The nuclear hormone-receptor family in the brain: classics and orphans.

Nuclear hormone receptors comprise a superfamily of over 40 transcription factors. About half of them are classical receptors for lipophilic ligands such as steroids and vitamins. Almost all of these true receptors are present in the brain, where they transduce chemical signals from endocrine organs or signals of nutritional origin into cellular responses. The other members resemble the classical receptors in structure, but have no known ligands, and are hence called 'orphan receptors'. The issue of whether ligands for nuclear orphan receptors exist is controversial. Evidence is emerging that orphan receptors might be activated by signal transduction pathways or might be constitutive enhancers or repressors that interact with the classical receptors. Thus, nuclear orphan receptors are placed in strategic positions in the regulation of gene expression in the nervous system.