Actions of sex steroids on kisspeptin expression and other reproduction-related genes in the brain of the teleost fish European sea bass

Kisspeptins are well known as mediators of the coordinated communication between the brain-pituitary axis and the gonads in many vertebrates. To test the hypothesis that gonadal steroids regulate kiss1 and kiss2 mRNA expression in European sea bass (a teleost fish), we examined the brains of gonad-intact (control) and castrated animals, as well as castrated males (GDX) and ovariectomized females (OVX) that received testosterone (T) and estradiol (E₂) replacement, respectively, during recrudescence. In GDX males, low expression of kiss1 mRNA is observed by in situ hybridization in the caudal hypothalamus (CH) and the mediobasal hypothalamus (MBH), although hypothalamic changes in kiss1 mRNA levels were not statistically different among the groups, as revealed by real-time PCR. However, T strongly decreased kiss2 expression levels in the hypothalamus, which was documented in the MBH and the nucleus of the lateral recess (NRLd) in GDX T-treated sea bass males. Conversely, it appears that E₂ evokes low kiss1 mRNA in the CH, while there were cells expressing kiss2 in the MBH and NRLd in these OVX females. These results demonstrate that kisspeptin neurons are presumably sensitive to the feedback actions of sex steroids in the sea bass, suggesting that the MBH represents a major site for sex steroid actions on kisspeptins in this species. Also, recent data provide evidence that both positive and negative actions occur in key factors involved in sea bass reproductive function, including changes in the expression of gnrh-1/gonadotropin, cyp19b, er and ar genes and sex steroid and gonadotropin plasma levels in this teleost fish.

Cyp19a1 (aromatase) expression in the Xenopus brain at different developmental stages

Cytochrome P450 aromatase (P450arom; aromatase) is a microsomal enzyme involved in the production of endogeneous sex steroids by converting testosterone into oestradiol. Aromatase is the product of the cyp19a1 gene and plays a crucial role in the sexual differentiation of the brain and in the regulation of reproductive functions. In the brain of mammals and birds, expression of cyp19a1 has been demonstrated in neuronal populations of the telencephalon and diencephalon. By contrast, a wealth of evidence established that, in teleost fishes, aromatase expression in the brain is restricted to radial glial cells. The present study investigated the precise neuroanatomical distribution of cyp19a1 mRNA during brain development in Xenopus laevis (late embryonic to juvenile stages). For this purpose, we used in situ hybridisation alone or combined with the detection of a proliferative (proliferating cell nuclear antigen), glial (brain lipid binding protein, Vimentin) or neuronal (acetylated tubulin; HuC/D; NeuroβTubulin) markers. We provide evidence that cyp19a1 expression in the brain is initiated from the very early larval stage and remains strongly detected until the juvenile and adult stages. At all stages analysed, we found the highest expression of cyp19a1 in the preoptic area and the hypothalamus compared to the rest of the brain. In these two brain regions, cyp19a1-positive cells were never detected in the ventricular layers. Indeed, no co-labelling could be observed with radial glial (brain lipid binding protein, Vimentin) or dividing progenitors (proliferating cell nuclear antigen) markers. By contrast, cyp19a1-positive cells perfectly matched with the distribution of post-mitotic neurones as shown by the use of specific markers (HuC/D, acetylated tubulin and NeuroβTubulin). These data suggest that, similar to that found in other tetrapods, aromatase in the brain of amphibians is found in post-mitotic neurones and not in radial glia as reported in teleosts.

Distribution of GABA-immunoreactive neurons in the forebrain of the goldfish, Carassius auratus

The distribution of gamma-aminobutyric acid (GABA) immunoreactivity was studied in the forebrain (tel- and diencephalon) of the goldfish by means of immunocytochemistry on Vibratome sections using antibodies against GABA. Positive perikarya were detected in the olfactory bulbs and in all divisions of the telencephalon, the highest density being found along the midline. In the diencephalon, GABA-containing cell bodies were found in the hypothalamus, in particular in the preoptic and tuberal regions. The inferior lobes, the nucleus recessus lateralis, and more laterodorsal regions, such as the nucleus glomerulosus and surrounding structures, also exhibited numerous GABA-positive perikarya. Cell bodies were also noted in the thalamus, in particular in the dorsomedial, dorsolateral and ventromedial nuclei. The relative density of immunoreactive fibers was evaluated for each brain nucleus and classified into five categories. This ubiquitous distribution indicates that, as in higher vertebrates, GABA most probably represents one of the major neurotransmitters in the brain of teleosts.

Endocrine disrupters: a review of some sources, effects, and mechanisms of actions on behaviour and neuroendocrine systems

Some environmental contaminants interact with hormones and may exert adverse consequences as a result of their actions as endocrine disrupting chemicals (EDCs). Exposure in people is typically a result of contamination of the food chain, inhalation of contaminated house dust or occupational exposure. EDCs include pesticides and herbicides (such as dichlorodiphenyl trichloroethane or its metabolites), methoxychlor, biocides, heat stabilisers and chemical catalysts (such as tributyltin), plastic contaminants (e.g. bisphenol A), pharmaceuticals (i.e. diethylstilbestrol; 17α-ethinylestradiol) or dietary components (such as phytoestrogens). The goal of this review is to address the sources, effects and actions of EDCs, with an emphasis on topics discussed at the International Congress on Steroids and the Nervous System. EDCs may alter reproductively-relevant or nonreproductive, sexually-dimorphic behaviours. In addition, EDCs may have significant effects on neurodevelopmental processes, influencing the morphology of sexually-dimorphic cerebral circuits. Exposure to EDCs is more dangerous if it occurs during specific 'critical periods' of life, such as intrauterine, perinatal, juvenile or puberty periods, when organisms are more sensitive to hormonal disruption, compared to other periods. However, exposure to EDCs in adulthood can also alter physiology. Several EDCs are xenoestrogens, which can alter serum lipid concentrations or metabolism enzymes that are necessary for converting cholesterol to steroid hormones. This can ultimately alter the production of oestradiol and/or other steroids. Finally, many EDCs may have actions via (or independent of) classic actions at cognate steroid receptors. EDCs may have effects through numerous other substrates, such as the aryl hydrocarbon receptor, the peroxisome proliferator-activated receptor and the retinoid X receptor, signal transduction pathways, calcium influx and/or neurotransmitter receptors. Thus, EDCs, from varied sources, may have organisational effects during development and/or activational effects in adulthood that influence sexually-dimorphic, reproductively-relevant processes or other functions, by mimicking, antagonising or altering steroidal actions.

Early regulation of brain aromatase (cyp19a1b) by estrogen receptors during zebrafish development

Early expression of estrogen receptors (esr) and their role in regulating early expression of cyp19a1b encoding brain aromatase were examined in the brain of zebrafish. Using in toto hybridization and quantitative reverse transcriptase-polymerase chain reaction (RT-PCR), a significant increase in the expression of esr1, esr2a, and esr2b was observed between 24 and 48 hours postfertilization (hpf). In toto hybridization demonstrated that esr2a and esr2b, but not esr1, are found in the hypothalamus. Using real-time RT-PCR, an increase in cyp19a1b mRNAs occurs between 24 and 48 hpf, indicating that expression of cyp19a1b is temporally correlated with that of esr. This increase is blocked by the pure anti-estrogen ICI182,780. Furthermore, E2 treatment of cyp19a1b-GFP (green fluorescent protein) transgenic embryos results in appearance of GFP expression in the brain as early as 25 hpf. These results indicate that basal expression of cyp19a1b expression in the brain of developing zebrafish most likely relies upon expression of esr that are fully functional before 25 hpf.

GnRH and GnRH receptors in metazoa: a historical, comparative, and evolutive perspective

About 50years after Harris's first demonstration of its existence, GnRH has strongly stimulated the interest and imagination of scientists, resulting in a high number of studies in an increasing number of species. For the endocrinologist, GnRH, via its actions on the synthesis and release of pituitary gonadotrophins, is first an essential hormone for the initiation and maintenance of the reproductive axis, but recent data suggest that GnRH emerged in animals lacking a pituitary. In this context, this review intends to explore the current status of knowledge on GnRH and GnRH receptors in metazoa in order to see if it is possible to draw an evolutive scenario according to which GnRH actions progressively evolved from the control of simple basic functions in early metazoa to an indirect mean of controlling gonadal activity in vertebrates through a sophisticated network of finely tuned neurons developing in a rather fascinating way. This review also intends to provide an evolutive scenario based on the recent advances of whole genome sequencing possibly explaining the number of GnRH and GnRH receptor variants according to the 2R and 3R theories accompanied by gene losses.

Analysis of the estrogen regulation of the zebrafish estrogen receptor (ER) reveals distinct effects of ERalpha, ERbeta1 and ERbeta2

We have previously cloned and characterized three estrogen receptors (ER) in the zebrafish (zfERalpha, zfERbeta1 and zfERbeta2). We have also shown that they are functional in vitro and exhibit a distinct expression pattern, although partially overlapping, in the brain of zebrafish. In this paper, we have shown that the hepatic expression of these zfER genes responds differently to estradiol (E2). In fact, a 48-h direct exposure of zebrafish to E2 resulted in a strong stimulation of zfERalpha gene expression while zfERbeta1 gene expression was markedly reduced and zfERbeta2 remained virtually unchanged. To establish the potential implication of each zfER in the E2 upregulation of the zfERalpha gene, the promoter region of this gene was isolated and characterized. Transfection experiments with promoter-luciferase reporter constructs together with different zfER expression vectors were carried out in different cell contexts. The data showed that in vivo E2 upregulation of the zfERalpha gene requires ERalpha itself and a conserved transcription unit sequence including at least an imperfect estrogen-responsive element (ERE) and an AP-1/ERE half site at the proximal transcription initiation site. Interestingly, although in the presence of E2 zfERalpha was the most potent at inducing the expression of its own gene, the effect of E2 mediated by zfERbeta2 represented 50% of the zfERalpha activity. In contrast, zfERbeta1 was unable to upregulate the zfERalpha gene whereas this receptor form was able to tightly bind E2 and activate a reporter plasmid containing a consensus ERE. Altogether, these results indicated that the two ERbeta forms recently characterized in teleost fish could have partially distinct and not redundant functions.

The GnRH system in the European sea bass (Dicentrarchus labrax)

The cDNA sequences encoding three GnRH forms, sea bream GnRH (sbGnRH), salmon GnRH (sGnRH) and chicken GnRH II (cGnRH II), were cloned from the brain of European sea bass, Dicentrarchus labrax. Comparison of their deduced amino acid sequences to the same forms in the gilthead sea bream, Sparus aurata, and striped bass, Morone saxatilis, revealed high homology of the prepro-cGnRH II (94% and 98% respectively), and prepro-sGnRH (92% to both species). The sbGnRH exhibited dissimilar identities, with high homology to the striped bass (93%), and lower homology (59%) to the gilthead sea bream. Two transcript types were identified for the GnRH-associated peptide (GAP)-sGnRH as well as for the GAP-cGnRH II, which suggests a possible alternative splicing followed by the addition of an early stop codon. In order to obtain antibodies specific for the three GnRH precursors, recombinant GAP proteins were produced. The differential expression of the three GnRHs previously reported in the brain by means of in situ hybridization, using riboprobes corresponding to the GAP-coding regions, was fully confirmed by immunocytochemistry using antibodies raised against the recombinant GAP proteins, indicating that the transcripts are translated into functional proteins. Moreover, this approach allowed us to follow, for the first time, the specific projections of the different cell groups: sGAP fibers are distributed mainly in the forebrain with few projections reaching the pituitary, sbGAP fibers are mainly present in the preoptic area, mediobasal hypothalamus and predominantly project to the pars distalis of the pituitary, whereas cGnRH II fibers have a widespread distribution primarily in the posterior brain, and do not project to the pituitary. These new tools will be extremely useful to study further the development, regulation and functional significance of three independent GnRH systems in the brain of vertebrate species.

Tissue-specific expression of two structurally different estrogen receptor alpha isoforms along the female reproductive axis of an oviparous species, the rainbow trout

In oviparous species, in addition to a full-length estrogen receptor alpha (ER alpha), another ER alpha isoform lacking the A domain and exhibiting a ligand-independent transactivation function has been consistently reported. Although both isoforms are expressed in the liver, their respective sites of expression in other potential target tissues are unknown. In contrast to the situation in Xenopus and chicken, the two isoforms of rainbow trout (Oncorhynchus mykiss) are generated from two classes of transcripts with different 5' untranslated sequences issued from the same gene by alternative splicing and promoter usage. The aim of this study was to take advantage of the unique organization of the rainbow trout ER alpha gene to investigate the tissue distribution of these two messenger species along the reproductive axis of female trout. The S1 nuclease assay and in situ hybridization were used, with probes specific for each of the transcripts. Reverse transcription polymerase chain reaction (RT-PCR) with primers specific for each of the isoforms also was performed. The data indicated that the full-length ER alpha is expressed in liver, brain, pituitary, and ovary, whereas expression of the isoform with the truncated A domain is restricted to the liver, demonstrating a tissue-specific expression of these two ER alpha isoforms. The presence of a short liver-specific isoform in oviparous species suggests its role in the development and/or maintenance of the unique function of the liver in the vitellogenesis process.

Differential expression of three different prepro-GnRH (gonadotrophin-releasing hormone) messengers in the brain of the european sea bass (Dicentrarchus labrax)

The expression sites of three prepro-gonadotrophin-releasing hormones (GnRHs), corresponding to seabream GnRH (sbGnRH: Ser(8)-mGnRH, mammalian GnRH), salmon GnRH (sGnRH: Trp(7)Leu(8)-mGnRH), and chicken GnRH-II (cGnRH-II: His(5)Trp(7)Tyr(8)-mGnRH) forms were studied in the brain of a perciform fish, the European sea bass (Dicentrarchus labrax) by means of in situ hybridization. The riboprobes used in this study correspond to the three GnRH-associated peptide (GAP)-coding regions of the prepro-GnRH cDNAs cloned from the same species (salmon GAP: sGAP; seabream GAP: sbGAP; chicken GAP-II: cIIGAP), which show little oligonucleotide sequence identity (sGAP versus sbGAP: 42%; cIIGAP versus sbGAP: 36%; sGAP versus cIIGAP: 41%). Adjacent paraffin sections (6 mm) throughout the entire brain were treated in parallel with each of the three anti-sense probes and the corresponding sense probes, demonstrating the high specificity of the hybridization signal. The results showed that both sGAP and sbGAP mRNAs had a broader expression in the olfactory bulbs, ventral telencephalon, and preoptic region, whereas cIIGAP mRNA expression was confined to large cells of the nucleus of the medial longitudinal fascicle. In the olfactory bulbs, both the signal intensity and the number of positive cells were higher with the sGAP probe, whereas sbGAP mRNA-expressing cells were more numerous and intensely stained in the preoptic region. Additional isolated sbGAP-positive cells were detected in the ventrolateral hypothalamus. These results demonstrate a clear overlapping of sGAP- and sbGAP-expressing cells in the forebrain of the European sea bass, in contrast to previous reports in other perciforms showing a clear segregation of these two cell populations.

Cloning, tissue distribution, and central expression of the gonadotropin-releasing hormone receptor in the rainbow trout (Oncorhynchus mykiss)

A full-length cDNA encoding a GnRH receptor (GnRH-R) has been obtained from the brain of rainbow trout. This cDNA encodes a protein of 386 amino acids (aa) exhibiting the typical arrangement of the G-protein-coupled receptors in seven transmembrane domains. However, a second ATG could give rise to a receptor with a 30-aa longer extracellular domain. As already shown in other fish and Xenopus, this protein possesses an intracellular domain, in contrast with its mammalian counterparts. In the case of rainbow trout, this intracellular carboxy-terminal tail consists of 58 residues. Northern blotting experiments carried out in the brain, the pituitary, and the liver only resulted in a single band of 1.9-2 kilobases in the pituitary, although reverse transcription-polymerase chain reaction amplification products were found in the brain, the pituitary, the retina, and the ovary. In situ hybridization using a probe corresponding to the full-length coding region of the receptor was performed on vitellogenic or ovulating females and allowed to detect a weak but specific signal in the proximal pars distalis of the pituitary, the preoptic region, the mediobasal hypothalamus, and the optic tectum. However, the strongest signal was consistently detected in a mesencephalic structure, the nucleus lateralis valvulae, the significance of which is presently open to speculation.

Peptide YY (PYY) and fish pancreatic peptide Y (PY) expression in the brain of the sea bass (Dicentrarchus labrax) as revealed by in situ hybridization

Tetrapod vertebrates express three neuropeptide Y (NPY)-related peptides: NPY, peptide YY (PYY), and pancreatic polypeptide (PP). Both NPY and PYY mRNA have been localized in the brain of tetrapods whereas PP expression is restricted to the pancreas. Some teleost fish commonly produce NPY and PYY but pancreatic peptide Y (PY) instead of PP. Both NPY and PYY mRNAs are widely distributed in the brain of non-tetrapod species, but no information about PY central expression is available. In the present study, molecular riboprobes were used to study PYY and PY mRNA central distribution in the sea bass (Dicentrarchus labrax). PYY and PY gene expression was predominantly detected within the sea bass forebrain. Telencephalic PYY gene expression was restricted to the ventral part of the ventral telencephalon, and no PY expression was detected in the cerebral hemispheres. Both PYY and PY mRNAs were found within the preoptic area and lateral hypothalamus. Distinct PY or PYY mRNA cell groups were localized in the pretectal area and synencephalon or posterior tubercle, respectively. Caudally, PY gene expression was found in the medial reticular formation, whereas PYY transcripts were localized within the vagal lobe. The results demonstrate that vertebrate brain expresses three NPY-related genes and further support the hypothesis that PP and PY arose by independent gene duplications from PYY. The receptor system of the NPY family as well as gene expression within the main hypophysiotropic and feeding behavior areas suggest an involvement of both peptides in the control of food intake and pituitary secretion.

Characterization of neuropeptide Y expression in the brain of a perciform fish, the sea bass (Dicentrarchus labrax)

The distribution of neuropeptide Y (NPY) gene expression was mapped in the brain of the sea bass (Dicentrarchus labrax) by in situ hybridization with 35S-UTP labeled cRNA probes. Gene expression was mainly detected within the forebrain, although NPY mRNA transcripts were also localized in the tectum and tegmentum mesencephali and posterior brain. New NPY-expressing nuclei were found in the dorsal and ventral telencephalon, preoptic area, tuberal hypothalamus, synencephalon, tegmentum mesencephali and posterior brain. The profuse NPY gene expression within the main neuroendocrine areas of the teleost fish further supports a physiological role in the control of the pituitary secretion. In addition, NPY gene was expressed within the primary visual, olfactory and gustatory circuits of teleost which, subsequently, project to hypothalamic feeding center in teleost fish. Our results extend the NPY-expressing areas known in teleost species.

Cloning and sequencing of the gilthead sea bream estrogen receptor cDNA

We report here the complete nucleotide sequence of a cDNA clone containing the full-coding sequence of the Sparus aurata estrogen receptor (ER) isolated from an expression library prepared from gilthead sea bream liver poly A+ RNA. The library was screened using a single strand rainbow trout ER cDNA probe, corresponding to the C-D domain. The cDNA sequence containing an insert of 2369 nucleotides was found to encode a protein of 579 amino acids. The 5'- and 3'-untranslated regions of the message are 186 and 392 nucleotides long, respectively. The gilthead sea bream ER shows the higher homology with the ER of another perciform, Chrysophrys major (93%), moderate to high homology with Oreocromis aureus (78%) medaka (77%) and rainbow trout (70.7%) ERs and lower homology with japanese eel (45%), amphibian (47%), avian (48.5%) and mammalian (47-47.5%) ERs. The sequence homologies and phylogenetic analysis of the various ERs suggest that gilthead sea bream ER should be considered as a ER alpha-like.

Expression of clock gene in the brain of rainbow trout: comparison with the distribution of melatonin receptors

To identify brain structures potentially acting as biological clocks in rainbow trout (Oncorhynchus mykiss), the expression sites of a trout homolog of the mouse clock gene were studied and compared with that of melatonin receptors (Mel-R). For this purpose, a partial sequence of the trout clock gene, including a PAS domain, was obtained by reverse transcription-polymerase chain reaction and used to perform in situ hybridization. The highest density of clock transcripts was observed in the periventricular layer (SPV) of the optic tectum, but a weaker expression was detected in some pretectal nuclei, such as the posterior pretectal nucleus (PO) and the periventricular regions of the diencephalon. Comparison of the hybridization signal in fish sacrificed at 08:00 and 17:00 did not indicate major changes in clock expression levels. Comparison of adjacent sections alternatively treated with clock and Mel-R probes suggests that both messengers are probably expressed in the same cells in the SPV and PO. In addition, in situ hybridization with a glutamate decarboxylase 65 probe, demonstrates that cells expressing clock and Mel-R in the optic tectum are gamma-aminobutyric acid neurons. The tight overlapping between the expression of Mel-R and clock transcripts in cells of the PO and SPV suggests a functional link between these two factors. These results indicate that the optic tectum and the pretectal area of the rainbow trout are major sites of integration of the melatonin signal, express the clock gene, and may act as biological clocks to influence behavioral and endocrine responses in trout.

Transcriptional interference between glucocorticoid receptor and estradiol receptor mediates the inhibitory effect of cortisol on fish vitellogenesis

In oviparous species, the synthesis of vitellogenin (Vg) takes place in the liver according to a strictly estrogen-dependent mechanism that first involves an up-regulation of the estrogen receptor (ER) by its own ligand. However, reports from the literature indicate that in trout stress or cortisol may cause a reduction of cytosolic E2-binding sites in the liver and a decrease in plasma Vg levels. To investigate the mechanisms underlying these effects, in vivo and in vitro experiments were designed in rainbow trout (Oncorhynchus mykiss). The results demonstrate that cortisol implanted into maturing females caused a marked decrease of rainbow trout ER (rtER) and rainbow trout Vg (rtVg) mRNA levels in the liver. In vitro experiments on hepatocyte aggregates also showed that dexamethasone (Dex) caused a strong decrease in the basal and E2-stimulated rtER mRNA and to a lesser extent rtVg mRNA. These effects were specific as no other hormones were able to mimic the inhibitory action of Dex. A study of rtER mRNA stability indicated that the effects of glucocorticoids are likely to take place at the transcriptional level. This was further indicated by transfection experiments in CHO-K(1) cells, which showed that rainbow trout glucocorticoid receptor (rtGR) strongly inhibited the E2-stimulated transcriptional activity of the rtER promoter. Taken together, these results indicate that the rtGR exerts a transcriptional interference on the expression of the rtER that may explain some of the negative effects of stress or cortisol on vitellogenesis.

Effects of melatonin on liver estrogen receptor and vitellogenin expression in rainbow trout: an in vitro and in vivo study

Although melatonin is believed to mediate many seasonal and circadian effects of photoperiod on reproduction in salmonids, the precise mechanisms underlying such effects are still largely unknown. Recent data of the literature indicate a relationship between melatonin and expression of estrogen receptors (ER) in various tissues. In this study, the effects of melatonin on estrogen receptor and/or vitellogenin expression were studied by a combination of in vivo and in vitro experiments. In yeast stably expressing ER and transfected with an estrogen-responsive element-beta-galactosidase reporter gene, melatonin had no effect on basal or E2-stimulated ER expression. Incubation of hepatocyte aggregates with melatonin (10(-8) to 10(-4)) for 16 or 48 h did not modify the E2-stimulated ER and vitellogenin mRNA, as measured by dot blots. Finally, neither pinealectomy nor melatonin implants caused any effect on basal or E2-stimulated ER and vitellogenin mRNA contents in the liver. Altogether, these results suggest that, although we cannot exclude potential effects at the brain or pituitary levels, melatonin has no or little effects on estrogen receptor in the liver.

The inhibitory effects of (gamma)-aminobutyric acid (GABA) on growth hormone secretion in the goldfish are modulated by sex steroids

Double-labelling studies at the electron microscopic level demonstrated that gamma-aminobutyric acid (GABA)-immunoreactive nerve endings are associated with growth-hormone-secreting cells in the proximal pars distalis of the goldfish pituitary gland, suggesting that GABA may be important for the control of growth hormone release in this species. An in vitro assay for GABA-transaminase activity demonstrated that the pituitary is a site for the metabolism of GABA to succinic acid. In vitro, GABA or the GABA antagonists bicuculline and saclofen did not affect the rate of growth hormone release from dispersed pituitary cells in static incubation. In contrast, intracerebroventricular injection of GABA reduced serum growth hormone levels within 30 min. During the seasonal gonadal cycle, intraperitoneal injection of GABA was without effect in sexually regressed goldfish, but caused a significant decrease in serum growth hormone levels in sexually recrudescent animals. Intraperitoneal implantation of solid silastic pellets containing oestradiol increased serum GH levels fivefold in sexually regressed and recrudescent goldfish; in both groups, GABA suppressed the oestradiol-stimulated increase in circulating growth hormone levels. The effect of oestradiol on basal serum growth hormone levels was specific since progesterone and testosterone were without effect. However, in recrudescent animals treated with progesterone and testosterone, the inhibitory effects of GABA on serum growth hormone levels were absent, indicating a differential role for these steroids in growth hormone release. Taken together, these results demonstrate that GABA has an inhibitory effect on growth hormone release in goldfish.

Distribution of glutamic acid decarboxylase mRNA in the forebrain of the rainbow trout as studied by in situ hybridization

By using degenerate primers designed from glutamate decarboxylase (GAD) sequences of mammals, Xenopus and Drosophila, a 270-bp cDNA fragment was cloned by reverse transcriptase-polymerase chain reaction (RT-PCR) from cerebellum total RNA of rainbow trout. This partial cDNA shows 90% identity with mammalian GAD 65 and presents the Asn-Pro-His-Lys (NPHK) sequence corresponding to the pyridoxal-binding region of porcine DOPA decarboxylase or mammalian GAD. The distribution of GAD 65 mRNA-expressing neurons in the forebrain of the trout was studied by in situ hybridization using either digoxigenin- or 35S-labeled probes. The results demonstrate that gamma-amino butyric acid (GABA) neurons are widely distributed throughout the forebrain, with a high density in the periventricular regions. In this study, we report their precise distribution in the telencephalon and diencephalon. GAD mRNA-expressing cells were particularly abundant in the preoptic region and the mediobasal hypothalamus, two major neuroendocrine and estrogen-sensitive regions in fish. The presence of GAD mRNA-expressing neurons was observed in visually related structures such as the suprachiasmatic nucleus, the pretectal region, and the thalamus. Immunohistochemistry with antibodies directed against mouse GAD failed to demonstrate the presence of immunoreactive cell bodies, but showed a very high concentration of GAD-immunoreactive fibers in many brain regions, notably in the preoptic area, hypothalamus, and neurohypophyseal digitations of the pituitary, in particular in the proximal pars distalis. These results indicate that GABA neurons are ideally placed to modulate neuroendocrine activities at the hypothalamic and pituitary levels and to participate in the processing of sensorial information.

Central melatonin receptors in the rainbow trout: comparative distribution of ligand binding and gene expression

To better define the role of melatonin in fish, we have compared in detail the distribution of 2-[125I]iodomelatonin binding sites with gene expression for melatonin receptor subtypes in a widely studied seasonal species, the rainbow trout. Three distinct partial sequences of the melatonin receptor gene were cloned from trout genomic DNA. Two of the sequences corresponded to the Mella receptor subtype, and one corresponded to the Mellb receptor subtype. Analysis of numerous clones failed to find a sequence equivalent to the Mel1c receptor subtype. Comparison of receptor gene expression with 2-[125I]iodomelatonin binding distribution indicated dendritic transport of the receptor. Melatonin receptors were associated predominantly with visually related areas of the trout brain, such as the thalamic region, the pretectal area, and the optic tectum. The pituitary was devoid of 2-[125I]iodomelatonin binding, and melatonin receptor gene expression was not detectable. It would appear from the results of the present study that melatonin in this species is involved primarily in the processing of visual signals. How melatonin interacts with circannual rhythms of growth and reproduction is unclear, although a direct interaction between melatonin and the hypothalamo-pituitary axis is not clearly indicated.

Involvement of gamma-aminobutyric acid in the control of GTH-1 and GTH-2 secretion in male and female rainbow trout

The potential role of the neurotransmitter gamma-aminobutyric acid (GABA) in the control of the secretion of the two pituitary fish gonadotropins (GTH-1 and GTH-2) was investigated in male and female rainbow trout (Oncorhynchus mykiss). The presence of glutamate decarboxylase-positive fibers in the neurohypophyseal digitations adjacent to the gonadotropic cells was demonstrated by means of double immunohistochemistry, providing a morphofunctional support for potential GABA-gonadotropin interactions in both sexes. In spermiating males, in vivo treatment with GABA did not affect basal gonadotropin release, but stimulated GTH-1 release when coadministered with a gonadotropin-releasing hormone analogue (GnRHa), and potentiated GnRHa-stimulated GTH-2 release. In vitro, using dispersed pituitary cells, GABA stimulated basal GTH-1 and GTH-2 secretion, in a dose-dependent manner, and potentiated salmon GnRH effect on both hormones. In mature females, GABA induced in vivo a strong elevation of plasma GTH-2 levels after 2- 6 h of injection, but had no effect in vitro. GABA treatment in vivo was also stimulatory in recrudescent females, slightly increasing plasma GTH-2 levels in both saline- and GnRHa-treated fish (GnRHa alone has no effect at this stage). Immature fish were unresponsive to GABA/GnRHa treatments but, after steroid implantation [testosterone (T) or estradiol] for 13 days, injection of GABA stimulated GTH-2 release in vivo (also GTH-1 slightly in T-implanted fish). In conclusion, GABA has an overall stimulatory action on GTH-1 and GTH-2 secretion in rainbow trout, which depends on the sex and the reproductive stage of the fish. The stimulatory action of GABA might be exerted, at least in part, directly onto the gonadotropes, as it stimulates basal and GnRH-induced GTH-1 and GTH-2 secretion from dispersed pituitary cells.

Glucocorticoid receptor immunoreactivity in neurons and pituitary cells implicated in reproductive functions in rainbow trout: a double immunohistochemical study

In order to identify the nature of the glucocorticoid receptor (GR)-expressing neurons and pituitary cells that potentially mediate the negative effects of stress on reproductive performance, double immunohistochemical stainings were performed in the brain and pituitary of the rainbow trout (Oncorhynchus mykiss). To avoid possible cross-reactions during the double staining studies, combinations of primary antibodies raised in different species were used, and we report here the generation of an antibody raised in guinea pig against the rainbow trout glucocorticoid receptor (rtGR). The results obtained in vitellogenic females showed that GnRH-positive neurons in the caudal telencephalon/anterior preoptic region consistently exhibited rtGR immunoreactivity. Similarly, in the anterior ventral preoptic region, a group of tyrosine hydroxylase-positive neurons, known for inhibiting gonadotropin (GTH)-2 secretion during vitellogenesis, was consistently shown to strongly express GR. Finally, we show that a large majority of the GTH-1 (FSH-like) and GTH-2 (LH-like) cells of the pituitary exhibit rtGR immunoreactivity. These results indicate that cortisol may affect the neuroendocrine control of the reproductive process of the rainbow trout at multiple sites.

Immunohistochemical localization and biochemical characterization of two novel decapeptides derived from POMC-A in the trout hypothalamus

Several vertebrate species which underwent duplication of their genome, such as trout, salmon and Xenopus, possess two proopiomelanocortin (POMC) genes. In the trout, one of the POMC molecules, called POMC-A, exhibits a unique C-terminal extension of 25 amino acids which has no equivalent in other POMCs characterized so far. This C-terminal peptide contains three pairs of basic residues, suggesting that it may be the source of novel regulatory peptides. The aim of the present study was to investigate the occurrence of these peptides in the brain of the trout Oncorhynchus mykiss by using specific antibodies raised against two epitopes derived from the C-terminal extension of POMC-A, i.e., EQWGREEGEE and YHFQ-NH2. Immunohistochemical labeling of brain sections revealed the presence of EQWGREEGEE- and YHFQ-NH2-immunoreactive cell bodies in the anterior part of the nucleus lateralis tuberis of the hypothalamus. Immunoreactive fibers were observed in the dorsal hypothalamus, the thalamus, the telencephalon, the optic tectum and the medulla oblongata. In contrast, no labeling was detected using antibodies against the non-amidated peptide YHFQG. Biochemical characterization was performed by combining high-performance liquid chromatography (HPLC) analysis with radioimmunoassay (RIA) quantification. Two peptides exhibiting the same retention time as synthetic EQWGREEGEE and ALGERKYHFQ-NH2 were resolved. However, no peptide co-eluting with YHFQ-NH2 or YHFQG could be detected. These results demonstrate that, in the trout brain, post-translational processing of POMC-A generates the two decapeptides EQWGREEGEE and ALGERKYHFQ-NH2. The wide distribution of immunoreactive fibers in the diencephalon, telencephalon, optic tectum and medulla oblongata suggests that these peptides may exert neurotransmitter and/or neuromodulator activities.

Immunohistochemical localization of glucocorticoid receptors in the forebrain of the rainbow trout (Oncorhynchus mykiss)

The distribution of glucocorticoid receptor-expressing cells was studied in the forebrain of the rainbow trout by means of antibodies produced against a fusion protein made of the NH2-terminal fragment of the rainbow trout glucocorticoid receptor fused in frame with glutathione-S-transferase. The results indicate that glucocorticoid receptor-expressing cells are located in many brain regions from the telencephalon to the spinal cord, with the highest density in the neuroendocrine component of the brain, the preoptic region and the mediobasal hypothalamus, and in the periventricular zone of the optic tectum. In virtually all cases, the labeling was located in the nucleus of the cells, although on very rare occasions, a slight labeling of the cytoplasm was detected. Concerning the preoptic region, the most striking feature was the high density of glucocorticoid receptors in the magnocellular preoptic nucleus, known to contain corticotrophin-releasing factor (CRF)-, vasotocin-, and isotocin-expressing cells. Colocalization experiments showed that 100% of the CRF-immunoreactive neurons in the preoptic nucleus express glucocorticoid receptors. In the mediobasal hypothalamus, the highest expression was found in the nucleus lateralis tuberis and parts of the nucleus recessus lateralis. Concerning the pituitary, the glucocorticoid receptor was consistently found in the rostral pars distalis, with the exception of the prolactin cells, and in the proximal pars distalis, which in trout contains thyrotrophs, gonadotrophs, and somatotrophs. In the hindbrain, expression of glucocorticoid receptors were localized mainly in the periventricular regions.

Identification of potential sites of cortisol actions on the reproductive axis in rainbow trout

The full length cDNA encoding a rainbow trout glucocorticoid receptor (rtGR) has been obtained from rainbow trout liver and intestine libraries. Northern blot analysis showed that the corresponding messengers are detected in the brain of trout with a size 7.5 kb similar to the size of rtGR mRNA in other target tissues. The distribution of the rtGR mRNA and protein was studied in the forebrain of the trout by means of both in situ hybridization and immunohistochemistry and compared with that of the oestrogen receptor (rtER). The GR and ER mRNAs and proteins were detected with a strong overlapping mainly in the: (a) preoptic region; (b) mediobasal hypothalamus; and (c) anterior pituitary, confirming their implication in the neuroendocrine control of pituitary functions. In both diencephalon and pituitary, the peptidergic phenotype of some neuron or cell categories expressing either type of receptors could be determined by double staining. Furthermore, double staining studies have demonstrated colocalization of the two receptors in the same neurons or pituitary cells. The rtER and rtGR were found to be co-expressed in the dopaminergic neurons inhibiting GTH2 secretion and in pituitary cells of the anterior lobe--notably the gonadotrophs. Given that the promoter of the ER gene contains several potential glucocorticoid-responsive elements (GRE) and that cortisol inhibits the oestradiol-stimulated ER expression in the liver, the possibility exists for modulation of ER gene expression by GR in the hypothalamo-pituitary complex. This could explain some of the well documented effects of stress on the reproductive performance in salmonids.

Rainbow trout glucocorticoid receptor overexpression in Escherichia coli: production of antibodies for western blotting and immunohistochemistry

Fragments of cDNA that encode the N-terminal and DNA-binding domains (DBD) of the rainbow trout glucocorticoid receptor (rtGR) were expressed in Escherichia coli as fusion proteins with glutathione-S-transferase (GST). The fusion proteins induced by IPTG could readily be detected as 45- and 40-kDa bands, respectively, in crude extracts, as well as in proteins purified on glutathione-agarose. These purified hybrid proteins were used to immunize rabbits. The antisera produced were tested for specificity by Western blot analysis using extracts from COS-1 cells transfected with an rtGR expression vector and from trout liver cells. The antisera raised against the DBD domain did not detect any bands on Western blots, even at low antiserum dilution. However, the purified DBD fusion protein specifically bound GRE-containing DNA fragments in gel-shift assays, and the retarded complexes were supershifted by these antibodies. The antisera raised against the N-terminal domain consistently detected two protein bands at 104 and 100 kDa in the two cell extracts and allowed specific immunohistochemical staining in fish brain and pituitary. For the first time in fish, these antibodies will allow analysis of GR expression in different cortisol target tissues.

The torus longitudinalis in the gilthead seabream: an undescribed fiber tract link with the valvula cerebelli

In this paper, we present a brief anatomical description of the torus longitudinalis and the valvula cerebelli in a percomorph fish, the gilthead seabream (Sparus aurata) based on the analysis of serial brain sections stained by paraldehyde fuchsin, Groat's hematoxylin-picroindigocarmin and cresyl violet. The existence of a small undescribed fiber tract directly bridging the ventral torus longitudinalis and the granular layer of the rostral valvula cerebelli is also reported. This small fiber tract was observed in its integrity on a few transverse sections providing that the angle of sectioning was appropriate. The existence of the anatomical link between the TL and the VC described in this paper might sustain the role of the TL as part of an ascending cerebello-tectal circuit, at least in gilthead seabream. However, these fibers might also represent fibers of passage originating from other brain structures.

Release of pituitary gonadotrophins GtH I and GtH II in the rainbow trout (Oncorhynchus mykiss): modulation by estradiol and catecholamines

The present study focused on the role of catecholaminergic neurons and estrogens on the release of gonadotropins I and II in immature and early vitellogenic female rainbow trout. The ovariectomy-induced increase of GtH I blood levels (from about 10 to 15 ng/ml) was prevented in vitellogenic fish by E2 supplementation. E2 implantation of immature fish decreased blood GtH I levels (from about 6 to 1 ng/ml). Blood levels of GtH II were low (about 0.5 ng/ml) and not altered by ovariectomy and E2 treatment. These data demonstrate that estrogens exert a negative feedback on the release of GtH I in trout. A treatment with alpha-methyl-p-tyrosine (MPT), an inhibitor of catecholamine synthesis, increased blood GtH II levels of sham-operated vitellogenic fish and ovariectomized fish implanted with E2, but had no effects in ovariectomized fish. MPT did not modify blood GtH I levels in any experimental group. A treatment of E2-implanted immature or vitellogenic fish with the dopamine antagonist pimozide also increased blood GtH II levels, but did not significantly change blood GtH I levels. These data demonstrate that release of GtH II, but not of GtH I, depends on an E2-activated DA inhibitory tone.

Distribution and expression of glucocorticoid receptor mRNA in the forebrain of the rainbow trout

The expression and distribution of glucocorticoid receptor mRNA was studied in the forebrain of mature female and immature undifferentiated rainbow trout (Oncorhynchus mykiss) by means of Northern blotting and in situ hybridization. A single mRNA species of 7.5 kb was detected in mRNA polyA+ prepared from the anterior brain. In situ hybridization was carried out using a 35S-labelled riboprobe corresponding to the A/B-domain (between nucleotides 1224 and 1763) of the recently cloned rainbow trout glucocorticoid receptor cDNA. Comparison of adjacent sections hybridized with the sense and antisense probes allowed detection of a specific signal with a similar distribution pattern in all animals studied. In the telencephalon, a specific hybridization was detected in scattered cells of the dorsal telencephalic hemisphere, but the stronger signal was consistently observed in the dorsal nucleus, and to a lesser degree in the ventral nucleus of the ventral telencephalon. Heavy hybridization staining was consistently observed in all subdivisions of the preoptic nucleus and the nucleus lateralis tuberis, which are the main hypophysiotrophic regions in fish. A weaker signal was detected in the nucleus anterioris periventricularis, nucleus suprachiasmaticus and thalamic region. The presence of a strong signal in virtually all magnocellular neurons of the nucleus preopticus, known for producing vasotocin, isotocin and corticotropin-releasing factor favors a direct function of cortisol in regulating its own secretion.

Spontaneous and gonadotropin-releasing hormone-stimulated cytosolic calcium rises in individual goldfish gonadotrophs

The cytosolic free calcium concentration, [Ca2+]i, was monitored in single isolated goldfish gonadotrophs with the fluorescent probe Indo-1. It was found that goldfish gonadotrophs exhibit both spontaneous and secretagogue-induced [Ca2+]i rises. Spontaneous [Ca2+]i transients showed striking kinetic features and a sensitivity to external Ca2+ suggesting that they were the consequence of transient Ca2+ entries. Two kinetically distinct patterns of [Ca2+]i rises were generated in response to the two native forms of gonadotropin-releasing hormone (GnRH), salmon GnRH (sGnRH) and chicken GnRH-II (cGnRH-II). In a part of the gonadotrophs, GnRHs triggered a plateau [Ca2+]i rise whereas in other responsive cells they induced a series of [Ca2+]i bursts, each consisting of grouped [Ca2+]i transients. Both plateau and burst [Ca2+]i response patterns were due to Ca2+ entry through plasma membrane Ca2+ channels, inasmuch as they were suppressed with external Ca2+ removal. No contribution of Ca2+ release from thapsigargin-sensitive stores was observed in either response pattern. While in mammalian gonadotrophs GnRH rises [Ca2+] by mostly acting on internal Ca2+ sequestering stores, our results show that GnRH-stimulated goldfish gonadotrophs rapidly increase Ca2+ entry to enhance their [Ca2+]i levels.

Three forms of gonadotropin-releasing hormone in a perciform fish (Sparus aurata): complementary deoxyribonucleic acid characterization and brain localization

Three forms of GnRH-salmon (sGnRH), seabream (sb-GnRH), and chicken (cGnRH-II)-have been described in the gilthead seabream (Sparus aurata) brain, and the cDNA encoding the sbGnRH precursor was recently isolated. In the present study, the cDNAs encoding the sGnRH and cGnRH-II were isolated and characterized, and the neurons producing the three GnRHs were localized in the seabream brain. Fragments of sGnRH and cGnRH-II cDNAs were amplified by polymerase chain reaction and used as probes to isolate the full-length cDNAs from a brain cDNA library. The cDNA encoding the cGnRH-II precursor is 573 nucleotides (nt) long, and the cDNA encoding the sGnRH precursor is 1971 nt in length with an unusually long 5' untranslated region. Specific single-strand DNA probes for in situ detection of mRNA were designed according to nonconserved regions among the three GnRH c-DNAs. Localization of GnRH mRNA-producing cells in the brain revealed five distinct populations of cells: sGnRH-producing cells in the ventromedial olfactory bulbs and the terminal nerve, sbGnRH-producing cells in the preoptic area and the ventral thalamus, and cGnRH-II-producing cells in the midbrain tegmentum. The discrete sites of expression of the three forms of GnRH indicate that only sbGnRH is directly involved in the control of gonadotropin secretion.

Development of enzyme immunoassays for 3,5,3'-triiodo-L-thyronine and L-thyroxine: time-course studies on the effect of food deprivation on plasma thyroid hormones in two marine teleosts, sea bass (Dicentrarchus labrax L.) and sea bream (Sparus aurata L.)

The effects of short-term food deprivation and photoperiod on plasma thyroid hormone levels of sea bass and sea bream were studied. Animals were acclimated under constant photoperiod regime (15L/9D) and feeding times (2 hr after light onset and 2 hr before light offset). Time-course studies involved monitoring plasma hormone levels every 4 hr throughout 1.5 24-hr cycles. Plasma 3,5, 3'-Triiodo-L-thyronine (T3) and L-thyroxine (T4) were assayed using a newly developed competitive enzyme immunoassay, utilizing acetylcholinesterase as a label of enzymatic tracers. Enzyme immunoassays had sensitivities of 1.25-0.02 and 62.5-0.2 ng/ml for T3 and T4, respectively, and reproducibilities of 3.7 and 5.6% intraassay variation for T3 and T4, respectively; interassay variations for T3 and T4 assays respectively were 1.6%, 11% and 6.6%, 8% for sea bass and sea bream plasma similar to RIA. In sea bass, 3 days of food deprivation resulted in depressed plasma T3 and T4, overriding significant diel variations seen during the second day of starvation. Sea bream displayed a slight decrease of T4 plasma levels while T3 levels remained constant for the whole sampling period. Both thyroidal systems responded to photoperiod with a significant increase in plasma T4 level at the time of light onset. In addition, sea bass also displayed increased T3 levels and decreases in both hormone levels coinciding with "lightoff." Data show different responses of the sea bass and sea bream thyroidal systems to both nutritional state and photoperiod in that the latter state is influenced by the former. Data suggest plasma thyroid levels can be used as a rapid indicator of nutritional status.

Regulation of growth hormone secretion by amino acid neurotransmitters in the goldfish (I): Inhibition by N-methyl-D, L-aspartic acid

High levels of the amino acid neurotransmitter glutamate were found in the goldfish hypothalamus and pituitary using high performance liquid chromatography with fluorometric detection. A specific polyclonal antibody to glutamate was generated in the rabbit for immunocytochemistry. Localization studies demonstrated that glutamatergic neurons of undetermined origin innervate the particular part of the goldfish adenohypophysis where somatotrophs and gonadotrophs are located. Intraperitoneal and brain third ventricle injection of the glutamate agonist N-methyl-D,L-aspartic acid (NMA) inhibited GH release in vivo. The gonadal steroid estradiol plays an important role in regulating GH secretion by stimulating basal serum GH levels and enhancing the inhibitory effects of NMA on GH secretion. Taken together, these results demonstrate that glutamate is an important regulator of GH secretion in goldfish.

Estrogen receptors are expressed in a subset of tyrosine hydroxylase-positive neurons of the anterior preoptic region in the rainbow trout

A double immunocytochemical procedure, with two different chromogens, was used to compare the respective distribution of estrogen receptor-immunoreactive cells and tyrosine hydroxylase-immunoreactive neurons on the same sections of the preoptic region of adult female rainbow trout (Oncorhynchus mykiss). Estrogen receptor-immunoreactive cells were observed in the anterior preoptic region surrounding the preoptic recess and its large lateral extensions. Tyrosine hydroxylase-immunoreactive cells were consistently detected in the ventral and ventrolateral walls of the preoptic recess, in an area that was named nucleus preopticus pars anteroventralis. Dopamine immunohistochemistry and Dil retrograde transport studies indicated that part of these catecholaminergic neurons are dopaminergic and could project to the pituitary. Double staining studies showed consistently that most estrogen receptor-positive cells located ventral to the large extensions of the preoptic recess are also tyrosine hydroxylase-positive, indicating that this region is a major target for estradiol feedback. The results are discussed in relation to the role of the nucleus preopticus pars anteroventralis in mediating the negative feedback actions of estradiol on the secretion of gonadotrophin (GTH2) secretion. A hypothesis is drawn in order to explain the synchronizing role of estradiol at the time of ovulation in rainbow trout.

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

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

Expression and distribution of two gonadotropin-releasing hormones in the catfish brain

The expression of prepro-catfish GnRH mRNA and prepro-chicken GnRH-II mRNA was investigated by means of in situ hybridization. The differential distribution of cells expressing the respective mRNAs was compared with the distribution of cells immunoreactive for (1) catfish (cf) GnRH and chicken (c) GnRH-II and (2) both GnRH-associated peptides (GAPs). It was found that the prepro-cfGnRH mRNA expressing cells were located in the ventral forebrain, with a similar distribution of the cfGnRH- and cfGAP-immunoreactive perikarya. The prepro-cGnRH-II mRNA expressing cells were exclusively located in the midbrain tegmentum, at the same position as a group of large cGnRH-II- and CIIGAP-immunoreactive perikarya. It was concluded that the peptidergic neurons in the ventral forebrain contain cfGnRH, whereas cGnRH-II perikarya are restricted to the midbrain. The proximal pars distalis of the pituitary, containing the gonadotropin cells, is innervated by fibers immunoreactive for both cfGnRH and cfGAP and originating from the cfGnRH neurons in the ventral forebrain. We could, however, not detect fibers innervating the pituitary that were immunoreactive for cIIGAP.

Characterization of trout galanin and its distribution in trout brain and pituitary

Galanin was purified from an extract of the stomach of the rainbow trout, Oncorhynchus mykiss, and its primary structure was established as Gly-Trp-Thr-Leu-Asn-Ser- Ala-Gly-Tyr-Leu10-Leu-Gly-Pro-His-Gly-Ile-Asp-Gly-His-Arg20- Thr-Leu-Ser-Asp- Lys-His-Gly-Leu-Ala. Trout galanin shows six amino acid substitutions compared with pig galanin, but the N-terminal region (residues 1-14) has been fully conserved. The distribution of galanin-immunoreactive (GAL-IR) structures in the trout brain and pituitary was studied via immunohistochemistry. GAL-IR cell bodies were observed only in the caudal telencephalon, the preoptic region, and the mediobasal hypothalamus. GAL-IR fibers, however, are widely distributed throughout the brain, with a much lower density in the midbrain and posterior brain than in the tel- and diencephalon. Particularly dense innervation of the mediobasal hypothalamus, the ventral and supracommissuralis parts of the caudal telencephalon, and the region above and below the anterior commissure was observed. A heavy innervation of the pituitary was consistently detected. GAL-IR fibers were present in neurohypophyseal digitations of both the anterior and intermediate lobes with highest density in the region of the proximal pars distalis, where growth hormone and gonadotropic cells are located. Fibers were also seen in digitations of the rostral pars distalis, in particular between the prolactin follicles. The distribution of GAL-IR neurons in the central nervous system and pituitary of the trout suggests that the peptide may exercise an important role in the regulation of neuroendocrine functions, particularly those related to reproduction.

Comparative distribution of neurotensin-like immunoreactivity in the brain of a teleost (Carassius auratus), an amphibian (Hyla meridionalis), and a reptile (Gallotia galloti)

The distribution of neurotensin (NT) was studied in the brain of three species belonging to the three major classes of cold-blooded vertebrates: teleost fishes (Carassius auratus), anuran amphibians (Hyla meridionalis), and reptiles (Gallotia galloti; Lacertidae). By using antibodies directed against synthetic bovine NT in the three species, immunoreactive cell bodies were discovered mostly in the telencephalon and diencephalon, in particular at the level of the preoptic region the mediobasal hypothalamus, and the thalamus. In the frog and the lizard, additional immunoreactive (ir) structures were observed in the optic tectum and the tegmentum of the mesencephalon. In the goldfish pituitary, an extensive innervation was consistently observed at the level of the rostral pars distalis, whereas in both frog and lizard, positive fibers were only detected in the external layer of the median eminence. In the three species there is a striking overlap between the distribution of the NT-ir cell bodies and that of the target cells for sexual steroids. The results are discussed in relation with those reported in birds and mammals, and with the possible interactions among NT, sexual steroids, and the neuroendocrine control of pituitary hormone release, in particular prolactin and gonadotrophin.

Immunocytochemical localization of mammalian GnRH (gonadotropin-releasing hormone) and chicken GnRH-II in the brain of the European silver eel (Anguilla anguilla L.)

Using specific antibodies for the two molecular forms of gonadotropin-releasing hormone (GnRH) present in the European eel, Anguilla anguilla, (mammalian GnRH, mGnRH, and chicken GnRH II, cGnRH-II), we employed immunocytochemistry to determine the distribution of these two peptides in the brain and in the pituitary. The results indicate that mGnRH and cGnRH-II are localized in different neurons: mGnRH-immunoreactive (ir) perikaria were observed in the olfactory bulbs, the junction between olfactory bulbs and telencephalon (nucleus olfactoretinalis), the telencephalon, the preoptic region and the mediobasal hypothalamus. These cell bodies are located along a continuum of ir-fibers that could be traced from the olfactory nerve to the pituitary. Mammalian GnRH-ir fibers were detected in many parts of the brain (olfactory bulbs, ventral telencephalon, hypothalamus, optic tectum, mesencephalon) and in the pituitary. Chicken GnRH-II-ir cell bodies were detected in the nucleus of the medial longitudinal fasciculus of the midbrain tegmentum, but only scattered fibers could be detected in different parts of the brain. The pituitary exhibited very few cGnRH-II-ir fibers, contrasting with an extensive mGnRH innervation. These results are in agreement with our previous data obtained in the same species using specific radioimmunoassays for mGnRH and cGnRH-II. They demonstrate a differential distribution of the two forms of GnRH in the brain of the eel, as in the brain of some other vertebrate species, and suggest differential physiological roles for the two GnRH forms in the eel. They also provide information concerning the evolution of the GnRH systems in vertebrates.

Distribution of estrogen receptor-immunoreactive cells in the brain of the rainbow trout (Oncorhynchus mykiss)

Using antibodies against the hormone binding domain of the trout estrogen receptor (ER), the distribution of ER-immunoreactive (ER-IR) cells was studied in the brain of maturing diploid and triploid female rainbow trout using a streptavidin-biotin-peroxidase method followed by a nickel-intensified diaminobenzidine reaction. This technique resulted in an excellent signal/background ratio allowing unambiguous identification of positive cells. In all animals, ER-IR cells were consistently located in three brain regions, the ventral telencephalon, the anterior ventral preoptic region, and the mediobasal hypothalamus. About 250 ER-IR cells were observed in the ventral and dorsal parts of the ventral telencephalon. In the anterior nucleus preopticus periventricularis, about 2400 ER-IR cells were observed surrounding the preoptic recess. In the posterior hypothalamus, approximately 2700 ER-IR cells were located in the anterior, posterior and inferior divisions of the nucleus lateralis tuberis and in the nucleus saccus vasculosus. In these regions cell nuclei exhibiting different densities of staining were observed and absolutely no labeling of cytoplasmic processes was detected. These results are in partial agreement with those obtained either after injection of tritiated-estradiol in other teleots species or in situ hybridization of ER mRNAs in trout. In particular, no immunoreactivity was observed in the thalamic region nor in the nucleus posterioris periventricularis. These data indicate that target cells for estradiol are essentially located in brain regions involved in the neuroendocrine control of pituitary functions and having direct connections with the hypophysis.

An enzyme immunoassay for salmon gonadotropin-releasing hormone and its application to the study of the effects of diet on brain and pituitary GnRH in the sea bass, Dicentrarchus labrax

The effects of two different diets [diet 1 (D1), high protein-low carbohydrate; diet 2 (D2), low protein-high carbohydrate] on brain and pituitary gonadotropin-releasing hormone (GnRH) contents, as well as circulating steroids and vitellogenin, were studied over the reproductive period of the sea bass. Salmon GnRH was measured using a newly developed competitive enzyme immunoassay with an enzymatic tracer made of sGnRH covalently coupled to acetylcholinesterase from the electric organ of the eel Electrophorus electricus. The pituitary GnRH content of animals of both sexes fed D1 was significantly reduced at the time of spawning compared with the pre- and postspawning stages, whereas fish fed D2 did not exhibit such changes. In the brain only minor differences in the GnRH content were observed between the two diets. It is concluded that GnRH release rather than synthesis is affected in fish fed a low protein-high carbohydrate regimen. Plasma sex steroids and vitellogenin were not greatly affected by the diet.

Overexpression of rainbow trout estrogen receptor domains in Escherichia coli: characterization and utilization in the production of antibodies for immunoblotting and immunocytochemistry

Complementary DNA fragments that encode central and C-terminal domains of rainbow trout estrogen receptor (rtER) were expressed in Escherichia coli as fusion proteins with glutathione-S-transferase (GST). Both fusion proteins were induced by IPTG and could readily be detected as a 53-55 kDa band in crude extracts or in insoluble fraction after polyacrylamide gel electrophoresis and Coomassie blue staining. These recombinant proteins were solubilized and partially purified (ca. 60-75%) using centrifugation and different concentrations of urea. Gel mobility shift assays revealed that the hybrid protein containing ER central domain forms a specific complex with a synthetic estrogen-response-element. Similarly, we showed by steroid-binding assays that the hybrid protein containing the ER C-terminal domain binds specifically estrogen and not other steroids. These hybrid receptors were further isolated by electroelution after electrophoresis and used to immunize rabbits. Polyclonal antibodies from each antiserum were purified using GST-rtER fusion proteins. The specificity of these purified antibodies was confirmed by Western blot analysis using extracts from yeast and COS-1 cells transfected with rtER cDNA expression vectors. In these cells, rtER level was about 300-500 fmol/mg of protein, and the receptor was found as a single band migrating as a 65 kDa polypeptide. Interestingly, Western blot analysis with both purified antibodies directed against central or C-terminal regions of rtER revealed two receptor forms in trout liver nuclear extracts: a major form migrating as 65 kDa protein also observed in transfected cells, and a minor band at 71 kDa specific to the liver. Both receptor form levels were strongly induced by estradiol whereas they were virtually undetectable in untreated male trout livers. Immunocytochemistry performed on brain and pituitary of female trout revealed the presence of rtER in neurons located in the ventral telencephalon, preoptic area and mediobasal hypothalamus, as well as cells in the proximal pars distalis of the pituitary.

Tachykinergic synaptic inputs to neurons of the medial preoptic region which project to the rat arcuate nucleus

Anatomical relationships between tachykinin-containing terminals and neurons of the medial preoptic area that innervate the arcuate nucleus were studied using silver staining of the retrograde tracer wheat germ agglutinin-apoperoxidase-gold (WGA-ApoHRP-gold) complex injected in the arcuate nucleus and pre-embedding immunocytochemistry for neurokinin A (NKA). At the histological level, retrogradely labeled cells not stained for NKA were seen to be surrounded by numerous NKA-immunopositive punctate profiles, in particular in the dorsal part of the medial preoptic area. At the ultrastructural level, retrogradely labeled cell bodies and dendritic profiles displayed highly electron-dense silver particle accumulations over the cytoplasm. The were seen in synaptic contact with one or several NKA-immunoreactive axon terminals containing small clear vesicles and dense-cored vesicles. Such synapses were either symmetrical or asymmetrical. The occurrence of synaptic contacts between tachykinin terminals and cells innervating the arcuate nucleus in the medial preoptic region provides a morphological support for a tachykinergic regulation of preoptic afferences to the arcuate nucleus. These results suggest that tachykinins are implicated in the indirect control of neuronal activity in the arcuate nucleus notably via the preoptic area. Consequently, tachykinins are potentially able to regulate indirectly numerous neuroendocrine events involving the tuberoinfundibular system.

Ultrastructural evidence for synaptic inputs of enkephalinergic nerve terminals to target neurons in the rat arcuate nucleus

The morphological support of interactions between enkephalins and three systems--beta-endorphin (beta-END), tyrosine hydroxylase (TH), or neuropeptide Y (NPY)--well represented in the arcuate nucleus, was examined by using an electron microscopic double immunostaining combining two sensitive chromogens, diaminobenzidine (DAB) and tetramethylbenzidine (TMB). The first step consisted of visualizing Metenkephalinergic terminals with DAB reaction product, and the second one involved detecting the antigens TH, beta-END, and NPY in their respective neurons with TMB reaction product. Ultrastructural analysis revealed enkephalinergic terminals presynaptic to TH-immunopositive cells and dendrites, principally in the dorsal portion of the arcuate nucleus. Enkephalinergic nerve terminals also contacted synaptically ventrolaterally located beta-END-immunoreactive cells. In the ventromedial arcuate nucleus, few synaptic contacts were observed between enkephalinergic boutons and NPY neurons, which were principally in close apposition with glial processes. Enkephalin-immunoreactive synapses were more frequently seen on TH-immunopositive neurons. This TH neuronal group is known to correspond to the dopaminergic tuberoinfundibular neurons implicated in the control of reproductive functions. The pattern of distribution of the different synapses within the arcuate nucleus (TH dorsal, beta-END ventrolaterally; NPY ventromedially) suggests that enkephalins may play a role in the neuroendocrine regulation of gonadotropin and prolactin secretion. The results provide evidence that enkephalins, in the arcuate nucleus, exert a postsynaptic action on the beta-END cells in addition to the presynaptic regulation previously demonstrated in the mediobasal hypothalamus, related to beta-END release. Moreover, the arcuate nucleus is a site of intercellular relationships between enkephalins and dopamine and between enkephalins and other peptides such as NPY.

Comparative distribution of mammalian GnRH (gonadotrophin-releasing hormone) and chicken GnRH-II in the brain of the immature Siberian sturgeon (Acipenser baeri)

The brain of the sturgeon has recently been shown to contain at least two forms of GnRH (gonadotropin-releasing hormone), mammalian GnRH (mGnRH) and chicken GnRH-II (cGnRH-II). In this study, we compared the distribution of immunoreactive (ir) mGnRH and cGnRH-II in the brain of immature Siberian sturgeons (Acipenser baeri). The overall distribution of mGnRH was very similar to the distribution of sGnRH in teleosts such as salmonids or cyprinids. mGnRH-ir perikarya were observed in the olfactory nerves and bulbs the telencephalon, the preoptic region, and the mediobasal hypothalamus. All these cell bodies are located along a continuum of ir-fibers that could be traced from the olfactory nerve to the hypothalamopituitary interface. No ir-fibers were observed in the anterior lobe of the pituitary, but a few were seen to enter the neurointermediate lobe. mGnRH-ir fibers were detected in many parts of the brain, particularly in the forebrain. mGnRH-ir cerebrospinal fluid-contacting cells were observed in the telencephalon, the preoptic region, and the mediobasal hypothalamus. In contrast, cGnRH-II was present mainly in the posterior brain, although a few ir axons were seen in the above-mentioned territories. In particular, cGnRH-II-ir cells bodies, negative for mGnRH, were consistently observed in the nucleus of the medial longitudinal fasciculus of the midbrain tegmentum. The cGnRH-II innervation in the optic tectum, cerebellum, vagal lobe, and medulla oblongata was more abundant than the mGnRH innervation in the same areas. This study provides evidence that the organization of the GnRH systems in a primitive bony fish is highly similar to that reported in teleosts and further documents the differential distribution of two forms of GnRH in the brain of vertebrates.

Purification and primary structure of alligator neurotensin

The gastrointestinal neurohormones of reptiles have been poorly characterized structurally. Neurotensin has been purified to apparent homogeneity from an extract of the small intestine of the alligator, Alligator mississipiensis. The primary structure of the peptide (pGlu-Leu-His-Val-Asn-Lys-Ala-Arg-Arg-Pro-Tyr-Ile-Leu) is identical to that of chicken neurotensin. The data provide further evidence for a close phylogenetic relationship between crocodilians and birds.