Agonist

An agonist is a ligand that binds to a protein target (e.g. a receptor) and activates the target. An agonist has both affinity for, and efficacy at, its target receptor.

For example, if the target is an ion channel, an agonist would bind to the channel protein and would open the channel to allow ions to flow into or out of a cell. As a result of activating (i.e. changing the behaviour of) the protein, the physiology of the cell or tissue on which that protein is located can be altered. Acetylcholine is an agonist at muscarinic receptors. When acetylcholine binds to these receptors, it triggers an intracellular signal transduction process that may cause an increase in intracellular calcium, which then triggers calcium-dependent processes such as muscle contraction. Most endogenous ligands for receptors are agonists. Further, most endogenous agonists are full agonists, which means they are able to evoke a maximal response from a tissue. This is discussed further under the manual entry for efficacy.