Mapping functional connectivity in barrel-related columns reveals layer- and cell type-specific microcircuits

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Abstract
Synaptic circuits bind together functional modules of the neocortex. We aim to clarify in a rodent model how intra- and transcolumnar microcircuits in the barrel cortex are laid out to segregate and also integrate sensory information. The primary somatosensory (barrel) cortex of rodents is the ideal model system to study these issues because there, the tactile information derived from the large facial whiskers on the snout is mapped onto so called barrel-related columns which altogether form an isomorphic map of the sensory periphery. This allows to functionally interpret the synaptic microcircuits we have been analyzing in barrel-related columns by means of whole-cell recordings, biocytin filling and mapping of intracortical functional connectivity with sublaminar specificity by computer-controlled flash-release of glutamate. We find that excitatory spiny neurons (spiny stellate, star pyramidal, and pyramidal cells) show a layer-specific connectivity pattern on top of which further cell type-specific circuits can be distinguished. The main features are: (a) strong intralaminar, intracolumnar connections are established by all types of excitatory neurons with both, excitatory and (except for layer Vb- intrinsically burst-spiking-pyramidal cells) inhibitory cells; (b) effective translaminar, intracolumnar connections become more abundant along the three main layer compartments of the canonical microcircuit, and (c) extensive transcolumnar connectivity is preferentially found in specific cell types in each of the layer compartments of a barrel-related column. These multiple sequential and parallel circuits are likely to be suitable for specific cortical processing of “what” “where” and “when” aspects of tactile information acquired by the whiskers on the snout.