Making memories last: the synaptic tagging and capture hypothesis

Sleep spindles track cortical learning patterns for memory consolidation

Memory consolidation-the transformation of labile memory traces into stable long-term representations-is facilitated by post-learning sleep. Computational and biophysical models suggest that sleep spindles may play a key mechanistic role for consolidation, igniting structural changes at cortical sites involved in prior learning. Here, we tested the resulting prediction that spindles are most pronounced over learning-related cortical areas and that the extent of this learning-spindle overlap predicts behavioral measures of memory consolidation. Using high-density scalp electroencephalography (EEG) and polysomnography (PSG) in healthy volunteers, we first identified cortical areas engaged during a temporospatial associative memory task (power decreases in the alpha/beta frequency range, 6-20 Hz). Critically, we found that participant-specific topographies (i.e., spatial distributions) of post-learning sleep spindle amplitude correlated with participant-specific learning topographies. Importantly, the extent to which spindles tracked learning patterns further predicted memory consolidation across participants. Our results provide empirical evidence for a role of post-learning sleep spindles in tracking learning networks, thereby facilitating memory consolidation.

Unrelenting Fear Under Stress: Neural Circuits and Mechanisms for the Immediate Extinction Deficit

Therapeutic interventions for disorders of fear and anxiety rely on behavioral approaches that reduce pathological fear memories. For example, learning that threat-predictive stimuli are no longer associated with aversive outcomes is central to the extinction of conditioned fear responses. Unfortunately, fear memories are durable, long-lasting, and resistant to extinction, particularly under high levels of stress. This is illustrated by the "immediate extinction deficit," which is characterized by a poor long-term reduction of conditioned fear when extinction procedures are attempted within hours of fear conditioning. Here, I will review recent work that has provided new insight into the neural mechanisms underlying resistance to fear extinction. Emerging studies reveal that locus coeruleus norepinephrine modulates amygdala-prefrontal cortical circuits that are critical for extinction learning. These data suggest that stress-induced activation of brain neuromodulatory systems biases fear memory at the expense of extinction learning. Behavioral and pharmacological strategies to reduce stress in patients undergoing exposure therapy might improve therapeutic outcomes.

Neuronal mTOR Outposts: Implications for Translation, Signaling, and Plasticity

The kinase mTOR is a signaling hub for pathways that regulate cellular growth. In neurons, the subcellular localization of mTOR takes on increased significance. Here, we review findings on the localization of mTOR in axons and offer a perspective on how these may impact our understanding of nervous system development, function, and disease. We propose a model where mTOR accumulates in local foci we term mTOR outposts, which can be found in processes distant from a neuron’s cell body. In this model, pathways that funnel through mTOR are gated by local outposts to spatially select and amplify local signaling. The presence or absence of mTOR outposts in a segment of axon or dendrite may determine whether regional mTOR-dependent signals, such as nutrient and growth factor signaling, register toward neuron-wide responses. In this perspective, we present the emerging evidence for mTOR outposts in neurons, their putative roles as spatial gatekeepers of signaling inputs, and the implications of the mTOR outpost model for neuronal protein synthesis, signal transduction, and synaptic plasticity.

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.

Fading memories in aging and neurodegeneration: Is p75 neurotrophin receptor a culprit?

Aging and age-related neurodegenerative diseases have become one of the major concerns in modern times as cognitive abilities tend to decline when we get older. It is well known that the main cause of this age-related cognitive deficit is due to aberrant changes in cellular, molecular circuitry and signaling pathways underlying synaptic plasticity and neuronal connections. The p75 neurotrophin receptor (p75^NTR) is one of the important mediators regulating the fate of the neurons in the nervous system. Its importance in neuronal apoptosis is well documented. However, the mechanisms involving the regulation of p75^NTR in synaptic plasticity and cognitive function remain obscure, although cognitive impairment has been associated with a higher expression of p75^NTR in neurons. In this review, we discuss the current understanding of how neurons are influenced by p75^NTR function to maintain normal neuronal synaptic strength and connectivity, particularly to support learning and memory in the hippocampus. We then discuss the age-associated alterations in neurophysiological mechanisms of synaptic plasticity and cognitive function. Furthermore, we also describe current evidence that has begun to elucidate how p75^NTR regulates synaptic changes in aging and age-related neurodegenerative diseases, focusing on the hippocampus. Elucidating the role that p75^NTR signaling plays in regulating synaptic plasticity will contribute to a better understanding of cognitive processes and pathological conditions. This will in turn provide novel approaches to improve therapies for the treatment of neurological diseases in which p75^NTR dysfunction has been demonstrated.

Behavioral and Cellular Tagging in Young and in Early Cognitive Aging

The ability to maintain relevant information on a daily basis is negatively impacted by aging. However, the neuronal mechanism manifesting memory persistence in young animals and memory decline in early aging is not fully understood. A novel event, when introduced around encoding of an everyday memory task, can facilitate memory persistence in young age but not in early aging. Here, we investigated in male rats how sub-regions of the hippocampus are involved in memory representation in behavioral tagging and how early aging affects such representation by combining behavioral training in appetitive delayed-matching-to-place tasks with the “cellular compartment analysis of temporal activity by fluorescence in situ hybridization” technique. We show that neuronal assemblies activated by memory encoding were also partially activated by novelty, particularly in the distal CA1 and proximal CA3 subregions in young male rats. In early aging, both encoding- and novelty-triggered neuronal populations were significantly reduced with a more profound effect in encoding neurons. Thus, memory persistence through novelty facilitation engages overlapping hippocampal assemblies as a key cellular signature, and cognitive aging is associated with underlying reduction in neuronal activation.

A Simplified Plasticity Model Based on Synaptic Tagging and Capture Theory: Simplified STC

The formation and consolidation of memory play a vital role for survival in an ever-changing environment. In the brain, the change and stabilization of potentiated and depressed synapses are the neural basis of memory formation and maintenance. These changes can be induced by rather short stimuli (only a few seconds or even less) but should then be stable for months or years. Recently, the neural mechanism of conversion from rapid change during the early phase of synaptic plasticity into a stable memory trace in the late phase of synaptic plasticity is more and more clear at the protein and molecular levels, among which synaptic tagging and capture (STC) theory is one of the most popular theories. According to the STC theory, the change and stabilization of synaptic efficiency mainly depend on three processes related to calcium concentration, including synaptic tagging, synthesis of plasticity-related product (PRP), and the capture of PRP by tagged synapse. Based on the STC theory, several computational models are proposed. However, these models hardly take simplicity and biological interpretability into account simultaneously. Here, we propose a simplified STC (SM-STC) model to address this issue. In the SM-STC model, the concentration of calcium ion in each neuronal compartment and synapse is first calculated, and then the tag state of synapse and PRP are updated, and the coupling effect of tagged synapse and PRP is further considered to determine the plasticity state of the synapse, either potentiation or depression. We simulated the Schaffer collaterals pathway of the hippocampus targeting a multicompartment CA1 neuron for several hours of biological time. The results show that the SM-STC model can produce a broad range of experimental phenomena known in the physiological experiments, including long-term potentiation induced by high-frequency stimuli, long-term depression induced by low-frequency stimuli, and cross-capture with two stimuli separated by a delay. Thus, the SM-STC model proposed in this study provides an effective learning rule for brain-like computation on the premise of ensuring biological plausibility and computational efficiency.

Spatial regulation of coordinated excitatory and inhibitory synaptic plasticity at dendritic synapses

The induction of synaptic plasticity at an individual dendritic glutamatergic spine can affect neighboring spines. This local modulation generates dendritic plasticity microdomains believed to expand the neuronal computational capacity. Here, we investigate whether local modulation of plasticity can also occur between glutamatergic synapses and adjacent GABAergic synapses. We find that the induction of long-term potentiation at an individual glutamatergic spine causes the depression of nearby GABAergic inhibitory synapses (within 3 μm), whereas more distant ones are potentiated. Notably, L-type calcium channels and calpain are required for this plasticity spreading. Overall, our data support a model whereby input-specific glutamatergic postsynaptic potentiation induces a spatially regulated rearrangement of inhibitory synaptic strength in the surrounding area through short-range heterosynaptic interactions. Such local coordination of excitatory and inhibitory synaptic plasticity is expected to influence dendritic information processing and integration.

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LTP of individual dendritic spines causes iLTD at neighboring GABAergic synapses

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Interaction between single-spine LTP and iLTD occurs in the spatial range of ±3 μm

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This iLTD depends on the local dendritic calcium increase and calpain activation

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iLTD is associated with reduced gephyrin clustering and increased GABAAR mobility

Preserved motor memory in Parkinson's disease

Patients with Parkinson's disease, who lose the dopaminergic projections to the striatum, are impaired in certain aspects of motor learning. Recent evidence suggests that, in addition to its role in motor performance, the striatum plays a key role in the memory of motor learning. Whether Parkinson's patients have impaired motor memory and whether motor memory is modulated by dopamine at the time of initial learning is unknown. To address these questions, we measured memory of a learned motor sequence in Parkinson's patients who were either On or Off their dopaminergic medications at the time of initial learning. We compared them to a group of older and younger controls. Contrary to our predictions, motor memory was not impaired in patients compared to older controls, and was not influenced by dopamine state at the time of initial learning. To probe post-learning consolidation processes, we also tested whether learning a new sequence shortly after learning the initial sequence would interfere with later memory. We found that, in contrast to younger adults, neither older adults nor patients were susceptible to this interference. These findings suggest that motor memory is preserved in Parkinson's patients and raise the possibility that motor memory in patients is supported by compensatory non-dopamine sensitive mechanisms. Furthermore, given the similar performance characteristics observed in the patients and older adults and the absence of an effect of dopamine, these results raise the possibility that aging and Parkinson's disease affect motor memory in similar ways.

Forgetting as a form of adaptive engram cell plasticity

One leading hypothesis suggests that memories are stored in ensembles of neurons (or 'engram cells') and that successful recall involves reactivation of these ensembles. A logical extension of this idea is that forgetting occurs when engram cells cannot be reactivated. Forms of 'natural forgetting' vary considerably in terms of their underlying mechanisms, time course and reversibility. However, we suggest that all forms of forgetting involve circuit remodelling that switches engram cells from an accessible state (where they can be reactivated by natural recall cues) to an inaccessible state (where they cannot). In many cases, forgetting rates are modulated by environmental conditions and we therefore propose that forgetting is a form of neuroplasticity that alters engram cell accessibility in a manner that is sensitive to mismatches between expectations and the environment. Moreover, we hypothesize that disease states associated with forgetting may hijack natural forgetting mechanisms, resulting in reduced engram cell accessibility and memory loss.

Heterogeneity of glutamatergic synapses: cellular mechanisms and network consequences

Chemical synapses are commonly known as a structurally and functionally highly diverse class of cell-cell contacts specialized to mediate communication between neurons. They represent the smallest "computational" unit of the brain and are typically divided into excitatory and inhibitory as well as modulatory categories. These categories are subdivided into diverse types, each representing a different structure-function repertoire that in turn are thought to endow neuronal networks with distinct computational properties. The diversity of structure and function found among a given category of synapses is referred to as heterogeneity. The main building blocks for this heterogeneity are synaptic vesicles, the active zone, the synaptic cleft, the postsynaptic density, and glial processes associated with the synapse. Each of these five structural modules entails a distinct repertoire of functions, and their combination specifies the range of functional heterogeneity at mammalian excitatory synapses, which are the focus of this review. We describe synapse heterogeneity that is manifested on different levels of complexity ranging from the cellular morphology of the pre- and postsynaptic cells toward the expression of different protein isoforms at individual release sites. We attempt to define the range of structural building blocks that are used to vary the basic functional repertoire of excitatory synaptic contacts and discuss sources and general mechanisms of synapse heterogeneity. Finally, we explore the possible impact of synapse heterogeneity on neuronal network function.

Molecular Mechanisms of Memory Consolidation That Operate During Sleep

The role of sleep for brain function has been in the focus of interest for many years. It is now firmly established that sleep and the corresponding brain activity is of central importance for memory consolidation. Less clear are the underlying molecular mechanisms and their specific contribution to the formation of long-term memory. In this review, we summarize the current knowledge of such mechanisms and we discuss the several unknowns that hinder a deeper appreciation of how molecular mechanisms of memory consolidation during sleep impact synaptic function and engram formation.

The Hippocampal Horizon: Constructing and Segmenting Experience for Episodic Memory

How do we recollect specific events that have occurred during continuous ongoing experience? There is converging evidence from non-human animals that spatially modulated cellular activity of the hippocampal formation supports the construction of ongoing events. On the other hand, recent human oriented event cognition models have outlined that our experience is segmented into discrete units, and that such segmentation can operate on shorter or longer timescales. Here, we describe a unification of how these dynamic physiological mechanisms of the hippocampus relate to ongoing externally and internally driven event segmentation, facilitating the demarcation of specific moments during experience. Our cross-species interdisciplinary approach offers a novel perspective in the way we construct and remember specific events, leading to the generation of many new hypotheses for future research.

Post-training intra-basolateral complex of the amygdala infusions of clenbuterol enhance memory for conditioned place preference and increase ARC protein expression in dorsal hippocampal synaptic fractions

The basolateral complex of the amygdala (BLA) is critically involved in modulation of memory by stress hormones. Noradrenergic activation of the BLA enhances memory consolidation and plays a necessary role in the enhancing or impairing effects of stress hormones on memory. The BLA is not only involved in the consolidation of aversive memories but can regulate appetitive memory formation as well. Extensive evidence suggests that the BLA is a modulatory structure that influences consolidation of arousing memories through modulation of plasticity and expression of plasticity-related genes, such as the activity regulated cytoskeletal-associated (Arc/Arg 3.1) protein, in efferent brain regions. ARC is an immediate early gene whose mRNA is localized to the dendrites and is necessary for hippocampus-dependent long-term potentiation and long-term memory formation. Post-training intra-BLA infusions of the β-adrenoceptor agonist, clenbuterol, enhances memory for an aversive task and increases dorsal hippocampus ARC protein expression following training on that task. To examine whether this function of BLA noradrenergic signaling extends to the consolidation of appetitive memories, the present studies test the effect of post-training intra-BLA infusions of clenbuterol on memory for the appetitive conditioned place preference (CPP) task and for effects on ARC protein expression in hippocampal synapses. Additionally, the necessity of increased hippocampal ARC protein expression was also examined for long-term memory formation of the CPP task. Immediate post-training intra-BLA infusions of clenbuterol (4 ng/0.2 µL) significantly enhanced memory for the CPP task. This same memory enhancing treatment significantly increased ARC protein expression in dorsal, but not ventral, hippocampal synaptic fractions. Furthermore, immediate post-training intra-dorsal hippocampal infusions of Arc antisense oligodeoxynucleotides (ODNs), which reduce ARC protein expression, prevented long-term memory formation for the CPP task. These results suggest that noradrenergic activity in the BLA influences long-term memory for aversive and appetitive events in a similar manner and the role of the BLA is conserved across classes of memory. It also suggests that the influence of the BLA on hippocampal ARC protein expression and the role of hippocampal ARC protein expression are conserved across classes of emotionally arousing memories.

Spaced training enhances memory and prefrontal ensemble stability in mice

It is commonly acknowledged that memory is substantially improved when learning is distributed over time, an effect called the "spacing effect". So far it has not been studied how spaced learning affects the neuronal ensembles presumably underlying memory. In the present study, we investigate whether trial spacing increases the stability or size of neuronal ensembles. Mice were trained in the "everyday memory" task, an appetitive, naturalistic, delayed matching-to-place task. Spacing trials by 60 min produced more robust memories than training with shorter or longer intervals. c-Fos labeling and chemogenetic inactivation established the involvement of the dorsomedial prefrontal cortex (dmPFC) in successful memory storage. In vivo calcium imaging of excitatory dmPFC neurons revealed that longer trial spacing increased the similarity of the population activity pattern on subsequent encoding trials and upon retrieval. Conversely, trial spacing did not affect the size of the total neuronal ensemble or the size of subpopulations dedicated to specific task-related behaviors and events. Thus, spaced learning promotes reactivation of prefrontal neuronal ensembles processing episodic-like memories.

Dimensions and mechanisms of memory organization

Memory formation is dynamic in nature, and acquisition of new information is often influenced by previous experiences. Memories sharing certain attributes are known to interact so that retrieval of one increases the likelihood of retrieving the other, raising the possibility that related memories are organized into associative mnemonic structures of interconnected representations. Although the formation and retrieval of single memories have been studied extensively, very little is known about the brain mechanisms that organize and link related memories. Here we review studies that suggest the existence of mnemonic structures in humans and animal models. These studies suggest three main dimensions of experience that can serve to organize related memories: time, space, and perceptual/conceptual similarities. We propose potential molecular, cellular, and systems mechanisms that might support organization of memories according to these dimensions.

Molecular mechanism of hippocampal long-term potentiation - Towards multiscale understanding of learning and memory

Long-term potentiation (LTP) of synaptic transmission is considered to be a cellular counterpart of learning and memory. Activation of postsynaptic NMDA type glutamate receptor (NMDA-R) induces trafficking of AMPA type glutamate receptors (AMPA-R) and other proteins to the synapse in sequential fashion. At the same time, the dendritic spine expands for long-term and modulation of actin underlies this (structural LTP or sLTP). How these changes persist despite constant diffusion and turnover of the component proteins have been the central focus of the current LTP research. Signaling triggered by Ca²⁺-influx via NMDA-R triggers kinase including Ca²⁺/calmodulin-dependent protein kinase II (CaMKII). CaMKII can sustain longer-term biochemical signaling by forming a reciprocally-activating kinase-effector complex with its substrate proteins including Tiam1, thereby regulating persistence of the downstream signaling. Furthermore, activated CaMKII can condense at the synapse through the mechanism of liquid-liquid phase separation (LLPS). This increases the binding capacity at the synapse, thereby contributing to the maintenance of enlarged protein complexes. It may also serve as the synapse tag, which captures newly synthesized proteins.

Memory consolidation as an adaptive process

We rely on our long-term memories to guide future behaviors, making it adaptive to prioritize the retention of goal-relevant, salient information in memory. In this review, we discuss findings from rodent and human research to demonstrate that active processes during post-encoding consolidation support the selective stabilization of recent experience into adaptive, long-term memories. Building upon literatures focused on dynamics at the cellular level, we highlight that consolidation also transforms memories at the systems level to support future goal-relevant behavior, resulting in more generalized memory traces in the brain and behavior. We synthesize previous literatures spanning animal research, human cognitive neuroscience, and cognitive psychology to propose an integrative framework for adaptive consolidation by which goal-relevant memoranda are "tagged" for subsequent consolidation, resulting in selective transformations to the structure of memories that support flexible, goal-relevant behaviors.

Anticipation of novel environments enhances memory for incidental information

Novelty is a potent driver of learning, but little is known about whether anticipation of novelty can enhance memory for incidental information. Here, participants incidentally encountered objects while they actively navigated toward novel or previously familiarized virtual rooms. Across immediate and delayed surprise memory tests, participants showed superior recollection for incidental objects encountered while anticipating novel as compared with familiarized rooms. Furthermore, memory for incidental objects correlated positively with between-participants average curiosity about novel rooms but negatively with within-participants trial-specific curiosity. Our findings contribute to the growing literature on how salient processes impact memory for incidental material.

Dopamine modulations of reward-driven music memory consolidation

Music listening provides one of the most significant abstract rewards for humans because hearing music activates the dopaminergic mesolimbic system. Given the strong link between reward, dopamine, and memory, we aimed here to investigate the hypothesis that dopamine-dependent musical reward can drive memory improvements. Twenty-nine healthy participants of both sexes provided reward ratings of unfamiliar musical excerpts that had to be remembered following a consolidation period under three separate conditions: after the ingestion of a dopaminergic antagonist, a dopaminergic precursor, or a placebo. Linear mixed modeling of the intervention data showed that the effect of reward on memory-i.e., the greater the reward experienced while listening to the musical excerpts, the better the memory recollection performance-was modulated by both dopaminergic signaling and individual differences in reward processing. Greater pleasure was consistently associated with better memory outcomes in participants with high sensitivity to musical reward, but this effect was lost when dopaminergic signaling was disrupted in participants with average or low musical hedonia. Our work highlights the flexibility of the human dopaminergic system, which can enhance memory formation not only through explicit and/or primary reinforcers but also via abstract and aesthetic rewards such as music.

Emergence of local and global synaptic organization on cortical dendrites

Synaptic inputs on cortical dendrites are organized with remarkable subcellular precision at the micron level. This organization emerges during early postnatal development through patterned spontaneous activity and manifests both locally where nearby synapses are significantly correlated, and globally with distance to the soma. We propose a biophysically motivated synaptic plasticity model to dissect the mechanistic origins of this organization during development and elucidate synaptic clustering of different stimulus features in the adult. Our model captures local clustering of orientation in ferret and receptive field overlap in mouse visual cortex based on the receptive field diameter and the cortical magnification of visual space. Including action potential back-propagation explains branch clustering heterogeneity in the ferret and produces a global retinotopy gradient from soma to dendrite in the mouse. Therefore, by combining activity-dependent synaptic competition and species-specific receptive fields, our framework explains different aspects of synaptic organization regarding stimulus features and spatial scales.

RGS14 Regulation of Post-Synaptic Signaling and Spine Plasticity in Brain

The regulator of G-protein signaling 14 (RGS14) is a multifunctional signaling protein that regulates post synaptic plasticity in neurons. RGS14 is expressed in the brain regions essential for learning, memory, emotion, and stimulus-induced behaviors, including the basal ganglia, limbic system, and cortex. Behaviorally, RGS14 regulates spatial and object memory, female-specific responses to cued fear conditioning, and environmental- and psychostimulant-induced locomotion. At the cellular level, RGS14 acts as a scaffolding protein that integrates G protein, Ras/ERK, and calcium/calmodulin signaling pathways essential for spine plasticity and cell signaling, allowing RGS14 to naturally suppress long-term potentiation (LTP) and structural plasticity in hippocampal area CA2 pyramidal cells. Recent proteomics findings indicate that RGS14 also engages the actomyosin system in the brain, perhaps to impact spine morphogenesis. Of note, RGS14 is also a nucleocytoplasmic shuttling protein, where its role in the nucleus remains uncertain. Balanced nuclear import/export and dendritic spine localization are likely essential for RGS14 neuronal functions as a regulator of synaptic plasticity. Supporting this idea, human genetic variants disrupting RGS14 localization also disrupt RGS14’s effects on plasticity. This review will focus on the known and unexplored roles of RGS14 in cell signaling, physiology, disease and behavior.

Long-term plasticity in the hippocampus: maintaining within and 'tagging' between synapses

Synapses between neurons are malleable biochemical structures, strengthening and diminishing over time dependent on the type of information they receive. This phenomenon known as synaptic plasticity underlies learning and memory, and its different forms, long-term potentiation (LTP) and long-term depression (LTD), perform varied cognitive roles in reinforcement, relearning and associating memories. Moreover, both LTP and LTD can exist in an early transient form (early-LTP/LTD) or a late persistent form (late-LTP/LTD), which are triggered by different induction protocols, and also differ in their dependence on protein synthesis and the involvement of key molecular players. Beyond homosynaptic modifications, synapses can also interact with one another. This is encapsulated in the synaptic tagging and capture hypothesis (STC), where synapses expressing early-LTP/LTD present a 'tag' that can capture the protein synthesis products generated during a temporally proximal late-LTP/LTD induction. This 'tagging' phenomenon forms the framework of synaptic interactions in various conditions and accounts for the cellular basis of the time-dependent associativity of short-lasting and long-lasting memories. All these synaptic modifications take place under controlled neuronal conditions, regulated by subcellular elements such as epigenetic regulation, proteasomal degradation and neuromodulatory signals. Here, we review current understanding of the different forms of synaptic plasticity and its regulatory mechanisms in the hippocampus, a brain region critical for memory formation. We also discuss expression of plasticity in hippocampal CA2 area, a long-overlooked narrow hippocampal subfield and the behavioural correlate of STC. Lastly, we put forth perspectives for an integrated view of memory representation in synapses.

The effect of acute aerobic exercise on the consolidation of motor memories

Acute aerobic exercise performed prior to training may assist with motor skill acquisition through enhancement of motor cortical plasticity. In addition, high-intensity exercise performed after training improves retention, although the mechanisms of this are unclear. We hypothesized that acute continuous moderate-intensity exercise performed post-motor training would also assist with motor skill retention and that this behavioral change would be positively correlated with neural markers of training-related cortical adaptation. Participants [n = 33; assigned to an exercise (EXE) or control (CON) group] completed a single visuomotor training session using bilateral wrist movements while movement-related cortical potentials (MRCPs) were collected. After motor training, the EXE group exercised for 20 min [70% of heart rate reserve (HRR)] and the CON group read for the same amount of time. Both groups completed two post-training tests after exercise/rest: 10 min and ~ 30 min once heart rate returned to resting level in EXE. Retention and transfer tests were both completed 1 and 7 days later. MRCPs measured training-related neural adaptations during the first visit and motor performance was assessed as time and trajectory to the target. The EXE group had better performance than CON at retention (significant 7 days post-training). MRCP amplitudes increased from early to late motor training and this amplitude change was correlated with motor performance at retention. Results suggest that moderate-intensity exercise post-motor training helps motor skill retention and that there may be a relationship with motor training-related cortical adaptations that is enhanced with post-motor training exercise.

Emotional learning retroactively enhances item memory but distorts source attribution

An adaptive memory system should prioritize information surrounding a powerful learning event that may prove useful for predicting future meaningful events. The behavioral tagging hypothesis provides a mechanistic framework to interpret how weak experiences persist as durable memories through temporal association with a strong experience. Memories are composed of multiple elements, and different mnemonic aspects of the same experience may be uniquely affected by mechanisms that retroactively modulate a weakly encoded memory. Here, we investigated how emotional learning affects item and source memory for related events encoded close in time. Participants encoded trial-unique category exemplars before, during, and after Pavlovian fear conditioning. Selective retroactive enhancements in 24-h item memory were accompanied by a bias to misattribute items to the temporal context of fear conditioning. The strength of this source memory bias correlated with participants' retroactive item memory enhancement, and source misattribution to the emotional context predicted whether items were remembered overall. In the framework of behavioral tagging: Memory attribution was biased to the temporal context of the stronger event that provided the putative source of memory stabilization for the weaker event. We additionally found that fear conditioning selectively and retroactively enhanced stimulus typicality ratings for related items, and that stimulus typicality also predicted overall item memory. Collectively, these results provide new evidence that items related to emotional learning are misattributed to the temporal context of the emotional event and judged to be more representative of their semantic category. Both processes may facilitate memory retrieval for related events encoded close in time.

'Sleep-dependent' memory consolidation? Brief periods of post-training rest and sleep provide an equivalent benefit for both declarative and procedural memory

Sleep following learning facilitates the consolidation of memories. This effect has often been attributed to sleep-specific factors, such as the presence of sleep spindles or slow waves in the electroencephalogram (EEG). However, recent studies suggest that simply resting quietly while awake could confer a similar memory benefit. In the current study, we examined the effects of sleep, quiet rest, and active wakefulness on the consolidation of declarative and procedural memory. We hypothesized that sleep and eyes-closed quiet rest would both benefit memory compared with a period of active wakefulness. After completing a declarative and a procedural memory task, participants began a 30-min retention period with PSG (polysomnographic) monitoring, in which they either slept (n = 24), quietly rested with their eyes closed (n = 22), or completed a distractor task (n = 29). Following the retention period, participants were again tested on their memory for the two learning tasks. As hypothesized, sleep and quiet rest both led to better performance on the declarative and procedural memory tasks than did the distractor task. Moreover, the performance advantages conferred by rest were indistinguishable from those of sleep. These data suggest that neurobiology specific to sleep might not be necessary to induce the consolidation of memory, at least across very short retention intervals. Instead, offline memory consolidation may function opportunistically, occurring during either sleep or stimulus-free rest, provided a favorable neurobiological milieu and sufficient reduction of new encoding.

Sleep and Memory Consolidation Dysfunction in Psychiatric Disorders: Evidence for the Involvement of Extracellular Matrix Molecules

Sleep disturbances and memory dysfunction are key characteristics across psychiatric disorders. Recent advances have revealed insight into the role of sleep in memory consolidation, pointing to key overlap between memory consolidation processes and structural and molecular abnormalities in psychiatric disorders. Ongoing research regarding the molecular mechanisms involved in memory consolidation has the potential to identify therapeutic targets for memory dysfunction in psychiatric disorders and aging. Recent evidence from our group and others points to extracellular matrix molecules, including chondroitin sulfate proteoglycans and their endogenous proteases, as molecules that may underlie synaptic dysfunction in psychiatric disorders and memory consolidation during sleep. These molecules may provide a therapeutic targets for decreasing strength of reward memories in addiction and traumatic memories in PTSD, as well as restoring deficits in memory consolidation in schizophrenia and aging. We review the evidence for sleep and memory consolidation dysfunction in psychiatric disorders and aging in the context of current evidence pointing to the involvement of extracellular matrix molecules in these processes.

Behavioral tagging as a mechanism for aversive-memory formation under acute stress

The behavioral tagging (BT) hypothesis postulates that a weak learning experience, which only induces short-term memory, may benefit from another event that provides plasticity-related proteins (PRPs) to establish a long-lasting memory. According to BT, the weak experience sets a transient learning tag at specific activated sites, and its temporal and spatial convergence with the PRPs allows the long-term memory (LTM) formation. In this work, rats were subjected to a weak inhibitory avoidance (IAw) training and we observed that acute stress (elevated platform, EP) experienced 1 hr before IAw promoted IA-LTM formation. This effect was dependent on glucocorticoid-receptor activity as well as protein synthesis in the dorsal hippocampus. However, the same stress has negative effects on IA-LTM formation when training is strong, probably by competing for necessary PRPs. Furthermore, our experiments showed that EP immediately after training did not impair the setting of the learning tag and even facilitated IA-LTM formation. These findings reveal different impacts of a given acute stressful experience on the formation of an aversive memory that could be explained by BT processes.

Upregulation of eIF4E, but not other translation initiation factors, in dendritic spines during memory formation

Local translation can provide a rapid, spatially targeted supply of new proteins in distal dendrites to support synaptic changes that underlie learning. Learning and memory are especially sensitive to manipulations of translational control mechanisms, particularly those that target the initiation step, and translation initiation at synapses could be a means of maintaining synapse specificity during plasticity. Initiation predominantly occurs via recruitment of ribosomes to the 5' mRNA cap by complexes of eukaryotic initiation factors (eIFs), and the interaction between eIF4E and eIF4G1 is a particularly important target of translational control pathways. Pharmacological inhibition of eIF4E-eIF4G1 binding impairs formation of memory for aversive Pavlovian conditioning as well as the accompanying increase in polyribosomes in the heads of dendritic spines in the lateral amygdala (LA). This is consistent with a role for initiation at synapses in memory formation, but whether eIFs are even present near synapses is unknown. To determine whether dendritic spines contain eIFs and whether eIF distribution is affected by learning, we combined immunolabeling with serial section transmission electron microscopy (ssTEM) volume reconstructions of LA dendrites after Pavlovian conditioning. Labeling for eIF4E, eIF4G1, and eIF2α-another key target of regulation-occurred in roughly half of dendritic spines, but learning effects were only found for eIF4E, which was upregulated in the heads of dendritic spines. Our results support the possibility of regulated translation initiation as a means of synapse-specific protein targeting during learning and are consistent with the model of eIF4E availability as a central point of control.

Protein-protein interactions at the NMDA receptor complex: From synaptic retention to synaptonuclear protein messengers

N-methyl-d-aspartate receptors (NMDARs) are glutamate-gated ion channels that support essential functions throughout the brain. NMDARs are tetramers composed of the GluN1 subunit in complex with GluN2- and GluN3-type regulatory subunits, resulting in the formation of various receptor subtypes throughout the central nervous system (CNS), characterised by different kinetics, biophysical and pharmacological properties, and the abilities to interact with specific partners at dendritic spines. NMDARs are expressed at high levels, are widely distributed throughout the brain, and are involved in several physiological and pathological conditions. Here, we will focus on the GluN2A- and GluN2B-containing NMDARs found at excitatory synapses and their interactions with plasticity-relevant proteins, such as the postsynaptic density family of membrane-associated guanylate kinases (PSD-MAGUKs), Ca²⁺/calmodulin-dependent kinase II (CaMKII) and synaptonuclear protein messengers. The dynamic interactions between NMDAR subunits and various proteins regulating synaptic receptor retention and synaptonuclear signalling mediated by protein messengers suggest that the NMDAR serves as a key molecular player that coordinates synaptic activity and cell-wide events that require gene transcription. Importantly, protein-protein interactions at the NMDAR complex can also contribute to synaptic dysfunction in several brain disorders. Therefore, the modulation of the molecular composition of the NMDAR complex might represent a novel pharmacological approach for the treatment of certain disease states.

The Making of Long-Lasting Memories: A Fruit Fly Perspective

Understanding the nature of the molecular mechanisms underlying memory formation, consolidation, and forgetting are some of the fascinating questions in modern neuroscience. The encoding, stabilization and elimination of memories, rely on the structural reorganization of synapses. These changes will enable the facilitation or depression of neural activity in response to the acquisition of new information. In other words, these changes affect the weight of specific nodes within a neural network. We know that these plastic reorganizations require de novo protein synthesis in the context of Long-term memory (LTM). This process depends on neural activity triggered by the learned experience. The use of model organisms like Drosophila melanogaster has been proven essential for advancing our knowledge in the field of neuroscience. Flies offer an optimal combination of a more straightforward nervous system, composed of a limited number of cells, and while still displaying complex behaviors. Studies in Drosophila neuroscience, which expanded over several decades, have been critical for understanding the cellular and molecular mechanisms leading to the synaptic and behavioral plasticity occurring in the context of learning and memory. This is possible thanks to sophisticated technical approaches that enable precise control of gene expression in the fruit fly as well as neural manipulation, like chemogenetics, thermogenetics, or optogenetics. The search for the identity of genes expressed as a result of memory acquisition has been an active interest since the origins of behavioral genetics. From screenings of more or less specific candidates to broader studies based on transcriptome analysis, our understanding of the genetic control behind LTM has expanded exponentially in the past years. Here we review recent literature regarding how the formation of memories induces a rapid, extensive and, in many cases, transient wave of transcriptional activity. After a consolidation period, transcriptome changes seem more stable and likely represent the synthesis of new proteins. The complexity of the circuitry involved in memory formation and consolidation is such that there are localized changes in neural activity, both regarding temporal dynamics and the nature of neurons and subcellular locations affected, hence inducing specific temporal and localized changes in protein expression. Different types of neurons are recruited at different times into memory traces. In LTM, the synthesis of new proteins is required in specific subsets of cells. This de novo translation can take place in the somatic cytoplasm and/or locally in distinct zones of compartmentalized synaptic activity, depending on the nature of the proteins and the plasticity-inducing processes that occur. We will also review recent advances in understanding how localized changes are confined to the relevant synapse. These recent studies have led to exciting discoveries regarding proteins that were not previously involved in learning and memory processes. This invaluable information will lead to future functional studies on the roles that hundreds of new molecular actors play in modulating neural activity.

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.

CA2 beyond social memory: Evidence for a fundamental role in hippocampal information processing

Hippocampal region CA2 has received increased attention due to its importance in social recognition memory. While its specific function remains to be identified, there are indications that CA2 plays a major role in a variety of situations, widely extending beyond social memory. In this targeted review, we highlight lines of research which have begun to converge on a more fundamental role for CA2 in hippocampus-dependent memory processing. We discuss recent proposals that speak to the computations CA2 may perform within the hippocampal circuit.

Transient Astrocytic Gq Signaling Underlies Remote Memory Enhancement

Astrocytes elicit transient Ca2+ elevations induced by G protein-coupled receptors (GPCRs), yet their role in vivo remains unknown. To address this, transgenic mice with astrocytic expression of the optogenetic Gq-type GPCR, Optoα1AR, were established, in which transient Ca2+ elevations similar to those in wild type mice were induced by brief blue light illumination. Activation of cortical astrocytes resulted in an adenosine A1 receptor-dependent inhibition of neuronal activity. Moreover, sensory stimulation with astrocytic activation induced long-term depression of sensory evoked response. At the behavioral level, repeated astrocytic activation in the anterior cortex gradually affected novel open field exploratory behavior, and remote memory was enhanced in a novel object recognition task. These effects were blocked by A1 receptor antagonism. Together, we demonstrate that GPCR-triggered Ca2+ elevation in cortical astrocytes has causal impacts on neuronal activity and behavior.

Orexin/Hypocretin and MCH Neurons: Cognitive and Motor Roles Beyond Arousal

The lateral hypothalamus (LH) is classically implicated in sleep-wake control. It is the main source of orexin/hypocretin and melanin-concentrating hormone (MCH) neuropeptides in the brain, which have been both implicated in arousal state switching. These neuropeptides are produced by non-overlapping LH neurons, which both project widely throughout the brain, where release of orexin and MCH activates specific postsynaptic G-protein-coupled receptors. Optogenetic manipulations of orexin and MCH neurons during sleep indicate that they promote awakening and REM sleep, respectively. However, recordings from orexin and MCH neurons in awake, moving animals suggest that they also act outside sleep/wake switching. Here, we review recent studies showing that both orexin and MCH neurons can rapidly (sub-second-timescale) change their firing when awake animals experience external stimuli, or during self-paced exploration of objects and places. However, the sensory-behavioral correlates of orexin and MCH neural activation can be quite different. Orexin neurons are generally more dynamic, with about 2/3rds of them activated before and during self-initiated running, and most activated by sensory stimulation across sensory modalities. MCH neurons are activated in a more select manner, for example upon self-paced investigation of novel objects and by certain other novel stimuli. We discuss optogenetic and chemogenetic manipulations of orexin and MCH neurons, which combined with pharmacological blockade of orexin and MCH receptors, imply that these rapid LH dynamics shape fundamental cognitive and motor processes due to orexin and MCH neuropeptide actions in the awake brain. Finally, we contemplate whether the awake control of psychomotor brain functions by orexin and MCH are distinct from their “arousal” effects.

Synaptic plasticity as Bayesian inference

Learning, especially rapid learning, is critical for survival. However, learning is hard; a large number of synaptic weights must be set based on noisy, often ambiguous, sensory information. In such a high-noise regime, keeping track of probability distributions over weights is the optimal strategy. Here we hypothesize that synapses take that strategy; in essence, when they estimate weights, they include error bars. They then use that uncertainty to adjust their learning rates, with more uncertain weights having higher learning rates. We also make a second, independent, hypothesis: synapses communicate their uncertainty by linking it to variability in postsynaptic potential size, with more uncertainty leading to more variability. These two hypotheses cast synaptic plasticity as a problem of Bayesian inference, and thus provide a normative view of learning. They generalize known learning rules, offer an explanation for the large variability in the size of postsynaptic potentials and make falsifiable experimental predictions.

Memory consolidation and improvement by synaptic tagging and capture in recurrent neural networks

The synaptic-tagging-and-capture (STC) hypothesis formulates that at each synapse the concurrence of a tag with protein synthesis yields the maintenance of changes induced by synaptic plasticity. This hypothesis provides a biological principle underlying the synaptic consolidation of memories that is not verified for recurrent neural circuits. We developed a theoretical model integrating the mechanisms underlying the STC hypothesis with calcium-based synaptic plasticity in a recurrent spiking neural network. In the model, calcium-based synaptic plasticity yields the formation of strongly interconnected cell assemblies encoding memories, followed by consolidation through the STC mechanisms. Furthermore, we show for the first time that STC mechanisms modify the storage of memories such that after several hours memory recall is significantly improved. We identify two contributing processes: a merely time-dependent passive improvement, and an active improvement during recall. The described characteristics can provide a new principle for storing information in biological and artificial neural circuits.

Durable memories and efficient neural coding through mnemonic training using the method of loci

Mnemonic techniques, such as the method of loci, can powerfully boost memory. We compared memory athletes ranked among the world's top 50 in memory sports to mnemonics-naïve controls. In a second study, participants completed a 6-week memory training, working memory training, or no intervention. Behaviorally, memory training enhanced durable, longer-lasting memories. Functional magnetic resonance imaging during encoding and recognition revealed task-based activation decreases in lateral prefrontal, as well as in parahippocampal and retrosplenial cortices in both memory athletes and participants after memory training, partly associated with better performance after 4 months. This was complemented by hippocampal-neocortical coupling during consolidation, which was stronger the more durable memories participants formed. Our findings advance knowledge on how mnemonic training boosts durable memory formation through decreased task-based activation and increased consolidation thereafter. This is in line with conceptual accounts of neural efficiency and highlights a complex interplay of neural processes critical for extraordinary memory.

A predictive account of how novelty influences declarative memory

A rich body of studies in the human and non-human literature has examined the question how novelty influences memory. For a variety of different stimuli, ranging from simple objects and words to vastly complex scenarios, the literature reports that novelty improves memory in some cases, but impairs memory in other cases. In recent attempts to reconcile these conflicting findings, novelty has been divided into different subtypes, such as relative versus absolute novelty, or stimulus versus contextual novelty. Nevertheless, a single overarching theory of novelty and memory has been difficult to attain, probably due to the complexities in the interactions among stimuli, environmental factors (e.g., spatial and temporal context) and level of prior knowledge (but see Duszkiewicz et al., 2019; Kafkas & Montaldi, 2018b; Schomaker & Meeter, 2015). Here we describe how a predictive coding framework might be able to shed new light on different types of novelty and how they affect declarative memory in humans. More precisely, we consider how prior expectations modulate the influence of novelty on encoding episodes into memory, e.g., in terms of surprise, and how novelty/surprise affect memory for surrounding information. By reviewing a range of behavioural findings and their possible underlying neurobiological mechanisms, we highlight where a predictive coding framework succeeds and where it appears to struggle.

The biophysical basis underlying the maintenance of early phase long-term potentiation

The maintenance of synaptic changes resulting from long-term potentiation (LTP) is essential for brain function such as memory and learning. Different LTP phases have been associated with diverse molecular processes and pathways, and the molecular underpinnings of LTP on the short, as well as long time scales, are well established. However, the principles on the intermediate time scale of 1-6 hours that mediate the early phase of LTP (E-LTP) remain elusive. We hypothesize that the interplay between specific features of postsynaptic receptor trafficking is responsible for sustaining synaptic changes during this LTP phase. We test this hypothesis by formalizing a biophysical model that integrates several experimentally-motivated mechanisms. The model captures a wide range of experimental findings and predicts that synaptic changes are preserved for hours when the receptor dynamics are shaped by the interplay of structural changes of the spine in conjunction with increased trafficking from recycling endosomes and the cooperative binding of receptors. Furthermore, our model provides several predictions to verify our findings experimentally.

The Mechanical Basis of Memory - the MeshCODE Theory

One of the major unsolved mysteries of biological science concerns the question of where and in what form information is stored in the brain. I propose that memory is stored in the brain in a mechanically encoded binary format written into the conformations of proteins found in the cell-extracellular matrix (ECM) adhesions that organise each and every synapse. The MeshCODE framework outlined here represents a unifying theory of data storage in animals, providing read-write storage of both dynamic and persistent information in a binary format. Mechanosensitive proteins that contain force-dependent switches can store information persistently, which can be written or updated using small changes in mechanical force. These mechanosensitive proteins, such as talin, scaffold each synapse, creating a meshwork of switches that together form a code, the so-called MeshCODE. Large signalling complexes assemble on these scaffolds as a function of the switch patterns and these complexes would both stabilise the patterns and coordinate synaptic regulators to dynamically tune synaptic activity. Synaptic transmission and action potential spike trains would operate the cytoskeletal machinery to write and update the synaptic MeshCODEs, thereby propagating this coding throughout the organism. Based on established biophysical principles, such a mechanical basis for memory would provide a physical location for data storage in the brain, with the binary patterns, encoded in the information-storing mechanosensitive molecules in the synaptic scaffolds, and the complexes that form on them, representing the physical location of engrams. Furthermore, the conversion and storage of sensory and temporal inputs into a binary format would constitute an addressable read-write memory system, supporting the view of the mind as an organic supercomputer.

Analysis of memory modulation by conditioned stimuli

Conditioned stimuli (CS) have multiple psychological functions that can potentially contribute to their effect on memory formation. It is generally believed that CS-induced memory modulation is primarily due to conditioned emotional responses, however, well-learned CSs not only generate the appropriate behavioral and physiological reactions required to best respond to an upcoming unconditioned stimulus (US), but they also serve as signals that the US is about to occur. Therefore, it is possible that CSs can impact memory consolidation even when their ability to elicit conditioned emotional arousal is significantly reduced. To test this, male Sprague–Dawley rats trained on a signaled active avoidance task were divided into “Avoider” and “Non-Avoider” subgroups on the basis of percentage avoidance after 6 d of training. Subgroup differences in responding to the CS complex were maintained during a test carried out in the absence of the US. Moreover, the subgroups displayed significant differences in stress-induced analgesia (hot-plate test) immediately after this test, suggesting significant subgroup differences in conditioned emotionality. Importantly, using the spontaneous object recognition task, it was found that immediate post-sample exposure to the avoidance CS complex had a similar enhancing effect on object memory in the two subgroups. Therefore, to our knowledge, this is the first study to demonstrate that a significant conditioned emotional response is not necessary for the action of a predictive CS on modulation of memory consolidation.

Retrograde enhancement of episodic learning by a postlearning stimulus

Evidence suggests encoding of recent episodic experiences may be enhanced by a subsequent salient event. We tested this hypothesis by giving rats a 3-min unsupervised experience with four odors and measuring retention after different delays. Animals recognized that a novel element had been introduced to the odor set at 24 but not 48 h. However, when odor sampling was followed within 5 min by salient light flashes or bedding odor, the memory lasted a full 2 d. These results describe a retroactive influence of salience to promote storage of episodic information and introduce a unique model for studying underlying plasticity mechanisms.

Energetics of stochastic BCM type synaptic plasticity and storing of accurate information

Excitatory synaptic signaling in cortical circuits is thought to be metabolically expensive. Two fundamental brain functions, learning and memory, are associated with long-term synaptic plasticity, but we know very little about energetics of these slow biophysical processes. This study investigates the energy requirement of information storing in plastic synapses for an extended version of BCM plasticity with a decay term, stochastic noise, and nonlinear dependence of neuron’s firing rate on synaptic current (adaptation). It is shown that synaptic weights in this model exhibit bistability. In order to analyze the system analytically, it is reduced to a simple dynamic mean-field for a population averaged plastic synaptic current. Next, using the concepts of nonequilibrium thermodynamics, we derive the energy rate (entropy production rate) for plastic synapses and a corresponding Fisher information for coding presynaptic input. That energy, which is of chemical origin, is primarily used for battling fluctuations in the synaptic weights and presynaptic firing rates, and it increases steeply with synaptic weights, and more uniformly though nonlinearly with presynaptic firing. At the onset of synaptic bistability, Fisher information and memory lifetime both increase sharply, by a few orders of magnitude, but the plasticity energy rate changes only mildly. This implies that a huge gain in the precision of stored information does not have to cost large amounts of metabolic energy, which suggests that synaptic information is not directly limited by energy consumption. Interestingly, for very weak synaptic noise, such a limit on synaptic coding accuracy is imposed instead by a derivative of the plasticity energy rate with respect to the mean presynaptic firing, and this relationship has a general character that is independent of the plasticity type. An estimate for primate neocortex reveals that a relative metabolic cost of BCM type synaptic plasticity, as a fraction of neuronal cost related to fast synaptic transmission and spiking, can vary from negligible to substantial, depending on the synaptic noise level and presynaptic firing.

Age-related changes in hippocampal-dependent synaptic plasticity and memory mediated by p75 neurotrophin receptor

The plasticity mechanisms in the nervous system that are important for learning and memory are greatly impacted during aging. Notably, hippocampal-dependent long-term plasticity and its associative plasticity, such as synaptic tagging and capture (STC), show considerable age-related decline. The p75 neurotrophin receptor (p75NTR ) is a negative regulator of structural and functional plasticity in the brain and thus represents a potential candidate to mediate age-related alterations. However, the mechanisms by which p75NTR affects synaptic plasticity of aged neuronal networks and ultimately contribute to deficits in cognitive function have not been well characterized. Here, we report that mutant mice lacking the p75NTR were resistant to age-associated changes in long-term plasticity, associative plasticity, and associative memory. Our study shows that p75NTR is responsible for age-dependent disruption of hippocampal homeostatic plasticity by modulating several signaling pathways, including BDNF, MAPK, Arc, and RhoA-ROCK2-LIMK1-cofilin. p75NTR may thus represent an important therapeutic target for limiting the age-related memory and cognitive function deficits.

Further evidence that CP-AMPARs are critically involved in synaptic tag and capture at hippocampal CA1 synapses

The synaptic tag and capture (STC) hypothesis provides an important theoretical basis for understanding the synaptic basis of associative learning. We recently provided pharmacological evidence that calcium-permeable AMPA receptors (CP-AMPARs) are a crucial component of this form of heterosynaptic metaplasticity. Here we have investigated two predictions that arise on the basis of CP-AMPARs serving as a trigger of STC. Firstly, we compared the effects of the order in which we delivered a strong theta burst stimulation (TBS) protocol (75 pulses) and a weak TBS protocol (15 pulses) to two independent inputs. We only observed significant heterosynaptic metaplasticity when the strong TBS preceded the weak TBS. Second, we found that pausing stimulation following either the sTBS or the wTBS for ~20 min largely eliminates the heterosynaptic metaplasticity. These observations are consistent with a process that is triggered by the synaptic insertion of CP-AMPARs and provide a framework for establishing the underlying molecular mechanisms.

Roles of palmitoylation in structural long-term synaptic plasticity

Long-term potentiation (LTP) and long-term depression (LTD) are important cellular mechanisms underlying learning and memory processes. N-Methyl-d-aspartate receptor (NMDAR)-dependent LTP and LTD play especially crucial roles in these functions, and their expression depends on changes in the number and single channel conductance of the major ionotropic glutamate receptor α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR) located on the postsynaptic membrane. Structural changes in dendritic spines comprise the morphological platform and support for molecular changes in the execution of synaptic plasticity and memory storage. At the molecular level, spine morphology is directly determined by actin cytoskeleton organization within the spine and indirectly stabilized and consolidated by scaffold proteins at the spine head. Palmitoylation, as a uniquely reversible lipid modification with the ability to regulate protein membrane localization and trafficking, plays significant roles in the structural and functional regulation of LTP and LTD. Altered structural plasticity of dendritic spines is also considered a hallmark of neurodevelopmental disorders, while genetic evidence strongly links abnormal brain function to impaired palmitoylation. Numerous studies have indicated that palmitoylation contributes to morphological spine modifications. In this review, we have gathered data showing that the regulatory proteins that modulate the actin network and scaffold proteins related to AMPAR-mediated neurotransmission also undergo palmitoylation and play roles in modifying spine architecture during structural plasticity.