Quantitative Evaluation of the Blood-Brain Barrier Capacity to Form Dopamine from Circulating L-DOPA

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Abstract
The ability of the blood-brain barrier to form dopamine from increasing doses of systemically administered L-dopa was studied in rats by a combination of chemical determination of dopamine, and histochemical and cytofluorometric measurements of L-dopa and dopamine. The break-through of L-dopa from the circulation into the brain parenchyma via the enzymatic blood-brain barrier was estimated by comparing the amount of newly formed dopamine in the caudate nucleus-putamen and in the cerebellum. The capillaries efficiently trapped L-dopa in their walls, and an upper limit was reached (at an administered i.p. dose of 100 mg/kg of L-dopa). Approximately 3% of the total dose of L-dopa given was decarboxylated by the blood-brain barrier. The possible influence by the regional differences in perfusion of the 2 regions seen after administration of L-dopa was ruled out in measurements of local cerebral blood flow using the 14C-ethanol technique.