An agonist is a chemical that activates a receptor to produce a biological response. Receptors are cellular proteins whose activation causes the cell to modify what it is currently doing. In contrast, an antagonist blocks the action of the agonist, while an inverse agonist causes an action opposite to that of the agonist.
From the Greek αγωνιστής (agōnistēs), contestant; champion; rival < αγων (agōn), contest, combat; exertion, struggle < αγω (agō), I lead, lead towards, conduct; drive
Receptors can be activated by either endogenous agonists (such as hormones and neurotransmitters) or exogenous agonists (such as drugs), resulting in a biological response. A physiological agonist is a substance that creates the same bodily responses but does not bind to the same receptor.
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 or tissue type. Terms that describe this phenomenon are "functional selectivity", "protean agonism", or selective receptor modulators.
As mentioned above, agonists have the potential to bind in different locations and in different ways depending on the type of agonist and the type of receptor. The process of binding is unique to the receptor-agonist relationship, but binding induces a conformational change and activates the receptor. This conformational change is often the result of small changes in charge or changes in protein folding when the agonist is bound. Two examples that demonstrate this process are the muscarinic acetylcholine receptor and NMDA receptor and their respective agonists.
For the muscarinic acetylcholine receptor, which is a G protein-coupled receptor(GPCR), the endogenous agonist is acetylcholine. The binding of this neurotransmitter causes the conformational changes that propagate a signal into the cell. The conformational changes are the primary effect of the agonist, and are related to the agonist's binding affinity and agonist efficacy. Other agonists that bind to this receptor will fall under one of the different categories of agonist mentioned above based on their specific binding affinity and efficacy.
The NMDA receptor is an example of an alternate mechanism of action, as the NMDA receptor requires co-agonists for activation. Rather than simply requiring a single specific agonist, the NMDA receptor requires both the endogenous agonists, N-methyl-D-aspartate (NMDA) and glycine. These co-agonists are both required to induce the conformational change needed for the NMDA receptor to allow flow through the ion channel, in this case calcium. An aspect demonstrated by the NMDA receptor is that the mechanism or response of agonists can be blocked by a variety of chemical and biological factors. NMDA receptors specifically are blocked by a magnesium ion unless the cell is also experiencing depolarization.
These differences show that agonists have unique mechanisms of action depending on the receptor activated and the response needed. The goal and process remains generally consistent however, with the primary mechanism of action requiring the binding of the agonist and the subsequent changes in conformation to cause the desired response at the receptor. This response as discussed above can vary from allowing flow of ions to activating a GPCR and transmitting a signal into the cell.
Potency is the amount of agonist needed to elicit a desired response. The potency of an agonist is inversely related to its EC50 value. The EC50 can be measured for a given agonist by determining the concentration of agonist needed to elicit half of the maximum biological response of the agonist. The EC50 value is useful for comparing the potency of drugs with similar efficacies producing physiologically similar effects. The smaller the EC50 value, the greater the potency of the agonist, the lower the concentration of drug that is required to elicit the maximum biological response.
When a drug is used therapeutically, it is important to understand the margin of safety that exists between the dose needed for the desired effect and the dose that produces unwanted and possibly dangerous side-effects (measured by the TD50, the dose that produces toxicity in 50% of individuals). This relationship, termed the therapeutic index, is defined as the ratio TD50:ED50. In general, the narrower this margin, the more likely it is that the drug will produce unwanted effects. The therapeutic index emphasizes the importance of the margin of safety, as distinct from the potency, in determining the usefulness of a drug.