Abstract
Most excitatory synapses in the brain form on dendritic spines. Two-photon uncaging of glutamate is widely utilized to characterize the structural plasticity of dendritic spines in brain slice preparations in vitro. In the present study, glutamate uncaging was used to investigate spine plasticity, for the first time, in vivo. A caged glutamate compound was applied to the surface of the mouse visual cortex in vivo, revealing the successful induction of spine enlargement by repetitive two-photon uncaging in a magnesium free solution. Notably, this induction occurred in a smaller fraction of spines in the neocortex in vivo (22%) than in hippocampal slices (95%). Once induced, the time course and mean long-term enlargement amplitudes were similar to those found in hippocampal slices. However, low-frequency (1–2 Hz) glutamate uncaging in the presence of magnesium caused spine shrinkage in a similar fraction (35%) of spines as in hippocampal slices, though spread to neighboring spines occurred less frequently than it did in hippocampal slices. Thus, the structural plasticity may occur similarly in the neocortex in vivo as in hippocampal slices, although it happened less frequently in our experimental conditions.
Highlights
Most excitatory synapses in the brain form on dendritic spines
A yellow fluorescent protein (YFP)-expressing mouse line (H) or green fluorescent protein (GFP)expressing mouse line (M), in which a subset of pyramidal neurons are labelled in a layer 5/6 selective manner, were used
To activate N-methyl-d-aspartic acid (NMDA) receptors effectively, the recording chamber was superfused with artificial cerebrospinal fluid containing no magnesium (Mg2+) ions
Summary
The volume of dendritic spines is tightly correlated with the functional expression of glutamate receptors in the young hippocampal slice preparations[1,2,3,4,5,6] and in the adult mouse neocortex in vivo[7]. Spine volume changes accompany long-term potentiation and depression of synapses in hippocampal slices[8,9,10,11,12]. We previously established a glutamate uncaging method in vivo in which a caged glutamate compound was applied to the surface of the brain. This allowed the compound to spread to the superficial extracellular space of the neocortex by passive diffusion[7]. The present study extends this work to assess the structural plasticity of dendritic spines, for the first time, in vivo
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