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The Journal of Neuroscience, September 15, 2002, 22(18):8305-8311
Reactivation and Reconsolidation of Long-Term Memory in the Crab
Chasmagnathus: Protein Synthesis Requirement and
Mediation by NMDA-Type Glutamatergic Receptors
María Eugenia
Pedreira,
Luis
María
Pérez-Cuesta, and
Héctor
Maldonado
Laboratorio de Neurobiología de la Memoria, Departamento de
Fisiología y Biología Molecular, Pabellón II,
Facultad de Ciencias Exactas y Naturales (C1428EHA), Universidad de
Buenos Aires, Argentina
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ABSTRACT |
Experiments with invertebrates support the view that intracellular
events subserving the consolidation phase of memory are preserved
across evolution. Here, we investigate whether such evolutionary
persistence extends to reconsolidation mechanisms, which have recently
received special attention in vertebrate studies. For this purpose, the
memory model of the crab Chasmagnathus is used. A visual
danger stimulus (VDS) elicits crab escaping, which declines after a few
stimulus presentations. The long-lasting retention of this decrement,
called context-signal memory (CSM), is mediated by an association
between contextual cues of the training site and the VDS. The
present results show amnesia for CSM in crabs re-exposed at 24 hr (day
2) for 5 min to the learning context, 24 hr after training, and
injected with one of two amnesic agents, then tested 24 hr later.
Agents and timing were either 15 µg of cycloheximide given between 1 hr before and 4 hr after re-exposure or 1 µg/gm
(+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine given between 1 hr before and 2 hr after re-exposure. The amnesic effects are specific to behavior that occurs a long time after reactivation but not a short time after. No CSM deficit is produced by
such agents when crabs are exposed to a context different from that of
training. Findings are consistent with those reported for vertebrates,
with both showing that reactivation induces a recapitulation of the
postacquisition cascade of intracellular events. The agreement between
results from such phylogenetically disparate animals suggests that
evolution may have adopted a given molecular cascade as the preferred
means of encoding experiences in the nervous system.
Key words:
reactivation; reconsolidation; invertebrate; context reminder; cycloheximide; MK-801; memory; crab; crustacea
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INTRODUCTION |
Evidence from invertebrate species
such as Aplysia (Abel and Kandel, 1998 ; Alberini, 1999),
Drosophila (Tully et al., 1994; Tully, 1998), honeybee
(Müller, 2000; Menzel, 2001), and the crab
Chasmagnathus (for review, see Maldonado, 2002) lends
support to the view that both the memory consolidation phase after
acquisition and the cascade of intracellular events subserving
consolidation are preserved across evolution. Since the early work of
Misanin et al. (1968) more than three decades ago, there has been
renewed interest in another phase of memory, namely the
reminder-reactivated memory. An increasing number of studies with
vertebrates show that reactivated old memories become labile and
sensitive to amnesic treatment. Such vulnerability decreases over time,
indicating that reactivation is followed by a temporally graded
reconsolidation (Nader et al., 2000; Sara, 2000). In addition, it has
been proposed that reconsolidation uses many of the same cellular and
molecular mechanisms as consolidation (Nadel and Land, 2000). Although
previous studies have also demonstrated the phenomenon of
reconsolidation in an invertebrate, the garden slug Lymax
(Sekiguchi et al., 1997), no one has addressed the mechanisms mediating
reconsolidation in a simple system. Thus, the purpose of the present
article is to investigate the memory reconsolidation phase and the
mechanisms subserving it by using the memory model of the crab
Chasmagnathus, extensively studied at the behavioral and
mechanistic levels (Maldonado, 2002).
The associative learning paradigm of the crab is based on its escape
response, elicited by the presentation of a visual danger stimulus
(VDS) (an opaque rectangle passing overhead). With the iterative
presentation of the VDS, the crab's response declines and a strong
freezing is built up (Pereyra et al., 1999, 2000). The response
decrement lasts for at least 5 d (Lozada et al., 1990; Pedreira et
al., 1995). This memory is mediated by an association between the
environmental features of the training site (the context) and the
features of the screen moving overhead (the signal) (Tomsic et al.,
1998), so that it is called context-signal memory (CSM). Studies about
mechanisms underlying consolidation have shown that CSM consolidation
is cycloheximide (CHX)-sensitive (Pedreira et al., 1995, 1996; Hermitte
et al., 1999); is positively modulated by angiotensins (Delorenzi et
al., 1996, 2000); is selectively regulated by a muscarinic cholinergic
mechanism (Berón de Astrada and Maldonado, 1999); and is mediated
by the cAMP signal pathway (Romano et al., 1996a,b; Locatelli et al.,
2000, 2002), by nuclear factor- B transcription factor (Freudenthal
et al., 1998; Freudenthal and Romano, 2000; Merlo et al., 2002), and by
NMDA-like glutamatergic receptors (Troncoso and Maldonado, 2002).
Specifically, experiments here are aimed at determining whether the CSM
of the crab could be reactivated by short re-exposure to the context
after a period of being impervious to amnesic agents. If so, the next
step is to characterize the retrieval properties and to ascertain
whether reconsolidation requires the same cellular machinery as
consolidation, starting with the protein synthesis requirement and the
mediation of NMDA-type glutamatergic receptors.
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MATERIALS AND METHODS |
Animals
Animals were adult male Chasmagnathus crabs 2.7-3.0
cm across the carapace, weighing ~17.0 gm, collected from water <1 m
deep in the rías (narrow coastal inlets) of San Clemente del
Tuyú, Argentina, and transported to the laboratory, where they
were lodged in plastic tanks (35 × 48 × 27 cm) filled to 2 cm depth with diluted marine water to a density of 20 crabs per tank.
Water used in tanks and other containers during the experiments was prepared using hw-Marinex (Winex, Hamburg, Germany), salinity 10-14 , at a pH of 7.4-7.6, and maintained within a range of
22-24°C. The holding and experimental rooms were maintained on a 12 hr light/dark cycle (lights on 7:00 A.M. to 7:00 P.M.). Animals were fed rabbit pellets (Nutrientes S.A., Buenos Aires, Argentina) every
3 d, and the water was changed after feeding. Experiments were
performed within the first week after the animal's arrival, from
January to August, and between 8:00 A.M. and 6:00 P.M. Each crab was
used in only one experiment. Experimental procedures are in compliance
with the Guide for the Care and Use of Laboratory Animals
published by the National Institutes of Health.
Apparatus
The apparatus has been described in detail previously
(Maldonado, 2002). Briefly, the experimental unit was the actometer: a
bowl-shaped opaque container with a steep concave wall 12 cm high (23 cm top diameter and 9 cm floor diameter) covered to a depth of 0.5 cm
with marine water. The crab was lodged in the container, which was
suspended by three strings from an upper wooden framework (23 × 23 × 30 cm) and illuminated with a 10 W lamp placed 30 cm above
the animal. A motor-operated screen (an opaque rectangular strip of
25.0 × 7.5 cm) was moved horizontally over the animal's head,
cyclically from left to right and vice versa. A trial lasted nearly 9 sec and included two successive cycles of movement. Screen
displacements provoked a running response of the crab and subsequent
container vibrations. A stylus was cemented centrally to the bottom of
the container and connected to a piezoelectric transducer. Container
vibrations induced electrical signals proportional to the amplitude and
frequency of the vibrations through the transducer. These signals were
amplified, integrated during each 9 sec trial, and translated into
arbitrary numerical units ranging from 0 to 5000 before being processed
by computer. The activity of every crab was recorded during each entire
trial time. The experimental room had 40 actometers, separated from each other by partitions. A computer was used to program trial sequences, trial duration, and intertrial intervals and to monitor experimental events.
Experimental procedure and design
Each crab was moved from the holding room to one actometer in
the experimental room. Each experiment lasted 3 d and included three phases: training session, treatment session, and test session, each corresponding to 1 d. Two pairs of groups, 30-40 crabs each, were formed in each experiment, called pair a and pair
b, whose protocols differed in the treatment session (day 2).
Day 1: training session. Either pair a or pair
b included one untrained group (U), which was kept in the
actometers during the entire training session (~50 min) but
without being trained (i.e., without being presented with the VDS), and
one trained group (T), which, after 5 min in the actometer without a
VDS (adaptation time), received 15 trials with a VDS separated by an
intertrial interval of 3 min. The actometer used during the training
session is referred to as the standard context. Immediately
after the training session, crabs were moved from the standard context
to be housed individually in the resting containers (i.e., plastic boxes covered to a depth of 0.5 cm with water and kept inside dimly lit
drawers) for 24 hr.
Day 2: treatment session. The core of this phase was the
re-exposure of the crab for 5 min to the standard context without VDS
presentation or to a context unlike that of the training session, referred to as the different context. It consisted of a cylinder 15 cm
high and 15 cm in diameter, whose wall consisted of vertical black and
white bands, illuminated like the actometers. An injection with
physiological or drug solution was given at diverse times relative to
the 5 min context exposure, during the same day 2. Pair a
and pair b differed from each other in one item of the treatment: either the drug injected or the time interval between injection and context exposure.
Day 3: test session. After 24 hr in the resting containers,
all crabs were again placed in the standard context for 5 min, but this
time followed by the test trial (i.e., the VDS presentation).
Before animals were placed in the actometers to start an experiment,
they underwent a selection test: each crab was turned on its back, and
only animals that immediately returned to their normal position were
used. The rationale behind this selection is that crabs with a slow
righting reaction show a low responsiveness to a large diversity of
stimuli and, at a later time, they usually present unhealthy symptoms.
No more than 5% of tested crabs were eliminated.
Drugs and injection procedure
Crustacean saline solution (SAL) (Hoeger and Florey, 1989) was
used as the vehicle. Fifty microliters of saline or drug solution was
given through the right side of the dorsal cephalothoracic-abdominal membrane by means of a syringe fitted with a sleeve to control the
depth of penetration to 4 mm, thus ensuring that the injected solution
was released in the pericardial sac.
CHX and the NMDA receptor antagonist
(+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine
(MK-801) were purchased from Sigma (St. Louis, MO).
Data analysis
CSM retention was assessed by focusing data analysis on test
trial scores [i.e., by estimating the difference between the response
level of the trained group (T) and that of the respective untrained
group (U) of each pair]. A trained group is said to show memory
retention when its mean response level at test trial is statistically
lower than that of the respective untrained group. Rescorla (1988)
convincingly argued in favor of using this sort of analysis instead of
a paired training-testing comparison, stressing the need to clearly
distinguish between time of input (training session) and time of
assessment (testing session). This view is amply justified in the
present case, because it has been demonstrated that CSM retention in
the crab is independent of the escape response level at training
(Tomsic et al., 1991), a result consistent with similar findings in
other animals (Applewhite et al., 1969; Peeke and Veno, 1976).
In previous experiments at our laboratory, a significant difference
(t test; p < 0.05) between the T and U
groups was invariably disclosed at the test trial (T < U), 24 hr
after training, provided that each group consisted of  30 crabs each
and that they were given 15 training trials with 3 min of intertrial
interval. Such a significant difference was also found when crabs were
injected with saline before or after training. Accordingly, prediction is feasible for a significant difference (T < U) at test trial, and therefore, results here are analyzed using a priori
planned comparisons (Rosenthal and Rosnow, 1985; Howell, 1987). For
each experiment, which includes two U-T pairs of groups (pair
a and pair b), three comparisons were performed:
the first one between the two untrained groups, a second for the U
group versus the T group of pair a, and a third for the U
group versus the T group of pair b. Each set of planned
comparisons was performed according to a significant main effect in
one-way ANOVA (p < 0.05). All response scores are
represented as mean ± SEM. We analyzed the data using Statistica
'99 edition (Windows 6.1 software package; StatSoft Inc., Tulsa, OK).
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RESULTS |
Effect of CHX administered at diverse times relative to re-exposure
to the original learning context
It was demonstrated previously that 10-15 µg per crab of CHX,
which inhibits ~90% of protein synthesis in Chasmagnathus
for >2 hr, impairs newly acquired CSM when given from 1 hr before or
up to ~6 hr after training (Pedreira et al., 1995). Here, we test
whether the consolidated memory could be reactivated by a reminder and
converted to a labile state. For this purpose, a similar dose of CHX
(15 µg) was given at varying times relative to the crab's exposure
to a standard or to a different context.
In the following five experiments, presented in Figure
1A-E (left
panels), crabs were injected with either SAL or CHX at diverse times relative to 5 min of context exposure during day 2. Each experiment includes a U-T pair a (SAL-injected) and a pair
b (CHX-injected). Figure 1A presents test
results (day 3) corresponding to groups injected on day 2 at 1 hr
before the 5 min re-exposure to the standard context. Planned
comparisons (ANOVA; F(3,144) = 3.35; p < 0.02) (right bar chart) showed a
significant difference (memory retention) for the
SAL-injected group a (p < 0.003) but
no significant difference (memory impairment) for the CHX-injected
group b (p = 0.34). In contrast, when
groups were exposed to the different context (Fig.
1B), planned comparisons (ANOVA;
F(3,136) = 3.93; p < 0.0097) presented significant differences for both pairs of groups
(p < 0.01); i.e., CHX failed to disrupt memory.
The succeeding three experiments explored the effect of injections
given at diverse time intervals after 5 min of re-exposure to the
standard context on day 2. With respect to groups injected 2 hr later
(Fig. 1C), planned comparisons (ANOVA;
F(3,156) = 6.65; p < 0.003) revealed a significant difference for the SAL-injected pair
a (p < 0.0001) but not for the
CHX-injected pair b (p = 0.75). A
similar pattern of test results was found with groups injected 4 hr
after re-exposure to the standard context on day 2 (Fig.
1D), because planned comparisons (ANOVA;
F(3,140) = 2.91; p < 0.036) disclosed a significant difference for pair a
(p < 0.0001) but not for pair b
(p = 0.47). However, when the interval between
the 5-min re-exposure and injection was delayed to 6 hr (Fig.
1E), the CHX injection no longer impaired CSM. In
fact, planned comparisons (ANOVA;
F(3,152) = 3.30; p < 0.023) showed significant differences for both the SAL- and
CHX-injected pair of groups (p < 0.04 and
p < 0.01, respectively). Thus, the CSM acquired on day
1 and reactivated by 5 min of re-exposure to the training context on
day 2 was blocked by CHX injection administered 1 hr before or either 2 or 4 hr after but not 6 hr after reactivation. These results may be
considered the first suggestion in this article that stable and
consolidated memory could again become active and labile (reactivated)
by brief re-exposure to the original learning context.
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Figure 1.
Effect of CHX administered at diverse times
relative to re-exposure to the original learning context.
Left, Behavioral protocols. Icons stand for
the crabs that remained in a container during the time interval
indicated below the icon; white icon,
standard context; black icon, different context.
Arrow, An injection of 50 µl of physiological solution
(SAL) or an injection of 50 µl of CHX (15 µg per crab). Day
1, Training session in the standard context for 50 min; U-T
pair a or b in each experiment has a trained group (T)
with 15 trials separated by 3 min and an untrained group
(U) without training. Day 2,
Treatment session, in which U-T pair a of each
experiment is injected with SAL and U-T pair b is
injected with CHX; injections indicated by an arrow were
given 1 hr ( 1h) before re-exposure to the standard
context (A) or 1 hr (1h) before
exposure to the different context (B);
alternatively, injections indicated by an arrow were
given at 2 hr (2h) after re-exposure
(C), 4 hr (4h) after re-exposure
(D), or 6 hr (6h) after
re-exposure (E). Day 3, Testing
session; all groups stay for 5 min in the standard context followed by
test trial (Ts). Right, Test trial on day
3. Ordinate, Response (i.e., average of the escape
response scores for the test trial ± SEM). Open
bars, Groups of U-T pair a (SAL-injected);
gray bars, groups of U-T pair b
(CHX-injected). Planned comparisons: *p < 0.05, significant T < U difference; **p < 0.01, significant T < U difference (both memory retention).
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Two additional experiments, including one U-T pair of SAL-injected
groups and another of CHX-injected groups, were performed. The first
experiment (Fig. 2) was aimed at
determining whether the amnesic effect of CHX could be observed
immediately after 5 min of crab re-exposure to the standard context.
For this purpose, both U-T pairs were injected 1 hr before
re-exposure, but unlike other experiments of this series, were tested
immediately after re-exposure. No memory-disrupting effect was found in
either pair of groups. Planned comparisons (ANOVA;
F(3,144) = 9.6; p < 0.001) revealed significant differences for both pair a and
pair b (p < 0.0001 and
p < 0.028, respectively). The second experiment (Fig. 3) was performed to explore to what
extent CSM reactivation could be affected by doubling the time interval
between training and contextual re-exposure. For this purpose, the
protocol was the same as that used in the experiment shown in Figure
1C, but with the 5 min context re-exposure shifted to 48 hr
after training (day 3), so that the test trial was performed on day 4. Planned comparisons on these data (ANOVA;
F(3,156) = 6.59; p < 0.003) showed differences similar to those of Figure 1C
[i.e., memory retention (T < U) for the SAL-injected group
(p < 0.001) but memory impairment for the
CHX-injected pair (p = 0.99)].
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Figure 2.
Effect of CHX administered 1 hr
(1h) before re-exposure to the original learning
context and the test trial given immediately after re-exposure. All
symbols and the protocol are as described in Figure
1A, except the test trial (Ts)
given immediately after re-exposure on day 2. Right,
Test trial on day 2. Symbols are as described in Figure
1A. Asterisks are as in Figure
1.
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Figure 3.
Effect of CHX administered 2 hr
(2h) after re-exposure to the original learning context
during day 3 and tested on day 4. Left, Behavioral
protocols. All symbols and the protocol are as described in
Figure 1C, except the treatment session given on day 3 and the test session on day 4. Ts, Test trial.
Right, Test trial on day 3. Symbols are as
described in Figure 1C. Asterisks are as in
Figure 1.
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Effect of NMDAR antagonist MK-801 administered at diverse times
to re-exposure to the original learning context
The impairing effect of the vertebrate NMDAR antagonist MK-801 on
newly acquired CSM was assessed at diverse time intervals relative to
training (Troncoso and Maldonado, 2002). Here, we explored whether a
similar dose of MK-801 (1 µg/gm) acts on retrieved CSM.
In the four experiments presented in Figure
4A-D (left
panels), crabs were injected with either SAL or MK-801 at varying
times relative to the 5 min of context exposure (day 2) and tested on day 3. Each U-T pair a was SAL-injected, whereas each U-T
pair b was MK-801-injected. Figure 4A
presents test trial results (day 3) corresponding to groups injected 1 hr before 5 min of re-exposure to the standard context. Memory
retention for the SAL-injected pair a
(p < 0.009) and memory impairment for
MK-801-injected pair b (p = 0.46)
were shown by planned comparisons (ANOVA;
F(3,156) = 2.6; p < 0.04). In contrast, when the 5 min exposure was to the different
context (Fig. 4B), planned comparisons (ANOVA;
F(3,116) = 8.08; p < 0.0005) disclosed significant differences for both pairs of groups
(p < 0.001 for pair a and
p < 0.0004 for pair b); i.e., MK-801 failed
to disrupt memory when a context different from that used at training
was presented during the 5 min exposure on day 2. With respect to
groups injected 2 hr after 5 min of re-exposure to the standard
context (Fig. 4C), planned comparisons performed on test
scores (ANOVA; F(3,156) = 3.9;
p < 0.009) disclosed significant differences for
the SAL-injected pair (p < 0.003) but not
for the MK-801-injected pair (p = 0.6). However,
when the interval time between contextual re-exposure and injection was
delayed to 4 hr (Fig. 4D), the MK-801 injection no
longer impaired CSM. In fact, planned comparisons (ANOVA;
F(3,156) = 8.23; p < 0.0001) showed significant differences for both pair a and
pair b (p < 0.001 and
p < 0.002, respectively).
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Figure 4.
Effect of the NMDA receptor antagonist MK-801
administered at diverse times relative to re-exposure to the original
learning context. Left, Behavioral protocols. All
symbols and protocols are as described in Figure
1A-D, except on day 2, when U-T pair
a of each experiment is injected with SAL and U-T pair
b is injected with MK-801 (MK).
Ts, Test trial. Right, Test trial on day
3. Symbols are as described in Figure 1. Asterisks
are as in Figure 1.
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Finally, one experiment was performed to analyze the effect of MK-801
on CSM when administered 1 hr before the 5 min context re-exposure and
tested immediately after re-exposure on the same day 2. The design
included two U-T pairs of groups, namely one SAL-injected pair
a and one MK-801-injected pair b. Results are illustrated in Figure 5. No
memory-disrupting effect was found in either pair of groups. Planned
comparisons (ANOVA; F(3,116) = 6.98;
p < 0.0002) revealed significant differences for both pair a and pair b (p < 0.01 and 0.0002, respectively).
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Figure 5.
Effect of the NMDA receptor antagonist MK-801
(MK) administered 1 hr before re-exposure to
the original context and the test trial given immediately after
re-exposure. Left, Behavioral protocols. All
symbols and the protocol are as described in Figure
4A, except for the test trial (Ts)
given immediately after re-exposure on day 2. Right,
Test trial on day 3. Symbols are as described in Figure
1A. Asterisks are as in Figure
1.
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Thus, the CSM acquired on day 1 and tested on day 3 was impaired by
MK-801 administered 1 hr before or 2 hr after, but not 4 hr after,
re-exposure on day 2. Conversely, memory seems to be intact if MK-801
is administered 1 hr before context re-exposure and tested immediately
after re-exposure on the same day 2.
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DISCUSSION |
These experiments provide clear evidence that the robust CSM
acquired by the crab through spaced training (Pedreira et al., 1995;
Romano et al., 1996a,b; Freudenthal et al., 1998; Locatelli et al.,
2000, 2002) again becomes labile after 5 min of re-exposure to the
learning context, proving vulnerable to CHX or MK-801 injection. Results are interpreted according to the view, stemming from findings obtained in vertebrates (Nader et al., 2000), that memory retrieved by
a reminder passes from a dormant and stable stage to an active and
labile one (reactivation), which undergoes a time-dependent consolidation process (reconsolidation) (Przybyslawski and Sara, 1997;
Przybyslawski et al., 1999; Sara, 2000). The effect of the two amnesic
agents cannot be accounted for in terms other than CSM impairment.
First, no significant difference between untrained groups of a same
experiment could be found throughout. Therefore, when no difference is
disclosed for a U-T pair treated by CHX or MK-801, such result could
hardly be attributable to a depressing or enhancing effect of the
amnesic agent on test performance. Second, either CHX or MK-801 failed
to produce amnesia whenever a context different from that of learning
was used as reminder, indicating that faithfulness to the original
context is a necessary condition for the amnesic agent to induce
amnesia. Third, a temporal gradient of effectiveness was demonstrated
for either CHX or MK-801, a fact that is incompatible with an
explanation of results in terms of proactive effects on test performance.
Amnesia for CSM may be explained by reactivation hindrance, caused by
interference with reactivation mechanisms, straightforwardly by a
change in the original learning context because drug injection incorporates new internal cues, or by interference with the
reconsolidation process. However, the explanation in terms of
reactivation blockade should be ruled out, because retention of the old
memory is not affected when reactivation is hindered (Figs.
1B, 4B).
Administration of 15 µg of CHX per crab (CHX injection) produces
disruption of the context-reactivated memory within a time window
estimated between 1 hr before and 4 hr after reactivation. The time
window for the CHX amnesic effect seems to be similar for both
consolidation and reconsolidation (Pedreira et al., 1995; Hermitte et
al., 1999). The finding that CHX has no effect when tested immediately
after contextual re-exposure suggests that such amnesia is most likely
attributable to interference with the molecular mechanisms mediating
reconsolidation but not to nonspecific dysfunctional effects.
Conversely, the fact that CHX maintains its amnesic effect despite the
doubling of the time interval between training and contextual
re-exposure is consistent with results obtained in rats (Nader et al.,
2000).
Apart from our present results with the crab, two other studies, both
with rats, have also reported that administration of a protein
synthesis inhibitor given after memory reactivation causes amnesia for
the original learning (Judge and Quartermain, 1982; Nader et al.,
2000). Hence, the need to synthesize new proteins for reconsolidation
would be a tenet valid for most animal species, namely, a principle as
universal as the need of new proteins for consolidation (Alberini,
1999).
The effect of the NMDAR antagonist MK-801 on reconsolidation was
explored, because this drug (1 µg/gm) had been shown to induce amnesia for CSM when injected immediately before training or up to 4 hr
after training (Troncoso and Maldonado, 2002). The present results
revealed that the same dose of MK-801 produces disruption of the CSM
within a time window ranging from 1 hr before up to 2 hr after
reactivation, but no effect when tested immediately after contextual
re-exposure. Some previous experiments with vertebrates have also shown
that reactivation of a well established memory triggers cellular events
depending on NMDA receptors (Summers et al., 1997; Przybyslawski et
al., 1999), but other reports have indicated that the NMDA receptor
does not appear to be involved in memory retrieval (Steele and Morris,
1999; Shimizu et al., 2000).
According to an interpretive model of CSM retrieval (Tomsic et al.,
1998; Hermitte et al., 1999; Maldonado, 2002), re-exposure of a trained
crab to the learning context evokes a CSM representation that induces a
freezing response as soon as the animal is faced with the VDS. No
previous evidence has shown that such memory representation is already
present before VDS display; however, results of this study support the
proposal. Mere re-exposure to the original learning context, even in
the absence of VDS presentation, is quite enough for an amnesic agent
to impair reactivated memory. Thus, these findings also support the
associative nature of CSM and, specifically, the existence of an
associative link between signal and context as the basis of this memory process.
Given the parallelism between results with crabs and those with
vertebrates concerning certain features of the reactivated memory, the
probable adaptive value of such properties should be explored, in
particular, that of reconsolidation and subsequent relabilization;
specifically, what would be the adaptive value of reconsolidation and
therefore of memory relabilization that necessarily follows after
reactivation. Several speculative arguments have been advanced with
regard to experiments with vertebrates to account for the functionality
of such a memory phase. Reconsolidation was proposed as a process of
reorganization of past experience within the current cognitive context
(Spear and Mueller, 1984; Sara, 1985), during which new information is
integrated on the past background and some forgettable context
attributes are strengthened (Sara, 2000).
This article offers results in keeping with two main tenets of the
reactivation/reconsolidation hypothesis: first, reactivation converts
memory from a dormant-stable state to an active-labile one; and
second, the postacquisition cascade of intracellular events is to some
extent recapitulated whenever memory is reactivated. Both tenets were
grounded on experiments with vertebrates (Nader et al., 2000; Sara,
2000); therefore, the present findings with a crustacean suggest the
persistence through evolution of molecular mechanisms subserving both
consolidation and reconsolidation phases of memory. The shared
mechanisms would be the basic tools used by evolution to promote
adaptive changes through phylogenetically disparate animals (Carew,
2000).
| |
FOOTNOTES |
Received March 20, 2002; revised July 3, 2002; accepted July 3, 2002.
This work was supported by the Agencia Nacional Científica y
Tecnológica (PICT-1-06602). We thank Dr. Alejandro Delorenzi, Dr.
Arturo Romano, and Dr. Daniel Tomsic for reading this manuscript and
for helpful criticism and Angel Vidal for technical assistance.
Correspondence should be addressed to Dr. Héctor Maldonado,
Laboratorio de Neurobiología de la Memoria, Departamento de Fisiología y Biología Molecular, Pabellón II,
Facultad de Ciencias Exactas y Naturales (C1428EHA), Universidad de
Buenos Aires, Buenos Aires, Argentina.
| |
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ABSTRACT
INTRODUCTION
