Biological rhythms are synchronized to a 24 h day mostly by the presence of the light-dark cycle. Sudden changes in the timing of the light phase are known to cause disturbances in circadian physiology manifested as internal desynchronization and/or the jet lag syndrome, fatigue and impaired performance after rapid changes of time zones. The occurrence of absence seizures, characterized by reduced consciousness and the presence of 3 - 4 Hz (humans) and 7 - 11 Hz (rats) spikewave discharges (SWD) in EEG, shows daily rhythmic fluctuations. The rhythm of SWD in WAG/Rij rats, a validated, genetic model of absence epilepsy, is generated endogenously by the circadian timing system and shaped by the light-dark cycle, motor activity and a momentary state of vigilance. In constant dim light condition, the rhythmicity in the occurrence of SWD is still present, however, internal desynchronization from the rhythm of motor activity is observed (Smyk et al. 2011). The main goals of the study were to verify the role of the light-dark cycle in the synchronization of the rhythm of SWD in WAG/Rij rats and to deliver evidence for the presence of distinct mechanisms controlling rhythms of SWD and motor activity. Chronic EEG and motor activity recordings were made in adult WAG/Rij rats kept in 12:12 light-dark cycle. After 4 baseline days, the onset of light was delayed by 8 h and the recordings were made during 10 consecutive days. An immediate effect of the phase shift on both rhythms was observed. On the 1st post-shift day the acrophase of the rhythms was advanced of about 7.5 h. After that, it gradually returned to the baseline level, however, the course of the resynchronization of the two rhythms was different. In conclusion, an important role of the light-dark cycle in the resynchronization of the rhythm of absence seizures has been confirmed. The phase shift caused an aggravation of epileptic activity and uncoupling between rhythmic occurrence of SWD and motor activity.
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