External light activates hair follicle stem cells through eyes via an ipRGC–SCN–sympathetic neural pathway

Significance Intrinsically photosensitive retinal ganglion cells (ipRGCs) exhibit several important functions including the circadian photo entrainment, pupillary light reflex, alertness, and phototaxis. Whether ipRGCs regulate other physiological activities is unknown. We show that external light stimulation can activate hair follicle stem cells through the eyes via an ipRGC–suprachiasmatic nucleus–sympathetic nervous circuit. Immediately after ipRGCs are stimulated by light, the systemic sympathetic activities are activated. In skin, the local release of norepinephrine activates hair follicle stem cells. This neural circuit enables prompt communication between peripheral tissues and the external environment. Due to the systemic activation of sympathetic activities, this circuit can also allow for timely responses to external light in other organs. It also highlights a function of ipRGCs in regulating autonomic nervous activity.