Brief CommunicationAn active transport system in the blood–brain barrier may reduce levodopa availability
Introduction
Parkinson's disease is a progressive disorder of the central nervous system affecting approximately 1% of the United States population over 65 years of age (National Center for Health Statistics). The disease is characterized by a decrease in spontaneous movements, increased rigidity, and involuntary tremor. Parkinson's disease is caused by the degeneration of the pigmented neurons in the substantia nigra of the brain, resulting in decreased availability of the neurotransmitter dopamine (Dauer and Przedborski, 2003, Suchowersky, 2002).
Administration of the drug levodopa (l-3, 4-dihydroxyphenylalanine, an aromatic amino acid) is a frequently utilized treatment for Parkinson's disease. Once inside the brain, levodopa is converted to dopamine, which then supplements the insufficient amounts of dopamine in Parkinson's patients thereby relieving some symptoms of the disease. Treatment is complicated by the so-called “on–off” phenomenon—abrupt transient fluctuations in the patient's clinical state that occur frequently without warning and with no obvious relationship to dosing schedules (Dauer and Przedborski, 2003, Suchowersky, 2002).
Some strategies to avoid the “on–off” phenomenon are based on current concepts of the blood–brain barrier (BBB) (Ogawa, 2000). Levodopa crosses the BBB and enters the brain through a facilitative transport system (L1) that carries a range of large neutral amino acids (Oldendorf and Szabo, 1976). Levodopa must compete with these other amino acids to gain access to the brain. However, recently, it was shown that a Na+-dependent transport system exists on the abluminal side of the BBB (Na+-LNAA) (O'Kane and Hawkins, 2003) that transports many of the same amino acids as L1, but in the opposite direction—from brain to blood against a concentration gradient (O'Kane and Hawkins, 2003).
There are other energy-dependent systems (systems A, ASC, and N) on the abluminal membrane of the BBB that also are capable of coupling the Na+ gradient that exists between the brain's ECF (extracellular fluid) and the capillary endothelial cells to actively transport small neutral amino acids out of the brain (O'Kane and Hawkins, 2003, O'Kane et al., 2004) (Fig. 1). The combined activity of the four known Na+-dependent systems have the ability to actively transfer every naturally occurring neutral amino acid from the ECF to endothelial cells and hence to the circulation. In this study, it is demonstrated that levodopa is indeed a substrate for Na+-LNAA with the capacity to remove levodopa from the ECF.
Section snippets
Methods
Membrane vesicles from the brain endothelial cells were prepared as previously described by Sánchez del Pino et al. (1992). Fresh bovine brains were bought from Strauss Veal and Lamb International Inc. (Franklin, WI). The cows were killed for food under USDA supervision, and the meat was sold for human consumption. Microvessels (>90% capillaries) from bovine cerebral cortices were obtained by the method of Pardridge et al. (1985). The microvessels were digested with collagenase Type IA to
Results
In the absence of a Na+ gradient, the uptake of [3H]-levodopa followed the time course expected of facilitated transport (Fig. 2). The initial rate of clearance was 5 μl min−1 mg protein−1. A Na+ gradient enhanced transport of [3H]-levodopa by at least 500%, increasing the initial rate of clearance (net) by 25 μl min−1 mg protein−1 (Fig. 2). BCH (2-aminobicyclo(2,2,1)-heptane-2-carboxylic acid) is a specific substrate of the facilitative transporter L1 (Ki = 11 μM) (Christensen et al., 1969,
Discussion
The therapeutic use of levodopa has to overcome several obstacles to be effective. The first is extensive decarboxylation in liver and other tissues by l-amino acid decarboxylase (Hardman et al., 2001). It is estimated that only about 5% of orally administered levodopa reaches the circulation and less than 1% enters the brain (Hardman et al., 2001) because the BBB presents a significant impediment. Passage across the BBB on the facilitative transport system L1 is slow. This carrier transports
Acknowledgments
This work was supported by grants from the National Institute of Neurological Disorders and Stroke, NS 041405 (I.A.S.), and The International Glutamate Technical Committee (R.A.H.).
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