A global brain state underlies C. elegans sleep behavior

Neuronal basis of lethargy in worms How does the brain switch between wakefulness and sleep? Nichols et al. studied this question using brain-wide Ca2+ imaging at single-neuron resolution in nematodes. By changing O2 concentrations, they could rapidly switch the worms between behaviorally quiescent and active states. They observed a global quiescence brain state characterized by the systemic down-regulation of neuronal network dynamics. Signaling from O2 sensory neurons rapidly evoked and maintained active network dynamics. Conversely, in the absence of such arousing cues, network dynamics converged into the quiescent mode. Science, this issue p. eaam6851 Brain imaging in nematode mutants reveals how a brain switches between alert wakefulness and sleep. INTRODUCTION Global brain states such as sleep and wakefulness involve reconfigurations of neural circuit activity across the entire nervous system. Yet it is not understood how the brain can effectively switch between and maintain different states. Do dedicated brain centers control states via top-down mechanisms? And to what extent do self-organizing principles of neuronal networks play a role? To address these questions, it would be ideal to measure the contributions of all individual neurons to a global brain state. Unfortunately, this is currently not possible in mammals or other large organisms. Every animal thoroughly studied exhibits sleeplike behaviors, implying that sleep is an essential, primordial, and common function of neural networks. In mammals, sleep is defined at the physiological level by a characteristic electroencephalography (EEG) signal. Such a definition is missing for invertebrate models, which primarily rely on behavioral definitions. RATIONALE The nematode Caenorhabditis elegans is a tractable model organism with the potential to overcome these limitations: It has a stereotypic and mapped nervous system of only 302 neurons. Sleep is developmentally timed and occurs predominantly during lethargus periods of ~2 hours at the end of each larval stage. During wakefulness, the worm brain exhibits neuronal population dynamics that involve a large fraction (~40%) of neurons. These neuronal activities are highly coordinated across the neuronal population; that is, they share common activity patterns. This feature can be quantified with computational methods and visualized in low-dimensional brain state phase plots. The resulting brain state trajectory represents the action sequence of the animals. To control sensory-evoked switching between sleep and wakefulness, we established a behavioral genetics paradigm combined with controlled changes in oxygen (O2) concentration. This method, together with whole-brain imaging at single-cell resolution, enables us to observe brainwide neuronal activity dynamics during brain state transitions. RESULTS During lethargus, wild C. elegans strains prefer to sleep in social aggregates, and local O2 concentrations are a key underlying cue. In this study, we have shown that a neuropeptide receptor (NPR-1) expressed in a hub interneuron regulates information processing of the arousal cue. We could recapitulate these switches between sleep and wakefulness in immobilized animals while recording the activity of nearly all neurons in the brain via Ca2+ imaging. We found that sleep in C. elegans is a global brain state in which about 75% of neurons displaying activity during wakefulness become inactive. However, a few specific neurons retained activity during sleep, notably γ-aminobutyric acid–producing (GABAergic) and peptidergic head neurons such as the sleep-promoting interneuron RIS. Chemosensory circuits activated by atmospheric O2 rapidly evoked transitions to wakefulness by effectively activating neuronal population dynamics. In contrast, entries into sleep occurred spontaneously in the absence of arousing cues via convergence of neuronal activities toward the global quiescent state. Here, the sleep-active neurons retained stationary high activity. CONCLUSION Using computational analysis, we have shown that sleep is an emergent property of neuronal networks. When lethargus animals are in a favorable environment such as a social aggregate, sleep can evolve spontaneously in the absence of arousing cues; these, however, can rapidly reactivate dynamical brain activity. Our analysis reveals that neuronal networks feature properties of dynamic attractors during wakefulness, whereas during sleep these dynamics converge toward a fixed point. This attractor state mechanism could be a means to effectively switch between and maintain global brain states. Sleep is a global quiescence brain state. Social aggregates of worms create a preferred milieu of low oxygen. During the lethargus developmental stage, these conditions permit sleep. Fluorescence heat maps (rectangles) show that wakefulness is associated with brainwide activity, whereas during sleep most neurons are down-regulated. The brain state cycles on a low-dimensional trajectory [as displayed by computational analysis (phase plot)], which corresponds to the pictured action command sequence. At sleep onset, these dynamics converge toward a tangle representing a fixed-point attractor state. How the brain effectively switches between and maintains global states, such as sleep and wakefulness, is not yet understood. We used brainwide functional imaging at single-cell resolution to show that during the developmental stage of lethargus, the Caenorhabditis elegans brain is predisposed to global quiescence, characterized by systemic down-regulation of neuronal activity. Only a few specific neurons are exempt from this effect. In the absence of external arousing cues, this quiescent brain state arises by the convergence of neuronal activities toward a fixed-point attractor embedded in an otherwise dynamic neural state space. We observed efficient spontaneous and sensory-evoked exits from quiescence. Our data support the hypothesis that during global states such as sleep, neuronal networks are drawn to a baseline mode and can be effectively reactivated by signaling from arousing circuits.