An agonist is a drug that binds to and activates the receptor site, eliciting a cellular response. Agonist binding may activate a receptor that was resting or turn off a receptor that was activated.
The drug with the greatest affinity will bind to the receptor site. The result of agonist binding may mimic the effects produced by binding of normal body chemicals to their target receptor. An example is the binding of barbiturates to their drug-receptor site.
This mimics the effects produced by the neurotransmitter γ-aminobutyric acid (GABA), a chemical messenger of the nervous system, when it binds to its receptor site. Drug-receptor binding may also stimulate the release of a normal biological chemical.
For example, when the drug amantadine is administered it stimulates the release of the neurotransmitter dopamine. An increase in dopamine levels reduces the symptoms associated with Parkinson disease.
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