The importance of dopamine in neural transmission is emphasized by the number of major neurological diseases that are associated with improper dopamine regulation. The earliest indication of this type of defect was the finding that dopamine levels are abnormally low in a particular region of the brain of patients with Parkinsonism, a severe neurological disorder. Attempts to treat such patients with dopamine were futile, because after injection, dopamine does not cross the bloodbrain barrier. However, the dopamine precursor, dopa, does cross the bloodbrain barrier. For many individuals with Parkinsonism, daily doses of dopa have provided dramatic clinical improvement.

The biosynthetic pathway from tyrosine to dopa, and the other catecholamines is shown in Figure 21.32.