Abstract
Corticofugal projection neurons are key components in connecting the neocortex and the subcortical regions. In the barrel field, these neurons have various projection targets and play crucial roles in the rodent whisker sensorimotor system. However, the projection features of corticofugal projection neurons at the single-axon level are far from comprehensive elucidation. Based on a brain-wide positioning system with high-resolution imaging for Thy1-GFP M-line mice brains, we reconstructed and analyzed more than one hundred corticofugal projection neurons in both layer V and VI of barrel cortex. The dual-color imaging made it possible to locate the neurons’ somata, trace their corresponding dendrites and axons and then distinguish the neurons as L5 type I/II or L6 type. The corticofugal projection pattern showed significant diversity across individual neurons. Usually, the L5 type I neurons have greater multi-region projection potential. The thalamus and the midbrain are the most frequent projection targets among the investigated multidirectional projection neurons, and the hypothalamus is particularly unique in that it only appears in multidirectional projection situations. Statistically, the average branch length of apical dendrites in multi-region projection groups is larger than that of single-region projection groups. This study demonstrated a single-axon-level analysis for barrel corticofugal projection neurons, which could provide a micro-anatomical basis for interpreting whisker sensorimotor circuit function.
Highlights
The barrel cortex has been recognized as a classical model for studying the neural circuits that connect the neocortex and the subcortical region[1]
For the layer V pyramidal neurons, we classified them into type I and type II according to the morphological features of apical dendrites[22]
We analyzed the morphology of corticofugal projection neurons of a Thy1-GFP M-line mouse in layer V and VI of the barrel field by applying the brain-wide positioning system
Summary
The barrel cortex has been recognized as a classical model for studying the neural circuits that connect the neocortex and the subcortical region[1]. Besides the contralateral cortex projections through the corpus callosum (cc), the long-distance axons of major pyramidal neurons may project to a mass of subcortical regions, such as the striatum, the thalamus, the superior colliculus and the pons[4]. Those pyramidal neurons are defined as corticofugal projection neurons[5]. To resolve the neural circuit that connects the barrel field and the subcortical target at microscopic resolution, we resorted to our newly developed brain-wide positioning system (BPS)[21]. This work intends to bring a novel and precise perspective for investigating subcortical connectivity of the barrel field
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