Blood-brain barrier choline transport in the senescent rat

The brain response to peripheral insulin declines with age: a contribution of the blood-brain barrier?

Objectives

It is a matter of debate whether impaired insulin action originates from a defect at the neural level or impaired transport of the hormone into the brain. In this study, we aimed to investigate the effect of aging on insulin concentrations in the periphery and the central nervous system as well as its impact on insulin-dependent brain activity.

Methods

Insulin, glucose and albumin concentrations were determined in 160 paired human serum and cerebrospinal fluid (CSF) samples. Additionally, insulin was applied in young and aged mice by subcutaneous injection or intracerebroventricularly to circumvent the blood-brain barrier. Insulin action and cortical activity were assessed by Western blotting and electrocorticography radiotelemetric measurements.

Results

In humans, CSF glucose and insulin concentrations were tightly correlated with the respective serum/plasma concentrations. The CSF/serum ratio for insulin was reduced in older subjects while the CSF/serum ratio for albumin increased with age like for most other proteins. Western blot analysis in murine whole brain lysates revealed impaired phosphorylation of AKT (P-AKT) in aged mice following peripheral insulin stimulation whereas P-AKT was comparable to levels in young mice after intracerebroventricular insulin application. As readout for insulin action in the brain, insulin-mediated cortical brain activity instantly increased in young mice subcutaneously injected with insulin but was significantly reduced and delayed in aged mice during the treatment period. When insulin was applied intracerebroventricularly into aged animals, brain activity was readily improved.

Conclusions

This study discloses age-dependent changes in insulin CSF/serum ratios in humans. In the elderly, cerebral insulin resistance might be partially attributed to an impaired transport of insulin into the central nervous system.

Acetyl-CoA the key factor for survival or death of cholinergic neurons in course of neurodegenerative diseases

Glucose-derived pyruvate is a principal source of acetyl-CoA in all brain cells, through pyruvate dehydogenase complex (PDHC) reaction. Cholinergic neurons like neurons of other transmitter systems and glial cells, utilize acetyl-CoA for energy production in mitochondria and diverse synthetic pathways in their extramitochondrial compartments. However, cholinergic neurons require additional amounts of acetyl-CoA for acetylcholine synthesis in their cytoplasmic compartment to maintain their transmitter functions. Characteristic feature of several neurodegenerating diseases including Alzheimer’s disease and thiamine diphosphate deficiency encephalopathy is the decrease of PDHC activity correlating with cholinergic deficits and losses of cognitive functions. Such conditions generate acetyl-CoA deficits that are deeper in cholinergic neurons than in noncholinergic neuronal and glial cells, due to its additional consumption in the transmitter synthesis. Therefore, any neuropathologic conditions are likely to be more harmful for the cholinergic neurons than for noncholinergic ones. For this reason attempts preserving proper supply of acetyl-CoA in the diseased brain, should attenuate high susceptibility of cholinergic neurons to diverse neurodegenerative conditions. This review describes how common neurodegenerative signals could induce deficts in cholinergic neurotransmission through suppression of acetyl-CoA metabolism in the cholinergic neurons.

Dynamics of the response to variations in dietary choline content in rats

A mathematical model was developed based on the analysis of body response to variations in choline content in the ration of growing rats. The peculiarities of choline metabolism and the expression of α7-nicotinic receptors in the brain were studied. The information obtained can be useful for comparative analysis of choline administration for correction of certain cerebral pathologies.

Exploiting nutrient transporters at the blood-brain barrier to improve brain distribution of small molecules

The blood-brain barrier (BBB) is a major physiological barrier for drugs that target CNS receptors or enzymes. Several methods exist by which permeability to the CNS can be increased, one of which is using native nutrient transporters to carry these drugs through the endothelial cells of the BBB. In this review, we focus on work that characterizes the use of nutrient transporters of the BBB in delivering drugs to the CNS.

Bis-azaaromatic quaternary ammonium salts as ligands for the blood-brain barrier choline transporter

A series of bis-azaaromatic quaternary ammonium compounds containing flexible polymethylenic linkers as well as conformationally restricted linkers were evaluated for their affinity for the blood-brain barrier choline transporter (BBB-ChT). The preliminary structure-activity relationships obtained from this study suggest that incorporating a linear, conformationally restricted linker into the molecule improves affinity for the BBB-ChT.

Norchloro-fluoro-homoepibatidine (NCFHEB) - a promising radioligand for neuroimaging nicotinic acetylcholine receptors with PET

Cholinergic neurotransmission depends on the integrity of nicotinic acetylcholine receptors (nAChRs), and impairment of both is characteristic for various neurodegenerative diseases. Visualization of specific receptor subtypes by positron emission tomography (PET) has potential to assist with diagnosis of such neurodegenerative diseases and with design of suitable therapeutic approaches. The goal of our study was to evaluate in vivo the potential of (18)F-labelled (+)- and (-)-norchloro-fluoro-homoepibatidine ([(18)F]NCFHEB) in comparison to 2-[(18)F]F-A-85380 as PET tracers. In the brains of NMRI mice, highest levels of radioactivity were detected at 20 min post-injection of (+)-[(18)F]NCFHEB, (-)-[(18)F]NCFHEB, and 2-F-[(18)F]-A-85380 (7.45, 5.60, and 3.2% ID/g tissue, respectively). No marked pharmacological adverse effects were observed at 25 mug NCFHEB/kg. Uptake studies in RBE4 cells and in situ perfusion studies suggest an interaction of epibatidine and NCFHEB with the carrier-mediated choline transport at the blood-brain barrier. The data indicate that (+)- and (-)-[(18)F]NCFHEB have potential for further development as PET tracers.

Metabonomic studies on the physiological effects of acute and chronic psychological stress in Sprague-Dawley rats

The biochemical effects of acute and chronic psychological stress have been investigated in male Sprague-Dawley rats using a combination of 1H NMR spectral analysis of plasma and conventional hematological analyses. Animals were subjected to 35 consecutive days of 6-h sessions of stress, and following a 9 day break, were stressed for a further 6-h period. Plasma samples were collected at 0, 1, 3, and 6 h on days 1, 9, 21, 35, and 44, measured using 600 MHz 1H NMR spectroscopy, and analyzed by Principal Components Analysis. Time-dependent biochemical effects of psychological stress on a range of endogenous metabolites were evident and were correlated with the intensity of the stress response as defined by corticosterone and hematological parameters. Following acute stress, increases in the levels of glucose and ketone bodies, and decreases in the levels of acetate, alanine, isoleucine, lactate, leucine, valine, and lipoproteins, were observed. Chronic stress-induced increases in plasma levels of alanine, lactate (day 9), and leucine, valine, and choline (day 44) and decreases in acetate (day 9) and lipoprotein concentrations were observed. Positive correlations between plasma corticosterone level and glucose and glycerol, and between plasma lipoprotein concentrations and hemoglobin levels, were established using Projection to Latent Structures (PLS) analysis. This study indicates the potential of using NMR-based metabonomic strategies for the characterization of endogenous metabolic perturbations induced by psychological stressors and lifestyle choices.

An autoradiographic analysis of rat brain nicotinic receptor plasticity following dietary choline modification

Choline is known to be involved with numerous physiological functions of the nervous system and also acts as a direct acting agonist of alpha7 nicotinic acetylcholine receptors (nAChRs). The purpose of this study was to conduct a brain region-specific evaluation of changes in nAChR subtype expression following dietary choline modification. In addition, we assessed changes in body weight, food/water intake, as well as changes in spatial learning (Morris Water Maze) in response to dietary choline modification. Male Sprague-Dawley rats were exposed to standard, choline supplemented or choline deficient diets for periods of 14 or 28 days. Choline supplemented animals gained significantly less weight over the course of the experiment, in spite of the fact that there were minimal differences in food consumption between the dietary regimens. Spatial memory did not differ between animals maintained on a standard rat diet, and the choline supplemented food. Brains of the animals kept on the diets for 14 and 28 days were used for quantitative autoradiographic analysis of nicotinic receptor subtypes using 125I-Bungarotoxin (alpha7) and 125I-Epibatidine (non-alpha7). There were no significant differences in nicotinic receptor binding or physiologic parameters measured between animals fed standard and choline deficient diets. However 2 weeks of dietary choline supplementation caused significant up-regulation of alpha7 receptors without significant effect on the density of non-alpha7 nAChRs. Increases in BTX binding predominantly occurred in cortical and hippocampal brain regions and ranged between 14 and 30% depending on the brain region. The results of our study suggest that choline acts as a selective agonist at alpha7 nicotinic cholinergic receptors in the rat central nervous system.

Inhibition of choline uptake by N-cyclohexylcholine, a high affinity ligand for the choline transporter at the blood-brain barrier

The blood-brain barrier (BBB) choline transporter (CHT) may have utility as a drug delivery vector for drugs that act in the central nervous system. Previous studies suggested the importance of hydrophobic moieties on the cationic nitrogen of choline for improved affinity for this transporter. In a pilot study, we therefore designed five novel N-cycloalkyl derivatives of choline, one of which showed promising inhibition properties. This choline analogue had a cyclohexyl (UMBB-5) moiety substituting one of the methyl groups attached to the cationic nitrogen in choline. In situ experimental data were obtained from in situ rat brain perfusion studies. The binding affinity for the BBB-choline transporter found for UMBB-5 was K(i)=1.9 microM. Comparative molecular field analysis (CoMFA) suggested that the cyclohexyl moiety orientates towards a steric favourable area. Taken together, the results of these in situ and in silico studies provide further evidence or restrictions that occur with binding to this brain drug delivery vector.

Cation Transport Specificity at the Blood?Brain Barrier

The molecular identification, expression and cloning of membrane-bound organic cation transporters are being completed in isolated in vitro membranes. In vivo studies, where cation specificity overlaps, need to complement this work.

METHOD

Cross-inhibition of [3H]choline and [3H]thiamine brain uptake by in situ rat brain perfusion.

RESULTS

[3H]Choline brain uptake was not inhibited by thiamine at physiologic concentrations (100 nM). However, choline ranging from 100 nM to 250 microM inhibited [3H]thiamine brain uptake, though not below levels observed at thiamine concentrations of 100 nM.

CONCLUSION

(1) The molecular family of the blood-brain barrier (BBB) choline transporter may be elucidated in vitro by its interaction with physiologic thiamine levels, and (2) two cationic transporters at the BBB may be responsible for thiamine brain uptake.

Development of subtype-selective ligands as antagonists at nicotinic receptors mediating nicotine-evoked dopamine release

N-n-Alkylation of nicotine converts it from an agonist into an antagonist at neuronal nicotinic acetylcholine receptor subtypes mediating nicotine-evoked dopamine release. Conformationally restricted analogues exhibit both high affinity and selectivity at this site, and are able to access the brain due to their ability to act as substrates for the blood-brain barrier choline transporter.

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.

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.

Drug delivery to brain via the blood-brain barrier

1. Many neurodegenerative diseases, cancer and infections of the brain become more prevalent as populations become older. Despite major advances in neuroscience, the blood-brain barrier (BBB) ensures that many potential therapeutic cannot reach the central nervous system (CNS). The BBB is formed by the complex tight junctions between the endothelial cells of the brain capillaries and their low endocytic activity. This results in the capillary wall that behaves as a continuous lipid bilayer and prevents the passage of polar and lipid-insoluble substances. It is, therefore, the major obstacle to drugs that may combat diseases affecting the CNS. 2. Several strategies for delivering drugs to the CNS have been developed. These enhance the capacity of therapeutic molecules to cross the BBB by modifying the drug itself, or by coupling it to a vector for receptor-mediated or adsorption-mediated transcytosis. 3. The current challenge is to develop drug-delivery systems that ensure that drugs cross the BBB in a safe and effective manner. This review focuses on the strategies developed to enhance drug delivery across the BBB.

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.