Gap Junctions and NCA Cation Channels Are Critical for Developmentally Timed Sleep and Arousal in Caenorhabditis elegans

An essential characteristic of sleep is heightened arousalthreshold, with decreased behavioral response to external stimuli. The molecular and cellular mechanisms underlying arousalthreshold changes during sleep are not fully understood. We report that loss of UNC-7 or UNC-9 innexinfunction dramatically reduced sleep and decreased arousalthreshold during developmentally timed sleep in Caenorhabditis elegans . UNC-7 function was required in premotor interneurons and UNC-9 function was required in motor neurons in this paradigm. Simultaneous transient overexpression of UNC-7 and UNC-9 was sufficient to induce anachronistic sleep in adult animals. Moreover, loss of UNC-7 or UNC-9 suppressed the increased sleep of EGL-4 gain-of-function animals, which have increased cyclic-GMP–dependent protein kinase activity. These results suggest C. elegans gap junctionsmay act downstream of previously identified sleep regulators. In other paradigms, the NCA cation channels act upstream of gap junctions. Consistent with this, diminished NCA channel activity in C. elegans robustly increased arousalthresholds during sleep bouts in L4-to-adult developmentally timed sleep. Total time in sleep bouts was only modestly increased in animals lacking NCA channel auxiliary subunit UNC-79, whereas increased channel activity dramatically decreased sleep. Lossof EGL-4 or innexinproteins suppressed UNC-79 loss-of-function sleep and arousaldefects. In Drosophila , the ion channel narrow abdomen, an ortholog of the C. elegans NCA channels, drive the pigment dispersing factor(PDF) neuropeptide release, regulating circadian behavior. However, in C. elegans , we found that loss of the PDF receptor PDFR-1 did not suppress gain-of-function sleep defects, suggesting an alternative downstream pathway. This study emphasizes the conservation and importance of neuronal activity modulation during sleep, and unequivocally demonstrates that gap junctionfunction is critical for normal sleep.