Canadian Association of Neurosciences Review: Learning at a Snail's Pace

ArticleinThe Canadian journal of neurological sciences. Le journal canadien des sciences neurologiques 33(4):347-56 · December 2006with58 Reads
Impact Factor: 1.53 · DOI: 10.1017/S0317167100005291 · Source: PubMed
Abstract

While learning and memory are related, they are distinct processes each with different forms of expression and underlying molecular mechanisms. An invertebrate model system, Lymnaea stagnalis, is used to study memory formation of a non-declarative memory. We have done so because: (1) We have discovered the neural circuit that mediates an interesting and tractable behaviour; (2) This behaviour can be operantly conditioned and intermediate-term and long-term memory can be demonstrated; and (3) It is possible to demonstrate that a single neuron in the model system is a necessary site of memory formation. This article reviews how Lymnaea has been used in the study of behavioural and molecular mechanisms underlying consolidation, reconsolidation, extinction and forgetting.

Full-text

Available from: Ken Lukowiak, Dec 26, 2013
CLASSIFICATIONS OF LEARNING AND MEMORY
The ability to learn and form a memory enables or
ganisms to
adapt to a changing environment so as to be better able to
survive. Learning and memory are behavioural manifestations of
activity within individual neurons and neuronal circuits.
While
learning and memory are interrelated, they are separate processes
each with dif
ferent underlying molecular mechanisms and forms
of expression. Learning can be broadly defined as the acquisition
of a new behaviour, while memory is defined as the ability to
both store and recall the new information.
Learning and memory (or their correlates) have been studied
at behavioural, systems, neuronal, and sub-cellular levels in
organisms ranging from humans to worms. In this short review
we will focus on research in an invertebrate model system that
has provided insight into the underlying mechanisms of memory
formation.
While an understanding of the causal mechanisms of
learning and memory formation in snails is of heuristic interest,
our main reason for employing such a molluscan model system
is to gain insight as to how learning and memory occur in us. It
appears that the ‘substrates’
for learning and memory have been
fairly conserved in all or
ganisms throughout evolution.
ABSTRACT: While learning and memory are related, they are distinct processes each with different
forms of expression and underlying molecular mechanisms. An invertebrate model system,
Lymnaea
stagnalis
, is used to study memory formation of a non-declarative memory. We have done so because:
1) We have discovered the neural circuit that mediates an interesting and tractable behaviour; 2) This
behaviour can be operantly conditioned and intermediate-term and long-term memory can be
demonstrated; and 3) It is possible to demonstrate that a single neuron in the model system is a necessary
site of memory formation.
This article reviews how
L
ymnaea
has been used in the study of behavioural
and molecular mechanisms underlying consolidation, reconsolidation, extinction and forgetting.
RÉSUMÉ: L’apprentissage au pas d‘escargot. Bien que l’apprentissage et la mémoire soient deux fonctions
connexes, leurs processus sont distincts et chacun a des formes d’expression différentes et des mécanismes
moléculaires sous-jacents différents. Nous utilisons un système dans un modèle invertébré, la Lymnaea stagnalis,
pour étudier comment se forme une mémoire non déclarative. Nous avons utilisé ce modèle parce que : 1) Nous
avons découvert un circuit neural qui assure la médiation d’un comportement intéressant et observable; 2) Ce
comportement peut être conditionné en cours d’étude et la mémoire à moyen et à long terme peut être démontrée;
3) Il est possible de démontrer dans ce modèle qu’un seul neurone est nécessaire pour la formation de la mémoire.
Cet article revoit comment la Lymnaea a été utilisée pour étudier les mécanismes comportementaux et moléculaires
sous-jacents à la consolidation, à la reconsolidation, à l’extinction et à l’oubli.
Can. J. Neurol. Sci. 2006; 33: 347-356
THE CANADIAN JOURNAL OF NEUROLOGICAL SCIENCES 347
CANADIAN ASSOCIATION OF
NEUROSCIENCES REVIEW: Learning at a
Snail’s Pace
Kashif Parvez, David Rosenegger, Michael Orr, Kara Martens, Ken Lukowiak
From the
Hotchkiss Brain Institute, University of Calgary
, Calgary
,
AB, Canada.
R
ECEIVED MAY 11, 2006. ACCEPTED IN FINAL FORM AUGUST 18, 2006.
Reprint r
equests to:
Ken Lukowiak, Hotchkiss Brain Institute, University of Calgary
,
3330 Hospital Drive N.W
., Calgary
,
Alberta, T2N 4N1, Canada.
R
EVIEW ARTICLE
Moreover
, there are many experimental advantages (see below)
to using invertebrate model systems over mammalian
preparations.
Thus, gaining an understanding of causal
mechanisms of memory formation in a snail may help us better
understand how memory systems function in higher organisms
such as in humans. Since this paper will focus primarily on how
memory is formed following learning we first need to describe
how memory is classified.
Memory encompasses a broad spectrum of sub-types. Studies
on humans have examined both declarative and non-declarative
forms of memory. Declarative (or explicit) memory is the
memory of facts and events, while non-declarative (implicit or
procedural) memory is the memory of ‘how’
to do things (e.g.
3050
Page 1
motor memory). However, there are a number of complexities
associated with declarative memory that have made it more
difficult to study. For example, declarative memories are initially
formed in different neuronal circuits in the brain than the ones
where they are ultimately stored. The distributed nature of
declarative memory makes it difficult to determine if an
unexpressed memory has been forgotten (i.e. no longer stored) or
is not accessible at a specific time (i.e. a failure in retrieval).
Moreover, whether a memory is no longer stored (i.e. forgotten)
or just not able to be accessed at a specific time is problematic.
These problems may be avoided by studying non-declarative
memory. Non-declarative memories are stored in the circuit that
mediates the behavior.
1-3
Thus, we know ‘where’ the memory is
stored if we can determine which neurons are responsible for
generating the behavior. Since the memory is stored within the
circuit that mediates the behaviour, we eliminate inaccessibility
as an explanation for lack of memory expression.
Probably, the most basic and fundamental form of learning is
non-associative learning. Habituation and sensitization are two
well-known examples of non-associative learning. In the case of
habituation the animal learns and remembers not to respond to a
‘benign’ (i.e. harmless) stimulus that has been repeatedly
presented to it. The analysis of how such learning and memory
are encoded in neurons was greatly aided by the development of
a set of parametric characteristics of habituation that all animals
exhibited
4
and the development of model systems such as
Aplysia. Model organisms showed that habituation was similar
in
intact animals and in reduced semi-intact preparations, which
allowed simultaneous study of behaviour and neurophysiology.
5-7
Sensitization, on the other hand, can de defined as an increase in
response amplitude following the presentation of another distinct
non-contingent stimulus. Again, our understanding of how
sensitization is mediated at the neuronal level came from the
development of model systems where behaviour and neural
activity could be studied simultaneously.
8
Finally, with both
habituation and sensitization it was shown that long-lasting
memory was dependent on new protein synthesis and altered
gene activity.
5,9
A more complicated and important form of learning is
associative learning.
Associative learning can itself be divided
into two types; classical conditioning and operant
conditioning.
3,10,1
1
In classical conditioning (i.e. Pavlovian
conditioning) a ‘neutral’ stimulus, known as the conditional
stimulus (CS) is temporally paired with a stimulus, known as the
unconditional stimulus (US), that inevitably elicits a response
(the unconditional response; UR). With repeated CS-US
pairings, but not US-CS pairings (backwards conditioning), the
CS comes to elicit the UR. That is, the animal learns that the CS
predicts the occurrence of the US.
The most famous example of
this form of learning is that of Pavlov’s dogs who learned, after
a number of CS-US pairings, that the presentation of a tone,
signaled the presentation of a food-substance. Thus, the dogs
salivated (the UR) on hearing the tone (CS) even if the US (the
food-substance) was not presented.
In contrast to classical conditioning, operant conditioning
involves application of a reinforcement stimulus contingent upon
spontaneous performance of a specific behaviour.
12-14
With
repeated contingent presentation of the reinforcing stimulus the
occurrence of the behaviour changes.
10
The reinforcing stimulus
can result in either increasing the occurrence of the behaviour
(positive reinforcement) or decrease its occurrence (negative
reinforcement). In short, operant conditioning results in an
animal learning the consequence of its behaviour.
LYMNAEA STAGNALIS AS A MODEL SYSTEM
A vast variety of model systems ranging from invertebrates
(e.g.
Lymnaea,
1
5,16
C. elegans,
1
7
Aplysia,
1
8-20
Apis,
2
1
Drosophila,
22
Hermissenda
23
) to non-human primates are
available to researchers for the study of memory formation. Each
model system comes with benefits and drawbacks. For example,
one benefit of using a rodent model system is that they have all
the neural structures hypothesized to play key roles in human
declarative memory formation and storage (e.g. the
hippocampus, amygdala and cortex). The equivalent brain
structures in rodents are also necessary for memory formation
and recall. However, in rodents as in humans, declarative
memory is stored ‘somewhere’
(i.e. in multiple areas) in the
cortex. Attempts have been made to directly study how and
where the various components of the memory (e.g. visual,
olfactory, auditory, emotional, etc) are stored and accessed.
There has been progress in localizing auditory fear conditioning
(i.e. classical conditioning) to the lateral amygdala.
24,25
Another
form of associative learning, conditioned taste aversion (CTA),
has been localized to the insular and prepiriform cortices and the
amygdala.
2
6-28
However, as of yet the necessary molecular
changes in neurons that cause memory formation in the
aforementioned structures have not been elucidated.
With the above caveats in mind we have taken to using an
invertebrate model system to study memory formation of a non-
declarative memory. We have done so because: 1) We have
discovered the neural circuit that mediates an interesting,
tractable behaviour; 2) This behaviour can be operantly
conditioned and long-lasting memory can be demonstrated; and
3) It is possible to demonstrate that a single neuron in the model
system is a necessary site of memory formation.
THE CANADIAN JOURNAL OF NEUROLOGICAL SCIENCES
348
Figure 1: L
ymnaea with its pneumostome open, performing aerial
r
espiration. Adult Lymnaea measure approximately 25 mm in shell
length.
Page 2
Lymnaea stagnalis, a freshwater pond snail, is an ideal model
system for the study of learning and memory. There are many
advantages to studying learning and memory in this mollusc.
Lymnaea are bimodal breathers, meaning they can exchange O
2
and CO
2
cutaneously (through their skin) or aerially through an
o
rifice (i.e. pneumostome; Jones, 1961; Lukowiak et al, 1996).
To aerially respire, the snail approaches the water surface, opens
its pneumostome and begins gas exchange. The snail must come
to the air-water interface for aerial respiration and it is easy to
observe this behaviour.
29
Another advantage to Lymnaea is that it has a simple central
nervous system consisting of several thousand neurons. Some of
the unusually large neurons have been characterized and are
easily identified. Most importantly, a three neuron central pattern
generator (CPG) has been thoroughly characterized and proven
both necessary and sufficient for controlling aerial respiratory
behaviour in
Lymnaea.
30,31
CENTRAL PATTERN GENERATOR
The aerial respiratory CPG in Lymnaea consists of three
identified interneurons. Right Pedal Dorsal 1 (RPeD1) is a
dopaminergic interneuron that initiates the respiratory rhythm.
Right Pedal Dorsal 1 receives chemosensory inputs from the
pneumostome area
32
to modulate its activity. The remaining two
interneurons in the CPG are Visceral Dorsal 4 (VD4) and the
Input 3 Interneuron (IP3I). Visceral Dorsal 4 initiates
pneumostome closure by excitation of VK closer motor neurons.
Input 3 Interneuron initiates pneumostome opening by excitation
of VI/J opener motor neurons. All synaptic connections between
the 3 interneurons are monosynaptic. Right Pedal Dorsal 1
excites IP3I via a biphasic (inhibitory then excitatory) synaptic
connection. Input 3 Interneuron and VD4 have mutually
inhibitory synaptic connections as do RPeD1 and VD4. The
activities of VD4 and IP3I alternate to generate the rhythmic
opening and closing of the pneumostome (see Lukowiak
3
3
for a
description of how the emergent network properties cause
rhythmic activity in this circuit). The sufficiency and necessity of
the 3-neuron CPG in producing rhythmogenesis was tested by
reconstructing the CPG in vitro and performing cell killing and
transplantation experiments.
31
Thus Lymnaea was demonstrated to possess an easily
observable behaviour (aerial respiration) that is driven by a
‘known’ neural network; it only remained to be shown that the
behaviour could exhibit both associative learning and long-term
memory (LTM). This was accomplished by our laboratory in
1996
1
5
when it was shown that aerial respiratory behaviour could
be operantly conditioned and that L
TM resulted following
conditioning.
OPERANT CONDITIONING OF AERIAL RESPIRATORY BEHAVIOUR
IN LYMNAEA
Operant conditioning of aerial respiratory behaviour in
Lymnaea is performed by placing snails into a beaker filled with
hypoxic pond-water (PW). The PW is made hypoxic by bubbling
N
2
gas through it for 20 minutes. Hypoxia is used to increase the
snails’ drive to perform aerial respiration. In hypoxic conditions,
snails crawl to the air-water interface to open their pneumostome
and perform gas exchange with the external environment. When
they open their pneumostome we apply a negative reinforcement
in the form of a gentle tactile stimulus. This reinforcing stimulus
is of sufficient strength to cause pneumostome closure but does
not elicit the whole-animal withdrawal response. With repeated
application of the negative reinforcement, snails reduce the
number of attempted pneumostome openings when placed in the
hypoxic context.
16
We record the number of attempted pneumostome openings
during training sessions (TS1 and TS2) and a ‘savings-test’
(memory test; MT). We defined learning and memory
operationally
.
15,34,35
Learning is present if the number of
attempted pneumostome openings in the last training session is
significantly less than the number of attempted pneumostome
openings in the first training session. In order for memory to be
present when ‘savings’ is tested (memory test, MT), two criteria
must be met: 1) the number of attempted pneumostome openings
in the MT is not significantly greater than that of the last training
session and 2) the number of attempted pneumostome openings
in the MT is significantly less than that of the first training
session.
While the ‘savings-test’ is a suitable method to test for
memory, we also use a second memory assessment method (a
‘probe-test’). In the ‘probe-test’ method, we compare total
breathing time before and after training. Thus, memory is
assessed in the absence of the presentation of reinforcing stimuli.
There are both advantages and disadvantages to these two
memory assessment methods. The disadvantage of the ‘savings-
test method’ is that reinforcing stimuli are applied during the MT.
LE JOURNAL CANADIEN DES SCIENCES NEUROLOGIQUES
Volume 33, No. 4 – November 2006 349
Figure 2: A
schematic diagram illustrating the connectivity of the thr
ee
interneur
on CPG that contr
ols aerial r
espiration in L
ymnaea. Included
in the diagram is the neur
on that r
egulates the whole-body withdrawal
r
eflex (i.e. RPeD1
1) and the neur
ons r
esponsible for the opening (i.e. VK
motor neur
ons) and closing (i.e. VI/J motor neur
ons) of the
pneumostome.
Page 3
Additional learning may occur, creating the possibility that
reduction in aerial respiration is due to this additional
reinforcement and not the memory phenotype. The advantage of
the ‘savings-test’ method is that, following the MT, another
different context-MT (see below) can be given showing that the
change in behaviour is a bona fide reflection of memory. The
advantage of the ‘probe-test’ method is reinforcing stimuli are
not applied during the MT, but there are two disadvantages. The
first is that the pre-training observation session (OBS1) may
trigger latent inhibition and thus affect memory formation. The
second disadvantage is the post-training observation session
(OBS2) is an extinction session. If another observation session in
a new context is given after the post-training observation session,
we cannot be sure if naïve respiratory levels are due to context
specificity or occlusion by extinction. In our model system, both
the ‘savings-test’ method and ‘probe-test’ method yield the same
conclusions and are valid methods for the assessment of
memory. Shown below is a diagram illustrating different training
procedures and procedures used to assess forgetting. Figure 3A
and 3B show the ‘massed’
and ‘spaced’ training procedures used
to produce ITM and LTM, respectively, as measured by a
savings-test method. Figure 3C and 3D show the ‘massed’ and
‘spaced’ training procedures used to produce ITM and LTM,
respectively, as measured by the probe-test method. Forgetting
can be assessed by giving snails memory tests at various time
points after training has ceased.
In addition to using two memory assessment methods to
ensure the validity of our experimental findings, we use an
important control, the yoked control procedure. A yoked control
snail is given a non-contingent stimulus (i.e. a stimulus when the
pneumostome is not open) every time a snail from the
experimental cohort receives a contingent stimulus (i.e. a
stimulus when pneumostome opening is initiated). As a result,
the experimental snail associatively learns and forms memory,
but the yoked control snail cannot as there is no association
present.
15
The yoked control procedure serves to control for
phenomena that may alter behaviour or internal state of the snail
such as: 1) stress associated with stimulus application, 2)
handling and 3) the training environment.
In addition to the yoked control, a ‘change of context’
procedure is used to ensure the behaviour of snails is not altered
by experimental conditions (i.e. stress, drug exposure, handling
or the training environment). When trained snails are placed in a
novel context, they respond as if they were naïve.
That is, they
show more respiratory activity in the novel context than in the
trained context. Typically we train snails in hypoxic pond-water
that lacks the presence of an odorant (i.e. the standard context).
To create a ‘novel context’, N
2
is first bubbled through a flask
containing, for example, a slurry of carrot and water
.
The
resulting carrot-odorant N
2
is bubbled into the training beaker
prior to training or testing. For complete details see Haney and
Lukowiak
36
MEMORY CONSOLIDATION
As already mentioned, learning and memory, while related,
are two separate and distinct processes.
1
Behaviourally learning
is the acquisition of a new or altered behaviour
, while memory
refers to the retention of what is learned. There are also
fundamental molecular differences between learning and
memory and these will be discussed.
The process that leads to the
formation of memory following learning has come to be known
as consolidation.
The concept of consolidation stems from
clinical observations made by Ribot in 1882. He noted that after
traumatic brain injury, recent memories were more likely to be
forgotten than memories of earlier events.
37
The hypothesis
postulated that after learning, memory existed in a fragile state
THE CANADIAN JOURNAL OF NEUROLOGICAL SCIENCES
350
Figure 3: A diagram illustrating training procedures used to produce
memory in Lymnaea and to measure the forgetting of memory. (A) To
produce ITM, snails are given a ‘massed’ training procedure that
consists of two 0.5h training sessions separated by a 0.5h rest interval
and then memory is tested for at 3h (3h MT) using the ‘savings-test’
method. If snails are given a MT 24h after training (24h MT), memory is
no longer present and hence forgotten. (B) To produce LTM, snails are
given a ‘spaced’ training procedure that consists of two 0.5h training
sessions separated by a 1h rest interval and then memory is tested for at
24h (24h MT) using the ‘savings-test’ method. The procedure to assess
forgetting is performed by giving snails MTs at 24h, 48h and 72h (24h
MT, 48h MT and 72h MT, respectively). (C) An alternative method to the
‘savings-test’ method is to use the ‘probe-test’ method. A breathing
observation (OBS1) is performed to measure mean total breathing time
of a cohort of naïve snails. Twenty four hours later, the snails are given
‘massed’ training (i.e. ITM training procedure; two 0.5h training
sessions separated by a 0.5h r
est inter
val). Memor
y is tested at 3h by
performing another br
eathing obser
vation (3h OBS2) to measur
e mean
total br
eathing times. If memor
y is tested using the ‘pr
obe-test’
method at
24h after training (24h OBS2), memor
y is no longer pr
esent and hence
for
gotten. (D) Snails ar
e tr
eated as in C except they ar
e given ‘spaced’
training (i.e. L
TM training pr
ocedur
e; two 0.5h training sessions
separated by a 1h r
est inter
val) and memor
y is tested at 24h. The
pr
ocedur
e to assess for
getting is performed by giving snails br
eathing
obser
vations at 24h, 48h and 72h (24h OBS2, 48h OBS2 and 72h OBS2,
r
espectively).
Page 4
and was vulnerable to interference. With time the memory was
stabilized and became insusceptible to interference by amnesiac
treatments.
38
It was observed that the shorter the interval
between the training and the amnesiac treatment, the more
prominent the retrograde amnesia. Therefore, if enough time had
p
assed after training, the amnesiac treatment was no longer able
to disrupt memory. That is, the memory had become fixed and
stable and, hence ‘consolidated’.
39,40
We noted above that memories could be classified as
declarative or non-declarative. In addition memories may also be
distinguished by their temporal characteristics as follows: 1)
short-term memory (STM; lasting seconds to minutes), 2)
intermediate-term memory (ITM; lasting up to 3 hours); and 3)
long-term memory (LTM; lasting greater than 6 hours).
3,41,42
We
found that we could in Lymnaea differentially produce ITM and
LTM by specific training procedures. We can broadly classify the
two procedures as ‘massed’ (ITM-training) or ‘spaced’ (LTM-
training) training. For example, if we subject snails to two 30-
min training sessions separated by a 0.5h interval only ITM is
formed. However
, if we increase the interval between the
training sessions to 1h, LTM (persisting at least 1 day) forms.
Thus, depending on which training procedure we choose to use
we can get ITM or LTM.
1
6
Short-term memory does not require the synthesis of new
protein rather it results from the transient modification of already
existing proteins (e.g. phosphorylation of proteins).
Intermediate-term memory requires de novo protein synthesis
from pre-existing mRNA but ITM can also be protein synthesis
independent as shown in
Aplysia.
43,44
Long-term memory
requires both de novo protein synthesis and altered gene activity
(for review see Milner et al, 1998). We first determined if
memory in our
Lymnaea model system conformed to these rules.
To show that ITM and LTM require protein synthesis, snails are
subjected to procedures that inhibit or reduce their rate of protein
synthesis. For example, if snails are injected with the protein
synthesis inhibitor anisomycin, before training neither ITM nor
LTM results. As a control, snails injected with saline at the same
time continue to exhibit memory. If, however, snails are injected
prior to training with the transcription inhibitor, actinomycin D,
ITM forms but LTM formation is blocked. A further attribute of
our model system is that we are able to subject snails, without
harming them, to cooling to 4
o
C. Cooling has been used to
interfere with memory consolidation by other several labs.
45,46
Cooling interferes with both protein synthesis and gene
transcription. Thus, if snails are subjected to 1h of cooling
immediately after training, neither ITM nor LTM result. If
however, the same duration of cooling (1h) is delayed by 1h after
training memory formation is not interfered with.
47
One of the reasons we have used the Lymnaea model for
studying memory formation is that we have a very through
knowledge of the underlying neuronal circuitry. Thus, we should
be able to show that molecular events in CPG neurons are
necessary for memory formation. To do this we chose to utilize
an advantage of molluscs, namely the ability to surgically
remove the soma of an identified neuron while leaving behind a
functional primary neurite. Since the synaptic specializations
(pre and post) of invertebrate neurons all reside on the primary
neurite, removal of the soma does not interfere with synaptic
interactions. Moreover, the primary neurite is still capable of de
novo protein synthesis and is capable of surviving for weeks
after surgery.
2,48
What is missing, after this procedure is
performed, are the genes. Thus, we can directly ask whether
altered gene activity in the soma of a particular neuron is need
for LTM formation. We made a decision to first examine the role
played in memory formation by the RPeD1, the neuron that
initiates CPG rhythmogenesis. Another reason for choosing
RPeD1 is that we had already shown specific changes in neural
activity could be correlated with changes in behaviour following
memory formation.
4
9
We therefore removed either the soma of
RPeD1 or, as a control, the soma of a similar sized neuron
(LPeD1) that is not a member of the respiratory CPG from naïve
snails. We found that both groups (i.e. RPeD1 and LPeD1 soma
ablated snails) demonstrated learning and ITM. However, the
RPeD1-soma ablated snails did not exhibit LTM. That is,
removal of only the somata of a single neuron prevented LTM
memory formation without altering the ability of the snail to
learn or form ITM.
2
LPeD1 soma ablated animals were still
capable of forming and exhibiting LTM.
A
question that has arisen is whether LTM formation first
requires ITM formation (i.e. LTM formation occurs serially) or
can it occur without ITM formation (i.e. a parallel process). If
ITM and LTM are the result of parallel molecular processes one
could block the ITM producing process and not interfere with the
LTM molecular process. This strategy has been attempted by a
number of different laboratories.
50-56
For example in a one-trial
step down inhibitory avoidance task studied in rats, 11 different
treatments blocked what we have termed ITM but not LTM and
18 treatments either blocked or enhanced LTM alone and left
ITM unaffected.
50
However, if ITM and LTM are the result of parallel processes,
then an ITM training procedure no matter how many times
presented should only produce ITM and never result in the
formation of LTM. Studies in our lab and others have shed doubt
on the notion that ITM and LTM formation occur in parallel by
demonstrating that training procedures that would not normally
result in LTM (i.e. ITM-training) can in fact be made to produce
LTM.
57-62
If an ITM training protocol is given to snails, the
memory produced persists for 3h but not 24h (i.e. ITM). If the
parallel hypothesis was correct, then a subsequent ITM training
procedure the following day could only result only in ITM since
the molecular pathways underlying ITM and LTM formation
occur in parallel. However, we found that a subsequent bout of
ITM training a day after the first bout of ITM training produces
LTM. This suggests that the first ITM training protocol produced
a residual molecular memory trace that persists longer than the
behaviourally observable memory
. Activation of the residual
molecular memory trace with a second bout of ITM training was
able to boost this trace into L
TM consistent with the hypothesis
that ITM and LTM are serial processes.
58,63
Another demonstration of how the processes that lead to LTM
occur in series was demonstrated in
Drosophila. In Drosophila,
behavioural experiments have shown that olfactory conditioning
(i.e. a form of classical conditioning) can produce an associative
LTM. Training is performed by repeated pairings of an odor and
electrical shock and a different odor and no electrical shock. To
test for conditioned odor avoidance responses (i.e. memory) fruit
flies are placed in a
T-maze and allowed to distribute themselves
into the arms of the maze containing the two odors. Memory is
LE JOURNAL CANADIEN DES SCIENCES NEUROLOGIQUES
Volume 33, No. 4 – November 2006 351
Page 5
observed if fruit flies avoid the odor that was paired to electrical
shock. LTM is produced by multiple ‘spaced’ training sessions
separated by rest intervals, a training paradigm that we also use.
In contrast, ‘massed’ training is unable to produce LTM. To show
that they could boost memory, they created transgenic flies that
carried a heat shock inducible activator CREB2 isoform. Three
hours after heat shock, massed training produced LTM in the
transgenic flies. This demonstrated that activating CREB2 in
transgenic flies permitted LTM formation by a procedure that
would not normally produce LTM.
6
4
These findings suggest that
activation of the activator isoform of CREB2 is part of a serial
pathway that allows LTM formation to occur.
While the role of the activator isoform of CREB2 was
explored in LTM formation, the repressor isoform of CREB2
was also studied in another invertebrate model system. In
Aplysia, a repressor isoform of CREB was identified and
characterized and termed ApCREB2. The repressor isoform of
ApCREB2 is targeted for phosphorylation by protein kinases
such as PKA, PKC, MAPK and CAMK. Normally, a single pulse
of 5-HT
produces short-term facilitation that lasts minutes. But
when anti-ApCREB2 was injected into the nucleus of sensory
neurons to ‘inactivate’ the repressor isoform of
ApCREB2, a
single 5-HT pulse produced long-term facilitation.
6
5
This long-
term facilitation required protein synthesis and altered gene
activity. These experiments showed that induction of CREB2 de-
repression could permit the formation of long-term facilitation
by a procedure that would normally produce short-term
facilitation. These findings suggest that the de-repression of the
repressor isoform of CREB2 is part of a serial pathway that
allows LTM formation to occur.
While the previous two studies manipulated CREB activity,
another study activated an upstream kinase, PKA, to produce
LTM. In the honeybee, the associative olfactory conditioning
(i.e. classical conditioning) of the proboscis extension response
results in the formation of LTM that was associated with
persistent PKA activation. A single pairing of the odor stimulus
(carnation; CS) followed by presentation of sucrose solution
(US) does not result in LTM formation. However, multiple
pairings produce a LTM that was associated with persistent PKA
activation. If photo-release of caged cAMP
in the antennal lobes
is coupled with a single conditioning trial, LTM is formed.
66
These findings suggest that activation of the PKA pathway is
also a part of the serial pathway that allows LTM formation to
occur
.
All the above mentioned results in
Lymnaea, Drosophila,
Aplysia
and the honeybee are consistent with the hypothesis that
L
TM formation involves processes that occur in series.
Memor
y Reconsolidation
While the idea of memory consolidation has existed for a long
time, what happens to the memory afterwards has only been
under recent investigation. Upon activation, a stable and
consolidated memory becomes active and labile.
Reconsolidation, first observed by Misanin et al,
67
is the process
by which an activated memory undergoes another process of
stabilization to return it to an inactive state.
While undergoing
reconsolidation, the memory can be updated and/or changed to
incorporate new information.
68,69
More recent studies have
examined the mechanisms underlying reconsolidation.
46,70-76
There exist some similarities between consolidation and
reconsolidation. CREB activation has been shown to be a
requirement in both consolidation
7
7-82
and reconsolidation [83].
As well, both processes have been shown to require new protein
synthesis.
69,71,72,74,83,84
Finally, NMDA receptors have been
shown to be involved in consolidation,
7
7,85,86
and in
reconsolidation.
72,73,87
While it appears that some molecular
cascades are involved in both consolidation and reconsolidation,
some differences have been identified. BDNF has been shown to
be required for consolidation of contextual fear memory but not
its reconsolidation. In the same study, Zif268 was shown to be
involved in the reconsolidation of contextual fear memory but
not its consolidation.
88
C/EBP (CAAT/enhancer-binding protein)
is required for consolidation.
7
6,89
However C/EBP does not seem
to be needed for reconsolidation.
76
This suggests that
consolidation and reconsolidation may trigger similar initial
molecular cascades, resulting in the activation of CREB.
Downstream from CREB however, the various molecular
cascades and proteins involved may be different.
T
o demonstrate reconsolidation in Lymnaea, we activate a
previously formed memory by placing the snails in the same
context-training beaker, and then attempt to block newly labile
memory using a variety of experimental methods. Application of
protein synthesis inhibitors, RNA transcription blockers or
ablation of the soma of RPeD1 after activation of the memory
perturbs the reconsolidation process. In all of these scenarios,
long-term memory was no longer present or abolished. However,
if the memory is not activated before the application of these
interventions, LTM is still present.
74
Thus, perturbations of
protein synthesis and RNA transcription inhibit both
consolidation and reconsolidation suggesting that similar
molecular mechanisms underlie both processes. Interestingly,
when snails were over-trained to produce a well-rehearsed
memory, the memory eventually becomes independent of protein
synthesis and transcription. This suggests that, somehow,
repeated reconsolidation transforms the memory into a very
stable state to the point that it can no longer be modified.
90
EXTINCTION
Extinction typically occurs when reinforcing stimuli are no
longer applied.
91,92
Extinction research is of great interest as it
potentially can be used as a therapeutic tool to treat substance
addiction and fear disorders. A question that has arisen is
whether extinction is unlearning or is it new learning and
memory formation that occludes the original memory
.
93-98
W
e
have attempted to directly determine whether extinction is
unlearning.
T
o cause extinction, snails that possess LTM are
placed in the training context and are allowed to perform
pneumostome opening without any reinforcement. That is, they
learn that it is ‘acceptable’ to open their pneumostomes. If
extinction occurs snails will perform as if they are naïve snails.
This is what we have found.
96,99
However, this still does not
address the question as to whether extinction is unlearning. The
original studies on extinction performed by Pavlov
92
demonstrated the phenomenon, which he termed ‘spontaneous
recovery’.
That is, following extinction training, the original
memory was not seen. However
, with time the original memory
could be evoked again by presentation of the CS, suggesting that
extinction training is not unlearning of the original learning. We
have also seen spontaneous recovery in our studies.
99
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One theory of extinction postulates that extinction involves
the weakening of pre-existing connections. Support for this
theory of extinction assumed that the original conditioning was
erased.
9
1
However, a defining characteristic of extinction is that
spontaneous recovery can be observed.
9
2
If extinction training-
s
essions are given after LTM has formed, the extinction memory
occludes the original memory. That is, extinction is not
unlearning but a new memory that acts in opposition of the
original LTM. However, the extinction memory itself can be
forgotten causing the original memory to return (i.e. spontaneous
recovery).
ME
CHANISMS OF
EX
TINCTION
There is strong support to demonstrate that extinction is an
active process that involves new learning and, therefore, new
protein synthesis, to produce a memory that occludes the original
conditioning.
9
3-95
Extinction is an active process that involves
new learning to produce a memory that occludes the original
conditioning. Extinction has been observed in many model
systems.
1
7,96-100
Using a number of the experimental attributes of
our model system outlined above we set out to rigorously test the
hypothesis that extinction is new learning and memory that
occludes the older memory. We were able to show that the
process of extinction requires the somata of RPeD1 to be present
(just as LTM formation requires RPeD1’s somata).
99
While we
have identified a neuron necessary for extinction (RPeD1), no
one particular brain structure has been identified in mammalian
systems to be responsible for extinction.
9
7
Thus highlighting
another advantage to the
Lymnaea model system.
FORGETTING
Of all the topics that we deal with in our laboratory the one
that we are most often questioned about by non-neuroscientists
is forgetting. The most frequently asked question is, “How can I
stop forgetting things and improve my memory?” We often
answer, much to the chagrin of the questioner, that the biggest
problem with forgetting is ‘not forgetting’ but rather the inability
to forget! We first need to define what we mean by forgetting.
The Oxford English Dictionary defines the verb for
get as “To
lose remembrance of; to cease to retain in one's memory” or “T
o
fail to recall to mind; not to recollect.” Because we are dealing
with non-declarative memory that is stored within the neural
circuit that mediates the behaviour (i.e. the CPG
network)
2,49,101,102
we define for
getting as the obliteration of the
memory and therefore the associated learned behaviour.
While forgetting is correlated with time, it is not caused by
the passage of time.
103
There are several theories on forgetting
with the two most prominent being: 1) Failure to retrieve; and 2)
Retroactive interference. The retrieval failure hypothesis states
that the memory cannot be accessed and hence it is dubbed
‘forgotten’. Certainly, we have all experienced this form of
forgetting. While a failure to retrieve a memory might explain
how forgetting of a declarative memory may occur, retrieval
failure cannot explain for
getting of a non-declarative memory
.
We study a non-declarative memory that is formed and forgotten
both in the same circuit.
2
Since we know the circuit, we can test
for retrieval failure.
T
o test this, we can produce two different
context specific memories in the snail (i.e. standard and carrot).
If we use a training procedure (two 45 min training sessions on
two successive days) in the standard context to produce a LTM
that persists for seven days and a training procedure in the carrot
context to produce a LTM that persists for only one day (two 30
min sessions on one day) we can then directly show that memory
is not due to an inability to retrieve the memory. If the memory
in the carrot context was forgotten due to retrieval failure, then
placing the animals in the standard context should not manifest
the original long-lasting memory. If the animal still has the LTM
for the standard context, then we know that the memory is
accessible and forgetting is not due to retrieval failure. We
observe the latter finding that demonstrates forgetting is not due
to retrieval failure.
The interference theory states that related events that occur
after LTM formation cause forgetting. When they are trained,
snails make the association that pneumostome opening results in
the delivery of a tactile stimulus. When snails are not being
trained, they are free to perform aerially respiration
ad libitum.
Animals learn anew that pneumostome opening will not result in
the delivery of a tactile stimulus. Interfering events can be
prevented by submer
ging animals after training to prevent
spontaneous opening of their pneumostome. Thus, animals will
not learn the new association and therefore retain the original
association. We found that preventing interfering events
extended the persistence of LTM (i.e. delayed forgetting).
35
The
molecular mechanisms of forgetting are still unknown. We have
hypothesized that forgetting is an active process. That is,
forgetting requires altered gene activity and new protein
synthesis in order to obliterate the memory. Thus, cooling snails
after the completion of consolidation can extend the persistence
of memory (i.e. delay forgetting). Cooling reduces the snails’
metabolic rate and subsequently reduce the amount of protein
synthesis that occurs.
47,58
Most importantly, we can directly
demonstrate that forgetting resembles learning something new
and remembering it since if we remove the soma of RPeD1 after
the LTM consolidation process, the snails are unable to forget
(they are also unable to form new LTM if operantly conditioned
in a new context) indicating that forgetting requires access to the
genome.
104
FUTURE DIRECTIONS
It has been demonstrated that stress and emotions can be
strong modulators of memory formation; however
, experimental
results have been varied and often contradictory.
105
For example,
rats that were exposed to uncontrollable restraint and tail-shock
stress demonstrated normal learning during water maze training,
but animals showed impaired memory compared to unstressed
controls when tested the next day
.
106
However
, memories for
very traumatic or highly emotional events are often extremely
long-lasting.
107,108
Given the complexity of animal behaviour and
the many diverse ways different stressors could act on learning
and memory formation, the disagreement in the literature is not
surprising. Preliminary evidence has shown that a single, highly
aversive stimulus (submersion in KCl), contingent with
pneumostome opening, is sufficient to alter the breathing
behaviour of
Lymnaea.
109
Yoked controls, change of context
controls, and handling controls indicate that this change is a bona
fide example of single-trial operant conditioning.
Stressors such as predators, inter- and intra-specific
competition for food, habitat and procreational resources can
LE JOURNAL CANADIEN DES SCIENCES NEUROLOGIQUES
Volume 33, No. 4 – November 2006 353
Page 7
impart a high strain on the energy allocation of an organism.
Learning about predators is expected to have adaptive payoffs in
any species that can alter their behaviour during times of
predation risk.
1
10
Lymnaea respond to crayfish predators by
using predator-avoidance behaviours such as the full body
withdrawal response when under attack.
1
11
It is possible that
snails which can detect the presence of a predator, in a
presumably stressful situation in which they respond with
defensive behaviors, will show a change in their ability to learn
and remember when given an operant training paradigm in the
presence of that predator. Preliminary experiments in our
laboratory suggest when
Lymnaea are operantly conditioned in
the presence of a crayfish predator memory formation is
enhanced.
1
12
How this crayfish exposure affects the neural
substrates responsible for this behavior and to what extent this
occurs is currently under investigation. In future work, we hope
to investigate the alteration of memory formation by stress.
Another avenue of research we would like to pursue is to
explore anatomical studies associated with memory formation.
W
e have as yet to determine what changes in synapse profile and
cytoarchitecture occur throughout neurons that are involved in
memory formation (i.e. the 3 interneurons of the CPG mediating
aerial respiratory behaviour). However, we have undertaken a
functional proteomics approach to identify changes in protein
expression associated with LTM formation using the entire CNS
of the snail. Preliminary analyses have indicated that LTM
formation results in a reduction in some specific membrane
proteins and an increase in the expression of many specific
cytosolic proteins. Once these proteins have been identified, we
can begin to construct a possible picture of what changes occur
in the Central nervous system of
Lymnaea to form LTM.
ACKNOWLEDGEMENTS
This work was supported by a grant from CIHR to KL and the
lab. KP is supported by a scholarship from the Alberta Heritage
Foundation for Medical Research (AHFMR) and the
Neuroscience Canada Foundation. KM is supported by a
studentship from NSERC.
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