An agonist is a DRUG or chemical that is able to bind to a RECEPTOR and activate it, producing an appropriate response which may involve changes in ION CHANNEL opening or closing, activation of SECOND MESSENGERS and changes in neuronal FIRING. An agonist effectively mimics whatever effect the natural LIGAND for a receptor has. Such agonists are called DIRECT AGONISTS.
It is important to note that receptors may have multiple binding sites present on them: the principal site is that at which the neurotransmitter binds, but there may be other sites at which neuromodulators or co-transmitters bind (see NEUROMODULATION). An INDIRECT AGONIST is one which binds to a site on receptor other than the main biding site for the ligand. In doing so it interferes with the action of the receptor, facilitating actions there. An INVERSE AGONIST is a molecule that binds to a site on the receptor other than the main binding site to produce an effect opposite to that of the agonist: essentially it binds to the same receptor yet has an opposite effect to the agonist—it is a functional ANTAGONIST. A PARTIAL AGONIST is one which is unable to produce a maximal effect, whatever concentration is applied.
