Corticotropin-releasing hormone physiology

Corticotropin-releasing hormone (CRH), also known as corticotropin-releasing factor, is a highly conserved peptide hormone comprising 41 amino acid residues. Its name derives from its role in the anterior pituitary, where it mediates the release of corticotropin (ACTH) leading to the release of adrenocortical steroids. CRH is the major hypothalamic activator of the hypothalamic–pituitary– adrenal (HPA) axis. Major functions of the HPA include: (i) influencing fetal development of major organ systems including lung, liver, and gut, (ii) metabolic functions, including the maintenance of normal blood glucose levels during the fasting state via glycogenolysis and gluconeogenesis, (iii) modulation of immune function, and (iv) maintenance of cardiovascular tone. In addition, CRH, acting both directly and via the HPA, has a role in regulating several neuroendocrine functions including behavior, food intake, reproduction, growth, immune function, and autonomic function. CRH has been localized to the paraventricular nucleus (PVN) of the hypothalamus, which projects to the median eminence and other hypothalamic and midbrain targets. The CRH gene is composed of two exons. The CRH promoter contains a cAMP-response element, and the intron contains a restrictive element-1/neuron restrictive silencing element (RE-1/NRSE) sequence. Recently, a family of CRH-related peptides, termed the urocortins, has been identified. These peptides probably play a role in integrating multiple aspects of the stress-response, although their functions are largely unknown. Both CRH and the urocortins interact with two transmembrane G-protein-coupled cell surface receptors, CRH-R1, and CRH-R2, which differ in their patterns of tissue distribution. In addition, the binding affinities for CRH and the urocortins to the two receptors differ considerably, and may contribute to the different actions of these peptides.