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

Relationship between brain gonadotropin-releasing hormone and final reproductive period of the adult male sea lamprey, Petromyzon marinus

Gonadotropin-releasing hormone (GnRH) concentrations were measured in brains of adult male sea lamprey, Petromyzon marinus, during their final reproductive period. The lampreys were collected during their upstream migration in coastal New Hampshire rivers and sampled at the trap (referred to as Group A) or they were transferred to an artificial spawning channel (referred to as Group B). Plasma estradiol and progesterone were also measured, and histological examination of the gonadal stages was done as well. The concentrations of brain GnRH and plasma estradiol fluctuated significantly through time. There was a rise in brain concentrations of GnRH coincident with an increase in temperature just prior to spawning. In addition, there was a significant progressive correlation between increasing plasma estradiol and temperature in lampreys from Group B during the period studied. These studies provide evidence for progressive seasonal relationships between changes in brain GnRH and gametogenic and steroidogenic activity of the gonads in adult male sea lampreys during their final reproductive period.

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.

In vitro goldfish growth hormone responses to gonadotropin-releasing hormone: possible roles of extracellular calcium and arachidonic acid metabolism?

Two hours of incubation of primary static cultures of dispersed goldfish pituitary cells with 0.01 nM to 1 microM [Trp7,Leu8]-gonadotropin-releasing hormone (sGnRH) increased growth hormone (GH) secretion in a dose-dependent manner with an ED50 estimate of 0.13 +/- 0.04 nM. Addition of calcium ionophores, 1 to 100 microM A23187 and 5 to 100 microM ionomycin, significantly elevated GH release with ED50s of 0.84 +/- 0.38 and 4.34 +/- 1.02 microM, respectively. Replacement of normal calcium-containing media with calcium-deficient media (prepared without the addition of calcium salts) significantly depressed basal GH secretion, attenuated the A23187- and ionomycin-stimulated GH release, and completely abolished the GH response to sGnRH. Arachidonic acid (AA) at 1 to 50 microM also enhanced GH secretion with an ED50 of 4.72 +/- 1.52 microM. Coincubation with 1 and 10 microM of a lipoxygenase inhibitor, nordihydroguaiaretic acid (NDGA), 10 microM of the cyclooxygenase inhibitor, indomethacin, and 10 microM of eicosatetraynoic acid, an enzyme blocker with mixed activities on both the lipoxygenase and cyclooxygenase pathways, did not alter basal, AA-, and sGnRH-induced GH release. However, at 100 microM concentration, NDGA increased AA- and sGnRH-stimulated, as well as basal GH, responses. These results confirm the direct stimulatory action of GnRH on goldfish somatotropes and indicate the importance of extracellular calcium in mediating basal and GnRH-induced GH responses. Although AA stimulates GH secretion, its lipoxygenase and cyclooxygenase metabolites probably do not mediate sGnRH action on somatotropes.

Alterations in gonadotropin-releasing hormone immunoactivities in discrete brain areas of male goldfish during spawning behavior

In male goldfish, spawning behavior and gonadotropin (GtH) secretion are stimulated during exposure to females which were induced to perform spawning behavior by treatment with prostaglandin F2 alpha (PG). In this study, exposure to PG-treated females for 1 or 2 h significantly increased serum GtH levels, and GtH-releasing hormone (GnRH) concentrations in the olfactory bulbs, telencephalon and hypothalamus, indicating that spawning behavior can influence cellular events of the brain GnRH neuronal system and pituitary GtH secretion in male goldfish. To study the role of the olfactory system in the hormonal responses to PG-treated females, bilateral sectioning of olfactory tracts (OTX), medial (mOTX) or lateral (lOTX) olfactory tract were carried out in male goldfish. Both OTX and mOTX, but not lOTX, blocked the behavioral response of male goldfish to PG-treated females and abolished the increases in serum GtH and brain GnRH levels, suggesting that alterations in pituitary GtH secretion and brain GnRH levels are associated with a pheromonal activation of spawning behavior through the medial olfactory tracts in male goldfish. This behavioral activation of the GnRH neuronal system provides a useful physiological model for studying the regulation of GnRH system in male goldfish.

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.

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.

Neuropeptide Y (NPY) modulatesin vitro gonadotropin in release from rainbow trout pituitary glands

This work investigated the action of neuropeptide Y (NPY) on thein vitro pituitary release of the maturing gonadotropic hormone (GtH) of the rainbow trout using a perifusion system employing trout balanced salt solution (pH 7.5) at 15°C and a 12.5 ml/h flow rate. In vitellogenic females a 20 minutes NPY application (10(-7) M) induced a 20-30% decrease in GtH secretion. Removal of NPY was followed by a rebound in GTH secretion. On the contrary, in ovulated females, NPY (15 minutes, 10(-7) M) directly stimulated GTH secretion. The greatest stimulation was obtained the day of ovulation where the stimulatory effect of NPY was similar to those induced by s.GnRH in the same conditions, reaching 400% of the basal GTH level. In vitellogenic females treated with 1-4-6 androstadien 3-7 dione, an inhibitor of aromatase activity, the pituitary response to NPY was similar to that obtained in ovulated females. Thus thein vitro action of NPY might depend on thein vivo steroidogenic environment.

Alterations in pituitary GnRH and dopamine receptors associated with the seasonal variation and regulation of gonadotropin release in the goldfish (Carassius auratus)

Seasonal variations in the serum concentrations of gonadotropin (GtH) and the serum GtH response to intraperitoneal injection of domperidone, a specific dopamine receptor antagonist, were examined in goldfish. In addition, the effects of in vivo treatment of goldfish with a superactive analog of salmon gonadotropin-releasing hormone (sGnRH-A) and domperidone on the binding parameters of pituitary GnRH and dopamine receptors were investigated in goldfish. Serum concentrations of GtH and the maximum GtH response to domperidone increased in correlation with advancing gonadal maturation; values increased from those in sexually regressed fish in January to maximal levels observed in fish in late stages of gonadal recrudescence in March, followed by a decrease with gonadal regression. At all stages, injection of domperidone increased serum concentrations of GtH in a dose-related manner; however, the ED50 of domperidone did not vary significantly over the course of the reproductive cycle. Multiple injections of sGnRH-A caused a progressively increasing and more prolonged serum GtH response; as well, multiple sGnRH-A treatment significantly potentiated the serum GtH response to domperidone without altering the ED50 of domperidone. sGnRH-A treatment caused a significant increase in the number of dopamine/neuroleptic receptors in the goldfish pars distalis, accompanied by a nonsignificant increase in dopamine/neuroleptic receptors in the neurointermediate lobe, and significantly increased the number of high-affinity GnRH receptors in the goldfish pituitary. Treatment with domperidone also significantly increased pituitary high-affinity GnRH receptor numbers. Receptor affinities were not significantly altered by either sGnRH-A or domperidone treatment.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

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.