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Effect of plasma levels of large neutral amino acids and degree of parkinsonism on the blood-to-brain transport of levodopa in naive and MPTP parkinsonian monkeys

Department of Neurology, Jefferson Medical College, Philadelphia, PA.

We investigated the transport of levodopa from blood to brain in control and in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) parkinsonian monkeys by using intracerebral microdialysis. The degree of parkinsonism was established by the monkey's clinical score and its postmortem striatali dopamine level. We administered levodopa plus carbidopa either orally or by intravenous injection. We estimated the blood-to-brain transport of levodopa as the ratio of its concentration in the brain's extracellular fluid to that in arterial plasma. This ratio was reduced in the MPTP parkinsonian monkeys, and we found an inverse relationship between the monkeys' ability to transport levodopa from blood to brain and their degree of parkinsonism. The oral administration of a high-protein meal or the intravenous infusion of large neutral amino acids before the administration of the levodopa plus carbidopa reduced the transport of levodopa into the brain, as measured by microdialysate collected from the striatum. We found that in two animals an intraperitoneal injection of the ß-adrenergic agonist isoproterenol increased the blood-to-brain transport of levodopa. Pharmacologic manipulation of the transport of levodopa from blood to brain may offer a new strategy for the treatment of Parkinson's disease.

Address correspondence and reprint requests to Dr. Guillermo M. Alexander, Department of Neurology, Jefferson Medical College, 1025 Walnut Street, Suite 511 College Building, Philadelphia, PA 19107.

Supported by grant no. 5 RO1 NS27101 from the National Institutes of Health.

Received July 27, 1993. Accepted in final form February 10, 1994.

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