A tropomyosin-related kinase B ligand is required for ERK activation, long-term synaptic facilitation, and long-term memory in aplysia

Synaptic Mechanisms of Delay Eyeblink Classical Conditioning: AMPAR Trafficking and Gene Regulation in an In Vitro Model

An in vitro model of delay eyeblink classical conditioning was developed to investigate synaptic plasticity mechanisms underlying acquisition of associative learning. This was achieved by replacing real stimuli, such as an airpuff and tone, with patterned stimulation of the cranial nerves using an isolated brainstem preparation from turtle. Here, our primary findings regarding cellular and molecular mechanisms for learning acquisition using this unique approach are reviewed. The neural correlate of the in vitro eyeblink response is a replica of the actual behavior, and features of conditioned responses (CRs) resemble those observed in behavioral studies. Importantly, it was shown that acquisition of CRs did not require the intact cerebellum, but the appropriate timing did. Studies of synaptic mechanisms indicate that conditioning involves two stages of AMPA receptor (AMPAR) trafficking. Initially, GluA1-containing AMPARs are targeted to synapses followed later by replacement by GluA4 subunits that support CR expression. This two-stage process is regulated by specific signal transduction cascades involving PKA and PKC and is guided by distinct protein chaperones. The expression of the brain-derived neurotrophic factor (BDNF) protein is central to AMPAR trafficking and conditioning. BDNF gene expression is regulated by coordinated epigenetic mechanisms involving DNA methylation/demethylation and chromatin modifications that control access of promoters to transcription factors. Finally, a hypothesis is proposed that learning genes like BDNF are poised by dual chromatin features that allow rapid activation or repression in response to environmental stimuli. These in vitro studies have advanced our understanding of the cellular and molecular mechanisms that underlie associative learning.

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

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

Specificity of synapse formation in Aplysia: paracrine and autocrine signaling regulates bidirectional molecular interactions between sensory and non-target motor neurons

The formation of appropriate neural connections during development is critical for the proper wiring and functioning of the brain. Although considerable research suggests that the specificity of synapse formation is supported by complex intercellular signaling between potential presynaptic and postsynaptic partners, the extracellular factors and the intracellular signal transduction pathways engaged in this process remain largely unknown. Using the sensory-motor neural circuit that contributes to learning in defensive withdrawal reflexes in Aplysia californica, we investigated the molecular processes governing the interactions between sensory neurons and both target and non-target motor neurons during synapse formation in culture. We found that evolutionarily-conserved intercellular and intracellular signaling mechanisms critical for learning-related plasticity are also engaged during synaptogenesis in this in vitro model system. Our results reveal a surprising bidirectional regulation of molecular signaling between sensory neurons and non-target motor neurons. This regulation is mediated by signaling via both paracrine and autocrine diffusible factors that induce differential effects on transcription and on protein expression/activation in sensory neurons and in target and non-target motor neurons. Collectively, our data reveal novel molecular mechanisms that could underlie the repression of inappropriate synapse formation, and suggest mechanistic similarities between developmental and learning-related plasticity.

Mechanisms of protein kinase C-induced sustained activation of extracellular signal-regulated kinase in the hippocampus

Protein Kinase C (PKC) and extracellular signal-regulated kinase (ERK) regulate synaptic plasticity and memory. PKC activation enhances long-term potentiation (LTP) in the hippocampal slices. In addition, activation of PKC by phorbol 12,13-diacetate (PDA) induces ERK activation. However, the mechanisms involved in PDA-induced activation of ERK are not well understood. Using hippocampal slices, we report that PDA induces a sustained activation of ERK. PDA-induced sustained ERK activation critically requires protein synthesis as well as transcription, the cellular processes that play crucial roles in long-lasting LTP and memory. In addition, the mammalian target of rapamycin activity is required for PDA-induced sustained ERK activation. Further, we show that growth factor signalling plays a critical role in PDA-induced sustained ERK activation. These results suggest that sustained ERK activation may have an important role in LTP.

A role for Numb in Protein kinase M (PKM)-mediated increase in surface AMPA receptors during facilitation in Aplysia

There is considerable evidence from both vertebrates and invertebrates that persistently active protein kinases maintain changes in synaptic strength that underlie memory. In the hermaphrodite marine mollusk, Aplysia californica, truncated forms of protein kinase C (PKC) termed protein kinase Ms have been implicated in both intermediate- and long-term facilitation, an increase in synaptic strength between sensory neurons and motor neurons thought to underlie behavioural sensitization in the animal. However, few substrates have been identified as candidates that could mediate this increase in synaptic strength. PKMs have been proposed to maintain synaptic strength through preventing endocytosis of AMPA receptors. Numb is a conserved regulator of endocytosis that is modulated by phosphorylation. We have identified and cloned Aplysia Numb (ApNumb). ApNumb contains three conserved PKC phosphorylation sites and PKMs generated from classical and atypical Aplysia PKCs can phosphorylate ApNumb in vitro and in cells. Over-expression of ApNumb that lacks the conserved PKC phosphorylation sites blocks increases in surface levels of a pHluorin-tagged Aplysia glutamate receptor measured using live imaging after intermediate- or long-term facilitation. Over-expression of this form of ApNumb did not block increases in synaptic strength seen during intermediate-term facilitation, but did block increases in synaptic strength seen during long-term facilitation. There was no effect of over-expression of this form of ApNumb on other putative Numb targets as measured using increases in calcium downstream of neurotrophins or agonists of metabotropic glutamate receptors. These results suggest that in Aplysia neurons, Numb specifically regulates AMPA receptor trafficking and is an attractive candidate for a target of PKMs in long-term maintenance of synaptic strength. OPEN SCIENCE BADGES: This article has received a badge for *Open Materials* because it provided all relevant information to reproduce the study in the manuscript. The complete Open Science Disclosure form for this article can be found at the end of the article. More information about the Open Practices badges can be found at https://cos.io/our-services/open-science-badges/. Open Science: This manuscript was awarded with the Open Materials Badge For more information see: https://cos.io/our-services/open-science-badges/.

Intermediate-term memory in Aplysia involves neurotrophin signaling, transcription, and DNA methylation

Long-term but not short-term memory and synaptic plasticity in many brain areas require neurotrophin signaling, transcription, and epigenetic mechanisms including DNA methylation. However, it has been difficult to relate these cellular mechanisms directly to behavior because of the immense complexity of the mammalian brain. To address that problem, we and others have examined numerically simpler systems such as the hermaphroditic marine mollusk Aplysia californica. As a further simplification, we have used a semi-intact preparation of the Aplysia siphon withdrawal reflex in which it is possible to relate cellular plasticity directly to behavioral learning. We find that inhibitors of neurotrophin signaling, transcription, and DNA methylation block sensitization and classical conditioning beginning ∼1 h after the start of training, which is in the time range of an intermediate-term stage of plasticity that combines elements of short- and long-term plasticity and may form a bridge between them. Injection of decitabine (an inhibitor of DNA methylation that may have other actions in these experiments) into an LE sensory neuron blocks the neural correlates of conditioning in the same time range. In addition, we found that both DNA and RNA methylation in the abdominal ganglion are correlated with learning in the same preparations. These results begin to suggest the functions and integration of these different molecular mechanisms during behavioral learning.

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

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

Behavioral tagging: A novel model for studying long-term memory

New information acquired by our brain is stored in the form of two types of memories: short term memory (STM) and long term memory (LTM). Initially, Synaptic and Capture hypothesis has been proposed to describe the synaptic changes that occur during memory formation. However, recently Behavioral Tagging hypothesis was proposed that relies on the setting of a learning tag and the synthesis of plasticity related proteins (PRPs). Behavioral Tagging has its roots in Synaptic and Capture hypothesis. It seeks to explain that how a learning tag produced as a result of weak training can be paired up with PRPs (formed as a result of novelty) and can lead to long lasting memories. We have focused on describing behavioral paradigms that have been used for establishing the model of "Behavioral Tagging" and the molecules which qualify for potential PRP candidature.

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

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

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

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

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

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

Nonassociative learning in invertebrates

The simplicity and tractability of the neural circuits mediating behaviors in invertebrates have facilitated the cellular/molecular dissection of neural mechanisms underlying learning. The review has a particular focus on the general principles that have emerged from analyses of an example of nonassociative learning, sensitization in the marine mollusk Aplysia. Learning and memory rely on multiple mechanisms of plasticity at multiple sites of the neuronal circuits, with the relative contribution to memory of the different sites varying as a function of the extent of training and time after training. The same intracellular signaling cascades that induce short-term modifications in synaptic transmission can also be used to induce long-term changes. Although short-term memory relies on covalent modifications of preexisting proteins, long-term memory also requires regulated gene transcription and translation. Maintenance of long-term cellular memory involves both intracellular and extracellular feedback loops, which sustain the regulation of gene expression and the modification of targeted molecules.

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.

Activity-dependent inhibitory gating in molecular signaling cascades induces a novel form of intermediate-term synaptic facilitation in Aplysia californica

Mechanistically distinct forms of long-lasting plasticity and memory can be induced by a variety of different training patterns. Although several studies have identified distinct molecular pathways that are engaged during these different training patterns, relatively little work has explored potential interactions between pathways when they are simultaneously engaged in the same neurons and circuits during memory formation. Aplysia californica exhibits two forms of intermediate-term synaptic facilitation (ITF) in response to two different training patterns: (1) repeated trial (RT) ITF (induced by repeated tail nerve shocks [TNSs] or repeated serotonin [5HT] application) and (2) activity-dependent (AD) ITF (induced by sensory neuron activation paired with a single TNS or 5HT pulse). RT-ITF requires PKA activation and de novo protein synthesis, while AD-ITF requires PKC activation and has no requirement for protein synthesis. Here, we explored how these distinct molecular pathways underlying ITF interact when both training patterns occur in temporal register (an "Interactive" training pattern). We found that (1) RT, AD, and Interactive training all induce ITF; (2) Interactive ITF requires PKC activity but not de novo protein synthesis; and (3), surprisingly, Interactive training blocks persistent PKA activity 1 h after training, and this block is PKC-independent. These data support the hypothesis that sensory neuron activity coincident with the last RT training trial is sufficient to convert the molecular signaling already established by RT training into an AD-like molecular phenotype.

Growth factor signaling and memory formation: temporal and spatial integration of a molecular network

Growth factor (GF) signaling is critically important for developmental plasticity. It also plays a crucial role in adult plasticity, such as that required for memory formation. Although different GFs interact with receptors containing distinct types of kinase domains, they typically signal through converging intracellular cascades (e.g., Ras–MEK–MAPK) to mediate overlapping functional endpoints. Several GFs have been implicated in memory formation, but due to a high level of convergent signaling, the unique contributions of individual GFs as well as the interactions between GF signaling cascades during the induction of memory is not well known. In this review, we highlight the unique roles of specific GFs in dendritic plasticity, and discuss the spatial and temporal profiles of different GFs during memory formation. Collectively, the data suggest that the roles of GF signaling in long-lasting behavioral and structural plasticity may be best viewed as interactive components in a complex molecular network.

Cellular, molecular, and epigenetic mechanisms in non-associative conditioning: implications for pain and memory

Sensitization is a form of non-associative conditioning in which amplification of behavioral responses can occur following presentation of an aversive or noxious stimulus. Understanding the cellular and molecular underpinnings of sensitization has been an overarching theme spanning the field of learning and memory as well as that of pain research. In this review we examine how sensitization, both in the context of learning as well as pain processing, shares evolutionarily conserved behavioral, cellular/synaptic, and epigenetic mechanisms across phyla. First, we characterize the behavioral phenomenon of sensitization both in invertebrates and vertebrates. Particular emphasis is placed on long-term sensitization (LTS) of withdrawal reflexes in Aplysia following aversive stimulation or injury, although additional invertebrate models are also covered. In the context of vertebrates, sensitization of mammalian hyperarousal in a model of post-traumatic stress disorder (PTSD), as well as mammalian models of inflammatory and neuropathic pain is characterized. Second, we investigate the cellular and synaptic mechanisms underlying these behaviors. We focus our discussion on serotonin-mediated long-term facilitation (LTF) and axotomy-mediated long-term hyperexcitability (LTH) in reduced Aplysia systems, as well as mammalian spinal plasticity mechanisms of central sensitization. Third, we explore recent evidence implicating epigenetic mechanisms in learning- and pain-related sensitization. This review illustrates the fundamental and functional overlay of the learning and memory field with the pain field which argues for homologous persistent plasticity mechanisms in response to sensitizing stimuli or injury across phyla.

BDNF and memory processing

Brain-derived neurotrophic factor has emerged as one of the most important molecules involved in memory. Its wide role in different types of memories that depend on different structures as well as its involvement in distinct memory stages points at BDNF as one likely target to treat cognitive impairments and anxiety-related memory disorders. However, regulation of BDNF expression is very complex as well as its modes of action. Here we describe the latest research carried out on the function of BDNF in memory to illustrate such complexity. This article is part of the Special Issue entitled 'BDNF Regulation of Synaptic Structure, Function, and Plasticity'.

A single Aplysia neurotrophin mediates synaptic facilitation via differentially processed isoforms

Neurotrophins control the development and adult plasticity of the vertebrate nervous system. Failure to identify invertebrate neurotrophin orthologs, however, has precluded studies in invertebrate models, limiting our understanding of fundamental aspects of neurotrophin biology and function. We identified a neurotrophin (ApNT) and Trk receptor (ApTrk) in the mollusk Aplysia and found that they play a central role in learning-related synaptic plasticity. Blocking ApTrk signaling impairs long-term facilitation, whereas augmenting ApNT expression enhances it and induces the growth of new synaptic varicosities at the monosynaptic connection between sensory and motor neurons of the gill-withdrawal reflex. Unlike vertebrate neurotrophins, ApNT has multiple coding exons and exerts distinct synaptic effects through differentially processed and secreted splice isoforms. Our findings demonstrate the existence of bona fide neurotrophin signaling in invertebrates and reveal a posttranscriptional mechanism that regulates neurotrophin processing and the release of proneurotrophins and mature neurotrophins that differentially modulate synaptic plasticity.

Feedback mechanism in depolarization-induced sustained activation of extracellular signal-regulated kinase in the hippocampus

Phosphorylation plays important roles in several processes including synaptic plasticity and memory. The critical role of extracellular signal-regulated kinase (ERK) in these processes is well established. ERK is activated in a sustained manner by different stimuli. However, the mechanisms of sustained ERK activation are not completely understood. Here we show that KCl depolarization-induced sustained ERK activation in the hippocampal slices is critically dependent on protein synthesis and transcription. In addition, the sustained ERK activation requires receptor tyrosine kinase(s) activity. In support of a role for a growth factor in sustained ERK activation, KCl depolarization enhances the level of brain-derived neurotrophic factor (BDNF). Furthermore, BDNF antibody blocks KCl-induced sustained ERK activation. These results suggest a positive feed-back loop in which depolarization-induced BDNF maintains ERK activation in the sustained phase.

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.

Repetitive acute intermittent hypoxia increases expression of proteins associated with plasticity in the phrenic motor nucleus

Acute intermittent hypoxia (AIH) initiates plasticity in respiratory motor control, including phrenic long term facilitation (pLTF). Since pLTF is enhanced by preconditioning with repetitive exposure to AIH (rAIH), we hypothesized that a rAIH protocol consisting of 3 AIH exposures per week for 10 weeks (3×wAIH; AIH: 10, 5-min episodes of 10.5% O(2); 5-min normoxic intervals) would enhance expression of molecules that play key roles in pLTF within the phrenic motor nucleus. Immunohistochemical analyses revealed that 3×wAIH for 10 weeks increased serotonin terminal density in the C4 phrenic motor nucleus and serotonin 2A (5-HT(2A)) receptor expression in presumptive phrenic motor neurons. Immunoreactive brain derived neurotrophic factor (BDNF) and its high affinity receptor (TrkB) also increased following 3×wAIH. 3×wAIH also increased expression of another hypoxia-sensitive growth factor known to elicit phrenic motor facilitation, vascular endothelial growth factor (VEGF), and its receptor (VEGFR-2). Kinases "downstream" from TrkB and VEGFR-2 were up-regulated in or near presumptive phrenic motor neurons, including phosphorylated extracellular-signal regulated kinase (p-ERK) and protein kinase B (p-AKT). Thus, 3×wAIH up-regulates neurochemicals known to be associated with phrenic motor plasticity. Since 3×wAIH upregulates pro-plasticity molecules without evidence for CNS pathology, it may be a useful therapeutic tool in treating disorders that cause respiratory insufficiency, such as spinal injury or motor neuron disease.

Modeling signal transduction in classical conditioning with network motifs

Biological networks are constructed of repeated simplified patterns, or modules, called network motifs. Network motifs can be found in a variety of organisms including bacteria, plants, and animals, as well as intracellular transcription networks for gene expression and signal transduction processes in neuronal circuits. Standard models of signal transduction events for synaptic plasticity and learning often fail to capture the complexity and cooperativity of the molecular interactions underlying these processes. Here, we apply network motifs to a model for signal transduction during an in vitro form of eyeblink classical conditioning that reveals an underlying organization of these molecular pathways. Experimental evidence suggests there are two stages of synaptic AMPA receptor (AMPAR) trafficking during conditioning. Synaptic incorporation of GluR1-containing AMPARs occurs early to activate silent synapses conveying the auditory conditioned stimulus and this initial step is followed by delivery of GluR4 subunits that supports acquisition of learned conditioned responses (CRs). Overall, the network design of the two stages of synaptic AMPAR delivery during conditioning describes a coherent feed-forward loop (C1-FFL) with AND logic. The combined inputs of GluR1 synaptic delivery AND the sustained activation of 3-phosphoinositide-dependent protein-kinase-1 (PDK-1) results in synaptic incorporation of GluR4-containing AMPARs and the gradual acquisition of CRs. The network architecture described here for conditioning is postulated to act generally as a sign-sensitive delay element that is consistent with the non-linearity of the conditioning process. Interestingly, this FFL structure also performs coincidence detection. A motif-based approach to modeling signal transduction can be used as a new tool for understanding molecular mechanisms underlying synaptic plasticity and learning and for comparing findings across forms of learning and model systems.

Differential regulation of synaptic plasticity of the hippocampal and the hypothalamic inputs to the anterior thalamus

The hippocampus projects to the anterior thalamic nuclei both directly and indirectly via the mammillary bodies, but little is known about the electrophysiological properties of these convergent pathways. Here we demonstrate, for the first time, the presence of long-term plasticity in anterior thalamic nuclei synapses in response to high- and low-frequency stimulation (LFS) in urethane-anesthetized rats. We compared the synaptic changes evoked via the direct vs. the indirect hippocampal pathways to the anterior thalamus, and found that long-term potentiation (LTP) of the thalamic field response is induced predominantly through the direct hippocampal projections. Furthermore, we have estimated that that long-term depression (LTD) can be induced only after stimulation of the indirect connections carried by the mammillothalamic tract. Interestingly, basal synaptic transmission mediated by the mammillothalamic tract undergoes use-dependent, BDNF-mediated potentiation, revealing a distinct form of plasticity specific to the diencephalic region. Our data indicate that the thalamus does not passively relay incoming information, but rather acts as a synaptic network, where the ability to integrate hippocampal and mammillary body inputs is dynamically modified as a result of previous activity in the circuit. The complementary properties of these two parallel pathways upon anterior thalamic activity reveal that they do not have duplicate functions.

AMPA receptor trafficking and learning

In the last few years it has become clear that AMPA-type glutamate neurotransmitter receptors are rapidly transported into and out of synapses to strengthen or weaken their function. The remarkable dynamics of AMPA receptor (AMPAR) synaptic localization provides a compelling mechanism for understanding the cellular basis of learning and memory, as well as disease states involving cognitive dysfunction. Here, we summarize the evidence for AMPAR trafficking as a mechanism underlying a variety of learned responses derived from both behavioral and cellular studies. Evidence is also reviewed supporting synaptic dysfunction related to impaired AMPAR trafficking as a mechanism underlying learning and memory deficits in Alzheimer's disease. We conclude that emerging data support the concept of multistage AMPAR trafficking during learning and that a broad approach to include examination of all of the AMPAR subunits will provide a more complete view of the mechanisms underlying multiple forms of learning.

Cleavage of proBDNF to BDNF by a tolloid-like metalloproteinase is required for acquisition of in vitro eyeblink classical conditioning

The tolloid/bone morphogenetic protein-1 family of metalloproteinases have an important role in the regulation of embryonic pattern formation and tissue morphogenesis. Studies suggest that they participate in mechanisms of synaptic plasticity in adults, but very little is known about their function. Recently, we isolated a reptilian ortholog of the tolloid gene family designated turtle tolloid-like gene (tTll). Here, we examined the role of tTLL in an in vitro model of eyeblink classical conditioning using an isolated brainstem preparation to assess its role in synaptic plasticity during conditioning. Analysis by real-time reverse transcription-PCR shows that an extracellularly secreted form of tTLL, tTLLs, is transiently expressed in the early stages of conditioning during conditioned response acquisition, whereas a cytosolic form, tTLLc, is not. Short interfering RNA (siRNA)-directed gene knockdown and rescue of tTLL expression demonstrate that it is required for conditioning. Significantly, we show that tTLLs cleaves the precursor proBDNF into mature BDNF in cleavage assay studies, and application of recombinant tTLLs protein alone to preparations results in induction of mature BDNF expression. The mature form of BDNF is minimally expressed in preparations treated with anti-tTLL siRNA, and the synaptic incorporation of both GluR1- and GluR4-containing AMPA receptors is significantly reduced, resulting in suppression of conditioning. This is the first study to demonstrate that expression of an extracellularly secreted tolloid-like metalloproteinase is regulated in the early stages of classical conditioning and functions in the conversion of proBDNF to mature BDNF. The mature form of BDNF is required for synaptic delivery of AMPA receptors and acquisition of conditioned responses.

Selective activation of p38 mitogen-activated protein kinase in dopaminergic neurons of substantia nigra leads to nuclear translocation of p53 in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-treated mice

Parkinson's disease (PD) is a progressive neurodegenerative disease characterized by the degeneration of the dopaminergic neurons in the substantia nigra pars compacta (SNpc). Activation of the mixed lineage kinase and c-Jun N-terminal kinase (JNK) has been reported in models of PD. Our focus was to discern whether distinct pathways were activated in cell-specific manner within the SNpc. We now demonstrate the selective phosphorylation of p38 MAP kinase within the dopaminergic neurons, whereas JNK activation occurs predominantly in the microglia. p38 activation results in downstream phosphorylation of p53 and increased p53 mediated transcription of Bax and Puma in the ventral midbrain. Treatment with p38 inhibitor, SB239063 protected primary dopaminergic neurons derived from human progenitor cells from MPP(+) mediated cell death and prevented the downstream phosphorylation of p53 and its translocation to the nucleus in vivo, in the ventral midbrain. The increased staining of phosphorylated p38 in the surviving neurons of SNpc in human brain sections from patients with PD and in MPTP treated mice but not in the ventral tegmental area provides further evidence suggesting a role for p38 in the degeneration of dopaminergic neurons of SNpc. We thus demonstrate the cell specific activation of MAP kinase pathways within the SNpc after MPTP treatment emphasizing the role of multiple signaling cascades in the pathogenesis and progression of the disease. Selective inhibitors of p38 may therefore, help preserve the surviving neurons in PD and slow down the disease progression.

Coordinate action of pre- and postsynaptic brain-derived neurotrophic factor is required for AMPAR trafficking and acquisition of in vitro classical conditioning

Brain-derived neurotrophic factor (BDNF) has been implicated in mechanisms of synaptic plasticity such as long-term potentiation (LTP), but its role in associative learning remains largely unknown. In the present study, we investigated the function of BDNF and its receptor tropomyosin-related kinase B (TrkB) in an in vitro model of classical conditioning using pond turtles, Pseudemys scripta elegans. Conditioning resulted in a significant increase in BDNF and phospho (p)-Trk expression. Bath application of antibodies directed against TrkB, but not TrkA or TrkC, abolished acquisition of conditioning, as did a receptor tyrosine kinase inhibitor K252a and an inhibitor of nitric oxide synthase 7-nitroindazole. Significantly, injections of BDNF Ab into the nerve roots of presynaptic axonal projections or postsynaptic motor neurons prevented acquisition of conditioning, suggesting that BDNF is required on both sides of the synapse for modification to occur. The presynaptic proteins synaptophysin and synapsin I were increased upon conditioning or BDNF application. Furthermore, BDNF application alone mimicked conditioning-induced synaptic insertion of GluR1 and GluR4 AMPAR subunits into synapses, which was inhibited by co-application of BDNF and K252a. Data also show that extracellular signal-regulated kinase (ERK) was activated in BDNF-treated preparations. We conclude that coordinate pre- and postsynaptic actions of BDNF are required for acquisition of in vitro classical conditioning.

The evidence for hippocampal long-term potentiation as a basis of memory for simple tasks

Long-term potentiation (LTP) is the enhancement of postsynaptic responses for hours, days or weeks following the brief repetitive afferent stimulation of presynaptic afferents. It has been proposed many times over the last 30 years to be the basis of long-term memory. Several recent findings finally supported this hypothesis: a) memory formation of one-trial avoidance learning depends on a series of molecular steps in the CA1 region of the hippocampus almost identical to those of LTP in the same region; b)hippocampal LTP in this region accompanies memory formation of that task and of another similar task. However, CA1 LTP and the accompanying memory processes can be dissociated, and in addition plastic events in several other brain regions(amygdala, entorhinal cortex, parietal cortex) are also necessary for memory formation of the one-trial task, and perhaps of many others.

Synapsin Regulates Basal Synaptic Strength, Synaptic Depression, and Serotonin-Induced Facilitation of Sensorimotor Synapses in Aplysia

Synapsin is a synaptic vesicle-associated protein implicated in the regulation of vesicle trafficking and transmitter release, but its role in heterosynaptic plasticity remains elusive. Moreover, contradictory results have obscured the contribution of synapsin to homosynaptic plasticity. We previously reported that the neuromodulator serotonin (5-HT) led to the phosphorylation and redistribution of Aplysia synapsin, suggesting that synapsin may be a good candidate for the regulation of vesicle mobilization underlying the short-term synaptic plasticity induced by 5-HT. This study examined the role of synapsin in homosynaptic and heterosynaptic plasticity. Overexpression of synapsin reduced basal transmission and enhanced homosynaptic depression. Although synapsin did not affect spontaneous recovery from depression, it potentiated 5-HT–induced dedepression. Computational analysis showed that the effects of synapsin on plasticity could be adequately simulated by altering the rate of Ca2+-dependent vesicle mobilization, supporting the involvement of synapsin not only in homosynaptic but also in heterosynaptic forms of plasticity by regulating vesicle mobilization.

Reviews: BDNF and Memory Formation and Storage

During the past decade, a large body of evidence has implicated BDNF in synaptic plasticity. In this review, we focus on the newer experiments that involve BDNF in different aspects of learning and memory processing—in particular, in memory persistence and storage. NEUROSCIENTIST 14(2):147—156, 2008. DOI: 10.1177/1073858407305850

Protein kinase C regulates local synthesis and secretion of a neuropeptide required for activity-dependent long-term synaptic plasticity

Long-term facilitation (LTF) of sensory neuron synapses in Aplysia is produced by either nonassociative or associative stimuli. Nonassociative LTF can be produced by five spaced applications of serotonin (5-HT) and requires a phosphoinosotide 3-kinase (PI3K)-dependent and rapamycin-sensitive increase in the local synthesis of the sensory neuron neuropeptide sensorin and a protein kinase A (PKA)-dependent increase in the secretion of the newly synthesized sensorin. We report here that associative LTF produced by a single pairing of a brief tetanus with one application of 5-HT requires a rapid protein kinase C (PKC)-dependent and rapamycin-sensitive increase in local sensorin synthesis. This rapid increase in sensorin synthesis does not require PI3K activity or the presence of the sensory neuron cell body but does require the presence of the motor neuron. The secretion of newly synthesized sensorin by 2 h after stimulation requires both PKA and PKC activities to produce associative LTF because incubation with exogenous anti-sensorin antibody or the kinase inhibitors after tetanus plus 5-HT blocked LTF. The secreted sensorin leads to phosphorylation and translocation of p42/44 mitogen-activated protein kinase (MAPK) into the nuclei of the sensory neurons. Thus, different stimuli activating different signaling pathways converge by regulating the synthesis and release of a neuropeptide to produce long-term synaptic plasticity.