Specific pituitary vasopressin receptors involved in stress-induced ACTH secretion have been postulated for a long time. This hypothesis was recently confirmed by the cloning of a cDNA coding for a new subtype of vasopressin receptor, termed V3 or V1b (1, 2). This receptor belongs to the superfamily of seven transmembrane domain receptors coupled to G proteins. It is coupled to a Gq-like protein and activates phospholipase C. The pharmacological characteristics of the V3 receptor are different from the characteristics of the Via and V2 receptors. In humans, this receptor seems mainly expressed in the pituitary corticotrophs.
Various neuroendocrine tumors expressing the proopiomelanocortin (POMC) gene, such as bronchial carcinoids, small cell carcinomas of the lung (SCCL), pheochromocytomas, pancreatic carcinoids, medullary thyroid cancer and others are known to secrete ectopic ACTH and induce Cushing's syndrome. The bronchial carcinoids are usually highly differentiated and have a rather benign course. On the other hand,