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Journal of Biological Rhythms
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Effects of Photoperiod on Rat Motor Activity Rhythm at the Lower Limit of Entrainment

Trinitat Cambras

Departament de Fisiologia, Facultat de Farmacia, Universitat de Barcelona, cambras{at}ub.edu

Juan Chiesa

Departament de Fisiologia, Facultat de Farmacia, Universitat de Barcelona

John Araujo

Departamento de Fisiologia, Universidade federal do Rio Grande do Norte, Natal-RN, Brazil

Antoni Díez-Noguera

Departament de Fisiologia, Facultat de Farmacia, Universitat de Barcelona

The experiment described here studied the rat motor activity pattern as a function of the photoperiod of circadian light-dark cycles in the limits of entrainment (22-and 23-h periods). In most cases, the overt rhythm showed 2 circadian components: 1 that followed the external LD cycle and a 2nd rhythm that was free run. The expression of these components was directly dependent on the photoperiod, and there was a gradual transition in the manifestation of 1 or the other. The component with a period equal to that of the external cycle was more manifested under long photoperiods, while the other 1 was more expressed during short photoperiods. Also, the period of the free-running component was longer under T22 than T23. For each period, the free-running component was longer under a longer photoperiod. At first sight, the presence of these 2 components in most of the rats might appear to be due to the fact that in the limits of entrainment, some rats do not entrain and thus show a free-running rhythm plus masking. However, the gradation observed in the different patterns of the overt motor activity rhythm, especially those patterns related to the different balance between the 2 components and the length of the period of the free-running component under LD as a function of the photoperiod, suggests that the circadian system can be functionally dissociated.

Key Words: photoperiod • circadian rhythms • masking • entrainment

Journal of Biological Rhythms, Vol. 19, No. 3, 216-225 (2004)
DOI: 10.1177/0748730404264201
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