Final answer:
An agonist is a drug that mimics the action of a neurotransmitter at receptor sites, increasing neurotransmitter activity. Examples include dopamine agonists used in the treatment of Parkinson's disease.
Step-by-step explanation:
An agonist is a drug that increases the activity of specific neurotransmitters by mimicking their action at receptor sites. Agonists can promote neurotransmitter synthesis, reduce reuptake, or bind to receptors.
For example, in the case of Parkinson's disease, which is associated with low dopamine levels, dopamine agonists are commonly used as a treatment to mimic the effects of dopamine by binding to dopamine receptors.