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ISSN: 1025-3890 (print), 1607-8888 (electronic)
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Stress, Early Online: 1–8
! 2013 Informa UK Ltd. DOI: 10.3109/10253890.2013.791276
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SHORT COMMUNICATION
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Sex-specific impairment of spatial memory in rats following a reminder
of predator stress
Hanna M. Burke1, Cristina M. Robinson1, Bethany Wentz1, Jerel McKay1, Kyle W. Dexter1, Julia M. Pisansky1,
Jeffery N. Talbot2, and Phillip R. Zoladz1
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Department of Psychology and Sociology and 2Department of Pharmaceutical and Biomedical Sciences, Raabe College of Pharmacy,
Ohio
Northern University, Ada, OH 45810, USA
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Abstract
Keywords
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It has been suggested that cognitive impairments exhibited by people with post-traumatic
stress disorder (PTSD) result from intrusive, flashback memories transiently interfering with
ongoing cognitive processing. Researchers have further speculated that females are more
susceptible to developing PTSD because they form stronger traumatic memories than males,
hence females may be more sensitive to the negative effects of intrusive memories on
cognition. We have examined how the reminder of a naturalistic stress experience would affect
rat spatial memory and if sex was a contributing factor to such effects. Male and female
Sprague–Dawley rats were exposed, without contact, to an adult female cat for 30 min. Five
weeks later, the rats were trained to locate a hidden platform in the radial-arm water maze and
given a single long-term memory test trial 24 h later. Before long-term memory testing, the rats
were given a 30-min reminder of the cat exposure experienced 5 weeks earlier. The results
indicated that the stress reminder impaired spatial memory in the female rats only. Control
manipulations revealed that this effect was not attributable to the original cat exposure
adversely impacting learning that occurred 5 weeks later, or to merely exposing rats to a novel
environment or predator-related cues immediately before testing. These findings provide
evidence that the reminder of a naturalistic stressful experience can impair cognitive processing
in rats; moreover, since female rats were more susceptible to the memory-impairing effects of
the stress reminder, the findings could lend insight into the existing sex differences in
susceptibility to PTSD.
Animal model, flashbacks, intrusive memories,
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PTSD, trauma, water maze
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History
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Received 7 August 2012
Revised 13 March 2013
Accepted 27 March 2013
Published online 2 2 2
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Introduction
Individuals who experience life-threatening trauma are at
significant risk of developing post-traumatic stress disorder
(PTSD). People who develop PTSD experience several
debilitating symptoms, such as flashback memories, persistent anxiety, exaggerated startle, hyperarousal and diminished
extinction of conditioned fear (Nemeroff et al., 2006; Stam,
2007). Individuals with PTSD also display significant cognitive impairments, including impaired declarative and working
memory and deficits in attention and concentration (Buckley
et al., 2000; Gilbertson et al., 2001). It has been hypothesized
that the cognitive impairments observed in PTSD patients
could be the result of intrusive, flashback memories transiently interfering with their ability to process new information
(Brewin & Smart, 2005; McNally, 2005; Moradi et al., 1999;
Zoladz et al., 2010). While such speculation has been difficult
to investigate in human patients, given the ethical concerns
and inherent subjectivity involved with the reactivation of
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Correspondence: Phillip R. Zoladz, Ph.D., Department of Psychology
and Sociology, Ohio Northern University, 525 S. Main Street, Ada,
59 OH 45810, USA. Tel: +419 772 2142. Fax: +419 772 2746. E-mail:
60 p-zoladz@onu.edu
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traumatic memories, investigators have used rat models to test
such a prediction. For instance, it has been reported previously that placing a rat in a context where it had been shocked
(i.e. inhibitory avoidance apparatus) impaired the rat’s
memory on a completely different task, which in this case
was the location of a hidden platform in a water maze (Zoladz
et al., 2010). Importantly, these investigators found that the
reactivation of an inhibitory avoidance fear memory impaired
rat spatial memory in the water maze up to a year after
inhibitory avoidance training occurred. This finding supported the notion that invasive fear memories could hinder
cognitive processing on other tasks.
An interesting feature of PTSD is that females are
significantly more likely to develop the disorder than males
(Tolin & Foa, 2006). Although little is known as to why
females are more susceptible to the disorder, one speculation
is that they form stronger emotional memories than males,
which results in significantly greater potential for the
development of intrusive, traumatic memories. Relevant to
this speculation, recent work has reported that females
with PTSD exhibit greater fear conditioning than males
with PTSD (Inslicht et al., 2013). In non-clinical work,
women exhibit better recall of emotional information than
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H. M. Burke et al.
men (Bloise & Johnson, 2007; Canli et al., 2002), an effect
that has been linked to greater emotion-induced noradrenergic
activity in women (Segal & Cahill, 2009). In addition, a
recent study found that post-learning stress enhanced longterm memory for emotional information in females, but not
males (Felmingham et al., 2012). If females are more
susceptible to the memory-enhancing effects of emotion,
it is possible that they form stronger memories of a traumatic
event, rendering these memories more intrusive and
debilitating.
Sex differences with regards to emotional memory in
rodents have been relatively inconsistent and seem to depend
on the type of task that is used to assess learning (Dalla &
Shors, 2009; ter Horst et al., 2012). For instance, females
outperform males in classical eyeblink conditioning and
operant avoidance conditioning paradigms (Beatty & Beatty,
1970; Dalla et al., 2008; Wood & Shors, 1998), while males
outperform females on contextual and cue fear conditioning
assessments (Maren et al., 1994; Pryce et al., 1999). At least
some of these effects may be explained by sex differences in
how the animals respond to aversive stimuli. That is, male
rats are more inclined to react to aversive stimuli via passive
responses (e.g. freezing), while female rats are more inclined
to react to such stimuli via active responses (e.g. attempting
to escape shock) (Kirk & Blampied, 1985; van Haaren et al.,
1990). How acute stress affects learning in male and female
rats has also been inconclusive (Conrad et al., 2004; Park
et al., 2008). In many cases, what has been reported is that
stress causes effects in females that are opposite to the ones
observed in males (Shors et al., 2004). Also, there is
evidence to suggest that, at least in some situations, female
rodents are more sensitive to the effects of acute stress
manipulations on anxiety-like behavior and learning. Studies
have shown that female rats exhibit more vocalizations and
greater anxiety in the presence of a predator (Blanchard
et al., 1991), and they exhibit greater startle responses and a
greater impairment of spatial memory when tested at least a
week following predator stress (Adamec et al., 2006, 2008;
Mazor et al., 2009). These findings provide reasonable
evidence to speculate that sex differences in how the
memory of a stressor affects cognitive performance may
exist in rodents.
The purpose of the present study was 2-fold. First, the
previous study reporting that fear memory reactivation
impaired water maze memory (Zoladz et al., 2010) used the
memory of shock, an unnatural experience, as the intrusive
reminder. Thus, we wanted to examine how a reminder of a
more naturalistic stress experience (e.g. cat exposure) would
influence water maze memory in rats. Second, researchers
have yet to assess the influence of a stress reminder on
cognitive processing in males and females. Thus, we also
wanted to examine whether a reminder of the naturalistic
stress experience would differentially influence memory in
male versus female rats. We hypothesized that rats given the
reminder of the stress experience would exhibit impaired
spatial memory in the water maze. We also hypothesized that
such an effect might depend on the sex of the organism;
however, given the inconsistencies in the literature examining
sex differences in stress-memory interactions, this hypothesis
was non-directional.
Stress, Early Online: 1–8
Methods
Animals
Sixty-five 1-month-old male (N ¼ 32) and female (N ¼ 33)
Sprague–Dawley rats that were bred at Ohio Northern
University were used in the present experiment. The rats
were housed on a 12 h/12 h light–dark schedule (lights on at
07:00 h) in standard Plexiglas cages (three to four per cage)
with food and water provided ad libitum. Both male and
female rats were housed in the same room, but they were
placed on separate shelves within the room. All experiments
were carried out in accordance with the guidelines set forth by
the National Institute of Health’s Guide for the Care and Use
of Laboratory Animals, and the Institutional Animal Care and
Use of Committee at Ohio Northern University approved all
procedures.
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Apparatus
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Predator stress
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Rats that were assigned to a stress group were stressed by
exposing them to a live cat. During cat exposure, the rats were
placed in a perforated wedge-shaped Plexiglas enclosure
(Braintree Scientific; Braintree, MA; 20 cm 20 cm 8 cm),
with a maximum of 6 rats being placed in the enclosure at one
time. This Plexiglas enclosure was then placed on the floor of
a small room (2.41 m 1.5 m 2.26 m) with a spayed adult
female cat for 30 min (Diamond et al., 2006; Park et al.,
2008). The Plexiglas enclosure prevented any physical contact
between the cat and rats and exposed the rats to only the nontactile sensory stimuli associated with the cat. Canned cat
food was smeared on top of the Plexiglas enclosure to direct
cat activity toward the rats. A litter box and a large metal cage
(53 cm 51 cm 61 cm) were also in the room. The purpose
of including these stimuli (e.g. cat food, litter box and metal
cage) was to increase the number of salient cues in the
environment that could subsequently be used to reactivate the
rat’s memory of the cat exposure.
Rats that were assigned to a no stress group either
remained in their home cages for the 30 min time frame or
were exposed to the same room in which cat exposure took
place, except without the presence of the cat. The rats exposed
to the room were placed in the Plexiglas apparatus with the
cat food smeared on top, which was then placed in the room.
Prior to these ‘‘non-stress’’ sessions, previous odors and
remains (i.e. fur, urine and feces) left by the cat were removed
from the room, and the room was thoroughly cleaned and
sanitized.
Predator stress reminder
Prior to water maze testing (described below), the rats either
remained in their home cages for 30 min or were exposed to
the cat exposure room for 30 min without the presence of the
cat. Room exposure consisted of placing the rats in a standard
Plexiglas cage and smearing canned cat food on top, which
was then placed in the room. The room exposure served as a
reminder of the original cat exposure for rats that had been
previously exposed to the live cat. As before, prior to these
sessions, any previous odors and remains (i.e. fur, urine and
feces) left by the cat were removed from the room, and the
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room was thoroughly cleaned and sanitized by sweeping it
and wiping everything down with a bleach cleaning agent.
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Learning and memory
Rat learning and memory were assessed in the radial-arm
water maze (RAWM) (Park et al., 2008; Zoladz et al., 2006,
2010). The RAWM consisted of a black, galvanized, round
tank (168 cm diameter, 56 cm height, 43 cm depth) filled with
water (21–22 C). Using 6 V-shaped stainless steel walls, the
tank was divided into 6 arms radiating from an open central
area. A black, plastic platform (12 cm diameter) was placed
1 cm below the surface of the water at the end of one arm (the
‘‘goal arm’’). At the beginning of each trial, the rats were
released at the end of one arm (the ‘‘start arm’’) into the
center of the maze and given 60 sec to find the hidden
platform. If a rat did not locate the platform within 60 sec,
it was gently guided to the platform by the experimenter. Once
a rat found or was guided to the platform, it was left there
undisturbed for 15 sec. Spatial learning and memory were
measured by counting the number of arm entry errors the rats
made on each trial. An arm entry error was operationally
defined as a rat passing at least halfway down an arm that did
not contain the hidden platform or, rarely, when a rat entered
and exited the goal arm without climbing onto the platform.
Behavioral procedure and treatment groups
The timeline and procedure for the present experiment is
illustrated in Figure 1. Male and female rats were assigned to
stress (i.e. cat exposure) or no stress (i.e. home cage or room
exposure) groups. On the first day of the experiment, the rats
were given predator stress, room exposure or home cage
exposure for 30 min. Five weeks later, the rats were trained
in the RAWM. On the first day of training, the rats were
given 12 massed acquisition trials, followed 30 min later by
Figure 1. Experimental timeline (top) and
groups (bottom). One-month-old rats were
exposed to their home cages, a laboratory
room or a laboratory room with a cat for
30 min (Manipulation 1). Five weeks later,
rats learned to locate a hidden platform in the
RAWM; 24 h later, the rats either remained in
their home cages or were exposed to the
laboratory room for 30 min (Manipulation 2).
Exposure to the laboratory room served as a
reminder of the stressor for rats previously
exposed to the room with a cat; 30 min after
Manipulation 2, the rats were given a longterm memory test trial in the RAWM.
Experimental groups are outlined below the
timeline. Male (M) and female (F) rats were
tested.
Stress reminder impairs female memory
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6 massed short-term memory test trials. Twenty-four hours
later, the rats were exposed, for 30 min, to the room where cat
exposure had previously occurred or remained in their home
cages for the same period of time. Subsequently, long-term
spatial memory was assessed in the RAWM by giving the rats
a single test trial. The 5-week time-gap between the initial cat
exposure and the commencement of RAWM training was
implemented for two reasons. First, since we were particularly
interested in the effects of the stress memory on water maze
performance, we wanted to provide a relatively wide time-gap
between the predator stress experience and water maze
training to limit any negative effects that the stress experience, in and of itself, could have on learning and memory.
Second, in humans, PTSD symptoms are expressed long after
the original stress experience occurred; thus, we wanted to
assess the whether the memory of the cat exposure could exert
negative effects on water maze memory relatively long after
the cat exposure occurred.
A total of four experimental manipulations were performed
in the present study, as illustrated in Figure 1; each
manipulation was conducted on male and female rats,
resulting in a total of 8 groups (N ¼ 8–9 rats per group).
The first manipulation consisted of exposing rats to predator
stress on Day 1 and then, immediately prior to the water maze
memory test, re-exposing them to the room where predator
stress had previously occurred (Cat ! Room). The second
manipulation consisted of exposing rats, on Day 1 and
immediately prior to the water maze memory test, to the room
where cat exposure occurred in stressed rats, but without the
presence of a cat (Room ! Room). The third manipulation
consisted of exposing rats to predator stress on Day 1 and
then, prior to the water maze memory test, leaving them in
their home cages for 30 min (Cat ! Home Cage). This
manipulation controlled for the influence of predator stress,
in general, on learning and memory. The fourth manipulation
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Stress, Early Online: 1–8
consisted of exposing rats to their home cages on Day 1 and
then, immediately prior to the water maze memory test,
exposing them to the room where cat exposure had occurred
in stressed rats, but without the presence of a cat (Home
Cage ! Room). This manipulation controlled for the influence of cat-related stimuli (e.g. cat food, litter box and metal
cage) and novelty, in general, on learning and memory.
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Statistical analyses
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Separate analyses were performed for the arm entry errors
committed during the acquisition, short-term memory and
long-term memory trials in the RAWM. The arm entry errors
committed during the acquisition and short-term memory
trials were averaged across every two trials to create 2-trial
blocks for statistical analysis (Figure 2). Additionally, since
the two cat-exposed groups (i.e. Cat ! Room and
Cat ! Home Cage) had been exposed to identical manipulations up to the point of RAWM training, their acquisition and
short-term memory data were combined for statistical
analysis. A mixed-model ANOVA was used to analyze arm
entry errors made during the acquisition and short-term
memory trials, with sex and condition (cat, room, home cage)
serving as the between-subjects factors and trial block serving
as the within-subjects factor. Since we did not have a fully
crossed design with regards to the behavioral manipulations
performed prior to the long-term memory test (Figure 1), the
four manipulations that were included in the present study
were labeled as a ‘‘condition’’ variable for statistical analysis.
A two-way ANOVA was used to analyze arm entry errors
made during the long-term memory trial, with sex and
condition serving as the between-subjects factors. Alpha was
set at 0.05 for all analyses, and Holm–Sidak post hoc tests
were employed when the omnibus F test indicated the
presence of a statistically significant difference.
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Results
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Acquisition
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The analysis of arm entry errors during acquisition revealed a
significant effect of trial block, F(5,295) ¼ 69.36, p50.001,
indicating that the rats made fewer errors across trials
and thus successfully acquired the task (Figure 2). There
was also a significant condition trial block interaction,
F(10,295) ¼ 3.19, p50.05, indicating that rats exposed to the
room 5 weeks prior to water maze training made more errors
than the other groups on trial block 2. However, there were no
group differences on trial blocks 5 and 6, indicating that by
the end of training, all groups exhibited statistically equivalent performance. No other significant effects were observed.
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Figure 2. Arm entry errors committed in the radial-arm water maze
(RAWM) during acquisition and the short-term memory test trials in
male (A; top) and female (B; bottom) rats. The groups are identified in
the key by manipulations performed 5 weeks prior to training. The top
panel illustrates the experimental timeline, and the box depicts the time
point for data. Performance during acquisition (Trials 1–12) and the
short-term memory test trials (Trials 13–18) is presented in two-trial
blocks (indicated by B1–B9). All rats acquired the spatial learning task,
as evidenced by a significant decrease in arm entry errors across trials
(p50.05, ANOVA). The data are mean SEM. N ¼ 8–9 rats per group.
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room 5 weeks prior to water maze training. No other
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significant effects were observed.
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Short-term memory
The analysis of arm entry errors committed during the shortterm memory trials revealed a significant effect of trial block,
F(2,118) ¼ 3.90, p50.05, which again showed that the rats
made fewer errors across trials (Figure 2). There was also a
significant effect of condition, F(2,59) ¼ 3.49, p50.05,
indicating that rats exposed to their home cages 5 weeks
prior to water maze training made more errors on the
short-term memory trials, overall, than rats exposed to the
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Long-term memory
The analysis of arm entry errors during the long-term memory
test trial revealed significant effects of condition,
F(3,53) ¼ 8.20, and sex, F(1,53) ¼ 6.38, and a significant
condition sex interaction, F(3,53) ¼ 4.83 (all p50.05)
(Figure 3). Post hoc tests revealed that the female
Cat ! Room group committed more arm entry errors on the
long-term memory test trial than all other groups.
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DOI: 10.3109/10253890.2013.791276
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Stress reminder impairs female memory
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Figure 3. Arm entry errors committed in the
radial-arm water maze (RAWM) during the
24-h long-term memory test. Below the xaxis, the groups are identified by the
manipulations that were performed on them
during time point 1 (i.e. 5 weeks prior to
training) and time point 2 (i.e. immediately
prior to water maze testing). The top panel
illustrates the experimental timeline, and the
box depicts the time point for the data.
Female rats exposed to a cat during time
point 1 and then given a reminder of that
experience immediately prior to memory
testing made significantly more arm entry
errors than all other groups. Data are
mean SEM. N ¼ 8–9 rats per group.
*p50.05 versus all other groups, Holm–
Sidak post hoc test.
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Discussion
We have shown that the reminder of a naturalistic stress
experience (i.e. cat exposure) impairs long-term (24 h) spatial
memory retrieval in rats, a finding that extends on previous
work reporting a similar effect resulting from the reactivation
of fear memory for a shock (Zoladz et al., 2010). More
important, however, is the finding that, in the present study,
such an impairment was observed in female rats only. As
stated above, researchers have speculated that females may be
more susceptible to PTSD because they form stronger
emotional memories than males. This has been supported,
at least in part, by the finding of greater fear conditioning in
females with PTSD than males with PTSD (Inslicht et al.,
2013). The memory impairment that we observed could have
been limited to female rats because they formed a stronger
memory of the cat exposure that occurred 5 weeks earlier,
which led to a significantly greater stress response during the
reminder. However, this possibility remains purely speculative, as we did not assess the physiological or behavioral
responses of the rats to the reminder of the stress experience.
Future work will have to examine if female rats exhibit greater
physiological (e.g. corticosterone) and/or behavioral (e.g.
freezing) responses to such a reminder to determine whether
or not this is the case. Although, comparing in rats the
strength of memory for a stress experience across the sexes
could be difficult, since, as described above, male and female
rats exhibit different behavioral responses to aversive stimuli.
Another potential explanation for the present findings is that
female rats are more susceptible to the memory-impairing
effects of stress or a stress reminder, in general; however, the
fact that Zoladz et al. (2010) found a similar effect in males,
albeit for a different type of stress reminder, does not seem to
fit with this speculation. Either way, the present findings may
lend insight into why females are more susceptible to
developing PTSD. Females may form stronger traumatic
memories than males or be more sensitive to the effects of
traumatic reminders, either of which would render them more
susceptible to PTSD symptomatology and the deleterious
effects of intrusive memories on cognitive processing.
The present findings cannot be attributed to a general
impairment of spatial learning and memory that resulted
from cat exposure 5 weeks prior to training. Control groups
that were given cat exposure 5 weeks prior to training and
then left in their home cages before the 24-h long-term
memory test (i.e. male and female Cat ! Home Cage
groups) demonstrated intact learning and memory in the
RAWM (Figures 2 and 3). In addition, the impairment of
spatial memory in the female Cat ! Room group does not
seem to be due to an acute stress response elicited solely by
a novel environment or exposure to predator-related cues.
Additional control groups that were exposed, for the first
time, to the cat room and predator-related cues immediately
prior to the 24-h memory test exhibited intact long-term
memory. This finding is consistent with previous work
indicating that rat spatial memory is exceptionally resistant
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to impairment by arousing or distracting stimuli and that for
such impairment to occur, arousal in combination with fear
is necessary (Woodson et al., 2003; Zoladz et al., 2010).
Collectively, the findings from these control manipulations
further support the notion that it was the memory of the
stress experience that impaired water maze performance
selectively in female rats.
Our findings that the reminder of a stress experience
impaired water maze memory is consistent with an extensive
literature demonstrating that pre-retrieval stress impairs
hippocampus-dependent memory in both humans and rodents
(Diamond et al., 2007; Roozendaal et al., 2009; Sandi &
Pinelo-Nava, 2007; Schwabe et al., 2012). Research has
shown that stress impairs electrophysiological measures of
hippocampal function, such as long-term potentiation (LTP),
which seems to be associated with the observed stressinduced impairments of hippocampus-dependent memory
(Diamond et al., 2007; Joels & Krugers, 2007; Kim &
Diamond, 2002). Relevant to the present study is the finding
that re-exposing rats to a fear-conditioned environment can
impair hippocampal LTP and hippocampal memory as well
(Garcia et al., 1998; Li et al., 2005; Zoladz et al., 2010).
Collectively, these findings resonate with our observed
impairment of hippocampus-dependent memory following
the reminder of cat exposure in rats.
Importantly, different parts of the hippocampus are
differentially involved in cognitive- and fear-related behaviors
in rats. For instance, the dorsal hippocampus (DH) is
evidently more responsible for cognitive-related behaviors,
such as spatial cognition (e.g. water maze navigation), while
the ventral hippocampus (VH) is evidently more responsible
for fear-related behaviors, such as anxiety and defensive
behaviors (e.g. contextual fear and predatory threat responses)
(Maggio & Segal, 2012). Relevant to the current findings,
previous work (Pentkowski et al., 2006) has shown that
lesions of the VH, but not the DH, impair contextual fear
conditioning to a predator threat (althoughWang et al. (in
press) found that both the DH and VH play a role in predator
odor contextual fear conditioning). This raises the question of
why the reminder of a predator exposure, which would
theoretically activate the VH, would impair spatial memory, a
process dependent on the DH. We would argue that a chief
involvement of the VH in responding to the reminder of
predator exposure would not preclude negative effects of that
reminder on DH processing. Indeed, as mentioned above, the
reminder of cat exposure was likely a stressful experience for
the rats, and there is considerable evidence for such stressors
exerting deleterious effects on DH-dependent processes, such
as spatial memory. Moreover, researchers have speculated that
acute stress results in a region-specific shift in hippocampal
function. In particular, Segal and colleagues (Maggio &
Segal, 2012; Segal et al., 2010) have proposed that acute
stress reduces the threshold for neuroplasticity in the VH,
while at the same time increasing the threshold for
neuroplasticity in the DH. This speculation has been
supported empirically in several reports (Maggio & Segal,
2007, 2009). Thus, we would suggest that the cat exposure
reminder, while perhaps primarily activating the VH, also led
to an increased threshold for neuroplasticity in the DH, which
impaired spatial memory.
Stress, Early Online: 1–8
It is also likely that amygdala-induced modulation of
hippocampal function contributed to the impairment of water
maze memory that was observed following the stress
reminder. The amygdala is significantly involved in fear
conditioning and fear memory retrieval, and research has
shown that an intact amygdala is necessary for stress-induced
alterations of hippocampal synaptic plasticity and learning
and memory (Kim et al., 2001, 2005; Korz & Frey, 2005;
Zoladz et al., 2011). Moreover, direct electrical stimulation of
the amygdala has been shown to impair hippocampal LTP
(Akirav & Richter-Levin, 1999, 2002). When the rats were
reminded of the original stress experience in the present
study, it likely activated the amygdala, which, coupled with
the stress-induced increase of corticosteroids, norepinephrine
and a host of other transmitter substances (e.g. glutamate),
resulted in a saturation of hippocampal plasticity, which
rendered the water maze memory inaccessible (Diamond
et al., 2004, 2005, 2007).
Perhaps the most important question, particularly for
future research, is why females were more affected by the
stress reminder than males. This is not the first study to report
that females are more susceptible to the emotional modulation
of memory than males. Previous work in humans has shown
that females recall emotional information better than males
(Bloise & Johnson, 2007; Canli et al., 2002), an effect that has
been associated with greater noradrenergic activity in females
(Segal & Cahill, 2009). Another study reported that postlearning stress enhanced long-term emotional memory in
females, but not males (Felmingham et al., 2012). In addition,
researchers have emphasized important neurobiological differences in how males and females respond to emotional
information, such as differential amygdala responsiveness,
that may explain why females are more prone to remembering
emotionally charged material (Cahill, 2006). Research in
rodents has also reported sex differences in the neurobiology
of emotion; however, this area of research has been less
conclusive. One relatively consistent finding is that acute
stress differentially affects learning and memory in male and
female rats. For instance, acute stress enhances eyeblink
conditioning in male rats, while impairing it in female rats
(Shors, 2004). Importantly, such differential effects of stress
on learning and memory depend on an intact amygdala
(Waddell et al., 2008), and the effects in females have been
associated with ovarian hormones (Wood & Shors, 1998).
Acute stress also produces greater noradrenergic activation in
female rats, relative to males (Curtis et al., 2006; Valentino
et al., 2011). These findings indicate that differential neurochemical and neuroanatomical responses to stress across the
sexes may be involved in mediating our observed impairment
of spatial memory in females.
It is also possible that stress interacts with sex to influence
the type of learning strategy that is employed. Research has
shown that when spatial and non-spatial (e.g. cue-dependent)
strategies are available, male rats tend to adopt spatial
strategies, but the preference demonstrated by females can
vary, depending on stage of the estrous cycle (Bettis &
Jacobs, 2009; Korol et al., 2004; Pleil & Williams, 2010;
Tropp & Markus, 2001). Furthermore, research has previously
shown that stress or amygdala activation causes male rats to
adopt a non-spatial, rather than the preferred spatial, strategy
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DOI: 10.3109/10253890.2013.791276
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to learn and remember the location of a hidden platform in a
water maze (Elliott & Packard, 2008; Kim et al., 2001). Thus,
baseline differences in learning strategy between the sexes
and/or their interaction with stress could have led to
differential effects of the stress reminder on water maze
memory.
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Conclusions
In summary, we have reported that the reminder of a
naturalistic stress experience (i.e. cat exposure) impairs
long-term spatial memory in female, but not male, rats.
These findings may be explained by the presence of a stronger
fear memory in female rats that, when reactivated, impaired
their spatial memory. Future work is necessary to determine
whether or not this is the case and to assess whether or not
male and female rats exhibit differential neurobiological
profiles following an acute stress reminder. Our findings
establish the present paradigm as a model that may be used in
future research to develop a better understanding of the
neurobiological mechanisms underlying intrusive memories
and their role in the sex differences involved in PTSD.
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Acknowledgements
The authors would like to thank Robert Carrothers, Rebecca
Brooks, Kassidy Beck and Anna Krivenko for their valuable
contribution to the present study.
750
Declaration of interest
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The present study was supported by a Psi Chi undergraduate
research grant to HMB.
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References
Adamec R, Head D, Blundell J, Burton P, Berton O. (2006). Lasting
anxiogenic effects of feline predator stress in mice: sex differences in
vulnerability to stress and predicting severity of anxiogenic response
from the stress experience. Physiol Behav 88:12–29.
Adamec R, Holmes A, Blundell J. (2008). Vulnerability to lasting
anxiogenic effects of brief exposure to predator stimuli: sex, serotonin
and other factors-relevance to PTSD. Neurosci Biobehav Rev 32:
1287–92.
Akirav I, Richter-Levin G. (1999). Biphasic modulation of hippocampal
plasticity by behavioral stress and basolateral amygdala stimulation in
the rat. J Neurosci 19:10530–5.
Akirav I, Richter-Levin G. (2002). Mechanisms of amygdala modulation
of hippocampal plasticity. J Neurosci 22:9912–21.
Beatty WW, Beatty PA. (1970). Hormonal determinants of sex
differences in avoidance behavior and reactivity to electric shock in
the rat. J Comp Physiol Psychol 73:446–55.
Bettis TJ, Jacobs LF. (2009). Sex-specific strategies in spatial orientation
in C57BL/6J mice. Behav Processes 82:249–55.
Blanchard DC, Shepherd JK, De Padua Carobrez A, Blanchard RJ.
(1991). Sex effects in defensive behavior: baseline differences and
drug interactions. Neurosci Biobehav Rev 15:461–8.
Bloise SM, Johnson MK. (2007). Memory for emotional and neutral
information: gender and individual differences in emotional sensitivity. Memory 15:192–204.
Brewin CR, Smart L. (2005). Working memory capacity and suppression
of intrusive thoughts. J Behav Ther Exp Psychiatry 36:61–8.
Buckley TC, Blanchard EB, Neill WT. (2000). Information processing
and PTSD: a review of the empirical literature. Clin Psychol Rev 20:
1041–65.
Cahill L. (2006). Why sex matters for neuroscience. Nat Rev Neurosci 7:
477–84.
Stress reminder impairs female memory
7
Canli T, Desmond JE, Zhao Z, Gabrieli JD. (2002). Sex differences in the
neural basis of emotional memories. Proc Natl Acad Sci USA 99:
10789–94.
Conrad CD, Jackson JL, Wieczorek L, Baran SE, Harman JS, Wright
RL, Korol DL. (2004). Acute stress impairs spatial memory in male
but not female rats: influence of estrous cycle. Pharmacol Biochem
Behav 78:569–79.
Curtis AL, Bethea T, Valentino RJ. (2006). Sexually dimorphic
responses of the brain norepinephrine system to stress and
corticotropin-releasing factor. Neuropsychopharmacology 31:544–54.
Dalla C, Edgecomb C, Whetstone AS, Shors TJ. (2008). Females
do not express learned helplessness like males do.
Neuropsychopharmacology 33:1559–69.
Dalla C, Shors TJ. (2009). Sex differences in learning processes of
classical and operant conditioning. Physiol Behav 97:229–38.
Diamond DM, Campbell AM, Park CR, Halonen J, Zoladz PR. (2007).
The temporal dynamics model of emotional memory processing: a
synthesis on the neurobiological basis of stress-induced amnesia,
flashbulb and traumatic memories, and the Yerkes-Dodson law.
Neural Plast 2007:60803.
Diamond DM, Campbell AM, Park CR, Woodson JC, Conrad CD,
Bachstetter AD, Mervis RF. (2006). Influence of predator stress on the
consolidation versus retrieval of long-term spatial memory and
hippocampal spinogenesis. Hippocampus 16:571–6.
Diamond DM, Park CR, Campbell AM, Woodson JC. (2005).
Competitive interactions between endogenous LTD and LTP in the
hippocampus underlie the storage of emotional memories and stressinduced amnesia. Hippocampus 15:1006–25.
Diamond DM, Park CR, Woodson JC. (2004). Stress generates emotional
memories and retrograde amnesia by inducing an endogenous form of
hippocampal LTP. Hippocampus 14:281–91.
Elliott AE, Packard MG. (2008). Intra-amygdala anxiogenic drug
infusion prior to retrieval biases rats towards the use of habit
memory. Neurobiol Learn Mem 90:616–23.
Felmingham KL, Tran TP, Fong WC, Bryant RA. (2012). Sex differences
in emotional memory consolidation: the effect of stress-induced
salivary alpha-amylase and cortisol. Biol Psychol 89:539–44.
Garcia R, Tocco G, Baudry M, Thompson RF. (1998). Exposure to
a conditioned aversive environment interferes with long-term
potentiation induction in the fimbria-CA3 pathway. Neuroscience
82:139–45.
Gilbertson MW, Gurvits TV, Lasko NB, Orr SP, Pitman RK. (2001).
Multivariate assessment of explicit memory function in combat
veterans with posttraumatic stress disorder. J Trauma Stress 14:
413–32.
Inslicht SS, Metzler TJ, Garcia NM, Pineles SL, Milad MR, Orr SP,
Marmar CR, Neylan TC. (2013). Sex differences in fear conditioning
in posttraumatic stress disorder. J Psychiatr Res 47:64–71.
Joels M, Krugers HJ. (2007). LTP after stress: up or down? Neural Plast
2007:93202.
Kim JJ, Diamond DM. (2002). The stressed hippocampus, synaptic
plasticity and lost memories. Nat Rev Neurosci 3:453–62.
Kim JJ, Koo JW, Lee HJ, Han JS. (2005). Amygdalar inactivation blocks
stress-induced impairments in hippocampal long-term potentiation
and spatial memory. J Neurosci 25:1532–9.
Kim JJ, Lee HJ, Han JS, Packard MG. (2001). Amygdala is critical for
stress-induced modulation of hippocampal long-term potentiation and
learning. J Neurosci 21:5222–8.
Kirk RC, Blampied NM. (1985). Activity during inescapable shock and
subsequent escape avoidance learning: female and male rats
compared. NZ J Psychol 14:9–14.
Korol DL, Malin EL, Borden KA, Busby RA, Couper-Leo J. (2004).
Shifts in preferred learning strategy across the estrous cycle in female
rats. Horm Behav 45:330–8.
Korz V, Frey JU. (2005). Bidirectional modulation of hippocampal
long-term potentiation under stress and no-stress conditions in
basolateral amygdala-lesioned and intact rats. J Neurosci 25:
7393–400.
Li Z, Zhou Q, Li L, Mao R, Wang M, Peng W, Dong Z, et al. (2005).
Effects of unconditioned and conditioned aversive stimuli in an
intense fear conditioning paradigm on synaptic plasticity in the
hippocampal CA1 area in vivo. Hippocampus 15:815–24.
Maggio N, Segal M. (2009). Differential corticosteroid modulation of
inhibitory synaptic currents in the dorsal and ventral hippocampus.
J Neurosci 29:2857–66.
781
782
783
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785
786
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861
862
863
864
865
866
867
868
869
870
871
872
873
Q3
874
875
876
877
878
879
H. M. Burke et al.
Maggio N, Segal M. (2012). Steroid modulation of hippocampal
plasticity: switching between cognitive and emotional memories.
Front Cell Neurosci 6:12.
Maggio N, Segal M. (2007). Striking variations in corticosteroid
modulation of long-term potentiation along the septotemporal axis
of the hippocampus. J Neurosci 27:5757–65.
Maren S, De Oca B, Fanselow MS. (1994). Sex differences in
hippocampal long-term potentiation (LTP) and Pavlovian fear conditioning in rats: positive correlation between LTP and contextual
learning. Brain Res 661:25–34.
Mazor A, Matar MA, Kaplan Z, Kozlovsky N, Zohar J, Cohen H. (2009).
Gender-related qualitative differences in baseline and post-stress
anxiety responses are not reflected in the incidence of criterion-based
PTSD-like behaviour patterns. World J Biol Psychiatry 10:856–69.
McNally RJ. (2005). Debunking myths about trauma and memory.
Can J Psychiatry 50:817–22.
Moradi AR, Doost HT, Taghavi MR, Yule W, Dalgleish T. (1999).
Everyday memory deficits in children and adolescents with PTSD:
performance on the Rivermead Behavioural Memory Test. J Child
Psychol Psychiatry 40:357–61.
Nemeroff CB, Bremner JD, Foa EB, Mayberg HS, North CS, Stein MB.
(2006). Posttraumatic stress disorder: a state-of-the-science review.
J Psychiatr Res 40:1–21.
Park CR, Zoladz PR, Conrad CD, Fleshner M, Diamond DM. (2008).
Acute predator stress impairs the consolidation and retrieval of
hippocampus-dependent memory in male and female rats. Learn Mem
15:271–80.
Pentkowski NS, Blanchard DC, Lever C, Litvin Y, Blanchard RJ. (2006).
Effects of lesions to the dorsal and ventral hippocampus on defensive
behaviors in rats. Eur J Neurosci 23:2185–96.
Pleil KE, Williams CL. (2010). The development and stability of
estrogen-modulated spatial navigation strategies in female rats. Horm
Behav 57:360–7.
Pryce CR, Lehmann J, Feldon J. (1999). Effect of sex on fear
conditioning is similar for context and discrete CS in Wistar, Lewis
and Fischer rat strains. Pharmacol Biochem Behav 64:753–9.
Roozendaal B, McEwen BS, Chattarji S. (2009). Stress, memory and the
amygdala. Nat Rev Neurosci 10:423–33.
Sandi C, Pinelo-Nava MT. (2007). Stress and memory: behavioral effects
and neurobiological mechanisms. Neural Plast 2007:78970.
Schwabe L, Joels M, Roozendaal B, Wolf OT, Oitzl MS. (2012). Stress
effects on memory: an update and integration. Neurosci Biobehav Rev
36:1740–9.
Segal M, Richter-Levin G, Maggio N. (2010). Stress-induced dynamic
routing of hippocampal connectivity: a hypothesis. Hippocampus 20:
1332–8.
Segal SK, Cahill L. (2009). Endogenous noradrenergic activation
and memory for emotional material in men and women.
Psychoneuroendocrinology 34:1263–71.
Stress, Early Online: 1–8
Shors TJ. (2004). Learning during stressful times. Learn Mem 11:
137–44.
Shors TJ, Falduto J, Leuner B. (2004). The opposite effects of stress on
dendritic spines in male vs. female rats are NMDA receptordependent. Eur J Neurosci 19:145–50.
Stam R. (2007). PTSD and stress sensitisation: a tale of brain and body
Part 1: human studies. Neurosci Biobehav Rev 31:530–57.
ter Horst JP, de Kloet ER, Schachinger H, Oitzl MS. (2012). Relevance
of stress and female sex hormones for emotion and cognition. Cell
Mol Neurobiol 32:725–35.
Tolin DF, Foa EB. (2006). Sex differences in trauma and posttraumatic
stress disorder: a quantitative review of 25 years of research. Psychol
Bull 132:959–92.
Tropp J, Markus EJ. (2001). Sex differences in the dynamics
of cue utilization and exploratory behavior. Behav Brain Res 119:
143–54.
Valentino RJ, Reyes B, Van Bockstaele E, Bangasser D. (2011).
Molecular and cellular sex differences at the intersection of stress and
arousal. Neuropharmacology 62:13–20.
van Haaren F, van Hest A, Heinsbroek RP. (1990). Behavioral
differences between male and female rats: effects of gonadal
hormones on learning and memory. Neurosci Biobehav Rev 14:23–33.
Waddell J, Bangasser DA, Shors TJ. (2008). The basolateral nucleus of
the amygdala is necessary to induce the opposing effects of stressful
experience on learning in males and females. J Neurosci 28:5290–4.
Wang ME, Fraize NP, Yin L, Yuan RK, Petsagourakis D, Wann EG,
Muzzio IA. (in press). Differential roles of the dorsal and ventral
hippocampus in predator odor contextual fear conditioning.
Hippocampus.
Wood GE, Shors TJ. (1998). Stress facilitates classical conditioning in
males, but impairs classical conditioning in females through
activational effects of ovarian hormones. Proc Natl Acad Sci USA
95:4066–71.
Woodson JC, Macintosh D, Fleshner M, Diamond DM. (2003). Emotioninduced amnesia in rats: working memory-specific impairment,
corticosterone-memory correlation, and fear versus arousal effects
on memory. Learn Mem 10:326–36.
Zoladz PR, Campbell AM, Park CR, Schaefer D, Danysz W, Diamond
DM. (2006). Enhancement of long-term spatial memory in adult rats
by the noncompetitive NMDA receptor antagonists, memantine and
neramexane. Pharmacol Biochem Behav 85:298–306.
Zoladz PR, Park CR, Diamond DM. (2011). Neurobiological basis
of the complex effects of stress on memory and synaptic plasticity.
157–78.
Zoladz PR, Woodson JC, Haynes VF, Diamond DM. (2010). Activation
of a remote (1-year old) emotional memory interferes with the
retrieval of a newly formed hippocampus-dependent memory in rats.
Stress 13:36–52.
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