Previous studies suggest that neuronal activity may guide the development of synaptic connections in the central nervous system through mechanisms involving glutamate receptors and GTPase-dependent modulation of the actin cytoskeleton. Here we demonstrate by in vivo time-lapse imaging of optic tectal cells in Xenopus laevis tadpoles that enhanced visual activity driven by a light stimulus promotes dendritic arbor growth. The stimulus-induced dendritic arbor growth requires glutamate-receptor-mediated synaptic transmission, decreased RhoA activity and increased Rac and Cdc42 activity. The results delineate a role for Rho GTPases in the structural plasticity driven by visual stimulation in vivo.

We report an electroporation technique for targeting gene transfer to individual cells in intact tissue. Electrical stimulation through a micropipette filled with DNA or other macromolecules electroporates a single cell at the tip of the micropipette. Electroporation of a plasmid encoding enhanced green fluorescent protein (GFP) into the brain of intact Xenopus tadpoles or rat hippocampal slices resulted in GFP expression in single neurons and glia. In vivo imaging showed morphologies, dendritic arbor dynamics, and growth rates characteristic of healthy cells. Coelectroporation of two plasmids resulted in expression of both proteins, while electroporation of fluorescent dextrans allowed direct visualization of transfer of molecules into cells. This technique will allow unprecedented spatial and temporal control over gene delivery and protein expression.

The synaptotropic hypothesis, which states that synaptic inputs control the elaboration of dendritic (and axonal) arbors was articulated by Vaughn in 1989. Today the role of synaptic inputs in controlling neuronal structural development remains an area of intense research activity. Several recent studies have applied modern molecular genetic, imaging and electrophysiological methods to this question and now provide strong evidence that maturation of excitatory synaptic inputs is required for the development of neuronal structure in the intact brain. Here we critically review data concerning the hypothesis with the expectation that understanding the circumstances when the data do and do not support the hypothesis will be most valuable. The synaptotrophic hypothesis contributes at both conceptual and mechanistic levels to our understanding of how relatively minor changes in levels or function of synaptic proteins may have profound effects on circuit development and plasticity.

Decreased synthesis and increased degradation of MyHCs contribute to ICU-acquired muscle wasting. The rates and time frames suggest that pathogenesis of muscle failure is initiated very early during critical illness. The persisting reduction of MyHC suggests that sustained treatment is required.

The size and shape of neuronal dendritic arbors affect the number and type of synaptic inputs, as well as the complexity and function of brain circuits. In the intact brain, dendritic arbor growth and the development of excitatory glutamatergic synapse are concurrent. Consequently, it has been difficult to resolve whether synaptic inputs drive dendritic arbor development. Here, we test the role of AMPA receptor (AMPAR)-mediated glutamatergic transmission in dendrite growth by expressing peptides corresponding to the intracellular C-terminal domains of AMPAR subunits GluR1 (GluR1Ct) and GluR2 (GluR2Ct) in optic tectal neurons of the Xenopus retinotectal system. These peptides significantly reduce AMPAR synaptic transmission in transfected neurons while leaving visual system circuitry intact. Daily in vivo imaging over 5 days revealed that GluR1Ct or GluR2Ct expression dramatically impaired dendrite growth, resulting in less complex arbors than controls. Time-lapse images collected at 2-h intervals over 6 h show that both GluR1Ct and GluR2Ct decrease branch lifetimes. Ultrastructural analysis indicates that synapses formed onto neurons expressing the GluRCt are less mature than synapses onto control neurons. These data suggest that the failure to form complex arbors is due to reduced stabilization of new synapses and dendritic branches. Although visual stimulation increases dendritic arbor growth rates in control tectal neurons, a weak postsynaptic response to visual experience in GluRCt-expressing cells leads to retraction of branches. These results indicate that AMPARmediated transmission underlies experience-dependent dendritic arbor growth by stabilizing branches, and support a competitionbased model for dendrite growth.dendrites ͉ plasticity ͉ retinotectal ͉ synaptogenesis ͉ visual system D endrite arbor growth is characterized by the highly dynamic addition and retraction of branches (1-3). Although most new branches rapidly retract, a small fraction are maintained and become long-lasting components of the growing arbor (1, 3). One hypothesis to explain the dynamic behavior of new dendritic branches is that transient branches sample the local environment for appropriate presynaptic contact sites (4-7) and are then stabilized by the formation and maturation of synapses. During the development of glutamatergic synapses, which comprise the majority of fast excitatory synaptic contacts in the central nervous system, AMPA receptors (AMPAR) are added to synapses which are initially populated solely by the NMDA subtype of glutamate receptors, by mechanisms akin to longterm potentiation (8-10). The ''synaptotrophic'' model of dendrite growth suggests that addition of AMPAR to transient NMDA-only ''silent'' synapses stabilizes both the synapse and the associated dendritic branch. This model is supported by findings that accumulation of postsynaptic density proteins correlates with branch stability (3) and evidence that synapse strengthening and weakening correlate with changes in spine size and dynamics (11-13). Alth...

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The formation of CNS circuits is characterized by the coordinated development of neuronal structure and synaptic function. The activity-regulated candidate plasticity gene 15 (cpg15) encodes a glycosylphosphatidylinositol (GPI)-linked protein whose in vivo expression increases the dendritic arbor growth rate of Xenopus optic tectal cells. We now demonstrate that tectal cell expression of CPG15 significantly increases the elaboration of presynaptic retinal axons by decreasing rates of branch retractions. Whole-cell recordings from optic tectal neurons indicate that CPG15 expression promotes retinotectal synapse maturation by recruiting functional AMPA receptors to synapses. Expression of truncated CPG15, lacking its GPI anchor, does not promote axon arbor growth and blocks synaptic maturation. These results suggest that CPG15 coordinately increases the growth of pre- and postsynaptic structures and the number and strength of their synaptic contacts.

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