Answer to Question #81733 in Human Anatomy and Physiology for Sandra Joseph
Blood brain barrier(BBB) regulates the entry of molecules into the brain cells, acting like a buffer between the brain and systemic blood circulation. It is considered, that not all types of molecules can be transported into the brain, thus BBB's limited permeability maintain a homeostatic state of the brain.
Since dopamine in central nervous system is synthesized within the dopaminergic neuron from the precursor L-DOPA(Levodopa), there is no way dopamine itself could be transported from the outside of the brain cell. On the contrary, its precursor, L-DOPA, is able to pass through BBB by a special transporting system called LNAA- Large Neutral Aminoacid system, that is also found in the gut barrier, where it provides absorption of L-dopa after food intake.