Final answer:
An agonist activates a biological response, typically by mimicking the effects of a naturally occurring neurotransmitter and either directly opening ion channels or initiating signaling pathways that lead to metabolic changes within the cell.
Step-by-step explanation:
When studying pharmacodynamics involving receptors, an agonist produces the effect of activating a biological response and opens the ion channel. Agonists mimic the effects of neurotransmitters by binding to the receptor site and promoting an action similar to that of the natural ligand. For example, when acetylcholine binds to the nicotinic receptor, an ion channel is opened, allowing positively charged sodium ions to enter the postsynaptic cell, leading to depolarization.
In contrast, an antagonist would bind to a receptor and block or impede the action of the neurotransmitter, which is not the case with agonists. Furthermore, agonists can also bind to metabotropic receptors, initiating a sequence of events leading to metabolic changes in the cell, such as the activation of G proteins and the generation of second messengers like cAMP.