Parallel somatic and synaptic processing in the induction of intermediate-term and long-term synaptic facilitation in Aplysia

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.

Role of nitric oxide in the induction of the behavioral and cellular changes produced by a common aversive stimulus in Aplysia

Although it is well documented that exposure to aversive stimuli induces modulation of neural circuits and subsequent behavioral changes, the means by which an aversive stimulus concomitantly alters behaviors of different natures (e.g., defensive and appetitive) remains unclear. Here, we addressed this issue by using the learning-induced concurrent modulation of defensive and appetitive behaviors that occurs when the mollusk Aplysia is exposed to aversive stimuli. In Aplysia, aversive stimuli concomitantly enhance withdrawal reflexes (i.e., sensitization) and suppress feeding. Sensitization and feeding suppression, which are expressed in the short term and long term, depending on the training protocol, are accompanied by increased excitability of the tail sensory neurons (TSNs) controlling the withdrawal reflexes, and by decreased excitability of feeding decision-making neuron B51, respectively. Serotonin (5-HT) has been shown to mediate sensitization, but not feeding suppression. In this study, we examined which other neurotransmitter might be responsible for feeding suppression and its underlying cellular changes. Our results indicate that nitric oxide (NO) contributes to both short-term and long-term feeding suppression, as well as to the underlying decreased B51 excitability. NO was also necessary for the induction of long-term sensitization and for the expression of short-term increased TSN excitability in vitro, revealing a previously undocumented interaction between 5-HT and NO signaling cascades in sensitization. Overall, these results revealed a scenario in which multiple modulators contribute to the widespread changes induced by sensitizing stimuli in Aplysia.

A novel in vitro analog expressing learning-induced cellular correlates in distinct neural circuits

When presented with noxious stimuli, Aplysia exhibits concurrent sensitization of defensive responses, such as the tail-induced siphon withdrawal reflex (TSWR) and suppression of feeding. At the cellular level, sensitization of the TSWR is accompanied by an increase in the excitability of the tail sensory neurons (TSNs) that elicit the reflex, whereas feeding suppression is accompanied by decreased excitability of B51, a decision-making neuron in the feeding neural circuit. The goal of this study was to develop an in vitro analog coexpressing the above cellular correlates. We used a reduced preparation consisting of buccal, cerebral, and pleural-pedal ganglia, which contain the neural circuits controlling feeding and the TSWR, respectively. Sensitizing stimuli were delivered in vitro by electrical stimulation of afferent nerves. When trained with sensitizing stimuli, the in vitro analog expressed concomitant increased excitability in TSNs and decreased excitability in B51, which are consistent with the occurrence of sensitization and feeding suppression induced by in vivo training. This in vitro analog expressed both short-term (15 min) and long-term (24 h) excitability changes in TSNs and B51, depending on the amount of training administered. Finally, in vitro application of serotonin increased TSN excitability without altering B51 excitability, mirroring the in vivo application of the monoamine that induces sensitization, but not feeding suppression.

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.

Persistent long-term facilitation at an identified synapse becomes labile with activation of short-term heterosynaptic plasticity

Short-term and long-term synaptic plasticity are cellular correlates of learning and memory of different durations. Little is known, however, how these two forms of plasticity interact at the same synaptic connection. We examined the reciprocal impact of short-term heterosynaptic or homosynaptic plasticity at sensorimotor synapses of Aplysia in cell culture when expressing persistent long-term facilitation (P-LTF) evoked by serotonin [5-hydroxytryptamine (5-HT)]. Short-term heterosynaptic plasticity induced by 5-HT (facilitation) or the neuropeptide FMRFa (depression) and short-term homosynaptic plasticity induced by tetanus [post-tetanic potentiation (PTP)] or low-frequency stimulation [homosynaptic depression (HSD)] of the sensory neuron were expressed in both control synapses and synapses expressing P-LTF in the absence or presence of protein synthesis inhibitors. All forms of short-term plasticity failed to significantly affect ongoing P-LTF in the absence of protein synthesis inhibitors. However, P-LTF reversed to control levels when either 5-HT or FMRFa was applied in the presence of rapamycin. In contrast, P-LTF was unaffected when either PTP or HSD was evoked in the presence of either rapamycin or anisomycin. These results indicate that synapses expressing persistent plasticity acquire a "new" baseline and functionally express short-term changes as naive synapses, but the new baseline becomes labile following selective activations-heterosynaptic stimuli that evoke opposite forms of plasticity-such that when presented in the presence of protein synthesis inhibitors produce a rapid reversal of the persistent plasticity. Activity-selective induction of a labile state at synapses expressing persistent plasticity may facilitate the development of therapies for reversing inappropriate memories.

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.

Critical period of memory enhancement during taste avoidance conditioning in Lymnaea stagnalis

The present study investigated the optimal training procedure leading to long-lasting taste avoidance behavior in Lymnaea. A training procedure comprising 5 repeated pairings of a conditional stimulus (CS, sucrose), with an unconditional stimulus (US, a tactile stimulation to the animal's head), over a 4-day period resulted in an enhanced memory formation than 10 CS-US repeated pairings over a 2-day period or 20 CS-US repeated pairings on a single day. Backward conditioning (US-CS) pairings did not result in conditioning. Thus, this taste avoidance conditioning was CS-US pairing specific. Food avoidance behavior was not observed following training, however, if snails were immediately subjected to a cold-block (4°C for 10 min). It was critical that the cold-block be applied within 10 min to block long-term memory (LTM) formation. Further, exposure to the cold-block 180 min after training also blocked both STM and LTM formation. The effects of the cold-block on subsequent learning and memory formation were also examined. We found no long lasting effects of the cold-block on subsequent memory formation. If protein kinase C was activated before the conditioning paradigm, snails could still acquire STM despite exposure to the cold-block.

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

Although the importance of spaced training trials in the formation of long-term memory (LTM) is widely appreciated, surprisingly little is known about the molecular mechanisms that support interactions between individual trials. The intertrial dynamics of ERK/MAPK activation have recently been correlated with effective training patterns for LTM. However, whether and how MAPK is required to mediate intertrial interactions remains unknown. Using a novel two-trial training pattern which induces LTM in Aplysia, we show that the first of two training trials recruits delayed protein synthesis-dependent nuclear MAPK activity that establishes a unique molecular context involving the recruitment of CREB kinase and ApC/EBP and is an essential intertrial signaling mechanism for LTM induction. These findings provide the first demonstration of a requirement for MAPK in the intertrial interactions during memory formation and suggest that the kinetics of MAPK activation following individual experiences determines effective training intervals for LTM formation.

Postsynaptic protein synthesis is required for presynaptic enhancement in persistent forms of long-term potentiation

Long-term potentiation (LTP) in the hippocampus is a fundamental process underlying learning and memory in the brain. At CA3-CA1 synapses, three discrete forms of LTP (LTP1, 2, and 3) have been differentiated on the basis of their persistence, maintenance mechanisms, Ca²⁺ signaling pathways, expression loci, and electrophysiological requirements. We previously showed that LTP2 and LTP3 involve a presynaptic expression component that is established in a translation-dependent manner. Here we investigate the locus of translation required for presynaptic expression. Neurotransmitter release rate was estimated via FM 1-43 destaining from CA3 terminals in hippocampal slices from male Wistar rats (6–8 weeks). Destaining was measured at sites making putative contact with CA1 dendritic processes in stratum radiatum that had been filled with a membrane impermeable translation inhibitor and a fluorescent indicator. Our results suggest that inhibition of postsynaptic translation eliminates the enhanced release ordinarily observed at 160 min post-LTP induction, and that this effect is limited to sites closely apposed to the filled postsynaptic cell. We conclude that postsynaptic translation is required for the presynaptic component of LTP2 and LTP3 expression. These data considerably strengthen the mechanistic separation of LTP1, 2, and 3 and provide evidence for an expanded repertoire of communication between synaptic elements.

Local synaptic integration of mitogen-activated protein kinase and protein kinase A signaling mediates intermediate-term synaptic facilitation in Aplysia

It is widely appreciated that memory processing engages a wide range of molecular signaling cascades in neurons, but how these cascades are temporally and spatially integrated is not well understood. To explore this important question, we used Aplysia californica as a model system. We simultaneously examined the timing and subcellular location of two signaling molecules, MAPK (ERK1/2) and protein kinase A (PKA), both of which are critical for the formation of enduring memory for sensitization. We also explored their interaction during the formation of enduring synaptic facilitation, a cellular correlate of memory, at tail sensory-to-motor neuron synapses. We find that repeated tail nerve shock (TNS, an analog of sensitizing training) immediately and persistently activates MAPK in both sensory neuron somata and synaptic neuropil. In contrast, we observe immediate PKA activation only in the synaptic neuropil. It is followed by PKA activation in both compartments 1 h after TNS. Interestingly, blocking MAPK activation during, but not after, TNS impairs PKA activation in synaptic neuropil without affecting the delayed PKA activation in sensory neuron somata. Finally, by applying inhibitors restricted to the synaptic compartment, we show that synaptic MAPK activation during TNS is required for the induction of intermediate-term synaptic facilitation, which leads to the persistent synaptic PKA activation required to maintain this facilitation. Collectively, our results elucidate how MAPK and PKA signaling cascades are spatiotemporally integrated in a single neuron to support synaptic plasticity underlying memory formation.

Massed training-induced intermediate-term operant memory in aplysia requires protein synthesis and multiple persistent kinase cascades

The Aplysia feeding system with its high degree of plasticity and well characterized neuronal circuitry is well suited for investigations of memory formation. We used an operant paradigm, learning that food is inedible (LFI), to investigate the signaling pathways underlying intermediate-term memory (ITM) in Aplysia. During a single massed training session, the animal associates a specific seaweed with the failure to swallow, generating short-term (30 min) and long-term (24 h) memory. We investigated whether the same training protocol induced the formation of ITM. We found that massed LFI training resulted in temporally distinct protein synthesis-dependent memory evident 4-6 h after training. Through in vivo experiments, we determined that the formation of ITM required protein kinase A, protein kinase C, and MAPK. Moreover, the maintenance of ITM required PKA, PKM Apl III, and MAPK because inhibition of any of these kinases after training or before testing blocked the expression of memory. In contrast, additional experiments determined that the maintenance of long-term memory appeared independent of PKM Apl III. Using Western blotting, we found that sustained MAPK phosphorylation was dependent upon protein synthesis, but not PKA or PKC activity. Thus, massed training-induced intermediate-term operant memory requires protein synthesis as well as persistent or sustained kinase signaling for PKA, PKC, and MAPK. While short-, intermediate-, and long-term memory are induced by the same training protocol, considerable differences exist in both the combination and timing of signaling cascades that induce the formation and maintenance of these temporally distinct memories.

Traces of Drosophila memory

Studies using functional cellular imaging of living flies have identified six memory traces that form in the olfactory nervous system after conditioning with odors. These traces occur in distinct nodes of the olfactory nervous system, form and disappear across different windows of time, and are detected in the imaged neurons as increased calcium influx or synaptic release in response to the conditioned odor. Three traces form at or near acquisition and coexist with short-term behavioral memory. One trace forms with a delay after learning and coexists with intermediate-term behavioral memory. Two traces form many hours after acquisition and coexist with long-term behavioral memory. The transient memory traces may support behavior across the time windows of their existence. The experimental approaches for dissecting memory formation in the fly, ranging from the molecular to the systems, make it an ideal system for elucidating the logic by which the nervous system organizes and stores different temporal forms of memory.

The tail-elicited tail withdrawal reflex of Aplysia is mediated centrally at tail sensory-motor synapses and exhibits sensitization across multiple temporal domains

The defensive withdrawal reflexes of Aplysia californica have provided powerful behavioral systems for studying the cellular and molecular basis of memory formation. Among these reflexes the tail-elicited tail withdrawal reflex (T-TWR) has been especially useful. In vitro studies examining the monosynaptic circuit for the T-TWR, the tail sensory-motor (SN-MN) synapses, have identified the induction requirements and molecular basis of different temporal phases of synaptic facilitation that underlie sensitization in this system. They have also permitted more recent studies elucidating the role of synaptic and nuclear signaling during synaptic facilitation. Here we report the development of a novel, compartmentalized semi-intact T-TWR preparation that allows examination of the unique contributions of processing in the SN somatic compartment (the pleural ganglion) and the SN-MN synaptic compartment (the pedal ganglion) during the induction of sensitization. Using this preparation we find that the T-TWR is mediated entirely by central connections in the synaptic compartment. Moreover, the reflex is stably expressed for at least 24 h, and can be modified by tail shocks that induce sensitization across multiple temporal domains, as well as direct application of the modulatory neurotransmitter serotonin. This preparation now provides an experimentally powerful system in which to directly examine the unique and combined roles of synaptic and nuclear signaling in different temporal domains of memory formation.

Whereas short-term facilitation is presynaptic, intermediate-term facilitation involves both presynaptic and postsynaptic protein kinases and protein synthesis

Whereas short-term plasticity involves covalent modifications that are generally restricted to either presynaptic or postsynaptic structures, long-term plasticity involves the growth of new synapses, which by its nature involves both pre- and postsynaptic alterations. In addition, an intermediate-term stage of plasticity has been identified that might form a bridge between short- and long-term plasticity. Consistent with that idea, although short-term term behavioral sensitization in Aplysia involves presynaptic mechanisms, intermediate-term sensitization involves both pre- and postsynaptic mechanisms. However, it has not been known whether that is also true of facilitation in vitro, where a more detailed analysis of the mechanisms involved in the different stages and their interrelations is feasible. To address those questions, we have examined pre- and postsynaptic mechanisms of short- and intermediate-term facilitation at Aplysia sensory-motor neuron synapses in isolated cell culture. Whereas short-term facilitation by 1-min 5-HT involves presynaptic PKA and CamKII, intermediate-term facilitation by 10-min 5-HT involves presynaptic PKC and postsynaptic Ca(2+) and CamKII, as well as both pre- and postsynaptic protein synthesis. These results support the idea that the intermediate-term stage is the first to involve both pre- and postsynaptic molecular mechanisms, which could in turn serve as some of the initial steps in a cascade leading to synaptic growth during long-term plasticity.

Presynaptic and postsynaptic mechanisms of synaptic plasticity and metaplasticity during intermediate-term memory formation in Aplysia

Synaptic plasticity and learning involve different mechanisms depending on the following: (1) the stage of plasticity and (2) the history of plasticity, or metaplasticity. However, little is known about how these two factors are related. We have addressed that question by examining mechanisms of synaptic plasticity during short-term and intermediate-term behavioral sensitization and dishabituation in a semi-intact preparation of the Aplysia siphon-withdrawal reflex. Dishabituation differs from sensitization in that it is preceded by habituation, and is thus a paradigm for metaplasticity. We find that whereas facilitation during short-term sensitization by one tail shock involves presynaptic covalent modifications by protein kinase A (PKA) and CamKII, facilitation during intermediate-term sensitization by four shocks involves both presynaptic (PKA, CaMKII) and postsynaptic (Ca(2+), CaMKII) covalent modifications, as well as both presynaptic and postsynaptic protein synthesis. The facilitation also involves presynaptic spike broadening 2.5 min after either one or four shocks, but not at later times. Dishabituation by four shocks differs from sensitization in several ways. First, it does not involve PKA or CaMKII, but rather involves presynaptic PKC. In addition, unlike sensitization with the same shock, dishabituation by four shocks does not involve protein synthesis or presynaptic spike broadening, and it also does not involve postsynaptic Ca(2+). These results demonstrate that not only the mechanisms but also the site of plasticity depend on both the stage of plasticity and metaplasticity during memory formation.

Updating contextual information during consolidation as result of a new memory trace

Reconsolidation studies have led to the hypothesis that memory, when labile, would be modified in order to incorporate new information. This view has reinstated original propositions suggesting that short-term memory provides the organism with an opportunity to evaluate and rearrange information before storing it, since it is concurrent with the labile state of consolidation. The Chasmagnathus associative memory model is used here to test whether during consolidation it is possible to change some attribute of recently acquired memories. In addition, it is tested whether these changes in behavioral memory features can be explained as modifications on the consolidating memory trace or as a consequence of a new memory trace. We show that short-term memory is, unlike long-term memory, not context specific. During this short period after learning, behavioral memory can be updated in order to incorporate new contextual information. We found that, during this period, the cycloheximide retrograde amnesic effect can be reverted by a single trial in a new context. Finally, by means of memory sensitivity to cycloheximide during consolidation and reconsolidation, we show that the learning of a new context (CS) during this short-term memory period builds up a new memory trace that sustains the behavioral memory update.

Synapsin is selectively required for anesthesia-sensitive memory

Odor-shock memory in Drosophila melanogaster consists of heterogeneous components each with different dynamics. We report that a null mutant for the evolutionarily conserved synaptic protein Synapsin entails a memory deficit selectively in early memory, leaving later memory as well as sensory motor function unaffected. Notably, a consolidated memory component remaining after cold-anesthesia is not impaired, suggesting that only anesthesia-sensitive memory [ASM] depends on Synapsin. The lack of Synapsin does not further impair the memory deficit of mutants for the rutabaga gene encoding the type I adenylyl cyclase. This suggests that cAMP signaling, through a Synapsin-dependent mechanism, may underlie the formation of a labile memory component.

Presynaptic translation: stepping out of the postsynaptic shadow

The ability of the nervous system to convert transient experiences into long-lasting structural changes at the synapse relies upon protein synthesis. It has become increasingly clear that a critical subset of this synthesis occurs within the synaptic compartment. While this process has been extensively characterized in the postsynaptic compartment, the contribution of local translation to presynaptic function remains largely unexplored. However, recent evidence highlights the potential importance of translation within the presynaptic compartment. Work in cultured neurons has shown that presynaptic translation occurs specifically at synapses undergoing long-term plasticity and may contribute to the maintenance of nascent synapses. Studies from our laboratory have demonstrated that Fragile X proteins, which regulate mRNA localization and translation, are expressed at the presynaptic apparatus. Further, mRNAs encoding presynaptic proteins traffic into axons. Here we discuss recent advances in the study of presynaptic translation as well as the challenges confronting the field. Understanding the regulation of presynaptic function by local protein synthesis promises to shed new light on activity-dependent modification of synaptic architecture.

Translational control of long-lasting synaptic plasticity and memory

Long-lasting forms of synaptic plasticity and memory are dependent on new protein synthesis. Recent advances obtained from genetic, physiological, pharmacological, and biochemical studies provide strong evidence that translational control plays a key role in regulating long-term changes in neural circuits and thus long-term modifications in behavior. Translational control is important for regulating both general protein synthesis and synthesis of specific proteins in response to neuronal activity. In this review, we summarize and discuss recent progress in the field and highlight the prospects for better understanding of long-lasting changes in synaptic strength, learning, and memory and implications for neurological diseases.

Sustained CPEB-dependent local protein synthesis is required to stabilize synaptic growth for persistence of long-term facilitation in Aplysia

The time course of the requirement for local protein synthesis in the stabilization of learning-related synaptic growth and the persistence of long-term memory was examined using Aplysia bifurcated sensory neuron-motor neuron cultures. We find that, following repeated pulses of serotonin (5-HT), the local perfusion of emetine, an inhibitor of protein synthesis, or a TAT-AS oligonucleotide directed against ApCPEB blocks long-term facilitation (LTF) at either 24 or 48 hr and leads to a selective retraction of newly formed sensory neuron varicosities induced by 5-HT. By contrast, later inhibition of local protein synthesis, at 72 hr after 5-HT, has no effect on either synaptic growth or LTF. These results define a specific stabilization phase for the storage of long-term memory during which newly formed varicosities are labile and require sustained CPEB-dependent local protein synthesis to acquire the more stable properties of mature varicosities required for the persistence of LTF.

Postsynaptic regulation of long-term facilitation in Aplysia

Repeated exposure to serotonin (5-HT), an endogenous neurotransmitter that mediates behavioral sensitization in Aplysia[1-3], induces long-term facilitation (LTF) of the Aplysia sensorimotor synapse [4]. LTF, a prominent form of invertebrate synaptic plasticity, is believed to play a major role in long-term learning in Aplysia[5]. Until now, LTF has been thought to be due predominantly to cellular processes activated by 5-HT within the presynaptic sensory neuron [6]. Recent work indicates that LTF depends on the increased expression and release of a sensory neuron-specific neuropeptide, sensorin [7]. Sensorin released during LTF appears to bind to autoreceptors on the sensory neuron, thereby activating critical presynaptic signals, including mitogen-activated protein kinase (MAPK) [8, 9]. Here, we show that LTF depends on elevated postsynaptic Ca2+ and postsynaptic protein synthesis. Furthermore, we find that the increased expression of presynaptic sensorin resulting from 5-HT stimulation requires elevation of postsynaptic intracellular Ca2+. Our results represent perhaps the strongest evidence to date that the increased expression of a specific presynaptic neuropeptide during LTF is regulated by retrograde signals.

Okadaic acid-sensitive protein phosphatases constrain phrenic long-term facilitation after sustained hypoxia

Phrenic long-term facilitation (pLTF) is a serotonin-dependent form of pattern-sensitive respiratory plasticity induced by intermittent hypoxia (IH), but not sustained hypoxia (SH). The mechanism(s) underlying pLTF pattern sensitivity are unknown. SH and IH may differentially regulate serine/threonine protein phosphatase activity, thereby inhibiting relevant protein phosphatases uniquely during IH and conferring pattern sensitivity to pLTF. We hypothesized that spinal protein phosphatase inhibition would relieve this braking action of protein phosphatases, thereby revealing pLTF after SH. Anesthetized rats received intrathecal (C4) okadaic acid (25 nm) before SH (25 min, 11% O(2)). Unlike (vehicle) control rats, SH induced a significant pLTF in okadaic acid-treated rats that was indistinguishable from rats exposed to IH (three 5 min episodes, 11% O(2)). IH and SH with okadaic acid may elicit pLTF by similar, serotonin-dependent mechanisms, because intravenous methysergide blocks pLTF in rats receiving IH or okadaic acid plus SH. Okadaic acid did not alter IH-induced pLTF. In summary, pattern sensitivity in pLTF may reflect differential regulation of okadaic acid-sensitive serine/threonine phosphatases; presumably, these phosphatases are less active during/after IH versus SH. The specific okadaic acid-sensitive phosphatase(s) constraining pLTF and their spatiotemporal dynamics during and/or after IH and SH remain to be determined.

Defining memories by their distinct molecular traces

It is often stated that short-term memory is consolidated in a protein-synthesis-dependent manner into long-term memory. Alternatively, memories might consist of distinct molecular traces that last for different periods of time. These traces can be graded by their 'volatility'; traces encoded by activation of protein kinases are more volatile than traces encoded by morphological changes at preexisting synapses. The least volatile ('static') traces are due to the generation and stabilization of new synapses. Importantly, whereas at the cellular level these traces are generated independently of each other, they might be linked at the network level where volatile memory traces are required to set up a cellular network that is in turn required to induce the static memory trace.

Regulation of hippocampus-dependent memory by cyclic AMP-dependent protein kinase

The hippocampus is crucial for the consolidation of new declarative long-term memories. Genetic and behavioral experimentation have revealed that several protein kinases are critical for the formation of hippocampus-dependent long-term memories. Cyclic-AMP dependent protein kinase (PKA) is a serine-threonine kinase that has been strongly implicated in the expression of specific forms of hippocampus-dependent memory. We review evidence that PKA is required for hippocampus-dependent memory in mammals, and we highlight some of the proteins that have been implicated as targets of PKA. Future directions and open questions regarding the role of PKA in memory storage are also described.

New tricks for an old slug: the critical role of postsynaptic mechanisms in learning and memory in Aplysia

The marine snail Aplysia has served for more than four decades as an important model system for neurobiological analyses of learning and memory. Until recently, it has been believed that learning and memory in Aplysia were due predominately, if not exclusively, to presynaptic mechanisms. For example, two nonassociative forms of learning exhibited by Aplysia, sensitization and dishabituation of its defensive withdrawal reflex, have been previously ascribed to presynaptic facilitation of the connections between sensory and motor neurons that mediate the reflex. Recent evidence, however, indicates that postsynaptic mechanisms play a far more important role in learning and memory in Aplysia than formerly appreciated. In particular, dishabituation and sensitization depend on a rise in intracellular Ca(2+) in the postsynaptic motor neuron, postsynaptic exocytosis, and modulation of the functional expression of postsynaptic AMPA-type glutamate receptors. In addition, the expression of the persistent presynaptic changes that occur during intermediate- and long-term dishabituation and sensitization appears to require retrograde signals that are triggered by elevated postsynaptic Ca(2+). The model for learning-related synaptic plasticity proposed here for Aplysia is similar to current mammalian models. This similarity suggests that the cellular mechanisms of learning and memory have been highly conserved during evolution.

Transient mitogen-activated protein kinase activation is confined to a narrow temporal window required for the induction of two-trial long-term memory in Aplysia

Although it is commonly appreciated that spaced training is superior to massed training in memory formation, the molecular mechanisms underlying this feature of memory are largely unknown. We previously described the selective benefit of multiple spaced (vs massed) training trials in the induction of long-term memory (LTM) for sensitization in Aplysia californica. We now report that LTM can be induced with only two spaced training trials [tail shocks (TSs)] when the second TS is administered 45 min after the first. In contrast, spacing intervals of 15 and 60 min are ineffective. This surprisingly narrow permissive training window for two-trial LTM is accompanied by an equally narrow window of transient mitogen-activated protein kinase (MAPK) activation, a necessary signaling molecule for LTM induction, at 45 min after a single TS. Thus, the transient recruitment of MAPK following a single TS may provide a narrow molecular window for two-trial LTM formation.

Protein synthesis inhibition and memory: formation vs amnesia

Studies using protein synthesis inhibitors have provided key support for the prevalent view that memory formation requires the initiation of protein synthesis as a primary element of the molecular biology of memory. However, many other interpretations of the amnesia data have received far less attention. These include: (a) protein synthesis may play a constitutive role in memory formation, providing proteins prior to an experience that can be activated by training; (b) protein synthesis may be needed to replace proteins available prior to learning but 'consumed' by learning; (c) inhibition of protein synthesis impairs the well-being of neurons, leading to an inability to deliver resources needed for memory formation; and (d) inhibition of protein synthesis results in abnormal neural functions that interfere with memory. One of these, abnormal release of neurotransmitters after inhibition of protein synthesis, is detailed here, along with a review of many circumstances in which it appears that protein synthesis at the time of training is not required for the formation of new memories. Evidence of activation of cell signaling molecules and transcription factors is another form of support for a role of training-initiated protein synthesis in memory. However, recent findings suggest that many of these molecules are activated by training and remain activated for days after training, i.e. activated for times well beyond those typically invoked for memory consolidation processes. Reviewing these results, this paper suggests that the long-lasting molecular changes may be the basis of a form of intracellular memory, one responsible for up-regulating the probability that a neuron, once activated in this manner, will engage in future plasticity. This view melds ideas of modulation of memory with those of consolidation of memory.

Effects of antisense oligonucleotides to mRNA for the early gene zif268 on the mechanisms of synapse-specific plasticity

Acquisition of nociceptive sensitization in common snails was accompanied by long-term facilitation of the responses of defensive behavior command neuron LPl1 to sensory stimulation of chemoreceptors on the head and mechanoreceptors on the head and foot. Acquisition of sensitization during intracellular administration of antisense oligonucleotides to mRNA encoding the early gene zif268 showed suppression of synaptic facilitation in the responses of neuron LPl1 to tactile and chemical stimulation of the snail's head. Synaptic facilitation in the responses to tactile stimulation of the foot developed as in neurons of control sensitized animals. These results suggest that the early gene zif268 is selectively involved in the mechanisms of the specific regulation of the synaptic inputs of neuron LPl1 from sensory receptors on the snail's head.

Mechanisms of serotonergic facilitation of a command neuron

The lateral giant (LG) command neuron of crayfish responds to an attack directed at the abdomen by triggering a single highly stereotyped escape tail flip. Experimentally applied serotonin (5-hydroxytrptamine, 5-HT) can increase or decrease LG's excitability, depending on the concentration, rate, and duration of 5-HT application. Here we describe three physiological mechanisms that mediate serotonergic facilitation of LG. Two processes strengthen electrical coupling between the primary mechanosensory afferent neurons and LG: first, an early increase in the conductance of electrical synapses between primary afferent neurons and LG dendrites and second, an early increase in the membrane resistance of LG dendrites. The increased coupling facilitates LG's synaptic response and it promotes recruitment of weakly excited afferent neurons to contribute to the response. Third, a delayed increase in the membrane resistance of proximal regions of LG increases the cell's input resistance near the initial segment. Together these mechanisms contribute to serotonergic facilitation of LG's response.

Signaling at A-kinase anchoring proteins organizes anesthesia-sensitive memory in Drosophila

The ubiquitous cAMP-protein kinase A (PKA) signaling pathway exhibits complex temporal requirements during the time course of associative memory processing. This directly raises questions about the molecular mechanisms that provide signaling specificity to this pathway. Here, we use Drosophila olfactory conditioning to show that divergent cAMP signaling is mediated by functionally distinct pools of PKA. One particular pool is organized via the PKA regulatory type II subunit at the level of A-kinase anchoring proteins (AKAPs), a family of scaffolding proteins that provides focal points of spatiotemporal signal integration. This AKAP-bound pool of PKA is acting within neurons of the mushroom bodies to support a late phase of aversive memory. The requirement for AKAP-bound PKA signaling is limited to aversive memory, but dispensable during appetitive memory. This finding suggests the existence of additional mechanisms to support divergence within the cAMP-PKA signaling pathway during memory processing. Together, our results show that subcellular organization of signaling components plays a key role in memory processing.

Dendritic Protein Synthesis, Synaptic Plasticity, and Memory

Considerable evidence suggests that the formation of long-term memories requires a critical period of new protein synthesis. Recently, the notion that some of these newly synthesized proteins originate through local translation in neuronal dendrites has gained some traction. Here, we review the experimental support for this idea and highlight some of the key questions outstanding in this area.

A neuronal network for the logic of Limax learning

We construct a neuronal network to model the logic of associative conditioning as revealed in experimental results using the terrestrial mollusk Limax maximus. We show, in particular, how blocking to a previously conditioned stimulus in the presence of the unconditional stimulus, can emerge as a dynamical property of the network. We also propose experiments to test the new model.

Essential role of somatic and synaptic protein synthesis and axonal transport in long-term synapse-specific facilitation at distal sensorimotor connections in Aplysia

To investigate further the cellular mechanisms underlying long-term facilitation (LTF) and long-term synapse-specific facilitation (LTSSF), we studied the role of axonal transport and somatic and synaptic protein synthesis at proximal and distal synapses of Aplysia siphon sensory neurons (SNs). The long soma-synapse distances (2.5 to 3 cm) of the SN distal synapses impose important temporal and mechanistic constraints on long-term facilitation and on intracellular signaling. Excitatory postsynaptic potentials (EPSPs) evoked by SNs in central and peripheral siphon motor neurons were used to assay LTF 24-30 h after various pharmacological treatments. Inhibition of protein synthesis via anisomycin application at either the SN soma or distal synapses blocked the induction of LTF and LTSSF normally produced by synaptic application of the facilitating transmitter serotonin (5-hydroxytryptamine). Further, disruption of axonal transport by application of nocodazole to the isolated siphon nerve completely blocked LTF at distal synapses. These results indicate an essential role for somatic and synaptic protein synthesis and active axonal transport in LTSSF at distal synapses, and raise intriguing questions for current synaptic marking/capture models of synapse specificity and LTF.

Synaptic facilitation and behavioral dishabituation in Aplysia: dependence on release of Ca2+ from postsynaptic intracellular stores, postsynaptic exocytosis, and modulation of postsynaptic AMPA receptor efficacy

Sensitization and dishabituation of the defensive withdrawal reflex in Aplysia have been ascribed to presynaptic mechanisms, particularly presynaptic facilitation of transmission at sensorimotor synapses in the CNS of Aplysia. Here, we show that facilitation of sensorimotor synapses in cell culture during and after serotonin (5-HT) exposure depends on a rise in postsynaptic intracellular Ca(2+) and release of Ca(2+) from postsynaptic stores. We also provide support for the idea that postsynaptic AMPA receptor insertion mediates a component of synaptic facilitation by showing that facilitation after 5-HT offset is blocked by injecting botulinum toxin, an exocytotic inhibitor, into motor neurons before application of 5-HT. Using a reduced preparation, we extend our results to synaptic facilitation in the abdominal ganglion. We show that tail nerve shock-induced facilitation of siphon sensorimotor synapses also depends on elevated postsynaptic Ca(2+) and release of Ca(2+) from postsynaptic stores and recruits a late phase of facilitation that involves selective enhancement of the AMPA receptor-mediated synaptic response. To examine the potential role of postsynaptic exocytosis of AMPA receptors in learning in Aplysia, we test the effect of injecting botulinum toxin into siphon motor neurons on dishabituation of the siphon-withdrawal reflex. We find that postsynaptic injections of the toxin block dishabituation resulting from tail shock. Our results indicate that postsynaptic mechanisms, particularly Ca(2+)-dependent modulation of AMPA receptor trafficking, play a critical role in synaptic facilitation as well as in dishabituation and sensitization in Aplysia.

Dynamic properties of regulatory motifs associated with induction of three temporal domains of memory in aplysia

A model was developed to examine dynamical properties of regulatory motifs correlated with different temporal domains of memory. The model represents short-, intermediate-, and long-term phases of protein kinase A (PKA) activation, which appear related to corresponding phases of facilitation of the Aplysia sensorimotor synapse. The model also represents phosphorylation of the transcription factor CREB1 by PKA and consequent induction of the immediate-early gene Aplysia ubiquitin hydrolase (Ap-uch), which is essential for long-term synaptic facilitation (LTF). Simulations suggest mechanisms responsible for differing profiles of synaptic facilitation following massed vs. spaced exposures to 5-HT, and suggest a novel regulatory motif (gated positive feedback) is important for LTF. Simulations suggest zero-order ultrasensitivity may underlie a requirement of a threshold number of exposures to 5-HT for LTF induction. The model makes predictions for the dynamics of PKA activation and Ap-uch induction when MAP kinase is activated, or when repression of Ap-uch is relieved by inhibiting the transcription factor CREB2. This model may therefore be useful for understanding processes underlying memory formation in Aplysia and other systems.