条件恐惧记忆的分子机制研究进展

恐惧位于记忆和情感的交汇处,既是触发防御机制以适应危险的一种情绪反应,又是研究记忆的模型。恐惧调节障碍时常会导致焦虑。该文综述了条件恐惧记忆形成及维持的主要机制,总结了在恐惧记忆产生后,神经元响应钙离子内流时相关的分子生物学变化,如CaMKs、PKA、PKM-ζ、Rho GTPases等激酶的激活,以及下游MAPK信号的转导、转录因子的调控、相关基因的表达等方面的现有研究结果,试图理清恐惧记忆相关分子机制。鉴于恐惧调节与诸如创伤后应激障碍(PTSD)之类的某些情绪障碍有密切关系,条件恐惧记忆形成的分子机制研究可能为精神障碍药物的开发提供潜在的靶标。     

条件化恐惧记忆消退的神经振荡机制

恐惧作为个体应对内外界危险因素形成的自我保护机制的一部分,在生物体的生存中发挥着重要作用.但过度的恐惧不仅对个体生存无益,反而易引发创伤后应激障碍、焦虑等精神疾病,严重影响个体生活质量.临床上通常采用基于行为学研究结果的暴露疗法对恐惧相关疾病进行治疗,然而在患者处于治疗环境之外的时候,上述症状经常会复发.因此,解析恐惧记忆相关神经环路内信息处理的神经机制,对于理解这些疾病的发生发展,寻求切实有效的治疗方案至关重要.大量研究表明与恐惧记忆消退相关的脑区主要涉及杏仁核、内侧前额叶和海马.在恐惧消退的过程中,这3个脑区表现出特定的神经振荡模式,而且这些活动也具有同步性,构成了恐惧记忆成功消退的神经基础.未来可利用基于神经神经振荡的无创性脑刺激手段干预恐惧记忆消退的神经环路,以促进恐惧记忆的消退并避免复发,为恐惧相关障碍的临床治疗提供重要的科学依据.     

恐惧记忆消退的表观遗传学机制

恐惧是一种特殊的情感,形成恐惧记忆的能力对于动物及人类检测和应对危险至关重要。近几年研究发现,表观遗传学可以在多个层面调控恐惧记忆,如组蛋白乙酰化与DNA甲基化在恐惧记忆消退过程中发挥重要作用。动物模型中的许多研究表明,适度的表观遗传调节对于学习记忆过程是必要的,表观遗传学机制失调将导致行为水平的认知缺陷,并且表观遗传失调已被越来越多地认为是各种精神疾病的重要因素,如创伤后应激障碍(post-traumatic stress disorder,PTSD)。因此,理解记忆形成的机制具有巨大的科学意义以及社会和医疗效益。     

大麻素受体参与创伤后应激障碍记忆功能损害的研究进展

创伤后应激障碍(PTSD)是个体在经历严重创伤性事件后一种迟发出现和持续存在的心理障碍,创伤相关恐惧记忆的长期持续存在是PTSD形成的重要机制。既往研究发现PTSD恐惧记忆的形成和消退与脑内神经环路的可塑性变化密切相关,大麻素受体因其在调节神经元突触可塑性中的重要作用而受到广泛关注。本文对近年来大麻素受体参与条件恐惧记忆与工作记忆调节的研究进行概述,旨在为PTSD的预防和治疗提供理论支持。     

睡眠剥夺影响恐惧记忆巩固的认知神经机制:静息态低频波动振幅分析

恐惧是人类的基本情绪,在人类生存和适应中发挥着重要的作用.先前的研究表明,睡眠剥夺会对恐惧记忆巩固过程产生影响,然而睡眠剥夺影响恐惧记忆巩固的认知神经机制并不清楚.为此,本研究采用了功能性磁共振技术探究睡眠剥夺影响恐惧记忆巩固的认知神经基础.行为结果发现,相对于控制组,睡眠剥夺组在恐惧习得过程中有更大的主观恐惧和皮肤电反应;脑成像结果发现,恐惧记忆巩固中,睡眠剥夺组增强了杏仁核的活动,减弱了腹内侧前额叶的活动;进一步的相关分析表明,睡眠剥夺组杏仁核活动变化程度与个体恐惧习得效果呈现显著的正相关,而控制组腹内侧前额叶活动变化程度与个体恐惧习得效果呈现显著的正相关.这些结果表明,睡眠剥夺可能削弱了恐惧记忆巩固过程中腹内侧前额叶对杏仁核自上而下的调节能力.     

运动缓解创伤后应激障碍恐惧记忆泛化及成体海马神经再生机制研究进展

运动可通过成体海马神经再生(adult hippocampal neurogenesis, AHN)促进模式分离从而有效缓解创伤后应激障碍(post-traumatic stress disorder, PTSD)恐惧记忆泛化,即通过运动促进AHN可以减少PTSD患者的恐惧记忆对相似记忆的干扰,从而缓解PTSD患者的恐惧记忆泛化,这为运动干预有效治疗PTSD恐惧记忆泛化的可行性提供了理论支持。该文阐述了运动促进的AHN改善PTSD恐惧记忆泛化的研究进展,以期为PTSD基础研究及治疗提供新思路。     

发现了与恐惧相关的基因

与恐惧有关的神经环路位于杏仁核，该部位是形成恐惧记忆的关键。最近Shumyatsky等发现杏仁核侧部主要神经元可大量释放胃泌素分泌相关肽(GRP)，该肽类神经递质的受体则分布在同一区域释放γ-氨基丁酸(GABA)的中间神经元上。GRP可引起含有GRP受体的神经元释放更多的GABA，大量的GABA可增强对该区域主要神经元活动的抑制。敲除小鼠GRP受体的基因后，这种抑制作用消失，并导致另一与恐惧记忆有关的神经回路皮层长时程增强效应加强。这表明，缺乏GRP受体会导致杏仁核中间神经元对主要神经元活动抑制作用的缺失，并使皮层和杏仁核的神经元对长时程增强效应变得敏感，从而增强由恐惧刺激引起的记忆。GRP受体基因敲除的小鼠因此表现出更强和更长久的恐惧记忆行为。     

视觉功能减弱增强小鼠听觉恐惧条件反射（英文）

作为一种高级认知活动,视觉功能减弱是否影响听觉恐惧条件化学习目前还不清楚.本文以突变体rd/rd、cl/cl小鼠为视觉功能减弱组,研究视觉功能减弱是否对听觉巴甫洛夫条件化恐惧反应有影响.在恐惧条件化、恐惧消退和消除记忆再现阶段记录了僵直行为.研究结果表明,视觉功能的减弱更有利于小鼠听觉恐惧条件化的建立.文中讨论了出现此结果的可能神经机制.     

中枢神经系统中的胃泌素释放肽：在厌恶性情绪记忆中的作用及机制研究

胃泌素释放肽(gastrin-releasing peptide,GRP)是蛙皮素(bombesin,BB/BN)在哺乳动物中的同系物,在中枢神经系统中广泛分布,是一种重要的脑内神经调质,参与动物的多种生理功能和本能行为,在大脑的高级功能方面也发挥一定的作用.在神经系统中,随着GRP水平的改变,动物的记忆特别是与恐惧、焦虑相关记忆的形成、巩固和消退以及突触可塑性均发生不同程度的变化.GRP及其受体还被认为与神经系统性疾病有关,是潜在的神经系统性疾病的治疗靶点,但其相关的机制尚未明确.很多研究者基于不同实验方法提出了相关假设.本文从传统药理学、遗传学和电生理学方面对GRP系统在厌恶性情绪驱动的记忆、突触可塑性变化以及在中枢神经系统中的作用机制进行综述,希望为进一步明确GRP系统在中枢神经系统中的作用研究提供新的思路.     

杏仁复合体β受体参与条件性恐惧记忆

杏仁复合体是条件性恐惧记忆形成和储存的关键脑区。杏仁复合体β受体参与条件性恐惧记忆的巩固。β受体激活易化杏仁复合体内突触传递的长时程增强,增强条件性恐惧记忆的巩固;而阻断β受体则抑制杏仁复合体内突触传递的长时程增强,损害条件性恐惧记忆的巩固。     

Behavioral interventions to eliminate fear responses

Fear memory underlies anxiety-related disorders, including posttraumatic stress disorder(PTSD). PTSD is a fear-based disorder,characterized by difficulties in extinguishing the learned fear response and maintaining extinction. Currently, the first-line treatment for PTSD is exposure therapy, which forms an extinction memory to compete with the original fear memory. However,the extinguished fear often returns under numerous circumstances, suggesting that novel methods are needed to eliminate fear memory or facilitate extinction memory. This review discusses research that targeted extinction and reconsolidation to manipulate fear memory. Recent studies indicate that sleep is an active state that can regulate memory processes. We also discuss the influence of sleep on fear memory. For each manipulation, we briefly summarize the neural mechanisms that have been identified in human studies. Finally, we highlight potential limitations and future directions in the field to better translate existing interventions to clinical settings.     

Directional Theta Coherence in Prefrontal Cortical to Amygdalo-Hippocampal Pathways Signals Fear Extinction

Theta oscillations are considered crucial mechanisms in neuronal communication across brain areas, required for consolidation and retrieval of fear memories. One form of inhibitory learning allowing adaptive control of fear memory is extinction, a deficit of which leads to maladaptive fear expression potentially leading to anxiety disorders. Behavioral responses after extinction training are thought to reflect a balance of recall from extinction memory and initial fear memory traces. Therefore, we hypothesized that the initial fear memory circuits impact behavioral fear after extinction, and more specifically, that the dynamics of theta synchrony in these pathways signal the individual fear response. Simultaneous multi-channel local field and unit recordings were obtained from the infralimbic prefrontal cortex, the hippocampal CA1 and the lateral amygdala in mice. Data revealed that the pattern of theta coherence and directionality within and across regions correlated with individual behavioral responses. Upon conditioned freezing, units were phase-locked to synchronized theta oscillations in these pathways, characterizing states of fear memory retrieval. When the conditioned stimulus evoked no fear during extinction recall, theta interactions were directional with prefrontal cortical spike firing leading hippocampal and amygdalar theta oscillations. These results indicate that the directional dynamics of theta-entrained activity across these areas guide changes in appraisal of threatening stimuli during fear memory and extinction retrieval. Given that exposure therapy involves procedures and pathways similar to those during extinction of conditioned fear, one therapeutical extension might be useful that imposes artificial theta activity to prefrontal cortical-amygdalo-hippocampal pathways that mimics the directionality signaling successful extinction recall.     

Protocol for Studying Extinction of Conditioned Fear in Naturally Cycling Female Rats

Extinction of conditioned fear has been extensively studied in male rodents. Recently, there have been an increasing number of studies indicating that neural mechanisms for certain behavioral tasks and response behaviors are different in females and males. Using females in research studies can represent a challenge because of the variation of gonadal hormones during their estrous cycle. This protocol describes well-established procedures that are useful in investigating the role of estrogen in fear extinction memory consolidation in female rats. Phase of the estrous cycle and exogenous estrogen administration prior to extinction training can influence extinction recall 24 hr later. The vaginal swabbing technique for estrous phase identification described here aids the examination and manipulation of naturally cycling gonadal hormones. The use of this basic rodent model may further delineate the mechanisms by which estrogen can modulate fear extinction memory in females.     

How engram mediates learning,extinction, and relapse

Fear learning ensures survival through an expression of certain behavior as a conditioned fear response. Fear memory is processed and stored in a fear memory circuit, including the amygdala, hippocampus, and prefrontal cortex. A gradual decrease in conditioned fear response can be induced by fear extinction, which is mediated through the weakening of the original fear memory traces and the newly formed inhibition of those traces. Fear memory can also recover after extinction, which shows flexible control of the fear memory state. Here, we demonstrate how fear engram, which is a physical substrate of fear memory, changes during fear extinction and relapse by reviewing recent studies regarding engram.     

Genetic disruptions of Drosophila Pavlovian learning leave extinction learning intact

Individuals who experience traumatic events may develop persistent posttraumatic stress disorder. Patients with this disorder are commonly treated with exposure therapy, which has had limited long‐term success. In experimental neurobiology, fear extinction is a model for exposure therapy. In this behavioral paradigm, animals are repeatedly exposed in a safe environment to the fearful stimulus, which leads to greatly reduced fear. Studying animal models of extinction already has lead to better therapeutic strategies and development of new candidate drugs. Lack of a powerful genetic model of extinction, however, has limited progress in identifying underlying molecular and genetic factors. In this study, we established a robust behavioral paradigm to study the short‐term effect (acquisition) of extinction in Drosophila melanogaster. We focused on the extinction of olfactory aversive 1‐day memory with a task that has been the main workhorse for genetics of memory in flies. Using this paradigm, we show that extinction can inhibit each of two genetically distinct forms of consolidated memory. We then used a series of single‐gene mutants with known impact on associative learning to examine the effects on extinction. We find that extinction is intact in each of these mutants, suggesting that extinction learning relies on different molecular mechanisms than does Pavlovian learning.     

Blockade of amygdala metabotropic glutamate receptor subtype 1 impairs fear extinction

The metabotropic glutamate receptor subtype 1 (mGluR1) is thought to be crucial for several forms of memory, but its role in memory extinction has not been determined. Here, we examined a role of mGluR1 in the extinction of conditioned fear using microinjection of an mGluR1 antagonist, CPCCOEt, into the lateral amygdala (LA), a critical structure for fear conditioning and extinction. Intra-LA injection of 3 microg CPCCOEt impaired extinction that was initiated 48 h after the conditioning, but not that initiated 2h after the conditioning, indicating that the effectiveness of CPCCOEt depends upon the length of time since fear conditioning. The CPCCOEt injection failed to alter an mGluR1-like receptor (mGluR5)-dependent acquisition of fear memory, further supporting the specificity of the injected CPCCOEt on mGluR1. Together, our results suggest that amygdala mGluR1 plays a critical role in the extinction of learned fear, but not in the acquisition of fear memory.     

A NMDA Receptor Antagonist,MK-801 Impairs Consolidating Extinction of Auditory Conditioned Fear Responses in a Pavlovian Model

Background

In auditory fear conditioning, repeated presentation of the tone in the absence of shock leads to extinction of the acquired fear responses. The glutamate N-methyl-D-aspartate receptor (NMDAR) is thought to be involved in the extinction of the conditioned fear responses, but its detailed role in initiating and consolidating or maintaining the fear extinction memory is unclear. Here we investigated this issue by using a NMDAR antagonist, MK-801.

Methods/Main Findings

The effects of immediate (beginning at 10 min after the conditioning) and delayed (beginning at 24 h after conditioning) extinctions were first compared with the finding that delayed extinction caused a better and long-lasting (still significant on the 20^th day after extinction) depression on the conditioned fear responses. In a second experiment, MK-801 was intraperitoneally (i.p.) injected at 40 min before, 4 h or 12 h after the delayed extinction, corresponding to critical time points for initiating, consolidating or maintaining the fear extinction memory. i.p. injection of MK-801 at either 40 min before or 4 h after delayed extinction resulted in an impairment of initiating and consolidating fear extinction memory, which caused a long lasting increased freezing score that was still significant on the 7th day after extinction, compared with extinction group. However, MK-801 administered at 12 h after the delayed extinction, when robust consolidation has been occurred and stabilized, did not affect the established extinction memory. Furthermore, the changed freezing behaviors was not due to an alteration in general anxiety levels, since MK-801 treatment had no effect on the percentage of open-arm time or open-arm entries in an Elevated Plus Maze (EPM) task.

Conclusions/Significance

Our data suggested that the activation of NMDARs plays important role in initiation and consolidation but not maintenance of fear extinction memory. Together with the fact that NMDA receptor is very important for memory, our data added experimental evidence to the concept that the extinction of conditioned fear responses is a procedure of initiating and consolidating new memory other than simply “erasing” the fear memory.     

Neuropeptide S-mediated control of fear expression and extinction: role of intercalated GABAergic neurons in the amygdala

A deficient extinction of memory is particularly important in the regime of fear, where it limits the beneficial outcomes of treatments of anxiety disorders. Fear extinction is thought to involve inhibitory influences of the prefrontal cortex on the amygdala, although the detailed synaptic mechanisms remain unknown. Here, we report that neuropeptide S (NPS), a recently discovered transmitter of ascending brainstem neurons, evokes anxiolytic effects and facilitates extinction of conditioned fear responses when administered into the amygdala in mice. An NPS receptor antagonist exerts functionally opposing responses, indicating that endogenous NPS is involved in anxiety behavior and extinction. Cellularly, NPS increases glutamatergic transmission to intercalated GABAergic neurons in the amygdala via presynaptic NPS receptors on connected principal neurons. These results identify mechanisms of NPS in the brain, a key role of intercalated neurons in the amygdala for fear extinction, and a potential pharmacological avenue for treating anxiety disorders.     

Inhibition of Spontaneous Recovery of Fear by mGluR5 after Prolonged Extinction Training

Fear behavior is vital for survival and involves learning contingent associations of non-threatening cues with aversive stimuli. In contrast, excessive levels of fear can be maladaptive and lead to anxiety disorders. Generally, extensive sessions of extinction training correlates with reduced spontaneous recovery. The molecular mechanisms underlying the long-term inhibition of fear recovery following repeated extinction training are not fully understood. Here we show that in rats, prolonged extinction training causes greater reduction in both fear-potentiated startle and spontaneous recovery. This effect was specifically blocked by metabotropic glutamate receptor 5 (mGluR5), but not by mGluR1 antagonists and by a protein synthesis inhibitor. Similar inhibition of memory recovery following prolonged extinction training was also observed in mice. In agreement with the instrumental role of mGluR5 in the prolonged inhibition of fear recovery, we found that FMR1−/− mice which exhibit enhanced mGluR5-mediated signaling exhibit lower spontaneous recovery of fear after extinction training than wild-type littermates. At the molecular level, we discovered that prolonged extinction training reversed the fear conditioning-induced increase in surface expression of GluR1, AMPA/NMDA ratio, postsynaptic density-95 (PSD-95) and synapse-associated protein-97 (SAP97). Accordingly, delivery of Tat-GluR2_3Y, a synthetic peptide that blocks AMPA receptor endocytosis, inhibited prolonged extinction training-induced inhibition of fear recovery. Together, our results demonstrate that prolonged extinction training results in the mGluR5-dependent long-term inhibition of fear recovery. This effect may involve the degradation of original memory and may explain the beneficial effects of prolonged exposure therapy for the treatment of phobias.     

Epigenetics and memory: causes,consequences and treatments for post‐traumatic stress disorder and addiction

Understanding the interaction between fear and reward at the circuit and molecular levels has implications for basic scientific approaches to memory and for understanding the etiology of psychiatric disorders. Both stress and exposure to drugs of abuse induce epigenetic changes that result in persistent behavioral changes, some of which may contribute to the formation of a drug addiction or a stress‐related psychiatric disorder. Converging evidence suggests that similar behavioral, neurobiological and molecular mechanisms control the extinction of learned fear and drug‐seeking responses. This may, in part, account for the fact that individuals with post‐traumatic stress disorder have a significantly elevated risk of developing a substance use disorder and have high rates of relapse to drugs of abuse, even after long periods of abstinence. At the behavioral level, a major challenge in treatments is that extinguished behavior is often not persistent, returning with changes in context, the passage of time or exposure to mild stressors. A common goal of treatments is therefore to weaken the ability of stressors to induce relapse. With the discovery of epigenetic mechanisms that create persistent molecular signals, recent work on extinction has focused on how modulating these epigenetic targets can create lasting extinction of fear or drug‐seeking behavior. Here, we review recent evidence pointing to common behavioral, systems and epigenetic mechanisms in the regulation of fear and drug seeking. We suggest that targeting these mechanisms in combination with behavioral therapy may promote treatment and weaken stress‐induced relapse.