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Empirical and computational methods were combined to examine whether individual or dual-drug treatments can restore the deficit in long-term synaptic facilitation (LTF) of theAplysiasensorimotor synapse observed in a cellular model of Coffin–Lowry syndrome (CLS). The model was produced by pharmacological inhibition of p90 ribosomal S6 kinase (RSK) activity. In this model, coapplication of an activator of the mitogen-activated protein kinase (MAPK) isoform ERK and an activator of protein kinase A (PKA) resulted in enhanced phosphorylation of RSK and enhanced LTF to a greater extent than either drug alone and also greater than their additive effects, which is termed synergism. The extent of synergism appeared to depend on another MAPK isoform, p38 MAPK. Inhibition of p38 MAPK facilitated serotonin (5-HT)-induced RSK phosphorylation, indicating that p38 MAPK inhibits activation of RSK. Inhibition of p38 MAPK combined with activation of PKA synergistically activated both ERK and RSK. Our results suggest that cellular models of disorders that affect synaptic plasticity and learning, such as CLS, may constitute a useful strategy to identify candidate drug combinations, and that combining computational models with empirical tests of model predictions can help explain synergism of drug combinations.

Mechanisms of specific memory deficits associated with Rett syndrome are poorly understood, at least in part because mutations ofMECP2have confounding effects on nervous system development and basal synaptic transmission. To mitigate such empirical uncertainties, this study exploited technical advantages of theAplysiasensorimotor synapse to examine the potential role of MeCP2 in long-term synaptic plasticity. The results indicate MeCP2 may act as an inhibitory constraint on gene expression required for formation as well as maintenance of plasticity.

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.

The mitogen-activated protein kinase (MAPK) isoform extracellular signal–regulated kinase (ERK) is a key kinase involved in the induction of long-term synaptic facilitation (LTF) of the Aplysia sensorimotor synapse. Therefore, elucidating the dynamics of ERK activation after LTF-inducing protocols is critical for understanding the mechanisms underlying neuronal and synaptic plasticity. ERK activation has rich dynamic features. After a single stimulus, activation peaks 45-minute later and declines rapidly, but after two stimuli spaced 45 min apart, activation persists (Kopec et al. 2015). However, little is known about possible changes in ERK activation for periods beyond 3 h. Given its key role in long-term learning, understanding the dynamics of ERK at 3 h and beyond might provide insights into new protocols that could enhance memory retention. Three different protocols that induce LTF were used to probe the dynamics of ERK activation. The first, termed the Standard protocol, consists of five 5-min pulses of serotonin (5-HT) with regular interstimulus intervals (ISIs) of 20 min. The second, termed the Enhanced protocol, consists of five pulses of 5-HT with irregular ISIs identified with the use of computer simulations. This protocol induces greater and longer-lasting LTF than the Standard protocol. A third protocol, termed the two-pulse protocol, consists of just two 5-min pulses of 5-HT with an ISI of 45 min. Immunofluorescence revealed complex patterns of ERK activation up to 24 h after 5-HT treatment. The Standard and two-pulse protocols led to an immediate increase in active, phosphorylated ERK (pERK), which decayed within 5 h post treatment. A second wave of increased pERK was detected at 18 h post treatment. This late phase was blocked by RpcAMP (an inhibitor of protein kinase A), and by TrkB Fc and TGF-β RII F antagonists. The latter two are chimeras that act via receptor sequestration. These results suggest that complex interactions among kinase pathways and growth factor cascades contribute to the late increase of ERK activity after different LTF-inducing protocols. Interestingly, ERK activity returned to basal levels 24 h after the Standard or two-pulse protocol, but remained elevated 24 h after the Enhanced protocol. This finding may help explain, in part, why the Enhanced protocol is superior to the Standard protocol in inducing long-lasting LTF.

Learning requires the activation of protein kinases with distinct temporal dynamics. Nonassociative learning inAplysiacan be enhanced by a computationally designed learning protocol with intertrial intervals (ITIs) that maximize the interaction between fast-activated protein kinase A (PKA) and slow-activated extracellular signal-regulated kinase (ERK). We tested whether associative learning in mammals can be enhanced by an optimal learning protocol with irregular ITIs, predicted computationally to increase the overlap between PKA and ERK signaling in rat hippocampus. We simulated 1,000 training protocols with varying ITIs to predict this optimal protocol. With auditory fear conditioning, we found that male adult rats exposed to the optimal protocol with irregular ITIs exhibited impaired extinction memory acquisition within the session with a standard footshock intensity, and stronger fear memory retrieval and spontaneous recovery with a weaker footshock intensity, when compared to rats that received either massed or spaced conditioning protocols with the same number of footshocks and fixed ITIs. With fear extinction stimuli, we likewise observed that rats exposed to the optimal protocol showed improved extinction of contextual fear memory and attenuated spontaneous recovery, compared to rats that received standard extinction protocols. Immunohistochemistry confirmed that the optimal conditioning protocol increased phosphorylated cAMP responsive element binding (pCREB) protein levels in the dentate gyrus (DG) of the dorsal hippocampus, suggesting higher levels of LTP induction. These findings demonstrate the capacity of a behavioral intervention driven by a computational model of memory-related signaling pathways to enhance associative learning in mammals, and may provide greater insight into strategies to improve cognition in humans.

Despite numerous studies examining the mechanisms of operant conditioning (OC), the diversity of OC plasticity loci and their synergism have not been examined sufficiently. In the well-characterized feeding neural circuit ofAplysia,in vivoandin vitroappetitive OC increases neuronal excitability and electrical coupling among several neurons leading to an increase in expression of ingestive behavior. Here, we used thein vitroanalog of OC to investigate whether OC reduces the excitability of a neuron, B4, whose inhibitory connections decrease expression of ingestive behavior. We found OC decreased the excitability of B4. This change appeared intrinsic to B4 because it could be replicated with an analog of OC in isolated cultures of B4 neurons. In addition to changes in B4 excitability, OC decreased the strength of B4's inhibitory connection to a key decision-making neuron, B51. The OC-induced changes were specific without affecting the excitability of another neuron critical for feeding behavior, B8, or the B4-to-B8 inhibitory connection. A conductance-based circuit model indicated that reducing the B4-to-B51 synapse, or increasing B51 excitability, mediated the OC phenotype more effectively than did decreasing B4 excitability. We combined these modifications to examine whether they could act synergistically. Combinations including B51 synergistically enhanced feeding. Taken together, these results suggest modifications of diverse loci work synergistically to mediate OC and that some neurons are well suited to work synergistically with plasticity in other loci.SIGNIFICANCE STATEMENTThe ways in which synergism of diverse plasticity loci mediate the change in motor patterns in operant conditioning (OC) are poorly understood. Here, we found that OC was in part mediated by decreasing the intrinsic excitability of a critical neuron ofAplysiafeeding behavior, and specifically reducing the strength of one of its inhibitory connections that targets a key decision-making neuron. A conductance-based computational model indicated that the known plasticity loci showed a surprising level of synergism to mediate the behavioral changes associated with OC. These results highlight the importance of understanding the diversity, specificity and synergy among different types of plasticity that encode memory. Also, because OC inAplysiais mediated by dopamine (DA), the present study provides insights into specific and synergistic mechanisms of DA-mediated reinforcement of behaviors.

SUMMARYSome neurons express a plateau of spike activity greatly outlasting a stimulus, whereas others do not. This difference could be due to differential expression of ion channels (e.g., persistent Na+, and Ca2+) or similar expression of channels that differ in their properties. We compared three Aplysia neurons with varying capacity to generate plateau potentials: B51 with self-terminating plateaus, B64 with plateau potentials that do not self-terminate, and the regular spiking neuron B8. Our results indicate the three neurons expressed outward currents IA and ID, voltage-gated Ca2+ currents ICaL and ICaR, and persistent inward INaP. The most notable difference observed was a larger IA, ID, and IKCa currents in B8, and the plateau generating B64 did not express IKCa. Computational models suggest outward currents suppress and temporally constrain the plateau potential and that inward Ca2+ currents suppress plateau potentials when coupled with IKCa.

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.

A major limitation of large-scale neuronal recordings is the difficulty in locating homologous neurons in different subjects, referred to as the “correspondence” issue. This issue stems, at least in part, from the lack of a unique feature that unequivocally identifies each neuron. One promising approach to this problem is the functional neurocartography framework developed by Frady et al. (2016), in which neurons are identified by a semisupervised machine learning algorithm using a combination of multiple selected features. Here, the framework was adapted to the buccal ganglia of Aplysia. Multiple features were derived from neuronal activity during motor pattern generation, responses to peripheral nerve stimulation, and the spatial properties of each cell. The feature set was optimized based on its potential usefulness in discriminating neurons from each other, and then used to match putatively homologous neurons across subjects with the functional neurocartography software. An alternative matching method based on a cyclic matching algorithm was also developed, which allows for unsupervised extraction of groups of neurons and automated selection of high-quality matches. This improvement enabled unsupervised implementation of the machine learning algorithm, thereby enhancing scalability of the analysis. This study paves the way for investigating the roles of both well-characterized and previously uncharacterized neurons in Aplysia, and helps to adapt the neurocartography framework to other systems.

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.

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.

Operant reward learning of feeding behavior in Aplysia increases the frequency and regularity of biting, as well as biases buccal motor patterns (BMPs) toward ingestion-like BMPs (iBMPs). The engram underlying this memory comprises cells that are part of a central pattern generating (CPG) circuit and includes increases in the intrinsic excitability of identified cells B30, B51, B63, and B65, and increases in B63–B30 and B63–B65 electrical synaptic coupling. To examine the ways in which sites of plasticity (individually and in combination) contribute to memory expression, a model of the CPG was developed. The model included conductance-based descriptions of cells CBI-2, B4, B8, B20, B30, B31, B34, B40, B51, B52, B63, B64, and B65, and their synaptic connections. The model generated patterned activity that resembled physiological BMPs, and implementation of the engram reproduced increases in frequency, regularity, and bias. Combined enhancement of B30, B63, and B65 excitabilities increased BMP frequency and regularity, but not bias toward iBMPs. Individually, B30 increased regularity and bias, B51 increased bias, B63 increased frequency, and B65 decreased all three BMP features. Combined synaptic plasticity contributed primarily to regularity, but also to frequency and bias. B63–B30 coupling contributed to regularity and bias, and B63–B65 coupling contributed to all BMP features. Each site of plasticity altered multiple BMP features simultaneously. Moreover, plasticity loci exhibited mutual dependence and synergism. These results indicate that the memory for operant reward learning emerged from the combinatoric engagement of multiple sites of plasticity.

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.

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.

Genetic disorders such as Rubinstein-Taybi syndrome (RTS) and Coffin-Lowry syndrome (CLS) cause lifelong cognitive disability, including deficits in learning and memory. Can pharmacological therapies be suggested that improve learning and memory in these disorders? To address this question, we simulated drug effects within a computational model describing induction of late long-term potentiation (L-LTP). Biochemical pathways impaired in these and other disorders converge on a common target, histone acetylation by acetyltransferases such as CREB binding protein (CBP), which facilitates gene induction necessary for L-LTP. We focused on four drug classes: tropomyosin receptor kinase B (TrkB) agonists, cAMP phosphodiesterase inhibitors, histone deacetylase inhibitors, and ampakines. Simulations suggested each drug type alone may rescue deficits in L-LTP. A potential disadvantage, however, was the necessity of simulating strong drug effects (high doses), which could produce adverse side effects. Thus, we investigated the effects of six drug pairs among the four classes described above. These combination treatments normalized impaired L-LTP with substantially smaller individual drug 'doses'. In addition three of these combinations, a TrkB agonist paired with an ampakine and a cAMP phosphodiesterase inhibitor paired with a TrkB agonist or an ampakine, exhibited strong synergism in L-LTP rescue. Therefore, we suggest these drug combinations are promising candidates for further empirical studies in animal models of genetic disorders that impair histone acetylation, L-LTP, and learning.

Objective. Accurate inference of functional connectivity is critical for understanding brain function. Previous methods have limited ability distinguishing between direct and indirect connections because of inadequate scaling with dimensionality. This poor scaling performance reduces the number of nodes that can be included in conditioning. Our goal was to provide a technique that scales better and thereby enables minimization of indirect connections. Approach. Our major contribution is a powerful model-free framework, graphical directed information (GDI), that enables pairwise directed functional connections to be conditioned on the activity of substantially more nodes in a network, producing a more accurate graph of functional connectivity that reduces indirect connections. The key technology enabling this advancement is a recent advance in the estimation of mutual information (MI), which relies on multilayer perceptrons and exploiting an alternative representation of the Kullback–Leibler divergence definition of MI. Our second major contribution is the application of this technique to both discretely valued and continuously valued time series. Main results. GDI correctly inferred the circuitry of arbitrary Gaussian, nonlinear, and conductance-based networks. Furthermore, GDI inferred many of the connections of a model of a central pattern generator circuit in Aplysia, while also reducing many indirect connections. Significance. GDI is a general and model-free technique that can be used on a variety of scales and data types to provide accurate direct connectivity graphs and addresses the critical issue of indirect connections in neural data analysis.

For many types of learning, spaced training that involves repeated long inter-trial intervals (ITIs) leads to more robust memory formation than does massed training that involves short or no intervals. Several cognitive theories have been proposed to explain this superiority, but only recently has data begun to delineate the underlying cellular and molecular mechanisms of spaced training. We review these theories and data here. Computational models of the implicated signaling cascades have predicted that spaced training with irregular ITIs 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., Comment: 34 pages, 5 figures

The transition from early long-term potentiation (E-LTP) to late LTP (L-LTP) involves protein synthesis and degradation. L-LTP is blocked by inhibiting either protein synthesis or proteasome-dependent degradation prior to and during a tetanic stimulus, but paradoxically, L-LTP is not blocked when synthesis and degradation are inhibited simultaneously, suggesting counter-acting positive and negative proteins regulate L-LTP. To investigate this paradox, we modeled LTP at the Schaffer collateral synapse. Nine differential equations describe the levels of positive and negative regulator proteins (PP and NP) and transitions among five discrete synaptic states, a basal state (BAS), three E-LTP states (EP1, EP2, ED), and a L-LTP state (LP). A stimulus initiates the transition from BAS to EP1 and from EP1 to EP2, initiates the synthesis of PP and NP, and activates the ubiquitin-proteasome system (UPS). UPS mediates transitions of EP1 and EP2 to ED and the degradation of NP. The conversion of E-LTP to L-LTP is mediated by a PP-dependent transition from ED to LP. NP mediates reversal of EP2 to BAS. This model simulates empirical observations: 1) normal L-LTP, 2) block by either proteasome inhibitor or protein synthesis inhibitor alone, and 3) preservation of L-LTP when both inhibitors are applied together. Elements of this abstract model can be correlated with specific molecules and processes. Moreover, the model makes testable predictions, such as a unique synaptic state ED that precedes the transition to L-LTP, and a time window for the action of the UPS (during the transitions from EP1 and EP2 to ED). Tests of these predictions will provide insights into the processes of long-term synaptic plasticity., Comment: 23 pages, 5 figures. Accepted to Journal of Theoretical Biology

Memory impairment is often associated with disrupted regulation of gene induction. For example, deficits in cAMP response element-binding protein (CREB) binding protein (CBP; an essential cofactor for activation of transcription by CREB) impair long-term synaptic plasticity and memory. Previously, we showed that small interfering RNA (siRNA)-induced knockdown of CBP in individual sensory neurons significantly reduced levels of CBP and impaired 5-HT-induced long-term facilitation (LTF) in sensorimotor cocultures fromAplysia. Moreover, computational simulations of the biochemical cascades underlying LTF successfully predicted training protocols that restored LTF following CBP knockdown. We examined whether simulations could also predict a training protocol that restores LTF impaired by siRNA-induced knockdown of the transcription factor CREB1. Simulations based on a previously described model predicted rescue protocols that were specific to CREB1 knockdown. Empirical studies demonstrated that one of these rescue protocols partially restored impaired LTF. In addition, the effectiveness of the rescue protocol was enhanced by pretreatment with rolipram, a selective cAMP phosphodiesterase inhibitor. These results provide further evidence that computational methods can help rescue disruptions in signaling cascades underlying memory formation. Moreover, the study demonstrates that the effectiveness of computationally designed training protocols can be enhanced with complementary pharmacological approaches.

Visual Abstract, A key issue in neuroscience is understanding the ways in which neuromodulators such as dopamine modify neuronal activity to mediate selection of distinct motor patterns. We addressed this issue by applying either low or high concentrations of l-DOPA (40 or 250 μM) and then monitoring activity of up to 130 neurons simultaneously in the feeding circuitry of Aplysia using a voltage-sensitive dye (RH-155). l-DOPA selected one of two distinct buccal motor patterns (BMPs): intermediate (low l-DOPA) or bite (high l-DOPA) patterns. The selection of intermediate BMPs was associated with shortening of the second phase of the BMP (retraction), whereas the selection of bite BMPs was associated with shortening of both phases of the BMP (protraction and retraction). Selection of intermediate BMPs was also associated with truncation of individual neuron spike activity (decreased burst duration but no change in spike frequency or burst latency) in neurons active during retraction. In contrast, selection of bite BMPs was associated with compression of spike activity (decreased burst latency and duration and increased spike frequency) in neurons projecting through specific nerves, as well as increased spike frequency of protraction neurons. Finally, large-scale voltage-sensitive dye recordings delineated the spatial distribution of neurons active during BMPs and the modification of that distribution by the two concentrations of l-DOPA.

The mitogen-activated protein kinase (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 contribute to the efficacy of spaced stimulus protocols to induce LTF, a correlate of long-term memory. For example, decreased p38 MAPK activity ∼45 min after the first of two sensitizing stimuli might be an important determinant of an optimal inter-stimulus interval for LTF induction. Significance Statement Mitogen-activated protein kinase (MAPK) pathways play critical roles in mediating diverse forms of synaptic plasticity. Activation of the ERK isoform is required for long-term synaptic facilitation (LTF), whereas the p38 MAPK isoform is required for long-term synaptic depression. Here we used isolated Aplysia sensory neurons (SNs) to confirm and extend previous studies delineating dynamics of ERK and p38 MAPK. We quantified p38 MAPK activity after application of the essential neurotransmitter 5-HT, and explored the crosstalk between p38 MAPK and ERK in SNs. These data were incorporated into a computational model for the induction of LTF. Simulations suggest that p38 MAPK together with ERK contribute to the efficacy of temporally spaced stimulus protocols to induce LTF, a correlate of long-term memory.

Modern theories of memory storage have focused on enduring experience-dependent modifications of synaptic efficacy. However, learning paradigms as well as patterns of electrical stimulation of neurons and neural pathways also produces persistent changes in intrinsic neuronal excitability. These changes in intrinsic excitability are primarily due to modifications of ion channels, which can be neuron wide or expressed locally in restricted membrane compartments, such as the dendrites. These intrinsic changes may function as part of the engram itself. They may also serve as adaptive mechanisms to shape the stimulus specificity of the learned response or as mechanisms through which a neural circuit is set to a permissive state to facilitate the occurrence of synaptic modifications necessary for memory formation and retrieval.

Mollusks have been used extensively as experimental systems to study learning and memory, allowing the dissection of learning mechanisms at the network, cellular, and molecular levels. Here we focus on three mollusks that have made particular contributions: the marine mollusk Aplysia , the freshwater pond snail Lymnaea , and the marine cephalopod Octopus .