Melatonin regulates LIN-42/PERIOD through its receptor PCDR-1 to promote dark adaptation in Caenorhabditis elegans

Organisms that live in lightless spatial niches (e.g., caves or soil) develop a daily clock using other senses, such as taste, smell, temperature, or mechanical senses. However, how these organisms, which experience only muted daily environmental cycles, use a circadian clock to adapt to darkness is worth exploring. In this study, we found that lin-42 /PERIOD was highly expressed in worms grown in darkness. lin-42 knockdown induced a phenotype of accelerated aging in worms under dark conditions. Melatonin inhibited the production of reactive oxygen species through its receptor, PCDR-1, thus maintaining lin-42 expression in darkness. Conversely, daylight inhibited lin-42 expression by ROS production due to suppression of melatonin synthesis. Moreover, we found that insufficient melatonin synthesis or lin-42 deficiency activated the transcription factor DAF-16/FOXO, which induced lipid accumulation in the worms. Our results suggest that the molecular network composed of melatonin and the LIN-42/PERIOD enables C. elegans to adapt to darkness.