Influence of Dietary Choline Availability and Neuronal Demand on Acetylcholine Synthesis by Rat Brain

The main objective of this study was to test the hypothesis that the chronic administration of choline supplements a bound pool of choline from which free choline can be mobilized and used to support acetylcholine synthesis when the demand for precursor is increased. For these experiments, brain slices from rats fed diets containing different amounts of choline were incubated in a choline-free buffer and acetylcholine synthesis was measured under resting conditions and in the presence of K+-induced increases in acetylcholine synthesis and release. Rats fed the choline-supplemented diet had circulating choline levels that were 52% greater than the controls, and striatal and cerebral cortical slices from this group produced significantly more free choline during the incubation than slices from the controls. However, the synthesis and release of acetylcholine by these tissues did not differ from those by controls, during either resting or K+-evoked conditions. In contrast, acetylcholine synthesis and release by striatal and hippocampal slices from choline-deficient rats, animals that had circulating choline levels that were 80% of control values, decreased significantly; the production of free choline by these tissues was also depressed. Results indicate that, despite an increased production of free choline by brain slices from choline-supplemented rats, the synthesis of acetylcholine was unaltered, even in the presence of an increased neuronal demand. In contrast, the choline-deficient diet led to a decreased release of free choline from bound stores and an impaired ability of brain to synthesize acetylcholine.     

Acute Choline Supplementation In Vivo Enhances Acetylcholine Synthesis In Vitro when Neurotransmitter Release Is Increased by Potassium

The main objective of these studies was to determine whether the acute administration of choline to rats provides supplemental precursor that can be used to support acetylcholine synthesis when the demand for choline is increased by increasing neurotransmitter release. For these experiments, hippocampal and striatal slices were prepared form rats that had received saline or an acute injection of choline. Slices were incubated in a choline-free buffer containing 4.74-35 mM KCl, and acetylcholine synthesis and release and choline production were measured. The initial tissue contents of acetylcholine and choline did not differ between experimental groups for either brain region. When hippocampal slices from the controls were incubated for 10 min with depolarizing concentrations of KCl, acetylcholine release increased and the tissue content decreased in a concentration-dependent fashion; no net synthesis of acetylcholine occurred. In contrast, hippocampal slices from the choline-injected animals maintained their tissue content in the presence of high concentrations of KCl, despite an increase in acetylcholine release that was similar in magnitude to that of the controls; positive net synthesis of acetylcholine resulted. Although the molar concentration of choline achieved in the incubation media at the end of the 10-min period did not differ between groups, the mobilization of free choline from bound stores was significantly greater in hippocampal slices from the choline-injected group than the controls. In addition, the synthesis of acetylcholine by hippocampal slices from the choline-injected group was prevented by the presence of hemicholinium-3 (1 microM) in the media.(ABSTRACT TRUNCATED AT 250 WORDS)     

Increase in tissue concentration of acetylcholine in guinea pigs in vivo induced by administration of choline

Administration of choline chloride (200 μmoles/kg) intravenously to guinea pigs caused an increase in the concentrations of choline and acetylcholine in adrenals, heart, kidneys, lungs, and liver within 2 min. These results suggest that raising the concentration of choline in plasma will accelerate the formation of acetylcholine in the organs cited. No significant increase in concentration of choline or acetylcholine occurred in brain.     

Seizures increase acetylcholine and choline concentrations in rat brain regions

Seizures induced by three convulsant treatment produced differential effects on the concentration of acetylcholine in rat brain. Status epilepticus induced by (i) coadministration of lithium and pilocarpine caused massive increases in the concentration of acetylcholine in the cerebral cortex and hippocampus, (ii) a high dose of pilocarpine did not cause an increase of acetylcholine, and (iii) kainate increased acetylcholine, but the magnitude was lower than with the lithium/pilocarpine model. The finding that the acetylcholine concentration increases in two models of status epilepticus in the cortex and hippocampus is in direct contrast with manyin vitro reports in which excessive stimulation causes depletion of acetylcholine. The concentration of choline increased during seizures with all three models. This is likely to be due to calcium- and agonist-induced activation of phospholipase C and/or D activity causing cleavage of choline-containing lipids. The excessive acetylcholine present during status epilepticus induced by lithium and pilocarpine was responsive to pharmacological manipulation. Atropine tended to decrease acetylcholine, similar to its effects in controls. The N-methyl-D-aspartate (NMDA) receptor antagonist, MK-801, reduced the excessive concentration of acetylcholine, especially in the cortex. Inhibition of choline uptake by hemicholinium-3 (HC-3) administered icv reduced the acetylcholine concentration in controls and when given to rats during status epilepticus. These results demonstrate that the rat brain concentrations of acetylcholine and choline can increase during status epilepticus. The accumulated acetylcholine was not in a static, inactive compartment, but was actively turning-over and was responsive to drug treatments. Excessive concentrations of acetylcholine and/or choline may play a role in seizure maintenance and in the neuronal damage and lethality associated with status epilepticus.     

THE EFFECTS OF BOTULINUM TOXIN ON ACETYLCHOLINE METABOLISM IN MOUSE BRAIN SLICES AND SYNAPTOSOMES

The effects of Type A botulinum toxin on acetylcholine metabolism were studied using mouse brain slice and synaptosome preparations. Brain slices that had been incubated with the toxin for 2h exhibited a decreased release of acetylcholine into high K⁺ media. Botulinum toxin did not affect acetylcholine efflux from slices in normal K⁺ media. When labeled choline was present during the release incubation, a‘newly-synthesized’pool of acetylcholine was formed in the tissue. In toxin-treated slices exposed to high K⁺, both the production and the release of this‘newly-synthesized’acetylcholine were depressed. A possible explanation for these actions of botulinum toxin would be via an inhibition of the high affinity uptake of choline. This hypothesis was tested by measuring the high affinity uptake of [³H]choline into synaptosomes prepared from brain slices. Previous exposure of slices to botulinum toxin caused a significant reduction in the accumulation of label by the synaptosomes. These data are discussed in terms of our current understanding of the mechanism of action of botulinum toxin and the toxin's interaction with the mechanisms regulating acetylcholine turnover.     

Decreased synthesis of acetylcholine accompanying impaired oxidation of pyruvic acid in rat brain minces.

The relation between pyruvate utilization and acetylcholine synthesis was investigated in minces of adult rat brain. The flux of pyruvate to acetylcholine was less than 1% of that to CO2; nevertheless, a number of agents which inhibited conversion of [1-14C]-pyruvate or [2-14C]pyruvate into 14CO2 were associated with corresponding decreases in the conversion of [2-14C]pyruvate into acetylcholine. The amount of acetylcholine produced by minces of whole rat brain, measured by g.l.c.-mass spectrometry, decreased similarly. Among the inhibitory compounds tested were 3-bromopyruvate, an irreversible inhibitor of pyruvate dehydrogenase; 2-oxobutyrate, a competitive inhibitor of pyruvate dehydrogenase; other 2-oxo acids; and amobarbital and pentobarbital. Linear-regression equations relating CO2 production to acetylcholine synthesis gave correlation coefficients of 0.89-0.93 for the combined observations. The inhibition of acetylcholine synthesis could not be attributed to inhibition of choline acetyltransferase. Incorporation of [2-14C]pyruvate into lipids, proteins and nucleic acids was effected less than that into acetylcholine. Under these experimental conditions, it was shown that pyruvate utilization can limit acetylcholine synthesis.     

Dietary choline augments associative memory function in Limax maximus

Previous clinical and experimental work has shown that increased dietary intake of choline elevates blood choline and brain acetylcholine levels. This change in neuronal acetylcholine concentration may augment learning and memory functions. We tested this prediction using the mollusc Limax maximus, an animal which can be readily conditioned to avoid food odors. In our experiments, initial learning of a food avoidance task was not augmented by the high choline diet. However, the duration of memory retention was prolonged. In previous studies, we have shown that intake of the choline enriched diet significantly increases blood choline and amplifies transmission at an identified cholinergic synapse in Limax. Together, these results support the involvement of cholinergic synapses in the memory retention mechanism.     

The Utilization of Choline and Acetyl Coenzyme A for the Synthesis of Acetylcholine

Acetylcholine synthesis in rat brain synaptosomes was investigated with regard to the intracellular sources of its two precursors, acetyl coenzyme A and choline. Investigations with α-cyano-4-hydroxycinnamate, an inhibitor of mitochondrial pyruvate transport, indicated that pyruvate must be utilized by pyruvate dehydrogenase located in the mitochondria, rather than in the cytoplasm, as recently proposed. Evidence for a small, intracellular pool of choline available for acetylcholine synthesis was obtained under three experimental conditions. (1) Bromopyruvate competitively inhibited high-affinity choline transport, perhaps because of accumulation of intracellular choline which was not acetylated when acetyl coenzyme A production was blocked. (2) Choline that was accumulated under high-affinity transport conditions while acetyl coenzyme A production was impaired was subsequently acetylated when acetyl coenzyme A production was resumed. (3) Newly synthesized acetylcholine had a lower specific activity than that of choline in the medium. These results indicate that the acetyl coenzyme A that is used for the synthesis of acetylcholine is derived from mitochondrial pyruvate dehydrogenase and that there is a small pool of choline within cholinergic nerve endings available for acetylcholine synthesis, supporting the proposal that the high-affinity transport and acetylation of choline are kinetically coupled.     

Effect of Increasing Choline, In Vivo and In Vitro, on the Synthesis of Acetylcholine in a Sympathetic Ganglion

Abstract: The experiments described in this paper were designed to test whether increasing choline availability over normal physiological levels increases acetylcholine synthesis in the cat's superior cervical ganglion. When ganglia were perfused with Krebs solution, an increase in the medium's choline concentration over physiological (10⁻³M) levels increased tissue choline but did not increase tissue acetylcholine or the release of acetylcholine from stimulated ganglia. However, increasing plasma choline in the whole animal increased ganglionic acetylcholine levels. The basis for this difference in the effects of in vivo and in Vitro exposure to elevated choline levels on the tissue acetylcholine content was found to involve plasma factor(s), rather than indirect actions of choline, and the acetylcholine content of isolated ganglia was increased when the tissue was perfused with plasma, instead of Krebs solution, containing 10⁻³M-choline. The extra acetylcholine generated by this procedure was associated with a subsequent transient increase in transmitter release during short intervals of stimulation, but most of the extra acetylcholine was not readily available for release from stimulated ganglia. It is concluded that increasing choline available to sympathetic ganglia over physiological concentration does not have a sustained effect on the turnover of releasable transmitter under the conditions of these experiments.     

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.     

Exogenous Choline Enhances the Synthesis of Acetylcholine Only Under Conditions of Increased Cholinergic Neuronal Activity

Abstract: The effect of choline (60 mg/kg, i.p.) on fluphenazine- and pentylenetetrazol-induced alterations in the concentration of acetylcholine (ACh) and/or the rate of sodium-dependent high-affinity choline uptake (HACU) in rat striatum and hippocampus was studied. Systemic administration of the dopamine receptor blocking agent fluphenazine hydrochloride (0.5 mg/kg, i.p.) decreased the concentration of ACh in the striatum; this effect was prevented by the prior administration of choline. The central nervous system stimulant pentylenetetrazol (30 mg/kg, i.p.) reduced the concentration of ACh in both striatum and hippocampus and increased the velocity of HACU in the hippocampus. Pretreatment with choline totally prevented the depletion of ACh induced by pentylenetetrazol in the striatum. In the hippocampus, prior administration of choline prevented the pentylenetetrazol-induced increase in the rate of HACU and attenuated the effect of pentylenetetrazol on the levels of ACh. Results indicate that the acute administration of choline antagonizes pharmacologically induced alterations in cholinergic activity as assessed by the rate of HACU and the steady-state concentration of ACh. Furthermore, data support the hypothesis that the administration of choline increases the ability of central cholinergic neurons to synthesize ACh under conditions of increased neuronal activity.     

Presynaptic effect of the aziridinium ion of acetylcholine mustard (methyl-2-acetoxyethyl-2'-chloroethylamine) on the phrenic nerve--rat diaphragm preparation.

The rat diaphragm has been used to investigate the neuromuscular blocking action of acetylcholine mustard which yields a potent nicotinic agonist, an aziridinium ion, in aqueous medium. Evidence was obtained that the acetylcholine mustard aziridinium ion impaired neuromuscular activity when the phrenic nerve was stimulated and that the ion did not directly inhibit muscle contraction. Impairment of neuromuscular activity was characterized by a latent period and depended both on the concentration of aziridinium ion and the frequency of stimulation of the phrenic nerve. Elevated concentrations of Ca-2+ and choline changed the response of the rat diaphragm to the aziridinium ion, the former increasing the rate of development of neuromuscular block and the latter protecting against neuromuscular block. These results indicated that the aziridinium ion may act either at the site of choline uptake or have an effect on acetylcholine synthesis in the nerve ending and that impairment of neuromuscular transmission in the rat diaphragm involved the availability of acetylcholine. Similar results were obtained with acetylcholine mustard aziridinium ion subjected to alkaline hydrolysis. This substance is thought to be choline mustard aziridinium ion. Although difficult to prove with the rat diaphragm it is possible that acetylcholinesterase of this preparation could hydrolyze acetylcholine mustard aziridinium ion at the neurotransmitter site and the resultant choline mustard aziridinium ion would interfere with the uptake of choline and eventually prevent neuromuscular transmission. This hemicholinium-like hypothesis for the mechanism of action of choline mustard aziridinium ion is compatible with reported date for toxicity of acetylcholine mustard aziridinium ion in the mouse.     

Cardiovascular effects produced by choline injected into the lateral cerebral ventricle of the unanesthetized rat

Intracerebroventricular (icv) injection of choline (50–150 μg) causes a transient increase in blood pressure and a more prolonged decrease in heart rate (HR) in conscious rats. The bradycardia results from a centrally mediated increase in vagal tone. The cardiovascular effects do not appear to involve endogenous brain acetylcholine since there is no significant difference in the responses induced by choline before and after icv injection of hemicholinium-3. Intracerebroventricular ventricular injection of atropine or mecamylamine, alone, failed to influence the choline effect. However, atropine and mecamylamine, given together, abolished the reduction of HR, but still failed to modify the pressor response. The changes in blood pressure and HR appear to be due to effects of choline on post-synaptic receptors in different brain regions.     

Quantitative analysis of acetylcholine release in depolarized hippocampal slices

Time course of the hippocampal slice acetylcholine content and the rate of acetylcholine release were studied during high K⁺-induced depolarization for 4 to 60 min. At the end of the potassium exposure, both the acetylcholine remaining in the tissue and appearing in the incubation medium were quantitatively determined by gas chromatography using a nitrogen-sensitive detector. During prolonged K⁺ incubation, the acetylcholine content of the slices decreased by 60%, reaching a steady state after 16 min. The increase in the acetycholine concentration of the depolarizing medium showed a biphasic pattern, with rate constants of 1.40 and 0.69 nmol/min/g in the early (0–16 min) and late (16–60 min) phase, respectively. K⁺-evoked acetylcholine release was Cal⁺-dependent, but addition of choline did not alter tissue levels of acetylcholine or the pattern of K⁺-evoked acetylcholine release. The rate of acetylcholine release was markedly decreased by inhibition of choline uptake with hemicholinium-3 or by addition of 4-(1-naphthylvinyl)pyridine which inhibits both ACh producing enzyme, choline acetyltransferase and choline uptake mechanism. These data confirm the essential role during depolarization of extracellular choline transport into the cholinergic terminals utilizing choline released by the slices during the incubation. It is concluded that drugs which can influence the processes of choline uptake and acetylcholine sythesis can alter the rate of acetylcholine release measured under similar conditions.     

THE LACK OF SPECIFICITY TOWARDS SALTS IN THE ACTIVATION OF CHOLINE ACETYLTRANSFERASE FROM HUMAN PLACENTA

Abstract— The effects of monovalent and divalent anions on the choline acetyltransferase reaction have been determined at high (5.0 mM) and low (0.58 mM) choline. At 0.58 mM-choline, both monovalent and divalent anions activate the enzyme ±9 fold; however, at 5.0mM-choline, monovalent anions activate the enzyme ±25 fold, while divalent anions activate ±9 fold. Both monovalent and divalent anions show uncompetitive activation with respect to choline. When either dimethylaminoethanol, N -(2-hydroxyethyl)- N -methyl piperidinium iodide, or N -(2-hydroxyethyl)- N -propyl pyrrolidinium iodide was substituted for choline, activation by monovalent or divalent anions was only 2.5-4 fold. With AcCoA as substrate the ChA reaction can be increased ±20 fold by increased salts; however, with acetyl dephosphoCoA as substrate, the reaction is insensitive to the salt concentration. Similar salt effects on the ChA reaction, as measured in the direction of acetylcholine synthesis, have been demonstrated in the reverse reaction. In addition, inhibition of the forward reaction by acetylcholine has been measured as a function of sodium chloride concentration. Although the K₁ for acetylcholine increases with increasing salt, this change in K ₁, parallels the increase in the K _m for choline. These results support the hypothesis that both monovalent and divalent anions activate choline acetyltransferase by the same singular mechanism; which is to increase the rate of dissociation of coenzyme A from the enzyme.     

SIMILARITIES OF β-BUNGAROTOXIN AND PHOSPHOLIPASE A2 AND THEIR MECHANISM OF ACTION

Abstract— β-Bungarotoxin, a presynaptic neurotoxin isolated from the venom of Bungarus multicinctus , has been shown to initially cause an increase in the frequency of miniature endplate potentials and subsequently block neuromuscular transmission by inhibiting nerve impulse induced release of acetylcholine. In rat brain synaptosomes it causes a Ca²⁺-dependent release of acetylcholine together, with a strong inhibition of the high affinity choline uptake system. In this report we demonstrate that β-bungarotoxin acts as a phospholipase A₂ (phosphatide 2-acyl hydrolase, EC 3.1.1.4), liberating fatty acids from synaptic membrane phospholipids. It also exhibits a striking similarity in a number of neurochemical properties with that of a purified phospholipase A₂ from Naja naja siamensis. In addition, both agents produce a marked depolarization of synaptosomal preparations as measured by a fluorescent dye. We propose that disruption of the membrane phospholipids by phospholipase activity can lead to depolarization of the synaptosomal preparation which promotes both transmitter release and inhibition of the energy-dependent high affinity choline uptake system. With this decreased supply of choline, the acetylcholine content of the cell would be gradually depleted leading to a decrease in transmission.     

Evaluation of acetate uptake and its relative conversion to acetylcholine in Torpedo electric organ synaptosomes under different ionic and metabolic conditions

The uptake of acetate and its incorporation into acetylcholine were measured under various conditions in nerve terminals isolated from the electric organ in order to characterize acetate uptake and to study the relationship between acetate uptake and acetylcholine synthesis in a pure cholinergic preparation. It was found that increasing extracellular choline up to 10^?4 M had no effect on either acetate uptake or the conversion of acetate to ACh, while the addition of hemicholinium-3 to the incubation medium led to decreases in both parameters. Hence, it appears that endogenous levels of choline are sufficient to support ongoing acetylcholine synthesis in this preparation and that this synthesis depends to some extent on the uptake of extracellular choline. Nonetheless, in the absence of choline uptake, both the uptake of acetate and the conversion of acetate to acetylcholine remained substantial, indicating that internal sources of choline as well can be used for acetylcholine synthesis.Acetate uptake displayed a marked requirement for external Na⁺ and was decreased following depolarization of the synaptosomes by an elevated K⁺ concentration. The conversion of acetate to acetylcholine followed a similar pattern, except that a small reduction in acetylcholine synthesis was observed in the absence of external Ca²⁺, while acetate uptake was unaffected. The addition of ATP, AMP-PNP or phosphate to the incubation medium caused an increase in both the uptake and incorporation of acetate, but adenosine had no effect on either of these functions. Choline uptake, meanwhile, was unchanged in the presence of ATP, phosphate or adenosine. Acetate uptake appears to be more closely linked to its intracellular metabolism than to the transmembrane movement of choline itself.The mechanism by which acetate crosses the nerve terminal membrane has not been established, but the possibility that acetate is a substrate for a monocarboxylate transport system such as has been described in other systems can be ruled out as inhibitors of anion permeability do not block acetate uptake in this preparation.     

SODIUM-DEPENDENT HIGH AFFINITY CHOLINE UPTAKE: A REGULATORY STEP IN THE SYNTHESIS OF ACETYLCHOLINE

The sodium-dependent high affinity choline uptake into synaptosomes from rat brain has been studied after in vivo treatments which would alter the activity of cholinergic neurons. We utilized a number of treatments to reduce the activity of cholinergc neurons in the brain. Administration of pentobarbital (65 mg/kg), chloral hydrate (40 mg/kg) and γbutyrelactone (750 mg/kg) caused a 50-80% reduction in sodium-dependent high affinity choline uptake in several brain regions (30 min). This depression was not found 24 h after injection. Interruption of the cholinergic septal-hippocampal or habenuleinterpeduncular tracts by lesions (10 min-1 h) also caused a similar, large reduction in sodium-dependent high affinity choline uptake in the hippocampus and the interpeduncular nucleus respectively. We reversed the inactivity after pentobarbital administration by direct electrical stimulation of the cholinergic septal-hippocampal tract. Stimulation (40 Hz) for 10-15 min completely reversed the depression in sodium-dependent high affinity choline uptake. Stimulation at lower frequencies or for shorter times caused a partial reversal. Administration of pentylenetetrazol (75 mg/kg), a convulsant, was utilized to increase the activity of central cholinergic neurons. After drug administration, we found a large (60%) increase in sodium-de-pendent high affinity choline uptake. This increase was not found in the hippocampus when cholinergic afferents were interrupted by septal lesion prior to drug administration. We also examined the uptake after administration of cholinergic drugs. Oxotremorine (0.75 mg/kg), a muscarinic agonist which reduces acetylcholine release and turnover, caused a reduction in uptake. On the other hand, administration of scopolamine (5 mg/kg), a cholinergic antagonist which increases acetylcholine turnover, caused an increase in sodium-dependent high affinity choline uptake. Addition of any drug utilized, drectly to uptake samples, did not alter uptake. We examined the conversion of [³H]choline to [³H]acetylcholine in hippocampal synaptosomes after septal lesion, pentylenetetrazol administration and in untreated controls. In all cases, 60-70% of the total sodium-dependent tritium content was present as [³H]acetylcholine. Evidence was presented that homoexchange is not or is less involved in choline uptake than in GABA uptake. A kinetic analysis of sodium-dependent high affinity choline uptake was performed after all treatments. We found changes in V_max, after all treatments, which were consistently in the same direction as the alterations in activity. The proposal is made that the sodium-dependent high affinity choline uptake is coupled to cholinergic activity in such a way as to regulate the entry of choline for the maintenance of acetylcholine synthesis. The findings also lead us to propose that sodium-dependent high affinity choline uptake in vitro be utilized as a rapid, relative measure of the activity of cholinergic nerve terminals in vivo.     

Alzheimer's disease through the eye of a mouse. Acceptance lecture for the 2001 Gayle A. Olson and Richard D. Olson prize

There is now ample evidence that beta-amyloid proteins decrease memory. The SAMP8 mouse (P8) develops an early decline in the ability to learn and to retain new information. The studies reviewed here suggest that this is due to overproduction of beta-amyloid. Both antibodies to beta-amyloid and specific antisense to the amyloid precursor protein reverse these deficits in the P8 mouse. This antisense can cross the blood brain barrier. It is hypothesized that the overproduction of beta-amyloid leads to a decline in Delta(9) desaturase activity with an alteration in membrane fatty acids. This results in altered membrane mobility leading to a decline in neurotransmitter activity and a decreased release of acetylcholine. This decreased cholinergic activity results in a decreased ability of the P8 mouse to learn and retain new information.     

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.