Carrier mediated blood-brain barrier transport of choline and certain choline analogs

Active transport of high-affinity choline and nicotine analogs into the central nervous system by the blood-brain barrier choline transporter

Cigarette smoking is strongly implicated in the development of cardiovascular disorders. Recently identified nicotinium analogs may have therapeutic benefit as smoking cessation therapies but may have restricted entry into the central nervous system by the blood-brain barrier (BBB) due to their physicochemical properties. Using the in situ perfusion technique, lobeline, choline, and nicotinium analogs were evaluated for binding to the BBB choline transporter. Calculated apparent K(i) values for the choline transporter were 1.7 microM N-n-octyl choline, 2.2 microM N-n-hexyl choline, 27 microM N-n-decylnicotinium iodide, 31.9 microM N-n-octylpyridinium iodide, 49 microM N-n-octylnicotinium iodide (NONI), 393 microM lobeline, and >/=1000 microM N-methylnicotinium iodide. Nicotine and N-methylpyridinium iodide, however, do not apparently interact with the BBB choline transporter. Given NONI's apparent K(i) value determined in this study and its ability to inhibit nicotine-evoked dopamine release from superfused rat brain slices, potential brain entry of NONI via the BBB choline transporter was evaluated. [(3)H]NONI exhibited a BBB transfer coefficient value of approximately 1.6 x 10(-3) ml/s/g and a K(m) of approximately 250 microM. Unlabeled choline addition to the perfusion fluid reduced [(3)H]NONI brain uptake. We hypothesize the N-n-octyl group on the pyridinium nitrogen of NONI facilitates brain entry via the BBB choline transporter. Thus, NONI may have utility as a smoking cessation agent, given its ability to inhibit nAChRs mediating nicotine-evoked dopamine release centrally, and to be distributed to brain via the BBB choline transporter.

Choline in the aging brain

Proton magnetic resonance spectroscopy has been increasingly utilized in brain research to monitor non-invasively metabolites such as N-acetyl aspartate (NAA), creatine (Cr) and choline (Cho). We present here studies of the effect of aging on the ratios of these metabolites measured in the rat brain in vivo and on choline transport and lipid synthesis in rat brain slices, in vitro. The in vivo studies indicated that the ratios of Cho/NAA and Cho/Cr increased in the aged hippocampus, whereas the ratio of Cr/NAA was similar in the aged and adult hippocampus. These three ratios remained similar in the cortex of adult and aged rats. The in vitro studies revealed that in the aged cortex and the aged hippocampus the activity of the low-affinity choline uptake increased, possibly compensating for a decrease in the high-affinity uptake activity and the rate of choline diffusion. The incorporation of choline into phospholipids exhibited high and low affinity kinetics which were not modified by aging.

The transport of choline

Choline has many physiological functions throughout the body that are dependent on its available local supply. However, since choline is a charged hydrophilic cation, transport mechanisms are required for it to cross biological membranes. Choline transport is required for cellular membrane construction and is the rate-limiting step for acetylcholine production. Transport mechanisms include: (1) sodium-dependent high-affinity uptake mechanism in synaptosomes, (2) sodium-independent low-affinity mechanism on cellular membranes, and (3) unique choline uptake mechanisms (e.g., blood-brain barrier choline transport). A comprehensive overview of choline transport studies is provided. This review article examines landmark and current choline transport studies, molecular mapping, and molecular identification of these carriers. Information regarding the choline-binding site is presented by reviewing choline structural analog (hemicholinium-3 and 15, and other nitrogen/methyl-hydroxyl compounds) inhibition studies. Choline transport in Alzheimer's disease, brain ischemic events, and aging is also discussed. Emphasis throughout the article is placed on targeting the choline transporter in disease and use of this carrier as a drug delivery vector.

Prediction of drug transport through the blood-brain barrier in vivo: a comparison between two in vitro cell models

PURPOSE

Studies were conducted to evaluate whether the use of an in vitro model of the blood-brain barrier (BBB) resulted in more accurate predictions of the in vivo transport of compounds compared to the use of a human intestinal cell line (Caco-2).

METHODS

The in vitro BBB model employs bovine brain capillary endothelial cells co-cultured with primary rat astrocytes. The Caco-2 cells originate from a human colorectal carcinoma. The rat was used as experimental animal for the in vivo studies.

RESULTS

Strong correlations (r = 0.93-0.95) were found between the results generated by the in vitro model of the BBB and two different methodologies to measure the permeability across the BBB in vivo. In contrast, a poor correlation (r = 0.68) was obtained between Caco-2 cell data and in vivo BBB transport. A relatively poor correlation (r = 0.74) was also found between the two in vitro models.

CONCLUSION

The present study illustrates the limitations of the Caco-2 model to predict BBB permeability of compounds in vivo. The results emphasize the fact that the BBB and the intestinal mucosa are two fundamentally different biologic barriers, and to be able to make accurate predictions about the in vivo CNS penetration of potential drug candidates, it is important that the in vitro model possesses the main characteristics of the in vivo BBB.

A critical evaluation of the brain efflux index method as applied to the nitric oxide synthase inhibitor, aminoguanidine

The Brain Efflux Index (BEI) method is an in vivo procedure designed to quantitate saturable efflux mechanisms resident at the blood--brain barrier (BBB). The present work utilized the BEI method to assess the BBB efflux mechanisms of [(14)C]aminoguanidine, a nitric oxide synthase inhibitor. The BEI for [(14)C]aminoguanidine was >100% (relative to [(3)H]inulin diffusion) over a range of 41-184 pmol after 40 min. The unusually high retention (>100%) of [(14)C]aminoguanidine suggested brain parenchymal sequestration, either by neuronal uptake or tissue protein binding. The uptake of [(14)C]aminoguanidine in dendritic neuronal endings (synaptosomes) showed a saturable concentration dependency, consistent with a carrier-mediated process. Nonlinear least-squares regression yielded the following Michaelis--Menten and diffusional (k(ns)) parameters for synaptosomal [(14)C]aminoguanidine uptake: V(max)=118.50 +/- 28.77 pmol x mg protein(-1)/3 min; K(m)=58.34 +/- 8.33 muM; k(ns)=0.15 +/- 0.029 pmol x mg protein(-1)/3 min/muM; mean +/- SEM; n=3 concentration profiles). Protein binding studies using brain tissue showed negligible binding. In summary, this work identified three principle findings: (1) An apparent lack of quantifiable aminoguanidine BBB efflux; (2) a previously undescribed synaptosomal accumulation process for aminoguanidine; and (3) an interesting limitation of the BEI technique where unusual brain parenchymal sequestration yields values >100%.

Ventricular choline transport: a role for organic cation transporter 2 expressed in choroid plexus

To determine whether organic cation transporter (OCT) family members might mediate choline transport in choroid plexus (CP), the handling of choline by cloned transporters and by intact CP isolated from the adult rat was investigated. Expression of OCT1 and OCT2 in Xenopus oocytes increased hemicholinium-3-sensitive choline uptake. In contrast, OCT3 did not mediate choline transport. Estimated K(m) values for choline in rOCT1-, rOCT2-, and hOCT2-expressing oocytes were 346 +/- 50, 441 +/- 67, and 102 +/- 80 microm, respectively. Membrane potential was the major driving force for choline uptake in rat and human OCT2-expressing oocytes and in intact CP in vitro. Lowering of medium pH (6 versus 7.4) was equally effective at inhibiting choline uptake in CP, suggesting that there might be a non-OCT component of choline uptake that is responsive to an H(+) gradient. However, choline efflux from CP was not stimulated by a trans-applied H(+) gradient. Choline uptake by CP was Na(+)-independent with an estimated K(m) of 183 microm. Reverse transcriptase-polymerase chain reaction detected OCT2 and OCT3, but not OCT1, mRNA expression in CP. Transfection of intact CP with a rOCT2/green fluorescent protein fusion construct resulted in strong apical membrane fluorescence with no detectable signal in the basal and lateral plasma membranes. These data indicate that OCT2 mediates choline transport across the ventricular membrane of CP.

Transport of choline and its relationship to the expression of the organic cation transporters in a rat brain microvessel endothelial cell line (RBE4)

The present study was undertaken to elucidate the functional characteristics of choline uptake and deduce the relationship between choline uptake and the expression of organic cation transporters in the rat brain microvessel endothelial cell line RBE4. Confluent RBE4 cells were found to express a high affinity choline uptake system. The system is Na(+)-independent and shows a Michaelis-Menten constant of approx. 20 microM for choline. The choline analogue hemicholinium-3 inhibits choline uptake in these cells with an inhibition constant of approx. 50 microM. The uptake system is also susceptible for inhibition by various organic cations, including 1-methyl-4-phenylpyridinium, 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine, clonidine, procainamide, and tetramethylammonium. The prototypical organic cation tetraethylammonium shows very little affinity for the choline uptake system in these cells. The inhibition of choline uptake by hemicholinium-3 is competitive. Northern analysis and RT-PCR show that these cells do not express the organic cation transporters OCT2 and OCT3. These cells do express, however, low levels of OCT1, but the functional characteristics of choline uptake in these cells are very different from the known properties of choline uptake via OCT1. The Na(+)-coupled high affinity choline transporter CHT1 is not expressed in these cells as evidenced by RT-PCR. This corroborates the Na(+)-independent nature of choline uptake in these cells. It is concluded that RBE4 cells express an organic cation transporter that is responsible for choline uptake in these cells and that this transporter is not identical to any of the organic cation transporters thus far identified at the molecular level in mammalian cells.

Characterization of the blood-brain barrier choline transporter using the in situ rat brain perfusion technique

Choline enters brain by saturable transport at the blood-brain barrier (BBB). In separate studies, both sodium-dependent and passive choline transport systems of differing affinity have been reported at brain capillary endothelial cells. In the present study, we re-examined brain choline uptake using the in situ rat brain perfusion technique. Saturable brain choline uptake from perfusion fluid was best described by a model with a single transporter (V:(max) = 2.4-3.1 nmol/min/g; K(m) = 39-42 microM) with an apparent affinity (1/Km)) for choline five to ten-fold greater than previously reported in vivo, but less than neuronal 'high-affinity' brain choline transport (K(m) = 1-5 microM). BBB choline uptake from a sodium-free perfusion fluid using sucrose for osmotic balance was 50% greater than in the presence of sodium suggesting that sodium is not required for transport. Hemicholinium-3 inhibited brain choline uptake with a K(i) (57 +/- 11 microM) greater than that at the neuronal choline system. In summary, BBB choline transport occurs with greater affinity than previously reported, but does not match the properties of the neuronal choline transporter. The V:(max) of this system is appreciable and may provide a mechanism for delivering cationic drugs to brain.

Verbal and visual memory improve after choline supplementation in long-term total parenteral nutrition: a pilot study

BACKGROUND

Previous investigations have demonstrated that choline deficiency, manifested in low plasma-free choline concentration and hepatic injury, may develop in patients who require long-term total parenteral nutrition (TPN). Preliminary studies have suggested lecithin or choline supplementation might lead to improved visual memory in the elderly and reverse abnormal neuropsychological development in children. We sought to determine if choline-supplemented TPN would lead to improvement in neuropsychological test scores in a group of adult, choline-deficient outpatients receiving TPN.

METHODS

Eleven subjects (8 males, 3 females) who received nightly TPN for more than 80% of their nutritional needs for at least 12 weeks before entry in the study were enrolled. Exclusion criteria included active drug abuse, mental retardation, cerebral vascular accident, head trauma, hemodialysis or peritoneal dialysis, (prothrombin time [PT] >2x control), or acquired immune deficiency syndrome (AIDS). Patients were randomly assigned to receive their usual TPN regimen (n = 6, aged 34.0 +/- 12.6 years) over a 12-hour nightly infusion or their usual TPN regimen plus choline chloride (2 g) (n = 5, aged 37.3 +/- 7.3 years). The following neuropsychological tests were administered at baseline and after 24 weeks of choline supplementation (or placebo): Weschler Adult Intelligence Scale-Revised (WAIS-R, intellectual functioning), Weschler Memory Scale-Revised (WMS-R, two subtests, verbal and visual memory), Rey-Osterrieth Complex Figure Test (visuospatial functioning and perceptual organization), Controlled Oral Word Association Test (verbal fluency), Grooved Pegboard (manual dexterity and motor speed), California Verbal Learning Test (CVLT, rote verbal learning ability), and Trail Making Parts A & B (visual scanning, psychomotor speed and set shifting). Scores were reported in terms of standard scores including z scores and percentile ranks. Mean absolute changes in raw scores were compared between groups using the Wilcoxon rank sum test, where p values < .05 constituted statistical significance.

RESULTS

Significant improvements were found in the delayed visual recall of the WMS-R (7.0 +/- 2.7 vs -.33 +/- 5.7, p = .028), and borderline improvements in the List B subset of the CVLT (1.0 +/- 0.8 vs -2.0 +/- 2.4, p = .06) and the Trails A test (-3.8 +/- 8.1 vs 3.7 +/- 4.5 seconds, p = .067). No other statistically significant changes were seen.

CONCLUSIONS

This pilot study indicates both verbal and visual memory may be impaired in patients who require long-term TPN and both may be improved with choline supplementation.

Characteristics of choline transport across the blood-brain barrier in mice: correlation with in vitro data

PURPOSE

We examined the functional properties of choline transport across the blood-brain barrier (BBB) in mice. We compared the kinetic parameters and transport properties with those found in our in vitro uptake experiments using mouse brain capillary endothelial cells (MBEC4).

METHODS

The permeability coefficient-surface area product (PS) values of [3H]choline at the BBB were estimated by means of an in situ brain perfusion technique in mice.

RESULTS

[3H]Choline uptake was well described by a two-component model: a saturable component and a nonsaturable linear component. The [3H]choline uptake was independent of pH and Na+, but was significantly decreased by the replacement of Na+ with K+. Various basic drugs, including substrates and inhibitors of the organic cation transporter, significantly inhibited the [3H]choline uptake. These in situ (in vivo) results corresponded well to the in vitro results and suggest that the choline transporter at the BBB is a member of the organic cation transporter (OCT) family.

CONCLUSION

The choline transport mechanism at the BBB is retained in MBEC4.

Blood-brain barrier transport studies of organic guanidino cations using an in situ brain perfusion technique

Blood-brain barrier (BBB) transport of essential polar substrates is mediated by specific, carrier-mediated transport proteins. The BBB transport mechanisms for polar compounds with terminal guanidino functional groups (R-NHC(NH)NH(2)) are not well defined. The goal of the present work was to investigate the BBB transport mechanism(s) for terminal guanidino substrates using an in situ brain perfusion technique. Brain region radiotracer influx clearance (Cl(in)) was calculated for representative guanidino substrates, [14C]L-arginine, [14C]aminoguanidine and [14C]guanidine, in the presence or absence of excess terminal guanidino analogues. The Cl(in) for [14C]L-arginine (0.21+/-0.0094 cm(3)/min/g wet brain weight, mean+/-S.E.M., n=four rats) was significantly decreased by 1000x concentrations of unlabeled L-arginine, N(G)-methyl-L-arginine, N(G)-,N(G)-dimethyl-L-arginine and N(G)-amino-L-arginine by approximately 83% (P<0.01; n=4-5), whereas 1000x concentrations of nitro-L-arginine, aminoguanidine and guanidine were without effect. In contrast, the respective Cl(in) of [14C]aminoguanidine and [14C]guanidine (0.0085+/-0.00039 and 0.015+/-0.0015 cm(3)/min/g, n=4, respectively) were not significantly decreased by 1000x concentrations of unlabeled aminoguanidine or guanidine. The Cl(in) values for all [14C]guanidino probes were significantly greater (P<0. 05) from that of [3H]inulin, a marker of cerebrovascular blood volume. These data suggest that the hydrophilic guanidino cations aminoguanidine and guanidine penetrate the BBB by a minor diffusional process with no appreciable transport via saturable processes. In contrast, BBB penetration of L-arginine occurs via the saturable basic amino acid transporter that has specificity for amino acid analogues possessing cationic terminal guanidino groups.

Choline uptake by mouse brain capillary endothelial cells in culture

Choline, a precursor of the neurotransmitter acetylcholine, is synthesized in only small amounts in the brain, so the choline concentration in the brain may vary depending on the plasma concentration and the transport rate across the blood-brain barrier. To elucidate the transport mechanism of choline, we carried out uptake experiments with mouse brain capillary endothelial cells in culture (MBEC4). [3H]Choline uptake was linear for up to 5 min. An examination of the concentration dependence of [3H]choline uptake revealed the operation of both saturable (Jmax = 423+/-27 pmol min(-1) (mg protein)(-1) and Kt = 20.0+/-3.1 microM) and non-saturable (kd = 1.23+/-0.045 microL min(-1)(mgprotein)-1) processes. The saturable process was independent of Na+ and pH, but was dependent on membrane potential as a driving force. Various basic drugs and endogenous substances, including substrates and inhibitors of the organic cation transporter, significantly inhibited the [3H]choline uptake. These data suggest that choline was taken up into the endothelial cells via two routes and that a membrane potential-dependent carrier-mediated transport system may participate in choline transport across the blood-brain barrier.

Choline uptake across the ventricular membrane of neonate rat choroid plexus

The uptake of [3H]choline from the cerebrospinal fluid (CSF) side of the rat neonatal choroid plexus was characterized in primary cultures of the choroidal epithelium grown on solid supports. Cell-to-medium concentration ratios were approximately 5 at 1 min and as high as 70 at 30 min. Apical choline uptake was facilitated; the Km was approximately 50 microM. Several organic cations (e.g., hemicholinium-3 and N1-methylnicotinamide) inhibited uptake. The reduction or removal of external Na+ or the addition of 5 mM LiCl had no effect on uptake. However, increasing external K+ concentration from 3 to 30 mM depolarized ventricular membrane potential (-70 to -15 mV) and reduced uptake to 45% of that for the control. Treatment with 1 mM ouabain or 2 mM BaCl2 reduced uptake 45%, and intracellular acidification reduced uptake to approximately 90% of that for controls. These data indicate that the uptake of choline from CSF across the ventricular membrane of the neonatal choroidal epithelium is not directly coupled to Na+ influx but is sensitive to plasma membrane electrical potential.

Vector-mediated drug delivery to the brain

Vector-mediated drug delivery to the brain employs the chimeric peptide technology, wherein a non-transportable drug is conjugated to a blood-brain barrier (BBB) transport vector. The latter is a modified protein or receptor-specific monoclonal antibody that undergoes receptor-mediated transcytosis through the BBB in vivo. Conjugation of drug to transport vector is facilitated with either chemical linkers, avidin-biotin technology, polyethylene glycol linkers, or liposomes. Multiple classes of therapeutics have been delivered to the brain with the chimeric peptide technology, including peptide-based pharmaceuticals, such as a vasoactive intestinal peptide analog or neurotrophins such as brain-derived neurotrophic factor, antisense therapeutics including peptide nucleic acids, and small molecules incorporated within liposomes. The successful delivery of a drug through the BBB in vivo requires special molecular formulation of the drug. Therefore, it is important to merge central nervous system drug discovery and delivery as early as possible in the overall CNS drug development process.

Formation of N-methylnicotinamide in the brain from a dihydropyridine-type prodrug: effect on brain choline

The enhancement of brain choline levels is a possible therapeutic option in neurodegenerative diseases; however, brain choline levels are held within narrow limits by homeostatic mechanisms including the rapid clearance of excess choline from the brain. The present study tests whether N-methylnicotinamide (NMN), an inhibitor of the outward transport of choline from the brain, can elevate brain choline levels in vivo. As NMN does not cross the blood-brain barrier, we synthesized and administered the brain-permeable prodrug, 1,4-dihydro-N-methyl-nicotinamide (DNMN), and tested its effect on the levels of NMN and choline in brain extracellular fluid, using the microdialysis procedure. Administration of DNMN (1 mmol/kg s.c.) caused a 4- and 9-fold increase in plasma and liver NMN levels, respectively, as determined by HPLC. Concomitantly, the brain tissue levels of NMN were increased by a factor of twenty. In brain extracellular fluid, the injection of DNMN (1-3 mmol/kg s.c.) elevated NMN levels by 3- to 10-fold to maximum levels of >10 microM. In spite of these enhanced NMN levels, the choline concentrations in the brain extracellular fluid and in the cerebrospinal fluid (4.7 microM) remained unchanged or were even slightly decreased. Microsomal incubations of DNMN indicated that cytochrome P-450 3A isoforms may be involved in NMN formation in the liver, but not in the brain. We conclude that DNMN, a brain-permeable prodrug of NMN, is efficiently oxidized to NMN in the brain, but a 10-fold increase in extracellular NMN levels is not sufficient to reduce the clearance of choline from the brain.

Brain choline has a typical precursor profile

Choline is product and precursor to both acetylcholine and membrane phospholipids, and, in the brain, is ultimately provided by the circulation. The brain is protected from excess choline and choline deprivation by a refined system of homeostatic mechanisms that maintain a level of extracellular choline that, for its role as precursor, meets saturation criteria under normal conditions. The kinetic and activity profiles of choline are typical for a biosynthetic precursor.

Sexually dimorphic activation of liver and brain phosphatidylethanolamine N-methyltransferase by dietary choline deficiency

Phosphatidylethanolamine N-methyltransferase (PEMT) activity was measured by a radioenzymatic assay in homogenates of brain and liver obtained from Sprague Dawley rats fed a choline-free or control (0.3 g/kg of choline chloride) diet for seven days. Choline deficiency increased PEMT activity in the liver of male rats by 34% but had no effect on hepatic PEMT in females. In contrast, brain PEMT activity was increased in brain of choline deficient females (by 49%) but was unaltered in males. Activation of the PE methylation pathway in female brain may constitute a compensatory mechanism to sustain PC synthesis during choline deficiency.

Regulation of free choline in rat brain: dietary and pharmacological manipulations

The present study analyses, comparatively, the kinetics of free choline in the brain of rats during dietary and pharmacological manipulations. Low-choline diet halved the choline plasma level but did not cause significant changes of CSF choline. High-choline diet, hypoxia and treatment with nicotinamide increased brain choline availability through a central site of action and increased the CSF choline concentration. CSF choline concentrations were more effectively elevated by nicotinamide treatment (20-25 microM) than by acute choline administration (13-15 microM). Increases of CSF choline, due to brain choline mobilization, were consistently associated with a net release of choline from the brain as reflected by strongly negative arterio-venous differences (AVD) of brain choline. The balance between release and uptake of brain choline was controlled by the arterial plasma choline level in all treatment groups; however, the normal 'reversal point' of 15 microM--representing the plasma choline level where uptake and release of brain choline are balanced--was shifted to more than 40 microM by high-choline diet and nicotinamide. In conclusion, our data characterize the release of choline into the venous blood as an important component of brain choline homeostasis. Furthermore, we demonstrate that the concentration of brain choline (e.g. as a precursor of acetylcholine) can be enhanced more efficiently by manipulating choline homeostatic mechanisms than by acute choline administration.

Organic cation transporters in intestine, kidney, liver, and brain

This review focuses on sodium-independent transport systems for organic cations in small intestine, liver, kidney, and brain. The roles of P-glycoproteins (MDR) and anion transporters (OATP) in organic cation transport are reported, and two members of the new transporter family OCT are described. The OCT transporters belong to a superfamily that includes multidrug-resistance proteins, facilitative diffusion systems, and proton antiporters. They mediate electrogenic transport of small organic cations with different molecular structures, independently of sodium and proton gradients. The current knowledge of the distribution and functional properties of cloned cation transport systems and of cation transport measured in intact plasma membranes is used to postulate identical or homologous transporters in intestine, liver, kidney, and brain.

Evidence for an asymmetrical uptake of L-carnitine in the blood-brain barrier in vitro

The transport of L-carnitine (4-N-trimethylammonium-3-hydroxybutyric acid) was studied with a primary culture of porcine brain capillary endothelial cells (BCEC) as an in vitro model of the blood-brain barrier. The measurements with suspended cells and cell monolayers allowed to distinguish a polarized transport phenomena. The part of the BCEC cells exposed to the medium (apical membrane) accumulated carnitine by a sodium-independent, saturable (Km=28 microM) system, with k=0.018 min-1. Exposure of the basolateral part revealed a presence of a facilitated diffusion process. Carnitine uptake through the saturable system was inhibited by butyrobetaine. Acylcarnitines and choline have no effect on the carnitine accumulation in suspended cells, a process diminished by phenylalanine, leucine, and L system inhibitor. This points to the possibility that carnitine enters through the basolateral membrane using amino acid transporting systems. A different, novel system is postulated to operate in the apical part of the plasma membrane of BCEC.

Age-related changes in the blood-brain barrier

Aging of the cerebral microcirculation results in significant alteration in the blood-brain barrier (BBB). The barrier function appears to remain intact in older animals, although it may be more susceptible to disruption by external factors (hypertension) and drugs (haloperidol). While overall transport processes do not change with age, aging animals and humans have altered BBB function of select carrier mediated transport systems including the transport of choline, glucose, butyrate and triiodothyronine. These age-related changes are the result of either alteration in the carrier molecules or the physiochemical properties of the cerebral microvessels. At the present time, it is not known whether changes in the BBB contribute to the age-related neurodegenerative diseases or are merely epiphenomena of aging.

Differential effect of CDP-choline on brain cytosolic choline levels in younger and older subjects as measured by proton magnetic resonance spectroscopy

Phosphatidylcholine (PtdCho), which is essential for membrane integrity and repair, is reduced in brain cell membranes with age. Evidence from both animal and in vitro studies indicates that cytidine 5' diphosphate choline (CDP-choline) can increase the synthesis of PtdCho; however, the effect of CDP-choline on brain choline metabolism has not previously been studied in human subjects. In this study, in vivo proton magnetic resonance spectroscopy (1H-MRS) was used to measure brain levels of cytosolic, choline-containing compounds before and after single oral doses of CDP-choline. Three hours after dosing, plasma choline increased similarly in younger (mean age 25 years) and older subjects (mean age 59 years). However, while the choline resonance in brain increased by 18% on average in younger subjects, it decreased by almost 6% in older subjects (P = 0.028). These results may be explained by a previously observed decrease in brain choline uptake, but not cytidine uptake, in older subjects. Additional intracellular cytidine following the administration of CDP-choline should lead to the increased incorporation of choline already present in brain into membrane PtdCho, which is not MRS-visible, consequently lowering the brain choline resonance below that of pre-treatment values. These results suggest that the cytidine moiety of CDP-choline stimulates phosphatidylcholine synthesis in human brain cell membranes in older subjects.

Determination of plasma choline by high-performance liquid chromatography with a postcolumn enzyme reactor and electrochemical detection

A method for the determination of choline in human plasma is described, involving rapid purification of plasma samples and analysis by high-performance liquid chromatography using an on-column enzyme reactor with electrochemical detection. The linearity of the method was tested at choline levels from 3.5 to 28.6 microM in plasma. The recovery was 86% and was independent of the analyte concentration. The inter-assay precision (as coefficient of variation) and accuracy (as the deviation of the concentration found from the theoretical value) were always below 12% in the whole concentration range. The method was applied to the determination of plasma choline levels in eight healthy volunteers after intramuscular administration of L-alpha-glycerophosphorylcholine (1 g) or a placebo. Mean plasma choline levels in the placebo group ranged from 10.6 to 12.0 microM. After drug administration, the plasma choline level reached 35.1 microM in 30 min, then decreased gradually. Plasma choline levels became comparable in the treated and placebo groups 6-8 h after administration.

Effect of choline on basal and stimulated acetylcholine release: an in vivo microdialysis study using a low neostigmine concentration

Using in vivo microdialysis, we examined the ability of choline (Ch) chloride (120 mg/kg i.p.) to amplify basal and stimulated acetylcholine (ACh) release from rat striatum in the presence of high (10(-5) M) and low (5 x 10(-8) M) neostigmine concentration. High concentrations might suppress ACh release, and thus Ch dependence, by excessively stimulating presynaptic cholinergic receptors; alternatively, they could enhance Ch dependence by depriving the cholinergic terminals of Ch that would otherwise be formed intrasynaptically from the hydrolysis of ACh. Both basal and stimulated ACh release were found to be tetrodotoxin (TTX) sensitive. The concentration of neostigmine in the microdialysis fluid positively affected basal ACh levels, but had no effect on Ch levels. Ch administration significantly increased ACh release (to 136% of basal values; P < 0.01) in the presence of the low neostigmine concentration, but failed to significantly increase ACh release following local electrical depolarization of striatal neurons. In contrast, Ch failed to affect basal ACh release in the presence of the high neostigmine concentration, but did increase electrically evoked release to 408% of basal values, as compared with 250% in rats receiving saline instead of the Ch (P < 0.05). Ch administration significantly increased microdialysate Ch levels in the presence of both of the neostigmine concentrations. Local administration of oxotremorine, a muscarinic agonist, to animals receiving the lower neostigmine concentration reduced basal ACh release and reduced the increase in basal release produced by Ch administration.(ABSTRACT TRUNCATED AT 250 WORDS)

Free choline and choline metabolites in rat brain and body fluids: sensitive determination and implications for choline supply to the brain

In the central nervous system, choline is an essential precursor of choline-containing phospholipids in neurons and glial cells and of acetylcholine in cholinergic neurons. In order to study choline transport and metabolism in the brain, we developed a comprehensive methodical procedure for the analysis of choline and its major metabolites which involves a separation step, selective hydrolysis and subsequent determination of free choline by HPLC and electrochemical detection. In the present paper, we report the levels of choline, acetylcholine, phosphocholine, glycerophosphocholine and choline-containing phospholipids in brain tissue, cerebrospinal fluid and blood plasma of the untreated rat. The levels of free choline in blood plasma (11.4 microM), CSF (6.7 microM) and brain intracellular space (64.0 microM) were sufficiently similar to be compatible with an exchange of choline between these compartments. In contrast, the intracellular levels of glycerophosphocholine (1.15 mM) and phosphocholine (0.59 mM) in the brain were considerably higher than their CSF concentrations of 2.83 and 1.70 microM, respectively. In blood plasma, glycerophosphocholine was present in a concentration of 4.58 microM while phosphocholine levels were very low or absent (< 0.1 microM). The levels of phosphatidylcholine and lyso-phosphatidylcholine were high in blood plasma (1267 and 268 microM) but very low in cerebrospinal fluid (< 10 microM). We concluded that the transport of free choline is the only likely mechanism which contributes to the supply of choline to the brain under physiological conditions.

Blood-brain barrier penetration abolished by N-methyl quaternization of nicotine

The present study determined the effect of organically quaternizing either of the two tertiary nitrogen sites of nicotine to assess the in vivo effects of the permanently ionized states of the synthesized N-[14C]methylnicotines on brain uptake in rat after intracarotid injection. Male Sprague-Dawley rats were used to measure the brain uptake index (BUI) by single-pass clearance in brain after rapid injection at pH 7.4 into the left common carotid artery (expressed as a percentage) relative to simultaneously injected 3HOH. The BUI of [14C]mannitol, a control for the method background, was measured to be 2.6 +/- 0.6. At physiological pH, in striking contrast to the [pyrrolidine-2-14C]nicotine BUI of 120 +/- 3, the N-[14C]-methylnicotines had a BUI of 3.0 +/- 0.6, which was not significantly different from the method background and which indicated abolition of blood-brain barrier penetration of nicotine with the sensitivity of the BUI method.

Effects of choline-free plasma induced by choline oxidase on regional levels of choline and acetylcholine in rat brain

Choline-free plasma (CFP) was induced in rats by intravenous (IV) injection of 56.0 x 10(2) units kg-1 of choline oxidase (ChO) which completely metabolized the free Ch circulating in the plasma for at least 15.0 h and caused subsequent significant decrease in the concentration of free Ch in the three brain regions examined, the striatum, hippocampus, and cortex. However, the treatment did not affect concentrations of acetylcholine (ACh) in these regions. By contrast, intraperitoneal (IP) injection of 1.0 mmol kg-1 Ch chloride resulted in a maximum concentration of free Ch in plasma in 5 min, after which tissue Ch in all regions examined increased (p < 0.001). Concomitant increases were observed in cortical and hippocampal ACh (p < 0.05) 20 min after the injection. It is thus suggested that the brain may possess compensative mechanisms to prevent the supply of free Ch from circulating to the brain during synthesis of ACh in the brain. It is also suggested that the CFP rat would be a useful and readily available animal model for future study.

Ouabain-sensitive choline transport system in capillaries isolated from bovine brain

In physiological conditions, there is a net transport of choline from brain to blood, despite the fact that the choline concentration is higher in plasma than in CSF. Because of the blood-brain barrier characteristics, such passage against the concentration gradient takes place necessarily through endothelial cells. To get a better understanding of this phenomenon, [3H]choline uptake properties have been analyzed in capillaries isolated from bovine brain. [3H]Choline uptake was linear with time for up to 1 h. Nonlinear regression analysis of the uptake rates at different substrate concentrations gave the best fit to a system of two components, one of which was saturable (Km = 17.8 +/- 4.8 microM; Vmax = 11.3 +/- 3.4 pmol/min/mg of protein) and the other of which was nonsaturable at concentrations up to 200 microM. The [3H]choline transport was significantly reduced in the absence of sodium and after incubation with 10(-4) M ouabain for 30 min. Ouabain also inhibited choline uptake in purified cerebral endothelial cells, but not in the endothelium isolated from bovine aorta. Accordingly, cerebral endothelial cells were able to concentrate [3H]choline, with this effect being abolished by ouabain, whereas in aortic endothelial cells the [3H]choline intracellular concentration was never higher than that of the incubation medium. These results suggest that the blood-brain barrier endothelium is specifically provided with an energy-dependent choline transport system, which may explain the choline efflux from the brain and the maintenance of a low choline concentration in the cerebral extracellular space.

Uptake and metabolism of choline by rat brain after acute choline administration

The present study is concerned with the uptake and metabolism of choline by the rat brain. Intraperitoneal administration of choline chloride (4-60 mg/kg) caused a dose-dependent elevation of the plasma choline concentration from 11.8 to up to 165.2 microM within 10 min and the reversal of the negative arteriovenous difference (AVD) of choline across the brain to positive values at plasma choline levels of greater than 23 microM. Net choline release and uptake were linearly dependent on the plasma choline level in the physiological range of 10-50 microM, whereas the CSF choline level was significantly increased only at plasma choline levels of greater than 50 microM. The bolus injection of 60 mg/kg of [3H]choline chloride caused the net uptake of greater than 500 nmol/g of choline by the brain as calculated from the AVD, which was reflected in a minor increase of free choline level and a long-lasting increase of brain phosphorylcholine content, which paralleled the uptake curve. Loss of label from phosphorylcholine 30 min to 24 h after choline administration was accompanied by an increase of label in phosphatidylcholine, an indication of a delayed transfer of newly taken-up choline into membrane choline pools. In conclusion, homeostasis of brain choline is maintained by a complex system that interrelates choline net movements into and out of the brain and choline incorporation into and release from phospholipids.

Recurrent and reciprocal inhibition of the human monosynaptic reflex shows opposite changes following intravenous administration of acetylcarnitine

A long-latency, long-lasting increase in the recurrent inhibitory effect on the soleus monosynaptic (Hoffmann, H) reflex was induced after intravenous administration of L-acetylcarnitine, a substance known to process central cholinergic activity. This effect was paralleled by disappearance of the H reflex inhibition (functionally disinhibition) induced by stimulation of Ia afferent fibres from the tibialis anterior (reciprocal inhibition) and gastrocnemius medialis muscle. Magnitude and time course of the L-acetylcarnitine-induced effects were significantly correlated. The data suggest that (1) the L-acetylcarnitine depression of the reciprocal inhibition is mediated by excitation of Renshaw cells impinging on Ia interneurones (INs), and (2) the inhibitory effect of GM Ia afferents onto Sol is mediated by INs subjected to Renshaw inhibition. The results point to the similarity in the wiring of the 'output stage' circuit between cat and humans, and provide a method for testing this network in man.

Uptake and storage of choline by rat brain: influence of dietary choline supplementation

In order to elucidate the regulation of the levels of free choline in the brain, we investigated the influence of chronic and acute choline administration on choline levels in blood, CSF, and brain of the rat and on net movements of choline into and out of the brain as calculated from the arteriovenous differences of choline across the brain. Dietary choline supplementation led to an increase in plasma choline levels of 50% and to an increase in the net release of choline from the brain as compared to a matched group of animals which were kept on a standard diet and exhibited identical arterial plasma levels. Moreover, the choline concentration in the CSF and brain tissue was doubled. In the same rats, the injection of 60 mg/kg choline chloride did not lead to an additional increase of the brain choline levels, whereas in control animals choline injection caused a significant increase; however, this increase in no case surpassed the levels caused by chronic choline supplementation. The net uptake of choline after acute choline administration was strongly reduced in the high-choline group (from 418 to 158 nmol/g). Both diet groups metabolized the bulk (greater than 96%) of newly taken up choline rapidly. The results indicate that choline supplementation markedly attenuates the rise of free choline in the brain that is observed after acute choline administration. The rapid metabolic choline clearance was not reduced by dietary choline load. We conclude that the brain is protected from excess choline by rapid metabolism, as well as by adaptive, diet-induced changes of the net uptake and release of choline.

Purification and characterization of metabolically active capillaries of the blood-brain barrier

Microvessels were isolated from bovine and rat cerebral cortex by simple procedures involving mechanical homogenization, differential and density-gradient centrifugation, and chromatography on a column of glass beads. The preparations were composed of short capillaries with a diameter of 1-10 microns. Both purifications were monitored by assaying the activity of the marker enzyme gamma-glutamyl transpeptidase (gamma-GTase). The final bovine and rat preparations were enriched 20- and 14-fold over the homogenate respectively. gamma-GTase activity was measured in different fractions after bovine and rat membranes were solubilized with 0.5% and 0.3% Triton X-100 respectively. Measurement of 5'-nucleotidase and acetylcholinesterase activities indicated very low levels of contamination of the microvessel preparations by glial cells and neurons. The integrity of the capillary membranes was confirmed by the assay of a cytosolic marker enzyme, lactate dehydrogenase. Viability of the microvessels was demonstrated by the presence of detectable levels of adenylates and by tissue respiration induced by glucose and succinate. Comparison of the proteins of homogenized bovine and rat brain cortex with those of purified capillaries separated by SDS/PAGE revealed enrichment of at least three predominant proteins of 14, 16 and 18 kDa in the capillary preparations. It is concluded that these methods allow rapid isolation of small blood vessels of the blood-brain barrier which are suitable for metabolic and structural studies in vitro.

Blood-brain barrier: transport studies in isolated brain capillaries and in cultured brain endothelial cells

The development of in vitro BBB models consisting of isolated brain capillaries and cultured brain microvessel endothelial cells has made possible the study of BBB transport phenomena at the cellular level. Basic characteristics of BBB transport of endogenous and exogenous solutes and their biochemical, pharmacological, ontogenic, and pathological regulation mechanisms have been investigated. This information has led not only to a better understanding of BBB transport but also to the construction of strategies for improving drug delivery to the CNS for diagnosis and therapeutics. To elucidate the complexity of BBB transport, in vivo studies are always necessary at some point; however, in vitro systems can be useful complements to the in vivo systems. The tissue culture systems seem to be especially important in the clarification of cellular, biochemical and molecular features of BBB transport. Appropriate systems should be selected or combined, depending on the purpose of the investigation.

Global in vivo replacement of choline by N-aminodeanol. Testing a hypothesis about progressive degenerative dementia: I. Dynamics of choline replacement

Severe disruption of certain cholinergic pathways is a characteristic feature of Alzheimer's disease. Attempts to establish animal models by interfering with cholinergic function have not been very successful. We now present data which show a substantial and progressive replacement of free and phospholipid-bound choline by the novel choline isostere N-amino-N,N-dimethylaminoethanol during its dietary administration in place of choline. Free choline in blood fell to approximately 20% of controls after 10 to 30 days on diet. Phospholipid-bound choline in plasma was reduced to less than 15%, and in erythrocytes to about 22%. After 120 days of diet free and bound choline were reduced in most tissues to approximately 30% of controls. Only liver retained more than 80% of free choline. Acetylcholine was decreased to 33 to 50% of control. Total true and false transmitter in experimental animals was in all tissues less that acetylcholine in controls, suggesting that muscarinic transmission would be impaired. Moderate reduction of choline acetyltransferase activity was seen in striatum and myenteric plexus, and of QNB-binding in hippocampus, striatum and myenteric plexus.

Small rises in plasma choline reverse the negative arteriovenous difference of brain choline

The concentrations of free choline in blood plasma from a peripheral artery and from the transverse sinus, in the CSF, and in total brain homogenate, have been measured in untreated rats and in rats after acute intraperitoneal administration of choline chloride. In untreated rats, the arteriovenous difference of brain choline was related to the arterial choline level. At low arterial blood levels (less than 10 microM) as observed under fasting conditions, the arteriovenous difference was negative (about -2 microM), indicating a net release of choline from the brain of about 1.6 nmol/g/min. In rats with spontaneously high arterial blood levels (greater than 15 microM), the arteriovenous difference was positive, implying a marked net uptake of choline by the brain (3.1 nmol/g/min). The CSF choline concentration, which reflects changes in the extracellular choline concentration, also increased with increasing plasma levels and closely paralleled the gradually rising net uptake. Acute administration of 6, 20, or 60 mg of choline chloride/kg caused, in a dose-dependent manner, a sharp rise of the arterial blood levels and the CSF choline, and reversed the arteriovenous difference of choline to markedly positive values. The total free choline in the brain rose only initially and to a quantitatively negligible extent. Thus, the amount of choline taken up by the brain within 30 min was stored almost completely in a metabolized form and was sufficient to sustain the release of choline from the brain as long as the plasma level remained low. We conclude that the extracellular choline concentration of the brain closely parallels fluctuations in the plasma level of choline.(ABSTRACT TRUNCATED AT 250 WORDS)

Choline uptake by cerebral capillary endothelial cells in culture

A passage of choline from blood to brain and vice versa has been demonstrated in vivo. Because of the presence of the blood-brain barrier, such passage takes place necessarily through endothelial cells. To get a better understanding of this phenomenon, the choline transport properties of cerebral capillary endothelial cells have been studied in vitro. Bovine endothelial cells in culture were able to incorporate [3H]choline by a carrier-mediated mechanism. Nonlinear regression analysis of the uptake curves suggested the presence of two transport components in cells preincubated in the absence of choline. One component showed a Km of 7.59 +/- 0.8 microM and a maximum capacity of 142.7 +/- 9.4 pmol/2 min/mg of protein, and the other one was not saturable within the concentration range used (1-100 microM). When cells were preincubated in the presence of choline, a single saturable component was observed with a Km of 18.5 +/- 0.6 microM and a maximum capacity of 452.4 +/- 42 pmol/2 min/mg of protein. [3H]Choline uptake by endothelial cells was temperature dependent and was inhibited by the choline analogs hemicholinium-3, deanol, and AF64A. The presence of ouabain or 2,4-dinitrophenol did not affect the [3H]choline transport capacity of endothelial cells. Replacement of sodium by lithium and cell depolarization by potassium partially inhibited choline uptake. When cells had been preincubated without choline, recently transported [3H]choline was readily phosphorylated and incorporated into cytidine-5'-diphosphocholine and phospholipids; however, under steady-state conditions most (63%) accumulated [3H]choline was not metabolized within 1 h.(ABSTRACT TRUNCATED AT 250 WORDS)

Choline acetyltransferase in bovine pineal gland

Recent studies from our laboratories have shown that the bovine pineal gland contains a muscarinic cholinergic receptor with a Kd value of 0.423 +/- 0.010 nM and a Bmax value of 69.75 +/- 20.91 fmol/mg protein. In order to substantiate further the possible existence of a pineal cholinergic transmission, we have measured the activity of choline acetyltransferase and delineated its kinetic properties in the bovine pineal gland. This enzyme exhibited an activity of 0.0339 +/- 0.0042 nmol/mg protein/min. Furthermore, the bovine pineal choline acetyltransferase possessed a Km value of 124.86 +/- 24.06 microM and a Vmax value of 0.0598 +/- 0.0034 nmol/mg protein/min for acetyl CoA, and a Km value of 3.11 +/- 0.94 mM and a Vmax value of 0.0155 +/- 0.0016 nmol/mg protein/min for choline. The presence of muscarinic cholinergic receptors along with a specific choline acetyltransferase are supportive evidences that the bovine pineal gland may receive cholinergic innervation.

Conversion of ethanolamine, monomethylethanolamine and dimethylethanolamine to choline-containing compounds by neurons in culture and by the rat brain

The incubation of neurons from chick embryos in primary culture with [3H]ethanolamine revealed the conversion of this base into monomethyl, dimethyl and choline derivatives, including the corresponding free bases. Labelling with [methyl-3H]monomethylethanolamine and [methyl-3H]dimethylethanolamine supported the conclusion that in chick neuron cultures, phosphoethanolamine appears to be the preferential substrate for methylation, rather than ethanolamine or phosphatidylethanolamine. The methylation of the latter two compounds, in particular that of phosphatidylethanolamine, was seemingly stopped at the level of their monomethyl derivatives. Fetal rat neurons in primary culture incubated with [3H]ethanolamine showed similar results to those observed with chick neurones. However, phosphoethanolamine and phosphatidylethanolamine and, to a lesser extent, free ethanolamine, appeared to be possible substrates for methylation reactions. The methylation of water-soluble ethanolamine compounds de novo was further confirmed by experiments performed in vivo by intraventricular injection of [3H]ethanolamine. Phosphocholine and the monomethyl and dimethyl derivatives of ethanolamine were detected in the brain 15 min after injection.