Short-wavelength violet light (420nm) stimulates melanopsin-dependent acute alertness responses in zebrafish

Sunlight throughout the day and seasons strongly influences our biological rhythms and activity. In recent years, it has become evident that night-time overexposure to bright light in urban environments can profoundly affect physiology and behaviour in humans and animals. In particular, the artificial emission of short-wavelength light has been shown to stimulate alertness in humans, but the mechanisms remain largely unknown. Utilising a diurnal larval zebrafish model, we identified instant, non-image-forming (NIF) responses to short-wavelength violet light (~420nm), which are activated only during light exposure, and are reminiscent of alertness, including increased heart rate, enhanced locomotor activity, and pectoral fin beating (for increased oxygen supply). We further determined that these responses are driven by sympathetic neuronal circuits and depend on the zebrafish melanopsin homologue Opn4a. We also found that these responses can be modulated by the sleep-regulatory hormone melatonin, but that melatonin is not essential. Our findings reveal a previously unknown mechanism for violet light-dependent acute alertness.