Isolation, characterization and expression of cDNAs encoding the catfish-type and chicken-II-type gonadotropin-releasing-hormone precursors in the African catfish

Molecular cloning and characterization of a gonadotropin-releasing hormone 2 precursor cDNA in the catfish Heteropneustes fossilis: Expression profile and regulation by ovarian steroids

Gonadotropin-releasing hormone 2 (Gnrh2) is one of the three classes of Gnrh distributed in vertebrates and is highly conserved. In the present study, the cDNA encoding Gnrh2 was isolated and characterized in the ostariophysan catfish Heteropneustes fossilis (hf). The cDNA is 611 bp long with an open reading frame (ORF) of 261 bp that encodes a highly conserved protein of 86 amino acids. The deduced Gnrh2 precursor protein clustered with the vertebrate Gnrh2 type. The sequence identity of hfgnrh2 is 94% with African catfish (Clarias gariepinus) gnrh2 mRNA (accession no. X78047). The hfgnrh2 transcripts were expressed only in the brain and gonads with a higher expression in the female brain and ovary in both resting and prespawning phases. The expression was higher in the prespawning phase than the resting phase. The gnrh2 expression in the brain and ovary showed significant seasonal variations but with opposite patterns. In the brain, the expression was the highest in the preparatory phase, decreased progressively to low levels in the postspawning and resting phases. In the ovary, the transcript level was low in the resting and preparatory phases, increased sharply in the prespawning phase reaching the peak level in the spawning phase and declined sharply in the postspawning phase. The gnrh2 mRNA showed the highest expression in the hind brain-medulla oblongata and moderate to low expression in forebrain regions and pituitary. Ovariectomy resulted in a duration-dependent inhibition of hfgnrh2 mRNA levels in the resting and prespawning phases. Steroid (E₂, testosterone and progesterone) replacement treatments (0.5 μg/g body weight) in the 3- week ovariectomized fish restored the inhibition due to ovariectomy, elevated the expression over and above the sham level in the resting phase (E₂ group), and raised the levels almost to that of the sham group (testosterone and progesterone groups) in the prespawning phase. In the sham control groups, the steroid replacement resulted in a significant reduction in the mRNA levels. The expression of the gnrh2 mRNA in the brain-pituitary-gonadal axis and its regulation by gonadal steroids suggest that Gnrh2 may have a reproductive role in the catfish.

Characterization of the gonadotropin-releasing hormone system in the Neotropical teleost, Steindachneridion parahybae during the annual reproductive cycle in captivity

This study evaluated by immunohistochemical and Western blot methods, the distribution of two distinct gonadotropin-releasing hormones (GnRHs), corresponding to catfish GnRH (cfGnRH or GnRH1) and chicken-II GnRH (cGnRH-II or GnRH2), in Steindachneridion parahybae females in captivity, focusing these analyses on the reproductive cycle by semi-quantification of optical density (OD). Further, we found that the GnRH neuronal systems co-localized with their respective GnRH-associated peptides (GAPs). A group of neurons immunoreactive (ir) to GnRH1 were identified along the ventral region of the olfactory bulb (vOB) in the telencephalon (vTel) and in the main areas of the diencephalon (especially the medial basal hypothalamus, HBM), including fibers extending into the pituitary gland. In contrast, GnRH2 neurons were confined to the midbrain tegmentum, close to the ventricular surface, without projections to the pituitary gland. Moreover, a cfGAP (GnRH1)-specific band (9 kDa) was identified in the brain and pituitary gland, while a cGAP-II (GnRH2)-specific band (26 kDa) was observed only in the brain extract. During the reproductive cycle, GnRH1-ir presented greater OD values at the vitellogenic and regression stages than at the previtellogenic stage and after artificially induced to spawn. Larger GnRH2-ir neurons were observed during the reproductive cycle, but a higher OD was identified only in the regression stage compared with the other maturation stages. Finally, GnRH1 axons were found to be directed towards the pituitary, and this GnRH type, which is probably the hypophysiotropic form, can contribute to the reproductive dysfunction that occurs in S. parahybae females in captivity, whereas GnRH2 may act as a neuromodulator and/or neurotransmitter.

Characterization and transcriptional profiles of Engraulis encrasicolus' GnRH forms

The European anchovy Engraulis encrasicolus, a member of the Clupeiformes order, holds a great biological and economical importance. In the past, this species was mostly investigated with the aim of assessing its reproductive biology, trophic ecology, population dynamics and the relations existing with the physical environment. At present days, though, an almost complete lack of information afflicts its neuroendocrinology and reproductive physiology. The hypothalamic-pituitary-gonadal (HPG) axis at its highest levels was herein investigated. In this study, the gonadotropin-releasing hormone (GnRH), a neuropeptide underlying many reproduction-related processes, the most critical of which is the stimulation of gonadotropin synthesis and secretion from the pituitary gland, was cloned. Three forms (salmon GnRH, chicken-II GnRH and the species-specific type) were characterized in their full-length open-reading frames and, in accordance with other Clupeiformes species, the distinctive one was found to be the herring-type GnRH. We qualitatively and semiquantitatively evaluated the localizations of expressions and the temporal transcription patterns of the three GnRH forms in male and female specimens throughout their reproductive cycle as well as described their phylogeny with regard to teleost GnRH lineages, and, specifically, to other Clupeiformes species.

Identification and distribution of three gonadotropin-releasing hormone (GnRH) isoforms in the brain of a clupeiform fish, Engraulis japonicus

To gain a better understanding of the reproductive endocrinology of a primitive order clupeiform fish (Japanese anchovy, Engraulis japonicus), cDNAs encoding three gonadotropin-releasing hormone (GnRH) isoforms were isolated from the brain, and their distribution was analyzed using insitu hybridization (ISH). The three GnRH isoforms include GnRH1 (herring GnRH), GnRH2 (chicken GnRH-ll) and GnRH3 (salmon GnRH), and their full-length cDNAs encode 88, 86, and 89 deduced amino acids (aa), respectively. Alignment analysis of Japanese anchovy GnRH isoforms showed lower identities with other teleost fish. The major population of GnRH1 neurons was localized in the ventral telencephalon (VT) and nucleus preopticus (NPO) of the preoptic area (POA) with minor population in the anterior olfactory bulb (OB). GnRH2 neurons were restricted to the midbrain tegmentum (MT), specific to the nucleus of the medial longitudinal fasciculus (nMLF). GnRH3 neurons were localized in the olfactory nerve (ON), ventral OB, and transitional area between OB and ON. Interestingly, GnRH1 neurons were also localized in the olfactory bulb, in addition to its major population in the preoptic area. These results indicate the differential distribution of three GnRH isoforms expressed in the brain of the Japanese anchovy.

Gonadotrophin-releasing hormone in winter flounder (Pseudopleuronectes americanus): molecular characterization, distribution and effects of fasting

Gonadotrophin-releasing hormone (GnRH) is primarily related to reproductive processes in vertebrates. However other physiological roles, including functions in food intake regulation and energy status, have been demonstrated for GnRH in animals. The ten amino acid active peptide is relatively conserved throughout chordates, more specifically in fish species. Teleosts generally have at least two variants of GnRH present in their genomes. GnRH2 (commonly termed chicken-GnRH) is common to all fish, whereas other prevalent forms include GnRH1 and/or GnRH3 (also known as salmon-GnRH). The mRNAs of all three forms were identified in winter flounder (Pseudopleuronectes americanus). Winter flounder GnRH1 appears to be ubiquitously and strongly expressed throughout the brain. GnRH2 mRNA is highly expressed in the optic tectum/thalamus. Finally, GnRH3 mRNA is expressed throughout the brain, but not in the pituitary, with apparent highest expression in the telencephalon/preoptic area. Flounder GnRH1 mRNA is found in most peripheral tissues examined, including the foregut, midgut and gonads. GnRH2 mRNA appears to be expressed throughout the periphery, with apparent highest transcript expression in male gonads. Finally, winter flounder GnRH3 transcript is found at low levels in the skin, heart, and gonads. The effect of fasting on the expression of each of the three isoforms was assessed. Fasting reduces GnRH2 and GnRH3 mRNA expression in the optic tectum/thalamus and hypothalamus, and telencephalon/preoptic area, respectively, compared with fed fish. GnRH1 mRNA expression does not appear to be altered by feeding status. GnRH mRNAs do not seem to regulate food intake peripherally through the gut based on our preliminary findings. Our preliminary results suggest that the GnRH system could play a central role in food intake regulation of winter flounder.

Discovery of a new reproductive hormone in teleosts: pituitary adenylate cyclase-activating polypeptide-related peptide (PRP)

Pituitary adenylate cyclase-activating polypeptide (PACAP)-related peptide (PRP) is a peptide encoded with PACAP in the same precursor protein. Non-mammalian PRPs were previously termed growth hormone-releasing hormone (GHRH)-like peptide, and was regarded as the mammalian GHRH homologue in non-mammalian vertebrates until the discovery of authentic GHRH genes in teleosts and amphibians. Although a highly specific receptor for PRP, which is lost in mammals, is present in non-mammals, a clear function of PRP in vertebrates remains unknown. Using goldfish as a model, here we show the expression of PRP and its cognate receptor in the brain-pituitary-gonadal (BPG) axis, thus suggesting a function of goldfish (gf) PRP in regulating reproduction. We found that gfPRP controls the expression of reproductive hormones in the brain, pituitary and ovary. Goldfish PRP exerts stimulatory effects on the expression of salmon gonadotropin-releasing hormone (sGnRH) in the brain, follicle-stimulating hormone (FSH) and luteinizing hormone (LH) in pituitary primary culture cells, but inhibits the expression of LH in the ovary. Using the same technique, we showed that gfPRP did not alter the mRNA level of growth hormone in the pituitary primary culture. In summary, we have discovered the first function of vertebrate PRP in regulating reproduction, which provides a new research direction in studying the neuroendocrine control of reproduction not only in teleosts, but also in other non-mammalian vertebrates.

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.

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.

A role forfoxd3andsox10in the differentiation of gonadotropin-releasing hormone (GnRH) cells in the zebrafishDanio rerio

Gonadotropin-releasing hormone (GnRH) is found in a wide range of vertebrate tissues, including the nervous system. In general, GnRH has two functions: endocrine, acting as a releasing hormone; and neuromodulatory,affecting neural activity in the peripheral and central nervous system. The best understood population of GnRH cells is that of the hypothalamus, which is essential for reproduction. Less well understood are the populations of GnRH cells found in the terminal nerve and midbrain, which appear to be neuromodulatory in function. The GnRH-containing cells of the midbrain are proposed to arise from the mesencephalic region of the neural tube. Previously, we showed that neuromodulatory GnRH cells of the terminal nerve arise from cranial neural crest. To test the hypothesis that neuromodulatory GnRH cells of the midbrain also arise from neural crest, we used gene knockdown experiments in zebrafish to disrupt neural crest development. We demonstrate that decrement of the function of foxd3 and/or sox10, two genes important for the development and specification of neural crest, resulted in a reduction and/or loss of GnRH cells of the midbrain, as well as a reduction in the number of terminal nerve GnRH cells. Therefore, our data support a neural crest origin for midbrain GnRH cells. Additionally, we demonstrate that knockdown of kallmann gene function resulted in the loss of endocrine GnRH cells of the hypothalamus, but not of neuromodulatory GnRH cells of the midbrain and terminal nerve, thus providing additional evidence for separate pathways controlling the development of neuromodulatory and endocrine GnRH cells.

Developmental Expression of Three Forms of Gonadotropin-Releasing Hormone and Ontogeny of the Hypothalamic-Pituitary-Gonadal Axis in Gilthead Seabream (Sparus aurata)1

To address the complexity of the origin of the GnRH system in perciforms, we investigated the ontogenic expression of three GnRHs in gilthead seabream. Using in situ hybridization, chicken (c) GnRH-II mRNA-expressing cells were detected in the hindbrain at 1.5 days postfertilization (DPF) and in the midbrain at 2 DPF and thereafter; the hindbrain signals became undetectable after 10 DPF. Salmon (s) GnRH mRNA-expressing cells were first seen in the olfactory placode at 3 DPF, started caudal migration at 14 DPF, and reached the preoptic areas at 59 DPF. Seabream (sb) GnRH mRNA-expressing cells were first detected in the terminal nerve ganglion cells (TNgc), ventral part of the ventral telencephalon, nucleus preopticus parvocellularis, and thalamus at 39 DPF, and extended to the nucleus preopticus magnocellularis at 43 DPF, ventrolateral hypothalamus at 51 DPF, and nucleus lateralis tuberis and posterior tuberculum at 59 DPF. Coexpression of sbGnRH and sGnRH transcripts was found in the TNgc. Using real-time fluorescence-based quantitative polymerase chain reaction, transcript levels of cGnRH-II and sGnRH were first detected at 1 and 1.5 DPF, respectively, and increased and remained high thereafter. Transcript levels of sbGnRH remained low after first detection at 1 DPF. Furthermore, these GnRH expression profiles were correlated with the expression profiles of reproduction-related genes in which at least four concomitant increases of GnRH, GnRH receptor, gonadotropin, gonadotropin receptor, and Vasa transcripts were found at 5, 8, 14, and 28 DPF. Our data provide an expanded view of the ontogeny of the GnRH system and reproductive axis in perciforms.

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.

Identification and characterization of the reptilian GnRH-II gene in the leopard gecko, Eublepharis macularius, and its evolutionary considerations

To elucidate the molecular phylogeny and evolution of a particular peptide, one must analyze not the limited primary amino acid sequences of the low molecular weight mature polypeptide, but rather the sequences of the corresponding precursors from various species. Of all the structural variants of gonadotropin-releasing hormone (GnRH), GnRH-II (chicken GnRH-II, or cGnRH-II) is remarkably conserved without any sequence substitutions among vertebrates, but its precursor sequences vary considerably. We have identified and characterized the full-length complementary DNA (cDNA) encoding the GnRH-II precursor and determined its genomic structure, consisting of four exons and three introns, in a reptilian species, the leopard gecko Eublepharis macularius. This is the first report about the GnRH-II precursor cDNA/gene from reptiles. The deduced leopard gecko prepro-GnRH-II polypeptide had the highest identities with the corresponding polypeptides of amphibians. The GnRH-II precursor mRNA was detected in more than half of the tissues and organs examined. This widespread expression is consistent with the previous findings in several species, though the roles of GnRH outside the hypothalamus-pituitary-gonadal axis remain largely unknown. Molecular phylogenetic analysis combined with sequence comparison showed that the leopard gecko is more similar to fishes and amphibians than to eutherian mammals with respect to the GnRH-II precursor sequence. These results strongly suggest that the divergence of the GnRH-II precursor sequences seen in eutherian mammals may have occurred along with amniote evolution.

Cell migration and evolutionary significance of GnRH subtypes

Hypothetically it can be assumed that in advanced teleost fishes, GnRH-III and GnRH-IV neurons migrate along the 'telencephalonic' (anterior) and 'diencephalonic' (posterior) migratory route, which perhaps fuses in primitive teleost fishes and land vertebrates to form the 'ancient migratory route' (in all probability = nervus terminalis; see Von Bartheld et al., 1988) of GnRH-I neurons. The difference in distribution pattern of GnRH forms in the vertebrate brain is due to distinct embryonic origins: (1) Cells of olfactory origin, which give rise to GnRH-I (salmon, catfish, chicken I, mammalian GnRH) are distributed along the olfactory system and the basal forebrain in primitive fishes and in land vertebrates; GnRH-I might be pivotal for LH/FSH synthesis-release, olfaction and metamorphosis in lower vertebrates. In advanced teleost fishes, neurons synthesizing GnRH-III ('salmon' GnRH) originate from the olfactory system; they are distributed along the basal olfactory bulbs, with distinct ganglia (NOR) at the caudalmost part of the olfactory bulbs and few scattered cells in the basal telencephalon. The NOR might function as a neuromodulator, hypophysiotropic hormone and regulate visual associated reproductive behaviors. (2) Cells of mesencephalonic origin, which give rise to GnRH-II (chicken-II GnRH) are evolutionarily conserved; might function as a neuromodulator involved in motor-associated reproductive behaviors and acid-base balance. (3) Cells of diencephalonic origin, which give rise to GnRH-IV (seabream, medaka GnRH); they are localized in the anterior-basal OVLT-POA area and present only in advanced teleost fishes. GnRH-IV has been implicated in gonadal sex differentiation, gonadal maturation, LH/FSH secretion and territorial behavior. Advance teleost fishes for yet unknown functions might have acquired GnRH-IV. Although all GnRH subtypes participate in some aspect of reproduction; the precise function of each GnRH form still remains unclear.

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.

Gonadotropin-releasing hormone does not directly stimulate luteinizing hormone biosynthesis in male African catfish

Besides gonadotropin release, GnRH stimulates gonadotropin subunit gene transcription and translation in gonadotrophs. In the African catfish, Clarias gariepinus, chicken GnRH-II (cGnRH-II: [His5,Trp7,Tyr8]-GnRH) and catfish GnRH (cfGnRH: [His5,Asn8]-GnRH) are two endogenous forms of GnRH. Studying their effects on LH subunit steady-state mRNA levels, LH de novo synthesis, and LH release in primary pituitary cell cultures of adult males, we found that cGnRH-II hardly influenced the steady-state levels of LH subunit mRNAs or LH de novo synthesis, although it stimulated LH release. Although cfGnRH stimulated LH secretion as well, high concentrations-although apparently still within the physiologic range-reduced LH transcript levels and de novo synthesis in primary pituitary cell cultures. In vivo experiments demonstrated a biphasic response of LH subunit transcript levels after a single GnRH injection: a decrease after 2 h was followed by an increase at 8 h. When the testes were removed before GnRH treatment, however, LH transcript levels remained depressed at 8 h after GnRH injection, indicating that the secondary increase in LH transcript levels depends on the presence of the testes. We conclude that the up-regulation of LH production subsequent to GnRH stimulation in adult male African catfish is mediated by factors originating from the testis. Previous work suggests that aromatizable androgens may play an important role in this context. Under the present experimental conditions, however, GnRHs had no, or an inhibitory, direct effect on LH production in catfish gonadotrophs.

Molecular cloning of three cDNAs encoding different GnRHs in the brain of barfin flounder

To examine the reproductive endocrinology of a large pleuronectiform fish, barfin flounder, Verasper moseri, a promising candidate for aquaculture and resource enhancement in northern Japan due to its high commercial value, three gonadotropin-releasing hormones (GnRHs) in the brain was identified by isolation of their cDNAs. This species had three molecular forms of GnRH; salmon GnRH (sGnRH), chicken GnRH-II (cGnRH-II), and seabream GnRH (sbGnRH). Each GnRH cDNA encoded a signal peptide (SP), GnRH, and a GnRH-associated peptide (GAP), which was connected to GnRH by a Gly-Lys-Arg sequence. The sGnRH cDNA encoded an SP composed of 23 amino acids and a GAP composed of 54 amino acids. The cGnRH-II cDNA encoded an SP of 23 amino acids and a GAP of 49 amino acids. The sbGnRH cDNA encoded an SP of 26 amino acids and a GAP of 57 amino acids. In situ hybridization showed that the genes for sGnRH, cGnRH-II, and sbGnRH are expressed in the ventromedial olfactory bulbs and the terminal nerve ganglion, the midbrain tegmentum, and the preoptic area, respectively. These results indicate that sbGnRH neurons in the preoptic area are involved in gonadotropin secretion in barfin flounder.

Evolutionary development of three gonadotropin-releasing hormone (GnRH) systems in vertebrates

Gonadotropin-releasing hormone (GnRH) is the neuropeptide that links the brain to the reproductive system. Most vertebrate species express two forms of GnRH, which differ in amino acid sequence, localization, distribution, and embryological origin. The GnRH system in the ventral forebrain produces a species-specific GnRH form and projects toward the gonadotropic cell in the pituitary. The GnRH neurons of this system originate from the olfactory placode and migrate into the brain during early development. The other GnRH system is localized in a nucleus in the midbrain, where large cells express chicken-GnRH-II, of which the function is still unclear. In modern teleosts, a third GnRH system is present in the terminal nerve, which contains salmon GnRH. The three GnRH systems appear at different times during fish evolution. Besides the two accepted lineages in GnRH evolution (of conserved chicken GnRH-II in the midbrain and of mammalian GnRH or species-specific GnRH in the hypophysiotropic system), we propose a third lineage: of salmon GnRH in the terminal nerve.

Development of three distinct GnRH neuron populations expressing two different GnRH forms in the brain of the African catfish (Clarias gariepinus)

The early development of both the catfish gonadotropin-releasing hormone (cfGnRH)- and the chicken GnRH-II (cGnRH-II) system was investigated in African catfish by immunocytochemistry by using antibodies against the GnRH-associated peptide (GAP) of the respective preprohormones. Weakly cfGnRH-immunoreactive (ir) neurons and fibers were present at 2 weeks after hatching (ph) but only in the ventral telencephalon and pituitary. Two weeks later, cfGnRH fibers and neurons were also observed in more rostral and in more caudal brain areas, mainly in the preoptic area and hypothalamus. Based on differences in temporal, spatial, and morphologic appearance, two distinct cfGnRH populations were identified in the ventral forebrain: a population innervating the pituitary (ventral forebrain system) and a so-called terminal nerve (TN) population. DiI tracing studies revealed that the TN population has no neuronal connections with the pituitary. The cGnRH-II system is present from 2 weeks ph onward in the midbrain tegmentum and only their size and staining intensity increased during development. Based on the comparison of GnRH systems amongst vertebrates, we hypothesize that during fish evolution, three different GnRH systems evolved, each expressing their own molecular form: the cGnRH-II system in the midbrain, a hypophysiotropic GnRH system in the hypothalamus with a species-specific GnRH form, and a salmon GnRH-expressing TN population. This hypothesis is supported by phylogenetic analysis of known GnRH precursor amino acid sequences. We hypothesize, because the African catfish is a less advanced teleost species, that it contains the cfGnRH form both in the ventral forebrain system and in the TN population.

Gonadal steroids and the maturation of the species-specific gonadotropin-releasing hormone system in brain and pituitary of the male African catfish (Clarias gariepinus)

The effect of testosterone (T), 11-ketotestosterone (KT) and estradiol (E(2)) on the development of the catfish gonadotropin-releasing hormone system (cfGnRH) of male African catfish (Clarias gariepinus), at the onset of puberty [between 10 and 12 weeks post hatching (ph)] was investigated. The cfGnRH neurons, located in the ventral forebrain, were visualized by immunofluorescence and their numbers were determined and the amounts of cfGnRH-associated peptide (cfGAP) in the pituitary were measured by RIA. Steroid treatments did not significantly alter the numbers of immunoreactive GnRH neurons. However, T and E(2) caused an increase in the amount of GnRH, demonstrated by the intensity of the immunostaining of GnRH neurons and fibers in the brain and the amount of cfGAP in the pituitary. Treatment with KT, the main circulating androgen in adult male catfish, neither changed the number of cfGnRH neurons, nor elevated the cfGnRH content in the pituitary. In previous experiments with younger, prepubertal fish (2-6 weeks ph), T caused an elevation of the number of cfGnRH neurons to the same level as present in pubertal fish of 12-14 weeks. We conclude that the onset of puberty in the male African catfish coincides with the completion of the steroid-dependent structural maturation of the cfGnRH system in the brain. T and/or E(2), however, are still able to exert a positive influence on the amounts of cfGnRH during the later stages of pubertal development, thus still playing a role in the control of the cfGnRH system.

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.

Molecular cloning, distribution and pharmacological characterization of a novel gonadotropin-releasing hormone ([Trp8] GnRH) in frog brain

To date nine structural variants of GnRH have been identified in vertebrates and two additional forms have been isolated from a tunicate. In amphibians only mammalian GnRH ([Arg8] GnRH) and type II GnRH (chicken GnRH II, [His5, Trp7, Tyr8] GnRH) have been identified. In the present study, a full-length cDNA encoding a novel type of GnRH was isolated from pituitary of Rana dybowskii. The GnRH gene encodes a GnRH peptide ([Trp8] GnRH) in which tryptophan is substituted for arginine of mammalian GnRH Northern blot analysis revealed the presence of a single 500 bp transcript for the [Trp8] GnRH precursor in forebrain but its absence in testis, ovary, kidney and liver. Restriction digests of genomic DNA demonstrated a single copy of the gene. The [Trp8] GnRH immunoreactive cells were identified in the preoptic area of the frog brain. Synthetic [Trp8] GnRH was tested for its ability to stimulate inositol phosphate production by COS-1 cells transfected with the cloned Xenopus pituitary GnRH receptor and the cloned human GnRH receptor. [Trp8] GnRH had a potency of about 60% compared with mammalian GnRH ([Arg8] GnRH) for the Xenopus receptor, whereas the potency of [Trp8] GnRH was approximately 5% compared with mammalian GnRH for the human receptor. Both mammalian GnRH and [Trp8] GnRH were 1000-fold less potent than type II GnRH for the Xenopus GnRH receptor. The similar potency of [Arg8] GnRH and the novel [Trp8] GnRH for the Xenopus pituitary receptor indicates that, unlike the human receptor, the Xenopus receptor does not discriminate between these amino acids in position eight thereby allowing substitution of the arginine in the mammalian GnRH.

Gonadotropin-releasing hormones in the brain and pituitary of the teleost, the white sucker

The present study investigated GnRH forms within the brain of a representative of the order Cypriniformes, the white sucker, Catostomus commersoni, using HPLC, RIA, and immunocytochemistry. Several immunoreactive (ir) GnRH forms were identified in the brain of the white sucker by chromatography and radioimmunoassay, including ir-salmon GnRH, ir-lamprey GnRH-I and -III, and ir-chicken GnRH-II. Results from immunocytochemical studies were consistent with multiple GnRH forms distributed in different patterns, particularly for fibers. Neuronal perikarya containing ir-salmon GnRH and ir-lamprey-like GnRH were found laterally within the preoptic area and rostral hypothalamus. Cells containing exclusively ir-salmon GnRH appeared slightly more rostrally, but in the same region. Fibers containing ir-salmon GnRH and ir-lamprey-like GnRH were seen throughout the caudal telencephalon and extended into the diencephalon, toward the pituitary. Fibers containing ir-chicken-II-like GnRH were also seen in the caudal telencephalon, but were concentrated more dorsally in the diencephalon. Within the pituitary, fibers containing ir-salmon GnRH and ir-lamprey-like GnRH entered the neurohypophysis, but differed in their destinations. Fibers containing ir-salmon GnRH remained within the neurohypophysis, while fibers containing ir-lamprey-like GnRH targeted adenohypophyseal tissue. These findings are consistent with the hypothesis that multiple GnRH forms with multiple functions exist within the brain and pituitary of teleosts and provide further evidence of a lamprey-like GnRH within an early evolved teleost species.

Gonadotropin-releasing hormone genes: phylogeny, structure, and functions

Gonadotropin-releasing hormone (GnRH, previously called leutinizing hormone-releasing hormone, LHRH) is the final common signaling molecule used by the brain to regulate reproduction in all vertebrates. Recently, genes encoding two other GnRH forms have been discovered. Here we present a phylogenetic analysis that shows that the GnRH genes fall naturally into three distinct branches, each of which shares not only a molecular signature but also characteristic expression sites in the brain. The GnRH genes appear to have arisen through gene duplication from a single ancestral GnRH whose origin predates vertebrates. Several lines of data support this suggestion, including the fact that all three genes share an identical exonic structure. The existence of three distinct GnRH families suggests a new, natural nomenclature for the genes, and in addition, we present a logical proposal for naming the peptide sequences. The two recently discovered GnRH genes are unusual because they encode decapeptides that are identical in all the species in which they have been found. The control of gene expression also differs among the three gene families as might be expected since they have had separate evolutionary trajectories for perhaps 500 million years.

Testosterone accelerates the development of the catfish GnRH system in the brain of immature African catfish (Clarias gariepinus)

The effects of two endogenous steroids on the maturation of the catfish GnRH and the chicken GnRH-II system in the African catfish were investigated. Immature fish (2 weeks of age, which is before sexual differentiation; thus male and female genotypes present) were fed with food pellets containing either testosterone (T), 11beta-hydroxyandrostenedione (OHA) or no steroid (control). After 2 and 4 weeks of treatment, the effects on the two GnRH systems were investigated immunocytochemically, using specific antibodies against the respective GnRH-associated peptides. By means of fluorescence microscopy the number of GnRH perikarya and the cell surfaces were determined. Confocal laser scanning microscopy was applied to verify spatial distribution and staining intensity. After 2 weeks of treatment no difference in any of the parameters between the groups was observed. However, 4 weeks T treatment resulted in significantly more cfGnRH-ir perikarya in the brain compared to the OHA and control groups. In addition, in the T group the number of immunoreactive fibers was markedly higher and the staining of the perikarya and axons was more intense. The distribution of cfGnRH-ir neurons over the ventral forebrain differed between the two age groups: in 4-week-old fish, the largest concentration of neurons was localized in the ventral telencephalon, while 2 weeks later the number of neurons in the supraoptic area had markedly increased, suggesting that the cfGnRH system is still undergoing developmental changes during this period. In 6-week-old fish the average volume of the cfGnRH perikarya (expressed as surface size in the microscopical sections) in both the OHA and the T group was significantly bigger than that in the control group. The cGnRH-II-ir neurons in the midbrain tegmentum showed strong immunoreactivity in all groups, both treated and nontreated. In contrast to the cfGnRH neurons, the staining intensity and the number of cGnRH-II neurons did not change after steroid treatment. The results of this study show that T is able to accelerate the development of the cfGnRH system, whereas OHA has only minimal effects; the cGnRH-II system develops independent from these steroids.

Evolution of neuroendocrine peptide systems: gonadotropin-releasing hormone and somatostatin

Nine vertebrate and two protochordate gonadotropin-releasing hormone (GnRH) decapeptides have been identified and sequenced. Multiple molecular forms of GnRH peptide were present in the brain of most species examined, and cGnRH-II generally coexists with one or more GnRH forms in all the major vertebrate groups. The presence of multiple GnRH forms has been further confirmed by the deduced GnRH peptide structure from cDNA and/or gene sequences in several teleost species and tree shrew. High conservation of the primary structure of GnRH decapeptides and the overall structure of GnRH genes and precursors suggests that they are derived from a common ancestor. Somatostatin (SRIF) is a phylogenetically ancient, multigene family of peptides. A tetradecapeptide, SRIF (SRIF14) has been conserved, with the same amino acid sequence, in representative species of all classes of vertebrate. Four molecular variants of SRIF14 have been identified. SRIF14 is processed from preprosomatostatin-I, which contains SRIF14 at its C-terminus; preprosomatostatin-I is also processed to SRIF28 in mammals and SRIF26 in bowfin. Teleost fish possess a second somatostatin precursor, preprosomatostatin-II, containing [Tyr7, Gly10]-SRIF14 at the C-terminus, that is mainly processed into large forms of SRIF.

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

We sequenced the complementary DNA (cDNA) encoding guinea pig LHRH from an expression library derived from the preoptic area-anterior hypothalamus. Data from in situ hybridization and RNase protection assays verified that the cloned cDNA was complementary to guinea pig LHRH messenger RNA. The architecture of the deduced precursor resembles that of LHRH precursors identified in other species. In contrast, the predicted sequence of the decapeptide differs from mammalian LHRH by two amino acid substitutions in positions 2 and 7. This is a novel finding, because the amino acid sequence that comprises LHRH decapeptide is identical in all mammals studied to date. Moreover, the predicted substitution in amino acid position 2 is unique among vertebrates. A second observation of potential significance is the existence of two subspecies of LHRH messenger RNA differing only in the length of their 3' untranslated regions. These two transcripts were verified by sequence analysis of positive clones from the cDNA library and by RNase protection analysis of preoptic area-anterior hypothalamus extracts, and their presence is consistent with the two polyadenylation signals identified in the untranslated regions of the LHRH gene. Future studies will examine LHRH gene expression in guinea pigs, which like primates but unlike rats, have a true luteal phase as a component of their reproductive cycle.

Distinct efficacies for two endogenous ligands on a single cognate gonadoliberin receptor

A cDNA encoding a putative gonadoliberin receptor was cloned from the pituitary of the African catfish. Conceptual translation predicts a protein of 379 amino acids which shows typical characteristics of GTP-binding-protein-coupled receptors. The isolated cDNA was stable expressed in human embryonic kidney (HEK) 293 cells which were used for studies on gonadoliberin-activated second messenger systems (inositol phosphate production; increase in cAMP and/or intracellular Ca2+). The isolated cDNA encoded a functional receptor, designated catfish gonadoliberin receptor (cfGnRH-R), which had an amino acid sequence similarity of 38% with mammalian gonadoliberin receptors. In contrast to its mammalian counterparts which lack an intracellular carboxy-terminal domain, the cfGnRH-R contains an additional 49 amino acid residues. From the two endogenous gonadoliberins in African catfish, chicken gonadoliberin-II had a several hundredfold higher potency than catfish gonadoliberin to activate cfGnRH-R-associated second messenger systems in transfected HEK 293 cells. This is in line with the previously determined higher gonadotropin-release capacity of chicken gonadoliberin-II in catfish. Stimulation of second messenger systems with chicken gonadoliberin-II, but not with catfish gonadoliberin, resulted in a biphasic effect and chicken gonadoliberin-II led to a higher maximum stimulation than catfish gonadoliberin. Challenging cfGnRH-R simultaneously with chicken gonadoliberin-II and catfish gonadoliberin did not lead to additive effects. In contrast, two types of mutual inhibitory effects were recorded. These data indicate that a single cognate cfGnRH-R couples with distinct efficacies to signal transduction systems upon stimulation by the two endogenous gonadoliberins which, in addition, may interact negatively.

Gonadotropin-releasing hormone gene expression in teleosts

Expression of multiple molecular forms of gonadotropin-releasing hormone (GnRH) mRNAs and GnRH peptides were examined in the brains of tilapia (Oreochromis mossambicus) and sockeye salmon (Oncorhynchus nerka), using in situ hybridization histochemistry and immunohistochemical techniques. After otherwise identical conditions, lesser background and stronger GnRH hybridization signals were observed on cryostat vs. paraffin sections. In both fresh and Bouin's-fixed paraffin-embedded tissues, there was a good correlation between the distribution of GnRH mRNA and GnRH peptide-containing cells. Although the brains of tilapia and the sockeye were immunoreactive to three forms of the GnRH molecule (salmon, mammal, chicken-II), GnRH mRNA expression was site-specific and species-specific. In the tilapia, ganglionic cells of the nucleus olfactoretinalis, basal telencephalon and the anteroventral preoptic area were immunoreactive to salmon-, and mammalian-GnRH peptide. Neurons of the nucleus olfactoretinalis expressed cichlid-GnRH I mRNA. The preoptic neurons, despite the immunoreactivity, expressed no hybridization signals. Midbrain neurons were immunoreactive to salmon-GnRH but expressed cichlid-GnRH II beta (= chicken-GnRH II) mRNA hybridization signals. In the sockeye, ganglionic cells along the extracerebral course of the nervus terminalis were immunoreactive to mammalian-, chicken-II and salmon-GnRH. These neurons expressed only salmon-GnRH mRNA hybridization signals. Intracerebral GnRH expression in the sockeye was delayed till smoltification. The basal telencephalon and midbrain neurons immunoreactive to salmon-GnRH, formed no hybridization signals with GnRH antisense probes. Oligonucleotide probes complementary to chicken-GnRH I and mammalian-GnRH revealed no hybridization signals in the tilapia and in the sockeye brain. Fibers, immunoreactive to salmon-, mammalian-, and chicken II-GnRH were seen in close association with growth hormone cells. Chicken-GnRH II-immunoreactive fibers were also seen in close proximity to somatolactin cells in the sockeye salmon.

Do gonadotrophin-releasing hormone neurons express estrogen receptors in the rainbow trout? A double immunohistochemical study

A double immunocytochemical procedure, with two different chromogens, was used to compare the respective distributions of estrogen receptor-immunoreactive cells and gonadotrophin-releasing hormone-immunoreactive neurons on the same sections of the brains of adult male and female rainbow trout (Oncorhynchus mykiss). Estrogen receptor-immunoreactive cells were observed in the ventral and lateral telencephalon, the preoptic region, the mediobasal hypothalamus, and the ventromedial thalamic nucleus. Gonadotrophin-releasing hormone-immunoreactive perikarya were detected in the olfactory bulbs, the ventral telencephalon, the preoptic area, and the mediobasal hypothalamus. Double-staining studies showed that, although some estrogen receptor-positive cells were in close proximity to gonadotrophin-releasing hormone-immunoreactive perikarya, careful examination of 550 gonadotrophin-releasing hormone-positive cells from five adult females and two adult males failed to demonstrate any evidence that gonadotrophin-releasing hormone neurons coexpress estrogen receptor in the brain of the rainbow trout. The present study provides, for the first time in teleosts, morphological evidence that gonadotrophin-releasing hormone neurons do not represent major direct targets for estradiol, suggesting that the positive feedback effects of estradiol onto the gonadotrophin-releasing hormone system are likely to be conveyed via other cell populations.

Characterization of the Pacific salmon gonadotropin-releasing hormone gene, copy number and transcription start site

Multiple forms of gonadotropin-releasing hormone (GnRH) have been shown to exist in all vertebrates examined except recently-evolved placental mammals. To study the origin and regulation of the GnRH genes in a Pacific salmon (Oncorhynchus nerka), we isolated and sequenced the salmon form of GnRH. The Southern blot shows a single band that strongly hybridizes to a probe for the gene reported here and weaker bands that may represent genes for related forms of GnRH. There is strong conservation of sequence in the hormone coding region and of the gene organization between fish and mammals. However, the GnRH-associated peptide (GAP) shows very little sequence identity with the mammalian GAPs, questioning its physiological role. We also show for the first time the transcriptional start site for a GnRH gene in a non-mammalian species. Interestingly, a large segment of 1152 nucleotides in the promoter region of the Pacific salmon GnRH gene is missing compared with the Atlantic salmon (Salmo salar) gene. These gene rearrangements suggest that these two salmonid species, which have been geographically separated for 8-15 million years, have evolved promoters with different mechanisms for control and transcription of GnRH.

Molecular cloning of three distinct cDNAs, each encoding a different adipokinetic hormone precursor, of the migratory locust, Locusta migratoria. Differential expression of the distinct adipokinetic hormone precursor genes during flight activity

Three distinct cDNAs encoding the preproadipokinetic hormones I, II, and III (prepro-AKH I, II, and III), respectively, of Locusta migratoria have been isolated and sequenced. The three L. migratoria AKH precursors have an overall architecture similar to that of other precursors of the AKH/red pigment-concentrating hormone (RPCH) family identified so far. The AKH I and II precursors of L. migratoria are highly homologous to the Schistocerca gregaria and Schistocerca nitans AKH precursors. Although the L. migratoria AKH III precursor appears to be the least homologous to the Manduca sexta, Drosophila melanogaster, and Carcinus maenas AKH/RPCH precursors, we favor the opinion that the L. migratoria AKH III precursor is evolutionary more related to the M. sexta, D. melanogaster, and C. maenas AKH/RPCH precursors than to the AKH I and II precursors of S. gregaria, S. nitans, or L. migratoria. In situ hybridization showed signals for the different AKH mRNAs to be co-localized in cell bodies of the glandular lobes of the corpora cardiaca. Northern blot analysis revealed the presence of single mRNA species encoding the AKH I precursor (approximately 570 bases), AKH II precursor (approximately 600 bases), and AKH III precursor (approximately 670 bases), respectively. Interestingly, flight activity increased steady-state levels of the AKH I and II mRNAs (approximately 2.0 times each) and the AKH III mRNA (approximately 4.2 times) in the corpora cardiaca.

Three gonadotropin-releasing hormone genes in one organism suggest novel roles for an ancient peptide

Gonadotropin-releasing hormone (GnRH) is known and named for its essential role in vertebrate reproduction. Release of this decapeptide from neurons in the hypothalamus controls pituitary gonadotropin levels which, in turn, regulate gonadal state. The importance of GnRH is underscored by its widespread expression and conservation across vertebrate taxa: five amino acids are invariant in all nine known forms, whereas two others show only conservative changes. In most eutherian mammals, only one form, expressed in the hypothalamus, is thought to exist, although in a recent report, antibody staining in developing primates suggests an additional form. In contrast, multiple GnRH forms and expression loci have been reported in many non-mammalian vertebrates. However, evidence based on immunological discrimination does not always agree with analysis of gene expression, since GnRH forms encoded by different genes may not be reliably distinguished by antibodies. Here we report the expression of three distinct GnRH genes in a teleost fish brain, including the sequence encoding a novel GnRH preprohormone. Using in situ hybridization, we show that this form is found only in neurons that project to the pituitary and exhibit changes in soma size depending on social and reproductive state. The other two GnRH genes are expressed in other, distinct cell populations. All three genes share the motif of encoding a polypeptide consisting of GnRH and a GnRH-associated peptide. Whereas the GnRH moiety is highly conserved, the GnRH-associated peptides are not, reflecting differential selective pressure on different parts of the gene. GnRH forms expressed in nonhypothalamic regions may serve to coordinate reproductive activities of the animal.

Evolutionary aspects of gonadotropin-releasing hormone and its receptor

1. Gonadotropin-releasing hormone (GnRH) was originally isolated as a hypothalamic peptide hormone that regulates the reproductive system by stimulating the release of gonadotropins from the anterior pituitary. However, during evolution the peptide was subject to gene duplication and structural changes, and multiple molecular forms have evolved. 2. Eight variants of GnRH are known, and at least two different forms are expressed in species from all vertebrate classes: chicken GnRH II and a second, unique, GnRH isoform. 3. The peptide has been recruited during evolution for diverse regulatory functions: as a neurotransmitter in the central and sympathetic nervous systems, as a paracrine regulator in the gonads and placenta, and as an autocrine regulator in tumor cells. 4. Evidence suggests that in most species the early-evolved and highly conserved chicken GnRH II has a neurotransmitter function, while the second form, which varies across classes, has a physiologic role in regulating gonadotropin release. 5. We review here evolutionary aspects of the family of GnRH peptides and their receptors.

Three forms of gonadotropin-releasing hormone characterized from brains of one species

Most vertebrate species have more than one form of gonadotropin-releasing hormone (GnRH) in their brains, but it is not clear whether each form has a distinct function. We report that sea bream (Sparus aurata) brains have three forms of GnRH, one of which is described herein and is called sea bream GnRH (sbGnRH). The primary structures of two forms were determined by Edman degradation and mass spectral analysis. The amino acid sequence of sbGnRH is pGlu-His-Trp-Ser-Tyr-Gly-Leu-Ser-Pro-Gly-NH2. The second peptide is identical to a form originally isolated from chicken brains (cGnRH-II): pGlu-His-Trp-Ser-His-Gly-Trp-Tyr-Pro-Gly-NH2. cGnRH-II is the most ancient form of GnRH identified to date in jawed fish and the most prevalent form throughout the vertebrates. The third form of GnRH has previously been identified as salmon GnRH by cDNA studies and is confirmed here by chromatographic and immunological studies. Phylogenetic distribution of GnRH peptides suggests sbGnRH arose in the perch-like fish as a gene duplication of the existing cGnRH-II or salmon GnRH genes. All three identified GnRH peptides were synthesized and shown to release gonadotropin in vivo in the sea bream. The dominant form of GnRH stored in the pituitary was sbGnRH. Not only was the content of sbGnRH 500-fold greater than that of salmon GnRH but also cGnRH-II was not detected in the pituitary. The latter evidence suggests that sbGnRH is the endogenous releaser of gonadotropin II.