Intergenerational transmission of emotional trauma
through amygdala-dependent mother-to-infant
transfer of specific fear
Jacek Debieca,b,c,1 and Regina Marie Sullivana,b
a
Emotional Brain Institute, Department of Child and Adolescent Psychiatry, New York University School of Medicine, New York, NY 10016; bThe Nathan Kline
Institute, Orangeburg, NY 10962; and cMolecular and Behavioral Neuroscience Institute and Department of Psychiatry, University of Michigan, Ann Arbor, MI 48109
Edited by Bruce S. McEwen, The Rockefeller University, New York, NY, and approved June 27, 2014 (received for review September 4, 2013)
Emotional trauma is transmitted across generations. For example,
children witnessing their parent expressing fear to specific sounds or
images begin to express fear to those cues. Within normal range, this
is adaptive, although pathological fear, such as occurs in posttraumatic
stress disorder or specific phobias, is also socially transmitted to
children and is thus of clinical concern. Here, using a rodent model, we
report a mother-to-infant transfer of fear to a novel peppermint odor,
which is dependent on the mother expressing fear to that smell in
pups’ presence. Examination of pups’ neural activity using c-Fos early
gene expression and 14C 2-deoxyglucose autoradiography during
mother-to-infant fear transmission revealed lateral and basal amygdala nuclei activity, with a causal role highlighted by pharmacological
inactivation of pups’ amygdala preventing the fear transmission. Maternal presence was not needed for fear transmission, because an
elevation of pups’ corticosterone induced by the odor of the frightened mother along with a novel peppermint odor was sufficient to
produce pups’ subsequent aversion to that odor. Disruption of axonal
tracts from the Grueneberg ganglion, a structure implicated in alarm
chemosignaling, or blockade of pups’ alarm odor-induced corticosterone increase prevented transfer of fear. These memories are acquired at younger ages compared with amygdala-dependent odorshock conditioning and are more enduring following minimal conditioning. Our results provide clues to understanding transmission
of specific fears across generations and its dependence upon maternal induction of pups’ stress response paired with the cue to
induce amygdala-dependent learning plasticity. Results are discussed within the context of caregiver emotional responses and
adaptive vs. pathological fears social transmission.
necklace glomeruli
[conditioned stimulus (CS)] is paired with a noxious event [unconditioned stimulus (US)]. Animal studies indicate that the
amygdala’s lateral and basal nuclei (LBA) play an important role
in FC (10). However, FC in infant rats is naturally attenuated
until postnatal day (PND) 10 due to low levels of the stress hormone corticosterone (CORT) during the stress hyporesponsive
period (11–15). This fear suppression continues in older pups
(until PND 16) in the mother’s presence due to social buffering
(attenuation) of the shock-induced CORT increase (15).
To study the intergenerational transmission of fear to specific
triggers, we developed a mother-to-infant social fear learning
paradigm. In social fear learning, an organism learns fear
through an exposure to a conspecific expressing fear to a discrete
CS. Social fear learning may thus serve as a model explaining
how defense responses to specific triggers are transmitted between individuals. Social fear learning has been demonstrated in
primates, including humans and in rodents, and involves the
amygdala (16–19).
Results
Social Transmission of Fear from Mothers to Pups. Before pregnancy, adult female rats were olfactory FC, in which a neutral
peppermint odor was paired with a mild electric foot shock US
(Materials and Methods; for a schematic diagram of the experiment, see Fig. 1A). After birth, these mothers were presented
with the CS odor to evoke fear in the presence of the pups.
Specifically, at PND 6 or 7, pups and the mothers that had
previously received fear conditioning (MFC) were either exposed
Significance
| pheromone | olfaction | PTSD | social referencing
Despite clinical evidence that specific fear is transmitted across
generations, we have little understanding of mechanisms. Here,
we model social transmission of mother-to-infant fear in
rodents. We show that maternal fear responses to a conditioned fear odor are sufficient to induce robust fear learning
throughout infancy, with robust retention. Assessment of
mechanism showed that maternal fear expression increases
pups’ stress hormone corticosterone and amygdala activation
to induce this cue-specific fear learning. Suppressing pups’
amygdala or preventing pups from mounting a stress response
blocked this fear learning. Specific fears may thus be transferred
across generations through maternal emotional communication
and infant’s associative learning mechanisms. Elucidating the
mechanisms of this transmission may inform the development
of novel therapeutic and preventive approaches.
C
hildren, including infants, use their parents’ emotions to guide
their behavior and learn about safety and danger (1–4). The
infant’s ability to regulate behavior in novel situations using the
caregiver’s emotional expression is known as social referencing and
occurs in humans and nonhuman primates (1). Although parental
physical presence itself or particular cues indicating parental presence, such as voice, touch, or smell typically signal safety for the
child, infants are especially responsive to the caregiver’s communication during threats (3–5). This social learning is critical for enhancing survival through an adaptation to the environment but also
provides transmission of pathological fears, such as occurs in posttraumatic stress disorder (PTSD) or in specific phobias (3–7).
Despite existing evidence that children are sensitive to parental fear and anxiety, the neurobiological mechanisms for the
transmission of parental specific fear to the offspring have
remained elusive (2–7). Animal studies investigating the impact
of parental stress on the offspring focused on the history of parental trauma, quality of maternal care, and resultant overall
behavioral alterations in the offspring (7, 8). However, to develop efficient survival strategies, progenies must learn about
specific environmental threats triggering parental fear (9).
Most of what we know about fear learning comes from studies
using fear conditioning (FC) (10). In FC, a neutral sensory cue
12222–12227 | PNAS | August 19, 2014 | vol. 111 | no. 33
Author contributions: J.D. and R.M.S. designed research, performed research, analyzed
data, and wrote the paper.
The authors declare no conflict of interest.
This article is a PNAS Direct Submission.
Freely available online through the PNAS open access option.
1
To whom correspondence should be addressed. Email: jdebiec@umich.edu.
This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.
1073/pnas.1316740111/-/DCSupplemental.
www.pnas.org/cgi/doi/10.1073/pnas.1316740111
to the CS (group MFC-CS) or not (group MFC-no CS). A third
group included pups from mothers that were preexposed to only
the odor CS (no shock) then reexposed to the same CS while
with their pups (CS alone-CS). Mothers of MFC-no CS pups
expressed some fear to context and served as control fearful
mothers with no CS present while interacting with pups (Table 1;
cue reexposure and maternal behavior). Seven days later, pups
received memory tests to the CS using a Y-maze (Fig. 1B). The
ANOVA for the Y-maze test revealed a significant effect [F(2,
15) = 10.5; P < 0.002]. Post hoc test indicated a significant difference in CS-odor choices between the MFC-CS group compared with the other groups (P < 0.05; Fig. 1B). This pattern of
findings shows that a co-occurrence of maternal fear and the
olfactory CS cue produces in pups subsequent aversive responses
to this CS as maternal fear alone (MFC-no CS) or the CS alone
had no effect on pups’ behavior. A similar pattern of results was
observed during the CS cue exposure test when the MFC-CS
pups expressed significantly more freezing than the two control
groups (Fig. S1).
However, these results do not rule out the possibility that
pups’ fear responses to the CS were due to nonlearning mechanisms, such as nonspecific maternal behavior (7, 8). Although
FC occurred before breeding, mothers previously fear-conditioned showed less nurturing behaviors and rougher handling of
pups during presentation of the CS (Table 1) and potentially
throughout rearing. To control for the possibility that these altered maternal behaviors mediated pups fear to the CS, we used
“substitute mothers,” a manipulation made possible by maternal
acceptance of all pups, as well as pups’ failure to distinguish
between their mother and a substitute mother matched for the
same postpartum period and the same diet (12–15). Substitute
mothers received olfactory FC (or were exposed to the CS
alone). On the following day, pups’ biological mother was
removed from the nest, and the substitute mother was placed in
Table 1. Maternal behavior during fear-inducing odor CS
Percent of observation periods in which
behaviors occurred
Maternal behaviors
Fearful/defensive
Freezing
Rough/abusive
Nurturing
Neutral
Mother and pup in nest
MFC-CS
MFC-no CS
CS alone-CS
78.0
80.5
6.14
33.0
18.4
56.66
30.22
95.6
0.0
37.0
74.05
62.5
0.0
0.0
0.0
68.8
39.77
75.0
Maternal behavior (%) during reexposure to MFC-CS and controls
including a MFC not presented with the CS (MFC-no CS) or a control mother
exposed to the odor CS without the US shock (CS alone) and later presented
with this odor while with her pups (CS alone-CS). Fearful/defensive behaviors
include freezing (the percent value of fearful/defensive behaviors accounted
for by freezing is displayed in brackets), startle, escaping, covering the
source of odor, and covering pups with bedding. Rough/abusive behaviors
include stepping/jumping on pup and throwing/dropping/dragging/pushing
away/rough handling pup. Nurturing behaviors include nursing, grooming/
licking/retrieving pup. Neutral behaviors include sleeping, resting, selfgrooming, eating, and drinking (for details, see SI Materials and Methods).
Supplemental analysis showed that maternal fear expression (CS cueinduced freezing) strongly correlated with pups’ subsequent CS-induced fear
and avoidance behavior (SI Supplemental Analysis).
the nest and permitted an hour to settle down and nurse the pups
at PND 13 before the CS-odor exposure. The first two groups
included pups with their substitute mothers previously FC; one
group was reexposed to the CS (MFC-CS), the other was not
(MFC-no CS). A third group included pups with the substitute
mother with prior exposure to the CS alone (no shock) and then
reexposed to this CS (CS alone-CS) (Table S1; maternal behavior during CS reexposure). Statistical analysis of the Y-maze
test on the following day revealed a significant effect [F(2, 15) =
7.636; P < 0.006]; post hoc test indicated a significant difference
in CS-odor choices between the MFC-CS group compared with
other groups (P < 0.05; Fig. 1C). This socially transmitted CSodor aversion persisted at least until adolescence (PND 43) and
did not generalize to a novel odor (Fig. S2). Older weaning-aged
pups (PND 18–19) also learned maternal fear through social
transmission (Fig. S3).
Fig. 1. Social transmission of fear from mothers to pups. (A) Schematic diagram illustrating experiments reported in B and C. The mothers were odor
fear conditioned (MFC). Next, the odor CS was presented to the mother in
the presence of her pups (MFC–CS). Control groups for mother–infant exposure include pups of MFC not presented with the CS (MFC-no CS) and pups
with the mother exposed the CS without the US shock (CS alone) and later
presented with this odor while with her pups (CS alone-CS; n = 6 per group).
Pups were then tested in a Y-maze to assess whether they showed aversion
to the CS. (B) PND 6–7 pups exposed to mother previously fear conditioned
and reexposed to the CS in pups presence (MFC-CS) avoid this CS during the
Y-maze test 7 d later (C). PND 13 pups were exposed to a substitute mother
frightened with a previously trained olfactory CS (MFC-CS) or controls (MFCCS and CS alone-CS) (n = 6 per group). At testing, 24 h later MFC-CS pups
expressed aversion to this CS but not controls. All bars indicate mean ± SEM.
***P < 0.006, ANOVAs.
Debiec and Sullivan
responses to a specific odor are acquired through coexposure to
fearful mothers and the CS. Earlier studies demonstrate that
social transmission of fear may occur through observational
learning in older animals (16–19), although infant rats lack
functional visual and auditory sensory systems until they enter
the third week of life (12). However, at birth, pups have a welldeveloped olfactory system, which supports olfactory learning
and infant–mother communication (12–15). Olfaction remains
important throughout life in rodents, including communicating
fear using an alarm odor to support social transmission of fear
(20–22). Thus, we explored whether the frightened mother’s
odor and its ability to increase pups’ CORT was important for
pups’ learning of socially transmitted fear. As illustrated in Fig. 2,
pups were physically isolated from their mother but received
the odor of a frightened mother (by exposure to the previously
trained CS cue) via an olfactometer (OFM-CS; Fig. 2A). Controls included pups exposed to the odor of a mother that was not
frightened (OM-CS) or a neutral odor alone (CS only). The next
day, pups were tested in a Y-maze and results showed a significant effect [F(2, 15) = 7.308; P < 0.007]. Post hoc means comparisons test indicated significant difference in CS-odor choices
between the OFM-CS group compared with the two other groups
(P < 0.05; Fig. 2B). This pattern of findings demonstrates that
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Odor of Frightened Mother Triggers Pup Stress Response and
Reinforces Aversive Learning. We have shown above that fear
pups may acquire an aversion to the CS through association with
the odor of the frightened mother and the potential release of
a maternal alarm odor (20–22).
Because the stress hormone CORT is increased by the alarm
odor (21) and is critical for pups’ fear learning (12–14), we next
explored the possibility that the alarm odor induced an increase
in pups’ CORT and was critical for pups socially transmitted FC.
It should be noted, however, that maternal presence typically
socially buffers pups to suppress the US-induced CORT increase
and blocks FC until PND 16, although these mothers did not
express fear (15). We hypothesized that maternal fear and associated release of the alarm odor was capable of overriding
social buffering and increased pups’ CORT levels to permit fear
learning. To this end, one group of pups at PND 11–12 underwent mother–pups transmission using OFM-CS, whereas the
control group was exposed to the odor of the (substitute) mother
that was not frightened paired with the CS cue (OM-CS) and
pups’ blood was collected for CORT RIA (SI Materials and
Methods). The t test with Welch’s correction for unequal variance revealed a significant effect of exposure [t(16, 18) = 2.133,
P < 0.05, with the OFM-CS group displaying significantly higher
CORT] (Fig. 2C). We then asked whether lowering pups’ CORT
levels would affect mother–infant transmission of fear. Prior (90
min) to the procedure for social transmission of fear using the
odor of the frightened (substitute) mother, pups at PND 11–12
were injected either with a CORT synthesis inhibitor metyrapone (50 mg/kg, i.p.) or saline and returned to the dam until
training. All pups received pairings of the odor of the frightened
mother paired with the olfactory CS (OFM-CS). The next day,
pups were tested with a Y-maze and showed that the CORT
block group failed to learn the odor aversion [t(14) = 5.465, P <
0.002] (Fig. 2D). This pattern of findings demonstrates that activation of the infant’s HPA axis, which is induced by the odor of
the frightened mother, plays an important role in the acquisition
of socially transmitted maternal fear responses.
Amygdala and Olfactory Autoradiography of Mother-to-Infant
Transfer of Fear. To assess neural activity during the trans-
mission of fear, pups were injected with 14C 2-deoxyglucose
(2-DG) (13) 5 min before the mother-to-pups social transmission
of fear. Groups included pups with their ‘substitute mother’
previously fear conditioned and then CS reexposed while with
pups (MFC-CS) or not (MFC-no CS). A third group included
pups with a substitute mother that had not received prior FC and
instead had been exposed to unpaired presentations of the CS and
the US, and then reexposed to the CS with pups (no MFC-CS;
SI Materials and Methods). Pups’ brains were removed and processed for autoradiography. Areas of interest were the amygdala,
a key structure for FC (10), and olfactory structures (20). Significant differences were found throughout amygdala nuclei: lateral
(LA) [F(2, 14) = 7.545; P < 0.008]; basal (BA) [F(2, 14) = 5.564;
P < 0.03]; central (CeA) [F(2, 14) = 13.68; P < 0.0006]; cortical
(CoA) [F(2, 14) = 5.032; P < 0.03]; medial (MeA) [F(2, 14) =
5.988; P < 0.02] (Fig. 3 A and B), and post hoc tests (P < 0.05)
revealed increased uptake in all MFC-CS amygdala nuclei compared with the two other groups. This pattern of findings indicates
that the infant’s amygdala plays a role in the mother-to-infant
transfer of fear. Analysis of the main olfactory bulb showed no
significant differences in the rostral portion (r-MOB; P = 0.24;
Fig. 3 C and D), although analysis of the caudal portion of the
MOB (c-MOB) showed a significant difference [F(2, 14) = 8.438;
P < 0.004] (Fig. 3 C and D), with post hoc tests revealing that the
MFC-CS group was significantly different from both controls. The
MFC-CS group showed increased 2-DG uptake located in the
isolated areas of the glomerular layer (c-MOB-iGl) forming a
characteristic ring encircling the c-MOB. This pattern of neural
activity resembles the location of the necklace glomeruli (NG),
which more rostrally form a ring around the anterior portion of
the accessory olfactory bulb (AOB) and moving posteriorly, encircle the entire trunk of the c-MOB (23). Interestingly, we observed
the highest activity in the ventral aspect of the c-MOB (Fig. 3D),
which has been shown to contain the largest NG (23). The robust
activity in the c-MOB-iGl suggests that this area is involved in the
transfer of fear, possibly through the Grueneberg ganglion (GG)NG olfactory subsystem, which was recently shown to be involved in
alarm odor processing (21) and which was further explored below.
Within the vomeronasal organ (VNO)–AOB subsystem (20), significant differences were found for the AOB [F(2, 14) = 6.090; P <
0.02]; post hoc tests showed increased AOB activity in both
groups with mothers expressing fear (Table 1 and Table S1)
compared with the no MFC-CS group (P < 0.05; Fig. 3 C and D)
suggesting that the infant’s AOB is responsive to maternal threat
communication.
Amygdala and Olfactory Expression of the Immediate Early Gene
c-Fos Following the Mother-to-Infant Transfer of Fear. To verify
Fig. 2. Odor of frightened mother triggers pup stress response and reinforces aversive learning. (A) Schematic diagram of experiment reported in
Fig. 2 B and C on PND 11–13 pups. A mother was placed within an olfactometer and presented with the fear inducing odor CS. The odor of the
frightened mother (OFM) was then presented to pups along the odor CS,
which was still neutral to pups (OFM-CS). Controls included pups exposed to
the odor of the mother (OM) that was not frightened paired with the
neutral CS (OM-CS) or pups exposed to the neutral odor only (CS only); n = 6
per group. The next day all pups were given a Y-maze test. (B). OFM-CS pups
compared with the two control groups (OM-CS and CS only) express subsequent aversion to this CS. (C) Exposure to the odor of the frightened
mother (OFM-CS pups; n = 12) elevates CORT levels compared with the
controls exposed to the odor of the calm mother (OM-CS; n = 8). (D) Pharmacological blockade of CORT synthesis (CORT block; n = 9) by metyrapone
impairs the mother-to-pups transmission of fear in OFM-CS pups compared
with the OFM-CS saline controls (control; n = 7). All bars indicate mean ±
SEM. *P < 0.05, t tests; ***P < 0.007, ANOVAs.
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our autoradiography data at a cellular resolution level, we measured neural activity using expression of the immediate early
c-Fos gene. In this experiment, pups underwent the procedure
for mother–pups social transmission of fear using the odor of the
frightened mother (Fig. 2A and SI Materials and Methods). One
group included pups exposed to the odor of the frightened
mother paired with the CS (OFM-CS), whereas another group
included pups exposed to the odor of the (unfrightened) mother
paired with the CS (OM-CS). Amygdala nuclei demonstrating
high 2-DG uptake during the mother-to-infant transmission of
fear also showed significant higher levels of c-Fos: [t test: LA
t(8) = 3.26, P < 0.02; BA t(8) = 2.33, P < 0.05; CeA t(8) = 3.72,
P < 0.006; CoA t(8) = 2.39, P < 0.05; MeA t(8) = 2.712, P < 0.03]
(Fig. 4 A and B). For the olfactory system, MOB analysis included the granule cell layer (MOB-Gr), associated with olfactory FC c-Fos changes in pups (24) as well as the AOB granule
(AOB-Gr) and mitral (AOB-Mi) cell layers because these areas
are involved in alarm odor signaling (20). Finally, we assessed
Debiec and Sullivan
Fig. 3. Amygdala and olfactory autoradiography of mother-to-infant
transfer of fear. PND 13–14 pups received injections of 2-DG radiographic
marker before exposure to the mother previously fear conditioned (MFC)
expressing fear in response to the CS odor (MFC-CS; n = 7). Controls included
pups exposed to MFC that did not receive CS exposure (MFC-no CS; n = 4)
and pups with mothers that instead of fear conditioning (no MFC) had been
subjected to unpaired presentations of the CS and a foot shock US and then
with pups were reexposed to the CS odor (no MFC-CS; n = 6). Following
exposure, 2-DG uptake was assessed in brains from all groups. (A) MFC-CS
pups display increased 2-DG uptake in the amygdala nuclei: LA, BA, CeA,
CoA, and MeA. (B, Upper Left) Location of the examined amygdala nuclei
followed by representative images (with outlined amygdala nuclei) from
each experiment. (C) All experimental groups display similar 2-DG uptake in
the r-MOB; MFC-CS and MFC-no CS pups display increased 2-DG uptake in
the AOB compared with the no MFC-CS group, and only the MFC-CS group
(compared with the two control groups) show increased uptake in the
c-MOB-iGl. (D) Representative images from each experimental group
showing 2-DG uptake in olfactory areas. (Top) From left: schematic diagram
of the coronal section of the r-MOB followed by the representative images
of the r-MOB. (Middle) From left: schematic diagram of the coronal
section of the c-MOB and AOB followed by the representative images (Gr,
granule cell layer; Gl, glomerular layer; ON, olfactory nucleus). (Bottom)
From left: magnified images from the representative MFC-CS brain displayed
in D showing augmented 2-DG uptake in the c-MOB-iGl. (E, Left) Sagittal
section of the rat brain showing distances from Bregma to coronal sections
where neural activity of the examined structures was measured. (Right)
Color gradation showing neural activity (pseudocolor images displayed using
ImageJ Red/Green Lookup Table: from light green (no activity) through dark
green to dark red (highest activity). All bars indicate mean ± SEM. **P <
0.03; ***P < 0.008; ****P < 0.0006, ANOVAs.
the glomerular layer of the c-MOB because our 2-DG data
showed high neural activity in this area (Fig. 3 C and D). We did
not observe any difference in c-Fos immunoreactivity in the
MOB-Gr (P < 0.05), AOB-Gr (P < 0.05), and AOB-Mi (P <
0.05; Fig. 4 C and D). However, we found increased c-Fos uptake
in the isolated glomeruli encircling the c-MOB (c-MOB-iGl),
a pattern similar to our 2-DG findings and consistent with the
NG location. For analysis of c-Fos expression, we used six
Debiec and Sullivan
Fig. 4. Amygdala and olfactory expression of the immediate early gene
c-Fos following the mother-to-infant transfer of fear. PND 13–14 pups were
subjected to the mother–pups fear transfer training through pairing of OFM
with group OFM-CS. Control group included pups exposed to the OM that
was calm paired with the CS (group OM-CS); n = 5 per group. (A) Increased
c-Fos expression in the OFM-CS group (compared with the OM-CS pups) in
the amygdala nuclei: LA, BA, CeA, CoA, and MeA. (B) Representative images
from each experimental group showing c-Fos expression (dark dots) in the
amygdala nuclei. (Top Right) Schematic diagram of the examined amygdala
nuclei (squares in each nucleus indicate the approximate position of the
representative microphotographs). (C) Similar levels of c-Fos expression in
both groups in the MOB-Gr, AOB-Gr, and AOB-Mi; increased c-Fos immunoreactivity in the MFC-CS group (compared with the OM-CS group) in the
c-MOB-iGl. All, c-Fos expression in all assessed c-MOB-iGl: six per brain; single, comparison of c-Fos immunoreactivity in the single isolated glomerulus
from the c-MOB-Gl area in the OFM-CS group with a corresponding glomerulus in the OM-CS group. (D) Representative images showing c-Fos immunoreactivity (dark dots) in the examined areas. (Upper) From left: c-Fos
expression in the MOB-Gr and the AOB (Gr and Mi); (Lower) Representative
images showing high c-Fos immunoreactivity in the isolated glomeruli
encircling the c-MOB in the OFM-CS group; From left: schematic drawing of
the coronal section of the c-MOB and the AOB (a and b show positions
where isolated glomeruli displayed on the right were located). All bars indicate mean ± SEM. *P < 0.05, **P < 0.03, ***P < 0.006, ****P < 0.0002,
unpaired Student t test; n.s., nonsignificant.
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isolated glomeruli (three from the left and three from the right
side) from each brain. The average score of c-Fos expression in
one isolated glomerulus for each brain was calculated. Data in
Fig. 4C were displayed as a sum of all average scores per glomerulus per brain (all iGls) and as an average score per glomerulus per brain (single iGl). The t test showed increased c-Fos
expression in the OFM-CS group [all iGls; t test: t(8) = 13.97,
P < 0.0002]. Overall, our analysis of c-Fos expression provides
further evidence that the MOB is not activated during the social
transmission of fear. Although we observed a trend of increased
c-Fos reactivity in the AOB-Mi, our data do not support the role
of the AOB in the mother-to-infant transmission of fear. Robust
c-Fos immunoreactivity in the amygdala and the isolated glomeruli encircling the c-MOB suggests the role of these structures
in the intergenerational transmission of fear in our paradigm.
Our behavioral results show that the odor of the frightened
mother triggers pup stress response and supports infant social
fear learning (Fig. 2). In addition, our 2-DG and c-Fos data show
increased neural activity in the isolated glomeruli encircling the
c-MOB, the location pattern similar to the organization of the
NG. Together, our behavioral and imaging data strongly suggest
the involvement of the maternal alarm odor and the GG-NG
pathway in the mother-to-infant transmission of fear.
Grueneberg Ganglion Axotomy Prevents Infant Fear Learning Using
Maternal Alarm Odor. To examine the role of the GG-NG pathway
in the social transmission of fear, PND 2–3 pups underwent either GG axotomy or sham surgery (SI Materials and Methods).
Previous work shows that disruption of GG axonal tracts abolishes fear responses to the alarm odor in adults (21). Twelve days
following the GG axotomy/sham procedure, pups were exposed
to the odor of the frightened mother paired with a neutral lemon
odor CS. The next day, pups were tested in a Y-maze (lemon
odor). To ensure that our surgical procedure left the non–GG-NG
fear learning intact (21), pups were olfactory FC to a novel CS
odor (peppermint) using UC shock and assessed by the Y-maze
test. After the completion of the experiment, GG axotomy lesions
were microscopically verified by an experimenter blind to the
treatment condition. Only data from subjects with bilateral lesions
(GG axotomy) or no disruption of the GG axonal tracts (sham)
were used for analysis. The t test revealed a significant effect of the
surgical procedure, t(10) = 2.401, P < 0.04, with the GG axotomy
group failing to show mother-to-infant transfer of fear, although
shock FC learning was left intact (P < 0.05; Fig. 5A). Thus, the
intact GG-NG pathway is critical for the social transmission of fear
using the odor of the frightened mother.
Amygdala Inactivation Prevents Mother-to-Infant Transfer of Fear.
Our data show robust infant amygdala activation during the social transmission of fear (2-DG, c-Fos) consistent with adult
social fear learning data (18, 25). To determine if the amygdala is
causally involved in social transmission of fear in pups, PND
11–13 pups were bilaterally implanted with intra-LBA amygdala
cannulae and 2–3 d later (14) had the amygdala suppressed
(using GABAA receptor agonist muscimol) or received vehicle
(control) infusion followed by the social transmission of fear
procedure. The next day, pups were tested in the Y-maze, and
then brains were removed to verify cannulae placement (Fig. S4).
Only data from animals with injector cannula tip bilaterally located in the LBA were used for analysis. The t test revealed
a significant effect of treatment, t(8) = 4.714, P < 0.02, with the
muscimol group failing to demonstrate the decreased number of
the CS-odor choices indicative of learning, as observed in controls (Fig. 5B). Thus, LBA inactivation prevented mother-to-pup
fear transmission.
Discussion
In this study we showed that maternal CS-specific fear responses
can be transmitted to the offspring in rats using a social fear
learning paradigm within the nest (Fig. 1). A possible history of
maternal rough handling alone was insufficient to produce specific fear responses in pups: the mothers that expressed contextual fear (Table 1 and Table S1: group MFC-no CS) during
interaction with pups (no CS presented) did not produce specific
fear to the CS in pups. Thus, pups only acquire the CS-specific
fear responses from their mother if she expresses CS-specific fear
in the presence of pups.
During the stress hyporesponsive period of pups (until PND
10), FC is physiologically suppressed and continues to be suppressed if mothers socially buffer the pups during FC until PND
16 (11–15). However, we show that pups as young as PND 6 are
capable of FC if this learning is reinforced by maternal fear
communication in the presence of the CS odor.
Furthermore, similarly to attachment-related odor learning,
this fear-related early learning is robustly retained. Research
shows that brief FC in infancy is usually short-lived, a phenomenon known as infantile amnesia or infantile forgetting, although
repeated sessions induce robust retention (26). In contrast, our
data demonstrate that one session of maternally transmitted fear
learning at PND 13 lasts through early adolescence, indicating
retention for at least 30 d (Fig. S2A). The ability to acquire
maternally transmitted fear during early infancy and before the
development of infant amygdala-dependent odor-shock conditioning (12), combined with the lasting character of these socially transferred memories, suggests a unique characteristic of
12226 | www.pnas.org/cgi/doi/10.1073/pnas.1316740111
Fig. 5. GG- NG pathway and the amygdala are involved in mother-to-infant
transmission of fear. (A) GG axotomy prevents infant fear learning using
maternal alarm odor. PND 2–3 pups underwent GG axotomy (n = 7) or sham
procedure (n = 5). Twelve days later, pups received the mother–pups transfer
of fear using the OFM paired with the odor CS followed 24 h later by the
Y-maze test. On the following day, pups received olfactory fear conditioning
to a novel odor, and 24 h later, Y-maze test to this odor. GG axotomy pups
did not acquire aversive responses to the CS odor paired with the OFM (Left);
both groups acquired aversion to a distinct odor cue trained during fear
conditioning (Right). (B) Amygdala inactivation prevents mother-to-infant
transfer of fear. PND 13–15 pups with implanted cannulae received bilateral
intra-LBA infusions of GABAA receptor agonist muscimol or equivalent volume of saline (control) before the mother–pups transmission of fear using
the odor of the frightened mother paired with the odor CS (n = 5 per group).
Muscimol group did not display CS odor aversion during the Y-maze test 24 h
later. All bars indicate mean ± SEM. *P < 0.04.
maternally transmitted emotional learning. Infants can learn
from their mothers about potential environmental threats before their sensory and motor development allows them a comprehensive exploration of the surrounding environment.
Similarly to odor-shock FC in pups, the role of CORT is important. Pups precociously learn FC if reared by a maltreating
mother, a procedure that increases CORT levels and permits
amygdala learning plasticity (12). Here we show that, without
early life stress, the mother can quickly raise pups’ CORT levels
by expressing fear, and this increase in CORT supports amygdala
plasticity (see SI Materials and Methods for an expanded discussion on CORT).
The critical role of the amygdala in this socially transmitted
fear was demonstrated through the inactivation of the LBA by
muscimol, which prevented mother-to-infant transmission of
fear; this suggests that for the social transmission of fear, pups
use a network similar to that required by shock supported FC
and social transmission of fear in adults. Notably, however, this
social transmission of fear functionally emerges at a younger age
compared with shock-supported FC. We have shown that in our
model, maternal physical presence is not necessary for fear
learning to occur. Pairing of just an odor of the frightened
mother with the neutral (to pups) smell of the CS is sufficient to
produce subsequent aversive responses in pups that are odor
specific (Fig. 2B). Additionally, our GG-related increases of
neural activity and GG axotomy data suggest the GG is important in fear communication via maternal alarm odor (similar to
odor fear communication in adults) and mother–pups transfer of
fear. However, this observation does not rule out the possibility
that other sensory modalities are involved in the intergenerational transfer of fear at later stages of development (18,
19). Consistent with existing alarm odor signaling literature, our
data suggest that through excitation of pups’ GG-NG (and
possibly VNO-AOB) pathway, maternal olfactory threat information reaches the MeA and CoA and activates the LA, BA,
and CeA known to support fear learning and expression (20–22).
However, the distinct neural and molecular mechanisms through
which maternal alarm odor instructs fear learning in the pups’
Debiec and Sullivan
Materials and Methods
A detailed description of materials and methods is provided in SI Materials
and Methods.
Subjects. Male and female Long–Evans rats were born and bred in our
colony. All animal care and experimental procedures were conducted in
accordance with National Institutes of Health guidelines and were approved by the Nathan Kline Institute’s Institutional Animal Care and Use
Committee.
Odor Cue Delivery. CS odor (pure peppermint or lemon; McCormick) was
delivered by a flow dilution olfactometer controlled by FreezeFrame software (2 L/min flow rate, 1:10 CS odor:air).
Fear Conditioning of Mothers. Mothers were conditioned before breeding or
during lactation. Standard Coulborn conditioning boxes were used. Six
pairings of a 30-s olfactory CS and electric foot shock US (0.5 s; 0.6 mA) were
controlled by FreezeFrame.
Fear Conditioning of Pups. Pups were placed individually in 600-mL clear
plastic beakers and were given five pairings of the 30-s olfactory CS and
electric tail shock US (1 s; 0.5 mA; Lafayette shock generator) controlled and
recorded using EthoVision.
Mother–Pups Transmission of Fear Using the Odor of the Frightened Mother.
Mother was placed in a visually and sound-isolated container connected
through an olfactometer (allowing continuous flow of air). The odor was
delivered to the plastic beakers that contained individual pups. Maternal fear
was elicited by four CS presentations that was accompanied by simultaneous
CS presentations to each pup (Figs. 2, 4, and 5).
Y-Maze Testing of Pups’ Aversion Learning. Pups were given five trials to
choose between two arms, one containing the CS odor and the other containing a familiar odor (clean wood bedding).
RIA. Immediately following the socially transmitted FC, trunk blood samples
of 300–400 μL were collected and analyzed (Kit brand).
CORT Blockade. Metyrapone (50 mg/kg) or vehicle was injected i.p. 90 min
before training.
Cannulae Implantation/Drug Infusions. Pups were implanted and recovered for
2–3 d. LBA pharmacological inactivation occurred with bilateral muscimol
(2 mM) or vehicle at 0.1 μL/min for 5 min (total infusion volume 0.5 μL).
GG Axotomy and Verification of Axonal Tract Lesions. GG axotomy was conducted as in previous work (21).
2-DG Autoradiography. Pups were injected with 14C 2-DG (20 μCi/100 g, s.c.)
5 min before the mother–pups transmission FC, the brain removed 45 min
later, processed for autoradiography, and analyzed (ImageJ software; National Institutes of Health).
Immediate Early Gene c-Fos Expression. Pup brains were removed 90 min after
socially transmitted FC and processed using standard procedures (SI Materials
and Methods).
Statistical Analysis. Statistical analysis was performed using Student t test or
ANOVA followed by post hoc Newman–Keuls test. Differences were considered significant when P < 0.05.
Mother–Pups Social Transmission of Fear. Socially transmitted fear was done in
the pup’s home cage with four CS presentations controlled by FreezeFrame
and videotaped (Figs. 1 and 3 and Figs. S1–S3).
ACKNOWLEDGMENTS. We thank Drs. D. Wilson, H. Akil, C. Cain, C. A. Turner,
and E. Brehman for comments on the manuscript, and Dr. C. Raineki,
L. Michelson, K. Szyba, L. Salstein, C. Hochman, E. Brehman, and O. Barbu for
technical assistance. This work was supported by National Institutes of Health
Grant DC 009910, National Institute of Mental Health Grant MH091451 (to
R.M.S.), NARSAD Young Investigator Award from the Brain and Behavior
Research Foundation, and University of Michigan funds (to J.D.).
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PNAS | August 19, 2014 | vol. 111 | no. 33 | 12227
NEUROSCIENCE
amygdala remain to be determined (see SI Materials and Methods
for an expanded discussion).
A recent study shows that parental traumatic experience may
induce neuroanatomical adaptations and related cue-specific
behavioral predispositions in offspring (9). Our results, however,
demonstrate that parental specific fear behaviors may be transferred to infants through emotional communication and associative learning mechanisms producing lasting memories. These
findings provide a model characterizing how parental adaptive
and pathological fear may be transmitted to their offspring, such
as in PTSD (6, 27, 28) and specific phobias (2). Understanding
of the neural and molecular mechanisms controlling intergenerational transmission of fear will help to develop better
preventive and therapeutic methods.