In both vertebrates and invertebrates, sleep-active neurons depolarize at sleep onset and, with their interplay with wake-promoting centers, guarantee fast and reliable switches between global brain states. However, how sleep homeostatic or circadian pressure influences their activity remains elusive1. While quiescent states in C. elegans are typically accompanied by downregulation of most neuronal activity2,3, the GABAergic and peptidergic interneuron RIS is both sleep-active and sleep-inducing4,5. Several evidences also point to a function of RIS more generally in sleep promotion2,3,6. These aspects represent discrete non-mutual exclusive hypotheses about RIS function, as either a final effector of the brain-state switch or a mediator of sleep pressure. The upregulation of RIS was first observed during lethargus sleep, but recent evidences show similar modulations occurring in different types of sleep, both in adulthood and during the larval stages3,7. Here, we first report about a previously uncharacterized type of spontaneous sleep in adult C. elegans. Then, through Ca2+ imaging of RIS in freely-moving animals, we draw a comparison between RIS dynamics seen during immobility periods in L4 lethargus and in adulthood. By using an arousing stimulation paradigm - O2 concentration shifts in worms with an
npr-1 mutant background - we compare basal RIS activity patterns with those following sleep-deprivation. With this approach, we aim to disentangle to what extent sleep pressure and sleep induction are represented in RIS activity. 1: H Bringmann (2018) Genetics 2: ALA Nichols et al (2017) Science 3: S Skora et al (2018) Cell Reports 4: M Turek et al (2013) Current Biology 5: M Turek et al (2016) Elife 6: J Spies & H Bringmann (2018) Scientific Reports 7: Y Wu et al (2018) Current Biology