Cortical layer 6b mediates state-dependent changes in brain activity and effects of orexin on waking and sleep

One of the most distinctive features of the mammalian cerebral cortex is its laminar structure. Of all cortical layers, layer 6b (L6b) is by far the least-studied, despite exhibiting direct sensitivity to orexin and having widespread connectivity, suggesting an important role in regulating cortical oscillations and brain state. We performed chronic electroencephalogram (EEG) recordings in mice in which a subset of L6b neurons was conditionally silenced, during undisturbed conditions, after sleep deprivation (SD), and after intracerebroventricular (ICV) administration of orexin. While the total amount of waking and sleep or the response to SD were not altered, L6b-'silenced' mice showed a slowing of theta-frequency (6-9 Hz) during wake and REM sleep, and a marked reduction of total EEG power, especially in NREM sleep. The infusion of orexin A increased wakefulness in both genotypes, but the effect was more pronounced in L6b-silenced mice, while the increase in theta-activity by orexin B was attenuated in L6b silenced animals. In summary, we show the role of cortical L6b in state-dependent brain oscillations and global vigilance state control, which could be mediated by orexinergic neurotransmission. Our findings provide new insights in the understanding of abnormal regulation of arousal states in neurodevelopmental and anxiety disorders.