The frequency of the spontaneous behavioral response in Paramecium tetraurelia is simultaneously modulated by both ultradian and circadian rhythms

The behavioral response of Paramecium tetraurelia is due to changes in the direction of swimming, which is initiated by the generation of an action potential that causes the re-orientation of the ciliary beat. The frequency of spontaneous behavioral responses displays an ultradian rhythm, with a periodicity of approximately 50 min. Furthermore, this rhythm is disrupted by the addition of LiCl, due to the interruption of the inositol signaling pathway. The present work is an observation of the frequency of the behavioral response for a 72-h period. The frequency of the behavioral response is influenced by a circadian clock, in that the frequency of behavioral responses changes during a 24-h period, while the periodicity of the ultradian clock remains unchanged. The addition of LiCl also disrupts the circadian rhythm, lengthening the circadian period from 24 h to 27.4 h. However, unlike the ultradian rhythm, where the addition of myo-inositol inhibits the LiCl effect, myo-inositol has no effect on the disruption of the circadian rhythm. In order to ascertain the effect of LiCl on the circadian clock, cells were grown in the presence of indirubin, a compound known to inhibit glycogen synthase kinase 3 (GSK3); this enzyme is also inhibited by LiCl. When cells are incubated with indirubin for several hours, the circadian rhythm is lengthened to approximately 27 h, while the ultradian rhythm is unaffected. Furthermore, a decrease in GSK3 by RNA interference gives similar results. Therefore, the frequency of the behavioral response in Paramecium appears to be controlled by two different biological rhythms.