Local synthesis of proteins at synaptic sites on dendrites: role in synaptic plasticity and memory consolidation?

Long-lasting forms of memory are thought to be mediated by modifications in synapses that are induced by particular patterns of activity, take time to be established (consolidated), and involve new gene expression. The molecular mechanisms underlying such long-lasting synaptic modifications remain to be defined. Here, we focus on new findings regarding synapse-specific gene expression and especially the intracellular transport and synaptic targeting of the mRNA for a recently identified immediate early gene called activity-regulated cytoskeleton-associated protein (Arc). Arc mRNA is transported into dendrites after episodes of neuronal activation. The newly synthesized Arc mRNA localizes selectively at synapses that have experienced particular patterns of activity, and this targeting depends on NMDA receptor activation. Arc protein is assembled into the postsynaptic junction, and Arc expression is strongly induced by brief behavioral experiences in select populations of neurons. These features of Arc induction and trafficking reveal novel cellular mechanisms that are well suited to mediate long-term synapse-specific modifications. We will review findings from other laboratories that both LTP and memory are disrupted when Arc induction is blocked, and we will discuss the possibility that Arc protein may play a key role in activity-dependent synaptic modification.     

Synaptic tagging, evaluation of memories, and the distal reward problem

Long-term synaptic plasticity exhibits distinct phases. The synaptic tagging hypothesis suggests an early phase in which synapses are prepared, or "tagged," for protein capture, and a late phase in which those proteins are integrated into the synapses to achieve memory consolidation. The synapse specificity of the tags is consistent with conventional neural network models of associative memory. Memory consolidation through protein synthesis, however, is neuron specific, and its functional role in those models has not been assessed. Here, using a theoretical network model, we test the tagging hypothesis on its potential to prolong memory lifetimes in an online-learning paradigm. We find that protein synthesis, though not synapse specific, prolongs memory lifetimes if it is used to evaluate memory items on a cellular level. In our model we assume that only "important" memory items evoke protein synthesis such that these become more stable than "unimportant" items, which do not evoke protein synthesis. The network model comprises an equilibrium distribution of synaptic states that is very susceptible to the storage of new items: Most synapses are in a state in which they are plastic and can be changed easily, whereas only those synapses that are essential for the retrieval of the important memory items are in the stable late phase. The model can solve the distal reward problem, where the initial exposure of a memory item and its evaluation are temporally separated. Synaptic tagging hence provides a viable mechanism to consolidate and evaluate memories on a synaptic basis.     

BDNF: a key regulator for protein synthesis-dependent LTP and long-term memory?

It is generally believed that late-phase long-term potentiation (L-LTP) and long-term memory (LTM) require new protein synthesis. Although the full complement of proteins mediating the long-lasting changes in synaptic efficacy have yet to be identified, several lines of evidence point to a crucial role for activity-induced brain-derived neurotrophic factor (BDNF) expression in generating sustained structural and functional changes at hippocampal synapses thought to underlie some forms of LTM. In particular, BDNF is sufficient to induce the transformation of early to late-phase LTP in the presence of protein synthesis inhibitors, and inhibition of BDNF signaling impairs LTM. Despite solid evidence for a critical role of BDNF in L-LTP and LTM, many issues are not resolved. Given that BDNF needs to be processed in Golgi outposts localized at the branch point of one or few dendrites, a conceptually challenging problem is how locally synthesized BDNF in dendrites could ensure synapse-specific modulation of L-LTP. An interesting alternative is that BDNF-TrkB signaling is involved in synaptic tagging, a prominent hypothesis that explains how soma-derived protein could selectively modulate the tetanized (tagged) synapse. Finally, specific roles of BDNF in the acquisition, retention or extinction of LTM remain to be established.     

Genetic approaches to the study of synaptic plasticity and memory storage

Long-term memory is believed to depend on long-lasting changes in the strength of synaptic transmission known as synaptic plasticity. Understanding the molecular mechanisms of long-term synaptic plasticity is one of the principle goals of neuroscience. Among the most powerful tools being brought to bear on this question are genetically modified mice with changes in the expression or biological activity of genes thought to contribute to these processes. This article reviews how strains of mice with alterations in the cyclic adenosine monophosphate/protein kinase A/cyclic adenosine monophosphate-response element-binding protein signaling pathway have advanced our understanding of the biological basis of learning and memory.     

A nitric oxide-independent and beta-adrenergic receptor-sensitive form of metaplasticity limits theta-frequency stimulation-induced LTP in the hippocampal CA1 region

The induction of long-term potentiation (LTP) and long-term depression (LTD) at excitatory synapses in the hippocampus can be strongly modulated by patterns of synaptic stimulation that otherwise have no direct effect on synaptic strength. Likewise, patterns of synaptic stimulation that induce LTP or LTD not only modify synaptic strength but can also induce lasting changes that regulate how synapses will respond to subsequent trains of stimulation. Collectively known as metaplasticity, these activity-dependent processes that regulate LTP and LTD induction allow the recent history of synaptic activity to influence the induction of activity-dependent changes in synaptic strength and may thus have an important role in information storage during memory formation. To explore the cellular and molecular mechanisms underlying metaplasticity, we investigated the role of metaplasticity in the induction of LTP by υ-frequency (5-Hz) synaptic stimulation in the hippocampal CA1 region. Our results show that brief trains of υ-frequency stimulation not only induce LTP but also activate a process that inhibits the induction of additional LTP at potentiated synapses. Unlike other forms of metaplasticity, the inhibition of LTP induction at potentiated synapses does not appear to arise from activity-dependent changes in NMDA receptor function, does not require nitric oxide signaling, and is strongly modulated by β-adrenergic receptor activation. Together with previous findings, our results indicate that mechanistically distinct forms of metaplasticity regulate LTP induction and suggest that one way modulatory transmitters may act to regulate synaptic plasticity is by modulating metaplasticity.     

Synaptic plasticity and translation initiation

It is widely accepted that protein synthesis, including local protein synthesis at synapses, is required for several forms of synaptic plasticity. Local protein synthesis enables synapses to control synaptic strength independent of the cell body via rapid protein production from pre-existing mRNA. Therefore, regulation of translation initiation is likely to be intimately involved in modulating synaptic strength. Our understanding of the translation-initiation process has expanded greatly in recent years. In this review, we discuss various aspects of translation initiation, as well as signaling pathways that might be involved in coupling neurotransmitter and neurotrophin receptors to the translation machinery during various forms of synaptic plasticity.     

Coincident induction of long-term facilitation at sensory-motor synapses in Aplysia: presynaptic and postsynaptic factors

Induction of long-term synaptic changes at one synapse can facilitate the induction of long-term plasticity at another synapse. Here we show that if Aplysia sensory neuron (SN) somata and their remote motor neuron (MN) synapses are simultaneously exposed to serotonin (5HT) pulses, which at either site alone are insufficient to induce long-term facilitation (LTF), processes activated at these sites interact to induce LTF. Coincident induction of LTF requires: (1) that the synaptic pulse occurs within a brief temporal window of the somatic pulse and (2) that local protein synthesis occurs immediately at the synapse, followed by delayed protein synthesis at the soma. LTF at the SN-MN synapses can also be induced with cell-wide application of repeated pulses of 5HT. However, these two forms of LTF differ mechanistically: (1) coincident LTF requires protein synthesis in the postsynaptic motor neuron, whereas repeated 5HT LTF does not, and (2) repeated 5HT LTF is accompanied by intermediate-term (3 h) facilitation, whereas coincident LTF is not. Thus LTF expressed in the same temporal domain can result from different underlying mechanisms.     

Chronic nicotine exposure induces a long-lasting and pathway-specific facilitation of LTP in the amygdala

Nicotine, in the form of tobacco, is the most commonly used drug of abuse. In addition to its rewarding properties, nicotine also affects many cognitive and emotional processes that involve several brain regions, including hippocampus and amygdala. Long-term changes in synaptic strength in these brain regions after drug exposure may be importantly correlated with behavioral changes induced by nicotine. Here, we study the effect of chronic oral administration of nicotine on the long-term synaptic potentiation in the amygdala, a key structure for emotional memory. We find that oral administration of nicotine for 7 d produces a significant enhancement of LTP in the amygdala. This facilitation is pathway specific: Nicotine selectively facilitates LTP in the cortical-lateral amygdala pathway, but not the thalamic-lateral and the lateral-basolateral synaptic pathway. The synaptic facilitation induced by a 7-d exposure to nicotine is long-lasting, it persists for 72 h after cessation of nicotine but decays 8 d after its cessation. In contrast, a shorter exposure of nicotine (24 h) induces only a short-lasting facilitation of synaptic plasticity that dissipates 24 and 72 h after cessation of nicotine. The facilitation of LTP in the amygdala after exposure to nicotine is mediated by removal of GABAergic inhibition, is dependent on the activation NMDA receptors, and can be prevented by blocking either α7 or β2 nACh receptors. Our results indicate that chronic exposure to nicotine can promote the induction of long-lasting modifications of synapses in a specific pathway in the amygdala.These changes in synaptic plasticity may contribute to the complex neural adaptations and behaviors caused by nicotine.     

Removal of S6K1 and S6K2 leads to divergent alterations in learning, memory, and synaptic plasticity

Protein synthesis is required for the expression of enduring memories and long-lasting synaptic plasticity. During cellular proliferation and growth, S6 kinases (S6Ks) are activated and coordinate the synthesis of de novo proteins. We hypothesized that protein synthesis mediated by S6Ks is critical for the manifestation of learning, memory, and synaptic plasticity. We have tested this hypothesis with genetically engineered mice deficient for either S6K1 or S6K2. We have found that S6K1-deficient mice express an early-onset contextual fear memory deficit within one hour of training, a deficit in conditioned taste aversion (CTA), impaired Morris water maze acquisition, and hypoactive exploratory behavior. In contrast, S6K2-deficient mice exhibit decreased contextual fear memory seven days after training, a reduction in latent inhibition of CTA, and normal spatial learning in the Morris water maze. Surprisingly, neither S6K1- nor S6K2-deficient mice exhibited alterations in protein synthesis-dependent late-phase long-term potentiation (L-LTP). However, removal of S6K1, but not S6K2, compromised early-phase LTP expression. Furthermore, we observed that S6K1-deficient mice have elevated basal levels of Akt phosphorylation, which is further elevated following induction of L-LTP. Taken together, our findings demonstrate that removal of S6K1 leads to a distinct array of behavioral and synaptic plasticity phenotypes that are not mirrored by the removal of S6K2. Our observations suggest that neither gene by itself is required for L-LTP but instead may be required for other types of synaptic plasticity required for cognitive processing.     

Synaptic Reliability Correlates with Reduced Susceptibility to Synaptic Potentiation by Brain-Derived Neurotrophic Factor

Recent studies have implicated brain-derived neurotrophic factor (BDNF) in use-dependent modification of hippocampal synapses. BDNF can rapidly potentiate synaptic transmission at glutamatergic synapses by enhancing transmitter release. Using simultaneous perforated patch recording from pairs and triplets of glutamatergic hippocampal neurons, we have examined how the initial state of the glutamatergic synapse determines its susceptibility to synaptic modification by BDNF. We found that the degree of synaptic potentiation by BDNF depends on the initial reliability and strength of the synapse: Relatively weak connections were strongly potentiated, whereas the effect was markedly reduced at stronger synapses. The degree of BDNF-induced potentiation strongly correlated with the initial coefficient of variation (CV) of the amplitude of excitatory postsynaptic currents (EPSCs) and inversely correlated with the initial paired–pulse facilitation, suggesting that synapses with lower release probability (P_r) are more susceptible to the action of BDNF. To determine whether saturation of P_r could have masked the potentiation effect of BDNF in the stronger synapses, we lowered the initial P_r either by reducing the extracellular Ca²⁺ concentration ([Ca²⁺]_o) or by bath application of adenosine. Synapses that were initially strong remained unaffected by BDNF under these conditions of reduced P_r. Thus, the lack of BDNF effect on synaptic efficacy cannot simply be accounted for by saturation of P_r, but rather may be due to intrinsic changes associated with synaptic maturation that might covary with P_r. Finally, the dependence on initial synaptic strength was also found for divergent outputs of the same presynaptic neuron, suggesting that synaptic terminals with different degrees of responsiveness to BDNF can coexist within in the same neuron.     

Long-term plasticity of intrinsic excitability: learning rules and mechanisms

Spatio-temporal configurations of distributed activity in the brain is thought to contribute to the coding of neuronal information and synaptic contacts between nerve cells could play a central role in the formation of privileged pathways of activity. Synaptic plasticity is not the exclusive mode of regulation of information processing in the brain, and persistent regulations of ionic conductances in some specialized neuronal areas such as the dendrites, the cell body, and the axon could also modulate, in the long-term, the propagation of neuronal information. Persistent changes in intrinsic excitability have been reported in several brain areas in which activity is elevated during a classical conditioning. The role of synaptic activity seems to be a determinant in the induction, but the learning rules and the underlying mechanisms remain to be defined. We discuss here the role of synaptic activity in the induction of intrinsic plasticity in cortical, hippocampal, and cerebellar neurons. Activation of glutamate receptors initiates a long-term modification in neuronal excitability that may represent a parallel, synergistic substrate for learning and memory. Similar to synaptic plasticity, long-lasting intrinsic plasticity appears to be bidirectional and to express a certain level of input or cell specificity. These nonsynaptic forms of plasticity affect the signal propagation in the axon, the dendrites, and the soma. They not only share common learning rules and induction pathways with the better-known synaptic plasticity such as NMDA receptor dependent LTP and LTD, but also contribute in synergy with these synaptic changes to the formation of a coherent engram.     

Eye-blink conditioning is associated with changes in synaptic ultrastructure in the rabbit interpositus nuclei

Eye-blink conditioning involves the pairing of a conditioned stimulus (usually a tone) to an unconditioned stimulus (air puff), and it is well established that an intact cerebellum and interpositus nucleus, in particular, are required for this form of classical conditioning. Changes in synaptic number or structure have long been proposed as a mechanism that may underlie learning and memory, but localizing these changes has been difficult. Thus, the current experiment took advantage of the large amount of research conducted on the neural circuitry that supports eye-blink conditioning by examining synaptic changes in the rabbit interpositus nucleus. Synaptic quantifications included total number of synapses per neuron, numbers of excitatory versus inhibitory synapses, synaptic curvature, synaptic perforations, and the maximum length of the synapses. No overall changes in synaptic number, shape, or perforations were observed. There was, however, a significant increase in the length of excitatory synapses in the conditioned animals. This increase in synaptic length was particularly evident in the concave-shaped synapses. These results, together with previous findings, begin to describe a sequence of synaptic change in the interpositus nuclei following eye-blink conditioning that would appear to begin with structural change and end with an increase in synaptic number.     

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

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

Conditioned taste aversion modifies persistently the subsequent induction of neocortical long-term potentiation in vivo

The ability of neurons to modify their synaptic strength in an activity-dependent manner has a crucial role in learning and memory processes. It has been proposed that homeostatic forms of plasticity might provide the global regulation necessary to maintain synaptic strength and plasticity within a functional dynamic range. Similarly, it is considered that the capacity of synapses to express plastic changes is itself subject to variation dependent on previous experience. In particular, training in several behavioral tasks modifies the possibility to induce long-term potentiation (LTP). Our previous studies in the insular cortex (IC) have shown that induction of LTP in the basolateral amygdaloid nucleus (Bla)-IC projection previous to conditioned taste aversion (CTA) training enhances the retention of this task. The aim of the present study was to analyze whether CTA training modifies the ability to induce subsequent LTP in the Bla-IC projection in vivo. Thus, CTA trained rats received high frequency stimulation in the Bla-IC projection in order to induce LTP 48, 72, 96 and 120 h after the aversion test. Our results show that CTA training prevents the subsequent induction of LTP in the Bla-IC projection, for at least 120 h after CTA training. We also showed that pharmacological inhibition of CTA consolidation with anisomycin (1 μl/side; 100 μg/μl) prevents the CTA effect on IC-LTP. These findings reveal that CTA training produces a persistent change in the ability to induce subsequent LTP in the Bla-IC projection in a protein-synthesis dependent manner, suggesting that changes in the ability to induce subsequent synaptic plasticity contribute to the formation and persistence of aversive memories.     

ERK1/2 activation is necessary for BDNF to increase dendritic spine density in hippocampal CA1 pyramidal neurons

Brain-derived neurotrophic factor (BDNF) is a potent modulator of synaptic transmission and plasticity in the CNS, acting both pre- and postsynaptically. We demonstrated recently that BDNF/TrkB signaling increases dendritic spine density in hippocampal CA1 pyramidal neurons. Here, we tested whether activation of the prominent ERK (MAPK) signaling pathway was responsible for BDNF's effects on spine growth. Slice cultures were transfected with enhanced yellow fluorescent protein (eYFP) by particle-mediated gene transfer, and CA1 pyramidal neurons were imaged by laser-scanning confocal microscopy. We confirmed that BDNF (24 h) increases spine density in apical dendrites of CA1 neurons. The MEK (ERK kinase) inhibitors PD98059 and U0126 completely prevented the increase in spine density induced by BDNF, without having an effect on spine density by themselves. In contrast to its actions on cortical pyramidal neurons, BDNF had minor and rather localized effects on dendritic complexity in hippocampal pyramidal neurons, increasing the total length, but not the branching of apical dendrites within CA1 stratum radiatum, without affecting basal dendrites in stratum oriens. Our results support the hypothesis that the ERK-signaling pathway not only mediates long-term synaptic plasticity and hippocampal-dependent learning, but it is also involved in the structural remodeling of excitatory spine synapses triggered by neurotrophins.