SIMULTANEOUS MANIFESTATION OF FREE-RUNNING AND ENTRAINED RHYTHMS IN THE RAT MOTOR-ACTIVITY EXPLAINED BY A MULTIOSCILLATORY SYSTEM

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 d ays. 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 w ith periods shorter than 25 hours also presented a nonentrained circad ian component. Moreover, in all cases, the effect of masking was prese nt, manifested as a reactivity to the light-dark transitions and as a reduction of activity induced by light. Our results demonstrate that m asking, entrainment, and free-running rhythm can be present at the sam e time. The simultaneous presence of the light-entrained component and the nonentrained component can be explained in terms of a multioscill atory system, in which some oscillators could be entrained to the ligh t/dark cycles while other oscillators could be nonentrained at the sam e time. Thus, the present study indicates that the circadian system is not necessarily entirely entrained, and that the degree of entrainmen t may depend on the number of oscillators involved in generating the e ntrained rhythm.