Alzheimer's disease and acetylcholine receptors Takeshi Kihara and Shun Shimohama
Department of Neurology, Graduate School of Medicine, Kyoto University,54 Shogoin-Kawaharacho, Sakyoku, Kyoto 606-8507, Japan
Abstract. Cholinergic abnormalities, alongside senile plaques, neurofibrillary tangles, and extensive neuronal loss, are the major characteristics in Alzheimer's disease (AD). Both nicotinic and muscarinic acetylcholine
receptors are decreased in AD, and it has been shown that the reduction in thenumber of acetylcholine receptors precedes other pathologic changes. Anti-cholinergic drugs induce amnesia, which can be reversed by withdrawalof the medication. Inhibition of the down-regulation of acetylcholine is,therefore, a strategy for the treatment of AD because it could augmentacetylcholine levels within synaptic clefts. In this context,acetylcholinesterase inhibitors, which improve cognitive functions, arecurrently approved for the treatment of AD. Stimulation of acetylcholinereceptors, nicotinic or muscarinic, is another strategy; some drugs arecurrently under investigation, and reported to be effective. In addition,nicotinic stimulation exerts a neuroprotective effect, and reduces the amyloidburden. Cholinergic therapy may counter the symptoms and progress of AD.
The correspondence should beaddressed to S. Shimohama, Email:
Key words: Alzheimer's disease, amyloid-b, nicotinic acetylcholine receptor,
a4b2, a7, muscarinic acetylcholine receptor
INTRODUCTION
Epidemiological studies have suggested that smok-
ing is associated with a lower incidence of AD (Hillier
Alzheimer's disease (AD) is one of the neurodegener-
and Salib 1997, Lee 1994, van Duijn and Hofman
ative diseases presenting with dementia, and there are
1991), and an inverse association between smoking and
no definitive treatments or prophylactic agents. The
Alzheimer's disease has been suggested by some stud-
presence of two types of abnormal deposits, senile
ies. Nicotinic cholinergic stimulation might counter or
plaques and neurofibrillary tangles, and extensive
delay the development of AD. However, these ideas are
neuronal loss characterize the pathology of AD.
Cholinergic abnormalities have been observed in AD
In this review, the association of cholinergic abnor-
brains (Shimohama et al. 1986, Whitehouse et al. 1986).
malities and AD, and implications for the treatment of
It has been reported that the protein level of acetylcho-
line receptors is reduced in AD (Nordberg 2001), andthat dysfunction of cholinergic signal transmission
EPIDEMIOLOGICAL DATA OF AD
could be responsible for the symptoms of AD. In addi-tion, anti-cholinergic drugs, used for the treatment of
It is well known that aging is the most important risk
Parkinson's disease, induce amnesia, which clinically
factor for dementia including AD. The prevalence and
resembles the symptoms of AD (Bymaster et al. 1993).
incidence of dementia doubles every 5 years in persons
The amnesia induced by anti-cholinergic drugs can be
reversed by withdrawal of the drugs. This phenomenon
A family history of dementia in first-degree relatives
implies that augmentation of the concentration of ace-
has been shown to increase the risk of developing de-
tylcholine within the surviving synaptic clefts could
mentia. Genetic abnormalities have been found in some
families with autosomal dominant inheritance of AD.
Anti-cholinergic drug-induced amnesia is thought to be
Amyloid-b protein precursor (AbPP), presenilin 1 and
due to the blockade of muscarinic acetylcholine recep-
2, have been associated with early onset of autosomal
tors since it has also been reported that muscarinic
dominant inherited AD. It has been shown that these ge-
agonists improve memory (Terry Jr. et al. 2002).
netic abnormalities lead to the over-production of amy-
Conversely, it has been shown that smoking im-
loid-b, which is the major protein component of senile
proves arousal and attention, and memory. Nicotinic
acetylcholine receptor stimulation might enhance the
It is controversial whether smoking is associated with
formation of memory (Potter et al. 1999) alongside its
the incidence of AD. Some reports have indicated the
protective effect against the development of AD (Kihara
negative relationship between smoking and AD (Hillier
et al. 1997, 1998, 2001, Shimohama et al. 1996). Sub-
and Salib 1997, Lee 1994, van Duijn and Hofman
cutaneous administration of nicotine significantly im-
1991). Van Duijn and Hofman (1991) reported that the
risk of Alzheimer's disease decreased with the increase
Conners’ continuous performance test (CPT) (White
in the daily number of cigarettes smoked before onset of
and Levin 1999). It has been reported that a significant
disease (relative risk 0.3 in those smoking greater than
reduction in errors of omission in the CPT occurred
21/day vs. 1 in non-smokers). They concluded that there
throughout the period of chronic nicotine administra-
is an inverse association between smoking and AD, al-
tion. No improvement in motor or memory function
though smoking cannot be advocated for other health
was observed. Nicotine is known to act on presynaptic
reasons. Hebert et al. (1992) suggested that smoking
nicotinic acetylcholine receptors (nAChR) to enhance
does not increase the risk of AD. Lee (1994) reviewed
glutamatergic transmission. Nicotine from tobacco is
19 case-control studies on the association between AD
thought to influence cognition by enhancing synaptic
and smoking, and showed a highly significant
transmission. Conversely, decreased efficacy in trans-
(P<0.001) negative association (ever/never smokers,
mission may account for the deficits associated with the
relative risk (RR) 0.64, 95% confidence interval (CI)
loss of cholinergic innervation during AD. It is clear that
0.54-0.76). This negative or inverse association
smoking cannot be advocated for a variety of health rea-
suggests that nicotine protects against AD.
sons, but these data imply that nicotine could protect
Letenneur et al. (1994) showed, however, that al-
though current smokers and past smokers had a lower
AD and ACh receptors 101
risk of cognitive deficit than non-smokers, this signifi-
fusiform and bulbous, are surrounded by reactive glial
cant relationship disappeared after adjustment for po-
tential confounding factors such as occupational
A reduction in the number of nAChR in the cerebral
category. They suggested that the apparent protective
cortex of AD patients has been detected using ligand
effect of smoking habits on cognitive abilities could be
due to a confounding effect of occupational category.
Shimohama et al. (1986) showed that, not only nicotinic
Launer et al. (1999) reported that smoking did not pro-
receptors, but also muscarinic acetylcholine receptors
tect against AD, contrary to previous reports. Wang et
are decreased in AD brains. The number of [ H]nicotine
al. (1999) also observed that smoking does not seem
binding sites in the AD brain was significantly reduced
protective against AD or dementia. Other reports have
in the putamen and the nucleus basalis of Meynert.
suggested that smoking is one of the risk factors of AD
[ H]QNB binding was significantly reduced in the hip-
(Doll et al. 2000, Meyer et al. 1999). Almeida et al.
pocampus and nucleus basalis of Meynert. These find-
(2002) recently reviewed case-control and cohort stud-
ings suggest that there are significant changes in the
ies, and concluded that the nature of the association be-
level of both nicotinic and muscarinic cholinergic re-
tween smoking and AD remains unclear. Overall,
ceptors in selected regions of AD brains. Perry et al.
epidemiological data on the association between nico-
(1995) examined high-affinity nicotine binding, consid-
tinic acetylcholine dysfunction and AD is unclear, and
ered to primarily reflect the presence of CNS
much more information is required to establish the facts.
Muscarinic cholinergic abnormalities are also sus-
autoradiographically in the brain regions most severely
pected in AD brains. The cholinergic antagonist, sco-
affected by AD pathology. Abnormalities in the nico-
polamine, leads to memory impairment in humans
tinic receptor were closely associated with primary
(Drachman and Leavitt 1974, Ghoneim and Mewaldt
1977). It was reported that learning and acquisition of
neurofibrillary tangles in subicular and entorhinal areas
new information were impaired, and this phenomenon
in AD brains. Therefore, it is notable that abnormalities
is similar to the early amnesic symptoms of AD. An-
in nicotinic receptors, especially alpha4beta2 (a4b2)
other cholinergic antagonist, trihexyphenidyl, is pre-
nAChR, occur in the early stages of the pathological
scribed for the treatment of Parkinson's disease, and this
process, not only in AD but also in other neurodegenera-
drug sometimes induces amnesia in these patients.
tive diseases. It is possible that nicotinic receptor
These findings suggest that both nicotinic and
down-regulation precedes neurodegeneration and the
muscarinic acetylcholine receptors may be involved in
difference between the diseases might depend upon the
AD pathogenesis, although the precise mechanism re-
distribution of the abnormal nicotinic receptors.
Nordberg (2001) showed that the protein content of
alpha4 (a4), alpha3 (a3), and alpha7 (a7) nAChR is re-
PATHOLOGY ASSOCIATED WITH
duced in AD brains. The regional pattern of messenger
ACETYLCHOLINE RECEPTORS
RNA (mRNA) for nAChR does not strictly follow theregional distribution of nAChR ligand-binding sites in
The presence of two deposits, senile plaques (SPs) and
the human brain. Alpha4 and alpha3 mRNA levels were
neurofibrillary tangles (NFTs), and extensive neuronal
not changed in AD brains and the mRNA level of the
death characterize the pathology of AD. Cholinergic ab-
alpha7 nAChR was increased in the hippocampus.
normalities, such as the loss of presynaptic cholinergic
These findings indicate that the subunit-specific
markers in the cerebral cortex, are also found in AD brains.
changes in gene expression and the consequent loss of
SPs are extracellular structures composed of
nicotinic-binding sites are not due to alterations at the
congophilic, fibrillar amyloid. There are two major
transcription level. PET studies revealed deficits in
plaque types: diffuse plaques and neuritic plaques. They
nAChRs as early phenomena in AD, stressing the im-
are found in AD, and also in some non-demented elderly
portance of nAChRs, which is consistent with the patho-
persons. Plaques with more amyloid and containing
logically predicted data (Perry et al. 2000). Also,
more abundant dystrophic neurites are called neuritic
nAChRs are considered to be a potential target for drug
plaques. In these, the amyloid cores form fibrils staining
intervention. The discrepancy between the protein level
with thioflavin-S. Neuritic plaques with neurites, both
and mRNA level of nAChRs implies that translational
and/or posttranslational modification might be damaged
mation. CREB, cAMP-regulatory element binding pro-
in AD brains. One possibility is the posttranslational
tein, is thought to be one of the most important molecu-
modification of nAChR by amyloid beta (Ab).
lar components for hippocampus-dependent memoryformation in mammals. From this point of view, block-
AMYLOID HYPOTHESIS ON
ade of the association between a7 nAChR and Ab might
MEMORY DEFICIT AND nAChR
be a strategy for the treatment of AD.
AD pathology is characterized by SPs which are
AMYLOID-INDUCED TOXICITY
composed of Ab. In addition, several mutations found in
AND nAChR
familial AD are involved in amyloidogenesis. It hasbeen shown that familial AD mutations in presenilin 1
There is still controversy over the role of Ab in the
(PS-1) enhance the generation of Ab1-42, indicating that
neuronal loss found in AD brains. However, evidence is
PS-1 is involved in amyloidogenesis (Citron et al.
accumulating that Ab causes neuronal death in many
1992). Ab must contribute to the pathogenesis, espe-
culture systems. Amyloid accumulation is one of the
earliest changes in AD pathology, and this peptide may
Walsh et al. (2002) reported that Ab oligomers in-
cause neuronal death in the CNS. The precise mecha-
hibit hippocampal long-term potentiation (LTP). Ab
nism of Ab -induced cytotoxicity remains unknown, al-
oligomers, in the absence of monomers and amyloid fi-
though various hypotheses have been suggested. It has
brils, disrupted synaptic plasticity at concentrations
been reported that oxidative stress or free radical gener-
found in human brain and cerebrospinal fluid. This phe-
ation may mediate Ab -induced cytotoxicity (Behl et al.
nomenon might be one of the major mechanisms of
1994). Ab stimulates nitric oxide (NO) production in
memory disturbance found in the early stages of AD.
astrocyte culture (Akama et al. 1998) and also calcium
Itoh et al. (1999) showed that nicotinic signaling was
entry, triggered by activated N-methyl-D-aspartate
impaired in Ab-infused rats using an extracellular re-
(NMDA)-gated channels (Le et al. 1995). This might
cording technique on hippocampal slices. LTP in CA1
cause peroxynitrite generation and lead to cell death.
pyramidal cells was also impaired in the Ab-infused
Other reports suggested that Ab inhibited glutamate up-
rats, and it was suggested that this dysfunction may be
take (Harris et al. 1996). These reports imply that Ab
-induced cytotoxicity might be mediated via glutamate
Recently it was reported that Ab, especially fragment
toxicity. There are also some reports suggesting that Ab
1-42, binds to a7 nAChR. Binding of Ab inhibits a7
enhances the toxicity induced by excitotoxin (Dornan et
nAChR-dependent calcium influx, which could explain
al. 1993, Morimoto et al. 1998). We also showed that
the cognitive deficit of AD (Wang et al. 2000).
Ab25-35 activity is mediated via NMDA receptors, and
Immunocytochemical studies on human sporadic Alz-
that Ab1-40 and Ab1-42 enhance glutamate neurotoxicity,
heimer's disease brains have demonstrated that Ab1-42
which was mediated via NMDA receptors. An imbal-
and a7 nAChR are both present in neuritic plaques and
ance in glutamate signals leading to survival or death is
co-localize in individual cortical neurons. Ab1-42 and a7
the point of glutamate-induced neuronal death, and Ab
nAChR can be co-immunoprecipitated, suggesting that
alters this balance to make neurons vulnerable to gluta-
they are tightly associated. Receptor binding experi-
ments confirmed this association. Human neuro-
Nicotinic receptor stimulation inhibits Ab toxicity
blastoma cells with a7 nAChR are killed by Ab1-42, and
(Kihara et al. 1997, 1998, 2001) and glutamate toxicity
nicotine or epibatidine inhibited this death. In addition,
(Shimohama et al. 1996), and a7 receptors, in particu-
Ab1-42 inhibits a7 nAChR-dependent calcium activa-
lar, contribute to PI3K-Akt phosphorylation, which is
tion and acetylcholine release, which may be involved
important for protection. In addition, the Bcl-2 family
exists downstream of the PI3K-Akt cascade and works
Dineley et al. (2001) indicated that Ab activated the
as an anti-neuronal death factor. Furthermore, nicotine
mitogen-activated protein kinase (MAPK) cascade via
modulates signal transduction to maintain the PI3K cas-
a7 nAChR. Ab-induced activation through a7 nAChR cade which might be down-regulated by Ab . Glutamatemight downregulate the MAPK-CREB phosphorylation
also activates the PI3K system, which might protect
system, which leads to the dysfunction of memory for-
cells from radical formation-induced injury. We hy-
AD and ACh receptors 103
pothesized that Ab-induced collapse of this system is
(Clark and Karlawish 2003, Kapaki et al. 2003). Four
the cause of vulnerability. The precise mechanism re-
AChEI have been approved: tacrine (Cognex ),
mains unknown, but nicotinic stimulation might up-reg-
donepezil (Aricept ), galantamine (Reminyl ), and
ulate the PI3K cascade, which would contribute to
rivastigmine (Excelon ). These drugs produce the same
maintain viability. Ab binds to a7 nicotinic receptors
degree of modest improvement in approximately
(Wang et al. 2000), which might cause vulnerability be-
30-40% of patients with mild to moderate AD. The ef-
cause of the reduction in nicotinic signal transduction.
fect of AChEIs may depend on augmented acetylcho-
Also, competitive stimulation of a7 nicotinic receptors
line levels. However, it has been reported that
might rescue cells from glutamate or NMDA recep-
galantamine allosterically modulates nicotinic acetyl-
choline receptors in addition to the effect of AChEI
Neuronal loss is one of the characteristics of AD pa-
(Maelicke and Albuquerque 2000). Direct stimulation
thology, and neuronal death may be induced by Ab.
of the nAChR might enhance the improvement of cogni-
Stimulation of nAChR could inhibit neuronal death,
tion. Recently, it has been shown that donepezil has a
which would counter the progress of AD pathogenesis.
protective effect on glutamate-induced neuronal deaththrough a7 nAChR (Takada et al. submitted). In our
CLINICAL TRIAL
study, galantamine also exerted a neuroprotective effecton Ab- and glutamate-induced neurotoxicity (Kihara et
Cholinergic abnormalities are found in AD brains as de-
al. submitted). There may be many more possibilities for
scribed above. Bymaster et al. (1993) showed that the
these drugs, and some clues might be found for the devel-
muscarinic antagonists scopolamine and trihexyphenidyl
opment of more effective drugs for the treatment of AD.
bind muscarinic acetylcholine receptors, and that these,but not a nicotinic antagonist, impaired memory perfor-
CONCLUSIONS
mance in a spatial alternation task in rats. In particular, ithas been reported that M1 receptors are associated with
Cholinergic replacement therapy, using AChEIs, is
muscarinic antagonist-induced amnesia. In addition, it has
currently available and effective for the treatment of
been shown that M1 receptors are involved in memory
AD. It is, however, controversial whether acetylcholine
processes using M1 receptor mutant mice (Anagnostaras
receptor agonists, nicotinic or muscarinic, would im-
et al. 2003). Conversely, muscarinic agonists could im-
prove the symptoms of AD. Recent data has indicated
prove working memory. However, some reports showed
that AChEIs possess a nicotinic receptor modulating ef-
little effect of muscarinic agonists on memory formation
fect, which might enhance the cognition improving ef-
fect. Abnormalities of the cholinergic system are
Alternatively, it has been reported that nicotinic re-
prominent findings in AD, beside senile plaques,
ceptor stimulation improves memory. The selective
neurofibrillary tangles and neuronal loss. Appropriate
cholinergic channel activator (nicotinic agonist),
and timely stimulation of acetylcholine receptors is nec-
ABT-418, significantly improved recall failure on a ver-
essary for the treatment of AD and it is important to
bal learning task in AD patients (Potter et al. 1999).
develop such drugs as soon as possible.
Qualitatively similar improvements were seen innon-verbal learning tasks such as spatial learning and
ACKNOWLEDGEMENTS
memory, and repeated acquisition. Stimulation of cen-tral nicotinic receptors is shown to have an acute
This work was supported in part by Grants-in-Aid for
Scientific Research from the Ministry of education,
Both nicotinic and muscarinic receptors seem to be
Culture, Sports, Science and Technology of Japan, and
associated with memory disturbance, and stimulation of
grants from the Ministry of Health, Labor and Welfare
these receptors may be efficacious for the treatment of
of Japan and the Smoking Research Foundation.
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Received 12 May 2003, accepted 21 May 2003
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