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
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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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    ABSTRACT: eLife digest In 1909, a German scientist called Korbinian Brodmann published the first map of the outer layer of the human brain. After staining neurons with a dye and studying the structures of the cells and how they were organized, he realized that he could divide the cortex into 43 numbered regions. Most Brodmann areas can be divided into a number of horizontal layers, with layer 1 being closest to the surface of the brain. Neurons in the different layers form distinct sets of connections, and the relative thickness of the layers has implications for the function carried out by that area. It is thought, for example, that the motor cortex does not have a layer 4, which suggests that the neural circuitry that controls movement differs from that in charge of vision, hearing, and other functions. Yamawaki et al. now challenge this view by providing multiple lines of evidence for the existence of layer 4 in the motor cortex in mice. Neurons at the border between layer 3 and layer 5A in the motor cortex possess many of the same properties as the neurons in layer 4 in sensory cortex. In particular, they receive inputs from a brain region called the thalamus, and send outputs to neurons in layers 2 and 3. Yamawaki et al. go on to characterize some of the properties of the neurons in the putative layer 4 of the motor cortex, finding that they do not look like the specialized ‘stellate’ cells that are found in some other areas of the cortex. Instead, they resemble the ‘pyramidal’ type of neuron that is found in all layers and areas of the cortex. The discovery that the motor cortex is more similar in its circuit connections to other area of the cortex than previously thought has important implications for our understanding of this region of the brain. DOI:
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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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