Electrotonic spread of dendritic potentials in feline pyramidal cells

In pyramidal cells synaptic activation of the entire apical dendritic tree distal to the branch point of the major shaft can dominate the neuronal firing pattern. Uniform synaptic activation of distant parts of the dendritic tree (~ 750 microns from the soma) would produce potential changes at the soma of 2 to 3 percent of the magnitude of the dendritic potential changes. Even these small somatic potential changes could modulate the frequency of firing of neurons depolarized close to or above firing level by more proximal synaptic inputs.     

Regulation of dendritic protein synthesis by miniature synaptic events

We examined dendritic protein synthesis after a prolonged blockade of action potentials alone and after a blockade of both action potentials and miniature excitatory synaptic events (minis). Relative to controls, dendrites exposed to a prolonged blockade of action potentials showed diminished protein synthesis. Dendrites in which both action potentials and minis were blocked showed enhanced protein synthesis, suggesting that minis inhibit dendritic translation. When minis were acutely blocked or stimulated, an immediate increase or decrease, respectively, in dendritic translation was observed. Taken together, these results reveal a role for miniature synaptic events in the acute regulation of dendritic protein synthesis in neurons.     

Combined analog and action potential coding in hippocampal mossy fibers

In the mammalian cortex, it is generally assumed that the output information of neurons is encoded in the number and the timing of action potentials. Here, we show, by using direct patchclamp recordings from presynaptic hippocampal mossy fiber boutons, that axons transmit analog signals in addition to action potentials. Excitatory presynaptic potentials result from subthreshold dendritic synaptic inputs, which propagate several hundreds of micrometers along the axon and modulate action potential-evoked transmitter release at the mossy fiber-CA3 synapse. This combined analog and action potential coding represents an additional mechanism for information transmission in a major hippocampal pathway.     

Androgens regulate the dendritic length of mammalian motoneurons in adulthood

Sex steroid hormones have been thought to alter behaviors in adulthood by changing the activity of neural circuits rather than by inducing major structural changes in these pathways. In a group of androgen-sensitive motoneurons that mediate male copulatory functions, decreases in androgen levels after castration of adult rats produced dramatic structural changes, decreasing both the dendritic length and soma size of these motoneurons. These changes were reversed by androgen replacement. These results imply a surprising degree of synaptic plasticity in adult motoneurons and suggest that normal changes in androgen levels in adulthood are associated with significant alterations in the structure and function of these neurons.     

The spread of Ras activity triggered by activation of a single dendritic spine

In neurons, individual dendritic spines isolate N-methyl-d-aspartate (NMDA) receptor-mediated calcium ion (Ca2+) accumulations from the dendrite and other spines. However, the extent to which spines compartmentalize signaling events downstream of Ca2+ influx is not known. We combined two-photon fluorescence lifetime imaging with two-photon glutamate uncaging to image the activity of the small guanosine triphosphatase Ras after NMDA receptor activation at individual spines. Induction of long-term potentiation (LTP) triggered robust Ca2+-dependent Ras activation in single spines that decayed in approximately 5 minutes. Ras activity spread over approximately 10 micrometers of dendrite and invaded neighboring spines by diffusion. The spread of Ras-dependent signaling was necessary for the local regulation of the threshold for LTP induction. Thus, Ca2+-dependent synaptic signals can spread to couple multiple synapses on short stretches of dendrite.     

Localization and mobility of omega-conotoxin-sensitive Ca2+ channels in hippocampal CA1 neurons

Voltage-dependent Ca2+ channels (VDCCs) are modulators of synaptic plasticity, oscillatory behavior, and rhythmic firing in brain regions such as the hippocampus. The distribution and lateral mobility of VDCCs on CA1 hippocampal neurons have been determined with biologically active fluorescent and biotinylated derivatives of the selective probe omega-conotoxin in conjunction with circular dityndallism, digital fluorescence imaging, and photobleach recovery microscopy. On noninnervated cell bodies, VDCCs were found to be organized in multiple clusters, whereas after innervation the VDCCs were concentrated and immobilized at synaptic contact sites. On dendrites, VDCC distribution was punctate and was interrupted by extensive bare regions or abruptly terminated. More than 85% of the dendritic VDCCs were found to be immobile by fluorescence photobleach recovery. Thus, before synaptic contact, specific mechanisms target, segregate, and immobilize VDCCs to neuronal cell bodies and to specialized dendritic sites. Regulation of this distribution may be critical in determining the firing activity and integrative properties of hippocampal CA1 neurons.     

PSD-95 involvement in maturation of excitatory synapses

PSD-95 is a neuronal PDZ protein that associates with receptors and cytoskeletal elements at synapses, but whose function is uncertain. We found that overexpression of PSD-95 in hippocampal neurons can drive maturation of glutamatergic synapses. PSD-95 expression enhanced postsynaptic clustering and activity of glutamate receptors. Postsynaptic expression of PSD-95 also enhanced maturation of the presynaptic terminal. These effects required synaptic clustering of PSD-95 but did not rely on its guanylate kinase domain. PSD-95 expression also increased the number and size of dendritic spines. These results demonstrate that PSD-95 can orchestrate synaptic development and are suggestive of roles for PSD-95 in synapse stabilization and plasticity.     

Locally synchronized synaptic inputs

Synaptic inputs on dendrites are nonlinearly converted to action potential outputs, yet the spatiotemporal patterns of dendritic activation remain to be elucidated at single-synapse resolution. In rodents, we optically imaged synaptic activities from hundreds of dendritic spines in hippocampal and neocortical pyramidal neurons ex vivo and in vivo. Adjacent spines were frequently synchronized in spontaneously active networks, thereby forming dendritic foci that received locally convergent inputs from presynaptic cell assemblies. This precise subcellular geometry manifested itself during N-methyl-D-aspartate receptor-dependent circuit remodeling. Thus, clustered synaptic plasticity is innately programmed to compartmentalize correlated inputs along dendrites and may reify nonlinear synaptic integration.     

Targeted protein degradation and synapse remodeling by an inducible protein kinase

Synaptic plasticity involves the reorganization of synapses at the protein and the morphological levels. Here, we report activity-dependent remodeling of synapses by serum-inducible kinase (SNK). SNK was induced in hippocampal neurons by synaptic activity and was targeted to dendritic spines. SNK bound to and phosphorylated spine-associated Rap guanosine triphosphatase activating protein (SPAR), a postsynaptic actin regulatory protein, leading to degradation of SPAR. Induction of SNK in hippocampal neurons eliminated SPAR protein, depleted postsynaptic density-95 and Bassoon clusters, and caused loss of mature dendritic spines. These results implicate SNK as a mediator of activity-dependent change in the molecular composition and morphology of synapses.     

High-resolution protein structure determination by serial femtosecond crystallography

Structure determination of proteins and other macromolecules has historically required the growth of high-quality crystals sufficiently large to diffract x-rays efficiently while withstanding radiation damage. We applied serial femtosecond crystallography (SFX) using an x-ray free-electron laser (XFEL) to obtain high-resolution structural information from microcrystals (less than 1 micrometer by 1 micrometer by 3 micrometers) of the well-characterized model protein lysozyme. The agreement with synchrotron data demonstrates the immediate relevance of SFX for analyzing the structure of the large group of difficult-to-crystallize molecules.     

Reward-predictive cues enhance excitatory synaptic strength onto midbrain dopamine neurons

Using sensory information for the prediction of future events is essential for survival. Midbrain dopamine neurons are activated by environmental cues that predict rewards, but the cellular mechanisms that underlie this phenomenon remain elusive. We used in vivo voltammetry and in vitro patch-clamp electrophysiology to show that both dopamine release to reward predictive cues and enhanced synaptic strength onto dopamine neurons develop over the course of cue-reward learning. Increased synaptic strength was not observed after stable behavioral responding. Thus, enhanced synaptic strength onto dopamine neurons may act to facilitate the transformation of neutral environmental stimuli to salient reward-predictive cues.     

Early experience effects upon cortical dendrites: a proposed model for development

We studied the effects of environmental stimulation on the development of rat cortical pyramidal cell synaptic loci (dendritic spines) and the number of such cells staining by the rapid Golgi technique. Stimulation three to five times a day from the day of birth increased the number of spines per micrometer in 8-day-old animals and increased the number of neurons stanining at 8 to 16 days of age. This effect of afferent input upon development of the dendritic spine may represent the neuroanatomical basis for the influence of early experience on subsequent behavior. The number of neurons staining by the rapid Golgi technique appears to be related to those that are functionally involved at the time of tissue preparation.     

Dependence of EPSP efficacy on synapse location in neocortical pyramidal neurons

Neurons receive thousands of synaptic inputs throughout elaborate dendritic trees. Here we determine the somatic impact of excitatory postsynaptic potentials (EPSPs) generated at known dendritic sites in neocortical pyramidal neurons. As inputs became more distal, somatic EPSP amplitude decreased, whereas use-dependent depression increased. Despite marked attenuation (>40-fold), when coactivated within a narrow time window (approximately 10 milliseconds), distal EPSPs could directly influence action potential output following dendritic spike generation. These findings reveal that distal EPSPs are ineffective sources of background somatic excitation, but through coincidence detection have a powerful transient signaling role.     

Target-dependent structural changes accompanying long-term synaptic facilitation in Aplysia neurons

The mechanisms underlying structural changes that accompany learning and memory have been difficult to investigate in the intact nervous system. In order to make these changes more accessible for experimental analysis, dissociated cell culture and low-light-level video microscopy were used to examine Aplysia sensory neurons in the presence or absence of their target cells. Repeated applications of serotonin, a facilitating transmitter important in behavioral dishabituation and sensitization, produced growth of the sensory neurons that paralleled the long-term enhancement of synaptic strength. This growth required the presence of the postsynaptic motor neuron. Thus, both the structural changes and the synaptic facilitation of Aplysia sensorimotor synapses accompanying long-term behavioral sensitization can be produced in vitro by applying a single facilitating transmitter repeatedly. These structural changes depend on an interaction of the presynaptic neuron with an appropriate postsynaptic target.     

Spine stems on tectal interneurons in jewel fish are shortened by social stimulation

Spined pyriform interneurons in community-reared jewel fish have more dendritic branches and spines in the deep tectal layers than those in isolates reared without visual-tactile contact with conspecifics. Furthermore, in the same dendritic loci in which the community-reared fish had more spines, the spine stems were shorter. The findings suggest that social stimulation induces localized formation of spines, which swell with synaptic activation. Shortening of the spine stem through elongated swelling of the spine head is likely to alter synaptic effectiveness through changes in electrotonic conductance.     

Activity- and mTOR-dependent suppression of Kv1.1 channel mRNA translation in dendrites

Mammalian target of rapamycin (mTOR) is implicated in synaptic plasticity and local translation in dendrites. We found that the mTOR inhibitor, rapamycin, increased the Kv1.1 voltage-gated potassium channel protein in hippocampal neurons and promoted Kv1.1 surface expression on dendrites without altering its axonal expression. Moreover, endogenous Kv1.1 mRNA was detected in dendrites. Using Kv1.1 fused to the photoconvertible fluorescence protein Kaede as a reporter for local synthesis, we observed Kv1.1 synthesis in dendrites upon inhibition of mTOR or the N-methyl-d-aspartate (NMDA) glutamate receptor. Thus, synaptic excitation may cause local suppression of dendritic Kv1 channels by reducing their local synthesis.     

维生素C对新生大鼠海马神经细胞生长的影响

取新生大鼠海马组织进行原代体外分散培养,用MTT比色法检测不同浓度维生素C对神经细胞生长的影响,并观察其形态学变化和分化程度。结果表明,2和20nmol/L维生素C组对神经细胞生长的影响与对照组无显著差别(P>0.05);200nmol/L维生素C组能促进神经细胞的存活,与对照组相比差异显著(P<0.01),进一步观察发现,该处理的细胞突起数目、长度、胞体面积均显著高于对照组(P<0.01);2000nmol/L维生素C组可显著减少细胞的存活数,能引起细胞凋亡。提示200nmol/L维生素C对神经细胞生长发育有一定的促进作用。     

Signaling for growth orientation and cell differentiation by surface topography in uromyces

The dimensions of the topographical signals for growth orientation and infection structure formation, a cell differentiation event that includes nuclear division, were determined for the stomatal penetrating rust fungus Uromyces appendiculatus. The differentiation signal was found to be a simple ridge on the substrate surface that had a markedly optimum height of 0.5 micrometer. Such ridges were microfabricated on silicon wafers by using electron-beam lithography. A similar ridge, in the form of a stomatal lip, was found associated with the stomatal guard cells of the bean (Phaseolus vulgaris) leaf. Ridge elevations greater than 1.0 micrometer or less than 0.25 micrometer did not serve as effective signals. Germ tubes of the fungus were highly oriented by ridge spacings of 0.5 to 6.7 micrometers. The data indicate that the fungus is able to distinguish uniquely minute differences in leaf surface topography in order to infect the host plant.     

Diversity and dynamics of dendritic signaling

Communication between neurons in the brain occurs primarily through synapses made onto elaborate treelike structures called dendrites. New electrical and optical recording techniques have led to tremendous advances in our understanding of how dendrites contribute to neuronal computation in the mammalian brain. The varied morphology and electrical and chemical properties of dendrites enable a spectrum of local and long-range signaling, defining the input-output relationship of neurons and the rules for induction of synaptic plasticity. In this way, diversity in dendritic signaling allows individual neurons to carry out specialized functions within their respective networks.     

Synchronous bursts of action potentials in ganglion cells of the developing mammalian retina

The development of orderly connections in the mammalian visual system depends on action potentials in the optic nerve fibers, even before the retina receives visual input. In particular, it has been suggested that correlated firing of retinal ganglion cells in the same eye directs the segregation of their synaptic terminals into eye-specific layers within the lateral geniculate nucleus. Such correlations in electrical activity were found by simultaneous recording of the extracellular action potentials of up to 100 ganglion cells in the isolated retina of the newborn ferret and the fetal cat. These neurons fired spikes in nearly synchronous bursts lasting a few seconds and separated by 1 to 2 minutes of silence. Individual bursts consisted of a wave of excitation, several hundred micrometers wide, sweeping across the retina at about 100 micrometers per second. These concerted firing patterns have the appropriate spatial and temporal properties to guide the refinement of connections between the retina and the lateral geniculate nucleus.