Internal desynchronization facilitates seizures

Summary Purpose:  The occurrence of spike‐wave discharges (SWDs) in WAG/Rij rats is modulated by the circadian timing system and is shaped by the presence of a light–dark cycle, motor activity, and state of vigilance. Here it is investigated whether the response to a phase shift is different between the SWDs and general motor activity rhythm. The process of reentrainment of both rhythms and its effect on number of absences was compared after a phase shift in the light–dark cycle, a condition known to induce internal desynchronization in the circadian timing system. Methods:  Chronic electroencephalographic and motor activity recordings were made in adult WAG/Rij rats, kept in the 12:12 h light–dark cycle. After four baseline days, rats were exposed to an 8‐h phase delay by shifting the light onset. Recordings were continuously made for another 10 consecutive days. Key Findings:  An immediate effect of the phase shift on both rhythms was observed: the acrophases were 7.5 h advanced. Next, they gradually returned to the baseline level, however, with a different speed. The more robust motor activity rhythm stabilizes first, whereas the weaker rhythm of SWDs adapted more slowly. The phase shift caused a prolonged aggravation of epileptic activity, observed mostly during the light phase. Significance:  Different speed and character of reentrainment suggests that the occurrence of seizures and motor activity are controlled by distinct circadian oscillators. The prolonged increase in absences after the phase shift has immediate practical consequences.