The tagging and capture hypothesis from synapse to memory

Ventral tegmental area dopamine projections to the hippocampus trigger long-term potentiation and contextual learning

In most models of neuronal plasticity and memory, dopamine is thought to promote the long-term maintenance of Long-Term Potentiation (LTP) underlying memory processes, but not the initiation of plasticity or new information storage. Here, we used optogenetic manipulation of midbrain dopamine neurons in male DAT::Cre mice, and discovered that stimulating the Schaffer collaterals - the glutamatergic axons connecting CA3 and CA1 regions - of the dorsal hippocampus concomitantly with midbrain dopamine terminals within a 200 millisecond time-window triggers LTP at glutamatergic synapses. Moreover, we showed that the stimulation of this dopaminergic pathway facilitates contextual learning in awake behaving mice, while its inhibition hinders it. Thus, activation of midbrain dopamine can operate as a teaching signal that triggers NeoHebbian LTP and promotes supervised learning.

Overnight neuronal plasticity and adaptation to emotional distress

Expressions such as 'sleep on it' refer to the resolution of distressing experiences across a night of sound sleep. Sleep is an active state during which the brain reorganizes the synaptic connections that form memories. This Perspective proposes a model of how sleep modifies emotional memory traces. Sleep-dependent reorganization occurs through neurophysiological events in neurochemical contexts that determine the fates of synapses to grow, to survive or to be pruned. We discuss how low levels of acetylcholine during non-rapid eye movement sleep and low levels of noradrenaline during rapid eye movement sleep provide a unique window of opportunity for plasticity in neuronal representations of emotional memories that resolves the associated distress. We integrate sleep-facilitated adaptation over three levels: experience and behaviour, neuronal circuits, and synaptic events. The model generates testable hypotheses for how failed sleep-dependent adaptation to emotional distress is key to mental disorders, notably disorders of anxiety, depression and post-traumatic stress with the common aetiology of insomnia.

Is there selective retroactive memory enhancement in humans?: a meta-analysis

Memory is an adaptive and flexible system that preferentially stores motivationally relevant information. However, in some cases information that is initially irrelevant can become relevant at a later time. The question arises whether and to what extent the memory system can retroactively boost memories of the initially irrelevant information. Experimental studies in animals and humans have provided evidence for such retroactive memory boosting. Additionally, these studies suggest that retroactive memory enhancement (RME) can be selective to the semantic meaning of the material. Nonetheless, recent experimental work could not replicate these findings, posing the question whether the selective RME effect is reliable. To synthesize the available evidence, we conducted meta-analyses of 14 experiments. Although the classical meta-analytic procedure suggested a small selective RME effect, Cohen's dz = 0.16, when accounting for small-study bias using robust Bayesian meta-analysis the null hypothesis was supported, Cohen's dz = 0.02, BF01 = 3.03. Furthermore strong evidence was found for a bias due to small-study effects, BF10 = 11.39. Together, this calls the reliability of a selective RME effect into question.

Memory capacity and prioritization in female mice

Our brain's capacity for memory storage may be vast but is still finite. Given that we cannot remember the entirety of our experiences, how does our brain select what to remember and what to forget? Much like the triage of a hospital's emergency room, where urgent cases are prioritized and less critical patients receive delayed or even no care, the brain is believed to go through a similar process of memory triage. Recent salient memories are prioritized for consolidation, which helps create stable, long-term representations in the brain; less salient memories receive a lower priority, and are eventually forgotten if not sufficiently consolidated (Stickgold and Walker in Nat Neurosci 16(2):139-145, 2013). While rodents are a primary model for studying memory consolidation, common behavioral tests typically rely on a limited number of items or contexts, well within the memory capacity of the subject. A memory test allowing us to exceed an animal's memory capacity is key to investigating how memories are selectively strengthened or forgotten. Here we report a new serial novel object recognition task designed to measure memory capacity and prioritization, which we test and validate using female mice.

A behavioral tagging account of kinase contribution to memory formation after spaced aversive training

Long-term memory (LTM) can be induced by repeated spaced training trials. Using the weak inhibitory avoidance (wIA) task, we showed that one wIA session does not lead to a 24-h LTM, whereas two identical wIA sessions spaced by 15 min to 6 h induce a 24-h LTM. This LTM promotion depends both on hippocampal protein synthesis and the activity of several kinases. In agreement with the behavioral tagging (BT) hypothesis, our results suggest that the two training sessions induce transient learning tags and lead, via a cooperative effect, to the synthesis of plasticity-related proteins (PRPs) that become available and captured by the tag from the second session. Although ERKs1/2 are needed for PRPs synthesis and CaMKs are required for tag setting, PKA participates in both processes. We conclude that the BT mechanism accounts for the molecular constraints underlying the classic effect of spaced learning on LTM formation.

Cognitive rescue in aging through prior training in rats

Cognitive decline in spatial memory is seen in aging. Understanding affected processes in aging is vital for developing methods to improve wellbeing. Daily memory persistence can be influenced by events around the time of learning or by prior experiences in early life. Fading memories in young can last longer if a novel event is introduced around encoding, a process called behavioral tagging. Based on this principle, we asked what processes are affected in aging and if prior training can rescue them. Two groups of aged rats received training in an appetitive delayed matching-to-place task. One of the groups additionally received prior training of the same task in young and in mid-life, constituting a longitudinal study. The results showed long-term memory decline in late aging without prior training. This would reflect affected encoding and consolidation. On the other hand, short-term memory was preserved and novelty at memory reactivation and reconsolidation enabled memory maintenance in aging. Prior training improved cognition through facilitating task performance, strengthening short-term memory and intermediate memory, and enabling encoding-boosted long-term memory. Implication of these findings in understanding brain mechanisms in cognitive aging and in beneficial effects of prior training is discussed.

Making memories last using the peripheral effect of direct current stimulation

Most memories that are formed are forgotten, while others are retained longer and are subject to memory stabilization. We show that non-invasive transcutaneous electrical stimulation of the greater occipital nerve (NITESGON) using direct current during learning elicited a long-term memory effect. However, it did not trigger an immediate effect on learning. A neurobiological model of long-term memory proposes a mechanism by which memories that are initially unstable can be strengthened through subsequent novel experiences. In a series of studies, we demonstrate NITESGON's capability to boost the retention of memories when applied shortly before, during, or shortly after the time of learning by enhancing memory consolidation via activation and communication in and between the locus coeruleus pathway and hippocampus by plausibly modulating dopaminergic input. These findings may have a significant impact for neurocognitive disorders that inhibit memory consolidation such as Alzheimer's disease.

Dopaminergic regulation of hippocampal plasticity, learning, and memory

The hippocampus is responsible for encoding behavioral episodes into short-term and long-term memory. The circuits that mediate these processes are subject to neuromodulation, which involves regulation of synaptic plasticity and local neuronal excitability. In this review, we present evidence to demonstrate the influence of dopaminergic neuromodulation on hippocampus-dependent memory, and we address the controversy surrounding the source of dopamine innervation. First, we summarize historical and recent retrograde and anterograde anatomical tracing studies of direct dopaminergic projections from the ventral tegmental area and discuss dopamine release from the adrenergic locus coeruleus. Then, we present evidence of dopaminergic modulation of synaptic plasticity in the hippocampus. Plasticity mechanisms are examined in brain slices and in recordings from in vivo neuronal populations in freely moving rodents. Finally, we review pharmacological, genetic, and circuitry research that demonstrates the importance of dopamine release for learning and memory tasks while dissociating anatomically distinct populations of direct dopaminergic inputs.

Contributions of extracellular-signal regulated kinase 1/2 activity to the memory trace

The ability to learn from experience and consequently adapt our behavior is one of the most fundamental capacities enabled by complex and plastic nervous systems. Next to cellular and systems-level changes, learning and memory formation crucially depends on molecular signaling mechanisms. In particular, the extracellular-signal regulated kinase 1/2 (ERK), historically studied in the context of tumor growth and proliferation, has been shown to affect synaptic transmission, regulation of neuronal gene expression and protein synthesis leading to structural synaptic changes. However, to what extent the effects of ERK are specifically related to memory formation and stabilization, or merely the result of general neuronal activation, remains unknown. Here, we review the signals leading to ERK activation in the nervous system, the subcellular ERK targets associated with learning-related plasticity, and how neurons with activated ERK signaling may contribute to the formation of the memory trace.

Tag and capture: how salient experiences target and rescue nearby events in memory

The long-term fate of a memory is not exclusively determined by the events occurring at the moment of encoding. Research at the cellular, circuit, and behavioral levels is beginning to reveal how neurochemical activations in the moments surrounding an event can retroactively and proactively rescue weak memory for seemingly mundane experiences. We review emerging evidence showing enhancement of weakly formed memories encoded minutes to hours before or after a related motivationally relevant experience. We discuss proposed neurobiological mechanisms for strengthening weak memories formed in temporal proximity to a strong event, and how this knowledge could be leveraged to improve memory for information that is prone to forgetting.

Persistence of spatial memory induced by spaced training involves a behavioral-tagging process

Spaced training, which involves long inter-trial intervals, has positive effects on memories. One of the main attributes of long-term memories (LTM) is persistence. Here, to identify the process that promotes LTM persistence by spaced learning, we used the spatial object recognition (SOR) task. The protocol consisted of a first strong training session that induced LTM formation (tested 1 day after training), but not LTM persistence (tested 7 or 14 days after training); and a second weak training session that promoted memory persistence when applied 1 day, but not 7 days, after the first training. We propose that the promotion of memory persistence is based on the Behavioral Tagging (BT) mechanism operating when the memory trace is retrieved. BT involves the setting of a tag induced by learning which gives rise to input selectivity, and the use of plasticity-related proteins (PRPs) to establish the mnemonic trace. We postulate that retraining will mainly retag the sites initially activated by the original learning, where the PRPs needed for memory expression and/or induced by retrieval would be used to maintain a persistent mnemonic trace. Our results suggest that the mechanism of memory expression, but not those of memory reinforcement or reconsolidation, is necessary to promote memory persistence after retraining. The molecular mechanisms involve ERKs1/2 activity to set the SOR learning tag, and the availability of GluA2-containing AMPA receptor. In conclusion, both the synthesis of PRPs and the setting of a learning tag are key processes triggered by retraining that allow SOR memory persistence.

Neuropsin-dependent and -independent behavioral tagging

Aim

The consolidation of short‐term memories into long‐term memories is promoted by associations with novel environmental stimuli. This phenomenon is known as behavioral tagging. Neuropsin, a plasticity‐related serine protease in the hippocampus and amygdala, is involved in memory formation. This study investigated how neuropsin affects associative long‐term memory.

Methods

Short‐term and long‐term memory were assessed in control and neuropsin‐deficient mice by investigating their performance in inhibitory avoidance and spatial object recognition tasks. The effect of exposure to novelty on the conversion of short‐term memory to associative long‐term memory was also examined.

Results

The consolidation of task‐related short‐term memories into long‐term memories was facilitated by exposing the animals to a novel environment 1 hour before training. However, this long‐term memory conversion was impaired in neuropsin‐deficient mice performing the inhibitory avoidance task but not the spatial object recognition task.

Conclusion

Behavioral tagging occurs via neuropsin‐dependent and neuropsin‐independent processes for different behavioral tasks.

The consolidation of task‐related short‐term memories into long‐term memories was facilitated by exposing the animals to a novel environment 1 hour before training. However, this long‐term memory conversion was impaired in neuropsin‐deficient mice performing the inhibitory avoidance task but not the spatial object recognition task. Behavioral tagging occurs via neuropsin‐dependent and neuropsin‐independent processes for different behavioral tasks.

Behavioral tagging underlies memory reconsolidation

Memory reconsolidation occurs when a retrieving event destabilizes transiently a consolidated memory, triggering thereby a new process of restabilization that ensures memory persistence. Although this phenomenon has received wide attention, the effect of new information cooccurring with the reconsolidation process has been less explored. Here we demonstrate that a memory-retrieving event sets a neural tag, which enables the reconsolidation of memory after binding proteins provided by the original or a different contiguous experience. We characterized the specific temporal window during which this association is effective and identified the protein kinase A (PKA) and the extracellular signal-regulated kinase 1 and 2 (ERK 1/2) pathways as the mechanisms related to the setting of the reconsolidation tag and the synthesis of proteins. Our results show, therefore, that memory reconsolidation is mediated by a "behavioral tagging" process, which is common to different memory forms. They represent a significant advance in understanding the fate of memories reconsolidated while being adjacent to other events, and provide a tool for designing noninvasive strategies to attenuate (pathological/traumatic) or improve (education-related) memories.

Spatial-Memory Formation After Spaced Learning Involves ERKs1/2 Activation Through a Behavioral-Tagging Process

The superiority of spaced over massed learning is an established fact in the formation of long-term memories (LTM). Here we addressed the cellular processes and the temporal demands of this phenomenon using a weak spatial object recognition (wSOR) training, which induces short-term memories (STM) but not LTM. We observed SOR-LTM promotion when two identical wSOR training sessions were spaced by an inter-trial interval (ITI) ranging from 15 min to 7 h, consistently with spaced training. The promoting effect was dependent on neural activity, protein synthesis and ERKs1/2 activity in the hippocampus. Based on the “behavioral tagging” hypothesis, which postulates that learning induces a neural tag that requires proteins to induce LTM formation, we propose that retraining will mainly retag the sites initially labeled by the prior training. Thus, when weak, consecutive training sessions are experienced within an appropriate spacing, the intracellular mechanisms triggered by each session would add, thereby reaching the threshold for protein synthesis required for memory consolidation. Our results suggest in addition that ERKs1/2 kinases play a dual role in SOR-LTM formation after spaced learning, both inducing protein synthesis and setting the SOR learning-tag. Overall, our findings bring new light to the mechanisms underlying the promoting effect of spaced trials on LTM formation.

Subthreshold Fear Conditioning Produces a Rapidly Developing Neural Mechanism that Primes Subsequent Learning

Learning results in various forms of neuronal plasticity that provide a lasting representation of past events, and understanding the mechanisms supporting lasting memories has been a primary pursuit of the neurobiological study of memory. However, learning also alters the capacity for future learning, an observation that likely reflects its adaptive significance. In the laboratory, we can study this essential property of memory by assessing how prior experience alters the capacity for subsequent learning. Previous studies have indicated that while a single weak fear conditioning trial is insufficient to support long-term memory (LTM), it can facilitate future learning such that another trial delivered within a protracted time window results in a robust memory. Here, we sought to determine whether or not manipulating neural activity in the basolateral amygdala (BLA) using designer receptors exclusively activated by designer drugs (DREADDs) during or after the initial learning trial would affect the ability of the initial trial to facilitate subsequent learning. Our results show that inhibiting the BLA in rats prior to the first trial prevented the ability of that trial to facilitate learning when a second trial was presented the next day. Inhibition of the BLA immediately after the first trial using DREADDs was not effective, nor was pharmacological inhibition of protein kinase A (PKA) or the mitogen-activated protein kinase (MAPK). These findings indicate that the neural mechanisms that permit an initial subthreshold fear conditioning trial to alter later learning develop rapidly and do not appear to require a typical post-learning consolidation period.

Locus Coeruleus Optogenetic Light Activation Induces Long-Term Potentiation of Perforant Path Population Spike Amplitude in Rat Dentate Gyrus

Norepinephrine (NE) in dentate gyrus (DG) produces NE-dependent long-term potentiation (NE-LTP) of the perforant path-evoked potential population spike both in vitro and in vivo. Chemical activators infused near locus coeruleus (LC), the source of DG NE, produce a NE-LTP that is associative, i.e., requires concurrent pairing with perforant path (PP) input. Here, we ask if LC optogenetic stimulation that allows us to activate only LC neurons can induce NE-LTP in DG. We use an adeno-associated viral vector containing a depolarizing channel (AAV8-Ef1a-DIO-eChR2(h134r)-EYFP-WPRE) infused stereotaxically into the LC of TH:Cre rats to produce light-sensitive LC neurons. A co-localization of ~62% in LC neurons was observed for these channels. Under urethane anesthesia, we demonstrated that 5-10 s 10 Hz trains of 30 ms light pulses in LC reliably activated neurons near an LC optoprobe. Ten minutes of the same train paired with 0.1 Hz PP electrical stimulation produced a delayed NE-LTP of population spike amplitude, but not EPSP slope. A leftward shift in the population spike input/output curve at the end of the experiment was also consistent with long-term population spike potentiation. LC neuron activity during the 10 min light train was unexpectedly transient. Increased LC neuronal firing was seen only for the first 2 min of the light train. NE-LTP was more delayed and less robust than reported with LC chemo-activation. Previous estimates of LC axonal conduction times suggest acute release of NE occurs 40-70 ms after an LC neuron action potential. We used single LC light pulses to examine acute effects of NE release and found potentiated population spike amplitude when a light pulse in LC occurred 40-50 ms, but not 20-30 ms, prior to a PP pulse, consistent with conduction estimates. These effects of LC optogenetic activation reinforce evidence for a continuum of NE potentiation effects in DG. The single pulse effects mirror an earlier report using LC electrical stimulation. These acute effects support an attentional role of LC activation. The LTP of PP responses induced by optogenetic LC activation is consistent with the role of LC in long-term learning and memory.

Spatial object recognition memory formation under acute stress

Stress is known to have a critical impact on memory processes. In the present work, we focus on the effects of an acute stress event closely associated to an unrelated learning task. Here, we show that acute stress (elevated platform [EP] session) experienced 1 hr after a weak spatial object recognition (SOR) training, which only induces a short-term memory (STM), promoted the formation of SOR-long term memory (SOR-LTM) in rats. The effect induced by stress was dependent on the activation of glucocorticoid- and mineralocorticoid-receptors, brain-derived neurotrophic factor (BDNF) and protein synthesis in the dorsal hippocampus. In contrast, EP after a strong SOR impaired SOR-LTM probably by interfering with the use of necessary resources. Moreover, we show that the EP session before training induced anterograde interference, which it was not reversed by a subsequent exposure to an open field. Our findings provide novel insights into the impact of stress on LTM formation in rodents and they are discussed under the behavioral analogue of the synaptic tagging and capture hypothesis.

Memory Allocation: Mechanisms and Function

Memories for events are thought to be represented in sparse, distributed neuronal ensembles (or engrams). In this article, we review how neurons are chosen to become part of a particular engram, via a process of neuronal allocation. Experiments in rodents indicate that eligible neurons compete for allocation to a given engram, with more excitable neurons winning this competition. Moreover, fluctuations in neuronal excitability determine how engrams interact, promoting either memory integration (via coallocation to overlapping engrams) or separation (via disallocation to nonoverlapping engrams). In parallel with rodent studies, recent findings in humans verify the importance of this memory integration process for linking memories that occur close in time or share related content. A deeper understanding of allocation promises to provide insights into the logic underlying how knowledge is normally organized in the brain and the disorders in which this process has gone awry.

Memory Synapses Are Defined by Distinct Molecular Complexes: A Proposal

Synapses are diverse in form and function. While there are strong evidential and theoretical reasons for believing that memories are stored at synapses, the concept of a specialized “memory synapse” is rarely discussed. Here, we review the evidence that memories are stored at the synapse and consider the opposing possibilities. We argue that if memories are stored in an active fashion at synapses, then these memory synapses must have distinct molecular complexes that distinguish them from other synapses. In particular, examples from Aplysia sensory-motor neuron synapses and synapses on defined engram neurons in rodent models are discussed. Specific hypotheses for molecular complexes that define memory synapses are presented, including persistently active kinases, transmitter receptor complexes and trans-synaptic adhesion proteins.

Sleep deprivation impairs synaptic tagging in mouse hippocampal slices

Metaplasticity refers to the ability of experience to alter synaptic plasticity, or modulate the strength of neuronal connections. Sleep deprivation has been shown to have a negative impact on synaptic plasticity, but it is unknown whether sleep deprivation also influences processes of metaplasticity. Therefore, we tested whether 5 h of total sleep deprivation (SD) in mice would impair hippocampal synaptic tagging and capture (STC), a form of heterosynaptic metaplasticity in which combining strong stimulation in one synaptic input with weak stimulation at another input allows the weak input to induce long-lasting synaptic strengthening. STC in stratum radiatum of area CA1 occurred normally in control mice, but was impaired following SD. After SD, potentiation at the weakly stimulated synapses decayed back to baseline within 2 h. Thus, sleep deprivation disrupts a prominent form of metaplasticity in which two independent inputs interact to generate long-lasting LTP.

Retrovirus-like Gag Protein Arc1 Binds RNA and Traffics across Synaptic Boutons

Arc/Arg3.1 is required for synaptic plasticity and cognition, and mutations in this gene are linked to autism and schizophrenia. Arc bears a domain resembling retroviral/retrotransposon Gag-like proteins, which multimerize into a capsid that packages viral RNA. The significance of such a domain in a plasticity molecule is uncertain. Here, we report that the Drosophila Arc1 protein forms capsid-like structures that bind darc1 mRNA in neurons and is loaded into extracellular vesicles that are transferred from motorneurons to muscles. This loading and transfer depends on the darc1-mRNA 3' untranslated region, which contains retrotransposon-like sequences. Disrupting transfer blocks synaptic plasticity, suggesting that transfer of dArc1 complexed with its mRNA is required for this function. Notably, cultured cells also release extracellular vesicles containing the Gag region of the Copia retrotransposon complexed with its own mRNA. Taken together, our results point to a trans-synaptic mRNA transport mechanism involving retrovirus-like capsids and extracellular vesicles.

Behavioral and neural mechanisms by which prior experience impacts subsequent learning

Memory is often thought about in terms of its ability to recollect and store information about the past, but its function likely rests with the fact that it permits adaptation to ongoing and future experience. Thus, the brain circuitry that encodes memory must act as if stored information is likely to be modified by subsequent experience. Considerable progress has been made in identifying the behavioral and neural mechanisms supporting the acquisition and consolidation of memories, but this knowledge comes largely from studies in laboratory animals in which the training experience is presented in isolation from prior experimentally-controlled events. Given that memories are unlikely to be formed upon a clean slate, there is a clear need to understand how learning occurs upon the background of prior experience. This article reviews recent studies from an emerging body of work on metaplasticity, memory allocation, and synaptic tagging and capture, all of which demonstrate that prior experience can have a profound effect on subsequent learning. Special attention will be given to discussion of the neural mechanisms that allow past experience to affect future learning and to the time course by which past learning events can alter subsequent learning. Finally, consideration will be given to the possible significance of a non-synaptic component of the memory trace, which in some cases is likely responsible for the priming of subsequent learning and may be involved in the recovery from amnestic treatments in which the synaptic mechanisms of memory have been impaired.

Inverse synaptic tagging: An inactive synapse-specific mechanism to capture activity-induced Arc/arg3.1 and to locally regulate spatial distribution of synaptic weights

Long-lasting forms of synaptic plasticity such as long-term potentiation (LTP) and long-term depression (LTD) are fundamental cellular mechanisms underlying learning and memory. The synaptic tagging and capture (STC) hypothesis has provided a theoretical framework on how products of activity-dependent genes may interact with potentiated synapses to facilitate and maintain such long-lasting synaptic plasticity. Although Arc/arg3.1 was initially assumed to participate in STC processes during LTP, accumulating evidence indicated that Arc/arg3.1 might rather contribute in weakening of synaptic weights than in their strengthening. In particular, analyses of Arc/Arg3.1 protein dynamics and function in the dendrites after plasticity-inducing stimuli have revealed a new type of inactivity-dependent redistribution of synaptic weights, termed "inverse synaptic tagging". The original synaptic tagging and inverse synaptic tagging likely co-exist and are mutually non-exclusive mechanisms, which together may help orchestrate the redistribution of synaptic weights and promote the enhancement and maintenance of their contrast between potentiated and non-potentiated synapses during the late phase of long-term synaptic plasticity. In this review, we describe the inverse synaptic tagging mechanism that controls synaptic dynamics of Arc/Arg3.1, an immediate early gene product which is captured and preferentially targeted to non-potentiated synapses, and discuss its impact on neuronal circuit refinement and cognitive function.

'Tagging' along memories in aging: Synaptic tagging and capture mechanisms in the aged hippocampus

Aging is accompanied by a general decline in the physiological functions of the body with the deteriorating organ systems. Brain is no exception to this and deficits in cognitive functions are quite common in advanced aging. Though a variety of age-related alterations are observed in the structure and function throughout the brain, certain regions show selective vulnerability. Medial temporal lobe, especially the hippocampus, is one such preferentially vulnerable region and is a crucial structure involved in the learning and long-term memory functions. Hippocampal synaptic plasticity, such as long-term potentiation (LTP) and depression (LTD), are candidate cellular correlates of learning and memory and alterations in these properties have been well documented in aging. A related phenomenon called synaptic tagging and capture (STC) has been proposed as a mechanism for cellular memory consolidation and to account for temporal association of memories. Mounting evidences from behavioral settings suggest that STC could be a physiological phenomenon. In this article, we review the recent data concerning STC and provide a framework for how alterations in STC-related mechanisms could contribute to the age-associated memory impairments. The enormity of impairment in learning and memory functions demands an understanding of age-associated memory deficits at the fundamental level given its impact in the everyday tasks, thereby in the quality of life. Such an understanding is also crucial for designing interventions and preventive measures for successful brain aging.

A decrease in eukaryotic elongation factor 2 phosphorylation is required for local translation of sensorin and long-term facilitation in Aplysia

Mechanistic target of rapamycin complex 1 (mTORC1)-dependent protein synthesis is required for many forms of synaptic plasticity and memory, but the downstream pathways important for synaptic plasticity are poorly understood. Long-term facilitation (LTF) in Aplysia is a form of synaptic plasticity that is closely linked to behavioral memory and an attractive model system for examining the important downstream targets for mTORC1 in regulating synaptic plasticity. Although mTORC1-regulated protein synthesis has been strongly linked to translation initiation, translation elongation is also regulated by mTORC1 and LTF leads to an mTORC1-dependent decrease in eukaryotic elongation factor 2 (eEF2) phosphorylation. The purpose of this study is to test the hypothesis that the decrease in eEF2 phosphorylation is required for mTORC1-dependent translation and plasticity. We show that the LTF-induced decrease in eEF2 phosphorylation is blocked by expression of an eEF2 kinase (eEF2K) modified to be resistant to mTORC1 regulation. We found that expression of this modified kinase blocked LTF. LTF requires local protein synthesis of the neuropeptide sensorin and importantly, local sensorin synthesis can be measured using a dendra fluorescent protein containing the 5' and 3' untranslated regions (UTRs) of sensorin. Using this construct, we show that blocking eEF2 dephosphorylation also blocks the increase in local sensorin synthesis. These results identify decreases in eEF2 phosphorylation as a critical downstream effector of mTOR required for long-term plasticity and identify an important translational target regulated by decreases in eEF2 phosphorylation.

Stress as a mnemonic filter: Interactions between medial temporal lobe encoding processes and post-encoding stress

Acute stress has been shown to modulate memory for recently learned information, an effect attributed to the influence of stress hormones on medial temporal lobe (MTL) consolidation processes. However, little is known about which memories will be affected when stress follows encoding. One possibility is that stress interacts with encoding processes to selectively protect memories that had elicited responses in the hippocampus and amygdala, two MTL structures important for memory formation. There is limited evidence for interactions between encoding processes and consolidation effects in humans, but recent studies of consolidation in rodents have emphasized the importance of encoding "tags" for determining the impact of consolidation manipulations on memory. Here, we used functional magnetic resonance imaging in humans to test the hypothesis that the effects of post-encoding stress depend on MTL processes observed during encoding. We found that changes in stress hormone levels were associated with an increase in the contingency of memory outcomes on hippocampal and amygdala encoding responses. That is, for participants showing high cortisol reactivity, memories became more dependent on MTL activity observed during encoding, thereby shifting the distribution of recollected events toward those that had elicited relatively high activation. Surprisingly, this effect was generally larger for neutral, compared to emotionally negative, memories. The results suggest that stress does not uniformly enhance memory, but instead selectively preserves memories tagged during encoding, effectively acting as mnemonic filter. © 2016 Wiley Periodicals, Inc.

Mechanisms underlying long-term fear memory formation from a metaplastic neuronal state

We previously showed that a single weak fear conditioning trial, that does not produce a long-term fear memory (LTM), appeared to prime memory formation such that when a second trial followed within a circumscribed time window a robust and long-lasting fear memory was formed. We also showed that this priming effect could be blocked if we interfered with protein kinase A (PKA) signaling in the amygdala during the first conditioning trial. The goals of the current study were to determine if LTM formation after the second trial depends on PKA signaling in the amygdala and to characterize the underlying memory processes engaged during the second trial that allows for LTM formation. Our interpretation of the original findings is that the second conditioning trial triggers LTM from a metaplastic state that is engaged by the first conditioning trial. However, it is also possible that the second conditioning trial acts as a reminder of the first and engages a reconsolidation-like process. Several experiments were conducted to distinguish between these two possibilities. We show that interfering with PKA signaling during the second conditioning trial disrupts memory formation. However, if a third trial follows the second or if the second trial was presented without shock, the PKA inhibitor was no longer effective. Our findings demonstrate that the induction of fear memory from a metaplastic state involves new learning that is distinct from retrieval-dependent updating of memories.

Pain Perception in Buddhism Perspective

Dhamma, which Lord Buddha has presented to people after his enlightenment, analyzes every phenomenon and objects into their ultimate elements. The explanation of sensory system is also found in a part of Dhamma named Abhidhammapitaka, the Book of the Higher Doctrine in Buddhism. To find out the relationship between explanation of pain in the present neuroscience and the explanation of pain in Abhidhamma, the study was carried out by the use of a comprehensive review. The comparisons were in terms of peripheral stimulation, signal transmission, modulation, perception, suffering, determination and decision making for the responding to pain. We found that details of the explanation on pain mechanism and perception in Abhidhamma could associate well with our present scientific knowledge. Furthermore, more refinement information about the process and its function in particular aspects of pain perception were provided in Abhidhammapitaka.

Behavioral tagging: A novel model for studying long-term memory

New information acquired by our brain is stored in the form of two types of memories: short term memory (STM) and long term memory (LTM). Initially, Synaptic and Capture hypothesis has been proposed to describe the synaptic changes that occur during memory formation. However, recently Behavioral Tagging hypothesis was proposed that relies on the setting of a learning tag and the synthesis of plasticity related proteins (PRPs). Behavioral Tagging has its roots in Synaptic and Capture hypothesis. It seeks to explain that how a learning tag produced as a result of weak training can be paired up with PRPs (formed as a result of novelty) and can lead to long lasting memories. We have focused on describing behavioral paradigms that have been used for establishing the model of "Behavioral Tagging" and the molecules which qualify for potential PRP candidature.

Fear Memory

Fear memory is the best-studied form of memory. It was thoroughly investigated in the past 60 years mostly using two classical conditioning procedures (contextual fear conditioning and fear conditioning to a tone) and one instrumental procedure (one-trial inhibitory avoidance). Fear memory is formed in the hippocampus (contextual conditioning and inhibitory avoidance), in the basolateral amygdala (inhibitory avoidance), and in the lateral amygdala (conditioning to a tone). The circuitry involves, in addition, the pre- and infralimbic ventromedial prefrontal cortex, the central amygdala subnuclei, and the dentate gyrus. Fear learning models, notably inhibitory avoidance, have also been very useful for the analysis of the biochemical mechanisms of memory consolidation as a whole. These studies have capitalized on in vitro observations on long-term potentiation and other kinds of plasticity. The effect of a very large number of drugs on fear learning has been intensively studied, often as a prelude to the investigation of effects on anxiety. The extinction of fear learning involves to an extent a reversal of the flow of information in the mentioned structures and is used in the therapy of posttraumatic stress disorder and fear memories in general.

Adaptive memory systems for remembering the salient and the seemingly mundane

In an adaptive memory system, events should be prioritized in memory based on their own significance, as well as the significance of preceding or following events. Here we argue that tag-and-capture models complement the GANE (glutamate amplifies noradrenergic effects) model by describing a mechanism that supports the transfer of memory benefits from one event to the next.

Histone deacetylase 3 inhibition re-establishes synaptic tagging and capture in aging through the activation of nuclear factor kappa B

Aging is associated with impaired plasticity and memory. Altered epigenetic mechanisms are implicated in the impairment of memory with advanced aging. Histone deacetylase 3 (HDAC3) is an important negative regulator of memory. However, the role of HDAC3 in aged neural networks is not well established. Late long-term potentiation (late-LTP), a cellular correlate of memory and its associative mechanisms such as synaptic tagging and capture (STC) were studied in the CA1 area of hippocampal slices from 82–84 week old rats. Our findings demonstrate that aging is associated with deficits in the magnitude of LTP and impaired STC. Inhibition of HDAC3 augments the late-LTP and re-establishes STC. The augmentation of late-LTP and restoration of STC is mediated by the activation of nuclear factor kappa B (NFκB) pathway. We provide evidence for the promotion of associative plasticity in aged neural networks by HDAC3 inhibition and hence propose HDAC3 and NFκB as the possible therapeutic targets for treating age -related cognitive decline.

Evidence for glycinergic GluN1/GluN3 NMDA receptors in hippocampal metaplasticity

Hebbian, or associative, forms of synaptic plasticity are considered the molecular basis of learning and memory. However, associative synaptic modifications, including long-term potentiation (LTP) and depression (LTD), can form positive feedback loops which must be constrained for neural networks to remain stable. One proposed constraint mechanism is metaplasticity, a process whereby synaptic changes shift the threshold for subsequent plasticity. Metaplasticity has been functionally observed but the molecular basis is not well understood. Here, we report that stimulation which induces LTP recruits GluN2B-lacking GluN1/GluN3 NMDA receptors (NMDARs) to excitatory synapses of hippocampal pyramidal neurons. These unconventional receptors may compete against conventional GluN1/GluN2 NMDARs to favor synaptic depotentiation in response to subsequent "LTP-inducing" stimulation. These results implicate glycinergic GluN1/GluN3 NMDAR as molecular brakes on excessive synaptic strengthening, suggesting a role for these receptors in the brain that has previously been elusive.

Evidence of Maintenance Tagging in the Hippocampus for the Persistence of Long-Lasting Memory Storage

The synaptic tagging and capture (STC) hypothesis provides a compelling explanation for synaptic specificity and facilitation of long-term potentiation. Its implication on long-term memory (LTM) formation led to postulate the behavioral tagging mechanism. Here we show that a maintenance tagging process may operate in the hippocampus late after acquisition for the persistence of long-lasting memory storage. The proposed maintenance tagging has several characteristics: (1) the tag is transient and time-dependent; (2) it sets in a late critical time window after an aversive training which induces a short-lasting LTM; (3) exposing rats to a novel environment specifically within this tag time window enables the consolidation to a long-lasting LTM; (4) a familiar environment exploration was not effective; (5) the effect of novelty on the promotion of memory persistence requires dopamine D1/D5 receptors and Arc expression in the dorsal hippocampus. The present results can be explained by a broader version of the behavioral tagging hypothesis and highlight the idea that the durability of a memory trace depends either on late tag mechanisms induced by a training session or on events experienced close in time to this tag.

Naturalizing semiotics: The triadic sign of Charles Sanders Peirce as a systems property

The father of pragmatism, Charles Sanders Peirce, gave in 1903 the following definition of a sign: "A Sign, or Representamen, is a First which stands in such a genuine triadic relation to a Second, called its Object, as to be capable of determining a Third, called its Interpretant, to assume the same triadic relation to its Object in which it stands itself to the same Object. The triadic relation is genuine, that is its three members are bound together by it in a way that does not consist in any complexus of dyadic relations". Despite its cult status and its pragmatic foundation, the Peircean sign has never revealed its true potential by being integrated into a formal system. In the present report, a reconstruction of the sign model is presented, which may at first appear somewhat obvious and superficial. However by use of the reconstructed model, the above statement and the majority of Peirce's other statements about the nature of signs fall into place. Instead of defining three links between Object (O), Representamen (R), and Interpretant (I), the sign is described as having a single three-dimensional link, specifying its location in a three dimensional (O,R,I) linkage space. To understand and explain sign function, the process of sign utilization (semiosis) has to be divided into two temporally separated phases, a sign-establishment phase where a three-dimensional link (Ψ(O,R,I)) is formed between three sign elements, and a later sign-interpretation phase where the established linkage is used for inferring significance to a novel phenomenon, if this satisfies the criteria for being a Representamen for the sign. Numerous statements from Peirce indicate that he used a two-staged semiosis paradigm although he did not state that explicitly. The three-dimensional model was primarily constructed for use in biosemiotics, as an exploratory frame for mapping the evolutionary establishment of sign links, which logically must have preceded the fixation of any regulatory process in molecular biological systems. It became clear, however, that the model is able to clarify many of the difficult explanations offered by Peirce about his sign model. I make no claim that Peirce used a similar type of three-dimensional model, because he explicitly used the chemical atom as naturalization (natural scientific explanation) for his sign model, an interesting but problematic analogy. In order to discuss common versus specific semiotic scaffolds for molecular biosemiotics, biosemiotics and semiotics proper, I start with a generic definition of the three-dimensional sign system, using human semiosis as examples. After this, the major part of the paper, I define the specific biochemical and evolutionary scaffolds that is used for obtaining the evolutionary memory that is needed for sign establishment. To exemplify semiosis according to the present model I present a typical situation where a Representamen (RE) and an object (OE) in the establishment phase are frequently encountered together by a sign interpreter. The process that links specific Representamens to specific Objects will first involve the recognition of the specific traits that distinguish the two sign elements. Subsequently the establishment process leads to the creation of a specific systems-state, called the Interpretant, which links the two traits in a way that allows retrieval of the information (a memory function). During a later interpretation phase, a hypothetical Object will be inferred by the interpreter when a Representamen (RI) harboring the required characteristics is encountered. This inference happens through a memory retrieval process, irrespective of the fact that relevant Objects of the sign may never be encountered after establishment. A simplified scheme for computer neural network algorithms is introduced as an example of such a system. Since the Peircean sign according to this definition is a systems property, there can be no sign without a sign interpreting systems or without some kind of memory function. A sign interpreter will thus harbor a semiotic scaffold that consists of at least an input sensor and an interpreting system coupled to a memory function. Further border conditions for semiotic scaffolds will be introduced. Peirce published a comprehensive sign definition system, but he allowed only ten sign classes, selected from the twenty-seven sign classes that result from his three main subdivisions, each containing three classes. His allowed sign classes are here identified as those which do not infer more significance during interpretation than was warranted during establishment. The excluded sign classes are either undefinable in his system or are of such a nature that the objects during interpretation are inferred to be much more significant than what was warranted during establishment. Occult signs are of these forbidden free-wheeling types, and it is postulated that they were omitted because Peirce defined his sign classes for use in a novel sign based logical system, where such over-signification would be detrimental.

Synaptic plasticity at the interface of health and disease: New insights on the role of endoplasmic reticulum intracellular calcium stores

Work from the past 40years has unraveled a wealth of information on the cellular and molecular mechanisms underlying synaptic plasticity and their relevance in physiological brain function. At the same time, it has been recognized that a broad range of neurological diseases may be accompanied by severe alterations in synaptic plasticity, i.e., 'maladaptive synaptic plasticity', which could initiate and sustain the remodeling of neuronal networks under pathological conditions. Nonetheless, our current knowledge on the specific contribution and interaction of distinct forms of synaptic plasticity (including metaplasticity and homeostatic plasticity) in the context of pathological brain states remains limited. This review focuses on recent experimental evidence, which highlights the fundamental role of endoplasmic reticulum-mediated Ca(2+) signals in modulating the duration, direction, extent and type of synaptic plasticity. We discuss the possibility that intracellular Ca(2+) stores may regulate synaptic plasticity and hence behavioral and cognitive functions at the interface between physiology and pathology.