Aims: 

Motility of dendritic spines underlie the brains remarkable ability to adapt to experience, and stability of synapses are thought to be necessary for long-lasting memories. Functional rewiring of cortical circuits are thought to rely on structural plasticity with synapse formation and elimination. While plasticity is most pronounced during a period of hightened plasticity, the critical period during postnatal development, most studies of spine dynamics have been performed in adult animals. Furthermore, studies of spine dynamics have mostly focused on apical dendrites of layer V neurons in layer Is. The aim of the current study was to investigate spine dynamics in layers II/III, in which neurons are more likely to be directly involved in processing of visual input from thalamocortical projection reaching visual cortex in juvenile animals.

Methods: 

In order obtain sparse and strong labeling of excitatory neurons in layer II/III we used in utero electroporation and single-cell electroporation (Kitamura et al., 2008) of C57Bl mice. Mice, P19-P21, was implanted with a cranial window. Before the peak of the critical period, P28-P32, the binocular region of V1 was determined by use of intrinsic optical imaging and dendritic branches from neurons within the binocular region was identified. Chronic two-photon laser-scanning microscopy was used to repeatedly image the same dendritic branches and spines.

Results: 

Neurons expressing fluorescent proteins by use of single-cell electroporation seemed to be healthy and could be followed for up to 90 days. Dendrites of neurons expressing fluorescent proteins after in utero electroporation were followed repeatedly during a brief period of eyeclosure during the peak of the critical period.