Regulation of REM Sleep Onset and Homeostasis by Preoptic Glutamatergic Neurons

Abstract

The preoptic area of the hypothalamus is key for the control of sleep onset and sleep homeostasis. Although traditionally considered exclusively somnogenic, recent studies identified a group of preoptic glutamatergic neurons that promote wakefulness. Specifically, our previous investigations demonstrated that chemogenetic stimulation of glutamatergic neurons within the medial-lateral preoptic area (MLPO_VGLUT2) promotes wakefulness, fragments non-rapid eye movement sleep (NREMs), and suppresses REM sleep (REMs). This evidence is further supported by recent work showing that preoptic glutamatergic neurons are activated during microarousals that fragment sleep in response to stress, and optogenetic stimulation of these neurons promotes microarousals and wakefulness. Thus, while the wake-promoting function of MLPO_VGLUT2 is clear, their role in sleep homeostasis has not been assessed. We tested the hypothesis that MLPO_VGLUT2 are wake-active, and their activation will increase wakefulness and disrupt sleep homeostasis via projections to arousal-promoting systems. Using fiber photometry, we found that MLPO_VGLUT2 were highly active during REMs, wakefulness and brief arousals, and remained minimally active during NREMs. Chemogenetic stimulation of MLPO_VGLUT2 inhibited REMs onset and suppressed the REMs homeostatic response after total sleep deprivation. Chemogenetic inhibition of MLPO_VGLUT2 increased REMs time (during the light phase only) but did not influence REMs and NREMs homeostasis. Anterograde projection mapping revealed that MLPO_VGLUT2 innervate central regions that promote wakefulness and inhibit REMs. We conclude that MLPO_VGLUT2 powerfully suppress REMs and that exogenous --and possibly pathologic-- activation of these neurons disrupts REMs recovery, presumably by directly or indirectly activating REMs-inhibitory mechanisms. SIGNIFICANCE STATEMENTThe preoptic area of the hypothalamus has been extensively studied and its role in sleep regulation is well-established. Importantly, recent work identified a group of preoptic glutamatergic neurons (MLPO_VGLUT2) that are wake-active and promote wakefulness. However, whether these neurons influence sleep homeostasis remains unknown. We demonstrate that MLPO_VGLUT2 are maximally active during REM sleep (REMs), wakefulness and brief arousals from sleep, and innervate wake-promoting and REMs-inhibitory regions. MLPO_VGLUT2 stimulation inhibits REMs and REMs rebound after sleep deprivation, whereas their inactivation increases REMs but does not alter REMs homeostatic response. We thus identified a preoptic mechanism that powerfully suppresses REMs, which we propose may engage during normal sleep-to-wake transitions to block REMs intrusions into subsequent wakefulness.