The effects depend upon what receptor has been stimulated as well as the organ/tissue from which it was activated. An agonist could cause increased heart rate, relaxation of muscles, reduced pain, altered moods or changed how a specific cell behaves.
How does an agonist work?
It may be helpful to think about an agonist by understanding what makes receptors function.
Receptors are unique structures that allow specific molecules to bind to them. These binding molecules are often referred to as ligands. If an endogenous signaling molecule (the “natural” or normal signal) attaches itself to its appropriate receptor on the surface of a cell, then this interaction can cause a series of responses inside the cell.
For example, morphine is an agonist of certain opioid receptors. When it binds to these receptors, it produces effects such as pain relief. The body’s own opioid peptides can also activate these receptors, but they are chemically different from morphine.
The strength and type of response can vary between different agonists. Some produce a strong response, while others activate the receptor to a lesser degree.
Full agonists and partial agonists
Not all agonists activate receptors to the same extent.
A full agonist can produce the maximum response that the receptor system is capable of producing under the conditions being studied. A classic example is morphine at the μ-opioid receptor, although the actual response also depends on the tissue and experimental system.
A partial agonist activates the receptor but produces a lower maximum response, even when it occupies a large proportion of the available receptors.
This difference becomes particularly important when two drugs interact with the same receptor. A partial agonist can at times diminish the effect of a full agonist. This is because it attaches to the receptors but causes less of a response. For example, buprenorphine is a partial agonist at the μ-opioid receptor and has different pharmacological effects than full opioid agonists.
So the difference between full and partial agonists is more than just how strongly a drug binds. Binding affinity and receptor activation are related but distinct concepts.
Read the full explanation
What is an inverse agonist?
An inverse agonist is different from a conventional agonist.
Some receptors have a degree of activity even when no external agonist is present. This phenomenon is sometimes described as constitutive activity. An inverse agonist binds to the receptor and reduces this baseline activity.
This baseline activity is different from an antagonist. An antagonist binds to a receptor but does not activate it in the usual sense. Instead, it can prevent an agonist from producing its effect.
This simplifies the concepts:
- An agonist is a substance that binds to and activates a receptor.
- A partial agonist is a substance that binds to a receptor and activates it to a limited extent.
- A substance that binds to a receptor but does not activate it is known as an antagonist. A substance that binds to a receptor and inhibits the activation of the receptor by another substance is also called an antagonist.
- An inverse agonist is a substance that reduces the activity of a receptor.
Keep in mind that these definitions are rather plain and that in the field of study, reality may not correspond to these definitions.
Throughout the practice of medicine and pharmacy, agonists have been and continue to be used extensively. This is because many drugs and medicines are developed to activate or stimulate certain body functions and responses.
An example is salbutamol, which is a medicine containing β₂- adrenoceptor agonist. It activates β₂- adrenoceptors present in the airways and therefore relieves bronchospasm (wheezing) in conditions such as asthma.
Another example is the use of dopamine agonists in the treatment of disorders of the movement system. The use of drugs that stimulate the dopamine system may alleviate the symptoms of neurological disorders.
Agonists are very useful in advanced research. The role of a receptor may be studied by using an agonist. The effect of an agonist on a receptor may also be used to study the actions of a new drug under development.
Agonist potency and efficacy
Two terms often appear alongside agonist: potency and efficacy.
Potency describes how much of a substance is needed to produce a particular level of response. A more potent agonist can produce a given effect at a lower concentration than a less potent one, assuming the comparison is made under appropriate conditions.
Efficacy refers to the maximum response an agonist can produce through a particular receptor system.
These terms should not be confused. A drug can be highly potent without having the highest efficacy. Likewise, a less potent drug can still produce a strong maximum response when enough of it is present.
These characteristics are studied by scientists in concentration-response studies. The data from these studies give an indication of the nature of the interaction of the agonist with the receptor and the level of biological action elicited by the agonist. The effect can depend on the receptor subtype, the number of receptors present, the tissue involved and the signalling pathways connected to the receptor.
The same compound may therefore behave differently in different biological systems. Experimental conditions are also factors that can impact the observed response.
While reviewing data from the lab for the purpose of new drug development, we consider many possible scenario or conditions. The fact that a strong response was observed in a cell-based assay may have little or no bearing on the outcome in clinical trials. A number of aspects play a role, including the route of administration, the presence of other competing substances in the body, the integrity of the receptors, the dose, and many others.
Agonist: a simple definition
Pharmacologically, an agonist is a substance that is capable of binding to a receptor and thereby generating a biological response.
Some examples of agonists are body-produced substances and pharmacological agents. The effects of an agonist are determined by the receptor that it stimulates and the signaling pathway that is activated.
For the purpose of pharmacology, it is important to define agonists as most drugs work by stimulating receptors to evoke a biological response. To predict the pharmacological action of a drug, information such as the potency and efficacy of the drug, the receptor that it targets and the presence of other competing molecules, plays an important role.
Research Use Only. This glossary explains terminology used in published research. It is not medical advice and does not describe any product for human or animal use.