Identification of compartment- and process-specific molecules required for "synaptic tagging" during long-term potentiation and long-term depression in hippocampal CA1

Molecular constraints on synaptic tagging and maintenance of long-term potentiation: a predictive model

Protein synthesis-dependent, late long-term potentiation (LTP) and depression (LTD) at glutamatergic hippocampal synapses are well characterized examples of long-term synaptic plasticity. Persistent increased activity of protein kinase M ζ (PKMζ) is thought essential for maintaining LTP. Additional spatial and temporal features that govern LTP and LTD induction are embodied in the synaptic tagging and capture (STC) and cross capture hypotheses. Only synapses that have been “tagged” by a stimulus sufficient for LTP and learning can “capture” PKMζ. A model was developed to simulate the dynamics of key molecules required for LTP and LTD. The model concisely represents relationships between tagging, capture, LTD, and LTP maintenance. The model successfully simulated LTP maintained by persistent synaptic PKMζ, STC, LTD, and cross capture, and makes testable predictions concerning the dynamics of PKMζ. The maintenance of LTP, and consequently of at least some forms of long-term memory, is predicted to require continual positive feedback in which PKMζ enhances its own synthesis only at potentiated synapses. This feedback underlies bistability in the activity of PKMζ. Second, cross capture requires the induction of LTD to induce dendritic PKMζ synthesis, although this may require tagging of a nearby synapse for LTP. The model also simulates the effects of PKMζ inhibition, and makes additional predictions for the dynamics of CaM kinases. Experiments testing the above predictions would significantly advance the understanding of memory maintenance.

A fundamental problem in neurobiology is to understand how memories are maintained for up to years. Long-term potentiation (LTP), an enduring increase in the strength of specific connections (synapses) between neurons, is thought to comprise, at least in part, the substrate of learning and memory. What processes transduce brief stimuli into persistent LTP? Persistent increased activity of an enzyme denoted protein kinase M ζ (PKMζ) is thought essential for maintaining LTP. Only synapses that have been “tagged” by a stimulus, such as stimuli needed for LTP and learning, can “capture” PKMζ. We developed a model simulating dynamics of key molecules required for LTP and its opposite, long-term depression (LTD). The model concisely represents relationships between tagging, capture, LTD, and LTP maintenance. It makes testable predictions concerning the dynamics of PKMζ. The maintenance of LTP and memory is predicted to require positive feedback in which PKMζ enhances its own synthesis at potentiated synapses. Without synaptic capture of PKMζ, no positive feedback would occur. LTD induction is also predicted to increase PKMζ synthesis. The model also makes predictions about regulation of PKMζ synthesis. Experiments testing the above predictions would advance the understanding of memory maintenance.

Long-lasting LTP requires neither repeated trains for its induction nor protein synthesis for its development

Current thinking about LTP triggered in the area CA1 of hippocampal slices is ruled by two “dogmas”: (1) A single train of high-frequency stimulation is sufficient to trigger short-lasting LTP (1 – 3 h), whereas multiple trains are required to induce long-lasting LTP (L-LTP, more than 4 h). (2) The development of the late phase of L-LTP requires the synthesis of new proteins. In this study, we found that a single high-frequency train could trigger an LTP lasting more than 8 h that was not affected by either anisomycin or cycloheximide (two inhibitors of protein synthesis). We ascertained that the induction of this L-LTP made use of the same mechanisms as those usually reported to be involved in LTP induction: it was dependent on NMDA receptors and on the activation of two “core” kinases, CaMKII and PI3K. These findings call into question the two “dogmas” about LTP.

CaMKII regulation in information processing and storage

The Ca(2+)/Calmodulin(CaM)-dependent protein kinase II (CaMKII) is activated by Ca(2+)/CaM, but becomes partially autonomous (Ca(2+)-independent) upon autophosphorylation at T286. This hallmark feature of CaMKII regulation provides a form of molecular memory and is indeed important in long-term potentiation (LTP) of excitatory synapse strength and memory formation. However, emerging evidence supports a direct role in information processing, while storage of synaptic information may instead be mediated by regulated interaction of CaMKII with the NMDA receptor (NMDAR) complex. These and other CaMKII regulation mechanisms are discussed here in the context of the kinase structure and their impact on postsynaptic functions. Recent findings also implicate CaMKII in long-term depression (LTD), as well as functional roles at inhibitory synapses, lending renewed emphasis on better understanding the spatiotemporal control of CaMKII regulation.

Switching off LTP: mGlu and NMDA receptor-dependent novelty exploration-induced depotentiation in the rat hippocampus

Both electrically induced synaptic long-term potentiation (LTP) and long-term depression have been extensively studied as models of the cellular basis of learning and memory mechanisms. Recently, considerable interest has been generated by the possibility that the activity-dependent persistent reversal of previously established synaptic LTP (depotentiation) may play a role in the time- and state-dependent erasure of memory. Here, we examined the requirement for glutamate receptor activation in experience-induced reversal of previously established LTP in the CA1 area of the hippocampus of freely behaving rats. Continuous exploration of non-aversive novelty for ~30 min, which was associated with hippocampal activation as measured by increased theta power in the electroencephalogram, triggered a rapid and persistent reversal of high frequency stimulation-induced LTP both at apical and basal synapses. Blockade of metabotropic glutamate (mGlu) receptors with mGlu5 subtype-selective antagonists, or N-methyl-D-aspartate (NMDA) receptors with GluN2B subunit-selective antagonists, prevented novelty-induced depotentiation. These findings strongly indicate that activation of both mGlu5 receptors and GluN2B-containing NMDA receptors is required for experience-triggered induction of depotentiation at CA3-CA1 synapses. The mechanistic concordance of the present and previous studies of experience-induced and electrically induced synaptic depotentiation helps to integrate our understanding of the neurophysiological underpinnings of learning and memory.

Interaction between long-term potentiation and depression in CA1 synapses: temporal constrains, functional compartmentalization and protein synthesis

Information arriving at a neuron via anatomically defined pathways undergoes spatial and temporal encoding. A proposed mechanism by which temporally and spatially segregated information is encoded at the cellular level is based on the interactive properties of synapses located within and across functional dendritic compartments. We examined cooperative and interfering interactions between long-term synaptic potentiation (LTP) and depression (LTD), two forms of synaptic plasticity thought to be key in the encoding of information in the brain. Two approaches were used in CA1 pyramidal neurons of the mouse hippocampus: (1) induction of LTP and LTD in two separate synaptic pathways within the same apical dendritic compartment and across the basal and apical dendritic compartments; (2) induction of LTP and LTD separated by various time intervals (0–90 min). Expression of LTP/LTD interactions was spatially and temporally regulated. While they were largely restricted within the same dendritic compartment (compartmentalized), the nature of the interaction (cooperation or interference) depended on the time interval between inductions. New protein synthesis was found to regulate the expression of the LTP/LTD interference. We speculate that mechanisms for compartmentalization and protein synthesis confer the spatial and temporal modulation by which neurons encode multiplex information in plastic synapses.

TrkB as a potential synaptic and behavioral tag

Late-phase long-term potentiation (L-LTP), a cellular model for long-term memory (LTM), requires de novo protein synthesis. An attractive hypothesis for synapse specificity of long-term memory is "synaptic tagging": synaptic activity generates a tag, which "captures" the PRPs (plasticity-related proteins) derived outside of synapses. Here we provide evidence that TrkB, the receptor of BDNF (brain-derived neurotrophic factor), may serve as a "synaptic tag." TrkB is transiently activated by weak theta-burst stimulation (TBS) that induces only early-phase LTP (E-LTP). This TrkB activation is independent of protein synthesis, and confined to stimulated synapses. Induction of L-LTP by strong stimulation in one synaptic pathway converts weak TBS-induced E-LTP to L-LTP in a second, independent pathway. Transient inhibition of TrkB around the time of weak TBS to the second pathway diminished L-LTP in that pathway without affecting the first one. Behaviorally, weak training, which induces short-term memory (STM) but not LTM, can be consolidated into LTM by exposing animals to novel but not familiar environment 1 h before training. Inhibition of TrkB during STM training blocked such consolidation. These results suggest TrkB as a potential tag for synapse-specific expression of L-LTP and LTM.

Mechanisms of dendritic mRNA transport and its role in synaptic tagging

The localization of RNAs critically contributes to many important cellular processes in an organism, such as the establishment of polarity, asymmetric division and migration during development. Moreover, in the central nervous system, the local translation of mRNAs is thought to induce plastic changes that occur at synapses triggered by learning and memory. Here, we will critically review the physiological functions of well-established dendritically localized mRNAs and their associated factors, which together form ribonucleoprotein particles (RNPs). Second, we will discuss the life of a localized transcript from transcription in the nucleus to translation at the synapse and introduce the concept of the 'RNA signature' that is characteristic for each transcript. Finally, we present the 'sushi belt model' of how localized RNAs within neuronal RNPs may dynamically patrol multiple synapses rather than being anchored at a single synapse. This new model integrates our current understanding of synaptic function ranging from synaptic tagging and capture to functional and structural reorganization of the synapse upon learning and memory.

Identification of transmitter systems and learning tag molecules involved in behavioral tagging during memory formation

Long-term memory (LTM) consolidation requires the synthesis of plasticity-related proteins (PRPs). In addition, we have shown recently that LTM formation also requires the setting of a "learning tag" able to capture those PRPs. Weak training, which results only in short-term memory, can set a tag to use PRPs derived from a temporal-spatial closely related event to promote LTM formation. Here, we studied the involvement of glutamatergic, dopaminergic, and noradrenergic inputs on the setting of an inhibitory avoidance (IA) learning tag and the synthesis of PRPs. Rats explored an open field (PRP donor) followed by weak (tag inducer) or strong (tag inducer plus PRP donor) IA training. Throughout pharmacological interventions around open-field and/or IA sessions, we found that hippocampal dopamine D1/D5- and β-adrenergic receptors are specifically required to induce PRP synthesis. Moreover, activation of the glutamatergic NMDA receptors is required for setting the learning tags, and this machinery further required α-Ca(2+)/calmodulin-dependent protein kinase II and PKA but not ERK1/2 activity. Together, the present findings emphasize an essential role of the induction of PRPs and learning tags for LTM formation. The existence of only the PRP or the tag was insufficient for stabilization of the mnemonic trace.

Diversity of neuropsin (KLK8)-dependent synaptic associativity in the hippocampal pyramidal neuron

Hippocampal early (E-) long-term potentiation (LTP) and long-term depression (LTD) elicited by a weak stimulus normally fades within 90 min. Late (L-) LTP and LTD elicited by strong stimuli continue for >180 min and require new protein synthesis to persist. If a strong tetanus is applied once to synaptic inputs, even a weak tetanus applied to another synaptic input can evoke persistent LTP. A synaptic tag is hypothesized to enable the capture of newly synthesized synaptic molecules. This process, referred to as synaptic tagging, is found between not only the same processes (i.e. E- and L-LTP; E- and L-LTD) but also between different processes (i.e. E-LTP and L-LTD; E-LTD and L-LTP) induced at two independent synaptic inputs (cross-tagging). However, the mechanisms of synaptic tag setting remain unclear. In our previous study, we found that synaptic associativity in the hippocampal Schaffer collateral pathway depended on neuropsin (kallikrein-related peptidase 8 or KLK8), a plasticity-related extracellular protease. In the present study, we investigated how neuropsin participates in synaptic tagging and cross-tagging. We report that neuropsin is involved in synaptic tagging during LTP at basal and apical dendritic inputs. Moreover, neuropsin is involved in synaptic tagging and cross-tagging during LTP at apical dendritic inputs via integrin β1 and calcium/calmodulin-dependent protein kinase II signalling. Thus, neuropsin is a candidate molecule for the LTP-specific tag setting and regulates the transformation of E- to L-LTP during both synaptic tagging and cross-tagging.

Different compartments of apical CA1 dendrites have different plasticity thresholds for expressing synaptic tagging and capture

The consolidation process from short- to long-term memory depends on the type of stimulation received from a specific neuronal network and on the cooperativity and associativity between different synaptic inputs converging onto a specific neuron. We show here that the plasticity thresholds for inducing LTP are different in proximal and distal compartments of apical dendrites. In addition, we show interactions between the proximal and distal compartments of the apical dendrites by providing evidence that even a subthreshold stimulus can activate plasticity-related proteins, such as PKMζ, enabling associativity between two distinct dendritic compartments in apical dendrites to occur.

Cooperation between cholinergic and glutamatergic receptors are essential to induce BDNF-dependent long-lasting memory storage

The induction of long-lasting memory storage depends on the behavioral state of humans and animals. This behavioral state is mediated by neuromodulatory systems, like the cholinergic-septum-hippocampal circuit. Cholinergic neurotransmission is known to affect short-term activity-dependent plasticity in various brain areas, including the hippocampus. We could show here that a chemical late-long-term potentiation (LTP) could be induced in the basal dendrites by the coapplication of the cholinergic receptor agonist, carbachol, and the phosphodiesterase type 4 (PDE4)-inhibitor, rolipram at a concentration that by itself has no effect on basal synaptic transmission. This chemical late-LTP was similar to electrical late-LTP in that it is dependent on protein synthesis, cAMP, and NMDA-receptor activation. Occlusion experiments demonstrated that saturation of three tetanus (TET) late-LTP occluded carbachol-rolipram-LTP, indicating that they share similar properties. This cholinergic modulation of LTP in the basal dendrites was mediated by both muscarinic and nicotinic receptors. Carbachol also reinforced an early form of LTP into a long-lasting LTP. Most interestingly, these two forms of LTP could participate in the functional plasticity processes like synaptic tagging and capture (STC). In addition, we studied whether a cooperation between cholinergic and glutamatergic receptors is essential to induce functional synaptic-plasticity. Indeed, we could show that coactivation of acetylcholine/PDE4 inhibition must coincide with the release of glutamate to induce a long-lasting plasticity, showing a functional convergence of the two neuromodulatory systems. Moreover, we could also show that both chemical late-LTP and carbachol-reinforced early-LTP-induced STC processes are mediated by the neurotrophin BDNF.

Metaplasticity governs compartmentalization of synaptic tagging and capture through brain-derived neurotrophic factor (BDNF) and protein kinase Mzeta (PKMzeta)

Activity-dependent synaptic plasticity is widely accepted to be the cellular correlate of learning and memory. It is believed that associativity between different synaptic inputs can transform short-lasting forms of synaptic plasticity (<3 h) to long-lasting ones. Synaptic tagging and capture (STC) might be able to explain this heterosynaptic support, because it distinguishes between local mechanisms of synaptic tags and cell-wide mechanisms responsible for the synthesis of plasticity-related proteins (PRPs). STC initiate storage processes only when the strength of the synaptic tag and the local concentration of essential proteins are above a certain plasticity threshold. We present evidence that priming stimulation through the activation of metabotropic glutamate receptors substantially increases the "range of threshold" for functional plasticity by producing protein kinase Mζ (PKMζ) as a PRP through local protein synthesis. In addition, our results implicate BDNF as a PRP which is mandatory for establishing cross-capture between synaptic strengthening and weakening, whereas the newly generated PKMζ specifically establishes synaptic tagging of long-term potentiation. Most intriguingly, we show here that STC are confined to specific dendritic compartments and that these compartments contain "synaptic clusters" with different plasticity thresholds. Our results suggest that within a dendritic compartment itself a homeostatic process exists to adjust plasticity thresholds. The range in which these clusters operate can be altered by processes of metaplasticity, which will operate on the cluster independently of other clusters at the same dendrite. These clusters will then prepare the synaptic network to form long-term memories.

Making memories last: the synaptic tagging and capture hypothesis

The synaptic tagging and capture hypothesis of protein synthesis-dependent long-term potentiation asserts that the induction of synaptic potentiation creates only the potential for a lasting change in synaptic efficacy, but not the commitment to such a change. Other neural activity, before or after induction, can also determine whether persistent change occurs. Recent findings, leading us to revise the original hypothesis, indicate that the induction of a local, synapse-specific 'tagged' state and the expression of long-term potentiation are dissociable. Additional observations suggest that there are major differences in the mechanisms of functional and structural plasticity. These advances call for a revised theory that incorporates the specific molecular and structural processes involved. Addressing the physiological relevance of previous in vitro findings, new behavioural studies have experimentally translated the hypothesis to learning and the consolidation of newly formed memories.

Protein kinase Mzeta is essential for the induction and maintenance of dopamine-induced long-term potentiation in apical CA1 dendrites

Dopaminergic D1/D5-receptor-mediated processes are important for certain forms of memory as well as for a cellular model of memory, hippocampal long-term potentiation (LTP) in the CA1 region of the hippocampus. D1/D5-receptor function is required for the induction of the protein synthesis-dependent maintenance of CA1-LTP (L-LTP) through activation of the cAMP/PKA-pathway. In earlier studies we had reported a synergistic interaction of D1/D5-receptor function and N-methyl-D-aspartate (NMDA)-receptors for L-LTP. Furthermore, we have found the requirement of the atypical protein kinase C isoform, protein kinase Mζ (PKMζ) for conventional electrically induced L-LTP, in which PKMζ has been identified as a LTP-specific plasticity-related protein (PRP) in apical CA1-dendrites. Here, we investigated whether the dopaminergic pathway activates PKMζ. We found that application of dopamine (DA) evokes a protein synthesis-dependent LTP that requires synergistic NMDA-receptor activation and protein synthesis in apical CA1-dendrites. We identified PKMζ as a DA-induced PRP, which exerted its action at activated synaptic inputs by processes of synaptic tagging.

Properties of subsequent induction of long-term potentiation and/or depression in one synaptic input in apical dendrites of hippocampal CA1 neurons in vitro

The hippocampus is a prominent structure to study mechanisms of learning and memory at the cellular level. Long-term potentiation (LTP) as well as long-term depression (LTD) are the major cellular models which could underlie learning and memory formation. LTP and LTD consist of at least two phases, an early protein synthesis-independent transient stage (<4 h; E-LTP, E-LTD) as well as a prolonged phase (>4 h; L-LTP, L-LTD) requiring the synthesis of new proteins. It is known that during E-LTP the further induction of longer lasting LTP is precluded. However, if E-LTP is transformed into L-LTP, the same synapses now allow the induction of LTP again. We reproduced the LTP-results first and then investigated whether hippocampal LTP or LTD also prevents the establishment of subsequent LTD-induction in the same synaptic input. We show that the prior induction of LTP or LTD does not prevent a short-term depression (STD) but occludes LTD in apical dendrites of CA1 neurons in hippocampal slices in vitro during the early phase of LTP or LTD. However, LTD can again be induced in addition to STD after the establishment of L-LTP or L-LTD, that is about 4 h after the induction of the first event in the same synaptic input. We suggest that the neuronal input preserves the capacity for STD immediately after an initial potentiation or depression, but for the onset of additional longer lasting LTD in the same synaptic input, the establishment of the late plasticity form of the preceding event is critical.

In situ visualization and dynamics of newly synthesized proteins in rat hippocampal neurons

Protein translation has been implicated in different forms of synaptic plasticity but direct in situ visualization of new proteins is limited to one or two proteins at a time. Here we describe a metabolic labeling approach based upon incorporation of non-canonical amino acids into proteins followed by chemo–selective fluorescent tagging via click chemistry. Following brief incubation with azidohomoalanine or homopropargylglycine, a robust fluorescent signal was detected in somata and dendrites. Pulse–chase–like application of azidohomoalanine and homopropargylglycine allowed visualization of proteins synthesized in two sequential time periods. This technique can be used to detect changes in protein synthesis and to evaluate the fate of proteins synthesized in different cellular compartments. Moreover, using strain–promoted cycloaddition, we explored the dynamics of newly synthesized membrane proteins using single particle tracking and quantum dots. The newly synthesized proteins exhibited a broad range of diffusive behaviors as expected if the pool of labeled proteins was heterogeneous.

Synaptic tagging and capture: differential role of distinct calcium/calmodulin kinases in protein synthesis-dependent long-term potentiation

Weakly tetanized synapses in area CA1 of the hippocampus that ordinarily display long-term potentiation lasting approximately 3 h (called early-LTP) will maintain a longer-lasting change in efficacy (late-LTP) if the weak tetanization occurs shortly before or after strong tetanization of an independent, but convergent, set of synapses in CA1. The synaptic tagging and capture hypothesis explains this heterosynaptic influence on persistence in terms of a distinction between local mechanisms of synaptic tagging and cell-wide mechanisms responsible for the synthesis, distribution, and capture of plasticity-related proteins (PRPs). We now present evidence that distinct CaM kinase (CaMK) pathways serve a dissociable role in these mechanisms. Using a hippocampal brain-slice preparation that permits stable long-term recordings in vitro for >10 h and using hippocampal cultures to validate the differential drug effects on distinct CaMK pathways, we show that tag setting is blocked by the CaMK inhibitor KN-93 (2-[N-(2-hydroxyethyl)]-N-(4-methoxybenzenesulfonyl)amino-N-(4-chlorocinnamyl)-N-methylbenzylamine) that, at low concentration, is more selective for CaMKII. In contrast, the CaMK kinase inhibitor STO-609 [7H-benzimidazo(2,1-a)benz(de)isoquinoline-7-one-3-carboxylic acid] specifically limits the synthesis and/or availability of PRPs. Analytically powerful three-pathway protocols using sequential strong and weak tetanization in varying orders and test stimulation over long periods of time after LTP induction enable a pharmacological dissociation of these distinct roles of the CaMK pathways in late-LTP and so provide a novel framework for the molecular mechanisms by which synaptic potentiation, and possibly memories, become stabilized.

Functional differences between and across different regions of the apical branch of hippocampal CA1 dendrites with respect to long-term depression induction and synaptic cross-tagging

The hippocampus is an ideal system to study synaptic plasticity in the context of learning and memory. The induction, expression, and interaction of long-term potentiation (LTP) as well as long-term depression (LTD) are essential elements for the functioning of complex networks in information processing and storage. Here we investigated whether different loci at the apical dendritic branch of CA1 pyramidal neurons are characterized by different capabilities to induce, express, and interact with LTP and LTD in hippocampal slices in vitro. We found that high-frequency stimulation resulted in longer-lasting forms of LTP in proximal and distal parts of the apical dendrites, whereas low-frequency stimulation induced longer-lasting LTD in distal but not at proximal parts. Interestingly, processes of "synaptic cross-tagging" could be described for any form of LTP transformation from early-stage LTP (E-LTP) into late-phase LTP (L-LTP) in distal and proximal parts, but for LTD, only at the distal part but not for the proximal part, although low-frequency stimulation at the proximal input, which resulted here only in a short-term depression, was paradoxically able to reinforce E-LTP into L-LTP at distal parts. We have identified protein kinase Mzeta (PKMzeta) as the LTP-specific, synthesized plasticity-related protein transforming E-LTP into L-LTP by strong low-frequency stimulation in the apical CA dendrite by cross-tagging mechanisms.

Temporal sensitivity of protein kinase a activation in late-phase long term potentiation

Protein kinases play critical roles in learning and memory and in long term potentiation (LTP), a form of synaptic plasticity. The induction of late-phase LTP (L-LTP) in the CA1 region of the hippocampus requires several kinases, including CaMKII and PKA, which are activated by calcium-dependent signaling processes and other intracellular signaling pathways. The requirement for PKA is limited to L-LTP induced using spaced stimuli, but not massed stimuli. To investigate this temporal sensitivity of PKA, a computational biochemical model of L-LTP induction in CA1 pyramidal neurons was developed. The model describes the interactions of calcium and cAMP signaling pathways and is based on published biochemical measurements of two key synaptic signaling molecules, PKA and CaMKII. The model is stimulated using four 100 Hz tetani separated by 3 sec (massed) or 300 sec (spaced), identical to experimental L-LTP induction protocols. Simulations show that spaced stimulation activates more PKA than massed stimulation, and makes a key experimental prediction, that L-LTP is PKA-dependent for intervals larger than 60 sec. Experimental measurements of L-LTP demonstrate that intervals of 80 sec, but not 40 sec, produce PKA-dependent L-LTP, thereby confirming the model prediction. Examination of CaMKII reveals that its temporal sensitivity is opposite that of PKA, suggesting that PKA is required after spaced stimulation to compensate for a decrease in CaMKII. In addition to explaining the temporal sensitivity of PKA, these simulations suggest that the use of several kinases for memory storage allows each to respond optimally to different temporal patterns.

The hippocampus is a part of the cerebral cortex intimately involved in learning and memory behavior. A common cellular model of learning is a long lasting form of long term potentiation (L-LTP) in the hippocampus, because it shares several characteristics with learning. For example, both learning and long term potentiation exhibit sensitivity to temporal patterns of synaptic inputs and share common intracellular events such as activation of specific intracellular signaling pathways. Therefore, understanding the pivotal molecules in the intracellular signaling pathways underlying temporal sensitivity of L-LTP in the hippocampus may illuminate mechanisms underlying learning. We developed a computational model to evaluate whether the signaling pathways leading to activation of the two critical enzymes: protein kinase A and calcium-calmodulin-dependent kinase II are sufficient to explain the experimentally observed temporal sensitivity. Indeed, the simulations demonstrate that these enzymes exhibit different temporal sensitivities, and make a key experimental prediction, that L-LTP is dependent on protein kinase A for intervals larger than 60 sec. Measurements of hippocampal L-LTP confirm this prediction, demonstrating the value of a systems biology approach to computational neuroscience.

The roles of CaMKII and F-actin in the structural plasticity of dendritic spines: a potential molecular identity of a synaptic tag?

Ca2+/calmodulin-dependent protein kinase II (CaMKII) and actin are two crucial molecules involved in long-term potentiation (LTP). In addition to its signaling function, CaMKII plays a structural role via direct interaction with actin filaments, thus coupling functional and structural plasticity in dendritic spines. The status of F-actin, regulated by CaMKII, determines the postsynaptic protein binding capacity and thus may act as a synaptic tag that consolidates LTP.

NMDA receptor-independent control of transcription factors and gene expression

Consolidation of synaptic plasticity seems to require transcription, but how the nucleus is informed in this context remains unknown. As NMDA receptor antagonists have been shown to interfere with action potential generation, the issue of whether or not a synaptically generated signal is required for nuclear signaling is currently unresolved. Here, we show that pharmacological maintenance of action potentials during NMDA receptor blockade allows for NMDA receptor-independent transcription factor binding and arc gene expression, both of which were previously thought to be NMDA receptor dependent. These data suggest that types of signaling in the nucleus previously attributed to NMDA-receptor-dependent synapse-to-nucleus signals can be initiated in the absence of NMDA receptor-dependent synaptic plasticity.

Spike-based reinforcement learning in continuous state and action space: when policy gradient methods fail

Changes of synaptic connections between neurons are thought to be the physiological basis of learning. These changes can be gated by neuromodulators that encode the presence of reward. We study a family of reward-modulated synaptic learning rules for spiking neurons on a learning task in continuous space inspired by the Morris Water maze. The synaptic update rule modifies the release probability of synaptic transmission and depends on the timing of presynaptic spike arrival, postsynaptic action potentials, as well as the membrane potential of the postsynaptic neuron. The family of learning rules includes an optimal rule derived from policy gradient methods as well as reward modulated Hebbian learning. The synaptic update rule is implemented in a population of spiking neurons using a network architecture that combines feedforward input with lateral connections. Actions are represented by a population of hypothetical action cells with strong mexican-hat connectivity and are read out at theta frequency. We show that in this architecture, a standard policy gradient rule fails to solve the Morris watermaze task, whereas a variant with a Hebbian bias can learn the task within 20 trials, consistent with experiments. This result does not depend on implementation details such as the size of the neuronal populations. Our theoretical approach shows how learning new behaviors can be linked to reward-modulated plasticity at the level of single synapses and makes predictions about the voltage and spike-timing dependence of synaptic plasticity and the influence of neuromodulators such as dopamine. It is an important step towards connecting formal theories of reinforcement learning with neuronal and synaptic properties.

Interfering with the actin network and its effect on long-term potentiation and synaptic tagging in hippocampal CA1 neurons in slices in vitro

Long-term potentiation (LTP) is a cellular correlate for memory formation, which requires the dynamic changes of the actin cytoskeleton. As shown by others, the polymerization of the actin network is important for early stages of LTP. Here, we investigated the role of actin dynamics in synaptic tagging and particularly in the induction of protein synthesis-dependent late-LTP in the CA1 region in hippocampal slices in vitro. We found that the inhibition of actin polymerization affects protein synthesis-independent early-LTP, prevents late-LTP, and interferes with synaptic tagging in apical dendrites of hippocampal CA1. The transformation of early-LTP into late-LTP was blocked by the application of the structurally different actin polymerization inhibitors latrunculin A or cytochalasin D. We suggest that the actin network is required for early "housekeeping" processes to induce and maintain early-LTP. Furthermore, inhibition of actin dynamics negatively interacts with the setting of the synaptic tagging complex. We propose actin as a further tag-specific molecule in apical CA1 dendrites where it is directly involved in the tagging/capturing machinery. Consequently, inhibition of the actin network prevents the interaction of tagging complexes with plasticity-related proteins. This results in the prevention of late-LTP by inhibition of the actin network during LTP induction.

A systematic investigation of the protein kinases involved in NMDA receptor-dependent LTD: evidence for a role of GSK-3 but not other serine/threonine kinases

BACKGROUND

The signalling mechanisms involved in the induction of N-methyl-D-aspartate (NMDA) receptor-dependent long-term depression (LTD) in the hippocampus are poorly understood. Numerous studies have presented evidence both for and against a variety of second messengers systems being involved in LTD induction. Here we provide the first systematic investigation of the involvement of serine/threonine (ser/thr) protein kinases in NMDAR-LTD, using whole-cell recordings from CA1 pyramidal neurons.

RESULTS

Using a panel of 23 inhibitors individually loaded into the recorded neurons, we can discount the involvement of at least 57 kinases, including PKA, PKC, CaMKII, p38 MAPK and DYRK1A. However, we have been able to confirm a role for the ser/thr protein kinase, glycogen synthase kinase 3 (GSK-3).

CONCLUSION

The present study is the first to investigate the role of 58 ser/thr protein kinases in LTD in the same study. Of these 58 protein kinases, we have found evidence for the involvement of only one, GSK-3, in LTD.

DHPG activation of group 1 mGluRs in BLA enhances fear conditioning

Group 1 metabotropic glutamate receptors are known to play an important role in both synaptic plasticity and memory. We show that activating these receptors prior to fear conditioning by infusing the group 1 mGluR agonist, (R.S.)-3,5-dihydroxyphenylglycine (DHPG), into the basolateral region of the amygdala (BLA) of adult Sprague-Dawley rats enhances freezing normally supported by a weak footshock. This effect of DHPG was blocked when it was co-infused with either the general group 1 mGluR1 antagonist, (R,S)-1-aminoindan-1,5 dicarboxylic acid (AIDA), or with the selective mGluR5 antagonist, 2-methyl-6-(phenylethynyl)-pyridine (MPEP). These results support previous findings by Rodrigues and colleagues that mGluR5s in the lateral region of the amygdala make an import contribution to fear conditioning. More importantly, they support the general ideas embedded in the concept of metaplasticity, as per Abraham, and the synaptic-tagging hypothesis per Frey and Morris-that the processes that specify the content of experience can be experimentally separated from those needed to acquire the memory.

Priming of short-term potentiation and synaptic tagging/capture mechanisms by ryanodine receptor activation in rat hippocampal CA1

Activity-dependent changes in synaptic strength such as long-term potentiation (LTP) and long-term depression (LTD) are considered to be cellular mechanisms underlying learning and memory. Strengthening of a synapse for a few seconds or minutes is termed short-term potentiation (STP) and is normally unable to take part in the processes of synaptic tagging/capture due to its inability to set the "synaptic tags." Here, we report that priming of synapses with ryanodine receptor agonists such as ryanodine (10 microM) or caffeine (10 mM) facilitates subsequent synaptic tagging/capture, enabling an STP protocol to establish a late-LTP in response to strong tetanization of a heterosynaptic input. We identified calcium/calmodulin-dependent protein kinase II (CaMKII) as mediating the primed synaptic tag setting, which persisted for 1 h. We also identified protein kinase Mzeta (PKMzeta), presumably captured from the strongly tetanized heterosynaptic input, as a plasticity-related protein maintaining the LTP at the tagged synapses. In addition, synaptic tags in primed STP were erased or interfered with by delivering low-frequency depotentiating stimulation 5 or 10 min after its induction, thus preventing capture of newly synthesized proteins. These data reveal a novel form of metaplasticity, whereby ryanodine receptor activation lowers the threshold for subsequent synaptic tagging/capture, thus priming weakly activated synapses for heterosynaptic interactions that promote long-term functional plasticity.

State based model of long-term potentiation and synaptic tagging and capture

Recent data indicate that plasticity protocols have not only synapse-specific but also more widespread effects. In particular, in synaptic tagging and capture (STC), tagged synapses can capture plasticity-related proteins, synthesized in response to strong stimulation of other synapses. This leads to long-lasting modification of only weakly stimulated synapses. Here we present a biophysical model of synaptic plasticity in the hippocampus that incorporates several key results from experiments on STC. The model specifies a set of physical states in which a synapse can exist, together with transition rates that are affected by high- and low-frequency stimulation protocols. In contrast to most standard plasticity models, the model exhibits both early- and late-phase LTP/D, de-potentiation, and STC. As such, it provides a useful starting point for further theoretical work on the role of STC in learning and memory.

Tag-trigger-consolidation: a model of early and late long-term-potentiation and depression

Changes in synaptic efficacies need to be long-lasting in order to serve as a substrate for memory. Experimentally, synaptic plasticity exhibits phases covering the induction of long-term potentiation and depression (LTP/LTD) during the early phase of synaptic plasticity, the setting of synaptic tags, a trigger process for protein synthesis, and a slow transition leading to synaptic consolidation during the late phase of synaptic plasticity. We present a mathematical model that describes these different phases of synaptic plasticity. The model explains a large body of experimental data on synaptic tagging and capture, cross-tagging, and the late phases of LTP and LTD. Moreover, the model accounts for the dependence of LTP and LTD induction on voltage and presynaptic stimulation frequency. The stabilization of potentiated synapses during the transition from early to late LTP occurs by protein synthesis dynamics that are shared by groups of synapses. The functional consequence of this shared process is that previously stabilized patterns of strong or weak synapses onto the same postsynaptic neuron are well protected against later changes induced by LTP/LTD protocols at individual synapses.

Metaplasticity: new insights through electrophysiological investigations

The term synaptic plasticity describes the ability of excitatory synapses to undergo activity-driven long-lasting changes in the efficacy of basal synaptic transmission. This change may be expressed as a long-term potentiation (LTP) or as a long-term depression (LTD). Metaplasticity is a higher-order form of synaptic plasticity that regulates the expression of both LTP and LTD through processes that are initiated by cellular activity that precedes a later bout of plasticity-inducing synaptic activity. Activation by prior synaptic activity and later expression as a facilitation or inhibition of activity-dependent synaptic plasticity are fundamental properties of metaplasticity. The intracellular mechanisms which support metaplasticity appear to be closely linked to those of synaptic plasticity, hence there are significant technical challenges to overcome in order to elucidate those mechanisms specific to metaplasticity. This review will examine the progress in the characterization of metaplasticity over the last decade or so with a focus on findings gained using electrophysiological techniques. It will look at the techniques applied, the brain regions investigated and the knowledge gained from the application of a wide range of protocols designed to examine the influence of varied forms of prior synaptic activity on later, activity-induced, synaptic plasticity.

CaMKII activation state underlies synaptic labile phase of LTP and short-term memory formation

BACKGROUND

The labile state of short-term memory has been known for more than a century. It has been frequently reported that immediate postlearning intervention can readily disrupt newly formed memories. However, the molecular and cellular mechanisms underlying the labile state of new memory are not understood.

RESULTS

Using a bump-and-hole-based chemical-genetic method, we have rapidly and selectively manipulated alpha CaMKII activity levels in the mouse forebrain during various stages of the short-term memory processes. We find that a rapid shift in the alpha CaMKII activation status within the immediate 10 min after learning severely disrupts short-term memory formation. The same manipulation beyond the 15 min after learning has no effect, suggesting a critical time window for CaMKII action. We further show that during this same 10 min time window only, shifting in CaMKII activation state is capable of altering newly established synaptic weights and/or patterns.

CONCLUSION

The initial 10 min of memory formation and long-term potentiation are sensitive to inducible genetic upregulation of alphaCaMKII activity. Our results suggest that molecular dynamics of CaMKII play an important role in underlying synaptic labile state and representation of short-term memory during this critical time window.

Dendritic excitability and synaptic plasticity

Most synaptic inputs are made onto the dendritic tree. Recent work has shown that dendrites play an active role in transforming synaptic input into neuronal output and in defining the relationships between active synapses. In this review, we discuss how these dendritic properties influence the rules governing the induction of synaptic plasticity. We argue that the location of synapses in the dendritic tree, and the type of dendritic excitability associated with each synapse, play decisive roles in determining the plastic properties of that synapse. Furthermore, since the electrical properties of the dendritic tree are not static, but can be altered by neuromodulators and by synaptic activity itself, we discuss how learning rules may be dynamically shaped by tuning dendritic function. We conclude by describing how this reciprocal relationship between plasticity of dendritic excitability and synaptic plasticity has changed our view of information processing and memory storage in neuronal networks.

An analysis of the stimulus requirements for setting the molecular switch reveals a lower threshold for metaplasticity than synaptic plasticity

The requirements for the synaptic activation of metabotropic glutamate (mGlu) receptors and for the induction of metaplasticity in the hippocampus are not known. In the present study, we have investigated the synaptic activation of mGlu5 receptors and the setting of the molecular switch, a form of metaplasticity, at CA1 synapses in the mouse hippocampus. We find that as few as eight stimuli (delivered at 100Hz) are sufficient to set the molecular switch, since a subsequent tetanus delivered to the same input is able to induce long-term potentiation (LTP) in the presence of the mGlu receptor antagonist MCPG ((S)-alpha-methyl-4-carboxyphenylglycine). In addition, we find that the molecular switch can be activated over a wide frequency range. When 10 shocks were delivered the threshold frequency was 4Hz. The ability of 10 shocks (delivered at 100Hz) to set the molecular switch was lost in the mGlu5 knockout. These data show that mGlu5 receptors can be activated synaptically and metaplasticity can be induced by relatively few stimuli. Indeed, metaplasticity was induced by stimuli that were subthreshold for the induction of LTP per se. Thus, metaplasticity has a lower threshold than the synaptic plasticity that it regulates.

Metaplasticity: tuning synapses and networks for plasticity

Synaptic plasticity is a key component of the learning machinery in the brain. It is vital that such plasticity be tightly regulated so that it occurs to the proper extent at the proper time. Activity-dependent mechanisms that have been collectively termed metaplasticity have evolved to help implement these essential computational constraints. Various intercellular signalling molecules can trigger lasting changes in the ability of synapses to express plasticity; their mechanisms of action are reviewed here, along with a consideration of how metaplasticity might affect learning and clinical conditions.

Distinct single but not necessarily repeated tetanization is required to induce hippocampal late-LTP in the rat CA1

The protein synthesis-dependent form of hippocampal long-term potentiation (late-LTP) is thought to underlie memory. Its induction requires a distinct stimulation strength, and the common opinion is that only repeated tetani result in late-LTP whereas as single tetanus only reveals a transient early-LTP. Properties of LTP induction were compared to learning processes where repetition is often the prerequisite for a long-lasting memory. However, also single events can lead to manifested memory. If LTP subserves processes of learning, similar results should be detectable for LTP. Here we show that a single tetanus is sufficient to induce late-LTP requiring dopaminergic co-transmission during induction.

'Synaptic tagging' and 'cross-tagging' and related associative reinforcement processes of functional plasticity as the cellular basis for memory formation

We focus on new properties of cellular and network processes of memory formation involving 'synaptic tagging' and 'cross-tagging' during long-term potentiation (LTP) and long-term depression (LTD) as well as associative heterosynaptic interactions, the latter of which are characterized by a time-window of about 1h. About 20 years ago we showed for the first time that the maintenance of LTP, like memory storage, depends on intact protein synthesis and thus consists of at least two temporal phases. Later, similar properties for LTD were shown by our own and other laboratories. Here we describe the requirements for the induction of the transient early-LTP/LTD and of the protein synthesis-dependent late-LTP/LTD. Late-LTP/LTD depend on the associative activation of heterosynaptic inputs, i.e. the synergistic activation of glutamatergic and modulatory reinforcing inputs within specific, effective time-windows during their induction. The induction of late-LTP/LTD is characterized by novel, late-associative properties such as 'synaptic tagging', 'cross-tagging' and 'late-associative reinforcement'. All of these phenomena require the associative setting of synaptic tags as well as the availability of plasticity-related proteins (PRPs) and they are restricted to functional dendritic compartments, in general. 'Synaptic tagging' guarantees input specificity, 'cross-tagging' determines the interaction between LTP and LTD in a neuron, and thus both are required for the specific processing of afferent signals for the establishment of late-LTP/LTD. 'Late-associative reinforcement' describes a process where early-LTP/LTD by the co-activation of modulatory inputs can be transformed into late-LTP/LTD in activated synapses where a tag is set. Recent experiments in the freely moving rat revealed a number of modulatory brain structures involved in the transformation of early-plasticity events into long-lasting ones. Further to this, we have characterized time-windows and activation patterns to be effective in the reinforcement process. Studies using a combined electrophysiological and behavioural approach revealed the physiological relevance of these reinforcement processes, which is also supported by fMRI studies in humans, which led to the hypothesis outlined here on cellular and system memory-formation by late-associative heterosynaptic interactions at the cellular level during functional plasticity events.

Multiple Gq-coupled receptors converge on a common protein synthesis-dependent long-term depression that is affected in fragile X syndrome mental retardation

Gq-coupled, M1 muscarinic acetylcholine receptors (mAChRs) facilitate hippocampal learning, memory, and synaptic plasticity. M1 mAChRs induce long-term synaptic depression (LTD), but little is known about the underlying mechanisms of mAChR-dependent LTD and its link to cognitive function. Here, we demonstrate that chemical activation of M1 mAChRs induces LTD in hippocampal area CA1, which relies on rapid protein synthesis, as well as the extracellular signal-regulated kinase and mammalian target of rapamycin translational activation pathways. Synaptic stimulation of M1 mAChRs, alone, or together with the Gq-coupled glutamate receptors (mGluRs), also results in protein synthesis-dependent LTD. New proteins maintain mAChR-dependent LTD through a persistent decrease in surface AMPA receptors. mAChRs stimulate translation of the RNA-binding protein, Fragile X mental retardation protein (FMRP) and FMRP target mRNAs. In mice without FMRP (Fmr1 knock-out), a model for human Fragile X syndrome mental retardation (FXS), both mGluR- and mAChR-dependent protein synthesis and LTD are affected. Our results reveal that multiple Gq-coupled receptors converge on a common protein synthesis-dependent LTD mechanism, which is aberrant in FXS. These findings suggest novel therapeutic strategies for FXS in the form of mAChR antagonists.

Genetic disruption of protein kinase A anchoring reveals a role for compartmentalized kinase signaling in theta-burst long-term potentiation and spatial memory

Studies of hippocampal long-term potentiation (LTP), a cellular model of memory storage, implicate cAMP-dependent protein kinase (PKA) in presynaptic and postsynaptic mechanisms of LTP. The anchoring of PKA to AKAPs (A kinase-anchoring proteins) creates compartmentalized pools of PKA, but the roles of presynaptically and postsynaptically anchored forms of PKA in late-phase LTP are unclear. In this study, we have created genetically modified mice that conditionally express Ht31, an inhibitor of PKA anchoring, to probe the roles of anchored PKA in hippocampal LTP and spatial memory. Our findings show that at hippocampal Schaffer collateral CA3-CA1 synapses, theta-burst LTP requires presynaptically anchored PKA. In addition, a pool of anchored PKA in hippocampal area CA3 is required for spatial memory. These findings reveal a novel and significant role for anchored PKA signaling in cellular mechanisms underlying memory storage.

STDP in a bistable synapse model based on CaMKII and associated signaling pathways

The calcium/calmodulin-dependent protein kinase II (CaMKII) plays a key role in the induction of long-term postsynaptic modifications following calcium entry. Experiments suggest that these long-term synaptic changes are all-or-none switch-like events between discrete states. The biochemical network involving CaMKII and its regulating protein signaling cascade has been hypothesized to durably maintain the evoked synaptic state in the form of a bistable switch. However, it is still unclear whether experimental LTP/LTD protocols lead to corresponding transitions between the two states in realistic models of such a network. We present a detailed biochemical model of the CaMKII autophosphorylation and the protein signaling cascade governing the CaMKII dephosphorylation. As previously shown, two stable states of the CaMKII phosphorylation level exist at resting intracellular calcium concentration, and high calcium transients can switch the system from the weakly phosphorylated (DOWN) to the highly phosphorylated (UP) state of the CaMKII (similar to a LTP event). We show here that increased CaMKII dephosphorylation activity at intermediate Ca(2+) concentrations can lead to switching from the UP to the DOWN state (similar to a LTD event). This can be achieved if protein phosphatase activity promoting CaMKII dephosphorylation activates at lower Ca(2+) levels than kinase activity. Finally, it is shown that the CaMKII system can qualitatively reproduce results of plasticity outcomes in response to spike-timing dependent plasticity (STDP) and presynaptic stimulation protocols. This shows that the CaMKII protein network can account for both induction, through LTP/LTD-like transitions, and storage, due to its bistability, of synaptic changes.