Final answer:
An agonist is a drug that has affinity for a target cell recipient and produces a functional change in the cell. It increases the activity of particular neurotransmitters by promoting their synthesis, reducing their reuptake, or mimicking their action by binding to their receptors.
Step-by-step explanation:
An agonist is a drug that has affinity for a target cell recipient and produces a functional change in the cell. It increases the activity of particular neurotransmitters by promoting their synthesis, reducing their reuptake, or mimicking their action by binding to their receptors. For example, cocaine impairs the reuptake of dopamine and causes more dopamine to be released into the synaptic cleft, resulting in increased dopamine activity. Therefore, cocaine would be classified as an agonist because it increases the activity of dopamine. Other examples of agonists include drugs used to treat Parkinson's disease, such as dopamine agonists that mimic the effects of dopamine by binding to dopamine receptors.