In biochemistry, agonists are any chemicals, including drugs, that show an affinity for a receptor on the cell membrane. Due to the affinity, the agonist can bind to the receptor and affect its activity within the cell. At the cellular level, receptors occur on the surface of the cell membrane and are usually an exposed part of a membrane protein. When a substance binds to a receptor, it causes a change in the receptor molecule, which can initiate or inhibit activity. Receptor agonists can either positively or negatively affect the activity of the receptor they bind to.
Within the body, receptors can be stimulated or inhibited by chemicals produced by the body, endogenous agonists; or those that are foreign or produced elsewhere, exogenous agonists. Examples of endogenous agonists include naturally occurring hormones, such as insulin, and neurotransmitters. Neurotransmitters are chemicals produced by the body that are released by nerve cells to transmit nerve impulses from one nerve cell to another. Examples of neurotransmitters include adrenalin and dopamine.
The ability of receptor agonists to affect the activity of the receptor is what makes them different from receptor antagonists. Receptor antagonists can also bind to receptors, but they do not affect the receptor or its activity in any way. The amount that a receptor agonist affects the activity of its target receptor is called its efficacy. There is a wide spectrum of efficacy in receptor agonists.
Across the efficacy spectrum, there are four levels of different receptor agonists, which are classified based how much they affect the receptor when they bind to it. From most to least, the groups are: superagonist, full agonist, partial agonist and inverse agonist. A superagonist is usually an exogenous receptor agonist. When it binds to the receptor and activates it, it causes a greater response than the endogenous agonist for that receptor. In other words, the response of the cell is greater than 100% when a superagonist binds to the target receptor.
Full receptor agonists cause full efficacy, or activity, of the cell when they bind to the receptor. These types of agonists can be endogenous or exogenous. Examples of endogenous and exogenous agonists that are full agonists are endorphins and morphine, respectively. Endorphins are natural pain killers produced by the body, which bind to opioid receptors found within the central nervous system. Morphine is a potent painkiller derived from opium poppies that mimics the action of endorphins and activates the opioid receptors also.
Partial agonists bind to the target receptor, but only cause a partial increase in activity of the cell when compared to the full or endogenous agonists. Finally, an inverse agonist binds to the receptor, but instead of activating it, it causes the reverse to occur. Inverse agonists act in complete opposition to full or endogenous agonists by causing the opposite effects to occur within a cell compared to when the full or endogenous agonist activated the receptor.