Shedding Light on Brain Function and Mood: A Role for the Retinoraphe Pathway in Regulating Serotonin

Light has the capacity for immediate and long-term modulation of complex neuronal systems and brain function. While correct timing of light exposure is essential for optimal physical and mental performance, insufficient light or light at the wrong time has been associated with mood-related detriments. In humans, affective disorders have been linked to a dysfunction of the serotonergic system, but the neuronal correlates linking mood and serotonin with effects of light remain elusive. In this review, we discuss the potential importance of the retinoraphe pathway, a direct neuronal connection between the retina and the dorsal raphe, the largest serotonergic nucleus in the brainstem. We discuss current knowledge on the cellular composition, innervation patterns, working mechanisms, and functional outputs of the retinoraphe system across different animal species. While this connection has not been extensively investigated in humans, indirect evidence suggests that the retinoraphe pathway may be at the center of the mechanistic wiring that mediates the effects of light on physiology and mood.