Channelrhodopsin-2-assisted circuit mapping of long-range callosal projections

The functions of cortical areas depend on their inputs and outputs, but the detailed circuits made by long- range projections are unknown. We show that the light- gated channel channelrhodopsin- 2 ( ChR2) is delivered to axons in pyramidal neurons in vivo. In brain slices from ChR2- expressing mice, photostimulation of ChR2- positive axons can be transduced reliably into single action potentials. Combining photostimulation with whole- cell recordings of synaptic currents makes it possible to map circuits between presynaptic neurons, defined by ChR2 expression, and postsynaptic neurons, defined by targeted patching. We applied this technique, ChR2- assisted circuit mapping ( CRACM), to map long- range callosal projections from layer ( L) 2/3 of the somatosensory cortex. L2/3 axons connect with neurons in L5, L2/3 and L6, but not L4, in both ipsilateral and contralateral cortex. In both hemispheres the L2/3- to- L5 projection is stronger than the L2/3- to- L2/3 projection. Our results suggest that laminar specificity may be identical for local and longrange cortical projections.