Petreanu L, Huber D, Sobczyk A, Svoboda K. Channelrhodopsin-2-assisted circuit mapping of long-range callosal projections. Nat Neurosci 10: 663-668

Howard Hughes Medical Institute, Janelia Farm Research Campus, 19700 Helix Drive, Ashburn, Virginia 20147, USA.
Nature Neuroscience (Impact Factor: 16.1). 06/2007; 10(5):663-8. DOI: 10.1038/nn1891
Source: PubMed


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 long-range cortical projections.

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Available from: Leopoldo Petreanu, May 15, 2014
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    • "Temporal windowing was used to detect photostimulation sites where the postsynaptic neuron's dendrites were directly stimulated (defined as excitatory events arriving within 7 msec post-stimulus) (Schubert et al., 2001), and these sites were excluded from analysis (shown in the figures as black pixels). Optogenetic photostimulation in brain slices was performed as previously described (Kiritani et al., 2012; Hooks et al., 2013), exploiting the retained photoexcitability of ChR2-expressing longrange axons in slices (Petreanu et al., 2007) and using conditions (in particular, tetrodotoxin and 4-aminopyridine in the bath solution) that isolate monosynaptic inputs (Petreanu et al., 2009). "
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    • "Conversely, a large number of synapses in a given cortical volume is " unaccounted for " (Binzegger et al. 2004). They have mostly been attributed to thalamo-cortical, inter-hemispheric (Cauller et al. 1998; Petreanu et al. 2007; Rubio-Garrido et al. 2009) or cortico-cortical horizontal projections connecting neighboring regions via far reaching axon-collaterals in supra-and sub-granular layers (Tucker and Katz 2003; Buzás et al. 2006; Larsen et al. 2007; Wester and Contreras 2012) but also to intra-areal horizontal projections. On a structural level, these long-range, often patchy projections can simultaneously minimize the wiring length and the average shortest path between a pair of neurons (Voges et al. 2011). "
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