HOW ANTICHOLINERGIC DRUGS MIGHT PROMOTE ALZHEIMER’S DEMENTIA:
MORE ABETA AND LESS PHOSPHATIDYLCHOLINE
Richard J. Wurtman, MD, Massachusetts Institute of Technology, 77 Mass Ave., Cambridge,
MA 02139 USA
Corresponding author:
Richard J. Wurtman
77 Mass Ave., Bldg. 46-5009, Cambridge, MA 02139
Phone: 617-253-6731; FAX 617-253-6992
Emal: dick@mit.edu
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ABSTRACT
Drugs that block muscarinic cholinergic neurotransmission in the brain can, as a consequence,
increase the formation of Abeta, and decrease brain levels of phosphatidylcholine (by slowing its
synthesis and accelerating its turnover). Both of these effects might cause a decrease in brain
synapses, as characterizes and probably underlies the memory disorder of early Alzheimer’s
disease.
Key words: acetylcholine, choline, amyloid, neurotransmission, synaptic membrane, synapses
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INTRODUCTION
A decrease in cortical and hippocampal synapses is an early and robust finding in AD
brains (1), and generates memory impairments by weakening the connections among brain
regions that underlie memory (2). This decrease is widespread, however it is most pronounced
among cholinergic synapses - numbers of which may decline by 90% (3,4) - and is ultimately
accompanied by the loss of cholinergic parikarya in the basal forebrain and septum (5).
The decrease in synaptic number could result from either of two processes, or from a
combination of both: an acceleration in their breakdown, perhaps caused by an endogenous
neurotoxin like Abeta42 polymers (6), or a slowing in their formation (which, in normal brains,
occurs continuously, replacing synapses that disappear after their short life-spans of days to
weeks [7]). The decrease in synapses in AD brain is associated with a parallel decline in the
numbers of dendritic spines - the essential anatomic precursors for excitatory brain synapses; this
has been observed both in animal models of AD (8) and in brains of AD patients (9).
A recent epidemiologic study (10) has shown that the high cumulative use of
anticholinergic drugs may promote the development of AD and other dementing diseases. It had
been known that single doses of muscarinic receptor blockers, like scopolamine, could acutely
impair short-term memory, and that the impairment would subside if subjects were also given an
acetylcholinesterase inhibitor to raise intrasynaptic acetylcholine levels (11). However the
possibility that anticholinergics could also produce long-term damage was generally
unrecognized. This neurotoxic effect apparently is independent of an anticholinergic agent's
chemical family or its other pharmacological properties, occurring with drugs as diverse as
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antihistamines, antispasmodics, antiemetics, and antipsychotics: Apparently all that is required is
that the drug block cholinergic receptors (10). Hence the repeated blockade of central cholinergic
neurotransmission, like AD itself, probably decreases cortical and hippocampal synapses,
probably also by accelerating their breakdown or slowing their production or both.
How might the chronic drug-induced blockade of central cholinergic transmission
promote the development of AD? At least two established biochemical mechanisms, both
affecting synaptic number, could underlie such an effect. One would involve increasing the
production of potentially-toxic Abeta peptides and their oligomers (12) and decreasing the AChstimulated hydrolysis of the amyloid precursor protein (APP) (13-16) which forms nonamyloidogenic intermediates. The other mechanism, operating principally in cholinergic brain
neurons, would impair synaptogenesis by lowering the levels of phosphatidylcholine (17,18) and
other, related constituents of synaptic membrane. As discussed below, the activation of m1 and
m3 muscarinic receptors (or other receptors that generate diacylglycerol (DAG) from
phosphatidylinositol [PI]) is known to inhibit the cleavage of APP by beta- and gamma-secretase
enzymes which would form Abeta and its oliogomers, and to enhance the activity of alphasecretase enzymes which cleave APP to form non-amyloidogenic metabolites (13-16). Thus, as
might be expected, the blockade of m1 and m3 receptors by muscarinic antagonists enhances
Abeta synthesis and suppresses alpha-secretase activity (12).
The depolarization of cholinergic neurons also affects, indirectly, the breakdown of the
PC in neuronal membranes (17 18) and slows its synthesis. PC breakdown liberates free choline,
most of which is diverted for ACh synthesis and away from regenerating PC (17). It thereby
reduces levels of PC and other membrane phosphatides, thus diminishing the quantity of neural
membrane available for synaptogenesis. In AD the major loss of cortico-hippocampal
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cholinergic synapses very likely increases the firing frequencies of the surviving terminals;
moreover their firing is further increased if a muscarinic antagonist has blocked postsynaptic
responses to the ACh. The higher firing frequency enhances PC breakdown and conversion of
the liberated choline to ACh, not PC (17). And if, in addition, a muscarinic receptor antagonist
has been administered, the resulting blockade of presynaptic receptors will further increase the
loss of ACh and choline from the terminal, and shunt free choline into forming ACh and not
PC (17,20,21). Hence PC levels fall, - as has been observed in frontal and parietal regions of
AD brain [19 ]. The enhanced turnover of membrane PC to liberate choline which is then used
for ACh synthesis reflects a kind of "autocannibalism" [18- 21])
This review briefly describes these two mechanisms, and suggests ways of ameliorating their
consequences in AD patients.
CHOLINE, CHOLINERGIC TRANSMISSION, AND THE PC IN SYNAPTIC
MEMBRANE
The enzymes choline acetyltransferase (CAT), which acetylates choline to form ACh,
and choline kinase (CK), which phosphorylates choline to initiate PC synthesis, both have
unusually low affinities for their choline substrate; their Km's are 540 uM and 2.6 mM
respectively, while the brain's choline concentration is only about 35uM (22,23). Hence both
enzymes are highly unsaturated in vivo, and even small changes in brain choline levels can have
important effects on the rates at which it is acetylated or phosphorylated. As discussed below,
the conversion of choline to PC involves a three-step process: first CK converts the choline to
phosphocholine; that compound then combines with cytidylyltriphosphate (CTP) to form CDPcholine; and the CDP-choline then combines with particular diacylglycerol (DAG) molecules
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which contain an omega-3 acid (i.e., DHA or EPA) to yield the PC. The CTP and the DHAcontaining DAG species needed for this pathway are formed in brain from circulating uridine
and omega-3 fatty acids, by enzymes that are also unsaturated with these substrates (24,25).
Circulating choline derives both from dietary sources (e.g., egg yolks, in the form of PC)
and, probably to a greater extent, from the hepatic synthesis and hydrolysis of PC (25), formed
by the sequential, B-vitamin-dependent methylation of phosphatidylethanolamine [PE] ) (26),
The recommended dietary intake of choline by normal adults is about half a gram per day,
however many women and older people of both sexes fail to attain this intake. Moreover basal
choline requirements may be significantly greater among individuals who, unknowingly, have
common variations in gene alleles for some of the hepatic enzymes involved in PE methylation
or folic acid metabolism (27). An increased need for dietary choline - e.g., for people carrying
such alleles, or in AD patients who might benefit from accelerating synaptogenesis, as described
below - can, to some extent, be satisfied by providing oral choline. However doses in excess of
500 mg cause many users to develop an unacceptable "fishy body odor" caused by choline's
intra-intestinal breakdown to trimethylamine, Thus the liver’s contribution to plasma choline
may also need to be enhanced, by administering the B-vitamins that promote the methylation of
PE (26).
Circulating choline enters the brain principally via a concentration-dependent, facilitated
diffusion mechanism, mediated by a transport protein in endothelial cells that line the brain's
capillaries. PC and lysoPC are not substrates for a transport protein, and enter the brain only
poorly. Once in the brain's extracellular fluid choline enters all cells by a low-affinity transport
system, or is taken up into terminals of ACh neurons by a high-affinity uptake mechanism. As
discussed below, much of the choline used for ACh synthesis in rapidly-firing neurons, or
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following administration of an anticholinergic drug, derives from the local breakdown of
membrane PC (17).
The initial experimental evidence that choline used for ACh synthesis can derive from
the breakdown of membrane PC came from studies on electrically-stimulated, superfused slices
of rat corpus striatum that were exposed to various concentrations of free choline (17, 18).
Stimulation of slices in a choline-free medium for four or six 20-minute periods caused 4-fold
increases in ACh release, with no reduction in tissue ACh levels. The total amount of ACh
released during the three-hour superfusion period was about three times the initial amount in the
tissues, and the amount of free choline released was even greater, i.e. 45 times the initial or final
amounts in the tissues.
Stimulation also caused significant 14% reductions in tissue PC levels, and comparable
reductions in levels of protein (10.5%) and in the other principal membrane phosphatides (PE
and PS). This indicated that the fall in PC represented the loss of both major membrane
constituents, phosphatides and proteins (17). The reductions in phosphatide levels and the
increase in ACh release were all blocked by addition of tetrodotoxin to the medium, indicating
that they were mediated by neuronal depolarization. Moreover slices of cerebellum - a tissue
with, unlike striatum, minimal cholinergic innervation - neither released significant amounts of
ACh during stimulation nor exhibited post-stimulation reductions in phosphatide or protein
contents. This suggested that most of the reductions in membrane constituents in stimulated
striatal tissue (and most of the choline used for ACh synthesis) had originated in synaptic
membranes of cholinergic neurons.
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Addition of choline (10-40 uM) to the media partially or completely blocked the
stimulation-induced reductions in striatal phophatides and protein, the highest choline
concentrations actually increasing tissue levels of all of the phosphatides. This suggested that,
during stimulations, the tissue retained the capacity to produce neural membranes, but that,
without supplemental choline, most of the newly-synthesized membrane was broken down to
provide choline for ACh synthesis.
It can be hypothesized that chronic treatment with anticholinergic drugs has effects on
PC breakdown and choline utilization that are similar to those observed after electrical
stimulation of striatal slices, since blockade of postsynaptic muscarinic receptors in vivo would
cause a reflex increase in the firing frequency of the presynaptic cholinergic neurons, and
blockade of the presynaptic receptors on those neurons would very much increase their release
of ACh and consequent loss of choline.
CHOLINERGIC TRANSMISSION AND ABETA FORMATION
The activation of muscarinic receptors that are coupled to phosphatidylinositol (PI)
breakdown, DAG formation, and protein kinase C (PKC) activation enhances the activity of
alpha-secretase enzymes that convert APP to non-amyloidogenic peptides, and suppresses the
formation by beta- and gamma-secretases of the amyloidogenic Abeta peptides (6). Thus
administration of carbachol (13), a general muscarinic agonist, or drugs like AF 102B which
specifically target M1 (28) or M3 (29) receptors, decreases Abeta levels and increases nonamyloidogenic peptides in brain. While the general muscarinic antagonist scopolamine (12) or
the selective M1 antagonist dicyclomine (30) promote amyloidogenic APP processing (31).
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It can thus be understood that chronic frequent administration of muscarinic antagonists might
cause prolonged increases in brain Abeta formation, and thereby, as Gray et al have shown (10)
constitute a risk factor for Alzheimer's disease and possibly other dementias.
SOME IMPLICATIONS OF THESE RELATIONSHIPS
Clearly, though AD brains ultimately show major reductions in most varieties of
synapses and neurons, the decrease in cholinergic synapses may be of special importance, both
because muscarinic cholinergic neurotransmission is essential for normal memory and because,
when impaired, this has effects on phosphatide availability and Abeta formation which may
extend the synaptic loss. And if cholinergic transmission is further compromised by chronically
administering anticholinergic drugs, the likelihood of developing dementia is thereby enhanced
(10). In contrast, a cholinergic agonist might be expected to have opposite and beneficial effects:
It might act postsynaptically to decrease Abeta production and, by slowing the neuron’s firing
frequency, increase the levels of phosphatides that can be used to generate synaptic membranes.
And acting presynaptically the agonist might further reduce the loss of choline to ACh
production, thus allowing more to be used to form PC.
This hypothesis might be tested by determining whether the chronic administration of
presently-utilized acetylcholinesterase (AcChE) inhibitors - which are presumed to increase
intrasynaptic ACh – actually elevate ACh release into cerebrospinal fluid and, if they do,
whether the dose-response relationship of the ACh increase parallels that for reducing CSF
Abeta levels. Higher drug doses might be required for these effects than the doses presently used
to treat patients with early AD, and even those relatively low current doses are sometimes
associated with unacceptable peripheral side-effects.
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Another implication of the ability of cholinergic antagonists to promote dementing
diseases is that sufficient choline needs to be provided so that adequate PC production can
continue. This might most effectively be done by administering the choline as a component of a
nutrient mixture that contains all three of the PC precursors which limit PC’s rate of synthesis –
uridine as its monophosphate; DHA or EPA; and choline (24). This mixture has been shown to
diminish Abeta formation in experimental animals (32), and could have the added benefit of
partially restoring the deficient brain synapses.
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