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The Journal of Neuroscience, September 1, 1998, 18(17):6704-6712
Increased Thresholds for Long-Term Potentiation and Contextual
Learning in Mice Lacking the NMDA-type Glutamate Receptor  1 Subunit
Yuji
Kiyama1,
Toshiya
Manabe2,
Kenji
Sakimura3, 4,
Fumiko
Kawakami2,
Hisashi
Mori1, and
Masayoshi
Mishina1, 4
Departments of 1 Molecular Neurobiology and
Pharmacology and 2 Neurophysiology, School of Medicine,
University of Tokyo, Tokyo 113-0033, Japan, 3 Department of
Cellular Neurobiology, Brain Research Institute, Niigata University,
Niigata 951-8585, Japan, and 4 Core Research for
Evolutional Science and Technology, Japan Science and Technology
Corporation, Saitama 332-0012, Japan
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ABSTRACT |
The NMDA-type glutamate receptor (GluR) channel, composed of the
GluR and GluR subunits, plays a key role in synaptic plasticity in the CNS. The mutant mice lacking the GluR1 subunit
exhibited a reduction in hippocampal long-term potentiation (LTP), but
a stronger tetanic stimulation restored the impairment and the
saturation level of LTP was unaltered. These results suggest an
increase of threshold for LTP induction in the GluR1 mutant mice.
After a series of backcrosses we established a GluR1 mutant mouse
line with a 99.99% pure C57BL/6 genetic background. The performance of
the mutant mice in tone- and context-dependent fear conditioning tests
was comparable with that of the wild-type mice. However, a significant
difference in the extent of contextual learning became apparent when
the chamber exposure time before footshock was shortened. Furthermore,
there was a significant difference in freezing responses immediately
after footshock on the conditioning day between the wild-type and
mutant mice, and the difference was not restored by longer chamber
exposure in contrast to the contextual learning on the next day of the
conditioning. These results suggest that the GluR1 subunit of the
NMDA receptor channel is a determinant of thresholds for both
hippocampal LTP and contextual learning and plays differential roles in
two forms of contextual fear memories.
Key words:
NMDA receptor channel; GluR1 subunit; contextual
learning; fear conditioning; LTP; threshold; memory; hippocampus
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INTRODUCTION |
The NMDA subtype of the glutamate
receptor (GluR) channel plays roles in synaptic plasticity as a
molecular coincidence detector (Bliss and Collingridge, 1993 ; Malenka
and Nicoll, 1993). Studies with selective antagonists have revealed
diverse physiological roles of NMDA receptor channels in learning,
memory, and neural development (Morris et al., 1986; Cline et al.,
1987; Kleinschmidt et al., 1987). NMDA receptor channels are composed
of the GluR (NR2) and GluR (NR1) subunits. There are four GluR
subunit genes (Ikeda et al., 1992; Kutsuwada et al., 1992; Meguro et
al., 1992; Monyer et al., 1992; Nagasawa et al., 1996), although
GluR subunit variants are derived from a single gene (Moriyoshi et
al., 1991; Yamazaki et al., 1992; Hollmann et al., 1993). The molecular
composition of NMDA receptor channels varies, depending on brain
regions and developmental stages (Watanabe et al., 1992). The four
GluR subunits are also distinct in functional properties and
regulations (Seeburg, 1993; Mori and Mishina, 1995). Thus, multiple
GluR subunits are key determinants of the NMDA receptor channel
diversity.
The physiological significance of the molecular diversity of the NMDA
receptor channel has been examined by gene-targeting techniques. We
showed that disruption of the GluR1 (NR2A) gene resulted in the
reduction of hippocampal long-term potentiation (LTP) and impairment of
Morris water maze learning (Sakimura et al., 1995). GluR2 (NR2B)
mutant mice died shortly after birth and failed to form the
whisker-related neural pattern (barrelettes) in the brainstem
trigeminal complex (Kutsuwada et al., 1996), similar to GluR1 (NR1)
mutant mice (Forrest et al., 1994; Li et al., 1994). The ablation of
the GluR2 subunit also impaired synaptic plasticity in the
hippocampus (Kutsuwada et al., 1996; Ito et al., 1997). GluR4 (NR2D)
mutant mice exhibited reduced spontaneous behavioral activity (Ikeda et
al., 1995), whereas GluR3 (NR2C) mutant mice showed little obvious
deficit (Ebralidze et al., 1996; Kadotani et al., 1996; Sprengel et
al., 1998). These analyses suggest that the GluR1 and GluR2
subunits play important roles in development, synaptic plasticity,
learning, and memory. By intraventricular infusion of
D-2-amino-5-phosphonovalerate (APV), Morris et al. (1986)
first suggested a link between hippocampal LTP and spatial learning.
Recently, Tsien et al. (1996) showed by a region-specific gene
targeting that the NMDA receptor channel in the hippocampal CA1 region
was essential for LTP and spatial learning. However, NMDA receptor
channel-dependent hippocampal LTP may not be essential for spatial
memory itself, although required for some component of water maze
learning, because pretraining eliminated the APV inhibition (Bannerman
et al., 1995; Saucier and Cain, 1995). In the present investigation we
used fear conditioning, a rapidly acquired and persistent form of
simple associative learning (Fanselow, 1980, 1984), to investigate the
role of the GluR1 subunit in learning and memory. The genetic
background of the GluR1 mutant mice that were used was highly
homogeneous after a series of backcrosses. Furthermore, we
characterized the impairment of hippocampal LTP in GluR1
mutant mice.
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MATERIALS AND METHODS |
Animals. C57BL/6 and CBA mice were purchased from
Clea (Tokyo, Japan) and Charles River (Yokohama, Japan), respectively.
The mutant mouse lacking the GluR1 subunit of the NMDA receptor
channel was produced by homologous recombination in TT2 embryonic stem (ES) cells derived from C57BL/6 × CBA mouse, using a targeting vector composed of the GluR1 subunit gene from C57BL/6 mouse as
described (Sakimura et al., 1995). The chimeric mouse derived from the
recombinant ES cells was crossed to C57BL/6 mouse to yield heteromeric
F2 mice with a 75% pure C57BL/6 genetic background. Heterozygous mice
for the GluR1 subunit gene were crossed successively to
C57BL/6 mice to yield next generations with a purer C57BL/6 genetic
background. The F13 heterozygous mice were crossed to each other to
yield the homozygous GluR1 subunit mutant mice ( /) with a
99.99% pure C57BL/6 genetic background and the wild-type littermates
(+/+). The genotypes of mice were determined by tail biopsy and PCR,
using primers E1P1, 5'-TCTGGGGCCTGGTCTTCAACAATTCTGTGC-3' (the
nucleotide residues 1766-1795 of GluR1 cDNA; Meguro et al., 1992),
E1P2, 5'-CTTCTTGTCACTGAGGCCAGTCACTTGGTC-3' (complementary to the
residues 1921-1950), and NeoP1,
5'-GCCTGCTTGCCGAATATCATGGTGGAAAAT-3'. Southern blot hybridization
analysis of the genomic DNA was performed by using the 1.2 kb
SmaI fragment from GluR1 genomic DNA (probe A) as
described (Sakimura et al., 1995). Animal care was performed in
accordance with institutional guidelines. Mice were fed ad libitum with standard laboratory chow and water in standard animal cages under a 12 hr light/dark cycle. Electrophysiological and behavioral analyses of mice were done in a blind manner.
Electrophysiology. The wild-type and GluR1 mutant F2 mice
9-17 weeks old were decapitated under halothane anesthesia, and hippocampi were removed quickly. Transverse hippocampal slices (400 µm) were cut with a vibratome tissue slicer and placed in a holding
chamber for at least 1 hr. Then a single slice was transferred to a
recording chamber and submerged beneath a continuously perfusing medium
that had been saturated with 95% O2/5%
CO2. The medium contained (in mM) 119 NaCl, 2.5 KCl, 1.3 MgSO4, 2.5 CaCl2, 1.0 NaH2PO4, 26.2 NaHCO3,
and 11 glucose plus 0.1 µM picrotoxin. All experiments
were performed at room temperature (22-25°C). Recordings were made
in the hippocampal CA1 region, using an Axopatch 1D amplifier with a
glass pipette filled with 3 M NaCl. Field EPSPs (fEPSPs)
were evoked by stimulating Schaffer collateral/commissural fibers in
the stratum radiatum at 0.1 Hz with a bipolar tungsten stimulating
electrode and were analyzed and stored on an IBM-compatible personal
computer. The stimulus strength was adjusted to give initial EPSP
slopes of 0.15-0.20 mV/msec (defined as the "standard" stimulus
strength in this paper) unless otherwise stated. LTP was induced by
high-frequency stimulation (100 Hz for 1 sec, a single train). In
two-pathway recordings the independence of the two pathways was
determined by the lack of paired-pulse facilitation between them. When
the stimulus strength was increased, independence was examined as above
just before tetanic stimulation was applied to that pathway.
Furthermore, when we applied tetanic stimulation in one pathway, we did
not observe post-tetanic potentiation in the other pathway in any
experiments, indicating that the two pathways were independent.
Fear conditioning. For behavioral analyses we used F13
GluR1 mutant mice and their littermate wild-type mice from postnatal day 35 (P35) to P48. The average body weights were 16.9 ± 0.2 gm
(mean ± SEM; n = 104) for the wild-type mice and
16.2 ± 0.2 gm (n = 162) for the mutant mice.
Naive adult mice were housed individually for at least 1 week before
behavioral testing and were handled for 1 min every day to reduce
stress. Fear conditioning was conducted in a small rodent chamber
(10 × 10 × 10 cm) with clear polyvinyl chloride boards and
a stainless steel rod floor (CL-MI; O'Hara, Tokyo, Japan). Before
experiments the chamber was cleaned with 1% acetic acid. The
conditioning chamber was surrounded by a sound-attenuating chest with
an observation window and was illuminated by a 13 W
fluorescent lamp; masking noise of 65 dB was provided by a
ventilation fan. Scrambled shock (1 sec, 0.5 mA) was delivered by a
shock generator/scrambler to the grid floor that was composed of 14 stainless steel rods 2 mm in diameter spaced 7 mm apart (center to
center). Freezing was monitored continuously by an observer blind to
mouse genotype and was recorded on a chart via a switch (see Fig. 3).
Freezing was defined as the absence of any visible movement of the body
and vibrissae except for movement necessitated by respiration
(Fanselow, 1984). Behaviors of mice were recorded on videotapes.
Freezing time was summated, and the percentage of freezing was
calculated per minute. The data were analyzed by ANOVA.
Tone-dependent fear conditioning. Mice were placed in the
conditioning chamber for 2 min and then presented with a loud tone of
~75 dB and 800 Hz for 1 min through a speaker on the side wall of the
conditioning chamber. At the end of the tone presentation the mice were
given a footshock (1 sec, 0.5 mA). Freezing responses were monitored
for 1 min more after footshock, and then the animals were returned to
their home cages. On the next day the mice were placed in a novel
chamber (reformed from a mouse home cage with clear polycarbonate
boards, 12.5 × 12.0 × 11.0 cm) with contexts different from
those of the conditioning chamber to minimize the freezing caused by
contextual fear conditioning; freezing was scored for 3 min before the
tone presentation and subsequently for 3 min in the presence of the
tone.
Contextual fear conditioning. Mice were placed in the
conditioning chamber for various times (from 0 to 9 min) and then given a footshock (1 sec, 0.5 mA). Freezing responses were monitored for 1 min more after the footshock, and then the animals were returned to
their home cages. On the next day the mice were placed in the
conditioning chamber and freezing was scored for 6 min.
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RESULTS |
LTP induction in GluR1 mutant mice
We have shown that synaptic responses mediated by NMDA receptor
channels and LTP in the hippocampal CA1 region are reduced in GluR1
mutant mice (Sakimura et al., 1995). Although the reduced LTP is likely
to be attributable to a decrease in influx of Ca2+
through postsynaptic NMDA receptor channels, it has not yet been examined whether the reduced NMDA receptor channel activity primarily causes the impairment of LTP. It is also possible that the absence of
the GluR1 subunit secondarily gives rise to any disturbance in the
LTP-inducing intracellular processes after NMDA receptor channel
activation. We examined this issue by manipulating the activity of NMDA
receptor channels during high-frequency stimulation. If the reduction
of LTP in the mutant mice is entirely attributable to the reduced
number of existing NMDA receptor channels (or attributable to the
increased threshold for LTP induction), stronger activation of the NMDA
receptor channel should restore the reduced LTP.
We first examined the effects of increased stimulus strength during
tetanic stimulation on LTP in the hippocampal CA1 region of the
wild-type (+/+) and GluR1 mutant (/) mice, which would cause
stronger depolarization of the postsynaptic cell, allowing a larger
influx of Ca2+ through NMDA receptor channels. To
compare the effect of the stimulus strength of tetanic stimulation
precisely, we made two-pathway recordings in a single slice. Thus, two
independent afferent inputs were stimulated alternately and a standard
tetanus was delivered to one pathway, while a strong tetanus, which
evoked twofold larger EPSPs in slope value than the baseline EPSPs, was
given to the other pathway. In the mutant mice, tetanic stimulation
with the standard stimulus strength gave rise to quite small magnitude LTP (111 ± 7% of control, n = 7) (Fig.
1A, open
circles), as reported previously (Sakimura et al., 1995), whereas
a stronger tetanus with the same stimulation pattern restored the
impaired LTP to the normal level (130 ± 5% of control;
p < 0.01, paired Student's t test) (Fig.
1A, filled circles). The standard
conditioning produced normal LTP in the wild-type mice (135 ± 6%
of control, n = 4) (Fig. 1B,
open circles), and increasing the stimulus strength during
the tetanus had little effect on LTP (132 ± 10% of control) (Fig. 1B, filled circles). These results
suggest that the stronger depolarization caused by the stronger tetanic
stimulation, which would activate NMDA receptor channels more
efficiently, can rescue the reduced LTP and that the impairment of LTP
is likely to be the result of an increased threshold for the LTP
induction process in the GluR1 mutant mice.
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Figure 1.
Stronger tetanic stimulation restores the impaired
LTP in GluR1 mutant mice. A, B, Time
courses of LTP in the mutant (A) and wild-type
(B) mice. Two independent pathways were
stimulated alternately. In one pathway a standard tetanus was applied
at time 0 (open circles;
n = 7 in A and n = 4 in B). In the other pathway a stronger tetanus was
delivered with some time lag (filled circles).
The pathway to which the stronger tetanic stimulation was given was
selected at random for each experiment. Sample traces represent EPSPs
recorded at the times indicated by the numbers in the
graph. Stimulus artifacts are truncated. EPSP slopes are normalized by
the averaged slope value of control EPSPs before tetanic
stimulation.
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If the impairment of LTP is attributable solely to the increased
threshold for the LTP induction and is not attributable to the
perturbation of LTP expression mechanisms, the saturation level of LTP
in the mutant mice is expected to be the same as that in the
wild-type mice. To saturate LTP, we repeatedly applied high-frequency stimulation until no more potentiation was observed (Fig. 2A). The
saturation level of LTP was not significantly different (p = 0.36, unpaired t test) between
the wild-type (209 ± 18% of control, n = 7) and
mutant (200 ± 13% of control, n = 7) mice (Fig.
2B). These results further suggest that the
impairment of LTP induced by a single train of high-frequency
stimulationmost likely is attributable to the increased threshold for
the induction and that the LTP expression mechanisms are intact.
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Figure 2.
Saturation level of LTP. A,
Examples of LTP saturation in the wild-type (open
circles) and GluR1 mutant (filled
circles) mice. B, Summary of LTP saturation
measurements.
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GluR1 mutant mice with a highly homogenous
genetic background
It is well established that the genetic background significantly
affects the behaviors of mice (Crawley, 1996; Gerlai, 1996; Lathe,
1996). Among mouse strains, C57BL/6 performs various learning tasks
well, such as the Morris water maze and contextual fear conditioning
(Owen et al., 1997). Because we produced GluR1 mutant mice by
homologous recombination in TT2 ES cells derived from C57BL/6 × CBA F1 hybrid (Yagi et al., 1993) by using a targeting vector with
genomic DNA from C57BL/6 mouse (Sakimura et al., 1995), we used a
backcross strategy to examine the effect of the disruption of the
GluR1 gene on learning behaviors under a pure C57BL/6 genetic
background. After 12 backcrosses to C57BL/6 strain, we obtained F13
GluR1 mutant mouse with a calculated C57BL/6 genetic background of
99.99%.
Fear conditioning and genetic background
We used fear conditioning tests to examine the learning ability of
GluR1 mutant mice with a highly homogeneous genetic background. Fear
conditioning is a rapidly acquired and persistent form of simple
associative learning between an aversive footshock and a conditional
stimulus such as a tone and an experimental chamber (Fanselow, 1984).
The freezing response is characterized by an immobile, crouching
posture and is the dominant defense reaction to the conditional fear in
rodents (Fanselow, 1984).
For the assessment of the freezing response we judged the behaviors of
the experimental mice to be freezing or not at every moment by using a
switch recorder, as exemplified in Figure
3. By this switch-recording method, we
obtained the entire profile of the freezing behavior and calculated the
percentage of the total freezing time as an index of the extent of the
freezing response. In the following analyses we used wild-type and
GluR1 mutant mice from P35 to P48.
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Figure 3.
Recording of freezing responses. Samples of
records from the wild-type and mutant mice are shown. Freezing is
upward, and moving is downward.
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To assess the effect of the homogeneity of the genetic background on
the contextual fear conditioning, we compared the wild-type littermates
of F13 mice (calculated C57BL/6 genetic background, 99.99%) with those
of F2 mice (75%), together with parental C57BL/6 and CBA mice (Fig.
4). Mice were placed in the conditioning
chamber and were given one footshock at 3 min after placement. One
minute after footshock the mice were returned to home cages. On the
next day the animals were returned to the conditioning chamber, and their freezing behaviors were monitored for 6 min. C57BL/6 mice showed
high levels of freezing time (43 ± 3%, n = 21),
as described previously (Paylor et al., 1994; Owen et al., 1997),
whereas CBA mice exhibited a very low level of freezing responses
(2.2 ± 0.6%, n = 12), and other defense
reactions, such as tail ratting and a crouching position, were
observed. Thus, there was a significant difference in contextual
freezing responses between the two strains (F(1,31) = 93.6; p < 0.001).
The extent of freezing of F13 mice (38 ± 8%, n = 10) was as high as that of C57BL/6 mice (F(1,29) = 0.48; p = 0.49), whereas that of F2 mice (19 ± 4%, n = 29) was significantly lower
(F(1,48) = 23; p < 0.001).
There was no significant difference between C57BL/6 and F13 mice. Thus,
the backcrosses yielded the GluR1 mutant mice with a high ability to
perform fear conditioning.
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Figure 4.
Effects of genetic backgrounds on freezing
responses in contextual fear conditioning. Mice were given a footshock
3 min after placement in the conditioning chamber. The freezing
responses were monitored for 6 min on the next day. The wild-type
littermates of F2 and F13 GluR1 mice were compared for their ability
of contextual learning with parental C57BL/6 and CBA mice.
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Tone-dependent fear conditioning
There are two paradigms in fear conditioning (Kim and Fanselow,
1992; Phillips and LeDoux, 1992). One form is the pairing of an
aversive footshock with specific cues, such as a tone, which is
insensitive to hippocampal lesions. The other is the pairing with
nonspecific cues, such as the context of an experimental chamber, which
is sensitive to hippocampal lesions. First, we tested the mutant mice
in the tone-dependent fear conditioning task. Mice were placed in the
conditioning chamber. After 2 min, a tone was presented for 1 min and a
footshock was given at the end of the tone presentation. Figure
5A shows the percentage of the
freezing time averaged every minute during conditioning. Both the
wild-type and mutant mice exhibited a slight freezing during tone
presentation (+/+, 2.4 ± 0.6%, n = 7; /,
2.0 ± 1.0%, n = 5) and after the footshock (+/+,
7.5 ± 2.6%, n = 7; /, 6.7 ± 3.1%,
n = 5). There were no differences in the extent of
freezing between the two genotypes (during tone presentation,
F(1,10) = 0.15, p = 0.71; after
footshock, F(1,10) = 0.051, p = 0.83). On the next day the animals were placed in a chamber with novel
contexts, and the tone was presented 3 min after placement (Fig.
5B). Until the tone was presented, the wild-type and mutant
mice displayed only a weak freezing behavior in the novel chamber (+/+,
4.9 ± 1.5%, n = 7; /, 4.2 ± 0.5%,
n = 5). When the tone was presented, both the wild-type
and mutant mice showed strong freezing responses (+/+, 55 ± 8%,
n = 7; /, 56 ± 6%, n = 5),
and the extent of the freezing did not differ significantly between the
two groups (F(1,10) = 0.003; p = 0.96). Thus, GluR1 mutant mice were apparently normal in the
performance of the tone-dependent fear response.
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Figure 5.
Freezing responses of the wild-type (open
circles) and GluR1 mutant (filled
circles) mice in tone-dependent fear conditioning.
A, Freezing responses on the conditioning day. Two
minutes after placement in the conditioning chamber a tone was
presented for 1 min (solid line), and mice were given a
footshock (arrow) at the end of the tone presentation.
B, Freezing responses on the next day. Three minutes
after placement in a testing chamber with novel contexts, a tone was
presented (solid line).
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Because pain sensitivity would affect freezing responses, we measured
current thresholds for three reactions of mice to nociceptive shock,
namely, flinch, vocalization, and jump (Kim et al., 1991). Mice were
given footshocks of increasing strength ranging from 0.1 to 0.5 mA in a
stepwise manner by 0.05 mA. Mice of both genotypes exhibited flinch
responses at 0.1 mA (+/+, n = 12; /,
n = 19) except for one mutant mouse (0.15 mA). There
were no significant differences between the wild-type and mutant mice
in pain thresholds for vocal (+/+, 0.15 ± 0.01 mA,
n = 12; /, 0.15 ± 0.01 mA, n = 19; F(1,29) = 0.034; p = 0.85)
and jump reactions (+/+, 0.40 ± 0.01 mA, n = 12;
/, 0.41 ± 0.01 mA, n = 19;
F(1,29) = 1.04; p = 0.32) (Fig.
6).
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Figure 6.
Current thresholds of the wild-type (open
boxes) and GluR1 mutant (filled boxes)
mice for flinch, vocal, and jump reactions.
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Contextual fear conditioning
We then examined the contextual fear conditioning that is
sensitive to hippocampal lesions (Kim and Fanselow, 1992; Phillips and
LeDoux, 1992). Mice were placed in the conditioning chamber and were
given one footshock 3 min after placement (Fig.
7E). Mice were removed 1 min
after footshock and returned to home cages. On the next day the animals
were returned to the chamber, and their freezing behaviors were
monitored for 6 min (Fig. 7F).
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Figure 7.
Freezing responses of the wild-type (open
circles) and GluR1 mutant (filled
circles) mice on the conditioning day (A,
C, E) and on the next day
(B, D, F) in
contextual fear conditioning. A footshock (arrows)
was given immediately (A), 20 sec
(C), or 3 min (E) after
placement of the mice in the conditioning chamber. Freezing responses
were monitored for 1 min after shock on the conditioning day and for 6 min on the next day.
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On the conditioning day the mice placed in the conditioning chamber
showed little or no freezing responses (Fig. 7E). During the
1 min immediately after the footshock both the wild-type and mutant
mice exhibited slight freezing responses (+/+, 8.1 ± 2.7%, n = 10; /, 2.4 ± 1.0%, n = 24). However, there was a significant difference in the extent of the
freezing immediately after shock (FIAS) between the wild-type and
mutant mice (F(1,32) = 6.4; p = 0.016). When returned to the conditioning chamber on the next day (Fig.
7F), both the wild-type and mutant mice showed strong freezing responses during the 6 min period (+/+, 38 ± 8%,
n = 10; /, 29 ± 3%, n = 24),
and there was no significant difference in the extent of freezing
between these animals (F(1,32) = 1.75; p = 0.20). Thus, the GluR1 mutation affected FIAS on
the conditioning day.
These observations tempted us to examine further the contextual fear
conditioning under weaker conditioning stimuli by decreasing the time
interval between the placement in the conditioning chamber (the
exposure time to the chamber) and footshock. When a footshock was given
immediately after placement in the chamber, both the wild-type and
mutant mice exhibited little or no freezing responses on the
conditioning day (Fig. 7A) and on the next day (Fig.
7B). Thus, a certain exposure time to the chamber before
footshock is essential for both FIAS and freezing responses on the next day, indicating that both types of fear responses are associative in
nature, as described for rats (Fanselow, 1986).
When a footshock was given at 20 sec after placement, the wild-type
mice showed a weak but significant FIAS (3.9 ± 1.2%,
n = 22) (Fig. 7C). On the other hand, the
mutant mice exhibited less FIAS (0.8 ± 0.4%, n = 26) than the wild-type mice (F(1,46) = 7.4;
p < 0.01). On the next day of conditioning the
wild-type mice exhibited strong freezing responses (26 ± 4%,
n = 22), indicating that the chamber exposure time of
20 sec was sufficient for the wild-type mice to be fear-conditioned
(Fig. 7D). In contrast, the mutant mice showed only a weak
response (5.5 ± 2.4%, n = 26), and there was a
significant difference in the contextual fear conditioning between the
wild-type and mutant mice (F(1, 46) = 19.1;
p < 0.001). These results suggest that the contextual
learning of the GluR1 mutant mice is impaired under a weak
conditional stimulus (short chamber exposure time).
Then we further examined the extent of the contextual fear conditioning
by giving a footshock at various time intervals after placement in the
conditioning chamber (Fig. 8). On the
conditioning day both the wild-type and mutant mice exhibited slight
freezing responses during the 1 min after the footshock, and the extent of FIAS gradually increased when the time interval between the placement in the conditioning chamber and the footshock became longer
(Fig. 8A). However, GluR1 mutant mice showed
significantly less FIAS than the wild-type mice at time intervals of 20 sec (F(1,46) = 7.4; p < 0.01),
30 sec (F(1,54) = 5.0; p = 0.03), 1 min (F(1,39) = 9.3; p < 0.01), 3 min (F(1,32) = 6.4;
p = 0.02), and 9 min (F(1,22) = 23; p < 0.001). Because FIAS is associative in nature
and the mutant mice performed normally the tone-dependent freezing
response (see above), these results suggest that associative learning
immediately after the contextual conditioning is impaired in GluR1
mutant mice.
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Figure 8.
Effects of chamber exposure time on the freezing
responses of the wild-type (open circles) and GluR1
mutant (filled circles) mice in contextual fear
conditioning. A footshock was given at 0, 10, 20, and 30 sec and 1, 3, or 9 min after placement of the mice in the conditioning chamber. The
freezing responses during the 1 min immediately after shock on the
conditioning day (A) and those during 6 min after
placement in the conditioning chamber on the next day
(B) were plotted as a function of the time
interval between the placement in the chamber and footshock on the
conditioning day.
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The contextual freezing responses on the next day of the conditioning
also increased when the time interval between the placement in the
conditioning chamber and the footshock became longer (Fig. 8B). However, the chamber exposure time (conditioning
stimulus)-freezing response curve for the mutant mice shifted
rightward as compared with that of the wild-type mice, and there was a
significant difference in the contextual freezing responses between the
wild-type and mutant mice at time intervals of 20 sec
(F(1,46) = 19; p < 0.001), 30 sec (F(1,54) = 9.1; p < 0.01),
and 1 min (F(1,39) = 9.3; p < 0.001). The saturation level of the freezing response was comparable between the wild-type and mutant mice (at 3 min,
F(1,32) = 1.75, p = 0.20; at 9 min, F(1,22) = 0.47, p = 0.50).
These results suggest that the threshold for the contextual learning
increases in GluR1 mutant mice. Furthermore, the difference in the
extent of FIAS on the conditioning day does not necessarily correlate
to the contextual fear memory on the next day.
Freezing immediately after shock
Because GluR1 mutant mice showed an impairment of FIAS,
we tested the effect of additional shocks (Fig.
9). Mice were placed in the conditioning
chamber for 3 min, and then three footshocks were given at 1 min
intervals. Mice were removed from the chamber 1 min after the last
footshock and returned to home cages. The extent of the freezing
responses increased after every footshock in both the wild-type and
mutant mice (Fig. 9A). Even after three footshocks, however,
the extent of the freezing response of the GluR1 subunit mutant mice
during 1 min (23 ± 11%, n = 5) was significantly
less than that of the wild-type mice (64 ± 6%, n = 6) (F(1,9) = 14.4; p = 0.004).
In contrast, there was no significant difference in the extent of
contextual freezing responses on the next day between the wild-type and
mutant mice (+/+, 73 ± 5%, n = 6; /, 57 ± 5%, n = 5) (F(1,9) = 6.9;
p = 0.28) (Fig. 9B).
View larger version (22K):
[in this window]
[in a new window]
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Figure 9.
Freezing responses of the wild-type (open
circles) and GluR1 mutant (filled
circles) mice on the conditioning day (A)
and on the next day (B) when footshocks
(arrows) were given at 3, 4, and 5 min after placement
in the conditioning chamber. Freezing responses were monitored for 1 min after the last shock on the conditioning day and for 6 min on the
next day.
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|
We then asked how long the difference in FIAS between the wild-type and
mutant mice was retained. Mice were placed in the conditioning chamber
and given a footshock 3 min after placement in the conditioning
chamber. Freezing responses of a group of mice were monitored for 6 min
after the footshock (Fig. 10,
circles). There was a significant difference in FIAS
monitored for 6 min between the wild-type (open circles,
18 ± 5%, n = 12) and mutant (filled
circles, 3.7 ± 0.6%, n = 13) mice
(F(1,23) = 6.7; p = 0.017).
Another group of mice was removed from the conditioning chamber 1 min
after the footshock, placed in home cages, and returned to the
conditioning chamber at 20 min after the footshock; freezing responses
were monitored for 6 min. The freezing responses of the mutant mice 20 min after conditioning (21 ± 4%, n = 9) were significantly higher than the FIAS of the mutant mice
(F(1,20) = 35; p < 0.001) and
became comparable with those of the wild-type mice (24 ± 7%,
n = 8) (F(1,15) = 0.14;
p = 0.71). Even when mice were returned to the
conditioning chamber 2 min after conditioning (Fig. 10,
triangles), the freezing responses of the mutant mice during
the 6 min period (filled triangles, 19 ± 4%,
n = 13) were significantly higher than the FIAS of the
mutant mice (F(1,24) = 15; p < 0.001) and were comparable with those of the wild-type mice (open
triangles, 29 ± 6%, n = 11)
(F(1,22) = 2.0; p = 0.17).
View larger version (20K):
[in this window]
[in a new window]
|
Figure 10.
Freezing responses of the wild-type (open
symbols) and GluR1 mutant (filled
symbols) mice immediately and 2 min after footshock. Mice were
given a footshock (arrow) 3 min after placement in the
conditioning chamber. Freezing responses of one group of mice
(circles) were monitored for 8 min after the footshock,
and those during the initial 6 min are shown for comparison; the extent
of freezing during the last 2 min was essentially the same as that
during the initial 6 min. Another group of mice
(triangles) was removed from the conditioning chamber 1 min after the footshock, returned to the conditioning chamber at 2 min
after the footshock, and monitored for freezing responses for 6 min.
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DISCUSSION |
Increased threshold for LTP induction
It generally is agreed that postsynaptic NMDA receptor channels
(Collingridge et al., 1983) and Ca2+ influx through
the channels (Lynch et al., 1983; Malenka et al., 1988) play crucial
roles in the induction of LTP in the CA1 region of the hippocampus. The
degree of NMDA receptor channel activation determines the threshold for
LTP induction (Malenka, 1991), and the amount of
Ca2+ influx through the channel regulates the
magnitude and direction of changes in synaptic efficacy (Perkel et al.,
1993; Cummings et al., 1996). We have shown that the disruption of the
GluR1 subunit causes a partial reduction of LTP in the hippocampal
CA1 region (Sakimura et al., 1995). In these mice the size of synaptic responses mediated by NMDA receptor channels is approximately one-half
that of the wild-type mice, suggesting that the GluR1 subunit
contributes, together with the GluR1 subunit, to one-half of the
NMDA synaptic transmission and that the remaining NMDA synaptic
responses are mediated exclusively by the GluR2/1 channel. Because the expression of GluR2 subunit proteins is unchanged in
GluR1 mutant mice (Sakimura et al., 1995), the reduction of LTP
magnitudes in the mutant mice can be explained most easily by the
decrease in the number of active NMDA receptor channels, which would
increase the threshold for LTP.
In the present study we have examined these issues more directly by
manipulating the activation of NMDA receptor channels in the mutant
mice. When NMDA receptor channels are activated more strongly (by
stronger tetanic stimulation) or repeatedly (by multiple trains of
tetanic stimulation), resulting LTP seems to be normal in the mutant
mice. These results strongly suggest that the impairment of LTP
observed in the mutant mice is caused by the reduction of
Ca2+ influx attributable to the absence of the
GluR1 subunit and that LTP expression mechanisms after
Ca2+ influx are not affected by the ablation of the
GluR1 subunit. Furthermore, they also indicate that, although the
GluR1 subunit is involved in the LTP induction, the GluR2 subunit
can substitute for the GluR1 subunit in the process of synaptic
plasticity in the hippocampus.
We did not observe any enhancement of LTP by tetanic stimulation with
the stronger stimulus strength in the wild-type slices. It is possible
that Ca2+ influx during the standard tetanus may
already reach saturation level for LTP induction. Alternatively, the
larger depolarization by the stronger tetanus may reduce the driving
force for Ca2+ influx through NMDA receptor
channels, resulting in no apparent changes in Ca2+
influx. On the other hand, in the mutant slices, depolarization during
the standard tetanus may not be sufficient to remove a voltage-dependent Mg2+ block of NMDA receptor
channels completely, and stronger tetanus may depolarize the cells to
the level more favorable for LTP induction.
Increased threshold for contextual learning
Because various inbred mouse strains exhibit wide variations in
their performance of learning behaviors (Crawley, 1996; Gerlai, 1996;
Lathe, 1996; Owen et al., 1997), it is important to examine the effect
of eliminating any specific gene on learning behaviors under a
homogeneous genetic background. We produced a GluR1 mutant mouse
line with a highly homogeneous C57BL/6 genetic background (calculated
to be 99.99% pure) by successive backcrosses. The performance of
contextual fear conditioning of the wild-type littermates of the
established F13 line was as high as that of C57BL/6 mice, whereas the
wild-type F2 littermates with a genetic background of 75%
C57BL/6 exhibited a lower level of freezing responses. Thus, the mutant
mice with a highly homogeneous genetic background will provide a
valuable tool to investigate the physiological role of the GluR1
subunit of the NMDA receptor channel. Furthermore, monitoring the
freezing responses of mice at every moment with a switch recorder made
it possible to obtain exact and entire profiles of the freezing
responses.
GluR1 mutant mice performed two types of fear conditioning tests
under single conditioning shock, although they showed impairment in the
performance of the Morris water maze task (Sakimura et al., 1995).
Similarly, a transgenic mouse with a mutant
Ca2+/calmodulin kinase II showed impairment in
hippocampal LTP and spatial learning, but not in contextual learning
(Bach et al., 1995). However, deficiency in contextual learning of
GluR1 mutant mice became evident when the conditioned stimulus was
weakened by shortening the time interval between the placement in the
conditioning chamber and the footshock. Systematic analyses of
contextual learning under various chamber exposure times revealed that
the conditioning stimulus-freezing response curve shifted rightward in
the mutant mice, whereas the saturation level of the freezing response
was comparable with that of the wild-type mice. These results suggest that the deprivation of the GluR1 subunit increases the threshold for contextual learning. Thus, the present investigation provides evidence that the GluR1 subunit of the NMDA receptor channel is an important determinant of thresholds for both LTP and contextual learning. These findings suggest that the increased threshold for LTP
induction results in a need for longer context exposure before the
conditioning for robust learning can occur, implying a strong
correlation between synaptic plasticity and contextual learning.
Recently, Sprengel et al. (1998) argued that the mutant mice expressing
the GluR1 subunit lacking most of the large intracellular C-terminal
region phenotypically resembled the GluR1 null mutant mice and that
the C-terminal domain was important for signal transduction. However,
the impairment of hippocampal LTP and contextual fear conditioning
seems to be more severe in the C-terminal-truncated GluR1 mutant
mice than in the null mutant mice. The truncated GluR1 subunit may
exert some abnormal dominant-negative effects. In fact, these mutant
mice exhibit deficits in motor coordination, but such defects are not
found in the null mutant mice (Kadotani et al., 1996).
Impairment of freezing immediately after shock
We found that GluR1 ablation strongly reduced FIAS on the
conditioning day in the fear conditioning test. Because mice showed little freezing when the shock was given immediately after placement in
the chamber and the extent of FIAS increased as the chamber exposure
time became longer, this freezing response was associative in nature,
as described in rats (Fanselow, 1986). Assuming that FIAS is
associatively based (Kim et al., 1991), these results suggest that the
GluR1 subunit of the NMDA receptor channel is critical for an
associative memory immediately after conditioning.
When the interval between the placement in the conditioning chamber and
footshock was lengthened, the deficit of contextual freezing on the
next day was restored, but FIAS remained impaired in GluR1 mutant
mice. Thus, FIAS does not relate directly to the contextual fear memory
after 24 hr of conditioning. These results suggest that there are two
forms of contextual learning in mice, one immediately after
conditioning and the other 1 d after conditioning. The former
depends strongly on the GluR1 subunit of the NMDA receptor
channel, whereas the latter requires the GluR1 subunit when the
conditioning stimulus is weak. The differential effects of the GluR1
subunit on the two forms of fear conditioning suggest that different
neural systems may underlie the two forms of memories.
Intracerebroventricular infusion of APV in rats produced selective
deficits on the acquisition of contextual fear conditioning expressed
24 hr after training but immediate postshock freezing was not affected
(Kim et al., 1991, 1992), whereas intra-amygdaloid APV infusion
impaired both (Maren et al., 1996). Thus, different brain regions may
be responsible for the two forms of contextual learning. It is well
established that the nuclei of the amygdala are essential for fear
conditioning (Miserendino et al., 1990; LeDoux, 1995; Maren and
Fanselow, 1996). In addition, the contextual fear conditioning is also
sensitive to hippocampal lesions (Kim and Fanselow, 1992; Phillips and
LeDoux, 1992), whereas post-training lesions of the dorsal hippocampus attenuated contextual freezing but had no effect on fear-potentiated startle (McNish et al., 1997). Among four GluR subunits of the NMDA
receptor channel, the GluR1 and GluR2 subunits are expressed strongly in the amygdala and the hippocampus of the adult brain (Watanabe et al., 1992, 1993, 1998). Fine differences in functional properties (Seeburg, 1993; Mori and Mishina, 1995), localization, or
contribution to synapses (Ito et al., 1997) between the two subunits
may underlie the differential contribution of the GluR1 subunit to
the two forms of contextual memories.
It is believed that the two forms of contextual fear memories are
temporally distinct processes in rats: short-term conditional fear and
long-term conditional fear (Kim et al., 1991, 1992). When GluR1
mutant mice were returned to the conditioning chamber 2 min after
conditioning under sufficient chamber exposure time (3 min, Fig. 10),
the freezing response of the mutant mice became significantly higher
than FIAS to be comparable with that of the wild-type mice. Because the
periods of measurement were greatly overlapped between two groups of
mutant mice, the procedure of removal and placement again into the
conditioning chamber, rather than the time after shock, appears to be
responsible for the difference in the freezing responses of the
GluR1 mutant mice measured immediately after shock and those 2 min
after shock. Thus, the two forms of contextual fear memories we
observed in mice may be procedurally, rather than temporally,
distinct.
| |
FOOTNOTES |
Received April 29, 1998; revised June 15, 1998; accepted June 17, 1998.
This work was supported by research grants from Core Research for
Evolutional Science and Technology of Japan Science and Technology
Corporation, the Ministry of Education, Science, Sports, and Culture of
Japan, and the Asahi Glass Foundation. We thank Dr. T. Takahashi for
advice and encouragement; Mses. R. Natsume, Y. Asaka, M. Yoshida, and
M. Furuya for help in breeding mice; and Ms. M. Senbonmatsu for help in
the preparation of this manuscript.
Correspondence should be addressed to Dr. Masayoshi Mishina, Department
of Molecular Neurobiology and Pharmacology, School of Medicine,
University of Tokyo, Hongo 7-3-1, Bunkyo-ku, Tokyo 113-0033, Japan.
| |
REFERENCES |
-
Bach ME,
Hawkins RD,
Osman M,
Kandel ER,
Mayford M
(1995)
Impairment of spatial but not contextual memory in CaMKII mutant mice with a selective loss of hippocampal LTP in the range of
 frequency.
Cell
81:905-915[Web of Science][Medline]. -
Bannerman DM,
Good MA,
Butcher SP,
Ramsay M,
Morris RG
(1995)
Distinct components of spatial learning revealed by prior training and NMDA receptor blockade.
Nature
378:182-186[Medline].
-
Bliss TV,
Collingridge GL
(1993)
A synaptic model of memory: long-term potentiation in the hippocampus.
Nature
361:31-39[Medline].
-
Cline HT,
Debski EA,
Constantine-Paton M
(1987)
N-methyl-D-aspartate receptor antagonist desegregates eye-specific stripes.
Proc Natl Acad Sci USA
84:4342-4345[Abstract/Free Full Text].
-
Collingridge GL,
Kehl SJ,
McLennan H
(1983)
Excitatory amino acids in synaptic transmission in the Schaffer collateral-commissural pathway of the rat hippocampus.
J Physiol (Lond)
334:33-46[Abstract/Free Full Text].
-
Crawley JN
(1996)
Unusual behavioral phenotypes of inbred mouse strains.
Trends Neurosci
19:181-182[Web of Science][Medline].
-
Cummings JA,
Mulkey RM,
Nicoll RA,
Malenka RC
(1996)
Ca2+ signaling requirements for long-term depression in the hippocampus.
Neuron
16:825-833[Web of Science][Medline].
-
Ebralidze AK,
Rossi DJ,
Tonegawa S,
Slater NT
(1996)
Modification of NMDA receptor channels and synaptic transmission by targeted disruption of the NR2C gene.
J Neurosci
16:5014-5025[Abstract/Free Full Text].
-
Fanselow MS
(1980)
Conditioned and unconditional components of post-shock freezing.
Pavlov J Biol Sci
15:177-182[Web of Science][Medline].
-
Fanselow MS
(1984)
What is conditioned fear?
Trends Neurosci
7:460-462[Web of Science].
-
Fanselow MS
(1986)
Associative vs topographical accounts of the immediate shock-freezing deficit in rats: implications for the response selection rules governing species-specific defensive reactions.
Learn Motiv
17:16-39[Web of Science].
-
Forrest D,
Yuzaki M,
Soares HD,
Ng L,
Luk DC,
Sheng M,
Stewart CL,
Morgan JI,
Connor JA,
Curran T
(1994)
Targeted disruption of NMDA receptor 1 gene abolishes NMDA response and results in neonatal death.
Neuron
13:325-338[Web of Science][Medline].
-
Gerlai R
(1996)
Gene-targeting studies of mammalian behavior: is it the mutation or the background genotype?
Trends Neurosci
19:177-181[Web of Science][Medline].
-
Hollmann M,
Boulter J,
Maron C,
Beasley L,
Sullivan J,
Pecht G,
Heinemann S
(1993)
Zinc potentiates agonist-induced currents at certain splice variants of the NMDA receptor.
Neuron
10:943-954[Web of Science][Medline].
-
Ikeda K,
Nagasawa M,
Mori H,
Araki K,
Sakimura K,
Watanabe M,
Inoue Y,
Mishina M
(1992)
Cloning and expression of the 4 subunit of the NMDA receptor channel.
FEBS Lett
313:34-38[Web of Science][Medline].
-
Ikeda K,
Araki K,
Takayama C,
Inoue Y,
Yagi T,
Aizawa S,
Mishina M
(1995)
Reduced spontaneous activity of mice defective in the 4 subunit of the NMDA receptor channel.
Mol Brain Res
33:61-71[Medline].
-
Ito I,
Futai K,
Katagiri H,
Watanabe M,
Sakimura K,
Mishina M,
Sugiyama H
(1997)
Synapse-selective impairment of NMDA receptor functions in mice lacking NMDA receptor 1 or 2 subunit.
J Physiol (Lond)
500:401-408[Abstract/Free Full Text].
-
Kadotani H,
Hirano T,
Masugi M,
Nakamura K,
Nakao K,
Katsuki M,
Nakanishi S
(1996)
Motor discoordination results from combined gene disruption of the NMDA receptor NR2A and NR2C subunits, but not from single disruption of the NR2A or NR2C subunit.
J Neurosci
16:7859-7867[Abstract/Free Full Text].
-
Kim JJ,
Fanselow MS
(1992)
Modality-specific retrograde amnesia of fear.
Science
256:675-677[Abstract/Free Full Text].
-
Kim JJ,
DeCola P,
Landeira-Fernandez J,
Fanselow MS
(1991)
N-methyl-D-aspartate receptor antagonist APV blocks acquisition but not expression of fear conditioning.
Behav Neurosci
105:126-133[Web of Science][Medline].
-
Kim JJ,
Fanselow MS,
DeCola JP,
Landeira-Fernandez J
(1992)
Selective impairment of long-term but not short-term conditional fear by the N-methyl-D-aspartate antagonist APV.
Behav Neurosci
106:591-596[Web of Science][Medline].
-
Kleinschmidt A,
Bear MF,
Singer W
(1987)
Blockade of "NMDA" receptors disrupts experience-dependent plasticity of kitten striate cortex.
Science
238:355-358[Abstract/Free Full Text].
-
Kutsuwada T,
Kashiwabuchi N,
Mori H,
Sakimura K,
Kushiya E,
Araki K,
Meguro H,
Masaki H,
Kumanishi T,
Arakawa M,
Mishina M
(1992)
Molecular diversity of the NMDA receptor channel.
Nature
358:36-41[Medline].
-
Kutsuwada T,
Sakimura K,
Manabe T,
Takayama C,
Katakura N,
Kushiya E,
Natsume R,
Watanabe M,
Inoue Y,
Yagi T,
Aizawa S,
Arakawa M,
Takahashi T,
Nakamura Y,
Mori H,
Mishina M
(1996)
Impairment of suckling response, trigeminal neuronal pattern formation, and hippocampal LTD in NMDA receptor 2 subunit mutant mice.
Neuron
16:333-344[Web of Science][Medline].
-
Lathe R
(1996)
Mice, gene targeting, and behavior: more than just genetic background.
Trends Neurosci
19:183-186[Web of Science][Medline].
-
LeDoux JE
(1995)
Emotion: clues from the brain.
Annu Rev Psychol
46:209-235[Web of Science][Medline].
-
Li Y,
Erzurumlu RS,
Chen C,
Jhaveri S,
Tonegawa S
(1994)
Whisker-related neuronal patterns fail to develop in the trigeminal brainstem nuclei of NMDAR1 knock-out mice.
Cell
76:427-437[Web of Science][Medline].
-
Lynch G,
Larson J,
Kelso S,
Barrionuevo G,
Schottler F
(1983)
Intracellular injections of EGTA block induction of hippocampal long-term potentiation.
Nature
305:719-721[Medline].
-
Malenka RC
(1991)
Postsynaptic factors control the duration of synaptic enhancement in area CA1 of the hippocampus.
Neuron
6:53-60[Web of Science][Medline].
-
Malenka RC,
Nicoll RA
(1993)
NMDA receptor-dependent synaptic plasticity: multiple forms and mechanisms.
Trends Neurosci
16:521-527[Web of Science][Medline].
-
Malenka RC,
Kauer JA,
Zucker RS,
Nicoll RA
(1988)
Postsynaptic calcium is sufficient for potentiation of hippocampal synaptic transmission.
Science
242:81-84[Abstract/Free Full Text].
-
Maren S,
Fanselow MS
(1996)
The amygdala and fear conditioning: has the nut been cracked?
Neuron
16:237-240[Web of Science][Medline].
-
Maren S,
Aharonov G,
Stote DL,
Fanselow MS
(1996)
N-methyl-D-aspartate receptors in the basolateral amygdala are required for both acquisition and expression of conditional fear in rats.
Behav Neurosci
110:1365-1374[Web of Science][Medline].
-
McNish KA,
Gewirtz JC,
Davis M
(1997)
Evidence of contextual fear after lesions of the hippocampus: a disruption of freezing but not fear-potentiated startle.
J Neurosci
17:9353-9360[Abstract/Free Full Text].
-
Meguro H,
Mori H,
Araki K,
Kushiya E,
Kutsuwada T,
Yamazaki M,
Kumanishi T,
Arakawa M,
Sakimura K,
Mishina M
(1992)
Functional characterization of a heteromeric NMDA receptor channel expressed from cloned cDNAs.
Nature
357:70-74[Medline].
-
Miserendino MJ,
Sananes CB,
Melia KR,
Davis M
(1990)
Blocking of acquisition but not expression of conditioned fear-potentiated startle by NMDA antagonists in the amygdala.
Nature
345:716-718[Medline].
-
Monyer H,
Sprengel R,
Schoepfer R,
Herb A,
Higuchi M,
Lomeli H,
Burnashev N,
Sakmann B,
Seeburg PH
(1992)
Heteromeric NMDA receptors: molecular and functional distinction of subtypes.
Science
256:1217-1221[Abstract/Free Full Text].
-
Mori H,
Mishina M
(1995)
Structure and function of the NMDA receptor channel.
Neuropharmacology
34:1219-1237[Web of Science][Medline].
-
Moriyoshi K,
Masu M,
Ishii T,
Shigemoto R,
Mizuno N,
Nakanishi S
(1991)
Molecular cloning and characterization of the rat NMDA receptor.
Nature
354:31-37[Medline].
-
Morris RG,
Anderson E,
Lynch GS,
Baudry M
(1986)
Selective impairment of learning and blockade of long-term potentiation by an N-methyl-D-aspartate receptor antagonist, AP5.
Nature
319:774-776[Medline].
-
Nagasawa M,
Sakimura K,
Mori KJ,
Bedell MA,
Copeland NG,
Jenkins NA,
Mishina M
(1996)
Gene structure and chromosomal localization of the mouse NMDA receptor channel subunits.
Mol Brain Res
36:1-11[Medline].
-
Owen EH,
Logue SF,
Rasmussen DL,
Wehner JM
(1997)
Assessment of learning by the Morris water task and fear conditioning in inbred mouse strains and F1 hybrids: implications of genetic background for single gene mutations and quantitative trait loci analyses.
Neuroscience
80:1087-1099[Web of Science][Medline].
-
Paylor R,
Tracy R,
Wehner J,
Rudy JW
(1994)
DBA/2 and C57BL/6 mice differ in contextual fear but not auditory fear conditioning.
Behav Neurosci
108:810-817[Web of Science][Medline].
-
Perkel DJ,
Petrozzino JJ,
Nicoll RA,
Connor JA
(1993)
The role of Ca2+ entry via synaptically activated NMDA receptors in the induction of long-term potentiation.
Neuron
11:817-823[Web of Science][Medline].
-
Phillips RG,
LeDoux JE
(1992)
Differential contribution of amygdala and hippocampus to cued and contextual fear conditioning.
Behav Neurosci
106:274-285[Web of Science][Medline].
-
Sakimura K,
Kutsuwada T,
Ito I,
Manabe T,
Takayama C,
Kushiya E,
Yagi T,
Aizawa S,
Inoue Y,
Sugiyama H,
Mishina M
(1995)
Reduced hippocampal LTP and spatial learning in mice lacking NMDA receptor 1 subunit.
Nature
373:151-155[Medline].
-
Saucier D,
Cain DP
(1995)
Spatial learning without NMDA receptor-dependent long-term potentiation.
Nature
378:186-189[Medline].
-
Seeburg PH
(1993)
The molecular biology of mammalian glutamate receptor channels.
Trends Neurosci
16:359-365[Web of Science][Medline].
-
Sprengel R,
Suchanek B,
Amico C,
Brusa R,
Burnashev N,
Rozov A,
Hvalby O,
Jensen V,
Paulsen O,
Andersen P,
Kim JJ,
Thompson RF,
Sun W,
Webster LC,
Grant SG,
Eilers J,
Konnerth A,
Li J,
McNamara JO,
Seeburg PH
(1998)
Importance of the intracellular domain of NR2 subunits for NMDA receptor function in vivo.
Cell
92:279-289[Web of Science][Medline].
-
Tsien JZ,
Huerta PT,
Tonegawa S
(1996)
The essential role of hippocampal CA1 NMDA receptor-dependent synaptic plasticity in spatial memory.
Cell
87:1327-1338[Web of Science][Medline].
-
Watanabe M,
Inoue Y,
Sakimura K,
Mishina M
(1992)
Developmental changes in distribution of NMDA receptor channel subunit mRNAs.
NeuroReport
3:1138-1140[Web of Science][Medline].
-
Watanabe M,
Inoue Y,
Sakimura K,
Mishina M
(1993)
Distinct distributions of five N-methyl-D-aspartate receptor channel subunit mRNAs in the forebrain.
J Comp Neurol
338:377-390[Web of Science][Medline].
-
Watanabe M,
Fukaya M,
Sakimura K,
Manabe T,
Mishina M,
Inoue Y
(1998)
Selective scarcity of NMDA receptor channel subunits in the stratum lucidum (mossy fiber-recipient layer) of the mouse hippocampal CA3 subfield.
Eur J Neurosci
10:478-487[Web of Science][Medline].
-
Yagi T,
Tokunaga T,
Furuta Y,
Nada S,
Yoshida M,
Tsukada T,
Saga Y,
Takeda N,
Ikawa Y,
Aizawa S
(1993)
A novel ES cell line, TT2, with high germline-differentiating potency.
Anal Biochem
214:70-76[Web of Science][Medline].
-
Yamazaki M,
Mori H,
Araki K,
Mori KJ,
Mishina M
(1992)
Cloning, expression, and modulation of a mouse NMDA receptor subunit.
FEBS Lett
300:39-45[Web of Science][Medline].
Copyright © 1998 Society for Neuroscience 0270-6474/98/18176704-09$05.00/0
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November 11, 2009;
29(45):
14086 - 14099.
[Abstract]
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K. Akashi, T. Kakizaki, H. Kamiya, M. Fukaya, M. Yamasaki, M. Abe, R. Natsume, M. Watanabe, and K. Sakimura
NMDA Receptor GluN2B (GluR{varepsilon}2/NR2B) Subunit Is Crucial for Channel Function, Postsynaptic Macromolecular Organization, and Actin Cytoskeleton at Hippocampal CA3 Synapses
J. Neurosci.,
September 2, 2009;
29(35):
10869 - 10882.
[Abstract]
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F. Longordo, C. Kopp, M. Mishina, R. Lujan, and A. Luthi
NR2A at CA1 Synapses Is Obligatory for the Susceptibility of Hippocampal Plasticity to Sleep Loss
J. Neurosci.,
July 15, 2009;
29(28):
9026 - 9041.
[Abstract]
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T. Muller, D. Albrecht, and C. Gebhardt
Both NR2A and NR2B subunits of the NMDA receptor are critical for long-term potentiation and long-term depression in the lateral amygdala of horizontal slices of adult mice
Learn. Mem.,
May 27, 2009;
16(6):
395 - 405.
[Abstract]
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R. Kimura and N. Matsuki
Protein kinase CK2 modulates synaptic plasticity by modification of synaptic NMDA receptors in the hippocampus
J. Physiol.,
July 1, 2008;
586(13):
3195 - 3206.
[Abstract]
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D. M. Bannerman, B. Niewoehner, L. Lyon, C. Romberg, W. B. Schmitt, A. Taylor, D. J. Sanderson, J. Cottam, R. Sprengel, P. H. Seeburg, et al.
NMDA Receptor Subunit NR2A Is Required for Rapidly Acquired Spatial Working Memory But Not Incremental Spatial Reference Memory
J. Neurosci.,
April 2, 2008;
28(14):
3623 - 3630.
[Abstract]
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J. L. Brigman, M. Feyder, L. M. Saksida, T. J. Bussey, M. Mishina, and A. Holmes
Impaired discrimination learning in mice lacking the NMDA receptor NR2A subunit
Learn. Mem.,
January 28, 2008;
15(2):
50 - 54.
[Abstract]
[Full Text]
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J.-P. Zhao and M. Constantine-Paton
NR2A / Mice Lack Long-Term Potentiation But Retain NMDA Receptor and L-Type Ca2+ Channel-Dependent Long-Term Depression in the Juvenile Superior Colliculus
J. Neurosci.,
December 12, 2007;
27(50):
13649 - 13654.
[Abstract]
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Y. Zhou, E. Takahashi, W. Li, A. Halt, B. Wiltgen, D. Ehninger, G.-D. Li, J. W. Hell, M. B. Kennedy, and A. J. Silva
Interactions between the NR2B Receptor and CaMKII Modulate Synaptic Plasticity and Spatial Learning
J. Neurosci.,
December 12, 2007;
27(50):
13843 - 13853.
[Abstract]
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A. Fontan-Lozano, J. L. Saez-Cassanelli, M. C. Inda, M. de los Santos-Arteaga, S. A. Sierra-Dominguez, G. Lopez-Lluch, J. M. Delgado-Garcia, and A. M. Carrion
Caloric Restriction Increases Learning Consolidation and Facilitates Synaptic Plasticity through Mechanisms Dependent on NR2B Subunits of the NMDA Receptor
J. Neurosci.,
September 19, 2007;
27(38):
10185 - 10195.
[Abstract]
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E. H. Norris and S. Strickland
Modulation of NR2B-regulated contextual fear in the hippocampus by the tissue plasminogen activator system
PNAS,
August 14, 2007;
104(33):
13473 - 13478.
[Abstract]
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O. Bukalo, M. Schachner, and A. Dityatev
Hippocampal Metaplasticity Induced by Deficiency in the Extracellular Matrix Glycoprotein Tenascin-R
J. Neurosci.,
May 30, 2007;
27(22):
6019 - 6028.
[Abstract]
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Y. Sato, N. Seo, and E. Kobayashi
Ethanol-Induced Hypnotic Tolerance Is Absent in N-Methyl-d-Aspartate Receptor {varepsilon}1 Subunit Knockout Mice.
Anesth. Analg.,
July 1, 2006;
103(1):
117 - 120.
[Abstract]
[Full Text]
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T. J. Ha, A. B. Kohn, Y. V. Bobkova, and L. L. Moroz
Molecular Characterization of NMDA-Like Receptors in Aplysia and Lymnaea: Relevance to Memory Mechanisms
Biol. Bull.,
June 1, 2006;
210(3):
255 - 270.
[Abstract]
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T. Shinoe, M. Matsui, M. M. Taketo, and T. Manabe
Modulation of Synaptic Plasticity by Physiological Activation of M1 Muscarinic Acetylcholine Receptors in the Mouse Hippocampus
J. Neurosci.,
November 30, 2005;
25(48):
11194 - 11200.
[Abstract]
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S. Berberich, P. Punnakkal, V. Jensen, V. Pawlak, P. H. Seeburg, O. Hvalby, and G. Kohr
Lack of NMDA Receptor Subtype Selectivity for Hippocampal Long-Term Potentiation
J. Neurosci.,
July 20, 2005;
25(29):
6907 - 6910.
[Abstract]
[Full Text]
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A. B. Petrenko, T. Yamakura, N. Fujiwara, A. R. Askalany, H. Baba, and K. Sakimura
Reduced Sensitivity to Ketamine and Pentobarbital in Mice Lacking the N-Methyl-D-Aspartate Receptor GluR{epsilon}1 Subunit
Anesth. Analg.,
October 1, 2004;
99(4):
1136 - 1140.
[Abstract]
[Full Text]
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D. Owen, E. Setiawan, A. Li, L. McCabe, and S. G. Matthews
Regulation of N-Methyl-D-Aspartate Receptor Subunit Expression in the Fetal Guinea Pig Brain
Biol Reprod,
August 1, 2004;
71(2):
676 - 683.
[Abstract]
[Full Text]
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Y. Sato, E. Kobayashi, Y. Hakamata, M. Kobahashi, T. Wainai, T. Murayama, M. Mishina, and N. Seo
Chronopharmacological studies of ketamine in normal and NMDA {epsilon}1 receptor knockout mice{dagger}
Br. J. Anaesth.,
June 1, 2004;
92(6):
859 - 864.
[Abstract]
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T. A. Simeone, R. M. Sanchez, and J. M. Rho
Molecular Biology and Ontogeny of Glutamate Receptors in the Mammalian Central Nervous System
J Child Neurol,
May 1, 2004;
19(5):
343 - 360.
[Abstract]
[PDF]
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G. Kohr, V. Jensen, H. J. Koester, A. L. A. Mihaljevic, J. K. Utvik, A. Kvello, O. P. Ottersen, P. H. Seeburg, R. Sprengel, and O. Hvalby
Intracellular Domains of NMDA Receptor Subtypes Are Determinants for Long-Term Potentiation Induction
J. Neurosci.,
November 26, 2003;
23(34):
10791 - 10799.
[Abstract]
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A. Fleischmann, O. Hvalby, V. Jensen, T. Strekalova, C. Zacher, L. E. Layer, A. Kvello, M. Reschke, R. Spanagel, R. Sprengel, et al.
Impaired Long-Term Memory and NR2A-Type NMDA Receptor-Dependent Synaptic Plasticity in Mice Lacking c-Fos in the CNS
J. Neurosci.,
October 8, 2003;
23(27):
9116 - 9122.
[Abstract]
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M. Inoue, M. Mishina, and H. Ueda
Locus-Specific Rescue of GluR{epsilon}1 NMDA Receptors in Mutant Mice Identifies the Brain Regions Important for Morphine Tolerance and Dependence
J. Neurosci.,
July 23, 2003;
23(16):
6529 - 6536.
[Abstract]
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M. Fagiolini, H. Katagiri, H. Miyamoto, H. Mori, S. G. N. Grant, M. Mishina, and T. K. Hensch
Separable features of visual cortical plasticity revealed by N-methyl-D-aspartate receptor 2A signaling
PNAS,
March 4, 2003;
100(5):
2854 - 2859.
[Abstract]
[Full Text]
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M. Miura, M. Watanabe, S. Offermanns, M. I. Simon, and M. Kano
Group I Metabotropic Glutamate Receptor Signaling via Galpha q/Galpha 11 Secures the Induction of Long-Term Potentiation in the Hippocampal Area CA1
J. Neurosci.,
October 1, 2002;
22(19):
8379 - 8390.
[Abstract]
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Y. Miyamoto, K. Yamada, Y. Noda, H. Mori, M. Mishina, and T. Nabeshima
Lower Sensitivity to Stress and Altered Monoaminergic Neuronal Function in Mice Lacking the NMDA Receptor epsilon 4 Subunit
J. Neurosci.,
March 15, 2002;
22(6):
2335 - 2342.
[Abstract]
[Full Text]
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G. Woodhall, D. I. Evans, M. O. Cunningham, and R. S. G. Jones
NR2B-Containing NMDA Autoreceptors at Synapses on Entorhinal Cortical Neurons
J Neurophysiol,
October 1, 2001;
86(4):
1644 - 1651.
[Abstract]
[Full Text]
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Y. Miyamoto, K. Yamada, Y. Noda, H. Mori, M. Mishina, and T. Nabeshima
Hyperfunction of Dopaminergic and Serotonergic Neuronal Systems in Mice Lacking the NMDA Receptor {epsilon}1 Subunit
J. Neurosci.,
January 15, 2001;
21(2):
750 - 757.
[Abstract]
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E. Morikawa, H. Mori, Y. Kiyama, M. Mishina, T. Asano, and T. Kirino
Attenuation of Focal Ischemic Brain Injury in Mice Deficient in the epsilon 1 (NR2A) Subunit of NMDA Receptor
J. Neurosci.,
December 1, 1998;
18(23):
9727 - 9732.
[Abstract]
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Top
Abstract
Introduction
