Stringent structural plasticity of dendritic spines revealed by two-photon glutamate uncaging in adult mouse neocortex in vivo

Two-photon uncaging of glutamate has been widely utilized to characterize the structural plasticity in brain slice preparations in vitro. Here, we investigated, for the first time, the spine plasticity using glutamate uncaging in the neocortex of adult mice in vivo. The spine enlargement was induced in a smaller fraction (22%) of spines than in the young hippocampal slices (95%) even under a low magnesium condition. Once induced, the time-course and the mean amplitudes (81%) of long-term enlargement were much the same as those in slices. Low frequency (1-2 Hz) glutamate uncaging resulted in spine shrinkage in a similar fraction (34%) of spines as in vitro, but spread to the neighboring spines to a less degree than in vitro. Thus, we have found that the adult neocortex in vivo shows similar plasticity of spines as in young hippocampus in vitro, while only a smaller subset of spines show rapid enlargement.