Somatostatins and their receptors in fish

Loss of function in somatostatin receptor 5 has no impact on the growth of medaka fish due to compensation by the other paralogs

Somatic growth in vertebrates is regulated endocrinologically by the somatotropic axis, headed by the growth hormone (GH) and the insulin growth factor-I (IGF-I). Somatostatin (Sst), a peptide hormone synthesized in the hypothalamus, modulates GH actions through its receptors (Sstr). Four Sstr subtypes (Sstr 1-3 and 5) have been identified in teleosts. However, little is known about whether they have a specific function or tissue expression. The aim of this study was to determine the role of sstr2 and sstr5 in the growth of the medaka (Oryzias latipes). The assessed expression pattern across diverse tissues highlighted greater prevalence of sstr1 and sstr3 in brain, intestine and muscle than in pituitary or liver. The expression of sstr2 was high in all the tissues tested, while sstr5 was predominantly expressed in the pituitary gland. A CRISPR/Cas9 sstr5 mutant with loss of function (sstr5-/-) was produced. Assessment of sstr5-/- indicated no significant difference with the wild type regarding growth parameters such as standard length, body depth, or peduncle depth. Furthermore, the functional loss of sstr5 had no impact on the response to a nutritional challenge. The fact that several sstr subtypes were upregulated in different tissues in sstr5-/- medaka suggests that in the mutant fish, there may be a compensatory effect on the different tissues, predominantly by sstr1 in the liver, brain and pituitary, with sstr2 being upregulated in pituitary and liver, and sstr3 only presenting differential expression in the brain. Analysis of the sstr subtype and the sstr5-/- fish showed that sstr5 was not the only somatostatin receptor responsible for Sst-mediated Gh regulation.

Neuropeptides and hormones in hypothalamus-pituitary axis of Chinese sturgeon (Acipenser sinensis)

The hypothalamus and pituitary serve as important neuroendocrine center, which is able to secrete a variety of neuropeptides and hormones to participate in the regulation of reproduction, growth, stress and feeding in fish. Chinese sturgeon is a basal vertebrate lineage fish with a special evolutionary status, but the information on its neuroendocrine system is relatively scarce. Using the transcriptome data on the hypothalamus-pituitary axis of Chinese sturgeon as reference, we found out 46 hypothalamus neuropeptide genes, which were involved in regulation of reproduction, growth, stress and feeding. The results of sequence alignment showed that the neuroendocrine system of Chinese sturgeon evolves slowly, which confirms that Chinese sturgeon is a species with a slow phenotypic evolution rate. In addition, we also isolated six pituitary hormones genes from Chinese sturgeon, including reproductive hormones: follicle-stimulating homone (FSH) and luteinizing hormone (LH), growth-related hormones: growth hormone (GH)/prolactin (PRL)/somatolactin (SL), and stress-related hormone gene: proopiomelanocortin (POMC). Similar to teleost, immunostaining localization analysis in Chinese sturgeon pituitary showed that LH and FSH were located in the pituitary proximal pars distalis, SL was located in the pituitary rostral pars distalis, and POMC was located in the pituitary pars intermedia and pituitary rostral pars distalis. This study will give a contribution to enrich our information on the neuroendocrine system in Chinese sturgeon.

Differential expression of five prosomatostatin genes in the central nervous system of the catshark Scyliorhinus canicula

Five prosomatostatin genes (PSST1, PSST2, PSST3, PSST5, and PSST6) have been recently identified in elasmobranchs (Tostivint et al., General and Comparative Endocrinology, 2019, 279, 139-147). In order to gain insight into the contribution of each somatostatin to specific nervous systems circuits and behaviors in this important jawed vertebrate group, we studied the distribution of neurons expressing PSST mRNAs in the central nervous system (CNS) of Scyliorhinus canicula using in situ hybridization. Additionally, we combined in situ hybridization with tyrosine hydroxylase (TH) immunochemistry for better characterization of PSST1 and PSST6 expressing populations. We observed differential expression of PSST1 and PSST6, which are the most widely expressed PSST transcripts, in cell populations of many CNS regions, including the pallium, subpallium, hypothalamus, diencephalon, optic tectum, midbrain tegmentum, and rhombencephalon. Interestingly, numerous small pallial neurons express PSST1 and PSST6, although in different populations judging from the colocalization of TH immunoreactivity and PSST6 expression but not with PSST1. We observed expression of PSST1 in cerebrospinal fluid-contacting (CSF-c) neurons of the hypothalamic paraventricular organ and the central canal of the spinal cord. Unlike PSST1 and PSST6, PSST2, and PSST3 are only expressed in cells of the hypothalamus and in some hindbrain lateral reticular neurons, and PSST5 in cells of the region of the entopeduncular nucleus. Comparative data of brain expression of PSST genes indicate that the somatostatinergic system of sharks is the most complex reported in any fish.

Characterization and distribution of GHRH, PACAP, TRH, SST and IGF1 mRNAs in the green iguana

The somatotropic axis (SA) regulates numerous aspects of vertebrate physiology such as development, growth, and metabolism and has influence on several tissues including neural, immune, reproductive and gastric tract. Growth hormone (GH) is a key component of SA, it is synthesized and released mainly by pituitary somatotrophs, although now it is known that virtually all tissues can express GH, which, in addition to its well-described endocrine roles, also has autocrine/paracrine/intracrine actions. In the pituitary, GH expression is regulated by several hypothalamic neuropeptides including GHRH, PACAP, TRH and SST. GH, in turn, regulates IGF1 synthesis in several target tissues, adding complexity to the system since GH effects can be exerted either directly or mediated by IGF1. In reptiles, little is known about the SA components and their functional interactions. The aim of this work was to characterize the mRNAs of the principal SA components in the green iguana and to develop the tools that allow the study of the structural and functional evolution of this system in reptiles. By employing RT-PCR and RACE, the cDNAs encoding for GHRH, PACAP, TRH, SST and IGF1 were amplified and sequenced. Results showed that these cDNAs coded for the corresponding protein precursors of 154, 170, 243, 113, and 131 amino acids, respectively. Of these, GHRH, PACAP, SST and IGF1 precursors exhibited a high structural conservation with respect to its counterparts in other vertebrates. On the other hand, iguana's TRH precursor showed 7 functional copies of mature TRH (pyr-QHP-NH₂), as compared to 4 and 6 copies of TRH in avian and mammalian proTRH sequences, respectively. It was found that in addition to its primary production site (brain for GHRH, PACAP, TRH and SST, and liver for IGF1), they were also expressed in other peripheral tissues, i.e. testes and ovaries expressed all the studied mRNAs, whereas TRH and IGF1 mRNAs were observed ubiquitously in all tissues considered. These results show that the main SA components in reptiles of the Squamata Order maintain a good structural conservation among vertebrate phylogeny, and suggest important physiological interactions (endocrine, autocrine and/or paracrine) between them due to their wide peripheral tissue expression.

Effects of dietary vitamin E on growth performance as well as intestinal structure and function of channel catfish (Ictalurus punctatus, Rafinesque 1818)

To evaluate the impact of dietary vitamin E supplementation on growth performance, the intestinal structure and function of channel catfish (Ictalurus punctatus, Rafinesque 1818) was investigated. A total of 900 healthy channel catfish (weight, 5.20±0.15 g) were divided into four groups, which received experimental diets with different vitamin E content (0, 50, 100 or 1,000 mg/kg). At the end of the feeding trial (after 15 weeks), the growth and gut performance of the animals was determined. The digestive enzyme activity in hepatopancreas and gut was also detected. In addition, the height of intestinal fold, the thickness of the mucous membrane and the number of somatostatin-positive cells was examined by histological analysis. Dietary vitamin E supplementation at 50 and 100 mg/kg significantly improved the growth and gut performance, which also increased the activity of several digestive enzymes compared to that in animals without vitamin E supplementation (P<0.05). In addition, vitamin E supplementation also significantly increased the height of intestinal fold and the thickness of the mucous membrane (P<0.05). Fish with dietary vitamin E supplementation at appropriate doses also had more somatostatin-positive cells in than those without vitamin E supplementation (P<0.05). In conclusion, dietary vitamin E supplementation at 50 and 100 mg/kg was shown to improve the growth performance as well as intestinal structure and function of channel catfish.

Molecular cloning of the cDNAs encoding three somatostatin variants in the dogfish (Scylorhinus canicula)

It has been recently shown that the somatostatin gene family was likely composed of at least three paralogous genes in the common ancestor of all extant jawed vertebrates. These three genes, namely SS1, SS2 and SS5, are thought to have been generated through the two rounds of whole-genome duplications (2R) that took place early during the vertebrate evolution. In the present study, we report the cloning of three distinct somatostatin cDNAs from the dogfish Scylorhinus canicula, a member of the group of cartilaginous fish. We decided to call these cDNAs, at least provisionally, SSa, SSb and SSc, respectively. Two of them, SSa and SSb, encode proteins that both contain the same tetradecapeptide sequence at their C-terminal extremity (AGCKNFFWKTFTSC). This putative peptide is identical to that generated by the SS1 gene in other vertebrate species. The last cDNA, SSc, encodes a protein that contains at its C-terminal extremity the same peptide sequence as that generated by the SS2 gene in teleosts (APCKNFFWKTFTSC). Phylogenetic analysis showed that the SSa and SSc genes likely correspond to the dogfish counterparts of the SS1 and SS2 genes, respectively. In contrast, the phylogenetic status of the SSb gene is less clear. Several lines of evidence suggest that it could correspond to the SS5 gene, but this view will need to be confirmed, for example by synteny analysis. Finally, RT-PCR analysis revealed that SSa, SSb and SSc genes are differentially expressed in dogfish tissues, suggesting that the corresponding peptides may exert distinct functions.

Stimulatory effects of neuropeptide Y on the growth of orange-spotted grouper (Epinephelus coioides)

Neuropeptide Y (NPY) is a member of the pancreatic polypeptide family which is a potent orexigenic peptide known to date in mammals and teleost. This study was carried out to investigate the effects of NPY on food intake and growth of orange-spotted grouper (Epinephelus coioides). Synthetic grouper NPY (gNPY) was given orally at the dose of 0.5, 1.0 and 2.0 μg/g feed for 50 days, results showed that NPY treatment (1.0 and 2.0 μg/g feed) significantly increased growth rate, weight gain, feed conversion efficiency (FCE) and pituitary growth hormone (GH) mRNA level than the control group (p<0.05). Furthermore, high level secretion of gNPY was expressed and purified in the Pichia pastoris expression system. The bioactivity of recombinant gNPY was confirmed by its ability to up-regulate GH mRNA expression in vivo and in vitro and down-regulate preprosomatostatin I (PSSI) mRNA expression in vivo. These results demonstrate that NPY has stimulatory effects on food intake as well as growth of grouper as in other teleost fish, also indicate that recombinant gNPY from P. pastoris has the same bioactivity as synthetic gNPY and has the potential to be used as a feed additive for both research and aquatic application.

Calcium and other signalling pathways in neuroendocrine regulation of somatotroph functions

Relative to mammals, the neuroendocrine control of pituitary growth hormone (GH) secretion and synthesis in teleost fish involves numerous stimulatory and inhibitory regulators, many of which are delivered to the somatotrophs via direct innervation. Among teleosts, how multifactorial regulation of somatotroph functions are mediated at the level of post-receptor signalling is best characterized in goldfish. Supplemented with recent findings, this review focuses on the known intracellular signal transduction mechanisms mediating the ligand- and function-specific actions in multifactorial control of GH release and synthesis, as well as basal GH secretion, in goldfish somatotrophs. These include membrane voltage-sensitive ion channels, Na(+)/H(+) antiport, Ca(2+) signalling, multiple pharmacologically distinct intracellular Ca(2+) stores, cAMP/PKA, PKC, nitric oxide, cGMP, MEK/ERK and PI3K. Signalling pathways mediating the major neuroendocrine regulators of mammalian somatotrophs, as well as those in other major teleost study model systems are also briefly highlighted. Interestingly, unlike mammals, spontaneous action potential firings are not observed in goldfish somatotrophs in culture. Furthermore, three goldfish brain somatostatin forms directly affect pituitary GH secretion via ligand-specific actions on membrane ion channels and intracellular Ca(2+) levels, as well as exert isoform-specific action on basal and stimulated GH mRNA expression, suggesting the importance of somatostatins other than somatostatin-14.

Regulation of preprosomatostatin 1 (PSS1) gene expression by 17beta-estradiol and identification of the PSS1 promoter region in orange-spotted grouper (Epinephelus coioides)

In the present paper the effects of 17beta-estradiol on the expression of the preprosomatostatin 1 (PSS1) in the orange-spotted grouper hypothalamus and ovary were investigated. Results from in vivo of intraperitoneal injection and in vitro static cultures showed that estradiol increased the mRNA expression of PSS1 gene in both hypothalamus and ovary. To investigate the molecular basis of the estrogen regulation on PSS1 gene expression, we cloned the upstream region of 848bp from the translation initiation codon of the grouper PSS1 gene. The TATA-box and putative transcription factor binding sites were identified using computer analysis. Transient transfections with promoter-luciferase reporter constructs together with hER expression vector were carried out in MCF-7 cell line. The results suggest that the region from -848 to -373bp, containing five putative ERE half sites, may contribute to the promoter activity induced by estradiol. These results represent the first demonstration at the molecular level of the regulation of PSS1 gene by 17beta-estradiol in fish.

Understanding the multifactorial control of growth hormone release by somatotropes: lessons from comparative endocrinology

Control of postnatal growth is the main, but not the only, role for growth hormone (GH) as this hormone also contributes to regulating metabolism, reproduction, immunity, development, and osmoregulation in different species. Likely owing to this variety of group-specific functions, GH production is differentially regulated across vertebrates, with an apparent evolutionary trend to simplification, especially in the number of stimulatory factors governing substantially GH release. Thus, teleosts exhibit a multifactorial regulation of GH secretion, with a number of factors, from the newly discovered fish GH-releasing hormone (GHRH) to pituitary adenylate cyclase-activating peptide (PACAP) but also gonadotropin-releasing hormone, dopamine, corticotropin-releasing hormone, and somatostatin(s) directly controlling somatotropes. In amphibians and reptiles, GH secretion is primarily stimulated by the major hypothalamic peptides GHRH and PACAP and inhibited by somatostatin(s), while other factors (ghrelin, thyrotropin-releasing hormone) also influence GH release. Finally, in birds and mammals, primary control of GH secretion is exerted by a dual interplay between GHRH and somatostatin. In addition, somatotrope function is modulated by additional hypothalamic and peripheral factors (e.g., ghrelin, leptin, insulin-like growth factor-I), which together enable a balanced integration of feedback signals related to processes in which GH plays a relevant regulatory role, such as metabolic and energy status, reproductive, and immune function. Interestingly, in contrast to the high number of stimulatory factors impinging upon somatotropes, somatostatin(s) stand(s) as the main primary inhibitory regulator(s) for this cell type.

Cloning, tissue distribution and effects of food deprivation on pituitary adenylate cyclase activating polypeptide (PACAP)/PACAP-related peptide (PRP) and preprosomatostatin 1 (PPSS 1) in Atlantic cod (Gadus morhua)

Full-length complementary deoxyribonucleic acid sequences encoding pituitary adenylate cyclase activating polypeptide (PACAP)/PACAP-related peptide (PRP) and preprosomatostatin 1 (PPSS 1) were cloned from Atlantic cod (Gadus morhua) hypothalamus using reverse transcription and rapid amplification of complementary deoxyribonucleic acid ends. Semi-quantitative reverse transcriptase polymerase chain reaction shows that PRP/PACAP mRNA and PPSS 1 mRNA are widely distributed throughout cod brain. During development, PRP/PACAP and PPSS 1 were detected at the 30-somite stage and pre-hatching stage, respectively, and expression levels gradually increased up to the hatched larvae. PPSS 1, but not PRP/PACAP, appeared to be affected by food availability during early development. In juvenile cod, PPSS 1 expression levels increased and remained significantly higher than that of control fed fish throughout 30 days of starvation and during a subsequent 10 days refeeding period. In contrast, PRP/PACAP expression levels were not affected by 30 days of food deprivation, but a significant increase in expression levels was observed during the 10 days refeeding period in the experimental food-deprived group as compared to the control fed group. Our results suggest that PRP/PACAP and PPSS 1 may be involved in the complex regulation of growth, feeding and metabolism during food deprivation and refeeding in Atlantic cod.

Changes in brain mRNA levels of gonadotropin-releasing hormone, pituitary adenylate cyclase activating polypeptide, and somatostatin during ovulatory luteinizing hormone and growth hormone surges in goldfish

In goldfish, circulating LH and growth hormone (GH) levels surge at the time of ovulation. In the present study, changes in gene expression of salmon gonadotropin-releasing hormone (sGnRH), chicken GnRH-II (cGnRH-II), somatostatin (SS) and pituitary adenylate cyclase activating polypeptide (PACAP) were analyzed during temperature- and spawning substrate-induced ovulation in goldfish. The results demonstrated that increases in PACAP gene expression during ovulation are best correlated with the GH secretion profile. These results suggest that PACAP, instead of GnRH, is involved in the control of GH secretion during ovulation. Increases of two of the SS transcripts during ovulation are interpreted as the activation of a negative feedback mechanism triggered by high GH levels. The results showed a differential regulation of sGnRH and cGnRH-II gene expression during ovulation, suggesting that sGnRH controls LH secretion, whereas cGnRH-II correlates best with spawning behavior. This conclusion is further supported by the finding that nonovulated fish induced to perform spawning behavior by prostaglandin F2α treatment increased cGnRH-II expression in both forebrain and midbrain, but decreased sGnRH expression in the forebrain.

Somatostatin signaling and the regulation of growth and metabolism in fish

The study of the somatostatins (SS) signaling system in fish has provided important information about the structure, function, and evolution of SSs and their receptors. The SS signaling system elicits widespread biological actions via multiple hormone variants, numerous receptor subtypes, and a variety of signal transduction pathways. SSs alter growth via both direct and indirect actions, including inhibiting growth hormone release at the pituitary, decreasing hepatic GH sensitivity, and lowering plasma IGF-I levels. Metabolism also is significantly influenced by SSs. SSs stimulate the breakdown of energy stores and influences digestion, food intake, nutrient absorption, and food conversion both directly and through the modulation of other hormonal systems. The study of fish, which display a diversity of habitat types and life history forms, reveals that the SS signaling system helps regulate energy partitioning and integrate metabolism with growth and other biological processes.

Distribution of somatostatin-like immunoreactivity in the brain of the caecilian Dermophis mexicanus (Amphibia: Gymnophiona): comparative aspects in amphibians

The organization of the somatostatin-like-immunoreactive (SOM-ir) structures in the brain of anuran and urodele amphibians has been well documented, and significant differences were noted between the two amphibian orders. However, comparable data are not available for the third order of amphibians, the gymnophionans (caecilians). In the present study, we analyzed the anatomical distribution of SOM-ir cells and fibers in the brain of the gymnophionan Dermophis mexicanus. In addition, because of its known relationship with catecholamines in other vertebrates, double immunostaining for SOM and tyrosine hydroxylase was used to investigate this situation in the gymnophionan. Abundant SOM-ir cell bodies and fibers were widely distributed throughout the brain. In the telencephalon, pallial and subpallial cells were labeled, being most numerous in the medial pallium and amygdaloid region. Most of the SOM-ir neurons were found in the preoptic area and hypothalamus and showed a clear projection to the median eminence. Less conspicuously, SOM-ir structures were found in the thalamus, tectum, tegmentum, and reticular formation. Both SOM-ir cells and fibers were demonstrated in the spinal cord. The double-immunohistofluorescence technique revealed that catecholaminergic neurons and SOM-ir cells are largely intermingled in many brain regions but form totally separated populations. Many differences were found between the distribution of SOM-ir structures in Dermophis and that in anurans or urodeles. Some features were shared only with anurans, such as the abundant pallial SOM-ir cells, whereas others were common only to urodeles, such as the organization of the hypothalamohypophysial SOM-ir system. In addition, some characteristics were found only in Dermophis, such as the localization of the SOM-ir spinal cells and the lack of colocalization of catecholamines and SOM throughout the brain. Therefore, any conclusions concerning the SOM system in amphibians are incomplete without considering evidence for gymnophionans.

Neuroendocrine control of growth hormone in fish

The biological actions of growth hormone (GH) are pleiotropic, including growth promotion, energy mobilization, gonadal development, appetite, and social behavior. Accordingly, the regulatory network for GH is complex and includes many endocrine and environmental factors. In fish, the neuroendocrine control of GH is multifactorial with multiple inhibitors and stimulators of pituitary GH secretion. In fish, GH release is under a tonic negative control exerted mainly by somatostatin. Sex steroid hormones and nutritional status influence the level of brain expression and effectiveness of some of these GH neuroendocrine regulatory factors, suggesting that their relative importance differs under different physiological conditions. At the pituitary level, some, if not all, somatotropes can respond to multiple regulators. Therefore, ligand- and function-specificity, as well as the integrative responses to multiple signals must be achieved at the level of signal transduction mechanisms. Results from investigations on a limited number of stimulatory and inhibitory GH-release regulators indicate that activation of different but convergent intracellular pathways and the utilization of specific intracellular Ca(2+) stores are some of the strategies utilized. However, more work remains to be done in order to better understand the integrative mechanisms of signal transduction at the somatotrope level and the relevance of various GH regulators in different physiological circumstances.

Insulin and growth hormone stimulate somatostatin receptor (SSTR) expression by inducing transcription of SSTR mRNAs and by upregulating cell surface SSTRs

This study examined the effects of insulin (INS) and growth hormone (GH) on mRNA and functional expression of somatostatin receptors (SSTRs). Rainbow trout liver was used as a model system to evaluate the direct effects of INS and GH on mRNA expression of three SSTR subtypes characterized previously from this species: SSTR1A, SSTR1B, and SSTR2. INS and GH directly stimulated steady-state levels of all SSTR mRNAs in a concentration- and time-dependent manner; however, the pattern of expression was hormone and SSTR subtype specific. INS stimulated SSTR2 expression to a greater extent than SSTR1A or SSTR1B expression, whereas GH stimulated SSTR2 and SSTR1B expression to a similar extent, with SSTR2 and SSTR1B expression being more responsive to GH than SSTR1A. Whether INS- or GH-stimulated SSTR expression resulted from altered rates of transcription and/or changes in mRNA stability also was investigated. Formation of nascent SSTR transcripts in nuclei isolated from rainbow trout hepatocytes was significantly stimulated by INS and GH. Neither INS nor GH, however, affected the stability of SSTR mRNAs. Functional expression of SSTRs was studied in Chinese hamster ovary (CHO-K1) cells stably transfected with SSTR1A or SSTR1B. Surface expression of functional SSTRs was stimulated by INS and GH. These findings indicate that INS and GH stimulate SSTR expression by regulating transcription of SSTR mRNAs and by increasing functional SSTRs on the cell surface, and they suggest that regulation of SSTRs may be important for the coordination of growth, development, and metabolism of vertebrates.

Feedback regulation of growth hormone synthesis and secretion in fish and the emerging concept of intrapituitary feedback loop

Growth hormone (GH) is known to play a key role in the regulation of body growth and metabolism. Similar to mammals, GH secretion in fish is under the control of hypothalamic factors. Besides, signals generated within the pituitary and/or from peripheral tissues/organs can also exert a feedback control on GH release by effects acting on both the hypothalamus and/or anterior pituitary. Among these feedback signals, the functional role of IGF is well conserved from fish to mammals. In contrast, the effects of steroids and thyroid hormones are more variable and appear to be species-specific. Recently, a novel intrapituitary feedback loop regulating GH release and GH gene expression has been identified in fish. This feedback loop has three functional components: (i) LH induction of GH release from somatotrophs, (ii) amplification of GH secretion by GH autoregulation in somatotrophs, and (iii) GH feedback inhibition of LH release from neighboring gonadotrophs. In this article, the mechanisms for feedback control of GH synthesis and secretion are reviewed and functional implications of this local feedback loop are discussed. This intrapituitary feedback loop may represent a new facet of pituitary research with potential applications in aquaculture and clinical studies.

Polygenic expression of somatostatin in orange-spotted grouper (Epinephelus coioides): molecular cloning and distribution of the mRNAs encoding three somatostatin precursors

In the present study, three preprosomatostatin (PSS) cDNAs were characterized from hypothalamus of orange-spotted grouper Epinephelus coioides. The first cDNA encodes a 123-amino acid protein (PSSI) that contains the SS14 sequence at its C-terminal extremity and that is identical to that of PSSI of human and other vertebrates. The second cDNA encodes a 127-amino acid protein (PSSII) that contains the SS28 sequence with [Tyr7, Gly10]-SS14 at its C-terminus. The third cDNA encodes a 110-amino acid protein (PSSIII) that contains the somatostatin variant [Pro2]-SS14 at its C-terminal extremity. All these three PSS mRNAs were expressed in brain and pituitary with different mRNA levels. In peripheral tissues, PSSII was more widely distributed than PSSI and PSSIII. High mRNA levels of PSS were found in stomach, intestine and ovary. PSS mRNAs were detected throughout embryogeny and early larval development. Its levels increased with the embryonic development and maintained a higher level during larva developing. The mRNA distribution suggests that the three grouper PSS products play important physiological functions in adult fish as well as in cell growth and organ differentiation in embryo and larva development.

Regulation of somatostatins and their receptors in fish

The multifunctional nature of the somatostatin (SS) family of peptides results from a multifaceted signaling system consisting of many forms of SS peptides that bind to a variety of receptor (SSTR) subtypes. Research in fish has contributed important information about the components, function, evolution, and regulation of this system. Somatostatins or mRNAs encoding SSs have been isolated from over 20 species of fish. Peptides and deduced peptides differ in their amino acid chain length and/or composition, and most species of fish possess more than one form of SS. The structural heterogeneity of SSs results from differential processing of the hormone precursor, preprosomatostatin (PPSS), and from the existence of multiple genes that give rise to multiple PPSSs. The PPSS genes appear to have arisen through a series of gene duplication events over the course of vertebrate evolution. The numerous PPSSs of fish are differentially expressed, both in terms of the distribution among tissues and in terms of the relative abundance within a tissue. Accumulated evidence suggests that nutritional state, season/stage of sexual maturation, and many hormones [insulin (INS), glucagon, growth hormone (GH), insulin-like growth factor-I (IGF-I), and 17beta-estradiol (E2)] regulate the synthesis and release of particular SSs. Fish and mammals possess multiple SSTRs; four different SSTRs have been described in fish and several of these occur as isoforms. SSTRs are also wide spread and are differentially expressed, both in terms of distribution of tissues as well as in terms of relative abundance within tissues. The pattern of distribution of SSTRs may underlie tissue-specific responses of SSs. The synthesis of SSTR mRNA and SS-binding capacity are regulated by nutritional state and numerous hormones (INS, GH, IGF-I, and E2). Accumulated evidence suggests the possibility of both tissue- and subtype-specific mechanisms of regulation. In many instances, there appears to be coordinate regulation of PPSS and of SSTR; such regulation may prove important for many processes, including nutrient homeostasis and growth control.

Regulation of pancreatic somatostatin gene expression by insulin and glucagon

Rainbow trout were used as a model system to study the effects of insulin and glucagon on the expression of preprosomatostatins (PPSS). We previously showed that the endocrine pancreas of trout contains three mRNAs that encode for distinct somatostatin-containing peptides: PPSS I, which contains somatostain-14 (SS-14) at its C-terminus, and two separate PPSS IIs, PPSS II' and PPSS II'', each containing [Tyr7, Gly10]-SS-14 at their C-terminus. Rainbow trout injected (100 ng/g body weight) with insulin displayed elevated expression of PPSS II' and PPSS II'' mRNAs. Glucagon-injected (100 ng/g body weight) animals displayed elevated pancreatic expression of all PPSS mRNAs compared to saline-injected control animals. Insulin directly stimulated the expression of pancreatic PPSS II' and PPSS II'' mRNAs in vitro in a dose-dependent manner in the presence of 4mM glucose. Glucagon, in the presence of 10mM glucose, directly stimulated the expression of all PPSS mRNAs in a dose-dependent manner in vitro. These results indicate that the pancreatic expression of PPSS mRNAs is differentially regulated by insulin and glucagon and that the regulatory pattern is dependent on glucose concentration.

Different somatostatin receptor subtypes are operating in the brain of the teleost fish,Coris julis

Characterization of somatostatinergic (sst) neuronal activity through the application of nonpeptidyl agonists L-779,976 and L-817,818 which are highly specific for the sst receptors (sstr) sstr(2) and sstr(5), respectively, shows for the first time that sstr2, 5-like subtypes are the two major sstr subtypes operating in the brain of the teleost sea wrasse, Coris julis. A somewhat high but heterogeneous distribution pattern (> 30 < 180 fmol/mg wet tissue weight) of neurons expressing sstr2, 5 was reported in the different diencephalic regions plus in mesencephalon and telencephalon while low values were obtained in the cerebellum. Application of the above nonpeptidyl agonists permitted us to identify sstr2-like as the predominant subtype in telencephalic areas such as the entopeduncular nucleus (E) and postcommissural nucleus of the ventral telencephalon (Vp) as well as in hypothalamic and thalamic areas. At the same time high levels of neurons expressing sstr5-like, that greatly overlap those of sstr2-like in the diencephalic areas such as the anteroventral part of the preoptic nucleus (NPOav), the dorsal habenular nucleus (NHd) and the ventrolateral thalamic nucleus (VL), indicate that sstr2-like is very likely not the only sstr subtype acting in this fish brain. The predominance of sstr5-like in other brain areas is confirmed by the high quantities of this subtype in mesencephalic areas such as the torus longitudinalis (TLo). Overall, the discriminately differing densities of neurons expressing both subtypes seem to point to this system as a key molecular basis accounting for the distinct neurophysiological and behavioral sst-dependent activities in Coris julis.

Growth hormone secretion from the Arctic charr (Salvelinus alpinus) pituitary gland in vitro: effects of somatostatin-14, insulin-like growth factor-I, and nutritional status

This study investigated the influence of nutritional status on the growth hormone (GH)/insulin-like growth factor-I (IGF-I) axis in Arctic charr (Salvelinus alpinus). The objectives were to study the regulation of GH secretion in vitro by somatostatin-14 (SRIF) and hIGF-I, and to determine whether pituitary sensitivity to these factors is dependent upon nutritional status. Arctic charr were fed at three different ration levels (0, 0.35, and 0.70% BWd(-1)), and pituitary glands were harvested at 1, 2, and 5 weeks for in vitro study. Both SRIF and hIGF-I inhibited GH secretion from Arctic charr pituitary tissue in long-term (18 h) static hemipituitary culture, as well as after acute exposure in a pituitary fragment perifusion system. This response appeared to be dose-dependent for SRIF in static culture over the range of 0.01-1 nM, but not for hIGF-I. The acute inhibitory action of hIGF-I on GH release in the perifusion system suggests an action that is initially independent of any effects on GH gene expression or protein synthesis. Nutritional status did not affect the sensitivity of Arctic charr pituitary tissue to either SRIF or hIGF-I in vitro, indicating that changes in abundance of pituitary SRIF or IGF-I receptors may not explain the alterations in plasma GH levels found during dietary restriction.

Somatostatin receptor subtype 1 and subtype 2 mRNA expression is regulated by nutritional state in rainbow trout (Oncorhynchus mykiss)

Somatostatin receptors (sst) mediate the numerous physiological actions (e.g., aspects of growth, development, and metabolism) of the somatostatin family of peptides. In this study, we used rainbow trout (Oncorhynchus mykiss) to establish the pattern of sst subtype 1A, 1B, and 2 mRNA expression in selected tissues (optic tectum of brain, endocrine pancreas, and liver) and to evaluate nutritional regulation of sst expression. Quantitative real-time reverse transcription-PCR, sensitive to less than 100 copies, revealed that sst1s and sst2 was differentially expressed, both in terms of distribution among the tissues of study and in terms of relative abundance within a particular tissue. Under normal physiological (fed) conditions, sst1B levels were two times greater than those of sst1A in all tissues examined and levels of sst2 were 2-5 times greater those of sst1B, except in optic tectum, in which sst1B and sst2 mRNA levels appeared equal. Nutritional state modulated the pattern of sst1 and sst2 mRNAs expression. Fasting for 2 or 6 weeks reduced the expression of sst mRNAs in optic tectum; whereas, fasting increased the expression of sst mRNAs in both pancreas and liver. Re-feeding animals for 2 weeks following a 4-week fast restored mRNA levels to near those in tissues from animals which were fed continuously. These findings indicate that the pattern of sst expression in optic tectum, pancreas, and liver is regulated by nutritional state.

The role of somatostatins in the regulation of metabolism in fish

Somatostatins (SS) are a structurally and functionally diverse family of peptide hormones. Somatostatins possess a wide variety of biological functions, including numerous secretotropic, developmental, and metabolic effects. Studies on fish have revealed considerable insight into the role of SS on the regulation of intermediary metabolism. Somatostatins promote both lipid and carbohydrate breakdown in fish and lamprey. Such actions are mediated by secretotropic effects of SS. For example, SS inhibit insulin (INS); insulin deficiency favors lipolysis and glycogenolysis over lipogenesis and glycogenesis. Somatostatins also directly stimulate the breakdown of stored triacylglycerols (TG) and glycogen in storage tissues. In addition, SS interact with the growth and reproductive axes of fish, findings that suggest SS serve to modulate energy partitioning among various growth, development and reproductive processes.

Brain mapping of three somatostatin encoding genes in the goldfish

In the present study the brain distribution of three somatostatin (SRIF)-encoding genes, PSS-I, PSS-II, and PSS-III, was analyzed by in situ hybridization (ISH) in the goldfish. The PSS-I mRNA showed the widest distribution throughout the brain, whereas PSS-II transcripts were restricted to some hypothalamic nuclei. On the other hand, PSS-III presents an intermediate distribution pattern. All SRIF encoding genes are expressed in hypophysiotropic nuclei supporting the idea that, in addition to SRIF-14, [Pro(2)] SRIF-14, and gSRIF-28 have pituitary-controlling functions. Moreover, each of the genes is expressed in nuclei directly associated with feeding behavior, suggesting a role for SRIF peptides in the central control of food intake and energy balance. Alternatively, they might have a role in processing sensory information related with feeding behavior, since PSS genes are expressed in the main gustatory, olfactory, and visual centers, which project to the hypothalamic feeding center in teleost fish.

The two subtype 1 somatostatin receptors of rainbow trout, Tsst1A and Tsst1B, possess both distinct and overlapping ligand binding and agonist-induced regulation features

In the present study, two isoforms of somatostatin receptor subtype one, previously obtained from the brain of rainbow trout, Tsst1A and Tsst1B, were stably transfected in the Chinese hamster ovary cell line (CHO-K1) and their binding properties were characterized. High affinity binding of somatostatin by expressed receptors was saturable and ligand selective. Both Tsst1A and Tsst1B preferentially bound peptides derived from preprosomatostatin I (PPSS I; e.g., SS-14-I) over those derived from PPSS II (containing Tyr7, Gly10-SS-14-I at their C-terminus; e.g., SS-25-II). The rank order of ligand affinities for Tsst1A was SS-28-I>SS-14-I>SS-26-I?SS-28-II>SS-14-II>SS-25-II. The rank order for Tsst1B was SS-14-I>SS-28-I>SS-26-1?SS-28-II>SS-25-II>SS-14-II. Agonist-induced regulation of Tsst1A and Tsst1B was also investigated. After 30 min of SS-14-I exposure, both Tsst1A and Tsst1B underwent rapid internalization; ca. 60% of membrane Tsst1A was internalized and only about 40% of membrane Tsst1B was internalized. Prolonged agonist exposure (up to 48 h) induced up-regulation of membrane-expressed Tsst1A, but had no effect on Tsst1B. These results indicate that Tsst1s display both distinct and overlapping ligand binding and agonist-induced regulation features. Such features may form the basis of ligand-selection and have important consequences on target organ responsiveness.

Growth hormone and insulin-like growth factor-1 differentially stimulate the expression of preprosomatostatin mRNAs in the Brockmann bodies of rainbow trout, Oncorhynchus mykiss

We previously characterized three cDNAs obtained from the endocrine pancreas (Brockmann body) of rainbow trout that encode for distinct preprosomatostatin (PPSS) molecules: PPSS I containing somatostain-14 (SS-14) at its C-terminus and two separate PPSS IIs, PPSS II' and PPSS II'', containing [Tyr7,Gly10]-SS-14 at their C-termini. In this study, we examined the control of PPSS I, PPSS II', and PPSS II'' mRNA expression by growth hormone (GH) and insulin-like growth factor-1 (IGF-1). Rainbow trout implanted with GH for 21 days displayed elevated pancreatic expression of all PPSS mRNAs compared to control animals. Growth hormone directly stimulated the expression of all pancreatic PPSS mRNAs in vitro in a dose-dependent manner; however, GH was a more potent stimulator of PPSS II' expression than of PPSS I or PPSS II'' expression. Insulin-like growth factor-1 also directly stimulated the expression of PPSS mRNAs in a dose-dependent manner in Brockmann bodies incubated in vitro; IGF-1 was a more potent stimulator of PPSS I and PPSS II' expression than of PPSS II'' expression. These results indicate that the expression of PPSS mRNAs in the Brockmann body of trout is differentially regulated by GH and IGF-1 and suggest that SS mediate the feedback regulation of GH and IGF-1.

Cysteamine-a somatostatin-inhibiting agent-induced growth hormone secretion and growth acceleration in juvenile grass carp (Ctenopharyngodon idellus)

Effects of cysteamine hydrochloride (CSH)-a somatostatin-inhibiting agent on growth hormone (GH) secretion from pituitary fragments (PF) or hypothalamus plus pituitary fragments (HPF) under static incubation conditions, serum GH, 3,5,3(')-triiodothyronine (T(3)) and thyroxine (T(4)) levels, and growth in juvenile grass carp (Ctenopharyngodon idellus) were investigated. CSH (0.1, 1, and 10 mM) had no influences on GH release from PF after 1 and 6h incubation, but was effective in stimulating GH release from HPF in a dose-dependent manner after 1 and 6h incubation. Moreover, prolonged treatment of HPF with CSH decreased the magnitude of enhancement of GH levels in culture medium. CSH and neuropeptides [e.g., human GH-releasing hormone (hGHRH, 100 nM), luteinizing hormone-releasing hormone analog (LHRH-A, [D-Trp(6),Pro(9)]LHRH, 100 nM)], or salmon gonadotropin-releasing hormone analogue (sGnRH-A, [D-Ala(6),Pro(9)]LHRH, 100 nM), alone and in combination during static incubation stimulated GH release from HPF after 1h incubation; in addition, there was an additive, not a synergistic effect of CSH and neuropeptides on stimulation of GH release. Administration of CSH (2.5mg/g diet) in combination with LHRH-A (5 microg/g diet) in diet twice daily for 8 weeks resulted in higher serum GH, T(3), and T(4) levels, ratio of RNA/DNA in muscle, food conversion efficiency, and growth rate than CSH or LHRH-A alone. At trial termination, significant decreases in condition factors and body lipid levels were observed in fish fed with CSH and/or LHRH-A. No significant differences were recorded for viscero-somatic index, hepato-somatic index, and percent body moisture and protein in muscle. These findings, taken as a whole, strongly suggest that the action of CSH stimulating GH release in vitro appears to be mediated through hypothalamic pathways and dietary delivery of CSH directly or indirectly stimulates endogenous GH, T(3), and T(4) secretion, and subsequently leads to a increase in growth rate in grass carp.

Gene expression analysis of largemouth bass exposed to estradiol, nonylphenol, and p,p'-DDE

The purpose of this study was to determine the specific expression profile of 132 genes, some of which are estrogen responsive, in largemouth bass (LMB) following exposure to estradiol (E(2)), or to two hormonally active agents, 4-nonylphenol (4-NP) and 1,1-dichloro-2, 2-bis (p-chlorophenyl) ethylene (p,p'-DDE), using gene array technology. The results of these experiments show that LMB exposed to E(2) and 4-NP had similar, but not identical genetic signatures for the genes examined, some of which are known to be estrogen-responsive genes. The differences suggest that 4-NP may have additional modes of action that are independent of the estrogen receptor (ER). We have also shown that exposure of male LMB to p,p'-DDE results in an increase in some estrogen-responsive genes. But in female LMB, the observed changes were a down-regulation of the normally up-regulated estrogen responsive genes. Other genes were also down-regulated. These results suggest that p,p'-DDE may affect regulation of genes differently in male and female LMB. This study further suggests that gene arrays have the potential to map out the gene activation pathways of hormonally active compounds.