How can Levodopa cross the blood-brain barrier, while Dopamine cannot?

Exogenous Dopamine is unable to cross the tight junctions of the endothelial cells in the blood-brain barrier (BBB). Dopamine has no clinically significant endogenous transporter on the blood-brain barrier and is otherwise too polar of a molecule to move through the lipid membrane. Levodopa, however, utilizes a receptor on the BBB known as Large amino acid transporter (LAT)  which allows it to be transported across the blood-brain barrier and decarboxylated into Dopamine.

REFERENCES

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