Somatostatin is a peptide hormone which inhibits the secretion of growth hormone from the anterior pituitary gland. It regulates the endocrine system, affecting neurotransmission and cell proliferation via interaction with G-protein-coupled somatostatin receptors and inhibition of the release of numerous secondary hormones. Somatostatin consists of 14 amino acids, with two cysteine residues joining by a disulfide bond so that the peptide forms a ring structure. A larger variant of this peptide, called somatostatin-28, is secreted in some cells.
Somatostatin is synthesized and released 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. Somatostatin is carried in the blood stream through these blood vessels 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.