Layer-specific regulation of cortical neurons by interhemispheric inhibition.

Processing of sensory information from both sides of the body requires coordination of sensory input between the two hemispheres. This coordination is achieved by transcallosal (interhemispheric) fibers that course though the upper cortical layers. In a recent study by Palmer et al. (2012), we investigated the role of this interhemispheric input on the dendritic and somatic activity of cortical pyramidal neurons. This study showed that interhemispheric input evokes GABAB-mediated inhibition in the distal dendrites of layer 5 pyramidal neurons, decreasing the action potential output when paired with contralateral sensory stimulation. In contrast, layer 2/3 pyramidal neurons were not inhibited by interhemispheric input, possibly due to transcallosal fibers evoking more excitation in these neurons than layer 5 neurons. These results highlight both the precise nature of the microcircuitry of interhemispheric inhibition and how the balance between excitation and inhibition is different in the different layers of the cortex. Identifying the cellular and molecular elements involved in interhemipsheric inhibition is crucial not only for understanding higher brain function and but also dysfunction in the diseased brain.