Every day, parents observe the growing behavioural
repertoires of their infants and young children, and
the corresponding changes in cognitive and emotional
functions. These changes are thought to relate to normal
brain development, particularly the development of the
hippocampus, the amygdala and the frontal lobes, and
the complex circuitry that connects these brain regions.
At the other end of the age spectrum, we observe changes
in cognition that accompany aging in our parents. These
changes are related to both normal and pathological
brain processes associated with aging.
Studies in animals and humans have shown that
during both early childhood and old age the brain
is particularly sensitive to stress, probably because it
undergoes such important changes during these periods.
Furthermore, research now relates exposure to early-life
stress with increased reactivity to stress and cognitive
deficits in adulthood, indicating that the effects of stress
at different periods of life interact.
Stress triggers the activation of the hypothalamus-
pituitary-adrenal (HPA) axis, culminating in the pro-
duction of glucocorticoids by the adrenals (FIG. 1).
Receptors for these steroids are expressed throughout
the brain; they can act as transcription factors and so
regulate gene expression. Thus, glucocorticoids can have
potentially long-lasting effects on the functioning of the
brain regions that regulate their release.
This Review describes the effects of stress during pre-
natal life, infancy, adolescence, adulthood and old age on
the brain, behaviour and cognition, using data from ani-
mal (BOX 1) and human studies. Here, we propose a model
that integrates the effects of stress across the lifespan,
along with future directions for stress research.
Animal studies. In animals, exposure to stress early in
life has ‘programming’ effects on the HPA axis and the
brain1. A single or repeated exposure of a pregnant
female to stress2 or to glucocorticoids3 increases mater-
nal glucocorticoid secretion. A portion of these gluco-
corticoids passes through the placenta to reach the fetus,
increasing fetal HPA axis activity and modifying brain
development4. In rats prenatal stress leads to long-term
increases in HPA axis activity5. Controlling glucocor-
ticoid levels in stressed dams by adrenalectomy and
hormone replacement prevents these effects, indicating
that elevations in maternal glucocorticoids mediate the
prenatal programming of the HPA axis6.
Glucocorticoids are important for normal brain
maturation: they initiate terminal maturation, remodel
axons and dendrites and affect cell survival7; both sup-
pressed and elevated glucocorticoid levels impair brain
development and functioning. For example, admin-
istration of synthetic glucocorticoids to pregnant rats
delays the maturation of neurons, myelination, glia
and vasculature in the offspring, significantly altering
neuronal structure and synapse formation and inhibit-
ing neurogenesis4. Furthermore, juvenile and adult rats
exposed to prenatal stress have decreased numbers of
mineralocorticoid receptors (MRs) and glucocorticoid recep-
tors (GRs) in the hippocampus, possibly because of epi-
genetic effects on gene transcription8. The hippocampus
*Université de Montréal,
Mental Health Research
Centre, Fernand Seguin
Hôpital Louis-H Lafontaine,
Montreal, Quebec, H1N 3V2,
Rockefeller University, 1230
York Avenue, New York,
New York 10021, USA.
§Institute of Child
Development, University of
Minnesota 55455, USA.
||Department of Psychiatry,
Emory University, 101
Woodruff Circle, Suite 4000,
Atlanta, Georgia 30307, USA.
Correspondence to S.J.L.
Published online 29 April 2009
When an environmental factor
that acts during a sensitive
developmental period affects
the structure and function of
tissues, leading to effects that
persist throughout life.
Effects of stress throughout
the lifespan on the brain,
behaviour and cognition
Sonia J. Lupien*, Bruce S. McEwen‡, Megan R. Gunnar § and Christine Heim||
Abstract | Chronic exposure to stress hormones, whether it occurs during the prenatal
period, infancy, childhood, adolescence, adulthood or aging, has an impact on brain
structures involved in cognition and mental health. However, the specific effects on the
brain, behaviour and cognition emerge as a function of the timing and the duration of
the exposure, and some also depend on the interaction between gene effects and previous
exposure to environmental adversity. Advances in animal and human studies have made it
possible to synthesize these findings, and in this Review a model is developed to explain why
different disorders emerge in individuals exposed to stress at different times in their lives.
434 | JUNE 2009 | VOLUME 10
A receptor that is activated by
mineralocorticoids, such as
aldosterone and deoxycorti-
costerone, as well as
glucocorticoids, such as
cortisol and cortisone. It also
responds to progestins.
A receptor that is activated by
cortisol, corticosterone and
other glucocorticoids and is
expressed in almost every cell
in the body. It regulates genes
metabolism and the immune
inhibits HPA axis activity (FIG. 1), and a prenatal stress-
induced reduction in hippocampal MRs and GRs could
decrease this inhibition, with a resulting increase in basal
and/or stress-induced glucocorticoid secretion. In rhe-
sus monkeys, prenatal treatment with the synthetic GR
agonist dexamethasone causes a dose-dependent degen-
eration of hippocampal neurons, leading to a reduced
hippocampal volume at 20 months of age9.
Effects on other brain regions are also apparent.
Rats exposed to stress during the last week of gestation
have significantly decreased dendritic spine density in
the anterior cingulate gyrus and orbitofrontal cortex10.
Furthermore, prenatal exposure to glucocorticoids leads
to increased adult corticotropin-releasing hormone
(CRH) levels in the central nucleus of the amygdala, a
key region in the regulation of fear and anxiety11.
Exposure to prenatal stress has three major effects
on adult behaviour: learning impairments, especially
in aging rats12; enhanced sensitivity to drugs of abuse13;
and increases in anxiety- and depression-related behav-
iours14. The impaired learning is thought to result from
the effects of prenatal stress on hippocampal function15,
whereas the effects on anxiety are thought to be medi-
ated by prenatal stress-induced increases in CRH in the
amygdala11. Prenatal glucocorticoid exposure affects
the developing dopaminergic system, which is involved
in reward- or drug-seeking behaviour16, and it has been
suggested that the increased sensitivity to drugs of abuse
is related to the interaction between prenatal stress,
glucocorticoids and dopaminergic neurons16.
Human studies. In agreement with animal data, findings
from retrospective studies on children whose mothers
experienced psychological stress or adverse events or
received exogenous glucocorticoids during pregnancy
suggest that there are long-term neurodevelopmental
effects17. First, maternal stress or anxiety18, depression19
and glucocorticoid treatment during pregnancy17 have
been linked with lower birthweight or smaller size (for
gestational age) of the baby. More importantly, mater-
nal stress, depression and anxiety have been associated
with increased basal HPA axis activity in the offspring
at different ages, including 6 months20, 5 years21 and
Disturbances in child development (both neurologi-
cal and cognitive) and behaviour have been associated
with maternal stress23 and maternal depression dur-
ing pregnancy24, and with fetal exposure to exogenous
gluco corticoids in early pregnancy25. These behavioural
alterations include unsociable and inconsiderate behav-
iours, attention deficit hyperactivity disorder and sleep
disturbances as well as some psychiatric disorders,
including depressive symptoms, drug abuse and mood
and anxiety disorders. Very few studies have measured
Figure 1 | The stress system. When the brain detects a
threat, a coordinated physiological response involving
autonomic, neuroendocrine, metabolic and immune
system components is activated. A key system in the
stress response that has been extensively studied is the
hypothalamus-pituitary-adrenal (HPA) axis. Neurons in
the medial parvocellular region of the paraventricular
nucleus of the hypothalamus release corticotropin-
releasing hormone (CRH) and arginine vasopressin (AVP).
This triggers the subsequent secretion of adrenocortico-
tropic hormone (ACTH) from the pituitary gland, leading
to the production of glucocorticoids by the adrenal
cortex. In addition, the adrenal medulla releases
catecholamines (adrenaline and noradrenaline) (not
shown). The responsiveness of the HPA axis to stress is in
part determined by the ability of glucocorticoids to
regulate ACTH and CRH release by binding to two
corticosteroid receptors, the glucocorticoid receptor
(GR) and the mineralocorticoid receptor (MR). Following
activation of the system, and once the perceived stressor
has subsided, feedback loops are triggered at various
levels of the system (that is, from the adrenal gland to the
hypothalamus and other brain regions such as the
hippocampus and the frontal cortex) in order to shut the
HPA axis down and return to a set homeostatic point. By
contrast, the amygdala, which is involved in fear
processing142, activates the HPA axis in order to set in
motion the stress response that is necessary to deal with
the challenge. Not shown are the other major systems
and factors that respond to stress, including the
autonomic nervous system, the inflammatory cytokines
and the metabolic hormones. All of these are affected by
HPA activity and, in turn, affect HPA function, and they
are also implicated in the pathophysiological changes
that occur in response to chronic stress, from early
experiences into adult life.
NATURE REVIEWS | NEUROSCIENCE
VOLUME 10 | JUNE 2009 | 435
FOCUS ON STRESS
changes in the brain as a function of prenatal stress in
humans. However, a recent study showed that low birth-
weight combined with lower levels of maternal care was
associated with reduced hippocampal volume in adult-
hood26. This finding is consistent with evidence that
effects of prenatal stress in humans are often moderated
by the quality of postnatal care, which in turn is consist-
ent with the protracted postnatal development of the
Animal studies. Although in rodents the postnatal
period is relatively hyporesponsive to stress (BOX 2), one
of the most potent stressors for pups is separation from
the dam. Long separation periods (3 h or more each day)
activate the pups’ HPA axis, as evidenced by increased
plasma levels of adrenocorticotropic hormone and
glucocorticoids27. Protracted maternal separation also
reduces pituitary CRH binding sites28, and low levels of
maternal care reduce GR levels in the hippocampus29.
The effects of maternal deprivation extend beyond
the HPA axis. Early prolonged maternal separation in
rats increases the density of CRH binding sites in the
prefrontal cortex, amygdala, hypothalamus, hippo-
campus and cerebellum, as measured post-infancy28. In
the hippocampus CRH mediates stress-related loss of
branches and spines30, and in the amygdala and hypotha-
lamus elevated CRH levels are associated with increased
anxiety and HPA axis activity, respectively31. Thus, the
increase in CRH-binding sites induced by maternal sep-
aration might have negative effects over time. The long-
term effects of prolonged separation depend on the age
of the pup and the duration of the deprivation, with the
effects noted above generally being greater when these
separations occur earlier in infancy and last for longer
Although the rodent work provides a rich frame-
work for conceptualizing the impact of early-life stress,
the fact that the rodent brain is much less developed at
birth than the primate brain makes translation of the
findings to humans somewhat challenging (BOX 3). Non-
human primates have more human-like brain matura-
tion at birth and patterns of parent–offspring relations,
and so provide an important bridge in the translation of
the rodent findings. Studies in monkeys have shown that
repeated, unpredictable separations from the mother33,
unpredictable maternal feedings34 or spontaneous mater-
nal abusive behaviour35 increases CRH concentrations
in the cerebrospinal fluid and alters the diurnal activity
of the HPA axis for months or even years after the period of
adversity: cortisol levels are lower than normal early in
the morning (around wake-up) and slightly higher than
normal later in the day, an effect that seems to reverse
over time in the absence of continued, ongoing psy-
chosocial stress35. These diurnal effects have not been
noted in rodents, but the effects on higher brain regions
seem to be comparable to the rodent findings and
include heightened fear behaviour36, exaggerated startle
responses33, hippocampal changes such as an increase in
the intensity of non-phosphorylated neurofilament pro-
tein immunoreactivity in the dentate gyrus granule cell
layer37, and atypical development of prefrontal regions
involved in emotion and behaviour control38.
Human studies. A human equivalent of the rodent
maternal separation paradigms might be studies of
children who attend full-day, out-of-home day care
centres. Studies have reported that glucocorticoid levels
rise in these children over the day, more so in toddlers
than in older preschool-aged children39,40. However, it is
important to note that the elevated glucocorticoid levels
observed are less pronounced than those observed in
rodents and monkeys exposed to maternal separation.
Moreover, although age accounts for most of the varia-
tion in the rise in glucocorticoid levels by late afternoon,
the quality of care is also important, with less supportive
care producing larger increases, especially for children
who are more emotionally negative and behaviour-
ally disorganized39. So far, there is no evidence that the
elevated glucocorticoid levels associated with being in
day care affect development; however, children who are
exposed to poor care for long hours early in develop-
ment have an increased risk of behaviour problems later
Parent–child interactions and the psychological state
of the mother also influence the child’s HPA axis activity.
Beginning early in the first year, when the HPA system
of the infant is quite labile, sensitive parenting is associ-
ated with either smaller increases in or less prolonged
activations of the HPA axis to everyday perturbations42.
Maternal depression often interferes with sensitive and
supportive care of the infant and young child; there is
increasing evidence that offspring of depressed mothers,
Box 1 | Models to study stress in animals and humans
The hypothalamus-pituitary-adrenal axis can be activated by a wide variety of stressors.
Some of the most potent are psychological or processive stressors (that is, stressors that
involve higher-order sensory cognitive processing), as opposed to physiological or
systemic stressors. Many psychological stressors are anticipatory in nature — that is,
they are based on expectation as the result of learning and memory (for example,
conditioned stimuli in animals and the anticipation of threats, real or implied, in humans)
or on species-specific predispositions (for example, avoidance of open space in
rodents or the threat of social rejection and negative social evaluations in humans).
Animal studies allow the development of experimental protocols in which animals
are submitted to acute and/or chronic stress and the resulting effects on brain and
behaviour are studied. Experimental stressful ‘early-life’ manipulations in animals can
be broadly split into prenatal and postnatal manipulations. Prenatal manipulations
involve maternal stress, exposing the mother to synthetic glucocorticoids or maternal
nutrient restriction. Postnatal manipulations include depriving the animal of
maternal contact, modifying maternal behaviour and exposing the animal to synthetic
glucocorticoids. In these protocols, the cause–effects relationship between stress and
its impact on the brain can be demonstrated. By contrast, and because of ethical issues,
the cause–effects impact of stress on the brain cannot be studied in humans, and most
human studies are correlational by nature. However, there are some ‘experiments of
nature’ that can be used to inform scientists about the effects of chronic exposure to
early adversity on brain development and of adulthood and late-life stress effects on
the brain. Intrauterine under-growth and low birth weight are considered indices of
prenatal stress (including malnutrition) in humans. In terms of postnatal stress, low
socio-economic status, maltreatment and war are considered adverse events. In adults
and older adults, studies of chronic caregivers (spouses of patients with brain
degenerative disorders, parents of chronically sick children and health-care
professionals) provide a human model of the impact of chronic stress on the brain,
behaviour and cognition.
436 | JUNE 2009 | VOLUME 10
especially those who were clinically depressed in the
child’s early years, are at risk of heightened activity of
the HPA axis43 or of developing depression during ado-
lescence (controlling for maternal depression during
adolescence)44. However, it should be noted that it can
be difficult to exclude potentially confounding genetic
factors in such studies. Furthermore, preschool-aged
children of depressed mothers exhibit electroencephalo-
graphic alterations in frontal lobe activity that corre-
late with diminished empathy and other behavioural
In contrast to findings of elevated glucocorticoid lev-
els in conditions of low parental care, studies in human
children exposed to severe deprivation (for example,
in orphanages or other institutions), neglect or abuse
report lower basal levels of glucocorticoids, similar to
what has been observed in primates39. One proposed
mechanism for the development of hypocortisolism is
downregulation of the HPA axis at the level of the pitui-
tary in response to chronic CRH drive from the hypoth-
alamus46, whereas a second possible mechanism is target
tissue hypersensitivity to glucocorticoids47. Importantly,
this hypocortisolism in humans in response to severe
stress may not be permanent: sensitive and supportive
care of fostered children normalizes their basal gluco-
corticoid levels after only 10 weeks48. Another impor-
tant finding comes from a recent study which showed
that exposure to early adversity is associated with epi-
genetic regulation of the GR receptor, as measured in the
post-mortem brains of suicide victims49.
Stress in adolescence
Animal studies. In rodents the period of adolescence has
three stages: a prepubescent or early adolescent period
from day 21 to 34, a mid-adolescent period from day
34 to 46 and a late adolescent period from day 46 to 59
(REF. 50). In humans, adolescence is often considered
to demarcate the period of sexual maturation (that is,
starting with menarche in girls).
Although adolescence is a time of significant brain
development, particularly in the frontal lobe51, there has
been relatively little research on stress during this period
in rodents. In adolescent rodents, HPA function is char-
acterized by a prolonged activation in response to stres-
sors compared with adulthood. Moreover, prepubertal
rats have a delayed rise of glucocorticoid levels and
prolonged glucocorticoid release in response to several
types of stressors compared with adult rats52, owing to
incomplete maturation of negative-feedback systems53.
In contrast to adult rats, which show a habituation of
the stress response with repeated exposure to the same
stressor54, juvenile rats have a potentiated release of
adrenocorticotropic hormone and glucocorticoids after
repeated exposure to stress55, suggesting that the HPA
axis responses to acute and chronic stress depend on
the developmental stage of the animal. Compared with
exposure to stress in adulthood alone, exposure to stress
as both a juvenile and an adult increases basal anxiety
levels in the adult56. Moreover, exposure to juvenile
stress results in greater HPA axis activation than a dou-
ble exposure to stress during adulthood56, and this effect
is long-lasting. These results suggest that repeated stress
in adolescence leads to greater exposure of the brain to
glucocorticoids than similar experiences in adulthood.
The fact that the adolescent brain undergoes vigor-
ous maturation and the fact that, in rats, the hippocam-
pus continues to grow until adulthood suggest that the
adolescent brain may be more susceptible to stressors
and the concomitant exposure to high levels of gluco-
corticoids than the adult brain. Consistent with this
hypothesis are findings that increased glucocorticoid
levels before but not after puberty alter the expression of
genes for NMDA (N-methyl-d-aspartate) receptor sub-
units in the hippocampus57. In addition, chronic, vari-
able stress during the peripubertal juvenile period results
in reduced hippocampal volume in adulthood, which
is accompanied by impairments in Morris water maze
navigation and delayed shutdown of the HPA response
to acute stress58. These differences became evident only
in adulthood58, suggesting that stress in adolescence
reduces hippocampal growth. Finally, the effects of juve-
nile stress are long-lasting: adult rats exposed to juvenile
stress exhibit reduced exploratory behaviour and poor
avoidance learning59. Moreover, stress in adolescence
increases susceptibility to drugs of abuse during the
adolescent period60 and in adulthood61.
Human studies. Interestingly, studies in human adoles-
cents also suggest that the adolescent period is associ-
ated with heightened basal and stress-induced activity
of the HPA axis62. This could be related to the dramatic
changes in sex steroid levels during this period, as these
steroids influence HPA axis activity50. However, studies
of stress in adolescent rats cannot be translated directly
to humans because the brain areas that are undergoing
development during adolescence differ between rats and
humans: although the rodent hippocampus continues to
Box 2 | The stress hyporesponsive period: from animals to humans
Despite there being clear evidence that corticotropin-releasing hormone-containing
neurons are present in the fetal rat139, in rodents noxious stimuli evoke only a
subnormal hypothalamus-pituitary-adrenal (HPA) axis response during the first 2 weeks
of life140. During this so-called stress hyporesponsive period (SHRP), baseline plasma
glucocorticoid levels are lower than normal and are only minimally increased by
exposure to a noxious stressor140. The SHRP is due to a rapid regression of the HPA axis
after birth140 and may have evolved in rodents to protect the rapidly developing brain
from the impact of elevated glucocorticoids.
Evidence is accumulating that in children there may be a comparable hyporesponsive
period that emerges in infancy and extends throughout most of childhood141. At birth,
glucocorticoid levels increase sharply in response to various stressors, such as a
physical examination or a heel lance. However, over the course of the first year the HPA
axis becomes more insensitive to stressors. No study has assessed the exact period over
which this human SHRP may occur, but in adolescents glucocorticoid levels can
become elevated in response to a psychosocial stressor141, which suggests that the
SHRP could extend throughout childhood.
In rodents the SHRP is maintained primarily by maternal care (that is, the presence of
the dam seems to suppress HPA axis activity); indeed, maternal separation is a potent
inducer of a stress response, even during the SHRP. Similarly, in humans the apparent
hyporesponsivity of the HPA axis might reflect the fact that during the first year of life
the HPA axis comes under strong social regulation or parental buffering141. Here again,
stressors that involve a lack of parental care or social contact can induce a stress
response in children.
NATURE REVIEWS | NEUROSCIENCE
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FOCUS ON STRESS
develop well into adulthood, in humans it is fully devel-
oped by 2 years of age63. The frontal cortex and amygdala
continue to develop in both species, but humans have
larger ontogenic bouts of development in frontal regions
than do rodents (BOX 3).
There are indications that the adolescent human brain
might be especially sensitive to the effects of elevated
levels of glucocorticoids and, by extension, to stress.
Recent studies on the ontogeny of MR and GR expres-
sion show that GR mRNA levels in the prefrontal cortex
are high in adolescence and late adulthood compared
with in infancy, young adulthood and senescence64. This
suggests that the cognitive and emotional processes that
are regulated by these brain areas might be sensitive to
GR-mediated regulation by glucocorticoids in an age-
dependent manner. Various forms of psychopathology,
including depression and anxiety, increase in prevalence
in adolescence65,66. Periods of heightened stress often
precede the first episodes of these disorders, raising the
possibility that heightened HPA reactivity during adoles-
cence increases sensitivity to the onset of stress-related
Adolescence is also a period in which the long-
lasting effects of earlier exposures to stress become evi-
dent. Adolescents who grew up in poor economic condi-
tions have higher baseline glucocorticoid levels67, as do
adolescents whose mothers were depressed in the early
postnatal period44. High early-morning glucocorticoid
levels that vary markedly from day to day during the
transition to adolescence are not associated with depres-
sive symptoms at that time, but they predict increased
risk for depression by age 16 (REF. 44).
Although early-life stress impairs hippocampal devel-
opment in rodents, there is currently little evidence
of comparable effects in humans. Children exposed
to physical or sexual abuse early in life do not exhibit
reduced hippocampal volume (relative to whole-brain
size) as adolescents, although adults with these histo-
ries do show volume reductions68. This finding holds
even when the abused children have been selected for
chronic post-traumatic stress disorder (PTSD), and even
though in some cases they exhibit overall reductions in
brain volume69. By contrast, alterations in grey matter
volume and the neuronal integrity of the frontal cortex,
and reduced size of the anterior cingulate cortex, have
been reported in adolescents exposed to early (and con-
tinued) adversity70. Together, these results suggest that in
humans the frontal cortex, which continues to develop
during adolescence, might be particularly vulnerable to
the effects of stress during adolescence. By contrast, the
hippocampus, which develops mainly in the first years
of life, might be less affected by exposure to adversity in
Stress in adulthood
Animal studies. Studies on adult stress in rodents have
delineated the effects of acute versus chronic stress
on brain and behaviour. The impact of acute stressors
depends on the level of glucocorticoid elevations, with
small increases resulting in enhanced hippocampus-
mediated learning and memory, and larger, prolonged
elevations impairing hippocampal function71. The
inverted-U-shaped effects of acute glucocorticoid ele-
vations might serve adaptive purposes by increasing
vigilance and learning processes during acute challenges.
The mechanism that underlies the acute bipha-
sic actions of glucocorticoids on cognition involves
the adrenergic system in the basolateral nucleus of the
amygdala. By enhancing noradrenergic function in
the amygdala, glucocorticoids have a permissive effect
on the priming of long-term potentiation in the den-
tate gyrus by the basolateral nucleus72. This modulation
of noradrenergic function by glucocorticoids has been
linked to the enhanced memory for emotional events
that occur under stress73.
Chronic stress or chronic exogenous administration
of glucocorticoids in rodents causes dendritic atrophy
in hippocampal CA3 pyramidal neurons74. However,
these changes take several weeks to develop and are
reversed by 10 days after the cessation of the stressor75.
Chronic stress in adult rats also inhibits neurogenesis
in the dentate gyrus76 and causes hippocampal volume
loss77. Importantly, this volume decrease is not associ-
ated with reduced neuron numbers and is not limited
to the dentate gyrus78, suggesting that reduced neuro-
genesis might not be the only contributing factor. The
morphological changes that take place in the hippocam-
pus after chronic stress have been related to changes in
spatial learning79, which are reversed following 21 days
of withdrawal from stress80. Here, it is interesting to note
that in contrast to the effects of chronic or severe stress
on the brain and behaviour earlier in life, which are long-
lasting, effects of adulthood stress — even chronic stress
— are reversed after a few weeks of non-stress. These
differences between the effects of early and adulthood
Box 3 | Stress effects on the brain: timing is crucial
In animals that give birth to relatively mature young (for example, primates, sheep and
guinea pigs), maximal brain growth and most of the neuroendocrine maturation occurs
in utero. However, in rats, rabbits and mice the mother gives birth to immature young
and most of the neuroendocrine development occurs in the postnatal period17. In
humans the hypothalamus-pituitary-adrenal axis is highly responsive at birth, but brain
development is not finished. The volume of the hippocampal formation increases
sharply until the age of 2 years, whereas amygdala volume continues to increase slowly
until the late 20s63. By contrast, the development of the frontal cortex in humans takes
place mostly between 8 and 14 years of age63. The late increase in prefrontal volumes is
consistent with data showing that this region develops latest in terms of myelination
and synaptic density in humans136.
Prenatal and postnatal stress can both thus have contrasting effects in different
species because perinatal manipulations will affect different stages of development as
a function of the species studied. Consequently, stress in the first week of the rodent’s
life is often developmentally equated with stress during the last trimester of human
Significant decreases in brain volume have been reported in aged animals and
humans, although most of the studies performed are cross-sectional. In men the
volume of the hippocampus starts to decrease by the second decade of life, whereas in
women this decrease is delayed until around 40 years of age, possibly owing to the
protective effects of oestrogen137. By contrast, amygdala volume decreases around the
sixth decade of life in humans63. In the frontal cortex, different subregions are
differentially affected by aging. For example, aging is associated with shrinking of the
dorsolateral and inferior frontal cortices, but no age effects have been reported for
the anterior cingulate cortex, the frontal pole or the precentral gyrus138.
438 | JUNE 2009 | VOLUME 10
stress might be related to differences in the severity of
stressors to which pups and adult rats are exposed or
in the development of the hippocampus at the time of
Pyramidal neurons in layers II/III of the prefron-
tal cortex also show dendritic retraction and a reduc-
tion in spine number81 in response to chronic stress in
adulthood — this can be observed 24 h after a single
forced-swim stress82 — but remodelling occurs after ces-
sation of the stressor83. In accordance with these find-
ings, glucocorticoid hypersecretion is associated with
reduced volume of at least the right anterior cingulate
cortex in rodents84. Contrary to the reduction in hip-
pocampal and frontal volumes, chronic stress in adult
rodents leads to dendritic hypertrophy in the baso lateral
amygdala85. Moreover, a recent study showed that
even a single acute administration of glucocorticoids
caused dend ritic hypertrophy in this area 12 days later86.
The dendritic hypertrophy was correlated with anxi-
ety in both the acute86 and the chronic85 administration
Human studies. In humans, studies of the effects of acute
stress confirm animal studies and report the presence
of an inverted-U-shaped relationship between gluco-
corticoid levels and cognitive performance87. However,
contrary to animal studies, in which most laboratory
tests for learning and memory involve a fear and/or an
emotional process88, tests of learning and memory in
humans can differentiate the effects of glucocorticoids
on the processing of neutral versus emotional informa-
tion. Most studies to date have shown that acute gluco-
corticoid elevations significantly increase memory for
emotional information, whereas they impair the retrieval
of neutral information89.
Only a few reports suggest that there is an association
between exposure to chronic stress and reduced hippo-
campal volume in individuals not suffering from men-
tal health disorders (for a review see REF. 90). However,
a recent study reported that low self-esteem, a potent
predictor of increased reactivity to stress in humans91, is
associated with reduced hippocampal volume92.
Most of the studies of chronic-stress effects on the
adult human brain have concentrated either on stress-
related psychopathologies or on the impact of early-life
stress on adult psychopathology. A large number of stud-
ies have reported elevated basal glucocorticoid levels in
individuals with some forms of depression93, whereas
reduced basal glucocorticoid concentrations have been
reported in patients with PTSD94, although this finding
has been controversial95. Given that low glucocorticoid
concentrations seem to develop in early childhood in
response to neglect or trauma, it is possible that low
cortisol predicts vulnerability to developing PTSD in
response to trauma in adulthood.
Studies of adults who suffered childhood abuse
also reveal hyper-reactivity of the HPA axis in abused,
depressed individuals96 and hypoactivity in those with
PTSD94. The changes in abused, depressed adults have
been associated with CRH-induced ‘escape’ of gluco-
corticoid secretion from suppression by treatment with
dexamethasone97, suggesting that the glucocorticoid
feedback of the HPA axis is impaired under conditions of
increased hypothalamic drive. Thus, a decreased capac-
ity of glucocorticoids to inhibit the HPA axis when it is
stimulated could further accentuate CNS responses to
stressors. In agreement with this suggestion, increased
cerebrospinal fluid CRH levels have been reported
in individuals who reported childhood stress98 and
Decreased hippocampal volume and function are
landmark features of depression and PTSD100,101. One
cross-sectional study102 found that a smaller hippo-
campus in women with major depression was associ-
ated with experiences of childhood trauma, whereas
depressed women without such trauma had hippocam-
pal volumes similar to healthy controls. This supports
the notion that certain brain changes in patients with
depression or PTSD could represent markers of vulner-
ability for the disorder rather than markers of the dis-
order itself. This finding is in line with results from a
twin study of Vietnam veterans103 which showed that
decreased hippocampal volume is not a consequence of
combat exposure or PTSD: decreased volume was also
present in unexposed co-twins, and thus it might be a
pre-existing risk factor for PTSD that could be genetic
or rooted early in life.
Stress in aging
Animal studies. Approximately 30% of aged rats have
basal glucocorticoid hypersecretion, which is correlated
with memory impairments and reduced hippocampal
volume104. If a middle-aged rat is exposed for a long
period to high levels of exogenous glucocorticoids, it
will develop memory impairments and hippocampal
atrophy105 similar to those observed in these 30% of aged
rats. Conversely, artificially keeping glucocorticoid lev-
els low in middle-aged rats prevents the emergence of
both memory deficits and hippocampal atrophy in old
age106. Several groups have also found that chronic stress
in aged rats can accelerate the appearance of biomarkers
of hippocampal aging (for example, frequency potentia-
tion and synaptic excitability thresholds) and that excess
endogenous or exogenous glucocorticoids induce hip-
pocampal dendritic atrophy and inhibit neurogenesis107.
Finally, in aged monkeys108 chronic glucocorticoid treat-
ment can increase amyloid-β pathology, similar to that
reported in Alzheimer’s disease.
These results have given rise to the glucocorticoid
cascade hypothesis109, which suggests that there is a rela-
tionship between cumulative exposure to high glucocor-
ticoid levels and hippocampal atrophy. It was recently
renamed the neurotoxicity hypothesis103, because the
proposed explanation for this relationship is that pro-
longed exposure to stress hormones reduces the ability of
neurons to resist insults, thus increasing the rate at which
they are damaged by other toxic challenges or ordinary
attrition109. Glucocorticoids might have a similar neuro-
toxic effect in the prefrontal cortex. A study demon-
strated an enhanced elevation of extracellular glutamate
levels post-stress in the hippocampus and medial pre-
frontal cortex of aged rats compared with young rats110.
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FOCUS ON STRESS
Stress in adolescence Stress in adulthoodStress in aging Postnatal stress
Birth28 18 306090
Programming effects Differentiation effects
Nature Reviews | Neuroscience
Increased glutamate levels after stress, and perhaps other
neurotoxic insults, might thus increase the vulnerability
of the aging brain to neuronal damage.
Human studies. Aging, healthy humans exhibit higher
mean diurnal levels of cortisol than younger individu-
als111, and a longitudinal study has found that elevated
plasma glucocorticoid levels over years in older adults
negatively correlates with hippocampal volume and
memory112. Given that aged individuals with Alzheimer’s
disease present both memory impairments and hippo-
campal atrophy, studies have assessed basal glucocor-
ticoid levels in this population and found that they are
higher than in controls113. In addition, chronic glucocor-
ticoid treatment has been shown to worsen cognition in
people with Alzheimer’s disease114.
The frontal lobe also seems to be sensitive to glucocor-
ticoid effects during human aging. Using a novel in vitro
post-mortem tracing method on human brain slices, Dai
et al.115 found an inverted-U-shaped effect of glucocor-
ticoids on axonal transport in prefrontal neurons with,
in most cases, a stimulating effect at low concentrations
and a depressing effect at high concentrations. Given
that axonal transport plays a crucial part in neuronal
survival and function, these results suggest that gluco-
corticoids potentially have negative effects on prefrontal
cortex neurons’ survival and/or function.
A model of stress effects throughout life
The data obtained in animals and humans suggest that
chronic or repeated exposure to stress has enduring
effects on the brain, through activation of the HPA axis
and the release of glucocorticoids, with the highest impact
on those structures that are developing at the time of the
stress exposure (in young individuals) and those that are
undergoing age-related changes (in adult and aged indi-
viduals). Stress in the prenatal period affects the devel-
opment of many of the brain regions that have a role in
regulating the HPA axis — that is, the hippocampus, the
frontal cortex and the amygdala (programming effects
(FIG. 2)). During childhood the hippocampus — which
continues to develop after birth — might be the brain
region that is most vulnerable to the effects of chronic
stress, possibly through a process of increased CRH
drive in the hippocampus116. Because it modulates HPA
axis activity, altered functioning of the hippocampus
Figure 2 | The life cycle model of stress. How the effects of chronic or repeated exposure to stress (or a single exposure
to severe stress) at different stages in life depend on the brain areas that are developing or declining at the time of the
exposure. Stress in the prenatal period affects the development of many of the brain regions that are involved in regulating
the hypothalamus-pituitary-adrenal (HPA) axis — that is, the hippocampus, the frontal cortex and the amygdala
(programming effects). Postnatal stress has varying effects: exposure to maternal separation during childhood leads to
increased secretion of glucocorticoids, whereas exposure to severe abuse is associated with decreased levels of
glucocorticoids. Thus, glucocorticoid production during childhood differentiates as a function of the environment
(differentiation effects). From the prenatal period onwards, all developing brain areas are sensitive to the effects of stress
hormones (broken blue bars); however, some areas undergo rapid growth during a particular period (solid blue bars). From
birth to 2 years of age the hippocampus is developing; it might therefore be the brain area that is most vulnerable to the
effects of stress at this time. By contrast, exposure to stress from birth to late childhood might lead to changes in amygdala
volume, as this brain region continues to develop until the late 20s. During adolescence the hippocampus is fully organized,
the amygdala is still developing and there is an important increase in frontal volume. Consequently, stress exposure during
this period should have major effects on the frontal cortex. Studies show that adolescents are highly vulnerable to stress,
possibly because of a protracted glucocorticoid response to stress that persists into adulthood (potentiation/incubation
effects). In adulthood and during aging the brain regions that undergo the most rapid decline as a result of aging (red bars)
are highly vulnerable to the effects of stress hormones. Stress during these periods can lead to the manifestation of
incubated effects of early adversity on the brain (manifestation effects) or to maintenance of chronic effects of stress
(maintenance effects). PTSD, post-traumatic stress disorder.
440 | JUNE 2009 | VOLUME 10
might cause glucocorticoid hyposecretion in cases
of severe abuse, or increased basal cortisol levels in
cases of maternal deprivation (differentiation effects
(FIG. 2)). By contrast, in adolescence the frontal cortex,
which undergoes major development at this stage, may be
most vulnerable to the effects of stress, possibly leading to
a protracted glucocorticoid response to stress that persists
into adulthood (potentiation/incubation effects (FIG. 2)).
In adulthood and old age the brain regions that undergo
the most rapid decline as a result of aging are highly vul-
nerable to the effects of stress hormones. For example,
in the hippocampus glucocorticoids affect neurogen-
esis, neuronal survival rate and dendritic arborization
(manifestation/maintenance effects (FIG. 2)).
The neurotoxicity and vulnerability hypotheses. The
data obtained in adults and older animals and humans
have led to the neurotoxicity hypothesis109, which sug-
gests that prolonged exposure to glucocorticoids reduces
the ability of neurons to resist insults, increasing the rate
at which they are damaged by other toxic challenges
or ordinary attrition109. This hypothesis implies that a
reduced hippocampal size is the end product of years
or decades of PTSD, depressive symptoms or chronic
stress. Although the neurotoxicity hypothesis has been
confirmed by various animal and human studies, it
does not explain the hyposecretion of glucocorticoids
that occurs in patients suffering from PTSD, who also
present reduced hippocampal volume.
Data obtained in children, adolescents or adult ani-
mals and humans exposed to acute or early-life trauma
have led to the vulnerability hypothesis103. In contrast to
the neurotoxicity hypothesis, the vulnerability hypoth-
esis suggests that reduced hippocampal volume in adult-
hood is not a consequence of chronic exposure to PTSD,
depression or chronic stress, but is a pre-existing risk fac-
tor for stress-related disorders that is induced by genetics
and/or early exposure to stress117. Unlike the neurotoxicity
hypothesis, the vulnerability hypothesis can explain gluco-
corticoid hyposecretion in patients with PTSD. Indeed,
studies in children facing significant adversity, such as
abuse, report the development of glucocorticoid hypo-
secretion39, which might last until adulthood and confer
vulnerability to developing PTSD as a result of trauma.
We think that the two hypotheses are not mutually
exclusive when viewed from a developmental perspec-
tive. Indeed, the data summarized in this Review suggest
that there might be early windows of vulnerability (or
sensitive periods68) during which specific regions of the
developing brain are most susceptible to environmental
influences, through a neurotoxicity process. Exposure
to stress and/or adversity during these key vulnerable
periods might slow the development of those brain
regions for the duration of the adversity. When meas-
ured in adulthood, the reduced volumes of these brain
regions could be a strong marker of the time of exposure
to early adversity rather than of the effects of specific
traumas on various brain regions. These windows of vul-
nerability could also be used to predict the nature of the
psychopathology that will result from exposure to stress
at different ages. Exposure to adversity at the time of
hippocampal development could lead to hippocampus-
dependent emotional disorders, which would be differ-
ent from disorders arising from exposure to adversity at
times of frontal cortex development. Two recent studies
support this hypothesis. The first reported that women
who experienced trauma before the age of 12 years had
increased risk for major depression, whereas women who
experienced trauma between 12 and 18 years of age more
frequently developed PTSD118. The second study reported
that repeated episodes of sexual abuse were associated
with reduced hippocampal volume if the abuse occurred
early in childhood, but with reduced prefrontal cortex
volume if the abuse occurred during adolescence119. These
results suggest that, similar to what has been observed in
animals120, there may be distinct structural, neuropsy-
chological and neuropsychiatric sequelae of early abuse,
depending in part on the age or developmental stage of
the brain when the insult occurred.
Besides slowing down the development of the brain
during the time of adversity, leading to reduced
brain volumes in adulthood, stress in early life could
modify the developmental trajectory of the brain. The
potential immediate benefit of such modifications is that
they might increase acute survival probability, but they
could have negative long-term effects. During child-
hood and adolescence the brain undergoes a period
of overproduction and pruning of synapses121. One of
the brain regions that shows the slowest development
over the lifespan is the amygdala (BOX 3). It is interest-
ing to note that contrary to the hippocampus and the
frontal lobe — which show volume reduction as a result
of chronic stress — the amygdala increases in volume under
chronic stress, owing to increased dendritic arborization.
Given that the amygdala plays a significant part in the
detection of fear and threat, it is possible that throughout
evolution increases in amygdala volume in response to
stress might have improved the detection of threatening
information and so increased survival probability. If this
is indeed the case, young children exposed to adversity
should also have increased amygdala volume, but no
study has yet examined this important question.
This acute effect of adversity on brain organization
could have negative long-term consequences. Stress at
key periods of synaptic organization could modify the
trajectories of connections, leading to an incubation
period, such that the effects of stress would not be appar-
ent at the time of adversity but would emerge later, when
the synaptic organization has been completed. Studies
showing protracted effects of early-life stress that emerge
at puberty support this suggestion44. Furthermore,
although depression is the most extensively documented
outcome of exposure to chronic sexual abuse in adults,
it is not a common occurrence in children suffering
abuse. Indeed, the average time from the onset of abuse
to the emergence of clinical depression is 11.5 years, with
the first major episode occurring during adolescence122.
It is thus conceivable that in susceptible individuals expo-
sure to early adversity during a window of vulnerability
sets into motion a series of events that lead to a hetero-
typic reorganization of synaptic development, resulting
in a protracted expression of depression or PTSD.
NATURE REVIEWS | NEUROSCIENCE
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FOCUS ON STRESS
This same process could also explain the develop-
ment of resilience in face of adversity. Environmental
enrichment in rodents is a potent inducer of changes
in neurogenesis and/or dendritic arborization in the
hippocampus, and has been documented to lead to
increases in hippocampal volume123. In children facing
early adversity, forms of environmental enrichment,
such as support from a family member, enriched day
care or school environment or social support from
members of the community, could induce a similar het-
erotypic reorganization of synaptic development, pro-
gramming of neurotrophic factors or changes in gene
expression that could lead to resilience later in life. If
this is the case, it could be suggested that any type of
intervention performed during the early years could not
only have a tremendous effect in preventing the deleteri-
ous impact of chronic stress and/or early abuse on the
developing brain, but could also help to prevent effects
on the brain of chronic stress occurring in adulthood or
Conclusions and future directions
Although studies on stress have provided a wealth of
data delineating the effects of acute and chronic stress
on the developing brain, much remains to be done to
fully understand how the brain develops pathology or
resilience in the face of adversity. We believe that three
main factors should receive special consideration in
future studies on stress in both animals and humans.
The first factor is sex and gender. Sex refers to the bio-
logical differences between males and females, whereas
gender refers to the different roles (gender role and gen-
der identity) that men and women may have during their
lifetime. Both sex and gender might have potent influ-
ences on stress reactivity in humans of all ages. However,
most studies of the effects of stress on the brain, behav-
iour and cognition have tested only male animals or
humans. This is a major issue considering that studies
in both animals50 and humans124 report sex differences in
response to stress, and considering the gender gap ratio
(two girls for one boy) that emerges in early adoles-
cence for the risk of depression125. To this day, a consist-
ent finding in the endocrine literature is that the risk of
depression in adolescent girls increases with decreasing
age at menarche126. An increased sensitivity of girls to
environmental and/or family adversity, along with inter-
actions between glucocorticoids and gonadal steroids,
could be a potential explanation for the increased risk
of depressive disorders in females. Recent results show-
ing an earlier age at menarche in girls exposed to early
adversity127 support this suggestion.
The second factor that should be considered in future
studies is exposure to environmental toxins. Today, chil-
dren in many cities are chronically exposed, at back-
ground levels, to a range of common toxins that are
environmentally persistent and that tend to be lipophilic
and bioaccumulate, such as lead and bisphenol A128.
These agents reach humans mainly through food and
food additives, and they can be transferred to the fetus
through the placenta and to infants through maternal
milk129. They have been shown to affect the endocrine
system in laboratory animals and in wildlife, and conse-
quently have been called ‘endocrine-disrupting chemi-
cals’ (REF. 130). A recent study showed that prenatal and
postnatal exposure to lead is associated with increased
glucocorticoid responses to acute stress in children131.
Also, perinatal exposure to endocrine-disrupting
chemicals is associated with an earlier age at menarche
among girls132. Taken together, these results suggest
that both the timing of sexual maturation and stress
reactivity may be sensitive to relatively low levels of
endocrine-disrupting chemicals in the environment.
The third factor that should receive greater attention
is circadian rhythmicity. Sleep deprivation, shift work
and jet lag all disrupt normal biological rhythms and
have major impacts on health. Interestingly, circadian
disorganization is often observed in stress-related dis-
orders such as depression133 and PTSD134. The discovery
of the molecular clock that is responsible for the genera-
tion of circadian rhythms135 provides new insights into
how rhythm abnormalities might lead to greater vulner-
ability to stress at various ages. Most studies performed
in animals and humans do not measure the circadian
fluctuations in glucocorticoid levels, but rather concen-
trate on specific time points across the day. Although
such measurements are easier, they do not provide
the full spectrum of circadian variations, which could
inform us about specific changes in circadian organi-
zation in response to chronic stress across the lifespan.
Consequently, studies assessing multiple time points
for glucocorticoid secretion across a whole day or sev-
eral days are needed in order to document the complex
relationships that exist between reactivity to stress and
Animal and human studies have provided a wealth
of results showing the negative effects of chronic expo-
sure to stress and/or adversity on the developing brain.
However, stress is not and should not be considered
as a negative concept only. Stress is a physiological
response that is necessary for the survival of the spe-
cies. The stress response that today can have negative
consequences for brain development and mental health
may have conferred the necessary tools to our ances-
tors in prehistorical times for surviving in the presence
of predators. Studies of modern individuals who have
developed resilience by facing significant adversity
should inform us about the physiological and psy-
chological mechanisms at the basis of vulnerability or
resilience to stress. Understanding these mechanisms,
which are possibly rooted in genes and modulated by
the family environment, is extremely important if one
wants to provide interventions early enough to individ-
uals who are the most likely to respond to them. This
article has reviewed the potential for early intervention
to prevent the deleterious effects of stress on the brain,
behaviour and cognition. After more than 30 years of
research on the negative effects of stress on the brain, it
is now time to turn our attention to the potential posi-
tive impact of early interventions on brain development.
These results could help us to develop social policies that
treat the problem of early-life stress at its root — that is,
in the family home.
442 | JUNE 2009 | VOLUME 10
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Sonia Lupien holds a Research Chair on Gender and Mental
Health by the Canadian Institutes of Health Research.
Sonia J. Lupien’s homepage: http://www.humanstress.ca
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