Evidence for activity-dependent cortical wiring: formation of interhemispheric connections in neonatal mouse visual cortex requires projection neuron activity.

Neuronal activity plays a pivotal role in shaping neuronal wiring. We investigated the role of neuronal activity in the formation of interhemispheric (callosal) axon projections in neonatal mouse visual cortex. Axonal labeling with enhanced green fluorescent protein (GFP) was used to demonstrate spatially organized pattern of callosal projections: GFP-labeled callosal axons from one hemisphere projected densely to a narrowly restricted region at the border between areas 17 and 18 in the contralateral hemisphere, in which they terminated in layers 1-3 and 5. This region- and layer-specific innervation pattern developed by postnatal day 15 (P15). To explore the role of neuronal activity of presynaptic and postsynaptic neurons in callosal connection development, an inwardly rectifying potassium channel, Kir2.1, was expressed in callosal projection neurons and their target postsynaptic neurons. Kir2.1 overexpression reduced the firing rate of cortical neurons. Kir2.1 overexpression in callosal projection neurons disturbed the growth of axons and their arbors that normally occurs between P7 and P13, whereas that in postsynaptic neurons had limited effect on the pattern of presynaptic callosal axon innervation. In addition, exogenous expression of a gain-of-function Kir2.1 mutant channel found in patients with a familial heart disease caused severe deficits in callosal axon projections. These results suggest that projection neuron activity plays a crucial role in interhemispheric connection development and that enhanced Kir2.1 activity can affect cortical wiring.