Corticotropin-releasing factor receptors: physiology, pharmacology, biochemistry and role in central nervous system and immune disorders.

Corticotropin-releasing factor (CRF) plays a major role in coordinating the endocrine, autonomic, behavioral and immune responses to stress through actions in the brain and the periphery. CRF receptors identified in brain, pituitary and spleen have comparable kinetic and pharmacological characteristics, guanine nucleotide sensitivity and adenylate cyclase-stimulating activity. Differences were observed in the molecular mass of the CRF receptor complex between the brain (58,000 Da) and the pituitary and spleen (75,000 Da), which appeared to be due to differential glycosylation of the receptor proteins. The recently cloned CRF receptor in the pituitary and the brain (designated as CRF1) encodes a 415 amino acid protein comprising seven putative membrane-spanning domains and is structurally related to the calcitonin/vasoactive intestinal peptide/growth hormone-releasing hormone subfamily of G-protein-coupled receptors. A second member of the CRF receptor family encoding a 411 amino acid rat brain protein with approximately 70% homology to CRF1 has recently been identified (designated as CRF2); there exists an additional splice variant of the CRF2 receptor with a different N-terminal domain encoding a protein of 431 amino acids. In autoradiographic studies, CRF receptors were localized in highest densities in the anterior and intermediate lobes of the pituitary gland, olfactory bulb, cerebral cortex, amygdala, cerebellum and the macrophage-enriched zones and red pulp regions of the spleen. CRF can modulate the number of CRF receptors in a reciprocal manner. For example, stress and adrenalectomy increase hypothalamic CRF secretion which, in turn, down-regulates CRF receptors in the anterior pituitary. CRF receptors in the brain and pituitary are also altered as a consequence of the development and aging processes. In addition to a physiological role for CRF in integrating the responses of the brain, endocrine and immune systems to physiological, psychological and immunological stimuli, recent clinical data implicate CRF in the etiology and pathophysiology of various endocrine, psychiatric, neurologic and inflammatory illnesses. Hypersecretion of CRF in the brain may contribute to the symptomatology seen in neuropsychiatric disorders, such as depression, anxiety-related disorders and anorexia nervosa. Furthermore, overproduction of CRF at peripheral inflammatory sites, such as synovial joints may contribute to autoimmune diseases such as rheumatoid arthritis. In contrast, deficits in brain CRF are apparent in neurodegenerative disorders, such as Alzheimer's disease, Parkinson's disease and Huntington's disease, as they relate to dysfunction of CRF neurons in the brain areas affected in the particular disorder. Strategies directed at developing CRF-related agents may hold promise for novel therapies for the treatment of these various disorders.