Mouna Maroun

The Role of the Medial Prefrontal Cortex-Amygdala Circuit in Stress Effects on the Extinction of Fear

Stress exposure, depending on its intensity and duration, affects cognition and learning in an adaptive or maladaptive manner. Studies addressing the effects of stress on cognitive processes have mainly focused on conditioned fear, since it is suggested that fear-motivated learning lies at the root of affective and anxiety disorders. Inhibition of fear-motivated response can be accomplished by experimental extinction of the fearful response to the fear-inducing stimulus. Converging evidence indicates that extinction of fear memory requires plasticity in both the medial prefrontal cortex and the amygdala. These brain areas are also deeply involved in mediating the effects of exposure to stress on memory. Moreover, extensive evidence indicates that gamma-aminobutyric acid (GABA) transmission plays a primary role in the modulation of behavioral sequelae resulting from a stressful experience, and may also partially mediate inhibitory learning during extinction. In this review, we present evidence that exposure to a stressful experience may impair fear extinction and the possible involvement of the GABA system. Impairment of fear extinction learning is particularly important as it may predispose some individuals to the development of posttraumatic stress disorder. We further discuss a possible dysfunction in the medial prefrontal cortex-amygdala circuit following a stressful experience that may explain the impaired extinction caused by exposure to a stressor.

Enhancement of conditioned fear extinction by infusion of the GABAA agonist muscimol into the rat prefrontal cortex and amygdala

In auditory fear conditioning, repeated presentation of the tone in the absence of the shock leads to extinction of the acquired fear response. Both the infra limbic prefrontal cortex (IL) and the basolateral amygdala (BLA) are involved in extinction. In this study, we examine the involvement of these two regions in extinction by manipulating the γ‐aminobutyric acid (GABA)ergic system, in the Sprague–Dawley rat. We microinfused a low dose of the GABAA agonist muscimol into the IL or BLA. Muscimol infused to IL before extinction training, but not after either a short (five‐trials) or long (15‐trials) extinction training, resulted in long‐term facilitation of extinction. Infusion of muscimol to the BLA following a short (five‐trial) extinction session facilitated extinction at least 48‐h post‐drug infusion. The differences in the temporal parameters of the effects of muscimol in the IL or BLA, suggest differential involvement of these structures in long‐term extinction of fear memory. We propose a facilitating role for GABAA neurotransmission in the IL in triggering the onset of fear extinction and its maintenance, whereas in the BLA, GABAA neurotransmission facilitates extinction consolidation. The involvement of GABAA receptors in fear extinction in the prefrontal cortex and amygdala is of particular interest, because of the role of these areas in emotional processes, and the role of the GABAA receptors in anxiety states.

Subfield-specific immediate early gene expression associated with hippocampal long-term potentiation in vivo.

It is not known whether NMDA receptor‐dependent long‐term potentiation (LTP) is mediated by similar molecular mechanisms in different hippocampal areas. To address this question we have investigated changes in immediate early gene and protein expression in two hippocampal subfields following the induction of LTP in vivo and in vitro. In granule cells of the dentate gyrus, LTP induced in vivo by tetanic stimulation of the perforant path was followed by strong induction of the immediate early genes (IEGs) Zif268, Arc and Homer. The increase in Zif268 mRNA was accompanied by an increase in protein expression. In contrast, we were unable to detect modulation of the IEGs Zif268, Arc, Homer and HB‐GAM following induction of LTP by high‐frequency stimulation of the commissural projection to CA1 pyramidal cells in vivo. In this pathway, we also failed to detect modulation of Zif268 protein levels. Zif268, Arc and Homer can be modulated in CA1 pyramidal cells approximately twofold after electroshock‐induced maximal seizure, which demonstrates potential responsiveness to electrical stimuli. When LTP was induced in vitro neither CA1 pyramidal cells nor granule cells showed an increase in Zif268, Arc or Homer mRNA. However, in the slice preparation, granule cells have a different transcriptional state as basal IEG levels are elevated. These results establish the existence of subfield‐specific transcriptional responses to LTP‐inducing stimulation in the hippocampus of the intact animal, and demonstrate that in area CA1‐enhanced transcription of Zif268, Arc and Homer is not required for the induction of late LTP.

Arousal and stress effects on consolidation and reconsolidation of recognition memory.

This study examined the effects of the arousal level of the rat and exposure to a behavioral stressor on consolidation and reconsolidation of a nonaversive learning paradigm, the object recognition task. Learning was tested under two arousal conditions: no previous habituation to the experimental context (high novelty stress/arousal level) or extensive prior habituation (reduced novelty stress/arousal level). Results indicated that in the habituated rats, exposure to an out-of-context stressor (ie, elevated platform stress) impaired long-term consolidation and reconsolidation of object recognition. RU-486, a glucocorticoid receptor (GR) antagonist, infused into the basolateral amygdala (BLA), reversed the impairing effects of the stressor. In contrast, the nonhabituated aroused rats were impaired when consolidation was examined, but their memory was intact following reactivation of the memory trace. Exposure of nonhabituated rats to an out-of-context stressor enhanced the long-term consolidation of recognition memory, but impaired reconsolidation, and the effects were reversed by a GR antagonist infused into the BLA. Additionally, nonhabituated control rats showed intact retrieval following microinfusion of propranolol to the BLA immediately after the training, suggesting an involvement of beta-adrenoceptors in the BLA in the arousal-induced impairment of consolidation. These findings demonstrate opposite effects, detrimental and facilitative, of arousal and stress on memory consolidation and reconsolidation. In addition, the data suggests that although some general features underlie consolidation and reconsolidation, there is a possible dissimilarity between the two processes, which is dependent on the arousal level of the animal during training.

Medial Prefrontal Cortex: Multiple Roles in Fear and Extinction

Anxiety disorders are among the most common mental health problems; deficits in extinction have been implicated as a possible risk factor for the development of these disorders. Fear extinction refers to the ability to adapt as situations change by learning to suppress a previously acquired fear. Attention is directed toward the medial prefrontal cortex (mPFC) and the interaction it has with the amygdala as this circuit has crucial roles in both the acquisition and the extinction of fear associations. Here, we review converging evidence from different laboratories pointing to multiple roles that the mPFC has in fear regulation. Research on rodents indicates opposing roles that the different subregions of the mPFC have in exciting and inhibiting fear. In addition, this review aims to survey the findings addressing the mechanisms by which the mPFC regulates fear. Data from our laboratory and others show that changes in plasticity in the mPFC could be one of the mechanisms mediating extinction of fear. Recent findings on rodents and nonhuman primates report that modifying plasticity in the mPFC alters fear and affects extinction, suggesting that targeting plasticity in the mPFC could constitute a therapeutic tool for the treatment of anxiety disorders.

Microinfusion of the D1 receptor antagonist, SCH23390 into the IL but not the BLA impairs consolidation of extinction of auditory fear conditioning

In auditory fear conditioning, repeated presentation of the tone in the absence of the shock leads to extinction of the acquired fear response. Both the medial prefrontal cortex (mPFC) and the basolateral amygdala (BLA) are involved in extinction. Here we examined this involvement by antagonizing D1 receptors in both regions, in the rat. We microinfused the D1 receptor antagonist, SCH23390, into the infra-limbic part of the mPFC (IL) or BLA at different time points. SCH23390 mircoinfused into the IL either before extinction acquisition or following short extinction training resulted in impairment of extinction consolidation. Microinfusion of SCH23390 into the BLA, prior to acquisition of extinction caused impairment in acquisition of extinction without affecting extinction consolidation. This is supported by the results showing that microinfusion of SCH23390 into the BLA following a short-training session did not affect consolidation. These results further strengthen the role of mPFC in consolidation of extinction while highlighting the role of the D1 receptors in this process.

Stress reverses plasticity in the pathway projecting from the ventromedial prefrontal cortex to the basolateral amygdala

We have previously shown that high‐frequency stimulation to the basolateral amygdala (BLA) induces long‐term potentiation (LTP) in the ventromedial prefrontal cortex (vmPFC) and that prior exposure to inescapable stress inhibits the induction of LTP in this pathway [ Maroun & Richter‐Levin (2003)J. Neurosci., 23, 4406–4409]. Here, we show that the reciprocal pathway projecting from the vmPFC to the BLA is resistant to the induction of LTP. Conversely, long‐term depression (LTD) is robustly induced in the BLA in response to low‐frequency stimulation to the vmPFC. Furthermore, prior exposure to inescapable stress reverses plasticity in this pathway, resulting in the promotion of LTP and the inhibition of LTD. Our findings suggest that, under normal and safe conditions, the vmPFC is unable to exert excitatory synaptic plasticity over the BLA; rather, LTD, which encodes memory of safety in the BLA, is favoured. Following stressful experiences, LTP in the BLA is promoted to encode memory of fear.

Olfactory Learning-Induced Long-Lasting Enhancement of Descending and Ascending Synaptic Transmission to the Piriform Cortex

Learning of a particularly difficult olfactory-discrimination (OD) task results in acquisition of rule learning. This remarkable enhancement in learning capability is accompanied by long-term enhancement of synaptic connectivity between piriform cortex (PC) pyramidal neurons. Because successful performance in the OD task requires integration of information about the identity and also about the reward value of odors, it is likely that a higher-order brain area would also be involved in rule learning acquisition and maintenance. The anterior PC (APC) receives a strong ascending input from the olfactory bulb, carrying information regarding olfactory cues in the environment. It also receives substantial descending input from the orbitofrontal cortex (OFC), which is thought to play an important role in encoding the predictive value of odor stimuli. Using in vivo recordings of evoked field postsynaptic potentials, we characterized the physiological properties of projections to APC from the OFC and examined whether descending and ascending synaptic inputs to the piriform cortex are modified after OD learning. We show that enhanced learning capability is accompanied by long-term enhancement of synaptic transmission in both the descending and ascending inputs. Long-term synaptic enhancement is not accompanied by modifications in paired-pulse facilitation, indicating that such modifications are likely postsynaptic. Predisposition for long-term potentiation induction was affected by previous learning, and surprisingly also by previous exposure to the odors and training apparatus. These data suggest that enhanced connectivity between the APC and its input sources is required for OD rule learning.

Enhanced Extinction of Aversive Memories by High-Frequency Stimulation of the Rat Infralimbic Cortex

Electrical stimulation of the rodent medial prefrontal cortex (mPFC), including the infralimbic cortex (IL), immediately prior to or during fear extinction training facilitates extinction memory. Here we examined the effects of high-frequency stimulation (HFS) of the rat IL either prior to conditioning or following retrieval of the conditioned memory, on extinction of Pavlovian fear and conditioned taste aversion (CTA). IL-HFS applied immediately after fear memory retrieval, but not three hours after retrieval or prior to conditioning, subsequently reduced freezing during fear extinction. Similarly, IL-HFS given immediately, but not three hours after, retrieval of a CTA memory reduced aversion during extinction. These data indicate that HFS of the IL may be an effective method for reducing both learned fear and learned aversion.

Fear extinction deficits following acute stress associate with increased spine density and dendritic retraction in basolateral amygdala neurons

Stress‐sensitive psychopathologies such as post‐traumatic stress disorder are characterized by deficits in fear extinction and dysfunction of corticolimbic circuits mediating extinction. Chronic stress facilitates fear conditioning, impairs extinction, and produces dendritic proliferation in the basolateral amygdala (BLA), a critical site of plasticity for extinction. Acute stress impairs extinction, alters plasticity in the medial prefrontal cortex‐to‐BLA circuit, and causes dendritic retraction in the medial prefrontal cortex. Here, we examined extinction learning and basolateral amygdala pyramidal neuron morphology in adult male rats following a single elevated platform stress. Acute stress impaired extinction acquisition and memory, and produced dendritic retraction and increased mushroom spine density in basolateral amygdala neurons in the right hemisphere. Unexpectedly, irrespective of stress, rats that underwent fear and extinction testing showed basolateral amygdala dendritic retraction and altered spine density relative to non‐conditioned rats, particularly in the left hemisphere. Thus, extinction deficits produced by acute stress are associated with increased spine density and dendritic retraction in basolateral amygdala pyramidal neurons. Furthermore, the finding that conditioning and extinction as such was sufficient to alter basolateral amygdala morphology and spine density illustrates the sensitivity of basolateral amygdala morphology to behavioral manipulation. These findings may have implications for elucidating the role of the amygdala in the pathophysiology of stress‐related disorders.