Autopharmacology relates to the scientific study of the regulation of body functions by the activity of its naturally existent (or endogenous) chemical factors of the tissues. A more restricted definition would consider substances that were first identified as external agents which had a documented action on physiological functions, but later were discovered as existing as endogenous factors. The best example is the class of endorphins, which, as it name implies, were discovered to exist in the brain and have specific receptors in it, by investigations on the mechanism of action of opioids, such as morphine.