TLDR: An agonist is a chemical that activates a receptor in our body to produce a response. It can be a natural substance produced by our body or a drug that we take. There are different types of agonists, such as full agonists that activate the receptor to its maximum response, partial agonists that have a weaker response, and co-agonists that work together with other substances to produce a desired effect. Agonists can have different mechanisms of action depending on the receptor they activate, and their potency and therapeutic index are important factors to consider in their use.

An agonist is a chemical that activates a receptor in our body to produce a biological response. Receptors are proteins in our cells that, when activated, cause the cell to change what it is currently doing. Think of receptors as little switches that turn on different functions in our body. An agonist is like the key that fits into the switch and turns it on.

There are different types of agonists. Some are naturally produced by our body and are called endogenous agonists. For example, serotonin is the endogenous agonist for serotonin receptors, and dopamine is the endogenous agonist for dopamine receptors. These substances bind to their respective receptors and activate them, causing a specific response in our body.

On the other hand, there are exogenous agonists, which are drugs or other substances that we take from outside the body. These exogenous agonists can also bind to receptors and activate them, producing a biological response. Some drugs act as full agonists, meaning they activate the receptor to its maximum response. Others may act as partial agonists, producing a weaker response even at maximum receptor occupancy.

There are also co-agonists, which work together with other substances to produce a desired effect. For example, the activation of NMDA receptors requires the binding of both glutamate and glycine as co-agonists. Calcium can also act as a co-agonist at the IP3 receptor.

The mechanism of action of agonists can vary depending on the receptor they activate. For example, the binding of an agonist to a muscarinic acetylcholine receptor causes conformational changes that propagate a signal into the cell. On the other hand, the NMDA receptor requires co-agonists to induce the conformational change needed for the receptor to allow the flow of ions.

The potency of an agonist refers to the amount of agonist needed to elicit a desired response. The smaller the amount needed, the more potent the agonist is. The therapeutic index is also important to consider when using agonists therapeutically. It refers to the margin of safety between the dose needed for the desired effect and the dose that produces unwanted side effects.

In summary, agonists are chemicals that activate receptors in our body to produce a response. They can be natural substances produced by our body or drugs that we take. Different types of agonists have different mechanisms of action and potencies. The therapeutic index is important to consider when using agonists therapeutically.