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The Journal of Neuroscience, February 15, 1998, 18(4):1595-1601
Modulation of Hippocampal Acetylcholine Release and Spontaneous
Alternation Scores by Intrahippocampal Glucose Injections
Michael E.
Ragozzino,
Shanthi N.
Pal,
Katharine
Unick,
Mark R.
Stefani, and
Paul E.
Gold
Department of Psychology, Gilmer Hall, University of Virginia,
Charlottesville, Virginia 22903
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ABSTRACT |
Recent evidence indicates that systemic glucose treatment enhances
memory while producing a corresponding increase in hippocampal acetylcholine (ACh) output. The present experiments examined whether unilateral intrahippocampal infusions of glucose would enhance spontaneous alternation performance and whether there would be a
corresponding increase in ACh output in the ipsilateral and contralateral hippocampus. Extracellular ACh was assessed in samples collected at 12 min intervals using in vivo microdialysis
with HPLC with electrochemical detection. Twelve minutes after a
unilateral infusion of artificial cerebrospinal fluid (CSF) or glucose
(6.6 mM), rats were tested in a cross maze for spontaneous
alternation behavior with concurrent microdialysis collection. In two
experiments, glucose infusions significantly increased alternation
scores (67.5 and 59%) compared with CSF controls (42.4 and 39.5%,
respectively). In both experiments, behavioral testing resulted in
increased ACh output in the hippocampus. Glucose administration at the
time of alternation tests enhanced ACh output beyond that of behavioral testing alone both ipsilateral (+93.8%) and contralateral (+85%) to
the infusion site, as compared with ACh output (+36.1% and +55.5%) of
CSF controls. Glucose infusions did not affect hippocampal ACh output
in rats kept in a holding chamber. These results suggest that glucose
may enhance alternation scores by modulating ACh release. The findings
also indicate that unilateral glucose infusions increase hippocampal
ACh output both ipsilateral and contralateral to the site of injection.
Furthermore, glucose increased ACh output only during maze testing,
suggesting that specific behavioral demands, perhaps requiring
activation of cholinergic neurons, determine the efficacy of glucose in
modulating ACh release.
Key words:
glucose; memory; learning; hippocampus; acetylcholine; microdialysis
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INTRODUCTION |
Systemic administration of glucose
enhances memory on a wide range of tests in humans and rodents (Gold,
1986 ; Messier and White, 1987; Hall et al., 1989; Manning et al., 1990,
1993; Packard and White, 1990). Such findings suggest that glucose may
directly modulate brain systems critical for mnemonic functioning. In
support of this view, glucose injections into the lateral ventricles or into specific brain areas, i.e., the medial septum and amygdala, modulate memory (Lee et al., 1988; Ragozzino et al., 1992; Ragozzino and Gold, 1994a).
Thus, although glucose can act directly on the brain to enhance
cognitive functions, the neural mechanisms underlying glucose enhancement of memory remain unclear. Circulating glucose readily crosses from blood to brain through a facilitated transport mechanism to serve as the major energy source for the CNS (Gibbs et al., 1942;
Pardridge, 1983). In addition to its critical metabolic functions,
glucose also serves as a substrate for the synthesis of certain
neurotransmitters. One possibility is that glucose acts on specific
neurotransmitter systems to facilitate memory. To unravel what
neurotransmitter systems glucose may modulate to augment memory,
several experiments have examined the effects of glucose administered
in combination with other pharmacological treatments on memory and
other behavioral and neural measures. One set of results indicates that
glucose may facilitate the actions of cholinergic neurons.
Specifically, systemic administration of glucose attenuates memory
impairments, paradoxical sleep deficits, and hyperactivity induced by
muscarinic cholinergic antagonists (Stone et al., 1987, 1988a, 1991,
1995).
Recent findings support the view that circulating glucose can
modulate central ACh release. For example, glucose injections at doses
that facilitate memory reverse the effects of morphine-induced decreases in hippocampal ACh output (Ragozzino et al., 1994b; Ragozzino
and Gold, 1995a). Conversely, glucose potentiates an enhancement in
hippocampal ACh output produced by scopolamine, a muscarinic
cholinergic antagonist (Durkin et al., 1992).
Recent evidence demonstrates that systemic glucose injections
concomitantly potentiate an increase in hippocampal ACh release and
enhance spontaneous alternation scores (Ragozzino et al., 1996). These
results suggest that glucose may act directly in the hippocampal
formation to augment ACh release and enhance memory. The first
experiment determined whether glucose can act directly in the
hippocampal formation to modulate ACh release by measuring the effects
of unilateral glucose infusions on hippocampal ACh output during
spontaneous alternation testing and while rats remain in a holding
chamber. Because there is a paucity of neurochemical results indicating
how neurotransmitter release in one hippocampal hemisphere influences
neurotransmitter release in the other hemisphere, the second experiment
determined whether modulation of hippocampal ACh release in one
hemisphere would affect ACh release in the contralateral hemisphere.
This experiment tested whether unilateral hippocampal infusions of
glucose would alter ACh output in the contralateral hemisphere during
behavioral testing and during resting conditions.
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MATERIALS AND METHODS |
Subjects. Male Sprague Dawley rats (Charles River
Breeding Laboratories, Wilmington, MA) weighing 300-350 gm at the time
of surgery served as subjects. Rats were housed individually with food
and water available ad libitum. A 12 hr light/dark cycle (lights on at 7 A.M.) was maintained.
Surgery. Twenty minutes before they were anesthetized with
sodium pentobarbital (50 mg/kg, i.p.), rats received atropine sulfate (0.2 ml of a 540 mg/ml solution, i.p.). After they were anesthetized, each rat was placed in a stereotaxic frame with the nose bar set at 5.0 mm above the interaural line according to the atlas of Pellegrino et
al. (1979). In the first experiment, a plastic guide cannula (CMA/12
type; Carnegie Medicin, Stockholm) was lowered into the hippocampal
formation at the stereotaxic coordinates 3.8 mm caudal to bregma, 5.0 mm lateral to the midline, and 3.8 mm ventral from dura. In the second
experiment, the procedure and stereotaxic coordinates were the same
except that guide cannulae were bilaterally implanted. Five jeweler's
screws were placed in the skull surrounding the cannula(e) and cemented
in place with dental acrylic (Plastics One, Roanoke, VA).
Microdialysis procedure. The microdialysis procedure
was similar to that described previously (Ragozzino et al., 1996).
Beginning 2 d after surgery, rats were handled each day for ~5
min. At least 1 week after surgery, rats were randomly assigned to
either a spontaneous alternation or a "resting" condition. All rats
were tested during their light phase (8 A.M.-4 P.M.). A 3 mm dialysis probe (CMA/12; Carnegie Medicin) was inserted through the guide cannula. The dialysis probe was perfused continuously at a rate of 2.1 ml/min with artificial cerebrospinal fluid (128 mM NaCl/2.5 mM KCl/1.3 mM CaCl2/2.1
mM MgCl2/21 mM
NaH2PO4/1.3 mM
Na2HPO4/3.3 mM glucose and
brought to pH 7.0 by NaOH) that contained the acetylcholinesterase inhibitor neostigmine (1 µM).
Spontaneous alternation condition. The spontaneous
alternation condition was identical for both experiments unless noted
otherwise. Rats were first placed in a black plexiglas chamber (23 × 30 × 30 cm), and microdialysis was then started. Perfusate
collected for the first 45-60 min was not analyzed, to allow
equilibration between the brain tissue and perfusion solution before
sampling. Subsequently, samples were collected at 12 min intervals.
Samples, assayed for ACh content, that were collected during a period
of 1 h (five total) served as baseline rates for ACh output. At
the beginning of the second hour, rats were randomly assigned to either the artificial CSF (aCSF) or glucose (6.6 mM) group. The
number of rats in each group was six. For the glucose group, aCSF with glucose at 6.6 mM was perfused for a total of 24 min (two
samples). In the aCSF group, the perfusion solution (standard aCSF)
remained unchanged, i.e., it contained 3.3 mM glucose.
At the beginning of the second postinjection sample (12 min after
injection), rats were removed from the holding chamber and placed in a
four-arm cross maze. The maze (85 cm height) was constructed of wood
painted gray and contained a central platform (25 cm diameter), from
which radiated four symmetrical arms (55 cm long × 10 cm wide),
with 12 cm walls. The treatment-test interval was based on past
behavioral experiments in which rats were tested 15 min after
intracranial injections (Ragozzino and Gold, 1994a; Ragozzino et al.,
1995b). After they were placed in the central platform, rats were
allowed to traverse the maze freely for 12 min. The number and sequence
of entries were recorded; an alternation was defined as entry into four
different arms on overlapping quintuple sets. Five consecutive arm
choices within the total set of arm choices made up a quintuple set. A
quintuple set consisting of arm choices B,A,C,B,D was considered an
alternation. A quintuple set consisting of arm choices B,A,D,B,A was
not considered an alternation. Using this procedure, possible
alternation sequences are equal to the number of arm entries minus
four. The percent alternation score is equal to the ratio of (actual
alternations/possible alternations) × 100; chance performance on this
task is 22%. Only data from rats that made at least 10 arm choices
were included in the behavioral and biochemical analyses.
After behavioral testing, rats were returned to the holding chamber.
Three additional microdialysis samples were collected. Thus, five
baseline and five postinjection samples were collected for a total of
10 samples.
Resting condition. The habituation, baseline collection, and
treatment groups (n = 6) were the same as in the
spontaneous alternation condition. After baseline collection,
microdialysis samples were collected for an additional 60 min (five
samples) while the rat remained in the Plexiglas chamber.
Acetylcholine assay. Samples (20 ml) were assayed for ACh
using HPLC with electrochemical detection. ACh was separated from choline by a reverse-phase analytical column (Chromspher 5 C18, 100 × 3 mm; Chrompack, Middleburg, The Netherlands).
Subsequently, an enzymatic postcolumn reactor containing
acetylcholinesterase (EC 3.1.1.7; Sigma VI-S, Sigma, St. Louis, MO) and
choline oxidase (EC 1.1.3.17; Sigma) converted the ACh to choline and
acetate and the choline to betaine. The final conversion to hydrogen
peroxide was electrochemically detected by a platinum electrode held at a potential of +525 mV. The mobile phase containing 0.2 mM
dibasic potassium phosphate, 1.0 mM tetramethylammonium
hydroxide, 0.3 mM EDTA, and 0.0005% Kathon CG was
delivered at a rate 0.6 ml/min by a solvent delivery system (PM-80;
Bioanalytical Systems, West Lafayette, IN). ACh and choline peaks were
quantified by comparison to peak heights of ACh and choline standard
solutions. The detection limit was 50 fmol, and the assay was completed
within 7 min.
Histology. After they were tested, rats received a lethal
dose of sodium pentobarbital. A dialysis probe dipped in ink was then
inserted through the hippocampal guide cannula. This procedure enhanced
determination of the dialysis probe location. This was followed by an
intracardial perfusion with 0.9% saline and a 10% formalin solution.
Brains were removed and placed in a 30% sucrose/formalin solution. The
brains were frozen and cut in coronal sections (40 mm) on a cryostat.
The sections were mounted onto slides and dried. The sections were
first analyzed unstained. Subsequently, the sections were stained with
cresyl violet. After staining, the sections were examined again to
determine whether any cellular damage had occurred during the
implantation.
Statistical analyses. For analysis of the microdialysis
data, the raw values were converted to percentages from the baseline output for each subject. The baseline output was calculated from the
mean of the first five samples for each subject. Baseline ACh output
did not vary significantly across groups in either Experiment 1 or 2 (F(3,19) = 1.12 and
F(3,18) = 1.09, respectively; p > 0.3). Mean baseline ACh output was 3.0 ± 0.2 pmol/20 µl
samples in Experiment 1 and 2.8 ± 0.2 pmol/20 µl samples in
Experiment 2.
The percent values from the groups were analyzed by a two-way ANOVA
with one-repeated measure. Simple main-effect tests were applied to
compare groups at specific times, and Dunnett's test was applied for
comparison to baseline levels.
Student t tests were used to compare differences in the
percent alternation scores and the number of arm entries.
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RESULTS |
Experiment 1: Effects of unilateral glucose infusions on
ipsilateral hippocampal ACh release and spontaneous alternation
performance.
Spontaneous alternation condition
The results describing ACh output during spontaneous
alternation testing are shown in Figure
1. CSF controls exhibited an increase in
hippocampal ACh output (+36.1%) during spontaneous alternation
testing. In CSF rats, ACh levels remained elevated above basal levels
12 min after testing but returned to basal levels by 24 min after
behavioral testing. Intrahippocampal infusions of glucose (6.6 mM) potentiated the increase in hippocampal ACh output
(+93.8%) during behavioral testing. As shown in Figure 1, CSF controls
exhibited a significant increase in hippocampal ACh release during
spontaneous alternation performance compared with basal levels
(p < 0.01). ACh output in CSF rats remained significantly enhanced (+35.3%) for the sample after behavioral testing (p < 0.01) but was not significantly
different from basal levels 24 min after behavioral testing
(p > 0.05).
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Figure 1.
Effects of unilateral hippocampal infusions of
glucose and artificial cerebrospinal fluid on ipsilateral hippocampal
ACh output during the behavioral condition. ACh output was
significantly enhanced during spontaneous alternation in CSF controls.
Glucose infusions (6.6 mM) significantly increased ACh
output during testing compared with CSF controls. ACh output in the
sample after testing remained significantly elevated in the CSF and
glucose groups; however, there was no significant difference in ACh
output between the groups. * p < 0.01 versus
baseline; ** p < 0.01 versus CSF.
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A unilateral perfusion of glucose (6.6 mM) during
behavioral testing significantly potentiated ACh output compared with
that of CSF controls (F(1,10) = 11.06;
p < 0.01). In glucose-treated rats, ACh release
remained significantly elevated above basal levels 12 min after
behavioral testing (p < 0.01) but did not differ significantly from ACh release in CSF controls
(F(1,10) = 0.29; p > 0.05).
Twenty-four minutes after spontaneous alternation testing, ACh levels
of glucose-treated rats were not significantly different from those of
basal levels (p > 0.05).
The spontaneous alternation scores are illustrated in Figure
2. Analysis of the percent alternation
scores indicated a significant difference between CSF- and
glucose-treated rats [t(10) = 8.18; p < 0.01]. CSF controls had a mean percent
alternation score of 42.5 ± 1.7 SEM compared with a mean percent
alternation score of 67.5 ± 2.4 in glucose-treated rats.
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Figure 2.
Unilateral hippocampal infusions of glucose
enhance spontaneous alternation performance. Glucose (6.6 mM) significantly increased spontaneous alternation
performance compared with that of CSF controls. ** p < 0.01 versus CSF.
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In contrast to the spontaneous alternation scores, analysis of
the number of arm choices did not reveal a significant difference in
the number of arm entries between the two groups
(t(10) = 1.20; p > 0.05). The
mean number of arms entered for CSF controls was 21.2 ± 1.4, and
for glucose-treated rats it was 25.3 ± 3.2.
Resting condition
ACh output in the two groups during the resting condition is
shown in Figure 3. The findings indicate
that neither glucose nor CSF treatment altered ipsilateral hippocampal
ACh output during this condition. An ANOVA indicated that there were no
significant differences between the groups
(F(1,9) = 1.65; p > 0.05),
across time (F(9,81) = 0.69; p > 0.05), nor was there a significant interaction between treatment and
time (F(9,81) = 0.57; p > 0.05).
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Figure 3.
Effects of unilateral hippocampal injections of
glucose and artificial cerebrospinal fluid on ipsilateral hippocampal
ACh output in the resting condition. Injections of glucose (6.6 mM) or CSF did not alter hippocampal ACh output in the
resting condition.
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Experiment 2: The effects of unilateral glucose infusions on
contralateral hippocampal ACh release and spontaneous alternation
performance
Spontaneous alternation condition
The results of contralateral hippocampal ACh output during the
behavioral testing condition are illustrated in Figure
4. Similar to the findings in Experiment
1, CSF controls showed an increase in contralateral hippocampal ACh
output during spontaneous alternation testing, but ACh output returned
to basal levels by 24 min after behavioral testing. A unilateral
glucose infusion into the hippocampal formation potentiated the
elevation in ACh release in the contralateral hippocampal formation.
During behavioral testing, CSF controls exhibited a significant
increase in hippocampal ACh release compared with basal levels
(p < 0.01). ACh levels in CSF controls remained significantly enhanced 12 min after behavioral testing
(p < 0.01) but were not significantly different
from baseline levels 24 min after testing (p > 0.05).
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Figure 4.
Effects of unilateral hippocampal infusions of
glucose and artificial cerebrospinal fluid on contralateral hippocampal
ACh output during the behavioral condition. ACh output was
significantly enhanced during spontaneous alternation in CSF controls.
Glucose infusions (6.6 mM) significantly increased ACh
output during testing compared with CSF controls. ACh output in the
sample after testing remained significantly elevated in the CSF and
glucose groups; however, there was no significant difference in ACh
output between the groups. * p < 0.01 versus
baseline; ** p < 0.05 versus CSF.
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Glucose treatment significantly potentiated contralateral hippocampal
ACh output compared with that of CSF controls during spontaneous
alternation testing (F(1,10) = 5.47;
p < 0.05). In the sample immediately after testing,
ACh output in glucose-treated rats was still significantly increased
compared with basal levels (p < 0.01) but not
significantly different compared with that of CSF controls
(F(1,10) = 4.26; p > 0.05).
Twenty-four minutes after testing, ACh output in glucose-treated rats
was not significantly different from baseline levels
(p > 0.05).
The results after spontaneous alternation testing are shown in Figure
5. As observed in Experiment 1, a
unilateral glucose infusion into the hippocampal formation
significantly enhanced spontaneous alternation performance
compared with that of CSF controls (t(10) = 2.46; p < 0.05). In contrast, the number of arms
entered was not significantly different between the groups (t(10) = 1.42; p > 0.05). The
mean number of arms entered for CSF controls was 16.0 ± 2.1 and
for glucose-treated rats was 21.7 ± 3.4.
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Figure 5.
Unilateral hippocampal infusions of glucose
enhance spontaneous alternation performance. Glucose (6.6 mM) significantly increased spontaneous alternation
performance compared with that of CSF controls. * p < 0.05 versus CSF.
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Resting condition
The findings of the treatments in the resting condition are
illustrated in Figure 6. Neither CSF nor
glucose treatment modified contralateral hippocampal ACh output. An
ANOVA indicated that there were no significant differences between
groups (F(1,10) = 2.61; p > 0.05), across time (F(9,90) = 1.69;
p > 0.05), nor was there a significant interaction
between treatment and time (F(9,90) = 0.71;
p > 0.05).
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Figure 6.
Effects of unilateral hippocampal injections of
glucose and artificial cerebrospinal fluid on contralateral hippocampal
ACh output in the resting condition. Injections of glucose (6.6 mM) or CSF did not alter hippocampal ACh output in the
resting condition.
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Correlations of behavior and chemistry: Experiments 1 and 2
When data of all groups tested for spontaneous alternation
performance in Experiments 1 and 2 were combined, there was a
significant correlation between the percent change in ACh output
(sample 7) and alternation scores performance (r = 0.62; df = 22; p < 0.01). However, this
correlation reflects the treatment difference between CSF and glucose
groups. Correlations within the CSF and glucose groups
(r = 0.37 and 0.17, respectively) did not approach
statistical significance. Also, for combined groups as well as within
the CSF and glucose groups, there were no significant correlations between number of arm entries and either the percent increases in ACh
output or alternation scores.
Histology: Experiments 1 and 2
As illustrated in Figure 7, the
dialysis probes were located within the hippocampal formation. There
was minor cellular damage that lined the dialysis probe and guide
cannula. Probes positions ranged from 4.8 to 6.0 mm posterior to bregma
in the Paxinos and Watson (1986) atlas. The probes were predominantly
located in the medial portions of the dentate gyrus and CA3 areas and
extended ventrally to a level of the rhinal fissure. Differences in
probe location within the hippocampal formation did not result in
differences in baseline ACh output or glucose-induced increases in ACh
output.
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Figure 7.
The stippled areas represent the range
of probe locations in the hippocampal formation for all rats included
in the data analyses for Experiments 1 and 2.
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DISCUSSION |
The present experiments demonstrated that a unilateral infusion of
glucose into the hippocampal formation potentiated the increase in
hippocampal ACh output during spontaneous alternation testing. This
potentiation of ACh output was observed both ipsilateral and
contralateral to the infusion. Finally, glucose infusions into the
hippocampal formation augmented spontaneous alternation performance in
a four-arm maze.
In CSF controls, hippocampal ACh output remained elevated for at least
12 min but returned to basal levels by 24 min after behavioral testing.
For the spontaneous alternation condition, both the time course and
magnitude of change in ACh output for CSF controls was comparable to
previous findings in rats (Ragozzino et al., 1996). The findings from
in vivo measurements of ACh output further support results
from analyses of high-affinity choline uptake, suggesting that
cholinergic neurons projecting to the hippocampal formation are
activated during learning and memory (Marighetto et al., 1989, 1994;
Wenk et al., 1984).
Previous studies have found that the septohippocampal cholinergic
system may be activated in tests that do not explicitly measure
learning and memory, i.e., immobilization and locomotor activity
(Gilad, 1987; Day et al., 1991). These findings open the possibility
that an increase in hippocampal ACh release during spontaneous
alternation testing may be the result of stress, novelty, or locomotor
activity. However, these explanations seem unlikely. Repeated
spontaneous alternation test sessions still produced increases in ACh
release comparable to those seen in the first test session (M. Ragozzino, K. Unick, P. Gold, unpublished observations). Furthermore,
as also noted previously (Ragozzino et al., 1996), there was no
relationship in any of the treatment groups between the number of arm
choices and the change in hippocampal ACh output in the present
experiments. Moreover, rats that receive multiple training sessions for
learning to swim to an escape platform exhibit changes in hippocampal
high-affinity choline uptake. In contrast, rats trained in a
swimming-only condition do not show changes in hippocampal
high-affinity choline uptake (Decker et al., 1988). Taken together, the
findings suggest that an activation of the septohippocampal cholinergic
system may be important for learning and memory.
A unilateral infusion of glucose into the hippocampal formation during
spontaneous alternation testing significantly augmented hippocampal ACh
output. The 6.6 mM concentration of glucose in the
treatment group is approximately double that estimated to be in the
extracellular fluid of the brain (Silver and Erecinska, 1994).
Interestingly, measurements of extracellular glucose levels in
particular brain areas indicate that extracellular glucose levels can
vary with different behavioral manipulations, i.e., doubling during
restraint stress (Fellows and Boutelle, 1993). Thus, the 6.6 mM concentration infused through the hippocampal formation
is within a physiological range that may occur under certain
conditions.
The potentiation of hippocampal ACh output by intrahippocampal glucose
infusions during behavioral testing is comparable to findings observed
with systemic injections of glucose (Ragozzino et al., 1996). Although
it is possible that systemic glucose injections potentiate hippocampal
ACh output by acting principally in the hippocampal formation, glucose
also affects behavioral performance when injected into other neural
regions (Ragozzino et al., 1992; Ragozzino and Gold, 1994a). Therefore,
the effects of systemic glucose injections on hippocampal ACh output
may be derived at least in part from glucose actions at multiple brain
sites.
The second experiment found that intrahippocampal infusions of glucose
also potentiated ACh output contralateral to the injection during
behavioral testing. On the basis of both anatomical and physiological
findings, there is substantial connectivity between the hemispheres of
the hippocampal formation (Swanson et al., 1978; Laurberg and
Sorsensen, 1981; Finnerty and Jefferys, 1993). However, we believe this
is the first demonstration indicating that modulation of
neurotransmitter release in one hippocampal hemisphere can alter the
release of that neurotransmitter in the other hemisphere during
behavioral testing. These neurochemical findings are important not only
in understanding the neural mechanisms underlying glucose effects on
memory, but also in building a comprehensive view of hippocampal
functions and their underlying neural mechanisms. On the basis of
histology indicating that the probes were within the hippocampal
formation, we believe that the contralateral potentiation of ACh output
is not caused by glucose diffusion into the ventricular system.
Possibly, contralateral increases in ACh output reflect glucose actions
on hippocampal neurons that project directly to the contralateral
hippocampal hemisphere via associational and commissural pathways.
Alternatively, unilateral glucose infusions may have altered neural
activity in other brain areas that project to the contralateral
hippocampal formation, leading subsequently to the potentiation of ACh
output seen in the contralateral hemisphere. An important example of
this latter possibility is that unilateral postsynaptic actions of
glucose provide feedback to the cholinergic projection neurons in the
medial septum, thereby changing the level of activity in these
neurons.
Intrahippocampal glucose injections did not modify hippocampal ACh
output under all conditions. When administered during spontaneous alternation testing, unilateral glucose infusions augmented the task-induced increase in ACh output. In marked contrast, when administered to rats at rest, glucose infusions did not modify hippocampal ACh output in the ipsilateral or contralateral side. These
findings add to others suggesting that glucose increases ACh output
only under conditions in which the activity of cholinergic neurons is
"significantly" altered (Stone et al., 1987, 1988b; Ragozzino et
al., 1996). For example, systemic glucose injections alone do not
induce tremors, but glucose coadministered with physostigmine potentiates the onset and severity of tremors (Stone et al., 1988b). The present results have a further implication, at least for studies of
learning and memory. Evaluation of biochemical effects of drug actions
should be performed within the context of the behavioral measures.
Without behavioral testing at the time that pharmacological measures
are obtained, assessments of CNS responses to drugs may potentially
produce misleading, or at least limited, results. In the present
instance, if glucose had been administered only "off-line" with
respect to behavioral testing, we would have concluded erroneously that
the treatment had no effect on ACh output.
Intrahippocampal glucose infusions not only augmented hippocampal ACh
output, but also produced a concomitant increase in spontaneous
alternation performance. Previous findings indicate that inserting
delays between arm choices or removing extra maze cues in this task
significantly reduces alternation scores (Ragozzino et al., 1996; M.E.
Ragozzino, unpublished observations), consistent with the idea that
this task has a spatial memory component and consistent with a
description of the task as a delayed non-matching-to-sample task guided
by unknown motivation. These findings suggest that enhancement of
memory in the cross maze produced by a unilateral hippocampal infusion
of glucose may be at least partially attributable to an increase in ACh
release in this structure. Because the unilateral infusions of glucose
produced bilateral augmentation of hippocampal ACh output, the findings
do not reveal whether unilateral potentiation of hippocampal ACh output
would be sufficient to enhance memory in the cross maze.
One mechanism by which glucose might directly affect the function of
cholinergic neurons is by providing a precursor for the synthesis of
acetyl-CoA, thereby increasing the availability of acetyl-CoA as a
substrate for ACh synthesis. Although high-affinity choline uptake is
generally the rate-limiting step for ACh synthesis (Simon et al.,
1976), the availability of acetyl-CoA may be rate-limiting in certain
conditions (Gibson and Blass, 1976; Bielarczyk and Szutowicz, 1989),
i.e., depolarized brain synaptosomes. Thus, glucose administration
might modulate ACh synthesis and release by increasing the availability
of acetyl-CoA in conditions in which cholinergic neurons are activated.
Importantly, however, hyperglycemia or glucose treatment also appears
to affect neurochemical systems implicated in learning and memory other
than the cholinergic system (Bradford, 1986; Brase et al., 1987; Stone
et al., 1991; During et al., 1995). For example, glucose is also
critical for the synthesis of glutamate and aspartate and may improve
learning and memory by modulating the release of glutamate and
aspartate under certain testing conditions (Bradford, 1986). In
addition, intrahippocampal infusions of glucose may enhance spontaneous performance through direct actions on several neurotransmitter systems,
perhaps by regulating neurotransmitter release via modulation of
K+-ATP channels (Amoroso et al., 1990; During et
al., 1995; Stefani et al., 1996). Although the mechanisms underlying
glucose effects on memory are still unknown, these findings add to
accumulating evidence suggesting that the effect of glucose on brain
functioning is not limited to its use as an energy source but that it
may also play an important role in synaptic transmission.
In conclusion, the present findings indicate that unilateral infusions
of glucose into the hippocampal formation enhance spontaneous alternation performance in a cross maze. Behavioral testing produces an
increase in hippocampal ACh output, which is potentiated by unilateral
glucose infusions into the hippocampal formation. The test-related
potentiation of hippocampal ACh output by glucose treatment occurs in
both the ipsilateral and contralateral hippocampal formation. In
contrast, unilateral glucose infusions do not modify hippocampal ACh
output during the resting condition. The results suggest that an
increase in hippocampal ACh output contributes to the memory
improvement in the cross maze. Furthermore, because glucose potentiated
ACh output in the behavioral but not the resting condition, glucose may
modify ACh output only in conditions in which there is an increase in
cholinergic activity.
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FOOTNOTES |
Received June 2, 1997; revised Nov. 21, 1997; accepted Dec. 1, 1997.
This research was supported by grants from the National Institute of
Neurological Disorders and Stroke (NS32914) and the National Institute
of Aging (AG07648).
Correspondence should be addressed to Dr. Paul E. Gold, Department of
Psychology, University of Virginia, Charlottsville, VA 22903.
Dr. Ragozzino's and Dr. Unick's present address: Department of
Psychology, University of Utah, Salt Lake City, UT
84112.
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REFERENCES |
-
Amoroso S,
Schmid-Antomarchi H,
Fosset M,
Lazdunski M
(1990)
Glucose, sulfonylureas, and neurotransmitter release: role of ATP-sensitive K+ channels.
Science
247:852-854[Abstract/Free Full Text].
-
Bielarczyk H,
Szutowicz A
(1989)
Evidence for the regulatory function of synaptoplasmic acetyl-CoA in acetylcholine synthesis in nerve endings.
Biochem J
262:377-380[Web of Science][Medline].
-
Bradford HF
(1986)
In: Chemical neurobiology. New York: Freeman.
-
Brase DA,
Han YH,
Dewey WL
(1987)
Effects of glucose and diabetes on binding of naloxone and dihydromorphine to opiate receptors in mouse brain.
Diabetes
36:1173-1177[Abstract].
-
Day J,
Damsma G,
Fibiger HC
(1991)
Cholinergic activity in the rat hippocampus, cortex and striatum correlates with locomotor activity: an in vivo microdialysis study.
Pharmacol Biochem Behav
38:723-729[Web of Science][Medline].
-
Decker MW,
Pelleymounter MA,
Gallagher M
(1988)
Effects of training on a spatial memory task on high affinity choline uptake in hippocampus and cortex in young adult and aged rats.
J Neurosci
8:90-99[Abstract].
-
During MJ,
Leone P,
Davis KE,
Kerr D,
Sherwin RS
(1995)
Glucose modulates rat substantia nigra GABA release in vivo via ATP-sensitive potassium channels.
J Clin Invest
95:2403-2408.
-
Durkin TP,
Messier C,
DeBoer P,
Westerwink BHC
(1992)
Raised glucose levels enhance scopolamine-induced acetylcholine overflow from the hippocampus: an in vivo microdialysis study in the rat.
Behav Brain Res
49:181-188[Web of Science][Medline].
-
Fellows LK,
Boutelle MG
(1993)
Rapid changes in extracellular glucose levels and blood flow in the striatum of the freely moving rat.
Brain Res
604:225-231[Web of Science][Medline].
-
Finnerty GT,
Jefferys JGR
(1993)
Functional connectivity from CA3 to the ipsilateral and contralateral CA1 in the rat dorsal hippocampus.
Neuroscience
56:101-108[Web of Science][Medline].
-
Gibbs EL,
Lennox WG,
Nims LF,
Gibbs FA
(1942)
Arterial and cerebral venous blood: arterial-venous differences in man.
J Biol Chem
144:325-332[Free Full Text].
-
Gibson GE,
Blass JP
(1976)
Impaired synthesis of acetylcholine in brain accompanying mild hypoxia and hypoglycemia.
J Neurochem
27:37-42[Web of Science][Medline].
-
Gilad GM
(1987)
The stress-induced response of the septo-hippocampal cholinergic system. A vectorial outcome of psychoneuroendocrinological interactions.
Psychoneuroendocrinology
12:167-184[Web of Science][Medline].
-
Gold PE
(1986)
Glucose modulation of memory storage processing.
Behav Neural Biol
45:342-349[Web of Science][Medline].
-
Hall JL,
Gonder-Frederick LA,
Chewning WW,
Silveira J,
Gold PE
(1989)
Glucose enhancement of performance on memory tests in young and aged humans.
Neuropsychologia
27:1129-1138[Web of Science][Medline].
-
Laurberg S,
Sorensen KE
(1981)
Associational and commissural collaterals of neurons in the hippocampal formation (hilus fasciae dentatae and subfield CA3).
Brain Res
212:287-300[Web of Science][Medline].
-
Lee MK,
Graham SN,
Gold PE
(1988)
Memory enhancement with posttraining intraventricular glucose injections in rats.
Behav Neurosci
102:591-595[Web of Science][Medline].
-
Manning CA,
Hall JL,
Gold PE
(1990)
Glucose effects on memory and other neuropsychological tests in elderly humans.
Psychol Sci
1:307-311. [Web of Science]
-
Manning CA,
Ragozzino ME,
Gold PE
(1993)
Glucose enhancement of memory in patients with Alzheimer's disease.
Neurobiol Aging
14:523-528[Web of Science][Medline].
-
Marighetto A,
Durkin T,
Toumane A,
Lebrun C,
Jaffard R
(1989)
Septal-noradrenergic antagonism in vivo blocks the testing-induced activation of septohippocampal cholinergic neurones and produces a concomitant deficit in working memory performance.
Pharmacol Biochem Behav
34:553-558[Web of Science][Medline].
-
Marighetto A,
Micheau J,
Jaffard R
(1994)
Effects of intraseptally injected glutamatergic drugs on hippocampal sodium-dependent high-affinity choline uptake in "naive" and "trained" mice.
Pharmacol Biochem Behav
49:689-699[Web of Science][Medline].
-
Messier C,
White NM
(1987)
Memory improvement by glucose, fructose, and two glucose analogs: a possible effect on peripheral glucose transport.
Behav Neural Biol
48:104-127[Web of Science][Medline].
-
Packard MG,
White NM
(1990)
Effect of posttraining injections of glucose on acquisition of two appetitive learning tasks.
Psychobiology
18:282-286.
-
Pardridge WM
(1983)
A perspective from the blood-brain barrier.
Physiol Rev
63:1481-1535[Free Full Text].
-
Paxinos G,
Watson C
(1986)
In: The rat brain in stereotaxic coordinates. Orlando, FL: Academic.
-
Pellegrino LJ,
Pellegrino AS,
Cushman AJ
(1979)
In: A stereotaxic atlas of the rat brain. New York: Plenum.
-
Ragozzino ME,
Parker ME,
Gold PE
(1992)
Spontaneous alternation and inhibitory avoidance impairments with morphine injections into the medial septum: attenuation by glucose administration.
Brain Res
597:241-249[Web of Science][Medline].
-
Ragozzino ME,
Gold PE
(1994a)
Task-dependent effect of intra-amygdala morphine injections: attenuation by intra-amygdala glucose injections.
J Neurosci
14:7478-7485[Abstract].
-
Ragozzino ME,
Wenk GL,
Gold PE
(1994b)
Glucose attenuates morphine-induced decrease in hippocampal acetylcholine output: an in vivo microdialysis study in rats.
Brain Res
655:77-82[Web of Science][Medline].
-
Ragozzino ME,
Gold PE
(1995a)
Glucose injections into the medial septum reverse the effects of intraseptal morphine infusions on hippocampal acetylcholine output and memory.
Neuroscience
68:981-988[Web of Science][Medline].
-
Ragozzino ME,
Hellems K,
Lennartz RC,
Gold PE
(1995b)
Pyruvate infusions into the septal area attenuate spontaneous alternation impairments induced by intraseptal morphine injections.
Behav Neurosci
109:1074-1080[Web of Science][Medline].
-
Ragozzino ME,
Unick KE,
Gold PE
(1996)
Hippocampal acetylcholine release during memory testing in rats: augmentation by glucose.
Proc Natl Acad Sci USA
93:4693-4698[Abstract/Free Full Text].
-
Silver IA,
Erecinska M
(1994)
Extracellular glucose concentration in mammalian brain: continuous monitoring of changes during increased neuronal activity and upon limitation in oxygen supply in normo-, hypo-, and hyperglycemic animals.
J Neurosci
14:5068-5076[Abstract].
-
Simon JR,
Atweh S,
Kuhar MJ
(1976)
Sodium-dependent high affinity choline uptake: a regulatory step in the synthesis of acetylcholine.
J Neurochem
26:909-922[Web of Science][Medline].
-
Stefani MR,
Nicholson GM,
Gold PE
(1996)
Enhancement of spontane- ous alternation scores in the rat by intra-septal injection of the K-ATP channel blocker glibenclamide.
Soc Neurosci Abstr
22:141.
-
Stone WS,
Cottrill KL,
Gold PE
(1987)
Glucose and epinephrine attenuation of scopolamine-induced increases in locomotor activity in mice.
Neurosci Res Commun
1:105-111.
-
Stone WS,
Croul CE,
Gold PE
(1988a)
Attenuation of scopolamine-induced amnesia in mice.
Psychopharmacology
96:417-420[Medline].
-
Stone WS,
Cottrill KL,
Walker DL,
Gold PE
(1988b)
Blood glucose and brain function: interactions with CNS cholinergic systems.
Behav Neural Biol
50:325-334[Web of Science][Medline].
-
Stone WS,
Walser B,
Gold SD,
Gold PE
(1991)
Scopolamine- and morphine-induced impairments of spontaneous alternation performance in mice: reversal with glucose and with cholinergic and adrenergic agonists.
Behav Neurosci
105:264-271[Web of Science][Medline].
-
Stone WS,
Rudd RJ,
Gold PE
(1995)
Glucose attenuation of atropine-induced deficits in paradoxical sleep and memory in rats.
Brain Res
694:133-138[Web of Science][Medline].
-
Swanson LW,
Wyss JM,
Cowan WM
(1978)
An autoradiographic study of the organization of intrahippocampal associational pathways in the rat.
J Comp Neurol
181:681-716[Web of Science][Medline].
-
Wenk G,
Hepler D,
Olton D
(1984)
Behavior alters the uptake of [3H]choline into acetylcholinergic neurons of the nucleus basalis magnocellularis and medial septal area.
Behav Brain Res
13:129-138[Web of Science][Medline].
Copyright © 1998 Society for Neuroscience 0270-6474/98/1841595-07$05.00/0
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Abstract
Introduction
