Hypothalamic peptides influencing growth hormone secretion in the goldfish,Carassius auratus

In vivo andin vitro techniques were used to examine the influence of various vertebrate peptides on growth hormone (GH) secretion in the goldfish. Tetradecapeptide somatostatin (SRIF-14) was found to inhibit GH secretionin vitro from perifused pituitary fragments, whereas similar concentrations of a salmonid SRIF peptide (sSRIF-25) did not affect GH secretion from the goldfish pituitary fragments. This indicates that SRIF receptors on the goldfish pituitary are very specific for SRIF-14-like peptides. Salmon gonadotropin (GTH)-releasing hormone (sGnRH) was found to elevate serum GH levels in male goldfish. The dopamine antagonist pimozide alone or injected in combination with sGnRH did not influence serum GH levels, although injection of pimozide alone significantly elevated serum GTH levels, in addition to potentiating the effects of sGnRH on GTH secretion. sGnRH stimulated GH secretion from goldfish pituitary fragmentsin vitro, indicating that sGnRH acts directly at the level of the pituitary to stimulate GH secretion in the goldfish. These results suggest that GnRH may also function as a GH-releasing factor in the goldfish, although the release-inhibitory factors for GH and GTH secretion do appear to be separate and distinct. Two human GH-releasing hormone (hGHRH) peptides were found to be ineffective in altering GH secretionin vitro from the perifused pituitary fragments. Consequently, a role for a mammalian GHRH-like peptide in the hypothalamic regulation of GH secretion in the goldfish remains questionable.     

Lack of growth hormone-releasing peptide-6 action on in vivo and in vitro growth hormone secretion in sexually immature grass carp (Ctenopharyngodon idellus)

Studies in mammals have shown that synthetic Met-enkephalin derivatives, called growth hormone-releasing peptides (GHRPs), stimulate growth hormone (GH) release. In the present study, GHRP-6 action on GH secretion was examined in vivo and in vitro in sexually immature grass carp. GHRP-6 injected intraperitoneally had no influences on serum GH levels in juvenile grass carp. Following intraperitonal injection of GHRP-6 and dopamine (DA) or cysteamine hydrochloride (CSH), alone and in combination into juvenile grass carp, DA and CSH were effective in elevating serum GH levels, but GHRP-6 was not effective in this respect; in addition, the synergistic action of GHRP-6 and DA or CSH on GH secretion was not seen. In this work, we had adapted and validated a perifusion system and a culture system for GH regulation studies. In a perifusion system, GHRP-6 (1000 to 0.1 nM), GHRP-6 (0.1 to 1000 nM), GHRP-6 (1 μM), and Hexarelin (an analog of GHRP, 1 μM) had no action on GH release from juvenile grass carp pituitary fragments or cells. Under static incubation conditions, GHRP-6 was inactive on GH release from juvenile grass carp pituitary fragments after 1 h and 6 h incubation, but human growth hormone-releasing hormone (hGHRH; 1 to 100 nM) as positive control could stimulate GH release in a dose-dependent manner. Furthermore, when GHRP-6 (100 nM) in combination static incubation with neuropeptides [e.g., hGHRH (100 nM), salmon gonadotropin-releasing hormone analogue (sGnRH-A) (100 nM), or D-Ala⁶,Pro⁹-NEt-luteinizing hormone-releasing hormone (D-Ala⁶,Pro⁹-NEt-LHRH, LHRH-A) (100nM)], GHRP-6 did not strengthen GH secretion actions of neuropeptides, and at the same time neuropeptides also did not modify the effects of GHRP-6 on GH secretion. The present results obtained using in vivo and in vitro techniques adapted for GH regulation studies show that GHRP-6 does not function as a GH-releasing factor in juvenile grass carp as it does in tilapia, amphibians, chickens, and mammals.     

Channel catfish, Ictalurus punctatus, leukocytes secrete immunoreactive adrenal corticotropin hormone (ACTH)

The interrelationships between the neuroendocrine and immune systems are becoming more understood, at least in mammalian systems. The most characterized of these relationships is that of hormonal signaling within the hypothalamo-pituitary-adrenal (HPA) axis. CNS-perceived signals stimulate the release of corticotropin releasing hormone (CRH) which in turn stimulates the release of pituitary corticotropin (ACTH) and ultimately the release of adrenal-cortex-derived corticosteroids. We demonstrate that channel catfish peripheral blood mononuclear cells, a channel catfish B-cell line (1G8) and a T-cell line (28S.1), constitutively and in response to CRF, secrete a molecule that is reactive with a mammalian RIA for ACTH (irACTH). The T-cell line was the most responsive to CRH and may provide a valuable model for understanding the interrelationships between the neuroendocrine and immune systems in lower vertebrates. Lymphoid derived ACTH, or ACTH-like products, in fish, as well as higher vertebrates, may represent a paracrine or autocrine control on lymphocyte function and immune regulation.     

Regulation of the interrenal of fishes: non-classical control mechanisms

The regulation of the interrenal of teleostean fishes is reviewed from the perspective of non-classical control mechanisms and new evidence is presented suggesting gonadotropic control of the interrenal. Cortisol secretion by the interrenal, in addition to regulation by ACTH, appears to be mediated by other hormones. Physiologically relevant, direct control of interrenal function by hydromineral factors is unclear.In vitro experiments with interrenals of coho salmon (Oncorhynchus kisutch) indicate that salmon gonadotropin is extremely corticotropic and both ACTH and gonadotropin stimulate the secretion of large quantities of androstenedione from the interrenal.     

Regulation of interrenal function in freshwater and sea water adapted tilapia (Oreochromis mossambicus)

In teleosts, cortisol is one of the key factors regulating the adaptation to environmental challenges, such as salinity changes. This paper compares interrenal function between fully adapted freshwater (FW) and sea water (SW) specimens of the euryhaline teleost Oreochromis mossambicus (tilapia), combining morphometric and biosynthetic approaches. Interrenal tissue and two tissues producing interrenal secretagogues (ACTH and ANP; atrial natriuretic peptide) were studied. The results demonstrate that sea water adaptation concurs with a sustained stimulation of the interrenal cells, as evidenced by a marked hyperplasia of the cells and the higher initial ex vivo cortisol release in seawater adapted tilapia. This difference was not reflected in ultrastructural differences in the pituitary corticotropes. Plasma ACTH levels were also similar in FW and SW adapted tilapia. Moreover, in vitro data indicate that the ACTH sensitivity of the interrenal cells of both groups was also similar. A second potential interrenal secretagogue (ANP) has recently been implicated in teleost ionic regulation during salinity changes. However, plasma immunoreactive ANP levels and in vitro production of the hormone were also indistinguishable between FW and SW tilapia. ANP pretreatment of tilapia head kidneys in vitro strongly inhibited the response to ACTH, an effect previously undocumented for teleosts. Whereas the sustained stimulation of the interrenal cells under sea water conditions corroborates results obtained with other teleost species, thereby supporting a hypoosmoregulatory role for cortisol, it also is evident that notable species differences exist regarding the regulation of the interrenal gland under these conditions.     

Immunohistochemical investigation of the pituitary of the sturgeon (Acipenser baeri,Chondrostei)

An immunohistochemical study of the sturgeon (Acipenser baeri) pituitary was undertaken using antisera directed against hormones from various classes of vertebrates, including the only pituitary hormone available from sturgeon, gonadotrophin. A positive reaction was obtained after application of antisera towards the following hormones 1–24 synthetic ACTH (1-24 ACTH), melanophore stimulating hormone (MSH), ovine prolactin (oPRL), ovine growth hormone (oGH), salmon growth hormone (sGH), carp gonadotrophin (cGTH) and its beta subunit (cGTH), sturgeon gonadotrophin (aciGTH), carp thyrotrophin (cTSH) and subunit of the human thyrotrophin (hTSH). The results demonstrate that, in general, the sturgeon pituitary resembles that of teleosts as regards the distribution of the different cell types: ACTH and PRL cells in the rostral pars distalis, GTH, TSH and GH cells in the proximal pars distalis and MSH and PAS-cells in pars intermedia. In addition to the topographical organization of the sturgeon pituitary, this study provides data on the immunological relationships between sturgeon pituitary hormones and those of other vertebrates.     

Studies on the mode of action of neuropeptide Y (NPY) on maturational gonadotropin (GtH) secretion from perifused rainbow trout pituitary glands

The action of neuropeptide Y (NPY) and gonadotropin releasing hormone (s-GnRH) have been compared on the release of gonadotropin (GtH) by perifused rainbow trout pituitary glands sampled from freshly ovulated female rainbow trout. We have already demonstrated that NPY stimulated the secretion of GtH in vitro. The pituitary responses to s-GnRH and NPY were similar either after repeated 10 min infusions or a one hour prolonged application. In both cases, after the first application, the pituitary did not responded to subsequent secretagogues stimulations, and appeared to be desensitized. The stimulatory action of s-GnRH was partly inhibited (60%) by LH-RH antagonist (DpGlu¹, DPhe², DTrp^3–6) LH-RH, which completely inhibited the response to NPY in perifused pituitary glands sampled from freshly ovulated females, but did not modify the response of pituitaries taken from vitellogenic animals in which NPY induced a transient inhibition of the GtH secretion. These results may indicate that the mode of action of NPY would differ between vitellogenic and matured animals. NPY also stimulated the GtH secretion from perifused pituitary dispersed cells prepared from pituitaries taken from freshly ovulated rainbow trout, indicating that NPY may act directly on the pituitary gonadotropic cells to stimulate GtH secretion.     

Dopamine functions as a growth hormone-releasing factor in the goldfish,Carassius auratus

In vivo and in vitro approaches have been used to examine the role of dopamine (DA) as a growth hormone (GH)-releasing factor in the goldfish. DA stimulated GH release from perifused pituitary fragments of goldfish in a dose-dependent manner. The GH-releasing effect of DA was seasonal, being the highest in sexually regressed fish, intermediate in recrudescent fish, and the lowest in sexually mature (prespawning) fish. The GH response to DA was blocked by the D1 antagonist (+)SCH23390, confirming the involvement of D1 receptors in DA-stimulated GH release. In studies using static incubation of pituitary cells, somatostatin, a known physiological GH-release inhibitor in the goldfish, abolished the GH response to DA. Intraperitoneal injection of apomorphine, a non-selective DA agonist, also increased the plasma GH levels and enhanced the linear body growth of goldfish. These results strongly suggest that DA, by acting through DA D1 receptors, functions as a GH-releasing factor in the goldfish.     

Tumors of the hypophysis as the cause of both Cushing's syndrome and diabetes insipidus in dogs]

Invasive tumors of the pituitary gland associated with Cushing's disease and diabetes insipidus are described in ten dogs. All patients showed typical clinical symptoms including polyuria/polydipsia and acanthosis nigricans. All tumors led to compression und partial destruction of the posterior lobe of the pituitary gland and the infundibular stalk; two of them infiltrated the hypothalamic region. All tumors produced ACTH and caused hyperadrenocorticism. In six cases, additional expression of beta-lipoprotein and MSH were found: in pars intermedia adenomas many cells stained strongly for MSH and/or beta-lipoprotein, whereas in tumors of the pars distalis only occasional cells stained positive. The purpose of the present study was to describe the neuropathological findings and the immunohistochemistry of hormone excretion in pituitary tumors in dogs resulting in Cushing's disease associated with D.i., to review the literature and to discuss the pathogenesis.     

Direct actions of serotonin on gonadotropin-II and growth hormone release from goldfish pituitary cells: interactions with gonadotropin-releasing hormone and dopamine and further evaluation of serotonin receptor specificity

In this study, the direct actions of serotonin (5HT) on gonadotropin (GTH)-II and growth hormone (GH) release in the goldfish were tested at the pituitary cell level. 5HT (10 nM - 10 µM) stimulated GTH-II but inhibited GH release from perifused goldfish pituitary cells in a dose-dependent manner. The minimal effective dose of 5HT tested to suppress basal GH secretion (10 nM) was 10-fold lower than that to stimulate GTH-II release (100 nM). The GTH-II releasing effect of 5HT was abolished by repeated 5HT treatment (10 µM) whereas the corresponding inhibition on GH release was unaffected. These results suggest that 5HT receptors on goldfish gonadotrophs and somatotrophs exhibit intrinsic differences in terms of sensitivity to stimulation and resistance to desensitization. Salmon GTH-releasing hormone (sGnRH, 100 nM) stimulated GTH-II and GH release from goldfish pituitary cells. The GTH-II releasing action of sGnRH was unaffected by simultaneous treatment of 5HT (1 µM). However, the corresponding GH response to sGnRH (100 nM) was inhibited. In the goldfish, dopamine is known to stimulate GH release through activation of pituitary D1 receptors. In the present study, the GH-releasing action of dopamine (1 µM) and the D1 agonist SKF38393 (1 µM) was significantly reduced by 5HT (1 µM). To examine the receptor specificity of 5HT action, the effects of 5HT1 and 5HT2 analogs on GTH-II and GH release were tested in goldfish pituitary cells. The 5HT1 agonist 8OH DPAT (0.1 and 1µM) and 5HT2 agonist methyl 5HT (0.1 1µM) mimicked the GTH-II releasing effect of 5HT. The 5HT1 agonist 8OH DPAT (0.1 and 1µM) also stimulated GH release but the 5HT2 agonist methyl 5HT (0.1 and 1µM) was inhibitory to basal GH secretion. In addition, 5HT (1µM) -stimulated GTH-II release was abolished by the 5HT1 antagonist methiothepin (10µM) and 5HT2 antagonist mianserin (10µM). Similarly, the inhibitory action of 5HT (1µM) on basal GH release was blocked by the 5HT2 antagonist mianserin (10µM). The 5HT1 antagonist methiothepin (10µM) was not effective in this regard. These results, taken together, indicate that 5HT exerts its regulatory actions on GTH-II and GH release in the goldfish directly at the pituitary cell level, probably through interactions with other regulators including sGnRH and dopamine. The GTH-II releasing action of 5HT is mediated through 5HT2 and possibly 5HT1 receptors. The inhibition of 5HT on basal GH release is mediated through 5HT2 receptors only. Apparently, 5HT1 receptors are not involved in this inhibitory action. In this study, a paradoxical stimulatory component of 5HT on GH release by activating 5HT1 receptors is also implicated.     

GnIH多肽对半滑舌鳎(Cynoglossus semilaevis)下丘脑生殖相关基因表达的影响

本研究首次通过下丘脑离体孵育的方法研究促性腺激素抑制激素(Gonadotropin-inhibitory hormone,GnIH)多肽对半滑舌鳎(Cynoglossus semilaevis)下丘脑中生殖相关基因的表达调控.研究结果显示,tsGnIH-1促进了gnrh2和gnih的表达,对gnrh3和kiss2的表达无影响;tsGnIH-2抑制了gnrh3的表达,对gnrh2、kiss2和gnih的表达无影响.GnIH多肽对生殖相关基因的不同调控表明同一前体蛋白编码的不同GnIH多肽在生殖调控中的作用不尽相同.本研究结果增加了对GnIH参与鱼类生殖调控机制的认识,为深入研究奠定了基础.     

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

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

Mechanisms of action of pituitary adenylate cyclase-activating polypeptide (PACAP) on growth hormone release from dispersed goldfish pituitary cells

The mechanisms of pituitary adenylate cyclase activating polypeptide (PACAP) action on goldfish growth hormone (GH) release were investigated by examining GH release responses from dispersed goldfish pituitary cells to a synthetic mammalian (m)PACAP₃₈ peptide. It was established that GH release stimulated by 2-h exposure to mPACAP₃₈ was concentration-dependent, attenuated by the PACAP receptor antagonist mPACAP_6–38, and subject to neuroendocrine modulation by somatostatin. Maximal mPACAP₃₈-stimulated GH release was not additive to the responses elicited by either the adenylate cyclase activator forskolin or the cyclic (c)AMP analog 8-bromo-cAMP. The GH responses to mPACAP₃₈, forskolin and 8-bromo-cAMP, either alone or in combination, were abolished by H89, a protein kinase A (PKA) inhibitor. SQ22536, an adenylate cyclase inhibitor, attenuated forskolin- and mPACAP₃₈-stimulated GH release. In contrast, mPACAP₃₈-stimulated GH release were additive to the responses to two protein kinase C (PKC) activators and unaffected by two PKC inhibitors. These results suggest that the stimulatory action of PACAP on GH secretion is mediated through a cAMP- / PKA-dependent mechanism, whereas the involvement of PKC appears unlikely. The ability of mPACAP₃₈ to further enhance maximal GnRH (PKC)-dependent GH release, but not dopamine D1 agonist (PKA)-dependent GH secretion, is consistent with this hypothesis. A possible involvement of Ca²⁺ in PACAP action is also suggested. Two inhibitors of voltage-sensitive Ca²⁺ channel reduced the GH responses to mPACAP₃₈ in static incubation; conversely, mPACAP₃₈ increased intracellular [Ca²⁺] in identified, single goldfish somatotropes.     

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

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

Hypothalamic control of prolactin release in the rainbow trout,Salmo gairdneri: in vitro studies

Hypothalamic control of prolactin (PRL) release in immature rainbow troutSalmo gairdneri was investigated using anin vitro perifusion system of the rostral pars distalis. Hypothalamic extract of trout induced a dose-dependent stimulation of PRL release. A similar effect was observed when infusing the medium from a 24h static incubation of the hypothalamus. Extracts from different control tissues (muscle, liver, gut) did not changein vitro release, thus confirming the specificity of this stimulatory effect. Hypothalamic extract from adult male rat, known to contain PRL release inhibiting factors, stimulatedin vitro PRL secretion in rainbow trout. This suggests that PRL cells are predominantly influenced by PRL releasing factors. Measurement of TRH and serotonin content in trout hypothalamus indicated consistent physiological levels of these two factors. HPLC studies of hypothalamic extract showed that immunoreactive — TRH eluted at the same place as labelled TRH standard. Moreover, pizotifen, a serotonin antagonist, partially inhibited the stimulation observed with trout hypothalamic extract. These results suggest that, in immature rainbow trout, PRL release is under stimulatory hypothalamic control and that serotonin and probably TRH play a major role in this control.     

Pituitary and plasma AVT content in the flounder (Platichthys flesus)

Radioimmunoassay measurement of pituitary AVT content and plasma AVT concentration indicated comparable levels in fully adapted sea water (SW) and fresh water (FW) flounders. Circulating AVT represented less than 0.1% of the pituitary AVT reserve. The urophysis contained AVT but the total content was only 2 or 3 fold that of circulating AVT. In fish adapted to hypertonic media, there was a close correlation between plasma AVT concentration and plasma Na⁺ concentration or osmolality. The present study examined the effects of acute osmotic challenge, associated with FW to SW transfer, and the influence of extracellular fluid volume status on AVT secretion. Short-term transfer of fish from FW to SW (up to 3 days) did not evoke a clear change in plasma AVT levels, though pituitary content was reduced at 24 h. During the first 3 days after transfer to SW, only small increments in plasma tonicity were apparent. The sensitivity of AVT secretion to osmotic stimuli may only be expressed when plasma osmolality has exceeded a specific threshold, which was probably not reached in these transfer studies. Fish in hypotonic media showed no relationship between plasma osmolality/tonicity and plasma AVT concentration. Acute extracellular fluid volume expansion of SW adapted fish also abolished the normally observed relationship between plasma osmolality and AVT concentration in these hypertonic media fish. This trend indicates that volume status may modulate the sensitivity of AVT secretion to osmotic stimuli as occurs in tetrapods.     

Activin stimulates goldfish FSH biosynthesis by enhancing FSHβ promoter activity

Follicle-stimulating hormone (FSH) and luteinizing hormone (LH) play critical roles in controlling vertebrate gonadal development and function. Activin, a dimeric growth factor initially identified in the gonads, is important in the differential regulation of the two gonadotropins in mammals. Using goldfish as a model, we have demonstrated that activin stimulates FSHβ but suppresses LHβ expression. The present study demonstrated that the 5′-flanking region of goldfish FSHβ gene is functional in the mouse gonadotrope cell line, LβT2 cells. Similar to its effect on the cultured pituitary cells, activin stimulated FSHβ promoter activity in the LβT2 cells and the effect could be blocked by its binding protein follistatin. Follistatin also significantly suppressed the basal FSHβ promoter activity, suggesting secretion of endogenous activin by the LβT2 cells. Further characterization of the cis-regulatory elements responsible for activin stimulation is now under way in our laboratory.     

Rat Ghrelin Stimulates GH Release and GH mRNA Expressionin the Pituitary of Orange-spotted Grouper, <Emphasis Type="BoldItalic">Epinephelus Coioides</Emphasis>

Ghrelin was recently demonstrated as an endogenous ligand of the growth hormone (GH) secretagogue receptor (GHS-R), which could promote the release of GH in mammal significantly. The present study conducted to determine whether ghrelin stimulate the release and synthesis of GH in orange-spotted grouper (Epinephelus coioides). Rat ghrelin was incubated with the pituitary fragments of grouper in static culture system. The culture medium was collected at 1, 6, 12, 18 and 24 h after incubation to detect the contents of GH by homologous radioimmunoassay. The level of GH mRNA in the pituitary fragments was measured by a sensitive chemiluminescent ribonuclease protection assay. The results showed that rat ghrelin not only stimulated the release of GH but also augmented the GH mRNA level in grouper. It suggested that the ghrelin-like peptide and the GHS-R involved in the regulation of GH synthesis and release in grouper. The present study would provide a better understanding of the regulatory mechanism of GH release in marine fish.     

A drop in ambient temperature results in a transient reduction of interrenal ACTH responsiveness in the gilthead sea bream (Sparus aurata, L.)

This study investigated the effects of a drop in water temperature (18 °C to 9 °C in 24 h) on the pituitary and interrenal hormones of the gilthead sea bream Sparus aurata. The in vitro sensitivity of the interrenal tissue to ACTH, plasma levels of cortisol, ACTH, -MSH, GH, glucose, lactate and ions were determined. In vitro ACTH, stimulated the release of cortisol from isolated interrenal glands from control gilthead sea bream in a concentration dependent fashion. However, the interrenal cells were less sensitive to ACTH as soon as 24 h following the onset of the temperature drop. At this time, plasma cortisol and ACTH levels were raised, and plasma GH concentrations were decreased, whereas no significant changes were found in plasma -MSH. After 96 h plasma ACTH levels had recovered whereas plasma cortisol levels were still higher than controls after 8 days of the beginning of the experiment. Interrenal sensitivity had recovered after 8 days. The results may help to clarify the relationship between the stress response and the aetiology of the winter syndrome in sea bream.     

Involvement of Gonadotropin-Releasing Hormone in Thyroxine Release in Three different forms of Teleost Fish: Barfin Founder, Masu Salmon and Goldfish

Effects of gonadotropin-releasing hormone (GnRH) on thyroxine (T₄) release in vivo and in vitro were studied in barfin flounder Verasper moseri, masu salmon Oncorhynchus masou and goldfish Carassius auratus. Seabream GnRH (sbGnRH) at a dose of 200 ng/50 g body weight (BW) significantly increased plasma T₄ levels 1 h after the in vivo injection in the barfin flounder, but thereafter the levels normalized. Salmon GnRH (sGnRH) significantly increased plasma T₄ levels l h after the injection with a significant return to initial levels in male masu salmon and male goldfish. In contrast, sGnRH and cGnRH-II in barfin flounder, and cGnRH-II in male masu salmon and male goldfish were not effective in stimulating T₄ release. To clarify direct involvement of GnRH in T₄ release, dissected lower jaw including scattered thyroid follicles was incubated with sbGnRH (1 μg/well) in barfin flounder, and with two doses (0.1 and 1 μg/well) of sGnRH in masu salmon and goldfish in vitro. T₄ concentrations of control were stable during 24 h. Incubation of lower jaw with high dose (1 μg/well) of GnRH significantly (P<0.05) increased T₄ concentrations of incubation medium at 1 h in all experimental fishes. These results indicate that direct stimulation of T₄ secretion by GnRH occurs widely in teleost fish.