Quantitative evaluation of the blood-brain barrier capacity to form dopamine from circulating L-DOPA

The ability of the blood-brain barrier to form dopamine from increasing doses of systemically administered L-DOPA has been 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 were found to efficiently trap L-DOPA in their walls, and an upper limit was reached (at an administered i.p. dose of 100 mg/kg of L-DOPA). It could be estimated that 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 two regions seen after administration of L-DOPA was ruled out in measurements of local cerebral blood flow using the 14C-ethanol technique.