Local calcium transients regulate the spontaneous motility of dendritic filopodia.

During development, dendrites, and in particular dendritic filopodia, undergo extensive structural remodeling, presumably to help establish synaptic contacts. Here, we investigated the role of calcium signaling in dendritic plasticity by simultaneously recording calcium dynamics and filopodial growth in rat hippocampal slice cultures. Local calcium transients occurred in dendritic filopodia and shafts, often at putative synaptic sites. These events were highly correlated with filopodial motility: comparatively rare when individual filopodia emerged from the dendrite, they became more frequent after filopodia started growing, finally causing them to halt. Accordingly, an experimental reduction of the frequency of local calcium transients elicited filopodial growth and, conversely, calcium uncaging reduced filopodial motility. Our observations suggest that low levels of local calcium transients facilitate filopodial outgrowth, whereas high levels inhibit the formation of filopodia and stabilize newly formed ones. This process may facilitate synapse formation and may serve as a homeostatic mechanism distributing synapses evenly along developing dendrites.