a precursor to the neurotransmitters dopamine, norepinephrine, and epinephrine (collectively known as the catecholamines). L-DOPA is produced as a drug called levodopa, which is used to treat Parkinson's disease. Parkinson's disease is characterized by the loss of dopamine neurons in the basal ganglia; when L-DOPA is administered to Parkinson's disease patients, the brain can use it to synthesize more dopamine to replenish depleted levels of the neurotransmitter.