Agonistsantagonists

Agonistsantagonists refers to the concepts of receptor ligands that either activate or inhibit receptor signaling. In pharmacology, an agonist is a molecule that binds to a receptor and stabilizes its active conformation, eliciting a cellular response. An antagonist binds to the same receptor but does not activate it, and primarily blocks or dampens the action of endogenous ligands or other agonists.

Agonists can be full, producing the maximum possible response, or partial, yielding a submaximal effect. Partial agonists can function as antagonists in the presence of full agonists, by competing for receptor binding and reducing overall activity. Inverse agonists go further by reducing constitutive (basal) receptor activity, producing effects opposite to those of agonists.

Antagonists are commonly described as competitive (reversible, sharing the same binding site as the agonist) or non-competitive (irreversible or allosteric, reducing maximal response). Allosteric antagonists bind at sites distinct from the orthosteric site and inhibit receptor activation. Some compounds exhibit mixed properties, acting as agonists under certain conditions and antagonists under others.

Clinical relevance: the balance of agonist and antagonist actions shapes drug effects, therapeutic efficacy, and safety. Understanding receptor efficacy, potency, and desensitization helps explain tolerance, withdrawal, and drug interactions. Receptor signaling can follow multiple pathways, and modern pharmacology recognizes biased agonism, where a ligand preferentially activates certain signaling routes over others.

Examples include morphine as a mu-opioid receptor agonist; naloxone as a competitive antagonist; buprenorphine as a partial agonist; and various antagonists such as propranolol (beta-adrenergic) and losartan (angiotensin II receptor, with inverse agonist properties).