Phototransduction by Retinal Ganglion Cells That Set the Circadian Clock

TLDR

Light synchronizes mammalian circadian rhythms by modulating retinal input to the SCN, and this entrainment does not require rods or cones. The study demonstrates that retinal ganglion cells projecting to the SCN are intrinsically photosensitive. These intrinsically photosensitive ganglion cells depolarize to light even without rod/cone input, and their response properties match the photic entrainment mechanism, indicating they are likely the primary photoreceptors for circadian entrainment.

Abstract

Light synchronizes mammalian circadian rhythms with environmental time by modulating retinal input to the circadian pacemaker—the suprachiasmatic nucleus (SCN) of the hypothalamus. Such photic entrainment requires neither rods nor cones, the only known retinal photoreceptors. Here, we show that retinal ganglion cells innervating the SCN are intrinsically photosensitive. Unlike other ganglion cells, they depolarized in response to light even when all synaptic input from rods and cones was blocked. The sensitivity, spectral tuning, and slow kinetics of this light response matched those of the photic entrainment mechanism, suggesting that these ganglion cells may be the primary photoreceptors for this system.