The Developmental Profile of Visual Cortex Astrocytes

SUMMARY Though once thought of as passive support cells for neurons, it is now clear that astrocytes signal via calcium (Ca 2+ ) to trigger gliotransmission that impacts surrounding neurons and synapses. We therefore investigated how astrocytes in layer-5 (L5) mouse visual cortex mature electrophysiologically and morphologically over postnatal days (P) 3 – 30 and how these relate to changes in spontaneous Ca 2+ events. Across this age range, resting membrane potential increased, and input resistance decreased. Astrocytes also revealed different membrane responses to voltage steps: notably, membrane responses became more passive with age. Two-photon (2p) imaging of dye-loaded patched cells and confocal imaging revealed that gap-junction coupling increased starting around P7. Morphological reconstructions revealed increased branch density but also shorter branches after P20, suggesting that astrocyte branches may get pruned as dense tiling is established. Finally, we visualized spontaneous Ca 2+ transients with 2p microscopy and found that Ca 2+ events decorrelated, became more frequent as well as briefer with age. As astrocytes mature, spontaneous Ca 2+ activity thus changes from relatively cell-wide, synchronous waves to local transients. Several astrocyte properties were stably mature from ~P15 onwards, although morphology continued to develop further. Our findings provide a descriptive foundation of astrocyte maturation, useful for the study of astrocytic impact on visual cortex critical period plasticity.