Simultaneous manifestation of free-running and entrained rhythms in the rat motor activity explained by a...

Seven groups of 6 young male rats were exposed for 60 calendar days to symmetrical light/dark cycles with different periods (T = 22, 23, 24, 25, 26, 27, and 28 h), and subsequently to constant darkness for 30 days. During exposure to the light/dark cycles, all the animals in all groups presented a motor activity component that was entrained to the external cycles, although the animals subjected to light/dark cycles with periods shorter than 25 hours also presented a nonentrained circadian component. Moreover, in all cases, the effect of masking was present, manifested as a reactivity to the light-dark transitions and as a reduction of activity induced by light. Our results demonstrate that masking, entrainment, and free-running rhythm can be present at the same time. The simultaneous presence of the light-entrained component and the nonentrained component can be explained in terms of a multioscillatory system, in which some oscillators could be entrained to the light/dark cycles while other oscillators could be nonentrained at the same time. Thus, the present study indicates that the circadian system is not necessarily entirely entrained, and that the degree of entrainment may depend on the number of oscillators involved in generating the entrained rhythm.