Consolidation of fear extinction requires NMDA receptor-dependent bursting in the ventromedial prefrontal cortex

Different fear states engage distinct networks within the intercalated cell clusters of the amygdala

Although extinction-based therapies are among the most effective treatments for anxiety disorders, the neural bases of fear extinction remain still essentially unclear. Recent evidence suggests that the intercalated cell masses of the amygdala (ITCs) are critical structures for fear extinction. However, the neuronal organization of ITCs and how distinct clusters contribute to different fear states are still entirely unknown. Here, by combining whole-cell patch-clamp recordings and biocytin labeling with full anatomical reconstruction of the filled neurons and ultrastructural analysis of their synaptic contacts, we have elucidated the cellular organization and efferent connections of one of the main ITC clusters in mice. Our data showed an unexpected heterogeneity in the axonal pattern of medial paracapsular ITC (Imp) neurons and the presence of three distinct neuronal subtypes. Functionally, we observed that the Imp was preferentially activated during fear expression, whereas extinction training and extinction retrieval activated the main ITC nucleus (IN), as measured by quantifying Zif268 expression. This can be explained by the IPSPs evoked in the IN after Imp stimulation, most likely through the GABAergic monosynaptic innervation of IN neurons by one subtype of Imp cells, namely the medial capsular-projecting (MCp)-Imp neurons. MCp-Imp neurons also target large ITC cells that surround ITC clusters and express the metabotropic glutamate receptor 1α. These findings reveal a distinctive participation of ITC clusters to different fear states and the underlying anatomical circuitries, hence shedding new light on ITC networks and providing a novel framework to elucidate their role in fear expression and extinction.

D-cycloserine facilitates extinction of cocaine self-administration in C57 mice

INTRODUCTION

Cocaine is a highly addictive drug of abuse for which there are currently no medications. In rats and mice d-cycloserine (DCS), a partial NMDA agonist, accelerates extinction of cocaine seeking behavior. Since cues delay extinction here, we evaluated the effects d-cycloserine in extinction with and without the presence of cues.

METHODS

Two doses of DCS (15 and 30 mg/kg) were studied in C57 mice. Mice self-administered cocaine (1 mg/kg) for 2 weeks and then underwent a 20-day extinction period where DCS was administered i.p. immediately following each daily session. Extinction was conducted in some mice with the presence of cocaine-paired cues; while others were in the absence of these cues.

RESULTS

DCS treated mice (either dose) showed significantly reduced lever pressing during extinction with cue exposures when compared with vehicle treated mice. Without cues, animals showed much lower levels of lever pressing but the differences between vehicle and DCS were not significant.

CONCLUSION

DCS accelerated extinction with the presence of cues, but there were no differences on extinction without cues as compared with vehicle. These findings are consistent with DCS disrupting the memory process associated with the cues. Since drug cues are significantly involved in relapse, these findings support research to assess the therapeutic potential of DCS in cocaine addiction.

Behavioral analysis of NR2C knockout mouse reveals deficit in acquisition of conditioned fear and working memory

N-methyl-D-aspartate (NMDA) receptors play an important role in excitatory neurotransmission and mediate synaptic plasticity associated with learning and memory. NMDA receptors are composed of two NR1 and two NR2 subunits and the identity of the NR2 subunit confers unique electrophysiologic and pharmacologic properties to the receptor. The precise role of NR2C-containing receptors in vivo is poorly understood. We have performed a battery of behavioral tests on NR2C knockout/nβ-galactosidase knock-in mice and found no difference in spontaneous activity, basal anxiety, forced-swim immobility, novel object recognition, pain sensitivity and reference memory in comparison to wildtype counterparts. However, NR2C knockout mice were found to exhibit deficits in fear acquisition and working memory compared to wildtype mice. Deficit in fear acquisition correlated with lack of fear conditioning-induced plasticity at the thalamo-amygdala synapse. These findings suggest a unique role of NR2C-containing receptors in associative and executive learning representing a novel therapeutic target for deficits in cognition.

The medial prefrontal and orbitofrontal cortices differentially regulate dopamine system function

The prefrontal cortex (PFC) is essential for top-down control over higher-order executive function. In this study we demonstrate that the medial prefrontal cortex (mPFC) and orbitofrontal cortex (OFC) differentially regulate VTA dopamine neuron activity, and furthermore, the pattern of activity in the PFC drastically alters the dopamine neuron response. Thus, although single-pulse activation of the mPFC either excites or inhibits equivalent numbers of dopamine neurons, activation of the OFC induces a primarily inhibitory response. Moreover, activation of the PFC with a pattern that mimics spontaneous burst firing of pyramidal neurons produces a strikingly different response. Specifically, burst-like activation of the mPFC induces a massive increase in dopamine neuron firing, whereas a similar pattern of OFC activation largely inhibits dopamine activity. Taken together, these data demonstrate that the mPFC and OFC differentially regulate dopamine neuron activity, and that the pattern of cortical activation is critical for determining dopamine system output.

Aging redistributes medial prefrontal neuronal excitability and impedes extinction of trace fear conditioning

Cognitive flexibility is critical for survival and reflects the malleability of the central nervous system (CNS) in response to changing environmental demands. Normal aging results in difficulties modifying established behaviors, which may involve medial prefrontal cortex (mPFC) dysfunction. Using extinction of conditioned fear in rats to assay cognitive flexibility, we demonstrate that extinction deficits reminiscent of mPFC dysfunction first appear during middle age, in the absence of hippocampus-dependent context deficits. Emergence of aging-related extinction deficits paralleled a redistribution of neuronal excitability across two critical mPFC regions via two distinct mechanisms. First, excitability decreased in regular spiking neurons of infralimbic-mPFC (IL), a region whose activity is required for extinction. Second, excitability increased in burst spiking neurons of prelimbic-mPFC (PL), a region whose activity hinders extinction. Experiments using synaptic blockers revealed that these aging-related differences were intrinsic. Thus, changes in IL and PL intrinsic excitability may contribute to cognitive flexibility impairments observed during normal aging.

Medial prefrontal cortex activation facilitates re-extinction of fear in rats

It has been suggested that reduced infralimbic (IL) cortical activity contributes to impairments of fear extinction. We therefore explored whether pharmacological activation of the IL would facilitate extinction under conditions it normally fails (i.e., immediate extinction). Rats received auditory fear conditioning 1 h before extinction training. Immediately prior to extinction, rats received microinfusions into the IL of the GABA(A) receptor antagonist, picrotoxin, or the NMDA receptor partial agonist, D-cycloserine. Although neither drug facilitated extinction, they both facilitated the subsequent re-extinction of fear when animals were trained in a drug-free state, suggesting that activating the IL primes behavioral extinction.

The use of cognitive enhancers in animal models of fear extinction

In anxiety disorders, such as posttraumatic stress disorders and phobias, classical conditioning pairs natural (unconditioned) fear-eliciting stimuli with contextual or discrete cues resulting in enduring fear responses to multiple stimuli. Extinction is an active learning process that results in a reduction of conditioned fear responses after conditioned stimuli are no longer paired with unconditioned stimuli. Fear extinction often produces incomplete effects and this highlights the relative permanence of bonds between conditioned stimuli and conditioned fear responses. The animal research literature is rich in its demonstration of cognitive enhancing agents that alter fear extinction. This review specifically examines the fear extinguishing effects of cognitive enhancers that act on gamma-aminobutyric acid (GABA), glutamatergic, cholinergic, adrenergic, dopaminergic, and cannabinoid signaling pathways. It also examines the effects of compounds that alter epigenetic and neurotrophic mechanisms in fear extinction. Of these cognitive enhancers, glutamatergic N-methyl d-aspartate (NMDA) receptor agonists, such as D-cycloserine, have enhanced fear extinction in a context-, dose- and time-dependent manner. Agents that function as glutamatergic α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor agonists, alpha2-adrenergic receptor antagonists (such as yohimbine), neurotrophic factors (brain derived neurotrophic factor or BDNF) and histone deacetylase inhibitors (valproate and sodium butyrate) also improve fear extinction in animals. However, some have anxiogenic effects and their contextual and temporal effects need to be more reliably demonstrated. Various cognitive enhancers produce changes in cortico-amygdala synaptic plasticity through multiple mechanisms and these neural changes enhance fear extinction. We need to better define the changes in neural plasticity produced by these agents in order to develop more effective compounds. In the clinical setting, such use of effective cognitive enhancers with cue exposure therapy, using compounds derived from animal model studies, provides great hope for the future treatment of anxiety disorders.

Pharmacological enhancement of memory and executive functioning in laboratory animals

Investigating how different pharmacological compounds may enhance learning, memory, and higher-order cognitive functions in laboratory animals is the first critical step toward the development of cognitive enhancers that may be used to ameliorate impairments in these functions in patients suffering from neuropsychiatric disorders. Rather than focus on one aspect of cognition, or class of drug, in this review we provide a broad overview of how distinct classes of pharmacological compounds may enhance different types of memory and executive functioning, particularly those mediated by the prefrontal cortex. These include recognition memory, attention, working memory, and different components of behavioral flexibility. A key emphasis is placed on comparing and contrasting the effects of certain drugs on different cognitive and mnemonic functions, highlighting methodological issues associated with this type of research, tasks used to investigate these functions, and avenues for future research. Viewed collectively, studies of the neuropharmacological basis of cognition in rodents and non-human primates have identified targets that will hopefully open new avenues for the treatment of cognitive disabilities in persons affected by mental disorders.

Pharmacological enhancement of drug cue extinction learning: translational challenges

Augmentation of cue exposure (extinction) therapy with cognitive-enhancing pharmacotherapy may constitute a rational strategy for the clinical management of drug relapse. While certain success has been reported for this form of therapy in anxiety disorders, in this paper we highlight several obstacles that may undermine the efficacy of exposure therapy for substance use disorders. We also review translational studies that have evaluated the facilitative effects of the cognitive enhancer D-cycloserine on extinction targeting drug-related cues. Finally, important considerations for the design and implementation of future studies evaluating exposure therapy combined with pharmacotherapy for substance use disorders are discussed.

Glutamate receptors in extinction and extinction-based therapies for psychiatric illness

Some psychiatric illnesses involve a learned component. For example, in posttraumatic stress disorder, memories triggered by trauma-associated cues trigger fear and anxiety, and in addiction, drug-associated cues elicit drug craving and withdrawal. Clinical interventions to reduce the impact of conditioned cues in eliciting these maladaptive conditioned responses are likely to be beneficial. Extinction is a method of lessening conditioned responses and involves repeated exposures to a cue in the absence of the event it once predicted. We believe that an improved understanding of the behavioral and neurobiological mechanisms of extinction will allow extinction-like procedures in the clinic to become more effective. Research on the role of glutamate-the major excitatory neurotransmitter in the mammalian brain-in extinction has led to the development of pharmacotherapeutics to enhance the efficacy of extinction-based protocols in clinical populations. In this review, we describe what has been learned about glutamate actions at its three major receptor types (N-methyl-D-aspartate (NMDA) receptors, α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors, and metabotropic glutamate receptors) in the extinction of conditioned fear, drug craving, and withdrawal. We then discuss how these findings have been applied in clinical research.

Development of anxiety: the role of threat appraisal and fear learning

Anxious individuals exhibit threat biases at multiple levels of information processing. From a developmental perspective, abnormal safety learning in childhood may establish threat-related appraisal biases early, which may contribute to chronic disorders in adulthood. This review illustrates how the interface among attention, threat appraisal, and fear learning can generate novel insights for outcome prediction. This review summarizes data on amygdala function, as it relates to learning and attention, highlights the importance of examining threat appraisal, and introduces a novel imaging paradigm to investigate the neural correlates of threat appraisal and threat-sensitivity during extinction recall. This novel paradigm can be used to investigate key questions relevant to prognosis and treatment. Depression and Anxiety, 2011.© 2010 Wiley-Liss, Inc.

Dissociable roles of prelimbic and infralimbic cortices, ventral hippocampus, and basolateral amygdala in the expression and extinction of conditioned fear

Current models of conditioned fear expression and extinction involve the basolateral amygdala (BLA), ventral medial prefrontal cortex (vmPFC), and the hippocampus (HPC). There is some disagreement with respect to the specific roles of these structures, perhaps due to subregional differences within each area. For example, growing evidence suggests that infralimbic (IL) and prelimbic (PL) subregions of vmPFC have opposite influences on fear expression. Moreover, it is the ventral HPC (vHPC), rather than the dorsal HPC, that projects to vmPFC and BLA. To help determine regional specificity, we used small doses of the GABA(A) agonist muscimol to selectively inactivate IL, PL, BLA, or vHPC in an auditory fear conditioning and extinction paradigm. Infusions were performed prior to extinction training, allowing us to assess the effects on both fear expression and subsequent extinction memory. Inactivation of IL had no effect on fear expression, but impaired the within-session acquisition of extinction as well as extinction memory. In contrast, inactivation of PL impaired fear expression, but had no effect on extinction memory. Inactivation of the BLA or vHPC impaired both fear expression and extinction memory. Post-extinction inactivations had no effect in any structure. We suggest a model in which amygdala-dependent fear expression is modulated by inputs from PL and vHPC, whereas extinction memory requires extinction-induced plasticity in IL, BLA, and/or vHPC.

Emotional processing in anterior cingulate and medial prefrontal cortex

Negative emotional stimuli activate a broad network of brain regions, including the medial prefrontal (mPFC) and anterior cingulate (ACC) cortices. An early influential view dichotomized these regions into dorsal-caudal cognitive and ventral-rostral affective subdivisions. In this review, we examine a wealth of recent research on negative emotions in animals and humans, using the example of fear or anxiety, and conclude that, contrary to the traditional dichotomy, both subdivisions make key contributions to emotional processing. Specifically, dorsal-caudal regions of the ACC and mPFC are involved in appraisal and expression of negative emotion, whereas ventral-rostral portions of the ACC and mPFC have a regulatory role with respect to limbic regions involved in generating emotional responses. Moreover, this new framework is broadly consistent with emerging data on other negative and positive emotions.

Projection-specific neuromodulation of medial prefrontal cortex neurons

Mnemonic persistent activity in the prefrontal cortex (PFC) constitutes the neural basis of working memory. To understand how neuromodulators contribute to the generation of persistent activity, it is necessary to identify the intrinsic properties of the layer V pyramidal neurons that transfer this information to downstream networks. Here we show that the somatic dynamic and integrative properties of layer V pyramidal neurons in the rat medial PFC depend on whether they project subcortically to the pons [corticopontine (CPn)] or to the contralateral cortex [commissural (COM)]. CPn neurons display low temporal summation and accelerate in firing frequency when depolarized, whereas COM neurons have high temporal summation and display spike frequency accommodation. In response to dynamic stimuli, COM neurons act as low-pass filters, whereas CPn neurons act as bandpass filters, resonating in the theta frequency range (3-6 Hz). The disparate subthreshold properties of COM and CPn neurons can be accounted for by differences in the hyperpolarization-activated cyclic nucleotide gated cation h-current. Interestingly, neuromodulators hypothesized to enhance mnemonic persistent activity affect COM and CPn neurons distinctly. Adrenergic modulation shifts the dynamic properties of CPn but not COM neurons and increases the excitability of CPn neurons significantly more than COM neurons. In response to cholinergic modulation, CPn neurons were much more likely to display activity-dependent intrinsic persistent firing than COM neurons. Together, these data suggest that the two categories of projection neurons may subserve separate functions in PFC and may be engaged differently during working memory processes.

Infralimbic D2 receptors are necessary for fear extinction and extinction-related tone responses

BACKGROUND

Fear extinction is dependent on plasticity in the infralimbic prefrontal cortex, an area heavily innervated by midbrain dopaminergic inputs. Dopamine D2 receptors are concentrated in infralimbic output neurons that are involved in the suppression of conditioned fear after extinction. Here, we examined the specific role of infralimbic D2 receptors in mediating associative learning underlying fear extinction using the selective D2 antagonist raclopride.

METHODS

Raclopride was administered systemically or infused into the infralimbic prefrontal cortex before fear extinction, and extinction retention was tested the following day. Rats were also prepared for single-unit recording in the infralimbic prefrontal cortex to assess the effect of raclopride on firing properties.

RESULTS

We found that systemic injection of raclopride given before extinction impaired retrieval of extinction when rats were tested drug-free the next day but also induced catalepsy during extinction training. To determine whether impaired extinction was due to impaired motor function or disruption of extinction consolidation, we infused raclopride directly into the infralimbic prefrontal cortex. Raclopride infused immediately before extinction training did not produce motor deficits but impaired recall of extinction when tested drug-free. Furthermore, in animals that underwent extinction training, systemic raclopride reduced the tone responsiveness of infralimbic prefrontal cortex neurons in layers 5/6, with no changes in average firing rate.

CONCLUSIONS

We suggest that D2 receptors facilitate extinction by increasing the signal-to-noise of infralimbic prefrontal cortex neurons that consolidate extinction.

The neural correlates and temporal sequence of the relationship between shock exposure, disturbed sleep and impaired consolidation of fear extinction

Consolidation of extinction learning is a primary mechanism disrupted in posttraumatic stress disorder (PTSD), associated with hypoactivity of the ventromedial prefrontal cortex and hippocampus. A role for rapid eye movement (REM) sleep disturbances in this failure to consolidate extinction learning has been proposed. We performed functional magnetic resonance imaging (fMRI) with simultaneous skin conductance response (SCR) measurements in 16 healthy participants during conditioning/extinction and later recall of extinction. The visual stimuli were basic geometric forms and electrical shocks functioned as the unconditioned stimulus. Between the conditioning/extinction and recall sessions, participants received a 90-min sleep window in the sleep laboratory. This daytime sleep was polysomnographically recorded and scored by professionals blind to the study design. Only seven out of 16 participants had REM sleep; participants without REM sleep had a significantly slower decline of both SCR and neural activity of the laterodorsal tegmentum in response to electrical shocks during conditioning. At recall of fear extinction, participants with preceding REM sleep had a reduced SCR and stronger activation of the left ventromedial prefrontal cortex and bilateral lingual gyrus in response to the extinguished stimulus than participants lacking REM sleep. This study indicates that trait-like differences in shock reactivity/habituation (mediated by the brainstem) are predictive of REM sleep disruption, which in turn is associated with impaired consolidation of extinction (mediated by the ventromedial prefrontal cortex). These findings help understand the neurobiological basis and the temporal sequence of the relationship between shock exposure, disturbed sleep and impaired consolidation of extinction, as observed in PTSD.

Retrieval induces reconsolidation of fear extinction memory

The nonreinforced expression of long-tem memory may lead to two opposite protein synthesis-dependent processes: extinction and reconsolidation. Extinction weakens consolidated memories, whereas reconsolidation allows incorporation of additional information into them. Knowledge about these two processes has accumulated in recent years, but their possible interaction has not been evaluated yet. Here, we report that inhibition of protein synthesis in the CA1 region of the dorsal hippocampus after retrieval of fear extinction impedes subsequent reactivation of the extinction memory trace without affecting its storage or that of the initial fear memory. Our results suggest that extinction memory is susceptible to a retrieval-induced process similar to reconsolidation in the hippocampus.

Activation of the infralimbic cortex in a fear context enhances extinction learning

Activation of the infralimbic region (IL) of the medial prefrontal cortex (mPFC) reduces conditioned fear in a variety of situations, and the IL is thought to play an important role in the extinction of conditioned fear. Here we report a series of experiments using contextual fear conditioning in which the IL is activated with the GABAa antagonist picrotoxin (Ptx) during a single extinction session in the fear context. We investigate the impact of this manipulation on subsequent extinction sessions in which Ptx is no longer present. First, we demonstrate that a single treatment with intra-IL Ptx administered in a conditioned fear context greatly accelerates the rate of extinction on the following days. Importantly, IL-Ptx also enhances extinction to a different fear context than the one in which IL-Ptx was administered. Thus, IL-Ptx primes extinction learning regardless of the fear context in which the IL was initially activated. Second, activation of the IL must occur in conjunction with a fear context in order to enhance extinction; the extinction enhancing effect is not observable if IL-Ptx is administered in a neutral context. Finally, this extinction enhancing effect is specific to the IL for it does not occur if Ptx is injected into the prelimbic region (PL) of the mPFC. The results indicate a novel persisting control of fear induced by activation of the IL and suggest that IL activation induces changes in extinction-related circuitry that prime extinction learning.

Chronic stress alters neural activity in medial prefrontal cortex during retrieval of extinction

Chronic restraint stress produces morphological changes in medial prefrontal cortex and disrupts a prefrontally mediated behavior, retrieval of extinction. To assess potential physiological correlates of these alterations, we compared neural activity in infralimbic and prelimbic cortex of unstressed versus stressed rats during fear conditioning and extinction. After implantation of microwire bundles into infralimbic or prelimbic cortex, rats were either unstressed or stressed via placement in a plastic restrainer (3 h/day for 1 week). Rats then underwent fear conditioning and extinction while activity of neurons in infralimbic or prelimbic cortex was recorded. Percent freezing and neural activity were assessed during all phases of training. Chronic stress enhanced freezing during acquisition of conditioned fear, and altered both prelimbic and infralimbic activity during this phase. Stress did not alter initial extinction or conditioned stimulus (CS)-related activity during this phase. However, stress impaired retrieval of extinction assessed 24 h later, and this was accompanied by alterations in neuronal activity in both prelimbic and infralimbic cortex. In prelimbic cortex, unstressed rats showed decreased activity in response to CS presentation, whereas stressed rats showed no change. In infralimbic cortex, neurons in unstressed rats exhibited increased firing in response to the CS, whereas stressed rats showed no increase in infralimbic firing during the tone. Finally, CS-related firing in infralimbic but not prelimbic cortex was correlated with extinction retrieval. Thus, the stress-induced alteration of neuronal activity in infralimbic cortex may be responsible for the stress-induced deficit in retrieval of extinction.

Learning increases the survival of newborn neurons provided that learning is difficult to achieve and successful

Learning increases neurogenesis by increasing the survival of new cells generated in the adult hippocampal formation [Shors, T. J. Saving new brain cells. Scientific American, 300, 46-52, 2009]. However, only some types of learning are effective. Recent studies demonstrate that animals that learn the conditioned response (CR) but require more trials to do so retain more new neurons than animals that quickly acquire the CR or that fail to acquire the CR. In these studies, task parameters were altered to modify the number of trials required to learn a CR. Here, we asked whether pharmacological manipulations that prevent or facilitate learning would decrease or increase, respectively, the number of cells that remain in the hippocampus after training. To answer this question, we first prevented learning with the competitive N-methyl-D-aspartate (NMDA) receptor antagonist (RS)-3-(2-carboxypiperazin-4-yl) propyl-1-phosphonic acid. As a consequence, training did not increase cell survival. Second, we facilitated learning with the cognitive enhancer D-cycloserine, which increases NMDA receptor activity via its actions at the glycine binding site. Administration of D-cycloserine each day before training increased the number of learned responses and the number of cells that survived. All animals that learned the CR retained more of the new cells, but those that learned very quickly retained fewer than those that required more training trials to learn. Together, these results demonstrate that NMDA receptor activation modifies learning and as a consequence alters the number of surviving cells in the adult hippocampus.

Strain difference in the effect of infralimbic cortex lesions on fear extinction in rats

The infralimbic division of the medial prefrontal cortex (IL) has been implicated in the consolidation and retention of extinction memories. However, the effects of IL lesions on the retention of extinction memory are inconsistent. In the present experiments, we examined whether rat strain influences the effects of IL lesions on extinction. In Experiment 1, Sprague-Dawley (SD) or Long-Evans (LE) rats received a standard auditory fear conditioning procedure, which was followed by an extinction session; freezing served as the index of conditional fear. Our results reveal that focal IL lesions impair the retention of extinction in SD, but not LE rats. In addition to the strain difference in sensitivity to IL lesions, LE rats exhibited significantly higher levels of contextual fear before the outset of extinction training than SD rats. In a second experiment we thus examined whether contextual fear influenced the sensitivity of extinction to IL lesions in LE rats. LE rats received the same conditioning as in Experiment 1, and then were either merely exposed to a novel context or administered unsignaled shocks in that context, followed by extinction and test sessions. Our results reveal that LE rats with IL lesions showed normal extinction regardless of the levels of contextual fear manifest before extinction. Thus, we conclude that rat strain is an important variable that influences the role of infralimbic cortex in fear extinction.

Impact of predatory threat on fear extinction in Lewis rats

Humans with post-traumatic stress disorder (PTSD) are deficient at extinguishing conditioned fear responses. A study of identical twins concluded that this extinction deficit does not predate trauma but develops as a result of trauma. The present study tested whether the Lewis rat model of PTSD reproduces these features of the human syndrome. Lewis rats were subjected to classical auditory fear conditioning before or after exposure to a predatory threat that mimics a type of traumatic stress that leads to PTSD in humans. Exploratory behavior on the elevated plus maze 1 wk after predatory threat exposure was used to distinguish resilient vs. PTSD-like rats. Properties of extinction varied depending on whether fear conditioning and extinction occurred before or after predatory threat. When fear conditioning was carried out after predatory threat, PTSD-like rats showed a marked extinction deficit compared with resilient rats. In contrast, no differences were seen between resilient and PTSD-like rats when fear conditioning and extinction occurred prior to predatory threat. These findings in Lewis rats closely match the results seen in humans with PTSD, thereby suggesting that studies comparing neuronal interactions in resilient vs. at-risk Lewis rats might shed light on the causes and pathophysiology of human PTSD.

M-type potassium channels modulate the intrinsic excitability of infralimbic neurons and regulate fear expression and extinction

Growing evidence indicates that the activity of infralimbic prefrontal cortex (IL) is critical for inhibiting inappropriate fear responses following extinction learning. Recently, we showed that fear conditioning and extinction alter the intrinsic excitability and bursting of IL pyramidal neurons in brain slices. IL neurons from Sprague Dawley rats expressing high fear had lower intrinsic excitability and bursting than those from rats expressing low fear, suggesting that regulating the intrinsic excitability and bursting of IL neurons would modulate fear expression. To test this, we combined patch-clamp electrophysiology, auditory fear conditioning, and IL infusions of M-type K(+) channel modulators. Patch-clamp recordings from IL neurons showed that the M-type K(+) channel blocker, XE-991, increased the number of spikes evoked by a depolarizing pulse and reduced the first interspike interval indicating enhanced bursting. To test whether pharmacological enhancement of IL excitability and bursting reduces fear expression and facilitates extinction, fear-conditioned rats were infused with XE-991 into IL before extinction training. XE-infused rats showed reduced freezing and facilitated extinction compared to vehicle-infused rats. The following day, recall of extinction memory was enhanced. Reducing IL excitability and bursting with the M-type K(+) channel agonist, flupirtine, had the opposite effect. Flupirtine reduced IL spike count and bursting in brain slices. Fear-conditioned rats infused with flupirtine into IL before extinction showed significantly higher levels of freezing, indicating that stimulation of M-channels enhanced fear expression. Our findings suggest that the intrinsic excitability and bursting of IL neurons regulate fear expression even before extinction.

Impaired extinction retention in adolescent rats: effects of D-cycloserine

The developmental trajectory of the prefrontal cortex (PFC) in both rats and humans is nonlinear, with a notable decline in synaptic density during adolescence, potentially creating a 'natural lesion' preparation at this age. Given that the PFC is critically involved in retention of extinction of learned fear in adult humans and rodents, the present study examined whether adolescent rats exhibit impaired extinction retention. The results of experiment 1 showed that adolescent rats were impaired in extinction retention, compared with both younger and older rats. The partial NMDA receptor agonist D-cycloserine (DCS) improved extinction retention in adolescent rats (experiment 2), but only if administered immediately after extinction training (experiment 3). In addition, providing extended extinction training improved extinction retention in adolescent rats in a manner similar to that of DCS (experiment 4). The results of this study show that adolescent rats exhibit impaired extinction retention, and that this can be reduced through either DCS or extended extinction training. These novel findings have potential implications for clinical treatments of fear and anxiety disorders in adolescent patients.

Memory for fear extinction requires mGluR5-mediated activation of infralimbic neurons

Consolidation of fear extinction involves enhancement of N-methyl D aspartate (NMDA) receptor-dependent bursting in the infralimbic region (IL) of the medial prefrontal cortex (mPFC). Previous studies have shown that systemic blockade of metabotropic glutamate receptor type 5 (mGluR5) reduces bursting in the mPFC and mGluR5 agonists enhance NMDA receptor currents in vitro, suggesting that mGluR5 activation in IL may contribute to fear extinction. In the current study, rats injected with the mGluR5 antagonist 2-methyl-6-(phenylethyl)-pyridine (MPEP) systemically, or intra-IL, prior to extinction exhibited normal within-session extinction, but were impaired in their ability to recall extinction the following day. To directly determine whether mGluR5 stimulation enhances the burst firing of IL neurons, we used patch-clamp electrophysiology in prefrontal slices. The mGluR5 agonist, (RS)-2-chloro-5-hydroxyphenylglycine (CHPG), increased intrinsic bursting in IL neurons. Increased bursting was correlated with a reduction in the slow after hyperpolarizing potential and was prevented by coapplication of MPEP. CHPG did not increase NMDA currents, suggesting that an NMDA receptor-independent enhancement of IL bursting via stimulation of mGluR5 receptors contributes to fear extinction. Therefore, the mGluR5 receptor could be a suitable target for pharmacological adjuncts to extinction-based therapies for anxiety disorders.

Single-unit activity in the medial prefrontal cortex during immediate and delayed extinction of fear in rats

Delivering extinction trials minutes after fear conditioning yields only a short-term fear suppression that fully recovers the following day. Because extinction has been reported to increase CS-evoked spike firing and spontaneous bursting in the infralimbic (IL) division of the medial prefrontal cortex (mPFC), we explored the possibility that this immediate extinction deficit is related to altered mPFC function. Single-units were simultaneously recorded in rats from neurons in IL and the prelimbic (PrL) division of the mPFC during an extinction session conducted 10 minutes (immediate) or 24 hours (delayed) after auditory fear conditioning. In contrast to previous reports, IL neurons exhibited CS-evoked responses early in extinction training in both immediate and delayed conditions and these responses decreased in magnitude over the course of extinction training. During the retention test, CS-evoked firing in IL was significantly greater in animals that failed to acquire extinction. Spontaneous bursting during the extinction and test sessions was also different in the immediate and delayed groups. There were no group differences in PrL activity during extinction or retention testing. Alterations in both spontaneous and CS-evoked neuronal activity in the IL may contribute to the immediate extinction deficit.

Regulation of extinction-related plasticity by opioid receptors in the ventrolateral periaqueductal gray matter

Recent work has led to a better understanding of the neural mechanisms underlying the extinction of Pavlovian fear conditioning. Long-term synaptic changes in the medial prefrontal cortex (mPFC) are critical for extinction learning, but very little is currently known about how the mPFC and other brain areas interact during extinction. The current study examined the effect of drugs that impair the extinction of fear conditioning on the activation of the extracellular-related kinase/mitogen-activated protein kinase (ERK/MAPK) in brain regions that likely participate in the consolidation of extinction learning. Inhibitors of opioid and N-methyl-d-aspartic acid (NMDA) receptors were applied to the ventrolateral periaqueductal gray matter (vlPAG) and amygdala shortly before extinction training. Results from these experiments show that blocking opioid receptors in the vlPAG prevented the formation of extinction memory, whereas NMDA receptor blockade had no effect. Conversely, blocking NMDA receptors in the amygdala disrupted the formation of fear extinction memory, but opioid receptor blockade in the same brain area did not. Subsequent experiments tested the effect of these drug treatments on the activation of the ERK/MAPK signaling pathway in various brain regions following extinction training. Only opioid receptor blockade in the vlPAG disrupted ERK phosphorylation in the mPFC and amygdala. These data support the idea that opiodergic signaling derived from the vlPAG affects plasticity across the brain circuit responsible for the formation of extinction memory.

Behavioral and neural analysis of GABA in the acquisition, consolidation, reconsolidation, and extinction of fear memory

The current review systematically documents the role of gamma-amino-butyric acid (GABA) in different aspects of fear memory-acquisition and consolidation, reconsolidation, and extinction, and attempts to resolve apparent contradictions in the data in order to identify the function of GABA(A) receptors in fear memory. First, numerous studies have shown that pre- and post-training administration of drugs that facilitate GABAergic transmission disrupt the initial formation of fear memories, indicating a role for GABA(A) receptors, possibly within the amygdala and hippocampus, in the acquisition and consolidation of fear memories. Similarly, recent evidence indicates that these drugs are also detrimental to the restorage of fear memories after their reactivation. This suggests a role for GABA(A) receptors in the reconsolidation of fear memories, although the precise neural circuits are yet to be identified. Finally, research regarding the role of GABA in extinction has shown that GABAergic transmission is also disruptive to the formation of newly acquired extinction memories. We argue that contradictions to these patterns are the result of variations in (a) the location of drug infusion, (b) the dosage of the drug and/or (c) the time point of drug administration. The question of whether these GABA-induced memory deficits reflect deficits in retrieval is discussed. Overall, the evidence implies that the processes mediating memory stability consequent to initial fear learning, memory reactivation, and extinction training are dependent on a common mechanism of reduced GABAergic neurotransmission.

Molecular mechanisms in hippocampus and basolateral amygdala but not in parietal or cingulate cortex are involved in extinction of one-trial avoidance learning

The establishment of extinction of one-trial avoidance involves the dorsal hippocampus (DH) and basolateral amygdala (BLA), two areas that participate in its original consolidation. The posterior parietal (PARIE) and posterior cingulate (CING) cortices also participate in consolidation of this task but their role in extinction has not been explored. Here we study the effect on the extinction of one-trial avoidance in rats of three different drugs infused bilaterally into DH, BLA, PARIE or CING 5min before the first of four daily unreinforced test sessions: The glutamate NMDA receptor antagonist, AP5 (5.0microg/side),and the inhibitors of calcium-calmodulin dependent kinase II (CaMKII), KN-93 (0.3microg/side), or of the cAMP-dependent protein kinase (PKA), Rp-cAMPs (0.5microg/side) hindered extinction when given into DH or BLA. Levels of pPKA and pCaMKII increased in DH after the first extinction trial; in BLA only the CaMKII increase was seen. Thus, this pathway appears to participate in extinction in BLA at the "basal" levels, and at enhanced levels in DH. None of the treatments affected extinction when given into PARIE or CING. The present findings indicate that: (1) the DH and BLA are important for the initiation of extinction at the time of the first unreinforced retrieval session; (2) both the CaMKII and the PKA signaling pathway are necessary for the development of extinction in the two regions; (3) PARIE and CING are probably unrelated to extinction.

Induction of fear extinction with hippocampal-infralimbic BDNF

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. Brain-derived neurotrophic factor (BDNF) is a key mediator of synaptic plasticity in multiple brain areas. In rats subjected to auditory fear conditioning, BDNF infused into the IL mPFC reduced conditioned fear for up to 48 hours, even in the absence of extinction training, which suggests that BDNF substituted for extinction. Similar to extinction, BDNF-induced reduction in fear required N-methyl-D-aspartate receptors and did not erase the original fear memory. Rats failing to learn extinction showed reduced BDNF in hippocampal inputs to the IL mPFC, and augmenting BDNF in this pathway prevented extinction failure. Hence, boosting BDNF activity in hippocampal-infralimbic circuits may ameliorate disorders of learned fear.

Lesion of infralimbic cortex occludes stress effects on retrieval of extinction but not fear conditioning

Chronic stress produces dendritic retraction in medial prefrontal cortex and impairs retrieval of extinction of conditioned fear, a behavior mediated by the infralimbic region (IL) of medial prefrontal cortex. To test the hypothesis that stress-induced changes in IL contribute to the stress-induced impairment in extinction retrieval, we performed an occlusion experiment in which we assessed the effects of stress alone, lesion of IL alone, and the combined effects of stress and lesion on extinction retrieval. If IL is the substrate upon which stress acts to produce deficits in extinction retrieval, then prior removal of IL should prevent the effect of stress on extinction retrieval. Rats received either sham or ibotenic acid lesions of IL. Rats in each group then remained unstressed or underwent daily restraint stress for 1week. Following the final day of restraint, rats received five habituation trials to a 30-s tone, followed by seven pairings of the tone with a 500-ms coterminating footshock. One hour later, rats received tone-alone extinction trials. On the following day, rats were given two extinction trials to test for extinction retrieval. Percent freezing was assessed throughout. Stress increased freezing during conditioning, and IL lesion did not block this effect. Either IL lesion alone or stress alone increased freezing on initial extinction trials. IL lesion did not attenuate the effect of stress during initial extinction. Similarly, IL lesion alone and stress alone produced deficits in extinction retrieval. However, stressed rats with IL lesions showed extinction retrieval comparable to that seen in unstressed, sham-lesioned rats. Thus, lesion of IL occluded the stress-induced impairment of extinction retrieval but failed to prevent the stress-induced facilitation of fear conditioning. This dissociation suggests that the effects of stress on these two aspects of emotion regulation are mediated at least in part by independent mechanisms, and that stress-induced changes in IL contribute to stress-induced deficits in extinction retrieval.

Plastic synaptic networks of the amygdala for the acquisition, expression, and extinction of conditioned fear

The last 10 years have witnessed a surge of interest for the mechanisms underlying the acquisition and extinction of classically conditioned fear responses. In part, this results from the realization that abnormalities in fear learning mechanisms likely participate in the development and/or maintenance of human anxiety disorders. The simplicity and robustness of this learning paradigm, coupled with the fact that the underlying circuitry is evolutionarily well conserved, make it an ideal model to study the basic biology of memory and identify genetic factors and neuronal systems that regulate the normal and pathological expressions of learned fear. Critical advances have been made in determining how modified neuronal functions upon fear acquisition become stabilized during fear memory consolidation and how these processes are controlled in the course of fear memory extinction. With these advances came the realization that activity in remote neuronal networks must be coordinated for these events to take place. In this paper, we review these mechanisms of coordinated network activity and the molecular cascades leading to enduring fear memory, and allowing for their extinction. We will focus on Pavlovian fear conditioning as a model and the amygdala as a key component for the acquisition and extinction of fear responses.

Prefrontal control of fear: more than just extinction

Although fear research has largely focused on the amygdala, recent findings highlight cortical control of the amygdala in the service of fear regulation. In rodent models, it is becoming well established that the infralimbic (IL) prefrontal cortex plays a key role in extinction learning, and recent findings are uncovering molecular mechanisms involved in extinction-related plasticity. Furthermore, mounting evidence implicates the prelimbic (PL) prefrontal cortex in the production of fear responses. Both IL and PL integrate inputs from the amygdala, as well as other structures to gate the expression of fear via projections to inhibitory or excitatory circuits within the amygdala. We suggest that dual control of the amygdala by separate prefrontal modules increases the flexibility of an organism's response to danger cues.

The infralimbic cortex regulates the consolidation of extinction after cocaine self-administration

The infralimbic cortex (IL) regulates the consolidation of extinction learning for fear conditioning. Whether the IL influences the consolidation of extinction learning for cocaine self-administration is unknown. To address this issue, male Sprague-Dawley rats underwent 2 wk of cocaine self-administration followed by extinction training. On the first 5 d of extinction, rats underwent brief (15- or 30-min) extinction sessions and received intra-IL microinjections immediately after each extinction session. On days 6-12 of extinction, rats underwent full-length (2-h) extinction sessions that were used to assess the retention of the extinction learning from the short sessions. IL inactivation via microinjections of the GABA agonists baclofen and muscimol (BM) immediately after the extinction sessions (days 1-5) impaired the retention of extinction learning. Control experiments demonstrated that this effect was not due to inactivation of the prelimbic cortex or due to effects of the drugs on the subsequent day's behavior. In contrast, post-training intra-IL microinjections of the allosteric AMPA receptor potentiator 4-[2-(phenylsulfonylamino)ethylthio]-2,6-difluorophenoxyacetamide (PEPA) enhanced retention of the extinction learning. As evidence suggests a role for the beta-adrenergic receptors in memory consolidation, other rats received microinjections of the beta(2)-adrenergic receptor agonist clenbuterol or antagonist ICI-118,551 (ICI). Post-training intra-IL administration of clenbuterol or pre-training administration of ICI enhanced or impaired, respectively, the retention of extinction learning. These data indicate that the IL, and specifically the glutamatergic and beta-adrenergic systems in the IL, regulates the consolidation of extinction of cocaine self-administration and that the IL can be manipulated to influence the retention of extinction.

Synchronized activity between the ventral hippocampus and the medial prefrontal cortex during anxiety

The ventral hippocampus, unlike its dorsal counterpart, is required for anxiety-like behavior. The means by which it acts are unknown. We hypothesized that the hippocampus synchronizes with downstream targets that influence anxiety, such as the medial prefrontal cortex (mPFC). To test this hypothesis, we recorded mPFC and hippocampal activity in mice exposed to two anxiogenic arenas. Theta-frequency activity in the mPFC and ventral, but not dorsal, hippocampus was highly correlated at baseline, and this correlation increased in both anxiogenic environments. Increases in mPFC theta power predicted avoidance of the aversive compartments of each arena and were larger in serotonin 1A receptor knockout mice, a genetic model of increased anxiety-like behavior. These results suggest a role for theta-frequency synchronization between the ventral hippocampus and the mPFC in anxiety. They are consistent with the notion that such synchronization is a general mechanism by which the hippocampus communicates with downstream structures of behavioral relevance.

Synaptic correlates of fear extinction in the amygdala

Anxiety disorders such as post-traumatic stress are characterized by an impaired ability to learn that cues previously associated with danger no longer represent a threat. However, the mechanisms underlying fear extinction remain unclear. Here we show in rats that extinction is associated with increased levels of synaptic inhibition in fear output neurons of the central amygdala (CEA). This increased inhibition results from a potentiation of fear input synapses to GABAergic intercalated amygdala neurons that project to CEA. Enhancement of inputs to intercalated cells required prefrontal activity during extinction training and involved a higher transmitter release probability coupled to an altered expression profile of ionotropic glutamate receptors. Overall, our results suggest that intercalated cells constitute a promising target for pharmacological treatments aiming to facilitate the treatment of anxiety disorders.

Alterations in fear response and spatial memory in pre- and post-natal zinc supplemented rats: remediation by copper

The role of zinc in the nervous system is receiving increased attention. At a time when dietary fortification and supplementation have increased the amount of zinc being consumed, little work has been done on the effects of enhanced zinc on behavior. Both zinc and copper are essential trace minerals that are acquired from the diet; under normal conditions the body protects against zinc overload, but at excessive dosages, copper deficiency has been seen. In order to examine the effect of enhanced metal administration on learning and memory, Sprague Dawley rats were given water supplemented with 10ppm Zn, 10ppm Zn+0.25ppm Cu, or normal lab water, during pre- and post-natal development. Fear conditioning tests at 4months showed significantly higher freezing rates during contextual retention and extinction and cued extinction for rats drinking water supplemented with zinc, suggesting increased anxiety compared to controls raised on lab water. During the MWM task at 9months, zinc-enhanced rats had significantly longer latencies to reach the platform compared to controls. The addition of copper to the zinc supplemented water brought freezing and latency levels closer to that of controls. These data demonstrate the importance of maintaining appropriate intake of both metals simultaneously, and show that long-term supplementation with zinc may cause alterations in memory.

Blockade of dopamine activity in the nucleus accumbens impairs learning extinction of conditioned fear

Three experiments used rats to investigate the role of dopamine activity in learning to inhibit conditioned fear responses (freezing) in extinction. In Experiment 1, rats systemically injected with the D2 dopamine antagonist, haloperidol, froze more across multiple extinction sessions and on a drug-free retention test than control rats. In Experiment 2, rats extinguished under an intracerebroventricular (ICV) infusion of haloperidol suppressed fear responses across extinction but froze more on a subsequent drug-free retention test than control rats. In Experiment 3, rats extinguished under an infusion of haloperidol in the nucleus accumbens were impaired in suppressing fear responses across extinction and froze more on subsequent drug-free retention test than control rats. These results show that learning to inhibit fear responses in extinction requires dopamine activity in the nucleus accumbens. They were interpreted to mean that dopaminergic activity in the nucleus accumbens regulates the prediction error required for learning to inhibit fear responses in extinction.

Role of the basolateral amygdala in the reinstatement and extinction of fear responses to a previously extinguished conditioned stimulus

Four experiments used rats to study the role of the basolateral amygdala (BLA) in the reinstatement and extinction of fear responses (freezing) to a previously extinguished conditioned stimulus (CS). In Experiment 1, BLA inactivation before pairing the extinguished CS with the shock unconditioned stimulus (US) or before US-alone exposure impaired the restoration and the reinstatement of fear responses to the extinguished CS. In Experiment 2, BLA inactivation before extinction impaired long-term inhibition of fear responses, but its inactivation before extinction of fear responses restored by CS-US pairing did not impair long-term inhibition. In Experiment 3, BLA inactivation before extinction of fear responses or before the extinction of fear responses reinstated by US-alone exposure impaired long-term inhibition. In Experiment 4, BLA inactivation did not impair long-term inhibition of fear responses reinstated by US-alone exposure if the context where the US-alone exposure occurred had been previously extinguished. These results confirm that the BLA is critical for both learning fear and fear inhibition, but not for relearning this inhibition. The results are consistent with the view that reinstatement is due to the extinguished CS being tested in a dangerous context and are discussed in terms of a contemporary neural model of fear inhibition.

Neuronal circuits of fear extinction

Fear extinction is a form of inhibitory learning that allows for the adaptive control of conditioned fear responses. Although fear extinction is an active learning process that eventually leads to the formation of a consolidated extinction memory, it is a fragile behavioural state. Fear responses can recover spontaneously or subsequent to environmental influences, such as context changes or stress. Understanding the neuronal substrates of fear extinction is of tremendous clinical relevance, as extinction is the cornerstone of psychological therapy of several anxiety disorders and because the relapse of maladaptative fear and anxiety is a major clinical problem. Recent research has begun to shed light on the molecular and cellular processes underlying fear extinction. In particular, the acquisition, consolidation and expression of extinction memories are thought to be mediated by highly specific neuronal circuits embedded in a large-scale brain network including the amygdala, prefrontal cortex, hippocampus and brain stem. Moreover, recent findings indicate that the neuronal circuitry of extinction is developmentally regulated. Here, we review emerging concepts of the neuronal circuitry of fear extinction, and highlight novel findings suggesting that the fragile phenomenon of extinction can be converted into a permanent erasure of fear memories. Finally, we discuss how research on genetic animal models of impaired extinction can further our understanding of the molecular and genetic bases of human anxiety disorders.

Extinction of drug- and withdrawal-paired cues in animal models: relevance to the treatment of addiction

Conditioned drug craving and withdrawal elicited by cues paired with drug use or acute withdrawal are among the many factors contributing to compulsive drug taking. Understanding how to stop these cues from having these effects is a major goal of addiction research. Extinction is a form of learning in which associations between cues and the events they predict are weakened by exposure to the cues in the absence of those events. Evidence from animal models suggests that conditioned responses to drug cues can be extinguished, although the degree to which this occurs in humans is controversial. Investigations into the neurobiological substrates of extinction of conditioned drug craving and withdrawal may facilitate the successful use of drug cue extinction within clinical contexts. While this work is still in the early stages, there are indications that extinction of drug- and withdrawal-paired cues shares neural mechanisms with extinction of conditioned fear. Using the fear extinction literature as a template, it is possible to organize the observations on drug cue extinction into a cohesive framework.

Deficiency of the 65 kDa isoform of glutamic acid decarboxylase impairs extinction of cued but not contextual fear memory

Extinction procedures are clinically relevant for reducing pathological fear, and the mechanisms of fear regulation are a subject of intense research. The amygdala, hippocampus, and prefrontal cortex (PFC) have all been suggested to be key brain areas in extinction of conditioned fear. GABA has particularly been implicated in extinction learning, and the 65 kDa isoform of glutamic acid decarboxylase (GAD65) may be important in elevating GABA levels in response to environmental signals. Extinction of conditioned fear was examined in Gad65(-/-) mice while recording local field potentials from the amygdala, hippocampus, and PFC simultaneously while monitoring behavior. Gad65(-/-) mice showed generalization of cued fear, as reported previously, and impaired extinction of cued fear, such that fear remained high across extinction training. This endurance in cued fear was associated with theta frequency synchronization between the amygdala and hippocampus. Extinction of contextual fear, however, was unaltered in Gad65(-/-) mice when compared with wild-type littermates. The data imply that GAD65 plays a critical role in regulating cued fear responses during extinction learning and that, during this process, GABAergic signaling is involved in modulating synchronized activity between the amygdala and hippocampus. In view of the more pronounced effect on cued versus contextual fear extinction, these influences may rely more on GABAergic mechanisms in the amygdala.

Changing fear: the neurocircuitry of emotion regulation

The ability to alter emotional responses as circumstances change is a critical component of normal adaptive behavior and is often impaired in psychological disorders. In this review, we discuss four emotional regulation techniques that have been investigated as means to control fear: extinction, cognitive regulation, active coping, and reconsolidation. For each technique, we review what is known about the underlying neural systems, combining findings from animal models and human neuroscience. The current evidence suggests that these different means of regulating fear depend on both overlapping and distinct components of a fear circuitry.

Noradrenergic modulation of extinction learning and exposure therapy

Memory consolidation is enhanced by emotional arousal, an effect mediated by noradrenergic beta-receptor signaling. Norepinephrine strengthens consolidation of both appetitive and aversive learning, and is implicated in extinction of conditioned responses. In this review, we summarize work on the noradrenergic mechanisms of extinction learning and implications for extinction-based exposure therapy. The evidence suggests that norepinephrine release evoked by conditioned stimuli during extinction strengthens extinction memory via beta-receptor signaling. The modulatory effect of norepinephrine during extinction depends on predictable presentation of conditioned stimuli and optimal levels of norepinephrine release. Mechanistically, norepinephrine acts to increase cellular excitability and enhance synaptic plasticity within extinction-related neural circuitry. Currently, drugs that modulate norepinephrine are being used to treat symptoms of anxiety disorders, and are now being tested as pharmacotherapeutic prophalactics in the prevention of chronic posttraumatic stress reactions and as adjuncts to extinction-based exposure therapy. Studies of these new applications of noradrenergic drugs show a converging pattern of results with basic science suggesting ways in which basic laboratory findings can be translated into procedures to enhance clinical outcomes.