Agonist – a type of chemical

An agonist is a type of chemical known as a ligand. It binds to a receptor. It is a compound that can bind to and cause activation of a receptor, thus mimicking an endogenous ligand or neurotransmitter. It activates (switches on) the receptor to produce a response. In Pharmacology, it is a chemical substance capable of activating a receptor to induce a full or partial pharmacological response.

An agonist causes an action. It is a substance that acts like another substance and therefore stimulates an action. There are also antagonists which block the action of the agonists, and there are inverse agonists which cause an action opposite to that of the agonist. It is a chemical that binds to a receptor and activates the receptor to produce a biological response.

Agonist is a drug or substance that binds to a receptor inside a cell or on its surface and causes the same action as the substance that normally binds to the receptor.

This is typical of the systems which regulate the body. Being able to control activity is the basis of homeostasis. It is a chemical substance capable of combining with a specific receptor on a cell and initiating the same reaction or activity typically produced by the binding endogenous substance.

Types of agonists

Receptors can be activated by either endogenous or exogenous agonists. Both types result in a biological response. Agonists and most natural ligands bind to receptors in their inactive state and quickly induce an active receptor conformation that initiates cell signaling. Endogenous agonists include hormones and neurotransmitters. Exogenous agonists include drugs.

A substance which causes the same bodily responses, but does not bind to the same receptor is called a physiological agonist. The active receptor state initiates signaling because of its structural complementariness with coupling proteins that activate signaling pathways, such as G proteins and G protein-coupled receptor kinases.


  • The endogenous agonist for serotonin receptors is serotonin, and the endogenous agonist for dopamine receptors is dopamine.
  • Morphine is an exogenous agonist. It mimics the actions of endorphins at certain receptors in the central nervous system.

Agonist is the opposite of antagonist. Antagonists and agonists are key players in the chemistry of the human body and in pharmacology. These are drugs or naturally occurring substances that activate physiologic receptors, whereas antagonists are drugs that block those receptors.

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