Gonadotrophin-releasing hormone signalling downstream of calmodulin

Gonadotrophin-releasing hormone (GnRH) regulates reproduction via binding a G-protein coupled receptor on the surface of the gonadotroph, through which it transmits signals, mostly via the mitogen-activated protein (MAPK) cascade, to increase synthesis of the gonadotrophin hormones: luteinising hormone (LH) and follicle-stimulating hormone (FSH). Activation of the MAPK cascade requires an elevation in cytosolic Ca(2+) levels, which is a result of both calcium influx and mobilisation from intracellular stores. However, Ca(2+) also transmits signals via an MAPK-independent pathway, through binding calmodulin (CaM), which is then able to bind a number of proteins to impart diverse downstream effects. Although the ability of GnRH to activate CaM was recognised over 20 years ago, only recently have some of the downstream effects been elucidated. GnRH was shown to activate the CaM-dependent phosphatase, calcineurin, which targets gonadotrophin gene expression both directly and indirectly via transcription factors such as nuclear factor of activated T-cells and Nur77, the Transducer of Regulated CREB (TORC) co-activators and also the prolyl isomerase, Pin1. Gonadotrophin gene expression is also regulated by GnRH-induced CaM-dependent kinases (CaMKs); CaMKI is able to derepress the histone deacetylase-inhibition of β-subunit gene expression, whereas CaMKII appears to be essential for the GnRH-activation of all three subunit genes. Asides from activating gonadotrophin gene expression, GnRH also exerts additional effects on gonadotroph function, some of which clearly occur via CaM, including the proliferation of immature gonadotrophs, which is dependent on calcineurin. In this review, we summarise these pathways, and discuss the additional functions that have been proposed for CaM with respect to modifying GnRH-induced signalling pathways via the regulation of the small GTP-binding protein, Gem, and/or the regulator of G-protein signalling protein 2.

Transcription of gonadotropin beta subunit genes involves cross-talk between the transcription factors and co-regulators that mediate actions of the regulatory hormones

The gonadotropins LH and FSH have distinct temporal patterns of expression as a result of differential regulation by hormones such as GnRH, steroids and activin. This specific regulation is due to diverse sets of transcription factors that are recruited to the promoters of these genes, and recruit specific co-activator complexes which function to stabilize interactions with the general transcription factors and RNA polymerase II, and also to induce covalent modifications of the histone tails at these gene loci. As these molecular mechanisms are elucidated, the nature of nuclear cross-talk between the various hormonally induced pathways is becoming evident, revealing both negative and positive effects of interacting transcription factors and co-regulators. This paper will review current knowledge on the transcriptional regulation of gonadotropin beta subunit gene expression in the chromatin setting, and will present new data pertaining to nuclear cross-talk between the various endocrine-induced pathways regulating gonadotropin gene transcription.

Gonadotropin response to GnRH during sexual ontogeny in the common carp, Cyprinus carpio

This study was designed to reveal whether gonadotropic response to GnRH in the common carp (Cyprinus carpio) changes during sexual ontogeny and whether the response of FSHbeta and LHbeta subunits is uniform or differential. The study comprised fish at the following stages: juveniles (4-month-old females with primary oocytes and early spermatogenic males); maturing (9-month-old previtellogenic females and advanced spermatogenic males); and mature (16-month-old postvitellogenic females and spermiating males). Fish were injected with superactive salmon GnRH analogue (sGnRHa; 25 microg/kg) and blood was sampled 6, 12 and 24 h later for cGtH (LH) and sex steroid levels. Pituitaries were taken for determination of FSHbeta and LHbeta mRNA levels by slot-blot hybridization and for cGTH content in the same glands by radioimmunoassay (RIA). Values were compared with the levels prior to sGnRHa administration and with control fish sampled at the same intervals. Juvenile fish did not respond at all to sGnRHa. In maturing females, FSHbeta mRNA increased by >300%, while that of LHbeta increased by 200%. In maturing males, FSHbeta mRNA did not change and only a slight increase occurred in that of LHbeta. In 16-month-old postvitellogenic females, there was no response of FSHbeta mRNA, while that of LHbeta dramatically increased. In spermiating males of the same age, mRNA of both FSHbeta and LHbeta increased following sGnRHa injection. Immunoreactive cGtH was present in the pituitary and plasma of all fish examined, but in juveniles it did not change following sGnRHa injection. In maturing and mature fish of both genders, sGnRHa administration was followed by a marked increase in circulating cGtH, concomitant with a decrease in its pituitary content, indicating the limited amount of the hormone stored in the gland. In conclusion, the response of the gonadotropin subunit mRNAs in the common carp was found to be differential and dependent on the gender and the phase of sexual ontogeny.

Tilapia glycoprotein hormone alpha subunit: cDNA cloning and hypothalamic regulation

The cDNA encoding the glycoprotein alpha (GPalpha) subunit of tilapia (Oreochromis mossambicus) was partially cloned using RACE-polymerase chain reaction (PCR) technique. The amplified cDNA was found to be 583 bases long, and to consist of a portion of the signal peptide, the full sequence encoding the mature peptide (94 amino acids) and the 3' untranslated region. Northern blot analysis revealed a single band of approximately 600 bp. Alignment of the deduced amino acids of the mature protein showed that the tilapia GPalpha subunit shares more than 80% identity with that of other perciform fish (i.e. striped bass, sea bream and yellowfin porgy) and less than 70% with that of more taxonomically remote fish and other vertebrates. Exposure of dispersed tilapia pituitary cells to salmon gonadotropin-releasing hormone (sGnRH) elevated GPalpha mRNA levels via both PKC and cAMP-protein kinase A (PKA) pathways. The transcript levels were also regulated by pituitary adenylate cyclase activating polypeptide (PACAP) and neuropeptide Y (NPY), both acting through PKC and PKA pathways. Moreover, a combined treatment of PACAP or NPY with GnRH seems to have an additive effect on the GPalpha subunit gene transcription. These results suggest that in tilapia the expression of GPalpha subunit is regulated by GnRH mainly via PKC and PKA pathways. Furthermore, PACAP and NPY can elevate the GnRH-stimulated GPalpha subunit transcription and can directly affect the subunit mRNA levels, via the same transduction pathways.

Regulation of gonadotropin subunit genes in tilapia

A steroidogenic tilapia gonadotropin (taGtH=LH) was purified from pituitaries of hybrid tilapia (Oreochromis niloticus x O. aureus) and a homologous RIA was established. This RIA enabled the study of the endocrine regulation of GtH release, the transduction pathways involved in its secretion and its profile during the spawning cycle. Discrepancies between steroid and taGtH peaks during the cycle led to the conclusion that an additional gonadotropin similar to salmonid FSH operates early in the cycle. In order to identify this hormone and to study the endocrine control of synthesis of all gonadotropin (GtH) subunits, a molecular approach was taken. The cDNA sequences and the entire gene sequences encoding the FSHbeta and LHbeta subunits, as well as an incomplete sequence of the glycoprotein hormone alpha subunit (GPalpha), were cloned. Salmon gonadotropin-releasing hormone (sGnRH) elevated mRNA steady-state levels of all three GtH subunits in cultured pituitary cells. Pituitary adenylate cyclase-activating polypeptide (PACAP) and neuropeptide Y (NPY) also stimulated the expression of these subunits and potentiated the effect of GnRH, except that NPY did not affect FSHbeta. The GnRH and NPY effects were found to be mediated mainly through protein kinase C (PKC), while protein kinase A (PKA) cascade was involved to a lesser extent. Mitogen-activated protein kinase (MAPK) cascade takes part in mediating GnRH effects, possibly via PKC. Testosterone (T) and estradiol (E2), but not 11-ketotestosterone (KT), are able to elevate GPalpha and LHbeta mRNAs in pituitary cells of early maturing or regressing males. Low levels of T exposure are associated with elevated FSHbeta mRNA in cells of mature fish, while higher levels suppress it, but elevate LHbeta mRNA. In vivo observations also showed the association of low T levels with increased FSHbeta mRNA and high T levels with elevated LHbeta mRNA. In accordance with these findings, analysis of LHbeta and FSHbeta 5' gene-flanking regions revealed on both gene promoters a GtH-specific element (GSE), half site estrogen response elements (ERE), cAMP response element (CRE) and AP1. In vitro experiments showed that recombinant human activin-A leads to higher levels of GPalpha, FSHbeta and LHbeta mRNAs in pituitary cell culture. Porcine inhibin marginally decreased the mRNA levels of GPalpha and FSHbeta, but at a low level (1 ng/ml) it stimulated that of LHbeta. These results shed some light on certain hypothalamic and gonadal hormones regulating the expression of GtH subunit genes in tilapia. In addition, they provide evidence for their differential regulation, and insight into their mode of action.

Characterization of tilapia FSHbeta gene and analysis of its 5' flanking region

The objective of the current study was to unveil molecular mechanisms underlying transcriptional regulation of the FSHbeta gene expression in the pituitary of tilapia (Oreochromis mossambicus). The full-length sequence of tilapia FSHbeta (tFSHbeta) gene was determined. Its transcriptional unit (2.7 kb) exhibits the conserved genomic organization, i.e. three exons and two introns. Primer extension and RT-PCR analysis revealed heterogeneity of the tFSHbeta transcripts, due to alternate mRNA splicing and multiple initiation sites for transcription. Examination of the 5' flanking region (5'FR) of the tFSHbeta gene identified potential CAAT and TATA promoter proximal elements as well as several sequences of cis-acting motifs known to dictate inducible and tissue-specific transcriptional regulation in other gonadotropin genes. Chimeric constructs containing 1.7 kb of the tFSHbeta 5'FR fused to a luciferase (LUC) reporter gene were transiently transfected into primary culture of tilapia pituitary cells. The tFSHbeta-LUC construct was efficiently expressed under basal conditions and was rapidly induced by GnRH stimulation. Our data indicate that the 5'FR contains a functional promoter, which is responsive to GnRH treatment. In addition, 5' deletion analysis showed that the 1.7 kb, DNA sequence of the FSHbeta 5'FR encompasses both positive and negative regulatory elements.

Changes along the pituitary-gonadal axis during maturation of the black carp, Mylopharyngodon piceus

The black carp, Mylopharyngodon piceus, is a late-maturing cyprinid reaching sexual maturity at the age of 6-7 years. The present work attempted to define nonfunctional sites along the pituitary-gonadal axis in immature fish utilizing in vivo and in vitro challenge experiments. Two- and 3-year old fish injected with salmon gonadotropin-releasing hormone analog (sGnRHa; 10 microg/kg) and metoclopramide (20 mg/kg) did not reveal any increase in circulating gonadotropin (cGtH) or estradiol (E(2)) level. Furthermore, cGtH release from cultured pituitary cells of fish at these ages did not increase in response to sGnRH (0.1 nM - 1 microM) but was augmented when exposed to TPA (12.5 nM). However, 4-year old female fish did respond to the above treatments both in vivo and in vitro. These results suggest the existence of nonfunctional site(s) proximal to the activation of PKC in the immature black carp gonadotrophs, probably at the level of GnRH receptors. These site(s) start to become functional in 4-year old fish. Two- and 3-year old fish injected with common carp pituitary extract (CPE) containing 350 microg cGtH/kg did not show any increase in circulating E(2). In addition, the estrogen secretion from fragments of the rudimentary gonads did not increase after exposure to CPE containing cGtH (0.5-4 microg/ml) but was elevated dose-dependently by exposure to dbcAMP (0.3-3 mM). However, the ovaries of 4-year old fish did respond to the gonadotropic stimulation, both in vivo and in vitro. These results suggest the existence of other non-functional site(s) in the immature black carp, proximal to the formation of cAMP in the gonads, probably at the level of GtH receptors. These site(s) start to become functional in 4-year old females. Another source of E(2) was discovered in the immature black carp: namely, the fat pad adjacent to the gonads. In contrast to the visceral adipose tissue, the fat pad secretes estrogen in response to cAMP elevation in 2- and 3-year old fish while in 4-year old fish it also responds to gonadotropic stimulation. Due to its large mass and high steroidogenic potency, it is assumed that the gonadal fat pad is involved in the process of puberty in the black carp. J. Exp. Zool. 286:405-413, 2000.

Reproductive development of male and female tilapia hybrids (Oreochromis niloticus x O. aureus) and changes in mRNA levels of gonadotropin (GtH) Ibeta and IIbeta subunits

A study was carried out in tilapia in order to see whether the gonadotropin (GtH) beta subunits show distinct patterns of expression at different stages of their reproductive development. Male and female tilapia hybrids (Oreochromis niloticus x O. aureus) were collected at various times of the year, and a number of parameters were measured in order to establish the reproductive state of the fish. Circulating testosterone (T), estradiol (E(2)) and 11 ketotestosterone (11KT) levels were assayed, gonads were removed for calculation of gonadosomatic index (GSI) values and histological studies, and RNA was extracted from the pituitaries for measurement of GtH Ibeta and IIbeta mRNA levels. In maturing fish of both sexes, the circulating steroid levels were positively correlated with each other (r(2) = 0.66-0.91) and in males, also with the GSI values (r(2) = 0.68). A positive correlation was also seen in these fish between GSI values and the prevalence of spermatocytes and spermatids (r(2) = 0.54). In maturing females, the maximal oocyte diameter was positively correlated with circulating E(2) levels (r(2) = 0.63), while GSI values showed no correlation; this presumably relates to the cycling nature of this asynchronous spawner. In regressing fish of both sexes, no clear correlation between these reproductive parameters was seen. In all fish, the GtH Ibeta mRNA levels were highest in fish with steroids ranging 1-10 ng T or E(2)/ml for males or females, respectively, and were lower in fish with steroids at higher or lower levels. In fish with high steroid levels, the IIbeta mRNA levels were also high, and in regressed males the increases were positively correlated. Exposure of cultured pituitary cells to either steroid (T at >10 nM, or E(2) at >1 nM) was followed by a decrease in the steady-state levels of the Ibeta transcript, while those of IIbeta were left unaltered. In situ hybridization studies revealed that in pituitaries of both sexes, the cells producing each of these mRNAs are located in a distinct location. These results suggest that gonadal steroids may exert differential feedback mechanisms at the level of the pituitary to control transcription of each GtH beta subunit in distinct cell types specific for each hormone.

Calcium ionophores lead to apoptotic-like changes in Tilapia pituitary cells

Influx or mobilization of Ca2+ plays an important part in the signal transduction mechanisms regulating release of gonadotropin (GtH) and growth hormone (GH) in teleost fish. In mammals it may also mediate a stimulatory effect on the transcription of the genes encoding these hormones (i.e., LHbeta, FSHbeta, and GH). In the present study, exposure of tilapia pituitary cells in primary culture to two ionophores, A23187 and ionomycin, increased GtH and GH secretion over 5-24 h but led to a significant drop in mRNA levels of GtH IIbeta and GH. The mRNA levels of beta actin were also reduced by this treatment, suggesting a general, nonspecific effect in these cells. The morphology of the ionophore-exposed cells also differed markedly; they lacked cytoplasmic extensions, appeared smaller, and were less aggregated than control cells. Staining the nuclei of these cells with 4,6-diamidino-2-phenyl-dihydrochloride revealed that they had undergone condensation and fragmentation, typical of programmed cell death. Extraction of DNA from the ionophore-exposed cells and its separation on ethidium bromide-stained gels revealed that, unlike in control cells, the DNA had been broken into fragments in multiples of approximately 180-200 bp, providing further evidence of apoptotic-like effects of the ionophores on the cells. It is speculated that Ca2+, which mediates stimulation of GtH and GH release by the hypothalamic regulatory hormones, may, under certain conditions, have apoptotic-like effects, which specifically regulate the sizes of gonadotroph and somatotroph cell populations. In addition, the fact that pituitary cells exposed to ionophores may become apoptotic should be borne in mind when experiments on signal transduction are carried out with these substances.

Possible interactions between gonadotrophs and somatotrophs in the pituitary of tilapia: apparent roles for insulin-like growth factor I and estradiol

The unique organization of the teleost pituitary, in which cells are grouped according to their characteristic hormone, makes this a suitable model for studying pituitary paracrine interactions. In a number of fish, including tilapia, there are variations in the circulating levels of the gonadotropins and GH, which are elevated during the reproductive season, suggesting interactions between the reproductive and growth axes. The aim of this study was to investigate paracrine interactions between the gonadotrophs and somatotrophs in the tilapia pituitary. Initially, dispersed pituitary cells were separated on a density gradient in which the gonadotrophs were found in the least dense fractions, and the somatotrophs were concentrated in the densest fraction. After 4 days in culture, cells in the least dense fractions showed characteristic cytoplasmic extensions not seen in the somatotrophs, which appeared small and failed to form aggregates; somatotrophs were found, however, attached to other non-GH cells. Staining of the nuclei with 4,6-diaminidino-2-phenyl-dihydrochloride revealed that the isolated somatotrophs had undergone nuclear condensation and fragmentation typical of apoptosis. Addition of either estradiol or human recombinant insulin-like growth factor I (IGF-I; 10 nM) to the somatotroph cultures increased the number of cell aggregates and reduced the number of condensed or fragmented nuclei. Immunocytochemical studies on pituitary sections revealed IGF-I immunoreactivity in regions of the proximal pars distalis that stain with gonadotropin IIbeta antisera and also in regions of the rostral pars distalis characteristic of corticotrophs; immunoreactive IGF-I was never seen in the region of the somatotrophs. Incubation of cells from the different fractions with testosterone (10 nM; 24 h) revealed that cells of the least dense fractions, which were rich in gonadotrophs, possessed aromatizing ability, which was absent in the somatotroph-enriched fraction. These results suggest that estradiol and IGF-I, both generated from cells other than the somatotrophs, may exert antiapoptotic effects and thus possibly control the size of this population of cells.

Endocrine regulation of gonadotropin and growth hormone gene transcription in fish

The pituitary of a number of teleosts contains two gonadotropins (GtHs) which are produced in distinct populations of cells; the beta subunit of the GtH I being found in close proximity to the somatotrophs, while the II beta cells are more peripheral. In several species the GtH beta subunits are expressed at varying levels throughout the reproductive cycle, the I beta dominating in early maturing fish, after which the II beta becomes predominant. This suggests differential control of the beta subunit synthesis which may be regulated by both hypothalamic hormones and gonadal steroids. At ovulation and spawning, changes also occur in the somatotrophs, which become markedly more active, while plasma growth hormone (GH) levels increase. In a number of species, GnRH elevates either the I beta or the II beta mRNA levels, depending on the reproductive state of the fish. In tilapia, the GnRH effect on the II beta appears to be mediated through both cAMP-PKA and PKC pathways. GnRH also stimulates GH release in both goldfish and tilapia, but it increases the GH transcript levels only in goldfish; both GnRH and direct activation of PKC are ineffective in altering GH mRNA in tilapia pituitary cells. Dopamine (DA) does not alter II beta transcript levels in cultured tilapia pituitary cells, but increases GH mRNA levels in both rainbow trout and tilapia, in a PKA-dependent manner. This effect appears to be through interactions with Pit-1 and also by stabilizing the mRNA. Somatostatin (SRIF) does not alter GH transcript levels in either tilapia or rainbow trout, although it may alter GH synthesis by modulation of translation. Gonadal steroids appear to have differential effects on the transcription of the beta subunits. In tilapia, testosterone (T) elevates I beta mRNA levels in cells from immature or early maturing fish (in low doses), but depresses them in cells from late maturing fish and is ineffective in cells from regressed fish. Similar results were seen in early recrudescing male coho salmon injected with T or E2. T or E2 administered in vivo has dramatic stimulatory effects on the II beta transcript levels in immature fish of a number of species, while less powerful effects are seen in vitro. A response is also seen in cells from early maturing rainbow trout or tilapia, or regressed tilapia, but not in cells from late maturing or spawning fish. These results are substantiated by the finding that the promoter of the salmon II beta gene contains several estrogen responsive elements (EREs) which react and interact differently when exposed to varying levels of E2. In addition, activator protein-1 (AP-1) and steroidogenic factor-1 (SF-1) response elements are also found in the salmon II beta promoter; the AP-1 site is located close to a half ERE, while the SF-1 acts synergistically with the E2 receptor. The mRNA levels of both AP-1 and SP-1 are elevated, at least in mammals, by GnRH, suggesting possible sites for cross-talk between GnRH and steroid activated pathways. Reports of the effects of T or E2 on GH transcription differ. No effect is seen in vitro in pituitaries of tilapia, juvenile rainbow trout or common carp, but T does increase the transcript levels in pituitaries of both immature and mature goldfish. Reasons for these discrepancies are unclear, but other systemic hormones may be more instrumental than the gonadal steroids in regulating GH transcription. These include T3 which increases both GH mRNA levels and de novo synthesis (in tilapia and common carp) and insulin-like growth factor-I (IGF-I) which reduces GH transcript levels as well as inhibiting GH release.

Differential effects of gonadotropin-releasing hormone, dopamine and somatostatin and their second messengers on the mRNA levels of gonadotropin II beta subunit and growth hormone in the teleost fish, tilapia

In cultured pituitary cells of tilapia, gonadotropin-releasing hormone (GnRH; 10 nM 4-24 h), elevation of cyclic AMP (by 10 microM forskolin or 0.2 mM 3-isobutyl-1-methylxanthine: IBMX 0.5-36 h) or activation of protein kinase C (PKC; by 12.5 nM tetradecanoyl phorbol-13-acetate: TPA, 0.5-24 h) all increased gonadotropin (GtH) II beta steady state mRNA levels by three to four-fold. The involvement of PKA and PKC in the GnRH stimulatory effect on both GtH release and GtH II beta mRNA levels was corroborated by use of the PKA and PKC inhibitors, H89 and GF109203X, respectively (100 nM) which attenuated the GnRH effect. Incubation with actinomycin D (8 microM, 4-21 h) after preexposure for 24 h to either forskolin (10 microM) or TPA (12.5 nM), revealed that rates of transcript degradation were slower in forskolin-treated cells (T 1/2 = 14.1 h) than in control or TPA-treated cells (T 1/2 = 8.47 or 8.38 h), suggesting a stabilizing effect on the mRNA. Dopamine (DA; 10 microM, 4-36 h) had no apparent effect on steady state mRNA levels of GtH II beta, but reduced GtH release by as much as 75%. Steady state levels of growth hormone (GH) mRNA were not affected by exposure to GnRH (10 nM, 4-24 h), although GH release was more than doubled. Similarly, activation of PKC (by TPA 12.5 nM, 1.5-36 h), which was shown to be essential for the GnRH-stimulatory effect on GH release, did not alter levels of the GH transcript, but increased GH release by more than fivefold. DA (10 microM, 4-24 h) moderately increased GH transcript levels (160%) with similar kinetics but lower potency than direct elevation of cAMP (by 10 microM forskolin or 0.2 mM IBMX, 0.5-36 h) which increased transcript levels by more than fourfold. The involvement of PKA in the DA effect was confirmed when the PKA inhibitor H89 (100 nM, 15 min prior to DA exposure) attenuated the DA effect on GH mRNA levels. Exposure of cells to actinomycin D (8 microM, 2-16 h) after treatment with forskolin (10 microM, 24 h) led to a slower rate of transcript degradation than in control cells (T 1/2 = 6.5 h vs. T 1/2 = 4.36 h), suggesting that cAMP also elicits a stabilizing effect on GH mRNA. Somatostatin (100 nM, 0.5-36 h) had no clear effect on GH transcript levels, but reduced GH release by as much as 90%. These results suggest that activation of either cAMP-PKA or PKC pathways can, possibly by different mechanisms, stimulate mRNA levels of the GtH II beta gene, but that only the cAMP-PKA pathway stimulates GH mRNA levels. It would appear therefore that GnRH, although stimulating GH release, does not regulate GH transcription in this fish.

Growth enhancement in transgenic tilapia by ectopic expression of tilapia growth hormone

The generation of transgenic fish with the transfer of growth hormone (GH) genes has opened new possibilities for the manipulation of growth in economically important fish species. The tilapia growth hormone (tiGH) cDNA was linked to the human cytomegalovirus (CMV) enhancer-promoter and used to generate transgenic tilapia by microinjection into one-cell embryos. Five transgenic tilapia were obtained from 40 injected embryos. A transgenic animal containing one copy of the transgene per cell was selected to establish a transgenic line. The transgene was stably transmitted to F1 and F2 generations in a Mendelian fashion. Ectopic, low-level expression of tiGH was detected in gonad and muscle cells of F1 transgenic tilapia by immunohystochemical analysis of tissue sections. Nine-month-old transgenic F1 progeny were 82% larger than nontransgenic fish at p = .001. These results showed that low-level ectopic expression of tiGH resulted in a growth acceleration in transgenic tilapia. Tilapia GH gene transfer is an alternative for growth acceleration in tilapia.

The effects of gonadal development and sex steroids on growth hormone secretion in the male tilapia hybrid (Oreochromis niloticus × O. aureus)

Profiles of plasma growth hormone (GH) in male tilapia hybrid (Oreochromis niloticus x O. aureus) were measured and compared at different times of the year. The profiles did not appear to be repetitive, however, differences in their nature were observed at the different seasons; the most erratic profiles were seen in the height of the reproductive season (July), while the peaks were more subdued in the spring and disappeared in the autumn. Peaks in male fish were more prominent than in the females when measured in July. Perifused pituitary fragments from fish with a high GSI responded to salmon gonadotropin-releasing hormone (sGnRH) analog (10 nM-1 μM), while those from fish with a low GSI barely responded to even the highest dose. Exposure of perifused pituitary fragments from sexually-regressed fish to carp growth hormone-releasing hormone (cGHRH; 0.1 μM) or sGnRH (I μM) stimulated GH release only after injection of the fish with methyl testosterone (MT; 3 injections of 0.4 mg kg (1)). The same MT pretreatment did not alter the response to dopamine (DA; 1 or 10 μM). GH pituitary content in MT-treated fish was lower than in control fish, which may be explained by the higher circulating GH levels in these fish, but does not account for the increased response to the releasing hormones. Castration abolished the response of cultured pituitary cells to sGnRH (I fM-100 nM) without altering either their basal rate of secretion or circulating GH levels. Addition of steroids to the culture medium (MT or estradiol at 10 nM for 2 days) enabled a GH response to sGnRH stimulation in cells from sexually regressed fish. Pituitary cells which had not been exposed to steroids failed to respond to sGnRH, although their response to forskolin or TPA was similar to that of steroid-exposed cells. It would appear, therefore, that at least one of the effects of the sex steroids on the response to GnRH is exerted proximally to the formation of cAMP, or PKC, presumably at the level of the receptor. An increase in the number of receptors to the GH-releasing hormones, following steroid exposure, would explain also the changing nature of the GH secretory profile in different stages of the reproductive season.

Hypothalamic and thyroidal regulation of growth hormone in tilapia

A radioimmunoassay (RIA) for recombinant tilapia growth hormone (GH) was established and validated. The ability of various hypothalamic factors to regulate GH secretion in the tilapia hybrid (Oreochromis niloticus x Oreochromis aureus) was studied. Somatostatin1-14 (SRIF1-14; 10-100 micrograms/kg) was found to reduce circulating GH levels in a dose-dependent manner. SRIF1-14 (0.1-1000 nM) inhibited GH release from perifused pituitary fragments (ED50 0.83 nM). Human growth hormone-releasing hormone fragment 1-29 (hGHRH1-29; 100 micrograms/kg) doubled circulating GH levels and modestly stimulated GH secretion in vitro. Carp growth hormone-releasing hormone (cGHRH) stimulated GH secretion in vitro to a similar degree at the same dose (1 microM). Injection of salmon gonadotropin-releasing hormone (sGnRH) superactive analog (10-100 micrograms/kg) increased plasma GH levels sixfold. sGnRH also stimulated GH release in vitro (ED50 142.56 nM). Dopamine (0.1-10 microM) and the D1 DA receptor agonist SKF 38393 increased GH secretion from perifused pituitary fragments dose-relatedly. Thyrotropin-releasing hormone (TRH) had no effect on GH secretion from perifused pituitary fragments, but increased plasma GH levels, as did bovine thyroid stimulating hormone (bTSH). The increased plasma GH in the bTSH-treated fish coincided with a dramatic increase in T4; however, TRH increased GH without changing T4 levels. T3 increased the synthesis of GH by isolated pituitaries (incorporation of [3H]leucine). SRIF1-14 seems to be a most potent hypothalamic regulator of GH secretion in tilapia; sGnRH and DA both increased GH secretion, although sGnRH elicited considerably greater responses at lower doses. Two forms of GHRH increased GH levels, although the unavailability of the homologous peptide prevented an accurate evaluation of its importance in regulating GH secretion. The thyroid axis (TRH, TSH, and T3) stimulates both synthesis and release of GH, although TRH did not appear to have a direct effect on the level of the pituitary.

Isolation of growth hormone and in vitro translation of mRNA isolated from pituitaries of the gilthead sea bream Sparus aurata

Growth hormone (GH) polypeptide was purified from pituitary glands of the gilthead sea bream (Sparus aurata) by a two-step procedure involving gel filtration on Sephadex G-100 and reverse-phase high-performance liquid chromatography (rpHPLC). At each stage of purification, fractions were monitored by SDS-polyacrylamide gel electrophoresis (SDS-PAGE) and by immunoblotting using anti-bonito GH antiserum. The molecular weight of the sea bream GH was estimated by SDS-PAGE to be 21 kDa when electrophoresed in the absence of beta-mercaptoethanol (nonreduced conditions) and 22 kDa when electrophoresed under reduced conditions (in the presence of 1% beta-mercaptoethanol). Pituitary RNA was used to direct cell-free translation. When specific immunoisolation from 35S-labeled proteins was conducted, using antisera against Sparus or tilapia GH, a larger prehormone was immunoprecipitated. The size of the pre-GH was estimated to be 27-28 kDa under reduced conditions and 26-27 kDa under nonreduced conditions, in agreement with the calculated molecular weight of Sparus pre-GH of 26,296 based on the deduced amino acid sequence of Sparus GH cDNA. The specificity of the immunoprecipitation reaction was demonstrated by the ability of recombinant tilapia GH to compete with the radioactively labeled translation product. No such competition was found after the addition of BSA. Our results demonstrate that the sea bream GH is similar in its size to other purified fish GHs and provide direct evidence for the synthesis of GH as a prepeptide, thus supporting the conclusions presented earlier by GH cDNA cloning.