Absence of extraocular photoreception in diurnal and nocturnal rodents exposed to direct sunlight

Abstract

1. 1. 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 out-of-doors and exposed to direct sunlight (average light intensity = 55,000 lx). 2. 2. Sighted animals entrained their activity rhythms to the natural photoperiod; the mice were nocturnal and the squirrels diurnal in their locomotor activity. 3. 3. 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 hr for squirrels and < 24 hr for grasshopper mice. 4. 4. Failure of entrainment in blind ground squirrels and grasshopper mice indicates that these species lack functional extraocular photoreceptors; they thus differ from non-mammalian vertebrates in which extraocular photoreception is well documented. 5. 5. The significance of exclusive reliance by mammals on ocular photoreceptors for entrainment of circadian rhythms is discussed.