THE NUCLEAR HORMONE-RECEPTOR FAMILY IN THE BRAIN - CLASSICS AND ORPHANS

Nuclear hormone receptors comprise a superfamily of over 40 transcript ion factors. About half of them are classical receptors for lipophilic ligands such as steroids and vitamins. Almost all of these true recep tors are present in the brain, where they transduce chemical signals f rom endocrine organs or signals of nutritional origin into cellular re sponses. The other members resemble the classical receptors in structu re, but have no known ligands, and are hence called 'orphan receptors' . The issue of whether ligands for nuclear orphan receptors exist is c ontroversial. Evidence is emerging that orphan receptors might be acti vated by signal transduction pathways or might be constitutive enhance rs or repressors that interact with the classical receptors. Thus, nuc lear orphan receptors are placed in strategic positions in the regulat ion of gene expression in the nervous system.