Retrograde tracing with recombinant rabies virus reveals correlations between projection targets and dendritic architecture in layer 5 of mouse barrel cortex

Abstract

A recombinant rabies virus was used as a retrograde tracer to allow complete fi lling of the axonal and dendritic arbors of identifi ed projection neurons in layer 5 of mouse primary somatosensory cortex (S1) in vivo. Previous studies have distinguished three types of layer 5 pyramids in S1: tall-tufted, tall-simple, and short. Layer 5 pyramidal neurons were retrogradely labeled from several known targets: contralateral S1, superior colliculus, and thalamus. The complete dendritic arbors of labeled cells were reconstructed to allow for unambiguous classifi cation of cell type. We confi rmed that the tall-tufted pyramids project to the superior colliculus and thalamus and that short layer 5 pyramidal neurons project to contralateral cortex, as previously described. We found that tall-simple pyramidal neurons contribute to corticocortical connections. Axonal reconstructions show that corticocortical projection neurons have a large superfi cial axonal arborization locally, while the subcortically projecting neurons limit axonal arbors to the deep layers. Furthermore, reconstructions of local axons suggest that tall-simple cell axons have extensive lateral spread while those of the short pyramids are more columnar. These differences were revealed by the ability to completely label dendritic and axonal arbors in vivo and have not been apparent in previous studies using labeling in brain slices.

Longitudinal in vivo Imaging Reveals Balanced and Branch-Specific Remodeling of Mature Cortical Pyramidal Dendritic Arbors after Stroke

“…Normally, the apical dendrites of pyramidal neurons are relatively symmetric (Larsen and Callaway, 2006;Larsen et al, 2007). Previous work from our laboratory examining Golgi-Cox-stained tissue (Brown et al, 2008) or single time-point imaging of dendrites after stroke (Brown et al, 2007) has shown that peri-infarct neurons become highly asymmetric with their arbors preferentially oriented away from the site of infarction.…”

“…This expression pattern raises the possibility that there may be something unique about this population of fluorescently labeled pyramidal cells. From an anatomic perspective, GFP-or yellow fluorescent protein (YFP)-labeled cortical neurons (under the Thy-1 promoter) have a dendritic morphology and spine structure (e.g., spine length and density; (Enright et al, 2007;Holtmaat et al, 2009)) that is comparable with cortical pyramidal neurons identified with Golgi-Cox, biocytin staining, or viral transfection (Brown et al, 2008;Larsen and Callaway, 2006;Larsen et al, 2007;Nimchinsky et al, 2001). Similarly, mapping of local circuits with laser scanning photostimulation has shown that GFP-positive cortical neurons in layer 5 receive synaptic inputs that are indistinguishable from non-GFP-positive cortical neurons (Holtmaat et al, 2005).…”

Three-dimensional axon morphologies of individual layer 5 neurons indicate cell type-specific intracortical pathways for whisker motion and touch

“…By using artificial whisking in anesthetized rats, it was suggested that the POm (i.e., paralemniscal) pathway may carry information on whisker motion, whereas the VPM (i.e., lemniscal) pathway may encode whisker touch (in addition to whisker motion). Thus, cell type-and state-specific spiking in L5 during active somatosensation (28, 29) could be mediated by these two functionally and anatomically segregated thalamocortical pathways (23).Finally, functional and morphological studies of slender-and thick-tufted neurons indicate that these output neurons also contribute to intracortical circuits (17,(30)(31)(32)(33). Here, we reconstructed the entire 3D axon projections in the vibrissal area of individual, in vivo filled slender-and thick-tufted neurons in rat vibrissal cortex by using a semiautomated tracing technique (34).…”

“…Finally, functional and morphological studies of slender-and thick-tufted neurons indicate that these output neurons also contribute to intracortical circuits (17,(30)(31)(32)(33). Here, we reconstructed the entire 3D axon projections in the vibrissal area of individual, in vivo filled slender-and thick-tufted neurons in rat vibrissal cortex by using a semiautomated tracing technique (34).…”

Selective Manipulation of Neural Circuits

“…Retrograde viruses have been more difficult to create. The first solution to this problem was to use viruses known to cause disease through retrograde transport, such as rabies and herpes simplex virus [57][58][59][60]. Although these viruses can be modified to prevent transmission to the experimenter, they can be difficult to produce and can cause inflammation and injury to nervous tissue.…”