The anterolateral barrel subfield differs from the posteromedial barrel subfield in the morphology and cell density of parvalbumin-positive GABAergic interneurons.

Layer 4 of the rodent somatosensory cortex has unitary structures called barrels that receive tactile information from individual vibrissae. Barrels in the anterolateral barrel subfield (ALBSF) are much smaller and have gained less attention than larger barrels in the posteromedial barrel subfield (PMBSF), though the former outnumber the latter. We compared the morphological features of barrels between the ALBSF and PMBSF in male mice using deformation-free tangential sections and confocal optical slice-based, precise reconstructions of barrels. The average volume of a single barrel in the ALBSF was 34.7% of that in the PMBSF, but the numerical density of parvalbumin (PV)-positive interneurons in the former was 1.49 times higher than that in the latter. Moreover, PV neuron density in septa was 2.08 times higher in the ALBSF than that in the PMBSF. The proportions of PV neuron number to both all neuron number and all GABAergic neuron number in the ALBSF were also higher than those in the PMBSF. Somata of PV neurons in barrels and septa in the ALBSF received 1.64 and 1.50 times more vesicular glutamate transporter Type 2-labeled boutons than those in the PMBSF, suggesting more potent feed-forward inhibitory circuits in the ALBSF. The mode of connectivity through dendritic gap junctions among PV neurons also differed between the ALBSF and PMBSF. Clusters of smaller unitary structures containing a higher density of representative GABAergic interneurons with differential morphological features in the ALBSF suggest division of functional roles in the two vibrissae-barrel systems, as has been demonstrated by behavioral studies. Significance Statement The somatosensory cortex of rodents contains unique unitary structures called barrels that receive tactile information from individual vibrissae, a group of specialized thick hair around the nose. This vibrissa-barrel system enables animals to search and recognize surrounding objects. Barrels are located in two subfields, anterolateral barrel subfield (ALBSF) and posteromedial barrel subfield (PMBSF). The ALBSF contains much smaller barrels than the PMBSF and have gained little attention thus far in spite of dense packing of much more barrels. We show here that the ALBSF is further characterized by a higher density of parvalbumin-positive GABAergic interneurons, differential thalamocortical connectivity, and a distinctive mode of gap junction coupling. All these suggest different roles of the two vibrissae-barrel systems as reflected in behaviors.