Spike-based synaptic plasticity and the emergence of direction selective simple cells: simulation results

Direction selectivity (DS) of simple cells in the primary visual cortex was recently suggested to arise from short-term synaptic depression in thalamocortical afferents (Chance F, Nelson S, Abbott L (1998), J. Neuroscience 18(12): 4785–4799). In the model, two groups of afferents with spatially displaced receptive fields project through either depressing and non-depressing synapses onto the V1 cell. The degree of synaptic depression determines the temporal phase advance of the response to drifting gratings. We show that the spatial displacement and the appropriate degree of synaptic depression required for DS can develop within an unbiased input scenario by means of temporally asymmetric spike-timing dependent plasticity (STDP) which modifies both the synaptic strength and the degree of synaptic depression. Moving stimuli of random velocities and directions break any initial receptive field symmetry and produce DS. Frequency tuning curves and subthreshold membrane potentials akin to those measured for non-directional simple cells are thereby changed into those measured for directional cells. If STDP is such that down-regulation dominates up-regulation the overall synaptic strength adapts in a self-organizing way such that eventually the postsynaptic response for the non-preferred direction becomes subthreshold. To prevent unlearning of the acquired DS by randomly changing stimulus directions an additional learning threshold is necessary. To further protect the development of the simple cell properties against noise in the stimulus, asynchronous and irregular synaptic inputs are required.     

Spike-based synaptic plasticity and the emergence of direction selective simple cells: mathematical analysis

In the companion paper we presented extended simulations showing that the recently observed spike-timing dependent synaptic plasticity can explain the development of simple cell direction selectivity (DS) when simultaneously modifying the synaptic strength and the degree of synaptic depression. Here we estimate the spatial shift of the simple cell receptive field (RF) induced by the long-term synaptic plasticity, and the temporal phase advance caused by the short-term synaptic depression in response to drifting grating stimuli. The analytical expressions for this spatial shift and temporal phase advance lead to a qualitative reproduction of the frequency tuning curves of non-directional and directional simple cells. In agreement with in vivo recordings, the acquired DS is strongest for test gratings with a temporal frequency around 1–4 Hz. In our model this best frequency is determined by the width of the learning function and the time course of depression, but not by the temporal frequency of the training stimuli. The analysis further reveals the instability of the initially symmetric RF, and formally explains why direction selectivity develops from a non-directional cell in a natural, directionally unbiased stimulation scenario.     

Transmission,Development, and Plasticity of Synapses

Chemical synapses are sites of contact and information transfer between a neuron and its partner cell. Each synapse is a specialized junction, where the presynaptic cell assembles machinery for the release of neurotransmitter, and the postsynaptic cell assembles components to receive and integrate this signal. Synapses also exhibit plasticity, during which synaptic function and/or structure are modified in response to activity. With a robust panel of genetic, imaging, and electrophysiology approaches, and strong evolutionary conservation of molecular components, Drosophila has emerged as an essential model system for investigating the mechanisms underlying synaptic assembly, function, and plasticity. We will discuss techniques for studying synapses in Drosophila, with a focus on the larval neuromuscular junction (NMJ), a well-established model glutamatergic synapse. Vesicle fusion, which underlies synaptic release of neurotransmitters, has been well characterized at this synapse. In addition, studies of synaptic assembly and organization of active zones and postsynaptic densities have revealed pathways that coordinate those events across the synaptic cleft. We will also review modes of synaptic growth and plasticity at the fly NMJ, and discuss how pre- and postsynaptic cells communicate to regulate plasticity in response to activity.     

在体大鼠海马CA1区侧枝/联合纤维通路和TA通路的不同EPSP空间整合特性

在麻醉Wistar大鼠上,结合脑室给药,应用双电极刺激技术刺激海马独立的两条侧枝／联合纤维通路、TA通路,并在CA1区放射层记录兴奋性突触后电位（EPSP）,对海马CA1区锥体细胞近、远端树突EPSP的空间整合进行了初步探讨。结果表明,海马CA1区锥体细胞近、远端树突的空间整合都是亚线性的;近端树突的空间整合不受期望值大小的影响,但远端树突的空间整合随期望值增加而减小（更趋于亚线性）。此外,荷包牡丹碱没有影响EPSP的空间整合;但瞬时A型钾通道（IAK^＋）的拮抗剂氨基吡啶-4却使得近端树突的空间整合趋于线性发展。本研究表明,海马CA1锥体细胞近、远端树突不同的被动、主动特征使它们具有了不同的空间整合特性。由于近端树突接受海马内部侧枝／联合纤维投射的信息,远端树突通过TA通路接受内嗅皮层投射的信息,由此提示,CA1区锥体细胞对来自海马内部和直接来自皮层的信息输入采用了不同的整合方式。     

Computer simulations of synchrony and oscillations evoked by two coherent inputs

Coherent oscillations have been reported in multiple cortical areas. This study examines the characteristics of output spikes through computer simulations when the neural network model receives periodic/aperiodic spatiotemporal spikes with modulated/constant populational activity from two pathways. Synchronous oscillations which have the same period as the input are observed in response to periodic input patterns regardless of populational activity. The results confirm that the output frequency of synchrony is essentially determined by the period of the repeated input patterns. On the other hand, weak periodic outputs are observed when aperiodic spikes are input with modulated populational activity. In this case, higher firing rates are necessary to input for higher frequency oscillations. The spike-timing-dependent plasticity suppresses the spikes which do not contribute to the synchrony for periodic inputs. This effect corresponds to the experimental reports that learning sharpens the synchrony in the motor cortex. These results suggest that spatiotemporal spike patterns should be entrained on modulated populational activity to transmit oscillatory information effectively in the convergent pathway.     

Light-Triggered Action Potentials and Changes in Quantum Efficiency of Photosystem II in <Emphasis Type="Italic">Anthoceros</Emphasis> Cells

Capillary microelectrodes and pulse amplitude-modulated microfluorometry were used to study light-triggered changes in cell membrane potential, chlorophyll fluorescence, and photochemical yield of PSII in chloroplasts of a hornwort Anthoceros sp. The action potential was generated by illuminating the plant sample for a few seconds. It was accompanied by a reversible decrease in quantum efficiency of PSII and by nonphotochemical quenching of fluorescence that continued as long as 10 min after the light stimulus. The presence of ammonium ions (2 mM) enhanced the amplitude and prolonged the duration of dark changes of fluorescence parameters in accordance with the reported increase in duration and amplitude of the light-triggered action potential in the presence of NH ₄ ⁺ . A rapid retardation of PSII activity within the first seconds of illumination was also evident from absorbance changes at 810 nm reflecting the redox conversions of chlorophyll P700. The PSII-dependent stage of reduction in the induction curves of P700 absorbance was strongly suppressed, and the amplitudes of signals induced by white and far-red light (717 nm) differed insignificantly. It is concluded that a short-term irradiation triggers the generation of ΔpH at the thylakoid membranes, which is accompanied by inhibition of the plasma membrane H⁺ pump and by reversible inactivation of PSII due to increased thermal dissipation of chlorophyll excitations.     

Respiration and Mitochondrial Membrane Potential Are not Required for Apoptosis and Anti-apoptotic Action of Bcl-2 in HeLa Cells

The release of cytochrome c from intermembrane space of mitochondria into cytosol is one of the critical events in apoptotic cell death. The important anti-apoptotic oncoprotein Bcl-2 inhibits this process. In the present study it was shown that apoptosis and release of cytochrome c induced by staurosporine or by tumor necrosis factor- in HeLa cells were not affected by inhibitors of respiration (rotenone, myxothiazol, antimycin A) or by uncouplers (CCCP, DNP) that decrease the membrane potential at the inner mitochondrial membrane. The inhibitors of respiration and the uncouplers did not affect also the anti-apoptotic activity of Bcl-2.     

Ubiquitination of MBNL1 Is Required for Its Cytoplasmic Localization and Function in Promoting Neurite Outgrowth

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Relocation of an Extrasynaptic GABAA Receptor to Inhibitory Synapses Freezes Excitatory Synaptic Strength and Preserves Memory

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Modeling of Substance P and 5-HT Induced Synaptic Plasticity in the Lamprey Spinal CPG: Consequences for Network Pattern Generation

Consequences of synaptic plasticity in the lamprey spinal CPG are analyzed by means of simulations. This is motivated by the effects substance P (a tachykinin) and serotonin (5-hydroxytryptamin; 5-HT) have on synaptic transmission in the locomotor network. Activity-dependent synaptic depression and potentiation have recently been shown experimentally using paired intracellular recordings. Although normally activity-dependent plasticity presumably does not contribute to the patterning of network activity, this changes in the presence of the neuromodulators substance P and 5-HT, which evoke significant plasticity. Substance P can induce a faster and larger depression of inhibitory connections but potentiation of excitatory inputs, whereas 5-HT induces facilitation of both inhibitory and excitatory inputs. Changes in the amplitude of the first postsynaptic potential are also seen. These changes could thus be a potential mechanism underlying the modulatory role these substances have on the rhythmic network activity.The aim of the present study has been to implement the activity dependent synaptic depression and facilitation induced by substance P and 5-HT into two alternative models of the lamprey spinal locomotor network, one relying on reciprocal inhibition for bursting and one in which each hemicord is capable of oscillations. The consequences of the plasticity of inhibitory and excitatory connections are then explored on the network level.In the intact spinal cord, tachykinins and 5-HT, which can be endogenously released, increase and decrease the frequency of the alternating left-right burst pattern, respectively. The frequency decreasing effect of 5-HT has previously been explained based on its conductance decreasing effect on K _Ca underlying the postspike afterhyperpolarization (AHP). The present simulations show that short-term synaptic plasticity may have strong effects on frequency regulation in the lamprey spinal CPG. In the network model relying on reciprocal inhibition, the observed effects substance P and 5-HT have on network behavior (i.e., a frequency increase and decrease respectively) can to a substantial part be explained by their effects on the total extent and time dynamics of synaptic depression and facilitation. The cellular effects of these substances will in the 5-HT case further contribute to its network effect.     

Adducin Is an In Vivo Substrate for Protein Kinase C: Phosphorylation in the MARCKS-related Domain Inhibits Activity in Promoting Spectrin–Actin Complexes and Occurs in Many Cells,Including Dendritic Spines of Neurons

Adducin is a heteromeric protein with subunits containing a COOH-terminal myristoylated alanine-rich C kinase substrate (MARCKS)-related domain that caps and preferentially recruits spectrin to the fast-growing ends of actin filaments. The basic MARCKS-related domain, present in α, β, and γ adducin subunits, binds calmodulin and contains the major phosphorylation site for protein kinase C (PKC). This report presents the first evidence that phosphorylation of the MARCKS-related domain modifies in vitro and in vivo activities of adducin involving actin and spectrin, and we demonstrate that adducin is a prominent in vivo substrate for PKC or other phorbol 12-myristate 13-acetate (PMA)-activated kinases in multiple cell types, including neurons. PKC phosphorylation of native and recombinant adducin inhibited actin capping measured using pyrene-actin polymerization and abolished activity of adducin in recruiting spectrin to ends and sides of actin filaments. A polyclonal antibody specific to the phosphorylated state of the RTPS-serine, which is the major PKC phosphorylation site in the MARCKS-related domain, was used to evaluate phosphorylation of adducin in cells. Reactivity with phosphoadducin antibody in immunoblots increased twofold in rat hippocampal slices, eight- to ninefold in human embryonal kidney (HEK 293) cells, threefold in MDCK cells, and greater than 10-fold in human erythrocytes after treatments with PMA, but not with forskolin. Thus, the RTPS-serine of adducin is an in vivo phosphorylation site for PKC or other PMA-activated kinases but not for cAMP-dependent protein kinase in a variety of cell types. Physiological consequences of the two PKC phosphorylation sites in the MARCKS-related domain were investigated by stably transfecting MDCK cells with either wild-type or PKC-unphosphorylatable S716A/S726A mutant α adducin. The mutant α adducin was no longer concentrated at the cell membrane at sites of cell–cell contact, and instead it was distributed as a cytoplasmic punctate pattern. Moreover, the cells expressing the mutant α adducin exhibited increased levels of cytoplasmic spectrin, which was colocalized with the mutant α adducin in a punctate pattern. Immunofluorescence with the phosphoadducin-specific antibody revealed the RTPS-serine phosphorylation of adducin in postsynaptic areas in the developing rat hippocampus. High levels of the phosphoadducin were detected in the dendritic spines of cultured hippocampal neurons. Spectrin also was a component of dendritic spines, although at distinct sites from the ones containing phosphoadducin. These data demonstrate that adducin is a significant in vivo substrate for PKC or other PMA-activated kinases in a variety of cells, and that phosphorylation of adducin occurs in dendritic spines that are believed to respond to external signals by changes in morphology and reorganization of cytoskeletal structures.     

Localization of proteins in the inner and outer membranes of Caulobacter crescentus

Cytoplasmic and outer membranes of Caulobacter crescentus were separated by isopycnic sucrose gradient centrifugation into two peaks with buoyant densities 1.22 and 1.14 g/cm³. These peaks were identified as outer and cytoplasmic membranes by the enrichment of malate dehydrogenase and NADH oxidase in the lower density peak and the presence of flagellin, a cell surface protein, in the heavier peak. The identity of the heavier peak as outer membrane was confirmed by labeling of cells with diazotized [³⁵S]sulfanilic acid, a reagent that does not penetrate intact cells. Under these conditions only outer membrane proteins were substituted by the sulfanilic acid. The distribution of proteins between the cytoplasmic and outer membranes were examined by the analysis of [³⁵S]methionine-labeled membranes by SDS-polyacrylamide and two-dimensional gel electrophoresis. These results showed that the inner and outer membranes contain approximately equal numbers of proteins, and that the distribution of these proteins between the two layers is highly asymmetric. Although many of the proteins could be assigned to one or the other membrane fraction, a number of the outer membrane proteins in the 32 000–100 000 molecular weight range frequently contaminate the inner membrane fractions. The implications of these results for membrane isolation and separation in C. crescentus are discussed.     

Localization and Subcellular Distribution of N-Copine in Mouse Brain

Abstract : N -Copine is a novel protein with two C2 domains. Its expression is brain specific and up-regulated by neuronal activity such as kainate stimulation and tetanus stimulation evoking hippocampal CA1 long-term potentiation. We examined the localization and subcellular distribution of N -copine in mouse brain. In situ hybridization analysis showed that N -copine mRNA was expressed exclusively in neurons of the hippocampus and in the main and accessory olfactory bulb, where various forms of synaptic plasticity and memory formation are known to occur. In immunohistochemical analyses, N -copine was detected mainly in the cell bodies and dendrites in the neurons, whereas presynaptic proteins such as synaptotagmin I and rab3A were detected in the regions where axons pass through. In fractionation experiments of brain homogenate, N -copine was associated with the membrane fraction in the presence of Ca²⁺ but not in its absence. As a GST-fusion protein with the second C2 domain of N -copine showed Ca²⁺ -dependent binding to phosphatidylserine, this domain was considered to be responsible for the Ca²⁺ -dependent association of N -copine with the membrane. Thus, N -copine may have a role as a Ca²⁺ sensor in postsynaptic events, in contrast to the known roles of "double C2 domain-containing proteins," including synaptotagmin I, in presynaptic events.     

Optical Mapping of Action Potentials and Calcium Transients in the Mouse Heart

The mouse heart is a popular model for cardiovascular studies due to the existence of low cost technology for genetic engineering in this species. Cardiovascular physiological phenotyping of the mouse heart can be easily done using fluorescence imaging employing various probes for transmembrane potential (V_m), calcium transients (CaT), and other parameters. Excitation-contraction coupling is characterized by action potential and intracellular calcium dynamics; therefore, it is critically important to map both V_m and CaT simultaneously from the same location on the heart^1-4. Simultaneous optical mapping from Langendorff perfused mouse hearts has the potential to elucidate mechanisms underlying heart failure, arrhythmias, metabolic disease, and other heart diseases. Visualization of activation, conduction velocity, action potential duration, and other parameters at a myriad of sites cannot be achieved from cellular level investigation but is well solved by optical mapping^1,5,6. In this paper we present the instrumentation setup and experimental conditions for simultaneous optical mapping of V_m and CaT in mouse hearts with high spatio-temporal resolution using state-of-the-art CMOS imaging technology. Consistent optical recordings obtained with this method illustrate that simultaneous optical mapping of Langendorff perfused mouse hearts is both feasible and reliable.     

Myosin IIb-dependent Regulation of Actin Dynamics Is Required for N-Methyl-d-aspartate Receptor Trafficking during Synaptic Plasticity

N-Methyl-d-aspartate receptor (NMDAR) synaptic incorporation changes the number of NMDARs at synapses and is thus critical to various NMDAR-dependent brain functions. To date, the molecules involved in NMDAR trafficking and the underlying mechanisms are poorly understood. Here, we report that myosin IIb is an essential molecule in NMDAR synaptic incorporation during PKC- or θ burst stimulation-induced synaptic plasticity. Moreover, we demonstrate that myosin light chain kinase (MLCK)-dependent actin reorganization contributes to NMDAR trafficking. The findings from additional mutual occlusion experiments demonstrate that PKC and MLCK share a common signaling pathway in NMDAR-mediated synaptic regulation. Because myosin IIb is the primary substrate of MLCK and can regulate actin dynamics during synaptic plasticity, we propose that the MLCK- and myosin IIb-dependent regulation of actin dynamics is required for NMDAR trafficking during synaptic plasticity. This study provides important insights into a mechanical framework for understanding NMDAR trafficking associated with synaptic plasticity.     

Thalamic synaptic transmission of sensory information modulated by synergistic interaction of adenosine and serotonin

The thalamic synapses relay peripheral sensory information to the cortex, and constitute an important part of the thalamocortical network that generates oscillatory activities responsible for different vigilance (sleep and wakefulness) states. However, the modulation of thalamic synaptic transmission by potential sleep regulators, especially by combination of regulators in physiological scenarios, is not fully characterized. We found that somnogen adenosine itself acts similar to wake‐promoting serotonin, both decreasing synaptic strength as well as short‐term depression, at the retinothalamic synapse. We then combined the two modulators considering the coexistence of them in the hypnagogic (sleep‐onset) state. Adenosine plus serotonin results in robust synergistic inhibition of synaptic strength and dramatic transformation of short‐term synaptic depression to facilitation. These synaptic effects are not achievable with a single modulator, and are consistent with a high signal‐to‐noise ratio but a low level of signal transmission through the thalamus appropriate for slow‐wave sleep. This study for the first time demonstrates that the sleep‐regulatory modulators may work differently when present in combination than present singly in terms of shaping information flow in the thalamocortical network. The major synaptic characters such as the strength and short‐term plasticity can be profoundly altered by combination of modulators based on physiological considerations.

     

Spatial Distribution of the Potential on a Cylindrical Surface Simulating the Somatic Membrane: Model Studies

In our study, we represent the theoretical and numerical analysis of a stochastic version of the Hodgkin–Huxley model applied to a two-dimensional spatial cylindrical area simulating the neuronal somatic membrane. We characterized the spatiotemporal dynamics of the membrane potential by its local value V _m (x, y, t) and the integral of this value with respect to time F(x, y, T) within an interval [0, T]. Analysis of the model showed that (i) there are nonzero gradients of F(x, y, T) at any distribution of ion channels; (ii) the maximum gradient F(x, y, T) decreases down to zero with the time T, if the channels are distributed homogeneously, and acquire some positive constant value, if the channels are distributed inhomogeneously; the gradient F(x, y, T) depends on the gradient of spatial distribution of the channels; and (iii) under conditions of spatial redistribution of the channels with preservation of their number, the dynamics of V _m (x, y, t) does not change.     

BK通道在细胞膜上的单分子定位及空间分布

目的：研究大电导、钙离子和电压激活的钾离子通道（BK通道）在HEK293细胞膜上的单分子定位及其总体空间分布情况。方法：分别用mEos2、Dronpa等荧光蛋白标记BK通道的α亚基和辅助性β2亚基,将这些质粒在HEK293细胞内瞬时转染以表达通道蛋白,然后用激光共聚焦荧光显微成像、全内反射荧光显微成像、光敏定位荧光成像等技术观察BK通道的亚细胞定位及单分子分布,并用电生理实验技术检测荧光蛋白对BK通道有影响。结果：激光共聚焦荧光显微成像和全内反射荧光显微成像技术只能在亚细胞水平定位通道蛋白,BK通道在细胞膜上聚集并形成不规则的蛋白簇,它的仅亚基和β2亚基在细胞膜上完全共定位;光敏定位荧光成像技术成功定位BK通道蛋白簇里面的单分子,虽然α和β2亚基紧紧靠在一起,它们之间依然存在空间距离;BK通道的质膜表达和功能特性不受荧光蛋白的影响。结论：BK通道蛋白簇里面包含大量的α和β2亚基的蛋白单分子,它们紧密地聚集在一起,但是并没有完全共定位,在分子水平上揭示了BK通道α和p亚基功能耦合的结构基础,为以后研究大分子蛋白质间的相互作用机制提供了很好的分子模型,光敏定位荧光成像技术作为一种全新的单分子荧光成像手段,在基因表达、信号通路、蛋白质相互作用等许多重要生命活动的研究中发挥重要作用。