- 124 - RADIOISOTOPIC STUDIES OP BRAIN UPTAKE . WOldendorH . f Vfedsworth Veterans Administration Hospital Los Angeles, California, U.S.A. ABSTRACT Measurement uptake th f radioactivf o eo s e substancee th n si brain tissues after their administratio y injectionb inhalar o n - tion provid n atraumatia e c approac stude th f bloo o yo t h d flow d metabolian c processe braine th n i .s This paper reviewe th s anatomical,physiologica d physicaan l l problems arisine th n gi measuremen f radioactivito t braine th e factor n Th .i y s governing the passage of various classes of substances through the brain capillaries and their transport through the brain tissues are first considered. The physical problems arising in the measure- men radioactivitf o t braie th e the nar n i ny discussed maie Th .n difficulties in such measurements is shown to arise from the con- tributio e observeth o t n d counting rate from radioactivite th n i y scal d skullan p . This contributiof o e minimizee b us e n th ca n y b d special collimators designed to view only a part of the brain but to include in their field of view a minimum of non-neural tissue. A further possibility arises with radioisotopes such as In which emit characteristi X radiatioc y radiatios wel na s a l n since the contribution of the former to the total observed counting rate is almost entirely due to radioactivity in the superficial tissues whereas that of the latter is due to radioactivity in the super- ficial e braintissueth y recordind B .an s e countingth g raten i s appropriate channel photoe th f o ns spectru s thui t si m possiblo t e correct the results for radioactivity in the scalp and skull.- With radioisotope c whicS s suc r morho s a hemieo tw photont 7"5 n i s cascade, coincidence counting techniques offer still a further possibilit minimizo t y e contributioth e n from radioactivite th n i y superficial tissues. Various potential applications of these techniques are described. - 5 12 - Although most solutes exchange slowly "between "blood and brain, "brain uptake measurement varieta f o s substancef o y s revea wida l e rang f freedoo e m of exchange. Some generalizations relating chemical composition to the rate of exchange can be made. blooe . 1 th Wate dd gaseran s exchange freely between plasm braind an a . 2. There is a restriction of all of the common electrolytes found in plasma. 3. Most small molecule plasman i s , suc ures ha d creatinine aan , showa restricted passage related in some degree to molecular weight. 4. Large molecules, such as the plasma proteins, show an extreme re- strictio passagf o n e into brain healthn I .nearls i t ,i y impossibl mako t e a e meaningful measurement of large molecular uptake because this uptake is so low. The exchang largf o e e molecules between plasm braid an amors i n e tha n ordea n r of magnitude less than that of small molecules such as sucrose or mannitol. This extreme restrictio largf o n e molecules result considerabla n si e dependence f capillaro y permeabilit smalo t y l molecules upon their completenesf o s bindin plasmo gt a proteins A substanc. e which might otherwis e exchangeb e d freely wil e markedlb l y restricte s firmli t i y f bouni d plasmo t d a proteins. 5- Lipid-soluble substance plasmn si a generally exchange freely with the brain. Non-electrolytes penetrate into brain tissue with variable ease and this variability usually is related, in addition to molecular size, to their lipid-solubility. This probably pertains to the solubility of the substanc plasme lipie th th f ao dn i emembran e capillarth f o es i y t i cell f I . solublplasme th n ai e membran n readile cellca th t f ,i o e y pass inte th o interio e capillarth f o r yexternae th cell t ou ,l plasma membrane d throug,an h the basement membrane. This free penetratio lipid-solublf o n e substances ordinarily doe expost sno e brain cell harmfuo t s l substances because there ar e essentailly no natural, lipid-soluble noxious substances present in plasma which are not bound firmly to plasma proteins. Thus, bilirubin which could be seriously neurotoxic, particularly when blood levels are elevated, achieves very little penetration of the brain capillary presumably because of its protein binding. Substances suc antipyrines ha , barbiturates, ethyl alcohol, anesthetic agents and many central nervous system drugs are lipid-soluble and thus freely penetrate from blood into brain. - 126 - 6. In addition to these relatively non-specific exchanges "between "blood and brain, certain metabolites suc somd glucoss ha an e ) amin(l e o} 3 acid , (2 s apparently pass throug capillare th h y wall n specifio s c carrier systemse Th . entry of both glucose and amino acids are quite efficient at low plasma concentrations. This transpor glucosf o t d aminan e o acid s saturablsi d an e simplo t e edu thu diffusiont sno . This transpor s specifii t r thesfo c e metabolites with restrictio f otheo n r solutes, even thoug f vero h y simila, - r molecular structure. Capillary permeability The mechanisms of the restricted permeability of brain capillaries are not completely understood t certai,bu n aspect f thio s s blood brain barrier (BBB) have been clarified. The capillary wall of most other tissues exhibits the following characteristics* t 1. Water and blood gases are freely exchanged. 2. Lipid-soluble substances are very rapidly exchanged; their rates of exchange approaching that of water. 3. Molecules less than about 20,00 40,00o 0t 0 molecular weight equrli- brate quite rapidly between plasma and extracellular fluid (4)» exhibiting an equilibration half-time less than one minute. This is considerably less rapid thar lipid-solublfo n e substances. 4. Substances with a molecular weight over about 40,000 exhibit a passage through the capillary wall -more than an order of magnitude less efficiently thar small molecules. Even very large macromolecules, however, continu o past e s through capillary walls wit a rath passagf o e e tha roughls i t y independenf o t their molecular size (4). Ther e probablar e y three route passagf o s e throug capillare th h y walf o l most tissues (Fig.l.). 1 Directl ye plasm wallo througth tw f ao e s membranth h capillare th f o e j cell with passage through the car-illary cellular cytoplasm. 2. Passage through clefts (about 90 A width) between the capillary cells whic e attachear h d onl point a y t junctions. Mucopolysaccharide e cleftth n i s may assis filterinn i t largt gou ebrain i molecule t nbu thi ) unlikels i s(5 s y since intraventricular horseradish peroxidase (M.W. 42,000) passeS sEC froe th m of brai ne tigh levee intth clefe th tf th o o lo intercellula t r junction (6). - 127 - 3. Pinocytosis (fluid phagocytosis) of small portions of "blood plasma which are transported through the cytoplasm, isolated from the cytoplasm "by the fragemen plasmf o t a membrane surroundin pinocytotie gth c vesicle. They are exuded into the extracellular space (ECS) at the basement membrane surroundi the capillary. These speculation e compatiblsar e witconcepe th h t that water, blood gases, and lipid-soluble substances pass directly throug membrane walle th th hf o sf o e the capillary cell. The intercellular clefts are the probable major route for the less efficiently exchanged substances suc s plasmha a electrolytes, urea, glucose, amino acids etc. The relatively inefficient transport of macromolecule independent of their molecular size could be attributed to pinocytosis (?)• Sinc pinocytotie th e c vesicl substantialls i e y largemacroe th f ro - thay an n molecules in blood, this mode of transport would be independent of molecular l plasmal siz d aan e solutes coul e expectedb o traverst d e capillarth e y celli at similar rates. There probably is no specific, carrier-mediated capillary transport in tissues other than nervous system. Such transport would not be required because intercellular non-specific passage woul e adequatb d meeo t e t most metabolic exchange requirements. If present, it would be inconspicuous against a back- ground of efficient non-specific transport. It is possible that a system of very small pores (less than 10 A) in the plasma membran f generao e l capillary cellmajoa s i spassage r th rout r eefo of some ions and very small molecules such as water and urea. Although these postulated pores havt beeeno n visualize y electrob d n microscopy s stili t i ,l possible that transcellular transport of certain small molecules takes place by this route. The issue of how much capillary transport takes place through the the cells and how much between capillary cells is still open. The extensive recent literature on membrane transport has been reviewed (8). Brain transport mechanisms In the brain there probably is passage directly through the capillary cell of water, blood gases and lipid-soluble substances. The junctions between brain capillary cells have been show y electronb n microscop e differenb o t y t from capillary cells elsewhere in that they constitute a continuous strip of fusion between adjacent plasma membranes (9) welding the entire capillary into what - 128 - is, in effect, a continuous plasma membrane.