Dopamine might compete with l-dopa and dopamine precursors in passing thru the small intestine and the BLOOD BRAIN BARRIER. not sure if dopamine might block the transfer sites in the BBB even though it can't pass thru.
Not sure if dopamine is broken down into another amino acid that can pass the BBB.

the fact that we eat far more protein than we need and that you can fast for days without developing pd systems tells me that the body tightly regulates, at least in a non pder, how much dopamine moves freely in the brain. if it didn't we wouldn't be able to eat any high protein foods, an egg would cause dyskinesias.