Structure-activity relationships of mammalian, chicken, and salmon gonadotropin releasing hormones in vivo in goldfish

Metabolic Changes During Growth and Reproductive Phases in the Liver of Female Goldfish (Carassius auratus)

Hormones of the brain-pituitary-peripheral axis regulate metabolism, gonadal maturation, and growth in vertebrates. In fish, reproduction requires a significant energy investment to metabolically support the production of hundreds of eggs and billions of sperms in females and males, respectively. This study used an LC-MS-based metabolomics approach to investigate seasonally-related changes in metabolic profile and energy allocation patterns in female goldfish liver. We measured basal metabolic profile in female goldfish at three phases of the reproductive cycle, including 1) Maximum growth period in postovulatory regressed phase, 2) mid recrudescence in fish with developing follicles, and 3) late recrudescence when the ovary contains mature ovulatory follicles. We also investigated changes in the liver metabolism following acute treatments with GnRH and GnIH, known to be involved in controlling reproduction and growth in goldfish. Chemometrics combined with pathway-driven bioinformatics revealed significant changes in the basal and GnRH/GnIH-induced hepatic metabolic profile, indicating that metabolic energy allocation is regulated to support gonadal development and growth at different reproductive cycles. Overall, the findings support the hypothesis that hormonal control of reproduction involves accompanying metabolic changes to energetically support gonadotropic and somatotropic activities in goldfish and other oviparous vertebrates.

Seasonally related metabolic changes and energy allocation associated with growth and reproductive phases in the liver of male goldfish (Carassius auratus)

Reproduction and growth follow a seasonal pattern in many fish species involving changes in gonadal development, growth, and metabolism. Significant metabolic energy is needed during gametogenesis in both female and male to produce hundreds of eggs and billions of sperms. Seasonal variations are controlled by the hormones of brain-pituitary-peripheral axis and are accompanied by significant metabolic changes. There is evidence that GnRH and GnIH are among the key neurohormones that regulate the reciprocal control of growth and reproduction. The objective of this study was to investigate changes in metabolic profile and energy allocation patterns at different stages of reproduction, using goldfish as a model organism and LC-MS as analytical platform for metabolic analysis. Goldfish undergoes a clear seasonal cycle of growth and reproduction. In vivo experiments were conducted at three different time point of the annual cycle: regressed gonadal phase (peak growth phase), mid gametogenesis and late gametogenesis. Emphasis is placed on changes in liver metabolic pathways to energetically sustain the physiological processes related to growth and reproduction. Moreover, we tested the hypothesis that GnRH and GnIH may play a role in the regulation of metabolism by investigating acute effects of these peptides at different stages of reproductive cycle. SIGNIFICANCE: The findings in this paper provide novel information on the seasonal changes in basal metabolism during different stages of reproductive cycle, and evidence for differential allocation of energy during reciprocal control of reproduction and growth in goldfish. Chemometrics combined with pathway-driven bioinformatics elucidated a shift in the metabolic profile, indicating distinct patterns of energy allocation in the reproductive and growth seasons. Furthermore, to our knowledge this is the first study to provide evidence for a possible regulatory role of GnRH and GnIH in liver metabolism and energy allocation patterns associated with growth and reproductive processes. Together our findings present a framework for better understanding of the hormonally induced changes in metabolism to energetically sustain growth and reproduction in fish and other oviparous species undergoing seasonal cycle.

Nesfatin-1-like peptide suppresses hypothalamo-pituitary-gonadal mRNAs, gonadal steroidogenesis, and oocyte maturation in fish†

Nucleobindin (Nucb)-1 and Nucb2 are DNA and Ca2+ binding proteins with multiple functions in vertebrates. Prohormone convertase-mediated processing of Nucb2 results in the production of biologically active nesfatin-1. Nesfatin-1 is involved in the regulation of reproduction in many vertebrates, including fish. Our lab originally reported a nesfatin-1-like peptide (Nlp) encoded in Nucb1 that exhibits nesfatin-1-like metabolic effects. We hypothesized that Nlp has a suppressive role in the reproductive physiology of fish. In this research, whether Nlp regulates reproductive hormones and oocyte maturation in fish were determined. Single intraperitoneal (IP) injection of goldfish Nlp (50 ng/g body weight) suppressed salmon and chicken gonadotropin-releasing hormone (sgnrh and cgnrh2), gonadotropin-inhibiting hormone (gnih) and its receptor (gnihr), and kisspeptin and brain aromatase mRNA expression in the hypothalamus of both male and female goldfish. In the pituitary, Nlp decreased mRNAs encoding lhb, fshb and kisspeptin and its receptor, while a significant increase in gnih and gnihr was observed. In the gonads, lh (only in male fish) and fsh receptor mRNAs were also significantly downregulated in Nlp-injected fish. Sex-specific modulation of gnih, gnihr, and kisspeptin system in the gonads was also observed. Nlp decreased sex steroidogenic enzyme encoding mRNAs and circulating levels of testosterone and estradiol. In addition, incubation of zebrafish ovarian follicles with Nlp resulted in a reduction in oocyte maturation. These results provide evidence for a robust role for Nlp in regulating reproductive hormones in goldfish and oocyte maturation in zebrafish, and these effects resemble that of nesfatin-1.

Multifactorial control of reproductive and growth axis in male goldfish: Influences of GnRH, GnIH and thyroid hormone

Reproduction and growth are under multifactorial control of neurohormones and peripheral hormones. This study investigated seasonally related effects of GnIH, GnRH, and T3 on the reproductive and growth axis in male goldfish at three stages of gonadal recrudescence. The effects of injection treatments with GnRH, GnIH and/or T3 were examined by measuring serum LH and GH levels, as well as peripheral transcript levels, using a factorial design. As expected, GnRH elevated serum LH and GH levels in a seasonally dependant manner, with maximal elevations of LH in late stages of gonadal recrudescence (Spring) and maximal increases in GH in the regressed gonadal stage (Summer). GnIH injection increased serum LH and GH levels only in fish at the regressed stage but exerted both stimulatory and inhibitory effects on GnRH-induced LH responses depending on season. T3 treatment mainly had stimulatory effects on circulating LH levels and inhibitory effects on serum GH concentrations. In the liver and testes, we observed seasonal differences in thyroid receptors, estrogen receptors, vitellogenin, follicle-stimulating hormone receptor, aromatase and IGF-I transcript levels that were tissue- and sex-specific. Generally, there were no clear correlation between circulating LH and GH levels and peripheral transcript levels, presumably due to time-related response and possible direct interaction of GnRH and GnIH at the level of liver and testis. The results support the hypothesis that GnRH and GnIH are important components of multifactorial mechanisms that work in concert with T3 to regulate reciprocal control of reproduction and growth in goldfish.

Seasonal Related Multifactorial Control of Pituitary Gonadotropin and Growth Hormone in Female Goldfish: Influences of Neuropeptides and Thyroid Hormone

Female reproduction is under multifactorial control of brain-pituitary-peripheral origin. The present study provides information on seasonal changes in circulating LH and GH concentrations, as well as transcript levels for a number of genes involved in the regulation of reproduction and growth in female goldfish. We also provide information on the effects of treatments with GnRH and/or GnIH, and their interaction with T3, at three stages of gonadal recrudescence. Maximum basal concentration of LH was observed at late recrudescence (Spring) while no seasonal changes in basal serum GH levels was detected. Serum LH and GH levels were stimulated by GnRH as expected, depending on the season. GnIH stimulated basal GH concentrations in gonadally regressed fish. GnIH inhibitory action on GnRH-induced LH response was observed in late, but not in mid recrudescence. T3 actions on basal and GnRH- or GnIH-induced GH secretion were generally inhibitory, depending on season. Administration of T3 attenuated GnRH-induced LH responses in mid and late stages of gonadal recrudescence, and the presence of GnIH abolished inhibitory actions of T3 in fish at mid recrudescence. Our results also demonstrated seasonal patterns in basal and GnRH- and/or GnIH-induced transcript levels for ERα, ERβI, FSHR, aromatase, TRαI, TRβ, IGF-I, and Vtg in the liver and ovary. However, there were no clear correlations between changes in transcript levels and circulating levels of LH and GH. The results support the hypothesis that GnRH, GnIH, and T3 are contributing factors in complex reciprocal control of reproduction and growth in goldfish.

Effects of the fish spawning inducer ovaprim on vasotocin receptor gene expression in brain and ovary of the catfish Heteropneustes fossilis with a note on differential transcript expression in ovarian follicles

Ovaprim (OVP), a commercial formulation of a salmon GnRH analogue and the dopamine receptor-2 blocker domperidone, is a successful spawning inducer for fish breeding. It induces a preovulatory surge in LH, which stimulates the synthesis of a maturation-inducing steroid (MIS, 17,20β-dihydroxy-4-pregnen-3-one) that initiates germinal vesicle breakdown (GVBD) and ovulation. Coincidently, the OVP treatment also stimulates vasotocin (VT) secretion in the brain and ovary of the catfish Heteropneustes fossilis that also stimulates the synthesis of the MIS. VT mediates its effect through V1- and V2-type receptors. In the present study in the catfish, we report that OVP stimulates the expression of VT receptor genes v1a1, v1a2 and v2a in the brain and ovary. A single intraperitoneal administration of OVP (0.5μL/g body weight) or incubation of post-vitellogenic ovarian follicles with 5μL/mL OVP, for 0, 4, 8, 12, 16, and 24h stimulated ovulation and GVBD, respectively, in a time-dependent manner. The OVP treatment in vivo stimulated brain VT receptor transcript levels 4h onwards. The peak expression was noticed at 12h (v1a1), 8 and 12h (v1a2), and 8, 12 and 16h (v2a), coinciding with FOM and ovulation. The VT receptor genes are expressed in the ovarian follicles compartmentally; both v1a1 and v1a2 are expressed in the isolated follicular layer (theca and granulosa) but absent in denuded oocytes. V2a is expressed in the denuded oocytes and not in the follicular layer. The OVP injection stimulated the v1a1 and v1a2 expression from 4h onwards in both intact follicle and isolated follicular layer, the peak expression was observed at 16h. The v2a expression was up-regulated in both intact follicles and denuded oocytes at 4h (denuded oocytes) or 8h (intact follicle) onwards with the peak expression at 12h and 16h (denuded oocytes) or at 16h (intact follicles). Under in vitro conditions, the OVP incubations elicited similar pattern of changes with the peak stimulation at 16h for all the genes. In conclusion, the VT receptor genes are differentially expressed in the ovarian follicles and OVP induced periovulatory stimulation of the VT receptor genes, coinciding with FOM and ovulation.

Effects of ovaprim, a commercial spawning inducer, on vasotocin and steroid hormone profiles in the catfish Heteropneustes fossilis: in vivo and in vitro studies

Ovaprim (OVP) is used as an effective spawning inducer for artificial breeding of fishes and contains a salmon gonadotropin-releasing hormone analogue and a dopamine receptor-2 antagonist, domperidone. Previously, we have shown that vasotocin (VT) stimulates ovarian final oocyte maturation, hydration, and ovulation through a mechanism involving induction of a steroidogenic shift, favouring the production of a maturation-inducing hormone (MIH). In the present study, we demonstrated that OVP stimulated brain, plasma and ovarian VT levels, suggesting multiple sites of action, apart from its well established role in the induction of a preovulatory LH surge. An intraperitoneal injection of 0.5μL/g body weight of OVP for different time intervals (0, 4, 8, 12, 16 and 24h) induced ovulation as well as increased significantly brain and plasma VT levels in a time-dependent manner. Plasma steroids were differentially altered; the levels of estradiol-17β (E2) and testosterone (T) decreased, and the MIH (17, 20β-dihydroxy-4-pregnen-3-one; 17, 20β-DP) level increased time-dependently. In order to demonstrate whether OVP acts at the level of the ovary directly, in vitro experiments were conducted. The incubation of ovarian slices/follicles with OVP (1, 5 and 10μL/mL) for different time points (0, 4, 8, 12, 16 and 24h) induced germinal vesicle breakdown (GVBD) in a concentration- and time-dependent manner. Ovarian VT increased significantly in a concentration- and time-dependent manner with a maximal increment at 16h. Ovarian T and E2 levels decreased concurrently with the rise in the MIH level, dose- and duration-dependently. The results show that OVP stimulates VT at the brain and ovarian level. The direct OVP-VT cascade has the potential to stimulate FOM and ovulation, sidelining the pituitary glycoprotein hormone (LH) surge.

Dopamine D1 receptor blockage potentiates AMPA-stimulated luteinising hormone release in the goldfish

Previous microarray analyses of the goldfish hypothalamus led us to hypothesise that dopamine could potentially inhibit the excitatory effects of glutamate on luteinising hormone (LH). Post-spawning female goldfish were pre-treated (-4.5 h) with either saline (C; control), SCH 23390 (S; D(1) -receptor antagonist) or sulpiride (L; D(2) -receptor antagonist), followed by an i.p. injection, at -0.5 h, of saline or the glutamate agonist AMPA (A, SA or LA). Blood, hypothalamus and telencephalon tissues were collected. Serum LH was not affected in the S, L, A, or LA groups relative to control as determined by radioimmunoassay. The SA group, however, showed a 289% (P<0.0005) increase in serum LH compared to either treatment alone or control. Real-time reverse transcriptase-polymerase chain reaction identified the mRNAs for ionotropic (Gria2a, Gria4) glutamate receptor subunits, activin βa, isotocin, and cGnRH-II as being significantly affected by some of the treatments. The same experiment conducted with sexually-regressed female fish showed a very different LH profile, indicating that this mechanism is seasonally-dependent. We also show that i.p. injection of 1 μg/g isotocin was able to increase LH levels by 167% in sexually regressed female fish relative to controls. Taken together, these results demonstrate that blockage of the D(1) receptor primes post-spawning goldfish for AMPA-stimulated LH release, and provides further insights into the central regulation of reproduction.

Innervation and control of the adenohypophysis by hypothalamic peptidergic neurons in teleost fishes: EM immunohistochemical evidence

Previous light microscopic studies have revealed neuropeptide-immunoreactive neurosecretory fibers in the teleostean neurohypophysis, and ultrastructural work has reported direct innervation of endocrine cells by the terminals of fibers penetrating the adenohypophysis. This paper reviews our recent data from ultrastructural, immunohistochemical, receptor localization, and superfusion studies, which suggest a role for neuropeptides in the control of teleost pituitary secretion. We have used a combination of pre- and post-embedding electron microscopic immunolabeling methods to determine which neuropeptides are present in fibers innervating the pituitaries of three species: Poecilia latipinna, Dicentrarchus labrax, and Clarias gariepinus. Numerous axon profiles with immunoreactivity for the neurosecretory peptides vasotocin and isotocin formed large Herring bodies and terminal-like boutons in contact with corticotropic, growth hormone, thyrotropic, and pars intermedia cells. Numerous melanin-concentrating hormone-immunoreactive fibers and scarcer neurotensin and corticotropin-releasing factor-immunoreactive fibers showed similar distributions, terminating close to pars intermedia and corticotropic cells. Somatostatin, cholecystokinin, galanin, substance P, neuropeptide Y, growth hormone-releasing factor, thyrotropin-releasing hormone, and gonadotropin-releasing hormone-immunoreactivities were found in small calibre fibers penetrating among growth hormone, thyrotropic, and gonadotropic cells. These morphological findings have been supplemented by autoradiographic studies, which showed the distribution of binding sites for vasotocin, isotocin, galanin, and neuropeptide Y ligands over specific groups of pituitary cells, and superfusion studies that showed growth hormone release was stimulated by growth hormone-releasing factor and thyrotropin-releasing hormone, but inhibited by somatostatin. The implications of these results for neuropeptidergic control of teleostean pituitary secretions are discussed.

Levels of the native forms of GnRH in the pituitary of the gilthead seabream, Sparus aurata, at several characteristic stages of the gonadal cycle

Brains of the gilthead seabream, Sparus aurata, contain three different forms of gonadotropin-releasing hormone (GnRH): seabream (sb) GnRH, chicken (c) GnRH-II, and salmon (s) GnRH. In the present study, we developed three specific enzyme-linked-immunosorbent assays (ELISA) for sbGnRH, cGnRH-II, and sGnRH and used them to measure the levels of each GnRH form in the pituitary of male and female seabream at different stages of gametogenesis. The sensitivity was 6 pg/well for the sbGnRH assay, 7 pg/well for the cGnRH-II assay, and 2 pg/well for the sGnRH assay. Levels of each of the three GnRH forms were measured in pituitaries from fish sampled at the beginning of gonadal recrudescence and during the spawning season. Of the three forms, only sbGnRH and cGnRH-II were detected in the pituitary, irrespective of reproductive state or sex. Recrudescent fish had similar levels of sbGnRH and cGnRH-II in the pituitary. In sexually mature fish, the levels of sbGnRH were higher than those in recrudescent fish while pituitary cGnRH-II content remained unchanged. Consequently, sbGnRH levels were 3- to 17-fold higher than cGnRH-II levels in mature fish. Positive correlations also existed between pituitary sbGnRH content and pituitary and plasma gonadotropin (GtH) II levels. Surprisingly, mature 1-year-old males had significantly higher levels of sbGnRH in the pituitary than mature 3-year-old males, while pituitary and plasma GtH II levels were similar between these two groups. Although the reason for this difference in sbGnRH levels is unclear, a possible role of sbGnRH in the processes of puberty or sex-inversion is implied. Based on the present results, it can be suggested that in the gilthead seabream, sbGnRH is the most relevant form of GnRH in the control of reproduction.

Structural modifications of non-mammalian gonadotropin-releasing hormone (GnRH) isoforms: design of novel GnRH analogues

Three natural forms of vertebrate gonadotropin-releasing hormone (GnRH) provided the structural basis upon which to design new GnRH agonists: [His5,Trp7,Leu8]-GnRH, dogfish (df) GnRH; [His5,Asn8]-GnRH, catfish (cf) GnRH; and [His5,Trp7,Tyr8]-GnRH, chicken (c) GnRH-II. The synthetic peptides incorporated the position 6 dextro (D)-isomers D-arginine (D-Arg) or D-naphthylalanine (D-Nal) in combination with an ethylamide substitution of position 10. The in vitro potencies for LH and FSH release of these analogues were assessed using static cultures of rat anterior pituitary cells. Efficacious peptides were examined for their gonadotropin-II and growth hormone releasing abilities from perifused goldfish pituitary fragments. Rat LH and FSH release was measured using homologous radioimmunoassays, whereas goldfish growth hormone and gonadotropin-II release were determined using heterologous carp hormone radioimmunoassays. The receptor binding of the most potent analogues was determined in bovine pituitary membrane preparations. Substitution of D-Nal6 into [His5,Asn8]-GnRH increased the potency over 2200-fold compared with the native ligand (cfGnRH) in cultured rat pituitary cells. This was equivalent to a 55-fold greater potency than that of the native mammal (m) GnRH peptide. Substitution of D-Nal6 or D-Arg6 into dfGnRH or cGnRH-II resulted in potencies that were related to the overall hydrophobicity of the analogues. The [D-Nal6,Pro9NEt]-cfGnRH bound to the bovine membrane preparation with an affinity statistically similar to that of [D-Nal6,Pro9NEt]-mGnRH (kd = 0.40 +/- 0.04 and 0.55 +/- 0.10 nM, respectively) in cultured rat pituitary cells. All analogues tested released the same ratio of FSH to LH. In goldfish, the analogues did not possess superagonistic activity but instead desensitized the pituitary fragments at lower analogue doses than that of the sGnRH standard suggesting differences in receptor affinity or signal transduction.

Multiple gonadotropin-releasing hormone (GnRH)-immunoreactive systems in the brain of the dwarf gourami, Colisa lalia: immunohistochemistry and radioimmunoassay

The present study characterizes gonadotropin-releasing hormone (GnRH) neuronal groups that are located in several different brain regions by investigating GnRH molecular species and projection patterns in an anabantid fish, Colisa lalia. First, we examined the molecular species of GnRHs in extracts of the brain and the pituitary by reverse-phase high-performance liquid chromatography followed by radioimmunoassays. We found salmon GnRH (sGnRH), chicken GnRH-II (cGnRH-II), and an unfamiliar GnRH-like substance. Next, to examine the distribution of each GnRH molecule in different GnRH neuronal groups, we performed immunohistochemistry using four kinds of antisera and an antibody. Furthermore, we performed brain lesioning experiments of terminal nerve (TN) cells, the most conspicuous GnRH-immunoreactive cells in Colisa lalia. Comparisons of immunoreactive structures between TN-lesioned fish and untreated fish elucidated the projection area of each neuronal group. Three major neuronal groups were observed. TN-GnRH cells, which are located in the transitional area between the olfactory bulb and the telencephalon, showed strong sGnRH and weaker cGnRH-II immunoreactivity. TN-GnRH cells projected to wide areas of the central nervous system from the olfactory bulb to the spinal cord. The second group, located in the preoptic area, showed only sGnRH immunoreactivity and projected only to the pituitary. The third one, located in the midbrain tegmentum, exhibited strong cGnRH-II and weaker sGnRH immunoreactivity. This cell group projected mainly to brain regions posterior to the hypothalamus and the spinal cord. These different projection patterns suggest functional differentiation of each GnRH neuronal group.

In vitro bioactivities of various forms of GnRH in relation to their susceptibility to degradation at the pituitary level in the rainbow trout, Oncorhynchus mykiss

In vitro potencies of native and modified forms of salmon and mammalian gonadotropin-releasing hormone (GnRH) were studied in relation with their susceptibility to degradation by intact pituitary cells maintained in culture. The kinetics of degradation and the origin of the proteases involved in this process were examined. All the molecules tested (native and modified forms) were equipotent at doses between 10(-6) and 10(-7) M in inducing GtH release by cultured pituitary cells. On the other hand, their effectiveness differed at 10(-9) and 10(-8) M leading to the establishment of the following hierarchy of bioactivity: the native forms, LHRH and sGnRH, were the less potent, the fish analogues (DAla6Pro9Net)sGnRH and (DArg6Pro9Net)sGnRH were the more potent, and mammalian analogues with substitutions at position 6 and/or 10 were intermediate in potency. The native form sGnRH was weakly degraded while no degradation of the modified molecules was observed. The degradation of the native sGnRH occurred after 12 and 24 hr of incubation and the results indicate that the peptidases involved are released from the cells into the incubation medium.

A second endogenous molecular form of mammalian hypothalamic luteinizing hormone-releasing hormone (LHRH), (hydroxyproline9)LHRH, releases luteinizing hormone and follicle-stimulating hormone in vitro and in vivo

In vitro and in vivo release of pituitary hormones were studied in the presence of (hydroxyproline9)LHRH ((Hyp)LHRH), a newly characterized endogenous molecular form of LHRH. Results were compared to those obtained with LHRH itself. (Hyp)LHRH, as LHRH, stimulated both luteinizing hormone (LH) and follicle-stimulating hormone (FSH) release in a homothetic manner. The hydroxylated compound was, however, 24 times (in vitro) and 5 times (in vivo) less potent than LHRH. The lower activity of (Hyp)LHRH than of LHRH in the in vitro assay correlated well with a 28-fold lesser potency in a binding test using pituitary membrane preparations. The higher relative potency and the prolonged effect of (Hyp)LHRH in the in vivo test were related to a lesser susceptibility of the hydroxylated form to proteolytic degradation. Effects of LHRH and of (Hyp)LHRH were not additive, both peptides were equally able to desensitize gonadotrophs to a subsequent challenge by the other. Taken together, these observations suggest that both forms of LHRH act at the same receptor site. The lesser affinity of the hydroxylated compound is compensated to a certain extent by its higher resistance to enzymatic degradation. It is concluded that in spite of its lesser potency, (Hyp)LHRH may participate in the regulation of gonadotropins.

Activity of vertebrate gonadotropin-releasing hormones and analogs with variant amino acid residues in positions 5, 7 and 8 in the goldfish pituitary

All non-mammalian vertebrates as well as marsupial mammals have two or more forms of gonadotropin-releasing hormone (GnRH) in the brain. Goldfish brain and pituitary contains two molecular forms of GnRH, salmon GnRH ([Trp7, Leu8]m-GnRH; s-GnRH) and chicken GnRH-II ([His5, Trp7, Tyr8]m-GnRH; cII-GnRH). Both sGnRH and cII-GnRH stimulate gonadotropin (GtH) as well as growth hormone (GH) release from the goldfish pituitary. The purpose of the present study was to study the activity of the five known forms of GnRHs as well as analogs of mammalian GnRH (m-GnRH) with variant amino acid residues in positions 5, 7 and 8 in terms of binding to GnRH receptors, and release of GTH and GH from the perifused fragments of goldfish pituitary in vitro. All five vertebrate GnRH peptides stimulated both GtH and GH release in a dose-dependent manner, although their potencies were very different. cII-GnRH was somewhat more active than s-GnRH in releasing GtH, whereas s-GnRH tended to have a greater potency than cII-GnRH in terms of GH release. Both chicken GnRH-I (cI-GnRH) and lamprey GnRH (l-GnRH) were significantly less potent than mGnRH, s-GnRH and cII-GnRH in releasing GtH and GH. cII-GnRH binds with higher affinity for the high affinity binding sites compared to all other native peptides. The activity of [Trp7]-GnRH was similar to both s-GnRH and cII-GnRH in releasing GtH and GH. Substitution of His5 resulted in a significant decrease in GtH releasing potencies compared to mGnRH, sGnRH and cII-GnRH. [His5]-GnRH also had lower GH releasing potency than mGnRH and sGnRH. Tyr8, His8 and Leu8 substitutions caused significant decreases in GtH releasing potencies compared to mGnRH, s-GnRH and cII-GnRH, but did not cause a significant change in GH releasing potency. The combination of [His5, Trp7]-GnRH had GtH and GH releasing activities similar to m-GnRH, s-GnRH and cII-GnRH.(ABSTRACT TRUNCATED AT 400 WORDS)

Metabolism of gonadotropin-releasing hormone in goldfish: serum clearance and tissue uptake studies

The metabolic clearance rate (MCR) and initial half-disappearance time (T(1/2)i) of salmon gonadotropin-releasing hormone (sGnRH) and its agonist analog [D-Arg6,Pro9-NEt]-sGnRH (sGnRH-A) were investigated in goldfish, following a single intraarterial injection of radioiodinated sGnRH and sGnRH-A. The tissue uptake of radioiodinated sGnRH-A was also investigated. 125I-sGnRH had a MCR of 0.01173 ml min-1 35 g-1 and a (T(1/2)i) of 32.38 min; 125I-sGnRH-A had a MCR of 0.0192 ml min-1 35 g-1 and a (T(1/2)i) of 72.95 min. In the tissue uptake experiments, high levels of accumulated labeled sGnRH-A were generally found in the gills, kidney, liver, and pituitary compared to air bladder, brain, gonad, eyes, and muscle. Coinjection of excess amount of cold sGnRH-A caused decreased uptake of labeled sGnRH-A only in pituitary, but increased accumulation of labeled sGnRH-A in some other tissues. Our results show a correlation between the T(1/2)i and the affinity of the peptides for a serum GnRH binding protein present in the goldfish, suggesting that the formation of a hormone-binding protein complex may decrease the MCR of GnRH in the circulation in goldfish. Our data also indicate specific uptake of 125I-sGnRH-A by the pituitaries of both male and female goldfish, confirming the pituitary as a major target organ of GnRH in goldfish.

Degradation of gonadotropin-releasing hormones in the gilthead seabream, Sparus aurata. II. Cleavage of native salmon GnRH, mammalian LHRH, and their analogs in the pituitary, kidney, and liver

The pattern and kinetics of degradation of native salmon gonadotropin-releasing hormone (sGnRH), mammalian luteinizing hormone-releasing hormone (LHRH), and some of their analogs by cytosolic enzymes of pituitary, kidney, and liver were studied in the gilthead seabream, Sparus aurata. The native peptides sGnRH and LHRH are rapidly degraded by all three tissues, LHRH being degraded faster than sGnRH. The kinetics of production of the peptide fragments suggest that initial cleavage of sGnRH and LHRH in the three studied tissues occurs at the 5-6 and 9-10 bonds. This indicates the initial activity of a Tyr5-Gly6 endopeptidase and a Pro9-Gly10NH2 peptidase or postproline cleaving enzyme. Secondary degradation of the main initial fragments (1-5, 6-10, and 1-9) is more intensive in the kidney than in the pituitary or liver. Substitution of the position 6 amino acid glycine by a dextrorotatory (D) amino acid such as in the D-Trp6-LHRH renders the 5-6 bond resistant to cleavage. However, whereas [D-Trp6]-LHRH is intensively cleaved at the Pro9-Gly10NH2 bond by the pituitary, its cleavage at this site by the kidney and liver is slow. This suggests a low activity of the Pro9-Gly10NH2 peptidase in the kidney and liver as compared to the pituitary. When, in addition to the position 6 substitution, the carboxy terminus Pro9-Gly10NH2 is modified to Pro9NET, such as in the [D-Ala6-Pro9NET]-LHRH and the [D-Arg6-Pro9NET]-sGnRH, the 9-10 cleavage site is also blocked, resulting in GnRH analogs highly resistant to degradation. The relationships between susceptibility of the different forms of GnRH to enzymatic degradation by the pituitary, kidney, and liver and their relative biological activities in S. aurata are discussed. We conclude that increased resistance of GnRH analogs to enzymatic degradation contributes to their superactivity.

Use of a pituitary cell dispersion method and primary culture system for the studies of gonadotropin-releasing hormone action in the goldfish, Carassius auratus. II. Extracellular calcium dependence and dopaminergic inhibition of gonadotropin responses

Primary static cultures of dispersed goldfish pituitary cells obtained by controlled trypsinization released gonadotropin (GTH) in response to 2-hr stimulations of 0.1 nM to 1 microM [Trp7,Leu8]-gonadotropin-releasing hormone (sGnRH), [D-Arg6,Pro9-N-ethylamide]-sGnRH (sGnRHa), and [His5,Trp7,Tyr8]-GnRH (cGnRH-II) in a dose-dependent manner. Coincubation with 10 to 1000 nM of a dopamine agonist, apomorphine, dose dependently reduced the GTH response to increasing concentrations of sGnRH. Apomorphine at 1 microM completely abolished the dose-dependent GTH response to sGnRHa and cGnRH-II, but only partially inhibited the GTH-releasing action of high concentrations of sGnRH. Addition of calcium ionophores, 1 to 100 microM A23187 and 10 to 100 microM ionomycin, significantly increased GTH release. The ED50S of the GTH response to A23187 and ionomycin were 0.88 +/- 0.15 and 13.67 +/- 2.76 microM, respectively. Incubation with Ca2(+)-deficient media (media prepared without the addition of Ca2+ salts) did not significantly affect basal GTH release, but severely decreased the hormone response to increasing concentrations of sGnRH, A23187, and ionomycin. These results confirm the direct inhibitory dopaminergic influence on GTH release in goldfish and further suggest that extracellular Ca2+ plays a role in mediating GnRH action on gonadotropes in fish.

Use of a pituitary cell dispersion method and primary culture system for the studies of gonadotropin-releasing hormone action in the goldfish, Carassius auratus. I. Initial morphological, static, and cell column perifusion studies

Two cell dispersion methods for excised goldfish pituitary glands were tested, and a cultured dispersed cell system based on trypsin enzymatic tissue digestion was developed and characterized. Controlled trypsin/DNase treatment of goldfish pituitary gland yielded dispersed cells of high viability (trypsin blue exclusion test) that responded to gonadotropin (GTH)-releasing hormone (GnRH) challenges with GTH secretion in a time- and dose-dependent manner following overnight culture. Electron microscopy revealed that cell preparations produced by the trypsin dispersion were free of cell debris and nerve terminals. The dispersed pituitary cells also retained distinct morphological and immunological identities. Under static incubation conditions, 2-hr treatments with 0.1 nM to 1 microM [Trp7,Leu8]-GnRH (sGnRH) and [D-Arg6,Pro9-N-ethylamide]-sGnRH (sGnRHa) stimulated GTH release with similar efficacy, but with ED50S of 1.92 +/- 0.48 and 0.19 +/- 0.08 nM, respectively. [His5,Trp7,Tyr8]-GnRH (cGnRH-II) stimulated GTH release in a nonsigmoidal, but dose-dependent manner, and with a higher efficacy than sGnRH. In contrast, sGnRH, sGnRHa, and cGnRH-II were equipotent in inducing growth hormone (GH) secretion in static culture studies and with ED50S of 0.29 +/- 0.13, 0.18 +/- 0.11, and 0.19 +/- 0.17 nM, respectively. When trypsin/DNase-dispersed cells cultured overnight with cytodex beads were tested in a cell column perifusion system, dose-related increase in GTH secretion, as well as GH release, were also observed with 0.5 to 50 nM sGnRH. These results suggest that trypsin-dispersed goldfish pituitary cells can be used effectively to study the actions of GnRH on teleost pituitary either in short-term static incubation or column perifusion studies. Differences in the GTH and GH responses to the two native GnRH forms, sGnRH and cGnRH-II, are also indicated.

A presumptive role for GABA in the stimulatory effects of Des-Gly10, [D-Ala6]-LHRH-ethylamide and pimozide on the gonadotropin release in carp

To investigate the effect of endogenous gamma-aminobutyric acid (GABA) on the blood maturating gonadotropin (GtH) levels, or to study its interaction with pimozide (dopamine antagonist) and a luteinizing hormone-releasing hormone analog (LHRH-a), sexually mature male and female carps were treated with drugs that may either inhibit GABA biosynthesis or GABA degradation. In females the irreversible inhibitor of GABA-transaminase, gamma-vinyl GABA (GVG), which was to increase the endogenous GABA-ergic tone, had no influence on GtH release. On the other hand, the increased GtH response to the combination of pimozide (PIM) and LHRH-a was clearly enhanced by the administration of 3-mercaptopropionic acid (MPA), an inhibitor of the rate limiting enzyme of GABA-biosynthesis. In males the GABA-ergic compound, valproic acid (DPA) decreased LHRH-a stimulated GtH levels. In male carps that received PIM to diminish the dopaminergic inhibition of GtH release, the spermiating response to LHRH-a was increased by administration of MPA. These data suggest that GABA interacts with the action of dopamine and the gonadotropin releasing hormone (GnRH) on the release of GtH.

Participation of arachidonic acid metabolism in gonadotropin-releasing hormone stimulation of goldfish gonadotropin release

Two intraperitoneal injections of a mammalian gonadotropin-releasing hormone (GnRH) analog, [D-Ala6, Pro9-N-ethylamide]-GnRH (mGnRHa; 0.1 micrograms/g), at 12-hr intervals increased serum gonadotropin (GTH) levels in sexually mature and sexually regressed female goldfish 2 and 6 hr after the second injection. This serum GTH response was decreased by the coinjection of a lipoxygenase enzyme inhibitor, nordihydroguaiaretic acid (NDGA: 0.1 to 10 micrograms/g) at the time of the second mGnRHa application. In static cultures of dispersed goldfish pituitary cells, 1-100 microM arachidonic acid (AA) and 0.1-1000 nM [Trp7, Leu8]-GnRH (salmon GnRH, sGnRH) and [D-Arg6, Pro9-N-ethylamide]-sGnRH (sGnRHa) caused dose-dependent increases in GTH release. Additions of 1-40 microM NDGA reduced the sGnRH-stimulated GTH release in a dose-dependent manner, and completely inhibited the GTH response to increasing concentrations of AA. NDGA 40 microM also decreased the elevated GTH levels induced by sGnRHa treatment. Exposure to 10 microM 5,8,11,14-eicosatetraynoic acid, an inhibitor with mixed action on lipoxygenase and cyclooxygenase enzymes, reduced the dose-dependent GTH response to sGnRH and AA. In contrast, coincubation with another cyclooxygenase blocker, indomethacin, at 10 microM, did not alter AA and sGnRH-induced GTH release. These results provide in vivo and in vitro evidence for the participation of AA metabolism in mediating GnRH-stimulated GTH release in the goldfish. The importance of AA metabolism through the lipoxygenase pathway is also indicated.

Effect of a teleost GnRH analog on steroidogenesis by the follicle-enclosed goldfish oocytes, in vitro

The influence of an agonist analog of teleost GnRH [(D-Arg6, Trp7, Leu8, Pro9-NEt)-GnRH; tGnRH-A] on steroidogenesis was studied in prophase-I arrested, follicle-enclosed, goldfish oocytes in vitro. Incubation of the follicles with carp gonadotropin (GtH) significantly increased production of 17 alpha-hydroxyprogesterone (HP) and testosterone following 24 hr of incubation in vitro. Concomitant incubation with tGnRH-A (10(-7) M) significantly attenuated the dose-related increase in GtH-induced testosterone production, but was without effect on the GtH-induced HP level. Time course studies indicated that tGnRH-A exerted its maximum inhibitory action on the GtH-induced testosterone production during the initial 8 hr of incubation in vitro. The inhibition of GtH-induced testosterone production by tGnRH-A was dose dependent with an ED50 of 1.39 +/- 2.88 nM. A significantly higher testosterone level was obtained in the incubation media containing HP as substrate; concomitant treatment with tGnRH-A reduced the conversion of HP to testosterone. The incubation media also contained low, but measurable levels of 17 alpha-hydroxy-20 beta-dihydroprogesterone (DHP), which increased in the presence of 3-isobutyl-methyl-xanthine; lower levels of DHP were obtained in the groups incubated with tGnRH-A. In view of our present findings and previous observations concerning inhibitory effects of tGnRH-A on the progestogen and GtH-induced reinitiation of meiosis in the follicle-enclosed goldfish oocytes (H. R. Habibi, G. Van Der Kraak, E. Bulanski, and R. E. Peter, Amer. J. Physiol. 255, R268-R273 (1988] the influence of testosterone on the GtH- and DHP-induced meiosis in vitro was also studied. Testosterone (1 micrograms/ml) enhanced both GtH- and DHP-induced oocyte meiosis in the goldfish oocytes. Testosterone alone was also found to significantly increase oocyte meiosis in the goldfish oocytes in a dose-related fashion. The present findings demonstrate an inhibitory effect of a GnRH agonist on GtH-induced testosterone production in goldfish oocytes and suggest that tGnRH-A might influence oocyte meiosis in part by influencing steroidogenesis.

Pituitary gonadotropin-releasing hormone (GnRH) receptor activity in goldfish and catfish: seasonal and gonadal effects

The goldfish pituitary contains two classes of gonadotropin-releasing hormone (GnRH) binding sites, a high affinity/low capacity site and a low affinity/high capacity site (Habibiet al. 1987a), whereas the catfish pituitary contains a single class of high affinity GnRH binding sites (De Leeuwet al. 1988a). Seasonal variations in pituitary GnRH receptor binding parameters, and the effect of castration on pituitary GnRH receptor binding were investigated in goldfish and catfish, respectively. In goldfish, GnRH receptors undergo seasonal variation with the highest pituitary content of both high and low affinity sites occurring during the late stages of gonadal recrudescence. The observed changes in pituitary GnRH receptor content correlate closely with responsiveness to a GnRH agonistin vivo in terms of serum gonadotropin (GTH) levels. In catfish, castration results in a two-fold increase in pituitary GnRH receptor content, which can be reversed by concomitant treatment with androstenedione, but not by the non-aromatizable androgen 11β-hydroxyandrostenedione; changes observed in GnRH receptor content correlate with variations in serum GTH levels and responsiveness to a GnRH agonist. In summary, the present study provides a clear evidence for seasonal variation in pituitary GnRH receptor activity in goldfish, and demonstrates a gonadal feedback mechanism regulating GnRH receptor activity in the catfish pituitary.

The bioactivity of gonadotropin releasing hormones and its regulation in the gilthead seabream,Sparus aurata: in vivo andin vitro studies

Thein vivo andin vitro potency of native and modified forms of gonadotropin releasing hormone (GnRH) to release gonadotropin (GtH) was studied inSparus aurata and correlated with their relative susceptibility to degradation by cytosolic-bound enzymes of the pituitary, kidney, and liver. Salmon (s) GnRH and luteinizing hormone-releasing hormone (LHRH) are equipotent whereas analogs of these peptides ((D-Arg(6)-Pro(9)NET)-sGnRH, (D-Ala(6)-Pro(9)NET)-LHRH, (D-Trp(6))-LHRH) are superactive in inducingin vivo GtH release (at 10 µg/kg body weight). In anin vitro superfusion system of pituitary fragments all analogs are equipotent to the native peptides (at 10(-10) to 2.5 × 10(-7)M). sGnRH and LHRH are rapidly degraded by cytosolic peptidases of the pituitary, liver, and kidney. The preferred site of cleavage is the Tyr(5)-Gly(6) bond. Substitution of the position 6 glycine by D-amino acids renders the 5-6 bond resistant to degradation and shifts the main site of cleavage to the Pro(9)-Gly(10)NH2 bond. Substitution at position 6 (as above) and at position 10 with Pro(9)NET results in analogs that are resistant to degradation. We propose that enzymatic cleavage terminates GnRH bioactivityin vivo and thus increased resistance to degradation is a major determinant of GnRH analog superactivity.

Immunocytochemical demonstration of close relationships between neuropeptidergic nerve fibers and hormone-producing cell types in the adenohypophysis of the sea bass (Dicentrarchus labrax)

Light microscopic double immunocytochemical stainings, performed on sea bass hypothalamo-hypophysial sections, revealed the projection of different neuropeptide-immunoreactive neurons innervating the hormone-producing cell populations in the pituitary gland. In the rostral pars distalis (PD) the ACTH cells were found in close proximity to fibers immunoreactive for somatostatin (SRIF), growth hormone-releasing hormone (GRF), corticotropin-releasing hormone (CRF), vasotocin (VT), isotocin (IT), substance P (SP), neurotensin, and galanin (GAL), while the PRL cell zone seemed only innervated by nerve fibers immunopositive for GAL. In the proximal PD, fibers immunoreactive for SRIF, GRF, VT, IT, cholecystokinin, SP, neuropeptide Y, and GAL formed a close relationship with the growth hormone cells. The gonadotrophs were observed near nerve fibers immunostained for gonadotropin-releasing hormone, IT, and less obviously GRF and VT, while fibers positive for GRF, CRF, VT, IT, SP, and GAL penetrated between and formed a close association with the thyrotrophs. In the pars intermedia the MSH cells and the PAS-positive (PAS+) cells seemed both innervated by separate nerve fibers immunoreactive for GRF, CRF, melanin concentrating hormone, VT, IT, and SP. All these results suggest a functional role of the neuropeptides in the adenohypophysis of the sea bass, possibly in the synthesis and/or release of hypophysial hormones from the different cell types.

Studies of the biological activity of LHRH analogs in the rainbow trout, landlocked salmon, and the winter flounder

Studies of gonadotropic hormone (GtH) release bioactivity by mammalian and submammalian varieties of LHRH and LHRH analog were primarily conducted in vivo in testosterone-primed yearling (TPY) rainbow trout, a convenient test animal for LHRH bioassays in fish. Validation of these results, using sexually mature fish, was accomplished by examining LHRH agonist activities on release of GtH in vivo in spermiating landlocked salmon and by studying LHRH peptide hormone binding affinities using a flounder pituitary LHRH radioreceptor assay. Our surveys of LHRH analog bioactivity in vivo in TPY trout and salmon demonstrated that all types of fish, bird, and mammalian LHRH agonists possess superactive properties on the fish pituitary. The most active group of LHRH analogs, based upon both LHRH receptor binding affinity and in vivo release of gonadotropin, was judged to include [D-Nal(2)6,Pro9-NHEt]LHRH, [D-Nal(2)6-AzaGly10]LHRH, [D-Ala6,Pro9-NHEt]-LHRH, and the fish LHRH analogs, [D-Arg6,Trp7,Leu8,Pro9-NHEt]LHRH, [D-hArg(Et2(6),Trp7,Leu8,Pro9-NHEt]LHRH.

Differential distribution of two molecular forms of gonadotropin-releasing hormone in discrete brain areas of goldfish (Carassius auratus)

Two molecular forms of gonadotropin-releasing hormone (GnRH) were identified in the extracts of various brain areas, spinal cord and pituitary in female and male goldfish and had chromatographic and immunological properties similar to [His5, Trp7, Tyr8]-GnRH (cGnRH-II) and [Trp7,Leu8]-GnRH (sGnRH). Radioimmunoassay using different GnRH antisera after high pressure liquid chromatography did not reveal significant peaks of mammalian GnRH, [Gln8]-GnRH and [Tyr3,Leu5,Glu6,Trp7,Lys8]-GnRH in the brain extracts. The proportion of cGnRH-II-like immunoactivity to sGnRH-like immunoactivity was higher in the caudal brain areas compared to the rostral areas. The differential distribution of two GnRH forms suggest that the different GnRH forms may have different physiological functions.

Characterization of the receptor for gonadotropin-releasing hormone in the pituitary of the African catfish,Clarias gariepinus

Receptors for gonadotropin-releasing hormone (GnRH) were characterized using a radioligand prepared from a superactive analog of salmon GnRH (sGnRH), D-Arg(6)-Pro(9)-sGnRH-NEt (sGnRHa). Binding of(125)I-sGnRHa to catfish pituitary membrane fractions reached equilibrium after 2 h incubation at 4°C. Displacement experiments with several GnRH analogs as well as other peptides, demonstrated the specificity of(125)I-sGnRHa binding. Specific binding was enhanced in the presence of the cation chelator ethylene bis (oxyethylenenitrilo) tetra-acetic acid (EGTA), indicating an inhibitory effect of cations on GnRH-receptor binding. The binding of(125)I-sGnRHa to pituitary membranes was found to be saturable at radioligand concentrations of 5 nM and above. A Scatchard analysis of the saturation data suggested the presence of a single class of high-affinity binding sites (Ka=0.901±0.06×10(9)M(-1), Bmax=1678±150 fmol/mg protein). A comparative study on(125)I-sGnRHa binding to pituitary membrane fractions of male and female catfish, indicated that there were no differences in binding affinity and binding capacity between both sexes. The results demonstrate the presence of specific, saturable GnRH receptors in the African catfish pituitary.

A study of LH-RH receptors in the pituitary gland of the winter flounder (Pseudopleuronectes americanus Walbaum)

A study using an iodinated [D-Ser(tBu)6,Pro9-NHEt]LH-RH (Buserelin), demonstrated the presence of a single class of high-affinity (KD = 2.90 nM), high-capacity LH-RH binding sites in pituitaries obtained from sexually mature male and female winter flounder. Displacement curves for unlabeled Buserelin and other preparations of mammalian and fish LH-RH, but a lack of competition for structurally unrelated peptide hormones, indicated the hormone specific nature of the fish pituitary LH-RH receptor preparation. Compared with native mammalian LH-RH and salmon LH-RH, Buserelin and an analog of salmon LH-RH, [D-Arg6,Trp7,Leu8,Pro9-NHEt]LH-RH, had significantly higher binding affinities for the flounder pituitary receptor correlating with results of previous studies demonstrating the superagonist biological activity of LH-RH analogs in trout and goldfish.

Evidence for a gonadotropin-releasing hormone binding protein in goldfish (Carassius auratus) serum

The binding of salmon gonadotropin-releasing hormone (sGnRH) and its superactive analog, [D-Arg6, Pro9-NEt]-sGnRH, to a macromolecular component in goldfish serum was studied, using 125I-[D-Arg6, Pro9-NEt]-sGnRH and 125I-sGnRH as labeled ligands. Bound was separated from free labeled ligand by gel filtration with Sephadex G-50. The binding of labeled ligand to goldfish serum was dose-dependent. The results indicate a single class of binding site having low affinity and high capacity. The existence of a GnRH binding protein in serum may, in part, contribute to the long-lasting pharmacological action of GnRHs in goldfish.

Effects of [D-Arg6, Trp7, Leu8, Pro9NEt]-luteinizing hormone-releasing hormone (sGnRH-A) and [D-Ala6, Pro9NEt]-luteinizing hormone-releasing hormone (LHRH-A), in combination with pimozide or domperidone, on gonadotropin release and ovulation in the Chinese loach and common carp

The effects of LHRH-A and sGnRH-A alone and in combination with the dopamine receptor antagonists pimozide (PIM) and domperidone (DOM) on stimulation of gonadotropin (GtH) secretion and ovulation in Chinese loach (Paramisgurnus dabryanus) and common carp (Cyprinus carpio) were studied. When tested in the absence of a dopamine receptor antagonist, sGnRH-A was about 10-fold more potent than LHRH-A in stimulating GtH secretion in loach, whereas these peptides were of similar effectiveness in common carp. Both peptides were effective alone in inducing ovulation in loach, but were ineffective in common carp. PIM and DOM stimulate a small but significant increase in serum GtH, and greatly potentiate the effects of LHRH-A and sGnRH-A on GtH secretion and ovulation in both species. DOM was of equal or somewhat greater potency than PIM in potentiating the action of releasing hormone agonists in loach; however, DOM had markedly greater effectiveness in common carp. sGnRH-A in the presence of DOM was more effective than LHRH-A in stimulating GtH secretion and ovulation in common carp. The present studies suggest that there are marked differences in the relative contribution of GnRH and dopamine in the control of GtH secretion in loach and common carp, and indicate in particular that the more intense dopaminergic inhibitory control of GtH release in common carp modifies the responsiveness of the pituitary to GnRH peptides.

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.

Specificity of amphibian and reptilian pituitaries for various forms of gonadotropin-releasing hormones in vitro

In vitro perifusion was employed to compare the potencies of mammalian, avian, salmon, and lamprey gonadotropin-releasing hormones (GnRHs) on the release of luteinizing hormone (LH) from the pituitaries of an amphibian (Rana pipiens) and a reptile (Chrysemys picta). The chicken-I and salmon GnRH variants were equipotent with mammalian GnRH in both the frog and the turtle glands. By contrast, the lamprey GnRH was inactive (less than 1% as potent as the others). Lamprey GnRH also failed to stimulate LH release or to induce GnRH priming when administered chronically to the frog gland. These results support the hypothesis that the GnRH receptors on nonmammalian pituitary cells are much less specific than those of the mammal with regard to the amino acid at position 8 of the GnRH molecule. These data suggest that the native GnRH variant or the one most like that found in the brain of a species is not necessarily the most potent biologically in that species. However, the nonmammalian pituitary does show some specificity with regard to the structure of natural GnRHs in that none of the tetrapod species studied is responsive to lamprey GnRH.

Gonadotropin production and release in female goldfish (Carassius auratus) after administration of pimozide and an LHRH analogue as studied by electron microscopy and radioimmunoassay

The effect of pimozide and an LHRH-analogue (LHRH-A) on gonadotropic cells of the goldfish pituitary gland were described qualitatively and quantitatively. A scale of four categories was devised to reflect various ultrastructural appearances of the cells. Experimental animals were divided into a control group, a group injected with LHRH-A alone, pimozide alone, and groups receiving these two substances in combination. Fish injected with the single substance were killed 12 h after injection while the groups receiving the combined treatments were killed at 4, 12 and 48 h. Serum levels of gonadotropin measured by radioimmunoassay were used to indicate whether an increase in hormone release had occurred. An immunocytochemical technique, the protein A-gold procedure, assured that the cells studied were gonadotropes. The control group showed variation in the profiles of gonadotropic cells. The single treatment groups showed some increase in secretory inclusions. At 4 h after injection the combined treatment caused a significant increase in hormone granules; at 12 and 48 h there was a gradual decrease in content of secretory products, and an increase in vacuolization. The results indicate that the combined pimozide and LHRH-A treatment stimulated gonadotropin production as well as release.

Activity of position-8-substituted analogs of mammalian gonadotropin-releasing hormone (mGnRH) and chicken and lamprey gonadotropin-releasing hormones in goldfish

Several vertebrate gonadotropin-releasing hormones (GnRH) and analogs were tested for activity in vivo in goldfish. Each peptide was administered intraperitoneally to goldfish, pretreated with pimozide or vehicle for pimozide, and changes in serum levels of gonadotropin were determined. Pimozide potentiates the activity of GnRH in vivo in goldfish by blocking the endogenous gonadotropin release-inhibitory activity of dopamine; relative potencies of GnRH peptides become evident in vivo in goldfish pretreated with pimozide (R. Peter et al. (1985), Gen. Comp. Endocrinol. 58, 231-242). Mammalian GnRH (mGnRH) was used as reference standard. [Try3, Leu5, Glu6, Trp7, Lys8]-GnRH (lamprey GnRH), [Gln8]-GnRH (chicken GnRH-I), and [His5, Trp7, Try8]-GnRH (chicken GnRH-II) caused increases in serum gonadotropin level similar in magnitude to mGnRH. [His5, D-Arg6, Trp7, Tyr8]-GnRH is superactive in the goldfish. [Asn8]-, [Met8]-, [Phe8]-, and [Ser8]-GnRH had activity similar to mGnRH in goldfish; [His8]-, [Ile8]-, and [Leu8]-GnRH had a lower level of activity; [Glu8]-GnRH had no apparent activity. The results indicate that there is no particular requirement for a hydrophobic or hydrophilic amino acid, or for a positively charged amino acid in position 8 of mGnRH for activity in vivo in the goldfish; a negatively charged amino acid in position 8 is detrimental for activity.

Effects of [D-Ala6, Pro9-NEt]-LHRH and catecholaminergic drugs on gonadotropin secretion and ovulation in the Chinese loach (Paramisgurnus dabryanus)

The effects of [D-Ala6,Pro9-NEt]-LHRH (LHRH-A) alone and in combination with drugs which influence the actions of dopamine or the synthesis of catecholamines on gonadotropin (GtH) secretion and ovulation in the loach (Paramisgurnus dabryanus) were investigated. LHRH-A alone stimulated an increase in serum GtH levels in the loach, but was a relatively ineffective treatment for the induction of ovulation. Injection of the dopamine receptor antagonist pimozide caused a marked potentiation of the GtH-release response to LHRH-A, and combined injections of pimozide and LHRH-A were an effective treatment for the induction of ovulation. Reserpine, a drug which causes depletion of catecholamines from presynaptic terminals, also caused a marked potentiation of the GtH-release response to LHRH-A and combined treatment induced ovulation. Similarly, administration of alpha-methyl-para-tyrosine to block conversion of tyrosine to L-dopa, or carbidopa to block conversion of L-dopa to dopamine, potentiated the GtH-release response to LHRH-A and induced ovulation. In contrast, the use of diethyldithiocarbamate, to block conversion of dopamine to norepinephrine, failed to augment the action of LHRH-A on GtH release and ovulation. The present results provide further evidence to suggest that dopamine functions as a gonadotropin release-inhibitory factor in teleosts, and demonstrate that the use of drugs which block either the synthesis or the actions of dopamine potentiates the action of LHRH-A in teleosts.

Gonadotropin-releasing hormone (GnRF), molluscan cardioexcitatory peptide (FMRFamide), enkephalin and related neuropeptides affect goldfish retinal ganglion cell activity

Recently gonadotropin-releasing hormone (GnRF)-like and molluscan cardioexcitatory peptide (FMRFamide)-like compounds have been colocalized immunocytochemically to the terminal nerve, a presumed olfactoretinal efferent system in goldfish. In the present study these and related neuropeptides were shown to affect ganglion cell activity, recorded extracellularly, when applied to the isolated superfused goldfish retina. GnRF was usually excitatory. Salmon GnRF (sGnRF) was 10-30x more potent than chicken or mammalian GnRF. FMRFamide and enkephalin also were often excitatory but caused more varied responses than sGnRF. Met5-enkephalin-Arg6-Phe7-NH2 (YGGFMRFamide), which contains both enkephalin and FMRFamide sequences, tended to act like both of these peptides but with mainly enkephalin-like properties. Neuropeptide Y and the C-terminal hexapeptide of pancreatic polypeptides, whose C-terminus (-Arg-Tyr-NH2) is closely related to that of FMRFamide (-Arg-Phe-NH2), gave no consistent responses. Threshold doses were equivalent to: 0.1 microM for sGnRF; 0.5 microM for YGGFMRFamide; 1.5 microM for FMRFamide and enkephalin. Rapid, complete and irreversible desensitization was induced by single, 10-20x threshold doses of sGnRF; but desensitization was infrequent and limited with the other peptides. In general, all peptides tested affected the spatially and chromatically antagonistic receptive field components similarly, but selective actions were seen in a few cases with FMRFamide and with the opioid antagonist, naloxone. Responses, especially to sGnRF and FMRFamide, tended to be most frequently obtained and pronounced in winter and spring, suggesting a correlation with seasonally regulated sexual and reproductive activity. Our observations provide further evidence for transmitter-like roles of neuropeptides related to sGnRF and FMRFamide in the teleostean terminal nerve. The actions of agonists and antagonists, singly and in combination, imply strongly that there are distinctive postsynaptic receptors and/or neural pathways for GnRF-, FMRFamide- and enkephalin-like peptides in the goldfish retina.

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.

The integrated development and application of controlled reproduction techniques in Pacific salmonid aquaculture

During the last several years there has been rapid progress in the development and application of controlled reproduction and associated biotechnologies to the culture of salmonids. This review focuses on the development of those techniques for Pacific salmon (Oncorhynchus species) and rainbow trout (Salmo gairdneri). These include methodologies for the induction of ovulation, storage of gametes, chromosome set manipulation, i.e., induced gynogenesis and triploidy, hormonal feminization, hormonal sterilization and hormonal masculinization of genotypic females to produce milt containing only female spermatozoa.The current and future applications of these methodologies to the commercial mariculture of salmonids and to the enhancement and management of ocean and fresh water stocks are described. The implementation of each individual methodology for the control of reproduction can in itself have a significant impact on the efficiency of culture systems. It is, however, from the combination of specific complementary methods in production systems that the greatest gains will ultimately be made.It is concluded that the utilization of single or paired controlled reproduction techniques by themselves or in combination with other biotechnologies (e.g. sterilization plus growth acceleration) offer significant advantages for both the commercial mariculture of salmonids and the rearing of salmonids by government agencies for release into the oceanic or lacustrine environment.

Effects of (D-Ala(6), Pro (9)N ethylamide) - LHRH on plasma levels of gonadotropin, 17α,20β-dihydroxy-4-pregnen-3-one and testosterone in male goldeye (hiodon alosoides Rafineque)

Male goldeye were treated with (D-Ala(6), Pro(9)-N ethylamide) - LHRH (LHRH-A) in saline or a silastic pellet (100 µg.kg(-1) body weight) and changes in plasma gonadotropin (GtH), 17α,20β-dihydroxy-4-pregnen-3-one (17α,20βP) and testosterone (T) levels were determined by radioimmunoassay. LHRH-A increased plasma GtH levels, with the response to LHRH-A in saline being of much greater magnitude and duration than the response to silastic pellet implants. Seasonal differences were found in the response to LHRH-A. Spermiated fish were the most responsive, recrudescing fish the least, and regressed fish displayed an intermediate response. Plasma 17α,20βP levels were elevated in response to LHRHA in fish of all sexual stages although the magnitude of the increase was not related to the magnitude of the increase in GtH levels. Treatment with LHRH-A also resulted in a transitory increase in plasma T levels. The endocrine control of GtH and steroid secretion in goldeye is discussed in relation to studies in cyprinids and salmonids.

Use of pituitary cells in primary culture to study the regulation of gonadotropin hormone (GtH) secretion in rainbow trout: setting up and validating the system as assessed by its responsiveness to mammalian and salmon gonadotropin releasing hormone

To study the regulation of gonadotropin secretion in rainbow trout in vitro, a method for preparing primary cultures of dispersed pituitary cells is described. Cells were dispersed by collagenase 0.1% in Hank's saline solution for 20 hr at 12 degrees and a high yield of viable cells was obtained. Attempts to improve cell functioning were made by varying culture conditions (density of cells initially plated, age of the culture). Cell functioning was assessed by their ability to respond to increasing doses of mammalian and salmon GnRH. Pituitaries were collected from spermiating males whose pituitaries are known to be sensitive to mammalian GnRH in vivo. Using 96-well plates, optimal conditions for good biological activity, are initial plating with 6.2 X 10(4) cells, incubation with GnRH for 24 hr on the third day after plating. In these conditions mammalian analog and salmon GnRH induced an increase in GtH release for doses ranging from 10(-9) to 10(-6) M. The GtH released during the GnRH incubation period does not decrease the sensitivity of the system since addition of 20 ng of GtH at the beginning of incubation does not modify the response profile.