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THE JOURNAL OF PHARMACOLOGY AND EXPERIMENTAL THERAPEUTICS
Copyright © 1997 by The American Society for Pharmacology and Experimental Therapeutics
JPET 281:1408 –1414, 1997
Vol. 281, No. 3
Printed in U.S.A.
Functional Activation of Cerebral Blood Flow Abolished by
Scopolamine Is Reversed by Cognitive Enhancers Associated
with Cholinesterase Inhibition: A Positron Emission
Tomography Study in Unanesthetized Monkeys
HIDEO TSUKADA, TAKEHARU KAKIUCHI, ICHIRO ANDO and YASUOMI OUCHI
Central Research Laboratory, Hamamatsu Photonics K.K., Shizuoka, Japan (H.T., T.K., I.A.), Subfemtomole Biorecognition Project, Japan
Science and Technology Corporation, Osaka, Japan (H.T.), and Positron Medical Center, Hamamatsu Medical Center, Shizuoka, Japan (Y.O.)
ABSTRACT
The effects of somatosensory stimulation on the regional cerebral blood flow (rCBF) response were studied in unanesthetized
monkeys before and after treatment with scopolamine and
three cognitive enhancers (physostigmine, E2020 and tacrine)
that inhibit cholinesterase, using 15O-labeled water and highresolution positron emission tomography. Under control conditions, somatosensory stimulation induced a significant increase in the rCBF response in the contralateral somatosensory
cortex of monkey brain. Intravenous administration of scopolamine (50 mg/kg) resulted in abolishment of the rCBF response
to stimulation. The rCBF response abolished by pretreatment
The tight coupling between neuronal activity, rCBF and
rCMR has been reported under physiological conditions. It is
assumed that both glucose and oxygen demand increase the
rCBF of the activated brain region. This provides the basis
for activation studies using PET and functional magnetic
resonance imaging, in which neuronal activation can be monitored as an increased rCBF response (for review, see Roland,
1993). On the other hand, the rCBF response to stimulation
is very rapid (Lindauer et al., 1993), and the rCMR of oxygen
response is much less than the rCBF response during physiological stimulation (Fox and Raichle, 1986; Fox et al.,
1988b). These results suggest the involvement of neuronal
mechanisms for the coupling of neuronal activity and rCBF
during stimulation. Several animal studies demonstrated
that the cortical neuronal activity was influenced by cholinergic projections from the basal forebrain (Kiyosawa et al.,
1989; Kurosawa et al., 1989; Ouchi et al., 1996; Sato and
Sato, 1992). Also, cholinergic muscarinic agonists were reported to increase rCBF in the cerebral cortex or pial vessel
Received for publication September 23, 1996.
with scopolamine was recovered by administration of physostigmine (1 or 10 mg/kg), E2020 (10 or 100 mg/kg) or tacrine
(100 or 1000 mg/kg), in a dose-dependent manner. The effect of
E2020 (100 mg/kg) on the rCBF response lasted for .4 hr,
whereas the effects of physostigmine and tacrine were of
shorter duration. These findings suggest that these compounds
reversed the scopolamine-abolished rCBF response to somatosensory stimulation via enhancement of cholinergic neurotransmission, which was mainly induced by cholinesterase
inhibition.
dilation with no increase in rCMR (Scremin et al., 1982).
Recently, it was reported that the systemic administration of
scopolamine, a muscarinic cholinergic receptor antagonist,
abolished the rCBF response to somatosensory stimulation
(Ogawa et al., 1994; Tsukada et al., 1997). The abolishment of
the rCBF response by scopolamine was recovered by administration of physostigmine, a cholinesterase inhibitor
(Tsukada et al., 1997), whereas the rCMR response was not
affected by scopolamine (Ogawa et al., 1994) or physostigmine (Tsukada et al., 1997) administration. It was reported
that this reactive rCBF increase was regulated by the intrinsic cholinergic neurons, which might have an important role
in mediating the neuronal activity level in the blood vessels
and dilating vessels locally to adjust glucose and oxygen
supply (Fukuyama et al., 1996). Taken together, these results
strongly suggested cholinergic regulation of rCBF responses
to physiological stimulation.
In experimental animals, alterations of cholinergic systems by pharmacological blockade or lesions of the cholinergic projection neurons are accompanied by cognitive deficits. The muscarinic antagonist scopolamine is the most
ABBREVIATIONS: AChE, acetylcholinesterase; AD, Alzheimer’s disease; BuChE, butyrylcholinesterase; E2020, 1-benzyl-4-[(5,6-dimethoxy-1indanon)-2-yl]methylpiperidine hydrochloride; PET, positron emission tomography; rCBF, regional cerebral blood flow; rCMR, regional cerebral
metabolic rate; ROI, region(s) of interest.
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Accepted for publication January 29, 1997
1997
Cholinergic System and rCBF Response
Materials and Methods
Animals and drugs. Four young-adult male rhesus monkeys
(Macaca mulatta) weighing 5 to 8 kg were used for the PET measurements. Monkeys were maintained and handled in accordance
with the recommendations of the United States National Institutes
of Health and the guidelines of the Central Research Laboratory,
Hamamatsu Photonics. They were trained to sit in a chair twice each
week for .3 months. At least 1 month before the PET study, an
acrylic plate, with which the monkey was fixed to a monkey chair,
was attached to the head of each monkey, as described previously
(Onoe et al., 1994).
Scopolamine hydrobromide and ketamine hydrochloride were purchased from Kyorin Pharmaceutical Co. (Tokyo, Japan) and Sankyo
Co. (Tokyo, Japan), respectively. Physostigmine sulfate and tacrine
were obtained from Kanto Chemical Co. (Tokyo, Japan) and Aldrich
(Milwaukee, WI), respectively. E2020 was synthesized according to
the previously described procedure (Iimura et al., 1989; Sugimoto et
al., 1992). All chemicals were dissolved in physiological saline solution (0.9% NaCl).
Physiological monitoring. During PET scanning, heart rate
and body temperature were continuously monitored using a life
monitoring system (Nihon Kohden, Tokyo, Japan). The results
shown were obtained 30 min after each drug treatment. Because of
the technical limitations with the use of unanesthetized monkeys,
PaCO2, PaO2, pH and blood pressure of arterial blood were measured
under pentobarbital anesthesia. Arterial blood samples were obtained 30 min after each drug administration, via a cannula in a
femoral artery, and monitored with a STAT PROFILE blood gas
analyzer (NOVA Biomedical, Waltham, MA).
PET experiment. Data were collected by using a high-resolution
PET scanner (SHR-2400; Hamamatsu Photonics, Hamamatsu, Japan), with transaxial resolution of 3.0-mm full width at half-maximum in the center of the scan field and a center-to-center distance of
6.5 mm (Watanabe et al., 1992). Animals were lightly sedated with
i.m. injections of ketamine (5 mg/kg) and were seated in the monkey
chair. During the sedation period, the head of the monkey was fixed
to the chair, stereotaxically aligned parallel to the orbitomeatal line.
Four hours after ketamine injection, at which time the monkey
showed complete recovery from the effects of ketamine, the study
was started under dim light.
PET experiments were performed in accordance with the method
reported previously (Tsukada et al., 1997). Monkeys were treated
with i.v. administration of saline or scopolamine (50 mg/kg) 30 min
before the PET measurements. In the experiments with cholinesterase inhibitors, physostigmine (1 or 10 mg/kg), E2020 (10 or 100
mg/kg) or tacrine (100 or 1000 mg/kg) was administered after the PET
measurements with scopolamine, and 30 min was allowed before
PET measurements were restarted. In separate experiments, physostigmine (10 mg/kg), E2020 (100 mg/kg) or tacrine (1000 mg/kg) was
given to the monkey after the PET measurements with saline, and
PET measurements were repeated.
For the duration studies, physostigmine (10 mg/kg), E2020 (100
mg/kg) or tacrine (1000 mg/kg) was given to the monkey 30 min after
administration of scopolamine, and PET measurements were performed 0 (just before administration), 1, 2 and 4 hr after the administration of cholinesterase inhibitors. In the case of scopolamine
alone, PET measurements were performed 1, 2 and 4 hr after administration of scopolamine.
A bolus injection of [15O]H2O (1.2 GBq in 3.5 ml of saline) was
delivered through a venous cannula placed into a sural vein, using a
specially designed automatic injector (Hamamatsu Photonics). We
used vibrotactile stimulation of the right forepaw for stimulation.
Stimulation was given at the start of [15O]H2O injection. Twenty
seconds after injection, when radioactivity reached the brain, PET
scanning was started and continued for 1 min. Three scans in each of
the nonstimulated and stimulated states were performed randomly
under each condition.
Data analysis. Data analysis of PET experiments was performed
in accordance with the method reported previously (Tsukada et al.,
1997). PET images were reconstructed using data integrated
throughout the scans for 1 min, determined from all slices from
reconstructed images in each scan, and these values were used to
normalize all data from the experiments of that day (Fox and Mintun, 1989; Fox et al., 1988a; Takechi et al., 1994). The images were
then smoothed with a median filter with four pixels next to each
pixel. Stimulation and control studies performed within 30 min were
paired. Three sets of stimulation minus resting subtracted images
were averaged and smoothed with a core r 5 2 median filter. The
averaged pixel values and the S.D. were calculated from all pixel
values in each slice of subtracted images. The pixels exceeding a
statistical criterion of P , .05 (two-tailed), corresponding to a Z score
of 1.96 or greater, were superimposed on magnetic resonance images
of the same subject, which were obtained with a Toshiba MRT-50A/II
(0.5-T) scanner. The stereotaxic coordinates for PET and magnetic
resonance imaging were adjusted based on the orbitomeatal line,
with a specially designed head-holder (Takechi et al., 1994). Because
three subtracted image pairs were obtained under each condition,
pixel-by-pixel statistical analysis was not performed (Friston et al.,
1990; Worsley et al., 1992). To statistically compare the data between
conditions, the ROI in the contralateral somatosensory cortex corresponding to a Z score of 1.96 or greater was obtained from the
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frequently used drug in studies of cognitive dysfunction. Administration of scopolamine produces transient cognitive impairment in various learning paradigms in both animals and
humans, and several cognitive enhancers have been reported
to prevent the scopolamine-induced disruption of memory
function (Honer et al., 1987; Sitaram et al., 1978; Summers et
al., 1986). Many compounds have been developed as centrally
acting acetylcholine enhancers to improve cognitive deficits
in subpopulations of patients with AD. In particular, cholinesterase inhibitors such as physostigmine and tacrine (tetrahydroaminoacridine) were reported to improve learning
and memory (Davis and Mohs, 1982; Summers et al., 1986).
Long-term administration of tacrine induced neurochemical
changes such as increased nicotinic receptor binding and
increased glucose metabolism, as measured by PET, in the
temporal and frontal cortices of patients with AD, in whom
neurochemical changes were paralleled by improvements in
neuropsychological tests (Nordberg et al., 1992). Furthermore, long-term treatment with tacrine might not only improve symptoms but also delay disease progression (Nordberg et al., 1992). In young and aged rats, metabolic studies
showed that physostigmine and tacrine increased rCMR in
several regions of the brain, which topographically overlapped with the distribution of M2 muscarinic receptors and
cholinesterase activity (Bassant et al., 1993, 1995). The
mechanisms of the effects of cognitive enhancers are still
unclear. However, they might show their therapeutic effects,
at least in part, by affecting cholinergic transmission (for
review, see Freeman and Dawson, 1991).
In the present study, the influences of scopolamine and
cholinesterase inhibitors on the rCBF response to somatosensory stimulation were evaluated in the somatosensory cortex
of unanesthetized monkeys using [15O]H2O and a high-resolution animal PET scanner. The drugs tested here were physostigmine, E2020 (Iimura et al., 1989; Sugimoto et al., 1992)
and tacrine, all of which have been reported to improve
learning and memory in experimental animals and also to
show cholinesterase inhibitory activity, facilitating their actions on the cholinergic system.
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subtracted image obtained after saline administration. The actual
sizes of the ROI ranged from 70 to 80 mm2. ROI were placed symmetrically and bilaterally in the contralateral and ipsilateral somatosensory cortical regions, and average counts associated with
[15O]H2O were determined in bilateral regions. The same ROI were
used for different conditions in each monkey. The ratios (contralateral/ipsilateral) of radioactivities in the regions were analyzed statistically (Ogawa et al., 1994; Tsukada et al., 1997). Comparison
between treatments was carried out using the unpaired two-tailed t
test, and a probability level of ,5% (P , .05) was considered to be
statistically significant.
Results
Discussion
The rCBF response to vibrotactile stimulation that was
abolished by systemic administration of scopolamine was
recovered by administration of cholinesterase inhibitors
(physostigmine, E2020 and tacrine) in the somatosensory
cortex of unanesthetized monkeys. It is of interest that, when
evaluated within 1 hr after administration of cholinesterase
inhibitors, the order of dose ranges needed for the recovery of
functional rCBF responses correlated well with that of cholinesterase inhibitory activity measured in vitro; that is,
E2020 was 15 times more potent than tacrine and 10 times
less potent than physostigmine when tested in vitro (Yamanishi et al., 1990). Physostigmine, a classical cholinesterase
inhibitor with the highest inhibitory activity among compounds examined, has been shown to improve memory performance in patients with AD (Davis and Mohs, 1982). However, peripheral side effects of physostigmine have limited its
use for treatment of AD. Tacrine was reported to exhibit
hepatotoxicity. These side effects are partially due to the low
selectivity of the effects of these drugs for AChE and BuChE.
The brain contains exclusively AChE, whereas peripheral
tissues contain BuChE rather than AChE. Physostigmine
and tacrine inhibit AChE in the brain as well as BuChE in
peripheral tissues, whereas E2020 inhibits AChE in the
brain (Sugimoto et al., 1992; Yamanishi et al., 1990). Other
reasons why the use of physostigmine for patients with AD is
limited are its short duration of action and low bioavailability. The recovery effect of physostigmine on the rCBF re-
TABLE 1
Physiological effects of scopolamine and cholinesterase inhibitors
Scopolamine (50 mg/kg)
Saline
Heart rate (beats/min)
Body temperature (°C)
pH of arterial blood
PaCO2 (mm Hg)
PaO2 (mm Hg)
Systolic blood pressure (mm Hg)
Diastolic blood pressure (mm Hg)
142.0 6 3.0
36.3 6 0.2
7.4 6 0.1
40.1 6 3.5
89.4 6 5.9
157.3 6 5.0
91.3 6 6.0
No inhibitor
Physostigmine (1 mg/kg)
E2020 (10 mg/kg)
Tacrine (100 mg/kg)
141.0 6 1.7
36.8 6 0.6
7.4 6 0.0
37.5 6 2.8
87.3 6 0.6
147.3 6 7.0
87.0 6 6.2
138.7 6 3.8
36.7 6 0.3
7.4 6 0.1
36.0 6 6.7
85.9 6 2.4
149.7 6 4.6
87.7 6 6.0
138.7 6 2.1
36.0 6 0.1
7.4 6 0.2
36.2 6 3.3
94.3 6 2.6
168.3 6 9.9
101.3 6 6.9
138.7 6 6.1
36.0 6 0.1
7.4 6 0.0
37.2 6 6.2
94.8 6 5.3
160.7 6 7.0
101.0 6 1.0
Physiological indices were measured 30 min after each drug administration. Heart rate and body temperature were monitored during PET scanning, and the other
parameters were measured under anesthetized conditions. Value are means 6 S.D. of three studies.
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Administration of scopolamine (50 mg/kg) alone or of scopolamine plus physostigmine (10 mg/kg), E2020 (100 mg/kg)
or tacrine (1000 mg/kg) did not produce any significant
changes in physiological indices, as shown in table 1. The
heart rate and body temperature were monitored 30 min
after each drug injection, during PET scanning. In the
present study, PaCO2, PaO2, pH and blood pressure of arterial blood were not monitored during PET scanning, because
of the technical limitations of using conscious monkeys.
When monitored in anesthetized animals with the same protocol as used in PET studies, PaCO2, PaO2, pH and blood
pressure of arterial blood were not altered significantly under each condition (table 1).
After saline injection, vibrotactile stimulation significantly
increased the rCBF response, represented as the ratio of
radioactivities of contralateral and ipsilateral cortices, to
134% of the resting condition (figs. 1– 4). Systemic administration of scopolamine at a dose of 50 mg/kg induced complete
abolishment of the rCBF response in the somatosensory cortex (figs. 1– 4). The rCBF response abolished by scopolamine
(50 mg/kg) was recovered by administration of physostigmine,
in a dose-dependent manner (123 and 146% of each resting
condition at 1 and 10 mg/kg, respectively) (figs. 1 and 2).
These findings were partially consistent with our previous
results (Tsukada et al., 1997). Administration of E2020 at
doses of 10 and 100 mg/kg induced the dose-dependent recovery of the rCBF response abolished by pretreatment of scopolamine (118 and 132% of each resting condition, respectively) (figs. 1 and 3). Much higher doses of tacrine were
required (120 and 124% of each resting condition at 100 and
1000 mg/kg, respectively) for recovery of the abolished rCBF
response, compared with physostigmine and E2020 (figs. 1
and 4). Administration of physostigmine (10 mg/kg), E2020
(100 mg/kg) or tacrine (1000 mg/kg) alone after saline treatment did not significantly affect the rCBF response induced
by vibrotactile stimulation (figs. 1– 4). The ratios (contralateral/ipsilateral) of rCBF were approximately 1.0 under all
conditions without vibrotactile stimulation (figs. 2– 4).
The recovery effect of physostigmine (10 mg/kg) on the
rCBF response abolished by scopolamine showed a peak with
the largest magnitude (140% of resting condition) among
three inhibitors at 1 hr, and the magnitude returned almost
to the basal level (contralateral/ipsilateral 5 1) from 2 hr
after administration and thereafter (fig. 5). Tacrine (1000
mg/kg) also showed a peak (123% of resting condition) at 1 hr,
followed by a gradual reduction, and the recovery effect was
not observed 4 hr after the administration (fig. 5). The recovery effect of E2020 (100 mg/kg) was comparable (136% of
resting condition) to that of physostigmine at 1 hr, and this
level was maintained up to 4 hr after administration (fig. 5).
The abolished rCBF response induced by scopolamine (50
mg/kg) was observed during PET measurement up to 4 hr
after systemic administration (fig. 5).
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Cholinergic System and rCBF Response
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Fig. 1. Typical images showing
the rCBF response affected by
scopolamine and cholinesterase
inhibitors. Saline, scopolamine (50
mg/kg), physostigmine (1 or 10 mg/
kg), E2020 (10 or 100 mg/kg) or
tacrine (100 or 1000 mg/kg) was
administered i.v. 30 min before
PET scanning. Three stimulationresting pairs of scans were averaged for each condition, and the
averaged PET images were superimposed on magnetic resonance
images of the same animals. The
range of Z scores for PET data is
color-coded.
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Tsukada et al.
Fig. 4. Effects of scopolamine and tacrine on rCBF responses to vibrotactile stimulation in monkey brain. Saline, scopolamine (50 mg/kg)
or tacrine (100 or 1000 mg/kg) was administered i.v. 30 min before PET
scanning. The ratios (contralateral/ipsilateral) of rCBF for each condition were analyzed statistically as described for figure 2. Values are
means 6 S.D. for four monkeys. *P , .05 vs. respective resting condition.
Fig. 3. Effects of scopolamine and E2020 on rCBF responses to vibrotactile stimulation in monkey brain. Saline, scopolamine (50 mg/kg)
or E2020 (10 or 100 mg/kg) was administered i.v. 30 min before PET
scanning. The ratios (contralateral/ipsilateral) of rCBF for each condition were analyzed statistically as described for figure 2. Values are
means 6 S.D. for four monkeys. *P , .05 vs. respective resting condition.
Fig. 5. Duration of the effects of cholinesterase inhibitors on the rCBF
response to the vibrotactile stimulation abolished by scopolamine in
monkey brain. Scopolamine (50 mg/kg), physostigmine (10 mg/kg),
E2020 (100 mg/kg) or tacrine (1000 mg/kg) was administered i.v., and
PET measurements were performed 1, 2 and 4 hr after administration.
Data at time 0 were obtained just before administration of cholinesterase inhibitors. The ratios (contralateral/ipsilateral) of rCBF for each
condition were analyzed statistically as described for figure 2. Values
are means 6 S.D. of four monkeys. E, scopolamine alone (50 mg/kg); F,
scopolamine plus physostigmine (10 mg/kg); Ç, scopolamine plus
E2020 (100 mg/kg); å, scopolamine plus tacrine (1000 mg/kg). *P , .05
vs. time 0 value for each drug treatment; #P , .05 vs. scopolaminealone condition at the respective time.
sponse abolished by scopolamine reached a peak 1 hr after
administration, and the effect disappeared after 2 hr. Tacrine
also showed a short-term action, with no effect observed 4 hr
after injection. In contrast, the recovery effect of E2020
lasted during PET measurements up to 4 hr after administration. These results seem to reflect the different durations
of cholinesterase inhibition by physostigmine and E2020
measured in vitro; that is, E2020 (10 mg/kg) significantly
inhibited cholinesterase in rat brain by 80% of control up to
8 hr after administration, whereas the inhibitory effect of
physostigmine (10 mg/kg) on cholinesterase diminished
within 2 hr (Sugimoto et al., 1992). The differences in dura-
tions of cholinesterase inhibition among these three compounds might, at least in part, be attributable to their differences in stability in vivo.
In addition to cholinesterase inhibition, other effects of
these drugs in the central nervous system could not be excluded. Tacrine has been reported to modulate the release of
acetylcholine (Nilsson et al., 1987), to block monoamine release and uptake (Drukarch et al., 1988) and to inhibit monoamine oxidase (Adem et al., 1989). Also, it has been demonstrated that tacrine directly blocks ion channels (Rogawski,
1987; Shaw et al., 1985). E2020 binds to s receptors (Eisai
company report), and s receptor-binding agonists have been
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Fig. 2. Effects of scopolamine and physostigmine on rCBF responses
to vibrotactile stimulation in monkey brain. Saline, scopolamine (50
mg/kg) or physostigmine (1 or 10 mg/kg) was administered i.v. 30 min
before PET scanning. When the stimulation was given, the ROI in the
contralateral somatosensory cortex corresponding to a Z score of 1.96
or greater was obtained from subtracted images after saline administration. Without (resting) and with vibration, ROI were placed symmetrically and bilaterally in the contralateral and ipsilateral somatosensory
cortical regions, and average counts associated with [15O]H2O were
determined in bilateral regions. The same ROI were examined under
different conditions in each monkey. The ratios (contralateral/ipsilateral)
of rCBF in the regions were analyzed statistically. Values are means 6
S.D. for four monkeys. *P , .05 vs. respective resting condition.
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1997
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different from those in humans, reflecting species differences
in the proportions of cholinergic receptor subtypes or in their
affinities for each modulator (Blin et al., 1995). Different
kinds of anesthesia induced different effects on cerebral functions such as rCBF and rCMR (Gjedde et al., 1980; Sokoloff,
1981), functional hemodynamic-metabolic coupling (Crosby
et al., 1983) and neurotransmission, as measured by ligandreceptor binding in vivo (Kobayashi et al., 1995; Onoe et al.,
1994). In the present study, we used unanesthetized monkeys as an experimental model to minimize the species differences and the effects of anesthesia. The combined use of
conscious monkeys and PET, a method for noninvasive determination of cerebral biochemistry and physiology in vivo,
can offer a unique bridge between animal and human experimental protocols, especially to predict therapeutic effects in
humans from the results of animal studies.
In conclusion, the present study suggested that compounds
with cholinesterase inhibitory activity can reverse the scopolamine-abolished coupling between neuronal activity and
rCBF responses to somatosensory stimulation via enhancement of cholinergic neurotransmission. In addition, this
study strongly supported the hypothesis that cholinergic
mechanisms might be involved in regulation of the coupling
mechanism in functional activation. This experimental procedure may provide the easiest method to evaluate compounds designed as cognitive enhancers, before clinical trials
for treatment of patients with dementia. Among the three
cholinesterase inhibitors examined here, the longer acting
and less harmful compound E2020 may be ideal for therapeutic use, if applied with care.
Acknowledgments
The excellent technical assistance of S. Nishiyama and S. Nakanishi in [15O]H2O synthesis and magnetic resonance imaging data
acquisition, respectively, is gratefully acknowledged.
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reported to increase acetylcholine release in the frontal cortex of rats (Matsuno et al., 1993). Some of these effects might
be involved in the activation of neuron activity. However, the
higher activity of cholinesterase inhibition provided a stronger recovery effect on the scopolamine-abolished rCBF response to stimulation, suggesting that the modulatory effect
might be attributable, at least in part, to activation of the
cholinergic neuronal system induced by cholinesterase inhibition.
Our previous reports demonstrated that the rCBF response abolished by scopolamine was recovered by administration of physostigmine (Tsukada et al., 1997), whereas the
rCMR response, as measured with [18F]-2-fluoro-2-deoxy-Dglucose, was not affected by administration of scopolamine
(Ogawa et al., 1994) or physostigmine (Tsukada et al., 1997).
These previous results indicated that the recovery effects
observed with the other two cholinesterase inhibitors, E2020
and tacrine, were also due to normalization of the coupling
mechanism between neuronal activity and the rCBF response by enhancement of acetylcholine transmission and
not to activation of the somatosensory neuronal response
itself.
It has been reported that the rCBF response to stimulation
might be influenced by changes in global cerebral blood flow
induced by changes in arterial PaCO2 (Shimosegawa et al.,
1995; Wilder, 1953). Human studies indicated that scopolamine administration resulted in decreased cortical blood
flow (Honer et al., 1987) and glucose metabolism (Blin et al.,
1995). In the present study, we did not monitor PaCO2 during PET scanning under unanesthetized conditions. It was
impossible for us to obtain arterial blood samples during PET
scans, because of the difficulty of placing and maintaining
the cannula in femoral arteries of monkeys under unanesthetized conditions. When monitored under anesthesia with
the same protocol, however, PaCO2 levels were not significantly affected by scopolamine, physostigmine, E2020 or tacrine. The physiological indices monitored during PET scanning suggested that scopolamine and the cholinesterase
inhibitors did not alter the heart rate or body temperature of
monkeys. Our previous study indicated that PaCO2 was not
significantly affected by scopolamine or physostigmine, and
the present results of the rCBF response obtained with scopolamine and physostigmine were also consistent with our
previous observations (Tsukada et al., 1997). It was also
reported that scopolamine butylbromide (100 mg/kg), which
does not penetrate the blood-brain barrier, did not abolish
the rCBF response to stimulation (Tsukada et al., 1997).
Taken together, the observed changes, induced by scopolamine and cholinesterase inhibitors, in the rCBF response to
vibrotactile stimulation can be attributed to modulation of
the central cholinergic system acting as a cerebrovascular
dilator and not to simple changes in the global cerebral blood
flow caused by changes in physiological conditions.
The roles of cholinergic neuronal systems in cognition,
learning and memory have been studied in experimental
animals (mainly rats and mice), to explain the pathogenesis
of human disorders associated with dementia. However, it is
difficult to obtain clear answers relevant to human disorders
from results obtained in rodents, because of the species differences and because most animal studies have been performed with anesthesia. The effects of cholinergic modulators
on rCMR in rats were sometimes similar to and sometimes
Cholinergic System and rCBF Response
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