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First published online May 19, 2008
Journal of Experimental Biology 211, 1747-1756 (2008)
Published by The Company of Biologists 2008
doi: 10.1242/jeb.014886
Determining friend vs foe through sensory cues |
The perception of stress alters adaptive behaviours in Lymnaea stagnalis
Hotchkiss Brain Institute, University of Calgary, 3330 Hospital Drive NW, Calgary, Alberta, Canada, T2N 4N1
* Author for correspondence (e-mail: lukowiak{at}ucalgary.ca)
Accepted 12 February 2008
Summary
Stress can alter adaptive behaviours, and as well either enhance or diminish learning, memory formation and/or memory recall. We show here that two different stressors have the ability to alter such behaviours in our model system, Lymnaea stagnalis. One, a naturally occurring stressor – the scent of a predator (crayfish) – and the other an artificially controlled one – 25 mmoll–1 KCl – significantly alter adaptive behaviours. Both the KCl stressor and predator detection enhance long-term memory (LTM) formation; additionally predator detection alters vigilance behaviours. The predator-induced changes in behaviour are also accompanied by specific and significant alterations in the electrophysiological properties of RPeD1 – a key neuron in mediating both vigilance behaviours and memory formation. Naive lab-bred snails exposed to crayfish effluent (CE; i.e. the scent of the predator) prior to recording from RPeD1 demonstrated both a significantly reduced spontaneous firing rate and fewer bouts of bursting activity compared with non-exposed snails. Importantly, in the CE experiments we used laboratory-reared snails that have not been exposed to a naturally occurring predator for over 250 generations. These data open a new avenue of research, which may allow a direct investigation from the behavioral to the neuronal level as to how relevant stressful stimuli alter adaptive behaviours, including memory formation and recall.
Key words: Lymnaea, instinct, aerial respiration, long-term memory, crayfish predator, vigilance behaviours
Introduction
The perception of stress by sensory systems modulates adaptive behaviours
including memory formation and/or its recall. This has been noted in the
scientific literature since the time of Bacon
(Bacon, 1620
) but it is
probably best summarized in the so-called Yerkes–Dotson `law'
(Yerkes and Dotson, 1908;
Shors, 2004). This `law' is
depicted graphically in Fig. 1.
As can be seen stress and/or attention is an important element in determining
both whether and how `good' information becomes stored as long-term memory
(LTM). Since every organism internalizes and retains details of its
surroundings to increase its chance of survival, it is not surprising that
memory is demonstrated in all animals. However, it is impractical to encode
all events into memory. Thus, organisms should only expend the `neuronal cost'
(e.g. altered gene activity and new protein synthesis) necessary to form LTM
to `relevant' events. One very important factor that helps determine whether a
specific occurrence will be encoded into memory is the level of stress of the
organism at the time of the event. Because memory is dynamic, stress and
traumatic events have substantial modulatory effects on memory, including
false memory and post-traumatic stress syndrome
(Kim and Diamond, 2002;
Lukowiak et al., 2007;
Yehuda and LeDoux, 2007).
These effects have been studied in a number of different model organisms, with
sometimes contradictory results (Shors,
2004). That is, in some instances memory is enhanced whilst in
others its formation or its recall is blocked. Given the complexities of the
vertebrate brain and animal behaviour, and the diverse ways stressors act on
memory formation, disagreement in the literature is not surprising.
|
; Lukowiak et al.,
2003b; Brembs,
2003; Parvez et al.,
2006b) and the majority of these snails have been derived from
snails originally collected (in the 1950s) from canals in a polder in Utrecht
Provence in the Netherlands. Subsequently these were maintained at Vrije
Universeit in Amsterdam and then distributed to various labs throughout the
world. However, it is important to remember that in the wild, Lymnaea
respond to a variety of predator-released kairomones (a chemical secreted and
released by an organism that, when detected by an organism of another species,
evokes a response that adaptively favours the latter) and, depending on the
specific predator detected, utilize a functionally appropriate response (e.g.
sheltering under crevices for fish or crawling above the waterline for
crayfish (Dalesman et al.,
2006; Jacobsen and Stabell,
2004; Covich et al.,
1994; Rundle and Bronmark,
2001). Thus, snails follow the age-old axiom that the most
effective way to avoid becoming prey is to avoid the predator. Many examples
of species-typical behaviours labelled `vigilance' or `risk assessment'
following predator detection have been demonstrated
(Apfelbach et al., 2005). Prey
species assess the risk of predation and adjust their behavioural and/or
anatomical phenotype appropriately
(Hoverman and Relyea, 2007;
Kats and Dill, 1998;
Turner et al., 2000;
Orr et al., 2007). Here we
tested whether lab-reared Lymnaea have maintained the ability to both
detect and respond appropriately (i.e. make the correct decision) to the scent
of a crayfish predator, even though they have not experienced a crayfish
predator for over 250 generations. Since the vast majority of previous reports
(e.g. Chivers et al., 1996;
Hazlett et al., 2002;
Langerhans and Dewitt, 2002;
Rochette et al., 1998)
concerning how gastropods (including Lymnaea) respond to predator
detection (via kairomones interacting with sensory neurons) utilized
wild-caught animals it makes it difficult to determine whether
predator-induced defence responses were innate or the result of prior
experience (Dalesman et al.,
2006). However, because we were using lab-bred snails we were able
to surmount this problem.
In this review we will primarily concentrate on one specific behaviour in
Lymnaea when we look at memory formation and stress; aerial
respiratory behaviour. Lymnaea is a bimodal breather that satisfies
its respiratory needs either cutaneously, through the skin, or aerially
through the pneumostome, the respiratory orifice
(Lukowiak et al., 1996).
Aerial respiration, which involves opening the pneumostome at the water's
surface to allow atmospheric gas exchange, is driven by a three interneuron
central pattern generator (CPG) whose sufficiency and necessity has been
directly demonstrated (Syed et al.,
1990; Syed et al.,
1992). In hypoxic conditions the frequency of aerial respiration
increases and can be modified by operant conditioning
(Lukowiak et al., 1996;
Lukowiak et al., 1998;
Lukowiak et al., 2003a;
Lukowiak et al., 2003b;
Parvez et al., 2006b).
Briefly, a tactile stimulus is applied to the pneumostome area each time the
snail begins to open it for gas exchange. Depending on the training procedure
used, intermediate-term memory (ITM; persists for up to 3 h and depends on
de novo protein synthesis) or LTM (persists for more than 6 h and
depends on both altered gene activity and de novo protein synthesis)
can be formed (Lukowiak et al.,
2000; Scheibenstock et al.,
2002; Sangha et al.,
2003a; Sangha et al.,
2003b). In fact, molecular changes in one of the three CPG
neurons, RPeD1, have been shown to be absolutely necessary for LTM formation,
extinction, memory reconsolidation and forgetting
(Scheibenstock et al., 2002;
Sangha et al., 2003c;
Sangha et al., 2003d;
Sangha et al., 2005;
McComb et al., 2005a;
Lattal et al., 2006). We will
thus have the opportunity to determine how stress in Lymnaea alters
LTM formation and vigilance behaviours at the single neuron level.
To evoke a standardized, repeatable, acute stress response in
Lymnaea, the snails were exposed to a noxious stimulus, 25 mmol
l–1 KCl [i.e. the KCl bath; see Martens et al.
(Martens et al., 2007a;
Martens et al., 2007b) for
full details]. The other stressor that will be discussed is exposure to the
`smell' of a crayfish predator. To obtain this `smell', which we call crayfish
effluent (CE), the crayfish were maintained in aquaria and we used the water
taken from the aquarium to train the snails in. Thus, snails did not come into
direct contact with the predator but only came into contact with water taken
from the aquarium where the crayfish were maintained (i.e. the CE).
KCl as a stressor
A single 30 min training session results in a memory that persists for
approximately 3 h (i.e. ITM) but not for 24 h
(Lukowiak et al., 1998;
Parvez et al., 2005;
Parvez et al., 2006a;
Parvez et al., 2006b).
However, we found (Martens et al.,
2007a) that if we stressed the snails with a KCl bath either
immediately before or just after (see
Martens et al., 2007a) the 30
min training session, LTM resulted (Fig.
2). That is, memory now persisted for at least 24 h. Thus, it
appears that the perception of a noxious, stressful stimulus (i.e. the KCl
bath) that elicits the whole-body withdrawal response
(Inoue et al., 1996) is
sufficient to cause a training procedure (i.e. a single 30·min training
session) that normally does not result in LTM formation to now result in LTM.
However, a number of controls had to be performed before we could conclude
that the KCl stressor enhanced LTM formation.
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We also found that the perception of the stressful stimulus itself did not lead to LTM formation (Fig. 3, bottom). That is, placing snails in the KCl bath just prior to a yoked control training session (non-contingent application of the stimuli) did not result in LTM formation.
The perception of a stressful situation immediately before an attempt to
recall memory can be detrimental to recall (see
Shors, 2004). To determine
whether the stressor used above (i.e. the KCl bath) would be detrimental to
memory recall, snails were trained with a protocol previously shown to result
in LTM (Lukowiak et al., 1998;
Taylor and Lukowiak, 2000;
Sangha et al., 2003a); two 30
min tactile training sessions, between which the snails were returned to the
home aquarium for 1 h. Twenty-four hours later these snails were given the KCl
bath, then placed in the hypoxic training beaker and tested for LTM. We found
that LTM was present (data not shown) (see
Martens et al., 2007a). Thus,
in Lymnaea this stressor did not block memory recall.
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)]. Our lab has previously demonstrated that ITM training,
which leaves no evidence of memory at 24 h, results in a `memory substrate'
that an additional ITM training session can build upon, resulting in LTM 24 h
later (Parvez et al., 2005;
Parvez et al., 2006a;
Parvez et al., 2006b). Here,
it appears that the application of a KCl bath before the MT salvages the
`residual memory substrate' to allow LTM formation.
We had previously demonstrated
(Scheibenstock et al., 2002)
that the soma (i.e. where the genes are) of RPeD1 had to be present for LTM
formation when a training procedure (two 30 min training sessions separated by
a 1 h interval) was used. We wished to determine whether the same requirement
for RPeD1's soma was necessary for the memory enhancement brought about by the
KCl stressor. These data are shown in Fig.
5. As can be seen in those snails where RPeD1's soma had been
ablated 2 days previously, the KCl stressor did not cause LTM formation. In
the control groups ablation of another neuron's soma, VD1, which is
approximately the same size as RPeD1 but which is not involved in driving
aerial respiratory behaviour, did not block KCl's ability to enhance LTM
formation nor did the sham-operated control. Thus, for the KCl stressor to
enhance LTM formation the soma of RPeD1 must be intact.
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Is too much stress a bad thing, as regards memory enhancement, as predicted by the Yerkes–Dodson 1908 `law'? We tested this question in two ways. The first way was to increase the concentration of the KCl bath; but when we used 100 mmoll–1 KCl the snails just got unresponsive (i.e. sick). So, instead, we subjected snails to the KCl bath immediately before and immediately after the 30 min training session. A single application of this stressor either before or after the single 0.5 h training session enhanced LTM formation; however, when applied both before and after training, LTM formation was blocked (Fig. 6) showing that too much stress prevented LTM formation. As a control we substituted a PW bath for one of the KCl baths and LTM was still present. Thus, too much stress blocked LTM formation.
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Crayfish effluent (CE) as a stressor
We hypothesized that our lab-reared Lymnaea would still respond to the scent of this predator (crayfish are sympatric predators of Lymnaea in The Netherlands where our snails originally came from) and that detection of this predator would significantly alter defensive vigilance behaviours. Therefore, we asked the question: when placed in a vulnerable position (i.e. with the ventral part of the foot exposed and away from the substrate), do snails alter their righting behaviour in CE? We found that snails exposed to CE significantly decreased their righting time compared with snails in pond water (PW) or boiled CE (BC) control groups. That is, when snails were placed inverted upon their shells, they took a significantly shorter time to flip over and regain their foothold on the substrate when in CE compared with PW or BC (Fig. 7).
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). We hypothesized
that this defensive behaviour would be altered by exposure of Lymnaea
to CE. Snails were placed in 500 ml of hypoxic CE or PW and allowed to
acclimate, after which the shadow treatment began
(Fig. 8, bottom). We found that
snails in CE elicited a full pneumostome withdrawal more often when presented
with a passing shadow than did snails in PW. Thus, CE exposure (i.e. detection
of a predator) enhanced this defensive withdrawal response.
Collectively these data demonstrate that lab-reared Lymnaea are
capable of detecting the presence of a crayfish predator (i.e. CE is detected)
and responding in an appropriate manner. Predator detection significantly
altered defensive vigilance behaviours (for details, see
Orr et al., 2007).
We next determined whether aerial respiratory behaviour was altered in
Lymnaea exposed to CE (Fig.
9). Previous reports indicate that when pulmonate snails are in
the presence of a crayfish predator they tend to spend more time near the
surface of the water (Dalesman et al.,
2006; Turner et al.,
2000; Turner and Montgomery,
2003). Presumably this is another defensive behaviour as crayfish
are `bottom' dwellers and if the snail tends to stay at the surface it would
be less likely to be preyed on. We further hypothesized that if
Lymnaea spent more time at the air–water interface as a result
of CE detection, they may show a significant alteration in aerial respiratory
behaviour. We therefore measured the number of pneumostome openings and the
total breathing time in PW, CE and BC. These data are plotted in
Fig. 9. The number of
pneumostome openings (Fig. 9A)
and the total breathing time (Fig.
9B) were significantly increased in CE (P<0.01,
N=65) compared with controls. Interestingly, the mean breathing time
for each pneumostome opening was not significantly different as a result of CE
exposure (Fig. 9C,
P=0.144, N=65). We conclude that these laboratory-reared
snails are capable of detecting CE (which in the wild would signal that a
crayfish predator is somewhere in the area), migrate to the air–water
interface and increase the number of times they open their pneumostome to
breathe, and thereby increase the total breathing time when exposed to CE.
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|
As can be easily seen above, predator detection (i.e. placing snails in CE)
alters a range of behaviours that possibly lessen the probability of being
preyed on. We next wished to determine whether this predator detection would
be reflected in a change in electrophysiological activity of a key neuron that
is involved in the mediation of aerial respiratory behaviour. It has
previously been found in Aplysia that there is often a lack of
correlation between the activity of a neuron that is involved with a
peripheral structure (e.g. the gill) and the behaviour of that organ
(Colebrook and Lukowiak, 1988).
Thus, we examined the activity of RPeD1 in semi-intact preparations that had
previously been treated with PW, CE or BC
(Fig. 10). We chose to examine
RPeD1 because this neuron initiates the rhythmic activity that drives aerial
respiratory behaviour, it receives sensory input from the pneumostome area and
it is inhibited during the defensive full-body withdrawal response.
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We next asked what would happen to the snails' ability to form LTM
following exposure to CE. We first examined what effect, if any, training in
CE would have on memory formation when snails were subjected to a single 0.5 h
training session. A naive cohort of snails was given this training in PW and
then tested 24 h later (Fig.
11). As already demonstrated
(Lukowiak et al., 2000;
Taylor and Lukowiak, 2000;
Lukowiak et al., 2003a;
Lukowiak et al., 2003b;
Rosenegger et al., 2004;
Parvez et al., 2006a), these
snails did not exhibit memory when tested 24 h later. We then asked whether
similar training of snails in CE would result in augmented memory and, if so,
how long would the memory persist? To answer this question, a new cohort of
snails was given training in CE water and tested for memory (in PW) 24, 48 and
72 h later. Yoked controls were also performed in CE and tested in PW. To our
amazement snails exposed to CE during training exhibited LTM when tested 24
and 48 h later but not at 72 h (Fig.
11). That is, the mean number of attempted pneumostome openings
was significantly decreased 24 and 48 h after training but not at 72 h. In
yoked control experiments in CE there was no statistical difference between
training and memory test sessions. Thus, we conclude that a single 0.5 h
training session in CE is sufficient to cause LTM formation that persists for
at least 48 h.
Knowing that exposure to CE alone produces an effect on RPeD1 that lasts 2
h (Fig. 10) but not 24 h
(Orr and Lukowiak, 2008), we
next asked whether the electrophysiological profile of RPeD1 in snails
subjected to the 0.5 h training session in CE was also altered. We therefore
examined the properties of RPeD1 48 and 72 h after training in CE. As in the
behavioural experiments we also examined a yoked control group at 48 h. We
found that snails trained in CE demonstrated significantly reduced spontaneous
firing frequency (measured by the number of spikes per 600 s), spikes per
burst and burst duration in the 48 h operantly conditioned group, but not in
the 48 h yoked or the 72 h operant groups
(Fig. 12, bottom three traces
and C, right three bars). From these behavioural and electrophysiological
data, we conclude that exposure to CE during the 0.5 h training session
enhances LTM formation. These data also suggest that for predator-induced
enhancement of LTM, the electrophysiological changes in RPeD1 associated with
these memories parallel the duration of the behavioural phenotype.
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Because memory is a dynamic process, it is modifiable
(Lukowiak et al., 2007). We
first showed that the perception of a noxious stressor (the KCl bath) that
elicits the whole-body withdrawal response enhances LTM formation. Here we
have only focused our attention on the effects of the noxious KCl stressor but
it needs to be emphasized that Martens et al.
(Martens et al., 2007b) also
show that similar results could be obtained using a more behaviourally
relevant stimulus, namely garlic. That the perception of garlic would alter
adaptive behaviour of a snail is a phenomenon that certainly deserves more
thorough study!
KCl's enhancing effect on LTM formation appears to be mediated via sensory pathways, and not via a direct result of a physical action on the CNS. By that we mean that an increase in [K+]o in the snails haemolymph as a result of K+ diffusing across the skin of the snail while it sits in the KCl bath as the cause of LTM enhancement seems highly unlikely. If this was indeed the case then we should have seen an enhancement of memory in the yoked control experiment or when we challenged snails with a change of context test, which we did not. Moreover, we did not see an enhancement of memory when we used 25 mmoll–1 NaCl as a possible stressor. Thus, an as yet unidentified sensory pathway was stimulated by the KCl, and activation of this pathway resulted in the whole-body withdrawal response and enhancement of LTM formation.
Although the literature describing the effect of stress (both physical and emotional) on memory is extensive, there are few examples of stress modulating adaptive behaviours in a relatively simple invertebrate model such as ours. Furthermore, this is the first example that we are aware of in an invertebrate model system where stress has caused a significant enhancement in LTM memory formation. Moreover, since LTM of the behaviour studied here has been shown to require the molecular processes in RPeD1; we are in an excellent position to begin a direct investigation into the specific alterations in this neuron that are correlated with, if not cause, this phenomenon.
As we have stated previously, even in the laboratory, let alone in its
natural environment, the probability of a snail encountering a KCl bath in its
everyday existence is pretty low. However, in its natural environment the
snail has a reasonably high probability of either sensing or coming into
direct contact with a predator, such as a crayfish. Thus, we were motivated to
determine whether lab-reared snails would still have the ability to both
recognize and respond appropriately to the presence of the predator or the
scent of the predator. As we have earlier reported
(Orr et al., 2007) and have
shown here, laboratory-reared Lymnaea (some 250 generations since the
early 1950s without contact with crayfish) have maintained their capacity to
detect prey via a kairomone, as evidenced by significant changes in
their defensive behaviours and changes in electrophysiological parameters in a
key neuron, RPeD1. We have shown that Lymnaea significantly increase
aerial respiratory activity (which occurs when snails are at the
air–water interface) when exposed to the effluent of Procambarus
clarkii scent (i.e. CE; Fig.
4). This finding is consistent with the observation that snails
often crawl out of the water when presented with shell-crushing predators such
as crayfish (Alexander and Covich,
1991; Levri, 1998;
McCarthy and Fisher, 2000;
Turner, 1997). However, we did
not detect any change in heart rate for snails exposed to CE
(Orr et al., 2007), suggesting
that the increase in aerial respiratory behaviour may simply be a result of
spending more time near the air–water interface and not due to increased
respiratory demand.
Our data suggest that once snails detect the presence of a predator, here
by sensing CE, they `decide' to alter their behavioural activities in a manner
that would prove beneficial to survivorship (i.e. `keeping a low profile' or
getting out of harm's way quicker). In other words, on detecting the presence
of a predator they make a risk assessment and take the appropriate actions to
reduce that risk. This is not surprising given that predator detection
via kairomones not only gives information regarding predator presence
but also potentially gives information regarding the proximity, physiological
state and even diet of potential predators
(Dalesman et al., 2006;
Kats and Dill, 1998;
Wisenden, 2000).
Interestingly, this `choice' is reflected in significant alterations in the
activity of RPeD1, a key neuron in the mediation of memory formation.
Exposure to CE caused a significant increase in both the `righting' and the
shadow response compared with controls
(Fig. 8). These data show that
snails actively reduce the duration of vulnerability on perceiving the
presence of a predator. Increases in anti-predator responses when the presence
of the predator is perceived have been demonstrated in many aquatic organisms
in which prey respond appropriately to factors such as predator density
(Wiackowski and Staronska,
1999), distance (Turner and
Montgomery, 2003), size of predator and prey vulnerability
(Alexander and Covich, 1991;
Cotton et al., 2004;
Dewitt et al., 1999).
Lymnaea are capable of altering their defensive responses
appropriately according the perceived predator threat. That is, depending on
`who' the predator is (e.g. fish vs crayfish), a different defensive
strategy will be taken. Differential habitat use under multiple predator
systems demonstrates both increased vigilance and the differentiation between
predator threats, allowing for functional tradeoffs in predator defence
(Dalesman et al., 2006;
Dewitt and Langerhans, 2003;
Orr et al., 2007).
Here we have shown that in addition to behavioural changes brought about by
CE exposure, significant changes in the electrophysiological activity in RPeD1
were also observed. For example, naive snails exposed to CE before dissection
showed a significant decrease in spontaneous firing activity and bursting
activity compared with control snails
(Fig. 10). To our knowledge
this investigation provides the first evidence of neurobiological changes
associated with predator detection in pulmonates. RPeD1 has been shown to be
both necessary and sufficient to drive the aerial respiratory behaviour of
Lymnaea (Syed et al.,
1990; Syed et al.,
1992) and is inhibited by the defensive full-body withdrawal
behaviour (Inoue et al.,
1996). It is therefore not too surprising that the activity
pattern of this neuron is altered when the crayfish predator is detected.
However, the data presented in
Figs9 and
10 appear to be contradictory.
That is, CE exposure results in a significant increase in total breathing time
and the number of pneumostome openings, yet CE exposure also results in a
significant decrease in RPeD1 activity, the neuron that initiates
rhythmogenesis within the neural circuit that drives aerial respiration. This
apparent conflict may be explained by an up-regulation in the efficacy of
peripheral inputs onto downstream components of the respiratory network, which
would therefore require less input from RPeD1 to initiate the respiratory
rhythm. Further investigation into both the location and activity of these
chemosensory receptors is ongoing in our laboratory. Previously it was
demonstrated that there is an age-dependent change in suppressive input from
the pneumostome area to CNS neurons, such as RPeD1
(McComb et al., 2005b). Our
working hypothesis is that CE is detected by sensory neurons in the
pneumostome and/or osphradial ganglion and that this activity in the
peripheral nervous system modulates aerial respiratory behaviours. The
interaction between the central and peripheral nervous systems of molluscs,
especially as regards mediation of adaptive behaviours involving respiratory
organs, is complicated, interesting and controversial
(Lukowiak and Colebrook, 1988;
Lukowiak and Jacklet, 1972).
Which neuron(s) in the Lymnaea CNS actually `makes the decision' to
alter both the various behaviours and activity in RPeD1 remains to be
determined.
With respect to the `Yerkes–Dotson memory curve', any
predator–prey encounter where the prey is aware of a predator presence,
yet escapes the interaction with its life, should fall within a range close to
the `optimal stress intensity' for memory formation and, therefore, should
augment memory formation. Unfortunately, attempts to confirm this theory
experimentally have yielded mixed results (see
Kim and Diamond, 2002;
Shors, 2004). We showed here,
using snails that have not experienced a natural predator for over 250
generations, that after exposure to CE, LTM formation was significantly
enhanced compared with the typical memory in PW. In PW a 0.5 h training
session only results in a memory persisting for 3 h. Here, CE exposure
increased the duration of memory persistence following the single 0.5 h
training session to 48 h.
Neural correlates of the CE-enhanced memory were also obtained in RPeD1. We
chose to record from this neuron as it is a necessary site for LTM formation,
memory reconsolidation, extinction and forgetting
(Scheibenstock et al., 2002;
Lukowiak et al., 2003a;
Sangha et al., 2003a;
Sangha et al., 2003b;
Lattal et al., 2006).
Moreover, we and others (Lowe and Spencer,
2006) have previously shown that significant alterations in
various electrophysiological parameters in this neuron adequately reflect the
significant behavioural changes that occur following training in either intact
or initially naive semi-intact preparations. Thus, we were not too surprised
to find that the enhanced LTM caused by training in CE was reflected in
altered RPeD1 activity. Our working hypothesis is that CE exposure alters the
molecular machinery in neurons (e.g. RPeD1) that are responsible for forming
and maintaining the memory.
The identity of the component(s) of the stress response responsible for
memory enhancement is presently unknown but previous investigations have
highlighted a number of stress hormones and neural modulators that are
expressed in molluscs and have been implicated in modulating memory. Our
present working hypothesis, based on very preliminary data, is that serotonin
plays a major role in the mediation of stress in Lymnaea. Our model
system is very tractable in determining whether this transmitter/modulator is
necessary for the stress-induced changes in adaptive behaviour since it is
relatively easy to alter the serotoninergic tone of the Lymnaea CNS
by injecting serotonin precursors, blockers or toxins that alter the serotonin
content in a predictable fashion (e.g.
Gadotti et al., 1986).
Our data unequivocally show that detection of a predator, an instinctive behaviour, has been maintained in our lab-reared snails over many generations and may allow us to determine at the neuronal level how such instinct is both mediated and maintained. These behaviours are robust and repeatable in the laboratory setting and support both laboratory and field investigations demonstrating that Lymnaea stagnalis not only detect predator kairomones but also respond in the appropriate manner to decrease the probability of predation.
Together, all the data presented above demonstrate causal links between
ecologically relevant (some more than others) behaviours and neural substrates
driving these behaviours. We are now beginning experiments to examine whether
CE will alter behaviours in populations of Lymnaea where crayfish are
not sympatric predators. That is, crayfish are not historically present in
Alberta yet Lymnaea stagnalis are. Will these Alberta
Lymnaea respond to CE in the same manner as our lab-bred snails? For
that matter, will freshly collected snails from The Netherlands (`wild' Dutch
snails) respond in the same manner to CE as their lab-bred cousins? Crayfish
are sympatric predators of Lymnaea in The Netherlands. Thus, it may
be possible to study at the neuronal level in a neuron such as RPeD1, which
plays a necessary role in memory formation, reconsolidation, extinction and
forgetting (for reviews, see Lattal et
al., 2006; Parvez et al.,
2006b), how an ecologically relevant stress stimulus that has been
maintained in laboratory-rearing conditions affects learning and memory.
Acknowledgments
We acknowledge CIHR, NSERC and AHFMR for support of our work.
References
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