 |
 Previous Article | Next Article 
The Journal of Neuroscience, October 15, 2002, 22(20):9113-9121
L-Type Voltage-Gated Calcium Channels Are Required for
Extinction, But Not for Acquisition or Expression, of Conditional Fear
in Mice
Chris K.
Cain1,
Ashley
M.
Blouin2, and
Mark
Barad2, 3
1 Interdepartmental Program in Neuroscience,
2 Department of Psychiatry and Biobehavioral Sciences, and
3 Brain Research Institute and Neuropsychiatric Institute,
University of California, Los Angeles, Los Angeles, California
90095
 |
ABSTRACT |
It has been shown recently that extinction of conditional fear does
not depend acutely on NMDA-type glutamate receptors, although other
evidence has led to the hypothesis that L-type voltage-gated calcium
channels (LVGCCs) play a role in conditional fear. We therefore tested
the role of LVGCCs in the acquisition, expression, and extinction of
conditional fear of cue and context in mice. Using systemic injections
of two LVGCC inhibitors, nifedipine and nimodipine, which both
effectively cross the blood-brain barrier, we show that LVGCCs are
essential for the extinction, but not for the acquisition or
expression, of conditional fear in mice.
Key words:
fear; acquisition; extinction; expression; freezing; L-type; calcium channel; nifedipine; nimodipine; state
dependence
| |
INTRODUCTION |
Fear conditioning is an important
form of behavioral plasticity that has been correlated to changes in
synaptic strength in the amygdala (Rogan and LeDoux, 1995 ; Rogan et
al., 1997). Both the acquisition and extinction of conditional fear are
forms of active learning. The acquisition of conditional fear requires the establishment of a novel association by pairing an initially neutral conditional stimulus (CS), such as a tone, with an
intrinsically aversive unconditional stimulus (US), typically a mild
foot shock. Although extinction, the reduction of conditional
responding after repeated exposures to the CS alone, might initially
appear to be a passive decay or erasure of this association, many
observations indicate that extinction is new inhibitory learning, which
leaves the original memory intact. Conditional fear responding shows the following: spontaneous recovery with time (Baum, 1988),
reinstatement after unpaired US presentations (Rescorla and Heth,
1975), and renewal with context change (Bouton and King, 1983). These
observations indicate that the original association is not lost during
extinction but rather is suppressed by new, context-dependent learning,
which is likely attributable to plasticity at separate synapses
from those mediating acquisition.
Many recent studies have investigated the molecular basis of fear
conditioning and its extinction. Accumulating evidence indicates that
fear acquisition and expression require NMDA-type glutamate receptor
(NMDAR) activity in the amygdala (Miserendino et al., 1990; Kim et al.,
1991, 1992; Falls et al., 1992; Tang et al., 1999; Rodrigues et al.,
2001). There is also evidence that extinction of fear depends on
NMDARs (Falls et al., 1992; Baker and Azorlosa, 1996; Tang et al.,
1999). However, the NMDAR inhibitors used in these experiments may also
have altered basal synaptic transmission in the amygdala, and thus,
fear expression, which may be required for extinction (Li et al., 1995;
Maren et al., 1996; Lee and Kim, 1998). A recent experiment
demonstrated that NMDAR activity is necessary for the long-term
retention of extinction memories but is not required for the generation
of extinction acutely (Santini et al., 2001). Thus, although it seems
clear that NMDAR activity is required for the formation of fear
acquisition memories, considerably less is known about the molecules
that mediate the induction of fear extinction.
Recently, a form of NMDAR-independent long-term potentiation (LTP) has
been described in synapses between thalamic afferents and neurons in
the lateral amygdala (Weisskopf et al., 1999). This LTP depends,
instead, on L-type voltage-gated calcium channels (LVGCCs). Because
these synapses have also been implicated in auditory fear conditioning
(Rogan and LeDoux, 1995), amygdaloid LVGCC-LTP is an important
candidate for a mechanism that may underlie some aspect of conditional
fear (Blair et al., 2001; Bauer et al., 2002). In this paper, we tested
the specific hypotheses that LVGCC activity is required for the (1)
acquisition, (2) expression, and (3) extinction of conditional fear in
mice. To do so, we used systemic injections of two dihydropyridine
LVGCC antagonists with good penetration through the blood-brain
barrier, nifedipine and nimodipine. Our results indicate that LVGCCs
are necessary for the extinction of conditional fear but are not
required for its acquisition or expression.
| |
MATERIALS AND METHODS |
Subjects
Naïve 12- to 20-week-old C57BL/6 male mice (Taconic
Farms, Germantown, NY) were housed four per cage, were maintained on a
12 hr light/dark schedule, and were allowed access to food and water
ad libitum. All testing was conducted during the light phase in illuminated testing rooms following protocols approved by the Institutional Animal Care and Use Committee of the University of
California, Los Angeles.
Drugs
The LVGCC antagonists nifedipine (1.25-80 mg/kg) and nimodipine
(4-16 mg/kg; Sigma, St. Louis) were sonicated into 100% Cremophor EL
(BASF, Mt. Olive, NJ). PBS was added to make the final vehicle 10%
Cremophor-90% PBS. The highest nifedipine doses were partly suspensions, and care was taken to thoroughly mix the drugs before injecting them. Mice were injected subcutaneously 20 or 50 min before
behavioral testing (10 ml/kg). Drug pretreatment times were chosen
based on pilot studies and previous reports of systemic administration
in rodents (Janicki et al., 1988; Larkin et al., 1992).
Conditioning apparatus
Two contexts (A and B), in separate rooms, were used for all
behavioral fear testing. Shuttle box compartments (Med Associates, St.
Albans, VT) measuring 20.3 × 15.9 × 21.3 cm served as
context A, and conditioning boxes (Med Associates) measuring 30.5 × 24.1 × 21 cm served as context B. Both contexts had two
transparent walls and stainless steel grid floors (3.2 mm in diameter,
8 mm centers); however, the grid floors in context B were covered with flat white acrylic inserts to minimize context generalization. Context
A was wiped down before testing with 10% ethanol, and context B was
wiped down with 10% methanol. Individual video cameras were mounted in
the ceiling of each chamber and connected via a quad processor to a
standard video cassette recorder and monitor for videotaping and
scoring of freezing. Grid floors were connected to a scrambled shock
source (Med Associates). Auditory stimuli (Med Associates) were
delivered via a speaker in the chamber wall. Delivery of stimuli was
controlled with a personal computer and Med-PC software through a
SmartCTL Interface System (DIG-716; Med Associates). Background white
noise was maintained at 62 dB throughout behavioral testing.
Open field
Spontaneous locomotor activity was monitored by placing mice in
one of four chambers (40 × 40 × 40 cm) and allowing them to freely explore for 60 min. Chambers had white floors and two white walls; the remaining two walls were transparent. A video camera was
mounted above the chambers, and total distance traveled was tracked
with a personal computer and software (Poly-Track Video Tracking
System, Chromotrack version 4.02; San Diego Instruments, San Diego, CA)
and expressed in arbitrary units.
Conditional fear testing
Cue fear acquisition. Experiments investigating the
effects of LVGCC antagonists on the acquisition of cue fear consisted of two phases: fear acquisition (context A) and testing (context B).
After injections, conditional fear was induced by presenting audible
cues (CS: white noise, 2 min, 80 dB) that coterminated with mild foot
shocks (US: shock, 2 sec, 0.7 mA). Two minute stimulus-free periods
preceded, separated, and followed the pairings. Most experiments used
five CS-US pairings; however, experiments using one (Fig. 1F) or two (Figs.
2E-G,
3B,C)
pairings were also conducted to ensure that LVGCC blockers did not
impair fear acquisition with weaker training protocols. After allowing
1 d for memory consolidation, cue fear was tested by presenting
one, continuous, 2 min CS after a 2 min acclimation. In two cases in
which reductions in fear acquisition were observed, a second test was
conducted 3 hr later to determine whether the reductions reflected
state-dependent memory retrieval (Fig.
2D,G). For these state-dependence
tests, mice were reinjected and subjected to an identical test of cue fear. In a separate test of state dependence, using a 2 × 2 intersubject design, mice were injected with vehicle or nimodipine (16 mg/kg) (Fig. 3) 20 min before both training and testing for fear
expression 24 hr later.
View larger version (19K):
[in this window]
[in a new window]
|
Figure 1.
Nifedipine does not affect cue fear acquisition.
A, Experimental design (7-8 mice per group).
Open circles, Vehicle; filled circles,
nifedipine. B, Freezing during the five cues that
preceded each shock for mice injected with nifedipine (40 mg/kg dose
shown, 20 min pretreatment) or vehicle. C, Freezing 24 hr later during a 2 min, drug-free CS test. D, Freezing
during the five cues that preceded each shock for mice injected with
nifedipine (40 mg/kg dose shown, 50 min pretreatment) or vehicle. Also
included is freezing during the 2 min stimulus-free period after the
last CS-US pairing (Ctxt). E, Freezing
24 hr later during a 2 min acclimation period (Pre-CS),
followed by a 2 min, drug-free CS test. F, Freezing
during a 2 min, drug-free CS test, 24 hr after injections (50 min
pretreatment) and a single cue-shock pairing. *p < 0.05 versus vehicle.
|
|
View larger version (29K):
[in this window]
[in a new window]
|
Figure 2.
Nimodipine does not affect cue fear acquisition,
although recall appears to be state dependent. A,
Experimental design (8 mice per group). B, Freezing during
the five cues that preceded each shock for mice injected with
nimodipine (15 mg/kg, 20 min pretreatment) or vehicle.
C, Freezing 24 hr later during a 2 min, drug-free CS
test. D, Freezing during a second state-dependence test
(3 hr after test 1) after reinjections of drug or vehicle (20 min
pretreatment). E-G, Repeat of the above experiment
(A-C) using a weaker training protocol (2 cue-shock pairings). *p < 0.05 versus
vehicle.
|
|
View larger version (26K):
[in this window]
[in a new window]
|
Figure 3.
Recall of fear is partially state dependent with
nimodipine. A, Experimental design (8 mice per group;
between-subjects design). B, Freezing during the two
cues that preceded each shock for mice injected with nimodipine (16 mg/kg, 20 min pretreatment; n = 16) or vehicle
(n = 16). Freezing 24 hr later during a 2 min CS
test, 20 min after drug or vehicle injections. One-half of the mice
from each of the treatment groups of the previous day received
nimodipine (16 mg/kg) or vehicle. *p < 0.05 versus
0-0 group; +p < 0.05 versus 16-0 group.
|
|
Cue fear extinction. Experiments investigating the effects
of LVGCC antagonists on cue fear extinction consisted of three phases:
fear acquisition (context A), fear extinction (context B), and testing
(context B), each separated by 1 d to allow for memory
consolidation. In all experiments, cue fear was induced in nondrugged,
naïve mice with the five-pairing protocol described above. Mice
were matched into equivalent treatment groups based on freezing during
the third training CS. One day later, after injections, mice were
placed in context B and allowed to acclimate for 2 min. Extinction was
induced with 60 2 min CS presentations [5 sec intertrial interval
(ITI)]. Additionally, nonextinguished (retention control) mice
were injected with vehicle and placed in context B for an equivalent
period of time but not exposed to any CS presentations. One day after
extinction, all mice were returned to context B in the drug-free state.
After a 2 min acclimation, freezing was assessed during a 2 min,
continuous CS presentation. In a subsequent experiment (see Fig. 7)
examining the potential for state-dependent recall of extinction
memories, fear acquisition, extinction, and testing were conducted as
described above; however, mice were reinjected 20 min before the final
test with drug or vehicle.
Context fear acquisition. Experiments investigating the
effects of LVGCC antagonists on context fear acquisition consisted of
two phases, both conducted in context B with the white inserts removed.
After injections, mice were placed in the chambers in which they
received five 0.7 mA, 2 sec unsignaled foot shocks. Two minute
stimulus-free periods preceded, separated, and followed the foot
shocks. Twenty-four hours later, mice were returned to the same
chambers for a 5 min test of context fear.
Context fear extinction. The experiment investigating the
effects of LVGCC antagonists on context fear extinction consisted of three phases, all conducted in context B with the white inserts removed. Context fear was induced in naïve, untreated mice with the five-shock protocol described above. Mice were then matched into
equivalent treatment groups based on freezing during the 2 min period
after the fifth shock. One day later, mice were injected and returned
the conditioning chambers for a 120 min shock-free session.
Nonextinguished retention control mice were injected with vehicle and
placed in different chambers (see Materials and Methods, Open field)
for 120 min. One day after extinction, all mice were returned to the
context B chambers for a 5 min drug-free test session.
Statistical analyses
Behavioral freezing, the absence of all nonrespiratory
movements, was rated during all phases by a blinded, experienced
investigator using a 5 sec instantaneous time sampling technique.
Percentage of freezing scores were calculated for each mouse, and data
represent mean ± SEM freezing percentages for groups of mice
during specified time bins. Total session means and individual CS
exposures were analyzed with one-way ANOVA and planned post
hoc Dunnett's test comparisons. Student's t tests
were used to analyze experiments with only two treatment groups.
Multiple trial data were analyzed with matched two-way ANOVA and
Bonferroni's post hoc tests to compare individual time
points. Differences were considered significant if p < 0.05.
| |
RESULTS |
LVGCC inhibitors do not prevent the acquisition or retention of
conditional cue fear
To test whether LVGCC activity is required for cue fear
acquisition, we injected nifedipine 20 min before a moderate training protocol (five CS-US pairings). We generated a dose-response curve for nifedipine (5-80 mg/kg) and assessed freezing during acquisition and, 24 hr later, during a drug-free retention test. Acute acquisition of freezing was unaffected by nifedipine administration; mice injected
with vehicle or nifedipine (40 mg/kg) froze identically during the CS
preceding each shock (for drug,
F(4,65) = 3.22, p = 0.08; for drug × trial interaction,
F(1,65) = 1.05, p = 0.39) (Fig. 1B). After allowing 24 hr for
consolidation of learning, the mice were presented with the CS in
a novel context. None of the drug doses reduced freezing during this
test (F(5,42) = 1.64, p = 0.17) (Fig. 1C). In fact, mice injected
previously with 10 mg/kg nifedipine froze slightly, but significantly,
more than mice injected previously with vehicle
(p < 0.05).
Because the rate of absorption and brain penetration of systemically
administered nifedipine varies depending on its vehicle (Larkin et al.,
1992), we also generated an abbreviated dose-response curve with a 50 min pretreatment (2.5-40 mg/kg). Again, we saw no impairment of fear
acquisition or retention with nifedipine administration [for 40 mg/kg
nifedipine during acquisition, F(1,35) = 0.88, p = 0.36; for the drug × trial
interaction, F(4,35) = 0.57, p = 0.69 (Fig. 1D); for the retention
test, F(3,28) = 0.48, p = 0.70 (Fig. 1E)]. To confirm that
confounding effects of context conditioning or generalization were not
somehow obscuring a blockade of acquisition, we also scored context
freezing in this experiment, both in the last 2 min stimulus-free
period of training and during the 2 min pre-CS period of testing on day
2 in context B. There was no evidence that nifedipine impaired
conditioning to the training context during CS-US pairings. In fact,
nifedipine-treated mice froze nearly twice as much as control mice
during the final 2 min stimulus-free period of training
(p < 0.05 vs vehicle) (Fig. 1D, Ctxt). Because nifedipine does not
induce freezing or potentiate context conditioning when the shocks are
unsignaled (Fig. 4), these data suggest
that nifedipine may retard negative contingency learning (i.e., that
the CS, not the context, predicts US delivery). During the retention
test, all groups showed a small amount of context generalization.
However, freezing before the CS delivery was low and statistically
undistinguishable for all groups
(F(3,28) = 1.30, p = 0.29) (Fig. 1E,
Pre-CS) and is unlikely to have obscured a blockade of
long-term cue fear acquisition. Last, to eliminate the possibility that
our lack of effect with nifedipine was attributable to an overly strong
training protocol, we injected the drug (2.5-40 mg/kg) 50 min before a
single CS-US pairing. This protocol does not allow the measurement of
short-term acquisition, but again we detected no significant reduction
in retained conditional fear 24 hr later (F(3,
28) = 1.19, p = 0.33) (Fig.
1F).
View larger version (24K):
[in this window]
[in a new window]
|
Figure 4.
LVGCC inhibitors do not prevent acquisition of
context fear. A, Experimental design (7-8 mice per
group). B, Freezing shown in 2 min blocks during the
entire 12 min session for mice injected with nifedipine (40 mg/kg, 50 min pretreatment) or vehicle. Unsignaled foot shocks occurred at the
2nd, 4th, 6th, 8th, and 10th minutes. C, Freezing 24 hr
later during a 5 min drug-free context exposure. D,
E, Identical experiment conducted with nimodipine (16 mg/kg, 20 min pretreatment).
|
|
To ensure that our failure to block acquisition was not specific to
nifedipine, we tested nimodipine (15 mg/kg, 20 min pretreatment), another LVGCC antagonist. Nimodipine had no effect on the acute acquisition of fear measured by freezing during the CS that preceded each of the five shocks (for drug,
F(1,35) = 0.07, p = 0.79; for drug × trial interaction,
F(4,35) = 0.65, p = 0.63) (Fig. 2B). When tested for retention of
conditional fear on the next day in a drug-free state, animals treated
with nimodipine froze slightly less than vehicle-treated controls;
however, this effect was not significant
(t(14) = 1.35, p = 0.20) (Fig. 2C). State-dependent effects on learning have
been reported for a number of pharmacological agents (Connelly et al.,
1975; Jackson et al., 1992; Blokland et al., 1998), suggesting that
such drugs provide a salient internal context that contributes to the
learned cue association. We therefore assayed the same mice for
freezing again, 3 hr after the first test, 20 min after reinjections.
Nimodipine-treated animals now showed the same levels of freezing as
vehicle-treated animals (t(14) = 0.58, p = 0.57) (Fig. 2D). Thus, nimodipine
did not interfere with the process of fear conditioning, although our
data indicates that it made the recall of fear partially state
dependent. Again, to rule out the possibility that the five-shock
training protocol represented overtraining that might obscure a subtle
role for LVGCC in the acquisition of conditional fear, we also tested
the role of nimodipine on a weaker training protocol, using only two CS-US pairings. There were no differences in freezing during the CS that preceded each shock (for drug,
F(1,14) = 0.11, p = 0.74; for drug × trial interaction,
F(1,14) = 0.84, p = 0.38) (Fig. 2E), and nimodipine again decreased
freezing slightly, although this time significantly, when assayed
1 d after training in the drug-free state
(t(14) = 2.32, p < 0.05) (Fig. 2F). Again, the difference disappeared
when the animals were reinjected before a second fear assay
(t(14) = 1.69, p = 0.11) (Fig. 2G).
We had not anticipated state-dependent recall of cue fear with
nimodipine, and the initial experiments were not designed to test this.
Therefore, we next assessed state dependence directly in a
between-subjects design, using two CS-US pairings for training followed after 1 d by a test CS presentation in a novel context. Eight mice per group received an injection before both training and
testing, in a 2 × 2 design: either nimodipine (16 mg/kg) before training and then nimodipine before testing, or vehicle both days, or
vehicle first followed by nimodipine, or vice versa. As seen before,
nimodipine had no effect on freezing during the tones that preceded
each shock during acquisition (for drug,
F(1,30) = 0.03, p = 0.86; for drug × trial interaction,
F(1,30) = 0.01, p = 0.93) (Fig. 3B). Again, nimodipine treatment before
acquisition reduced freezing 24 hr later compared with vehicle-treated
mice (0-0 vs 16-0, p < 0.01) (Fig. 3C).
However, freezing in mice that received nimodipine before both the
acquisition and test sessions was indistinguishable from vehicle
treated mice (0-0 vs 16-16, p > 0.05), supporting
the hypothesis that nimodipine makes cue fear recall partially state
dependent. Importantly, nimodipine injections before testing did not
increase freezing in mice that were trained with vehicle (0-0 vs
0-16, p > 0.05). Thus, although recall of fear is
state dependent with nimodipine, expression of fear is unaltered.
LVGCC inhibitors do not prevent the acquisition or retention of
conditional context fear
We also examined the effects of nifedipine (40 mg/kg) and
nimodipine (16 mg/kg) on acquisition of context fear in separate experiments (Fig. 4). Acutely, mice injected with either nifedipine or
vehicle acquired context fear (for time,
F(5,78) = 10.12, p < 0.01), although nifedipine-treated mice appeared to learn at a slightly
slower rate, and the effect of drug treatment was statistically significant (F(1,78) = 8.33, p < 0.01) (Fig. 4B). However, the effect was small because freezing by nifedipine-treated mice was never
statistically different at any single time point (p
values >0.05), and the group × time interaction was
statistically insignificant (F(5,78) = 0.87, p = 0.51). Nimodipine-treated mice were
indistinguishable from vehicle-treated mice in the acquisition of
context fear (for time, F(5,84) = 17.27, p < 0.01; for drug,
F(1,84) = 2.46, p = 0.12; for the drug × time interaction,
F(5,84) = 0.83, p = 0.54) (Fig. 4D). When tested drug-free 24 hr later,
both nifedipine- and nimodipine-treated mice froze the same as
vehicle-treated mice, indicating that retention of context fear was
unimpaired by LVGCC blockade (t(13) = 0.74, p = 0.47 and
t(14) = 0.29 , p = 0.78, respectively) (Fig.
4C,E).
LVGCC inhibitors block extinction but not expression of conditional
cue fear
Using five CS-US pairings for training groups of naïve
mice, we next tested the effect of LVGCC inhibitors on the expression and extinction of cue fear. One day after training, mice were injected
with drug or vehicle, placed in a novel context, and exposed to 60 2 min CS (5 sec ITI). Retention control mice were injected with vehicle
and placed in the extinction chambers for an equivalent period of time
but were not exposed to any CS. Expression of conditional fear was
assessed by measuring freezing during the first CS exposure of the day
2 session, before any extinction could occur. Acute extinction was
assessed during the first 15 CS exposures, and retained extinction was
assessed during a single 2 min CS exposure in the same context 1 d
later. The 60 CS protocol generated substantial persistent extinction
in vehicle-treated mice compared with retention controls
(p values <0.05) (Figs. 5C,E,
6C,E).
Neither inhibitor affected the expression of conditional fear during
the first 2 min exposure to the cue (p values
>0.05 compared with vehicle) (Figs.
5B,D,
6B,D). Furthermore, nifedipine did
not affect pre-CS freezing to the novel context before cue exposure
began (t14 = 1.1, p = 0.28) (Fig. 5D, Pre) or pre-CS freezing on the
test of extinction (F(2,21) = 1.64, p = 0.22) (Fig. 5E, Pre-CS).
However, whereas vehicle-treated mice showed a progressive decline in
freezing with repeated CS exposures, freezing by mice treated with
LVGCC blockers remained elevated in all four experiments. Although mice
treated with 40 mg/kg nifedipine (20 min pretreatment) appear to begin
to extinguish, they rapidly return to initial freezing levels, whereas
controls show continuing declines (for drug,
F(1,105) = 39, p < 0.01; for drug × trial interaction,
F(14,105) = 2.27, p < 0.01) (Fig. 5B). Furthermore, freezing during each of the CS
was never significantly less than freezing during the first CS
(F(14,105) = 1.61, p = 0.09). In contrast, vehicle controls show a significant reduction in
freezing by the seventh CS presentation
(F(14,105) = 4.71, p < 0.01). Nevertheless, a trend toward some early extinction suggested
that nifedipine may be more efficient with a longer pretreatment, and
we repeated both our extinction experiment and our acquisition
experiments (see above) with longer pretreatments. When injected with
nifedipine (40 mg/kg) 50 min before extinction began, there was no hint
of acute extinction in the nifedipine-treated group (for drug,
F(1,105) = 228, p < 0.01; for drug × trial,
F(14,105) = 2.05, p < 0.05) (Fig. 5D). To verify that the blockade of extinction
was not particular to nifedipine, we also tested nimodipine (15 mg/kg),
and it, too, blocked acute extinction entirely (for drug,
F(1,165) = 220, p < 0.01; for drug × trial,
F(14,165) = 5.05, p < 0.01) (Fig. 6B). A dose-response curve was then
generated for nimodipine (4-16 mg/kg), and an identical result was
also obtained for the 16 mg/kg dose (for drug,
F(1,165) = 179, p < 0.01; for drug × trial,
F(14,165) = 1.72, p = 0.06) (Fig. 6D); acute extinction was blocked
entirely.
View larger version (29K):
[in this window]
[in a new window]
|
Figure 5.
Nifedipine blocks extinction, but not expression,
of cued fear. A, Experimental design. Extinction
sessions began 24 hr after cue fear acquisition (5 cue-shock
pairings). B, Freezing during the first 15 CS
presentations, after injections of nifedipine (40 mg/kg dose shown, 20 min pretreatment) or vehicle. C, Freezing 24 hr later
during a 2 min, drug-free CS test (n values = 8 for
extinction groups). Retention control mice (RC;
n = 16) were injected with vehicle and placed in
the extinction chamber on day 2 but were not exposed to any CS.
D, E, Identical experiment with a longer
pretreatment (50 min) of a single nifedipine dose (40 mg/kg) or vehicle
(n values = 8). Freezing was also scored during a 2 min acclimation period (Pre-CS). *p < 0.05 versus retention control; +p < 0.05 versus
vehicle-extinction.
|
|
View larger version (32K):
[in this window]
[in a new window]
|
Figure 6.
Nimodipine blocks extinction, but not expression,
of cued fear. A, Experimental design (12 mice per
group). Extinction sessions began 24 hr after cue fear acquisition (5 cue-shock pairings). B, Freezing during the first 15 CS
presentations, after injections of nimodipine (15 mg/kg, 20 min
pretreatment) or vehicle. C, Freezing 24 hr later during
a 2 min, drug-free CS test. Retention control mice (RC)
were injected with vehicle and placed in the extinction chamber on day
2 but were not exposed to any CS. D, E,
Identical experiment with several doses of nimodipine. Only the 16 mg/kg dose is shown for acute extinction. *p < 0.05 versus retention control; +p < 0.05 versus
vehicle-extinction.
|
|
Consistent with the acute results, when the same mice were tested drug
free after 1 d for consolidation, both LVGCC antagonists had
completely blocked the extinction seen in vehicle-treated mice
(p < 0.01 for 40 mg/kg nifedipine, 20 min
pretreatment; p < 0.01 for 40 mg/kg nifedipine, 50 min
pretreatment; p values <0.05 for 15 and 16 mg/kg
nimodipine, 20 min pretreatment). Additionally, freezing in these
groups was statistically equivalent to nonextinguished retention
controls (p > 0.05 for 20, 40, and 80 mg/kg
nifedipine, 20 min pretreatment; p > 0.05 for 40 mg/kg
nifedipine, 50 min pretreatment; and p values >0.05 for
nimodipine 4, 8, 15, and 16 mg/kg) (Figs.
5C,E,
6C,E).
It has been shown previously that extinction generated in the presence
of benzodiazepines can be state dependent (Bouton et al., 1990). Thus,
although no persistent extinction is seen when rats extinguished in the
presence of benzodiazepines are tested in the absence of drugs,
extinction can be uncovered by administering benzodiazepines again
before the extinction test. We therefore tested whether such
state-dependent extinction might occur with LVGCC antagonist treatment,
by both extinguishing and testing after drug injections. There was no
evidence of state-dependent extinction; mice given extinction training
in the presence of nifedipine or nimodipine and then reinjected with
drug before testing showed freezing no lower than retention controls
(p values >0.05) and froze significantly more than
mice extinguished and tested in the presence of vehicle
(p values <0.05) (Fig.
7). Nifedipine-treated animals showed a
trend to more freezing than retention controls in the final drugged
test; however, this effect was not statistically significant
(p > 0.05).
View larger version (17K):
[in this window]
[in a new window]
|
Figure 7.
Extinction with LVGCC blockers is not state
dependent. A, Experimental design (7-8 mice per group).
The 3 d experiment was identical to previous experiments
(Figs. 5, 6), with the exception that drugs and vehicle were injected
20 min before both day 2 extinction and the day 3 test.
B, Freezing during a 2 min CS test, 24 hr after
extinction. *p < 0.05 versus retention control
(RC); +p < 0.05 versus
vehicle-extinction.
|
|
LVGCC inhibitors block extinction but not expression of conditional
context fear
To further explore the effects of LVGCC blockade, we also tested
the effects of nifedipine (40 mg/kg) and nimodipine (16 mg/kg) directly
on the expression and extinction of context fear (Fig. 8). Neither inhibitor altered the
expression of freezing as measured during the first 5 min block of the
extinction session (p values >0.05 vs vehicle) (Fig.
8B). Freezing was assessed during the first 30 min of
the 120 min extinction session. As with cue fear, both LVGCC inhibitors
blocked the acute extinction of context fear evident in vehicle-treated
mice (nifedipine: for group, F(1,90) = 45.7, p < 0.01; for time,
F(5,90) = 10.9, p < 0.01; for the group × time interaction,
F(5,90) = 8.3, p < 0.01; nimodipine: for group, F(1,90) = 54.2, p < 0.01; for time,
F(5,90) = 19.6, p < 0.01; for the group × time interaction,
F(5,90) = 5.7, p < 0.01) (Fig. 8B). Likewise, vehicle-treated mice
showed significant long-term extinction compared with retention control
mice when tested 24 hr later (for group,
F(1,150) = 20.7, p < 0.01) (Fig. 8C). Treatment during context
exposure with nifedipine or nimodipine completely blocked long-term
extinction of context fear; freezing in these groups was
indistinguishable from nonextinguished retention control mice
(nifedipine vs vehicle, F(1,150) = 33.1, p < 0.01; nimodipine vs vehicle,
F(1,145) = 22.1, p < 0.01; nifedipine vs retention control,
F(1,150) = 0.24, p = 0.62; nimodipine vs retention control,
F(1,145) = 0.27, p = 0.60). All mice showed extinction during this final 5 min
drug-free test (main effect for time, F(4,295) = 15.9, p < 0.01); however, there was no indication that the groups extinguished at
different rates during this test (main effect for group × time
interaction, F(12,295) = 0.31, p = 0.99).
View larger version (24K):
[in this window]
[in a new window]
|
Figure 8.
LVGCC inhibitors block extinction, but not
expression, of context fear. A, Experimental design (16 mice per group). Extinction was conducted 24 hr after context fear
acquisition (5 unsignaled foot shocks, 2 min ITI). B,
Freezing in 5 min blocks for the first 30 min of context exposure (120 total). Mice were injected with nifedipine (40 mg/kg, 50 min
pretreatment), nimodipine (16 mg/kg, 20 min pretreatment), or vehicle.
Retention control mice were injected with vehicle and placed in
dissimilar chambers for 120 min. C, Freezing 24 hr later
during a 5 min drug-free context fear test (data were lost for 1 mouse
in the nimodipine group attributable to a camera failure).
|
|
Spontaneous locomotor activity and LVGCC blockers
To rule out the possibility that our behavioral effects with the
LVGCC inhibitors were attributable to gross reductions in movement, we
next assessed the effects of nifedipine and nimodipine on spontaneous
locomotor activity in a novel open field. Mice were injected with
vehicle, nifedipine (40 mg/kg), or nimodipine (16 mg/kg) 20 min before
a 60 min session. One day later, mice were returned to the same
chambers and allowed to freely explore again for 60 min, although now
in the drug-free state. Total distance traveled (arbitrary units) was
recorded in each session. The distance traveled by nifedipine-treated
mice was no different from vehicle-treated mice in either session
(p values >0.05) (Fig.
9). Nimodipine-treated mice showed
suppressed locomotion acutely (test 1, p < 0.01) but normal locomotion the next day (test 2, p > 0.05).
View larger version (22K):
[in this window]
[in a new window]
|
Figure 9.
Effects of nifedipine and nimodipine on
spontaneous locomotor activity in an open field. A,
Experimental design (6 mice per group). B,
Left, Total distance traveled during a 60 min session in
a novel chamber (arbitrary units). Mice were injected 20 min before the
session with nifedipine (40 mg/kg), nimodipine (16 mg/kg), or vehicle.
Right, An identical test of the same mice 24 hr later in
the drug-free state. p < 0.05 versus
vehicle.
|
|
| |
DISCUSSION |
We presented data that point to a distinction in the molecular
mechanisms underlying acquisition and extinction of conditional fear in
mice. Blockade of LVGCCs effectively prevents extinction in a
dose-related manner (Figs. 5-8), both acutely (during extinction exposures) and persistently (1 d after exposures). These effects are
not state dependent; no hidden extinction is uncovered by treating mice
with the same drugs at testing as during extinction exposures (Fig. 7).
LVGCC activity thus appears to be essential to induce conditional fear
extinction. To our knowledge, this is the first intervention reported
to completely block the induction of extinction. Because it has been
shown recently that NMDAR activity is not required for extinction
induction (Santini et al., 2001), this work provides a candidate
induction mechanism: calcium entry through LVGCCs may initiate
plasticity underlying extinction memories.
On the other hand, doses of LVGCC inhibitors that completely block
extinction fail to prevent conditional fear acquisition (Figs. 1-4).
For cue fear, drug-injected animals showed a pattern of increasing
freezing with training trials identical to that of vehicle-treated
controls, indicating that acute fear learning is independent of LVGCC
activity. Across a wide range of nifedipine doses and two pretreatment
durations, conditional fear 1 d later was at least as great as
that of controls. These data indicate that long-term retention of fear
learning does not require LVGCC activity and that the failure to block
acquisition was not attributable to delayed absorption or early
metabolism of the drug. Whereas drug-free freezing was sometimes
significantly lower in the long-term retention test 1 d later in
animals treated with nimodipine, this was clearly a state-dependent
effect on recall (Figs. 2, 3). When nimodipine was reinjected (Fig. 2)
or injected before both training and testing (Fig. 3), mice showed
conditional freezing identical to that expressed by animals doubly
injected with vehicle. Normal fear acquisition occurred with both
moderate and weak training protocols, indicating that it was not
overtraining that prevented detection of a role for LVGCCs in fear acquisition.
LVGCC inhibitors also failed to block context fear acquisition (Figs.
1, 4). The rate of acute context fear acquisition in nimodipine-treated
mice was indistinguishable from vehicle-treated mice.
Nifedipine-treated mice appear to acquire context fear more slowly in
the absence of auditory CS, but the difference in acquisition rate was
not significant, and animals achieved the same final freezing levels.
The 24 hr retention test was unambiguous; neither LVGCC inhibitor
prevented the long-term acquisition of context fear (Fig. 4), and
nifedipine caused no significant increase in context generalization of
fear (Fig. 1). This last finding may be especially relevant to the
present studies, because extinction expression is context dependent
(Bouton and Bolles, 1979). Our context experiments indicate that LVGCC
inhibitors do not impair context-dependent learning and that their
blockade of extinction is probably not by preventing associations with
the context of extinction.
Several of our findings make it clear that LVGCCs are also not required
for the expression of conditional fear. Vehicle- and drug-treated mice
acquire freezing to the tone at the same rate (Figs. 1-4).
Furthermore, initial freezing levels during the extinction sessions
were equivalent for drug-injected and vehicle-injected mice (Figs. 5,
6, 8). Finally, in the state-dependence test of acquisition, animals
injected first with vehicle and then with nimodipine show no difference
in freezing from those injected with vehicle twice (Fig. 3). These
findings argue strongly that the drugs interfere neither with the
detection of the CS or US nor with expression of conditional fear (freezing).
We also tested whether nonspecific effects of the drugs on locomotion
could account for our results. In an open field test, nimodipine, but
not nifedipine, acutely decreased total distance traveled, but neither
affected locomotion the next day (Fig. 9). Thus, affects on locomotion
cannot account for the blockade of extinction in our drug-free tests.
It is also unlikely that our blockade of acute extinction is a result
of reduced locomotion because nifedipine blocks extinction acutely but
does not reduce locomotion. In addition, nimodipine-injected animals
were never scored with increased freezing acutely compared with
vehicle-injected animals (Figs. 2-4, 6, 8). These results confirm that
freezing is behaviorally distinct from reduced locomotion and that the
acute effects of nimodipine on locomotion neither confounded our
freezing scores nor accounted for the persistent blockade of extinction.
Forms of LVGCC-dependent synaptic modification have been described in a
number of synapses in the brain (Johnston et al., 1992; Huang and
Malenka, 1993; Christie and Abraham, 1994; Huber et al., 1995; Zhuo and
Hawkins, 1995; Kurotani et al., 1996; Izumi and Zorumski, 1998; Kapur
et al., 1998; Morgan and Teyler, 2001; Zakharenko et al., 2001).
Usually, these synapses also show NMDAR-dependent LTP. However, LTP at
synapses between thalamic afferents and neurons in the lateral amygdala
is NMDAR independent (Weisskopf et al., 1999). This LTP depends,
instead, on LVGCCs, because it can be blocked by nifedipine. Consistent
with this, few previous reports indicate a clear dependence of learning
on LVGCCs (Lee and Lin, 1991; Deyo et al., 1992; Borroni et al., 2000).
To the contrary, many studies indicate that LVGCC blockade promotes
learning rather than blocking it (Disterhoft et al., 1993; Vetulani et
al., 1993; Fulga and Stroescu, 1997; Quevedo et al., 1998; Quartermain
et al., 2001). We also show a significantly increased acquisition of
fear with 10 mg/kg nifedipine (Fig. 1C) and trends toward
better learning at other doses in acquisition and in extinction (Figs. 1C, 4C). Previous investigators hypothesize
various explanations for the paradoxical enhancement of learning by
LVGCC blockade, including compensatory cellular changes (Quevedo et
al., 1998), low concentrations of antagonists acting to hold channels
open rather than closing them (Fulga and Stroescu, 1997),
or nonspecific vasodilatory effects (Vetulani et al., 1993).
Importantly, this learning enhancement has been observed repeatedly to
disappear as the dose of LVGCC inhibitors increases, suggesting that it results from modulation rather than complete blockade of the channels.
However, in the present studies, we did not pursue the nootropic
effects of low doses of LVGCC inhibitors, because we wanted to
determine when LVGCC-dependent plasticity was required in fear learning. We, therefore, chose high drug doses to maximally inhibit LVGCCs. The results demonstrate a robust blockade of one type of
inhibitory learning (extinction) with no effect on a type of excitatory
learning (acquisition). The fact that LVGCCs are implicated in
extinction but not in acquisition of conditional fear, whereas NMDARs
are implicated in both (Falls et al., 1992; Baker and Azorlosa, 1996;
Tang et al., 1999), raises questions about the need for this extra
molecule in extinction learning. We hypothesize that LVGCCs are needed
in extinction but not in acquisition, because no CS-US pairing occurs
during extinction. LVGCCs may allow plasticity to occur after
presentation of CS alone, a hypothesis we hope to test using other
forms of CS-alone learning, such as latent inhibition (Lubow,
1973).
Although systemic injections cannot support any anatomical hypothesis
about the sites at which these inhibitors have their effect on
extinction, other evidence suggests that the amygdala may be the
relevant location. First, whereas long-lasting extinction may depend on
areas of prefrontal cortex (Morgan et al., 1993), the induction of
extinction proceeds normally in animals with frontal lesions (Quirk et
al., 2000). Similarly, extinction induction occurs normally in animals
with hippocampal lesions (Frohardt et al., 2000). The amygdala, on the
other hand, clearly plays a role in extinction, because
intraparenchymal infusions of NMDAR or mitogen-activated protein
kinase inhibitors there block extinction (Falls et al., 1992; Lu et
al., 2001). Furthermore, the identification of LVGCC-dependent, but
NMDAR-independent, LTP in the thalamo-amygdala pathway (Weisskopf et
al., 1999) has led to the hypothesis that this LTP is crucial for fear
conditioning (Blair et al., 2001), and a very recent paper from this
group shows an attenuation of cue fear acquisition with the LVGCC
blocker verapamil (Bauer et al., 2002). We cannot account entirely for
the inconsistency of these results with our own. Our data argue
strongly against the importance of LVGCC-dependent LTP, whether in
amygdala or elsewhere in the brain, in the acquisition of conditional
fear. Two potential confounds may account for the inconsistency. First,
it is difficult to compare intraparenchymal infusions of verapamil with
our systemic administrations, and, second, no test of state-dependent
recall was performed by Bauer et al. Because our data clearly indicate that LVGCCs participate in the extinction of conditional fear at doses
that do not affect acquisition, we expect that intra-amygdala infusions
of LVGCC inhibitors will also block extinction of conditional fear at
doses that fail to block acquisition. We are currently testing this
hypothesis with intraparenchymal administrations and performing
state-dependence controls to resolve the apparent inconsistency.
However, it should be noted that LVGCCs are ubiquitous in the brain,
and LVGCC-dependent plasticity outside the amygdala may well make the
relevant contribution to extinction memory formation.
The demonstration that LVGCCs are required for extinction but not
acquisition of conditional fear suggests that it may be possible to
identify cells, synapses, or molecular pathways specific to extinction.
Because extinction is the explicit model for behavior therapy (Wolpe,
1969), the most efficacious treatment for human anxiety disorders, this
discovery also holds out hope for the development of new drugs that can
make such therapy easier and more effective by selectively facilitating
the extinction of fear.
| |
FOOTNOTES |
Received April 11, 2002; revised July 5, 2002; accepted July 9, 2002.
This work was supported in part by a National Alliance for Research on
Schizophrenia and Depression Young Investigator Award and by the Forest
Award of the West Coast College of Biological Psychiatry (M.B.). We
thank Aaron Blaisdell, Gene Gurkoff, Kelsey Martin, Tom O'Dell, and
Alcino Silva for helpful comments on this manuscript.
Correspondence should be addressed to Dr. Mark Barad, Department of
Psychiatry and Biobehavioral Sciences, University of California, Los
Angeles, 3506 Gonda, 695 Charles Young Drive South, Los Angeles, CA
90095-1761. E-mail: mbarad{at}mednet.ucla.edu.
| |
REFERENCES |
-
Baker JD,
Azorlosa JL
(1996)
The NMDA antagonist MK-801 blocks the extinction of Pavlovian fear conditioning.
Behav Neurosci
110:618-620[Web of Science][Medline].
-
Bauer EP,
Schafe GE,
LeDoux JE
(2002)
NMDA receptors and L-type voltage-gated calcium channels contribute to long-term potentiation and different components of fear memory formation in the lateral amygdala.
J Neurosci
22:5239-5249[Abstract/Free Full Text].
-
Baum M
(1988)
Spontaneous recovery from the effects of flooding (exposure) in animals.
Behav Res Ther
26:185-186[Medline].
-
Blair HT,
Schafe GE,
Bauer EP,
Rodrigues SM,
LeDoux JE
(2001)
Synaptic plasticity in the lateral amygdala: a cellular hypothesis of fear conditioning.
Learn Mem
8:229-242[Abstract/Free Full Text].
-
Blokland A,
Prickaerts J,
Honig W,
de Vente J
(1998)
State-dependent impairment in object recognition after hippocampal NOS inhibition.
NeuroReport
9:4205-4208[Medline].
-
Borroni AM,
Fichtenholtz H,
Woodside BL,
Teyler TJ
(2000)
Role of voltage-dependent calcium channel long-term potentiation (LTP) and NMDA LTP in spatial memory.
J Neurosci
20:9272-9276[Abstract/Free Full Text].
-
Bouton ME,
Bolles RC
(1979)
Contextual control of the extinction of conditioned fear.
Learn Motiv
10:445-466.
-
Bouton ME,
King DA
(1983)
Contextual control of the extinction of conditioned fear: tests for the associative value of the context.
J Exp Psychol Anim Behav Process
9:248-265[Web of Science][Medline].
-
Bouton ME,
Kenney FA,
Rosengard C
(1990)
State-dependent fear extinction with two benzodiazepine tranquilizers.
Behav Neurosci
104:44-55[Web of Science][Medline].
-
Christie BR,
Abraham WC
(1994)
L-type voltage-sensitive calcium channel antagonists block heterosynaptic long-term depression in the dentate gyrus of anaesthetized rats.
Neurosci Lett
167:41-45[Web of Science][Medline].
-
Connelly JF,
Connelly JM,
Phifer R
(1975)
Disruption of state-dependent learning (memory retrieval) by emotionally-important stimuli.
Psychopharmacologia
41:139-143[Medline].
-
Deyo RA,
Nix DA,
Parker TW
(1992)
Nifedipine blocks retention of a visual discrimination task in chicks.
Behav Neural Biol
57:260-262[Medline].
-
Disterhoft JF,
Moyer Jr JR,
Thompson LT,
Kowalska M
(1993)
Functional aspects of calcium-channel modulation.
Clin Neuropharmacol
16:S12-S24.
-
Falls WA,
Miserendino MJ,
Davis M
(1992)
Extinction of fear-potentiated startle: blockade by infusion of an NMDA antagonist into the amygdala.
J Neurosci
12:854-863[Abstract].
-
Frohardt RJ,
Guarraci FA,
Bouton ME
(2000)
The effects of neurotoxic hippocampal lesions on two effects of context after fear extinction.
Behav Neurosci
114:227-240[Web of Science][Medline].
-
Fulga IG,
Stroescu V
(1997)
Experimental research on the effect of calcium channel blockers nifedipine and verapamil on the anxiety in mice.
Rom J Physiol
34:127-136[Medline].
-
Huang YY,
Malenka RC
(1993)
Examination of TEA-induced synaptic enhancement in area CA1 of the hippocampus: the role of voltage-dependent Ca2+ channels in the induction of LTP.
J Neurosci
13:568-576[Abstract].
-
Huber KM,
Mauk MD,
Kelly PT
(1995)
Distinct LTP induction mechanisms: contribution of NMDA receptors and voltage-dependent calcium channels.
J Neurophysiol
73:270-279[Abstract/Free Full Text].
-
Izumi Y,
Zorumski CF
(1998)
LTP in CA1 of the adult rat hippocampus and voltage-activated calcium channels.
NeuroReport
9:3689-3691[Web of Science][Medline].
-
Jackson A,
Koek W,
Colpaert FC
(1992)
NMDA antagonists make learning and recall state-dependent.
Behav Pharmacol
3:415-421[Medline].
-
Janicki PK,
Siembab D,
Paulo EA,
Krzascik P
(1988)
Single-dose kinetics of nifedipine in rat plasma and brain.
Pharmacology
36:183-187[Medline].
-
Johnston D,
Williams S,
Jaffe D,
Gray R
(1992)
NMDA-receptor-independent long-term potentiation.
Annu Rev Physiol
54:489-505[Web of Science][Medline].
-
Kapur A,
Yeckel MF,
Gray R,
Johnston D
(1998)
L-Type calcium channels are required for one form of hippocampal mossy fiber LTP.
J Neurophysiol
79:2181-2190[Abstract/Free Full Text].
-
Kim JJ,
DeCola JP,
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[Medline].
-
Kurotani T,
Higashi S,
Inokawa H,
Toyama K
(1996)
Protein and RNA synthesis-dependent and -independent LTPs in developing rat visual cortex.
NeuroReport
8:35-39[Web of Science][Medline].
-
Larkin JG,
Thompson GG,
Scobie G,
Forrest G,
Drennan JE,
Brodie MJ
(1992)
Dihydropyridine calcium antagonists in mice: blood and brain pharmacokinetics and efficacy against pentylenetetrazol seizures.
Epilepsia
33:760-769[Medline].
-
Lee EH,
Lin WR
(1991)
Nifedipine and verapamil block the memory-facilitating effect of corticotropin-releasing factor in rats.
Life Sci
48:1333-1340[Medline].
-
Lee H,
Kim JJ
(1998)
Amygdalar NMDA receptors are critical for new fear learning in previously fear-conditioned rats.
J Neurosci
18:8444-8454[Abstract/Free Full Text].
-
Li XF,
Phillips R,
LeDoux JE
(1995)
NMDA and non-NMDA receptors contribute to synaptic transmission between the medial geniculate body and the lateral nucleus of the amygdala.
Exp Brain Res
105:87-100[Web of Science][Medline].
-
Lu KT,
Walker DL,
Davis M
(2001)
Mitogen-activated protein kinase cascade in the basolateral nucleus of amygdala is involved in extinction of fear-potentiated startle.
J Neurosci
21:RC162[Abstract/Free Full Text](1-5).
-
Lubow RE
(1973)
Latent inhibition.
Psychol Bull
79:398-407[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].
-
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].
-
Morgan MA,
Romanski LM,
LeDoux JE
(1993)
Extinction of emotional learning: contribution of medial prefrontal cortex.
Neurosci Lett
163:109-113[Web of Science][Medline].
-
Morgan SL,
Teyler TJ
(2001)
Electrical stimuli patterned after the theta-rhythm induce multiple forms of ltp.
J Neurophysiol
86:1289-1296[Abstract/Free Full Text].
-
Quartermain D,
deSoria VG,
Kwan A
(2001)
Calcium channel antagonists enhance retention of passive avoidance and maze learning in mice.
Neurobiol Learn Mem
75:77-90[Medline].
-
Quevedo J,
Vianna M,
Daroit D,
Born AG,
Kuyven CR,
Roesler R,
Quillfeldt JA
(1998)
L-type voltage-dependent calcium channel blocker nifedipine enhances memory retention when infused into the hippocampus.
Neurobiol Learn Mem
69:320-325[Medline].
-
Quirk GJ,
Russo GK,
Barron JL,
Lebron K
(2000)
The role of ventromedial prefrontal cortex in the recovery of extinguished fear.
J Neurosci
20:6225-6231[Abstract/Free Full Text].
-
Rescorla RA,
Heth CD
(1975)
Reinstatement of fear to an extinguished conditioned stimulus.
J Exp Psychol Anim Behav Process
1:88-96[Medline].
-
Rodrigues SM,
Schafe GE,
LeDoux JE
(2001)
Intra-amygdala blockade of the NR2B subunit of the NMDA receptor disrupts the acquisition but not the expression of fear conditioning.
J Neurosci
21:6889-6896[Abstract/Free Full Text].
-
Rogan MT,
LeDoux JE
(1995)
LTP is accompanied by commensurate enhancement of auditory-evoked responses in a fear conditioning circuit.
Neuron
15:127-136[Web of Science][Medline].
-
Rogan MT,
Staubli UV,
LeDoux JE
(1997)
Fear conditioning induces associative long-term potentiation in the amygdala.
Nature
390:604-607[Medline]. [Erratum (1998) 391:818]
-
Santini E,
Muller RU,
Quirk GJ
(2001)
Consolidation of extinction learning involves transfer from NMDA-independent to NMDA-dependent memory.
J Neurosci
21:9009-9017[Abstract/Free Full Text].
-
Tang YP,
Shimizu E,
Dube GR,
Rampon C,
Kerchner GA,
Zhuo M,
Liu G,
Tsien JZ
(1999)
Genetic enhancement of learning and memory in mice.
Nature
401:63-69[Medline].
-
Vetulani J,
Battaglia M,
Castellano C,
Sansone M
(1993)
Facilitation of shuttle-box avoidance behaviour in mice treated with nifedipine in combination with amphetamine.
Psychopharmacology
113:217-221[Medline].
-
Weisskopf MG,
Bauer EP,
LeDoux JE
(1999)
L-type voltage-gated calcium channels mediate NMDA-independent associative long-term potentiation at thalamic input synapses to the amygdala.
J Neurosci
19:10512-10519[Abstract/Free Full Text].
-
Wolpe J
(1969)
In: The practice of behavior therapy, Ed 1. New York: Pergamon.
-
Zakharenko SS,
Zablow L,
Siegelbaum SA
(2001)
Visualization of changes in presynaptic function during long-term synaptic plasticity.
Nat Neurosci
4:711-717[Web of Science][Medline].
-
Zhuo M,
Hawkins RD
(1995)
Long-term depression: a learning-related type of synaptic plasticity in the mammalian central nervous system.
Rev Neurosci
6:259-277[Medline].
Copyright © 2002 Society for Neuroscience 0270-6474/02/22209113-09$05.00/0
This article has been cited by other articles:
|
|
|
|
 
J. Xu, Y. Zhu, A. Contractor, and S. F. Heinemann
mGluR5 Has a Critical Role in Inhibitory Learning
J. Neurosci.,
March 25, 2009;
29(12):
3676 - 3684.
[Abstract]
[Full Text]
[PDF]
|
|
|
|
|
|
|
R. Kimura, A. J. Silva, and M. Ohno
Autophosphorylation of {alpha}CaMKII is differentially involved in new learning and unlearning mechanisms of memory extinction
Learn. Mem.,
October 30, 2008;
15(11):
837 - 843.
[Abstract]
[Full Text]
[PDF]
|
|
|
|
|
|
|
K. Hefner, N. Whittle, J. Juhasz, M. Norcross, R.-M. Karlsson, L. M. Saksida, T. J. Bussey, N. Singewald, and A. Holmes
Impaired Fear Extinction Learning and Cortico-Amygdala Circuit Abnormalities in a Common Genetic Mouse Strain
J. Neurosci.,
August 6, 2008;
28(32):
8074 - 8085.
[Abstract]
[Full Text]
[PDF]
|
|
|
|
|
|
|
A. Suzuki, T. Mukawa, A. Tsukagoshi, P. W. Frankland, and S. Kida
Activation of LVGCCs and CB1 receptors required for destabilization of reactivated contextual fear memories
Learn. Mem.,
May 29, 2008;
15(6):
426 - 433.
[Abstract]
[Full Text]
[PDF]
|
|
|
|
|
|
|
G. E. Schafe
Rethinking the role of L-type voltage-gated calcium channels in fear memory extinction
Learn. Mem.,
April 25, 2008;
15(5):
324 - 325.
[Full Text]
[PDF]
|
|
|
|
|
|
|
B. C. McKinney, W. Sze, J. A. White, and G. G. Murphy
L-type voltage-gated calcium channels in conditioned fear: A genetic and pharmacological analysis
Learn. Mem.,
April 25, 2008;
15(5):
326 - 334.
[Abstract]
[Full Text]
[PDF]
|
|
|
|
|
|
|
R. Waltereit, S. Mannhardt, S. Nescholta, C. Maser-Gluth, and D. Bartsch
Selective and protracted effect of nifedipine on fear memory extinction correlates with induced stress response
Learn. Mem.,
April 25, 2008;
15(5):
348 - 356.
[Abstract]
[Full Text]
[PDF]
|
|
|
|
|
|
|
P. Busquet, A. Hetzenauer, M. J. Sinnegger-Brauns, J. Striessnig, and N. Singewald
Role of L-type Ca2+ channel isoforms in the extinction of conditioned fear
Learn. Mem.,
April 25, 2008;
15(5):
378 - 386.
[Abstract]
[Full Text]
[PDF]
|
|
|
|
|
|
|
C. L. Wellman, A. Izquierdo, J. E. Garrett, K. P. Martin, J. Carroll, R. Millstein, K.-P. Lesch, D. L. Murphy, and A. Holmes
Impaired Stress-Coping and Fear Extinction and Abnormal Corticolimbic Morphology in Serotonin Transporter Knock-Out Mice
J. Neurosci.,
January 17, 2007;
27(3):
684 - 691.
[Abstract]
[Full Text]
[PDF]
|
|
|
|
|
|
|
C. Isiegas, A. Park, E. R. Kandel, T. Abel, and K. M. Lattal
Transgenic Inhibition of Neuronal Protein Kinase A Activity Facilitates Fear Extinction
J. Neurosci.,
December 6, 2006;
26(49):
12700 - 12707.
[Abstract]
[Full Text]
[PDF]
|
|
|
|
|
|
|
M. Barad
Divide and conquer: An L-type voltage-gated calcium channel subtype finds a role in conditioned fear.
Learn. Mem.,
September 1, 2006;
13(5):
560 - 561.
[Full Text]
[PDF]
|
|
|
|
|
|
|
B. C. McKinney and G. G. Murphy
The L-Type voltage-gated calcium channel Cav1.3 mediates consolidation, but not extinction, of contextually conditioned fear in mice.
Learn. Mem.,
September 1, 2006;
13(5):
584 - 589.
[Abstract]
[Full Text]
[PDF]
|
|
|
|
|
|
|
K. Kamprath, G. Marsicano, J. Tang, K. Monory, T. Bisogno, V. D. Marzo, B. Lutz, and C. T. Wotjak
Cannabinoid CB1 receptor mediates fear extinction via habituation-like processes.
J. Neurosci.,
June 21, 2006;
26(25):
6677 - 6686.
[Abstract]
[Full Text]
[PDF]
|
|
|
|
|
|
|
Z. Callaerts-Vegh, T. Beckers, S. M. Ball, F. Baeyens, P. F. Callaerts, J. F. Cryan, E. Molnar, and R. D'Hooge
Concomitant deficits in working memory and fear extinction are functionally dissociated from reduced anxiety in metabotropic glutamate receptor 7-deficient mice.
J. Neurosci.,
June 14, 2006;
26(24):
6573 - 6582.
[Abstract]
[Full Text]
[PDF]
|
|
|
|
|
|
|
R. Ponnusamy, H. A. Nissim, and M. Barad
Systemic blockade of D2-like dopamine receptors facilitates extinction of conditioned fear in mice
Learn. Mem.,
July 1, 2005;
12(4):
399 - 406.
[Abstract]
[Full Text]
[PDF]
|
|
|
|
|
|
|
C. K. Cain, B. P. Godsil, S. Jami, and M. Barad
The L-type calcium channel blocker nifedipine impairs extinction, but not reduced contingency effects, in mice
Learn. Mem.,
May 1, 2005;
12(3):
277 - 284.
[Abstract]
[Full Text]
[PDF]
|
|
|
|
|
|
|
J. M. Power and P. Sah
Intracellular calcium store filling by an L-type calcium current in the basolateral amygdala at subthreshold membrane potentials
J. Physiol.,
January 15, 2005;
562(2):
439 - 453.
[Abstract]
[Full Text]
[PDF]
|
|
|
|
|
|
|
K. Robleto, A. M. Poulos, and R. F. Thompson
Brain Mechanisms of Extinction of the Classically Conditioned Eyeblink Response
Learn. Mem.,
September 1, 2004;
11(5):
517 - 524.
[Abstract]
[Full Text]
[PDF]
|
|
|
|
|
|
|
M. Barad, A. M. Blouin, and C. K. Cain
Like Extinction, Latent Inhibition of Conditioned Fear in Mice Is Blocked by Systemic Inhibition of L-Type Voltage-Gated Calcium Channels
Learn. Mem.,
September 1, 2004;
11(5):
536 - 539.
[Abstract]
[Full Text]
[PDF]
|
|
|
|
|
|
|
A. Cannich, C. T. Wotjak, K. Kamprath, H. Hermann, B. Lutz, and G. Marsicano
CB1 Cannabinoid Receptors Modulate Kinase and Phosphatase Activity During Extinction of Conditioned Fear in Mice
Learn. Mem.,
September 1, 2004;
11(5):
625 - 632.
[Abstract]
[Full Text]
[PDF]
|
|
|
|
|
|
|
D. S. Charney
Psychobiological Mechanisms of Resilience and Vulnerability: Implications for Successful Adaptation to Extreme Stress
Focus,
July 1, 2004;
2(3):
368 - 391.
[Abstract]
[Full Text]
[PDF]
|
|
|
|
|
|
|
A. Suzuki, S. A. Josselyn, P. W. Frankland, S. Masushige, A. J. Silva, and S. Kida
Memory Reconsolidation and Extinction Have Distinct Temporal and Biochemical Signatures
J. Neurosci.,
May 19, 2004;
24(20):
4787 - 4795.
[Abstract]
[Full Text]
[PDF]
|
|
|
|
|
|
|
C. K. Cain, A. M. Blouin, and M. Barad
Adrenergic Transmission Facilitates Extinction of Conditional Fear in Mice
Learn. Mem.,
March 1, 2004;
11(2):
179 - 187.
[Abstract]
[Full Text]
[PDF]
|
|
|
|
|
|
|
D. S. Charney
Psychobiological Mechanisms of Resilience and Vulnerability: Implications for Successful Adaptation to Extreme Stress
Am J Psychiatry,
February 1, 2004;
161(2):
195 - 216.
[Abstract]
[Full Text]
[PDF]
|
|
|
|
|
|
|
C.-H. Lin, S.-H. Yeh, H.-Y. Lu, and P.-W. Gean
The Similarities and Diversities of Signal Pathways Leading to Consolidation of Conditioning and Consolidation of Extinction of Fear Memory
J. Neurosci.,
September 10, 2003;
23(23):
8310 - 8317.
[Abstract]
[Full Text]
[PDF]
|
|
|
|
|
TOP
ABSTRACT
INTRODUCTION
