Diverse Long-Range Axonal Projections of Excitatory Layer 2/3 Neurons in Mouse Barrel Cortex

Takayuki Yamashita,Aurélie Pala,Katia Galan,Sara S A Petersen,Carl C H Petersen,Angeliki Vavladeli,Sylvain Crochet

doi:10.3389/fnana.2018.00033

Abstract

Excitatory projection neurons of the neocortex are thought to play important roles in perceptual and cognitive functions of the brain by directly connecting diverse cortical and subcortical areas. However, many aspects of the anatomical organization of these inter-areal connections are unknown. Here, we studied long-range axonal projections of excitatory layer 2/3 neurons with cell bodies located in mouse primary somatosensory barrel cortex (wS1). As a population, these neurons densely projected to secondary whisker somatosensory cortex (wS2) and primary/secondary whisker motor cortex (wM1/2), with additional axon in the dysgranular zone surrounding the barrel field, perirhinal temporal association cortex and striatum. In three-dimensional reconstructions of 6 individual wS2-projecting neurons and 9 individual wM1/2-projecting neurons, we found that both classes of neurons had extensive local axon in layers 2/3 and 5 of wS1. Neurons projecting to wS2 did not send axon to wM1/2, whereas a small subset of wM1/2-projecting neurons had relatively weak projections to wS2. A small fraction of projection neurons solely targeted wS2 or wM1/2. However, axon collaterals from wS2-projecting and wM1/2-projecting neurons were typically also found in subsets of various additional areas, including the dysgranular zone, perirhinal temporal association cortex and striatum. Our data suggest extensive diversity in the axonal targets selected by individual nearby cortical long-range projection neurons with somata located in layer 2/3 of wS1.

Highlights

The mouse primary somatosensory barrel cortex is an anatomically-defined brain region specialized in processing whisker sensory information (Woolsey and Van der Loos, 1970; Petersen, 2007; Diamond et al, 2008; Feldmeyer et al, 2013)
In agreement with previous reports (Harris et al, 2014; Madisen et al, 2015), we found that tdTomato-expressing cells were almost exclusively restricted to L2/3 with a large fraction of neurons being labeled in that cortical layer (Figure 1A)
Hotspots of long-range axonal projections from these L2/3 neurons were identified across mice in striatum and various cortical regions (Figures 1, 2)

Summary

Introduction

The mouse primary somatosensory barrel cortex (wS1) is an anatomically-defined brain region specialized in processing whisker sensory information (Woolsey and Van der Loos, 1970; Petersen, 2007; Diamond et al, 2008; Feldmeyer et al, 2013). Using a brain-wide imaging system (Gong et al, 2016), the whole axonal structure of more than 100 cortico-fugal projection neurons in layers 5 and 6 (L5/6) of wS1 were reconstructed at the single-cell level to reveal their axonal projection patterns and target preferences (Guo et al, 2017). The target locations and density of long-range axonal arborizations of wS1 L2/3 projection neurons remain to be elucidated

Methods

Results

Conclusion