Orphan receptors of the nuclear receptor group are apparent receptors that have a similar structure as other identified receptors but whose ligand is not yet known. Their function is to be elucidated. Once the ligands of the orphan receptors were found, they were then referred as adopted ophan receptor. For example, liver X receptor(LXR), farnesoid X receptor(FXR), and peroxisome proliferator-activated receptor(PPAR) are adopted orphan receptors. Nuclear receptors are a class of intracellular receptors which function as ligand activated transcription factors which up or down regulate the expression of genes. ... In chemistry, a ligand is an atom, ion, or molecule (see also: functional group) that generally donates one or more of its electrons through a coordinate covalent bond to, or shares its electrons through a covalent bond with, one or more central atoms or ions (these ligands act as a... The liver X receptor (LXR) is a nuclear hormone receptor with activity similar to that seen in other steroid receptors such as estrogen or progesterone but more similar in form to PPAR, FXR and RXR. There are two isoforms known as LXR-alpha and LXR-beta. ... The farnesoid X receptor (FXR) is a nuclear hormone receptor with activity similar to that seen in other steroid receptors such as estrogen or progesterone but more similar in form to PPAR, LXR and RXR. Its not as good as PXR. Chenodeoxycholic acid and other bile acids are the... PPAR gamma In cell biology, peroxisome proliferator-activated receptors (PPARs) are a group of nuclear receptor isoforms that exist across biology. ...
receptor was accepted as a member of the "family" of opioid receptors on the basis of its structural homology towards the classical types, there is no corresponding pharmacological homology.
However, different responses evoked in different cell types in response to activation of different opioid receptors or even in response to activation of the same receptor are likely to reflect changes in the expression of G proteins and effector systems between cell types rather than any inherent differences in the properties of the receptors themselves.
Among the receptors for the many neuropeptides that exist in the nervous system, the opioid receptors are unique in that there existed before the discovery of the natural agonists, an abundance of non-peptide ligands with which the pharmacology of the receptors was already defined.
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