554 DYNAMIC ASPECTS — PERMEABILITY AND TRANSPORT 
CEREBRAL PASSAGE OF EREE AMIN© ACIDS 
ABEL LAJTHA 
The New York State Psychiatric Institute and Department of Biochemistry, 
College of Physicians and Surgeons, Columbia University, New York, N.Y. (U.S.A.) 
The question whether it is justified to treat cerebral transport mechanisms separately 
from those of other organs cannot be answered unequivocally at the present time. 
There is little doubt that because of its isolated circulation and anatomical localiza- 
tion the brain offers a number of convenient possibilities for the experimental 
approach to organ transport, although its structural complexity adds difficulties of 
interpretation. It is not certain whether transport processes operate in the brain that 
are unique for this organ; however, it has been shown that cerebral permeability is 
considerably lower than that of most organs. 
Quantitative differences between the brain and other organs in penetration of 
substances have been amply documented for several classes of substances (for 
reviews discussing the subject see!~®). It is tempting to speculate that if net increase 
of some substances in organs is inhibited by means of active transport processes 
(partially transporting substances out of the organ), then at least quantitatively 
these processes have to be more active in brain than in most other organs. In 77 vitro 
experiments employing tissue slices, differences between the brain and other organs 
have been pointed out; for example, accumulation of a-aminobutyrate against a 
concentration gradient could be shown only in brain, among the tissues tried’ 8. On 
the other hand, glutamate uptake against a concentration gradient was found in 
several, though not in all, of the examined tissues®. It is likely that processes closely 
related to transport mechanisms studied in other tissues will be found in the central 
nervous system. Some aspects of this question have been reexamined lately by 
GUROFF AND UDENFRIEND!®, 4, and Dr GurRorF will discuss this point in more 
detail elsewhere in this volume. 
EXCHANGE OF AMINO ACIDS BETWEEN PLASMA AND BRAIN 
Our interest in cerebral amino acid transport was started with our observation, 
made in studying the amino acid and protein metabolism of the brain, that although 
net increase in brain levels is restricted if plasma concentrations of a number of amino 
acids are greatly increased, a rapid exchange between plasma and brain takes place. 
This finding, made with acidic, basic, and neutral amino acids such as glutamic 
acid!?, 18, lysine!4, 15, and leucine?® 17, made it unlikely that amino acid passage in 
the brain is governed by passive diffusion only. 
By studying this exchange in more detail!®, !® it was found that rapid exchange 
occurred between plasma and brain even when after the elevation of brain amino 
References p. 563 
