Critical period for activity-dependent elimination of corticospinal synapses in vitro

There is no in vitro model of the critical periods for developmental plasticity, the time windows of plastic changes during development, which may hinder in-depth mechanistic analysis. We have shown previously that the corticospinal tract with synaptic connections can be reconstructed in in vitro co-cultures using slices of the sensorimotor cortex and spinal cord of the rat. In our in vitro system, corticospinal synapses form widely over spinal gray matter during early development, after which those on the ventral side are eliminated in an activity and N-methyl-d-aspartate (NMDA)-dependent manner. A detailed quantitative analysis of the time course of sensitivity to an NMDA blocker was made with this system. Synapse distribution was evaluated by recording field excitatory post-synaptic potentials evoked by deep cortical layer stimulation. Corticospinal axon terminal distribution was examined by anterograde labeling with biocytin. We showed that the d-2-amino-5-phosphonovaleric acid (APV) effect is irreversible for at least the length of culture. When APV was removed from the medium before 6 days in vitro(DIV) or after 11 DIV, elimination of ventral synapses was not blocked. APV sensitivity showed a clearly defined time window. A 6-11 DIV application was necessary and sufficient for the full, irreversible block of synapse elimination. From 6-11 DIV, APV sensitivity seems to decrease gradually but not linearly. This system provides an in vitro model of critical periods for developmental plasticity of central synapses which up to now has not been available.