Neurons and molecules involved in noxious light sensation in Caenorhabditis elegans
Ultraviolet (UV) light is dangerous to unpigmented organisms, inducing photodamage of cells and DNA. The transparent nematode Caenorhabditis elegans, despite having no eyes, detects light and exhibits negative phototaxis in order to evade sunlight. UV absorption is detected by the photosensor protein LITE-1, that also responds to reactive oxygen species. We investigated which neurons act as noxious photosensors and how they transmit the sensation to the nervous system to evoke escape behavior. We identified the interneuron AVG as a main focus of LITE-1 function in mediating the noxious light evoked escape behavior, with minor roles of the interneuron PVT, the sensory ASK neurons and touch receptor neurons. AVG is activated by blue light, and its optogenetic stimulation causes escape behavior. Signaling from AVG involves chemical neurotransmission, likely directly to premotor interneurons, and to other cells, by extrasynaptic signaling through the neuropeptide NLP-10. NLP-10 signaling is not required for the acute response, but for maintaining responsiveness to repeated noxious stimuli. The source of NLP-10 is largely AVG, however, also other cells contribute, possibly PVT, expressing both LITE-1 and NLP-10. This work uncovers entry points of sensory information to the neuronal circuits mediating the behavioral response to noxious UV/blue light.