Circadian Locomotor Rhythms Research Articles

Circadian Locomotor Rhythms of Rodents in the Arctic

In both the arctic and subarctic regions of Alaska during the spring and summer pronounced daily periodicity was observed in the field and running-wheel activity of the following diurnal and "nocturnal" rodents: Spermophilus undulatus, Clethrionomys rutilus, Lemmus trimucronatus, Microtus miurus, and M. oeconomus. Animals whose phases were shifted to abnormal times of the day by artificial light cycles regained their normal phases when exposed to the local field conditions. However, difficulty in retaining proper phase was observed in several of these nocturnal species at the time of the summer solstice, i.e., at the time when the amplitude of the daily cycle of illumination is lowest. The usually nocturnal L. trimucronatus centered their activity about noon at this time of year. Temperate zone rodents (Ammospermophilus leucurus and Peromyscus leucopus) taken to Point Barrow, Alaska, also showed pronounced daily periodicity under arctic conditions, although the latter species did not show locomotor activity until August, when the sun moved below the horizon near midnight. All of the species studied from both the arctic and temperate zones were found to have endogenous circadian rhythms of activity. Laboratory experiments showed that those rhythms could be entrained by square wave and approximately sinusoidal cycles of illumination even when the amplitude of the entraining light was as small as that encountered at Point Barrow at the summer solstice (max:min = 23:1). However, individuals differed in their ability to entrain successfully to some light cycles. These experiments and field observations support the conclusion that in the arctic, even during the summer, the environmental light cycle is the principal entraining agent of the endogenous rhythm. When the amplitude of the light cycle was insufficient to entrain an animal "oscillatory free-runs" were frequently observed, i.e., as an organism's rhythm free-ran across the light cycle its period changed systematically. The functional significance of a pronounced circadian locomotor rhythmicity in the arctic may be attributed to two factors: (1) its usefulness during that restricted part of the year when light and dark occur every day, and (2) its historical derivation from temperate zone ancestors, its ability to entrain to low amplitude light cycles being a part of that inheritance.

Entrainment of circadian rhythms by sound in Passer domesticus.

The circadian locomotor rhythm of house sparrows was entrained by a sound stimulus. The birds were maintained at a constant temperature in, dim green light. The entraining agent was 4 (1/2) 12 hours of tape-recorded bird song ,played each day. Variations in the response to this stimulus have been correlated with individual variations in free-running period. This is the first clear demonstration that a biological clock can be influenced by sound stimuli.

Photoreception and Entrainment of Cockroach Activity Rhythms

In the cockroach there are two distinct sets of photoreceptors-the compound eyes and the ocelli-which may function as a sensory input whereby circadian locomotor rhythms are entrained by environmental light cycles. Surgical removal of the ocelli did not interfere with normal entrainment, but covering over the compound eyes effected a loss of entrainment.