Layer 6b Is Driven by Intracortical Long-Range Projection Neurons

Abstract

Summary: Layer 6b (L6b), the deepest neocortical layer, projects to cortical targets and higher-order thalamus and is the only layer responsive to the wake-promoting neuropeptide orexin/hypocretin. These characteristics suggest that L6b can strongly modulate brain state, but projections to L6b and their influence remain unknown. Here, we examine the inputs to L6b ex vivo in the mouse primary somatosensory cortex with rabies-based retrograde tracing and channelrhodopsin-assisted circuit mapping in brain slices. We find that L6b receives its strongest excitatory input from intracortical long-range projection neurons, including those in the contralateral hemisphere. In contrast, local intracortical input and thalamocortical input were significantly weaker. Moreover, our data suggest that L6b receives far less thalamocortical input than other cortical layers. L6b was most strongly inhibited by PV and SST interneurons. This study shows that L6b integrates long-range intracortical information and is not part of the traditional thalamocortical loop. : Zolnik et al. find that layer 6b, the deepest and least-studied neocortical layer, is an input node for cortical long-range intercommunication. Mapping the input connections of L6b reveals that this layer receives significantly stronger input from long-range intracortical neurons compared with other types of neurons, such as thalamic neurons. Keywords: layer 6, layer 6b, cortex, thalamus, long-range