Somatostatin receptors in pituitary function, diagnosis and therapy

Recent studies have demonstrated that human pituitary adenomas express multiple somatostatin receptor (sst) subtypes. The expression of sst subtypes in human pituitary adenomas is highly variable. This variability in sst subtype expression may explain the variable responsiveness of patients with pituitary adenomas to medical treatment with the sst2-preferring SS-analogs octreotide and lanreotide. In human GH-secreting pituitary adenomas, both sst2 and sst5 are involved in the regulation of GH secretion. In prolactinomas, sst5 receptors are the key receptors in regulating responsiveness to SS. The low abundance of sst2 in prolactinomas explains the lack of efficacy of octreotide in lowering the elevated PRL levels in prolactinoma patients. Octreotide and lanreotide successfully suppress TSH levels, including normalization of thyroid hormone levels in the large majority of patients with TSH-secreting pituitary adenomas, probably due to the high level of sst2, expression in the adenomas. Although sst2 receptors are expressed by a significant proportion of gonadotroph and clinically non-functioning pituitary adenomas, the overall efficacy of sst2-preferring SS-analogs seems low in this type of patients. Finally, corticotroph adenomas may express multiple sst subtypes as well. Octapeptide SS-analogs do not lower circulating ACTH levels in patients with untreated pituitary-dependent Cushing's disease, whereas SS and the SS-analog octreotide suppress pathological ACTH release in some patients with Nelson's syndrome. This review discusses the expression and potential role of sst subtypes in the different types of human pituitary adenomas.