General Aspects of the Biology and Function of Somatostatin

Somatostatin signalling coordinates energy metabolism allocation to reproduction in zebrafish

BACKGROUND

Energy allocation between growth and reproduction determines puberty onset and fertility. In mammals, peripheral hormones such as leptin, insulin and ghrelin signal metabolic information to the higher centres controlling gonadotrophin-releasing hormone neurone activity. However, these observations could not be confirmed in lower vertebrates, suggesting that other factors may mediate the energetic trade-off between growth and reproduction. A bioinformatic and experimental study suggested co-regulation of the circadian clock, reproductive axis and growth-regulating genes in zebrafish. While loss-of-function of most of the identified co-regulated genes had no effect or only had mild effects on reproduction, no such information existed about the co-regulated somatostatin, well-known for its actions on growth and metabolism.

RESULTS

We show that somatostatin signalling is pivotal in regulating fecundity and metabolism. Knock-out of zebrafish somatostatin 1.1 (sst1.1) and somatostatin 1.2 (sst1.2) caused a 20-30% increase in embryonic primordial germ cells, and sst1.2-/- adults laid 40% more eggs than their wild-type siblings. The sst1.1-/- and sst1.2-/- mutants had divergent metabolic phenotypes: the former had 25% more pancreatic α-cells, were hyperglycaemic and glucose intolerant, and had increased adipocyte mass; the latter had 25% more pancreatic β-cells, improved glucose clearance and reduced adipocyte mass.

CONCLUSIONS

We conclude that somatostatin signalling regulates energy metabolism and fecundity through anti-proliferative and modulatory actions on primordial germ cells, pancreatic insulin and glucagon cells and the hypothalamus. The ancient origin of the somatostatin system suggests it could act as a switch linking metabolism and reproduction across vertebrates. The results raise the possibility of applications in human and animal fertility.

Somatostatin Containing δ-Cell Number Is Reduced in Type-2 Diabetes

Recent developments suggest that increased glucagon and decreased somatostatin secretion from the pancreas contribute to hyperglycaemia in type-2 diabetes (T2D) patients. There is a huge need to understand changes in glucagon and somatostatin secretion to develop potential anti-diabetic drugs. To further describe the role of somatostatin in the pathogenesis of T2D, reliable means to detect islet δ-cells and somatostatin secretion are necessary. In this study, we first tested currently available anti-somatostatin antibodies against a mouse model that fluorescently labels δ-cells. We found that these antibodies only label 10-15% of the fluorescently labelled δ-cells in pancreatic islets. We further tested six antibodies (newly developed) that can label both somatostatin 14 (SST14) and 28 (SST28) and found that four of them were able to detect above 70% of the fluorescent cells in the transgenic islets. This is quite efficient compared to the commercially available antibodies. Using one of these antibodies (SST10G5), we compared the cytoarchitecture of mouse and human pancreatic islets and found fewer δ-cells in the periphery of human islets. Interestingly, the δ-cell number was also reduced in islets from T2D donors compared to non-diabetic donors. Finally, with the aim to measure SST secretion from pancreatic islets, one of the candidate antibodies was used to develop a direct-ELISA-based SST assay. Using this novel assay, we could detect SST secretion under low and high glucose conditions from the pancreatic islets, both in mice and humans. Overall, using antibody-based tools provided by Mercodia AB, our study indicates reduced δ-cell numbers and SST secretion in diabetic islets.

PET Imaging of Pheochromocytoma

Pheochromocytomas are tumors derived from chromaffin cells of the adrenal medulla that synthesize, store, metabolize, and usually, but not always, secrete catecholamines. Although pheochromocytomas are the cause of hypertension in only a small number of patients, they can precipitate life-threatening hypertension or cardiac arrhythmias caused by excessive and episodic catecholamine secretion. This article reviews the genetics, clinical presentation, and imaging of pheochromocytoma, with special emphasis on new positron emission tomographic radiopharmaceutical agents.

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.

Deficiency of somatostatin (SST) receptor type 5 (SSTR5) is associated with sexually dimorphic changes in the expression of SST and SST receptors in brain and pancreas

The actions of somatostatin (SST) are mediated through five somatostatin receptor subtypes, termed SSTR1-5. Although SSTRs commonly display an overlapping pattern of tissue distribution, subtype-selective responses have been shown to occur in the same tissue. In the present study, we have investigated the changes in SSTR subtypes at the cellular and molecular level in both the brain and the pancreatic islets of mice deficient in SSTR5 (SSTR5KO). Expression levels of insulin and glucagon were also determined in the pancreas of these mice. Semi-quantitative RT-PCR and Western blot analysis showed significant increases in the expression of SSTR2 and 3 with a corresponding reduction in SSTR4 in the brains of female SSTR5KOs, while no changes were observed in male KOs. Strikingly, SST mRNA and SST-like immunoreactivity (SST-LI) were reduced in the brain of male KO animals but not in their female counterparts. In male SSTR5KO islets, there was an increase in the number of cells immunoreactive for SSTR1-3, whereas in female islets only SSTR3 expression was increased. Pancreatic SST-LI and SST mRNA, as well as immunoreactivity for insulin were reduced in male but not in female KO mice. These data indicate that deficiency of SSTR5 leads to subtype-selective sexually dimorphic changes in the expression of both brain and pancreatic SSTRs.

Chronic somatostatin treatment affects pituitary gonadotrophs, ovaries and onset of puberty in rats

The effects of chronic somatostatin (SRIH-14) treatment on the pituitary gonadotrophs (FSH and LH cells) and ovaries of female Wistar rats were examined. Females were given 20 microg/100 g b.w. twice per day from the immature (23rd day) till the adult period of life (71st day). The onset of puberty was determined by daily examination for vaginal opening. The peroxidase-antiperoxidase immunocytochemical procedure was used to study the gonadotrophs. Changes in the number per unit area (mm2), cell volume and volume densities of LH- and FSH-immunoreactive cells were evaluated by morphometry and stereology. Ovaries were analysed by simple point counting of follicles and corpora lutea (CL). Follicles were divided by size according to the classification of Gaytán and Osman. The mitotic indexes of granulosa and theca cells in the follicles were estimated at all stages of folliculogenesis. The number, volume and the volume density of FSH- and LH-immunoreactive cells decreased after chronic SRIH-14 treatment, particularly the latter. In the ovary, SRIH-14 treatment decreased the number of healthy follicles at all phases of folliculogenesis, lowered the mitotic indexes of granulosa and theca cells but increased the number of atretic follicles. Healthy CL were fewer in number, while regressive CL were increased. Vaginal opening occurred at a later age in treated females. It can be concluded that chronic SRIH-14 treatment markedly inhibited LH cells and to a lesser extent FSH cells. In the ovary SRIH-14 inhibited folliculogenesis, enhanced atretic processes and lowered proliferative activity of granulosa and theca cells. It also delayed puberty onset.

Nuclear medicine imaging of neuroendocrine tumours

Different tracers have been proposed in nuclear medicine to visualize neuroendocrine tumours: the majority are based on specific uptake mechanisms while some are aspecific. Among the most important gamma-emitting tracers, radiolabelled metaiodobenzylguanidine (123I or 121I-MIBG) and 111In-pentetreotide should be mentioned. In particular. good results can be obtained with 111In-pentetreotide scanning, which visualizes more than 70% of all neuroendocrine tumours and in some indications, as in gastro-entero-pancreatic (GEP) tumours, has a diagnostic sensitivity superior to that of conventional radiological imaging. Radiolabelled monoclonal antibodies have at present only a storical value, while a series of new peptides represent interesting subjects in areas currently being regarded. Positron emission tomography (PET) is a successful modality to detect cancer and recent years, has demonstrated a great diagnostic value in a large series of tumour types. 18F-deoxyglucose (FDG)-PET has also been used to diagnose tumours of neuroendocrine origin. Even if 18F-FDG has been successfully and widely employed in oncology, it has not demostrated a significant uptake in well differentiated neuroendocrine tissues. On the contrary, other positron emitter tracers seem to be more promising. A serotonin precursor 5-hydroxytryptophan (5-HTP) labelled with 11C has shown an increased uptake in carcinoids. This uptake seems to be selective and some clinical evidence has demonstrated that it allows the detection of more lesions with PET than with CT or octreotide scintigraphy. Another radiopharmaceutical in development for PET is 11C L-DOPA, which seems to be useful in visualizing endocrine pancreatic tumours. This Review summarizes the potential of several nuclear medicine techniques in the diagnosis of neuroendocrine tumours and stresses the renewed role of nuclear -medicine in the management of this disease.

Receptor-mediated internalization of somatostatin in rat cortical and hippocampal neurons

Binding of neuropeptides to their receptors usually results in internalization of receptor-ligand complexes. This process serves a crucial role in receptor downregulation, resensitization, and transmembrane signaling. It has mainly been investigated in cells ectopically expressing recombinant receptors. In the present study, we investigated whether rat central neurons and astrocytes naturally expressing somatostatin (SRIF) receptors internalized this neuropeptide. We demonstrated that 29% of cortical and 45% of hippocampal neurons in culture expressed the SRIF receptor sst(2A) and that 40-50% of the neurons internalized fluorescent SRIF. Similarly, an important proportion of astrocytes expressed sst(2A) (up to 60% in cortical cultures) and internalized fluo-SRIF. Competition experiments using the sst(2)/sst(5)-preferring agonist SMS 201-995 (octreotide) showed that a subpopulation of neurons internalized fluo-SRIF via sst(2) and/or sst(5) receptors, but that others also did so via other subtypes. Fluo-SRIF labeling was barely competed for by the sst(1)-selective agonist CH-275, indicating that sst(1) was unlikely to be mediating SRIF internalization in hippocampal and cortical neurons. Given the paucity of sst(5) receptors in cerebral cortex and hippocampus and the poor yield of sst(4) internalization in transfected cells, we conclude that sst(2) and sst(3) subtypes are the most likely to be responsible for SRIF internalization in our culture systems.

Receptors for dopamine and somatostatin: formation of hetero-oligomers with enhanced functional activity

Somatostatin and dopamine are two major neurotransmitter systems that share a number of structural and functional characteristics. Somatostatin receptors and dopamine receptors are colocalized in neuronal subgroups, and somatostatin is involved in modulating dopamine-mediated control of motor activity. However, the molecular basis for such interaction between the two systems is unclear. Here, we show that dopamine receptor D2R and somatostatin receptor SSTR5 interact physically through hetero-oligomerization to create a novel receptor with enhanced functional activity. Our results provide evidence that receptors from different G protein (heterotrimeric guanine nucleotide binding protein)-coupled receptor families interact through oligomerization. Such direct intramembrane association defines a new level of molecular crosstalk between related G protein-coupled receptor subfamilies.

The expression of preprosomatostatin II mRNAs in the Brockmann bodies of rainbow trout, Oncorhynchus mykiss, is regulated by glucose

We previously characterized two cDNAs that encode for distinct preprosomatostatin molecules containing [Tyr(7), Gly(10)]-somatostatin-14 at their C-termini (PPSS II' and PPSS II") and found that these cDNAs were differentially expressed in the endocrine pancreas (Brockmann body) of rainbow trout, Oncorhynchus mykiss. In this study, we examined the control of PPSSII' mRNA and PPSS II" mRNA expression by glucose. Fish injected with glucose displayed elevated plasma levels of glucose in association with nearly three-fold higher levels of PPSS II mRNAs compared to saline-injected control animals. Glucose directly stimulated the expression of both PPSS II mRNAs in vitro in a dose-dependent manner; however, glucose was a more potent stimulator of PPSS II" expression than of PPSS II' expression. The hexoses, mannose, galactose, and fructose, as well as glucose, all induced the expression of PPSS II mRNAs, whereas, sucrose and the glucose analogs, 3-o-methylglucose and 2-deoxyglucose, were without effect. In addition, the expression of PPSS II mRNAs was stimulated by dihydroxyacetone, pyruvate, lactate, acetate, and citrate. Furthermore, the expression of PPSS II mRNAs was inhibited by iodoacetate, an inhibitor of glycolysis, but was stimulated by dichloroacetate, a stimulator of Krebs cycle flux via pyruvate dehydrogenase activation. Finally, glucose-stimulated PPSS II expression was inhibited by actinomycin. These results indicate that the expression of PPSS II mRNAs in the Brockmann body of trout is regulated by nutrients such as glucose and suggest that glucose-stimulated expression of PPSS II mRNAs requires the uptake and subsequent metabolism of the sugar and is transcription sensitive.

Characterization and distribution of somatostatin binding sites in goldfish brain

Somatostatin (SRIF) binding sites were characterized in goldfish brain. Binding of (125)I-[Tyr(11)]-SRIF-14 to a brain membrane preparation was found to be saturable, reversible, and time-, temperature-, and pH-dependent. Binding was also displaceable by different forms of SRIF. Under optimal conditions (22 degrees C, pH 7.2), the equilibrium binding of (125)I-[Tyr(11)]-SRIF-14 to goldfish brain membranes was achieved after 60 min incubation. Analysis of saturable equilibrium binding revealed a one-site model fit with K(a) of 1.3 nM. SRIF-14, mammalian SRIF-28, and salmon SRIF-25 displaced (125)I-[Tyr(11)]-SRIF-14 binding with similar affinity, whereas other neuropeptides, e.g., substance P, were unable to displace (125)I-[Tyr(11)]-SRIF-14. Autoradiography studies demonstrated that (125)I-[Tyr(11)]-SRIF-14 binding sites are found throughout the goldfish brain. A high density of (125)I-[Tyr(11)]-SRIF-14 binding sites was found in the forebrain, including the nucleus preopticus, nucleus preopticus periventricularis, nucleus anterioris periventricularis, nucleus lateralis tuberis, nucleus dorsomedialis thalami, nucleus dorsolateralis thalami, nucleus ventromedialis thalami, and nucleus diffusus lobi inferioris. In midbrain, (125)I-[Tyr(11)]-SRIF-14 binding sites were found in the optic tectum. The facial and vagal lobes and the mesencephalic-cerebellar tract were found to have a high density of binding sites. This study provides the first characterization and distribution of specific binding sites for SRIF in a fish brain.

Rainbow trout (Oncorhynchus mykiss) possess two somatostatin mRNAs that are differentially expressed

Previously, we isolated a 624-bp cDNA encoding for a 115-amino acid preprosomatostatin containing [Tyr7,Gly10]-somatostatin (SS)-14 (now designated PPSS-II') obtained from the endocrine pancreas (Brockmann bodies) of rainbow trout. In this study we have characterized a second cDNA obtained from trout pancreas that is 600-bp in length and encodes for a 111-amino acid precursor containing [Tyr7,Gly10]-SS-14 (PPSS-II''). The nucleotide and amino acid identity between the two cDNAs is 82.3 and 80.5%, respectively. Both PPSS-II' and PPSS-II'' mRNA were present in esophagus, pyloric ceca, stomach, upper and lower intestine, and pancreas, whereas only SS-II" mRNA was present in brain. PPSS-II'' mRNA was more abundant than PPSS-II' mRNA in pancreas, whereas PPSS-II' mRNA was more abundant than PPSS-II" mRNA in stomach. Fasting increased pancreatic PPSS-II'' mRNA levels but had no effect on the levels of PPSS-II' mRNA. These results indicate the existence of two nonallelic pancreatic SS-II genes that are differentially expressed, both in terms of distribution among tissues and in terms of relative abundance within the tissues.

Immunohistochemical distribution of the somatostatin receptor subtype 5 in the adult rat brain: predominant expression in the basal forebrain

Somatostatin exerts its actions by means of a family of G protein-coupled receptors, five of which have so far been cloned. Whereas mRNAs for receptor subtypes sst(1)-sst(4) have been unequivocally localized in the brain, the data concerning the fifth subtype, sst(5), are contradictory. Moreover, whereas sst(1) and sst(2A) receptor proteins have been localized by immunohistochemistry, the distribution of sst(3)-sst(5) receptor proteins and/or subtype-specific binding remains to be determined in the central nervous system. In the present study, we investigated the distribution of immunoreactive sst(5) in adult rat brain and pituitary and demonstrated the presence of this receptor protein in the central nervous system by using an affinity-purified antibody generated against the C-terminus of the receptor. The specificity of the antibody for sst(5) was established by immunoblotting experiments on membranes prepared from cells transfected with cDNA encoding different somatotropin release inhibiting (SRIF) receptor subtypes as well as from anterior pituitary. In both systems, the antibody specifically recognized a band at approximately 50 kDa molecular mass, corresponding well to the reported size of the cloned receptor (48 kDa). Immunofluorescence in COS-7 cells transfected with individual SRIF receptor subtypes as well as in sections of rat pituitary demonstrated the antibody's applicability to the immunohistochemical detection of sst(5) receptors. In rat brain sections, sst(5) immunoreactivity was predominantly associated with neuronal perikarya and primary dendrites. Immunolabeling was most prominent in the olfactory tubercle, islands of Calleja, diagonal band of Broca, substantia innominata, and magnocellular preoptic nucleus of the basal forebrain as well as in the reticular nucleus of the thalamus. Other, less intensely labeled areas included the cerebral cortex, hippocampus, amygdala, preoptic area as well as the lateroanterior nucleus of the hypothalamus. The present findings provide the first characterization of the anatomic distribution of sst(5) receptors in the rat brain. They demonstrate a prominent expression of these receptors in the basal forebrain, suggesting that they may be involved in the mediation of somatostatin effects on the sleep-wake cycle through their association with cortically projecting subcortical systems.

Somatostatin and its receptor family

Somatostatin (SST), a regulatory peptide, is produced by neuroendocrine, inflammatory, and immune cells in response to ions, nutrients, neuropeptides, neurotransmitters, thyroid and steroid hormones, growth factors, and cytokines. The peptide is released in large amounts from storage pools of secretory cells, or in small amounts from activated immune and inflammatory cells, and acts as an endogenous inhibitory regulator of the secretory and proliferative responses of target cells that are widely distributed in the brain and periphery. These actions are mediated by a family of seven transmembrane (TM) domain G-protein-coupled receptors that comprise five distinct subtypes (termed SSTR1-5) that are endoded by separate genes segregated on different chromosomes. The five receptor subtypes bind the natural SST peptides, SST-14 and SST-28, with low nanomolar affinity. Short synthetic octapeptide and hexapeptide analogs bind well to only three of the subtypes, 2, 3, and 5. Selective nonpeptide agonists with nanomolar affinity have been developed for four of the subtypes (SSTR1, 2, 3, and 4) and putative peptide antagonists for SSTR2 and SSTR5 have been identified. The ligand binding domain for SST ligands is made up of residues in TMs III-VII with a potential contribution by the second extracellular loop. SSTRs are widely expressed in many tissues, frequently as multiple subtypes that coexist in the same cell. The five receptors share common signaling pathways such as the inhibition of adenylyl cyclase, activation of phosphotyrosine phosphatase (PTP), and modulation of mitogen-activated protein kinase (MAPK) through G-protein-dependent mechanisms. Some of the subtypes are also coupled to inward rectifying K(+) channels (SSTR2, 3, 4, 5), to voltage-dependent Ca(2+) channels (SSTR1, 2), a Na(+)/H(+) exchanger (SSTR1), AMPA/kainate glutamate channels (SSTR1, 2), phospholipase C (SSTR2, 5), and phospholipase A(2) (SSTR4). SSTRs block cell secretion by inhibiting intracellular cAMP and Ca(2+) and by a receptor-linked distal effect on exocytosis. Four of the receptors (SSTR1, 2, 4, and 5) induce cell cycle arrest via PTP-dependent modulation of MAPK, associated with induction of the retinoblastoma tumor suppressor protein and p21. In contrast, SSTR3 uniquely triggers PTP-dependent apoptosis accompanied by activation of p53 and the pro-apoptotic protein Bax. SSTR1, 2, 3, and 5 display acute desensitization of adenylyl cyclase coupling. Four of the subtypes (SSTR2, 3, 4, and 5) undergo rapid agonist-dependent endocytosis. SSTR1 fails to be internalized but is instead upregulated at the membrane in response to continued agonist exposure. Among the wide spectrum of SST effects, several biological responses have been identified that display absolute or relative subtype selectivity. These include GH secretion (SSTR2 and 5), insulin secretion (SSTR5), glucagon secretion (SSTR2), and immune responses (SSTR2).

Neuroendocrine control of growth hormone secretion

The secretion of growth hormone (GH) is regulated through a complex neuroendocrine control system, especially by the functional interplay of two hypothalamic hypophysiotropic hormones, GH-releasing hormone (GHRH) and somatostatin (SS), exerting stimulatory and inhibitory influences, respectively, on the somatotrope. The two hypothalamic neurohormones are subject to modulation by a host of neurotransmitters, especially the noradrenergic and cholinergic ones and other hypothalamic neuropeptides, and are the final mediators of metabolic, endocrine, neural, and immune influences for the secretion of GH. Since the identification of the GHRH peptide, recombinant DNA procedures have been used to characterize the corresponding cDNA and to clone GHRH receptor isoforms in rodent and human pituitaries. Parallel to research into the effects of SS and its analogs on endocrine and exocrine secretions, investigations into their mechanism of action have led to the discovery of five separate SS receptor genes encoding a family of G protein-coupled SS receptors, which are widely expressed in the pituitary, brain, and the periphery, and to the synthesis of analogs with subtype specificity. Better understanding of the function of GHRH, SS, and their receptors and, hence, of neural regulation of GH secretion in health and disease has been achieved with the discovery of a new class of fairly specific, orally active, small peptides and their congeners, the GH-releasing peptides, acting on specific, ubiquitous seven-transmembrane domain receptors, whose natural ligands are not yet known.

Polygenic expression of somatostatin in rainbow trout, Oncorhynchus mykiss: evidence of a preprosomatostatin encoding somatostatin-14

Previously we reported the existence of two distinct cDNAs in rainbow trout that encode for separate preprosomatostatins (PPSS), each containing [Tyr7, Gly10]-somatostatin-14. In the present study, we used rainbow trout to further characterize the polygenic origin of somatostatins (SSs), a peptide hormone important in the regulation of growth, development, and metabolism of vertebrates. A two-phase rapid amplification of cDNA ends (RACE)-PCR was used for the isolation of selected cDNAs. We amplified and sequenced a ca. 350-bp 3' RACE-PCR fragment. Based upon this sequence we designed a second gene-specific primer for 5' RACE-PCR which yielded a 452-bp fragment. Sequence analysis revealed a 745-bp cDNA containing the complete 5'-untranslated region, a single initiation site 118 bases from the most 5' end, and a single putative polyadenylation site 17 bases from the most 3' end that was terminated with a polyadenylated tail. The deduced protein is a 114-amino acid PPSS molecule that contains a number of putative processing sites, potentially yielding a 26-amino acid peptide that could be processed further to a 14-amino acid peptide identical in structure to mammalian SS-14. Northern analysis revealed that PPSS-I was expressed in the pancreas, stomach, intestine, and brain of rainbow trout. These results suggest a polygenic origin of SS, possibly resulting from gene duplication events prior to the divergence of teleosts.

Somatostatin binding to hepatocytes isolated from rainbow trout, Oncorhynchus mykiss, is modulated by insulin and glucagon

Previously we reported somatostatin-14 (SS-14) binding in the liver of rainbow trout and that fasting enhanced SS-14-binding capacity. In this study, we used hepatocytes isolated from rainbow trout to study aspects of SS-14 processing and to evaluate the basis of fasting-associated changes in SS-14 binding. Hepatocytes specifically bound 5-8% of the total 125I-SS-14 added. Scatchard analysis revealed the existence of two classes of binding sites. The high-affinity site had a dissociation constant (Kd) of 23.6 +/- 1.1 nM and a binding capacity (Bmax) of 3459 +/- 134 receptors/cell. The low-affinity site had a Kd of 764 +/- 27 nM and a Bmax of 17,432 +/- 345 receptors/cell. 125I-SS-14 dissociation was hastened by the presence of 10(-6) M SS-14 . The internalization of 125I-SS-14 was time dependent; preincubation of hepatocytes with 10(-6) M SS-14 reduced internalization of 125I-SS-14. The number of high-affinity binding sites was also reduced by 10(-6) M SS-14. Because plasma levels of insulin (INS) decline relative to those of glucagon (GLU) during fasting of trout, we also investigated the effects of these hormones on SS-14 binding. The number of high-affinity SS-14-binding sites was reduced by 10(-6) M INS and was increased by 10(-6) M and 10(-8) M GLU. These results indicated that SS-14 regulates aspects of SS-14 binding and processing and suggests that INS and GLU play a role in fasting-associated changes in SS-14 binding.

Effects of somatostatin on lipid metabolism of larvae and metamorphosing landlocked sea lamprey, Petromyzon marinus

This study was designed to examine the role of somatostatin in regulating changes in lipid metabolism of larvae and metamorphosing landlocked sea lamprey, Petromyzon marinus. Larvae and animals in late metamorphosis (stage 6 on a 7-stage scale) were injected intraperitoneally once per day for 2 days with either saline (0.6%) or somatostatin-14 (SS-14; 500 ng/g body wt). Injection of SS-14 into larval and stage 6 metamorphosing animals resulted in elevated plasma fatty acids levels. In larvae, SS-14-induced hyperlipidemia was supported by enhanced lipolysis, as indicated by increased triacylglycerol lipase (TGL) activity in the liver and kidney. Mobilization of larval renal lipid was accompanied by reduced TG synthesis, as indicated by decreased diacylglycerol acyltransferase (DGAT) activity. In stage 6 metamorphosing lamprey, SS-14 did not significantly affect TGL activity; however, SS-14 significantly reduced fatty acid synthesis, as measured by acetyl-CoA carboxylase activity, in kidney, liver, and muscle, as well as muscular TG synthesis. SS-14-stimulated lipid depletion is reminiscent of the pattern of lipid metabolism displayed by P. marinus during their spontaneous metamorphosis-an observation which suggests that somatostatin may play a role in metamorphosis-associated changes in lipid metabolism in this species.

Somatostatin-14, somatostatin-28, and prosomatostatin[1-10] are independently and efficiently processed from prosomatostatin in the constitutive secretory pathway in islet somatostatin tumor cells (1027B2)

We have characterized the biosynthetic origin of somatostatin-14 (SS-14), SS-28, and pro-SS[1-10] from pro-SS (PSS) in 1027B2 rat islet tumor cells. Because these cells lack regulated secretion and show unresponsiveness of the SS gene to cAMP, we have additionally carried out morphological and functional studies to elucidate the molecular defect in cAMP signalling and to localize the sites of PSS maturation along the secretory pathway. Cell extracts and secretion media were analysed by high performance liquid chromatography and specific C- and N-terminal radioimmunoassays. Electron microscopic sampling of 1027B2 cell cultures showed that most cells had very few dense core secretory granules for heterogeneous sizes. The cells expressed the endoproteases furin, PC1, and PC2 and contained large quantities of fully processed SS-14 and SS-28 with very little unprocessed PSS (ratio SS-14:SS-28:PSS = 39:51:10%). They secreted high concentrations of SS-14, SS-28, and PSS[1-10] constitutively along with PC1 and PC2. Pulse-chase studies demonstrated that PSS is rapidly (within 15 min), and efficiently processed to SS-14, SS-28, and PSS[1-10] via separate biosynthetic pathways: PSS --> SS-14 + 8 kDa; PSS --> SS-28 + 7 kDa; PSS --> PSS[1-10]. Monensin reduced intracellular SS-like immunoreactivity without altering processing efficiency. Transfection with the catalytic subunit of protein kinase A (PKA-C) activated SS promoter-CAT activating indicating that the defect in cAMP-dependent signaling in 1027B2 cells lies at the level of PKA-C. PKA-C overexpression failed to alter the ratio of processed SS-14 and SS-28. These results demonstrate that SS-14, SS-28, and PSS[1-10] are independently synthesized from PSS and that efficient precursor processing can occur within the constitutive secretory pathway in the relative absence of dense core secretory vesicles.

Acute effects of D1- and D2-receptor agonist and antagonist drugs on somatostatin binding, inhibition of adenylyl cyclase activity and accumulation of inositol 1,4,5-trisphosphate in the rat striatum

A recent study carried out by our group demonstrated that exogenous dopamine increases the somatostatin (SS) receptor-effector system in the rat striatum. The present study examined the participation of the D1- and D2-dopaminergic systems in the modulation of the rat striatal SS receptor-effector system by use of the D1-receptor agonist and antagonist SKF 38393 and SCH 23390, respectively, and the D2-receptor agonist and antagonist bromocriptine and raclopride, respectively. In view of the rapid onset of dopamine action, the effect of dopaminergic agents on the SS mechanism of action were studied 3 h after their administration. SKF 38393 (4 mg/kg i.p.) or bromocriptine (2 mg/kg i.p.) administered to male Wistar rats increased the number of 125I-Tyr3-SMS receptors in the striatum (52 and 30%, respectively) without changing the affinity constant. The effect of SKF 38393 on 125I-Tyr3-SMS binding was antagonized by the D1-specific antagonist SCH 23390 (0.25 mg/kg i.p.) whereas the effect of bromocriptine was abolished by the D2-specific antagonist raclopride (5 mg/kg i.p.). No change in binding was produced when SKF 38393 or bromocriptine were added directly to the incubation medium. The acute systemic administration of SCH 23390 or raclopride alone had no effect on the binding of 125I-Tyr3-SMS to its receptors. The increase of the number of 125I-Tyr3-SMS receptor induced by SKF 38393 or bromocriptine was accompanied by an increase in the capacity of SMS 201-995 to inhibit basal and forskolin (FK)-stimulated adenylyl cyclase (AC) activity when compared to the control groups. In addition, the effect of SMS 201-995 on the mass accumulation of inositol 1,4,5-trisphosphate (IP3) was investigated. SKF 38393 as well as bromocriptine increased the capacity of SMS 201-995 to accumulate IP3 in the rat striatum although this effect was only statistically significant in the case of SKF 38393. These results suggest that the activation of D1 and D2 receptors increases the activity of the SS receptor-effector system, the effect being greater in the case of D1 receptors. These findings are consistent with a functional interaction between dopamine and SS in the rat striatum.

Cortisol enhances non-REM sleep and growth hormone secretion in elderly subjects

Aging is accompanied by a continuous decline in slow wave sleep (SWS) and in growth hormone (GH) secretion, particularly during the sleeping period. Because short-term pulsatile administration of cortisol increases GH release and SWS in young adults, we wondered whether similar effects can be induced also in elderly men. Hourly injections of cortisol between 1700 and 600 h increased stage 2 and SWS and decreased rapid eye movement sleep. Spectral analysis revealed significant increases in delta and theta power. Cortisol infusions increased the GH secretion prior to sleep onset, but remained largely unchanged during sleep. Thus, sleep EEG and GH release are modulated by cortisol administration in a manner similar to that in young subjects, but to a lesser extent. The stimulatory effect of cortisol on both GH release and SWS points to a mechanism involving glucocorticoid-enhanced production and release of GH-releasing hormone that activates pituitary GH release and simultaneously antagonizes the effects of corticotropin-releasing hormone and somatostatin.

Expression of somatostatin in neurons of the central posterior/prepacemaker nucleus projecting to the preglomerular nucleus: immunohistochemical evidence for a non-synaptic function

In the diencephalon of the weakly electric gymnotiform fish Apteronous leptorhynchus, part of the central posterior/prepacemaker nucleus innervates the preglomerular nucleus. A minor population of these neurons expresses immunoreactivity against somatostatin, as has been shown by combining peptide immunohistochemistry with an in vitro tract-tracing technique. In contrast to the expectation, however, this neuropeptide does not appear to be transported along the axons to the projection site, as somatostatin-like immunoreactivity could not be detected in the preglomerular nucleus. It is, therefore, likely that somatostatin expressed in these neurons exerts a non-synaptic function in the region of the central posterior/prepacemaker nucleus itself.

Pancreatic development and maturation of the islet B cell. Studies of pluripotent islet cultures

Pancreas organogenesis is a highly regulated process, in which two anlage evaginate from the primitive gut. They later fuse, and, under the influence of the surrounding mesenchyme, the mature organ develops, being mainly composed of ductal, exocrine and endocrine compartments. Early buds are characterized by a branching morphogenesis of the ductal epithelium from which endocrine and exocrine precursor cells bud to eventually form the two other compartments. The three compartments are thought to be of common endodermal origin; in contrast to earlier hypotheses, which suggested that the endocrine compartment was of neuroectodermal origin. It is thus generally believed that the pancreatic endocrine-lineage possesses the ability to mature along a differentiation pathway that shares many characteristics with those of neuronal differentiation. During recent years, studies of insulin-gene regulation and, in particular, the tissue-specific transcriptional control of insulin-gene activity have provided information on pancreas development in general. The present review summarizes these findings, with a special focus on our own studies on pluripotent endocrine cultures of rat pancreas.

Molecular biology of somatostatin receptor subtypes

Somatostatin (SRIF) receptors (ssts) comprise a family of heptahelical membrane proteins encoded by five related genes that map to separate chromosomes and which, with the exception of sst1, are intronless. The ssts1-4 display weak selectivity for SRIF-14 binding, whereas sst5 is SRIF-28-selective. Based on structural similarity and reactivity for octapeptide and hexapeptide sst analogs, ssts2,3 and sst5 belong to a similar sst subclass; ssts1-4 react poorly with these analogs and belong to a separate subclass. All five ssts are functionally coupled to inhibition of adenylyl cyclase via pertussis toxin-sensitive guanosine triphosphate (GTP)-binding proteins. mRNA for ssts1-5 is widely expressed in brain and peripheral organs and displays an overlapping but characteristic pattern that is subtype-selective and tissue- and species-specific. All pituitary cell subsets express sst2 and sst5, with sst5 being more abundant. Individual pituitary cells coexpress multiple sst subtypes. The binding pocket for SRIF-14 ligand lies deep within the membrane in transmembrane domains (TMDs) 3 to 7. Except for extracellular loop 2, it does not involve the other exofacial structures. Human (h)sst2A and hsst5 undergo agonist-mediated desensitization, associated with receptor internalization. The C-tail segment of hsst5 displays positive molecular internalization signals. The ssts inhibit the growth of tumor cells directly, through blockade of mitogenic signaling leading to growth arrest and through induction of apoptosis. This process is associated with translocation of phosphotyrosine phosphatase (PTP) 1C from the cytosol to the membrane.

Effects of octreotide on cardiovascular hormones and haemodynamics in conscious dogs

Octreotide inhibits the secretion of several hormones and exerts vasopressor effects. To clarify the mechanism of atrial natriuretic factor (ANF) secretion and to assess the cardiovascular effects of octreotide in relation to changes in vasoactive peptide secretion, four groups of conscious dogs were studied: group I (n = 11) received saline infusion after placebo, group II (n = 10), the same infusion after octreotide, group III (n = 10), placebo only and group IV (n = 10) octreotide injection only. Saline (10% body wt) was infused over 40 min after subcutaneous injection of placebo or octreotide (1 microgram/kg). Saline produced a rise (p < 0.001) of plasma ANF from 32.4 +/- 4.1 to 59.0 +/- 8.5 pM after placebo and from 35.6 +/- 5.5 to 77.0 +/- 12.6 pM after octreotide. This rise, not significantly different between groups I and II paralleled a 4-5-fold increase (p < 0.005) of right and left atrial pressures. With a higher dose of octreotide (4 micrograms/kg) injected in 4 dogs, plasma ANF increased by 27.5 +/- 5 pM. During hypervolemia, plasma endothelin-1 remained unchanged but plasma angiotensin II and epinephrine decreased (p < 0.05) approximately by 80% without being affected by octreotide. Octreotide did not influence the basal secretion of ANF, endothelin-1, angiotensin II and catecholamines. However, in basal conditions, octreotide injection resulted in a 9% increase (p < 0.005) of left ventricular systolic pressure, unobserved after placebo. Plasma glucose decreased (p < 0.005) in groups receiving octreotide. Thus, octreotide does not impair the stretch-mediated release of ANF which implies a release mechanism independent from somatostatin receptors and consequent changes in intracellular c-AMP. Octreotide has also a pressor effect, unrelated to changes in vasoactive peptide production.

Molecular pharmacology of somatostatin receptors

Somatostatin was discovered for its ability to inhibit growth hormone (GH) secretion. Later, it was found to be widely distributed in other brain regions, in which it fulfills a neuromodulatory role, and in several organs of the gastrointestinal tract where it can act as a paracrine factor or as a true circulating factor. In mammals, two molecules of 14 (somatostatin 14) and 28 (somatostatin 28) amino acids are the only biologically active members of the family. They originate from a single gene which gives rise to a single propeptide alternately cleaved in different tissues. In 1992, a major breakthrough in our understanding of somatostatin functions was made with the cloning of five different receptor genes (sstr1 to sstr5) which belong to the seven transmembrane domain receptor family. Their closer relatives are opioid receptors. In first approximation, the tissular expression of the sstrs matches quite well with the distribution of somatostatin binding sites in the "classical" targets of the peptide ie brain, pituitary pancreatic islets and adrenals. The pharmacology of GH inhibition is very close to sstr2 binding but other actions of somatostatins have not yet been attributed clearly to a single receptor subtype. All clinically relevant agonists tested so far (octreotide, lanreotide and vapreotide) are selective of sstr2 being less potent on sstr3 and inactive for sstr1 and sstr4. Surprisingly, rat sstr5 displays nanomolar affinities for octreotide and vapreotide while these agonists are only active at much higher concentrations on human sstr5. All five receptors can be more or less efficiently coupled to inhibition of adenylate cyclase activity in transfected cell systems.(ABSTRACT TRUNCATED AT 250 WORDS)

Choline acetyltransferase and somatostatin levels in aged Microcebus murinus brain

beta-Amyloid protein (beta-AP) deposits, analoguous to those found in Alzheimer's disease (AD) are observed in the brain of aging Microcebus murinus. Because choline acetyltransferase (ChAT) activity and somatostatin (SRIH) content are consistently decreased in AD, we tested whether such changes could be observed in middle aged to aged Microcebus cerebral cortex and whether they were accompanied by beta-AP deposits. A positive correlation was observed between age and ChAT activity. By HPLC, SRIH immunoreactivity eluted as four peaks, two of which being identical with SRIH-28 and SRIH-14 while the other two likely represented precursor forms. Cortical SRIH content was not significantly affected by age. ChAT activity and SRIH content were not significantly correlated. Amyloid angiopathy was observed in every brain examined and the presence of cortical lesions analoguous to senile plaques observed in the oldest case only which did not demonstrate important alterations in ChAT and somatostatin levels.