Cell diversity and connection specificity between callosal projection neurons in the frontal cortex.

Recent advances have established that intralaminar and interlaminar excitatory networks between neocortical pyramidal cells are specialized into subnetworks. Here, we have investigated how the commissural system organizes the intracortical excitatory subnetworks to communicate between cortical hemispheres. Whole-cell recordings were obtained from callosal projection neurons [commissural (COM) cells], identified by in vivo injection of retrograde fluorescent tracer into one hemisphere, in rat frontal cortical slices. We found that layer V (L5) COM cells were heterogeneous in physiological and morphological properties that correlated with projection patterns to contralateral and ipsilateral cortical areas. The probability of synaptically connected pairs of L5 COM cells was higher in cell pairs of the same firing subtypes than that in different cell subtype pairs. In interlaminar connections, layer II/III (L2/3) COM cells preferentially innervated L5 COM cells. Moreover, pairs of the same L5 COM subtypes were more likely to share inputs from L2/3 COM cells than were different COM subtype cell pairs. In addition, common inputs from L2/3 COM cells were frequently observed in L5 pairs of corticopontine cells and given firing subtypes of COM cells. Our results suggest that callosal communications are achieved via several distinct COM cell subnetworks differentiated according to the ipsilateral corticocortical and subcortical projection patterns.