Oscillatory population-level activity of dorsal raphe serotonergic neurons sculpts sleep structure

Dorsal raphe (DR) 5HT neurons are involved in regulating sleep/wake transitions. Previous studies demonstrated that single unit activity of DR 5HT neurons is high during wakefulness, decreases during non rapid eye movement (NREM) sleep, and ceases during rapid eye movement (REM) sleep. However, characteristics of the population level activity of DR 5HT neurons, which can influence the entire brain, are largely unknown. Here we measured population activities of 5 HT neurons in male and female mouse DR across the sleep/wake cycle by a ratiometric fiber photometry system. We found a slow oscillatory activity of compound intracellular Ca2+ signals during NREM sleep. The trough of concave 5HT activity increased along with sleep progression, but the 5HT activity level always returned to that seen in wake periods. When the trough reached the minimum level and remained there, REM sleep initiated. We also found a unique coupling of the oscillatory 5HT activity and EEG power fluctuation, suggesting that EEG fluctuation is a proxy for 5HT activity. Optogenetic activation of 5HT neurons during NREM sleep triggered a high EMG power and induced wakefulness. Optogenetic inhibition induced REM sleep or sustained NREM with an EEG power increase and EEG fluctuation. These manipulations demonstrated a causal role of DR 5HT neurons in sculpting sleep/wake structure. We also observed EEG fluctuations in human males during NREM sleep, implicating the existence of 5HT oscillatory activity in humans. We propose that NREM sleep is not a monotonous state, but that it is dynamically regulated by the oscillatory population activity of DR 5HT neurons.