agonist

thumb|400 px|Dose response curves of a full agonist, partial agonist, neutral antagonist, and inverse 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.

== Etymology == The word originates from the Greek word (agōnistēs), "contestant; champion; rival" 1A. Partial agonists (such as buspirone, aripiprazole, buprenorphine, or norclozapine) also bind and activate a given receptor, but have only partial efficacy at the receptor relative to a full agonist, even at maximal receptor occupancy. Agents like buprenorphine are used to treat opiate dependence for this reason, as they produce milder effects on the opioid receptor with lower dependence and abuse potential. An inverse agonist is an agent that binds to the same receptor binding-site as an agonist for that receptor and inhibits the constitutive activity of the receptor. Inverse agonists exert the opposite pharmacological effect of a receptor agonist, not merely an absence of the agonist effect as seen with an antagonist. An example is the cannabinoid inverse agonist rimonabant. A superagonist is a term used by some to identify a compound that is capable of producing a greater response than the endogenous agonist for the target receptor. It might be argued that the endogenous agonist is simply a partial agonist in that tissue. An irreversible agonist is a type of agonist that binds permanently to a receptor through the formation of covalent bonds. A biased agonist is an agent that binds to a receptor without affecting the same signal transduction pathway. Oliceridine is a μ-opioid receptor agonist that has been described to be functionally selective towards G protein and away from β-arrestin2 pathways.