PROOF COVER SHEET
Author(s):
Boyd R. Rorabaugh, Anna Krivenko, Eric D. Eisenmann, Albert D. Bui, Sarah Seeley, Megan E. Fry,
Joseph D. Lawson, Lauren E. Stoner, Brandon L. Johnson, and Phillip R. Zoladz
Article title: Sex-dependent effects of chronic psychosocial stress on myocardial sensitivity to ischemic injury
Article no:
ISTS_A_1087505
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Given name(s)
Surname
1
Boyd R.
Rorabaugh
2
Anna
Krivenko
3
Eric D.
Eisenmann
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Albert D.
Bui
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Sarah
Seeley
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Megan E.
Fry
7
Joseph D.
Lawson
8
Lauren E.
Stoner
9
Brandon L.
Johnson
10
Phillip R.
Zoladz
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http://informahealthcare.com/sts
ISSN: 1025-3890 (print), 1607-8888 (electronic)
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Stress, Early Online: 1–9
! 2015 Taylor & Francis. DOI: 10.3109/10253890.2015.1087505
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ORIGINAL RESEARCH REPORT
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Sex-dependent effects of chronic psychosocial stress on myocardial
sensitivity to ischemic injury
Boyd R. Rorabaugh1, Anna Krivenko2, Eric D. Eisenmann2, Albert D. Bui1, Sarah Seeley1, Megan E. Fry2,
Joseph D. Lawson2, Lauren E. Stoner2, Brandon L. Johnson2, and Phillip R. Zoladz2
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Department of Pharmaceutical & Biomedical Sciences and 2Department of Psychology, Sociology & Criminal Justice, Ohio Northern University,
Ada,
OH, USA
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Abstract
Keywords
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Individuals with post-traumatic stress disorder (PTSD) experience many debilitating symptoms,
including intrusive memories, persistent anxiety and avoidance of trauma-related cues. PTSD
also results in numerous physiological complications, including increased risk for cardiovascular
disease (CVD). However, characterization of PTSD-induced cardiovascular alterations is lacking,
especially in preclinical models of the disorder. Thus, we examined the impact of a psychosocial
predator-based animal model of PTSD on myocardial sensitivity to ischemic injury. Male and
female Sprague–Dawley rats were exposed to psychosocial stress or control conditions for 31
days. Stressed rats were given two cat exposures, separated by a period of 10 days, and were
subjected to daily social instability throughout the paradigm. Control rats were handled daily
for the duration of the experiment. Rats were tested on the elevated plus maze (EPM) on day
32, and hearts were isolated on day 33 and subjected to 20 min ischemia and 2 h reperfusion on
a Langendorff isolated heart system. Stressed male and female rats gained less body weight
relative to controls, but only stressed males exhibited increased anxiety on the EPM. Male, but
not female, rats exposed to psychosocial stress exhibited significantly larger infarcts and
attenuated post-ischemic recovery of contractile function compared to controls. Our data
demonstrate that predator stress combined with daily social instability sex-dependently
increases myocardial sensitivity to ischemic injury. Thus, this manipulation may be useful for
studying potential mechanisms underlying cardiovascular alterations in PTSD, as well as sex
differences in the cardiovascular stress response.
Animal model, cardiovascular, heart, ischemia,
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PTSD, stress
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History
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Received 18 June 2015
Revised 20 August 2015
Accepted 24 August 2015
Published online 2 2 2
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Introduction
Traumatized individuals who develop post-traumatic stress
disorder (PTSD) are at increased risk for developing cardiovascular disease (CVD) (Boscarino, 2011; Buckley et al.,
2013; Coughlin, 2011; Edmondson & Cohen, 2013;
Edmondson et al., 2013). This association appears to be
independent of comorbid depression, genetic influences and
several other potentially confounding psychosocial factors
(Edmondson et al., 2013; Vaccarino et al., 2013). According
to Edmondson and colleagues (2013), the risk for CVD in
PTSD patients could be greater in females, relative to males,
but the limited number of CVD studies involving female
subjects has underpowered such statistical comparisons. The
scientific literature suggests that the link between PTSD and
CVD may be causal in nature and related to the numerous
cardiovascular abnormalities observed in PTSD patients. For
instance, people with PTSD exhibit lower heart rate variability (HRV), reduced baroreflex sensitivity and increased QT
57
Correspondence: Phillip R. Zoladz, Department of Psychology,
Sociology, & Criminal Justice, Ohio Northern University, 525 S. Main
59 St., Ada, OH 45810, USA. Tel: +1 419 772 2142.
60 E-mail: p-zoladz@onu.edu
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variability (Cohen et al., 1998, 2000; Rozanski et al., 2005),
each of which has been linked to greater CVD incidence or
cardiovascular-related mortality (Bigger et al., 1992;
Piccirillo et al., 2007; Rozanski et al., 2005). PTSD patients
also demonstrate abnormally elevated heart rate (HR), blood
pressure (BP) and norepinephrine (NE) levels at baseline and
in response to emotional stimuli or events (for a review, see
Zoladz & Diamond, 2013). Coupled with reports of reduced
HRV and parasympathetic nervous system (PNS) activity in
PTSD patients, this exaggerated sympathetic nervous system
(SNS) activity suggests that PTSD causes autonomic
dysregulation.
CVD has been described as a disease of inflammation
(Libby et al., 2009), and PTSD patients exhibit elevated levels
of pro-inflammatory cytokines, such as tumor necrosis factor
a and IL-1b (von Kanel et al., 2007, 2010), as well as
increased incidence of autoimmune diseases (O’Donovan
et al., 2015). These observations may be associated with
baseline alterations of hypothalamus–pituitary–adrenal (HPA)
axis function in PTSD, as a common finding in PTSD patients
is abnormally low levels of baseline cortisol (Daskalakis
et al., 2013; Yehuda, 2009; Yehuda & Seckl, 2011). Because
corticosteroids act as major anti-inflammatory agents,
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insufficient HPA axis activity, coupled with exaggerated SNS
activity, could promote inflammation and negative health
outcomes.
One of the present authors was previously involved
in developing a psychosocial predator-based animal model
that has been shown to emulate several core symptoms of
PTSD (Roth et al., 2011; Zoladz et al., 2008, 2012, 2013,
2015). In the model, rats are immobilized and placed in close
proximity to a cat (i.e. predator stress) on two separate
occasions. The stressed rats are also exposed to daily
social instability (randomized changing of cage mates)
throughout the duration of the model to increase the
likelihood of producing long-lasting sequelae in the rats.
Thus, the model incorporates elements, such as uncontrollability, unpredictability, a lack of social interaction and a reexperiencing of the stress that, in people, significantly
increases the likelihood of PTSD onset and maintenance
following trauma exposure.
Previous work has shown that exposing rats to this
psychosocial stress manipulation results in a number of
physiological and behavioral abnormalities that are similar to
those observed in people with PTSD. Three weeks after the
second predator exposure, psychosocially stressed rats exhibit
a robust fear memory for the cat exposures (evidenced by
greater immobility in response to cat-associated context and
cues), heightened anxiety on an EPM (evidenced by less time
spent exploring open arms), an exaggerated startle response
and impaired memory for newly learned information (Zoladz
et al., 2008, 2012, 2013). The psychosocially stressed rats also
display physiological changes indicative of elevated SNS
activity and HPA axis abnormalities, such as greater cardiovascular and hormonal reactivity to an acute stressor, an
exaggerated physiological and behavioral response to the a2adrenergic receptor antagonist, yohimbine, abnormally low
levels of baseline corticosterone and enhanced negative
feedback of the HPA axis (Zoladz et al., 2008, 2012, 2013).
Others have replicated and extended the findings from this
model, revealing that the chronic psychosocial stress results in
decreased serotonin, increased norepinephrine and increased
measures of oxidative stress and inflammation in the brain,
adrenal glands and systemic circulation (Wilson et al., 2013,
2014a). More recently, research has shown that some effects
induced by the psychosocial stress manipulation persist for
four months following the initiation of stress (Zoladz et al.,
2015).
There has been limited research investigating the cardiovascular consequences of animal models of PTSD and none
have assessed whether a rodent model of PTSD could
influence myocardial ischemic injury. There have also been
few animal models of PTSD that include females, despite the
fact that females are reportedly more likely to develop the
disorder (Tolin & Foa, 2006). Based on previous work
demonstrating cardiovascular-related abnormalities and
increased inflammatory markers in male rats exposed to the
psychosocial predator-based animal model of PTSD, we
examined whether male and female rats exposed to this
psychosocial stress manipulation would exhibit greater myocardial sensitivity to ischemic injury and whether female rats
would demonstrate the anxiogenic and physiological effects
previously observed in males.
Stress, Early Online: 1–9
Methods and materials
Animals
Experimentally naı̈ve adult male and female Sprague–Dawley
rats (approximately 200–300 g at the beginning of the experiment) part of an established breeding colony (established from
Sprague–Dawley rats that were obtained from Charles River
laboratories) were used in the present experiment. The rats
were housed on a 12-h light/dark schedule (lights on at 07:00)
in standard Plexiglas cages (two per cage) with free access to
food and water. All procedures were approved by the
Institutional Animal Care and Use Committee at Ohio
Northern University and followed those recommended by the
Guide for the Care and Use of Laboratory Animals provided by
the National Institute of Health.
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Psychosocial stress procedure
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Rats were assigned to ‘‘psychosocial stress’’ (males: N ¼ 10;
females: N ¼ 14) or ‘‘no stress’’ (males: N ¼ 11; females:
N ¼ 10) groups. On day 1, rats in the psychosocial stress
groups were immobilized in plastic DecapiCones (Braintree
Scientific; Braintree, MA) and placed in a perforated wedgeshaped Plexiglas enclosure (Braintree Scientific; Braintree,
MA; 20 20 8 cm). This enclosure was then taken to a cat
housing room and placed in a metal cage (61 53 51 cm)
with an adult female cat for 1 h. The Plexiglas enclosure
prevented any contact between the cat and rats, but the rats
were still exposed to all non-tactile sensory stimuli associated
with the cat. Canned cat food was smeared on top of the
enclosure to direct the cat’s attention toward the rats. An hour
later, the rats were returned to their home cages. Rats in the
no stress groups remained in their home cages during the 1-h
stress period.
Rats in the psychosocial stress groups were given two acute
stress sessions, which were separated by 10 days. The first
stress session took place during the light cycle (between 08:00
and 13:00), and the second stress session took place during the
dark cycle (between 19:00 and 21:00). The stress sessions took
place during different times of the day to add an element of
unpredictability as to when the rats might re-experience the
predator exposure, as per previously-employed methodology
(Roth et al., 2011; Zoladz et al., 2008, 2012, 2013, 2015).
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Daily social stress
Beginning on the day of the first stress session (day 1), rats in
the psychosocial stress groups were exposed to unstable housing
conditions until behavioral testing (day 32). Rats in the
psychosocial stress groups were housed two per cage, but
every day, their cohort pair combinations were changed.
Therefore, no rats in the psychosocial stress groups had the
same cage mate on two consecutive days during the 31-day
stress period. Rats in the no stress groups were housed with the
same cohort pair throughout the experiment; these rats were
handled daily to control for handling effects on stressed animals.
Behavioral testing
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Three weeks following the second stress session (day 32), rats 238
were tested on the EPM to assess anxiety-like behavior. Rats 239
were placed on the EPM for 5 min, and their behavior was 240
Cardiac injury in animal model of PTSD
DOI: 10.3109/10253890.2015.1087505
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videotaped by a JVC hard disk camera hanging above the
EPM and scored offline by two separate investigators who
were blind to the experimental conditions of the animals.
Investigators assessed time spent in the open arms and
number of entries into the open and closed arms. Rats were
removed from data analysis if they fell off of the EPM during
testing.
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Langendorff heart preparation and measurement of
250 cardiac function
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Q2
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Measurement of infarct size
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On Day 33, rats were anesthetized with an intraperitoneal
injection of pentobarbital (75 mg/kg). Hearts were rapidly
removed and cannulated while bathed in ice-cold Krebs–
Henseleit solution (118 mM NaCl, 4.7 mM KCl, 1.2 mM
MgSO4, 25 mM NaHCO3, 1.2 mM KH2PO4, 0.5 mM
Na2EDTA, 11 mM glucose, 2.5 mM CaCl2, pH 7.4). Krebs
solution was perfused through the aortic cannula at a constant
pressure of 80 mmHg. Contractile function of the left
ventricle was measured using an intraventricular balloon
connected to a pressure transducer. The balloon was inflated
to an end diastolic pressure of 4–8 mmHg, and data were
continually recorded using a Powerlab 4SP data acquisition
system (AD Instruments, Colorado Springs, CO). Hearts were
equilibrated for 25 min prior to the onset of 20 min ischemia
and 2 h reperfusion. Hearts were excluded after the 25-min
equilibration period if developed pressure was less than
100 mmHg, coronary flow rate was greater than 20 ml/min or
if they had a persistent irregular rhythm. Ischemia was
induced by closing a valve which stopped the flow of Krebs
solution through the aortic cannula. Reperfusion was initiated
by reopening the valve and allowing Krebs solution to resume
flow through the cannula (Granville et al., 2004; Waterson
et al., 2011). The heart was submerged in Krebs solution to
maintain the temperature of the heart at 37.5 ± 0.5 C
throughout the experiment, and temperature was continuously
monitored using a thermocouple placed on the surface of the
heart. Pre-ischemic contractile function was measured immediately prior to ischemia. Post-ischemic recovery of contractile function was measured following 1 h of reperfusion.
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Following 20 min ischemia and 2 h reperfusion, hearts were
perfused with 1% triphenyltetrazolium chloride for 8 min (at a
rate of 7.5 ml/min) and then submerged in 1% triphenyltetrazolium chloride and incubated at 37 C for 15 min. Hearts
were subsequently frozen at 80 C, sliced into approximately 1 mm sections, soaked in 10% neutral buffered
formalin and then photographed with a Nikon SMZ 800
microscope equipped with a Nikon DS-Fi1 digital camera.
The infarcted surface area and total surface area of each slice
were measured using NIH Image J software. Infarct size was
expressed as a percentage of the area at risk (AAR). The AAR
was defined as the entire ventricular myocardium since hearts
were exposed to global ischemia.
Body and organ weights
Rats were weighed on the days of the first and second stress
299 sessions, as well as on the day of EPM testing. Growth rates
300 (grams per day) were calculated for statistical analysis.
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In females, adrenal and thymus glands were removed and
weighed to compare to previously-reported findings in
psychosocially stressed males. Organ weights were expressed
as mg/100 g body weight for statistical analysis.
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Statistical analyses
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Male and female data were analyzed separately. Most data
(body weight, EPM behavior, infarct sizes, adrenal/thymus
weights) were compared via independent samples t tests with
treatment (psychosocial stress, no psychosocial stress) serving
as the between-subjects factor. Parameters of contractile
function were compared with two-way, mixed-model
ANOVAs with treatment (psychosocial stress, no psychosocial stress) serving as the between-subjects factor and time
(pre-ischemic, post-ischemic) serving as the within-subjects
factor. Tukey post-hoc tests were employed when necessary,
and alpha was set at 0.05 for all analyses. Data points more
than three standard deviations beyond exclusive group means
were eliminated from statistical analyses.
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Results
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Preliminary characterization of psychosocial stressed
rats
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Males
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Psychosocially stressed males spent significantly less time in
the open arms of the EPM than control males [t(16) ¼ 2.17,
p ¼ 0.046; Figure 1A], indicating a heightened state of
anxiety. This effect was not attributable to reduced locomotor
activity in the psychosocially stressed males, as both groups
exhibited a statistically equivalent number of total arm entries
[t(16) ¼ 0.60, p ¼ 0.56; Figure 1B]. Psychosocial stress also
led to significantly reduced growth rate in male rats over the
31-day period [t(19) ¼ 2.39, p ¼ 0.028; Figure 1C].
Females
Examination of the physiological and behavioral data from
females revealed inconsistent findings. Although, psychosocially stressed females exhibited significantly reduced
growth rate over the 31-day period [t(22) ¼ 2.93, p ¼ 0.008;
Figure 2C], they spent a statistically equivalent amount of
time in the open arms of the EPM as control rats [t(20) ¼ 0.19,
p ¼ 0.86; Figure 2A], suggestive of no anxiogenic effect of
the psychosocial stress manipulation. Adrenal [t(22) ¼ 1.75,
p ¼ 0.09; Figure 2D] and thymus gland [t(20) ¼ 2.04,
p ¼ 0.055; Figure 2E] comparisons were not significant.
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Effect of psychosocial stress on cardiac function and
myocardial ischemic injury
Males
Psychosocial stress had no effect on pre-ischemic contractile
function. Pre-ischemic rate pressure product, +dP/dT, dP/
dT and end diastolic pressure were similar in psychosocially
stressed males and controls (Figure 3). However, 20 min of
ischemia induced more myocardial injury in psychosocially
stressed males, relative to controls. Hearts from psychosocially stressed males exhibited significantly larger infarcts
compared to controls [t(15) ¼ 2.54, p ¼ 0.023; Figure 3A].
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B. R. Rorabaugh et al.
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Figure 1. Physiological and behavioral effects of psychosocial stress in male rats. Psychosocially stressed males spent significantly less time in the 448
open arms of the elevated plus maze (A), despite making an equivalent numbers of overall arm entries in the maze (B). The stressed males also 449
390 exhibited significantly reduced growth rates (C) relative to controls. Sample sizes were 9–11 rats per group. Data are presented as means ± SEM. 450
*p50.05 versus no stress.
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In addition, post-ischemic recovery of contractile function
was significantly attenuated in the hearts from psychosocially
stressed males. Recovery of the rate pressure product
[significant Group Time interaction: F(1, 14) ¼ 5.38,
p ¼ 0.036; Figure
3B] and
+dP/dT [significant
Group Time interaction: F(1, 14) ¼ 5.90, p ¼ 0.029;
Figure 3C] was significantly reduced in psychosocially
stressed males, relative to controls, indicating that recovery
of systolic function was worsened by the psychosocial stress.
Recovery of dP/dT was decreased 33% in psychosocially
stressed male hearts compared to control hearts, but this did
not reach statistical significance [Group Time interaction:
F(1, 14) ¼ 3.24, p ¼ 0.094; planned comparison: t(14) ¼ 2.08,
p ¼ 0.056; Figure 3D]. Post-ischemic recovery of end diastolic pressure was significantly elevated in psychosocially
stressed males [significant Group Time interaction:
F(1, 13) ¼ 8.26, p ¼ 0.013; Figure 3E], indicating that recovery of diastolic function was also worsened by psychosocial
stress. Collectively, these data indicate that chronic psychosocial stress occurring prior to the onset of ischemia increases
cell death and decreases post-ischemic recovery of contractile
function in male rats.
Females
Psychosocial stress had no effect on pre-ischemic contractile
419 function in females. In contrast to male rats, infarct size
420 [t(22) ¼ 1.51, p ¼ 0.15); Figure 4A] and post-ischemic
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recovery of rate pressure product [Group Time interaction:
F(1, 21) ¼ 0.19, p ¼ 0.67; Figure 4B], +dP/dT [Group Time
interaction:
F(1, 21) ¼ 0.04,
p ¼ 0.84],
dP/dT
[Group Time interaction: F(1, 21) ¼ 0.13, p ¼ 0.73] and
end
diastolic
pressure
[Group Time
interaction:
F(1, 22) ¼ 0.07, p ¼ 0.79] were unaffected by psychosocial
stress in females.
Discussion
Despite extensive evidence of increased risk for CVD in
PTSD patients, few animal models of the disorder have
examined cardiovascular endpoints. In the present study, we
have shown that a psychosocial stress manipulation involving
predator exposure and daily social instability, one that has
previously been shown to emulate several core symptoms of
PTSD, increases myocardial sensitivity to ischemic injury in
male, but not female, rats. Following ischemia, hearts from
psychosocially stressed males exhibited significantly less
recovery of contractile function and demonstrated larger
infarct sizes than hearts from controls. Hearts from psychosocially stressed females, on the other hand, exhibited no
differences in infarct size or recovery of contractile function,
relative to controls. Moreover, psychosocially stressed
females did not show increased anxiety on the EPM, despite
exhibiting a reduced growth rate. Overall, our findings
provide evidence of a sex-dependent increase in myocardial
sensitivity to ischemic injury as a result of predator stress and
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DOI: 10.3109/10253890.2015.1087505
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Figure 2. Physiological and behavioral effects of psychosocial stress in female rats. Psychosocially stressed females spent a similar amount of time in 582
the open arms of the elevated plus maze as control rats (A) and made a similar number of overall arm entries in the maze (B), despite exhibiting 583
524 significantly reduced growth rates (C) relative to controls. There were no significant differences between the adrenal (D) or thymus (E) gland weights 584
of stressed and non-stressed female rats. Sample sizes were 10–14 rats per group. Data are presented as means ± SEM. *p50.05 versus no stress.
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chronic social instability, suggesting that this model may be
528 used to explore mechanisms underlying cardiovascular
529 changes that result from intense stress, at least in males.
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Cardiovascular effects
PTSD promotes hypertension, atherosclerosis and the production of inflammatory markers that are detrimental to
cardiovascular function (Boscarino, 2011; Buckley et al.,
2013; Coughlin, 2011; Edmondson & Cohen, 2013;
Edmondson et al., 2013; von Kanel et al., 2007, 2010). The
underlying cause of CVD in PTSD patients is thought to be an
acceleration of atherosclerosis and subsequent blockage of
coronary arteries (Ahmadi et al., 2011). It is important to
note, however, that myocardial ischemia in the present study
was not induced by the atherosclerotic occlusion of coronary
arteries but was artificially induced by termination of
coronary flow. The observation that stressed males exhibited
greater ischemic injury than male controls in the absence of
atherosclerotic occlusion of the coronary vasculature suggests
that PTSD-induced atherosclerotic blockade of coronary
arteries may not be the only factor that contributes to the
increased risk of myocardial infarction in patients with PTSD.
The chronic psychosocial stress manipulation utilized in
the present study provides several important advantages over
studying the impact of PTSD on myocardial infarction in a
clinical setting. One advantage is that the psychosocial stress
manipulation avoids factors that are commonly comorbid with
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Figure 3. Myocardial infarct sizes, pre-ischemic contractile function and post-ischemic recovery of contractile function in male hearts exposed to
myocardial ischemia. Hearts from psychosocially stressed males exhibited significantly larger infarcts than control hearts (A; insets depict
representative myocardial slices with white areas indicative of infarction). There were no differences in pre-ischemic parameters of contractile function
between hearts from psychosocially stressed and control males. Recovery of rate pressure product (B) and +dP/dT (C) were significantly attenuated in
hearts from psychosocially stressed males, relative to those from controls. Recovery of dP/dT (D) was borderline significant in hearts from
psychosocially stressed males. Recovery of end diastolic pressure (E) was significantly elevated in psychosocially stressed males relative to controls.
Samples sizes were 7–9 hearts per group. Data are presented as means ± SEM. *p50.05 relative to no stress.
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PTSD (depression, smoking, drug and alcohol abuse, etc.) and
can independently alter cardiovascular function. It also
provides a mechanism to compare the extent of myocardial
injury in stressed and non-stressed hearts under conditions
where the ischemic insult is identical between subjects.
Clinical data (death rates, hospitalization rates, rates of
coronary revascularization, etc.) have been important for
establishing that PTSD increases the likelihood of experiencing a myocardial infarction. However, clinical studies have
been unable to establish whether or not PTSD increases the
extent of myocardial injury. In addition to the common
comorbid factors mentioned above, ischemic conditions in
patients who are experiencing heart attacks are heterogeneous. For example, occlusion of a large coronary artery that
delivers blood to a wide region of myocardial tissue induces
ischemia over a much larger region of the heart than occlusion
of a smaller vessel that delivers blood to a more localized
area. In contrast, the psychosocial stress paradigm used here
enables stressed and non-stressed hearts to be made uniformly
ischemic so that the anatomical location and amount of
myocardial tissue that is subjected to ischemia is consistent
between individual hearts. One fundamental difference
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DOI: 10.3109/10253890.2015.1087505
Cardiac injury in animal model of PTSD
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Figure 4. Myocardial infarct sizes, pre-ischemic contractile function and post-ischemic recovery of contractile function in female hearts exposed to
myocardial ischemia. Psychosocial stress had no effect on infarct sizes (A; insets depict representative myocardial slices with white areas indicative of 819
760 infarction) or pre-ischemic contractile function. In addition, post-ischemic recovery of rate pressure product (B), +dp/dT (C), dP/dT (D) and end 820
761 diastolic pressure (E) were unaffected by psychosocial stress. Sample sizes were 10–14 hearts per group. Data are presented as means ± SEM.
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between the psychosocial stress paradigm used here and a
clinical heart attack is that atherosclerotic blockage of
coronary arteries develops over a long period of time and
may be accompanied by the formation of collateral vessels or
other myocardial compensatory changes as ischemic heart
disease progresses. In contrast, the fact that the present
psychosocial stress manipulation induces myocardial ischemia through a mechanism that is independent of atherosclerotic coronary occlusion represents an important distinction
(and perhaps a limitation) from the clinical scenario.
Our finding that chronic psychosocial stress increases
myocardial sensitivity to ischemic injury is consistent with
previous work exploring the impact of chronic social,
psychological or emotional stress on myocardial sensitivity
to ischemic injury. For instance, Mercanoglu et al. (2008)
reported that consecutive exposure to a series of multiple
stressors (e.g. cage crowding, isolation, changes in light/dark
cycle, changes in cage mates, cage tilting, food/water
restriction) increased infarct size in the rat heart. Chronic
restraint stress produces a similar effect (Scheuer & Mifflin,
1998). In contrast, acute stress immediately prior to the onset
of ischemia protects the heart from ischemic injury
(Moghimian et al., 2012). Thus, the duration and timing of
stress appear to play a critical role in how ischemia impacts
the heart.
The present psychosocial stress paradigm induces multiple
physiological changes that may underlie the increased sensitivity of the male heart to ischemic injury. Wilson and
colleagues reported that male rats exposed to this psychosocial stress exhibit increased expression of inflammatory
cytokines and other protein markers of inflammation in the
brain (Wilson et al., 2013, 2014b,c). These rats also have
elevated concentrations of reactive oxygen species in blood,
brain and adrenal glands (Wilson et al., 2013). In addition,
male rats exposed to the psychosocial stress exhibit increased
norepinephrine release in the hippocampus and prefrontal
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cortex (Wilson et al., 2014a). Elevated blood pressure and
heart rate in psychosocial stressed rats are suggestive of
increased SNS stimulation (Zoladz et al., 2008, 2013).
Inflammation and oxidative stress play prominent roles in
modulating ischemia/reperfusion injury (Frangogiannis,
2014; Ling et al., 2013; Sandanger et al., 2013). Chronic
b-adrenergic receptor stimulation also worsens ischemic
injury (Hu et al., 2006). Thus, pro-inflammatory conditions,
increased oxidative stress and chronic b-adrenergic receptor
stimulation that pre-exist prior to the onset of ischemia may
all contribute to the psychosocial stress-induced increase in
myocardial sensitivity to ischemic injury.
Sex differences
Investigators have reported that traumatized females are twice
as likely as traumatized males to develop PTSD (Tolin & Foa,
2006). However, in a recent review paper, Zoladz & Diamond
(2013) discussed issues with this position, noting that for
several types of trauma, the incidence of PTSD is statistically
equivalent across the sexes. According to Zoladz & Diamond
(2013), most research reporting a greater prevalence of PTSD
in females, relative to males, has involved individuals whose
trauma included some element of assault. Indeed, for nonassault trauma, there appears to be no difference in the
prevalence of PTSD across men and women (e.g. Kessler
et al., 1995). Moreover, the authors raise the issue of female
hormone influences on emotional memory and, thus, traumatic memory formation. Basic and applied research studies
have shown that the levels of progesterone and estradiol
significantly influence emotional memories and their extinction (Bryant et al., 2011; Felmingham et al., 2012a,b). Thus,
the point at which the trauma occurs during the menstrual
cycle can significantly influence a single female’s likelihood
of PTSD development. To suggest that being female, per se,
presents an inherent vulnerability to PTSD may be an
oversimplification of a much more complicated process. It
is likely that the female sex interacts with genetics, trauma
type, personality characteristics and other environmental
factors to affect PTSD susceptibility.
Most basic science research on stress has been conducted
in males, and an established animal model of PTSD including
females is non-existent. The present study included an initial
attempt to extend the psychosocial, predator-based model to
females. However, we observed no anxiogenic or cardiovascular effects of psychosocial stress in female rats, despite
stressed females exhibiting reduced growth rate compared to
non-stressed females. One possibility for the lack of
anxiogenic or cardiovascular effects in stressed females is
that the psychosocial stress manipulation is less ‘‘stressful’’ to
females. Additionally, as suggested in the previous paragraph,
extensive work has shown that female hormone levels
significantly influence the stress response and emotional
memory formation. Thus, it is possible that the random times
of predator stress and physiological/behavioral testing in the
present study masked any estrous-dependency of the observed
effects. On the other hand, a recent meta-analysis showed that,
across thousands of behavioral, morphological, physiological
and molecular traits, there was no greater variability in the
data obtained from female rats that were examined without
Stress, Early Online: 1–9
estrous stage monitoring than was observed in data obtained
from male rats (Prendergast et al., 2014). In fact, the authors
of this meta-analysis observed significantly more variability
in the data from male rats for certain dependent variables.
Thus, it is also possible that, in the present study, estrous stage
did not introduce anymore variability in the behavioral or
myocardial measures than was observed in males. We did not
assess the estrous cycle in females because we did not want
the stress associated with estrous testing to confound our
results. To conclusively determine whether females are less
stressed by this psychosocial stress manipulation or if estrous
stage influences female responses to the psychosocial stress,
future research is necessary.
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Conclusions
The psychosocial predator-based model used here provides a
useful tool to study the cardiac impact of intense stress. We
anticipate that future work will further our understanding of
the mechanisms by which PTSD-like sequelae in rats increase
myocardial ischemic injury. This could provide new therapeutic strategies to protect the hearts of PTSD patients from
myocardial infarction and other ischemic heart diseases.
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Declaration of interest
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The authors report no conflicts of interest.
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