An agonist is a term used to describe a type of ligand or drug that binds and alters the activity of a receptor. The ability to alter the activity of a receptor, also known as the agonist's efficacy is a property that distinguishes it from antagonists, a type of receptor ligand which also bind a receptor but which do not alter the activity of the receptor. The efficacy of an agonist may be positive, causing an increase in the receptor's activity or negative causing a decrease in the receptor's activity.
An inverse agonist is an agent which binds to the same receptor binding-site as an agonist for that receptor and reverses constitutive activity of receptors. Inverse agonists exert the opposite pharmacological effect of a receptor agonist.
A co-agonist works with other co-agonists to produce the desired effect together. NMDA receptor activation requires the binding of both of its glutamate and glycine co-agonists. An antagonist blocks a receptor from activation by agonists.
A selective agonist is selective for one certain type of receptor. It can additionally be of any of the aforementioned types.
A physiological agonist is a substance that creates the same bodily responses, but does not bind to the same receptor.
Receptors can be activated or inactivated either by endogenous (such as hormones and neurotransmitters) or exogenous (such as drugs) agonists and antagonists, resulting in stimulating or inhibiting a biological response. To see how an agonist may activate a receptor see this link
New findings that broaden the conventional definition of pharmacology demonstrate that ligands can concurrently behave as agonist and antagonists at the same receptor, depending on effector pathways. Terms that describe this phenomenon are "functional selectivity" or "protean agonism".
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