Somatostatin

Somatostatin is a peptide hormone comprising two peptides, one built of 14 amino acids, the other of 28 amino acids. Like all peptide hormones, it is packaged in large dense core secretory vesicles ahich are secreted by calcium-dependent exocytosis. Somatostatin is secreted by [[neurosecretory (neuroendocrine neurons of the hypothalamus, and also by delta cells of stomach, intestine, and pancreas. It acts via specific, high affinity G-protein coupled receptors.

Actions

Somatostatin

• Inhibits the release of growth hormone (GH) and thyroid-stimulating hormone (TSH)Somatostatin is a functional antagonist of Growth hormone-releasing hormone (GHRH)
• Inhibits the secretion of gastrointestinal hormones (specifically Gastrin, Cholecystokinin,

Secretin, Motilin, Vasoactive intestinal peptide, Gastric inhibitory polypeptide and Enteroglucagon)

• Lowers the rate of gastric emptying, and reduces smooth muscle contractions and blood flow within the intestine.
• Suppress the secretion of the pancreatic hormones insulin and glucagon

Synthetic substitutes

Octreotide (brand name Sandostatin, Novartis Pharmaceuticals]]) is an octopeptide that mimics natural somatostatin pharmacologically, but is a more potent inhibitor of growth hormone, glucagon, and insulin than the natural hormone. The FDA has approved the use of a form of this peptide octreotide acetate, for the treating acromegaly, and for treating diarrhea and flushing episodes associated with carcinoid syndrome, and treatment of diarrhea in patients with vasoactive intestinal peptide-secreting tumors. Octreotide has also been used off-label for treating severe, refractory diarrhea from other causes. It is also used for treating prolonged recurrent hypoglycemia after sulfonylurea overdose.

Somatostatin in the brain

Somatostatin is produced by neuroendocrine neurons of the periventricular nucleus of the hypothalamus. These neurons project to the median eminence, where somatostatin is released from neurosecretory nerve endings into the hypothalamo-hypophysial portal circulation. These blood vessels carry somatostatin to the anterior pituitary gland, where somatostatin inhibits the secretion of growth hormone from somatotrope cells. The somatostatin neurons in the periventricular nucleus mediate negative feedback effects of growth hormone on its own release; the somatostatin neurons respond to high circulating concentrations of growth hormone and somatomedins by increasing the release of somatostatin, so reducing the rate od secretion of growth hormone.

Somatostatin is also produced by several other populations that project centrally - i.e. to other areas of the brain, and somatostatin receptors are expressed at many different sites in the brain. In particular, there ae populations of somatostatin neurons in the arcuate nucleus, the hippocamus and the brainstem nucleus of the solitary tract.