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 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 express LITE-1 and act as noxious photosensors and how they transmit this 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 also its optogenetic stimulation causes escape behavior, while its optogenetic inhibition reduced escape. 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 somewhat required for the acute response, yet is more important for maintaining responsiveness to repeated noxious stimuli. The source of NLP-10 in this context is largely AVG, yet also other cells contribute, possibly ASK. We uncover entry points of sensory information to neuronal circuits mediating noxious UV/blue light responses.