In mammals, sleep is comprised of two distinct stages, rapid eye movement (REM) and non-REM (NREM) sleep. The regulation of the sleep-wake cycle involves many brains regions that influence the probability of each state. Since many brain regions contain a mixed population of wake-promoting and sleep-promoting neurons, it remains a challenge to dissect and interrogate the neural circuits of sleep-wake regulation.
In Chapter 2, I describe the mapping of neural circuits of the basal forebrain (BF) known to regulate wake and NREM sleep. The BF contains a spatially intermingled diverse population of neurons that plays key roles in multiple brain functions, including sleep-wake regulation, attention, and learning/memory. For sleep-wake regulation, the cholinergic, glutamatergic, and parvalbumin-expressing (PV) GABAergic neurons have been shown to promote wakefulness, whereas the somatostatin-expressing (SOM) GABAergic neurons promote NREM sleep. To better understand the functional specialization of these cell types, we performed whole-brain mapping of both inputs and outputs of these four BF cell types in the mouse brain. Using rabies virus-mediated monosynaptic retrograde tracing to label the inputs and adeno-associated virus to trace axonal projections, we identified numerous brain areas connected to the BF. The inputs to different cell types were qualitatively similar, but the output projections showed marked differences. The connections to glutamatergic and SOM+ neurons were strongly reciprocal, while those to cholinergic and PV+ neurons were more unidirectional. These results reveal the long-range wiring diagram of the BF circuit with highly convergent inputs and divergent outputs and point to both functional commonality and specialization of different BF cell types.
In Chapter 3, I describe the interrogation of the midbrain ventrolateral periaqueductal gray (vlPAG), known to be important for gating REM sleep. We demonstrated that activating vlPAG GABAergic neurons in mice suppressed the initiation and maintenance of REM sleep while consolidating NREM sleep, partly through their projection to the dorsolateral pons. Cell-type-specific recording and calcium imaging showed that most vlPAG GABAergic neurons are strongly suppressed at REM sleep onset and activated at its termination. In addition to the rapid changes at brain state transitions, their activity decreases gradually between REM sleep and is reset by each REM episode in a duration-dependent manner, mirroring the accumulation and dissipation of REM sleep pressure. Thus, vlPAG GABAergic neurons powerfully gate REM sleep, and their firing rate modulation may contribute to the ultradian rhythm of REM/NREM alternation.