Agonist

An agonist is a chemical that activates a receptor to produce a biological response. Receptors are cellular proteins whose activation causes the cell to modify what it is currently doing. In contrast, an antagonist blocks the action of the agonist, while an inverse agonist causes an action opposite to that of the agonist. From the Greek αγωνιστής (agōnistēs), contestant; champion; rival < αγων (agōn), contest, combat; exertion, struggle < αγω (agō), I lead, lead towards, conduct; drive Receptors can be activated by either endogenous agonists (such as hormones and neurotransmitters) or exogenous agonists (such as drugs), resulting in a biological response. A physiological agonist is a substance that creates the same bodily responses but does not bind to the same receptor. An endogenous agonist for a particular receptor is a compound naturally produced by the body that binds to and activates that receptor. For example, the endogenous agonist for serotonin receptors is serotonin, and the endogenous agonist for dopamine receptors is dopamine. Full agonists bind to and activate a receptor with the maximum response that an agonist can elicit at the receptor. One example of a drug that can act as a full agonist is isoproterenol, which mimics the action of adrenaline at β adrenoreceptors. Another example is morphine, which mimics the actions of endorphins at μ-opioid receptors throughout the central nervous system. However, a drug can act as a full agonist in some tissues and as a partial agonist in other tissues, depending upon the relative numbers of receptors and differences in receptor coupling. A co-agonist works with other co-agonists to produce the desired effect together. NMDA receptor activation requires the binding of both glutamate, glycine and D-serine co-agonists. Calcium can also act as a co-agonist at the IP3 receptor. A selective agonist is selective for a specific type of receptor. E.g. buspirone is a selective agonist for serotonin 5-HT1A.