Entrainment of the rat motor activity rhythm: effects of the light-dark cycle and physical exercise.

The circadian system is believed to be composed of a population of oscillators that couple together and generate a single rhythm. If this coupling is not strong enough, the circadian system can be dissociated into two or more groups of oscillators, and this is manifested in a dissociation of the overt rhythm into at least two circadian components. This study aims to examine the influence of factors, such as the difference in impact between T and tau, light intensity, and access to a running wheel, on the distribution of motor activity throughout the light-dark (LD) cycle and the dissociation of the rhythm. Rats were submitted to LD cycles of 23 h (T23) or 25 h. For each such cycle, half the rats were submitted to high light intensity and the other half to low light intensity. For each of these conditions, half the rats were kept in small cages, and the other half were in cages with a running wheel. Rats were maintained first under LD cycles and afterwards under constant darkness (DD). Motor activity was recorded throughout the whole experiment by means of activity meters with infrared beams. Results show that the distribution of motor activity throughout the cycle and the after effects observed in the rhythm under DD depended on light intensity and access to the wheel. Moreover, under T23, some rats showed two simultaneous circadian components whose manifestation also depended on the experimental conditions. The results indicate that the strength of circadian entrainment to LD cycles in the rat depends on three factors: the period length of the LD cycle, light intensity used during the light phase, and access to a running wheel.