Major depression likely involves imbalances in serotonin, dopamine, and norepinephrine, with individual variations.

“So it's very clear that 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. So for instance, epinephrin or norepinephrine is thought to relate to the so-called psychomotor defects, sometimes called psychomotor retardation. This is the lethargy, this is the exhaustion, this is the inability to get out of bed in the morning. Dopamine is thought to relate to the anhedonia, or I should say 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.”