Relationship between short- and long-term memory and short- and long-term extinction

GluN2B and GluN2A-containing NMDAR are differentially involved in extinction memory destabilization and restabilization during reconsolidation

Extinction memory destabilized by recall is restabilized through mTOR-dependent reconsolidation in the hippocampus, but the upstream pathways controlling these processes remain unknown. Hippocampal NMDARs drive local protein synthesis via mTOR signaling and may control active memory maintenance. We found that in adult male Wistar rats, intra dorsal-CA1 administration of the non-subunit selective NMDAR antagonist AP5 or of the GluN2A subunit-containing NMDAR antagonist TCN201 after step down inhibitory avoidance (SDIA) extinction memory recall impaired extinction memory retention and caused SDIA memory recovery. On the contrary, pre-recall administration of AP5 or of the GluN2B subunit-containing NMDAR antagonist RO25-6981 had no effect on extinction memory recall or retention per se but hindered the recovery of the avoidance response induced by post-recall intra-CA1 infusion of the mTOR inhibitor rapamycin. Our results indicate that GluN2B-containing NMDARs are necessary for extinction memory destabilization whereas GluN2A-containing NMDARs are involved in its restabilization, and suggest that pharmacological modulation of the relative activation state of these receptor subtypes around the moment of extinction memory recall may regulate the dominance of extinction memory over the original memory trace.

mTOR inhibition impairs extinction memory reconsolidation

Fear-motivated avoidance extinction memory is prone to hippocampal brain-derived neurotrophic factor (BDNF)-dependent reconsolidation upon recall. Here, we show that extinction memory recall activates mammalian target of rapamycin (mTOR) in dorsal CA1, and that post-recall inhibition of this kinase hinders avoidance extinction memory persistence and recovers the learned aversive response. Importantly, coadministration of recombinant BDNF impedes the behavioral effect of hippocampal mTOR inhibition. Our results demonstrate that mTOR signaling is necessary for fear-motivated avoidance extinction memory reconsolidation and suggests that BDNF acts downstream mTOR in a protein synthesis-independent manner to maintain the reactivated extinction memory trace.

Neuroprotective Methodologies of Co-Enzyme Q10 Mediated Brain Hemorrhagic Treatment: Clinical and Pre-Clinical Findings

Cerebral brain hemorrhage is associated with the highest mortality and morbidity despite only constituting approximately 10-15% of all strokes classified into intracerebral and intraventricular hemorrhage where most of the patients suffer from impairment in memory, weakness or paralysis in arms or legs, headache, fatigue, gait abnormality and cognitive dysfunctions. Understanding molecular pathology and finding the worsening cause of hemorrhage will lead to explore the therapeutic interventions that could prevent and cure the disease. Mitochondrial ETC-complexes dysfunction has been found to increase neuroinflammatory cytokines, oxidative free radicals, excitotoxicity, neurotransmitter and energy imbalance that are the key neuropathological hallmarks of cerebral hemorrhage. Coenzyme Q10 (CoQ10), as a part of the mitochondrial respiratory chain can effectively restore these neuronal dysfunctions by preventing the opening of mitochondrial membrane transition pore, thereby counteracting cell death events as well as exerts an anti-inflammatory effect by influencing the expression of NF-kB1 dependent genes thus preventing the neuroinflammation and energy restoration. Due to behavior and biochemical heterogeneity in post cerebral brain hemorrhagic pattern different preclinical autologous blood injection models are required to precisely investigate the forthcoming therapeutic strategies. Despite emerging pre-clinical research and resultant large clinical trials for promising symptomatic treatments, there are very less pharmacological interventions demonstrated to improve post operative condition of patients where intensive care is required. Therefore, in current review, we explore the disease pattern, clinical and pre-clinical interventions under investigation and neuroprotective methodologies of CoQ10 precursors to ameliorate post brain hemorrhagic conditions.

Short- and long-term memory and age in a traditional tribe (Dani of Papua) and a modern population (Poland)

The different environmental conditions in which people live might challenge memory in different ways. Moreover, the frequency of usage can be a source of improvement of both short- and long-term memory. The aim of our study was to investigate the effects of environmental differences on short- and long-term memory in a traditional versus a contemporary population (Dani of Papua, n = 62; Polish, n = 134). We found that both short- and long-term memory varied in the two populations, living in totally distinct surroundings. However, there were no age differences between Polish and Dani participants in either short- or long-term memory tasks, indicating that culture was not a significant moderator of the memory differences between populations. The differences in short- and long-term memory between the two populations are consistent with the argument that short-term memory plays a more significant role in contemporary societies because of technical developments, the electronic revolution, and reading ability. The lack of an age difference appears to support the assumption that it is age, not culture, that plays a crucial role in the memory performance.

A Single Dose of Methylprednisolone Improves Aversive Memory Consolidation and Extinction in Rats

Aversive memory is essential for survival, but in some situations its exacerbation can be potentially dangerous. There are several ways to modulate memory, among them, through stress-related hormones physiological release or administration of exogenous substances analogous to them. Recently, our group shown that a chronic treatment with a low dose of methylprednisolone (MP) is able to promote memory persistence in rats. Herein, we evaluate if a single intraperitoneal (IP) dose of MP (5 mg/kg) is able to modulate aversive memory consolidation and promote memory persistence and extinction in rats. For this, two experiments were carried out. In the first one, we demonstrated that a single IP MP administration in specific times after inhibitory avoidance (IA) training improved memory consolidation and persistence. In the second experiment, we verified that a single IP MP administration 2 h after IA extinction training promoted memory extinction. This results suggest a possible new clinical applicability for MP on the aversive memory disorders, as post-traumatic stress.

Prior Learning of Relevant Nonaversive Information Is a Boundary Condition for Avoidance Memory Reconsolidation in the Rat Hippocampus

Reactivated memories can be modified during reconsolidation, making this process a potential therapeutic target for posttraumatic stress disorder (PTSD), a mental illness characterized by the recurring avoidance of situations that evoke trauma-related fears. However, avoidance memory reconsolidation depends on a set of still loosely defined boundary conditions, limiting the translational value of basic research. In particular, the involvement of the hippocampus in fear-motivated avoidance memory reconsolidation remains controversial. Combining behavioral and electrophysiological analyses in male Wistar rats, we found that previous learning of relevant nonaversive information is essential to elicit the participation of the hippocampus in avoidance memory reconsolidation, which is associated with an increase in theta- and gamma-oscillation power and cross-frequency coupling in dorsal CA1 during reactivation of the avoidance response. Our results indicate that the hippocampus is involved in memory reconsolidation only when reactivation results in contradictory representations regarding the consequences of avoidance and suggest that robust nesting of hippocampal theta-gamma rhythms at the time of retrieval is a specific reconsolidation marker.SIGNIFICANCE STATEMENT Posttraumatic stress disorder (PTSD) is characterized by maladaptive avoidance responses to stimuli or behaviors that represent or bear resemblance to some aspect of a traumatic experience. Disruption of reconsolidation, the process by which reactivated memories become susceptible to modifications, is a promising approach for treating PTSD patients. However, much of what is known about fear-motivated avoidance memory reconsolidation derives from studies based on fear conditioning instead of avoidance-learning paradigms. Using a step-down inhibitory avoidance task in rats, we found that the hippocampus is involved in memory reconsolidation only when the animals acquired the avoidance response in an environment that they had previously learned as safe and showed that increased theta- and gamma-oscillation coupling during reactivation is an electrophysiological signature of this process.

The consolidation of inhibitory avoidance memory in mice depends on the intensity of the aversive stimulus: The involvement of the amygdala, dorsal hippocampus and medial prefrontal cortex

Several studies using inhibitory avoidance models have demonstrated the importance of limbic structures, such as the amygdala, dorsal hippocampus and medial prefrontal cortex, in the consolidation of emotional memory. However, we aimed to investigate the role of the amygdala (AMG), dorsal hippocampus (DH) and medial prefrontal cortex (mPFC) of mice in the consolidation of step-down inhibitory avoidance and whether this avoidance would be conditioned relative to the intensity of the aversive stimulus. To test this, we bilaterally infused anisomycin (ANI-40μg/μl, a protein synthesis inhibitor) into one of these three brain areas in mice. These mice were then exposed to one of two different intensities (moderate: 0.5mA or intense: 1.5mA) in a step-down inhibitory avoidance task. We found that consolidation of both of the aversive experiences was mPFC dependent, while the AMG and DH were only required for the consolidation of the intense experience. We suggest that in moderately aversive situations, which do not represent a severe physical risk to the individual, the consolidation of aversive experiences does not depend on protein synthesis in the AMG or the DH, but only the mPFC. However, for intense aversive stimuli all three of these limbic structures are essential for the consolidation of the experience.

Neural structures mediating expression and extinction of platform-mediated avoidance

Individuals use both passive and active defensive responses to environmental threats. Much is known about the neural circuits of passive defensive responses (e.g., freezing), but less is known about the substrates of active defensive responses (e.g., avoidance). We developed an active avoidance task in which rats learn to avoid a tone-signaled footshock by stepping onto a nearby platform. An advantage of this task is that freezing, which can interfere with avoidance, is reduced, thereby facilitating comparison of the effects of manipulations on avoidance versus freezing. After 10 d of avoidance training, rats were infused with muscimol to pharmacologically inactivate the prelimbic cortex (PL), infralimbic cortex (IL), ventral striatum (VS), or basolateral amygdala (BLA). Inactivating PL, VS, or BLA all impaired avoidance expression, but these areas differed with respect to freezing. Inactivating BLA decreased freezing consistent with loss of the tone-shock association, whereas inactivation of VS increased freezing consistent with loss of avoidance memory. Inactivation of PL had no effect on freezing. Inactivation of IL did not impair avoidance expression but did impair avoidance extinction. Our findings suggest that active avoidance is mediated by prefrontal-striatal circuits, which may be overactive in individuals suffering from trauma-related disorders.

Study on action mechanism of 1-(4-methoxy-2-methylphenyl)piperazine (MMPP) in acquisition, formation, and consolidation of memory in mice

In the present study, the mechanism of action of MMPP (1-(4-methoxy-2-methylphenyl) piperazine) in the acquisition (pretraining administration), formation (posttraining administration), and consolidation (pretest administration) of memory was assessed in the passive avoidance test using a short- and long-term memory protocol in mice. MMPP modified avoidance in the acquisition and formation of memory protocols but not in the consolidation protocol. Scopolamine (0.1 mg/kg i.p.), dizocilpine (0.003 mg/kg i.p.), and buspirone (0.1 mg/kg i.p.) completely inhibited MMPP-induced effects on memory acquisition and partially inhibited memory formation in the short-term but not long-term paradigm. This suggested that cholinergic, N-methyl-D-aspartate (NMDA) and 5-hydroxytryptamine-1A (5-HT1A ) receptors were implicated in the MMPP-induced improvements in memory. The sedative, anxiolytic, motor impairment, myorelaxant, and anticonvulsive (pentylenetetrazole-induced seizures) properties of MMPP were also assessed with the compound only showing a nondose-dependent myorelaxation. These results suggest that MMPP can enhance acquisition and formation, but not consolidation, of memory in short-term and long-term protocol via cholinergic, NMDA-glutamatergic, and 5-HT1A receptors.

Neuroactive effects of cotinine on the hippocampus: behavioral and biochemical parameters

The present work evaluated the effects of nicotine (NIC), cotinine (COT), mecamylamine (MEC), methyllycaconitine (MLA) and dihydro-beta-eritroidine (DHβE) on memory extinction and the following biochemical parameters of the hippocampus: lipid peroxidation (LPO), antioxidant capacity (AC) and the phosphorylation of Extracellular-Signal-Regulated Kinase (ERK 1/2). Young male rats that were implanted bilaterally with cannulae were submitted to memory extinction tests sessions, and their hippocampi were dissected for biochemical assays. The extinction of fear memory was significantly improved by both nicotine and its metabolite. Cotinine significantly increased LPO, while nicotine significantly decreased it. Antioxidant capacity was increased by all treatments. Our results showed that cotinine, unlike nicotine, may increase oxidative stress in the hippocampus, but this increase depends upon the dose used and happens without causing corresponding impairments in cognitive function. Cotinine also increased the phosphorylation of ERK 1/2 in a similar fashion as nicotine. Considering these results, it is plausible to wonder to what extent nicotine-attributed effects are really due to the actions of this alkaloid and whether they could be due instead to cotinine or to cotinine-nicotine interactions within the brain.

Histamine facilitates consolidation of fear extinction

Non-reinforced retrieval induces memory extinction, a phenomenon characterized by a decrease in the intensity of the learned response. This attribute has been used to develop extinction-based therapies to treat anxiety and post-traumatic stress disorders. Histamine modulates memory and anxiety but its role on fear extinction has not yet been evaluated. Therefore, using male Wistar rats, we determined the effect of the intra-hippocampal administration of different histaminergic agents on the extinction of step-down inhibitory avoidance (IA), a form of aversive learning. We found that intra-CA1 infusion of histamine immediately after non-reinforced retrieval facilitated consolidation of IA extinction in a dose-dependent manner. This facilitation was mimicked by the histamine N-methyltransferase inhibitor SKF91488 and the H2 receptor agonist dimaprit, reversed by the H2 receptor antagonist ranitidine, and unaffected by the H1 antagonist pyrilamine, the H3 antagonist thioperamide and the antagonist at the NMDA receptor (NMDAR) polyamine-binding site ifenprodil. Neither the H1 agonist 2-2-pyridylethylamine nor the NMDAR polyamine-binding site agonist spermidine affected the consolidation of extinction while the H3 receptor agonist imetit hampered it. Extinction induced the phosphorylation of ERK1 in dorsal CA1 while intra-CA1 infusion of the MEK inhibitor U0126 blocked extinction of the avoidance response. The extinction-induced phosphorylation of ERK1 was enhanced by histamine and dimaprit and blocked by ranitidine administered to dorsal CA1 after non-reinforced retrieval. Taken together, our data indicate that the hippocampal histaminergic system modulates the consolidation of fear extinction through a mechanism involving the H2-dependent activation of ERK signalling.

The role of the dorsal hippocampus on the Ginkgo biloba facilitation effect of fear extinction as assessed with fear-potentiated startle

RATIONALE

Ginkgo biloba extract, EGb761, is widely used as herbal supplements throughout Western society. It has been used in the treatment of various common geriatric complaints including short-term memory loss. Our previous study has shown that acute systemic administration of EGb761 enhanced extinction of fear-potentiated startle (FPS) in rats. Little is known about the behavioral effects of hippocampally administered EGb761 on the extinction of FPS.

OBJECTIVE

The current study was performed to evaluate the involvement of the dorsal hippocampus (DH) in the EGb761 facilitation effect on the extinction of FPS.

METHODS AND RESULT

Male adult SD rats were used. EGb761 (28 ng/side, bilaterally) was infused into DH bilaterally 10 min prior to extinction training. Animals were then tested for FPS 24 h later. Results showed that intra-hippocampal infusion of EGb761 prior to extinction training facilitated extinction, which was not due to impairments of expression of conditioned fear. Intra-hippocampal injection of ERK1/2 inhibitor PD98059 partially attenuates the above EGb761 effect. Therefore, acute EGb761 administration modulated extinction of conditioned fear, which might be mediated by more than one signal cascade.

CONCLUSIONS

These results suggest that DH may participate in the facilitation effect of EGb761 on the extinction of conditioned fear. In addition to ERK1/2, another signal cascade may also be involved in the EGb761 facilitation effect on extinction.

Dimebon enhances hippocampus-dependent learning in both appetitive and inhibitory memory tasks in mice

Pre-clinical and clinical studies on dimebon (dimebolin or latrepirdine) have demonstrated its use as a cognitive enhancer. Here, we show that dimebon administered to 3-month-old C57BL6N mice 15 min prior to training in both appetitive and inhibitory learning tasks via repeated (0.1 mg/kg) and acute (0.5 mg/kg) i.p. injections, respectively, increases memory scores. Acute treatment with dimebon was found to enhance inhibitory learning, as also shown in the step-down avoidance paradigm in 7-month-old mice. Bolus administration of dimebon did not affect the animals' locomotion, exploration or anxiety-like behaviour, with the exception of exploratory behaviour in older mice in the novel cage test. In a model of appetitive learning, a spatial version of the Y-maze, dimebon increased the rate of correct choices and decreased the latency of accessing a water reward after water deprivation, and increased the duration of drinking behaviour during training/testing procedures. Repeated treatment with dimebon did not alter the behaviours in other tests or water consumption. Acute treatment of water-deprived and non-water-deprived mice with dimebon also did not affect their water intake. Our data suggest that dimebon enhances hippocampus-dependent learning in both appetitive and inhibitory tasks in mice.

Glutamate receptors in extinction and extinction-based therapies for psychiatric illness

Some psychiatric illnesses involve a learned component. For example, in posttraumatic stress disorder, memories triggered by trauma-associated cues trigger fear and anxiety, and in addiction, drug-associated cues elicit drug craving and withdrawal. Clinical interventions to reduce the impact of conditioned cues in eliciting these maladaptive conditioned responses are likely to be beneficial. Extinction is a method of lessening conditioned responses and involves repeated exposures to a cue in the absence of the event it once predicted. We believe that an improved understanding of the behavioral and neurobiological mechanisms of extinction will allow extinction-like procedures in the clinic to become more effective. Research on the role of glutamate-the major excitatory neurotransmitter in the mammalian brain-in extinction has led to the development of pharmacotherapeutics to enhance the efficacy of extinction-based protocols in clinical populations. In this review, we describe what has been learned about glutamate actions at its three major receptor types (N-methyl-D-aspartate (NMDA) receptors, α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors, and metabotropic glutamate receptors) in the extinction of conditioned fear, drug craving, and withdrawal. We then discuss how these findings have been applied in clinical research.

Autophosphorylation of alphaCaMKII is differentially involved in new learning and unlearning mechanisms of memory extinction

Accumulating evidence indicates the key role of alpha-calcium/calmodulin-dependent protein kinase II (alphaCaMKII) in synaptic plasticity and learning, but it remains unclear how this kinase participates in the processing of memory extinction. Here, we investigated the mechanism by which alphaCaMKII may mediate extinction by using heterozygous knock-in mice with a targeted T286A mutation that prevents the autophosphorylation of this kinase (alphaCaMKII(T286A+/-)). Remarkably, partial reduction of alphaCaMKII function due to the T286A(+/-) mutation prevented the development of extinction without interfering with initial hippocampus-dependent memory formation as assessed by contextual fear conditioning and the Morris water maze. It is hypothesized that the mechanism of extinction may differ depending on the interval at which extinction training is started, being more akin to "new learning" at longer intervals and "unlearning" or "erasure" at shorter intervals. Consistent with this hypothesis, we found that extinction conducted 24 h, but not 15 min, after contextual fear training showed spontaneous recovery (reappearance of extinguished freezing responses) 21 d following the extinction, representing behavioral evidence for new learning and unlearning mechanisms underlying extinction 24 h and 15 min post-training, respectively. Importantly, the alphaCaMKII(T286A+/-) mutation blocked new learning of contextual fear memory extinction, whereas it did not interfere with unlearning processes. Our results demonstrate a genetic dissociation of new learning and unlearning mechanisms of extinction, and suggest that alphaCaMKII is responsible for extinguishing memories specifically through new learning mechanisms.

mGluR7 facilitates extinction of aversive memories and controls amygdala plasticity

Formation and extinction of aversive memories in the mammalian brain are insufficiently understood at the cellular and molecular levels. Using the novel metabotropic glutamate receptor 7 (mGluR7) agonist AMN082, we demonstrate that mGluR7 activation facilitates the extinction of aversive memories in two different amygdala-dependent tasks. Conversely, mGluR7 knockdown using short interfering RNA attenuated the extinction of learned aversion. mGluR7 activation also blocked the acquisition of Pavlovian fear learning and its electrophysiological correlate long-term potentiation in the amygdala. The finding that mGluR7 critically regulates extinction, in addition to acquisition of aversive memories, demonstrates that this receptor may be relevant for the manifestation and treatment of anxiety disorders.

Extinction learning: neurological features, therapeutic applications and the effect of aging

Extinction learning consists of the usually gradual inhibition of the retrieval of a previously learned response or behavior. It is widely used for the treatment of syndromes of learned fear, such as phobias and post-traumatic stress disorder. It relies on well-identified molecular processes in the hippocampus, basolateral amygdala, ventromedial prefrontal cortex (vmPFC) and entorhinal cortex. In humans, thickness of the orbital cortex, vmPFC and the anterior cingulate cortex correlates with the capacity to extinguish. The three regions are functionally inter-related (see below). The development of learned fear syndromes in humans is viewed by many as being due to a deficit of extinction, and so of the capacity to deal with fear. Blockade of NMDA receptors, inhibition of protein synthesis in the vmPFC or blockade of protein synthesis or of various molecular signaling cascades in the hippocampus, amygdala or entorhinal cortex impairs extinction. d-cycloserine, a partial agonist at NMDA receptors, enhances extinction in animals and humans and may help extinction to exert its therapeutic effect. Cannabinoids also enhance extinction, acting through CB1 receptors, but their therapeutic use is not warranted.

The role of the entorhinal cortex in extinction: influences of aging

The entorhinal cortex is perhaps the area of the brain in which neurofibrillary tangles and amyloid plaques are first detectable in old age with or without mild cognitive impairment, and very particularly in Alzheimer's disease. It plays a key role in memory formation, retrieval, and extinction, as part of circuits that include the hippocampus, the amygdaloid nucleus, and several regions of the neocortex, in particular of the prefrontal cortex. Lesions or biochemical impairments of the entorhinal cortex hinder extinction. Microinfusion experiments have shown that glutamate NMDA receptors, calcium and calmodulin-dependent protein kinase II, and protein synthesis in the entorhinal cortex are involved in and required for extinction. Aging also hinders extinction; it is possible that its effect may be in part mediated by the entorhinal cortex.

Inhibition of c-Jun N-terminal kinase in the CA1 region of the dorsal hippocampus blocks extinction of inhibitory avoidance memory

Step-down inhibitory avoidance (IA) memory formation involves association of stepping-down from a platform present in a training box (conditioned stimulus) with a footshock (unconditioned stimulus). A single short training session is enough to induce a lasting and strong memory trace expressed as an increase in step-down latency. Repeated nonreinforced retrieval, however, induces extinction of the IA response, a process involving a new learning that overrules the original one to indicate that the conditioned stimulus no longer predicts the unconditioned stimulus. Although the molecular requirements of IA memory consolidation are well understood, comparatively less is known about the signaling pathways involved in its extinction. Here we report that, when given into dorsal CA1 immediately but not 180 min after daily nonreinforced retrieval sessions, SP60015, a specific inhibitor of the mitogen-activated protein kinase, c-Jun N-terminal kinase, impaired IA memory extinction in a dose-dependent manner without producing any motor or perceptual impairment or damaging the hippocampal formation. Our results suggest that, as happens during consolidation, extinction of IA long-term memory also requires c-Jun N-terminal kinase activity in the CA1 region of the dorsal hippocampus.

Mechanisms of fear extinction

Excessive fear and anxiety are hallmarks of a variety of disabling anxiety disorders that affect millions of people throughout the world. Hence, a greater understanding of the brain mechanisms involved in the inhibition of fear and anxiety is attracting increasing interest in the research community. In the laboratory, fear inhibition most often is studied through a procedure in which a previously fear conditioned organism is exposed to a fear-eliciting cue in the absence of any aversive event. This procedure results in a decline in conditioned fear responses that is attributed to a process called fear extinction. Extensive empirical work by behavioral psychologists has revealed basic behavioral characteristics of extinction, and theoretical accounts have emphasized extinction as a form of inhibitory learning as opposed to an erasure of acquired fear. Guided by this work, neuroscientists have begun to dissect the neural mechanisms involved, including the regions in which extinction-related plasticity occurs and the cellular and molecular processes that are engaged. The present paper will cover behavioral, theoretical and neurobiological work, and will conclude with a discussion of clinical implications.

Short-term memory formation and long-term memory consolidation are enhanced by cellular prion association to stress-inducible protein 1

Cellular prion protein (PrP(C)) is a cell surface glycoprotein that interacts with several ligands such as laminin, NCAM (Neural-Cell Adhesion Molecule) and the stress-inducible protein 1 (STI1). PrP(C) association with these proteins in neurons mediates adhesion, differentiation and protection against programmed cell death. Herein, we used an aversively motivated learning paradigm in rats to investigate whether STI1 interaction with PrP(C) affects short-term memory (STM) formation and long-term memory (LTM) consolidation. Blockage of PrP(C)-STI1 interaction with intra-hippocampal infusion of antibodies against PrP(C) or STI1 immediately after training impaired both STM and LTM. Furthermore, infusion of PrP(C) peptide 106-126, which competes for PrP(C)-STI1 interaction, also inhibited both forms of memory. Remarkably, STI1 peptide 230-245, which includes the PrP(C) binding site, had a potent enhancing effect on memory performance, which could be blocked by co-treatment with the competitive PrP(C) peptide 106-126. Taken together, these results demonstrate that PrP(C)-STI1 interaction modulates both STM and LTM and suggests a potential use of ST11 peptide 230-245 as a pharmacological agent.

Inhibition of mRNA synthesis in the hippocampus impairs consolidation and reconsolidation of spatial memory

Using two different mRNA synthesis inhibitors, we show that blockade of hippocampal gene expression during restricted posttraining or postretrieval time windows hinders retention of long-term spatial memory for the Morris water maze task, without affecting short-term memory, nonspatial learning, or the functionality of the hippocampus. Our results indicate that spatial memory consolidation induces the activation of the hippocampal transcriptional machinery and suggest the existence of a gene expression-dependent reconsolidation process that operates in the dorsal hippocampus at the moment of retrieval to stabilize the reactivated mnemonic trace.

The interaction between prion protein and laminin modulates memory consolidation

Cellular prion protein (PrPc) has a pivotal role in prion diseases. PrPc is a specific receptor for laminin (LN) gamma1 peptide and several lines of evidence indicate that it is also involved in neural plasticity. Here we investigated whether the interaction between PrPc and LN plays a role in rat memory formation. We found that post-training intrahippocampal infusion of PrPc-derived peptides that contain the LN binding site (PrPc163-182 and PrPc173-192) or of anti-PrPc or anti-LN antibodies that inhibit PrPc-LN interaction impaired inhibitory avoidance memory retention. The amnesic effect of anti-PrPc antibodies and PrPc173-192 peptide was reversed by co-infusion of a LN gamma1 chain-derived peptide containing the PrPc-binding site, suggesting that PrPc-LN interaction is indeed crucial for memory consolidation. In addition, PrPc173-192 peptide and anti-PrPc or anti-LN antibodies also inhibited the activation of hippocampal cAMP-dependent protein kinase A (PKA) and extracellular regulated kinase (ERK1/2), two kinases that mediate the up-regulation of signaling pathways needed for consolidation of inhibitory avoidance memory. Our findings show that, through its interaction with LN, hippocampal PrPc plays a critical role in memory processing and suggest that this role is mediated by activation of both PKA and ERK1/2 signaling pathways.

The extinction of conditioned fear: structural and molecular basis and therapeutic use

OBJECTIVE: Through association, a large variety of stimuli acquire the property of signaling pleasant or aversive events. Pictures of a wedding or of a plane disaster may serve as cues to recall these events and/or others of a similar nature or emotional tone. Presentation of the cues unassociated with the events, particularly if repeated, reduces the tendency to retrieve the original learning based on that association. This attenuation of the expression of a learned response was discovered by Pavlov 100 years ago, who called it extinction. In this article we review some of the most recent findings about the behavioral and biochemical properties of extinction. RESULTS AND DISCUSSION: It has been shown that extinction is a new learning based on a new link formed by the cues and the absence of the original event(s) which originated the first association. Extinction does not consist of the erasure of the original memory, but of an inhibition of its retrieval: the original response reappears readily if the former association is reiterated, or if enough time is allowed to pass (spontaneous recovery). Extinction requires neural activity, signaling pathways, gene expression and protein synthesis in the ventromedial prefrontal cortex and/or basolateral amygdala, hippocampus, entorhinal cortex and eventually other areas. The site or sites of extinction vary with the task. CONCLUSIONS: Extinction was advocated by Freud in the 1920's for the treatment of phobias, and is used in cognitive therapy to treat diseases that rely on conditioned fear (phobias, panic, and particularly posttraumatic stress disorder). The treatment of learned fear disorders with medications is still unsatisfactory although some have been shown useful when used as adjuncts to behavioral therapy.

On the participation of hippocampal p38 mitogen-activated protein kinase in extinction and reacquisition of inhibitory avoidance memory

Inhibitory avoidance (IA) learning relies on the formation of an association between stepping down from a platform present in a certain context (conditioned stimulus; CS) with an aversive unconditioned stimulus (US; i.e. a footshock). A single CS-US pairing establishes a robust long-term memory expressed as an increase in step-down latency at testing. However, repeated retrieval of the avoidance response in the absence of the US induces extinction of IA memory. That is, recurring presentation of the CS alone results in a new learning indicating that the CS no longer predicts the US. Although the signaling pathways involved in the consolidation of IA and other fear-motivated memories have been profusely studied, little is known about the molecular requirements of fear memory extinction. Here we report that, as happens with its consolidation, extinction of IA long-term memory requires activity of the p38 subfamily of mitogen-activated protein kinases (MAPK) in the CA1 region of the dorsal hippocampus. Moreover, we found that inhibition of hippocampal p38MAPK blocked memory reacquisition after extinction without affecting either the increase in IA memory retention induced by a second training session or animal's locomotor/exploratory activity and anxiety state.

Histamine enhances inhibitory avoidance memory consolidation through a H2 receptor-dependent mechanism

Several evidences suggest that brain histamine is involved in memory consolidation but the actual contribution of the hippocampal histaminergic system to this process remains controversial. Here, we show that when infused into the CA1 region of the dorsal hippocampus immediately after training in an inhibitory avoidance task, but not later, histamine induced a dose-dependent promnesic effect without altering locomotor activity, exploratory behavior, anxiety state or retrieval of the avoidance response. The facilitatory effect of intra-CA1 histamine was mimicked by the histamine N-methyltransferase inhibitor SKF-91844 as well as by the H2 receptor agonist dimaprit and it was blocked completely by the H2 receptor antagonist ranitidine. Conversely, the promnesic action of histamine was unaffected by the H1 receptor antagonist pyrilamine, the H3 receptor antagonist, thioperamide, and the NMDAr polyamine-binding site antagonist ifenprodil. By themselves, ranitidine, pyrilamine, thioperamide, and ifenprodil did not affect IA memory consolidation. Our data indicate that, when given into CA1, histamine enhances memory consolidation through a mechanism that involves activation of H2 receptors; however, endogenous CA1 histamine does not seem to participate in the consolidation of IA memory at least at the post-training times analyzed.

Reduced extinction of hippocampal-dependent memories in CPEB knockout mice

CPEB is a sequence-specific RNA binding protein that regulates translation at synapses. In neurons of CPEB knockout mice, synaptic efficacy is reduced. Here, we have performed a battery of behavioral tests and find that relative to wild-type animals, CPEB knockout mice, although similar on many baseline behaviors, have reduced extinction of memories on two hippocampal-dependent tasks. A corresponding microarray analysis reveals that about 0.14% of hippocampal genes have an altered expression in the CPEB knockout mouse. These data suggest that CPEB-dependent local protein synthesis may be an important cellular mechanism underlying extinction of hippocampal-dependent memories.