Bi-directional modulation of AMPA receptor unitary conductance by synaptic activity

Activity-dependent diffusion trapping of AMPA receptors as a key step for expression of early LTP

This review focuses on the activity-dependent diffusion trapping of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (AMPARs) as a crucial mechanism for the expression of early long-term potentiation (LTP), a process central to learning and memory. Despite decades of research, the precise mechanisms by which LTP induction leads to an increase in AMPAR responses at synapses have been elusive. We review the different hypotheses that have been put forward to explain the increased AMPAR responsiveness during LTP. We discuss the dynamic nature of AMPAR complexes, including their constant turnover and activity-dependent modifications that affect their synaptic accumulation. We highlight a hypothesis suggesting that AMPARs are diffusively trapped at synapses through activity-dependent interactions with protein-based binding slots in the post-synaptic density (PSD), offering a potential explanation for the increased synaptic strength during LTP. Furthermore, we outline the challenges still to be addressed before we fully understand the functional roles and molecular mechanisms of AMPAR dynamic nanoscale organization in LTP. This article is part of a discussion meeting issue 'Long-term potentiation: 50 years on'.

Tonic activin signaling shapes cellular and synaptic properties of CA1 neurons mainly in dorsal hippocampus

Dorsal and ventral hippocampus serve different functions in cognition and affective behavior, but the underpinnings of this diversity at the cellular and synaptic level are not well understood. We found that the basal level of activin A, a member of the TGF-β family, which regulates hippocampal circuits in a behaviorally relevant fashion, is much higher in dorsal than in ventral hippocampus. Using transgenic mice with a forebrain-specific disruption of activin receptor signaling, we identified the pronounced dorsal-ventral gradient of activin A as a major factor determining the distinct neurophysiologic signatures of dorsal and ventral hippocampus, ranging from pyramidal cell firing, tuning of frequency-dependent synaptic facilitation, to long-term potentiation (LTP), long-term depression (LTD), and de-potentiation. Thus, the strong activin A tone in dorsal hippocampus appears crucial to establish cellular and synaptic phenotypes that are tailored specifically to the respective network operations in dorsal and ventral hippocampus.

A computational model of current control mechanism for long-term potentiation (LTP) in human episodic memory based on gene-gene interaction

The establishment of long-term potentiation (LTP) is a prime process for the formation of episodic memory. During the establishment of LTP, activations of various components are required in the signaling cascade of the LTP pathway. Past efforts to determine the activation of components relied extensively on the cellular or molecular level. In this paper, we have proposed a computational model based on gene-level cascading and interaction in LTP signaling for the establishment and control of current signals for achieving the desired level of activation in the formation of episodic memory. This paper also introduces a model for a generalized signaling pathway in episodic memory. A back-propagation feedback mechanism is used for updating the interaction levels in the signaling cascade starting from the last stage and ending at the start stage of the signaling cascade. Simulation of the proposed model has been performed for the LTP signaling pathway in the context of human episodic memory. We found through simulation that the qualifying genes correction factors of all stages are updated to their maximum limit. The article explains the signaling pathway for episodic memory and proves its effectiveness through simulation results.

NMDA and AMPA receptor physiology and role in visceral hypersensitivity: a review

N-methyl-d-aspartate receptors (NMDARs) and alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid receptors (AMPARs) are excitatory neurotransmission receptors of the central nervous system and play vital roles in synaptic plasticity. Although not fully elucidated, visceral hypersensitivity is one of the most well-characterized pathophysiologic abnormalities of functional gastrointestinal diseases and appears to be associated with increased synaptic plasticity. In this study, we review the updated findings on the physiology of NMDARs and AMPARs and their relation to visceral hypersensitivity, which propose directions for future research in this field with evolving importance.

PKA drives an increase in AMPA receptor unitary conductance during LTP in the hippocampus

Long-term potentiation (LTP) at hippocampal CA1 synapses can be expressed by an increase either in the number (N) of AMPA (α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid) receptors or in their single channel conductance (γ). Here, we have established how these distinct synaptic processes contribute to the expression of LTP in hippocampal slices obtained from young adult rodents. LTP induced by compressed theta burst stimulation (TBS), with a 10 s inter-episode interval, involves purely an increase in N (LTP_N). In contrast, either a spaced TBS, with a 10 min inter-episode interval, or a single TBS, delivered when PKA is activated, results in LTP that is associated with a transient increase in γ (LTP_γ), caused by the insertion of calcium-permeable (CP)-AMPA receptors. Activation of CaMKII is necessary and sufficient for LTP_N whilst PKA is additionally required for LTP_γ. Thus, two mechanistically distinct forms of LTP co-exist at these synapses.

Learning-induced enduring changes in inhibitory synaptic transmission in lateral amygdala are mediated by p21-activated kinase

In this study we explored whether learning leads to enduring changes in inhibitory synaptic transmission in lateral amygdala (LA). We revealed that olfactory discrimination (OD) learning in rats led to a long-lasting increase in postsynaptic GABAA channel-mediated miniature inhibitory postsynaptic currents (mIPSCs) in LA. Olfactory fear conditioning, but not auditory fear conditioning, also led to enduring enhancement in GABAA-mediated mIPSCs. Auditory fear conditioning, but not olfactory fear conditioning or OD learning, induced an enduring reduction in the frequency but not the current of mIPSC events. We found that p21-activated kinase (PAK) activity is needed to maintain OD and olfactory fear conditioning learning-induced enduring enhancement of mIPSCs. Further analysis revealed that OD led to an increase in GABAA channel conductance whereas olfactory fear conditioning increased the number of GABAA channels. These alterations in GABAA channels conductance and level are controlled by PAK activity. Our study shows that the learning-induced increase in postsynaptic inhibitory transmission in LA is specific to the sensory modality. However, the mechanism that mediates the increase in inhibitory transmission, namely the increase in the conductance or in the level of GABAA channel, is learning specific.

NEW & NOTEWORTHY Here we studied whether learning leads to long-lasting alterations in inhibitory synaptic transmission in lateral amygdala (LA). We revealed that learning led to enduring changes in inhibitory synaptic transmission in LA that are affected by the sensory modality (auditory or olfaction) used during learning. However, the mechanism that mediated the changes in inhibitory transmission (alterations in GABAA channel level or conductance) depended on the type of learning. These long-lasting alterations are maintained by p21-activated kinase.

Differential sensitivity of three forms of hippocampal synaptic potentiation to depotentiation

Theta-burst stimulation (TBS) induces short-term potentiation (STP) plus two types of transcriptionally-independent forms of long-term potentiation (LTP), termed LTP1 and LTP2. We have compared the susceptibility of these three types of synaptic plasticity to depotentiation, induced by low frequency stimulation (LFS; 2 Hz for 10 min) at the Schaffer collateral-commissural pathway in area CA1 of adult rat hippocampal slices. In interleaved experiments, STP and LTP were induced by three episodes of either compressed or spaced TBS (cTBS or sTBS). LFS had a more pronounced effect on the LTP induced by the cTBS. One traditional interpretation of these results is a difference in the time-dependent immunity against depotentiation. We suggest an alternative explanation: LFS rapidly reverses STP to reveal a slowly developing LTP. The cTBS protocol induces LTP1 that is moderately sensitive to depotentiation. The sTBS induces an additional component of LTP (LTP2) that is resistant to depotentiation.

AMPA-Type Glutamate Receptor Conductance Changes and Plasticity: Still a Lot of Noise

Twenty years ago, we reported from the Collingridge Lab that a single-channel conductance increase through α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)-type ionotropic glutamate receptors (AMPARs) could mediate one form of plasticity associated with long-term potentiation (LTP) in the hippocampus (Benke et al., Nature 395:793-797, 1998). Revealed through peak-scaled non-stationary fluctuation analysis (PS-NSFA, also known as noise analysis), this component of LTP could be exclusively mediated by direct increases in channel conductance or by increases in the number of high conductance synaptic AMPARs. Re-evaluation of our original data in the light of the molecular details regarding AMPARs, conductance changes and plasticity suggests that insertion of high-conductance GluA1 homomers can account for our initial findings. Any potential cost associated with manufacture or trafficking of new receptors could be mitigated if pre-existing synaptic AMPARs also undergo a modest conductance change. The literature suggests that the presence of high conductance AMPARs and/or GluA1 homomers confers an unstable synaptic state, suggesting state transitions. An experimental paradigm is proposed to differentiate these possibilities. Validation of this state diagram could provide insight into development, disease pathogenesis and treatment.

Learning-induced modulation of the effect of endocannabinoids on inhibitory synaptic transmission

Endocannabinoids are key modulators that regulate central brain functions and behaviors, including learning and memory. At the cellular and molecular levels, endocannabinoids are potent modulators of excitatory and inhibitory synaptic function. Most effects of cannabinoids are thought to be mediated via G protein-coupled cannabinoid receptors. In particular, cannabinoids released from postsynaptic neurons are suggested to act as retrograde messengers, activating presynaptic type-1 cannabinoid receptors (CB1Rs), thereby inducing suppression of synaptic release. Another central mechanism of cannabinoid-induced action requires activation of astroglial CB1Rs. CB1Rs are also implicated in self-modulation of cortical neurons. Rats that are trained in a particularly difficult olfactory-discrimination task show a dramatic increased ability to acquire memories of new odors. The memory of the acquired high-skill acquisition, termed “rule learning” or “learning set,” lasts for many months. Using this behavioral paradigm, we show a novel function of action for CB1Rs, supporting long-term memory by maintaining persistent enhancement of inhibitory synaptic transmission. Long-lasting enhancement of inhibitory synaptic transmission is blocked by a CB1R inverse agonist. This effect is mediated by a novel purely postsynaptic mechanism, obtained by enhancing the single GABAA channel conductance that is PKA dependent. The significant role that CB1R has in maintaining learning-induced long-term strengthening of synaptic inhibition suggests that endocannabinoids have a key role in maintaining long-term memory by enhancing synaptic inhibition.

NEW & NOTEWORTHY In this study we show a novel function and mechanism of action for cannabinoids in neurons, mediated by activation of type-1 cannabinoid receptors, supporting long-term memory by maintaining persistent enhancement of inhibitory synaptic transmission on excitatory neurons. This effect is mediated by a novel purely postsynaptic mechanism, obtained by enhancing the single GABAA channel conductance that is PKA dependent. Thus we report for the first time that endocannabinoids have a key role maintaining learning-induced synaptic modification.

Real Time Multiplicative Memory Amplification Mediated by Whole-Cell Scaling of Synaptic Response in Key Neurons

Intense spiking response of a memory-pattern is believed to play a crucial role both in normal learning and pathology, where it can create biased behavior. We recently proposed a novel model for memory amplification where the simultaneous two-fold increase of all excitatory (AMPAR-mediated) and inhibitory (GABA_AR-mediated) synapses in a sub-group of cells that constitutes a memory-pattern selectively amplifies this memory. Here we confirm the cellular basis of this model by validating its major predictions in four sets of experiments, and demonstrate its induction via a whole-cell transduction mechanism. Subsequently, using theory and simulations, we show that this whole-cell two-fold increase of all inhibitory and excitatory synapses functions as an instantaneous and multiplicative amplifier of the neurons’ spiking. The amplification mechanism acts through multiplication of the net synaptic current, where it scales both the average and the standard deviation of the current. In the excitation-inhibition balance regime, this scaling creates a linear multiplicative amplifier of the cell’s spiking response. Moreover, the direct scaling of the synaptic input enables the amplification of the spiking response to be synchronized with rapid changes in synaptic input, and to be independent of previous spiking activity. These traits enable instantaneous real-time amplification during brief elevations of excitatory synaptic input. Furthermore, the multiplicative nature of the amplifier ensures that the net effect of the amplification is large mainly when the synaptic input is mostly excitatory. When induced on all cells that comprise a memory-pattern, these whole-cell modifications enable a substantial instantaneous amplification of the memory-pattern when the memory is activated. The amplification mechanism is induced by CaMKII dependent phosphorylation that doubles the conductance of all GABA_A and AMPA receptors in a subset of neurons. This whole-cell transduction mechanism enables both long-term induction of memory amplification when necessary and extinction when not further required.

Amplifying the strength of a neuronal assembly that underlies a behavioral choice can lead to a particularly long lasting dominant memory. We report experimental and theoretical evidence for a long-term mechanism that amplifies the response of a neuronal assembly which we termed “memory amplification mechanism”. The amplification mechanism is mediated by doubling the strength of all inhibitory and all excitatory synapses in the cell and is induced by whole-cell phosphorylation of all inhibitory and excitatory synaptic receptors in a subset of cells, via a process that is distinct from memory formation. Computationally, the inherent scaling of both excitation and inhibition yields a robust and stable amplifier of the neuron’s response. When such an amplifier is induced in a set of cells that compose a memory-pattern, it can selectively amplify the response of this memory. The memory amplification mechanism is independent from associative learning. Thus, while associative learning forms a memory that encodes new associations, the amplification mechanism can promote an already formed memory to a dominant memory.

Differential Modulation of GABAA Receptors Underlies Postsynaptic Depolarization- and Purinoceptor-Mediated Enhancement of Cerebellar Inhibitory Transmission: A Non-Stationary Fluctuation Analysis Study

Cerebellar GABAergic inhibitory transmission between interneurons and Purkinje cells (PCs) undergoes a long-lasting enhancement following different stimulations, such as brief depolarization or activation of purinergic receptors of postsynaptic PCs. The underlying mechanisms, however, are not completely understood. Using a peak-scaled non-stationary fluctuation analysis, we therefore aimed at characterizing changes in the electrophysiological properties of GABA_A receptors in PCs of rat cerebellar cortex during depolarization-induced “rebound potentiation (RP)” and purinoceptor-mediated long-term potentiation (PM-LTP), because both RP and PM-LTP likely depend on postsynaptic mechanisms. Stimulation-evoked inhibitory postsynaptic currents (eIPSCs) were recorded from PCs in neonatal rat cerebellar slices. Our analysis showed that postsynaptic membrane depolarization induced RP of eIPSCs in association with significant increase in the number of synaptic GABA_A receptors without changing the channel conductance. By contrast, bath application of ATP induced PM-LTP of eIPSCs with a significant increase of the channel conductance of GABA_A receptors without affecting the receptor number. Pretreatment with protein kinase A (PKA) inhibitors, H-89 and cAMPS-Rp, completely abolished the PM-LTP. The CaMKII inhibitor KN-62 reported to abolish RP did not alter PM-LTP. These results suggest that the signaling mechanism underlying PM-LTP could involve ATP-induced phosphorylation of synaptic GABA_A receptors, thereby resulting in upregulation of the channel conductance by stimulating adenylyl cyclase-PKA signaling cascade, possibly via activation of P2Y₁₁ purinoceptor. Thus, our findings reveal that postsynaptic GABA_A receptors at the interneuron-PC inhibitory synapses are under the control of two distinct forms of long-term potentiation linked with different second messenger cascades.

Modulation of Synaptic Plasticity by Exercise Training as a Basis for Ischemic Stroke Rehabilitation

In recent years, rehabilitation of ischemic stroke draws more and more attention in the world, and has been linked to changes of synaptic plasticity. Exercise training improves motor function of ischemia as well as cognition which is associated with formation of learning and memory. The molecular basis of learning and memory might be synaptic plasticity. Research has therefore been conducted in an attempt to relate effects of exercise training to neuroprotection and neurogenesis adjacent to the ischemic injury brain. The present paper reviews the current literature addressing this question and discusses the possible mechanisms involved in modulation of synaptic plasticity by exercise training. This review shows the pathological process of synaptic dysfunction in ischemic roughly and then discusses the effects of exercise training on scaffold proteins and regulatory protein expression. The expression of scaffold proteins generally increased after training, but the effects on regulatory proteins were mixed. Moreover, the compositions of postsynaptic receptors were changed and the strength of synaptic transmission was enhanced after training. Finally, the recovery of cognition is critically associated with synaptic remodeling in an injured brain, and the remodeling occurs through a number of local regulations including mRNA translation, remodeling of cytoskeleton, and receptor trafficking into and out of the synapse. We do provide a comprehensive knowledge of synaptic plasticity enhancement obtained by exercise training in this review.

EGFR signaling upregulates surface expression of the GluN2B-containing NMDA receptor and contributes to long-term potentiation in the hippocampus

N-methyl-d-aspartate receptors (NMDARs) have been known to be regulated by various receptor tyrosine kinases. Activation of epidermal growth factor receptor (EGFR) specifically increases NMDAR-mediated currents and enhances long-term potentiation (LTP) in the hippocampus. However, the mechanism through which EGFR regulates NMDARs remains to be elucidated. In this study we found that EGFR was highly expressed in the hippocampus and mainly localized in the non-synaptic region including the soma and neurites of cultured hippocampal neurons. EGFR activation led to an increase in ifenprodil-sensitive NMDAR currents. Consistent with this, we also observed that surface expression of GluN2B-containing NMDAR was upregulated. Our biochemical data from hippocampal slices and hippocampal cultured neurons demonstrated that EGF treatment in vitro significantly increased phosphorylation of the GluN2B subunit at Y1472 with a coincidental activation of Src family kinases (SFKs). EGFR blockade with a specific antagonist BIBX-1382 attenuated an increase of GluN2B in the postsynaptic density during high-frequency stimulation (HFS)-induced LTP. Moreover, BIBX blockade significantly impaired HFS-induced LTP. In conclusion, our findings suggest that EGFR signaling upregulates NMDARs through modification of the GluN2B subunit, and is required for HFS-induced LTP in the hippocampus.

Persistent CaMKII activation mediates learning-induced long-lasting enhancement of synaptic inhibition

Training rats in a particularly difficult olfactory-discrimination task results in acquisition of high skill to perform the task superbly, termed "rule learning" or "learning set." Such complex learning results in enhanced intrinsic neuronal excitability of piriform cortex pyramidal neurons, and in their excitatory synaptic interconnections. These changes, while subserving memory maintenance, must be counterbalanced by modifications that prevent overspreading of activity and uncontrolled synaptic strengthening. Indeed, we have previously shown that the average amplitude of GABAA-mediated miniature IPSCs (mIPSCs) in these neurons is enhanced for several days after learning, an enhancement mediated via a postsynaptic mechanism. To unravel the molecular mechanism of this long-term inhibition enhancement, we tested the role of key second-messenger systems in maintaining such long-lasting modulation. The calcium/calmodulin-dependent kinase II (CaMKII) blocker, KN93, significantly reduced the average mIPSC amplitude in neurons from trained rats only to the average pretraining level. A similar effect was obtained by the CaMKII peptide inhibitor, tatCN21. Such reduction resulted from decreased single-channel conductance and not in the number of activated channels. The PKC inhibitor, GF109203X, reduced the average mIPSC amplitude in neurons from naive, pseudo-trained, and trained animals, and the difference between the trained and control groups remained. Such reduction resulted from a decrease in the number of activated channels. The PKA inhibitor H89 dihydrochloride did not affect the average mIPSC amplitude in neurons from any of the three groups. We conclude that learning-induced enhancement of GABAA-mediated synaptic inhibition is maintained by persistent CaMKII activation.

Conditional deletion of α-CaMKII impairs integration of adult-generated granule cells into dentate gyrus circuits and hippocampus-dependent learning

New granule cells are continuously integrated into hippocampal circuits throughout adulthood, and the fine-tuning of this process is likely important for efficient hippocampal function. During development, this integration process is critically regulated by the α-calcium/calmodulin-dependent protein kinase II (α-CaMKII), and here we ask whether this role is conserved in the adult brain. To do this, we developed a transgenic strategy to conditionally delete α-CaMKII from neural progenitor cells and their progeny in adult mice. First, we found that the selective deletion of α-CaMKII from newly generated dentate granule cells led to an increase in dendritic complexity. Second, α-CaMKII deletion led to a reduction in number of mature synapses and cell survival. Third, consistent with altered morphological and synaptic development, acquisition of one-trial contextual fear conditioning was impaired after deletion of α-CaMKII from newly generated dentate granule cells. Previous work in Xenopus identified α-CaMKII as playing a key role in the stabilization of dendritic and synaptic structure during development. The current study indicates that α-CaMKII plays a plays a similar, cell-autonomous role in the adult hippocampus and, in addition, reveals that the loss of α-CaMKII from adult-generated granule cells is associated with impaired hippocampus-dependent learning.

Maximum likelihood estimation of biophysical parameters of synaptic receptors from macroscopic currents

Dendritic integration and neuronal firing patterns strongly depend on biophysical properties of synaptic ligand-gated channels. However, precise estimation of biophysical parameters of these channels in their intrinsic environment is complicated and still unresolved problem. Here we describe a novel method based on a maximum likelihood approach that allows to estimate not only the unitary current of synaptic receptor channels but also their multiple conductance levels, kinetic constants, the number of receptors bound with a neurotransmitter, and the peak open probability from experimentally feasible number of postsynaptic currents. The new method also improves the accuracy of evaluation of unitary current as compared to the peak-scaled non-stationary fluctuation analysis, leading to a possibility to precisely estimate this important parameter from a few postsynaptic currents recorded in steady-state conditions. Estimation of unitary current with this method is robust even if postsynaptic currents are generated by receptors having different kinetic parameters, the case when peak-scaled non-stationary fluctuation analysis is not applicable. Thus, with the new method, routinely recorded postsynaptic currents could be used to study the properties of synaptic receptors in their native biochemical environment.

Inhibitory interactions between phosphorylation sites in the C terminus of α-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid-type glutamate receptor GluA1 subunits

The C terminus of AMPA-type glutamate receptor (AMPAR) GluA1 subunits contains several phosphorylation sites that regulate AMPAR activity and trafficking at excitatory synapses. Although many of these sites have been extensively studied, little is known about the signaling mechanisms regulating GluA1 phosphorylation at Thr-840. Here, we report that neuronal depolarization in hippocampal slices induces a calcium and protein phosphatase 1/2A-dependent dephosphorylation of GluA1 at Thr-840 and a nearby site at Ser-845. Despite these similarities, inhibitors of NMDA-type glutamate receptors and protein phosphatase 2B prevented depolarization-induced Ser-845 dephosphorylation but had no effect on Thr-840 dephosphorylation. Instead, depolarization-induced Thr-840 dephosphorylation was prevented by blocking voltage-gated calcium channels, indicating that distinct Ca(2+) sources converge to regulate GluA1 dephosphorylation at Thr-840 and Ser-845 in separable ways. Results from immunoprecipitation/depletion assays indicate that Thr-840 phosphorylation inhibits protein kinase A (PKA)-mediated increases in Ser-845 phosphorylation. Consistent with this, PKA-mediated increases in AMPAR currents, which are dependent on Ser-845 phosphorylation, were inhibited in HEK-293 cells expressing a Thr-840 phosphomimetic version of GluA1. Conversely, mimicking Ser-845 phosphorylation inhibited protein kinase C phosphorylation of Thr-840 in vitro, and PKA activation inhibited Thr-840 phosphorylation in hippocampal slices. Together, the regulation of Thr-840 and Ser-845 phosphorylation by distinct sources of Ca(2+) influx and the presence of inhibitory interactions between these sites highlight a novel mechanism for conditional regulation of AMPAR phosphorylation and function.

Regulation of GluA1 α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor function by protein kinase C at serine-818 and threonine-840

Three residues within the AMPA (α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid) receptor subunit GluA1 C terminus (Ser818, Ser831, Thr840) can be phosphorylated by Ca(2+)/phospholipid-dependent protein kinase (PKC). Here, we show that PKC phosphorylation of GluA1 Ser818 or Thr840 enhances the weighted mean channel conductance without altering the response time course or agonist potency. These data support the idea that these residues constitute a hyper-regulatory domain for the AMPA receptor. Introduction of phosphomimetic mutations increases conductance only at these three sites within the proximal C terminus, consistent with a structural model with a flexible linker connecting the distal C-terminal domain to the more proximal domain containing a helix bracketed by Ser831 and Thr840. NMR spectra support this model and raise the possibility that phosphorylation can alter the configuration of this domain. Our findings provide insight into the structure and function of the C-terminal domain of GluA1, which controls AMPA receptor function and trafficking during synaptic plasticity in the central nervous system.

A novel whole-cell mechanism for long-term memory enhancement

Olfactory-discrimination learning was shown to induce a profound long-lasting enhancement in the strength of excitatory and inhibitory synapses of pyramidal neurons in the piriform cortex. Notably, such enhancement was mostly pronounced in a sub-group of neurons, entailing about a quarter of the cell population. Here we first show that the prominent enhancement in the subset of cells is due to a process in which all excitatory synapses doubled their strength and that this increase was mediated by a single process in which the AMPA channel conductance was doubled. Moreover, using a neuronal-network model, we show how such a multiplicative whole-cell synaptic strengthening in a sub-group of cells that form a memory pattern, sub-serves a profound selective enhancement of this memory. Network modeling further predicts that synaptic inhibition should be modified by complex learning in a manner that much resembles synaptic excitation. Indeed, in a subset of neurons all GABA_A-receptors mediated inhibitory synapses also doubled their strength after learning. Like synaptic excitation, Synaptic inhibition is also enhanced by two-fold increase of the single channel conductance. These findings suggest that crucial learning induces a multiplicative increase in strength of all excitatory and inhibitory synapses in a subset of cells, and that such an increase can serve as a long-term whole-cell mechanism to profoundly enhance an existing Hebbian-type memory. This mechanism does not act as synaptic plasticity mechanism that underlies memory formation but rather enhances the response of already existing memory. This mechanism is cell-specific rather than synapse-specific; it modifies the channel conductance rather than the number of channels and thus has the potential to be readily induced and un-induced by whole-cell transduction mechanisms.

PKCα is required for inflammation-induced trafficking of extrasynaptic AMPA receptors in tonically firing lamina II dorsal horn neurons during the maintenance of persistent inflammatory pain

Persistent inflammation promotes internalization of synaptic GluR2-containing, Ca(2+)-impermeable AMPA receptors (AMPARs) and insertion of GluR1-containing, Ca(2+)-permeable AMPARs at extrasynaptic sites in dorsal horn neurons. Previously we have shown that internalization of synaptic GluR2-containing AMPARs requires activation of spinal cord protein kinase C alpha (PKCα), but molecular mechanisms that underlie altered trafficking of extrasynaptic AMPARs are unclear. Here, using antisense (AS) oligodeoxynucleotides (ODN) that specifically knock down PKCα, we found that a decrease in dorsal horn PKCα expression prevents complete Freund's adjuvant (CFA)-induced increase in functional expression of extrasynaptic Ca(2+)-permeable AMPARs in substantia gelatinosa (SG) neurons of the rat spinal cord. Augmented AMPA-induced currents and associated [Ca(2+)](i) transients were abolished, and the current rectification 1 day post-CFA was reversed. These changes were observed specifically in SG neurons characterized by intrinsic tonic firing properties, but not in those that exhibited strong adaptation. Finally, dorsal horn PKCα knockdown produced an antinociceptive effect on CFA-induced thermal and mechanical hypersensitivity during the maintenance period of inflammatory pain, indicating a role for PKCα in persistent inflammatory pain maintenance. Our results indicate that inflammation-induced trafficking of extrasynaptic Ca(2+)-permeable AMPARs in tonically firing SG neurons depends on PKCα, and that this PKCα-dependent trafficking may contribute to persistent inflammatory pain maintenance.

PERSPECTIVE

This study shows that PKCα knockdown blocks inflammation-induced upregulation of extrasynaptic Ca(2+)-permeable AMPARs in dorsal horn neurons and produces an antinociceptive effect during the maintenance period of inflammatory pain. These findings have potential implications for use of PKCα gene-silencing therapy to prevent and/or treat persistent inflammatory pain.

GluA1 promotes the activity-dependent development of motor circuitry in the developing segmental spinal cord

The neuronal dendritic tree is a key determinant of how neurons receive, compute, and transmit information. During early postnatal life, synaptic activity promotes dendrite elaboration. Spinal motor neurons utilize GluA1-containing AMPA (2-amino-3-(3-hydroxy-5-methyl-isoxazol-4-yl) propanoic acid) receptors (AMPA-R) to control this process. This form of developmental dendrite growth can occur independently of N-methyl-d-aspartate receptors (NMDA-R). This review focuses on the mechanism by which the GluA1 subunit of AMPA-R transforms synaptic activity into dendrite growth, and describes the essential role of the GluA1 binding partner SAP97 (synapse-associated protein of 97 kDa molecular weight) in this process. This work defines a new mechanism of activity-dependent development, which might be harnessed to stimulate the recovery of function following insult to the central nervous system.

Mechanisms of CaMKII action in long-term potentiation

Long-term potentiation (LTP) of synaptic strength occurs during learning and can last for long periods, making it a probable mechanism for memory storage. LTP induction results in calcium entry, which activates calcium/calmodulin-dependent protein kinase II (CaMKII). CaMKII subsequently translocates to the synapse, where it binds to NMDA-type glutamate receptors and produces potentiation by phosphorylating principal and auxiliary subunits of AMPA-type glutamate receptors. These processes are all localized to stimulated spines and account for the synapse-specificity of LTP. In the later stages of LTP, CaMKII has a structural role in enlarging and strengthening the synapse.

Posttranslational regulation of AMPA receptor trafficking and function

In the mammalian central nervous system, the majority of fast excitatory synaptic transmission is mediated by glutamate acting on AMPA-type ionotropic glutamate receptors. The abundance of AMPA receptors at the synapse can be modulated through receptor trafficking, which dynamically regulates many fundamental brain functions, including learning and memory. Reversible posttranslational modifications, including phosphorylation, palmitoylation and ubiquitination of AMPA receptor subunits are important regulatory mechanisms for controlling synaptic AMPA receptor expression and function. In this review, we highlight recent advances in the study of AMPA receptor posttranslational modifications and discuss how these modifications regulate AMPA receptor trafficking and function at synapses.

AMPA receptor regulation during synaptic plasticity in hippocampus and neocortex

Discovery of long-term potentiation (LTP) in the dentate gyrus of the rabbit hippocampus by Bliss and Lømo opened up a whole new field to study activity-dependent long-term synaptic modifications in the brain. Since then hippocampal synapses have been a key model system to study the mechanisms of different forms of synaptic plasticity. At least for the postsynaptic forms of LTP and long-term depression (LTD), regulation of AMPA receptors (AMPARs) has emerged as a key mechanism. While many of the synaptic plasticity mechanisms uncovered in at the hippocampal synapses apply to synapses across diverse brain regions, there are differences in the mechanisms that often reveal the specific functional requirements of the brain area under study. Here we will review AMPAR regulation underlying synaptic plasticity in hippocampus and neocortex. The main focus of this review will be placed on postsynaptic forms of synaptic plasticity that impinge on the regulation of AMPARs using hippocampal CA1 and primary sensory cortices as examples. And through the comparison, we will highlight the key similarities and functional differences between the two synapses.

Decreasing temperature shifts hippocampal function from memory formation to modulation of hibernation bout duration in Syrian hamsters

Previous studies in hibernating species have characterized two forms of neural plasticity in the hippocampus, long-term potentiation (LTP) and its reversal, depotentiation, but not de novo long-term depression (LTD), which is also associated with memory formation. Studies have also shown that histamine injected into the hippocampus prolonged hibernation bout duration. However, spillover into the ventricles may have affected brain stem regions, not the hippocampus. Here, we tested the hypothesis that decreased brain temperature shifts the major function of the hippocampus in the Syrian hamster (Mesocricetus auratus) from one of memory formation (via LTP, depotentiation, and de novo LTD) to increasing hibernation bout duration. We found reduced evoked responses in hippocampal CA1 pyramidal neurons following low-frequency stimulation in young (<30 days old) and adult (>60 days old) hamsters, indicating that de novo LTD was generated in hippocampal slices from both pups and adults at temperatures >20°C. However, at temperatures below 20°C, synchronization of neural assemblies (a requirement for LTD generation) was markedly degraded, implying that de novo LTD cannot be generated in hibernating hamsters. Nonetheless, even at temperatures below 16°C, pyramidal neurons could still generate action potentials that may traverse a neural pathway, suppressing the ascending arousal system (ARS). In addition, histamine increased the excitability of these pyramidal cells. Taken together, these findings are consistent with the hypothesis that hippocampal circuits remain operational at low brain temperatures in Syrian hamsters and suppress the ARS to prolong bout duration, even though memory formation is muted at these low temperatures.

Mechanism of Ca2+/calmodulin-dependent kinase II regulation of AMPA receptor gating

The function, trafficking and synaptic signalling of AMPA receptors are tightly regulated by phosphorylation. CaMKII phosphorylates the GluA1 AMPA subunit at Ser831 to increase single channel conductance. We show for the first time that CaMKII increases the conductance of native heteromeric AMPA receptors in mouse hippocampal neurons via phosphorylation of Ser831. In addition, co-expression of TARPs with recombinant receptors is required for phosphoSer831 to increase conductance of heteromeric GluA1/GluA2 receptors. Finally, phosphorylation of Ser831 increases the efficiency with which each subunit can activate, independent of agonist efficacy, thereby increasing the likelihood that more receptor subunits will be simultaneously activated during gating. This underlies the observation that phosphoSer831 increases the frequency of openings to larger conductance levels rather than altering unitary conductance. Together, these findings suggest that CaMKII phosphorylation of GluA1-Ser831 decreases the activation energy for an intrasubunit conformational change that regulates the conductance of the receptor when the channel pore opens.

Long-term depression in the CNS

Long-term depression (LTD) in the CNS has been the subject of intense investigation as a process that may be involved in learning and memory and in various pathological conditions. Several mechanistically distinct forms of this type of synaptic plasticity have been identified and their molecular mechanisms are starting to be unravelled. Most studies have focused on forms of LTD that are triggered by synaptic activation of either NMDARs (N-methyl-D-aspartate receptors) or metabotropic glutamate receptors (mGluRs). Converging evidence supports a crucial role of LTD in some types of learning and memory and in situations in which cognitive demands require a flexible response. In addition, LTD may underlie the cognitive effects of acute stress, the addictive potential of some drugs of abuse and the elimination of synapses in neurodegenerative diseases.

Regulation of hippocampal long-term potentiation and long-term depression by diacylglycerol kinase ζ

Diacylglycerol (DAG) is an important signaling molecule at neuronal synapses. Generation of synaptic DAG is triggered by the activation of diverse surface receptors including N-methyl-D-aspartate (NMDA) receptors and metabotropic glutamate receptors. The action of DAG is terminated by enzymatic conversion of DAG to phosphatidic acid (PA) by DAG kinases (DGKs). DGKζ, one of many mammalian DGKs, is localized to synapses through direct interaction with the postsynaptic scaffolding protein PSD-95, and regulates dendritic spine maintenance by promoting DAG-to-PA conversion. However, a role for DGKζ in the regulation of synaptic plasticity has not been explored. We report here that Schaffer collateral-CA1 pyramidal synapses in the hippocampus of DGKζ-knockout (DGKζ(-/-) ) mice show enhanced long-term potentiation (LTP) and attenuated long-term depression (LTD). The attenuated LTD at DGKζ(-/-) synapses involves both NMDA receptors and metabotropic glutamate receptors. These changes in LTP and LTD were reversed by phospholipase C inhibition, which blocks DAG production. Similar reversals in both LTP and LTD were also induced by inhibition of protein kinase C, which acts downstream of DAG. These results suggest that DGKζ regulates hippocampal LTP and LTD by promoting DAG-to-PA conversion, and establish that phospholipase C and protein kinase C lie upstream and downstream, respectively, of DGKζ-dependent regulation of hippocampal LTP and LTD.

Glutamate receptor ion channels: structure, regulation, and function

The mammalian ionotropic glutamate receptor family encodes 18 gene products that coassemble to form ligand-gated ion channels containing an agonist recognition site, a transmembrane ion permeation pathway, and gating elements that couple agonist-induced conformational changes to the opening or closing of the permeation pore. Glutamate receptors mediate fast excitatory synaptic transmission in the central nervous system and are localized on neuronal and non-neuronal cells. These receptors regulate a broad spectrum of processes in the brain, spinal cord, retina, and peripheral nervous system. Glutamate receptors are postulated to play important roles in numerous neurological diseases and have attracted intense scrutiny. The description of glutamate receptor structure, including its transmembrane elements, reveals a complex assembly of multiple semiautonomous extracellular domains linked to a pore-forming element with striking resemblance to an inverted potassium channel. In this review we discuss International Union of Basic and Clinical Pharmacology glutamate receptor nomenclature, structure, assembly, accessory subunits, interacting proteins, gene expression and translation, post-translational modifications, agonist and antagonist pharmacology, allosteric modulation, mechanisms of gating and permeation, roles in normal physiological function, as well as the potential therapeutic use of pharmacological agents acting at glutamate receptors.

Calcium/calmodulin-dependent protein kinase II mediates hippocampal glutamatergic plasticity during benzodiazepine withdrawal

Benzodiazepine withdrawal anxiety is associated with potentiation of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate receptor (AMPAR) currents in hippocampal CA1 pyramidal neurons attributable to increased synaptic incorporation of GluA1-containing AMPARs. The contribution of calcium/calmodulin-dependent protein kinase II (CaMKII) to enhanced glutamatergic synaptic strength during withdrawal from 1-week oral flurazepam (FZP) administration was further examined in hippocampal slices. As earlier reported, AMPAR-mediated miniature excitatory postsynaptic current (mEPSC) amplitude increased in CA1 neurons from 1- and 2-day FZP-withdrawn rats, along with increased single-channel conductance in neurons from 2-day rats, estimated by non-stationary noise analysis. Input-output curve slope was increased without a change in paired-pulse facilitation, suggesting increased AMPAR postsynaptic efficacy rather than altered glutamate release. The increased mEPSC amplitude and AMPAR conductance were related to CaMKII activity, as intracellular inclusion of CaMKIINtide or autocamtide-2-related inhibitory peptide, but not scrambled peptide, prevented both AMPAR amplitude and conductance changes. mEPSC inhibition by 1-naphthyl acetyl spermine and the negative shift in rectification index at both withdrawal time points were consistent with functional incorporation of GluA2-lacking AMPARs. GluA1 but not GluA2 or GluA3 levels were increased in immunoblots of postsynaptic density (PSD)-enriched subcellular fractions of CA1 minislices from 1-day FZP-withdrawn rats, when mEPSC amplitude, but not conductance, was increased. Both GluA1 expression levels and CaMKII alpha-mediated GluA1 Ser(831) phosphorylation were increased in PSD-subfractions from 2-day FZP-withdrawn rats. As phospho-Thr(286)CaMKII alpha was unchanged, CaMKII alpha may be activated through an alternative signaling pathway. Synaptic insertion and subsequent CaMKII alpha-mediated Ser(831) phosphorylation of GluA1 homomers contribute to benzodiazepine withdrawal-induced AMPAR potentiation and may represent an important hippocampal pathway mediating both drug-induced and activity-dependent plasticity.

The function of GluR1 and GluR2 in cerebellar and hippocampal LTP and LTD is regulated by interplay of phosphorylation and O-GlcNAc modification

Long-term potentiation (LTP) and long-term depression (LTD) are the current models of synaptic plasticity and widely believed to explain how different kinds of memory are stored in different brain regions. Induction of LTP and LTD in different regions of brain undoubtedly involve trafficking of AMPA receptor to and from synapses. Hippocampal LTP involves phosphorylation of GluR1 subunit of AMPA receptor and its delivery to synapse whereas; LTD is the result of dephosphorylation and endocytosis of GluR1 containing AMPA receptor. Conversely the cerebellar LTD is maintained by the phosphorylation of GluR2 which promotes receptor endocytosis while dephosphorylation of GluR2 triggers receptor expression at the cell surface and results in LTP. The interplay of phosphorylation and O-GlcNAc modification is known as functional switch in many neuronal proteins. In this study it is hypothesized that a same phenomenon underlies as LTD and LTP switching, by predicting the potential of different Ser/Thr residues for phosphorylation, O-GlcNAc modification and their possible interplay. We suggest the involvement of O-GlcNAc modification of dephosphorylated GluR1 in maintaining the hippocampal LTD and that of dephosphorylated GluR2 in cerebral LTP.

Experimental and computational aspects of signaling mechanisms of spike-timing-dependent plasticity

STDP (spike-timing-dependent synaptic plasticity) is thought to be a synaptic learning rule that embeds spike-timing information into a specific pattern of synaptic strengths in neuronal circuits, resulting in a memory. STDP consists of bidirectional long-term changes in synaptic strengths. This process includes long-term potentiation and long-term depression, which are dependent on the timing of presynaptic and postsynaptic spikings. In this review, we focus on computational aspects of signaling mechanisms that induce and maintain STDP as a key step toward the definition of a general synaptic learning rule. In addition, we discuss the temporal and spatial aspects of STDP, and the requirement of a homeostatic mechanism of STDP in vivo.

Delivery of AMPA receptors to perisynaptic sites precedes the full expression of long-term potentiation

Trafficking of AMPA subtype glutamate receptors (AMPARs) from intracellular compartments to synapses is thought to be a major mechanism underlying the expression of long-term potentiation (LTP), a cellular substrate for learning and memory. However, it remains unclear whether the AMPAR trafficking that takes place during LTP is due to a targeted insertion of AMPARs directly into the synapse or delivery to extrasynaptic sites followed by translocation into the synapse. Here, we provide direct physiological evidence that LTP induced by a theta-burst pairing and tetanic stimulation protocols causes the rapid delivery of AMPARs to a perisynaptic site. Perisynaptic AMPARs do not normally detect synaptically released glutamate but can do so when the glial glutamate transporter EAAT1 is inhibited. AMPARs can be detected at this perisynaptic site before, but not after, the full expression of LTP. The appearance of perisynaptic AMPARs requires postsynaptic exocytosis, PKA signaling, and the C-terminal region of GluR1 subunit of AMPARs but not actin polymerization. Actin polymerization after LTP induction is required to retain AMPARs at the perisynaptic site after their appearance. Low-frequency stimulation given shortly after LTP induction leads to activity-dependent removal of perisynaptic AMPARs and suppresses the subsequent expression of LTP. These results demonstrate that AMPARs are rapidly trafficked to perisynaptic sites shortly after LTP induction and suggest that the delivery and maintenance of perisynaptic AMPARs may serve as a checkpoint in the expression of LTP.

Recruitment of calcium-permeable AMPA receptors during synaptic potentiation is regulated by CaM-kinase I

Ca(2+)-permeable AMPA receptors (CP-AMPARs) at central glutamatergic synapses are of special interest because of their unique biophysical and signaling properties that contribute to synaptic plasticity and their roles in multiple neuropathologies. However, intracellular signaling pathways that recruit synaptic CP-AMPARs are unknown, and involvement of CP-AMPARs in hippocampal region CA1 synaptic plasticity is controversial. Here, we report that intracellular infusion of active CaM-kinase I (CaMKI) into cultured hippocampal neurons enhances miniature EPSC amplitude because of recruitment of CP-AMPARs, likely from an extrasynaptic pool. The ability of CaMKI, which regulates the actin cytoskeleton, to recruit synaptic CP-AMPARs was blocked by inhibiting actin polymerization with latrunculin A. CaMK regulation of CP-AMPARs was also confirmed in hippocampal slices. CA1 long-term potentiation (LTP) after theta bursts, but not high-frequency tetani, produced a rapid, transient expression of synaptic CP-AMPARs that facilitated LTP. This component of TBS LTP was blocked by inhibition of CaM-kinase kinase (CaMKK), the upstream activator of CaMKI. Our calculations show that adding CP-AMPARs numbering <5% of existing synaptic AMPARs is sufficient to account for the potentiation observed in LTP. Thus, synaptic expression of CP-AMPARs is a very efficient mechanism for rapid enhancement of synaptic strength that depends on CaMKK/CaMKI signaling, actin dynamics, and the pattern of synaptic activity used to induce CA1 LTP.

AKAP79 selectively enhances protein kinase C regulation of GluR1 at a Ca2+-calmodulin-dependent protein kinase II/protein kinase C site

Enhancement of AMPA receptor activity in response to synaptic plasticity inducing stimuli may arise, in part, through phosphorylation of the GluR1 AMPA receptor subunit at Ser-831. This site is a substrate for both Ca(2+)-calmodulin-dependent protein kinase II (CaMKII) and protein kinase C (PKC). However, neuronal protein levels of CaMKII may exceed those of PKC by an order of magnitude. Thus, it is unclear how PKC could effectively regulate this common target site. The multivalent neuronal scaffold A-kinase-anchoring protein 79 (AKAP79) is known to bind PKC and is linked to GluR1 by synapse-associated protein 97 (SAP97). Here, biochemical studies demonstrate that AKAP79 localizes PKC activity near the receptor, thus accelerating Ser-831 phosphorylation. Complementary electrophysiological studies indicate that AKAP79 selectively shifts the dose-dependence for PKC modulation of GluR1 receptor currents approximately 20-fold, such that low concentrations of PKC are as effective as much higher CaMKII concentrations. By boosting PKC activity near a target substrate, AKAP79 provides a mechanism to overcome limitations in kinase abundance thereby ensuring faithful signal propagation and efficient modification of AMPA receptor-mediated responses.

NMDA receptor activation dephosphorylates AMPA receptor glutamate receptor 1 subunits at threonine 840

Phosphorylation-dependent changes in AMPA receptor function have a crucial role in activity-dependent forms of synaptic plasticity such as long-term potentiation (LTP) and long-term depression (LTD). Although three previously identified phosphorylation sites in AMPA receptor glutamate receptor 1 (GluR1) subunits (S818, S831, and S845) appear to have important roles in LTP and LTD, little is known about the role of other putative phosphorylation sites in GluR1. Here, we describe the characterization of a recently identified phosphorylation site in GluR1 at threonine 840. The results of in vivo and in vitro phosphorylation assays suggest that T840 is not a substrate for protein kinases known to phosphorylate GluR1 at previously identified phosphorylation sites, such as protein kinase A, protein kinase C, and calcium/calmodulin-dependent kinase II. Instead, in vitro phosphorylation assays suggest that T840 is a substrate for p70S6 kinase. Although LTP-inducing patterns of synaptic stimulation had no effect on GluR1 phosphorylation at T840 in the hippocampal CA1 region, bath application of NMDA induced a strong, protein phosphatase 1- and/or 2A-mediated decrease in T840 phosphorylation. Moreover, GluR1 phosphorylation at T840 was transiently decreased by a chemical LTD induction protocol that induced a short-term depression of synaptic strength and persistently decreased by a chemical LTD induction protocol that induced a lasting depression of synaptic transmission. Together, our results show that GluR1 phosphorylation at T840 is regulated by NMDA receptor activation and suggest that decreases in GluR1 phosphorylation at T840 may have a role in LTD.

Spike timing-dependent plasticity: a learning rule for dendritic integration in rat CA1 pyramidal neurons

Long-term plasticity of dendritic integration is induced in parallel with long-term potentiation (LTP) or depression (LTD) based on presynaptic activity patterns. It is, however, not clear whether synaptic plasticity induced by temporal pairing of pre- and postsynaptic activity is also associated with synergistic modification in dendritic integration. We show here that the spike timing-dependent plasticity (STDP) rule accounts for long-term changes in dendritic integration in CA1 pyramidal neurons in vitro. Positively correlated pre- and postsynaptic activity (delay: +5/+50 ms) induced LTP and facilitated dendritic integration. Negatively correlated activity (delay: -5/-50 ms) induced LTD and depressed dendritic integration. These changes were not observed following positive or negative pairing with long delays (> +/-50 ms) or when NMDA receptors were blocked. The amplitude-slope relation of the EPSP was facilitated after LTP and depressed after LTD. These effects could be mimicked by voltage-gated channel blockers, suggesting that the induced changes in EPSP waveform involve the regulation of voltage-gated channel activity. Importantly, amplitude-slope changes induced by STDP were found to be input specific, indicating that the underlying changes in excitability are restricted to a limited portion of the dendrites. We conclude that STDP is a common learning rule for long-term plasticity of both synaptic transmission and dendritic integration, thus constituting a form of functional redundancy that insures significant changes in the neuronal output when synaptic plasticity is induced.

Aging effects on the limits and stability of long-term synaptic potentiation and depression in rat hippocampal area CA1

Altered hippocampal synaptic plasticity may underlie age-related memory impairment. In acute hippocampal slices from aged (22-24 mo) and young adult (1-12 mo) male Brown Norway rats, extracellular excitatory postsynaptic field potentials were recorded in CA1 stratum radiatum evoked by Schaffer collateral stimulation. We used enhanced Ca(2+) to Mg(2+) ratio and paired-pulse stimulation protocol to induce maximum changes in the synaptic plasticity. Six episodes of theta-burst stimulation (TBS) or nine episodes of paired low-frequency stimulation (pLFS) were used to generate asymptotic long-term potentiation (LTP) and long-term depression (LTD), respectively. In addition, long-term depotentiation (LTdeP) or de-depression (LTdeD) from maximal LTP and LTD were examined using two episodes of pLFS or TBS. Multiple episodes of TBS or pLFS produced significant LTP or LTD in aged and young adult rats; this was not different between age groups. Moreover, there was no significant difference in the amount of LTdeP or LTdeD between aged and young adult rats. Our results show no age differences in the asymptotic magnitude of LTP or LTD, rate of synaptic modifications, development rates, reversal, or decay after postconditioning. Thus impairment of the basic synaptic mechanisms responsible for expression of these forms of plasticity is not likely to account for decline in memory function within this age range.

A new principle for information storage in an enzymatic pathway model

Strong experimental evidence indicates that protein kinase and phosphatase (KP) cycles are critical to both the induction and maintenance of activity-dependent modifications in neurons. However, their contribution to information storage remains controversial, despite impressive modeling efforts. For instance, plasticity models based on KP cycles do not account for the maintenance of plastic modifications. Moreover, bistable KP cycle models that display memory fail to capture essential features of information storage: rapid onset, bidirectional control, graded amplitude, and finite lifetimes. Here, we show in a biophysical model that upstream activation of KP cycles, a ubiquitous mechanism, is sufficient to provide information storage with realistic induction and maintenance properties: plastic modifications are rapid, bidirectional, and graded, with finite lifetimes that are compatible with animal and human memory. The maintenance of plastic modifications relies on negligible reaction rates in basal conditions and thus depends on enzyme nonlinearity and activation properties of the activity-dependent KP cycle. Moreover, we show that information coding and memory maintenance are robust to stochastic fluctuations inherent to the molecular nature of activity-dependent KP cycle operation. This model provides a new principle for information storage where plasticity and memory emerge from a single dynamic process whose rate is controlled by neuronal activity. This principle strongly departs from the long-standing view that memory reflects stable steady states in biological systems, and offers a new perspective on memory in animals and humans.

Regulatory mechanisms of AMPA receptors in synaptic plasticity

Activity-dependent changes in the strength of excitatory synapses are a cellular mechanism for the plasticity of neuronal networks that is widely recognized to underlie cognitive functions such as learning and memory. AMPA (alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid)-type glutamate receptors (AMPARs) are the main transducers of rapid excitatory transmission in the mammalian CNS, and recent discoveries indicate that the mechanisms which regulate AMPARs are more complex than previously thought. This review focuses on recent evidence that alterations to AMPAR functional properties are coupled to their trafficking, cytoskeletal dynamics and local protein synthesis. These relationships offer new insights into the regulation of AMPARs and synaptic strength by cellular signalling.

A unified model of the presynaptic and postsynaptic changes during LTP at CA1 synapses

Long-term potentiation (LTP) has been studied extensively at CA1 synapses of the hippocampus, and there is evidence implicating both postsynaptic and presynaptic changes in this process. These changes include (i) addition of AMPA channels to the extrasynaptic membrane and diffusional equilibrium of extrasynaptic receptors with synaptic receptors, (ii) sudden addition of AMPA channels to the synapse in large groups, (iii) a change in the mode of glutamate release (presumably from kiss-and-run to full fusion), and (iv) a delayed increase in the number of vesicles released. However, it remains unclear whether (or how) these changes work together. We have incorporated all of these processes into a structural model of the synapse. We propose that the synapse is composed of transsynaptic modules that function quasi-independently in AMPA-mediated transmission. Under basal conditions, synapses are partially silent; some modules are AMPA-silent (but contribute to NMDA-mediated transmission), whereas others are functional (and contribute to both AMPA- and NMDA-mediated transmission). During LTP, there is both a rapid change in the mode of vesicle fusion and a rapid insertion of a postsynaptic complex (a hyperslot) containing many proteins (slots) capable of binding AMPA channels. The combined effect of these pre- and postsynaptic changes is to convert AMPA-silent modules into functional modules. Slot filling is transiently enhanced by a rapid increase in extrasynaptic GluR1, a form of the AMPA-type receptor. A slower transsynaptic growth process adds AMPA-silent modules to the synapse, enhancing the number of vesicles released and thereby enhancing the NMDA response. This model accounts for a broad range of data, including the LTP-induced changes in quantal parameters. The model also provides a coherent explanation for the diverse effects of GluR1 knockout on basal transmission, LTP, and distance-dependent scaling.

Synaptic plasticity and phosphorylation

A number of neuronal functions, including synaptic plasticity, depend on proper regulation of synaptic proteins, many of which can be rapidly regulated by phosphorylation. Neuronal activity controls the function of these synaptic proteins by exquisitely regulating the balance of various protein kinase and protein phosphatase activity. Recent understanding of synaptic plasticity mechanisms underscores important roles that these synaptic phosphoproteins play in regulating both pre- and post-synaptic functions. This review will focus on key postsynaptic phosphoproteins that have been implicated to play a role in synaptic plasticity.

Synaptic Incorporation of AMPA Receptors during LTP Is Controlled by a PKC Phosphorylation Site on GluR1

Incorporation of GluR1-containing AMPA receptors into synapses is essential to several forms of neural plasticity, including long-term potentiation (LTP). Numerous signaling pathways that trigger this process have been identified, but the direct modifications of GluR1 that control its incorporation into synapses are unclear. Here, we show that phosphorylation of GluR1 by PKC at a highly conserved serine 818 residue is increased during LTP and critical for LTP expression. GluR1 is phosphorylated by PKC at this site in vitro and in vivo. In addition, acute phosphorylation at GluR1 S818 by PKC, as well as a phosphomimetic mutation, promotes GluR1 synaptic incorporation. Conversely, preventing GluR1 S818 phosphorylation reduces LTP and blocks PKC-driven synaptic incorporation of GluR1. We conclude that the phosphorylation of GluR1 S818 by PKC is a critical event in the plasticity-driven synaptic incorporation of AMPA receptors.

Temperature modifies potentiation but not depotentiation in bidirectional hippocampal plasticity of Syrian hamsters (Mesocricetus auratus)

Previous studies have shown that one form of neuroplasticity, population spike (PS) potentiation, can be established in the hamster hippocampus at temperatures above 20 degrees C. Here, we tested three related hypotheses; namely, that in Syrian hamsters: (1) PS potentiation can be elicited below 20 degrees C and that at any constant temperature, potentiation can be described by a pair of sigmoidal functions matched to input/output curves; (2) potentiation can be partially reversed by depotentiation (a second and distinctive form of neuroplasticity); and (3) tetanus evokes long-term potentiation in slices from animals housed under conditions corresponding to various stages of the annual hibernation cycle. To test these hypotheses, we measured PS amplitudes and fEPSP slopes in CA1 pyramidal cells in hippocampal slices. We found that sigmoidal functions before and after tetanus showed PS enhancement at 18 degrees C and a larger enhancement at 28 degrees C, thereby supporting hypothesis 1. We also found that low-frequency stimulation reduced the amplitude of the potentiated PS by approximately 29% at both 18 degrees C and 28 degrees C, consistent with hypothesis 2; and that slices from nonhibernating hamsters on long and short photoperiods and from hamsters in hibernation all showed at least 40% increases in fEPSP slope following tetanus at a slice temperature of 23 degrees C, supporting hypothesis 3. Thus, bidirectional plasticity is present in hamsters. That is, both potentiation and depotentiation were readily evoked at 28 degrees C; potentiation was muted, while depotentiation (the reversal of the potentiation) remained robust at 18 degrees C. Moreover, potentiated responses could be elicited in slices from animals housed under diverse conditions.

Developmental changes in AMPA and kainate receptor-mediated quantal transmission at thalamocortical synapses in the barrel cortex

During the first week of life, there is a shift from kainate to AMPA receptor-mediated thalamocortical transmission in layer IV barrel cortex. However, the mechanisms underlying this change and the differential properties of AMPA and kainate receptor-mediated transmission remain essentially unexplored. To investigate this, we studied the quantal properties of AMPA and kainate receptor-mediated transmission using strontium-evoked miniature EPSCs. AMPA and kainate receptor-mediated transmission exhibited very different quantal properties but were never coactivated by a single quantum of transmitter, indicating complete segregation to different synapses within the thalamocortical input. Nonstationary fluctuation analysis showed that synaptic AMPA receptors exhibited a range of single-channel conductance (gamma) and a strong negative correlation between gamma and functional channel number, indicating that these two parameters are reciprocally regulated at thalamocortical synapses. We obtained the first estimate of gamma for synaptic kainate receptors (<2 pS), and this primarily accounted for the small quantal size of kainate receptor-mediated transmission. Developmentally, the quantal contribution to transmission of AMPA receptors increased and that of kainate receptors decreased. No changes in AMPA or kainate quantal amplitude or in AMPA receptor gamma were observed, demonstrating that the developmental change was attributable to a decrease in the number of kainate synapses and an increase in the number of AMPA synapses contributing to transmission. Therefore, we demonstrate fundamental differences in the quantal properties for these two types of synapse. Thus, the developmental switch in transmission will dramatically alter information transfer at thalamocortical inputs to layer IV.