Dopamine inhibits luteinizing hormone synthesis and release in the juvenile European eel: a neuroendocrine lock for the onset of puberty

In various adult teleost fishes, LH ovulatory peak is under a dual neurohormonal control that is stimulatory by GnRH and inhibitory by dopamine (DA). We investigated whether DA could also be involved in the inhibitory control of LH at earlier steps of gametogenesis by studying the model of the European eel, Anguilla anguilla, which remains at a prepubertal stage until the oceanic reproductive migration. According to a protocol previously developed in the striped bass, eels received sustained treatments with GnRH agonist (GnRHa), DA-receptor antagonist (pimozide), and testosterone (T) either alone or in combination. Only the triple treatment with T, GnRHa, and pimozide could trigger dramatic increases in LH synthesis and release as well as in plasma vitellogenin levels and a stimulation of ovarian vitellogenesis. Thus, in the prepubertal eel, removal of DA inhibition is required for triggering GnRH-stimulated LH synthesis and release as well as ovarian development. To locate the anatomical support for DA inhibition, the distribution of tyrosine hydroxylase (TH) in the brain and pituitary was studied by immunocytochemistry. Numerous TH-immunoreactive cell bodies were observed in the preoptic anteroventral nucleus, with a dense tract of immunoreactive fibers reaching the pituitary proximal pars distalis, where the gonadotrophs are located. This pathway corresponds to that mediating the inhibition of LH and ovulation in adult teleosts. To our knowledge, this is the first demonstration of a pivotal role for DA in the control of LH and puberty in a juvenile teleost. These data support the view that DA inhibition on LH secretion is an ancient evolutionary component in the neuroendocrine regulation of reproduction that may have been partially maintained throughout vertebrate evolution.     

Duplicated Leptin Receptors in Two Species of Eel Bring New Insights into the Evolution of the Leptin System in Vertebrates

Since its discovery in mammals as a key-hormone in reproduction and metabolism, leptin has been identified in an increasing number of tetrapods and teleosts. Tetrapods possess only one leptin gene, while most teleosts possess two leptin genes, as a result of the teleost third whole genome duplication event (3R). Leptin acts through a specific receptor (LEPR). In the European and Japanese eels, we identified two leptin genes, and for the first time in vertebrates, two LEPR genes. Synteny analyses indicated that eel LEPRa and LEPRb result from teleost 3R. LEPRb seems to have been lost in the teleost lineage shortly after the elopomorph divergence. Quantitative PCRs revealed a wide distribution of leptins and LEPRs in the European eel, including tissues involved in metabolism and reproduction. Noticeably, leptin1 was expressed in fat tissue, while leptin2 in the liver, reflecting subfunctionalization. Four-month fasting had no impact on the expression of leptins and LEPRs in control European eels. This might be related to the remarkable adaptation of silver eel metabolism to long-term fasting throughout the reproductive oceanic migration. In contrast, sexual maturation induced differential increases in the expression of leptins and LEPRs in the BPG-liver axis. Leptin2 was strikingly upregulated in the liver, the central organ of the reproductive metabolic challenge in teleosts. LEPRs were differentially regulated during sexual maturation, which may have contributed to the conservation of the duplicated LEPRs in this species. This suggests an ancient and positive role of the leptin system in the vertebrate reproductive function. This study brings new insights on the evolutionary history of the leptin system in vertebrates. Among extant vertebrates, the eel represents a unique case of duplicated leptins and leptin receptors as a result of 3R.     

Influence of age and juvenile hormone on brain dopamine level in male honeybee (Apis mellifera): association with reproductive maturation

Dopamine (DA) is a major functional biogenic amine in insects and has been suggested to regulate reproduction in female honeybees. However, its function has not been investigated in male drones. To clarify developmental changes of DA in drones, brain DA levels were investigated at various ages and showed a similar pattern to the previously reported juvenile hormone (JH) hemolymph titer. The DA level was lowest at emergence and peaked at day 7 or 8, followed by decline. Application of JH analog increased brain DA levels in young drones (2-4-days-old), suggesting regulation of DA by JH in drones. In young drones, maturation of male reproductive organs closely matched the increase in brain DA. The dry weight of testes decreased and that of seminal vesicles increased from emergence to day 8. The dry weight of mucus glands increased up to day 4. Consequently, DA regulated by JH might have reproductive behavior and/or physiological functions in drones.     

Access of dopamine to the median eminence and brain throughout local vascular pathways in sheep

In female sheep, estradiol-dependent dopaminergic inhibition exerted by the A15 nucleus during long days (LD) results in a blockade of reproductive activity. This effect could involve the GnRH cell bodies or their terminals in the median eminence (ME). However, a vast majority of terminals of the A15 nucleus are located in neurohypophysis and only a few in the ME. Previously we demonstrated that tritiated dopamine (DA) was transferred from the venous blood of the cavernous sinus to the arterial blood supplying the brain. In the present paper, we tested the hypothesis that the transferred dopamine could reach further the brain and ME. Using isolated sheep heads harvested on short days vs. long days, we examined radioactivity in brain tissues following infusion of tritiated dopamine into the cavernous sinus. The experiment was performed in ovariectomized ewes treated with estradiol (E2) or vehicle. The mean level of radioactivity in brain was affected by season (p<0.001) and E2 (p<0.05) and was the highest during LD in E2-treated animals. In the next experiment on isolated sheep head we measured dopamine and its metabolites levels in blood and pituitary after infusion of non-radiolabeled dopamine. We observed an increase (p<0.01) in dopamine concentration in arterial blood but not in the brain. The pituitary was the only structure examined in which a tendency (p=0.06) towards increased dopamine concentration following dopamine infusion was observed. Thus, even if part of DA released from terminals within the posterior and intermediate lobes of the pituitary reaches the vessels of the ME through local vascular pathways, it is unlikely that it could affect the LHRH terminals located in ME. In addition, our results suggest that brain capillaries in the isolated head are able to maintain a functional blood brain barrier.     

Effects of acute 17alpha-methyltestosterone,acute 17beta-estradiol,and chronic 17alpha-methyltestosterone on dopamine,norepinephrine and serotonin levels in the pituitary,hypothalamus and telencephalon of rainbow trout (Oncorhynchus mykiss)

This study investigated: (a) the effects of acute 17alpha-methyltestosterone (MT) or 17beta-estradiol (E(2)) administration on norepinephrine (NE), dopamine (DA), serotonin (5-HT), 3,4, dihydroxyphenylacetic acid (DOPAC), and 5-hydroxyindoleacetic acid (5-HIAA) contents in the hypothalamus, telencephalon and pituitary of previtellogenic female rainbow trout Oncorhynchus mykiss, and (b) the effects of chronic MT administration on the levels of these neurotransmitters in these brain regions in immature male rainbow trout. The acute administration of MT induced a significant decrease in pituitary levels of DOPAC as well as in the DOPAC/DA ratio. On the other hand, the acute administration of E(2) induced an increase in pituitary 5-HT levels as well as a decrease in the 5-HIAA/5-HT ratio. In a second experiment, 20 mg MT per kilogram body weight was implanted for 10, 20 or 40 days into sexually immature male rainbow trout. Implanted rainbow trout showed increased testosterone and decreased E(2) levels. In the pituitary, MT induced long-term decreases in NE, DA, DOPAC and 5-HT levels, as well as in the DOPAC/DA ratio. Hypothalamic and telencephalic DA, NE and 5-HT levels were not affected by MT implantation. However, 5-HIAA levels and the 5-HIAA/5-HT ratio were reduced by MT implantation in both brain regions. These results show that chronic treatment with MT exerts both long-term and region-specific effects on NE, DA, and 5-HT contents and metabolism, and thus that this androgen could inhibit pituitary catecholamine and 5-HT synthesis. A possible role for testosterone in the control of pituitary dopaminergic activity and gonadotropin II release is also discussed.     

Single doses of catecholamines in the rat: catecholaminotropic, but not hyperglycemic

1. As in two "lower" vertebrates, the lamprey and the eel, single intravascular injections of physiological doses (2.5 micrograms/kg) of epinephrine (E) into the rat immediately increased levels of plasma dopamine (DA) and norepinephrine (NE). 2. Single doses of DA (5 micrograms/kg) enhanced circulating NE and E, while NE (5 micrograms/kg) had no clear impact on the plasma levels of the other two catecholamines (CAs). 3. These data are at variance with findings in the eel, where all three CAs are mutually stimulatory; and in the lamprey, where only E stimulates release of the other two CAs. 4. It appears that E-stimulated CA release is widespread or ubiquitous among vertebrates, and that complex interactions between circulating CAs must be considered under experimental, physiological, and clinical conditions. 5. None of the injections had a significant hyperglycemic effect.     

Differential expression of neural and gonadal aromatase enzymatic activities in relation to gonadal development in Japanese eel, Anguilla japonica

The objectives of the present study were to investigate the enzymatic characteristics of brain aromatase in Japanese eel, Anguilla japonica, as well as the correlations between aromatase activities in various tissues (brain regions, pituitary, and gonads) and ovarian development. Eel brain aromatase exhibited a K(m) of 75 nM and a V(max) of 1.14 fmol/min mg protein (91.5 fmol/h mg protein), indicating an enzymatic activity much lower than in other teleosts but similar to that in mammals. This supports the hypothesis of the occurrence of a single aromatase gene in eels (representative of an ancient group of teleosts, Elopomorphs), as in mammals, but unlike what is observed in more recent teleosts. Aromatase inhibitors could significantly suppress brain and pituitary aromatase activity. There was no significant sex difference in aromatase activity in the forebrain, midbrain, hindbrain, or pituitary, but there was in the gonads, where aromatase could be detected in the ovaries but not in the testes, in accordance with the role of this enzyme in ovarian differentiation. Comparison with another teleost, black porgy (Percomorph), under the same experimental conditions, further confirmed the low activity of aromatase in the eel. We investigated variations in brain, pituitary, and gonad aromatase activity in relation to ovarian development in control female eels (gonadosomatic index, GSI, 0.1-1.6%) as well as in eels treated with pituitary extract (experimental ovarian maturation; GSI up to 25%). Differential expression of neural and gonadal aromatase was observed in relation to the course of gonadal development. Pituitary aromatase activity increased with GSI at all stages. Brain (specially forebrain) aromatase activity significantly increased in early vitellogenic control eels (GSI>0.8%) and in treated eels. The low activity of eel aromatase may be related to the characteristic life cycle of the eel, in which there is a long delay of the onset of puberty before oceanic reproductive migration.     

The reproduction and development of sharks,skates, rays and ratfishes: introduction,history, overview,and future prospects

References This volume had its origin in aSymposium on the Reproduction and Development of Cartilaginous Fishes that was held at the annual meetings of the American Elasmobranch Society and the American Society of Ichthyologists and Herpetologists in Charleston, South Carolina in June 1990. The aim of this symposium was to bring together many of those scientists interested in chondrichthyan reproduction and development in order to assess the current state of knowledge in these fields.The chondrichthyan fishes occupy a pivotal position in comparative and evolutionary studies of vertebrate reproduction and development. They are the oldest surviving group of jawed vertebrates and they possess both the adult vertebrate Bauplan and the vertebrate program of embryonic development. The major features of the female reproductive system, including its embryonic origin, structure, physiological function, and biochemistry, apparently were established early in vertebrate evolution and are fully developed in chondrichthyan fishes. These features of the female reproductive system have been retained during the evolution of the other classes of vertebrates. Much the same can be said for the male reproductive system. Moreover, viviparity, placental nourishment of developing embryos, and the hormonal regulation of these events made an initial appearance in this group.The twenty-two articles contained in this volume bring together a wide variety of complementary research by investigators from seven countries. It is hoped that presentation of this disparate body of research and thought in one place will provide perspective on current research activity, call attention to those areas in which the research endeavour is deficient, and identify opportunities for future study. The appearance at this time of a volume on the reproduction and development of cartilaginous fishes is quite opportune. The continued existence of these fishes, which survived the great extinction events of Earth"s history, is now threatened by over-exploitation unless immediate steps for their conservation are undertaken. Knowledge of their reproduction and development not only is an end in itself, but is of critical importance in devising successful conservation and resource management strategies.     

Activins and their receptors in female reproduction.

Activins are growth and differentiation factors belonging to the transforming growth factor-beta superfamily. They are dimeric proteins consisting of two inhibin beta subunits. The structure of activins is highly conserved during vertebrate evolution. Activins signal through type I and type II receptor proteins, both of which are serine/threonine kinases. Subsequently, downstream signals such as Smad proteins are phosphorylated. Activins and their receptors are present in many tissues of mammals and lower vertebrates where they function as autocrine and (or) paracrine regulators of a variety of physiological processes, including reproduction. In the hypothalamus, activins are thought to stimulate the release of gonadotropin-releasing hormone. In the pituitary, activins increase follicle-stimulating hormone secretion and up-regulate gonadotropin-releasing hormone receptor expression. In the ovaries of vertebrates, activins are expressed predominantly in the follicular layer of the oocyte where they regulate processes such as folliculogenesis, steroid hormone production, and oocyte maturation. During pregnancy, activin-A is also involved in the regulation of placental functions. This review provides a brief overview of activins and their receptors, including their structures, expression, and functions in the female reproductive axis as well as in the placenta. Special effort is made to compare activins and their receptors in different vertebrates.     

Gonadotrophin-releasing hormone (GnRH) in the animal kingdom

As a major actor of the brain-pituitary-gonad axis, GnRH has received considerable attention, mainly in vertebrates. Biochemical, molecular, neuroanatomical, pharmacological and physiological studies have mainly focused on the role of GnRH as a gonadotrophin-releasing factor and have led to a detailed knowledge of the hypophysiotrophic GnRH system, primarily in mammals, but also in fish. It is now admitted that the corresponding neurons develop from the olfactory epithelium and migrate into the forebrain during embryogenesis to establish connections with the median eminence in tetrapods or the pituitary in teleost fish. However, all vertebrates possess a second GnRH system, expressing a variant known as chicken GnRH-II in neurons of the synencephalon, whose functions are still under debate. In addition, many fish species express a third form, salmon GnRH, whose expression is restricted to neurons of the olfactory systems and the ventral telencephalon, with extensive projections in the brain and a minor contribution to the pituitary. In vertebrates, GnRHs are also expressed in the gonads where they act on cell proliferation and steroidogenesis in males, and apoptosis of granulosa cells and reinititaion of meiosis in females. These functions could possibly represent the primitive roles of GnRH-like peptides, as an increasing number of studies in invertebrate classes point to a more or less direct connection between GnRH-producing sensory neurons and the gonads. According to recent studies, GnRHs appear as very ancient peptides that emerged at least in the cnidarians, the first animals with a nervous system. GnRH-like peptides have been partially characterized in several classes of invertebrates notably in molluscs, echinoderms and prochordates in which effects on the reproductive functions, notably gamete release and steroidogeneis, have been evidenced. It is possible that, with the increasing complexity of metozoa, GnRH neurons have lost their direct connection with the gonad to specialize in the control of additional regulatory centers such as the hypophysis in vertebrates or the optic gland in cephalopods. However, reminiscent effects of GnRH functions at the gonadal level would have persisted due to local production of GnRHs in the gonad itself. Altogether, these data indicate that GnRHs were involved in the control of reproduction long before the appearance of pituitary gonadotrophs.     

Interhemispheric regulation of dopaminergic ascending systems.

There are previous evidences of biochemical and functional interhemispheric asymmetry for central dopaminergic systems. The purpose of the current study was to establish an evidence of the existence of mechanisms for interhemispheric regulation of dopaminergic systems. The initial hypothesis was that to compensate the spontaneous interhemispheric asymmetry, the brain side with lower dopamine (DA) concentration show a higher DA turnover. The ratio of metabolite/neurotransmitter was computed as an index of presynaptic turnover rate. Both striatum and hippocampus showed a DOPAC/DA index higher in the brain side with a lower DA concentration. In addition, we found an inverse relation between the interhemispheric index of DA and interhemispheric index of DA turnover. Taken together the present data provide evidence of an interhemispheric regulation for the DA-innervation of both striatum and hippocampus in normal unlesioned rats.     

Phospholipid liposomes and prolactin secretion: Effects on spontaneous and drug-induced hyperprolactinaemia

Bovine brain phospholipid liposomes (BC-PL) reduce plasma prolactin (PRL) levels in humans after acute administration and counteract the metoclopramide- and sulpiride-induced hyperprolactinaemia. However, BC-PL, like nomifensine, a dopaminergic compound, does not influence TRH-induced hyperprolactinaemia. Moreover BC-PL and nomifensine reduce plasma PRL levels in hyperprolactinaemic PCO syndromes but not in PRL secreting pituitary adenomas. The results obtained indicate that BC-PL antagonizes the DA blockade-induced hyperprolactinaemia and that the main site of action of BC-PL seems to be at the hypothalamic level; however a concomitant pituitary effect cannot be ruled out.     

Decrease in dopamine and norepinephrine concentration in different brain regions of mice during progression of Sarcoma 180 tumour

The distribution and concentration of dopamine (DA) and norepinephrine (NE), the catecholamine neurotransmitters, were studied in discrete brain areas of Sarcoma 180 tumour bearing mice. With the progression of tumour, marked depletion of DA and NE concentration was observed in some brain areas richly innervated with dopaminergic and noradrenergic neurons suggesting an inverse relationship between brain CA and tumour growth. Since brain CA influence different important physiological activities like hormonal and immunological functions, it's alteration in brain areas during malignant growth suggests the possibility that the hormonal and immunological alterations during tumour growth is at the level of brain CA.     

Effect of environmental temperature on growth- and reproduction-related hormones gene expression in the female blue gourami (Trichogaster trichopterus)

Fish are ectothermic vertebrates, and their gonadal development and spawning are affected by changes in environmental temperature. Recent global temperature changes have increased the importance of studying the effect of temperature on reproduction. The aim of this paper was to study the effect of temperature on oogenesis and hormone gene expression related to reproduction and growth in the blue gourami female maintained under non-reproductive and reproductive conditions. In females under non-reproductive conditions, vitellogenic oocytes, gonadotropin-releasing hormone 3 (GnRH3), β luteinizing hormone (βLH) and growth hormone (GH) mRNA levels were affected by temperature changes. In females maintained under reproductive conditions with non-reproductively active males, a percentage of females in the final oocyte maturation (FOM) stage, pituitary adenylyl cyclase activating polypeptide (PACAP and PRP-PACAP), gonadotropins and GH mRNA levels were affected due to temperature changes. In females maintained under reproductive conditions with reproductively active males, also GnRH3 and insulin-like growth factor 1 (IGF-1) were affected by temperature changes. In conclusion, in blue gourami females, changes in environmental temperature affect oogenesis through changes in brain and pituitary hormone mRNA levels.     

Changes in the gene expression profiles of the brains of male European eels (Anguilla anguilla) during sexual maturation

Background

The vertebrate brain plays a critical role in the regulation of sexual maturation and reproduction by integrating environmental information with developmental and endocrine status. The European eel Anguilla anguilla is an important species in which to better understand the neuroendocrine factors that control reproduction because it is an endangered species, has a complex life cycle that includes two extreme long distance migrations with both freshwater and seawater stages and because it occupies a key position within the teleost phylogeny. At present, mature eels have never been caught in the wild and little is known about most aspects of reproduction in A. anguilla. The goal of this study was to identify genes that may be involved in sexual maturation in experimentally matured eels. For this, we used microarrays to compare the gene expression profiles of sexually mature to immature males.

Results

Using a false discovery rate of 0.05, a total of 1,497 differentially expressed genes were identified. Of this set, 991 were expressed at higher levels in brains (forebrain and midbrain) of mature males while 506 were expressed at lower levels relative to brains of immature males. The set of up-regulated genes includes genes involved in neuroendocrine processes, cell-cell signaling, neurogenesis and development. Interestingly, while genes involved in immune system function were down-regulated in the brains of mature males, changes in the expression levels of several receptors and channels were observed suggesting that some rewiring is occurring in the brain at sexual maturity.

Conclusions

This study shows that the brains of eels undergo major changes at the molecular level at sexual maturity that may include re-organization at the cellular level. Here, we have defined a set of genes that help to understand the molecular mechanisms controlling reproduction in eels. Some of these genes have previously described functions while many others have roles that have yet to be characterized in a reproductive context. Since most of the genes examined here have orthologs in other vertebrates, the results of this study will contribute to the body of knowledge concerning reproduction in vertebrates as well as to an improved understanding of eel biology.

Electronic supplementary material

The online version of this article (doi:10.1186/1471-2164-15-799) contains supplementary material, which is available to authorized users.     

Dopaminergic neurons modulate GABA neuron migration in the embryonic midbrain

Neuronal migration, a key event during brain development, remains largely unexplored in the mesencephalon, where dopaminergic (DA) and GABA neurons constitute two major neuronal populations. Here we study the migrational trajectories of DA and GABA neurons and show that they occupy ventral mesencephalic territory in a temporally and spatially specific manner. Our results from the Pitx3-deficient aphakia mouse suggest that pre-existing DA neurons modulate GABA neuronal migration to their final destination, providing novel insights and fresh perspectives concerning neuronal migration and connectivity in the mesencephalon in normal as well as diseased brains.     

Biogenic amines in the ponerine ant Harpegnathos saltator: serotonin and dopamine immunoreactivity in the brain

Many studies suggest a role for biogenic amines in a variety of insect behaviors including intraspecific aggression. In ants, despite a rich behavioral repertoire and prominent aggressive interactions, little is known about the potential impact of biogenic amines. This may partly be due to the general lack of information about aminergic systems in the ant brain. The present study investigates serotonergic and dopaminergic neuronal systems in the brain of the ponerine ant Harpegnathos saltator. In H. saltator, intraspecific aggression is important for the regulation of reproduction. This species, therefore, is amenable to comparative studies of aminergic neuronal effects on long-term changes in aggression. Using immunocytochemistry and confocal microscopy, we found that in the brains of sterile workers, the distributions of serotonergic and dopaminergic neuronal processes differed substantially. In addition, branching patterns of serotonergic neurons showed marked differences between males and females. Brains of workers after 3 days and 3 weeks of aggressive interactions revealed no marked differences in serotonergic and dopaminergic neurons compared to those of reproductive and non-aggressive individuals. We conclude that different levels of intraspecific aggression do not involve profound anatomical changes in serotonergic and dopaminergic neurons. Subtle changes may be masked by inter-individual variances.