Synapse-specific, long-term facilitation of aplysia sensory to motor synapses: a function for local protein synthesis in memory storage

Facilitation of taste memory acquisition by experiencing previous novel taste is protein-synthesis dependent

Very little is known about the biological and molecular mechanisms that determine the effect of previous experience on implicit learning tasks. In the present study, we first defined weak and strong taste inputs according to measurements in the behavioral paradigm known as latent inhibition of conditioned taste aversion. We then demonstrated that a strong novel taste input facilitated acquisition of the memory of subsequent weak taste input in inverse correlation with the time interval between the inputs. However, not only was a strong taste input unable to rescue an immediately subsequent strong taste input when the gustatory cortex was under the influence of the protein-synthesis inhibitor, anisomycin, but the effect of the interaction was to reduce the variation among individual taste memories. Taken together, these results demonstrate that taste memory facilitation, induced by previously experiencing a different unimodal taste input, depended on time, novelty, and directionality. Moreover, the results imply that learning is enhanced on the level of acquisition but not of molecular consolidation.

Living within the cellular envelope: subjectivity and self from an evolutionary neuropsychoanalytic perspective

A growing literature has been exploring the implications of reconciling psychoanalytic understandings of human behavior with the research findings of neuroscience. This essay proposes a new linking perspective--neurodarwinian psychoanalysis--as a way to revise the predominantly disembodied nature of existing analytic theory by grounding it in the biological realities of human nature, development, and psychopathogenesis. Beginning with a focus on the evolutionary significance of the cellular envelope within which all living organisms exist, it provides theoretical and clinical examples of how evolved neural assemblies in the brain play a key role in the representational depictions of both typical and atypical human predicaments. Conventional psychoanalytic concepts of such theoretical entities as the self and tripartite concretizations of intrapsychic tropes are reformulated in terms of naturally selected neural innervations. Accordingly, dynamically unconscious functioning and psychoanalytically-informed therapeutic process are considered as crucial adaptations that warrant natural selection.

Beyond transcription factors: the role of chromatin modifying enzymes in regulating transcription required for memory

One of the alluring aspects of examining chromatin modifications in the role of modulating transcription required for long-term memory processes is that these modifications may provide transient and potentially stable epigenetic marks in the service of activating and/or maintaining transcriptional processes. These, in turn, may ultimately participate in the molecular mechanisms required for neuronal changes subserving long-lasting changes in behavior. As an epigenetic mechanism of transcriptional control, chromatin modification has been shown to participate in maintaining cellular memory (e.g., cell fate) and may underlie the strengthening and maintenance of synaptic connections required for long-term changes in behavior. Epigenetics has become central to several fields of neurobiology, where researchers have found that regulation of chromatin modification has a significant role in epilepsy, drug addiction, depression, neurodegenerative diseases, and memory. In this review, we will discuss the role of chromatin modifying enzymes in memory processes, as well as how recent studies in yeast genetics and cancer biology may impact the way we think about how chromatin modification and chromatin remodeling regulate neuronal function.

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.

Transcriptional regulation of long-term memory in the marine snail Aplysia

Whereas the induction of short-term memory involves only covalent modifications of constitutively expressed preexisting proteins, the formation of long-term memory requires gene expression, new RNA, and new protein synthesis. On the cellular level, transcriptional regulation is thought to be the starting point for a series of molecular steps necessary for both the initiation and maintenance of long-term synaptic facilitation (LTF). The core molecular features of transcriptional regulation involved in the long-term process are evolutionally conserved in Aplysia, Drosophila, and mouse, and indicate that gene regulation by the cyclic AMP response element binding protein (CREB) acting in conjunction with different combinations of transcriptional factors is critical for the expression of many forms of long-term memory. In the marine snail Aplysia, the molecular mechanisms that underlie the storage of long-term memory have been extensively studied in the monosynaptic connections between identified sensory neuron and motor neurons of the gill-withdrawal reflex. One tail shock or one pulse of serotonin (5-HT), a modulatory transmitter released by tail shocks, produces a transient facilitation mediated by the cAMP-dependent protein kinase leading to covalent modifications in the sensory neurons that results in an enhancement of transmitter release and a strengthening of synaptic connections lasting minutes. By contrast, repeated pulses of 5-hydroxytryptamine (5-HT) induce a transcription- and translation-dependent long-term facilitation (LTF) lasting more than 24 h and trigger the activation of a family of transcription factors in the presynaptic sensory neurons including ApCREB1, ApCREB2 and ApC/EBP. In addition, we have recently identified novel transcription factors that modulate the expression of ApC/EBP and also are critically involved in LTF. In this review, we examine the roles of these transcription factors during consolidation of LTF induced by different stimulation paradigms.

Drosophila memory: will Orb(2) predict the future?

Genetically tractable organisms with relatively simple nervous systems offer a realistic platform to understand how and where memories are formed and stored in defined neural circuits. Recent work in Drosophila provides promise that this analysis may soon reach the resolution of identifiable synapses.

Local translation in primary afferent fibers regulates nociception

Recent studies have demonstrated the importance of local protein synthesis for neuronal plasticity. In particular, local mRNA translation through the mammalian target of rapamycin (mTOR) has been shown to play a key role in regulating dendrite excitability and modulating long-term synaptic plasticity associated with learning and memory. There is also increased evidence to suggest that intact adult mammalian axons have a functional requirement for local protein synthesis in vivo. Here we show that the translational machinery is present in some myelinated sensory fibers and that active mTOR-dependent pathways participate in maintaining the sensitivity of a subpopulation of fast-conducting nociceptors in vivo. Phosphorylated mTOR together with other downstream components of the translational machinery were localized to a subset of myelinated sensory fibers in rat cutaneous tissue. We then showed with electromyographic studies that the mTOR inhibitor rapamycin reduced the sensitivity of a population of myelinated nociceptors known to be important for the increased mechanical sensitivity that follows injury. Behavioural studies confirmed that local treatment with rapamycin significantly attenuated persistent pain that follows tissue injury, but not acute pain. Specifically, we found that rapamycin blunted the heightened response to mechanical stimulation that develops around a site of injury and reduced the long-term mechanical hypersensitivity that follows partial peripheral nerve damage--a widely used model of chronic pain. Our results show that the sensitivity of a subset of sensory fibers is maintained by ongoing mTOR-mediated local protein synthesis and uncover a novel target for the control of long-term pain states.

Local Gene Expression in Axons and Nerve Endings: The Glia-Neuron Unit

Neurons have complex and often extensively elongated processes. This unique cell morphology raises the problem of how remote neuronal territories are replenished with proteins. For a long time, axonal and presynaptic proteins were thought to be exclusively synthesized in the cell body, which delivered them to peripheral sites by axoplasmic transport. Despite this early belief, protein has been shown to be synthesized in axons and nerve terminals, substantially alleviating the trophic burden of the perikaryon. This observation raised the question of the cellular origin of the peripheral RNAs involved in protein synthesis. The synthesis of these RNAs was initially attributed to the neuron soma almost by default. However, experimental data and theoretical considerations support the alternative view that axonal and presynaptic RNAs are also transcribed in the flanking glial cells and transferred to the axon domain of mature neurons. Altogether, these data suggest that axons and nerve terminals are served by a distinct gene expression system largely independent of the neuron cell body. Such a local system would allow the neuron periphery to respond promptly to environmental stimuli. This view has the theoretical merit of extending to axons and nerve terminals the marginalized concept of a glial supply of RNA (and protein) to the neuron cell body. Most long-term plastic changes requiring de novo gene expression occur in these domains, notably in presynaptic endings, despite their intrinsic lack of transcriptional capacity. This review enlightens novel perspectives on the biology and pathobiology of the neuron by critically reviewing these issues.

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.

Rapid consolidation to a radish and protein synthesis-dependent long-term memory after single-session appetitive olfactory conditioning in Drosophila

In Drosophila, formation of aversive olfactory long-term memory (LTM) requires multiple training sessions pairing odor and electric shock punishment with rest intervals. In contrast, here we show that a single 2 min training session pairing odor with a more ethologically relevant sugar reinforcement forms long-term appetitive memory that lasts for days. Appetitive LTM has some mechanistic similarity to aversive LTM in that it can be disrupted by cycloheximide, the dCreb2-b transcriptional repressor, and the crammer and tequila LTM-specific mutations. However, appetitive LTM is completely disrupted by the radish mutation that apparently represents a distinct mechanistic phase of consolidated aversive memory. Furthermore, appetitive LTM requires activity in the dorsal paired medial neuron and mushroom body alpha'beta' neuron circuit during the first hour after training and mushroom body alphabeta neuron output during retrieval, suggesting that appetitive middle-term memory and LTM are mechanistically linked. Last, experiments feeding and/or starving flies after training reveals a critical motivational drive that enables appetitive LTM retrieval.

The role of protein synthesis in memory consolidation: progress amid decades of debate

A major component of consolidation theory holds that protein synthesis is required to produce the synaptic modification needed for long-term memory storage. Protein synthesis inhibitors have played a pivotal role in the development of this theory. However, these commonly used drugs have unintended effects that have prompted some to reevaluate the role of protein synthesis in memory consolidation. Here we review the role of protein synthesis in memory formation as proposed by consolidation theory calling special attention to the controversy involving the non-specific effects of a group of protein synthesis inhibitors commonly used to study memory formation in vivo. We argue that molecular and genetic approaches that were subsequently applied to the problem of memory formation confirm the results of less selective pharmacological studies. Thus, to a certain extent, the debate over the role of protein synthesis in memory based on interpretational difficulties inherent to the use of protein synthesis inhibitors may be somewhat moot. We conclude by presenting avenues of research we believe will best provide answers to both long-standing and more recent questions facing field of learning and memory.

Translational control of gene expression: a molecular switch for memory storage

A critical requirement for the conversion of the labile short-term memory (STM) into the consolidated long-term memory (LTM) is new gene expression (new mRNAs and protein synthesis). The first clues to the molecular mechanisms of the switch from short-term to LTM emerged from studies on protein synthesis in different species. Initially, it was shown that LTM can be distinguished from STM by its susceptibility to protein synthesis inhibitors. Later, it was found that long-lasting synaptic changes, which are believed to be a key cellular mechanism by which information is stored, are also dependent on new protein synthesis. Although the role of protein synthesis in memory was reported more than 40 years ago, recent molecular, genetic, and biochemical studies have provided fresh insights into the molecular mechanisms underlying these processes. In this chapter, we provide an overview of the role of translational control by the eIF2alpha signaling pathway in long-term synaptic plasticity and memory consolidation.

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.

Signaling mechanisms linking neuronal activity to gene expression and plasticity of the nervous system

Sensory experience and the resulting synaptic activity within the brain are critical for the proper development of neural circuits. Experience-driven synaptic activity causes membrane depolarization and calcium influx into select neurons within a neural circuit, which in turn trigger a wide variety of cellular changes that alter the synaptic connectivity within the neural circuit. One way in which calcium influx leads to the remodeling of synapses made by neurons is through the activation of new gene transcription. Recent studies have identified many of the signaling pathways that link neuronal activity to transcription, revealing both the transcription factors that mediate this process and the neuronal activity-regulated genes. These studies indicate that neuronal activity regulates a complex program of gene expression involved in many aspects of neuronal development, including dendritic branching, synapse maturation, and synapse elimination. Genetic mutations in several key regulators of activity-dependent transcription give rise to neurological disorders in humans, suggesting that future studies of this gene expression program will likely provide insight into the mechanisms by which the disruption of proper synapse development can give rise to a variety of neurological disorders.

Visualizing mRNA localization and local protein translation in neurons

Fluorescent proteins (FPs) have been successfully used to study the localization and interactions of proteins in living cells. They have also been instrumental in analyzing the proteins involved in the localization of RNAs in different cell types, including neurons. With the development of methods that also tag RNAs via fluorescent proteins, researchers now have a powerful tool to covisualize RNAs and associated proteins in living neurons. Here, we review the current status of the use of FPs in the study of transport and localization of ribonucleoprotein particles (RNPs) in neurons and provide key protocols used to introduce transgenes into cultured neurons, including calcium-phosphate-based transfection and nucleofection. These methods allow the fast and efficient expression of fluorescently tagged fusion proteins in neurons at different stages of differentiation and form the basis for fluorescent protein-based live cell imaging in neuronal cultures. Additional protocols are given that allow the simultaneous visualization of RNP proteins and cargo RNAs in living neurons and aspects of the visualization of fluorescently tagged proteins in neurons, such as colocalization studies, are discussed. Finally, we review approaches to visualize the local synthesis of proteins in distal dendrites and axons.

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

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

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.

Molecular memory traces

To understand the essence of memory, one must examine the working of the brain on many levels. It is important to find the appropriate level to study the particular aspect of memory under investigation. In this review, I will focus on insights gained from examining memory at the molecular level. I will illustrate these insights with specific examples from examining the molecular and cellular mechanisms underlying long-term facilitation in the marine mollusk Aplysia and long-term potentiation, studied mainly in rodents. In particular, I will discuss how molecular memory traces are formed and focus in detail on what role increasing the level of proteins through protein synthesis and gene expression plays in memory formation. I will point out three important constraints from molecular work that should impact on cognitive modeling of the nervous system: (i) the induction of plasticity depends on the 'state' of the synapse; (ii) there are multiple independent molecular traces formed after experience with different half-lives; and (iii) the requirement for the conjunction of synaptic activation and new protein synthesis implies that new conjunctions are required to induce long-term memory formation.

Locally dynamic synaptic learning rules in pyramidal neuron dendrites

Long-term potentiation (LTP) of synaptic transmission underlies aspects of learning and memory. LTP is input-specific at the level of individual synapses, but neural network models predict interactions between plasticity at nearby synapses. Here we show in mouse hippocampal pyramidal cells that LTP at individual synapses reduces the threshold for potentiation at neighbouring synapses. After input-specific LTP induction by two-photon glutamate uncaging or by synaptic stimulation, subthreshold stimuli, which by themselves were too weak to trigger LTP, caused robust LTP and spine enlargement at neighbouring spines. Furthermore, LTP induction broadened the presynaptic-postsynaptic spike interval for spike-timing-dependent LTP within a dendritic neighbourhood. The reduction in the threshold for LTP induction lasted approximately 10 min and spread over approximately 10 microm of dendrite. These local interactions between neighbouring synapses support clustered plasticity models of memory storage and could allow for the binding of behaviourally linked information on the same dendritic branch.

The role of rapid, local, postsynaptic protein synthesis in learning-related synaptic facilitation in aplysia

The discovery that dendrites of neurons in the mammalian brain possess the capacity for protein synthesis stimulated interest in the potential role of local, postsynaptic protein synthesis in learning-related synaptic plasticity. But it remains unclear how local, postsynaptic protein synthesis actually mediates learning and memory in mammals. Accordingly, we examined whether learning in an invertebrate, the marine snail Aplysia, involves local, postsynaptic protein synthesis. Previously, we showed that the dishabituation and sensitization of the defensive withdrawal reflex in Aplysia require elevated postsynaptic Ca(2+), postsynaptic exocytosis, and functional upregulation of postsynaptic alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)-type glutamate receptors. Here, we tested whether the synaptic facilitation that underlies dishabituation and sensitization in Aplysia requires local, postsynaptic protein synthesis. We found that the facilitatory transmitter, serotonin (5-HT), enhanced the response of the motor neuron to glutamate, the sensory neuron transmitter, and this enhancement depended on rapid protein synthesis. By using individual motor neurites surgically isolated from their cell bodies, we showed that the 5-HT-dependent protein synthesis occurred locally. Finally, by blocking postsynaptic protein synthesis, we disrupted the facilitation of the sensorimotor synapse. By demonstrating its critical role in a synaptic change that underlies learning and memory in a major model invertebrate system, our study suggests that local, postsynaptic protein synthesis is of fundamental importance to the cell biology of learning.

Serotonin catabolism in the central and enteric nervous systems of rats upon induction of serotonin syndrome

Serotonin, a well-known neurotransmitter in mammals, has been linked to a number of neurological and gastrointestinal disorders. One of these disorders, serotonin syndrome, is a potentially deadly condition caused by increased levels of serotonin in the extracellular space. Information on the neurochemical effects of serotonin syndrome on serotonin catabolism is lacking, particularly in relation to the enteric system of the gastrointestinal tract. Here the catabolism of serotonin is monitored in rats with pharmacologically induced serotonin syndrome, with the catabolites characterized using a specialized capillary electrophoresis system with laser-induced native fluorescence detection. Animals induced with serotonin syndrome demonstrate striking increases in the levels of serotonin and its metabolites. In the brain, levels of serotonin increased 2- to 3-fold in animals induced with serotonin syndrome. A major serotonin metabolite, 5-hydroxyindole acetic acid, increased 10- to 100-fold in experimental animals. Similar results were observed in the gastrointestinal tissues; in the small intestines, serotonin levels increased 4- to 5-fold. Concentrations of 5-hydroxyindole acetic acid increased 32- to 100-fold in the intestinal tissues of experimental animals. Serotonin sulfate showed surprisingly large increases, marking what may be the first time the compound has been reported in rat intestinal tissues.

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.

RNA trafficking in axons

A substantial number of studies over a period of four decades have indicated that axons contain mRNAs and ribosomes, and are metabolically active in synthesizing proteins locally. For the most part, little attention has been paid to these findings until recently when the concept of targeting of specific mRNAs and translation in subcellular domains in polarized cells emerged to contribute to the likelihood and acceptance of mRNA targeting to axons as well. Trans-acting factor proteins bind to cis-acting sequences in the untranslated region of mRNAs integrated in ribonucleoprotein (RNPs) complexes determine its targeting in neurons. In vitro studies in immature axons have shown that molecular motors proteins (kinesins and myosins) associate to RNPs suggesting they would participate in its transport to growth cones. Tau and actin mRNAs are transported as RNPs, and targeted to axons as well as ribosomes. Periaxoplasmic ribosomal plaques (PARPs), which are systematically distributed discrete peripheral ribosome-containing, actin-rich formations in myelinated axons, also are enriched with actin and myosin Va mRNAs and additional regulatory proteins. The localization of mRNAs in PARPs probably means that PARPs are local centers of translational activity, and that these domains are the final destination in the axon compartment for targeted macromolecular traffic originating in the cell body. The role of glial cells as a potentially complementary source of axonal mRNAs and ribosomes is discussed in light of early reports and recent ultrastructural observations related to the possibility of glial-axon trans-endocytosis.

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.

Serotonin Induces Memory-Like, Rapamycin-Sensitive Hyperexcitability in Sensory Axons ofAplysiaThat Contributes to Injury Responses

The induction of long-term facilitation (LTF) of synapses of Aplysia sensory neurons (SNs) by serotonin (5-HT) has provided an important mechanistic model of memory, but little is known about other long-term effects of 5-HT on sensory properties. Here we show that crushing peripheral nerves results in long-term hyperexcitability (LTH) of the axons of these nociceptive SNs that requires 5-HT activity in the injured nerve. Serotonin application to a nerve segment induces local axonal (but not somal) LTH that is inhibited by 5-HT–receptor antagonists. Blockade of crush-induced axonal LTH by an antagonist, methiothepin, provides evidence for mediation of this injury response by 5-HT. This is the first demonstration in any axon of neuromodulator-induced LTH, a phenomenon potentially important for long-lasting pain. Methiothepin does not reduce axonal LTH induced by local depolarization, so 5-HT is not required for all forms of axonal LTH. Serotonin-induced axonal LTH is expressed as reduced spike threshold and increased repetitive firing, whereas depolarization-induced LTH involves only reduced threshold. Like crush- and depolarization-induced LTH, 5-HT–induced LTH is blocked by inhibiting protein synthesis. Blockade by rapamycin, which also blocks synaptic LTF, is interesting because the eukaryotic protein kinase that is the target of rapamycin (TOR) has a conserved role in promoting growth by stimulating translation of proteins required for translation. Rapamycin sensitivity suggests that localized increases in translation of proteins that promote axonal conduction and excitability at sites of nerve injury may be regulated by the same signals that increase translation of proteins that promote neuronal growth.

Axon viability and mitochondrial function are dependent on local protein synthesis in sympathetic neurons

(1) Axons contain numerous mRNAs and a local protein synthetic system that can be regulated independently of the cell body. (2) In this study, cultured primary sympathetic neurons were employed, to assess the effect of local protein synthesis blockade on axon viability and mitochondrial function. (3) Inhibition of local protein synthesis reduced newly synthesized axonal proteins by 65% and resulted in axon retraction after 6 h. Acute inhibition of local protein synthesis also resulted in a significant decrease in the membrane potential of axonal mitochondria. Likewise, blockade of local protein transport into the mitochondria by transfection of the axons with Hsp90 C-terminal domain decreased the mitochondrial membrane potential by 65%. Moreover, inhibition of the local protein synthetic system also reduced the ability of mitochondria to restore axonal levels of ATP after KCl-induced depolarization. (4) Taken together, these results indicate that the local protein synthetic system plays an important role in mitochondrial function and the maintenance of the axon.

Characterization of the adaptor protein ARH expression in the brain and ARH molecular interactions

Previously, pA134 was identified as one of the mRNAs present in the squid giant axon. Comparative sequence analyses revealed that the pA134 gene product manifested significant similarity to the mammalian lipoprotein receptor adaptor protein also known as ARH (autosomal recessive hypercholesterolemia). ARH mRNA and protein displayed very similar pattern of expression throughout the mouse brain. Significant levels of expression were observed in cells with a predominantly neuronal profile in the cerebellum, brainstem, olfactory bulb, hippocampus, and cortex. A yeast two hybrid screen for ARH protein interactions in mouse brain identified the following binders: amyloid precursor-like protein 1, low density lipoprotein receptor-related protein (LRP) 1, LRP8, and GABA receptor-associated protein-like 1. The interactions of ARH with LRP1 and GABA receptor-associated protein-like 1 were subsequently verified by co-immunoprecipitation of the protein complexes from transfected human embryonic kidney cells. The presence of ARH mRNA in axon of primary sympathetic neurons was established by RT-PCR analyses and confirmed by in situ hybridization. Taken together, our data suggest that ARH is a multifunctional protein whose spectrum of function in the brain goes beyond the traditionally known metabolism of lipoproteins, and that ARH may be locally synthesized in the axon.

All-trans-retinoic acid stimulates translation and induces spine formation in hippocampal neurons through a membrane-associated RARalpha

Differentiation and patterning in the developing nervous system require the vitamin A metabolite all-trans-retinoic acid (atRA). Recent data suggest that higher cognitive functions, such as creation of hippocampal memory, also require atRA and its receptors, RAR, through affecting synaptic plasticity. Here we show that within 30 min atRA increased dendritic growth approximately 2-fold, and PSD-95 and synaptophysin puncta intensity approximately 3-fold, in cultured mouse hippocampal neurons, suggesting increased synapse formation. atRA (10 nM) increased ERK1/2 phosphorylation within 10 min. In synaptoneurosomes, atRA rapidly increased phosphorylation of ERK1/2, its target 4E-BP, and p70S6K, and its substrate, ribosome protein S6, indicating activation of MAPK and mammalian target of rapamycin (mTOR). Immunofluorescence revealed intense dendritic expression of RARalpha in the mouse hippocampus and localization of RARalpha on the surfaces of primary cultures of hippocampal neurons, with bright puncta along soma and neurites. Surface biotinylation confirmed the locus of RARalpha expression. Knockdown of RARalpha by shRNA impaired atRA-induced spine formation and abolished dendritic growth. Prolonged atRA stimulation reduced surface/total RARalpha by 43%, suggesting internalization, whereas brain-derived nerve growth factor or bicuculline increased the ratio by approximately 1.8-fold. atRA increased translation in the somatodendritic compartment, similar to brain-derived nerve growth factor. atRA specifically increased dendritic translation and surface expression of the alpha-amino-3-hydroxyl-5-methyl-4-isoxazole propionate receptor (AMPAR) subunit 1 (GluR1), without affecting GluR2. These data provide mechanistic insight into atRA function in the hippocampus and identify a unique membrane-associated RARalpha that mediates rapid induction of neuronal translation by atRA.

Cognitive and emotional information processing: protein synthesis and gene expression

Recent findings suggest that functional plasticity phenomena such as long-term potentiation (LTP) and long-term depression (LTD) - cellular processes underlying memory - are restricted to functional dendritic compartments. It was also shown, however, that a relatively strong activation of a synaptic input can abolish compartment restrictions. Our data support these findings and we present one cellular pathway responsible for uncompartmentalization of the normally localized plasticity processes by the action of rolipram, an inhibitor of type 4 phosphodiesterases. In contrast with compartment-restricted information processing, uncompartmentalization requires transcription. In the search for system relevance of compartmentalization versus uncompartmentalization we describe firstly data which show that more cognitive information processing in rats' behaviour may follow rules of compartmentalization, whereas stressful, more life-threatening, inputs abolish compartment-restricted information processing involving transcription. Our findings allow us to suggest that consolidation of processes which take place during the cognitive event most probably depend on local protein synthesis, whereas stress immediately induces gene expression in addition, resulting in a compartment-unspecific up-regulation of plasticity-related proteins (PRPs), providing the entire neuron with a higher level of 'reactiveness'. These data would provide a specific functional cellular mechanism to respond differentially and effectively to behaviourally weighted inputs.

Nuclear translocation of CAM-associated protein activates transcription for long-term facilitation in Aplysia

Repeated pulses of serotonin (5-HT) induce long-term facilitation (LTF) of the synapses between sensory and motor neurons of the gill-withdrawal reflex in Aplysia. To explore how apCAM downregulation at the plasma membrane and CREB-mediated transcription in the nucleus, both of which are required for the formation of LTF, might relate to each other, we cloned an apCAM-associated protein (CAMAP) by yeast two-hybrid screening. We found that 5-HT signaling at the synapse activates PKA which in turn phosphorylates CAMAP to induce the dissociation of CAMAP from apCAM and the subsequent translocation of CAMAP into the nucleus of sensory neurons. In the nucleus, CAMAP acts as a transcriptional coactivator for CREB1 and is essential for the activation of ApC/EBP required for the initiation of LTF. Combined, our data suggest that CAMAP is a retrograde signaling component that translocates from activated synapses to the nucleus during synapse-specific LTF.

Cytoplasmic polyadenylation element binding protein 1-mediated mRNA translation in Purkinje neurons is required for cerebellar long-term depression and motor coordination

The ability of neurons to modify synaptic connections is critical for proper brain development and function in the adult. It is now clear that changes in synaptic strength are often accompanied by changes in synaptic morphology. This synaptic plasticity can be maintained for varying lengths of time depending on the type of neuronal activity that first induced the changes. Long-term synaptic plasticity requires the synthesis of new proteins, and one mechanism for the regulation of experience-induced protein synthesis in neurons involves cytoplasmic polyadenylation element binding protein (CPEB1). CPEB1 can bidirectionally regulate mRNA translation, first repressing translation, and then activating translation after the phosphorylation of two critical residues (T171 and S177). To determine the full extent of CPEB1-mediated protein synthesis in synaptic function, we engineered a line of mice expressing CPEB1 with these phosphorylation sites mutated to alanines (mCPEB1-AA) exclusively in cerebellar Purkinje neurons (PNs). Thus, mRNAs bound by mCPEB1-AA would be held in a translationally dormant state. We show that mCPEB1-AA localizes to synapses in cerebellum and resulted in a loss of protein synthesis-dependent phase of parallel fiber-PN long-term depression. This was accompanied by a change in spine number and spine length that are likely attributable in part to the dysregulation of IRSp53, a protein known to play a role in synaptic structure. Finally, mCPEB1-AA mice displayed a significant impairment of motor coordination and a motor learning delay.

A new mechanism of synapse-specific neuronal plasticity

According to current concepts, long-term memory is based on structural-functional changes in particular synaptic connections between neurons in the brain (synapse-specific plasticity), which depend on the processes of translation and transcription. Studies on neurons in the mollusk Aplysia and the mammalian hippocampus have addressed a mechanism of synapse-specific plasticity which does not require synapse-specific molecular genetic processes. Stimulation of a synapse has been shown to lead to activation of intracellular second messengers in the synapse as well as "synaptic tagging"--the formation of mechanisms "recognizing" transcription products. In the neuron body, second messengers induce the synthesis of RNA and protein molecules which are widely distributed in neuron processes and which are inserted selectively only into stimulation-tagged synapses, evoking long-term changes in their functional and morphological characteristics. The results of our studies on common snail defensive behavior command neurons LPl1 and RPl1 suggest the existence of another mechanism controlling synapse-specific plasticity. On acquisition of sensitization, a number of second messengers and the genes controlled by them are involved in supporting the plasticity of defined synaptic inputs of these neurons in snails. The processes of induction of long-term facilitation in the sensory inputs of neurons from chemoreceptors on the head have been shown to involve cAMP and cAMP-dependent transcription factors of the immediate early gene C/EBP (CAAT/enhancer binding protein), while the mechanisms controlling the other sensory input of neurons LPl1 and RPl1--from mechanoreceptors on the head--involve protein kinase C and protein kinase C-dependent transcription factor SRF (serum response factor). The immediate early gene zif268 is involved in controlling the inputs from both chemo-and mechanoreceptors on the head. These results are regarded as experimental support for the hypothesis that the molecular mechanisms of synapse-specific plasticity during learning may form on the basis of a selective neurochemical "projection" of the synaptic connections onto defined genes in the neuron.

The transport of Staufen2-containing ribonucleoprotein complexes involves kinesin motor protein and is modulated by mitogen-activated protein kinase pathway

There is increasing evidence showing that mRNA is transported to the neuronal dendrites in ribonucleoprotein (RNP) complexes or RNA granules, which are aggregates of mRNA, rRNA, ribosomal proteins, and RNA-binding proteins. In these RNP complexes, Staufen, a double-stranded RNA-binding protein, is believed to be a core component that plays a key role in the dendritic mRNA transport. This study investigated the molecular mechanisms of the dendritic mRNA transport using green fluorescent protein-tagged Staufen2 produced employing a Sindbis viral expression system. The kinesin heavy chain was found to be associated with Staufen2. The inhibition of kinesin resulted in a significant decrease in the level of dendritic transport of the Staufen2-containing RNP complexes in neurons under non-stimulating or stimulating conditions. This suggests that the dendritic transport of the Staufen2-containing RNP complexes use kinesin as a motor protein. A mitogen-activated protein kinase inhibitor, PD98059, inhibited the activity-induced increase in the amount of both the Staufen2-containing RNP complexes and Ca(2+)/calmodulin-dependent protein kinase II alpha-subunit mRNA in the distal dendrites of cultured hippocampal neurons. Overall, these results suggest that dendritic mRNA transport is mediated via the Staufen2 and kinesin motor proteins and might be modulated by the neuronal activity and mitogen-activated protein kinase pathway.

Temporal and spatial control of gene expression in early embryos of farm animals

A gradual transition from oocyte-derived mRNA and proteins to full embryonic transcription characterises early embryonic development. Messenger RNAs and proteins of maternal origin are accumulated into the oocyte throughout its growth inthe ovary. Upon fertilisation, sev eral mechanisms ar e activated that controlthe appropriate use of such material and prepare for the synthesis of new products. The present review will describe some of the mechanisms active in early embryos of domestic species. Data will be presented on the control of gene expression by the 3' untranslated regions and their interaction with specialised sequences at the 5' cap end. The process of RNA sorting and localisation, initially described in different cell types and in oocytes of lower species, will also be discussed, particularly in relation to its possible role in regulating early pig development. Finally, specific genes involved in the activation of cattle embryonic transcription will be described. This brief overview will provide some suggestions on how these different mechanisms may be integrated and cooperate to ensure the correct initiation of embryonic development.

Transcription factor serum response factor is selectively involved in the mechanisms of long-term synapse-specific plasticity

Our previous studies demonstrated that acquisition of nociceptive sensitization in common snails is accompanied by long-term facilitation of the responses of defensive behavior command neurons LPl1 and RPl1 to sensory stimuli, this being dependent on the processes of translation and transcription. The mechanism of induction of long-term synaptic facilitation at the sensory inputs of neurons from chemoreceptors on the head involves cAMP and the immediate early gene transcription factor C/EBP (CCAAT-enhancer-binding protein), while regulation of the other sensory input of neurons LPl1 and RPl1 - from mechanoreceptors on the head - depends on protein kinase C. The present report describes studies of the involvement of the transcription factor serum response factor (SRF) in the processes of the synapse-specific plasticity of neuron LPl1 during the acquisition of sensitization in snails. The acquisition of sensitization during intracellular administration of oligonucleotides specifically inhibiting SRF led to the selective suppression of synaptic facilitation in the responses of neuron LPl1 to tactile stimulation of the snail's head. Synaptic facilitation of responses to chemical stimulation of the head and tactile stimulation of the foot developed just as in neurons in control sensitized animals. The results were assessed in relation to a hypothesis postulating that synapse-specific plasticity on learning may occur because of selective neurochemical "projection" of synaptic connections to various genes within neurons.

In vivo regulation of an Aplysia glutamate transporter, ApGT1, during long-term memory formation

Regulation of glutamate transporters often accompanies glutamatergic synaptic plasticity. We investigated the mechanisms responsible for the increase in glutamate uptake associated with increased glutamate release at the Aplysia sensorimotor synapse during long-term sensitization (LTS) and long-term facilitation. An increase in the V(max) of transport, produced by LTS training, suggested that the increased glutamate uptake was due to an increase in the number of transporters in the membrane. We cloned a high-affinity, Na(+)-dependent glutamate transporter, ApGT1, from Aplysia central nervous system that is highly enriched in pleural sensory neurons, and in pleural-pedal synaptosome and cell/glial fractions. ApGT1, expressed in Xenopus oocytes, demonstrated a similar pharmacological profile to glutamate uptake in Aplysia synaptosome and cell/glial fractions (strong inhibition by threo-beta-benzyloxyaspartate and weak inhibition by dihydrokainate) suggesting that ApGT1 may be the primary glutamate transporter in pleural-pedal ganglia. Levels of ApGT1 and glutamate uptake were increased in synaptosomes 24 h after induction of LTS by electrical stimulation or serotonin. Regulation of ApGT1 during LTS appears to occur post-transcriptionally and results in an increased number of transporters in synaptic membranes. These results suggest that an increase in levels of ApGT1 is responsible, at least in part, for the long-term increase in glutamate uptake associated with long-term memory.

Function and translational regulation of mRNA in developing axons

The capacity to synthesize proteins in axons is limited to early stages of neuronal development, while axons are undergoing elongation and pathfinding. Although the roles of local protein synthesis are not fully understood, it has been implicated in regulating the morphological plasticity of growth cones. Recent studies have identified specific mRNAs that are translated in growth cones in response to specific extracellular signals. In this review, we discuss the functional relevance of axonal protein translation for developing axons, the differences in translational capacity between developing and mature vertebrate axons, and possible pathways governing the specific translational activation of axonal mRNAs.

Cytoplasmic remodelling and the acquisition of developmental competence in pig oocytes

The progression of oocyte meiosis is accompanied by major changes in the ooplasm that play a key role in the completion of a coordinate nuclear and cytoplasmic maturation. We review evidence from the literature and present data obtained in our laboratory on different aspects of pig oocyte cytoplasm compartmentalization during maturation and early embryo development. In particular, we will discuss the changes in adenosine triphosphate (ATP) concentration and distribution taking place during the maturation process and their possible significance for oocyte developmental competence. We describe two important aspects of cytoplasmic streaming: mitochondrial distribution patterns in oocytes and early embryos and the complex rearrangements of cytoplasmic microtubule networks, while discussing their possible correlations with ooplasm compartmentalization. Recent evidence indicates that the cytoskeleton is used to shuttle not only organelles but also mRNAs to specific sites within the oocyte cytoplasm. Localization is driven by specific molecular motors belonging to the kinesin superfamily and requires the involvement of the RNA targeting molecule Staufen. We present recent experimental evidence, obtained in our laboratory, on the pig orthologues for kinesin KIF5B and Staufen, describe their expression patterns and discuss their possible role in oocyte maturation.

Ca2+ -stimulated adenylyl cyclases regulate ERK-dependent activation of MSK1 during fear conditioning

The cAMP and ERK/MAP kinase (MAPK) signal transduction pathways are critical for hippocampus-dependent memory, a process that depends on CREB-mediated transcription. However, the extent of crosstalk between these pathways and the downstream CREB kinase activated during memory formation has not been elucidated. Here we report that PKA, MAPK, and MSK1, a CREB kinase, are coactivated in a subset of hippocampal CA1 pyramidal neurons following contextual fear conditioning. Activation of PKA, MAPK, MSK1, and CREB is absolutely dependent on Ca(2+)-stimulated adenylyl cyclase activity. We conclude that adenylyl cyclase activity supports the activation of MAPK, and that MSK1 is the major CREB kinase activated during training for contextual memory.

Persistence of long-term memory storage requires a late protein synthesis- and BDNF- dependent phase in the hippocampus

Persistence is the most characteristic attribute of long-term memory (LTM). To understand LTM, we must understand how memory traces persist over time despite the short-lived nature and rapid turnover of their molecular substrates. It is widely accepted that LTM formation is dependent upon hippocampal de novo protein synthesis and Brain-Derived Neurotrophic Factor (BDNF) signaling during or early after acquisition. Here we show that 12 hr after acquisition of a one-trial associative learning task, there is a novel protein synthesis and BDNF-dependent phase in the rat hippocampus that is critical for the persistence of LTM storage. Our findings indicate that a delayed stabilization phase is specifically required for maintenance, but not formation, of the memory trace. We propose that memory formation and memory persistence share some of the same molecular mechanisms and that recurrent rounds of consolidation-like events take place in the hippocampus for maintenance of the memory trace.

Rapid redistribution of synaptic PSD-95 in the neocortex in vivo

Most excitatory synapses terminate on dendritic spines. Spines vary in size, and their volumes are proportional to the area of the postsynaptic density (PSD) and synaptic strength. PSD-95 is an abundant multi-domain postsynaptic scaffolding protein that clusters glutamate receptors and organizes the associated signaling complexes. PSD-95 is thought to determine the size and strength of synapses. Although spines and their synapses can persist for months in vivo, PSD-95 and other PSD proteins have shorter half-lives in vitro, on the order of hours. To probe the mechanisms underlying synapse stability, we measured the dynamics of synaptic PSD-95 clusters in vivo. Using two-photon microscopy, we imaged PSD-95 tagged with GFP in layer 2/3 dendrites in the developing (postnatal day 10–21) barrel cortex. A subset of PSD-95 clusters was stable for days. Using two-photon photoactivation of PSD-95 tagged with photoactivatable GFP (paGFP), we measured the time over which PSD-95 molecules were retained in individual spines. Synaptic PSD-95 turned over rapidly (median retention times τ_r ~ 22–63 min from P10–P21) and exchanged with PSD-95 in neighboring spines by diffusion. PSDs therefore share a dynamic pool of PSD-95. Large PSDs in large spines captured more diffusing PSD-95 and also retained PSD-95 longer than small PSDs. Changes in the sizes of individual PSDs over days were associated with concomitant changes in PSD-95 retention times. Furthermore, retention times increased with developmental age (τ_r ~ 100 min at postnatal day 70) and decreased dramatically following sensory deprivation. Our data suggest that individual PSDs compete for PSD-95 and that the kinetic interactions between PSD molecules and PSDs are tuned to regulate PSD size.

Using two-photon microscopy and photoactivation of a fluorescently tagged synaptic protein (PSD-95), the authors demonstrated rapid turnover of these molecules in dendritic spines of the mouse sensory cortex in vivo.

Formation of microtubule-based traps controls the sorting and concentration of vesicles to restricted sites of regenerating neurons after axotomy

Transformation of a transected axonal tip into a growth cone (GC) is a critical step in the cascade leading to neuronal regeneration. Critical to the regrowth is the supply and concentration of vesicles at restricted sites along the cut axon. The mechanisms underlying these processes are largely unknown. Using online confocal imaging of transected, cultured Aplysia californica neurons, we report that axotomy leads to reorientation of the microtubule (MT) polarities and formation of two distinct MT-based vesicle traps at the cut axonal end. Approximately 100 microm proximal to the cut end, a selective trap for anterogradely transported vesicles is formed, which is the plus end trap. Distally, a minus end trap is formed that exclusively captures retrogradely transported vesicles. The concentration of anterogradely transported vesicles in the former trap optimizes the formation of a GC after axotomy.

Ribosomal distributions in axons of mammalian myelinated fibers

The distribution of ribosomes and polysomes in uninjured myelinated axons of rat sciatic nerve was analyzed. Ribosomes were identified by immunocytochemistry at the light and electron microscopic levels. A polyclonal antibody developed against ribosomes recognized both rRNA and ribosomal proteins. The distribution of the immunoreaction product was similar to that obtained with human anti-ribosomal P protein. The immunoreaction product distributions were of two types in axons: 1) periodic localization in the cortical region of axoplasm that appeared as a compact structural aggregate, consistent with that described as a periaxoplasmic ribosomal plaques (PARP) domain (Koenig et al. [2000] J. Neurosci. 20:8390-8400), and 2) scattered small immuno-reactive clusters of varying sizes (RNP) within the central core of the axon. The latter observation suggested the possibility that RNP-like particles could be associated with the axonal transport system and in transit. Immunoreaction product was also associated with a novel structural inclusion, possibly multi-vesicular in makeup that was located in the axon and at the myelin-axon interface, and visible at the light and EM levels. The potential significance of this structural peculiarity is considered.

The late maintenance of hippocampal LTP: Requirements, phases, ‘synaptic tagging’, ‘late-associativity’ and implications

Our review focuses on the mechanisms which enable the late maintenance of hippocampal long-term potentiation (LTP; >3h), a phenomenon which is thought to underlie prolonged memory. About 20 years ago we showed for the first time that the maintenance of LTP - like memory storage--depends on intact protein synthesis and thus, consists of at least two temporal phases. Here we concentrate on mechanisms required for the induction of the transient early-LTP and of the protein synthesis-dependent late-LTP. Our group has shown that the induction of late-LTP requires the associative activation of heterosynaptic inputs, i.e. the synergistic activation of glutamatergic and modulatory, reinforcing inputs within specific, effective time windows. The induction of late-LTP is characterized by novel, late-associative properties such as 'synaptic tagging' and 'late-associative reinforcement'. Both phenomena require the associative setting of synaptic tags as well as the availability of plasticity-related proteins (PRPs) and they are restricted to functional dendritic compartments, in general. 'Synaptic tagging' guarantees input specificity and thus the specific processing of afferent signals for the establishment of late-LTP. 'Late-associative reinforcement' describes a process where early-LTP by the co-activation of modulatory inputs can be transformed into late-LTP in activated synapses where a tag is set. Recent evidence from behavioral experiments, which studied processes of emotional and cognitive reinforcement of LTP, point to the physiological relevance of the above mechanisms during cellular and system's memory formation.