Distribution of somatostatin receptors in normal and tumor tissue

Specific receptors for somatostatin (SS), mediating the various actions of this peptide, have been described in SS target tissues in animal and man. Using homogenate binding assays, as well as receptor autoradiography, the presence of SS receptors has been demonstrated in various regions of the brain (cortex, limbic system, basal ganglia), the anterior pituitary, the endocrine and exocrine pancreas, the gastrointestinal tract, and the adrenals. There are species-, as well as age-, related variations. Furthermore, there is evidence for different SS receptor subtypes. Interestingly, a large variety of human tumors also contain SS receptors with similar characteristics as those found in normal tissue; in numerous tumors, the SS receptor density is even higher than in healthy tissue counterparts. Most GH- and TSH-producing pituitary adenomas, but also a subgroup of endocrine inactive pituitary adenomas, have SS receptors; most carcinoids and islet cell carcinomas, as well as their metastases, also contain SS receptors. Several differentiated (usually EGF receptor negative) glia tumors possess SS receptors, whereas undifferentiated (EGF receptor positive) glia tumors lack such receptors. Furthermore, a subgroup of breast tumors, usually steroid receptor positive and neuroendocrine-differentiated, contain SS receptors. Finally, small cell lung carcinomas, but not non-small cell carcinomas, often possess SS receptors. These receptors are likely to be functional since in 11 acromegalics and 18 gastroenteropancreatic tumor patients, a positive correlation was observed between their SS receptor status and their hormone secretion sensitivity to Sandostatin.