![]() Neurotrophins are potent modulators of neuronal morphology,influencing not only axonal arborization( Cohen-Cory and Fraser, 1995 Singh and Miller, 2005) but also dendritic arbor growth ( Horch and Katz, 2002 Lom et al.,2002 McAllister,2000). Cell-surface adhesion molecules, inductive signals, secreted factors and signaling molecules actively participate in cell-to-cell communication, making neurons receptive to form synapses ( Chen and Ghosh,2005 Ethell and Pasquale,2005 Waites et al.,2005). Proper neuronal connections are thus specified as axons and dendrites make initial contacts and establish synapses that are often transient. Postsynaptic dendritic arbors are also dynamic, and it is through coordinated interactions between axon and dendritic filopodia that target recognition and synapse formation take place( Cline, 2001 Cohen-Cory, 2002 Dailey and Smith, 1996 Deng and Dunaevsky, 2005 Jontes and Smith, 2000 Niell et al., 2004 Trachtenberg et al., 2002). In vivo imaging studies demonstrate that developing axon terminals are highly dynamic and actively participate in synaptogenesis ( Alsina et al.,2001 O'Rourke and Fraser,1990 Witte et al.,1996). In the developing central nervous system, functional neuronal circuitry is established as axon terminals arborize, recognize their target neurons and form precise synaptic connections. Thus, BDNF influences synaptic connectivity in multiple ways: it enhances axon arbor complexity expanding the synaptic territory of the axon, while simultaneously coordinating synapse formation and stabilization with individual postsynaptic cells. These results, together with the timescale of the response by tectal neurons, suggest that the effects of BDNF on dendritic synaptic connectivity are secondary to its effects on presynaptic RGCs. The BDNF-elicited increase in synapse number complements the previously observed increase in presynaptic sites on RGC axons. BDNF function-blocking antibodies had opposite effects. Microinjection of BDNF into the optic tectum significantly increased synapse number in tectal neuron dendritic arbors within 24 hours, without significantly influencing arbor morphology. Time-lapse confocal microscopy of individual, double-labeled neurons revealed a coincident, activity-dependent mechanism of synaptogenesis and axon and dendritic arbor growth, which is differentially modulated by BDNF. ![]() Immunoelectron microscopy confirmed that PSD95-GFP predominantly localized to ultrastructurally identified synapses. Individual neurons were co-labeled with DsRed2 and a GFP-tagged postsynaptic density protein (PSD95-GFP) to visualize dendritic morphology and postsynaptic specializations simultaneously in vivo. Here, we have further explored the mechanisms by which BDNF shapes synaptic connectivity by imaging tectal neurons, the postsynaptic partners of RGCs. Increasing BDNF levels in the optic tectum of Xenopus tadpoles significantly increases both axon arborization and synapse density per axon terminal within a few hours of treatment. In the visual system, BDNF modulates the development of neuronal connectivity by influencing presynaptic retinal ganglion cell (RGC) axons. Neuronal connections are established through a series of developmental events that involve close communication between pre- and postsynaptic neurons.
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