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S. Harvey

Introduction

Growth hormone (GH) secretion has traditionally been considered to be under dual hypothalamic control, being stimulated by a GH-releasing factor (GRF) and suppressed by somatostatin (SRIF), an inhibitory releasing factor (Müller, 1987). These hypothalamic peptides are released into hypophysial circulation in response to stimuli in the internal and external environment, and act at receptors on somatotroph cells to regulate GH synthesis and release. Hypophysial portal plasma, however, also transports other hypophysiotrophic factors to the pituitary gland, and somatotrophs are undoubtedly exposed to other putative GRFs.

Thyrotrophin-releasing hormone (TRH; pGlu-His-Pro-NH2) was the first hypophysiotrophic peptide to be isolated and synthesized chemically and was called TRH because it was found to stimulate thyrotrophin (TSH) release from the pituitary gland (Nelson, 1982). However, since its discovery, TRH has been found to be synthesized in numerous locations throughout the 'diffuse neuroendocrine system', and in addition to its neuroendocrine role in the regulation of

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Raúl M Luque and Rhonda D Kineman

Neuronostatin, a somatostatin gene-encoded peptide, exerts important physiological and metabolic actions in diverse tissues. However, the direct biological effects of neuronostatin on pituitary function of humans and primates are still unknown. This study used baboon (Papio anubis) primary pituitary cell cultures, a species that closely models human physiology, to demonstrate that neuronostatin inhibits basal, but not ghrelin-/GnRH-stimulated, growth hormone (GH) and luteinizing hormone (LH) secretion in a dose- and time-dependent fashion, without affecting the secretion of other pituitary hormones (prolactin, ACTH, FSH, thyroid-stimulating hormone (TSH)) or changing mRNA levels. Actions of neuronostatin differs from somatostatin which in this study reduced GH/PRL/ACTH/LH/TSH secretion and GH/PRL/POMC/LH gene expression. Remarkably, we found that inhibitory actions of neuronostatin are likely mediated through: (1) the orphan receptor GPCR107 (found to be highly expressed in pituitary compared to somatostatin-receptors), (2) common (i.e. adenylyl cyclase/protein kinase A/MAPK/extra-/intracellular Ca2+ mobilization, but not phospholipase C/protein kinase C/mTOR) and distinct (i.e. PI3K) signaling pathways than somatostatin and; (3) dissimilar molecular mechanisms than somatostatin (i.e. upregulation of GPCR107 and downregulation of GHS-R/Kiss1-R expression by neuronostatin and, upregulation of sst1–5 expression by somatostatin). Altogether, the results of this study provide the first evidence that there is a functional neuronostatin signaling circuit, unique from somatostatin, which may work in concert with somatostatin to fine-tune hormone release from somatostropes and gonadotropes.

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R. P. MILLAR

U.C.T./M.R.C. Protein Research Unit, Department of Chemical Pathology, Medical School, Observatory 7925, Cape Town, Republic of South Africa

(Received 26 January 1978)

Higher molecular weight (HMW) immunoreactive forms of somatostatin have been reported in extracts of ovine hypothalami (Vale, Ling, Rivier, Villarreal, Rivier, Douglas & Brown, 1976), rat pancreas and stomach (Arimura, Sato, Dupont, Nishi & Schally, 1975) and human pancreatic somatostatinoma (Larsson, Hirsch, Holst, Ingemansson, Kühl, Jensen, Lundquist & Rehfeld, 1977). However, the possibility that the HMW immunoreactive substances were oligomers of somatostatin or somatostatin bound to larger molecules was not excluded. The present study was undertaken to establish that authentic HMW immunoreactive somatostatin is present in the ovine hypothalamus and to glean information on the structural relationship of the HMW species with somatostatin.

Sheep hypothalami were extracted and subjected to gel permeation chromatography as described previously by Millar, Aehnelt & Rossier (1977). Fractions were assayed for somatostatin immunoreactivity

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A. S. Tischler, Y. C. Lee, D. Costopoulos, G. Nunnemacher, R. A. DeLellis, M. J. Van Zwieten, H. J. Wolfe and S. R. Bloom

ABSTRACT

A continuous line of somatostatin-producing medullary thyroid carcinoma cells was established from a transplantable tumour in BALB/c mice. Virtually all of the somatostatin immunoreactivity co-chromatographed with somatostatin 14. The tumour cells replicated in spinner cultures with a doubling time of approximately 4 days, and the concentration of somatostatin released into the culture medium increased in proportion to the number of cells. Two-to threefold increases in amounts of stored and released somatostatin were observed after treatment of the cells with bromodeoxyuridine. This cell line might be valuable for studies of somatostatin regulation in normal and neoplastic C-cells, and for other studies of C-cell biology which require a mouse model.

J. Endocr. (1986) 110, 309–313

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YUZURU KATO, KAZUO CHIHARA, SHOZO OHGO and HIROO IMURA

Recent studies in animals have demonstrated that growth hormone (GH) secretion is controlled by GH releasing factor (GHRF) and GH inhibiting factor (somatostatin). Somatostatin has not only been purified from the ovine hypothalamus, but also synthesized (Brazeau, Vale, Burgus, Ling, Butcher, Rivier & Guillemin, 1973). It has been demonstrated recently that synthetic somatostatin suppresses the spontaneous secretion of GH and the increase in plasma GH induced by i.v. injection of pentobarbitone in the rat (Brazeau, Rivier, Vale & Guillemin, 1974). We have previously reported that a single i.v. injection of isoprenaline or chlorpromazine causes a significant increase in plasma GH in the rat (Kato, Dupre & Beck, 1973). In the present experiment, we examined the effect of synthetic somatostatin (kindly supplied by Dr N. Yanaihara) on the response of plasma GH to isoprenaline or chlorpromazine in rats.

Wistar strain male rats, weighing 180–220 g, were housed in an air-conditioned room

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Ch. Foltzer, S. Harvey and P. Mialhe

ABSTRACT

Variations in the concentrations of plasma and pituitary GH were determined in ducks for 66 and 87 days after hatch, and compared with somatostatin-like immunoreactivity (SLI) in the plasma, hypothalamus and neural lobe. Plasma GH levels gradually decreased during growth, while pituitary GH content increased. The concentration of pituitary GH increased during the first 3 weeks of age and remained relatively constant thereafter. The decline in plasma GH concentration was paralleled by a similar fall in the level of plasma SLI. While the content of hypothalamic SLI increased during development, the SLI concentration was maximal at 14 days of age and lowest in adults. The content and concentration of SLI in the neural lobe, in contrast, increased progressively during development. Gel filtration of hypothalamic and neural lobe extracts demonstrated that both young and older birds had two main peaks of SLI, corresponding to somatostatin-14 and somatostatin-28, and a third, larger form. The elution pattern of plasma SLI was similar in young and older birds and was principally composed of a large molecular species ('big' somatostatin), although an additional small peak eluting between somatostatin-28 and somatostatin-14 was eluted from a large pool of plasma from 90-day-old ducks. These results suggest that increased plasma GH levels in young birds do not result from a hypothalamic somatostatin deficiency nor from variations in molecular forms of SLI, and that the age-related decline in plasma GH concentration is not due to a deficiency in pituitary GH content. The decline in the circulating GH level during growth is probably due to an increase in hypothalamic somatostatin release.

J. Endocr. (1987) 113, 57–63

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S. H. Shin, R. L. Heisler and M. S. Szabo

ABSTRACT

Patterns of prolactin release were examined using stimulating and inhibiting agents. Primary cultured pituitary cells primed with oestrogens were used for perifusion experiments. TRH (100 nmol/l) increased the peak prolactin concentration to 360% of the basal concentration, while TRH, under inhibition by 1 nmol somatostatin/l, raised the peak prolactin concentration to 185% of the basal levels. When the somatostatin concentration was increased to 10, 100 and 1000 nmol/l, TRH still stimulated prolactin release to 128%, 121% and 140% respectively, indicating that concentrations of somatostatin of 10 nmol/l or higher did not further suppress the stimulatory effect of TRH. TRH (1 μmol/l) stimulated prolactin release under the influence of 0 (control), 1, 10, 100 and 1000 nmol dopamine/l (plus 0·1 mmol ascorbic acid/l) to 394, 394, 241, 73 and 68% of the basal concentration respectively, showing that the dopamine concentrations and peak prolactin concentrations induced by TRH have an inverse linear relationship in the range 1–100 nmol dopamine/l. The stimulatory effect of dibutyryl cyclic AMP (dbcAMP) on prolactin release was also tested. The relationship between dbcAMP and somatostatin was similar to that between TRH and somatostatin. When adenohypophyses of male rats were used for perifusion experiments, somatostatin (100 nmol/l) did not inhibit basal prolactin release from the fresh male pituitary in contrast with the primary cultured pituitary cells, but dopamine (1 μmol/l) effectively inhibited prolactin release.

In conclusion, (1) oestrogen converts the somatostatin-insensitive route into a somatostatin-sensitive route for basal prolactin release, (2) TRH-induced prolactin release passes through both somatostatin-sensitive and -insensitive routes, (3) dopamine blocks both somatostatin-sensitive and -insensitive routes and (4) cAMP activates both somatostatin-sensitive and -insensitive routes.

Journal of Endocrinology (1991) 130, 79–86

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Alejandro Ibáñez-Costa and Márta Korbonits

Classic somatostatin analogues aimed at somatostatin receptor type 2, such as octreotide and lanreotide, represent the mainstay of medical treatment for acromegaly. These agents have the potential to decrease hormone secretion and reduce tumour size. Patients with a germline mutation in the aryl hydrocarbon receptor-interacting protein gene, AIP, develop young-onset acromegaly, poorly responsive to pharmacological therapy. In this review, we summarise the most recent studies on AIP-related pituitary adenomas, paying special attention to the causes of somatostatin resistance; the somatostatin receptor profile including type 2, type 5 and truncated variants; the role of G proteins in this pathology; the use of first and second generation somatostatin analogues; and the role of ZAC1, a zinc-finger protein with expression linked to AIP in somatotrophinoma models and acting as a key mediator of octreotide response.

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N. H. GRANT, D. SARANTAKIS and J. P. YARDLEY

Somatostatin, the growth hormone release inhibitory hormone (Brazeau, Vale, Burgus, Ling, Butcher, Rivier & Guillemin, 1973), NH2-Ala-Gly-Cys-Lys-Asn-Phe-Phe-Trp-Lys-Thr-Phe-Thr-Ser-Cys-Co2H, is the first hypothalamic hormone shown unequivocally to exert a negative control over pituitary secretion. Its inhibition of growth hormone release has been confirmed in normal and diabetic men (Hansen, Ørskov, Seyer-Hansen & Lundbaek, 1973).

The present study was undertaken to determine whether somatostatin can also alter prolactin release. Cultures of secreting pituitary cells offer the least ambiguous model for such a question, because of the enormous population of viable secreting units, the expectation of direct interaction between receptor and hormone, and a level of basal secretion sufficiently high to show a range of changes by radioimmunoassay. Monolayer cultures of enzymically dispersed cells were prepared from whole pituitaries of normal Charles River CD male rats, using Eagle's minimal essential medium for growth (Grant, Clark & Rosanoff, 1973). After test

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M J Pesek and M A Sheridan

Abstract

Somatostatins are a diverse family of peptides that influence various aspects of animal growth, development, and metabolism. Recent work in our laboratory has shown that somatostatins stimulate hepatic lipolysis in rainbow trout. In this study we characterized somatostatin-binding sites in trout hepatic membrane preparations. We also examined changes in binding characteristics brought about by food deprivation. Binding of [Tyr11]-somatostatin-14 (SS-14) was saturable, reversible, and time- and temperature-dependent. Under optimal conditions, [Tyr11]-SS-14 specific binding averaged 5·7 ± 0·3%. While SS-14 and SS-28 (an N-terminally extended form of SS-14 and derived from the same gene as SS-14) displaced [Tyr11]-SS-14 specific binding (ED50 values of approximately 50 nm and 100 nm respectively), salmon SS-25 (containing [Tyr7,Gly10]-SS-14 at its C terminus and presumably derived from a gene different from that giving rise to SS-14/SS-28), except at pharmacological concentrations, did not. Significant specific binding was also detected in brain, esophagus, stomach, upper and lower intestine, pancreas, and adipose tissue. Scatchard analysis suggested the existence of two classes of hepatic somatostatin-binding sites: a high-affinity site with a K d of 23 nm and Bmax of 1·4 pmol/mg protein and a low-affinity site with a K d of 379 nm and Bmax of 4·9 pmol/mg protein. Fasting resulted in reduced growth and elevated plasma levels of SS-14 compared with fed animals. SS-14 binding capacity of the high-affinity class in liver membranes isolated from fasted fish increased by 120% over that from fed counter-parts. No difference in K d for the high-affinity binding class or in either K d or Bmax of the low-affinity class was noted between fasted and fed animals. These data support the role of the liver as a target of somatostatin and suggest that fasting enhances hepatic sensitivity to SS-14 binding.

Journal of Endocrinology (1996) 150, 179–186