A wake-active locomotion circuit depolarizes a sleep-active neuron to switch on sleep

Abstract

Sleep-active neurons depolarize during sleep to suppress wakefulness circuits. Wakeactive wake-promoting neurons in turn shut down sleep-active neurons, thus forming a bipartite flip-flop switch. However, how sleep is switched on is unclear because it is not known how wakefulness is translated into sleep-active neuron depolarization when the system is set to sleep. Using optogenetics in Caenorhabditis elegans, we solved the presynaptic circuit for depolarization of the sleep-active RIS neuron during developmentally regulated sleep, also known as lethargus. Surprisingly, we found that RIS activation requires neurons that have known roles in wakefulness and locomotion behavior. The RIM interneuronswhich are active during and can induce reverse locomotion-play a complex role and can act as inhibitors of RIS when they are strongly depolarized and as activators of RIS when they are modestly depolarized. The PVC command interneurons, which are known to promote forward locomotion during wakefulness, act as major activators of RIS. The properties of these locomotion neurons are modulated during lethargus. The RIMs become less excitable. The PVCs become resistant to inhibition and have an increased capacity to activate RIS. Separate activation of neither the PVCs nor the RIMs appears to be sufficient for sleep induction; instead, our data suggest that they act in concert to activate RIS. Forward and reverse circuit activity is normally mutually exclusive. Our data suggest that RIS may be activated at the transition between forward and reverse locomotion states, perhaps when both forward (PVC) and reverse (including RIM) circuit activity overlap. While RIS is not strongly activated outside of lethargus, altered activity of the locomotion interneurons during lethargus favors strong RIS activation and thus sleep. The control of sleep-active neurons by locomotion circuits suggests that sleep control may have evolved from locomotion control. The flip-flop sleep switch in C. elegans thus requires an additional component, wake-active sleep-promoting neurons that translate wakefulness into the depolarization of a sleep-active neuron when the worm is sleepy. Wake-active sleep-promoting circuits may also be required for sleep state switching in other animals, including in mammals.

Behavioral States

“…Mechanistic details of RIS activation are beginning to emerge. RIS, like ALA (see below), responds to the epidermal growth factor EGF (Konietzka et al 2020), but is also activated by excitatory input from several other neurons (Maluck et al 2020). These include the forward command interneuron PVC, suggesting a mechanism by which locomotion circuit activity during wake behavior can influence sleep (Maluck et al 2020).…”

“…Activation of the DAF-7 receptor DAF-1 in the tyraminergic RIM neuron and the octopaminergic RIC neuron promotes satiety quiescence (Gallagher et al 2013). How DAF-1 signaling in RIM and RIC promotes behavioral quiescence remains unclear, though it presumably involves RIS activation (Wu et al 2018;Maluck et al 2020).…”

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

“…We compared activity of several neurons during sleep and awake states of unrestrained animals. The RIS interneuron is active at the onset of spontaneous adult sleep, as has been shown during developmentally-timed lethargus sleep ( Turek et al, 2013 ; Maluck et al, 2020 ). The automated closed-loop stimulation system, which requires no user input, further allows unbiased comparison of stimulus-evoked neural responses during alternating sleep and awake bouts within the same animal.…”

“…RIS interneuron activity correlates with the onset of developmentally-timed sleep ( Maluck et al, 2020 ; Turek et al, 2013 ) and quiescent behavior in adults ( Steuer Costa et al, 2019 ). To demonstrate neural imaging during spontaneous sleep-wake cycles in the microfluidic device, we recorded activity in the RIS interneuron expressing GCaMP3 ( Fig.…”

Antimicrobial peptides signal need for sleep from peripheral wounds to the nervous system

“…FLP and PVC neurons synapse onto RIS [57], and, intriguingly, the PVC neurons have recently been shown to be key activators of RIS [58]. Single-cell sequencing showed that NPR-12 is expressed also in RIM neurons [59], which are also involved in RIS activation [58]. However, NPR-12 is not expressed in RIS itself [55].…”