Sleep Analysis in AdultC. elegansReveals State-Dependent Alteration of Neural and Behavioral Responses

Abstract

Sleep, a state of quiescence associated with growth and restorative processes, is conserved across species. Invertebrates including the nematode Caenorhabditis elegans exhibit sleep-like states during development, satiety, and stress. Here, we describe behavior and neural activity during sleep and awake states in adult C. elegans hermaphrodites using new microfluidic methods. We observed effects of fluid flow, oxygen, feeding, odors, and genetic perturbations on long-term sleep behavior over 12 h. We developed a closed-loop sleep detection system to automatically deliver chemical stimuli to assess sleep-dependent changes to evoked neural responses in individual animals. Sleep increased the arousal threshold to aversive stimulation, yet the associated sensory neuron and first-layer interneuron responses were unchanged. This localizes adult sleep-dependent neuromodulation within interneurons presynaptic to the premotor interneurons, rather than afferent sensory circuits. However, sleep prolonged responses in appetitive chemosensory neurons, suggesting that sleep modulates responsiveness specifically across sensory systems rather than broadly damping global circuit activity. SIGNIFICANCE STATEMENT Much is known about molecular mechanisms that facilitate sleep control. However, it is unclear how these pathways modulate neural circuit-level sensory processing or how misregulation of neural activity contributes to sleep disorders. The nematode Caenorhabditis elegans provides the ability to study neural circuitry with single-neuron resolution, and recent studies examined sleep states between developmental stages and when stressed. Here, we examine an additional form of spontaneous sleep in adult C. elegans at the behavioral and neural activity levels. Using a closed-loop system, we show that delayed behavioral responses to aversive chemical stimulation during sleep arise from sleep-dependent sensorimotor modulation localized presynaptic to the premotor circuit, rather than early sensory circuits.

Phenotyping single-cell motility in microfluidic confinement

“…Microorganisms inhabiting different ecological niches likely evolved divergent strategies for sensorimotor integration, despite conserved cilia-based motility. For example, CR swims according to a more continuous repertoire, whereas PO motility was episodic or excitable, fluctuating stochastically between fast swimming and behavioural quiescence, reminiscent of transitions between ‘wakeful’ and ‘sleep states’ observed in many animals [46]. Future work should seek to map the shape space of permissible ciliary waveforms [35] and any multiciliary actuation patterns [6], to the resulting species-specific motility behaviours.…”

Sleep is required for odor exposure to consolidate memory and remodel olfactory synapses

“…For a similar understanding of sleep architecture in C. elegans , it will be useful to observe both muscle and neuronal activity in animals that show the hallmarks of sleep. Lawler and colleagues have begun these studies and verified that sleeping worms show a more flaccid posture, though they twitch the anterior portions of their body during bouts of spontaneous sleep (Lawler et al ., 2021). Detailed analysis of calcium currents in post-training sleep may allow us to better understand sleep architecture in memory consolidating animals.…”

“…Indeed, synaptic transmission at GABAergic neuromuscular junctions decreased in animals sleeping during developmental lethargus, but UNC-49 GABA receptor immunostaining was not reduced (Dabbish and Raizen, 2011). Studies of unrestrained, sleeping animals showed that the neural dynamics of the AVA backward command neuron is severely blunted, while AWA, an appetitive sensory neuron that has an ON response, rather than an OFF response like AWCs, has a prolonged response to odor when sleeping (Lawler et al, 2021). A similar analysis of the AWC sensory circuit during post-training sleep is required to understand the neural dynamics in the sleeping worm, so that the relationship between neuronal activity and synapse size can be determined.…”

Monitoring neural activity during sleep/wake events in adult C. elegans by automated sleep detection and stimulation

“…This closed-loop system is flexible and can be configured to detect any visible events, such as behavior patterns or optical reporters, and measure corresponding evoked neural responses. For complete details on the use and execution of this protocol, please refer to Lawler et al. (2021) .…”

“…For complete details on the use and execution of this protocol, please refer to Lawler et al. (2021) .…”