The frequency of the behavioral response in Paramecium tetraurelia displays an ultradian rhythm: a regulatory role for the inositol signaling pathway

Paramecium tetraurelia displays a behavioral response to environmental stimuli that involves changes in the direction and speed of swimming. This paper demonstrates that there are periodic variations in the frequency of behavioral responses of individual cells. The frequency of the backward swimming response undergoes repeated oscillations with a period (tau) between 40 and 50 minutes; these oscillations are only slightly affected by the temperature at which the cells have been grown. These data indicate the presence of an ultradian clock that underlies this cellular process. However, growing cells in the presence of low concentrations of LiCl perturbs the ultradian rhythm, with the periodicity being reduced to approximately 11 minutes. The LiCl effect is seen within 10-45 minutes, depending on the concentration of LiCl used, and is completely reversible. In order to determine if the LiCl effect was due to a depletion of inositol, cells were grown in 10mM myo-inositol for 24 hours prior to the addition of LiCl. It was found that the presence of excess inositol inhibited the perturbation of the periodicity caused by the LiCl. These results signify that Paramecium tetraurelia possesses an endogenous ultradian clock that influences the strength of the behavioral response, and the inositol signaling pathway is most likely involved in the generation of the ultradian rhythm. Therefore, the Paramecium behavioral response is an excellent model system for elucidation of the molecular mechanisms underlying the ultradian rhythms.