ABSENCE OF EXTRA-OCULAR PHOTORECEPTION IN DIURNAL AND NOCTURNAL RODENTS EXPOSED TO DIRECT SUNLIGHT

Entrainment of circadian wheel-running activity was monitored over a period of several months for northern grasshopper mice (Onychomys leucogaster) and golden mantled ground squirrels (Spermophilus lateralis), housed outdoors and exposed to direct sunlight (average light intensity = 55,000 lx). Sighted animals entrained their activity rhythms to the natural photoperiod; the mice were nocturnal and the squirrels diurnal in their locomotor activity. The activity rhythms of all blind animals from each species failed to entrain to the light-dark cycle, and free-ran with endogenous periods > 24 h for squirrels and < 24 h for grasshopper mice. Failure of entrainment in blind ground squirrels and grasshopper mice indicates that these species lack functional extraocular photoreceptors; they, thus, differ from non-mammalian vertebrate in which extraocular photoreception is well documented. The significance of exclusive reliance by mammals on ocular photoreceptors for entrainment of circadian rhythms is discussed.