Neuropeptide somatostatin (SST) is a cyclic neuropeptide containing a disulfide bond and is produced by specialized cells in a large number of human organs and tissues. SST primarily acts as inhibitor of endocrine and exocrine secretion via the activation of five G-protein-coupled receptors (GPCR-A4 subfamily), named SST1-5. SST is ubiquitously expressed in humans, with high concentrations in brain, liver, lungs, pancreas, thyroid, gastrointestinal tract, and adrenal gland mainly acting as an inhibitor of exocrine and endocrine secretions on target organs. SST suppresses GH, prolactin, and TSH production from pituitary gland, insulin, glucagon and exocrine secretions from pancreas, and several gastrointestinal peptides. In the brain, SST acts as neuromodulator, with physiological effects on neuroendocrine, motor, and cognitive functions, and as neurotransmitter, exerting both stimulatory and inhibitory effects,.
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