MAPK establishes a molecular context that defines effective training patterns for long-term memory formation

The massed-spaced learning effect in non-neural human cells

The massed-spaced effect is a hallmark feature of memory formation. We now demonstrate this effect in two separate non-neural, immortalized cell lines stably expressing a short-lived luciferase reporter controlled by a CREB-dependent promoter. We emulate training using repeated pulses of forskolin and/or phorbol ester, and, as a proxy for memory, measure luciferase expression at various points after training. Four spaced pulses of either agonist elicit stronger and more sustained luciferase expression than a single "massed" pulse. Spaced pulses also result in stronger and more sustained activation of molecular factors critical for memory formation, ERK and CREB, and inhibition of ERK or CREB blocks the massed-spaced effect. Our findings show that canonical features of memory do not necessarily depend on neural circuitry, but can be embedded in the dynamics of signaling cascades conserved across different cell types.

Network motifs exhibiting a differential response to spaced and massed inputs

One characteristic of long-term memory is the existence of an inverted U-shaped response to increasing intervals between training sessions, and consequently, an optimal spacing that maximizes memory formation. Current models of this spacing effect focus on specific molecular components and their interactions. Here, we computationally study the underlying network architecture, in particular, the potential of motif dynamics in qualitatively capturing the spacing effect in a manner that is independent of the animal model, biomolecular components, and the timescales involved. We define a common training and test protocol, and computationally identify network topologies that can qualitatively replicate the experimentally observed characteristics of the spacing effect. For 41 motifs derived from fundamental network architectures such as autoregulation, feedback, and feedforward motifs, we tested their capacity to manifest the spacing effect in terms of an inverted U-shaped response curve, using different combinations of stimulation protocols, response metrics, and kinetic parameters. Our findings indicate that positive feedback motifs where the stimulus enhances conversion reaction in the loop replicate the spacing effect across all response metrics, while feedforward motifs exhibit a metric-specific spacing effect. For some parameter combinations, linear cascades of activation and conversion reactions were found sufficient to qualitatively exhibit spacing effect characteristics.

Isolation of the differential effects of chronic and acute stress in a manner that is not confounded by stress severity

Firm conclusions regarding the differential effects of the maladaptive consequences of acute versus chronic stress on the etiology and symptomatology of stress disorders await a model that isolates chronicity as a variable for studying the differential effects of acute versus chronic stress. This is because most previous studies have confounded chronicity with the total amount of stress. Here, we have modified the stress-enhanced fear learning (SEFL) protocol, which models some aspects of posttraumatic stress disorder (PTSD) following an acute stressor, to create a chronic variant that does not have this confound. Comparing results from this new protocol to the acute protocol, we found that chronic stress further potentiates enhanced fear-learning beyond the nonassociative enhancement induced by acute stress. This additional component is not observed when the unconditional stimulus (US) used during subsequent fear learning is distinct from the US used as the stressor, and is enhanced when glucose is administered following stressor exposure, suggesting that it is associative in nature. Furthermore, extinction of stressor-context fear blocks this additional associative component of SEFL as well as reinstatement of generalized fear, suggesting reinstatement of generalized fear may underlie this additional SEFL component.

Pattern detection in the TGFβ cascade controls the induction of long-term synaptic plasticity

Transforming growth factor β (TGFβ) is required for long-term memory (LTM) for sensitization in Aplysia. When LTM is induced using a two-trial training protocol, TGFβ inhibition only blocks LTM when administrated at the second, not the first trial. Here, we show that TGFβ acts as a "repetition detector" during the induction of two-trial LTM. Secretion of the biologically inert TGFβ proligand must coincide with its proteolytic activation by the Bone morphogenetic protein-1 (BMP-1/Tolloid) metalloprotease, which occurs specifically during trial two of our two-trial training paradigm. This paradigm establishes long-term synaptic facilitation (LTF), the cellular correlate of LTM. BMP-1 application paired with a single serotonin (5HT) pulse induced LTF, whereas neither a single 5HT pulse nor BMP-1 alone effectively did so. On the other hand, inhibition of endogenous BMP-1 activity blocked the induction of two-trial LTF. These results suggest a unique role for TGFβ in the interaction of repeated trials: during learning, repeated stimuli engage separate steps of the TGFβ cascade that together are necessary for the induction of long-lasting memories.

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

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

Specific patterns of neural activity in the hippocampus after massed or distributed spatial training

Training with long inter-session intervals, termed distributed training, has long been known to be superior to training with short intervals, termed massed training. In the present study we compared c-Fos expression after massed and distributed training protocols in the Morris water maze to outline possible differences in the learning-induced pattern of neural activation in the dorsal CA1 in the two training conditions. The results demonstrate that training and time lags between learning opportunities had an impact on the pattern of neuronal activity in the dorsal CA1. Mice trained with the distributed protocol showed sustained neuronal activity in the postero-distal component of the dorsal CA1. In parallel, in trained mice we found more active cells that tended to constitute spatially restricted clusters, whose degree increased with the increase in the time lags between learning trials. Moreover, activated cell assemblies demonstrated increased stability in their spatial organization after distributed as compared to massed training or control condition. Finally, using a machine learning algorithm we found that differences in the number of c-Fos positive cells and their location in the dorsal CA1 could be predictive of the training protocol used. These results suggest that the topographic organization and the spatial location of learning activated cell assemblies might be critical to promote the increased stability of the memory trace induced by distributed training.

Impact of Pseudomonas aeruginosa, Bacillus subtilis, Staphylococcus aureus, and Escherichia coli Oral Infusions on Cognitive Memory Decline in Mild Cognitive Impairment

Synaptic plasticity is a result of changes in the neuronal circuits which may result in the formation of protein-dependent (long-term memory (LTM) formation) and protein-independent (short-term memory (STM) formation) memories. This STM formation is based on existing proteins, but LTM formation depends on RNA and protein synthesis within the neuronal cells. This RNA and protein synthesis may depend on stimulus exposure like odour, taste, and other environmental stimuli. The present study is aimed to show the impact of oral bacterial infusions on cognitive memory formation through pre and post-infusive behavioural analysis. The results of the study revealed that oral infusions of Pseudomonas aeruginosa, Bacillus subtilis, Staphylococcus aureus and Escherichia coli result in impaired cognitive learning and memory formation. This impaired cognitive memory formation is shown with the help of two-step (pre and post-infusive) behavioural analysis. Pre-infusive behavioural study shows no decline in cognitive learning and memory formation before oral microbial infusions in a serene habituated environment. After oral microbial infusions, a post-infusive behavioural analysis may reveal a memory decline in the treated group. Comparative two-step behavioural analysis indicates that P. aeruginosa infusions strongly impact cognitive memory decline compared to the other three groups. This cognitive memory decline may happen due to the production of primary/secondary metabolites within the animal gut and their transportation to the CNS  through the blood-brain barrier. The outcome of the present study states that poor oral hygiene plays a significant role in cognitive memory decline concerning mild cognitive impairment (MCI).

AKT and MAPK signaling pathways in hippocampus reveals the pathogenesis of depression in four stress-induced models

Major depressive disorder (MDD) is a highly heterogeneous psychiatric disorder. The pathogenesis of MDD remained unclear, and it may be associated with exposure to different stressors. Most previous studies have focused on molecular changes in a single stress-induced depression model, which limited the identification of the pathogenesis of MDD. The depressive-like behaviors were induced by four well-validated stress models in rats, including chronic unpredictable mild stress, learned helplessness stress, chronic restraint stress and social defeat stress. We applied proteomic and metabolomic to investigate molecular changes in the hippocampus of those four models and revealed 529 proteins and 98 metabolites. Ingenuity Pathways Analysis (IPA) and Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis identified differentially regulated canonical pathways, and then we presented a schematic model that simulates AKT and MAPK signaling pathways network and their interactions and revealed the cascade reactions. Further, the western blot confirmed that p-AKT, p-ERK12, GluA1, p-MEK1, p-MEK2, p-P38, Syn1, and TrkB, which were changed in at least one depression model. Importantly, p-AKT, p-ERK12, p-MEK1 and p-P38 were identified as common alterations in four depression models. The molecular level changes caused by different stressors may be dramatically different, and even opposite, between four depression models. However, the different molecular alterations converge on a common AKT and MAPK molecular pathway. Further studies of these pathways could contribute to a better understanding of the pathogenesis of depression, with the ultimate goal of helping to develop or select more effective treatment strategies for MDD.

Formation characteristics of long-term memory in Bactrocera dorsalis

Studies on insects have contributed significantly to a better understanding of learning and memory, which is a necessary cognitive capability for all animals. Although the formation of memory has been studied in some model insects, more evidence is required to clarify the characteristics of memory formation, especially long-term memory (LTM), which is important for reliably storing information. Here, we explored this question by examining Bactrocera dorsalis, an agricultural pest with excellent learning abilities. Using the classical conditioning paradigm of the olfactory proboscis extension reflex (PER), we found that paired conditioning with multiple trials (>3) spaced with an intertrial interval (≥10 min) resulted in stable memory that lasted for at least 3 d. Furthermore, even a single conditioning trial was sufficient for the formation of a 2-d memory. With the injection of protein inhibitors, protein-synthesis-dependent memory was confirmed to start 4 h after training, and its dependence on translation and transcription differed. Moreover, the results revealed that the dependence of memory on protein translation exhibited a time-window effect (4-6 h). Our findings provide an integrated view of LTM in insects, suggesting common mechanisms in LTM formation that play a key role in the biological basis of memory.

Dynamics and Mechanisms of ERK Activation after Different Protocols that Induce Long-Term Synaptic Facilitation in Aplysia

Phosphorylation of the MAPK family member extracellular signal-regulated kinase (ERK) is required to induce long-term synaptic plasticity, but little is known about its persistence. We examined ERK activation by three protocols that induce long-term synaptic facilitation (LTF) of the Aplysia sensorimotor synapse - the standard protocol (five 5-min pulses of 5-HT with interstimulus intervals (ISIs) of 20 min), the enhanced protocol (five pulses with irregular ISIs, which induces greater and longer-lasting LTF) and the two-pulse protocol (two pulses with ISI 45 min). Immunofluorescence revealed complex ERK activation. The standard and two-pulse protocols immediately increased active, phosphorylated ERK (pERK), which decayed within 5 h. A second wave of increased pERK was detected 18 h post-treatment for all protocols. This late phase was blocked by inhibitors of protein kinase A, TrkB and TGF-β. These results suggest that complex interactions among kinase pathways and growth factors contribute to the late increase of pERK. ERK activity returned to basal 24 h after the standard or two-pulse protocols, but remained elevated 24 h for the enhanced protocol. This 24-h elevation was also dependent on PKA and TGF-β, and partly on TrkB. These results begin to characterize long-lasting ERK activation, plausibly maintained by positive feedback involving growth factors and PKA, that appears essential to maintain LTF and LTM. Because many processes involved in LTF and late LTP are conserved among Aplysia and mammals, these findings highlight the importance of examining the dynamics of kinase cascades involved in vertebrate long-term memory.

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

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

Precise timing of ERK phosphorylation/dephosphorylation determines the outcome of trial repetition during long-term memory formation

Two-trial learning in Aplysia reveals nonlinear interactions between training trials: A single trial has no effect, but two precisely spaced trials induce long-term memory. Extracellularly regulated kinase (ERK) activity is essential for intertrial interactions, but the mechanism remains unresolved. A combination of immunochemical and optogenetic tools reveals unexpected complexity of ERK signaling during the induction of long-term synaptic facilitation by two spaced pulses of serotonin (5-hydroxytryptamine, 5HT). Specifically, dual ERK phosphorylation at its activating TxY motif is accompanied by dephosphorylation at the pT position, leading to a buildup of inactive, singly phosphorylated pY-ERK. Phosphorylation and dephosphorylation occur concurrently but scale differently with varying 5HT concentrations, predicting that mixed two-trial protocols involving both "strong" and "weak" 5HT pulses should be sensitive to the precise order and timing of trials. Indeed, long-term synaptic facilitation is induced only when weak pulses precede strong, not vice versa. This may represent a physiological mechanism to prioritize memory of escalating threats.

Sex-Dependent Effects of Chronic Microdrive Implantation on Acquisition of Trace Eyeblink Conditioning

Neuroscience techniques, including in vivo recording, have allowed for a great expansion in knowledge; however, this technology may also affect the very phenomena researchers set out to investigate. Including both female and male mice in our associative learning experiments shed light on sex differences on the impact of chronic implantation of tetrodes on learning. While previous research showed intact female mice acquired trace eyeblink conditioning faster than male and ovariectomized females, implantation of chronic microdrive arrays showed sexually dimorphic effects on learning. Microdrive implanted male mice acquired the associative learning paradigm faster than both intact and ovariectomized females. These effects were not due to the weight of the drive alone, as there were no significant sex-differences in learning of animals that received "dummy drive" implants without tetrodes lowered into the brain. Tandem mass tag mass spectrometry and western blot analysis suggest that significant alterations in the MAPK pathway, acute inflammation, and brain derived neurotrophic factor may underlie these observed sex- and surgery-dependent effects on learning.

The neural substrate of spatial memory stabilization depends on the distribution of the training sessions

Distributed training has long been known to lead to more robust memory formation as compared to massed training. Using the water maze, a well-established task for assessing memory in laboratory rodents, we found that distributed and massed training differentially engage the dorsolateral and dorsomedial striatum, and optogenetic priming of dorsolateral striatum can artificially increase the robustness of massed training to the level of distributed training. Overall, our findings demonstrate that spatial memory consolidation engages different neural substrates depending on the training regimen, identifying a therapeutic avenue for memory enhancement.

Distributed training is known to lead to more robust memory formation as compared to training experiences with short intervals. Although this phenomenon, termed distributed practice effect, ubiquitous over a wide variety of tasks and organisms, has long been known by psychologists, its neurobiological underpinning is still poorly understood. Using the striatum as a model system here we tested the hypothesis that the ability of distributed training to optimize memory might depend upon the recruitment of different neural substrates compared to those engaged by massed training. First, by contrasting the medial and the lateral domains of the dorsal striatum after massed and distributed training we demonstrated that neuronal activity, as assessed using c-Fos expression, is differentially affected by the training protocol in the two striatal subregions. Next, by blocking the AMPA receptors before recall we provide evidence to support a selective role of the medial and the lateral striatum in the storage of information acquired by massed and distributed training, respectively. Finally, we found that optogenetic stimulation of the dorsolateral striatum during massed training enables the formation of an enduring memory similar to what is observed with distributed learning. Overall, these findings identify a possible mechanism for the distributed practice effect, a still poorly understood aspect of learning.

Some Twist of Molecular Circuitry Fast Forwards Overnight Sleep Hours: A Systematic Review of Natural Short Sleepers' Genes

This systematic review focuses on different genetic mutations identified in studies on natural short sleepers, who would not be ill-defined as one type of sleep-related disorder. The reviewed literature is from databases such as PubMed, PMC, Scopus, and ResearchGate. Due to the rare prevalence, the number of studies conducted on natural short sleepers is limited. Hence, searching the search of databases was done without any date restriction and included animal studies, since mouse and fly models share similarities with human sleep behaviors. Of the 12 articles analyzed, four conducted two types of studies, animal and human (cross-sectional or randomized-controlled studies), to testify the effects of human mutant genes in familial natural short sleepers via transgenic mouse or fly models. The remaining eight articles mainly focused on one type of study each: animal study (four articles), cross-sectional study (two articles), review (one article), and case report (one article). Hence, those articles brought different perspectives on the natural short sleep phenomenon by identifying intrinsic factors like DEC2, NPSR1, mGluR1, and β1-AR mutant genes. Natural short sleep traits in either point-mutations or single null mutations in those genes have been examined and confirmed its intrinsic nature in affected individuals without any related health concerns. Finally, this review added a potential limitation in these studies, mainly highlighting intrinsic causes since one case study reported an extrinsically triggered short sleep behavior in an older man without any family history. The overall result of the review study suggests that the molecular mechanisms tuned by identified sleep genes can give some potential points of therapeutic intervention in future studies.

Quantitative description of the interactions among kinase cascades underlying long-term plasticity of Aplysia sensory neurons

Kinases play critical roles in synaptic and neuronal changes involved in the formation of memory. However, significant gaps exist in the understanding of how interactions among kinase pathways contribute to the mechanistically distinct temporal domains of memory ranging from short-term memory to long-term memory (LTM). Activation of protein kinase A (PKA) and mitogen-activated protein kinase (MAPK)-ribosomal S6 kinase (RSK) pathways are critical for long-term enhancement of neuronal excitability (LTEE) and long-term synaptic facilitation (LTF), essential processes in memory formation. This study provides new insights into how these pathways contribute to the temporal domains of memory, using empirical and computational approaches. Empirical studies of Aplysia sensory neurons identified a positive feedforward loop in which the PKA and ERK pathways converge to regulate RSK, and a negative feedback loop in which p38 MAPK inhibits the activation of ERK and RSK. A computational model incorporated these findings to simulate the dynamics of kinase activity produced by different stimulus protocols and predict the critical roles of kinase interactions in the dynamics of these pathways. These findings may provide insights into the mechanisms underlying aberrant synaptic plasticity observed in genetic disorders such as RASopathies and Coffin-Lowry syndrome.

Insula to mPFC reciprocal connectivity differentially underlies novel taste neophobic response and learning in mice

To survive in an ever-changing environment, animals must detect and learn salient information. The anterior insular cortex (aIC) and medial prefrontal cortex (mPFC) are heavily implicated in salience and novelty processing, and specifically, the processing of taste sensory information. Here, we examined the role of aIC-mPFC reciprocal connectivity in novel taste neophobia and memory formation, in mice. Using pERK and neuronal intrinsic properties as markers for neuronal activation, and retrograde AAV (rAAV) constructs for connectivity, we demonstrate a correlation between aIC-mPFC activity and novel taste experience. Furthermore, by expressing inhibitory chemogenetic receptors in these projections, we show that aIC-to-mPFC activity is necessary for both taste neophobia and its attenuation. However, activity within mPFC-to-aIC projections is essential only for the neophobic reaction but not for the learning process. These results provide an insight into the cortical circuitry needed to detect, react to- and learn salient stimuli, a process critically involved in psychiatric disorders.

Two spaced training trials induce associative ERK-dependent long term memory in Neohelice granulata

Memory formation depends upon several parametric training conditions. Among them, trial number and inter-trial interval (ITI) are key factors to induce long-term retention. However, it is still unclear how individual training trials contribute to mechanisms underlying memory formation and stabilization. Contextual conditioning in Neohelice granulata has traditionally elicited associative long-term memory (LTM) after 15 spaced (ITI = 3 min) trials. Here, we show that LTM in crabs can be induced after only two training trials by increasing the ITI to 45 min (2t-LTM) and maintaining the same training duration as in traditional protocols. This newly observed LTM was preserved for at least 96 h, exhibiting protein synthesis dependence during consolidation and reconsolidation as well as context-specificity. Moreover, we demonstrate that 2t-LTM depends on inter-trial and post-training ERK activation showing a faster phosphorylation after the second trial compared to the first one. In summary, we present a new training protocol in crabs through a reduced number of trials showing associative features similar to traditional spaced training. This novel protocol allows for intra-training manipulation and the assessment of individual trial contribution to LTM formation.

Efficacy of spaced learning in adaptation of optokinetic response

INTRODUCTION

The superiority of spaced training, in which repeated training sessions are given with resting intervals, over massed training in learning efficacy has been well established. However, longer duration of total training time has been required for spaced training than massed training because spacing intervals intervene between training sessions in spaced training. Thus, the learning efficacy may not be simply compared between spaced and massed training in terms of "time efficiency." The aim of the present study was to investigate the efficacy of spaced and massed training using adaptation of horizontal optokinetic reflex (hOKR) in mice.

METHODS

Training paradigms were categorized into seven groups according to the duration of spacing interval, keeping total duration of hOKR training including spacing almost equal in all training paradigms.

RESULTS

The amount of short-term hOKR gain increase immediately after the 60 min hOKR training was not significantly different among seven training paradigms. The hOKR adaptation was still in progress during a spacing interval, and the increment in hOKR gain tended to be greater with the longer spacing interval. The increase in hOKR gain was maintained until 48 hr after the end of training in both massed and spaced training.

CONCLUSION

The short-term learning effect was not significantly different among training paradigms regardless of spacing interval in hOKR adaptation, which suggests that the spacing effect is robust enough to overcome the shortage of optokinetic training cycles in hOKR adaptation.

ELAV Proteins Bind and Stabilize C/EBP mRNA in the Induction of Long-Term Memory in Aplysia

Long-term memory (LTM) formation is a critical survival process by which an animal retains information about prior experiences to guide future behavior. In the experimentally advantageous marine mollusk Aplysia, LTM for sensitization can be induced by the presentation of two aversive shocks to the animal's tail. Each of these training trials recruits distinct growth factor signaling systems that promote LTM formation. Specifically, whereas intact TrkB signaling during Trial 1 promotes an initial and transient increase of the immediate early gene apc/ebp mRNA, a prolonged increase in apc/ebp gene expression required for LTM formation requires the addition of TGFβ signaling during Trial 2. Here we explored the molecular mechanisms by which Trial 2 achieves the essential prolonged gene expression of apc/ebp We find that this prolonged gene expression is not dependent on de novo transcription, but that apc/ebp mRNA synthesized by Trial 1 is post-transcriptionally stabilized by interacting with the RNA-binding protein ApELAV. This interaction is promoted by p38 MAPK activation initiated by TGFβ. We further demonstrate that blocking the interaction of ApELAV with its target mRNA during Trial 2 blocks both the prolonged increase in apc/ebp gene expression and the behavioral induction of LTM. Collectively, our findings elucidate both when and how ELAV proteins are recruited for the stabilization of mRNA in LTM formation. Stabilization of a transiently expressed immediate early gene mRNA by a repeated training trial may therefore serve as a "filter" for learning, permitting only specific events to cause lasting transcriptional changes and behavioral LTM.SIGNIFICANCE STATEMENT: In the present paper, we significantly extend the general field of molecular processing in long-term memory (LTM) by describing a novel form of pretranslational processing required for LTM, which relies on the stabilization of a newly synthesized mRNA by a class of RNA binding proteins (ELAVs). There are now compelling data showing that important processing can occur after transcription of a gene, but before translation of the message into protein. Although the potential importance of ELAV proteins in LTM formation has previously been reported, the specific actions of ELAV proteins during LTM formation remained to be understood. Our new findings thus complement and extend this literature by demonstrating when and how this post-transcriptional gene regulation is mediated in the induction of LTM.

Optimization and validation of a modified radial-arm water maze protocol using a murine model of mild closed head traumatic brain injury

Cognitive impairments can be a significant problem after a traumatic brain injury (TBI), which affects millions worldwide each year. There is a need for establish reproducible cognitive assays in rodents to better understand disease mechanisms and to develop therapeutic interventions towards treating TBI-induced impairments. Our goal was to validate and standardize the radial arm water maze (RAWM) test as an assay to screen for cognitive impairments caused by TBI. RAWM is a visuo-spatial learning test, originally designed for use with rats, and later adapted for mice. The present study investigates whether test procedures, such us the presence of extra-maze cues influences learning and memory performance. C57BL/6 mice were tested in an 8-arm RAWM using a four-day protocol. We demonstrated that two days of training, exposing the mice to extra-maze cues and a visible platform, influenced learning and memory performance. Mice that did not receive training performed poorer compared to mice trained. To further validate our RAWM protocol, we used scopolamine. We, also, demonstrated that a single mild closed head injury (CHI) caused deficits in this task at two weeks post-CHI. Our data supported the use of 7 trials per day and a spaced training protocol as key factor to unmask memory impairment following CHI. Here, we provide a detailed standard operating procedure for RAWM test, which can be applied to a variety of mouse models including neurodegenerative diseases and pathology, as well as when pharmacological approaches are used.

Repeating or spacing learning sessions are strategies for memory improvement with shared molecular and neuronal components

Intellectual disability is a common feature in genetic disorders with enhanced RAS-ERK1/2 signaling, including neurofibromatosis type 1 (NF1) and Noonan syndrome (NS). Additional training trials and additional spacing between trials, respectively, restores memory deficits in animal models of NF1 and NS. However, the relationship between the underlying mechanisms in these strategies remain obscure. Here, we developed an approach to examine the effect of adding training trials or spacing to a weak training protocol and used genetic and behavioral manipulations in Drosophila to explore such question. We found that repetition and spacing effects are highly related, being equally effective to improve memory in control flies and sharing mechanistic bases, including the requirement of RAS activity in mushroom body neurons and protein synthesis dependence. After spacing or repeating learning trials, memory improvement depends on the formation of long-term memory (LTM). Moreover, a disease-related gain-of-function RasV152G allele impaired LTM. Using minimal training protocols, we established that both learning strategies were also equally effective for memory rescue in the RasV152G mutant and showed non-additive interaction of the spacing and repetition effects. Memory improvement was never detected after Ras inhibition. We conclude that memory improvement by spacing or repeating training trials are two ways of using the same molecular resources, including RAS-ERK1/2-dependent signaling. This evidence supports the concept that learning problems in RAS-related disorders depend on the impaired ability to exploit the repetition and the spacing effect required for long-term memory induction.

Postsynaptic effects of Aplysia cysteine-rich neurotrophic factor in the induction of activity-dependent long-term facilitation in Aplysia californica

The spatial and temporal coordination of growth factor signaling is critical for both presynaptic and postsynaptic plasticity underlying long-term memory formation. We investigated the spatiotemporal dynamics of Aplysia cysteine-rich neurotrophic factor (ApCRNF) signaling during the induction of activity-dependent long-term facilitation (AD-LTF) at sensory-to-motor neuron synapses that mediate defensive reflexes in Aplysia We found that ApCRNF signaling is required for the induction of AD-LTF, and for training-induced early protein kinase activation and late forms of gene expression, exclusively in postsynaptic neurons. These results support the view that ApCRNF is critically involved in AD-LTF at least in part through postsynaptic mechanisms.

Role of p90 ribosomal S6 kinase in long-term synaptic facilitation and enhanced neuronal excitability

Multiple kinases converge on the transcription factor cAMP response element-binding protein (CREB) to enhance the expression of proteins essential for long-term synaptic plasticity and memory. The p90 ribosomal S6 kinase (RSK) is one of these kinases, although its role is poorly understood. The present study exploited the technical advantages of the Aplysia sensorimotor culture system to examine the role of RSK in long-term synaptic facilitation (LTF) and long-term enhancement of neuronal excitability (LTEE), two correlates of long-term memory (LTM). Inhibition of RSK expression or RSK activity both significantly reduced CREB1 phosphorylation, LTF, and LTEE, suggesting RSK is required for learning-related synaptic plasticity and enhancement in neuronal excitability. In addition, knock down of RSK by RNAi in Aplysia sensory neurons impairs LTF, suggesting that this may be a useful single-cell system to study aspects of defective synaptic plasticity in Coffin-Lowry Syndrome (CLS), a cognitive disorder that is caused by mutations in rsk2 and associated with deficits in learning and memory. We found that the impairments in LTF and LTEE can be rescued by a computationally designed spaced training protocol, which was previously demonstrated to augment normal LTF and LTM.

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

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

Neurotropic and modulatory effects of insulin-like growth factor II in Aplysia

Insulin-like growth factor II (IGF2) enhances memory in rodents via the mannose-6-phosphate receptor (M6PR), but the underlying mechanisms remain poorly understood. We found that human IGF2 produces an enhancement of both synaptic transmission and neurite outgrowth in the marine mollusk Aplysia californica. These findings were unexpected since Aplysia lack the mammal-specific affinity between insulin-like ligands and M6PR. Surprisingly, this effect was observed in parallel with a suppression of neuronal excitability in a well-understood circuit that supports several temporally and mechanistically distinct forms of memory in the defensive withdrawal reflex, suggesting functional coordination between excitability and memory formation. We hypothesize that these effects represent behavioral adaptations to feeding that are mediated by the endogenous Aplysia insulin-like system. Indeed, the exogenous application of a single recombinant insulin-like peptide cloned from the Aplysia CNS cDNA replicated both the enhancement of synaptic transmission, the reduction of excitability, and promoted clearance of glucose from the hemolymph, a hallmark of bona fide insulin action.

How can memories last for days, years, or a lifetime? Proposed mechanisms for maintaining synaptic potentiation and memory

With memory encoding reliant on persistent changes in the properties of synapses, a key question is how can memories be maintained from days to months or a lifetime given molecular turnover? It is likely that positive feedback loops are necessary to persistently maintain the strength of synapses that participate in encoding. Such feedback may occur within signal-transduction cascades and/or the regulation of translation, and it may occur within specific subcellular compartments or within neuronal networks. Not surprisingly, numerous positive feedback loops have been proposed. Some posited loops operate at the level of biochemical signal-transduction cascades, such as persistent activation of Ca²⁺/calmodulin kinase II (CaMKII) or protein kinase Mζ. Another level consists of feedback loops involving transcriptional, epigenetic and translational pathways, and autocrine actions of growth factors such as BDNF. Finally, at the neuronal network level, recurrent reactivation of cell assemblies encoding memories is likely to be essential for late maintenance of memory. These levels are not isolated, but linked by shared components of feedback loops. Here, we review characteristics of some commonly discussed feedback loops proposed to underlie the maintenance of memory and long-term synaptic plasticity, assess evidence for and against their necessity, and suggest experiments that could further delineate the dynamics of these feedback loops. We also discuss crosstalk between proposed loops, and ways in which such interaction can facilitate the rapidity and robustness of memory formation and storage.

ERK1/2: A Key Cellular Component for the Formation, Retrieval, Reconsolidation and Persistence of Memory

Extracellular regulated kinase 1/2 (ERK1/2) has been strongly implicated in several cellular processes. In the brain ERK1/2 activity has been primarily involved in long-term memory (LTM) formation and expression. Here, we review earlier evidence and describe recent developments of ERK1/2 signaling in memory processing focusing the attention on the role of ERK1/2 in memory retrieval and reconsolidation, and in the maintenance of the memory trace including mechanisms involving the protection of labile memories. In addition, relearning requires ERK1/2 activity in selected brain regions. Its involvement in distinct memory stages points at ERK1/2 as a core element in memory processing and as one likely target to treat memory impairments associated with neurological disorders.

Emergent memory in cell signaling: Persistent adaptive dynamics in cascades can arise from the diversity of relaxation time-scales

The mitogen-activated protein kinase (MAPK) signaling cascade, an evolutionarily conserved motif present in all eukaryotic cells, is involved in coordinating crucial cellular functions. While the asymptotic dynamical behavior of the pathway stimulated by a time-invariant signal is relatively well-understood, we show using a computational model that it exhibits a rich repertoire of transient adaptive responses to changes in stimuli. When the signal is switched on, the response is characterized by long-lived modulations in frequency as well as amplitude. On withdrawing the stimulus, the activity decays over long timescales, exhibiting reverberations characterized by repeated spiking in the activated MAPK concentration. The long-term persistence of such post-stimulus activity suggests that the cascade retains memory of the signal for a significant duration following its removal. The molecular mechanism underlying the reverberatory activity is related to the existence of distinct relaxation rates for the different cascade components. This results in the imbalance of fluxes between different layers of the cascade, with the reuse of activated kinases as enzymes when they are released from sequestration in complexes. The persistent adaptive response, indicative of a cellular “short-term” memory, suggests that this ubiquitous signaling pathway plays an even more central role in information processing by eukaryotic cells.

Signaling Pathways for Long-Term Memory Formation in the Cricket

Unraveling the molecular mechanisms underlying memory formation in insects and a comparison with those of mammals will contribute to a further understanding of the evolution of higher-brain functions. As it is for mammals, insect memory can be divided into at least two distinct phases: protein-independent short-term memory and protein-dependent long-term memory (LTM). We have been investigating the signaling pathway of LTM formation by behavioral-pharmacological experiments using the cricket Gryllus bimaculatus, whose olfactory learning and memory abilities are among the highest in insect species. Our studies revealed that the NO-cGMP signaling pathway, CaMKII and PKA play crucial roles in LTM formation in crickets. These LTM formation signaling pathways in crickets share a number of attributes with those of mammals, and thus we conclude that insects, with relatively simple brain structures and neural circuitry, will also be beneficial in exploratory experiments to predict the molecular mechanisms underlying memory formation in mammals.

Unfamiliar faces in recognition memory: spaced learning enhances subsequent recognition memory by reducing repetition priming

Although the spacing effect is one of most robust effects in learning, its cognitive and neural mechanisms are still under investigation. Whether the spacing effect is achieved by reducing neural repetition priming or depends on learning experience is still unclear. In this event-related potential study, participants were asked to memorize 140 novel faces, half under the massed learning condition and the other half under the spaced learning condition. The afterwards recognition tests indicated that participants recognized more items under the spaced learning condition than under the massed learning condition. The electroencephalography data suggested that spaced learning was associated with a reduced familiarity effect in frontal N400. Remembered faces showed smaller repetition priming than forgotten faces under both learning conditions and spaced learning significantly reduced repetition suppression. Although no direct association was found between repetition priming and episodic memory, the difference in quantity between spaced learning and massed learning in the repetition priming can predict the different quantities in the recognition memory. These results suggest that the neural mechanism of the spacing effect is influenced by experience; however, the impact is mainly repetition priming and the spacing effect is still very robust.

Superior long-term synaptic memory induced by combining dual pharmacological activation of PKA and ERK with an enhanced training protocol

Developing treatment strategies to enhance memory is an important goal of neuroscience research. Activation of multiple biochemical signaling cascades, such as the protein kinase A (PKA) and extracellular signal-regulated kinase (ERK) pathways, is necessary to induce long-term synaptic facilitation (LTF), a correlate of long-term memory (LTM). Previously, a computational model was developed which correctly predicted a novel enhanced training protocol that augmented LTF by searching for the protocol with maximal overlap of PKA and ERK activation. The present study focused on pharmacological approaches to enhance LTF. Combining an ERK activator, NSC, and a PKA activator, rolipram, enhanced LTF to a greater extent than did either drug alone. An even greater increase in LTF occurred when rolipram and NSC were combined with the Enhanced protocol. These results indicate superior memory can be achieved by enhanced protocols that take advantage of the structure and dynamics of the biochemical cascades underlying memory formation, used in conjunction with combinatorial pharmacology.

Long days enhance recognition memory and increase insulin-like growth factor 2 in the hippocampus

Light improves cognitive function in humans; however, the neurobiological mechanisms underlying positive effects of light remain unclear. One obstacle is that most rodent models have employed lighting conditions that cause cognitive deficits rather than improvements. Here we have developed a mouse model where light improves cognitive function, which provides insight into mechanisms underlying positive effects of light. To increase light exposure without eliminating daily rhythms, we exposed mice to either a standard photoperiod or a long day photoperiod. Long days enhanced long-term recognition memory, and this effect was abolished by loss of the photopigment melanopsin. Further, long days markedly altered hippocampal clock function and elevated transcription of Insulin-like Growth Factor2 (Igf2). Up-regulation of Igf2 occurred in tandem with suppression of its transcriptional repressor Wilm’s tumor1. Consistent with molecular de-repression of Igf2, IGF2 expression was increased in the hippocampus before and after memory training. Lastly, long days occluded IGF2-induced improvements in recognition memory. Collectively, these results suggest that light changes hippocampal clock function to alter memory, highlighting novel mechanisms that may contribute to the positive effects of light. Furthermore, this study provides insight into how the circadian clock can regulate hippocampus-dependent learning by controlling molecular processes required for memory consolidation.

The Spacing Effect for Structural Synaptic Plasticity Provides Specificity and Precision in Plastic Changes

In contrast to trials of training without intervals (massed training), training trials spaced over time (spaced training) induce a more persistent memory identified as long-term memory (LTM). This phenomenon, known as the spacing effect for memory, is poorly understood. LTM is supported by structural synaptic plasticity; however, how synapses integrate spaced stimuli remains elusive. Here, we analyzed events of structural synaptic plasticity at the single-synapse level after distinct patterns of stimulation in motoneurons of Drosophila We found that the spacing effect is a phenomenon detected at synaptic level, which determines the specificity and the precision in structural synaptic plasticity. Whereas a single pulse of stimulation (massed) induced structural synaptic plasticity, the same amount of stimulation divided in three spaced stimuli completely prevented it. This inhibitory effect was determined by the length of the interstimulus intervals. The inhibitory effect of the spacing was lost by suppressing the activity of Ras or mitogen-activated protein kinase, whereas the overexpression of Ras-WT enhanced it. Moreover, dividing the same total time of stimulation into five or more stimuli produced a higher precision in the number of events of plasticity. Ras mutations associated with intellectual disability abolished the spacing effect and led neurons to decode distinct stimulation patterns as massed stimulation. This evidence suggests that the spacing effect for memory may result from the effect of the spacing in synaptic plasticity, which appears to be a property not limited to neurons involved in learning and memory. We propose a model of spacing-dependent structural synaptic plasticity.SIGNIFICANCE STATEMENT Long-term memory (LTM) induced by repeated trials spaced over time is known as the spacing effect, a common property in the animal kingdom. Altered mechanisms in the spacing effect have been found in animal models of disorders with intellectual disability, such as Noonan syndrome. Although LTM is sustained by structural synaptic plasticity, how synapses integrate spaced stimuli and decode them into specific plastic changes remains elusive. Here, we show that the spacing effect is a phenomenon detected at the synaptic level, which determines the properties of the response in structural plasticity, including precision of such response. Whereas suppressing or enhancing Ras/mitogen-activated protein kinase signaling changed how synapses decode a pattern of stimuli, a disease-related Ras allele abolished the spacing effect for plastic changes.

Biphasic Regulation of p38 MAPK by Serotonin Contributes to the Efficacy of Stimulus Protocols That Induce Long-Term Synaptic Facilitation

The MAPK isoforms ERK and p38 MAPK are believed to play opposing roles in long-term synaptic facilitation (LTF) induced by serotonin (5-HT) in Aplysia. To fully understand their roles, however, it is necessary to consider the dynamics of ERK and p38 MAPK activation. Previous studies determined that activation of ERK occurred ∼45 min after a 5-min pulse of 5-HT treatment. The dynamics of p38 MAPK activation following 5-HT are yet to be elucidated. Here, the activity of p38 MAPK was examined at different times after 5-HT, and the interaction between the ERK and p38 MAPK pathways was investigated. A 5-min pulse of 5-HT induced a transient inhibition of p38 MAPK, followed by a delayed activation between 25 and 45 min. This activation was blocked by a MAPK kinase inhibitor, suggesting that similar pathways are involved in activation of ERK and p38 MAPK. ERK activity decreased shortly after the activation of p38 MAPK. A p38 MAPK inhibitor blocked this decrease in ERK activity, suggesting a causal relationship. The p38 MAPK activity ∼45 min after different stimulus protocols was also characterized. These data were incorporated into a computational model for the induction of LTF. Simulations and empirical data suggest that p38 MAPK, together with ERK, contributes to the efficacy of spaced stimulus protocols to induce LTF, a correlate of long-term memory (LTM). For example, decreased p38 MAPK activity ∼45 min after the first of two sensitizing stimuli might be an important determinant of an optimal interstimulus interval (ISI) for LTF induction.

The Contribution of Spatial and Temporal Molecular Networks in the Induction of Long-term Memory and Its Underlying Synaptic Plasticity

The ability to form long-lasting memories is critical to survival and thus is highly conserved across the animal kingdom. By virtue of its complexity, this same ability is vulnerable to disruption by a wide variety of neuronal traumas and pathologies. To identify effective therapies with which to treat memory disorders, it is critical to have a clear understanding of the cellular and molecular mechanisms which subserve normal learning and memory. A significant challenge to achieving this level of understanding is posed by the wide range of distinct temporal and spatial profiles of molecular signaling induced by learning-related stimuli. In this review we propose that a useful framework within which to address this challenge is to view the molecular foundation of long-lasting plasticity as composed of unique spatial and temporal molecular networks that mediate signaling both within neurons (such as via kinase signaling) as well as between neurons (such as via growth factor signaling). We propose that evaluating how cells integrate and interpret these concurrent and interacting molecular networks has the potential to significantly advance our understanding of the mechanisms underlying learning and memory formation.

Sustained Hypoxia Elicits Competing Spinal Mechanisms of Phrenic Motor Facilitation

Acute intermittent hypoxia (AIH) induces phrenic long-term facilitation (pLTF), a form of spinal motor plasticity. Competing mechanisms give rise to phrenic motor facilitation (pMF; a general term including pLTF) depending on the severity of hypoxia within episodes. In contrast, moderate acute sustained hypoxia (mASH) does not elicit pMF. By varying the severity of ASH and targeting competing mechanisms of pMF, we sought to illustrate why moderate AIH (mAIH) elicits pMF but mASH does not. Although mAIH elicits serotonin-dependent pLTF, mASH does not; thus, mAIH-induced pLTF is pattern sensitive. In contrast, severe AIH (sAIH) elicits pLTF through adenosine-dependent mechanisms, likely from greater extracellular adenosine accumulation. Because serotonin- and adenosine-dependent pMF interact via cross talk inhibition, we hypothesized that pMF is obscured because the competing mechanisms of pMF are balanced and offsetting during mASH. Here, we demonstrate the following: (1) blocking spinal A2A receptors with MSX-3 reveals mASH-induced pMF; and (2) sASH elicits A2A-dependent pMF. In anesthetized rats pretreated with intrathecal A2A receptor antagonist injections before mASH (PaO2 = 40-54 mmHg) or sASH (PaO2 = 25-36 mmHg), (1) mASH induced a serotonin-dependent pMF and (2) sASH induced an adenosine-dependent pMF, which was enhanced by spinal serotonin receptor inhibition. Thus, competing adenosine- and serotonin-dependent mechanisms contribute differentially to pMF depending on the pattern/severity of hypoxia. Understanding interactions between these mechanisms has clinical relevance as we develop therapies to treat severe neuromuscular disorders that compromise somatic motor behaviors, including breathing. Moreover, these results demonstrate how competing mechanisms of plasticity can give rise to pattern sensitivity in pLTF.

SIGNIFICANCE STATEMENT

Intermittent hypoxia elicits pattern-sensitive spinal plasticity and improves motor function after spinal injury or during neuromuscular disease. Specific mechanisms of pattern sensitivity in this form of plasticity are unknown. We provide evidence that competing mechanisms of phrenic motor facilitation mediated by adenosine 2A and serotonin 2 receptors are differentially expressed, depending on the pattern/severity of hypoxia. Understanding how these distinct mechanisms interact during hypoxic exposures differing in severity and duration will help explain interesting properties of plasticity, such as pattern sensitivity, and may help optimize therapies to restore motor function in patients with neuromuscular disorders that compromise movement.

Administration of the TrkB receptor agonist 7,8-dihydroxyflavone prevents traumatic stress-induced spatial memory deficits and changes in synaptic plasticity

Post-traumatic stress disorder (PTSD) occurs after exposure to traumatic situations and it is characterized by cognitive deficits that include impaired explicit memory. The neurobiological bases of such PTSD-associated memory alterations are yet to be elucidated and no satisfactory treatment for them exists. To address this issue, we first studied whether a single exposure of young adult rats (60 days) to immobilization on boards (IMO), a putative model of PTSD, produces long-term behavioral effects (2-8 days) similar to those found in PTSD patients. Subsequently, we investigated whether the administration of the TrkB agonist 7,8-dihydroxyflavone (DHF) 8 h after stress (therapeutic window) ameliorated the PTSD-like effect of IMO and the associated changes in synaptic plasticity. A single IMO exposure induced a spatial memory impairment similar to that found in other animal models of PTSD or in PTSD patients. IMO also increased spine density and long-term potentiation (LTP) in the CA3-CA1 pathway. Significantly, DHF reverted both spatial memory impairment and the increase in LTP, while it produced no effect in the controls. These data provide novel insights into the possible neurobiological substrate for explicit memory impairment in PTSD patients, supporting the idea that the activation of the BDNF/TrkB pathway fulfils a protective role after severe stress. Administration of DHF in the aftermath of a traumatic experience might be relevant to prevent its long-term consequences. © 2016 Wiley Periodicals, Inc.

Transforming growth factor β recruits persistent MAPK signaling to regulate long-term memory consolidation in Aplysia californica

In this study, we explore the mechanistic relationship between growth factor signaling and kinase activity that supports the protein synthesis-dependent phase of long-term memory (LTM) consolidation for sensitization of Aplysia. Specifically, we examine LTM for tail shock-induced sensitization of the tail-elicited siphon withdrawal (T-SW) reflex, a form of memory that requires both (i) extracellular signal-regulated kinase (ERK1/2; MAPK) activity within identified sensory neurons (SNs) that mediate the T-SW and (ii) the activation of transforming growth factor β (TGFβ) signaling. We now report that repeated tail shocks that induce intermediate-term (ITM) and LTM for sensitization, also induce a sustained post-training phase of MAPK activity in SNs (lasting at least 1 h). We identified two mechanistically distinct phases of post-training MAPK: (i) an immediate phase that does not require ongoing protein synthesis or TGFβ signaling, and (ii) a sustained phase that requires both protein synthesis and extracellular TGFβ signaling. We find that LTM consolidation requires sustained MAPK, and is disrupted by inhibitors of protein synthesis and TGFβ signaling during the consolidation window. These results provide strong evidence that TGFβ signaling sustains MAPK activity as an essential mechanistic step for LTM consolidation.

Importin-7 mediates memory consolidation through regulation of nuclear translocation of training-activated MAPK in Drosophila

Translocation of signaling molecules, MAPK in particular, from the cytosol to nucleus represents a universal key element in initiating the gene program that determines memory consolidation. Translocation mechanisms and their behavioral impact, however, remain to be determined. Here, we report that a highly conserved nuclear transporter, Drosophila importin-7 (DIM-7), regulates import of training-activated MAPK for consolidation of long-term memory (LTM). We show that silencing DIM-7 functions results in impaired LTM, whereas overexpression of DIM-7 enhances LTM. This DIM-7-dependent regulation of LTM is confined to a consolidation time window and in mushroom body neurons. Image data show that bidirectional alteration in DIM-7 expression results in proportional changes in the intensity of training-activated MAPK accumulated within the nuclei of mushroom body neurons during LTM consolidation. Such DIM-7-regulated nuclear accumulation of activated MAPK is observed only in the training specified for LTM induction and determines the amplitude, but not the time course, of memory consolidation.

Stochasticity and bifurcations in a reduced model with interlinked positive and negative feedback loops of CREB1 and CREB2 stimulated by 5-HT

The cyclic AMP (cAMP)-response element-binding protein (CREB) family of transcription factors is crucial in regulating gene expression required for long-term memory (LTM) formation. Upon exposure of sensory neurons to the neurotransmitter serotonin (5-HT), CREB1 is activated via activation of the protein kinase A (PKA) intracellular signaling pathways, and CREB2 as a transcriptional repressor is relieved possibly via phosphorylation of CREB2 by mitogen-activated protein kinase (MAPK). Song et al. [18] proposed a minimal model with only interlinked positive and negative feedback loops of transcriptional regulation by the activator CREB1 and the repressor CREB2. Without considering feedbacks between the CREB proteins, Pettigrew et al. [8] developed a computational model characterizing complex dynamics of biochemical pathways downstream of 5-HT receptors. In this work, to describe more simply the biochemical pathways and gene regulation underlying 5-HT-induced LTM, we add the important extracellular sensitizing stimulus 5-HT as well as the product Ap-uch into the Song's minimal model. We also strive to examine dynamical properties of the gene regulatory network under the changing concentration of the stimulus, [5-HT], cooperating with the varying positive feedback strength in inducing a high state of CREB1 for the establishment of long-term memory. Different dynamics including monostability, bistability and multistability due to coexistence of stable steady states and oscillations is investigated by means of codimension-2 bifurcation analysis. At the different positive feedback strengths, comparative analysis of deterministic and stochastic dynamics reveals that codimension-1 bifurcation with respect to [5-HT] as the parameter can predict diverse stochastic behaviors resulted from the finite number of molecules, and the number of CREB1 molecules more and more preferentially resides near the high steady state with increasing [5-HT], which contributes to long-term memory formation.

The right time to learn: mechanisms and optimization of spaced learning

For many types of learning, spaced training, which involves repeated long inter-trial intervals, leads to more robust memory formation than does massed training, which involves short or no intervals. Several cognitive theories have been proposed to explain this superiority, but only recently have data begun to delineate the underlying cellular and molecular mechanisms of spaced training, and we review these theories and data here. Computational models of the implicated signalling cascades have predicted that spaced training with irregular inter-trial intervals can enhance learning. This strategy of using models to predict optimal spaced training protocols, combined with pharmacotherapy, suggests novel ways to rescue impaired synaptic plasticity and learning.

Synaptic generation of an intracellular retrograde signal requires activation of the tyrosine kinase and mitogen-activated protein kinase signaling cascades in Aplysia

Cellular changes underlying memory formation can be generated in an activity-dependent manner at specific synapses. Thus an important question concerns the mechanisms by which synaptic signals communicate with the cell body to mediate these cellular changes. A monosynaptic circuit that is enhanced by sensitization in Aplysia is well-suited to study this question because three different subcellular compartments: (i) the sensorimotor SN-MN synapses, (ii) the SN projections to MNs via axonal connections, (iii) the SN cell bodies, can all be manipulated and studied independently. Here, we report that activity-dependent (AD) training in either the entire SN-MN circuit or in only the synaptic compartment, activates MAPK in a temporally and spatially specific pattern. Specifically, we find (i) MAPK activation is first transiently generated at SN-MN synapses during training, (ii) immediately after training MAPK is transiently activated in SN-MN axonal connections and persistently activated in SN cell bodies, and finally, (iii) MAPK is activated in SN cell bodies and SN-MN synapses 1h after training. These data suggest that there is an intracellularly transported retrograde signal generated at the synapse which is later responsible for delayed MAPK activation at SN somata. Finally, we find that this retrograde signal requires activation of tyrosine kinase (TK) and MEK signaling cascades at the synapses.

Latent memory facilitates relearning through molecular signaling mechanisms that are distinct from original learning

A highly conserved feature of memory is that it can exist in a latent, non-expressed state which is revealed during subsequent learning by its ability to significantly facilitate (savings) or inhibit (latent inhibition) subsequent memory formation. Despite the ubiquitous nature of latent memory, the mechanistic nature of the latent memory trace and its ability to influence subsequent learning remains unclear. The model organism Aplysia californica provides the unique opportunity to make strong links between behavior and underlying cellular and molecular mechanisms. Using Aplysia, we have studied the mechanisms of savings due to latent memory for a prior, forgotten experience. We previously reported savings in the induction of three distinct temporal domains of memory: short-term (10min), intermediate-term (2h) and long-term (24h). Here we report that savings memory formation utilizes molecular signaling pathways that are distinct from original learning: whereas the induction of both original intermediate- and long-term memory in naïve animals requires mitogen activated protein kinase (MAPK) activation and ongoing protein synthesis, 2h savings memory is not disrupted by inhibitors of MAPK or protein synthesis, and 24h savings memory is not dependent on MAPK activation. Collectively, these findings reveal that during forgetting, latent memory for the original experience can facilitate relearning through molecular signaling mechanisms that are distinct from original learning.

Enhanced memory consolidation in mice lacking the circadian modulators Sharp1 and -2 caused by elevated Igf2 signaling in the cortex

The bHLH transcription factors SHARP1 and SHARP2 are partially redundant modulators of the circadian system. SHARP1/DEC2 has been shown to control sleep length in humans and sleep architecture is also altered in double mutant mice (S1/2(-/-)). Because of the importance of sleep for memory consolidation, we investigated the role of SHARP1 and SHARP2 in cognitive processing. S1/2(-/-) mice show enhanced cortex (Cx)-dependent remote fear memory formation as well as improved reversal learning, but do not display alterations in hippocampus (Hi)-dependent recent fear memory formation. SHARP1 and SHARP2 single null mutants do not display any cognitive phenotype supporting functional redundancy of both factors. Molecular and biochemical analyses revealed elevated insulin-related growth factor 2 (IGF2) signaling and increased phosphorylation of MAPK and S6 in the Cx but not the Hi of S1/2(-/-) mice. No changes were detected in single mutants. Moreover, adeno-associated virus type 2-mediated IGF2 overexpression in the anterior cingulate cortex enhanced remote fear memory formation and the analysis of forebrain-specific double null mutants of the Insulin and IGF1 receptors revealed their essential function for memory formation. Impaired fear memory formation in aged S1/2(-/-) mice indicates that elevated IGF2 signaling in the long term, however, has a negative impact on cognitive processing. In summary, we conclude that the bHLH transcription factors SHARP1 and SHARP2 are involved in cognitive processing by controlling Igf2 expression and associated signaling cascades. Our analyses provide evidence that the control of sleep and memory consolidation may share common molecular mechanisms.

Distinct Growth Factor Families Are Recruited in Unique Spatiotemporal Domains during Long-Term Memory Formation in Aplysia californica

Several growth factors (GFs) have been implicated in long-term memory (LTM), but no single GF can support all of the plastic changes that occur during memory formation. Because GFs engage highly convergent signaling cascades that often mediate similar functional outcomes, the relative contribution of any particular GF to LTM is difficult to ascertain. To explore this question, we determined the unique contribution of distinct GF families (signaling via TrkB and TGF-βr-II) to LTM formation in Aplysia. We demonstrate that TrkB and TGF-βr-II signaling are differentially recruited during two-trial training in both time (by trial 1 or 2, respectively) and space (in distinct subcellular compartments). These GFs independently regulate MAPK activation and synergistically regulate gene expression. We also show that trial 1 TrkB and trial 2 TGF-βr-II signaling are required for LTM formation. These data support the view that GFs engaged in LTM formation are interactive components of a complex molecular network.

Doxorubicin attenuates serotonin-induced long-term synaptic facilitation by phosphorylation of p38 mitogen-activated protein kinase

Doxorubicin (DOX) is an anthracycline used widely for cancer chemotherapy. Its primary mode of action appears to be topoisomerase II inhibition, DNA cleavage, and free radical generation. However, in non-neuronal cells, DOX also inhibits the expression of dual-specificity phosphatases (also referred to as MAPK phosphatases) and thereby inhibits the dephosphorylation of extracellular signal-regulated kinase (ERK) and p38 mitogen-activated protein kinase (p38 MAPK), two MAPK isoforms important for long-term memory (LTM) formation. Activation of these kinases by DOX in neurons, if present, could have secondary effects on cognitive functions, such as learning and memory. The present study used cultures of rat cortical neurons and sensory neurons (SNs) of Aplysia to examine the effects of DOX on levels of phosphorylated ERK (pERK) and phosphorylated p38 (p-p38) MAPK. In addition, Aplysia neurons were used to examine the effects of DOX on long-term enhanced excitability, long-term synaptic facilitation (LTF), and long-term synaptic depression (LTD). DOX treatment led to elevated levels of pERK and p-p38 MAPK in SNs and cortical neurons. In addition, it increased phosphorylation of the downstream transcriptional repressor cAMP response element-binding protein 2 in SNs. DOX treatment blocked serotonin-induced LTF and enhanced LTD induced by the neuropeptide Phe-Met-Arg-Phe-NH2. The block of LTF appeared to be attributable to overriding inhibitory effects of p-p38 MAPK, because LTF was rescued in the presence of an inhibitor (SB203580 [4-(4-fluorophenyl)-2-(4-methylsulfinylphenyl)-5-(4-pyridyl)-1H-imidazole]) of p38 MAPK. These results suggest that acute application of DOX might impair the formation of LTM via the p38 MAPK pathway.

Understanding intellectual disability through RASopathies

Intellectual disability, commonly known as mental retardation in the International Classification of Disease from World Health Organization, is the term that describes an intellectual and adaptive cognitive disability that begins in early life during the developmental period. Currently the term intellectual disability is the preferred one. Although our understanding of the physiological basis of learning and learning disability is poor, a general idea is that such condition is quite permanent. However, investigations in animal models suggest that learning disability can be functional in nature and as such reversible through pharmacology or appropriate learning paradigms. A fraction of the cases of intellectual disability is caused by point mutations or deletions in genes that encode for proteins of the RAS/MAP kinase signaling pathway known as RASopathies. Here we examined the current understanding of the molecular mechanisms involved in this group of genetic disorders focusing in studies which provide evidence that intellectual disability is potentially treatable and curable. The evidence presented supports the idea that with the appropriate understanding of the molecular mechanisms involved, intellectual disability could be treated pharmacologically and perhaps through specific mechanistic-based teaching strategies.

A novel cysteine-rich neurotrophic factor in Aplysia facilitates growth, MAPK activation, and long-term synaptic facilitation

Neurotrophins are critically involved in developmental processes such as neuronal cell survival, growth, and differentiation, as well as in adult synaptic plasticity contributing to learning and memory. Our previous studies examining neurotrophins and memory formation in Aplysia showed that a TrkB ligand is required for MAPK activation, long-term synaptic facilitation (LTF), and long-term memory (LTM) for sensitization. These studies indicate that neurotrophin-like molecules in Aplysia can act as key elements in a functionally conserved TrkB signaling pathway. Here we report that we have cloned and characterized a novel neurotrophic factor, Aplysia cysteine-rich neurotrophic factor (apCRNF), which shares classical structural and functional characteristics with mammalian neurotrophins. We show that apCRNF (1) is highly enriched in the CNS, (2) enhances neurite elongation and branching, (3) interacts with mammalian TrkB and p75^NTR, (4) is released from Aplysia CNS in an activity-dependent fashion, (5) facilitates MAPK activation in a tyrosine kinase dependent manner in response to sensitizing stimuli, and (6) facilitates the induction of LTF. These results show that apCRNF is a native neurotrophic factor in Aplysia that can engage the molecular and synaptic mechanisms underlying memory formation.