Two GnRHs fluctuate in correlation with androgen levels in the male frog Rana esculenta

Celebrating 50+ years of research on the reproductive biology and endocrinology of the green frog: An overview

This issue is dedicated to the late Professor Giovanni Chieffi, and this article is an overview of the research on Comparative Endocrinology of reproduction using Rana esculenta (alias Pelophylax esculentus) as a model system. Starting from the early 1970s till today, a large quantity of work have been conducted both in the fields of experimental endocrinology and in the definition of the diffuse neuroendocrine system, with a major focus on the increasing role of regulatory peptides. The various aspects investigated concerned the histological descriptions of principal endocrine glands of the hypothalamic-pituitary-gonadal (HPG) axis, the localization and distribution in the HPG of several different substances (i.e. neurosteroids, hypothalamic peptide hormones, pituitary gonadotropins, gonadal sex steroids, and other molecules), the determination of sex hormone concentrations in both serum and tissues, the hormone manipulations, as well as the gene and protein expression of steroidogenic enzymes and their respective receptors. All together these researches, often conducted considering different periods of the annual reproductive cycle of the green frog, allowed to understand the mechanism of cascade control/regulation of the HPG axis of R. esculenta, characterizing the role of different hormones in the two sexes, and testing the hypotheses about the function of single hormones in different target organs. It becomes evident from the review that, in their simplest form, several features of this species are specular as compared to those of other vertebrate species and that reproduction in this frog species is either under endogenous multi-hormonal control or by a wide array of different factors. Our excursus of this research, spanning almost five decades, shows that R. esculenta has been intensively and successfully used as an animal model in reproductive endocrinology as well as several field studies such as those involving environmental concerns that focus on the effects of endocrine disruptors and other environmental contaminants.

Estradiol and reproduction in the South American toad Rhinella arenarum (Amphibian, Anura)

Rhinella arenarum is a South American toad with wide geographic distribution. Testes of this toad produce high amount of androgens during the non reproductive season and shift steroid synthesis from androgens to 5α-pregnanedione during the breeding. In addition, plasma estradiol (E₂) in males of this species shows seasonal variations but, since testes of R. arenarum do not express aromatase, the source of plasma E₂ remained unknown for several years. However, the Bidder's organ (BO), a structure located at one pole of each testis, is proposed to be the main source of E₂ in male's toads since it expresses several steroidogenic enzymes and is able to produce E₂ from endogenous substrates throughout the year. In addition, there were significant correlations between plasma E₂ and total activity of BO aromatase, and between plasma E₂ and the amount of hormone produced by the BO in vitro. In the toad, apoptosis induced by in vitro treatment with E₂ was mostly detected in spermatocytes during the breeding and in spermatids during the post-reproductive season, suggesting that this steroid has an important role in controlling spermatogenesis. However, in vitro treatment with E₂ had no effect on proliferation. This evidence suggests that the mechanism of action of E₂ on amphibian spermatogenesis is complex and more studies are necessary to fully understand the role of estrogens regulating the balance between cellular proliferation and apoptosis. In addition, in R. arenarum in vitro studies suggested that E₂ has no effect on CypP450c17 protein levels or enzymatic activity, while it reduces 3β-hydroxysteroid dehydrogenase/isomerase (3β-HSD/I) activity during the post reproductive season. As well, E₂ regulates FSHβ mRNA expression all over the year suggesting a down regulation process carried out by this steroid. The effect on LHβ mRNA is dual, since during the reproductive season estradiol increases the expression of LHβ mRNA while in the non-reproductive season it has no effect. In conclusion, the effect of E₂ on gonadotropins and testicular function is complex, not clearly understood and probably varies depending on the species. The aim of the current article is to review evidence on reproductive endocrinology and on the role of estradiol regulating reproduction in amphibians, with emphasis on the South American species Rhinella arenarum.

Kisspeptin regulates steroidogenesis and spermiation in anuran amphibian

Kisspeptin (Kp) system has a recognized role in the control of gonadotropic axis, at multiple levels. Recently, a major focus of research has been to assess any direct activity of this system on testis physiology. Using the amphibian anuran, Pelophylax esculentus, as animal model, we demonstrate - for the first time in non-mammalian vertebrate - that testis expresses both Kiss-1 and Gpr54 proteins during the annual sexual cycle and that ex vivo 17B-estradiol (E₂, 10^-6 M) increases both proteins over control group. Since the interstitium is the main site of localization of both ligand and receptor, its possible involvement in the regulation of steroidogenesis has been evaluated by ex vivo treatment of testis pieces with increasing doses of Kp-10 (10^-9-10^-6 M). Treatments have been carried out in February - when a new wave of spermatogenesis occurs - and affect the expression of key enzymes of steroidogenesis inducing opposite effects on testosterone and estradiol intratesticular levels. Morphological analysis of Kp-treated testes reveals higher number of tubules with spermatozoa detached from Sertoli cells than control group and the expression of connexin 43, the main junctional protein in testis, is deeply affected by the treatment. In spite of the effects on spermatozoa observed ex vivo, in vivo administration of Kp-10 has been unable to induce sperm release in cloacal fluid. In conclusion, we demonstrate Kp-10 effects on steroidogenesis with possible involvement in the balance between testosterone and estradiol levels, and report new Kp-10 activities on spermatozoa-Sertoli cell interaction.

Gonadotropin-inhibitory hormone (GnIH) in the amphibian brain and its relationship with the gonadotropin releasing hormone (GnRH) system: An overview

It is well known that the hypothalamic neuropeptide gonadotropin-releasing hormone (GnRH) plays an important role as a primary factor regulating gonadotropin secretion in reproductive processes in vertebrates. The discovery of the presence of a gonadotropin-inhibitory hormone (GnIH) in the brains of birds has further contributed to our understanding of the reproduction control by the brain. GnIH plays a key role in inhibition of reproduction and acts on the pituitary gland and GnRH neurons via a novel G protein-coupled receptor (GPR147). GnIH decreases gonadotropin synthesis and release, thus inhibiting gonadal development and maintenance. The GnRH and GnIH neuronal peptidergic systems are well reported in mammals and birds, but limited information is available regarding their presence and localization in the brains of other vertebrate species, such as reptiles, amphibians and fishes. The aim of this review is to compile and update information on the localization of GnRH and GnIH neuronal systems, with a particular focus on amphibians, summarizing the neuroanatomical distribution of GnIH and GnRH and emphasizing the discovery of GnIH based on RFamide peptides and GnIH orthologous peptides found in other vertebrates and their functional significance.

Anandamide acts via kisspeptin in the regulation of testicular activity of the frog, Pelophylax esculentus

In the frog Pelophylax esculentus, the endocannabinoid anandamide (AEA) modulates Gonadotropin Releasing Hormone (GnRH) system in vitro and down-regulates steroidogenic enzymes in vivo. Thus, male frogs were injected with AEA ± SR141716A, a cannabinoid receptor 1 (CB1) antagonist, to evaluate possible effects on GnRH and Kiss1/Gpr54 systems, gonadotropin receptors and steroid levels. In frog diencephalons, AEA negatively affected both GnRH and Kiss1/Gpr54 systems. In testis, AEA induced the expression of gonadotropin receptors, cb1, gnrh2 and gnrhr3 meanwhile reducing gnrhr2 mRNA and Kiss1/Gpr54 proteins. Furthermore, aromatase (Cyp19) expression increased in parallel to testosterone decrease and estradiol increase. In vitro treatment of testis with AEA revealed direct effects on Cyp19 and induced the expression of the AEA-degrading enzyme Faah. Lastly, AEA effects on Faah were counteracted by the antiestrogen ICI182780, indicating estradiol mediated effect. In conclusion, for the first time we show in a vertebrate that AEA regulates testicular activity through kisspeptin system.

Endocannabinoids and endovanilloids: a possible balance in the regulation of the testicular GnRH signalling

Reproductive functions are regulated both at central (brain) and gonadal levels. In this respect, the endocannabinoid system (eCS) has a very influential role. Interestingly, the characterization of eCS has taken many advantages from the usage of animal models different from mammals. Therefore, this review is oriented to summarize the main pieces of evidence regarding eCS coming from the anuran amphibian Rana esculenta, with particular interest to the morphofunctional relationship between eCS and gonadotropin releasing hormone (GnRH). Furthermore, a novel role for endovanilloids in the regulation of a testicular GnRH system will be also discussed.

Anandamide modulates the expression of GnRH-II and GnRHRs in frog, Rana esculenta, diencephalon

In the hypothalamus, endocannabinoids affect neuroendocrine activity by means of Gonadotropin-Releasing-Hormone-I (GnRH-I) inhibition. Since most vertebrates, human included, possess at least two GnRH molecular forms, the aim of this work was to investigate the effect of endocannabinoids on GnRH molecular forms other than GnRH-I and on GnRHRs. Thus, we cloned GnRH precursors as well as GnRH receptors (GnRHR-I, GnRHR-II, GnRHR-III) from the diencephalons of the anuran amphibian, Rana esculenta. GnRH-II expression was evaluated in pituitary, whole brain, spinal cord, hindbrain, midbrain and forebrain during the annual sexual cycle. Then, in post-reproductive period (May), GnRH-I, GnRH-II and GnRHRs expression was evaluated by quantitative real time (qPCR) after incubation of diencephalons with the endocannabinoid anandamide (AEA). AEA significantly decreased GnRH-I and GnRH-II expression, up regulated GnRHR-I and GnRHR-II mRNA and it had no effect upon GnRHR-III expression. These effects were counteracted by SR141716A (Rimonabant), a selective antagonist of type I cannabinoid receptor (CB1). In conclusion our results demonstrate a CB1 receptor dependent modulation of GnRH system expression rate (both ligands and receptors) in frog diencephalons. In particular, we show that AEA, besides GnRH-I, also acts on GnRH-II expression.

The contribution of lower vertebrate animal models in human reproduction research

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

Cannabinoids and Reproduction: A Lasting and Intriguing History

Starting from an historical overview of lasting Cannabis use over the centuries, we will focus on a description of the cannabinergic system, with a comprehensive analysis of chemical and pharmacological properties of endogenous and synthetic cannabimimetic analogues. The metabolic pathways and the signal transduction mechanisms, activated by cannabinoid receptors stimulation, will also be discussed. In particular, we will point out the action of cannabinoids and endocannabinoids on the different neuronal networks involved in reproductive axis, and locally, on male and female reproductive tracts, by emphasizing the pivotal role played by this system in the control of fertility.

CB1 activity in male reproduction: mammalian and nonmammalian animal models

The importance of the endocannabinoid system (ECBS) and its involvement in several physiological processes is still increasing. Since the isolation of the main active compound of Cannabis sativa, Delta(9)-THC, several lines of research have evidenced the basic roles of this signaling system mainly considering its high conservation during evolution. In this chapter the attention is focussed on the involvement of the ECBS in the control of male reproductive aspects at both central and local levels which are both considered from a comparative point of view.

Interplay between the endocannabinoid system and GnRH-I in the forebrain of the anuran amphibian Rana esculenta

The morphofunctional relationship between the endocannabinoid system and GnRH activity in the regulation of reproduction has poorly been investigated in vertebrates. Due to the anatomical features of lower vertebrate brain, in the present paper, we chose the frog Rana esculenta (anuran amphibian) as a suitable model to better investigate such aspects of the reproductive physiology. By using double-labeling immunofluorescence aided with a laser-scanning confocal microscope, we found a subpopulation of the frog hypothalamic GnRH neurons endowed with CB1 cannabinoid receptors. By means of semiquantitative RT-PCR assay, we have shown that, during the annual sexual cycle, GnRH-I mRNA (formerly known as mammalian GnRH) and CB1 mRNA have opposite expression profiles in the brain. In particular, this occurs in telencephalon and diencephalon, the areas mainly involved in GnRH release and control of the reproduction. Furthermore, we found that the endocannabinoid anandamide is able to inhibit GnRH-I mRNA synthesis; buserelin (a GnRH agonist), in turn, inhibits the synthesis of GnRH-I mRNA and induces an increase of CB1 transcription. Our observations point out the occurrence of a morphofunctional anatomical basis to explain a reciprocal relationship between the endocannabinoid system and GnRH neuronal activity.

Type-1 cannabinoid receptor expression in the frog,Rana esculenta, tissues: A possible involvement in the regulation of testicular activity

Endogenous cannabinoids and type-1 cannabinoid receptor (CB1) are widely produced and distributed in the central nervous system (CNS) and peripheral nerves in mammals. In addition, the detection of endocannabinoids and corresponding receptors in non nervous peripheral tissues indicates an involvement of the system in the control of a wide range of physiological activities, including reproduction. Recently, the existence of CB1 was also observed in lower vertebrates and in urochordate suggesting that the endocannabinoid system is phylogenetically conserved. Using RT-PCR, CB1 mRNA expression profiles were characterized in a wide range of tissues of the anuran amphibian, the frog, Rana esculenta. Besides a strong expression in the CNS, CB1 was also present in testis, kidney, liver, ovary, muscle, heart, spleen, and pituitary. The CB1 expression pattern has been characterized in both testis and CNS during the annual sexual cycle. In testis, CB1 is poorly expressed during the winter stasis of the spermatogenesis rising during the breeding season and resumption period. An expression profile mismatching to that observed in testis was detected in whole-brain preparations during the sexual cycle; in particular in the diencephalon, the encephalic area mainly involved in the control of reproductive functions. Furthermore, fluctuations inside isolated encephalic areas and spinal cord were observed all over the reproductive cycle. In conclusion, CB1 receptor is expressed in R. esculenta CNS and testis. As far as the gonad it concerns, our results suggest the involvement of the endocannabinoids in the control of reproductive function.

Jun localization in cytosolic and nuclear compartments in brain-pituitary system of the frog, Rana esculenta: an analysis carried out in parallel with GnRH molecular forms during the annual reproductive cycle

The presence of c-jun like mRNA was assessed in the brain of the frog, Rana esculenta, during the annual sexual cycle. In parallel, Jun protein and GnRH molecular form (mammalian and chicken II also indicated as GnRH1 and GnRH2, respectively) activity was studied in order to establish possible relationships. Northern blot analysis of total RNA reveals the presence of a 2.7 kb c-jun-like mRNA. Western blots, carried out on cytoplasmic and nuclear protein extracts, show the presence of Jun immunoreactive band of 39 kDa in brain and pituitary. Fluctuations of c-jun-like mRNA and Jun immunoreactive protein (cytoplasmic and nuclear) levels in brains during the year indicate relationships among transcription, translation, and nuclear activity. In particular, mRNA levels increase gradually from September until November when Jun protein concentration peaks in cytosolic extracts. Conversely, the nuclear protein reaches highest concentration in July when the cytosolic level shows low values. Immunocytochemical studies confirm the presence of Jun immunoreactivity in both cytoplasmic and nuclear compartments of several brain areas, including those primarily involved in gonadotropin discharge (e.g., anterior preoptic area and preoptic nucleus). GnRH molecular forms and Jun are colocalized in anterior preoptic area and preoptic nucleus. Moreover, during the period characterized by GnRH release, Jun levels strongly decrease in nuclei. Finally, we show that treatments with a GnRH analog (buserelin, Hoechst, Frankfurt) increase Jun levels in brain nuclear extracts.

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.

Complementary deoxyribonucleic acid cloning, gene expression, and ligand selectivity of a novel gonadotropin-releasing hormone receptor expressed in the pituitary and midbrain of Xenopus laevis

We have cloned the full-length complementary DNA (cDNA) for a GnRH receptor from Xenopus laevis pituitary cDNA and determined its gene structure. The cDNA encodes a 368-amino acid protein that has a 46% amino acid identity to the human GnRH receptor. The X laevis GnRH receptor has all of the amino acids identified in the mammalian GnRH receptors as sites of interaction with the GnRH ligand. However, this receptor cDNA shares the same distinguishing structural features of the GnRH receptor that have been characterized from other nonmammalian vertebrates. These include the pair of aspartate residues in the transmembrane domains II and VII compared with the aspartate/asparagine arrangement in mammalian receptors, the amino acid PEY motif in extracellular loop III (SEP in mammals), and the presence of a carboxyl-terminal tail. Previous studies have reported that mammalian GnRH was equipotent to other naturally occurring GnRH subtypes in stimulating LH release from the amphibian pituitary. However, in this study we show that the X. laevis GnRH receptor has ligand selectivity for the naturally occurring GnRHs similar to other nonmammalian GnRH receptors. The order of potency of the GnRHs in stimulating inositol phosphate production in COS-1 cells transiently transfected with the X. laevis GnRH receptor cDNA was chicken GnRH II>salmon GnRH>mammalian GnRH. Transcripts of this GnRH receptor are expressed in the pituitary and midbrain of X. laevis.

Differences in mGnRH and cGnRH-II contents in pituitaries and discrete brain areas of Rana rugulosa W. according to age and stage of maturity

(1) In tadpoles, chicken-II gonadotropin-releasing hormone (cGnRH-II) could be measured in the brains before metamorphosis, but mammalian gonadotropin-releasing hormone (mGnRH) did not appear until the stage of metamorphosis, i.e. cGnRH-II appeared earlier than mGnRH during ontogenesis. (2) During the metamorphic climax, mGnRH content increased more rapidly than cGnRH-II; the content of mGnRH was about two times of that of cGnRH-II. (3) In juveniles and adults, the content of mGnRH and cGnRH-II, and the distribution pattern of mGnRH (but not cGnRH-II) in the brains and pituitaries changed with age and stages of gonadal development. mGnRH mainly distributed in the rostral brain areas, whereas cGnRH-II had a widespread distribution in the brain. (4) Both mGnRH and cGnRH-II were present in the pituitaries at each stage of maturity. The gonadotropin-releasing hormone (GnRH) content at sexually maturity was significantly higher than that at other stages of gonadal development, and the content of mGnRH was about 15-18 times of that of cGnRH-II. (5) These results suggest that both mGnRH and cGnRH-II are potentially involved in the direct regulation of pituitary gonadotropes, and mGnRH may be the major active form, cGnRH-II may also serve as a neurotransmitter or neuromodulator in the brain.

Fos localization in cytosolic and nuclear compartments in neurones of the frog, Rana esculenta, brain: an analysis carried out in parallel with GnRH molecular forms

C-fos activity was determined in the brain of the frog, Rana esculenta, during the annual sexual cycle. The localization of GnRH molecular forms (mammalian- and chicken-GnRHII) was also carried out to determine whether or not the proto-oncogene and the peptides showed a functional relationship. Northern blot analysis of total RNA revealed the presence of a single strong signal of c-fos like mRNA of 1.9 Kb during February and April. This was followed by expression of c-Fos protein (Fos) in several brain areas during March and July shown by immunocytochemistry. In particular, the olfactory region, the lateral and medial pallium, the nucleus lateralis septi, the ventral striatum, the caudal region of the anterior preoptic area, the suprachiasmatic nucleus, the ventral thalamus, tori semicircularis and ependymal layers of the tectum were immunostained. There was no overlap between Fos immunoreactive perikarya and GnRH immunoreactive perikarya (e.g. gonadotrophin-releasing hormone (GnRH) in the rostral part and Fos in the caudal region of the anterior preoptic area). Interestingly, a cytoplasmic localization of Fos was also observed by immunocytochemistry and gel retardation experiments supported this observation. Cytoplasmic extracts from September-October animals bound the AP1 oligonucleotide. The complex was not available in the nuclear extracts from the same preparation, suggesting that, besides Fos, Jun products were also present. Conversely, nuclear but not cytosolic binding was detected in the brain of animals collected in July. In conclusion, we show that Fos and GnRH activity does not correlate in the frog brain and, for the first time in a vertebrate species, we give evidence of a cytoplasmic AP1 complex in neuronal cells.

Distribution of gonadotropin-releasing hormone immunoreactivity in the brain of Ichthyophis beddomei (Amphibia: Gymnophiona)

From a comparative viewpoint, we have investigated the presence and neuroanatomical distribution of gonadotropin-releasing hormone (GnRH)-immunoreactive material in the brain of a gymnophione amphibian, Ichthyophis beddomei. Immunocytochemical analysis of the adult brain and terminal nerves in both sexes shows the presence of neurons and fibers containing mammalian GnRH (mGnRH)- and chicken GnRH-II (cGnRH-II)-like peptides. With respect to GnRH-immunoreactive material, there are two distinct neuronal systems in the brain: one containing mGnRH, which is located in the forebrain and terminal nerve, and the other containing cGnRH-II, which is restricted to the midbrain tegmentum. Basically, this distribution pattern parallels that of many species of anurans and a urodele. Whereas the presence of cGnRH-II-immunoreactive fibers in the dorsal pallium of L. beddomei is a feature in common with a urodele amphibian, the total absence of cGnRH-II-like material in the median eminence is unique to this species. It is suggested here that the distribution profile of GnRH-like material within the brain and terminal nerve of I. beddomei represents a primitive pattern.

Enhancement by prolactin of the GnRH-induced release of LH from dispersed anterior pituitary cells of the bullfrog (Rana catesbeiana)

The response of enzymatically dispersed anterior pituitary cells of the bullfrog (Rana catesbeiana) to gonadotropin-releasing hormone (GnRH) was studied by monitoring the release of luteinizing hormone (LH) into the culture medium. The cells responded to GnRH by releasing LH according to the incubation time and to the GnRH concentration. The responsiveness to GnRH became less conspicuous as the cell density was reduced. Addition of prolactin (PRL) to the medium enhanced the responsiveness to the secretagogue, and addition of antiserum against PRL lowered the responsiveness to a certain extent. Immunohistochemical studies of sectioned pituitaries revealed that PRL cells most frequently located in contact with LH cells. The possibility that PRL acts directly on gonadotrophs to enhance their responsiveness to GnRH was suggested.

Detection of GnRH molecular forms in brains and gonads of the crested newt, Triturus carnifex

Gonadotrophin-releasing hormone (GnRH) immunoreactivity is detectable in the brain, ovary, and testis of the newt, Triturus carnifex, collected during February (reproductive phase), May, and July (nonreproductive phase). In the brain of May animals, chicken GnRH-II positive cell bodies are located within the terminal nerve, the anterior preoptic area, and the preoptic nucleus, which appears to be devoid of immunoreactive mammalian GnRH cell bodies. During February and July, both chicken GnRH-II and mammalian GnRH are detected only within the terminal nerve and anterior preoptic area. Generally, in the reproductive as well as the nonreproductive periods, chicken GnRH-II fibers are widely distributed in the brain; however, the distribution of fibers of both molecular forms suggests that they exert hypophysiotropic activity. High-pressure liquid chromatography (HPLC) coupled with radioimmunoassay indicates the presence of an early-eluting GnRH peak in brains and gonads but not in plasma. Using chicken GnRH-II antiserum, immunoreactivity is observed in spermatocytes, spermatozoa, and the external theca layer. Seasonal changes of the GnRH-like material are observed in both sexes, and its high concentration detectable during February is in good correlation with the timing of reproduction.

Immunoreactive mammalian and chicken-II GnRHs in Rana esculenta brain during development

Two forms of gonadotropin-releasing hormone (mammalian, mGnRH and chicken-II, cGnRH-II) were measured by radioimmunoassay in the nasal area (containing peripheral terminal nerve), brain and pituitary of Rana esculenta during larval development, metamorphosis, and until prior to becoming reproductively active. Small amounts of both forms of GnRH were first detected in the brain extract of early tadpoles (stage 26-27, when hindlimbs begin to develop). Later, there was a gradual, but constant, stage-dependent increase in the brain content of GnRHs, with the most remarkable increase recorded at postclimax and in young frogs. In tadpoles, postclimax froglets, and young frogs, the brain concentration of mGnRH was higher than that of cGnRH-II, with a ratio of approximately 2:1 in favor of mGnRH. In juveniles, however, the brain extract contained more cGnRH-II than mGnRH. No GnRH immunoreactivity was detected in the nasal area until stage 31. In successive stages of development, however, only mGnRH was present in the nasal area, and this confirmed our previous immunohistochemical analysis which showed that the peripheral terminal nerve contains only mGnRH-immunoreactive neurons and fibers. Although both GnRH forms were detected in the anterior (telencephalon, diencephalon) and posterior (mesencephalon, rhombencephalon) brain halves from juveniles, mGnRH content predominated in the anterior half, whereas in the posterior half cGnRH-II was present in greater amounts. Pituitaries from male and female postclimax froglets and young frogs contained both forms of GnRH in a ratio of approximately 10:1 in favor of mGnRH. This finding may shed light on the question of which GnRH(s) regulate gonadotropin release from the pituitary. The developmental changes in GnRH-immunoreactive content of the brain and pituitary have been discussed in the light of functional maturation of the brain-pituitary-gonad axis.

Localization of GnRH molecular forms in the brain, pituitary, and testis of the frog, Rana esculenta

In the amphibian brain four molecular forms of GnRH have been identified so far: mammalian GnRH (m- and hydroxyproline9m-), chicken II GnRH (cII), and a salmon (s) GnRH-like peptide. In Rana esculenta, cII- and s-GnRH-like molecules have been partially characterized in the brain extracts using HPLC combined with radioimmunoassay. Moreover, since cII-GnRH-like material has been detected in Rana esculenta testis, the present study describes the localization of the above peptides in the brain and testis of the frog. Immunoreactive cII-GnRH and m-GnRH neurons and fibers were identified in the anterior preoptic area (APOA) and in the median septal area (MSA). A population of cells located on the dorsal side of the caudal preoptic region was also stained. Immunopositive fibers were seen to overlap the median eminence before ending within the pars nervosa. Moreover, densely packed fibers made close contact with the vascular complex in the median eminence. Conversely, immunoreactive s-GnRH-like material was absent in APOA and MSA, but weakly scattered elements were detected by the anti-s-GnRH serum in the dorsal side of the caudal preoptic region. Using m-GnRH antiserum, a strong immunopositivity was observed in the median eminence but not within the pars nervosa, indicating that, besides cII-GnRH and s-GnRH-like material, also m-GnRH-like material is present in Rana esculenta brain. In the testis, cells of the interstitial and germinal compartment were detected by anti-cII-GnRH during different periods of the annual cycle. In particular, in October and February interstitial tissue was intensely stained, coinciding with periods of increased androgen production and the onset of the new spermatogenic wave, respectively.

Induction of S-phase entry by a gonadotropin releasing hormone agonist (buserelin) in the frog, Rana esculenta, primary spermatogonia

In the testis of the frog, Rana esculenta, mitotic activity of primary spermatogonia is regulated by gonadotropins and synergistically by testosterone. In addition GnRH-like material directly stimulates gonadal activity. Intact animals were treated with a GnRH agonist (GnRHa, buserelin, Hoechst) and/or a GnRH antagonist giving injections intraperitoneally on alternate days for 15 days. Moreover, testes were treated in vitro for 24 hr with GnRHa. 3H-thymidine and colchicine were used to assess the labelling and the mitotic index (LI and MI) of primary spermatogonia. Both LI and MI were increased by the treatment with GnRHa but the rate of cells measured by LI was significantly higher than that of cells measured by MI. Therefore, our results confirm the role of GnRH-like material as local regulator of the testicular activity in vertebrates and show its involvement in promoting the G1-S transition of spermatogonial cell cycle in the frog, Rana esculenta.

Distribution of two molecular forms of gonadotropin-releasing hormone (GnRH) in the central nervous system of the frog Rana ridibunda

Two molecular forms of gonadotropin-releasing hormone (GnRH) have been recently characterized in the brain of the frog Rana ridibunda i.e. mammalian GnRH (mGnRH) and chicken GnRH-II (cGnRH-II). Using highly specific antisera against each form of GnRH, we have investigated the distribution of these two neuropeptides in the frog brain by the indirect immunofluorescence and the peroxidase-antiperoxidase techniques. mGnRH-immunoreactive cell bodies were restricted to a well defined region corresponding to the septal-anterior preoptic area. mGnRH-containing fibers projected through the ventral diencephalon and ended in the median eminence. In contrast, cGnRH-II-immunoreactive structures were widely distributed in the frog brain. In the telencephalon cGnRH-II-positive elements formed a ventromedial column extending from the olfactory bulb to the septal area, a pathway which corresponds to the terminal nerve. A dense accumulation of cGnRH-II-immunoreactive cell bodies was also found in the septal-anterior preoptic area; these neurons sent processes towards the median eminence via the hypothalamus. Double immunostaining revealed that, in this area, mGnRH- and cGnRH-II-like immunoreactivity co-existed in the same neurons. In the mid-diencephalon, numerous cGnRH-II-immunoreactive perikarya were found, surrounding the third ventricle, in the posterior preoptic and infundibular areas. Many of these neurons sent processes towards the ventricular cavity. More caudally, a dense population of cGnRH-II-immunoreactive perikarya was also observed in the nucleus of the paraventricular organ and the posterior tubercle. Dorsally, the thalamus, the tegmentum, the tectum and the granular layer of the cerebellum were richly innervated by cGnRH-II-positive fibers. In the medulla oblongata, numerous cGnRH-II-immunoreactive perikarya were seen in several cranial nerve nuclei. Ventrally, a dense plexus of immunoreactive fibers projected rostrocaudally into the spinal cord. The occurrence of mGnRH- and cGnRH-II-like immunoreactivity in the septal-anterior preoptic area and the hypothalamo-pituitary pathway supports the view that both peptides act as hypophysiotropic neurohormones. The widespread distribution of cGnRH-II-immunoreactive elements in the central nervous system of the frog strongly suggests that this peptide may also exert neuromodulator and/or neurotransmitter activities.

Development and distribution of gonadotropin-releasing hormone neuronal systems in the frog (Rana esculenta) brain: immunohistochemical analysis

The ontogenesis of the gonadotropin-releasing hormone (GnRH) neuronal systems was studied in the brain of the frog, Rana esculenta. Attention was also focussed on the differential distribution of molecular forms of GnRH during development. The first GnRH-immunoreactive neurons appear in the mesencephalon of posterior limb-stage tadpoles. These neurons are shown to contain only chicken [His5,Trp7,Tyr8]GnRH (cGnRH-II). Later in development, mammalian [Tyr5,Leu7,Arg8] GnRH (mGnRH)-like peptide-containing neurons appear simultaneously in the terminal nerve as well as in the anterior preoptic area of the telencephalon. Subsequently, only after metamorphosis, mGnRH-containing neurons appear in the medial septal area of the posterior telencephalon. It is here shown that neurons containing the two forms of GnRH are distributed in distinct brain areas during development and in the adult: mGnRH-immunoreactive neurons in the terminal nerve, olfactory bulb, mediobasal telencephalon, medial septal area, anterior preoptic area, ventrolateral thalamus and infundibulum, whereas cGnRH-II neurons are located in the mesencephalon. We hypothesize that the terminal nerve/forebrain-located GnRH neurons express immunohistochemically late in development and originate extracranially migrating centrally, along the terminal nerve, during development, whereas those located in the mesencephalon express earlier and may have an intracranial site of origin.

Chicken GnRH-II and salmon GnRH effects on plasma and testicular androgen concentrations in the male frog, Rana esculenta, during the annual reproductive cycle

In the frog, Rana esculenta, two molecular forms of GnRH, coeluting with chicken (c) GnRH-II and salmon (s) GnRH, have been detected using HPLC and radioimmunoassay. Mammalian (m) GnRH seems to be also present. In amphibians the role of cGnRH-II seems to be primarily the involvement in the regulation of neuroendocrine processes and, while the mGnRH has been postulated to act as a neurotransmitter and/or neuromodulator, the activity of sGnRH-like material has not been investigated. Therefore, we have treated the frogs with single or multiple injections of cGnRH-II or sGnRH (6 micrograms) or both peptides (6 micrograms of each) to detect differences in the response measured as testicular or plasma androgen (testosterone plus 5 alpha-dihydrotestosterone) concentration during the annual reproductive cycle. The basal profile of testicular and plasma androgen shows that the spring peak disappeared in control animals given multiple injections and kept in short-term captivity. We show in the treatment with cGnRH-II and/or sGnRH that the effects of the peptides depend on the season, the experimental design, and the tissue in which androgen levels were measured. In particular, both peptides strongly stimulate androgen production during the autumn-winter period, the time of the greater response to the GnRHs when basal levels of steroids are highest.

Detection and localization of gonadotrophin-releasing hormone (GnRH)-like material in the frog, Rana esculenta, ovary

GnRH-like material has been identified using HPLC followed by RIA in the ovary of Rana esculenta. During the reproductive cycle three immunoreactive GnRH peaks were eluted. One of them coeluted with s-GnRH, the other two forms between GnRH and cII-GnRH. During the recovery phase s-GnRH immunoreactivity disappears. By immunocytochemistry, cII-GnRH immunostaining was localized to granulosa cells while s-GnRH was present in the perinuclear zone of the oocytes.