The effect of activating serotonergic neurons in the dorsal raphe nucleus on control of vigilance state

Based on results from genetic, neuropharmacological and electrophysiological studies, serotonin (5-HT) neurons in the dorsal raphe nucleus (DRN) apparently function predominantly to promote wakefulness and inhibit REM sleep. However, the effects of 5-HT on sleep architecture are complicated and not fully understood. The present study focused on the specific contribution of DRN serotonergic neurons to vigilance state switch. We used an optogenetic approach, which allowed us to manipulate the activity of cell type-specific neuronal populations with high temporal and spatial precision in freely moving animals. Channelrhodopsin-2 (ChR2 [C128S]) is a 470nm blue light-sensitive cation channel, which evokes action potentials in targeted cells under photostimulation. ChR2 was incorporated into central 5-HT neurons by breeding Tph2-tTA and tetO-ChR2 (C128S)/EYFP mice (Tanaka et al., Cell Rep. 2012, 397-406). Whole-cell patch clamp recordings from acute brain slices confirmed that photostimulation with blue light pulses increased the frequency of action potentials in ChR2-expressing 5-HT neurons. Optogenetic activation of 5-HT neurons in the DRN in vivo induced a transition from NREM (non-rapid eye movement) sleep to active wakefulness within seconds and the state was maintained throughout the photostimulation period. Finally, we activated 5-HT neurons in the medulla to eliminate the possibility that light propagating through the brain tissue could activate these medullary 5-HT neurons, but their activation did not affect the vigilance state. Our results suggest that DRN serotonergic neuronal activity plays an important role in the control of vigilance state.