There are at least three major chemical systems in the brain—norepinephrine, dopamine and serotonin—that relate to and can adjust the symptoms of depression ... Norepinephrine is thought to relate to the so-called psychomotor defects ... This is the lethargy ... Lack of dopamine in depressive patients is thought to lead to the anhedonia, the lack of ability to experience pleasure. And serotonin is thought to relate to the grief, the guilt, some of the more cognitive or more emotional aspects of depression.