Spatio-temporal dynamics of neocortical presynaptic terminal development using multi-photon imaging of the corpus callosum in vivo.

Within the developing central nervous system, the dynamics of synapse formation and elimination are insufficiently understood. It is ideal to study these processes in vivo, where neurons form synapses within appropriate behavioral and anatomical contexts. In vivo analysis is particularly important for long-range connections, since their development cannot be adequately studied in vitro. The corpus callosum (CC) represents a clinically-relevant long-range connection since several neurodevelopmental diseases involve CC defects. Here, we present a novel strategy for in vivo longitudinal and rapid time-lapse imaging of CC presynaptic terminal development. In postnatal mice, the time-course of CC presynaptic terminal formation and elimination was highly variable between axons or groups of axons. Young presynaptic terminals were remarkably dynamic - moving, dividing to generate more boutons, and merging to consolidate small terminals into large boutons. As synaptic networks matured, presynaptic mobility decreased. These rapid dynamics may be important for establishing initial synaptic contacts with postsynaptic partners, refining connectivity patterns or modifying synapse strength during development. Ultimately, this in vivo imaging approach will facilitate investigation of synapse development in other long-range connections and neurodevelopmental disease models.