The origin of somatostatin fibers in the median eminence and neural lobe of Rana temporaria

Summary A light microscopic immunocytochemical study of the brain of frogs with hypothalamic lesions was performed in order to obtain evidence concerning the origin of somatostatin fibers in the median eminence and neural lobe of the hypophysis. The results indicate that the somatostatin fibers of the neural lobe originate from somatostatin perikarya located in the prechiasmatic part of the hypothalamus and possibly also in the telencephalon. The somatostatin neurons of the pars ventralis tuberis cinerei do not send axons to the neural lobe. The frog median eminence contains axon terminals of somatostatin neurons located in the pars ventralis of the tuber cinereum. Many other somatostatin fibers of the frog median eminence originate from somatostatin neurons located outside the tuber cinereum. Most of these neurons probably lie in the preoptic hypothalamic region.This investigation was supported by a grant from the Belgian Nationaal Fonds voor Geneeskundig Wetenschappelijk Onderzoek     

Reciprocal paracrine pathways link atrial natriuretic peptide and somatostatin secretion in the antrum of the stomach

Atrial natriuretic peptide (ANP) as well as its receptor, NPR-A, have been identified in gastric antral mucosa, suggesting that ANP may act in a paracrine fashion to regulate gastric secretion. In the present study, we have superfused antral mucosal segments obtained from rat stomach to examine the paracrine pathways linking ANP and somatostatin secretion in this region.ANP (0.1 pM to 0.1 microM) caused a concentration-dependent increase in somatostatin secretion (EC(50), 0.3 nM). The somatostatin response to ANP was unaffected by the axonal blocker tetrodotoxin but abolished by addition of the selective NPR-A antagonist, anantin. Anantin alone inhibited somatostatin secretion by 18+/-3% (P<0.005), implying that endogenous ANP, acting via the NPR-A receptor, stimulates somatostatin secretion. Somatostatin (1 pM to 1 microM) caused a concentration-dependent decrease in ANP secretion (EC(50), 0.7 nM) that was abolished by addition of the somatostatin subtype 2 receptor (sst2) antagonist, PRL2903. Neutralization of ambient somatostatin with somatostatin antibody (final dilution 1:200) increased basal ANP secretion by 70+/-8% (P<001), implying that endogenous somatostatin inhibits ANP secretion. We conclude that antral ANP and somatostatin secretion are linked by paracrine feedback pathways: endogenous ANP, acting via the NPR-A receptor, stimulates somatostatin secretion, and endogenous somatostatin, acting via the sst2 receptor, inhibits ANP secretion.     

Immunocytochemical localization of somatostatin-containing neurons in the rat hypothalamus

Summary The rat hypothalamus was studied at the light microscopic level with the use of single and double immunocytochemical staining methods. It was shown that the rat supraoptic and paraventricular hypothalamic nuclei, and their accessory neurosecretory nuclei, do not contain magnocellular somatostatin neurons. The distribution of the hypothalamic parvocellular somatostatin cells is described. The parvocellular component of the rat hypothalamic paraventricular nucleus is, at least partly, composed of somatostatin cells: they form a fairly well circumscribed periventricular cell mass. The rat suprachiasmatic nuclei contain separate somatostatin neurons and vasopressin neurons. Scattered somatostatin cells are present in the entire arcuate nucleus. In addition to the periventricular somatostatin cells located in the preopticanterior hypothalamic area and in the arcuate nucleus, the rat hypothalamus also contains numerous scattered somatostatin cells located distant from the third ventricle.This investigation was supported by a grant from the Belgian Nationaal Fonds voor Geneeskundig Wetenschappelijk Onderzoek     

Active immunoneutralization of somatostatin in the sheep: effects on gastrointestinal somatostatin expression, storage and secretion.

In the absence of somatostatin antagonists, somatostatin antisera administered acutely or animals chronically immunized against somatostatin have been used to define the functions of somatostatin. However, the circulating immunoglobulins from immunized animals may contain substantial quantities of endogenous hormones. This has not been examined for somatostatin. We have measured the amount of free somatostatin bound to circulating immunoglobulins in somatostatin-immunized animals and the effect of this sequestering of the free peptide on somatostatin secretion and gastric somatostatin synthesis and storage. The average concentration of somatostatin bound to the antisera was 6.9 nmol/l, about 1000-fold higher than normal circulating levels. Compared to control animals, there was a doubling of somatostatin mRNA in the fundus and a 4-fold increase in fundic somatostatin peptide. Similar increases were seen in pancreas, but the antrum was not significantly affected providing further evidence of distinct regulatory mechanisms between the antrum and fundus. We suggest that withdrawal of active somatostatin activates a regulatory loop to increase fundic somatostatin biosynthesis and storage. The data support the concept that somatostatin autoregulates its own expression at both the RNA and peptide level.     

Distribution and molecular forms of peptides containing somatostatin immunodeterminants in extracts from the entire gastrointestinal tract of man and pig

Specimens from human porcine mucosal and muscular tissue from the entire gastrointestinal tract were extracted in acid ethanol, subjected to chromatography and analysed for somatostatin-like immunoreactivity by region-specific radioimmunoassays. The concentration of somatostatin-like immunoreactivity from man and pig ranged from 1.13 +/- 0.37 to 101.15 +/- 33.93 pmol/g wet weight, and from 7.64 to 159.48 +/- 23.79 pmol/g wet weight, respectively. In both species the highest concentrations were found in the jejunum. The immunoreactivity in intestinal mucosal extracts was distributed among four major peaks, two of which were identified by HPLC as somatostatin 1-28 and somatostatin 1-14, respectively. A peak of approx. 10 kDa was resolved by ion exchange plus HPLC into three components, two containing at least part of the somatostatin 1-14 sequence as well as (part of) the somatostatin 1-28(1-14) sequence (but differing in charge), the third containing only the 1-28(1-14) sequence. These peptides probably represent uncleaved and partially cleaved prosomatostatin. The fourth component to be identified by gel filtration was slightly larger than somatostatin 1-14. Extracts from the antrum, the pancreas and from muscular tissues contained almost exclusively somatostatin 1-14, and very little somatostatin 1-28, indicating that the somatostatin precursor is processed differently at these sites. Furthermore, extracts of porcine gastric antrum, analysed for somatostatin 1-28(1-14) immunoreactivity, showed two immunoreactive forms in the mucosa and three major forms in the muscular layers.     

Development of immunoreactive somatostatin in C-cell complexes in the thyroid gland of the dog

Summary In connection with our previous finding that an intense immunoreaction to somatostatin transiently appears in thyroid C cells of the dog during early fetal periods, the present study investigated C-cell complexes in thyroid glands from early fetuses to adults in an attempt to clarify whether the transient appearance of immunoreactivity to somatostatin is dependent on the degree of differentiation of C cells. C-cell complexes retain their fetal characteristics; even in the complexes of postnatal dogs, there are numerous undifferentiated cells, immature C cells and primitive follicular cells, which are not yet organized into follicles. Neither the degree of differentiation of C cells nor that of other constituent elements of the complexes affected the developmental pattern of somatostatin immunoreactivity in C cells. The C cells located in complexes displayed the same pattern of developmental changes in immunoreactivity to somatostatin as the cells in thyroid parenchyma. In the C-cell complexes of early fetal dogs a very intense immunoreactivity for somatostatin was observed; almost all calcitonin-positive cells were also somatostatin positive. The immunoreactivity to somatostatin progressively decreased with age. In the postnatal complexes the number of somatostatin-positive cells was very small compared with that of calcitoninpositive cells.     

Glucose regulation of arginine-induced pancreatic somatostatin release from the isolated perfused rat pancreas

The effects of glucose alone, combinations of glucose with arginine or tolbutamide and either arginine or tolbutamide alone, on somatostatin, insulin, and glucagon secretion were investigated using the isolated perfused rat pancreas. When glucose alone was raised in graded increments at 15-min intervals from an initial concentration of 0 mM to a maximum of 16.7 mM, somatostatin as well as insulin in the perfusate increased with the glucose, while glucagon decreased. The similarity of the glucose stimulated somatostatin and insulin release was especially evident when the perfusate glucose was increased from an initial dose of 4.4 mM rather than 0 mM to 8.8 mM or 16.7 mM. In addition, glucose at concentrations varying from 4.4 mM to 11 mM dose-dependently enhanced arginine-induced somatostatin and insulin release and suppressed glucagon release dose-dependently as before. Arginine in the absence of glucose was not capable of stimulating somatostatin secretion whereas tolbutamide, in contrast, was capable of stimulating somatostatin secretion even in the absence of glucose.     

The morphology of somatostatin-immunoreactive neurons in the lateral nucleus of the rat amygdala

Three types of somatostatin-immunoreactive neurons are described in the lateral nucleus of the rat amygdala. These three types closely correspond to neurons previously reported in Golgi preparations of the lateral nucleus. Class I somatostatin neurons have triangular- or piriform-shaped somata with large primary dendrites and spiny secondary dendrites. Class II somatostatin neurons have small to medium-sized oval perikarya and are fusiform or multipolar in shape. Class III somatostatin neurons have small spheroid somata with small thinner relatively aspinous dendrites. Class I somatostatin neurons give rise to axons which project outside the lateral nucleus whereas class II and III neurons innervate other somatostatin-positive and non-somatostatin neurons within the lateral nucleus. Somatostatin neurons within the lateral nucleus are hypothesized to function as part of a network of somatostatin neurons extending from cortical regions through the amygdala to basal telencephalic and lower brain stem regions.     

Permeability of epidural somatostatin and morphine into the intrathecal space of dogs

In an in vivo saline perfusate of the intrathecal space of 6 dogs, the concentration of somatostatin was determined by radioimmunoassay before and over 2 h after epidural administration of 3 mg somatostatin. The total recovered amount of somatostatin was negligible, about 0.02%. However, within 50 min after the bolus epidural injection of somatostatin, the concentration per ml perfusate increased from 0.1 +/- 0.02 ng/ml to 138 +/- 102 ng/ml (P less than 0.001) and declined to 4 +/- 1.7 ng/ml after 120 min. This increase of the somatostatin concentration by 3 orders of magnitude might explain why epidurally administered somatostatin is effective in treatment of acute and chronic pain. In a control investigation with epidural morphine in another 6 dogs to prove the feasibility of the method, the total recovered amount of morphine in the intrathecal perfusate over 2 h was about 12%.     

Macrophages within the granulomas of murine Schistosoma mansoni are a source of a somatostatin 1-14-like molecule

Inflammatory cells secrete some neuropeptides that function as immunomodulators. Somatostatin has immunoregulatory properties. It is unknown whether immune cells make somatostatin. Intricate intercellular communications govern the granulomas of murine Schistosoma mansoni. These granulomas synthesize several neuropeptides. Thus, it was determined whether somatostatin is in these inflammatory lesions. Granulomas, which were isolated from the livers of infected mice, contained somatostatin 1-14 as shown by radioimmunoassay and chromatography. Immunostaining localized immunoreactive somatostatin and pre-prosomatostatin to granuloma macrophages. Granuloma macrophages cultured in vitro released immunoreactive somatostatin. Calcium ionophore A23187, which promotes macrophage secretion, increased somatostatin in the culture supernatants. Thus, the granulomas have a molecule with somatostatin 1-14-like properties that is possibly a secretory product of the granuloma macrophage.     

Immunohistochemical localization of somatostatin in the human hypothalamus

Summary In order to identify clearly the nervous structures containing somatostatin in the human hypothalamus, an immunohistochemical localization of this neurohormone was performed at light-microscopic level. Using a antiserum specific to somatostatin and the unlabeled antibody peroxidase-antiperoxidase technique, we have found somatostatin in neurons with cell bodies in an area in the anterior hypothalamus corresponding to the infundibular nucleus. Somatostatin-containing fibers were also detected in the neurovascular zone of the pituitary stalk, suggesting that somatostatin is released in that region to reach the capillaries in the pituitary portal plexus. A large bundle of somatostatin fibers extending from the anterior part of the paraventricular nucleus up to the posterior portion of the mammillary bodies has also been detected. The role of these fibers still remains to be clarified.     

Ontogeny and ultrastructure of somatostatin and calcitonin cells in the thyroid gland of the rat

Summary Calcitonin cells are relatively numerous in the thyroid gland of the rat. In contrast, somatostatin cells are very scarce except at the time of birth and a few days thereafter, when they are conspicuously numerous. Somatostatin cells of the thyroid gland, which are ultrastructurally similar to somatostatin cells in gut and pancreas, also contain immunoreactive calcitonin. It is not clear whether somatostatin cells in the rat thyroid gland produce calcitonin or accumulate calcitonin from the environment.     

Effect of somatostatin infusion on VIP-induced transport changes in the human jejunum

This study was designed to elucidate the mechanism by which somatostatin administration ameliorates or abolishes diarrhea in pancreatic cholera syndrome (PCS). Absorption (or secretion) of water and electrolytes was measured in 30-cm segments of jejunum of 18 healthy volunteers in whom PCS was mimicked by intravenous infusion of VIP. Using the triple-lumen tube technique, the intestine was perfused with a plasma-like electrolyte solution while administering intravenous saline (control), VIP (400 pmol/kg/hr), somatostatin (5000 pmol/kg/hr), or VIP plus somatostatin. VIP infusion abolished water and electrolyte absorption and somatostatin had no effect on these VIP-induced transport changes regardless of whether somatostatin infusion was started before or after VIP infusion. Somatostatin infusion had no effect on VIP plasma concentration when elevated by intravenous VIP infusion (control: 10 +/- 1 pmol/l; during VIP infusion: 108 +/- 6). In a patient with pancreatic cholera syndrome identical perfusion experiments showed jejunal water secretion (93 ml/30 cm/hr) which changed to absorption (65 ml/30 cm/hr) when somatostatin was infused (5000 pmol/kg/hr). Plasma VIP concentration fell from 145 to 74 pmol/l (normal less than 50) during somatostatin infusion. Stool weight fell from 3722 g to 819 g per 24 hours when somatostatin was given at a dose of 2500 pmol/kg/hr for two days. Our observations in healthy subjects show that somatostatin has no effect on intestinal transport at the mucosal level when circulating VIP concentration is elevated.(ABSTRACT TRUNCATED AT 250 WORDS)     

The nature of big plasma somatostatin: implications for the measurement of somatostatin-like immunoreactivity in human plasma

Big plasma somatostatin (BPS) represents an artifact of measurement. High-molecular-weight globulins (α, β, and γ) in human plasma inhibit, in a concentration-dependent manner, the binding of radiolabeled somatostatin analogs to antibody directed against somatostatin. The magnitude of inhibition varies with antibody and plasma sample and is greatest for the α-globulin fraction. The mechanism of inhibition involves binding of plasma globulins to antibody, thereby blocking tracer-binding sites, and does not involve inhibition by somatostatin bound noncovalently to plasma proteins or tracer degradation. Thus BPS arises from a property of plasma rather than of somatostatin and so it is suggested that this mechanism may account for the presence of other “big” forms of hormones in plasma.     

Somatostatin, insulin and glucagon secretion by the perfused pancreas from the cysteamine-treated rat

In rats, administration of a single dose of cysteamine (300 mg/kg, intragastrically) induces a depletion of pancreatic somatostatin content (approximately 60%) without modifying pancreatic insulin or glucagon content. In perfused pancreases from cysteamine-treated rats, there was a lack of somatostatin response to glucose, arginine or tolbutamide. In the absence of stimulated somatostatin release, the secretory responses of insulin and glucagon to glucose, to arginine, and to tolbutamide were not significantly different from those observed in pancreases from control rats. Our data do not support the concept that pancreatic somatostatin plays a major role in the control of insulin and glucagon release.     

The expression of the phosphotyrosine phosphatase DEP-1/PTPeta dictates the responsivity of glioma cells to somatostatin inhibition of cell proliferation

Here we characterize the intracellular effectors of the antiproliferative activity of somatostatin in glioma cell lines and post-surgical specimens. The responsiveness to somatostatin correlated with the expression of the phosphotyrosine phosphatase DEP-1/PTPeta, identified in C6 and U87MG cells, in which somatostatin inhibited cell growth. The expression of a dominant negative mutant of DEP-1/PTPeta in C6 cells abolished somatostatin effects, confirming the involvement of this phosphotyrosine phosphatase in such effects. Somatostatin treatment increased the activity of DEP-1/PTPeta and inhibited ERK1/2 activation. Conversely, basic fibroblast growth factor-dependent MEK phosphorylation was not affected, suggesting a direct effect on ERK1/2. In vitro experiments showed that PTPeta was able to interact and dephosphorylate ERK1/2 activated by basic fibroblast growth factor. Furthermore, by transfecting PTPeta in the somatostatin-unresponsive, DEP-1/PTPeta-deficient U373MG cells, the somatostatin-dependent control of cell proliferation was recovered. Finally we evaluated the requirement for DEP-1/PTPeta in somatostatin inhibition of cell proliferation in post-surgical specimens derived from different grade human gliomas. Although all of the glioma analyzed expressed somatostatin receptor mRNA, DEP-1/PTPeta expression was limited to 8 of 22 of the tumors. Culturing seven gliomas, a correlation between the expression of DEP-1/PTPeta and the somatostatin antiproliferative effects was identified. In conclusion we propose that the expression and activation of DEP-1/PTPeta is required for somatostatin inhibition of glioma proliferation.     

Self-replication of somatostatin cells in the antral mucosa of rodents

The possibility that antral somatostatin cells have a self-replicating activity has been studied in three species of rodents: mice, rats and guinea-pigs, after a flash tritiated thymidine injection. The immunocytochemical staining of somatostatin cells, using specific antiserum, was combined with radioautographic procedures. The labelling index for somatostatin cells--and for gastrin cells identified on serial sections--was established after counting a large number of cells at the optical microscope level, on parallel tissue strips removed throughout the entire antrum. A significant percentage of the somatostatin cell population synthesized DNA. Values were similar for the three species of rodents ranging from 0.8 to 1.1%, that is slightly higher than the percentage of labelled gastrin cells, which was 0.67-0.7%. After a 36-hr continuous infusion of radioactive precursor in one rat, the labelling index observed remained low; 2.33% for somatostatin cells and 1.68% for gastrin cells. Colchicine injection in mice allowed the observation of mitotic figures in well differentiated somatostatin cells. Four hours after that injection, the mitotic index was estimated roughly at 0.3%. Thus, evidence has been presented that in rodents a fraction of the antral somatostatin cell population is capable of dividing, similar to the situation in gastrin cells.     

Receptor-mediated internalization is critical for the inhibition of the expression of growth hormone by somatostatin in the pituitary cell line AtT-20.

The inhibitory effect of the neuropeptide somatostatin on the expression of growth hormone was measured by quantitative polymerase chain reaction in the pituitary cell line AtT-20. We demonstrate that this effect is dependent on the internalization of somatostatin-receptor complexes and that it is totally independent from the peptide-induced inhibition of adenylate cyclase. Indeed, the inhibitory effect of the peptide on growth hormone mRNA levels was totally insensitive to pertussis toxin treatment but was totally abolished under conditions which block somatostatin receptor internalization. Comparative confocal microscopic imaging of fluorescent somatostatin sequestration and fluorescence immunolabeling of sst1, sst2A, and sst5 receptors suggests that sst2A is most probably responsible of the inhibitory effect of somatostatin on growth hormone expression.     

Effect of somatostatin on intestinal absorption of nutrients the rat

Effects of somatostatin on absorption of D-glucose, L-leucine and triacylglycerols by the small intestine were studied in rats after treatment with the peptide in vivo and in everted jejunal segments in vitro.Absorption of glucose was not affected in vitro by somatostatin or the analogue [D-Trp⁸, D-Cys¹⁴]somatostatin at concentrations up to 0.006 mM. Addition of various peptidase inhibitors had no influence, suggesting that failure of somatostatins to inhibit absorption was not due to inactivation by peptidases. Glucose absorption in vitro by jejunum from rats treated with high doses of somatostatin in vivo was not different from that of untreated rats. The biguanide phenformin inhibited glucose absorption, whether added in vitro (IC₅₀ ≈ 1 mM) of after treatment in vivo (3–100 mg/kg per os). The blood glucose increase following oral glucose administration in fasted rats was not affected by somatostatin, but significantly suppressed by phenformin.Absorption of leucine in vitro was not affected by somatostatin (up to 0.03 mM) or [D-Trp⁸, D-Cys¹⁴]somatostatin (0.01 mM), but inhibited by phenformin (IC₅₀ = 2 mM).Absorption of acylglycerols (glycerol tri[1-¹⁴C]oleate) administered orally was significantly inhibited by somatostatin (twice 5 mg/kg subcutaneously) and phenformin (100 mg/kg per os).In rats — apparently in contrast to man — somatostatin does not decrease role of somatostatin in carbohydrate absorption remains controversial. Investigations in healthy [9] and diabetic [20] human subjects suggest that the peptide inhibits (directly or indirectly) the intestinal absorption of glucose in man. On the other hand, our results and those of others obtained in experiments in rats [4,11,21] and Rhesus monkeys [7] clearly do not support such a role in these species. Further studies are therefore needed to resolve this problem.     

Localization of somatostatin-, bombesin-, and serotonin-like immunoreactivity in the lung of the fetal Rhesus monkey

Summary Immunoreactivity of regulatory peptides has been demonstrated in the fetal lung of Macaca mulatta by the peroxidase anti-peroxidase method. Serotonin-immunoreactive neuroepithelial bodies are distributed in the airways from the bronchi to the alveolar ducts. Many neuroepithelial bodies also show bombesin-like immunoreactivity; a very few are immunoreactive to somatostatin antiserum. Four populations of neuroepithelial bodies were identified which contain immunoreactivity for 1) serotonin alone, 2) serotonin and bombesin, 3) serotonin and somatostatin, and 4) serotonin, bombesin, and somatostatin. Since bombesin and somatostatin have been demonstrated to have opposite effects on the release of other peptide hormones, it seems likely that the presence of these same peptides in neuroepithelial bodies may have a similar regulatory role in the lung.