Hormones usually bind to receptors on target cells in the body. The receptors may be located in the cell membrane (catecholamines, polypeptide hormones), the cytoplasm (steroids), or the nucleus (thyroid hormones and steroids). The binding of the hormone to the receptor leads to a series of changes in the cell that results in the metabolic action of the hormone. In the case of catecholamines and polypeptide hormones, there is activation of adenylyl cyclase, which stimulates intracellular production of cyclic adenosine monophosphate (cAMP). cAMP acts as an internal (second) messenger, effecting the specific biochemical change dictated by the hormone on the target cell. Other hormones such as corticosteroids and thyroid hormone cause increased messenger ribonucleic acid (mRNA) synthesis, leading to protein (enzyme) synthesis.