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

Kisspeptin and GnRH interactions in the reproductive brain of teleosts

It is well known that gonadotropin-releasing hormone (Gnrh) has a key role in reproduction by regulating the synthesis and release of gonadotropins from the anterior pituitary gland of all vertebrates. About 25 years ago, another neuropeptide, kisspeptin (Kiss1) was discovered as a metastasis suppressor of melanoma cell lines and then found to be essential for mammalian reproduction as a stimulator of hypothalamic Gnrh and regulator of puberty onset. Soon after, a kisspeptin receptor (kissr) was found in the teleost brain. Nowadays, it is known that in most teleosts the kisspeptin system is composed of two ligands, kiss1 and kiss2, and two receptors, kiss2r and kiss3r. Even though both kisspeptin peptides, Kiss1 and Kiss2, have been demonstrated to stimulate gonadotropin synthesis and secretion in different fish species, their actions appear not to be mediated by Gnrh neurons as in mammalian models. In zebrafish and medaka, at least, hypophysiotropic Gnrh neurons do not express Kiss receptors. Furthermore, kisspeptinergic nerve terminals reach luteinizing hormone cells in some fish species, suggesting a direct pituitary action. Recent studies in zebrafish and medaka with targeted mutations of kiss and/or kissr genes reproduce relatively normally. In zebrafish, single gnrh mutants and additionally those having the triple gnrh3 plus 2 kiss mutations can reproduce reasonably well. In these fish, other neuropeptides known to affect gonadotropin secretion were up regulated, suggesting that they may be involved in compensatory responses to maintain reproductive processes. In this context, the present review explores and presents different possibilities of interactions between Kiss, Gnrh and other neuropeptides known to affect reproduction in teleost fish. Our intention is to stimulate a broad discussion on the relative roles of kisspeptin and Gnrh in the control of teleost reproduction.

Multimodal hypothalamo-hypophysial communication in the vertebrates

The vertebrate pituitary is arguably one of the most complex endocrine glands from the evolutionary, anatomical and functional perspectives. The pituitary plays a master role in endocrine physiology for the control of growth, metabolism, reproduction, water balance, and the stress response, among many other key processes. The synthesis and secretion of pituitary hormones are under the control of neurohormones produced by the hypothalamus. Under this conceptual framework, the communication between the hypophysiotropic brain and the pituitary gland is at the foundation of our understanding of endocrinology. The anatomy of the connections between the hypothalamus and the pituitary gland has been described in different vertebrate classes, revealing diverse modes of communication together with varying degrees of complexity. In this context, the evolution and variation in the neuronal, neurohemal, endocrine and paracrine modes will be reviewed in light of recent discoveries, and a re-evaluation of earlier observations. There appears to be three main hypothalamo-pituitary communication systems: 1. Diffusion, best exemplified by the agnathans; 2. Direct innervation of the adenohypophysis, which is most developed in teleost fish, and 3. The median eminence/portal blood vessel system, most conspicuously developed in tetrapods, showing also considerable variation between classes. Upon this basic classification, there exists various combinations possible, giving rise to taxon and species-specific, multimodal control over major physiological processes. Intrapituitary paracrine regulation and communication between folliculostellate cells and endocrine cells are additional processes of major importance. Thus, a more complex evolutionary picture of hypothalamo-hypophysial communication is emerging. There is currently little direct evidence to suggest which neuroendocrine genes may control the evolution of one communication system versus another. However, studies at the developmental and intergenerational timescales implicate several genes in the angiogenesis and axonal guidance pathways that may be important.

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.

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.

Effects of lamprey PQRFamide peptides on brain gonadotropin-releasing hormone concentrations and pituitary gonadotropin-β mRNA expression

Within the RFamide peptide family, PQRFamide peptides that include neuropeptide FF and AF possess a C-terminal Pro-Gln-Arg-Phe-NH(2) motif. We previously identified PQRFamide peptides, lamprey PQRFa, PQRFa-related peptide (RP)-1 and -RP-2 by immunoaffinity purification in the brain of lamprey, one of the most ancient vertebrate species [13]. Lamprey PQRFamide peptide precursor mRNA was expressed in regions predicted to be involved in neuroendocrine regulation in the hypothalamus. However, the putative function(s) of lamprey PQRFamide peptides (PQRFa, PQRFa-RP-1 and PQRFa-RP-2) were not examined nor was the distribution of PQRFamide peptides examined in other tissues besides the brain. The objective of this study was to determine tissue distribution of lamprey PQRFamide peptide precursor mRNA, and to examine the effects of PQRFamide peptides on brain gonadotropin-releasing hormone (GnRH)-I, -II, and -III protein concentrations, and pituitary gonadotropin (GTH)-β mRNA expression in adult lampreys. Lamprey PQRFamide peptide precursor mRNA was expressed in the eye and the brain. Lamprey PQRFa at 100 μg/kg increased brain concentrations of lamprey GnRH-II compared with controls. PQRFa, PQRFa-RP-1 and PQRFa-RP-2 did not significantly change brain protein concentrations of either lamprey GnRH-I, -III, or lamprey GTH-β mRNA expression in the pituitary. These data suggest that one of the PQRFamide peptides may act as a neuroregulator of at least the lamprey GnRH-II system in adult female lamprey.

Evolutionary origin of the structure and function of gonadotropin-inhibitory hormone: insights from lampreys

Gonadotropin (GTH)-inhibitory hormone (GnIH) is a novel hypothalamic neuropeptide that inhibits GTH secretion in mammals and birds by acting on gonadotropes and GnRH neurons within the hypothalamic-pituitary-gonadal axis. GnIH and its orthologs that have an LPXRFamide (X = L or Q) motif at the C terminus (LPXRFamide peptides) have been identified in representative species of gnathostomes. However, the identity of an LPXRFamide peptide had yet to be identified in agnathans, the most ancient lineage of vertebrates, leaving open the question of the evolutionary origin of GnIH and its ancestral function(s). In this study, we identified an LPXRFamide peptide gene encoding three peptides (LPXRFa-1a, LPXRFa-1b, and LPXRFa-2) from the brain of sea lamprey by synteny analysis and cDNA cloning, and the mature peptides by immunoaffinity purification and mass spectrometry. The expression of lamprey LPXRFamide peptide precursor mRNA was localized in the brain and gonad by RT-PCR and in the hypothalamus by in situ hybridization. Immunohistochemistry showed appositions of lamprey LPXRFamide peptide immunoreactive fibers in close proximity to GnRH-III neurons, suggesting that lamprey LPXRFamide peptides act on GnRH-III neurons. In addition, lamprey LPXRFa-2 stimulated the expression of lamprey GnRH-III protein in the hypothalamus and GTHβ mRNA expression in the pituitary. Synteny and phylogenetic analyses suggest that the LPXRFamide peptide gene diverged from a common ancestral gene likely through gene duplication in the basal vertebrates. These results suggest that one ancestral function of LPXRFamide peptides may be stimulatory compared with the inhibitory function seen in later-evolved vertebrates (birds and mammals).

Seasonal changes of brain GnRH-I, -II, and -III during the final reproductive period in adult male and female sea lamprey

Sea lampreys are anadromous and semelparous, i.e., they spawn only once in their lifetime, after which they die. Sexual maturation is thus a synchronized process coordinated with the life stages of the lamprey. Recently, a novel gonadotropin-releasing hormone, lamprey GnRH-II (lGnRH-II), was identified in lampreys and suggested to have a hypothalamic role in reproduction (Kavanaugh et al., 2008). To further understand the role of lGnRH-II, changes in ovarian morphology, brain gonadotropin-releasing hormone (lGnRH-I, -II, and -III), and plasma estradiol were examined during the final two months of the reproductive season of adult male and female sea lamprey. The results showed significant correlations between water temperature, fluctuation of brain GnRHs, plasma estradiol and reproductive stages during this time. In males, lGnRH-I concentration increased early in the season, peaked, then declined with a subsequent increase with the final maturational stages. In comparison, lGnRH-II and -III concentrations were also elevated early in the season in males, dropped and then peaked in mid-season with a subsequent decline of lGnRH-II or increase of lGnRH-III at spermiation. In females, lGnRH-III concentration peaked in mid-season with a drop at ovulation while lGnRH-I remained unchanged during the season. In contrast, lGnRH-II concentrations in females were elevated at the beginning of the season and then dropped and remained low during the rest of the season. In summary, these data provide evidence that there are seasonal and differential changes of the three GnRHs during this final reproductive period suggesting specific roles for each of the GnRHs in male and female reproduction.

Inhibitory effect of chicken gonadotropin-releasing hormone II on food intake in the goldfish, Carassius auratus

Gonadotropin-releasing hormone (GnRH) is an evolutionarily conserved neuropeptide with 10 amino acid residues, which possesses some structural variants. A molecular form known as chicken GnRH II ([His(5) Trp(7) Tyr(8)] GnRH, cGnRH II) is widely distributed in vertebrates, and has recently been implicated in the regulation of sexual behavior and food intake in an insectivore, the musk shrew. However, the influence of cGnRH II on feeding behavior has not yet been studied in model animals such as rodents and teleost fish. In this study, therefore, we investigated the role of cGnRH II in the regulation of feeding behavior in the goldfish, and examined its involvement in food intake after intracerebroventricular (ICV) administration. ICV-injected cGnRH II at graded doses, from 0.1 to 10 pmol/g body weight (BW), induced a decrease of food consumption in a dose-dependent manner during 60 min after treatment. Cumulative food intake was significantly decreased by ICV injection of cGnRH II at doses of 1 and 10 pmol/g BW during the 60-min post-treatment observation period. ICV injection of salmon GnRH ([Trp(7) Leu(8)] GnRH, sGnRH) at doses of 0.1-10 pmol/g BW did not affect food intake. The anorexigenic action of cGnRH II was completely blocked by treatment with the GnRH type I receptor antagonist, Antide. However, the anorexigenic action of cGnRH II was not inhibited by treatment with the corticotropin-releasing hormone (CRH) 1/2 receptor antagonist, *-helical CRH((9-41)), and the melanocortin 4 receptor antagonist, HS024. These results suggest that, in the goldfish, cGnRH II, but not sGnRH, acts as an anorexigenic factor, as is the case in the musk shrew, and that the anorexigenic action of cGnRH II is independent of CRH- and melanocortin-signaling pathways.

In vitro and in vivo effects of GABA, muscimol, and bicuculline on lamprey GnRH concentration in the brain of the sea lamprey (Petromyzon marinus)

gamma-Aminobutyric acid (GABA) is a neurotransmitter with a demonstrated neuroregulatory role in reproduction in most representative species of vertebrate classes via the hypothalamus. The role of GABA on the hypothalamus-pituitary axis in lampreys has not been fully elucidated. Recent immunocytochemical and in situ hybridization studies suggest that there may be a neuroregulatory role of GABA on the gonadotropin-releasing hormone (GnRH) system in lampreys. To assess possible GABA-GnRH interactions, the effects of GABA and its analogs on lamprey GnRH in vitro and in vivo were studied in adult female sea lampreys (Petromyzon marinus). In vitro perfusion of GABA and its analogs at increasing concentrations (0.1-100 microM) was performed over a 3-h time course. There was a substantial increase of GnRH-I and GnRH-III following treatment of muscimol at 100 microM. In in vivo studies, GABA or muscimol injected at 200 microg/kg significantly increased lamprey GnRH concentration in the brain 0.5 h after treatment compared to controls in female sea lampreys. No significant change in lamprey GnRH-I or GnRH-III was observed following treatment with bicuculline. These data provide novel physiological data supporting the hypothesis that GABA may influence GnRH in the brain of sea lamprey.

Seasonal Profiles of Brain and Pituitary Gonadotropin-Releasing Hormone and Plasma Luteinizing Hormone in Relation to Sex Change of Protandrous Black Porgy, Acanthopagrus schlegeli1

Three molecular variants of GnRH in the brain (sbGnRH, sGnRH, and cGnRH-II) and two forms in the pituitary (sbGnRH and sGnRH) were detected in protandrous black porgy, Acanthopagrus schlegeli using chromatographic and immunological methods. In juvenile fish, brain sbGnRH, sGnRH, and cGnRH-II levels increased in May and reached their highest levels in July and August (the nonspawning season) and in January through March (the spawning season). In fish aged 1 yr and older, high levels of brain sbGnRH and sGnRH were detected in September, November, and February-March, but the levels of brain cGnRH-II remained constant. A gradual increase in pituitary sbGnRH was detected in juvenile fish from July to March. In fish aged 1+ yr, pituitary sbGnRH levels were high in September and March-May, but low in January-February. A close correlation between pituitary sbGnRH and plasma LH levels was found in juvenile fish and in those aged 1+ yr. In fish aged 2+ yr, significantly lower levels of plasma LH was detected during the nonspawning period in fish that changed sex compared with the fish that remained as males. Higher plasma LH levels were detected in the sex-changing fish from artificially sex-reversed female to male. FSH receptor and LH receptor transcripts were higher in bisexual testicular tissue than in ovarian tissue in 2+-yr-old fish. Direct effects of hCG on sex change were studied and the results show that exogenous hCG did not stimulate gonadal aromatase activity in 2+-yr-old fish. Therefore, it is suggested that high and basal levels of plasma LH during the nonspawning season correlate with the development of male and female gonad, respectively, in black porgy. This important role of the neuroendocrine system in sex change (for male direction) is proposed in hermaphroditic fish.

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.

Reproduction phase-related expression of GnRH-like immunoreactivity in the olfactory receptor neurons, their projections to the olfactory bulb and in the nervus terminalis in the female Indian major carp Cirrhinus mrigala (Ham.)

The reproductive biology of the Indian major carp Cirrhinus mrigala is tightly synchronized with the seasonal changes in the environment. While the ovaries show growth from February through June, the fish spawn in July-August to coincide with the monsoon; thereafter the fish pass into the postspawning and resting phases. We investigated the pattern of GnRH immunoreactivity in the olfactory system at regular intervals extending over a period of 35 months. Although no signal was detected in the olfactory organ of fish collected from April through February following year, distinct GnRH-like immunoreactivity appeared in the fish collected in March. Intense immunoreactivity was noticed in several olfactory receptor neurons (ORNs) and their axonal fibers as they extend over the olfactory nerve, spread in the periphery of the olfactory bulb (OB), and terminate in the glomerular layer. Strong immunoreactivity was seen in some fascicles of the medial olfactory tracts extending from the OB to the telencephalon. Some neurons of the ganglion cells of nervus terminalis showed GnRH immunostaining during March; no immunoreactivity was detected at other times of the year. Plexus of GnRH immunoreactive fibers extending throughout the bulb represented a different component of the olfactory system; the fiber density showed a seasonal pattern that could be related to the status of gonadal maturity. While it was highest in the prespawning phase, significant reduction in the fiber density was noticed in the fish of spawning and the following regressive phases. Taken together the data suggest that the GnRH in the olfactory system of C. mrigala may play a major role in translation of the environmental cues and influence the downstream signals leading to the stimulation of the brain-pituitary-ovary axis.

Somatostatin-like receptors in goldfish: cloning of four new receptors

In this study, four somatostatin-like receptor (Sst) cDNAs were identified from goldfish pituitary, using RT-PCR screening and rapid amplification of cDNA ends (RACE) strategies. These include two type-five like Sst (Sst(5B) and Sst(5C)) and two type-three like Sst receptors (Sst(3A) and Sst(3B)), designated based on their amino acid sequence similarities to the known mammalian and fish Sst(5) and Sst(3). Both Sst(5C) and Sst(3A) mRNAs are widely expressed in all brain regions and pituitary; however, Sst(3B) expression is restricted to forebrain and Sst(5B) expression is mainly detected in pituitary and spinal cord.

Isolation and structure-function studies of a glucagon-like peptide 1 receptor from goldfish Carassius auratus: identification of three charged residues in extracellular domains critical for receptor function

A better understanding of the molecular mechanism of ligand-receptor interaction of glucagon-like peptide 1 (GLP-1) receptors (GLP-1Rs) is useful for the design of potent GLP-1 analogs that could potentially be used as a treatment for diabetic patients. Changes in the ligand and receptor sequences during evolution provide invaluable clues to evaluate the functional motifs of the receptor that are responsible for ligand interaction. For these reasons, in the present study, we have isolated and functionally characterized a GLP-1R from goldfish. Its amino acid sequence shows 50.8% and 52.3% identity with the human glucagon (hGLU) and GLP-1Rs, respectively, and 84.1% with the zebrafish GLP-1R (the only other GLP-1R isolated from teleost fish). Peptides that are structurally different from goldfish (gf)GLP-1, such as gfGLU and hGLU and human GLP-1 (7-36)amide, are also capable of stimulating this receptor, albeit with lower potencies than gfGLP-1. gfGLP-1 stimulates the formation of cAMP through the recombinant gfGLP-1R with EC(50) = 0.18 nM, whereas EC(50) values for gfGLU, human GLP-1 (7-36)amide, and hGLU are 0.53 nM, 0.9 nM, and 1.2 nM, respectively. These results indicate that the gfGLP-1R is structurally more flexible than its mammalian counterpart and that its binding pocket can accommodate a wider spectrum of peptide ligands. Previous studies demonstrated that the charged residues in the extracellular domains of mammalian GLP-1R, particularly those found in the N-terminal domain and the first exoloop, are important for ligand binding. We investigated the roles of the conserved charged residues in the function of the gfGLP-1R. Eleven mutant receptors were constructed, and the effects of mutations were determined by functional assays. Our results demonstrated that three charged residues (D(113), R(197), and D(205)) present in the extracellular domains are critical for receptor function.

Maturational changes in brain contents of salmon GnRH in rainbow trout as measured by a newly developed time-resolved fluoroimmunoassay

A newly developed time-resolved fluoroimmunoassay (TR-FIA) for salmon gonadotropin-releasing hormone (sGnRH) was applied to investigate changes in sGnRH content in discrete brain areas at three different gonadal stages in the rainbow trout, Oncorhynchus mykiss. The sensitivity (6.8 pg/well), specificity, intraassay coefficients of variation (<7.4%), and interassay coefficients of variation (<10.3%) of the assay system were almost the same as those for the radioimmunoassay. Displacement curves of serially diluted brain extracts of nine teleost fish (freshwater fish and seawater fish) including rainbow trout paralleled that of the sGnRH standard, indicating that the sGnRH TR-FIA is widely applicable to the measurement of the brain sGnRH contents of various fishes. The sGnRH content in female hypothalamus decreased during final gonad maturation, whereas the sGnRH levels in pituitary were highest at the time of spermiating in males or ovulating in females, decreasing significantly thereafter. In contrast, there were no changes in the sGnRH contents of olfactory bulbs, telencephalon, optic tectum + thalamus, and cerebellum + medulla oblongata during final maturation, except for olfactory bulbs of males. Changes in sGnRH contents in the hypothalamus and the pituitary indicate that sGnRH is involved in final maturation (ovulation or spermiation) in the rainbow trout.

Seasonal changes in beta-endorphin-like immunoreactivity in the olfactory system of the female catfish, Clarias batrachus (Linn)

In the olfactory system of the catfish Clarias batrachus, beta-endorphin-like immunoreactivity was seen in several olfactory receptor neurons (ORN) and their fiber projections extending caudally over the olfactory nerve to the olfactory bulb (OB). With beta-endorphin-like immunoreactivity as a cellular marker, the olfactory system in the female fish was investigated at different stages of its annual reproductive cycle. The reproductive cycle of the fish is divisible into four distinct phases: preparatory (February-April), prespawning (May-June), spawning (July-August), and postspawning (September-January). The gonosomatic index and the immunocytochemical profile of beta-endorphin-like immunoreactivity showed distinct changes as the fish progressed from one phase to another. In the preparatory phase, limited immunoreactivity was seen in the periphery of the bulb. However, the immunoreactivity showed a robust increase as the immunolabeled fibers extended progressively deeper into the bulb toward the mitral cell layer during the prespawning and spawning phases. Significant reduction in the immunoreactivity was noticed in the olfactory nerve layer of the fish in the postspawning phase. Several granule cells showed poor to moderate immunoreactivity during the spawning phase, although no immunoreactivity was seen in the inner cell layer during the rest of the year. The beta-endorphin-like immunoreactivity in the ORN also showed season-related changes, although these were less distinct. Whereas weak immunoreactivity confined to a few ORN was noticed in the fish collected in the preparatory phase, those in the prespawning phase showed conspicuous augmentation in immunoreactivity. During the spawning phase, the sensory layer of the olfactory epithelium showed reduced, homogenous immunoreactivity. In the postspawning phase, several ORN revealed distinct granular immunoreactivity, suggesting possibilities of de novo synthesis. These annual cyclic changes in the beta-endorphin-like immunoreactivity were consistently observed over a 30-month study period that spanned three consecutive spawning phases. The results suggest that the beta-endorphin-containing ORN, their fiber projections to the OB, and several granule cells in the inner cell layer may be involved in the processing of reproduction/reproductive behavior-related signals.

Alterations in the GnRH-LH system in relation to gonadal stage and Aroclor 1254 exposure in Atlantic croaker

Exposure of Atlantic croaker (Micropogonias undulatus) to the polychlorinated biphenyl mixture (Aroclor 1254, PCB; 1 mg/kg body wt/day for 30 days) during the early-recrudescence phase of the gonadal cycle results in the impairment of LH secretion and gonadal growth. In order to determine whether impairment was due to disruption of the stimulatory GnRH neuroendocrine pathway, we compared various parameters of the GnRH-LH system in early recrudescing vs. spermiating (mature) fish. Seabream GnRH (GnRH) content in the preoptic anterior hypothalamic area (POAH) and pituitary, pituitary GnRH receptor concentrations, and basal and GnRH analog (GnRHa)-induced LH secretion were significantly higher in gonadally mature croaker compared to early-recrudescing fish. In a subsequent experiment, the effects of PCB on the same neuroendocrine indices were investigated during the gonadal recrudescence phase of croaker. PCB exposure during the period of testicular maturation prevented the natural increase in GnRH content in the POAH but not in the pituitary. This finding suggests that PCB may impair GnRH synthesis in the POAH. The number of pituitary GnRH receptors also remained significantly lower in the PCB-exposed group, which was likely due to an impairment of GnRH release. The GnRH content in the POAH, number of pituitary GnRH receptors, and LH secretion in the PCB-exposed group were comparable to those in early-recrudescing fish, suggesting an impairment of normal maturation of the GnRH-LH system during the gonadal recrudescence phase. This impairment may be due to a direct action of PCB on GnRH neurons and/or indirectly via interference with other neurotransmitter pathways that modulate GnRH function.

Theory on the evolutionary history of lamprey metamorphosis: role of reproductive and thyroid axes

Metamorphosis is a developmental strategy used by only a small number of extant fishes and little is known about its phylogenetic development during the evolution history of this large group of vertebrates. The present report provides a putative evolutionary history of metamorphosis in the lamprey, an extant agnathan with direct descendancy from some of the oldest known vertebrates. The study reviews recent data on the role of the thyroid gland and its hormones in metamorphosis, summarizes some recent views on the evolution of the endostyle/follicular thyroid in lampreys, and provides new data on the content of two gonadotropin-releasing hormones (GnRH-I and -III) in brain during goitrogen-stimulated, precocious metamorphosis. These new data support an earlier viewpoint of a relationship between thyroid and reproductive axes during metamorphosis. It is proposed that the earliest lampreys were paedomorphic larvae and they lived in a marine environment; as such, they resembled in many ways the larvae from which the ancient protochordates, Larvacea, are derived. The iodide-concentrating efficiency of the endostyle was a critical factor in the evolution of metamorphosis and this gland was replaced by a follicular thyroid, for postmetamorphic animals needed to store iodine following their invasion of freshwater. Larval growth and postmetamorphic reproduction in freshwater became fixtures in the lamprey life cycle; a non-parasitic adult life-history type appeared later. The presence among extant lampreys of two different adult life-history types, and examples of the lability of the timing of sexual maturation in some species, imply that there has been a complex interplay between the thyroid and reproductive axes during the evolution of metamorphosis in lampreys. This proposal is consistent with what we know of interplay of these axes in extant adult lampreys and with the long-held viewpoint that thyroid function and sexual maturation are an association with an ancient history.

Evidence for lamprey GnRH-I and -III-like molecules in the brains of the southern hemisphere lampreys Geotria australis and Mordacia mordax

The present study has characterized gonadotropic releasing hormone (GnRH)-like molecules in the brains of representatives of the two southern hemisphere families of lampreys, Geotriidae and Mordaciidae. Chromatographic and immunocytochemical evidence showed that the brains of Geotria australis and Mordacia mordax contain two forms of GnRH-like molecules. These two forms correspond to lamprey GnRH-I and -III, which were first sequenced from the brain of the anadromous sea lamprey Petromyzon marinus, a representative of the family Petromyzontidae that is found only in the northern hemisphere. In chromatographic studies (HPLC) using lamprey GnRH-I and -III antiserum, two early eluting GnRH forms coeluted with synthetic lamprey GnRH-I and -III standards. Our studies thus indicate that, despite their apparently long period of separation, the three families of extant lampreys have each retained both of the lamprey GnRH (-I and -III forms) molecules. Moreover, immunocytochemical localization of lamprey GnRH indicated that the pattern of its distribution in the adult brain of at least one of these southern hemisphere lampreys (G. australis) is similar to that previously described for P. marinus. Distribution of GnRH in the brain of larval G. australis was not as extensive as that in larval P. marinus, which may account for the later gonadal development in the former species. The fact that lamprey GnRH-I and -III are the dominant GnRH forms in all three families of lampreys implies that these neurohormones have an ancient origin.

Gonadotropin-releasing hormones in the brain and pituitary of the teleost, the white sucker

The present study investigated GnRH forms within the brain of a representative of the order Cypriniformes, the white sucker, Catostomus commersoni, using HPLC, RIA, and immunocytochemistry. Several immunoreactive (ir) GnRH forms were identified in the brain of the white sucker by chromatography and radioimmunoassay, including ir-salmon GnRH, ir-lamprey GnRH-I and -III, and ir-chicken GnRH-II. Results from immunocytochemical studies were consistent with multiple GnRH forms distributed in different patterns, particularly for fibers. Neuronal perikarya containing ir-salmon GnRH and ir-lamprey-like GnRH were found laterally within the preoptic area and rostral hypothalamus. Cells containing exclusively ir-salmon GnRH appeared slightly more rostrally, but in the same region. Fibers containing ir-salmon GnRH and ir-lamprey-like GnRH were seen throughout the caudal telencephalon and extended into the diencephalon, toward the pituitary. Fibers containing ir-chicken-II-like GnRH were also seen in the caudal telencephalon, but were concentrated more dorsally in the diencephalon. Within the pituitary, fibers containing ir-salmon GnRH and ir-lamprey-like GnRH entered the neurohypophysis, but differed in their destinations. Fibers containing ir-salmon GnRH remained within the neurohypophysis, while fibers containing ir-lamprey-like GnRH targeted adenohypophyseal tissue. These findings are consistent with the hypothesis that multiple GnRH forms with multiple functions exist within the brain and pituitary of teleosts and provide further evidence of a lamprey-like GnRH within an early evolved teleost species.

Differences in mGnRH and cGnRH-II contents in pituitaries and discrete brain areas of Rana rugulosa W. according to age and stage of maturity

(1) In tadpoles, chicken-II gonadotropin-releasing hormone (cGnRH-II) could be measured in the brains before metamorphosis, but mammalian gonadotropin-releasing hormone (mGnRH) did not appear until the stage of metamorphosis, i.e. cGnRH-II appeared earlier than mGnRH during ontogenesis. (2) During the metamorphic climax, mGnRH content increased more rapidly than cGnRH-II; the content of mGnRH was about two times of that of cGnRH-II. (3) In juveniles and adults, the content of mGnRH and cGnRH-II, and the distribution pattern of mGnRH (but not cGnRH-II) in the brains and pituitaries changed with age and stages of gonadal development. mGnRH mainly distributed in the rostral brain areas, whereas cGnRH-II had a widespread distribution in the brain. (4) Both mGnRH and cGnRH-II were present in the pituitaries at each stage of maturity. The gonadotropin-releasing hormone (GnRH) content at sexually maturity was significantly higher than that at other stages of gonadal development, and the content of mGnRH was about 15-18 times of that of cGnRH-II. (5) These results suggest that both mGnRH and cGnRH-II are potentially involved in the direct regulation of pituitary gonadotropes, and mGnRH may be the major active form, cGnRH-II may also serve as a neurotransmitter or neuromodulator in the brain.

cDNA cloning and expression of a novel estrogen receptor beta-subtype in goldfish (Carassius auratus)

We have isolated a second goldfish estrogen receptor (ER) beta-subtype (gfER-beta2) cDNA which is distinct from the liver-derived ER-beta (gfER-beta1) cDNA reported previously. The 2650-bp cDNA, isolated from a goldfish pituitary and brain cDNA library, encodes a 610 amino acid (aa) protein which shows only a 53% aa sequence identity with gfER-beta1 in overall structure. RT-PCR analysis showed that mRNA of gfER-beta2, in contrast to that of gfER-beta1, was predominantly expressed in pituitary, telencephalon and hypothalamus as well as in liver of female goldfish. The existence of a second distinct ER-beta subtype opens new dimensions for studying tissue-specific regulation of gene expression by estrogen in the tetraploid goldfish.

Distribution and seasonal variations in levels of three native GnRHs in the brain and pituitary of perciform fish

Specific and sensitive radioimmunoassays (RIAs) were newly developed for two types of gonadotropin-releasing hormone (GnRH), namely, seabream (sb) GnRH and chicken (c) GnRH-II. We employed these two RIAs together with a previously reported RIA for salmon (s) GnRH to study the presence and regional distribution of these three GnRHs in the brains and pituitaries of four perciform fishes (red seabream, Pagrus major; black seabream, Acanthopagrus schlegeli; striped knifejaw, Oplegnathus fasciatus; and Nile tilapia, Oreochromis niloticus), as well as clarify seasonal changes in levels of these GnRHs in the brain and pituitary of red seabream. All three GnRHs were found in brains of all fishes examined, with regional distributions in the brains of the three GnRHs being rather similar. sbGnRH was abundant in telencephalon and hypothalamus. cGnRH-II was concentrated from the middle to posterior part of the brain and distributed throughout the brain. sGnRH was concentrated in the olfactory bulb and distributed all over the brain, as was cGnRH-II. The dominant form of GnRH in the pituitary was sbGnRH, with levels 500- to 2400-fold higher than those of sGnRH, while cGnRH-II was undetectable in all four species. In the brain and pituitary of female red seabream, levels of both brain and pituitary sbGnRH increased from October (immature phase) and reached a peak in April (spawning phase), reflecting the increase in gonadosomatic index and vitellogenesis. However, levels of sbGnRH remained high only in the pituitary of completely regressed fish in June. Levels of both sGnRH and cGnRH-II in the brain were higher in the regressed phase and remained lower during the spawning phase. From these and previous results, it appears that sbGnRH is physiologically the most important form of GnRH in reproduction in red seabream and, probably, in other perciforms also.

mRNA expression of gonadotropin-releasing hormones (GnRHs) and GnRH receptor in goldfish

In goldfish (Carassius auratus), two distinct forms of gonadotropin-releasing hormone (GnRH), namely, salmon GnRH (sGnRH) and chicken GnRH-II (cGnRH-II), have been identified in the brain using chromatographic, immunological, and molecular cloning approaches. These two native GnRHs act on specific receptors in the anterior pituitary to stimulate the synthesis and release of gonadotropins and growth hormone in goldfish. To evaluate the potential roles of sGnRH and cGnRH-II in both neural and reproductive tissues in goldfish, we studied the mRNA expression of sGnRH, cGnRH-II, and GnRH receptor (GnRH-R) in discrete brain areas, pituitary, ovary, and testis by a combined reverse transcription-polymerase chain reaction (RT-PCR) and Southern blot analysis. Total RNA was extracted from various tissues of sexually recrudescent male and female goldfish and RT-PCR was performed with primers specific for GnRH-R complementary DNA (cDNA), sGnRH cDNA, cGnRH-II cDNA-1, and cDNA-2. Results showed that GnRHs and GnRH-R mRNAs are differentially distributed in the brain. In the goldfish brain, sGnRH mRNA was predominantly expressed in the forebrain areas (olfactory bulb, telencephalon, and hypothalamus) whereas cGnRH-II mRNA-1 were expressed in all brain areas including olfactory bulbs and optic tectum-thalamus. The expression level of cGnRH-II mRNA-2 was much lower than that of cGnRH-II mRNA-1 in the brain. On the other hand, GnRH-R mRNA was expressed in all brain regions and pituitary. In the ovary and testis, GnRH-R mRNA, sGnRH mRNA, and cGnRH-II mRNA-1, but not cGnRH-II mRNA-2, are expressed. Sequence analysis of the PCR products showed that nucleotide sequences of GnRH-R in gonads are identical with that in the brain and pituitary. The coexistence of GnRHs and GnRH-R mRNAs in both neural and gonadal tissues supports the notion that sGnRH and cGnRH-II may act as neurotransmitters and/or neuromodulators in the brain and as autocrine and/or paracrine hormones in gonadal tissues in addition to their established neuroendocrine roles at the pituitary of goldfish.

Chromatographic and immunological identification of GnRH (gonadotropin-releasing hormone) variants. Occurrence of mammalian and a salmon-like GnRH in the forebrain of an eutherian mammal: Hydrochaeris hydrochaeris (Mammalia, Rodentia)

The molecular variants of Gonadotropin releasing hormone (GnRH) in brain extracts of the eutherian mammal Hydrochaeris hydrochaeris (Mammalia, Rodentia) were characterized. An indirect method combining reverse-phase high-performance liquid chromatography (RP-HPLC) and radioimmunoassay (RIA) with different antisera was used. Two different forebrain regions (olfactory bulbs and preoptic-hypothalamic region) were analyzed. Characterization of RP-HPLC fractions from preoptic-hypothalamic extracts with three different RIA systems revealed two immunoreactive GnRH (ir-GnRH) peaks coeluting with mammalian GnRH (mGnRH) and salmon GnRH (sGnRH) synthetic standards. These results were additionally supported by serial dilution studies with specific antisera. Similar results were obtained from olfactory bulb extracts with the same methodology. However, a third ir-GnRH peak in a similar position to that of chicken GnRH II (cIIGnRH) synthetic standard was revealed. As far as we know, this is the first report showing chromatographic and immunological evidences for the presence of a second GnRH variant in the forebrain of an eutherian mammal.

GnRH molecular variants in the brain and pituitary gland of pejerrey, Odontesthes bonariensis (Atheriniformes). Immunological and chromatographic evidence for the presence of a novel molecular variant

Gonadotropin-releasing hormone (GnRH) molecular variants in the brain and pituitary gland of pejerrey, Odontesthes bonariensis (Atheriniformes), were characterized by gradient reverse phase high performance liquid chromatography (RP-HPLC). Eluted fractions were tested in radioimmunoassays with different antisera. The results show that the brain extract contains three forms of GnRH: one is immunologically and chromatographically similar to cIIGnRH (chicken II), and another is similar to sGnRH (salmon). A third GnRH appears to be chromatographic and immunologically different from the nine other known forms of the vertebrate hormone. This is the only variant present in the pituitary gland.

Identification of immunoreactive mammalian gonadotropin-releasing hormone in the brain of metamorphic larvae of Bufo arenarum Hensel (Amphibia: Anura)

Gonadotropin-releasing hormone (GnRH) immunoreactivity in brain extracts of Bufo arenarum tadpoles were investigated by high-performance liquid chromatography, followed by radioimmunoassay analysis using two different antisera raised against different GnRH variants. Only one immunoreactive peak was identified, eluting in the same position as synthetic mammalian GnRH. This result was further confirmed by serial dilution studies using more specific mammalian GnRH antisera. Our results suggest that mammalian GnRH is most likely an endogenous peptide in the brain of the developing larvae and froglets of Bufo arenarum and quite likely it is the only GnRH variant present during those development stages. The distribution and density of cell bodies and fibers were analysed by immunocytochemical procedures. Immunoreactive cell bodies appeared in the olfactory epithelium and across the olfactory nerve at late prometamorphic larval stages. Near the metamorphic climax and in froglets, perikarya and fibers were detected in basal forebrain, preoptic and hypothalamic areas. No immunoreaction was observed at midbrain, hindbrain and spinal cord levels. This study suggests that mammalian GnRH is most likely an endogenous peptide and is probably the only GnRH variant in the brain of the developing larvae and froglets of Bufo arenarum.

Rapid separation of gonadotropin-releasing hormone molecular forms by isocratic high-performance liquid chromatography on an ion-exchange column

The purpose of the present work was to develop a chromatographic system for the separation of five molecular forms of the gonadotropin-releasing hormone (GnRH); mammalian GnRH (mGnRH) (LHRH), salmon GnRH (sGnRH), chicken I GnRH (cIGnRH), chicken II GnRH (cIIGnRH) and lamprey GnRH I (IGnRH-I). By using an ion-exchange HPLC column and isocratic elution, it was possible to separate properly the five peptides in approximately 20 min. The utility of the system in determining the GnRHs forms present in the brain of two species of vertebrates was examined.

Distribution of two molecular forms of gonadotropin-releasing hormone (GnRH) in the central nervous system of the frog Rana ridibunda

Two molecular forms of gonadotropin-releasing hormone (GnRH) have been recently characterized in the brain of the frog Rana ridibunda i.e. mammalian GnRH (mGnRH) and chicken GnRH-II (cGnRH-II). Using highly specific antisera against each form of GnRH, we have investigated the distribution of these two neuropeptides in the frog brain by the indirect immunofluorescence and the peroxidase-antiperoxidase techniques. mGnRH-immunoreactive cell bodies were restricted to a well defined region corresponding to the septal-anterior preoptic area. mGnRH-containing fibers projected through the ventral diencephalon and ended in the median eminence. In contrast, cGnRH-II-immunoreactive structures were widely distributed in the frog brain. In the telencephalon cGnRH-II-positive elements formed a ventromedial column extending from the olfactory bulb to the septal area, a pathway which corresponds to the terminal nerve. A dense accumulation of cGnRH-II-immunoreactive cell bodies was also found in the septal-anterior preoptic area; these neurons sent processes towards the median eminence via the hypothalamus. Double immunostaining revealed that, in this area, mGnRH- and cGnRH-II-like immunoreactivity co-existed in the same neurons. In the mid-diencephalon, numerous cGnRH-II-immunoreactive perikarya were found, surrounding the third ventricle, in the posterior preoptic and infundibular areas. Many of these neurons sent processes towards the ventricular cavity. More caudally, a dense population of cGnRH-II-immunoreactive perikarya was also observed in the nucleus of the paraventricular organ and the posterior tubercle. Dorsally, the thalamus, the tegmentum, the tectum and the granular layer of the cerebellum were richly innervated by cGnRH-II-positive fibers. In the medulla oblongata, numerous cGnRH-II-immunoreactive perikarya were seen in several cranial nerve nuclei. Ventrally, a dense plexus of immunoreactive fibers projected rostrocaudally into the spinal cord. The occurrence of mGnRH- and cGnRH-II-like immunoreactivity in the septal-anterior preoptic area and the hypothalamo-pituitary pathway supports the view that both peptides act as hypophysiotropic neurohormones. The widespread distribution of cGnRH-II-immunoreactive elements in the central nervous system of the frog strongly suggests that this peptide may also exert neuromodulator and/or neurotransmitter activities.

Gonadotropin-releasing hormones, including a novel form, in snook Centropomus undecimalis, in comparison with forms in black sea bass Centropristis striata

The molecular forms of gonadotropin-releasing hormone (GnRH) in brain-pituitary extracts were determined for snook Centropomus undecimalis and black sea bass Centropristis striata. The extracts were analyzed in both isocratic and gradient high performance liquid chromatography (HPLC) programs. Eluted fractions were tested in radioimmunoassays with 4 different antisera made against 3 distinct GnRH peptides. Results show that snook contain 3 forms of GnRH, all of which are present in males and females irrespective of the stage of the reproductive cycle. Larger quantities of these GnRH peptides are present in snook in the nonreproductive phase than in snook in the reproductive phase. One form of snook GnRH is immunologically and chromatographically similar to salmon GnRH, and a second form is similar to chicken GnRH-II. However, the third snook GnRH appears to be distinct from the 7 known forms of the vertebrate hormone. In contrast, sea bass contain only the salmon GnRH-like and chicken GnRH-II-like forms of GnRH and, hence, appear to match the more usual pattern of GnRH peptides in teleosts. We speculate that one of the GnRH genes was duplicated and then altered in a fish ancestral to snook but not sea bass, even though both species of fish are in the recently evolved Perciformes order.

The reproductive brain in fish

In fish as in other vertebrates, the brain is actively involved in the control of reproduction, first by participating, under the influence of external factors, in the establishment of an appropriate endocrine status, but also by allowing synchronization of the partners by the time of spawning. It is now well established that the pituitary gonadotropic function is controlled by multiple stimulatory and inhibitory factors, originating mainly from the preoptic region and the mediobasal hypothalamus, both target regions for sexual steroids. Little is known about the mechanisms involved in the mediation of external and internal factors, however there is indication that internal factors, such as androgens and melatonin, known to trigger particular behavioural and endocrine responses, act both at the level of neuroendocrine territories, but also on sensorial systems, which are the actual sites of action for external factors. This paper represents an attempt to summarize and integrate the recent literature devoted to the different aspects of the brain as a major participant in the complex endocrine and behavioural mechanisms of reproduction in fish.

Multiple molecular forms of gonadotropin-releasing hormone in the brain of an elasmobranch: evidence for IR-lamprey GnRH

These studies investigated brains of skate, Raja erinacea (order Rajiformes, class Chondrichthyes), for gonadotropin-releasing hormone (GnRH) peptides by chromatograph and immunoreactivity with region-specific antisera raised against mammalian GnRH and lamprey GnRH. The region-specific antibody to lamprey GnRH-I was produced following conjugation to bovine serum albumin using the bis-diazotized benzidine method. This antibody was characterized by assaying a range of increasing dilutions of the known vertebrate GnRHs, as well as analogs to lamprey GnRH-I. Two analogs, lamprey [Phe2]GnRH-I and lamprey [Leu7]GnRH-I, were synthesized by solid phase peptide synthesis using a benzhydrylamine resin as the supporting medium and purified by chromatography. This antibody demonstrated less than 0.01% cross-reactivity with all GnRH peptides tested, suggesting a highly specific antibody with a region of amino acids 2-8 that appears essential for binding. In the skate brain, five immunoreactive (IR) GnRH forms were detected, four of which eluted in the same positions as synthetic mammal and chicken GnRH-I (which coelute): lamprey GnRH-I, salmon and chicken GnRH-II, and one that was an unidentified form. A minor peak coeluted with lamprey GnRH-III. The major form in the skate brain is considered to have eluted with synthetic mammalian GnRH. These studies confirm an earlier report of an IR-mammalian GnRH peptide and provide new evidence for IR-lamprey GnRH in the brain of an elasmobranch.

Seasonal variation of neuropeptide Y actions on growth hormone and gonadotropin-II secretion in the goldfish: effects of sex steroids

The effects of neuropeptide Y (NPY) on growth hormone (GH) and gonadotropin-II (GtH-II) release in different reproductive stages were studied using perifused pituitary fragments of female goldfish. The GH and GtH-II release responses to 5-min pulses of NPY were relatively small in sexually regressed fish (July), intermediate in recrudescent fish (December), and maximal in sexually mature (= prespawning) fish (May). To test if sex steroids can modulate NPY action, the effects of in vivo implantation of 17 beta-estradiol (E2) and testosterone (T) (both at 100 micrograms/g dosage) on NPY-induced GH and GtH-II secretion were examined. In sexually regressed goldfish, implantation of T significantly enhanced NPY-induced GH and GtH-II release from perifused pituitary fragments; implantation of E2 potentiated the NPY-induced GtH-II, but not GH release. However, steroid implantation did not affect responses to NPY when this experiment was repeated using pituitaries from sexually mature fish. To test the hypothesis that steroids may act directly at the level of the pituitary to potentiate NPY action, pituitary fragments taken from sexually regressed goldfish were incubated with 100 nM T for 24 h, and the GH and GtH-II responses to 5-min challenges of NPY assessed in the presence of T. Both GH and GtH-II responses to NPY were not affected by treatment with T in vitro, suggesting that T does not act directly at the level of the pituitary.(ABSTRACT TRUNCATED AT 250 WORDS)

Differential distribution of chicken-I and chicken-II GnRH in the turtle brain

Chicken-I and chicken-II gonadotropin-releasing hormone (cI-GnRH and cII-GnRH) were shown to be differentially distributed in the brain of a turtle, Trachemys scripta, by HPLC and specific radioimmunoassays. The cI-GnRH was most concentrated in the median eminence (ME), while cII-GnRH was most concentrated in the caudal brain regions, especially medulla and cerebellum. The ratio of cI- to cII-GnRH in the ME of adults was 8:1. Age- and sex-related differences in GnRH concentrations were observed exclusively in the ME: adult females had significantly higher cI-GnRH than younger females and adult males; adult females also had significantly higher cII-GnRH than hatching females. Their differential distribution and sex- and age-related differences suggest that the two peptides may have distinct physiological roles; cI-GnRH is likely the form responsible for stimulating gonadotropin release.

1-Methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), and gamma-vinyl-gamma-aminobutyric acid (gamma-vinyl GABA) alter neurotransmitter concentrations in the nervous tissue of the goldfish (Carassius auratus) but not the cockroach (Periplaneta americana)

1-Methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), 1-methyl-4-phenylpyridinium iodide (MPP+) and gamma-vinyl-gamma-aminobutyric acid (gamma-vinyl GABA) are drugs demonstrated to alter catecholamine or gamma-aminobutyric acid (GABA) concentrations in vertebrate nervous tissue. MPTP and MPP+, which are potent and selective vertebrate neurotoxins, are effective in depleting noradrenaline and dopamine concentrations in goldfish. However, only MPP+ depletes dopamine in the central nervous tissues of the cockroach, and only when injected directly into the nervous tissue. Systemic injection of gamma-vinyl GABA, a selective GABA transaminase inhibitor in vertebrates, increases GABA concentrations in goldfish but not cockroach nervous tissue. Incubations of both goldfish hypothalamus and cockroach nervous tissue demonstrated the presence of GABA transaminase activity in vitro. However, the GABA transaminase activity obtained from goldfish tissues was much more sensitive to inhibition by gamma-vinyl GABA than that obtained from cockroach nervous tissue. These results demonstrate that MPTP, MPP+ and gamma-vinyl GABA are useful pharmacological tools which can alter neurotransmitter concentrations in a lower vertebrate. Unfortunately, they possess limited effectiveness in the cockroach.

Changes in brain gonadotropin-releasing hormone, pituitary and plasma gonadotropins, and plasma thyroxine during smoltification in chinook salmon (Oncorhynchus tschawytscha)

Concentrations of brain salmon gonadotropin-releasing hormone (sGnRH), plasma gonadotropin I (GTH I), and pituitary GTH I and GTH II were determined in yearling chinook salmon (Oncorhynchus tschawytscha) during the parr-smolt transformation in two successive seasons. There were significant elevations in brain sGnRH content from February to March in 1988, and from February to April in 1989. Increases in brain sGnRH content coincided with elevations in plasma thyroxine levels that occurred from February to March, 1988 and 1989. Plasma GTH levels were relatively constant (1-2 ng/ml) throughout the period of sampling. However, during 1988, plasma concentrations of GTH I decreased significantly between late March and early April. During 1989, plasma GTH I levels appeared to reach a peak (2 ng/ml) in mid-February, but otherwise remained near 1 ng/ml. Previous studies have shown that GTH II was not detectable in plasma at this stage. During 1989, pituitary GTH I concentrations were 50- to 70-fold higher than that of GTH II, and increased, though not significantly, from February through April. Although GTH II was detected in the pituitary by RIA, it is likely that the measurable levels are due to GTH I cross-reaction in the GTH II RIA. Histological examination of the gonads indicated that throughout smoltification the oocytes remained in the perinucleolar stage of oogenesis and the testes were in the spermatogonial stage of spermatogenesis. Although no observable changes in gametogenesis occurred, the changes in brain sGnRH content, plasma GTH I levels, and pituitary GTH content suggest that some changes in the hypothalamic-pituitary axis may occur during smoltification.

Two gonadotropin-releasing hormones from African catfish (Clarias gariepinus)

Two forms of gonadotropin-releasing hormone (GnRH) have been purified from brain extracts of the African catfish, Clarias gariepinus, using reverse-phase high performance liquid chromatography (HPLC) and radioimmunoassay (RIA). The amino acid sequences of both forms of African catfish GnRH were determined using Edman degradation after digestion with pyroglutamyl aminopeptidase. In addition, both GnRHs were studied by mass spectrometry. The primary structure of African catfish GnRH I is identical to Thai catfish GnRH I, pGlu-His-Trp-Ser-His-Gly-Leu-Asn-Pro-Gly-NH2, and the primary structure of African catfish GnRH II is identical to the widely distributed and highly conserved chicken GnRH II, pGlu-His-Trp-Ser-His-Gly-Trp-Tyr-Pro-Gly-NH2.

Amino Acid Neurotransmitters and Dopamine in Brain and Pituitary of the Goldfish: Involvement in the Regulation of Gonadotropin Secretion

An isocratic high-performance liquid chromatographic technique was developed to measure levels of gamma-aminobutyric acid (GABA), glutamate, and taurine in the brain and pituitary of goldfish. Accuracy of this procedure for quantification of these compounds was established by evaluating anesthetic and postmortem effects and by selectively manipulating GABA concentrations by intraperitoneal administration of the glutamic acid decarboxylase (GAD) inhibitor 3-mercaptopropionic acid or the GABA transaminase inhibitor gamma-vinyl GABA. The technique provided a simple, rapid, and reliable method for evaluating the concentrations of these amino acids without the use of complex gradient chromatographic systems. To investigate the relationship between neurotransmitter amino acids and the control of pituitary secretion of gonadotropin, the effects of injection of taurine, GABA, or monosodium glutamate on GABA, glutamate, taurine, and, in some instances, monoamine concentrations in the brain and pituitary were evaluated and related to serum gonadotropin levels. Injection of taurine caused an elevation in serum gonadotropin concentrations. In addition, injection of the taurine precursor hypotaurine but not the taurine catabolite isethionic acid elevated serum gonadotropin levels. Intracerebroventricular injection of either GABA or taurine also elevated serum gonadotropin concentrations. Pretreatment of recrudescent fish with alpha-methyl-p-tyrosine reduced pituitary dopamine concentrations and also potentiated the serum gonadotropin response to taurine. Injection of monosodium glutamate caused an increase of glutamate content in the pituitary at 24 h; this was followed by a decrease at 72 h after administration. Pituitary GABA, taurine, and dopamine concentrations underwent a transient depletion after monosodium glutamate administration, and this was associated with an elevation of serum gonadotropin content.(ABSTRACT TRUNCATED AT 250 WORDS)

A reduction in pituitary dopamine turnover is associated with sex pheromone-induced gonadotropin secretion in male goldfish

In goldfish, the gonadal steroid, 17 alpha,20 beta-dihydroxy-4-pregnen-3-one (17,20 beta-P), functions as a potent preovulatory female sex pheromone which stimulates rapid elevations in serum gonadotropin (GtH) levels and subsequent increases in milt production in males. GtH secretion in goldfish is known to be regulated by the stimulatory actions of gonadotropin-releasing hormone (GnRH) and the inhibitory actions of dopamine (DA). This study specifically examined whether the 17,20 beta-P-induced elevation in male GtH is caused by pheromone-mediated changes in DA inhibition at the level of the pituitary. First, we have demonstrated that dihydroxyphenylacetic acid (DOPAC) is the primary metabolite of DA catabolism in the brain and pituitary gland of goldfish. Second, we measured changes in circulating levels of GtH and changes in pituitary content of DA and its metabolite, DOPAC, as well as possible alterations in DA turnover rate (DOPAC/DA ratio) following short-term exposure of male goldfish to water-borne 17,20 beta-P. Water-borne 17,20 beta-P consistently increased serum GtH levels in males within 20 min of exposure and maintained elevated levels for up to 120 min. Although changes in pituitary DA content were not observed during periods of high GtH release, coincident reductions in pituitary levels of DOPAC were measured within 45 min of exposure to the pheromone. More importantly, there was a significant decrease in the rate of DA turnover in the pituitary, as assessed by comparing the ratio of DOPAC to DA present, at 20, 45, and 120 min of exposure. Since the reduction of DA turnover in the pituitary is inversely correlated with periods of increased GtH release, the present results suggest that water-borne 17,20 beta-P causes an abatement of DA release to the pituitary. Based on the latency of the GtH response to water-borne 17,20 beta-P, a rapid reduction of DA turnover in the pituitary appears to be at least part of the neuroendocrine trigger for 17,20 beta-P-induced GtH release in male goldfish.

Relationship between cyclic AMP-stimulated and native gonadotropin-releasing hormone-stimulated gonadotropin release in the goldfish

The relationship between drugs elevating intracellular cAMP levels and gonadotropin (GTH)-releasing hormone (GnRH) in the stimulation of GTH secretion in the goldfish was investigated using dispersed goldfish pituitary cells in primary culture. In static incubation experiments, activation of adenylyl cyclase by forskolin and the inhibition of cAMP phosphodiesterase by 3 isobutyl-1-methylxanthine (IBMX) increased cAMP release and stimulated GTH secretion. The addition of membrane permeant cAMP analogs, 8-bromoadenosine 3':5'-cyclic monophosphate (8Br-cAMP), and dibutyryl cAMP also increased GTH release, suggesting that elevation of cAMP levels can induce GTH secretion. In the goldfish, dopamine is a physiological inhibitor of GTH release. Application of the dopamine agonist apomorphine decreased the GTH responses to forskolin, 8Br-cAMP, and salmon GTH-releasing hormone (sGnRH). The ability of agents that elevate cAMP levels to mimic GnRH action on GTH release suggests that cAMP may mediate GnRH-stimulated GTH secretion in the goldfish; however, this possibility was not substantiated by results from further experiments. In 2-hr static incubation studies, the GTH responses to sGnRH and chicken GnRH-II (cGnRH-II) were enhanced by coincubations with forskolin, IBMX, and 8Br-cAMP. The magnitudes of these enhancements were at least additive, if not synergistic. The levels of cAMP released into the media were unaffected by treatment with sGnRH and cGnRH-II, either in the absence or in the presence of IBMX. Replacement of normal testing media with Ca(2+)-deficient media (without Ca2+ salts and in the presence of 0.1 mM EGTA) decreased sGnRH and cGnRH-II stimulation of GTH release but did not affect forskolin and 8Br-cAMP actions. These results indicate that sGnRH and cGnRH-II stimulation of short term (less than or equal to 2-h) GTH release in the goldfish is not mediated by cAMP. The kinetics of the interactions between sGnRH, forskolin, and IBMX were also investigated in cell column perifusion studies. Applications of 5-min pulses of forskolin and IBMX stimulated rapid increases in GTH release; the latencies of these responses were similar to that observed with sGnRH. The simultaneous applications of sGnRH with either forskolin or IBMX resulted in GTH responses that were of greater magnitude and longer duration than those in response to sGnRH alone. These results together indicate that elevation of cAMP levels can potentiate the GTH response to the native GnRHs by increasing the magnitude of the acute GTH release and by prolonging the duration of GnRH action; however, cAMP does not appear to be involved directly in mediating GnRH stimulation of GTH release.(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.

Lack of involvement of arachidonic acid metabolism in chicken gonadotropin-releasing hormone II (cGnRH-II) stimulation of gonadotropin secretion in dispersed pituitary cells of goldfish, Carassius auratus. Identification of a major difference in salmon GnRH and chicken GnRH-II mechanisms of action

Gonadotropin (GTH) release in static incubations of dispersed goldfish pituitary cells was stimulated by chicken GTH-releasing hormone II (cGnRH-II), salmon (s)GnRH, phospholipase A2, phospholipase C, phospholipase D, and arachidonic acid (AA). Coincubations with nordihydroguaiaretic acid (NDGA), 5,8,11,14-eicosatetraenoic acid, and indomethacin did not alter the GTH responses to cGnRH-II. In contrast, NDGA reduced sGnRH-stimulated GTH release. Incubation with Ca(2+)-deficient medium abolished the GTH responses to cGnRH-II, reduced sGnRH-stimulated GTH release, but did not alter AA actions on GTH secretion. Apomorphine, a dopamine agonists that had been shown to partially inhibit the GTH responses to sGnRH and to abolish those induced by cGnRH-II, did not affect the hormone response to AA. However, the partial inhibitory actions of NDGA and apomorphine on sGnRH-induced GTH release were additive. These findings suggest the existence of a major difference in cGnRH-II and sGnRH stimulation of GTH release--AA metabolism is not involved in cGnRH-II, as opposed to sGnRH actions. This difference in second messenger activation may also explain the differential sensitivity of the two GnRH peptides to dopamine inhibition and manipulations of extracellular Ca2+ availability. The results further suggest that dopamine and AA affect GTH release via non-overlapping signal transduction pathways.

Possible involvement of protein kinase C in gonadotropin and growth hormone release from dispersed goldfish pituitary cells

Static incubation with tumor-promoting 4 beta-phorbol esters, activators of the Ca2(+)- and phospholipid-dependent protein kinase C enzyme (PKC), caused dose-dependent increases in gonadotropin (GTH) and growth hormone (GH) secretion in primary cultures of dispersed goldfish pituitary cells. The estimated half-maximal effective doses (ED50) for stimulating GTH and GH release were 0.35 +/- 0.17 and 0.32 +/- 0.13 nM 12-O-tetradecanoyl phorbol 13 acetate (TPA), 3.71 +/- 1.30 and 1.37 +/- 0.76 nM 4 beta-phorbol 12,13-dibutyrate, 6.90 +/- 4.84 and 1.89 +/- 0.25 nM 4 beta-phorbol 12,13-dibenzoate, and 455 +/- 258 and 311 +/- 136 nM 4 beta-phorbol 12,13-diacetate, respectively. In contrast, treatments with up to 10 microM of the inactive 4 alpha-phorbol 12,13-didecanoate ester did not alter GTH and GH release. Additions of the synthetic diacylglycerol, dioctanoyl glycerol, also enhanced GTH and GH secretion in a dose-dependent manner and with ED50s of 1.73 +/- 0.83 and 1.73 +/- 1.19 microM, respectively. The GTH and GH responses to stimulation by TPA were attenuated by incubation with Ca2(+)-depleted medium containing EGTA or by treatment with the Ca2+ channel blocker verapamil. Coincubation with the PKC inhibitor H7 reduced the GTH and GH responses to TPA. As in previous studies, additions of salmon gonadotropin-releasing hormone (sGnRH) or chicken GnRH-II (cGnRH-II) induced GTH and GH release; these hormone responses to sGnRH and cGnRH-II were also decreased by the addition of H7. These results indicate that activation of PKC may stimulate GTH and GH release in goldfish and suggest that sGnRH and cGnRH-II actions on goldfish pituitary GTH and GH secretion are also mediated, at least partially, by PKC.

In vitro release of gonadotropin-releasing hormone from the brain preoptic-anterior hypothalamic region and pituitary of female goldfish

In vitro release of gonadotropin releasing hormone (GnRH) from slices of the preoptic-anterior hypothalamic (P-AH) region and fragments of the pituitary of goldfish was studied using a static incubation system. Release of GnRH from both tissue preparations was stimulated by depolarizing concentrations of extracellular potassium ions (K+). Other putative secretagogues, calcium ionophore A23187 (1 microM), forskolin (100 microM), and prostaglandin E2 1 microM) also stimulated release of GnRH from both tissue preparations. Omission of Ca2+, or chelating the remaining remaining Ca2+ by EGTA (0.1 mM), abolished the release of GnRH stimulated by high K+ concentrations (60 mM), but did not reduce spontaneous release. Verapamil (1 microM), a voltage-sensitive calcium channel blocker, abolished the release of GnRH stimulated by high K+ or A21387 from both tissue preparations. The GnRH released in vitro from both the P-AH region and pituitary was concentrated by Sep-Pak and then separated by high-performance liquid chromatography. The major peak of the GnRH immunoreactivity was found to coelute with synthetic salmon GnRH [( Trp7,Leu8]-GnRH) and the minor peak with chicken GnRH-II [( Gln8]-GnRH). Dopamine (10 and 100 microM) inhibited GnRH release from both P-AH slices and pituitary fragments, while serotonin (1-100 microM) stimulated release from both. Norepinephrine (10-100 microM) stimulated GnRH release from P-AH slices but not from pituitary fragments. The results demonstrate that the release of GnRH from goldfish P-AH slices and pituitary fragments in vitro in response to various secretagogues and monoamines can be studied using a static incubation system.