If the agonist is a key that opens the lock, the antagonist is a key that fits but does not turn: it occupies the site and blocks the right key. On its own it produces no response; its effect shows because it reduces or cancels that of the agonist.
There are two main types. A competitive antagonist competes for the same binding site as the agonist, so a high enough agonist concentration can displace it. A non-competitive antagonist acts on another site of the receptor or inactivates it in such a way that adding more agonist does not reverse the block.
There is also the inverse agonist, which not only blocks but reduces the baseline activity some receptors have even without a signal. In research, antagonists are key tools to show that an effect depends on a specific receptor: if the antagonist cancels it, the receptor is involved.
Many preclinical peptide studies use selective antagonists to test the mechanism. Checking which antagonist was used and whether it cancelled the effect is key to judging how solid the conclusion is.
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Definition for educational and scientific purposes. It is not medical advice or a recommendation for use. NeoPeptidos products are sold labelled for research use only (RUO).