Response Variation following Trauma:
A Translational Neuroscience Approach
to Understanding PTSD
Rachel Yehuda1,* and Joseph LeDoux2
1Division of Traumatic Stress Studies, Mount Sinai School of Medicine, James J. Peters Veteran Affairs, New York, NY 10468, USA
2Center for Neural Science, New York University, New York, NY 10003, USA
Exposure to traumatic stress is a requirement for the development of posttraumatic stress disorder
(PTSD). However, because the majority of trauma-exposed persons do not develop PTSD, examina-
tion of the typical effects of a stressor will not identify the critical components of PTSD risk or path-
ogenesis. Rather, PTSD representsa specific phenotypeassociated with afailure to recover from the
thatexplain thedevelopmentofthedisorder andthefailuretoreinstatephysiological homeostasis. In
articulate some of the gaps in knowledge that can be addressed by basic neuroscience research. We
emphasize how knowledge about individual differences related to genetic and epigenetic factors in
behavioral and brain responses to stress offers the hope of a deeper understanding of PTSD.
The Relationship between Traumatic Stress
Exposure and PTSD
The theoretical link between exposure to extreme stress
and the development of PTSD (APA, 1980) provided the
rationale for early hypotheses that PTSD-related biologi-
cal alterations would be similar in direction to those ob-
served acutely in animals exposed to stressors. When
subsequent findings indicated that only a minority of
et al., 1995), an alternative hypothesis was generated
proposing that PTSD involves a failure of mechanisms
involved in recovery and restitution of physiological
homeostasis, possibly resulting from individualistic pre-
disposition (Yehuda and McFarlane, 1995). It has been
challenging to interpret the extent to which biological
alterations that are consistent with normative conse-
quences of stress exposure in PTSD reflect pathogenesis.
In this review, we suggest that the clinical syndrome of
PTSD may describe several biological phenotypes (e.g.,
some characterized by exaggerated responses, some
by inadequate recovery mechanisms) that reflect individ-
ual variation originating from pretraumatic risk factors
and review the supporting evidence for this from animal
and human studies.
Definition and Description of PTSD
PTSD can occur in persons who experience fear, help-
lessness, or horror following threat of injury or death. It is
characterized by the presence of three distinct, but co-
occurring, symptom clusters. Reexperiencing symptoms
describe spontaneous, often insuppressible intrusions of
the traumatic memory in the form of images or nightmares
that are accompanied by intense physiological distress.
Avoidance symptoms involve restricting thoughts and
distancing oneself from reminders of the event, as well
as more generalized emotional and social withdrawal.
Hyperarousal symptoms reflect more overt physiological
manifestations, such as insomnia, irritability, impaired
concentration, hypervigilance, and increased startle re-
sponses. These symptoms must be severe enough to
impair social, occupational, or interpersonal function and
co-occur for at least 1 month. The impairment from PTSD
is amplified by poor coping strategies, substance abuse,
co-occurring mood and anxiety disorders, lack of social
support, and the accelerated development of stress-
related medical conditions (Yehuda, 2002a).
Prevalence and Longitudinal Course of PTSD
Approximately 6.8% of persons in the United States
develop PTSD at some time in their lives (Kessler et al.,
2005), yet estimates of the prevalence of trauma exposure
suggests that more than 75% are exposed to at least one
traumatic event (Breslau and Kessler, 2001). Experiences
that most often give rise to PTSD include rape, assault,
and combat, whereas natural disasters or man-made ac-
cidents result in PTSD far less frequently. The symptoms
of PTSD are present in almost all people in the days and
weeks following trauma exposure and are considered re-
flections of a universal response (McFarlane, 2000). Even
among those who develop PTSD (defined by sustained
symptoms for more than 1 month following exposure),
the most common trajectory is spontaneous remission,
with the most dramatic decline in symptoms occurring
by 3 months posttrauma (Kessler et al., 1995). Thus,
PTSD is best described as a condition in which the pro-
cess of recovery from trauma is impeded. Approximately
Neuron 56, October 4, 2007 ª2007 Elsevier Inc.
5% of persons follow a different trajectory in that they do
not develop PTSD symptoms immediately (Adams and
Boscarino, 2006). Whether the underlying mechanism of
delayed PTSD is similar to that in people who fail to
recover from early trauma is unknown. Moreover, those
who recover from PTSD can often experience a recrudes-
cence, usually triggered by an adverse life event or trau-
matic reminder, implying the involvement of mechanisms
of biological sensitization in the maintenance or initiation
of PTSD symptoms.
Risk Factors for PTSD
The relative rareness of PTSD in trauma-exposed people
has prompted an interest in identifying risk factors for this
disorder (Yehuda, 2004). These include event characteris-
tics (e.g., severity of trauma) and individual differences
(e.g., preexisting traits, pre- or posttraumatic life events).
These two domains are theoretically different from one
another, but may be linked in practice. For example, the
one’s risk by providing an increased ‘‘dosage’’ of trauma.
Yet, because exposure to interpersonal violence occurs
less randomly in populations than accidents, the link be-
socioeconomic, or even genetic predictors of event expo-
sure. One study demonstrated a higher concordance
between monozygotic than dizygotic twins for exposure
to interpersonal violence as well as for PTSD, implying
shared genetic risk factors for exposure and PTSD (Stein
et al., 2002). These findings raise the possibility of distinct
such subtypes have not been formally characterized.
Other risk factors for PTSD include a family history of
psychopathology, cognitive factors (such as lower IQ),
childhood adversity, preexisting avoidant personality or
behavioral problems, and poor social support (Bromet
et al., 1998; Yehuda et al., 2006). It is not currently known
how these risk factors interact or even whether they indi-
vidually or collectively reflect a genetic diatheses or re-
sponse to an even earlier life experience. Even factors as-
sociated with stable preexposure heritable parental
characteristics may increase risk for PTSD by increasing
exposure to neglect or abuse. Information about risk fac-
tors for PTSD has also been constrained by the fact that
most factors have been identified retrospectively, based
on comparing people with and without PTSD on many pa-
rameters, some of which might have been influenced by
posttraumatic factors. Regardless of our incomplete
knowledge about the etiology of risk factors, their pres-
ence constitutes important sources of individual variation
in stress responses and may underlie different biological
phenotypes of PTSD.
Peripheral Markers of Stress and PTSD
The physiological changes associated with acute expo-
sure to a stressor have been very well characterized and
include increases in sympathetic, and decreases in para-
sympathetic, tone and the release of ACTH, cortisol, and
catecholamines from the pituitary, adrenal cortex, and
adrenal medulla, respectively. These and related physio-
logical adjustments of autonomic nervous system (ANS)
end organs (i.e., changes in heart rate, blood pressure,
respiration, skin conductance) represent adaptive re-
sponses, as they help the body accommodate to an im-
mediate demand. A critical feature of the stress response
is the autoregulation initiated by cortisol negative-feed-
back inhibition that restores stress-related reactions to
baseline after the termination of the acutestressor (Munck
et al., 1984). In contrast, initial descriptions of combat vet-
erans suggested a chronic and sustained physiological
hyperarousal (Kardiner, 1941). Subsequent studies con-
firmed that veterans with chronic PTSD showed increases
in peripheral catecholamine levels (Yehuda et al., 1998a)
and other autonomic measures compared to controls
under baseline conditions and in response to traumatic
triggers (O’Donnell et al., 2004). Insofar as the actual
stressor (e.g., combat) was no longer occurring in reality,
it was not clear why physiological homeostasis had not
been achieved in trauma survivors with PTSD.
In 1986, Mason and colleagues reported that although
combat veterans with PTSD demonstrated sustained
elevations in urinary catecholamine levels, cortisol levels
were significantly lower in veterans with PTSD than those
with other psychiatric disorders (Mason et al., 1986).
These observations were later confirmed by carefully con-
trolled studies of plasma cortisol release over the diurnal
cycle (Yehuda et al., 1996a; Bremner et al., 2007). Cortisol
levels were lower in combat veterans with PTSD than con-
trols, despite evidence for increased hypothalamic CRF
release (Yehuda et al., 1996b). Furthermore, PTSD was
associated with an enhanced cortisol negative-feedback
inhibition that seemed to result from increased respon-
siveness of GR (reviewed in Yehuda, 2002b, 2005). The
profile of neuroendocrine alterations was different from
that observed in animal models of ongoing, chronic stress
and also from observations in depressed patients, in
which elevated CRF resulted in increased cortisol levels,
decreased GRresponsiveness, and weaker cortisol nega-
tive-feedback inhibition (Holsboer, 2003). Rather, the neu-
roendocrine altertations observed in PTSD suggested an
increased sympathetic and central CRF activation in the
face of reduced cortisol signaling.
Implicit in the model of risk for PTSD is that the disorder
recovery. Though cross-sectionalstudies of chronicPTSD
could not address the mechanisms underlying the neuro-
endocrine findings, results from prospective, longitudinal
studies of trauma survivors strongly suggested that
cortisol-related alterations in PTSD reflected preexisting
vulnerability factors. In rape victims, lower plasma cortisol
levels (Resnick et al., 1995) but higher levels of plasma
MHPG were associated with the risk factor of prior trau-
matization (Yehuda etal., 1998b). Studies of motor vehicle
accidents demonstrated that persons who subsequently
Neuron 56, October 4, 2007 ª2007 Elsevier Inc.
developed PTSD had lower cortisol levels within hours
after the accident than those who did not (Yehuda et al.,
1998c; Delahanty et al., 2003). In parallel studies of per-
suppression following DEX were noted in the adult off-
spring of Holocaust survivors with, compared to those
without, parental PTSD (Yehuda et al., 2007a, 2007b). Pa-
rental PTSD is a risk factor for PTSD because it produces
attributable to higher rates of trauma exposure in offspring
(Yehuda et al., 2001). Lower cortisol levels were also ob-
served in the infant offspring of mothers who developed
PTSD following exposure, while pregnant, to the WTC
attacks on 9/11, compared to those of mothers who did
not develop PTSD (Yehuda et al., 2005). In both of these
‘‘at risk’’ cohorts, neuroendocrine measures associated
with severity of parental PTSD symptoms. This was true
in the adult offspring of Holocaust survivors even after
controlling for mood and anxiety in the offspring. That
low cortisol is associated with PTSD risk may also explain
why not all persons with PTSD show identical neuroendo-
SNS response to stress, reduced cortisol signaling could
impede the reinstatement of physiologic homeostasis.
Becausethe releaseof adrenalinefacilitates consolidation
of the threat memory (McGaugh and Roozendaal, 2002),
failure to contain the SNS response might lead to more
strongly encoded, hence more subjectively distressing,
memories of the event. If low cortisol levels represent
a preexisting characteristic, reenforced by ‘‘overconsoli-
dation’’ at the time of the trauma, then failing to properly
contain the SNS response to traumatic reminders could
perpetuate the intrusive and hyperarousal symptoms of
PTSD, leading to the elaboration of avoidance symptoms
that commonly occurs in the disorder.
Searching for Brain Mechanisms of PTSD: Early
Focus on the Hippocampus
The hippocampus was examined as a region of central
importance in PTSD due to its prominent role in both the
neuroendocrine stress response and memory alterations
(McEwen et al., 1992), similar to those that have been ob-
served in PTSD (Golier et al., 2006). Many studies have
demonstrated smaller hippocampal volumes in PTSD
(for review see Rauch et al., 2006; Bremner, 2007). How-
ever, it has been difficult to attribute these findings to
glucocorticoid toxicity resulting from extreme trauma
exposure, oreventrauma exposure itself,ascortisollevels
aftermath of trauma nor demonstrated in association with
hippocampal alterations (Neylan et al., 2003; Yehuda
et al., 2006). Furthermore, prospective, longitudinal stud-
ies failed to show change in hippocampal volume over
time in persons followed in the acute aftermath of trauma
and longitudinally (Bonne et al., 2001). This led investiga-
tors to consider that smaller hippocampal volume repre-
sented a preexisting marker of vulnerability to PTSD.
The best evidence for this possibility is the strong asso-
ciation between hippocampal volume and identical twins
discordant for Vietnam combat exposure (Gilbertson
et al., 2002; Pitman et al., 2006). The risk hypothesis was
also supported by the demonstration of smaller hippo-
campal volume in veterans who developed PTSD follow-
ing their first traumatic exposure compared to those who
only developed PTSD in response to a subsequent event
(Yehuda et al., 2006). When all PTSD subjects were com-
pared to similarly exposed veterans without PTSD, no
changes in hippocampal volume were observed in the
PTSD group. Smaller hippocampal volume is correlated
with other constitutional factors, such as low IQ (Gurvits
et al., 1996; Gilbertson et al., 2001), that have also been
associated with increasing risk for the development of
PTSD in combat veterans (Macklin et al., 1998), but not
necessarily in other traumatized groups. If so, this would
explain why Holocaust survivors, who were certainly ex-
posed to severe and chronic trauma but had different
risk factors for their traumatic exposures, did not show
smaller hippocampal volumes relative to nonexposed
subjects (Golier et al., 2005).
If reduced hippocampal volume is related to cognitive
capacity or even cognitive deficits associated with PTSD
(Vasterling et al., 2001), it might confer risk by making it
more difficult for persons to contextualize and reinterpret
the experience of trauma in a way that can facilitate
recovery. A more limited cognitive flexibility could impede
posttraumatic recovery even in the absence of prior ex-
perience. Risk factors associated with reduced cortisol
signaling may be distinct from these (Yehuda and Flory,
2007), as they might result from early experience and con-
fer risk by interfering with the neurochemical response to
iological homeostasis. Yet, persons with both risk factors
may be even more vulnerable to PTSD than those with
Searching for Brain Mechanisms of PTSD: Fear
Conditioning and the Amygdala
As noted above, one of the limitations of stress theory was
that it could not explain the persistence of biological and
psychological fear responses in PTSD well after the end
of trauma. One idea that arose was that PTSD might re-
flect strong associative learning akin to Pavlovian fear
conditioning (e.g., Pitman, 1989; Charney and Deutch,
1996). In fear conditioning, a neutral conditioned stimulus
being associated with an unconditioned stressful stimulus
responses (URs) (e.g., Bolles and Fanselow, 1980; Le-
Doux, 1996). Translating to PTSD, individuals initially react
to a traumatic event (US) with arousal and fear (UCR) and
then continue to show arousal (CR) when confronted with
trauma-related cues (CS), long after the trauma.
Part of the attraction of fear conditioning was that much
was concurrently being learned about the neurobiology
of this behavioral paradigm from animal studies (LeDoux,
Neuron 56, October 4, 2007 ª2007 Elsevier Inc.
1996; Maren, 2001). In brief, fear conditioning occurs as
a result of the convergence of information from the CS
and US pathways in the lateral nucleus of the amygdala
(LA), where synaptic plasticity occurs. When the individual
to the central amygdala (directly and through indirect
nects to hypothalamic and brainstem areas that control
that help the organism cope with the threat. Threat pro-
cessing by the amygdala is the key step in the circuitry
through which catecholamines, ACTH, and cortisol are
released into the circulation.
In the 1990s, studies of the human brain began to show
a key role for the amygdala in fear conditioning as well
(reviewed in Phelps, 2006; Buchel and Dolan, 2000).
Thus, damage to the amygdala in humans prevented
fear conditioning,andexposuretoconditionedfear stimuli
led to functional activation of the amygdala, as measured
by fMRI. The amygdala is also activated by unlearned
tioned threats, it is not necessary to consciously process
the stimulus in order to react to it with physiological
responses (Dolan and Vuilleumier, 2003).
garding fear neurocircuitry, studies of PTSD patients have
generally demonstrated increased activation in the amyg-
dala compared to controls, including nontrauma controls
and trauma-exposed people who did not develop PTSD,
in response to threat stimuli (Rauch et al., 2006; Protopo-
pescu et al., 2005; Bremner, 2007). The fact that in healthy
persons threats elicit physiological responses when pro-
cessed unconsciously makes persons with a hyperactive
amygdala, as in PTSD, vulnerable to threats in ways that
are difficult for them to protect against. It is not known,
at this point, whether a hyperactive amygdala response
to threats preexisted and predisposed the development
of PTSD or whether it was a consequence of the disorder.
Extinction and PTSD
Conditioned fear responses can be reduced or extin-
guished by repeatedly presenting the CS without the US.
Extinction is an active process, often involving new learn-
ing (Myers and Davis, 2007; Sotres-Bayon et al., 2004). In
rodents, damage to the medial prefrontal cortex (mPFC)
interferes with extinction, as does pharmacological dis-
ruption of memory storage in the mPFC or amygdala
(Quirk and Beer, 2006; Sotres-Bayon et al., 2007; Myers
and Davis, 2007). Fear extinction in the human brain has
also been recently shown to involve regions of mPFC
and the amygdala (Phelps and LeDoux, 2005), and the
size of mPFC areas is related to extinction facility in
humans (Milad et al., 2005). Collectively, the animal and
human studies suggest that fear disorders may be related
to a malfunction of the mPFC that makes it difficult to
extinguish or otherwise regulate fears that have been
acquired (Morgan et al., 1993; Morgan and LeDoux,
1995; LeDoux, 1996; Quirk and Beer, 2006). These rela-
tionships are schematically portrayed in Figure 1. Recent
mPFC and amygdala circuits, causing dendritic hypertro-
phy in mPFC (Radley et al., 2004) and hypertrophy in
amygdala (Vyas et al., 2002). Thus, chronic exposures,
in particularly, can lead to both a hyperactive amygdala-
mediated fear response to threats and a weakened ability
of mPFC to regulate these responses. Alternatively, how-
ever, persons with a hyperactive amygdala may be more
likely to process neutral, unconscious, or implicit threats,
which would serve to even further weaken the ability of
mPFC to regulate these responses. Indeed, even healthy
persons elicit physiological responses to threatening
stimuli that are processed unconsciously. As discussed
further below, it is not known, at this point, whether a
hyperactive amygdala response to threats preexisted
and predisposed the development of PTSD, or whether
it was a consequence of the disorder.
As in rodents exposed to reminders of fear-producing
stimuli, humans with PTSD show an attenuated activation
Figure 1. The Amygdala’s Ability to
Control Fear Responses to Threatening
Stimuli Is Regulated by the
Hippocampus and Medial Prefrontal
The hippocampus adds contextual regulation,
allowing you to distinguish the difference in
threat level posed by a snake in the woods
versus in a zoo. The medial prefrontal cortex
regulates the degree to which the amygdala
expresses fear responses, including the regu-
lation that occurs during extinction of fear.
Alterations in information processing by these
three areas might account for symptoms in
anxiety disorders. However, possibly of greater
relevance are the sources of individual varia-
tion, either constitutional or environmental,
that can affect one of these target areas and
lead to different phenotypes with respect to
fear-related behavior or biological responses.
Neuron 56, October 4, 2007 ª2007 Elsevier Inc.
of mPFC (especially the subgenual ACC) in response to
personalized trauma scripts or combat sounds (Bremner
et al., 1999; Shin et al., 1999) and show reduced activa-
tion, compared to controls, to more generalized negative
stimuli in this region (Lanius et al., 2003). Particularly
important is that PTSD patients also show a negative
correlation between amygdala and mPFC activation in
response to fearful versus happy faces, suggesting a dis-
connect in the normal modulation of amygdala by mPFC
(Shin et al., 2005). Thus, in PTSD, there is an increased
activation of the amygdala in response to fear-related
triggers that is accompanied by an abnormally low
response in the brain regions that generally inhibit the
The above findings from rodents and humans are
consistent in demonstrating alterations in brain regions
thought to be important to fear acquisition and recovery.
Because PTSD is a clinical syndrome in which an initial
fear response does not abate, the neuroimaging findings
showing exaggerated amygdala responses recapitulate,
but do not explain, the nature of the brain disturbance in
PTSD. Indeed, as with stress findings, a limitation of the
standard fear conditioning model, and its emphasis on
interactions between the amydgala and mPFC, is that it
does not address why only some persons exposed to
fear develop the abnormality. A modified version of fear
conditioning focused on phenotypic differences in fear in
a population of subjects (rats or humans) offers a solution,
as described below.
Is Altered Fear Processing and Regulation
a Consequence of Trauma or Another Risk Factor
Oneofthechallengesoftranslating information aboutnor-
mal responses to fear and mental disorders in which the
emotion of fear may be expressed is that it becomes too
difficult to determine whether a noted biological change
is, in fact, an aspect of disease physiology. A hyperactive
amygdala or hypoactive mPFC may be part of a patho-
physiological process that causes or sustains PTSD
symptoms (e.g., involving difficulty in mobilizing brain re-
gions that dampen the fear response) or an adaptive one
(e.g., ‘‘permitting’’ the amygdala to attend to the stimulus
as dangerous because, previously, this was a dangerous
stimulus). The cross-sectional nature of most studies
does not allow a differentiation between whether findings
reflect a response to the focal trauma that initially pro-
duced PTSD, an ongoing adaptation to chronic symp-
toms, or a predisposing risk factor.
The fact that similar observations regarding the exag-
gerated amygdala activity have also been made in other
anxiety disorders, such as panic, specific phobia, and
generalized anxiety disorder (Rauch et al., 2003), implies
that enhanced activation of the amygdala in response to
provocation may be a general consequence of experienc-
ing fear or anxiety regardless of whether the anxiety is
anticipatory, based on a real threat, or the product of a
Particularly important will be studies that compare acti-
vation patterns in patients with different disorders using
the same behavioral paradigms. For example, exagger-
ated fear in PTSD and panic disorder may both involve
heightened amygdala activity and weakened mPFC regu-
lation, but different responses of the hippocampus or
other areas (decreased hippocampal function in PTSD
may make PTSD patients insensitive to the context in
which fear-arousing triggers occur, while heightened hip-
pocampal function in panic disorder may render them
of potentially threatening situations). This notion is based
on animal studies showing that the amygdala is regulated
by the context of a fear-related stimulus (LeDoux, 1996;
Maren, 2001), presumably via projections from the hippo-
campal formation. A failure of hippocampal contextual
processing in distinguishing safe from dangerous con-
texts could in part explain why patients with PTSD have
exaggerated responses to trauma-related triggers. Al-
though the degree to which such deficits in hippocampal
processing are linked to hippocampal volume and/or
baseline cognitive abilities is unknown, such mediation is
certainly possible. Moreover, hippocampally mediated
actions relevant to cognitive flexibility (and specifically,
the ability to form new associations) may be relevant to re-
covery from trauma and may similarly provide regulatory
influences that help contain excessive amygdala activa-
tion. Similarly, preexisting neuroendocrine alterations
may underlie differences in responses to traumatic stimuli
An important point is that the empirical finding of low
esis that elevated stress-induced glucocorticoid release
mediates the impairment of mPFC and enhancement of
amygdala function that occur in stress and that are
believed to be mediated by glucocorticoids. At the same
time, reduced exposure to glucocorticoids in the amyg-
dala in PTSD could explain in part the failure to adapt to
trauma, because glucocorticoid action in the amygdala
promotes adaptive cognitive functions such as arousal,
attention, and memory formation, thus enhancing survival
in threatening situations (Bohus and de Kloet, 1981;
McGaugh and Roozendaal, 2002).
Another important issue is the possibility that increased
or hippocampus themselves represent preexisting risk
factors. This possibility is supported by findings of individ-
ual variation in the activation of the amygdala and mPFC
negative emotion in the laboratory, by findings showing
that the size of mPFC is related to fear extinction, and to
the finding that natural variation in monoamine transporter
gene variants is related to fMRI-measured amygdala
responses to threatening faces (Hariri et al., 2003). Delib-
erate emotion regulation skill is a measurable trait associ-
ated with an ability to manipulate emotional responses
experience (Urry et al., 2006). Through instruction,
Neuron 56, October 4, 2007 ª2007 Elsevier Inc.
persons can reappraise emotional situations and increase
activity in prefrontal areas and decreased activity in the
amygdala (Phelps, 2006; Ochsner and Gross, 2005).
Thus, persons who have difficulty in emotional regulation
may be particularly vulnerable in highly stressful situations
and may acquire stronger fear responses or be impaired
at recovering from fear through normal homeostatic pro-
cesses, implicit regulation (as in extinction), or explicit
regulation (as in reappraisal).
Adapting Basic Neuroscience Approaches to
Achieve Maximal Translational Utility: Searching
for Phenotypic Differences Rather than Typical
Responses to Stress
In addition to the fear conditioning model already dis-
cussed, a number of animal models of PTSD, based on
responses to threats and other stressors, have been pro-
posed (Miller andMcEwen, 2006; Cohenand Zohar,2004;
Adamec et al., 2006; Mechiel Korte and De Boer, 2003;
Rau et al., 2005; Rittenhouse et al., 1992; Richter-Levin,
1998; Cohen et al., 2004). Like fear conditioning, these
are potentially very useful because of their ability to iden-
tify brain regions that may underpin PTSD symptoms.
However, for the most part, animal models, including
fear conditioning models, have examined the effects of
stressors on ‘‘normal’’ animals. As noted above, a major
limitation, and translational gap, in the basic science
approaches is that they have emphasized the normative
biological consequences of trauma exposure. Because
a critical question for PTSD is why only some people de-
velop the disorder, animal models that examine individual
differences, or phenotypic differences between sub-
groups in the population, in response to stressors might
be especially informative.
Like humans, outbred rat strains exhibit stable individ-
ual differences in a wide range of emotional behaviors
(Garcia and Armario, 2001), including fear or anxiety-
related responses (Cools et al., 1990; Cure and Rolinat,
1992). Studies of fear conditioning could also delineate
the range of responses to a uniform provocation rather
than collect information about the ‘‘prototypic’’ animal or
human by combining observations from groups assumed
to be homogeneous.
There are only a few examples of attempts to develop
animal models for PTSD based on the premise of examin-
ing individual differences to a uniform provocation that
yields long-term biobehavioral consequences analogous
to those in PTSD, though criteria for establishing animal
and Antelman, 1993). One example involves exposure of
rats to the scent of a predator (cat) and, subsequently, of
reminders (Cohen et al., 2006a). This study established
behavioral cut-off criteria based on both fearful/avoidant
behavior and hypervigilant/hyperalert responses, paralle-
ling aspects of PTSD symptoms in humans, allowing for
group differencesto the same provocation. This model es-
tablished proof of concept for several biological correlates
of PTSD (Cohen and Zohar, 2004) and also demonstrated
stress paradigm, associated with low cortisol responses
exogenous administration of cortisol to Lewis rats before
applying the stressor decreased the behavioral conse-
quences of fear conditioning (Matar et al., 2006). Further-
more, by manipulating conditions such as timing of the
stressors as well as features of early environment prior
to stress, this model yielded information demonstrating
able to adverse effects of fear conditioning, thus providing
a platform by which to compare effects of ‘‘early’’ versus
‘‘later’’ exposures in rats (Cohen et al., 2007a).
A slightly different approach attempted to distinguish
between two fundamentally important memory processes
strates (Siegmund and Wotjak, 2006). This was achieved
byexposing animals to a single stressorbut distinguishing
between behavioral responses in response to contextual
reminders (associative fear, relevant to reexperiencing
and avoidance) and sensitization (nonassociative fear, rel-
have yielded face validity, in that the behavioral ‘‘symp-
toms’’ of PTSD can be produced in a dose-dependent
manner by even a single exposure, persist for a consider-
able length of time, include behaviors from a wide range
of domains associated with PTSD, and show individual
variation. The model has predictive validity in that the
core features of the phenotype respond to common phar-
macological interventions, such as SSRI, and also utility,
because the stressor can be varied in intensity without
inducing habituation (Siegmund and Wotjak, 2007).
The two examples above are singled out because of the
care taken to model critical aspects of the disorder. How-
ever, little is known about the underlying brain mecha-
nisms involved in these animal models. In contrast, recent
studies of fear conditioning, especially cued fear condi-
tioning (as opposed to contextual fear conditioning, which
is less understood neurobiologically), offer a model in
which phenotypic differences in fear can be related to
specific brain circuits and cellular, synaptic, molecular,
and genetic mechanisms. Bush et al. (2007) examined
conditioned fear reactivity in a group of outbred rats (Fig-
ure 2). When tested 48 hr after conditioning, individual dif-
distribution. A typical study would focus on the middle of
this distribution (the average response). By comparing an-
imals in the middle, upper, and lower ends of the distribu-
tion, it is possible to study not just the average fear re-
sponse, but also individuals that are highly reactive
(possibly PTSD prone) and weakly reactive (possibly resil-
ient). However, as noted above, PTSD might reflect a fail-
ure to recover from normal fear learning. An additional
study therefore focused on rats that were highly reactive
and then subjected to extinction. Two additional pheno-
types emerged, one that recovered (extinguished fear)
Neuron 56, October 4, 2007 ª2007 Elsevier Inc.
quickly and one that was significantly slower to recover
of PTSD. Thus, examination of phenotypic variation in the
mation to PTSD, as it addresses the essential question of
why some persons demonstrate intense and prolonged
reactions to fear-provoking events (i.e., as in PTSD), while
others show more attenuated responses or easily recover
With distinct phenotypes established, gene function in
relevant brain circuits can be examined with microarray
analysis in order to identify relevant molecular processes
at least some differences in fear reactivity are heritable is
supported by observations in both people (Kagan, 1994;
hand, recent studies in monkeys using stress inoculation
training have suggested that differences in amygdala/
mPFC correlations can be modified by early environment
(Parker et al., 2005). The possibility offered by capturing
constitutional and acquired factors that explain variation
in amygdala and mPFC responses, and testing their rele-
vance to PTSD, represents important directions for the fu-
ture. As argued above, the fear conditioning model in ro-
dents offers many advantages for such work.
Genetic and Nongenetic Contributions
to PTSD Risk
The mostcompelling evidencefor anassociation between
genetic factors and PTSD has been findings of an in-
creased prevalence of PTSD among twinswho arediscor-
dant with respect to traumatic environmental exposures
(Stein et al., 2002; True et al., 1993; Koenen et al., 2002).
However, to date, very few genes for PTSD have been
identified. Significant associations were found with a vari-
able number tandem repeat (VNTR) polymorphism in an
untranslated region of the dopamine (DA) transporter
gene (Segman and Shalev, 2003) in 93 PTSD patients
ation was found between two GR polymorphisms (N363S
and BclI) and the diagnosis of PTSD in 118 PTSD patients
compared with 42 unaffected control subjects, though
PTSD patients homozygous for the BclI GG genotype
tended to show enhanced GR and displayed more severe
PTSD symptoms (Bachmann et al., 2005).
The limited number of gene-related findings in PTSD
may reflect the complexity of executing research in this
area. Alternatively, it may suggest that the enduring
pretraumatic changes are not associated with specific
genetic polymorphisms, but rather with gene-related
differences resulting from epigenetic alterations. Epi-
genetics refers to a transgenerationally transmissible
Figure 2. Phenotypic Differences in Fear
(A) Frequency histogram showing the distribu-
tion of fear reactivity test scores (n = 51). Data
are from the average percent freezing re-
sponses to the CS-alone presentations across
(B) Scatterplot showing the correlation be-
tweenaveragefreezing scores obtained during
fear conditioning (x axis, criterion data) and av-
erage freezing scores obtained during testing
(y axis, test data). Circled data points indicate
rats that formed the high and low fear reactivity
groups. The trend line indicates the correlation
between the criterion and test scores.
(C) Line graph shows that the phenotypic dif-
ferences in freezing (mean ± SEM) identified
during conditioning are stable across the con-
ditioning and two testing sessions (* indicates
significant difference between high/low fear
reactivity phenotypes, p < 0.01).
Reproduced from data in Bush et al. (2007).
Neuron 56, October 4, 2007 ª2007 Elsevier Inc.
functional change in the genome that can be altered by
environmental events and does not involve an alteration
of sequence (Novik et al., 2002). Such mechanisms offer
the possibility of defining concrete molecular pathways
by which environmental risk factors might directly alter
the expression of a gene, thus forming a basis for individ-
ual differences in a function relating to the gene and, per-
haps, vulnerability to disorder (Sutherland and Costa,
2003). These are likely to be relevant to PTSD and might
specifically explain the origin of glucocorticoid-related al-
terations associated with PTSD and PTSD risk.
Indeed, DNA methylation has been demonstrated as
a mechanism in programming the activity of genes regu-
lating HPA activity by early life events (i.e., differences in
maternal care) (Weaver et al., 2004) paralleling observa-
tions that early life events are associated both with the
development of PTSD (Nishith et al., 2000; Breslau et al.,
1999; Koenen et al., 2007) and the HPA axis alterations
(Yehuda, 2002b) described in this condition. Such
changes in the rat pups result in permanent changes in
hippocampal GR expression and HPA function (Francis
et al., 1999) and provide a clear molecular link between
early environment and gene expression and function.
The alterations observed are in the same direction as
those described in PTSD (i.e., increased GR sensitivity,
enhanced cortisol to DEX, lower cortisol), offering proof
of principle that environmental exposures can result in
in PTSD would explain the relationship between such
alterations and pretrauma risk and may be particularly
relevant to transgenerational transmission of risk from
mothers to offspring.
Other Challenges for the Translational Research
Also important to translational studies of PTSD is the use
of a developmental neurobiological approach, spanning
across the entire course of the illness. Symptom severity
in PTSD can wax and wane over several decades. Biolog-
ical alterations reflecting risk rather than pathophysiology
may not account for this phenomenon. On the other hand,
even putative risk factors such as glucocorticoid respon-
siveness and hippocampal volume show changes in re-
tion of illness, comorbidity, and aging. Thus, it is important
to understand whether risk factors influence, or are influ-
enced by, other parameters associated with PTSD.
A recent longitudinal studies of PTSD in aging subjects
demonstrated that cortisol levels in trauma survivors may
influence the longitudinal course of PTSD and/or inter-
actions between PTSD and age-related neuroendocrine
alterations (Yehuda et al., 2007c). At a 10 year follow-up,
there was a general decline in cortisol levels in Holocaust
survivors who maintained their diagnostic status or devel-
oped PTSD, but an increase in those who demonstrated
remission. Cortisol levels at the initial assessment
Figure 3. Phenotypic Differences in Fear
(A) Frequency histogram showing the distri-
bution of fear recovery test scores (n = 51).
Data are from the average percent freezing
responses to the first two CS-alone presenta-
tions during the extinction retrieval test.
(B) Scatterplot showing the correlation be-
tween averagefreezing scores obtained during
the early phase (trials 5 to 7) of fear extinction
training (x axis, criterion data) and average
freezing scores obtained during the first two
trials of the extinction test (y axis, test data).
Circled data points indicate rats that formed
the fast and slow fear reactivity groups. The
trend line indicates the correlation between
the criterion and test scores.
(C) Line graph shows that phenotypic differ-
ences in rate of fear recovery during extinction
training (mean ±SEM)predict thesustainability
of fear recovery in the extinction retrieval test.
Both groups showed comparable levels of
fear reactivity prior to extinction training and
comparable levels of fear reduction by the
end of extinction training (*indicates significant
difference between fast/slow fear recovery
phenotypes, p < 0.05).
Reproduced from data in Bush et al. (2007).
Neuron 56, October 4, 2007 ª2007 Elsevier Inc.
predicted remission or relapse, though they themselves
showed change over time (Yehuda et al., 2007c).
That risk factors are not immutable but may change
over time should be carefully considered in interpreting
biological studies in PTSD. For example, smaller hippo-
campal volumes have been noted more often in younger
cohorts with PTSD. However, two investigations of older
combat veterans failed to observe this association (Ye-
huda et al., 2006; Freeman et al., 2006). Because normal
aging is associated with hippocampal atrophy, it is possi-
ble that smaller hippocampal volumes in PTSD may be
particularly evident at a time at which healthy subjects
are not manifesting atrophy in this region.
Basic neuroscience research can help anticipate the
relevant systems that should be investigated using a de-
velopmental perspective. If biological alterations reflect-
ing a superimposition of PTSD and aging diverge from
normal patterns associated with either PTSD or aging,
such as they do with respect to both cortisol and hippo-
campal-related alterations, this information will provide
important insights for interpreting variables previously as-
sociated with risk, pathophysiology of PTSD, and chronic
effects of trauma exposure.
There have been several recent biological approaches to
PTSD prevention based on clinical neuroscience data.
The first builds on findings of lower cortisol levels in
PTSD asexerting permissive effects to facilitate increased
catecholamines in the immediate aftermath of trauma.
Accordingly, cortisone has been administered to patients
immediately after experiencing acute trauma (usually as-
sociated with critical illness) in several randomized clinical
trials. Cortisol treatment demonstrated specificity for the
prevention of recurring traumatic memories (Schelling
et al., 2006; de Quervain, 2006). Animal studies have
also confirmed the potential utility of cortisol-related treat-
ments in preventing ‘‘PTSD-related’’ consequences of
fear conditioning (Cohen et al., 2006c). In a recently devel-
oped paradigm in which rats are exposed to single-
prolonged stress, administration of a GR antagonist prior
to SPS exposure prevented the normally observed poten-
tiation of fear conditioning in the amygdala, impairment of
LTP in the hippocampus, enhanced inhibition of the HPA
axis, and increased expression of GR in the hippocampus
with this treatment (Kohda et al., 2007). Inactivating GR
receptors in the amygdala postretrieval similarly blocked
the ‘‘traumatic memory’’ (i.e., behavioral response to con-
ditioning) (Tronel and Alberini, 2007). Given the role of GR
pathophysiology, future approaches may include the use
of GR blockers, such as mifipristone, in the treatment of
A more direct containment of SNS in the immediate
aftermath of trauma can be accomplished using catechol-
aminergic drugs such as propranolol and guanfacine.
However, neither has been shown to prevent PTSD (Ney-
lan et al., 2006; Pitman et al., 2002; Vaiva et al., 2003). In
one randomized trial, propranolol impeded the develop-
ment of PTSD-related psychophysiological alterations
(Pitman et al., 2002). These approaches are promising,
yet the disconnection between what would be predicted
based on ameliorative effects on fear conditioning when
exposing animals to such drugs (Shinba et al., 2001;
Levy et al., 2001) and clinical trials serves as a cautionary
note for the increased complexity of humans. Ultimate
PTSD prevention with biological mechanisms may require
identifying a broader range of factors, including genetic or
epigenetic modifications that underlie failure of reinstate-
ment of physiological homeostasis.
On the basis of findings of SNS alterations in PTSD, it
might be predicted that catecholaminergic drugs would
be effective in treating this condition. Although findings
for a2-adrenergic antagonists have been mixed, treat-
ment with the a1-adrenergic antagonist prasozin has
been shown to be extremely effective, particularly in
reducing nightmares in PTSD (Raskind et al., 2003), with
effects confirmed in animal models (Manion et al., 2007).
More recently, efficacy has been achieved for chronic
PTSD symptoms using glucocorticoids (Aerni et al.,
2004). However, the rationale for positive glucocorticoid
effects in chronic PTSD appears to be more related to
glucocorticoid-induced inhibition of memory retrieval
rather than its role in containment of the stress responses
(de Quervain, 2006).
Advances in basic and clinical neuroscience studies of
fear may in the future prove to be relevant to providing
strategies for supplementing psychotherapeutic ap-
proaches. One common approach to the clinical treat-
tion through cognitive behavioral therapy. However, this
approach is often difficult to implement due to high
drop-out rates and the need for good adherence among
extinction is known to exhibit spontaneous recovery,
which means that the fear simply comes back (Myers
and Davis, 2007). If the extinction process could be facil-
tion of a drug in connection with the therapeutic session,
the success rate might increase. One possible agent is
the partial NMDA agonist d-cycloserine (DCS). Adminis-
tration of this drug facilitated extinction training in rats
and then produced a more rapid extinction of phobic
fear in anxiety disordered patients (Ressler et al., 2004).
A pilot controlled trial found that DCS had some efficacy
for the treatment of PTSD (Heresco-Levy et al., 2002).
Though promising, more work is needed to evaluate how
DCS might be best used in PTSD and how effective it
A different approach emerging from animal studies in-
volves the blockade of memory reconsolidation (Nader
et al., 2000; Nader and Wang, 2006). Although much of
the initial work in animals involved the use of protein
synthesis blockers, later studies in rats showed that
the b-adrenergic antagonist propranolol was also effec-
tive in blocking memory reconsolidation when given
Neuron 56, October 4, 2007 ª2007 Elsevier Inc.
systemically or directly in the amygdala (Debiec and
olol is believed to mimic the effects of protein synthesis
inhibitors by negatively modulating, via protein kinases,
to be somewhat effective in preventing the development
of PTSD (Pitman et al., 2002), the reconsolidation ap-
proach is potentially useful in chronic PTSD because it
only depends on pairing of the drug with the retrieval of
traumatic memory. A pilot study in PTSD patients demon-
strated some efficacy (Brunet et al., 2007).
Advances in understanding whether alterations in fear
conditioning in PTSD are related to preexisting traits
(e.g., ability to activate mPFC or inhibit amygdala in re-
sponse to negative stimuli) or state may one day provide
an elegant tool for helping to predict persons who are
most likely to benefit from cognitive behavioral therapies
and even persons who might particularly benefit from
pharmacological augmentation of psychotherapy. Possi-
bly, patients who respond to cognitive behavioral therapy
possess an enhanced ability to modulate activity in rele-
vant brain regions and fear circuits when exposed to tasks
involving emotion regulation or cognitive restructuring.
Alternatively, studying fear circuits following successful
treatment may help confirm that the manipulation of
such circuits is the active ingredient of such therapies,
designed to promote extinction.
Future Horizons: Delineating the Contribution
of Resilience to Individual Differences Associated
with Risk and PTSD
The literature regarding the homogeneous effects of
stressful or fear-provoking stimuli has allowed the field
of clinical neuroscience to define relevant circuits or sys-
tems that might be involved in PTSD, but at the same
time has fallen short of explaining the mechanisms
through which stress exposure directly contributes to
PTSD. This is because classic studies using animal
models of stress or fear have not explained the variation
in phenotypes that might explain why some people de-
velop PTSD while others do not. We have suggested
above that identification of neurobiological correlates of
PTSD requires an extension of prior translational ap-
proaches aimed at examining the contribution of stress
exposure to the development of this condition, so as to
focus on biological correlates of individual differences.
For simplicity, we have emphasized the distinction be-
consists of a diverse group who, even though unaffected
Indeed, the complexity of the relationship between stress
exposure and any psychopathology can be further illus-
trated by considering that not only does exposure to
stressful life events sometimes fail to contribute to psy-
chopathology, butthatdepending onthe timingandinten-
sity of the exposure(s) and/or other individual differences,
it may actually be protective or ‘‘inoculating.’’ An impor-
tant direction in translational neuroscience studies con-
cerns an examination of the bidirectional effects of stress,
with those at the extreme ends showing not only a lack of
a poor outcome, but the presence of a beneficial one.
Examples of animal models of stress that might be partic-
ularly helpful in this regard include studies of stress-
inoculated animals (Parker et al., 2005) that are generated
through the presentation of early maternal separations in
early development and animals exposed to maternal
If the effects of stress range beyond detrimental to neu-
tral and extend to include beneficial outcomes, this would
serve as a basis for understanding both resilience and
vulnerability, there might also be distinct, countervailing
resilience-related traits that may either contribute to re-
covery from stress, being resistant to stress effects, or
even using stressful experiences as a means of achieving
mastery. What remains unknown is whether the resilient
phenotype fails to demonstrate, or shows directionally
different changes in the same biological systems that
are altered in PTSD, or rather, manifests biological attri-
butes in different systems that serve to regulate, counter-
that impede recovery from trauma. The answers to these
questions will undoubtedly yield important insights into
prophylaxis and treatment of PTSD. Exploiting new devel-
opments in techniques of genotyping, microarray analy-
sis, methylation, molecular biology, and functional neuro-
imaging will allow for an expansion of relevant biological
markers. The opportunity to combine these technological
advancements with an approach aimed at elucidating
individual differences represents an exciting frontier for
translational studies of both stress and PTSD.
This work was supported by NIH (R01 MH064675-02, R01 MH64104-
01, R56MH077321), Department of Defense Grant W18XWH-06-2-
0032, and VA Merit funding (R.Y.) and by NIH (P50MH58911, R37
MH38774, R01 MH46516, K05 MH067048) (J.L.). The authors wish
to thank Dr. Julia Golier for reading several drafts of this manuscript.
We also thank Janelle Wohltmann for her assistance in the literature
review and manuscript preparation.
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