Intracortical connectivity of layer VI pyramidal neurons in the somatosensory cortex of normal and barrelless mice

Abstract

In mice, barrels in layer IV of the somatosensory cortex correspond to the columnar representations of whisker follicles. In barrelless (BRL) mice, barrels are absent, but functionally, a columnar organization persists. Previously we characterized the aberrant geometry of thalamic projection of BRL mice using axonal reconstructions of individual neurons. Here we proceeded with the analysis of the intracortical projections from layer VI pyramidal neurons, to assess their contribution to the columnar organization. From series of tangential sections we reconstructed the axon collaterals of individual layer VI pyramidal neurons in the C2 barrel column that were labelled with biocytin [controls from normal (NOR) strain, 19 cells; BRL strain, nine cells]. Using six morphological parameters in a cluster analysis, we showed that layer VI neurons in NOR mice are distributed into four clusters distinguished by the radial and tangential extent of their intracortical projections. These clusters correlated with the cortical or subcortical projection of the main axon. In BRL mice, neurons were distributed within the same four clusters, but their projections to the granular and supragranular layers were significantly smaller and their tangential projection was less columnar than in NOR mice. However, in both strains the intracortical projections had a preference for the appropriate barrel column (C2), indicating that layer VI pyramidal cells could participate in the functional columnar organization of the barrel cortex. Correlative light and electron microscopy analyses provided morphometric data on the intracortical synaptic boutons and synapses of layer VI pyramidal neurons and revealed that projections to layer IV preferentially target excitatory dendritic spines and shafts.

Synaptic Microcircuits in the Barrel Cortex

“…The axonal arbour of CT L6A pyramidal neurons with its marked collateralisation at the level of a L4 barrel (see Fig. 4.2c) suggests that these neurons form synaptic connections with L4 spiny neurons [56,57]. This has been demonstrated for visual cortex [58,59] and very recently also for barrel cortex [49,60].…”

“…The majority of axonal collaterals of CC L6A pyramidal cells remains predominantly within layers 5 and 6 of the S1 cortex [56,57,128]. The axon of CC L6A pyramidal cells is very extensive and projects over many 'barrel columns' thereby mediating transcolumnar interactions in the infragranular layers of the barrel cortex ( Fig.…”

“…4.5d). In addition, a small population of L6A pyramidal cells with apical dendrites ascending to layer 1 or those with obliquely oriented main dendrites were also found [49,56,57,127]. L6A pyramidal cells can be subdivided into at least two major groups: those with a predominantly intracortical axonal projection pattern and those with an axon that projects back to the thalamic nuclei [55-57, 128, 129; see also [130] for a review).…”

“…The axon is significantly longer than that of VPM-projecting L6A pyramidal cells; it terminates either in layer 5A or 4 in several barrel-related clusters that ramify profusely. These neurons may also project laterally within layer 6 of the barrel cortex [57]. CT L6A pyramidal cells that innervate exclusively POm neurons are relatively rare and have not been described in detail [56,119]; their axons reside largely within layer 6 with a few collaterals ascending to layers 5 and 4.…”

“…CT L6A pyramidal cells can be further subdivided with respect to the innervated thalamic nucleus i.e. whether they target only VPM, POm or both [56,57,128]. L6A pyramidal cells projecting exclusively to VPM reside mostly in the upper half of layer 6.…”