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.     

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.     

Choline Administration Elevates Brain Phosphorylcholine Concentrations

Abstract: The phosphorylcholine concentration of rat brain rises and falls in response to parallel changes in the concentration of circulating choline. A single oral dose of choline chloride (20 mmol/kg) elevated whole-brain concentrations of both choline and phosphorylcholine 5 h after administration; a greater proportion of exogenously administered choline was retained by the brain in its phosphorylated form than as the free arnine. Striatal phosphorylcholine concentrations were elevated within 2 h of choline administration and continued to be significantly greater than control values for up to 34 h after treatment. The response of striatal choline levels to exogenous choline was of shorter duration than that of phosphorylcholine and was correlated with a significant increase in striatal acetylcholine concentrations. The consumption of a choline-free diet for 7 days lowered both serum choline and striatal phosphorylcholine concentrations, but had no effect on striatal choline or acetylcholine. These results suggest that choline kinase is unsaturated by its substrate in vivo and may thus serve to modulate the response of brain choline concentrations to alterations in the supply of circulating choline.     

Phosphorylation and regulation of choline kinase from Saccharomyces cerevisiae by protein kinase A

The CKI1-encoded choline kinase (ATP:choline phosphotransferase, EC 2.7.1.32) from Saccharomyces cerevisiae was phosphorylated in vivo on multiple serine residues. Activation of protein kinase A activity in vivo resulted in a transient increase in the phosphorylation of choline kinase. This phosphorylation was accompanied by a stimulation in choline kinase activity. In vitro, protein kinase A phosphorylated choline kinase on a serine residue with a stoichiometry (0.44 mol of phosphate/mol of choline kinase) consistent with one phosphorylation site/choline kinase subunit. The major phosphopeptide derived from the enzyme phosphorylated in vitro by protein kinase A was common to one of the major phosphopeptides derived from the enzyme phosphorylated in vivo. Protein kinase A activity was dose- and time-dependent and dependent on the concentrations of ATP (Km 2.1 microM) and choline kinase (Km 0.12 microM). Phosphorylation of choline kinase with protein kinase A resulted in a stimulation (1.9-fold) in choline kinase activity whereas alkaline phosphatase treatment of choline kinase resulted in a 60% decrease in choline kinase activity. The mechanism of the protein kinase A-mediated stimulation in choline kinase activity involved an increase in the apparent Vmax values with respect to ATP (2.6-fold) and choline (2.7-fold). Overall, the results reported here were consistent with the conclusion that choline kinase was regulated by protein kinase A phosphorylation.     

Increase in Exogenous Choline Fails to Elevate the Content or Turnover Rate of Cortical, Striatal, or Hippocampal Acetylcholine

Abstract: The present experiments were designed to test whether increasing the availability of choline to rat brain increases the rate of acetylcholine synthesis in that organ. The content of choline and acetylcholine and the turnover rate of acetylcholine in striatum, hippocampus, and cerebral cortex were measured following changes in dietary choline, intraperitoneal choline, or intravenous infusion of choline. Increasing plasma choline caused some increase in tissue choline but did not increase acetylcholine levels nor acetylcholine turn-over rate in any of the areas of brain studied. Indeed, in hippocampus, choline decreased the turnover rate of acetylcholine.     

Carrier-mediated inhibition of choline acetyltransferase

Incubation of rat forebrain synaptosomes with choline mustard aziridinium ion in a sodium-rich medium caused a time-dependent inhibition of the high-affinity transport of choline, as well as a significant decrease in intrasynaptosomal choline acetyltransferase activity. In the absence of added sodium choline uptake by a sodium-independent mechanism was also blocked in a time-dependent manner but intrasynaptosomal choline acetyl-transferase activity was unaltered. Neither monoethylcholine nor hemicholinium-3 changed intrasynaptosomal choline acetyl-transferase activity but competitively inhibited the transport of choline. The results indicate that there may be a fraction of choline acetyltransferase that is closely associated with the sodium-dependent high-affinity choline transport system and that this fraction can be irreversibly inhibited by choline mustard aziridinium ion, perhaps indirectly mediated by alkylation of the carrier.     

CHOLINE AND ACETYLCHOLINE IN RATS: EFFECT OF DIETARY CHOLINE

Abstract– The concentration of free choline in peripheral tissues (duodenum, heart, kidney, liver, stomach and plasma) of rats was found to be related to the amount of free choline in the diet. Under steady-state conditions, the concentration of free choline in plasma varied from a minimum of approx 6 nmol/ml (in rats fed a choline-deficient diet) to a maximum value not exceeding 21 nmol/ml. The concentration of plasma choline was elevated above 21 nmol/ml for a short time after parenteral administration of choline chloride or one of its precursors (CDP choline or phosphorylcholine), but was not affected by stress, endocrine manipulations, drug treatments or the time of day when rats were killed. The metabolism of intravenously administered [methyl-³H] choline was accelerated in peripheral tissues (except plasma) of choline-deficient rats, indicating that free choline is not preserved during choline deficiency by a reduction in its rate of turnover. Furthermore, the decrease in concentration of plasma choline that occurred in rats fed a choline-deficient diet was prevented by addition of deanol (dimethylaminoethanol) to the diet. These results indicate that free choline in peripheral tissues of rats is derived from both free choline in the diet, and from precursors of choline present within the diet. In contrast to the effects in peripheral tissues, the concentration of free choline in brain was not reduced by dietary deprivation of free choline; however, the increase in free choline that occurred when rats were decapitated was reduced in brains by deficiency of choline, suggesting a decrease in the concentration of esterified forms of cerebral choline. The concentration of acetylcholine was not reduced in the brain, duodenum, heart, kidney or stomach of 21-week old rats raised from birth on a choline-deficient diet, in the duodenum of rats given a choline-deficient diet for 1, 5 or 11 days, or in brains of rats deprived of free choline for 1 or 11 days. However, the rate of in vivo synthesis of ACh from [methyl-³H]choline was accelerated in cholinergic tissues that were depleted of free choline (i.e. duodenum, heart and stomach).     

Effect of ethyl choline mustard on choline dehydrogenase and other enzymes of choline metabolism

The effect of ethyl choline mustard (ECMA), and effective irreversible inhibitor of choline transport, was investigated on the enzymes of choline metabolism. ECMA at concentrations of 50 microM hardly affected choline acetyltransferase and caused only a 20% inhibition of choline kinase at a concentration of 1 mM. However, the mustard was an extremely effective inhibitor of choline dehydrogenase, producing 50% inhibition at concentrations of 6 microM. The inhibition was prevented by incubation in the presence of choline or by prior reaction of the mustard with thiosulphate. Separation of the components of the ECMA solution on TLC suggested that only the compound with an aziridine ring was an effective inhibitor of choline dehydrogenase. The inhibition was resistant to the washing out of excess unreacted mustard. The rate constant of inhibition was 395 M-1 X S-1. By the use of [3H]ECMA a single polypeptide in the enzyme preparation having a MW of 67,000 was labelled. The labelling was thiosulphate-sensitive and prevented by incubation with choline. It is concluded that ECMA is an irreversible inhibitor of choline dehydrogenase. It is at least as effective an inhibitor of choline dehydrogenase as of the choline transport system, although it does not appreciably inhibit choline acetyltransferase or choline kinase in the micromolar range.     

Cortical choline transporter function measured in vivo using choline-sensitive microelectrodes: clearance of endogenous and exogenous choline and effects of removal of cholinergic terminals

The capacity of the high-affinity choline transporter (CHT) to import choline into presynaptic terminals is essential for acetylcholine synthesis. Ceramic-based microelectrodes, coated at recording sites with choline oxidase to detect extracellular choline concentration changes, were attached to multibarrel glass micropipettes and implanted into the rat frontoparietal cortex. Pressure ejections of hemicholinium-3 (HC-3), a selective CHT blocker, dose-dependently reduced the uptake rate of exogenous choline as well as that of choline generated in response to terminal depolarization. Following the removal of CHTs, choline signal recordings confirmed that the demonstration of potassium-induced choline signals and HC-3-induced decreases in choline clearance require the presence of cholinergic terminals. The results obtained from lesioned animals also confirmed the selectivity of the effects of HC-3 on choline clearance in intact animals. Residual cortical choline clearance correlated significantly with CHT-immunoreactivity in lesioned and intact animals. Finally, synaptosomal choline uptake assays were conducted under conditions reflecting in vivo basal extracellular choline concentrations. Results from these assays confirmed the capacity of CHTs measured in vivo and indicated that diffusion of substrate away from the electrode did not confound the in vivo findings. Collectively, these results indicate that increases in extracellular choline concentrations, irrespective of source, are rapidly cleared by CHTs.     

Choline mustard: an irreversible ligand for use in studies of choline transport mechanisms at the cholinergic nerve terminal

It has been shown in our laboratory that choline mustard aziridinium ion is a potent and irreversible inhibitor of choline transport into rat brain synaptosomes; this compound showed selectivity for the sodium-dependent, high affinity carrier in that it was 30 times more potent as an inhibitor when compared with the effect on sodium-independent, low affinity choline uptake. In the present study, this mustard analogue did not inhibit synaptosomal uptake of 5-hydroxytryptamine, noradrenaline, or gamma-aminobutyric acid, thereby confirming further the specificity of this compound for the choline carrier. Studies of the effect of depolarization of the nerve terminals on the inactivation of choline carriers by choline mustard were performed. It was determined that alkylation of the carrier was significantly increased in nerve endings previously depolarized. The enhancing effect of depolarization on choline transport velocity and on the alkylation of choline carriers by choline mustard was dependent upon the presence of sodium in the external medium. Possible mechanisms for the enhanced inactivation of choline carriers by choline mustard aziridinium ion are proposed, and kinetic interactions of choline mustard with the high affinity choline carrier and with choline acetyltransferase are reviewed and discussed.     

Inhibition of choline transport into human erythrocytes by choline mustard aziridinium ion.

Acetylcholine mustard aziridinium ion inhibited the transport of [3H]choline into human erythrocytes. Treatment of the erythrocytes with 1 X 10(-4) M tetraethylpyrophosphate prevented the inhibition of [3H]choline transport by acetylcholine mustard aziridinium ion. Hydrolyzed acetylcholine mustard aziridinium ion inhibited choline transport both in the presence and absence of 1 X 10(-4) M tetraethylpyrophosphate. The product of hydrolysis was equipotent with acetylcholine mustard in its ability to inhibit choline transport; incubation of this product with sodium thiosulfate prevented inhibition of choline transport thereby indicating the presence of an aziridinium ion. The hydrolysis product is likely to be choline mustard aziridinium ion. Results on the efflux of [3H]choline from erythrocytes in the presence of the proposed choline mustard aziridinium ion showed that the mustard moiety was transported into the red cells on the choline carrier. The rate of efflux of [3H]choline produced by choline mustard aziridinium ion was 55% of that produced by the same concentration of choline. It is concluded that acetylcholinesterase (EC 3.1.1.7) of red cells rapidly hydrolyzes acetylcholine mustard aziridinium ion to acetate and choline mustard aziridinium and the latter compound can act as a potent inhibitor of choline transport. This finding would indicate that the hemicholinium-like toxicity of acetylcholine mustard in the mouse is due to the formation of choline mustard aziridinium ion.     

Determination of intracellular choline levels by an enzymatic assay

A sensitive enzymatic assay for the measurement of intracellular choline is described. The separation of choline from choline-containing phospholipids is accomplished by a minor modification of the Folch technique. The method is based on the specific oxidation of choline by choline oxidase. Phenol and 4-aminoantipyrine in the presence of hydrogen peroxide generated by the oxidation of choline and peroxidase form a red quinone dye which can be detected spectrophotometrically. The assay was useful between 12.5 and 100 nanomoles of choline. The recovery of standard choline in liver homogenates averaged 102 +/- 1.6%. Structurally similar compounds produced minimal interference.     

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)     

Stimulation of photosynthesis and growth of photoautotrophically cultured plant cells by choline and its analogs

The effects of choline and its analogs, allylcholine and benzylcholine, on the photosynthesis and on the cell growth were examined using photoautotrophically, photomixotrophically and heterotrophically cultured cells. The addition of choline and its analogs stimulated the cellular photosynthetic activity and enhanced the dry weight increase in both photoautotrophic and photomixotrophic cells. However, the growth of heterotrophic cells did not increase by the addition of choline and choline analogs. The photosynthetic electron transport activity in thylakoid membrane was enhanced when cells were treated with choline and choline analogs, suggesting that thylakoid membranes are the initial site of the stimulation of cellular photosynthesis. The stimulatory effect of choline and choline analogs was sustained even after 3 week-culture. Among the choline analogs tested, benzylcholine showed the most quick effect and was effective at a lower concentration (1 mg/l) than choline (10 mg/l).Abbreviations GA₃ gibberellin A₃     

Tissue Choline Studied Using a Simple Chemical Assay

Abstract: An enzymatic-radioisotopic assay was used to measure free choline in unextracted tissue. The lowest concentration of free choline in any tissue studied was present in human cerebrospinal fluid (mean, 5.7 μM; range, 1.8–31.2 μM). A postmortem increase in concentration of free choline occurred in blood (O.2 nmol/min ml), kidney (13 nmol/min·g), and liver (22 nmol/min·g) of mice. The concentration of free choline in these tissues was estimated by extrapolation to be 5, 77, and 29 nmol/g (or ml), respectively. Several treatments were found to increase the concentration of free choline. For example, intraperitoneal administration of choline or 2-amino-2-methyl-propanol (a choline oxidase inhibitor) induced an increase in the level of choline in blood, kidneys, liver, and brain of mice, and administration of 2-dimethylaminoethanol (deanol) caused an increase in kidney and liver choline. The level of choline in blood was increased when rats were treated orally with either antibiotics or esters of choline such as phosphorylcholine, glycerylphos-phorylcholine, laroylcholine, or propionylcholine. The results show that the concentration of free choline may be regulated by intestinal metabolism, availability of esterified precursors, and activity of enzymes that metabolize choline.     

Choline transport into rat liver mitochondria. Characterization and kinetics of a specific transporter.

Rat liver mitochondria possess a specific choline transporter in the inner membrane. The transporter shows saturable kinetics at high membrane potential with a Km of 220 microM and a Vmax of 0.4 nmol/mg of protein/min at pH 7.0 and 25 degrees C. At physiological concentrations of choline, the rate of choline uptake by the transporter shows a linear dependence on membrane potential; uptake is distinct from the nonspecific cation diffusion process. Hemicholinium-3, hemicholinium-15, quinine, and quinidine, all analogues of choline, are high affinity competitive inhibitors of choline transport with Ki values of 17, 55, 15, and 127 microM, respectively. The choline transporter is distinct from other known mitochondrial transporters. Rat heart mitochondria do not appear to possess a choline transporter. Evidence suggests that the transporter is an electrophoretic uniporter. Analogue studies have shown that the hydroxyl and the quaternary ammonium groups of choline are necessary for binding to the transporter. A comparison of molecular models of choline and the high affinity inhibitors has provided evidence for the preferred conformation of choline for binding to the transporter. The presence of a choline transporter in the mitochondrial inner membrane provides a potential site for control of choline oxidation and hence supply of endogenous betaine.     

The spatial and temporal concentrations of choline in the lumen contents of the small intestine of uninfected and Hymenolepis diminuta infected rats

The spatial and temporal concentrations of free choline in the lumen of the small intestine of the uninfected and Hymenolepis diminuta-infected rat were investigated. In the unfed infected or uninfected rat, the choline concentrations ranged from approximately 500 microM in the duodenum to approximately 20 microM in the posterior ileum, with some segments in infected rats containing significantly higher choline levels than in uninfected rats. Following feeding, choline levels were significantly elevated to approximately 3 mM by 6 h, although these concentrations fell rapidly in transit down the intestine. By 12 h the choline levels were similar to those in the unfed rat. An initial small shift in the worm biomass toward the duodenum after feeding was followed by a redistribution of biomass along the length of the small intestine. The worm biomass, however, had little or no effect on the choline levels. The high concentrations of free choline observed in the anterior regions of the intestine are postulated to be predominantly determined by nutritional intake while the concentrations in the posterior region may in part be determined by blood choline levels. The high levels of choline indicate that choline is not limiting to support the growth of the worms.     

Phosphatidylcholine and the CDP–choline cycle

The CDP–choline pathway of phosphatidylcholine (PtdCho) biosynthesis was first described more than 50 years ago. Investigation of the CDP–choline pathway in yeast provides a basis for understanding the CDP–choline pathway in mammals. PtdCho is considered as an intermediate in a cycle of synthesis and degradation, and the activity of a CDP–choline cycle is linked to subcellular membrane lipid movement. The components of the mammalian CDP–choline pathway include choline transport, choline kinase, phosphocholine cytidylyltransferase, and choline phosphotransferase activities. The protein isoforms and biochemical mechanisms of regulation of the pathway enzymes are related to their cell‐ and tissue-specific functions. Regulated PtdCho turnover mediated by phospholipases or neuropathy target esterase participates in the mammalian CDP–choline cycle. Knockout mouse models define the biological functions of the CDP–choline cycle in mammalian cells and tissues. This article is part of a Special Issue entitled Phospholipids and Phospholipid Metabolism.     

Regulation of phospholipid metabolism in differentiating cells from rat brain cerebral hemispheres in culture. Patterns of acetylcholine phosphocholine, and choline phosphoglycerides labeling from (methyl-14C)choline.

The uptake and metabolism of [14C]choline in dissociated rat brain embryo cell cultures was examined as a function of the extracellular choline concentration. Choline uptake did not follow normal Michaelis-Menten kinetics, but rather exhibited two components with apparent Km of 0.016 mM and 0.96 mM. At low choline concentrations (high affinity uptake) most of the [14C]choline label was present in the phosphocholine fraction prior to the appearance of label in phospholipids. At high choline concentrations (low affinity uptake) a large proportion of the radioactivity was converted into acetylcholine. The dissimilarities between the formation of phosphocholine and acetylcholine as a function of choline concentration might be explained by the existence of two mutually independent enzymatic activities with different Km affinities for choline. Kinetic data augmented by double label studies, suggested that formation of choline phosphoglyceride proceeds entirely via a phosphocholine intermediate. Nearly all radioactivity in the lipid fraction is incorporated into choline phosphoglycerides. A higher turnover rate of choline incorporation into choline phosphoglycerides, accompanied by an increase in the levels of glycerophosphocholine, was observed in older cultures as compared to younger cultures. The metabolic implications of these findings in cultured brain cells in comparison with other in vitro systems are discussed.     

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.