Presence and distribution of radioimmunoassayable LHRH in the European eel, Anguilla anguilla

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

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

Changes in brain GnRH mRNA and pituitary GnRH peptide during testicular maturation in barfin flounder

The pleuronectid barfin flounder (Verasper moseri) expresses three forms of gonadotropin-releasing hormone (GnRH) in the brain. To clarify the physiological roles of the respective forms during testicular maturation, changes in brain GnRH mRNA levels and pituitary GnRH peptide levels were examined by real-time quantitative PCR and time-resolved fluoroimmunoassay, respectively. Fish hatched in April 2000. The gonadosomatic index remained low until October 2001 and then rapidly increased in January 2002. Fish continued to grow from hatching through testicular maturation. Fish spermiated in March 2002. The amount of seabream GnRH (sbGnRH) mRNA per brain significantly increased in January 2002 and remained at high levels in March 2002. The amounts of salmon GnRH (sGnRH) and chicken GnRH-II (cGnRH-II) mRNA per brain did not show significant changes during the experimental periods. Pituitary sbGnRH peptide content significantly increased in March 2002. Pituitary sGnRH peptide and cGnRH-II peptide contents were extremely low compared to sbGnRH peptide levels and showed no significant changes during the experiment. These results indicate that sbGnRH is involved in the testicular maturation of barfin flounder.

Distribution, characterization, and growth hormone-releasing activity of pituitary adenylate cyclase-activating polypeptide in the European eel, Anguilla anguilla

The complementary DNA encoding pituitary adenylate cyclase-activating polypeptide (PACAP) has been cloned from two species of teleost fishes, the Sockeye salmon and the Thai catfish, and the amino acid sequence of PACAP has been determined in another teleost, the stargazer. However, to date, the detailed distribution of PACAP immunoreactivity has never been investigated in the fish brain. In the present study, we have determined the localization of PACAP-immunoreactive neurons in the central nervous system of a primitive teleost fish, the European eel Anguilla anguilla, using an antiserum raised against PACAP27. PACAP-positive perikarya were exclusively observed in the diencephalon, i.e. in the preoptic nucleus of the hypothalamus and in the dorsal and ventral nuclei of the thalamus. PACAP-immunoreactive fibers were detected in various areas of the brain, notably in the ventral telencephalon, the diencephalon, the mesencephalon, the cerebellar valvula, and the medulla oblongata. In addition, a dense accumulation of PACAP-containing nerve terminals was found in the pars distalis of the pituitary. The PACAP-like immunoreactivity contained in the eel brain was characterized by HPLC analysis combined with RIA quantification. The major form of PACAP-immunoreactive material coeluted with mammalian PACAP38. Molecular cloning of the PACAP precursor has previously shown that in fish, PACAP and GH-releasing hormone (GHRH) originate from the same precursor. We have thus investigated the effects of PACAP and GHRH on GH secretion from eel pituitary cells in primary culture. Dose-response experiments revealed that PACAP27 and PACAP38 possessed the same efficacy, but PACAP38 was 12 times more potent than PACAP27 in stimulating GH release (ED50 = 4.3 x 10(-10) and 3.5 x 10(-9) M, respectively). In contrast, GHRH, even at a high concentration (10(-6) M), had no effect on GH release. Taken together, these data indicate that in the eel, PACAP may play a significant role in the regulation of somatotrope cells: 1) PACAP-immunoreactive neurons are exclusively located in the diencephalon and send numerous projections in the pars distalis; and 2) PACAP, but not GHRH, dose dependently stimulates GH secretion from cultured eel pituitary cells.

Differential distribution and response to experimental sexual maturation of two forms of brain gonadotropin-releasing hormone (GnRH) in the European eel, Anguilla anguilla

Using specific radioimmunoassays for the two GnRH molecular forms present in the European eel, Anguilla anguilla, (mGnRH and cGnRH II), we compared their distributions in the pituitary and different parts of the brain of female silver eels, as well as the modifications of their levels in experimentally matured female eels (treated with carp pituitary extract). In control eels, mGnRH levels were higher than cGnRH II levels in the pituitary, olfactory lobes and telencephalon, di- and mesencephalon, while the opposite was found in the posterior part of the brain (met- and myelencephalon). Experimental sexual maturation of the gonads significantly increased mGnRH levels in the pituitary and anterior parts of the brain; such a positive effect was not observed on the low cGnRH II levels, which were, in contrast, reduced. These data indicate that the positive feedback of gonadal hormones on GnRH, that we previously demonstrated, would specifically affect the mGnRH form. The differential distribution and control of mGnRH and cGnRH II suggest that these two forms have different physiological roles in the eel. The large increase in mGnRH during sexual maturation suggests the prime implication of this form in the neuroendocrine control of reproduction.

Gonadotropin-releasing hormone (GnRH) in bony fish that are phylogenetically ancient: reedfish (Calamoichthys calabaricus), sturgeon (Acipenser transmontanus), and alligator gar (Lepisosteus spatula)

Three species of fish that are phylogenetically older than other members of the bony fish lineage were selected to determine if gonadotropin-releasing hormone (GnRH) is present in their brains. Brain extracts were prepared from each species and found to contain immunoreactive (ir) GnRH. To further characterize the molecular forms of GnRH in each species, the extracts were injected into a high pressure liquid chromatograph (HPLC). The elution time of each GnRH-like form was compared to those of the synthetic forms of the five known GnRHs. Several antisera were used to detect both the synthetic and unknown GnRHs in the HPLC fractions. All three species of fish had two forms of GnRH: a dominant form that is mammalian GnRH-like (mGnRH), and a minor form of irGnRH material that is similar to chicken GnRH-II (cGnRH-II). The other known forms of GnRH (salmon, lamprey, and chicken-I) were not detected. The appearance in these ancient bony fish of a mammalian-like form of GnRH, which has not been found in the jawless or cartilaginous fish studied to date, suggests that mGnRH arose in a common phylogenetic ancestor of the bony fish and tetrapods. This mGnRH-like molecule is known to have been conserved in the amphibian and mammalian lineage, but not in the reptilian or avian line. In addition, the presence of a cGnRH-II-like molecule in the bony fish examined here, and in the cartilaginous fish studied earlier, implies that this form of GnRH may have been present in an ancestor common to both of these classes of fish.

Differences in salmon GnRH and chicken GnRH-II contents in discrete brain areas of male and female rainbow trout according to age and stage of maturity

We have developed sensitive and specific radioimmunoassays (RIA) for salmon gonadotropin-releasing hormone (sGnRH) and chicken GnRH-II (cGnRH-II). Synthetic sGnRH and cGnRH-II(2-10) were conjugated to bovine serum albumin and injected into rabbits to raise specific antisera. The antiserum against sGnRH showed cross-reactivities of 1.58 and 0.08% for cGnRH-II and lamprey GnRH, respectively. The antiserum against cGnRH-II showed cross-reactivities of 0.05 and 0.01% for sGnRH and lamprey GnRH, respectively. Both antisera were observed not to cross-react with mammalian GnRH and cGnRH-I or other peptide hormones. Synthetic sGnRH and cGnRH-II were iodinated using the chloramine-T method. The iodinated GnRH was purified by HPLC using a reverse-phase C18 column. The RIA system was developed as a double antibody method. Brain extracts of rainbow trout showed displacement curves which were parallel to the sGnRH and cGnRH-II standards in each RIA. HPLC analysis followed by RIA has revealed that rainbow trout brain contains two types of GnRH: sGnRH and cGnRH-II. Total sGnRH content in the brain was about three-fold higher than that of cGnRH-II. In the olfactory bulbs, telencephalon, optic tectum-thalamus, hypothalamus, and pituitary, sGnRH content (per region) was higher than cGnRH-II content, whereas cerebellum and medulla oblongata contained much more cGnRH-II than sGnRH. sGnRH content in the optic tectum-thalamus and pituitary was the highest in 1-year-old immature fish and 3-year-old mature fish, respectively. Medulla oblongata showed the highest cGnRH-II content in all groups. sGnRH concentrations (per milligram of protein) were high in the pituitary and intermediate in the olfactory bulbs, hypothalamus, and telencephalon. In all groups, the cGnRH-II concentration was high in the medulla oblongata, whereas the concentration in the olfactory bulbs and pituitary gland was below the detectable limit in most individuals.

Chromatographic and immunological evidence for mammalian GnRH and chicken GnRH II in eel (Anguilla anguilla) brain and pituitary

Gonadotropin-releasing hormone (GnRH) peptides in the brain and pituitary of the European eel (Anguilla anguilla) were investigated by reverse phase high performance liquid chromatography (HPLC) and radioimmunoassay with region-specific antisera. Two GnRH molecular forms were demonstrated in brain and pituitary extracts. One form eluted in the same position as synthetic mammalian GnRH on HPLC and was recognized by antibodies directed against the NH2 and COOH termini of mammalian GnRH as well as by antibodies to the middle region. The second form eluted in the same position as synthetic chicken GnRH II and was recognized by specific antibodies to this molecule. Salmon GnRH and chicken GnRH I were not detected. The occurrence of mammalian GnRH in teleost fish suggests that this molecular form is more ancient than was previously suspected and arose earlier than in primitive tetrapods, or that it has arisen in the eel through random mutation of salmon GnRH. The lack of salmon GnRH in the eel brain indicates that this molecular form is not common to all teleost species. The finding in eel brain of chicken GnRH II, which has previously been described in species of Mammalia, Aves, Reptilia, Amphibia, Osteichthyes, and Chondrichthyes, supports our hypothesis that this widespread structural variant may represent an early evolved and conserved form of GnRH.

The GnRH systems in the brain and pituitary of normal and hCG treated European silver eels

The distribution of immunoreactive GnRH was studied in the brain and pituitary gland of normal and human chorionic gonadotrophin (hCG) injected silver eels. It was found that the general organization of GnRH systems in this species is similar to that reported in other teleosts. Cell bodies were present in the olfactory bulbs, ventral telencephalon, periventricular hypothalamus and dorsal tegmentum. No positive perikarya could be detected in the preoptic region. Only scarce fibers were observed in the proximal neurohypophysis. Treatment with hCG does not modify the distribution of GnRH but it increases the density of positive structures, in particular at the level of the pituitary. The results are discussed in relation with the present status of knowledge of the mechanisms underlying the blockage of sexual maturation in the European eel at the silver stage.

Positive feedback control by the gonads on gonadotropin (GTH) and gonadoliberin (GnRH) levels in experimentally matured female silver eels,Anguilla anguilla

Treatment of sham-operated female silver eels with carp pituitary extract stimulated ovarian development and induced increases in pituitary gonadotropin (GTH) and gonadoliberin (GnRH) contents. Both effects of carp pituitary extract were abolished in ovariectomized eels, indicating the involvement of the gonads. Endogenous sexual steroids, the secretion of which was increased during sexual maturation, should be responsible for the stimulation of GTH and GnRH levels. Ovariectomy itself had no significant effect on pituitary GTH and GnRH contents, reflecting the fact that, at the silver stage, sexual steroid levels are too low to exert any significant effect on pituitary GTH and GnRH. The positive feedback control exerted by the gonads on GTH and GnRH levels during sexual maturation, in the eel as well as in some other teleosts, would produce an amplification of the pubertal stimulation of the hypothalamo-pituitary-gonadal axis.

Brain distribution of radioimmunoassayable gonadotropin-releasing hormone in female goldfish: seasonal variation and periovulatory changes

A radioimmunoassay (RIA) for [Trp7, Leu8]gonadotropin-releasing hormone (sGnRH) was developed to determine the gonadotropin-releasing hormone (GnRH) content in discrete brain areas of female goldfish at different stages of ovarian development. Temporal changes in serum gonadotropin (GtH) and GnRH concentrations in discrete brain areas were measured during spontaneous ovulation. There were no clear parallel changes in brain GnRH with seasonal ovarian development in goldfish. However, under a 10 degrees temperature acclimation regimen, the GnRH content in the hypothalamus and pituitary decreased as the ovary progressed from the regressed to the mature condition; on the other hand. GnRH content in the spinal cord increased in sexually mature fish compared with that in regressed fish. Significant decreases in GnRH concentration were observed in certain brain areas (olfactory bulbs, telencephalon, hypothalamus, and pituitary) of fish undergoing spontaneous ovulation compared with those of nonovulatory fish. The simultaneous changes of GnRH concentration in these brain areas suggested that the GnRH neuronal system may function as an integrated unit for the activation of GtH secretion during ovulation in goldfish.

Central regulation of reproduction in teleosts

As in other vertebrates, reproduction in teleosts depends upon interactions taking place along the brain-pituitary-gonads axis. At the central level, these interactions involve at least three types of factors:A gonadotrophin-releasing factor which has recently been isolated from chum salmon brain extracts. This decapeptide, whose structure is (Trp(7)-Leu(8))-LHRH, appears to have a widespread distribution among teleosts, and is less active that LHRH or LHRH analogues in releasing gonadotrophin from the teleost pituitary. Immunohistochemical and quantitative studies have demonstrated that Gn-RH neurons are mainly located in the ventral telencephalon and the preoptic area, while projections are found in the entire brain and the pituitary gland.A gonadotrophin release-inhibiting factor has been demonstrated in the anterior preoptic region of the goldfish and a large set of data suggests that dopamine has GRIF activity in goldfish, and in other teleost species, by direct action on the gonadotrophs. Accordingly, a dopaminergic preoptico-hypophyseal pathway could be demonstrated in the goldfish brain.Sex steroids exert, depending on the dosages, either a negative feedback in sexually mature fish or a positive feedback in immature fish. Such a positive feedback is caused by estrogens and aromatizable androgens. Accordingly, the brain of teleosts contains high levels of aromatase activity in particular in the telencephalon and anterior hypothalamus. The distribution of estrogens concentrating cells within the brain is consistent with possible interactions with Gn-RH or catecholaminergic neurons at the level of certain brain territories.These data are discussed in relation with the functional significance of different brain areas where interactions between these different factors possibly take place, in particular the terminal nerve, the ventral telencephalon, the preoptic area and nucleus lateralis tuberis.

Diverse molecular forms of gonadotropin-releasing hormone in an elasmobranch and a teleost fish

Immunoreactive and biologically active gonadotropin-releasing hormones (GnRHs) in dogfish (Poroderma africanum) and teleost (Coris julis) brain extracts were studied by high-performance liquid chromatography (HPLC), radioimmunoassay with region-specific antisera, and assessment of luteinizing hormone (LH)-releasing activity in a chicken dispersed pituitary cell bioassay. In dogfish brain extract, seven GnRH molecular forms with LH-releasing activity were demonstrated. Three of these forms coeluted with synthetic mammalian GnRH; His5,Trp7,Tyr8-GnRH; and Trp7,Leu8-GnRH on HPLC. The peaks coincident with His5,Trp7,Tyr8-GnRH and Trp7,Leu8-GnRH had immunological and biological properties identical to those of the synthetic peptides. However, the molecular form coeluting with mammalian GnRH had immunological and biological properties different from those of mammalian GnRH and is thus a novel molecular variant of GnRH. The four remaining forms are also novel GnRHs or structurally unrelated peptides with LH-releasing activity. Dogfish systemic blood contained immunoreactive GnRH. In teleost brain extract, three biologically active GnRH forms with LH-releasing activity were present. The major peak of GnRH immunoreactivity coeluted with Trp7,Leu8-GnRH, and a second immunoreactive form coeluted with His5,Trp7,Tyr8-GnRH. The third biologically active peak is a novel, early-eluting molecular variant of GnRH or a structurally unrelated peptide with LH-releasing activity.

Immunoreactive gonadotropin-releasing hormone-like material in the brain and the pituitary gland during the periovulatory period in the brown trout (Salmo trutta L.): relationships with the plasma and pituitary gonadotropin

In fish there are few data on the gonadotropin-releasing hormone (Gn-RH) neurosecretory activity, which could explain long- and short-term variations of the gonadotropin secretion. There is no biological species specificity between mammal and fish Gn-RH; although there is a structural difference, they are, on the contrary, characterized by a high immunological specificity which does not allow measurement of fish Gn-RH using radioimmunoassay for LH-RH. We have synthesized salmon Gn-RH according to the formula recently proposed by Sherwood (N. Sherwood, L. Eiden, M. Brownstein, J. Spies, J. Rivier, and W. Vale, 1983. Proc. Natl. Acad. Sci. USA 80, 2794-2798). Its activity has been tested by its ability to stimulate the gonadotropin hormone (GtH) secretion in vivo in testosterone-implanted juvenile rainbow trout, and for the recognition of synthesized Gn-RH (s-Gn-RH) perykaria by a specific antibody raised against the s-Gn-RH in regions of the brain described as containing LH-RH immunoreactive-like material. A radioimmunoassay has been developed for the salmon Gn-RH, and its specificity to measure trout Gn-RH has been tested. Using this assay, the brain and pituitary Gn-RH contents have been measured throughout the final phases of maturation and ovulation. Brain Gn-RH increases from the end of vitellogenesis (8.9 +/- 0.76 ng/brain) to ovulation (more than 15 ng/brain). Pituitary Gn-RH is lower (1.58 +/- 0.69 ng/pituitary) at the end of vitellogenesis and follows a similar profile as in the brain, except for a significant decrease just prior the beginning of oocyte maturation. The correlations between Gn-RH levels and GtH pituitary and plasma levels show that total brain Gn-RH is never correlated to the GtH, suggesting that the increase in the brain Gn-RH content is related to a Gn-RH system closely related to maturation and ovulation, which remains to be investigated. On the contrary, pituitary Gn-RH levels are well correlated with pituitary and plasma GtH levels, indicating that pituitary Gn-RH levels might represent a good index of the Gn-RH neurosecretory activity in the fish hypothalamohypophysial complex, given the absence of a portal system in teleost.

Increase in brain and pituitary radioimmunoassayable gonadotropin releasing hormone (GnRH) in the European silver eel treated with sexual steroid or human chorionic gonadotropin

In the female silver eel, a single estradiol 17 beta (E2) injection significantly increased radioimmunoassayable GnRH (IRGnRH) in the di- and mesencephalon and also in the telencephalon and olfactory lobes, during the first following days; after a chronic estradiol treatment, the pituitary IRGnRH was doubled. In the male silver eel, a single injection of human chorionic gonadotropin (hCG), which is able to induce a progressive testicular development and a durable increase in androgens production, produced a long-term effect on IRGnRH: IRGnRH was significantly increased in the same brain areas as in E2-treated females; a more important rise (10-fold) was observed for pituitary IRGnRH, probably reflecting the accumulation of GnRH in the axonal endings which directly innervate the pituitary in teleosts. These results suggest a positive effect of sexual steroids on GnRH synthesis but not release in the silver eel.

Multiple molecular forms of gonadotropin-releasing hormone in teleost fish brain

Gonadotropin-releasing hormone (GnRH) immunoreactive peptides in extracts of hake (Merluccius capensis) and tilapia (Tilapia sparrmanii) brain were investigated by high performance liquid chromatography (HPLC) and radioimmunoassay with region-specific antisera. In hake brain, content and concentration of GnRH was higher in the pituitary gland than in the hypothalamic lobes or extrahypothalamic brain. Hake pituitary gland GnRH was purified by six consecutive HPLC systems. The major GnRH molecular form co-eluted with salmon brain GnRH (Trp7, Leu8-GnRH) in four different HPLC systems which were specifically designed to separate the four natural vertebrate GnRHs (mammalian, salmon, chicken I and II). The immunoreactive peak in the final purification step had a retention time identical to that of Trp7, Leu8-GnRH and an UV absorbance (280 nm) peak appropriate for two tryptophan residues in the peptide, as in Trp7, Leu8-GnRH. Six additional less hydrophobic forms of GnRH were detected. Tilapia brain extract contained two major GnRH molecular forms which had identical retention times to chicken GnRH I (Gln8-GnRH) and Trp7, Leu8-GnRH in an HPLC system which separates the natural vertebrate GnRHs. The immunological properties of these two immunoreactive peaks, determined by relative interaction with four region-specific GnRH antisera raised against vertebrate GnRHs, were identical to those of Gln8-GnRH and Trp7, Leu8-GnRH. Additional GnRH molecular forms were also detected. In summary, these findings indicate that a major GnRH molecule in hake pituitary gland is Trp7, Leu8-GnRH, while tilapia brain contains both Trp7, Leu8-GnRH and Gln8-GnRH. Additional GnRH molecular forms were detected in both species.(ABSTRACT TRUNCATED AT 250 WORDS)

Immunocytochemical localization of LHRH in the median eminence, infundibular stalk, and neurohypophysis. Evidence for multiple sites of releasing hormone secretion in humans and other mammals

The distribution of luteinizing hormone-releasing hormone (LHRH) was studied by light-microscopic immunocytochemistry in the hypothalamo-pituitary complex of humans, monkeys, ferrets, bats, and rats. LHRH-immunoreactive fibers were identified in the median eminence of all these species, but the precise location of these fibers varied. In rats, the vast majority of LHRH fibers in the median eminence was confined to the external zone. In contrast, in bats, most of the LHRH fibers were located in the internal zone. While these two species represent opposite extremes in distribution of LHRH fibers within the median eminence, intermediate conditions were found in humans, monkeys, and ferrets, as considerable numbers of fibers occurred in both internal and external zones. In addition to fibers in the median eminence, large numbers of LHRH-immunoreactive fibers were identified traversing the infundibular stalk and entering the neural lobe of the pituitary in all species examined except the rat. In rats, only occasional fibers were observed in the infundibular stalk, and they did not project into the neural lobe. However, in humans, monkeys, ferrets, and bats, groups of LHRH-immunoreactive fibers extended well into the substance of the posterior pituitary. Most of these fibers appeared to terminate near the adenohypophysis, but others coursed away from the anterior lobe and penetrated deeper portions of the neural lobe. These observations, made in several mammalian species, indicate that multiple routes may exist in the median eminence/stalk/pituitary complex for the delivery of LHRH to the anterior pituitary.