During development, competition between axons causes permanent removal of synaptic connections, but the dynamics have not been directly observed. Using transgenic mice that express two spectral variants of fluorescent proteins in motor axons, we imaged competing axons at developing neuromuscular junctions in vivo. Typically, one axon withdrew progressively from postsynaptic sites and the competing axon extended axonal processes to occupy those sites. In rare instances when the remaining axon did not reoccupy a site, the postsynaptic receptors rapidly disappeared. Interestingly, the progress and outcome of competition was unpredictable. Moreover, the relative areas occupied by the competitors shifted in favor of one axon and then the other. These results show synaptic competition is not always monotonic and that one axon's contraction in synaptic area is associated with another axon's expansion.
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