Dicer and eIF2c are enriched at postsynaptic densities in adult mouse brain and are modified by neuronal activity in a calpain-dependent manner

We have hypothesized that small RNAs may participate in learning and memory mechanisms. Because dendritic spines are important in synaptic plasticity and learning, we asked whether dicer, the rate-limiting enzyme in the formation of small RNAs, is enriched within dendritic spines. In adult mouse brain, dicer and the RNA-induced silencing complex (RISC) component eIF2c were expressed in the somatodendritic compartment of principal neurons and some interneurons in many regions, and dicer was enriched in dendritic spines and postsynaptic densities (PSDs). A portion of dicer and eIF2c were associated with each other and with fragile X mental retardation protein (FMRP), as assessed by co-immunoprecipitation. Calpain I treatment of recombinant dicer or immunopurified brain dicer caused a marked increase in RNAse III activity. Purified PSDs did not exhibit RNAse III activity, but calpain caused release of dicer from PSDs in an enzymatically active form, together with eIF2c. NMDA stimulation of hippocampal slices, or calcium treatment of synaptoneurosomes, caused a 75 kDa dicer fragment to appear in a calpain-dependent manner. The findings support a model whereby acute neuronal stimulation at excitatory synapses increases intracellular calcium, which activates calpain, which liberates dicer and eIF2c bound to PSDs. This supports the hypothesis that dicer could be involved in synaptic plasticity.     

Protein kinase D promotes plasticity-induced F-actin stabilization in dendritic spines and regulates memory formation

Actin turnover in dendritic spines influences spine development, morphology, and plasticity, with functional consequences on learning and memory formation. In nonneuronal cells, protein kinase D (PKD) has an important role in stabilizing F-actin via multiple molecular pathways. Using in vitro models of neuronal plasticity, such as glycine-induced chemical long-term potentiation (LTP), known to evoke synaptic plasticity, or long-term depolarization block by KCl, leading to homeostatic morphological changes, we show that actin stabilization needed for the enlargement of dendritic spines is dependent on PKD activity. Consequently, impaired PKD functions attenuate activity-dependent changes in hippocampal dendritic spines, including LTP formation, cause morphological alterations in vivo, and have deleterious consequences on spatial memory formation. We thus provide compelling evidence that PKD controls synaptic plasticity and learning by regulating actin stability in dendritic spines.     

神经元的突触可塑性与学习和记忆

大量研究表明,神经元的突触可塑性包括功能可塑性和结构可塑性,与学习和记忆密切相关.最近,在经过训练的动物海马区,记录到了学习诱导的长时程增强(long term potentiation,LTP),如果用激酶抑制剂阻断晚期LTP,就会使大鼠丧失训练形成的记忆.这些结果指出,LTP可能是形成记忆的分子基础.因此,进一步研究哺乳动物脑内突触可塑性的分子机制,对揭示学习和记忆的神经基础有重要意义.此外,在精神迟滞性疾病和神经退行性疾病患者脑内记录到异常的LTP,并发现神经元的树突棘数量减少,形态上产生畸变或萎缩,同时发现,产生突变的基因大多编码调节突触可塑性的信号通路蛋白,故突触可塑性研究也将促进精神和神经疾病的预防和治疗.综述了突触可塑性研究的最新进展,并展望了其发展前景.     

alpha5 integrin signaling regulates the formation of spines and synapses in hippocampal neurons

The actin-based dynamics of dendritic spines play a key role in synaptic plasticity, which underlies learning and memory. Although it is becoming increasingly clear that modulation of actin is critical for spine dynamics, the upstream molecular signals that regulate the formation and plasticity of spines are poorly understood. In non-neuronal cells, integrins are critical modulators of the actin cytoskeleton, but their function in the nervous system is not well characterized. Here we show that alpha5 integrin regulates spine morphogenesis and synapse formation in hippocampal neurons. Knockdown of alpha5 integrin expression using small interfering RNA decreased the number of dendritic protrusions, spines, and synapses. Expression of constitutively active or dominant negative alpha5 integrin also resulted in alterations in the number of dendritic protrusions, spines, and synapses. alpha5 integrin signaling regulates spine morphogenesis and synapse formation by a mechanism that is dependent on Src kinase, Rac, and the signaling adaptor GIT1. Alterations in the activity or localization of these molecules result in a significant decrease in the number of spines and synapses. Thus, our results point to a critical role for integrin signaling in regulating the formation of dendritic spines and synapses in hippocampal neurons.     

Enhancement of Morphological Plasticity in Hippocampal Neurons by a Physically Modified Saline via Phosphatidylinositol-3 Kinase

Increase of the density of dendritic spines and enhancement of synaptic transmission through ionotropic glutamate receptors are important events, leading to synaptic plasticity and eventually hippocampus-dependent spatial learning and memory formation. Here we have undertaken an innovative approach to upregulate hippocampal plasticity. RNS60 is a 0.9% saline solution containing charge-stabilized nanobubbles that are generated by subjecting normal saline to Taylor-Couette-Poiseuille (TCP) flow under elevated oxygen pressure. RNS60, but not NS (normal saline), PNS60 (saline containing a comparable level of oxygen without the TCP modification), or RNS10.3 (TCP-modified normal saline without excess oxygen), stimulated morphological plasticity and synaptic transmission via NMDA- and AMPA-sensitive calcium influx in cultured mouse hippocampal neurons. Using mRNA-based targeted gene array, real-time PCR, immunoblot, and immunofluorescence analyses, we further demonstrate that RNS60 stimulated the expression of many plasticity-associated genes in cultured hippocampal neurons. Activation of type IA, but not type IB, phosphatidylinositol-3 (PI-3) kinase by RNS60 together with abrogation of RNS60-mediated upregulation of plasticity-related proteins (NR2A and GluR1) and increase in spine density, neuronal size, and calcium influx by {"type":"entrez-nucleotide","attrs":{"text":"LY294002","term_id":"1257998346","term_text":"LY294002"}}LY294002, a specific inhibitor of PI-3 kinase, suggest that RNS60 upregulates hippocampal plasticity via activation of PI-3 kinase. Finally, in the 5XFAD transgenic model of Alzheimer’s disease (AD), RNS60 treatment upregulated expression of plasticity-related proteins PSD95 and NR2A and increased AMPA- and NMDA-dependent hippocampal calcium influx. These results describe a novel property of RNS60 in stimulating hippocampal plasticity, which may help AD and other dementias.     

Dynamic microtubules promote synaptic NMDA receptor-dependent spine enlargement

Most excitatory synaptic terminals in the brain impinge on dendritic spines. We and others have recently shown that dynamic microtubules (MTs) enter spines from the dendritic shaft. However, a direct role for MTs in long-lasting spine plasticity has yet to be demonstrated and it remains unclear whether MT-spine invasions are directly influenced by synaptic activity. Lasting changes in spine morphology and synaptic strength can be triggered by activation of synaptic NMDA receptors (NMDARs) and are associated with learning and memory processes. To determine whether MTs are involved in NMDAR-dependent spine plasticity, we imaged MT dynamics and spine morphology in live mouse hippocampal pyramidal neurons before and after acute activation of synaptic NMDARs. Synaptic NMDAR activation promoted MT-spine invasions and lasting increases in spine size, with invaded spines exhibiting significantly faster and more growth than non-invaded spines. Even individual MT invasions triggered rapid increases in spine size that persisted longer following NMDAR activation. Inhibition of either NMDARs or dynamic MTs blocked NMDAR-dependent spine growth. Together these results demonstrate for the first time that MT-spine invasions are positively regulated by signaling through synaptic NMDARs, and contribute to long-lasting structural changes in targeted spines.     

High speed two-photon imaging of calcium dynamics in dendritic spines: consequences for spine calcium kinetics and buffer capacity

Rapid calcium concentration changes in postsynaptic structures are crucial for synaptic plasticity. Thus far, the determinants of postsynaptic calcium dynamics have been studied predominantly based on the decay kinetics of calcium transients. Calcium rise times in spines in response to single action potentials (AP) are almost never measured due to technical limitations, but they could be crucial for synaptic plasticity. With high-speed, precisely-targeted, two-photon point imaging we measured both calcium rise and decay kinetics in spines and secondary dendrites in neocortical pyramidal neurons. We found that both rise and decay kinetics of changes in calcium-indicator fluorescence are about twice as fast in spines. During AP trains, spine calcium changes follow each AP, but not in dendrites. Apart from the higher surface-to-volume ratio (SVR), we observed that neocortical dendritic spines have a markedly smaller endogenous buffer capacity with respect to their parental dendrites. Calcium influx time course and calcium extrusion rate were both in the same range for spines and dendrites when fitted with a dynamic multi-compartment model that included calcium binding kinetics and diffusion. In a subsequent analysis we used this model to investigate which parameters are critical determinants in spine calcium dynamics. The model confirmed the experimental findings: a higher SVR is not sufficient by itself to explain the faster rise time kinetics in spines, but only when paired with a lower buffer capacity in spines. Simulations at zero calcium-dye conditions show that calmodulin is more efficiently activated in spines, which indicates that spine morphology and buffering conditions in neocortical spines favor synaptic plasticity.     

Structural Plasticity Can Produce Metaplasticity

Background

Synaptic plasticity underlies many aspect of learning memory and development. The properties of synaptic plasticity can change as a function of previous plasticity and previous activation of synapses, a phenomenon called metaplasticity. Synaptic plasticity not only changes the functional connectivity between neurons but in some cases produces a structural change in synaptic spines; a change thought to form a basis for this observed plasticity. Here we examine to what extent structural plasticity of spines can be a cause for metaplasticity. This study is motivated by the observation that structural changes in spines are likely to affect the calcium dynamics in spines. Since calcium dynamics determine the sign and magnitude of synaptic plasticity, it is likely that structural plasticity will alter the properties of synaptic plasticity.

Methodology/Principal Findings

In this study we address the question how spine geometry and alterations of N-methyl-D-aspartic acid (NMDA) receptors conductance may affect plasticity. Based on a simplified model of the spine in combination with a calcium-dependent plasticity rule, we demonstrated that after the induction phase of plasticity a shift of the long term potentiation (LTP) or long term depression (LTD) threshold takes place. This induces a refractory period for further LTP induction and promotes depotentiation as observed experimentally. That resembles the BCM metaplasticity rule but specific for the individual synapse. In the second phase, alteration of the NMDA response may bring the synapse to a state such that further synaptic weight alterations are feasible. We show that if the enhancement of the NMDA response is proportional to the area of the post synaptic density (PSD) the plasticity curves most likely return to the initial state.

Conclusions/Significance

Using simulations of calcium dynamics in synaptic spines, coupled with a biophysically motivated calcium-dependent plasticity rule, we find under what conditions structural plasticity can form the basis of synapse specific metaplasticity.     

Investigating Sub-Spine Actin Dynamics in Rat Hippocampal Neurons with Super-Resolution Optical Imaging

Morphological changes in dendritic spines represent an important mechanism for synaptic plasticity which is postulated to underlie the vital cognitive phenomena of learning and memory. These morphological changes are driven by the dynamic actin cytoskeleton that is present in dendritic spines. The study of actin dynamics in these spines traditionally has been hindered by the small size of the spine. In this study, we utilize a photo-activation localization microscopy (PALM)–based single-molecule tracking technique to analyze F-actin movements with ∼30-nm resolution in cultured hippocampal neurons. We were able to observe the kinematic (physical motion of actin filaments, i.e., retrograde flow) and kinetic (F-actin turn-over) dynamics of F-actin at the single-filament level in dendritic spines. We found that F-actin in dendritic spines exhibits highly heterogeneous kinematic dynamics at the individual filament level, with simultaneous actin flows in both retrograde and anterograde directions. At the ensemble level, movements of filaments integrate into a net retrograde flow of ∼138 nm/min. These results suggest a weakly polarized F-actin network that consists of mostly short filaments in dendritic spines.     

Biphasic Synaptic Ca Influx Arising from Compartmentalized Electrical Signals in Dendritic Spines

Excitatory synapses on mammalian principal neurons are typically formed onto dendritic spines, which consist of a bulbous head separated from the parent dendrite by a thin neck. Although activation of voltage-gated channels in the spine and stimulus-evoked constriction of the spine neck can influence synaptic signals, the contribution of electrical filtering by the spine neck to basal synaptic transmission is largely unknown. Here we use spine and dendrite calcium (Ca) imaging combined with 2-photon laser photolysis of caged glutamate to assess the impact of electrical filtering imposed by the spine morphology on synaptic Ca transients. We find that in apical spines of CA1 hippocampal neurons, the spine neck creates a barrier to the propagation of current, which causes a voltage drop and results in spatially inhomogeneous activation of voltage-gated Ca channels (VGCCs) on a micron length scale. Furthermore, AMPA and NMDA-type glutamate receptors (AMPARs and NMDARs, respectively) that are colocalized on individual spine heads interact to produce two kinetically and mechanistically distinct phases of synaptically evoked Ca influx. Rapid depolarization of the spine triggers a brief and large Ca current whose amplitude is regulated in a graded manner by the number of open AMPARs and whose duration is terminated by the opening of small conductance Ca-activated potassium (SK) channels. A slower phase of Ca influx is independent of AMPAR opening and is determined by the number of open NMDARs and the post-stimulus potential in the spine. Biphasic synaptic Ca influx only occurs when AMPARs and NMDARs are coactive within an individual spine. These results demonstrate that the morphology of dendritic spines endows associated synapses with specialized modes of signaling and permits the graded and independent control of multiple phases of synaptic Ca influx.     

Effect of the Initial Synaptic State on the Probability to Induce Long-Term Potentiation and Depression

Long-term potentiation (LTP) and long-term depression (LTD) are the two major forms of long-lasting synaptic plasticity in the mammalian neurons, and are directly related to higher brain functions such as learning and memory. Experimentally, they are characterized by a change in the strength of a synaptic connection induced by repetitive and properly patterned stimulation protocols. Although many important details of the molecular events leading to LTP and LTD are known, experimenters often report problems in using standard induction protocols to obtain consistent results, especially for LTD in vivo. We hypothesize that a possible source of confusion in interpreting the results, from any given experiment on synaptic plasticity, can be the intrinsic limitation of the experimental techniques, which cannot take into account the actual state and peak conductance of the synapses before the conditioning protocol. In this article, we investigate the possibility that the same experimental protocol may result in different consequences (e.g., LTD instead of LTP), according to the initial conditions of the stimulated synapses, and can generate confusing results. Using biophysical models of synaptic plasticity and hippocampal CA1 pyramidal neurons, we study how, why, and to what extent the phenomena observed at the soma after induction of LTP/LTD reflects the actual (local) synaptic state. The model and the results suggest a physiologically plausible explanation for why LTD induction is experimentally difficult to obtain. They also suggest experimentally testable predictions on the stimulation protocols that may be more effective.     

Boosting of synaptic potentials and spine Ca transients by the peptide toxin SNX-482 requires alpha-1E-encoded voltage-gated Ca channels

The majority of glutamatergic synapses formed onto principal neurons of the mammalian central nervous system are associated with dendritic spines. Spines are tiny protuberances that house the proteins that mediate the response of the postsynaptic cell to the presynaptic release of glutamate. Postsynaptic signals are regulated by an ion channel signaling cascade that is active in individual dendritic spines and involves voltage-gated calcium (Ca) channels, small conductance (SK)-type Ca-activated potassium channels, and NMDA-type glutamate receptors. Pharmacological studies using the toxin SNX-482 indicated that the voltage-gated Ca channels that signal within spines to open SK channels belong to the class Ca(V)2.3, which is encoded by the Alpha-1E pore-forming subunit. In order to specifically test this conclusion, we examined the effects of SNX-482 on synaptic signals in acute hippocampal slices from knock-out mice lacking the Alpha-1E gene. We find that in these mice, application of SNX-482 has no effect on glutamate-uncaging evoked synaptic potentials and Ca influx, indicating that that SNX-482 indeed acts via the Alpha-1E-encoded Ca(V)2.3 channel.     

Actin in dendritic spines: connecting dynamics to function

Dendritic spines are small actin-rich protrusions from neuronal dendrites that form the postsynaptic part of most excitatory synapses and are major sites of information processing and storage in the brain. Changes in the shape and size of dendritic spines are correlated with the strength of excitatory synaptic connections and heavily depend on remodeling of its underlying actin cytoskeleton. Emerging evidence suggests that most signaling pathways linking synaptic activity to spine morphology influence local actin dynamics. Therefore, specific mechanisms of actin regulation are integral to the formation, maturation, and plasticity of dendritic spines and to learning and memory.     

A Calcium-Dependent Plasticity Rule for HCN Channels Maintains Activity Homeostasis and Stable Synaptic Learning

Theoretical and computational frameworks for synaptic plasticity and learning have a long and cherished history, with few parallels within the well-established literature for plasticity of voltage-gated ion channels. In this study, we derive rules for plasticity in the hyperpolarization-activated cyclic nucleotide-gated (HCN) channels, and assess the synergy between synaptic and HCN channel plasticity in establishing stability during synaptic learning. To do this, we employ a conductance-based model for the hippocampal pyramidal neuron, and incorporate synaptic plasticity through the well-established Bienenstock-Cooper-Munro (BCM)-like rule for synaptic plasticity, wherein the direction and strength of the plasticity is dependent on the concentration of calcium influx. Under this framework, we derive a rule for HCN channel plasticity to establish homeostasis in synaptically-driven firing rate, and incorporate such plasticity into our model. In demonstrating that this rule for HCN channel plasticity helps maintain firing rate homeostasis after bidirectional synaptic plasticity, we observe a linear relationship between synaptic plasticity and HCN channel plasticity for maintaining firing rate homeostasis. Motivated by this linear relationship, we derive a calcium-dependent rule for HCN-channel plasticity, and demonstrate that firing rate homeostasis is maintained in the face of synaptic plasticity when moderate and high levels of cytosolic calcium influx induced depression and potentiation of the HCN-channel conductance, respectively. Additionally, we show that such synergy between synaptic and HCN-channel plasticity enhances the stability of synaptic learning through metaplasticity in the BCM-like synaptic plasticity profile. Finally, we demonstrate that the synergistic interaction between synaptic and HCN-channel plasticity preserves robustness of information transfer across the neuron under a rate-coding schema. Our results establish specific physiological roles for experimentally observed plasticity in HCN channels accompanying synaptic plasticity in hippocampal neurons, and uncover potential links between HCN-channel plasticity and calcium influx, dynamic gain control and stable synaptic learning.     

Loss of the actin regulator cyclase-associated protein 1 (CAP1) modestly affects dendritic spine remodeling during synaptic plasticity

Dendritic spines form the postsynaptic compartment of most excitatory synapses in the vertebrate brain. Morphological changes of dendritic spines contribute to major forms of synaptic plasticity such as long-term potentiation (LTP) or depression (LTD). Synaptic plasticity underlies learning and memory, and defects in synaptic plasticity contribute to the pathogeneses of human brain disorders. Hence, deciphering the molecules that drive spine remodeling during synaptic plasticity is critical for understanding the neuronal basis of physiological and pathological brain function. Since actin filaments (F-actin) define dendritic spine morphology, actin-binding proteins (ABP) that accelerate dis-/assembly of F-actin moved into the focus as critical regulators of synaptic plasticity. We recently identified cyclase-associated protein 1 (CAP1) as a novel actin regulator in neurons that cooperates with cofilin1, an ABP relevant for synaptic plasticity. We therefore hypothesized a crucial role for CAP1 in structural synaptic plasticity. By exploiting mouse hippocampal neurons, we tested this hypothesis in the present study. We found that induction of both forms of synaptic plasticity oppositely altered concentration of exogenous, myc-tagged CAP1 in dendritic spines, with chemical LTP (cLTP) decreasing and chemical LTD (cLTD) increasing it. cLTP induced spine enlargement in CAP1-deficient neurons. However, it did not increase the density of large spines, different from control neurons. cLTD induced spine retraction and spine size reduction in control neurons, but not in CAP1-KO neurons. Together, we report that postsynaptic myc-CAP1 concentration oppositely changed during cLTP and cTLD and that CAP1 inactivation modestly affected structural plasticity.     

Morphological constraints on calcium dependent glutamate receptor trafficking into individual dendritic spine

Glutamate receptor trafficking into dendritic spines is a pivotal step in synaptic plasticity, yet the relevance of plasticity-producing rise of [Ca2+]i and of spine morphology to subsequent delivery of glutamate receptors into dendritic spine heads are still not well understood. Following chemical induction of LTP, an increase in eGFP-GluR1 fluorescence in short but not long dendritic spines of cultured hippocampal neurons was found. Repeated flash photolysis of caged calcium, which produced a transient rise of [Ca2+]i inside spine heads caused a selective, actin and protein synthesis dependent increase of eGFP-GluR1 in these spines. Strikingly, GluR1 increase was correlated with the ability of a calcium transient generated in the spine head to diffuse into the parent dendrite, and inversely correlated with the length of the spine: short spines were more likely to raise GluR1 than long ones. These observations link, for the first time, calcium transients in dendritic spines with spine morphology and its ability to undergo synaptic plasticity.     

Neuritin Attenuates Cognitive Function Impairments in Tg2576 Mouse Model of Alzheimer's Disease

Neuritin, also known as CPG15, is a neurotrophic factor that was initially discovered in a screen to identify genes involved in activity-dependent synaptic plasticity. Neuritin plays multiple roles in the process of neural development and synaptic plasticity, although its binding receptor(s) and downstream signaling effectors remain unclear. In this study, we found that the cortical and hippocampal expression of neuritin is reduced in the brains of Alzheimer''s disease (AD) patients and demonstrated that viral-mediated expression of neuritin in the dentate gyrus of 13-month-old Tg2576 mice, an AD animal model, attenuated a deficit in learning and memory as assessed by a Morris water maze test. We also found that neuritin restored the reduction in dendritic spine density and the maturity of individual spines in primary hippocampal neuron cultures prepared from Tg2576 mice. It was also shown that viral-mediated expression of neuritin in the dentate gyrus of 7-week-old Sprague-Dawley rats increased neurogenesis in the hippocampus. Taken together, our results demonstrate that neuritin restores the reduction in dendritic spine density and the maturity of individual spines in primary hippocampal neurons from Tg2576 neurons, and also attenuates cognitive function deficits in Tg2576 mouse model of AD, suggesting that neuritin possesses a therapeutic potential for AD.     

Structural and functional plasticity of dendritic spines – root or result of behavior?

Dendritic spines are multifunctional integrative units of the nervous system and are highly diverse and dynamic in nature. Both internal and external stimuli influence dendritic spine density and morphology on the order of minutes. It is clear that the structural plasticity of dendritic spines is related to changes in synaptic efficacy, learning and memory and other cognitive processes. However, it is currently unclear whether structural changes in dendritic spines are primary instigators of changes in specific behaviors, a consequence of behavioral changes, or both. In this review, we first examine the basic structure and function of dendritic spines in the brain, as well as laboratory methods to characterize and quantify morphological changes in dendritic spines. We then discuss the existing literature on the temporal and functional relationship between changes in dendritic spines in specific brain regions and changes in specific behaviors mediated by those regions. Although technological advancements have allowed us to better understand the functional relevance of structural changes in dendritic spines that are influenced by environmental stimuli, the role of spine dynamics as an underlying driver or consequence of behavior still remains elusive. We conclude that while it is likely that structural changes in dendritic spines are both instigators and results of behavioral changes, improved research tools and methods are needed to experimentally and directly manipulate spine dynamics in order to more empirically delineate the relationship between spine structure and behavior.     

Increased expression of phospho-cofilin in CA1 and subiculum areas after theta-burst stimulation of Schaffer collateral-commissural fibers in rat hippocampal slices

Activity-dependent structural plasticity of dendritic spines of pyramidal neurons in the central neuron system has been proposed to be a cellular basis of learning and memory. Long-term potentiation (LTP) is accompanied by changes in synaptic morphology and structural remodeling of dendritic spines. However, there is considerable uncertainty as to the nature of the adjustment. The present study tested whether immunoreactive phospho-cofilin, an index of altered actin filament assembly, could be increased by theta-burst stimulations (TBS), which is an effective stimulation pattern for inducing LTP in the hippocampus. The slope of fEPSPs evoked by TBS to Schaffer collateral-commissural fibers in hippocampal slices was measured, and p-cofilin expression was examined using immunofluorescence techniques. Results indicated that saturated L-LTP was produced by multiple TBS episodes to Schaffer collateral-commissural fibers in the hippocampal CA1 area, and TBSs also increased immunoreactive p-cofilin expression in the stratum radiatum of the hippocampal CA1 area and pyramidal layer of the subiculum. D-2-amino-5-phosphonovalerate (D-APV) prevented LTP and expression of p-cofilin immunoreactive induced by multiple TBS episodes in the stratum radiatum of the hippocampal CA1 area. Two paired-pulse low-frequency stimulation (PP-LFS) episodes to Schaffer collateral-commissural fibers induced long-term depression (LTD), and did not affect p-cofilin expression in the stratum radiatum of the hippocampal CA1 area. These results suggest that LTP induction is associated with altered actin filament assembly. Moreover, the CA1 and subiculum areas of the hippocampal formation possibly cooperate with each other in important physiological functions, such as learning and memory, or in pathological diseases, such as epilepsy.     

Isoflurane Reversibly Destabilizes Hippocampal Dendritic Spines by an Actin-Dependent Mechanism

General anesthetics produce a reversible coma-like state through modulation of excitatory and inhibitory synaptic transmission. Recent evidence suggests that anesthetic exposure can also lead to sustained cognitive dysfunction. However, the subcellular effects of anesthetics on the structure of established synapses are not known. We investigated effects of the widely used volatile anesthetic isoflurane on the structural stability of hippocampal dendritic spines, a postsynaptic structure critical to excitatory synaptic transmission in learning and memory. Exposure to clinical concentrations of isoflurane induced rapid and non-uniform shrinkage and loss of dendritic spines in mature cultured rat hippocampal neurons. Spine shrinkage was associated with a reduction in spine F-actin concentration. Spine loss was prevented by either jasplakinolide or cytochalasin D, drugs that prevent F-actin disassembly. Isoflurane-induced spine shrinkage and loss were reversible upon isoflurane elimination. Thus, isoflurane destabilizes spine F-actin, resulting in changes to dendritic spine morphology and number. These findings support an actin-based mechanism for isoflurane-induced alterations of synaptic structure in the hippocampus. These reversible alterations in dendritic spine structure have important implications for acute anesthetic effects on excitatory synaptic transmission and synaptic stability in the hippocampus, a locus for anesthetic-induced amnesia, and have important implications for anesthetic effects on synaptic plasticity.