What type of drug initiates a physiological response when bound to a receptor?

An agonist is a type of drug that, when bound to a receptor, initiates a physiological response. This means that an agonist activates the receptor it binds to, leading to a biological effect. For example, when an agonist interacts with a neurotransmitter receptor, it mimics the action of a natural ligand, resulting in similar physiological responses, such as increased heart rate or enhanced mood depending on the type of receptor targeted.

In contrast, antagonists block receptors and prevent the natural ligand from binding, thereby inhibiting the usual effect. Inhibitors typically work by decreasing the activity of enzymes or proteins rather than directly affecting receptor activation. Modifiers, while they might suggest altering responses, do not specifically indicate a mechanism involving receptor activation in the same direct way that agonists do. Therefore, understanding the role of agonists is crucial for comprehending many pharmacological effects and therapeutic approaches.