The group could include the drug levodopa as well as the dopamine agonists. Levodopa is a precursor of dopamine. While dopamine does not pass the blood-brain barrier, the levodopa does and it is transformed in dopamine as soon as it passes the barrier becoming free dopamine to be used in the brain - it is the lack of it that is the base of the Parkinson's disease. The dopamine agonists work alike, binding directly to the dopamine receptors, mimetising its action.