Pre‐synaptic and post‐synaptic neuronal activity supports the axon development of callosal projection neurons during different post‐natal periods in the mouse cerebral cortex

Callosal projection neurons, one of the major types of projection neurons in the mammalian cerebral cortex, require neuronal activity for their axonal projections [H. Mizuno et al. (2007) J. Neurosci. , 27, 6760–6770; C. L. Wang et al. (2007) J. Neurosci. , 27, 11334–11342]. Here we established a method to label a few callosal axons with enhanced green fluorescent protein in the mouse cerebral cortex and examined the effect of pre‐synaptic/post‐synaptic neuron silencing on the morphology of individual callosal axons. Pre‐synaptic/post‐synaptic neurons were electrically silenced by Kir2.1 potassium channel overexpression. Single axon tracing showed that, after reaching the cortical innervation area, green fluorescent protein‐labeled callosal axons underwent successive developmental stages: axon growth, branching, layer‐specific targeting and arbor formation between post‐natal day (P)5 and P9, and the subsequent elaboration of axon arbors between P9 and P15. Reducing pre‐synaptic neuronal activity disturbed axon growth and branching before P9, as well as arbor elaboration afterwards. In contrast, silencing post‐synaptic neurons disturbed axon arbor elaboration between P9 and P15. Thus, pre‐synaptic neuron silencing affected significantly earlier stages of callosal projection neuron axon development than post‐synaptic neuron silencing. Silencing both pre‐synaptic and post‐synaptic neurons impaired callosal axon projections, suggesting that certain levels of firing activity in pre‐synaptic and post‐synaptic neurons are required for callosal axon development. Our findings provide in‐vivo evidence that pre‐synaptic and post‐synaptic neuronal activities play critical, and presumably differential, roles in axon growth, branching, arbor formation and elaboration during cortical axon development.