Column-like Ca2+ clusters in the mouse neonatal neocortex revealed by three-dimensional two-photon Ca2+ imaging in vivo

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Neuronal network activity in the developing brain is generated in a discontinuous manner. In the visual cortex during the period of physiological blindness of immaturity, this activity mainly comprises retinally triggered spindle bursts or Ca2+ clusters thought to contribute to the activity-dependent construction of cortical circuits. In spite of potentially important developmental functions, the spatial structure of these activity patterns remains largely unclear. In order to address this issue, we here used three-dimensional two-photon Ca2+ imaging in the visual cortex of neonatal mice at postnatal days (P) 3–4 in vivo. Large-scale voxel imaging covering a cortical depth of 200 μm revealed that Ca2+ clusters, identified as spindle bursts in simultaneous extracellular recordings, recruit cortical glutamatergic neurons of the upper cortical plate (CP) in a column-like manner. Specifically, the majority of Ca2+ clusters exhibit prominent horizontal confinement and high intra-cluster density of activation involving the entire depth of the upper CP. Moreover, using simultaneous Ca2+ imaging from hundreds of neurons at single-cellular resolution, we demonstrate that the degree of neuronal co-activation within Ca2+ clusters displays substantial heterogeneity. We further provide evidence that co-activated cells within Ca2+ clusters are spatially distributed in a non-stochastic manner. In summary, our data support the conclusion that dense coding in the form of column-like Ca2+ clusters is a characteristic property of network activity in the developing visual neocortex. Such knowledge is expected to be relevant for a refined understanding of how specific spatiotemporal characteristics of early network activity instruct the development of cortical circuits.

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