SST (growth hormone-inhibiting hormone, somatostatin, SMST) is an inhibitory cyclic peptide produced by somatostatinergic neurons in the brain and delta cells in pancreatic islets, the duodenum and the pyloric antrum of the stomach. SST functions to regulate the endocrine system, neurotransmission and cell proliferation by inhibiting neurotransmitter and hormone release upon binding to various SST receptors. Deregulated SST is associated with excitation/inhibition balance in various neurodegenerative and neuropsychiatric disorders. Cognitive deficits in Alzheimer’s disease correlate with a decrease in SST expression in the brain. A decrease in SST expression has also been reported in Parkinson’s disease, major depressive disorder and schizophrenia. Somatostatin release also correlates with seizures, and it is highly expressed in regions of the brain related to seizures. Finally, pretreatment with SST may help prevent dopaminergic neuronal degeneration in Parkinson’s disease. In immunohistochemistry of normal tissue, SST has secretory and extracellular positivity in the hypothalamus, adrenal medulla and cortex in the brain, in islets of Langerhans in the pancreas, and in gastrointestinal neuroendocrine cells. References: PLoS One. 2019 May 28;14(5):e0217392, PMID: 31136617; Mol Med Rep. 2015 Jul;12(1):1002-8, PMID: 25777539; Front Endocrinol (Lausanne). 2012 Dec 6;3:154, PMID: 23230430;