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The Journal of Neuroscience, August 15, 2001, 21(16):6413-6422
The Contribution of Activity-Dependent Synaptic Plasticity to
Classical Conditioning in Aplysia
Igor
Antonov1,
Irina
Antonova1,
Eric R.
Kandel1, 2, 3, and
Robert D.
Hawkins1, 2
1 Center for Neurobiology and Behavior, College of
Physicians and Surgeons, Columbia University, 2 New York
State Psychiatric Institute, and 3 Howard Hughes Medical
Institute, New York, New York 10032
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ABSTRACT |
Plasticity at central synapses has long been thought to be the most
likely mechanism for learning and memory, but testing that idea
experimentally has proven to be difficult. For this reason, we have
developed a simplified preparation of the Aplysia siphon
withdrawal reflex that allows one to examine behavioral learning and
memory while simultaneously monitoring synaptic connections between
individual identified neurons in the CNS. We previously found that
monosynaptic connections from LE siphon sensory neurons to LFS
siphon motor neurons make a substantial contribution to the reflex in
the siphon withdrawal preparation (Antonov et al., 1999a ). We have now
used that preparation to assess the contribution of various cellular
mechanisms to classical conditioning of the reflex with a siphon tap
conditioned stimulus (CS) and tail shock unconditioned stimulus (US).
We find that, compared with unpaired training, paired training with the
CS and US produces greater enhancement of siphon withdrawal and evoked
firing of LFS neurons, greater facilitation of the complex PSP elicited
in an LFS neuron by the siphon tap, and greater facilitation of the
monosynaptic PSP elicited by stimulation of a single LE neuron.
Moreover, the enhanced facilitation of monosynaptic LE-LFS PSPs is
greater for LE neurons that fire during the siphon tap and correlates
significantly with the enhancement of siphon withdrawal and evoked
firing of the LFS neurons. These results provide the most direct
evidence to date that activity-dependent plasticity at specific central synapses contributes to behavioral conditioning and support the idea
that synaptic plasticity is a mechanism of learning and memory more generally.
Key words:
Aplysia; classical conditioning; synaptic
plasticity; learning; siphon withdrawal; monosynaptic PSP
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INTRODUCTION |
Since the writings of Ramon y Cajal
(1911) and Sherrington (1906) nearly 100 years ago, plasticity at
synapses in the CNS has been thought to be the most likely mechanism
for learning and memory. However, testing that idea experimentally has
proven to be extremely difficult, primarily because of the immense
complexity of the mammalian CNS. Thus, although studies on mammalian
systems have provided support for the idea that synaptic plasticity
contributes to learning (Mayford et al., 1996; Tsien et al., 1996;
Rogan et al., 1997), the evidence has been somewhat inconsistent
(Zamanillo et al., 1999), perhaps because there is not a direct
relationship between changes at specific synapses and behavior
(Hawkins, 1997). For this reason, invertebrate preparations such as
Aplysia are advantageous, and previous studies have
suggested that plasticity at sensory neuron-motor neuron synapses
contributes to several simple forms of learning in Aplysia
(for review, see Carew and Sahley, 1986; Byrne, 1987; Hawkins et al.,
1993).
The Aplysia gill and siphon withdrawal reflex undergoes
several nonassociative forms of learning, such as habituation and sensitization, as well as an associative form of learning, classical conditioning (Carew et al., 1981). Conditioning of the withdrawal reflex has many of the behavioral features characteristic of
conditioning in mammals, including stimulus specificity (Carew et al.,
1983), response specificity (Hawkins et al., 1989; Walters, 1989),
temporal specificity and effects of contingency (Hawkins et al., 1986), as well as several effects of context (Colwill et al., 1988a,b) and
both forward and simultaneous second-order conditioning (Hawkins et
al., 1998). These behavioral results suggest that conditioning in
Aplysia and in mammals may share common cellular and
molecular mechanisms. Cellular studies in the isolated nervous system
of Aplysia have shown that monosynaptic connections between
sensory and motor neurons that contribute to the reflex exhibit an
analog of conditioning with temporal parameters similar to the
behavioral conditioning (Hawkins et al., 1983; Walters and Byrne, 1983;
Carew et al., 1984; Clark et al., 1994; Murphy and Glanzman, 1996,
1997, 1999). Moreover, studies of those synapses in isolated cell
culture have revealed that they undergo two different types of
activity-dependent, associative plasticity that could contribute to the
conditioning: enhancement of presynaptic facilitation (Eliot et al.,
1994; Bao et al., 1998) and Hebbian long-term potentiation (Lin and
Glanzman, 1994a,b, 1997; Bao et al., 1997).
However, these studies in the isolated nervous system or in culture
have not been able to address the contribution of the cellular
mechanisms to behavioral conditioning. Furthermore, studies of
simplified gill withdrawal preparations have suggested that other sites
and mechanisms of plasticity might also contribute (Lukowiak, 1986;
Colebrook and Lukowiak, 1988; Lukowiak and Colebrook, 1988), so that
the contribution of each mechanism has remained uncertain. To address
these issues, we developed a simplified preparation consisting of the
siphon, tail, and CNS of Aplysia that allows one to examine
behavior while simultaneously monitoring synaptic connections between
identified neurons in the CNS (Antonov et al., 1999a). We have now used
that preparation to assess the contribution of activity-dependent
synaptic plasticity and other cellular mechanisms to classical
conditioning of the siphon withdrawal reflex.
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MATERIALS AND METHODS |
The experimental preparation (Fig. 1A) has
been described previously (Antonov et al., 1999a). Briefly, the siphon
tail and CNS of Aplysia californica (100-150 gm; Marinus,
Long Beach, CA) were dissected out in 50% MgCl2
and 50% artificial seawater and then pinned to the floor of a
recording chamber filled with circulating, aerated artificial seawater
at room temperature. The siphon was partially split, and one-half was
left unpinned. In electrophysiological experiments, the abdominal
ganglion was partially desheathed, and double-barreled microelectrodes
(filled with 2.5 M KCl) were inserted in an LE
siphon sensory neuron (Byrne et al., 1974) and an LFS siphon motor
neuron (Frost and Kandel, 1995) for intracellular stimulation and recording.
The conditioned stimulus (CS; a tap of ~20
gm/mm2, 500 msec duration, produced by a
controlled force stimulator; Cohen et al., 1997) was delivered to the
pinned half of the siphon either within the receptive field of the LE
neuron ("on-field") or outside the receptive field of the neuron
("off-field"). Withdrawal of the unpinned half of the siphon was
recorded with a low-mass isotonic movement transducer (Harvard
Apparatus, South Natick, MA) attached by a silk suture. The peak
amplitude of siphon withdrawal and both the amplitude and area of PSPs
in the motor neuron were measured using a laboratory interface to a
microcomputer and commercially available software (Hilal Associates,
Englewood, NJ), which also controlled the stimulation. The
unconditioned stimulus (US; an AC electric shock of 25 mA, 1 sec
duration) was delivered to the tail via a fixed capillary electrode.
Preparations were considered unhealthy or damaged and not used if the
first shock produced a siphon withdrawal of <3 mm (the maximal
withdrawal was usually ~7 mm).
The preparation rested for at least 1 hr before the beginning of
training. There were three blocks of four training trials each, with a
5 min interval between trials in a block and a 20 min rest between
blocks (Fig. 1B). The response to the CS was measured
in a pretest 5 min before the first block (Pre), in test trials 15 min
after each block (T1-T3), and in a final post-test 45 min after the
last block (Post). Experiments were continued only if the siphon
withdrawal on the pretest was between 0.5 and 5 mm and were excluded if
there was any visible evidence of damage to the siphon at the end of
the experiment. During paired training, the CS began 500 msec before
the US on each trial. During unpaired training, the interstimulus
interval was 2.5 min. Animals were randomly assigned to the training conditions.
On each test trial in the electrophysiological experiments, we measured
the monosynaptic PSP from the LE neuron to the LFS neuron, siphon
withdrawal and firing of the LE and LFS neurons evoked by the siphon
tap, and the withdrawal produced by direct stimulation of the LFS
neuron. In some experiments we hyperpolarized the LFS neuron during the
siphon tap and measured the complex PSP instead of evoked LFS firing.
In those experiments, we also measured the input resistance of the LE
and LFS neurons before the tap. If the LE or LFS neuron was lost before
the final post-test, results for measures involving that neuron were
not included. The data for each measure were analyzed using two- or
three-way ANOVAs with one repeated measure (test), followed by planned
comparisons of the experimental groups at each test.
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RESULTS |
Classical conditioning of the Aplysia
siphon-withdrawal reflex
We have previously demonstrated habituation, dishabituation, and
sensitization in the simplified siphon withdrawal preparation (Antonov
et al., 1999a). We now tested classical conditioning by comparing
changes in the withdrawal reflex in four groups that received paired
training, unpaired training, training with the CS alone, or training
with the US alone (Fig. 1). Paired
training produced a significant increase in the response to the CS
compared with the pretest (p < 0.01 on the
final post-test). By contrast, training with the US alone produced a
smaller and shorter-lasting increase (sensitization) that was not
significant with the stimulus parameters used in these studies, and
unpaired training produced a slight decrease. Training with the CS
alone produced a significant decrease (habituation) so that the effects
of paired or unpaired training (which include repeated stimulation with
the CS) should be judged in comparison with that habituation as well as
with their own pretest scores. The habituation would tend to oppose sensitization produced by the US, and these competing processes might
account for the small decrease in responding in the unpaired group.
There was a significant overall difference between the four groups
(F(3,66) = 7.93; p < 0.001), and paired training produced a greater enhancement of siphon
withdrawal than each of the other training conditions
(p < 0.01 on the final post-test in each case). These results demonstrate classical conditioning of the siphon withdrawal reflex in the simplified preparation.
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Figure 1.
Classical conditioning of the
Aplysia siphon withdrawal reflex (SWR) in
the simplified preparation. A, Experimental
preparation. G., Ganglion; S.N.,
sensory neuron; M.N., motor neuron. B,
Protocol for paired training. See Materials and Methods for details.
ITI, Intertrial interval. C, Examples of
the siphon withdrawal produced by a tap to the siphon (the CS) on the
pretest and final post-test after either paired
(P) training, unpaired (UP)
training, training with the CS alone, or training with the US (tail
shock) alone. D, Average results in experiments such as
those shown in C. Paired training produced a greater
increase in the amplitude of siphon withdrawal than unpaired training,
training with the CS alone, or training with the US alone.
*p < 0.05; **p < 0.01 compared with paired. Responses have been normalized to the average
values on the pretest, which were 3.2 mm (paired), 2.7 mm (unpaired),
2.7 mm (CS alone), and 3.1 mm (US alone), not significantly different
by a one-way ANOVA. The average unconditioned responses to the first
tail shock were 7.5 mm (paired), 5.9 mm (unpaired), and 7.8 mm (US
alone), also not significantly different.
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Increase in evoked firing of LFS siphon motor neurons and LE siphon
sensory neurons during classical conditioning
We next began a cellular analysis of the conditioning by recording
evoked firing of an identified LFSB siphon motor
neuron and an LE siphon sensory neuron in the abdominal ganglion
simultaneously with the behavior (Fig.
2). There are ~25 LE neurons, of which ~5 are activated by the CS (Byrne et al., 1974; Hickie et al., 1997),
and 4 LFSB neurons, of which 2 or 3 contribute to
the response measured in this preparation (Frost and Kandel, 1995;
Antonov et al., 1999a). Previous studies have shown that monosynaptic connections between these two classes of cells mediate approximately one-third of the reflex response in this preparation (Antonov et al.,
1999a). The remainder of the response is mediated by other, unidentified sensory neurons (Frost et al., 1997), polysynaptic inputs
onto the LFS neurons from excitatory and inhibitory interneurons (Frost
and Kandel, 1995), and peripheral motor neurons (Perlman, 1979), which
also receive monosynaptic input from the LE neurons (Bailey et al.,
1979).
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Figure 2.
Increase in evoked firing of LFS siphon motor
neurons and LE siphon sensory neurons during classical conditioning.
A, Examples of the behavioral and cellular responses to
the conditioned stimulus on the pretest and the final post-test after
paired and unpaired training. In some cases the spike amplitudes have
been attenuated because of the limited frequency response of the
recording. B, Average siphon withdrawal and evoked
firing of LFS and LE neurons recorded in the same experiments. Paired
training produced a greater increase in the amplitude of siphon
withdrawal, which was accompanied by a greater increase in evoked
firing of the LFS and LE neurons in the first 1 sec.
*p < 0.05; **p < 0.01 compared with unpaired in this and subsequent figures. The average
values on the pretest were 2.4 mm (paired) and 2.7 mm (unpaired) for
siphon withdrawal, 13.7 Hz (paired) and 16.3 Hz (unpaired) for LFS
firing, and 5.2 Hz (paired) and 3.9 Hz (unpaired) for LE firing, not
significantly different by t tests. The average
unconditioned responses to the first tail shock were 4.3 mm (paired)
and 4.9 mm (unpaired), not significantly different. C,
There was a significant correlation between the increase in siphon
withdrawal and the increase in evoked firing of the LFS neurons. The
solid line indicates the linear regression, and the
dashed lines indicate the 95% confidence intervals for
the regression. SWR, Siphon withdrawal reflex.
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In these experiments, there were two groups that received either paired
or unpaired training. Again, paired training produced significantly
greater enhancement of siphon withdrawal than unpaired training overall
(F(1,24) = 17.39; p < 0.001) and on each of the last three tests (p < 0.01 in each case). Figure 2A shows examples of the
firing of LE and LFS neurons, and Figure 2B shows the
average evoked firing during the first 1 sec after the start of the
response to siphon stimulation, which included the peak of the siphon
withdrawal on most trials (the average time to peak was 927 ± 28 msec overall and did not change by >200 msec during conditioning).
Evoked firing of LFS motor neurons changed approximately in parallel
with the changes in siphon withdrawal, with paired training producing a significantly greater enhancement of LFS firing than unpaired training
overall (F(1,24) = 13.51;
p < 0.01) and on the last three tests
(p < 0.01 in each case). Moreover, the increase
in evoked LFS firing correlated significantly with the increase in
siphon withdrawal (r = 0.856; p < 0.001 on the final post-test; Fig. 2C), and pairing did not
have any significant additional effect on withdrawal when this
correlation was factored out in an analysis of covariance. These
results suggest that pairing-specific changes in evoked LFS firing make
an important contribution to conditioning of the siphon withdrawal
reflex in this preparation.
There was also an increase in evoked firing of LE sensory neurons, with
paired training again producing a significantly greater increase than
unpaired training overall (F(1,20) = 6.16; p < 0.05) and on the last two tests
(p < 0.05 in each case). However, whereas LFS
firing and siphon withdrawal increased only after paired training, LE
firing increased after either paired or unpaired training. Furthermore,
although there was a trend for the changes in LE firing to correlate
with the changes in LFS firing (r = 0.26) and siphon
withdrawal (r = 0.25), pairing still had significant effects when these correlations were factored out in analyses of
covariance. These results suggest that the increase in LE firing contributes to the changes in LFS firing and siphon withdrawal during
conditioning, but that other mechanisms probably also contribute.
Another mechanism that might contribute to conditioning is a change in
the pattern of firing of either the LFS or LE cells, which could
produce a change in their effectiveness. To examine the pattern of
firing, we calculated the average number of spikes in each 100 msec
interval after the onset of the response to the tap on each test (Fig.
3). As illustrated in the examples in
Figure 2A and the average results in Figure 3, the
pattern of firing of the LFS cells on the pretest had four components
similar to those described previously (Antonov et al., 1999a): a peak
at the onset of the tap, a lower level of sustained firing during the
tap, a second peak around the offset of the tap, and a gradual decline
after the tap. Also, as described previously (Antonov et al., 1999a),
evoked firing of the LE cells on the pretest had two components that
corresponded to the first two components of LFS firing, so that PSPs
from LE cells contributed directly to LFS firing during the tap but
only indirectly to LFS firing after the tap. After either paired or
unpaired training, there was a substantial increase in firing of the LE
cells during and also after the tap, so that PSPs from the LE cells
could contribute directly to LFS firing at that time as well. However,
there was no significant change in the overall pattern of LE firing,
with approximately equal increases in firing in all of the time
intervals and no significant interaction involving time in an ANOVA.
Similarly, there were no significant changes in the overall pattern of
LFS firing. These results suggest that changes in the pattern of firing of either the LE or LFS cells probably do not contribute significantly to the conditioning.
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Figure 3.
Average pattern of firing of LFS motor neurons and
LE sensory neurons on the pretest and final post-test after paired and
unpaired training in the same experiments as in Figure 2. The number of
spikes in each 100 msec interval has been normalized to the total
number of spikes on the pretest in each experiment. The average values
on the pretest were 15.8 spikes (paired) and 18.8 spikes
(unpaired) for LFS firing and 5.2 spikes (paired) and 3.9 spikes
(unpaired) for LE firing, not significantly different. The
horizontal bar below the x-axis indicates
the approximate duration of the siphon tap.
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No change in peripheral effectiveness of the motor neurons
during conditioning
In addition to changes in the CNS, changes in the peripheral
effectiveness of the motor neurons contribute to dishabituation and
sensitization of siphon withdrawal (Antonov et al., 1999a) and
classical conditioning of gill withdrawal (Colebrook and Lukowiak, 1988; Lukowiak and Colebrook, 1988) in simplified mantle organ preparations. We therefore investigated whether similar peripheral changes also contribute to classical conditioning of siphon withdrawal by measuring the withdrawal produced by intracellular stimulation of an
LFS motor neuron ~30 sec after the siphon tap on each test. As shown
in Figure 4, there was a decrease in the
withdrawal after either paired or unpaired training and no significant
difference between them, although there was a trend for paired training
to produce a smaller decrease. There was also no significant
correlation between the changes in siphon withdrawal elicited by the
LFS stimulation and by siphon stimulation (r = 0.26).
These results suggest that changes in the peripheral effectiveness of
the LFS neurons do not make an important contribution to conditioning
in the siphon withdrawal preparation.
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Figure 4.
Classical conditioning did not involve significant
changes at sites distal to the synapses onto the motor neurons.
A, Examples of siphon withdrawal produced by constant
current intracellular stimulation of an LFS neuron
(ILFS) 30 sec after the pretest and the final
post-test after paired and unpaired training. B, Average
siphon withdrawal and number of spikes produced by intracellular
stimulation of an LFS neuron in experiments like those shown in
A, as well as the spontaneous firing rate of the LFS
neuron 5 sec before each test. There was no significant difference in
siphon withdrawal and also no differences in excitability or
spontaneous firing of the motor neuron. The average values on the
pretest were 0.7 mm (paired) and 0.8 mm (unpaired) for siphon
withdrawal, 23.1 spikes (paired) and 26.2 spikes (unpaired) for
LFS spikes, and 1.6 Hz (paired) and 1.6 Hz (unpaired) for LFS
spontaneous firing, not significantly different by t
tests. SWR, Siphon withdrawal reflex.
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In these experiments, the LFS neuron was stimulated intracellularly
with a constant current pulse, which produced an approximately constant
number of LFS spikes on each test (Fig. 4B),
indicating that there was no significant change in the excitability of
the motor neuron. There was also no significant change in either the spontaneous firing rate of LFS neurons (Fig. 4B) or
their input resistance measured with a hyperpolarizing intracellular
current pulse 10 sec before the siphon tap (paired mean ± SEM = 99.5 ± 3.8%; n = 14; unpaired
mean ± SEM = 96.7 ± 3.6%; n = 11).
These results indicate that classical conditioning of the siphon
withdrawal reflex is not accompanied by significant changes in the
membrane properties of the LFS neurons.
Increase in the complex PSP in LFS motor neurons
during conditioning
To investigate changes in the synaptic input to the LFS motor
neurons during conditioning, we hyperpolarized the LFS neuron for a few
seconds to prevent it from spiking and recorded the complex PSP
produced by the siphon tap on each test (Fig.
5). With the LFS cell effectively removed
from the circuit, the average siphon withdrawal on the pretest was
reduced by ~30% compared with experiments in which the LFS cell was
not hyperpolarized, which agrees fairly well with previous estimates of
the contribution of a single LFS cell to the reflex response in this
preparation (Antonov et al., 1999a). Despite the functional removal of
the neuron from the circuit, behavioral conditioning was fairly normal, suggesting that firing of the remaining motor neurons undergoes plasticity similar to that of the one that was hyperpolarized.
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Figure 5.
Facilitation of the complex PSP in LFS motor
neurons during classical conditioning. A, Examples of
the complex PSP produced in an LFS motor neuron by the siphon tap on
the pretest and the final post-test after paired and unpaired training.
In these experiments, the motor neuron was hyperpolarized to
approximately  90 mV for a few seconds to prevent it from firing
during the tap. B, Average magnitudes of siphon
withdrawal and the complex PSP recorded in the same experiments. Paired
training produced a greater increase in the amplitude of siphon
withdrawal that was accompanied by a greater increase in the area of
the complex PSP in the first 1 sec after its onset. The average values
on the pretest were 1.8 mm (paired) and 1.7 mm (unpaired) for siphon
withdrawal, and 36,369 mVmsec (paired) and 34,516 mVmsec (unpaired) for
PSP area, not significantly different by t tests. The
average unconditioned responses to the first tail shock were 3.7 mm
(paired) and 4.1 mm (unpaired), not significantly different.
C, There was a significant correlation between the
increase in the amplitude of siphon withdrawal and the increase in the
area of the complex PSP.
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Because the complex PSP in an LFS neuron has a complicated shape with
multiple peaks (Figs. 5A, 6), we measured the total area
under the PSP in the first 1 sec after its onset. As shown in Figure
5B, the area of the PSP changed approximately in parallel with the change in siphon withdrawal, with paired training producing a
significantly greater increase in the PSP than unpaired training overall (F(1,23) = 20.79;
p < 0.001) and on each test (p < 0.01 in each case). Moreover, the increase in the area of the
complex PSP correlated significantly with the increase in siphon
withdrawal (r = 0.643; p < 0.001 on
the final post-test). These results suggest that changes in synaptic
input to the LFS motor neurons make an important contribution to
classical conditioning of the siphon withdrawal reflex.
To examine possible changes in the shape of the complex PSP, we
calculated the average area in each 50 msec interval after the onset of
the response to the tap on each test (Fig.
6). As illustrated in the examples in
Figure 5A and the average results in Figure 6, the complex PSP on the
pretest had four components similar to those described previously
(Antonov et al., 1999a), which correspond to the four components of
firing of the motor neuron: a peak at the beginning of the tap, a
smaller sustained depolarization during the tap, a second peak around
the offset of the tap, and a gradual decline after the tap. There was a
significant change in the shape of the complex PSP after paired but not
unpaired training (F(29,667) = 1.74;
p < 0.01 for the pairing × test × time
interaction), with increases in each time interval but a greater
increase around the end of the tap. This pairing-specific change in the
shape of the complex PSP could in principle contribute to conditioning,
although it did not result in a similar change in the pattern of firing
of the LFS motor neurons when they were not hyperpolarized (Fig.
3).
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Figure 6.
The average shape of the complex PSP in LFS motor
neurons on the pretest and the final post-test after paired and
unpaired training in the same experiments as in Figure 5. The PSP in
each 50 msec interval has been normalized to the total area on the
pretest in each experiment. The average values on the pretest were
44,018 mVmsec (paired) and 41,132 mVmsec (unpaired), not significantly
different.
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Increase in monosynaptic PSPs from on-field LE sensory neurons
during conditioning
The complex PSP in an LFS neuron includes monosynaptic PSPs from
identified LE siphon sensory neurons as well as unidentified sensory
neurons (Frost et al., 1997) and a polysynaptic input from excitatory
and inhibitory interneurons (Frost and Kandel, 1995). To examine
changes in the monosynaptic component from LE neurons during
conditioning, we used an intracellular depolarizing current pulse to
fire a single action potential in an LE neuron ~10 sec before the
siphon tap on each test and measured both the amplitude and area of the
unitary, monosynaptic PSP in the LFS neuron (Fig.
7). On average, the area of the
monosynaptic PSP on the pretest was 2.8 ± 0.3% of the area of
the complex PSP produced by siphon stimulation 10 sec later in the same
experiments (n = 21). When the siphon tap was within
the receptive field of the LE cell, it fired on average 4.6 spikes
during the tap (Fig. 2), and taps of this strength are thought to
activate approximately five LE cells (Byrne et al., 1974; Hickie et
al., 1997). These results suggest that if monosynaptic PSPs from the LE
cells added linearly, they would contribute ~64% of the area of the
complex PSP, which is in good agreement with the previous estimate by Antonov et al. (1999a) based on similar methods. This is probably an
overestimate, because the PSPs do not add linearly as they approach
their reversal potential, and they also tend to undergo homosynaptic
depression during a burst of spikes in a single sensory neuron (cf.
Murphy and Glanzman, 1996). However, these effects might be partially
offset by heterosynaptic facilitation caused by recruitment of
modulatory neurons during a siphon tap (Hawkins and Schacher, 1989;
Mackey et al., 1989), so that the linear estimate may serve as an
approximation.
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Figure 7.
Facilitation of the monosynaptic PSP from an LE
neuron to an LFS neuron during classical conditioning.
A, Examples of the monosynaptic PSP produced in an LFS
neuron by intracellular stimulation of an on-field
(A1) and off-field (A2)
LE neuron ~10 sec before the siphon tap on the pretest and the final
post-test after paired and unpaired training. B, Average
modulation of the monosynaptic PSPs during paired and unpaired
training. Paired training produced a greater increase than unpaired
training in the amplitude and area of PSPs from LE neurons that fired
during the siphon tap (on-field; B1). The
increase in PSPs from on-field LE neurons during paired training was
also significantly greater than the increase in PSPs from LE neurons
that did not fire during the siphon tap (off-field;
B2). The average values on the pretest were 10.8 mV and 820 mVmsec (paired, on-field), 13.0 mV and 1040 mVmsec
(unpaired, on-field), 10.6 mV and 633 mVmsec (paired, off-field), and
14.2 mV and 1031 mVmsec (unpaired, off-field), not significantly
different by ANOVAs.
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The LE neurons in these experiments were classified as either on-field
if the siphon tap was within the receptive field of the cell and caused
it to fire and participate in the reflex or off-field if the tap was
outside the receptive field of the cell and did not cause it to fire.
Both the amplitude and area of monosynaptic PSPs from on-field LE
neurons changed approximately in parallel with changes in the complex
PSP and evoked firing of the LFS neurons during conditioning, whereas
there were much smaller changes in PSPs from off-field LE neurons. For
the on-field LE neurons, paired training produced a significantly
greater increase in the monosynaptic PSPs than unpaired training
overall (amplitude, F(1,37) = 18.75; p < 0.001; area,
F(1,37) = 15.60; p < 0.001) and on the last three tests (p < 0.01 in
each case). Furthermore, the effect of pairing was significantly
different for on- and off-field LE neurons (amplitude, F(1,37) = 8.79; p < 0.01; area, F(1,37) = 3.78;
p < 0.05, one-tailed test for the pairing × receptive field interaction), with paired training producing
significantly greater increases in the PSPs from on- than off-field LE
neurons on the final post-test (p < 0.05).
These results are all consistent with the idea that conditioning involves changes in monosynaptic PSPs from LE sensory neurons to LFS
motor neurons, and that those changes in turn are attributable to
activity-dependent, associative synaptic plasticity, which occurs when
spikes in the sensory neuron are temporally paired with the US.
The increase in the area of monosynaptic PSPs from on-field LE neurons
correlated significantly with the increases in both siphon withdrawal
(r = 0.684; p < 0.001 on the final
post-test) and LFS firing (r = 0.777; p < 0.01) (Fig. 8). Furthermore, pairing did not have any significant additional effect on LFS firing when this
correlation was factored out in an analysis of covariance. By contrast,
the increase in the area of PSPs from off-field LE neurons did not
correlate significantly with the increases in either siphon withdrawal
(r = 0.016) or LFS firing (r = 0.203). Results on the amplitude of the PSPs were similar (data not shown). These results suggest that increases in monosynaptic PSPs from on-field
LE neurons make an important contribution to the increases in LFS
firing and siphon withdrawal during conditioning in this preparation.
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Figure 8.
Correlations between on-field monosynaptic PSPs
and siphon withdrawal or LFS firing during classical conditioning.
A, Examples of the monosynaptic PSP recorded ~10 sec
before siphon withdrawal and evoked firing of an LFS neuron on the
pretest and the final post-test in a single experiment.
B, There were significant correlations between the
increase in the area of on-field monosynaptic PSPs and the increases in
the amplitude of siphon withdrawal or number of evoked spikes in an LFS
motor neuron. SWR, Siphon withdrawal reflex.
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Increase in input resistance of on-field LE sensory neurons
during conditioning
Cellular analogs of conditioning produce pairing-specific changes
in the membrane properties of the LE sensory neurons (Hawkins et al.,
1983; Clark et al., 1994; Eliot et al., 1994). These changes are
thought to be attributable to a reduction in
K+ current, which is reflected in an
increase in input resistance of the neurons. We therefore investigated
changes in the input resistance of the LE neurons by measuring the
hyperpolarization produced by an intracellular current pulse ~10 sec
before the monosynaptic PSP on each test in some of the experiments
shown in Figure 7. As shown in the example in Figure
9A and the average results in
Figure 9B, the input resistance of the LE neurons changed approximately in parallel with changes in the monosynaptic PSPs during
conditioning. Paired training produced a significantly greater increase
in the input resistance of on-field LE neurons than unpaired training
on the final post-test (p < 0.01), whereas there was no significant difference for off-field LE neurons. Paired
training also produced a significantly greater increase in the input
resistance of on- than off-field LE neurons on the final post-test
(p < 0.05, one-tailed). Moreover, the increase in input resistance of on-field LE neurons correlated significantly with the increase in the area of the monosynaptic PSP
(r = 0.695; p < 0.05 on the final
post-test; Fig. 9C), and pairing did not have any
significant additional effect on the PSP when this correlation was
factored out in an analysis of covariance. These results suggest that
pairing-specific changes in the membrane properties of on-field LE
neurons make an important contribution to changes in the monosynaptic PSPs during conditioning. The increase in input resistance of on-field
LE neurons also correlated significantly with the increase in evoked
firing of the LE neurons (r = 0.622; p < 0.05), suggesting that changes in LE membrane properties contribute
to changes in evoked LE firing as well.
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|
Figure 9.
Increase in input resistance of LE sensory neurons
during classical conditioning. A, Examples of the input
resistance of an LE neuron measured with a hyperpolarizing
intracellular current pulse (ILE) ~10 sec
before recording the monosynaptic PSP on the pretest and the final
post-test in a single experiment. B, Average change in
input resistance in experiments like the one shown in A.
Paired training produced a greater increase than unpaired training in
the input resistance of on-field LE neurons. There were no significant
changes in the input resistance of off-field LE neurons.
C, There was a significant correlation between the
increase in the area of the monosynaptic PSP and the increase in the
input resistance of on-field LE neurons. Rm, Membrane
resistance.
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|
| |
DISCUSSION |
We have used the simplified siphon withdrawal preparation to
analyze cellular mechanisms contributing to classical conditioning. Behavioral conditioning in this preparation is similar to conditioning of siphon withdrawal in intact Aplysia (Carew et al., 1981)
and conditioning of gill withdrawal in another simplified preparation (Hawkins et al., 1998), both of which display additional higher-order features that are characteristic of conditioning in vertebrates (Carew
et al., 1983; Hawkins et al., 1986, 1989, 1998; Colwill et al.,
1988a,b; Walters, 1989). With the siphon withdrawal preparation, it is
relatively easy to examine behavioral conditioning while simultaneously
monitoring PSPs between identified sensory and motor neurons that
contribute to the behavior. Previous studies have estimated that
~55% of the siphon withdrawal reflex in the simplified preparation
is mediated through the CNS, that most of the central component of the
reflex is mediated by two or three LFS siphon motor neurons, and that
~60% of the synaptic input to the LFS neurons is provided by
monosynaptic PSPs from LE siphon sensory neurons (Antonov et al.,
1999a). Our results are in good agreement with those estimates. In
addition, we have found that during conditioning the monosynaptic
LE-LFS PSPs change approximately in parallel with complex PSPs in the
LFS motor neurons, evoked firing of the LFS neurons, and behavior (Fig.
10). Changes in the monosynaptic PSPs
also correlate significantly with changes in LFS firing and behavior.
These results provide perhaps the strongest evidence yet obtained in
any system that changes in monosynaptic PSPs contribute to associative
learning.
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Figure 10.
Summary of results of the
electrophysiological experiments. The siphon withdrawal reflex
(SWR) in the simplified preparation undergoes classical
conditioning that is accompanied by pairing-specific increases in
evoked firing of both LFS motor neurons and LE sensory neurons, the
complex PSP in LFS neurons, the monosynaptic PSP from on-field LE
neurons to LFS neurons, and the input resistance of on-field LE neurons
(**p < 0.01; *p < 0.05;
 p < 0.05, one-tailed compared with either
unpaired or off-field). Moreover, there were significant correlations
between most of these measures. These results provide direct evidence
that conditioning is attributable in part to activity-dependent
plasticity of monosynaptic sensory neuron-motor neuron PSPs.
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|
We also obtained evidence for two additional mechanisms, although their
contribution to conditioning appears to be smaller. First, there was an
increase and prolongation of evoked firing of the LE sensory neurons,
and this effect was greater after paired than unpaired training (Fig.
2). A prolongation of sensory neuron firing has previously been
described for sensitization of the tail withdrawal reflex (Walters et
al., 1983) but not for sensitization of the siphon withdrawal reflex
(Antonov et al., 1999a). However, the increase in evoked LE firing had
low correlations with the increases in LFS firing and siphon
withdrawal, and pairing still had significant effects when those
correlations were factored out in analyses of covariance. Second, there
was a change in the shape of the complex PSP after paired but not
unpaired training, with the largest increase in the PSP occurring
around the end of the tap (Fig. 6). A similar change occurs during
sensitization in the siphon withdrawal preparation (Antonov et al.,
1999a) and might be attributable to either plasticity in the
interneurons or changes in short-term plasticity (metaplasticity) of
monosynaptic PSPs from LE sensory neurons during the siphon tap
(Fischer et al., 1997; Phares and Byrne, 1999). However, this change in
the shape of the complex PSP did not result in a similar change in the
pattern of firing of the LFS motor neurons when they were not
hyperpolarized (Fig. 3).
In similar experiments on conditioning in a simplified gill withdrawal
preparation, Colebrook and Lukowiak (1988) and Lukowiak and Colebrook
(1988) found a dissociation between firing of the motor neuron and
behavior, which they attributed to a change in the peripheral
effectiveness of the gill motor neurons. We found no significant change
in the peripheral effectiveness of the LFS siphon motor neurons after
conditioning, although there is such a change shortly (2.5 min) after
sensitization in the simplified siphon withdrawal preparation (Antonov
et al., 1999a). Such peripheral effects may have been more important in
the gill withdrawal preparation of Colebrook and Lukowiak (1988) and
Lukowiak and Colebrook (1988), because part of the reflex in that
preparation is mediated through peripheral ganglia (the genital and
gill ganglia). By contrast, most of the peripheral component of the
siphon withdrawal reflex is mediated through direct synaptic
connections from siphon sensory neurons to peripheral siphon motor
neurons, which undergo many of the same types of plasticity as the
central synapses of the sensory neurons (Bailey et al., 1979; Clark and
Kandel, 1984).
Lukowiak (1986) also observed an increase in PSPs from off-field
sensory neurons during conditioning in a simplified gill withdrawal
preparation. The mechanism of this effect is not known, but it could
involve a pairing-specific enhancement of the effectiveness of the US
(Hawkins et al., 1983). There was a similar trend in our results (Fig.
7B), although it was not significant. However, in addition
to this modest increase in PSPs from off-field LE neurons, we observed
a significantly greater pairing-specific increase in monosynaptic PSPs
from on-field LE neurons (Fig. 7A). This difference between
on- and off-field LE neurons is consistent with either of two known
activity-dependent mechanisms of associative synaptic plasticity:
activity-dependent enhancement of presynaptic facilitation, which
occurs when presynaptic spike activity is temporally paired with a
facilitatory neurotransmitter (Hawkins et al., 1983, 1993; Walters and
Byrne, 1983), and Hebbian long-term potentiation, which occurs when
presynaptic spike activity is temporally paired with postsynaptic
depolarization (Kelso et al., 1986; Wigström et al., 1986). Both
of these types of plasticity occur at Aplysia sensory-motor
neuron synapses in isolated cell culture (Eliot et al., 1994; Lin and
Glanzman, 1994a,b; Bao et al., 1997, 1998; Schacher et al., 1997). They
could also both occur during behavioral conditioning of the siphon
withdrawal reflex, because the siphon tap causes spike activity in the
LE sensory neurons, and the tail shock causes both the release of facilitatory neurotransmitters (Hawkins and Schacher, 1989; Mackey et
al., 1989) and depolarization of the LFS motor neurons.
It will now be interesting to examine the roles of these two
activity-dependent mechanisms in conditioning of the siphon withdrawal reflex. Preliminary evidence suggests that they may both contribute. On
the one hand, postsynaptic manipulations that block Hebbian long-term
potentiation, such as BAPTA injection and NMDA receptor antagonists,
also reduce a cellular analog of conditioning in the isolated nervous
system (Murphy and Glanzman, 1996, 1997, 1999) and behavioral
conditioning in the simplified preparation (Antonov et al., 1999b).
However, those manipulations may also block activity-dependent
enhancement of presynaptic facilitation (Bao et al., 1998). On the
other hand, the changes in the monosynaptic PSPs during conditioning
are accompanied by and correlated with changes in the input resistance
of the LE sensory neurons (Fig. 9). The changes in input resistance are
clearly presynaptic and are consistent with activity-dependent
presynaptic facilitation, which is thought to involve a
pairing-specific decrease in K+ current,
leading to broadening of action potentials and enhanced transmitter
release from the sensory neurons (Hawkins et al., 1983, 1993). Such a
decrease in K+ current could also account
for the pairing-specific increase in evoked firing of the sensory
neurons (Fig. 2). Changes in input resistance and evoked firing of the
LE sensory neurons are not incompatible with Hebbian long-term
potentiation and might be expected if a postsynaptic site of induction
is accompanied by a presynaptic site of expression. Indeed, we have
recently found that the pairing-specific increases in evoked firing and
input resistance of the LE neurons are blocked by injection of BAPTA into the LFS neuron, consistent with the idea that conditioning involves a hybrid presynaptic and postsynaptic mechanism and retrograde signaling (Antonov et al., 2000). Regardless of the exact cellular mechanism, however, our present results provide the most direct evidence so far that plasticity of monosynaptic sensory neuron-motor neuron PSPs contributes to classical conditioning of the siphon withdrawal reflex and support the idea that synaptic plasticity is a
mechanism of learning and memory more generally.
| |
FOOTNOTES |
Received Feb. 28, 2001; revised May 30, 2001; accepted May 30, 2001.
This research was supported by National Institute of Mental Health
Grant MH26212. We thank J. Koester, I. Kupfermann, and M. Rogan for
their comments, H. Ayers and M. Pellan for typing this manuscript, and
C. Lam for help with the figures.
Correspondence should be addressed to Dr. Robert D. Hawkins, Center for
Neurobiology and Behavior, Columbia University College of
Physicians and Surgeons, 1051 Riverside Drive, New York, NY 10032. E-mail: rhawkins{at}pi.cpmc.columbia.edu.
| |
REFERENCES |
-
Antonov I,
Kandel ER,
Hawkins RD
(1999a)
The contribution of facilitation of monosynaptic PSPs to dishabituation and sensitization of the Aplysia siphon withdrawal reflex.
J Neurosci
19:10438-10450[Abstract/Free Full Text].
-
Antonov I,
Antonova I,
Hawkins RD
(1999b)
Activity-dependent facilitation of monosynaptic sensory neuron-motor neuron PSPs contributes to classical conditioning of the Aplysia siphon-withdrawal reflex in a simplified preparation.
Soc Neurosci Abstr
25:1129.
-
Antonov I,
Kandel ER,
Hawkins RD
(2000)
Contribution of pre- and postsynaptic mechanisms to activity-dependent facilitation during classical conditioning of the Aplysia siphon-withdrawal reflex.
Soc Neurosci Abstr
26:1523.
-
Bailey CH,
Castellucci VF,
Koester J,
Kandel ER
(1979)
Cellular studies of peripheral neurons in siphon skin of Aplysia californica.
J Neurophysiol
42:530-557[Abstract/Free Full Text].
-
Bao J-X,
Kandel ER,
Hawkins RD
(1997)
Involvement of pre- and postsynaptic mechanisms in posttetanic potentiation at Aplysia synapses.
Science
275:969-973[Abstract/Free Full Text].
-
Bao J-X,
Kandel ER,
Hawkins RD
(1998)
Involvement of presynaptic and postsynaptic mechanisms in a cellular analog of classical conditioning at Aplysia sensory-motor neuron synapses in isolated cell culture.
J Neurosci
18:458-466[Abstract/Free Full Text].
-
Byrne J
(1987)
Cellular analysis of associative learning.
Physiol Rev
6:329-439.
-
Byrne JH,
Castellucci V,
Kandel ER
(1974)
Receptive fields and response properties of mechanoreceptor neurons innervating skin and mantle shelf of Aplysia.
J Neurophysiol
37:1041-1064[Free Full Text].
-
Carew TJ,
Sahley CL
(1986)
Invertebrate learning and memory: from behavior to molecules.
Annu Rev Neurosci
9:435-487[Web of Science][Medline].
-
Carew TJ,
Walters ET,
Kandel ER
(1981)
Classical conditioning in a simple withdrawal reflex in Aplysia californica.
J Neurosci
1:1426-1437[Abstract].
-
Carew TJ,
Hawkins RD,
Kandel ER
(1983)
Differential classical conditioning of a defensive withdrawal reflex in Aplysia californica.
Science
219:397-400[Abstract/Free Full Text].
-
Carew TJ,
Hawkins RD,
Abrams TW,
Kandel ER
(1984)
A test of Hebb's postulate at identified synapses which mediate classical conditioning in Aplysia.
J Neurosci
4:1217-1224[Abstract].
-
Clark GA,
Kandel ER
(1984)
Branch-specific heterosynaptic facilitation in Aplysia siphon sensory cells.
Proc Natl Acad Sci USA
81:2577-2581[Abstract/Free Full Text].
-
Clark GA,
Hawkins RD,
Kandel ER
(1994)
Activity-dependent enhancement of presynaptic facilitation provides a cellular mechanism for the temporal specificity of classical conditioning in Aplysia.
Learn Mem
1:243-257[Abstract/Free Full Text].
-
Cohen TE,
Kaplan SW,
Kandel ER,
Hawkins RD
(1997)
A simplified preparation for relating cellular events to behavior: mechanisms contributing to habituation, dishabituation, and sensitization of the Aplysia gill-withdrawal reflex.
J Neurosci
17:2886-2899[Abstract/Free Full Text].
-
Colebrook E,
Lukowiak K
(1988)
Learning by the Aplysia model system: lack of correlation between gill and gill motor neuron responses.
J Exp Biol
135:422-429.
-
Colwill RM,
Absher RA,
Roberts ML
(1988a)
Context US learning in Aplysia californica.
J Neurosci
8:4434-4439[Abstract].
-
Colwill RM,
Absher RA,
Roberts ML
(1988b)
Conditional discrimination learning in Aplysia californica.
J Neurosci
8:4440-4444[Abstract].
-
Eliot LS,
Hawkins RD,
Kandel ER,
Schacher S
(1994)
Pairing-specific, activity-dependent presynaptic facilitation at Aplysia sensory-motor neuron synapses in isolated cell cultures.
J Neurosci
14:368-383[Abstract].
-
Fischer TM,
Blazie DE,
Priver NA,
Carew TJ
(1997)
Metaplasticity at identified inhibitory synapses in Aplysia.
Nature
389:860-865[Medline].
-
Frost L,
Kaplan SW,
Cohen TE,
Henzi V,
Kandel ER,
Hawkins RD
(1997)
A simplified preparation for relating cellular events to behavior: Contribution of LE and unidentified siphon sensory neurons to mediation and habituation of the Aplysia gill and siphon withdrawal reflex.
J Neurosci
17:2900-2913[Abstract/Free Full Text].
-
Frost WN,
Kandel ER
(1995)
Structure of the network mediating siphon-elicited siphon withdrawal in Aplysia.
J Neurophysiol
73:2413-2427[Abstract/Free Full Text].
-
Hawkins RD
(1997)
LTP and learning: let's stay together.
Behav Brain Sci
20:620-621.
-
Hawkins RD,
Schacher S
(1989)
Identified facilitator neurons L29 and L28 are excited by cutaneous stimuli used in dishabituation, sensitization, and classical conditioning of Aplysia.
J Neurosci
9:4236-4245[Abstract].
-
Hawkins RD,
Abrams TW,
Carew TJ,
Kandel ER
(1983)
A cellular mechanism of classical conditioning in Aplysia: activity-dependent amplification of presynaptic facilitation.
Science
219:400-415[Abstract/Free Full Text].
-
Hawkins RD,
Carew TJ,
Kandel ER
(1986)
Effects of interstimulus interval and contingency on classical conditioning of the Aplysia siphon withdrawal reflex.
J Neurosci
6:1695-1701[Abstract].
-
Hawkins RD,
Lalevic N,
Clark GA,
Kandel ER
(1989)
Classical conditioning of the Aplysia siphon-withdrawal reflex exhibits response specificity.
Proc Natl Acad Sci USA
86:7620-7624[Abstract/Free Full Text].
-
Hawkins RD,
Kandel ER,
Siegelbaum SA
(1993)
Learning to modulate transmitter release: themes and variations in synaptic plasticity.
Annu Rev Neurosci
16:625-665[Web of Science][Medline].
-
Hawkins RD,
Greene W,
Kandel ER
(1998)
Classical conditioning, differential conditioning, and second-order conditioning of the Aplysia gill-withdrawal reflex in a simplified mantle organ preparation.
Behav Neurosci
112:636-645[Web of Science][Medline].
-
Hickie C,
Cohen LB,
Balaban PM
(1997)
The synapse between LE sensory neurons and gill motoneurons makes only a small contribution to the Aplysia gill-withdrawal reflex.
Eur J Neurosci
9:627-636[Web of Science][Medline].
-
Kelso SR,
Ganong AH,
Brown TH
(1986)
Hebbian synapses in hippocampus.
Proc Natl Acad Sci USA
83:5326-5330[Abstract/Free Full Text].
-
Lin XY,
Glanzman DL
(1994a)
Long-term potentiation of Aplysia sensorimotor synapses in cell culture: regulation by postsynaptic voltage.
Proc R Soc Lond [Biol]
255:113-118[Medline].
-
Lin XY,
Glanzman DL
(1994b)
Hebbian induction of long-term potentiation of Aplysia sensorimotor synapses: partial requirement for activation of a NMDA-related receptor.
Proc R Soc Lond [Biol]
255:215-221[Medline].
-
Lin XY,
Glanzman DL
(1997)
Effect of interstimulus interval on pairing-induced LTP of Aplysia sensorimotor synapses in cell culture.
J Neurophysiol
77:667-674[Abstract/Free Full Text].
-
Lukowiak K
(1986)
In vitro classical conditioning of a gill withdrawal reflex in Aplysia: neural correlates and possible neural mechanisms.
J Neurobiol
17:83-101[Web of Science][Medline].
-
Lukowiak K,
Colebrook E
(1988)
Classical conditioning alters the efficacy of identified gill motor neurons in producing gill withdrawal movements in Aplysia.
J Exp Biol
140:273-285[Abstract/Free Full Text].
-
Mackey SL,
Kandel ER,
Hawkins RD
(1989)
Identified serotonergic neurons LCB1 and RCB1 in the cerebral ganglia of Aplysia produce presynaptic facilitation of siphon sensory neurons.
J Neurosci
9:4227-4235[Abstract].
-
Mayford MA,
Bach ME,
Huang Y-Y,
Wang L,
Hawkins RD,
Kandel ER
(1996)
Control of memory formation through regulated expression of a CaMKII transgene.
Science
274:1678-1683[Abstract/Free Full Text].
-
Murphy GG,
Glanzman DL
(1996)
Enhancement of sensorimotor connections by conditioning-related stimulation in Aplysia depends on postsynaptic Ca2+.
Proc Natl Acad Sci USA
93:9931-9936[Abstract/Free Full Text].
-
Murphy GG,
Glanzman DL
(1997)
Mediation of classical conditioning in Aplysia californica by long-term potentiation of sensorimotor synapses.
Science
278:467-471[Abstract/Free Full Text].
-
Murphy GG,
Glanzman DL
(1999)
Cellular analog of differential classical conditioning in Aplysia: disruption by the NMDA receptor antagonist DL-2-amino-5-phosphono-valerate.
J Neurosci
19:10595-10602[Abstract/Free Full Text].
-
Perlman AJ
(1979)
Central and peripheral control of siphon-withdrawal reflex in Aplysia californica.
J Neurophysiol
42:510-529[Abstract/Free Full Text].
-
Phares GA,
Byrne JH
(1999)
Differential modulation of Aplysia sensorimotor EPSPs during bursts of sensory neuron action potentials.
Soc Neurosci Abstr
25:1611.
-
Ramon y Cajal S
(1911)
In: Histologie du systeme nerveux de l'homme et des vertebres. Paris: Maloine.
-
Rogan MT,
Staubli UV,
LeDoux JE
(1997)
Fear conditioning induces associative long-term potentiation in the amygdala.
Nature
390:604-607[Medline].
-
Schacher S,
Wu F,
Sun Z-Y
(1997)
Pathway-specific synaptic plasticity: activity-dependent enhancement and suppression of long-term heterosynaptic facilitation at converging inputs on a single target.
J Neurosci
17:597-606[Abstract/Free Full Text].
-
Sherrington CS
(1906)
In: The integrative action of the nervous system. New Haven, CT: Yale UP.
-
Tsien JZ,
Huerta PT,
Tonegawa S
(1996)
The essential role of hippocampal CA1 NMDA receptor-dependent synaptic plasticity in spatial memory.
Cell
87:1327-1338[Web of Science][Medline].
-
Walters ET
(1989)
Transformation of siphon responses during conditioning of Aplysia suggests a model of primitive stimulus-response association.
Proc Natl Acad Sci USA
86:7616-7619[Abstract/Free Full Text].
-
Walters ET,
Byrne JH
(1983)
Associative conditioning of single sensory neurons suggests a cellular mechanism for learning.
Science
219:405-408[Abstract/Free Full Text].
-
Walters ET,
Byrne JH,
Carew TJ,
Kandel ER
(1983)
Mechanoafferent neurons innervating tail of Aplysia. II. Modulation by sensitizing stimuli.
J Neurophysiol
50:1543-1559[Abstract/Free Full Text].
-
Wigström H,
Gustafsson B,
Huang Y-Y,
Abraham WC
(1986)
Hippocampal long-lasting potentiation is induced by pairing single afferent volley with intracellularly injected depolarizing current pulses.
Acta Physiol Scand
126:317-319[Web of Science][Medline].
-
Zamanillo D,
Sprengel R,
Hvalby O,
Jensen V,
Burnashev N,
Rozov A,
Kaiser KM,
Koster HJ,
Borchardt T,
Worley P,
Lubke J,
Frotscher M,
Kelly PH,
Sommer B,
Andersen P,
Seeburg PH,
Sakmann B
(1999)
Importance of AMPA receptors for hippocampal synaptic plasticity but not for spatial learning.
Science
284:1805-1811[Abstract/Free Full Text].
Copyright © 2001 Society for Neuroscience 0270-6474/01/21166413-10$05.00/0
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[Full Text]
[PDF]
|
|
|
|
|
|
|
B. D. Burrell and C. L. Sahley
Serotonin Mediates Learning-Induced Potentiation of Excitability
J Neurophysiol,
December 1, 2005;
94(6):
4002 - 4010.
[Abstract]
[Full Text]
[PDF]
|
|
|
|
|
|
|
Q. Li, A. C. Roberts, and D. L. Glanzman
Synaptic Facilitation and Behavioral Dishabituation in Aplysia: Dependence on Release of Ca2+ from Postsynaptic Intracellular Stores, Postsynaptic Exocytosis, and Modulation of Postsynaptic AMPA Receptor Efficacy
J. Neurosci.,
June 8, 2005;
25(23):
5623 - 5637.
[Abstract]
[Full Text]
[PDF]
|
|
|
|
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J. Xu, N. Kang, L. Jiang, M. Nedergaard, and J. Kang
Activity-Dependent Long-Term Potentiation of Intrinsic Excitability in Hippocampal CA1 Pyramidal Neurons
J. Neurosci.,
February 16, 2005;
25(7):
1750 - 1760.
[Abstract]
[Full Text]
[PDF]
|
|
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|
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V. A. Straub, B. J. Styles, J. S. Ireland, M. O'Shea, and P. R. Benjamin
Central localization of plasticity involved in appetitive conditioning in Lymnaea
Learn. Mem.,
November 1, 2004;
11(6):
787 - 793.
[Abstract]
[Full Text]
[PDF]
|
|
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|
|
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R. Mozzachiodi, H. A. Lechner, D. A. Baxter, and J. H. Byrne
In Vitro Analog of Classical Conditioning of Feeding Behavior in Aplysia
Learn. Mem.,
November 1, 2003;
10(6):
478 - 494.
[Abstract]
[Full Text]
[PDF]
|
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G. A. Phares, E. G. Antzoulatos, D. A. Baxter, and J. H. Byrne
Burst-Induced Synaptic Depression and Its Modulation Contribute to Information Transfer at Aplysia Sensorimotor Synapses: Empirical and Computational Analyses
J. Neurosci.,
September 10, 2003;
23(23):
8392 - 8401.
[Abstract]
[Full Text]
[PDF]
|
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D. Barbas, L. DesGroseillers, V. F. Castellucci, T. J. Carew, and S. Marinesco
Multiple Serotonergic Mechanisms Contributing to Sensitization in Aplysia: Evidence of Diverse Serotonin Receptor Subtypes
Learn. Mem.,
September 1, 2003;
10(5):
373 - 386.
[Abstract]
[Full Text]
[PDF]
|
|
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R. Nargeot
Voltage-Dependent Switching of Sensorimotor Integration by a Lobster Central Pattern Generator
J. Neurosci.,
June 15, 2003;
23(12):
4803 - 4808.
[Abstract]
[Full Text]
[PDF]
|
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|
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C. T. Tamse, Y. Xu, H. Song, L. Nie, and E. N. Yamoah
Protein Kinase A Mediates Voltage-Dependent Facilitation of Ca2+ Current in Presynaptic Hair Cells in Hermissenda crassicornis
J Neurophysiol,
March 1, 2003;
89(3):
1718 - 1726.
[Abstract]
[Full Text]
[PDF]
|
|
|
|
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|
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G. E. Spencer, M. H. Kazmi, N. I. Syed, and K. Lukowiak
Changes in the Activity of a CPG Neuron After the Reinforcement of an Operantly Conditioned Behavior in Lymnaea
J Neurophysiol,
October 1, 2002;
88(4):
1915 - 1923.
[Abstract]
[Full Text]
[PDF]
|
|
|
|
|
|
|
J. R. Wolpaw
Memory in neuroscience: rhetoric versus reality.
Behav Cogn Neurosci Rev,
June 1, 2002;
1(2):
130 - 163.
[Abstract]
[PDF]
|
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TOP
ABSTRACT
INTRODUCTION
