title: Effects of photoperiod on rat motor activity rhythm at the lower limit of entrainment.
abstract: 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.
authors.affiliation: Departament de Fisiologia, Facultat de Farmacia, Universitat de Barcelona, Spain. cambras@ub.edu
authors.forename: Trinitat
authors.initials: T
authors.lastname: Cambras
authors.forename: Juan
authors.initials: J
authors.lastname: Chiesa
authors.forename: John
authors.initials: J
authors.lastname: Araujo
authors.forename: Antoni
authors.initials: A
authors.lastname: Díez-Noguera
mesh_terms: D000818:Animals; D001683:Biological Clocks / Q000502:physiology; D002940:Circadian Rhythm / Q000502:physiology*; D003624:Darkness; D005260:Female; D008027:Light; D008297:Male; D009043:Motor Activity / Q000502:physiology*; D017440:Photoperiod; D051381:Rats; D017208:Rats, Wistar