- For other meanings of 'agonist' or 'agonism', see agonism
(disambiguation).
In pharmacology an agonist is a substance that binds to a specific
receptor and triggers a response in the cell. It mimics the action of an endogenous ligand (such as hormone or neurotransmitter) that binds to the same receptor.
Types
Full agonists bind (have affinity for) and activate a receptor,
displaying full efficacy at that receptor.
Partial agonists (such as buspirone, aripiprazole, buprenorphine, or norclozapine) also bind and activate a given receptor, but have only partial efficacy at the receptor relative to a full agonist. They may also be considered as a ligand which displays
both agonistic and antagonistic effects, as in the presence of an full agonist, a partial agonist actually acts as a competitive
antagonist.[1] A co-agonist works with other
co-agonists to produce the desired effect together. An antagonist blocks a receptor
from activation by agonists.
Receptors can be activated or inactivated either by endogenous (such as hormones and neurotransmitters) or exogenous (such as drugs) agonists and antagonists, resulting in
stimulating or inhibiting a biological response. To see how an agonist may activate a receptor see this link.
New findings that broaden the conventional definition of pharmacology demonstrate that ligands can concurrently behave as
agonist and antagonists at the same receptor, depending on effector pathways. Terms that describe this phenomenon are
"functional selectivity" or "protean agonism".[2][3]
Activity (EC50)
The potency of an agonist is usually defined by its EC50 value. This can be calculated for a given agonist by determining
the concentration of agonist needed to elicit half of the maximum biological response of the agonist. Elucidating an
EC50 value is useful for comparing the potency of drugs with similar efficacies. The
lower the EC50, the greater the potency of the agonist the lower the concentration of drug that is required to elicit
the maximum biological response
Etymology
From the Greek αγωνιστής (agōnistēs), contestant; champion; rival <
αγων (agōn), contest, combat; exertion, struggle < αγω (agō), I lead, lead towards, conduct; drive
See also
References
- ^ Principles and Practice of Pharmacology for Anaesthetists By Norton Elwy
Williams, Thomas Norman Calvey Published 2001 Blackwell Publishing ISBN 0632056053
- ^ Kenakin T. (2001). Inverse, protean, and ligand-selective agonism: matters
of receptor conformation. FASEB J. 15:598-611. PMID 11259378. Fulltext
- ^ Urban J.D. et al. (2007). Functional selectivity and classical concepts of
quantitative pharmacology. J. Pharmacol. Exp. Ther. 320:1-13. PMID 16803859.
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