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BOR - Papers in Press, published online ahead of print June 30, 2004.
Biol Reprod 2004, 10.1095/biolreprod.104.030627
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BIOLOGY OF REPRODUCTION 71, 1491–1500 (2004)
DOI: 10.1095/biolreprod.104.030627
© 2004 by the Society for the Study of Reproduction, Inc.

Neuroendocrinology

Dopamine Inhibits Luteinizing Hormone Synthesis and Release in the Juvenile European Eel: A Neuroendocrine Lock for the Onset of Puberty1

Bernadette Vidal3, Catherine Pasqualini4, Nadine Le Belle3, M. Claire H. Holland5, Miskal Sbaihi3, Philippe Vernier4, Yonathan Zohar5, and Sylvie Dufour2,3

USM 0401,3 UMR 5178 CNRS/MNHN/UPMC Biologie des Organismes Marins et Ecosystèmes, DMPA, Muséum National d'Histoire Naturelle, 75231 Paris Cedex 05, France DEPSN,4 UPR CNRS 2197, Institut de Neurobiologie Alfred Fessard, CNRS, 91198 Gif-sur-Yvette Cedex, France Center of Marine Biotechnology,5 University of Maryland Biotechnology Institute, Baltimore, Maryland 21202


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
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.

dopamine, gonadotropin-releasing hormone, luteinizing hormone, neuroendocrinology, puberty


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
The European eel, Anguilla anguilla, exhibits a striking life cycle, with a long delay before sexual maturation, and provides a powerful model for experimental studies of regulatory controls of puberty [1, 2]. Furthermore, its phylogenetical position, as a member of the group of Elopomorphs, which is considered to be an early emerging group among teleosts [3], may provide information concerning regulations in vertebrates. The present study aimed at investigating the neuroendocrine control of the onset of puberty in the eel. This is a crucial point, because eels are extensively fished in many parts of the world and natural stocks are dramatically declining [4]. Because of the prepubertal blockade of sexual maturation, artificial reproduction has not yet been successful, and no spawning or even maturing eels have ever been observed in natural conditions.

The European eel spawns in unknown areas of the Sargasso Sea, from which larvae drift back toward the European coasts following oceanic currents, such as the Gulf Stream. After metamorphosis of the larvae into glass eels, the juvenile growth phase in continental habitats lasts for many years (from several to more than 20 years, according to latitude and hydrosystems) and ends up with a second metamorphosis (silvering), which prepares the future genitors (silver eels) for the oceanic reproductive migration. However, silver eels are still sexually immature when they leave the continental habitats. Furthermore, they remain blocked at this prepubertal stage as long as the reproductive migration is prevented. The silver prepubertal stage therefore is the last known stage of the eel biological cycle in natural conditions, because maturing or spawning eels have never been caught in the wild (for review, see [5]).

Early studies [6, 7] showed that sexual maturation can be induced in silver male or female eels by administration of exogenous gonadotropins, demonstrating that the prepubertal blockade resulted from a deficient gonadotropic function. However, it is much more difficult to stimulate endogenous gonadotropic function. Synthesis of LH can be stimulated by administration of sexual steroids in the eel [8], as in other juvenile teleosts (for review, see [9]), revealing the positive-feedback control of LH by gonadal steroids in immature fish. However, despite a strong accumulation of LH in the eel pituitary, no stimulation of LH release and, consequently, no significant ovarian development could be observed [8]. Moreover, even in steroid-pretreated females, a GnRH treatment alone is unsuccessful in inducing LH release [10].

Numerous physiological and anatomical data now indicate that in various adult teleosts, gonadotropic function is under dual neuroendocrine control: In addition to the positive control exerted by GnRH, a negative control is exerted by dopamine (DA), which overrides the effects of GnRH on LH release [11]. In the goldfish [12, 13] and the trout [14], this dopaminergic inhibitory control was shown to be produced by a group of neurons originating from the anteroventral preoptic region (NPOav) and projecting to the proximal pars distalis (PPD) of the pituitary. Dopamine clearly participates in control of the final steps of gametogenesis, ovulation, and spermiation by inhibiting LH release in mature teleosts, as originally demonstrated and largely documented in cyprinids (for review, see [11, 15]) and also evidenced in silurids [16], salmonids [17], and some percomorphs (e.g., tilapia [18]). The intensity of the dopaminergic inhibition varies among species, possibly related to the wide diversity of their biological cycles. In some percomorphs, DA appears to play no inhibitory role (e.g., gilthead seabream [19]) or even a possible stimulatory role (e.g., Atlantic croaker [20]) in LH secretion.

Involvement of DA in the regulation of ovulation and spermiation has been established in a certain number of adult teleosts, but very little is known about its role in the earlier stages of gametogenesis. A role for DA in the onset of puberty has been hypothesized in the juvenile spadefish, a decrease in dopaminergic activity being observed in the hypothalamus at the time of puberty [21]. In contrast, results with the juvenile striped bass refute a role for DA in the prepubertal control of gonadotropins [22]. Similarly, in red seabream juvenile females, the results of recent studies argue against a role for DA in the control of gonadotropin secretion, GnRH alone being able to trigger precocious puberty in this species [23]. In the immature European eel, preliminary experiments from our laboratory showed that after a heavy pituitary loading in LH by a long-term treatment with estradiol (E2), a combined treatment with both GnRH and DA antagonist was required to induce LH release and ovarian development [10]. These data suggested a possible role for DA as early as the prepubertal stages. However, E2 has been reported to increase the inhibitory dopaminergic tone in adult teleosts [24] (for review, see [9, 25]), raising the possibility of an artifactual reinforcement of the dopaminergic inhibitory tone in long-term E2-treated eels.

To clarify further the actual involvement of DA in the control of puberty in the European eel, we used the same protocols as those previously developed for the juvenile female striped bass (Morone saxatilis) [22] and which led to a role for DA at puberty to be ruled out in this species. Moreover, to locate the anatomical substrate of the inhibition exerted by DA on the pituitary release of gonadotropins in the eel, we analyzed the distribution of tyrosine hydroxylase (TH) protein, the rate-limiting enzyme in catecholamine biosynthesis, in the juvenile eel brain and pituitary.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 REFERENCES
 
Animals

Female freshwater European eels (Anguilla anguilla L.) were netted in ponds in the north of France during their downstream migration (silver stage). The animals were transferred to the laboratory (MNHN, Paris, France) and kept in running aerated freshwater (3–4 eels/100-L tank). Because eels undergo a natural starvation period at the silver stage, they were not fed. Animal manipulations were performed according to the recommendations of the French ethical committee and under the supervision of authorized investigators.

In Vivo Treatments

Eels received treatments with GnRH agonist (GnRHa), testosterone (T), and DA antagonist (pimozide; Sigma Chemical Co., Saint-Quentin Fallavier, France), either alone or in combination, according to the protocol previously developed in the striped bass [22]. Sustained treatment (repeated injections, silastic capsules, emulsions, pellets, microspheres) with long-acting GnRHa is a current practice for stimulating gonadotropin synthesis and release in teleosts (for review, see [26]). In the present study, we chose the biweekly injection of GnRHa-containing microspheres, previously shown to be efficacious in striped bass [22]. However, GnRHa alone may not be sufficient to trigger gonadotropin synthesis and release as well as gonadal development in juvenile teleosts. Indeed, in various juvenile teleosts, this can only be achieved by a sustained treatment with GnRHa and T (for review, see [26]). Thus, we chose the biweekly injection of T-containing microspheres that was previously shown to be efficacious in striped bass [22]. Finally, because this combined treatment was still not sufficient in the eel (experiment 1), we tested the effect of biweekly injections of the long-acting DA-antagonist pimozide, which has been shown to prevent dopaminergic inhibition of GnRH action in various adult teleosts (for review, see [11]).

Microspheres (sustained-release delivery systems) containing T or GnRHa (D-Ala6, Pro9NEt-GnRH) were prepared at the Center of Marine Biotechnology and administered as previously described [22]. Biodegradable, T-containing microspheres were prepared according to a modified solvent-evaporation method [27]. The GnRHa-containing microspheres were prepared at 3% loading using a copolymer of fatty acid dimer and sebacic acid [28]. The long-acting DA-antagonist pimozide was suspended in a vehicle of 0.15 M NaCl with 0.02% acetic acid.

Experiment 1: effects of T and/or GnRHa on the reproductive axis Twenty-four female silver eels (body weight [BW], 458 ± 9 g, mean ± SEM) were randomly distributed in four experimental groups (n = 6 eels/ group). They were treated for 8 wk with four i.m. injections (one injection every 14 days) of one of the following microsphere treatments: T (4 mg/ kg BW), GnRHa (0.3 mg/kg BW), T and GnRHa (a combination of both types of microspheres at a dose of 4 mg of T and 0.3 mg/kg BW of GnRHa), or microspheres devoid of any hormone (controls).

Experiment 2: effects of T, GnRHa, and pimozide on the reproductive axis Fifty-six female silver eels (BW, 422 ± 4 g, mean ± SEM) were randomly distributed in eight experimental groups (n = 7 eels/group). They were treated for 6 wk with three i.m. injections (one injection every 14 days) of one of the following treatments: T (4 mg/kg BW), GnRHa (0.3 mg/kg BW), or pimozide (10 mg/kg BW), T and GnRHa, T and pimozide, GnRHa and pimozide, T and GnRHa, and pimozide. Control animals received injections of the pimozide solvent as well as of microspheres devoid of any hormone.

Sampling Procedure

In both series of experiments, fish were weighed and killed by decapitation 2 wk after the last injection. Blood was collected in heparinized tubes, and plasma samples obtained after centrifugation were aliquoted and kept frozen (–20°C) until LH RIA, steroid (T, E2) RIAs, and vitellogenin (Vg) immunoenzymatic assay. Pituitaries were quickly removed and kept frozen (–20°C) in 0.15 M NaCl (0.5 ml/pituitary) until extraction and LH RIA. Ovaries, liver, and digestive tract were dissected out and weighed for calculation of the gonadosomatic index (GSI; [gonad weight/ BW] x 100%), hepatosomatic index (HSI; [liver weight/BW] x 100%) and digestive tract-somatic index (DTSI; [digestive tract weight/BW] x 100%), respectively.

Gonad Histology

Ovary samples were quickly fixed in Bouin fluid, dehydrated, embedded in paraffin, cut in section (thickness, 8 µm), rehydrated, and stained by the Cleveland-Wolf method as described by Gabe [29].

RIA for Eel LH

Individual eel pituitaries were extracted by sonication (Bioblock Scientific, Illkirch, France), and supernatants were collected after centrifugation. Pituitary extracts and plasma samples were assayed in a RIA previously established for the ß subunit of carp LH and validated for the measurement of eel LH [30].

RIAs for T and E2

Concentrations of T and E2 in the plasma samples were measured using commercially available kits for plasma T (kit Testo-CT2; Cis Bio International, Gif sur Yvette, France) and plasma E2 (kit Estr-US-CT; Cis Bio International).

Immunoenzymatic Assay (ELISA) for Eel Vg

Vitellogenin levels in plasma samples were measured using an ELISA developed previously for the eel [31].

Immunocytochemical Study of Brain and Pituitary TH

To layout the anatomical support of the inhibition exerted by DA on the pituitary production of LH, we analyzed the brain and pituitary distribution of TH, the rate-limiting enzyme for catecholamine synthesis. Ten female silver eels were anesthetized by immersion in a 3-aminobenzoic acid ethyl ester solution (0.2%; Sigma) and perfused intracardially with 0.6% NaCl in 0.1 M PBS (pH 7.4) and then with 4% paraformaldehyde in PBS. The skull was removed, and the brain and pituitary were carefully dissected out and then stored overnight in fresh fixative at 4°C. On the following day, the brains were rinsed in PBS and immersed in 15% sucrose in PBS at 4°C. For cryostat sectioning, the tissues were frozen in cold isopentane and stored at –80°C.

Cryostat transversal or longitudinal sections (thickness, 16 µm) of eel brain and pituitary were incubated overnight in a 1:500 dilution of a rabbit anti-rat TH antibody (Institut Jacques Boy, Reims, France) in Coons buffer (pH 7.2) with 0.1% Triton (Sigma) at room temperature. The sections were then rinsed three times for 10 min each time in Coons buffer and exposed to peroxidase-conjugated goat anti-rabbit Fab fragments (1:100; Biosys, Compiègne, France) for 2 h [32]. Peroxidase activity was visualized (blue) with 4-chloro-1-naphtol (0.03%; Sigma) containing 0.03% hydrogen peroxide (Sigma). Finally, the sections were mounted in 0.1M glycerol-PBS buffer (pH 7.4) and analyzed by light microscopy.

Controls were carried out by omission of the primary or secondary antibodies or by replacement of the primary antiserum by nonimmune rabbit serum.

Statistical Analysis

For in vivo experiments, groups of six or seven eels were used for each treatment, and the results are expressed as the mean ± SEM. Statistical analyses were performed using Instat (Graph-Pad Software, Inc., San Diego, CA). Homogeneity of variance was assessed by the Bartlett test, and data were compared by one-way ANOVA followed by the Tukey-Kramer multiple-comparison test. When standard deviations were not equal, the Kruskal-Wallis nonparametric test was applied, followed by the Dunn multiple-comparison test.


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 REFERENCES
 
Effects of T and GnRHa on Biometric and Endocrine Parameters of the Female Eel

The effects of T and GnRHa were investigated in the eel in the same experimental conditions as previously developed in the striped bass [22].

Treatment with T-containing microspheres significantly increased plasma T levels (15 ± 1 ng/ml) compared to control eels (<0.1–0.5 ng/ml). Plasma E2 levels remained low and unchanged in T-treated eels (<0.1–1 ng/ml in T-implanted as well as in control eels), indicating no significant aromatization. The administration of T-containing microspheres during 8 wk induced various changes in female eel biometric parameters, with significant increases in GSI (x1.6, P < 0.01 vs. controls) and HSI (x1.4, P < 0.05) and a decrease in DTSI (x0.66, P < 0.05) (Fig. 1). Treatment with T also significantly increased pituitary LH content (x40, P < 0.05) (Fig. 2A), whereas plasma LH levels remained less than 0.4 ng/ml and plasma Vg levels were unchanged (Fig. 2B).



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  		FIG. 1. Effects of T and GnRHa on biometric parameters of the female eel. Eels received four i.m. injections over 8 wk of T (4 mg/kg BW)-containing microspheres, GnRHa (0.3 mg/kg BW)-containing microspheres, both T- and GnRHa-containing microspheres, or of microspheres devoid of hormone (controls). Internal biometric parameters were measured. Values are presented as the mean ± SEM (n = 6 eels/group). A) GSI. B) HSI. C) DTSI. *P < 0.05, **P < 0.01 versus controls (ANOVA)



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  		FIG. 2. Effects of T and GnRHa on endocrine parameters of the female eel. For eel treatments, see legend of Figure 1. Pituitary LH content was measured by RIA (A) and plasma Vg level by ELISA (B). Values are presented as the mean ± SEM (n = 6 eels/group). *P < 0.05 versus controls (Kruskal-Wallis nonparametric test)

In contrast, an 8-wk treatment with GnRHa-containing microspheres did not induce any significant change in any biometric parameters (GSI, HSI, DTSI) (Fig. 1) compared with control eels. Pituitary LH content was not modified (Fig. 2A), plasma LH levels remained undetectable (<0.4 ng/ml), and plasma Vg levels were unchanged (Fig. 2B). The combined treatment with T- and GnRHa-containing microspheres did not induce any further change in biometric or endocrine parameters compared to the treatment with T-containing microspheres alone (Figs. 1 and 2).

Effects of T, GnRHa, and DA-Receptor Antagonist (Pimozide) on Biometric and Endocrine Parameters of the Female Eel

Because GnRHa treatments, either alone or in combination with T, had no effects on the pituitary-gonadal axis of the eel, the contribution of DA was investigated by the administration of the D2-type DA receptor-antagonist pimozide.

Treatments with GnRHa or pimozide, either alone or in combination, for 6 wk did not induce any significant change in any of the biometric (GSI, HSI, DTSI) (Fig. 3) or endocrine parameters (pituitary LH content, plasma LH level, plasma Vg level) (Fig. 4) when compared with control eels.



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  		FIG. 3. Effects of T, GnRHa, and DA antagonist (pimozide) on biometric parameters of the female eel. Eels received three i.m. injections over 6 wk of T (4 mg/kg BW)-containing microspheres, GnRHa (0.3 mg/kg BW)-containing microspheres, pimozide (10 mg/kg BW), or combined treatments. Control fish received injections of vehicle alone. Internal biometric parameters were measured. Values are presented as the mean ± SEM (n = 7 eels/group). A) GSI. B) HSI. C) DTSI. *P < 0.05, **P < 0.01 versus controls (ANOVA)



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  		FIG. 4. Effects of T, GnRHa, and DA antagonist (pimozide) on endocrine parameters of the female eel. For eel treatments, see legend of Figure 3. The pituitary LH content (A) and plasma LH level (B) were measured by RIA and plasma Vg level (C) by ELISA. Values are presented as the mean ± SEM (n = 7 eels/group). ND, Not detectable. *P < 0.05, **P < 0.01, ***P < 0.001 versus controls (Kruskal-Wallis nonparametric test)

Treatment with T-containing microspheres for 6 wk induced various changes in biometric and endocrine parameters, which is in agreement with the results of the experiment described above: Compared to control eels, T induced slight increases in GSI and HSI (P < 0.05), a decrease in DTSI (P < 0.05) (Fig. 3), an increase in pituitary LH content (x21, P < 0.05), but no significant changes in plasma LH levels (<0.04 ng/ml) or Vg levels (Fig. 4).

Combined treatment with T- and GnRHa-containing microspheres did not induce any further change in biometric or in endocrine parameters compared to the treatment with T alone (Figs. 3 and 4), which is in agreement with the results of the experiment described above. Similarly, combined treatment with T and pimozide did not induce any further change in biometric or in endocrine parameters compared to the treatment with T-containing microspheres alone (Figs. 3 and 4).

In contrast, the combined treatment with T, GnRHa, and pimozide induced a marked increase in GSI that reached values (up to 4.7%) never observed in any control silver European eels (GSI, 1.2%–2.2%) (for review, see [5]) (P < 0.01 vs. controls (Fig. 3A). Although it did not induce any further change in the other biometric parameters (HSI, DTSI) compared to T alone (Fig. 3, B and C), the triple treatment with T, GnRHa, and pimozide induced a dramatic increase in the pituitary LH content, which reached levels (14.4 ± 4 µg/pituitary) 232-fold higher than those in control eels (0.06 ± 0.01 µg /pituitary, P < 0.001) (Fig. 4A). Moreover, the triple treatment induced a large release of LH, the plasma levels of which reached 5.7 ± 2.2 ng/ml, whereas they remained undetectable (<0.4 ng/ml) in all the other experimental groups (Fig. 4B). Plasma Vg levels were also largely increased by the triple treatment, compared to all the other groups (Fig. 4C), reaching a value of 25 ± 0.9 µg/ml, or 44-fold higher than those in control eels (0.57 ± 0.39 µg/ml, P < 0.01).

From all these data, it appears that the triple treatment with T, GnRHa, and DA antagonist was remarkably more potent than any of the other treatments in stimulating various parameters of the eel pituitary-gonadal axis: LH pituitary content and plasma levels, plasma Vg levels, and GSI.

Effects of T, GnRHa, and Pimozide on Ovarian Histology

The effect of the various treatments on ovarian development was further assessed by histological observation of the ovaries. Oocytes of control silver eels (Fig. 5A) had a large nucleus with small nucleoli at a peripheral position and numerous lipidic vesicles in the ooplasm. This feature typically corresponds to the oil-droplet stage of early vitellogenesis [33], also called endogenous vitellogenesis. Oocytes from eels treated with T (Fig. 5B), T and GnRHa (Fig. 5C), or T and pimozide (Fig. 5D) remained at the same stage (oil-droplet stage). In contrast, in the eels who received the triple treatment with T, GnRHa, and pimozide (Fig. 5E) the oocytes were enlarged and contained, in addition to the lipidic vesicles, large yolk granules deeply stained with Orange-G resulting from the incorporation of Vg. This stage corresponds to the yolk stage of vitellogenesis, also called exogenous vitellogenesis. These oocytes were surrounded by a thickened zona radiata and easily observable, numerous, and enlarged follicular cells, features that are also characteristics of an advanced stage of exogenous vitellogenesis (Fig. 5F).



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  		FIG. 5. Effects of T, GnRHa, and DA antagonist (pimozide) on eel ovarian histology. For eel treatments, see legend of Figure 3. Oocytes from control silver eels (A) showed small nucleoli (n) at the periphery of the nucleus (N) and contained numerous lipid vesicles (LV) in the ooplasma, a feature characteristic of the early vitellogenic stage (oil-droplet stage). Oocytes from T-treated eels (B), T- and GnRHa-treated eels (C), and T- and pimozide-treated eels (D) were at the same stage. Oocytes from eels treated with T, GnRHa, and pimozide (E) were enlarged and contained, in addition to large LVs, deeply stained yolk granules (black arrow) resulting from the incorporation of Vg. Higher magnification of the oocyte from T-, GnRHa-, and pimozide-treated eels (F) shows a thickened zona radiata (black arrowhead) and visible follicular cells (white arrowhead). These features (E and F) are characteristics of the yolk stage of vitellogenesis. Bar = 20 µm

Neuroanatomical Substrate for the Inhibition of Gonadotropin Secretion: Preopticohypophysial Dopaminergic Neuronal Tract

To describe the anatomical support of the inhibition exerted by DA on the eel pituitary gonadotropins, TH protein distribution was analyzed in the brain and pituitary. On transverse sections at the level of the anterior preoptic region (Fig. 6A), numerous TH-immunoreactive neurons were observed in the preoptic anteroventral nucleus, in the area ventral and ventrolateral to the preoptic recess, just above the anterior aspect of the optic chiasma, which is a nucleus referred to as the NPOav by Kah et al. [13] in the adult goldfish. From there, as shown on Figure 6B, axons form a dense TH-immunoreactive tract projecting posteriorly, ventrally to the preoptic recess, and then turning around the optic chiasma (Fig. 6B, longitudinal section) and reaching the mediobasal hypothalamus. At the pituitary level (Fig. 6C, transverse section), strongly labeled TH-immunoreactive axon terminals were observed innervating the PPD of the pituitary, the region where gonadotroph cells are located.



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  		FIG. 6. Characterization by immunocytochemistry of the preopticohypophysial dopaminergic neuronal pathway. Immunoreactive (ir) neurons were labeled with an antibody to TH. A) Transverse section at the level of the preoptic recess (RPO). Numerous ir cell bodies and fibers are located in the anterior preoptic area, in the nucleus preopticus anteroventralis (NPOav). B) Longitudinal section at the level of the RPO. A dense ir axonal tract (arrow) originating from the ir neurons of the NPOav project posteriorly, ventrally to the RPO and contouring the optic nerve (ON). C) Transverse section at the level of the medial basal hypothalamus (MBH) and PPD of the pituitary. Numerous ir axonal endings (arrow) innervate the PPD, the region where the gonadotroph cells are located. Bar = 100 µm


   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
REFERENCES
 
The present study, which is based on both physiological and anatomical approaches, demonstrates the pivotal inhibitory role exerted by DA on the gonadotropic function and onset of puberty in the juvenile eel.

Hormonal treatments were administered as previously developed in the juvenile striped bass [22]. Implantation of T-containing microspheres resulted in plasma T levels of approximately 15 ng/ml in the eel. Because no data are available concerning natural mature eels, these T values can be considered as physiological, as they are in the range of those (7–23 ng/ml; unpublished data) observed in naturally maturing female conger eels, Conger conger, which is a species phylogenetically close to Anguilla species. No significant increase in plasma E2 levels was observed in T-treated eels, which is in agreement with low aromatase activity in the immature eel [34]. Sustained treatments of juvenile eels with T significantly increased pituitary LH content, an effect that was enhanced with duration of T treatment, as shown by comparison between experiment 2 (6 wk) and experiment 1 (8 wk). A similar stimulatory effect of T on pituitary LH was observed in the juvenile female striped bass [22]. Increases in pituitary LH have also been observed in various immature fish treated with sex steroids, revealing the occurrence of a positive steroid feedback in juvenile teleosts (for review, see [9]). In the eel, the effect of T may result from a direct action on the pituitary gonadotrophs. Indeed, we previously demonstrated that the in vitro treatment with T of primary cultures of eel pituitary cells stimulated LH synthesis in a dose- and time-dependent manner [35]. This effect involved a selective increase in mRNA levels for the LH ß, but not {alpha}, subunit [2, 35].

As in the juvenile female striped bass [22], T also induced an increase in GSI in the eel, an effect that was enhanced with duration of T treatment. However, in both species, plasma LH or Vg levels remained undetectable. Histological observations indicated that the oocytes were still in the early vitellogenic (lipid droplets) stage in T-treated eels, and just as in control eels, no sign of vitellus incorporation could be observed. Analysis of the other biometric parameters showed an increase in HSI together with a decrease in DTSI. Regression of the digestive tract was shown previously to occur during experimental sexual maturation of the eel [36]. The present study suggests that androgens could mediate the mobilization of metabolic stores from the digestive tract toward the liver, which would participate in the metabolic processes required for gonadal growth and reproductive migration.

Our present study demonstrates that in the immature eel, a sustained, combined treatment with T and GnRH analog did not further increase pituitary LH levels compared to T treatment alone. Just as with T alone, plasma LH levels remained undetectable, Vg levels were unchanged, GSI did not further increase, and oocytes remained at the same early vitellogenic stage. This is in striking contrast to the situation in the juvenile striped bass [22, 37] or in juvenile salmonids [38], in which a combined treatment with T and GnRH has a synergistic effect on pituitary and plasma LH levels as well as on gonadal development.

Sustained treatments with GnRHa is a current practice for stimulating gonadotropin synthesis and release in teleosts, which, in contrast to mammals, do not require GnRH pulses. The lack of effect of GnRHa, even in T-treated eels, reinforces the hypothesis of an inhibitory factor acting at the silver stage to prevent the onset of puberty. Our present data confirm that DA is this factor. We demonstrated that pimozide, an antagonist of DA D2-like receptors, should be administered together with GnRHa in T-treated juvenile females to induce a very large increase in pituitary LH content. Notably, this combined treatment was the only treatment that was also able to trigger the plasma release of LH, to increase plasma Vg levels, and to result in a marked ovarian development, as evidenced by the strong increase of the GSI. Histological observations of the ovaries clearly demonstrated that the oocytes have reached a further step in vitellogenesis (yolk stage) that is characterized by the presence of yolk granules in the ooplasm resulting from the incorporation of Vg in the oocyte (exogenous vitellogenesis). Exogenous vitellogenesis is a gonadotropin-regulated process, as previously demonstrated in the eel [39] and in the other teleosts. Thus, whereas it had no further effect on the other biometric parameters (HSI, DTSI) compared to T alone, the triple treatment specifically activated the pituitary-gonadal axis, dramatically stimulating both LH synthesis and release and, consequently, Vg production and incorporation.

Thus, not only a deficiency in GnRH production, as shown in some other juvenile teleosts, but also a strong inhibition by DA of GnRH action impedes the onset of puberty in the eel. The present data support the conclusions of our former study, which showed an induction of LH release and ovarian vitellogenesis by GnRHa and DA antagonist after a large stimulation of LH synthesis by long-term E2 pretreatment [10]. These previous data were at variance with those obtained in other juvenile teleosts in which DA was shown to play no role in the prepubertal control of gonadotropins [22, 23, 37]. However, the use of very different experimental protocols (nature, dose, duration of hormonal treatments) from one study to another makes comparison difficult. One reason to put our first results to the test was that E2 had been reported to enhance the inhibitory dopaminergic tone in adult teleosts [9, 24, 25], leading to the possibility that DA inhibition in the eel could be the result of the long-term E2 pretreatment. In fact, in adult teleosts, the stimulatory action of E2 on DA partly mediates its negative feedback on LH [9, 25, 40]. Therefore, such a regulation is rather unlikely in juvenile fish, including eel, in which sexual steroids exert a positive feedback on LH. No data are yet available concerning regulation of DA activity by steroids in juvenile teleosts. Preliminary studies from our group indicate no stimulatory effect of E2 on the expression of TH in any part of the eel brain (unpublished data). Nevertheless, in the present study, we choose to use the same protocol that previously allowed us to rule out a role of DA in the control of puberty in the striped bass [22] to compare the two species under the same experimental conditions. The present results now allow us to firmly assess the key role of DA inhibition in the juvenile eel. The strength of this inhibitory mechanism in the eel likely is related to its biological cycle, requiring a true lock of reproduction until the occurrence of oceanic migration. We can hypothesize that environmental factors of the oceanic reproductive migration, such as hydrostatic pressure or metabolic challenge (for review, see [5]) may participate in the relief from DA inhibition.

It is important to note that in juvenile eels, neither GnRH nor pimozide, either alone or in combination, were able to induce a significant increase in the pituitary content of LH or in its release in the absence of T treatment. As in the striped bass [22], T was necessary to observe the effect of GnRH. Thus, in both species, T appears to play a facilitatory role, likely by increasing pituitary sensitivity to GnRH, as it does in various teleosts [38, 41, 42]. Similarly, in various juvenile teleosts, GnRHa is efficacious only when administered in combination with T (for review, see [26]). The mechanisms underlying this androgen-induced increased pituitary sensitivity to GnRH may involve an increase in GnRH-receptor gene expression and/or a modulation of GnRH-receptor coupling and signaling pathway, as demonstrated in the goldfish [43]. Thus, in addition to directly stimulating LHß gene expression in the eel [2, 35], T would have the ability to increase pituitary sensitivity to GnRH. In agreement with our present data in the eel, several lines of evidence indicate a key role of androgens in activation of the brain-pituitary gonadal axis at puberty in juvenile teleosts (for review, see [44]). In the eel, T appears to play additional roles at puberty in facilitating the reproductive migration, such as mobilization of metabolic stores as shown in the present work or adaptation of the visual system to deep-sea conditions as demonstrated by other studies (for review, see [45]).

Involvement of DA in the regulation of ovulation and spermiation has been demonstrated in a certain number of adult teleosts, but few data are available regarding prepubertal fish. To the best of our knowledge, only one study, which was carried out in the juvenile spadefish, previously suggested a role for DA in the prepubertal blockade of reproduction in a teleost, showing a reduction of DA turnover in the hypothalamus at the time of puberty [21]. In striking contrast are the cases of the striped bass [22, 37] and the red seabream [23], in which DA has been clearly demonstrated not to be involved in the control of puberty. In the female Japanese eel, Anguilla japonica, one group reported that a chronic treatment with T alone could induce a large ovarian development, with no further effect of GnRHa or DA antagonist [46]. However, in spite of the massive doses of steroid administered (20-fold more elevated than in the present study), the authors could not observe any increase in plasma LH level, suggesting that different mechanisms were involved at these extraphysiological T doses. Such an effect of T also was not found by other authors in the same species using lower doses of steroids [47].

Studies in adult teleosts have shown that DA, acting on D2-like receptors on the gonadotrophs [48], may oppose GnRH actions through a down-regulation of pituitary GnRH receptors [49, 50] or acutely restrain LH release by interfering with intracellular GnRH signal-transducing pathways (for review, see [18]). Beside these direct pituitary effects, DA was also shown in the goldfish to exert additional inhibitory effects on GnRH neurons, blocking the synthesis of the peptide or inhibiting its release from the pituitary nerve terminals [51, 52]. In the eel, our data demonstrate that DA exerts a major inhibitory action at the level of pituitary gonadotrophs, antagonizing the actions of GnRH even if we cannot rule out the possibility of additional targets of DA inhibition at the level of GnRH neurons, either directly or through other interneurons. Our results clearly demonstrate the potent inhibitory role exerted by DA through D2-like receptors on LH release in the immature eel. Moreover, as shown by its large effects on both plasma and pituitary LH levels, we demonstrate here that DA counteracts GnRH action not only on LH release but also on LH synthesis. To our knowledge, this is the first demonstration in a teleost of DA inhibitory control of GnRH-induced LH synthesis. Indeed, this has never been reported in adult fish (for review, see [18]).

Our physiological data on the dopaminergic blockade of puberty are validated by our anatomical data. In the prepubertal silver eel, as shown in the present study, the PPD receives an abundant dopaminergic innervation: Numerous TH-positive cells were observed in the preoptic anteroventral nucleus, in the area ventral and ventrolateral to the preoptic recess, which is a nucleus referred to as the NPOav [13] and is responsible for the inhibition of LH release in adult teleosts [13, 14]. These perikarya have been clearly identified as dopaminergic in the eel, and called nucleus preopticus parvocellularis anterior [53], using a specific anti-DA antibody. The present study shows that these neurons project posteriorly, via bilateral tracts in the preoptic region, toward the hypothalamus. At the pituitary level, strongly labeled, TH-immunoreactive axon terminals were observed innervating the PPD, the region where gonadotroph cells are located [54]. In teleost fish, the hypothalamohypophysial portal system is absent, and hypothalamic nerve fibers run through the pituitary stalk, discharging their neurohormones in close proximity to the secretory cells in the adenohypophysis. This direct innervation of the PPD is the functional equivalent of the median eminence in higher vertebrates. Therefore, the LH-secreting cells in the eel are directly innervated by both GnRH neurons [55] and dopaminergic ones (present study). Thus, a preopticohypophysial TH-immunoreactive pathway, similar to that described in adult teleost as responsible for the DA inhibition of LH secretion, is evidenced here in the juvenile eel. Furthermore, the high density and strong immunoreactivity of the TH-positive fibers innervating the eel PPD are in good agreement with the strength of the blockade that DA exerts on the eel gonadotropic function.

The DA inhibitory actions on LH and reproduction, which are found in a wide phylogenetic range of adult teleosts (see Introduction) is not restricted to fish. In amphibians, some data in the frog, Rana temporaria, point to an important role of DA inhibition in the regulation of seasonal reproduction, particularly during hibernation [56]. In birds, similarly, evidence indicates an inhibitory role of DA on LH release and ovulation [57, 58]. In mammals, DA has been shown in several species to be involved in the modulation of pulsatile LH release through direct or indirect actions. This has been extensively studied in sheep, in which DA participates in the inhibition of gonadotrophic activity before puberty as well as in the adult during seasonal anestrus by acting on GnRH neurons [59–61]. A direct inhibitor action of DA at the pituitary level was reported in the rabbit, in which DA inhibits GnRH-stimulated LH release in normal as well as in stalk-sectioned animals [62]. Finally, in humans, numerous observations suggest that a direct inhibition by DA on LH release may occur under physiological and pathological conditions [63]. The inhibitory role of DA in the control of reproduction may even be more ancient than the origin of vertebrates. Indeed, in crustaceans, DA inhibits ovarian development by inhibiting the release of gonad-stimulating hormone [64].

In conclusion, our data in the eel, which is considered to be a primitive teleost, support the view that DA inhibition is an ancient evolutionary component of the neuroendocrine regulation of reproductive function, which appears to be a powerful counterpart to the well-known stimulatory role of GnRH. In fact, DA acts as an inhibitor of reproduction in representatives of all classes of vertebrates, but this inhibitory role has been differentially conserved throughout vertebrate evolution. Thus, the intensity of DA inhibition, its involvement at puberty or during adulthood, and its main site of action may differ between classes of vertebrates as well as within a smaller phylogenetic unit, such as teleosts or mammals. The present data demonstrate that in the eel, DA acts directly at the pituitary level to counteract both GnRH-stimulated LH synthesis and release and that these inhibitory actions represent a true lock for puberty.


   ACKNOWLEDGMENTS
 
We are grateful to Dr. E. Burzawa-Gérard (CNRS, MNHN, Paris, France) for standard and antiserum for eel Vg ELISA. We thank D. Rooke (European Programme Leonardo da Vinci, University of Keele, UK/ MNHN, France) for English corrections, and Dr. F.A. Weltzien (MNHN, Marie Curie European Fellowship) for fruitful discussions.


   FOOTNOTES
 
1 Supported in part by research grant 3900 from CNRS, France/NSF, USA (to S.D. and Y.Z.). Back

2 Correspondence: Sylvie Dufour, USM 0401, UMR 5178 CNRS, DMPA, Muséum National d'Histoire Naturelle, 7 rue Cuvier, 75231 Paris Cedex 05, France. FAX: 33 1 40793618; dufour{at}mnhn.fr Back

Received: 8 April 2004.

First decision: 3 May 2004.

Accepted: 14 June 2004.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 

  1. Huang YS, Rousseau K, Le Belle N, Vidal B, Burzawa-Gérard E, Marchelidon J, Dufour S. Insulin-like growth factor stimulates gonadotrophin production from eel pituitary cells: a possible metabolic signal for induction of puberty. J Endocrinol 1998 159:43-52[Abstract]
  2. Huang YS, Rousseau K, Sbaihi M, Le Belle N, Schmitz M, Dufour S. Cortisol selectively stimulates pituitary gonadotropin ß-subunit in a primitive teleost, Anguilla anguilla. Endocrinology 1999 140:1228-1235[Abstract/Free Full Text]
  3. Lauder GV, Liem KF. Patterns of diversity and evolution in ray-finned fishes. In: Northcutt RC, David RE (eds.), Fish Neurobiology, 1st ed. Ann Arbor, MI: University of Michigan Press; 1983:2–24
  4. Stone R. Freshwater eels are slip-sliding away. Science 2003 302:221-222[Abstract/Free Full Text]
  5. Dufour S, Burzawa-Gérard E, Le Belle N, Sbaihi M, Vidal B. Reproductive endocrinology of the European eel, Anguilla anguilla. In: Aida K, Tsukamoto K, Yamauchi K (eds.), Eel Biology. Tokyo: Springer Verlag; 2003:373–383
  6. Fontaine M. Sur la maturation complète des organes génitaux de l'anguille mâle et l'émission spontanée de ses produits sexuels. C R Acad Sci Paris 1936 202:1312-1315
  7. Fontaine M, Bertrand E, Lopez E, Callamand O. Sur la maturation des organes génitaux de l'anguille femelle (Anguilla anguilla L.) et l'émission spontanée des oeufs en aquarium. C R Acad Sci Paris 1964 259:822-824
  8. Dufour S, Delerue-Le Belle N, Fontaine YA. Effects of steroid hormones on pituitary immunoreactive gonadotropin in European freshwater eel, Anguilla anguilla L. Gen Comp Endocrinol 1983 52:190-197[CrossRef][Medline]
  9. Goos HJ. Steroid feedback on pituitary gonadotropin secretion. In: Idler DR, Crim LW, Walsh J (eds.), Proceedings of the Third International Symposium on Reproductive Physiology of Fish. St. John's University, St. John, NS, Canada: Fish Symposium; 1987:16–20
  10. Dufour S, Lopez E, Le Menn F, Le Belle N, Baloche S, Fontaine YA. Stimulation of gonadotropin release and of ovarian development, by the administration of a gonadoliberin agonist and of dopamine antagonists, in female silver eel pretreated with estradiol. Gen Comp Endocrinol 1988 70:20-30[CrossRef][Medline]
  11. Peter RE, Chang JP, Nahorniak CS, Omeljaniuk RJ, Sokolowska M, Shih SH, Billard R. Interactions of catecholamines and GnRH in regulation of gonadotropin secretion in teleost fish. Recent Prog Horm Res 1986 42:513-548
  12. Peter RE, Paulencu CR. Involvement of the preoptic region in the gonadotropin release-inhibition in the goldfish. Neuroendocrinology 1980 31:133-141[Medline]
  13. Kah O, Dulka JG, Dubourg P, Thibault J, Peter RE. Neuroanatomical substrate for the inhibition of gonadotrophin secretion in goldfish: existence of a dopaminergic preopticohypophyseal pathway. Neuroendocrinology 1987 45:451-458[Medline]
  14. Linard B, Anglade I, Corio M, Navas JM, Pakdel F, Saligaut C, Kah O. Estrogen receptors are expressed in a subset of tyrosine hydroxylase-positive neurons of the anterior preoptic region in the rainbow trout. Neuroendocrinology 1996 63:156-165[Medline]
  15. Peter RE, Lin HR, Van Der Kraak G. Induced ovulation and spawning of cultured freshwater fish in China: advances in application of GnRH analogues and dopamine antagonists. Aquaculture 1988 74:1-10[CrossRef]
  16. Van Asselt LA, Goos HJ, Smit-van Dijk W, Speetjens PAM, Van Oordt PG. Evidence for the involvement of D2 receptors in the dopaminergic inhibition of gonadotropin release in the African catfish, Clarias gariepinus. Aquaculture 1988 72:369-378[CrossRef]
  17. Van der Kraak G, Donaldson EM, Chang JP. Dopamine involvement in the regulation of gonadotropin secretion in coho salmon. Can J Zool 1986 64:1245-1248[CrossRef]
  18. Yaron Z, Gur G, Melamed P, Rosenfeld H, Elizur A, Levavi-Sivan B. Regulation of fish gonadotropins. Int Rev Cytol 2003 225:131-185[Medline]
  19. Zohar Y, Harel M, Hassin S, Tandler A. Broodstock management and manipulation of spawning in the gilthead seabream, Sparus aurata. In: Bromage N, Roberts RJ (eds.), Broodstock Management and Egg and Larval Quality. London: Blackwell Scientific Press; 1995:94–117
  20. Copeland PA, Thomas P. Control of gonadotropin release in the Atlantic croaker (Micropogonias undulatus): evidence for lack of dopaminergic inhibition. Gen Comp Endocrinol 1989 74:474-483[CrossRef][Medline]
  21. Marcano D, Guerrero HY, Gago N, Cardillo E, Requena M, Ruiz L. Monoamine metabolism in the hypothalamus of the juvenile teleost fish, Chaetodipterus faber. In: Goetz FW, Thomas P (eds.), Proceedings of the Fifth International Symposium on the Reproductive Physiology of Fish. University of Texas, Austin, TX: Fish Symposium; 1995;95:64–66
  22. Holland MC, Hassin S, Zohar Y. Effects of long-term testosterone, gonadotropin-releasing hormone agonist, and pimozide treatments on gonadotropin II levels and ovarian development in juvenile female striped bass (Morone saxatilis). Biol Reprod 1998 59:1153-1162[Abstract/Free Full Text]
  23. Kumakura N, Okuzawa K, Gen K, Kagawa H. Effects of gonadotropin-releasing hormone agonist and dopamine antagonist on hypothalamus-pituitary-gonadal axis of prepubertal female red seabream (Pagrus major). Gen Comp Endocrinol 2003 131:264-273[CrossRef][Medline]
  24. Linard B, Bennani S, Saligaut C. Involvement of estradiol in a catecholamine inhibitory tone of gonadotropin release in rainbow trout (Oncorhyncus mykiss). Gen Comp Endocrinol 1995 99:192-196[CrossRef][Medline]
  25. Saligaut C, Linard B, Breton B, Anglade I, Bailhache T, Kah O, Jego P. Brain aminergic systems in salmonids and other teleosts in relation to steroid feedback and gonadotropin release. Aquaculture 1999 177:13-20[CrossRef]
  26. Crim L. Hormonal manipulation of fish seasonal reproductive cycles. In: Scott AP, Sumpter JP, Kime DE, Rolfe MS (eds.), Reproductive Physiology of Fish. Sheffield, UK: Fish Symposium; 1991:43–47
  27. Benita S, Benoit JP, Puisieux F, Thies C. Characterization of drug-loaded poly(D,L-lactide) microspheres. J Pharm Sci 1984 73:1721-1724[CrossRef][Medline]
  28. Mylonas CC, Tabata Y, Langer R, Zohar Y. Preparation and evaluation of polyanhydride microspheres containing gonadotropin-releasing hormone (GnRH), for inducing ovulation and spermiation in fish. J Controlled Release 1995 35:25-34
  29. Gabe M. Techniques Histologiques. Paris: Masson & Cie; 1968
  30. Dufour S, Delerue-Le Belle N, Fontaine YA. Development of a heterologous radioimmunoassay for eel (Anguilla anguilla) gonadotropin. Gen Comp Endocrinol 1983 49:404-413[CrossRef][Medline]
  31. Burzawa-Gérard E, Nath P, Baloche S, Peyon P. ELISA (enzyme-linked immunosorbent assay) for vitellogenin and vitellus in the eel (Anguilla anguilla) and in the Indian major carp (Labeo rohita). In: Scott AP, Sumpter JP, Kime DE, Rolfe MS (eds.), Reproductive Physiology of Fish. University of East Anglia, Sheffield, UK: Fish Symposium; 1991:319
  32. Boularand S, Faucon-Biguet NF, Vidal B, Veron M, Mallet J, Vincent JD, Dufour S, Vernier P. Tyrosine hydroxylase in the European eel (Anguilla anguilla): cDNA cloning, brain distribution, and phylogenetic analysis. J Neurochem 1998 71:460-470[Medline]
  33. Ohta H, Kagawa H, Tanaka H, Okuzawa K, Iinuma N, Hirose K. Artificial induction of maturation and fertilization in the Japanese eel, Anguilla japonica. Fish Physiol Biochem 1997 17:163-169[CrossRef]
  34. Leloup-Hatey J, Hardy A, Nahoul K, Quérat B, Zohar Y. Influence of a gonadotrophic treatment upon the ovarian steroidogenesis in European silver eel. In: Reproduction in Fish: Basic and Applied Aspects in Endocrinology and Genetics. Paris: INRA; 1988:127–130
  35. Huang YS, Schmitz M, Le Belle N, Chang CF, Quérat B, Dufour S. Androgens stimulate gonadotropin-II ß subunit in eel pituitary cells in vitro. Mol Cell Endocrinol 1997 131:157-166[CrossRef][Medline]
  36. Pankhurst NW, Sorensen PW. Degeneration of the alimentary tract in sexually maturing European Anguilla anguilla (L.) and American eels Anguilla rostrata (LeSueur). Can J Zool 1984 62:1143-1149
  37. Hassin S, Holland MCH, Zohar Y. Early maturity in the male striped bass: FSH and LH gene expression and their regulation by GnRHa and testosterone. Biol Reprod 2000 63:1691-1697[Abstract/Free Full Text]
  38. Crim LW, Evans DM. Influence of testosterone and/or luteinizing-hormone releasing hormone analogue on precocious sexual development in the juvenile rainbow trout. Biol Reprod 1983 29:137-142[Abstract]
  39. Burzawa-Gerard E, Dumas-Vidal A. Effects of 17ß-estradiol and carp gonadotropin on vitellogenesis in normal and hypophysectomized European silver female eel (Anguilla anguilla L.) employing a homologous radioimmunoassay for vitellogenin. Gen Comp Endocrinol 1991 84:264-276[CrossRef][Medline]
  40. Vétillard A, Atteke C, Saligaut C, Jego P, Bailhache T. Differential regulation of tyrosine hydroxylase and estradiol receptor expression in the rainbow trout brain. Mol Cell Endocrinol 2003 199:37-47[CrossRef][Medline]
  41. Trudeau VL, Lin HR, Peter RE. Testosterone potentiates the serum gonadotropin response to gonadotropin-releasing hormone in the common carp (Cyprinus carpio) and Chinese loach (Paramisgurnus dabryanus). Can J Zool 1991 96:2480-2484
  42. Lo A, Chang JP. In vitro application of testosterone potentiates gonadotropin-releasing hormone-stimulated gonadotropin-II secretion from cultured goldfish pituitary cells. Gen Comp Endocrinol 1998 111:334-346[CrossRef][Medline]
  43. Lo A, Chang JP. In vitro action of testosterone in potentiating gonadotropin-releasing hormone-stimulated gonadotropin-II secretion in goldfish pituitary cells: involvement of protein kinase C, calcium, and testosterone metabolites. Gen Comp Endocrinol 1998 111:318-333[CrossRef][Medline]
  44. Goos H, Schulz R. Gonadal steroid hormones drive puberty in fish. In: Kawashima S, Kikuyama S (eds.), Advances in Comparative Endocrinology. Bologna, Italy: Monduzzi Editore; 1997:1429–1433
  45. Lokman PM, Rohr DH, Davie PS, Young G. The physiology of silvering in anguillid eels: androgens and control of metamorphosis from the yellow to silver stage. In: Aida K, Tsukamoto K, Yamauchi K (eds.), Eel Biology. Tokyo: Springer Verlag; 2003:331–349
  46. Lin H-R, Zhang ML, Zhang SM, Van Der Kraak G, Peter RE. Stimulation of pituitary gonadotropin and ovarian development by chronic administration of testosterone in female Japanese silver eel, Anguilla japonica. Aquaculture 1991 96:87-95
  47. Jeng SR, Chen GR, Lai JY, Huang YS, Dufour S, Chang CF. Regulation of pituitary gonadotropin II and growth hormone content by sex steroids and pituitary extract in the aquacultured Japanese eel, Anguilla japonica. Aquaculture 2002 209:319-332[CrossRef]
  48. Chang JP, Yu KL, Wong AO, Peter RE. Differential actions of dopamine receptor subtypes on gonadotropin and growth hormone release in vitro in goldfish. Neuroendocrinology 1990 51:664-674[Medline]
  49. De Leeuw R, Van't Veer C, Goos HJ, Van Oordt PG. The dopaminergic regulation of gonadotropin-releasing hormone receptor binding in the pituitary of the African catfish, Clarias gariepinus. Gen Comp Endocrinol 1988 72:408-415[CrossRef][Medline]
  50. De Leeuw R, Habibi HR, Nahorniak CS, Peter RE. Dopaminergic regulation of pituitary gonadotropin-releasing hormone receptor activity in the goldfish (Carassius auratus). J Endocrinol 1989 121:239-247[Abstract/Free Full Text]
  51. Yu KL, Peter RE. Dopaminergic regulation of brain gonadotropin-releasing hormone in male goldfish during spawning behavior. Neuroendocrinology 1990 52:276-283[Medline]
  52. Peter RE, Trudeau V, Sloley BD, Peng C, Nahorniak CS. Actions of catecholamines, peptides and sex steroids in regulation of gonadotropin-II in the goldfish. In: Scott AP, Sumpter JP, Kime DE, MS Rolfe (eds.), Reproductive Physiology of Fish. University of East Anglia, Sheffield, UK: Fish Symposium; 1991:30–34
  53. Roberts BL, Meredith GE, Maslam S. Immunocytochemical analysis of the dopamine system in the brain and spinal cord of the European eel, Anguilla anguilla. Anat Embryol 1989 180:401-412[CrossRef][Medline]
  54. Olivereau M. Nagahama Y. Immunocytochemical study of eel and trout gonadotrophs. Gen Comp Endocrinol 1982 46:365-366
  55. Montero M, Vidal B, King JA, Tramu G, Vandesande F, Dufour S, Kah O. Immunocytochemical localization of mammalian GnRH (gonadotropin-releasing hormone) and chicken GnRH-II in the brain of the European silver eel (Anguilla anguilla L). J Chem Neuroanat 1994 7:227-241[CrossRef][Medline]
  56. Sotowska-Brochocka J, Martynska L, Licht P. Dopaminergic inhibition of gonadotropic release in hibernating frogs, Rana temporaria. Gen Comp Endocrinol 1994 93:192-196[CrossRef][Medline]
  57. El Halawani ME, Burke WH. Brain monoamine metabolism of turkey hens in various stages of their reproductive cycle. Biol Reprod 1976 15:254-259[Abstract]
  58. Sharp PJ, Talbot RT, Macnamee MC. Evidence for the involvement of dopamine and 5-hydroxytryptamine in the regulation of the preovulatory release of luteinizing hormone in the domestic hen. Gen Comp Endocrinol 1989 76:205-213[CrossRef][Medline]
  59. Brango CW, Whisnant CS, Goodman R. A role of catecholaminergic neurons in the suppression of pulsatile luteinizing hormone secretion in the prepubertal ewe lamb. Neuroendocrinology 1990 52:448-454[Medline]
  60. Thiéry JC, Gayrard V, Le Corre S, Viguié C, Martin GB, Chemineau P, Malpaux B. Dopaminergic control of LH secretion by the A15 nucleus in anestrous ewes. J Reprod Fertil 1995 49:285-296
  61. Bertrand F, Thiéry J, Picard S, Malpaux B. Implication of D2-like dopaminergic receptors in the median eminence during the establishment of long-day inhibition of LH secretion in the ewe. J Endocrinol 1999 163:243-254[Abstract]
  62. Dailey RA, Tsou RC, Tindall GT, Neill JD. Direct hypophysial inhibition of luteinizing hormone release by dopamine in the rabbit. Life Sci 1978 22:1491-1498[CrossRef][Medline]
  63. Huseman CA, Kugler JA, Schneider IG. Mechanism of dopaminergic suppression of gonadotropin secretion in men. J Clin Endocrinol Metab 1980 51:209-214[Abstract/Free Full Text]
  64. Chen YN, Fan HF, Hsieh SL, Kuo CM. Physiological involvement of DA in ovarian development of the freshwater giant prawn, Macrobrachium rosenbergii. Aquaculture 2003 228:383-395[CrossRef]



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