<Ol> <Li> Ligands </Li> <Li> Receptors </Li> <Li> Secondary messengers </Li> </Ol> <P> In biochemistry and pharmacology, a receptor is a protein molecule that receives chemical signals from outside a cell . When such chemical signals bind to a receptor, they cause some form of cellular / tissue response, e.g. a change in the electrical activity of a cell . In this sense, a receptor is a protein - molecule that recognizes and responds to endogenous chemical signals, e.g. an acetylcholine receptor recognizes and responds to its endogenous ligand, acetylcholine . However, sometimes in pharmacology, the term is also used to include other proteins that are drug targets, such as enzymes, transporters, and ion channels . </P> <P> Receptor proteins can be classified by their location . Transmembrane receptors include ion channel - linked (ionotropic) receptors, G protein - linked (metabotropic) hormone receptors, and enzyme - linked hormone receptors . Intracellular receptors are those found inside the cell, and include cytoplasmic receptors and nuclear receptors . A molecule that binds to a receptor is called a ligand, and can be a protein or peptide (short protein), or another small molecule such as a neurotransmitter, hormone, pharmaceutical drug, toxin, or parts of the outside of a virus or microbe . The endogenously designated - molecule for a particular receptor is referred to as its endogenous ligand . E.g. the endogenous ligand for the nicotinic acetylcholine receptor is acetylcholine but the receptor can also be activated by nicotine and blocked by curare . </P>

Any substance that binds to a receptor is known as a
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