Making memories last: the synaptic tagging and capture hypothesis

New waves in dendritic spine actin cytoskeleton: From branches and bundles to rings, from actin binding proteins to post-translational modifications

Dendritic spines are small actin-rich protrusions from neuronal dendrites that form the postsynaptic part of most excitatory synapses. Changes in the number or strength of synapses are physiological mechanisms behind learning. The growth and maturation of dendritic spines and the activity-induced changes to their morphology are all based on changes to the actin cytoskeleton. In this review, we will discuss the regulation of the actin cytoskeleton in dendritic spine formation and maturation, as well as in synaptic strengthening. Concerning spine formation, we will focus on spine initiation, which has received less attention in the literature. We will also examine the recently revealed regulation of the actin cytoskeleton through post-translational modifications of actin monomers, in addition to the conventional regulation of actin via actin-binding proteins.

'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.

Cell-Specific PKM Isoforms Contribute to the Maintenance of Different Forms of Persistent Long-Term Synaptic Plasticity

Multiple kinase activations contribute to long-term synaptic plasticity, a cellular mechanism mediating long-term memory. The sensorimotor synapse of Aplysia expresses different forms of long-term facilitation (LTF)-nonassociative and associative LTF-that require the timely activation of kinases, including protein kinase C (PKC). It is not known which PKC isoforms in the sensory neuron or motor neuron L7 are required to sustain each form of LTF. We show that different PKMs, the constitutively active isoforms of PKCs generated by calpain cleavage, in the sensory neuron and L7 are required to maintain each form of LTF. Different PKMs or calpain isoforms were blocked by overexpressing specific dominant-negative constructs in either presynaptic or postsynaptic neurons. Blocking either PKM Apl I in L7, or PKM Apl II or PKM Apl III in the sensory neuron 2 d after 5-hydroxytryptamine (5-HT) treatment reversed persistent nonassociative LTF. In contrast, blocking either PKM Apl II or PKM Apl III in L7, or PKM Apl II in the sensory neuron 2 d after paired stimuli reversed persistent associative LTF. Blocking either classical calpain or atypical small optic lobe (SOL) calpain 2 d after 5-HT treatment or paired stimuli did not disrupt the maintenance of persistent LTF. Soon after 5-HT treatment or paired stimuli, however, blocking classical calpain inhibited the expression of persistent associative LTF, while blocking SOL calpain inhibited the expression of persistent nonassociative LTF. Our data suggest that different stimuli activate different calpains that generate specific sets of PKMs in each neuron whose constitutive activities sustain long-term synaptic plasticity.SIGNIFICANCE STATEMENT Persistent synaptic plasticity contributes to the maintenance of long-term memory. Although various kinases such as protein kinase C (PKC) contribute to the expression of long-term plasticity, little is known about how constitutive activation of specific kinase isoforms sustains long-term plasticity. This study provides evidence that the cell-specific activities of different PKM isoforms generated from PKCs by calpain-mediated cleavage maintain two forms of persistent synaptic plasticity, which are the cellular analogs of two forms of long-term memory. Moreover, we found that the activation of specific calpains depends on the features of the stimuli evoking the different forms of synaptic plasticity. Given the recent controversy over the role of PKMζ maintaining memory, these findings are significant in identifying roles of multiple PKMs in the retention of memory.

A saturation hypothesis to explain both enhanced and impaired learning with enhanced plasticity

Across many studies, animals with enhanced synaptic plasticity exhibit either enhanced or impaired learning, raising a conceptual puzzle: how enhanced plasticity can yield opposite learning outcomes? Here, we show that the recent history of experience can determine whether mice with enhanced plasticity exhibit enhanced or impaired learning in response to the same training. Mice with enhanced cerebellar LTD, due to double knockout (DKO) of MHCI H2-K^b/H2-D^b (K^bD^b−/−), exhibited oculomotor learning deficits. However, the same mice exhibited enhanced learning after appropriate pre-training. Theoretical analysis revealed that synapses with history-dependent learning rules could recapitulate the data, and suggested that saturation may be a key factor limiting the ability of enhanced plasticity to enhance learning. Optogenetic stimulation designed to saturate LTD produced the same impairment in WT as observed in DKO mice. Overall, our results suggest that the recent history of activity and the threshold for synaptic plasticity conspire to effect divergent learning outcomes.

DOI:http://dx.doi.org/10.7554/eLife.20147.001

All animals can learn from their experiences. One of the main ideas for how learning occurs is that it involves changes in the strength of the connections between neurons, known as synapses. The ability of synapses to become stronger or weaker is referred to as synaptic plasticity. High levels of synaptic plasticity are generally thought to be good for learning, while low levels of synaptic plasticity make learning more difficult.

Nevertheless, studies have also reported that high levels of synaptic plasticity can sometimes impair learning. To explain these mixed results, Nguyen-Vu, Zhao, Lahiri et al. studied mice that had been genetically modified to show greater synaptic plasticity than normal mice. The same individual mutant animals were sometimes less able to learn an eye-movement task than unmodified mice, and at other times better able to learn exactly the same task. The main factor that determined how well the mice could learn was what the mice had experienced shortly before they began the training.

Nguyen-Vu et al. propose that some experiences change the strength of synapses so much that they temporarily prevent those synapses from undergoing any further changes. Animals with these “saturated” synapses will struggle to learn a new task, even if their brains are normally capable of high levels of synaptic plasticity. Notably, even normal activity appears to be able to put the synapses of the mutant mice into a saturated state, whereas this saturation would only occur in normal mice under a restricted set of circumstances. Consistent with this idea, Nguyen-Vu et al. showed that a specific type of pre-training that desaturates synapses improved the ability of the modified mice to learn the eye-movement task. Conversely, a different procedure that is known to saturate synapses impaired the learning ability of the unmodified mice.

A future challenge is to test these predictions experimentally by measuring changes in synaptic plasticity directly, both in brain slices and in living animals. The results could ultimately help to develop treatments that improve the ability to learn and so could provide benefits to a wide range of individuals, including people who have suffered a brain injury or stroke.

DOI:http://dx.doi.org/10.7554/eLife.20147.002

Screening the Molecular Framework Underlying Local Dendritic mRNA Translation

In the last decade, bioinformatic analyses of high-throughput proteomics and transcriptomics data have enabled researchers to gain insight into the molecular networks that may underlie lasting changes in synaptic efficacy. Development and utilization of these techniques have advanced the field of learning and memory significantly. It is now possible to move from the study of activity-dependent changes of a single protein to modeling entire network changes that require local protein synthesis. This data revolution has necessitated the development of alternative computational and statistical techniques to analyze and understand the patterns contained within. Thus, the focus of this review is to provide a synopsis of the journey and evolution toward big data techniques to address still unanswered questions regarding how synapses are modified to strengthen neuronal circuits. We first review the seminal studies that demonstrated the pivotal role played by local mRNA translation as the mechanism underlying the enhancement of enduring synaptic activity. In the interest of those who are new to the field, we provide a brief overview of molecular biology and biochemical techniques utilized for sample preparation to identify locally translated proteins using RNA sequencing and proteomics, as well as the computational approaches used to analyze these data. While many mRNAs have been identified, few have been shown to be locally synthesized. To this end, we review techniques currently being utilized to visualize new protein synthesis, a task that has proven to be the most difficult aspect of the field. Finally, we provide examples of future applications to test the physiological relevance of locally synthesized proteins identified by big data approaches.

Microfilament-coordinated adhesion dynamics drives single cell migration and shapes whole tissues

Cell adhesion to the substratum and/or other cells is a crucial step of cell migration. While essential in the case of solitary migrating cells (for example, immune cells), it becomes particularly important in collective cell migration, in which cells maintain contact with their neighbors while moving directionally. Adhesive coordination is paramount in physiological contexts (for example, during organogenesis) but also in pathology (for example, tumor metastasis). In this review, we address the need for a coordinated regulation of cell-cell and cell-matrix adhesions during collective cell migration. We emphasize the role of the actin cytoskeleton as an intracellular integrator of cadherin- and integrin-based adhesions and the emerging role of mechanics in the maintenance, reinforcement, and turnover of adhesive contacts. Recent advances in understanding the mechanical regulation of several components of cadherin and integrin adhesions allow us to revisit the adhesive clutch hypothesis that controls the degree of adhesive engagement during protrusion. Finally, we provide a brief overview of the major impact of these discoveries when using more physiological three-dimensional models of single and collective cell migration.

Chelation of hippocampal zinc enhances long-term potentiation and synaptic tagging/capture in CA1 pyramidal neurons of aged rats: implications to aging and memory

Aging is associated with decline in cognitive functions, prominently in the memory consolidation and association capabilities. Hippocampus plays a crucial role in the formation and maintenance of long‐term associative memories, and a significant body of evidence shows that impairments in hippocampal function correlate with aging‐related memory loss. A number of studies have implicated alterations in hippocampal synaptic plasticity, such as long‐term potentiation (LTP), in age‐related cognitive decline although exact mechanisms underlying are not completely clear. Zinc deficiency and the resultant adverse effects on cognition have been well studied. However, the role of excess of zinc in synaptic plasticity, especially in aging, is not addressed well. Here, we have investigated the hippocampal zinc levels and the impairments in synaptic plasticity, such as LTP and synaptic tagging and capture (STC), in the CA1 region of acute hippocampal slices from 82‐ to 84‐week‐old male Wistar rats. We report increased zinc levels in the hippocampus of aged rats and also deficits in the tetani‐induced and dopaminergic agonist‐induced late‐LTP and STC. The observed deficits in synaptic plasticity were restored upon chelation of zinc using a cell‐permeable chelator. These data suggest that functional plasticity and associativity can be successfully established in aged neural networks by chelating zinc with cell‐permeable chelating agents.

Optogenetic Control of Synaptic Composition and Function

The molecular composition of the postsynaptic membrane is sculpted by synaptic activity. During synaptic plasticity at excitatory synapses, numerous structural, signaling, and receptor molecules concentrate at the postsynaptic density (PSD) to regulate synaptic strength. We developed an approach that uses light to tune the abundance of specific molecules in the PSD. We used this approach to investigate the relationship between the number of AMPA-type glutamate receptors in the PSD and synaptic strength. Surprisingly, adding more AMPA receptors to excitatory contacts had little effect on synaptic strength. Instead, we observed increased excitatory input through the apparent addition of new functional sites. Our data support a model where adding AMPA receptors is sufficient to activate synapses that had few receptors to begin with, but that additional remodeling events are required to strengthen established synapses. More broadly, this approach introduces the precise spatiotemporal control of optogenetics to the molecular control of synaptic function.

The Yin and Yang of Memory Consolidation: Hippocampal and Neocortical

While hippocampal and cortical mechanisms of memory consolidation have long been studied, their interaction is poorly understood. We sought to investigate potential interactions with respect to trace dominance, strengthening, and interference associated with postencoding novelty or sleep. A learning procedure was scheduled in a watermaze that placed the impact of novelty and sleep in opposition. Distinct behavioural manipulations-context preexposure or interference during memory retrieval-differentially affected trace dominance and trace survival, respectively. Analysis of immediate early gene expression revealed parallel up-regulation in the hippocampus and cortex, sustained in the hippocampus in association with novelty but in the cortex in association with sleep. These findings shed light on dynamically interacting mechanisms mediating the stabilization of hippocampal and neocortical memory traces. Hippocampal memory traces followed by novelty were more dominant by default but liable to interference, whereas sleep engaged a lasting stabilization of cortical traces and consequent trace dominance after preexposure.

Methylphenidate during early consolidation affects long-term associative memory retrieval depending on baseline catecholamines

RATIONALE

Synaptic memory consolidation is thought to rely on catecholaminergic signaling. Eventually, it is followed by systems consolidation, which embeds memories in a neocortical network. Although this sequence was demonstrated in rodents, it is unclear how catecholamines affect memory consolidation in humans.

OBJECTIVES

Here, we tested the effects of catecholaminergic modulation on synaptic and subsequent systems consolidation. We expected enhanced memory performance and increased neocortical engagement during delayed retrieval. Additionally, we tested if this effect was modulated by individual differences in a cognitive proxy measure of baseline catecholamine synthesis capacity.

METHODS

Fifty-three healthy males underwent a between-subjects, double-blind, placebo-controlled procedure across 2 days. On day 1, subjects studied and retrieved object-location associations and received 20 mg of methylphenidate or placebo. Drug intake was timed so that methylphenidate was expected to affect early consolidation but not encoding or retrieval. Memory was tested again while subjects were scanned three days later.

RESULTS

Methylphenidate did not facilitate memory performance, and there was no significant group difference in activation during delayed retrieval. However, memory representations differed between groups depending on baseline catecholamines. The placebo group showed increased activation in occipito-temporal regions but decreased connectivity with the hippocampus, associated with lower baseline catecholamine synthesis capacity. The methylphenidate group showed stronger activation in the postcentral gyrus, associated with higher baseline catecholamine synthesis capacity.

CONCLUSIONS

Altogether, methylphenidate during early consolidation did not foster long-term memory performance, but it affected retrieval-related neural processes depending on individual levels of baseline catecholamines.

Procedural Memory Consolidation in Attention-Deficit/Hyperactivity Disorder Is Promoted by Scheduling of Practice to Evening Hours

In young adults without attention-deficit/hyperactivity disorder (ADHD) training on a novel movement sequence results not only in large within-session (online) gains in task performance but also in additional (delayed, off-line) gains in the performance, expressed after an interval of sleep. In contrast, young people with ADHD, given an identical practice, were shown to improve online but expressed much smaller delayed gains overnight. As delayed gains in performance are taken to reflect procedural ("how to") memory consolidation processes, this may explain skill learning deficits in persons with ADHD. However, motor training is usually provided in morning sessions, and, given that persons with ADHD are often evening types, chronobiological constraints may constitute a hidden factor. Here, we tested the hypothesis that evening training, compared to morning training, would result in larger overnight consolidation gains following practice on a novel motor task in young women with ADHD. Participants with (N = 25) and without (N = 24) ADHD were given training on a finger opposition sequence tapping task, either in the morning or at evening. Performance was assessed before and immediately after training, overnight, and at 2 weeks post-training. Individuals with ADHD reported a general preference for evening hours. Evening training was equally effective in participants with and without ADHD, both groups showing robust consolidation gains in task performance overnight. However, the ability to express delayed gains overnight was significantly reduced in participants with ADHD if trained in the morning. Typical peers were as effective in expressing overnight consolidation phase gains irrespective of the time-of-day wherein the training session was afforded. Nevertheless, even after morning training, participants with ADHD fully retained the gains acquired within the first 24 h over an interval of about 2 weeks. Our results suggest that procedural memory consolidation processes are extant and effective in ADHD, but require that specific biobehavioral conditions be met. The affordance of training in the evening hours can relax some of the constraints on these processes in ADHD. The current results are in line with the notion that the control of what is to be retained in procedural memory is atypical or more stringent in ADHD.

Continual Learning Through Synaptic Intelligence

While deep learning has led to remarkable advances across diverse applications, it struggles in domains where the data distribution changes over the course of learning. In stark contrast, biological neural networks continually adapt to changing domains, possibly by leveraging complex molecular machinery to solve many tasks simultaneously. In this study, we introduce intelligent synapses that bring some of this biological complexity into artificial neural networks. Each synapse accumulates task relevant information over time, and exploits this information to rapidly store new memories without forgetting old ones. We evaluate our approach on continual learning of classification tasks, and show that it dramatically reduces forgetting while maintaining computational efficiency.

Reward retroactively enhances memory consolidation for related items

Reward motivation has been shown to modulate episodic memory processes in order to support future adaptive behavior. However, for a memory system to be truly adaptive, it should enhance memory for rewarded events as well as for neutral events that may seem inconsequential at the time of encoding but can gain importance later. Here, we investigated the influence of reward motivation on retroactive memory enhancement selectively for conceptually related information. We found behavioral evidence that reward retroactively enhances memory at a 24-h memory test, but not at an immediate memory test, suggesting a role for post-encoding mechanisms of consolidation.

Dopamine neurons encode performance error in singing birds

Many behaviors are learned through trial and error by matching performance to internal goals. Yet neural mechanisms of performance evaluation remain poorly understood. We recorded basal ganglia-projecting dopamine neurons in singing zebra finches as we controlled perceived song quality with distorted auditory feedback. Dopamine activity was phasically suppressed after distorted syllables, consistent with a worse-than-predicted outcome, and was phasically activated at the precise moment of the song when a predicted distortion did not occur, consistent with a better-than-predicted outcome. Error response magnitude depended on distortion probability. Thus, dopaminergic error signals can evaluate behaviors that are not learned for reward and are instead learned by matching performance outcomes to internal goals.

SorCS2 is required for BDNF-dependent plasticity in the hippocampus

SorCS2 is a member of the Vps10p-domain receptor gene family receptors with critical roles in the control of neuronal viability and function. Several genetic studies have suggested SORCS2 to confer risk of bipolar disorder, schizophrenia and attention deficit-hyperactivity disorder. Here we report that hippocampal N-methyl-d-aspartate receptor-dependent synaptic plasticity is eliminated in SorCS2-deficient mice. This defect was traced to the ability of SorCS2 to form complexes with the neurotrophin receptor p75NTR, required for pro-brain-derived neurotrophic factor (BDNF) to induce long-term depression, and with the BDNF receptor tyrosine kinase TrkB to elicit long-term potentiation. Although the interaction with p75NTR was static, SorCS2 bound to TrkB in an activity-dependent manner to facilitate its translocation to postsynaptic densities for synaptic tagging and maintenance of synaptic potentiation. Neurons lacking SorCS2 failed to respond to BDNF by TrkB autophosphorylation, and activation of downstream signaling cascades, impacting neurite outgrowth and spine formation. Accordingly, Sorcs2-/- mice displayed impaired formation of long-term memory, increased risk taking and stimulus seeking behavior, enhanced susceptibility to stress and impaired prepulse inhibition. Our results identify SorCS2 as an indispensable coreceptor for p75NTR and TrkB in hippocampal neurons and suggest SORCS2 as the link between proBDNF/BDNF signaling and mental disorders.

Computational model of a positive BDNF feedback loop in hippocampal neurons following inhibitory avoidance training

Inhibitory avoidance (IA) training in rodents initiates a molecular cascade within hippocampal neurons. This cascade contributes to the transition of short- to long-term memory (i.e., consolidation). Here, a differential equation-based model was developed to describe a positive feedback loop within this molecular cascade. The feedback loop begins with an IA-induced release of brain-derived neurotrophic factor (BDNF), which in turn leads to rapid phosphorylation of the cAMP response element-binding protein (pCREB), and a subsequent increase in the level of the β isoform of the CCAAT/enhancer binding protein (C/EBPβ). Increased levels of C/EBPβ lead to increased bdnf expression. Simulations predicted that an empirically observed delay in the BDNF-pCREB-C/EBPβ feedback loop has a profound effect on the dynamics of consolidation. The model also predicted that at least two independent self-sustaining signaling pathways downstream from the BDNF-pCREB-C/EBPβ feedback loop contribute to consolidation. Currently, the nature of these downstream pathways is unknown.

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.

Linking Memories across Time via Neuronal and Dendritic Overlaps in Model Neurons with Active Dendrites

Memories are believed to be stored in distributed neuronal assemblies through activity-induced changes in synaptic and intrinsic properties. However, the specific mechanisms by which different memories become associated or linked remain a mystery. Here, we develop a simplified, biophysically inspired network model that incorporates multiple plasticity processes and explains linking of information at three different levels: (1) learning of a single associative memory, (2) rescuing of a weak memory when paired with a strong one, and (3) linking of multiple memories across time. By dissecting synaptic from intrinsic plasticity and neuron-wide from dendritically restricted protein capture, the model reveals a simple, unifying principle: linked memories share synaptic clusters within the dendrites of overlapping populations of neurons. The model generates numerous experimentally testable predictions regarding the cellular and sub-cellular properties of memory engrams as well as their spatiotemporal interactions.

Time-Dependent Effects of Cardiovascular Exercise on Memory

We present new evidence supporting the hypothesis that the effects of cardiovascular exercise on memory can be regulated in a time-dependent manner. When the exercise stimulus is coupled temporally with specific phases of the memory formation process, a single bout of cardiovascular exercise may be sufficient to improve memory.

Test Expectation Enhances Memory Consolidation across Both Sleep and Wake

Memory consolidation benefits from post-training sleep. However, recent studies suggest that sleep does not uniformly benefit all memory, but instead prioritizes information that is important to the individual. Here, we examined the effect of test expectation on memory consolidation across sleep and wakefulness. Following reports that information with strong “future relevance” is preferentially consolidated during sleep, we hypothesized that test expectation would enhance memory consolidation across a period of sleep, but not across wakefulness. To the contrary, we found that expectation of a future test enhanced memory for both spatial and motor learning, but that this effect was equivalent across both wake and sleep retention intervals. These observations differ from those of least two prior studies, and fail to support the hypothesis that the “future relevance” of learned material moderates its consolidation selectively during sleep.

Novelty during a late postacquisition time window attenuates the persistence of fear memory

Learning to avoid threats in the environment is highly adaptive. However, sometimes a dysregulation of fear memories processing may underlie fear-related disorders. Despite recent advances, a major question of how to effectively attenuate persistent fear memories in a safe manner remains unresolved. Here we show experiments employing a behavioural tool to target a specific time window after training to limit the persistence of a fear memory in rats. We observed that exposure to a novel environment 11 h after an inhibitory avoidance (IA) training that induces a long-lasting memory, attenuates the durability of IA memory but not its formation. This effect is time-restricted and not seen when the environment is familiar. In addition, novelty-induced attenuation of IA memory durability is prevented by the intrahippocampal infusion of the CaMKs inhibitor KN-93. This new behavioural approach which targets a specific time window during late memory consolidation, might represent a new tool for reducing the durability of persistent fear memories.

Computational principles of synaptic memory consolidation

Memories are stored and retained through complex, coupled processes operating on multiple timescales. To understand the computational principles behind these intricate networks of interactions, we construct a broad class of synaptic models that efficiently harness biological complexity to preserve numerous memories by protecting them against the adverse effects of overwriting. The memory capacity scales almost linearly with the number of synapses, which is a substantial improvement over the square root scaling of previous models. This was achieved by combining multiple dynamical processes that initially store memories in fast variables and then progressively transfer them to slower variables. Notably, the interactions between fast and slow variables are bidirectional. The proposed models are robust to parameter perturbations and can explain several properties of biological memory, including delayed expression of synaptic modifications, metaplasticity, and spacing effects.

Sniff-Like Patterned Input Results in Long-Term Plasticity at the Rat Olfactory Bulb Mitral and Tufted Cell to Granule Cell Synapse

During odor sensing the activity of principal neurons of the mammalian olfactory bulb, the mitral and tufted cells (MTCs), occurs in repetitive bursts that are synchronized to respiration, reminiscent of hippocampal theta-gamma coupling. Axonless granule cells (GCs) mediate self- and lateral inhibitory interactions between the excitatory MTCs via reciprocal dendrodendritic synapses. We have explored long-term plasticity at this synapse by using a theta burst stimulation (TBS) protocol and variations thereof. GCs were excited via glomerular stimulation in acute brain slices. We find that TBS induces exclusively long-term depression in the majority of experiments, whereas single bursts ("single-sniff paradigm") can elicit both long-term potentiation and depression. Statistical analysis predicts that the mechanism underlying this bidirectional plasticity involves the proportional addition or removal of presynaptic release sites. Gamma stimulation with the same number of APs as in TBS was less efficient in inducing plasticity. Both TBS- and "single-sniff paradigm"-induced plasticity depend on NMDA receptor activation. Since the onset of plasticity is very rapid and requires little extra activity, we propose that these forms of plasticity might play a role already during an ongoing search for odor sources. Our results imply that components of both short-term and long-term olfactory memory may be encoded at this synapse.

The interplay between neuronal activity and actin dynamics mimic the setting of an LTD synaptic tag

Persistent forms of plasticity, such as long-term depression (LTD), are dependent on the interplay between activity-dependent synaptic tags and the capture of plasticity-related proteins. We propose that the synaptic tag represents a structural alteration that turns synapses permissive to change. We found that modulation of actin dynamics has different roles in the induction and maintenance of LTD. Inhibition of either actin depolymerisation or polymerization blocks LTD induction whereas only the inhibition of actin depolymerisation blocks LTD maintenance. Interestingly, we found that actin depolymerisation and CaMKII activation are involved in LTD synaptic-tagging and capture. Moreover, inhibition of actin polymerisation mimics the setting of a synaptic tag, in an activity-dependent manner, allowing the expression of LTD in non-stimulated synapses. Suspending synaptic activation also restricts the time window of synaptic capture, which can be restored by inhibiting actin polymerization. Our results support our hypothesis that modulation of the actin cytoskeleton provides an input-specific signal for synaptic protein capture.

Emerging pathways driving early synaptic pathology in Alzheimer's disease

The current state of the AD research field is highly dynamic is some respects, while seemingly stagnant in others. Regarding the former, our current lack of understanding of initiating disease mechanisms, the absence of effective treatment options, and the looming escalation of AD patients is energizing new research directions including a much-needed re-focusing on early pathogenic mechanisms, validating novel targets, and investigating relevant biomarkers, among other exciting new efforts to curb disease progression and foremost, preserve memory function. With regard to the latter, the recent disappointing series of failed Phase III clinical trials targeting Aβ and APP processing, in concert with poor association between brain Aβ levels and cognitive function, have led many to call for a re-evaluation of the primacy of the amyloid cascade hypothesis. In this review, we integrate new insights into one of the earliest described signaling abnormalities in AD pathogenesis, namely intracellular Ca²⁺ signaling disruptions, and focus on its role in driving synaptic deficits - which is the feature that does correlate with AD-associated memory loss. Excess Ca²⁺release from intracellular stores such as the endoplasmic reticulum (ER) has been well-described in cellular and animal models of AD, as well as human patients, and here we expand upon recent developments in ER-localized release channels such as the IP₃R and RyR, and the recent emphasis on RyR2. Consistent with ER Ca²⁺ mishandling in AD are recent findings implicating aspects of SOCE, such as STIM2 function, and TRPC3 and TRPC6 levels. Other Ca²⁺-regulated organelles important in signaling and protein handling are brought into the discussion, with new perspectives on lysosomal regulation. These early signaling abnormalities are discussed in the context of synaptic pathophysiology and disruptions in synaptic plasticity with a particular emphasis on short-term plasticity deficits. Overall, we aim to update and expand the list of early neuronal signaling abnormalities implicated in AD pathogenesis, identify specific channels and organelles involved, and link these to proximal synaptic impairments driving the memory loss in AD. This is all within the broader goal of identifying novel therapeutic targets to preserve cognitive function in AD.

Talking to the neighbours: The molecular and physiological mechanisms of clustered synaptic plasticity

Synaptic connectivity forms the basis for neuronal communication and the storage of information. Experiences and learning of new abilities can drive remodelling of this connectivity and promotes the formation of spine clusters; dendritic segments with a higher spine density. Spines located within these segments are frequently co-activated, undergo different dynamics than synapses located outside of this dendritic compartment and have, in general, a longer lifetime. Several lines of evidence have shown that chemical synapses located close to each other share or compete for intracellular signalling molecules and structural resources. This sharing and competition directly influences spine dynamics. Spines can grow, shrink, increase or decrease the surface expression of receptors, channels and adhesion molecules or remain stable and unchanged over extended periods of time. Here we summarize recent discoveries and provide a closer look at spine clustering, dendritic segment-specific signalling and potential molecular mechanisms underlying associative and heterosynaptic plasticity.

Locus coeruleus and dopaminergic consolidation of everyday memory

The retention of episodic-like memory is enhanced, in humans and animals, when something novel happens shortly before or after encoding. Using an everyday memory task in mice, we sought the neurons mediating this dopamine-dependent novelty effect, previously thought to originate exclusively from the tyrosine-hydroxylase-expressing (TH+) neurons in the ventral tegmental area. Here we report that neuronal firing in the locus coeruleus is especially sensitive to environmental novelty, locus coeruleus TH+ neurons project more profusely than ventral tegmental area TH+ neurons to the hippocampus, optogenetic activation of locus coeruleus TH+ neurons mimics the novelty effect, and this novelty-associated memory enhancement is unaffected by ventral tegmental area inactivation. Surprisingly, two effects of locus coeruleus TH+ photoactivation are sensitive to hippocampal D1/D5 receptor blockade and resistant to adrenoceptor blockade: memory enhancement and long-lasting potentiation of synaptic transmission in CA1 ex vivo. Thus, locus coeruleus TH+ neurons can mediate post-encoding memory enhancement in a manner consistent with possible co-release of dopamine in the hippocampus.

Induction and Consolidation of Calcium-Based Homo- and Heterosynaptic Potentiation and Depression

The adaptive mechanisms of homo- and heterosynaptic plasticity play an important role in learning and memory. In order to maintain plasticity-induced changes for longer time scales (up to several days), they have to be consolidated by transferring them from a short-lasting early-phase to a long-lasting late-phase state. The underlying processes of this synaptic consolidation are already well-known for homosynaptic plasticity, however, it is not clear whether the same processes also enable the induction and consolidation of heterosynaptic plasticity. In this study, by extending a generic calcium-based plasticity model with the processes of synaptic consolidation, we show in simulations that indeed heterosynaptic plasticity can be induced and, furthermore, consolidated by the same underlying processes as for homosynaptic plasticity. Furthermore, we show that by local diffusion processes the heterosynaptic effect can be restricted to a few synapses neighboring the homosynaptically changed ones. Taken together, this generic model reproduces many experimental results of synaptic tagging and consolidation, provides several predictions for heterosynaptic induction and consolidation, and yields insights into the complex interactions between homo- and heterosynaptic plasticity over a broad variety of time (minutes to days) and spatial scales (several micrometers).

Identification of Stable Spike-Timing-Dependent Plasticity from Spiking Activity with Generalized Multilinear Modeling

Characterization of long-term activity-dependent plasticity from behaviorally driven spiking activity is important for understanding the underlying mechanisms of learning and memory. In this letter, we present a computational framework for quantifying spike-timing-dependent plasticity (STDP) during behavior by identifying a functional plasticity rule solely from spiking activity. First, we formulate a flexible point-process spiking neuron model structure with STDP, which includes functions that characterize the stationary and plastic properties of the neuron. The STDP model includes a novel function for prolonged plasticity induction, as well as a more typical function for synaptic weight change based on the relative timing of input-output spike pairs. Consideration for system stability is incorporated with weight-dependent synaptic modification. Next, we formalize an estimation technique using a generalized multilinear model (GMLM) structure with basis function expansion. The weight-dependent synaptic modification adds a nonlinearity to the model, which is addressed with an iterative unconstrained optimization approach. Finally, we demonstrate successful model estimation on simulated spiking data and show that all model functions can be estimated accurately with this method across a variety of simulation parameters, such as number of inputs, output firing rate, input firing type, and simulation time. Since this approach requires only naturally generated spikes, it can be readily applied to behaving animal studies to characterize the underlying mechanisms of learning and memory.

Organization and dynamics of the actin cytoskeleton during dendritic spine morphological remodeling

In the central nervous system, most excitatory post-synapses are small subcellular structures called dendritic spines. Their structure and morphological remodeling are tightly coupled to changes in synaptic transmission. The F-actin cytoskeleton is the main driving force of dendritic spine remodeling and sustains synaptic plasticity. It is therefore essential to understand how changes in synaptic transmission can regulate the organization and dynamics of actin binding proteins (ABPs). In this review, we will provide a detailed description of the organization and dynamics of F-actin and ABPs in dendritic spines and will discuss the current models explaining how the actin cytoskeleton sustains both structural and functional synaptic plasticity.

Synaptic and extrasynaptic traces of long-term memory: the ID molecule theory

It is generally assumed at the time of this writing that memories are stored in the form of synaptic weights. However, it is now also clear that the synapses are not permanent; in fact, synaptic patterns undergo significant change in a matter of hours. This means that to implement the long survival of distant memories (for several decades in humans), the brain must possess a molecular backup mechanism in some form, complete with provisions for the storage and retrieval of information. It is found below that the memory-supporting molecules need not contain a detailed description of mental entities, as had been envisioned in the 'memory molecule papers' from 50 years ago, they only need to contain unique identifiers of various entities, and that this can be achieved using relatively small molecules, using a random code ('ID molecules'). In this paper, the logistics of information flow are followed through the steps of storage and retrieval, and the conclusion reached is that the ID molecules, by carrying a sufficient amount of information (entropy), can effectively control the recreation of complex multineuronal patterns. In illustrations, it is described how ID molecules can be made to revive a selected cell assembly by waking up its synapses and how they cause a selected cell assembly to ignite by sending slow inward currents into its cells. The arrangement involves producing multiple copies of the ID molecules and distributing them at strategic locations at selected sets of synapses, then reaching them through small noncoding RNA molecules. This requires the quick creation of entropy-rich messengers and matching receptors, and it suggests that these are created from each other by small-scale transcription and reverse transcription.

Differential involvement of Ca2+/calmodulin-dependent protein kinases and mitogen-activated protein kinases in the dopamine D1/D5 receptor-mediated potentiation in hippocampal CA1 pyramidal neurons

Dopaminergic neurotransmission modulates and influences hippocampal CA1 synaptic plasticity, learning and long-term memory mechanisms. Investigating the mechanisms involved in the slow-onset potentiation induced by the dopamine D1/D5 receptor agonists in hippocampal CA1 region, we have reported recently that it could play a role in regulating synaptic cooperation and competition. We have also shown that a sustained activation of MEK/MAP kinase pathway was involved in the maintenance of this long-lasting potentiation (Shivarama Shetty, Gopinadhan, & Sajikumar, 2016). However, the molecular aspects of the induction of dopaminergic slow-onset potentiation are not known. Here, we investigated the involvement of MEK/MAPK pathway and Ca²⁺-calmodulin-dependent protein kinases (CaMKII and CaMKIV) in the induction and maintenance phases of the D1/D5 receptor-mediated slow-onset potentiation. We report differential involvement of these kinases in a dose-dependent manner wherein at weaker levels of dopaminergic activation, both CaMKII and MEK1/2 activation is necessary for the establishment of potentiation and with sufficiently stronger dopaminergic activation, the role of CaMKII becomes dispensable whereas MEK activation remains crucial for the long-lasting potentiation. The results are interesting in view of the involvement of the hippocampal dopaminergic system in a variety of cognitive abilities including memory formation and also in neurological diseases such as Alzheimer's disease and Parkinson's disease.

Acute Exercise and Motor Memory Consolidation: The Role of Exercise Timing

High intensity aerobic exercise amplifies offline gains in procedural memory acquired during motor practice. This effect seems to be evident when exercise is placed immediately after acquisition, during the first stages of memory consolidation, but the importance of temporal proximity of the exercise bout used to stimulate improvements in procedural memory is unknown. The effects of three different temporal placements of high intensity exercise were investigated following visuomotor skill acquisition on the retention of motor memory in 48 young (24.0 ± 2.5 yrs), healthy male subjects randomly assigned to one of four groups either performing a high intensity (90% Maximal Power Output) exercise bout at 20 min (EX90), 1 h (EX90+1), 2 h (EX90+2) after acquisition or rested (CON). Retention tests were performed at 1 d (R1) and 7 d (R7). At R1 changes in performance scores after acquisition were greater for EX90 than CON (p < 0.001) and EX90+2 (p = 0.001). At R7 changes in performance scores for EX90, EX90+1, and EX90+2 were higher than CON (p < 0.001, p = 0.008, and p = 0.008, resp.). Changes for EX90 at R7 were greater than EX90+2 (p = 0.049). Exercise-induced improvements in procedural memory diminish as the temporal proximity of exercise from acquisition is increased. Timing of exercise following motor practice is important for motor memory consolidation.

miRNAs in NMDA receptor-dependent synaptic plasticity and psychiatric disorders

The identification and functional delineation of miRNAs (a class of small non-coding RNAs) have added a new layer of complexity to our understanding of the molecular mechanisms underlying synaptic plasticity. Genome-wide association studies in conjunction with investigations in cellular and animal models, moreover, provide evidence that miRNAs are involved in psychiatric disorders. In the present review, we examine the current knowledge about the roles played by miRNAs in NMDA (N-methyl-D-aspartate) receptor-dependent synaptic plasticity and psychiatric disorders.

Physical Exercise Performed Four Hours after Learning Improves Memory Retention and Increases Hippocampal Pattern Similarity during Retrieval

Persistent long-term memory depends on successful stabilization and integration of new memories after initial encoding [1, 2]. This consolidation process is thought to require neuromodulatory factors such as dopamine, noradrenaline, and brain-derived neurotrophic factor [3-7]. Without the release of such factors around the time of encoding, memories will decay rapidly [3, 5, 6, 8]. Recent studies have shown that physical exercise acutely stimulates the release of several consolidation-promoting factors in humans [9-14], raising the question of whether physical exercise can be used to improve memory retention [15-17]. Here, we used a single session of physical exercise after learning to exogenously boost memory consolidation and thus long-term memory. Three groups of randomly assigned participants first encoded a set of picture-location associations. Afterward, one group performed exercise immediately, one 4 hr later, and the third did not perform any exercise. Participants otherwise underwent exactly the same procedures to control for potential experimental confounds. Forty-eight hours later, participants returned for a cued-recall test in a magnetic resonance scanner. With this design, we could investigate the impact of acute exercise on memory consolidation and retrieval-related neural processing. We found that performing exercise 4 hr, but not immediately, after encoding improved the retention of picture-location associations compared to the no-exercise control group. Moreover, performing exercise after a delay was associated with increased hippocampal pattern similarity for correct responses during delayed retrieval. Our results suggest that appropriately timed physical exercise can improve long-term memory and highlight the potential of exercise as an intervention in educational and clinical settings.

Evidence of VTA and LC control of protein synthesis required for the behavioral tagging process

Several works have shown that the formation of different long-term memories relies on a behavioral tagging process. In other words, to establish a lasting memory, at least two parallel processes must occur: the setting of a learning tag (triggered during learning) that defines where a memory could be stored, and the synthesis of proteins, that once captured at tagged sites will effectively allow the consolidation process to occur. This work focused in studying which brain structures are responsible of controlling the synthesis of those proteins at the brain areas where memory is being stored. It combines electrical activation of the ventral tegmental area (VTA) and/or the locus coeruleus (LC), with local pharmacological interventions and weak and strong behavioral trainings in the inhibitory avoidance and spatial object recognition tasks in rats. The results presented here strongly support the idea that the VTA is a brain structure responsible for regulating the consolidation of memories acting through the D1/D5 dopaminergic receptors of the hippocampus to control the synthesis of new proteins required for this process. Moreover, they provide evidence that the LC may be a second structure with a similar role, acting independently and complementary to the VTA, through the β-adrenergic receptors of the hippocampus.

A shared neural ensemble links distinct contextual memories encoded close in time

Recent studies suggest that a shared neural ensemble may link distinct memories encoded close in time. According to the memory allocation hypothesis, learning triggers a temporary increase in neuronal excitability that biases the representation of a subsequent memory to the neuronal ensemble encoding the first memory, such that recall of one memory increases the likelihood of recalling the other memory. Here we show in mice that the overlap between the hippocampal CA1 ensembles activated by two distinct contexts acquired within a day is higher than when they are separated by a week. Several findings indicate that this overlap of neuronal ensembles links two contextual memories. First, fear paired with one context is transferred to a neutral context when the two contexts are acquired within a day but not across a week. Second, the first memory strengthens the second memory within a day but not across a week. Older mice, known to have lower CA1 excitability, do not show the overlap between ensembles, the transfer of fear between contexts, or the strengthening of the second memory. Finally, in aged mice, increasing cellular excitability and activating a common ensemble of CA1 neurons during two distinct context exposures rescued the deficit in linking memories. Taken together, these findings demonstrate that contextual memories encoded close in time are linked by directing storage into overlapping ensembles. Alteration of these processes by ageing could affect the temporal structure of memories, thus impairing efficient recall of related information.

A Model of Synaptic Reconsolidation

Reconsolidation of memories has mostly been studied at the behavioral and molecular level. Here, we put forward a simple extension of existing computational models of synaptic consolidation to capture hippocampal slice experiments that have been interpreted as reconsolidation at the synaptic level. The model implements reconsolidation through stabilization of consolidated synapses by stabilizing entities combined with an activity-dependent reservoir of stabilizing entities that are immune to protein synthesis inhibition (PSI). We derive a reduced version of our model to explore the conditions under which synaptic reconsolidation does or does not occur, often referred to as the boundary conditions of reconsolidation. We find that our computational model of synaptic reconsolidation displays complex boundary conditions. Our results suggest that a limited resource of hypothetical stabilizing molecules or complexes, which may be implemented by protein phosphorylation or different receptor subtypes, can underlie the phenomenon of synaptic reconsolidation.

Motivated encoding selectively promotes memory for future inconsequential semantically-related events

Neurobiological models of long-term memory explain how memory for inconsequential events fades, unless these happen before or after other relevant (i.e., rewarding or aversive) or novel events. Recently, it has been shown in humans that retrospective and prospective memories are selectively enhanced if semantically related events are paired with aversive stimuli. However, it remains unclear whether motivating stimuli, as opposed to aversive, have the same effect in humans. Here, participants performed a three phase incidental encoding task where one semantic category was rewarded during the second phase. A memory test 24h after, but not immediately after encoding, revealed that memory for inconsequential items was selectively enhanced only if items from the same category had been previously, but not subsequently, paired with rewards. This result suggests that prospective memory enhancement of reward-related information requires, like previously reported for aversive memories, of a period of memory consolidation. The current findings provide the first empirical evidence in humans that the effects of motivated encoding are selectively and prospectively prolonged over time.

Effects of pre-natal alcohol exposure on hippocampal synaptic plasticity: Sex, age and methodological considerations

The consumption of alcohol during gestation is detrimental to the developing central nervous system (CNS). The severity of structural and functional brain alterations associated with alcohol intake depends on many factors including the timing and duration of alcohol consumption. The hippocampal formation, a brain region implicated in learning and memory, is highly susceptible to the effects of developmental alcohol exposure. Some of the observed effects of alcohol on learning and memory may be due to changes at the synaptic level, as this teratogen has been repeatedly shown to interfere with hippocampal synaptic plasticity. At the molecular level alcohol interferes with receptor proteins and can disrupt hormones that are important for neuronal signaling and synaptic plasticity. In this review we examine the consequences of prenatal and early postnatal alcohol exposure on hippocampal synaptic plasticity and highlight the numerous factors that can modulate the effects of alcohol. We also discuss some potential mechanisms responsible for these changes as well as emerging therapeutic avenues that are beginning to be explored.

The Effect of an Acute Bout of Moderate-Intensity Aerobic Exercise on Motor Learning of a Continuous Tracking Task

INTRODUCTION

There is evidence for beneficial effects of acute and long-term exercise interventions on several forms of memory, including procedural motor learning. In the present study we examined how performing a single bout of continuous moderate intensity aerobic exercise would impact motor skill acquisition and retention in young healthy adults, compared to a period of rest. We hypothesized that exercise would improve motor skill acquisition and retention, compared to motor practice alone.

MATERIALS AND METHODS

Sixteen healthy adults completed sessions of aerobic exercise or seated rest that were immediately followed by practice of a novel motor task (practice). Exercise consisted of 30 minutes of continuous cycling at 60% peak O2 uptake. Twenty-four hours after practice, we assessed motor learning with a no-exercise retention test (retention). We also quantified changes in offline motor memory consolidation, which occurred between practice and retention (offline). Tracking error was separated into indices of temporal precision and spatial accuracy.

RESULTS

There were no differences between conditions in the timing of movements during practice (p = 0.066), at retention (p = 0.761), or offline (p = 0.966). However, the exercise condition enabled participants to maintain spatial accuracy during practice (p = 0.477); whereas, following rest performance diminished (p = 0.050). There were no significant differences between conditions at retention (p = 0.532) or offline (p = 0.246).

DISCUSSION

An acute bout of moderate-intensity aerobic exercise facilitated the maintenance of motor performance during skill acquisition, but did not influence motor learning. Given past work showing that pairing high intensity exercise with skilled motor practice benefits learning, it seems plausible that intensity is a key modulator of the effects of acute aerobic exercise on changes in complex motor behavior. Further work is necessary to establish a dose-response relationship between aerobic exercise and motor learning.

Proteomics of the Synapse--A Quantitative Approach to Neuronal Plasticity

The advances in mass spectrometry based proteomics in the past 15 years have contributed to a deeper appreciation of protein networks and the composition of functional synaptic protein complexes. However, research on protein dynamics underlying core mechanisms of synaptic plasticity in brain lag far behind. In this review, we provide a synopsis on proteomic research addressing various aspects of synaptic function. We discuss the major topics in the study of protein dynamics of the chemical synapse and the limitations of current methodology. We highlight recent developments and the future importance of multidimensional proteomics and metabolic labeling. Finally, emphasis is given on the conceptual framework of modern proteomics and its current shortcomings in the quest to gain a deeper understanding of synaptic plasticity.

RNA polymerase I transcription is modulated by spatial learning in different brain regions

Long-term memory is accompanied by changes in neuronal morphology and connectivity. These alterations are thought to depend upon new gene expression and protein synthesis over a distributed network of brain structures. Although much evidence supports the idea that the creation of stable, persistent memory traces requires synthesis of new proteins, the role of rRNA transcription and nucleolar activity in learning and memory has hardly been explored. rRNAs needed for protein synthesis result from the activity of two different RNA polymerases, RNA polymerase I and RNA polymerase III, transcribing for 47S RNA and 5S RNA, respectively. In this study, we first investigated the effects of spatial training in the Morris water maze on 47S RNA transcription in the central nervous system, demonstrating bidirectional modulation of its expression over a distributed neural network. We found learning-induced increases in the nucleolar organizer regions in the hippocampus. Finally, we demonstrated that intrahippocampal administrations of CX-5461 (0.6 μg/side), the specific RNA Polymerase I inhibitor, impair the ability of mice to locate the platform in the same task. These results suggest that de novo rRNA transcription is a necessary step for spatial memory consolidation, and that after learning, it occurs in several brain regions with a complex spatiotemporal dynamic. In this study, we demonstrate for the very first time that spatial learning modulates ribosomal RNA transcription in a wide brain circuit, with anatomical specificities in the dynamic of modulation. Together with pharmacological evidences, data presented here support the hypothesis of a necessary role of RNA Pol-I transcription during spatial memory formation. Read the Editorial Highlight for this article on page 673.

PV plasticity sustained through D1/5 dopamine signaling required for long-term memory consolidation

Long-term consolidation of memories depends on processes occurring many hours after acquisition. Whether this involves plasticity that is specifically required for long-term consolidation remains unclear. We found that learning-induced plasticity of local parvalbumin (PV) basket cells was specifically required for long-term, but not short/intermediate-term, memory consolidation in mice. PV plasticity, which involves changes in PV and GAD67 expression and connectivity onto PV neurons, was regulated by cAMP signaling in PV neurons. Following induction, PV plasticity depended on local D1/5 dopamine receptor signaling at 0-5 h to regulate its magnitude, and at 12-14 h for its continuance, ensuring memory consolidation. D1/5 dopamine receptor activation selectively induced DARPP-32 and ERK phosphorylation in PV neurons. At 12-14 h, PV plasticity was required for enhanced sharp-wave ripple densities and c-Fos expression in pyramidal neurons. Our results reveal general network mechanisms of long-term memory consolidation that requires plasticity of PV basket cells induced after acquisition and sustained subsequently through D1/5 receptor signaling.