Facilitated transport of melphalan at the rat blood-brain barrier by the large neutral amino acid carrier system.

Melphalan has been reported to be actively transported into tumor cells by two amino acid carrier systems. As amino acids are transported across cerebral capillaries by a facilitated mechanism, studies were undertaken to assess whether or not melphalan was transported similarly, and additionally to determine melphalan's plasma and brain pharmacokinetics. The brain uptake of [14C]melphalan was measured by an in situ brain perfusion technique in the anesthetized rat utilizing [14C]-melphalan. The cerebrovascular permeability-surface area product of [14C]melphalan was calculated at cold melphalan concentrations from O to 16.3 mumol/ml. The permeability-surface area product was concentration dependent and decreased from 10.8 +/- 0.6 (+/- SE) X 10(-4)S-1 at 0.02 mumol/ml melphalan to 5.4 +/- 0.3 X 10(-4)S-1 at 16.3 mumol/ml. The system became saturated at a concentration in excess of 0.1 mumol/ml. The Michaelis-Menten parameters Vmax and Km, determined by nonlinear regression analysis of the permeability-surface area product data, equaled 0.9 +/- 0.3 X 10(-4) mumol/s/g and 0.15 +/- 0.06 mumol/ml, respectively, for the saturable component of melphalan's brain uptake. The Kd of the nonsaturable component was 5.3 +/- 0.03 X 10(-4)S-1. Addition of the amino acid 1-phenylalanine to the brain perfusate inhibited the saturable component of melphalan's brain uptake. The analysis of the plasma and brain concentrations of melphalan by high-performance liquid chromatography, following i.v. melphalan administration, demonstrated that approximately 15% of the drug that was present in plasma entered the brain. These data suggest that the brain uptake of melphalan is facilitated, demonstrating concentration-dependent uptake, saturation, and inhibition, and that melphalan shares the large neutral amino acid carrier system at the blood-brain barrier.

Rapid high-affinity transport of a chemotherapeutic amino acid across the blood-brain barrier.

The therapeutic efficacy of many anticancer drugs against intracerebral tumors is limited by poor uptake into the central nervous system. One way to enhance brain delivery is to design agents that are transported into the brain by the saturable nutrient carriers of the blood-brain barrier. In this paper, we describe a nitrogen mustard amino acid, DL-2-amino-7-bis[(2-chloroethyl)amino/bd-1,2,3,4-tetrahydro-2-napthoi c acid, that is taken up into brain with high affinity by the large neutral amino acid carrier of the blood-brain barrier. Brain transport of DL-2-amino-7-bis[(2-chloroethyl)aminol-1,2,3,4-tetrahydro-2-naphth oic acid in the rat was found to be rapid (cerebrovascular permeability-surface area product approximately 2 x 10(-2) ml/s/g), saturable and inhibitable by large neutral amino acids. Maximal influx rate (Vmax) and half-saturation (Km) constants equaled 0.26 nmol/min/g and 0.19 microM, respectively, in the parietal cortex. Regional brain uptake of acid exceeded that of the clinical analogue, melphalan, by greater than 20-fold. The results demonstrate that drug modification to produce high-affinity ligands for the cerebrovascular nutrient carriers is a viable means to enhance drug delivery to brain for the treatment of brain tumors and other central nervous system disorders.

Prevention of nerve edema and increased blood-nerve barrier permeability-surface area product in galactosemic rats by aldose reductase or thromboxane synthetase inhibitors.

Nerve water content and the permeability-surface area product (PA) to [3H]-or [14C]sucrose at the blood-nerve barrier were determined in unanesthetized control rats fed a normal diet and in rats fed galactose with or without an aldose reductase inhibitor (Statil or AL 1576) or a thromboxane synthetase inhibitor (CGS 12970). Nerve water content was determined by taking the difference between dry and wet weights of whole tibial nerves. PA was determined by an intravenous bolus injection of radiotracer with multiple-time-point graphic and quantitative autoradiographic methods. The mean nerve water content in galactosemic rats was 15% higher than in control rats after 7-11 mo on the diet. Statil and AL 1576 prevented nerve edema, but CGS 12970 was only partially effective in preventing an increase in nerve water content in galactose-fed rats. In galactosemic rats, the mean PA to sucrose at the blood-nerve barrier, calculated from nerve dry weight, was twofold higher than in control rats. Treatment with Statil, AL 1576, or CGS 12970 prevented increased PA. Our results suggest that nerve edema and increased blood-nerve barrier PA are secondary to polyol production and can be prevented by inhibiting aldose reductase.

Altered blood-nerve barrier permeability to small molecules in experimental diabetes mellitus.

Permeability-surface area products (PA) were determined with a quantitative in vivo injection technique at the blood-nerve barrier of tibial nerve, and at the blood-brain barrier, in control and streptozotocin-induced diabetic rats. The PA product for [14C]mannitol at the blood-nerve barrier was increased by 100% in diabetic animals, 3.12 +/- 0.15 X 10(-5) ml X s-1 X g-1, compared with controls, 1.61 +/- 0.10 X 10(-5) ml X s-1 X g-1. In contrast, PA for [14C]mannitol at the blood-brain barrier was unaltered in the diabetic animals. Following intravenous injection, no leakage of microperoxidase across the perineurium or endoneurial vessels of diabetic rats could be demonstrated by morphological techniques. Nerve blood-space, as determined with intravenous [3H]inulin, and blood-nerve barrier surface area as determined by morphometric methods, did not differ in diabetic when compared to control animals. Thus, the calculated permeability coefficient for [14C]mannitol at the blood-nerve barrier was about 100% greater in diabetic nerve compared to control nerves. The increased permeability was accompanied by a 7% increase in nerve-water content and a 32% decrease in motor-nerve conduction velocity. The results demonstrate a specific vulnerability of nerve as compared to brain in an animal model of diabetes mellitus. Chronically altered permeability to small water-soluble molecules reduces the protective effect of salt impermeability at the blood-nerve barrier against nerve edema, and may be an important pathogenic mechanism in diabetic neuropathy.

Regional cerebrovascular transport of leucine as measured by the in situ brain perfusion technique.

Leucine influx into six brain regions was determined in anesthetized rats with the in situ brain perfusion technique using either saline or plasma perfusate. This technique has several advantages over other methods such as the brain uptake index (BUI) technique. The concentration dependence of L-leucine influx was best described by a model with a saturable and a nonsaturable component. For the saturable component, Vmax equaled 10.67 +/- 0.21 X 10(-4) mumol s-1 g-1 and Km equaled 0.0257 +/- 0.0009 mumol ml-1, whereas the constant of nonsaturable diffusion (Kd) equaled 0.957 +/- 0.067 X 10(-4) s-1 in the parietal lobe during saline perfusion. Vmax was higher in the cortical lobes than in other brain areas, probably owing to a greater capillary surface area. There were no regional differences in Km or Kd. The apparent Km for L-leucine during plasma perfusion was 20 times greater than the Km during saline perfusion, and 3 to 4 times greater than the plasma leucine concentration, owing to competitive inhibition of leucine transport by other large neutral amino acids in plasma. These results for Vmax, Km, and Kd differ by three- to fourfold from previous estimates obtained with the BUI technique. The high apparent Km during plasma perfusion indicates that leucine influx is a linear function of plasma concentration up to 0.5 mumol ml-1 when the plasma concentrations of other amino acids remain constant, whereas influx would be approximately constant when plasma concentrations of all large neutral amino acids increased or decreased by a constant fraction.

L-methionine uptake by human cerebral cortex: maturation from infancy to old age.

Age-associated changes in amino acid transport from blood to normal frontal cortex were studied using positron emission tomography (PET). Seventeen patients, 1.8-71 yr, were injected intravenously with tracer doses of [11C] L-methionine and a baseline PET scan was obtained. To assess competitive inhibition of [11C]L-methionine uptake, patients received either oral L-phenylalanine or an i.v. infusion of amino acids 1 hr before a second PET study. Uptake of [11C]L-methionine by frontal cortex decreased seven-fold between 1.8 and 71 yr (r = -0.71; p less than 0.05). Blood-to-brain transfer of [11C]L-methionine, at 4.5 yr, exceeded mean adult values by more than five-fold. Competitive inhibition reduced L-methionine uptake in all patients older than 4.6 yr. These developmental changes parallel findings in animals. The neutral amino acid transport system may modulate human brain amino acid levels to meet changing developmental metabolic needs.

Fatty acid uptake and incorporation in brain: studies with the perfusion model.

The contributions of individual components of blood to brain [14C]palmitate uptake and incorporation were studied with the in situ brain perfusion technique in the pentobarbital-anesthetized rat. With whole-blood perfusate, brain unacylated [14C]palmitate uptake was linear with time and extrapolated to zero at T = 0 s of perfusion. Tracer accumulated in brain with a blood-to-brain transfer coefficient of 1.8 +/- 0.1 x 10(-4) mL/s/g (whole cerebral hemisphere). Incorporation into brain lipids was rapid such that approximately 40% of tracer in brain at 45 s of perfusion was in cerebral phospholipids and neutral lipids. Similar rates of uptake were obtained during unacylated [14C]palmitate perfusion in whole rat plasma, serum, or artificial saline containing 2-3% albumin, suggesting that albumin has a key role in determining [14C]palmitate uptake in brain. The excellent match in brain uptake rates between whole blood and albumin-containing saline fluid suggests that the perfusion technique will be useful method for quantifying the individual contributions of blood constituents and albumin binding on brain [14C]palmitate uptake.

Kinetics of amino acid transport at the blood-brain barrier studied using an in situ brain perfusion technique.

The in situ brain perfusion technique of Takasato et al. allows accurate measurements of regional amino-acid influx across the blood-brain barrier. Influx can be determined in the absence of competing amino acids, using saline perfusate, or in the presence of physiological concentrations of amino acids, using plasma or blood perfusate. The concentration dependence of cerebrovascular LNAA transport is best described by a model with a Michaelis-Menten saturable and a nonsaturable component. Vmax values for four LNAAs are similar and average 10 X 10(-4) mumol X sec-1 X g-1. In contrast, Km values differ by up to 10-fold with the lowest value for leucine. Competitive inhibition increases the apparent Km for transport from plasma by approximately 20-fold. These data provide accurate new estimates of the kinetic constants that describe amino-acid transport across the blood-brain barrier. In addition, they indicate that the cerebrovascular transport capacity (Vmax) and affinity (1/Km) are greater than previously reported with the brain uptake index technique.

Facilitated brain uptake of 4-chlorokynurenine and conversion to 7-chlorokynurenic acid.

7-Chlorokynurenic acid (7-Cl-KYNA) and 5,7-dichlorokynurenic acid (5,7-Cl2-KYNA) are of therapeutic interest as potent glycine/N-methyl-D-aspartate NMDA) receptor antagonists, but are excluded from brain by the blood-brain barrier. We examined whether these compounds could be delivered to brain through their respective precursors, L-4-chlorokynurenine (4-Cl-KYN) and L-4,6-dichlorokynurenine (4,6-Cl2-KYN), which are amino acids. 4-Cl-KYN was shown to be rapidly shuttled into the brain by the large neutral amino acid transporter of the blood-brain barrier (K(m) = 105 +/- 14 microM, Vmax = 16.9 +/- 2.3 nmol min-1 g-1) and to be converted intracerebrally to 7-Cl-KYNA. 4,6-Cl2-KYN also expressed affinity for the transporter, but four-fold less than that of 4-Cl-KYN. In summary, the results show that because of their facilitated uptake 4-Cl-KYN and 4,6-Cl2KYN might be useful prodrugs for brain delivery of glycine-NMDA receptor antagonists.

Affinity of antineoplastic amino acid drugs for the large neutral amino acid transporter of the blood-brain barrier.

The relative affinity of six anticancer amino acid drugs for the neutral amino acid carrier of the blood-brain barrier was examined in rats using an in situ brain perfusion technique. Affinity was evaluated from the concentration-dependent inhibition of L-[14C]-leucine uptake into rat brain during perfusion at tracer leucine concentrations and in the absence of competing amino acids. Of the six drugs tested, five, including melphalan, azaserine, acivicin, 6-diazo-5-oxo-L-norleucine, and buthionine sulfoximine, exhibited only low affinity for the carrier, displaying transport inhibition constants (Ki, concentrations producing 50% inhibition) ranging from 0.09 to 4.7 mM. However, one agent - D,L-2-amino-7-bis[(2-chloroethyl)amino]- 1,2,3,4-tetrahydro-2-naphthoic acid (D,L-NAM) - demonstrated remarkably high affinity for the carrier, showing a Ki value of approximately 0.2 microM. The relative affinity (1/Ki) of D,L-NAM was greater than 100-fold that of the other drugs and greater than 10-fold that of any compound previously tested. As the blood-brain barrier penetrability of most endogenous neutral amino acids is related to their carrier affinity, the results suggest that D,L-NAM may be a promising agent which may show enhanced uptake and distribution to brain tumors.

Brain uptake and anticancer activities of vincristine and vinblastine are restricted by their low cerebrovascular permeability and binding to plasma constituents in rat.

Unidirectional blood-brain barrier transfer of the lipophilic anticancer agents vincristine and vinblastine was studied in anesthetized rats, using an isolated, in situ brain perfusion technique. Drug binding to plasma constituents was also measured. Despite the high lipophilicity of these agents (the log octanol/physiological saline partition coefficient equalled 2.14 and 1.68, respectively), the cerebrovascular permeability-surface area product, PA, of vincristine in plasma was only 0.49 x 10(-4) ml s-1 g-1 for parietal cerebral cortex, whereas that of vinblastine was too low for determination. These values are similar to those of water-soluble, poorly diffusible nonelectrolytes. The PAs were significantly higher in the absence of plasma protein, being 1.24 x 10(-4) and 5.36 x 10(-4) ml s-1 g-1, respectively. Even these values, determined by brain perfusion of protein-free buffer, were lower than would be expected from the lipophilicity of the agents. The results suggest that additional factors, such as steric hindrance and molecular charge distribution, related to the chemical and geometric structure and the large size of vincristine and vinblastine (molecular weight, 825 and 814 daltons, respectively) restrict their passage across the blood-brain barrier. As a consequence of their paradoxically low permeability at the blood-brain barrier and restrictive binding to plasma and blood constituents, doses of both agents that cause significant inhibition of extracerebral Walker 256 carcinosarcoma tumor implants in rat have no effect on tumor located in the brain.

Chloride efflux from isolated choroid plexus.

Chloride efflux was analyzed in adult rat lateral ventricle choroid plexus (LVCP) incubated in artificial CSF (aCSF) at 37 degrees C. Following steady-state loading of 36Cl in LVCP, the tracer release from plexus to aCSF was quantified by the efflux coefficient (k, s-1), equal to ln 2/t/2. Cl efflux could be described by a 2-component model, with a t/2 for the 'fast' component matching well that for [3H]sucrose (extracellular marker) and a slower, drug-inhibitable component of 36Cl release thought to reflect cellular washout. The cellular Cl efflux was more than twice as fast as 37 degrees C than at 15 degrees C. There was progressively more rapid efflux (k) of 36Cl from cells as the aCSF was altered over a range of several pH values from 6.7 (k = 0.026 s-1) to 8.2 (0.070 s-1). CSF medium anion replacement (isethionate and HEPES for Cl and HCO3, respectively) reduced the k for 36Cl by 57%. Acetazolamide (0.1 mM) and other Cl transport inhibitors (disulfonic stilbenes and loop diuretic) reduced Cl efflux by 35-55%. Acetazolamide inhibited Cl release from LVCP into aCSF whether the latter contained Cl and HCO3, or not. Overall, the findings suggest that Cl extrusion from choroid plexus is by way of an anion exchanger and via channels.

Kinetic analysis of L-leucine transport across the blood-brain barrier.

Unidirectional L-leucine influx across cerebral capillaries was measured at different concentrations with an in situ rat brain perfusion technique, which has several advantages over presently-used methods such as the Brain Uptake Index (BUI) technique. The maximal influx rate (Vmax) and half-saturation concentration (Km) equaled 1.07 +/- 0.02 X 10(-3) mumol X s-1 X g-1 and 0.026 +/- 0.002 mumol X ml-1, respectively, for the saturable component, and the constant for non-saturable influx equaled 6.8 +/- 1.4 X 10(-5) s-1. These values differ by 3-4-fold from respective values obtained with the BUI technique.

Calcium influxes into brain and cerebrospinal fluid are linearly related to plasma ionized calcium concentration.

Unidirectional Ca influxes into brain and cerebrospinal fluid (CSF) were measured at different plasma concentrations of ionized Ca ([Ca]i) in pentobarbital-anesthetized rats. Plasma [Ca]i was varied acutely from 0.6 to 3.0 mumol/ml by intravenous infusion of EGTA, NaCl or CaCl2 or by thyroparathyroidectomy. Ca influx was determined from the 15-min uptake of 45Ca after intravenous injection. There were significant regional differences in 45Ca uptake into the CNS, with a approximately 20-fold greater rate into ventricular CSF than into frontal cortex. Autoradiographs of 45Ca uptake demonstrated that uptake into frontal cortex reflects primarily transport across the cerebral capillaries, whereas uptake into ventricular CSF reflects transport across the choroid plexuses. At both sites, Ca influx was a linear function of plasma [Ca]i and extrapolated to zero at [Ca]i = 0. Infusion of EGTA or CaCl2 did not alter the integrity of the blood-brain barrier, as determined by the permeability to [14C]sucrose. These results indicate that Ca influx into the CNS is not regulated by a saturable mechanism that is sensitive to acute changes in plasma [Ca]i. The proportionality between influx and concentration is suggestive of passive diffusional transport. The brain is protected from acute changes in plasma [Ca]i by the low cerebrovascular permeability to Ca, approximately 5 X 10(-8) cm/s.

Uptake and concentrations of calcium in rat choroid plexus during chronic hypo- and hypercalcemia.

The choroid plexus has been implicated in the regulation of cerebrospinal fluid (CSF) [Ca], but little information is available concerning Ca transport by this epithelium. We determined the transfer coefficients for 45Ca uptake into choroid plexus from blood, as well as tissue [Ca], in weanling Fischer-344 rats fed low, normal, or high Ca diets for 8 weeks. Plasma [Ca] decreased by 45% with low Ca diet and increased by 25% with high Ca diet. Choroid plexus 45Ca uptake varied inversely with plasma [Ca]. This relation was due largely to changes in extracellular Ca binding rather than to entry from blood, as the transfer coefficient was independent of plasma [Ca]. The extracellular Ca distribution in choroid plexus, the intercept of a plot of tissue 45Ca distribution against time, was reciprocally related to plasma [Ca]. Changes in total cell [Ca] during hypercalcemia were equivalent to those in plasma, and in hypocalcemia were 70% of those in plasma. These findings indicate that regulation of CSF [Ca] does not involve saturable transport of Ca into the choroid epithelium from blood, and that the apical membrane of the choroid epithelium is involved in homeostasis of CSF [Ca].

Kinetic analysis of [36Cl]-, [22Na]- and [3H]mannitol uptake into the in vivo choroid plexus-cerebrospinal fluid brain system: ontogeny of the blood brain and blood-CSF barriers.

The kinetics of penetration of radioactive [36Cl]-, [22Na]- and [3H]mannitol into the choroid plexus-CSF brain system was investigated in 1-week, 2-week and adult Sprague-Dawley rats. For adult rats (5 weeks), 36Cl and 22Na uptake by the choroid plexus of lateral ventricle (LVCP) and fourth ventricle (4VCP) resolved into a fast component (t1/2 0.02 - 0.05 h) representing isotope distribution within the extracellular and residual erythrocyte compartments, and a slow component (t1/2 0.8 - 1.9 h) representing isotope movement into the epithelial cell compartment. From steady-state distribution data, choroid cell [Cl] in both LVCP and 4VCP was calculated to be 67 mmol/kg cell H2O, a level nearly 4 times greater than that predicted by the membrane potential for passive distribution. In 1-week immature rats cell [Cl] and [Na] in the choroid plexuses were even greater than the corresponding levels in adults, probably because ion transport across the basolateral membrane is not yet coupled with ion movement from cell to CSF. In mature rats the 36Cl and 22Na uptake into the CSF resolved into 2 components (t1/2 0.18 h, fractional volume 0.24 and t1/2 1.2 h, fractional volume 0.76); however, the fast component of CSF uptake, which likely represents isotope movement across the choroid plexuses, was negligible in the 1-week animals. Permeability-surface area products (PA) were determined for the blood-CSF barrier (i.e. the choroid plexuses) as well as the blood-brain barrier (cerebral cortex and cerebellum). The PA values for 36Cl and 22Na as determined by the fast component of CSF uptake (choroid plexus secretion?) were an order of magnitude less in the 1-week rats than in adults. In contrast, the effective permeability of the blood-CSF barrier as well as the blood-brain barrier, as evaluated by changes in PA of [3H]mannitol, decreased steadily with advancing age.

Kinetics and distribution volumes for tracers of different sizes in the brain plasma space.

Regional brain and plasma concentrations were determined for a series of radiotracers that differ in molecular weight and size in pentobarbital-anesthetized rats at 1, 5 and 30 min after i.v. injection. The tracers, [3H]inulin (mol. wt. 5000 Da, radius 1.5 nm), 5 [3H]dextrans (10,000-200,000 Da, 2.3-9.5 nm) and [51Cr]transferrin (79,000 Da, 3.8 nm), are not taken up into erythrocytes and do not measurably cross the blood-brain barrier in 30 min. Results were expressed as a brain distribution volume, defined as (dpm/g brain)/(dpm/ml plasma). Within 1 min after injection, all tracers attained an initial distribution volume which varied regionally from 0.4 to 1.6 X 10(-2) ml/g. The volumes remained constant between 1 and 30 min for tracers with radii greater than or equal to 3.8 nm, whereas the volumes increased up to 90% for tracers with radii less than or equal to 3.1 nm. Rates of equilibration for tracers with radii less than or equal to 3.1 nm were size dependent with smaller tracers equilibrating before larger tracers. These results indicate that the brain distribution volume for plasma tracers consists of two compartments: one which is quickly filled (less than or equal to 1 min) by all tracers and comprises approximately 60% of the total volume, and one which allows only tracers with radii less than or equal to 3.1 nm and comprises 40% of the total volume. The inverse relation between the rate of equilibration in the second compartment and molecular size may indicate a diffusion limitation.(ABSTRACT TRUNCATED AT 250 WORDS)

Calcium deficiency enhances cadmium accumulation in the central nervous system.

Weanling male rats were administered 1 of 4 diets for 40 days: control (CONT), low Ca (LOCA), control plus Cd (CONT + Cd) or low Ca plus Cd (LOCA + Cd). After 40 days, Cd was analyzed in 7 brain regions, spinal cord, serum, liver, kidney, muscle and femur by atomic absorption spectrophotometry with Zeeman background correction. No significant difference in Cd between CONT and LOCA was found except in femur, where it was increased. In CONT + Cd rats, peripheral tissues showed an increase in Cd of 30-71 fold above CONT rats. Brain regions exhibited a more modest 7-10 fold change, and serum Cd was 8.5 times above control. LOCA + Cd rats showed a 25-fold increase of Cd above LOCA in serum, 25-100 fold in peripheral tissues, and a 14-20 times in brain. These findings show that brain Cd is increased during Ca deficiency, but that central nervous system Cd changes less than peripheral tissue Cd. This increase in brain Cd could alter brain function.

Brain uptake of a food dye, erythrosin B, prevented by plasma protein binding.

Although food colors have been held responsible for several behavioral disorders and do affect neuronal function when directly applied, there is no information on whether significant quantities of the dyes appear in the brain after consumption or parenteral administration. [14C]erythrosin B was administered directly into the circulation of mature rats and radioactivity was measured thereafter in brain regions at several times. Although insignificant parenchymal radioactivity was detected in brains perfused with dye in whole blood, significant concentrations of [14C]erythrosin B were detected in all brain regions when perfused with protein-free Ringers, as predicted from the octanol-water partition coefficient of the dye. Thus, significant brain uptake of intravascular dye is normally prevented by its binding to plasma protein (greater than 99% bound) and by the blood-brain barrier impermeability to the dye-protein complex. Sensitivity to food dyes such as erythrosin B in some individuals may reflect altered plasma protein binding capacity, which can vary with age and disease.

Blood-brain barrier choline transport in the senescent rat.

Choline is an important membrane phospholipid constituent and a neurotransmitter precursor that is minimally synthesized in brain. The long-term maintenance of brain choline concentration is dependent on uptake from plasma, which occurs via saturable transporter at the blood-brain barrier. Previous studies have suggested that brain choline uptake declined with age. To reevaluate this, brain choline uptake in 3, 12, 24, and 28-month-old Fischer-344 rats was evaluated using the in situ brain perfusion technique. Minimal differences were found with uptake parameters differing by approximately 10% between aged and adult rats for tracer levels while similar trends were observed at higher choline concentrations. Further, estimated Vmax and Km values differed by <30% between the groups. The results suggest that blood-brain barrier choline uptake changes minimally with aging in the rat.