NMDA受体在海马习得性长时程突触增强形成中的作用

本文应用慢性埋植技术以电生理学结合行为学的方法,在大鼠条件性饮水反应的建立中,于每实验日训练开始前给海马CA_3区微量注射NMDA受体拮颃剂2-amino-5-phosphonovalerate(APV),观察它对海马CA_s区突触效应及与之相关的条件性行为反应的影响。发现注射APV后对CA_3区突触效应的习得性长时程增强的产生有显著的抑制性影响,相应地条件性饮水反应的建立也受到显著的抑制,提示NMDA受体参与习得性LTP的产主。     

同一学习中海马不同区的习得性长时程突触增强的形成

本研究探查大鼠分辨学习中海马齿状回(DG)和CA_3区突触效应的变化及其相互关系,以探讨同一学习中海马不同区突触可塑性变化的特点及其相互关系,进一步论证习得性长时程增强(LTP)是学习和记忆的神经基础。发现:(1)穿通纤维的单个刺激可在DG和CA_3同时记录到潜伏期、峰值和波形各异的群体峰电位(PS);在一定范围内PS随刺激强度的增加而增大,但DG的PS增大程度比CA_3的小;据此,检测刺激的强度应选在该范围内。(2)随着行为训练,DG和CA_3区的突触传递同时产生LTP,两者LTP的发展是显著正相关关系(P<0.01),PS峰值同时达到最高水平,且先于分辨反应达学会标准,首次表明在同一学习中大鼠海马的DG和CA_3的习得性LTP是同时产生的。本研究还表明,在慢性实验下同时记录和分析行为学习中海马两个区突触效应的变化及其相互关系是可能的,这给从神经环路角度研究学习和记忆的突触机制提供了有用的模型。     

不同学习方式下习得性长时程突触增强形成的特点

一般认为,突触传递的长时程增强(Long-term potentiation,LTP)可能是学习和记忆的神经基础。我室的系列工作表明,大鼠在条件性行为的建立中,海马齿状回、CA_3、CA_1区均出现LTP样变化,并将之称为习得性LTP(Learning-de-pendent LTP),而且发现每次训练作业后突触效应的增强,经一段时间(4小时)后就     

海马齿状回突触效应的习得性长时程增强

本实验应用慢性埋植电极技术以电生理学给合行为学的方法,观察大白鼠海马齿状回在以视觉和味觉信号为条件信号的条件反应的建立、巩固和消退过程中突触效应的变化。结果表明在明暗辨别学习中出现突触效应长时程增强(LTD),其保持时间的长短与巩固性训练的多少呈正相关,LTP消退,习得行为也消退。对照实验证明它并非测试本身所引起,提示此习得性LTP与学习和记忆活动是密切相关的。但在建立条件性味觉厌恶中,却无LTP出现。这就从突触水平上进一步证明我们提出的海马在不同感觉信息的记忆活动中其作用是不同的及海马不仅在记忆的形成,而且在其保持和再现中也有重要作用的观点。     

不同发展阶段的习得性长时程突触增强对新学习的影响

在慢性实验条件下观察到大鼠由学习训练所产生的海马CA_3区习得性长时程突触增强,它的不同发展阶段对新学习任务的习得有不同的作用:(1)在它的形成阶段及巩固阶段对新学习均有易化作用,虽然两个阶段突触效应增强的程度都处于相同的高水平,但后者的易化作用相对要大些,提示这两个发展阶段突触的可塑性变化是有差异的;还有,新学习任务的训练并没有引起突触效应的进一步增强,表明突触效应的长时程增强有“饱和”现象。(2)在它消退后,对新学习任务的习得没有影响。上述结果提示学习的组织形式不同,可从突触机制上影响学习的效果,并进一步证明习得性长时程突触增强是学习和记忆的神经基础。     

海马CA_3区在长时记忆的保持中的作用

本工作将M-胆碱能受体阻断剂东莨菪碱,r-氨基丁酸及其受体阻断剂印防己毒素,经埋植的慢性导管注入家兔双侧海马CA_3区,观察上述药物对已巩固的操作式食物性条件反应的影响。发现东莨菪碱(每侧45微克/5微升)、r-氨基丁酸(每侧3毫克/5微升)均使条件反应抑制;若注r-氨基丁酸之前先加注印防己毒素,则抑制时间缩短,且剂量愈大时间更为缩短。抑制期间动物的摄食和运动功能正常,抑制后条件反应皆恢复;注生理盐水作自身对照则均无抑制效应。这表明海马CA_3区在长时记忆的保持上起着重要作用,且此种作用有胆碱能突触和GABA能突触参与。     

去甲肾上腺素对习得性长时程突触增强的影响

在大鼠条件性饮水反应的建立和巩固过程中,在每天训练之前,通过预先与记录电极一起埋植在海马齿状回的注药管,微量注射去甲肾上腺素(NE),观察其对海马齿状回的习得性长时程突触增强的影响。结果表明:NE能增强突触效应;训练作业前注NE易化了习得性长时程突触增强的形成,但对其巩固无影响;同时相应地易化了动物条件反应的建立。提示NE参与海马齿状回的习得性长时程增强的形成。本文对其可能的作用机制进行了讨论。     

海马CA_3区习得性LTP的进一步探讨

分辨学习中大鼠ＰＰ－ＣＡ_３突触效应有习得性ＬＴＰ产生,进一步观察到ＭＦ－ＣＡ_３及Ｃｏｍｍ－ＣＡ_３也同步地产生习得性ＬＴＰ,表明几种输入突触都产生习得性ＬＴＰ,而ＭＦ－ＣＡ_３突触效应增强的程度则较Ｃｏｍｍ－ＣＡ_３的为大（Ｐ＜０．０１）。海马ＣＡ_３注入印防己毒素能易化ＰＰ－ＣＡ_３突触习得性ＬＴＰ的产生,工作发现一侧海马ＣＡ_３注印防己毒素,易化对侧ＭＦ－ＣＡ_３突触习得性ＬＴＰ的产生,表明两侧海马ＣＡ_３区在其习得性ＬＴＰ的形成上是相互协同的。     

中枢N-甲基-D-天冬氨酸受体在应激所致行为改变中的作用

应激所致行为效应的脑机制研究是目前生理心理学研究的热点领域。近年来,对于参与应激所致行为效应的神经递质研究从5-HT、多巴胺和去甲肾上腺素的范畴,逐渐发展到关注脑内含量最为丰富的谷氨酸能神经元所产生的兴奋性递质,包括谷氨酸、天冬氨酸及其相应受体NMDAR可能在应激性行为效应的中枢机制中的作用。近十年来的研究表明,中枢NMDAR是学习记忆的关键物质,在兴奋性突触传递、突触可塑性和脑发育过程中扮演重要的角色。不同类型的应激能导致动物的与行为密切相关脑区如杏仁核,海马的兴奋性氨基酸及NMDAR数量增多,活性增高。突触间隙增多的兴奋性氨基酸与NMDAR结合后,通过激活NMDAR促进糖皮质激素的相关性释放,共同产生的兴奋毒性作用引起上述脑区的神经元细胞缺失和变性;或干扰其他中枢神经递质在动物行为的脑内奖赏机制中的正常功能;或通过持续激活NMDAR,导致细胞内Ca2＋超载,损害其信号传导途径下游的蛋白激酶级联反应,使其底物蛋白的磷酸化或去磷酸化作用发生改变,影响突触可塑性和神经细胞间的信号传递,导致动物出现相应的行为障碍。应激前给动物的上述脑区注射NMDAR阻滞剂,可以减轻动物的应激性焦虑和抑郁行为。而NMDAR依赖性LTP下游途径的新信号分子,神经颗粒素,参与了脑内多种蛋白信号传导,可能是应激性行为效应的另一重要中枢机制。     

睡眠对恐惧学习的影响及其认知神经机制

睡眠问题可能会诱发恐惧相关情绪障碍(焦虑、创伤性应激障碍、恐怖症等),研究睡眠影响恐惧学习的认知神经机制,有助于增强对恐惧相关情绪障碍的预测、诊断和治疗。以往研究表明睡眠剥夺影响恐惧习得和消退主要是通过抑制vmPFC活动,阻碍其与杏仁核的功能连接,从而导致恐惧习得增强或是消退学习受损。进一步研究发现睡眠不同阶段对恐惧学习相关脑区有独特的影响:剥夺(缺乏)快速眼动睡眠会抑制vmPFC活动、增强杏仁核、海马激活,导致恐惧习得增强,消退学习受损,此外边缘皮层的功能连接减少破坏了记忆巩固(恐惧记忆和消退记忆);而慢波睡眠主要与海马变化有关,慢波睡眠期间进行目标记忆重激活可促进恐惧消退学习。未来研究需要增加睡眠影响恐惧泛化的神经机制研究、及昼夜节律中断对恐惧消退的影响,以及关注动物睡眠研究向人类睡眠研究转化中存在的问题。     

乳头体对海马齿状回习得性长时程突触增强的影响

用高频电脉冲刺激大鼠乳头体（ＭＢ）,可一过性抑制由刺激穿通纤维（ＰＰ）诱发的海马齿状回（ＤＧ）群体锋电位（ＰＳ）。训练作业前先刺激ＭＢ,导致ＤＧ的习得性长时程突触增强（ＬＤＬＴＰ）的形成显著延缓,相应地延缓了条件性饮水反应的建立。电刺激ＭＢ对ＰＰ─ＤＧＰＳ的这种抑制作用在训练所产生的ＰＳ增大达最高水平时显著减弱,刺激ＭＢ对ＬＤＬＴＰ的保持及相应地对条件反应的巩固没有明显影响。结果表明ＭＢ对ＤＧＬＤＬＴＰ的形成有抑制性调制作用。     

Mutation at the TrkB PLC{gamma}-docking site affects hippocampal LTP and associative learning in conscious mice

Previous in vitro studies have characterized the electrophysiological properties and molecular events associated with long-term potentiation (LTP), but as yet there are no in vivo data from molecular-level dissection that directly identify LTP as the biological substrate for learning and memory. Understanding whether the molecular pathways required for learning are also those generating LTP when measured directly on the relevant circuit of a learning animal is clearly important, although so far has proved technically difficult. Here, for the first time, we combine highly defined genetic mouse models with behavior and in vivo recordings. We recorded the activity-dependent changes taking place at the CA3-CA1 synapses during the acquisition and extinction of a simple form of an associative learning task in mice carrying point mutations on specific docking sites of TrkB receptors (trkB(SHC), trkB(PLC)). The learning task consisted of a classical eyeblink conditioning using a trace paradigm. The conditioned stimulus (CS) consisted of a tone and was followed by a periorbital electrical shock as an unconditioned stimulus (US). The US started 500 msec after the end of the CS. We show that a single pulse presented to the Schaffer collateral-commissural pathway during the CS-US interval evoked a monosynaptic field excitatory postsynaptic potential (fEPSP) at the CA1 pyramidal cells, with a slope linearly related to learning evolution in controls and trkB(SHC) mutants, but the relationship was impaired in trkB(PLC) mice. These data support a link between the PLCgamma-docking site downstream of TrkB and the activity-dependent synaptic changes evoked at the CA3-CA1 synapses during associative learning in conscious mice, and indicate that TrkB PLCgamma-site-activated molecular pathway(s) underlie both associative learning and LTP triggered at the CA3-CA1 synapse.     

Post-training administration of corticosterone enhances consolidation of contextual fear memory and hippocampal long-term potentiation in rats

This study was designed to examine the effect of corticosterone on consolidation of contextual fear memory and hippocampal long-term potentiation (LTP) in rats. In Experiment 1, dose–response effects of corticosterone on consolidation of contextual fear memory were determined. Immediately after training in contextual fear conditioning task, rats received different doses of corticosterone. Testing 24 h later, it revealed that corticosterone enhanced memory consolidation in an inverted U shape as evidenced in increased freezing behavior of corticosterone-treated animals. The most effective dose was 3 mg/kg. In Experiment 2, LTP was examined in rats whose memory consolidation has been enhanced with corticosterone. The rats were trained as the above and received corticosterone (3 mg/kg) immediately after training. Immediately or up to one day after retention test, rats were anesthetized with urethane for LTP experiments. For LTP induction, three episodes of high frequency stimuli, 30 s apart, were delivered to the perforant path, each consisting of 10 stimuli at 250 Hz. LTP was assessed by measuring the increase in the initial slope of the population excitatory post-synaptic potentials and the amplitude of the population spikes. Data indicated that animals whose memory has been enhanced by corticosterone, also displayed enhanced hippocampal LTP. The above findings suggest that glucocorticoids may enhance contextual fear memory consolidation via enhancing hippocampal LTP.     

L-type voltage-gated calcium channels in conditioned fear: a genetic and pharmacological analysis

Using pharmacological approaches, others have suggested that L-type voltage-gated calcium channels (L-VGCCs) mediate both consolidation and extinction of conditioned fear. In the absence of L-VGCC isoform-specific antagonists, we have begun to investigate the subtype-specific role of LVGCCs in consolidation and extinction of conditioned fear using a molecular genetics approach. Previously, we used this approach to demonstrate that the Ca(v)1.3 isoform mediates consolidation, but not extinction, of contextually conditioned fear. Here, we used mice in which the gene for the L-VGCC pore-forming subunit Ca(v)1.2 was conditionally deleted in forebrain excitatory neurons (Ca(v)1.2(cKO) mice) to address the role of Ca(v)1.2 in consolidation and extinction of conditioned fear. We demonstrate that Ca(v)1.2(cKO) mice consolidate and extinguish conditioned fear as well as control littermates. These data suggest that Ca(v)1.2 is not critical for these processes and together with our previous data argue against a role for either of the brain-expressed L-VGCCs (Ca(v)1.2 or Ca(v)1.3) in extinction of conditioned fear. Additionally, we present data demonstrating that the L-VGCC antagonist nifedipine, which has been used in previous conditioned fear extinction studies, impairs locomotion, and induces an aversive state. We further demonstrate that this aversive state can enter into associations with conditioned stimuli that are present at the time that it is experienced, suggesting that previous studies using nifedipine were likely confounded by drug toxicity. Taken together, our genetic and pharmacological data argue against a role for Ca(v)1.2 in consolidation of conditioned fear as well as a role for L-VGCCs in extinction of conditioned fear.     

Facilitation of memory for extinction of drug-induced conditioned reward: role of amygdala and acetylcholine

These experiments examined the effects of posttrial peripheral and intra-amygdala injections of the cholinergic muscarinic receptor agonist oxotremorine on memory consolidation underlying extinction of amphetamine conditioned place preference (CPP) behavior. Male Long-Evans rats were initially trained and tested for an amphetamine (2 mg/kg) CPP. Rats were subsequently given limited extinction training, followed by immediate posttrial peripheral or intrabasolateral amygdala injections of oxotremorine. A second CPP test was then administered, and the amount of time spent in the previously amphetamine-paired and saline-paired apparatus compartments was recorded. Peripheral (0.07 or 0.01 mg/kg) or intra-amygdala (10 etag/0.5 microL) postextinction trial injections of oxotremorine facilitated CPP extinction. Oxotremorine injections that were delayed 2 h posttrial training did not enhance CPP extinction, indicating a time-dependent effect of the drug on memory consolidation processes. The findings indicate that memory consolidation for extinction of approach behavior to environmental stimuli previously paired with drug reward can be facilitated by posttrial peripheral or intrabasolateral amygdala administration of a cholinergic agonist.     

Blocking of inhibitory conditioning within a serial conditioned stimulus-conditioned inhibitor compound: Maintenance of acquired behavior without an unconditioned stimulus

Well-trained classically conditioned stimuli, presented unreinforced, were protected from extinction when they were followed by a signal of the omission of the reinforcer (conditioned inhibitor Konorskian type) in eight cats. An aversive classical conditioning paradigm with shock as the reinforcer was used. Of several behavioral (leg flexion, vocalization) and organismic arousal (heart rate, respiration rate, respiration amplitude) measures of conditioned responses, the respiration amplitude changes were found to be most informative for the continuous assessment of elicited arousal of low and medium intensity. In all subjects conditioned stimuli presented during extinction in serial compound with the conditioned inhibitor elicited larger responses than did conditioned stimuli presented alone during extinction. The mechanism of protection from extinction in a paradigm in which the elicitor of learned behavior occurs prior to the conditioned inhibitor provides the organism with the mechanism for the maintenance of learned behavior in the absence of a reinforcer.     

RESPONDING MAINTAINED BY INTERMITTENT REINFORCEMENT: IMPLICATIONS FOR THE USE OF EXTINCTION WITH PROBLEM BEHAVIOR IN CLINICAL SETTINGS

Results of basic research have demonstrated that behavior maintained on an intermittent schedule of reinforcement (INT) will be extinguished more slowly than behavior maintained on a continuous schedule (CRF). Although these findings suggest that problem behaviors may be difficult to treat with extinction if they have been maintained on INT rather than on CRF schedules, few applied studies have examined this phenomenon with human behavior in clinical settings. The purpose of this study was to determine whether problem behavior maintained on CRF schedules would be extinguished more rapidly than behavior maintained on INT schedules. Three individuals diagnosed with profound mental retardation participated after results of pretreatment functional analyses had identified the sources of reinforcement that were maintaining their self-injury, aggression, or disruption. Subjects were exposed to extinction following baseline conditions with CRF or INT schedules alternated within reversal or multielement designs. Results suggested that problem behaviors may not be more difficult to treat with extinction if they have been maintained on INT rather than CRF schedules. However, switching from an INT to a CRF schedule prior to extinction may lower the baseline response rate as well as the total number of responses exhibited during extinction.     

Amygdala upregulation of NCAM polysialylation induced by auditory fear conditioning is not required for memory formation, but plays a role in fear extinction

There is much interest to understand the mechanisms leading to the establishment, maintenance, and extinction of fear memories. The amygdala has been critically involved in the processing of fear memories and a number of molecular changes have been implicated in this brain region in relation to fear learning. Although neural cell adhesion molecules (NCAMs) have been hypothesized to play a role, information available about their contribution to fear memories is scarce. We investigate here whether polysialylated NCAM (PSA-NCAM) contributes to auditory fear conditioning in the amygdala. First, PSA-NCAM expression was evaluated in different amygdala nuclei after auditory fear conditioning at two different shock intensities. Results showed that PSA-NCAM expression was increased 24 h post-training only in animals subjected to the highest shock intensity (1mA). Second, PSA-NCAM was cleaved in the basolateral amygdaloid complex through micro-infusions of the enzyme endoneuraminidase N, and the consequences of such treatment were investigated on the acquisition, consolidation, remote memory expression, and extinction of conditioned fear memories. Intra-amygdaloid cleavage of PSA-NCAM did not affect acquisition, consolidation or expression of remote fear memories. However, intra-amygdaloid PSA-NCAM cleavage enhanced fear extinction processes. These results suggest that upregulation of PSA-NCAM is a correlate of fear conditioning that is not necessary for the establishment of fear memory in the amygdala, but participates in mechanisms precluding fear extinction. These findings point out PSA-NCAM as a potential target for the treatment of psychopathologies that involve impairment in fear extinction.     

Memory is not extinguished along with CS presentation but within a few seconds after CS-offset

Prior work with the crab's contextual memory model showed that CS-US conditioned animals undergoing an unreinforced CS presentation would either reconsolidate or extinguish the CS-US memory, depending on the length of the reexposure to the CS. Either memory process is only triggered once the CS is terminated. Based on these results, the following questions are raised. First, when is extinction memory acquired, if not along extinction training, and how long does it take? Second, can acquisition and consolidation of extinction memory be pharmacologically dissected? Here we address these questions performing three series of experiments: a first one aimed to study systematically the relationship between extinction and increasing periods of unreinforced CS presentations, a second one to determine the time boundaries of the extinction memory acquisition, and the third one to assay the requirement for protein synthesis and NMDA-like receptors of acquisition and consolidation of extinction memory. Our results confirm that it is CS-offset and not the mere retrieval (CS-onset) that triggers acquisition of extinction memory and that it is completed in less than 45 sec after CS-offset. In addition, protein synthesis is required for consolidation but not for acquisition of this memory and, conversely, NMDA-like receptor activity is required for its acquisition but not for its consolidation. Finally, we offer an interpretative scheme of our results and we discuss to what extent it could apply to multitrial extinction.     

THE USE OF POSITIVE AND NEGATIVE REINFORCEMENT IN THE TREATMENT OF ESCAPE-MAINTAINED DESTRUCTIVE BEHAVIOR

We identified 3 clients whose destructive behavior was sensitive to negative reinforcement (break from tasks) and positive reinforcement (access to tangible items, attention, or both). In an instructional context, we then evaluated the effects of reinforcing compliance with one, two, or all of these consequences (a break, tangible items, attention) when destructive behavior produced a break and when it did not (escape extinction). For 2 clients, destructive behavior decreased and compliance increased when compliance produced access to tangible items, even though destructive behavior resulted in a break. For 1 client, extinction was necessary to reduce destructive behavior and to increase compliance. Subsequently, when the schedule of reinforcement for compliance was faded for all clients, destructive behavior was lower and fading proceeded more rapidly when compliance produced multiple functional reinforcers (i.e., a break plus tangible items or attention) and destructive behavior was on extinction. The results are discussed in terms of the effects of relative reinforcement value and extinction on concurrent operants.