Download Document

Memorie di paura condizionata:
formazione ed estinzione
L’utilizzo di protocolli di condizionamento in
cui uno stimolo inizialmente neutro (stimolo
condizionato, SC) viene ripetutamente
associato ad uno stimolo incondizionato (SI)
doloroso provoca la formazione di una
risposta di paura allo SC molto robusta e
duratura.
Esseri umani ed altri animali, sottoposti ad un protocollo
di condizionamento come quello descritto, apprendono
due associazioni:
Primo, apprendono l’associazione fra lo stimolo
incondizionato e l’ambiente in cui è avvenuto
l’apprendimento.
Una volta che questa associazione si è formata, la visita
all’ambiente di condizionamento determinerà il
manifestarsi della risposta di paura, anche in assenza
dello stimolo incondizionato e dello stimolo condizionato.
Questa associazione fra stimolo incondizionato e
ambiente dà origine al condizionamento alla paura al
contesto (contextual fear conditioning).
La seconda associazione appresa è fra
stimolo condizionato (suono, stimolo
luminoso) e stimolo incondizionato: lo
stimolo condizionato (predice lo stimolo
incondizionato)
Quindi, anche quando il suono o lo stimolo
luminoso vengono presentati in un
ambiente nuovo, il soggetto mostrerà una
chiara risposta di paura allo stimolo
condizionato.
L’associazione suono (luce)-stimolo
incondizionato dà origine al
condizionamento alla paura allo stimolo
(cued fear conditioning).
In both context and cued fear conditioning, retention
of the learned associations is assessed by
measuring fear responses, such as freezing,
following conditioning.
It is important to use a novel environment when
measuring retention of the tone-shock association in
order to dissociate cued conditioning from context
conditioning.
Importantly, this simple tone-US pairing is able to elicit
conditioned fear in many types of environments, and is
thus context independent.
That is, the animal learns the association between
tone and shock, not that the tone predicts shock
only in a certain time or place. (LeDoux, 2011)
This is a form of elemental learning.
The presence of the fear response during
retention tests reflects the extent to which
context-US associations have been
consolidated.
Once formed, these memories can
persist for days, weeks, or even years
(Golet et al 1986).
Cued fear conditioning in rodents
45 days later, only CS (tone in new context)
Baseline freezing
Contextual fear conditioning in rodents
45 days later, only training context
Baseline freezing
Tali protocolli, detti di “acquisizione
di paura condizionata”, o di
“condizionamento alla paura”, sono
utilizzati come modello per la
formazione di memorie emotive
negative associate a stimoli o eventi
traumatici e per la loro estinzione
non solo nell’animale ma anche
nell’uomo.
Primo passo: quali strutture sono coinvolte
nell’acquisizione e nel recupero a lungo termine di
memorie di paura condizionata?
Studi classici di lesione e di attivazione hanno indicato
l’amigdala come struttura cruciale per la formazione
ed il richiamo di memorie di paura condizionata
“cued”usando come uscita la risposta emotiva di
paura (freezing, aumento pressione arteriosa,
conduttanza cutanea, …).
L’ amigdala fa parte del sistema limbico ed è
costituita da diverse suddivisioni (laterale, basale,
intercalata, centrale)
Regioni dell'amigdala messe in evidenza con differenti metodi colorazione. Nelle immagini
in alto troviamo la colorazione di Nissl, sinistra, e la colorazione per l'acetilcolinesterasi
utile per mettere in evidenza i differenti nuclei a destra. Nelle immagini sottostanti a sinistra
abbiamo le connessioni tra l'amigdala laterale (LA), basale (B), accessorio basale (AB) e
centrale (CE), l'immagine di destra sottolinea il fatto che ogni nucleo può essere suddiviso
in sotto-nuclei. PIR sta per corteccia piriforme, e CPU caudato-putamen (LeDoux 2000).
PFC
A
Corteccia
prefrontale
Aree corticali associative
Corteccia cingolata
Nuclei talamici
Ippocampo
Ipotalamo
Amigdala
Fig. 2. Schema del circuito neurale proposto da Papez come base neurale per le emozioni
con le successive integrazioni di Maclean. In grigio le strutture che oggi sappiamo essere
maggiormente coinvolte nelle risposte emozionali. Le connessioni tracciate con le linee
tratteggiate rosse fra l’amigdala, le aree corticali prefrontali e la corteccia cingolata non
erano previste nel circuito originale. Le abbiamo aggiunte in base alle attuali conoscenze.
L’amigdala è cruciale per il provare e manifestare emozioni,
non solo per apprendere relazioni fra stimoli o contesti ed
eventi emozionalmente “carichi”.
La stimolazione elettrica dell’amigdala provoca nell’uomo un forte
stato di paura.
Lesioni dell’amigdala in animali producono docilità ed assenza di
reazioni di paura in risposta a stimoli che normalmente le
inducono.
Pazienti con lesioni che includono l’amigdala (paziente SM)
mostrano difficoltà a reagire e a giudicare le espressioni facciali di
felicità, paura, disgusto o tristezza anche se non hanno alcuna
difficoltà a riconoscere l’identità delle stesse facce (questa
capacità è invece danneggiata da lesioni alle aree visive
inferotemporali).
Esperimenti di neuroimmagine hanno evidenziato come, in
soggetti normali cui vengono mostrate facce con espressione
impaurita o felice, l’attivazione dell’amigdala, ed in particolare
dell’amigdala sinistra, è significativamente maggiore per le facce
impaurite che per quelle felici
L’attivazione
dell’amigdala è
correlata con la
“paurosità” della foto
Lesioni dell’amigdala impediscono la
formazione di tracce di memoria di paura
condizionata.
Questo significa che soggetti con lesioni
dell’amigdala non imparano a reagire in maniera
appropriata ad uno stimolo la cui apparizione
dovrebbe invece avere valore predittivo per
l’accadere di un evento negativo.
Differenze amigdala ippocampo (memoria
implicita/esplicita)
Papez attribuiva all’ippocampo un ruolo centrale nella regolazione delle
emozioni, questo ruolo in realtà appartiene all’amigdala.
Esempio nell’uomo
Compito: Venivano mostrati campi colorati, uno dei colori era associato un
fastidioso suono, intenso e di alta frequenza.
Lesionati amigdala: ricordo dei colori mostrati e del fatto che alcuni fossero
seguiti dal suono, ma nessuna associazione del colore con risposte emotive
sistemiche (es. frequenza cardiaca).
Lesionati ippocampo: mostravano risposte emotive sistemiche in risposta ai
colori associati al suono, ma non si ricordavano che era avvenuta una
associazione colore-suono
L’ippocampo è importante per le memorie di paura
condizionata al contesto
Cued fear conditioning
ASSOCIAZIONE
Amigdala
Contextual Fear Conditioning
ASSOCIAZIONE
Amigdala e ippocampo
L’accoppiamento
CS-US è
necessario e, una
volta appresa, la
risposta di paura
si manifesta anche
in contesti diversi
In a typical auditory fear conditioning procedure, rats are habituated to the conditioning chamber but given no stimuli.
During the conditioning session, the electric shock (US) is paired with the auditory-conditioned stimulus (CS) several
times (usually 1–5). The effects of conditioning are then assessed in a test session during which the conditioned
stimulus is presented alone. Most studies measure “freezing” behavior, which is an innate defensive response
elicited by the conditioned stimulus after conditioning. An unpaired control group in which the conditioned stimulus
and unconditioned stimulus are presented in a nonoverlapping manner is often used. The conditioned stimulus elicits
little or no freezing prior to conditioning (not shown). Both the paired and unpaired group freeze during the training
session due to the shock presentation. In the test session, the paired group exhibits considerably more conditioned
stimulus-elicited freezing than the unpaired. Differences between the paired and unpaired group reflect the
association that is learned as a result of conditioned-unconditioned stimuli pairing.
Corteccia acustica
primaria
Cortecce associative
polimodali
Corteccia acustica
associativa
Nucleo colinergico del
prosencefalo basale
Nucleo genicolato
mediale
(parte ventrale)
Stimolo
acustico
emozionale
Nucleo genicolato
mediale
(parte mediale)
NC
Amigdala
NBL
Sistemi di controllo delle risposte
emozionali
(risposte comportamentali, risposte del
SNA, risposte del sistema endocrino
Schema semplificato delle vie coinvolte nell’elaborazione di uno stimolo sensoriale (in questo caso acustico) con
significato emozionale. Notare che l’informazione sensoriale è inviata al nucleo basolaterale (NBL) dell’amigdala
attraverso due vie, una rapida e diretta dai nuclei sensoriali talamici ed una più lenta, ma accompagnata dalla
cosciente percezione dello stimolo, che arriva dalle aree corticali acustiche. Particolarmente per le risposte a
stimoli paurosi l’importanza dell’informazione che arriva all’amigdala dal talamo sta nel fatto che innesca risposte
rapide che possono essere importanti in situazioni di pericolo. Il NBL proietta al nucleo centrale (NC), che
costituisce l’uscita dell’amigdala e proietta all’ipotalamo ed a diversi nuclei del tronco dell’encefalo che controllano
le risposte emozionali. Esso proietta anche ai nuclei colinergici del prosencefalo basale le cui proiezioni alla
corteccia determinano una attivazione corticale generalizzata, detta arousal, e facilitano la modificazione
dell’efficacia sinaptica (plasticità sinaptica). Adattata da LeDoux, 1992.
Quindi, l’amigdala è in grado di mediare la
componente corporea delle emozioni tramite le
proiezioni del nucleo centrale: la maggior attività nel
nucleo centrale si può pensare che determini, nel
condizionamento alla paura, la maggior risposta allo
stimolo condizionato.
L’amigdala, attraverso le sue proiezioni all’ipotalamo ed
al tronco dell’encefalo media le reazioni corporee, la
parte inconscia di uno stato emozionale.
L’amigdala è anche importante per l’esperienza
cosciente delle emozioni: essa proietta infatti alle aree
corticali associative, ed in particolare alla corteccia
cingolata anteriore ed alla corteccia orbitofrontale
E’ possibile evidenziare nell’uomo risposte emozionali a stimoli
“non percepiti”?
In un lavoro su soggetti normali, (Morris et al., 1999) è stata
usata una procedura di rapida presentazione di due stimoli visivi
in successione. In questo modo, la presentazione del secondo
stimolo previene la percezione cosciente del primo (procedura di
masking).
Gli stimoli erano costituiti da facce, alcune delle quali erano state
accoppiate con un forte rumore per provocare una risposta di
paura condizionata.
Ebbene, la presentazione di queste facce evocava sudorazione
delle mani (risposta emozionale) sia quando esse venivano
coscientemente percepite sia quando la loro percezione cosciente
era prevenuta dal masking.
Presentation
Presentation
Perception
Perception
Presentation
Perception
SCR
CS+
Presentation
CS+
Perception
SCR
Quindi, è possibile evocare una risposta emotiva con uno stimolo
“non visto”.
In questo caso si attivava l’amigdala destra in correlazione con
l’attivazione del collicolo superiore e del pulvinar (nucleo
talamico) ed in correlazione negativa con la corteccia
orbitofrontale e con l’area delle facce nella corteccia
inferotemporale.
Quando invece lo stimolo condizionato veniva coscientemente
percepito, l’attivazione dell’amigdala destra correlava
positivamente con l’attivazione dell’ippocampo e del cervelletto e
negativamente con quella del pulvinar.
L’amigdala sinistra non mostrava correlazione con l’attività del
pulvinar.
Cosa accade nell’amigdala quando si forma
una memoria di paura condizionata?
Convergence of the auditoryconditioned stimulus and nociceptive
unconditioned stimulus in the amygdala
is essential for fear conditioning.
Convergence of conditioned and
unconditioned stimuli occurs in
lateral nucleus of the amygdala (LA),
especially in the dorsal subnucleus
(LAd), leading to synaptic plasticity
in LA.
Plasticity may also occur in the central
nucleus (CE) and in the auditory
thalamus.
LA connects with CE directly and
indirectly by way of connections in
the basal (B), accessory basal (AB),
and intercalated cell masses (ICM).
CE connects with hypothalamic and
brainstem areas that control the
expression of conditioned fear
responses, including freezing and
autonomic (ANS) and hormonal
responses.
LeDoux, 2011
In a Hebbian model of fear conditioning, strong
depolarization of LA pyramidal cells evoked by the aversive
unconditioned stimulus leads to strengthening of coactive
conditioned stimulus inputs onto the same neurons
(remember inception of false memories, Tonegawa’s paper
2014).
Existing data support the idea that LA associative plasticity and
fear memory formation are triggered by unconditioned stimulusinduced activation of LA neurons.
Thus, unconditioned stimulus-evoked depolarization is necessary
for the enhancement of conditioned stimulus-elicited neural
responses in LA after conditioned-unconditioned stimuli pairing
(Rosenkranz and Grace, 2002), and pairing a conditioned
stimulus with direct depolarization of LA pyramidal neurons as an
unconditioned stimulus supports fear conditioning (Johansen
et al., 2010).
Though there is evidence that Hebbian plasticity in LA
may not entirely explain fear conditioning, it is clear that
synaptic plasticity at conditioned stimulus input
pathways to the LA does occur with fear
conditioning.
Supporting this, in vivo studies demonstrate an
enhancement of auditory stimulus-evoked responses in
LA neurons after fear conditioning.
Potenziali d’azione/sec
Il condizionamento alla paura comporta variazioni della
trasmissione sinaptica fra i neuroni nell’amigdala
Working Model of
Molecular Processes in the
amygdala Mediating
Acquisition and
Consolidation of Fear
Memories
All dotted lines denote hypothetical pathways.
Molecules and processes in blue are
known to be involved in the acquisition
of fear conditioning.
Molecules and process in black are
known to be involved specifically in
the consolidation or maintenance of
fear conditioning.
Purple labels denote molecules or elements
whose role is not established for fear
conditioning but are part of an established
intracellular signaling pathway.
Abbreviations: AC, adenyl cyclase; AKAP, A-kinase anchoring protein; Arc, activity-regulated cytoskeletal-associated protein; β-AR, βadrenergic receptor; BDNF, brain-derived neurotrophic factor; Ca2+, calcium; CaMKII, Ca2+/calmodulin (Cam)-dependent protein
kinase II; CREB, cAMP response element (CRE) binding protein; GluA1, glutamate AMPA receptor subunit 1; GluA2/3, glutamate
AMPA receptor subunit 2 and 3 heteromer; IP3, inositol 1,4,5-triphosphate; MAPK, mitogen-activated protein kinase; mGluR,
metabotropic glutamate receptor; mTOR, mammalian target of rapamycin; NF-kB, nuclear factor κ light-chain enhancer of activated B
cells; NMDAR, N-methyl-d-aspartate glutamate receptor; NO, nitric oxide; NOS, nitric oxide synthase; PI3-K, phosphatidylinositol-3
kinase; PKA, protein kinase A; PKC, protein kinase C; PKG, cGMP-dependent protein kinase; PKMζ, protein kinase M ζ; TrkB,
tyrosine kinase B; VGCC, voltage-gated calcium channel.
Monoamine Neuromodulatory-Dependent Mechanisms
Involved in Fear Learning
Though Hebbian plasticity may indeed occur during learning, it
does not fully explain learning — especially learning in highly
charged emotional situations.
It is generally thought that monoamine transmitters such as
norepinepherine (NE) and dopamine (DA) that are released in
emotional situations regulate glutamatergic transmission and
Hebbian plasticity (for review, see [Bailey et al., 2000] and
[McGaugh, 2000] , and Tully and Bolshakov, 2010).
The modulation of Hebbian (or activity-dependent) plasticity by
neuromodulators (such as monoamines) or plasticity that is
independent of postsynaptic activity is called heterosynaptic
plasticity.
This is in contrast with purely activity-dependent
Hebbian plasticity, which is referred to as
homosynaptic plasticity (Bailey et al., 2000).
Indeed, in a variety of model systems, it has been
shown that monoamines modulate plasticity
underlying memory formation (Carew et al., 1984]
and [Bailey et al., 2000] , and Glanzman, 2010 for
review).
Neuromodulators also contribute to fear
conditioning.
Tono noradrenergico nell’amigdala alto:
maggior probabilità che si inneschi e consolidi un
potenziamento delle risposte dell’amigdala allo
stimolo condizionato appaiato allo stimolo
incondizionato.
Tono noradrenergico basso: si verifica l’opposto.
A quale stato comportamentale corrisponde un tono
noradrenergico alto?
L’apprendimento di risposte di paura condizionata
coinvolge quindi cambiamenti duraturi
dell’efficacia sinaptica di tipo potenziamento a
lungo termine (LTP) nel nucleo laterale.
L’ipotesi è che l’attività evocata nel nucleo
laterale dalla presentazione dello stimolo
condizionato, quando appaiata con l’attività
evocata dallo stimolo incondizionato, provochi un
potenziamento della risposta allo stimolo
condizionato (proprietà associativa di LTP).
Il nucleo laterale dell’amigdala proietta al nucleo
centrale, che costituisce la principale uscita
dell’amigdala.
La maggior risposta del nucleo laterale allo
stimolo condizionato determinerà una maggior
attivazione del nucleo centrale.
Le proiezioni del nucleo centrale sono dirette
all’ipotalamo e alle strutture del tronco
dell’encefalo coinvolte nel controllo del sistema
nervoso autonomo: in questo modo l’amigdala
controlla sia il sistema nervoso autonomo che il
sistema endocrino che l’asse ipotalamo-ipofisisurrene.
Indurre risposte di paura
in assenza di stimoli
condizionati (o
incondizionati)
attraverso la semplice
stimolazione
optogenetica
dell’amigdala centrale
mediale
Coronal section of the mouse brain indicating the location of the central amygdala (CEA). CEl/CEm, lateral/medial
subdivisions of CEA. Numbers indicate the antero-posterior coordinates caudal to bregma. b, Red fluorescent
neurons in CEm infected with AAV-ChR22A-tdimer. Scale bar, 100 μm. c, Left: example experiment
illustrating rapid and reversible freezing induced by bilateral stimulation of ChR2-expressing
CEm neurons with 10 s of blue light (inter-stimulation intervals, 30–60 s). Right: summary data demonstrating
significant light-induced freezing responses in AAV-ChR22A-tdimer infected animals, but not in sham-operated
controls.
Circuiti nei quali si manifesta
plasticità nell’acquisizione e
nell’estinzione del “fear
conditioning”
PKC-d+
Fear
CS presentation induced an increase (in CElon neurons) or a
decrease (in CEloff neurons) in firing.
S Ciocchi et al. Nature 468, 277-282 (2010)
Is freezing a suitable behavioural parameter to
assess contextual fear conditioning?
Fin qui 19 novembre
Studies from many laboratories have reported
that in rats, lesions or inactivation of the
basolateral amygdala complex (BLC; the set of
lateral, basal, and accessory basal nuclei of the
amygdala) decrease the immobility or
“freezing” (defined as lack of movement except
for respiration) displayed in the presence of
specific cues or contexts previously paired with
footshock
Such findings have suggested that the BLC may be critical for storing cue–
footshock or context–footshock learned associations and that the BLC may be
a locus of the neuroplasticity that mediates fear conditioning.
A major difficulty with these interpretations is that lesions and other treatments
that disrupt amygdala functioning also decrease unlearned fear, or anxiety.
(Vazdarijanova e McGaugh, 1998)
I comportamenti rintracciabili dopo un training di
apprendimento che possono essere presi in
considerazione come indici di un
apprendimento nel paradigma del contextual
fear condition sono (Vazdarjanova McGaugh 1998):
1.La latenza della prima entrata nella zona
dell’apparato in cui è avvenuto
l’apprendimento (memoria esplicita);
2.Il numero di entrate nei singoli bracci
(memoria esplicita);
3.Il freezing definito come l’assenza totale di
movimenti eccetto quelli respiratori (memoria
implicita);
La componente esplicita ed implicita della memoria
emotiva hanno un decorso di ritenzione diverso
LATENZA D'INGRESSO "braccio shock"
GRUPPO ST. SHOCK
GRUPPO ST.CONTROLLO
140,00
latenza d'ingresso sec.
120,00
100,00
80,00
60,00
40,00
20,00
0,00
Abituazione
Test
7° giorno
14°giorno
21°giorno
28°giorno
35°giorno
TEMPO SPESO IN FREEZING"braccio Shock"
GRUPPO STANDARD SHOCK
GRUPPO STANDARD CONTROLLO
8,00
7,00
6,00
freezing sec.
5,00
4,00
3,00
2,00
1,00
0,00
-1,00
-2,00
Abituazione
Test
7°giorno
14°giorno
21°giorno
28°giorno
35°gionro
Recall of remote fear memories
Visual, acoustic, and olfactory stimuli
associated with a highly charged emotional
situation take on the affective qualities of
that situation.
Where the emotional meaning of a given
sensory experience is stored is a matter of
debate.
Secondary auditory cortex
Remote
Sacchetti 2010
L’amigdala si attiva anche per il richiamo di memorie remote
Fin qui 16 n0vembre
Secondary auditory cortex and fear memory
Secondary cortices that perform
high-level sensory analysis combine sensory processing and
memory plasticity to encode the behavioral salience of
perceiving stimuli.
Such information becomes widely distributed throughout the
cortex, each secondary sensory cortex coding the valence of
stimuli of a specific modality.
Such a memory storage mechanism results in a synaptic
strengthening of corticocortical
connections that may provide the integrated view of the
whole emotional experience during memory recall.
Dysregulation of the fear system is at the
core of many disorders.
Despite recent advances, the question of
how to persistently weaken aversive CSUS associations, or dampen traumatic
memories in pathological cases, remains
a major dilemma.
The term ‘emotion regulation’ refers to the different
types of regulatory processes that can control the
physiological, behavioral, and experiential
components of our affective responses (Gross and
Thompson, 2007).
These include automatic forms of regulation that
flexibly alter our emotional responses as we learn about
changing stimulus outcome contingencies in our
environment, as well as intentionally deployed
techniques.
We may change how we think about an
emotion-evoking stimulus, or shift our focus of
attention to diminish an undesired emotion.
We may also take action to avoid or cope with a
distressing situation or to bring about a positive
outcome.
Effective regulation of emotion through these
various processes is essential for both our
mental and physical well being.
Cancellare o ridurre l’impatto di memorie traumatiche
intrusive
Strategie utilizzate:
1. Sfruttare il riconsolidamento
2. Usare protocolli di estinzione
3. Cognitive regulation
4. Active coping
During extinction, fear is diminished through
learning that a previously threatening stimulus no
longer signals danger.
Cognitive emotion regulation involves using
various mental strategies to modify a fear
response.
In active coping, fear is regulated through the
performance of behaviors that reduce exposure
to a fear-evoking stimulus.
Finally, a fear memory can be disrupted after it is
recalled through pharmacological or behavioral
manipulations that block its reconsolidation.
Our understanding of the neurocircuitry
underlying the control of fear stems from
research across species clarifying the
mechanisms by which we learn and
modify emotional associations, as well
as studies exploring forms of cognitive
emotion regulation that are uniquely
human.
Failure to properly regulate fear responses
has been associated with various forms of
psychopathology.
For example, some forms of anxiety
disorders are thought to involve dysfunction
in the neural systems underlying the
extinction of fear learning (see Rauch et al,
2006) and are treated with extinctionbased therapies.
“Facilitating emotion regulation through
intentional cognitive mechanisms is
a primary aim of cognitive-behavioral
psychotherapy, a successful approach
to the treatment of
depression and other psychological
disorders.”
Hartley and Phelps, 2010
“Active coping may be used to
attenuate learned fear responses and
mitigate the functional impairments
engendered by a fear evoking
stimulus.”
Hartley and Phelps, 2010
Finally, reconsolidation may permit
the permanent modification of the
pathological traumatic memories.
Hartley and Phelps, 2010
“An improved understanding of the
neurocircuitry of normal emotion
regulation sheds light on the potential
mechanisms underlying specific
disorders, and may aid in the
development of more effective
treatments for these conditions.”
Hartley and Phelps, 2010
Il riconsolidamento
Il riconsolidamento
The classic view of memory suggests that
immediately after learning, there is a period of
time during which the memory is fragile and
labile, but after sufficient time has passed the
memory is, more or less, permanent.
During this consolidation period, it is possible to
disrupt the formation of the memory, but once this
time window has passed, the memory may be
modified or inhibited, but not eliminated.
Il riconsolidamento
However, recent studies support an alternative
view of memory in which every time a memory
is retrieved, the underlying memory trace is
once again labile and fragile, requiring
another consolidation period called
reconsolidation.
Given that fear memories can, at times, be
maladaptive, contributing to fear or anxiety
disorders, the possibility of disrupting an
earlier acquired fear memory by blocking
reconsolidation could have significant clinical
implications.
The interest in reconsolidation was sparked by
a finding by Nader et al (2000), showing that
conditioned fear can be eliminated by
blocking reconsolidation in the amygdala
of a reactivated fear memory.
Fear memories undergo
reconsolidation only when
they are retrieved and this
reconsolidation process
can be disrupted,
essentially eliminating
the earlier learned fear.
Blockade of reconsolidation is specific to the fear memory
reactivated, leaving other memories intact (Doyere et al,
2007).
Retrieval and reconsolidation alter memories via synaptic
plasticity in lateral amygdala (LA).
Changes in short-latency
AEFPs in lateral amygdala. A
selective additional potentiation
was observed during PR-STM
(left panel, P < 0.05, contrast
analysis) after reactivation with
the pure tone conditioned
stimulus. U0126 produced a
depotentiation of lateral
amygdala responses to the
reactivated conditioned
stimulus (CSr), regardless of
tone type (right panel, P < 0.05,
two-way ANOVA).
(c) Changes in AEFPs in the
auditory thalamus. No
differential effect of reactivation
or intra–lateral amygdala
infusion of U0126 on either
memory test was observed.
Fear memories erased by disrupting reconsolidation
processes do not return with passage of time, or the
alteration of contextual cues, or additional stress
(Duvarci and Nader, 2004).
Owing to the lack of return of fear, blocking reconsolidation
provides an important advantage over other techniques that
might be used to regulate emotion.
If, as suggested, the memory trace is permanently altered, the
inhibition of the fear memory through extinction, regulation, or
adaptive action is not necessary.
Research on the blockade of the reconsolidation
of fear memories in humans has been slow to
emerge for a few reasons.
One primary reason is that the initial findings
showing the blockade of reconsolidation relied on
the administration of protein synthesis inhibitors,
which is not a viable technique in humans.
Recently, however, two techniques that can be
translated to humans have been identified.
The first uses a beta-adrenergic antagonist, propranolol, which is safe to administer to
humans, to block reconsolidation. Debiec and LeDoux (2004) showed that either intraamygdala or systemic administration of propranolol immediately after reactivation of the
CS blocked the reconsolidation of conditioned fear and prevented the return of fear.
A recent finding by Kindt et al (2009) found that the administration of
propranolol can block the return of fear in human participants.
“From an evolutionary perspective, it is extremely functional to never forget the most
important events in life. However, the putative indelibility of emotional memory can also
be harmful and maladaptive, such as in some trauma victims who suffer from dreadful
memories and anxiety. If emotional memory could be weakened or even erased, then
we might be able to eliminate the root of many disorders, such as post-traumatic stress
disorder.” Kindt et al., 2009
Testing included different phases across 3 d:
fear acquisition (day 1), memory reactivation
(day 2), and extinction followed by a
reinstatement procedure and a test phase
(day 3)
The conditioned fear response was measured as potentiation
of the eyeblink startle reflex to a loud noise (40 ms, 104 dB) by
electromyography of the right orbicularis oculi muscle.
Stronger startle responses to the loud noise during the fearconditioned stimulus (CS1+) as compared with the control
stimulus (CS2-) reflects the fearful state of the participant
elicited by CS1+.
Startle potentiation taps directly into the amygdala, and fearconditioning procedures yield highly reliable and robust
startle potentiation
Mean startle potentiation to the
fear-conditioned stimulus
(CS1), the control stimulus
(CS2) and noise alone (NA)
trials during acquisition (trial
1–8), extinction (trial 1–10) and
test (trial 1–5) for the placebo
(n = 20, a,b), propranolol
reactivation (n = 20, c,d) and
propranolol without
reactivation (n = 20, e,f) group.
CS1+ refers to the fear
conditioned stimulus during
acquisition,
CS1- refers to the fear
conditioned stimulus during
extinction and test,
CS1-R refers to the reactivation
of the fear conditioned stimulus
and CS2- refers to the control
stimulus during all phases of the
experiment. Error bars represent
s.e.m.
In the procedure of Kindt et al (2009), propranolol was given
before reactivation.
If propranolol blocks the expression and return of fear memories,
this suggests a potentially promising clinical intervention.
Earlier research has suggested that propranolol might be clinically
useful if administered immediately after a traumatic event.
It is suggested that propranolol might impair the initial
consolidation of fear memories, and thus prevent the development
of PTSD in the future (Pitman and Delahanty, 2005).
However, this treatment is not applicable to someone who
already suffers from PTSD.
To explore whether the administration of propranolol during
the reconsolidation period might be useful in the treatment of
clinical disorders, Brunet et al (2008) tested whether it could
prevent the return of older fear memories in patients
suffering from PTSD.
The patients were asked to describe their traumatic events to
reactivate their traumatic memories.
Immediately after reactivation, patients were given either
propranolol or placebo.
A week later during a script-driven memory task, physiological
responses to these memories were assessed.
The patients who received propranolol showed
diminished physiological fear responses for most of
the measures assessed.
Although this initial clinical study had a small sample,
moderate effects, and lacked some important control
conditions, it provides preliminary evidence that
propranolol could be an important tool in the treatment of
anxiety disorders by targeting the reconsolidation
process.
The studies examining means to block the reconsolidation of
fear memories to date have primarily used pharmacological
manipulations.
However, reconsolidation is not a process that evolved via
pharmacological manipulation in the service of treating
clinical disorders.
Rather reconsolidation is a natural memory
process that allows older memories to be
updated to incorporate new information each
time they are retrieved.
On account of this, it is possible that presenting specific
new information during the reconsolidation window can
provide another means to alter or block older fear
memories.
To explore whether the reconsolidation of fear memories
can be influenced without pharmacological manipulation,
Monfils et al (2009) developed a behavioral
intervention aimed at blocking reconsolidation.
To understand this work, we need to examine first the
other approach, extintion, because the Monfils paper
uses the reconsolidation approach coupled with
extintion.
L’estinzione consiste nella riduzione della risposta emotiva
condizionata, determinata dall’apprendimento dell’associazione
tra uno stimolo condizionato (CS) ed uno incondizionato (US), in
seguito alla ripetuta esposizione allo CS in assenza dello US.
L’estinzione non ha caratteristiche costanti durante l’ontogenesi.
Apprendimento in contesto A, estinzione in contesto B
Extinction learning involves the formation of a novel
stimulus-outcome association.
The CS that earlier predicted danger now predicts safety. This
new extinction memory does not erase or overwrite the
memory for the original CS–US association. This is evidenced
by the re-emergence of the CR in certain circumstances including
a shift in context (renewal), unsignaled presentation of the US
(reinstatement), or the mere passage of time (spontaneous
recovery)
(context B)
Livello di freezing al termine dell’estinzione 10 giorni prima
(context A)
Mechanisms of fear extintion
When the CS is present, the LA excites the central nucleus (CE),
which controls passive forms of expression of the CR through
descending projections to the brainstem and hypothalamus.
The LA also has indirect projections to the CE, through the basal
nucleus (B) and the intercalated (ITC) cell groups, clusters of
inhibitory GABAergic neurons.
The B itself also projects directly to the ITC.
These pathways provide multiple potential circuits for gating
fear expression.
Knowledge of the fear conditioning circuitry has
allowed researchers to investigate functional
changes that occur during extinction.
Research in rats using lesions, pharmacological manipulations,
and electrophysiology are providing an increasingly detailed
model of the neural circuitry of fear extinction.
Recent studies in human beings have been consistent with the
animal literature.
This body of research suggests that interaction
between the amygdala, the ventromedial prefrontal
cortex (vmPFC), and the hippocampus supports the
acquisition, storage, retrieval, and contextual
modulation of fear extinction
Acquisition and consolidation of extintion
Amygdala
Recent studies have shown that blockade of NMDA (SotresBayon et al, 2007) or glutamate (Kim et al, 2007) receptors within
the basolateral amygdala complex (BLA) impaired extinction
learning and that the blockade of mitogen-activated protein
kinase (MAPk) activity in the basolateral nucleus entirely
prevented the acquisition of extinction (Herry et al, 2006).
Furthermore, several studies suggest that morphological
changes in the BLA synapses after extinction training support
the consolidation of extinction learning (Lin et al, 2003;
Chatwal et al, 2005, 2006; Markram et al, 2007).
Consistent with the idea that extinction constitutes new
learning, not erasure of the original fear memory, Repa
et al (2001) identified a population of neurons in the
lateral amygdala whose response to the CS
decreased during extinction training, as well as a
second population in which the CS response
remained high despite a decrease in the expression of
conditioned fear.
This finding provides further evidence that the amygdala
supports the maintenance of the original fear
memory when simultaneously facilitating extinction
learning.
Although the amygdala seems to be critical for the
acquisition of extinction learning, convergent
evidence suggests that the vmPFC is necessary for
the retention and recall of extinction.
In line with the well-documented observation in human
beings and primates that damage to the PFC leads to
perseverative behavior (see Sotres-Bayon et al, 2006 for
a review), Morgan et al (1993) observed that rats with
vmPFC lesions required many more unreinforced
presentations of the CS to extinguish conditioned
responding.
A subsequent study pointed to the infralimbic (IL) region of the
vmPFC as a potential site of extinction consolidation, reporting
that pre-training IL lesions left within-session acquisition of
extinction intact, but impaired extinction retrieval on the
following day (Quirk et al, 2000).
Infusion studies showing that impairing long term plasticity
(disruption of protein synthesis (Santini et al, 2004), MAPk
blockade (Hugues et al, 2006), and administration of an NMDA
antagonist (Burgos-Robles et al, 2007)) within the vmPFC all
impair retrieval of extinction suggest that the plasticity in this
region supports extinction consolidation
mPFC could act on inhibitory lateral amygdala neurons or on
intercalated (inhibitory) neurons.
The results is a decrease of central amygdala output.
Effetto del blocco dei recettori NMDA nella PFC sull’estinzione
Infusion
Burgos-Robles et a., 2007
Bursting activity may reflect hippocampal input to
mPFC
Paré found that fear extinction in rats was associated
with a potentiation of fear input synapses from BLA
to GABAergic intercalated amygdala neurons that
project to the CEA.
Enhancement of inputs to intercalated cells
required prefrontal activity during extinction
training.
a) Experimental procedure.
(b) Intensity-dependence of BLA-evoked
responses in ITC cells from the vehicle (n = 15)
and muscimol (n = 11) groups (average ± s.e.m.).
Dashed line indicates data from unpaired group
Inset, extent of fluorophore-conjugated muscimol
diffusion in the infralimbic cortex.
LA
Extintion
Acquisition
BLA
CEA
ITC
mPFC
Fear
Relapse (spontaneous retrieval of fear memory,
reinstatement, renewal)
Since extintion does not erase the original fear memory, the
CS has an ambiguous valence, it could be fear eliciting or
not, depending on:
1. the level of activation of the US representation during non
reinforced trials,
2. the decreases in attention, which can influence conditioned
performance during extinction,
3. contexts.
La risposta emotiva può perciò riapparire a causa del
recupero spontaneo, della ripresentazione dello SI o del
rinnovo (se lo SC è presentato in un contesto differente da
quello dell’estinzione).
After extinction, contextual information has a
critical function in determining whether the
original fear memory or the new extinction
memory should control fear expression.
Evidence suggests that hippocampal
projections to the vmPFC and the amygdala
mediate the context-dependent expression of
extinction (Fanselow, 2000; Ji and Maren,
2005).
Conditioning day
Extintion day
Test day
Effects of Dorsal Hippocampus lesions on
AAA/ABA renewal.
(A) Mean (±SEM) percentage of freezing exhibited on
the conditioning day, with 3 min prior to tone CS onset
(baseline) and 1 min post-CS for five trials. All rats were
fear conditioned in context A.
(B) Extinction to the tone CS. Mean (±SEM) percentage
of freezing for the first four CS presentations across the
3 d of extinction in contexts A and B.
(C) Mean (±SEM) percentage of freezing for the first 4
min after CS onset during test. Rats were tested for fear
of CS in the conditioning context, either in the extinction
context (SAME; AAA; open bars) or outside of the
extinction context (DIFF; ABA; filled bars). Electrolytic
DH (DH-Pre) or sham (SH) lesions were made before
conditioning. Ji and Maren, 2005
Furthermore, inactivation of the hippocampus before
extinction learning impairs extinction recall on the subsequent
day (Corcoran et al, 2005), suggesting that the hippocampus
regulates fear expression both outside and within the extinction
context.
One suggestion is that the hippocampus controls the contextspecific retrieval of extinction through projections to the
vmPFC (Corcoran and Quirk, 2007).
Another possibility is that the hippocampus gates fear expression
directly through projections to the LA.
Although clarification of the precise circuitry
requires further investigation, there is strong
evidence that the hippocampus, through
communication with the vmPFC and the
amygdala, regulates the contextual
modulation of fear expression during
extinction retrieval.
Ruolo della neurogenesi ippocampale nell’espressione di
comportamenti ansiosi e nel pattern separation
Ipotesi sul ruolo della neurogenesi ippocampale:
Formazione di cluster temporali di memorie (retrieval episodico)
(Aimone e Gage)
Separazione di contesti diversi (Sahai et al., 2011)
Meno neurogenesi, più generalizzazione (Fear Conditioning in
contesto A, test in contesto B, mancanza di discriminazione, come
nel sistema olfattivo)
Ippocampo ventrale e dorsale: ruoli diversi
Ippocampo dorsale cognition of context;
ippocampo ventrale connesso con il sistema limbico, espressione
di ansietà (blocco con chRh dei neuroni dell’ippocampo ventrale,
animali esplorano i bracci aperti dell’EPM)
Legame con PTSD: eccessiva generalizzazione è un
evento maladattivo, un singolo reminder del contesto
originale può provocare il richiamo dell’intero contesto
traumatico anche in un contesto “sicuro”
B
A
Menta
Contesto del
condizionamento
Menta
Contesto sicuro
Research examining extinction learning in humans has
been largely consistent with the findings from rodent
studies.
Initial fMRI studies of extinction learning in humans
reported an increase in amygdala activation during
early extinction after the shift in the CS–US
contingency (LaBar et al, 1998; Gottfried and Dolan,
2004; Knight et al, 2004; Phelps et al, 2004).
Amygdala activation decreased as extinction
progressed and remained low during extinction
The magnitude of the decrease in amygdala
activation during extinction learning correlated with
the degree of participants’ extinction retrieval
(Phelps et al, 2004).
These findings further support a function for the
amygdala in extinction learning and confirm that
amygdala activity is reduced during extinction
retrieval.
The first fMRI study examining both extinction learning and
subsequent retrieval in humans supported the function of the
vmPFC in extinction retrieval (Phelps et al, 2004).
Fear conditioning fMRI studies have typically reported decreases
in the blood oxygen level-dependent (BOLD) signal in the vmPFC
below the resting baseline when the CS
is presented during fear acquisition (Gottfried and Dolan, 2004;
Phelps et al, 2004).
BOLD signal in this region increases toward
baseline during initial extinction learning, and
increases further during extinction retrieval (Phelps et
al, 2004). Subsequent studies have also observed increased
vmPFC activation during extinction retrieval (Kalisch et al,
2006; Milad et al, 2007).
The degree of increase in activity in the
vmPFC as well as the thickness of the
cortex in this region are correlated with
the degree of extinction success during
recall (Milad et al, 2005, 2007).
Anterior cingulate region has been proposed as
a potential human homologue of the rodent IL
region (Kim et al, 2003), and may inhibit
conditioned fear expression during extinction
retrieval, as suggested by the animal literature.
Extinction-based therapies have been widely
explored as potential avenues of treatment for
anxiety disorders (Barlow, 2002; Garakani et al,
2006).
Posttraumatic stress disorder (PTSD), in which patients
display spontaneous or cue induced re-experiencing of
a traumatic event, may result from failure to
consolidate and retrieve extinction learning (see
Rauch et al, 2006 for a review).
Consistent with this theory, PTSD patients exhibit
deficits in extinction retention (Orr et al, 2000),
along with reduced vmPFC and hippocampal
volume and activity and increased amygdala
activity (Gilbertson et al, 2002; Bremner, 2006;
Liberzon and Martis, 2006; Shin et al, 2006).
fMRI studies examining the context-dependent recall of
extinction in humans point to an important function for the
hippocampus (Kalisch et al, 2006; Milad et al, 2007).
In such studies, context is manipulated through changes in the
background color or the visual scene in which the CS is
presented.
Extinction learning takes place only in one of the
contexts, allowing activation correlated with context dependent
retrieval to be examined.
Increased hippocampal activation was observed during
extinction retrieval (Kalisch et al, 2006; Milad et al, 2007).
Activation in the hippocampus was positively
correlated with vmPFC activation, providing further
support for the notion that the hippocampus may
mediate context-dependent extinction recall through
connections with the vmPFC.
Finally, confirming the function of the hippocampus in
contextual reinstatement, a human lesion study found
that contextual reinstatement of the CR was impaired
in individuals with hippocampal lesions (LaBar and
Phelps, 2005), similar to the findings observed in
rodents (Wilson et al, 1995).
Consistent with studies in animal models, extinction
learning in humans seems therefore to depend on the
integrated functioning of a neural circuit that includes
the amygdala, the vmPFC, and the hippocampus.
This convergent evidence across species suggests that the
neural mechanisms supporting fear extinction are highly
phylogenetically conserved.
Hippocampus, amygdala and mPFC are often
disfunctional in PTSD
L’esperire eventi traumatici e paurosi può determinare
nell’uomo l’insorgenza di un disturbo d’ansia, la cui cura
è ostacolata dal riaffiorare delle memorie a valenza
negativa legate all’evento.
Alcuni disturbi, come il disturbo post traumatico da
stress (PTSD), sono associati ad un deficit di estinzione
(Milad et al., 2009).
A clinical characteristic of posttraumatic stress disorder (PTSD) is
persistently elevated fear responses to stimuli associated with the
traumatic event.
The objective of the Milad paper is to determine whether
extinction of fear responses is impaired in PTSD and whether
such impairment is related to dysfunctional activation of
brain regions known to be involved in fear extinction, e.g.,
amygdala, hippocampus, ventromedial prefrontal cortex (vmPFC),
and dorsal anterior cingulate cortex (dACC).
Individuals with PTSD typically show exaggerated amygdala
and diminished hippocampal activation relative to controls.
The dACC has emerged as another brain region that appears
hyperactive in PTSD.
Most studies have shown that vmPFC is hypoactive in this
disorder, but a few have reported hyperactivity
The objective of the Milad study was to examine the
neurobiological basis of deficient extinction recall in PTSD with a
focus on the above-mentioned brain regions.
While in a 3T fMRI scanner, PTSD and trauma-exposed, nonPTSD control (TENC) subjects underwent a two-day Pavlovian
fear conditioning and extinction procedure previously used
in healthy and PTSD subjects.
Skin conductance response (SCR), a commonly used measure in
human fear studies served as the dependent measure of
conditioned responding.
On day 1, subjects underwent fear conditioning to two pictures of
differently colored lamps, followed by extinction for one of them.
Day 2 tested recall of the extinction that had been learned the
previous day by contrasting responses to the previously
extinguished and unextinguished stimuli.
A total of 19 PTSD patients and 20 trauma-exposed non-PTSD control (TENC)
subjects were recruited from the community.
Late extinction learning (day 1)
Early extinction recall (day 2)
These findings support the hypothesis that fear
extinction is impaired in PTSD.
They further suggest that dysfunctional activation in
brain structures that mediate fear extinction learning,
and especially its recall, underlie this impairment.
Disturbi d’ansia e persistenza di memorie emotive hanno
una prevalenza familiare, suggerendo la presenza di
componenti genetiche come fattori di rischio
nell’insorgenza di disturbi.
A Genetic Variant BDNF Polymorphism Alters Extinction
Learning in Both Mouse and Human (Soliman et al., 2010)
BDNF mediates synaptic plasticity associated with learning and
memory specifically in fear learning and extinction.
BDNF-dependent forms of fear learning have known
biological substrates, and lie at the core of a number of clinical
disorders associated with the variant BDNF.
Fear learning paradigms require the ability to recognize and
remember cues that signal safety or threat and to extinguish
these associations when they no longer exist.
As we have seen, these abilities are impaired in anxiety
disorders such as posttraumatic stress disorder and phobias
and behavioral treatments for these disorders rely on basic
principles of extinction learning.
Understanding the effect of the BDNF Met allele on these
forms of learning can provide insight into the
mechanism of risk for anxiety disorders.
The objective of this study was to test if the Val66Met
genotype could impact extinction learning in a mouse model,
and if such findings could be generalized to human
populations.
The neuroimaging findings of diminished ventromedial
prefrontal activity and elevated amygdala activity during
extinction are reminiscent of those reported in patients with
anxiety disorders and depression when presented with empty
threat or aversive stimuli (e.g., fearful faces).
Understanding the effect of the BDNF Met allele on specific
components of a simple form of learning provides insight into risk
for anxiety disorders and has important implications for the
efficacy of treatments for these disorders that rely on extinction
mechanisms.
These findings suggest that the BDNF Val66Met polymorphism
may play a key role in the efficacy of such treatments and
may ultimately guide personalized medicine for related clinical
disorders.
BDNF agisce dove?
Induction of Fear Extinction with Hippocampal-Infralimbic
BDNF (Peters et al., 2010)
As we have seen, the extinction of conditioned fear memories
requires plasticity in the infralimbic medial prefrontal cortex (IL
mPFC), but little is known about the molecular mechanisms
involved.
Epigenetic regulation within the IL mPFC of the gene
encoding BDNF correlates with fear extinction. Because
BDNF is a major molecular mediator of memory
consolidation, the authors hypothesized that BDNF is
responsible for consolidating
extinction memory within the ILmPFC.
If true, it should be possible to enhance extinction via direct
application of BDNF to the ILmPFC
Research showing the facilitation of extinction learning through
pharmacological means (Walker et al, 2002; Ressler et al, 2004;
see Anderson and Insel, 2006; Quirk and Mueller, 2008 for
reviews) suggests that drug administration may enhance the
efficacy of such extinction based therapies.
As human beings with the low-expressing short allele variant of
BDNF gene exhibit decreased functional connectivity between the
vmPFC and amygdala (Pezawas et al, 2005), this suggests a
possible genetic basis for individual differences in extinction
learning, as well as a potential risk factor for the development of
anxiety disorders.
NORADRENERGIC ENHANCEMENT OF RECONSOLIDATION
IN THE AMYGDALA IMPAIRS EXTINCTION OF CONDITIONED
FEAR IN RATS—A POSSIBLE MECHANISM FOR THE
PERSISTENCE OF TRAUMATIC MEMORIES IN PTSD
Debiec et al., 2011
Posttraumatic stress disorder (PTSD) is associated with
enhanced noradrenergic activity.
Animal and human studies demonstrate that noradrenergic
stimulation augments consolidation of fear learning.
Retrieval of well-established memories by presenting a learned
fear cue triggers reconsolidation processes during which
memories may be updated, weakened, or strengthened.
Noradrenergic blockade in the rat amygdala impairs
reconsolidation of fear memories.
Noradrenergic enhancement on reconsolidation of learned
fear?
Clinical research indicates that the persistence and severity of
PTSD symptoms is associated with increased noradrenergic
activity long after the traumatic event.
It is possible that norepinephrine is involved not only in the original
encoding but also in the maintenance and exacerbation of
symptoms associated with traumatic memories.
The major finding of this study was that beta-adrenergic
receptor agonist isoproterenol infused into the lateral
amygdala following retrieval of a conditioned fear
impaired its extinction 48 hr later
Noradrenergic enhancement of reconsolidation may also
serve as a model for delayed-onset PTSD, whereas
PTSD symptoms emerge and exacerbate over time.
Hippocampal dysfunction effects on context memory:
Possible etiology for posttraumatic stress disorder.
Acheson DT, Gresack JE, Risbrough VB.
Hippocampal volume reductions and functional impairments are
reliable findings in posttraumatic stress disorder (PTSD) imaging
studies.
However, it is not clear if and how hippocampal dysfunction
contributes to the etiology and maintenance of PTSD.
Individuals with PTSD are often described as showing fear
responses to trauma reminders outside of contexts in which these
cues would reasonably predict danger. Animal studies suggest
that the hippocampus is required to form and recall associations
between contextual stimuli and aversive events.
Conversely, the hippocampus is not required for
associations with discrete cues.
In animal studies, if configural memory is disrupted,
learning strategies using discrete cue associations
predominate.
These data suggest that poor hippocampal function
could bias the organism toward forming multiple
simple cue associations during trauma, thus
increasing the chances of fear responses in multiple
environments (or contexts) in which these simple
cues may be present.
2. Neural circuit abnormalities reported in PTSD
2.1. Cortical and subcortical findings
Structural MRI and functional MRI studies in PTSD report
differences in volume and activation of limbic circuitry
mediating stress responding, fear memory, and emotional
modulation ( [Garfinkel and Liberzon, 2009] and [Rauch et al.,
2006] ).
A recent meta-analysis of functional neuroimaging studies in
anxiety disorders supports the notion of dysfunction in
prefrontal-limbic areas in PTSD, showing modest amygdala
hyperactivation along with more pronounced hypoactivation in
the medial prefrontal cortex as well as the rostral and dorsal
anterior cingulate cortex (ACC; Etkin and Wager, 2007).
Another robust finding in neuroimaging research is reduced gray matter volume in the
ACC in patients with PTSD ( [Yamasue et al., 2003] , [Corbo et al., 2005] and [Kitayama
et al., 2006] ).
Kasai et al. (2008) found that combat-exposed twins with PTSD had lower gray matter
volumes in the subgenual ACC compared to their trauma-exposed non-PTSD co-twins.
This finding suggests that subgenual ACC volume reductions are an acquired
feature of PTSD, rather than a vulnerability factor.
However, in another study, Shin et al. (2009) used positron emission tomography
(PET) to assess resting metabolic activity in the dorsal ACC in veterans with PTSD and
their non-combat-exposed co-twins.
They found that veterans with PTSD and their co-twin both exhibited higher
resting metabolic rates than veterans without PTSD and their co-twins. These
results suggest that metabolic abnormalities in the dorsal ACC represent a preexisting vulnerability factor for the development of PTSD subsequent to trauma.
More research is needed to clarify the nature of
prefrontal/ACC abnormalities in PTSD, and how they may
contribute to the development or maintenance of the
disorder.
In order to understand how hippocampal abnormalities contribute
to development and/or maintenance of PTSD, an important
question is whether this reduced volume is a trait in vulnerable
individuals or, rather, a manifestation of the disease state after
experiencing trauma.
Recent research is beginning to support a conceptualization of
these abnormalities as a pre-existing vulnerability factor for
development of the disorder.
[Gilbertson et al., 2002] and [Gilbertson et al., 2007] examined
hippocampal volume and function in a sample of monozygotic
twins discordant for trauma exposure. In the first study, the
authors compared hippocampal volume, as assessed by structural
MRI, across twins with and without PTSD, and also correlated
volume with PTSD severity in the exposed twin (Gilbertson et al.,
2002).
Findings supported smaller, predominately right,
hippocampal volume in severe PTSD vs. traumaexposed non-PTSD subjects.
Interestingly, twins of severe PTSD subjects showed
hippocampal volumes comparable to their brothers
and significantly lower than both trauma-exposed
non-PTSD patients as well as their non-traumaexposed twins.
Disorder severity in PTSD subjects was negatively
correlated with both their own hippocampal volumes and
with those of their trauma-discordant twin.
Thus, hippocampal volume may be a vulnerability
trait that contributes to the development of PTSD
upon trauma exposure.
Support for the alternative idea that hippocampal reductions
manifest only after trauma (i.e. “state”) is predominantly based on
evidence for stress- and glucocorticoid signaling-induced
reductions of hippocampal volume
It has been hypothesized that that trauma exposure induces HPA
axis dysregulation, resulting in hippocampal atrophy in vulnerable
individuals (Elzinga and Bremner, 2002). Measures of circulating
cortisol levels in PTSD patients have been inconsistent however,
predominantly with reports of normal or low levels of cortisol and
increased negative feedback (for review see Yehuda, 2009).
Recent findings of alterations in second messenger systems
involved in glucocorticoid signaling (Binder, 2009) suggest that,
although glucocorticoid levels may be relatively normal, patient’s
ability to regulate signaling at the cellular level may be impaired.
It remains to be determined if these downstream modifiers of
glucocorticoid signaling are linked to the hippocampal
abnormalities reported in PTSD patients.
A recent human structural MRI study, while replicating the finding
of smaller hippocampal volume in chronic PTSD patients, found no
differences between Gulf War veterans without PTSD and those
who had successfully recovered from the disorder (Apfel et al.,
2011).
The authors suggest that this pattern of findings may indicate that
low hippocampal volume is a state factor.
However, the cross-sectional design of the study precludes any
causal inference in this regard.
It is also possible, as the authors mention, that measurable
structural abnormalities are a trait factor predisposing individuals
toward a more severe, treatment resistant form of the disorder, or
that hippocampal neurogenesis is a mediating factor in successful
recovery.
Longitudinal imaging studies are necessary to disentangle
these competing hypotheses.
Remember the effects of poor hippocampal functioning
Normal hippocampal function
A conjunctive representation comprised of many individual
elements present in the environment is encoded as a whole. In
this case the individual elements comprising the environment are
bound into one representation that defines a place (i.e. context)
where the event occurs (Rudy et al., 2004) and a conditioned
response would only occur in the presence of an environment with
stimuli that matches the full representation, with no single cue
sufficient to induce a conditioned response. Conjunctive
representation of the context is also referred to as a cognitive
map (O’Keefe and Nadel, 1978)
Poor hippocampal function leads to poor cognitive mapping,
generalization and shift to elemental representation (each element
of the traumatic context is associated with the aversive event and
can evoke a fear response even in a safe context)
Ontogenesi ed estinzione
Come già visto, la risposta emotiva può riapparire dopo estinzione a causa del recupero
spontaneo, della ripresentazione dello SI o del rinnovo (se lo SC è presentato in un
contesto differente).
Nell’età giovanile l’estinzione sembra invece avere un effetto permanente che
porta alla cancellazione della traccia associativa precedentemente acquisita (Kim
e Richardson, 2008)
Il cambiamento molecolare che porta alla
mancanza di effetto duraturo dell’estinzione
potrebbe essere costituito dalla maturazione della
matrice extracellulare (ECM) nell’amigdala
(Gogolla et al., 2009).
Non è escluso un ruolo della maturazione della
corteccia prefrontale, che prenderebbe, con il
procedere della maturazione, il ruolo di guida nel
processo di estinzione, conducendo al processo
di inibizione, ma non di cancellazione, della
traccia di memoria di paura condizionata.
I topi link (Crl1 KO)
mancano delle reti
perineuronali della
matrice
extracellulare
Attivazione dell’amigdala dopo il primo giorno di estinzione
Range of Zif positive cells in
pseudoconditioned subjects (unpaired
CS-US)
Attivazione dell’amigdala dopo il primo giorno di estinzione
Range of Zif positive cells in
pseudoconditioned subjects (unpaired
CS-US)
La riduzione dell’attivazione è a livello dell’ingresso all’amigdala, ovvero ad LA
Estinzione duratura nei topi Crtl1 KO
Estinzione duratura nei topi Crtl1 KO
Effetto arricchimento ambientale sull’estinzione??
Latenze Braccio Shock
GRUPPO ST. SHOCK
GRUP. AA SHOCK
160,00
140,00
Latenza sec.
120,00
100,00
80,00
60,00
40,00
20,00
0,00
Abituazione
Test
7°giorno
14°giorno
21°giorno
28°giorno
35°giorno
Tempo medio speso in freezing "braccio shock"
GRUPPO STAN. SHOCK
GRUP. AA SHOCK
12,00
Tempo Freezing. Sec.
10,00
8,00
6,00
4,00
2,00
0,00
Abituazione
Test
7°giorno
14°giorno
21°giorno
28°giorno
35°giorno
Tempo speso in freezing durante la fase di apprendimento.
Gruppo Standard
Gruppo Arricchito
5,00
Tempo freezing sec.
4,00
3,00
2,00
1,00
0,00
-1,00
-2,00
entrata
15 sec
30 sec.
75 sec.
90.sec
Considerazioni finali sull’estinzione
Combinare estinzione e riconsolidamento
Science. 2009 May 15;324(5929):951-5. Epub
2009 Apr 2.
Extinction-reconsolidation boundaries: key to
persistent attenuation of fear memories.
Monfils MH, Cowansage KK, Klann E, LeDoux
JE.
Extinction-Reconsolidation Boundaries: Key to Persistent
Attenuation of Fear Memories
The possibility that reactivated memories may be modifiable was
proposed many years ago, and since then, numerous studies
have demonstrated that blockade of the updating process
engaged during retrieval—usually via pharmacological
intervention within the reconsolidation window—prevents memory
restorage and produces amnesia (loss of the specific memory
that was reactivated in the presence of the drug or access to it).
Thus, in the case of aversive memories, blocking reconsolidation
weakens the emotional impact of a once fear-inducing stimulus by
altering the molecular composition of the memory trace.
This process generally requires the use of drugs that often cannot
be readily administered to humans. (Monfils et al., 2009)
In contrast, fear extinction—a paradigm in which the conditioned
stimulus (CS) is repeatedly presented in the absence of the
unconditioned stimulus (US)—leads to progressive reduction in
the expression of fear, but is not permanent because extinction
does not directly modify the existing memory but instead leads to
the formation of a new memory that suppresses activation of the
initial trace.
The efficacy of this inhibition, however, is strongly contingent on
spatial, sensory, and temporal variables.
Specifically, the reemergence of a previously extinguished fear is
known to occur, in rodents and humans alike, under three general
conditions:
(i) renewal, when the CS is presented outside of the extinction
context;
(ii) reinstatement, when the original US is given unexpectedly;
(iii) spontaneous recovery, when a substantial amount of time has
passed
Monfils et al devised an effective, drug-free paradigm for the
persistent reduction of learned fear, capitalizing on differences
between reconsolidation and extinction.
Given that extinction training reduces the threatening
value of the CS, they reasoned that when applied
within the reconsolidation window (after the memory
is rendered unstable by presenting an isolated retrieval
trial), extinction training would result in the storage
of the new nonthreatening meaning of the CS and
prevent renewal, reinstatement, and spontaneous
recovery, thus resulting in a more enduring reduction in
fear relative to extinction training conducted outside the
reconsolidation window.
Specifically, they predicted that an extinction
session presented after an isolated retrieval trial
would lead to a persistent revaluation of the CS
as less threatening, and/or a weakening of the
stored trace or access to it, and thus would
prevent the return of fear in the three
aforementioned tests.
Six experiments were conducted.
1) Test whether their behavioral paradigm could prevent the return
of fear on a spontaneous recovery test, and if so, whether the
observed effect was the result of an update during reconsolidation.
They specifically designed this experiment on the basis of the
premise that the lability window engaged at the time of retrieval is
temporary—in rat fear conditioning, it closes within 6 hours—at
which time the memory is thought to be reconsolidated.
They posited that if the interval between the isolated retrieval cue
and extinction training was brief enough to enable the repeated
unreinforced CSs to be presented within the lability window, then
the new interpretation of the CS as no longer threatening should
be incorporated during reconsolidation.
Rats were fear-conditioned using three tone-shock pairings, and
were then divided into five experimental groups. Two groups had
a retrieval-extinction interval within the reconsolidation window
[10 min (n = 8) and 1 hour (n = 8)] and two groups outside the
reconsolidation window [6 hours (n = 8) and 24 hours (n = 8)].
In addition to these four retrieval (Ret) groups, a fifth group (No
Ret) was exposed to context but did not receive a CS retrieval (n
= 12).
All procedures were conducted in context A (grid floor). All groups
showed equivalent freezing for the last four trials of extinction
Finite lability window to prevent return of fear via post-retrieval extinction. (A) Rats were fear-conditioned (Fear Cond) with three tone-shock pairings. After
24 hours, they were exposed either to an isolated cue retrieval trial (Ret) or context only (No Ret) followed by extinction training. The time interval between the
retrieval trial (or context exposure, n = 12) and the extinction was either within (10 min, n = 8; 1 hour, n = 8) or outside (6 hours, n = 8; 24 hours, n = 8) the
reconsolidation window. Twenty-four hours after extinction, all groups were tested for LTM, and 1 month later for spontaneous recovery. The gray shading
represents context A. (B) All groups were equivalent for the last four trials of extinction and at the 24-hour LTM test. One month later, the Ret groups with an
interval outside the reconsolidation window as well as the No Ret group showed increased freezing (spontaneous recovery) relative to the 24-hour test
Attenuation of fear memory by presenting a single isolated retrieval trial followed by an extinction session
prevents renewal. (A) Rats were fear-conditioned in context A. Twenty-four hours later, they were exposed either
to an isolated cue retrieval trial (Ret, n = 8) or context only (No Ret, n = 8) in context B, followed 1 hour later by
extinction training in context B. Twenty-four hours after extinction, they were tested for LTM in context B. The gray
shading represents context A; the blue shading represents context B (smooth black floor and peppermint scent)
(28). (B) Rats from both experimental groups froze equivalently during the LTM test (all ANOVAs, P > 0.1). When
they were placed back in the acquisition context, the No Ret group (black) showed fear renewal (P = 0.012), but
the Ret group (red) did not (P > 0.1), relative to their respective LTM tests
Presenting a single isolated retrieval trial before an extinction session prevents
reinstatement. (A) Rats were fear-conditioned. The next day, they were exposed either to
an isolated cue retrieval trial (Ret, n = 8) or context only (No Ret, n = 8), followed 1 hour
later by extinction training. Twenty-four hours after extinction, they received five
unsignaled footshocks, and the next day were tested for reinstatement. The gray shading
represents context A. (B) The No Ret and Ret groups froze equivalently to the last four
CSs of extinction; however, 24 hours after the unsignaled footshocks, the No Ret group
(black) showed increased freezing (reinstatement) (P < 0.05), but the Ret group (red) did
not (P > 0.05).
Nature 463, 49-53 (7 January 2010)
Preventing the return of fear in humans using
reconsolidation update mechanisms
Schiller et al., 2010
“Research on changing fears has highlighted several techniques,
most of which rely on the inhibition of the learned fear response.
An inherent problem with these inhibition techniques is that the
fear may return, for example with stress.
Recent research on changing fears targeting the reconsolidation
process overcomes this challenge to some extent. During
reconsolidation, stored information is rendered labile after being
retrieved, and pharmacological manipulations at this stage result
in an inability to retrieve the memories at later times, suggesting
that they are either erased or persistently inhibited.”
Remember that the hypothesis is that reconsolidation is an
adaptive update mechanism by which new information is
incorporated into old memories.
By introducing new information during the reconsolidation period,
it may be possible to permanently change the fear memory.
In the present study, Schiller et al provide evidence in humans
that old fear memories can be updated with non-fearful
information provided during the reconsolidation window.
As a consequence, fear responses are no longer expressed.
If an old fear memory could be restored while
incorporating neutral or more positive information
provided at the time of retrieval, it may be possible to
permanently modify the fearful properties of this
memory.
Although this approach captures the very essence of
reconsolidation, it has been surprisingly neglected in
emotion research in humans and other animals.
The authors designed two experiments examining whether extinction training conducted
during the reconsolidation window would block the return of extinguished fear.
In the first study, three groups of subjects underwent fear conditioning using a
discrimination paradigm with partial reinforcement. Two coloured squares were used.
One square (conditioned stimulus+, hereafter termed CS+) was paired with a mild shock
to the wrist (unconditioned stimulus) on 38% of the trials, whereas the other square was
never paired with shock (CS-).
A day later, all three groups underwent extinction training in which the two conditioned
stimuli were repeatedly presented without the unconditioned stimulus.
In two groups the fear memory was reactivated before extinction using a single
presentation of the CS+. One group (n = 20) received the reminder trial 10 min before
extinction (within the reconsolidation window), whereas the second group (n = 23) was
reminded 6 h before extinction (outside the reconsolidation window).
The third group (n = 22) was not reminded of the fear memory before extinction training.
Twenty-four hours later, all three groups were presented again with the conditioned
stimuli without the unconditioned stimulus (re-extinction) to assess spontaneous fear
recovery.
The measure of fear was the skin conductance response (SCR).
At each stage, the differential fear response was calculated by subtracting responses to
the CS- from responses to the CS+.
a, Experimental design and
timeline. b, Mean differential
SCRs (CS+ minus CS-) during
acquisition (late phase),
extinction (last trial) and reextinction (first trial) for each
experimental group (10-min
reminder, 6-h reminder and no
reminder). The three groups
showed equivalent fear
acquisition and extinction.
Spontaneous recovery (first
trial of re-extinction versus
the last trial of extinction)
was found in the group that
had not been reminded or
that was reminded 6 h before
extinction.
In contrast, there was no
spontaneous recovery in
the group reminded 10
min before extinction.
a, Experimental design and
timeline. US, unconditioned
stimulus. b, Mean SCRs (CSa+,
CSb+ and CS-) during acquisition
(late phase), extinction (last trial)
and re-extinction (first trial).
Subjects had equivalent levels of
acquisition and extinction of
conditioned fear to the two
conditioned stimuli. The index of
fear recovery was the first trial of
re-extinction (after reinstatement)
minus the last trial of extinction
(before reinstatement).
Fear reinstatement was found
only to CSb+ (not reminded
before extinction training), but
not to CSa+ (reminded 10 min
before extinction training).
The present findings suggest a new technique to target specific
fear memories and prevent the return of fear after extinction
training.
Using two recovery assays, the authors demonstrated that
extinction conducted during the reconsolidation window of an old
fear memory prevented the spontaneous recovery or the
reinstatement of fear responses, an effect that was maintained a
year later.
Moreover, this manipulation selectively affected only the
reactivated conditioned stimulus while leaving fear memory to the
other non-reactivated conditioned stimulus intact.
The current results also suggest that timing may have a more
important role in the control of fear than previously appreciated.
Standard extinction training, without previous memory
reactivation, also triggers the fear memory.
Given this, one might expect mere extinction training to have
similar effects. That is, the first trial of extinction might serve as
the reminder cue triggering the reconsolidation cascade, which is
immediately followed by extinction. However, there is abundant
evidence that during standard extinction training the nonreinforced presentations of the fear-eliciting cue induce new
inhibitory learning, which competes for expression with the initial
fear learning, resulting in the recovery of fear responses in some.
Schiller et al findings indicate that the timing of extinction
relative to the reactivation of the memory can capitalize on
reconsolidation mechanisms.
Two factors may be important determinants in this
process:
the timing of extinction training relative to retrieval,
and/or the chunking of the conditioned stimulus
presentations during extinction relative to reactivation
(that is, the fact that they are massed relative to the
single retrieval trial during the reconsolidation phase).
Further studies are required to disentangle these
possibilities.
The present study proposes that invasive techniques are
not necessary to reduce conditioned fear responses.
Using a more natural intervention that captures the
adaptive purpose of reconsolidation allows a safe and
easily implemented way to prevent the return of fear.
Let us discuss this
Proc Natl Acad Sci U S A. 2013 Nov 25.
Extinction during reconsolidation of threat memory diminishes
prefrontal cortex involvement.
Schiller D, Kanen JW, Ledoux JE, Monfils MH, Phelps EA.
Abstract
Controlling learned defensive responses through extinction does
not alter the threat memory itself, but rather regulates its expression
via inhibitory influence of the prefrontal cortex (PFC) over
amygdala.
Individual differences in amygdala-PFC circuitry function have been
linked to trait anxiety and posttraumatic stress disorder.
This finding suggests that exposure-based techniques may actually
be least effective in those who suffer from anxiety disorders.
A theoretical advantage of techniques influencing
reconsolidation of threat memories is that the threat
representation is altered, potentially diminishing reliance
on this PFC circuitry, resulting in a more persistent
reduction of defensive reactions.
Schiller, Phelps and LeDoux hypothesized that timing
extinction to coincide with threat memory reconsolidation
would prevent the return of defensive reactions and
diminish PFC involvement.
Two conditioned stimuli (CS) were paired with shock and
the third was not.
A day later, one stimulus (reminded CS+) but not the
other (nonreminded CS+) was presented 10 min before
extinction to reactivate the threat memory, followed by
extinction training for all CSs.
The recovery of the threat memory was tested 24 h later.
Extinction of the nonreminded CS+ (i.e., standard
extinction) engaged the PFC, as previously shown,
but extinction of the reminded CS+ (i.e., extinction
during reconsolidation) did not.
Moreover, only the nonreminded CS+ memory recovered
on day 3.
These results suggest that extinction during
reconsolidation prevents the return of defensive
reactions and diminishes PFC involvement.
Reducing the necessity of the PFC-amygdala
circuitry to control defensive reactions may
help overcome a primary obstacle in the longterm efficacy of current treatments for anxiety
disorders
Cognitive regulation of emotions (testable in
humans)
Ochsner et al. (2002) had participants view emotionally
negative pictures (e.g., woman crying on steps of
church) and either “attend” (focus on your natural
feelings) or “reappraise” (reinterpret the picture in a less
negative context; e.g., the woman is crying at a
wedding) the stimulus.
Behaviorally, the reappraisal technique was
successful in decreasing the negative affect
as measured through subjective ratings.
An examination of changes in the BOLD signal
contrasting the reappraisal and attend conditions
revealed enhanced BOLD signal in the left
dorsolateral prefrontal cortex (dlPFC) and decreased
BOLD signal in the amygdala.
Several other neuroimaging studies have reported
similar modulation of the amygdala response with
emotion regulation, along with involvement of regions of
the dlPFC as well as anterior cingulate and other mPFC
regions (Ochsner and Gross, 2005).
Cognitive regulation of emotions
Using an emotion regulation strategy, Phelps examine
the neural mechanisms of regulating conditioned fear
using fMRI and compare the resulting activation pattern
with that observed during classic extinction.
The results suggest that the lateral PFC
regions engaged by cognitive emotion
regulation strategies may influence the
amygdala, diminishing fear through similar
vmPFC connections that are thought to inhibit
the amygdala during extinction.
These findings further suggest that humans
may have developed complex cognition that
can aid in regulating emotional responses
while utilizing phylogenetically shared
mechanisms of extinction.
The dlPFC regions often observed in studies of emotion
regulation have also been implicated in other higher
cognitive functions, such as executive processing and
working memory, or the active maintenance of on-line
information (Smith and Jonides, 1999).
The finding that the cognitive regulation of responses to
emotional scenes alters functional activity in the dlPFC
and amygdala could suggests that these brain regions
are functionally interconnected.
However, anatomical connectivity studies have failed to
find direct connections between the amygdala and the
dlPFC (Barbas, 2000; McDonald et al., 1996; Stefanacci
and Amaral, 2002).
Phelps proposes that the dlPFC regions
linked with the cognitive regulation of
emotion may take advantage of the
evolutionarily shared mechanisms of
extinction to diminish emotional responses.
Although these dlPFC regions do not have direct projections
to the amygdala, they do project to ventral and medial PFC
regions that are more directly connected with the
amygdala (Amaral, 2002; Groenewegen et al., 1997; McDonald et
al., 1996).
It is possible, therefore, that cognitive emotion regulation
strategies, which recruit dlPFC regions, may diminish
emotional response through connections with vmPFC, which
has been shown to inhibit the amygdala in fear extinction
(Milad and Quirk, 2002).
Meno deattivata è la mPFC, più inibita è l’amigdala.
La mPFC è meno deattivata nel caso della regolazione
In other words, humans may have
developed complex cognition that can aid in
the regulation of emotional responses, but
these processes utilize phylogenetically
older mechanisms of extinction for
diminishing fears that are no longer
adaptive.