Somatostatin

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D cell is visible at upper right, and somatostatinis represented by middle arrow pointing left
Somatostatin
Identifiers
Symbol(s)	SST
Entrez	6750
OMIM	182450
RefSeq	NM_001048
UniProt	P61278
Other data
Locus	Chr. 3 q28

Somatostatin is a peptide hormone that regulates the endocrine system and affects neurotransmission and cell proliferation via interaction with G-protein-coupled somatostatin receptors and inhibition of the release of numerous secondary hormones. Somatostatin has two active forms produced by alternative cleavage of a single preproprotein: one of 14 amino acids, the other of 28 amino acids.

Somatostatin is secreted not only by cells of the hypothalamus but also by delta cells of stomach, intestine, and pancreas. It binds to somatostatin receptors.

[edit] Actions

Somatostatin is classified as an inhibitory hormone, whose main actions are to:

Inhibit the release of growth hormone (GH)
Inhibit the release of thyroid-stimulating hormone (TSH)
Suppress the release of gastrointestinal hormones
- Gastrin
- Cholecystokinin (CCK)
- Secretin
- Motilin
- Vasoactive intestinal peptide (VIP)
- Gastric inhibitory polypeptide (GIP)
- Enteroglucagon (GIP)
Lowers the rate of gastric emptying, and reduces smooth muscle contractions and blood flow within the intestine.
Suppress the release of pancreatic hormones
- Inhibit the release of insulin
- Inhibit the release of glucagon
Suppress the exocrine secretory action of pancreas.

Somatostatin opposes the effects of Growth Hormone-Releasing Hormone (GHRH)

[edit] Synthetic substitutes

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

[edit] 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 of 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 are populations of somatostatin neurons in the arcuate nucleus, the hippocampus and the brainstem nucleus of the solitary tract.

Hormones and endocrine glands - edit

Hypothalamus: - TRH - CRH - GnRH - GHRH - somatostatin - dopamine | Posterior pituitary: vasopressin - oxytocin - lipotropin | Anterior pituitary: GH - ACTH - TSH - LH - FSH - prolactin - MSH - endorphins - lipotropin