The fear circuit revisited: contributions of the basal amygdala nuclei to conditioned fear

The lateral nucleus (LA) is the input station of the amygdala for information about conditioned stimuli (CSs), whereas the medial sector of the central nucleus (CeM) is the output region that contributes most amygdala projections to brainstem fear effectors. However, there are no direct links between LA and CeM. As the main target of LA and with its strong projection to CeM, the basomedial amygdala (BM) constitutes a good candidate to bridge this gap. Consistent with this notion, it was reported that combined posttraining lesions of the basal nuclei [BM plus basolateral nucleus (BL)] abolish conditioned fear responses, whereas selective BL inactivation does not. Thus, we examined the relative contribution of BM and BL to conditioned fear using unit recordings and inactivation with muscimol microinfusions in rats. Approximately 30% of BM and BL neurons acquired robust responses to auditory CSs predicting footshocks. While most BL cells stopped firing at CS offset, BM responses typically outlasted the CS by ≥ 40 s, paralleling the persistence of conditioned fear after the CS. This observation suggests that BM neurons are not passive relays of rapidly adapting LA inputs about the CS. Surprisingly, independent inactivation of either BM or BL with muscimol did not cause a reduction of conditioned freezing even though an extinction recall deficit was seen the next day. In contrast, combined BL-BM inactivation did. Overall, there results support the notion that the basal nuclei are involved in conditioned fear expression and extinction but that there is functional redundancy between them.     

Hippocampal NMDA receptor blockade impairs CREB phosphorylation in amygdala after contextual fear conditioning

In contextual fear conditioning (CFC), hippocampus is thought to process environmental stimuli into a configural representation of the context and send it to amygdala nuclei, which current evidences point to be the site of CS‐US association and fear memory storage. If it is true, hippocampus should influence learning‐induced plasticity in the amygdala nuclei after CFC acquisition. To test this, we infused wistar rats with saline or AP5, a NMDA receptor antagonist, in the dorsal hippocampus just before a CFC session, in which they were conditioned to a single shock, exposed to the context with no shocks or received an immediate shock. The rats were perfused, their brains harvested and immunohistochemically stained for cAMP element binding protein (CREB) phosphorylation ratio (pCREB/CREB) in lateral (LA), basal (B) and central (CeA) amygdala nuclei. CFC showed a learning‐specific increase in pCREB ratio in B and CeA, in conditioned‐saline rats compared to context and immediate shocked ones. Further, conditioned rats that received AP5 showed a decrease in pCREB ratio in LA, B and CeA. Our results support the current ideas that the role of hippocampus in contextual fear conditioning occurs by sending contextual information to amygdala to serve as conditioned stimulus. © 2013 Wiley Periodicals, Inc.     

Conditioned suppression and freezing as measures of aversive Pavlovian conditioning: effects of discrete amygdala lesions and overtraining

Freezing and suppression are measures of conditioned fear that correlate in unlesioned animals. Both the basolateral (BLA) and central (CeN) nuclei of the amygdala are required for conditioned freezing, though there can be recovery with overtraining. The neuroanatomical substrates of conditioned suppression are less clear, with evidence both for a specific requirement of the CeN and for disruption by BLA lesions. The present study investigated the impact of selective excitotoxic lesions of the BLA and CeN upon the acquisition and expression of conditioned fear, measured by freezing and both on-baseline and off-baseline conditioned suppression in the same rats. BLA and CeN lesions both abolished all measures of conditioned fear after 9 trials of fear conditioning. However, when conditioning was extended to 33 trials, whereas rats with combined lesions of both the BLA and CeN continued to show no conditioned fear responses, there was a pattern of recovery observed after selective lesions. There was a partial recovery of freezing with both lesions, and full recovery of conditioned suppression, except for off-baseline suppression in CeN lesioned rats. These results indicate that with few conditioning trials, both the BLA and CeN are required in a serial manner for conditioned fear responses, but that overtraining can mitigate such impairments, likely involving parallel pathways in and through the amygdala.     

Extinction of cued fear memory involves a distinct form of depotentiation at cortical input synapses onto the lateral amygdala

The amygdala is known to be a critical storage site of conditioned fear memory. Among the two major pathways to the lateral amygdala (LA), the cortical pathway is known to display a presynaptic long‐term potentiation which is occluded with fear conditioning. Here we show that fear extinction results in a net depression of conditioning‐induced potentiation at cortical input synapses onto the LA (C‐LA synapses). Fear conditioning induced a significant potentiation of excitatory postsynaptic currents at C‐LA synapses compared with naïve and unpaired controls, whereas extinction apparently reversed this potentiation. Paired‐pulse low‐frequency stimulation (pp‐LFS) induced synaptic depression in the C‐LA pathway of fear‐conditioned rats, but not in naïve or unpaired controls, indicating that the pp‐LFS‐induced depression is specific to associative learning‐induced changes (pp‐LFS‐induced depotentiation_{ex vivo}). Importantly, extinction occluded pp‐LFS‐induced depotentiation_{ex vivo}, suggesting that extinction shares some mechanisms with the depotentiation. pp‐LFS‐induced depotentiation_{ex vivo} required NMDA receptor (NMDAR) activity, consistent with a previous finding that blockade of amygdala NMDARs impaired fear extinction. In addition, pp‐LFS‐induced depotentiation_{ex vivo} required activity of group II metabotropic glutamate receptors (mGluRs), known to be present at presynaptic terminals, but not AMPAR internalization, consistent with a presynaptic mechanism for pp‐LFS‐induced depotentiation_{ex vivo}. This result is in contrast with another form of ex vivo depotentiation in the thalamic pathway that requires both group I mGluR activity and AMPAR internalization. We thus suggest that extinction of conditioned fear involves a distinct form of depotentiation at C‐LA synapses, which depends upon both NMDARs and group II mGluRs.     

Synaptic transmission changes in fear memory circuits underlie key features of an animal model of schizophrenia

Non-competitive antagonists of the N-methyl-d-aspartate receptor (NMDA) such as phencyclidine (PCP) elicit schizophrenia-like symptoms in healthy individuals. Similarly, PCP dosing in rats produces typical behavioral phenotypes that mimic human schizophrenia symptoms. Although schizophrenic behavioral phenotypes of the PCP model have been extensively studied, the underlying alterations of intrinsic neuronal properties and synaptic transmission in relevant limbic brain microcircuits remain elusive. Acute brain slice electrophysiology and immunostaining of inhibitory neurons were used to identify neuronal circuit alterations of the amygdala and hippocampus associated with changes in extinction of fear learning in rats following PCP treatment. Subchronic PCP application led to impaired long-term potentiation (LTP) and marked increases in the ratio of NMDA to 2-amino-3(5-methyl-3-oxo-1,2-oxazol-4-yl)propionic acid (AMPA) receptor-mediated currents at lateral amygdala (LA) principal neurons without alterations in parvalbumin (PV) as well as non-PV, glutamic acid decarboxylase 67 (GAD 67) immunopositive neurons. In addition, LTP was impaired at the Schaffer collateral to CA1 hippocampal pathway coincident with a reduction in colocalized PV and GAD67 immunopositive neurons in the CA3 hippocampal area. These effects occurred without changes in spontaneous events or intrinsic membrane properties of principal cells in the LA. The impairment of LTP at both amygdalar and hippocampal microcircuits, which play a key role in processing relevant survival information such as fear and extinction memory concurred with a disruption of extinction learning of fear conditioned responses. Our results show that subchronic PCP administration in rats impairs synaptic functioning in the amygdala and hippocampus as well as processing of fear-related memories.     

Amygdaloid metabotropic glutamate receptor subtype 7 is involved in the acquisition of conditioned fear

The metabotropic glutamate receptor subtype 7 (mGluR7) is presynaptically located and modulates transmitter release. An earlier study from our group demonstrated that systemic administration of N,N'-dibenzyhydryl-ethane-1,2-diamine dihydrochloride (AMN082), a selective allosteric mGluR7 agonist, attenuates the acquisition of conditioned fear measured by fear-potentiated startle. Aim of this study was to explore whether this effect is mediated by the basolateral amygdala, a crucial brain structure for acquisition of conditioned fear. Therefore, AMN082 was locally injected into the basolateral amygdala of rats and the effects of these injections on the acquisition of conditioned fear was measured. Our data clearly show that intra-amygdala injection of AMN082 impairs fear acquisition. This finding demonstrates that amygdaloid mGluR7 controls the learning of conditioned fear.     

Dynamic amino acid increases in the basolateral amygdala during acquisition and expression of conditioned fear

Glutamate and gamma-aminobutyric acid (GABA) release in the amygdala are thought to be crucial for the acquisition and expression of fear memories, but the time course of amino acid changes during conditioning is unknown. We used rapid-sampling microdialysis with 14 s temporal resolution to address this issue. During auditory fear conditioning, large, rapid and transient increases in glutamate and GABA were detected, but only during the first noise-shock pairing. In contrast, rats receiving unsignaled shocks during contextual fear conditioning showed no changes in GABA and less glutamate release for the initial shock, but increased glutamate release during later shocks. Expression of conditioned fear to either a white noise or the context previously paired with shock produced similar rapid and transient increases in many amino acids in the amygdala. These experiments demonstrate glutamate and GABA levels in the amygdala are differentially modulated during auditory and contextual fear learning, and are transiently increased during the expression of fear memories.     

Inhibition of the amygdala central nucleus by stimulation of cerebellar output in rats: a putative mechanism for extinction of the conditioned fear response

The amygdala and the cerebellum serve two distinctively different functions. The amygdala plays a role in the expression of emotional information, whereas the cerebellum is involved in the timing of discrete motor responses. Interaction between these two systems is the basis of the two‐stage theory of learning, according to which an encounter with a challenging event triggers fast classical conditioning of fear‐conditioned responses in the amygdala and slow conditioning of motor‐conditioned responses in the cerebellum. A third stage was hypothesised when an apparent interaction between amygdala and cerebellar associative plasticity was observed: an adaptive rate of cerebellum‐dependent motor‐conditioned responses was associated with a decrease in amygdala‐dependent fear‐conditioned responses, and was interpreted as extinction of amygdala‐related fear‐conditioned responses by the cerebellar output. To explore this hypothesis, we mimicked some components of classical eyeblink conditioning in anesthetised rats by applying an aversive periorbital pulse as an unconditioned stimulus and a train of pulses to the cerebellar output nuclei as a cerebellar neuronal‐conditioned response. The central amygdala multiple unit response to the periorbital pulse was measured with or without a preceding train to the cerebellar output nuclei. The results showed that activation of the cerebellar output nuclei prior to periorbital stimulation produced diverse patterns of inhibition of the amygdala response to the periorbital aversive stimulus, depending upon the nucleus stimulated, the laterality of the nucleus stimulated, and the stimulus interval used. These results provide a putative extinction mechanism of learned fear behavior, and could have implications for the treatment of pathologies involving abnormal fear responses by using motor training as therapy.     

Prevention of stress-impaired fear extinction through neuropeptide s action in the lateral amygdala

Stressful and traumatic events can create aversive memories, which are a predisposing factor for anxiety disorders. The amygdala is critical for transforming such stressful events into anxiety, and the recently discovered neuropeptide S transmitter system represents a promising candidate apt to control these interactions. Here we test the hypothesis that neuropeptide S can regulate stress-induced hyperexcitability in the amygdala, and thereby can interact with stress-induced alterations of fear memory. Mice underwent acute immobilization stress (IS), and neuropeptide S and a receptor antagonist were locally injected into the lateral amygdala (LA) during stress exposure. Ten days later, anxiety-like behavior, fear acquisition, fear memory retrieval, and extinction were tested. Furthermore, patch-clamp recordings were performed in amygdala slices prepared ex vivo to identify synaptic substrates of stress-induced alterations in fear responsiveness. (1) IS increased anxiety-like behavior, and enhanced conditioned fear responses during extinction 10 days after stress, (2) neuropeptide S in the amygdala prevented, while an antagonist aggravated, these stress-induced changes of aversive behaviors, (3) excitatory synaptic activity in LA projection neurons was increased on fear conditioning and returned to pre-conditioning values on fear extinction, and (4) stress resulted in sustained high levels of excitatory synaptic activity during fear extinction, whereas neuropeptide S supported the return of synaptic activity during fear extinction to levels typical of non-stressed animals. Together these results suggest that the neuropeptide S system is capable of interfering with mechanisms in the amygdala that transform stressful events into anxiety and impaired fear extinction.     

NMDA receptors are essential for the acquisition, but not expression, of conditional fear and associative spike firing in the lateral amygdala

We examined the contribution of N-methyl-D-aspartate (NMDA) receptors (NMDARs) to the acquisition and expression of amygdaloid plasticity and Pavlovian fear conditioning using single-unit recording techniques in behaving rats. We demonstrate that NMDARs are essential for the acquisition of both behavioral and neuronal correlates of conditional fear, but play a comparatively limited role in their expression. Administration of the competitive NMDAR antagonist +/--3-(2-carboxypiperazin-4-yl) propyl-1-phosphonic acid (CPP) prior to auditory fear conditioning completely abolished the acquisition of conditional freezing and conditional single-unit activity in the lateral amygdala (LA). In contrast, CPP given prior to extinction testing did not affect the expression of conditional single-unit activity in LA, despite producing deficits in conditional freezing. Administration of CPP also blocked the induction of long-term potentiation in the amygdala. Together, these data suggest that NMDARs are essential for the acquisition of conditioning-related plasticity in the amygdala, and that NMDARs are more critical for regulating synaptic plasticity and learning than routine synaptic transmission in the circuitry supporting fear conditioning.     

Two types of social buffering differentially mitigate conditioned fear responses

In a phenomenon known as 'social buffering' in various species, signals from a conspecific animal can mitigate stress responses. This buffering can be achieved either by 'pair-housing' after a stressful event or by 'pair-exposure' to an acute stressor with a conspecific animal. In this study, we compared the impacts of these two types of social buffering on auditory conditioned fear responses in male rats. When subjects were exposed to an auditory conditioned stimulus (CS) that had been paired with foot shocks on the previous day, they clearly exhibited behavioral (freezing), autonomic (aggravated stress-induced hyperthermia) and neural (Fos expression) responses. Pair-housing for 24 h with an unfamiliar rat following fear conditioning resulted in a suppressed autonomic, but not behavioral, response, with Fos expression in the lateral nucleus of the amygdala and ventrolateral periaqueductal gray. On the other hand, pair-exposure to the CS with an unfamiliar rat eliminated the behavioral, but not the autonomic, response, with Fos expression in the basal nucleus of the amygdala and infralimbic region of the prefrontal cortex. Furthermore, subjects that had been pair-housed and then pair-exposed showed no behavioral, autonomic or neural responses, suggesting that the combination of the two procedures can completely block the fear conditioning sequence. These results demonstrate that two types of social buffering differentially relieve conditioned fear responses, by influencing different neural pathways in the amygdala.     

Stability of presynaptic vesicle pools and changes in synapse morphology in the amygdala following fear learning in adult rats

Changes in synaptic strength in the lateral amygdala (LA) that occur with fear learning are believed to mediate memory storage, and both presynaptic and postsynaptic mechanisms have been proposed to contribute. In a previous study we used serial section transmission electron microscopy (ssTEM) to observe differences in dendritic spine morphology in the adult rat LA after fear conditioning, conditioned inhibition (safety conditioning), or naïve control handling (Ostroff et al. [2010] Proc Natl Acad Sci U S A 107:9418–9423). We have now reconstructed axons from the same dataset and compared their morphology and relationship to the postsynaptic spines between the three training groups. Relative to the naïve control and conditioned inhibition groups, the ratio of postsynaptic density (PSD) area to docked vesicles at synapses was greater in the fear‐conditioned group, while the size of the synaptic vesicle pools was unchanged. There was significant coherence in synapse size between neighboring boutons on the same axon in the naïve control and conditioned inhibition groups, but not in the fear‐conditioned group. Within multiple‐synapse boutons, both synapse size and the PSD‐to‐docked vesicle ratio were variable between individual synapses. Our results confirm that synaptic connectivity increases in the LA with fear conditioning. In addition, we provide evidence that boutons along the same axon and even synapses on the same bouton are independent in their structure and learning‐related morphological plasticity. J. Comp. Neurol. 520:295–314, 2012. © 2011 Wiley Periodicals, Inc.     

NMDA receptor antagonism in the basolateral but not central amygdala blocks the extinction of Pavlovian fear conditioning in rats

Glutamate receptors in the basolateral complex of the amygdala (BLA) are essential for the acquisition, expression and extinction of Pavlovian fear conditioning in rats. Recent work has revealed that glutamate receptors in the central nucleus of the amygdala (CEA) are also involved in the acquisition of conditional fear, but it is not known whether they play a role in fear extinction. Here we examine this issue by infusing glutamate receptor antagonists into the BLA or CEA prior to the extinction of fear to an auditory conditioned stimulus (CS) in rats. Infusion of the α‐amino‐3‐hydroxyl‐5‐methyl‐4‐isoxazole‐propionate (AMPA) receptor antagonist, 2,3‐dihydroxy‐6‐nitro‐7‐sulfamoyl‐benzo[f]quinoxaline‐2,3‐dione (NBQX), into either the CEA or BLA impaired the expression of conditioned freezing to the auditory CS, but did not impair the formation of a long‐term extinction memory to that CS. In contrast, infusion of the N‐methyl‐d ‐aspartate (NMDA) receptor antagonist, d,l ‐2‐amino‐5‐phosphonopentanoic acid (APV), into the amygdala, spared the expression of fear to the CS during extinction training, but impaired the acquisition of a long‐term extinction memory. Importantly, only APV infusions into the BLA impaired extinction memory. These results reveal that AMPA and NMDA receptors within the amygdala make dissociable contributions to the expression and extinction of conditioned fear, respectively. Moreover, they indicate that NMDA receptor‐dependent processes involved in extinction learning are localized to the BLA. Together with previous work, these results reveal that NMDA receptors in the CEA have a selective role acquisition of fear memory.     

Plasticity of inhibitory synaptic network interactions in the lateral amygdala upon fear conditioning in mice

After fear conditioning, plastic changes of excitatory synaptic transmission occur in the amygdala. Fear‐related memory also involves the GABAergic system, although no influence on inhibitory synaptic transmission is known. In the present study we assessed the influence of Pavlovian fear conditioning on the plasticity of GABAergic synaptic interactions in the lateral amygdala (LA) in brain slices prepared from fear‐conditioned, pseudo‐trained and naïve adult mice. Theta‐burst tetanization of thalamic afferent inputs to the LA evoked an input‐specific potentiation of inhibitory postsynaptic responses in projection neurons; the cortical input was unaffected. Philanthotoxin (10 µm ), an antagonist of Ca²⁺‐permeable AMPA receptors, disabled this plastic phenomenon. Surgical isolation of the LA, extracellular application of a GABA_B receptor antagonist (CGP 55845A, 10 µm ) or an NMDA receptor antagonist (APV, 50 µm ), or intracellular application of BAPTA (10 mm ), did not influence the plasticity. The plasticity also showed as a potentiation of monosynaptic excitatory responses in putative GABAergic interneurons. Pavlovian fear conditioning, but not pseudo‐conditioning, resulted in a significant reduction in this potentiation that was evident 24 h after training. Two weeks after training, the potentiation returned to control levels. In conclusion, a reduction in potentiation of inhibitory synaptic interactions occurs in the LA and may contribute to a shift in synaptic balance towards excitatory signal flow during the processes of fear‐memory acquisition or consolidation.     

Protein synthesis in the amygdala, but not the auditory thalamus, is required for consolidation of Pavlovian fear conditioning in rats

The amygdala is an essential neural substrate for Pavlovian fear conditioning. Nevertheless, long-term synaptic plasticity in amygdaloid afferents, such as the auditory thalamus, may contribute to the formation of fear memories. We therefore compared the influence of protein synthesis inhibition in the amygdala and the auditory thalamus on the consolidation of Pavlovian fear conditioning in Long-Evans rats. Rats received three tone-footshock trials in a novel conditioning chamber. Immediately after fear conditioning, rats were infused intra-cranially with the protein synthesis inhibitor, anisomycin. Conditional fear to the tone and conditioning context was assessed by measuring freezing behaviour in separate retention tests conducted at least 24 h following conditioning. Post-training infusion of anisomycin into the amygdala impaired conditional freezing to both the auditory and contextual stimuli associated with footshock. In contrast, intra-thalamic infusions of anisomycin or a broad-spectrum protein kinase inhibitor [1-(5'-isoquinolinesulphonyl)-2-methylpiperazine, H7] did not affect conditional freezing during the retention tests. Pre-training intra-thalamic infusion of the NMDA receptor antagonist 2-amino-5-phosphonopentanoic acid (APV), which blocks synaptic transmission in the auditory thalamus, produced a selective deficit in the acquisition of auditory fear conditioning. Autoradiographic assays of cerebral [14C]-leucine incorporation revealed similar levels of protein synthesis inhibition in the amygdala and thalamus following intra-cranial anisomycin infusions. These results reveal that the establishment of long-term fear memories requires protein synthesis in the amygdala, but not the thalamus, after auditory fear conditioning. Forms of synaptic plasticity that depend on protein synthesis, such as long-term potentiation, are likely candidates for the encoding and long-term storage of fear memories in the amygdala.     

Impaired fear conditioning in Alzheimer's disease

Classical conditioning of the fear response is a basic form of nondeclarative (nonconscious) memory that mediates both normal and pathological responses to aversive stimuli. Because fear conditioning critically depends on the amygdala, a medial temporal lobe structure that frequently undergoes significant pathological changes early in the course of Alzheimer's disease (AD), we hypothesized that fear conditioning would be impaired in patients with mild to moderate AD. We examined simple classical fear conditioning in a group of 10 patients with probable AD and 14 demographically matched, neurologically intact elderly controls. During conditioning, one stimulus (e.g. a green rectangle, the conditioned stimulus (CS+)), was paired with an aversive stimulus (a loud noise, the unconditioned stimulus (US)) using a partial reinforcement conditioning schedule. The opponent color (e.g. red rectangle), the CS-, was never paired with the US. The elderly controls acquired robust fear responses as demonstrated by their differential skin-conductance responses to the CS+ and CS-. In contrast, the AD group showed a marked impairment in conditioning, failing to exhibit significant conditioned fear responses. This failure to acquire conditioned responses could not be attributed to diminished responding by patients, relative to controls, to the aversive US. The results indicate that fear conditioning, an amygdala-dependent form of memory, is impaired in AD. These findings complement previous reports of impairments in declarative emotional memory in AD by demonstrating that a basic form of nondeclarative emotional memory is also impaired in AD.     

Impaired extinction of learned fear in rats selectively bred for high anxiety – evidence of altered neuronal processing in prefrontal‐amygdala pathways

The impaired extinction of acquired fear is a core symptom of anxiety disorders, such as post‐traumatic stress disorder, phobias or panic disorder, and is known to be particularly resistant to existing pharmacotherapy. We provide here evidence that a similar relationship between trait anxiety and resistance to extinction of fear memory can be mimicked in a psychopathologic animal model. Wistar rat lines selectively bred for high (HAB) or low (LAB) anxiety‐related behaviour were tested in a classical cued fear conditioning task utilizing freezing responses as a measure of fear. Fear acquisition was similar in both lines. In the extinction trial, however, HAB rats showed a marked deficit in the attenuation of freezing responses to repeated auditory conditioned stimulus presentations as compared with LAB rats, which exhibited rapid extinction. To gain information concerning the putatively altered neuronal processing associated with the differential behavioural response between HAB and LAB rats, c‐Fos expression was investigated in the main prefrontal‐amygdala pathways important for cued fear extinction. HAB compared to LAB rats showed an attenuated c‐Fos response to repeated conditioned stimulus presentations in infralimbic and cingulate cortices, as well as in the lateral amygdala, but facilitated the c‐Fos response in the medial part of the central amygdala. In conclusion, the present results support the notion that impaired extinction in high anxiety rats is accompanied by an aberrant activation profile in extinction‐relevant prefrontal‐amygdala circuits. Thus, HAB rats may represent a clinically relevant model to study the mechanisms and potential targets to accelerate delayed extinction processes in subjects with enhanced trait anxiety.     

Temporal changes in c-Fos activation patterns induced by conditioned fear

Mechanisms underlying shock-induced conditioned fear - a paradigm frequently used to model posttraumatic stress disorder, PTSD - are usually studied shortly after shocks. Some of the brain regions relevant to conditioned fear were activated in all the c-Fos studies published so far, but the overlap between the activated regions was small across studies. We hypothesized that discrepant findings were due to dynamic neural changes that followed shocks, and a more consistent picture would emerge if consequences were studied after a longer interval. Therefore, we exposed rats to a single session of footshocks and studied their behavioral and neural responses one and 28 days later. The neuronal activation marker c-Fos was studied in 24 brain regions relevant for conditioned fear, e.g. in subdivisions of the prefrontal cortex, hippocampus, amygdala, hypothalamic defensive system, brainstem monoaminergic nuclei and periaqueductal gray. The intensity of conditioned fear (as shown by the duration of contextual freezing) was similar at the two time-points, but the associated neuronal changes were qualitatively different. Surprisingly, however, Multiple Regression Analyses suggested that conditioned fear-induced changes in neuronal activation patterns predicted the duration of freezing with high accuracy at both time points. We suggest that exposure to electric shocks is followed by a period of plasticity where the mechanisms that sustain conditioned fear undergo qualitative changes. Neuronal changes observed 28 days but not 1 day after shocks were consistent with those observed in human studies performed in PTSD patients.     

Theta activity in neurons and networks of the amygdala related to long-term fear memory

With a combined in vitro/in vivo electrophysiological and behavioral approach, we have correlated conditioned fear behavior to electrophysiological activities in the lateral amygdala and the hippocampal formation in rodents. Data indicate that projection neurons in the lateral amygdala display a continuum of spike patterns including accommodating patterns, regular firing, and oscillatory activity at theta frequencies. The firing pattern is controlled to an important part by the intracellular cAMP system, in that an increase in intracellular cAMP concentration facilitates regular firing and theta oscillations. Oscillatory electrical activity, in turn, provides an important cellular element of synchronized theta activity at 4-8 Hz (indicating atropine-sensitive type 2 theta) occurring in amygdalo-hippocampal pathways during conditioned fear responses. This type of rhythmic network activity is associated with the retrieval of long-term fear memory following cued and contextual fear conditioning, but is not related to the expression of fear behavior per se or to short-term fear memory. Synchronization at theta frequencies is suggested to represent activity in amygdalo-hippocampal pathways associated with system consolidation of fear memory, which is supported by the cholinergic system.     

The striatal-enriched protein tyrosine phosphatase gates long-term potentiation and fear memory in the lateral amygdala.

BACKGROUND: Formation of long-term memories is critically dependent on extracellular-regulated kinase (ERK) signaling. Activation of the ERK pathway by the sequential recruitment of mitogen-activated protein kinases is well understood. In contrast, the proteins that inactivate this pathway are not as well characterized. METHODS: Here we tested the hypothesis that the brain-specific striatal-enriched protein tyrosine phosphatase (STEP) plays a key role in neuroplasticity and fear memory formation by its ability to regulate ERK1/2 activation. RESULTS: STEP co-localizes with the ERKs within neurons of the lateral amygdala. A substrate-trapping STEP protein binds to the ERKs and prevents their nuclear translocation after glutamate stimulation in primary cell cultures. Administration of TAT-STEP into the lateral amygdala (LA) disrupts long-term potentiation (LTP) and selectively disrupts fear memory consolidation. Fear conditioning induces a biphasic activation of ERK1/2 in the LA with an initial activation within 5 minutes of training, a return to baseline levels by 15 minutes, and an increase again at 1 hour. In addition, fear conditioning results in the de novo translation of STEP. Inhibitors of ERK1/2 activation or of protein translation block the synthesis of STEP within the LA after fear conditioning. CONCLUSIONS: Together, these data imply a role for STEP in experience-dependent plasticity and suggest that STEP modulates the activation of ERK1/2 during amygdala-dependent memory formation. The regulation of emotional memory by modulating STEP activity may represent a target for the treatment of psychiatric disorders such as posttraumatic stress disorder (PTSD), panic, and anxiety disorders.