Regulation of gephyrin and GABAA receptor binding within the amygdala after fear acquisition and extinction

Rethinking Extinction

Extinction serves as the leading theoretical framework and experimental model to describe how learned behaviors diminish through absence of anticipated reinforcement. In the past decade, extinction has moved beyond the realm of associative learning theory and behavioral experimentation in animals and has become a topic of considerable interest in the neuroscience of learning, memory, and emotion. Here, we review research and theories of extinction, both as a learning process and as a behavioral technique, and consider whether traditional understandings warrant a re-examination. We discuss the neurobiology, cognitive factors, and major computational theories, and revisit the predominant view that extinction results in new learning that interferes with expression of the original memory. Additionally, we reconsider the limitations of extinction as a technique to prevent the relapse of maladaptive behavior and discuss novel approaches, informed by contemporary theoretical advances, that augment traditional extinction methods to target and potentially alter maladaptive memories.

Effects of a Flavonoid-Rich Fraction on the Acquisition and Extinction of Fear Memory: Pharmacological and Molecular Approaches

The effects of flavonoids have been correlated with their ability to modulate the glutamatergic, serotoninergic, and GABAergic neurotransmission; the major targets of these substances are N-methyl-D-aspartic acid receptor (NMDARs), serotonin type1A receptor (5-HT_1ARs), and the gamma-aminobutyric acid type A receptors (GABA_ARs). Several studies showed that these receptors are involved in the acquisition and extinction of fear memory. This study assessed the effects of treatment prior to conditioning with a flavonoid-rich fraction from the stem bark of Erythrina falcata (FfB) on the acquisition and extinction of the conditioned suppression following pharmacological manipulations and on gene expression in the dorsal hippocampus (DH). Adult male Wistar rats were treated before conditioned fear with FfB, vehicle, an agonist or antagonist of the 5-HT_1AR, GABA_ARs or the GluN2B-NMDAR or one of these antagonists before FfB treatment. The effects of these treatments on fear memory retrieval, extinction training and extinction retrieval were evaluated at 48, 72, and 98 h after conditioning, respectively. We found that activation of GABA_ARs and inactivation of GluN2B-NMDARs play important roles in the acquisition of lick response suppression. FfB reversed the effect of blocking GluN2B-NMDARs on the conditioned fear and induced the spontaneous recovery. Blocking the 5-HT_1AR and the GluN2B-NMDAR before FfB treatment seemed to be associated with weakening of the spontaneous recovery. Expression of analysis of DH samples via qPCR showed that FfB treatment resulted in the overexpression of Htr1a, Grin2a, Gabra5, and Erk2 after the retention test and of Htr1a and Erk2 after the extinction retention test. Moreover, blocking the 5-HT1ARs and the GluN2B-NMDARs before FfB treatment resulted in reduced Htr1a and Grin2b expression after the retention test, but played a distinct role in Grin2a and Erk2 expression, according session evaluated. We show for the first time that the serotoninergic and glutamatergic receptors are important targets for the effect of FfB on the conditioned fear and spontaneous recovery, in which the ERK signaling pathway appears to be modulated. Further, these results provide important information regarding the role of the DH in conditioned suppression. Taken together, our data suggest that FfB represents a potential therapy for preventing or treating memory impairments.

Stress and Fear Extinction

Stress has a critical role in the development and expression of many psychiatric disorders, and is a defining feature of posttraumatic stress disorder (PTSD). Stress also limits the efficacy of behavioral therapies aimed at limiting pathological fear, such as exposure therapy. Here we examine emerging evidence that stress impairs recovery from trauma by impairing fear extinction, a form of learning thought to underlie the suppression of trauma-related fear memories. We describe the major structural and functional abnormalities in brain regions that are particularly vulnerable to stress, including the amygdala, prefrontal cortex, and hippocampus, which may underlie stress-induced impairments in extinction. We also discuss some of the stress-induced neurochemical and molecular alterations in these brain regions that are associated with extinction deficits, and the potential for targeting these changes to prevent or reverse impaired extinction. A better understanding of the neurobiological basis of stress effects on extinction promises to yield novel approaches to improving therapeutic outcomes for PTSD and other anxiety and trauma-related disorders.

Cholinergic neuronal lesions in the medial septum and vertical limb of the diagonal bands of Broca induce contextual fear memory generalization and impair acquisition of fear extinction

Previous research has shown that the ventral medial prefrontal cortex (vmPFC) and hippocampus (Hipp) are critical for extinction memory. Basal forebrain (BF) cholinergic input to the vmPFC and Hipp is critical for neural function in these substrates, which suggests BF cholinergic neurons may be critical for extinction memory. In order to test this hypothesis, we applied cholinergic lesions to different regions of the BF and observed the effects these lesions had on extinction memory. Complete BF cholinergic lesions induced contextual fear memory generalization, and this generalized fear was resistant to extinction. Animals with complete BF cholinergic lesions could not acquire cued fear extinction. Restricted cholinergic lesions in the medial septum and vertical diagonal bands of Broca (MS/vDBB) mimicked the effects that BF cholinergic lesions had on contextual fear memory generalization and acquisition of fear extinction. Cholinergic lesions in the horizontal diagonal band of Broca and nucleus basalis (hDBB/NBM) induced a small deficit in extinction of generalized contextual fear memory with no accompanying deficits in cued fear extinction. The results of this study reveal that MS/vDBB cholinergic neurons are critical for inhibition and extinction of generalized contextual fear memory, and via this process, may be critical for acquisition of cued fear extinction. Further studies delineating neural circuits and mechanisms through which MS/vDBB cholinergic neurons facilitate these emotional memory processes are needed. © 2015 Wiley Periodicals, Inc.

HDAC inhibitors as cognitive enhancers in fear, anxiety and trauma therapy: where do we stand?

A novel strategy to treat anxiety and fear-related disorders such as phobias, panic and PTSD (post-traumatic stress disorder) is combining CBT (cognitive behavioural therapy), including extinction-based exposure therapy, with cognitive enhancers. By targeting and boosting mechanisms underlying learning, drug development in this field aims at designing CBT-augmenting compounds that help to overcome extinction learning deficits, promote long-term fear inhibition and thus support relapse prevention. Progress in revealing the role of epigenetic regulation of specific genes associated with extinction memory generation has opened new avenues in this direction. The present review examines recent evidence from pre-clinical studies showing that increasing histone acetylation, either via genetic or pharmacological inhibition of HDACs (histone deacetylases) by e.g. vorinostat/SAHA (suberoylanilide hydroxamic acid), entinostat/MS-275, sodium butyrate, TSA (trichostatin A) or VPA (valproic acid), or by targeting HATs (histone acetyltransferases), augments fear extinction and, importantly, generates a long-term extinction memory that can protect from return of fear phenomena. The molecular mechanisms and pathways involved including BDNF (brain-derived neurotrophic factor) and NMDA (N-methyl-D-aspartate) receptor signalling are just beginning to be revealed. First studies in healthy humans are in support of extinction-facilitating effects of HDAC inhibitors. Very recent evidence that HDAC inhibitors can rescue deficits in extinction-memory-impaired rodents indicates a potential clinical utility of this approach also for exposure therapy-resistant patients. Important future work includes investigation of the long-term safety aspects of HDAC inhibitor treatment, as well as design of isotype(s)-specific inhibitors. Taken together, HDAC inhibitors display promising potential as pharmacological adjuncts to augment the efficacy of exposure-based approaches in anxiety and trauma therapy.

Sensory inputs to intercalated cells provide fear-learning modulated inhibition to the basolateral amygdala

Increasing evidence suggests that parallel plastic processes in the amygdala involve inhibitory elements to control fear and extinction memory. GABAergic medial paracapsular intercalated cells (mpITCs) are thought to relay activity from basolateral nucleus (BLA) and prefrontal cortex to inhibit central amygdala output during suppression of fear. Recently, projection diversity and differential behavioral activation of mpITCs in distinct fear states suggest additional functions. Here, we show that mpITCs receive convergent sensory thalamic and cortical inputs that undergo fear learning-related changes and are dynamically modulated via presynaptic GABAB receptors recruited by GABA released from the mpITC network. Among mpITCs, we identify cells that inhibit but are also mutually activated by BLA principal neurons. Thus, mpITCs take part in fear learning-modulated feedforward and feedback inhibitory circuits to simultaneously control amygdala input and output nuclei. Our findings place mpITCs in a unique position to gate acquired amygdala-dependent behaviors via their direct sensory inputs.

Pharmacology of cognitive enhancers for exposure-based therapy of fear, anxiety and trauma-related disorders

Pathological fear and anxiety are highly debilitating and, despite considerable advances in psychotherapy and pharmacotherapy they remain insufficiently treated in many patients with PTSD, phobias, panic and other anxiety disorders. Increasing preclinical and clinical evidence indicates that pharmacological treatments including cognitive enhancers, when given as adjuncts to psychotherapeutic approaches [cognitive behavioral therapy including extinction-based exposure therapy] enhance treatment efficacy, while using anxiolytics such as benzodiazepines as adjuncts can undermine long-term treatment success. The purpose of this review is to outline the literature showing how pharmacological interventions targeting neurotransmitter systems including serotonin, dopamine, noradrenaline, histamine, glutamate, GABA, cannabinoids, neuropeptides (oxytocin, neuropeptides Y and S, opioids) and other targets (neurotrophins BDNF and FGF2, glucocorticoids, L-type-calcium channels, epigenetic modifications) as well as their downstream signaling pathways, can augment fear extinction and strengthen extinction memory persistently in preclinical models. Particularly promising approaches are discussed in regard to their effects on specific aspects of fear extinction namely, acquisition, consolidation and retrieval, including long-term protection from return of fear (relapse) phenomena like spontaneous recovery, reinstatement and renewal of fear. We also highlight the promising translational value of the preclinial research and the clinical potential of targeting certain neurochemical systems with, for example d-cycloserine, yohimbine, cortisol, and L-DOPA. The current body of research reveals important new insights into the neurobiology and neurochemistry of fear extinction and holds significant promise for pharmacologically-augmented psychotherapy as an improved approach to treat trauma and anxiety-related disorders in a more efficient and persistent way promoting enhanced symptom remission and recovery.

Diffusion dynamics of synaptic molecules during inhibitory postsynaptic plasticity

The plasticity of inhibitory transmission is expected to play a key role in the modulation of neuronal excitability and network function. Over the last two decades, the investigation of the determinants of inhibitory synaptic plasticity has allowed distinguishing presynaptic and postsynaptic mechanisms. While there has been a remarkable progress in the characterization of presynaptically-expressed plasticity of inhibition, the postsynaptic mechanisms of inhibitory long-term synaptic plasticity only begin to be unraveled. At postsynaptic level, the expression of inhibitory synaptic plasticity involves the rearrangement of the postsynaptic molecular components of the GABAergic synapse, including GABA_A receptors, scaffold proteins and structural molecules. This implies a dynamic modulation of receptor intracellular trafficking and receptor surface lateral diffusion, along with regulation of the availability and distribution of scaffold proteins. This Review will focus on the mechanisms of the multifaceted molecular reorganization of the inhibitory synapse during postsynaptic plasticity, with special emphasis on the key role of protein dynamics to ensure prompt and reliable activity-dependent adjustments of synaptic strength.

Ganaxolone improves behavioral deficits in a mouse model of post-traumatic stress disorder

Allopregnanolone and its equipotent stereoisomer, pregnanolone (together termed ALLO), are neuroactive steroids that positively and allosterically modulate the action of gamma-amino-butyric acid (GABA) at GABA_A receptors. Levels of ALLO are reduced in the cerebrospinal fluid of female premenopausal patients with post-traumatic stress disorder (PTSD), a severe, neuropsychiatric condition that affects millions, yet is without a consistently effective therapy. This suggests that restoring downregulated brain ALLO levels in PTSD may be beneficial. ALLO biosynthesis is also decreased in association with the emergence of PTSD-like behaviors in socially isolated (SI) mice. Similar to PTSD patients, SI mice also exhibit changes in the frontocortical and hippocampal expression of GABA_A receptor subunits, resulting in resistance to benzodiazepine-mediated sedation and anxiolysis. ALLO acts at a larger spectrum of GABA_A receptor subunits than benzodiazepines, and increasing corticolimbic ALLO levels in SI mice by injecting ALLO or stimulating ALLO biosynthesis with a selective brain steroidogenic stimulant, such as S-norfluoxetine, at doses far below those that block serotonin reuptake, reduces PTSD-like behavior in these mice. This suggests that synthetic analogs of ALLO, such as ganaxolone, may also improve anxiety, aggression, and other PTSD-like behaviors in the SI mouse model. Consistent with this hypothesis, ganaxolone (3.75–30 mg/kg, s.c.) injected 60 min before testing of SI mice, induced a dose-dependent reduction in aggression toward a same-sex intruder and anxiety-like behavior in an elevated plus maze. The EC₅₀ dose of ganaxolone used in these tests also normalized exaggerated contextual fear conditioning and, remarkably, enhanced fear extinction retention in SI mice. At these doses, ganaxolone failed to change locomotion in an open field test. Therefore, unlike benzodiazepines, ganaxolone at non-sedating concentrations appears to improve dysfunctional emotional behavior associated with deficits in ALLO in mice and may provide an alternative treatment for PTSD patients with deficits in the synthesis of ALLO. Selective serotonin reuptake inhibitors (SSRIs) are the only medications currently approved by the FDA for treatment of PTSD, although they are ineffective in a substantial proportion of PTSD patients. Hence, an ALLO analog such as ganaxolone may offer a therapeutic GABAergic alternative to SSRIs for the treatment of PTSD or other disorders in which ALLO biosynthesis may be impaired.

Fear extinction learning can be impaired or enhanced by modulation of the CRF system in the basolateral nucleus of the amygdala

The neuropeptide corticotropin-releasing factor (CRF) is released during periods of anxiety and modulates learning and memory formation. One region with particularly dense concentrations of CRF receptors is the basolateral nucleus of the amygdala (BLA), a critical structure for both Pavlovian fear conditioning and fear extinction. While CRF has the potential to modify amygdala-dependent learning, its effect on fear extinction has not yet been assessed. In the present study, we examined the modulatory role of CRF on within-session extinction and fear extinction consolidation. Intra-BLA infusions of the CRF binding protein ligand inhibitor CRF(6-33) which increases endogenous levels of free CRF, or intra-BLA infusions of exogenous CRF made prior to fear extinction learning did not affect either fear expression or within-session extinction learning. However, when these animals were tested twenty-four hours later, drug free, they showed impairments in extinction memory. Conversely, intra-BLA infusions of the CRF receptor antagonist α-helical CRF(9-41) enhanced memory of fear extinction. These results suggest that increased CRF levels within the BLA at the time of fear extinction learning actively impair the consolidation of long-term fear extinction.

Amygdala microcircuits controlling learned fear

We review recent work on the role of intrinsic amygdala networks in the regulation of classically conditioned defensive behaviors, commonly known as conditioned fear. These new developments highlight how conditioned fear depends on far more complex networks than initially envisioned. Indeed, multiple parallel inhibitory and excitatory circuits are differentially recruited during the expression versus extinction of conditioned fear. Moreover, shifts between expression and extinction circuits involve coordinated interactions with different regions of the medial prefrontal cortex. However, key areas of uncertainty remain, particularly with respect to the connectivity of the different cell types. Filling these gaps in our knowledge is important because much evidence indicates that human anxiety disorders results from an abnormal regulation of the networks supporting fear learning.

Mechanisms to medicines: elucidating neural and molecular substrates of fear extinction to identify novel treatments for anxiety disorders

The burden of anxiety disorders is growing, but the efficacy of available anxiolytic treatments remains inadequate. Cognitive behavioural therapy for anxiety disorders focuses on identifying and modifying maladaptive patterns of thinking and behaving, and has a testable analogue in rodents in the form of fear extinction. A large preclinical literature has amassed in recent years describing the neural and molecular basis of fear extinction in rodents. In this review, we discuss how this work is being harnessed to foster translational research on anxiety disorders and facilitate the search for new anxiolytic treatments. We begin by summarizing the anatomical and functional connectivity of a medial prefrontal cortex (mPFC)-amygdala circuit that subserves fear extinction, including new insights from optogenetics. We then cover some of the approaches that have been taken to model impaired fear extinction and associated impairments with mPFC-amygdala dysfunction. The principal goal of the review is to evaluate evidence that various neurotransmitter and neuromodulator systems mediate fear extinction by modulating the mPFC-amygdala circuitry. To that end, we describe studies that have tested how fear extinction is impaired or facilitated by pharmacological manipulations of dopamine, noradrenaline, 5-HT, GABA, glutamate, neuropeptides, endocannabinoids and various other systems, which either directly target the mPFC-amygdala circuit, or produce behavioural effects that are coincident with functional changes in the circuit. We conclude that there are good grounds to be optimistic that the progress in defining the molecular substrates of mPFC-amygdala circuit function can be effectively leveraged to identify plausible candidates for extinction-promoting therapies for anxiety disorders.

Combination of fluoxetine and extinction treatments forms a unique synaptic protein profile that correlates with long-term fear reduction in adult mice

The antidepressant fluoxetine induces synaptic plasticity in the visual and fear networks and promotes the structural remodeling of neuronal circuits, which is critical for experience-dependent plasticity in response to an environmental stimulus. We recently demonstrated that chronic fluoxetine administration together with extinction training in adult mice reduced fear in a context-independent manner. Fear conditioning and extinction alter excitatory and inhibitory transmissions within the fear circuitry. In this study, we investigated whether fluoxetine, extinction or their combination produced distinct long-lasting changes in the synaptic protein profile in the amygdala, hippocampus and prefrontal cortex of conditioned mice. We determined that extinction induced synaptophysin expression and down-regulated the GluA1:GluA2 ratio throughout the fear network in water- and fluoxetine-treated mice, suggesting a common fluoxetine-independent mechanism for increased synaptic transmission and re-arrangement of AMPA-receptors by extinction training. In contrast to common changes, the presynaptic vesicular neurotransmitter transporters VGAT and Vglut1 were upregulated after extinction in water- and fluoxetine-treated mice, respectively. The cortical levels of the GABA transporter Gat1 were reduced in high-freezing water-drinking mice, suggesting a maladaptive increase of GABA spillover at cortical inhibitory synapses. Fear conditioning decreased, and extinction induced the expression of GABA-receptor alpha1 and alpha2 subunits in water- and fluoxetine-treated mice, respectively. Only a combination of fluoxetine with extinction enhanced GluN2A expression in the amygdala and hippocampus, emphasizing the role of this NMDA-receptor subunit in the successful erasure of fear memories. Our finding provides novel data that may become helpful in developing beneficial pharmacological fear-reducing treatment strategies.

Synaptic recruitment of gephyrin regulates surface GABAA receptor dynamics for the expression of inhibitory LTP

Postsynaptic long-term potentiation of inhibition (iLTP) can rely on increased GABAA receptors (GABA(A)Rs) at synapses by promoted exocytosis. However, the molecular mechanisms that enhance the clustering of postsynaptic GABA(A)Rs during iLTP remain obscure. Here we demonstrate that during chemically induced iLTP (chem-iLTP), GABA(A)Rs are immobilized and confined at synapses, as revealed by single-particle tracking of individual GABA(A)Rs in cultured hippocampal neurons. Chem-iLTP expression requires synaptic recruitment of the scaffold protein gephyrin from extrasynaptic areas, which in turn is promoted by CaMKII-dependent phosphorylation of GABA(A)R-β3-Ser(383). Impairment of gephyrin assembly prevents chem-iLTP and, in parallel, blocks the accumulation and immobilization of GABA(A)Rs at synapses. Importantly, an increase of gephyrin and GABA(A)R similar to those observed during chem-iLTP in cultures were found in the rat visual cortex following an experience-dependent plasticity protocol that potentiates inhibitory transmission in vivo. Thus, phospho-GABA(A)R-β3-dependent accumulation of gephyrin at synapses and receptor immobilization are crucial for iLTP expression and are likely to modulate network excitability.

Drugs currently in Phase II clinical trials for cocaine addiction

INTRODUCTION

There are no FDA-approved pharmacotherapies for treating cocaine addiction; thus, developing drugs to treat cocaine dependence is an unmet critical need. Fortunately, there are a number of drugs that are currently in Phase II clinical trial/s. This is due in part to the advances from in vivo imaging in humans which provided a roadmap of the neurochemistry of the cocaine-dependent brain. Most drugs currently in Phase II clinical trials attempt to modulate the disturbed neurochemistry in cocaine dependents to resemble those of healthy individuals. These predominantly modulate dopamine, serotonin, glutamate, GABA or noradrenaline signalling.

AREAS COVERED

This review summarizes the therapeutic potential of each drug as evidenced by clinical and preclinical studies. It also discusses their utility in terms of bioavailability and half-life.

EXPERT OPINION

Amphetamine salts and topiramate clearly stand out in terms of their potential efficacy in treating cocaine addiction. The efficacy of topiramate was closely associated with regular cognitive-behavioural therapy (CBT), which highlights the importance of a combined effort to promote abstinence and enhance retention via CBT. Cognitive/psychological screening appears necessary for a more symptom-based approach with more reasonable outcomes other than abstinence (e.g., improved quality of life) in treating cocaine addiction.

Neocortical Tet3-mediated accumulation of 5-hydroxymethylcytosine promotes rapid behavioral adaptation

5-hydroxymethylcytosine (5-hmC) is a novel DNA modification that is highly enriched in the adult brain and dynamically regulated by neural activity. 5-hmC accumulates across the lifespan; however, the functional relevance of this change in 5-hmC and whether it is necessary for behavioral adaptation have not been fully elucidated. Moreover, although the ten-eleven translocation (Tet) family of enzymes is known to be essential for converting methylated DNA to 5-hmC, the role of individual Tet proteins in the adult cortex remains unclear. Using 5-hmC capture together with high-throughput DNA sequencing on individual mice, we show that fear extinction, an important form of reversal learning, leads to a dramatic genome-wide redistribution of 5-hmC within the infralimbic prefrontal cortex. Moreover, extinction learning-induced Tet3-mediated accumulation of 5-hmC is associated with the establishment of epigenetic states that promote gene expression and rapid behavioral adaptation.

Fear conditioning and extinction across development: evidence from human studies and animal models

The ability to differentiate danger and safety through associative processes emerges early in life. Understanding the mechanisms underlying associative learning of threat and safety can clarify the processes that shape development of normative fears and pathological anxiety. Considerable research has used fear conditioning and extinction paradigms to delineate underlying mechanisms in animals and human adults; however, little is known about these mechanisms in children and adolescents. The current paper summarizes the empirical data on the development of fear conditioning and extinction. It reviews methodological considerations and future directions for research on fear conditioning and extinction in pediatric populations.

M1-muscarinic receptors promote fear memory consolidation via phospholipase C and the M-current

Neuromodulators released during and after a fearful experience promote the consolidation of long-term memory for that experience. Because overconsolidation may contribute to the recurrent and intrusive memories of post-traumatic stress disorder, neuromodulatory receptors provide a potential pharmacological target for prevention. Stimulation of muscarinic receptors promotes memory consolidation in several conditioning paradigms, an effect primarily associated with the M1 receptor (M1R). However, neither inhibiting nor genetically disrupting M1R impairs the consolidation of cued fear memory. Using the M1R agonist cevimeline and antagonist telenzepine, as well as M1R knock-out mice, we show here that M1R, along with β2-adrenergic (β2AR) and D5-dopaminergic (D5R) receptors, regulates the consolidation of cued fear memory by redundantly activating phospholipase C (PLC) in the basolateral amygdala (BLA). We also demonstrate that fear memory consolidation in the BLA is mediated in part by neuromodulatory inhibition of the M-current, which is conducted by KCNQ channels and is known to be inhibited by muscarinic receptors. Manipulating the M-current by administering the KCNQ channel blocker XE991 or the KCNQ channel opener retigabine reverses the effects on consolidation caused by manipulating β2AR, D5R, M1R, and PLC. Finally, we show that cAMP and protein kinase A (cAMP/PKA) signaling relevant to this stage of consolidation is upstream of these neuromodulators and PLC, suggesting an important presynaptic role for cAMP/PKA in consolidation. These results support the idea that neuromodulatory regulation of ion channel activity and neuronal excitability is a critical mechanism for promoting consolidation well after acquisition has occurred.

Target-specific vulnerability of excitatory synapses leads to deficits in associative memory in a model of intellectual disorder

Intellectual disorders (IDs) have been regularly associated with morphological and functional deficits at glutamatergic synapses in both humans and rodents. How these synaptic deficits may lead to the variety of learning and memory deficits defining ID is still unknown. Here we studied the functional and behavioral consequences of the ID gene il1rapl1 deficiency in mice and reported that il1rapl1 constitutive deletion alters cued fear memory formation. Combined in vivo and in vitro approaches allowed us to unveil a causal relationship between a marked inhibitory/excitatory (I/E) imbalance in dedicated amygdala neuronal subcircuits and behavioral deficits. Cell-targeted recordings further demonstrated a morpho-functional impact of the mutation at thalamic projections contacting principal cells, whereas the same afferents on interneurons are unaffected by the lack of Il1rapl1. We thus propose that excitatory synapses have a heterogeneous vulnerability to il1rapl1 gene constitutive mutation and that alteration of a subset of excitatory synapses in neuronal circuits is sufficient to generate permanent cognitive deficits.

Gephyrin plays a key role in BDNF-dependent regulation of amygdala surface GABAARs

Brain-derived neurotrophic factor (BDNF) is critically involved in synaptic plasticity and neurotransmission. Our lab has previously found that BDNF activation of neurotrophic tyrosine kinase, receptor, type 2 (TrkB) is required for fear memory formation and that GABAA receptor (GABAAR) subunits and the GABAA clustering protein gephyrin are dynamically regulated during fear memory consolidation. We hypothesize that TrkB-dependent internalization of GABAARs may partially underlie a transient period of amygdala hyperactivation during fear memory consolidation. We have previously reported that BDNF modulates GABAAR α1 subunit sequestration in cultured hippocampal and amygdala neurons by differential phosphorylation pathways. At present, no studies have investigated the regulation of gephyrin and GABAAR α1 subunits following BDNF activation in the amygdala. In this study, we confirm the association of GABAAR α1 and γ2 subunits with gephyrin on mouse amygdala neurons by coimmunoprecipitation and immunocytochemistry. We then demonstrate that rapid BDNF treatment, as well as suppression of gephyrin protein levels on amygdala neurons, induced sequestration of surface α1 subunits. Further, we find that rapid exposure of BDNF to primary amygdala cultures produced decreases in gephyrin levels, whereas longer exposure resulted in an eventual increase. While total α1 subunit levels remained unchanged, gephyrin was downregulated in whole cell homogenates, but enhanced in complexes with GABAARs. Our data with anisomycin suggest that BDNF may rapidly induce gephyrin protein degradation, with subsequent gephyrin synthesis occurring. Together, these findings suggest that gephyrin may be a key factor in BDNF-dependent GABAAR regulation in the amygdala. This work may inform future studies aimed at elucidating the pathways connecting BDNF, GABAA systems, gephyrin, and their role in underlying amygdala-dependent learning.

The involvement of the GABAergic system in the formation and expression of the extinction memory in the crab Neohelice granulata

There is growing interest in the neurobiological mechanisms involved in the extinction of aversive memory. This cognitive process usually occurs after repeated or prolonged presentation of a conditioned stimulus that was previously associated with an unconditioned stimulus. If extinction is considered to be a new memory, the role of the γ-aminobutyric acid system (GABAergic system) during extinction memory consolidation should be similar to that described for the original trace. It is also accepted that negative modulation of the GABAergic system before testing can impair extinction memory expression. However, it seems possible to speculate that inhibitory mechanisms may be required in order to acquire a memory that is inhibitory in nature. Using a combination of behavioral protocols, such as weak and robust extinction training procedures, and pharmacological treatments, such as the systemic administration of GABAA agonist (muscimol) and antagonist (bicuculline), we investigated the role of the GABAergic system in the different phases of the extinction memory in the crab Neohelice granulata. We show that the stimulation of the GABAergic system impairs and its inactivation facilitates the extinction memory consolidation. Moreover, fine variations in the GABAergic tone affect its expression at testing. Finally, an active GABAergic system is necessary for the acquisition of the extinction memory. This detailed description may contribute to the understanding of the role of the GABAergic system in diverse aspects of the extinction memory.

ABA and ABC renewal of conditioned magazine approach are not impaired by dorsal hippocampus inactivation or lesions

Three experiments investigated the role of the dorsal hippocampus (DH) in renewal of conditioned and then extinguished magazine approach responding in rats. Experiments 1 and 2 found no effect of muscimol inactivation of the DH during testing on ABA and ABC renewal, respectively. However, subjects from these studies were subsequently found to be impaired on a delayed non-matching-to-place task following muscimol but not saline infusions. Experiment 3 found no effects of post-training excitotoxic lesions of the DH on ABA and ABC renewal. Lesioned subjects were, however, impaired on the delayed non-matching-to-place task compared to control subjects. These findings suggest that the DH may not play a similar role in Pavlovian extinction in appetitive learning tasks as has previously been reported in aversive learning.

Post-natal treatment by a blood-brain-barrier permeable calpain inhibitor, SNJ1945 rescued defective function in lissencephaly

Toward a therapeutic intervention of lissencephaly, we applied a novel calpain inhibitor, SNJ1945. Peri-natal or post-natal treatment with SNJ1945 rescued defective neuronal migration in Lis1^+/− mice, impaired behavioral performance and improvement of ¹⁸F-FDG uptake. Furthermore, SNJ1945 improved the neural circuit formation and retrograde transport of NFG in Lis1^+/− mice. Thus, SNJ1945 is a potential drug for the treatment of human lissencephaly patients.

Long-term neural correlates of reversible fear learning in the lateral amygdala

Fear conditioning and extinction are behavioral models that reflect the association and dissociation of environmental cues to aversive outcomes, both known to involve the lateral amygdala (LA). Accordingly, responses of LA neurons to conditioned stimuli (CS) increase after fear conditioning and decrease partially during extinction. However, the long-term effects of repeated fear conditioning and extinction on LA neuronal firing have not been explored. Here we show, using stable, high signal-to-noise ratio single-unit recordings, that the ensemble activity of all recorded LA neurons correlates tightly with conditioned fear responses of rats in a conditioning/extinction/reconditioning paradigm spanning 3 d. This CS-evoked ensemble activity increased after conditioning, decreased after extinction, and was repotentiated after reconditioning. Cell-by-cell analysis revealed that among the LA neurons that displayed potentiated responses after initial fear conditioning, some exhibited weakened CS responses after extinction (extinction-susceptible), whereas others remained potentiated (extinction-resistant). The majority of extinction-susceptible neurons exhibited strong potentiation after reconditioning, suggesting that this distinct subpopulation (reversible fear neurons) encodes updated CS-unconditioned stimulus (US) association strength. Interestingly, these reversible fear neurons displayed larger, more rapid potentiation during reconditioning compared with the initial conditioning, providing a neural correlate of savings after extinction. In contrast, the extinction-resistant fear neurons did not show further increases after reconditioning, suggesting that this subpopulation encodes persistent fear memory representing the original CS-US association. This longitudinal report on LA neuronal activity during reversible fear learning suggests the existence of distinct populations encoding various facets of fear memory and provides insight into the neuronal mechanisms of fear memory modulation.

Biological studies of post-traumatic stress disorder

Post-traumatic stress disorder (PTSD) is the only major mental disorder for which a cause is considered to be known: that is, an event that involves threat to the physical integrity of oneself or others and induces a response of intense fear, helplessness or horror. Although PTSD is still largely regarded as a psychological phenomenon, over the past three decades the growth of the biological PTSD literature has been explosive, and thousands of references now exist. Ultimately, the impact of an environmental event, such as a psychological trauma, must be understood at organic, cellular and molecular levels. This Review attempts to present the current state of this understanding on the basis of psychophysiological, structural and functional neuroimaging, and endocrinological, genetic and molecular biological studies in humans and in animal models.

Ex vivo depotentiation of conditioning-induced potentiation at thalamic input synapses onto the lateral amygdala requires GluN2B-containing NMDA receptors

We have previously characterized the ex vivo depotentiation (depotentiation(ex vivo)) of conditioning-induced synaptic potentiation at thalamic input synapses onto the lateral amygdala (T-LA synapses) as a potential cellular substrate for fear extinction: both depotentiation(ex vivo) and fear extinction require NMDA receptors, mitogen-activated protein kinases, metabotropic glutamate receptor 1, de novo protein synthesis and AMPA receptor internalization in the amygdala. Surprisingly, as shown in our and other previous studies, ifenprodil, an antagonist of GluN2B-containing NMDA receptors, fails to inhibit depotentiation(ex vivo) at a saturating concentration (10μM), although it has been suggested that GluN2B-containing NMDA receptors are required for fear extinction. Because ifenprodil is also known to act on other molecular targets in addition to GluN2B-containing NMDA receptors, especially at high concentrations (i.e., ≥10μM), the ineffectiveness of 10μM of ifenprodil may be due to its side effects. Therefore, in the present study, we tested Ro25-6981, a more specific antagonist of GluN2B-containing NMDA receptors, and a lower concentration (3μM) of ifenprodil, which may reduce any possible side effects. Ro25-6981 (3μM) blocked both depotentiation(ex vivo) and late-phase long-term potentiation at T-LA synapses. While 10μM ifenprodil failed to inhibit depotentiation(ex vivo), a lower concentration (3μM) of ifenprodil blocked depotentiation(ex vivo). Together, our findings suggest that depotentiation(ex vivo) requires GluN2B-containing NMDA receptors.

Using the conditioned fear stress (CFS) animal model to understand the neurobiological mechanisms and pharmacological treatment of anxiety

SUMMARY

The mechanisms underlying the etiology and pathophysiology of anxiety disorders - the most prevalent class of mental disorders - remain unclear. Over the last 30 years investigators have used the animal model of conditioned fear stress (CFS) to investigate the brain structures and neurotransmitter systems involved in aversive emotional learning and memory. Recent studies have focused on the neuronal circuitry and cellular mechanisms of fearful emotional experiences. This review describes the CFS paradigm, discusses the neural circuit and neurotransmission underlying CFS, and explains the mechanism of action of pharmacological treatments of CFS. The focus of the review is on the molecular mechanisms of fear extinction, a phenomenon directly implicated in the clinical treatment of anxiety. Based on our assessment of previous work we will conclude by considering potential molecular targets for treating symptoms of anxiety and fear.

L-type voltage-gated calcium channels in the basolateral amygdala are necessary for fear extinction

L-type voltage-gated calcium channels (L-VGCCs) in the basolateral nucleus of the amygdala (BLA) are necessary for long-term memory of fear conditioning, where they are thought to drive the activation of protein kinases and to initiate gene transcription. However, their role in fear extinction learning is unclear given that systemic injections of VGCC antagonists induce a protracted stress response. Here we tested the effects of local infusions of the L-VGCC antagonists verapamil and nifedipine on both within-session extinction and fear extinction consolidation. Intra-BLA infusions of verapamil or nifedipine did not affect the expression of fear conditioning or the amount of within-session extinction but impaired extinction memory when rats were tested 24 h later drug-free. L-VGCC antagonists also prevented the increase in phosphorylated mitogen-activated protein kinase (MAPK) normally seen in the BLA following extinction learning. These results suggest that activation of L-VGCCs in the BLA at the time of fear extinction learning is necessary for the long-term retention of fear extinction via the MAPK pathway.

Neural and cellular mechanisms of fear and extinction memory formation

Over the course of natural history, countless animal species have evolved adaptive behavioral systems to cope with dangerous situations and promote survival. Emotional memories are central to these defense systems because they are rapidly acquired and prepare organisms for future threat. Unfortunately, the persistence and intrusion of memories of fearful experiences are quite common and can lead to pathogenic conditions, such as anxiety and phobias. Over the course of the last 30 years, neuroscientists and psychologists alike have attempted to understand the mechanisms by which the brain encodes and maintains these aversive memories. Of equal interest, though, is the neurobiology of extinction memory formation as this may shape current therapeutic techniques. Here we review the extant literature on the neurobiology of fear and extinction memory formation, with a strong focus on the cellular and molecular mechanisms underlying these processes.

Phase-dependent synaptic changes in the hippocampal CA1 field underlying extinction processes in freely moving rats

Recent studies focus on the functional significance of a novel form of synaptic plasticity, low-frequency stimulation (LFS)-induced synaptic potentiation in the hippocampal CA1 area. In the present study, we elucidated dynamic changes in synaptic function in the CA1 field during extinction processes associated with context-dependent fear memory in freely moving rats, with a focus on LFS-induced synaptic plasticity. Synaptic transmission in the CA1 field was transiently depressed during each extinction trial, but synaptic efficacy was gradually enhanced by repeated extinction trials, accompanied by decreases in freezing. On the day following the extinction training, synaptic transmission did not show further changes during extinction retrieval, suggesting that the hippocampal synaptic transmission that underlies extinction processes changes in a phase-dependent manner. The synaptic potentiation produced by extinction training was mimicked by synaptic changes induced by LFS (0.5 Hz) in the group that previously received footshock conditioning. Furthermore, the expression of freezing during re-exposure to footshock box was significantly reduced in the LFS application group in a manner similar to the extinction group. These results suggest that LFS-induced synaptic plasticity may be associated with the extinction processes that underlie context-dependent fear memory. This hypothesis was supported by the fact that synaptic potentiation induced by extinction training did not occur in a juvenile stress model that exhibited extinction deficits. Given the similarity between these electrophysiological and behavioral data, LFS-induced synaptic plasticity may be related to extinction learning, with some aspects of neuronal oscillations, during the acquisition and/or consolidation of extinction memory.

Hippocampal and prefrontal projections to the basal amygdala mediate contextual regulation of fear after extinction

Knowing when and where to express fear is essential to survival. Recent work in fear extinction paradigms reveals that the contextual regulation of fear involves a neural network involving the hippocampus, medial prefrontal cortex, and amygdala. The amygdaloid basal nuclei (BA) receive convergent input from the ventral hippocampus (VH) and prelimbic (PL) prefrontal cortex and may integrate VH and PL input to regulate fear expression. To examine the functional organization of this neural circuit, we used cellular imaging of c-fos expression in anatomically defined neuronal populations and circuit disconnections to identify the pathways involved in the contextual control of extinguished fear. Before behavioral testing, we infused a retrograde tracer into the amygdala to label BA-projecting neurons in VH and PL. Rats then underwent fear conditioning and extinction and were tested for their fear to the extinguished conditioned stimulus (CS) in either the extinction context or in another context; freezing behavior served as the index of conditional fear. CS presentation outside the extinction context renewed conditional freezing and was associated with significantly more c-fos expression in BA-projecting neurons in the VH and PL than that induced by CS presentation in the extinction context. We next examined whether direct or indirect projections of VH to BA mediate fear renewal. Interestingly, disconnections of the VH from either the BA or PL eliminated renewal. These findings suggest that convergent inputs from both the VH and PL in the BA mediate the contextual control of fear after extinction.

Genome-wide association for fear conditioning in an advanced intercross mouse line

Fear conditioning (FC) may provide a useful model for some components of post-traumatic stress disorder (PTSD). We used a C57BL/6J × DBA/2J F(2) intercross (n = 620) and a C57BL/6J × DBA/2J F(8) advanced intercross line (n = 567) to fine-map quantitative trait loci (QTL) associated with FC. We conducted an integrated genome-wide association analysis in QTLRel and identified five highly significant QTL affecting freezing to context as well as four highly significant QTL associated with freezing to cue. The average percent decrease in QTL width between the F(2) and the integrated analysis was 59.2%. Next, we exploited bioinformatic sequence and expression data to identify candidate genes based on the existence of non-synonymous coding polymorphisms and/or expression QTLs. We identified numerous candidate genes that have been previously implicated in either fear learning in animal models (Bcl2, Btg2, Dbi, Gabr1b, Lypd1, Pam and Rgs14) or PTSD in humans (Gabra2, Oprm1 and Trkb); other identified genes may represent novel findings. The integration of F(2) and AIL data maintains the advantages of studying FC in model organisms while significantly improving resolution over previous approaches.

Cannabinoids prevent the development of behavioral and endocrine alterations in a rat model of intense stress

Cannabinoids have recently emerged as a possible treatment of stress- and anxiety-related disorders such as post-traumatic stress disorder (PTSD). Here, we examined whether cannabinoid receptor activation could prevent the effects of traumatic stress on the development of behavioral and neuroendocrine measures in a rat model of PTSD, the single-prolonged stress (SPS) model. Rats were injected with the CB1/CB2 receptor agonist WIN55,212-2 (WIN) systemically or into the basolateral amygdala (BLA) at different time points following SPS exposure and were tested 1 week later for inhibitory avoidance (IA) conditioning and extinction, acoustic startle response (ASR), hypothalamic-pituitary-adrenal (HPA) axis function, and anxiety levels. Exposure to SPS enhanced conditioned avoidance and impaired extinction while enhancing ASR, negative feedback on the HPA axis, and anxiety. WIN (0.5 mg/kg) administered intraperitoneally 2 or 24 h (but not 48 h) after SPS prevented the trauma-induced alterations in IA conditioning and extinction, ASR potentiation, and HPA axis inhibition. WIN microinjected into the BLA (5 μg/side) prevented SPS-induced alterations in IA and ASR. These effects were blocked by intra-BLA co-administration of the CB1 receptor antagonist AM251 (0.3 ng/side), suggesting the involvement of CB1 receptors. These findings suggest that (i) there may be an optimal time window for intervention treatment with cannabinoids after exposure to a highly stressful event, (ii) some of the preventive effects induced by WIN are mediated by an activation of CB1 receptors in the BLA, and (iii) cannabinoids could serve as a pharmacological treatment of stress- and trauma-related disorders.

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.

Fear conditioning and extinction: emotional states encoded by distinct signaling pathways

Conditioning and extinction of fear have traditionally been viewed as two independent learning processes for encoding representations of contexts or cues (conditioned stimuli, CS), aversive events (unconditioned stimuli, US), and their relationship. Based on the analysis of protein kinase signaling patterns in neurons of the fear circuit, we propose that fear and extinction are best conceptualized as emotional states triggered by a single CS representation with two opposing values: aversive and non-aversive. These values are conferred by the presence or absence of the US and encoded by distinct sets of kinase signaling pathways and their downstream targets. Modulating specific protein kinases thus has the potential to modify emotional states, and hence, may emerge as a promising treatment for anxiety disorders.

Reversible plasticity of fear memory-encoding amygdala synaptic circuits even after fear memory consolidation

It is generally believed that after memory consolidation, memory-encoding synaptic circuits are persistently modified and become less plastic. This, however, may hinder the remaining capacity of information storage in a given neural circuit. Here we consider the hypothesis that memory-encoding synaptic circuits still retain reversible plasticity even after memory consolidation. To test this, we employed a protocol of auditory fear conditioning which recruited the vast majority of the thalamic input synaptic circuit to the lateral amygdala (T-LA synaptic circuit; a storage site for fear memory) with fear conditioning-induced synaptic plasticity. Subsequently the fear memory-encoding synaptic circuits were challenged with fear extinction and re-conditioning to determine whether these circuits exhibit reversible plasticity. We found that fear memory-encoding T-LA synaptic circuit exhibited dynamic efficacy changes in tight correlation with fear memory strength even after fear memory consolidation. Initial conditioning or re-conditioning brought T-LA synaptic circuit near the ceiling of their modification range (occluding LTP and enhancing depotentiation in brain slices prepared from conditioned or re-conditioned rats), while extinction reversed this change (reinstating LTP and occluding depotentiation in brain slices prepared from extinguished rats). Consistently, fear conditioning-induced synaptic potentiation at T-LA synapses was functionally reversed by extinction and reinstated by subsequent re-conditioning. These results suggest reversible plasticity of fear memory-encoding circuits even after fear memory consolidation. This reversible plasticity of memory-encoding synapses may be involved in updating the contents of original memory even after memory consolidation.

Kappa opioid receptor signaling in the basolateral amygdala regulates conditioned fear and anxiety in rats

BACKGROUND

The kappa opioid receptor (KOR) system contributes to the prodepressive and aversive consequences of stress and is implicated in the facilitation of conditioned fear and anxiety in rodents. Here, we sought to identify neural circuits that mediate KOR system effects on fear and anxiety in rats.

METHODS

We assessed whether fear conditioning induces plasticity in KOR or dynorphin (the endogenous KOR ligand) messenger RNA (mRNA) expression in the basolateral (BLA) and central (CeA) nuclei of the amygdala, hippocampus, or striatum. We then assessed whether microinfusions of the KOR antagonist JDTic (0-10 μg/side) into the BLA or CeA affect the expression of conditioned fear or anxiety. Finally, we examined whether fear extinction induces plasticity in KOR mRNA expression that relates to the quality of fear extinction.

RESULTS

Fear conditioning upregulated KOR mRNA in the BLA by 65% and downregulated it in the striatum by 22%, without affecting KOR levels in the CeA or hippocampus, or dynorphin levels in any region. KOR antagonism in either the BLA or CeA decreased conditioned fear in the fear-potentiated startle paradigm, whereas KOR antagonism in the BLA, but not the CeA, produced anxiolytic-like effects in the elevated plus maze. Effective fear extinction was associated with a 67% reduction in KOR mRNA in the BLA.

CONCLUSIONS

These findings suggest that fear conditioning and extinction dynamically regulate KOR expression in the BLA and provide evidence that the BLA and CeA are important neural substrates mediating the anxiolytic-like effects of KOR antagonists in models of fear and anxiety.

Modulation of fear memory by retrieval and extinction: a clue for memory deconsolidation

Memories are fragile and easily forgotten at first, but after a consolidation period of hours to weeks, are inscribed in our brains as stable traces, no longer vulnerable to conventional amnesic treatments. Retrieval of a memory renders it labile, akin to the early stages of consolidation. This phenomenon has been explored as memory reactivation, in the sense that the memory is temporarily 'deconsolidated', allowing a short time window for amnesic intervention. This window closes again after reconsolidation, which restores the stability of the memory. In contrast to this 'transient deconsolidation' and the short-spanned amnesic effects of consolidation blockers, some specific treatments can disrupt even consolidated memory, leading to apparent amnesia. We propose the term 'amnesic deconsolidation' to describe such processes that lead to disruption of consolidated memory and/or consolidated memory traces. We review studies of these 'amnesic deconsolidation' treatments that enhance memory extinction, alleviate relapse, and reverse learning-induced plasticity. The transient deconsolidation that memory retrieval induces and the amnesic deconsolidation that these regimes induce both seem to dislodge a component that stabilizes consolidated memory. Characterizing this component, at both molecular and network levels, will provide a key to developing clinical treatments for memory-related disorders and to defining the consolidated memory trace.

The role of cannabinoids in modulating emotional and non-emotional memory processes in the hippocampus

Cannabinoid agonists generally have a disruptive effect on memory, learning, and operant behavior that is considered to be hippocampus-dependent. Nevertheless, under certain conditions, cannabinoid receptor activation may facilitate neuronal learning processes. For example, CB₁ receptors are essential for the extinction of conditioned fear associations, indicating an important role for this receptor in neuronal emotional learning and memory. This review examines the diverse effects of cannabinoids on hippocampal memory and plasticity. It shows how the effects of cannabinoid receptor activation may vary depending on the route of administration, the nature of the task (aversive or not), and whether it involves emotional memory formation (e.g., conditioned fear and extinction learning) or non-emotional memory formation (e.g., spatial learning). It also examines the memory stage under investigation (acquisition, consolidation, retrieval, extinction), and the brain areas involved. Differences between the effects of exogenous and endogenous agonists are also discussed. The apparently biphasic effects of cannabinoids on anxiety is noted as this implies that the effects of cannabinoid receptor agonists on hippocampal learning and memory may be attributable to a general modulation of anxiety or stress levels and not to memory per se. The review concludes that cannabinoids have diverse effects on hippocampal memory and plasticity that cannot be categorized simply into an impairing or an enhancing effect. A better understanding of the involvement of cannabinoids in memory processes will help determine whether the benefits of the clinical use of cannabinoids outweigh the risks of possible memory impairments.

Seeking a spotless mind: extinction, deconsolidation, and erasure of fear memory

Learning to contend with threats in the environment is essential to survival, but dysregulation of memories for traumatic events can lead to disabling psychopathology. Recent years have witnessed an impressive growth in our understanding of the neural systems and synaptic mechanisms underlying emotional memory formation. As a consequence, interest has emerged in developing strategies for suppressing, if not eliminating, fear memories. Here, I review recent work employing sophisticated behavioral, pharmacological, and molecular tools to target fear memories, placing these memories firmly behind the crosshairs of neurobiologically informed interventions.

Dynamic changes in neurexins' alternative splicing: role of Rho-associated protein kinases and relevance to memory formation

The three neurexins genes (NRXN1/2/3) encode polymorphic synaptic membrane proteins that are involved in cognitive functioning. Neurexins' selectivity of function is presumably conferred through differential use of 2 promoters and 5 alternative splicing sites (SS#1/2/3/4/5). In day-old rat brain neurons grown in culture, activation (depolarization) induces reversible, calcium dependent, repression of NRXN2α SS#3 insert. The effects of depolarization on NRXN1/2/3α splicing and biochemical pathways mediating them were further studied in these neurons. NRXN1/2/3α splicing in the course of memory formation in vivo was also explored, using fear conditioning paradigm in rats in which the animals were trained to associate an aversive stimulus (electrical shock) with a neutral context (a tone), resulting in the expression of fear responses to the neutral context.

In the cultured neurons depolarization induced, beside NRXN2α SS#3, repression of SS#3 and SS#4 exons in NRXN3α but not NRXN1α. The repressions were mediated by the calcium/protein kinase C/Rho-associated protein kinase (ROCK) pathway. Fear conditioning induced significant and transient repressions of the NRXN1/2/3α SS#4 exons in the rat hippocampus. ROCK inhibition prior to training attenuated the behavioral fear response, the NRXN1/2/3α splicing repressions and subsequent recovery and the levels of excitatory (PSD95) and inhibitory (gephyrin) synaptic proteins in the hippocampus. No such effects were observed in the prefrontal cortex. Significant correlations existed between the fear response and hippocampal NRXN3α and NRXN2α SS#4 inserts as well as PSD95 protein levels. Hippocampal NRXN1α SS#4 insert and gephyrin levels did not correlate with the behavioral response but were negatively correlated with each other.

These results show for the first time dynamic, experience related changes in NRXN1/2/3α alternative splicing in the rat brain and a role for ROCK in them. Specific neurexins' transcripts may be involved in synaptic remodeling occurring at an intermediate (hours) time scale in the course of memory formation.

Modulation of conditioned fear, fear-conditioned analgesia, and brain regional c-Fos expression following administration of muscimol into the rat basolateral amygdala

Evidence suggests that gamma-aminobutyric acid (GABA) signalling in the basolateral amygdala (BLA) is involved in pain, fear, and fear-conditioned analgesia (FCA). In this study, we investigated the effects of intra-BLA administration of the GABA(A) receptor agonist muscimol on the expression of conditioned-fear, formalin-evoked nociception, and fear-conditioned analgesia in rats, and the associated alterations in brain regional expression of the immediate early gene product and marker of neuronal activity, c-Fos. Formalin-evoked nociceptive behavior, conditioned-fear and fear-conditioned analgesia were apparent in animals receiving intra-BLA saline. Intra-BLA muscimol suppressed fear behavior and prevented fear-conditioned analgesia, but had no significant effect on the expression of formalin-evoked nociception. The suppression of fear behavior by intra-BLA muscimol was associated with increased c-Fos expression in the central nucleus of the amygdala (CeA) and throughout the periaqueductal grey (PAG). These intra-BLA muscimol-induced increases in c-Fos expression were abolished in rats receiving intraplantar formalin injection. These data suggest that alterations in neuronal activity in the CeA and PAG as a result of altered GABAergic signalling in the BLA may be involved in the behavioral expression of fear and associated analgesia. Furthermore, these alterations in neuronal activity are susceptible to modulation by formalin-evoked nociceptive input in a state-dependent manner.

PERSPECTIVE

The expression of learned fear and associated analgesia are under the control of GABA(A) receptors in the basolateral amygdala, through a mechanism which may involve altered neuronal activity in key components of the descending inhibitory pain pathway. The results enhance our understanding of the neural mechanisms subserving fear-pain interactions.