRH-I-ir. In the ovary, lGnRH-III-ir was found primarily in oogonia and stage I oocytes, whereas tGnRH-ir was observed in stage I oocytes and some stage II oocytes. These results indicate that GnRH produced in neural ganglia may act in neural signaling. Alternatively, GnRH may also be synthesized locally in the ovary where it could induce oocyte development.

Gonadotropin-releasing hormone-like molecule is not an acute reproductive activator in the gastropod, Aplysia californica

Gonadotropin-releasing hormone (GnRH) is indispensable for reproductive activation in all vertebrates. Although several GnRH-like molecules have been isolated from non-chordates, the function of GnRH in these taxa remains unclear. We previously isolated the full-length cDNA sequence of a prohormone containing a GnRH-like molecule, termed ap-GnRH, from the gastropod mollusk, Aplysia californica. In this study, we characterized the distribution and quantity of ap-GnRH peptide in several central and peripheral tissues of A. californica. Further, we performed in vivo and in vitro studies to explore the function of ap-GnRH in these animals. Immunohistochemistry and radioimmunoassay using specific antisera against ap-GnRH showed that pedal ganglia contained the highest level of ap-GnRH peptide, followed by cerebral ganglia, abdominal ganglia, and then buccal ganglia. Ovotestis did not contain detectable levels of ap-GnRH peptide. Injection of sexually mature and immature animals with synthetic ap-GnRH over a course of 10-14 days had no effects on ovotestis mass, reproductive tract mass, egg-laying, and penile eversion. ap-GnRH also failed to alter oocyte growth and egg-laying hormone accumulation and secretion. Interestingly, ap-GnRH injection triggered acute behavioral responses including the stimulation of parapodial opening, inhibition of feeding, and promotion of substrate attachment. Our results showed that in A. californica, ap-GnRH could modulate a wide range of behavioral attributes. Most strikingly, ap-GnRH is not involved in the acute activation of reproduction in a fashion similar to vertebrate GnRH.

GnRH in the brain and ovary of Sepia officinalis

We have cloned from brain, ovary and eggs of the cephalopod Sepia officinalis a 269-bp PCR product, which shares 100% sequence identity with the open reading frame of GnRH isoform isolated from Octopus vulgaris. Similar to Octopus, this sequence encodes a peptide that is organized as a preprohormone from which, after enzymatic cleavage, a dodecapeptide is released. Apart from its length, this peptide shares all the common features of vertebrate GnRHs. Reverse transcriptase-polymerase chain reaction (RT-PCR) analyses followed by sequencing have confirmed that the same peptide transcript is also present in the ovary, as well as in eggs released in the mantle cavity. The use of an antibody made specifically against the oct-GnRH has revealed that the peptide is localized in the dorso-lateral basal and olfactory lobes, the two neuropeptidergic centers controlling the activity of the gonadotropic optic gland. Immunoreactive nerve endings are also present on the glandular cells of the optic glands. These results confirm the fact that, regardless of the evolutionary distances among animal phyla, GnRH is an ancient peptide present also in invertebrates, and also reinforce the notion that, despite the name "gonadotropin releasing-hormone" was attributed according to its role in vertebrates, probably this family of peptides always had a role in the broad context of animal reproduction. The divergence and spread of several different isoforms of this peptide among animals seem to be balanced, in both invertebrates and vertebrates, by the class-specificity of the GnRH isoform involved in reproductive processes.

The emergence and loss of gonadotropin-releasing hormone in protostomes: orthology, phylogeny, structure, and function

Gonadotropin-releasing hormone (GNRH) is a neuropeptide critical for reproductive activation and maintenance in vertebrates. The recent elucidation of molluscan GNRH-like sequences led to several important questions regarding the evolution of the GNRH family. For instance, are molluscan and chordate GNRHs true orthologs? Has GNRH been retained in most protostomian lineages? What was the function of the ancestral GNRH? The goal of this review is to provide a critical analysis of GNRH evolution based on data available from the known forms of protostomian GNRH. Judging from the orthology between chordate and protostomian GNRH receptors, conservation of several structural motifs on the GNRH peptide, and exon/intron arrangement conserved between protostomian and chordate GNRH genomic sequences, we conclude that chordate and protostomian GNRHs likely share a common ancestor. Based on our analysis of phylogenetic distribution, we also hypothesize that GNRH may have been lost in the ecdysozoan lineage but preserved in lophotrochozoans. Lastly, we propose that the ancestral function of GNRH is to serve as a general neural regulator, and its considerable specialization in reproduction seen in chordates is a consequence of neofunctionalization following gene duplication.

Molecular cloning, expression pattern, and immunocytochemical localization of a gonadotropin-releasing hormone-like molecule in the gastropod mollusk, Aplysia californica

Successful reproduction in vertebrates depends upon the actions of gonadotropin-releasing hormone (GnRH). Despite the wide presence of GnRH in Phylum Chordata, GnRH has not been isolated in protostomes other than the common octopus. To provide information on the evolution of this critical hormone, we isolated the full-length cDNA of a GnRH-like molecule from the central nervous system of a gastropod mollusk, the sea hare Aplysia californica. The open reading frame of this cDNA encodes a protein of 147 amino acids. The molecular architecture of the deduced protein is highly homologous to that reported for the prepro-octopus GnRH (oct-GnRH) and consists of a putative signal peptide, a GnRH dodecapeptide, a downstream processing site, and a GnRH-associated peptide (GAP). The deduced amino acid sequence of the Aplysia GnRH (ap-GnRH) is QNYHFSNGWYAG and differs from oct-GnRH by only two amino acids. The transcript for ap-GnRH is widely expressed in the central nervous system (CNS), the ovotestis, and the atrial gland, an exocrine gland. Immunocytochemistry (ICC) using an antiserum against oct-GnRH detected immunoreactive neurons in all CNS ganglia examined, and the staining was abolished by the preadsorption of the antiserum with synthetic ap-GnRH. In sum, ap-GnRH sequence is the first gastropod GnRH-like molecule to be elucidated. Further, it represents one of the only two GnRH-like molecules found outside Phylum Chordata. These data refute the possibility that oct-GnRH arose singly in cephalopods by convergent evolution and provide valuable support for an ancient origin of GnRH during metazoan evolution.

The identification and distribution of gonadotropin-releasing hormone-like peptides in the central nervous system and ovary of the giant freshwater prawn, Macrobrachium rosenbergii

In the present study, we demonstrated the existence of GnRH-like peptides in the central nervous system (CNS) and ovary of the giant freshwater prawn, Macrobrachium rosenbergii using immunocytochemistry. The immunoreactivity (ir) of lamprey (l) GnRH-III was detected in the soma of medium-sized neurons located in neuronal cluster number 11 in the middle part of supraesophageal ganglion (deutocerebrum), whereas ir-octopus (oct) GnRH was observed in the soma of both medium-sized and large-sized neurons in thoracic ganglia, as well as in the fibers innervating the other medium-sized and large-sized neuronal cell bodies in the thoracic ganglia. In addition, ir-lGnRH-I was observed in the cytoplasm of late previtellogenic oocyte and early vitellogenic oocyte. These data suggest that M. rosenbergii contain at least three isoforms of GnRH: two GnRH isoforms closely related to lGnRH-III and octGnRH in the CNS, whereas another isoform, closely related to lGnRH-I, was localized in the ovary. This finding provides supporting data that ir-GnRH-like peptide(s) may exist in this decapod crustacean.

The presence and distribution of gonadotropin-releasing hormone-liked factor in the central nervous system of the black tiger shrimp, Penaeus monodon

The distribution and presence of gonadotropin-releasing hormone (GnRH) in the central nervous system (CNS) of Penaeus monodon were examined by immunocytochemistry, high performance liquid chromatography (HPLC), and radioimmunoassay (RIA). We demonstrated the existence of octopus (oct)GnRH-liked immunoreactivity (ir-octGnRH) and lamprey (l)GnRH-III-liked immunoreactivity (ir-lGnRH-III) in cell bodies of medium-sized neurons of the anterior part (protocerebrum) of the supraesophageal ganglion (brain). In addition, only the ir-octGnRH was detected in the nerve fibers located in the brain and segmental ganglia (subesophageal, thoracic, and abdominal ganglia). Moreover, some branches of these fibers also innervated the neurons in the middle (deutrocerebrum), posterior (tritocerebrum) brain and segmental ganglia. There was no ir-lGnRH-I and ir-salmon (s)GnRH detected in the shrimp CNS. The results from HPLC and RIA showed ir-GnRH in the CNS using anti-lGnRH-III, but not with anti-mammalian (m)GnRH. The data from immunocytochemistry, HPLC and RIA suggest that ir-GnRH in shrimp may be more similar to octGnRH and lGnRH-III than the other forms. These findings support the hypothesis that GnRH-liked factor(s) may be an ancient peptide that also exists in this decapod crustacean.

Endocrine disruption in aquatic pulmonate molluscs: few evidences, many challenges

As compared to other groups of aquatic gastropods, documented examples of endocrine disruption in pulmonates are rather limited. This is quite surprising because the endocrine control of physiological functions has been extensively studied in these animals. In the model-species Lymnaea stagnalis, the neurohormonal regulation of reproduction has been thoroughly investigated, and the primary structure of several peptides and receptors involved in endocrine processes has been established. However, the use of this knowledge has been fairly limited in the context of ecotoxicology, to investigate the effects of endocrine-disrupting chemicals. The present review summarizes the main and more recent findings on the neuroendocrine control of reproduction in aquatic pulmonate snails (Basommatophora). It then comprehensively describes selected in vivo laboratory and semi-field studies which provide evidence for possible endocrine disrupting effects of estrogenic and androgenic test compounds [e.g., ethynylestradiol, methyltestosterone (MT)], and of environmental contaminants [e.g., cadmium (Cd), tributyltin (TBT), and nonylphenol (NP), pesticides]. Finally, challenging perspectives for future research are discussed.

Involvement of GnRH neuron in the spermatogonial proliferation of the scallop, Patinopecten yessoensiss

The aim of this study was to quantitatively analyze a pattern of proliferation of gonial cells and to demonstrate neural involvement in spermatogonial proliferation of the scallop by the in vitro experiment. Immunocytochemistry for incorporated BrdU was used to identify mitotically active gonial cells. The pattern of proliferation of gonial cells was divided into two phases: phase I; oogonia and spermatogonia slowly proliferate through the growing stage: phase II; oogonia develop into oocytes and spermatogonia start to proliferate rapidly from the mature stage through the spawning stage. The neurons detected with anti-mammalian (m)GnRH antibody were distributed sparsely in the pedal ganglion and predominantly in the cerebral ganglion of both sexes at the growing stage. The extracts from the cerebral and pedal ganglion (CPG) of both sexes collected at the growing stage promoted proliferation of spermatogonia in the in vitro culture of the testicular tissue as well as mGnRH. However, CPG extract had no effect on oogonial proliferation. The increased mitotic activity induced by CPG and mGnRH was abolished by the addition of mGnRH antagonists and anti-mGnRH antibody, suggesting that the spermatogonial proliferation is regulated by GnRH-like peptide in CPG of the scallop. The same mitotic activity as CPG extract and mGnRH was observed in the hemocyte lysate, but not in the serum. These findings suggest that the spermatogonial proliferation at phase II in the scallop may be under the neuroendocrine control by GnRH neuron in CPG.

Tissue preferential expression of estrogen receptor gene in the marine snail, Thais clavigera

Sex steroid hormones have been widely detected in molluscs, and experiments have shown the importance of sex steroids in sex determination, gonadal tissue maturation and gametogenesis. Nevertheless, the signaling pathways of sex steroids in invertebrates have not yet been elucidated. In order to gain insights into the mechanism of sex steroid signaling in molluscs, we have, therefore, tried to isolate molluscan estrogen receptors from the prosobranch mollusc Thais clavigera. Cerebral ganglia of T. clavigera (Mollusca, Gastropoda, Prosobranchia) were subjected to RNA extraction, and degenerate primers for amino acid sequences conserved in vertebrate estrogen receptors were designed. PCR amplification using cerebral RNA and degenerate primers followed by 5'- and 3'-RACE identified the cDNA encoding T. clavigera estrogen receptor 1 (tcER1). The deduced amino acid sequence showed 93% identity in the DNA-binding domain and 72% identity in the ligand binding domain when compared to Aplysia estrogen receptor. Reporter gene assay revealed that tcER1 is constitutively active and unresponsive to estrogen. Quantitative analysis of the tcER1 mRNA level demonstrated the preferential expression in the ovary. Furthermore, cerebral ganglia expressed tcER1 at a high level in the spring followed by subsequent enlargement of the ovary in later seasons. These results suggest importance of tcER1 in the seasonal development of reproductive organs in T. clavigera.

Gonadotropin-releasing hormone in invertebrates: structure, function, and evolution

Gonadotropin-releasing hormone (GnRH) is central to the initiation and maintenance of reproduction in vertebrates. GnRH is found in all major groups of Phylum Chordata, including the protochordates. Studies on functional and structural evolution of GnRH have, in the past, focused exclusively on chordates. However, the recent structural elucidation of an octopus GnRH-like molecule and increasing evidence that GnRH-like substances are present in multiple invertebrate phyla suggest GnRH is an ancient peptide that arose prior to the divergence of protostomes and deuterostomes. The extraordinary conservation of GnRH structure and function raises interesting questions regarding the functional role assumed by GnRH over the course of evolution. This review will focus on the current understanding of GnRH structure and function in non-chordate invertebrates. Special emphasis will be placed upon the possible and speculated functions of GnRH in mollusks.

Evolution of GnRH ligands and receptors in gnathostomata

Gonadotropin-releasing hormone (GnRH) is the final common signaling molecule used by the brain to regulate reproduction in all vertebrates. Until now, a total of 24 GnRH structural variants have been characterized from vertebrate, protochordate and invertebrate nervous tissue. Almost all vertebrates already investigated have at least two GnRH forms coexisting in the central nervous system. Furthermore, it is now well accepted that three GnRH forms are present both in early and late evolved teleostean fishes. The number and taxonomic distribution of the different GnRH variants also raise questions about the phylogenetic relationships between them. Most of the GnRH phylogenetic analyses are in agreement with the widely accepted idea that the GnRH family can be divided into three main groups. However, the examination of the gnathostome GnRH phylogenetic relationships clearly shows the existence of two main paralogous GnRH lineages: the ''midbrain GnRH" group and the "forebrain GnRH" group. The first one, represented by chicken GnRH-II forms, and the second one composed of two paralogous lineages, the salmon GnRH cluster (only represented in teleostean fish species) and the hypophysotropic GnRH cluster, also present in tetrapods. This analysis suggests that the two forebrain clades share a common precursor and reinforces the idea that the salmon GnRH branch has originated from a duplication of the hypophysotropic lineage. GnRH ligands exert their activity through G protein-coupled receptors of the rhodopsin-like family. As with the ligands, multiple GnRHRs are expressed in individual vertebrate species and phylogenetic analyses have revealed that all vertebrate GnRHRs cluster into three main receptor types. However, new data and a new phylogenetic analysis propose a two GnRHR type model, in which different rounds of gene duplications may have occurred in different groups within each lineage.

The presence and ancestral role of gonadotropin-releasing hormone in the reproduction of scleractinian coral, Euphyllia ancora

The objectives of this study were to investigate the presence of immunoreactive GnRH (irGnRH) in scleractinian coral, Euphyllia ancora, study its seasonal variation, and evaluate its biological activity. irGnRH was detected and quantified in coral polyps. The biological activity of coral irGnRH was tested on pituitary cells from black porgy by evaluating its ability to stimulate LH release. Coral extracts (10(-9)-10(-5) M irGnRH) as well as mammalian (m) GnRH agonist (10(-10)-10(-6) M) had a similar dose-dependent effect on LH release. Furthermore, GnRH receptor antagonist dose-dependently inhibited the stimulation of LH release in response to coral extracts (10(-5) M irGnRH) and mGnRH agonist (10(-6) M). Peak levels of irGnRH (10-fold increase) were observed during the spawning period in a 3-yr investigation. Significantly higher aromatase activity and estradiol (E2) levels were also detected during the period of spawning compared with the nonreproductive season. In in vivo experiments, mGnRH agonist time- and dose-dependently stimulated aromatase activity as well as the concentrations of testosterone and E2 in free and glucuronided forms in coral. In conclusion, our data indicate that irGnRH does exist in coral, with its ability to stimulate LH release in fish. Seasonal variations of coral irGnRH, with a dramatic increase during the spawning period, concomitant to that in aromatase and E2, as well as the ability of mGnRH agonist to stimulate coral aromatase, steroidogenesis, and steroid glucuronization suggest that irGnRH plays an important role in the control of oocyte growth and mass spawning in corals.

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

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.

Localization of gonadotropin-releasing hormone in the central nervous system and a peripheral chemosensory organ of Aplysia californica

Gonadotropin-releasing hormone (GnRH) is a neurohormone crucial for the regulation of reproductive and neural functions in vertebrates. Recent discoveries of GnRH immunoreactivity (IR) in a number of invertebrates raised the possibility that GnRH may be an ancient molecule that had arisen before the emergence of Phylum Chordata. We previously demonstrated the presence of a GnRH IR similar to the mammalian (m) and tunicate I (tI) forms of GnRH in the hemolymph and ovotestis of an opisthobranch mollusk, Aplysia californica; however, the presence of GnRH in the central nervous system (CNS) of A. californica could not be detected with the available antisera against various forms of chordate GnRH. In the present study, we performed immunohistochemistry (IHC) to localize the presence of GnRH in the CNS and a peripheral chemosensory organ, the osphradium, of A. californica. A newly generated antiserum against tI-GnRH revealed the strong expression of GnRH IR in neurons of all CNS ganglia. A notable asymmetry in immunostaining was detected in the left and right abdominal hemiganglia. The CNS is rich in tI-GnRH immunoreactive neurons but lacks mGnRH IR, whereas the osphradium contains abundant mGnRH immunoreactive neurons but lacks tI-GnRH IR. The extract of CNS failed to stimulate the release of LH from mouse pituitary, demonstrating that the A. californica GnRH IR is structurally different from what is required to bind and activate mammalian GnRH receptor. Together, these results indicate the presence of at least two distinct GnRH systems in A. californica. The presence of GnRH in the osphradium is consistent with the long-standing anatomical relationship between GnRH and the chemosensory system observed in vertebrates.

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.

Isolation and characterization of a GnRH-like peptide from Octopus vulgaris

Gonadotropin-releasing hormone (GnRH) is the key peptide in the hypothalamo-hypophysial-gonadal axis, the core of regulation of reproduction in vertebrates. In this study, an octopus peptide with structural features similar to vertebrate GnRHs was isolated from brains of Octopus vulgaris. This peptide showed luteinizing hormone-releasing activity in quail anterior pituitary cells. A cDNA encoding the precursor protein was cloned. The RT-PCR transcripts were expressed in the supraesophageal and subesophageal brains, peduncle complex, and optic gland. The presence of the peptide in the different brain region was confirmed with enzyme-linked immunosorbent assay and time-of-flight mass spectrometric analysis. Immunoreactive neuronal cell bodies and fibers were observed in the subpedunculate lobe that controls the optic-gland activity. Optic gland nerves and glandular cells in the optic gland were immunostained. The isolated peptide may be octopus GnRH that contributes to octopus reproduction not only as a neurohormone but also as an endocrine hormone.

Presence of two neuropeptides in the fusiform ganglion and reproductive ducts of Octopus vulgaris: FMRFamide and gonadotropin-releasing hormone (GnRH)

We have found evidence of FMRFamide-like and cGnRH-I-like immunoreactivity in the central nervous system (CNS) and in the reproductive ducts of both female and male cephalopod Octopus vulgaris. Cell bodies and fibers were immunolocalized in the fusiform ganglion from which the nerves that reach the female and male reproductive ducts arise. FMRFamide-like and cGnRH-I-like immunoreactive nerve endings were present in the oviduct, and in the oviducal gland of the female and in the seminal vesicle of the male. The GnRH-like peptide from the reproductive ducts has been partially characterized by HPLC. The retention time of the Octopus vulgaris GnRH-like peptide was similar to the retention time of cGnRH-I. Based on these observations we suggest that FMRFamide-like and a novel GnRH-like peptide are involved in the control of reproductive ducts of Octopus vulgaris. One possibility is that the peptides affect gamete transport. Another possibility is that they regulate secretory products such as mucus and mucilaginous substances from the oviducal gland and the seminal vesicle. Our data provide further evidence to support the hypothesis of the existence of a central and peripheral peptidergic control of reproduction of Octopus vulgaris.

Evidence for gonadotropin-releasing hormone-like peptides in a cnidarian nervous system

There is increasing evidence that peptides of the gonadotropin-releasing hormone (GnRH) family, long considered a vertebrate preserve, are also present in invertebrate (molluscan) nervous systems. The possibility was examined that GnRHs are present and bioactive in cnidarians, considered to be representatives of the most primitive animals possessing a nervous system. Immunoreactive GnRH was detected in endodermal neurons of two anthozoans, the sea pansy Renilla koellikeri and the sea anemone Nematostella vectensis. In the sea pansy, immunoreactivity was detected throughout the autozooid polyps, including gamete-producing endoderm. High-performance liquid chromatography and radioimmunoassays of extracts from whole sea pansy colonies yielded two elution peaks exhibiting GnRH immunoreactivity with antisera raised against shark or mammalian GnRH. Vertebrate GnRHs as well as the two sea pansy GnRH-like factors inhibited the amplitude and frequency of peristaltic contractions in the sea pansy, and these actions were blocked by the LHRH analog [D-pGlu(1),D-Phe(2),D-Trp(3,6)]-LHRH. These results suggest that the GnRH family of neuropeptides is more widespread in metazoans than previously thought. Although our physiological data are preliminary, they point to a role for GnRHs as inhibitory modulators of neuromuscular transmission in the sea pansy.

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.

Mammalian and chicken I forms of gonadotropin-releasing hormone in the gonads of a protochordate, Ciona intestinalis

Two forms of gonadotropin-releasing hormone (GnRH) were isolated from the gonads of the tunicate, Ciona intestinalis. The primary structure of the purified peptides was determined by MS and chemical sequence analysis. Both GnRH forms have blocked NH(2) and COOH termini, and their primary structures are identical to mammalian (mGnRH) and chicken I (cGnRH-I) forms reported previously in vertebrates. A total of 1.2 mg of purified cGnRH-I and 0.98 mg of mGnRH was obtained from 100 g of Ciona gonads. The physiological effects of native GnRHs included the induction of synthesis and secretion of sex steroids from ciona gonads and the secretion of luteinizing hormone from rat pituitary. These results suggest that the primary structure and functional roles of mGnRH and cGnRH-I have been highly conserved throughout evolution of chordates.

Gonadotropin-releasing hormone in mulberry cells of Saccoglossus and Ptychodera (Hemichordata: Enteropneusta)

Mulberry cells are epidermal gland cells bearing a long basal process resembling a neurite and are tentatively regarded as neurosecretory cells. They occur scattered through the ectoderm of the proboscis, collar, and anterior trunk regions of the acorn worms Saccoglossus, usually in association with concentrations of nervous tissue. They contain secretion granules that appear from electron micrographs to be released to the exterior. The granules are immunoreactive with antisera raised against mammalian and salmon gonadotropin-releasing hormone (GnRH). Similar results were obtained with another enteropneust, Ptychodera bahamensis, using antisera raised against tunicate-1 and mammalian GnRH. Mulberry cells were not found in either Cephalodiscus or Rhabdopleura (Hemichordata: Pterobranchia). Extracts of tissues from 4200 Saccoglossus contain an area of immunoreactive GnRH that is detected by an antiserum raised against lamprey GnRH when characterized by high-performance liquid chromatography and radioimmunoassay. This is the first report of the occurrence of GnRH in hemichordates, probably the most primitive group clearly belonging to the chordate lineage. The physiological function of GnRH in enteropneusts is unknown, but an exocrine function appears more likely than an endocrine or neurotransmitter role.

Gonadotropin-releasing hormone neuronal system of the freshwater snails Helisoma trivolvis and Lymnaea stagnalis: possible involvement in reproduction

Peptides of the gonadotropin-releasing hormone (GnRH) family are present in neural and nonneural tissues throughout the chordate phylum. Although GnRH peptides have been implicated in nonreproductive functions, their primary function is to control reproduction by regulating sexual behaviors and inducing gonadotropin hormone release from the pituitary. Evidence suggesting the presence of a similar peptide in the central nervous system (CNS) of the gastropod mollusc Helisoma trivolvis has recently been provided. In the present study, we examined the tissue distribution of the peptide and found that it is likely restricted to the nervous system. The neuronal system containing the endogenous GnRH-like peptide is described further and is shown, in part, to innervate the male reproductive tract. Immunostaining in the closely related snail, Lymnaea stagnalis, showed a conservation in the locations of some immunoreactive neurons. Notably, staining occurred in and adjacent to the lateral lobes of both snails. Because these lobes contain neurons involved in the stimulation of egg laying and GnRH staining occurred in additional areas in the Helisoma CNS that are involved in reproduction, we suggest that the endogenous GnRH-like peptide plays a role in regulating reproduction in freshwater snails.

Effects of five natural gonadotropin-releasing hormones on cell suspensions of marine bivalve gonad: stimulation of gonial DNA synthesis

Gonadotropin-releasing hormones (GnRH) constitute a family of neuropeptides which are important regulators of reproduction in vertebrates. The effect of mammalian GnRH (mGnRH), salmon GnRH, chicken GnRH-I, chicken GnRH-II, and lamprey GnRH-I on [3H]thymidine incorporation into DNA of dissociated gonadal cells of marine bivalves has been studied. The incorporation of [3H]thymidine is linear between 1.5 and 8 h of incubation. All five GnRHs significantly increased DNA synthesis in gonial cells of Crassostrea gigas. The maximal activation was about of 135-140% above control. The activation is dose dependent, over the range 10(-11) to 10(-6) M, but is modulated by the physiological condition of the cells and the stage of sexual maturity of the gonad. mGnRH has also a mitogenic effect in dissociated mantle cells of Mytilus edulis. The effect of mGnRH is blocked by a GnRH antagonist ([D-pGlu1,D-Phe2, D-Trp3,6]GnRH, 5 x 10(-6)M) in C. gigas as well as in M. edulis, suggesting that the GnRH action in the gonad is mediated by specific receptors for GnRH or GnRH-like peptides. The existence of GnRH-immunoreactive neurons and fibers in the cerebral and pedal ganglia of M. edulis was demonstrated by immunocytochemistry. They are located principally in the anterior internal area of the cerebral ganglia, close to the cerebral commissure and in the posterior part of the pedal ganglia. The presence of GnRH-responsive cells and GnRH-like immunoreactive material suggests that peptides of the GnRH-like family are present and functional in bivalve molluscs.

Neuropeptidergic control of the optic gland of Octopus vulgaris: FMRF-amide and GnRH immunoreactivity

In cephalopods, the endocrine optic glands on the optic tract control the maturation of the gonads. The glands are innervated by the optic gland nerve, which originates in the central nervous system. To explore the involvement of neuropeptides in the nervous control of the optic gland of Octopus vulgaris, the presence and distribution of Phe-Met-Arg-Phe-NH2 (FMRF-amide)-like and gonadotropin releasing homone (GnRH)-like peptides were examined in the central nervous system and optic gland by immunohistochemistry. For GnRH immunodetection, antibodies against four different forms of GnRH were used: cGnRH-I, cGnRH-II, sGnRH, and mGnRH. The optic gland nerve provides direct and indirect signals coming from the centres of integration of chemical, visual, and olfactive stimuli to modulate the glandular activity. In these centres, the subpedunculate area, the olfactory and optic lobes, and FMRF-amide-like and GnRH-like immunoreactivities were detected. The subpedunculate area seems to be the source of the FMRF-amide-like peptide, whereas the posterior olfactory lobule is the source of the GnRH-like peptide. The immunoreactive fibres for both neuropeptides leave their sources and directly enter the optic gland nerve. FMRF-amide- and GnRH-immunoreactive nerve endings are seen on the glandular cells. The evidence of a possible neuropeptidergic control of optic gland activity reinforces the analogies and the functional parallels in the octopus, insect, crustacean, and vertebrate hormonal systems.

Mammalian gonadotropin-releasing hormone (GnRH) identified by primary structure in Russian sturgeon, Acipenser gueldenstaedti

The mammalian form of gonadotropin-releasing hormone (GnRH) was purified from the brains of Russian sturgeon, Acipenser gueldenstaedti, using reversed-phase high pressure liquid chromatography (HPLC). The total concentration of mGnRH within these fish was 5.4 ng/brain. Small amounts of immunoreactive chicken GnRH-II like molecules were also detected but at insufficient quantities for purification. The primary structure of mGnRH was determined using automated Edman degradation. Because sequence data could not be obtained until after digestion by bovine pyroglutamyl amino-peptidase, it was determined that the amino-terminal residue was modified. Furthermore, mass spectrometric data and co-elution with synthetic mGnRH on HPLC confirmed that the carboxy-terminal residue was amidated. The amino acid sequence of sturgeon GnRH is pGlu-His-Trp-Ser-Tyr-Gly-Leu-Arg-Pro-Gly-NH2.