Author: Charles Frank

AGONIST Definition & Usage Examples

A physiological agonist is a substance that creates the same bodily responses but does not bind to the same receptor. A co-agonist requires the combination of two or more agonists to elicit a particular biological response. For example, the activation of infected macrophages to produce nitric oxide is dependent on the binding of bacterial ligands, IFN-gamma, and TNF, to their respective receptors. There are several types of agonists, which include endogenous, exogenous, physiological, superagonists, full, partial, inverse, irreversible, selective, and co-agonists.

They mimic the natural agonists and trigger the receptors, producing the desired response, or in some cases, a much stronger action. Full agonists are able to fully bind to and activate their cognate receptor, thereby inducing the complete response capable of that receptor. In contrast, partial agonists also bind to the cognate receptor; however, they only induce a partial response. Partial agonists are useful for the treatment and avoidance of drug dependencies, as they induce a similar effect, albeit less potent and addictive. A non-competitive antagonist binds at an allosteric site (a site other than the true binding site).

If someone has taken a potentially fatal heroin overdose, naloxone (an opioid receptor antagonist) can reverse the effects. Naloxone (brand name Narcan) works by blocking or occupying all the opioid receptors, preventing morphine or heroin from binding and activating them. An overdose victim who is unconscious and near death can become fully conscious quite dramatically within seconds of receiving naloxone. Scientists have developed drug agonists to behave with partial or full efficacy [1-3]. An example of this behavior can be seen in opioid drugs and their effects on the opioid receptors in the brain.

Hopefully, in the coming decades scientists will develop drugs with high selectivity and fewer adverse effects. 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.

Receptors are protein molecules present on the cell surface in the human body. This information comes from other molecules such as hormones, neurotransmitters, and drugs. An inverse agonist binds to the same receptor as an agonist; however, it exerts the opposite biological response of an agonist.

Satellite Cells

Imagine that ligands are keys that fit specific locks, which are receptors. Drug mechanics are quite incredible, and understanding them has a lot to do with receptors, agonists, and antagonists. Please continue reading to learn more, including the main difference between antagonist & agonist. In other words, an antagonist works by blocking the activity of an agonist.

In contrast, an antagonist blocks the action of the agonist, while an inverse agonist causes an action opposite to that of the agonist. Irreversible antagonist drugs bind strongly to the receptor through covalent bonds and cannot be displaced or washed out. They permanently modify the receptor and prevent the binding of the natural ligand. An inverse agonist is a drug that produces the opposite effect by binding to a receptor.

Every Letter Is Silent, Sometimes: A-Z List of Examples

Everlywell uses telehealth to give you access to providers and works with a network of labs to get you access to various tests. Another example is the ghrelin receptor, known as the growth hormone secretagogue receptor. The ghrelin receptor exerts various physiological functions, including appetite regulation, alcohol consumption, adipocyte metabolism, and glucose homeostasis [1,2,5].

1. An agonist is a chemical that activates a receptor to produce a biological response.
2. Naloxone is a competitive antagonist for the opioid receptor, and it prevents a natural ligand like morphine or heroin from binding to the receptor.
3. An agonist drug that binds to a receptor and produces the opposite pharmacological effect that would be made by an agonist is referred to as an inverse agonist [1,2].
4. However, if the concentration of the natural ligand increases, it can suppress the effect of a competitive antagonist.
5. Some agonists can act as a partial antagonist, but an antagonist drug cannot act as an agonist drug.

When the key fits the keyhole, the receptor can be activated to work and stimulate a particular response. In this case, an agonist drug will mimic a naturally occurring chemical molecule to fit a specific receptor. The receptor cannot distinguish between the natural chemical and the agonist drug. They refer to  drugs or chemical agents that work in opposite ways in terms of how they function and produce effects.

Some agonists can act as a partial antagonist, but an antagonist drug cannot act as an agonist drug. Also called direct binding agonist drugs, they bind directly to the receptor at the same binding site where natural ligands bind. Methadone, which is used to treat opioid addiction, is a full opioid agonist. Also called indirect binding agonist drugs, they promote the binding of the natural ligand to the receptor site.

White Blood Cell

However, if the concentration of the natural ligand increases, it can suppress the effect of a competitive antagonist. Naloxone is a competitive antagonist for the opioid receptor, and it prevents a natural ligand like morphine or heroin from binding to the receptor. Another good example of a competitive antagonist is naltrexone, which is also used to treat opioid addiction.

Meaning of agonist in English

Agonists can produce a maximal or partial activation of a receptor [1-3]. You can think of partial agonists as drugs that can bind to a receptor but will only have limited efficacy. Whereas maximal agonists will produce the greatest response with the most effectiveness. Receptors can be activated by either endogenous agonists (such as hormones and neurotransmitters) or exogenous agonists (such as drugs), resulting in a biological response.

In contrast, exogenous agonists are external factors which bind to various receptors and induce a biological response. The pain medication morphine (and the illegal drug heroin) are artificial agonists of opioid receptors. They produce pain relief or a “high” by mimicking the action of the natural agonist. The main difference between an agonist and antagonist is that they have opposite actions. An agonist drug always produces a specific action and triggers the receptor to produce a natural response. On the other hand, antagonist drugs block or oppose the natural action or response of a receptor.

An agonist is a molecule that can bind and activate a receptor to induce a biological reaction. The activity mediated by agonists are opposed by antagonists, which inhibit the biological response induced by an agonist. The level of agonist required to induce a desired biological response is referred to as potency. Agonist potency is derived by measuring the concentration of agonist required to induce half of the maximum response, called the EC50 value. Therefore, agonists with greater potency will have smaller EC50 values.

An inverse agonist differs from an antagonist in that rather than simply inhibiting the response of the agonist, the opposing response is induced. Serotonin, a natural neurotransmitter or chemical messenger in the brain, is a natural agonist for the 5-HT2A receptors. The synthetic hallucinogenic drug LSD is an artificial agonist of the 5-HT2A receptors. G-protein coupled receptors (GPCR) are good examples of protein receptors that can exhibit an inverse agonism [1,2,4]. GPCR transmits information or signals inside a cell and can be modulated above or below its basal activity levels. If you thought an agonist drug was interesting, they have another exciting property called an inverse agonist.