Dietary Phosphatidylcholine As a Precursor of Brain Acetylcholine

Lipids in Modern Nutrition, edited by M. Horisberger and U. Bracco. Nestle Nutrition, Vevey/Raven Press, New York © 1987. Dietary Phosphatidylcholine as a Precursor of Brain Acetylcholine Jean Mauron and Peter Leathwood Nestle Research Centre, Nestec Ltd., CH-1000 Lausanne 26, Switzerland Degeneration of cholinergic neurons and decreases in cholinergic function occur in several clinical syndromes. In tardive dyskinesia, senile dementia of the Alz- heimer type (SDAT), Huntington's disease, myasthenia gravis, and even in the normal process of growing old, more or less specific cholinergic deficits have been described (1-4). In some cases, treatment with cholinomimetic drugs such as phy- sostigmine have brought improvement (5-7), but the unpleasant side effects, short duration of action, and extreme variability of individual sensitivity to cholinomi- metics means that their use is clinically impractical. Recently, an alternative strat- egy for increasing cholinergic function has been suggested. Several research groups (8-12) have shown in animals that, under appropriate circumstances, treatment with choline or phosphatidylcholine can accelerate acetylcholine synthesis. This has led to the hope that ingestion of these neurotransmitter precursors might influence cholinergic function, without producing the side effects seen with drugs. Studies on elderly animals have produced quite promising results (13—15), but so far, clinical trials have not been very successful (16). Some improvements have been seen in patients with tardive dyskinesia (17) and Alzheimer disease (18), but most have been negative. It is, however, worth remembering that initial trials with L-dopa in the treatment of Parkinson's disease were not very successful either (see ref. 19 for a review). In this chapter, we will examine the rationale for using phosphatidylcholine in the treatment of cholinergic deficiency (with particular reference to treating senile mental decline and senile dementia) and critically review some of the clinical trials. SYNTHESIS OF ACETYLCHOLINE Synthesis of the neurotransmitter acetylcholine is catalyzed by the enzyme choline acetyltransferase (Fig. 1). Reported values of the KM for choline and for acetyl coenzyme A are 400 to 600 |XM and 6.6 to 18 JJLM, respectively (20). Whole brain levels of choline are 133 134 PHOSPHATIDYLCHOUNE AND ACETYLCHOLINE RCETYLCOENZYME R CHOLINE Choiine Rcetyltransferase COENZYME H * ^ RCETYLCHOLINE FIG. 1. Synthesis of acetylcholine. The reaction between acetyl coenzyme A and choline is catalyzed by the enzyme choline acetyltransferase. For precursor control to operate, the Mi- chaelis constant (KM) of the enzyme for choline must be similar to or greater than the normal intracellular concentration of choline. usually about 30 |XM, while acetyl coenzyme A varies between 2 and 20 (XM. This combination of observations has suggested to some that choline acetyltransferase must therefore be unsaturated in vivo so that the rate of acetylcholine synthesis will be precursor-dependent (21,22); others disagree (23). As Leathwood and Schlosser (24) have pointed out, the uncertainty about the real values of substrate concentration and kinetic constants within the microenvironment of the cell pre- cludes any strong conclusion from enzyme kinetic analyses about the possibility (or not) of precursor control of acetylcholine synthesis. SOURCES OF CHOLINE FOR THE CHOLINERGIC NEURON The cholinergic neuron can obtain choline from (a) de novo synthesis of choline in the brain; (b) reuptake of choline produced in the synaptic cleft by hydrolysis of released acetylcholine; and (c) uptake from the systemic circulation (via the interstitial fluid). Synthesis It has recently been confirmed that the brain can methylate phosphatidyletha- nolamine to phosphatidylcholine, which can then be broken down to yield choline. The estimated V^ for the synthesis is between 15 and 100 pmol/min/g of brain (25). This is far less than the average rate of synthesis of acetylcholine (about 6 nmol/min/g) (26) or the estimated rate of efflux of choline from the brain (about 4 nmol/min/g) (27). The phosphatidylcholine formed by this synthesis can be broken down to release choline by the action of phospholipase D, by base exchange, or by stepwise deacylation and subsequent breakdown of lysophosphatidylcholine or phosphocholine. The relative importance of these pathways in vivo is still not PHOSPHATIDYLCHOUNE AND ACEJYLCHOLINE 135 clear (Fig. 2) (25). Blusztajn et al. (21) have speculated that in Alzheimer's disease the (presumed) depletion of intracellular choline due to the high rate of acetylcholine synthesis in the remaining neurons may lead to autocannibalism of struc- tural phosphatidylcholine. This might explain the specific vulnerability of cholinergic neurons. Reuptake Acetylcholine released into the synaptic cleft is rapidly hydrolyzed and the choline taken up again by a high affinity (KT = 1 to 3 JAM) transport system and reuti- lized (28). Speth and Yamamura (29) have estimated that, under resting conditions, this cycle provides most of the choline needed for synthesis of acetylcholine. When the neuron is firing rapidly, reuptake is no longer sufficient. Uptake from the Interstitial Fluid The cholinergic neuron also contains a low affinity transport system for choline. At first it was thought that this system was only used to transport choline into the cell body for phospholipid synthesis. Later it was shown that in synaptosomes maintained under depolarizing conditions (to ensure that they were releasing acetylcholine rapidly), choline taken up by the low-affinity system also contributes to acetylcholine synthesis (30). Evidence that the low-affinity system feeds acetylcholine synthesis when the neuron is firing rapidly comes from several sources. Bierkamper and Goldberg (8) using the stimulated rat phrenic nerve-hemidia- phragm preparation showed that adding 30 or 60 JAM choline to the medium led to a dose-dependent increase in acetylcholine release. Adding choline to the unstimu- lated preparation had no effect on acetylcholine release. In slices of rat striatum, addition of 5 or 20 JAM choline to the bathing fluid produces a dose-dependent increase in acetylcholine release (21). Evidence for a similar relationship between firing frequency and precursor responsiveness in the whole brain has been obtained by unilaterally destroying some striatal neurons with kainic acid. After this treatment, systemic choline administration (sufficient to raise brain choline) had no effect on brain acetylcholine levels in the intact striatum but doubled them in the kainate-lesioned side (11). Similarly, as Wecker and Schmidt (12) showed, atro- pine (5-7 mg/kg) depletes caudal and hippocampal acetylcholine levels by 10% to 15%, presumably by accelerating release to such an extent that resynthesis cannot keep pace. Treatment with choline completely restores acetylcholine levels. TRANSPORT OF CHOLINE ACROSS THE BLOOD-BRAIN BARRIER Choline is carried bidirectionally across the blood-brain barrier (BBB) by a specific transport system (26). The carrier has a low affinity for choline with a KT of 0.44 mM so that it is unsaturated in the physiologically possible range of plasma Phos Phos p p •co 3" 3" 3- O a id o (B a y o \ CO CO T o M B > T > z •^ dyl z i - c • - | I— e ro UOUI U|p- S y n a p t I c FIG. 2. Sources of choline for brain neurons. The choline in cholinergic neurons is shown as deriving from three main sources: (i) Choline in extracellular fluid, which is in equilibrium with circulating choline and the choline present in other brain cells and which presumably enters the neuron by a low-affinity transport mechanism (1); (ii) intrasynaptic choline formed by ace- tylcholinesterase (2) from the acetylcholine released by the neuron (3), and then taken up into it by a high-affinity transport mechanism (4); (iii) neuronal phosphatidylcholine, from which the choline is liberated by base exchange (5)—the enzymatic substitution of a serine or an etha- nolamine for the choline—or phospholipase-mediated hydrolyses (6-13). (Other potential PHOSPHATIDYLCHOUNE AND ACETYLCHOUNE 137 choline concentrations. There is also abundant evidence that increasing plasma choline levels can raise brain choline. In rats, it is generally found that a twofold rise in plasma choline (from 10 u-M to 20 U,M) will produce a 20% to 50% rise in brain choline (i.e., from 30 nmol/g to 40 or 50 nmol/g). The exact values vary according to the details of the experimental design and the brain region used (see ref. 24 for a review). It is important to note that the BBB choline transport system is generally thought to produce a net efflux of choline from the brain (25,27,31)—but all au- thors do not agree (26)—and that increasing plasma choline levels raises brain choline by slowing the output. It is not clear where the excess choline leaving the brain actually comes from. The brain can synthesize choline but the amounts are small (usually estimated at less than 10% of the efflux) (26,31). One suggestion is that phosphatidylcholine or lysophosphatidylcholine in plasma may contribute to brain choline (27,31), but Pardridge et al. (26) claim that these molecules cannot be transported across the BBB. They suggest that the "efflux" of choline might be an artefact produced by contamination of venous (jugular) blood from noncerebral sources. Thus, the net loss of choline from the brain (if it really occurs) still lacks a convincing explanation. EFFECT OF INGESTED PHOSPHATIDYLCHOLINE ON PLASMA CHOLINE LEVELS Plasma choline is influenced both by phosphatidylcholine (and choline) in food and by synthesis and breakdown of choline in the liver. The diet usually furnishes 0.1 to 1 g of choline per day (22,32), with most in the form of phosphatidylcholine or sphingomyelin and less than 5% as free choline (Fig. 3). sources not shown include membrane plasmalogens and sphingomyelin.) The phosphatidylcholine in neurons is shown as being formed either by the incorporation of preexisting choline [by way of base-exchange (5) or cytidine diphosphatidyl-choline (14-16) pathways] or by its de novo synthesis.

Load more